Fundamentals of semi-crystalline poly(vinylidene fluoride) membrane formation and its prospects for biofuel (ethanol and acetone) separation via pervaporation

The separation of acetone and ethanol binary mixture derived from the fermentation broth using pervaporation membranes is very challenging, most likely due to their small molecular size differences. In addition, the characteristics of acetone as a strong solvent which can easily swell and dissolve polymer materials, further increases the difficulty of separation. Flat asymmetric PVDF membranes with strong chemical resistance and good mechanical strength fabricated via mixed solvent systems show a great potential for the aforementioned binary separation via pervaporation. A mixed solvent system (50/50wt%), i.e. NMP/THF or NMP/Acetone, coupled with the evaporation of volatile latent solvent at elevated temperatures, i.e. 40-100°C, has effectively suppressed macrovoids and promoted dense skin layer formation in PVDF membranes as compared to that of pure NMP system. By altering the demixing process at selected evaporation temperatures, i.e. 60-80°C, the pore structure of membranes cast from mixed solvent systems can be further designed into the desired interconnected open pore structure rather than globular structure with weaker mechanical strengths. From the trade-off line of acetone/ethanol separation via pervaporation, it can be revealed that PVDF membranes cast from mixed solvent systems at higher evaporation temperatures, i.e. 60-100°C, exhibit superior separation factors (up to 4) with reasonably high fluxes (1-4kg/m²h) as compared to those of pure NMP system. This separation performance is comparable or higher than that of commercially available PDMS membranes.