89 2
(TUT ROTARY DIGESTER IN
WOOL SACCI-1ARIEICATION,
j
. September 1 44
-'
:
No. 8145 6
UNITED STATES DEPARTMENT OF AGRICULTUR E
FOREST SERVIC E
a,s,FOREST PRODUCTS LABORATOR Y
Madison, Wisconsi n
In Cooperation with the University of Wisconsi n
;,k
}
THE ROTARY DIGESTER iN WOODSACCHARIFICATICN l'By .
R . H . PLOW, Technologist _
JEROME F ., SAED,AN, Chemis t
H . DALE TURNER, Chemical Enginee r
an d
E .-C . SHERRARD, Chemis t
Forest Products Laboratorys3 Forest Servic e
U . S . Department of Agricultur e
This investigation into the use :of the rotary digester in woo d
'saccharification was begun at the Forest Products Laboratory in the fall o f
1 9213 at the request of the Office of Production Research and Development o f
the War Production Board . Its purpose was to serve as a collateral program
to the study of the Scholler process (3) that was being conducted b y
members of the ' Laboratory staff with the Cliffs-Dow Chemical Company .
À
ar atu s
A schematic cdia .ram of the equipment used in this investigation i s
shown in figure 1 and two views in actual operation in figures 2 and 3 .
In
figure 1, the digester (A) is a 30-inch diameter sphere constructed o f
copper silicon alloy sheet of suffi,ient weight to allow a working pressur e
of 300 pounds per square inch . It is supported by two hollow- trunnions ,
mounted in babbit bearings and rotated at a speed of 12 revolul,ions .pe r
minute . This was later reduced to 21- revolutions per minute in order t o
decrease the attrition of the residue . 'Th5 digester opening is closed by a
steel plate (B), fared with heavy copper, fastened' down by 10 swing bolts .
There are two openings through the plate, one for a 1-inch wort draw-of f
line (C), closed off by a plug valve (D), and ending in a half union (E l ) ,
the other for 4 thermometer well (not shcrwia) which extends down into th e
filtq heat (F) . After two unsuccessful designs, -the firer head finally
tOok a fork, shown in figure 4, Vast fits into the neck of the digester waif
.ma y
a.a d on studies of the. U . S . Forest Products Laboratory at l a ieon ; Wis . ,
in cooperation with the Office of Production Research and Development Of .
the War Production Board ,
-Presented before the Wood-sugar Symposium of the American Chemical Society ,
New York City, September 11-15,
1944 .
-Maintained at Madison, Wis ., in cooperation with the University of Wisconsin .
RIk56
•1 -
about 1/8-inch clearance on all sides . The perforated filtering surfac e
projects into the digester body so that any residue adhering to the hea d
after drawing wort will be swept clear by the charge during subsequen t
rotation .
Steam, controlled by valve (G), is introduced through one hollo w
trunnion . Internal pressures and temperature are manually regulated according to the pressure shown on gage (H) . Acid solution is injected through
the sane trunnion by a steam actuated, automatic injector (I), after bein g
mixed. tc proper concentration in metering tank (J) .
To discharge wort, the digester is stopped in an . inverted positio n
and the half union(E l ) connected with the other half (E 2 ) at the end of a
length of flexible metal tubing . The wort then passes'through the sigh t
glass (K) to the . flash tank (L) . The vapor flashed from, (L) goes throug h
flash condenser (M) and • is collected at its base . The wort drops from . th e
bottom of the flash tank to a tared earthenware crock (N) which rests on stale (0) . After weighing and sampling, this wort is poured into woode n
collecting tank (P) . The -sample of tile . composite liquors from each run i s
taken from this collecting tank after suitable agitation (Q) .
After the completion of the run,, the pressure in the digester i s
released through condenser (RQ . The cover and -filter head are removed, th e
digester inverted, and the drip-free residue dumped on the concrete floo r
for sampling and weighing .
Abed used for Hydrolysis '
Two types of chips were used in this work ; one a 3/ k inch pulp chip ,
the other'a chip approximately 1/4 inch in the fiber airection . The onl y
wood used was a Douglas-fir lumber and no bark was present . .After chipping ;
during which mixing ' of the chips occurred, the chips were sacked . Befor e
each series of runs, the chips .were spread out on the floor to a depth of 4
inches . After standing overnight to come to an equilibrium moisture content ,
the chips were mixed and piled together, a sample taken =from the fou r
quarters of the pile, e.nrd the wood again sacked . In this way a well-mixe d
composite material was obtained . The sample was analyzed for potentia l
sugar (6) and for moisture content . The potential sugar content'of all th e
wood used was between 66 .55 4 .id 68 .5 percent . Samples from individual logs ,
hO;rrever, shod much 1a,r per v ia. ious• (6) .
Salauof the
Wort samples were obtained by thoroughly mixing the product of an
entire cycle or the composite of all cycles .
No samples were take n
directly from the wort draw-off line .
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Sampling of the Residue
After a11 ' free wort had been removed, the-residue was 'mixed in th e
digester by tumbling, dumped on the floor, further mixed, and shoveled int o
a pile . A 3-pound sample was taken for analysis . On reaching the
Laboratory, this sample was again thoroughly mixed in a porcelain dish . t o
insure against any concentration of liquid'in the bottom of the sample .
-Methods of Analysis
.
The potential sugar cofit 'ent of the original wood and wood residue
was determined by a method described elsewhere (6) ; the reducing sugar ;'by '
the Shaffer and Somogyi method, using reagent No . 50 a.nd a 30-minute , •
heating period (7) ; The fermentable sugar , was .determined by measuring-the ,
sugar sorbed by yeast . :in high concentration from a to tion of •th wood.
.
sugar .-containing :1 .5 milligrams of reducing sugar• peu• •milliliter (7) .
Alcohol yields were estimated by multiplying the yield of fermentable _quga r
by 0,47, a value which represents .the weight of alcohol obtalihe'si per •unit .
weight" of fermentable sugar . :This value was established experimentally by .
comparing the yield of alcohol obtained from 8-liter fermentations with the
fermentable-sugar content found by yeast sorption'() .
' The reddc-ing sugar content of the wet hydr61yted-wood reelidues :was
determined. by mixing a 100-gram sample pf the residue 'and about ,4bQ sgilliliters of'water in a "Waring Blendor" for 10 minutes . The macerate d
residue was washed on a Buchner funnel until nearly 2 liters had bee n
collected . This was made upto volume in a 2-liter, volumetric flask . The
well-ground residue remaining was air dried, weighed, and its moisture con tent determined .- It'was'then analyzed for its potential sugar content .
This use of the Waring Blendor saves much time over the older method o f
washing the intact hydrolyzed chips' . A ground sample is obtained at the
same time for subsequent analyses . ... '
The Investigati4n .of 'Wood+#>ydrolysi s
intom` RptarvDigester
od hydrolysis is a complicated:. study because :
1. Wood is not homogeneous in either its' vh-ysical or it s
Structure,
2. The physical and cAemical character of residual o& changes as .
hydrolysis progresses .
3. The retention-rof sugar :elution b..wood. is =high ; ,eno-half to '
two-thirds of the sugar produced in each cycle remains with the wood ire- •the '
digester .
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1
.1
4. Interdependent variables, governing wood hydrolysis, that mus t
be studied both individually and as interrelated functions are : (a) time ,
(b) temp erature, (c) acid concentration, (d) liquid-solid ratio, (e) particle size of charge .
The rotary digester program has included three main lines o f
investigation :
1. Multistage hydrolysis in which an effort was made to obtain a
maximum yield of alcohol . In such operations, yields exceeding 55 gallon s
per ton were obtained .
2. Single-stage hydrolysis in which an attempt was made to complet e
the hydrolysis in a minimum of time . Yields of 30 gallons per ton wer e
obtained .
3. Limited multistage hydrolysis giving yields between those obtained in (1) and (2) in the shortest time consistent with economica l
oferat ion .
. MULTISTAGE HYDROLYSI S
With the exception of the work begun in 1922 by H . Scholler, an d
the development of his commercially successful process, the multistag e
hydrolysis of wood with dilute acids has not been very thoroughly studied .
G . Meunier conducted a study at about the same time as Scholler but wa s
unable to commercialize his process successfully . In 1927, when studying
the effects of successive hydrolyses on the components of`wood, Sherrar d
and Davidson attained a sugar yield of about 32 percent, calculated a s
glucose, on a dry-wood basis .
In 1943, Harris and Beg-linger made a series of multistage cooks i n
the spherical rotary digester of the Forest Products Laboratory at Madison ,
Wisconsin, in which were used the conditions recommended by H . Scholler .
In spite of deficiencies in apparatus, alcohol yields of 47 .5 gallons pe r
ton of dry softwood chips were obtained .
The present program was authorized :
1. To investigate the advantages and disadvantages, both chemica l
and mechanical, of the rotary digester as compared with the stationar y
percolator .
2.
To investigate the effects of agitation on sugar yields .
3. As a more flexible piece of equipment, to make basic studies o f
wood hydrolysis under conditions suggested by work in the stationary equipment .
81456
4 . To act as a basic unit for studies of subequent,-steps , in th e
le
process of producing ethyl alcohol from wood, such as neutraliAetion-,
filtration, and fermentation of wood hydrolysis worts- .4 ;~
_~ •
The first program for studying multistage hydrolysis involved complete cooks, using constant conditions throughout, a nd .varingk only one fac tor in each cook . Such variations were over ranges considered by past
experience to be effective . The results of this, series were not definitiv e
and will not be included
this paper . This phase of the problem, however ,
merits further study because of the advantages offered by the simpiifiq-ati :o n
. of hydrolysis under constant conditions . The program that was .finally , used_
involved a cyc1p-by-cycle study of the conversion of the cellulose to sugar . -
Cycle-by-cycle Stud y
In devising a schedule for the multistage hydrolysis of -wood it wa s
considered that, as - the cook progressed, it would be necessary to chang e
one or more t of the .f actors affecting the severity of hydrolysis-of the
succeeding cycles . This was to compensate for the changing chemical natur e
of the hydrolyzed-wood residue .
ti-
Since wood contains hemicellulose, a relatively easily hydrolyzabl e
carbohydrate fraction, the conditions used for the hydrolysis of the firs t
cycle should . be less severe than those required far succeed'ing . cycles Thi s
'
was found to be true by Scholler• .•and other workers .
In order, , tco .determine what . conditions were sufficiently severe, an d
yet efficient, for the hydrolysis of the first cycle, samples-of finel y
divided wood together with dilute acid were sealed in glass bombs an d
"hydrolyzed by heating with direct-,steam in a,rotating autoclave accordin g
to the methods described in another report (8) . This work, as well as some
done earlier (3), showed -thatif the hydrolytic conditions- were no t
sufficient ly severe , much of the hemicellulose was rendered o1u'ble bdt wa s
not hydrolyzed tcq jonosaceharides . This source of potential sugar was
r
therefore lost to . tide , process . Liquors obtained In sudh• inadequatel y
hydrolyze first• cy l e , s can be rehydrolyzed by . the addition of more acrd an d
further heating . • In commercia l ' operation this would be an,undeS :i-Table 'top .
In order to reduce the acid, lime, and time consumption to a minimum ,
hoy vex, it is• desirable to us e - no more than the amo .pt o, acid . arequired fo r
efficient hydrolysis .
The small-scale study showed that an acid concentration of 0 .25
percent and a temperature of 170° C . maintained for 10 minutes is adequate ,
fo the satisfactory hydrolysis of the hemicellulose . At a temperatur e
g
10° C . lower than thip, there is a marked decrease in the,ratio o-f reducin g
-Much of this work was done with the assistance of the engineering staff o f
The Vulcan Copper and Supply Company of Cincinnati, Ohio, the designer s
of the commercial unit under construction .
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-5-
sugar produced to potential sugar removed from the wood, due to the presenc e
of incompletely hydrolyzed hemicellulose in the wort .
The hydrolysis of wood was then studied in the rotary digester unde r
controlled conditions . The measurement of some of the factors involve d
require interpretation .
Time of digest is defined as the interval between the time at whic h
the prescribed hydrolysis temperature was reached and the time at which th e
valve was opened to draw off wort . The hydrolysis of cellulose proceede d
during the wort withdrawal period .
Acid concentration is the concentration in the digester during the
digest period . The dilution from condensed steam was calculated and checke d
experimentally . This is in contrast to p rocedures commonly used in whic h
the acid concentration cited is that of the hydrolysis liquid introduced ,
thus ignoring the dilution effect of the condensed steam . The amount o f
! hydrolysis solution remaining sorbed by the wood after wort withdrawal wa s
arrived at from the wet residue of the cycle-by-cycle studies and allowe d
for when computing the amounts of water and acid to be added for the subsequent cycle .
Liquid-solid ratio was calculated on the same basis as the acid concentration, In multistage operation it is desirable to take off as much a s
possible of the sugar produced in each cycle . The higher the water-woo d
ratio, the greater will be the amount but as this ratio increases, the aci d
requirement increases, and the wort concentration decreases . On the basi s
of earlier experience, it was decided to use a constant liquid-solid rati o
of 3 to 1 . This represents a satisfactory compromise between stea m
consumption, acid requirements, sugar yield, and wort concentration .
A series of hydrolyses of 50-pound portions of wood were undertake n
in the rotary digester using 0 .25 percent sulfuric acid and various temperatures and times . The water-wood ratio was held at 3 to 1 in the digester .
The total sugar produced in the hydrolysis was determined by an analysis o f
the drawn wort and of the wort sorbed in the wet residue . The potential sugar content of the washed and dried residue was also determined . B y
subtracting the potential sugar in the residue from that in the origina l
wood, a measure of the gross-sugar production was obtained . The ratio o f
the net to the gross sugar production was a measure of the efficiency of th e
hydrolysis . On the basis of such studies it was decided that a temperatur e
of 170° C . with 0 .25 percent eulfuric acid for 10 minutes was satisfactor y
for the first-cycle hydrolysis . !
Some of the residue from this first-cycle cook was washed, dried ,
and ground, and then subjected to a series of experiments using smal l
. ..Subsequent work has indicated that a temperature of 160° C . wit h ' the sam e
acid concentration and time would be even more satisfactory i n
multistage hydrolysis .
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-6-
samples, as described previously, tol . define the conditi©ns -required for the
second cycle . The residue froth the two-e-ale cook was used to deterthdhe =
the optimum conditions of hydrolysis for the third cycle and so on . .
,operatio n
The operation of the rotary digester during the hydrolysis proceede d
as follows :
r
,. Before charging, the digester was heated to the'•tempefature used fo r
the first cycle, the steam . condensate removed, and the sampled, weighe d
charge of chips introduced . The digester and charge were again brought t o
temperature and the predeterm n .ed amount of sulfuric acid. soluttifi inje6'ted .
The -cook wat-timed, the rotation of the digester halted shortiy ' befbre fns
digest time elapsed, and the wort discharge line Cdr .nected thr-oiigii i~Y e
flexible tubing to the :.flash tank . At the end of edigest ; period ; stead was
shift off, tide discharge .,valve was opened . and theedischarged wort was- '
collected in a tared crock . It was -weighed, sampled,e and then dumped •i't o
a wooden collecting tank . The flash was cellecterd!'froni'the 'flash condenser ,
weighed, and sampled . As soon as the sight glass showed the passa6t .'of''' -'
clear vapor, indicating the end of the wort discharge, the discharge valv e
was_ smut off,- the flexible tubing disconnected, rotation .start-g- ;-and the
proper amount of acid solution injected to meet the r'egi1irements rfor the
next„cycle .• The digester was then brought . to the proper temperature-tor : '
this next cycle -and the' program repeated . At the end of the run, after
wort removal ;- the %digester was blown down to atmospheric pressure through •
the blow-down c9ndRnser, the, digester cover . and filter head - removed, anti
the drip-free cone 8d mped ;.onto .the concrete floor-'where a repre .sente;tive
sample vias, taken- aAd ;thn : residue.- weighed .
% '- • _
~:
Yip- 4s f rom = uterrupted Mult gtage Htydroltiyse s
'i1
i
f
The• .conditionsrused for the -hydrolysis of wood in the Various-cycle s
were? as fo;1llws :
l0 _ninuteb for` the f -rst `cycle , 6_-min rtes for the second, aril
4 mt iu ;ti for : subde .quent cycles.
- :r`
Tempere,tur.4 -, 17D° C . . for'the first cycle ; 1800 C . for the second '
and • 1850 :•:C - ;f ,r ubse4uent cyel'es .
. Acid concentration : 0 . ;25 percent sulfuric acid for : the first cycle
and 0 .60 'per rit for all subsOuent"cycles .
Liquid-solid ratio :
3 to 1 throughout .
- iesults are shown in table 1 and figure 5 .
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. ._w:
t
e
The experimental errors involved in the determination of th e
difference in the potential sugar content of the original wood and of th e
residue made it difficult to determine accurately the change in efficienc y
with different methods-0f operation .
After this cycle'-by-cycle study was completed and further information was obtained on the theory of wood hydrolysis, it became apparent that
wood cellulose doss not become more easily hydlrolyzecl as the length o f
exposure to hydrolytic conditions increases . This was shown in th e
following way :
'The-washed and dried .residues from each of the cooks listed in table
1 were hydrolyzed with 0 .8 percent sulfuric acid at 180 0 -C ., according t o
the technique described in another paper (8) and the rate of hydrolysis O f
the residual cellulose in each sample was determined . A sample of the
original wood was included for comparison . The results of this ' experiment ,
are given in table 2 and fire 6 . It is seen that there is no significan t
change in the rate of hydrolysis of the cellulose in the various residue s
; even though'tho total residue obtained from the 11-cycle cook contained '
only 12 .5 percent of the carbohydrate initially present in the wood, Thi s
indicates that after the hydrolysis of the hemicellu5 .ose is complete, constant conditions of hydrylysis can be used for the succeeding cycles . Thi s
means that the conversion of the cellulose proceeds at a constant rat e
based on the amount present at any time : If it is desired to complete a,
hydrolysis in less time when a small , amoun t of cellulose remains, more
severe conditions may be used .
To show that - the cycle-by-cycle method of investigation was
reproducible, a curve •was' made (f4g . 7)_ o,f the cumulative sugar productio n
of-a coitt)lete cook and of interrupted cooks in which identical condition s
were used . This figure also shows the cumulative amounts of (a) reducing
sugar recovered, (b) reducing sugar sorbed in residue, (c) reducing sugar
destroyed,-and (d) potential reducing sugar in residual cellulose ,
This curve also shows the most impprtant limitation on the yield o f
sugar obtainable. from multistage hydrolysis -- : a large percentage of the
sugar present in one cycle is retp .ined by the wood to be recgoked in th e
succeeding cycle . This sugar, therefore, is subject to much longer
exp osure to destructive , conditions than if more complete repiov,al .was possiblo ,
CompletedMultistage Hydrolyse s
the first complete coo}, No . 72, . employed conditions selected from
the cycle-by-cycle study that were the same as those used for the cook s
in Vale 1 except that run No . 72 was carried to completion .
The time consumed in this cook and the yields are shown in table 3 ..
The cumulative reducing-sugar production on a time basis is shown i n
figure 8 .
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-8..
.
In an endeavor to decrease the total time consumed, cook 74 was mad e
in which the temperature was raised to 190 0 C . in the fifth cycle and maintained at that point throughout the succeeding cycles . All other conditions
remained the same as in No . 72 . The time of operation was reduced but th e
yield remained about the same as in No . 72 as can be seen in table 3 an d
figure 8 .
In another attempt to shorten the time consumed and improve yields ,
cook 75 wae made in which the acid concentration in the digester wa s
increased at the fifth cycle to 0 .90 percent and maintained at that leve l
throughout the remainder of the cook . The temperatures used in No . 72 wer e
resumed, however . Yields were substantially improved thereby as is seen i n
table 3 and figure 8 .
This improvement in yield puts the choice on the basis of economics ;
whether a 10 percent increase in alcohol production plus a 16 .5 percen t
reduction in time consumed per cook outweigh a 30 percent increase in tota l
acid needed plus the increase in the amount of lime necessary fo r
neutralization . This decision can be made only by reviewing local condition s
at a proposed plant site .
The amount of sugar destroyed in a solution is in direct ratio to the
amount present in the solution at any given period of time (8) . In the
multistage hydrolysis of wood, a maximum concentration is reached in the
first cycle when the hemicelluloses are converted to sugar . It seemed ,
therefore, that the introduction of a wash cycle between the first an d
second hydrolysis cycles would decrease the amount of, .sugar retained in th e
residue and so reduce the amount decom_oose d . To accomplish this, 75 pound s
of water were injected after the first wort draw-off, the digester wa s
rotated for 6 minutes without any application of heat, and the wash wate r
was drawn off . The amount of liquor withdrawn was weighed, sampled, an d
dumped into the collecting tank with the other wort . This was done in two
cooks, No . 73 and 79, which duplicated in all other respects cooks 72 an d
75, respectively . As is seen in table 4, the yields were substantiall y
improved .
The
comparative course of the cooks is shown graphically in figure 9 .
The reduction in sugar concentration in the wort is an importan t
factor if this method for improving the yields is to be considered for
commercial operation .
All the work so far described was done with Douglas-fir chip s
measuring about 3/4 inch in the fiber direction . To study the effect o f
particle size, two more cooks were made, one (No . 80) with sawdust an d
another (No . 83) with chips measuring 1/4 inch in the fiber direction .
These cooks were made under the same conditions as cook 75 . The yield s
were as shown in table 5 .
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_9-
Little difference in yield occurred, but an inspection of the comparative-production curves in figure 10 exhibits a somewhat more rapi d
release of the wood-sugar wort when 1/4-inch chips were used . This may b e
of importance when selecting the cycle beyond which the financial return i s
marginal . The advantages which sawdust might show in more rapid diffusio n
were apparently counteracted by the higher retentive capacity of sawdust i n
comparison with that of the denser chips .
Discussion and Conclusion s
The yields of alcohol attainable from the multistage hydrolysis o f
Douglas-fir chips in a rotary digester are similar to those obtained in th e
stationary, vertic-al p ercolator . Sugar concentration in the wort, however ,
is lower and thus requires greater fermentation and distillation capacit y
and also higher steam consumption for the rectification of the alchol . The
superiority of the percolator arises from the fact that it - is an extractio n
,column as well as a digester . Fresh dilute acid pumped in the top washe s
the sugar toward the bottom and creates a marked vertical concentratio n
gradient . The removal of wort at each cycle, therefore, removes a greate r
quantity of augar than would be the case if this concentration gradien t
did not exist .
Stationary equipment is cheaper both in original cost and upkeep .
There is no danger of plugging of the mass in the rotary digeste r
as sometimes happens in the percolator (3), but the discharge is les s
convenient than the blow-out system of the percolator .
The data acquired in these studies emphasize that the yields o f
sugar from multistage hydrolysis with dilute acid in rotary or stationar y
equipment are limited by the retention of the sugar solution by th e
hydrolyzed wood . Attempts were made to reduce this limitation in severa l
ways : by the choice of a proper particle size of the wood, by a close stud y
of the hydrolysis efficiency at different points of the hydrolysis, and b y
the inclusion of one or more extraction cycles . These, however, have brought
about little improvement . It is difficult to foresee any positive cure fo r
this inherent characteristic .
The conclusion arrived at by Neumann CO, and repeated by othe r
workers, that 173° to 175 0 C . is the maximum temperature for the efficient
conVersion of cellulose to sugars is at variance with the findings of th e
present i nvestigation . These findings indicate that temperatures up t o
190° 0 . . (the limit set by the available steam supply) give equally efficien t
conversion if the hydrolysis time is sufficiently shortened .
It is impossible to select one optimum set of conditions for th e
economic conversion of wood to sugar by multistage hydrolysis . In order t o
use the equipment at maximum efficiency the time should be reduced to a
practical minimum, The time required for a complete hydrolysis is almos t
R1456
-10- .
exclusively dependent on temperature and acid concentration . These tw o
factors are partially compensating in effect and their manipulation is a
matter of relative costs and plant design .
'Another factor in multistage hydrolysis which can be establishe d
only by economic considerations is the extent to which the hydrolysis can `
be carried . The sugar yields from the last cycles are low and a thoroug h
review of current alcohol prices, equipment, and operating costs is necessary to determine the exact point at which production costs exceed return .
II .
SINGLE-STAGE HYDROLYSI S
. During the period extending from 1910 to about 1922, two wood sugar p lants were in operation in this country using the single-stag e
hydrolysis with dilute acid described by nth (2), Demuth (1), Tomlinso n
(9), and Krtssman (4) . In plant-scale operation, 22 gallons of alcohol6
were produced per ton of dry wood containing a maximum of 10 percent bark . '
The chief advantage of this process was its simplicity ; large variations in operational technique had little effect on the sugar production .
.The chief disadvantage of the process was the low yield ; hence only the
largest sawmills could supply enough wood waste to maintain a plant in
economical operation .
The process as outlined by Kressman (4) consisted in heating 10 0
parts of wood, 125 parts of water, and 1,8 to 2 .5 parts of sulfuric acid '
with direct steam in a rotary digester, she time for a complete cook was divided as follows :
5 minute s
15 to 20 minute s
15 to 20 minute s
5 to 8 minute s
5 to 7 minute s
Charge
Heating
Hydrolysis period Blowdown
Discharge
The drip-free residue so obtained was washed free of sugars in a
battery ,
diffusio n
In the light of present knowledge of wood hydrolysis, this proces s
was open to number of improvements :
1 . The time required for a hydrolysis can be reduced by decreasin g
the time consumed in coming to temperature . This can be done by injecting
the dilute acid at the reaction temperature into the charged digester, thus
avoiding the expense of using the digester as a' water heater .
.Certain plant losses, primarily in distillation, caused this recovery t o
be low .
R1456
-11-
2. The time of digestion can be reduced by increasing the aci d
concentration, the temperature, or both .
3. The yield can be increased . From data presented in another pape r
from this Laboratory (8) it is apparent that the two factors that increas e
the speed of hydrolysis also increase the efficiency of conversion o f
cellulose to sugar .
4. Wood sugar; of higher fermentability can be obtained by a
proper choice of conditions .
In an effort to complete the digestions in a minimum of time ,
experiments were first made with the injection of hot dilute acid from a n
auxiliary autoclave . The experimental difficulties encountered in thi s
type of operation with the available equipment were such that it was considered better to inject cold dilute acid and have better control over th e
variables involved, The injection of hot dilute acid, while important fo r
efficient plant operation, can be neglected in experimental work, since onl y
12 minutes are required to raise the pressure from 70 pound s , per square inch ,
below which point attack on the cellulose is relatively slow, to 165 pound s
per square inch, the highest pressure used .
include
reagent
tion in
for the
The substances in wood-sugar worts that appear as unfermentable suga r
(1) xylose, (2) sugar decomposition p roducts that affect the suga r
but are unfermentable, and (3) oligosaccharides of unknown composimultistage hydrolyses when conditions are not sufficiently sever e
hydrolysis of the hemicellulose .
The changes that occur in the fermentability of wood hydrolyzate s
from one-stage cooks as a function of time are shown in figure 11 . The dat a
for this graph were obtained by the hydrolysis of wood in glass bombs b y
techniques described elsewhere (8) . The fermentability is initially low ,
for hemicellulose yields about one part of unfermentable sugar to two part s
of fermentable sugar . As the unfermentable sugar is destroyed and replace d
by fermentable sugar from the resistant portion of the cellulose, th e
fermentability rises . If there were only two factors involved, the ferment ability should approach 100 percent as hydrolysis continues, but due to th e
accumulation of decomposition products that affect the sugar reagent, the
fermentability passes through a maximum and then declines . In multistag e
hydrolyzates obtained by percolation or from the rotary digester, th e
fermentability may be 60 percent initially . The sugar produced in the late r
cycles of the multistage cooks, however, rises to a fermentability of ove r
95 percent . A smaller amount of decomposition products is formed, and a
smaller amount accumulates in such a cook than in the single-stage operations .
In this paper will be described the investigation . of the single stage hydrolysis of Douglas-fir by several methods :
1 . The American process, used at the Fullerton plant during Worl d
War I under she conditions given at the beginning of this section .
R1456
-12-
2. A modified American process, in which higher temperatures an d
.acid concentrattolks are . used .
.
.
3. Co▪ nstant temperature wort removal method, in which hydrolysis
temperature is maintained during and for a short time after the wort draw - .
off period ;
American Process.
The results of the hydrolysis of Douglas-fir • hydrolyzed according .
to the method used commercially in this country 25 years ago are given i n
In cooks 121, 123, 124, and 125, 3/4- and 1/4-inch chip s
table 6, series A .
were hydrolyzed for 15 and 25 minutes, using the lowest recommended acid.
wood ratio and a water-wood ratio only slightly higher than that require d
to obtain a drip-free residue . In order to obtain a representative sample'.
of the wet residue, it is essential that a minimum of wort be present . Eve n
in those runs that would be considered drip free by industrial standards ,
any free wort was removed from the residue by the injection of compressed '
air after the digester had been blown down to atmospheric pressure . This . '
also. insured the removal of any free wort that might have been trapped i n
the' filter head . The yields, 22 .4 gallons per ton and lower, were les s
•
thin those reported by Kressman (4) . With higher acid-wood ratios, the
.
When
the
water-yield increased to 25 .2 gallons per ton`(runs 138 and 139)
" wood ratio was decreased below that used in'runs - 138 ' and"139, and the acid concentration was increased, a further slight improvement in yield occurre d
• (runs 149 and 150) ._ These results are considered a sat sfagtAry confirmation of the earlier work .
.
•
.r
Modified American p roces s
In this method of. :hydrolysis, higher temperatures and higher acid concentrations were used than those'employed in the old American process .
The pressure was raised as rapidly as possible and, after completion o f
the digestion period, was quickly blown down .
e
▪
Jr?. the first experiments, series B,,an attempt was made to hold th e
water-wood ratio at 1 .5 to 1 and the a. cid :.concentra,tion at 2 percent ., The
results,are given in table 6 . Maximum yields . of alcohol were obtained with',
digestion pericidsranging from ;4 to 8 minutes . The yields, above .27 g4l1a
per ton, were higher than those obtained in series A and by earlier worker s
(4) for the•American process with Douglas-fir wood .
• •
In series C are shown the results of hydrolysis using a 2 to 1
liquid-solid ratio and 1 .percent sulfuric .acid . The yields were siTilar t o
those obtained .in series B . The .acid requirement was lower, but twice as .
much time was needed to '-reduce the same yields .
R1456
-13 -
In series D, conditions similar to those used for series C wer e
employed, except that the temperature was increased 5° C . The yield wa s
increased by 1 gallon per ton . The time required was expected to decrease ,
but the data obtained did not show a time decrease .
In series E and F, the hydrolysis of wood with a 2 to 1 liquid-soli d
ratio and 2 percent acid is shown . Maximum yields of 28 gallons per to n
were obtained in approximately 5 minutes .
Series G shows that when the acid-wood ratio was kept at 4 to 100 ,
as in series E and F, and the water-wood ratio was increased to 3 to 1, the
yield of alcohol was essentially the same as that found in series E and F ,
but the time required was doubled .
In series H, when a 3 to 1 liquid-solid ratio was used with 2 per cent acid, the yield rose to 29 gallons per ton with a scarcely significan t
decrease in time . The acid requirement in this series was 6 percent on th e
basis of the wood .
It would have been interesting to have continued the study of single stage hydrolysis, using higher temperatures and correspondingly shorte r
times, but further work along these lines was limited by the equipment an d
the steam pressure available .
Constant Temperature Wort Removal Metho d
By the introduction of a small modification, the yield of sugar i n
single-stage hydrolysis has been significantly increased . After a digestio n
period, the free wort is removed from the charge through the digeste r
filter head . The charge with the adherent wort is maintained at digestio n
temp erature for an additional period, and then discharged . This results i n
a higher total production of reducing sugar than if all the sugar produce d
in the cook is kept at digester temperature throughout the entire heating
period . Sugar production has been raised in this way to more than 3 1
gallons per ton in 10 minutes . The data for this series of runs are give n
in series I, table 6 .
In runs 106 to. 108, inclusive, and 114 to 117, inclusive, the woo d
was hydrolyzed for 5, 10, and 15 minutes ; and the free wort was withdraw n
during an additional 5 minutes . The total time of digestion and wort
withdrawal is given in table 10 . In run 114, the water-wood ratio was to o
low to take advantage of this ty pe of operation . A maximum yield of 31 . 2
gallons per ton was obtained in a-total time of 10 minutes at reactio n
temperature .
In runs 180, 181, and 182 the same acid-wood ratio was used as that
used in the runs previously described, but 50 percent more water was used .
This was expected to give better results because a higher proportion of th e
Rl456
-14-
sugar was removed in the hot wort withdrawal . Ire run '180, the digestio n
was o xried on .for 1 minute, and the wort withdrawal -occiupied 5 minutes I n
rur 18 1 . a 4-minute digestionPeriod.was used, wort withdrawal required 2
minutes, and the residue was held at'the digestion temperature for .2 minute s
after wort withdrawal . Run 182 'was similar to 181 . except that the initia l
digestion period ;was 8 minutes . The maximum yield found in this series wa s
31 .6 gallons per_ - ..to n., and the required time was 12 minutes, . The resit a.l
potential sugar indicated that more severe hydrolysis might have resulted i n
higher yields .
- Indications were obtained in the work on hot wort withdrawal, tha t
lar gger chips•_(3/4-inch pulp chipq) give, lower yields than those obtained ,
from 1/4-inch chips . This fact has not yet been established, with certainty .
.
Discussion and Conclusion s
By il4creasing the acid concentration and the temperature, the yiel d
of fermentable, .sugar from one-stage cooks-was sign1ficantly increased .
Alcohol yields above 30 gallons per ton . were. obtainable from one-'' '
stage hydrolyses if the wort was removed at the digester temperature .atd i f
the residue was subjected to a small additional holding period .
While the rotary digester -was used for this!•workr there is ever y
reason to believe that stationary equipment would be as satisfactory . The
schedule used in this work on single-stage hydrolysis was such that the .
digestion period occupied a small percentage of the total time required t o
charge, heat, . and discharg e . available industrial . equipment : This.
indicates :the need of :new equipment designed for continuous operation ., .
.a .
LIMITED MULTISTAO HYDROLYSI S
Thus• fax in this paper two general methods` of operation for woo d
hydrolysis have been described : (1) a multistage hydrelysis, .giving high
yields but requiring an involved operating schedule and a long time, an d
(2) a single=stage hydrolysis requiring• a short time :and simpler -equipment ,
and . giving. much lower yields . •
'
A thira genera,l method of operation was studied, which ,offers a Com promise between the techniques previously described .. Constant .temperature•. '
and cycles of equal length are employed . Only one-fourth to •one-half . as • .
many cycles are required as are commonly used in multistage operation . ,
Yields above 40 gallons of 95 percent alcohol per ton are easily obtained .
R1456
'r-15-
In all the runs listed in table 7 a 3 to 1 liquid-solid ratio wit h
0,25 percent acid was used for the first cycle . In the succeeding cycle s
the water added, plus the steam condensate, was equal to the original weight
of the wood . The acid concentration in this heated water was 1 .5 percent .
The acid concentration of this solution was lowered in the digester by th e
residual liquor from the preceding cycle . A total of three or four injec tions were used . After the last injection and wort draw-off the cookin g
temperature was maintained for a time equal to the length of the cycle .
This resulted in the production of more sugar which, together with tha t
remaining from the last cycle, had to be leached from the residue .
A series of 3-cycle hydrolyses were made : Nos . 183, 1g4, and 185 at
185° C ., and Nos . lg6 and 188 at 190° C . The yield varied from 37 .7 to 40. 4
gallons per ton, always increasing with the severity of the cook as measure d
by the potential reducing sugar in the residue .
In runs 187 and 189, four 31-minute cycles were used at 190° and 185° ,
respectively . Yields of 41 .5 and 44 .0 gallons per ton were obtained. It i s
considered that such hydrolyses, while not obtaining a maximum yield, make
it efficient use of the relatively expensive apparatus required for high pressure wood saccharification . Because this process is essentially a highl y
abbreviated multistage cook, it is believed that important advantages i n
yield and wort concentration might be gained by carrying out this operation
in a Scholler percolator .
General Conclusion s
The yield of alcohol obtainable in single-stage hydrolysis wa s
significantly increased by control of time, temperature, and acid concen trations .
Contrary to the older view, temperatures in excess of 173° C . wer e
proven advantageous .
The use of the rotary digester in multistage wood saccharificatio n
has little apparent advantage and certain serious disadvantages over th e
vertical stationary percolator .
The choice between multistage hydrolysis, either to completion or
a limited degree, and single-stage hydrolysis is governed by economi c
conditions .
R1456
716-
to
IitteraOre Cite d
,' .
1 Demuth von, R ., Zeitachr . f Angew Chemi .s Aui'satzteil ,
"(1913) .
2 ." Foth
37 :1221-1222, 1297-1298,41913)'.
3 ; Harris, E .
. orest Products LaboratPPY mimeo graph :No . R1446 ,
1-58 (1944) . "
Kressman, F . W ., U .
• "b .
.
Dept . Agr . Bull . 983:1-100 0120,-
Nauman, J . ; Diss . ' Desden (.1910) ,
6 .. Saeman, J . F .,
Ann,
Ed; in press) .(1944) . "
7. .''Saeman, J F
Harfis' E 7
.! .Ed .'(Ip.'press) (1944) :
8. Saeman,
9.
Ind,.--Eng .. . Chem ; Anal .
..
Hayrig, F ., ..
.
Kline, A . A
Ind . Eng . Chem . Ailal.
.% Ind . and Eng . Chem . (In Pres s)• ( lk-4) . .
Tomlinson, C . It ., Ihd . and Eng . Chem . 10 :859 (1918) .
R1456
,
Table 1 .--Reducing sugarfrom interrupted multistage hydrolyse s
Total : Net-reducing sugar : Gross reducing sugar : Effic- : Pounds dry residu e
cycles : Dry-wood substance : Dry-wood substance : ' iency :
Pounds dryer
in cook :
wood substanc e
-:Percent
:
Percent
_Percent
Percent
.
1
1
1
2
3
4
17,8
22 .3
27 .1
31 .2
:
5
• 34 .3
6
36 .7
7
11
t
15,4
27 .2
34 .1
38 .1
.
:
':
:
:
44 .0
48 .4
51 .9
62 .4
40 .7
47 .0
86 '
82
80
82
78
76
78
76
C
:
:
:
.
76 . 6
68 . 4
62 . 6
58 . 7
. 52 . 2
50 . 0
47 . 9
40 .8
:
„*
1
Table-2 .--Resiauea of cooks of table 1 hydrolyzed with 0 . 8
percent sulfuric aci d
Sample
: Potential :
First order . . : . Half-life of the
: reducing : reaction constant : resistant portio n
sugar
k(minutes -1 )
: of the cellulos e
Percent
:
4
Minute s
Original wood
:
First-cycle residue . . . . :
Second-cycle residue . . . :
68 .0
63 .4
57 .9
.
0 .0260
.0262
.0265
26 . 6
25 . 4
26 . 2
Third-cycle residue . . . . :
Seventh-cycle residue . . :
Eleventh-cycle residue . :
53 .6
37,6 . :
22 .2
.0278
.0266
.0282
24, 9
26 . 0
24 . 6
- R1456
:
Table 3 .--Yield of alcohol from complete cook s
Cook :Total : Total :Wort per : Reducing : Reducing sugar :Ferment- :95 percen t
No . :number :; time :100 pound, : sugar in : Dry-wood substance ability : alcoho l
:per to n
:
of
: dry-wood :
wort
:
dry-woo d
:cycles :
: substance :
. substanc e
:Minutes :
Pounds
:Grams per :
:300 milli- :
liter s
:Percent : Gallon s
Percent
72 :
17
:
197
:
1,180
:
3 .79
43 .1
:
81 .5
:
52 . 8
74 :
14
:
150
:
1,040=
:
4 .09
42 .8
:
80 .7
.
51 . 7
75 :
15
:
151
:
1,090
:
4 .21
45 .8
:
83 .0
:
57 . 0
Table 4 .--Effect of wash cycle on yield s
Cook :
: Reducing sugar : 95 percent
Reducing sugar
No . : Dry-wood substance : in composite : alcoho l
: per to n
----------------
•
72
73
75
79
Percent
:
:
:
:
----------- -
Gallon s
' Grams per 100
milliliter s
43 .1
46 .4
45 .8
48 .0
3 .8
3 .5
4 .2
3 .4
52 . 8
56 . 5
57 . 0
60 .0
:
t
Table 5 .--Effect of particle size on yield s
Cook : Chip size
Reducing sugar
: Ferment- : 95 percen t
No . :
1 Dry-wood substance : ability : alcoho l
. per to n
75
80
83
R145 6
: 3/4 inch
: Sawdust
: 1/4 inch
:
:
:
Percent
: Percen t :
Gallon s
45,8
45 .9
47 .7
83 .0
83 .0
80 .5
57 . 0
57 . 0
57 . 6
:
:
:
.1
II
▪
Table 6.-9ydroly.leof Onu2la0-firbythe A0rtoen prooeee . themo41fle4Marlow. prooeee . end the~ 'oo0'Sta20-2e26erature-0ort-re0ov6l metho d
11012 0
fat
ary ;220201061 202662191 ! ►erment-s
eedu .ing : 2210Vy
.
220,21,612 02-01tl0---J
tdWor t
20012,., leugrr 1021 2281101ry3
ability t
0ufI~ 1oI
Fermentable
: 95 pero3at
,
Water-
Ip .u 100
wca•66 y24 .-20o2 1 D22-6*06r 2••122el auger Sn t
IPe4uo200sugar
Ohip i 91mr T02mppew-IAG1a
ro saws '
sugar
t 00601 per
i
i •6uye 10990022 :2,•y-road A1422202. 4P0nnda
rebatann
l
aubuanue
l
heat
wy-road
eubatmod(
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1
IErot102 1
fddyy^2006
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2201222 1 Ory-2006 '
'
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I
euDetenc e
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13. 8
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Dater-wood ratio was 2
determination of the peroent roduelag sugar by weight requires that a oo2reotlo2 be made for the .2 .oiflo gravity of the wort .
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Liquid-solid ratio ras 10. to 1 .
-
240
270