- U. S . Department of Agriculture, Forest Servic e
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsi n
MADISON, WISCONSIN
CHEMISTRY OF THE SULPHITE PROCESS
V. Effect of Various Compositions of Aci d
Upon Yield and Quality of Pulp
By R. N. MILLER
Chemist in Forest Product s
an d
W. H . SWANSO N
Assistant Wood Technologist
2enrinted from Paper Trade Journal, October 11, 192 3
CHEMISTRY OF THE STTLPHITE PROCESS *
V . Effect of Various Compositions of Aci d
IToon Yield and Quality of Pul p
By
R . Y . Willer' and W . H . Swanson2
The results of the tests herein described show that unde r
the conditions of cooking used the presence of calcium base above a
certain minimum is without appreciable effect upon the yield of pul p
obtained by the sulphite cooking process, and in the acid it is tha t
part of the so-called "tree" that is present as excess sulphurous aci d
which has the greatest effect upon the yield of pulp .
411
In the industry manufacturing sulphite wood gulp a consider able diversity of opinion exists as to the most desirable strengt h
and composition of acid to use . The advocates of a ='.high free sul phur
dioxide content and a low combined . sulphur dioxide content are probabl y
in the majority, but there are advocates of low free and hi gh combined .
The diversity of opinion is founded upon individual e xp erienc e
in different mills and is difficult to reconcile ?because t .e different
conditions under which the opinion have been developed . . `_:re series o f
cooks considered here was made in an endeavor to determine, in a
carefully controlled system, the influence of variations in the aci d
composition rpon the yield and quality of the pulp produced .
*Presented at the fall meeting of T_4PPI, October 26, 1923 at Appleton, Vie .
1
Chemist in forest products, Forest Products Laboratory, Madison ,
2.
Assistant wood technologist, Forest Products Laboratory, Madison, is .
R9
Equipment and Manipulatio n
The cooks upon which this study is based were mace i•ea the
experimental di=gester at the Forest Products Laboratory.. In. tha t
digester because of the greater ratio of surface to contents than
exists in a commercial digester it has been found impractical to mak e
a cook under strictly Ritter- ealaer or direct steam ciancQit ns .. Some
heat must be maintained on the coils to prevent e,xcessive' .eondensation .
.
The rate of steam flo* irnto the digester and colsegaxent dilutio n
was approximated by determining the rate of oocldensation when a©!'_
a digester filled with water over a temperature rise similar to tha t
of a sulphite cook . The rate of condensation obtained with
$Xw
openings of a needle valve was noted and in actual cooks -t ie gAPAK
valve was set at the point that would give a rate of cen4e=:tT;' i6
comparable to that in a commercial cook . In the hater oooks of the serie s
tb.e
We.artainty as to the volume of liquor preen, wag. iminated by cooking entirely with the coil and forcing in m-ea0
: rantities
of water in the right amount to produce the dilution effesent in
commercial cooking by airect steam .
The relief was handled as was n
to keen the rm mum
cooking pressure at 75 pounds Der square inch over a cooking temperatur e ture curve that has given the best results to date with jack pine .
This curve is shown in Fig . 1 and was followed in all cooks .
Material s
The wood was well seasoned jack pine from northern ;Risconsin, ,
The acid used was made up to eive a uniform free sulphur dioxide con tent in the first three coo'' :s . In these the combined sulphur dioxid e
was 2,00 per cent, 1 .60 p er cent, and 1 .20 ner cent .
the secdhd three eoo rres the txcems sulri
'eit to . constant value than in tlb . first three a- t
varied ag in the first three ea•okx . The base
ad wail 1
calcium_ lime .
rib
Table 1 g ives the cone-entr~; on of e e
and combined sulphur d_ioxida and al . the cs eot
sulphurous acid in the acid after diDiutie- by ire.
tit
e was kep t
bine d
de
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-
et *xcess ,
mod .
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Results and Discussio n
Table 1 also gives the yield from the wood `; . The uniform ity of yield is noteworthy and if one has preconceived ideas as t o
the effect of acid composition upon yield it is rather d .isconcerti na .
Quite obviously one cannot show any variation in yield attributabl e
to any of the factors in acid composition commonly considered . Total ,
free and combined as ordinarily understood in commercial practice d o
not appear to have any direct influence upon the yield from the woo d
when comparison is made on coos carried on over the some temperatur e
rise .
Table 2 . -- Excess sulphur dioxide .
Cook :
Ten and one halt Lour s
:
Raw : Six : Nine
Ten
hours
hours : hours
acid
664-2
:
2 .56 .
1 .85 . 1 .03
: 0 .77
:
0,54
665-I
:
2 .93 .
2 .04
1 .00
:
.51
,
.2 2
3,22 ,
1 .96 , 1 .08
.
. 41
'To .
666-I
668-I
.
3 .15 .
1 .96 . 1 .18
669-I
:
3 .45 ,
2 .38 . 1 .10
671-I
,
3 .0 0
1 .26 . 1 .25
.24
.51
.
.4 5
:
.65
,
.1 3
:
.57
,
.2 7
Table 3 . -- Total sulphur dioxid e
664-1
:
6 .40
A .93 .
2 .45 .
1 .59 :
0 .5 4
665-I
.
6 .07
^ .00 ,
2 .24 .
1 .15 ,
.3 4
666-I
:
5 .62
3 .36 :
1 .80 :
0 .35
.3 0
668-I
:
7 .18 .
4 .35 .
2 .66 :
C .37 .
.6 5
669-I
.
6 .67 .
4,22
2 .34 .
0 .85 :
.2 9
671-I
.
5 .53 :
3 .28 :
2 .03 .
0 .77 .
.27
3 Chemical analysis by V . W . Bray and T . M . Andrews, Forest Products Lab oratory, Iradison, Wisconsin .
R9
Table 4 . -- Free sulphur dioxide .
Six
hours
664-I
4 .48
2 .89
1 .74 :
1 .18 :
665-1
4 .50 :
3 .02
1 .62
0 .83
666--I .
4 .42 .
2 .66 .
1 .44 .
.63 .
:2 7
668-I
5 .17
3 .16 :
1 .92
.69 :
.5 5
669-I
5 .06
3 .30
1 .72
.73
.2 1
671-I :
4 .26 :
2 .62 .
1 .64 .
.67 .
.2 7
:
.
Nine
hours
Ten : Ten and one hours : half hour s
Raw
acid
Cook
No .
0 .54
.28
Table 5, -- Combined sulphur dioxide ,
664-I :
1 .92
1 .04
665-I
1 .57
0 .98
666-I
1 .20
.70
668-I
2 .02 :
669-I
671-I
0 .71
0 .41
0 .00
.62 :
.32
.06
.36
.22
.0 3
1 .20 :
.74 :
.13 :
.1 0
1 .51
0 .92
.62 :
.10 :
.0 8
1 .26
0 .65
.39
.10
.00
Further attention is directed toward the values for exces s
S02, that is the excess over that necessary to combine with the bas e
as bisulphite . This should be thought of as sulphurous acid . In
Table 2 this value is seen to vary markedly in the raw acid but upo n
further examination of the table it is seen that at and after th e
sixth hour this constituent of the cooking liquor is nearly constan t
in all cooks . In view of the remarkable uniformity of yield of screened pulp and cellulose the attention is centereR at once upon this th e
only factor in acid strength and composition that shows a correspond ing uniformity as is seen upon examining tables 3, 4 and. 5 . The
reason for this uniformity is not far to seek if one considers the cook ing acid to be composed of sulphurous acid and a bisulphite, in this cas e
calcium bisulphite .
a
a
as i water as is the case when
T1pon heating a sole sig
sulphurous acid is considered, *Ns elements ph icc lead one to expec t
a driving out of the gas, and if the heatii be carried on in a close d
vessel, a generation of pressure . Ftiir Mer, tie more gas is dissolved ,
the greater will be, the pressure at any tet nature ;. 'pie numerical
s(,Alue for the presalare generated depends upon the e:
itions of th e
eNperim*nt and will not be taken up here ex pt to reproduce the curves
obtained. when heating sulphurous acid in a
.ir ptacle in which
the volume of the se:1iution was 90 per cent e-f tkr
closed volume .
(Fig . 2 .) 'lb determ.rhe roughly . the effect o-f
►bined SQa- bisulphite
$02 Won the system a farther a ri+nae_nt was m ie by heattft o cooking
a cid of 1 .10
ant combined.. '.the variatio?. in the, patens •r,'e fro m
1 t of
og sulphurous acid cantatjn3.iag 'ti so
s concentratio n
in
the
eking
acid.
The excess sulphurous
of an• o-rdAtOcr essure gauge ,
*10timge of s.en$itiven
00 * *ill be cons isle lead. that th e
* t:
gas pressure generated during cooking results entirely from the exces s
sulphurous acid .
closed
e tote
SOW 4P
ss
acid
The explanation of the uniformity in concentratic-i of exces s
sulphurous acid follows simply enough from the before-m
ro : o points .
In the cooking of sulphite pulp the p ressure is usually held 60 a
certain maximum . This pressure is made up of steam pr?ssure,
s
pressure and at some stages hydrostatic pressure . In system In which
tha ah3Q
*0104
tressure are fixed as they are in commercia l
cook
y "pip ■
ane concentration of excess srRpl urous aci d
at every te`
r.:perafi,re . In other words, when a digester l
reached 120 °
MP any other temperature along the rise the concentration of exces s
sulphurous acid will be the same at the maximum p vessmjrc ragrdless o f
the concentration of excess sulphurous acid in the raw acid . ,mss
statement is strictly true when certain factors such as quantity o f
liquil
rAb cliges
are constant and equilibrium is maintained an d
in commercial practice it is only possible
that the concentratio n
of excess sulphurous ac s'.d is approximately the same at the =tee temperature and pressure marinun . The closeness of approximation ke be expected is seen in Table 2 2 that of the concentration of excess sulphurou s
+►t
In
*,w
er
rftii
to say
fair*
The addition of quamilOOk atcium base above that required
does not appear to increase the yield from the wood ant when a certai n
maximum is exceeded is detrimental because of precipitation of bas e
La the digT . !tit
;t'Rn E7as exceeded in the cooks made with 1 .60
4010 combined or more and in all these the base precipitated betwee n
oration of the view
,~
]industry and expressed
+MINA Oat'~ lb
.. t
-
at•ory by S . B . Lunak *.at additional
M thpu6vlic tion,s of t ta-iss la:
desirable . Th e
-base abov*
minim s is t nnecP
ercial
practieW
s
variable an d
co
!
l value
minimum in
.
dioxide .
.40
per
cent
combine*
tent and 1
E-)
r
.a WWI.
sift*
In the previous publications in this series it has been point ed out that a high free acid was necessary to p roduce a well cooke d
easy-bleaching pulp under the cooking conditions outlined, that is, a
relatively fast cook, and that the reactions involving the combinatio n
of sulphur and base 'with the wood material were of secondary importanc e
from the standpoint of °yield and quality of pulp . The results obtaine d
more definite statement ,
in this series make it p cssible to make a
Porely, that as rega r ds acid corm c.#,]
t~tt part, of the so- .ea,,llea
M free" which is -ore-sent as excess sulphur dioxide is the .factor havin g
the greatest influence on the character and Yield of pulp and tha t
under the conditions stheied, the amount or concentration of calciu m
base is without great influence if above a certain mrrnium .
still
The'opinion has been long held by-well-informed srlnhit e
chemists that an im p ortant reaction in the pul p ing of eood by th e
sulphite p rocess was a -_rd .,ol-tic reaction . This is o 'ten lost sight o f
in commercial cookie in the perplexity caused by the nfluence of variables of a physical n n t .re . The data in this and precious paper o f
this series strengthens the opinion as to the importance of a'idering
the sulphite pulping process, from on* point of view, as an a
hydrolysis of wood .
4
U. S . Department of Ag riculture Bulletin r?o . 620 .
R9
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