Continuous direct solvent extraction of butanol in a fermenting fluidized-bed bioreactor with immobilized Clostridium acetobutylicum

Immobilized Clostridium acetobutylicum was used to ferment glucose into acetone and butanol in a fluidized-bed bioreactor. A nontoxic immiscible solvent, oleyl alcohol, was added to, and removed directly from, the fermenting columnar reactor and extracted the majority of the inhibitory butanol from the aqueous broth. The extracting solvent had a distribution coefficient of near 3 for butanol. Nonfermenting system tests indicated that equilibrium between the phases could be reached in one pass through the column. Steady-state results are presented for the fermentation with and without the extractive solvent addition. One run, with a continuous aqueous feedstream containing 40 g/L glucose, was operated for 23 d. A steady state was established with just the aqueous feedstream. Approximately half of the glucose was consumed, and the pH fell to 4.5 from 6.5. Then, during multiple intervals, the flow of organic extractive solvent (oleyl alcohol) was begun into the fermenting columnar reactor. A new apparent steady state was reached in about 4 h. The final aqueous butanol concentration was lowered by more than half. The total butanol production rate increased by 50-90% during the solvent extraction, as the organic-to-aqueous ratio increased from 1 to 4, respectively. There was an observed maximum volumetric productivity of 1.8 g butanol h^-1L^-1 in this nonoptimized system. The butanol yield apparently improved because of the removal of the inhibition. More substrate is going to the desired product, butanol, and less to maintenance or acid production, resulting in 10-20% increases in the ratio of butanol relative to all products.