Comparison of hydrogen fluoride saccharification of lignocellulosic materials with other saccharification technologies

Energy in Agriculture, 2 (1983) 219--244 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

219

COMPARISON OF HYDROGEN FLUORIDE SACCHARIFICATION OF LIGNOCELLULOSIC MATERIALS WITH OTHER SACCHARIFICATION TECHNOLOGIES

MARTIN C. HAWLEY 1 , SUSAN M. SELKE l and DEREK T.A. LAMPORT 2

1Department of Chemical Engineering, Michigan State University, 389 Engineering Building, East Lansing, MI 48824 (U.S.A.) 2MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI (U.S.A.) (Accepted 13 January 1983)

ABSTRACT Hawley, M.C., Selke, S.M. and Lamport, D.T.A., 1983. Comparison of hydrogen fluoride saccharification of lignocellulosic materials with other saccharification technologies. Energy Agric., 2: 219--244. Laboratory investigations have been conducted for the past year in the MSU-DOE Plant Research Laboratory in collaboration with Chemical Engineering at Michigan State University on the hydrogen fluoride (HF) saccharification of cellulose and wood. Our work is an extension of the work done by Fredenhagen in Germany reported in 1933. We have produced evidence that HF saccharification of lignocellulosic materials when compared with hydrolysis saccharification techniques has the important features of: (1) high conversion of cellulose and hemicellulose to simple sugars; (2) recycle of HF in the process thereby eliminating the need for major acid neutralization requirements; (3) a valuable lignin byproduct for chemical use; and (4) no elaborate pretreatment requirements. High sugar yields and chemically useful lignin are essential in the long term if biomass is to replace petroleum and later coal as sources of chemical raw materials and liquid fuels. There are several laboratory investigations recommended in order to more fully evaluate the potential of HF saccharification of lignocellulosic materials. These investigations include studies on reaction conditions versus yield, removal and recycle methods, post-hydrolysis conditions versus oligomer distribution, sugar separation methods, and basic studies on uses for xylose and lignin. Results of these studies could be utilized to develop a process design and economic evaluation of the HF technology and to identify those processing steps which require pilot scale experimentation.

INTRODUCTION The conversion of cellulose to sugar and then alcohol has a long history. B r a c o n n o t m a d e the discovery t h a t c o n c e n t r a t e d acid could c o n v e r t cellulose to simple sugars in 1819. S i m o n s e n , in 1 8 9 4 , appears to have b e e n the first to successfully use dilute acid to saccharify w o o d . Classen, in 1899, u s e d s u l f u r o u s a c i d i n a p r o c e s s w h i c h was w i d e l y p a t e n t e d a n d w a s e m p l o y e d

220 in an experimental plant. Ewen and Tomlinson modified the Classen process, using sulfuric acid, and this American process was used successfully in two plants until sometime after World War I, when higher costs for wood waste left them unable to compete economically with alcohol from blackstrap molasses. World War II brought a resurgence of interest in the saccharification of wood, and a number of wood hydrolysis plants operated in Europe during this period. The availability of cheap ethanol from ethylene after World War II again rendered wood alcohol uneconomical. More recently, the Arab oil embargo and the inevitability of oil depletion and rising oil prices have led to a resurgence of interest in alcohol from biomass. Processes for the conversion of lignocellulosic materials to sugars can be divided into three major categories: concentrated acid, dilute acid, and enzymatic. These processes are discussed and compared in the sections which follow. Saccharification of wood using concentrated hydrofluoric acid (HF) was investigated by Fredenhagen and others in the 1930's, but has n o t received much attention since that time. HF saccharification appeared to have some basic advantages over other saccharification methods, leading us to do some laboratory investigation for use as a basis of comparison of HF with other saccharification technologies. These investigations, as will be shown, do suggest t h a t HF has advantages relative to other saccharification technologies. It is clear, therefore, that more work on HF saccharification is justified to bring this m e t h o d to a status of technological development similar to that of the alternative processes. Several specific research problems related to HF saccharification will be suggested for study.

CONCENTRATED ACID PROCESSES Concentrated acid processes fall into three groups: sulfuric acid, hydrochloric acid, and hydrogen fluoride. Because hydrogen fluoride saccharification has a number of unique features, it will be discussed in a later section. Sulfuric acid and hydrochloric acid processes have several c o m m o n features. The wood or other feedstock is first subjected to a pretreatment (often with dilute acid) to hydrolyze the hemicellulose. This is typically followed by a drying process to prevent dilution of the concentrated acid, and then the main hydrolysis with concentrated acid. Because monomeric sugars undergo reversion in concentrated acid solutions, forming sugar oligomers, a post-hydrolysis step utilizing dilute acid follows the main hydrolysis. These steps are outlined in Fig. 1. Concentrated acid processes have the advantage of high yields of sugars, with relatively short reaction times at essentially ambient conditions. Disadvantages include the large amounts of acid required, which render processes uneconomical unless the acid can be recycled. Corrosion problems require special materials of construction. Products are obtained in dilute

221

DILUTEACID OR STEAM,ETC.

I CONCENTRATEDHCI JOR H2SO4

I ~ I..MAJN.-.J LIGNOCELLULOSICI,I PRETREAT MENT ~~YUNULY~=~ MATERIAL I I EXCESS ~, WATER SUGAR-ACID PENTOSESFROM SOLUTION HEMICELLULOSE TO PENTOSE UTILIZATION ACID ] DILUTEACIDTO REMOVALl RECONCENTRATI~)N

[

FILTRATIONL

LIGNIN

1 I POSTHYDROLYSISI

LIME r] NEUTRALIZATIONJ

IFILTRATIONI PRECIPITATE SUGARSOLUTION TO FERMENTATION Fig. 1. Concentrated acid hydrolysis processes.

solutions, and the lignin is left in a highly condensed nonreactive form, suitable for little except burning. Because of the need for acid recycling, and the fact that hydrochloric acid is volatile and sulfuric acid is not, most recent research in concentrated acid hydrolysis has dealt with hydrochloric acid rather than sulfuric acid. H o k k a i d o process

The Hokkaido process is a concentrated sulfuric acid process developed in Japan in 1948, which culminated in a full-scale plant which opened in 1963 with a capacity of 100 t per day of dry wood. Special features included a process for spraying the dried powdered wood and concentrated acid to-

222

gether in a thin film and then immediately filtering and washing the product under pressure, thus minimizing sugar degradation. 80% of the total sulfuric acid was recovered by diffusion dialysis using an ion exchange resin. Overall sugar yields were reported as 280--290 kg of crystalline glucose from 1 t of dry w o o d (Oshima, 1965; O'Neil et al., 1978). A modification of the Hokkaido process was the thorough-drying process developed by J. Kobayashi. Instead of adding concentrated acid to the prehydrolyzed wood, it is immersed in dilute acid and then dried at 40--50°C, which concentrates the acid and completes the main hydrolysis (O'Neil et al., 1978). The Nippon Process is another modification of the Hokkaido process in which the sulfuric acid is n o t recycled, b u t instead converted to high quality gypsum and marketed. A small-scale plant using 5 t of dry w o o d per day was established in 1956 (Locke and Garnum, 1961; Oshima, 1965; O'Neil et al., 1978). Peoria process -- Northern Regional Research Laboratory

The U.S. Department of Agriculture's Northern Regional Research Laboratory developed a concentrated sulfuric acid process for the hydrolysis of agricultural residues in the early 1940's. The main hydrolysis reaction t o o k place at 40°C for 2 min under a pressure of 11.9 atm. Sugar yields were reported to be 85.5% of theoretical (Dunning and Lathrop, 1945). University o f Missouri

A recent process using concentrated sulfuric acid was developed by O.C. Sitton and others at the University of Missouri in Rolla, MO. It employs ground corn stover as feedstock. The sulfuric acid is recovered by electrodialysis. R e p o r t e d yields are 94% of xylose and 89% of glucose (Sitton et al., 1979). Bergius-Rheinau process

The Bergius-Rheinau process was the first commercial hydrolysis process using hydrochloric acid. The first pilot plant was constructed in 1925, and the first large-scale plant in 1932, followed by another large plant in 1940, using a modified process. Both plants closed after World War II. A b o u t 80% of the hydrochloric acid was recovered by distillation under reduced pressure. In the second plant, at Regensburg, calcium chloride was added to the dilute acid solution to increase the concentration of hydrogen chloride in the gas phase (Hall et al., 1956; Oshima, 1965; O'Neil et al., 1978; Ladisch, 1979).

223 Udic-Rheinau process The Udic-Rheinau process was a post-war modification of the BergiusRheinau process. A semi-works plant in 1960 provided for the recovery of 94--95% of the acid by vacuum distillation and reconcentration in two rectification columns operating at different pressures. Wood particles 15 mm in size were used as feedstock. A unique feature was the hydrogenation of the mother liquors from the glucose and xylose crystallization to produce polyalcohol solutions. It was also planned to use the lignin as a raw material for plastics (Locke and Garnum, 1961; Oshima, 1965; O'Neil et al., 1978). Dow Chemical company The D o w Chemical c o m p a n y recently reinvestigated the liquid hydrochloric acid hydrolysis process, concentrating on the development of a recovery system for concentrated hydrochloric acid using a solvent extraction process (Forster et al., 1980). Battelle Another recent reinvestigation of the liquid hydrochloric acid has been carried o u t at the Battelle Memorial Institute. Their process involves the repeated immersion of lignocellulosic material in concentrated hydrochloric acid. A tubular rotary reactor is utilized, with the solid material passing through in a helical path. L o w temperature and pressure are employed, and provision is made for recycle of some of the acid (Battelle Mem. Inst., 1980). Noguchi-Chisso The Noguchi-Chisso process employs hydrogen chloride gas rather than a concentrated hydrochloric acid solution. A pilot plant using this process operated from 1958--1962. After the prehydrolysis and drying, the solid material is cooled to 10°C and exposed to cold HC1 gas, absorbing it to a concentration of 42%. Next it is rapidly heated to 40--45°C for the hydrolysis reaction, which requires 10--30 min for completion, depending on particle size. Next the p r o d u c t is heated in a flash process to vaporize most of the hydrogen chloride in the material, which is recycled after the removal of water. After posthydrolysis, the remaining acid can be recovered by dialysis. An anti-sticking agent is used to reduce the stickiness of the intermediate p r o d u c t (Locke and Garnum, 1961; Oshima, 1965; O'Neil et al., 1978; Kusama, 1979). Lawrence Berkeley Laboratory A high pressure (13.6--27.2 atm) process involving hydrogen chloride gas has recently been investigated at the Lawrence Berkeley Laboratory. 10

224 weight % glass beads are used to fluidize the w o o d particles during the reaction. Yields are reported to be 75% after dilute-acid posthydrolysis (U.S. Dep. Energy, 1980a, b).

DILUTE ACID PROCESSES Dilute acid processes generally use sulfuric acid. Historically, dilute sulfuric acid processes are the only biomass saccharification processes which have operated successfully commercially. A number of plants were in operation in Germany and other countries during World War II, and the Soviet Union has a number of plants in operation today. Because there is substantial sugar degradation in dilute acid solutions at the high temperatures and pressures necessary for hydrolysis, a prehydrolysis under milder conditions to remove xylans is required. Failure to remove them leads to considerable contamination of the product sugar solution with furfural and other degradation products which make it difficult to ferment. Even after prehydrolysis and with careful attention to reaction time and temperature to maximize glucose yields, typical yields are a b o u t 50%. Posthydrolysis is n o t required. The acid is generally removed after the main hydrolysis by neutralization with lime. A typical reaction scheme is shown in Fig. 2. DILUTE ACID OR STEAM, ETC.

"" MATERIAL

I

I

/ DILUTE ACID

1

HYDROLYSIS -I (HIGHT,P)

PENTOSES FROM HEMICELLULOSE TO PENTOSE UTI LIZATION

FILITRATION

]

LIGNIN '

I SUGAR- ACID SOLUTION (ALSO DEGRADATION PRODUCTS)

[

NEUTRALIZATIONIA

LIME

I-

FILITRATION

I PRECIPITATE =,

SUGAR SOLUTION ( + DEGRADATION PRODUCTS) TO FERMENTATION

Fig. 2. Dilute

acid hydrolysis processes.

225

Scholler process

The first commercial plant using the Scholler process began operation in 1931. By 1941 at least 20 plants using this process were in operation in Germany alone. The reaction vessel was a percolator, through which successive batches of 0.5% sulfuric acid were forced, at temperatures of 130--180°C and pressure ranging from 11.2--12.2 atm. Total hydrolysis time was about 2 h with yields of a b o u t 200 1 of 95% alcohol per 1000 kg oven-dried w o o d (Faith, 1945; Oshima, 1965; Hokanson and Katzen, 1978). Madison process

The Madison process was developed by the Forest Products Laboratory at their Marquette pilot plant in 1941--1945. It was a modification of the Scholler process which reduced the total hydrolysis time to 6 h. Yields were 200--270 1 of 95% ethanol per 1000 kg oven-dried w o o d chips (Harris et al., 1945; Plow et al., 1945; Oshima, 1965; Hokanson and Katzen, 1978). T V A process

The TVA process is a more recent modification of the Madison process, resulting from the Tennessee Valley Authority pilot plant. It incorporates recycling of the dilute hydrolysis solution from a previous batch to the reactor, and a total percolation time of 145--190 min. Pentose sugars were collected after fermentation of hexoses, and sold as a feed supplement or converted to furfural (H0kanson and Katzen, 1978). Grethlein process (Dartmouth)

Dr. Hans Grethlein of D a r t m o u t h has developed an isothermal plug flow reactor for the hydrolysis of a refuse slurry (5--13.5% solids) using dilute sulfuric acid. An initial prehydrolysis at 180--200°C for xylose recovery is followed by the main hydrolysis at 230--240°C. In the main hydrolysis, cold acid is injected and thoroughly mixed with the slurry, which is then suddenly brought to the reaction temperature by the direct injection of steam. Yields of 50--57% are reported (Grethlein, 1975, 1978a, b; O'Neil et al., 1978; Converse and Grethlein, 1979; Thompson and Grethlein, 1979; U.S. Dep. Energy, 1980b). Brenner process (New York University)

Dr. Walter Brenner of New York University has built a 900 kg per day demonstration plant for the hydrolysis of sawdust or pulped newspaper with dilute sulfuric acid in a twin barrel extruder, operating at 34 atm and 450°C. Actual reaction time is a b o u t 20 s, after which the mixture is

226 quenched by sudden cooling on leaving the extruder. Glucose yields are 50--55% (Chem. Eng. News, 1979b; U.S. Dep. Energy, 1980b; Chem. Eng., 1981).

Georgia Tech (GITprocess) Investigators at Georgia Tech are developing a hydrolysis process which can employ either steam explosion, delignification by solvent extraction, prehydrolysis with dilute acid, or some combination as a pretreatment, followed by the main hydrolysis with dilute acid. Yields are expected to be 80--85% (O'Neil et al., 1979; Biomass Digest, 1980; U.S. Dep. Energy, 1980a, b; Chem. Eng., 1981).

ENZYMATIC PROCESSES A great deal of research has been done in the last few years on enzymatic processes. The major advantages are that the products are obtained in a pure form, as the reaction is specific and there is no formation of degradation products, and that no high temperatures or pressures or special corrosion problems are encountered, as in the case of acid processes. The major disadvantages are that fairly extensive pretreatments are required, and yields are generally in the 50% range after all but the most drastic pretreatments. Further, long reaction times are required, enzyme requirements are high, little recycling is possible, and enzyme production costs are high. Enzymatic processes are divided into two categories. In the first, cellulose is first converted to sugars, and the sugars then fermented to alcohol or put to other uses. In the second, simultaneous saccharification and fermentation, cellose is converted to sugars and the sugars simultaneously fermented to alcohol. Outlines of these processes are presented in Figs. 3 and 4. The major avenues of current research are directed toward the developm e n t of less expensive and more effective pretreatments and the develm e n t of less expensive and more productive enzyme systems.

Natick The Natick process is the oldest and most developed of the enzymatic hydrolysis processes. Trichoderma viride (reesei) is used for enzyme production. Hydrolysis of Kraft pulp is successful without pretreatment, but use of other substrates requires ball milling or two-roll milling. A 60% yield of glucose has been obtained from ball-milled newspaper after a 48 h hydrolysis (Brandt et al., 1973; Mandels et al., 1974, 1978; Mandels and Stemberg, 1976; Spano et al., 1976; Gallo et al., 1978; Allen et al., 1979; Mandel, 1979; U.S. Dep. Energy, 1980a, b).

227

CELLULOSIC MATERIAL IENZYM E NUTRIENTS

LIGNOCELLULOSIC_ I MATERIAL -- I PRETREATMENT

I

~

]

'~,, HYDROLYSIS

I

XYLOSE SOLUTION

I

I

FILTRATION

I LIGNIN I

SUGAR SOLUTION TO FILTRATION

Fig. 3. Enzymatic hydrolysis processes.

ENZYME AND/OR ORGANISM PRODUCTION ENZYME

ALCOHOL~,-

I LIGNOCELLULOSI._cclPRETREATMENT I MATERIAL- I

I

1

I FERMENTATION ,

XYLOSE I SOLUTION

1 i

1

FILTRATION

I UGNm

I SUGAR AND ALCOHOL SOLUTION TO SEPARATION Fig. 4. Simultaneous saccharification and fermentation.

228 Wilke processes (Berkeley) Dr. C.R. Wilke has developed enzymatic hydrolysis processes with T. viride for shredded and hammer-milled newsprint and for prehydrolyzed (dilute sulfuric acid) wheat straw and other agricultural residues. Cellulose conversion is about 50%, with the product obtained in a 4% solution. Approximately 34% of the enzyme is recovered for recycling. After fermentation of the glucose to ethanol, the remaining xylose and cellobiose are converted to single cell protein by Torula yeast, or alternatively utilized in anaerobic digestion to methane. Fermentation of xylose to ethanol by Fusarium oxysporum has also been investigated (Brandt, 1975; Wilke and Mitra, 1975; Wilke and Yang, 1975; Wilke et al., 1976, 1979, 1981; Batter and Wilke, 1977; Wilke, 1977; U.S. Dep. Energy, 1980b). Indian Institute o f Technology The Indian Institute of Technology in New Delhi has developed a modification of the Natick process for hydrolysis of newsprint in which the sugar is recovered by dialysis followed by fractional crystallization to separate the glucose from the cellobiose (O'Neil et al., 1978). Rutgers Researchers at Rutgers University are investigating the production of hypercellulolytic mutants of T. reesei which are able to grow at elevated temperatures on inexpensive carbon and nitrogen sources (Montenecourt and Eveleigh, 1978; U.S. Dep. Energy, 1980a). Swedish Forest Products Laboratory Work at the Swedish Forest Products Laboratory involves the investigation of a white rot fungus which can decompose lignin as well as cellulose and hemicellulose (O'Neil et al., 1978). Iotech In the process developed by the Iotech Corporation of Ottawa, Ont., hardwood chips (poplar or aspen) are treated with saturated steam at 240--300°C and 34--38 atm in a "gun reactor" for 5 s to 5 min. The wood is then " e x p l o d e d " by depressurization and subjected to enzymatic attack, followed by a conventional ethanol fermentation. Lignin is recovered by filtration at the end of the process. A proprietary catalyst is used in the explosion pretreatment. A commercial plant is planned to begin operation by the fall of 1982 (Chem. Eng., 1981).

229

General Electric -- University o f Pennsylvania The G.E. University of Pennsylvania process is a simultaneous saccharification and fermentation process which involved delignification of biomass (poplar chips or urban refuse) by n-butanol at 150--175°C for 15--30 min as a pretreatment. This is followed by a simultaneous saccharification by Thermoactinomyces and fermentation to ethanol by Clostridium thermocellum at 55--60°C. The alcohol is recovered under low pressure. Hemicellulose is fermented to butanol and acetone with C. acetobutylicum. About 90% sugar utilization is reported (O'Neil et al., 1978; Su, 1978; Pye and Humphrey, 1 9 7 9 , U.S. Dep. Energy, 1980a, b).

General Electric (GE/RCD) The General Electric Corporate Research and Development process employs a pretreatment of poplar fibers by steaming at 16.3 atm (205°C) for approximately 15 min in the presence of sulfur dioxide, followed by rapid decompression. The next step is simultaneous saccharification and fermentation at 60°C by a mixed culture of C. thermocellum and C. thermosaccharolyticum. Both 5-carbon and 6-carbon sugars are fermented by this method. Lignin is used as fuel. Overall yields are reported to be about 60%. Inclusion of a dilute acid prehydrolysis before the steaming raised yields to 68% (Brooks et al., 1979; U.S. Dep. Energy, 1980a, b).

Massachusetts Institute of Technology The MIT process involves anaerobic saccharification and simultaneous fermentation of corn stover to ethanol and acetic acid, by Clostridium thermocellum at 60°C. A strain of C. thermocellurn which has a higher ethanol tolerance and a favorable ethanol to acetic acid production ratio has been isolated. Studies on C. thermosaccharolyticum and C. thermohydrosulfuricum which can also metabolize pentoses are also being conducted (Gordon et al., 1978; O'Neil et al., 1978; Wang et al., 1978, 1979; Cooney et al., 1979; Kusama, 1979; Avgerinos and Wang, 1980; U.S. Dep. Energy, 1980a, b).

Gulf The Gulf Oil Chemical Co. process involves the simultaneous saccharification of cellulose by T. reesei and fermentation to ethanol by Saccharomyces cerevisiae, Candida brassicae and Saccharornyces carlsbergensis. Conversions up to 90% have been reported on pretreated cellulose. Ethanol is recovered by steam stripping. Poplar, corn stover, and other waste cellulose have been used as feedstocks. A commercial scale plant is scheduled to open in 1983

230 (Blotkamp et al., 1978; Chem. Eng. News, 1979a; Emert, 1979; U.S. Dep. Energy, 1980a, b; Emert and Katzen, 1980; Emert et al., 1980). Natick

In addition to the enzymatic saccharification investigations described earlier, investigators at Natick have studied the saccharification of cellulose by T. viride coupled with fermentation by Candida utilis. Saccharification is carried out for 20 h at 45°C, then the temperature lowered to 40°C and the yeast cells added. A b o u t 3.5 h later when the dissolved oxygen concentration has decreased to a negligible level, the temperature is further reduced to 30°C. 45% yields were obtained (Meyers, 1978). PRETREATMENTS

A great deal of current research is directed towards developing less expensive and more effective pretreatments, both for the dilute acid and enzymatic hydrolysis processes. The scope of this work is too broad to cover here, but there is one process which deserves special mention. The solvent extraction process developed by George Tsao at Purdue can be used as a pretreatment in either enzymatic or dilute acid processes, and is reported to produce glucose yields of 80% after a 40 h enzymatic hydrolysis of corn stover, compared to a 30% yield without extraction. This process involves a dilute acid prehydrolysis to remove hemicellulose, followed by treatment of the lignin-cellulose residue with a cellulose solvent. The cellulose is then reprecipitated, becoming highly reactive to acid or enzymatic hydrolysis. Dr. Tsao has also investigated the fermentation of pentose sugars to ethanol and butanediol (Ladisch et al., 1978, 1979; Tsao, 1978, 1979; Tsao et al., 1978; Bose et al., 1979). HF SACCHARIFICATION The ability of hydrofluoric acid to dissolve cellulose was first investigated by Helferich and Bottger (1929), although it had been noted as early as 1869 t h a t HF transformed paper and other cellulosic materials into "glutinous substances" and dissolved them (Gore, 1869). The mechanism of the reaction of HF with filter paper was investigated by Fredenhagen and Cadenbach (1933) who discovered that it involved the formation of glucosyl fluorides, which in HF solution react with even small amounts of water to produce glucose and regenerate HF. On precipitation or evaporation of HF, the glucose monomers can recombine to form polyglucans. Fredenhagen and Cadenbach suggested saccharification of wood by HF as a quick m e t h o d for determining the lignin and carbohydrate contents of woods, straws, reeds, and other lignocellulosic materials. They also investigated the use of gaseous HF, which was found to saccharify cellulose if the temperature was low

231 enough for a liquid adsorption phase to be formed. Yields of up to 95% were obtained with one part wood to one part gaseous HF (by weight). The HF was released by vacuum or flue evaporation at a temperature of 100°C (Fredenhagen and Cadenbach, 1933). Two Australian inventors, Hoch and Bohunek, around 1937, patented a process using gaseous HF under reduced pressure, in which a liquid adsorption phase was believed not to be required. A pilot plant using their process was constructed in Germany and ran for 6 months. 90% of the cellulose reacted to form sugars, of which they recovered 80% for further processing. The reaction temperature was 35--40°C, pressure 30 mm Hg, and reaction time 30 min. 40 kg of 95% HF were used for 100 kg of dry wood (2--3% water content). HF was removed by raising the temperature to 62 ° C, and recycled after contaminating acetic acid was removed (Luers, 1937, 1938). With the exception of some Russian investigations in the late 1950's (Rogovin and Pogosov, 1958, 1959), HF saccharification was ignored until fairly recently. Currently research on HF saccharification is being carried out by our group at Michigan State University, Andrew Mort and Susan Parker at the Charles F. Kettering Research Laboratory, (Mort and Parker, 1982) and by a group in France and Denmark (Defaye et al., 1981). Our work at Michigan State University began in September 1979. We are currently investigating the liquid phase reaction of anhydrous HF and HF containing specified water contents with aspen chips (Populus grandidentata). Nearly quantitative sugar yields have been obtained from filter paper (pure cellulose), and yields of 70--85% of theoretical from wood chips. HF was successfully removed by vacuum evaporation, with total fluoride retention in the sugar and lignin fractions of approximately 4 mg/g wood. Thus the HF can be successfully recycled. Removal of HF leads to the formation of sugar oligomers by reversion, as in other concentrated acid processes. These oligomers have been successfully converted to monomers by a posthydrolysis in dilute acid. No prehydrolysis is required. Successful fermentation of the wood sugars has been demonstrated. The lignin byproduct obtained is not significantly fluorinated and may be valuable as a chemical raw material. Because anhydrous hydrogen fluoride can be handled in carbon steel vessels (as is c o m m o n practice in the HF alkylation industry) corrosion problems should be substantially less than in other concentrated acid processes. We have also identified several process flow alternatives to be considered (Lamport et al., 1981; Hardt and Lamport, 1982; Selke et al., 1982). An outline of the HF saccharification flow scheme is presented in Fig. 5. COMPARISON OF SACCHARIFICATION TECHNOLOGIES Some characteristics of representative concentrated acid hydrolysis processes are presented in Table I. As has been described, concentrated acid processes have the advantage of higher sugar yields than dilute acid or en-

232

I ANHYDROUS HF

EXCESS ~I MOISTURE LIGNOCELLULOSIC I DRYING ~ . ~ MATERIAL -- I

SACCHARIFICATION ]

HF TO WATERREMOVAL AND RECYCLING WATERI ~ASHmNGAND I LIGNIN

-- I FILTRATION

I SUGARSOLUTION POSTHYD.ROLYSISI

CaCO3~-IINEUTRALIZATIONI

I

CaFTO REGENERATIONOF HF .ira WITH H2SO4 AND RECYCLE SUGARSOLUTION TO FERMENTATION OR OTHERUSE

FILTRATION

Fig. 5. HF saccharification.

zymatic processes, short reaction times, and the use of essentially ambient conditions. Their disadvantages are high consumption of acid, expensive recycling methods, and severe corrosion problems. Both pre- and posthydrolysis are generally required, along with drying of the material after the dilute acid prehydrolysis. Products are obtained in dilute solutions, and lignin is left in a highly condensed nonreactive form, especially after sulfuric acid hydrolysis. HC1 lignin is of considerably higher quality, b u t is significantly chlorinated.

233 Some characteristics of representative dilute acid hydrolysis processes are presented in Table II. These processes have the advantage of generally being cheaper and having a considerable shorter reaction time than enzymatic hydrolysis processes. The substrate need not be dried. Prehydrolysis can be omitted if desired. Post-hydrolysis is n o t required. The disadvantages are the requirement for high temperature and pressure, and the occurrence of considerable sugar degradation, which contaminates the p r o d u c t solution and leads to maximum yields of a b o u t 55% unless drastic pretreatments of the feedstock, such as solvent extraction or solvent delignification, are employed. Characteristics of representative enzymatic hydrolysis processes are presented in Table III. These processes have the advantages of producing pure products, and of not requiring high temperatures or pressures. The substrate need n o t be dried. The disadvantages are extensive pretreatment requirements, long reaction times, yields of only a b o u t 50% after most pretreatments, high enzyme requirements, and a high production cost for the enzymes, resulting in high sugar costs. Hydrogen fluoride saccharification shares the advantages of high sugar yield, short reaction times, and the use of essentially ambient conditions with the other concentrated acid processes. In contrast to them, it does n o t require a prehydrolysis with dilute acid, and thus does n o t require drying of a very wet prehydrolyzed material. A relatively dry feedstock is required, however. Because H F is readily removed by vaporization, recycling of H F will be less expensive than for the sulfuric acid or liquid hydrochloric acid processes. Corrosion problems with HF will also be significantly less than for other concentrated acid processes. The lignin b y p r o d u c t may be of high value. A preliminary comparison of chemical and feedstock costs for HF saccharification versus enzymatic and dilute acid hydrolysis showed these costs to be l l . 9 c / k g for H F versus 13--20c/kg for dilute acid and enzymatic processes (Selke et al., 1982). The major disadvantage of the HF saccharification process for lignocellulosic materials is that it has been studied for a much shorter period of time than have the alternative hydrolysis processes, which have generally reached or are nearing the pilot plant stage. Because the comparisons we can make between the alternative technologies indicate that H F saccharification may well be the most advantageous technology for production of chemical raw materials and liquid fuels from lignocellulosic materials, it is very important that the necessary laboratory and pilot-scale investigations be carried o u t to bring H F saccharification to the same level of development attained for the other concentrated acid, dilute acid, and enzymatic technologies. A more complete economic analysis can then be performed to reliably compare HF saccharification with the alternative hydrolysis processes.

234 TABLE I Summary of representative concentrated acid hydrolysis processes Sulfuric acid

Status Feed Yield Byproducts

Reaction

conditions

Acid recovery

Other chemicals Energy

Water Costs

Hokkaido

Nippon

full-scale plant, 1963 hardwood

5 t/day laboratory pilot plant sawdust, waste cornstalks timber, charcoal probably same as 89% of glucose Hokkaido molasses, gypsum 94% of xyiose

83--85% as crystalline glucose furfural, xylose

Mo. Univ.

prehydrolysis, 1.2--1.5% acid, 140--150°C, dried, crushed; main hydrolysis, 80% acid, r o o m temperature; posthydrolysis, 100 rain, 100°C ion exchange membrane; 80% recovery as 25--35% solution; concentration by evaporation

prehydrolysis, 1.2--1.5% acid, 140--150 ° C, dried, crushed; main hydrolysis, 80% acid, r o o m temperature; posthydrolysis, 100 min, 100°C none-neutralized

prehydrolysis, 50 rain, 4.4% acid, dry; impregnation 85% acid, dilute to 8%, 10 min, 110 ° C

NaC1, lime

lime

lime

electrodialysis

21.7 million kcal/h required, 26.7 million available (to alcohol) (25 and 31 MW, respectively) $7.531 million capital investment; $4.4 million/year operation including $381,200/year utility cost; $0.26/1 alcohol; 17 million 1/year (1979)

235

HC1 liquid

HCI gas

N.Reg.Res.Lab.

Udic-Rheinau

Noguchi-Chisso

laboratory

semi-works plant, 1960 logs and wastewood chipped to 15 mm mixed dextrosexylose lignin, polyalcohol solution, xylitol, xylose dry, prehydrolysis, 35% acid, 20 ° C; hydrolysis 41%, 20°C; evaporation; posthydrolysis, 100 ° C

pilot plant, 1953--59 logs and wastewood, reduced to sawdust 90%, 300 kg dextrose per t of wood xylose

94--95% recovery ; vacuum distillation; reconcentration; deionization after posthydrolysis

heat to high temperature; deionization aftel posthydrolysis

ground cornstalks 85--90% glucose 25% xylose, 69% furfural from remaining xylose prehydrolysis, 50--185 min, 4.9--9.8% acid, 100°C, impregnation 85%, 40°C dilute to 8%, 45 min, 110°C

none-neutralized

lime

prehydrolysis, dilute acid, cool; HC1 gas absorption, heated off; posthydrolysis

236 TABLE II Summary of representative dilute acid hydrolysis processes Dilute sulfuric acid

Status Feed Yield

Scholler

Madison

plants, 1931, Europe wood chips, sawdust

Marquette pilot laboratory plant, 1945 wood chips, wood, other biomass 200 t/day 0.22--0.27 l 190 80--85% proof alcohol/kg

200 1 190 proof alcohol/ODT wood

GIT

wood, 50% of sugar 59--68 t/day preheated with steam, 3.4 atm, increased to 10.2 atm; total time hydrolysis 0.5% acid, 150 ° C : 6 h, 0.4--0.85% acid

Byproducts Reaction conditions

furfural compressed and preheated with steam, 129 ° C, 0.5% acid, 12.2 atm; forced through in up to 24 batches, 45 rain per batch

Acid recovery

none--consumes 0.28 kg 76% acid/1 alcohol lime, 0.26 kg/1 alcohol

none--consumes 12 t / d a y acid lime

12.9 atm steam 25.3 kg/1 alcohol, 15.8 We/1 , 1590 1 compressed air/1

501.7 t of 12.9 atm steam/ day, 671 kWe, 48000 1 compressed air/rain 7.2 million 1/day $2.247 million (1945) capital investment for 43500 1/day alcohol

Other chemicals Energy

Water Costs

42.3 l/1 alcohol

lignin steam explosion, lignin extraction with ethanol, recycled; prehydrolysis, 150°C; hydrolysis, 0.5% acid, 190°C: 5% sugar concentration to 13--20% by evaporation none - - neutralized Ca(OH)2 or other alkali

Estimated selling price for 95% ethanol: 45 c/1

RECOMMENDED LABORATORY INVESTIGATIONS In order to bring HF saccharification to a level of development closer to t h a t o f t h e a l t e r n a t i v e h y d r o l y s i s t e c h n o l o g i e s , o n e o f t h e m a j o r a r e a s in n e e d o f s t u d y is f u r t h e r d e t e r m i n a t i o n o f t h e r e a c t i o n k i n e t i c s a n d o p t i m a l reaction conditions. In particular, studies of yield versus reaction time, temperature, w a t e r c o n t e n t , a n d p a r t i c l e size a r e r e q u i r e d . F u r t h e r inv e s t i g a t i o n o f p o s t h y d r o l y s i s c o n d i t i o n s is n e e d e d , e s p e c i a l l y f o c u s i n g o n t h e p o s s i b i l i t y o f u s e o f d i l u t e H F in t h e p o s t h y d r o l y s i s a n d t h e e f f e c t o f H F

237

T.V.A.

NYU--Brenner

Grethlein

pilot plant

900 kg/day demo plant, 1979 newspaper pulp, dust 60% glucose

laboratory refuse slurry

0.53% acid, average, percolated through wood chip bed, max. 196°C, 14.6 atm, 145--190 min

0.5% acid, 232 ° C with steam, 34 atm in twin screw extruder

1% acid, 20 s residence time, 240°C isothermal plug flow reactor

none

none

none

wood chips

lime

max. 56% glucose predicted

lime 890 kcal/kg cellulose, 940 kcal/kg sugar (3.7 and 3.9 MJ/kg)

$0.50/1 alcohol for 94.6 million l/year, 15% ROI, $7.70/t wood waste (1975)

7.5 c/kg sugar, 227 t/day plant 5 c/kg for 908 t/day plant (1974)

p o s t h y d r o l y s i s on lignin if it has n o t p r e v i o u s l y been r e m o v e d f r o m the r e a c t i o n system. F u r t h e r c h a r a c t e r i z a t i o n o f lignin is n e e d e d to verify its r e a c t i v i t y a n d estimate its value as a c h e m i c a l f e e d s t o c k . These investigations are in progress, b u t c o n s i d e r a b l e w o r k remains. T w o m a j o r areas w h e r e l a b o r a t o r y w o r k has n o t y e t b e g u n are developm e n t o f H F r e m o v a l - r e g e n e r a t i o n - r e c y c l e systems, and investigation o f t h e gas-phase H F s a c c h a r i f i c a t i o n reactions. C o m p l e t i o n o f the investigations o u t l i n e d a b o v e will allow us to specify a p r o c e s s design a n d u n d e r t a k e a m o r e c o m p l e t e e v a l u a t i o n o f t h e process

238 TABLE III Summary of representative enzymatic hydrolysis processes Enzymatic hydrolysis Wilke

Wilke

Natick

laboratory newsprint, 803 t/day plus 60 for enzyme production 50% conversion, 216 t/day glucose, 4% solution

laboratory corn stover (wheat stover), 1.25 t/day 40% conversion

pilot plant newsprint, delignified pulp

Byproducts

Lignin for fuel

Reaction conditions

shredding, hammer-milling, 40 h, 45°C, 1:20 sol:liq ratio

10 900 1 methane/day from corn stover, single cell protein from wheat prehydrolysis, 1.1 h, 1% acid, 100 ° C, 5-stage hydrolysis over 40 h, 45°C

Enzyme and recovery

T. viride, 3.5 FPA, 34% recovery

Other chemicals

enzyme fermentation nutrients

Energy

8060 kWe, 16.3 t/h steam

Water

314 000 1/h

Costs

11.4 c/kg sugar + substrate cost, $23.39 million fixed capital (1975)

Status Feed Yield

60% yield from newsprint, in 15% solution; 85% yield from pulp

ball milling for newsprint, hydrolysis 48 h for newsprint, 72 h for pulp, 40--50 ° C, 1 atm, pH 4.79--4.89, hydrolysis solution recycled T. viride, 3.5 FPA, T. viride 58% recovery estimated, high lime, fermentation formalin, fermentanutrients tion nutrients 4.08 t delignified cellulose/day 9875 kWe, 93.4 t/h steam (corn stover) 742 000 l/h (corn stover) 35--40 e/kg sugar 10.3 c/kg sugar from wheat from pulp (1975), 22 c/kg glucose, 11.2 c/kg xylose from corn (1978) + suhstrate cost, $29.407 million capital

239

Simultaneous saccharification and fermentation GE/Pa. Univ.

MIT

Gulf

laboratory

laboratory Solka floc, corn residue

laboratory pure cellulose (Avicel & Solka floc)

low

low; 0.42--0.65 g product/g cellulose degraded

low

acetic acid

delignification with n-butanol, alcohol recovered under reduced pressure, anaerobic

anaerobic

anaerobic, 40 ° C

Thermoactinomyces, Clostridium thermocellum nutrients

Clostridium thermocellum

T. reesei S. cerevisiae

nutrients

nutrients

240 economics. In addition, during the course of our investigations we will identify the process components which will require investigation on a larger scale. CONCLUSIONS We have previously outlined the long-term requirements for a conversion process for biomass to replace petroleum and later coal as a source of chemical raw materials and liquid fuels (Selke et al., 1982). These requirements are two-fold. First, the process must provide a high yield of sugars from both the cellulose and hemicellulose fractions of the biomass. The sugars can be used directly as chemical feedstocks or can be fermented to ethanol, which can be used as a liquid fuel or further converted to ethylene or butadiene for use as a chemical raw material. Secondly, the lignin fraction of the biomass must be recovered in a chemically valuable. It can then be used to produce phenol or a variety of other aromatic compounds, including polymers. Table IV presents a brief comparison of the various hydrolysis technologies. It is apparent that HF saccharification is the only biomass conversion technology that appears able to produce both high sugar yields and a chemically valuable lignin at a reasonable cost. Concentrated hydrochloric and sulfuric acid processes produce considerable degradation of the lignin, and are also plagued by high costs due to expensive acid recovery and severe corrosion problems. Dilute acid processes also cause lignin degradation, and in addition face low yields due to extensive sugar degradation. Enzymatic processes can produce a very high quality lignin, but have long reaction times, expensive enzyme production, and generally low yields. Some of the newer pretreatments being developed, such as solvent extraction and steam explosion, offer substantial improvements in yield from dilute acid and TABLE IV Comparison of characteristics of typical lignocellulosic hydrolysis technologies

Glucose yield (%) Acid consumption

Dilute acid

Concentrated acid

50 medium

85--90 high low-medium high pilot

Lignin

reactivity Enzyme and

low moderate

Enzymatic

HF

50 none

85--95 very low

high high

? low

pilot

laboratory

chemical requirements

Status of technology

commercial in Soviet Union

241 enzymatic processes, but are generally expensive. HF saccharification can produce high sugar yields and a valuable lignin byproduct without any pretreatment of the substrate other than chipping and drying. Acid recovery a n d r e c y c l e is e x p e c t e d t o b e f a i r l y e a s y a n d e c o n o m i c a l . C o r r o s i o n p r o b l e m s will b e s u b s t a n t i a l l y less t h a n f o r o t h e r c o n c e n t r a t e d a c i d p r o c e s s e s . Because HF saccharification has advantages over alternative hydrolysis t e c h n o l o g i e s , it is i m p o r t a n t t h a t i t b e f u r t h e r i n v e s t i g a t e d . W h e n w e h a v e completed the remaining laboratory investigations that are prerequisite to t h e d e v e l o p m e n t o f a p r o c e s s d e s i g n , w e will b e a b l e t o m o r e r e a l i s t i c a l l y e v a l u a t e t h e e c o n o m i c s o f t h e u s e o f H F in t h e c o n v e r s i o n o f b i o m a s s t o chemical raw materials and liquid fuels. Based on our present knowledge, we believe this evaluation will be favorable.

REFERENCES Allen, A.L., Blodgett, C.R. and Nystrom, J.M., 1979. Pilot plant conversion of cellulose to glucose. In: V. Venkatesh (Editor), Advances in the Utilization and Processing of Forest Products. AIChE Symp. Ser. No. 184, American Institute of Chemical Engineers New York, NY, pp. 20--23. Avgerinos, G.C. and Wang, D.I.C., 1980. Direct microbial conversion of cellulosics to ethanol. In: D. Perlman (Editor), Annual Reports on Fermentation Processes, Vol. 4. Academic Press, New York, NY, pp. 27--47. Battelle, Mem. Inst., 1980. Process and apparatus for continuous acid hydrolysis and saccharification. U.K. Patent 1-562-682, Patent Office, London, 8 pp. Batter, T.R. and Wilke, C.R., ]977. A study of the fermentation of xylose to ethanol by Fusarium o x y s p o r u m . LBL-6351, Lawrence Berkeley Laboratory, Berkeley, CA, 29 pp. (Unpublished). Biomass Digest, 1980, Vol. 2, No. 7, Technical Insights, F o r t Lee, NJ, 10 pp. Blotkamp, P.J., Tagaki, M., Pemberton, M.S. and Emert, G.H., 1978. Enzymatic hydrolysis of cellulose and simultaneous fermentation to alcohol. In: J.M. Nystrom and S.M. Barnett (Editor), Biochemical Engineering Renewable Sources of Energy and Chemical Feedstocks. AIChE Syrup. Ser. No. 181, American Institute of Chemical Engineers New York, NY, pp. 85--90. Bose, A., Bao, Y., Ladisch, M.R. and Tsao, G.T., 1979. Loop of cellulose as a function of solvent pretreatment and its relation to hydrolysis. Presented at American Institute of Chemical Engineers, 72nd Annual Meeting, San Francisco, CA, 22 pp. (Unpublished). Brandt, D., 1975. Remarks on the process economics of enzymatic conversion of cellulose to glucose. In: C.R. Wilke (Editor), Cellulose as a Chemical and Energy Resource. Biotechnol. Bioeng. Symp. 5, pp. 275--277. Brandt, D., Hontz, L., and Mandels, M., 1973. Engineering aspects of the enzymatic conversion of waste cellulose to glucose. In: W.S. Kaghan (Editor), Forest Products and the Environment. AIChE Symp. Ser. No. 133, American Institute of Chemical Engineers, New York, NY, pp. 127--133. Brooks, R., Su, T., Brennan, M. and Frick, J., 1979. Bioconversion of plant biomass to ethanol. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SERI/TP-33-285, Solar Energy Research Institute, pp. 275--280. Chem. Eng., 1981. Wood-to-ethanol methods edge closer to fruition. Chemical Engineering, 88(2): 51--55. Chem. Eng. News, 1979a. Pilot plants to make ethanol from biomass. Chemical Engineering News, 16 April, pp. 38--39.

242 Chem. Eng. News, 1979b. Continuous cellulose-to-glucose process. Chemical Engineering News, 8 October, pp. 19--20. Converse, A.O. and Grethlein, H.E., 1979. Acid hydrolysis of cellulosic biomass. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SERI/TP, TP-33-285, Solar Energy Research Institute, pp. 91--95. Cooney, C.L., Wang, D.I.C., Wang, S., Gordon, J. and Jiminez, M., 1979. Simultaneous cellulose hydrolysis and ethanol production by a cellulolytic anaerobic bacterium. In: C.D. Scott (Editor), Biotechnology in Energy Production and Conservation. Biotechnol. Bioeng. Symp. 8, pp. 103--114. Defaye, J., Gadelle, A. and Pedersen, C., 1981. Degradation of cellulose with hydrogen fluoride. In: W. Palz, P. Chartier and D.O. Hall (Editors), Energy from Biomass. Applied Science Publishers, London, pp. 319--323. Dunning, J.W. and Lathrop, E.C., 1945. The saccharification of agricultural residues, a continuous process. Ind. Eng. Chem., 37 : 24--29. Emert, G., 1979. Economic update of the bioconversion of cellulose to ethanol. Presented at American Institute of Chemical Engineers, 72nd Annual Meeting, San Francisco, CA, 16 pp. (Unpublished). Emert, G.H. and Katzen, R., 1980. Gulf's cellulose-to-ethanol process. Chemtech, 10: 610--614. Emert, G.H., Katzen, R., and Kaupisch, K.F., 1980. Economic update of the Gulf cellulose alcohol process. Chem. Eng. Progr., 76(9): 47--52. Faith, W.L., 1945. Development of the Scholler process in the United States. Ind. Eng. Chem., 37: 9--11. Forster, A.V., Martz, L.E. and Leng, D.E., 1980. Process for separating and recovering concentrated hydrochloric acid from the crude product obtained from the acid hydrolysis of cellulose. U.S. Patent 4-237-110, 6 pp. Fredenhagen, K. and Cadenbach, G., 1933. Der abbau der cellulose durch Fluorwasserstoff und ein neues Verfahren der Holzverzuckerung durch hochkonzentrierten Fluorwasserstoff. Angew. Chem., 46: 113--117. Gallo, B.J., Andreotti, R., Roche, C., Ryu, D. and Mandels, M., 1978. Cellulose production by a new mutant strain of Trichoderma reesei MCG77. In: C.D. Scott (Editor), Biotechnology in Energy Production and Conservation. Biotechnol Bioeng. Symp. 8, pp. 89--101. Cordon, J., Jiminez, M., Cooney, C.L. and Wang, D.I.C., 1978. Sugar accumulation during enzyme hydrolysis and fermentation of cellulose. In: J.M. Nystrom and S.M. Barnett (Editors), Biochemical Engineering: Renewable Sources of Energy and Chemical Feedstocks. AIChE Symp. Serr. No. 181, American Institute of Chemical Engineers New York, NY, pp. 91--97. Gore, G., 1869. On hydrofluoric acid. J. Chem. Soc., 22: 396--406. Grethlein, H., 1975. The acid hydrolysis of refuse. In: C.R. WIlke (Editor) Cellulose as a Chemical and Energy Resource, Biotechnol. Bioeng. Symp. 5, pp. 303--318. Grethlein, H., 1978a. Comparison of the economics of acid and enzymatic hydrolysis of newsprint. Biotechnol. Bioeng., 20: 503--525. Grethlein, H., 1978b. Chemical breakdown of cellulosic materials. J. Appl. Chem. Biotech., 28: 296--308. Hall, J.A., Saeman, J.F. and Harris, J.F., 1956. Wood saccharification - - a summary statement. Unasylva, 10: 7--15. Hardt, H. and Lamport, D.T.A., 1982. Hydrogen fluoride saccharification of wood: lignin fluoride content, isolation of ~-D-glucopyranosyl fluoride and posthydrolysis of reversion products. Biotechnol. Bioeng., 24 : 903--918. Harris, E.E., Beglinger, E., Hajny, G.J. and Sherrard, E.C., 1945. Hydrolysis of wood, treatment with sulfuric acid in a stationary digester. Ind. Eng. Chem., 37: 12--23. Helferich, B. and Bottger, 8., 1929. Die Einwirkung von Fluorwasserstoff auf Cellulose. Justus Liebigs Annal. Chem., 476: 150--170. Hokanson, A.E. and Katzen, R., 1978. Chemicals from wood waste. Chem. Eng. Progr., 74(1): 67--71.

243 Kusama, J., 1979. Cellulose saeeharifieation process with HC1 gas-flash saccharification process. Chem. Econ. Eng. Rev., 11(6): 33--36. Ladisch, M.R., 1979. Fermentable sugars from cellulosic residues. Process Biochem., 14(1): 21--25. Ladisch, M.R., Ladisch, C.M. and Tsao, G.T., 1978. Cellulose to sugars: new p a t h gives quantitative yield. Science, 201 : 743--745. Ladisch, M.R., Flickinger, M.C. and Tsao, G.T., 1979. Fuels and chemicals from biomass. Energy, 4: 263--275. Lamport, D.T.A., Hardt, H., Smith, G., Mohrlok, S., Hawley, M.C., Chapman, R. and Selke, S., 1981. HF saccharification: the key to ethanol from wood? In: W. Palz, P. Chattier and D.O. Hall (Editors), Energy from Biomass. Applied Science Publishers, London, pp. 292--297. Locke, E.G. and Garnum, E., 1961. Working party on wood hydrolysis. For. Prod. J., 11: 380--382. Luers, H., 1937. Der heutige Stand der Holzverzuckerung. Holz Roh-Werkstoff, 1: 35--40. Luers, H., 1938. Das Holzverzuckerungsverfahren mit Fluorwasserstoff yon Hoch und Bohunek. Holz Roh-Werkstoff, 1: 342--344. Mandels, M., 1979. Enzymatic saccharification of waste cellulose. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SERI/TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 281--289. Mandels, M. and Sternberg, D., 1976. Recent advances in cellulose technology. J. Ferment. Technol., 54: 267--286. Mandels, M., Hontz, L. and Nystrom, J., 1974. Enzymatic hydrolysis of waste cellulose. Biotechnol. Bioeng., 16: 1471--1493. Mandels, M., Dorval, S. and Medeiros, J., 1978. Saccharification of cellulose with Trichoderma cellulase. Presented at the Second Annual Fuels from Biomass Symposium, June 20--22, 1978, Rensselaer Polytechnic Institute, Troy, NY, 42 pp. (Unpublished). Meyers, S.G., 1978. Ethanolic fermentation during enzymatic hydrolysis of cellulose. In: J.M. Nystrom and S.M. Barnett (Editors), Biochemical Engineering: Renewable Sources of Energy and Chemical Feedstocks. AIChE Symp. Ser. No. 181, American Institute of Chemical Engineers, New York, NY, pp. 79--84. Montenecourt, B.S. and Eveleigh, D.E., 1978. Hypercellulolytic mutants and their role in saccharification. Presented at Second Annual Fuels from Biomass Symposium, June 20--22, 1978, Rensselaer Polytechnic Institute, Troy, NY, 13 pp. (Unpublished). Mort, A. and Parker, S., 1982. The use of anhydrous HF solvolysis in conversion of biomass to glucose. In: Biotechnology R and D for Fuels and Chemicals from Biomass. I.P.C. Press (In press). O'Neil, D.J., Colcord, A.R., Bery, M.K., Day, S.W., Roberts, R.S., El-Barbary, I.A., Havlicek, S.C., Anders, M.E. and Sondhi, D., 1978. Design, Fabrication and Operation of a Biomass Fermentation Facility. Re. DSE-3060-T1, U.S. Department of Energy, Washington, DC, 139 pp. O'Neil, D.J., Bery, M.K., Colcord, A.R., Roberts, R.S. and Sondhi, D., 1979. Conceptual design of a biomass fermentation facility. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SER/TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 515--546. Oshima, M., 1965. Wood chemistry process engineering aspects. Chemical Process Monograph Series 11, Noyes Development Corp., New York, NY, 157 pp. Plow, R.H., Saeman, J.F., Turner, H.D. and Sherrard, E.C., 1945. The rotary digester in wood saccharification. Ind. Eng. Chem., 37 : 36--43. Pye, E.K. and Humphrey, A.E., 1979. Production of liquid fuels from cellulosic biomass. In: Annual Biomass Energy Systems Conference Proceedings, Third Tech. Rep. SERI! TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 69--75. Rogovin, Z.A. and Pogosov, Y.L., 1958. Gidroliz. i. Lesokhim. Prom., 11(1): 4 (Chem. Abstr., 52: 8453i). Rogovin, Z.A. and Pogosov, Y.L., 1959. Nauch Doklady Vysshei Shkoly, Khim, i. Khim. Tekhnol., 1959, No. 2, p. 368 (Chem. Abstr., 53: 22912h).

244 Selke, S.M., Hawley, M.C., Hardt, H., Lamport, D.T.A., Smith, G., Smith, J., 1982. Chemicals from wood via HF. Ind. Eng. Chem. Prod. Res. Dev. 21: 11--16. Sitton, O.C., Foutch, G.L., Book, N.L. and Gaddy, J.L., 1979. Ethanol from agricultural residues. Chem. Eng. Progr., 75(12): 52--57. Spano, L.A., Medeiros, J. and Mandels, M., 1976. Enzymatic hydrolysis of cellulosic wastes to glucose. In: Conference on Capturing the Sun through Bioconversion, Council on Solar Biofuels, Washington, DC, pp. 541--566. Su, T.M., 1978. Bioconversion of plant biomass to ethanol. In: J.M. Nystrom and S.M. Barnett (Editors), Biochemical Engineering: Renewable Sources of Energy and Chemical Feedstocks.AIChE Symp. Set. No. 181, American Institute of Chemical Engineers, New York, NY, pp. 75--78. Thompson, D.R. and Grethlein, H.E., 1979. Design and evaluation of a plug flow reactor for acid hydrolysis of cellulose. Ind. Eng. Chem. Prod. Res. Dev., 18:(3) 166--169. Tsao, G., 1978. Fermentable sugars from cellulosic wastes as a natural resource. Presented at the Second Annual Fuels from Biomass Symposium, June 20--22, 1978, Rensselaer Polytechnical Institute, Troy, NY, 27 pp. (Unpublished). Tsao, G., 1979. Selective solvent extraction in utilization of stored solar energy in cellulosic biomass. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SERI/TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 77r -78. Tsao, G.T., Ladisch, M., Ladisch, C., Hsu, T.A., Dale, B. and Chou, T., 1978. Fermentation substrates from cellulosic materials: production of fermentable sugars from cellulosic materials. In: D. Perlman (Editor), Annual Reports on Fermentation Processes Vol. 2. Academic Press, New York, NY, pp. 1--21. U.S. Dep. Energy, 1980a. Biomass Refining Newsletter, Summer, 1980. Solar Energy Research Institute, U.S. Department of Energy, Washington, DC, 53 pp. U.S. Dep. Energy, 1980b. Alcohol Fuels Process RID Newsletter, Winter. Solar Energy Research Institute U.S. Department of Energy, Washington, DC, 72 pp. Wang, D.I.C., Cooney, C.L., Wang, S., Gordon, J. and Wang, G.Y., 1978. Anaerobic biomass degradation to produce sugars, fuels and chemicals. Presented at Second Annual Fuels from Biomass Symposium, June 20--22, 1978, Rensselaer Polytechnic Institute, Troy, NY, 33 pp. (Unpublished). Wang, D.I.C., Biocic, I., Fang, H. and Wang, S., 1979. Direct microbiological conversion of cellulosic biomass to ethanol. In: Annual Biomass Energy Systems Conference, Proceedings, Third. Tech. Rep. SERI/TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 61-67. Wilke, C.R., 1977. Production of sugars and ethanol based on the enzymatic hydrolysis of cellulose. In: J.T. Pfeffer and J.J. Stukel (Editors), Fuels from Biomass SymposiumProceedings. University of Illinois, Urbana, IL, pp. 115--144. Wilke, C.R. and Mitra, G., 1975. Process development studies on the enzymatic hydrolysis of cellulose. In: C.R. Wilke (Editor), Cellulose as a Chemical and Energy Resource. Biotechnol. Bioeng. Symp., 5, pp. 253--274. Wilke, C.R. and Yang, R.D., 1975. Process-development studies of the enzymatic hydrolysis of newsprint. In: T.E. Timell (Editor), Proceedings of the Eighth Cellulose Conference. Appl. Polymer Syrup., 28, pp. 175--188. Wilke, C.R., Yang, R.D. and Stockar, U.V., 1976. Preliminary cost analyses for enzymatic hydrolysis of newsprint. In: E.L. Gaden, Jr., M.H. Mandels, E.T. Reese and L.A. Spano (Editors), Enzymatic Conversion of Cellulosic Materials: Technology and Applications. Biotechnol. Bioeng. Syrup. 6, pp. 155--175. Wilke, C.R., Blanch, H.W., Sciamanna, A.F., Roseuberg, S.L., Tangnu, S.K. and Freitas, R.P., 1979. Process development studies on the bioconversion of cellulose and production of ethanol. In: Annual Biomass Energy Systems Conference Proceedings, Third. Tech. Rep. SERI/TP, TP-33-285, Solar Energy Research Institute, Golden, CO, pp. 79--84. Wilke, C.R., Yang, R.D., Sciamanna, A.F. and Freitas, R.P., 1981. Raw materials evaluation and process development studies for conversion of biomass to sugars and ethanol. Biotechnol. Bioeng., 23: 163--183.