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We explored the influence of dilution rate and pH in continuous cultures of Clostridium acetobutylicum. A 200-mL fibrous bed bioreactor was used to produce high cell density and butyrate concentrations at pH 5.4 and 35degreesC. By feeding glucose and butyrate as a cosubstrate, the fermentation was maintained in the solventogenesis phase, and the optimal butanol productivity of 4.6 g/(L h) and a yield of 0.42 g/g were obtained at a dilution rate of 0.9 h-1 and pH 4.3. Compared to the conventional acetone-butanol-ethanol fermentation, the new fermentation process greatly improved butanol yield, making butanol production from corn an attractive alternative to ethanol fermentation.

An energy-efficient hollow-fiber membrane extraction process was successfully developed to separate and recover lactic acid produced in fermentation. Although many fermentation processes have been developed for lactic acid production, an economical method for lactic acid recovery from the fermentation broth is still needed. Continuous extraction of lactic acid from a simulated aqueous stream was achieved by using Alamine 336 in 2-octanol contained in a hollow-fiber membrane extractor. In this process, the extractant was simultaneously regenerated by stripping with NaOH in a second membrane extractor, and the final product is a concentrated lactate salt solution. The extraction rate increased linearly with an increase in the Alamine 336 content in the solvent (from 5 to 40%). Increasing the concentration of the undissociated lactic acid in the feed solution by either increasing the lactate concentration (from 5 to 40 g/L) or decreasing the solution pH (from 5.0 to 4.0) also increased the extraction rate. Based on these observations, a reactive extraction model with a first-order reaction mechanism for both lactic acid and amine concentrations was proposed. The extraction rate also increased with an increase in the feed flow rate, but not the flow rates of solvent and the stripping solution, suggesting that the process was not limited by diffusion in the liquid films or membrane pores. A mathematical model considering both diffusion and chemical reaction in the extractor and back extractor was developed to simulate the process. The model fits the experimental data well and can be used in scale up design of the process.

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