Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 237-241
pH, P A , p v = partial pressure of HCl, CzH2,and vinyl chloride.
237
Box, G. E. P., Hill, W. J., Technometrics, 16(3), 385 (1974). Doraiswamy, L. K.. N.C.L., Poona, India, personal communication, Jan 1978. Draper, N. R., Smith. H.."Applied Regression Analysis", Wiley, New York, 1967. Gel'bshtein, A. I., Slin'ko, M., Shcheglova, G. G., Yabbnskii. G. S.,Timoshenko, V. I.. Kamenko, 8. L., Kinet. Catal. ( f n g l . Trans/.), 13, 634 (1972). Gel'bshtein, A. I., Siling. M. I., Sergeeva, G. A., Shcheglova, G. G., Khomenko, A. A., Kinet. Catal. (Engl. Trans/.), 4, 123, 262, 543 (1963), Kipling, J. J., "Adsorption from Solution of Non-electrolytes", Academic Press, London, 1965. McPherson, R. W., Starks, C. M., Fryar, G. J., Hydrocarbon Process., 75 (Mar 1979). Sanderson, J. T., "Chemical Periodicity", Reinhold. New York, 1966. Shankar, H. S.,Ph.D. Thesis, Monsah University, Clayton, Australia, 1976. Weisz, P. B., Prater, C. D., Adv. Catal., 6, 143 (1954). Wesselhoft, R. D., Woods, J. M., Smith, J. M., A I C M J . , 5(3), 361 (1959). Weston, D. F., Agnew, J. B., Indian Chem. Eng., 15(3). 37 (1973). Weston, D. F., Ph.D. Thesis, Monash university, Clayton, Australia, 1970.
atm R = gas constant, kcal mol-' K-' r = reaction rate, mol h-l (g of HgC12)F' Greek L e t t e r s p = percent w/w HgCI,, in catalyst u I 2 = percent variance explained by model u2 = percent deviation between measured and predicted data u3 = percent root variance between measured and predicted data u4 = percent deviation between sum of squares and sum of squares due to pure error L i t e r a t u r e Cited
Received f o r reuieu; August 23, 1979 Resubmitted February 13, 1980
Bond, G. C., "Principles of Catalysis' , pp 36-39, Chemical Society, London, 1972
GENERAL ARTICLES Utilization of Cellulosic Feedstock in the Production of Fuel Grade Ethanol Jackson Yu' and Steven F. Miller' Bechtel National, Inc., San Francisco, California 94 1 19
Recent interest in producing ethanol from renewable resources has focused on the use of lignocellulose as a possible feedstock. Ethanol production could become a compatible addition to integrated forest products operations. This paper outlines the status of the various process steps available for liberating fermentable sugars from the lignocellulose, for ferimenting the sugars to alcohol, and for recovering alcohol and byproducts. Process and laboratory studies associ,ated with these steps are discussed. Finally, this paper outlines some of the developmental activities which will lead toward commercialization.
The production of motor fuel quality ethanol from renewable resources has attracted great interest for extending existing petroleum based motor fuel. Fermentation alcohol has been used as a motor fuel in a number of countries and is being used currently in Brazil, South Africa, and certain midwestern states in the United States. Most of the fermentation ethanol produced today is based on sugars, grains, or cassava. Lignocellulose is being seriously considered as an alternative feedstock candidate. However, in order to use lignocellulose, fermentable sugars must be first liberated and separated from the lignin and hemicellulose fractions of the feedstock. The production of ethanol from cellulose is not new. In fact, much of the developmental work was carried out in the 1940's and 1950's. Commercial facilities in the United States and abroad have used cellulosic sugars for ethanol production. However, most current processes are energy intensive. In general glucose yield is low. Figure 1 shows in a block diagram form the basic steps involved in producing ethanol from lignocellulose. The cellulose fraction must be separated and converted to Cutter Laboratories, Inc., Berkeley, Calif. 94710. 0196-4321/80/1219-0237$01.00/0
glucose to provide substrate for the fermentation. The ethanol thus produced must be concentrated from the fermentation beer to produce anhydrous fuel quality material. There are a number of approaches to the liberation of sugars. The three most important process categories are weak acid, strong acid, and enzymatic hydrolysis. Weak acid hydrolysis processes generally use 0.5 % sulfuric acid at a relatively low temperature of 140 of 190 " C to degrade the cellulose. The Madison-Scholler process, as practiced today in the Soviet Union, yields about 50% of theoretical sugars as a 4% glucose solution along with significant degradation of glucose to undesirable byproducts. The low concentration glucose is fermented to only 2% ethanol, thus requiring energy intensive ethanol distillation and byproduct evaporation procedures. Experimental work conducted recently at a higher temperature and much shorter residence time has resulted in a higher glucose yield and reduced glucose degradation (Brenner, 1978; Bender, 1978). At about 500 " C , glucose yield was approximately 70% of theoretical and about a 20 70 post-hydrolysis glucose concentration. This final glucose concentration is accomplished with residence times in seconds rather than the hours required a t the lower
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r-
11 FERMENT
SUGARS
SUGARS
rRECOL ER ETHANOL
*
1 I
I I
Figure 1. Ethanol from lignocellulose.
temperatures. However, the high temperature and short residence time hydrolysis in the presence of hemicellulose sugars produces certain byproduct materials which inhibit ethanol fermentation. To combat this problem catalysts are added prior to the hydrolysis. Strong acid hydrolysis processes, originally developed primarily to produce crystalline glucose at greater than 90% yield, are of increasing interest today. For the strong acid processes 70 to 80% sulfuric acid or 35 to 42% hydrochloric acid is used at a relatively low temperature of between 10 and 45 OC (Oshima, 1965). In order to effectively use these strong acid processes the acid must be recovered and reused. The acid recovery operations investigated to date generally involve extensive energy consumption. An area of concern with the acid processes is the problem of the corrosive environment. Recent advances in materials of construction will likely minimize the corrosive impact on the processes. In order to increase the post-hydrolysis glucose concentration, removal of hemicellulose prior to the acid attack is desirable. A number of methods have been developed for the prehydrolysis hemicellulose removal. Hemicellulose can be depolymerized by weak acids and the sugars removed by leaching. Alternately, hemicellulose can be degraded by steam with direct in situ furfural production. Generally the prehydrolysis hemicellulose removal leaves the lignocellulose matrix intact for easy recovery by filtration. Enzymatic hydrolysis of cellulose has recently been the subject of considerable interest in the United States and abroad. In order for enzyme to attack the cellulose, effective pretreatment of lignocellulose is required. Chemical pulping processes which produce cellulose fibers relatively free of hemicellulose and lignin seem to have considerable merit. However in general, chemical pulping is relatively expensive. Developmental activities are being carried out whereby the fermentation and enzyme hydrolysis are combined into one single operation (Blotkamp et al., 1978). The combined enzyme hydrolysis/fermentation approach currently requires large vessels to provide adequate residence time in order to overcome the compromise in operating temperatures. The temperature which may be optimal for microorganism to convert glucose to ethanol is likely to be suboptimal for enzyme hydrolysis. Genetic improvement of microorganisms may result in increasingly effective conversion systems. A number of fermentation schemes utilizing saccharomyces cereuessie ferment hexose sugars with approximately the same yields of ethanol. Schemes that retain and reuse cell mass have reported higher yield compared to batch fermentation. Batch fermentation assures minimum contamination hazard and simplest operation but generally requires large reactor volume and is labor intensive. Since semicontinuous and continuous cell recycle type operation can have relatively high rates, these schemes would generally be more cost effective. Continuous cascade of open vats is currently applied to ethanol production from sulfite waste liquor. This type
of operation seems to have better utilization of the sugars. In the open vat fermentors approximately 3% of the ethanol product is lost with the C 0 2 leaving the open vat. Covered vats may prove ideal for larger scale operations. Ethanol produced in the fermentor at 2-12% concentration can be readily concentrated from the fermentation beer to 50 to 80% overhead product. Further concentration to the azeotropic composition is energy intensive. The production of absolute alcohol from the azeotrope typically requires the use of either an azeotropic or an extractive agent. The production of ethanol from lignocellulose results in a significant quantity of lignin and hemicellulose byproducts. These byproducts must be used to generate energy or be marketable in order to influence the total process economics in a favorable manner. Based on various current research and development activities (all entries in the Literature Cited section), we prepared four conceptual process flow diagrams which could be the basis for further developmental work. Figure 2 demonstrates the method of high temperature and short residence time lignocellulose degradation. In this scheme undried wood particles are deaerated and conveyed to an explosive defibrator along with a catalyst. High-pressure steam addition which raises particle temperature and vessel pressure is accompanied by the addition of dilute acid. The combined stream is discharged through an extrusion dye at 3000 to 4000 ft/s destructing the lignocellulosic structure and hydrolyzing the cellulose. Explosive defibrators (e.g., Masonite guns) were originally designed for up to 1000 psi of steam, although current commercial practice uses about 500 psi. This equipment has also been used commercially for many years to pretreat human food grains and cellulosic biomass to increase digestability. Energy requirements are high but the steam latent heat can be effectively recaptured and cascaded in a well-designed full scale integrated conversion process. The discharge from the explosive defibrator is fed to a cyclone where the unused steam and some furfural are removed overhead. The underflow of the cyclone is discharged into a mixing tank where the spent acid is neutralized. The discharge from the neutralization tank is steam stripped to further remove additional furfural. The bottoms from the stripper containing lignin, pentoses, and glucose is clarified by a centrifuge. The glucose is fed to a series of cascading fermentors. Two seed fermentors and four cascading fermentors are depicted in the diagram. Fermentor effluent is centrifuged and the cells are washed and recycled employing a modified Melle-Boinot procedure. Ethanol is recovered from the clarified beer still. Direct steam is used in lieu of a reboiler, and a pasturization section is indicated at the top. The ethanol rich overhead product is dehydrated and recovered in a pair of extractive distillation columns. The relative volatility of water to ethanol is reversed and water is recovered overhead from the second column. Ethanol is recovered from the extractive solvent in the third column, and the solvent is recycled. In commercial operation where the ethanol is to be used with gasoline. it is suggested that gasoline can be used directly as the extractive agent (Chambers, 1978) and under such circumstances the regeneration of the extractive agent would then be eliminated. Thus the ethanol will leave the plant as an ethanol/gasoline mixture. Energy requirements for dehydration and recovery of ethanol may be significantly reduced. In addition, ethanol will leave the operation denatured, greatly simplifying the enforcement of government tax regulations.
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F U R F U R A L RICH BVPRDDUCTS
WATER
I
I
WOOD
JETRi
’
-I
I DISCHARGE
HEAT RECOVERY
uq
I
I
FERMENTERS
FEEDER
FURFURAL COLUMN
n DEFIBRATOR
CVCLONE
OIOki*
NEUTRALIZATION VESSEL
Figure 2. Weak acid hydrolysis of lignocellulose by explosive defibration, continuous cascade fermentation, and extractive distillation recovery.
n
STRIPPING COLUMN
VEDEL
Figure 3. Ethanol pumping, enzymatic hydrolysis/fermentation of cellulose azeotropic distillation recovery.
Figure 3 shows the second scheme in which a pulping agent is used to digest the lignocellulose. The undried wood particles are steam deaerated and fed into a pressure feeder along with recycling cooking liquor into a continuous pulping tower digester. The chips fall on top of a continuous moving bed which descends down the tower. The exit is mechanically assisted from below. The lignin and hemicellulose components of the wood are first depolymerized, then eluted by countercurrent flow of the cooking liquor. At the base of the column fresh cooking liquor
enters to wash the final cellulose fiber matrix free of the lignin and hemicellulose byproduct. Cellulose exiting the digester is defiberized and recovered with a rotary vacuum filter. The filter cake is reslurried with water for enzymatic hydrolysis. The pulping agent in this case is a 50% ethanol-50% water mixture. The cooking liquor, rich in hemicellulose and lignin, exits the top of the digester and passes through multiple effect evaporators where ethanol is recovered as a condensate. The concentrated ethanol from the first few
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“EFSEL
Figure 4. Cellulose solvent pretreatment, enzymatic hydrolysis/fermentation of cellulose, and azeotropic distillation recovery
VESSEL
EXTRACTION AGENT
Figure 5. Strong acid hydrolysis, cascade fermentation, electrodialysis membrane acid recovery
evaporator effects can be recycled directly along with the vacuum filtrate and fresh pulping agent to the base of the digester. The lignin-hemicelldosewater concentrate from the evaporators is separated in a clarifier. Lignin is discharged as the underflow. The hemicellulose-water-ethanol supernate, contaminated by dissolved lignin, is steam stripped to remove residual ethanol. The dilute ethanol condensate is conveniently reconcentrated in the plant beer still for reuse as fresh pulping agent. The ethanol solvent pulping operation has been developed in a 2 m tall pulping column (Kleinert, 1971). Further process development effort would be required before commercialization. The slip stream of cellulose is fed to the first fermentor as substrate for trichoderma uiride to produce cellulase enzyme. The enzyme thus produced discharges from this fermentor and is fed to the next three fermentors where simultaneous saccharification and fermentation of the
main stream cellulose is carried out in a continuous cascade. Yeast cells are recycled. A yeast makeup stream produced in a second seed fermentor is added to 1he cell recycle. The ethanol produced in this scheme is shown concentrated by azeotropic distillation. The third conceptual process involving a cellulose solvent (Tsao, 1978) is depicted in Figure 4. In this case hemicellulose is first separated from the lignocellulose. Undried lignocellulose particles are prehydrolyzed with dilute acid in a countercurrent digestion column. A hemicellulose sugar stream containing the dilute acid leaves the top of the digester, is neutralized, and then concentrated by evaporation. The dried hemicellulose is produced as a byproduct. The lignin/cellulose leaving the first digester is waterwashed and fed through a pressure feeder into a second digester column along with recycle “cellulose solvent”
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manipulation of microorganisms may significantly improve the efficiency of enzymatic hydrolysis. Continuous fermentation and combined hydrolysis/fermentation are being studied to maximize productivity per unit fermentor volume, thus reducing fermentation costs. Novel ethanol recovery dehydration techniques require development and demonstration to improve process energy efficiency. Extractive distillation with gasoline, liquidliquid extraction, and hydrophobic molecular sieves are examples of potentially energy efficiency processes which require further study. Overall process energy yields are of extreme importance in converting cellulose to ethanol fuel. Further effort could significantly reduce the energy requirement for well designed commercial processes. In particular, energy requirement for liberation of sugar, ethanol recovery, and byproduct utilization must be carefully examined. Although research and development efforts still remain, lignocellulose will increasingly play an important role in the near future in extending existing petroleum-based motor fuel. The effective utilization of lignocellulose as the feedstock in the production of ethanol will come about when the concepts illustrated in this paper and others are proven to be cost effective and energy efficient. The fact that many of the basic concepts associated with these processes have already been practiced by related industries will likely shorten the lead time before commercialization of this alternative energy production route. Literature Cited
stream. The lignin/cellulose particles move down through the second digester countercurrent to the rising cellulose solvent solution. The cellulose is eluted and leaves the top of the digester with the solvent. The cellulose is precipitated by dilution of the solvent with water and then recovered by sedimentation and centrifugation. The cellulose thus collected is slurried in water for enzymatic hydrolysis as in the previous scheme. The lignin matrix leaving the base of the second digester is separated from the cellulose solvent by vacuum filtration and washing. Finally, the last sclheme is shown in Figure 5. In this scheme strong acid hydrolysis is used to produce fermentable sugars. This scheme which is based on the Hokkaido Strong Acid Hydrolysis Pretreatment Process (Oshima, 1965) first received undried wood particles in a prehydrolysis digester column in which dilute sulfuric acid is used to remove the hemicellulose as in the previous case. Again the lignin-cellulose particles enter a pressurized feeder and are transported by a recycling strong acid hydrolysis solution to the top of the second digester. In the second countercurrent column the cellulose is hydrolyzed a t room temperature by 'i0--80% sulfuric acid. The glucose-sulfuric acid solution leaving the top of the column is separated by electrodialysis membranes. The glucose retained by the membrane is neutralized and deionized in preparation for fermentation. The sulfuric acid permeated from the electrodialysis is evaporated and reconcentrated for recycle to the second digester. Lignin is separated from the strong acid exiting the bottom of the second digester by filtration and washing. This last scheme represents one of several strong acid processes previously developed but warranting further study in light of recent improvements in materials of construction and in membrane technology. Individual process steps require development in order to synthesize the most cost efficient overall process for effectively utilizing lignocellulose. While several diverse methods of sugar liberation have been presented here, the best alternative is not yet apparent. Weak acid hydro1,ysis at high temperature and short residence times needs further development. Enzymatic hydrolysis processes also require further study to select the most compatable pretreatment in order to maximize yield and minimize energy consumption. Also, genetic
Bender, J., Stake Industries, personal communication, May 1978. Blotkamp, P. J.. Takagi, M., Pamberton. M. S., Emert, G. H.. AIChE Symp. Ser. No. 787, 74, 85-90 (1978). Brenner, W., New York University, personal communication, May 1978. Chambers, R., University of Illinois, personal communication, May 1978. De Long, T., IOTECH, personal communication, May 1978. Kleinert, T., US. Patent 3585 104 (1971). Nystrom, J., Bioeng. Bioeng. Biotechnol., 5 , 221 (1975). Oshima, M., "Wood Chemistry: Process Engineering Aspects", Noyes Data Co., CPM No. 11, 1965. Tsao, G., "Fermentable Sugars From Cellulose via Solvent Pretreatment", 2nd Annual Fuels From Biomass Symposium, RPI, June 21, 1978.
Received f o r review April 24, I979 A c c e p f e d January 17, 1980 Paper presented a t American Chemical Society/Chemical Society of Japan, Chemical Congress, Apr 1 4 , 1 9 7 9 , Division of Industrial and Engineering Chemistry.
Silicon Carbide-Tungsten Heat Pipes for High-Temperature Service Lynn B. Lundberg Los Al'amos Scientific Laboratory, Los Alamos, New Mexico 87545
Silicon carbide is being considered for use as the structural material in liquid metal heat pipes to be used in high-temperature, waste-heat recovery from plants operating with fossil fuels. I n order to protect the Sic from attack by the sodium or lithium metal working fluid, the SIC must be coated internally with tungsten. Experiments have been performed to determine the rate of reaction (layer growth) between SIC and tungsten in a vacuum over the temperature range 1875 to 2075 K. This reaction rate is similar to that observed for the tungsten-carbon reaction forming W2C. The reaction layer developed from the Sic-W reaction consists of two phases, W5Si, and W2C. Based on the observed reaction rate, it is estimated that for heat pipe operating lifetimes of 20 years and tungsten coating thickness of 0.25 mm, the practical steady operating temperature limit is about 1575 K.
Introduction Today there is a growing need for energy conservation methods and equipment, and heat pipe based recuperators 0196-4321/80/1219-0241$01.00/0
are presently satisfying some of this demand, especially in the 250-750 K temperature range. Heat pipe recuperators operating in this temperature range are currently
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1980 American Chemical Society
