CATALYTIC SELECTIVITY IN ALCOHOL ORGANOSOLV PULPING
OF SPRUCE WOOD
By
DOMTNGGUS YAWALATA
In, Pattimura University, Ambon - Indonesia, 1987 M.Sc., The University of British Columbia, 1996
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
in
THE FACULTY OF GRADUATE STUDIES
(Faculty of Forestry)
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
February 2001
© Dominggus Yawalata, 2001 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of 7&#&7~/Qy
The University of British Columbia Vancouver, Canada
Date M#*/3 c$GO/
DE-6 (2/88) ABSTRACT
Development of a more efficient and environmentally friendly pulping process is seen as a necessity to cope with problems currently faced by pulp and paper industries.
The organosolv pulping process seems to be the right choice to deal with the problems.
Although many organosolv pulping process variants have been developed and are at various stages of commercialization, most processes are incapable or struggle with pulping softwoods. Furthermore, employment of a catalyst in organosolv pulping liquor appears to be essential for softwood pulping. However, systematic studies of catalyst effectiveness, especially for softwood pulping by organosolv means, are lacking.
Therefore, the objectives of this research were to investigate the effect of catalysts on pulping behavior, selectivity, pectin removal and delignification leading to fiber separation in a catalysed alcohol organosolv pulping process.
Under the specified pulping conditions, it was found that not all catalysts used in the organosolv pulping of softwood were able to liberate the fibers. The pulping outcomes were different, depending on the catalyst species. Different species of cations and anions had an impact on cooking liquor pH that subsequently controlled the pulping process and its outcomes. Generally, the pH of the cooking liquor controlled the pulping selectivity and fiber separation. Furthermore, pulping selectivity and topochemistry, and not necessarily delignification alone, control both fiber liberation and fiber quality. For high fiber yield and quality, control of the final pH between 3.8 and 4.2 is a necessity. In some cases, the use of two different catalysts in the cooking liquor showed a synergistic
ii effect on the improvement of fiber liberation.
It is concluded that neither mono- nor trivalent acidic inorganic salts, nor simple organic acids in themselves, except citric acid, can be used as catalysts for effective fiber liberation in organosolv pulping, due largely to the uncontrolled drop of pH below 3.5, at which point delignification becomes significantly retarded. The mystery of the unique neutral alkali earth metal (NAEM) salt catalyst effectiveness is now fully explained and described. Investigation of NAEM catalysed pulps showed total loss of arabinose and galactose from among the carbohydrates of the pulp as if their presence controlled the process of fiber liberation.
Early fiber liberation resulting in high pulp yield and viscosity can only be accomplished with a cooking liquor possessing high chemical and topochemical selectivity for lignin. In this respect, effective removal of the middle lamella compounds is seen as the key factor for fiber liberation. On the other hand, the removal of middle lamella compounds is found to be an antagonistic process, in which the removal of esterified pectin preferred less acidic pH due to the P-elimination mechanism while more acidic cooking liquor (pH <4.5) was required to enhance and accelerate delignification. At low pH (<3.8), however, carbohydrate degradation and removal are also enhanced due to hydrolysis. The enhancement of carbohydrate degradation and loss grows with increasing cooking liquor acidity, while delignification decreases or even stops. The removal of middle lamella compounds, to liberate the fibers, basically followed 2 different pathways:
1) the removal of pectin and some hemicelluloses, followed by delignification, as in the neutral alkali earth metal salt catalysed process, and 2) the removal of hemicelluloses,
iii followed by delignification and pectin removal, as in the citric acid catalysis. It was also found that in all organosolv cooks, fibers can not be completely separated without removing all arabinose and galactose.
Extending the cooking beyond the fiber liberation point provides no benefit at all, because the extent of additional delignification is small and is attained at the expense of carbohydrate degradation and loss, and lignin re-precipitation. Therefore, highest yield is obtained if delignification is halted at the fiber liberation point. Unexpectedly, the effectiveness of citric acid as a catalyst was found to be comparable to that of the NAEM salt catalysed organosolv pulping process. In both NAEM salt and citric acid catalysed organosolv pulping with 80 % methanol, the loss of hemicelluloses comprising xylose and mannose increased when the degree of delignification exceeded 60 %.
The mechanism of fiber liberation in the NAEM salt catalysed organosolv pulping process can now be described as "versatile" being able to affect both initial removal of the pectin and hemicelluloses (arabinogalactan) followed by bulk and residual delignification.
NAEM salt catalysts therefore play a double role : 1) effectively promote the initial pectin removal at pH >5, and 2) effectively buffer the cooking liquor against uncontrolled pH drops as more and more protons are liberated due to ion exchange with carboxyls and liberation of acetyls. The role of pectin dissolution in controlling effective fiber liberation in organosolv pulping of softwood is described for the first time. The topochemistry of pectin removal from spruce wood was followed by immunolabelling followed by fluorescent microscopy of thin cross-sections.
iv TABLE OF CONTENTS
ABSTRACT ii
TABLE OF CONTENTS v
LIST OF TABLES viii
LIST OF FIGURES xi
LIST OF ABBREVIATIONS xv
ACKNOWLEDGEMENT xvi
1. INTRODUCTION 1 1.1. Background 1 1.2. Objectives 4 1.3. Hypotheses. 5 1.4. Outline of Experiment 6
2. LITERATURE REVIEW 9 2.1. Chemical Constituents of Wood 9 2.1.1. Lignin 10 2.1.2. Cellulose 12 2.1.3. Hemicelluloses 13 2.1.4. Extractives 14 2.1.5. Pectic substances 16 2.1.6. Distribution of the main chemical constituents in the cell wall... 18 2.1.7. Carboxyl group in wood 19 2.2. Strength of Acidic Protons 24 2.3. Organosolv Pulping 26 2.3.1. Alcohol-based solvent pulping . . 27 2.3.1.1. Acidic alcohol-based solvent pulping 27 2.3.1.1.1. ALCELL process 32 2.3.1.1.2. NAEM process 36 2.3.1.2. Alkaline alcohol-based solvent pulping 39 2.3.1.2.1. ASAM process 40 2.3.1.2.2. ORGANOCELL™ process 43 2.3.2. Organic acid and ester pulping 46 2.3.3. Other solvent pulping processes 50
v 2.4. Factors Affecting Delignification 51 2.4.1. Effect of type of lignin and linkages 51 2.4.2. Effect of solvent 54 2.4.3. Proton catalysis 56 2.5. Topochemistry of Delignification 61 2.6. Pectin Degradation and Dissolution 64
3. MATERIALS AND METHODS 67 3.1. Raw Material 67 3.2. Methods 67 3.2.1. Pulping 67 3.2.1.1. Kraft pulping 67 3.2.1.2. Organosolv pulping 69 3.2.1.2.1. Pulping conditions and procedures 69 3.2.1.2.2. Pulp washing 70 3.2.1.2.3. Pulp screening 70 3.2.2. Chemical analyses on the raw material and/or pulp 70 3.2.2.1. Extractives 70 3.2.2.2. Klason lignin (acid-insoluble lignin) 72 3.2.2.3. Acid-soluble lignin 73 3.2.2.4. Sugar analysis 74 3.2.2.4.1. Sample preparation 74 3.2.2.4.2. HPLC analysis 74 3.2.2.4.3. Establishment of the standard curve 75 3.2.2.4.4. Quantification of sugars 76 3.2.2.5. Holocellulose 76 3.2.2.6. Alpha-cellulose 77 3.2.2.7. Kappa number 78 3.2.2.8. Viscosity 79 3.2.3. Acidity and buffer capability of cooking liquor 79 3.2.4. Immunocytochemical labelling of pectin for fluorescent microscopy 81 3.2.4.1. Sample preparation and fixation 81 3.2.4.2. Dehydration, embedding and sectioning 82 3.2.4.3. Protocol of immunolabelling 82 3.2.4.4. Visualization of slides with a fluorescent microscope . .83
4. RESULTS 85 4.1. Chemical Composition of Spruce Wood 85 4.2. Pulp Yield 86 4.3. Residual Lignin and Viscosity of the Pulp 96 4.4. Monitoring the pH of Cooking Liquor 105 4.5. Sugar Analysis Ill
vi 4.6. Delignification and Carbohydrate Removal 118 4.7. Immunocytochemical Study 133
5. DISCUSSION 146 5.1. Effect of Catalysts on Pulp Production Capability in Organosolv Pulping 146 5.1.1. Uncatalysed organosolv pulping 147 5.1.2. Catalysed organosolv pulping 148 5.1.2.1. Organosolv pulping with inorganic acid catalysts . . .149 5.1.2.2. Organosolv pulping with organic acid catalysts . . . .150 5.1.2.3. Organosolv pulping with monovalent metal ion catalysts 151 5.1.2.4. Organosolv pulping with divalent metal ion catalysts 152 5.1.2.5. Organosolv pulping with trivalent metal ion catalysts 154 5.1.2.6. Organosolv pulping with Mg-salt of different anion species catalysts ....155 5.1.2.7. Organosolv pulping with combination catalysts . . . . 155 5.2. Effect of Extended Cooking 157 5.3. Effect of Cooking Liquor Acidity 159 5.4. Pulping Selectivity 162 5.5. Behavior of Delignification 165 5.6. Carbohydrate Degradation and Removal 169 5.7. The Role of Pectin in Fiber Liberation 172 5.7.1. Removal of pectins 175 5.7.2. Mechanism of pectin removal in organosolv pulping 178 5.8. Mechanism of Middle Lamella Removal in Organosolv Pulping 180 5.9. Fiber Liberation : An Antagonistic Process 181 5.10. Mechanism of Fiber Liberation in the NAEM Process 184
6. CONCLUSIONS AND FUTURE RESEARCH . . . 187 6.1. Conclusions 187 6.2. Reasons for Lower Strength of Organosolv Pulp : A Suggestion for a Further Study 190
REFERENCES 193
APPENDICES 212
vii LIST OF TABLES
1. Chemical composition of some wood species 10 2. Some common fatty acids 23 3. pKa values for some acids 25 4. Comparative paper strength properties of hardwood ALCELL and kraft pulps at 300 mL CSF 35 5. Comparative paper strength properties of various types of unbleached pulps 38 6. Cooking conditions in the AS AM pulping process 41 7. Cooking conditions of the modified ORGANOCELL process 44 8. Three-stage MTLOX pulping conditions for hardwood and softwood, two-stage pulping conditions for non-wood species, and properties of the pulps 49 9. Proportions of types of linkages connecting phenylpropane units in spruce (Picea abies) and birch (Betula verrucosa) lignins (MWL) 53 10. Chemical composition of spruce wood 85 11. Sugar composition of spruce wood 85 12. Pulp yields and reject contents in organosolv pulping with and without catalysts of chloride salts of monovalent cations 89 13. Pulp yields and reject contents in organosolv pulping with catalysts of chloride salts of divalent cations 89 14. Pulp yields and reject contents in organosolv pulping with catalysts of chloride salts of trivalent cations 90 15. Pulp yields and reject contents in organosolv pulping with Mg-salt catalysts 90 16. Pulp yields and reject contents in organosolv pulping with inorganic acid catalysts. .91 17. Pulp yields and reject contents in organosolv pulping with organic acid catalysts.. .91 18. Pulp yields and reject contents in organosolv pulping with combination catalysts.. .92 19. Effect of catalysts on pulp production capability in organosolv pulping of spruce wood at 60 min cook in 80 % methanol 93 20. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with and without catalysts of chloride salts of monovalent cations 99 21. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of divalent cations 99 22. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of trivalent cations 100 23. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with magnesium salt catalysts 100 24. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with inorganic acid catalysts 101 25. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with organic acid catalysts 101
viii 26. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with combination catalysts 102 27. pH of cooking liquor with and without catalysts of chloride salts of monovalent cations and the estimated amount of proton generated during the cooking. .106 28. pH of cooking liquor with catalysts of chloride salts of divalent cations and the estimated amount of proton generated during the cooking 106 29. pH of cooking liquor with catalysts of chloride salts of trivalent cations 107 30. pH of cooking liquor with magnesium-salt catalysts and the estimated amount of proton generated during the cooking.. 107 31. pH of cooking liquor with inorganic acid catalysts 108 32. pH of cooking liquor with organic acid catalysts 108 33. pH of cooking liquor with combination catalysts and the estimated amount of proton generated during the cooking 109 34. Sugar composition of pulps and / or undefibrated cooked chips pulped with and without catalysts of chloride salts of monovalent cations Ill 35. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of divalent cations 112 36. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of trivalent cations 112 37. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of magnesium salts 113 38. Sugar composition of pulps and / or undefibrated cooked chips pulped with inorganic acid catalysts 113 39. Sugar composition of pulps and / or undefibrated cooked chips pulped with organic acid catalysts 114 40. Sugar composition of pulps and / or undefibrated cooked chips pulped with combination catalysts 115 41. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.050 M CaCl2 catalyst at different cooking time 116 42. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.025 M CaCl2 + 0.025 M Mg(N03)2 catalyst at different cooking times. . . .116 43. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.025 M A1C13 catalyst at different cooking times 117 44. Sugar composition of pulps and / or undefibrated cooked chips pulped with 0.050 M citric acid catalyst at different cooking times 117 45. Sugar composition of kraft pulp and / or undefibrated cooked chips pulped at different cooking times 118 46. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with and without catalysts of chloride salts of monovalent cations in 80% methanol 122 47. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with catalysts of chloride salts of divalent cations in 80 % methanol 122
ix 48. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with catalysts of chloride salts of trivalent cations in 80 % methanol 123 49. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with magnesium salt catalysts in 80 % methanol 123 50. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with inorganic acid catalysts in 80 % methanol 124 51. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with organic acid catalysts in 80 % methanol 124 52. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with combination catalysts in 80 % methanol 125 53. Summary of relationship between pH and pectin removal 135
x LIST OF FIGURES
1. (l-»-4)-a-D galacturonan 16 2. An acidic hemicellulose (xylan) 21 3. Locations of carboxyl groups in an anhydroglucose unit 22 4. The resin acids 23 5. Methylated structure of a benzylalcohol group in the lignin molecule 40 6. Types of linkages connecting phenylpropane units in lignin 53 7. Delignification as a function of the solubility parameter of solvent/water mixtures 55 8. The proposed mechanism of proton generation during NAEM-catalysed organosolv pulping 57 9. The mechanism of hydrolysis and competing condensation reactions of a-aryl ether bonds in lignin 59 10a. Hydrolysis of P-aryl ether bonds (Pathway A) 60 10b. Release of formaldehyde from y-carbons (Pathway B) 60 11. The mechanism of rearrangement reactions caused by homolytic cleavage of P-aryl ether bonds in phenolic lignin structures 60 12. The relationship between the percentage of lignin removed from the middle lamella (ML) and secondary wall (S) and the percentage of lignin removed from whole wood by kraft, acid sulfite, neutral sulfite, acid chlorite, NAEM process and the ORGANOCELL process 63 13. Mechanism of P-elimination in depolymerization of a partially esterified galacturonan chain 64 14. Relationship between cooking time and total pulp yield of kraft and organosolv pulping with different catalysts 94 15. Relationship between cooking time and screened pulp yield of kraft and organosolv pulping with different catalysts 95 16. Relationship between cooking time and lignin-free pulp yield of kraft and organosolv pulping with different catalysts 95 17. Relationship between cooking time and screen reject of kraft and organosolv pulping with different catalysts 96 18. Relationship between cooking time and Klason lignin of kraft and organosolv pulps cooked with different catalysts 103 19. Relationship between cooking time and acid-soluble lignin of kraft and organosolv pulps cooked with different catalysts 103 20. Relationship between cooking time and total residual lignin of kraft and organosolv pulps cooked with different catalysts 104 21. Effect of cooking time on viscosity of kraft and organosolv pulps cooked with different catalysts 104 22. pH behavior of catalysed organosolv cooking liquor HQ
xi 23. Predicted buffer capability of cooking liquor 110 24. Effect of cooking time on the degree of delignification in kraft and organosolv pulping with different catalysts 127 25. Effect of cooking time on the degree of carbohydrate removal in kraft and organosolv pulping with different catalysts 127 26. Relationship between cooking time and the ratio of lignin/carbohydrate removal in kraft and organosolv pulping with different catalysts 128 27. Relationship between cooking time and arabinose removal in kraft and organosolv pulping with different catalysts 128 28. Relationship between cooking time and galactose removal in kraft and organosolv pulping with different catalysts 129 29. Relationship between cooking time and glucose removal in kraft and organosolv pulping with different catalysts 129 30. Relationship between cooking time and xylose removal in kraft and organosolv pulping with different catalysts 130 31. Relationship between cooking time and mannose removal in kraft and organosolv pulping with different catalysts 130
32. Relationship between delignification and sugar removal in 0.050 M CaCl2 catalysed organosolv pulping 131 33. Relationship between delignification and sugar removal in combination of
0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulping 131
34. Relationship between delignification and sugar removal in 0.025 M A1C13 catalysed organosolv pulping 132 35. Relationship between delignification and sugar removal in 0.050 M citric acid catalysed organosolv pulping 132 36. Relationship between delignification and sugar removal in kraft pulping 133 37. Fluorescent photomicrograph of spruce wood labelled with monoclonal antibody JTM5 showing the presence and location of acidic pectin in the wood matrix (x 400) 136 38. Fluorescent photomicrograph of spruce wood labelled with monoclonal antibody JIM7 showing the presence and location of esterified pectin in the wood matrix (x 400) 136
39. Fluorescent photomicrograph of 30 min 0.025 M A1C13 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing the presence and location of residual acidic pectin (x 400) 137
40. Fluorescent photomicrograph of 30 min 0.025 M A1C13 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing the presence and location of residual esterified pectin (x 400) 137 41. Fluorescent photomicrograph of 60 min 0.025 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing the presence and location of residual acidic pectin (x 800) 138
xii 42. Fluorescent photomicrograph of 60 min 0.025 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing the presence and location of residual esterified pectin (x 800) 138 43. Fluorescent photomicrograph of 60 min 0.0125 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing the presence and location of residual acidic pectin (x 600) 139 44. Fluorescent photomicrograph of 60 min 0.0125 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing the presence and location of residual esterified pectin (x 600) 139 45. Fluorescent photomicrograph of 70 min 0.050 M citric acid catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM5 showing disappearance of acidic pectin (x 400) 140 46. Fluorescent photomicrograph of 50 min 0.050 M citric acid catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400) 140
47. Fluorescent photomicrograph of 40 min 0.050 M CaCl2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing disappearance of acidic pectin (x 400) 141
48. Fluorescent photomicrograph of 40 min 0.050 M CaCl2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing disappearance of esterified pectin (x 400) 141
49. Fluorescent photomicrograph of 40 min 0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody J1M5 showing disappearance of acidic pectin (x 400) 142
50. Fluorescent photomicrograph of 40 min 0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400) 142 51. Fluorescent photomicrograph of 60 min uncatalysed organosolv pulped spruce wood labelled with monoclonal antibody JLM5 showing disappearance of acidic pectin (x 400) 143 52. Fluorescent photomicrograph of 60 min uncatalysed organosolv pulped spruce wood labelled with monoclonal antibody JLM7 showing disappearance of esterified pectin (x 400) 143 53. Fluorescent photomicrograph of 60 min 0.050 M NaCl catalysed organosolv pulped spruce wood labelled with monoclonal antibody JLM5 showing disappearance of acidic pectin (x 400) 144 54. Fluorescent photomicrograph of 60 min 0.050 M NaCl catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400) 144 55. Fluorescent photomicrograph of 90 min kraft pulped spruce wood labelled with monoclonal antibody JIM5 showing disappearance of acidic pectin (x 400) 145
xiii 56. Fluorescent photomicrograph of 90 min kraft pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400) 145 57. Block diagram showing sequential removal of pectins, hemicelluloses and lignin by the NAEM process 186
xiv LIST OF ABBREVIATIONS
ADMT air-dry metric ton AOX adsorbable organic halides AQ anthraquinone ASAM Alkaline Sulfite Anthraquinone Methanol ASTM American Society for Testing and Materials BSA Bovine Serum Albumin COD chemical oxygen demand CSF Canadian Standard Freeness CTH controlled temperature and humidity Cuen cupriethylenediamine DP degree of polymerization ECF elemental chlorine free FLP fiber liberation point HPLC High Performance Liquid Chromatography LCC lignin-carbohydrate complex number average molecular weights Hv weight average molecular weights MWL milled wood lignin MT metric ton NAEM Neutral Alkali Earth Metal OD oven dry ODW oven-dry wood OZP oxygen, ozone, hydrogen peroxide (bleaching) OZEP oxygen, ozone, extraction, hydrogen peroxide (bleaching)
OZERP oxygen, ozone, extraction (reductive), hydrogen peroxide (bleaching) PAD Pulsed Amperiometric Detector SEM-EDXA Scanning Electron Microscopy-Energy Dispersive X-ray Analysis TAPPI Technical Association of the Pulp and Paper Industry TCF totally chlorine free TMP thermomechanical pulp UV ultraviolet W/L wood to liquor (ratio) ZEP ozone, extraction, hydrogen peroxide (bleaching)
XV ACKNOWLEDGEMENT
First of all, I want to thank God for His Grace and Blessing. I would like to express my special thanks and great appreciation to Dr. Laszlo Paszner, my academic and research supervisor, for his invaluable advice and guidance during my graduate study at this university. I would also like to thank the members of my supervisory committee :
Drs. C. Breuil (Wood Science), S.C. Ellis (Wood Science) and K.L. Pinder (Chemical
Engineering), for their suggestions and reviewing my thesis. I also want to extend my thanks to Dr. K. Robert (John Innes Institute, Norwich, U.K.) for kindly providing me with the monoclonal antibodies, JIM5 and JTM7.
To wood science committee on graduate fellowships and scholarships, I thank for awarding me the 1997/1998 MacMillan Bloedel Limited Roger Wiewil Fellowship in
Wood Chemistry. Great appreciation and thanks are also extended to my former sponsor, the Eastern Indonesia University Development Project (ERJDP), for allowing me to access the tool kit that helped me in finishing up my Ph.D. program. Finally, to all my friends and my family, I thank them for their spiritual support, prayers and encouragement during my graduate study at this great University.
In Whom are hidden all the treasures of wisdom and knowledge (Col. 2:3). But seek first His. Kingdom and His Righteousness, and all these things will be given to you as well (Mat. 6:33) (NIV).
xvi 1. INTRODUCTION
1.1. Background
Demographics and redistribution of wealth drive the global demand for pulp and paper over the long term. On the other hand, the pulp and paper industries are constantly under pressure to comply with more and more stringent environmental regulations that are being implemented in many parts of the world (Anon., 1994b, 1995a). In addition, especially in chemical pulp production, the conventional (kraft) pulping process requires high capital investment and large quantities of natural resources, i.e., raw materials, water and energy. Furthermore, the pulp yield is low and the pulp is difficult to bleach. A decline in wood supply, the main raw material for pulp and paper, has been reported in many countries, including Canada (Swann, 1994; Forrest, 1995), one of the wood-rich countries in the world. Therefore, developing a more efficient and environmentally friendly pulping process is needed and has been initiated to overcome or at least to alleviate the above problems.
Concomitantly, the new pulping process must be able to pulp any kind of lignocellulosic materials, i.e., hardwoods, softwoods and non-wood fibers, and produce pulp at higher yields, yet easy to bleach and possesses comparable strength properties to kraft pulp. Besides, for logistics reasons the pulp mill should be built at small economic scale, requiring low capital investment and a smaller supply of natural resources. Among the sulfur-free chemical pulping processes, organosolv pulping has been identified as the right choice for further development that potentially can meet all the above criteria
1 2
(Akhtar and Young, 1998). Especially for alcohol organosolv pulping, other advantages
are valuable and marketable by-products, such as fermentable sugars, utilizable lignin, furfural and acetic acid among other products (Lora et al, 1989; Hergert, 1998).
Many types of solvents have been evaluated as pulping agents for various species
of lignocellulosic materials. However, without adding catalyst(s) to the cooking liquor,
which is usually a solvent/water mixture, most solvents are inert or incapable of
delignifying the wood to the level where individual fibers are completely separated. Thus,
pulping results are different for each solvent and species of lignocellulosics. In case of
solvent type, lower molecular weight aliphatic alcohols seem to be the most desirable
solvents.
Alcohol-based solvent pulping processes, such as ALCELL, ORGANOCELL and
AS AM, are now in various stages of commercialization (Stockburger, 1993). The interest
of using a solvent in pulp production has become more obvious. Alcohol has been used
not only as per definition for solvent pulping, but also as an additive to the kraft pulping
liquor (Norman, etal, 1993; Yoon, etal, 1997; Yoon and Labosky, 1998) and
bleaching processes (Solinas and Murphy, 1996; Baeza, etal, 1999; Bouchard, 2000) in
order to improve the process performance, such as delignification selectivity, yield and
pulping time, and the pulp properties.
Many organosolv pulping processes, including the ALCELL process which is the
autocatalysed alcohol organosolv pulping process, developed so far have difficulties in
pulping softwoods. The ALCELL process does not work satisfactorily on softwoods
(Aronovsky and Gortner, 1936; Baumeister and Edel, 1980; Lange etal, 1981; 3.
Lonnberg et al, 1987; Argyropoulos, 1999). The capability of a pulping process to pulp
softwoods is seen to be an important aspect of the process, as softwoods are a major fiber
source with the best fiber strength properties for papermaking. In this regard, the NAEM
(Neutral Alkali Earth Metal) salt catalysed organosolv pulping seems to be better than
others, as more than 25 species of softwoods, hardwoods and non-wood fiber sources
have been successfully pulped with this process without any difficulties.
Topochemical selectivity of a pulping process, preferential for removing middle
lamella compounds, is another crucial pulping aspect that affects the resulting pulp yield
and quality. All pulping studies in the past seemed to have focussed only on
delignification. However, lignin is not the only compound in the middle lamella. In fact,
by using mercurization with the SEM-EDXA (Scanning Electron Microscopy-Energy
Dispersive X-ray Analysis) technique, which was implicitly claimed to be a more reliable
method as compared to the previously developed methods, i.e., UV (ultraviolet)
microscopy and Bromination SEM-EDXA, Westermark, et al. (1988) found that the lignin
concentration in the true middle lamella of spruce (Picea abies) was 55 % and 58 % for
earlywood and latewood, respectively. Thereby, more than 40 % of the middle lamella
substance consists of non-lignin compounds. Accordingly, the process of fiber liberation
is not exclusively dependent on delignification, but also concurrent removal of non-lignin
components of the middle lamella must be initiated.
Recent evidence shown by Van Der Veen and Van Den Ent (1994), Westermark
and Vennigerholz (1995), confirmed that pectins were found to be mainly located in the
middle lamella. The concentration of pectins in the compound middle lamella could be as 4 high as 20 % (Rydholm, 1985). As a result, assuming a role for pectin removal as a contributing factor in fiber liberation is inevitable. The critical importance of pectin removal, particularly in catalysed organosolv pulping of softwoods, was hitherto
overlooked. Therefore, in this research, the observations were not focussed only on
delignification aspects, but were also extended to investigations of the role of pectins and their removal from the middle lamella on the fiber liberation process.
1.2. Objectives
The use of a catalyst in organosolv pulping of softwoods appears to be essential in
directing pulping selectivity and is expected to promote the removal of middle lamella
compounds leading to fiber liberation. Therefore, in general, the objectives of the research
were to investigate the effect of catalysts on pulping behavior, selectivity, pectin removal
and delignification, leading to fiber separation in organosolv pulping of spruce wood.
Specifically, the objectives of the research are directed to the following :
1) Investigate the effect of different cations, anions and acids as the catalyst on the
pulping selectivity in the catalysed alcohol organosolv pulping of spruce wood.
2) Investigate the impact of the catalysts on the removal of pectins from middle lamella of
spruce wood in elucidation of the role of pectins during delignification leading to fiber
separation in the catalysed alcohol organosolv pulping process.
3) Elucidate the uniqueness of the NAEM salts as catalysts in their capability of
facilitating fast and selective removal of the middle lamella compounds leading to fiber
liberation in the catalysed alcohol organosolv pulping process. 5
4) Try to produce a low kappa number chemical pulp by extended cooking in the
catalysed alcohol organosolv pulping under specified pulping conditions and procedures.
1.3. Hypotheses
In order to accomplish the research objectives, the following hypotheses were
established.
1) Different cations and anions are expected to exhibit varying pulping selectivity during
delignification.
2) Divalent metal ions, especially alkali earth metals, provide unique catalytic conditions
and result in selective topochemical delignification to take place in the middle lamella.
3) A synergistic effect on the rate and selectivity of delignification can be obtained by
employing more than one catalyst in the cooking liquor.
4) Under the specified pulping conditions and procedures, extended cooks in the
catalysed alcohol organosolv pulping process can be expected to produce low kappa
number pulp of spruce wood.
5) Dissolution of pectin during the early stage of cooking will enhance delignification and
allow early fiber separation.
6) Selective and rapid delignification, facilitated by pectin removal taking place in the
middle lamella, are the key factors for early fiber liberation by the NAEM catalysed
organosolv pulping process.
7) Fiber liberation in the mineral acid catalysis systems is caused by extensive
delignification in the middle lamella without preceding dissolution of pectin. 6
1.4. Outline of Experiment
The following outline is provided to make the experiments designed for the research more easily understood. The detailed experimental procedures are described in
Section 3.
I. Kraft pulping
The pulping was conducted under standardized conditions for batch cooking for a series of cooking times, keeping all other conditions constant.
II. Organosolv pulping
All organosolv cooks were made with 80/20 methanol/water and the W/L ratio was 1/10. The cooking series were grouped based on the catalyst types used. The catalysts can be grouped as follows:
1. Uncatalysed organosolv pulping
The uncatalysed cooks were conducted at a fixed cooking time of 60 min,
including heating-up time, without addition of any chemical to the cooking liquor.
2. Catalysed organosolv pulping
The cooks were conducted under both fixed and varied cooking times.
a) For fixed cooking time of 60 min, including heating-up time, the
following series of cooks were conducted :
1) Chloride salts of mono- (NaCl, KC1), di- (CaCl2, MgCl2, MnCl2,
ZnCl2) and trivalent cations (A1C13, FeCl3) were used. The catalyst concentrations were 0.025 M, 0.050 M and 0.075 M. 7
2) Magnesium salts of different anions, i.e., MgCl2, MgS04,
Mg(N03)2, Mg(CH3COO)2. The catalyst concentrations were
0.025 M, 0.050 M and 0.075 M (Note : particularly MgS04, it was found to be difficult to dissolve at concentration >0.025 M. Some of the dissolved catalyst re-precipitated).
3) Mineral acids, i.e., HC1, H2S04, HN03. The catalyst concentrations were 0.00625 M, 0.00833 M, 0.0125 M and
0.025 M for HC1, 0.0050 M and 0.0100 M for H2S04, and
0.050 M and 0.075 M for HN03.
4) Organic acids, i.e., acetic, oxalic, malonic, malic, citric acids. The catalyst concentrations were 0.025 M, 0.050 M and 0.075 M.
5) Combination of two different catalysts, i.e., CaCl2 + Mg(N03)2,
CaCl2 + MgCl2, CaCl2 + MnCl2, CaCl2 + ZnCl2, CaCl2 + NaCl,
NaCl + HC1, NaCl + HN03, NaCl + MgS04, NaCl + Mg(N03)2,
NaCl + MgCl2. The concentration of each catalyst was 0.025 M, except HC1, which was 0.00625 M.
b) In the varied cooking time series, the following cooks were conducted:
1) 0.050 MCaCl2
2) 0.025 M CaCl2 + 0.025 M Mg(N03)2
3) 0.025 MAIC13
4) 0.050 M citric acid.
The series of cooking times applied for the above cooks was 10,
20, 30, 40, 50, 60, 70, 90, 120 and 150 min, except for A1C13, which was
10, 20, 30, 40, 50 and 60 min, including the heating-up time. 8
ELT. Immunocytochemical study of pectin removal
The pectin distribution and disappearance from the middle lamella and / or cell wall during the course of pulping was studied by selective labelling of the pectin on cooked chip cross-sections with monoclonal antibodies JTM5 and JTM7. The monoclonal antibody
JTM5 was used to identify acidic or low esterified pectin (polygalacturonan), whereas
JTM7 was used to identify methyl-esterified polygalacturonan (esterified pectin) with a
degree of methylesterification of 35 - 90 % (Knox etal., 1990; Vennigerholz, 1992;
Westermark and Vennigerholz, 1995). Detailed immunolabelling procedures are described
in Section 3.2.4. 2. LITERATURE REVIEW
2.1. Chemical Constituents of Wood
Lignin, cellulose, hemicelluloses and extractives are four important wood
components, common to all lignocellulosic materials. Lignin, cellulose and hemicellulose
are the main wood components and part of the cell wall while extractives are considered
minor components and not part of the cell wall (Fengel and Wegener, 1984). The
proportion of these components varies with the species. Quantitatively, the cellulose
content in wood is not much different between softwoods and hardwoods, but there are
significant differences in their hemicelluloses and lignin contents among the species. In
general, hardwoods contain 18-25 % lignin, 40 - 50 % cellulose, 2 - 5 % hemicelluloses
in form of (galacto)glucomannans and 15 - 30 % in form of xylans, and 1 - 5 %
extractives. Softwoods contain 25 - 35 % lignin, 45 - 50 % cellulose, 20 - 25 %
hemicellulose in form of (galacto)glucomannans and 5 - 10 % in form of xylans, and 3-8
% extractives. In addition to these organic components, wood also contains a small
amount of inorganic material, i.e., ash, amounting to 0.4 - 0.8 % for hardwoods and 0.2 -
0.5 % for softwoods (Biermann, 1993). The chemical composition of some wood species
is given in Table 1. In addition to the chemical constituents mentioned above, wood also
contains small amount of pectic substances, known as pectin. The pectin content in the
compound middle lamella of wood may be up to 20 % and is rarely recorded (Rydholm,
1985) or largely overlooked as to its possible importance in controlling fiber liberation
during pulping.
9 10
Table 1. Chemical composition of some wood species.
Species Holocel- a-cel• Klason Extract• Ash lulose lulose lignin ives*
Hardwoods : Acer rubrum (Red maple) 77 47 21 2 0.4 Betula papyrifera (Paper birch) 78 45 18 3 0.3 Fagus grandifolia (American beech) 77 49 22 2 0.4 Quercus rubra (Northern red oak) 69 46 24 5 0.4 Ulmus americana (American elm) 73 50 22 2 0.4 Eucalyptus saligna (Sydney blue gum) 74 50 27 1 0.2 Shorea sp. (White meranti) 69 50 30 4 0.5 Canarium indicum (Galip) 70 46 28 1 0.9 Dipterocarpus sp. (Keruing) 74 55 29 3 0.9 Aleuritas moluccana 78 46 20 1 2.1
Softwoods : Abies concolor (White fir) 66 49 28 2 0.4 Picea engelmanni (Engelman spruce) 69 45 28 2 0.2 Pinus banksiana (Jack pine) 66 43 27 5 0.3 Pinus elliottii (Slash pine) 64 46 27 4 0.2 Pseudotsuga menziesii (Douglas-fir) 66 45 27 4 0.2 Tsuga heterophylla(y^estern hemlock) 67 42 29 4 0.4 Pinus merkusii (Mindoro pine) 65 - 28 4 0.3 Note: *ethanol/benzene; The values are in percentage. Source: Pettersen (1984).
2.1.1. Lignin
In the cell wall, lignin functions as an incrusting substance and provides rigidity to
the fibers. Lignin is a highly branched, three dimensional incrusting polymer comprised of
aromatic phenylpropane building units. Therefore, structurally it is believed to be totally
amorphous and dispersed throughout but not an integral part of the fibrillar system of
carbohydrate polymers (Janes, 1969). It is believed that lignin is formed by enzymatic
dehydrogenation of phenylpropane units followed by random radical coupling. The main
structure type and composition of lignin found in different lignocellulosic species is not the 11 same. Softwood lignin consists mainly of guaiacyl units originating from trans-coniferyl alcohol as the precursor, while hardwood lignin is composed of two different types, namely : guaiacyl and syringyl units, originating from trans-coniferyl and trans-synapyl alcohols, respectively. The ratio of guaiacyl-to-syringyl units is 4:1 to 1:2 (Parham,
1983). Grass lignin consists of p-hydroxyphenyl units derived from trans-coumaryl
alcohol units besides being mixed with the foregoing two precursors. Basically, almost all
plants contain more or less these three types of lignins : guaiacyl, syringyl and p-
hydroxyphenyl moieties (Sakakibara, 1991).
As a complex polymer, lignin is built by a number of chemical bonding sequences.
The most common and dominant type of bond is the ether bond, found to link more than two-third of the lignin monomeric units in the lignin molecule, and the rest are carbon-to-
carbon bonds (Sjostrom, 1981). Lignin with a high proportion of carbon-to-carbon bonds
is usually more difficult to degrade into the smaller fragments (Sakakibara, 1991). This
type of lignin may cause difficulties during the pulping and bleaching processes.
Furthermore, delignification could become more difficult and complicated by the chemical
complexity of bonding between lignin and carbohydrates, known as the lignin-
carbohydrate complex (LCC) (Eriksson and Lindgren, 1977). The benzyl ether bond was
found to be the main type of linkage between sugar and lignin fragments (Koshijima et al,
1989). Therefore, lignin bears importance in paper production in that it seems to be a
controlling factor in pulp production through delignification, fiber liberation and pulp
brightness. In this case, selective delignification is required for high yield pulp production.
The theoretical pulp yield for any species is set by its holocellulose content which for 12 hardwoods is 60 - 80 % and for softwoods is 60 - 71 % (Pettersen, 1984). The holocellulose content of non-wood fiber sources, e.g., sisal, may be as high as 83 %
(Yawalata, 1996).
2.1.2. Cellulose
Cellulose is a homopolysaccharide, built up of D-glucopyranose chains in which
glucose is linked by (W4)-|3-glycosidic bonds. The degree of polymerization (DP) is
1000 to 15000, depending on source and extent of degradation of the cellulose and also
the method used for determining the DP (Janes, 1969). Cellulose is considered a linear
polymer and capable of having intra- and intermolecular hydrogen bonds. Aggregation of
cellulose molecules through hydrogen bonds forms microfibrils, in which the cellulose
chains are arranged in a highly ordered manner, known as the crystalline regions.
Crystalline zones alternate with less ordered ones, known as the amorphous regions.
Microfibrils build fibrils and finally cellulose fibers. Because of its fibrous structure and
strong hydrogen bonds, cellulose possesses high tensile strength and is insoluble in most
solvents and dilute acids and alkali (Sjostrom, 1981). By use of small angle X-ray
scattering, it is found that the length of the crystalline portion is about 600 A and the
width is 50 - 150 A. These values agree with the size of microfibrils or micelles observed
by electron microscopy (Stamm, 1964). The crystallinity of wood pulp is 45 - 65 % while
cotton is up to 90 %, ramie, cotton linters and hemp are up to 80 % crystalline (Mark,
1952). The crystalline portion is known to be the most resistant part of the cellulose
molecule against degradation or attack during chemical treatment. 13
In pulping, more than 90 % of the cellulose survives the high temperature and harsh chemical environment. Cellulose forms the skeletal substance of pulp fibers with the state of cellulose having a substantial effect on the physical and mechanical properties of papers.
2.1.3. Hemicelluloses
Unlike cellulose, hemicelluloses are heteropolysaccharides consisting of D-xylose,
D-mannose, D-galactose, D-glucose, L-arabinose and L-rhamnose, 4-O-methyl-D-
glucuronic acid, D-glucuronic acid, and galacturonic acid as components. D-xylose, D-
mannose, D-galactose, D-glucose and L-arabinose are the most common sugar
components found in most wood species, while L-rhamnose exists only in some hardwood
species (Shimizu, 1991). The hemicelluloses isolated from softwoods and hardwoods are
different from each other. The principal hemicelluloses in softwoods are
galactoglucomannans (about 20 %, in which 5 - 10 % is water-soluble, Timell, 1967),
arabinoglucuronoxylan (5-10 %), arabinogalactan (minor amount, except in heartwood
of larches), and other polysaccharides (minor quantities). The main hemicelluloses of
hardwoods are glucuronoxylan (15-30 %), followed by glucomannan (2-5 %), and
other polysaccharides (minor quantities). The DP of hemicelluloses is mostly 100 - 200,
which is much lower than that of cellulose. Hemicelluloses are branched, two dimensional
polymers, easily dissolved in alkali solution and will even partially dissolve in water
(Sjdstrom, 1981; Timell, 1967). Because of their solubility in water and alkali solutions,
hemicelluloses are readily removed during the chemical pulping or bleaching processes. 14
Therefore, attempts to retain the hemicelluloses by employing a high solvent content in the process to discourage their dissolution during the organosolv delignification process, is seen to be a pulping strategy to increase pulp yield beyond the a-cellulose content of wood.
2.1.4. Extractives
Wood extractives, mostly found in parenchyma cells and resin canals, are not considered part of the cell wall components. They consist primarily of organic, non- polymeric compounds that can be extracted from wood with water and neutral organic solvents and / or volatilized by steam. In general, the extractive content in woods is only 2
- 8 % for softwoods and 2 - 4 % for hardwoods, based on the oven-dry wood (Janes,
1969). However, extractives consist of a large number of chemical compounds which can be sorted into three main groups, i.e., 1) aliphatic compounds, which are mainly fats and waxes, 2) terpenes and terpenoids, and 3) phenolic compounds (Sjostrom, 1981).
Although, in general, the quantity of extractives in woods is considered small, in many cases, their presence has a significant effect on wood utilization. Extractives often affect wood characteristics, such as durability (resistance to insect or fungal attack), color, odor, taste, inflammability, toxicity and even certain other physical properties, such as ultraviolet-fluorescence and density (Wise, 1952). Extractives also have a significant effect in pulp manufacture, i.e., increasing chemical consumption, inhibiting the pulping reactions by reducing penetrability, reducing lignin solubility and decomposing the cooking liquor, causing equipment corrosion (tropolones, polyphenols and organic acids) 15 affecting the pulp properties, such as color and bleachability, wettability and sticking at the press, and causing some problems during spent liquor recovery, such as concentration and burning difficulties, foaming during concentration and oxidation (Gardner and Hillis,
1962). The effect seems to be more significant in the acid sulfite pulping than in the alkaline (soda and kraft) processes. It was reported that the presence of less than 1 % of dihydric phenol (pinosylvin) in sulfite pulping can inhibit the pulping process of pine heartwood (Wise, 1952). Some constituents, such as abienol, geranyllinalool, thunbergol, thunbergene and other highly unsaturated terpenoids, are easily oxidized and polymerized to insoluble sticky products which can subsequently cause pitch problem particularly in
acid sulfite pulping (Kimland and Norin, 1972; Gustafsson et al, 1957).
On the other hand, there is no report on pitch problems in organosolv pulping so
far. Research conducted by Quinde and Paszner (1992) on the behaviour of the major
resin- and fatty acids of slash pine (Pinus elliottii Engelm.) during neutral alkali earth
metal salt catalysed organosolv pulping found that the pulp retained only 11.7 % and 8.2
% of the resin- and fatty acids originally present, respectively, while most extractives (86
%) were found in the black liquor and precipitated with the lignin. Therefore, it does not
seem likely that a pitch problem will occur in organosolv pulping, particularly in the
NAEM process. Freedom from the pitch problem seems to be another advantage that can
be added to the list of advantages of the organosolv pulping processes. 2.1.5. Pectic substances
Pectic substances, generally also called "pectin", are a group of polysaccharides
(heteropolysaccharides) which are an important structural component of all higher plants
(Hassid, 1970). Pectin is a known component of the connecting tissue (middle lamella) between wood fibers (Raven et al, 1986). The basic building unit of pectin is D- galacturonic acid and / or D-galacturonic acid methyl ester (Pigman and Geopp, 1948).
These basic units are connected by (l-»4)-a linkages (Fig. 1). Carboxyl groups of pectin are methyl-esterified to various degrees. Variations in the pectin properties are determined by the differences in molecular size, degree of esterification of the carboxyl groups and the amount and types of accompanying polysaccharide impurities, mainly D- galactan and L-arabinan (Pigman and Geopp, 1948; Hassid, 1970). In a review on the pectin structure conducted by Jarvis (1984), it was reported that the main chain of galacturonan is often interrupted and bent by rhamnose units, to where most of side-chains attach. The side-chains are mainly arabinan and galactan but xylan could be also found.
COOR COOR COOR COOR
H OH H OH H OH H OH
Fig. 1. (l->4)-a-D galacturonan (Note : R = H or CH3). 17
By using ruthenium red staining and fluorescent microscopy techniques, Van Der
Veen and Van Den Ent (1994) have recently confirmed that the main location of pectic substances in wood is the middle lamella and cell corners. Lesser amounts of pectin were found in the primary and secondary walls. In addition, pectin was also reported to be found in ray cells and the tori of the bordered pits (Westermark and Vennigerholz, 1995).
Regarding the type and distribution of pectin found in wood, particularly in spruce, the dominant type of pectin is in the methyl-esterified form. This type of pectin locates in the compound middle lamella, in the ray cells and in the tori of the bordered pits while the acidic form of pectin locates in the ray cells, especially at the pores connecting ray cells to fibers, and in the tori of bordered pits. However, lesser amount of the acidic pectin is also found in the compound middle lamella and occasionally also occurs in the cell corners
(Westermark and Vennigerholz, 1995).
At the early stages of cell growth and development, before lignification, intercellular pectin is the substance which holds the cells together (Raven et al, 1986).
During the maturation of the cells, pectic substances (polygalacturonic acid, galactan and arabinan which constitute two-third of the carbohydrates) rapidly diminish (mostly degrade, Westermark et al, 1986), whereas the contents of cellulose, glucomannan and glucuronoarabinoxylan increase (Rydholm, 1985). As the cells mature, the pectic substances remaining constitute only <1 to < 4 % of hardwoods and <1 to <3 % of softwoods based on total mass (Saka, 1991). The inner bark, particularly of white birch
(Betulapapyri/era Marsh), contained 3 - 4 % pectins with the ratio of 66:7:27, respectively for D-galacturonic acid, galactose and L-arabinose (Timell and Mian, 1961). 18
2.1.6. Distribution of the main chemical constituents in the cell wall
The distribution gradient of lignin and polysaccharides across the cell wall is found to be an opposite pattern (Janes, 1969). Lignin is found in high concentration in the middle lamella (90 %) and continuously declines in concentration across the cell wall from
the primary wall (P) (70 %) to the secondary wall Sj (40 %), S2 (15 %) and S3 (5 % or less). Whereas polysaccharides, i.e., cellulose, hemicelluloses and pectins, are distributed in the opposite trend across the cell wall (note : the amount of hemicelluloses and pectins are counted together as a single entity). Their quantities increase from the primary wall to the secondary wall. In the middle lamella, none / or little, if any, cellulose, and only 10 % hemicelluloses and pectins are found. In the primary wall, 10 % cellulose, and 20 % hemicelluloses and pectins are found. In the secondary wall, 35 % cellulose, and 25 %
hemicelluloses and pectins in Sl5 55 % cellulose, and 30 % hemicelluloses and pectins in S2
and 55 % cellulose, and 40 % hemicelluloses and pectins in S3 are located.
Although lignin is highly concentrated in the middle lamella, most of the lignin in
wood is found in the secondary wall, mainly in the S2 layer because the middle lamella is
much thinner (0.2 - 1.0 ptm) than the secondary wall (1-5 pirn, S2). According to Fengel et al. as reported by Westermark et al (1986) and Hafren et al (2000), the proportion of middle lamella is about 2 - 4 % of the total wood substance. Therefore, only 20 - 25 % of the total lignin in wood is located in the middle lamella (Sjostrom, 1981). However, due to different methods used for its determination, there are some discrepancies in terms of lignin concentration in the middle lamella, as reported in the literature. As it was pointed out by Westermark, et al. (1988), by using previously developed methods, the lignin 19 concentration in the middle lamella was found to be 85 - 100 % by the UV microscopy technique, and 40 - 60 % by the bromination SEM-EDXA method. By introducing a correction factor of 1.7, due to lower reactivity of bromine with the middle lamella lignin, the lignin concentration became 70 - 90 %. Due to some drawbacks of the methods mentioned above, Westermark, et al. (1988) used another technique, mercurization with
SEM-EDXA, which was implicitly claimed to be a more reliable technique. By using that technique, lignin concentration in the middle lamella of spruce (Picea abies) was found to be 55 % and 58 %, which was about 2.5 times as high as the lignin content in the secondary wall for earlywood and latewood, respectively.
Due to the high concentration of lignin in the middle lamella, it must play a significant role in fiber liberation and ultimately affect the pulp yield, in that a topochemically driven, highly selective delignification process can liberate the fibers before removal of carbohydrates in the secondary wall occurs by degradation. The residual middle lamella and cell wall lignins can then be removed by more selective oxidative bleaching such as chlorine dioxide, oxygen, ozone and hydrogen peroxide. Thereby, super high chemical pulp yields (>65 %) can be achieved.
2.1.7. Carboxyl group in wood
The acidic protons in wood presumably originate from the carboxyl groups that are found as constituent functional groups on all main wood components, i.e., lignin (Dence,
1992; Ekman and Lindberg, 1960), cellulose, hemicelluloses (Browning, 1967; Shimizu,
1991), extractives (Mutton, 1962) and acidic pectin (Westermark and Vennigerholz, 20
1995). The carboxyl group is the functional group of carboxylic acid, therefore, the carboxyl protons may undergo ion exchange with cations, generating carboxylic salt and proton. In this case, the carboxyl groups in the wood act as cation exchangers (Ant-
Wuorinen and Visapaa, 1954). In general, the mechanism of the reaction can be represented by the following equations :
RCOOH * RCOCV + H+ (1)
RCOO" + M~ « RCOOM (2) where M+ is a monovalent cation, although divalent cations can also be used (Browning,
1967; Abubakr et al, 1997). Therefore, the quantity of carboxyl group in wood is a good indicator of the ion exchange capacity of the wood chemical components.
The existence of carboxyl groups in native lignin has not been clearly explained.
However, Ekman and Lindberg (1960) were convinced that the band in the infra-red spectrum at the 1720 cm"1 region in lignin is due to the carboxyl-carbonyl stretching vibrations even though it was notified that the quantity of the carboxyl groups in the lignin structure is very small. The locations of the carboxyl groups in the lignin molecule were not known with assurance. However, they may be associated with ferulic and /?-coumaric acids residues as occurring in grass lignin (Dence and Lin, 1992). These acids may be considered residues of the biochemical synthesis of lignin precursors during formation and development of wood cells (Sarkanen, 1971).
Carboxyl groups are also found in cellulose and hemicelluloses in form of uronic acids. The quantity of uronic acid units (uronic anhydride) in wood is about 4 to 5 %.
Glucuronic, usually as a monomethyl ether, and galacturonic acids are the uronic acids 21 commonly found in wood. Glucuronic acid units are predominant in the hemicelluloses of mature wood, whereas galacturonic acid units are related to pectins (Browning, 1967).
4-O-methyl-D-glucuronic acid characterizes the acidic hemicelluloses. It was reported by
Saarnio et al. (1954) that in birch wood holocellulose, each xylan molecule is built up by
20 xylose units and one 4-O-methyl-D-glucuronic acid, which is linked to the C2 position of the xylose unit (Fig. 2). The exact location of carboxyl group in the anhydroglucose unit is not always known although many oxidants exhibit considerable selectivity in their attack (Browning, 1967). However, an investigation conducted by Sihtola (1954) indicated that the carboxyl group can be found at carbon atom 2, 3 or 6 of an anhydroglucose unit (Fig. 3). It was also reported that the acidity of the carboxyl group at
C6 is stronger than those at C2 and C3.
H OH H OH
Fig. 2. An acidic hemicellulose (xylan) (Saarnio et al, 1954). 22
H OH OH H
Fig. 3. Locations of carboxyl groups in an anhydroglucose unit (Sihtola, 1954).
In extractives the carboxyl groups are found in resins and fatty acids. The resin
acids are diterpene acids with the general formula C2oH3002, and classified into two types : pimaric and abietic types (see Fig. 4). The fatty acids are long straight chain aliphatic monocarboxylic acids and could be in saturated and unsaturated forms. The double bonds of the unsaturated fatty acids could be found in conjugated or unconjugated structures.
Some more important natural fatty acids are listed in Table 2 (Mutton, 1962).
The importance of carboxyl groups in wood pulping may be two-fold. In kraft and soda pulping, carboxyl groups consume alkali, thus weakening the cooking liquor.
Apparently, they are not desired and are without function in those pulping processes. On the other hand, in organosolv pulping, carboxyl groups may provide "internally generated protons" which are suspected as the true catalysts in acidic solvent delignification of both hardwoods and especially softwoods (Paszner and Cho, 1989). 23
Pimoric type
d«xtropimaric acid itod«xtropimaric acid
Abictic typ«
CH3 CH3 CH3 patustric acid dthydroabi«tic acid dihydroabiatic acid
Fig. 4. The resin acids (Mutton, 1962).
Table 2. Some common fatty acids.
Name Formula Saturated:
Laurie CnH23COOH
Myristic C13H27COOH
Palmitic C15H31COOH
Stearic C17H35COOH
Arachidic C19H39COOH
Behenic C21H43COOH
Lignoceric C23H47COOH Monounsaturated:
Palmitoleic CH3(CH2)5CH=CH-(CH2)7COOH
Oleic CH3(CH2)7CH=CH-(CH2)7COOH Polyunsaturated:
Linoleic CH3(CH2)4CH=CH-CH2-CH=CH-(CH2)7-COOH
Linolenic CH3CH2CH=CH-CH2-CH=CH-CH2-CH=CH-(CH2)7-COOH
Eleo stearic CH3(CH2)3-CH=CH-CH=CH-CH=CH-(CH2)7-COOH Source: Mutton(1962). 24
2.2. Strength of Acidic Protons
By Lowry-Bronsted's definition, an acid is a substance that can donate a proton to another substance. The strength of an acid depends on the dissociation constant (Ka) of the acidic proton of the acid molecule. Therefore, the acid strength is expressed by Ka or pKa value which is - log Ka. The lower the pKa value, the stronger the acid, meaning the greater the amount of ionization. Basically, the strength of an acid is determined by the relative stability of the acid and its anion. In this respect, the structure of the acid molecule and the location of the acidic proton in the molecule play important roles in determining the strength of the acid. Fessenden and Fessenden (1990) outlined the factors affecting the acid strength as follows :
1) electronegativity, e.g., ITF > ROH > R2NH > RH,
2) size, e.g., HI > HBr > HC1 > HF, and
3) hybridization, e.g., =CH > =CH2, -CH3.
An increase in acid strength can be caused by the inductive effect of electron-withdrawing
groups, e.g., ClCH2COOH > CH3COOH, and the resonance stabilization of the anion, e.g., RCOOH > ArOH > ROH. The anion can be partially stabilized by solvation. In this case, acid strength increases with increased solvation.
For polyprotic acids, having more than one acidic proton, the acid strength of the first proton is stronger than the subsequent proton, as indicated by their pKa values (see
Table 3). This means that the first acidic proton easily dissociates compared to the subsequent acidic protons. 25
Table 3. pKa values for some acids.
Acid name Structure PK, pK3 Hydrochloric HC1 -6.1 - -
Nitric HN03 -1.38 - -
Sulfuric H2S04 ~-3 1.99 -
Formic HC02H 3.75 - -
Acetic CH3C02H 4.75 - -
Oxalic H02C-C02H 1.27 4.27 -
Malonic H02CCH2C02H 2.83 5.7 -
^^COOH o-phthalic 2.95 5.41 -
CH2—COOH Citric 1 3.13 4.76 6.40 HO-C-COO1 H
CH2—COOH
Source: Lange's Handbook of Chemistry 13th ed. (1985).
In organosolv pulping, under acidic conditions, it appears that selective delignification occurs in a very narrow range of pH (4.0 + 0.4), in which cellulose degradation seems to be minimum while the delignification rate is optimum. Therefore, controlling pH at this range is essential to produce high pulp yield and quality. Since the pKa value is the measure of an acid's strength, which is determined by the degree of dissociation of the acid, and for polyprotic acids, the pKa values for the first proton are not the same as that of the subsequent protons, therefore, polyprotic acids are expected to be better in controlling the cooking liquor pH during delignification than monovalent acids. 26
2.3. Organosolv Pulping
Since Kleinert and Tayenthal in 1931 obtained their patent on organosolv pulping,
many researchers in this field have tried a number of solvents to pulp lignocellulosic
materials, mostly woods, although non-wood fibers have also been tried. Based on the
solvent used, the organosolv pulping processes could be categorized as alcohol, phenol,
cresol, organic acids, ester, amine, ketone, etc. pulping processes. Basically, these
solvents can not perform good delignification without addition of a catalyst. So far,
catalysts, such as HC1, H2S04, S02, oxalic acid, salicylic acid, A1C13, A12(S04)3, BF3,
MgCl2, NaOH, NaHS03, ammonia, ammonium sulfide (Sarkanen, 1990), NH4C1 (Lange et
al, 1981) and neutral alkali earth metal (NAEM) salts (Paszner et al, 1980's), have been
used with aqueous solvents. However, not all catalysts listed above work universally on
all wood species and result in high pulp yield and with the desired pulp quality. NAEM
salt catalysts in aqueous alcohol (Chang and Paszner, 1986) appear to make the best
organosolv pulping process catalysts among the other organosolv processes, proposed so
far. Furthermore, most of the proposed solvent pulping processes do not work
satisfactorily on softwoods. Alcohol, e.g., methanol and ethanol, based-solvent pulping
processes seem to be the most successful, as shown by practical applications, such as in
ORGANOCELL, ALCELL and ASAM processes (Stockburger, 1993). 27
2.3.1. Alcohol-based solvent pulping
Alcohols, especially the lower-molecular weight aliphatic alcohols, are the most frequently used solvents in organosolv pulping. Regarding the type of alcohol, Aronovsky and Gortner (1936) found that normal primary alcohols were better pulping agents than the secondary or tertiary alcohols, although the mixtures of «-butyl-alcohol-water appeared to be the most efficient in removing lignin from wood. However, due to low cost and ease of recovery, methanol and ethanol seem to be the most favoured alcohols for alcohol-based solvent pulping today, as shown by their potential commercial applications recently pursued. An advantage of employing methanol or ethanol as pulping liquor is in their low boiling point, ease of recovery by simple distillation (Sarkanen, 1990) with concomitant low energy requirement for their recovery. They are low cost and also fully miscible with water. Ethanol losses in pulping may be readily replenished from fermentation of the dissolved sugars, i.e., glucose and mannose. Whereas, methanol losses may be compensated by limited demethylation of lignin during the high-temperature cooking. Alcohol-based solvent pulping processes can be conducted under both acidic and alkaline conditions, although simpler lignin recovery can be had under acidic conditions.
2.3.1.1. Acidic alcohol-based solvent pulping
In acidic alcohol-based solvent pulping of hardwoods, the cooking can be carried out with or without catalysts. In the uncatalysed process, such as the ALCELL process, delignification is actually promoted by the acetic acid released from wood (Sarkanen, 28
1990), without an externally added catalyst, therefore, this process is also known as the autocatalysed organosolv pulping process. However, without addition of a catalyst this application is limited in regards of wood species.
Historically, it was reported in 1893 by Klason that delignification of wood occurred by treatment with hot ethyl alcohol. He found that aqueous ethanol containing small amounts of hydrochloric acid (5 %) could extract lignin from wood (Brauns, 1952).
Kleinert and Tayenthal (1931) recognizing the potentials used a similar approach for pulping purposes and filed a patent, in which a mixture of alcohol-water (20 - 75 %), with small amounts of acid or base (< 0.1 %) as the catalyst, was described to delignify wood under pressure and at high temperature (>150 °C).
Employing acids, especially strong mineral acids, such as HC1 and H2S04, as catalysts in organosolv pulping can cause the loss of pulp yield and lower the strength properties of the cellulose fibers, unless the pH is tightly controlled within a very narrow range, where the highest specificity for delignification occurs. Kleinert (1971) described that the preferred pH was between 4 and 10. In this range the cooking time is short and the viscosity and strength properties of the pulp produced are relatively high. However, a
mixture of aqueous alcohol with small amount of mineral acid, e.g., HC1 or H2S04, becomes a strong hydrolysing agent for cellulose, especially at high temperature (> 180 °C)
(Schlapfer and Silberman, 1960; Garves, 1988), thereby, together with the autocatalytically generated acetic acid from wood, the pH may drop below 3.0 causing severe degradation of cellulose and loss of 95 % of the hemicelluloses from the pulp.
Hence, acid catalysed pulps are low, nearly devoid, in hemicelluloses, thus, consist of 29 merely degraded a-cellulose. The system is similar to acidified aqueous acetone as it is used to hydrolyse wood and other lignocellulose to produce fermentable sugars (Paszner and Chang, 1983a, 1984; Paszner and Cho, 1988). Therefore, no useful softwood pulps could be produced with addition of mineral acids and acidic salts (Lora and Klein, 1990;
Chang and Paszner, 1986; Sarkanen, 1980, 1990).
Therefore, salts were preferred instead of acids. For instance, Lange et al. (1981)
used NH4C1 in 50 % aqueous ethanol as the cooking liquor to pulp spruce and beech
wood. The results indicated that NH4C1 improved the delignification but spruce pulp with a kappa number lower than 80 could not be obtained even though beech wood could be pulped to a kappa number of 22. However, for both cases the pulp yields were substantially decreased due to extensive cellulose degradation because of the hydrolysis effect. Apparently, the acidity of the cooking liquor during the pulping process of beech wood was much higher, caused by acetic acid generated during the cooking. The amount
of acetyl group in beech wood is 3.9 % (Rydholm, 1985). This indicated that NH4C1 was unable to buffer the solution to prevent a further pH drop.
Paszner et al. in 1980's came up with a better catalysis system, i.e., buffered acid catalysis, in which alkali earth metal salts were used with or without acidic compounds, preferably strong mineral acids, as secondary catalysts in high concentration (>70 %) aqueous alcohol solutions (Paszner and Chang, 1983b; Paszner and Behera, 1985, 1989;
Chang and Paszner, 1986; Paszner and Cho, 1989) to produce high-yield organosolv pulps of both softwoods and hardwoods. Recently, this process has been extended to pulp a number of non-wood fibrous materials successfully (Yawalata, 1996). Somehow, with 30 such a cooking liquor composition, a buffer system is created whereby the pH can be maintained in a very narrow range of nearly 4.0 (± 0.2) which provides the preferential pH range, assuring a strong topochemical effect. In this pH range, the highest specificity of delignification is initially limited to the middle lamella, cell corner areas and some "free cell wall lignin" (Behera, 1985; Paszner and Behera, 1985). Therefore, this process is capable of producing fully liberated, high-yield chemical pulps of softwoods, hardwoods and non- wood fibrous materials with viscosities higher than 50 cPs. (Yawalata, 1996). The strength properties of the pulps are comparable to kraft pulps, depending on species
(Paszner and Behera, 1985; Yawalata, 1996). A further brief review for this process will be given in section 2.3.1.1.2.
Another buffered solvent pulping system was suggested by Faass et al. (1989) in which sodium bicarbonate was used as a buffer to maintain the ethanol-water- methylanthraquinone pulping liquor at relatively neutral pH (~ 7.2) at 180 °C. It was reported that at this pH range the cook required high temperature of 200 to 240 °C but short cooking time. The delignification was reportedly excellent and the pulp quality was good. However, the results were reported only for tulip poplar (Lirodendron tulipfera), a hardwood species.
In uncatalysed-alcohol-based pulping, the results found so far among researchers are controversial, especially in respect of the capability of pulping softwoods. Kleinert
(1974, 1975) showed that without adding any catalyst, various wood species, including softwoods, i.e., spruce and pine, could be pulped with mixtures of 45 % by weight ethanol-water alone at a temperature of 185 °C in 30 to 60 min total cooking time. The 31 reported screened pulp yields were 53 - 56 % with 1.5 - 2.8 % rejects. In contrast,
Aronovsky and Gortner (1936), Baumeister and Edel (1980), Lange et al. (1981),
Lonnberg et al. (1987) and Pye and Lora (1991) found that without catalyst, the mixture of alcohol-water was unable to delignify softwoods and no fiber liberation could be achieved. Argyropoulos (1999) states that "the ALCELL® process represents one such organosolv alternative with the inherent limitation that it can not be used for the pulping of softwoods", assuming that lignin condensation reactions during the ALCELL process lead to carbon-carbon condensed residual lignin structures in softwood pulping by the
ALCELL process. A similar finding was also reported by Sabatier et al. (1989) on pulping sugarcane bagasse/rind in which, without catalyst, the ethanol-water mixture required high temperature and pressure cooks, yet produced only low quality semichemical grade pulps. However, an organosolv pulping process in this category, known as the ALCELL process, has been proposed for commercial stage of development
(Gibbens, 1993) although the process is limited to selected hardwoods only. A brief review of this process is given in the following section.
In addition to methanol and ethanol, which are most frequently used, other alcohols have also been tried for pulping purposes. For instance, as mentioned before,
Aronovsky and Gortner (1936) used a mixture of 50/50 (v/v) «-butyl alcohol and water to pulp aspen, a hardwood species. The pulp yield was reportedly high with low rejects, while the strength properties were reported similar to those obtained for the commercial alkaline aspen pulps. No detailed data on the strength properties were presented. Later,
April et al. (1979) and Bowers and April (1977) used the same type of solution, i.e., 50/50 32
v/v aqueous w-butanol, to delignify southern yellow pine. Again the pulp properties were
not described. Nelson (1977) used glycol solutions with salicylic acid derivatives to pulp
E. regnans, P. radiata and P. elliottii at temperature 170 0 and 195 °C. This process also
faced difficulties in pulping the softwoods, suffering from high percentage of reject
residues. While the strength properties of the pulps produced by this process were lower
than those of kraft pulps made from the same species, the pulp yield in general was higher
than that obtainable by the kraft process. Polyhydric alcohols, i.e., propylene glycol and
ethylene glycol, with HC1 or H2S04 as the catalyst has also been tried to pulp softwoods
(Uraki and Sano, 1999). The highest bleached pulp yield was 44.7 % with very high a-
cellulose content and crystallinity indicating extensive loss of hemicelluloses. Some
mechanical properties were reported to be close to those of kraft pulps.
2.3.1.1.1. ALCELL process
The ALCELL process actually takes its origin from the Kleinert (1974) APR
(Alcohol Pulp Recovery) process. Subsequent material flow modifications, based on a
multivessel qwas/'-continuous batch extraction process, were described by Katzen et al.
(1980) and patented by Diebold et al. (1978). The reported screened pulp yield of the
APR process for birch and red oak was 43 - 52 %, with kappa numbers of 14 - 38 and
viscosities 10-43 mPa.s, respectively, while for aspen, the yield was higher at 53 - 58 %,
with kappa numbers of 27 - 36 and viscosities 32 - 38 mPa.s. The reject content ranged
from 0.1 to 1.1 % (Lora and Aziz, 1985).
This process is an uncatalysed, also known as autocatalysed, organosolv pulping 33 process using 50 - 60 % aqueous ethanol (w/w) as the cooking liquor, at temperature 195
°C and pressure about 400 psig. For operational reasons, nitrogen is used to maintain a slight overpressure in the vessel (Pye and Lora, 1991). Thereby, no external catalyst is added to the cooking liquor. Basically, without catalyst the delignification seems to be limited, even at high temperature (>190 °C). Delignification in this process is promoted by the autocatalytically generated acetic acid as the catalyst, which is released from wood during the pulping process as a result of the hydrolysis of acetyl groups. It is well known that, in general, softwoods contain about less than half as many acetyl groups (1-2 %) than hardwoods (3-5 %) do (Rydholm, 1985), therefore the acetic acid generated during this pulping process is also less from softwoods than hardwoods. The inadequate amount of acetic acid, as the catalyst in the system, was thought to cause the inability of the process to pulp softwoods. However, externally added acetic acid could not make up for the deficiency. Addition of acetic acid to the ALCELL cook in pulping white birch conducted by Girard and Heiningen (2000), did not improve the delignification rate or pulp yield. No useful softwood pulps could be produced by this modified ALCELL process. The inability of the process to pulp softwoods can be seen as a major drawback, although the process can pulp most low to medium density hardwoods.
Pulping of some non-wood fibrous materials, such as kenaf, sugarcane bagasse and rind (Winner etal, 1991), and wheat straw (Lora, 1994) by the ALCELL process was also reported. The pulp yield of sugarcane bagasse/rind reported by Winner et al. (1991) was
48 %. Later Lora (1994) claimed that with certain modification, the ALCELL process can produce sugarcane rind/bagasse pulp higher than 70 % yield, although the author did not 34 present an authentic evidence of the claim. Process conditions appear to be critical and species dependent for the ALCELL process.
The ethanol concentration seemed to be an important factor in determining the delignification rate and selectivity of this process. A 60 % ethanol concentration (v/v) was reported as the desirable concentration for cooks at 195 °C, the typical cooking temperature for the ALCELL process (Goyal et al, 1992), although nearly all reports by the company used less than 60 % alcohol in the cooking liquor (Hergert, 1998). The cooking time was found to be a function of pH, temperature, alcohol concentration and type of wood (Pye and Lora, 1991). According to Pye and Lora (1991), this process could produce fully bleachable aspen chemical pulps with low AOX level in the effluent.
Development of ozone43ased bleaching for hardwood ALCELL pulp, with and without ethanol assistance in the ozone stage, was reported by Ni and Heiningen (1998), and Ooi and Ni (1998). Although the ALCELL pulp in general has a higher kappa number than similar kraft pulps, ALCELL pulps gave a higher brightness with the same bleaching sequences (Lora and Klein, 1990).
Pye and Lora (1991) also reported that oversized chips and knots were cooked effectively. The pulp yield was reported at least 2 % higher than that by the kraft process and the optical and physical properties of the pulp were claimed to be equivalent to those of kraft pulps for the same species. However, according to the data presented by Hergert
(1998), as shown in Table 4, the ALCELL pulp is a little weaker than kraft pulp. 35
Table 4. Comparative paper strength properties of hardwood ALCELL and kraft pulps at 300 mL CSF. Type of pulp Breaking Tear index, Burst index, length, km mN.m2/g kPa.m2/g
ALCELL1} 6.47 6.46 4.28
Eastern Canadian Hardwoods, Kraft2) 7.80 8.9 5.1
Central Canadian Hardwoods, Kraft2) 8.00 7.5 4.7 Sources : >' Hergert (1998). 2) Liebergott et al. (1981).
In spite of the drawback of inability to pulp softwoods, the ALCELL process has its main advantages :
1) the simplicity of its chemical recovery system as compared to either kraft,
AS AM or ORGANOCELL processes (Stockburger, 1993) and
2) valuable / marketable by-products.
In an approved commercial plant of 142,000 ADMT/year (Gibbens, 1993), the
ALCELL pulp mill was designed to co-produce 49,300 MT high-purity lignin, 4,300 MT modified lignin, 7,500 MT furfural and 8,300 MT acetic acid (Hergert, 1998).
Physical and chemical properties of the ALCELL lignin are different from those of kraft and sulfite lignins. It was reported by Lora et al. (1989) that the ALCELL lignin has a low molecular weight. M„ < 1040 g/mol is 80 %, in which 27 % has 1VL = 714 or M„
= 480 g/mol. (Thring et al, 1996). It is highly hydrophobic and insoluble in neutral or acidic aqueous media, but soluble in moderate to strong alkaline solutions and certain organic solvents. Therefore, this type of lignin has potential applications in many industries, i.e., 1) Wood adhesives, 2) Molding compounds, 3) Flame retardants, 4)
Diesel fuel additive, 5) Papermaking additives, 6) Slow release agricultural, veterinary 36 and pharmaceutical chemicals, 7) Insulation materials, 8) Friction materials, 9)
Surfactants, 10) Asphalt extender, 11) Rubber reinforcement, 12) Medical applications,
13) Engineering plastics, 14) Antioxidants, and 15) Lignin-derived chemicals (Lora et al., 1989). Particularly in its utilization as a wood adhesive, it had been demonstrated that at least 35 % of the solid resin of a phenol-formaldehyde resin could be successfully replaced with organosolv hardwood lignin (Cook and Sellers Jr., 1989).
2.3.1.1.2. NAEM process
The NAEM process is a catalysed alcohol-based solvent pulping process, in which neutral alkali earth metal salts are used as primary catalysts, with or without added acidic compounds. The acid additive would preferably be a strong mineral acid, as secondary catalyst, employed in the high concentration (> 70 %) aqueous alcohol solvent. This process was first introduced and developed by Paszner et al. in 1980's. To date, all pulping trials were still at laboratory scale or at 16 L-pilot scale conducted in 1983.
However, this process has shown its potential for commercialization as indicated by results of pulping trials obtained for over a decade. During that period of time, this process has successfully pulped a large number of lignocelluloses, more than 25 wood and agricultural residue species (Paszner and Behera, 1985; Chang and Paszner, 1986;
Paszner and Cho, 1989; Quinde and Paszner, 1992; Yawalata, 1996; Yawalata and
Paszner, 1997). The species pulped were :
1) Hardwoods : aspen, birch, Eucalyptus grandis, Eucalyptus saligna, mangrove
(Rhizophora sp), rubber wood, 37
2) Softwoods : radiata pine, ponderosa pine, Pinus oocarpa, slash pine (Pinus elliottii), spruce, western hemlock, western red cedar, Douglas-fir,
3) Non-wood fibers : sugarcane rind, sisal, tebu-tebu (local Indonesian reed-like material), rice straw, switch grass, canary reed grass, bamboo, giant mineral reed, jute, empty fruit bunch of palm oil trees, wheat straw, kenaf (bast fibers), hemp and bitter cane.
The pulp yields were considerably higher, compared to the kraft pulp yields of the same species. However, the pulp yield was found to vary depending on pulping conditions applied and degree of optimization of the cooking conditions for the various species. For instance, with pulping condition as used by Yawalata (1996), spruce chips yielded 56 % screened pulp at <1 % rejects, with kappa number 57. With different pulping conditions, high pressure cooks, as used by Chang and Paszner (1986), a pulp yield as high as 78 % with reject content of 6 - 8 % was reported but the kappa number was also very high, i.e.,
112. Viscosities of the pulps of higher than 50 cPs could be easily obtained, depending on species (Paszner and Cho, 1989; Paszner and Chang, 1983b; Yawalata, 1996). The strength properties of NAEM pulps in comparison to conventional chemical pulps of various species are shown in Table 5.
In case of NAEM catalysts, Aziz and Sarkanen (1989) reported that the effectiveness of the NAEM catalysts was confirmed by numerous researchers but they failed to reproduce the high pulp yield and strength properties as claimed by Paszner and
Behera (1985). It seems that the optimum pulping conditions were not exactly met by these trials. It should be evident that, in addition to the NAEM catalysts, the process has several distinguishing features, among which the high alcohol content (>70 %) liquor, 38 over-pressure (>700 psi), high cooking temperature (>180 °C) and good pH control in the cooking liquor become major factors for the short cooking times (18-22 min at temperature) which result in high pulp yield and viscosity.
Table 5. Comparative paper strength properties of various types of unbleached pulps.
Type of pulp Freeness, Breaking Tear index, Burst index, Double mL csf length, km mN.m2/g kPa.m2/g fold
Radiata pine, NAEM1} 287 11.97 7.8 6.9 na Radiata pine, Kraft1} 289 12.32 8.2 7.8 na Spruce, NAEM2) 300 9.22 6.37 6.48 971 E.C.Sw., kraft3) 300 11.80 9.2 9.5 2084 W.C.Sw., kraft3) 300 11.40 11.2 9.4 2470 Spruce, kraft4) 300 11.76 11.3 9.70 na Spruce, soda4) 300 8.87 10.9 6.68 na Hemlock, sulfite5) 300 8.70 5.40 4.9 1260
Mangrove, NAEM2) 300 4.83 8.21 2.65 15 E.C.Hw., kraft3) 300 7.80 8.9 5.1 110 C.C.Hw., kraft3) 300 8.00 7.5 4.7 136
Bagasse, NAEM2) 300 7.46 6.40 4.39 446 Bagasse, kraft6) 300 5.50-7.10 5.88-7.55 3.92-4.80 400 Bagasse, soda7) 33°SR 5.00 5.61 4.51 86 Bagasse, soda-AQ7) 33°SR 5.30 5.50 4.88 102
Sisal, NAEM2) 300 3.96 5.02 2.45 9 Sisal, soda8) 55°SR 6.47* 30.4* 4.97* na
Tebu-tebu, NAEM2) 300 6.14 7.12 3.25 42 Reeds, kraft6) 300 5.00 4.41 2.94 350 Notes : *Grammage = 67 g/m2; E.C.Sw. = Eastern Canadian Softwoods; W.C.Sw. = Western Canadian Softwoods (Interior B.C.); E.C.Hw. = Eastern Canadian Hardwoods; C.C.Hw. = Central Canadian Hardwoods. Sources: ^Paszner and Behera (1985); 2) Yawalata (1996); 3)Liebergotte* al. (1981); 4)Marton and Granzow(1982); 5)Bublitz and Hull (1981); ^Atchison (1987); 7)Saade/a/. (1988); s) Gerischer and Bester (1993). 39
2.3.1.2. Alkaline alcohol-based solvent pulping
Basically, alkaline solvent pulping can be considered as a modification of existing alkaline, i.e., kraft, soda or sulfite, pulping by introducing a solvent into the processes. In general, the researchers in this field claimed that by introducing alcohols into the alkaline pulping processes, such as soda (Marton and Granzow, 1982; Green and Sanyer, 1982), kraft (Norman etal, 1993; Yoon, etal, 1997; Yoon and Labosky, 1998) and alkaline sulfite anthraquinone (ASAM) (Kordsachia and Patt, 1988), higher delignification rates and better selectivity were obtained. The screened pulp yield was reportedly higher (4 to
5 %, Nakano et al, 1977) than that of the kraft process and the strength properties were comparable to kraft pulps (Nakano et al, 1976, 1977; Marton and Granzow, 1982;
Kordsachia and Patt, 1988; Norman et al, 1993). According to Daima et al. (1978) the high rate of delignification in the alkali-methanol system was due to the prevention of lignin recondensation at a-carbon through the methylation of active benzylalcohol groups in the lignin molecule (see Fig. 5), although extensive methylation of lignin during alcohol pulping has not been observed under the currently used cooking condition. Today, two alkali-alcohol pulping processes are in initial stages of commercialization. These processes are the ASAM and ORGANOCELL processes. 40
OH
Fig. 5. Methylated structure of a benzylalcohol group in the lignin molecule.
2.3.1.2.1. ASAM process
ASAM (Alkaline Sulphite Anthraquinone Methanol) process actually can be viewed as a modification of the alkaline sulphite process in which anthraquinone and methanol are added to improve the delignification rate and pulping selectivity. As it is known, the sulphite process has some limitations compared to the kraft process such as inferior pulp strength and limited species capability, although it gives high unbleached pulp brightness, 52 - 56 % ISO (Kordsachia and Patt, 1988).
Anthraquinone and methanol were added into the alkaline sulfite process to improve the process performance. Anthraquinone serves the function as a delignification redox catalyst while methanol serves as an organic solvent not only to dissolve extractives and lignin, but also to improve the solubility of anthraquinone, suppress free radical formation and reduce cooking chemical ionization (Fuchs etal, 1991). It was claimed that the ASAM process can pulp almost all kinds of lignocellulosic materials (Fuchs,
1990), produce pulps of equal or superior strength and yield to that possible by the kraft 41 process. The ASAM pulps can be bleached to high brightness without chlorine additives, i.e., ECF (elemental chlorine free) and TCF (totally chlorine free) bleaching sequences
(Fuchs et al, 1991; Teubner et al, 1994). Typical cooking conditions in the ASAM process are presented in Table 6.
Table 6. Cooking conditions in the ASAM pulping process.
Process parameter Range Chemical charge (calculated as NaOH), % on oven dry wood 15-25
Alkali ratio (NajSOj / NaOH or Na^Oj) 85/15 - 70/30
Anthraquinone (AQ), % on oven dry wood 0.05-0.15
Methanol content in the cooking liquor (by volume), % 15 - 30
Liquor to wood ratio 3-5 : 1
Cooking temperature, °C 170-180
Cooking time at maximum temperature, min 60- 150
Digester pressure, bar* 10- 14 Sources: Black (1991), *Teubner et al. (1994).
This process has shown success in pulping of a number of lignocellulosic materials.
Pulping trials with a quite large number of softwoods, hardwoods and annual plants were reported by Patt, et al. (1998). The results in pulping five species of wood, which were spruce, pine, beech, birch and poplar, were reported by Fuchs (1990) and indicated that the pulp yields by the ASAM process were higher and the kappa numbers were lower than for pulps produced by the kraft process from the same species. In pulping softwood, the pulp yield by the ASAM process was 2 - 4 % higher than that by the kraft process at kappa number of 30. It was claimed that the pulp yield differences may be up to 10 % 42 greater at low kappa numbers (<20). It was also claimed that the ASAM process was capable of delignifying softwoods to a kappa number below 20 and hardwoods to below
10 while retaining high yield and strength values (Fuchs, 1990). The screened pulp yield obtained for Douglas-fir was 47 % with kappa number of 22.1 and reject content of 1.4 %
(Zimmermann et al, 1991). The screened birch pulp yield was 54.5 % with kappa number
17.5 and 6.3 % reject (Kordsachia etal, 1990), whereas the total pulp yield of pine
(Pinus sylvestris) was 52.5 % with kappa number 27 and 2.6 % rejects (Kordsachia and
Patt, 1988). It is interesting to note that in the ASAM process even though a kappa number as low as 8.9 was achieved, total fiber liberation was not achieved as indicated by the relatively high amount of rejects (2.2 %) (Kordsachia et al, 1990). It seems that liquor penetration may be a problem in the ASAM process causing non-uniform cooking of the chips. This is in strong contrast to the results found with the NAEM salt catalysed organosolv pulping process. By the results shown in Yawalata (1996), the fibers could be totally liberated even at higher kappa numbers of 25 to 55. These results were explained by the specific topochemical effect which was found to operate during NAEM delignification of softwoods (Behera, 1985).
The ASAM pulp was also found to be much easier to bleach than the kraft pulp since it possessed very low lignin content and high initial brightness. ZEP or OZP sequences could bleach hardwood ASAM pulps to a final brightness of 90 % ISO, and the
OZEP sequence was used to bleach the softwood pulp to approximately 90 % ISO brightness. In a comparative study the results showed that the brightness and strength factor (square root of tensile • tear) of ASAM pulps were higher than those of the kraft 43 pulp, the tensile strength being higher and tear strength about the same (Black, 1991).
Studies on the bleachability of ASAM pulps of Douglas-fir (Zimmermann et al, 1991) and pine (Kordsachia and Patt, 1988) in comparison to kraft pulps found that the ASAM pulps were easier to bleach by ECF and TCF sequences than the equivalent kraft pulps.
In these studies the chemical consumption for the ASAM pulps was lower than that required by the kraft pulps.
From an environmental point of view, it was claimed that the ASAM process is considered an environmentally friendly technology. In a bleaching study conducted by
Kordsachia and Patt (1988), using a totally chlorine-free bleaching sequence (OZERP), it was concluded that there was no environmental impact, as shown by zero release of AOX and low levels of COD. However, commercialization of the ASAM process has been held up by the lack of a suitable low cost chemical recovery process. Design and demonstration of a working commercial size chemical recovery process for the ASAM process has now been accepted by Voest-Alpine Industrieanlagenbau and Kraftanlagen
Heilderberg (Anon., 1994c).
2.3.1.2.2. ORGANOCELL™ process
Originally, the ORGANOCELL process was a two-stage pulping process, in which the cooking liquor was aqueous methanol and sodium hydroxide. In the first stage, wood chips were extracted with 50/50 methanol/water at a temperature up to 195 °C for 20 - 25 min. In the second stage, the chips were then cooked with 18-22 % NaOH on wood for up to 60 min at a temperature 165 - 175 °C. The details of the process were described by 44
Dahlmann and Schroeter (1990) while the ORGANOCELL lignin recovery and characteristics were described and studied by Lindner and Wegener (1988, 1989, 1990a,
1990b) and Schroeter (1991).
The process as described above had some difficulties with pulping softwoods as reported by Pappens (1990). This report was contradictory to what was reported by
Dahlmann and Schroeter (1990). However, the process was later modified by eliminating the first stage, adding anthraquinone as a catalyst and reducing the methanol concentration in the cooking liquor (Young, 1992; Stockburger, 1993). The typical pulping conditions of the modified ORGANOCELL process version are outlined in Table 7.
Table 7. Cooking conditions of the modified ORGANOCELL process.
Process parameter Range Sodium hydroxide (NaOH), % on wood 17-22
Anthraquinone (AQ), % on wood 0.1
Methanol content in cooking liquor (by volume), % 25-30
Liquor-to-wood ratio 4.2 : 1
Cooking temperature, °C 155 - 170
Cookingjime at maximum temperature, min. 60 - 120 Source: Stockburger(1993).
It was reported that this process can pulp any kind of raw material, e.g., bagasse, straw, hardwoods, softwoods, including pine, etc. (Pappens, 1990). However, Ldnnberg et al. (1987) tried to reproduce an ORGANOCELL cook but the screened unbleached pulp yield obtained was only 47.2 - 50.7 % with kappa numbers 28.2 - 51.2 and reject 0 - 45
5.2 %, respectively in 1 - 3 h cooks, excluding 65 min heating up time. In addition, it was also claimed that the pulp quality was excellent, similar to that of kraft pulps with outstanding bleaching properties (Pappens, 1990; Stockburger, 1993). In terms of strength properties of the pulps, claims by Dahlmann and Schroeter (1990) could not be confirmed.
To the contrary, it was reported recently that the ORGANOCELL plant in
Kelheim was shut down due to many driving forces, one of them being the poor pulp quality (Gottsching, 1994). If the poor pulp quality problem can be resolved this process could be considered viable since the process has less environmental impact as shown by low AOX levels (Pappen, 1990), even though in terms of chemical recovery, the process may be as complicated as that of a kraft or the ASAM processes. Therefore, due to this aspect, the process seems to gain only marginal advantages over the kraft process.
On the other hand, it seemed that the main reason for the mill closure was that the pulp markets collapsed during 1995-1996 and the mill operated at a substantial loss for nearly 8 months. Further, the mill failed to operate as expected, as some mill components in the fiber line malfunctioned, due to the mill design deficiencies and construction problems (Hergert, 1998). Therefore, it was suggested that the mill should be redesigned and reconstructed in order to revive this process, otherwise, the ORGANOCELL process will remain "history". To date the fate of the process, as that of the commercial plant at
Kelheim, is uncertain and may be committed to doom. 46
2.3.2. Organic acid and ester pulping
Some organic acids, such as acetic acid (Kin, 1990; Vazquez et al, 1995; Yasuda etal, 1991; Davis and Young, 1991; Parajoefa/., 1993; Young al, 1986; Nimz, et al, 1989) and formic acid (Baeza et al, 1991) or peroxyformic acid (Hording et al,
1991; Sundquist and Poppius-Levlin, 1998), and ester (Young, 1989; Azis and
McDonough, 1987; Young etal, 1987) have also been tried for pulping purposes. The pulping results varied, depending on the type of solvents, wood species and pulping conditions.
Organosolv pulping with acetic acid can be carried out with or without addition of
catalysts. HC1 and H2S04 were more frequently used as catalyst in these pulping trials, but
HBr has also been tried by Benar and Schuchardt (1994) and demonstrated to be more effective than HC1. The Acetosolv process is an HC1 catalysed acetic acid organosolv pulping process. Nimz, et al. (1989) carried out pulping trials with this process, in which the raw material was cooked at 110 °C with highly concentrated aqueous acetic acid (93
%) containing 0.1 - 0.2 % HC1. The kappa numbers of the pulp were <20 for softwoods and <10 for hardwoods. A similar pulping trial on beech wood was also conducted by Kin
(1990). In this trial, the wood chips (pins) were cooked with 80 % acetic acid with small
amount of HC1 or H2S04 as the catalyst at 100 - 105 °C for 4 h under a reflux condenser.
The reported pulp yield was 45 - 49 % and the kappa numbers measured were 20 - 40 at best.
As reported by Young (1998), a pilot plant, built in southern Germany, to evaluate the Acetosolv process developed by Nimz and co-workers demonstrated massive 47 corrosion problems. To overcome the above problems, according to Young (1998),
Gotlieb and co-workers discontinued the use of acid catalysts in the cooking liquor while maintaining high acetic acid concentrations. However, due to these modifications, the cooking must be conducted at higher temperature, 170 - 190 °C, for 2 -3 h. This uncatalysed acetic acid pulping process is known as the ACETOCELL process. This process can produce pulps at yields of 46 - 53 % with kappa numbers of 10 - 25, depending on the species (Young, 1998). Another modification in acetic acid-based pulping was proposed by replacing the mineral acid catalyst with about 10-20 % formic acid. This modified version is known as the FORMACELL process (Puis, et al, 1999;
Saake, etal, 1998).
Besides acetic acid, formic acid is another low molecular weight carboxylic acid which has been intensively investigated for its potential to be used for pulping purposes.
Basically, the cooking liquor is highly concentrated formic acid with a small amount of
HC1 as catalyst. In an investigation conducted by Erismann et al. (1994), it was found that by using 79 - 92 % (v/v) formic acid with 0.22 % (w/v) HC1 as the catalyst to pulp
Eucalyptus grandis, the reported pulp yield was in the range of 44 - 52 % with kappa numbers of 30 to 40. Another experiment, conducted by Baeza et al. (1991) to pulp
Eucalyptus globulus under more or less similar pulping conditions, found that the pulp yield was 56 % with a kappa number 22 and reject content 1.6 %. This yield was obtained after a pretreatment by preheating and soaking the chips in water for 72 h.
Another type of formic acid-based solvent pulping process is the MILOX process, developed in Finland. In this process hydrogen peroxide is added into highly concentrated 48
(>70 %) formic acid as the cooking liquor. Thereby, the MJJLOX process can be considered as a hydrogen peroxide-reinforced formic acid (peroxyformic acid) pulping process. This process has been tried for pulping of several softwoods and hardwoods
(Hortling etal, 1991; Ruggiero etal, 1998; Puis, etal, 1999), also non-wood fibers
(Seisto and Poppius-Levlin, 1997; Seisto, etal, 1997). As described by Sundquist and
Poppius-Levlin (1998), the MILOX process is a multiple-stage pulping process, e.g., three stages for woods and two stages for non-wood fiber sources. In the first stage, the dry
raw material was cooked with formic acid with addition of H202, in the second stage with
only formic acid, and in the third stage with formic acid with addition of H202 again. For non-wood fibers, the first stage was eliminated. More detailed pulping conditions and the pulp properties are presented in Table 8. 49
Table 8. Three-stage MTLOX pulping conditions for hardwood and softwood, two-stage pulping conditions for non-wood species, and properties of the pulps. Pulping parameters / pulp properties Hardwood Softwood Nonwood First Stage Time, h 1.5-3.0 1.5-3.0 Temp., °C (max.) 80 80
H202, % on pulp 1.0 2.0-2.5 L : W 4 : 1 4 : 1 Second Stage Time, h 2.0-3.0 1.5 1.0 Temp., °C 107 130-140 107 Third Stage Time, h. 1.5-3.0 1.5-3.0 1.5-3.0 Temp., °C (max.) 80 80 80
H202, % on pulp 1.0 2.0-2.5 1.5 Screened yield, % 40-43 44-45 40-46 Rejects, % 0.1 - 1 0.2- 1.6 <1 Kappa No. 3.5 - 12 30-39 14-24 Brightness, % 47-30 17- 18 29-33 Viscosity, g/mL 1470 - 1100 820 - 850 950 - 960 Source: Sundquist and Poppius-Levlin (1998).
Ester pulping is another type of solvent pulping, derived from carboxylic acid basis. The type of ester mostly used in the pulping trials was methyl, ethyl, propyl and butyl acetate. Basically, the ester was used to substitute some proportions of acetic acid to enhance delignification (Young, 1998). Thereby, the process could be viewed as a modification of an acetic acid-based pulping process. Results of some pulping trials with the process were reported by Young (1989) as follows. The pulp yields were found to range from 46 to 55 %, for hardwoods at kappa numbers from 11 to 20 and 42 to 44 % for softwoods, i.e., spruce and pine, at kappa numbers from 29 to 22. In addition to the 50 wood species, the type and proportion of ester in the cooking liquor appeared to have a significant impact on the pulping results. Propyl acetate and butyl acetate showed the highest pulp yields of 53 % for aspen at 33 % ester concentration (the rest was equal amount of acetic acid and water) with 2 h cooking time at 170 °C. Kappa numbers as low as 5 and 7, respectively, were obtained (Young, 1989). Later Davis and Young (1991) reported that with a liquor composition of 15/70/15 ethyl acetate/acetic acid/water, the pulp yield was 58 % on aspen with a kappa number of 15. However, when the same liquor composition was applied to Loblolly pine (Pinus taeda L.) the yield dropped to 50
% with a kappa number 25.
2.3.3. Other solvent pulping processes
Some other solvents have also been tried for pulping purposes, e.g., amines (Green and Sanyer, 1982; Abbot and Bolker, 1984; Zargarian etal, 1988), chloroethanol (Nimz etal, 1986), cresols (Sakakibaraef a/., 1984), phenol (April etal, 1979), acetone/ oxygen (Deineko et al, 1989; Zarubin et al, 1989), and xylene sulfonic acid (Springer and Zoch, 1971). In general, almost all the authors claimed that the pulp yield was higher, but only by a few percents, and the strength properties were reported satisfactory or even comparable to that of kraft pulps from the same species. However, most of these processes had difficulties in pulping softwoods and resulted in inferior quality of softwood pulps. 51
2.4. Factors Affecting Delignification
Differences in pulp yields and quality are strongly correlated with the chemical environment existing in the various pulping liquors. Particularly in solvent pulping, the selection of solvent types and catalyst species has a significant impact. The chemistry of lignin in the fiber source, especially the types of phenylpropane units and chemical bonds connecting the units, is also an important factor in the delignification process. In addition, cooking time and temperature are important variables that also affect the rate and extent of delignification. Tirtowidjojo et al. (1988) showed that in acid catalysed organosolv pulping, the delignification rate constant was found to be doubled with every 10 °C cooking temperature increase over the range of 130 - 170 °C. Therefore, the outcome of a chemical pulping process is much dependent on those factors mentioned above.
2.4.1. Effect of type of lignin and linkages
Type of lignin and linkages connecting phenylpropane units in the lignin seem to play an important role in delignification. A study conducted by Tsutsumi et al. (1995) using syringyl and guaiacyl types of P-aryl ether lignin model compounds found that in an alkaline pulping system, syringyl type lignin was more reactive (cleaved faster) than guaiacyl type lignin. This may contribute to the higher delignification rate of hardwoods than softwoods. In kraft pulping, it was found that the higher the proportion of syringyl type lignin, the greater the rate of delignification (Chang and Sarkanen, 1973). Therefore, apart from the greater accessibility through vessels, the delignification rate of hardwoods is generally higher than that of softwoods due to their higher content of syringyl lignin as compared to softwoods. In addition, softwoods may have higher proportions of condensed lignin units which are resistant to hydrolysis to smaller soluble fragments. In hardwoods, middle lamella and cell corner areas of fibers contain mixtures of syringyl and guaiacyl lignin whereas their cell wall contains syringyl type lignin, exclusively (Sjostrom,
1981).
In addition to the types of lignin, types of linkages among phenylpropane units in the lignin molecule are also important in delignification. In a lignin molecule, there are numerous kinds of chemical bonds, as shown in Table 9 and Fig. 6, however, the ether bond is the dominant bond type. Therefore, the rate of cleavage of ether bonds will determine the rate of delignification in a pulping process. In organosolv pulping, where the success depends on the effectiveness of breaking down lignin macromolecules into fragments ready to dissolve in the cooking liquor, the rate and degree of delignification are mostly dependent on the rate of ether bond cleavage (Sarkanen, 1990). In acid catalysed organosolv pulping, the delignification is governed primarily by the solvolytic splitting of a-ether linkages although P-ether cleavage was also observed and seemed to increase with more strongly acidic systems (Sarkanen, 1990; McDonough, 1993). In spruce wood
(Picea abies), it was found that the middle lamella lignin contained more a-0-4 and also structures with p-P linkages, but fewer structures contained P-O-4 linkages (Sorvari et al., 1986). 53
Table 9. Proportions of types of linkages connecting phenylpropane units in spruce (Picea abies) and birch (Betula verrucosa) lignins (MWL). Proportions (%) Fig. 6 Types of linkages Spruce Birch Gua. Syr. Total A Arylglycerol-P-aryl ether 48 22-28 34-39 60 B Glyceraldehyde-2-aryl ether 2 2 C Noncyclic benzyl aryl ether 6-8 6-8 D Phenylcoumaran 9-12 6 E Structures condensed in 2- or 6-positions 2.5-3 1-1.5 0.5-1 1.5-2.5 F Biphenyl structures 9.5-11 4.5 4.5 G Diphenyl ether structures 3.5-4 1 5.5 6.5 H 1,2-diarylpropane structures 7 7 J P-P linked structures 2 3 K Quinone ketal structures traces Source: Adler(1977).
F (troces) F (Irocesl
Ghroces)
Fig. 6. Types of linkages connecting phenylpropane units in lignin (Adler, 1977). 54
2.4.2. Effect of solvent
The type of solvent selected for an organosolv pulping process is a critical parameter as it affects the pulping results due to the different pulping environments that they provide for lignin solubilization. The capability of solvents to dissolve lignin depends on the hydrogen bonding capacities and Hildebrand's solubility parameters (8) of the solvents. The solubility parameter was defined by Hildebrand as the square root of the internal pressure or cohesive energy density of the substance. The value of solubility parameter (5) is calculated from the formula (Hildebrand and Scott, 1950) :
8 = (-E/vy*, where : 8 is Hildebrand solubility parameter, -E is the energy of vaporation to a gas at zero pressure, and V7 is the molal volume of the liquid.
It should be noted that E and V are temperature dependent, therefore, 8-values must be calculated for each temperature used. The importance of knowing the 8-value is its use for semi-quantitative prediction of the lignin dissolving power of solvents.
Schuerch (1952) showed that the solubilizing power of the solvents increases with increasing hydrogen bonding capacities whereas solvents with 8-values of 10 - 14 are good solvents for some isolated lignins. Solvents, such as dioxane, acetone, methanol, ethanol and ethylene glycol, were found among the solvents having the above range of 8- values. However, the extent of lignin dissolution is also affected by molecular weight of the lignin. It was also shown that the lignin dissolution decreased as the molecular weight increased. 55
In case of using solvents as delignifying agents for organosolv pulping, the solvents as such are generally powerless to remove lignin from the wood matrix in separating the fibers individually. Therefore, the solvents must be mixed with water and mostly with
additional protons of an acid, e.g., HC1 or H2S04, as catalysts. By altering solvent/water ratios, the Hildebrand solubility parameter, as a measure of delignifying power of the mixture, is also altered. In this regard, Bose and Francis (1999) demonstrated a relationship between delignification power and the solubility parameter of solvent/water mixtures of a number of different solvents, as shown in Fig. 7. Especially for ethanol-
water mixtures, increasing water proportions in the mixture increases the 6-values (6ethanol
3 0,5 -12, 6water -22.5 (cal/cm ) ) concurrently with fiber swelling (Ni and Heiningen, 1997).
110 T
+ Water
Methanol 90 + -•— Ethanol
—±— Propanol
° Acetone
o Dioxane ai Q 50 + A Dimethyl Sulphoxide
30 +
10 10 15 20 25
Solubility Parameter, (cal/cm3)
Fig. 7. Delignification as a function of the solubility parameter of solvent/water mixtures (Bose and Francis, 1999). 56
Particularly, in alcohol organosolv pulping, the type of alcohols and the alcohol/ water ratio have a significant impact on delignification (Goyal, et al, 1992; Uraki and
Sano, 1999; Bose and Francis, 1999; Girard and Heiningen, 2000). In autocatalysed organosolv pulping of hardwoods, Goyal, et al. (1992) demonstrated that delignification increased with increasing the ethanol concentration, particularly from 50 to 70 % (v/v), even though the proton concentration in the cooking liquor was reversed. On the other
hand, the results of a 0.005 M H2S04 catalysed organosolv pulping of Norway spruce
TMP fibers, using lower molecular weight alcohols, i.e., methanol, ethanol and propanol, in varied ratios to water, conducted by Bose and Francis (1999) showed that the highest achievable degree of delignification was obtained by ethanol (81 % delignification), followed by propanol (76 % delignification) and then methanol (73 % delignification). In terms of the alcohol/water ratio, the study indicated that around 90 % alcohol (v/v) was the maximum alcohol concentration, regardless of the types. No further delignification occurred beyond 90 %, whereby the delignification even declined. However, there was no apparent delignification improvement at alcohol concentrations higher than 76 % for ethanol and 79 % for propanol.
2.4.3. Proton catalysis
The function of a catalyst in a chemical reaction is to increase the reaction rate by lowering the activation energy. In case of organosolv delignification under acidic condition, the effective break-down of lignin macromolecules to soluble fragments depends on the availability of protons as the catalyst (Sarkanen, 1990). In this organosolv 57 pulping the source of proton could be from the carboxyl groups found in the wood components themselves, as reviewed in Section 2.1.7., and / or externally added.
Particularly for the internally generated protons, the protons are generated from carboxyl groups through an ion exchange mechanism (Abubakr et al, 1997). Therefore,
Paszner and Cho (1989) proposed that in the NAEM catalysed organosolv pulping, the protons could be generated through formation of a "calcium bridge" between two adjacent cellulose chains, as shown in Fig. 8. However, the Paszner and Cho's proposal has not been proven experimentally but the formation of such calcium bridge occurring naturally had been reported in the literature (Fry, 1989). The proton generation by ion exchange has been observed at room temperature on treatment of both softwoods and hardwoods
with the cooking liquor, consisting of 80 % aqueous methanol with 0.05 M CaCl2 /
Mg(N03)2 catalyst. pH drops of up to 1.0 unit, from 6.3 to 5.2, were observed with various species of wood and non-wood fiber sources.
Fig. 8. The proposed mechanism of proton generation during NAEM-catalysed organosolv pulping by Paszner and Cho (1989). 58
Lignin degradation in proton catalysed delignification is mainly due to the hydrolysis of a - and P-aryl ether bonds in lignin molecules (Meshgini and Sarkanen, 1989;
Balogh et al, 1992). As indicated by their activation energy, i.e., 80 - 118 kJ/mol for a- aryl ether and 148 - 151 kJ/mol for P-aryl ether bonds, under the same acidity and reaction temperature, the cleavage rate of a-aryl ether bonds is much higher than that of
P-aryl ether bonds (Meshgini and Sarkanen, 1989). Therefore, it can be assumed that due to the higher cleavage rate and lower activation energy, early stage delignification in organosolv pulping under acidic conditions must be dominated by a-aryl ether bond cleavages. The mechanism of a-aryl ether bond cleavage, as shown in Fig. 9, is triggered by protonation of the a-carbon to form a benzyl carbonium ion intermediate. This intermediate could either react with water, resulting in a benzyl alcohol, or with an alcohol solvent, forming a benzyl ether. Meanwhile, a condensation reaction with an aromatic nucleus also takes place (Sarkanen, 1990).
In addition to a-aryl ether bond cleavage, the cleavage of P-aryl ether bonds, requiring higher acidity, also takes place (Bose and Francis, 1999). However, since almost half of the interunit linkages present in a lignin molecule (see Table 9 for spruce) is
P-aryl ether, the extent of hydrolytic cleavage of these bonds determines the extent of lignin degradation to the soluble fragments (Hoo et al., 1983; Bose and Francis, 1999), and subsequently controls the fiber liberation. The hydrolysis of P-aryl ether bonds occurs by two different pathways, A and B, in which the cleavage in pathway A is faster than B
(Fig. 10). As shown in Fig. 10b, the reaction also involves the release of formaldehyde from the y -carbon that tends to be condensed with aromatic moieties. At the same time, 59 the phenolic P-aryl ether bonds could undergo homolytic cleavage resulting in more condensed lignin structures (Fig. 11) (Sarkanen, 1990). Simultaneous lignin recondensation reactions are seen as a drawback of the delignification under acidic conditions and prevention of such reactions could be also a major strategy in the selection of solvent concentration and catalysts for organosolv pulping.
CH.OH I CH CH-N
CH.OH I CH H* I CH ©
OMe —O —0
RO- = a- elky! or HN ether linkage OMe CH.OH CH- —o I CHOR
HN-NudeophNe,tudiaa MeOH, phenol, or H^SO)
Fig. 9. The mechanism of hydrolysis and competing condensation reactions of a-aryl ether bonds in lignin (Sarkanen, 1990) (Correction : reaction of benzyl carbonium ion intermediate with H20 results in a benzyl alcohol, therefore, N at the a-carbon should be replaced with OH, while in the reaction with HN, a nucleophile, the OR should be replaced with N). 60
Fig. 10a. Hydrolysis of P-aryl ether bonds (Pathway A) (Sarkanen, 1990).
Fig. 10b. Release of formaldehyde from y-carbons (Pathway B) (Sarkanen, 1990).
Fig. 11. The mechanism of rearrangement reactions caused by homolytic cleavage of P-aryl ether bonds in phenolic lignin structures (Sarkanen, 1990). 61
An investigation on the role of P-O-4 cleavage in acidic organosolv pulping of softwoods, conducted by Bose and Francis (1999), indicated that there was an association between the extent of P-O-4 cleavage and degree of delignification. It was shown that a low degree of P-0-4 cleavage resulted in a low degree of delignification, whereas a high degree of P-0-4 cleavage resulted in a high degree of delignification. Although the investigation on P-0-4 cleavage was conducted on a model compound, the pulping conditions applied were identical.
2.5. Topochemistry of Delignification
Topochemistry of delignification is a very important aspect of delignification that determines the outcome of a pulping process, i.e., pulp yield and property. With liquor compositions, such as those described in Section 2.3.1.1.2., an acidic buffer system with pH ~ 6.3 is created to control pH at the desired range for delignification. Thereby, the preferred topochemical delignification, allowing fast removal of lignin in the middle lamella and cell corner, is able to take place earlier, as indicated in the topochemical study conducted by Behera (1985). It was found that the delignification in the NAEM-catalysed organosolv pulping process containing 78 % methanol proceeded in two distinctive stages.
Bulk delignification occurred in the first stage, where lignin removal was fast. In this stage, about 70 % of the lignin was removed, including complete loss of cell corner and middle lamella lignins, causing early and total fiber liberation from the wood matrix.
During the residual delignification, in the second stage, the rate of delignification proceeded at a slower rate, and lignin removal was limited to the secondary wall region of 62 the wood matrix. Continued second stage delignification was usually associated with substantial carbohydrate, mainly hemicelluloses, losses from the cell wall and hence the continued pulp yield loss.
The topochemistry of delignification by the NAEM catalysed organosolv process appears to be opposite to that of other processes, as shown in Fig. 12, including the
ORGANOCELL process (Fig. 12b). As can be seen in Fig. 12, more lignin is removed from the secondary wall with most processes (Wood et al, 1972; Fengel et al, 1989;
Paszner and Behera, 1989). In addition to those described in Fig. 12, soda/AQ pulping of
Douglas-fir was found to be more selective in removing middle lamella lignin than either soda or kraft pulping of the same species (Saka et al, 1982). 63
KRAFT ACID SULFITE NEUTRAL SULFITE ACIO CHLORITE ORGANOSOLV
fl • O o N. ML tS 3 A \ s N. \ • N\ -i eo 80 : ^ •
0 20 40 80 80 0 20 40 80 80 0 20 40 60 80 0 20 40 so eo c 20 40 eo eo
Fig. 12a.
Progress of the delignillca- lion during a two-stage (left) and a one-stage (right) organosolv pulp• ing of spruce wood shown by the relative bromine concentration (above) and the lignin percentage (below). CC =cell corner, ML = middle lamella (including SI and the outer part ol" the S2), SW = secondary wall; W = wood, un• treated, M =meihanol-water stage, MA = methanol-alkali stage
Fig. 12b. Fig. 12. The relationship between the percentage of lignin removed from the middle lamella (ML) and secondary wall (S) and the percentage of lignin removed from whole wood by kraft, acid sulfite, neutral sulfite, acid chlorite (Wood et al, 1972), NAEM process (Paszner and Behera, 1989) (Fig. 12a) and the ORGANOCELL process (Fig. 12b)(Fengel etal, 1989). 64
2.6. Pectin Degradation and Dissolution
Pectin degradation followed by dissolution is due to glycosidic bond cleavage through two main mechanisms, i.e., 1) P-elimination, and 2) hydrolysis. The rate of degradation is pH dependent. Since there are two different types of pectin, acidic and esterified pectins, the glycosidic splitting involved is not the same for these two types of pectin. The degradation of esterified pectin prevails by the P-elimination mechanism (Fig.
13), and at pH above 3.8 the degradation rate is enhanced (Albersheim et al, 1960;
Keijbets and Pilnik, 1974; Keijbets, 1974), whereas the degradation of the acidic pectin proceeds through acid hydrolysis at a much lower rate (Smidsrad et al, 1966; Krall and
McFeeters, 1998). It was also found that at pH lower than 3.5 the hydrolysis rate of polypectate was higher than that of the acidic (35 % methylation) or esterified (70 % methylation) pectins. In addition, it was also reported that acid hydrolysis of pectin was not inhibited by the presence of calcium ions (Krall and McFeeters, 1998).
HO H
r I n
+ HP
Fig. 13. Mechanism of P-elimination in depolymerization of a partially esterified galacturonan chain (Keijbets, 1974). 65
In case of the cleavage of pectin glycosidic bonds through the p-elimination
mechanism, the presence of a methylester at C6, next to which chain cleavage occurs, is crucial to trigger the glycosidic splitting (Albersheim et al, 1960; Keijbets, 1974). The rate of the esterified pectin degradation is enhanced by increased degree of esterification
(Krall and McFeeters, 1998). The P-elimination mechanism is also pH dependent, it requires a higher pH range, from 3.8 to nearly neutral, 6.8. Keijbets (1974) reported that the reaction rate of P-elimination of pectic galacturonan at boiling temperature was
affected by pH, i.e., hydroxyl ion concentration, the presence of a methylester at C6 and the nature and quantity of anions and cations in the medium surrounding the pectin.
Keijbets (1974) conducted an investigation on the effect of cations, anions and pH on pectin solubilization in association with potato tuber cell wall separation. The result of the investigation can be summarized as follows. It was found that cations, anions and pH of the medium greatly influenced the pectin solubility. However, the extent of the solubilization was depending on the ion species and the ratio of the ion to carboxylate ion
(COO"). It was shown that if the ratio of Ca+7COO" was > 2, pectin solubility was enhanced but if the ratio was < 2, pectin solubility was retarded. Other divalent metal ions, i.e., copper, iron and magnesium, were also investigated. Copper and iron showed similar behaviour to that of calcium when the cations to COO" ratio was < 1. At these ratios, pectin solubilization was progressively inhibited. Unfortunately, the investigation was not carried out on the cation to COO' ratios >1. On the other hand, potassium showed a greater effect in encouraging pectin solubilization than that of calcium when the ratio of K7COO" was < 10 but when the ratio was > 10, the effect of calcium surpassed 66 that of potassium. In case of magnesium, it was thought to aid in the insolubility of native pectic substances (protopectin) by polyvalent ion bonding. Its function was suspected to be similar to that of calcium, copper and iron, however, it was found that magnesium exerted no insolubilizing influence during boiling on the pectic galacturonan of potato tuber cell wall, as compared to potassium. In case of the effect of anions, it was found that citrate ion had a greater effect in encouraging pectin solubilization than that of the chloride ion. On the other hand, pH also has a great influence in pectin solubility. It was shown that there was a steady increase in the pectin solubility when pH of the buffer solution was raised from 6.1 to 6.5. Based on this result, there is an indication that high pH (alkaline) medium favours pectin removal from wood due to P-elimination.
Regarding the relationship between pectin solubility and cell separation as reported above by Keijbets (1974), with tuber cells it could be observed that cell separation improved with increasing pectin solubility. In this particular case, it indicates that removal of pectin from the middle lamella by degradation and solubilization plays an important role in cell separation. Although the above investigation was mostly conducted on the cell wall of potato tubers, the results might be comparable to wood since the potato tuber is the modification and enlargement of an underground stem (Langer and Hill, 1991). 3. MATERIALS AND METHODS
3.1. Raw Material
Spruce wood (Picea engelmanni) was the wood species used in this study. The wood chips were prepared manually from the lower part of a fresh-cut 95 year-old log.
The log diameter was 12 in (-30 cm), taken from the research forest of the University of
British Columbia. The size of the chips was set as % in (-20 mm) long, V4 in (-13 mm) wide and VB in (-3 mm) thick. After chipping, the wood chips were well mixed. The wood chips were air-dried and the moisture content was 10.5 %. A small amount of the wood chips was also sampled and ground into 40 - 60 mesh (0.25 - 0.40 mm) sawdust for determination of extractives, lignin, holocellulose, a-cellulose contents and sugar composition in the wood, as background information against which the pulping results can be evaluated.
3.2. Methods
3.2.1. Pulping
3.2.1.1. Kraft pulping
A standard kraft cook was carried out to prepare the kraft pulp samples. The resulting kraft pulp was used as background information against which the results of organosolv pulping can be compared. The pulping was conducted in a 450-mL high pressure stainless steel vessel without using a glass liner. The same vessel was also used for organosolv cooking. The type of cook was a batch or stationary type conducted in a
67 68 high temperature oil bath. The cooking temperature was set at 170 °C, at which temperature the pressure developed was 120 psi. The pulping time applied was 30, 60,
90, 120, 150 and 180 min. These cooking times included the heating-up time with 25 °C registered as the starting point of measuring the cooking time. The time-temperature profile of the cooking can be seen in Appendix 1.
The chemical charge was 21 % active alkali as NajO on oven-dry wood basis and the sulfidity was 26 % as NajO. The wood-to-liquor (W/L) ratio was 1 : 7, whereby 35 g oven-dry equivalent of air-dry wood chips was charged with 245 mL of cooking liquor.
At this specified cooking liquor composition and W/L ratio, 28.65 g of NaOH (as NaOH) and 9.81 g of NajS (as Na2S) were required to make one liter solution.
After the predetermined cooking time was reached, the vessel was immediately removed from the thermal oil bath and cooled under cold tap water. The black liquor was decanted. Several cooked wood chips were taken, washed, air-dried and kept separately for immunocytochemical study for detection of the distribution of residual pectin in the wood chips. The rest of the chips were disintegrated with a standard pulp disintegrator in
2 L of water for 15 min (about 50,000 revolutions) and the pulp was strained on a filter paper on a Buchner funnel under vacuum. The pulp was continuously washed with distilled water under vacuum until all the black liquor was removed. The washed pulp was air dried and the unscreened (total) pulp yield, including the chips saved for immunocytochemical study, was determined. Subsequently, the pulp was screened as . described in Section 3.2.1.2.3. 69
3.2.1.2. Organosolv pulping
3.2.1.2.1. Pulping conditions and procedures
The solvent used for all organosolv pulping cooks was 80/20 methanol/water
(v/v). The pulping was conducted in several series of experiments with a number of different catalysts at different concentrations and cooking times as outlined in Section 1.4.
All organosolv cooks were conducted by the single-stage stationary vessel, batch method.
For the purpose, a 450-mL high pressure stainless steel vessel, i.e., the same vessel used for kraft cooking, equipped with a glass liner, was used. The vessel was also equipped with a pressure gauge to monitor the pressure, a thermocouple, to monitor the temperature and a relief needle valve to control the pressure inside the vessel. The amount of wood charged into the vessel was 31.0 g on an oven-dry basis for all cooks. Cooking liquor charged was 310 mL, giving an effective liquor to wood ratio of 10 : 1 (mL / g oven-dry wood). The cooking temperature was measured inside the vessel with the aid of a thermocouple. 25 °C was the starting point of the cooking time. The cooking temperature was fixed for all cooks at 205 °C. Under these conditions, the pressure during the cooking was allowed to rise and was maintained at 1500 psi.
After the desired cooking time was reached, the vessel was removed immediately from the thermal oil bath and cooled under cold tap water. The spent liquor was decanted, and the pH was measured. Several cooked wood chips were taken, washed, air- dried and kept separately for immunocytochemical study for the distribution of residual pectin in the wood chips. The rest of the chips were slushed in a standard pulp disintegrator and washed. 70
3.2.1.2.2. Pulp washing
The cooked chips were disintegrated in a standard pulp disintegrator in 2 L of
70/30 methanol/water (v/v) for 15 min (about 50,000 revolutions) and then the excess washing liquor (2 L) was filtered off on a Buchner funnel. The resulting pulp was washed with 3 x 500 mL of 70/30 methanol/water (v/v) under vacuum. Between each washing stage, the pulp was soaked for 15 min before the washing liquor was removed by vacuum filtration. The pulp was allowed to air-dry and the unscreened yield was determined.
3.2.1.2.3. Pulp screening
The pulp was screened on a laboratory scale portable screen using No. 9 screen plate. The portion of pulp retained on the screen, the reject, was collected and dried in the oven at 105 °C. The amount of the rejects is expressed as the percentage of oven-dry wood and oven-dry pulp basis. On the other hand, the portion of pulp passing through the screen, the screened pulp, was caught on a No. 325-mesh sieve. After removing the excess water by vacuum filtration and rinsing the pulp several times with distilled water, the pulp was air-dried and the screened pulp yield determined. The air-dry pulp was ready for evaluation and chemical analysis. All chemical analyses were conducted in duplicate.
3.2.2. Chemical analyses on the raw material and/or pulp
3.2.2.1. Extractives
The extractives in the wood were determined by Soxhlet extraction of the sawdust samples with ethanol, benzene and water. The extraction was sequential, first with a 71 mixture of 1/2 ethanol/benzene (v/v), followed by ethanol and then water (Tappi T 264 om-88).
5.0 g OD equivalent of the sawdust sample was weighed into an extraction thimble. The thimble containing the sawdust was placed in the Soxhlet apparatus and extracted with 200 mL of solvent for 6 to 8 h with the refluxing rate being no less than 4 siphons per hour, thus at least 24 extractions in total. After extraction with ethanol- benzene, the thimble and its content were washed with ethanol to remove the benzene.
The thimble was then returned to the Soxhlet apparatus and extracted with 95 % ethanol for 4 h or longer until the alcohol siphoned over was colorless. When the extraction was completed, the sample was washed with distilled water to remove the ethanol and then transferred into a 1000-mL Erlenmeyer flask, and 500 mL of boiling distilled water was added. The flask was heated for 1 h in a hot, boiling water bath. After the extraction was completed, the sample was oven-dried to constant weight at 105 ± 3 °C. The amount of extractives reported was the average of triplicate determinations. The amount of extractives was calculated as follows:
% Extractives = {(ODj - ODf) / ODJ x 100,
where : OD; = oven-dry unextracted wood weight, and
ODf = oven-dry extracted wood weight.
The sample required for the other chemical analyses was identically extracted according to the above procedure but has never been oven-dried. 72
3.2.2.2. Klason lignin (acid-insoluble lignin)
Sample preparation and determination of Klason lignin and sugar analyses were done simultaneously. Klason lignin content was determined on both wood chips before and after pulping, i.e., pulp or undefibrated cooked chips. For undefibrated cooked chips, the sample was ground into sawdust passing No. 40 mesh. Klason lignin is the lignin
obtained by dissolving all carbohydrates of the wood with 72 % H2S04 (sulfuric acid)
(TAPPIT222 om-88).
1.0 g oven-dry equivalent of air-dry pulp or sawdust sample of wood or ground undefibrated cooked chips was placed in a glass digestion cylinder, and 15 mL (for wood
and undefibrated cooked chips) or 20 mL (for pulp) of 72 % H2S04 was added and mixed with a glass stirring rod. The mixture was allowed to stand with frequent stirring for 2 h at room temperature. At the end of the required reaction time, the mixture was transferred into a 1-L Erlenmeyer flask and diluted with 420 mL or 560 mL, respectively, of boiling, hot distilled water to produce a 4 % sulfuric acid solution. Thus, the total volume of acid would be 435 mL or 580 mL, respectively, and then autoclaved for 1 h at
121±3 °C (Anon, 1995b / ASTM E 1758 - 95). The dark brown insoluble material is the
Klason lignin. After the insoluble material settled, the solution was filtered through a tared medium porosity crucible (30M) under vacuum. The Klason lignin collected in the crucible was subsequently washed with 300 mL of hot, distilled water, and then dried at
105±3 °C for 4 h. The clean supernatant filtrate was kept for acid-soluble lignin and sugar analyses described in the following sections. 73
3.2.2.3. Acid-soluble lignin
The acid-soluble lignin content of the wood raw material, pulp and undefibrated cooked chips was determined spectrometrically on the clean supernatant filtrate by measuring the absorbance of ultra-violet radiation at 205 nm wavelength (TAPPI um-
250). The filtrates were diluted and the volumes were adjusted to 575 mL for wood and
undefibrated cooked chips, and to 770 mL for pulp to produce the filtrate at 3 % H2S04.
A 3 % H2S04 solution was also prepared for reference. The filtrate was placed in a special cuvet and the absorbance was measured with a UV spectrophotometer at 205 nm.
The accepted absorbance values ranged from 0.2 to 0.7. The filtrates having absorbencies
higher than 0.7 were further diluted with 3 % H2S04 until the absorbance fell within the accepted range. The acid-soluble lignin content (B) in the filtrate (g/L) was calculated as follows :
110 where : A = absorbance,
D = dilution factor of the filtrate, expressed as Vp/Vo, where VD is the volume of
diluted filtrate and V0 is the original volume of the filtrate taken.
The formula used to calculate the lignin content in the samples
B x V x 100 Lignin (%) 1000 x W where : B = lignin content in the filtrate(g/L) , V = total volume of filtrate,i.e. , 575 mL; 770 mL, W = oven-dry weight of original material (sample). 74
3.2.2.4. Sugar analysis
3.2.2.4.1. Sample preparation
All samples, i.e., raw material, pulp and undefibrated cooked chips, were analysed for sugar composition. The sugar solutions were obtained as part of the Klason lignin procedure (see Section 3.2.2.2). They were the supernatant filtrates. After dilution and adjustment of the supernatant filtrate volumes (see Section 3.2.2.3), the filtrates were ready to be analysed directly by High Performance Liquid Chromatography (HPLC) for their sugar composition. Before the sugar solutions were injected into the HPLC, fructose was added as the internal standard. The samples were injected without any pretreatment or neutralization.
3.2.2.4.2. HPLC analysis
Sugar analysis was carried out by using a computer controlled High Performance
Liquid Chromatograph (HPLC) built by Dionex corp. This HPLC unit is equipped with a post column pump and an autosampler. The Carbopack PA-1 column used is sold by
Dionex Corp., and the detector was a pulsed amperiometric detector (PAD). The eluent was degassed deionized distilled water. The elution rate was set to 1.0 mL/min. The total analytical time required for each sample was 55 min, consisting of several events (Jeong,
1994):
1) min 0 - 8.5, regenerating the column after each run with 0.25 M NaOH at a flow rate
of 1.0 mL/min.
2) min 8.6-24.5, washing the column with the eluent at a flow rate of 1.0 mL/min. The 75
elution was continued until the end of analysis. Also in this event, the post column
pump was activated to pump in 0.50 M NaOH to be mixed with the separated sugar
solution before reaching the detector cell (electrode). The pumping rate was 1.0
mL/min, and the pump remained active until the end of analysis.
3) min 24.6 - 25.5, sampling event consisting of sample injection into the column.
4) min 25.6 - 55.0, separation of sugar components.
3.2.2.4.3. Establishment of the standard curve
A standard sugar solution was prepared by dissolving high purity sugar standards, i.e., arabinose, galactose, glucose, xylose and mannose, in 3 % sulfuric acid. In addition to these monomelic sugars, fructose was also added into the standard solution as the internal standard. The standard sugar solution was prepared in 3 different concentrations
(three-point calibration). The areal ratio between each monomeric sugar and the internal standard was used as the dependent variable to construct a simple regression line. The amount of each monomeric sugar found in the experimental samples was quantified based on the constructed regression lines. The general equation for the regression line is :
y = a + bx, where : y = the area ratio of each monomeric sugar to the internal standard (fructose), x = the standard sugar concentration (mg/mL), a = the intercept, and b = the regression coefficient. 76
3.2.2.4.4. Quantification of sugars
The monomeric sugars resulting from the HPLC analysis were quantified in their polymeric form as percentage of the sample. Therefore, each sugar quantity was corrected for hydration taking place during hydrolysis and the loss because of degradation during autoclaving. The correction factor for hydration is 0.9 for hexoses, and 0.88 for pentoses. The correction for the losses were found to vary depending on the type of sugar and the extent of degradation due to the temperature variation in the autoclave. The amount of sugars lost during autoclaving was estimated by including a standard sugar solution of the same concentration as the concentration of sugars in the wood. The difference in the standard sugar concentration, before and after autoclaving, was considered the amount of sugar lost or degraded.
3.2.2.5. Holocellulose
The holocellulose content of wood was determined by the single-stage chlorite addition method, a modification of the Wise et al.'s method (Wise et al, 1946). 2 g extracted wood sawdust sample was weighed into a 50-mL glass reaction tube equipped
with a screw cap. 12 mL of 20 % NaC102 (sodium chlorite) and 28 mL buffer solutions were added into the reaction tube. The buffer solution is a mixture of 60 mL glacial acetic acid and 1.3 g NaOH per liter aqueous solution. The initial pH of the reaction solution was 3.6, which was within the desired range of 3.2 - 3.8. The reaction tubes containing the sample, chlorite and buffer solution were placed in a shaker bath at 70 °C and shaken for 6 h. The digestion was stopped by quenching the tubes in a cold water bath (<5 °C). 77
The contents of the cold tubes were transferred into tared sintered glass crucibles (30C) using 100 mL of 1 % acetic acid solution, and then washed with an additional 100 mL, followed by 100 mL distilled water, and finally washed with 2 x 25 mL portions of acetone. The filtration was concluded by applying suction for 5 min. The holocellulose was conditioned in the CTH room (20 °C, 50 % RH) for 1 week, weighed and dried in the oven at 105±3 °C for 4 h. The oven-dry holocellulose was weighed and expressed as the percentage of oven-dry extracted raw material.
3.2.2.6. Alpha-cellulose
Alpha-cellulose is the 17.5 % sodium hydroxide insoluble fraction of cellulose
(Browning, 1967). The amount of alpha-cellulose is indicative of the quality of the pulps or lignocellulosics, especially in utilization as dissolving pulp. The alpha-cellulose content in wood was determined after chloriting as described in Section 3.2.2.5. The air-dry holocellulose, never been oven-dried, obtained in Section 3.2.2.5., was washed with 50 mL 17.5 % NaOH in 10 mL portions, added in 10 min intervals while stirring with a glass rod after each addition. The total time required was 60 min. After the last alkali addition, the alpha-cellulose was washed with 2 x 50 mL distilled water, 2 x 100 mL 1 % acetic acid, 2x50 mL distilled water, and 2x50 mL acetone. The alpha-cellulose was firstair - dried and then oven-dried at 105±3 °C for 4 h. The amount of alpha-cellulose was expressed as the percentage of oven-dry, extractives-free raw material. 3.2.2.7. Kappa number
Kappa number is defined as the number of cubic centimetres of 0.1 N potassium permanganate solution consumed by one gram of moisture-free pulp under the specified conditions. This is an indirect method used to determine the amount of residual lignin in pulp. Therefore, it is a very useful tool for evaluation of the extent of delignification in a pulping process. The method used in this experiment is TAPPIUM-246 known as Micro
Kappa Number Method, which is a modified method of the Standard Kappa Number
Method, T-236. In principle, both methods are the same, but in UM-246 the volume of water and reagents, and indeed the amount of sample, are scaled down proportionally while the permanganate concentration and acidity are maintained the same as in T-236.
The amount of pulp sample was weighed in such a way, usually by trial and experience, that it consumed 50 % of the permanganate solution as closely as possible. In practice, values between 30 - 70 % are acceptable. The pulp samples were disintegrated in 70 mL distilled water in the Warring blender for 2 min, transferred into a 250-mL
Erlenmeyer flask using 10 mL distilled water, and conditioned in a water bath to 25 °C.
Exactly 10 mL 0.1 N KMn04 and 10 mL 4 N H2S04 were pipetted into the flask at 25 °C, and a stopwatch was started simultaneously while the mixture was continuously stirred.
The reaction was stopped after exactly 10 min by quickly adding 2 mL 1.0 N KI. The mixture was immediately titrated with standard 0.1 N Na^Oj (sodium thiosulfate) to a straw-yellow colour, at which point 5 drops of starch or thyodene indicator were added into the mixture, and continuously titrated to the disappearance of the blue colour. The kappa number, K was calculated as follows : 79
K = (P x f) / w, where : P = the cubic centimetres of 0.1 N permanganate solution consumed by the test specimen, f = the correction factor to 50 % permanganate consumption (from the table dependent on the value of P), w = the weight of moisture-free specimen (in gram).
3.2.2.8. Viscosity
The viscosity of the pulp was determined by using the Capillary Viscometer
Method (TAPPI T230 om-89). The viscosity was measured with a Cannon-Fenske capillary viscometer on 0.5 % cellulose (pulp) solutions in 0.5 M cupriethylenediamine
(Cuen) as the solvent, at the temperature of 25 °C. Dispersion of the pulp sample requires hand shaking of the sample mixture. The viscosity procedure used is known as the Closed
Bottle Procedure. The viscosity of the pulp is calculated by using the formula :
V = Ctd, where : V = viscosity of cupriethylenediamine solution at 25 °C, mPa.s (cP), C = viscometer constant found by calibration, e.g., 0.1 for viscometer size of 200, t = average efflux time, s, d = density of pulp solution, g/cm3 (=1.052).
3.2.3. Acidity and buffer capability of cooking liquor
The acidity of cooking liquors was monitored throughout the study. The pH of cooking liquors was measured before and after cooking. In addition to the pH measurement, the amount of proton generated during the cooking was also estimated by titration with 0.15 N NaOH (Sihtola, 1954). However, this procedure could only be 80 applied to the spent liquor without catalyst and with catalysts of chloride salts of mono- and divalent metal ions. The titration procedure was adapted and modified from the method of Ant-Wuorinen and Visapaa (1954).
The titration was conducted on the spent liquor after removing the methanol by evaporation. After evaporating the methanol, the dissolved lignin was precipitated and removed. Thereby, the proton concentration in the spent liquor was increased five times and the titration was carried out at that concentration. Before the titrant was applied, the methanol-free spent liquor was degassed with nitrogen while stirring for 10 min to remove
any dissolved C02 (carbon-dioxide). The titration was done under nitrogen atmosphere in
order to avoid the influence of C02. The end point was the initial pH, i.e., the pH of fresh cooking liquor without the presence of methanol. Thereby, the amount of NaOH consumed during the titration was assumed to be equal to the amount of proton generated during the cooking. The quantity of protons generated was expressed in rnM/kg oven-dry wood (ODW).
In addition to the spent liquor titration, the fresh liquor, without catalyst and with
0.050 M catalysts of chloride salt of mono- and divalent metal ions, was also titrated with
0.001 M HC1. The purpose of this titration was to estimate the buffering capability of each catalyst within a certain pH range. 25 mL of fresh liquor was titrated with 0.001 M
HC1. The pH and amount of HC1 added into the liquor were recorded and plotted to show the relationship. 81
3.2.4. Immunocytochemical labelling of pectin for fluorescent microscopy
The sample preparation and labelling procedures for fluorescent microscopy were adapted from Vennigerholz (1992). The primary antibodies were the monoclonal antibodies JTM5 and JTM7. The antibody JIM5 is from the IgG2a class and JTM7 is from
IgA class (Knox et al., 1990). These monoclonal antibodies were produced in rat. The antibodies were kindly provided by Dr. K. Roberts, from John Innes Institute, Norwich,
U.K. These antibodies have been proved to specifically recognize the two types of pectin epitopes, i.e., JTM5 to recognize unesterified or low esterified (<35 % esterification) and
JTM7 to recognize methyl-esterified (35 - 90 % esterification) pectins (Knox etal., 1990;
Vennigerholz, 1992; Westermark and Vennigerholz, 1995).
The labelling was conducted on the wood chips before and after cooking. The
cooks selected for this study were the time series cooks of 0.025 M A1C13, 0.050 M CaCl2,
combination of 0.025 M CaCl2 and 0.025 M Mg(N03)2, 0.050 M citric acid and Kraft, 60 min cooks without catalyst, 0.050 M NaCl and HC1 at various concentrations. The selection was made to cover a wide range of starting and ending pH of the cooking liquors and represent all catalyst groups used in this research.
3.2.4.1. Sample preparation and fixation
Wood chips, before and after pulping, were cut into small pieces of about 1 mm thick and 2.5 mm long, and then fixed in 2.5 % glutaraldehyde in 0.05 M phosphate buffer
(pH 7.2) for 3 h. After the desired fixing time was reached, the samples were washed in the buffer for 3 x 30 min. The samples were then subjected to post-fixing in 1 % osmium 82 tetroxide for lh. The 1 % osmium tetroxide solution was made by diluting a higher concentration with Tris-HCI buffer (pH 7.3). After post-fixing, the samples were rinsed with deionized distilled water several times.
3.2.4.2. Dehydration, embedding and sectioning
Dehydration of the fixed samples was carried out in a sequentially increasing ethanol series of 30, 50, 70, 85, 95 % and 3 x 100 %, for 2 h in each stage, except for 70
% which was run overnight. After dehydration, the samples were embedded in LR-white resin. In order to facilitate better resin penetration, the samples were soaked in a series of ethanol: resin mixtures at ratios of 3:1, 1:1, 1:3 (v/v), and finally in 100 % resin overnight. The resin-soaked samples were placed in small centrifuge tubes, filled with a small amount of resin and placed in an oven at 60 °C for 24 h to cure the resin. The resin- embedded samples were sliced cross-sectionally with a rotary microtome, using a glass knife, into 1.25 um thick sections. The sections were collected and placed on glass slides, ready for immunolabelling.
3.2.4.3. Protocol of immunolabelling
Before applying the primary antibodies, i.e., the monoclonal antibodies JTM5 and
JTM7, the sections were pre-incubated in 10 mM Tris-HCI buffer (pH 7.3) containing 150 mM NaCl, 1.0% BSA and 0.05 % Tween 20 (pre-incubation buffer), for 3 x 20 min. The sections were incubated with the primary antibodies, JTM5 or JTM7 for 2 h.
The primary antibodies were diluted to 1 : 5 with the pre-incubation buffer (Tris- 83
HC1, pH 7.3, containing 150 mM NaCl, 0.05 % Tween 20 and 1.0 % BSA). After the incubation, the sections were washed with pure 10 mM Tris-HCI (pH 8.0) for 4 x 10 min.
The sections were blocked with pre-incubation buffer for 2 x 20 min, followed by 1-hour application of secondary antibody, i.e., goat anti-rat IgG conjugated with biotin, from
SIGMA, which was diluted to 1 : 200 with pre-incubation buffer. The sections were then washed again in 10 mM Tris-HCI (pH 8.0) buffer for 4x10 min, and reblocked with pre• incubation buffer for 2 x 20 min. The biotin-avidin system was used for fluorescent microscopy. Therefore, avidin coupled to Texas Red™ sulfonyl chloride diluted to 1 :
200 with pre-incubation buffer, was used to bind the biotin. The reaction was conducted in the dark for 1 h. From this point on, the rest of the treatment was conducted in the dark, not under direct light.
The sections were washed in the Tris-HCI buffer followed by distilled water for 4 x
10 min, respectively. Finally, the sections were mounted on the glass slides with mounting medium, a mixture of 2.5 % DABCO (l,4-Diazabicyclo[2.2.2]octane, an anti-fading agent), 90 % glycerol and Tris-HCI buffer (pH 8.0). Before microscopic examination, the slides were kept temporarily in a refrigerator in the dark.
3.2.4.4. Visualization of slides with a fluorescent microscope
The sections were viewed with a Zeiss epifluorescent microscope using an excitation filter BP 546/12 nm and emission filter LP 590 nm. This was conducted in a dark room. The immunolabelled pectin parts of the cell wall appear brighter as compared to the non-fluorescent cell wall background. The microscope was attached with a 84 computerized digital camera. The light source for the microscope was a mercury vapor lamp. The images were captured with the camera and saved as a file in TIF format. These images were printed out with a photographic printer. 4. RESULTS
4.1. Chemical Composition of Spruce Wood
Analytical results of the main chemical components of spruce wood (Picea engelmannf) are tabulated in Table 10, while the sugar composition can be seen in Table
11. The results are also expressed as percentage of unextracted wood sample, which is used as the basis for further analysis of pulping results.
Table 10. Chemical composition of spruce wood.
%, based on oven-dry wood Wood component extractive-free sample unextracted sample
Extractives _ 3.6 Lignin : 27.0 26.6 - Klason (acid-insoluble) 26.5 26.1 - Acid-soluble 0.5 0.5 Holocellulose 70.9 69.8 a-Cellulose 43.1 42.5 Note: Ash content = 0.3 % on extractive-free wood (Ramalingam and Timell, 1964).
Table 11. Sugar composition of spruce wood.
%, based on oven-dry wood Type of sugar extractive-free sample unextracted sample
Arabinose 1.26 1.21 Galactose 1.60 1.54 Glucose 50.49 48.67 Xylose 6.38 6.15 Mannose 12.72 12.26 Total 72.45 69.83 Note: Acetyl content = 1.3 % on extractive-free wood (Ramalingam and Timell, 1964).
85 4.2. Pulp Yield
A wide variety of catalysts have been used in this research. Under the specified pulping conditions in which the cooking time was fixed to 60 min and the catalyst concentration varied, the ability of each catalyst to liberate the fibers, as indicated by the pulp yields and reject contents, can be seen in Table 12 - 18. The reject content was calculated based on the oven-dry wood and unscreened oven-dry pulp after pulping. The purpose of the former is to show how much wood (in percent) remained undefibrated, whereas the purpose of the latter is to show the relative proportion of defibrated and undefibrated fibers after cooking, thus, 100 % rejects means no fiber liberation. In addition, lignin-free yield was also calculated for the screened pulp and expressed as the percentage of oven-dry wood. Theoretically, the lignin-free yield can be used for predicting the maximum fully bleached pulp yield, assuming that bleaching will be conducted on the screened pulp. The results of these calculations are also presented in
Table 12-18. In addition, a summarized result, listing the catalysts in terms of their capability to pulp spruce wood in alcohol organosolv pulping process, is tabulated in
Table 19.
The results of pulping with and without catalysts of NaCl and KC1, as shown in
Table 12, indicated that organosolv pulping with these systems was unable to liberate the fibers, as no fibers were liberated under the specified pulping conditions and less than 15
% of the wood mass was lost in the process. In Table 13, where the results of pulping
with divalent metal ion catalysts, in form of chloride salts, i.e., CaCl2, MgCl2, MnCl2,
ZnCl2, are tabulated, it indicated that all of these catalysts were working and the pulping 87 results were varied by the catalyst species and concentrations. In general, these catalysts were capable of producing organosolv spruce pulp at 60 - 61 % screened yield at a reject content <2 %, based on oven-dry wood. The screen reject <2 % was arbitrarily defined as the fiber liberation point, thus, at this point, the process of fiber liberation is considered complete. It was predicted that the maximum bleached pulp yield could be 52 %. In
pulping with A1C13 and FeCl3, the results, presented in Table 14, showed that these two catalysts were also working but the pulp yields were very low, 45 - 47 % at best, compared to the pulp yields of divalent metal ion catalysts.
In pulping with Mg-salt of four different anions, i.e., chloride, nitrate, sulfate and acetate, the pulping results indicated that only chloride and nitrate were capable to liberate the fibers. However, between these species of anions, chloride was better than nitrate. As
shown in Table 15, with 0.025 M MgCl2 as the catalyst, this organosolv catalysed process can produce chemical pulp at 59.9 % screened yield with 1.4 % reject. The lignin-free yield was 52.2 %. On the other hand, the highest screened pulp yield obtained by using
Mg(N03)2 was 57.9 %, which was only 2 % lower, but the screen reject was 6.0 %,
considered high, and further, this was achieved by 0.075 M Mg(N03)2, as high as three
times the catalyst concentration required for MgCl2.
Table 16 contains the pulping results with inorganic acid catalysts, i.e., HC1, HN03
and H2S04. Among these acids, only HN03 was not capable of liberating the fibers.
However, between HC1 and H2S04, HC1 seemed to be better. The highest screened yield at 0 % reject was 52.1 %, which was obtained by applying 0.0125 M HC1. With 0.005 M
H2S04, the total pulp yield was 56.5 % but the reject content was very high, 17.8 %. 88
When doubling the catalyst concentration to 0.010 M in order to reduce the amount of reject, the result was devastation. All fibers were completely destroyed.
Table 17 presents the results of pulping with organic acid catalysts, i.e., acetic, oxalic, malonic, malic and citric acids. Acetic and malonic acids showed no sign of being capable of liberating the fibers. The best result was obtained by 0.050 M citric acid, in which the total pulp yield was 65.0 % with 1.4 % reject. At this pulping yield, it is estimated that the bleached pulp yield as high as 54.2 % could be achieved.
The pulping results with combination of two different catalysts are tabulated in
Table 18. Among the catalyst combinations, only the combination of 0.025 M NaCl and
0.00625 M HC1 was not able to liberate the fibers. The pulp yields were various, depending on the catalyst combinations. At fiber liberation point, the screened pulp yields ranged from 58.0 to 62.0 %. The highest screened pulp yield of 62.0 % with 1.9 % reject
was obtained by the combination of 0.025 M CaCl2 and 0.025 M ZnCl2, while the
combination of 0.025 M CaCl2 and 0.025 M Mg(N03)2, the most common catalysts used by Paszner and coworkers, produced 59.0 % screened pulp yield but the screen reject was only 0.3 %. 89
Table 12. Pulp yields and reject contents in organosolv pulping with and without catalysts of chloride salts of monovalent cations. % Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue) No catalyst 88.1 N/A 0 N/A 100 0.025 M NaCl 86.9 N/A 0 N/A 100 0.050 M NaCl 86.3 N/A 0 N/A 100 0.075 M NaCl 85.4 N/A 0 N/A 100 0.025 M KC1 87.0 N/A 0 N/A 100 0.050 MKC1 86.3 N/A 0 N/A 100 0.075 M KC1 86.1 N/A 0 N/A 100 Note: N/A = not applicable.
Table 13. Pulp yields and reject contents in organosolv pulping with catalysts of chloride salts of divalent cations. % Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue)
0.025 M CaCl2 65.9 8.5 57.4 49.0 12.9
0.050 M CaCl2 61.7 1.2 60.6 52.0 1.9
0.075 M CaCl2 59.6 0.4 59.2 51.7 0.7
0.025 M MgCl2 61.3 1.4 59.9 52.2 2.3
0.050 M MgCl2 56.7 0.2 56.5 50.1 0.3
0.075 M MgCl2 54.8 0 54.8 49.1 0
0.025 MMnCl2 65.4 5.2 60.2 52.0 7.9
0.050 MMnCl2 60.9 1.1 59.8 52.3 1.8
0.075 MMnCl2 58.2 0.3 57.9 50.9 0.5
0.025 M ZnCl2 71.0 26.5 44.5 37.4 37.4
0.050 MZnCl2 65.3 3.4 61.9 53.0 5.2
0.075 M ZnCl2 62.2 1.2 61.1 52.7 1.9 90
Table 14. Pulp yields and reject contents in organosolv pulping with catalysts of chloride salts of trivalent cations. % Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue)
0.025 M A1C13 36.0 0 36.0 31.5 0 0.050 M AICI3 27.7 0 27.7 - 0 0.050 M AICI3 (40,)1) 47.3 0 47.3 - 0
0.025 MFeCl3 45.2 0 45.2 39.2 0
0.050 MFeCl3 42.7 0 42.7 35.4 0
2) 0.075 M FeCl3 - - - - - Notes : ''The cook was reduced to 40 min. 2)The pulp was completely damaged (like mud).
Table 15. Pulp yields and reject contents in organosolv pulping with Mg-salt catalysts.
% Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue)
0.025 M MgCl2 61.3 1.4 59.9 52.2 2.3
0.050 M MgCl2 56.7 0.2 56.5 50.1 0.3
0.075 MMgCl2 54.8 0 54.8 49.1 0
0.025 MMg(N03)2 66.6 10.4 56.2 47.2 15.6
0.050 MMg(N03)2 64.8 8.5 56.3 47.7 13.3
0.075 M Mg(N03)2 63.9 6.0 57.9 49.1 9.2
0.025 MMgS04 88.4 N/A 0 N/A 100
1) 0.050 MMgS04 88.7 N/A 0 N/A 100 0.075 M MgSO^ 88.2 N/A 0 N/A 100
0.025 M Mg(CH3COO)2 90.0 N/A 0 N/A 100
0.050 M Mg(CF£3COO)2 90.7 N/A 0 N/A 100
0.075 M Mg(CFL,COO)7 91.2 N/A 0 N/A 100 Notes : ''Not permanently dissolved (some reprecipitated). N/A = not applicable. Table 16. Pulp yields and reject contents in organosolv pulping with inorganic acid catalysts. % Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue) 0.00625 M HC1 66.3 25.6 40.7 35.5 38.6 0.00833 MHC1 59.2 6.9 52.3 45.8 11.7 0.0125 MHC1 52.1 0 52.1 46.5 0 0.025 M HC1 45.5 0 45.5 39.1 0
0.050 M HN03 78.1 N/A 0 N/A 100
0.075 M HN03 77.8 N/A 0 N/A 100
0.005 M H2S04 56.5 17.8 38.7 35.0 31.5 0.010 MH.SO;' - - - - - Notes : ''Fibers were completely damaged (like mud). N/A = not applicable.
Table 17. Pulp yields and reject contents in organosolv pulping with organic acid catalysts. % Oven dry wood Reject, Catalyst %OD Total Reject Screened Lignin-free total pulp yield yield yield (residue) 0.025 M acetic acid 87.6 N/A 0 N/A 100 0.050 M acetic acid 87.3 N/A 0 N/A 100 0.075 M acetic acid 87.3 N/A 0 N/A 100 0.025 M oxalic acid 80.5 N/A 0 N/A 100 0.075 M oxalic acid 67.0 6.6 60.4 50.5 9.9 0.025 M malonic acid 87.2 N/A 0 N/A 100 0.075 M malonic acid 86.7 N/A 0 N/A 100 0.050 M malic acid 70.2 16.0 54.2 45.4 22.8 0.075 M malic acid 66.8 6.1 60.7 51.2 9.1 0.025 M citric acid 69.8 15.9 53.9 45.2 22.7 0.050 M citric acid 65.0 1.4 63.6 54.2 2.2 0.075 M citric acid 61.6 0.2 61.4 52.8 0.3 Note: N/A = not applicable. 92
Table 18. Pulp yields and reject contents in organosolv pulping with combination catalysts. % Oven dry wood Reject, %OD
1 Catalyst * Total Reject Screen Lig-free total pulp yield yield yield (residue)
CaCl2 + Mg(N03)2 59.3 0.3 59.0 50.8 0.5
CaCl2 + MgCl2 58.2 0.1 58 50.1 0.2
CaCl2 + MnCl2 61.7 0.7 61 52.5 1.1
CaCl2 + ZnCl2 63.8 1.9 62 53.0 3
CaCl2 + NaCl 66 11.4 54.6 45.9 17.2 NaCl + HC1 85.3 N/A 0 - 100 NaCl + HC1 (90? > 80.1 N/A 0 - 100
NaCl + HN03 66.7 20.8 45.9 38.6 31.2
3) NaCl + MgS04 73.5 40.6 32.8 27.1 55.3
) NaCl + MgS04 (90? 64.8 12.6 52.2 44.6 19.4
NaCl + Mg(N03)2 65.4 5.1 60.3 50.8 7.8 NaCl + MgCL, 62.5 2 60.5 51.6 3.2 Notes: ''Concentration of each catalyst was 0.025 M, except HC1, which was 0.00625 M. 2)The cook was extended to 90 min. 3)Not permanently dissolved (reprecipitated). N/A = not applicable. 93
Table 19. Effect of catalysts on pulp production capability in organosolv pulping of spruce wood at 60 min cook in 80 % methanol. Catalysts Group of catalysts Able Unable - - No catalyst Monovalent cations - NaCl, KC1
Divalent cations CaCl2, MgCl2, MnCl2, ZnCl2, Mg(N03)2 MgS04, Mg-acetate
Trivalent cations A1C13, FeCl3 -
Inorganic acids HC1, H2S04 HNO3 Organic acids oxalic, malic, citric acetic, malonic
CaCl2 + {Mg(N03)2, NaCl, MgCl2, MnCl2,
Combination ZnCl2}; NaCl + {HN03, MgCl2, MgS04, NaCl + HC1
Mg(N03)?}
Among the effective pulping catalysts, four were chosen to be used in organosolv pulping at fixed concentration and varied cooking time. Kraft cooking was used as control for the series. The results of pulping, in terms of pulp yield and reject content, are presented in Fig. 14 - 17. Note, the data points in the graphs herein were connected with smoothed lines using Microsoft Excel 2000 package. The pulp yields and rejects were calculated based on oven-dry wood.
From Fig. 15, it should be noted that fiber liberation just started taking place after
10 min for 0.025 M A1C13, 30 min for 0.050 M CaCl2 and the combination of 0.025 M
CaCl2 and 0.025 M Mg(N03)2, 40 min for citric acid and 60 min for the Kraft cooks.
Therefore, what was shown in Fig. 14, before fiber liberation started taking place, is a mass loss during that initial period of time. On the other hand, Fig. 17 plots only the reject contents after fiber liberation started taking place. This figure also shows that the cooking 94 times required to achieved complete fiber liberation were depending on the species of the
catalysts. At this point, the cooking time required for 0.025 M A1C13 was 30 min, the
combination of 0.025 M CaCl2 and 0.025 M Mg(N03)2 was 50 min, 0.050 M CaCl2 was
60 min, 0.050 M citric acid was 60 min and kraft was 150 min. Fig. 15 shows that at fiber liberation point, the highest screened pulp yields of 46.3 %, 61.1 %, 62.9 %, 64.1 % and
48 %, respectively for A1C13, CaCl2, the combination of CaCl2 and Mg(N03)2, citric acid and kraft, were obtained. Fig. 16 shows that at fiber liberation point, the lignin-free yields
were 41.2 %, 51.1 %, 52.5 %, 53.8 % and 45.0 %, for A1C13, CaCl2, the combination of
CaCl2 and Mg(N03)2, citric acid and kraft, respectively.
30 4; ' • 1 • 1 1 • • 1 ' • 1 • • 1 • ' 1 0 30 60 90 120 150 180 Cooking time, min
Fig. 14. Relationship between cooking time and total pulp yield of kraft and organosolv pulping with different catalysts (the arrows show the FLP). 95
180 Cooking time, min
Fig. 15. Relationship between cooking time and screened pulp yield of kraft and organosolv pulping with different catalysts (the arrows show the FLP).
25 + 1 1 1 1 ' 1 1 1 1 1 1 1 ' 1 1 1 1 0 30 60 90 120 150 180 Cooking time, min
Fig. 16. Relationship between cooking time and lignin-free pulp yield of kraft and organosolv pulping with different catalysts (the arrows show the FLP). 96
25 -#-CaCI2 -»-CaCI2 + Mg(N03)2 20 + -*-AICI3 —e— Citric acid
5 +
0 0 60 90 120 150 180
130 Cooking time, min
Fig. 17. Relationship between cooking time and screen reject of kraft and organosolv pulping with different catalysts.
4.3. Residual Lignin and Viscosity of the Pulp
Residual lignin content of the pulps obtained by pulping with various catalysts and catalyst concentrations at fixed cooking time was determined directly, i.e., Klason lignin plus acid soluble lignin, and indirectly, i.e., kappa number. Kappa number and viscosity were determined only for the pulps, whereas Klason lignin and acid-soluble lignin were determined for all samples. However, it should be noted that the viscosity of the pulp was determined as such, without any additional treatment. The results of the analyses are presented in Tables 20-26.
It should be noted that when pulping with and without monovalent cations as the catalysts, no fibers were liberated, therefore, Table 20 does not present the kappa numbers 97 and viscosities of the pulps. The residual lignin content of the undefibrated cooked chips was very high, i.e., higher than 25 %, indicating very low degree of delignification. Table
21 presents the results of chemical analyses of the pulps cooked with divalent metal ion catalysts. The residual lignin content of the pulp obtained under the specified catalyst concentration was ranging from 10.3 % to 15.9 %, or in terms of kappa number, 69.4 to
92.3. At the same catalyst concentration, the residual lignin content of the MgCl2 pulp was always lower than that obtained with other catalysts of the divalent cation group. The residual lignin content in the pulps was not drastically affected by the catalyst concentrations, whereas the viscosities tended to decrease with an increase of the catalyst
concentration. Table 22 shows that when pulping with A1C13 and FeCl3, the resulting pulps could contain lignin as high as 17.2 %, or equivalent to 111.8 in kappa number. The viscosities of the pulps were extremely low, lower than 2.5 cPs.
The residual lignin contents and viscosities of the pulps resulted from pulping with different anions of Mg-salts as the catalysts are shown in Table 23. As reported before
that there was no fiber liberation in pulping with catalysts of MgS04 or Mg(CH3COO)2, therefore, in Table 23, there are no values for kappa numbers and viscosities shown for these catalysts. The analysis results showed that the lowest residual lignin content of the
pulps was obtained with 0.075 M MgCl2, whereas the highest viscosity of 19.9 cPs was
obtained with 0.025 M MgCl2. These viscosities are lower than those determined for spruce pulps in a previous study by Yawalata (1996).
Table 24 presents the residual lignin contents and viscosities of the pulps obtained with inorganic acid catalysts. The residual lignin content as low as 9.5 % could be 98
obtained with 0.005 M H2S04, while the highest viscosity of 13.6 cPs was obtained with
0.00625 M HC1. However, it should be noted that at these catalyst concentrations, the pulps contained very high amount of reject, but the analysis was conducted with screened pulps. Table 25, where the analysis results of organic acid catalysed pulps are tabulated, shows that the lowest residual lignin of 14.1 %, equivalent to the kappa number of 87.4, was obtained with 0.075 M citric acid, concurrently with the highest viscosity of 18.4 cPs.
The residual lignin contents and viscosities of the pulps, resulting from pulping with combinations of two different catalysts, are presented in Table 26. The combination that produced pulp with the lowest residual lignin content of 13.6 % or 81.9 in kappa
number was the combination of 0.025 M CaCl2 and 0.025 M MgCl2, but with the highest
viscosity of 25.1 cPs was obtained with the combination of 0.025 M CaCl2 and Mg(N03)2.
Basically, the variation in the residual lignin content from the latter (13.9 %) to the former
(13.6 %) mentioned combinations was minor, which was only 0.3 % or equivalent to 1.5 in kappa number. 99
Table 20. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with and without catalysts of chloride salts of monovalent cations. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin No catalyst 25.64 0.61 26.25 - -
0.025 MNaCl 24.91 0.80 25.71 - _ 0.050 M NaCl 24.90 0.82 25.72 - - 0.075 MNaCl 24.60 0.85 25.45 - - 0.025 M KC1 25.26 0.42 25.68 - - 0.050 MKC1 24.97 0.46 25.43 - - 0.075 MKC1 24.80 0.46 25.26 - -
Table 21. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of divalent cations. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin
1 0.025 M CaCl2 14.43 0.28 14.71 91.0 17.0 *
0.050 M CaCl2 13.84 0.28 14.12 90.4 22.6
0.075 M CaCl2 12.47 0.27 12.74 79.8 17.6
0.025 M MgCl2 12.39 0.43 12.82 83.2 19.9
0.050 MMgCl2 10.92 0.43 11.35 75.7 11.8
0.075 M MgCl2 9.91 0.41 10.32 69.4 8.4
0.025 M MnCl2 13.34 0.31 13.65 89.0 21.1
0.050 M MnCl2 12.22 0.30 12.52 81.5 17.2
0.075 M MnCl2 11.82 0.30 12.12 74.6 11.6
1 0.025 M ZnCl2 15.58 0.29 15.87 92.3 10.3 *
0.050 M ZnCl2 14.10 0.28 14.38 86.4 20.8
0.075 M ZnCl2 13.42 0.27 13.69 85.9 19.8 Note: ''Not well dissolved in the Cuen solution. 100
Table 22. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of trivalent cations. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin
0.025 M AlClj 12.15 0.25 12.4 92.0 ND1)
J) 0.050 M A1C13 ND ND ND ND ND 0.050 M AICI3 (40')2) ND ND ND ND NDJ)
0.025 M FeCl3 12.98 0.36 13.34 84.7 2.2
1} 0.050 MFeCl3 16.87 0.34 17.21 111.8 ND Notes: ''Fibers were completely damaged (like mud). 2)The cook was reduced to 40 min. ND = not determined.
Table 23. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with magnesium salt catalysts. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin
0.025 MMgCl2 12.39 0.43 12.82 83.2 19.9
0.050 M MgCl2 10.92 0.43 11.35 75.7 11.8
0.075 M MgCl2 9.91 0.41 10.32 69.4 8.4
1 0.025 M Mg(N03)2 15.73 0.29 16.02 100.2 13.4 *
1 0.050 M Mg(N03)2 15.03 0.30 15.33 98.9 18.3 )
0.075 M Mg(N03)2 14.88 0.29 15.17 99.8 19.1
0.025 MMgS04 24.83 0.55 25.38
2) 0.050 MMgS04 24.62 0.55 25.17 - -
2) 0.075 M MgS04 24.85 0.56 25.41
0.025 M Mg(CH3COO)2 26.83 0.70 27.53
0.050 M Mg(CH3COO)2 26.67 0.72 27.39 - -
0.075 M Mg(CH3COO)2 26.72 0.71 27.43 Notes : ''Not well dissolved in the Cuen solution. 2)Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. 101
Table 24. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with inorganic acid catalysts. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin 0.00625 M HC1 12.55 0.32 12.87 80.7 13.6
0.00833 MHC1 12.24 0.28 12.52: 82.4 11.8 0.0125 MHC1 10.46 0.26 10.72 71.0 5.3 0.025 M HC1 13.80 0.21 14.01 91.4 2.2
0.050 MHN03 22.22 0.67 22.89 ND ND 0.075 MHNO3 22.34 0.71 23.05 ND ND
0.0050 MH2S04 9.24 0.29 9.53 61.9 4.2 0.0100 ME^SCV* ND ND ND ND ND Notes : ''Fibers were completely damaged (like mud). ND = not determined.
Table 25. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with organic acid catalysts. % Oven dry pulp (residue) Catalyst Kappa Viscosity Klason Acid-sol. Total No. (cPs) lignin lignin lignin
0.025 M acetic acid 25.20 0.51 25.71 _ _ 0.050 M acetic acid 25.18 0.50 25.68 - - 0.075 M acetic acid 25.29 0.51 25.80 - -
0.025 M oxalic acid 22.23 0.58 22.81 _ _ 0.075 M oxalic acid 16.17 0.26 16.43 101.3 7.3X)
0.025 M malonic acid 25.11 0.60 25.71 _ _ 0.075 M malonic acid 24.83 0.60 25.43 - - 0.050 M malic acid 15.90 0.27 16.17 102.3 - 0.075 M malic acid 15.38 0.26 15.64 99.6 10.21*
0.025 M citric acid 15.53 0.58 16.11 99.9 8.21* 0.050 M citric acid 14.27 0.57 14.84 92.7 15.71* 0.075 M citric acid 13.53 0.56 14.09 87.4 18.4 Note: ''Not well dissolved in the Cuen solution. 102
Table 26. Residual lignin and viscosity of the pulps and / or undefibrated cooked chips pulped with combination catalysts. % Oven dry pulp (residue) Kappa Vise. Catalyst1' Klason Acid- sol. Total No. (cPs) lignin lignin lignin
CaCl2 + Mg(N03)2 13.64 0.30 13.94 83.4 25.1
CaCl2 + MgCl2 13.26 0.30 13.56 81.9 16.4
CaCl2 + MnCl2 13.62 0.29 13.91 85.6 21.8
CaCl2 + ZnCl2 14.28 0.31 14.59 92 20.3
4) CaCl2 + NaCl 15.67 0.31 15.98 97.2 16.1 NaCl + HC1 24.80 0.57 25.37 - - NaCl + HC1 (90')2) 23.11 0.57 23.68
4) NaCl + HN03 15.68 0.31 15.99 97.8 16.5
3) NaCl + MgS04 16.91 0.34 17.25 101.6
2) 4) NaCl + MgSO4*(90') 14.34 0.32 14.66 88.5 20.1
NaCl + Mg(N03)2 15.37 0.31 15.67 95.9 17.4
NaCl + MgCl2 14.35 0.31 14.66 91.8 20.6 Notes: ''Concentration of each catalyst was 0.025 M, except HC1, which was 0.00625 M. 2)The cook was extended to 90 min. 3) Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. 4)Not well dissolved in the Cuen solution.
Residual lignin content of the pulps and undefibrated cooked chips from pulping with fixed catalyst concentrations at various cooking times was determined directly by
Klason lignin plus acid-soluble lignin. The relationships between cooking time and the residual lignin contents are plotted in Fig. 18-20. The figures indicate that the residual lignin contents remained relatively unchanged after fiber liberation points were reached,
except the pulp made with A1C13 catalyst. The relationship between cooking time and viscosities of the pulps, plotted in Fig. 21, indicates that the viscosities decreased after fiber liberation point was reached. Note, the NAEM and citric acid pulps obtained in less than 60 min and 90 min cooks, respectively, did not dissolve well in the Cuen solution. 103
30
5 +
1 1 1 1 1 o -F • 1 1 • < 1 • • 1 • 1 • 1 0 30 60 90 120 150 180
Cooking time, min.
Fig. 18. Relationship between cooking time and Klason lignin of kraft and organosolv pulps cooked with different catalysts (the arrows show the FLP).
0.9
0.2 -F ' ' 1 ' ' 1 ' ' 1 1 ' h 0 30 60 90 120 150 180
Cooking time, min
Fig. 19. Relationship between cooking time and acid-soluble lignin of kraft and organosolv pulps cooked with different catalysts. 104
30
o 4—> • 1 • > 1 • < 1 « • 1 • • 1 • • 1 0 30 60 90 120 150 180
Cooking time, min
Fig. 20. Relationship between cooking time and total residual lignin of kraft and organosolv pulps cooked with different catalysts (the arrows show the FLP).
Fig. 21. Effect of cooking time on viscosity of kraft and organosolv pulps cooked with different catalysts (the arrows show the FLP). 105
4.4. Monitoring the pH of Cooking Liquor
The pH of the cooking liquor, before and after cooking, was monitored throughout the experiment. From the recorded pH presented in Table 27 - 33, it can be seen that the behavior of cooking liquor pH was depending on the species and concentration of the catalysts. In pulping with salts of mono- or divalent cations, the pH of cooking liquor dropped after cooking, as shown in Table 27, 28, 30 and 33. The magnitude of dropping was depending on the catalyst species and concentrations. The dropping of pH in this case indicated that a certain amount of protons was generated during the pulping process.
Therefore, in some select cases, the spent liquor was titrated with NaOH to estimate the amount of protons generated during the course of pulping, especially for pulping with catalysts of chloride salts of mono- and divalent cations. The amount of protons generated was found to vary depending on the catalysts. On the other hand, in pulping with acid catalysts, as the results show in Table 31 and 32, the pH of the cooking liquor was always increased after cooking. Whereas pulping with trivalent cations, as shown in
Table 29, the pH of cooking liquor was decreased if using A1C13 but increased if using
FeCl3. In the time series cooks, the pH of the cooking liquor was also recorded and plotted in Fig. 22. Particularly in pulping with the NAEM salt catalysts, the pH change after 10 min into the cook was small.
In addition, 25 mL of some fresh cooking liquors of different catalysts were also sampled and titrated with 0.001 M HC1. The pH and the amount of titrant added were recorded and plotted in Fig. 23. The purpose of this exercise was to get a rough idea about relative buffering capacities of certain catalysts in the cooking liquor. The results 106 indicated that there was a difference in buffering capabilities among the catalysts, e.g., KC1 and NaCl have a higher buffer capacity than that of the NAEM salts.
Table 27. pH of cooking liquor with and without catalysts of chloride salts of monovalent cations and the estimated amount of proton generated during the cooking. pH of cooking liquor Estimated amount Catalyst of H+generated Fresh Spent (mmol/kg ODW) 0 M catalyst 6.24 4.83 88 0.025 M NaCl 6.60 4.86 87 0.050 M NaCl 6.65 4.85 95 0.075 M NaCl 6.66 4.77 96 0.025 M KC1 6.41 4.87 104 0.050 MKC1 6.53 4.88 103 0.075 MKC1 6.58 4.87 102
Table 28. pH of cooking liquor with catalysts of chloride salts of divalent cations and the estimated amount of proton generated during the cooking. pH of cooking liquor Estimated amount Catalyst of FT generated Fresh Spent (mmol/kg ODW)
0.025 M CaCl2 6.10 4.20 127
0.050 M CaCl2 5.99 3.81 123
0.075 M CaCl2 5.96 3.85 124
0.025 M MgCl2 6.18 4.20 160
0.050 M MgCl2 6.04 4.00 156
0.075 MMgCl2 5.96 3.80 174
0.025 M MnCl2 5.89 4.03 74
0.050 M MnCl2 5.73 3.73 102
0.075 M MnCl2 5.54 3.60 112
0.025 M ZnCl2 5.28 3.55 119
0.050 MZnCl2 5.14 3.33 188
0.075 MZnCl2 4.98 3.19 137 Table 29. pH of cooking liquor with catalysts of chloride salts of trivalent cations.
pH of cooking liquor Catalyst Fresh Spent
0.025 M AICI3 2.76 2.26 0.050 M AICI3 2.51 2.08
0.025 MFeCl3 1.89 2.63
0.050 M FeCl3 1.90 2.35
Table 30. pH of cooking liquor with magnesium-salt catalysts and the estimated amount of proton generated during the cooking. pH of cooking liquor Estimated amount Catalyst of H+generated Fresh Spent (mmol/kg ODW)
0.025 M MgCl2 6.18 4.20 160
0.050 MMgCl2 6.04 4.00 156
0.075 M MgCl2 5.96 3.80 174
0.025 M Mg(N03)2 6.21 4.82
0.050 M Mg(N03)2 6.11 5.21 -
0.075 M Mg(N03)2 6.06 5.49
0.025 M MgS04 6.80 4.85 0.050 M MgSCV' 6.92 4.83 0.075 M MgSOV' 6.90 4.84
0.025 M Mg(CH3COO)2 8.56 7.85
0.050 M Mg(CH3COO)2 8.74 8.35 -
0.075 M Mg(CH3COO)2 8.82 8.45 Note: ''Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. Table 31. pH of cooking liquor with inorganic acid catalysts.
pH of cooking liquor Catalyst Fresh Spent 0.00625 M HC1 2.32 3.78 0.00833 M HC1 2.22 3.57 0.0125 MHC1 2.06 3.23 0.025 M HC1 1.84 2.77 0.050 MHNOj 1.58 4.35 0.075 MHNO3 1.40 4.33
0.0050 M H2S04 2.39 3.08
0.0100 MH2S04 2.17 2.46
0.0125 MH2S04 2.06 2.31
0.0250 M H2S04 1.85 2.01
Table 32. pH of cooking liquor with organic acid catalysts.
pH of cooking liquor Catalyst Fresh Spent 0.025 M acetic acid 3.83 4.70 0.050 M acetic acid 3.74 4.63 0.075 M acetic acid 3.67 4.59 0.025 M oxalic acid 2.34 4.18 0.075 M oxalic acid 2.11 3.87 0.025 M malonic acid 2.96 4.66 0.075 M malonic acid 2.76 4.56 0.050 M malic acid 3.13 3.82 0.075 M malic acid 3.04 3.79 0.025 M citric acid 3.16 3.86 0.050 M citric acid 2.99 3.68 0.075 M citric acid 2.93 3.52 109
Table 33. pH of cooking liquor with combination catalysts and the estimated amount of proton generated during the cooking. pH of cooking liquor Estimated amount Catalyst1* of Ffgenerated Fresh Spent (mmol/kg ODW)
0 M catalyst 6.24 4.83 88
CaCl2 + Mg(N03)2 6.3 4.02 -
CaCl2 + MgCl2 6.21 3.9 188
CaCl2 + MnCl2 6.03 3.78 160
CaCl2 + ZnCl2 5.61 3.68 148
CaCl2 + NaCl 6.4 4.24 129 NaCl + HC1 2.96 4.62 - NaCl + HC1 (90*)2) 2.96 4.60
NaCl + HN03 1.77 4.24 -
3) NaCl + MgS04 6.70 4.54 -
2) NaCl + MgS04 (90') 6.70 4.40
NaCl + Mg(N03)2 6.26 4.48 -
NaCl + MgCl2 6.41 4.23 153 Notes : ''Concentration of each catalyst was 0.025 M, except HC1, which was 0.00625 M. 2)The cook was extended to 90 min. 3)Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. 110
-CaCI2 -CaCI2 + Mg(N03)2 -AICI3 -Citric acid
60 90 120 150 180
Cooking time, min
Fig. 22. pH behavior of catalysed organosolv cooking liquor.
I I I I I I • No catalyst -•-NaCl -A-KCI \ - -*-CaCI2 4.5 -e-MgCI2 -«—ZnCI2 X a. -B-MnCI2
3.5 3 4 5 6 7 8 9 10 11 12 13 14 15
0.001 M HCI consumed by cooking liquor, mL
Fig. 23. Predicted buffer capability of cooking liquor. Ill
4.5. Sugar Analysis
The results of sugar analyses on pulps or cooked chips are presented in Tables 34 -
45, whereas some HPLC chromatograms can be seen in Appendix 2. Tables 34 - 40 indicate that the type of catalysts has an effect on the sugar retention in organosolv pulping. The type of sugars most affected were arabinose and galactose. Depending on the catalysts, arabinose and galactose could be completely removed. Particularly in
pulping with A1C13 or FeCl3 in extended cooks, the residual carbohydrate component of the resulting pulps appeared to be mere glucose, obviously of cellulose. Tables 41-44 show that arabinose and galactose are completely removed before the fiber liberation point is reached. The analysis results in Table 45 indicate that in kraft pulping, the resulting pulp still contained all sugar types that were originally present in the raw material.
Table 34. Sugar composition of pulps and / or undefibrated cooked chips pulped with and without catalysts of chloride salts of monovalent cations. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total No catalyst trace 1.15 50.57 6.43 13.11 71.26 0.025 MNaCl trace 1.05 51.06 5.95 13.17 71.23 0.050 MNaCl trace 1.04 48.93 5.89 12.96 68.82 0.075 MNaCl trace 1.01 48.88 5.73 12.92 68.54
0.025 M KC1 trace 0.99 47.55 5.51 11.73 65.78 0.050 MKC1 trace 0.89 49.60 5.54 11.92 67.95 0.075 M KC1 trace 0.79 50.50 5.38 11.93 68.6 112
Table 35. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of divalent cations. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total
- _ 0.025 M CaCl2 65.49 4.72 9.21 79.42
0.050 M CaCl2 - - 69.94 4.32 7.46 81.72
0.075 M CaCl2 - - 73.86 3.91 6.66 84.43
0.025 MMgCl2 - - 69.62 4.62 7.26 81.50
0.050 MMgCl2 - - 75.69 3.81 5.08 84.58
0.075 M MgCl2 - - 80.05 3.25 3.97 87.27 - - 0.025 MMnCl2 68.53 4.41 10.43 83.37
0.050 M MnCl2 - - 72.65 3.83 8.25 84.73
0.075 MMnCl2 - - 75.45 3.28 6.82 85.55 - - 0.025 M ZnCl2 67.76 5.39 12.42 85.57
0.050 M ZnCl2 - - 73.41 4.85 11.12 89.38
0.075 M ZnCl2 - - 77.39 4.27 9.35 91.01
Table 36. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of chloride salts of trivalent cations. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total
0.025 M A1C13 94.44 94.44 0.050 M AlCL.1* _ _ _ _
0.025 MFeCl3 89.08 89.08
0.050 MFeCl3 - - 88.35 - - 88.35
1} 0.075 M FeCl3 Note: ''Fibers were completely damaged (like mud). 113
Table 37. Sugar composition of pulps and / or undefibrated cooked chips pulped with catalysts of magnesium salts. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total - 0.025 MMgCl2 69.62 4.62 7.26 81.50
0.050 MMgCl2 - - 75.69 3.81 5.08 84.58
0.075 MMgCl2 - - 80.05 3.25 3.97 87.27 - - 0.025 M Mg(N03)2 65.26 5.07 9.45 79.78
0.050 M Mg(N03)2 - - 65.47 4.72 8.55 78.74
0.075 M Mg(N03)2 - - 65.14 4.40 8.08 77.62
0.025 M MgS04 trace 1.04 49.82 5.81 12.52 69.19 0.050 M MgSO^ trace 0.99 49.32 5.73 12.64 68.68 0.075 M MgSO^ trace 1.07 49.55 5.66 12.64 68.92
0.025 M Mg(CH3COO)2 0.58 1.09 46.59 6.42 11.69 66.37
0.050 M Mg(CH3COO)2 0.69 1.04 47.51 6.38 11.68 67.30
0.075 M Mg(CH3COO)7 0.75 0.99 47.53 6.31 11.66 67.24 Note: ''Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor.
Table 38. Sugar composition of pulps and / or undefibrated cooked chips pulped with inorganic acid catalysts. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total 0.00625 MHC1 - 74.83 5.07 6.61 86.51 0.00833 MHC1 - - 74.76 4.27 4.71 83.74 0.0125 MHC1 - - 81.68 2.49 1.71 85.88 0.025 M HC1 - - 82.94 0.55 0.12 83.61
0.050 M HN03 trace 0.57 57.16 5.53 11.70 74.96
0.075 M HN03 trace 0.56 56.39 5.48 11.39 73.82
0.0050 MH2S04 - 88.14 2.31 2.19 92.64 0.0100 MHjSO^ _ Note: ''Fibers were completely damaged. Table 39. Sugar composition of pulps and / or undefibrated cooked chips pulped with organic acid catalysts. % Oven dry pulp (cooked chips) Catalyst Ara Gal Glu Xyl Man Total 0.025 M acetic acid trace 1.03 51.59 6.24 13.18 72.04 0.050 M acetic acid trace 1.05 51.86 6.23 13.28 72.42 0.075 M acetic acid trace 1.03 51.62 6.25 13.17 72.07 0.025 M oxalic acid trace 0.72 55.05 6.07 12.30 74.14 0.075 M oxalic acid - trace 64.81 5.48 8.06 78.35 0.025 M malonic acid trace 1.14 50.91 6.47 13.12 71.64 0.075 M malonic acid trace 1.07 52.61 6.57 13.64 73.89 0.050 M malic acid - - 64.41 6.03 9.21 79.65 0.075 M malic acid - - 65.66 5.94 8.52 80.12 0.025 M citric acid - - 64.34 6.22 8.79 79.35 0.050 M citric acid - - 68.83 6.01 7.62 82.46 0.075 M citric acid - - 69.17 5.57 6.49 81.23 115
Table 40. Sugar composition of pulps and / or undefibrated cooked chips pulped with combination catalysts. % Oven dry pulp (cooked chips) Catalyst1* Ara Gal Glu Xyl Man Total
CaCl2 + Mg(N03)2 - - 76.96 4.59 6.51 88.06
CaCl2 + MgCl2 - - 76.63 4.24 6.17 87.04
CaCl2 + MnCl2 - - 74.46 4.38 8.50 87.34
CaCl2 + ZnCl2 - - 74.74 4.95 9.35 89.04
CaCl2 + NaCl - - 72.20 5.28 10.20 87.68 NaCl + HC1 trace 0.87 52.05 6.15 12.90 71.97 NaCl + HC1 (9p')2) trace 0.64 54.72 5.81 12.86 74.03
NaCl + HN03 - - 68.53 5.35 8.51 82.39
3) NaCl + MgS04 - 66.04 5.67 10.88 82.59 2) _ NaCl + MgSO4(90') 70.90 5.21 9.26 85.37
NaCl + Mg(N03)2 - - 69.44 5.23 9.46 84.13
NaCl + MgCl2 - - 75.64 5.05 8.12 88.81 Notes : ''Concentration of each catalyst was 0.025 M, except HC1, which was 0.00625 M. 2)The cook was extended to 90 min. 3)Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. 116
Table 41. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.050 M CaCl2 catalyst at different cooking time. Cooking time, Type of sugar, % on OD pulp (cooked chips) mm. Ara Gal Glu Xyl Man Total 10 1.20 1.48 45.52 6.58 12.06 66.84 20 0.59 1.25 47.68 6.38 12.53 68.43 30 0.32 0.90 52.45 5.88 12.14 71.69 40 - 0.24 64.97 5.36 10.79 81.36 50 - - 67.43 4.91 9.58 81.92 60* - - 71.61 4.63 8.41 84.65 70 - - 71.75 3.89 7.28 82.92 90 - - 76.54 2.95 5.63 85.12 120 - - 81.65 2.16 4.15 87.96 150 - - 80.07 1.58 2.84 84.49 Note : *Fiber liberation point.
Table 42. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.025 M CaCl? + 0.025 M Mg(NOQ? catalyst at different cooking times. Cooking time, Type of sugar, % on OD pulp (cooked chips) min. Ara Gal Glu Xyl Man Total 10 1.12 1.31 46.53 6.38 12.46 67.80 20 0.61 1.16 48.41 6.52 12.65 69.35 30 - 0.83 52.14 5.53 12.20 70.70 40 - 0.26 65.88 5.47 10.59 82.20 50* - - 70.22 5.36 8.67 84.25 60 - - 73.11 4.85 7.32 85.28
70 - -< 79.32 4.00 5.77 89.09 90 - - 81.66 3.35 4.59 89.60 120 - - 83.85 2.35 3.01 89.21
150 - - 83.64 1.73 2.11 87.48 Note: *Fiber liberation point. 117
Table 43. Sugar composition of pulps and / or undefibrated cooked chips pulped with
0.025 M A1C13 catalyst at different cooking times. Cooking time, Type of sugar, % on OD pulp (cooked chips) min. Ara Gal Glu Xyl Man Total 10 0.53 1.09 47.28 5.42 11.59 65.91 20 - - 69.58 3.34 4.21 77.13 30* - - 88.92 1.01 1.04 90.97 40 - - 92.44 - - 92.44 50 - - 88.23 - - 88.23 60 - - 94.44 - - 94.44 Note: *Fiber liberation point.
Table 44. Sugar composition of pulps and / or undefibrated cooked chips pulped with 0.050 M citric acid catalyst at different cooking times. Cooking time, Type of sugar, % on OD pulp (cooked chips) min. Ara Gal Glu Xyl Man Total 10 1.07 1.49 46.17 5.78 11.97 66.48 20 0.50 1.13 47.03 5.70 12.14 66.50 30 0.25 1.01 51.71 5.59 12.11 70.67
40 0.20 0.69 56.27 5.84 11.70 74.70 50 - 0.24 67.89 5.94 10.17 84.24 60* - - 70.09 5.80 9.21 85.10 70 - - 70.52 5.75 8.23 84.50 90 - - 72.71 5.52 7.50 85.73 120 - - 74.41 5.35 6.37 86.13 150 - - 74.79 5.27 5.83 85.89 Note: *Fiber liberation point. 118
Table 45. Sugar composition of kraft pulp and / or undefibrated cooked chips pulped at different cooking times. Cooking time, Type of sugar, % on OD pulp (cooked chips) mm. Ara Gal Glu Xyl Man Total 30 0.91 0.92 56.78 6.53 7.30 72.44 60 0.64 0.76 65.60 6.38 6.66 80.04 90 0.54 0.64 73.31 6.16 6.88 87.53 120 0.35 0.45 78.55 6.48 6.57 92.40 150* 0.38 0.41 80.61 6.80 6.38 94.58 180 0.34 0.38 79.57 6.82 6.09 93.20 Note: *Fiber liberation point.
4.6. Delignification and Carbohydrate Removal
The amount of lignin and carbohydrate removed during the pulping process was calculated based on the original amount of each component found in the wood and expressed in percentage. Since the wood chips were pulped without being previously extracted, the calculation was based on the unextracted wood sample as control (see Table
10 and 11). On the other hand, the ratio of lignin and carbohydrate removal (L/C) was calculated as mass loss (g/g) of each component based on its original amount in the wood on an oven-dry basis. Thereby, L/C ratio is the actual mass ratio for lignin and carbohydrates removed (g/g). An L/C ratio < 1 indicates a greater proportion of carbohydrate removed than lignin, whereas a ratio of L/C = 1 indicates equal amounts of lignin and carbohydrate removal. Further, the ratio L/C > 1 indicates that more lignin was removed than carbohydrate. The results of calculations are presented in the following tables and figures. 119
In organosolv pulping with and without catalysts of NaCl and KC1, as shown in
Table 46, it is found that the degree of delignification obtainable with these catalysis systems was very low, i.e., 13.1 % for uncatalysed, and 16.0 % - 18.3 % for NaCl and
KC1. At these degrees of delignification, the carbohydrate removal ranged from 10.1 % to
18.1 %, giving L/C ratios of 0.34 to 0.54, and indicating more carbohydrate removed than lignin lost. Increased catalyst concentrations did not improve the delignification at all.
Thus, no fibers were liberated with these monovalent cation catalysed organosolv pulping processes.
In Table 47, the degree of delignification achieved by pulping with divalent metal
ion catalysts, i.e., CaCl2, MgCl2, MnCl2 and ZnCl2, ranged from 57.6 % to 78.7 %. The carbohydrate removal was found to range from 13.0 % to 31.5 %, giving L/C ratios of
1.69 to 0.92. The results showed that the highest L/C ratios of 1.37 - 1.69 were obtained
with ZnCl2, whereas with the NAEM salts, the ratios were 0.92 - 0.97. However, it should be noted that at the same catalyst concentration, the reject content of the NAEM
pulp was always lower than that of the ZnCl2 catalysed pulp (see also Table 13). In case of the effect of catalyst concentrations, it can be observed that in general the removal of lignin and carbohydrate increased with increasing the catalyst concentrations.
The results in Table 48 show that in pulping with A1C13 and FeCl3, the degree of delignification was found to be 72.4 % - 83.2 %. The carbohydrate removal was 42.3 % -
51.3 %, thus, the L/C ratios were 0.60 - 0.70, which is considered low. It should be noted that the resulting pulp was badly degraded or even completely destroyed.
In pulping with Mg-salt of four different anions, i.e., chloride, nitrate, sulfate and 120 acetate, the results in Table 49 indicate that the degree of delignification achieved by the
NAEM salts, i.e., MgCl2 and Mg(N03)2, were 59.9 % - 78.7 %, which were much higher
than that achieved by MgS04 or Mg-acetate. Particularly with Mg-acetate, the degree of delignification was extremely low, at 6.0 % - 6.9 %. At this level of delignification, carbohydrate removal was 12.2 % - 14.5 %, giving very low L/C ratios of 0.18 - 0.20.
These ratios indicated that the amount carbohydrate removed was at least five times as high as the amount of lignin removed. However, noticing the spent liquor pH, i.e., 7.85 -
8.45, these cooks were already on the alkaline side. The apparently high degree of carbohydrate removal was due to loss of the alkali soluble hemicellulose fraction, removal of which can not be avoided in alkaline pulping. Furthermore, there were no fibers
liberated with either of the MgS04 or Mg-acetate catalysis system.
Table 50 contains the analysis results of pulping with inorganic acids, i.e., HC1,
HN03 and H2S04. Among these acids, the degree of delignification reached by HN03 was only 32.6 % - 32.8 %. This was too low to liberate fibers. On the other hand, the degree
of delignification achievable by HC1 and H2S04 was nearly 80 %. The carbohydrate removal ranged from 17.9 % to 45.5 %, and the L/C ratios were 0.64 - 1.45. Especially, in pulping with HC1 catalyst, it is interesting to observe that the carbohydrate removal increased steadily with increasing the catalyst concentration or acidity, but when the acidity increased over a certain limit (0.0125 M), the degree of delignification decreased.
Table 51 presents the analysis results of pulping with organic acid catalysts, i.e., acetic, oxalic, malonic, malic and citric acids. The degree of delignification achieved by acetic and malonic acid catalysts was only 15.3 % - 17.1 %, insufficient to liberate the 121 fibers. The best result was obtained with >0.050 M citric acid, in which the degree of delignification was 63.7 % - 67.4 % and the carbohydrate removal was 23.2 % - 28.3 %, giving the L/C ratios of 0.91 - 1.04. At these degrees of delignification the fibers were well separated.
In pulping with combination of two different catalysts, the degree of delignification, carbohydrate removal and the L/C ratio were found to vary widely, as shown in Table 52. The degree of delignification achieved by the combination of 0.025 M
NaCl and 0.00625 M HC1 was very low, 18.6 % - 28.7 %, which was insufficient to liberate the fibers. The degree of delignification achieved by other combinations ranged from 52.3 % to 70.3 %, whereas the carbohydrate removal ranged from 13.1 % to 27.5
%, giving the L/C ratios of 0.98 - 1.53. However, not all of these catalyst combinations could carry out delignification to the fiber liberation point. Only those with degree of
delignification >65.0 %, i.e., the combinations of CaCl2 + Mg(N03)2, CaCl2 + MgCl2,
CaCl2 + MnCl2, CaCl2 + ZnCl2 and NaCl + MgCl2, were able to liberate the fibers to achieve a reject content <2.0 %. Furthermore, the L/C ratios of these combinations,
except for CaCl2 + MgCl2, were found to be higher than 1.0, an implication of good chemical selectivity of delignification. 122
Table 46. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with and without catalysts of chloride salts of monovalent cations in 80 % methanol. Removal of: Remarks; Catalyst pH L/C1} Reject, % ODW Lignin, % Carbo., % No catalyst 4.83 13.06 10.10 0.49 NFL 0.025 M NaCl 4.86 16.01 11.36 0.54 NFL 0.050 M NaCl 4.85 16.56 14.95 0.42 NFL 0.075 M NaCl 4.77 18.29 16.18 0.43 NFL 0.025 MKC1 4.87 16.01 18.05 0.34 NFL 0.050 MKC1 4.88 17.50 16.02 0.42 NFL 0.075 MKC1 4.87 18.24 15.42 0.45 NFL Notes : ''The mass ratio of Lignin/Carbohydrate removal. NFL = no fiber liberation.
Table 47. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with catalysts of chloride salts of divalent cations in 80 % methanol. Removal of: Remarks; Catalyst pH L/C15 Reject, % ODW Lignin, % Carbo., %
0.025 M CaCl2 4.20 63.56 25.05 0.97 8.5
0.050 M CaCl2 3.81 67.25 27.79 0.92 1.2
0.075 M CaCl2 3.85 71.45 27.94 0.97 0.4
0.025 MMgCl2 4.20 70.46 28.46 0.94 1.4
0.050 MMgCl2 4.00 75.81 31.32 0.92 0.2
0.075 MMgCl2 3.80 78.74 31.51 0.95 0
0.025 MMnCl2 4.03 66.44 21.92 1.15 5.2
0.050 M MnCl2 3.73 71.34 26.11 1.04 1.1
0.075 M MnCl2 3.60 73.48 28.70 0.98 0.3
0.025 M ZnCl2 3.55 57.64 13.00 1.69 26.5
0.050 M ZnCl2 3.33 64.70 16.42 1.50 3.4
0.075 MZnCl2 3.19 67.99 18.93 1.37 1.2 Note: ''The mass ratio of Lignin/Carbohydrate removal. 123
Table 48. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with catalysts of chloride salts of trivalent cations in 80 % methanol. Removal of: Remarks; Catalyst pH L/C1* Reject, % ODW Lignin, % Carbo., %
0.025 M A1C13 2.26 83.22 51.31 0.62 0 0.050 M AICI3 2.08 - - - mud 0.050 M AICI3 (40')2) - - - - mud
0.025 M FeCl3 2.63 77.33 42.34 0.70 0
0.050 M FeCl3 2.35 72.37 45.98 0.60 0 Notes : ''The mass ratio of Lignin/Carbohydrate removal. 2)The cook was reduced to 40 min.
Table 49. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with magnesium salt catalysts in 80 % methanol. Removal of: Remarks; Catalyst pH L/C1} Reject, % ODW Lignin, % Carbo., %
0.025 MMgCl2 4.20 70.46 28.46 0.94 1.4
0.050 M MgCl2 4.00 75.81 31.32 0.92 0.2
0.075 M MgCl2 3.80 78.74 31.51 0.95 0
0.025 M Mg(N03)2 4.82 59.89 23.91 0.95 10.4
0.050 M Mg(N03)2 5.21 62.77 27.16 0.88 8.5
0.075 M Mg(N03)2 5.49 63.56 28.97 0.84 6.0
0.025 MMgS04 4.85 15.65 12.41 0.48 NFL
2) 0.050 MMgS04 4.83 16.07 12.76 0.48 NFL
2) 0.075 M MgS04 4.84 15.75 12.95 0.46 NFL
0.025 M Mg(CH3COO)2 7.85 6.85 14.46 0.18 NFL
0.050 M Mg(CH3COO)2 8.35 6.61 12.59 0.20 NFL
0.075 M Mg(CH3COO;j2 8.45 5.95 12.18 0.19 NFL Notes : ''The mass ratio of Lignin/Carbohydrate removal. 2)Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. NFL = no fiber liberation. 124
Table 50. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with inorganic acid catalysts in 80 % methanol. Removal of: Remarks; Catalyst pH L/C1' Reject, % ODW Lignin, % Carbo., % 0.00625 M HC1 3.78 67.92 17.86 1.45 25.6 0.00833 MHC1 3.57 72.14 29.01 0.95 6.9 0.0125 MHC1 3.23 79.00 35.93 0.84 0 0.025 M HC1 2.77 76.04 45.52 0.64 0
0.050 MHN03 4.35 32.79 16.16 0.77 NFL 0.075 MHNO3 4.33 32.58 17.75 0.70 NFL
0.005 MH2S04 3.08 79.76 25.04 1.21 17.8
2) 0.010 MH2S04 2.46 - - - - Notes : ''The mass ratio of Lignin/Carbohydrate removal. 2)Fibers were completely damaged (like mud). NFL = no fiber liberation.
Table 51. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with organic acid catalysts in 80 % methanol. Removal of: Remarks; Catalyst pH L/C1' Reject, % ODW Lignin, % Carbo., % 0.025 M acetic acid 4.70 15.33 9.63 0.61 NFL 0.050 M acetic acid 4.63 15.72 9.46 0.63 NFL 0.075 M acetic acid 4.59 15.33 9.90 0.59 NFL 0.025 M oxalic acid 4.18 30.97 14.53 0.81 NFL 0.075 M oxalic acid 3.87 58.62 24.83 0.90 6.6 0.025 M malonic acid 4.66 15.72 10.54 0.57 NFL 0.075 M malonic acid 4.56 17.11 8.26 0.79 NFL
0.050 M malic acid 3.82 57.33 19.93 1.10 16.0 0.075 M malic acid 3.79 60.72 23.36 0.99 6.1 0.025 M citric acid 3.86 57.73 20.68 1.06 15.9 0.050 M citric acid 3.68 63.74 23.24 1.04 1.4 0.075 M citric acid 3.52 67.37 28.34 0.91 0.2 Notes : ''The mass ratio of Lignin/Carbohydrate removal. NFL = no fiber liberation. 125
Table 52. Spent liquor pH and Lignin/Carbohydrate removal in organosolv pulping with combination catalysts in 80 % methanol. Removal of: Remarks; Catalyst1' pH L/C2' Reject, % ODW Lignin, % Carbo., %
CaCl2 + Mg(N03)2 4.02 68.92 25.22 1.04 0.3
CaCl2 + MgCl2 3.90 70.33 27.46 0.98 0.1
CaCl2 + MnCl2 3.78 67.74 22.83 1.13 0.7
CaCl2 + ZnCl2 3.68 64.95 18.52 1.34 1.9
CaCl2 + NaCl 4.24 60.35 17.13 1.34 11.4
NaCl + HC1 4.62 18.64 12.09 0.59 NFL NaCl + HC1 (90')4) 4.60 28.69 15.08 0.72 NFL
NaCl + HN03 4.24 59.90 21.30 1.07 20.8
3) NaCl + MgS04 4.54 52.34 13.07 1.53 40.6
4) NaCl + MgSO4(90') 4.40 64.29 20.78 1.18 12.6
NaCl + Mg(N03)2 4.48 61.45 21.21 1.10 5.1
NaCl + M MgCl2 4.23 65.55 20.51 1.22 2.0 Notes : ''Concentration of each catalyst was 0.025 M, except for HC1, which was 0.00625 M. 2)The mass ratio of Lignin/Carbohydrate removal. 3)Catalyst not permanently dissolved (some reprecipitation) in the cooking liquor. 4)The cook was extended to 90 min. NFL = no fiber liberation.
Degree of delignification, carbohydrate removal and L/C ratio of pulping with some selected catalysts at various cooking times were calculated and plotted in Fig. 24 -
26. Fig. 24 indicates that the rate and degree of delignification in pulping with the A1C13 catalyst was higher than that of citric acid or the NAEM catalysts. The degree of
delignification was drastically reduced following 30 min cooking with A1C13, 40 min with
NAEM and 50 min with citric acid catalysts. On the other hand, Fig. 25 shows that the
carbohydrate removal in A1C13 catalysed cooks was also higher than that observed of the
NAEM or citric acid catalysed cooks. As a consequence, Fig. 26 shows that the L/C ratio
of the A1C13 catalysed cooks was lower than that of the NAEM or citric acid catalysed 126
cooks. Thus, this shows that the A1C13 catalyst was worse than the NAEM or citric acid catalysts in terms of chemical selectivity during the course of pulping.
For more detail, the loss of each monomeric sugar as a function of the cooking time can be observed Fig. 27 - 31. Fig. 27 and 28 indicate that arabinose and galactose were removed completely in the catalysed organosolv pulping processes, while they were retained in kraft pulping even after 180 min cooking. Fig. 29 shows that glucose loss was
drastic in the A1C13 catalysed cooks as compared to other catalysis and kraft pulping. Fig.
30 and 31 indicate that in the A1C13 catalysed cooks, xylose and mannose could be removed completely with an extended cook of 40 min or longer. In general, the loss of xylose and mannose was found to be higher in the NAEM than in the citric acid catalysed or kraft pulping, being more pronounced in extended cooks.
The relationship between delignification and carbohydrate removal for each catalyst used in organosolv pulping and for kraft cooks can be seen in Fig. 32 - 36. In general, it can be seen that in organosolv pulping, arabinose and galactose were completely removed before delignification reached 60 %, whereas the removal of xylose and mannose were enhanced when delignification was higher than 60 %. On the other
hand, glucose seemed to be more resistant, except for A1C13 catalyst. As indicated by Fig.
34, at 80 - 85 % delignification, an apparent reduction in percentage delignification due to lignin reprecipitation occurred while the glucose removal continued. Note, each data point in the graphs was bound to the cooking time. 127
90
180 Cooking time, min
Fig. 24. Effect of cooking time on the degree of delignification in kraft and organosolv pulping with different catalysts (the arrows show the FLP).
Fig. 25. Effect of cooking time on the degree of carbohydrate removal in kraft and organosolv pulping with different catalysts (the arrows show the FLP). 128
1.4
180 Cooking time, min
Fig. 26. Relationship between cooking time and the ratio of lignin/carbohydrate removal in kraft and organosolv pulping with different catalysts (the arrows show the FLP).
Cooking time, min
Fig. 27. Relationship between cooking time and arabinose removal in kraft and organosolv pulping with different catalysts. 129
0 30 60 90 120 150 180 Cooking time, min
Fig. 28. Relationship between cooking time and galactose removal in kraft and organosolv pulping with different catalysts.
35
0 -F ' ' 1 1 ' 1 ' ' 1 1 1 1 ' ' 1 ' ' 1 0 30 60 90 120 150 180
Cooking time, min
Fig. 29. Relationship between cooking time and glucose removal in kraft and organosolv pulping with different catalysts (the arrows show the FLP). 130
0 30 60 90 120 150 180
Cooking time, min
Fig. 30. Relationship between cooking time and xylose removal in kraft and organosolv pulping with different catalysts (the arrows show the FLP).
0 30 60 90 120 150 180
Cooking time, min
Fig. 31. Relationship between cooking time and mannose removal in kraft and organosolv pulping with different catalysts (the arrows show the FLP). 131
Fig. 32. Relationship between delignification and sugar removal in 0.050 M CaCl2 catalysed organosolv pulping (the arrows show the FLP).
-•-Ara -«-Gal -A-Glu -»-Xyl -*-Man Poly. (Total) I R2 = 0.9952 Fig. 33. Relationship between delignification and sugar removal in combination of
0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulping (the arrows show the FLP). 132
Fig. 35. Relationship between delignification and sugar removal in 0.050 M citric acid catalysed organosolv pulping (the arrows show the FLP). 133
100
20 30 40 50 60 70 80 90 100 Delignification, % y = 0.0002x3 - 0.0277x2 + 1.4927x -1.1933 |—•—Ara -•— Gal —*-Glu —•— Xyl -*— Man Poly. (Total) | R2 = Q.962 Fig. 36. Relationship between delignification and sugar removal in kraft pulping (the arrows show the FLP).
4.7. Immunocytochemical Study
The results of immunolabelling are presented in Fig. 37 - 56. It should be noted that in the time series cooks, the images presented were intended to show the end point of pectin disappearance under specified pulping conditions, unless the pectins have not disappeared even after achieving fiber liberation. However, the removal of pectins in the middle lamella was of the main interest of observations. The labelling on the fresh wood chips has served as the control. The results of labelling, as presented in Fig. 37 and 38, indicate that the main location of both acidic and methyl-esterified pectins was in the middle lamella or compound middle lamella. It appeared that the pectins found were more abundant in the radial than in the tangential direction. Consistently, it could be observed 134 that there was an indication that spruce wood contained more esterified pectin than acidic pectin.
The labelling results of cooked chips saved from 30 min organosolv pulping with
0.025 M A1C13 catalyst are presented in Fig. 39 and 40. It is evident that some amounts of both acidic and esterified pectins were still retained after cooking, even though at this cooking stage the fiber liberation point had been surpassed. Fig. 41-44 also show that in organosolv pulping with HC1 catalyst, some amounts of both types of pectins were retaining in the compound middle lamella after reaching fiberliberatio n point. Those figures also show that a reduction or doubling of the HC1 catalyst concentrations did not seem to have a great effect on the pectin retention.
The results obtained from labelling citric acid catalysed cooked chips indicate that the removal of acidic and esterified pectins did not occur within the same cooking time, suggesting that their removal occurred at different rates. As shown in Fig. 45, the acidic pectin was removed at 70 min cook, whereas Fig. 46 shows that the esterified pectin was removed at 50 min cook. On the other hand, in the NAEM catalysed organosolv pulping, as shown in Fig. 47 - 50, the removal of both types of pectins took place at 40 min cook, before all fiberswer e liberated. Fig. 51 - 54 indicate that the pectins were not present in
60 min uncatalysed and NaCl catalysed cookings, whereas Fig. 55 and 56 indicate that in
90 min cooked kraft chips, the pectin could no longer be detected.
Based on the above results, it seemed that the removal of pectins was affected by the cooking liquor pH provided by each catalyst. Therefore, in addition to the images presented in Fig. 37 - 56, a table is provided to summarize the labelling results in regard to 135 the pH characteristics of the cooking liquors. The entries of the table were arranged in the manner of spent liquor acidity (see Table 53). It appeared that the removal of pectins was easier with lower cooking (spent) liquor acidity.
Table 53. Summary of relationship between pH and pectin removal.
pH of cooking Type of pectin Catalyst liquor : present : Shown by Fig. Fresh Spent Acidic Esterified
0.025 M A1C13 (30 min) 2.76 2.54 Yes Yes 39, 40 0.025 MHC1 (60 min) 1.84 2.77 Yes Yes 41, 42 0.0125 MHC1 (60 min) 2.06 3.23 Yes Yes 43, 44 0.050 M Citric acid (50 min) 2.99 3.60 Yes No' 46 0.050 M Citric acid (70 min) 2.99 3.65 No No 45
0.050 M CaCl2 (40 min) 5.99 3.98 No No 47, 48
1 CaCl2 + Mg(N03)2 (40 min) ) 6.30 4.14 No No 49, 50 No catalyst (60 min) 6.24 4.83 No No 51, 52 0.050 M NaCl (60 min) 6.65 4.85 No No 53, 54 Kraft (90 min) - - No No 55, 56 Note : 1)0.025 M each catalyst. 136
Fig. 37. Fluorescent photomicrograph of spruce wood labelled with monoclonal antibody JTM5 showing the presence and location of acidic pectin in the wood matrix (x 400).
Fig. 38. Fluorescent photomicrograph of spruce wood labelled with monoclonal antibody JIM7 showing the presence and location of esterified pectin in the wood matrix (x 400). 137
Fig. 39. Fluorescent photomicrograph of 30 min 0.025 M A1C13 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM5 showing the presence and location of residual acidic pectin (x 400).
Fig. 40. Fluorescent photomicrograph of 30 min 0.025 M A1C13 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JJJV17 showing the presence and location of residual esterified pectin (x 400). 138
Fig. 41. Fluorescent photomicrograph of 60 min 0.025 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM 5 showing the presence and location of residual acidic pectin (x 800).
Fig. 42. Fluorescent photomicrograph of 60 min 0.025 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing the presence and location of residual esterified pectin (x 800). Fig. 43. Fluorescent photomicrograph of 60 min 0.0125 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM5 showing the presence and location of residual acidic pectin (x 600).
Fig. 44. Fluorescent photomicrograph of 60 min 0.0125 M HC1 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing the presence and location of residual esterified pectin (x 600). 140
Fig. 45. Fluorescent photomicrograph of 70 min 0.050 M citric acid catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM5 showing disappearance of acidic pectin (x 400).
Fig. 46. Fluorescent photomicrograph of 50 min 0.050 M citric acid catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing disappearance of esterified pectin (x 400). 141
Fig. 47. Fluorescent photomicrograph of 40 min 0.050 M CaCl2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing disappearance of acidic pectin (x 400).
Fig. 48. Fluorescent photomicrograph of 40 min 0.050 M CaCl2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing disappearance of esterified pectin (x 400). 142
Fig. 49. Fluorescent photomicrograph of 40 min 0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing disappearance of acidic pectin (x 400).
Fig. 50. Fluorescent photomicrograph of 40 min 0.025 M CaCl2 and 0.025 M Mg(N03)2 catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing disappearance of esterified pectin (x 400). 143
Fig. 51 Fluorescent photomicrograph of 60 min uncatalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM5 showing disappearance of acidic pectin (x 400).
Fig. 52. Fluorescent photomicrograph of 60 min uncatalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM7 showing disappearance of esterified pectin (x 400). 144
Fig. 53. Fluorescent photomicrograph of 60 min 0.050 M NaCl catalysed organosolv pulped spruce wood labelled with monoclonal antibody JIM5 showing disappearance of acidic pectin (x 400).
Fig. 54. Fluorescent photomicrograph of 60 min 0.050 M NaCl catalysed organosolv pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400). 145
Fig. 55. Fluorescent photomicrograph of 90 min kraft pulped spruce wood labelled with monoclonal antibody JIM5 showing disappearance of acidic pectin (x 400).
Fig. 56. Fluorescent photomicrograph of 90 min kraft pulped spruce wood labelled with monoclonal antibody JTM7 showing disappearance of esterified pectin (x 400). 5. DISCUSSION
5.1. Effect of Catalysts on Pulp Production Capability in Organosolv Pulping
Many cases of alcohol organosolv pulping trials, except for those conducted with the neutral alkali earth metal (NAEM) salt catalysts, resulted in failure to produce useful pulps from softwoods. Both acidic and alkaline catalysis systems had been tried.
Although low density hardwoods, such as aspen and poplar, could be pulped without catalyst addition (autocatalytic cooks), softwoods remained recalcitrant to organosolv pulping (Aronovsky and Gortner, 1936; Baumeister and Edel, 1980; Lange et al, 1981;
Lonnberg et al, 1987; Argyropoulos, 1999).
Throughout this entire research, it was found that the employment of catalysts in the cooking liquor had a remarkable effect on pulp production capability in the alcohol organosolv pulping of spruce wood. It is re-confirmed that without a suitable catalyst, alcohol pulping did not lead to individual fiber separation from the softwood matrix.
However, the effectiveness of catalysts used in this research varied widely. Pulping selectivity (L/C ratio) varied with the catalyst type, as did the cooking liquor's buffering capacity and ability to remove the pectin from the compound middle lamella. Due to the rather extensive number of compounds which may be soluble in aqueous alcohol and considered as "catalysts" in organosolv pulping, the current study was limited to chloride compounds of mono-, di- and trivalent salts, some Mg-salts and some acids.
146 147
5.1.1. Uncatalysed organosolv pulping
Uncatalysed organosolv pulping is also known as autocatalysed organosolv pulping. In this case, there is no catalyst added into the cooking liquor albeit progressive acidification of the spent liquor takes place due to high-temperature hydrolysis of acetyls from hemicelluloses. The ALCELL process was developed based on this principle.
Autocatalysis for hardwoods works only with <60 % alcohol. Particularly in this study, alcohol organosolv pulping was also conducted without catalyst, with 80/20 (v/v) methanol/water as the cooking liquor. The cooks were conducted at 205 °C for 60 min, including the heating-up time. Time-temperature profile can be seen in Appendix 1. The results showed that under these conditions no fibers were liberated after the specified pulping time even though almost 12 % of wood mass was lost during the process (see
Table 12). Further analysis indicated that under these circumstances the degree of delignification was only 13.1 % based on the original amount of lignin content in the wood, while the carbohydrate loss was 10.1 % based on the original amount of carbohydrate content in the wood. Therefore, the ratio of L/C (lignin/carbohydrate) removal was 0.49 (see Table 46) indicating inadequate delignification selectivity in the autocatalysed pulping of spruce wood. The L/C ratio in this case indicates that a two-fold amount of carbohydrates was removed compared to the lignin dissolved.
Observing the pH of cooking liquor, it was found that the pH decreased from 6.24 to 4.83 after cooking. The decline of pH clearly indicated that a certain amount of acid was generated during the cooking. However, it seemed that the protons generated during the cooking were insufficient, or not strong enough, to bring delignification to the point 148 where the fibers could be liberated. The acid generated in this pulping process was believed to be acetic acid which became the catalyst for the limited delignification (Aziz and Sarkanen, 1989). Therefore, additional acetic acid was externally added into the cooking liquor to make up the apparent deficiency. However, the addition of acetic acid up to 0.075 M, pH 3.67, was also unable to bring delignification to the fiber liberation point. In fact, even though with it the starting cooking liquor pH dropped to 3.67, the low starting pH resulted in no improvement at all since the final spent liquor pH increased to
4.59. The insignificant improvement in delignification by addition of extra acetic acid to the cooking liquor was also reported by Girard and Heiningen (2000). Organosolv delignification is not only depending on the availability of protons, but also requires a high degree of pH control of the cooking liquor during the cook. This aspect has not been recognized to date and will be discussed further in the section detailing the importance of an initial high pH (>4.5) for pectin removal.
5.1.2. Catalysed organosolv pulping
As briefly described in Section 1.4, a wide variety of catalysts was used in this study. Although there was a remarkable impact of the catalysts on the capability of the pulping process to delignify and liberate the fibers, not all catalysts employed in this experiment were capable of pulping the wood chips under the specified pulping conditions. The capability of each catalyst, as measured by its ability to liberate the fibers, remove lignin and thereby affect the pulp yield, was different. Some catalysts were found to be very effective, some were mediocre, whereas others were incapable or too 149 aggressive in hydrolysing the wood and thereby, completely destroyed the fibers.
5.1.2.1. Organosolv pulping with inorganic acid catalysts
The results of pulping with acid catalysts show that not all type of acids employed as catalysts in this study were capable of liberating the fibers under the specified pulping
conditions. Among the inorganic acids, nitric acid (HN03) was ineffective. Although 22
% of wood mass has been lost during the cooking (see Table 16), no fibers were liberated.
The degree of delignification achieved was only 33 % while the carbohydrate loss was 16 -
18 %, in which case the L/C ratio was 0.70 - 0.77 (see Table 50). This ratio indicated that more carbohydrate was lost than lignin. In other words, the pulping or delignification selectivity was poor. Based on visual observation of the cooked chips, it seemed that nitric acid might eventually be able to liberate the fibers but required longer cooking time
(>60 min) and / or higher acid concentration (>0.075 M). However, it should be noted that increasing the acid concentration and / or extending the cooking time will compromise pulp yield and viscosity due to the enhanced cellulose degradation.
The use of hydrochloric acid (HC1) and sulfuric acid (H2S04) as catalysts afforded limited success in pulping spruce wood. In general, hydrochloric acid gave better results as compared to sulfuric acid. As can be seen in Table 16 and 24, the highest pulp yield of
52.1 % at 0 % reject was obtained from pulping with 0.0125 M HC1 as the catalysts. The lignin-free yield was 46.5 % which was 1.5 % higher than by the kraft process at fiber liberation although the viscosity of the HC1 catalysed organosolv pulp was too low (5.3 cPs.) and the kappa number was high at 71.0, or equivalent to 10.7 % residual lignin. On 150
the other hand, pulping with 0.005 M H2S04 gave a pulp yield of 56.5 % at 17.8 % reject content whereby, the screened yield became only 38.7 %, rather low. The viscosity of the pulp was 4.2 cPs and the kappa number was 61.9, or equivalent to 9.5 % residual lignin.
The effect of catalyst concentration required was noticeable in these two types of inorganic acids. The degree of fiber liberation increased by increasing the catalyst concentration, while the pulp viscosity decreased further, as expected. The results indicate that increasing the catalyst concentration beyond that required for the fiber liberation point, i.e., increasing from 0.0125 M HC1 to 0.025 M HC1 (see Table 24), the residual lignin content increased as a result of reprecipitation of dissolved lignin onto the fibers. In addition to this drawback, degradation of cellulose was also increased, as indicated by further drop of viscosity to 2.2 cPs, which is unacceptably low. A similar phenomenon was also observed on pulping with sulfuric acid as the catalyst. At the low pulp viscosities, the integrity of the fibers was totally destroyed as the acid concentration was increased from 0.005 M to >0.01 M. In such case, observing the pulp yield only is deceiving as the resulting "pulp" has no papermaking value at all.
From these inorganic acid catalysed cooks, it is evident that they are ineffective as true pulping catalysts and the eventual fiber liberation is brought about by incomplete destruction of the wood matrix by acid hydrolysis.
5.1.2.2. Organosolv pulping with organic acid catalysts
In pulping with organic acid catalysts, citric acid appeared to be the best in terms of achieving early fiber liberation. A pulp yield as high as 65 % with only 1.4 % screen 151 reject could be obtained by using 0.05 M citric acid as the catalyst (see Table 17). The kappa number of the pulp was 92.7, or 14.8 % residual lignin (see Table 25). The viscosity could be as high as 26 cPs (see Fig. 21). It was estimated that employment of
0.05 M citric acid as the catalyst can produce a fully bleached pulp as high as 54 % yield.
The other organic acids, i.e., oxalic and malic acids, showed potential to be able to pulp softwoods but it seemed that these two types of acids required high (>0.075 M) catalyst concentrations and / or longer cooking times (>60 min). Under the predetermined pulping conditions, malic and oxalic acid can produce pulp at 67 % yield but the reject contents were higher than 6 % (see Table 17). The kappa number of the oxalic acid pulp was
101.3, or 16.4 % residual lignin, and that of malic acid 99.6, or 15.6 % residual lignin (see
Table 25). In contrast, malonic and acetic acids did not show any potential of being able to catalyse the pulping of softwood although more than 12 % wood mass was lost during the cooking.
5.1.2.3. Organosolv pulping with monovalent metal ion catalysts
Sodium chloride (NaCl) and potassium chloride (KC1) are monovalent salt catalysts used in this series of cooks. The pulping results, as shown in Table 12, indicate that NaCl and KC1 were not capable of pulping or bringing about fiber liberation of spruce wood. Wood mass lost during the pulping process was only 13-15 %, in which the degree of delignification was only 16 - 18.3 %, respectively. The degree of delignification achieved was not high enough to liberate the fibers. Apparently, the deficiency is due to insufficient proton generation, i.e., available protons in the cooking liquor, during the 152 delignification process as indicated by the high final spent liquor pH of 4.77 - 4.88 (see
Table 27). Alternatively, the failure to liberate the fibers could be caused by a higher buffering capacity (see Fig. 23). Throughout the entire experiment, it was observed that complete fiber liberation would not take place if the final pH was higher than 4.2, regardless of the catalyst species. Therefore, the catalysts employed in the cooking liquor should possess close to neutral pH at the start of the cook and be able to moderate the drop of pH during the course of pulping to below 4.2 in order to ensure delignification to be carried out to the point where all individual fibers in the wood matrix are liberated.
5.1.2.4. Organosolv pulping with divalent metal ion catalysts
Among the catalysts used in this research, chloride salts of divalent metal ions, i.e.,
Mg++, Ca++, Mn++ and Zn++, showed high capability of delignification selectivity and pulping to produce chemical grade pulps at high yield from spruce wood. All catalysts in this group were found to be able to delignify the wood to reach the fiber liberation point at various pulp yields.
In terms of the species and concentration of the catalysts in this group, it was found that the species and the concentration of metal ions have an effect in achieving fiber liberation (see Table 13). The higher the concentration, the higher the degree of fiber liberation, as indicated by the lower reject content. As can be seen in Table 13, 0.025 M
MgCl2 was able to liberate the fiber at a pulp yield of 61.3 % and reject content of 1.4 %,
but for CaCl2 and MnCl2 the catalyst concentration had to be doubled to 0.050 M in order to obtain a pulp yield of 61.7 % and 60.9 % at reject contents of 1.2 % and 1.1%, 153
respectively. With ZnCl2, the concentration required was tripled to 0.075 M in order to reach fiber liberation, where the pulp yield was 62.2 % at a reject content of 1.2 %. These findings indicate that there is a hierarchy of ionic strengths with these cations for delignifying the softwood to the level where all fibers can be freed. The sequence from the stronger to the weaker cations can be written as : Mg++ > Ca++ = Mn++ > Zn++.
Regardless of the concentration, at fiber liberation point this group of catalysts were capable of producing chemical grade pulps with screened yields >60.0 %. This yield is at least 12 % higher than that achievable by the kraft process on this wood species. At this level, the organosolv pulping process is potentially capable of producing a fully bleached chemical pulp at 52.0 % yield.
The pulp properties, such as kappa number and viscosity, were also found to be affected by the metal ion species and concentration (see Table 21). The kappa numbers and viscosities tended to be lowered by increasing catalyst concentrations. At fiber liberation point, the kappa numbers were 90.4, 85.9, 83.2 and 81.5, and the viscosities
were 22.6, 19.8, 19.9 and 17.2 cPs, respectively for CaCl2, ZnCl2, MgCl2 and MnCl2.
Apparently, the residual lignin content in these organosolv pulps was much higher than in the kraft pulp. Even though at higher kappa number, the organosolv pulps are found to bleach easier (Cronlund and Powers, 1992; Lora et al, 1993), and irrespective of the high residual lignin content, organosolv pulps still produce at least 7.0 % more fully bleached pulp than obtained from lower residual lignin-containing kraft pulp. Demonstrably, the higher kappa number will not be an obstacle in solvent pulp bleaching. 154
5.1.2.5. Organosolv pulping with trivalent metal ion catalysts
The chloride salts of trivalent metal ions, i.e., AT++ and Fe+++, were also tried for pulping the spruce wood. These two catalysts were found to be able to pulp the wood but resulted in low pulp yield and badly degraded pulp (see Table 14 and 22). The catalyst concentrations were found to greatly affect the extent of fiber damage. The pulping
outcome was a mud-like material when using >0.05 M A1C13 or FeCl3. In other words, the
fiber structures were completely destroyed. In time serial cooks with 0.025 M A1C13 as the catalyst, the highest pulp yield obtained at fiber liberation point was 46.3 % with 0.0 %
reject (see Fig. 14 - 17). The delignification rate in pulping with A1C13 was very high, as it was noted that such cooks required only 30 min, including heating up time (see Appendix
1), to completely liberate the fibers. At this point, the residual lignin content of the pulp was 11.1 %. Surprisingly, the viscosity of the pulp was only 3.0 cPs, indicating a strong hydrolytic degradation of the pulp. These trivalent catalysts are too aggressive and poor in delignification selectivity, attacking both lignin and carbohydrate at the same time.
Therefore, they are not good candidates as catalysts in organosolv pulping for production of good quality chemical pulps. In this case, an alternative may be that the pulp be used for producing microcrystalline cellulose. As it can be seen in Table 43, 40 min or longer cooks produce pure cellulose, without a significant hemicellulose contaminant, at a near theoretical yield as high as 37.8 % based on oven-dry wood. This carbohydrate yield is
4.7 % below the a-cellulose yield recorded in Table 10. Another potential use of trivalent metal ion catalysts is for chemical conversion (hydrolysis) of wood residues, however, this requires a study of the fate of the dissolved sugars and lignin in such a highly acidic spent 155 liquor, pH 2.3.
5.1.2.6. Organosolv pulping with Mg-salt of different anion species catalysts
As indicated in Section 5.1.2.4., Mg++ provided the highest cation activity among the divalent catalysts included in this study. Therefore, in this series of cooks, Mg-salts of
= chloride (CI"), nitrate (N03), sulfate (S04 ) and acetate (CH3COO") were used as the catalysts. The purpose of this series was to investigate the effect of the anions on the pulping catalytic effect, particularly in proton generation and delignification specificity during cooking. As expected, the pulping results showed that there was an impact of the
anion species on fiber liberation (Table 15) and pulp yield. Only CI" and N03" counter ions
= were able to liberate the fibers, whereas S04 and CH3COO" were largely ineffective.
However, the CI" ion seemed to be more effective than N03", as indicated by the higher
degree of fiber liberation. As shown in Table 15, MgCl2 was capable of bringing delignification to the point where complete fiber liberation (0 % reject) could take place,
while Mg(N03)2 was not so effective under the same conditions. Additionally, the kappa
number of the pulp cooked with Mg(N03)2 catalyst was much higher (around 100) than
that obtained with MgCl2 (69.4 - 83.2) (see Table 23).
5.1.2.7. Organosolv pulping with combination catalysts
The employment of combinations of two different catalysts is expected to provide synergistic effects in the catalysis systems that can result in higher pulping rates, better pulping selectivity and better pH buffering capacity which subsequently can produce 156 higher pulp yield with better fiber quality. In general, the results as shown in Table 18 indicate that there can be improvements in the degree of fiber liberation. In terms of pulp yield, improvements are also observed in some cases. However, the extent of improvement varied depending on the combination of the catalysts (see Table 18).
In the case of the NAEM catalysis system, particularly in the combination of 0.025
M CaCl2 with 0.025 M Mg(N03)2 at 60 min cook, it was found that the screen reject was only 0.3 %. It showed that there was improvement in the degree of fiber liberation,
especially compared to using 0.050 M Mg(N03)2 alone, in which the reject content was
8.5 % (see Table 15). The screened pulp yield was also slightly increased. If compared to
using 0.050 M CaCl2 alone, there was still an improvement in the degree of fiber liberation but not in pulp yield. However, it should be noted that the reject content obtained by
using 0.050 M CaCl2 alone was already less than 2.0 %, i.e., 1.2 %, at which level the fiber liberation was considered complete. Therefore, any attempt to increase fiber liberation beyond this point seems to sacrifice pulp yield (this matter will be discussed in more detail in the next section) although the kappa number of the pulp, made by the
combination of 0.025 M CaCl2 and 0.025 M Mg(N03)2, was found to be lower (83.4) (see
Table 26) than that obtained by 0.050 M CaCl2 alone (90.4) (see Table 21).
An improvement in fiber liberation due to the combination of more than one catalyst was also observed in pulping with the combination of 0.025 M NaCl and 0.025 M
HN03. As can be seen in Table 12 and 16, when using either NaCl or HN03 alone, no fibers were liberated even at the catalyst concentrations of 0.075 M, which was already higher than that of the combination catalysts of 0.050 M. Although complete fiber 157
liberation was not achieved by the NaCl/HN03 catalyst combination under the specified pulping conditions, it is expected that complete fiber liberation could be reached by increasing the catalyst concentrations further and / or extending the cooking time beyond
60 min. However, based on the result given in Table 18, this catalyst combination is not expected to produce a pulp at the yield and quality as high as those obtained by the divalent metal ions or the NAEM salt catalysed cooks.
5.2. Effect of Extended Cooking
As briefly described in Section 1.4, organosolv cooks with CaCl2, combination of
CaCl2 and Mg(N03)2, citric acid and A1C13 catalysts, in addition to kraft cooks, were selected to be carried out with different extended cooking times. The pulping results indicated that the highest screened pulp yield was always obtained at fiber liberation point
(see Fig. 15). The cooking time required for complete chemical defibration was found to be dependent on the catalyst species. As a result, different catalyst systems required different cooking times to reach the fiber liberation point (see Fig. 17).
As mentioned above, a number of cooks have been extended beyond the fiber liberation point in the expectation that extended cooks would produce pulps with lower kappa number, as once became a practice in kraft pulping (Mera and Chamberlin, 1988;
Andrews, 1989; Jiang etal, 1992). In contrast, it was found that the residual lignin content of organosolv pulps remained more or less unchanged (see Fig. 20) while yield loss occurred continuously as the cooks were more and more prolonged (see Fig. 14 - 16).
The pulp yield loss was more pronounced in NAEM salt (CaCl2 and CaCl2 + Mg(N03)2) 158 catalysis (see Fig. 15 and 16) as compared to that of citric acid catalysis. It appears that the yield loss is associated with a slight subsequent drop of pH (see Fig. 22). Especially, in the 150 min cook, the lignin-free pulp yields were even lower than those of the kraft pulp (Fig. 16) although the residual lignin content of the organosolv pulp remained much higher than that of the kraft pulp (Fig. 20). It was the result of imbalance between degree of delignification achieved and the carbohydrate lost. As shown in Fig. 24, delignification beyond the fiber liberation point is drastically decreased whereas the carbohydrate removal continues steadily (Fig. 25). Consequently, with prolonged cooking time more carbohydrate is removed than lignin. Therefore, in extended organosolv cooks the L/C
(lignin/carbohydrate removal) ratio is even lower than that found for the kraft process
(Fig. 26) while it appears that the pulping selectivity becomes poorer and poorer when the cook is extended beyond the fiber liberation point.
Another disadvantage of extending the cook was that the reprecipitation of lignin increased with increasing cooking time. Lignin reprecipitation was more noticeable with
the A1C13 catalyst (Fig. 20), the system being more acidic than either the CaCl2 or the
combination of CaCl2 and Mg(N03)2 catalysts. It should be noted that reprecipitated, or recondensed, lignin is not soluble in the aqueous alcohol solvent. However, this reprecipitated lignin can be removed with acetone, tetrahydrofuran (THF), dimethylsulfoxide (DMSO) or 3 - 5 % hot alkali washing (Paszner and Cho, 1989).
Therefore, preventing its formation is very important to avoid introduction of other, albeit more powerful solvent types as mentioned above in the pulp washing, which may cause complications in solvent handling in the mill. 159
Another problem with extended cooking in organosolv pulping is the loss of pulp viscosity. Particularly, beyond fiber liberation in NAEM catalysed pulp, the pulp viscosity is dropping with cooking time by extending the cook (see Fig. 21). This is assumed to be due to the drop in pH below 4.0 where the buffering capacity of the catalyst is exhausted, the delignification selectivity is rapidly diminished as aggressive hydrolysis takes over.
Therefore, it can be concluded that extended cooking beyond the fiber liberation point provides no benefit in organosolv pulping. In short, the disadvantages can be summarized as follows :
1) Yield loss due to more carbohydrate dissolution,
2) Drastically reduced delignification rate,
3) Increased lignin reprecipitation, and
4) Decreased pulp viscosity.
For these reasons, extended cooking for the production of chemical grade pulp by organosolv pulping is not recommended. The cook has to be quenched at the fiber liberation point in order to guarantee the production of high yield and high quality pulp.
The residual lignin can be destroyed by a suitable ECF or TCF bleaching (Solinas and
Murphy, 1996; Wang, et al, 1997; Ni and Heiningen, 1998; OoiandNi, 1998).
5.3. Effect of Cooking Liquor Acidity
As indicated by the cooking liquor pH, all organosolv cooks, except for those with magnesium acetate as catalyst, were conducted under acidic conditions, in which the pH ranged from 1.4 to 6.9 (Table 27 - 33). The pH of fresh cooking liquors ranged widely 160 from very acidic, e.g., mineral acids and chloride salts of AT++ and Fe+++, to nearly neutral,
e.g., MgS04. Particularly, pulping with and without a salt catalyst of mono- and divalent metal ions showed that after cooking, the pH of the cooking liquor always dropped, indicating that additional protons were generated during the cook (see Table 27 -30 and
33). The extent of pH drop varied, depending on the catalyst and length of cooking time.
In pulping with salt catalysts, it is found that a high degree of fiber liberation, or the fiber liberation point, consistently occurred at a spent liquor pH < 4.20. When the pH remained higher than 4.20, no fibers were liberated or the degree of fiber liberation was very low, as indicated by high reject content. The optimum pH for a high degree of fiber liberation and a high pulp yield seemed to depend on the type of catalyst. For divalent cations, the spent liquor pH ranged from 3.2 to 4.2, in which the order of cation acidity followed Zn++ > Mn++ > Ca++ > Mg++ (higher to lower acidity) (see Table 28). For the
NAEM salt catalysts, the optimum pH fell into a very narrow range of 4.0 ± 0.2. Kleinert
(1977) specified a pH 4.0 but was unable to control it consistently. It seemed that in organosolv pulping, regardless of the catalyst species, the spent liquor pH should be 4.2 or lower to ensure that delignification is carried out to the point where the fibers can be liberated to result in a low reject content pulp. Otherwise, at higher pH the fibers are only partially liberated, resulting in a high reject content pulp, or none at all, as occurred for the uncatalysed and monovalent cation-catalysed pulping trials (see Table 12 and 27).
Especially, for the NAEM catalysed organosolv pulping cooks, the pH drop took place mainly in the first 10 min of the cook (see Fig. 22), or already during the heating-up period (see Appendix 1). The pulping system required about 40 min to reach the pulping 161 temperature of 205 °C. It means that most of the protons, which are the true catalyst for delignification, are generated at the beginning of the cook. As the cook is continued and extended, the pH decreases steadily resulting in the increase of cooking liquor acidity.
Concurrently, these "secondary" protons cause yield and viscosity losses, and result in pulp degradation. At low pH (<3.5) lignin reprecipitation is also increased. Therefore, the sequence of cooking liquor acidity development becomes a crucial parameter in the process of fiber separation and pulping selectivity that ultimately determines the pulp yield and properties.
In pulping with acid catalysts, it was interesting to observe the behavior of pH.
The trend of pH change is in the opposite direction to that of pulping with salt catalysts.
The starting pH is always lower and the final pH is always higher, even higher than the final pH of NAEM salt catalysed cooks under the same pulping conditions. This indicates a fundamental difference that in pulping with acid catalysts, protons are consumed as indicated by the pH increases, whereas in pulping with salt catalysts, the protons are generated. For this latter case, the protons are generated through an ion exchange mechanism (Browning, 1967; Abubakr etal, 1997; Paszner and Cho, 1989). The protons generated by this means could be defined as "internally generated protons", whereas the protons provided by the addition of an acid to the cooking liquor, e.g., pulping with acid catalysts, could be defined as "externally added protons". On the other hand, the protons generated without addition of catalyst, thus, uncatalysed or autocatalysed organosolv pulping process, could be defined as "autogenerated protons".
It was also observed that when the final cooking liquor pH was > 4.2, there was no fiber 162
liberation taking place, as for example with HN03 (Table 31), acetic, malonic and oxalic
(0.025 M) acids (Table 32) as catalysts.
In terms of proton generation by salt catalysts, it was found that at the same molar concentration divalent cations generated more protons than monovalent cations (Table 27 and 28). Perhaps, monovalent cations used in this experiment (Na+ and K+) were unable to exchange enough protons to bring down the pH to lower than 4.2. Neither was pH control evident in the autocatalysed process. Another possibility for the inability of NaCl and KC1 to generate protons may be their higher buffering capability as compared to the
NAEM salts, i.e., CaCl2 and MgCl2 (see Fig. 23). Consequently, fewer protons were available in the cooking liquor to catalyse delignification. Therefore, for these catalyst types, delignification is not extensive enough to facilitate fiber liberation.
Over all, the acidity of the cooking liquor, as indicated by the pH, becomes the key factor in controlling the pulping selectivity, fiber separation, extent of delignification and carbohydrate removal which ultimately imprint their impact on the pulp yield and pulp properties. However, in the NAEM catalysed organosolv pulping process, the pH can be regulated and controlled by the cation species and concentration of catalyst employed in the cooking liquor.
5.4. Pulping Selectivity
In a chemical pulping process, selectivity of the pulping process is a very important factor that ultimately determines the pulp yield and properties. Pulping selectivity in terms of preference of removing a certain main wood component can be evaluated by calculating 163 the ratio of Lignin/Carbohydrate (L/C) removal. The higher the L/C ratio, the better the pulping selectivity. It means that more lignin is removed than carbohydrates. Herein, pulping selectivity is referred to as delignification selectivity in the pulping process since the focus of chemical pulping is on delignification. In this case, it refers to chemical selectivity. However, in addition to chemical selectivity, there is another type of pulping selectivity, known as topochemical selectivity. This latter selectivity is also an essential factor in determining the pulping outcome. Topochemical selectivity is the pulping selectivity with respect to the location where pulping activity preferentially takes place.
High topochemical selectivity directed to the compound middle lamella invariably guarantees a high pulp yield, as in the case of the NAEM catalysed organosolv process, whereas lack of topochemical selectivity, as with the kraft process, leads to lower pulp yield.
Therefore, in terms of delignification, pulping selectivity could be defined by two different terms : 1) Chemical selectivity, and 2) Topochemical selectivity. Chemical selectivity refers to the preference of removing lignin, regardless of its location, or origin, whereas topochemical selectivity leads to preferred removal of lignin from strategic locations. In this case, a high L/C ratio indicates high chemical selectivity of delignification. However, a high L/C ratio does not guarantee early fiber liberation and somehow does not guarantee high pulp yield either. It could merely guarantee a lower kappa number pulp. On the other hand, topochemically selective delignification may guarantee early fiber liberation and likely high pulp yield, too, if the delignification is preferentially taking place in the middle lamella. However, residual lignin content of the 164
pulp may remain very high. For instance, in case of using 0.005 M H2S04 as the catalyst, the L/C ratio was 1.21 (Table 50), which was much higher than that of NAEM salt catalysts, i.e., <1.04 (see Table 47 and 52). However, the degree of fiber liberation for the
H2S04 catalysed cook was low, as indicated by high reject content of 17.8 %, even though the pulp kappa number was also lower than that of NAEM salt (see Table 21 and 24).
The implication is that topochemically sulfuric acid was more active in the delignification of the secondary cell wall than in the middle lamella. Since the fibers need to be liberated as early as possible, the high topochemical selectivity concentrated in the cell wall provides no useful advantage in pulp production. Residual lignin removal is better done by oxidative bleaching (Wang, etal., 1997).
The L/C ratios, as tabulated in Table 46 - 52, were found to be a function of the species and concentration of the catalysts. In general, the catalysts that were incapable of
liberating the fibers or were too aggressive, e.g., A1C13 and FeCl3, showed lower L/C values than those capable of early fiber liberation. It is desirable to have higher L/C ratios, meaning that more lignin is removed than carbohydrate, but only if the higher L/C ratio also provides higher degree of fiber liberation. In some cases, it was found that the L/C ratio was high but the reject content of the pulp was also high. This indicated that delignification was more preferably taking place in the cell wall. On the other hand, it was also found that the L/C ratio was low but the reject content was also low. In this case, the preference of delignification was in the middle lamella, a beneficial topochemical effect.
These phenomena suggested that each catalyst had its own preferential location where it was more active. In other words, wide topochemical differences are created by the 165 catalysts. The results showed that generally, there was a tendency that with more (very) acidic species or circumstances, the catalyst was likely to be more active in the cell wall than in the middle lamella. Therefore, the employment of two catalysts of different topochemical selectivity could lead to a synergistic effect and result in the improvement of the degree of fiber liberation. This seems to be accomplished by several combinations,
including the combination of CaCl2 and Mg(N03)2, the NAEM process, as indicated in
Table 52.
In extended cooking, beyond the fiber liberation point, the L/C ratio tended to decline with longer cooking time (Fig. 26). Particularly, with the NAEM salt catalyst, a decline of the L/C ratio seemed to be associated with a slight increase of cooking liquor acidity. However, the effect of cooking liquor acidity on the L/C ratio was more noticeable with acid catalysts, especially strong mineral acids, such as HC1, in which case the L/C ratio tended to be lowered when the acidity increased. Therefore, it can be concluded that pulping selectivity is regulated and controlled by catalysts through the management and modulation of cooking liquor pH.
5.5. Behavior of Delignification
As shown in Fig. 24, delignification in organosolv pulping proceeds in two distinctive phases : 1) bulk delignification, and 2) residual delignification. Bulk delignification proceeds at a very high rate while residual delignification seems to progress at an extremely slow rate. In bulk delignification the selectivity of delignification increased with the cooking time since it reached the highest point at about the fiber liberation point. 166
Selectivity then decreased during the residual delignification. During the bulk delignification stage, about 60 % of the lignin is removed and this includes most of the middle lamella lignin. The results show that complete fiber liberation is achieved at the stage (Fig. 17) when the bulk delignification starts levelling off and the residual delignification begins to take place (Fig. 24). In this case, it can be concluded that the fiber liberation point is the transition stage between bulk delignification and residual delignification at which point a sharp break in the delignification curve is evident (see Fig.
24). Therefore, residual delignification occurs after fiber liberation takes place. On this basis, it can be concluded that topochemically residual delignification must take place exclusively in the secondary wall. Since almost all carbohydrates in wood are concentrated in the secondary wall and the cell wall lignin is associated with hemicelluloses, known as the lignin-carbohydrate complex (Eriksson, et al, 1980;
Iversen, 1985; Tanabe and Kobayashi, 1987; Karlsson, etal, 1999), hydrolytic delignification in the secondary wall without carbohydrate loss is unavoidable. A pulping process which possesses extremely high selectivity by attacking only lignin and / or lignin- carbohydrate bonds has not been discovered as yet. Even organosolv pulping processes, including the NAEM salt catalysed organosolv pulping process, do not posses such selectivity beyond fiber liberation. As can be seen in Fig. 26, the selectivity in extended cooks by the organosolv processes was worse than that of the kraft process beyond the fiber liberation point, where delignification is taking place exclusively in the cell wall.
Therefore, in organosolv pulping, it is critical to quench the cooking when the fiber liberation is achieved, whereby production of high pulp yield with good properties can be 167
preserved. In this case, delignification can be completed by the more selective 02, 03 and
H202 oxidation process.
Under predetermined organosolv pulping conditions, it was indicated that the extent and rate of delignification appear to be controlled by catalysts through regulating the pH of the cooking liquor. Therefore, the achievable degree of delignification varies depending on the catalyst species and its concentration. It also shows that, regardless of the catalyst species and concentration, delignification could not be carried out to the point where all individual fibers were completely separated unless the pH of cooking liquor dropped to < 4.2. Under such conditions, the fibers start to be liberated when the degree of delignification achieved is 50 % or higher (see Table 46 - 52, Fig. 17 and 24).
Otherwise, at less than 50 % lignin removal no fibers are liberated at all. On the other hand, the fibers could be completely liberated when delignification exceeded 60 %. If compared to the kraft process, no fibers can be liberated at such relatively low degrees of delignification. For fiber liberation by the kraft process, lignin removal in excess of 80 % is required. Complete fiber liberation for the kraft process is accomplished only at 88 % or higher degree of delignification. The differential rates in fiber liberation are caused by the difference in topochemistry of delignification (see Fig. 12). In the kraft process, cell wall delignification prevails, whereas in organosolv pulping, i.e., the NAEM process, the middle lamella delignification prevails up to the fiber liberation point. However, it should be noted that not all types of organosolv pulping processes behave this way. Because lignin is the major component of the middle lamella, the capability of catalysts to produce pulp is determined by the extent of delignification achieved by the catalyst. As indicated 168 above, not all catalysts used in this study had sufficient delignifying power to achieve the required degree of delignification for fiber liberation. Some catalysts, such as NaCl, KC1,
MgS04, Mg-acetate, acetic acid and malonic acid, have very weak delignifying powers under the conditions applied, whereby less than 19 % of the lignin was removed.
Apparently, the inability of these catalysts to provide the desired pH range of 4.0 ± 0.2 for delignification is responsible for their ineffectiveness as catalysts. Therefore, the delignification with such catalysts was observed to proceed at a very slow rate. In
contrast, very acidic species, such as A1C13 and FeCl3, and strong mineral acids, such as
HC1 and H2S04, exhibited a very strong delignifying power. The rate and achievable degree of delignification were the highest by these catalysts as compared to the other
catalysts studied. The delignification degree could reach 85 %, especially with A1C13 (see
Fig. 24). However, the drawback in employing such catalysts is the high rate of carbohydrate degradation (hydrolysis), and that lignin reprecipitation increases as the cooking liquor acidity drops below the optimum level. As was observed in using 0.005 M
H2S04 as the catalyst, lignin reprecipitation seemed to be severe and occurred very fast while the dissolved lignin was still in the wood chip. The reprecipitated lignin created a blockage to further delignification. As a consequence, such blockage limited or probably even halted further liquor penetration into the inner part of the chips. Therefore, the inner portion of the chip became inaccessible to the cooking liquor. The result was retarded delignification and curtailed fiber liberation. Eventually, the inaccessible undelignified portion of the chip ends up as the reject, i.e., bundles of fibers that are not separated. As evidence, it was observed during the experiments that the reject amount of 17.8 % 169
resulting from 0.005 M H2S04 catalysed cooks (see Table 16) were solely coming from the center portions of the wood chips indicating restricted penetration of the chips by the cooking liquor. This phenomenon seemed to be the main obstacle in employing strong
mineral acids, particularly the use of H2S04, as the catalyst in the alcohol organosolv pulping process. Since mineral acid catalysed pulps are badly degraded or even completely destroyed, such pulps have very little utility other than microcrystalline cellulose following bleaching. However, bleachability of such degraded pulps would have to be demonstrated since Lachenal and co-workers (1999) found that the lower lignin content pulps produced by extended delignification were far more difficult to bleach than high kappa number pulps.
5.6. Carbohydrate Degradation and Removal
The trend of carbohydrate degradation and removal in organosolv pulping was found to follow closely the acidity of the cooking liquor. Obviously, carbohydrate losses are due to their hydrolysis by acids. Since the acidity of cooking liquor was to be controlled by the catalysts, thus, indirectly the extent and rate of carbohydrate degradation and removal could be managed by the catalyst selection. As can be seen in Table 46 - 52, in general, the amount of carbohydrates removed varied according to the catalyst species and their concentration. Unlike delignification, although carbohydrate loss was gradual from the start, the loss and degradation of carbohydrates became more intensified at high acidity. In this case, as expected, the loss of carbohydrates was enhanced as the cooking liquor acidity increased. This was clearly observed when HC1 was used as the catalyst at 170 various concentrations (Table 50). Furthermore, the carbohydrate degradation and loss tended to progress steadily as the cook was extended beyond the fiber liberation point, as shown in Fig. 21 and 25.
Among the carbohydrates, hemicelluloses were readily removed. Cellulose was found to be more resistant, unless at very low pH (~ 2.0) where the cellulose loss became more noticeable. Apparently, the loss of carbohydrates associated with lignin is not avoidable during pulping. Some carbohydrates are already removed prior to delignification (see Fig. 32 - 35). However, based on the sugar analysis (Table 34 - 45, see also Fig. 27 - 31), a substantial difference among the hemicellulose components, in terms of their ease in removal in organosolv pulping, can be observed. Arabinan followed by galactan are the easiest hemicellulose components to be removed even in high alcohol- content cooking liquor. Both arabinan and galactan are completely removed and signal imminent fiber liberation (see Table 41 - 44, and Fig. 27 and 28). Their rapid loss or absence in fibers indicates a control of the fiber liberation process in organosolv pulping.
Surprisingly, it was also found that in all catalysed organosolv pulping trials, without complete removal of arabinan and galactan, delignification could not be carried out to the point of fiber liberation (see Table 12, 15 - 18 and 34, 37 - 40). Apparently, due to their presence in high proportions in the middle lamella and the primary wall
(Rydholm, 1985), the removal of arabinan and galactan is required to facilitate delignification to take place in the middle lamella in order to separate the fibers completely. Therefore, the sugar analysis shows that well separated organosolv pulp fibers do not contain arabinose and galactose. These pulps contain only glucose, xylose 171 and mannose. In contrast, the kraft pulp still contained all five sugar types, i.e., arabinose, galactose, glucose, xylose and mannose, that were originally present in the wood.
Residual arabinose and galactose in kraft pulp may indicate incomplete removal of pectin in kraft pulping although neither pectin type remained evident on staining delignified kraft chips which could be completely defibrated to individual fibers in water (Fig. 55 and 56).
Probably, the difference in sugar composition between organosolv and kraft pulps might be one of the sources of difference in the paper strength properties, especially inter-fiber bonding. However, it should be noted that arabinan and galactan are not only part of hemicelluloses, but also constitute the side chains of pectin (Jarvis, 1984). Therefore, to some extent, disappearance of these polysaccharides could be used as an indicator of disappearance of pectin from the compound middle lamella, as well. The implication is that fiber liberation in a chemical pulping process is not solely controlled by delignification but also by the removal of pectin and hemicelluloses. Furthermore, these results also imply that the location of arabinan and galactan is restricted to the compound middle lamella.
The extent of carbohydrate degradation, especially that of cellulose, is a main concern in pulping because it is a crucial factor in determining the pulp yield and quality.
Particularly, in extended cooking of the NAEM catalysed organosolv pulping, the carbohydrate removal, especially hemicelluloses, taking place beyond the fiber liberation point was remarkable and resulted in low-yield pulps. The pulp yield loss at these cooking stages was due to the loss of hemicelluloses containing xylan and mannan.
The mode of carbohydrate degradation in the weakly acidic organosolv and alkaline kraft pulping processes appears to be different. In the organosolv pulping process 172 under acidic conditions, carbohydrate degradation is due to a random hydrolytic cleavage of the glycosidic bonds whereas in kraft pulping, the mechanism is defined as the alkaline end-wise peeling reaction. It appears that the differences inflicted on the pulping outcomes are indicated by the respective pulp yields and viscosities. Theoretically, random splitting would cause a drastic loss of viscosity but less mass wasting, in terms of pulp yield, i.e., in carbohydrate component, especially cellulose loss, unless the applied pulping conditions are so severe that cellulose fragments with DP < 10 ensue (Rydholm,
1985). On the other hand, the impact of the peeling reaction on viscosity could be minimal but hard on the pulp yield, whereby the losses could be more intensified, as evident in Fig. 16 and 21, because progressive peeling means mass loss. The implications are that the kraft pulping process may produce a high viscosity pulp but at lower yield, whereas the organosolv pulping process, e.g., the NAEM process, is capable of producing a higher yield pulp but at lower viscosity unless the pulping conditions are tightly controlled. In this case, the organosolv cooking must be halted at the fiber liberation point whereby high pulp yield >54 % bleached yield with high viscosity >25 cPs will be obtained.
5.7. The Role of Pectin in Fiber Liberation
In terms of quantity, the presence of pectin in wood is considered minor, as compared to the major wood components, i.e., lignin, cellulose and hemicelluloses.
However, the location of pectin is important and quite strategic in affecting fiber separation. As result of the immunocytochemical studies shown in Fig. 37 and 38, the 173 middle lamella or more likely the compound middle lamella contains high concentrations of both acidic and methyl-esterified pectins. Immunocytochemical evidence suggests that the fibers in the wood matrix are separated and encapsulated by a shared layer of pectin as if it were completely encapsulating and separating the fibers. Total continuity of the pectin layer encapsulating the fibers is only assumed but not evident in photomicrographs.
The research objective was not specifically aiming to elucidate the form of pectin distribution among other cell wall components in the wood matrix. Access of the pectin in the middle lamella is assumed through the bordered pits. The tori of the bordered pits seem to be incrusted by pectin. Therefore, the removal of the tori pectin may destroy the tori themselves. For aspirated pits, the destruction of the tori can ease and facilitate liquor penetration to the middle lamella. Thereby, the removal of middle lamella compounds could be enhanced to accelerate early fiberliberation . Results and evidence obtained in this research are consistent with findings reported by Westermark and Vennigerholz
(1995), Westermark et al. (1986), Veen and Ent (1994), and Minor (1991).
In regard to fiber separation, Harlow (1952) reported that in Kerr and Bailey's experiments, completely delignified wood sections did not macerate, i.e., the cells did not separate from their adjacent cells, without treatment with ammonium oxalate, which is known as a pectin solvent. Recently, a similar finding was also reported by Hafren et al.
(2000) that extensively delignified fibers with only 1.3 % Klason lignin content were not separated after acid sodium chlorite treatment. When this delignified wood (Pinus thunbergii) sample was examined with a TEM (transmission electron microscope), it revealed a thin network-like structure connecting two adjacent fibers together. However, 174 this structure was invisible in the lignified sample. With an unlignified sample, before lignification had taken place during cell wall development, the network-like structure appeared more prominently. A similar feature in unlignified middle lamella of Eucalyptus tereticornis had also been observed earlier by Fujino and Itoh (1998). The chemical composition of the network-like structure in the unlignified middle lamella was pectin and hemicellulose (Hafren, 2000). In the cambial tissue the pectin content could be more then
90 % (Simson and Timell, 1978). Even though most of the pectin was reported to be removed or degraded after lignification (Westermark, 1986), the retained amount may still have a significant impact on fiber separation as implied by the works of Kerr and Bailey, and Hafren et al. (2000) mentioned above. Furthermore, Itoh et al. (1998) also found that substantial amounts of parenchyma cells can be removed from abaca and kenaf pulps by pectinase treatment. These evidences seem to be enough to conclude that the removal of pectin from the middle lamella plays a crucial role in fiber separation although the magnitude remains unknown. Furthermore, Minor (1991) reported the existence of linkages between pectin and lignin through arabinose and galactose. Therefore, the removal of pectin is considered essential in facilitating sustainable delignification for early fiber liberation. Because of their association with pectin, theoretically, the removal of arabinan and galactan could be an indicator of the removal of pectin. However, practically, some limitations may apply because arabinan and galactan are also part of hemicelluloses, such as in arabinogalactan, galactoglucomannan and arabinoglucuronoxylan (Sjostrom, 1981). The lack of arabinan and galactan in well separated organosolv pulps and the established association of arabinose and galactose in 175 mixed sugar hemicellulose fractions needs further study especially as to the distribution of the latter types within the cell wall. Both arabinose and galactose may also be implicated in some types of readily hydrolysable lignin-carbohydrate complexes.
5.7.1. Removal of pectins
In general, the results showed that pectin removal is dictated by pH modulation during the course of pulping. However, the pH modulation, as discussed earlier, is certainly controlled by the catalyst employed for the cooking liquor make-up. Therefore, the extent of pectin removal from the middle lamella can be seen as a primary step in the fiber liberation mechanism indirectly requiring catalyst management in organosolv pulping.
In this case, there is an indirect impact of catalyst type through its pH modulation on the removal of pectin. Throughout this study of organosolv pulping catalysis, two distinctive modes of pectin removal were observed, each to follow pH modulation that can be described as follows.
1) When employing acids, e.g., HC1, citric acid, or acidic salts, e.g., A1C13, FeCl3, as catalysts, the starting pH is very low and the cook may end with higher or even lower pH. In this acidic environment the rate of pectin dissolution is slow and the pectin removal could not be completed even though fiber liberation may be achieved, except for citric acid. For instance, in cooking with 0.0125 M HC1 for 60 min, in which case the starting pH was 2.06, and the end pH became 3.23 (see Table 31), although fiber liberation had been achieved (see Table 16), the presence of both types of residual pectin was still detected by immunolabelling with JTM5 and JTM7, as can be seen in Fig. 43 and 176
44. Doubling the acidity to 0.025 M HC1 under the same cooking conditions, but with the starting pH at 1.84 and end pH at 2.77, pectin removal was still incomplete, as shown in
Fig. 41 and 42. Similar results were also obtained when applying 0.025 M A1C13, in which case the starting pH was 2.76 and the end pH was 2.54 for the 30 min cook. Although at that point fiber liberation had taken place, the presence of both acidic and esterified pectin was still detected by JTM5 and JTM7, as can be seen in Fig. 39 and 40. On the other hand, when applying a milder acid, 0.050 M citric acid, with starting pH of 2.99 and ending with higher pH levels depending on the cooking time (see Table 32 and Fig. 22), the pectins could be removed. However, it was found that the two types of pectin were not removed at the same rate. The methyl-esterified pectin was removed faster than the acidic pectin.
As shown in Fig. 46, JIM7 no longer detected the presence of methylated pectin after the
50 min cook, in which the final pH was 3.60, although at this point the fiber liberation point had not been achieved. On the other hand, JTM5 did not detect the presence of acidic pectin after a 70 min cook (Fig. 45), in which case the final pH was 3.65, or slightly
(0.05) increased compared to that of 50-min cook (Table 53). At that point, fiber liberation had been achieved. Therefore, by noticing the pH behavior in the catalysis system mentioned above, it can be concluded that the pH of the cooking liquor plays an important role as a determining factor for pectin removal from the middle lamella.
Apparently, pectin removal preferentially occurs at higher starting pH or lower acidity.
2) When pulping was conducted with chloride salts of mono- and divalent metal ions as catalyst, the starting pH was only mildly acidic. Some were even very close to neutral pH, whereas the end pH was lower even though it could be still considered "mild" 177
(see Table 27 and 28). Following a 60 min cook with 0.05 M NaCl, the presence of pectins could no longer be detected, as can be seen in Fig. 53 and 54. In a 60 min uncatalysed cook, the pectins might be removed, as indicated in Fig. 51 and 52. On the
other hand, pulping with 0.05 M CaCl2, in which the starting pH was 5.99 and end pH was
3.98, or with the combination of 0.025 M CaCl2 and 0.025 M Mg(N03)2, in which the starting pH was 6.3 and end pH was 4.14, rapid pectin removal was observed after a 40 min cook (see Fig. 47 - 50) even though fiber liberation had not yet been achieved. This indicates that the pectins were removed before fiber liberation was completed, which was
60 min for 0.05 M CaCl2 and 50 min for the combination of 0.025 M CaCl2 and 0.025 M
Mg(N03)2 catalyst cooks. Therefore, if compared to cooking in a more acidic environment, such as with acid or acidic salt species, pectin removal in cooks at higher starting pH seemed to be faster and complete long before the fiberliberatio n point was reached. Apparently, pectin dissolution requires a mildly acidic final environment (pH
>3.8).
By relating spent liquor pH to the dissolution / disappearance of pectins throughout all cooking trials mentioned above, it is evident that the removal of pectins from the middle lamella may be controlled by pH modulation. It can be clearly observed that as the spent liquor pH is shifted toward higher pH, but still slightly on the acidic side, the pectins are removed rapidly and completely. Furthermore, there is also an indication that pectins do not survive kraft pulping, as it is shown in Fig. 55 and 56 wherein pectins were not detected after a 90 min cook. In contrast, when the spent liquor pH was shifted toward the more acidic side, the pectin removal is found to be slower or even halted. This 178 phenomenon of pH modulation seems to be related to the mechanism of pectin degradation as reported by Smidsrod, et al. (1966), Keijbets (1974), Krall and McFeeters
(1998).
5.7.2. Mechanism of pectin removal in organosolv pulping
As mentioned before, the immunocytochemical study indicates that spruce wood contains both acidic and esterified pectins. Degradation of these two different types of pectins, due to the splitting of glycosidic bonds, leads to their dissolution and removal from the middle lamella or compound middle lamella. This process is pH dependent, also to some extent affected by the presence of anions and cations, and follows different mechanisms. Based on the results and evidence obtained throughout the experiments and also supported by literature, as reviewed in Section 2.6., a mechanism of pectin removal from the middle lamella in organosolv pulping can be proposed.
The splitting mechanism of glycosidic bonds of the esterified pectin in catalysed organosolv pulping seems to follow the P-elimination mechanism (Albersheim etal,
1960; Keijbets and Pilnik, 1974). In the p-elimination mechanism, the cleavage of the glycosidic bond is initiated by attacking or eliminating of the P-hydrogen by the hydroxyl ion (OH") as shown in Fig. 13. Therefore, availability of the hydroxyl ion in the cooking liquor is crucial, not only for controlling the rate, but also for the extent of esterified pectin removal under specified cooking conditions in time. Availability of the hydroxyl ion is practically dependent on the pH. As the pH declines, availability of OH" in the cooking liquor is decreased, therefore, the pectin removal is slowed or even stopped, as evidence 179
shows in Fig. 39 - 44, i.e., A1C13 and HC1 catalysis. In contrast, as the pH increases, availability of OH- ions in the cooking liquor is also increased, so that the pectin is removed faster and more completely, as evidenced, for instance, in uncatalysed, NAEM catalysed organosolv and kraft pulping (Fig. 47 -56).
Smidsred, et al. (1966) reported that carbohydrate containing carboxyl groups
(acidic carbohydrate) hydrolysed at lower rate at low pH but the hydrolysis rate increased with increasing pH. In this regard, the cleavage of glycosidic bonds of acidic pectin under acid catalysis seems to follow acid hydrolysis at a very low rate. As evidence, in pulping with HC1 catalyst for 60 min, the presence of acidic pectin was still detected by the monoclonal antibody JIM 5 (see Fig. 41 and 43), and in pulping with citric acid, a milder acid, the acidic pectin disappeared after 70 min cook (see Fig. 45).
In mono- and divalent metal ion catalysis, hydrolytic cleavage of the glycosidic bonds of acidic pectin could undergo as such and / or as polypectate (salt form), e.g.,
reacting with CaCl2 will form Ca-pectate (Sundberg et al, 1998). The formation of polypectate could be by ion exchange between the carboxyl group and the cations added as the catalyst, as described in Section 2.1.7. In this case, the hydrolysis of the acidic pectin and its salt form is suspected to take place simultaneously. As a result, the removal of pectin under the metal ion catalysis effect is enhanced. Furthermore, the mild pH range under this salt catalysis system provides a favourable condition for the higher rate of hydrolysis if compared to using acids as the catalysts. As evidence shown in Fig. 47, 49 and 53, the acidic pectin was not detected by JTM5 in 40 min cooks with NAEM catalyst and 60 min cooks with NaCl catalyst. 180
As mentioned above, hydrolysis of acidic pectin / polypectate (not subject to P- elimination, Krall and McFeeters, 1998) and the P-elimination mechanism of esterified pectin is pH dependent. Unfortunately, a quantitative correlation between pH of spent liquor and disappearance of pectins could not be established under the present circumstances. However, qualitatively the relationship can be seen in Table 53, in which the trend of pectin removal in association with the pH modulation can be clearly observed.
5.8. Mechanism of Middle Lamella Removal in Organosolv Pulping
In principle, the liberation of individual fibers from the wood matrix will take place when the middle lamella is selectively removed. In chemical pulping, the middle lamella is removed by dissolution in the cooking liquor at elevated temperature. Since it is known that the middle lamella is mainly composed of lignin, hemicelluloses and pectin, topochemical selectivity of the cooking liquor to simultaneously dissolve these three chemical compounds will determine the stage and extent of fiber liberation, pulp yield, and quality. Unfortunately, only few pulping liquors posses such varied solvolytic powers.
Throughout these experiments, it was found that the NAEM salt catalysis system was the only system that demonstrates such an ability in promoting the dissolution of middle lamella compounds effectively, thereby, it shows an unusually high topochemical selectivity for removing the middle lamella components. In this case as well, the composition of the cooking liquor determines the extent of middle lamella dissolution.
Consequently, such specificity also determines the degree of fiber liberation.
The results also showed that in the catalysed alcohol organosolv pulping process, 181 the behavior of the cooking liquor toward the dissolution of the middle lamella compounds was broadly controlled by the pH of the cooking liquor. However, variation of pH was managed by selection of the catalysts. As evident in Table 27 - 33 and Fig. 22, two different modes of pH changes develop during the course of cooking, i.e., 1) increasing pH of the cooking liquor when acids or acidic salt catalyst species were used, and 2) decreasing pH during pulping when neutral salt catalysts were used. The pH changes have been shown to affect the behavior and dissolution tendencies of middle lamella compounds. There is evidence that the removal of pectin and hemicelluloses at the beginning of a cook improved the rate of delignification, however, the high rate of fiber liberation, on the other hand, is due to the synergistic effects of pectin removal, delignification and limited hydrolysis of the hemicelluloses in the middle lamella.
Therefore, the extent of delignification and simultaneous pectin removal in the middle lamella ultimately determine the degree of fiber liberation or pulp reject content since together lignin and pectin are the major components of the middle lamella, as they both literally extend over the full length of the fiber surfaces. It seems that failing to remove one or the other completely may result in inseparable fiber bundles or shives.
5.9. Fiber Liberation : An Antagonistic Process
Based on the evidence obtained throughout the experiments described herein, it can be concluded that the process of middle lamella removal and fiber liberation in organosolv pulping is an antagonistic process which consists of two different mechanisms, i.e., p-elimination of methyl-esterified pectin and hydrolysis of the middle lamella lignin 182 and carbohydrates. Removal of the esterified pectin from the middle lamella was found to follow the p-elimination mechanism, which requires hydroxyl ions that determine the rate and extent of the pectin removal (see Fig. 13). Thus, this phase of the fiber liberation mechanism requires higher pH (>4.5) to provide more hydroxyls in the cooking liquor. In contrast, the simultaneous removal of hemicelluloses and depolymerization of the lignin occur through an hydrolysis mechanism, in which protons play a decisive role. Therefore, availability of protons in the cooking liquor determines the rate and extent of hemicellulose and lignin removal from the middle lamella. However, it must be considered that hemicellulose removal and delignification may occur in two different phases. Access to the middle lamella itself must be created by dissolution and removal of pectin. Ion exchange of the catalyst cations with pectin is then responsible for generation of acidic protons required for hemicellulose and lignin hydrolysis in the middle lamella region. As the proton concentration increases, hemicellulose removal and delignification increase. At a certain level of acidity (pH -3.2), delignification is slowed down, or even halted, due to lignin recondensation in highly acid solutions while the extent of hemicellulose and also cellulose removal, due to hydrolysis, is steadily increased more rapidly than delignification in the cell wall. Therefore, to guarantee an effective removal of all middle lamella compounds, the cooking liquor must provide optimum amounts of both hydroxyls and protons. However, the optimum amounts of these two antagonistic ions needed in order to carry out an effective removal of the middle lamella can not be provided at the same time from the same catalyst source. This problem could now be seen as the major obstacle in organosolv pulping. Employment of a suitable catalyst in the cooking liquor is found to 183 be essential to tackle this delicate problem. In this case, the catalyst(s) employed must be able to manage the availability of hydroxyls and protons at optimum levels as they are required for effective removal of each middle lamella component. Neutral alkali earth
metal salts, e.g., CaCl2, MgCl2 or combination of CaCl2 and Mg(N03)2, are among the catalysts that can regulate and modulate the amounts of protons and hydroxyls in the cooking liquor at the right time and in the proportion required to promote removal of the middle lamella compounds effectively. As result, the fibers can be separated earlier to guarantee high pulp yield and better pulp quality. The conditions required for removal of the residual lignin from the cell wall other than by oxidative means remains a real challenge.
It seems that the real problem in organosolv pulping of softwoods is not in the crucial differences in the lignin structure, e.g., an abundance of condensed phenolic structures (Argyropoulos, 1999), but rather the requirement of gradual controlled decrease of pH from near neutral (pH >6.0) to allow degradation of the pectin and subsequent hydrolysis of the lignin and hemicelluloses in the middle lamella. The limited, selective cell wall delignification, without excessive carbohydrate removal, signals a different problem unlike that observed for fiber liberation and may be limited by accessibility or penetration of high alcohol-content cooking liquor into the densely packed
S2 lamellar structure. 184
5.10. Mechanism of Fiber Liberation in the NAEM Process
The mechanism of early fiber liberation by the NAEM salt catalysed organosolv pulping process can be seen as a process that proceeds in several stages. Based on the cooking time flow, starting from the beginning to the end of the cook, there are some important events that can be marked out and grouped into 3 subsequent stages :
Stage J. At the beginning of the cook, the first 20 min of the cook, this stage could be called preconditioning stage, in which the wood chips are made ready for delignification to proceed. At this cooking stage, the following important events take place :
1) The wood chips are fully impregnated with cooking liquor.
2) The cooking temperature reaches 175 - 180 °C, the minimum temperature required to
trigger delignification for softwoods.
3) Hydroxyls are rapidly depleted by proton generation through ion exchange with the
wood components.
4) Most of the pectins and some hemicelluloses are removed. The removal is seen to be
crucial for facilitating delignification. Actually, some hemicelluloses, such as arabinan
and galactan, are already removed during the first 10 min but delignification has not
started.
5) Protons needed for delignification are mostly generated in the first 10 min of the cook.
6) Delignification is just starting to take off.
Stage 2. During the next 20 min, the cook can be divided into two 10-rnin periods.
This stage, as a whole, must be considered as the main stage which determines whether 185 delignification can be carried out to complete fiber liberation. In this stage, proton generation proceeds gradually and facilitates bulk delignification, in which stage 50 % of lignin in the wood is removed. However, in the first 10-min period, only 14 % of the lignin was removed with an L/C ratio of 0.90. In the second 10-min period, some crucial events take place, i.e.,
1) Delignification takes place at a high rate as the cooking liquor pH decreases to 4.0 -
4.2. As a result, an additional 36 % of lignin is removed with a very high L/C ratio of
1.50, the lowest rate of carbohydrate removal relative to delignification. It seems that
at this stage delignification is localized in the middle lamella.
2) The ideal cooking temperature of 200 - 205 °C for organosolv pulping of softwoods is
reached (Paszner, 1998) and maintained at 205 °C.
3) The pectins are completely removed.
Stage 3. The next 5-10 min, or the 45th - 50th min from the starting point. In this stage :
1) Bulk delignification is ended.
2) Residual delignification largely in the cell wall begins and as a consequence,
3) Enhanced cell wall carbohydrate removal begins as the liquor buffering capacity is
exhausted by liberation of acetyls from hemicelluloses.
4) Complete fiber separation is achieved in the early stages of residual delignification.
Overall, it can be summarized that the effective fiber separation with the NAEM salt catalysed alcohol organosolv pulping process is due to the effectiveness of the removal of middle lamella compounds. This is accomplished by the following 186 mechanisms that occur in an overlapping fashion, i.e.,
1) Proton generation, through an ion exchange mechanism between the alkali earth metal
salts and acidic functional groups in the wood, e.g., pectins, hemicelluloses and lignin.
2) Removal of esterified pectin, through the fJ -elimination mechanism, whereas removal
of acidic pectin, through hydrolysis with and / or without being firstly forming a
polypectate, e.g., Ca++ salt, through an ion exchange mechanism under mild acid pH
(~5).
3) Partial hemicellulose removal, by a hydrolysis mechanism through in situ generated
acidic protons.
4) Delignification, by a hydrolysis mechanism driven by medium range acidic protons.
Diagrammatically, the sequence of pectin, hemicellulose and lignin removals from the middle lamella, and to some degree from the cell wall, can be envisaged as shown in Fig.
57. It should be noted that relative to the delignification, more hemicelluloses are removed at the beginning of the cook and after the fiber liberation point.
Pectin removal
Hemicellulose removal
Delignification Cook, Start > Fiber liberation
Fig. 57. Block diagram showing sequential removal of pectins, hemicelluloses and lignin by the NAEM process. 6. CONCLUSIONS AND FUTURE RESEARCH
6.1. Conclusions
Results of the pulping trials described herein indicate that there is an impact of the catalysts on pulping behavior and selectivity, pectin removal, fiber liberation and carbohydrate retention in catalytic organosolv pulping of the softwood. In this regard, some conclusions can be drawn as follows :
1) Not all catalysts tried in this study were able to liberate the fibers.
2) Different types of cations and anions have an impact on cooking liquor pH that
subsequently controlled the pulping process and the outcomes.
3) The successive pH changes of the cooking liquor controlled pulping selectivity, pectin
removal, fiber liberation and carbohydrate retention.
4) Control of the final pH between 3.8 and 4.2 is necessary to retain good quality fibers
and avoid excessive carbohydrate losses.
5) In some cases, employment of two different catalysts can show a synergistic effect on
early fiber liberation.
6) Early fiber liberation with high pulp yield and viscosity can only be accomplished by
catalysed cooking liquor possessing high chemical and topochemical selectivities.
7) Removal of the middle lamella compounds was found to be an antagonistic process, in
which the removal of esterified pectin preferred less acidic pH (4.2 - 5.6) due to
the requirement by the P -elimination mechanism which involves hydroxyl ions
while more acidic (pH <4.2) cooking liquor was required to enhance
187 188
delignification. However, carbohydrate degradation was also enhanced due to
hydrolysis at lower pH, whereas the removal of acidic pectin occurs also by
hydrolysis through splitting of glycosidic bonds as such and / or by forming
polypectate before hydrolysis.
8) The removal of the middle lamella to liberate the fibers from the wood matrix basically
followed 2 different pathways :
a) Removal of pectin and hemicelluloses followed by delignification, such as in the
NAEM process.
b) Removal of hemicelluloses followed by delignification and pectin removal, such
as in citric acid catalysed pulping.
9) In all organosolv cooks, regardless of catalyst species, fibers can not be completely
separated without removing arabinose and galactose.
10) The removal of pectin and hemicelluloses at the beginning of a cook contributed to
enhanced fiber liberation. The magnitude of these individual effects was
undetermined in this study.
11) No benefit was gained by extending the cooking beyond the fiber liberation point
because rate of removal of the residual lignin became marginal and occurred at the
expense of carbohydrate degradation and lignin reprecipitation. Therefore,
maximum pulp yield and quality are obtained if delignification is halted at the fiber
liberation point. Residual lignin removal by hydrolysis requires a different set of
pulping conditions. 189
12) A different mechanism, which has not been investigated, may apply to selective
removal of the residual cell wall lignin by hydrolytic, other than oxidative, means.
Possible impediments to residual lignin removal may be access in the cell wall and
lignin-carbohydrate complexing.
13) Unexpectedly, the catalytic effectiveness of citric acid to cause fiber liberation was
found to be comparable to that of the NAEM salt catalysed organosolv pulping
process.
14) In the NAEM salt and citric acid catalysed organosolv pulping, the loss of
hemicelluloses comprising xylose and mannose became greater when
delignification was higher than 60 %.
15) The mechanism of fiber liberation in the NAEM salt catalysed organosolv pulping
process starts with removal of pectin and hemicelluloses consisting of arabinose
and galactose followed by bulk and residual delignification.
16) NAEM salt catalysts, therefore, play a double role : effectively promote initial pectin
removal at pH >5 and buffer the cooking liquor to pH 4.0 ±0.2 as more and more
protons are liberated by ion exchange between the metal ions and acidic
components of the cell wall.
17) Finally, it can also be concluded that the basic principle of producing high yield and
high quality organosolv pulp is based on creating a cooking liquor that possesses
high chemical and topochemical selectivities in removal of the middle lamella
effectively. To posses such a capability, the cooking liquor must have a pH that
can modulate the cooking process in such a way that at the beginning of the cook 190
it can remove pectin and hemicelluloses found in the middle lamella to facilitate
better liquor accessibility to the middle lamella lignin while controllably increasing
the acidity to enhance delignification. Such dual function cooking liquor can be
created by employing NAEM salts as the catalysts.
6.2. Reasons for Lower Strength of Organosolv Pulp : A Suggestion for a Further Study
The strength of paper is a very complicated matter. It is a result of the interaction of many factors that can be summarized as follows : morphological (fiber length, fiber width and cell wall thickness), physical (degree of polymerization of cellulose and strength of individual fibers), chemical (cellulose, hemicelluloses and lignin content), topochemical
(distribution of various chemical components within the cell wall layer), physicochemical
(swelling properties of pulps) (Jayme, 1958; Aurell, 1964a, 1964b; Horn, 1978;
Paavilainen, 1990, 1994; Hatton and Cook, 1992; Goyal etal, 1999; Snowman et al,
1999). These factors basically determine the intrinsic strength and inter-fiber bonding strength of individual fibers that are originally derived from the fiber source itself and the process used to produce the fibers. Therefore, the strength of paper may be considered as the resultant of the intrinsic strength of individual fibers and inter-fiber bonding strength.
Hydrogen bonding is believed to be the main contributor to the inter-fiber bonding strength (Nissan and Batten Jr., 1990).
Regarding the pulping process used to prepare the pulp, the kraft process is known as the developer of the strongest pulp. In general, organosolv pulp seems to be inferior to 191 kraft pulp although it can be ascertained that the integrity of organosolv pulps, as indicated by viscosity and zero-span tensile, may be superior to that of kraft pulps. In some studies (Paszner and Behera, 1985; Johansson et al., 1987; Yawalata, 1996;
Yawalata and Paszner, 1997), it was found that organosolv pulps can be upgraded by lixiviation (Paszner and Behera, 1985) to be comparable to kraft pulp depending on the fiber species and type of strength property being considered. Many other forms of fiber treatments may be developed to upgrade the sheetmaking properties of organosolv pulps, but particular attention must be paid to tear strength (Paszner and Behera, 1985).
In the case of the chemical composition and their distribution in the cell wall of pulp fibers, the presence of hemicelluloses contributes to improvement of the strength properties, especially inter-fiber bonding strength (Scott and Trosset, 1989). However, it is not clear what type of hemicelluloses or components of hemicelluloses play the role or hold the key to strength improvement by inter-fiber bonding. The results of sugar analyses
(see Table 34 - 45) indicate that there is a significant difference in carbohydrate composition between organosolv and kraft pulps. All organosolv pulps, including NAEM catalysed, do not contain arabinose and galactose whereas kraft pulp does. Furthermore, arabinan and galactan were found to be the dominant among the hemicellulose components in the middle lamella and primary wall of spruce (Picea abies) (Rydholm,
1985). Therefore, theoretically, in the kraft process, when the fibers are liberated, presumably due to complete removal of the middle lamella, the primary wall becomes exposed with residues of arabinan and galactan. On the other hand, in the organosolv process, when the fibers are liberated, all arabinan and galactan are also removed 192 completely. Thus, no arabinan and galactan remain on the surface of organosolv pulps whereas they do on the kraft pulp. Presumably, the primary wall is retained in kraft pulping after fiber separation. Therefore, based on the evidence obtained, a hypothesis can be proposed for a future research as follows :
The presence of arabinan and galactan on the surface of kraft pulp may significantly contribute to the inter-fiber bonding strength that makes it the superior pulp as far as the strength properties are concerned. Therefore, the lower strength of organosolv pulps making them inferior to kraft pulp in some applications is caused by the absence of arabinan and galactan on the surface of the fibers. In addition, the lower strength of organosolv pulp is also suspected to be caused by cell wall dehydration, or swellability or lack of it, and fewer fibers per unit weight due to the higher pulp yield. So far, no structured research has been conducted to explain and tackle these problems. REFERENCES
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Appendix 1. Time-temperature profiles of organosolv and kraft cooks.
20 30 40 50 60 Cooking time, min
212 213
Appendix 2. HPLC Chromatograms.
Appendix 2a. HPLC chromatograms showing disappearance of arabinose and galactose in the NAEM (0.05 M CaCl2) catalysed organosolv pulping. 214
Appendix 2. HPLC Chromatograms (continued).
Apendix 2b. HPLC chromatograms showing disappearance of arabinose and galactose in
the NAEM (0.025 M CaCl2 + 0.025 M Mg(N03)2) catalysed organosolv pulping. 215
Appendix 2. HPLC Chromatograms (continued).
1.6e+004H
Apendix 2c. HPLC chromatograms showing disappearance of arabinose, galactose,
xylose and mannose in the 0.025 M AlCl3 catalysed organosolv pulping. 216
Appendix 2. HPLC Chromatograms (continued).
Appendix 2d. HPLC chromatograms showing disappearance of arabinose and galactose in the 0.05 M citric acid catalysed organosolv pulping. 217
Appendix 2. HPLC Chromatograms (continued).
^.6e•*m
2500
Minutes
Appendix 2e. HPLC chromatograms showing the presence of arabinose and galactose in the kraft pulping. 218
Appendix 2. HPLC Chromatograms (continued).
Appendix 2f. HPLC chromatograms showing disappearance of arabinose and galactose in catalysed organosolv pulping with different catalysts at 60 min cook. 219
Appendix 2. HPLC Chromatograms (continued).
1.66+004
nA
2500
Minutes
Appendix 2g. HPLC chromatograms showing the presence of arabinose and galactose in undefibrated organosolv cooked chips with different catalysts at 60 min cook.