Mass Transfer Performance Of PDMS Membrane In A Continuous Ethanol Fermentation-Pervaporation Process

Process principles and researches in a continuous ethanol fermentation-pervaporation system have shown that PDMS membrane bioreactor with continuous ethanol fermentation has many technological advantages and wide application prospects. At present, the applications and researches of pervaporation used in continuous ethanol fermentation are still under experimental stage, and many research efforts are needed. In this work, a membrane bioreactor with a composite silicone rubber membrane was developed. Separation performances of PDMS membrane in the process and the effects of flow patterns in the different modules on boundary layer mass transfer were investigated, and the extended resistance-in-series model was established for the membrane mass transfer process.The author prepared a silicon rubber membrane and experimentally studied its pervaporation performances at the temperatures from 30C to 40 C, the glucose concentrations from 0g/L to 100g/L and the yeast cell concentrations of 0g/L and 55g/L. The results show that higher temperature promotes the pervaporation but brings down the ethanol generation rate so that the solution flux through the membrane increases while the ethanol flux keeps nearly unchanged. With the increase of glucose concentration from 0 to 100g/L, the total flux decreases 25% but the separation factor for ethanol increases from 7.71 to 9.99, suggesting that higher glucose concentration inhibits water molecules from being adsorbed to the membrane or, in other words, enhances the selectivity of membrane to ethanol. Also, the existence of cells stimulates the mass transfers on the membrane surface and, therefore, increases the flux or pervaporation of ethanol through the membrane.The mass transfer performances in the boundary layer of the membrane were analyzed under the different flow patterns, formed by different structures and feedways of modules. The variations of mass transfer coefficient with flow rate show that with the increase of feed flow rate, the thickness of boundary layer becomes thinner and convective mass transfer on the surface of membrane intensifies, leading to the increase of mass transfer coefficient in some degree. The results indicate that for the better mass transfer, it is very important in the design of modules to keep the fluid distributing uniformly on the membrane surface.This paper presented an extended resistance-in-series model to describe the pervaporation of dilute solution, which encompasses support layer resistance, membrane swelling and component concentration in the vapor mixture. By defining an enhancement factor to diffusion coefficient in the membrane top layer, the model equations were developed with the form similar to conventional heat and mass transfer equation. This result has an instructional significance not only for the further applicants and researches, but also for the process design.