Structure And Transfer Performances Of Polyamide Composite Menbrane

Membrane technology is a new high-tech,and is more than one interdisciplinary science and technology.Membrane separation processes has become an important industrial process in food processing, pharmaceutical industry,chemical industry,water treatment,and other important production.Membrane structure and performances are the main determining factor on all kinds of membrane separation process. Therefore,membrane material and membrane-forming conditions,how to control its structure,as well as membrane transfer process are important parts of membrane science and technology.In recent years,It has been systematic studied that the aromatic polyamide(PA) composite membrane for reverse osmosis(RO) desalination as the support-layer coated with polyvinyl alcohol(PVA) to prepare PVA/PA composite membrane,or prepared multilayer PA composite membrane by interfacial polymerization(IP),which used for organic/water mixture separation by pervaporation(PV) in food products and pharmaceutical industry,and other areas.However,the performance of those composite membranes is restricted by single composite technology and/or single material.Therefore,it is a technical innovation that the high-performance composite membranes with multilayer separation structure——multilayer composite membrane(MLCM) were prepared by combination of two different composite techniques.Preparation of new polyamide/polyvinyl alcohol/polyamide (PA/PVA/PA) MLCM by the hybrid process of the dip-coating in polyvinyl alcohol solution and the interfacial polymerization(IP) of m-phenyldiamine(m-PDA) and trimesoyl chloride(TMC) with the PA membrane as support for organics mixtures separation was presented in this article.Both PVA and PA are highly hydrophilic,and have ability to separate organics mixtures.The PA composite membrane which performance is excellent can be obtained by dip-coating and IP process. The method is not only without special equipment and too many materials,but also easy to operate and control.In the experiments,the membrane surface structures were observed and analyzed through Atom Force Microscope(AFM) and Scanning Electronic Microscope(SEM).The results showed that the composition of PA/PVA/PA MLCM on the PA membrane surface was feasible. Moreover,the morphology and the performance of MLCM in separating IPA/water mixtures by PV were satisfying.The PA layer which prepared by IP technique possesses three-dimensional structure,and improved membrane specific surface area.This layer has up-dense and down-loose asymmetric structure in the effective penetration area.The PVA layer played three roles,firstly,as support structure for IP reaction;secondly,as a good transportation access for permeate which priority transported through the layer by PV; thirdly,as further permselective separation layer for permeate.The combination composite methods solved the problems about the composite membrane performance depends on a single material and/or single method,and improved membrane permeability.PVA/PA double-layer hydrophilic structure solved the thickness problem of a single layer,and was conducive to increase the flux.The effects of the membrane-forming conditions and the operation conditions on the separation performances of PA/PVA/PA MLCM were emphatically investigated.The following conclusions can be drawn from the experimental results.(1) The performance of MLCM which was prepared by the following conditions is satisfying.The concentration of water in the permeation(C_p-H2O) of MLCM was more than 99wt%for separate IPA/water mixtures by PV. The preferred dip-coating process conditions are:the concentration of PVA was 2wt%,the concentration of crosslinking agent——GA was 3wt%,the temperature and time of heat treatment were 150℃and 1h, respectively.The preferred IP process conditions were:the concentrations of m-PDA and TMC were 0.2wt%and 0.07wt%respectively,the soaks time of the support in aqueous phase and organic phase was 20min and 20s respectively,the temperature and time of heat reaction were 50℃and 5min respectively,then IP 2 times.(2) The flux(J) and separation factor(α) of PA/PVA/PA MLCM were closely related to the concentration of IPA in the feed(C_F-IPA) and the operation temperature(T).With the increasing of C_F-IPA and T,C_P-H2O was always more than 99wt%.Through the regression of experimental results,the forecast equations of the flux of water and isopropanol transported through the PA/PVA/PA MLCM were established.For flux of water:J_w = 0.059(C_F-IPA)²-9.3179C_F-IPA+434.11For flux of IPA:J_IPA = 0.0007(C_F-IPA)²-0.1133(C_F-IPA)+4.5859For total flux:J= 67171.21exp(-1940.6/T)(3) The composite technique has preferable stability and extensive applicability,and many kinds of porous membrane could be used as support to prepare PA/PVA/PA MLCM.(4) PV performance of PA/PVA/PA MLCM was stable for separating IPA/water mixtures.(5) PA/PVA/PA MLCM was suitable for separation of IPA/water mixture with various concentrations by PV.Simulation experiments in integration process of distillation and PV showed that:the flux of MLCM reached 200～270g/m².h and C_P-H2O was always more than 99.4wt%, could possess excellent separation efficiency and economic benefits. Apparently,PA/PVA/PA MLCM could be applied to the separation of IPA/water mixtures in food,biological and pharmaceutical engineering.PA/PVA/PA MLCM with high separation performance revealed its important application values in the actual separation projects.