In a product recovery technique termed pervaporation, a membrane that directly comes in contact with fermentation broth is used to selectively remove volatile compounds such as ethanol and butanol [219, 222]. The volatile compounds diffuse through the membrane as vapour and are then collected by condensation. To facilitate volatilization of permeates into vapour, a partial pressure difference across the membrane is usually maintained by apply­ing a vacuum or inert gas (e. g. N2) across the permeate side of the membrane [219]. Polydi- methylsiloxane (PDMS) is the current material of choice for the membrane, but other materials such as poly(l-trimethylsilyl-l-propyne) (PTMSP), hydrophobic zeolite mem­branes, and composite membranes have also been investigated [225].

1.7. Gas stripping

Gas stripping is an attractive product recovery method for gas fermentation because the exit gas stream from the bioreactor can be used for in situ/online product recovery [219]. Following product recovery via condensation, the effluent and gas can be recycled back into the bioreactor. In sugar-based fermentation using C. beijerinckii mutant strain BA101, in situ gas stripping was shown to improve ABE productivity by 200%, complete sub­strate utilization and also complete acid conversion into solvents, when compared to non-integrated process [226].