Fabrication Of Two-active-layer Composite Membranes For Pervaporation Dehydration Of Ethanol

According to solution-diffusion theory, key steps of pervaporation include thesolution step on a membrane surface and the diffusion step through the membranematrix. Based on that, polyacrylonitrile (PAN) supported polyelectrolytetwo-active-layer pervaporation membranes, whose active layer consists of a cappinglayer and an inner layer, was designed and fabricated. The capping layer selectsmaterials with strong water retention capacity and high sorption selectivity, in orderto enhance the solution step; while the inner layer selects materials with highdiffusion selectivity and less mass transfer resistance, in order to enhance thediffusion step. Pervaporation dehydration performance of ethanol of membraneswith different kinds of polymer as the capping layer and the inner layer wasinvestigated.HA/Alg/PAN and Alg/HA/PAN membranes were prepared by two spin-coatingof hyaluronic acid (HA) and sodium alginate (NaAlg). NaAlg was cross-linked byCa2+. Water contact angle showed HA and alginate were both coated on surfaces ofHA/Alg/PAN and Alg/HA/PAN membranes, respectively. The distinct boundary wasalso observed in the scanning electron microscope (SEM) pictures, which provedthat the active layer possessed two-active-layer structure. The HA/Alg/PANmembrane showed good pervaporation performance with a flux of973g/(m2h) andseparation factor of1130for dehydration of90wt.%ethanol at80℃, while theAlg/HA/PAN membrane possessed similar flux and lower separation factor of527. Itsuggested that active layer structure of HA/Alg/PAN membrane was better at thisoperation condition. Effect of operation conditions on membrane structure was alsostudied by calculation of permeance and selectivity. At the condition of80℃,selectivity of the HA/Alg/PAN membrane was higher at low water concentration (<20wt.%), while selectivity of the Alg/HA/PAN membrane was higher at high waterconcentration (≥20wt.%).HA/CS/PAN and CS/HA/PAN membranes were prepared by sequentialspin-coating and self-assembly of HA and chitosan (CS). The inner layer of theactive layer was prepared by spin-coating; while the capping layer was prepared byself-assembly. HA was successfully coated on HA/CS/PAN membranes, which wasproved by infrared spectra and X-ray photoelectron spectroscopy. HA/CS/PANmembranes showed better pervaporation performance than CS/HA/PAN membranes for dehydration of90wt.%ethanol at80℃, which was caused by differences oftheir sorption selectivity and diffusion selectivity as well as swelling behavior. Effectof prepare conditions (HA concentration and degree of cross-linking of CS) andoperation conditions (temperature and water concentration in feed) were studied. Atthe condition of80℃and10wt.%water concentration in feed, the HA/CS/PANmembrane with HA cross-linked by glutaraldehyde and no cross-linked CS showedgood pervaporation performance with a flux of1447g/(m2h) and separation factorof704. The long-time pervaporation test showed that HA/CS/PAN membranes werestable for dehydration of90wt.%ethanol at80℃.Finally, comparison of pervaporation data between our work and previousliterature showed the two-active-layer membranes in this study obtained a goodcombination of flux and separation factor.