The Environmentally Benign Pulping Process of Non-Wood Fibers

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Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 105

THE ENVIRONMENTALLY BENIGN PULPING PROCESS OF NON-WOOD FIBERS

Waranyou Sridach Received: Dec 16, 2009; Revised: Mar 11, 2010; Accepted: Mar 15, 2010

Abstract The increasing demand for paper has raised the need for low-cost raw materials and also for the development of new process in order to boost production. Non-wood fibers, for example agricultural residues and annual plants, are considered an effective alternative source of cellulose for producing pulp and paper sheets with acceptable properties. This paper reviews some physical and chemical properties of non-wood pulps which have effects on the making of paper. The less polluting pulping processes that use organic solvents are of interest for pulp production. The delignification of the Organosolv pulping process depends on the type of Organosolv methods and cellulosic sources used. The chemicals and cooking conditions, such as the catalysts, solvent concentration, cooking temperature, cooking time, and liquor to raw material ratio, all influence the properties of the pulp and paper. Keywords: Non-wood fiber, organosolv, alcohol pulping, solvent-based pulping, delignification

Introduction Pulp and paper production is one of the high been evolved. demand sectors in the world of industrial Cleaner technology is applied to achieve production. The total global consumption increased production with minimum effect from paper-making was projected to increase on the environment, and to save, utilize, and from 316 million tons in 1999 and 351 recycle expensive and scarce chemicals and million tons in 2005 to about 425 million raw materials. This technology is also called tons by 2010 (García et al., 2008). Progress in low and non-waste technology (Müller, 1986). pulp and paper technology has overcome most The technology lessens the disposal costs, of the related environmental problems. The stability risks and resource costs results in environmental problems have brought forth a reduced burden on the natural environment the cleaner technology now involved in paper and increases profits. New technology is making. New raw materials have replaced essential for a clean industry, but this option traditional wood raw materials with non-wood is largely suppressed because of the costs of and residual materials, and less polluting the technology required. Some studies have cooking and pulp bleaching processes have looked specifically into the environmental

Department of Material Product Technology, Faculty of Agro-Industry, Prince of Songkla University. E-mail: [email protected]

Suranaree J. Sci. Technol. 17(2):105-123 106 The Environmentally Benign Pulping Process of Non-wood Fibers consequences of pulp and paper production this objective. Mechanical pulps are most using wood as the feedstock (Young often produced from softwood sources, such as and Akhtar, 1998; Thompson et al., 2001; spruce and pine. The smaller, thinner hardwood Environment Canada, 2003; Sadownic et al., fibers are more severely damaged during 2005; Avşar and Demirer, 2008). mechanical pulping and yield a finer, more Wood is the most widely used raw flour-like material that forms an exceedingly material for production of pulp and paper weak sheet. in the world. It is used as part or all of fiber composition in practically every type of paper Chemical Pulping and constitutes approximately 90% of virgin pulp fiber used by the world’s paper and The two principal methods of chemical board industry (Feng and Alén, 2001). Wood pulping process are the alkaline process, such pulp is pulp manufactured either by mechanical as kraft process (Figure 1), and the acidic or chemical means or both from softwood or process, such as sulfite process (Figure 2). hardwood trees. The pulping processes used over the years, Pulping is the process by which wood is both for woody and non-woody fibers, have reduced to a fibrous mass. It is the means of been mainly chemical based (Wegener, 1992). rupturing the bond within the wood structure. The world pulp production statistics reveals The commercial processes are generally that most of the chemical pulps produced classified as mechanical, chemical or semichemical today are made by the kraft process (Dahlmann pulping. and Schroeter, 1990). Kraft pulping produces a stronger pulp, but it too is feeling the pressure Mechanical Pulping of environmental regulations on emissions from manufacturing plants, such as total The most common method of mechanical reduced sulfur compound (TRS), sulfur dioxide, pulping is the groundwood process, where a suspended solids, and wastewater pollution block (or bolt) of wood is pressed lengthwise (UNEP, 1997). Sulfite pulping has been in a against a roughened grinding stone revolving steady decline for many years due to the at peripheral speeds of 1000 to 1200 m/min. environmental concerns and the inferior Fibers are torn out of the wood, abraded, and physical properties of the pulp. washed away from the stone surface with The increasing of environmental concerns, water. A recent development in mechanical uncertain future availability of wood fiber and pulping involves shredding and grinding chip potential increases in wood costs have caused of wood between the rotating discs of a device the pulp and paper industry to search for called a refiner. The product of this process is alternative fiber sources, such as non-wood known as refiner mechanical pulp (RMP). fibers. RMP usually retains more long fibers than stone groundwood and yields stronger paper. Non-wood Fibers Most new installations now employ thermal (and /or chemical) presoftening of the chips to There is a growing interest in the use of modify both the energy requirement and the non-wood such as annual plants and agricultural resultant pulp properties, e.g., thermomechanical residues as a raw material for pulp and paper. pulp (TMP). TMP is usually much stronger Non-wood raw materials account for less than than RMP and contains very little screen 10% of the total pulp and paper production reject materials. worldwide (El-Sakhawy et al., 1996). This is Mechanical pulping processes have the made up of 44% straw, 18% bagasse, 14% advantage of converting up to 95% of the dry reeds, 13% bamboo and 11% others (Figure 3). weight of the wood into pulp, but require The production of non-wood pulp mainly prodigious amounts of energy to accomplish takes place in countries with a shortage of Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 107

wood, such as China and India (Oinonen and applications can give additional income to the Koskivirta, 1999). China accounts for more farmer from food crops or cattle production than two thirds of the non-wood pulp produced (Rousu et al., 2002; Kissinger et al., 2007; worldwide (Hammett et al., 2001). Rodríguez et al, 2007). The utilization of non-wood fibers is an Non-wood fibers are used for all kinds ethically sound way to produce pulp and of paper. Writing and printing grades produced paper compared to the clear-cutting of rain from bleached non-wood fiber are quite forests or primeval forests. The benefits of common. Some non-wood fibers are also used non-wood plants as a fiber resource are their for packaging. This reflects the substantially fast annual growth and the smaller amount of increased use of non-wood raw materials, lignin in them that binds their fibers together. from 12,000 tons in 2003 to 850,000 tons in Another benefit is that non-wood pulp can be 2006 (FAO, 2009; López et al., 2009). Given produced at low temperatures with lower that world pulp production is unlikely to chemical charges. In addition, smaller mill increase dramatically in near future, there is a sizes can be economically viable, giving a practical need for non-wood pulp to supplement simplified process. Non-wood pulps are also the use of conventional wood pulp (Diesen, more easily refined. Moreover, non-food 2000). According to their origin, non-wood fibers are divided into three main types: (1) Wood chip agricultural by-products; (2) industrial crops; White liquor Na 2 S + NaOH and (3) naturally growing plants (Rowell Causticizing and Cook, 1998; Svenningsen et al., 1999). Kraft cooking Agricultural by products are the secondary Waste water pollution Suspended products of the principal crops (usually cereals solid and grains) and are characterized by low raw material price and moderate quality, such as Pulp Evaporation Green liquor H2S burning Na2S + Na2CO3 rice straw and wheat straw (Navaee-Ardeh et al., 2003; Deniz et al., 2004). Industrial crops, such as hemp, sugarcane and kenaf, can CO 2 produce high quality pulps with high expense Figure 1. Kraft process cost of raw materials. However, the source of the pulp is limited and these materials come from crops planted specifically to yield fiber (Kaldor et al., 1990; Zomers et al., 1995). Sulfur + Air Naturally growing plants are also used for the NH + H O 3 2 production of high quality pulps. This includes Cumbustion bamboo and some grass fibers, such as elephant Pressure chamber Wood chip Accumulator

SO2 +CO2

11% others Sulfite cooking Sulfur 13% bamboo SO2 compounds 44% straw

14% reeds Pulp Blow pit 18% bagsse Acid and Wastewater pollution Figure 3. Consumption of non-wood pulp Figure 2. Sulfite process in paper production 108 The Environmentally Benign Pulping Process of Non-wood Fibers grass, reed and sabai grass (Walsh, 1998, non-fibrous materials may be removed by the Poudyal, 1999; Shatalov and Pereira, 2002; pre-treatment of the raw material, which has a Salmela et al., 2008). The specific physical positive influence on the ash content and the and chemical characteristics of non-wood pulp and paper properties. Table 1 shows the fibers have an essential role in the technical average results of the chemical and physical aspects involved in paper production. On the analyses of some non-wood fibers (Hurter, other hand, the technical issues involved are 1988; Chen et al., 1987; Rodrίguez et al., related to the economic, environmental and 2008). The standard deviations of the three ethical contexts and vice versa. replicates in each test with respect to the means were always less than 10%. Properties of Non-wood Fibers Short fiber length, high content of fines and low bulk density are the most important The chemical compositions of non-wood physical features of non-wood raw materials materials have tremendous variations in (Oinonen and Koskivirta, 1999; Paavilainen, chemical and physical properties compared to 2000). The large amount of fines and the short wood fibers (Gümuüşkaya and Usta, 2002; fiber length (< 2 mm.) especially affect the Rezayati-Charani et al., 2006). They vary, drainage properties of pulp. Apart from the depending on the non-wood species and the operation of the pulp mill itself, these local conditions, such as soil and climate properties also affect dewatering in the paper (Bicho et al., 1999; Jacobs et al., 1999). The machine. Due to the wide range of different non-wood materials generally have higher non-wood species and their different physical silicon, nutrient and hemicellulose contents properties, substantial differences in dewatering than wood (Hurter, 1988). Some parts of the behavior may arise. (Cheng and Paulapuro,

Table 1. Physical and chemical properties of some non-woods used for Papermaking

Properties Unit Rice Wheat Bagasse Reed Bamboo Jute Hemp Kenaf straw straw grass (bast) (bast)

Avg. fiber mm 1.41 1.48 1.70 1.5 1.36- 2.5 20 2.74 length 4.03 Avg. diameter μm 8 13 20 20 8-30 18 22 20

L/D ratio 175:1 110:1 85:1 75:1 135- 139:1 1000:1 135:1

175:1 Alpha % 28-36 29-35 32-44 45 26-43 61 55-65 31-39 cellulose Lignin % 12-16 16-21 19-24 22 21-31 11.5 2-4 15-18 Pentosan % 23-28 26-32 27-32 20 15-26 24 4-7 21-23 HWS % 7.3 12.27 4.4 5.4 4.8 3.7 20.5 5.0 ABS % 0.56 4.01 1.7 6.4 2.3 2.4 2.6 2.1 SS % 57.7 43.58 33.9 34.8 24.9 28.5 - 28.4 Ash % 15-20 4-9 1.5-5 3 1.7-5 1.6 5-7 2-5 Silica % 9-14 3-7 0.7-3 2 1.5-3 <1 <1 -

HWS: hot water solubility, ABS: alcohol benzene solubility, SS: 1%sodium hydroxide solubility Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 109

1996a, b). The low bulk density affects the year on environmental projects in an attempt logistics of non-wood raw materials. This to resolve some of the problems associated would make the amount of cellulose handled with the pulping process. Solving these comparable to wood. motivates much the research and development The production of pulp from non-wood in relation to new pulping technologies. resources has many advantages such as easy In chemical pulping, the raw materials pulping capability, excellent fibers for the are cooked with appropriate chemicals in an special types of paper and high-quality bleached aqueous solution at an elevated temperature pulp. They can be used as an effective substitute and pressure. The objective is to degrade and for the forever decreasing forest wood resources dissolve away the lignin and leave behind (El-Sakhawy et al., 1995; 1996; Jiménez et al., most of the cellulose and hemicelluloses in 2007). In addition to their sustainable nature, the form of intact fibers. In practice, chemical other advantages of non-wood pulps are their pulping methods are successful in removing easy pulping and bleaching capabilities. These most of the lignin; they also degrade and allow the production of high-quality bleached dissolve a certain amount of the cellulose and pulp by a less polluting process than hardwood hemicelluloses (Smook, 1994). pulps (Johnson, 1999) and the reduced energy Non-wood pulping processes generate requirements (Rezayati-Charani et al., 2006). large volumes of black liquor as by-products However, some mineral substances in their and wastes. Black liquor wastewater is a composition, including K, Ca, Mn, Cu, Pb, and mixture of organic and inorganic materials, Fe, may have negative effects on the different with very high amounts of total dissolved steps of pulp and paper manufacturing, solids (TDS). The total dissolved solids in the especially the bleaching process. Metals may black liquor are composed of lignin derivatives, interfere during the bleaching with hydrogen low molecular weight organics, and the rest peroxide or ozone. The transition elements being made up of chemicals from the digesting form radicals that react unselectively with the liquor (Huang et al., 2007). In delignification, pulp when the pulp is bleached without the relatively high amount of silicon present chlorine chemicals (Gierer, 1997). Furthermore, in non-wood material is dissolved together bleaching is accompanied by the formation of with lignin into cooking liquor, This has led to oxalic acid. Calcium reacts with oxalic acid to difficulties in the recovery of cooking chemicals. form calcium oxalate, which deposits easily. This situation makes black liquor one of the Thus, effluent-free bleaching will obviously most difficult materials to handle in wastewater be difficult to achieve in the bleaching plant treatment processes. (Dexter and Wang, 1998). Generally, alkaline non-wood pulps contain much hemicellulose while their fibers Non-wood Pulping are short. This impairs the dewatering properties in different unit processes, the adhesive forces Traditionally, non-wood material is in the paper machine, and paper quality. Then cooked with hybrid chemimechanical and the hemicellulose content of the pulp should alkali-based chemicals (Goyal et al., 1992; be controlled to avoid these problems. However, Jahan et al., 2007). Hybrid chemimechanical when using the alkaline pulping processes, the pulps, which were once thought of as a logical hemicellulose content of the pulp cannot be replacement for chemical pulps, simply do not easily controlled without losses in pulp quality provide the purity necessary for high grade (Rousu et al., 2002). and dissolving pulps. Chemimechanical pulps The conventional alkaline pulping cannot be used in grades that do not allow process is not suitable for many non-wood fiber-containing furnishes due to brightness raw materials and caused serious environmental reversion, brightness levels, or simply customer problems. Therefore, throughout the world many insistence. Much more money is spent each alternative pulping processes have been 110 The Environmentally Benign Pulping Process of Non-wood Fibers introduced. One group of the most promising due to their high purity, low molecular weight, alternative processes is called the Organosolv and easily recoverable organic reagents processes. These cooking methods are based (Mcdonough, 1993, Dapía et al., 2002; Pan on cooking with organic solvents such as et al., 2005). alcohols or organic acids. Methanol and In recent years, research into the ethanol are common alcohols used and the Organosolv pulping processes has led to the organic acids are normally formic acid and development of several Organosolv methods acetic acid. High cooking temperatures and capable of producing pulp with properties associated high pressures are needed when near those of Kraft pulp. Prominent among alcohols are used in cooking. However, organic the processes that use alcohols for pulping are acids require lower temperatures and the those of Kleinert (Aziz and Sarkanen, 1989), pressure is closer to atmospheric pressure. Alcell (Lönnberg et al., 1987; Aziz and Other more unusual solvents include various Sarkanen, 1989; Stockburger, 1993), MD phenols, amines, glycols, nitrobenzene, dioxane, Organocell (Lönnberg et al., 1987; Aziz and dimethylsulfoxide, sulfolane, and liquid carbon Sarkanen, 1989; Stockburger, 1993), Organocell dioxide (Sunquist, 2000). (Lönnberg et al., 1987; Dahlmann and Schroeter, 1990; Stockburger, 1993), ASAM Organosolv Pulping of Non-wood (Lönnberg et al., 1987; Black, 1991), and ASAE (Kirci et al., 1994). Other processes The Organosolv process has certain based on other chemicals also worthy of advantages. It makes possible the breaking up special note are ester pulping (Aziz and of the lignocellulosic biomass to obtain McDonough, 1987; Young, 1989), phenol cellulosic fibers for pulp and papermaking, pulping (Aziz and Sarkanen, 1989; Funaoka high quality hemicelluloses and lignin and Abe, 1989), Acetocell (Neumann and degradation products from generated black Balser, 1993), Milox (Poppius-Levlin et al., liquors, thus avoiding emission and effluents 1991, Sundquist and Poppius-Levlin, 1992; (Aziz and Sarkanen, 1989; Hergert, 1998; Sundquist and Poppius-Levlin 1998), Formacell Paszner, 1998; Sidiras and Koukios, 2004). (Saake et al., 1995) and NAEM (Paszner and The Organosolv processes use either Cho, 1989). low-boiling solvents (for example methanol, Organosolv pulping processes, by ethanol, acetone), which can be easily recovered replacing much or all of the water with an by distillation or high-boiling solvents (for organic solvent, delignify by chemical example ethyleneglycol, ethanolamine), which breakdown of the lignin prior to dissolving it. can be used at a low pressure and hence at The cleavage of ether linkages is primarily available facilities currently used in classical responsible for lignin breakdown in Organosolv pulping processes. Thus, it is possible to use pulping. The chemical processing in Organosolv the equipment used in the classic processes, pulping is fairly well understood (McDonough, for example the soda and Kraft processes, 1993). High cooking temperature and thus hence saving capital costs. (Muurinen, 2000; high pressures are needed when alcohols are Lavarack et al., 2005; López et al., 2006; used in cooking. However, organic acids require Rodríguez and Jiménez, 2008). Using this lower temperatures and the pressure is closer process, pulps with properties such as high- to atmospheric. yield, low residual lignin content, high The ethanol Organosolv process was brightness and good strength can be produced originally designed to produce clean pulping (Shatalov and Pereira, 2004; Yawalata and and was further developed into the Alcell® Paszner, 2004). Moreover, valuable byproducts process for pulp production (Pye and Lora, include hemicelluloses and sulphur-free lignin 1991). The Alcell® process is a solvent-pulping fragments. These are useful for the production process that employs a mixture of water and of lignin-based adhesives and other products ethanol (C2H5OH) as the cooking medium. Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 111

The process can be viewed as three separate 0.05%-0.1% by weight for fibrous materials. operations: extraction of lignin to produce The liquor-to-raw material ratio is 3-5:1, and pulp; lignin and liquor recovery; and by- the cooking temperature and time are 175°C product recovery (Stockburger, 1993). The and 60-150 min, respectively. Anthraquinone raw materials are cooked in a 50:50 mixture serves as a catalyst to increase the reaction of water and ethanol at around 175-195°C for rate. Methanol is added to assist in dissolving 1hour. The typical liquid to biomass solid the lignin and acts as a buffer, prevents lignin ratio is 4-7 and a liquor pH of about 2-3. The from condensing and stabilizes the carbohydrates system employs liquor-displacement washing (Muurinen, 2000). Methanol also improves at the end of the cooking to separate the the solubility of the anthraquinone. The strength extracted lignin. The sulfur-free lignin produced properties of ASAM pulps have been found to with this process has very high purity and has be equivalent to Kraft pulps while at a higher the potential of high-value applications. yield and lower residual lignin content. It is Furthermore, this process generates the more environmentally benign, since the process furfural which is used as the solvent for is free of the reduced sulfur compounds lubricating oil production. It is claimed that produced in the Kraft process. Unbleached the process produces pulps with a higher yield ASAM pulps also have higher initial brightness that bleach more easily and are free of sulfur and thus lend themselves well to totally emissions. The Alcell® process enjoys a chlorine-free bleaching sequences. Methanol significant capital cost advantage compared improves the impregnation of the chemicals. with the Kraft process, since it does not The Organocell process is a solvent pulping require a recovery furnace or other traditional process that uses sodium hydroxide, methanol, chemical recovery equipment (such as lime and catalytic amounts of anthraquinone as the kilns and causticizers). pulping chemicals (Stockburger, 1993). The The methanol Organosolv process Organocell process was originally a two-stage has been used in the alkaline sulphite- process. The first stage is cooking with aqueous anthraquinone-methanol process (ASAM) and methanol, a 50% methanol solution, at 190°C the soda pulping method with methanol for 20-50 min. This stage operates at a mildly (Organocell). The ASAM process is basically acidic condition due to a deacetylation of the alkaline sulfite pulping with the addition of raw material. The main part of the sugars and anthraquinone (AQ) and methanol (CH3OH) 20 % of lignin is dissolved in this stage. The to achieve a higher delignification level second stage involves the addition of sodium (Stockburger, 1993). The process has been hydroxide at an 18-22% concentration at successful in the pulping of softwood, hardwood temperatures of 160-170°C (Kinstrey, 1993). and also non-wood material. The active cooking For the new Oganocell pulping process, chemicals of the ASAM process are sodium the first stage was eliminated from the hydroxide, sodium carbonate and sodium process. The resultant single stage process is sulphite. The addition of methanol to the operated with sodium hydroxide, methanol, alkaline sulphite cooking liquor considerably and catalytic amounts of anthraquinone as improves delignification, and the process cooking chemicals. The concentration of produces pulp with better strength properties, methanol in the cooking liquor is in the range higher yields and better bleachability compared of 25-30%. The one stage process is easier to to the Kraft process. control and the elimination of the first stage The ASAM process utilizes sodium results in stronger fibers than those from hydroxide, sodium carbonate, sodium sulfite the two stage process (Leponiemi, 2008). (Na2SO3), methanol, and small amounts of the Methanol improves the capacity of the catalyst anthraquinone. ASAM cooking liquor cooking liquor to penetrate into the fibrous normally contains about 10% methanol materials and renders the lignin more soluble. by volume. The anthraquinone dose is Anthraquinone functions in the same way as 112 The Environmentally Benign Pulping Process of Non-wood Fibers in soda cooking by stabilizing polysaccharides and Formacell processes. and accelerating lignin dissolution (Sundquist, The Milox process is an Organosolv 2000). Methanol is recovered by evaporation pulping process which uses peroxyformic acid and distillation. Lignin is precipitated in or peroxyacetic acid as the cooking chemical evaporation by decreasing the pH of the liquor (Leponiemi, 2008). Peroxyformic or and it can be separated using a centrifuge. peroxyacetic acids are simple to prepare Organocell pulps produced at a pilot operation by equilibrium reaction between hydrogen are almost as good as the corresponding Kraft peroxide and formic or acetic acids. These are pulps in yield and physical characteristics. highly selective chemicals that do not react The organocell pulps were also found to with cellulose or other wood polysaccharides bleach more easily. The process is suitable for in the same way as formic acid. The hydrogen hardwood, softwood and non-wood species. peroxide consumption is reduced by performing The process also is entirely free of the sulfur the process in two or three stages. The two- emissions found in the traditional Kraft and stage formic acid/peroxyformic acid process sulfite processes (Aziz and Sarkanen, 1989). can be used to produce high viscosity (> 900 Organic acid processes are alternative dm3/kg SCAN) and fully bleached (90 % ISO) methods of organosol pulping to delignify pulp with a reasonable yield (40-48 %). The lignocellulosic materials to produce pulp for pulping stages are carried out at atmospheric paper (Poppius et al., 1991; Jiménez et al., pressure and at temperatures below 100°C. 1998; Lam et al., 2001; Kham et al., 2005a,b). The resulting pulps have kappa numbers Typical organic acids used in the acid pulping between 5 and 35. (Muurinen, 2000). methods are formic acid and acetic acid. The The hydrogen peroxide charge needed process is based on acidic delignification can be reduced by using a three-stage cooking to remove lignin, a necessary part of the method. In the first stage, the temperature hemicellulose and nutrients, while silicon increases from 60°C to 80°C. The peroxyformic remains in the pulp. The pulping operation acid that forms is allowed to react with can be carried out at atmospheric pressure. the cellulosic material for 0.5-1 hours. The Acid used in pulping can be easily recovered temperature is raised to the boiling point of by distillation and re-used in the process the formic acid (ca. 105°C) and the cooking (Muurinen, 2000). Cellulose, hemicellulose proceeds for 2-3 h. The softened chips are and lignin can be effectively separated by then blown into another reactor, and the pulp degradation in aqueous acetic acid or formic is washed with pure formic acid. The washed acid. The cooking liquor is washed from the pulp is then reheated with peroxyformic acid pulp, and both cooking chemicals and water at 60°C at about 10% consistency. Peroxide is are recovered and recycled completely. Formic applied to the liquor at 1%-2% of the original acid can also be used to enhance acetic acid dry weight of the chips. After cooking, the pulping. The temperature and pressure can be pulp is washed with strong formic acid, lower when formic acid is used in pulping pressed to 30%-40% consistency, and washed compared to those used in alcohol or acetic under pressure with hot water at 120°C. This r acid pulping. Organic acid lignin is an optimal emoves the chemically bonded formic acid. feedstock for many value-added products, After washing and screening, the pulp is ready due to its lower molecular weight and higher for bleaching. reactivity (Kubo et al., 1998; Cetin and Ozmen, Unlike with wood species, the two-stage 2002). Another advantage of organic acid Milox pulping of agricultural plants is more pulping is the retention of silica on the pulp effective than three-stage cooking. The two- fiber that facilitates the efficient recovery of stage process uses cooking with formic acid cooking chemicals (Seisto and Poppius, 1997). alone, followed by treatment with formic acid The Organosolv pulping processes based on and hydrogen peroxide (Sundquist, 2000). organic acid cooking are the Milox, Acetosolv When the Milox method is used to delignify Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 113

agricultural plants, the resulting pulp contains Schone (1993) have invented a process where all the silicon present in the plant. This enables lignocellulosic material is delignified the use of a similar chemical recycling system under pressure with a mixture of acetic acid as in a corresponding wood pulping process. (50-95 w-%), formic acid (< 40 w-%) and The silica is dissolved during the alkaline water (< 50 w-%). The pulping temperature is peroxide bleaching. (Muurinen, 2000). The between 13°C and 190°C. Pulps with very two stage peroxyacetic acid process gives low residual lignin contents are produced and higher delignification than three-stage process they can be bleached to full brightness using and vice-versa with peroxyformic acid. The ozone and peroxyacetic acid. Azeotropic Milox process is a sulphur free process and distillation with butyl acetate is used to bleaching can be achieved totally without separate water from the acids. Low pulping chlorine chemicals (Sundquist, 2000). temperatures and high acetic acid concentrations Acetic acid was one of the first organic should be used in the Formacell process in acids used for the delignification of order to preserve hemicelluloses for paper lignocellulosic raw material to produce pulp grade pulps. The use of higher temperatures for paper. Processes based on the use of acetic and water concentrations in the pulping liquor acid as an organic solvent have been applied results in dissolving pulps with hemicellulose with success to hard and softwoods, and even contents below 3% (Saake et al. 1995). to non-wood materials (Pan and Sano, 2005). Formacell pulps produced from annual It can be used as a pulping solvent in plants have better strength properties than uncatalyzed systems (Acetocell method) or in corresponding soda pulps (Sundquist, 2000). catalyzed systems (the Acetosolv method) (Young and Davis, 1986; Kin, 1990; Parajó Factors of Delignification et al., 1993; Vázquez et al., 1995; Pan et al., 1999; Abad et al., 2003; Ligero et al., Important parameters controlling the 2005). The Acetosolv process is a hydrochloric delignification of the Organosolv pulping acid catalysed (0.1%-0.2%) acetic acid process. processes are the types of raw materials, the The cooking temperature is 110°C and the solvent properties, the chemical properties process can be conducted at atmospheric of catalysts and pulping conditions. The pressure, or above (Nimz, 1989). chemical composition of non-wood materials Acetic acid used in pulping can be easily varies, depending on the non-wood species. recovered by a stilling operation and reused in Non-wood materials generally have higher the process. Acetic acid lignin is an optimal silicon, nutrient and hemicellulose contents feedstock for many value-added lignin than wood (Hurter, 1988). By pre-treatment of products due to its lower molecular weight the raw materials, part of the leaves and non- and higher reactivity. The sugars from fibrous materials may be removed. This has a hemicellulose are readily convertible to positive influence on the ash content and the chemicals and fuels. It has already been pulp and paper properties; the chemical reported by a number of researchers that the composition of the fibers, however, still acetic acid pulping properties of woods are remains different from paper processed from comparable to conventional chemical processes. woods. They also have some advantages in comparison The solvent properties have effects to other Organosolv processes (Groote et al., on the delignification and pulp properties of 1993; Sahin and Young, 2008). non-wood fibers. In Organosolv pulping, The Formacell process was developed alcohols promote solvolysis reactions (Sarkanen, from the Acetosolv process. It is an Organosolv 1990; Schroeter, 1991; McDonough, 1993) pulping approach in which a mixture of but they also reduce the viscosity of the formic and acetic acid is used as the cooking pulping liquor. This makes possible a better chemical (Leponiemi, 2008). Nimz and penetration and the diffusion of chemicals 114 The Environmentally Benign Pulping Process of Non-wood Fibers into fibrous materials (Balogh et al., 1992; has some interesting features, such as easy Bendzala et al., 1995). Ethanol and methanol recovery by distillation, and has a lower are normally used as the pulping solvents. material cost than ethanol. However, the use Both alcohols show similar selectivity when of methanol may be hazardous since methanol pulp total yield is considered, but higher is a highly flammable and toxic chemical screened yield values can be obtained in (Oliet et al., 2002). ethanol pulping. Methanol shows better lignin Aranovsky and Gortner (1936) found dissolution on average. However, ethanol that primary alcohols were more selective pulping produces pulps with less lignin at delignifying agents than secondary or tertiary high-intensity cooking conditions, where alcohols. The monovalent and polyvalent Kappa numbers lower than 10 can be obtained. alcohols had higher pulping efficiencies in the The extent of delignification increases as the presence of water. The use of methanol resulted ethanol concentration is decreased (Oliet in less hemicellulose loss than with η-butanol. et al., 2002). The selectivity towards lignin The higher pulping efficiency (better fiber dissolution is similar for ethanol and separation) was associated with increased methanol. hemicellulose losses in the aqueous alcohol The most important differences are mixtures. those observed for pulp viscosity. Although When using organic acid solvents, the ethanol pulps have a higher viscosity on typical organic acid used as the pulping average, the best results are obtained from solvents are acetic and formic acid. Formic methanol pulping. Thus, viscosity values well acid can also be used to enhance acetic acid over 1000 ml/g are obtained for pulps with pulping. Temperature and pressure can be Kappa numbers between 20 and 30 in the lower when formic acid is used in pulping methanol system. These pulps, which are compared to that used in alcohol or acetic obtained under mild cooking conditions, are acid pulping (Rousu et al., 2002). The major of special interest since they can be bleached influence was the acidity or acid concentration. and are obtained at a good screened yield. Increasing acetic acid concentration reduced Ethanol provides lower viscosity pulp but at a yield and lignin content. The solvent slightly higher screened yield. In both cases, concentration had effects on the various acceptable pulp Kappa numbers can be mechanical properties (breaking length, burst, reached. tear index and folding endurance) of paper The interest in ethanol and methanol sheets obtained from each pulping process. pulping is not only justified in terms of cost. The extent of delignification was found to be The acceptable quality of the pulp produced associated with the system’s hydrogen ion and the ease of recovery of the solvent by content. rectification also make the use of ethanol and Hydrogen ion concentration plays a methanol attractive. Furthermore, some valuable very important role in solvent pulping. This is by-products, such as lignin and carbohydrates, because lignin dissolution is expected to be can be obtained during solvent recovery. preceded by the acid-catalyzed cleavage of The ethanol solvent has mainly been used in α-aryl and ß-aryl ether linkages in the lignin autocatalyzed pulping, the ALCELL process, macromolecule, and becomes soluble in and antraquinone catalyzed pulping (Aziz and the pulping liquor (Goyal et al., 1992). Sarkanen, 1989; Pye and Lora, 1991). The Delignification in cooking in high-alcohol focus of methanol use has been alkaline concentration can be improved by the addition pulping (Stockburger, 1993). However, it has of mineral acids. A lower alcohol concentration been shown that pulps with low lignin content favored faster delignification by virtue of a and acceptable viscosity can be obtained in an higher hydrogen ion concentration. Acidity acidic medium by methanol autocatalyzed increases at lower alcohol concentrations and pulping (Gilarranz et al., 1999). Methanol at lower liquor-to-raw material ratios, but this Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 115

did not translate into enhanced delignification, process to be performed at ambient pressure, probably because of some lignin redeposition. thus eliminating the need for a pressurized Acidity also increased with increasing reactor. A brief review of the pH effects on cook time. There are clear signs for the solvent pulping will help in understanding the recondensation and deposition of lignin that effect of catalysts to control delignification is common in the Organosolv processes. and fiber quality. The solvent type and pH However, organic acids, especially formic control are both important factors. A drop in acid, are highly corrosive and cause severe pH during solvent cooking is considered to be corrosion problems in the processing equipment responsible for a number of effects. These (Leponiemi, 2008). include the condensation of the solvolytically Operating conditions that lead to the liberated lignin, the extensive hydrolytic best pulp quality are important to the Organosolv dissolution of hemicelluloses, degradation of pulping of non-wood fibers. The actual cooking the cellulose, and formation of solvent insoluble condition required is a function of solvent condensation products in the cooking liquor concentration, cooking temperature, cooking (Aziz et al., 1988). time, type of non-wood being cooked and the The Organosolv pulping conditions type of organosolv pulping process. Table 2 have effects on the delignification of fibrous shows a range of pulping conditions from the materials. Acidic Organosolv pulping is literature and the pulp properties of Organosolv facilitated by the hydrolysis of ether linkages pulping processes. between lignin and carbohydrate. The dissolved Chemical catalysts is a critical factor in lignin decreases with increasing cooking time accelerating delignification in the Organosolv when the fibrous materials are pulped with pulping processes. Paszner and Cho (1989) organic acids, indicating that lignin condensation discovered that salts of alkaline earth metals, occurs during cooking. Condensation during such as calcium and magnesium chlorides, are pulping occurs to a greater extent with formic effective catalysts in Organosolv liquors with acid than with acetic acid, and to a greater high methanol and ethanol contents. They extent with acetic acid than with propionic have applied these catalysts to the Organosolv acid. Propionic acid was also observed to pulping of several softwood and hardwood delignify more effectively than either of the species as well as bagasse. Bleachable-grade other two acids. Lignin precipitation occurs in pulps with exceptionally high yields were the ethanol pulping process due to either the obtained in each case. The reported bleached reduction of the ethanol concentration in the pulp yields are 54-57% for softwoods, 57-62% washing process or the drop in temperature for hardwoods, and 55% for bagasse. The which causes a decrease in lignin solubility, or strength properties of these pulps were both. essentially equal to those of corresponding The influence of cooking conditions, bleached Kraft processes. as mentioned previously, have effects on Orth and Orth (1977) recommended the the properties of paper obtained from use of aluminium chloride (AlCl3 * 6 H2O) as each Organosolv pulping process. Sahin and the preferred catalyst in solvents of aqueous Young (2008) found that the pulping of jute glycol or glycol ether. They further indicated in acetic acid results in strength losses at that organic acids such as formic, acetic, higher temperatures and prolonged cooking. propionic, oxalic, malic, citric or phthalic acids The severe pulping conditions cause the were suitable catalysts, as did Paszner and depolymerization reactions of the carbohydrates. Chang (1983). The use of these organic acids The extended delignification, with increasing has proved to be a great improvement over the temperature, strongly affected the strength use of mineral acids because they facilitate properties of paper. Increasing temperature pulping. In the presence of an acid catalyst, and extending cooking time usually brings many high-boiling solvents allow the pulping about 10–50% reduced tear strength. 116 The Environmentally Benign Pulping Process of Non-wood Fibers .,

et al - - - - 107 180 45.8 52.9 2.52 4.38 12:1 2003 CIMV 20:60:20 (Rice straw) (FA:AA:water) Delmas .,

et al - - - - 107 180 28.2 49.4 3.21 4.23 10:1 2004 CIMV 30:55:15 (Bagasse) Lam (FA:AA:water) .,

et al - - - - 43 107 120 2.14 3.27 50.4 15:1 2005 straw) CIMV (wheat 60:20:20 Kham (FA:AA:Water) , et al

- - - 4.6 3.1 195 120 31.4 47.7 87.4 12:1 1995 EtOH 60 (EtOH) Zomers (Core Hemp) (Core Autocatalyzed Autocatalyzed

., et al

- - - - 60 30 60 4.4 9.9 200 1997 7-10:1 (Kenaf) ALCELL ALCELL 60 (EtOH) Winner Winner

- - - 60 76 24 1.1 8.0 7:1 195 42-56 (Jute) 90 (AA) Sahin and Acetic Acid Acetic Young, 2008 Young, .,

et al - 60 14 3.5 0.1 7.3 7:1 170 16.4 56.1 1999 3.2-4 ASAE straw) (Wheat 50 (EtOH) Usta ., et al

- - - - - 3-6 2000 3-5:1 16-18 61-63 60-150 ASAM 170-180 (Bagasse) 20 (MtOH) Shukry

/g /g 2 2

C % % % % % % O min Unit Nm/g kPa.m mN.m Some Organosolv pulping conditions of non-wood fibers Title charge 3 SO 2 NaOH charge Anthraquinone(AQ) Kappa No. Yield Screened index Tensile Burst index index Tear Reference L:M ratio Cooking temp. Time Cooking solvent Organic Na L:M : liquor to raw material ratio, MtOH: Methanol, EtOH: Ethanol, FA: Formic acid, AA: Acetic acid AA: Formic acid, L:M : liquor to raw material ratio, MtOH: Methanol, EtOH: Ethanol, FA: Table 2. Table Suranaree J. Sci. Technol. Vol. 17 No. 2; April - June 2010 117

For alkali ethanol pulping of rice straw pulping liquor which limits the emission of (Navaee-Ardeh, et al., 2004), the breaking volatile sulfur compounds into the atmosphere length, burst index and folding endurance of and gives efficient chlorine-free bleaching. paper sheets were more affected by ethanol These processes should be capable of pulping concentration than temperature. This was after all lignocellulose species with equal efficiency. a cooking time of 150 minutes. At a high Another major advantage of the Organosolv cooking temperature, these dependent variables process is the formation of useful by-products were much more sensitive to changes in time such as furfural, lignin and hemicelluloses. than in an ethanol concentration. Sabatier However, there are inherent drawbacks to the et al. (1989) found that the acid catalyzed Organosolv pulping methods. The dilution of ethanol pulping process is less selective than pulping liquor with water tends to reprecipitate the alkaline process. Soda ethanol pulping can the dissolved lignin on the pulp fibers. The produce paper-grade pulps of good strength digester leaks in Organosolv pulping can be with a saving of 50% of the sodium hydroxide inherent fire and explosion hazard. or more, compared with plain soda pulping. The pulps obtained by the alcohol soda References (NaOH-EtOH–H2O) method had a better quality and lower kappa number than those Abad, S., Santos, V., and Parajό, J.C. (2000). from ethanol solvent (EtOH-H2O). Adding Formic acid–peroxyformic acid pulping anthraquinone (AQ) as a catalyst to the pulping of aspen wood: an optimization study. by aqueous alcohol soda gave higher yields, Holzforschung, 54(6):544–552. lower kappa number and better strength Abad, S., Santos, V., and Parajό, J.C. (2003). (Physico-mechanical) properties. (El-Skhawy Two-stage acetosolv pulping of Eucalyptus et al., 1995). wood. Cellul. Chem. Technol., 35(3): 333–343. Conclusions Aranovsky, S.I. and Gortner, R.A. (1936). The cooking process IX. Pulping wood The increasing demands for paper and alcohols and other organic reagents. Ind. environmental concerns have increased the Eng. Chem., 28(11):1270-1276. need for non-wood pulp as a low-cost raw Avşar, E. and Demirer, G.N. (2008). Cleaner material for papermaking. This has also led to production opportunity assessment study the developing of alternative pulping technologies in SEKA Balikesir pulp and paper mill. that are environmentally benign. Annual plants J. Cleaner Prod., 16(4):422-431. and agricultural residues appear to be well Aziz, S. and Sarkanen, K. (1989). Organosolv suited for papermaking due to them being an pulping: A review., Tappi J., 72(3):169- abundant and renewable resource. However, 175. many factors influence the suitability of raw Aziz, S. and Mc Donough, T.J. (1987). Ester materials for use in papermaking. These pulping: A brief evaluation. Tappi J., include: the ease of pulping; the yield of 70(3):137-138. usable pulp; the cost of collection and Aziz, S., McDonough, T., Thompson, N., and transportation of the fiber source; the presence Doshi, M.R. (1988). Solvent pulping- of contaminants; and the availability of the Promise and Programs. Tappi J., 71(2): fiber supply. Additional factors include fiber 251-256. morphology, such as its composition and Balogh, D.T., Curvelo, A.A.S., and De Groote, strength, the fiber length and diameter. R.A.M.C. (1992). Solvent effects on The Organosolv pulping processes are Organosolv lignin from Pinus caribea alternatives to conventional pulping processes, hondurensis. Holzforschung., 46(4): and have environmental advantages. Organosolv 343–348. pulping features an organic solvent in the Bendzala, J., Pekarovicova, A., and Kokta, 118 The Environmentally Benign Pulping Process of Non-wood Fibers

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