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Hydrolysis occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. The first stage is operated under milder conditions to hydrolyze hemicellulose, while the second stage is optimized to hydrolyze the more resistant cellulose fraction. Liquid hydrolyzates are recovered from each stage, neutralized, and fermented to ethanol. BCI plans to use dilute acid hydrolysis during the startup phase of its facility in Jennings, Louisiana.
Background
Process Description
Commercial Status
References
Dilute acid hydrolysis of biomass is, by far, the oldest technology for converting biomass to ethanol. As indicated earlier, the first attempt at commercializing a process for ethanol from wood was done in Germany in 1898. It involved the use of dilute acid to hydrolyze the cellulose to glucose, and was able to produce 7.6 liters of ethanol per 100 kg of wood waste (18 gal per ton). The Germans soon developed an industrial process optimized for yields of around 50 gallons per ton of biomass. This process soon found its way to the United States, culminating in two commercial plants operating in the southeast during World War I. These plants used what was called "the American Process"—a one-stage dilute sulfuric acid hydrolysis. Though the yields were half that of the original German process (25 gallons of ethanol per ton versus 50), the throughput of the American process was much higher. A drop in lumber production forced the plants to close shortly after the end of World War I1. In the meantime, a small, but steady amount of research on dilute acid hydrolysis continued at the USDA's Forest Products Laboratory.
In 1932, the Germans developed an improved "percolation" process using dilute sulfuric acid, known as the "Scholler Process." These reactors were simple systems in which a dilute solution of sulfuric acid was pumped through a bed of wood chips. Several years into World War II, the United States found itself facing shortages of ethanol and sugar crops. The U.S. War Production Board reinvigorated research on wood-to-ethanol as an "insurance" measure against future worsening shortages, and even funded construction of a plant in Springfield, OR. The board directed the Forest Products lab to look at improvements in the Scholler Process2. Their work resulted in the Madison Wood Sugar process, which showed substantial improvements in productivity and yield over its German predecessor3. Problems with start up of the Oregon plant prompted additional process development work on the Madison process at TVA's Wilson Dam facility. TVA's pilot plant studies further refined the process by increasing yield and simplifying mechanical aspects of the process4. The dilute acid hydrolysis percolation reactor, culminating in the design developed in 1952, is still one of the simplest means of producing sugars from biomass. It is a benchmark against which we often compare our new ideas. In fact, such systems are still operating in Russia.
In the late 1970s, a renewed interest in this technology took hold in the United States because of the petroleum shortages experienced in that decade. Modeling and experimental studies on dilute hydrolysis systems were carried out during the first half of the 1980s. DOE and USDA sponsored much of this work. By 1985, most researchers recognized that, while the dilute acid percolation designs were well understood, these systems had reached the limits of their potential. Their comparatively high glucose yields (around 70%) were achieved at the expense of producing highly dilute sugar streams. Kinetic models, based on pseudo first order kinetics, and process design work showed that the most effective designs would require both high solids concentration and some form of countercurrent flow. The former is a consequence of equipment size and energy cost and the latter is a consequence of the reactor kinetics. Both requirements involve significant equipment design problems. Studies shifted to alternative designs, such as plug flow reactors5,6 and so-called progressing batch systems that mimicked countercurrent operation7. Optimal operation of the plug flow reactors required very short residence time (6 to 10 seconds) and high temperature (around 240°C)8. On scale up, these systems encountered some difficulties with solids handling, even at lower-than-optimal concentrations9. Plug flow systems in the lab and the pilot plant produced yields of glucose of around 50%. These yields are approaching the theoretical limits for such continuous reactor systems.
Fermentation Background
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After a century of research and development, the dilute acid hydrolysis process has evolved into the general concept outlined in Figure 1. The hydrolysis occurs in two stages to accommodate the differences between hemicellulose and cellulose10. The first stage can be operated under milder conditions, which maximize yield from the more readily hydrolyzed hemicellulose. The second stage is optimized for hydrolysis of the more resistant cellulose fraction. The liquid hydrolyzates are recovered from each stage and fermented to alcohol. Residual cellulose and lignin left over in the solids from the hydrolysis reactors serve as boiler fuel for electricity or steam production.
While a variety of reactor designs has been evaluated, the percolation reactors originally developed at the turn of the century are still the most reliable. Though more limited in yield than the percolation reactor, continuous cocurrent pulping reactors have been proven at industrial scale11. NREL recently reported results for a dilute acid hydrolysis of softwoods in which the conditions of the reactors were as follows:
- Stage 1: 0.7% sulfuric acid, 190°C, and a 3-minute residence time
- Stage 2: 0.4% sulfuric acid, 215°C, and a 3-minute residence time
Figure 1: General schematic of two-stage dilute acid hydrolysis process
These bench scale tests confirmed the potential to achieve yields of 89% for mannose, 82% for galactose and 50% for glucose. Fermentation with Saccharomyces cerevisiae achieved ethanol conversion of 90% of the theoretical yield12.
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There is quite a bit of industrial experience with the dilute acid process. As indicated earlier, Germany, Japan, and Russia have operated dilute acid hydrolysis percolation plants off and on over the past 50 years. However, these percolation designs would not survive in a competitive market situation. Today, companies are beginning to look at commercial opportunities for this technology, which combine recent improvements and niche opportunities to solve environmental problems.
BC International (BCI)
BCI and the DOE's Office of Fuels Development have formed a cost-shared partnership to develop a biomass-to-ethanol plant. The facility will initially produce 20 million gallons per year of ethanol. BCI will utilize an existing ethanol plant located in Jennings, LA. Dilute acid hydrolysis will be used to recover sugar from bagasse, the waste left over after sugar cane processing. A proprietary, genetically engineered organism will ferment the sugars from bagasse to ethanol13,14. Construction is expected to begin in the Summer of 2000 with ethanol production starting in February 2002.
Pulp and Paper Industry
Tembec and Georgia Pacific are operating sulfite pulp mills in North America, which utilize a dilute acid hydrolysis process to dissolve hemicellulose and lignin from wood, and produce specialty cellulose pulp. The hexose sugars in the spent sulfite liquor are fermented to ethanol. The lignin is either burnt to generate process steam or converted to value-added products such as dispersing agents, animal feed binders, concrete additives, drilling mud additives, and soil stabilizer.
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1Sherrard, E.C.; Kressman, F.W. "Review of Processes in the United States Prior to World War II." Industrial and Engineering Chemistry, Vol 37, No. 1, 1945, pp 5-8.
2Faith, W.L. "Development of the Scholler Process in the United States." Industrial and Engineering Chemistry, vol 37, No. 1, 1945, pp 9-11.
3Harris, E.E.; Beglinger, E. "Madison Wood-sugar Process." Industrial and Engineering Chemistry, Vol 38, No. 9, 1946, pp 890-895.
4Gilbert, N.; Hobbs, I.A.; Levine, J.D. "Hydrolysis of Wood Using Dilute Sulfuric Acid." Industrial and Engineering Chemistry, Vol. 44, No. 7, 1952, pp 1712-1720.
5Church, J.A.; Wooldridge, D. "Continuous High-Solids Acid Hydrolysis of Biomass in a 1½ inch Plug Flow Reactor. Industrial and Engineering Chemistry Product Research and Development, Vol 20, 1981, pp 371-378.
6Thompson, D.R.; Grethlein, H.E. "Design and Evaluation of a Plug Flow Reactor for acid Hydrolysis of Cellulose." Industrial and Engineering Chemistry Product Research and Development, Vol 18, 1979, pp 166-169.
7Bergeron, P.; Wright, J.D.; Werdene, P.J. "Progressing-Batch Hydrolysis Reactor Single-Stage Experiments." Biotechnology and Bioengineering Symposium, No. 17, 1986, pp 33-51.
8McParland, J.J.; Grethlein, H.E.; Converse, A.O. "Kinetics of Acid Hydrolysis of Corn Stover." Solar Energy, Vol 28, No. 1, 1982, pp 55-63.
9Brennan, A.H.; Hougland, W.; Schell, D.J. "High Temperature Acid Hydrolysis of Biomass Using an Engineering Scale Plug Flow Reactor: Results of Low Solids Testing." Biotechnology and Bioengineering, No. 17, 1986, pp 53-70.
10Harris, J.F.; Baker, A.J.; Conner, A.H.; Jeffries, T.W.; Minor, J.L.; Patterson, R.C.; Scott, R.W.; Springer, E.L.; Zorba, J. Two-Stage Dilute Sulfuric Acid Hydrolysis of Wood: An Investigation of Fundamentals. General Technical Report FPL-45, U.S. Forest Products Laboratory, Madison, Wisconsin, 1985.
11Torget, R. Milestone Completion Report: Process Economic Evaluation of the Total Hydrolysis Option for Producing Monomeric Sugars Using Hardwood Sawdust for the NREL Bioconversion Process for Ethanol Production. Internal Report, National Renewable Energy Laboratory, Golden, Colorado, 1996.
12Nguyen, Q. Milestone Completion Report: Evaluation of a Two-Stage Dilute Sulfuric Acid Hydrolysis Process. Internal Report, National Renewable Energy Laboratory, Golden, Colorado, 1998.
13Anonymous, "Bagasse-to-Ethanol Plant Proposed." Ethanol Report. January 8, 1998.
14Wald, M.L., "A New Bacterium Helps Turn Agricultural Waste Into Energy to Fuel Cars." The New York Times, October 25, 1998.
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Last updated: 03/03/03
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