Recovery of ethanol from fermentation broths using selective sorption-desorption

Industrialized nations face a critical problem in replacing the sources of liquid fuels that traditionally have been supplied by petroleum. One solution that has gained increasing support in this country is the use of ethanol produced by fermentation of renewable biomass as an extender in, or supplement to, gasoline for transportation fuel. Distillation, the present method of separating ethanol from the fermentation broth, is an energy-intensive one and frequently uses more energy than is available from the ethanol recovered. There are many investigations under way to find alternative, less energy-intensive techniques for the ethanol-water separation. The separations method described in this article involves the use of solid materials to preferentially remove ethanol from fermentation broths. Subsequent stripping of the ethanol from the sorbent with a dry gas reduces dramatically the energy required for the separation. Three solid sorbents have been investigated experimentally. Their sorption/desorption characteristics are described, and their incorporation in an ethanol recovery process is evaluated. Three sorbents were investigated: two commercially available divinylbenzene crosslinked polystyrene resins in bead form (one with a nominal surface area of 300 m(2)/g, the other with 750 m(2)/g) and an experimental proprietary molecular sieve with hydrophobic properties. Equilibrium adsorption isotherms for two of the sorbents were obtained at ambient temperature (21 degrees C) for ethanol-water solutions containing up to 12 wt. % ethanol. In addition, 40 degrees C isotherms were obtained for the polystyrene sorbents. Although different, the equilibrium isotherms for the sorbents indicated that ethanol could be preferentially sorbed from a dilute solution. Column breakthrough curves indicated very favorable kinetics. Desorption of the ethanol was readily effected with warm (60-80 degrees C), dry nitrogen.