Continuous Pulping Processes

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Continuous Pulping Processes

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Queensland University of Technology. Brisbane, Australia

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T A P P I 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.

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Johan Richter Pioneer in Continuous Pulping Technology

Born in Lier, Norway, in 1901

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Continuous Pulping Processes

12 Lectures

By Sven Rydholm Director of Research

Billeruds AB

SPECIAL TECHNICAL A S S O C I A T I O N P U B L I C A T I O N • STAP N O . 7

Gardens Point A22810250B

Continuous pulping processes : 12 lectures

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A22810250B

Technical Association of the Pulp and Paper Industry 360 Lexington Avenue, New York, N. Y. 10017

Library of Congress Catalog Card Number: 74-140131

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

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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 dominated 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 combination 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

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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, myself, 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, particularly, 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 development efforts in machine and process design, perhaps rewarded in the continued success of digester sales of Kamyr and in the successful operation 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 viii

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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

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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 digester. 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

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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 digester. 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

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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 discharge 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 advantage 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

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Table 1.1. Pioneering Mills in Kamyr Continuous Digester Systems

Year Mill 1948

1949

1951

1952

1954

1955

Fengersfors Bruks AB, Fengersfors Cartiera Vita Mayer,

Cairate Cartiera Burgo, Ferrara Ste". An. Progile, Condat-

le-Lardin Ohji Seishi KK, Kasugai Associated Pulp and Paper

Mills Ltd, Bumie Tasmania Wifstavarfs AB,

Vifstavarv Joutseno Pulp OY,

Joutseno Joutseno Pulp OY,

Joutseno Cellulose du Rhdne,

Tarascon N Z Forest Products,

Tokoroa Billeruds AB, Saffle Billeruds AB, Gruvon Billeruds AB, Jossefors Backhammars Bruk AB,

Bjorneborg North Western Pulp & Power

Co., Ltd., Hinton, Alberta North Western Pulp & Power Co., Ltd., Hinton, Alberta International Paper Co.,

Camden, Arkansas V Rosenlew & Cp. AB,

Bjorneborg Sudbrook Pulp Mill Ltd.,

Sudbrook Cellulose du Rh6ne,

Tarascon Ohji Seishi KK, Kasugai Gulf States Paper Co.,

Demopolis, Ala.

Weyerhaeuser Timber Co.,

Country Sweden Italy Italy France Japan Australia

Sweden Finland Finland France New Zealand Sweden Sweden Sweden Sweden Canada Canada USA Finland England France Japan USA USA

Capacity, tons/day 50 90 25 40 90 60

100 120 120 60 60 60 150 10 150 250 250 150 200 60 150 90 350 150

Process Kraft Kraft NSSC Kraft Kraft Soda

Kraft Kraft Kraft Soda Kraft NSSC Kraft Pilot Kraft Kraft Kraft Kraft Kraft NSSC Kraft Kraft Kraft Kraft

: Wood Softwood Softwood Straw Chestnut Softwood Eucalypt

Softwood Softwood Softwood Esparto Softwood Hardwood Softwood Softwood Softwood Softwood Softwood Softwood Hardwood Softwood Softwood, hardwood Softwood Softwood Longview, Wash.

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Table 1.1. Pioneering Mills in Kamyr Continuous Digester Systems Mill

Nippcm Pulp Kogyo KK, Yonago Jujyo Seshi KK,

Yataushiro Sanyo Pulp KK,

Iwakuni

Eastern Corp., Lincoln, Me.

Skogsagarnas Cellulosa AB, Monsteras Skogsagarnas Cellulosa AB,

Monsteras Kokusaku Pulp KK,

Asahigawa Dai Showa Seishi,

Fuji La Cellulose du Pin,

Facture La Cellulose du Pin,

Facture Continental Can Co.,

Nixon Station, Ga.

Celulosa Argentina Belisce Kombinat Celgar Ltd., Castlegar, B. C.

Celgar Ltd., Castlegar, B. C.

Fibreboard Paper Prod. Inc.

Antioch, Calif.

Oxford Paper Co., Rumford, Me.

Associated Pulp and Paper Mills Ltd., Burnie Usutu Pulp Co. Ltd.

AB Statens Skogsindustrier, Lovholmen AB Statens Skogsindustrier,

Lovholmen Tokai Pulp KK,

Shimada Techmashimport Techmashimport St. Regis Paper Co.,

Tacoma, Wash.

Canadian International Paper Country Japan Japan Japan USA Sweden Sweden Japan Japan France France USA Argentina Yugoslavia Canada Canada USA USA Australia Swaziland Sweden Sweden Japan USSR USSR USA Canada

tons/day 120 120 120 150 120 120 150 150 150 150 350 100 50 250 250 250 225 100 300 150 300 150 420 420 300 300

Process Kraft Kraft Kraft Kraft Kraft Kraft Kraft Kraft Kraft Kraft Kraft Cold Caustic NSSC Kraft Kraft Kraft Kraft Soda Kraft Kraft Kraft Kraft Kraft Kraft Kraft Kraft

Wood Softwood Softwood Softwood Hardwood Softwood Softwood Softwood, hardwood Softwood Softwood Softwood Softwood Eucalypt Hardwood Softwood Softwood Softwood Softwood Eucalypt Softwood Softwood Softwood Hardwood Softwood Softwood Softwood Softwood

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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 disturbances, 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

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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 circulation 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

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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 digester. Ray Sloman, who had ordered his first Kamyr digester for APPM, Burnie, Tasmania, as early as 1951, wanted to run his digester counter-

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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 circulation; (12) blow tank; (13) knotter; (14) washing filter.

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10 Lecture 1

currently, succeeded, and has done so ever since. There may still be different 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 counter- current 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.

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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 composition 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 S 02, which is generally 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 increasing temperature.

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Fig. 2 . 1 . Mechanism of delignification according to t w o concepts:

sulfonation followed either by sulfitolysis or by hydrolysis with subsequent further sulfonation in solution.

Fig. 2.2. Sulfite cooking acid concepts and composition.

12 Lecture 2

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Sulfite Cooking Process Theory 13 Temperature, C

25 70 100 110 120 130 140 150

Ka 0.0172 0.0046 0.0024 0.0016 0.0011 0.0008 0.0005 0.0003

pKa 1.8 2.3 2.6 2.8 3.0 3.1 3.3 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

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 magnesium base cooking liquors, up to pH 5. A straight bisulfite solution, of 4% total and 2% combined S 02, for example, has a pH at room temperature 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 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

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14 Lecture 2

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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 S 02, 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 S 02, 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 carbonate 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 indicates 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.

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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 previously 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-

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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.

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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 decomposition 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,

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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 process 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

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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 capacity demands).

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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).

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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

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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

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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 temperature conditions of stage I.

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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 bisulfite 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 degradation in the acid cook, whether deacetylated or not, at least under the

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Sulfite Cooking Process Theory

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.

27

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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 softwoods.

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 process 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

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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 softwoods, 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 problems 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 penetrated, 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 resistance 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

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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

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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, impregnation 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 damage to the wood. That mechanical damage in combination with acid sulfite 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 longitudinal 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.

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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 developments 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 exchangers, 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 uncooked 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 digester 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

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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 experimental pulp mill was created at Jossefors in 1956, it contained a digester designed accordingly (Fig. 3.2). In order to separate the two stages completely, 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 bottom 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 mechanical experience, e.g., with the material of the high-pressure feeder, corrosion in the digester top and behind blind strainer plates, where cooking acid decomposition took place and sulfuric acid corrosion was severe. 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 34 Lecture 3

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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 solutions of radioactive sodium carbonate. We put sight glasses on the digester 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 decomposition caused by back-water pockets in the digester, we succeeded in getting a proper delignification and deresination; but how about the carbo-

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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 conditions. The desired improvement was obtained when pH was decreased to

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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

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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 construction 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,

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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 liquor. 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

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Table 3.1. Pulp Quality Obtained from Spru e, Birch, and Eucalypt Rayon Pulps by the Sulfite Process, Using Batch and Continuous Cooking

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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 cooking 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 S 02. In a larger digester, the influence of seal water from circulation pumps and other water-sealed shafts is less pronounced, and a somewhat larger volume of cooking liquor can be tolerated yet to achieve such low liquor ratios. However, much higher concentrations of cooking liquor must be realized than the normal 1.2-1.3% combined 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 system will, however, have to allow for this demand in order to realize the possibilities of a better steam economy offered by continuous cooking.

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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 liquor 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 difference 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 dissolution 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

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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 accurate 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,

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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 cooking 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