Liquid-liquid extraction has been considered an important technique for the recovery of ABE from fermentation broths. Usually, a water-insoluble organic extractant is mixed with the fermentation broth. Butanol is more soluble in the organic (extractant) phase than in the aqueous (fermentation broth) phase. There­fore, butanol selectively concentrates in the organic phase. Since the extractant and fermentation broth are immiscible, the extractant can easily be separated from the fermentation broth after butanol extraction. Liquid-liquid extraction is able to remove fermentation products without removing substrates, water, or nutrients.

In order to improve substrate utilization and productivity, liquid-liquid extrac­tion must be integrated with butanol fermentation such that simultaneous fermen­tation and butanol removal from the fermentation broth is achieved. The choice of extractant is critical because an extractant with low partition coefficient will not be efficient in the recovery of butanol and a toxic extractant will inhibit or kill the bacterial cells. Unfortunately, most extractants with high partition coef­ficient are toxic to the clostridia. The extractant of choice among researchers has been oleyl alcohol because it is nontoxic and a good extractant as well (Evans and Wang, 1988, Groot et. al., 1990).