Continuous Pulping Processes

Home , Defibrator, Johan Richter (inventor)

Disclaimer: In some cases, the Million Book Project has been unable to trace the copyright owner. Items have been reproduced in good faith. We would be pleased to hear from the copyright owners. Queensland University of Technology. Brisbane, Australia TAPPI STAP SERIES

1 The Training of Supervisors in Corrugated Box Plants: Ten Lesson Plans; Lesson 11: Time Study 2 Petroleum Waxes: Characterization, Performance, and Addi tives 3 *Preparation, Circulation, and Storage of Corrugating Adhesives 4 *Operations Research and the Design of Management Informa tion Systems 5 Management Science in Planning and Control 6 Technical Evaluation of Petroleum Waxes * Out of print. Photocopy may be obtained from University Microfilms, Ann Arbor, Michigan 48106. Johan Richter Pioneer in Continuous Pulping Technology Born in Lier, Norway, in 1901 Continuous Pulping Processes

12 Lectures

By Sven Rydholm Director of Research Billeruds AB

SPECIAL TECHNICAL ASSOCIATION PUBLICATION • STAP NO. 7

Gardens Point A22810250B Continuous pulping processes : 12 lectures A22810250B

Library of Congress Catalog Card Number: 74-140131

Printed in the United States of America By Mack Printing Company, Easton, Pa. Preface

This book is a compilation of lectures given at the TAPPI Pacific Section Meeting in Seattle, Wash., in September 1968. They dealt with experiences in continuous pulping obtained over more than one decade at Billeruds AB in collaboration with AB Kamyr. One reason for my choice of topic was that Kamyr digesters have dom inated the most vital operation in our industry for more than ten years and still do, although some signs of healthy competition have appeared. A second reason was that the Kamyr digesters are now becoming quite di versified, and thus the lectures would have to cover all pulping processes needing a pressure vessel. A third reason was that I have been involved in developing many of these process variants during my past sixteen years in Billerud research. A final reason was that it was then 30 years since Johan Richter started the development of a continuous digester. Someone, perhaps outside his company, should tell the story of the devoted efforts from him and his associates to realize an idea, in which he firmly believed, in spite of innumerable troubles and a general disbelief from most people in this industry for a good many years. I am not able to do that initial story justice, but shall instead cover some experiences of continuous cooking gained during the last decade. In preparing the lectures, I have endeavored to give a brief background of wood chemistry and pulping chemistry, which simplifies the understanding of what goes on in the digester and what comes out of it. The theory is treated more extensively in my book "Pulping Processes" (Wiley, New York, 1965). The treatment of the continuous cooking itself includes the results of work undertaken by my company. I think I may make clear that continuous cooking is now ready for all pulping processes and offers possibilities for carrying out process modifications which only with difficulty are feasible in batch cooking. It gives a system available for the largest mill units conceivable, lends itself well to control and automation, offers advantages in combina tion with other mill operations, and yields pulps which meet the highest quality standards. It is one. of the weapons needed in this industry to meet the competition from other materials in serving the markets of today and tomorrow. vii viii Continuous Pulping Processes

Thanks are due to Billerud and Kamyr for allowing the comprehensive publication of all results which were previously unpublished or published only in scattered presentations at Scandinavian and American meetings and magazines. Actively involved in the administration of the Jossefors experimental pulp mill were G. Ojermark, S. Haglund, and W. Ameen of Billerud, and in the administration of the research program T. Bergek, L. JQrgensen, and myself from Billerud, K. Dahl, J. Richter, and T. Christen- son from Kamyr. Actively involved in carrying out the experimental work during different periods over the 16 years were, from Billerud, my self, G. Arnborger, S. Boren, T. Krantz, J. Grundstrom, U. Mohlin, E. Nilsson, G. Annergren, A. Haglund, K. Mattsson, S. Lokrantz, S. Wenneras, B. Dillner, and an experienced crew of 10-20 men. From Kamyr, particu larly, the following were involved in the experiments at varying periods: H. Ortqvist, L. Jansson, A. Backlund, S. Jungeblad, and L. Westerlurtd. The sparks necessary to carry the work over the critical periods and to yield the successful redesigns of the machinery were supplied by Johan Rich ter. With Knut Dahl, the managing director of Kamyr, and Ake Pihlgren and Gunnar Hindemark, former and present managing directors of Bil lerud, the financing of the program has rested. This has required from them substantial courage and belief in the soundness of research and de velopment efforts in machine and process design, perhaps rewarded in the continued success of digester sales of Kamyr and in the successful opera tion of continuous pulping processes at Billerud. Part of the work has also received substantial financial support from the Swedish development fund Malmfonden. Sven Rydholm Billeruds AB Saffie, Sweden Lectures 1 Historical Development of Continuous Kraft Cooking... 1 2 Sulfite Cooking Process Theory 11 3 Continuous Acid Sulfite Cooking 33 4 Continuous Bisulfite Cooking 51 5 Continuous Neutral Sulfite Cooking 63 6 Kraft Cooking Process Theory 75 7 Continuous Conventional Kraft Cooking 97 8 Continuous Prehydrolysis-kraft Cooking 105 9 Continuous Modified Kraft Cooking 121 10 Washing Process Theory 159 11 Continuous Digester Washing 173 12 Technical and Economic Aspects of Present and Future Developments of Continuous Pulping 179 Tndev 193 Lecture I

Historical Development of Continuous Kraft Cooking

The detailed story of the development of the continuous kraft digester began with a 5 ton/day pilot plant at Karlsborg in northernmost Sweden in 1938 and continued, (after an interlude during the war) in 1948 at Fengersfors, a small kraft mill in Central Sweden. During the period 1948-52, Fengersfors took the brave stride to continuous cooking from its 19th-century technique of stationary batch digesters, which opened at side lids near the bottom and were emptied manually into wheelbarrows to carry the pulp to the washing and screening departments. In this mill, the Kamyr enthusiasts introduced their first commercial unit, for 50 tons/day. The basic principle of a downflow digester with a balanced high-pressure pocket feeder (Fig. 1.1) was already established, but they had a long way to go. The high-pressure feeder is still the key feature of the Kamyr system. It solves the problem of introducing the wood chips into the high-pressure system without mechanical compression damage, without excessive wear on the feeder, and without losses of steam from the pressure room. The chips fall from the steaming vessel into the pocket of the feeder when the pocket is in vertical position, and are packed by a liquor circulation. The revolving plug of the feeder contains 2-4 such pockets at varying angles, so that always at least one pocket will receive chips. The plug, which is slightly conical, is a precision work of stainless steel in an iron housing with a monel sleeve. In turning, the plug delivers the pockets successively into a horizontal position, in contact with the high-pressure feeding line. By a circulation pump, the chips are then conveyed into the digest er. The pocket is thus emptied of chips but full of liquor when it arrives again at a vertical position. A liquor volume corresponding to the volume of the new chip charge must thus leave the system at an overflow in the chip chute and has to be pumped into the system again by a high-pressure pump, together with the small leakage volume from between plug and housing, and with the cooking liquor charge. The first commercial continuous kraft digester is outlined in Fig. 1.2. Its basic design is similar to that of the present digesters, but many improvements have been made. The chips are charged from a hopper 1 2 Lecture 1

Fig. 1.1. Principle of the balanced high-pressure pocket feeder. over a measuring wheel into a low-pressure feeder, which introduces the chips into the horizontal steaming vessel. After 3-5 min steaming, the chips fall into the high-pressure feeder and are pumped into the digest er. A top screw or separator keeps a strainer clean, through which the feeding hquor returns to the circulation pump. A torsional indicator at the end of the screw feels the chip level in the digester and gives an impulse to the discharge system for balancing the charge and discharge flows. Cooking liquor is fed into the digester top over the chip feeding circulation, and is heated in a new circulation somewhat further down the Historical Development of Continuous Kraft Cooking 3

BLOW STEAM RECOVERED FOR PRESTEAMING

Fig, 1.2. First commercial Kamyr kraft cooking system. digester. The circulation strainer is kept clean by the moving chip column, but there was the problem of distributing the heated liquor over the digester cross section. The initial arrangement was improved in co operation with the Fengersfors mill staff, of which the contributions of Ragnar Jonsson particularly should be mentioned. He also cooperated in the improvement of the bottom scraper and discharge problems, which were connected with the pulp quality. The distribution of the cooking liquor was solved by the introduction of the central pipe, ending just above the level of the circular strainer. The final solution of the dis charge was yet to come. In 1952, the Kamyr men thought they were ready for a larger unit in Sweden (100 tons/day), which was sold to Wifstavarf. However, in spite of their own efforts, and some very energetic ones from the mill personnel, this digester did not turn out to be a success, and it was finally closed down. This was watched by the entire Swedish forest industry, and many wise and experienced men had their doubts confirmed. The system was not ready for production, and what was, after all, the purpose and advan tage of continuous cooking? Johan Richter at that time had only one answer, which kept him going: "People used to ask the same question when we started to make bleaching continuous, and look what they are buying now, all of them." However, there were still people in industry fascinated with the idea of Table 1.1. Pioneering Mills in Kamyr Continuous Digester Systems Capacity, Year Mill Country tons/day Process: Wood 1948 Fengersfors Bruks AB, Sweden 50 Kraft Softwood Fengersfors 1949 Cartiera Vita Mayer, Italy 90 Kraft Softwood Cairate Cartiera Burgo, Ferrara Italy 25 NSSC Straw 1951 Ste". An. Progile, Condat- France 40 Kraft Chestnut le-Lardin Ohji Seishi KK, Kasugai Japan 90 Kraft Softwood Associated Pulp and Paper Australia 60 Soda Eucalypt Mills Ltd, Bumie Tasmania 1952 Wifstavarfs AB, Sweden 100 Kraft Softwood Vifstavarv Joutseno Pulp OY, Finland 120 Kraft Softwood Joutseno 1954 Joutseno Pulp OY, Finland 120 Kraft Softwood Joutseno Cellulose du Rhdne, France 60 Soda Esparto Tarascon N Z Forest Products, New Zealand 60 Kraft Softwood Tokoroa Billeruds AB, Saffle Sweden 60 NSSC Hardwood Billeruds AB, Gruvon Sweden 150 Kraft Softwood 1955 Billeruds AB, Jossefors Sweden 10 Pilot Backhammars Bruk AB, Sweden 150 Kraft Softwood Bjorneborg North Western Pulp & Power Canada 250 Kraft Softwood Co., Ltd., Hinton, Alberta North Western Pulp & Power Canada 250 Kraft Softwood Co., Ltd., Hinton, Alberta International Paper Co., USA 150 Kraft Softwood Camden, Arkansas V Rosenlew & Cp. AB, Finland 200 Kraft Softwood Bjorneborg Sudbrook Pulp Mill Ltd., England 60 NSSC Hardwood Sudbrook Cellulose du Rh6ne, France 150 Kraft Softwood Tarascon Ohji Seishi KK, Kasugai Japan 90 Kraft Softwood, hardwood Gulf States Paper Co., USA 350 Kraft Softwood Demopolis, Ala. Weyerhaeuser Timber Co., USA 150 Kraft Softwood Longview, Wash. Table 1.1. Pioneering Mills in Kamyr Continuous Digester Systems

Mill Country tons/day Process Wood Nippcm Pulp Kogyo KK, Japan 120 Kraft Softwood Yonago Jujyo Seshi KK, Japan 120 Kraft Softwood Yataushiro Sanyo Pulp KK, Japan 120 Kraft Softwood Iwakuni Eastern Corp., Lincoln, Me. USA 150 Kraft Hardwood Skogsagarnas Cellulosa AB, Sweden 120 Kraft Softwood Monsteras Skogsagarnas Cellulosa AB, Sweden 120 Kraft Softwood Monsteras Kokusaku Pulp KK, Japan 150 Kraft Softwood, Asahigawa hardwood Dai Showa Seishi, Japan 150 Kraft Softwood Fuji La Cellulose du Pin, France 150 Kraft Softwood Facture La Cellulose du Pin, France 150 Kraft Softwood Facture Continental Can Co., USA 350 Kraft Softwood Nixon Station, Ga. Celulosa Argentina Argentina 100 Cold Eucalypt Caustic Belisce Kombinat Yugoslavia 50 NSSC Hardwood Celgar Ltd., Castlegar, B. C. Canada 250 Kraft Softwood Celgar Ltd., Castlegar, B. C. Canada 250 Kraft Softwood Fibreboard Paper Prod. Inc. USA 250 Kraft Softwood Antioch, Calif. Oxford Paper Co., USA 225 Kraft Softwood Rumford, Me. Associated Pulp and Paper Australia 100 Soda Eucalypt Mills Ltd., Burnie Usutu Pulp Co. Ltd. Swaziland 300 Kraft Softwood AB Statens Skogsindustrier, Sweden 150 Kraft Softwood Lovholmen AB Statens Skogsindustrier, Sweden 300 Kraft Softwood Lovholmen Tokai Pulp KK, Japan 150 Kraft Hardwood Shimada Techmashimport USSR 420 Kraft Softwood Techmashimport USSR 420 Kraft Softwood St. Regis Paper Co., USA 300 Kraft Softwood Tacoma, Wash. Canadian International Paper Canada 300 Kraft Softwood 6 Lecture 1

Fig. 1.3. Kamyr kraft cooking system after the introduction of the cold blow. continuous cooking. The next two units, sold to Finland, to Joutseno, handled 120 tons/day each. There, much important work was done to improve the reliability of the system, and although there were still distur bances, production went on smoothly enough to let a very serious problem become evident: The pulp quality was not satisfactory. As seen from the list of early buyers (Table 1.1), there was no general breakthrough; only a few brave companies in various parts of the world were curious enough to test the idea of continuous cooking. The essential reasons for Historical Development of Continuous Kraft Cooking 7 this hesitation were the problems of production reliability and of pulp quality. Billerud belonged to the fairly early believers, and in 1954 we ordered three units simultaneously, one 150 tons/day kraft unit for Gruvon, one 75 tons/day unit to Saffle for the production of neutral sulfite birch pulp to bleached glassine, and one 10 tons/day pilot unit for all conceivable process variants, to be placed at an experimental pulp mill within the Jossefors sulfite pulp mill. This decision was taken after thorough consideration and several visits to kraft mills with existing Kamyr digesters. We also had had some direct experience, since we had carried out, jointly with Kamyr and Mo & Domsjo, the first effort in continuous sulfite cooking in a small, 1 ton/day unit, located at Domsjo, during 1950-53. My first job in Billerud was to attend to those trials, which were quite informative, but also quite un successful. With the kraft digester at Gruvon, we had a fair startup, and although there were many teething troubles, we became eventually very satisfied with the production reliability. The pulp quality was, however, unsatis factory, and we were glad to have a fairly large batch digester room besides the continuous unit. At that time, however, the solution to the quality problem was underway, thanks to Kamyr, particularly Lennart Jansson, in collaboration with the Finns at Joutseno, headed by Hannes Jans son. Some contributions were also made by the Central Laboratory of the Cellulose Industry in Stockholm with Lennart Stockman, and some by us at Gruvon, Saffle, and Jossefors. Like Joutseno, we found that the kraft pulp became degraded by the action of the discharge devices, when blown at full temperature, and that this also applied to the neutral sulfite and particularly to the acid sulfite process. Then the cold blow was introduced, first as a cooling of the bottom circula tion liquor, and later on, with the introduction of cool wash liquor to the bottom zone (Fig. 1.3). This again necessitated a change in the discharge system. Up to now, the discharge was done through an Asplund sluice, developed for the Defibrator process and consisting of a pressure room with two alternating valves. The pulp was sluiced into that small cham ber and then blown by its own thermal expansion when the second valve opened toward the blow tank. With the cold blow, no such expansion was possible. Instead, a blow-valve was developed which directly reduced the digester pressure. Contributing here were the French Progile mill at Condat, International Paper in Camden, and Billerud's mill at Saffle. The cold blow improved the quality by 10-20% on most paper strength properties, and the industrial progress of the continuous kraft digester Lecture 1

1950 1955 I960 1965 1970 YEAR Fig. 1.4. Development of Kamyr kraft cooking installations during the first 20 years of commercial production, million tons annual capacity.

could proceed. As seen in Fig. 1.4, the system had now its first major breakthrough. This also meant that there was experience accumulating in all parts of the world, not the least in North America. One of the most valuable ideas emerged from a stubborn Australian, who was ahead of even the Kamyr personnel in some thinking around the continuous digest er. Ray Sloman, who had ordered his first Kamyr digester for APPM, Burnie, Tasmania, as early as 1951, wanted to run his digester counter- Fig. 1.5. Kamyr continuous kraft cooking system with digester washing followed by filter washing. (1) Chip measuring wheel; (2) steaming vessel; (3) high-pressure feeder; (4) white liquor pump; (5) impregnation zone; (6) heating circulation; (7) cooking zone; (8) black liquor withdrawal; (9) flashing system; (10) "Hi-heat" washing zone; (11) washing liquor circula tion; (12) blow tank; (13) knotter; (14) washing filter. 10 Lecture 1

currently, succeeded, and has done so ever since. There may still be dif ferent opinions about the virtue of cooking countercurrently, but the mere proof that it was possible to let the liquor flow upwards and the pulp downwards gave rise to the second breakthrough, the high-heat countercurrent wash in the lower part of the digester (Fig. 1.5). This eliminated quite a few problems in the filter wash after the digester, where Kamyr had been temporarily less successful with its constructions, and which at one period had caused more production disturbances than the digester itself. With both those features, the cold blow and the digester wash, the system was quite competitive, technically and economically, and there after increasingly dominated the new capacity of the kraft pulp industry, in increasingly large units. The largest one on order at the moment is for 1150 metric tons/day, and this feature alone, the size, is proof enough of the foresight and soundness of Johan Richter's basic idea. Other advantages, such as heat economy and labor economy through facili tated combination of cooking with subsequent operations, will be further discussed in the last lecture. It is no overstatement to say that the continuous digester introduced on a massive scale the concepts of instru mentation and automatic control to the pulp industry, now being com pleted by digital control from computers. The happy situation of Kamyr, with a practical monopoly in the field of continuous kraft cooking, would have made many firms self-assured and lazy. It is commendable that the company continued the development efforts and extended them to adjacent fields, mainly sulfite pulping in 1957-64, prehydrolysis-kraft pulping in 1964-65, neutral sulfite pulping in 1965-66 and modified kraft pulping in 1965-68. Since that development has been performed mainly in collaboration with Billerud, at times even by Billerud alone or in collaboration with Swedish Cellulose Co., it is on this topic that I should be able to speak with some experience. However, I felt it was only fair to enlarge somewhat on the development of the kraft digester, thus paying my respects not only to the Kamyr men but to all pioneers of continuous cooking working in the industry, who have carried the double burden of technical difficulties of development and the respon sibility of the current production. Lecture 2

Sulfite Cooking Process Theory

The wine dealers of France have for a long time disinfected their barrels with sulfur dioxide. It is told that an observant American, Benjamin Tilghman, made his invention of the sulfite process by reflecting about the cause of wine barrels' becoming fiberized on the inside after repeated use and disinfections. Tilghman got his patents in the 1860's, but he did not succeed in making much money out of them, since he tried to carry out his sulfite cooking continuously and it took a decade to make the sulfite process work even on a batch system. This was done by a Swede, C. D. Ekman, who started the Bergvik sulfite mill in 1874. A few years later, in 1883, the first Billerud sulfite mill started production at Saffle. I must confess that the foresight and failure of Benjamin Tilghman was often in my mind when we started, almost a century later, to make the sulfite process continuous. Sometimes we thought we should go down in history with the same rather dubious fame-foresight and failure. And if you fail, somebody else has to prove whether you were really fore- sighted—or just on the wrong track. The basic reaction of the sulfite process is the introduction of hydrophilic sulfonate groups into a virtually hydrophobic substance, the lignin (Fig. 2.1). This is done by reacting the wood with bisulfite solutions, normally calcium bisulfite, but more recent ly ammonium, sodium, or magnesium bisulfite. The pioneers soon found that calcium required an excess of sulfurous acid, in order to prevent the precipitation of calcium sulfite at elevated temperature. This immediately introduces us to the concepts of cooking acid composi tion and to the peculiar behavior of sulfurous acid at elevated temperature (Fig. 2.2). Since the early days of the sulfite industry, the cooking acid has been visualized as a mixture of sulfite and sulfurous acid, the former named combined, the latter free S02. Chemically we know now that in these acidic solutions there is practically no sulfite, and instead twice as much bisulfite as the combined S02, and in addition S02, which is gen erally called excess S02. The excess S02 stands in equilibrium with water to form sulfurous acid, which is ionized into hydrogen ions and bisulfite ions. The hydration equilibrium is much influenced by temperature, as experienced by an increasing S02 pressure of the cooking acid with in creasing temperature. 11 12 Lecture 2

Fig. 2.1. Mechanism of delignification according to two concepts: sulfonation followed either by sulfitolysis or by hydrolysis with subse quent further sulfonation in solution.

Fig. 2.2. Sulfite cooking acid concepts and composition. Sulfite Cooking Process Theory 13

Temperature, C Ka pKa 25 0.0172 1.8 70 0.0046 2.3 100 0.0024 2.6 110 0.0016 2.8 120 0.0011 3.0 130 0.0008 3.1 140 0.0005 3.3 150 0.0003 3.5 Fig. 2.3 Temperature dependence of the apparent ionization constant of sulfurous acid. Evidently, the dissociation constant of sulfurous acid decreases rapidly with temperature. The constant is defined by the equation

Fig. 2.3 Temperature dependence of the apparent ionization constant of sulfurous acid. Evidently, the dissociation constant of sulfurous acid decreases rapidly with temperature. The constant is defined by the equation + + k= [H ] • [HSOj ] = [H ] • [HSO3 ]

[H2S03] + [S02] [total S02] - [HSO3 1 Measured in terms of hydrogen ion concentration, these changes are apparently reflected as changes in the ionization equilibrium (Fig. 2.3), though there is reason to believe that most of the sulfurous acid is ionized at all temperatures. Including all excess S02 in the denominator of the ionization equilibrium equation, sulfurous acid has a pKa of 1.8 at room temperature and 3.0-3.5 at cooking temperature. This means a pH of this order of magnitude for a normal cooking acid, containing after some gas relief about 4% total and 1% combined S02, i.e., equal parts, 2%, of excess S02 and bisulfite S02. When bisulfite ions are consumed during the cook (Fig. 2.4), more excess S02 becomes hydrated and then more hydrogen ions formed. There is thus an increase in acidity during the cook, after an initial decrease due to excess S02 disappearing by gas relief, and because of the temperature rise. The increase in acidity is pronounced only toward the end of certain cooks, where more bisulfite ions have been consumed than corresponding to the metal ions, or the "base." Such cooks are avoided in most cases with paper pulps, and only with rayon pulp is this acidity peak desired to speed up the hydrolytic degradation of the pulp to a controlled viscosity level. The cooking acid composition is with calcium base limited to such really acid cooks. The solubility of magnesium sulfite is higher than that of calcium sulfite, which means that a higher pH can be allowed for mag nesium base cooking liquors, up to pH 5. A straight bisulfite solution, of 4% total and 2% combined S02, for example, has a pH at room tempera ture of slightly above 4, and contains equal and small amounts of sulfite and sulfurous acid (Fig. 2.5). Cooks at pH 4 were investigated during the 1930's and introduced to industry during the 1950's with the introduction 14 Lecture 2 Sulfite Cooking Process Theory 15

Sulfite Cooking Process Theory 15 < Figure 2.4 Curves 1, 5.12% S02, k = 0.020 (20°C) 2,5.12% 0.005 (70 C) 3,5.12% 0.002 (105°C) 4,5.12% 0.001 (125 C) 5,2.56% 0.001 (125°C) 6, 2.56%S02,fc =0.0005 (140 C) . 7,1.28% 0.0005 (140°C) 8,0.64% 0.0005 (140°C) 9, 0 Dotted curves: technical sulfite cook conditions. Fig. 2.4. The relations of hydrogen and bisulfite ion concentrations to combined S02 (or to strong acid anions formed) at constant levels of total S02 and temperature. of soluble bases. Such cooks are called bisulfite cooks, whereas the tradi tional sulfite cooks should now be called acid sulfite cooks. To increase the pH still more requires ammonium or sodium base, which have easily soluble sulfites. The so-called mixed sulfite-bisulfite cooks of pH 6-7, with a cooking liquor composition of 4% total and 3% combined S02, contain about equal parts, 2% S02 of bisulfite and sulfite. They are also used industrially for very high-yield cooks. When the cooking liquor contains only sulfite, e.g., with 4% total and 4% combined S02, it becomes alkaline, with a pH of about 10.5. Such cooks are sometimes called monosulfite cooks. In order to save some chemicals, cooks of this type are more often carried out with a deficit of total S02, e.g., 3% total and 4% combined, the rest being a sodium carbon ate or rather bicarbonate buffer. They have a pH initially of 8.5-9.0 and end at pH 6-7. They are usually called neutral sulfite cooks and are, of course, widely used for hardwood semichemical pulps. Still more alkaline sulfite cooks have also been tried in the laboratory. The function of the sulfite in such cooks is not quite clear. In order to have them proceed with any rapidity, a considerable excess of alkali must be present, and the delignification is also speeded up by the presence of sulfide. This indi cates that the delignification in such cooks involves the alkaline hydrolysis of lignin bonds just as in ordinary kraft cooking. A decrease in the sulfite content likewise retards the delignification, however, and shows that also sulfonation plays a part. Other variants of the sulfite cook are the multistage processes, whereby attempts are made for effects not possible in one-stage cooks. These cooks are generally combinations of the cook types .previously mentioned, such as neutral sulfite-bisulfite or neutral sulfite—acid sulfite. I shall come back to those processes later on. 16 Lecture 2

Fig. 2.5. Bjerrum diagram showing pH and ion concentrations of various types of sulfite cooking liquors, assuming pKfl of sulfurous acid to be 1.75 at room temperature and 3.1 at 130°C, and pKfl of the bisulfite ion to be 7.0.

Before doing so, I shall finish off the inorganic chemistry of the sulfite cook by mentioning the decomposition reactions of the cooking acid. Some discouraging experiences of the early sulfite process pioneers with so-called burnt or black cooks have their cause in the catalytic decomposition of bisulfite ions into sulfate and thiosulfate (Fig. 2.6). That reaction is among other things catalyzed by thiosulfate, and will thus, once started, accelerate unless thiosulfate is removed. All sulfite cooks at a pH below neutrality form thiosulfate, and hence it is not possible to mix cooking acid with waste liquor to the same extent as has been done in the kraft industry to increase the solids content of the waste liquor for evaporation. If that is done with sulfite cooking acid and waste liquor, the initial thiosulfate content will be excessive and cause an accel erated decomposition of the fresh bisulfite.

The reactions forming thiosulfate during the cook are not only the previ ously mentioned inorganic reaction, but also reactions of bisulfite with sugars, as well as with terpenes, etc. (Fig. 2.7). In a normal acid sulfite cook, these amounts of thiosulfate are consumed in a reaction with lignin (Fig. 2.8), and the detrimental cooking acid decomposition is thus checked. When cooking at a higher pH, such as in the bisulfite cook, it appears that this reaction between lignin and thiosulfate does not occur to any extent. Then the sensitivity to thiosulfate contamination becomes accentuated, especially since the higher content of bisulfite ions tends to give increasing amounts of thiosulfate in reaction with the carbohy- Sulfite Cooking Process Theory 17

Fig. 2.7. Decomposition of sulfite cooking acid in absence of organic matter, as well as interaction of cooking liquor components and wood components in forming and consuming decomposition catalyst, thiosulfate. 18 Lecture 2

drates. This prevents the cooking down to pulps with lignin contents below 4% with the bisulfite process, as compared to less than 1% as the minimum in the acid sulfite cook. It also became evident in our trials with continuous bisulfite cooking that the decomposition reactions can create real technical problems, which I shall refer to later. On the alkaline side there is no similar spontaneous decomposition, and considerable amounts of thiosulfate can be tolerated in the neutral sulfite cook, for example. The lignin reactions of the sulfite cook, disregarding that often neglected one with thiosulfate, are predominantly sulfonation, hydrolysis, and condensation (Fig. 2.9). In all cases, the initial reaction is probably a protolysis of the dialkyl ether bonds of lignin, or protolysis at the benzyl alcohol groups. Various model reactions have illustrated the various reactivities of the possible configurations. Here it is sufficient to state that, in the course of delignification, lignin is at first sulfonated to an extent corresponding to one sulfonate group for every three monomers without dissolving. Continued acidic treatment without bisulfite ions will remove some of the lignin so sulfonated, but a continued sulfonation will introduce more sulfonate groups, to at least one on every two monomers, and results in a more complete delignification. If the sulfonation is interrupted at too early a stage, e.g., by the decom position of bisulfite ions to sulfate and thiosulfate, the reactive groups of the lignin not only can be hydrolyzed, but to a considerable extent, also condense with other reactive centers in the lignin molecule, particularly in the 5-position of the aromatic ring. This is called the self-condensation of lignin and leads to discoloration, screenings, and in bad cases to "burnt cooks." A similar cause of the same phenomena is where the heating of the cook has proceeded too rapidly in relation to the penetration of the cooking liquor. Then condensation can occur within the lignin before sulfonation has a chance to take place, and the end result will be nonuni form cooks, with "burnt centers" of the chips, where impregnation has been incomplete. A large amount of research work has been devoted to these phenomena, such as pretreatment in various buffer solutions. Part of the problems has also been ascribed to physical phenomena, such as "coalescence" of the lignin during the pretreatment, which should have the same consequences as condensation. There is also direct chemical evidence of condensation during such pretreatment, and it is likely that both condensation and coalescence play a role in the deactivation of the lignin. Another condensation of a similar type occurs betwen lignin and phenolic extractives, such as the pinosylvin compounds of pine heartwood, Sulfite Cooking Process Theory 19

Fig. 2.9. Mechanism of lignin sulfonation, etc., assuming proton activa tion: (1) hydrolysis (R' ,alkyl) or status quo (R'H); (2) sulfitolysis (R alkyl) or sulfonation (R H); (3) condensation. or the tannins of bark-damaged spruce surface wood (Fig. 2.10). This prevents the cooking of such wood by the original acid sulfite process and has led to the development of two-stage processes, such as the Stora pro cess for pine or the Kramfors process for tannin-damaged spruce (Fig. 2.11). If namely the initial treatment is carried out in a less acid medium, pH 4-10, sulfonation is favored and condensation suppressed sufficiently to allow complete delignification. In the neutral sulfite cook, carried out at higher temperatures than the acid sulfite and bisulfite versions, the predominant sulfonation reaction 20 Lecture 2

Fig. 2.11. Temperature schedules of some two-stage cooks with industrial application (cooking curves could be adjusted according to quality and ca pacity demands). Sulfite Cooking Process Theory 21 appears to be a sulfitolysis of the alkylaryl ether bond under formation of a styrene sulfonate which tends to polymerize (Fig. 2.12). Likewise, some sulfitolysis of methoxyl groups occurs, under formation of methane sulfonate (Fig. 2.13). 22 Lecture 2

The dominant carbohydrate reaction in acid sulfite cooking is hydrolysis of the glycosidic bonds (Fig. 2.14). The susceptibility to hydrolysis varies. The arabinose groups of the softwood xylan (Fig. 2.15), are the most sensitive ones, followed by the galactose groups of the galactoglucomannan (Fig. 2.16). Then follow the xylosidic bonds of the xylan chains, and the mannosidic and glucosidic bonds of the glucomannan chains. Also the glucosidic bonds of the cellulose chains (Fig. 2.17), are attacked, but less easily. That is largely the result of lower Sulfite Cooking Process Theory 23

accessibility to hydrolysis. The hemicelluloses are less well ordered than cellulose and are preferentially attacked. We found that the glucomannan molecules of softwoods can change their accessibility to hydrolysis when their acetyl groups are removed by a neutral or alkaline precook prior to the sulfite cook (Fig. 2.18). This preservation is not desirable in a rayon pulp cook, where it is endeavored to remove as much as possible of the hemicelluloses (Fig. 2.19). In a paper pulp cook, however, it is generally desired to preserve them, and a yield improvement of 4-7% is possible by applying the two-stage technique. In order to illustrate the location of the hemicelluloses around the elementary fibril of the cellulose in the secondary waE of those different pulps, schematic representations can be made (Figs. 2.20, 2.21). The pulp yields obtained with these various types of sulfite pulps also depend on the degree of delignification (Fig. 2.22). The more lignin left in the pulp, the higher is also the carbohydrate yield, since the industrial delignification is far from selective. The two-stage cook with initial deacetylation preserves a higher yield level throughout, and one-stage 24 Lecture 2

Fig. 2.18. Pulp yield, mannose content and acetyl content of two-stage sulfite pulps, cooked to constant ligniri content at varying pH and temper ature conditions of stage I. Sulfite Cooking Process Theory 25

Fig. 2.19. Yield of wood components on sulfite pulping of spruce and birch with or without a preceding neutral sulfite stage. high-yield semichemical cooks with deacetylating action likewise give a higher yield at equivalent lignin content than does the conventional bisul fite cook at pH 4. The yield effects with deacetylating cooks concern only softwoods. The hardwoods are low in glucomannan content, and it is not known whether that glucomannan is acetylated. The acetyl groups of hardwoods mainly belong to the xylan, that also contains branches of glucuronic acid (Fig. 2.23), which do not split off completely in any of the two stages. The hardwood xylan therefore remains accessible to degrada tion in the acid cook, whether deacetylated or not, at least under the

Sulfite Cooking Process Theory 27

PULP LIGNIN.% of Fig. 2.22. Dissolution of sprucewood components on sulfite pulping. Yield of components as a function of pulp yield or pulp lignin yield. Broken lines indicate constant hemicellulose yield ("iso-hemi lines") as suggested by Loschbrandt. GAX = glucuronoarabinoxylan; GGM = galactoglucomannan acetate. 28 Lecture 2

Fig. 2.24. Sulfonation and oxidative degradation of carbohydrates in acid sulfite and bisulfite cook.

conditions of the two-stage cook that gives the yield effects with soft woods. Another carbohydrate reaction in the sulfite cook is that of oxidation by bisulfite ions under formation of aldonic acids and thiosulfate (Fig. 2.24). Its importance for the cooking acid stability has been commented on already. It is also responsible for a yield decrease in the bisulfite cook, to an extent which offsets the advantage of a lower hydrolytic degrading action in that less acidic process variant. Bisulfite pulps are therefore obtained in about the same yields as are acid sulfite pulps at equivalent lignin content. Not only aldonic acids, but also sugarsulfonic acids are formed, and this appears to be the case also with neutral sulfite pulping, where, however, both hydrolytic and oxidative degradation are quite limited (Fig. 2.25). It is not necessary here to enlarge on the pulp properties obtained from the various types of sulfite cooks. It suffices to say that the sulfite pro cess generally gives a higher yield but a weaker paper pulp than the kraft process, and that its sensitivity to the type of wood, to the state of wood seasoning, to the extent of mechanical chip damage—as well as the less Sulfite Cooking Process Theory 29

Fig. 2.25. Sulfonation and oxidative degradation of carbohydrates in neutral sulfite cook. highly perfected engineering of the sulfite process—have given it a poorer competitive situation to-day. The efforts to improve this situation by process variants have not given sufficient results. Exceptions are the semichemical pulps of neutral sulfite from hardwoods and bisulfite from soft woods, as well as the normal grades of viscose rayon pulp, particularly from softwoods, where the acid sulfite process is still preferred. A few words should be said also on the impregnation of wood chips before concluding the lecture on the sulfite process. Impregnation prob lems were mentioned in connection with the acid self-condensation of lignin, where sulfonation had not taken place. The occurrence of burnt chip centers was often a problem to the sulfite industry before it learned the influence of chip size and the impregnation variables. Wood consists of a capillary system containing 50-75% of void spaces, filled with air or water. These spaces are mainly the luminae of the fibers, tracheids, and vessels. In softwoods, the luminae are interconnected over the pits, the membranes of which are perforated with holes of 0.03-lju in size. The pits are closed in pine heartwood, which is therefore difficult to pene trate. In hardwoods, the capillary system of the vessels is easily pene trated, unless blocked by so-called tylose formation, which sometimes occurs in the heartwood of some species, such as white oak. Even in those hardwoods which have easily penetrated vessels the fiber luminae appear to be accessible only by diffusion through the fiber walls. Rea sonable methods have been worked out to determine both the flow resis tance of the capillary system of the various species and the diffusion resistance of soaked wood. Both flow and diffusion are much more rapid in the longitudinal than in the transverse directions, and hence a critical factor for cooking acid penetration is the chip length. This is normally 30 Lecture 2

STEAMING, min Fig. 2.26. Degree of penetration vs. time of steaming for spruce chips treated at 75°C with an acid containing 5% total S02 and applying a pressure of 0.8, 5, and 9 atm (0.8 atm corresponding to the vapor pressure of the acid). Penetration after 2 min •, 5 min X, 10 min A, 15 min O, 30 min D, 45 min •. kept at about 20 mm, shorter chips leading to more mechanical damage than can be usually tolerated. A factor which complicates penetration is the air in the chips, which becomes trapped in the capillaries and prevents complete penetration. In the practical range of pulpwood moisture content, 30-50%, a combination of air and water pockets fill the capillaries, the most difficult situation from the standpoint of impregnation. Therefore, several methods of air removal have been devised, the most efficient of which is steam ing. Steaming causes the air pockets to expand thermally, and when steaming to 100°C at atmospheric pressure, the increased vapor pressure of water will force the air to leave the system. Steam shooks have been applied to increase this effect but have in general been found not to Sulfite Cooking Process Theory 31

improve the situation as compared to ordinary steaming of the same endurance. Provided most of the air has been expelled from the wood chips, impreg nation is aided considerably by hydraulic pressure. Figure 2.26 shows that penetration is rather incomplete even after considerable periods of steaming and impregnation if the hydraulic pressure of the system does not exceed the vapor pressure of the cooking acid. Increased hydraulic pressure gives almost complete penetration even after 5-10 min each of steaming and impregnation. I shall come back to this circumstance in connection with rapid continuous cooking. It was also mentioned that too short chips tend to give intolerable dam age to the wood. That mechanical damage in combination with acid sul fite cooking will cause degradation has been manifested in many ways. Sulfite cooking of mechanical pulp, either groundwood or the more well-preserved Defibrator fibers, yields a very degraded sort of sulfite pulp. It has been established that the mechanical damage of a longitu dinal compression of the wood is sufficient to cause the sulfite pulp to become degraded. Transversal compression is less harmful. It has also been demonstrated that the morphological disturbance which leads to the degradation is not cracks in the "lignin enamel," exposing cellulose to hydrolysis, but rather disturbances in the cellulose fibrillar structure, so- called slip planes. The phenomenon not only leads to damage at the bruised ends of the chips, and consequently efforts with new chipping principles, it has also necessitated caution as regards movements in the digester content during the sulfite cook or at the discharge. I shall later demonstrate that this applies also to continuous sulfite cooking. Lecture 3

Continuous Acid Sulfite Cooking

With the theoretical background just given for the sulfite process and its variants, I shall now proceed to describe our efforts to make the process continuous. Those efforts started contemporarily with the develop ments on soluble bases in the sulfite process and drew constantly from the results of the laboratory work in progress. The first approach was that of the 1 ton/day pilot unit at Domsjo in the early 1950's. This was mainly a stainless steel version of the contem porary continuous kraft digester (Fig. 3.1). Thus, it contained equipment for chip charging and steaming, to ensure efficient steaming there were two steaming vessels. The system further contained a high-pressure feeder with level tank, packing circulation, feeding circulation, and high- pressure pump for charging of cooking acid and compensating liquor, top screw, downflow digester body with strainers, circulations, and heat ex changers, and finally a discharge system with bottom scraper, Asplund sluice, and discharge circulation. Our first experience was that the size of 1 ton/day is too small for practical development work of this type. The discharge of knots or un cooked chips caused excessive troubles, a leaking valve unproportionate disturbances. The second experience was that calcium-based cooking acid is not practical for development work. Any cooking acid decomposition caused liming-up of the entire system and consequent cleaning of pipes and heat exchangers, which consumed much trial time and patience. The third and decisive experience was that the feeding system of a kraft digest er is quite unsuitable for a straight acid sulfite cook. A proper hot acid system gives a cooking acid of about 6% total S02 and an excess pressure of at least 3 atm at the usual storage temperature, 60-70°C. At 100-110°C, the vapor pressure is still higher. However, in the feeding system in question, the cooking liquor assumes the temperature of the steaming vessel, 100-110°C, whereas the pressure maintained by the low- pressure feeder is only 0.5-1.5 atm. It was therefore inevitable that the excess S02 tended to leave the cooking liquor already in the feeding circulation. This is undesirable from a process point of view, and also caused hammering in the high-pressure feeder and liming-up of the feeding circulation when using calcium-based acid.

33 34 Lecture 3

Fig. 3.1. First Kamyr pilot plant system for continuous sulfite cooking.

These experiences indicated a 10 ton/day pilot unit, a soluble base, and a two-stage cook with no excess S02 in the initial stage. When the experi mental pulp mill was created at Jossefors in 1956, it contained a digester designed accordingly (Fig. 3.2). In order to separate the two stages com pletely, the digester consisted of two bodies, one upflow and one downflow. There were the usual arrangements for charging and steaming the chips, the standard feeding circulation, and then the upflow digester, equipped with two chip-lifting circulations, a top scraper for transferring the chips to the second stage, with a transfer circulation as an additional acid. The second body was equipped with a heating circulation and the usual discharging system, with bottom scraper, discharge sluice, and bot tom circulation. A blow tank with subsequent filter received the pulp for further operations. The machinery development work continued in that equipment for two years, until it had been made to work properly, and then process studies continued for some years. During this period, we had considerable me chanical experience, e.g., with the material of the high-pressure feeder, corrosion in the digester top and behind blind strainer plates, where cook ing acid decomposition took place and sulfuric acid corrosion was se vere. We learned how to get the chip column moving upward in the first stage, and how to discharge through a small blow valve instead of the Continuous Acid Sulfite Cooking 35

Fig. 3.2. Two-body digester for acid sulfite pulping. sluice. We learned how to construct a top scraper to transfer the chips without damage, and reconstructed the bottom scraper to do the least possible damage. We traced the flow of the chips with radioactive copper wire bits, inserted in chips, and the flow of the liquid by injecting solu tions of radioactive sodium carbonate. We put sight glasses on the digest er tops to study the phenomena in the uppermost part of the digester and tried various designs for chip level control. We learned how to charge liquid S02 into the cooking acid circulation at a suitable rate, and how to avoid the plugging of the high-pressure feeder with sawdust. In short, we fought a great many troubles all around the clock, and at night we went up to the roof of the digester house to derive inspiration from the Sputniks and Explorers, which had just begun to encircle the planet. After all, our problems ought to be the easier ones. After a while, we got sulfite pulp on the subsequent filter. In the first development phase, we wanted dissolving pulp from spruce. Our analyses showed lower than normal Roe numbers and lower resin contents in the pulp at a certain viscosity level. After preventing the cooking acid de composition caused by back-water pockets in the digester, we succeeded in getting a proper delignification and deresination; but how about the carbo- 36 Lecture 3

Fig. 3.3. Alkali non-solubility - viscosity relation, spruce sulfite rayon pulps. Initial pH 7 in the continuous system. hydrate reactions? The alpha-cellulose content, or rather the nonsolubili- ties in 18 and 10% NaOH, Rj 8 and Rj 0, showed a level about 3% too low (Fig. 3.3). We eliminated a few obvious sources of mechanical damage to the chips and had some improvements, but the larger part of the difference remained. Could the very movement of the chips through the digester be the cause of the damage? Then continuous sulfite cooking would be prin cipally impossible. At that time, the construction material of the high-pressure feeder had now allowed us to run the feeding circulation on the acid side. Thus the cooking liquor for the initial stage was kept at pH 7. We now began to suspect that this deviation from normal cooking practice, 1.5 hr at 125°C and pH 7-6, though seemingly harmless, was the cause of our troubles. This initiated a laboratory investigation, which led to the dis covery that the glucomannan, for reasons then unknown, became resistant to hydrolysis when we ran the upflow digester under neutral condi tions. The desired improvement was obtained when pH was decreased to Continuous Acid Sulfite Cooking 37

VISCOSITY, cp(TAPPI) Fig. 3.4. Alkali non-solubility - viscosity relation, spruce sulfite rayon pulps. Initial pH 4 in the continuous system. pH 4 in the precook (Fig. 3.4). After introduction of the complete cold blow, we got a further improvement, which gave at last equivalent results with batch cooking. As" also shown in Fig. 3.4, an additional improvement in alpha-cellulose content was obtained, after the reconstruction of the digester to the "Mumin" version (Fig. 3.5) for other reasons, which I shall come back to. In this system, the feeding remains the conventional one, and so does the digester shell. What has been altered is the design of the digester top. The internal top separator has been removed, and the new external separator has been placed in an inverted and inclined position, with the strainer at its lower end. The chips are moved upward by a screw and discharged into the digester through an elbow. As the liquor level is controlled at a point below the overflow, the chips are drained and do not carry into the digester more liquor than that which has been absorbed by the chips, unless the process requires more liquor to be introduced. In that case there is also an overflow of liquor. Heating is conducted by direct steam at the elbow. This means that the chips become uniformly and almost individually heated. The chips are thus brought straight to maximum temperature, and by introduction of liquid S02 at the digester top, the desired acidity of the sulfite cook is also obtained, whereas the 38 Lecture 3

feed liquor contains the base and an amount of S02 corresponding to bisulfite. The excess S02 is later recovered in the flash of the waste liquor withdrawn from the digester and recirculated after liquification. (I feel I must give a brief explanation of the nickname "Mumin." The Kamyr men thus manifested the change of the profile of their digester caused by the external top separator. A charming figure from a modern Finnish fairy tale novel is a troll with a big nose, called Mumin. It is a brief and practical name for the system, but unofficial, and will probably disappear when the nose is eliminated. A somewhat less expensive con struction can be expected if the inverted top separator is placed inside the digester.) The improvement in alpha-cellulose content achieved with this system probably depends on the very short heating period (1.5 min) to maximum temperature and full S02 concentration. This results in a rather rapid hydrolytical degradation of the glucomannan before it becomes sta bilized. The results are now somewhat better than can be obtained by batch cooking with the heating periods of 3-5 hr necessary to get a uni form temperature distribution within the batch. During the first stage of development, we also studied dissolving pulp from birch in the continuous two-body digester. As in the case of spruce, Continuous Acid Sulfite Cooking 39

Fig. 3.6. Alkali non-solubilities and pentosan vs. viscosity for birch sulfite rayon pulp cooked by batch, continuous two-body and Mumin systems. AS-acid sulfite stage, BS-bisulfite, NS-neutral sulfite. we got initially a much lower alpha-cellulose content than with batch cooking (Fig. 3.6). As far as we knew then, this might have the same cause as with spruce, since we ran the first stage at pH 7 and consequently deacetylated the xylan, which could then have become less accessible to hydrolysis. However, decrease in pH to 4 did not improve the results. Some laboratory experiments indicated that the phenomenon instead had something to do with the lifting circulation of the first stage. Some of the xylan dissolved there could be adsorbed again when recirculating the li quor. This was confirmed by an increase in alpha-cellulose content when decreasing the temperature of the precook to 110°C and limiting the lifting circulation to the bottom part of the upflow digester. Such mea sures were, however, limited by the demands of the upflow movement of Table 3.1. Pulp Quality Obtained from Spru e, Birch, and Eucalypt Rayon Pulps by the Sulfite Process, Using Batch and Continuous Cooking the chips, which required a minimum length of the lifting zone and a certain minimum temperature to make the chips soft enough to exert a low friction against the digester walls. The final solution also here proved to be the Mumin digester. Table 3.1 shows the results obtained by the two-stage and the one-stage digester, as compared to batch mill scale and laboratory scale cooks to the same viscosity, using spruce, birch, beech, and eucalypt. It is quite evident that the final one-stage continuous cook ing gave completely satisfactory results with all wood species, at least as good as with batch cooking. It is also clearly demonstrated that experi menting with digester machinery often leads to theoretically unexpected results because of deviations from batch cooking conditions. This, of course, not only indicates troubles, but also possibilities. Table 3.2 shows the cooking conditions. The most striking feature of the successful one-stage cooking is the low liquor ratio, 2.2-2.5 (tons liquor/tons o.d. wood). This includes chip moisture, steam condensate, and seal water, as well as actual cooking liquor. Since the latter was only about 1 ton/ton o.d. wood, and the charge of combined S02 about 40 kg/ton o.d. wood (or 4%), the concentration of the cooking liquor had to be about 4% combined S02. In a larger digester, the influence of seal water from circulation pumps and other water-sealed shafts is less pro nounced, and a somewhat larger volume of cooking liquor can be tolerated yet to achieve such low liquor ratios. However, much higher concentra tions of cooking liquor must be realized than the normal 1.2-1.3% com bined S02 in batch cooking, where a large liquor volume is needed to cover the chips. Soluble base is a prerequisite for a high concentration, but then the concentration as such is no problem for either sodium or magnesium base, as seen from the table. The design of the recovery sys tem will, however, have to allow for this demand in order to realize the possibilities of a better steam economy offered by continuous cooking. 42 Lecture 3

At this point we may enlarge on the differences between liquor-to-wood ratio and degree of packing. I have found this a difficulty for most people used to batch thinking. In a batch digester, the chips are packed with suitable devices and then liquor is added into the digester to cover the chips. Depending on the degree of packing, the wood density, etc., the liquor ratio is about 4-5. In a continuous digester, there is about the same degree of chip packing and consequently a similar liquor ratio when the liquor has been introduced to cover the chips. However, in continuous cooking this liquor ratio is not of the same interest as in batch cooking for the steam consumption and waste liquor concentration. Instead, the li quor ratio of interest to continuous cooking is the flow ratio between liquor and wood chips, and this is the ratio to be called liquor ratio in continuous cooking. When this is 2.2, but the liquor ratio to cover the chips in the digester is 4.4, this means that the linear flow of the chips through the digester is about twice as fast as the linear flow of the liquor, and that the retention time of the liquor is twice that of the chips.

The actual retention time and the actual degree of packing was deter mined by tracer experiments, as illustrated by Fig. 3.7. The two-body digester thus gave a packing density of 0.18 tons o.d. wood/m3 digester volume in the upflow and 0.19 in the downflow part, at spruce with a wood density of 0.41. The downflow Mumin digester gave a slightly higher degree of packing. From these direct determinations of retention time at various zones of the digester, at a set chip flow, could be calculated the approximate cooking time for other conditions. An indirect check up could be made by determining the time required for chip filling of the empty digester at an upstart. This time was invariably somewhat shorter than the retention time for a chip at steady state, since the packing density is always lower for uncooked chips than for semicooked ones. The dif ference is, however, usually surprisingly small, indicating that the wall friction tends to prevent the additional packing made possible by the softening of the chips. In a larger digester with a larger diameter, the packing effect increases, but is still far from that corresponding to dissolu tion of wood substance. Another striking feature is the very short impregnation periods, 1-2 min, of the one-stage cooking, and yet the very satisfactory uniformity of the pulp with low screenings and low lignin content at a certain viscosity level. The impregnating liquor, at pH 5, penetrated the chips at a pressure of about 7 atm, applied after 3 min of steaming. This is just the favorable combination of steaming and pressure impregnation which has been found particularly effective in the laboratory experiments previously Continuous Acid Sulfite Cooking 43

Fig. 3.7. Radioactive tracer experiment for the determination of rentention time and packing density in the two-body digester system. quoted. However, this is the first time that this impregnation system has been proved in practical cooking and is now in production on a large mill scale in Germany, using beech chips. After impregnation in the feeding system, steam and S02 are directly introduced at the digester top. In principle, this is the only heating necessary, but a circulation for an accu rate adjustment of the cooking temperature is inserted a short distance below the top. The accuracy of the cooking temperature should be better than 0.5°C. This demand was first scoffed at by our instrument people but later on accepted. We could show, by means of accurate mercury ther mometers inserted in specially devised pockets, that 0.2°C can be felt and correlated with small viscosity variations. Naturally, other variations also occur, such as the base charge or the S02 charge, and these chemical flows must be carefully controlled in relation to the chip flow. Measurement of waste liquor color was also tried as an indication of the degree of cooking, 44 Lecture 3

Fig. 3.8. Viscosity variations during a 24-hour production of rayon pulp from spruce, birch, and beech, Mumin system.

but with proper control of chip, steam, and chemicals flows, the frequency of the viscosity variations was not higher than could be controlled by direct viscosity analysis on the pulp. This is demonstrated in Fig. 3.8, showing viscosity variations when cooking spruce, birch, and beech acid sulfite rayon pulp in the Mumin digester system. Of other quality data of Table 3.1, the favorably low contents of lignin and extractives should be pointed out. Compared to the corresponding batch pulps this means a decided saving in bleaching chemicals. The efforts on the continuous cooking of acid sulfite paper pulp were mainly concerned with the paper strength properties, after the initial cook ing acid decomposition problems had been solved and the delignification controlled by the same means as described for rayon pulp. The initial trials showed a paper strength which was only half that of batch pulps. This was a much more severe degradation than that experienced for kraft pulp during the hot blow period. Considerable efforts were made to locate the source of the damage by sampling from various levels of the two-body digester and continuing the cook- on the laboratory scale, followed by paper testing. This showed several degrading influences, com mencing at the bottom screw of the upflow digester, when that ran heavily loaded, continuing with a slight influence of the top scraper, and finally severe,degradation by the discharge devices. Table 3.3 shows the paper properties of pulps cooked to Roe No. 5 (lignin content 4-5%), using bisulfite precook at 130°C, followed by acid sulfite cooking at 135-140°C in the downflow digester body. The charge of combined S02 was 40-50 kg/ton o.d. wood (4-5%) and the excess S02 concentration in the cooking liquor of the second stage 2.5-3.0%. It can be seen that by unloading the bottom screw in the upflow digester through better lifting circulation and introducing the cold blow (this was done at the same time as in the kraft Continuous Acid Sulfite Cooking 45

Table 3.3. Paper Properties of Spruce Acid Sulfite Pulp, Two-body Pilot Digester

Process: Bisulfite-acid sulfite, combined SO2 charge 4.5% of o.d. wood, pulp Roe No. 5.0.

industry) most of the degradation was eliminated and a pulp of decent strength obtained. In fact, the pulp was somewhat better than the corre sponding mill scale batch pulp, but not quite up to laboratory stan dard. It was concluded that the two-body digester contained too many dangers to the acid sulfite paper pulp quality to be a safe unit for produc tion. After the reconstruction of the digester to the Mumin downflow system, interest was focused on straight bisulfite cooking for paper pulp, as will be described in the following lecture. However, two processes with an acid cooking stage were tried, namely, neutral sulfite—acid sulfite and alkali—acid sulfite cooking. Both two-stage cooking processes can be used for obtain ing an increased carbohydrate yield through deacetylation and stabiliza tion of the glucomannan prior to the acid sulfite cook. Figs. 3.9 and 3.10 show the systems used to produce this effect. In the first case, the chips were introduced with neutral sulfite cooking liquor of pH 7, and steam added at the digester top to give 155°C for 2 hr precook, followed by acidification withjiquid S02 for an acid sulfite cook at 142°C for 1.5 hr. This two-stage cook was tried with both spruce and pine, yielding acceptably low rejects, high brightness, and the desired glucomannan sta bilization (Table 3.4). These trials are also interesting from the stand point that there was no problem in maintaining the two cooking stages sufficiently well-defined in the downflow one-body digester, provided the temperature of the upper zone was kept higher than that of the acid Fig. 3.9. Kamyr digester with inclined top separator used for two-stage neutral sulfite - acid sulfite pulping. Continuous Acid Sulfite Cooking

Fig. 3.10. Kamyr digester with inclined top separator used for two-stage alkali - acid sulfite cooking. Lecture 3

Table. 3.4. Two-stage Neutral Sulfite-Acid Sulfite Pulping of Spruce and Pine in Mumin Pilot Digester Digester Pressure: 10 atm.

cooking. If a considerable temperature gradient was attempted, with 10-20°C lower temperature in the top, a gradual transition of the hotter zone toward the top was experienced through convection along the walls and a consequent acidulation of the top zone. The second system made use of our observation on the laboratory scale that deacetylation of the glucomannan can be achieved rapidly and at fairly low temperature, provided that the pH is high enough. This makes it possible to use sodium hydroxide or sodium carbonate as the feed liquor, in an amount corresponding to the normal quantity of base, 40-50 kg/ton o.d. wood (4-5%), calculated as combined S02. In one case only sodium hydroxide was introduced, in the other sodium hydroxide in mix ture with sodium sulfite. In equilibrium with S02 absorbed from the digester top to the feeding circulation, this gave liquors containing, respec tively, 20 and 70 molar percent of sodium sulfite and a pH greater than 13. After impregnation in this liquor for 1.5 min, the chips arrived at the digester top to meet steam and S02 to full cooking conditions directly. These are described in Table 3.5 together with the results of the pulp- Continuous Acid Sulfite Cooking

Table 3.5. Two-stage Alkali-Acid Sulfite Pulping of Spruce in Mumin Pilot Digester

Digester Pressure: 9 atm. Estimated yield improvement over acid sulfite 4%, wood basis. ing. It is seen that also in this case the screenings were very low and the major part of the glucomannan stabilization obtained. This elegant pro cess requires a recovery system that yields carbonate with some sulfite, but simplifies the cooking liquor preparation very much. From an operational point of view, both these two-stage cooks were thus successful. The quality obtained was also entirely satisfactory, equal to that of laboratory pulps, as shown in Table 3.6. There is every reason to believe that a straight acid sulfite cooking without glucomannan stabiliza tion would lead to laboratory quality also, although this was not tried. Thus, the Mumin version of the continuous digester appears to have eliminated the remaining slight disadvantages of the two-body digester as regards acid sulfite paper pulp quality, which rounds off our development work on continuous acid sulfite pulping very nicely. The system is good for both rayon and paper grades, for both straight one-stage and two-stage pulping. Lecture 4

Continuous Bisulfite Cooking

If the sulfite process is to have a future, three major improvements must be accomplished: all species must be pulpable, the yield must be improved, and the strength properties must be improved. Two-stage pulping with a neutral or alkaline precook may be the answer to the first two demands, but certainly not to the demand for strength, which is rather decreased than improved. The bisulfite process allows the pulping of all wood species and gives a definite strength improvement over acid sulfite pulp ing. It does not improve the carbohydrate yield, but the higher strength at equal lignin content can be utilized for pulping to higher lignin con tents. Then the strength improvement disappears, but instead a substan tial yield improvement is achieved. Two bisulfite pulp types are therefore of interest, namely a chemical bisulfite pulp for bleaching, of normal yield and higher strength, and a semichemical bisulfite pulp of 65-75% yield, with the strength of a chemical acid sulfite pulp. The semichemical bisul fite pulp cannot be economically bleached to high brightness because of its high lignin content, but it is important to obtain the highest possible brightness of the unbleached pulp, and it might also in some cases be of interest to brighten the pulp with dithionite or similar brightening agents. This pulp type is increasingly used as the long-fibered component in newsprint and some magazine grades, and holds good possibilities for use in various types of board. The efforts to produce these pulps by continuous cooking started in the two-body digester. From several aspects, bisulfite cooking is a very simple and straightforward process, which ought to be quite suitable for the trimming of a new digester system. That is at least what we thought when starting the trials in 1956-57. There is little or no excess S02 and hence no charging problems with liquid S02 and little gas problems. There is just one charge of chemicals, and heating to maximum temperature can be made as fast as desired. Yet we learned very quickly that the process was not a very easy one. The dominant problem was that of cooking acid stability. A disturbance in the process, such as a temporary stop for clearing a discharge problem, and 51 52 Lecture 4 soon the liquor color started to darken, thiosulfate concentration to rise, and the pulp to become discolored and shivy. The disease always spread from the bottom of the downflow digester, where the thiosulfate content is always the highest, but soon reached the cooking zone. Then the situation was normally out of hand, the chips became umpulpable even with fresh cooking acid,, and the digester content had to be dumped for a fresh start after thorough cleaning. This was the typical result of the autocatalytic decomposition of bisul fite ions to thiosulfate and sulfate, which was mentioned previously in the lecture on sulfite process theory. The thiosulfate always formed in side reactions is not sufficiently reactive with the lignin at pH 4-5 to become "neutralized," but takes instead full part in the further decomposition of bisulfite ions at an ever-increasing rate. The question was only: why does it seem to be impossible to run a continuous digester on bisulfite, when it is quite feasible to do it batch-wise? Batch bisulfite cooking is capable of yielding chemical bisulfite pulps down to a lignin content of 4-5%; if continued further, the decomposition reaction takes over and will cause discoloration also of the batch pulp. Thus, thiosulfate does form also in the batch bisulfite cook, but the difference is that batch pulping does not allow the spreading of the cooking liquor disease backward in time; contin uous pulping allows this phenomenon to spread to the simultaneously present initial phase of the cooking. In a batch cook, the thiosulfate con centration is 0.5 grams/liter in the beginning of the cook at maximum temperature and 2.5 grams/liter toward the end. We had 1.5 and 4 grams/liter, respectively, at the corresponding places of the digester before complete collapse occurred. At that point we could not be certain that continuous bisulfite pulping was even principally possible, but we at least knew what to look out for: places and opportunities for thiosulfate to form unnecessarily. We found several such places-partially blindfolded strainers, behind which the cook ing liquor had access, could decompose without contact with the chips and ooze out thiosulfate to poison the cook. Furthermore, since we ran the digester at hydraulic pressure, the volumes of cooking liquor were too large at the top of the two-body digester, which did not contain any chips and thus only served to prolong the retention of the liquor at full tempera ture. Finally, there were the cooking liquor circulations, which at least in the pilot digester tended to give excessive mixing of cooking liquor in various parts of the digester, as we could verify not only by thiosulfate analyses but also with injections of radioactive sodium carbonate solutions in the various circulations. With the two-body digester, it was therefore found almost impossible to Continuous Bisulfite Cooking

Fig. 4.1. One-body pilot plant digester for bisulfite cooking, produce the bleachable bisulfite grade of pulp at a steady state. Increas ing the liquor ratio helped to keep the system clean for short periods of time, during which pulp sampling for quality control was possible. Lim iting the delignification to high yield pulps of a lignin content of 10-13%, yields around 60%, or to semichemical pulp in the 65-90% yield range, gave production at steady-state conditions. This allowed us to study the quality of the various bisulfite pulps (Tables 4.1 and 4.2). We found, as expected, that the cold blow is important also for bisulfite pulp quality, although perhaps not to the same extent as with acid sulfite pulp. With the cold blow, some pulps, especially at the inter mediate or highest yields, were almost up to the standard of laboratory pulps. With that background two mills chose continuous digesters for their bisulfite pulping, one in Switzerland and one in Canada, yet of the standard, downflow type. We felt, however, that this result of the development work was not entirely satisfactory. The pilot digester was reconstructed to a long, downflow digester, equipped for both concurrent and countercurrent cooking, in which the bisulfite cooking experiments continued (Fig. 4.1). In this system, the mixing action of the lifting circulation in the former digester was eliminated. Unfortunately, the troubles of cooking acid decomposition continued, when cooking down to yields of bleachable bisulfite pulp. The digester was then run like a kraft digester, with a 54 Lecture 4

Table 4.1. Spruce Bisulfite Pulps of the Two-body Pilot Digester

Analytical data. Cooking conditions: 160-165 C, 4 hr, 8-10% combined S02, wood basis, initial pH 4.5.

Table 4.2. Spruce Bisulfite Pulps of the Two-body Pilot Digester Paper properties. Cooking conditions of Table 4.1.

Discharge temperature, C 125 50 Roe chlorine No. 10 7 11 7 Paper properties at 45 SR Beating time, min 22 25 24 28 Tensile strength, km 10.1 8.8 10.4 9.0 Tear factor 64 62 65 73

moderate-temperature zone followed by a heating circulation to give the maximum temperature, 160-170°C, of the cooking zone, prior to a countercurrent washing. The decomposition tendencies were probably- connected with excessive mixing of the liquor from various parts of the digester. This was partly caused by the circulations and partly by migra tion of hot liquor from the cooking zone along the walls to the upper, low-temperature part of the digester. The mixing and migration was also directly demonstrated by injection of radioactive bromide to the circula tions. The effect is probably less pronounced in the large mill-scale digest ers, but the development work on continuous bisulfite cooking could not be regarded as finished to satisfaction. A fresh approach was possible after the development of the Mumin digester system (Figs. 4.2 and 4.3). With that system, the temperature gradient at the upper part of the digester was eliminated, and likewise the mixing action of the heating circulation, provided the chip impregnation in the feeding system could be regarded as satisfactory. To eliminate any Continuous Bisulfite Cooking 55

possibility of liquor mixing, we started the cooking trials with a vapor phase down to the washing zone. This immediately gave what we wanted, a steady-state production of bleachable bisulfite pulp. However, at least on the pilot digester scale, the packing of the semicooked chips and the pulp became excessive in the vapor phase and tended to give hangings of the chip column in the digester, a phenomenon normally only encountered in exceptional cases of overcooking, etc. Therefore, the liquor level was raised to that of the chips, which resulted in a steady movement of the chip column and maintained steady cooking conditions without decompo sition tendencies of the cooking liquor. This allowed the production of bleachable bisulfite pulp down to lignin contents of 4-5%, at liquor ratios similar to those of batch cooking, or lower, 3.5-4.5. That the liquor ratio is important even in the Mumin system, when cooking bleachable bisulfite pulp, was shown by decreasing the liquor ratio down to 2.5, at which the decomposition reactions tended to get out of hand. With high-yield chemical and with semichemical pulping, there were no such trends even at the lowest liquor ratio. The most sensitive indication of the absence or presence of undesirable liquor decomposition is given by the pulp color. Figure 4.4 shows the brightness of unbleached bisulfite pulp at various yields. The batch curve shows a rise in brightness when cooking to increasingly lower yield, until at around Roe No. 5-6 the brightness starts to drop sharply. The bright ness of the pulp from the conventional downflow digester was better than batch in the higher-yield region, but tended to get lower as the cooking approached the chemical pulp yields. With the Mumin digester system, a stable pulp color was maintained down to the bleachable yields. The brightness level was high throughout the yield region and exceptionally high for green sprucewood, where it reached the 70-75% brightness level. The unorthodox system of instant heating of the bisulfite cook to maxi mum temperature had been tried already in the two-body digester, with out excessive screenings as a result. The question of whether the impregnation of the chips in the Mumin feeding system would be adequate or not is best answered by giving the screenings for chemical grade bisulfite pulp from spruce, pine, and birch. Table 4.3 shows those figures, together with the cooking condi tions and other pulp data. The uniformity of cooking is obviously en tirely satisfactory for spruce, and as good for pine and birch as is normally achieved in batch pulping of those species. The screenings did not contain uncooked or burned chips but mainly easily fiberized fiber bundles. The unusual brightness has already been commented on, and the remaining pulp data are quite normal.

Fig. 4.3. Schematic comparison between the feeding principles of conventional and Mumin feeding systems. 58 Lecture 4

0 5 10 15 20 25 ROE NUMBER Fig. 4.4. Brightness of spruce bisulfite pulps from various types of cooking. The difficulties of obtaining a bisulfite pulp with strength properties comparable to those of laboratory cooking were mentioned in connection with the pulps from the two-body digester. It was suspected, that part of those difficulties were connected with the tendencies for cooking liquor decomposition. Table 4.4 shows the strength properties of chemical bisul fite pulp cooked from spruce in the Mumin system at varying liquor ratios. It appears that a decrease in the liquor ratio will lower the pulp quality, as it tended to decrease the pulp brightness. Table 4.5 shows the strength properties of spruce bisulfite pulps cooked to three yield levels in the Mumin digester system, as compared to laboratory pulps. The labora tory pulps are a shade stronger throughout, but it can be said with some conviction that the bisulfite pulps from the Mumin digester are at least as good as any batch bisulfite pulp produced on the mill scale. The above results were obtained with sodium bisulfite. Magnesium bi- Continuous Bisulfite Cooking

Table 4.3. Bisulfite Pulping in the Mumin Pilot Digester Conditions and results. Digester pressure 10 atm.

Table 4.4. Bisulfite Pulping in the Mumin Pilot Digester Paper properties of chemical bisulfite pulps from spruce at varying liquor ratio. Roe chlorine No. 6.5. 60 Lecture 4

Table 4.5. Bisulfite Pulping in the Mumin Pilot Digester Paper properties at three yield levels as compared to laboratory pulps.

Pulp Beating Tensile yield, time, strength, Tear Sheet % Scale min km factor density

Paper properties at 25 SR 53 Lab. 13 8.7 109 0.67 Pilot 14 8.5 100 0.68 58 Lab. 15 9.8 100 0.66 Pilot 16 8.9 103 0.66 75 Lab. 33 8.8 80 0.64 Pilot 19 8.8 73 0.61 Paper properties at 45 SR 53 Lab. 26 9.8 98 0.74 Pilot 27 9.4 88 0.76 58 Lab. 27 10.5 86 0.75 Pilot 29 9.8 85 0.74 75 Lab. 49 10.0 66 0.64 Pilot 28 9.9 62 0.68

sulfite was also tried at the intermediate yield level and found to perform equally good as sodium bisulfite. The one-body downflow digester allowed countercurrent cooking opera tion. However, with bisulfite it is not surprising to find that countercurrent operation resulted in rapid cooking liquor decomposition. More interesting results with countercurrent operation were obtained with prehydrolysis-kraft and kraft pulping, which will be described later on. In order to achieve additional brightness with a chemical bisulfite pulp of 70% brightness, dithionite was added to the washing zone and the pulp thus subjected to a brightening action at 130°C. The result was dis appointing, since the brightness improvement was less than with dithionite at a subsequent conventional brightening stage. The idea of using the washing zone for brightening is, however, tempting, and might work out better after a more systematic effort made on a laboratory scale prior to mill scale trials. One remaining doubt regarding the sufficiency of the short impregnation period, when using sprucewood with excessive heartwood content, initi ated further trials. It was found that such wood from northern Sweden gave on cooking somewhat discolored centers in about 2% of the chips, as compared to none with spruce from central Sweden. The brightness of Continuous Bisulfite Cooking 61

Table 4.6. Cooking Conditions, Analytical Data and Paper Properties of Spruce Bisulfite Semichemical Pulps for Newsprint, Produced in the Mumin Pilot Digester, Without and With Pressure Preimpregnation at HO^C. Sodium Base

Mumin system Mumin system High-pressure Mumin impregnation preimpr.

Retention time, hr Impregnation 0.02 0.02 0.02 0.02 1.40 Cooking 0.83 2.00 3.50 4.85 2.00 Temperature, °C 169 160 152 145 156 Pressure, atm 9.2 9.2 7.4 8.0 8.9 Liquor ratio, m /ton o.d.wood 3.0 3.0 3.5 3.4 3.0 Total S02,% on wood 17.0 16.6 16.4 17.0 16.0 Combined S02,% on wood 8.5 8.3 8.2 8.5 8.0 Initial pH 4.4 4.4 4.5 4.6 4.3 Roe chlorine No. 24.2 23.6 24.3 23.5 22.7 Brightness, % SCAN' 64.4 62.4 60.9 62.1 65.7 Resin content, % 1.1 1.0 ...... 1.2 Carbohydrate analysis, % Galactose 1.4 1.2 1.1 1.0 1.0 Glucose 76.2 77.6 81.0 79.0 78.4 Mannose 14.8 14.0 11.9 12.7 13.2 Arabinose ...... Xylose 7.6 7.2 6.2 7.4 7.4 Cellulose content, % on carbohydr. 71 73 77 74 72 Estimated pulp yield, % on wood 77 75 72 74 76 Paper properties at 25 SR Beating time, min 24 16 21 21 321 Tensile strength, km 8.6 8.6 8.9 8.3 9.1 Tear factor 73 76 74 72 70 Sheet density, g/cm3 0.57 0.59 0.63 0.56 0.63 Paper properties at 45 SR Beating time, min 35 23 30 33 471 Tensile strength, km 9.5 9.4 9.7 9.4 10.2 Tear factor 62 64 67 60 59 Sheet density, g/cm3 0.66 0.68 0.70 0.65 0.74

Testing at a different laboratory, giving denser sheets. Same tensile-tear relation. 62 Lecture 4

the refined pulp was virtually unaffected by this phenomenon, and the only conceivable disadvantage would show up in a slightly higher shive content. Some trials with a preimpregnation vessel for high-pressure impregnation {cf. Lecture 9) prior to cooking (Table 4.6), indicated possibly an improvement, although the differences were fairly hard to detect, and a comparison with batch pulp of the same type gave better brightness and uniformity for the continuously cooked pulp. The table is of interest mainly to show that the cooking time and temperature can be varied considerably without quality problems. This indicates the flexibility in capacity of a system chosen initially for a retention time, for example, of 2 hi. To sum up the development efforts on continuous bisulfite pulping, it might be said that the Mumin digester system appears to allow bisulfite pulping at all yield levels with satisfactory result from both an operational and a quality standpoint. Lecture 5

Continuous Neutral Sulfite Cooking

When discussing the previous cooking processes, acid sulfite, two-stage sulfite, and bisulfite cooking, as well as the subsequent ones, prehydrolysis-kraft, kraft, and modified kraft cooking, there is no need to compare the Kamyr digester system with other designs, since attempts to present similar machinery by others have been few or none at all. With neutral sulfite cooking, the situation is rather the reverse. In particular Pandia, Defibrator, and M & D units have given satisfactory production of neutral sulfite pulp for many years. Of Kamyr units, only two were in operation in earlier years (on hardwoods), namely ours at Saffle and another unit in Great Britain. These two units have, furthermore, produced a somewhat different pulp, neutral sulfite for full bleaching to be used in white papers, whereas most of the others produced unbleached neutral sulfite pulp for corrugating medium. An Impco digester in the United States has also been used for neutral sulfite pulp for bleaching. Later on, a few NSSC digesters for corrugating medium were delivered by Kamyr to Yugoslavia, Australia, and Poland. They were all of the conventional design. I do not intend to compare the virtues of the various types of neutral sulfite digesters, since my experience with the other designs is limited to mill visits. I shall only describe our reasoning when choosing the Kamyr version for Saffle and our purpose for the development work on the new type of Kamyr neutral sulfite digester for corrugating medium at Gruvon. The neutral sulfite pulps are mainly produced from hardwoods, and are all semichemical. By using the mild chemical conditions of the neutral sulfite process and by limiting the delignification, the xylan of the hard woods is quite well preserved in the pulps. This gives the corresponding corrugating medium board the desired stiff structure, and the bleached neutral sulfite pulp grade becomes very easy-beating and suitable for dense papers, such as greaseproof and glassine. The cooking liquor is generally prepared from sodium carbonate, which is then reacted with S02 to give sodium sulfite with an excess of buffer in bicarbonate form. This gives a pH of about 8.5, and the function of the cooking liquor is partly to maintain pH above 6, and partly to sulfonate 63 Fig. 5.1. Schematic diagram of the first NSSC continuous cooking system at Billerud: (1) digester; (2) discharge pipe; (3) Asplund Defibrator; (3a) blow valve; (4) blow tank; (5) Davenport press; (6) Sprout-Waldron re finer; (7) to screening. the lignin. The buffer, 25-33% of the original carbonate, has mainly to take care of the uronic acid and acetyl groups of the xylan to give uronic acid sodium salt and sodium acetate, under formation of carbon diox ide. The total quantity of sodium carbonate charged is about 160 kg/ton o.d. wood (16%) when cooking to bleachable grades of 70-75% yield, and 110 kg/ton (11%) when cooking to corrugating medium grade of 77-83% yield. The cooking temperatures used vary between 160 and 190°C and the cooking time is adjusted according to the temperature used and the yield desired. It thus varies between 15 min and several hours. Our reason for choosing the Kamyr system for Saffle was that we desired maximum uniformity of cooking for the bleachable pulp grade. We did not feel convinced that the other systems would give an acceptable impreg nation because of short heating periods to maximum temperature and sometimes rather primitive methods of adding the liquor to the chips. To day, we know we were wrong about the heating period required for uni form cooking, but we might still have made the best choice from an impregnation point of view. The system chosen was the standard kraft digester, although with a shell lined with stainless steel and some auxiliary machinery attached (Fig. 5.1). Thus, before the blow valve and the disk strainer of the bottom circulation, a Defibrator was inserted, to fiberize the semicooked chips Continuous Neutral Sulfite Cooking 65

Fig._5.2. The effect of temperature on the tensile strength of (7) water- swollen sprucewood, (2) birchwood, and (.?) neutralisulfite-cooked birchwood, 75% yield. before blowing. The blow tank did not contain the conventional impeller at the bottom, but instead a low-pressure feeder to handle even very coarse material. The pulp was then to be discharged to a Davenport press prior to refining in a Sprout-Waldron 36-2 precision refiner. The intention with the Defibrator was partly to facilitate the blowing and subsequent han dling of the semicooked material, partly to save refining energy, and pos sibly to improve quality. Refining at high temperature of uncooked chips, as practiced in the Defibrator process, gives a less damaged fiber than groundwood and at a much lower power consumption, since the fiberizing is done above 170°C, in the temperature of plastic flow of the lignin (Fig. 5.2). One immediate experience with the system was that the bicarbonate buffer had to be abandoned and substituted with the more expensive 66 Lecture 5 sodium hydroxide. The carbon dioxide evolved from the bicarbonate gave rise to troubles in the upper part of the hydraulic digester and in the high-pressure feeder. The migration of gas bubbles to the top accentuated the thermal flow, which gave excessive temperatures in the feeding circula tion and steam hammering in the feeder. The hammering was accentuated by the presence of carbon dioxide gas bubbles. With sodium hydroxide buffer, the system worked quite well mechani cally, but gave a pulp quality much lower than that of the previous labora tory cooks. The pulp quality was then measured both on the unbleached pulp and after a standard bleaching procedure. This quality experience came in 1957, contemporaneously with the experience on kraft pulp qual ity in Gruvon and just prior to the quality efforts on acid sulfite pulp in. Jossefors. Thus, it was natural to look for degradation in combination with mechanical action, particularly from the digester arrangement. Perhaps the high-temperature fiberizing, in spite of giving a seemingly undamaged fiber in the Defibrator process, led to a quality degradation in combination with a chemical cooking, even the very mild one of the neutral sulfite process. The first attempt was therefore to make the fiber izing coarser. Table 5.1 shows that increasing plate setting of the Defibra tor from 0.3 to 2.0 or 5.0 mm only slightly improved the quality of the bleached pulp. Then, with the Defibrator maintained open, the effect of the temperature was investigated by charging varying amounts of cold water to the bottom circulation (Table 5.2). Cooling down to 110°C gave the desired considerable quality improvement on bleached pulp, as also shown in Fig. 5.3. In order to show whether that improvement corre sponded to undamaged pulp, a new series of runs were made, with sam pling both at the digester bottom and after the fiberizing and discharge. The plate setting of the Defibrator was varied at the extreme temperature levels. The results are shown in Table 5.3, with the strength properties of bleached pulps. The strength of unbleached pulps gave analogous results, showing that the quality decrease of fiberizing and discharge was sub stantial at 160°C, more with a tight plate setting, but intolerable with an open Defibrator. At 110°C the quality damage was eliminated whether the Defibrator was open or tightly set. Table 5.4 shows the analytical composition of the unbleached and bleached pulps. No differences in the composition of these pulps with such widely differing strength properties could be detected; hence it must be concluded that the damage of the hot blow is not connected with a dissolution of material, but rather with a local damage of the cellulose in the fiber wall. This may well be a combi nation of mechanical and chemical degradation, but with the chemical action not detectable by carbohydrate analysis. Continuous Neutral Sulfite Cooking

Table 5.1. Effect of Refiner Plate Setting on Quality of Bleached Neutral Sulfite Semichemical Pulp from the Saffle Digester. Refining Temperature 167°C

Paper properties at "greaseproof density'

Refiner Beating Tensile Slow plate setting, time, strength, Tear Sheet ness, mm mm km factor density °SR

0.3 19 7.9 48 0.98 73 2.0 21 8.2 46 0.98 69 5.0 15 8.2 48 0.99 74 Batch cooked and refined at low temperature 15 10.0 60 62

Table 5.2. Effect of Refining Temperature on Quality of Bleached Neutral Sulfite Semichemical Pulp from the Saffle Digester. Refiner Plate Setting 5.0 mm (wide open)

Paper properties at "greaseproof density"

Refining Beating Tensile temperature, time, strength, Tear Sheet Slow °C mm km factor density ness, °SR

162 15 8.2 48 0.99 74 130 15 8.8 51 0.99 68 120 23 9.2 55 0.97 61 110 13 10.0 61 61 Batch cooked and refined at low temperature 15 10.0 60 62

Table 5.3 also contains information on the CMT value of the unbleached pulps. This value indicates the corrugating medium properties. The re sults are somewhat bewildering, since they indicate more damage at 110°C than at 160°C. The average indicates some 10% quality decrease from the fiberizing and discharge equipment, as compared to fiberizing at low tem perature in the laboratory of samples removed from the digester bottom. After these experiences it could be concluded that continuous cooking of bleachable neutral sulfite pulp from birch is feasible in the normal Kamyr kraft system, and that a good quality can be obtained if the dis charge is made at 110°C or lower. The Defibrator was removed from the pressure system and the fiberizing done entirely with the Sprout-Waldron refiner at 60-80°C. Following this experience the British digester was 68 Lecture 5

VALLEY BEATING TO 45° SR SLOWNESS 10 cm3/sec POROSITY

DEFIBRATOR PLATE SETTING, mm Fig. 5.3. Effect of refining temperature on quality of bleached pulp. Refiner plate setting 5.0 mm.

also designed and started. The disadvantage of omitting the bicarbonate as buffer remained for this hydraulic digester system, but was accepted at Saffie and presented no problem with the British mill, where the cooking liquor was made up from byproduct sodium sulfite entirely. The problem of cooking corrugating medium grade pulp could be solved with the same system. However, with that grade, bicarbonate-buffered cooking must be feasible to get maximal economy, and further, it was an open question whether the Kamyr system was competitive with the other continuous digesters, with regard to investment costs. However, just as with kraft cooking, it is desirable to develop machinery for the very large production units. Until recently, the largest unit for the production of corrugating medium board pulp was about 150-200 tons/day, either lim iting the board production correspondingly, or necessitating 2 to 3 pulping lines for one fairly large board machine. We regarded the Kamyr digester to hold the "best possibilities for large-scale production, provided the sys- Continuous Neutral Sulfite Cooking 69

Table 5.3. Effect of Refining Temperature and Plate Setting on Quality of Neutral Sulfite Semichemical Pulp from the Saffle Digester

Paper properties at "greaseproof density," bl.

Refining Refiner Sam- CMT of Beating Tensile temp., plate set- pling at unbl. pulp, time, strength, Tear °C ting, mm discharge lb min km factor

110 0.3 Before 65 9 9.5 62 After 60 9 9.7 60 5.0 Before 63 11 9.7 60 After (54) 12 9.7 62 130 5.0 Before 68 9 9.2 64 After 64 9 9.3 58 160 5.0 Before 59 9 9.3 59 After 58 10 8.6 53 0.3 After ... 19 7.9 48 tem could be adapted to the demands of the bicarbonate-buffered neutral sulfite process. The possibilities of digester washing were a further plus for the system, with the increasing demands to minimize stream pollu tion. There was, however, no experience on whether digester washing could be applied with any material effect on semicooked chips in this high-yield range. The development work on corrugating medium grade of birch neutral sulfite pulp started after the reconstruction of the pilot digester to the Mumin system, with all the experience of the prehydrolysis-kraft, bisulfite, and acid sulfite cooking behind. We therefore knew that the system would be able to run continuously with a suitable C02 relief and had every chance to run well on bicarbonate-buffered neutral sulfite. It was thus rather quick development work. We tried vapor phase as well as liquor phase and found no particular advantage of running a vapor-phase cook, but possibly somewhat less, uniform pulp. All further development work thus concentrated on liquor-phase cooking. Sodium-based cooking as well as ammonia-buffered ammonium sulfite was tried, with practically the same results. Laboratory trials in connection with bleachable grades had already shown that ammonia base was quite feasible, though yielding a darker pulp with very slightly inferior yield and strength proper ties. Three different yield levels were chosen for the sodium-based cook ing, which were, as nearly as could be determined, 78, 80, and 82%. The cooking conditions and the analytical data of the corresponding pulps are ^

Table 5.4. Analytical Composition of Neutral Sulfite Semichemical Pulp from the Saffle Digester; Varied Refining Temperature and Plate Setting

Analytical composition Refining Refiner Sampling Pentosan Anatytical composition of of fraction soluble temp., plate set- at dis- content, pulp carbohydrates, % in 10% NaOH, % C ting, mm charge % Glucose Mannose Xylose Glucose Mannose Xylose Before 85.4 79.2 24.4 68.5 28.0 After 82.5 74.4 23.6 66.4 30.3 Before 85.8 80.6 23.1 71.0 27.5 After 85.9 79.7 24.8 69.8 28.4 130 Before 84.8 79.4 23.8 70.8 25.7 90.6 After 85.2 79.2 24.4 69.9 27.5 89.0 160 Before 84.7 78.2 23.4 69.2 28.4 After 85.7 80.5 22.6 69.4 28.0 Continuous Neutral Sulfite Cooking 71

Table 5.5. Cooking Conditions, Analytical Data and Paper Properties of NSSC Birch Pulps for Corrugating Medium Board, Produced in the Mumin Pilot Digester and in the Laboratory

Laboratory pulps Pilot plant pulps

Yield level, % on wood 82 78 72 82 80 781 76

Retention time, hr Impregnation 2.0 2.0 2.0 0.02 0.02 0.02 0.02 Cooking 1.5 3.7 4.0 1.75 1.75 1.75 1.75 Temperature, C 153 153 160 155 160 160 165 Liquor ratio, m /ton wood 4 4 4 1.7 1.8 2.0 1.8 Cooking chemicals charged Na2C03 , % on wood 120 120 120 110 103 103 116 S02,%onwood 48 48 48 44 40 40 46 Final pH 6.5 6.1 6.4 6.6 6.3 6.6 6.5 Roe chlorine No. 19.3 18.9 16.9 19.9 19.6 19.0 18.6 Brightness, % SCAN 48.1 47.1 46.4 53.5 50.2 49.0 46.6 Resin content, % 0.64 0.81 0.94 0.76 0.81 0.90 1.00 Carbohydrate analysis, % Galactose 0.6 Glucose 67.8 68.7 71.1 69.4 69.2 68.8 70.3 Mannose 2.2 3.2 1.9 3.2 3.6 5.2 2.2 Arabinose 0.4 Xylose 30.1 28.1 27.1 27.5 27.3 25.4 27.5 Paper properties at 25 SR Beating time, min 15 8 12 6 1 4 7 Tensile strength, km 4.4 4.9 6.2 4.9 5.4 5.3 6.0 Tear factor 61 65 70 73 77 80 70 Sheet density, g/cm3 0.51 0.56 0.63 0.52. 0.58 0.54 0.61 CMT at 127 g/m2, lb 37 38 39 38 36 40 42 Paper properties at 45 SR Beating time, min 27 14 20 12 6 9 12 Tensile strength, km 6.0 6.4 7.8 6.9 7.4 7.3 7.6 Tear factor 54 57 62 60 66 71 69 Sheet density, g/cm3 0.62 0.65 0.73 0.62 0.66 0.64 0.70 CMT at 127 g/m2, lb 47 48 49 48 46 48 48

Vapor phase cooking.

shown in Table 5.5. They show that the pulps obtained corresponded to laboratory quahty in every respect, and that they were obtained with a minimum of cooking chemicals and a favorable liquor ratio. The recovery of the waste liquor will be described in a subsequent lec ture on washing. The pulp produced in the pilot digester was taken to two paper-machine trials. One was with the experimental wet end of Fig. 5.4. NSSC pulping at Billeruds fluting mill of Gruvon.

Table 5.6. Corrugating Medium Board, Produced out of NSSC Birch Pulp from the Mumin Pilot Digester, as Well as from a Full-Scale Mumin Digester and from a Conventional Multiscrew NSSC Digester. Pine Kraft Addition to the Furnish 15% or Less

Pilot Commercial Commercial Mumin Mumin multiscrew

Pulp yield level, % 82 84 83 Slowness of furnish at headbox, °SR 38 35 Basis weight, g/m2 126 127 127 Sheet density, g/cm3 0.69 0.59 0.57 Tensile strength, km MD 7.7 8.8 7.9 CD 4.2 3.8 4.0 Elongation to break, % MD 1.8 2.1 2.2 CD 4.6 2.6 2.7 CMT, kp MD 28 31 25 CD 17 15 Continuous Neutral Sulfite Cooking 73

KMW, where it was run at the maximal speed of 600 m/min without wet-web breaks, with 15% pine kraft, and at 550 m/min with no addition of long-fibered pulp. The other trial was at the experimental paper ma chine of PCL (Central Laboratory of the Swedish Paper Industry) at Stockholm, where the pulp was run at moderate speed, but instead a finished corrugating medium was produced. The test results of this board are shown in Table 5.6 together with an average of mill-scale produced Scandinavian corrugating medium boards. The board was further run on the experimental corrugator of CL (Central Laboratory of the Finnish Pulp and Paper Industry) at Helsinki. There it showed no breaks up to the maximal speed of the corrugator, 300 m/min, and goQd glueability over the entire practical speed range. To judge from the experimental pulping, papermaking, and converting data, the Mumin digester system produced an excellent corrugating medi um board grade. This, together with the satisfactory washing results, con tributed to our decision to order a digester for Gruvon, for a corrugating medium board line which is the largest in the world so far. The design of the system, which will have an ultimate capacity of about 600 tons/day, is shown in Fig. 5.4. The Mumin digester is attached to a blow tank in the form of a live-bottom bin to allow a reliable feeding of the refiners and some drainage after the digester washing. This will, so far, at 300-400 tons/day, be the entire washing system, with high-density pumps after the refiners directly feeding the thick stock buffer tower with no intermediate washing or screening. Ultimately, with the increased cooking capacity, a pressing stage can be included prior to the refiners to complete the wash ing. We also considered omitting the live-bottom bin and feeding the refiners directly from the digester blow cyclones, then passing the stock through a continuous diffuser prior to the buffer storage towers. This may be the most elegant solution but it was not sufficiently developed at the stage of decision for us. As shown by Table 5.6, the quality experi ence of the pilot digester and experimental paper machine has been veri fied on the large scale. It remains to be added that an equally large digester has been ordered for Japan for the same purpose, and a smaller one for Sweden. It should also be added, that there is now also one large Defibrator digester unit coming into operation in Finland for the production of 300-400 tons/day which was considered by us but finally decided against because of limitations of expansion and absence of digester washing. Another NSSC digester of similar capacity, Esco, has been recently installed in North America. Lecture 6

Kraft Cooking Process Theory

Many of you will now think that I have spent more than enough time on processes other than kraft. I agree that the paper pulps of tomorrow are kraft and mechanical pulp, with some semichemical neutral sulfite and bisulfite for special purposes. However, I am convinced that the men of the kraft and groundwood pulping industries will have to draw from the entire pulping knowledge in order to introduce some new approaches to these two well-developed and standardized industries. The simplicity of the kraft process is of course a great asset. It used to be still more simple, before the discovery of the advantage of sulfide. The original process of Watt and Burgess, patented in 1853, concerned the cooking of wood with sodium hydroxide under pressure. With a suffi cient charge of alkali, free fibers were obtained from hardwoods. With softwoods, alkaline cooking with pure sodium hydroxide did not yield a chemical pulp very easily, and prolonged cooking to achieve fiber libera tion resulted in a weak and brittle pulp. Accidentally, it was discovered that sodium sulfide accelerated the cooking process. This was in 1879, when Dahl in Danzig tried to add sodium sulfate instead of carbonate as make-up chemical in the recovery process. The soda pulping became sulfate pulping, and eventually the high strength of that pulp motivated the name of kraft pulping. The new process had two snags: the pulp was brown and difficult to bleach. It could not be used in printing papers in unbleached form like sulfite pulp, and it could not be used in very white papers as bleached. The development of the packaging papers and boards created, however, a large market for unbleached kraft. Multistage bleaching in the beginning of the 1930's, complemented by chlorine dioxide bleaching 20 years later, solved the problem of kraft pulps for white papers and boards. The development of bleached hardwood kraft in the 1950's invaded the fine paper field, where bleached sulfite had one of its last strongholds, and the development of prehydrolysis-kraft rayon pulp during and after the last world war invaded another protected sulfite area. Today, the kraft process, together with groundwood, could cover the markets for wood pulp and paper almost entirely, and because of its 75 76 Lecture 6

well-developed technology, its quality and economy, it is gradually taking over the markets. The competition is, however, not confined to sulfite versus kraft, but also kraft versus kraft, and paper versus other materials. Therefore devel opment must continue to improve the economy and quality of the kraft process. The improvement of economy must come largely in three areas: decreased wood raw material costs, decreased other operating costs, and decreased investment costs. The ways to achieve this must be increased pulp yields and increased size of one-line units. As to quality improve ments, it can be stated that there is fairly little to achieve in cooking and bleaching today, when comparing with a well-run uniform cooking and a six-stage bleach plant including two chlorine dioxide stages. I know of only one process modification which gives a stronger pulp than chemical kraft, and then at the expense of pulp yield. With new raw materials coming in, such as certain eucalypts for hardwoods, radiata pine, and the light-weight softwoods of British Columbia, there are, of course, sub stantial quality effects to be achieved, but these are somewhat outside the scope of what can be achieved by process modifications. The quality aspect rather enters in a negative way with the process modifications: how much quality can be sacrificed in order to gain econ omy? This question must be interpreted not to imply that a real quality should be sacrificed, but rather what we have erroneously believed to be a quality factor without really knowing, and perhaps by overestimating its importance. Everybody will agree, for instance, that a double fold of 4000 is not a quality criterion to keep up for a liner board or sack paper pulp, if it costs money, as long as the scoring and creasing resistance of the products remains adequate. More discussion will arise if, instead of double folds, I mention burst, tensile, or tear strength, and yet none of the qualities have any direct bearing on the service strength of a box or a sack. We have to look for the directly relevant quality criteria, and to relate them back to the pulping process, and that is now increasingly being done. The principal lignin reaction- of the soda cook is the hydrolysis of the alkyl-aryl ether bond between the monomers, proceeding via epoxide for mation (Fig. 6.1). This ether bond is very difficult to split in the acid processes, and also with pure sodium hydroxide the reaction is not fast enough to prevent excessive carbohydrate degradation, particularly with softwoods. In the presence of sodium sulfide, the reaction is accelerated without much sulfide being really consumed. From many years' study of kraft pulping, lignin reactions, and reactions of model compounds, it ap pears likely that not sulfide, but rather the hydrosulfide ion reacts with Fig. 6.1. Alkaline hydrolysis of lignin. the vanillyl alcohol groups of lignin to form mercapto groups, which then give episulfide under splitting of the bond between two monomers (Fig. 6.2). The episulfide is subsequently hydrolyzed to alcohol groups in the lignin under reformation of hydrosulfide, whereas the sodium hydroxide is consumed by the lignin in the principal splitting reaction of the alkyl-aryl ether bond. That reaction not only decreases the molecular size of the lignin but also makes it water-soluble as a phenolate and thus brings it into solution. Only 0.1 -0.2 S/monomer remains in that lignin. Unfortunately, there are also other lignin reactions. There is condensa tion to larger molecules, which partly counteracts the degradation just mentioned. To some extent the condensation is favored by the formalde hyde formation occurring from the aliphatic part of the lignin monomer in the absence of sulfide (see Fig. 6.1). In the kraft cook, lignin condensa tion is not a factor to watch out for as in sulfite cooking, in the sense that it should prevent delignification if conditions are not chosen cor rectly. There is merely one phenomenon of importance, which is likely to OCH3 Fig. 6.2. Sulfide-induced alkaline hydrolysis of lignin. have connection with condensation, and that is color formation of the pulp lignin. It has had to be accepted that kraft pulp and papers are brown, but there are different shades, brighter or darker, reddish, yel lowish, or blueish. In the first approximation, a kraft pulp can be said to be the darker the more lignin it contains, as illustrated by Fig. 6.3. Then there is an influence from other colored compounds present, such as extractives, or the blue color from iron and bark tannins when pulping spruce containing substantial amounts of bark. Such influences may necessitate a more complete color measurement than brightness, such as with the tristimulus filters of the Elrepho meter. Unbleached kraft pulp brightness and color shades are of increasing importance, not only for kraft paper to envelopes, etc., but also to sacks and for liner board to boxes, which are increasingly being printed. The increase in pulp color with an increase in yield is therefore a serious limiting factor in our efforts to save wood. The cause of the discoloration of lignin in kraft pulping is therefore an urgent subject of research, since we know too little today to describe the reaction and define its vari ables. We can only suspect that it has to do with the formation of quinoid or semiquinoid structures. Demethylation of guaiacyl propane monomers leads to pyrocatechol structures, which in order to yield o-quinone structures must find a ready hydrogen acceptor. This may be dioxocyclohexene, likewise formed from pyrocatechol structures, which after hydrogenation and rearrangement ends up as hydroxycyclopentane- carboxylate, a proved reaction. This demethylation has been studied quite closely for a different reason: odor abatement. Softwood lignin contains a methoxyl group on almost every monomer, and hardwood lignin in addition has a second Kraft Cooking Process Theory 79

40 50 60 70 80 90 100 YIELD, % Fig. 6.3. Brightness of unbleached spruce pulps. methoxyl on every two monomers (Fig. 6.4). Just as the main inter connecting bond between the monomers is an alkylaryl ether bond, the methyl is attached to the benzene nucleus, and tends to split off on cooking in alkali, particularly if the adjacent position contains a free phenolic group (Fig. 6.5). We have also learned that the hydrosulfide ion is a more powerful demethylating agent than the hydroxide ion, whereby the reaction product is no longer methanol but the foul-smelling methyl mercaptan (Fig. 6.6). We have learned that methyl mercaptan is ionized during kraft cooking conditions, and that also the mercaptide ion is a more powerful demethylator than is the hydrosulfide ion, and yields dimethyl sulfide, which unfortunately smells almost as bad as mercaptan. The carbohydrate reactions of the kraft cook are partly similar to the lignin reactions. The demethylation of the methylglucurono groups of the xylan (Fig. 6.7) proceeds in parallel with the lignin demethyla-

Kraft Cooking Process Theory 81 tion. The splitting of the alkyl-aryl ether bond of lignin has its counter part in the scission of the glycosidic bonds of the carbohydrate chains, which appears to proceed over an epoxide (Fig. 6.8). It is thus under standable that the conditions of the delignification in the soda cook had to cause carbohydrate degradation, until a reaction was found to speed up the lignin degradation. As far as is known, it is not possible to accelerate the carbohydrate degradation with hydrolsulfide, and thus episulfide for mation is not an intermediate reaction in this case. The chain splitting reaction of the carbohydrates proceeds mainly at maximum temperature of the kraft cook, 160-180°C. At lower tempera ture, 60-160°C, an important carbohydrate reaction takes place, called the peeling reaction (Fig. 6.9). This starts at the aldehyde end groups of the chains, and after certain internal rearrangement reactions leads to the re moval of the terminating monomer in the form of the sodium salt of a saccharinic acid. The nature of this acid varies with the carbohydrate attacked, but it is usually a C6- or C5-isosaccharinic acid. In addition, some fragmentation of the end monomer occurs, to give acids of lower molecular weight, formic and lactic acid, also consuming alkali. After peeling of the end monomer, a new terminating unit appears, likewise

H OH Fig. 6.7. Demethylation of glucuronoxylan in .the kraft cook. ending with an aldehyde group and capable of the same reaction. In this way, monomer after monomer is peeled off under consumption of alkali, and the entire carbohydrate wouid be destroyed if there were not a stopping mechanism. There are several types, having in common the elim ination of the aldehyde group, which is the weak point of the molecule (Fig. 6.10). One stopping reaction would be a fragmentation of the end monomer, yielding a terminating unit of 3-5 carbon atoms and ending with an acidic or alcoholic group. This type has not been definitely proved but is rather likely. Another stopping reaction is an intermolecular rearrangement of Fig. 6.8. Alkaline hydrolysis of glycosidic bonds. the full terminating monomer, to give a metasaccharinic acid end mono mer. This rearrangement may be facilitated by a suitable substitution of the molecule, and it has been indicated that the arabinose units in the 3-position on the xylose monomers has such an influence. The glucuronoarabinoxylan of softwoods thus tends to stabilize toward alkali more easily than the glucomannan. Also the cellulose is subjected to peeling but is less accessible because of its ordered superstructure. These two types of stopping reactions occur spontaneously and decide the yield of the normal kraft cook. Attempts to bring about addi tional stopping of the alkaline peeling, in order to improve the carbohy drate yield, have led to modified kraft cooking methods. These employ either reduction or oxidation of the aldehyde end groups to alcohol or carboxyl groups (Fig. 6.11). The most well-known agents are borohydride (sodium tetrahydridoborate) and polysulfide, which, applied in suita ble amounts, lead to 3-7% yield increases (Fig. 6.12). The changes in carbohydrate composition shows that the main preservation occurs with softwood glucomannan and hardwood xylan but with polysulfide also some cellulose yield improvement is indicated (Figs. 6.13 and 6.14). Also other redox agents have been tried with some success, such as chlorite, hydroxylamine, hydrazine, and hydrogen sulfide. The last is the only one with some commercial possibilities, together with polysulfide, and both present complications in the cooking chemicals recovery which have not been completely solved yet. The reason for these complications, and also the cause of the economical unfeasibility of the other redox additions, is the low degree of utilization of the redox agents, because of their con temporary decomposition in side reactions. It is the object of the development work of the' cooking process variants to choose the condi tions so that these side reactions are limited. Fig. 6.9. Alkaline peeling and stopping reactions of carbohydrates.

CH2OH H^ ^CHOHOH \~/c\OH ACOOH H H Fig. 6.11. End group stabilization by redox reactions. In connection with the methods of increasing the carbohydrate yield should also be mentioned the method for diminishing it by prehydrolysis. By subjecting the wood to an acid hydrolysis, either with mineral acids at moderate temperature or with a steam or water treatment at 160-180°C, the carbohydrates are degraded to shorter molecules, which to a certain extent dissolve during the prehydrolysis but predominantly re main in the chips. Because of their shorter chains, they contain more aldehyde groups and therefore degrade more easily by alkaline peel ing. Since the cellulose is more protected against the prehydrolysis than are the hemicelluloses, because of its more well-ordered superstructure, the prehydrolysis-kraft cook yields a rather pure cellulose, suitable for the viscose process (Fig. 6.15). In order to illustrate how the main part of the fiber wall is influenced by these process modifications, Figs. 6.16-6.18 show the schematical carbohydrate distribution around the secondary wall elementary fibril, analogous to Figs. 2.20-2.21 for wood and sulfite pulps. Changes not only in the chemical reactivity but also in physical acces sibility to peeling may influence the carbohydrate yield of kraft pulp ing. The reprecipitatfon of xylan onto the fiber has been indicated to be of this kind. Anyway, it has been proved that some carbohydrates dis solve in the beginning of the kraft cook, reach a maximum concentration at the end of the impregnation period, and then eventually disappear again from the solution (Fig. 6.19). Some of them probably degrade in solu- 2.0 2.5

SULFUR, % on wood Fig. 6.12. Increase in unbleached kraft pulp yield, given in percent based on the wood, in relation to the amount of sodium borohydride (above), and polysulfide sulfur (below) added to the cooking liquor, given in per cent based on the wood. tion, but it has also been demonstrated that part is capable of becoming adsorbed to cellulose. This phenomenon is dependent on the alkali con centration of the liquor (Fig. 6.20), and therefore on the alkali charge. It is mainly xylan that is capable of readsorption, probably because it is more easily stabilized chemically than the glucomannan, which degrades rather quickly in solution. When the alkali charge of the cook is increased, the xylan content in the pulp decreases, the glucomannan content increases, and the pulp yield goes down for a constant degree of delignification. For the same reason, a decrease in pulp yield and xylan content must be expected of a countercurrent kraft cook, since that will leave the highest alkali concentration to the end of the cook. There are two more carbohydrate reactions of importance to the alkali consumption, namely, the direct neutralization of the glucuronic acid Kraft Cooking Process Theory

Fig. 6.13. Yield of wood components on pulping to Roe number 4-5, using the kraft process without additions (KR), polysulfide (PO) or tetrahydridoborate (BO) additions. Spruce, pine, birch.

groups of the xylan, and the deacetylation of the glucomannan of soft woods, and of the xylan of hardwoods (Fig. 6.21). Those two reactions occur instantaneously as soon as the wood comes into contact with the white liquor. Some of the peeling of the easily accessible hemicelluloses also occurs fairly quickly, whereas most of the peeling proceeds during the heating period and some also at full cooking temperature. The chain splitting mainly occurs at maximum temperature. With this background of lignin and carbohydrate reactions, I should like to comment on the alkali consumption of the kraft cook and the concepts of active and effective alkali (Fig. 6.22). A normal white liquor has a sulfidity of 30%, that is, the molar fraction of sulfide as related to the sum of hydroxide and sulfide. A normal charge of white liquor in Scandinavia is 200 kg/ton o.d. wood, or 20%, expressed as NaOH and active alkali. Expressed as Na20, this would mean 15.5%. Of these 200 kg, 60 kg would be sulfide and 140 kg hydroxide. The term "active alkali" includes the entire sulfide together with the hydroxide, and this assumes that all sulfide gives an equivalent amount of hydroxide in the cook. With POLYSULFIDE SULFUR, % Fig. 6.14. The effect of polysulfide on yield and composition of bleached loblolly pine pulps. the knowledge that the pH of the black liquor is about 12, or seldom, slightly lower, it is quite obvious from a Bjerrum diagram (Fig. 6.23), that only one of the two stages of hydrolysis of the sulfide is utilized, and that in principle only half of the sulfide should be regarded as effective alkali. In the given example, with 200 kg active alkali of 30% sulfidity, thus 140 + 0.5 x 60 = 170 kg are effective alkali. Normally, about 90% of the alkali charge is consumed, and in this example, thus, about 150 kg NaOH/ton o.d. wood. This cook gives an unbleached chemical kraft pulp at a yield of 47%, meaning that 3% extractives, 25% lignin, and 25% carbohydrates were dissolved. With about 0.8 phenolic groups per lignin monomer to be neutralized in the dissolved lignin, this means a consumption of about 40 kg NaOH/ton o.d. wood in lignin reactions, or roughly one fourth of the alkali. The acetyl groups will consume about 15 kg NaOH/ton o.d. wood, the uronic acids about 10 kg; the remaining 90 kg NaOH/ton o.d. wood have thus been consumed in peeling, which means about 1.6 moles of acid Fig. 6.16. Schematic representation of cross section of cellulose elementary fibril, surrounded by hemicelluloses, in main secondary wall of kraft pulp. Fig. 6.17. Schematic representation of cross section of cellulose elemen tary fibril surrounded by hemicelluloses, in main secondary wall of j>oly- sulfide or borohydride kraft pulp.

Fig. 6.18. Schematic representation of cross section of cellulose elementary fibril, surrounded by hemicelluloses, in the main sec ondary wall, prehydrolysis-kraft pulp. Kraft Cooking Process Theory

Fig. 6.20. Dissolution of lignin and carbohydrate components during the kraft cooking of pine; upper curves 0 g/liter NaOH, 20 g/liter Na2S; lower curves 35 g/liter NaOH, 10 g/liter Na2 S.

CH3COONa Fig. 6.21. Instantaneous carbohydrate reactions of the kraft cook. produced per hexose unit. This is what is found by direct reaction of carbohydrates with alkali. Thus the predominant part of the alkali is consumed by carbohy drates. The amount consumed at immediate contact between wood and cooking liquor at 100-110°C is about 70 kg NaOH/ton o.d. wood, 7 or 8% 92 Lecture 6 active alkali, expressed as NaOH, or almost half of the alkali consumption, consisting of about 25 kg with acetyl and uronic acid, 15 kg with native phenolic hydroxyls of lignin, and about 30 kg in rapid peeling of easily accessible carbohydrates, constituting about 8% of the wood. The yield at this stage is about 90%. It should be observed that the cooking-rate-controlling part of the alkali is that consumed toward the end of the cook by the lignin-degrading reaction and the unavoidable accompanying carbohydrate reactions. The alkali concentration at cooking temperature and time at temperature, and the cooking temperature itself, are thus the reaction variables of in terest. Of course, the alkali concentration at temperature is related to the liquor ratio and to the total alkali charge. The sulfide concentration does not materially influence the rate of cook ing, as long as it is kept above a certain minimum. The amount of sulfur involved in the lignin reactions would be about 40 kg/ton o.d. wood (4%), if the sulfur were consumed in the main reaction, the episulfide forma tion. However, since it is only some sulfur consumed in side reactions,

Composition, g/liter Na20: NaOH 76 lg5 ) |

Na2C03 19 ) M28 [13()

Na2S 33 » J

Na2S04 2 ' Concepts, g/liter Na20: Total alkali NaOH + Na2C03 + Na2S + Na2S04 130 Total titrable alkali NaOH + Na2C03 + Na2 S 128 Total active alkali NaOH + Na2 S 109 Total effective alkali NaOH + Vi Na2 S 93 %: Sulfldity Na2S/(Na2S + NaOH) 30 Causticity NaOH/(NaOH + Na2C03 + Na2S) 60 Causticizing efficiency NaOH/(NaOH + Na2C03) 80' Fig. 6.22. White liquor composition and concepts.

Fig. 6.23. Electrolyte systems of kraft cooking liquors. Kraft Cooking Process Theory 93

which really becomes fixed to the lignin, the amounts of organically com bined lignin toward the end of the cook is only 5-10 kg S/ton o.d. wood (0.5-1%). With an effective alkali charge of 170 kg NaOH/ton o.d. wood, 10 kg S only means 25 kg Na2S, or a sulfidity of about 13%. The normal sulfidity of 30% means 24 kg S, which thus leaves a considerable surplus above the sulfur consumed in side reactions. Extensive investigations on the role of the sulfidity in kraft pulping indicate a critical level of 15-20% and no major improvement in the rate of pulping above 25%. Although the above considerations are principally correct—that alkali concentration, temperature, and time are the major variables controlling the rate of pulping, provided that the sulfidity is high enough—it would be wrong to treat the kraft pulping as a homogeneous-phase system. Since a large part of the alkali is consumed immediately or almost immediately, it is an obvious possibility that some of the alkali has been consumed before impregnation is complete and that there exists an alkali concentration gradient in the chips. This means a danger of non-uniform cooking from different parts of the chips and different chip thicknesses. A dependence of the rate of cooking from the chip thickness has been well established (Fig. 6.24). Assuming that the chips can absorb 2 m3 of liquor per ton o.d. wood and have a moisture content of 50%, this means that 1 m3 of chip moisture and 1 m3 of white liquor can be absorbed, disregarding steaming condensate. That volume of white liquor of normal strength will contain 110-120 kg active alkali, of which about 80 kg will be consumed directly and 40 kg will be left over for the genuine cooking. Another 60-80 kg active alkali/ton o.d. wood will have to diffuse into the chips, and run the risk of being consumed on the way in, thus overcooking the exterior and undercooking the interior of the chips. Since the diffusion paths are normally only 1-2 mm to the interior from the outside, the effects are not catastrophic at normal chip thickness and normal chemical pulp grades. With nonuniform chips or with semichemical kraft pulp for liner board, the heterogeneity becomes quite noticeable and may limit the maximum yield level. This makes various systems for impregnation pretreatment of interest, in addition to the steaming and pressure-impreg nation treatments already described in connection with the sulfite pro cess. The figures quoted are valid for Scandinavian softwoods. The heavier southern pines absorb less liquor, and the diffusion of alkali into the wood becomes more important, whereas some of the lightweight soft- .woods of Canada may be able to absorb the entire alkali demand by good impregnation with white liquor of normal concentration. The properties of spruce kraft pulps at varying yield and process modi fications are shown in Fig. 6.25, together with the properties of sulfite 94 Lecture 6

Fig. 6.24. The charge of effective alkali required to reach Roe number 6 using different chip thicknesses and cook ing temperatures.

process variants and groundwood. It is seen that kraft pulp gives the strongest combination of the controversial tensile and tear strength prop erties. A strength maximum occurs at around 50% yield. Further cook ing reduces tensile and improves tear strength, largely along the beating curve, meaning that the additional lignin and hemicellulose removal made the pulp less easily beaten and less easy to give paper bonding at a certain drainage and porosity level. The same effect, though still more pro nounced, is obtained by additional hemicellulose removal with the prehydrolysis-kraft process. Increasing the yield by limiting the delignification will cause a decrease in strength, finally approaching the groundwood region. It is rather striking that kraft pulp at 75% yield has lost half its tensile strength but comparatively little tear, whereas a bisulfite pulp at 75% yield has lost only somewhat more than 10% of each property, as compared to the maximum strength yield level. Increasing the carbo- Kraft Cooking Process Theory

100 120 140 160 180 200 220 240 TEAR FACTOR Fig. 6.25. Tensile-tear strength relation in the 25-45°SR region for spruce pulps of various degrees of delignification from various processes. Numbers indicate pulp yield. hydrate yield of the kraft process by polysulfide or borohydride additions keeps up the tensile fairly well, but at the expense of tear strength. All those changes reflect the influence of the hemicelluloses to improve the paper bonding characteristics, at the same time tending to make the paper more sensitive to locally applied stresses, such as in the tear test. The figure also illustrates the strength advantage of kraft pulps as compared to acid sulfite, and the insufficiency of the sulfite process modifications to meet the strength demands that the market is used to put up because of kraft pulp. This picture would not be complete without showing the influence of wood species on kraft paper pulp strength (Fig. 6.26). Two main con clusions can be drawn from that figure and comparisons with the previous one. One is that hardwood kraft represents a widely varying quality con cept, with the poor members being filler pulps of little strength and the better ones approaching or surpassing spruce sulfite in strength. The same picture is obtained when bleached grades are compared and also when opacity is introduced as one of the quality criteria. The other main con clusion from the figure is the influence of softwood density, i.e., fiber wall thickness or summerwood content, on kraft pulp strength. A lightweight softwood, as grown in Scandinavia or Canada, gives a better paper bond- 96 Lecture 6

Fig. 6.26. Tensile-tear strength relation in the 25-45°SR region for kraft pulps of various wood species. Numbers at each curve indicate basic den sity/average fiber length for the respective wood species. ing, with a higher tensile and lower tear, than a heavier softwood such as slash pine. In general, these differences move along the general beating curve for softwood kraft, meaning that a southern pine kraft, beaten to the extreme, reaches similar paper characteristics, aside from porosity, as a light-beaten Scandinavian or Canadian softwood kraft. This terminates the process background for kraft pulping. As in the case of sulfite, it has had to be a rather sketchy one, serving as an introduc tion to the presentation of efforts on continuous cooking rather than to give a complete coverage of what is known today on the subject. Lecture 7

Continuous Conventional Kraft Cooking

This is a field where some of you may have considerably more experi ence than I have. I shall limit my presentation here to the quality experi ence we have had in our 150 tons/day digester at Gruvon, recently ex panded to 250 tons/day, and some experience from Jossefors, where the pilot digester was run on a few wood species, mainly as control runs when trying process modifications. I shall also add some information given to me by Kamyr, regarding the quality investigations made at other mills and laboratories, as well as some statistical information on digester per formance etc. In conventional kraft cooking there is now 20 years of mill experience with continuous cooking. We thus ought to know the quality we get and the reliability of the system. Yet it is not very easy to get objective comparisons. Not often has the pulp from a local digester been compared either to mill batch or laboratory batch pulp on identical raw material and comparable conditions. Gruvon is one of the places where this has been done. The objection against those data is, however, that Gruvon has one of the early digesters, which in spite of improvements lacks some features of a modern digester, especially the countercurrent digester wash, which ensures the best and most uniform cold blow. The Gruvon system is shown in Fig. 7.1, as it was originally and as it has been modified—except ing the last reconstruction, which was mainly an elongation of the digester body to allow a larger capacity. Instead, the final development of the cold blow, the countercurrent washing, is shown. This system is now standard and will be installed in the next 1000 tons/day digester at Gruvon. The Gruvon digester was designed in 1956 and started in spring 1957. It had a volume at the delivery of 145 m3 and is now 177 m3 by means of increased height from 20 to 24.5 m. The nominal capacity at delivery was 150 tons/day, it was run at 120 tons/day during the first few years and is now making up to 250 tons/day. The digester was run with a hot blow for one year. Limited bottom cooling was introduced in Janu ary 1958, with black liquor charged at the bottom connections. This 97

Continuous Conventional Kraft Cooking 99

brought the blow temperature down from 170 to 145°C. In August 1958, the bottom connections were prolonged inwardly to facilitate the cooling. Also there were some adjustments on the bottom scraper blades and reductions in the scraper speed from 3 to 2 rpm, then to 1 rpm, which was found to give discharging difficulties and again increased to 1.6 rpm. In Spring 1959, a screw press was installed between digester and blow tank to improve the black liquor recovery, and in February 1960, the cold blow system of strainers and flash tanks was introduced, allowing blow temperatures of about 100°C. With suitable intervals during the first five years, pulp samples were analyzed and tested for paper properties, whereby these changes in the system were reflected. Contemporary sampling of the pulp from the batch digesters, run on the same raw material, was done for similar test ing. The batch digesters have a volume of 85 m3 and run a straightfor ward kraft cook at 170°C and are blown at 5.5 atm. To complete the.

Table 7.1. Survey of Kraft Pulp Quality of Continuous and Batch Cooking Systems in Gruvon, 1957-68

Year and cooking system

1957 1958 1959 Batch Cont. Batch Cont. Batch Cont.

Speed of bottom scraper, rpm 3 2 1.6 Discharge temperature, C 170 145 145 Screw pressing + Number of tests per average 20 20 59 59 33 33 Paper properties at 45 SR Beating time, min 59 62 60 60 55 58 Tensile strength, km 10.3 9.9 10.5 10.4 10.1 9.9 Tear factor 114 101 114 102 111 97

1960-61 1967 1968 Batch Cont. Batch Cont. Batch Cont.

Speed of bottom scraper, rpm 1.6 1.6 1.6 Discharge temperature, C 100 100 100 Screw pressing + + + Number of tests per average 34 34 40 39 . 18 19 Paper properties at 45°SR Beating time, min 61 66 57 69 48 57 Tensile strength, km 10.5 10.6 10.0 10.0 10.2 10.2 Tear factor 112 113 124 122 121 125 Table 7.2. Kraft Pulp Quality at Various Stages of Processing, Hot or Cold Blow

Paper properties at 25 SR

Discharge Beating Tensile Sheet temp., Sampling time, strength, Tear density, °C station mm km factor g/cm3

145 Above scraper 39 9.5 131 After discharge 39 9.4 114 Batch pulp 39 9.5 125 100 Above scraper 42 10.2 144 0.67 After discharge 47 9.6 134 0.65 Batch pulp 39 9.6 132 0.67

Paper properties at 45 SR

Beating Tensile Sheet time, strength, Tear density, min km factor g/cm

Above scraper 10.4 119 After discharge 10.2 102 Batch pulp 10.3 113 Above scraper 11.3 129 0.71 After discharge 10.8 119 0.70 Batch pulp 10.6 119 0.73

information, some laboratory cooks on the same chips were performed, and further sampling was done at several places in the digester discharge system, namely above the bottom scraper, in the discharge line prior to the disk strainer, and before and after the screw press. Table 7.1 summarizes the results of the comparative testing of batch and continuously cooked pulp over the period. At a discharge temperature of 170°C there was a considerable strength loss as compared to batch. Al most the same loss was experienced at 145°C. At 100°C blow tempera ture, the quality appears to be well up to that of batch pulp. Table 7.2 shows the results of testing samples taken in the discharge system at blow temperatures of 145 and 100°C. Again, the strength loss in discharging is considerable at 145 and less severe at 100°C. The pulp above the bottom scraper appears to be a shade stronger than the batch pulp, enough to cover the slight strength loss in the discharge. A more extensive paper testing on the various pulp samples and also on laboratory pulp is shown in Table 7.3. Continuous Conventional Kraft Cooking 101

Table. 7.3. Kraft Pulp Quality at Various Stages of Processing; Extended Testing Discharge Temperature, 100°C

Paper properties at 25 SR

Beating Tensile Stretch Sheet Air Sampling time, strength, at break, Tear density, resist., station nun km % factor g/cm3 sec/100 ml Above scraper 38 9.9 4.7 139 0.67 29 After discharge 41 9.9 4.7 136 0.67 31 After screw press 42 9.5 4.9 134 0.65 26 Batch pulp 35 9.5 4.8 138 0.66 34 Laboratory pulp 31 9.6 4.5 146 0.67 31

Paper properties at 45 SR

Beating Tensile Stretch Sheet Air time, strength, at break, Tear density, resist., min km % factor g/cm3 sec/100 ml Above sciaper 58 11.3 4.9 124 0.72 335 After discharge 64 11.1 5.0 123 0.72 300 After screw press 65 10.8 5.1 119 0.71 270 Batch pulp 58 11.0 5.1 121 0.72 390 Laboratory pulp 54 10.9 4.9 130 0.73 400

These trials clearly demonstrated the necessity of the cold blow and showed that the continuously cooked pulp was up to mill batch pulp standard. They also indicated the possibility of surpassing it and reaching the laboratory pulp standard, and this has been realized elsewhere in con nection with the countercurrent washing, where the cooling of the pulp to blow temperature is more uniform than it can be in the rather primitive cooling system of Gruvon. We also sought to find out the reasons why the hot blow gave an inferior pulp quality. Table 7.4 shows the results of paper testing, fiber length measurements, chemical analysis, and DP determination of pulp and hemicellulose fractions, of the samples taken before discharge, after hot and semi-hot blows, and from batch cooking. No evidence whatsoever can be seen on any measurable property of those usually having an influence on paper strength. It is therefore concluded that the damage done by the hot blow is restricted to very limited regions of the fibers, such as the so-called slip planes of the fiber wall in the superstructure of cellu lose. The agreement with the previously reported experience on the hot blow of other pulp types is striking, as compiled in Table 7.5. Table 7.4. Effects of Hot Blow on the Pulp Carbohydrates in Continuous Kraft Pulping

Discharge temperature, C 20 145 170 Batch Paper properties at 45 SR Beating time, min 57 60 54 61 Tensile strength, km 10.5 10.2 9.8 10.3 Tear factor 117 •103 91 119 Fiber length, length average, mm 1.8 2.0 1.9 1-9 Analysis Viscosity, cp TAPPI 77 83 75 81 Alpha-cellulose content, % 87.3 86.8 86.7 86.7 Rio, % (nonsolubles, 10% NaOH) 87.1 86.6 86.3 86.5 ft o, % (precipitable solubles) 5.7 2.7 3.8 2.7 7io, % (nonprecipitable solubles) 8.6 10.2 9.9 10.0 DP of fro 141 180 142 DPof710 182 115 108 113 Pulp composition, % Cellulose 78 78 78 Glucomannan 7 8 8 Glucuronoarabinoxylan 15 14 14 0io composition, % Cellulose 4 4 4 Glucomannan 9 7 7 Glucuronoarabinoxylan 81 79 81 7i o composition, % Cellulose 0 0 0 Galactoglucomannan 30 31 31 Glucuronoarabinoxylan 70 69 69

The Jossefors experience of normal kraft is confined to a few runs on pine, birch, and eucalypt, which are of interest only so far as they have been performed with countercurrent washing and have been compared to laboratory cooks. Table 7.6 shows that in these cases, laboratory stan dard has been reached for all wood species. This confirms the con clusion that the standard system for continuous kraft cooking gives a satisfactory performance and quality. All experience on modified kraft cooking, whether modified in the cooking process or in the impregnation, will be dealt with in the two subsequent lectures. The digester performance is of great importance for the production effi ciency in a pulp and paper mill. Many claims have been made over the years that the continuous digesters endanger the reliability of the produc tion. Such claims were indisputable in the early days, before the machine elements had been sufficiently developed and enough running experience gathered. Likewise, every new mill with a new crew has to learn to run Continuous Conventional Kraft Cooking 103

Table 7.5. Development of Paper Pulp Quality in Continuous Cooking Using Various Processes

Paper properties, 45°SR Beating Tensile Discharge time, strength, Tear Pulping process Digester temp., C min km factor

Neutral sulfite, birch Cont., 1-body 160 7 8.0 57 (bleached grade) Cont., 1-body 130 9 8.6 62 Cont., 1-body 110 9 9.6 68 Batch, mill 6 9.4 70 Kraft, pine Cont., 1-body 170 62 9.9 101 Cont., 1-body 145 60 10.4 102 Cont., 1-body 100 66 10.9 117 Batch, mill 61 10.5 114 Bisulfite-acid sulfite, Cont., 2-body 120 20 7.5 59 spruce Cont., 2-body 50 22 9.0 68 Batch, mill 20 8.7 66 Batch, lab. 23 9.5 75 Neutral sulfite-acid Cont., 2-body 120 16 8.1 49 sulfite, spruce Cont., 2-body 50 17 8.3 55 Cont., Mumin 50 19 8.4 66 Batch, mill 21 9.3 53 Batch, lab, 14 8.6 64 Bisulfite, spruce Cont., 2-body 120 25 8.8 62 Cont, 2-body 50 24 10.4 65 Cont., Mumin 50 29 9.8 85 Batch, mill 28 10.3 75 Batch, lab. 27 10.5 86 their digester, as well as their paper machines. Many problems have also come up because the continuous one-line production requires a different approach to maintenance than a batch system with several digesters and chests. Systematic preventive maintenance is vital to the success of a continuous digester. By experience and skill, several mills have achieved a digester performance of 99%. In Gruvon, with the pulp mill attached to a paper mill, the average downtime of the continuous kraft digester during recent years has been about 5%, of which 3% occurred because of the paper mill, power failures, or steam shortage, 0.5-1% because of mis cellaneous disturbances, and 1-1.5% because of actual digester troubles. No major changes have been made in the machine parts. Low- pressure and high-pressure feeders are worn out in about two years, which appears to be general experience. Table 7.6. Kraft Pulp Quality from Pine, Birch, and Eucalypt in the Jossefors Pilot Digester, as Compared to Laboratory Pulping

Wood species

Pine Birch Eucalypt Pinus silvestris Betula verrucosa Eucalyptus globulus

Labo Pilot Pilot Labo Pilot Pilot Labo Pilot Pilot Pulping operation ratory hydraulic Mumin ratory hydraulic Mumin ratory hydraulic Mumin

Roe chlorine No. 5.2 5.4 5.1 3.0 2.2 3.0 2.2 2.2 2.6 Brightness, % SCAN 24.3 27.1 30.6 32 35.9 34.8 35.3 42.3 37.8 Resin content, % 0.34 0.18 0.19 0.9 0.51 0.51 0.27 0.40 0.25 Carbohydrate analysis, % Glucose 84.4 84.2 82.9 73.7 73.1 73.2 81.2 80.1 80.9 Mannose 6.2 7.5 9.1 0.5 0.0 1.7 0.3 0.3 Xylose 8.7 8.3 7.6 25.8 26.9 25.2 18.8 19.6 18.8 Paper properties at 25 SR Beating time, min 41 53 43 16 14 17 18 14 17 Tensile strength, km 9.5 9.9 9.4 7.0 6.6 7.1 5.7 5.5 5.4 Tear factor 156 143 149 88 90 103 95 93 100 Paper properties at 45 SR Beating time, min 66 82 67 25 26 31 37 28 33 Tensile strength, km 10.8 11.0 10.8 8.6 8.9 9.5 8.6 8.1 7.9 Tear factor 130 129 128 73 81 87 105 106 103 Lecture 8

Continuous Prehydrolysis-kraft Cooking

Our interest in rayon pulp has resulted in development work not only in continuous acid sulfite cooking but also in continuous prehydrolysis-kraft cooking. The advantages of the latter process are little sensitivity to wood species, better economy in producing the highly purified grades, quality advantages in the DP distribution of rayon cord pulp, and finally, the possibility of producing kraft pulp as the paper grade alternative of the mill. The principles of the prehydrolysis-kraft process were developed around 1930 in America, carried to mill production in Germany during the war, and resulted in large-scale production after the war in Sweden, USA, Russia, Japan and India. The theory behind the process was presented previously. The purpose of the acid prehydrolysis is to degrade the hemicelluloses with the least possible damage to the cellulose. The degradation partially dissolves the he mi celluloses in the prehydrolyzate, but also leaves shortchain molecules, which are efficiently degraded by peeling in the kraft cook. Whereas a straight kraft cook results in a final bleached pulp yield of 43-53%, depending on wood species, prehydrolysis-kraft yields 35-40%. Our experience on continuous prehydrolysis-kraft is limited to water and steam hydrolysis. Steam hydrolysis gives essentially the same effect as hydrolysis in liquid water, but there exists a series of practical differences which shall be commented on subsequently. The prehy drolysis used to be performed with dilute mineral acid in the first mills, but subsequent mills have favored water hydrolysis, i.e., hydrolysis at 160-180°C in water, whereby an acidity of pH 3-4 is developed by the wood constituents, particularly by acetic acid from the acetyl groups. The first company to study continuous prehydrolysis-kraft cooking with a Kamyr digester was the Italian SAICI. In the early 1950's, it initiated experiments with a small two-body unit in its Torviscosa dissolving pulp mill. This unit had a capacity of 1 ton/day and consisted of one upflow and one downflow tower of different pressures, separated by a rotary pocket feeder. As with the first continuous acid sulfite digester at Domsjo, this one also proved to be too small to give reliable experimental conditions. The initial Jossefors two-body digester was a simplified ver- 105 106 Lecture 8

Fig. 8.1. Two-body continuous digester for prehydrolysis-kraft cooking. sion of this unit, with both digester bodies operated at the same pressure (Fig. 8.1). The reason for having two digester bodies was to facilitate the operation of a two-stage process by maintaining well-defined, separate cooking stages. After presteaming, the chips were fed, together with water, by the con ventional feeding system into the upflow digester. Liquor withdrawn from the upper part of the digester and recycled just above the bottom screw helped to move the chip column upward, thereby reducing the load on the screw. This circulation was also heated to the desired temperature for prehydrolysis. The prehydrolyzed chips with their hydrolyzate were transferred over to the downflow digester by the top scraper, and then mixed with white liquor. The chips were then digested by the kraft process and the pulp was discharged according to the cold blow method. The movement of the chips in the upflow vessel was problematic. It was quickly discovered that prehydrolysis of wood species such as pine and birch resulted in pitch deposits on the strainers of the lifting circula tion, which plugged after 3-6 days of continuous operation. This proved to be crucial for the whole system—although an alkaline wash would re move the pitch deposits, a safe continuous operation was obviously pre vented. Experience accumulated from other cooking processes indicated Continuous Prehydrolysis-kraft Cooking 107

H.P. STEAM

Fig. 8.2. Downflow, liquor-phase continuous digester for prehydrolysis- kraft cooking. that there were other disadvantages with the upflow digester, as already described. Therefore, the system was redesigned. Experience with the two-body digester had proved that it was easier than anticipated to keep two different stages apart. Therefore the digester was reconstructed to the conventional downflow Kamyr system. The new digester (Fig. 8.2) was higher than the two-body system and was equipped with more strainers and central pipes than a standard kraft digester. The strainers and central pipes were arranged to allow cooking in two stages, followed by countercurrent washing. The digester volume was 27 m3 and had a capacity of 8 tons/day of airdry prehydrolysis-kraft pulp. This volume gave a total retention time of 5.5 hr, of which 1 hr was for heating to prehydrolysis temperature, 1 hr prehydrolysis at maximum tempera ture, 2 hr kraft cooking and 1.5 hr washing. The new system was similar in principle to a conventional kraft digester with a downward chip flow, and no trouble was expected with the chip movement. The main concern was whether the prehydrolysis and kraft cooking stages could be effectively separated and if deposits of pitch would occur. The system was tried separately on pine and eucalyptus using essentially the same prehydrolysis and kraft cooking con ditions. Chips and water for prehydrolysis were fed into the top of the 108 Lecture 8 digester, using the conventional Kamyr feeding equipment. After a period of gradual temperature rise, the digester was heated to full prehydrolysis temperature by indirect heating in the upper circulation zone. This temperature was maintained until the chips reached the second circulation zone, where the acid prehydrolysis liquor was neutralized by alkaline liquor for the kraft stage. The kraft cook was carried out either concurrently or countercurrently. In the concurrent cooks all the white liquor was added to the second circulation zone that formed the transition point between the pre hydrolysis and kraft stages. This circulation zone was also used for heat ing to cooking temperature. When cooking countercurrently, the white liquor was not introduced until the third circulation zone. Later, a modi fication was incorporated, wherein the white liquor was divided into two parts, with one part added at the second circulation zone and one part at the third zone. The total liquor flow, including dilution from the washing stage, was extracted at the second circulation zone and thus the liquor and chips moved countercurrently against each other below that zone. The concurrent as well as the countercurrent kraft cooking stages were fol lowed by a countercurrent high-heat wash and the prewashed pulp dis charged according to the cold blow method. The separation of the prehydrolysis and kraft cooking stages proved to be quite effective as long as there was a certain flow of free liquor from the prehydrolysis into the kraft cooking stage. The most favorable liquor-to-wood ratios achieved in the prehydrolysis were 3.0 for pine and 3.5 for eucalyptus. The reason for the higher liquor ratio in the latter case was an increased tendency for carbon dioxide evolution in the pre hydrolysis stage. This caused a flow of hot liquor to the top of the digester and resulted in disturbances in the feeding circulation. By intro ducing more water, these disturbances were avoided. An attempt was made to extract the prehydrolyzate before mixing with alkaline liquor, but when doing so the flow of free liquor into the kraft stage was reduced and the black liquor level raised. This caused precipitation of alkali lignin on the extraction strainer and in the pipelines, clogging the system. Deposition of pitch on strainers and in the heat exchanger of the pre hydrolysis stage proved to be about as extensive for the new system as for the two-body digester. Though in the downflow system the chip move ment was not disturbed, the pitch deposits reduced the liquor flow in the heating circulation and impeded the rate of heat transfer. Difficulty was thus encountered in maintaining the desired prehydrolysis temperature. The pitch problem was partially solved by using wider strainer slots and by direct steam heating in the prehydrolysis circulation. These innova- Continuous Prehydrolysis-kraft Cooking 109

tions permitted sufficiently long operation to allow considerable studies of the process and pulp quality, but presented no final solution. Another complication was pitch migrating into the feeding circulation, particularly with hardwoods. This necessitated the addition of considerable amounts of alkali to that circulation in order to prevent pitch deposition in the high-pressure feeder. The alkali added affected the pH of the prehydrolysis and thus the pulp quality. Considerable interest centered on the possible advantages of countercurrent cooking versus concurrent cooking. According to earlier experiences, countercurrent kraft cooking seemed to give a somewhat higher dissolu tion of xylans which might prove to be an advantage for dissolving grades. Also, in concurrent cooking a great deal of the active alkali was found to be consumed directly in the heating zone, where the white liquor was added. This gave a low concentration of chemicals which most likely caused deficiencies in the chemical distribution, particularly to the interior of the thicker chips. As a consequence, high percentages of screen rejects were obtained even though the pulp was of a low lignin content. In contrast, countercurrent operation resulted in a pulp with low screenings. These results are most likely explained by the difference in active alkali concentration gradients in concurrent and countercurrent cook ing. Figure 8.3 shows the concentration of active alkali in the liquor measured at different points in the cooking and washing zones. The curve for countercurrent cooking goes from a low concentration at the transition point between the prehydrolysis and kraft stages to a very high concentra tion at the end of the cooking zone. The corresponding curve for con current cooking starts at a medium concentration at the transition point and goes down to a low residual active alkali content at the end of the cooking stage. It is obvious that these differences must have a certain effect on the distribution of active alkali in the inner parts of the chips and also influence the rate of delignification. The latter is well illustrated by the cooking temperatures that were necessary. At the same production, a higher alkali charge and the same final pulp viscosity, concurrent cooking demanded 170°C in contrast to only 155°C for countercurrent cooking. Under certain conditions of countercurrent cooking there was a ten dency of too low an alkalinity at the transition point between the prehy drolysis and kraft cooking stages. This resulted in the precipitation of alkali lignin, which clogged the strainer plates and pipelines. To maintain control of the alkalinity it was found suitable to split the white liquor charge into two flows, one part added to the transition circulation and one part to the lower cooking circulation. This gave good control of the pH 110 Lecture 8

Fig. 8.3. Concentration of active alkali in liquor vs. digester height: , concurrent kraft stage, 25% active alkali (NaOH) on moisture-free wood added at A; —O— countercurrent kraft stage, 22% active alkali (NaOH) on moisture-free wood added at B; - A -, countercurrent kraft stage, 15% active alkali (NaOH) on moisture-free wood added at B, 7% active alkali (NaOH) on moisture-free wood added at A. of the extracted liquor and prevented any tendency to clogging, and yet the advantages of countercurrent cooking were preserved. Although operation of the downflow, liquor-phase system was managed under comparatively steady conditions, the result was not satis factory. Deposition of pitch, flow disturbances by gas evolution, and the relatively large amounts of water required to ensure good separation be tween the two stages, were all disadvantages which could render a full-scale operation difficult, uneconomical and not completely satisfactory from a quality standpoint. During the experiments with the two-body digester, it had been visualized that prehydrolysis-kraft cooking could be carried out in a downflow digester, where the prehydrolysis was to be performed in the vapor phase and the kraft cook in the liquor pnase. Such a cooking method would give an efficient separation of the two stages and result in a simple opera- Continuous Prehydrolysis-kraft Cooking 111

Fig. 8.4. Downflow, vapor-liquor-phase digester for prehydrolysis-kraft cooking. tion. The great problem was how to feed the chips into the digester. Al though there were several vapor-phase feeders, none was very satisfactory from all aspects such as operation, maintenance, and steam econ omy. The balanced liquor-phase feeder used in conventional kraft cook ing is superior in most respects. Furthermore, any prehydrolysis-kraft digester should preferably also be suitable for the production of high quality kraft pulp for paper grades, for which the standard high-pressure feeder is desirable. The final answer to these demands proved to be an inverted top sepa rator placed outside the digester shell. Figure 8.4 shows this digester version, the "Mumin" system, arranged for prehydrolysis-kraft cook ing. The feeding system remains the conventional one, and so does the digester shell, although a separate circulation for heating the prehydrolysis is no longer needed. What has been altered is the design of the digester top, as described earlier. As the liquor level is controlled at a point below the overflow, from the external, inclined top separator, the chips are drained and do not carry into the digester more water than that which has been absorbed by the chips. The direct steam meeting the chips heats them very uniformly and al- 112 Lecture 8

most individually, in contrast to the danger of local overheating in batch vapor-phase cooking—one of the major reasons why the existing mills run batch liquor-phase cooking. The steam flow is controlled by the temper ature in the prehydrolysis stage. Since the need of a prehydrolysis heating circulation is thus eliminated, so is the danger of resin problems there. The instant heating to maximum prehydrolysis temperature results in a digester volume not appreciably larger than that of a conventional kraft digester, since the prehydrolysis zone corresponds to the zone of gradual temperature rise in the conventional digester. Therefore the system has a great versatility and can be used for either rayon grade pulp or kraft paper pulp, with slight alterations. The liquor level can be kept at an arbitrary height in the prehydrolysis stage; but preferably it should be slightly above the transition circulation to ensure an efficient separation of the two stages. The cooking can be performed either concurrently or countercurrently and be followed by countercurrent washing. The main interest when starting the cooking trials after the last recon struction was to study the behavior of the inverted top separator and the combined vapor-liquor phase operation. After a slight adjustment of the top separator screw, this arrangement worked satisfactorily, with little or no heat flow from the vapor phase back to the feeding circulation. The liquor level in the top separator could be easily controlled, and no more liquor was carried over to the digester than what had been absorbed by the chips. Since the feeding liquor was not in contact with the hydrolyzate, there were no significant pitch troubles in the high-pressure feeder, in contrast to the previous systems. The heating with direct steam went smoothly and the carbon dioxide evolved caused no trouble, since it occurred in vapor phase. In fact, the C02 gas served as a pressure controller, since pressure in excess of the steam pressure is desirable for undisturbed liquor circulation in the kraft cooking zone. Thus there is no need for maintaining the same or higher temperature in the prehydrolysis than in the kraft stage, leaving the choice of temperature above a minimum level entirely to quality considera tions. The desired digester pressure is maintained by relief of excess gas through a control valve. The chip flow through the vapor phase zone presented no problems, and the pitch deposits on the digester walls in the prehydrolysis zone were inconsequential in the absence of liquor circulation. The transition from prehydrolysis to kraft cooking caused no particular problems. With countercurrent cooking, the addition of white liquor was done, as before, in two parts with about half added at the transition circulation and half at Continuous Prehydrolysis-kraft Cooking 113

the lower cooking circulation. By this method the pH of the extracted mixture of prehydrolyzate and black liquor could be carefully controlled and any precipitation of alkali lignin on the strainers avoided. The filtrate from a subsequent washer was introduced into the digester bottom as a wash liquor for the countercurrent wash. The disadvantage of having to heat this wash liquor to the full temperature of the countercurrent cook is partially offset by the increase in flash steam from the extracted black liquor. This steam can be used for presteaming and hot water preparation, the same as in conventional kraft cooking. A further compensation is the lower maximum temperature required with countercurrent cooking. The black liquor volume extracted should correspond to the sum of water in the chips, direct-steam condensate, white liquor addition, and dilution in the washing. Because of the small quantity of liquor used in the prehydrolysis stage, the extracted liquor has a comparatively high concentration of dissolved solids. When cooking eucalyptus, the liquor to the evaporators will have a solids content of 17-19%, depending on the white liquor strength, dilution of washing, etc. Heat economy is good with the downflow vapor-liquor phase system, and under normal conditions there will be no need for live low-pressure steam to the steaming vessel, since the flash steam from the extracted black liquor will be sufficient for the presteaming. The total high-pressure steam consumption for eucalypt prehydrolysis-kraft pulp will be about 1.4 ton/ton airdry pulp. About 40% of this quantity will be available as flash steam for hot water preparation. The consumption of high-pressure steam can be reduced further by preheating the white liquor with flash steam or with the extracted black liquor. The flow of the chips through the digester was studied by the conven tional radioactive tracer technique in order to determine the uniformity and retention in the various zones (Fig. 8.5). At a feeding rate of 25 tons of moisture-free wood per day, corresponding to a production of 11 tons a.d. pulp, the retention times were 2.5 hr in the prehydrolysis zone, 3.0 hr in the kraft cooking zone, and 2.5 hr in the washing zone. A 50% increase in the rate of feeding with a corresponding change in cooking conditions gave a reduction in retention to about 1.5 hr prehydrolysis, 2 hr kraft cooking and 1.5 hr washing. The continuous digester purchased by Bil- lerud for the Leirosa rayon pulp mill in Portugal has been designed for approximately the same retention times as those last mentioned, in order to obtain reasonable temperatures in the process. During the initial trials with the vapor-liquor phase system the digester was started up by carrying out a batch kraft cook without preceding 114 Lecture 8

Fig. 8.5. Radioactive tracer experiments to determine retention time and packing density in the one-body prehydrolysis-kraft pilot digester.

'hydrolysis. The startup pulp became mixed in the pulp storage chest with the subsequent prehydrolysis-kraft pulp. Such a method, however, is not acceptable, since for quality reasons, "no admixture of paper pulp in the rayon grades can be allowed. To avoid any mixing of kraft pulp and prehydrolysis-kraft, it is desirable to start up the digester with prehy- Continuous Prehydrolysis-kraft Cooking 115

Table 8.1. Cooking Conditions and Quality Data for Eucalypt Prehydrolysis-kraft Pulp, Mumin Pilot Digester

Retention time, hr Prehydrolysis 2.5 1.5 Kraft cooking 3.0 2.0 Washing 2.5 ' 15 Temperature, C Prehydrolysis 166 170 Kraft cooking 155 158 Washing 135 135 Active alkali charge, % Na2O on wood To transition circulation 8.6 8.5 To cooking circulation 9.4 9.8 Total 18.0 18.3

White liquor sulfidity, % 30 30 Roe No. 0.7 0.6 Screenings, % on pulp 0.9 0.9 Viscosity, cp TAPPI 65 56 Rl8.% 97.8 97.5 R10,% 96.4 96.4 R, % (= alpha-cellulose) 97.2 97.0 Brightness, unbleached, % SCAN 48 49

drolysis. This was achieved by feeding the first chips into the empty digester at full vapor-phase prehydrolysis conditions. After reaching a certain chip level, white liquor which was preheated to cooking tempera ture was introduced to the digester bottom and the liquor level succes sively raised. The discharge was started at the moment the chip level reached the digester top. This startup method gave a rayon grade pulp from the very beginning, and any contamination by paper grade kraft pulp was avoided. After the initial adjustments in equipment and operation, and a limited trial period with three wood species, a reUability test was performed to investigate the operation efficiency. During a test period of one month, the digester was in operation more than 99% of the time, disregarding a short stop because of a pipe welding failure. The remaining 0.8% down time was caused by tramp iron. Considering that this reliability test was carried out with a new system, the results must be regarded as quite satisfactory. Subsequent operation of the pilot digester for over half a year to produce trial quantities for customers fully confirmed these con clusions, and this can also on the whole be said about the commercial operation of the 300 ton/day digester in Portugal, after some necessary 116 Lecture 8

Table 8.2. Cooking Conditions and Quality Data for Prehydrolysis-kraft Pulps from Pine, Birch, and Eucalypt, Mumin Pilot Digester and Laboratory Cooking. Retention Times Not Identical

Prehydrolyzate withdrawn before charging white liquor. adjustments caused by scaling-up factors. Digesters have also been or dered for USA and Russia with identical design but twice the size. Table 8.1 shows digester conditions and quality data for eucalypt pre hydrolysis-kraft pulp produced at two different rates of production. The retention times were fairly long, though not excessive. The comparatively low temperatures in the kraft cooking stage indicate that the volume of this zone can be reduced. The necessary charge of active alkali, 230 kg/ton o.d. wood, as NaOH (18% Na20), is rather high owing to the fact that the prehydrolyzate is not extracted separately. In many cases, how ever, it is an advantage to include the solids dissolved by the prehydrolysis stage in the black liquor. This will give more heat from the recovery furnace and also minimize the water pollution problem. The pulp quality was uniform and on a high level. The high alpha-cellu lose content of 97% obtained was partly due to severe hydrolysis condi tions and partly to the fact that eucalyptus is a wood species well suited for high-grade rayon pulp. The effect of different wood species on final pulp quality is illustrated in Table 8.2, which shows the result of similar trials on pine, birch, and eucalyptus. The quality data obtained are satisfactory and quite comparable to those of laboratory-cooked batch pulp. Possibly the countercurrent principle in the kraft cook gave a shade higher purity than the corresponding batch cook, though the main reason for maintaining it is the greater uniformity obtained from the kraft cook. One can therefore conclude that the new continuous cooking system Continuous Prehydrolysis-kraft Cooking 1

Table 8.3. Quality Data for Eucalypt Prehydrolysis-kraft Pulp, Produced in Mumin Pilot Digester, Without and With Cold Alkali Purification Incorporated in the System

developed produces a prehydrolysis-kraft pulp of good uniformity and high quality. One further possibility of increasing the pulp purity exists, which is unique to the system. The highest grades of cord pulps are sold at an alpha-cellulose content of above 98%. They are produced by more or less complicated methods of cold alkali purification. The alkali con sumption is generally economically prohibitive, or anyway a severe eco nomic burden. It is therefore sometimes attempted to use white liquor for the cold alkali purification and then to use the waste liquor from the purification in the cooking. Normally, however, elaborate machinery would be necessary to realize this idea at a reasonable heat econ omy. Continuous cooking, particularly countercurrently, in combination with countercurrent digester washing, is ideal to accomplish cold alkali purification in a simple and economical way. As already shown in Fig. 8.3, the alkali concentration realized in pure countercurrent cooking at the end of the cooking zone is above 60 grams/liter even with washing liquor introduced into the digester bottom. For an efficient cold alkali purifica tion, 80 grams/liter NaOH and 30°C are required. If the white liquor required for cooking is introduced into the blow line (Fig. 8.6), the blow tank becomes the logical purification tower and the washing of the pulp from white liquor can be performed on the normal filter wash. The white water from the filters is carried countercurrently to the digester bottom and is used in the washing and cooking zones for the kraft

Continuous Prehydrolysis-kraft Cooking 119 cook. That white water has to be cooled before the introduction to the digester base, to ensure proper cold alkali purification temperature, and then subsequently heated in the digester as usual between the washing and cooking zones. Likewise the fresh white liquor should be cooled to ensure 30°C in the purification. A less good heat economy and a slightly reduced pulp yield are the limited disadvantages to obtain the increased cellulose purity shown in Table 8.3. The system has thus been tested on the pilot scale and will soon be tried on the large scale. It constitutes an elegant example of how the continuous cooking system can be advanta geously utilized in combination with a subsequent operation. I have described the development of prehydrolysis-kraft cooking fairly elaborately, in spite of its specificity, partly because that was the process which initiated the development of the flexible Mumin digester system, and partly because several of its elements may become useful in other process applications. The reliable performance of the system further shows that once a continuous system has been worked out, one should not fear to run a series of seemingly complicated operations, such as steaming, liquor impregnation, vapor-phase cooking, countercurrent liquor-phase cooking, and countercurrent washing, or even cold alkali purification. Complexity is not a virtue in itself, but in order to achieve the goals of quality and economy it may sometimes be necessary to accept it, and to make it work. Lecture 9

Continuous Modified Kraft Cooking

The targets for the development work on modified kraft cooking have been, to carry out some chemical process modifications in the continuous pilot digester, and also to investigate how the ideal impregnation system of a kraft digester should be designed. The work has concerned both chem ical kraft pulping to a lignin content of about 4% and semichemical kraft pulping to a lignin content of 10-15%. The corresponding pulp yield ranges would thus be about 47 and 53-58%, respectively, for normal pine kraft and 3-5% higher in some of the modified processes. Pine was pre dominantly used for the trials, but some runs were also made on birch and eucalyptus. As always, these trials were preceded by careful laboratory investigations, but some of the process modifications, particularly those of rapid heating and of countercurrent cooking, are not easily simulated in the laboratory. The development work on prehydrolysis-kraft pulping just described had led to a digester essentially the same as the standard kraft cooking system, with a few extra features. The major deviation in the machinery was in the digester top, which gave instant heating to maximum temperature instead of the usual 0.5-1.5 hr of gradual temperature rise in the upper part of the digester. The question now was whether this digester would allow satisfactory production of kraft paper pulp, which is the desired second grade in a prehydrolysis-kraft pulp mill, and sometimes growing to be the primary grade. There was always the possibility of purchasing a second top separator and changing separators when changing pulp grades, but this solution was regarded as too complicated for current produc tion. The Mumin digester had also proved to be admirably easy to oper ate, and it was thus desired to maintain this system if acceptable from a quality standpoint. We therefore launched a program to investigate the kraft pulping of eucalypt with instant heating in the Mumin system, and extended that later to the pulping of birch and pine. Figure 9.1 shows the system arranged for kraft pulping. The chips and white liquor are thus charged to the feeding system, and only 1-2 min are allowed for impregnation before live steam is added at the digester top. 121 122 Lecture 9

Fig. 9.1. Concurrent cooking of kraft by the Mumin system.

Impregnation pressure is digester pressure, 8-9 atm. After cooking, the black liquor is withdrawn and washing countercurrently performed in the lower part of the digester. The pressure is controlled by the top relief valve and an air compressor, and the liquor level by balancing the waste liquor withdrawal and the wash liquor introduced into the digester bot tom. From the previously related success with this impregnation system on acid sulfite, bisulfite, and neutral sulfite pulping, it would seem fairly sure that it would work also on kraft. The impregnation problems have always been more accentuated in sulfite than in kraft pulping. However, for reasons discussed in Lecture 6, the impregnation problems are somewhat different in kraft cooking, since so large a quantity of the chemicals are rapidly consumed, and since normal concentration of cooking liquor does not allow the introduction of all chemicals needed for the cook, even with complete penetration of the chips with liquor. This means that with normal white liquor concentrations some liquor has to flow over to the digester with the soaked chips from the top separator, and that the chem icals of the liquor must diffuse into the chips during the retention of the chips in the cooking zone. If that diffusion is too slow in relation to the reactions at maximum temperature, cooking with instant heating may lead to nonuniform pulp. Continuous Modified Kraft Cooking 123

Table 9.1. Cooking Conditions and Analytical Data for Chemical Kraft Pulps Produced with the Normal Hydraulic System and the System with Instant Heating (Mumin). Pine, Birch, Eucalypt

Hydraulic system Mumin system

Pine Birch Eucalypt Pine Birch Eucalypt

The results of the kraft paper pulp cooking in the Mumin digester system with instant heating are shown in Tables 9.1 and 9.2 ( cf. also Table 9.4), in comparison with normal hydraulic cooking with a heating period. In the case of eucalypt and birch, only chemical kraft pulps were produced, and the composition, strength, and screen rejects with the Mumin system were found to be entirely equivalent to normal hydraulic cooking. This conclusion has been further corraborated by the preparation of several hundred tons of trial quantities of bleached eucalypt paper pulp in sefors and by the 500 ton/day operation at Leirosa, Portugal. In the case of pine, some differences could be detected, seemingly insig nificant, perhaps, but important enough in view of the size of the pine kraft pulping of the world. The carbohydrate composition of the pulps from the cooking with instant heating appears to be slightly different, with somewhat less xylan and somewhat more glucomannan, indicating about 124 Lecture 9

Table 9.2. Paper Properties of Chemical Kraft Pulps Produced with Normal Hy draulic System and System with Instant Heating (Mumin). Pine, Birch, Eucalypt Hydraulic system Mumin system

Pine Birch Eucalypt Pine Birch Eucalypt

2% lower cellulose content and 1% higher yield. As shown in the tables, there is hardly any significant difference in paper strength for chemical grades, but the few results from the semichemical pulp testing appear to indicate a shade lower strength with instant heating "Mumin cooking." The screenings of the chemical grade pulps at a Roe No. level of 5, or about 4% lignin content, appears to be about 2% for hydraulic cooking and about 3% for Mumin cooking. No additional shiviness could be de tected in the latter pulp type. Some trials in more rapid Mumin cooking, to utilize the extended cooking zone for a higher capacity, increased the screenings to 5%. These cooks, however, had to be performed not at a higher capacity, because of the limitations in receiving capacity of the pulp plant, but at a lower chip level in the digester, which may have influenced the results. At Roe No. 6, hydraulic cooking gave 2% screenings at a certain white liquor charge, Mumin cooking 6% screenings on pulp. A 10% increase in the white liquor charge compensated for this disadvan tage. For kraft pulps of somewhat higher lignin content, somewhat larger differences in screenings appeared. The refining energy of the semichem ical pulps was also quite similar, within the considerable experimental error, with possibly a shade higher consumption in the cooks with instant heating. One interesting aspect is the unusually high brightness of the Mumin pulps, particularly the chemical grade. Taken together, the results on pine cannot be said to have indicated any important advantages for Mumin cooking, other than a higher brightness and a slightly higher yield, but neither were there any great disadvan tages. The extra white liquor needed for equal screenings level must, however, be considered as a drawback. The subsequent results on coun- Continuous Modified Kraft Cooking 125

Fig. 9.2. Countercurrent cooking of kraft by the Mumin system. tercurrent cooking, polysulfide cooking, and preimpregnation cooking give some additional aspects on these problems. Countercurrent soda cooking has been practiced for several years in Aus tralia on eucalypt. It works well mechanically and is claimed to give no disadvantages in quality. It was also tried with kraft cooking on radiata pine in New Zealand, and the investigation carried out by us for Kamyr on those pulps indicated a certain decrease in yield, beatability, tensile and bursting strength, and an increase in tearing strength. This could be ex pected from a loss in xylan due to the higher alkalinity toward the end of the cook. The brightness of the pulp was improved, however, which was also to be expected from the elimination of any lignin reprecipitation to ward the end of the cook, through the higher alkalinity. In connection with the radiata pine pulp characterization, we also car ried out a few trials with countercurrent cooking in the two-body diges ter. This was not very well equipped for the purpose, and the project had to await the two reconstructions of the digester system. Trials were then run with countercurrent cooking both hydraulically and in the Mumin digester system. Both the pure countercurrent version and the modified, partial countercurrent process were tried (Fig. 9.2). In the latter process, the white liquor charge is split between the top and bottom, but the black liquor is always withdrawn at the top, thus maintaining a proper counter- 126 Lecture 9

Fig. 9.3. Countercurrent cooking of kraft by the conventional system.

current flow principle in the digester. The white liquor charge at the top has the function of preventing the pH from becoming too low from wood acids, which leads to blockage of the black liquor extraction strainer. This was the experience with prehydrolysis-kraft, and it was again con firmed with countercurrent normal kraft. With pure countercurrent kraft cooking, the chips are charged to the impregnation system together with black liquor only. In the convention al, hydraulic digester, the chips will then move in black liquor during the entire zone of impregnation, which can hardly be an advantage (Fig. 9.3). A system without that zone, i.e., the Mumin digester system, would in this case be a particular advantage. As shown in Table 9.3, pure countercurrent kraft cooking on pine gave a chemical pulp with normal screenings, relatively high brightness, and low content of hemicelluloses, particularly of xylan. This corresponds to 3-4% lower yield, based on the wood. With partial countercurrent cook ing the screenings, hemicellulose content, and pulp yield were inter mediate, whereas the brightness decreased considerably. The paper properties of the pulps were changed by countercurrent cooking toward decreased beatability, lower tensile and burst, and higher tear and folding endurance, just as expected from the lower hemicellulose content. The partial countercurrent cooking gave intermediate results. Continuous Modified Kraft Cooking 127

Table 9.3. Cooking Conditions, Analytical Data, and Paper Properties for Pine Chemical Kraft Pulps, Produced Concurrently and Countercurrently. In all cases instant heating in the Mumin system.

The countercurrent cooking trials with semichemical pine kraft pulp (Table 9.4), showed essentially the same results, i.e., 2-3% lower yield at equal lignin content, mainly through a loss of xylan, and a certain increase in tear strength. The decrease in tensile and burst strength was not observed here, however; but neither was there a significant brightness improvement. An advantage appeared to be an observed decrease in fiberizing energy with countercurrent cooking. Since the corresponding laboratory refining on washed semicooked chip samples gave no differ ences, the result is interpreted so that the semicooked chips after digester wash have a higher pH with countercurrent cooking than with concurrent 128 Lecture 9

Table 9.4. Cooking Conditions, Analytical Data and Paper Properties of Semi- chemical Kraft Pulps, Produced Concurrently and Countercurrently. Hydraulic cooking as well as Mumin cooking Hydraulic cooking Mumin cooking

Partial Partial Con- counter- Counter- Con- countercurrent current current current current

Retention time, hr

cooking, and that this facilitates the fiberizing. Partial countercurrent cooking gave an intermediate and rather insignificant yield decrease. Partial countercurrent cooking of birch to chemical kraft pulp gave a pulp with similar screening rejects and not noticeably lower in hemicelluloses than concurrent cooking, but with slightly less good paper strength. Continuous Modified Kraft Cooking 129

Fig. 9.4. Polysulfide cooking by the Mumin system. In general, it can be concluded that countercurrent kraft cooking of pine and birch offers little advantage over concurrent operation, and that a decrease in yield and sometimes in paper strength occurs. Partial coun tercurrent cooking gives intermediate results. The possibility cannot be excluded, however, that further trials, particularly with decreased charge of white liquor, might eliminate at least part of the yield loss. Polysulfide cooking appears at first sight be a straightforward kraft pro cess, with sulfur added to the white liquor to give polysulfide and thus the desired end group oxidation of the carbohydrates and an increased yield. That is the way it has been practiced so far in the few mill scale runs with batch digesters. In principle, therefore, the normal Kamyr system should be applicable to polysulfide cooking, and this was also our first approach at Jossefors. Polysulfide cooking with rapid heating was another obvious variant, and both were tried with chemical as well as semichemical pine kraft (Fig. 9.4). In order to economize with the sulfur addition while maintaining the yield improvement, laboratory trials have been made with two-stage poly sulfide cooking, essentially employing a polysulfide cooking stage at lower Table 9.5. Cooking Conditions, Analytical Data, and Paper Properties of Chemical and Semichemical Kraft and Polysulfide Pulps, Produced by the Hydraulic or Mumin Systems

Mumin cooking

Hydraulic cooking Chemical

Chemical Semichemical Polysulfide Semichemical Poly- Poly- Poly- Kraft sulfide Kraft sulfide Kraft 1-stage 1-stage 2-stage Kraft sulfide

Retention time, hr Impregnation 2.5 2.5 2.5 2.5 0.02 0.02 0.02 2.0 0.02 0.02 Cooking 2.0 2.0 2.0 2.0 2.2 2.2 2.2 2.4 2.0' 2.0 Temperature, C Impregnation 125 125 125 125 105 105 105 122 105 105 Cooking 168 171 155 150 165 158 156 151 152 147 Pressure, atm 10 10 10 10 9 9 9 9 9 9 Active alkali charge, %Na20 on wood 14.5 18.0 13.5 15.0 16.0 17.0 18.5 18.5 14.0 14.0 Sulfur charge, % S on wood 0 4.4 0 3.8 0 1.0 4.3 4.0 0 3.2 Roe chlorine No. 5.4 5.2 14.5 14.5 5.1 4.4 5.2 5.5 14.5 14.5 Screenings, % on pulp 2 4 3 4 4 1 Brightness, % SCAN 27.1 21.9 17.9 15.7 30.6 29.6 26.7 26.5 19.2 18.0 Resin content, % 0.18 0.18 0.38 0.38 0.19 0.15 0.20 0.27 0.37 0.35 Carbohydrate analysis, % Galactose ...... 1.1 1.2 0.2 0.6 0.6 0.8 1.2 1.6 Glucose 84.2 83.2 79.9 78.1 82.9 83.1 "81.3 79.9 79.4 76.0 Mannose 7.5 8.7 8.5 11.0 9.1 10.3 11.4 12.7 10.5 13.7 Arabinose ... 0.9 1.7 1.7 0.2 0.6 1.0 0.7 1.5 1.7 Xylose 8.3 7.2 8.8 8.0 7.6 5.4 5.7 5.9 7.4 7.0 Table 9.5 Cont'd. Cooking Conditions, Analytical Data, and Paper Properties of Chemical and Semichemical Kraft and Polysulfide Pulps, Produced by the Hydraulic or Mumin Systems

Mumin cooking

Hydraulic cooking Chemical

Chemical Semichemical Polysulfide Semichemical Poly- Poly- Poly- Kraft sulfide Kraft sulfide Kraft 1-stage 1-stage 2-stage Kraft sulfide

Cellulose content, % on carbohydrate 81.7 80.3 77.1 74.4 79.9 79.7 77.5 75.7 75.9 71.4 Estimated pulp yield, % on wood 47.3 48.1 55.0 57.5 48.5 49.0 51.5 53.5 56.0 61.2 Paper properties at 25 SR Beating time, min 53 39 37 35 43 42 35 44 34 34 Tensile strength, km 9.9 9.6 7.6 7.6 9.4 9.2 9.0 8.8 7.4 7.2 Tear factor 143 146 148 129 149 129 131 124 135 116 3 Sheet density, g/cm o 0.70 0.67 0.57 0.56 0.68 0.66 0.65 0.66 0.55 0.55 Paper properties at 45 SR Beating time, min 82 65 58 51 67 62 53 63 51 48 Tensile strength, km 11.0 11.2 9.0 9.0 10.8 10.4 10.4 10.1 8.7 8.4 Tear factor 129 126 130 113 134 116 116 110 117 100 Sheet density, g/cm3 0.76 0.75 0.63 0.64 0.74 0.72 0.72 0.69 0.64 0.62

CeEulose, % on wood Galactoglucomannan Glucuronoarabinoxylan Lignin Total 132 Lecture 9

temperature to stabilize the carbohydrates, followed by a normal kraft cook for delignification. Table 9.5 shows the results of all polysulfide cooking variants. In com parison with the results shown for normal kraft with the hydraulic and the Mumin systems, the carbohydrate analyses prove a yield increase due to better preservation of glucomannan. Hydraulic cooking, however, gave with 4% S on wood only rather insignificant yield increases, 1-2%; whereas with instant heating, better results were achieved, about 3% for chemical and 5% for semichemical pulps, in addition to the extra 1% obtained by the instant heating without sulfur. This is at least as good as can be expected from laboratory trials, which gave about 4% yield increase for 4% polysulfide sulfur added, both on wood basis. It is somewhat difficult to calculate the yield increases from carbohydrate composition figures. The yield increase due to glucomannan stabilization can always be calculated, but in addition there is normally by the same end group mechanism also an increased cellulose yield, 1-2%, wood basis, at normal polysulfide cook ing conditions. It can thus be stated that normal hydraulic kraft cooking will not be an ideal procedure to give the expected yield improvement on sulfur addition to polysulfide cooking, whereas cooking with instant heating in the Mumin system is. The cause of this difference is probably differences in the cooking liquor concentrations. Figure 9.5 shows the concentrations of alkali and polysulfide sulfur found in the hydraulic and the Mumin sys tems. The cause of the low concentrations in the top of the hydraulic digester is probably the thermal convection flow from the hotter cooking zone. This phenomenon is likely to be less pronounced in the large diges ters. One very encouraging observation is that the pulp brightness, which with polysulfide cooking normally suffers by as great a percentage as the yield is improved—probably because of carbonyl group formation in the lignin—is not lower with the Mumin polysulfide pulps than for normal kraft pulps cooked hydraulically. Thus, one of the objections to polysul fide cooking is eliminated by continuous cooking with instant heating. Another serious objection to polysulfide cooking is the decrease in strength properties, as shown previously for laboratory cooks. That de crease was also observed in these continuous cooking trials, where a yield improvement was achieved. However, this general strength decrease is no more serious than that, for each pulp application, it must be considered whether this change in paper properties is acceptable or not. As seen by the tables, the screenings also increase, as does the consumption of active alkali, when changing to polysulfide cooking; but these disadvantages can Continuous Modified Kraft Cooking 133

Fig. 9.5. Concentration of active alkali and polysulfide sulfur vs. digester height, hydraulic and Mumin systems. be met. More serious, perhaps, is the fact that the refining energy of semichemical polysulfide may be 200 kW-hr/ton in excess of that of kraft at equal lignin content, with some reservation for the accuracy of that determination. The third objection to polysulfide cooking is that the addition of sul fur in quantities of about 4% on wood tends to increase the sulfidity of the white liquor stock from 30 to 60%. This is undesirable from an econom ical standpoint, and although there are methods devised for the recovery of elemental sulfur from the reclaimed chemicals, such methods are costly in both investment and operation. It is therefore very desirable to per form the cooking in a way which achieves maximum yield improvement at a minimum sulfur addition. With the Mumin system a yield increase was obtained of about 2% above normal kraft, wood basis, at a polysulfide sulfur charge as low as 1% (Table 9.5). Such a low charge is feasible without any special recovery arrangements other than using carbonate instead of sulfate for makeup chemical. A still greater efficiency in the polysulfide utilization is desirable, however. One of the few possibilities of improving it would be to react wood with a polysulfide solution prior to the kraft cooking. To design a corresponding cooking system, the Mumin digester was used in the manner described below (see Fig. 9.6). 134 Lecture 9

Fig. 9.6. Polysulfide-kraft two-stage cooking by the Mumin system.

The feeding circulation was charged with a limited quantity of white liquor, corresponding to less than half of the total charge, or about 80 kg/ton o.d. wood active alkali, expressed as NaOH (6.2% Na2 0). That is a quantity corresponding to the amount normally consumed by the chips at feeding conditions, 100-110°C for 1-2 min, and leaves a feeding circula tion liquor essentially consisting of sulfide-hydrosulfide. This solution is ideal for dissolving sulfur to polysulfide, and it can be bled off to a sulfur dissolving tank at the same rate as polysulfide solution is fed to the sys tem. It is also possible, however, to dissolve the sulfur in the white liquor with rather small sulfur losses, provided correct conditions are chosen. The chips, thus impregnated, will fall into the digester, and can be allowed to react with the polysulfide a suitable time at a suitable temperature, before white liquor is added to the digester to accomplish the kraft cook. To ensure that the stages are well separated, the liquor level is kept just above the white liquor addition, and the polysulfide stage carried out in vapor phase. The choice of the temperature in the first stage is not optional, unless special precautions are taken. If the temperature should be kept lower than that of the kraft cook, addition of a gas is necessary to keep the digester pressure above the vapor pressure of the kraft cooking liquor. Such pressure control with a compressed air cushion has been Continuous Modified Kraft Cooking 135

Table 9.6. Cooking Conditions, Analytical Data and Paper Properties of One-stage and Two-stage Kiaft and Polysulfide Pulps, Mumin System

Kraft Polysulfide 1-stage 2-stage1 1-stage 2-stage2 2-stage1

1 Impregnation followed by vapor-phase treatment about 125°C prior to cooking. All chemicals charged at impregnation. 2 Impregnation in a limited amount of white liquor plus all polysulfide sulfur, fol lowed by instant heating to maximum temperature. The remaining white liquor charged to the second half of the cooking zone. 136 Lecture 9 tried in the two-body digester and found to work well. In the two-stage polysulfide cookings, both 130 and 170°C were tried for the initial cook ing stage, the latter temperature being equal to that of the subsequent kraft cook. The results are shown in Table 9.6, indicating that the yield improvement of polysulfide cooking disappeared with two-stage cooking using two white liquor charge points. This rather astonishing result indi cates that the polysulfide oxidation of carbohydrate end groups requires a fairly high-alkalinity. As shown in the table, a straight polysulfide cook in the Mumin system, with all white liquor charged to the feeding circulation but a lower temperature, 122°C, kept in the upper part of the digester for the polysulfide reaction, gave an unusually high yield. The system must be arranged for vapor phase in the polysulfide reaction zone to prevent or limit thermal flow from the kraft cooking zone. The combined high alka linity and polysulfide concentration of the Mumin feeding system and the favorable temperature conditions of the vapor-liquor phase cooking ex plain the good polysulfide efficiency. This is in accordance with labora tory results. It should therefore be possible to limit the sulfur addition to reasonable amounts, 2% or less, to achieve a desired yield improvement of 4%, but regrettably, lower sulfur additions were not tried with the vapor- liquor phase system. Another favorable observation with the vapor-liquor phase system is the low screenings obtained with both kraft and polysul fide cooking. The trials with polysulfide cooking have thus shown that continuous cooking systems can be designed to fit advanced process requirements for optimal economy and quality. Before continuing with the presentation of additional process variants, I should like to sum up the experience on carbohydrate composition of pine pulps, cooked by the normal kraft process and the variants of instant heating, countercurrent cooking, and polysulfide cooking, for both chem ical and semichemical pulps (Tables 9.3-9.5). The sugar analyses have been recalculated to cellulose, galactoglucomannan, and glucuronoarabinoxylan (Fig. 9.7). Countercurrent cooking decreases primarily the xylan yield, whereas instant heating, as well as polysulfide, increases the glucomannan yield and causes a certain loss in xylan. Limiting the degree of cooking causes a gain not only in lignin yield, but also in xylan and particularly in glucomannan yield. A process variant of kraft which has attracted considerable attention recently is the so-called "Alkafide" process, which is essentially a hydrosulfide-sulfide cook. It is unique so far as it has been developed directly for continuous cooking by the M & D "piggyback" system, that is, a two-body digester for a two-stage cook. The essential virtue of the pro-

138 Lecture 9

cess is that it can utilize uncausticized liquor and that therefore the cal cium cycle of the recovery system can be eliminated. The snag is—so far -that the recovery furnace cannot be run to give 100% sulfidity in the green liquor, but that the carbonate must be separated from the sulfide, a process which has always been within reach of the kraft industry but not been regarded as a very attractive unit operation. Leaving the recovery aspects aside, the cooking process consists of a pretreatment at a some what lower temperature, where a limited amount of sodium sulfide is allowed to react with the wood chips to form at steady state a mixture of hydrosulfide and sulfide. In the second digester body, run at a higher temperature, the semicooked chips are allowed to react with more sulfide to give a pulp of essentially kraft pulp yield and strength. In order to run this process in a really large-scale new mill, or in those mills already having a Kamyr digester, it could be of interest to apply the Kamyr system to the Alkafide process. This could be accomplished in a similar manner as the two-stage polysulfide cooking just described, i.e., with a limited charge of sodium sulfide to the feeding circulation, to an extent corresponding to the first stage of the Alkafide cook. The alkali consumption in the feeding circulation will convert most of the sulfide to hydrosulfide and the chips will become impregnated with hydrosulfide, which may then react with the chips in the precook in the vapor phase, down to the liquor level, where more sulfide is added. There is also the obvious possibility of adding sulfur to the feeding liquor for polysulfide effect. In the first approximation it was tried to run a straight sodium sulfide cook, with all cooking liquor added to the chips, and in the second ap proximation, liquor from the middle of the digester was recycled back to the top, which also ought to give the desired hydrosulfide cooking medi um. The cooking temperature was in both cases constant throughout the digester, 168 resp. 174°C, and the sulfide charged 260 kg/ton o.d. wood, 26%, expressed as NaOH or Na2S. The liquor ratio was kept as low as possible, about 2.5, which gave a reasonably fast reaction. The chemical pulp obtained contained more screenings than previous pulps, but was in most other respects equivalent to kraft pulp (Table 9.7). The carbohy drate composition indicates a fairly normal yield, the strength was on the kraft pulp level, the brightness somewhat lower, and resin content some what higher, without extremes. Considering these trials to have been rather short and exploratory, the process can be said to have definite possibilities without having shown any advantages over kraft pulp, as also indicated by laboratory trials. The interest in the process thus appears to lie mainly on the recovery side. If the advantages there are found large Continuous Modified Kraft Cooking 139

enough to validate a development, the cooking process is likely to give no particular obstacles. The addition of sulfur to give a polysulfide yield increase was, however, not successful, as shown also in Table 9.7. This is another indication that a high alkalinity in the initial polysulfide treatment is essential to get the yield improvement. A process of both theoretical and possibly practical interest is the alka line sulfite cooking method. This has been tried in many variants since the original work on the Keebra process variants in the 1920's. The most interesting version is that containing sodium sulfite, sodium sulfide, and small quantities of sodium hydroxide or carbonate as the active cooking chemicals. Laboratory work by us and in other laboratories has shown that both the sulfite and the sulfide have a function to fulfill, since the cook is retarded when the charge of any of them is decreased. Substitut ing thiosulfate for them does not give the same result, although it seems unavoidable that thiosulfate is formed during a cook containing both sul fite and sulfide. As far as can be understood, both sulfonation and sulfidation reactions with the lignin occurs, but the reactions of the cook have not been more closely studied. With the pH considerably lower than for the kraft cooking varieties, the carbohydrate degradation appears to be less rapid, with almost no cellulose degradation occuring until the chemical pulp region is approached. The drawback with the process is the large amount of chemicals needed to accomplish pulping. The total charge of chemicals necessary to get a reasonable cook is about 30% Na20, wood basis, or almost twice that of kraft cooking, at a liquor ratio of 4.0. However, much of those chemicals appear to be unconsumed after the cook, indicating that concentrations of the cooking chemicals rather than absolute charges are important. A lower liquor ratio therefore ought to give considerable savings in chemi cals, such as the 2.7 reached in continuous cooking. It was therefore attempted to try alkaline sulfite pulping at low liquor ratios in the Mumin digester system, without as well as with pressure preimpregnation (for details, see below). The main reason for the interest in alkaline sulfite pulping is that it represents the only pulping process known which gives a significantly better strength than kraft cooking, or possibly higher yield at equivalent strength. The results of the trials in the pilot digester are shown in Table 9.8. On the whole, the laboratory results were confirmed, whereby it is possible to produce either the same Roe No. as kraft at a slightly higher yield and strength, but using large amounts of chemicals, or to produce a pulp of equal strength as semichemical kraft at a higher Roe No. and about 5% higher yield, using reasonable amounts of chemicals. The work presented so far gives the results of process modifications of Table 9.7. Cooking Conditions, Anayltical Data, and Paper Properties of Chemical Sodium Sulfide, Hydrosulfide, and Polysulfide Pulps (Mumin System)

1-stage 2-stage polysulfide polysulfide Sodium Normal High Normal High Kraft sulfide sulfidity sulfidity sulfidity sulfidity

Retention time, hr Impregnation 0.02 0.02 0.02 0.02 2.0 2.0 Cooking 2.2 2.2 4.2 4.4 2.4 2.4 Temperature, °C Impregnation 105 105 105 105 122 130 Cooking 165 172 156 177 151 168 Pressure, atm 9 8 9 9 9 8 Active alkali charge, %Na20 on wood 16.0 20.0 18.5 5.5/15.51 18.5 5.0/17.01 Sulfidity, % on total alkali 30 90 30 100 30 125 Sulfur charge, % S on wood 0 0 4.3 4.0 4.0 4.0 Roe chlorine No. 5.1 5.3 5.2 5.8 5.5 5.0 Screenings, % on pulp 3 8 4 2 0.7 5 Brightness, % SCAN 30.6 23.2 26.7 20 26.5 22 Resin content, % 0.19 0.35 0.20 ... 0.27 Table 9.7 Cont'd. Cooking Conditions, Analytical Data, and Paper Properties of Chemical Sodium Sulfide, Hydrosulfide, and Polysulfide Pulps (Mumin System)

1-stage 2-stage polysulfide polysulfide Sodium Normal High Normal High Kraft sulfide sulfidity sulfidity sulfidity suffidity Carbohydrate analysis, % Galactose 0.2 ... 0.6 ... 0.8 Glucose 89.9 85.4 81.3 83.4 79.9 83.5 Mannose 9.1 6.9 11.4 8.6 12.7 7.3 Arabinose 0.2 1.6 1.0 0.9 0.7 1.0 Xylose 7.6 6.1 5.7 7.2 5.9 8.2 Cellulose content, % on carbohydrate 79.9 83.1 77.5 80.6 75.7 81.1 Estimated pulp yield, % on wood 48.5 46.5 51.5 48.0 53.5 47.5 Paper properties at 25 SR Beating time, min 43 41 35 39 44 41 Tensile strength, km 9.4 9.5 9.0 10.0 8.8 10.1 Tear factor 149 145 131 155 124 152 Sheet density, g/cm3 0.68 0.66 0.65 0.70 0.66 0.68 Paper properties at 45 SR Beating time, min 67 66 53 65 63 64 Tensile strength, km 10.8 10.9 10.4 11.0 10d 11.2 Tear factor 134 129 116 136 110 137 Sheet density, g/cm3 0.74 0.73 0.72 0.77 0.69 0.74

1 Charged after 2 hi cooking. Table 9.8. Cooking Conditions, Analytical Data, and Paper Properties of Semichemical Kraft and Alkaline Sulfite Pulps (Mumin System)

Kraft Alkaline sulfite Mumin Mumin with Mumin with only no extra preimpr. pressure preimpr. Laboratory

Retention time, hr Impregnation 0.02 0.02 0.02 1.25 1.25 0.2 0.2 Cooking 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Temperature, C Impregnation 105 105 105 105 105 105 105 Cooking 152 170 175 170 175 170 175 Pressure, atm 9 9 9 9 9 9 9 Active alkali charge, 14.0 29.0 19.8 27.0 19.8 27.0 19.8 % Na20 on wood Roe chlorine No. 14.5 14.5 18.0 14.5 18.0 14.5 18.0 Brightness, % SCAN 19.2 20.0 14.5 18.9 14.9 17.0 14.0 Resin content, % 0.4 0.5 1.2 0.9 1.2 0.5 0.6 Carbohydrate analysis, % Galactose 1.2 0.8 0.92 1.2 1.3 1.0 1.0 Glucose 79.4 83.2 82.8 84.2 83.5 85.2 84.6 Mannose 10.5 7.6 8.0 6.9 7.5 5.5 5.8 Arabinose 1.5 1.4 1.4 1.3 1.3 1.3 1.3 Xylose 7.4 7.3 7.0 6.4 6.4 6.9 7.3 Cellulose content, % on carbohydrate 75.9 80.7 80.1 81.9 81.0 83.4 82.7 Table 9.8 Cont'd. Cooking Conditions, Analytical Data, and Paper Properties of Semichemical Kraft and Alkaline Sulfite Pulps (Mumin System)

Kraft Alkaline sulfite Mumin Mumin with Mumin with only no extra preimpr. pressure preimpr. Laboratory

Estimated pulp yield, 56.0 58.5 61.5 57.5 61.0 57 60.5 % on wood Paper properties at 25 SR Beating time, min 34 42 33 33 35 45 39 Tensile strength, km 7.4 8.5 7.5 8.1 7.7 8.6 7.6 Tear factor 135 132 121 140 140 150 142 Sheet density, g/cm 0.55 0.60 0.58 0.56 0.56 0.61 0.57 Paper properties at 45 SR Beating time, min 51 59 45 47 48 67 57 Tensile strength, km 8.7 9.5 8.5 9.6 9.0 10.0 9.2 Tear factor 117 116 109 123 124 135 125 Sheet density, g/cm3 0.64 0.67 0.64 0.64 0.63 0.67 0.63

preimpr. = preimpregnation. 144 Lecture 9

Fig. 9.8. Low-pressure preimpregnation system for Mumin kraft cooking. more or less radical chemical deviations from the kraft process. The work on instant heating has been included in that series of presentations, since so many of the real process modifications employed that system. I shall now describe the work done to investigate whether the impregnation of the kraft pulping can be improved or needs any improvements. In addi tion to normal hydraulic cooking and that of instant heating in the Mumin system, the following versions have been tried: Mumin system but with the vapor-liquor phase arrangement (see Fig. 9.6), to allow some time at lower temperature, preimpregnation in a vertical steaming vessel, the lower part of which is kept under liquor level at steaming pressure, followed by the Mumin digester system (Fig. 9.8), and finally preimpregnation in a hydraulic pressure vessel after a normal horizontal steaming vessel, fol lowed by the Mumin digester system (Fig. 9.9). The impregnation vessel in the latter two systems was originally designed by Kamyr and delivered for steaming at a bisulfite digester in Switzer land. It consists of a vertical cylinder, equipped with two alternative circulation strainers and a discharge table for the impregnated chips. For low-pressure impregnation, the upper part is used as a steaming vessel, attached to a low-pressure feeder, with the liquor level kept somewhat below the chip level in the vessel, and the discharge of chips and liquor attached to the packing circulation of the high-pressure feeder to the Continuous Modified Kraft Cooking 145 146 Lecture 9 Continuous Modified Kraft Cooking 147 148 Lecture 9

Fig. 9.10. Pulp screenings vs. white liquor charge at various kraft cooking systems.

Mumin system. For high-pressure impregnation, the vessel is simply at tached under hydraulic pressure in the Mumin system between the high¬ pressure feeder and the inclined top separator. The impregnation vessel will then need a conventional top separator. The virtue of the latter system as compared to the normal hydraulic system is that the tempera ture conditions of the impregnation can be well defined and thermal flow disturbances in the digester avoided, thus combining the advantages of the hydraulic and the Mumin systems at an extra investment cost. The results of the preimpregnation studies on pine (Table 9.9) are best illustrated in a series of curves for screen rejects of kraft pulp as a function of the white liquor charge at two lignin content levels (Fig. 9.10). Hydraulic cooking gave a uniformity equivalent to laboratory batch cooking with 2.5 hr heating time. Instant heating with the Mumin system gave, as previously mentioned, a clearly higher screenings content at equal white liquor charge, although the differences would be eliminated by increasing the white liquor charge. Preimpregnation in the low-pres sure system prior to Mumin cooking compensated for the difference to hydraulic cooking even at equal white liquor charge, and high-pressure preimpregnation for 2 hr at 105°C actually gave lower screenings than the hydraulic system. Such a system, however, adds considerably to the in vestment costs, and it is therefore interesting to find the same effect achieved by simply using the Mumin system but allowing the impregnated Continuous Modified Kraft Cooking 149 chips to prevail at 120-130°C for an hour in the vapor phase in the digester top before increasing the temperature to cooking in the liquor phase. Analogous results were obtained on polysulfide cooking with these various systems, i.e., less screenings through preimpregnation or prevailing in vapor phase at lower temperature before cooking. The results are summarized in Table 9.10, together with those of other modifications of chemical kraft cooking. Similarly, preimpregnation treatment prior to Mumin cooking of semichemical kraft pulp variants appeared to decrease the fiberizing energy, indicating better homogeneity of cooking (Table 9.11). Obviously, differences in the penetrability of the wood may influence the choice of the kraft pulping system (see experience on semichemical bisulfite pulping quoted in Lecture 4). For chemical kraft pulps from hardwoods, such as eucalypt and birch, there appears to be no reason for choosing elaborate impregnation equipment, and indeed the Mumin diges ter system is the simplest, fully adequate equipment to choose. For chemical kraft pulps from pine, the choice is more open to question. The ordinary hydraulic system appears to come fairly close to ideal uniformity, and gives somewhat better uniformity than instant heating. If the instant heating system is preferred for other reasons, and yet absolute unchanged uniformity desired, the steaming vessel should preferably be designed as the vertical impregnation chamber. With demands on improved unifor mity, the pressure impregnation vessel could be chosen. This is possibly the case with chemical pulping to higher lignin content than normal. A possible alternative is the simple Mumin system for vapor-liquor phase cooking. For semichemical pulping, the impregnation effects should be reflected in the fiberizing energy consumption and in the paper proper ties. As indicated in the previous tables, this is the case to only a very limited extent as far as paper properties are concerned, and the measure ment of fiberizing energy did not allow any fine gradation, although pre impregnation did seem to decrease the energy demand. The impregnation in the simple Mumin system with a white liquor of high concentration may in many cases yield equivalent results, and could motivate the choice of a simple digester system. However, the first steaming-impregnation vessels for the large-scale production of kraft pulps have already been delivered, to Italy, Japan, and Sweden, and further experience may bring out their possible advantages more clearly. The uniformity of impregnation and cooking of semichemical kraft must be important not only for the energy consumption of the fiberizing, but also for the shiviness and runnability of the stock on the paper ma chine. Particularly at the highest yields, this influence of pulp uniformity ought to be pronounced and a limiting factor in the efforts to decrease 150 Lecture 9 Continuous Modified Kraft Cooking 151 152 Lecture 9 Continuous Modified Kraft Cooking 153 154 Lecture 9

Fig. 9.11. Uniformity of semichemical kraft pulp, low-pressure preim¬ pregnation. Continuous Modified Kraft Cooking 155

Fig. 9.12. Uniformity of semichemical kraft pulp, high-pressure preim¬ pregnation. 156 Lecture 9

Fig. 9.13. Paper strength between 25 and 45°SR for pulps cooked contin uously in the experimental pulp plant, as compared to kraft pulps cooked by the batch and continuous systems on the full mill scale.

wood consumption and cost. A new technique for study of the unifor mity of semicooked kraft chips has been applied on four pulps from the trials with low-pressure and high-pressure preimpregnation. Step-wise laboratory turrnix treatment of the chips followed by lignin content de terminations on the pulp fractions liberated indicated (Figs. 9.11 and 9.12), considerably better uniformity by the high-pressure preimpregna tion. Further studies along these lines appear motivated, and the results suggest that a perfect impregnation prior to cooking may make higher yields possible. Several large-scale installations are under way. To sum up the results of the kraft pulping development work, it is fair to say that both the process and the machinery may deserve to be less stan dardized than they used to be. Systems have been designed for simpler operation, for higher process flexibility, and for improved uniformity. The process modifications indicated so far by research have all been realized with fair success in continuous cooking. Their large-scale realization will now depend on how the recovery problems can be solved, and whether the quality of the process variants can be accepted. My personal opinion is that the conventional kraft process will remain dominant for considerable time, and that the inroads of the process modifications will first come on special applications. The urge for less wood costs will, however, move the yields upward by limiting the delignification and by stabilizing the Continuous Modified Kraft Cooking 157

Fig. 9.14. Tear factor at equal tensile strength vs. pulp yield for the pulps produced in the experimental pulp plant by various processes and process variants. carbohydrates. Yields of 60% and above will then be realized. Figure 9.13 compiles the paper strength of the pulps produced in the experimental pulp plant by various processes and process variants in a diagram plotting tensile strength against tear strength. The large variety in strength properties makes those pulps useful in different areas. As shown in Fig. 9.14, the strength variation is largely a function of pulp yield, with the exception of acid sulfite pulping, where the acidity combined with the mechanical chip damage results in lower strength at equal yield. It is not irrelevant, however, how a certain pulp yield was obtained, since the drainage properties at a certain tensile strength will be quite different for example, for a semichemical kraft pulp and a chemical polysulfide pulp of equal yield. A combination of less delignification and less carbohydrate degradation may therefore prove to be the best alternative in some cases. Since it is now fairly well known how the process conditions should be chosen to obtain a certain yield and a certain combination of properties, I am personally convinced that the most important task for future research is not to improve the pulping processes as such (aside of their recovery systems), but to find out what properties are important for each essential application and then to apply existing knowledge to select the process and its conditions. Lecture 10

Washing Process Theory

The primary reason for washing is to obtain a pulp free of soluble impurities. Yet, as long as subsequent operations were run with an abun dance of water, the pulp was automatically cleaned from the soluble im purities by the repeated dilution and thickening operations. The solubles then went down the drain, and they still do in many places. This, how ever, is becoming unacceptable for two reasons: stream pollution and economics. The so-called waste liquor has a value because of its chemicals and combustible organic matter. The second reason for washing is thus to recover the waste liquor as completely as is economically possible. The costs of recovery are investment in equipment and current steam costs for the evaporation of the wash water added, or rather for that portion of the wash water which did not remain in the washed pulp but found its way to the recovered waste liquor and thereby diluted it. The interest of the washing operation and washing system design thus centers around the problem of how to recover a maximum of waste liquor at a minimum dilution and a minimum investment. The kraft industry was the first of the pulping industries to devote attention to the problem in this sense, and it introduced in the 1880's the diffuser, a construction originally designed for the sugar industry. The sulfite industry used less valuable chemicals and concentrated on the pri mary task, to rid the pulp of the waste liquor sufficiently to allow screen ing to take place without foaming. Blow pit wash or batch digester wash were the systems used. Filters were first introduced to the pulp industry as washers in the bleachery, but in the 1930's, the kraft industry began to use filter washers instead of diffusers. It was then found necessary to apply a series of 3-4 filters and the countercurrent washing principle, i.e., to introduce the wash water on the last filter and to pass the filtrate therefrom onto the previous filter as wash liquor, etc., until the filtrate from the first filter could be recovered in concentrated form and passed to the evaporators. The countercurrent principle as such had been applied already in the diffuser washing, but met with greater practical difficulties in the filter systems, 159 160 Lecture 10 because these allowed air to mix with liquor and to cause immense foam ing problems. These were only eventually solved by improving the filter design, adding large seal tanks with foam tanks and foam breakers. Thereby filter washing became an efficient operation but also required expensive equipment. With the increasing capacities of the kraft mills, the filter sizes have grown immensely (now approaching 15 m in width), until it has become evident that a different design principle is needed. The filter washing has contributed to the pulp washing technique in many ways. It introduced continuous operation and instrumentation to pulp washing, with an easy control and adjustment of the dilution wa ter. However, its design principle has many fundamental drawbacks. It involves too much of a complicated pulp transport and web-forming oper ation before it arrives at the proper washing phase; it allows fairly little time for diffusion during that phase; it introduces the entrainment of air to the pulp and liquor; it leaves to the atmosphere malodor and steam, necessitating ventilation hoods and odor abatement systems, it introduces an operation which requires attention by the operator to prevent over flowing of pulp slurry; and it requires indoor operation. Presses were more lately introduced to the washing operation. They do not allow diffusion nor displacement, but are quite efficient by also squeezing out liquor from the fiber lumen. In combination with filters or with several presses in series and intermediate dilution, satisfactory wash ing is obtained. However, a press stage does not give better results than a filter stage, requires a similar high investment, introduces air, and has similar operation problems. With the ever-increasing capacities of the pulp lines, press constructions have had still more difficulties than have filters in keeping pace with the development. The diffuser really represented a much sounder approach to washing than the filter or the press, because it involved fewer moving parts, gave a closed operation without air entrainment and ventilation, and allowed both liquor displacement and time for diffusion. Its essential drawback was the batch operation. For that reason, the continuous diffuser has been subject of many ef forts. However, it was only recently that a successful construction could be presented. The principal difficulty has been to ensure that the neces sary strainer is kept clean of the pulp mat which tends to form at the withdrawal of liquor from a pulp slurry. At fairly high consistency, this is not a problem, but then the resistance to liquor flow within the pulp is excessive instead. A fairly dilute pulp suspension, 5-10% consistency, must be used, and the strainers must be kept clean by movement. One Washing Process Theory 161 162 Lecture 10

construction tried rotating strainers in the bottom of an upflow tower, with wash water applied to the top. The final successful construction by Kamyr, (Fig. 10.1) makes use of concentric strainer rings, moving slowly upward with the pulp in an upflow tower, and then rapidly dropping back to move up again with another part of the pulp column. The wash water is applied in ring shape between the strainers. This construction is now being applied for both black liquor recovery and bleach tower washing. It is too early to judge whether it represents the final solution to the problem of the continuous diffuser and the end of the filter era, but there are considerable possibilities. A variant of the continuous diffuser was realized some years earlier, namely the continuous digester washing. The main problem of the con tinuous diffuser, the clogging of the strainers, is thereby avoided, since the fibers are not completely liberated and the pulp is still in the form of soft chips with a comparatively loose degree of packing, about 100-110 kg/m3 digester volume or 10-11% "consistency." In the lower part of all con tinuous Kamyr digesters delivered after 1961, there is provision for countercurrent washing, normally with wash liquor derived from a filter washer after the blow tank, but in some cases with hot water directly (Fig. 10.2). The washing zone is of varying length, corresponding to a reten tion of 1-4 hr. This, of course, corresponds to a digester volume as large as, or often larger than, that used for cooking. Therefore, and also be cause of the necessary additional strainer and circulation, continuous di gester washing involves considerable investment costs, and some steam is required to heat the wash liquor to the desired temperature, about 130°C. Yet it represents a very satisfactory technology for performing the washing, and is one of the keys to the success of the entire digester system. I shall come back to discussion of its efficiency after describing some of the underlying principles of washing. The washing process has been visualized in many different ways, and several concepts have been introduced to describe the phenomena in quantitative terms. It is quite easy to conceive that part of the waste liquor is more available to displacement by the washing liquor than the rest, which is entrained within the fibers or the chips. The solubles of that portion can join the recovered solution only by diffusion into the moving liquor, and that diffusion can take place only where there is a concentration gradient, i.e., after some displacement has taken place. Furthermore, when the waste liquor is displaced by the washing liquor, some mixing is unavoidable, which causes the recovery even of the displaceable waste liquor to be incomplete in undiluted form. One theory has treated the mathematics of partial volumes in displacement recovery, Washing Process Theory 163

Fig. 10.2. Kamyr continuous digester washing. assuming complete mixing between waste liquor and washing liquor within each one of them. Some concepts are commonly accepted, others are more dependent upon the manner of conceiving the washing process. The substance yield factor or degree of recovery is simply the percentage of solubles recovered in the washing. The concentration quotient is simply the ratio between the concentrations of the recovered liquor and of the original liquor. The relative liquor volume is the ratio between the recovered liquor volume and the original one. The dilution factor is the volume of wash water which found its way to the recovered liquor, expressed per weight of airdry pulp. Losses of solubles, generally sodium, and expressed as salt 164 Lecture 10

Fig. 10.3. U-f diagram for waste liquor recover in practice. (A) Sulfite industry, Ca base, digester washing; (B) kraft industry, diffuser washing. cake per ton of airdry pulp, are the part remaining in the washed pulp. Sulfite waste liquor recovery has generaUy been expressed as substance yield factor at a defined concentration quotient (Fig. 10.3), whereas kraft black liquor recovery is defined by the dilution curve, i.e., salt cake loss at varying dilution factor (Fig. 10.4). These two pairs of figures mainly describe the net result of the washing, not the contributions to the washing from the various submechanics as sumed. Sulfite pulp washing in digester or blow pit generaUy results in a liquor volume about as large as the original liquor, and thus has about equal numerical values for substance yield factor and concentration quo tient, between 0.8 and 0.9. With kraft pulp washing, with its more ex pensive cooking chemicals, it pays to recover a larger black liquor volume of 1.2-1.4, which gives a concentration quotient of 0.7-0.8 and a substance yield factor of 0.95-0.99, as indicated by the shadowed area in Fig. 10.3. This means something like a loss of 10-30 kg of salt cake per ton of airdry pulp and a dilution factor of 2-3 tons of water per ton of airdry pulp. To accomplish this, normally 3-4 filter washers in series are re- Washing Process Theory 165

1 = One-drum two-stage, spruce-balsam fir, permanganate no. 16 2 = Two-drum three-stage, hemlock-Douglas-fir, permanganate no. 22 3 = Three-drum five-stage, southern pine, permanganate no. 28 4 = Three-drum five-stage, southern pine, permanganate no. 34 5 = One-drum four-stage, southern pine 6 = One-drum three-stage, southern pine

Fig. 10.4. Dilution curves at brown stock filter washing, with various numbers of drums and stages, and with pulps of various types.

quired. Of the sodium loss, about 2-5 kg salt cake per ton of pulp is chemically combined with the pulp and cannot be recovered by washing unless the pulp is acidulated. Filter washing is often visualized as predominantly a displacement opera tion, since the retention times—only a few seconds—are considered too short to allow much diffusion.* In countercurrent continuous digester washing, on the other hand, with 1-4 hr, considerable diffusion must be expected, despite the considerable diffusion distances from the chip cen ters. Also, the slower the flow rates, the more efficient must the dis placement process be, and hence the less the turbulence and mixing be tween waste liquor and washing liquor. In crude terms, old-fashioned batch diffuser wash involved linear flow rates of about 1 m/hr, continuous *Assuming accessibility of the individual fibers, however, diffusion will take place because of the short diffusion distances. 166 Lecture 10

digester wash a relative linear flow rate of about 10 m/hr, and filter wash about 100 m/hr through the pulp pad. In all cases, channeling is obvi ously detrimental to the result. A concept widely used in filter washing is the displacement ratio, DR, which is defined as the ratio between the recovered solids in a volume corresponding to the liquor volume of the pulp leaving the washing opera tion, and the solids in a corresponding volume of liquor entering the washing stage. Assuming the same pulp consistency in and out of the washing, the DR becomes equal to U, the substance yield factor, but if there is an increase in consistency, the substance yield factor will be higher than the DR as a result of a thickening preceding the displacement. The DR is simply measured by the ratio of concentrations as follows:

where Cin and cout are the concentration of solubles in the liquor of the pulp entering and leaving the washing, and cwash, the solubles concentra tion of the wash liquor. This is a convenient way of measuring the wash ing result but tells nothing about the quality of the operation unless re lated to the dilution factor, since the DR must necessarily increase by an increasing volume of wash liquor. A related concept to the displacement ratio is the entrainment, E, or the amount of solids entrained by the pulp leaving the washing stage with a weight of W ton of liquor per ton of pulp. Obviously, E = W( 1 - DR). The continuous digester washing must obviously involve some diffusion from the interior of the chips to their surface, as long as the chip structure is preserved. There will always be a larger resistance to flow within the wood structure than between the chips, until complete fiber liberation has taken place. It can be assumed that all solutes are in equilibrium between the liquors inside and outside the chips, when these enter into the coun¬ tercurrent washing zone, and that the solutes in the outside liquor are withdrawn by displacement in the very beginning. The solutes in the interior of the chips, on the other hand, diffuse to the chip surface and into the outside liquor during the further course of the washing in the lower part of the digester, where a liquor of increasingly lower solute concentration passes the chips on the way upward through the washing zone. The diffusion of solutes through the chips occurs at a rate varying with the nature of the solute. The inorganics and low-molecular organics dif fuse faster than the carbohydrate and lignin polymers. The diffusion is slower in wood than in water, and slower in tangential-radial direction than in longitudinal direction of the wood. Particularly in the former Washing Process Theory 167

Fig. 10.5. Effect of pH on the effective capillary cross-sectional area of aspen.

Table 10.1. Experimental Diffusion Coefficients for Kraft Cooked Pine Chips

directions, the diffusion rate is also dependent on the pH of the liquor and the yield of cooking. One way of expressing the diffusion rate in wood relative to water is the so-called effective capillary cross-sectional area, ECCSA, which is the fractional area of the wood considered open for diffusion at the rate of diffusion in water. As shown in Figs. 10.5-10.7, this is about 50% in longitudinal direction and 5-10% in the tangential-ra dial direction for uncooked wood at all but alkaline solutions, where the diffusion in the latter two directions increases to 20 (pine)-40 (aspen) %. At a pH slightly higher than that of black liquor, the tangential-radial diffusion in pine increases from about 20% to 30-40% as the pulp yield drops from 100 to 65%, whereas the longitudinal diffusion remains at 50-60%. 168 Lecture 10

Fig. 10.6. Effective capillary cross-sectional area (ECCSA) vs. pH for the diffusion into softwood blocks. In crude terms, then, diffusion at kraft pulp washing conditions in the continuous digester proceeds at a rate half of that in water for the longi tudinal direction of the chips, and one quarter to one third in the trans versal directions. Since the chips are about 20 mm long and 3 mm thick on average, the predominant part of the solutes will diffuse out across the chips to the flat sides. The thickness distribution will thus be important for the result. It is an old experience that the washing result is improved by the use of hot water. Direct determination of the rate of sodium chloride diffusion in uncooked pine chips gave a diffusion constant of 0.5 x 10-5 cm2/sec at 20°C and 1.1 x 10-5 at 50°C for the longitudinal direction. Another investigation on the rate of sodium ion diffusion out of pine chips, cooked to normal-yield chemical kraft pulp, gave the diffusion constants shown in Table 10.1. The diffusion rate is thus doubled at a temperature increase of 30 C. The temperature dependence of the diffusion constant, calculated according to the formula 0.5 - D = A·T ·e E/RT with the experimentally determined activation energy (5000 cal/mol for transversal diffusion), allows us to compute the effect of increasing the temperature from that of hot water in filter washing (D = 0.7 x 10-5 at 65°C) to that of the cold blow (D = 1.4 x 10-5 at 100°C) and that of the countercurrent "high-heat" diffusion washing (D = 2.4 x 10-5 at 130°C). It is thus an advantage to increase the temperature of the kraft pulp washing to that of a pressure operation such as in the digester. Washing Process Theory 169

Fig. 10.7. ECCSA vs. pulp yield for blocks of partially digested softwood at a pH of 13.2. A mathematical model has been developed by McKibbins et al* to describe the diffusion subprocess in the countercurrent diffusion washing in the digester, based on the following simplifying assumptions: Subsequent to an initial displacement of the external black liquor, it is considered that the washing mechanism consists of diffusion from the chip interior to the chip surface, followed by local mixing with the outside liquor. The chips have all the same thickness, sufficiently low as com pared to the length to consider the diffusion as occurring exclusively across the thickness. The degree of packing is assumed to be no greater than that all chip surfaces are available to the wash liquor. The diffusion is slow compared to the local mixing of liquor outside the chips. The wash water-chip flow is countercurrent and uniform over the digester cross sec tion. Applying Fick's second law of diffusion, where c(x,t) is solute concentration in chip liquor at the time t after the start of washing and at distance x from the midplane of the chip in the direction of diffusion, an equation is finally derived for the system, which

*McKibbins, S. W., Tappi 43: 801 (1960); Williams, D. A., McKibbins, S. W.,and Riese, J. W., Tappi 48: 481 (1965). 170 Lecture 10

Fig. 10.8. Gross washing efficiency as a function of reduced time 2 (Dtr/L ) and wash liquor to chip liquor flow rate ratio (F) for counter¬ current washing. is only graphically illustrated here in Fig. 10.8. It employs on the abscissa the concept "reduced time," Dt/L2, which refers the retention time in the washing zone to a certain diffusion constant D and chip thickness L. On the ordinate, the gross diffusion washing efficiency G is employed, defined as G = 100E/[l — (cwash/cin)], where E is the diffusion-recovered solutes divided by initial solutes in the chip interior. The parameter F of the figure is the important flow ratio of wash liquor to liquor moving with the chips in their interior. From the figure, the following implications can be derived. Chip thickness is important, and a reduction to half the thickness should give the same washing result at a retention time just one-fourth of the previous one. An increase in temperature from 100 to 130°C, which nearly Washing Process Theory 171 doubles the diffusion constant, should reduce the required retention time by half. A flow ratio F much lower than 1 will cause the washing effi ciency to level off at a value corresponding to F even at rather long retention times, whereas a ratio much above 2 will not appreciably im prove the result, unless the retention times are very short. In the units of the figure, 1 hr retention at 130°C and a chip thickness of 3 mm should correspond to a reduced time of about 1.0. The least defined function is that of F. The upward flow of wash liquor ought to equal the dilution factor times the pulp capacity, but the downward flow of liquor inside the chips should be dependent on the extent of fiber swelling and chip swell ing, and thus to the species and to the degree of cooking, but possibly also to the degree of packing of the soft chips. It can be anything from 1 to 4 ton/ton of pulp and F thus anything from 0.2 to 2, as indicated in the figure, with a normal value of about 1.0. With both F and "reduced time" being about unity, the expected digester washing efficiencies ought to be considerable, about 90%, or even above, particularly if a prolonged washing zone of 3-4 hr is used. Lecture 11 will present some evidence from mill scale and the Jossefors pilot digester experience. Comparisons between the various types of washing equipment is desired for obvious reasons. A recent attempt has been made by Kommonen, based upon theoretical work by Norden. The latter introduced the con cept of efficiency factor, defined as the number of ideal mixing stages required to get the washing effect of the washing equipment in ques tion. Kommonens data indicate an efficiency factor of about 2.5 for a kraft brown stock filter washer and 7-9 for a complete multistage kraft filter washing station. The continuous diffuser has an efficiency factor of about 5, and the continuous high-heat diffusion wash in the digester gives an efficiency factor of 5-10, depending on washing zone retention time, 1-3 hr. A comparison has to take also into consideration the pulp consis tency from the washing stage, and ends with the result that the continuous diffuser is worth about 1.5 filters, continuous digester wash 2-3 filters, depending on retention time, 1.5 resp. 3 hr. The results are in agreement with the general opinion, that a 3 hr digester wash needs no additional filter, but that 1 hr digester wash will require at least one filter washer. Lecture 11

Continuous Digester Washing

In the previous lecture, the fundamentals of washing were outlined, and the present washing principles and equipment described. It was found that the washing technology is still under development, and that continu ous digester washing is likely to remain an important part of that technol ogy. In some cases it might constitute the sole washing operation; in others it will be complemented with filters, presses, or a continuous dif¬ fuser. The development of large one-line production units favors continu ous digester washing alone or complemented by a continuous diffuser. In this lecture, the experiences of continuous digester washing will be surveyed, not only that in the Jossefors pilot digester, but also some of the Kamyr experience on the mill scale. Figure 11.1 illustrates the principles of continuous digester washing. The cooked chips, still essentially retaining their original shape, enter the washing zone at full cooking temperature. To avoid continued reactions in the washing zone, some of which may be less desirable for the pulp quality, the pulp is cooled to washing temperature by a radial displace ment circulation. This circulation not only cools the entire cross section of the digester content but also accomplishes a washing by displacement of the original black liquor. This black liquor, in mixture with the excess wash liquor, is extracted at a strainer immediately above the displacement circulation strainer. The wash liquor is injected into the bottom of the digester and flows upward countercurrently to the chip flow. Slightly above the digester bottom the wash liquor is heated to 130°C in a circula tion. This improves the rate of diffusion in the washing zone and yet allows a discharge from the digester at below 100°C to avoid mechano¬ chemical damage to the pulp. The wash liquor introduced to the digester bottom usually comes from a filtrate tank at 70-80°C and the pulp leaves the digester at about 5°C higher temperature. The balance of the liquor introduced into the digester bottom less that leaving again with the pulp is the wash liquor passing countercurrently to the chip plug upward through the digester washing zone. Expressed on the basis of pulp weight, it corresponds to the dilution factor concept of Lecture 10. 173 174 Lecture 11

Fig. 11.1. Principal zones of a Kamyr kraft digester with countercurrent washing. The efficiency of the washing is a function of the dilution factor, the washing temperature, the chip retention time, the chip thickness distribu tion, and the liquor-to-wood ratio above the washing zone. The normal way of studying the digester washing efficiency experi mentally is to measure flows and concentrations of sodium and of total Continuous Digester Washing 175 solids in black liquor to flash respectively with the discharged pulp. In addition, the flow and concentration of the sodium and total solids enter ing the washing zone is calculated from the knowledge of charged chips and cooking liquor flows and the approximate pulp yield. Then the wash liquor concentration entering at the digester bottom must be determined, unless it is hot water only. If the digester wash is combined with a filter wash, finally the sodium and solids losses after the filter or filters are also measured. From these measurements, the displacement ratio DR, as defined in Lecture 10, and the wash efficiency or substance yield factor U are calculated. The experience from careful measurements on the mill scale production of pine kraft pulp is exemplified in Fig. 11.2 and Table 11.1. Those results are among the best obtained and should represent the best washing technology of today, for Scandinavia and America. Earlier instal lations appear to require two filter stages for equivalent results. There are several possible reasons for this difference: one is improvements in me chanical design, another the decreased or eliminated black liquor charge to the feeding circulation of the digester, which means less sodium to re cover. The flow conditions of the chip and liquor columns are also impor tant. There is likely to be a wall effect, where wash liquor may flow upward without receiving solutes by diffusion, and that effect should be proportionally more important in digesters with a smaller cross section and lower capacity. Some of those reasons, and particularly the wall effect, will apply to the Jossefors pilot digester and decrease its washing efficiency. Yet it was considered of interest to compare the influence of pulp yield and pulping process on the efficiency of continuous digester washing. Table 11.2 shows that the results were obtained at different liquor ratios and dilution factors and are thus not entirely comparable, but they correspond on the other hand to liquor ratios realistic for each process. Figure 11.3 shows that the washing efficiency on kraft pulp was less good than in the com mercial digesters quoted earlier, whereby it should be observed, however, that the retention time was considerably lower in the pilot digester wash ing. Another observation is that the digester washing of semichemical kraft or polysulfide pulp is almost as efficient as with chemical kraft pulp. As evident from Table 11.1, that is true also on the large scale, although the difference is quite noticeable, expressed in salt cake losses. Even bisulfite and neutral sulfite semichemical pulp in the yield range of 75-80% appears to be quite washable in the digester. An inter esting feature appears to be that it is easier to recover sodium than total solids for all pulps, but particularly so with the semichemical bisulfite and 176 Lecture 11

DILUTION FACTOR, †/†90pulp Fig. 11.2. Results of continuous digester washing of chemical pine kraft pulp on the mill scale (Kamyr).

Digester washing only 47 4 1.7 30 24 96.9 96.1 47 4 2.6 17 14 98.6 97.5 47 4 3.0 15 11 98.8 97.8 47 3 3.0 17 14 98.4 97.4 47 3-4 2.0 25 97 Digester plus one filter stage 47 2 2.5 11 (691) 5 (621) 99.5 99.0 47 2 1.8 13 8 98.8 98 54 1.5 2 30 20 96.8 95.9 Digester plus two filter stages 54 1.5 2 23 17 97.9 97.2

Salt cake losses with pulp discharged from digester. Continuous Digester Washing 177

Neutral 80 1.8 0.45 70 70 sulfite 80 1.8 0.82 84 75 (birch) 80 1.8 1.50 91 83 Bisulfite 75 5.0 3.0 87 79 (spruce) 58 5.0 3.0 87 86 Kraft 55 2.8 2.7 88 86 (pine) 47 2.8 2.7 88 86 Poly sulfide 59 2.8 2.7 87 82 (pine) 51 2.8 2.7 87 83 178 Lecture 11

neutral sulfite pulps. This may reflect both the diffusion hindrance of the chip structure and the fairly high-molecular structure of the organic solutes of those processes as compared to kraft. The conclusions of those trials are mainly that the continuous digester washing is called for at all yield levels and pulping processes, and since then commercial digesters with a countercurrent washing zone have started operation at all yield levels between 40 and 85%, including kraft, prehydrolysis-kraft, acid sulfite, bisulfite, and neutral sulfite pulping. The fol lowing design philosophy appears to be gaining ground: The digester vol ume is dimensioned according to a future capacity demand, with a large washing zone and no subsequent filter. When the mill is expanded, a filter or a continuous diffuser is added, and if necessary the cooking zone of the digester is extended downward at the expense of the washing zone. Thereby, the one-line operation economy can be maintained with only marginal investments. One advantage with digester washing is the minimization of foaming troubles. With a complete digester wash, no foaming is experienced, since there is no introduction of air to the system. Even with a combined system of digester and filter, the foaming troubles around the filter are considerably decreased because the dominant part of the tall soaps have been removed in the high-temperature digester wash. Particularly for the introduction of open precision refiners for semichemicaJ pulps, it is vital to have removed the waste liquor. Lecture 12

Technical and Economic Aspects of Present and Future Developments of Continuous Pulping

Continuous Pulping Processes Realized Now it is time to sum up what has been accomplished during the devel opment of continuous pulping and what remains to be done in the future. Continuous digester systems have been developed for most of the exist ing variants of chemical and semichemical pulping, namely: Kraft* Acid sulfite dissolving pulp* Acid sulfite paper pulp, one-stage* Acid sulfite paper pulp, two-stage Bisulfite chemical* Bisulfite semichemical* Neutral sulfite semichemical* Prehydrolysis-kraft dissolving pulp* Polysulfide one-stage Polysulfide two-stage Hydrosulfide-sulfide Alkaline sulfite Kraft, instant heating* Kraft, preimpregnation, low pressure* Kraft, preimpregnation, high pressure* Kraft, countercurrent*

Advantages of Continuous Pulping Systems With the modifications of the original kraft digester it has become pos sible for the industry to make use of the possibilities offered by modern pulping chemistry. With the exception of countercurrent cooking, how ever, the same variants can be carried out in batch digesters. Has there fore the development work only brought continuous cooking to the level

* Units in operation or under installation. 179 180 Lecture 12

of batch cooking? To some extent, the choice between batch and con tinuous remains a matter of philosophy. Yet, there are some good reasons why those in the pulping industry with few exceptions now choose contin uous cooking. 1. Large one-line units have been made possible. 2. Improved heat economy. 3. Ease of instrumentation, automation, and coordination with paper mill. 4. Ease of attaching subsequent operations, such as washing, refining, evaporation, odor abatement, etc. 5. Improved quality. I shall comment on each of these items and also point out what remains to be done to improve the systems.

Large One-line Units One of the most spectacular trends of the pulp and paper industry in recent years is the increased size of the mills. This increase has been stimulated by the severe competition in most sectors of the industry, and particularly those of market pulp, newsprint, liner board, corrugating medium board, and other packaging grades. Not long ago, 50,000 tons/ year was the output of a sizable mill, and 100,000 tons/year a large one. The units now constructed are for 200,000-500,000 tons/year. It is also important to observe that good economy demands few lines, prefer ably one, in those units to achieve maximum benefit from the large size. Machinery development in all sections of the pulp and paper mill has been required to meet the demands of the large one-lined mill. The most spectacular ones have been the digester, the recovery boiler, and the paper machine, where now 1000 ton/day units are available. The largest diges ters in operation or on order are as shown in Table 12.1. No doubt there are mills of similar sizes in all those categories equipped with batch digesters, but they tend to get fairly complicated and not very labor-saving. That is particularly true for two-stage processes, semichemical pulping, and acid sulfite pulping. That the present size of digester varies considerably with the process is more nearly ascribable to historical de velopment than to real technical limitations. If somebody for some less understandable reason would like to build an 800 ton/day acid sulfite dissolving pulp mill today, it would be technically quite feasible, although both seller and buyer of the digester would probably sleep better after having seen not only the present 275 ton/day unit in operation but also one or two of intermediate size. Kraft digesters have been through that Present and Future Developments of Continuous Pulping 181

Table 12.1. Largest Size Units, Delivered or on Order, for Kamyr Digesters on Various Pulping Processes

Capacity, metric tons/day

Kraft, bleached market pulp 850 Kraft, semichemical pulp for liner board 1150 Prehydrolysis-kraft, bleached market pulp 620 Neutral sulfite, semichemical pulp for corrugating medium 600 Bisufite, bleached market pulp 400 Bisulfite, semichemical pulp for newsprint 200 Acid sulfite, bleached market dissolving pulp 275 phase, and since the development work for the other processes has en deavored to make as much use as possible of the kraft digester details, it is likely that these processes will catch up in size quite rapidly. In fact, it took kraft 17 years and 17 units before reaching 300 tons/day, but pre hydrolysis-kraft and neutral sulfite less than one year and one unit before reaching the same size. It may be fascinating but fairly useless to speculate on the ultimate digester size. The present units are far from ultimate size, since the largest chip feeder in operation is capable of feeding chips for 1150 tons/day of liner board pulp and there is always the possibility of two feeding lines to one digester if necessary. The ultimate digester size is therefore likely to be decided by external factors, such as wood transportation costs or economical size of other machinery used. It is a good testimonial to the digester machinery development that it can be said that the pressure oper ation of pulp production will not become the bottleneck of the large mill for the foreseeable future. On the other hand, it must be admitted, that the large-capacity digesters have become rather monstrous, and that efforts should be made to de crease their size and costs. It is debatable though, whether a temperature increase alone will bring about an improvement, since the decrease in size will be accompanied by an increase in shell thickness due to the increased pressure. The elimination of the excess hydraulic pressure achieved through, the inverted external top separator is one of the better means of reducing shell costs and some circulations. It is also conceivable that the really large digesters of the future may have to minimize the volume spent on countercurrent washing. That a considerable part should be devoted to the task is an obvious advantage when considering the subsequent unit operations of liquor recovery and 182 Lecture 12

Fig. 12.1. Mass and heat balance for continuous cooking and washing plant. odor abatement. The development of the continuous diffuser is likely to offer an alternative to the extreme digester wash in cases where filters should be few or entirely avoided.

Heat Economy of Cooking and Subsequent Operations One of the original arguments in favor of continuous cooking, the su perior heat economy, still holds and has become accentuated through some refinements of the system. Figure 12.1 shows the heat flow to and around the digester system. The blow steam used to be passed to the steaming vessel for preheating the chips, which is a major improvement in heat economy as compared to batch cooking. With the introduction of the cold blow and countercurrent washing, an increased digester steam consumption was experienced, since the wash liquor is heated to about 130°C. A further increase in steam demand is experienced when cooking countercurrently, since all liquor, including the wash liquor, is then heated to cooking temperature, about 170°C. On the other hand, improvements in the liquor balance have contributed to reduce the digester steam demand. In the original system, considerable quantities of black liquor were added to the chip feeding system as so- called "compensating liquor." This served to keep up the hydraulic pressure of the digester, compensating for the drainage of liquor from the Present and Future Developments of Continuous Pulping 183

digester. When it was realized, that the compensating liquor could be added to the bottom, this reduced the liquor to be heated to maximum temperature to the actual cooking liquor plus the chip moisture. For a liner base pulp from Scots pine in a mill with a decent white liquor con centration, this means about 1.3 m3/ton o.d. wood plus 1 m3/ton o.d. wood for 50% chip moisture, or a total liquor-to-wood ratio of 2.3 as compared to about 3.5 in a normal batch kraft operation. As mentioned earlier, acid sulfite or bisulfite cooking has been carried out at similar low liquor ratios, 2.3, when cooking continuously in Jossefors, whereas batch operation for these processses require 4.0 or 4.5 in order to cover the chips of the digester entirely with liquor. When using the external inverted top separator, the liquor absorbed by the chips in the top separator will decide the liquor ratio, since the chips become completely immersed in the liquor of the feeding circulation but can then be transferred by the screw to the digester body under drain age. For neutral sulfite pulping of birch, a liquor ratio of 1.2 to 1.8 is thereby reached. In case the chips tend to absorb more liquor than re quired by the charge of cooking liquor at actual concentration, water or waste liquor must be added to maintain a liquor level in the top separa tor. This appears only with dry chips of lightweight wood. With wood of normal density and moisture, cooking liquor concentration is the factor deciding the liquor ratio. It is suggested that there is more to be done to improve the concentration of cooking liquor for several processes, and that this is more motivated than it used to be when the liquor was charged to batch digesters. With the preimpregnation system at low pressure, there are additional possibilities to utilize blow steam, since not only the wood, but also the cooking liquor, is preheated to steaming temperature, 110°C. An interesting feature of the continuous cooking system with countercurrent washing in the digester is that the wash liquor is obtained at almost full cooking temperature. This introduces two new possibili ties. One is the treatment of waste liquors, particularly black liquor, at high temperature without the use of much extra steam. This may become of importance in kraft odor abatement and will be discussed further later on. The other new possibility is to use the flash steam in the evaporation system rather than for preheating of the chips. In any event, the high- temperature flash will increase the black liquor concentration and thus improve the heat economy of evaporation. However, the flash can also be limited to a smaller temperature drop, to allow the flash steam to be utilized in pressure evaporation of subsequent evaporation stages (Fig. 12.2). 184 Lecture 12

Fig. 12.2. Evaporation system utilizing combination of flash heat evaporator and live steam evaporators.

The effect of all those modifications of the system on the heat economy of the various pulping processes, as compared to batch pulping, is not easy to show here. It will suffice to exemplify with heat balances and steam demands calculated for kraft, prehydrolysis-kraft, and neutral sulfite pulp ing, using the Kamyr system (Table 12.2).

Instrumentation and Automation With some exaggeration it could be said that continuous cooking brought a consistent instrumentation to the pulp and paper indus try. Certainly there were instruments before, scattered all over the pulp and paper mill, controlling a consistency here and a flow there. The continuous digester presented us a system, which had been logically ana lyzed for control loops and instrumented accordingly (Fig. 12.3). The early versions naturally contained less good solutions, and some of the early machine elements introduced oscillations to the control loops. For example, the high-pressure feeder, before it got large enough to have pockets in contact with the feeding circulation in all positions, introduced regular pressure variations, and so did the discharge sluice with its two alternating valves, which preceded the present continuous blow valve. The discharge conditions also created other disturbances. The consistency of the discharge was difficult to control, and this caused vari ations in the "compensating liquor" to keep the hydraulic pressure con stant. As long as the compensating liquor was added to the top, its varia tions then introduced temperature variations to be compensated by ad justment of the steam flow. The chip level control was not reliable in the early stages and introduced additional variations, to be compensated either Table 12.2. Heat Balances and Steam Consumption With Continuous Cooking of Various Pulp Types. Units per Ton Aitdry Pulp

Total Secondary Demand Production heat heat LP- HP- secondary demand, produced, Pulp type steam, ton steam, ton steam, ton Meal Meal

NSSC, hardwood (Mumin) 0.14 0.48 0.25 395 x106 155 x 106 Kraft Liner base, softwood 0.06 0.79 0.36 485 230 Chemical pulp, softwood 0.01 0.90 0.38 465 240 Chemical pulp, softwood1 0.36 0.91 0.22 755 140 Chemical pulp, hardwood (Mumin) 0.09 0.60 0.31 365 200 Prehydr.-kraft, hardwood (Mumin) 0.00 1.33 0.73 755 460

Black liquor to evaporation 128 C. Fig. 12.3. Instrumentation of the Kamyr cooking system. Present and Future Developments of Continuous Pulping 187

by the chip feed, which then required corresponding adjustments in the flow of cooking chemicals, or by the pulp discharge, which again intro duced the variations in pulp consistency, compensating liquor, and steam flow. These are only a few examples of the early instrumentation and control problems. The system allowed, however, almost from the start, some thing new to this industry: one could observe what happened in quantita tive terms and analyze what measures should be taken to eliminate the problems. The continuous blow valve, the adjustable speed of the bottom scraper, the compensating liquor to the bottom of the digester, the reliable gamma ray chip level indicators, and many other adjustments have now led to a fairly reliable and well-controlled system. A few things are still desired to make it perfect. A reliable chip mois ture meter would be advantageous to get the liquor balance completely covered. Efforts are underway here. (With the Mumin system we have found another way to observe the moisture variations, since they are re flected in the variations in liquor flow needed to keep the liquor level in the top separator constant.) A reliable indication of the alkali concentra tion is needed to allow adjustments in the white liquor flow to the chip feed, or perhaps directly to the cooking zone, to keep the degree of cooking constant. An automatic reader of the degree of cooking would naturally also be a good aid, but measurements on the final pulp leaving the digester will not, however rapid, be an ideal guide to adjustments of the cooking conditions, since the digester washing constitutes a consider able time lag. In general, therefore, material balances and feed-forward principles are better suited to the control of continuous digesters than feed-back principles. The indication of alkali concentration could be re garded as belonging to either control principle, but is a very direct and essential measurement at the central zone of reaction. We have tried conductivity measurements at Jossefors and installed a conductivity cell in one of our digesters. With a suitable construction it gives reliable readings but a higher sensitivity to alkali concentration would be desirable. An automatic sampler and titration analyzer has also been tried by us and is under further development elsewhere. This will give a higher sensitivity if the snags of the complicated equipment can be overcome. For special purposes, such as preventing the pH from dropping under a certain value in the combined prehydrolyzate and black liquor from the prehydrolysis- kraft digester, the conductivity cell is quite adequate. Instrumentation has also been introduced to batch digesters, predomi nantly temperature and pressure control and eventually also control of the charges of wood and chemicals. A few cases of rather elaborate automa- 188 Lecture 12 tion of batch digester rooms have also been seen, with remotely controlled valves and automatic charging and discharging. For some processes, instru mentation of the end point control of the cook has been attempted. In general, however, it can be said that batch cooking requires a more elaborate instrumentation and control to be fully automated, and particularly so with two-stage processes and other deviations from the simple kraft process. The large number of digesters in the batch system naturally increases the points of measurement, and the discontinuous op eration complicates the control of the system to give a uniform quality. The final stage of automation is to put the continuous digester on a computer control. This has already been done at some places, but the main development remains to be seen. For the running of a constant pulp grade, the general instrumentation already in use today allows the contin uous digester to be put on digital control, although the system analysis to give the process equations will have to be done specifically for each pro cess and digester type. We now attempt to do so for the simple case of neutral sulfite semichemical pulping. The principles for digital control of the digester switching from one grade to another have also been worked out jointly by Kamyr, International Business Machines and International Paper. It is to be expected that during the years to come, most new mills will run their digesters on a computer basis, but with the same computer as controls the other activities of the pulp and paper mill. We make efforts in this direction with our NSSC digester, which is directed from the same computer that controls the corrugating medium board machine and in termediate operations. This will also eliminate the traditional borderlines between the pulp and the paper mill as far as supervision and personnel organization is concerned. The continuous cooking principle has then made pulping an operation too simple to validate its own organization, and it will also have eliminated the pulp equalizing chests and minimized buffer storage facilities. This development, of course, owes much to the corresponding simplifications of the intermediate operations as well, but it should be stressed that their progress could not be fully utilized without the development of the continuous digester, as will be discussed now.

Combination of Continuous Cooking With Subsequent Operations The development of continuous pulp washing on filters preceded the development of continuous cooking. Considerable problems were en countered, particularly with foaming, which made the filter constructions sophisticated, instrumentation elaborate, and the liquor tanks volumi nous. The expenses were multiplied through the considerable number of Present and Future Developments of Continuous Pulping 189 stages necessary to make washing complete. With batch digesters, large blow tanks were necessary as capacity buffers between the discontinuous and continuous systems. Sad to say, the large blow tanks have not been eliminated even after the arrival of the continuous cooking principle, since they are considered nec essary to meet shutdowns for minor repairs in the system before or after the blow tank. The size of the blow tank can, however, now be kept more reasonable, which is of some importance with the present large mill units. Also, the size of the filters for washing is becoming rather monstrous with present-day capacities, as has been pointed out already, and a four- stage filter wash with adjacent tanks in a 300,000 tons/year mill is a massive sight. Here continuous cooking has made one of its major contributions, as elaborated in previous lectures. Countercurrent washing in the lower part of the digester not only eliminates the need of a certain number of filters, but also facilitates the operation of the remaining ones by removing the tall soap. At the expense of making the digester shell more than twice as big as is needed for cooking, the entire filter wash can be eliminated. Some mills consider this worthwhile, since the washing zone of the digester represents a possibility of future capacity expansion, if only space is allowed for the installation of a future outside washing. The development of the contin uous diffuser here introduces new aspects. With the elimination of the filters, the need of the blow tank is eliminated, since there will be only a simple screening equipment between the digester and the thick stock stor age for unbleached pulp. Combination of cooking with refining of semichemical pulps is also con siderably facilitated when cooking is done continuously. So far, a live bottom bin is still the safest solution for the storage between cooking and refining, but it can be kept fairly small after a continuous digester and consequently offers few problems. For semichemical kraft pulps of liner base type, the handling problems of the semicooked chips with the black liquor have been limiting the yield in the batch system. The normal blow tank has been equipped with fiberizing impeller, or with a breaker trap in the pipe from blow tank to washer. Normally also the final fiberizing has been carried out in closed pump-through refiners before the final wash ing. Continuous cooking here offers a possibility to apply a coarse fiber izing in the blow line prior to the blow tank, and this, in combination with the digester wash, and possibly with the continuous diffuser, will allow the final fiberizing in open precision refiners. Thereby, both handling and fiberizing arrangements will be able to take care of really semichemical 190 Lecture 12

kraft pulps and not just high-yield chemical kraft for liner board and similar purposes. The combination of cooking with evaporation was touched upon ear lier. Batch cooking delivers the black liquor to evaporation at a tempera ture of only 70-80°C, which means that the enthalpy drop from cooking temperature can be used only for hot water production. With continuous cooking, including digester washing, the black liquor is delivered at almost full cooking temperature, and the flash can be used in pressure evaporation in a suitable countercurrent system. We shall see more developments in that field. Closely connected with this train of thought is our idea of treating the black liquor at high temperature for odor abatement. Batch cooking delivers malodorous compounds in two ways: partly in the blow gases, partly in the black liquor. The collection of the blow gases causes a special problem, which has been solved at considerable expense in the so-called "Vaporspheres," gas buffers to receive the batch flow of gases and deliver a continuous flow of gases for treatment. Furthermore, the blow gases unavoidably become admixed with the air of the blow system, which also complicates their treatment. Here the first and fundamental virtue of continuous cooking is obvious. The second source of malodor ous compounds is the black liquor during evaporation, which delivers these compounds to the condensates of all stages. It is not impossible that a prolonged treatment of the black liquor at or above cooking temperature, will create all obnoxious volatiles at once and eliminate their successive formation during evaporation and combustion. Such a heat treatment, possibly combined with a suitable stripping arrangement, might therefore concentrate the odor problem to one convenient spot, where suitable means of destruction or by-product recovery could be arranged. Continu ous cooking is a prerequisite for doing this at a reasonable steam comsump- tion.

Quality Aspects of Batch and Continuous Cooking The quality aspects have been commented on in connection with each process. Therefore I just need to summarize that laboratory pulp quality has been achieved when pulping continuously, with the possible exception of acid sulfite paper pulp, where also batch pulping on the mill scale gives a somewhat lower quality than in the laboratory. Whether with this gen eral statement it can be maintained that continuous pulping yields a supe rior quality as compared to batch is thus mainly a question of batch pulping standard. At some instances, mill scale batch pulping yields a quality close to that of laboratory pulping, but in many cases, perhaps normally, some quality is sacrificed to gain production or production Present and Future Developments of Continuous Pulping 191 economy. This need more seldom be the case with continuous cooking, which thus normally yields a somewhat better quality than batch cooking, provided the modern modifications of cold blow and countercurrent wash ing are installed. The advanced process control likewise has generally re sulted in a superior uniformity of the pulp as compared to batch. Index

AB Statens Skogsindustrier 5 102-104,106, 116, 121, Accessibility 23, 39, 82, 85 123-124,128-129,156-157,177 Acetic acid 105 Bisulfite cook 15-16,51-62, 103 Acetyl groups 23-25, 28, 48, 64, Bisulfite liquor 11-31,51-53,55,60 87-88, 92, 105 Bisulfite, pulp 103, 175,177-178 Acid sulfite cook 15-16, 33-50 Bjerrum diagram 16, 88 second-stage 45-47,49,103,157 Black cook 16 Acid sulfite pulp washing 178 Black liquor 97, 108,113, 122, Active alkali 87, 109-110, 116,123, 125-126,164, 173, 175, 182-183, 127, 130, 132-135, 140, 142 190 Air compressor 122 Bleachable pulp {see also Brightness) Air in chips 29-31 51,53,55,63-64,66-68, 103, 123 Aldehyde end groups 81-82, 85 Blow gas 182-184,190 Aldonic acids 28-29 Blow pit wash 159,164 Alkafide process 136,138 Blow tank 64-65,73, 99, 117, 189 Alkali consumption 77, 80, 87-88, Blow temperature (see also Cold blow; 91-92, 109,117,122,132, 138 Hot blow) 100 Alkali lignin 108-109,113 Blow valve 7,34,64,187 Alkaline hydrolysis 77, 82-83 Board-grade pulp 51 Alkaline peeling reaction 81, 88, 92, Bonding strength 95 105 Borohydride 82, 85-87, 90 Alkaline sulfite cook 15-16,139, Bottom scraper 3, 35, 99 142-143, 152-153 Bottom screw 37, 44, 99 acid sulfite second-stage 45, 47, 49 Breaker trap 189 Alkalinity 109, 125, 136 Brightness 40, 45, 48-49, 51, 54-55, Alkalirresistant cellulose 36, 40, 54, 58-62,71,79,104,115,118, 70,102 123-126, 130,132, 135, 138,140, Alpha-cellulose 36-40,61,102, 142, 146, 150" 115-117,123-124,127-128,131, Bromide (radioactive) 54 135, 141-142, 147, 150 Buffer liquor 15, 18, 63-65, 68, 69 Ammonium bisulfite 11 Buffer storage 72,188 Ammonium sulfite 69 Burgess-Watt process 75 Arabinose 22,61,71,82,123, Burnt cook 16,18,29 127-128, 130, 135, 141-142, 147 By-product recovery 190 Arabinoxylan 22 Calcium-base cooking acid 33 Arabitol 84 Calcium bisulfite 11 Aspen 167 Calcium recovery cycle 138 Asplund Defibrator 64-67 Calcium sulfite 11,13 Asplund sluice 7 Canadian International Paper Co. 5 Associated Pulp & Paper Mills Ltd. Capillarity of wood 29-31,167-169 4-5,8 Carbohydrates (see also Arabinose, Automatic control 10, 180, 184, Galactose, Glucose, Mannose, 187-188 Xylose) 16-17, 22-29,40, 76, Backhammars Bruk AB 4 79-92,102,104,129,132, Back-water pockets 35 136-137,157 Bark extractives 19,78 Carbohydrate sulfonic acids 28-29 Batch cooking 40,99,187-188, Carbon dioxide 64,66, 69, 108, 112 190-191 Carbonyl groups 132 Batch diffuser 165 Cartiera Burgo 4 Batch digester wash 159 Cartiera Vita Meyer 4 Beating time, see Paper pulp quality Celgar Ltd. 5 Beech 43-44 Cellulose (see also Alpha-cellulose) Belisce Kombinat 5 22-23,26,31,82,87-90,105 Benzyl alcohol 18 Cellulose du Pin, La 5 Bergvik mill 11 Cellulose du Rh&ne, La 4 Beta-cellulose 102 Cell wall 23, 26, 29, 85, 89, 95, 101 BillerudsAB 4,7,10-11,64,113 Celulosa Argentina 5 Birch 7,38,40-41,44,55,59,65, Central Laboratory of the Cellulose 67, 69, 71, 72-73, 87, 89-90, Industry 7 194 Continuous Pulping Processes

Central Laboratory of the Finnish Pulp Deacetylation, see Acetyl groups & Paper Industry [CL] 73 Deaeration, see Air in chips Central Laboratory of the Swedish Defibrator 64, 67 Paper Industry [PCL] 73 Defibrator digester 63, 72 Channeling (in chips) 166 Defibrator process 65-66 Chemical consumption (see also Alkali Defibrator pulp 31 consumption; Sodium sulfate; Sulfur Degree of polymerization 101 charge) 122, 139 Delignification 11-31, 76-79 Chemical recovery 41, 82, 133, 138, 181 Demethylation, see Methoxyl groups Chip damage 1, 28, 30-31, 36, 44-45, Diffuser 98, 159-162, 173, 189 157 Diffusion 29-31, 122, 165-169, 178 Chip dimensions 29-31, 168-170, 174 Digester capacity 97, 112, 178, Chip feeding and flow 1-2, 106-108, 181-182 111-113, 187 Digester design 1-2, 8, 10, 33-50, Chip impregnation 18, 29-31, 42-43, 63-73, 97-99 55,62,64, 121-157, 166-171, Digester wash 8, 10, 70, 162, 166, 182-183 173-178 Chlorine number, see Roe Number Digital control 188 Chlorite oxidation 82 Dilution curve 164-165 Cold alkaline purification 117-119 Dilution factor 163-165, 173-177 Cold blow 6-7, 10, 37, 44, 53, 97-99, Dimethyl sulfide 79, 81 101, 106, 108, 182, 191 Dicyclohexene 78 Color (see also Brightness) 43, 52, 55, Direct steam heat 108,111-112 60, 69, 78 Discoloration, see Color Combined sulfur dioxide 11-12 Displacement ratio 166,175 Compensating liquor 182,187 Displacement washing 162, 165, 173 Compressed air 134 Dissolving pulp 13, 23, 35-36, 38, Computer control 188 40-41,44, 75, 85, 105-119 Concentration quotient 163-165 Dissolving tank 134 Concora Medium Test [CMT] 67, 69, Dithionite bleach 51,60 71-72 Down flow digester 42, 44-45, 53, 60, Concurrent cooking 53,108-109, 105-108, 110-113 112, 127-129 Downtime 103, 115 Condensate odor 190 Eastern Corp. 5 Conductivity cell 187 Economics of continuous cooking Consistency 160,162,171 75-76, 105,133, 136, 179-191 Continental Can Co. 5 Effective alkali 87, 94 Continuous diffuser, see Diffuser Effective capillary cross-sectional area Continuous digesters, see Digester 167-169 Control loops 184,187 Efficiency of washing factor 171, Convection 132 174-177 Cooking degree (see also Lignin; Roe Ekman, C. D. 11 Number; Yield) 43, 187 Electrolyte system 92 Cooking liquor composition, see Sulfite Energy consumption, see Fiberizing: cooking acid; White liquor Refining Cooking liquor flow 1-3,35,43,52, Entrainment 166 106-119 Episulfide formation 77, 92 Corrosion 34 Epoxide formation 76 Corrugating medium 63-64, 68, 69, Erythritol 84 71-72 ESCO digester 73 Countercurrent cooking 86, 108-110, Eucalyptus 40-41, 76, 89, 102, 104, 112-113,117,119,124-129, 136, 107-108, 115-117, 121,123-125 150-153 Evaporator system 159,183-184,190 Countercurrent-flow digester 8-9, 53, Excess sulfur, see Sulfur charge 60,182 Excess sulfure dioxide 11-12, 33, 38, Countercurrent washing 97, 101, 51 107-108, 112, 117, 119, 122, 165, Extractives 18-20, 78 173-178, 181-182, 191 Feed-forward control 187 Dahl 75 Feed liquor 48,134 Dai Showa Seishi 5 Feeder system 1-2, 33, 36, 57, 66, Davenport press 64-65 103, 106, 108-109, 111-112 Index 195

Fengersfors Bruks AB 1,3,4 105,126,128 Fiber length 101-102 High-heat washing 10,108, 168, 171, Fiber wall, see Cell wall 178 Fiberizing (see also Refining) 64-66, High-yield pulping 15, 25, 53 127-128, 149, 189 History of Fibreboard Paper Products Inc. 5 acid sulfite cook 33 Fick's diffusion law 169 bisulfite cook 51 Filter washing 117,159-160,165, cold blow 7 175,188 countercurrent digester 8,10 Fine papers 75 Hi-Heat washing 10 Flash steam 113,183-184 Keebra process 139 Flash tank 99 kraft continuous cook 1-10 Flow ratio 42,171 kraft process 75 Flow resistance 166 multistage bleach 75 Foam problems 160,178,188 Mumin digester 37-38 Formaldehyde 77 neutral sulfite cook 10 Formic acid 17, 81, 84 prehydrolysis kraft cook 10 Free sulfur dioxide 11-12 pulp washing 159 Friction 41-42 sulfite continuous cook 7, 10 Galactan 91 sulfite process 11 Galactoglucomannan 22, 102,131, Holocellulose 26 136-137 Hot blow 66,97-99, 101-102 Galactoglucomannan acetate 27 Hydraulic pressure 31, 52, 144,148, Galactose 22, 61, 71, 123, 127-128, 181 130, 135,141-142, 147 Hydrazine 82 Gamma-cellulose 102 Hydrogen sulfide 82 Gas bubbles 66,110 Hydrosulfide ion 76-77, 79 Glassine/greaseproof pulp 7, 63, 67, Hydrosulfide-sulfide cook 134, 136, 69 140-141 Gluability 73 Hydroxycyclopentane carboxylate 78 Glucan 91 Hydroxylamine 82 Glucomannan 22-23, 26, 36, 38, 45, Hydroxypyruvic acid 84 48, 50, 82-84, 86-89, 102, 123, 132 IMPCO digester 63 Glucometasaccharinic acid 84 Instrumentation 10, 43, 180, 184-186, Glucosaccharinic acid 83 187-188 Glucose 40-49,59,61,70-71,73 International Business Machines [IBM] 104, 123, 127-128, 130, 135, 141-142, 147, 150 International Paper Co. 4, 7, 188 Glucuronic acid 22, 25, 28, 86 Iron in pulp 78,115 Glucuronoarabinoxylan 27, 82, 88, Iso-hemi lines 27 102,131, 136-137 Isosaccharinic acid 81,83 Glucuronoxylan 26, 81, 87, 89 Jansson, Hannes 7 Glycolic acid 84 Jansson, Lennart 7 Glycoside hydrolysis 22-23, 81-92 Jonsson, Ragnar 3 Green liquor 138 Jossefors mill 7,34, 123, 173 Gross washing efficiency 170 Joutseno Pulp Oy. 4, 6, 7 Groundwood 31 Jujyo Seshi KK. 5 Guaiacylpropane 78 KamyrAB 3,97,188 Gulf States Paper Co. 4 Kamyr digester 1-10,46-47,63,67, Hardwood pulping 15, 25, 28-29, 105-108,138,161,163,174, 63-73, 95-96,109 180-181,184-188 Heat balance 182,186 Kamyr impregnating vessel 144 Heat economy 10, 43, 113, 119, Kamyr washing system 161,163 182-184 Karlsborg pilot plant 1 Heat exchangers 108 Keebra process 139 Heat recovery 116 KMW machine 73 Heat transfer 108 Knots 33 Heat-up time 18,37-38,51, 55, 64 Kokusaku Pulp KK. 5 107-108,112,121,132,135,148 Kommonen efficiency factor 171 Hemicellulose (see also Xylan, etc.) Kraft cooking 22-29,70, 87, 89-90, 94-95, 101, continuous 97-104 196 Continuous Pulping Processes

historical development 1-10 Norden efficiency factor 171 "modified" processes 82, 121-157 North Western Pulp & Power Co. 4 prehydrolysis process 105-119 NSSCpulp 63-73,175,177-178 semichemical 127-157 Oak 29 theory of 75-96 Odor control 78, 180, 182, 190 Kraft digester 1-10 Ohji-Seishi KK. 4 Kraft pulp 94-95,102-104,175-177 Opacity 95 Kramfors process 19-20 Oxford Paper Co. 5 Lanor economy 10 Packaging papers 75 Lactic acid 81, 84 Packing density 42-43, 55, 114, 169, Leirosa (Portugal) pulp mill 113, 123 171 Level control 2,35,112,122 Pandia digester 63 Lignin content, see Roe Number Paper pulp quality 28, 44-45, 50-51, Lignin enamel 31 54,59-61,63,67,69,71,94-95, Lignin reactions 11-12, 15-21, 29, 99-104, 121-157 76-81, 87-91, 139 Peeling reaction 81,88,92 Lining-up of feeders 33 Pentosans 54, 70 . Liquor balance 182 PH 13,16,36-37,63,105,109,113, Liquor ratio 41-42, 53, 55, 108, 127, 167-168, 187 138-139, 174-176 Phenolic extractives 18-20 Loschbrandt 27 Phenolic groups 77, 92 Luminae in wood elements 29 Piggyback digester 136 M&D digester 63, 136 Pine 18-19,45,48,55,59,76, McKibbins, S. W. 169 87-91, 96, 102-104, 107-108, 116, Magazine-grade paper pulp 51 121, 123-127, 129, 136, 146-148, Magnetite neutral process 20 156-157, 167,175-177 Magnesium-base liquor 41 Pinosylvin 18 Magnesium bisulfite 11,58-59 Pitch deposits 106-110,112 Magnesium sulfite 13 Pit membrane 29 Maintenance 103,111 Pocket feeder 1-2 Mannan 83, 91 Polysaccharide, see Carbohydrates Mannose 22-24, 40, 48-49, 59, 61, Polysulfide cook 82, 85-88, 90, 125, 70, 72, 104, 123, 127-128, 130, 129-136, 139-141, 146-147, 149-153 135, 141-142, 147, 150 157, 175, 177 Mass balance 182,187 Power demand (see also Fiberizing; Re Mechanical pulp 31 fining) 65,124,127,133,149,152 Mercaptide ion 79 Power failure 103 Mercapto groups 77 Prehydrolysis kraft cook 10, 85, 90, Metasaccharinic acid 82 94, 105-119, 126, 178 Methane sulfonate 21 Preimpregnation of chips 62,139, Methoxyl groups 21, 78, 80-81 143-145,148,154-155, 183 Methylglucuronoxylan 79 Press washers 160 Methyl glyoxal 84 Pressure control 112, 122, 134 Methyl mercaptan 79, 81 Pressure impregnation 139, 143-145 Modified kraft cooks 121-158 Production reliability 7, 102 Moisture control 187 Progil SA. 4, 7 Moisture in wood 28, 30 Propanediol 84 Monel 1 Protolysis 18-19 Monosulfite cook 15 Pulp chests 114,188 Mo och Domsjo AB 7 Pulp mill capacity 160, 180-181 Multistage bleach 75 Pulp quality (see also Paper pulp quality) Multistage pulping, see Two-stage 6-7, 40, 48-50, 62, 66, 96, 99-104, Mumin digester 37-45, 54-61, 115-117,137,190-191 69-73, 111,119, 121-157 Pumps 1, 72 Natchez process 20 Pyrocatechol 78 Neutral sulfite cook 7,10, 15-16, Quinones 78 19-21, 28-29, 63-73, 103 Radioactive tracers 35,42-43, 52, 54, acid sulfite 2nd-stage 45-46, 103 113-114 Newsprint stock 51,61 Rayon pulp, see Dissolving pulp New Zealand Forest Products 4 Reactivity 85 Nippon Pulp Kogyo KK. 5 Recovery system 41,116, 138, 181 Redox reactions 82, 85 87,95,103,156-157,177 Refining (see also Fiberizing) 65-69, Stainless steel 1, 33, 64 70,124,133,152,178,189 Startup 114-115 Relative liquor volume 163-164 Steam economy 41, 111, 114,182-183, Resin in pulps 27, 40, 48-49, 54, 59, 188 61, 71, 104, 117, 123, 127-ll8, 130, Steam flow 112 135,138,140,142, 146 Steam hammering 33, 66 Retention time 43, 113-114, 116, 123, Steam hydrolysis 105 127, 130, 140, 142, 146, 171, 174 Steam shock 30-31 Richter, Johan 3,10 Steaming 30-31,119 Roe Number 40,48-50,54,59,61,71, Steaming vessel 33, 144 94,104,115-117,123, 127-128, Stiffness 63 130, 135, 139-140, 142,146, 150, Stockman, Lennart 7 154-155 Stora process 19 Rosenlev, V., & Co. AB 4 Strainers 52, 99, 106-109, 113, 144, Runnability 149 160, 162,173 Saccharinic acids 81 Styrene sulfonate 21 SAICI (Italy) 105 Substance yield factor 163-165, 175 St. Regis Paper Co. 5 Sudbrook Pulp Mill Ltd. 4 Saltcake, see Sodium sulfate Sugars, see Carbohydrates Sanyo Pulp KK. 5 Sulfate formation 16-17 Sawdust 35 Sulfidation of lignin 13 9 Screen rejects 18,40, 45, 48-50, 55, Sulfide cook 15,75,134 59,109,115-116,123-124,126, Sulfidity 87,93,115,133,138, 128, 130, 132,135-136, 138, 140, 140-141 146, 148, 150 Sulfite-bisulfite mixed cook 15 Screw press 99-100 Sulfite cook 7,10-31 Secondary wall 23, 26, 85, 89-90 Sulfite cooking acid 11-17, 33,41, 48, Semichemical pulp 63-65, 68, 69 bisulfite 51-62 Sulfite pulp 26, 164 kraft 127-157,175-176 Sulfitolysis 12, 19, 21, 29 neutral sulfite 63-73 Sulfonation 11-12,15, 18-19, 21, poly sulfide 133 28-29, 63-64, 139 Separators 111-112,121, 148, 183 Sulfur charge 129-130,135-136, Shives 52, 62, 149 138-140, 146 Skogsagarnas Cellulosa AB 5 Sulfur dioxide 11-31,51 Skutskar process 20 Sulfuric acid 34 Slip planes 31,101 Sulfurous acid 11,14, 16 Sloman, Ray 8 Summerwood (latewood) 95 Soda process 75,90,125 Swedish Cellulose Co. 10 Sodium acetate 64 Swelling degree 171 Sodium-base liquor 41,70 Tall oil soap 178,189 Sodium bicarbonate buffer 15, 63, Tannins 19,78 65-66,68,69 Techmashimport 5 Sodium bisulfite 11, 16-17, 5 8 Temperature schedules 11-12, 20, 43, Sodium borohydride 82, 85-87, 90 64, 66-69, 81, 91, 98-100, 108-109, Sodium carbonate 15, 35, 48, 52, 132, 134,136, 140, 142, 146,168, 63-64, 138-139 171, 173-174 Sodium hydrosulfide 138 Terpenes 16 Sodium hydroxide 48, 66, 75, 139 Thermometry 43 Sodium sulfate 16-17, 75,163-165, Thiosulfate 16-17, 28, 52, 139 174-176 Tilghman, Benjamin 11 Sodium sulfide 15, 75-76,138-141, Tokai Pulp KK. 5 150-151 Top scraper 2-3, 35, 106 Sodium sulfite 63,68,139 Tramp iron 115 Softwoods (see also Pine; Spruce) 29, Two-body digester 34, 37-38, 43, 45, 95-96,167-169 51-52,105-107,110,125,136 Spent pulping liquor 159,164, 178 Two-stage polysulfide 129,134-136, Sprout-Waldron refiner 64-65, 67 138 Spruce 19, 25, 27, 30, 35,40-41, Two-stage sulfite cook 15, 19-20, 23-24, 44-45, 48-50, 54-55, 58-62, 65, 78, 44-50, 103 198 Continuous Pulping Processes

Tyloses 29 White Uquor 87, 92, 106-119, 121-122, Upflow digester 36, 39,44, 105-107 124-125, 129, 134-136, 148 Uronic acids 64,88,92 Whitewater 117-118 Usutu Pulp Co. Ltd. 5 Wifstavarfs AB 3-4 Vanillyl alcohol 77 Xylan 22, 25-26, 28, 39, 63-64, 79, 82, Vapor-phase cooks 55, 110, 112, 119, 85-87,91,109, 123,125-127,136 135-136,138, 144,146-149 Xylose 40, 48-49, 59, 61, 70, 71, 104, Vapor pressure 31 123,127-128,130, 135, 141-i42, Vaporspheres 190 147, 150 Viscose pulp, see Dissolving pulp Yield of Viscosity 40, 43-45, 54, 59, 102, acid sulfite pulp 49 115-117 alkaline sulfite pulp 143 Wall effect 178 bisulfite pulp 60-61 Washing 159-178,188-189 kraft pulp 78, 82, 86-87, 90 Wash liquor 117, 122,173, 175 modified kraft cooks 121-158 Wash zone bleaching 60 NSSCpulp 69,71-73 Water hydrolysis 105 prehydrolysis kraft cook 105,119 Water pollution 70, 116, 159 spruce pulp, strength affected by 95 Watt-Burgess process 75 sulfite pulps 23-25, 27-29 Weyerhaeuser Timber Co. 4 washing effect on 167,175-178