Biobutanol can be produced via acetone-butanol-ethanol, or ABE, fermentation. One of the challenges in industrializing this process is product inhibition. Organic products of the fermentation deter productivity of the cells above certain concentrations of products. These concentrations are typically very low (< 2%), thus resulting in low productivity and restricting fermentations to batch processes. Many researches have been conducted to address this problem by in situ product recovery. Of the various approaches, pervaporation has advantages in the fact that the separated permeate is very concentrated and that it is non-invasive to the cell culture. However, the pervaporation technology is currently limited by the effectiveness of the membranes. An optimal pervaporation membrane would have a high permeability and selectivity for the fermentation products. Simultaneously, it would have to have enough mechanical stability to be produced in a thin film. This is to minimize the time that it takes for the molecules to diffuse through the membrane.
We try to address these two design aspects by using a block copolymer membrane. We covalently joined polydimethylsiloxane(PDMS), which is the most widely used membrane material for this application of pervaporation, with two polystyrene(PS) blocks on either end, forming a PS-b-PDMS-b-PS(PSDS) tri block copolymer. The PS blocks are expected to provide the polymer with enhanced mechanical properties, replacing the chemical cross-links which is normally used as a way of tethering the PDMS. The PDMS blocks provide the transporting phase through which molecules selectively diffuse. We fabricated PSDS block copolymer membranes with comparable thicknesses to those of commercially available cross-linked PDMS membranes. Our binary butanol-water pervaporation experiments show that the PSDS membrane has both the higher butanol selectivity and higher butanol permeability than those of the commercial PDMS membrane.
We were also able test the PSDS and the commercial PDMS membranes in a pervaporation-incorporated ABE fermentation system. In our study, we show that the pervaporative-fermentations were able to deter product inhibition, resulting in the increase of ABE production. Also, PSDS membranes had higher permeabilities and selectivities for the products than those of the commercial PDMS membranes, which resulted in a further increase in ABE production for the PSDS pervaporative-fermentation.
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