Experimental And Theoretical Investigation Of Ethanol Fermentation/Vapor Permeation Process

Due to the energy crisis and environmental pollution caused by the combustion of fossil fuels, bioethanol as a renewable clean fuel has been receiving tremendous attentions in the recent decades. However, bioethanol production by the conventional process of batch fermentation suffers from low ethanol yield and productivity, and high production cost due to ethanol inhibition. In situ product recovery (ISPR) technique is widely used to overcome the product inhibition effect by continuous removing ethanol from broth during the process. Several common ethanol separation techniques can be integrated with fermentation as ISPR, including vacuum extraction, gas stripping, solvent extraction, and membrane technology, such as, pervaporation. However, there were some issues in these ISPR processes, which led to a low efficiency. Among those ISPR approaches, pervaporation (PV) is considered as a promising and efficient technology. Nevertheless, in PV/fermentation process, the direct contact of membrane with the fermentation broth leads to serious membrane fouling, which limits its industrial application.Vapor permeation (VP) is an alternative membrane technique, of which the feed is clean vapors, not the broth. Therefore, VP is proposed to integrate with ethanol fermentation as an ISPR technique in this work. Membrane fouling in the PV/fermentation process can be greatly avoided in VP/fermentation process, which results in a longer life of membrane and higher process efficiency. The three fermentation processes, namely, batch fermentation, PV/fermentation integrated process, internal VP/fermentation integrated process, are investigated, and the results revealed that membrane fouling in internal VP/fermentation process is much lighter than that in PV/fermentation process, and the ethanol productivities of both integrated process are much higher than that of bath fermentation. The performance of external VP/fermentation process is better than that of internal VP/fermentation process.Firstly, due to the low flux of PDMS membrane resultes from its high molecular weight, silanes with small molecule, such as, vinyltriethoxysilane (VTES), dimethyl diethoxysilane (DMDES), hydroxyl silicone oil (HSO), are proposed to prepare ultra-thin membrane by monomer polymerization. The thicknesses of these ultra-thin active layers are around 1?m. The fluxes of these membranes are about ten times higher than that of PDMS membrane, and separation factors of them are close to that of PDMS membrane, which greatly improves the economical efficiency of the integrated process. And the surface and internal structure of the membranes are characterized and the relationship of the membrane's structure and PV performance is discussed. PVTES-HSO membrane can be used in VP/fermentation process for ethanol recovery, and the total coupled time of the integrated process used this membrane is much shorter than that used PDMS membrane due to the high flux of PVTES-HSO membrane.Secondly, to improve the efficiency of the integrated process and lower the cost of ethanol production, the conditions of ethanol fermentation and vapor permeation process are optimized. The optimal temperature range of membrane module is from 35? to 45?. The optimal cycling flow of the vapor is 1.3 L·min-1; The optimal pressure in permeate side is 0.097MPa. At the optimal operation conditions, the continuous ethanol fermentation coupled with VP process is carried out successfully.The mass transfer behaviors of multi-components (ethanol, water, N2 and CO2) in the PDMS membrane are investigated. The results indicate that CO2 has a plasticizing effect on PDMS and water in low activity (?=0-0.3) has little effect on the behavior of ethanol vapor through the PDMS membrane. According to solution-diffusion mechanism and mass transfer behavior in the membrane, a multi-component vapor permeation model was established using UNIQUAC-HB and Long's model. The results show that the predicted values of the model are agreement with the experimental results very well.Based on ethanol fermentation kinetics and mass transfer kinetics of component through membrane, ethanol fermentation/vapor permeation coupled dynamics model was established and it correctly reflects the law of ethanol fermentation. This model predicted cell concentration and ethanol concentration in the fermentation broth accurately; but predicted glucose concentration not ideal due to the instability of VP in the early stage.Finally, according to the results of the small scale, the pilot scale of ethanol fermentation/VP process was conducted. Ethanol concentration, cell concentration, and glucose concentration in the broth were stable during the whole process (120 h). The ethanol productivity of this integrated process was higher than that of batch fermentation process.