Bio-ethanol production using a hybrid pervaporation-distillation system: Influence of fermentation by-products on membrane performance

In the light of further decreasing energy demands and costs of bio-ethanol production, a hybrid configuration of pervaporation followed by distillation was proposed. Since fermentation broths contain a variety of by-products which could hinder the purification, the influence of these components on the separation of ethanol from water during the pervaporation step was investigated. Two commercial hydrophobic PDMS membranes (unfilled and zeolite-filled) were used in this study. It was found that components either altered vapor/liquid equilibrium of an ethanol/water mixture, or interacted with the membrane material. Salts and sugars increased the relative volatility between ethanol and water, which resulted in a higher ethanol flux and selectivity factor in comparison to a pure ethanol/water solution. 2,3-Butanediol showed a high affinity towards a pure PDMS membrane, and thus decreased ethanol flux and selectivity. Carboxylic acids exhibited similar effect towards the unfilled PDMS membrane as observed for butanediol, but irreversibly enhanced the water flux through the zeolite-filled PDMS membrane. This membrane fouling was a result of interaction of weak acids with silanol groups of the zeolite, which rendered the membrane surface more hydrophilic, as found by measuring contact angle of the membrane. Membrane deterioration of the zeolite-filled PDMS membrane could be avoided by increasing the pH of the solution above the dissociation constant of the weak acid. It is thus advisable to increase the pH during fermentation processes in order to minimize the impact of organic acids on membrane performance.status: accepte