U.S. Department of Energy - Energy Efficiency and Renewable EnergyBiomass ProgramConcentrated Acid HydrolysisThis process is based on concentrated acid decrystallization of cellulose followed by dilute acid hydrolysis to sugars at near theoretical yields. Separation of acid from sugars, acid recovery, and acid reconcentration are critical unit operations. Fermentation converts sugars to ethanol. Arkenol and Masada Resource Group plan to use this process in their bioethanol plants. BackgroundThe concentrated acid process for producing sugars and ethanol from lignocellulosic biomass has a long history. The ability to dissolve and hydrolyze native cellulose in cotton using concentrated sulfuric acid followed by dilution with water was reported in the literature as early as 18831. The hydrolysis that occurs in the dilution step gives almost quantitative yields of sugar. The concentrated acid disrupts the hydrogen bonding between cellulose chains, converting it to a completely amorphous state. Once the cellulose has been decrystallized, it forms a homogeneous gelatin with the acid. The cellulose is extremely susceptible to hydrolysis at this point. Thus, dilution with water at modest temperatures provides complete and rapid hydrolysis to glucose, with little degradation. In fact, the use of concentrated sulfuric acid is an accepted test method for quantifying potential glucose content of cellulose2 and for quantifying lignin content. It seems as though most of the research on concentrated acid processes has been done using agricultural residues, particularly corncobs. In 1918, researchers at the U.S. Department of Agriculture (USDA) proposed a process scheme for production of sugars and other products from corn cobs based on a two-stage process. These researchers introduced the idea of using dilute acid pretreatment of the biomass to remove hemicellulose before decrystallization and hydrolysis of the cellulose fraction3. The ability to isolate hemicellulosic sugars from cellulosic sugars was an important improvement to the process, because the five-carbon sugars were not fermentable. In 1937, the Germans built and operated commercial concentrated acid hydrolysis plants based on the use and recovery of hydrochloric acid. Several such facilities were successfully operated. During World War II, researchers at USDA's Northern Regional Research Laboratory in Peoria, Illinois further refined the concentrated sulfuric acid process for corncobs. They conducted process development studies on a continuous process that produced a 15%-20% xylose sugar stream and a 10%-12% glucose sugar stream, with the lignin residue remaining as a byproduct. The glucose was readily fermented to ethanol at 85%-90% of theoretical yield. The Japanese developed a concentrated sulfuric acid process that was commercialized in 1948. The remarkable feature of their process was the use of membranes to separate the sugar and acid in the product stream. The membrane separation, a technology that was way ahead of its time, achieved 80% recovery of acid4. R&D based on the concentrated sulfuric acid process studied by USDA (and which came to be known as the "Peoria Process") picked up again in the United States in the 1980s, particularly at Purdue University5 and at Tennessee Valley Authority (TVA)6. Among the improvements added by these researchers were 1) recycling of dilute acid from the hydrolysis step for pretreatment, and 2) improved recycling of sulfuric acid. Minimizing the use of sulfuric acid and recycling the acid cost-effectively are critical factors in the economic feasibility of the process. Process DescriptionThe concentrated sulfuric acid technology being supported by DOE and NREL follows directly from the work done by USDA's Peoria lab in the 1940s and TVA's work during the 1980s. A generalized process scheme is shown in Figure 1.
Figure 1: Generalized schematic of concentrated sulfuric acid process This process flow diagram is based on information available from Arkenol7. It is one example of how a process based on concentrated acid might be configured. The heart of the process is the decrystallization followed by dilute acid hydrolysis. The original Peoria process, and a modified version proposed by Purdue, carry out dilute acid pretreatment to separate the hemicellulose before decrystallization. The biomass would then be dried to concentrate the acid absorbed in the biomass prior to addition of concentrated sulfuric acid. Purdue proposed recycling sulfuric acid by taking the dilute acid/water stream from the hydrolysis reactor and using it in the hemicellulose pretreatment step. In Arkenol's process, decrystalllization is carried out by adding 70%-77% sulfuric acid to biomass that has been dried to 10% moisture. Acid is added at a ratio of 1.25:1 (acid:cellulose+hemicellulose), and temperature is controlled at less than 50°C. Adding water to dilute the acid to 20%-30% and heating at 100°C for an hour results in the release of sugars. The gel from this reactor is pressed to remove an acid/sugar product stream. Residual solids are subjected to a second hydrolysis step. The use of a chromatographic column to achieve a high yield and separation of acid and sugar is a crucial improvement in the process that was first introduced by TVA and researchers at the University of Southern Mississippi8. The fermentation converts both the xylose and the glucose to ethanol at theoretical yields of 85% and 92%, respectively. A triple effect evaporator is required to reconcentrate the acid9,10. Arkenol claims that sugar recovery in the acid/sugar separation column is at least 98%, and acid lost in the sugar stream is not more than 3%11. Commercial StatusThe concentrated sulfuric acid process has been commercialized in the past, particularly in the former Soviet Union and Japan4. However, these processes were only successful during times of national crisis, when economic competitiveness of ethanol production could be ignored. Conventional wisdom in the literature suggests that the Peoria and TVA processes cannot be economical because of the high volumes of acid required12. Improvements in acid sugar separation and recovery have opened the door for commercial application. Two companies in the United States are currently working with DOE and NREL to commercialize this technology by taking advantage of niche opportunities involving the use of biomass as a means of mitigating waste disposal or other environmental problems. ArkenolArkenol holds a series of patents on the use of concentrated acid to produce ethanol. They are currently working with DOE to establish a commercial facility that will convert rice straw to ethanol. Arkenol plans to take advantage of opportunities for obtaining rice straw in the face of new regulations that would restrict the current practice of open field burning of rice straw. The economics of this opportunity are driven by the availability of a cheap feedstock that normally poses a disposal problem. Arkenol's technology further improves the economics of raw straw conversion by allowing for the recovery and purification of silica present in the straw. NREL is working with Arkenol under a CRADA to develop a recombinant Zymomonas Mobilis strain for the project. The facility would be located in Sacramento County13. Masada Resource GroupMasada holds several patents related to municipal solid waste (MSW)-to-ethanol conversion. DOE and NREL have been working with Masada to support their MSW-to-ethanol plant, which will be located in Middletown, NY. The plant will process the lignocellulosic fraction of municipal solid waste into ethanol using technology based on TVA's concentrated sulfuric acid process. The robustness of this process makes it well suited to complex and highly variable feedstocks like municipal solid waste. Masada's New York project takes advantage of relatively high tipping fees available in the area for collection and disposal of municipal solid waste. Masada is finalizing engineering and project financing, and expects to break ground on the plant. References1Harris, E.E. "Wood Saccharification." In Advances in Carbohydrate Chemistry, Vol 4, Academic Press, New York, 1949, pp 153-188. 2Fan, L.T.; Gharpuray, M.M.; Lee, Y-H. "Chapter 5: Design and Economic Evaluation of Cellulose Hydrolysis Processes." In Cellulose Hydrolysis. Springer-Verlag, New York, 1987, pp 149-169-187. 3LaForge, F.B.; Hudson, C.S. "The Preparation of Several Useful Substances from Corn Cobs." The Journal of Industrial and Engineering Chemistry Vol 10, No. 11, 1918, pp 925-927. 4Wenzl, H.F.J. "Chapter IV: The Acid Hydrolysis of Wood." In The Chemical Technology of Wood, Academic Press, New York, 1970, pp 157-252. 5Tsao, G.T.; Ladisch, M.R.; Voloch, M.; Bienkowski, P., "Production of Ethanol and Chemicals from Cellulosic Materials." Process Biochemistry, September/October 1982, pp 34-38. 6Broder, J.D.; Barrier, J.W.; Lightsey, G.R. "Conversion of Cotton Trash and Other Residues to Liquid Fuel." In Liquid Fuels from Renewable Resources: Proceedings of an Alternative Energy Conference (Cundiff, J.S., ed). American Society of Agricultural Engineers, St. Joseph, MI, 1992, pp 189-200. 7See Arkenol 8Nanguneri, D.R.; Hester, R.D. "Acid/Sugar Separation Using Ion Exclusion Resins: A Process Analysis and Design," Separation Science and Technology, Vol. 25, No. 13-15, 1990, pp 1829-1842. 9Process description taken from Yancey, M.A.; Kadam, K.L. Biomass to Ethanol Facility Design, Cost Estimate, and Financial Evaluation: Volume I. National Renewable Energy Laboratory, Golden, CO, 1997. 10Farone, W.A.; Cuzens, J.E., Method of Producing Sugars Using Strong Acid Hydrolysis of Cellulosic and Hemicellulosic Materials. U.S. Patent No. 5,562,777. October 8, 1996. This patent describes the option of including a drier between the first and a second stage to accommodate a second decrystallization step. 11Farone, W.A.; Cuzens, J.E. Method of Separating Acids and sugars Resulting from Strong Acid Hydrolysis. U.S. Patent No. 5,580,389. December 3, 1996. 12Wright, J.D.; d'Agincourt, C.G. "Evaluation of Sulfuric Acid Hydrolysis Processes for Alcohol Fuel Production." Biotechnology and Bioengineering Symposium, No 14, John Wiley and Sons, New York, 1984, pp 105-123. 13Anonymous, "Two New Cellulosic Ethanol Plants in Late Preconstruction Stages." New Fuels & Vehicle Report, March 14, 1997. |
