Silicone membrane contactor for selective volatile fatty acid and alcohol separation

The effect of pH and extraction temperature on flux, recovery, mass transfer coefficient and separation factor of volatile fatty acids (VFAs) and alcohols from synthetic solutions and cheese whey fermentate was investigated using a silicone membrane contactor with water as extractant. The silicone membrane allowed extraction of undissociated acids only, resulting in substantially higher recovery efficiencies at pH 3 than at pH 5. Furthermore, the non-porous silicone membrane favoured extraction of longer chain over shorter chain acids. Caproic acid was extracted with the highest flux of 1.30 (± 0.02) g m−2 h−1 in short time (32 h), with a 41.5 % recovery efficiency at pH 3 and 20 °C, indicating the feasibility of its selective separation from the VFA mixture. A similar trend was observed for alcohols, with butanol being extracted with a 39 % recovery efficiency at 40 °C, against 32 % and 19 % of propanol and ethanol, respectively, while the mass transfer coefficients were not affected by temperature. When applying the silicone membrane contactor to real cheese whey fermentate at pH 3, butyric and acetic acid were extracted with 21.5 % and 7% recovery efficiency, respectively, suggesting the feasibility of the contactor for VFA recovery from real fermentate.