M. T. Holtzapple, M. K. Ross, N.-S. Chang, V. S. Chang, S. K. Adelson,

and C. Brazel

Department of Chemical Engineering, Texas A&M University,
College Station, TX 77843-3122

The MixAlco Process converts biomass into mixed alcohol fuels. The biomass is first treated with lime to render it more digestible. Then, it is fed to a mixed culture of acid-forming microorganisms that produce salts of volatile fatty acids, such as calcium acetate, propionate, and butyrate. These salts are separated from the fermentation broth and thermally converted to ketones that are subsequently hydrogenated to alcohols, such as 2-propanol, 2-butanol, 2-pentanol, and 3-pentanol. Design data are presented related to the lime treatment, fermentation, thermal conversion, and hydrogenation. A preliminary economic evaluation indicates that alcohol fuels can be produced from negative-value biomass, such as municipal solid waste and sewage sludge, for about $0.19/L ($0.72/gal).

Figure 1 shows a schematic of the MixAlco Process which converts biomass (e. g. municipal solid waste, sewage sludge, agricultural residues, energy crops) into mixed alcohol fuels. To enhance digestibility, the biomass is treated with lime. Then, using a mixed population of acid-forming microorganisms such as those found in cattle rumen, the lime-treated biomass is converted to volatile fatty acids (VFA’s) such as acetic, propionic, and butyric acids. To prevent the pH from decreasing as the acids are formed, a neutralizing agent is added to the fermentor; thus VFA salts — such as calcium acetate, propionate, and butyrate — exit the fermentor. These salts are concentrated to dryness and then are thermally converted to mixed ketones (e. g., 2-propanone, 2-butanone, 2-pentanone, 3-pentanone) that are subsequently hydrogenated to mixed alcohols (e. g., 2-propanol, 2-butanol, 2-pentanol, 3-pentanol).

In the fermentor, both lime and calcium carbonate are possible neutralizing agents. Lime is a much stronger alkali and therefore can attain the optimal rumen pH of 6.7 whereas calcium carbonate can achieve a pH of only 5.8 to 6.2. Although lower pH slows the fermentation rate, it also discourages methanogens and thus can

© 1997 American Chemical Society

increase the selectivity toward VFA’s. This advantage, plus lower cost, favors calcium carbonate as the neutralization agent.

The fermentor temperature can be adjusted for mesophilic (~40°C) or thermophilic (~55°C) microorganisms. Of the total VFA’s, mesophilic microorganisms produce 60-70% acetic acid whereas thermophilic microorganisms produce 80-90%; thus, temperature is an important parameter that affects the product distribution. Higher fermentor temperatures have an advantage of reducing the cooling water required to remove metabolic heat; however, for fuel production, the greater energy content in the higher VFA’s favors lower fermentor temperatures.

A common approach to producing alcohol fuels from biomass is to enzymatically convert treated biomass with extracellular enzymes that hydrolyze polysaccharides to soluble sugars that are fermented to ethanol which is recovered by distillation. Compared to this approach, the MixAlco Process offers the following advantages:

• It is adaptable to a wide variety of feedstocks.

• Aseptic process conditions are not required.

• Inexpensive tanks can be employed.

• Expensive extracellular enzymes are not required.

• The fermenting organisms regenerate themselves.

• Cells and enzymes can be recycled without contamination risk.

• The fermenting organisms are stable.

• The process is robust.

Elements of the MixAlco Process have been investigated for many years. In 1914, Lefranc received a patent (7) on a process to convert waste biomass into butyric acid which was neutralized with calcium carbonate. The calcium butyrate was thermally converted to ketones for high-octane "Ketol" motor fuel (2). More recently, Playne (3) has further developed this technology by incorporating various pretreatments (4) and membrane separation techniques (5). In the U. S., only ethers
and alcohols may be added to fuel, so the MixAlco Process converts the ketones into alcohols.