Fermentative Butanol Production Using a Pervaporation Membrane Reactor

The integrated fermentation/pervaporation process was constructed and its performance for bio-butanol production was investigated. A novel tri-layer composite membrane, consisting of the active layer polydimethylsiloxane (PDMS, Sylgard® 184) and dual support layers of high porosity polyethylene (PE) and high mechanical stiffness perforated metal, was first developed and evaluated for the pervaporative separation of 1-butanol from butanol-water binary solutions. With the membrane of PDMS/perforated metal support as a control, it has been shown that total flux and separation factor were both increased by placing a layer of hydrophobic PE between the PDMS and the metal support layers. Hydrophobic, inter-connected, and micro-porous are three necessary properties for constructing the top layer of the dual support. ^ The PDMS/dual support pervaporation membrane was then applied in the integrated fermentation/pervaporation process for A-B-E fermentation. While butanol was simultaneously removed by pervaporation, the performance of A-B-E fermentation using the integrated process showed higher biomass concentrations and higher glucose consumption rates than those of the A-B-E fermentation without pervaporation. The solution-diffusion model, specifically the mass transfer equation based on Fick\u27s First Law, was first shown to be applicable to the undefined A-B-E fermentation culture solutions. ^ Batch, fed-batch, and continuous A-B-E fermentations were conducted and compared with pH controlled at 4.5, the optimal value for solvent production. It has been shown that the continuous mode was preferential in terms of butanol yield and productivity. The highest butanol yield and productivity found in the continuous fermentation at dilution rate of 0.1 h−1 were 0.21 g-butanol/g-glucose/h and 0.81 g/L/h, respectively. It has been shown in the continuous and fed-batch fermentation that the time needed for passing acidogenesis to solventogenesis was an intrinsic hindrance to higher butanol productivity. While 3:6:1 ratio of acetone, butanol, and ethanol is commonly observed from A-B-E batch fermentation by Clostridium acetobutylicum when the pH is uncontrolled, up to 94% of the produced solvent was butanol in the chemostat with pH controlled at 4.5. ^ It is concluded that the integrated fermentation/pervaporation process is feasible for A-B-E fermentation as being demonstrated by operating the batch and fed-batch A-B-E fermentation with the pervaporation membrane reactor. The optimization of the integrated fermentation/pervaporation process can be done by using the solution-diffusion model as the governing equation for the pervaporation process, and by using the continuous mode at the low dilution rates for A-B-E fermentation. Two difficulties are expected to operate continuous A-B-E fermentation with the pervaporation process: the stability of the clostridial fermentation culture and the capacity of the pervaporation process. ^