Comparison of Sweepgas and Vacuum Membrane Distillation as In-Situ Separation of Ethanol from Aqueous Solutions

The rising oil prices, as well as the desire to reduce the environmental impact of fossil fuels, increases the interest in fuels from renewable raw materials. Bioethanol production via starch fermentation is state of the art. Right now researchers focus on second generation biofuels from lignocellulose sources for example like wood and straw. Still investigations have to be done concerning fermentation yield and energy efficiency. Due to low product concentration in the fermentation broth latter downstream processing is quiet energy consuming. Many separation processes like Gas stripping, Liquid-Liquid Extraction, pervaporation and membrane distillation are investigated for an in-situ separation during continuous ethanol fermentation.
The aim of this work was to compare two membrane distillation configurations for ethanol separation from aqueous solutions. Two different process variations like vacuum membrane distillation (VMD) and sweepgas membrane distillation (SGMD) were examined. The influencing factors on the separation of the process such as feed temperature, feed concentration, permeate pressure at VMD and sweepgas flow at SGMD were investigated. The gained results were compared in order to find the best application and process conditions.
The initial feed concentration varied between 0.5 w% and 5 w% ethanol in the mixture and feed temperature was held at 20, 35 and 50 °C. During the VMD configuration the permeate pressure was kept at 25 and 50 mbar. Using the SGMD set up the sweepgas volume flow was varied between 360, 498, 900, 1,302 and 1,500 L/h. Membrane parameters like transmembrane fluxes and selectivity were calculated and discussed.
The results of the VMD configuration showed that transmembrane ethanol flux increased with rising feed temperature and lower vacuum pressure. A linear correlation between feed temperature and transmembrane ethanol flux was found. In comparison VMD showed much higher transmembrane fluxes than SGMD. This result corresponds with the theories, in which the driving force during VMD is much higher due to a larger partial pressure difference between feed and permeate side.
Comparison of selectivity results showed that VMD has better separation potential than SGMD. At a feed temperature of 50 °C selectivity was the same in both configurations. Feed concentration has a negligible influence on the selectivity in the investigated value range.
The result of this work showed that both membrane distillation configurations have the possibility to separate ethanol from aqueous solutions. VMD has a bigger separation potential due the higher driving force, but the applied vacuum makes permeate condensation more difficult. SGMD with selectivities of around 4 and low transmembrane fluxes shows advantages due to easier process configuration and lower energy consumption.