THE ROTARY DIGESTER in WOOD SACCHARIFICA TION
yieidr and improved convenion efficiency c m be obtained in ieEs time by higher temperatures and Acid concentretiom.
T
HE early work on wood hydrolysis was carried out in rotating sutochves ( I ,8, 4). Present commercial prsotice, outside of the United States, makes use of stationary equipment. This paper prescnts reinvestigation of the use of rotat.ing equipment. The digester, which WRR heated by steam admitted throush the trunnions into the reaction chamber, was oquipped for dilute seid injection and for wort removal while maintaining the charge at constant temperature. PhotoQaphs of the apparatus &re shown ss Figures 1, 2, 3; Figure 4 is a schomiitic diagram of the equipment. Several alternative methods of wood hydrolysis are described. A study is slm included of the prowess of hydrolysis of wood through B multistage'operation. Recaux of the size of ehnrge and convenience, this equipment w m better ndapted to the study of the pmgms of hydrolysk than WBB the stationary equipment used by ot.her workors of this Laboratory (9). Two t y p u of chips were used-ne 8 8/.ioel~pulp ohip, the other approrimatel? inch in the fiber direction. Doughs fu lumber was used and no bark was present. The ssmple was analyzed for moist.urc and potential sugar content (Y), which ranged between 66.5and 68.5%. hmpleg irom individual logs, howevcr; showed much larger variations (7). Analytical methob employed are described in detail elsewhere
(7, 8). Alcohol yields were estimsted by multiplying the yield of fermentahla mgar by 0.47, B d u e that represents the weight of alcohol obtained per unit weight of fermentable sugar. This vdue was established experimentally by comparing the yield of alcohol obtained from 8-liter fermentations with the fermentable-sugar content found by yeast sorption (5). The redwing sugar content of the wet hydrolyzed-wood residues was determined by mixing 100-gram sample of the residue and about 400 mi. of water in B Wsring Blendor for 10 minutes. The macerated residue w&s weshed on e. Biiehncr funnei until nearly 2 liters had been eolleoted. This filtrate was made up to volume in a %liter volumetric f h k . The well-ground residue remaining wes air-dried and weighed, and its moisture eontent determined. It w a then analyzed for potential sugm content. Sampling of the wet hydrolyzed wood must be done with care. Aftor all free wort had been removed, the residue w ~ l jmixed in the digester, dumped on the floor, further mixed, and shoveled into B pile. A 3-pound sample was taken and, before analysis, was again thoroughly mixed in s porcelain dish to ensure against any ooncentrstion of liquid in the bottom of the sample. Wood hydrolysis is 8 complieated study because wood is not homogeneous in either its physieal or its ehemieal structure, bR cauge the physical and chemical character of residual wood ohanges 8s hydrolysis progresses, and because the retention of
R. H. PLOW, JEROME F. SAEMAN, H. DALE TURNER, AND E. C. SHERRARD' U. S. Forest Rodvch Laboratory, Madison, Wis. 1 Dscwrrd
O d o k 6.1944.
The revults of the glair bomb study formed the basis for subsequent hydrolysis seherlulcs in the r o t a ~ ydigester. In the data given in this report, the dimeat period is the interval between the time %,hend i p t temperature w8s attained and when wort removal wss begun. Acid concentration and liquid-aotid ratio were determinod on the basis of oonditions during the digest
8ugm solution by wood i s high (one half to two thirds of the sugar
produced in each cycle remeins with the wwd in the digester). This study included three w i n lines of investigation: (a) Multistage hydrolysis in which an effort was made to obtain a maximum yield of alcohol. In such operations, yields exceeding 55 gallons per ton were obtained. Two additional studios were made: first, B cycle-by-cycle study of net and poss sugar production; second, s determination of the quantity of sugar retained hy the re$due and thitt removed in the wort. ( b ) Single stage hy&olysis in which an attempt w8s medo to complete the hydrolysis in a minimum of time. Yields of 30 gallons per ton were obtained. (c) k i t e d multistage hydrolysis giving yields between those obtained in (a) and (b) in the shortest time consistent with economical operation.
period.
In the muIti8tage operation it i8 desirable to remove M much of the sugar produced in each cycle BS possible. The higher the water-wood ratio, the greater will be the amount of sugar removed; but 8s the ratio incressos, the Beid requirement incresses and the wort concentration decreases. Hydrolyses of 50-pound chargee of wood were made in the rotary digester, using 0.25% sulfuric acid and various temperatures and times. The wster-bod ratio was held at 3 to 1 in the digester. The net sugw produced in the hydrolysis is that present in the wort plus that sorbed in the residue. By subtracting the potentinl sugar in the residue from that in the original wood, a meSSure of gross sugar production was obtained. The ratio of the net to the gross sugar production was a measure of the efficiency of hydrolysis. On the basis of such studies it w&s decided that a temperature of 170" C. with 0.25% sulfuric acid for 10 minutes was satisfactory for the first-cycle hydrolysis. (Subsequent work has indicated thst a temperature of 180' C. with the same mid concentration and time would be even more satisfactory in multistsge hydrolysis.) Samples of the residue were then hydrolyzed in glass bombs to define the conditions for the second cycle. The residue from
MULTISTAGEHYDROLYSS
The multiakge hydrolysis of wood in the rotary digester was investigated to determine whether such operation offered advantages over the Sehcller process. O p ~ l u n o r r . The rotary digester operated during hydrolysis 88 follows: Before chxging, the digester was hosted to the tempersture used for the first cycle, the weighed c h g e of chips introduced, and the sulfuric acid solution injected and brought to tempereture. At the end of the digest period, wort was 001leeted, weighed, sampled, and then put into a collecting tank. This injection snd hydrolysis w w repested for the required number of cycles. At the end of the run the digester was blown dawn to atmasoherie ~ r e a ~ u rtlrroueh e . the condenser, and the residue weighed and sampled. CYCLE-BY-CTCLE STUDY. In devising 8 schedule for the multistage hydrolysis of wood it n'as eonsidered that, as hydrolysis prqgrased, it would be necessary to change one or more of the factors stleeting the severity of hydrolysis of the succeeding cycles to compensate for the chsnging chemical nature of the hydrolysed wood residue. To determine whst conditions were most efficient fqr the hydrolysis of the first cycle in which the hemicellulose of the wood is attacked, samples of finely divided wood, together with dilute acid, were sealed in glass bombs and heeted with direct steam in a rotating autoclave according to the methods described in another report ( 6 ) . This work, as well 8s that carried out earlier (3), showed that, if the hydrolytic conditions were not sufficiently severe, much of the hernicellulaie "8s rendered soluble hut was not hydrolyzed to monosacchsridos. This source of potential sugar wss therefore lost to the process. Liquors obtsined in such inadequately hydrolyzed first cycles a n be rehydrolyzed by the addition of more acid and further hesting. In eommerrial operation this would be nn undesirable step. To roduee the aoid snd lime oonsumption to B minimum, however, it is desirable to use no more than the amount of acid required for Fisuw 4. Rotsry Digeater Showing Wort Drawoff Connections, Thermometer, efficient hydrolysis. Motor end hire, end lnirdor System 37
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 37, No. 1
involved in the determination of the difference in the potential 8 u content ~ of the ,,ricjnal w& -,j of the residue de it Nat Reducing Gross Rdueing Lb.DZ , difficult to determine aeourntely the change in efficiency with Total 6uqar/Dry B"g.X/D*7 hidma/ b C 01WYoodSubWoodsub- Effideory, Dr W d different method8 of operation. in 6 O O k stance. % stsnoe. % % suKetarm This cycle-hy-cycle study snd other work indicated that wood 1 17.8 20.8 88 76.8 cellulose does not become more easily hydrolysed as the hngth 2 22.3 27.2 82 68.4 3 27.1 34.1 80 82.8 of exposure to hydrolytic conditions increases. This was ahown 4 31.2 38.1 82 58.7 44.0 78 52.2 in the following way: The washed end dried residues fmm esoh 5 34.8 6 38.7 48.4 76 50.0 of the cooks I i h d in Table I were hydrolyzed with 0.8% sulfuric 7 40.7 51.9 78 47.9 11 47.0 62.4 78 40.8 mid at 180" C. according to the technique described in another paper (a), and the mte of hydrolysis of the residual cellulose in each sample was determined. A ssmple of the origirurl wood Table II. Reridues of Cooks of Table I Hydrolyzed with 0.8% was included for oomprison, The of this experiment Sulfuric Acid sre presented in Table I1 and Fl-ipure 6. Them is no significant Pot+iaI Ist-Order ReaaHslf-Life 01 ReReduotng tion Conarant. aia~en~ t o~tio ofn ehsnge in the rate of hydrolysis of the cellulase in the various SDmpla awr.% (Mi".-r' ceilu'onsb residues, even though the total residue obtained irom the elevenOriginal -o?d we 0.0260 26.6 cycle ewk contained only 12.5% of the carbohydrate initially 68.4 0.0262 25.4 l.t
