| Ethanol is the product of fermentation industry which is widely used and renewable, and fuel ethanol showed a strong trend that can replace petroleum-based fuels, so the ethanol fermentation process is the research focus at the moment. During the production process of biofuel by fermentation, the ethanol in dilute solution should be recovered to lower the product inhibition and ensure the continuous fermentation. The conventional seperation processes, including distillation, adsorption, extraction and so on, suffer from high engrgy consumption, pollution, high investment and poor economic performance. Pervaporation(PV) is a new type of membrane separation technology, which has been widely used in the separation of homogeneous mixtures such as azeotropic, closing-boiling and dilute concentration mixtures(ethanol/water). This paper, we made pervaporation composite membranes, integrated the membrane with the fermentation system, and optimized this process fully. The main experimental results and conclusions can be listed as followings:(1) The glucose was fermented with the microorganism of ThermopHilic Alcohol Active Dry Yeast(alcohol active dry yeast) to optimize the batch fermentation conditions. We used the orthogonal table L9 (3~4), regarded ethanol yield as investigated factors, focused on the most obvious impact on the fermentation of the three external factors, which were substrate concentration, fermentation temperature and pH. The research showed that the greatest impact on ethanol yield was glucose concentration, followed by fermentation temperature, and the last was pH. In batch fermentation under optimal conditions, ethanol yield was 0.257, which was greater than the above nine groups. So we can proceed the following continuous ethanol fermentation under such optimal conditions.(2) Four types of composite PDMS membranes were prepared by using CA, PAN, PS and PVDF as support materials to research the pervaporation of ethanol/water system. The results show that the influence of support layer of composite PDMS membrane on pervaporation performance was great. The experimental results evidently demonstrated that the total flux and ethanol flux of PDMS/PVDF was the largest and PDMS/CA was the lowest, as PVDF was the most hydrophobic and CA was the most hydrophilic. In addition, with increasing temperature, the flux of each composite PDMS membrane increased quickly while the selectivity decreased. The total flux of four composite PDMS membranes increased with increasing feed concentration, while the separation factor decreased slightly. When increasing flow rate, the flux increased, separation factor increased slightly, but almost the same. Next, we can use PDMS/PVDF in the following continuous ethanol fermentation .(3) Based on the best PV membrane, we constructed a continuous ethanol fermentation system. The fermenter was the continuous stirred tank reactor. Investigated the effect of PV temperature and circulation flow rate on concentration of each component in the fermentation broth and the performance of the composite PDMS membrane, we found that, the higher the temperature, the lower the cell concentration and ethanol concentration in the fermentation broth, but the permeate flux was higher and separation factor was lower. And the higher flow rate, the higher cell concentration, but no significant difference in ethanol concentration. at the same time, permeation flux and separation factor were all improved. In the intergrated process, the permeate flux of the PDMS composite membrane was 400-700g/(m~2·h) and the separation factor was 6-12. The mass fraction of permeate ethanol stabilized at around 30%(w/w), which can up to 42%(w/w). This result showed that the separation performance of the PDMS membrane was stable, and we also can get high concentration of ethanol solution. The ethanol product is almost free of any cells and other inorganic salt composition, which was beneficial for producing a higher concentration of product. |
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