CHEMISTRY OF THE SULPHITE PROCES S
VIII . Studies of the Acid Hydrolysis of Woo d
By
R . N . :fille r
Chemist in Forest Product s
an d
W . H . Swanso n
Assistant Wood Technologis t
The species of wood which serves most commonly a s
the raw material for the manufacture of sulphite wood pul p
is white spruce . Balsam fir and hemlock are also used, an d
other species in smaller quantities .
The composition of spruce wood, as recorded in th e
literature, varies considerably with the :methods employed i n
the analysis . Practical agreement is found as to the mai n
constituents, but Tlot as to their exact grouping . A typical
analysis by Klason- gives : Cellulose free from pentosans ,
53 per cent ; hemicellulose, 15 per cent ; lignin, 30 per cent ;
fat, resin, and proteins, 2 per cent .
Johnson and Hove y
find cellulose, 50 .33 per cent, lignin, 24 .44 per cent a s
constants for balsam fir . The methods of Schorger_ at the
Forest Products Laboratory show as a typical analysis o f
spruce wood : Cellulose, 55 .4 per cent ; lignin, 28 .3 pe r
cent ; pentosans, 11 .8 per cent ; soluble in 7 .14 per cent
sodium hydroxide, 18 .55 per cent ; soluble in 1 per cent
sodium hydroxide, 8 .8 per cent ; soluble in ether, 1 .24 pe r
cent . From these results it can be seen that while the distribution of constituents may vary, spruce wood may be take n
to consist of a more resistant carbohydrate called cellulose ,
certain less resistant carbohydrates (called variously hemicelluloses, pentosans, etc .), lignin, and extractives, whic h
1Svensk Pappers Tid ., 26, 319-22 (1923) .
-Paper, Vol . 21, 40 {1918) .
3J . Ind . Eng . Chem ., Vol . 9, 556 (1917) .
R33
latter class may be defined as materials consisting of resinous and fatty bodies soluble either in a mixture of benzen e
and alcohol or in ether .
The analytical methods employed in the presen t
study were : (1) Schorger's modification of Cross and Bevan' s
chlorination method for cellulose ; (2) lignin determination
by means of 72 per cent sulphuric acid ; and (3) Allihn'smodification of Fehling's method for determination of reducing sugars . It is appropriate to note here that when th e
terms cellulose, lignin, etc ., are used in connection wit h
experimental results, they refer only to the material as
defined by the analytical methods used .
Cross and Bevan- and Cross and Engelstadt6 hav e
advanced the idea that sulphurous acid is the active chemica l
in the sulphite process . More commonly the view is held ,
however, thet the part of the cooking liquor known as "free, "
that is, all acid in excess of a monosulphite compound of th e
base, is the active constituent . The concept of the reaction s
of the sulphite pulping process that has at times seemed t o
the authors of paramount importance is that it is, from on e
point of view, essentially an acid hydrolysis of wood . Th e
reaction from the standpoint of hydrolysis has therefor e
been made the subject of special study in the work reporte d
here, and the results are interpreted from that point of vie w
throughout .
The behavior of wood toward hydrolytic attack a s
been studied under a variety of conditions by Kressmann, L
Sherrard and Blanco, . Haggland,
Klason,?G and others .
Generalizing from their results it may be stated that spruc e
wood when subjected to hydrolytic attack will yield a variet y
of reducing sugars with more or less marked reduction in th e
amount of stable cellulose, and with little or no change i n
the lignin content of the wood except in the presence o f
sulphurous acid and bisulphites .
4J . Assn . Off . Agr . Chem ., pp . 107-108, (1916) ; see als o
Leach, Food inspection and analysis, 4th ed ., p . 633 .
- A Textbook of Papermaking, 5th ed ., p . 73 .
-J . Soc . Chem . Ind ., 43, 253T-357T ; also Paper, Vol, 34 ,
No, 18, Aug ., 1924 .
-U . S . Dept . of Agric . But . 983 .
J . Ind . and Eng . Chem ., Vol . 15, No . 6, p . 611, June, 1923 ,
gSvensk . Kern . Tids ., Vol . 35, pp . 2-24 (1923) .
--Paper, Vol . 35, No . 1, Oct . 23, 1924 .
R33
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Hydrolysis of Wood with Hydrochloric Aci d
It seemed rather desirable at the start of thi s
study to establish a conception of the acid hydrolysis o f
wood in terms of the analytical methods to be employed an d
under a range of conditions, as re7,ards destruction of cellulose, comparable with those obtaining in bisulphite cooking .
To this end hydrochloric acid in a graduated series of wea k
concentrations was selected as the hydrolytic reagent, an d
white spruce, ground to pass an 80-mesh but to remain on a
100-mesh screen and extracted with a mixture of benzene an d
alcohol in proportions of 2 to 1, as the wood to be hydrolyzed .
The hydrochloric acid was prepared in a series o f
concentrations ranging from 0 .05 to 3 .00 per cent . Five gram samples of the finely ground wood were heated wit h
100 cc . portions of the acid in a water bath at 96° 0 . ,
under a reflux condenser, for 5 hours . The loss in weigh t
suffered by the wood was determined and the residue Was examined for cellulose .
The extract from the wood was divided into tw o
parts . In one the reducing sugars were determined directly .
The other part was rehydrolyzed by boiling again under a
reflu condenser for 3 hours with 3 per cent hydrochlori c
acid, -L2- and the reducing sugars were determined as before .
The results obtained in both treatments are shown in Table 1 .
The loss in weight suffered by the wood under th e
primary hydrolytic treatment increases decidedly as the aci d
concentration is increased, until 0 .75 per cent is reached .
From that point on the loss in weight becomes nearly enoug h
constant to suggest either a marked decrease in the rate o r
reaction or else a complete cessation . The latter is unlikely ,
as it has been shown that mineral acids may bring about complete resolution of the carbohydrate material of wood upo n
treatment at high concentrations . Rather, an acid concentration of 0 .75 per cent seems the point of differentiatio n
between two parts of the carbohydrate material with respec t
to their resistance to such hydrolytic conditions as ar e
represented in the experiments .
11
--The quantities marked with an asterisk (*) in Table I
(fourth column) were obtained with 1 .5 per cent and 2 pe r
cent hydrochloric acid respectively and are therefor e
somewhat low .
R33
-3-
Table 1 .--Hydrolysis with hydrochloric aci d
(All results are expressed as percentages o f
the weight of the original wood . )
Con- : Weight : Reduc- ; Reduc-- ; Cellu- : Lignin : Cellu- : Mate cen- : loss :
ing
ing :lose in :
in
lose : rial
tra- :
on : sugar : sugar :residue :residue ;
loss : enac tion : hydro- : formed : after
on :counte d
of : lysis : during :rehydro- :
hydro- :
for
HC1 :
: hydro- : lysis .
. lysis .
. lysis .
Per
cent :
0 .05
.10
.15
.25
.50
.75
1 .00
1 .50
2 .00
2 .50
3 .00
Per .
cent
Original
9 .88
:
12
.72
.
: 14 .67
: 14 .75
: 18 .33
: 20 .13
: 20 .16
: 18 .78
: 19 .28
: 20 .17
: 20 .95
Per
cent :
wood
2 .31
:
3 .50
4 .54
:
7 .32
: 12 .52
: 16 .15
16 .37
: 14 .12
: 15 .24
: 16 .37
: 17 .54
:
:
:
:
:
:
:
:
:
:
:
Per
cent
: r Per
cent
6 .59 :
8 .57 :
12 .07 :
13 .15 :
16 .28 :
19 .00 :
18 .05
17 .34* :
17 .48** :
18 .67 :
19 .40 :
58 .88
51 .18
50 .96
50 .08
50 .89
49 .06
48 .31
48 .61
48 .74
48 .62
48 .79
48 .70
*Rehydrolyzed with 1 .5 per cent HC1 .
**Rehydroly
with 2 per cent HC1 .
.
:
:
:
:
:
:
:
:
:
:
:
:
Per
cent :
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
28 .55
Per : Pe r
cent : cent
:
:
7 .70 :
:
7 .92 :
:
8 .80 :
:
7 .99 :
:
9 .82
: 10 .57
: 10 .27
: 10 .14 :
10 .26 :
: 10 .09 :
: 10 .18 :
12 .5 7
13 .6 8
11 .9 2
9 .3 0
7 .41
66 .1 1
4 .1 4
4.7 9
5 .3 7
5 .35
3 .9 9
3 .35
The determination of reducing sugars (column 3 )
shows a reduction in rate of their formation at about th e
The de
same point as the reduction in rate of weight loss .
e
rehydrolysis
of
th
termination of reducing sugars after
4)
show
s
extract with 3 per cent hydrochloric acid (column
y
an approach to constancy quite comparable to that shown b
the loss in weight of wood ; not only so, but the reducin g
sugars now fall but little short of accounting for the whol e
loss in weight . From these results it seems that the carbohydrate material of wood most subject to removal by the mil d
acid hydrolysis represented is, in part at least, removed a s
a nonreducing body and is further broken down by the rehydrolysis .
This is in cc;formation of earlier observations of
Sherrard and Blanco . The values for cellulose in the residue (column 5 )
show a slow decrease in the earlier range of the hydrolysis .
The highest of the figures after hydrolysis is, however, i n
marked reduction from the initial value, 58 .88 per cent, obtained on the unhydrolyzed wood, and shows that part of th e
carbohydrate material included in the cellulose determination
in wood, as performed at this Laborato4y, is much more susceptible to hydro tic attack than the rest .
Similar effect s
are noted by Dore
and also by Johnson and Hovey in thei r
work on the analysis of wood . The former includes, in hi s
analytical summary, determinations of material soluble in 5
per cent caustic, besides mannans, galactans, and unaccounted for pentosans . The latter investigators provide a preliminary hydrolysis with acetic acid and glycerine as one ste p
in the determination of cellulose in balsam fir .
s
From the foregoing analysis of the data, it is evident that the carbohydrates that are removed from wood b y
acid hydrolysis of varying intensity, within the range examined, come from at least two different sources or groups o f
bodies, and with different degrees of reluctance .
Again, if it is assumed, as is done throughout thi s
paper, that the hydrolytic action does not alter the inerti a
of the cellulose toward the manipulations of the cellulos e
determination, it is interesting to observe that the reduction in cellulose in the experiment with the weakest aci d
is approximately equal to the reducing sugars formed, an d
that the reduction in cellulose does not increase greatl y
with the more drastic treatments . From Table 1 it can b e
12
T . Ind . and Eng . Chem ., 15, 1166 (1923) .
13
--J . Ind . and Eng . Chem ., 12, pp . 264 and 476 .
R33
-5-
seen that the material unaccounted for, obtained by addin g
together the analytical results for lignin, cellulose, and
reducing sugars after rehydrolysis, and subtracting from 100 ,
decreases slowly as the strength of the acid increases . The
loss in cellulose reaches a slow rate of change early in th e
series, whereas the reducing sugars in the rehydrolyze d
extract increase at a rate comparable to the reduction i n
amount of material unaccounted for .
It is, therefore, possible to conceive of the carbohydrate materials of wood a s
existing in three groups :
(1) Those most resistant to hydrolytic attack and, (2) those least resistant, both of whic h
are grouped as cellulose by the analytical method ; and ( 3 )
a class of intermediate resistance, referred to by variou s
writers as pentosans, carbohydrates other than cellulose ,
etc .
Hydrolysis with Sulphite Cooking Aci d
Next, for the investigation of the acid hydrolysi s
of wood under conditions related to the foregoing but als o
favorable for the removal of lignin, that is, with sulphit e
cooking liquor, a special apparatus was contrived and a technique developed .
Apparatus
An autoclave of bronze, capable of withstandin g
high pressures and the solvent action . of the cooking acid ,
was the digester used . Details of its construction are shown
in the accompanying sketch (fig . 3) . It was heated in a well insulated bath of oil .
Heat was supplied by two "Simplex st
electric imi ersion heaters of 440 watts capacity, operatin g
at 110 volts . One, the primary heater, was connected throug h
a rheostat with the source of current . This heater was use d
constantly, and with proper adjustment of the rheostat woul d
maintain the oil within 2° or 3° of the desired temperature .
The secondary heater, furnishing the heat required to maintai n
the bath at the exact temperature, worked intermittently .
It
was connected to the source of current through a relay circui t
breaker actuated by an adjustable mercury thermoregulato r
immersed in the oil .
Temperatures up to 150° C . could thu s
be maintained within +0 .5° C . of that required .
Eodifications of procedure were made to correct fo r
irregularities that appeared in the preliminary runs, and th e
experimental procedure as now used may be described in detai l
as follows :
R33
-6-
.r
4
r
l
F
5c&€~. f-inc h
BRONZE
AUTOCLAVE
FoR
SULPHITE COOKING
1
Preparation of Woo d
Spruce ground to 40--60 or 60-80 mesh size was th e
material cooked . It was extracted with a 67 per cent benzene ,
33 per cent alcohol mixture for 6 hours in a Soxhlet extracto r
in order to remove resins and waxes . Also, the wetting of th e
wood was thus enhanced . The sample for cooking was weighe d
approximately into a weighed aluminum can . Can and content s
were then dried at 105° C . for 5 hours, cooled in a desiccator ,
and weighed to determine the oven-dry weight of the charge .
About 15 grams of wood made a convenient portion . The woo d
was then transferred to a 250 cc . tall form pyrex beake r
cut off just below the lip, making a size which fitted conveniently within the autoclave .
•
41,
Liquor Preparatio n
The cooking liquor was made up in 250 cc . quantities, which amount was sufficient for one cook . Stock solu tions of sulphurous acid and sodium bisulphite make a satisfactory source, respectively, for the acid and salt used i n
the liquor making . The two solutions were analyzed in th e
usual manner with N/16 iodine solutio to determine the tota l
sulphur dioxide . The Sander analysisl4 was used to determin e
the oisulphite in the bisulphite solution . Five per cent
total sulphur dioxide for the sulphurous acid and 7 per cen t
for the bisulphite solution were convenient strengths for th e
stock solutions . From the analysis the correct quantities o f
each solution necessary to produce the desired concentratio n
of cooking acid when diluted with distilled water to 250 cc .
were calculated . After the acid was made up, its compositio n
was checked by analysis .
Two hundred cubic centimeters of the acid wer e
siphoned into the beaker containing the wood, and 40 cc . mor e
were poured into the autoclave around it .
A lead disc wa s
placed over the beaker, after which the autoclave cap wa s
tightened on .
Cocking
The oil bath having been brought to a temperatur e
8° to 10° C . above that at which cooking was to be carrie d
on, the autoclave was immersed, and when the temperatur e
had dropped to the desired point, usually in about 5 minutes ,
the thermoregulator was adjusted to maintain it there . The
time at which the desired temperature was reached was take n
. See Miller and Swanson, Paper . Trade J ., April 13, 1922 .
R33
-7-
as e starting point of the cook . When the autoclave ha d
`., 'e; ' -, been in the oil for the intended length of time, it was
withdrawn and cooled in a bucket of water .
was the n
ate ' opened, the beaker and contents were removed, and the pul p
was separated from the liquor on a filter folded to fit a
•_« ' Buchner funnel . The paper had previously been dried a t
.•. 105°
C . and weighed in a covered aluminum dish .
-,
a
se.
After filtration and washing with hot water th e
~x<< ,
pulp was air dried for 16 hours, oven dried for 5 hours a t
105° C . in the aluminum weighing dish, cooled, accuratel y
f
weighed, and the yield thus calculated . The pulp was the n
analyzed for lignin and cellulose in the manner noted earlie r
e-,. .* ;
in this paper .
.4
It
The results of the analyses are recorded in Table 2
and are shown graphically in Figures 1 and 2 . The figure s
for yield, or residue after cooking, follow a descendin g
curve with increasing time, but at a decreasing rate of chang e
that becomes quite slow in the later stages .
The curve showing the removal of so-called cellulose material (as define d
by the application of the Cross and Bevan method) indicate s
a rapid onset followed by a slow continuation in the late r
stages of the reaction . The rate of removal of lignin i s
- steady up to about the twelfth hour but decreases thereafter .
The relts confirm those previously obtained on a large
scale . The reaction involving the removal of carbohydrat e
material from the wood shows certain similarities to that i n
, the previous series in which hydrochloric acid was used . The
reduction in amount of material determined by the application
of Cross and Bevan's cellulose method to the residue after
cooking is, as just stated, rapid at first and reaches a
slowly changing value early in the series .
The quantity o f
so-called cellulose showing a resistance to prolonged aci d
eatment at the higher temperature used is very close t o
at which in the previous series showed resistance to in creased concentration of acid . That the resistance is no t
perfect is shown by the lower values appearing, especially ,
. 'With further reference to the analysis of wood acrding to Dore, or the results of Klason, attention is agai n
rected to the group of poorly defined materials of carbohydrate nature which are not included in the part often re ' erred to as cellulose . 7 In the first1 series of experiment s
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(Table 1) considerable proportions of these ordinarily unaccounted for substances were left in the wood residues afte r
hydrolysis . In the second series the more complete degradation of the wood has been accompanied by their more nearl y
complete removal . (See Table 2, last column . )
In the first series it was shown that a part of th e
carbohydrate material defined as cellulose by the analytica l
treatment was very susceptible to mild hydrolytic attack ,
and that the materials unaccounted for in the analytica l
methods used, and frequently defined as pentosans, are mor e
resistant .
The same conditions can be seen to exist in th e
second series . The values for cellulose based on weight o f
original wood show an immediate initial drop, while the portion of the residue unaccounted for in the analytical method s
used shows a continuous decrease, but without reaching zero .
The latter fact hould occasion no surprise, as it has bee n
shown by Sieber t and others that sulphite pulps always contain a portion of furfural-yielding bodies .
Of especial interest is the fact that the part o f
the cellulose most susceptible to hydrolytic attack is re moved so easily, with or without accompanying lignin removal .
or does there seem to be any interdependence whatever between the removal of lignin and that of the carbohydrate s
other than cellulose, the latter having been reduced practically
minimum before the lignin had been remove d
even to the extent of one-half .
to the
The general similarity of the results obtained i n
the two series of experiments indicates that the reactio n
upon the carbohydrate materials of the wood remains unaltere d
by the presence or absence of lignin ,
RelativeEffects of Acid Concentratio n
and of Temperature upon Cooking Tim e
The idea of a differentiation between more and les s
stable carbohydrate material in the wood having been forti fied by the results of the study thus far, it seemed desirable to examine the reactions under different but comparabl e
conditions of temperature, acid concentration, and time o f
,poking .
To this end the series of cooks, results for whic h
are tabulated in Table 3, was performed . The aim in all instances was to reduce the lignin to between 1 and 2 per cen t
of the weight of the wood .
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It may be remarked that the effect upon the rate o f
cooking shown by both temperature and acid concentratio n
change is in the direction which earlier exper gents on a
larger scale, already reported, had indicated _ The greate r
refinement of the later experiments, however, makes possibl e
a more exact estimate of the relative magnitude of the effect s
of temperature and acid concentration on the rate of cooking .
It has been hown, in this report, and also i n
earlier publicatiorfs, l2 that the removal of carbohydrat e
material from wood by sulphite cooking proceeds much mor e
rapidly than the removal of lignin . The factor controllin g
the rate of pulping, then, must be the rate at which ligni n
is removed . In the cooks here reported it is seen that a t
any particular acid concentration within the experimenta l
range (between I and 3 per cent excess sulphur dioxide) th e
rate of cooking is doubled by each successive 10° increas e
in temperature between 120 0 and 150° C .,1- with but sligh t
tendency toward greater loss of cellulose at the higher
acid concentrations . Also, at a particular temperature, a
change in acid concentration from 1 to 3 per cent is necessary to make an equivalent change in rate . As pointed ou t
in a previous paper, such a change in acid concentration i s
not possible in present commercial practice .
It is quite a fortuitous circumstance that the resistance of the desired fibrous carbohydrate material o f
wood to sulphurous acid hydrolysis should be such as to permit the removal of the lignin by the same reagent, and further remarkable that such a wide variation in cooking conditions should produce approximately the same amount of pul p
of approximately the same composition . The picture of th e
course of the sulphite pulping reactions that can be draw n
from the foregoing results is not that of a successive breaking down of the wood in various parts, but rather that of a
set of simultaneous reactions proceeding at different rates . $
l
Effect of Acids and Salts on Cooking Action
With reference to the changes that might be manifes t
in the qualities of the residual fibrous material as a resul t
of adding reagents to alter the rate of lignin removal, muc h
15 See Miller and Swanson, Paper Trade J ., Apr . 10, 1924 .
---Cf . R . Manisawa, Eng . Chem . Jour . (Japan), 1, 194-206 (1921) .
-$ See Sieber, loc . cit .
R33
- -12-
speculation has prevailed, and some experiments are on record .
The following cooks, the results of which are given in Table 4 ,
were performed to show the effects producible by the additio n
of a few simple chemicals to the cooking liquor .
As standards of comparison, the two cooks, Nos . 8 7
and 68, were made with 1 .05 and 0 .55 per cent excess sulphu r
dioxide, respectively . A 6-hour cooking period was allowe d
in all cases, and conditions were identical in all respect s
except for the addition or substitution of some reagent .
410
411
In cooks Nos . 63, 64, and 65 enough hydrochlori c
acid, sodium chloride, and acetic acid, respectively, wer e
added to produce 1/2 per cent concentration of each in a
cooking acid containing the usual 1 per cent combined sulphu r
dioxide and 1/2 per cent excess sulphur dioxide . The result s
in all three cases show a tendency toward the removal of mor e
lignin and cellulose than in cook No, 68 . The difference is
most decided in the case where hydrochloric acid was added .
Cook No . 71, in which sodium bisulphate was added, also show s
a tendency toward rapid removal of lignin and cellulose ,
while in cook No . 82 with sodium acetate, the same cellulos e
destruction is seen with less lignin removal . Cook No . 70 ,
with an extra 1/2 per cent of sodium bisulphate, is littl e
different from the control in cook No . 68 . In cooks Nos .
72 and 81 the only base present was that added as sodiu m
chloride and sodium acetate in amounts equivalent to th e
quantity of sodium bisulphate necessary to give 1 per cent
combined sulphur dioxide . In the case of cook No . 72 containing sodium chloride, a decided increase in the destruction of cellulose is•seen without accompanying acceleratio n
in lignin removal ; the pulp was decidedly burned . Coo k
No . 81, with sodium acetate, shows a decided and genera l
slowing up of the reaction .
It is unprofitable to generalize too widely o n
results with so small positive differences, but it may be observed that there is a tendency for strong acids and salt s
of strong acids to "speed up" the reaction ; while weak acid s
or salts of the same have a contrary effect, if any . Suc h
a generalization points directly to the theory of the pulpin g
process as an acid hydrolysis of wood, and to a consideratio n
of the influence of the concentration of the hydrogen ion
the pulping process . Results of experiments to establis h
e possible relation of hydrogen-ion concentration to th e
hite pulping process will be i .n _•_ d. in. a future pape r
-his ser s .
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Summar y
The results recorded in this paper show :
I.
That the same amount of carbohydrate material s
are removed from wood by mild hydrolysis with hydrochlori c
acid as are removed by sulphite digestion which reduces th e
lignin content of the residue below 2 per cent .
That the carbohydrate materials removed b y
2.
hydrolysis are, in part at least, removed as complex bodie s
that can be further hydrolyzed to reducing sugars .
410
3.
That the time of lignin removal by the sulphit e
reaction is reduced approximately one-half by each 10 0 inin temperature between 120° and 150° C .
4.
That a change in acid concentration from 1 t o
3 per cent is necessary to reduce the time of lignin removal
by one-half .
5. That the tendency of salts of strong acid s
added to the cooking liquor is to increase the rate of reaction, and for those of weak acids to decree the rate o f
reaction, suggesting a process dependent upon hydrogen-io n
concentration .
R33
-15-