Study On Structure Of Composite Silicone Rubber Membrane And Its Performance For Pervaporation

Owing to its high selective permeability and with the virtue of insulating other inorganic ions, the nonporous hydrophobic silicone rubber [polydimethysiloxane (PDMS)] membrane has been investigated for separation of volatile organic compounds from their aqueous solutions by pervaporation or vapor-permeation. In this work, the pervaporation performance of the composite silicone rubber membrane prepared in our laboratory and its application to separation of some practical liquid mixtures has been studied, and control experiments was conducted with a industrial PDMS composite membrane. We have explored the effect of the composite membrane structure on the pervaporation performance, and found some interesting features on this topic. In pervaporation experiments of dilute ethanol -water solution, the relation of the flux and the separation factor to the flow rate, temperature and concentration have been investigated. When the flow rate was constant, the overall flux rose with increasing temperature, while the separation factor declined slightly with increasing temperature. When the ethanol concentration in the feed decreased, the overall flux declined accordingly, but the separation factor increased slightly. In addition, the overall mass transfer coefficient and the flux increased with the increasing flow rate under similar temperature. The varied flow rates produced different flow patterns in the membrane module, and then changed the overall mass transfer coefficient. So the well-designed module on hydrodynamics is of importance for improving the transport performance of the membrane. The membrane module should have the uniform flow distribution and the high speed on the surface to reduce the thickness of the boundary layer and improve the mass transfer coefficient of the membrane. The composite PDMS membrane was used for pervaporation separation of Chinese spirits and wine. For Chinese spirits, the membrane exhibited good stabilities in high concentration of ethanol, high flux and separation factor. The trace content of aroma controlling the taste features of the liquor could be effectively separated and recovered in experiments. With re-mixing the separated compositions, we can get some new-type of spirits with better flavor than the original one. The qualified tasters for Chinese spirits gave high evaluation to the new-type of spirits. So it is valuable to further study this subject. For the selected wine, the separation produced two products: low alcohol of wine and high alcohol of brandy. The qualified tasters offered similarly high evaluation to the new wine and brandy, and predicted a better market prospect. The effect of the composite membrane structure on its performance has been analyzed. The pore of support layer and the binding structure between the active layer and the support layer influenced pervaporation performance of the membrane. The pore diameter directly affects the resistance of the downstream. If the pore diameter of support layer is as small as nanometer, like that in a commercial membrane, the transport in support layer may not be modeled with Poiseulle flow, and a high mass transfer resistance could be expected. This can probably be attributed to the growth of a new dense layer in the interface of the active layer and support layer. The dense interface layer might dominate the overall mass transfer coefficient and gave very high resistance. We can optimize the material of support layer and improve the membrane preparation technique after further work on this aspect. It is significant to develope the new-type composite membrane of high selective permeability and high permeate flux based on the observation.