S118: The use of expanded-bed adsorption for in situ butanol removal and development of a semi-continuous fermentation process

Due to its compatibility with the current transportation infrastructure, butanol is an attractive alternative biofuel.  Although several microorganisms can ferment renewable biomass-derived sugars into butanol, including Clostridium acetobutylicum, all conventional butanol-producers are inevitably limited by butanol’s toxicity at relatively dilute concentrations (~1.3% w/v).  This fact, which both limits volumetric productivity and leads to energy-intensive downstream product recovery, renders the entire process as uneconomical.  As has been previously demonstrated, the effects of butanol toxicity can be overcome via its in-situ removal from producing cultures.  This approach has been implemented in a modular fashion through the development of an expanded-bed adsorption (EBA) design operated in a simulated moving bed manner to enable the development of a semi-continuous butanol fermentation process.  The EBA module, through which the culture is circulated either continuously or intermittently without filtration, houses the hydrophobic polymer adsorbent Dowex Optipore L-493.  The effects on adsorbent dosage and circulation rate on butanol removal rate and extent were systematically investigated and the results were compared to the predictions of a mechanistic model.  The best results were obtained when the EBA column was operated at a superficial velocity of 9.5 cm/min using a resin fraction of 50 g/L. Biofouling was found to be minimal and did not reduce the separation potential of the adsorbent.  Finally, when applied to fed-batch fermentations, the use of EBA for in situ butanol removal enabled the volumetric productivity to be enhanced by at least 50% relative to conventional batch cultures after more than 5 days of continuous operation.