• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Illustrations
 Abstract
 Introduction
 A brief history of chemical...
 The chemical and physical composition...
 Sulfite chemistry and the sulfite...
 The status of the literature
 Definition of the project
 Experimental equipment
 Experimental procedures and presentation...
 Discussion of results
 Conclusions and recommendation...
 Summary
 Reference
 Appendix
 Biographical items
 Copyright














Title: study of the mechanism of sulfite pulping.
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
    List of Tables
        Page iv
    List of Illustrations
        Page v
        Page vi
    Abstract
        Page vii
        Page viii
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
    A brief history of chemical pulping
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
    The chemical and physical composition of wood
        Page 12
        Page 13
        Page 14
        Page 15
    Sulfite chemistry and the sulfite pulping process
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
    The status of the literature
        Page 25
        Page 26
        Page 27
    Definition of the project
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
    Experimental equipment
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
    Experimental procedures and presentation of the data
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
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        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
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        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
    Discussion of results
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
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        Page 111
        Page 112
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
    Conclusions and recommendations
        Page 119
    Summary
        Page 120
        Page 121
    Reference
        Page 122
        Page 123
    Appendix
        Page 124
        Page 125
        Page 126
        Page 127
        Page 128
    Biographical items
        Page 129
        Page 130
    Copyright
        Copyright
Full Text















A STUDY OF THE MECHANISM OF

SULFITE PULPING











By
MICHAEL REID SHAFFER


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY










UNIVERSITY OF FLORIDA


August, 1966
















ACKN WLEDGEMENT


The author wishes to express his indebtedness to Dr. J.

Nolan, Professor of Chemical Engineering, for his keen interest and

valuable guidance in this research work. The following is a partial

list of persons who assisted in equipment fabrication, operating pro-

cedures, chemical testing, and literature translation: L. Hughlett,

K. Kalloway, K. Hutchins, G. Rose, J. Strickland, W. Ferguson, R. Baxley,

E. Warshyk, and S. I. Chang. The author also wishes to thank the other

members of his committee, Prof. R. D. Walker, Dr. R. Fahien, Dr. W. H.

Beisler, Dr. P. M. Downey, Dr. D. B. Wilcox, the late Dr. H. Meyer,

and Dr. R. G. Blake for their interest and participation.














TABLE OF CONTENTS


Page


Acknowledgment . . . . . . .
List of Tables . . . . . . . .
List of Illustrations o . . . . .
Abstract of Dissertation . . . ...
Introduction . . . . . . .


Chapter


* 0 0

* .
. .
. .
. .
. .


I. A Brief History of Chemical Pulping . .
II. The Chemical and Physical Composition
of Wood . . . . . . . .
III. Sulfite Chemistry and the Sulfite
Pulping Process . . . . . .
IV. The Status of the literature . . . .
V. Definition of the Project . . . .
VI. Experimental Equipment . . . . . .
VII. Experimental Procedures and
Presentation of the Data . . . .
Wood Preparation . . . . . .
Liquor Preparation. . . . . .
Constant Temperature--Constant
Concentration Cooks . . . . .
Cooks with a Varied Ratio of "Free"
to "Combined" . . . . . .
Batch Type Cooks . . . . . .
Increasing Temperature--Falling
Concentration . . ...
Increasing Temperature--Constant
Concentration . . . . .
Pulp Handling and Treatment . . ..
VIII. Discussion of Results
General Equation for Reaction Rate . .
Prediction of Cellulose Degradation . .
The Effect of Varying the "Free" to
"Combined" Sulfur Dioxide . .
Batch Cooks . . . .. . ...
Observations Related to Pulping Theories


ii
iv
v
vii
1


C *

C *


Conclusions and Recommendations . . . . .
Summary .. . . . . . . . .
References Cited . . . .. . .
Appendix At Numerical Solution of the Rate Equations
Biographical Items . . . . . . . .


iii


* C

. C
* SO

* .

* C
C 0

* C
* C


63
66

66

72
72

75
96

104
108
113

119
120
122
124
129















LIST OF TABLES


Table Page
1. Key to Apparatus Sketch Numbers . . . . . 37

2. Constant Temperature Constant

Concentration Cooks ............. 50

3. Cooks at 6.0% Total SO2 with 3.5%

Free SO2 and 2.5% Free SO2 Respectively . . 64

4. Batch Cooks . . . . .. . . 67

5. Constant Concentration Cooks with

Batch Cooking Temperature Cycle . . . . 70

6. Calculation of K . ...... ..... 79

7. Computation of the Rate Coefficients Using

Equation 14 . . . . .a .* . 91

8. Solution of the Rate Equation . . . . . 93

9. Test of Degradation Prediction Equation . . . 102













LIST OF ILIUSTRATIOUS


F


igure Page
1. U. S. Chemical Pulp Production (1900 to 1960) . .. . 8

2. Flow Diagram, Sulfite Digester . . . . . . . 38

2a. Schematic Layout of Digester as Used in Constant
Concentration Cooking . . . . . . . ..

3. Sulfite Digester . . . . . . .... ... 39

4. Storage and Measuring Tanks, Sulfite Digester . ... 4.

5. Constant Temperature--Constant Concentration Cooks,
Nominal Concentration 2.5% Total S02 .. . . 54

6. Constant Temperature--Constant Concentration Cooks,
Nominal Concentration S.O. % Total SO2 ..... 55

7. Constant Temperature--Constant Concentration Cooks,
Nominal Concentration 7.5% Total S02 .. . 56

7a. Cross Plot of Data for Smoothing Purposes . .. . .57

8a. Cellulose Degradation as a Function of Cooking Time . . 9

8b. Cellulose Degradation as a Function of Cooking Time . . 60

8c. Cellulose Degradation as a Function of Cooking Time . . 61

9. The Effect of the Ratio of Free SO2 to Combined SO2 . . 6

10. Batch Cooks of Spruce ..... . . . . ... 68

11. Constant Temperature Cooks with Batch Temperature
Cycle . . . . . . . . . . . 71

12. Lignin Yield Related to Cooking Time . . ..... 77

13. Lignin Analysis Corrections . . . . . . . . 78

14. Constant Temperature--Constant Concentration Lignin
Related to Total Yield . . . . . . 85













Figure Page

15. Reaction Rate Constant Determination . . . . 87

16a. Simplified Representation of Degradation of
Cellulose with Cooking Time . . . . * 98

16b. Simplified Representation of Degradation of
Cellulose with Cooking Time . . . . . 99

16c. Simplified Representation of Degradation of
Cellulose with Cooking Time .... * . . 100

17. The Relationship Between the Reaction Rate
Coefficient and the Ratio of Free S02 to
Combined S2 . . . . 105

18. Lignin Yield Total Yield with a Varied Ratio
of Free SO2 to Combined SO2 . . . .. 106

19. Apparent Chemical Consumption and Waste Liquor
pH for Batch Pulping of Shredded Spruce . . 110

20. Lignin Yield of Batch Cooks . .. . 111













Abstract of Dissertation
Presented to the Graduate Council
in Partial Fulfillment of the Requirements of the
Degree of Doctor of Philosophy






A STUDY OF THE MECHANISM OF
SULFITE PULPING




by


Michael Reid Shaffer
August, 1966




Chairman Dr. William J. Nolan

Major Department: Chemical Engineering


An exploratory study of the mechanism of the bisulfite type

of sulfite pulping was made. By simulating constant concentration -

constant temperature conditions, kinetic data for pulping spruce wood

with sodium bisulfite over a sulfur dioxide concentration range of

from 2.5 to 7.5 per cent were obtained. Three temperature levels were

studied: 1600C, 1700C, and 1800C. From these data it is demonstrated

that a rate equation useful in reactor design or process control can

be developed. A method for predicting the accompanying degradation

of the cellulose has been shown.












Using batch type pulping methods, a measure of the chemical

consumption in bisulfite pulping was obtained. An interesting area

of further work was indicated by the rather unusual pH and chemical

consumption relationships observed.

A limited series of experiments was undertaken to observe the

effect of deviations from the bisulfite condition in the pulping chem-

ical. The amount of sulfur dioxide present for a given quantity of

cation was shown to have a very large effect on the reaction rate.

No difference in selectivity of lignin removal with varying ratios

of anion to cation was observed. This means that highly superior

special sulfite pulping processes are unlikely.

The over-all implications are that further development of the

moving interface theory of pulping should be undertaken. All observa-

tions made in this work indicate that more effort would be justified

in this area. Several methods of further testing and/or developing

the theory are suggested.


viii













INTRODUCTION


The timeliness of an engineering research project is always

of considerable concern since engineering, by its very nature, must be

built on a skeleton of economic considerations. After almost a century

of commercial application, the chemical pulping industry is again in a

state of technological upheaval. New processes are being proposed, old

processes are being restudied for new applications, and a new technology

is developing.

The area in which the greatest turmoil and uncertainty seems

to exist is in the acid or "sulfite" pulping field. A broad, engineering-

based study in this area would do much to define both the variables in-

volved and the benefits to be obtained as well as the true limitations

of the process. In commercial sulfite pulping four different "bases"

or cations are used to buffer the pulping solutions. There are single-

and multi-stage processes. The pH range is from about one and a half

to about four and a half, which, in terms of hydrogen ion concentration,

is three orders of magnitude. Each user and each investigator claim

higher yield, superior pulp properties, or application to difficult

wood species.

There are principally two reasons for all the concern. One is

that pulp properties are becoming more important. The revolution in the

container field resulting from such developments as steel foil, the com-

posite wall can, the multiwall bag with a coated polymer vapor barrier,
1










2

the printable display type shipping carton, the enormous cost reduction

in polymer films, to name a few, has forced a more vigorous evaluation

of pulp properties. The pulp and paper industry has had an undisputed

monopoly in the low cost packaging material line. To survive the on-

rushing competition it is having to evaluate and improve both properties

and costs.

The other side of the sulfite pulping industry's problem is

steam pollution. Some years ago it was a generally held opinion in the

industry that the sulfite mill effluent in North America was sufficient

to render the combined flows of the Columbia, MLississippi, and Hudson

rivers unfit for fish. Inasmuch as the "clean water" movement has steadily

gathered strength, even mills which have demonstrated that the sulfite

effluent they discharge is not sufficient to be noxious or to be harmful

to marine life find themselves being attacked on esthetic grounds. Recov-

ery of the old traditional calcium base sulfite process waste liquors can-

not be done economically today. Once a change to a more readily adapted

process is considered, the entire question of process choice is thrown

open. There are no clear-cut guides based on a sound technology to rely

on. Even complete abandonment of the acid pulping process, and in some

cases the mill site, is advanced as a solution to the problem.

Over the past ten years the most interesting and apparently com-

mercially successful change in the sulfite process that has occurred has

been in the area of increased pH. The accompanying higher concentrations

of pulping agent are claimed to have some advantages in both pulp proper-

ties and the pulping of species not adaptable to the traditional sulfite









3
process (1) (2). Such processes, "Magnefite" with a magnesium bisulfite

pulping agent and "Arbiso" with a sodium bisulfite liquor, have received

extensive pilot plant testing and some commercial application. At the

time this work was begun there were no available data on kinetics and

reaction rates in the bisulfite pulping area.

It is the purpose of this project to demonstrate that rate equa-

tions suitable for the ordinary chemical engineering type of reactor

design calculations may be obtained. Further, it is hoped by this

demonstration to establish, at least over a substantial portion of the

pulping range, the effects of some of the important process variables

on both pulping rate and cellulose quality. It is intended that some

insight into the mechanism of pulping be obtained and that methods will

be outlined for further work. To accomplish these goals a specialized

piece of equipment was required. This was specified, designed, and

built as a part of the project. Some discussion of the variables and

an outline of the dissertation will complete the introduction.

The sulfite pulping process uses the sulfite and bisulfite ions

to dissolve the lignin, the material which binds together the fibers in

wood or other cellulosic materials. This dissolution of lignin must

be done in such a manner that the relatively undamaged fibers can be

later felted into a sheet of paper or paperboard or dissolved to form

cellulose solutions. From the standpoint of chemical equilibria it

can be seen that the sulfite pulping solutions must contain bisulfite

ions, sulfite ions, and dissolved sulfur dioxide since the pulping

agent is primarily a solution of sulfur dioxide to which a metallic

cation has been added.









4

The dissolved sulfur dioxide will approach an equilibrium with

any vapor phase present above the solution. Sulfurous acid has been used

without the metallic cation in laboratory scale pulp preparations, but in

commercial practice and most laboratory work a metallic oxide, hydroxide,

or carbonate is added. Among other things, this lowers the vapor pressure

of sulfur dioxide over the solution.

In commercial practice it is obvious that a minimum chemical re-

quirement would be sought for economic reasons. This consideration then

links together both concentration and the amount of pulping liquor pro-

vided. In the traditional sulfite process this consideration is compli-

cated by the fact that sulfur dioxide gas is released throughout the

process to avoid excessive pressures.

The temperature used in most commercial sulfite pulping processes

is similarly ill-defined. The steaming rate is usually gauged by the

pressure and this is interrelated to the amount of dissolved sulfur diox-

ide present and the chemical composition of the liquid phase. The cur-

rently widely accepted theory on the mechanism of sulfite pulping calls

for a period of slowly rising temperature, or "warm-up," to allow the

liquor to penetratedt the wood structure. This is followed by a period

of approximately constant temperature in which the reaction is assumed

to take place.

In addition to the variables of pulping liquor composition, ini-

tial concentration and amount supplied, and the temperature, a number of

variables such as wood species, chip size, and chip moisture are also

important. While there is much uncertainty about the relative importance

of the sulfite pulping variables, a generalized list could be s:












1) The physical and chemical properties of the wood or

other fiber source

2) The physical form or subdivision of the wood

3) The chemical composition of the digesting agent or

pulping liquor

h) The ratio of "liquor" to wood, or the amount of

liquor supplied

5) The temperature and its variation

6) The cooking time


In this project the first two general variables listed were

held approximately constant (within the best technique obtainable) and

the effects of the last four general variables studied.

The experimenter has selected bisulfite pulping as an area

of interest. He felt that sodium base, although currently limited in its

area of application by waste recovery problems, was the most promising

since relatively low cost sodium compounds usable in the process are

available. Further, there are no solubility limitations with sodium base

for any expected pulping conditions. Magnesium base is more readily re-

coverable, but the magnesium used must be low in calcium to avoid scaling

of the recovery equipment, and magnesium sulfite and bisulfite have very

definite solubility restrictions that fall well within the possible

pulping range.

Spruce wood was selected as a species. This selection was made

to allow comparison with the work of others. Methods for control of wood












properties and subdivision are treated in detail in the portion on pro-

cedures.

The remainder of this work is divided in the following manner.

Chapters 2, 3, h, and 5 are background material intended to provide the

reader with sufficient grounding in the pulp and paper industry, its

nomenclature, literature, and principal processes such that an under-

standing of the project and its meaning and motivation may be achieved.

In Chapter 7 the apparatus designed and built for the project

and the procedures used are discussed in detail. Also, recommendations

for improvements that have resulted from experience are discussed. Pre-

sentation of experimental data is made in Chapter 8.

Chapter 9 is the discussion of the results and the conclusions

drawn from them. Recommendations for planning further work are included.














CHAPTER I


A BRIEF HISTORY OF CHEMICAL PULPING


The manufacture of paper is an ancient art. Developed by the

Chinese as a hand process, paper-making flourished for centuries almost

unchanged. The Industrial Revolution promoted change since it not only

supplied the technology necessary to mechanize the production of paper,

but also created an enormous new demand. Although the hand manufacture

of small quantities of paper for the communications of the era preceding

the Industrial Revolution met the needs of the day, the economic and

political changes that occurred in the Western world at the beginning

of the 19th Century brought about an enormous increase in both record

keeping and mass communication.

The development of the paper machine is a fascinating story,

but we are more concerned with one of the problems it created rather

than with those it solved.

Once paper-making was mechanized, the problem of pulp supply

became critical. The rag, straw, and bark sources were no longer ade-

quate. In the first 75 years of the 19th Century, three major processes

for preparing paper-making pulps from wood were developed. A mechanical

or "groundwood" process and two chemical processes, one alkaline and one

acid, appeared. A variation of the alkaline process gave pulps of high

strength but poor color. The acid process gave pulps of good color that

could be readily bleached where white papers were desired.

7



















































0 1 I ---- I I
1900 1910 1920 1930 1940 1950
Year


Figure 1. U. S. Chemical Pulp Production (1900 to 1960, NSSC Included) (3)


1960










9
Whereas the acid process used calcium bisulfite and sulfurous acid, the

alkaline process used sodium hydroxide and, in the improved process,

some sodium sulfide. withoutt recovery of the chemicals the acid process

was by far the more economical, and only the special pulp characteristics

and applicability to certain species of wood kept the alkaline process

alive.

In the 20th Century, two factors came into play that changed the

character of the chemical pulping industry. A chemical recovery system

for the modified alkaline or "Kraft" process was gradually developed, and

by about 1935 was good enough to change the economic picture completely.

This system not only recovered chemicals but also allowed the use of a

single lower cost material to supply the make-up chemicals, salt cake,or

crude sodium sulfate. A fortuitous distribution of losses allows a sin-

gle material, sodium sulfate, to supply both sodium and sulfur in proper

proportion, which is about 3:1 on a molar basis.

In addition to chemical recovery, the second factor that helped

change the chemical pulping industry is the fact that the Kraft process

has the advantage of being applicable to practically all wood species.

Pine was not readily pulped by the original acid process for reasons

which are presumed to be related to the presence of rosin and other

materials. But with the advent of the Kraft process, a tremendous impe-

tus was given to the production of chemical pulp in the United States

between 1930 and 1950. A whole new industry based on the Kraft process

grew up in the American South and West. Its effect on the national pulp

production figures is shown in Figure 1. A relatively modest growth is










10

shown for the acid or sulfite process while the expansion of Kraft is

enormous.

Two other factors now enter the picture: the development of a

new pulping process and the broadening of the pulping base in the sulfite

process to include other cations in addition to calcium.

The new process is known as the Neutral Sulfite Semi-Chemical

process (NSSC) and is used for making pulps of moderate strengths in high

yields from hardwoods. This process is a combination of chemical and

mechanical pulping that has proven to be economical in spite of the use

of relatively expensive chemicals, sodium sulfite and sodium carbonate.

Its high yield keeps the wood cost down and mades the process practical.

Recently, however, a demand has risen for the development of a chemical

recovery system for this process so that it will be less expensive and

will not contribute to the stream pollution problem, about which there

'is growing concern. There has been some success in this development (4).

The change in the sulfite process to other bases, such as ammonia,

magnesium, and sodium for acid bisulfite pulping is also contributing to

the growth of the pulping industry. For the magnesium base there is a

relatively simple recovery process and for sodium base the systems being

developed for NSSC would be applicable. Recovery of ammonia base liquors,

however, is complicated by the thermal decomposition of ammonia in the

combustion process, and requires some special treatment such as ion

exchange.

The chemistry of pulping, although much studied, is little

understood. Because of large areas of ignorance, many unsubstantiated










11

theories have been advanced that have interfered with a rational process

selection for industry growth. In addition, chemical pulping is pres-

ently undergoing a transition from a fundamentally batch process to a

continuous process. The necessary re-evaluation of the chemical pulping

processes that is accompanying this transition has resulted in the re-

assessment of much of the theory that has stood previously unchallenged.

There is sufficient concern for the relative merits of the vari-

ous processes to warrant such re-evaluation. In one case, a group of 30

companies retained one of the larger independent research organizations

to evaluate the future of the acid pulping process (5). Many other

studies are available in the literature. The consensus seems to be that

the old calcium base acid process is non-competitive. Almost all the

investigators recommend changing to a more soluble base and many recom-

mend eliminating excess sulfur dioxide beyond that necessary to form the

bisulfites.

There is a wide divergence of opinion on the relative superior-

ity of the more soluble bases. Much of the work has consisted of a few

mill cooks or relatively few experimental cooks followed by a mill trial.

In no case have the process variables been studied completely enough to

allow an evaluation of the process through a study of its kinetics.

Practically all the chemical pulping agents in use attack cellu-

lose and cause a loss in fiber strength when their application is not

regulated within certain bounds. Another aspect of the problem of process

evaluation thus involves the selection of agents that will do the pulping

job without excessive attack on the cellulose. The literature and some

work done in this laboratory seem to indicate that, for an acid process,

the most likely areas of success may lie in the use of sodium bisulfite.











CHAPTER II-


THE CHEMICAL AND PHYSICAL COMPOSITION OF WOOD

A very brief description of the chemical and physical structure

of wood, the principal paper-making material, will be given in an attempt

to clarify the background of the problem. For economic reasons, wood is
the principal source of paper-making fibers and some discussion of its

properties should be presented.

The chemistry of wood is complex and, as yet, little understood.

Much of the theory that does exist consists of definitions rather than

a clear chemical picture. The three chief constituents that make up about

90 per cent of the weight of wood are: cellulose; the hemicelluloses or

complex sugars, such as arabans, manans, and pentosans; and a material

called lignin, whose chemical structure is still largely unknown.

The chemistry of the cellulose is perhaps the best understood.

The structure of this polymer and the individual building blocks of cel-

lulosic units are fairly well established. The structure of the individual

units is given by various authors (6) (7):




H H
CH2 OH 0 0

0 0

H H H
SCH20H


Ho 0--- o/ C
H CH / \ / H O/r

0' H CHOH H
12












These units form a linear chain. Basically cellulose is a typical

polymer with such attendant properties as the solution viscosity being

proportional to the chain length. Chains lying near one another inter-

act by a mechanism of "secondary valences" to form ordered regions or

micelless." These exhibit X-ray diffraction patterns similar to those

of crystals, and the groupings are often called crystallitess." This

sub-microstructure is further arranged in the form of fibrils visible

with an optical microscope. These fibrils are wound spirally to form

a hollow porous tube, or "fiber" skeleton. These hollow tubes are

closed at each end. The fibers form the grain of the wood and are

usually one to three millimeters in length, with a length to diameter

ratio on the order of 100 to one.

The function and location of the hemicelluloses are not so

clearly understood. Some of the hemicelluloses, such as pentosan, are

known to improve the strength characteristics of a paper web; however,

their function in the wood is open to discussion. The existence of

"tertiary" bonds has been proposed (8). For paper-making pulps, it is

generally advantageous to retain as high a hemicellulose content as

possible. A more detailed discussion and a review of the work done up

to 1953 may be found in a discussion by Nolan (9) on alkaline pulping.

Since that time much of the work has been similar to that of Galley (8),

an attempt to explain the observed strength improvement by various

mechanisms.

While the arrangement and distribution of cellulose is well

understood, the distribution of the hemicellulose within the fiber wall

*is not so well known. Where some studies have fairly well determined the









14

distribution of lignin by dissolving the cellulose, locating the hemi-

cellulose by such direct methods is not possible. The fact that small

amounts of hemicellulose have such a marked effect on strength and the

fact that Findley (10) found that when the hollow fibers are impregnated

with pulping chemical in NSSC pulping, the yield of hemicellulose is

increased, indicate that the distribution of the hemicellulose is such

that it might be more concentrated immediately under the fiber's lignin

sheath or middle lamella.

The lignin, whose chemical composition is not yet well defined

(11), seems to.serve as a "cement" in wood, bonding the fibers together.

The areas between adjacent fibers are filled with lignin and there is

lignin in lesser quantities throughout the fiber wall. This material

makes up about 25 to 30 per cent of the dry weight of wood. It bonds

fibers to one another and cements the fibrils in the fiber wall, impart-

ing stiffness and strength to the fiber and, under conditions where the

lignin has not been modified by chemicals or temperature, it renders the

cell wall impervious to water.

The hollow fiber structure mentioned earlier is interconnected

by a series of openings in the fiber wall. Each fiber has openings re-

ferred to in the literature of wood structure as "bordered pits." The

name comes from the appearance of the openings under an optical micro-

scope. These openings are aligned with those in adjacent fibers. In addi-

tion there is a system of larger open-ended tubes called "vessels" in the

deciduous woods and "resin ducts" in the conifers. By applying a vacuum

or during successive heating and cooling, wood may be made to hold about

twice its dry weight of water. The specific gravity of the actual woody











material is almost twice that of water. A simple calculation shows that

approximately three quarters of the volume of wood is open space. A

further demonstration of the open structure of wood and the interconnec-

tions between openings is sometimes accomplished by connecting one end

of a log to a fire hydrant and showing the flow of water lengthwise

through the log. (Note: The arrangement is different in geometry from

most of the porous bed problems encountered by chemical engineers in

such applications as fixed beds of catalyst because the bulk of the

open space in wood is within the fibers rather than between them. A

realization of the actual physical arrangement is important to an under-

standing of the physical mechanism by which the pulping chemicals reach

the lignin with which they are to react.)

1ith such a structure, it is not surprising that a theory for

the pulping of wood should arise which would demand that the voids be

filled with the pulping agent as aprerequisite to the pulping reaction.

This "penetration" theory has been rather thoroughly refuted for Kraft

pulping (12). In the case of the acid pulping process, an extension of

the pepetration theory was used to account for "black" or "burned" cooks.

It was claimed that the insoluble dark material was "condensed" lignin

caused by excess acidity. This acidity was in turn supposed to have re-

sulted from a more rapid diffusion of sulfur dioxide gas than was possible

for the liquid phase. This sulfur dioxide redissolved in the water al-

ready present in the wood voids to form an unbuffered sulfurous acid.

Previous work done here at the University of Florida indicates that the

penetration theory is unsound even in the case of acid pulping (13) (14).













CHAPTER III


SULFITE CHEMISTRY AND THE SULFITE PULPING PROCESS


One of the chief problems in using sulfite pulping agents is the

instability of the sulfite ion. Even at room temperatures the metallic

sulfites and sulfurous acid slowly decompose by a sort of auto-oxidation-

reduction towards sulfur as the reduced form and sulfate as an oxidized

form (15). The complex reactions involved in such decomposition have

been studied by Marusawa (16) and Foerster (15) among others. A refer-

ence to a standard inorganic chemistry text (40) shows a large number of

possible sulfur acids that depend on the oxidation level of the sulfur.

The most reduced form of sulfur is the sulfide ion and the most oxidized

form of sulfur is the sulfate ion. The sulfite ion lies between these

states. Looking at the over-all oxidation-reduction picture, a portion

.of the sulfite is oxidized to the sulfate state and a part is reduced to

elemental sulfur. Reduction to the sulfide state does not seem to occur.

A number of intermediate levels of oxidation and the resulting formation

of ions seems to result. One of the principal intermediate products seems

to be the thiosulfate ion, but others seem to be present in smaller quan-

tities. In general, high temperature and high acidity promote the decom-

position reactions.

Another factor that must be considered is the gaseous phase in

equilibrium with sulfur dioxide solutions. The concentration of the

16










17

sulfurous acid in the solution and all equilibria such as those involving

the sulfite and bisulfite ions depend on the partial pressure of the

sulfur dioxide in the vapor phase. Considering the over-all solution

reaction,

SO2 + H20 H2SO 3 + HSO;

there is a common ion effect with

NaHSO3 = Na+ + HS3-,

and the ionization of the sulfite,

Na2SO 2Na + SO,".


Also under consideration is the further dissociation of the bisulfit

HSOj H + SO .


The fact that there is considerable vapor pressure of sulfur dioxide

associated with bisulfite solutions, which by industry nomenclature have

no "free" sulfur dioxide, can be seen from Johnstone's correlation (17).

In general, there seems to be some confusion and ill-defined

nomenclature in this area in the industry literature. Perhaps it would

be well to discuss the situation since it will arise in referencing the

literature and again in the discussion of results. By definition, under

industry nomenclature, there is only one variable, concentration, that de-

fines the pulping agent used in bisulfite pulping once the cation or base

has been specified. This sort of thinking arises out of considering that

equimolal solutions (in the case of monovalent cations) of base and sulfur

dioxide result in a fixed and distinct form such as one would find in a

crystal. Considering the chemical equilibria involved one can see that the










18

bisulfite state is merely one point on a continuous surface of con-

centration of base and sulfur dioxide and attributing special proper-

ties to this point to distinguish it from its neighbors is a purely

artificial distinction. This point would be largely pedantic if it

were not for a second consideration. The decomposition of the solu-

tion changes the relation between the sulfite ion and the base because,

not only is the amount of sulfite ion reduced, but different equilibria

exist in the sulfate-cation system that results.

Having established an awareness of the considerations, it is

now intended to restrict the experimental work undertaken in this pro-

ject to an approximation of "bisulfite" pulping as defined in industry

nomenclature. The reasons are twofold. First and most importantly,

bisulfite pulping, by its industry definition, is the most active field

of interest in acid pulping. Secondly, such a restriction keeps the

experimentation within reasonable scope. Without the use of special-

ized statistical techniques recently published by Box (18) the intro-

duction of another variable would have tripled the number of experi-

ments.

The industrial background of the experimenter tended to em-

phasize the first consideration. The entire sulfite pulping industry

considers bisulfite pulping as a clearly limited project. Without the

knowledge of high temperature equilibria in a system which includes

the soluble lignin fragments and a method of tracking the shift in the

equilibria, such as with high temperature pH electrodes, the experi-

menter is fairly well limited to specifying a starting condition.










19
Specifying that this starting condition should be approximately the

bisulfite then becomes merely a convention. Useful work may be done

within this convention, but the over-all complexity of the problem must

not be forgotten or suppressed by such a device. Many of the paradoxes

and unexplained phenomena in the pulping technology result from the mis-

use of such conventions and definitions. As mentioned in the historical

sketch, there are, in industry nomenclature, three broad classes of

chemical pulping! acid, neutral, and alkaline. The distinction is made

strictly on the pH of the pulping agent. The best justification for

consideration of such a classification here is that the literature of

the pulp and paper industry is organized on this basis with respect to

indexing and bibliographies.

On such a pH scale, as mentioned previously, the process selected

for study is a modified, or relatively high pH, acid process. This is a

rather recent variation in the field of acid pulping. For reference, a

brief description of the conventional "acid bisulfite" pulping process

will be given. In this frame of reference the real and the supposed

significance of the increase in pH in the modified process will be more

apparent.

In all chemical pulping processes, the debarked logs are mechan-

ically chipped and the chips are loaded into a "digester." The digester

is a large autoclave type vessel that contains normally from 3000 to

5000 cubic feet of chips. Loading is often accomplished with mechanical

loaders that use a steam jet or similar device to obtain a dense compact

filling to efficiently utilize the space available. After filling the

digester with chips, the operator or "cook" closes the digester with a
rf









20
bolted or clamped cover. The cooking chemicals are pumped in and the

air which surrounded the chips is vented. The pulping process is start-

ed by heating. This heating may be by means of the introduction of

steam into the digester, "direct heating," or by circulating the pulp-

ing chemical, or liquor, through a surface-type heat exchanger and back

into the digester, which method is referred to as "indirect heating."

Since the pulping chemical in acid bisulfite pulping is a mix-

ture of the solution of metallic bisulfites and sulfurous acid, there is

a substantial partial pressure of sulfur dioxide in any gaseous phase

in equilibrium with the liquid phase. As the temperature is increased

to accelerate the reaction, this partial pressure rises along with that

of the water vapor. Air from within the chips is added to the mixture.

The vessel is suitably vented to prevent the total pressure from exceed-

ing the normal operating limits of the vessel.

This venting of the digester is called "relief." Considerable

care is given to relief schedules in an attempt to most effectively dis-

card the air and recover the sulfur dioxide for reuse in preparing more

pulping chemicals, or "cooking acid." The details of the schedules and

collection points are beyond the interest of this project. The concern

that is important is the fact that the release of sulfur dioxide in such

considerable quantities changes the entire character of the liquid phase

remaining, and conditions approach those of "bisulfite" pulping.

During the filling process, a compact, dense bed had formed, the

interstices of which were almost completely filled with the pulping agent.

The weight of this pulping agent is about five times that of the chips.










21

If an individual chip were examined, as mentioned earlier, one would fine

that from two-thirds to three-quarters of the volume was void. In the

usual case about half of this space is filled with water and the remain-

der with air. As the temperature within the digester rises, the total

pressure increases. If the liquid pulping agent is to penetrate into

the chips, the pressure in the space surrounding the chip must be higher

than the pressure within the chips. There is also diffusion of the ions

in solution into the water within the chip as a result of the concentra-

tion gradient. This flow and diffusion must follow the available chan-

nels, and for the bulk of the fibers within the chip, that is, those

not adjacent to either a surface formed in chipping or a naturally occur-

ring opening such as the resin ducts in the conifers and the vessels in

deciduous woods, this means passage through preceding fibers and inter-

connecting border pits. The fact that these fibers are partially filled

with water and partially with air tends to complicate the process. The

partial pressures of air and water are temperature dependent. In the

fully expanded penetration theory of the pulping mechanism, where it is

used to explain blackened or "burned" sulfite cooks, it is hypothesized

that the gas phase diffusion is faster than the combined flow and diffu-

sion transfer in the liquid phase. Hence, sulfurous acid without the

benefit of the metallic ions would be formed. This is presumed to bring

about an irreversible "lignin condensation" which renders the lignin dark

and forever insoluble in the pulping chemicals. This explanation of

"burning" neglects the fact that pulping can, under carefully controlled

conditions, be accomplished using sulfuruous acid alone (19). The very

long, slow, temperature rise periods of early commercial sulfite pulping









22

were supposed to allow the relatively slow ionic diffusion process in the

liquid phase to take place.

A theory developed in this laboratory, while having certain

shortcomings, gives a more reasonable explanation of the physical mech-

anism, This theory states that the chip temperature will be only slightly

below that of the surrounding pulping agent. Since the chip has both air

and water in its interior, a balanced pressure system will develop. In

the interior of the chip, the pressure will be the sum of the vapor pres-

sure of the water and the pressure contributed,by the air contained in

the chip. The-external pressure will be the vapor pressure of the water

and sulfur dioxide and any hydrostatic pressure. Flow into the chip will

occur only until the air within the chip is compressed so that the inter-

nal pressure balances the external pressure. In systems where no material

such as sulfur dioxide is used, such as in alkaline pulping, it is prob-

able that the net flow is always outward as air is released from the chip.

Ionic diffusion rates are too slow to account for the observed reaction

rates and a process of sequential diffusion and reaction does not appear

possible.

Even where there is a considerable partial pressure of sulfur

dioxide, a pressure equilibrium must be obtained at some point, and fur-

ther penetration can occur only by the diffusion mechanism. This fact is

supported by the very long penetration times required where manipulation

of the temperature and/or pressure is not used to effect rapid penetration.

For alkaline pulping, Nolan showed that the deliberate penetra-

tion of the hollow structure of the wood with pulping chemical by such a









23

temperature manipulation produced pulps markedly inferior in cellulose

quality to those obtained under similar conditions by normal procedures

(12). This might be the natural result of placing the pulping chemicals

where they must diffuse through the cellulose to attack the lignin. An

earlier detailed study of the alkaline pulping mechanism by Kulkarni (20)

led to the hypothesis that an interface of reaction with untouched lignin

on one side and an increasing gradient of delignification on the other

moves from the surface into the chip. This interface might be considered

to be marked by the occasional cleavage of a lignin molecule. Toward

the surface of the chip these cleavages increase. As pulping proceeds,

the interface moves toward the center of the chip with fresh chemical

diffusing from.the exterior to replace that consumed in the cleavage re-

action. The reaction products tend to diffuse outwardly countercurrent

to the diffusion of fresh chemical. When sufficient lignin has been

attacked, the fiber is freed from its neighbors although there may still

be a considerable amount of lignin remaining on the surface of the fiber.

By the time that the fibers at the center of the chip are free from each

other, the fibers at the outside of the chip, which are in contact with

the highest concentration of pulping chemical, must be rather completely

delignified. Notice that, in this theory, the principal path of the

active chemical and the reaction products is around the fibers rather

than through them.

In alkaline pulping, this moving interface theory has been ad-

vanced by Nolan and his coworkers (12)(22)(23) and strongly attacked by

a Swedish group (22). In acid bisulfite pulping, Nolan's view that there











is a very strong indication that the moving interface theory is more

satisfactory than the penetration theory has been presented in several

papers (13)(14) even though the experimental evidence has not yet been

as completely assembled as in the case of alkaline pulping. One should

note that this discussion centers around the physical mechanism rather

than the chemical.

Several of the studies made in this laboratory, while support-

ing the physical concept of the moving interface theory, resulted in

better empirical rate equations by considering that the reaction of lignin

dissolution is-carried out by two successive chemical reactions. This

concept was used by Findley in Neutral Sulfite Semi-Chemical pulping (10)

and by Chapnerkar in Kraft pulping (23). This idea might be contrasted

with the two-stage concept of the penetration theory in which the first

stage is penetration followed by the second, or reaction stage.

The chemical reactions themselves are complex in that they in-

volve the cleavage of a polymer chain of unknown specific structure. It

is usually hypothesized that there are points in the chain where cleav-

age may take place by sulfonation and that the dissolution of the lignin

is the result of such cleavages followed by the solution of the resulting

salt formed by the lignosulfonic acid and the cation supplied.













CHAPTER IV


THE STATUS OF THE LITERATURE



There is an enormous literature of pulping, with sulfite pulp-

ing a major subject. Under these circumstances, since a complete re-

view would be sufficient material for an entire library based disser-

tation, only the material deemed most significant by the experimenter

will be included. This section then becomes a reflection of opinion

rather than a completely scholarly appraisal of a vast literature. Un-

fortunately there is no alternative.

The older but highly respected text of Hagglund (6) has 194 ref-

erences for the chapter on sulfite pulping. The textbook more recently

published by Casey (24) has 253 references for the corresponding chapter.

Each text has separate chapters on such subjects as the physical structure

of wood, the chemistry of cellulose, and other background material, with

each of these chapters having extensive references.

The late C. J. West was perhaps the industry's most extensive

bibliographer. On the use of ammonia, sodium, and magnesium in sulfite

pulping, he collected 304 references in the period before 1952 when this

was a relatively inactive field.

In addition to the extensive group of bibliographies compiled by

the late C. J. West, the Institute of Paper Chemistry issues monthly and

25










26

annual bibliographies under the direction of J. Weimer. For 1962 there

were 49 entries in the area of sulfite pulping.

The industry nomenclature, an appreciation of which is necessary

to an understanding of the literature, is based on stoichiometric con-

cepts. This results from building the system of nomenclature largely

from wet chemical analyses. As an example, "total sulfur dioxide" is

the result of an iodine titrationand "free sulfur dioxide" is a titra-

tion with standard base to the phenolphthalien end point. The difference

is called the "combined sulfur dioxide." Such a nomenclature suppresses

all dynamic considerations and equilibria. When even rather elementary

concepts of equilibria and dynamics are introduced, the nomenclature

becomes highly inconsistent. A serious effort to develop a more funda-

mentally correct system has been begun recently. As mentioned earlier,

there are extensive studies of the equilibria in the sulfur dioxide-

water system. There are two basic reasons why such studies have not

thrown more light on the fundamentals of the pulping reaction. With

such understanding, one could expect a more accurate terminology and a

better system of nomenclature.

The first reason is that there is not a clear understanding of

the reactions between the pulping agent and the wood. How these reactions

affect the sulfur dioxide-water system then cannot be quantitatively ex-

pressed. The lignosulfonic acids produced are strong acids (6) and cer-

tainly cannot be ignored.

The second reason is that only limited data on the decomposition

of the pulping agent are available. The decomposition reactions are










27
catalysed by the presence of selenium and elemental sulfur. In addition

the presence of an intermediate, thiosulfate, is also supposed to accel-

erate the reaction. The effects of the wood are unknown.

With reaction rate data for the pulping reaction, rate of de-

composition data on the sulfur dioxide system, and some reliable equilib-

rium relationships it should be possible to carry out a very interesting

theoretical study based on the changing distribution of sulfur during

the pulping process. A very brief attempt made with the limited informa-

tion available showed that the problem would not be practical for hand

calculation and would actually require the use of a good sized digital

computer, even with many simplifying assumptions which involve among

other things the important lignosulfonic acids and perhaps make the

study purely academic.

Several studies of the equilibria without reference to either

the reactions with wood or the decomposition problem are available. A.

recent example is the paper by Ingruber (25). This author has done ex-

tensive studies on pH in acid bisulfite pulping (26) (27) (28). An over-

all approach has not yet been accomplished, however.














CHAPTER V


DEFINITION OF THE PROJECT


It was the original intention of the author to satisfy himself

as to whether there was any good reason to continue work in sulfite pulp-

ing research and the related recovery of waste products, or if there were

such technical limitations to the sulfite process that it was obsolete.

As mentioned earlier, such evaluations have been made by others. Their

conclusions plus an observable trend in the industry support the view

that the classical calcium-base acid bisulfite pulping process is truly

obsolete. If sulfite pulping has any future, the initial conditions will

include a higher plH and a more soluble cation to achieve a concentration

range not possible with calcium.

The author's experience and a very superficial economic study

support the belief that from the point of view of both recovery of waste

liquor and solubility, sodium supplies the most practical cation or base.

Before proceeding more deeply into this discussion, it might be

well to review material already mentioned and introduce parenthetically

some further comments. To understand the problem completely, as seen

from the point of view of the experimenter, one must understand bisulfite

pulping defined both from a realistic point of view and in terms of in-

dustry nomenclature. The latter is necessary if one is to critically

evaluate this work in terms of the current literature of which it must

become a part.










29

In reality "bisulfite" pulping is merely relatively high pH sul-

fite pulping. It differs very little in a practical sense from the final

phases of the sulfite cook described earlier where much of the gaseous

sulfur dioxide has been vented to prevent excessive pressures. The

starting conditions are widely different. The pH in the so-called acid

bisulfite cook is usually below 2 at the start of the pulping process.

The concentration of sulfur dioxide in the cooking liquor is normally

between h and 6 per cent. There is enough cation or "base" so that on

a gross stoichiometric basis about 1 per cent sulfur dioxide might be

considered to be related to it, or "combined," as the metallic sulfite.

In the "bisulfite" pulping process considered in this study,

the initial pH was between 2.5 and h. A range of values of sulfur dioxide

between 1 and $ per cent was studied. Enough "base" (sodium ion) was

used to make the solution approximately the equivalent of sodium bisul-

fite with a slight tendency towards excess sulfur dioxide. In terms of

the equilibria this is merely a point on a continuum. In the literature

one finds that the bisulfite is a major bench mark in the terminology

of sulfite pulping to which special properties are assigned.

The sulfite pulping terminology is the result of taking a narrow

view of both wet analytical techniques and of stoichiometry. In general,

the amount of sulfur dioxide present in a solution is determined by

titration with iodine. The industry standard for analysis (32) tells

the analyst to report this as "total" sulfur dioxide. Then the entire

acid is titrated with standard base to the phenophthalein end point, or

a pH of about 8.3. The analyst is told to report the acid as "free"










30

sulfur dioxide. Further, he is told to subtract the "free" from the

"total" and report this as "combined."

In attempting to explain the results of analysis, as described

above, equilibria and many concepts so simple as to be included in a

basic chemistry course are ignored. It is said in the industry that

the "combined" represents the sulfite form of the base and that twice

the "combined" represents the bisulfite. Thus the condition with which

this project is concerned is the condition where the "combined" is

approximately equal to the "free." In a chemical sense, this point is

one of definition. In terms of the literature nomenclature, it is very

clearly fixed. From the experimental point of view, one degree of free-

dom, the relative amount of cation or base, is removed.

In reading the literature one becomes aware of the chaotic

condition of the nomenclature (29). The Technical Association of the

Pulp and Paper Industry, through the Sulfite Pulping Committee, as

mentioned earlier, is trying to rectify the problem. There is hope for

relief, but the current problem remains.

As conditions are changed from the bisulfite and the resulting

additional degree of freedom is used, (Two variables are required to

define concentration rather than one.) the terminology becomes confusing.

In addition to the terms "total," "free," and "combined" where three

terms are used to describe two variables and where the terms result from

the analytical method, some workers halve the value of the "free" and

call it "true free." The difference between the "total" and the "true

free" is the "true combined." This somewhat naive approach considers

that the "combined" represents the equivalent of the sulfite of the










31
"base" ion and that "true combined" represents the equivalent of the

bisulfite. To distinguish the original terms which resulted directly

from the analytical techniques, these may be referred to as "mill free"

and "mill combined," relating them to production terminology as opposed

to the more sophisticated terminology of research.

This situation has unfortunately been furthered by the technical

associations.- For example, The Technical Section, Canadian Pulp and Paper

Association in its Data Sheet C-OOc, June 1955, defines "total sulfur

dioxide" as the total amount of sulfur dioxide present in a sulfite

liquor and, although measured in grams per 100 milliliters, is expressed

as percentage. "Free sulfur dioxide" is the amount of free sulfur dioxide

present in a sulfite liquor and is equal to the "true free" plus one-half

of the sulfur dioxide in the form of bisulfite. "Combined sulfur dioxide"

is the difference between the "total sulfur dioxide" and the "free sulfur

dioxide" and represents one-half of the sulfur dioxide actually combined

with the base. This paraphrase of a longer definition illustrates the

manner in which the workers in the field can become entrapped in their

own nomenclature.

Obviously such definitions leave much to be desired from a the-

oretical point of view. Under such a system of nomenclature the pulping

solutions used in "bisulfite pulping" require only one variable in their

description and that is the amount of sulfur dioxide present. Although

commercially available, bisulfites are unstable, so that it is recom-

mended that one prepare one's own solutions (2). On preparing the solu-

tions one soon finds that not only are at least two concentration vari-

ables required to define the system, but that it is a dynamic system












with a very definite vapor pressure of sulfur dioxide. Closed storage

should be used for even such short periods of time as overnight. Under

the industry definition given above, such a bisulfite system has no "true

free" since the "free" or "mill free" is equal to the "combined" and is

also equal to one-half of the "total."

Even experienced workers in the field occasionally fall into

the nomenclature morass outlined when they try to compare their work with

that of others. A rational nomenclature with an accurate basis that is.:

simple enough in concept to achieve widespread acceptance would be an

enormous contribution to research in this field.

Returning to the definition of the project, of immediate need

in applying a pulping process is some method of predicting the effects of

changes in the independent variables on both reaction rate and pulp char-

acteristics. In general, this has been done in the industry by selecting

rather narrow working limits and carrying out sufficient experiments to

build a table. The chief shortcoming in this method is that often all

the variables have not been covered over a wide enough range to permit

interpolation and actual separation of effects has not been accomplished.

It might be better, especially in view of the advent of computer

control, if the tabulated results could be reduced to a set of equations

that would allow prediction. With such equations computer control can

take the measured history of a particular portion of the material in

process and adjust conditions to obtain a desired result. Such a'scheme

would be of value in the case of the present batch type operations or

in continuous digesters where large upsets or temporary stoppages might










33

occur. A method of approximating such a result is the subject of a recent

paper (30).

Within the limitations of university graduate research, such a

project may seem overly ambitious. With this in mind, some qualifications

should be made. The results are not expected by themselves to provide an

adequate basis for computer control in the modified sulfite pulping proc-

ess which is apparently coming into prominence in the pulping industry.

The results, however, should show the possibilities and provide a basic

structure on which more detailed studies can build the necessary refine-

ment. Thus it is hoped that an effective demonstration can be accomplished.

It is intended to obtain results in two forms. The first is to

demonstrate the practicality of a rate equation or equations which, when

integrated, will supply a predictor of the necessary time to accomplish

a given degree of delignification while pulping under a given set of

conditions. Since most pulping agents degrade cellulose while removing

lignin, a second or companion equation should be developed to predict the

effect on cellulose properties of any given set of conditions and reaction

time.

The first type of equation will be a quasi-theoretical one and

will involve the common technique of testing various orders of reaction

until one is found to fit the data. The Arrhenius type equation will be

used to introduce the effect of temperature. It is hoped to use an empir-

ical relationship reported by Nolan in Kraft pulping (21) to obtain the

second result.













CHAPTER VI


EXPERIMENTAL EQUIPMENT


The digester system built for this study is very similar to the

experimental digester already in use in this laboratory and described

previously (31). The over-all arrangement is shown in Figure 2 and some

of the details are illustrated in Figures 3 and h.

The digester shell itself $ (2, 3)* is fabricated from a piece

of 14-inch steel pipe and two heads made from 11-inch steel plate. The

shell contains three tubes 6 (3) made of h-inch Nionel pipe. The mul-

tiple tubes allow several experiments to be run concurrently. An expan-

sion joint 7 (3) is provided to compensate for the difference in coefficient

of thermal expansion of Nionel and steel.

Each end of the digester tube is closed by a flange 8, 10 (3).

On one flange 10 (3) is mounted a reducing bushing 11 (3) and a 2-inch

standard pipe flange 12 (3). This provides a mounting for a Durco

2-inch plug valve 13 (2) with a Teflon sleeve and a body of Type 25 alloy.

This valve serves as the "blow" valve for emptying the digester. On the

opposite flange 8 (3) a i-inch pipe mounting 9 (2) is used for a i-inch

Durco plug valve. This half-inch pipe outlet is covered with a screen

cage to allow passage of liquid while retaining the chips and pulp.



The underlined numbers designate components and the numbers in
parentheses indicate the figure where the component is shown.










35

The liquor supply system consists of a 210-liter storage tank

1 (2, 4) and related piping. For simulated high speed continuous digester

operation, where hot cooking liquor is required, a return tube-type heater

2 (2, 4) is supplied with steam to preheat the liquor. Where the liquor

is forced in cold, a nitrogen pressure system is available to supply the

pressure. The storage tank gauge glass is shown as 21 (2).

For cases where an amount of liquor must be measured under pres-

sure,'a jacketed measuring tank 3 (2, 4) is supplied. This tank has a

calibrated gauge glass 20 (2) to allow measured quantities of liquor to

be withdrawn. An appropriate arrangement of valves and venting connec-
F-
tions e.g., 23 (2) is provided to permit both tanks to be heated, with

circulation between the two. Appropriate valves may be closed and a

measured liquor charge withdrawn and introduced into the digester through

a flexible connection 22 (2). The digester may be positioned to allow the

connection of the hose 22 to the fitting 16.

Where liquor is to be stored cold and introduced under pressure,

rotameters 21 (2) are supplied to measure flow. Preheating is accom-

plished in jacketed coil-type heaters 15 (2). The flexible connections

to the digester, h (2), enter the body of the blow valves 13 (2) through

1/8-inch pipe fittings immediately upstream of the valve plug. This

insures liquor flow through the entire digester with minimum "dead"

space. When these connections are not used, the 1/8-inch openings in

the valve may be plugged.

The fitting 16 (2), which is a 1/2-inch pipe tee with the side

outlet reduced to 1/4-inch tubing, on the opposite end of the digester

from the blow valve, serves a number of purposes. First, when liquor












under pressure is supplied via the measuring tank ) (2), a flexible

connection provides the means of introducing the liquor through the

straight run of this fitting. As a second use, hot wash water to

clean the system after each blow is supplied through this connection.

Provisions are also made for mounting a pressure gauge at this point,

and a side tap leads waste liquor out through a coil cooler 17 (2) and

a sewer line 18 (2) when liquor is flowed through the system.

Emptying the digester tubes is accomplished through the two-

inch Durco "blow valves" and a flexible connection to a cyclone separator

14 (2) and hence to a canvas-lined trough 19 (2). Repeated "blows" with

hot wash water can be used through the provisions mentioned earlier to

insure that the system is clean.









TABLE 1

Key to Apparatus Sketch Numbers, Figures 2, 3, 4


1 Liquor Holding Tank

2 Head of Bayonet Type Heater

3 Measuring Tank

4 Flexible Connections

$ Digester Shell

6 Digester Tubes

7 Expansion Joint

8 Closure Flange, Waste, Charging and Wash End

9 Liquor and Wash Connection

10 Closure Flange Blow Valve End

11 Reducing Fitting

12 Standard 2-inch Flange

13 Blow Valves

1i Cyclone Separator

15 Liquor Preheaters

16 Waste Liquor Outlet, Hot Liquor Charging and Wash Fittings

17 Coolers

18 Cooling Water Outlet to Sewer

19 Receiving Trough

20 Measuring Tank Gauge Glass

21 Holding Tank Gauge Glass

22 Large Flexible Connection

23 Venting and Circulating Valves and Connections

24 Rotameters








































Steam --X - - -


Steam Q
Pressure -
I- -
I-
xm


I 0
I


-0I




--X- -- -


Flow Diagram Sulfite Digester


0


Regulator


Figure 2.
Figure 2.


















0
cY


Figure 32 Sulfite Digester

















































Storage Tank


3-3/8"




1
2. 421






---s T 2'



Measur10ng Tank
Measuring Tank


Storage and Measuring Tanks Sulfite Digester


Figure 4.














CHAPTER VII


EXPERIMENTAL PROCEDURES AND PRESENTATION OF THE DATA


This chapter consists of descriptions of wood preparation,

liquor preparation, detailed operating procedures for each type of

experiment and the results obtained, and a description of the methods

used in pulp handling.

Wood Preparation The wood used throughout these experiments was spruce

obtained from the Augusta, Maine, mill of the Hudson Pulp and Paper Com-

pany. This wood was shipped to Hudson's Palatka, Florida, mill and chip-

ped July 7, 1962. The logs were generally straight and free from knots

and ranged from about 8 inches to about 20 inches in diameter. No mois-

ture samples were taken. This only affected chipping since the chips

were dried for storage, to prevent mold, and rewet before pulping. The

chipper supervisor stated that the logs appeared to be of normal moisture

content. The chipper used was a 12-knife Carthage chipper with "Norman"

blades. The chips were passed over Hudson's screens, which removed any

unchipped slabs or long slivers, and then removed from the conveyors

leading from the screens to the chip silos. After an air-drying opera-

tion, which probably resulted in some loss of fines in handling, the

screen analysis of the chips, obtained on a Tyler "Rotap," was as follows:

Screen Opening Per Cent Retained on Screen
+ 1.05 inches 2 .
1.05 inches + 0.742 inches 3.8
0.742 inches + 0,.25 53.6










42

Screen Opening Per Cent Retained on Screen
0.525 inches + 0.371 inches 27.4
0.371 inches + 3 mesh 10.3
3 mesh + 4 mesh 2.2
4 mesh 0.4


A little over half of the green chips were separated before dry-

ing and shredded in a vertical attrition mill. This operation has been

extensively described by Nolan (12) who has patents on this method of

chip preparation. The E. D. Jones "Vertiflex" used was operated with

3/8-inch shims lifting the upper housing for additional plate spacing.

The lower adjustable plate was backed down 120 turns or 0.480 inches

for a total gap of 0.855 inches. The feed gate was 3/8 open.

The product from the attrition mill was screened on a Link-Belt

vibrating screen with two decks. The top deck had slots 3/8-inches wide

by 1/2-inches long, and the bottom screen was 8-mesh screen. The use

of the bouncing action and the slotted screen gave a product whose cross-

sectional dimension in at least one direction was less than 3/8-inches.

The length of the product in the grain direction was substantially the

same as that of the original chips, about 5/8-inches. About 7 per cent

of the material passed through the 8-mesh screen and was discarded.

About 32 per cent of the material was oversize and was run through the

attrition mill again with the plate spacing reduced to 0.735 inches.

On this second pass, about 81 per cent was accepted, while 15 per cent

was oversize and about 4 per cent undersize.

The Tyler "Rotap" screen analysis of the combined product

was as follows












Screen Opening Per Cent Retained on Screen
+ 2 mesh 14 .7
+ 3 mesh 2 mesh 27.3
+ 4 mesh 8 mesh 25.0
+ 8 mesh 10 mesh 23.2
+ 10 mesh 16 mesh 6.8
+ 16 mesh 3.3
Through 16 mesh .7


Liquor Preparation It was felt that making up liquor by bubbling SO2

gas through a solution of the base would be the most practical method

of preparation. Although bisulfites are available, these are reported

to be unstable (2). There were available several open tanks in the

150-liter to 180-liter capacity range and it was felt that the use of

one of these tanks and a bubbler system for introducing the gas would

be satisfactory.

When making liquor for earlier acid bisulfite pulping studies

in this laboratory, a large portion of the water used (20-30 per cent)

had been supplied in the form of ice. The melting ice took up the

heats of solution and formation and kept the solution cool and the

vapor pressure of sulfur dioxide low. After several attempts to do

without the ice, because of an inadequate appreciation of the problem

of the vapor pressure of sulfur dioxide over a bisulfite solutions, the

use of ice was resumed. The same procedure was used in this research.

When the liquor was being prepared, the tank was mounted on a

scale and the calculated amount of ice and water added. Then the base

was dissolved. It did not prove to be an easy matter to dissolve large

quantities of sodium hydroxide. Often a solid cake of sodium hydroxide

would form in the bottom of the tankwhich would then require several












hours to dissolve. Besides being an inconvenience, failure to notice

this cake led to erroneous corrections in the amount of.base, which led

to errors in preliminary experiments.

In spite of the cooling, there is sufficient vapor pressure of

sulfur dioxide over bisulfite solutions to require that they not be left

open to the atmosphere for any extended period of time. In addition,

with such a method of preparation, mixing can be a serious problem. When

a batch failed to show the required chemical composition on analysis,

often merely stirring it again and resampling gave satisfactory results.

This sampling problem is similar to that encountered in commercial proc-

esses where calculated analyses often exceed in accuracy results obtained

by sampling and analysis. Then this type of work is undertaken, it would

seem that a closed tank with mechanical agitation would be a valuable

asset and would avoid much confusion and waste of effort.

The Palmrose method of liquor analysis (32) was used throughout.

This method has clearly defined endpoints in its titrations and is appli-

cable to waste liquors. The choice was based on earlier work in this

laboratory (13).


PROCEDURE AND DATA--CONSTANT TEMPERATURE AND CONCENTRATION OF BISULFITE

LIQUOR

Constant Temperature--Constant Concentration Cooks These experiments

constituted the greater portion of the experimental work, and will be

covered by introductory comments, the description of some necessary

background work relating to the starting procedures, and a step-by-step

description of the procedure used.




Water
Watr Liquor to Sewer Figure 2a. Schematic Layout
of Digester as used in Constant

!, Cooling water to Sewer Concentration Cooking
Union




Valve



Screen







Wood


Cool Liquor y

2" Durco Valve

Steam
\ 2" Union
for "Blow"
Line

Liquor Hot Liquor /-
from ___
Storage
Tank -4 Condensate










146
To approximate constant temperature--constant concentration

conditions the digester tubes were loaded with the carefully prepared

and weighed charges of wood. The digester shell was heated rapidly.

Liquor from the storage tank 1 (all underlined equipment numbers refer

to Figure 2 in the preceding chapter, as well as to Figure 2a) was

brought through the rotameters 24 and preheaters where the liquor is

heated to the required temperature and introduced through flexible

connections to the body of the blow valve immediately inside of the

plug closure of the valve. This arrangement avoided "dead" spots.

After flowing up through.the digester tube, the used liquor, with a

relatively minor drop in concentration, passed out through a screens

which retained the chips and/or pulp, through the 1/2-inch connection

a coil type cooler, a needle valve, and then to the sewer. Col-

lection and periodic measurement at this point of each liquor stream

insured accurately controlled flows since the rotameters seemed to

have a tendency to stick.

By using a flowing stream of heated fresh liquor to give an

approximation of constant concentration and by approximating a con-

stant temperature environment, two of the principal pulping variables,

concentration and temperature, become known and fixed. By using one

species of wood and a common treatment, the important variables of

wood species and chip size, and the less important variables such as

chip moisture, etc., were fixed. The configuration or geometry of the

reaction zone, which includes actual lignin area exposed, diffusion

path lengths, etc., is a function of the starting conditions, the










47

reaction rates, the reaction times, etc. By selecting a range of known

times, it is possible to lump all these variables under the heading of

time in determining rate equations for any given starting conditions.

Perhaps the chief weakness in the procedure was in the method of approxi.

mating constant temperature. In the discussion of results in the next

chapter, several discrepancies will be pointed out which could have re-

sulted from a poor method of approximating constant temperature at the

start of the cooks.

Actually considerable care was given to developing a method of

approximating constant temperature, but the results indicate that further

work in this area might be needed, especially if the early stages of

pulping are to be studied. The method used in determining the starting

conditions involved a dummy run made with water so that temperature coulc

be directly inferred from pressure. A pressure gauge was installed on

the digester outlet. After filling the tube, the supply valve was closed

so that the pressure could be used as an indicator of temperature. The

time required to reach 100 psig starting from cold with a 150 psig heating

steam pressure was noted. A large number of such check runs should have

been made over the entire temperature range to be used and the tubes

should have been charged with wood. The indications from the results

are that either a higher temperature or a longer time should have been

used in preheating the digester.

The steps taken in making an actual experiment were as follows:

1) The storage and measuring tank were filled by the use of a

pump and hose arrangement with the prepared pulping chemicals.










48

2) Three batches of air-dried wood chips were weighed and a

moisture sample was taken to allow precise computation of the

weight of the wood used.

3) The batches were water soaked overnight by submergence

under a weighted screen.

4) The excess water was drained from the chips and the wet

drained chips were weighed to allow an estimate of the water

pick-up.

$) All three tubes of the digester were charged. Considerable

care was necessary since spillage would introduce errors in

the yield calculation.

6) A convenient starting time was selected.

7) Four minutes before the selected starting time, steam

was supplied to the jacket of the digester at 50 psig higher

pressure than desired during the pulping operation.

8) Che minute before the start, steam was supplied to the

liquor heaters to preheat them. Again, the higher pressure

steam was used to supply a steeper temperature gradient and

obtain more rapid heating.

9) At the preplanned starting time, the liquor (forced by

nitrogen pressure applied in the storage tank) was admitted

to the digester tubes. Approximately three minutes were

required to fill the tubes.

10) As soon as the air had been purged, the predetermined flow

rates were established7 using the rotameter provided, and check-

ing the flow by capturing timed samples at the sewer discharge.










49

The latter precaution was necessary since a few small pieces of

wood carried in the air from other work being done in the labo-

ratory seemed to find their way into the liquor, and these slivers

would jam the rotameter floats. (In any future work, a screen

in the liquor line would be useful.)

11) Five minutes after the start of the run, the steam pressure

was dropped back to the planned operating level.

12) The operation was monitored over the desired period of time

and any necessary adjustment made in pressure and flow rate.

13) At the end of the cook for each tube, the "blow line" was

attached and the contents of the digester "blown" into the

cyclone separator.

14) Two charges of hot water from the wash tank were "blown"

through the system to give a high degree of certainty that all

the pulp had been removed. Visual checking was used to insure

that all the pulp had been collected frcm the tubes.

15) The cyclone separator was washed into the collection trough

and the contents of the trough transferred to a canvas bag for

washing and pressing.

16) The product was screened using a Valley laboratory screen

with 0.008-inch slots and the screenings were dried and weighed.

17) To improve the accuracy of the yield determination, rather

than weighing the pressed accepted pulp and taking a moisture

sample, the entire quantity of pulp was made into heavy sheets

and dried on a sheet drier. This gave a very rapid drying proce-

dure and a minimum exposure to air and hence possible oxidation.







TABLE 2

CONSTANT TEMPERATURE--CONSTANT CONCENTRATION COOKS


TEMP. TIME TOTAL FREE
COOK C MIN. S02,% S02,%


COMBINED
S0 ,%


YIELD
% ORIG. WOOD
TOTAL SCREENED


PER CENT
TOTAL YIELD
SCREENINGS


PER CENT
LIGNIN
PULP SCREEN.


YIELD
PER CENT
ORIG. LIG.


-- 19.5

8.7 19.5

5.0 19.5

1.9 19.5

1.4 16.3

1.3 20.1

9.8 21.1


160

160

160

160

160

160

170

170

170

170

170

170

180

180


90

160

240

360

480

600

120

120

150

200

210

300

60

.90


2.54

2.54

2.43

2.43

2.43

2.54

2.39
2.51

2.51
2.51

2.39

2.39

2.37

2.48


D.P.
NUMBER


1.31

1.31

1.22

1.22

1.22

1.31

1.22

1.29

1.29

1.29

1.22

1.22

1.23

1.25


1.23

1.23

1.21

1.21

1.21

1.23

1.18

1.22

1.22

1.22

1.18

1.18

1.14

1.23


72.3

64.3

58.9

55 .4

54.0
50.0

63.9

63.2

59.6

54.3

55.2

49.5

77.4

58.5


0.0

33.4

37.5

42.3

46.3

48.7

33.8

37.2

40.5

43.4

47.1

48.6



37.2


100.0

49.0

36.4

23.8

10.8

2.7

47.4

40.7

30.6

20.1

24.5

1.6

100.0

37.4


55.7

35.4

24.0

13.14

6.3

3.6

38.5


9.2 17.6


3.7 23.1

1.9 19.3


--

1866
--

1541



1278

1874


0.5 23.9


22.3

11.6


17


19.2


1142



1682 'S


--










COOK

41

53

42

54

31

79

32

80

33
81

28

76

29

30


TEMP.
oC

180
180

180

180

160

160

160

160

160

160

170

170

170

170


TABLE 2 (Continued)

YIELD
COMBINED % ORIG. WOOD
SO2,$ TOTAL SCREENED


TIME TOTAL
MIN. S02,%

100 2.37
140 2.48

160 2.37

160 2.48

90 4.93

120 4.98

180 4.93

240 4.98

270 4.93

360 4.98

60 4.90

100 4.98

120 4.90

165 4.90


FREE
S02,%

1.23

1.25

1.23

1.25

2.48

2.56

2.48

2.56

2.48

2.56

2.45

2.51

2.45

2.45


PER CENT
[OTAL YIELD
SCREENINGS


1.14 58.2

1.23 50.3

1.14 48.4

1.23 48.3

2.45 78.2

2.43 70.6

2.45 59.8

2.43 64.7

2.45 56.0

2.43 51.2

2.45 72.0

2.47 63.4

2.45, 59.3

2.45 53.1


PER CENT
LIGNIN
PULP SCREEN.


0.3 19.0

0.5 19.5

-- 21.1



5.3 23.1


37.8

43.5

47.7

48.1

0.0

38.0

36.5

40.7

43.1

47.7

8.3

31.7

37.6

41.8


35.0

52.3

1.5

1.9

100.0

46.2

38.5

21.5

23.3

6.9

88.4

50.0

36.7

17.5


-- ---

3.5 18.8

1.5 16.8


YIELD
PER CENT
ORIG. LIG.


1.1

1.4

65.2



28.5



14.1

4.0

55.0


21.6

8.6


77 170 210 4.98 2.51 2.47 50.8 49.1


1199


2.4

1.1

13.4


19. 4

13.8

20.1


D.P.
NUMBER



1322

1160






1674



1453

1305




1416

1160


3.2


--







TABLE 2 (Continued)


TEMP.
COOK C


78

37

38

39

67

68

69

73

70

74

75

72

64

65


170

180

180

180

160

160

160

170

170

170

170

170

S180

180


TIME TOTAL


MIN.

260

30

90

15$0

120

240

360

90

120

150

210

360

45

75


so02%

4.98

4.89

4.89

4.89

6.96

6.96

6.95

7.18

7.35

7.18

7.18

7.35

7.22

7.22


YIELD
FREE COMBINED % ORIG. WOOD


SO,2 s02,% TOTAL


SCREENED


2.51 2.47 48.8 48.2

2.46 2.43 82.4 1.8

2.46 2.43 59.6 39.3

2.46 2.46 46.0 45.0

3.61 3.35 63.3 35.9

3.61 3.35 55.0 41.2

3.61 3.35 49.9 46.3

3.63 3.55 66.5 40.8

3.88 3.47 58.2 40.8

3.63 3.55 51.4 45.3

3.63 3.55 50.6 48.3

3.88 3.47 48.6 48.2

3.70 3.52 61.3 34.2

3.70 3.52 53.3 45.2


66 180 120 7.22 3.70 3.52 48.3 46.7


PER CENT
TOTAL YIEI
SCREENINGS


1.2

99.5

39.1

1.7

43.1

25.2

7.3

55.0

37.8

55.0
28.1

1.1

44.7

15.2

3.2
1


PER CENT YIELD
LD LIGNIN PER CENT
PULP SCREEN. ORIG. LIG.

0.1 22.3 1.9

17.1 23.5 77.6

3.1 18.6 21.1

0.8 17.4 2.4

4.7 22.3 30.9

0.8 20.1 12.3

0.9 15.5 3.9

7.2 19.7 36.8


0.7

4.7

2.6

0.6


--

22.2

18.1

14.9

18.7


1.7
25.4

9.3

2.2


D.P.
NUMBER

995


1294

917

1593

1238

1037


1370




100o

1300

1112

813










53
The data obtained in the constant temperature--constant concen-

tration cooks are given in Table 2. In order to obtain the maximum in-

formation, cooking curves were plotted. In Figures $ through Figure 7,

the data are presented in this form, grouping according to the "target"

value of total sulfur dioxide used in the pulping agent and the tempera-

ture. Both the total and the screened yield are plotted against time.

The three temperatures, 1600C, 1700C, and 1800C, and the concentration

levels of 2.5 per cent, 5.0 per cent, and 7.5 per cent total sulfur

dioxide were selected with the idea of covering the possible operating

range. In the direction of the third independent variable, time, the

values were adjusted to achieve a wide range of yields, varying from

raw cooks with very small screened yields to completely cooked pulps.

This form of experimental design was selected since it was

considered desirable to have data that could be compared with the work

of others,and since previous experience in both acid and alkaline pulping

had proved that the pulping procedures used gave generally reproducible

results with minimum experimental error. Some cooks were carried out

in duplicate to confirm this experience and to check the specific pro-

cedures and the operator.

In addition, the fact that the cooking curves are by nature

smooth, with a fixed starting point and a known general form, made it

seem reasonable to carry out the experiments as indicated. A perhaps

more statistically elegant technique is that recently published by Box

(18). Box's approach is designed specifically for this type of work

to give maximum reliability with minimum experimentation.




























































200


300 o00
Cooking Time Minutes


500


600 .


Constant Temperature--Constant Concentration Cooks
Nominal Concentration 2.5% Total SO2


100


Figure 5.








5 Per Cent Total SO
S\* 1600o
80 ~- X 1700C

0 180o

x
70-








60X
70 ~ ~_ ---- _ __------------,----,----











30 0






20 -0 -




10




0 100 200 300 400 500 600
Cooking Time Minues
Figure 6. Constant Temperature--Constant Concentration Cooks
Nominal Concentration 5.0% Total SO2
\Ix^ x






Figure 6. Contan-Tepe-tu----------Concntra-on -ook
Noia Cocnrton'TtlS
// /2
























































100


200 300 b
Cooking Time Minutes


6.9% 7.h% Total SO2


S1600c

X 17000
0 180oC


500


Figure 7. Constant Temperature--Constant Concentration Cooks
Nominal Concentration 6.9-7.4% SO2
















o
10
r0


-r4
'0


O 0

ro
0 t3
0

C,






0







4-)
1-4


ri
H 0

0%(
E-40

0





0










'4 ^
0
p4


Concentration Grams per


. '
0I Min.
--?-MO-Min.


rn

I n


I I
I I
-~I-

I --


Grams


Data from Figure 7



-I i



I I --


2.0 3.0 5.0 7.0
SO2 Concentration Grams per 100 Grams

Figure 7a. Cross Plot of Data for Smoothing Purposes


100


90

80


-- ~ ~ ~-------
S3.0 5n.

-4----


A 1I


10.0


0











58
The simulation or constant temperature--constant concentration

conditions was the most difficult problem undertaken. The apparatus used,

while capable of approximating these conditions, was a general purpose

experimental digester, rather than one designed specifically for this

purpose. For this purpose alone, a rather different design would be

superior. The problems associated with establishing the starting con-

ditions have been discussed and will be referenced again in the discussion

of results.

Using the raw data, an equation which was a reasonably good

predictor of over-all yield was developed. Before developing this

equation, the rather wide scatter of the data indicated that smoothing

of the raw data to arrive at reliable estimates of the cooking curves

might be desirable.

It was discovered that the plot of the logarithm of total

yield against concentration resulted in a family of parallel straight

lines. These plots (Figure 7a) not only gave an indication of the effect

of concentration, but provided a method of smoothing the data. The lines

drawn in Figures 5, 6, and 7 represent the result of this smoothing.

Included with the basic pulping data are measurements of the

degree of polymerization of the cellulose. These were made to serve

as guidesto the amount of cellulose degradation that accompanied each

experiment. The values given are computed from the Hercules Chart (33)

and the measurement of cupriethylene-diamine solution viscosities by

TAPPI Standard Method T230 SM 50. The relationships between D.P. num-

ber may be used as a relative guide.









Symbol Temp.
1600
x 170
S 1800


2000
x I


300
Cooking Time


o00
- Minutes


Degradation of Cellulose as a Function of Cooking Time


3000


Cone.
2.5
2.5
2.5


1000
900
800

700

600


500


100


200


600


;IF


I

I


Figure 8a.







30OC --------- --- ------------
Symbol Temp. Cone.
1600 5.0
+ 1700 5.0
O 1800 5.0

2000







\ -t-
+\

1000
900 __

800

700_----


600


500
0


100 200 300 400 500 600
Cooking Time Minutes
Figure 8b. Degradation of Cellulose as a Function of Cooking Time


-i_













Temp. Cone.
1600 7.5
170 7.5
1800 7o_:


500


Cooking Time Minutes


Figure 8c.


Degradation of Cellulose as a Function
of Cooking Time.


Symbol


3000


1000


900


800


700










62
In Figures 8a, 8b, and 8c these data are plotted in a repre-

sentation of Log D.P. vs. Cooking Time. This type of presentation has

been used by Nolan (21). These data will be used later to develop an

equation for predicting cellulose degradation.











PROCEDURE AND DATA--CONSTANT TEMPERATURE, CONSTANT CONCENTRATION WITH

VARYING RATIO OF FREE TO COMBINED SULFUR DIOXIDE


Cooks with a Varied Ratio of "Free" to "Combined Since it was intended

to study primarily bisulfite pulping where the ratio of "free" to "com-

bined" is fixed, this study was considered supplemental. Some deviations

in liquor analysis from a bisulfite with exactly equal values of "free"

to "combined" occurred, and some idea of the effect of these errors was

desirable. Further, it was hoped that some general concepts useful in

the over-all evaluation of the process of sulfite pulping might be obtained.

As an example of the possible errors, consider a case where 5.0

per cent total sulfur dioxide is desired. The free sulfur dioxide would

be 2.5 and the combined SO2 2.5 per cent. After preparation and adjustment,

the total sulfur dioxide is 5.03 per cent, the combined 2.48 per cent, and

the free 2.55 per cent. With an open liquor system and hand agitation, no

further work is justified since losses in handling and unavoidable delay

in use, and errors in sampling which are associated with hand mixing pre-

vent a closer approach to the target values. Had minor deviations made

major changes in reaction rate, the closed liquor preparation system with

mechanical agitation, recommended elsewhere, would have been a necessity.

The problem was to plan experiments which would give a simple

clear answer to the question "What is the effect of a change in concentra-

tion of sulfur dioxide?" Since changing one of the values of "free,"

"combined" or "total" requires changing at least one of the other, some

planning is required. The most extensive data had been obtained in the

63







TABLE 3

COOKS AT 6.0% TOTAL SO2 WITH 3.5% FREE SO2 AND 2.5% FREE S02 RESPECTIVELY


CONC. OF S02%
TOTAL FREE COMB. T


COOK

94

95

96

97

98

99
100

101

102

103

104

105


TEMP
oc

170

170

170

170

170

170

170

170

170

170

170

170


TIME
MIN.

60

'80

100

40

70

90

120

210

300

390

480

570


YIELD
% ORIG. WOOD
TOTALL SCREENED

59.2 33.3

53.9 38.6

49.2 47.0

62.3 29.5

55.9 40.2

50.9 47.9

89.2 0.0

78.1 0.0

70.5 14.1

66.0 30.5

60.7 41.5

55.1 42.1


PER CENT
TOTAL YIELD
SCREENINGS

40.6

27.8

3.7

55.5
28.2

5.9

100.0

100.0

80.0

54.4

31.6

23.6


PER CENT
LIGNIN
PULP SCREENINGS

3.2 12.7

1.2 18.7

1.6 18.9


6.19

6.19

6.19

6.01

6.01

6.01

5.90

5.90

5.90

5.90

5.90

5.90


3.72

3.72

3.72

3.55

3.55

3.55
2.68

2.68

2.68

2.67

2.67

2.67


2.47

2.47

2.47

2.46

2.46

2.46

3.22

3.22

3.22

3.23'

3.23

3.23


20.2


14.4

15.6


16.7


12.5

8.7

6.4

3.3


LIGNIN
YIELD %

22.0

12.8

4.3






67.4


41.7

32.6


14.7


--











100



90 -




80



o
O *
0




60




*i \
S70 _





5










* 30
r4
0







20




10 -
I I
I/


0 U(


Figure 9.
I~/
0)- -/












Figue9


) 200 300 400 o00 600
Cooking Time (min)
The Effect of the Ratio of Free SO2 with a Fixed
Total SO2

*Est. from 5.0 & 7.5% Total










66

vicinity of 5.0 per cent total sulfur dioxide, and additional data were

available at 7.5 per cent total sulfur dioxide. For the 5.0 per cent

total sulfur dioxide level, the "free" sulfur dioxide and the "combined"

sulfur dioxide were each 2.5 per cent. For one series of cooks,.the

"free" sulfur dioxide was increased to 3.5 per cent and in the other, the

"combined" sulfur dioxide was increased to 3.5 per cent.

This allows a direct comparison of the effect of increased

"free" and increased "combined" with the work done at 5.0 per cent total

sulfur dioxide. The increased total (6.0 per cent) can be accounted for

by comparison with a result interpolated from the actual experiments

made at 5.0 per cent total sulfur dioxide and at 7.5 per cent total

sulfur dioxide.

The results of the runs are given in Table 3 and plotted in

Figure 9.


BATCH TYPE COOKS--INCREASING TEMPERATURE, FALLING CONCENTRATION


Batch- Type Cooks. Two types of cooks were run on a batch temperature

cycle, falling concentration and constant concentration. The first case

is the commonly encountered batch cook similar to commercial practice.

In such truly batch-type cooks, there was no liquor flow through the

digester tubes and the entire shell was rotated to provide agitation and

uniformity. The flexible lines from the three liquor preheaters to the

digester tubes were removed and the connections in the base of the blow

valves plugged. The liquor coolers were removed from the outlet end of

the digester and their attachment openings similarly plugged. Rotation

was at approximately two revolutions per minute.








TABLE 4


BATCH COOKS 90 MIN. TO 1700C, LIQUOR 4.98% TOTAL, 2.59% FREE, 2.39% CCMB. SO2


COOK

82

85

83

86

87

84

88

91

89

92

90

93


TIME
MIN.

0
O

30

60

75

90

120

0

20

40

60

80

100


CLIP
FORM

SHRED

SHRED

SHRED

SHRED

SHRED

SHRED

MILL

MILL

MILL

MILL

MILL

MILL


YIELD %
TOTAL SCREENED

73.0 0.0

58.5 43.2

51.4 48.2

50.2 48.2

48.1 47.5

44.9 44.2

74.9 0.0

64.7 13.9

58.7 32.7

54.6 36.4

52.3 38.1

50.0 37.6


PER CENT
SCREENINGS

100.0

15.2

6.3

3.9

3.1

1.6

100.0

78.7

44.3

33.4

32.6

24.7


WASTE
TOTAL

2.33

1.61

1.26

0.90

0.63

0.0

2.37

1.82

1.49

1.21

0.55

0.0


LIQUOR
FREE

1.34

1.17

0.92

0.67

0.53

0.0

1.36

1.07

1.02

0.87

0.52

0.0


%502
COMB.

0.99

0.44

0.34

0.23

0.08

0.0

1.01

0.75

0.47

0.34

0.03

0.0


PRESS. WASTE
pSIG LIQ.pH

98 2.81

98 2.81

121 2.63

121 2.48

124 2.31

120 1.80

104 2.74

121 2.71

121 2.58

130 2.32

124 2.19

129 2.00


LIGNIN %
PULP SCREEN.

20.7

20.7

-- 14.9

1.8 16.1

-- 17.3

0.7 12.9

-- 26.5

20.2

20.3

2.8 19.9

-- 20.4

0.9 21.5


PULP


--

1378

975
--

--



1692

1467

1213







68

0* MFill Size Chips
X Shredded Chips
9Q \





*9 ____ ____
\


8C








------- ~. ------ --------
3U
7C

6 6



6-4
2( 1 x
o 6C ------\ .---------



x x X






I
I I /

3C _






I .


2c /


0 50 100 150 200 250 30)
Cooking Time Minutes
Figure 10. Batch Cooks of Spruce with 5.0% Total SO2
2.5 Combined SO2 and 2.5% Free SO2; A
Liquor Ratio of :1l, 90 Minutes to 1700C.










69

In this type of cooking, the liquor was stored in the pressure

tank to avoid changes in concentration due to the loss of sulfur dioxide.

Preliminary wood moisture value was obtained on the chips to permit more

accurate estimate of the liquor volume. The weighed chips were water

soaked overnight and loaded into the digester tubes. The measured amount

of liquor was added and the digester shell brought up to temperature in

small increments according to the desired schedule. A 4i-minute lag was

assumed to exist between the time a temperature change was made in the

shell and the time the change became effective in the digester. In other

words, steam pressures were set 41 minutes earlier than would be given

assuming an immediate temperature reaction. This was a convenient approxi-

mation and seems sufficiently good for this minor correction.

The batch type cooks were run with a bisulfite liquor of approxi-

mately 5 per cent total sulfur dioxide and, in the case of the falling

concentration, a liquor ratio of 4:1. This gave approximately 20 per

cent sulfur dioxide supplied on the basis of dry wood. On the basis of

the consumption figures obtained from these experiments, this was a little

higher than necessary.

The selected time to temperature of 90 minutes was chosen for

possible comparison with the work of others and with commercial appli-

cations. Nolan (14) had shown that, while there might be an optimum,

the effect of the length of the temperature rise period in acid bisulfite

pulping was small and even eliminating it completely was possible.

The data from the normal falling concentration batch cooks are

given in Table 4. The results are plotted in Figure 10 in the form of

yield curves.










TABLE 5

CONSTANT CONCENTRATION COOKS 1VITH BATCH COOK TEMP. CYCLE (90 MIN. TO 1700C)


CONCENTRATION
TOTAL FREE

4.85 2.45

4.85 2.45

4.85 2.45

4.93 2.53

4.93 2.53

4.93 2.53


YIELD
TOTAL

84.1

62.0

52.7

74.2

65.8

55.7


PER CENT
SCREENED

0.0

37.6

46.0

0.0

37.4

46.3


PER CENT
SCREENINGS

100.0

39.3

12.6

100.0

43.2

16.9


PER CENT LIGNIN
PULP SCREENINGS

-- ,,.


1.3
--


16.4

18.8


NUMBER

106

107

108

109

110

111.


TOTAL
TIME

90

180

24O

120

150

210


OF SO,%
COM B.

2.40

2.40

2.40

2.40

2.40

2.40


LIGNIN
YIELD







55.6


--







71

100 -




90-
1



80

0
70- --- - ------------

t*4*


71


6 60




4) 5



0 60 .
C5








o 30



20




10
I




to i
I
/
20 ____ __ ---- I------ ------ --______



0I

0 50 100 140 200 250 300
Cooking Time Minutes

Figure 11. Constant Concentration Cooks with Batch Temperature
Cycle (90 Minutes to 1700C)













BATCH TYPE COOKS--INCREASING TEPERATURE, CONSTANT CONCENTRATION

The batch temperature cycle cooks at constant concentration

combined the slow heating cycle with a constant supply of fresh liquor.

The tanks, heaters, and coolers were all used to supply fresh liquor at

approximately digester temperature to each tube and remove waste liquor

at a high enough rate to avoid any significant drop in concentration.

The procedure used in these cooks was identical to that used for the

constant temperature--constant concentration cooks except the same

temperature cycle used in the batch cooks was applied. The liquor pre-

heaters were operated at the same steam pressure as the digester shell

and, at the low flow rates needed, supplied liquor at approximately

digester temperature.

These cooks were an attempt to provide a means of checking

the temperature relationship in any over-all equation. With the con-

centration variable fixed and a known temperature variation, it was

hoped that a partial test could be made of the equation which would

express, in differential form, yield as a function of temperature,

concentration, and time.

The results of the constant concentration cooks using the batch

temperature cycle are shown in Table 5 and the yield curves are plotted

as Figure 11.


Pulp Handling and Treatment After the digester had been blown, the pulp

was dipped from the canvas trough collector under the cyclone into a

canvas bag. The bag was hung on a rack and hot water at approximately

1700 F was sprayed into the bag and over the pulp for at least four










73

hours. The washed pulp was screened on a Valley laboratory screen using

an "eight cut" plate (slotted holes of 0.008 inch width).

The pulp was collected in another canvas bag and the screenings

were dried and weighed. The pulp was made up in the form of thick sheets,

using a laboratory sheet mold, and then dried on an electrically heated

sheet drier. This technique was used to minimize degradative oxidation

and still obtain the convenience and accuracy of oven dry pulps.

Although the capacity of the digester had been selected with

the possibility of beater tests in mind, these were not undertaken. There

is enough uncertainty in.such tests to require duplication and very close

controls. Since the desired objective was of a broad nature, not in-

volving application, it was felt that obtaining information on the degra-

dation of the cellulose would not only be more practical but more useful.

The viscosity test to determine the degree of polymerization of the cel-

lulose can be a more sensitive index of the results since considerable

degradation can occur before sheet strength tests show any effect (34).

The capriethylehediamine viscosities used to obtain the D.P.

numbers shown in Table 1 were obtained by use of the procedure described

in Technical Association of the Pulp and Paper Industry (TAPPI) Standard

Method T230 am50, using 0.5 molar cupriethylenediamine and Ostwald

Fenske viscometers. The pulps were completely delignified before dis-

solution by using a treatment with sodium chlorite and acetic acid.

The calculation of the D.P. number was accomplished by the use of the

Hercules Conversion Chart of Cellulose Viscosities, Form 803, AM 11-48

published by the Hercules Powder Company, Wilmington, Delaware.









74

The lignin content reported in the various tables was obtained

by the TAPPI Standard Method T222 m54 using 72 per cent sulfuric acid.

The values obtained may be slightly high since no alcohol benzene ex-

traction was used to remove any possible acid resistant rosins. These

were probably not present in any appreciable quantity since a few of

the pulps were shown to be almost completely lignin free. Had there

been any large amounts of acid resistant rosins, an analysis showing

a lignin free condition would not have been possible.













CHAPTER VIII


DISCUSSION OF RESULT'S


In general, the discussion of results will be treated in two

parts. In the first, those aspects that can be treated in a quantita-

tive manner will be developed. These are 1) the general equation for

reaction rate, 2) the computation of the Arrhenius constants and their

comparison with the work of others, and 3) the development of an equa-

tion for the degree of degradation of the cellulose. In the second

part, observations which do not involve specific measurements (something

did or did not happen) will be introduced and discussed with reference

to the moving interface and penetration theories of the pulping mechan-

ism.


General Equation for Reaction Rate The data used in developing this

equation are those presented in Table 1. In general, the cooking curves

developed from these data, shown as Figures 5 to 7, are about as ex-

pected. The "rule of thumb" that a ten degree Centigrade rise in tem-

perature doubles the reaction rate and hence cuts in half the time to

maximum screened yield seems to apply. As would be expected, increasing

the concentration of the pulping agent increases the reaction rate.

Previous work done in other pulping areas in this laboratory

(10) (20) (23) has resulted in the type of equation sought. While such

equations have a general theoretical basis, they are empirical in nature.

75










76

The basic concepts call for fitting the obviously non-applicable case

for the irreversible reaction in a homogeneous mixture to the data for

constant concentration--constant temperature pulping experiments. This

approach has a practical justification in that it gives results in a

useful form.

The method used by Kulkarni (20) seems to be the most appli-

cable here. Practically any text dealing with chemical kinetics will

supply the form of the equation, for example, Smith (35). For a first

approximation assume a first order reaction with the equation of the

form for the constant temperature-constant concentration case


dL KL (1)
d9


Where L = Lignin per cent remaining

9 0 Time

K = a constant,

C = concentration of sulfur dioxide


In the constant concentration case

K = kC (2)

and the effect of concentration may be introduced.

It should be noted that K and, subsequently, k are proportion-

ality constants that keep the equation dimensionally consistent. The data

may be grouped so that there are sub-sets for which both temperature and

concentration are at a given value. Using this device and integrating

Equation 1,


-ln L = KG + B







90 2.5% Total, 160C
80 5_ .0% Total, 16000

70 Q17.5% Total, 1600C
60_ __x 2.5% Total, 17000C

0- + 5.0% Total, 1700C

S40 V 7.5% Total, 1700C
i2 2.5% Total, 1800C
30 0 5.0 Total, 18000
X\ 7.5% Total, 1800C

4. Cooking Time Minutei
20 __
0 x



\ \ \ *

10 \\__\ \ __


Cooking Time Minutes
Figure 12. Lignin Yield Related to Cooking Time




























Corrected Screened Yield Per Cent


Corrected Screenings Per Cent


Figure 13. Lignin Analysis Corrections








TABLE 6

CALCULATION OF K

CONSTANT CONCENTRATION CONSTANT TEMPERATURE COOKS


TOTAL SO2
CONC.

2.5


COOKING
TEMP.C

160


COOK
NUMBER

61

62

49

50

51

63

43

59

60

45

52

64


CORRECTED
L YIELD

55.1

36.0

24.4

13.3

4.8

2.6

28.5

18.3

9.4

1.7

26.6

1.9


1 n 1/L

0.594

1.021

1.411

2.091

2.035

3.543

1.254

1.670

2.522

4.070

1.522

3.965


1/9

0.01111

0.00625

o.00o17

0.00278

0.00208

0.00167

0.00834

0.00667

0.00500

0.00333

0.01111

0.00606


90


160

240

360

480

600

120

150

200

300

90

160

180


1.0 100.00 4.394


100/L

1.81

2.78

4.10

7.51

20.80

38.L7

3.51

5.32

10.42

58.83

3.76

53.81


170






180


K

0.00661

0.00638

0.00629

0.00583

0.00631

0.00393

0.01045

0.01113

0.01179

0.01359

0.01819

0.02037


0.00556 0.02440








TABLE 6 (Continued)


CORRECTED


TEMP. C 9 L YIELD


160






170






180




160




170


90

180

270

360

60

120

165

260

30

90

150

120

240

360

90


100/L 1 n 1/L


50.0

23.9

11.8

5.2

52.9

22.4

8.96

0.67

43.6

18.3

0.59

32.1

11.0

4.8

32.5


2.00

4.19

8.48

19.23

1.89

4.47

11.16

149.0

1.86

5.32

17.00

3.12

9.10

20.84

3.08


COOK
NUMBER


TOTAL S02
CONC.


COOKING


0.643

4.431

2.137

2.934

0.636

1.497

2.407

5.000

0.615

1.671

2.832

1.138

2.209

3.036

1.125


1/9

0.01111

0.00556

0.00373

0.00278

0.01667

0.00833

0.00625

0.00385

0.03333

0.01111

0.00667

0.00834

0.00417

0.00278

0.01111


0.00769

0.00794

0.00792

0.00842

0.010o9

0.01248

0.01506

0.01920

0.02064

0.02863

0.02892

0.00948

0.00938

0.00844

0.01359








TABLE 6 (Continued)


TOTAL SO2
CONC.

7.5


7.5


CORRECTED
L YIELD

15.9

3.0

37.0


3.6


COOK
NUMBER

70

75

6h

65

66


COOKING
TEMP .C

170


180


9

120

210

645

75

120


100/L

6.30

33.33

2.70

6.94

27.80


In 1/L

1.840

3.507

0.993

1.937

3.30%


1/9

0.00834

0.00476

0.02222

0.01333

0.00834


K

0.01517

0.01671

0.0292

0.0268

0.0296












where B = the constant of integration,

Considering L to be in percentage units and using the boundary

conditions,

where O 0

L = 100

B = In 100 .()

Solving for K in (4) the desired rate equation is obtained
K lrn( ) I 2n (100) ($)
100 Q JJ

In Figure 12 are shown a number of the data sets with logarithm

of lignin plotted against time. Where there are enough data to establish

a relationship, i.e., 2.5 per cent total sulfur dioxide at 1600C, it can

be seen that an equation of the form of Equation 6 could very well rep-

resent the data. For the more rigorous cooking conditions, the lines

through the data have to be forced to pass through 100 per cent lignin

at zero time, indicating the problems in establishing the constant tem-

perature approximation.

To calculate the constants in Equation 3 for all cases where

lignin data were available, Equation 3 was solved. As a first try, the

raw data were used and the results averaged. At the suggestion of Pro-

fessor Nolan, the smoothed yields (Figures 5, 6, ?) and smoothed lignin

data (Figure 13) were used. This technique made the rate coefficients

much more consistent but had little effect on the over-all averaged

results. The results are shown in Table 6. Compared to some of the

other areas investigated, the results of the constant-concentration--

constant temperature work are somewhat erratic.









83

An interesting paper on errors in linear approximations has been given

by Osburn (41) which shows why even with the difficulties shown, a rea-

sonable result can be obtained.

An interesting possibility is suggested by the difficulties en-

countered. Other workers have partially overcome a similar problem by

suggesting that two different rate controlling mechanisms exist, such

that in the course of the pulping reaction, control is transferred from

one mechanism to the other. A physical explanation of such a situation

could be made as follows. If the first mechanism had a chemical reaction

rate limitation,and the second was actually a question of diffusion,

which could be expressed as a chemical reaction, the two-mechanism proc-

ess would agree with the availability of lignin in the wood structure.

Findley (10) in NSSC pulping and Chapnerkar (23) in Kraft

pulping used two sequential reactions to correlate their data. If more

"early" or very high yield data had been taken here, such an approach

might have been profitable. Besides such a short range solution to the

problem, several attacks which might help separate diffusion and actual

chemical reaction would involve such things as deliberate prepenetration

of the pulping agent and the use of a range of bases such as lithium,

sodium, and potassium. The first technique was used by Nolan (12) to show

pulp damage in Kraft pulping resulting from the currently fashionable

"vapor phase" pulping. He also did some Kraft pulping with potassium

base and had an apparent slower rate of reaction. Such devices were not

considered in this exploratory approach and various techniques for hand-

ling the existing data will be explored.












In this type of approach, fractional orders of reaction are

allowed (35). In fact, in a complex mechanism of the type considered

here, fractional orders might also be expected. To ascertain if there

were any valid reasons for such an approach, a return to the general

equation was made.


dL kLnC, n 1. (6)
dG


Integrating
.(-n + 1)
L(n+ ) (-n + 1) Gk (7)

From this equation, it can be seen that if the device of linear-

izing such expressions by using the logarithmic form is applied,

(-n + 1) log L = Log 9 + log (-n +1) Ck (8)

then in such a plot, (-n + 1) will be the slope of the resulting straight

line relationship. Unfortunately, such a device failed when applied to

these data. When all the data were considered there was a distinct cur-

vature in the plot. At the higher lignin content conditions a value of

minus two for (-n + 1) would have been a fairly good approximation. How-

ever, the lower lignin content data represented a virtually flat area

with a slope of zero. About the only conclusion that could be drawn is

that the apparent order of reaction was between one and three.

In another attempt to evaluate the method and the data, the follow-

ing approach is used. The K values for the last four experiments at 1600

C and 2.5 per cent were averaged (this was selected because more data

were available for those conditions and the slower rates should better










160C
1700C
1800C
1600C
1700C
1800C
1600C
1700C
18000


Total Yield (%)


Figure 14.


Constant Temperature-Constant Concentration Lignin Yield
Related to Total Yield


2.5"
2.5%
2.5%
5.0%
$.0y
5.0%
7.5%
7.5%
7.5%


Total
Total
Total
Total
Total
Total
Total
Total
Total


so2,
S02,

So2,
502,
SO2,
SO2,
S02,










86

illustrate the issue.) and the yield calculated for the first two experi-

ments. The average value of the reaction rate coefficient (calculated)

from the data for 240, 360, 480, and 600 minutes) is 0.00593 and Equation

6 becomes


0.00593 In 100 (9)


L 100 e- (0.o00593)


Using this coefficient, the lignin yield for the 90-minute cook

would calculate to 57.1 per cent and 39.4 for the 160-minute cook. The

experimental results were 55.7 and 3$5.. If the lignin total yield re-

lationship shown as Figure H1 (and which will be discussed later) were

used, the predicted total yield in the first case would be 73 per cent

and in the second 66.5 per cent. The experimental values were 72.3 and

64.3 per cent.

No conclusive proof of a two-stage reaction results. It is

interesting to note that K is very sensitive to experimental error and

conversely, the use of average and hence approximate values of K gives

reasonably good predictions of the results.

To account for the temperature effect, the Arrhenius type equa-

tion will be used. Although this equation is empirical in nature, some

success has been achieved in obtaining the constants involved by purely

theoretical methods. A general form is

d In K =
dt RT_ (10)





















Variation of K with Temperature


2.5% 0
Total SO2 I


0.00220


0.00222


0.00224


~x 7.5% Total S02




--- I __ -- _--
-r 4 ,S


0.00226


0.00228 0.00230 0.00232


1/T


Figure 15. Reaction Rate Constant Determination


0.030

0.020



0. 010
0.008
0.006


I I












here A energy of activation
R = gas constant
T = absolute temperature
K = reaction rate coefficient.

If, as in many cases, A is approximately constant,


In K =-A D, (11)
RT
where D = constant of integration.

Both A and D are related to the particular reaction being studied.

A method used in checking the validity of the application obtained is the

use of a plot of log K versus I/T for the experimental results. For this

case, the.plot is shown in Figure 15. Averaged values of K were used.

In computing these average values, specific values of K obtained from

using short cooks (such as 2.5 per cent total SO2 and 1800C for 90 minutes)

were not used due to the unreliability of the data in the very early stages

of the cook. It is interesting to note at this point that the results

could be approximated by three parallel lines with a different line for

each liquor concentration used. While not supported by any theoretical

considerations this condition can be used to introduce the liquor concen-

tration effect into the approximate prediction equation. Since a differ-

ent intercept value D seems to exist for each concentration, it is only

necessary to find D as a function of concentration and introduce this re-

lationship into Equation 11. Since plotting D against C results in a

straight line, D can be empirically expressed in terms of concentration

as


D = 0.064 C + 24.L8


(12)












Substituting into Equation 10,

In K -12-900 + 0.06 O + 24.48 (13)
T

This equation provides a predictor that considers variations in

temperature and concentration. It is written in exponential form as


K n expQ ( -12,900 + 0.064 + 24.48) (14)
T

This equation is checked and compared to experimental values

in Table 7. (* "cxp" is used to abbreviate the expression of e to the

designated exponent.)

The energy of activation found here is 25,700 calories per

gram mol. Findley (10), who treated NSSC pulping as a two-stage reaction,

found a value of 35,400 calories per gram mol for his first reaction and

25,200 for his second. In calcium-base acid bisulfite pulping, Calhoun

et al. (36) report a value of 20,200 calories per gram mol. In "soda"

type alkaline pulping Laroque and Maas (37) report 32,000 calories per

gram mol and Kulkarni (20) reports 24,160 calories per gram mol for

Kraft pulping. The recent paper of Kubelka et al. (38) reports 25,000

calories per gram mol for a "semi-acid" cooking condition that appears

to have a slightly higher starting pH than the work done for this

project.

The second constant in the Arrhenius equation, D, or the constant

of integration in the nomenclature of this paper, varies with concentra-
10 -1
tion in this result and ranges from 2.5 to 10 minutes for 7.5 per

cent total sulfur dioxide to 3.5 x 1010 minutes -1 for 2.5 per cent total

sulfur dioxide. Findley reports 4.4 x 1041 for his first reaction and










90
10
5.9 x 10 for his second. The other investigators reported gave results

the order of magnitude of which was 10I for "soda" pulping, 1010 for

Kraft pulping, and 109 for acid bisulfite. The "semi-acid" work resulted

in a value of 109.

Since both the wood species and the pulping conditions and agents

used by the investigators mentioned above differed from the conditions used

in this study, direct comparison is not possible. However, it certainly

appears as though the values obtained are reasonable.















TABLE 7

COMPUTATION OF THE RATE COEFFICIENTS USING EQUATION 14

Temp. Cone. Calc. Avg. Expl.
TK C, _% K K
433 2.5 0.0058 0.0062
443 2.5 0.0112 0.0117
453 2.5 0.0230 0.0213
433 5.0 0.0067 0.0076
h43 5.0 0.0131 0.0141
453 5.0 0.0252 0.0242
733 7.5 0.0080 0.0092
h43 7.5 0.0155 0.0152
453 7.5 0.0290 0.0287


















Sample Solution of the Rate Equation


Example: Cook #61


2.54 %
4330K
90 minutes


,-12900
T


-dY
dd

-dY
-SQ


- (Y -. 8.7) e


= (Y 48.7) (


+ 0.064C + 2hL.8)


0.00580)


- In (Y 48.7) (0.00580) 9 + B1

In (Y 48.7) .538 + 3.938

Y = 48.7 + e3400


Y = 48.7 + 29.6 = 79.1




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