Stability of MFI zeolite-filled PDMS membranes during pervaporative ethanol recovery from aqueous mixtures containing acetic acid

Pervaporation is a potential process for recovering bioethanol produced from biomass fermentation. Fermentation broths contain ethanol, water, and a variety of other compounds, often including carboxylic acids. The effects of acetic acid on long-term pervaporation of aqueous ethanol mixtures through high-silica ZSM-5 zeolite-filled polydimethylsiloxane (PDMS; silicone rubber) membranes were investigated. Acetic acid was shown to reduce the ethanol removal effectiveness of these membranes. Initially after acetic acid addition, ethanol and water fluxes decreased due to acetic acid competing with ethanol and water for adsorption sites in the membrane. Longer-term exposure to acetic acid resulted in an irreversible, steady decline in ethanol/water separation factor because of declining ethanol flux. Increasing feed pH to above the dissociation constant (pKa) of acetic acid diminished the longer-term decline in ethanol flux and essentially eliminated the effect of competitive adsorption. Measurements of adsorption competition between ethanol, water, and either acetic acid or succinic acid on the zeolite particles suggested that other carboxylic acids would have qualitatively similar short-term effects on membrane performance as those observed for acetic acid.