(106e) Sustainable Production of Gasohol from Biomass

Ethanol, produced mainly by fermentation of biomass, including grains, e.g., corn, and cellulosic materials, e.g., switchgrass (panicum virgatum), is highly versatile as a fuel resource: It can serve as a stand-alone fuel, or can be variously blended with another liquid fuel, gasoline, for instance. If gasoline is mixed with 10 vol. % of anhydrous ethanol, the resultant blend has been generally termed gasohol. Ethanol, in gasohol, not only is a fuel substitute but also serves as an antiknock agent (octane enhancer) replacing MTBE (methyl tert-butyl ether), which, in turn, has replaced TEL (tetraethyllead).

The concentration of ethanol in fermentation beer is rather low, ranging from a few percent to 14 %. Enormous thermal energy and appreciable capital expenditure are required to generate anhydrous ethanol from such fermentation beer through a sequence of separation operations consisting of simple fractional distillation and azeotropic distillation.

Obviously, thermal energy and capital expenditure need to be reduced for the manufacture of gasohol to be sustainable. A highly promising alternative process has been proposed for producing gasohol; it combines simple fractional distillation and extraction of ethanol by gasoline, the latter of which replaces azeotropic distillation.

Performed in the current work is a systematic retrofitting of the conventional process for gasohol production by incorporating the direct extraction of ethanol from fermentation beer by gasoline. The retrofitting is holistically executed by means of an inordinately efficient graph-theoretic method for network synthesis based on process graphs (P-graphs). Moreover, thermodynamic analysis is carried out based on the law of mass conservation and the first and second laws of thermodynamics. By resorting to the mass, energy, entropy-dissipation, and available energy balances, the sources and magnitudes of the inefficiencies and irreversibilities are identified in terms of energy requirement, entropy increase, and available energy dissipation.

Discussion is given of possible adoption of gasoline extraction for other biochemical processes, which yields ethanol as one of the products, e.g., the biochemical production of butanol from grains, generating ethanol and acetone as by-products.