The Effects Of Additives On Performance Of PDMS/PAN Composite Membranes For Oxygen-enrichment

The oxygen-enriched air for combustion-supporting produced by the membrane separation,can reduce the energy consumption and the exhaust emission in the industry,which is to the great interest of energy-saving and emission-reduction strategy in our days.Therefore, it has brought considerable social and economic benefits,and thus,become a focus to the investigators.For oxygen-enrichment membranes to produce combustion air with low oxygen concentration of 25.0%to 35.0%,the oxygen permeation rate is more important than the selectivityof oxygen/nitrogen,which can be satisfied easily by poly(dimethylsiloxane)(PDMS) membranes.Therefore,it has been widely used to prepare oxygen-enrichment membranes for its excellent oxygen permeability coefficient.Nevertheless,its membrane-forming ability is so poor that its direct applications are limited.Consequently,PDMS has often been coated as the selective layer on porous polymeric substrates to form oxygen-enrichment composite membranes in practical applications.Moreover,not only the membrane material but also the structure of the substrate plays an important role in the performance of the composite membrane.As a result,to prepare the substrates with proper morphology is the key to the composite membrane with excellent performance.The composite membranes were prepared by coating PDMS on the surface of porous polyacrylonitrile(PAN) substrates,which were prepared via phase inversion process using PAN as the substrate material,N,N-dimethyl-formamide(DMF) as the solvent and water as the coagulant.To prepare the PAN substrate with proper pore size and porosity,which is helpful to form a thin defect-free PDMS dense membrane coated on the substrate surface,and to improve the oxygen permeation rate of the composite membranes,the organic additive of polyvinylpyrrolidone(PVP) and polyethylene glycol(PEG) with different molecular weight (400,600,1000,2000 g/mol) as well as the inorganic additive of zinc chloride(ZnCl₂) were used to control the structure of the substrate.Firstly,the membrane preparation conditions of the concentration of PAN,the coating method and the concentration of PDMS were investigated.It was shown that the composite membranes formed by coating PDMS solution of 4.0%on the substrates of 19.0%(wt.% following the same without any sepecial notes) PAN,exhibited good performance with the oxygen permeation rate of 292GPU,and the selectivity of oxygen/nitrogen of 2.07. Secondly,the effects of the additives on the thermodynamics properties and precipitation kinetics of the casting system were investigated.The cloudy points were measured and the experimental data were linearly regressed with the linearized cloud point(LCP) relation,by which,the theoretical binodal line was calculated.All the results showed that the systems in this study were in accord with the LCP relation and were suitable for membrane preparation. Moreover,the casting system became less thermodynamically stable for that the coagulation value decreased,with the increase of the concentration of PVP,PEG and the molecular weight of PEG,which improved the phase separation.However,when the concentration of the PVP, PEG,ZnCl₂ and the molecular weight of PEG increased further,the viscosity of the casting solution increased,and the precipitation rate decreased from the light transmittance curve, which delayed the phase separation.The structure of the membrane was the result of the both properties.Thirdly,the effects of the additives on the cross-section structure of the substrate membrane were characterized by scanning electron microscope(SEM),and the gas permeation performaces were measured with the oxygen and nitrogen pure gas.It was shown by the results that the composite membranes with 6.0%of PEG 400,4.0%of PEG 600,2.0%of PEG 1000, 2.0%of PEG 2000,3.0%of PVP,2.0%of ZnCl₂ as the additives exhibited good oxygen-enrichment performance,such as the oxygen permeation rate of 407,352,377,521, 410,490 GPU as well as the selectivity of oxygen/nitrogen of 2.07,2.10,2.09,2.07,2.01,2.03, respectively.(1GPU=10^-6cm³(STP)/cm²Â·s·cmHg)...