Study On Fixed-site Carrier Facilitated Transport Membrane For Separating Carbon Dioxide

Separation and removal of CO₂ using membrane technology is one of the most important subjects about energy and environment. Because of Robeson upper limitation, the commercially available polymeric membranes often have either high permeability or high selectivity, but not two together. Facilitated transport membranes selectively permeate a substance by means of the reversible reaction between the substance and the carriers in the membranes, and they possess high permeability as well as high selectivity. Fixed carrier membrane is a kind of promising separation membrane. However, CO₂ fixed carrier membrane is a new research field in which both the development of membrane material and the exploration of transport mechanism are urgent. This study made attempts to develop fixed carrier membranes with high performance for CO₂ removal and explore transport mechanism of CO₂ in the membranes.In view of the interactions of gases, polymeric membrane, carrier and water, the principles of selecting facilitated transport system, polymeric material and carrier were proposed. On the basis of these principles, copolymer of acrylamide(AM) and maleic acid anhydride(MA), 2-N,N-dimethyl aminoethyl methacrylate and crylic acid were prepared by radical polymerization. Copolymer of 2-(N,N-dimethyl aminothylme) and acrylonitrile was prepared by water-phase precipitation copolymerization(WPPCP). Effects of reaction condition on copolymerization were explored. The results indicate that the composition of copolymer was controlled by changing the composition in feed monomer mixture. The composite membranes were developed respectively with the above three kinds of materials as active layers and polysulfone, polyethersulfone, polyacrylonitrile , cellulose acetate ultra-filtration membranes and polytetrafluorinethylene as the supports. FTIR(Fourier Transform Infrared Spectroscopy), XRD(X-ray diffractions), GPC(Gel Permeation Chromatography), NMR(Nuclear Magnetic Resonance), elemental analysis, DSC(Differential Scanning Calorimetry), ESEM (Environmental Scanning Electron Microscope) and titration were employed to characterize the physical and chemical structure of the polymer and the morphology of the composite membrane. The results show that the active layers of the composite membranes are dense and smooth.The sorption performances of different homogeneous membranes were measured with pure CO₂ and CH4 gas. The membranes present excellent sorption performances. For example, at 100KPa of CO₂ or CH4 pressure and 298K, poly (AM-MA)homogeneous membrane displays CO2 saturated sorption capacity of copolymer 45⁷0mg(CO2)-(g(dry membrane) )"1, but CH4 saturated sorption capacity of copolymer only 4mg(CH4)-(g(dry membrane) )"*;At latm of CO2 or CH4 pressure and 298K, poly (DM-AN) homogeneous membrane displays CO2 saturated sorption capacity of copolymer 4.5⁷.5cm3(CO2)/cm3 (dry membrane),but saturated sorption capacity of copolymer only 1.8, poly (DM-AA) homogeneous membrane displays CO2 saturated sorption capacity of copolymer 7.0⁹.0cm3(CO2)/cm3 (dry membrane),but saturated sorption capacity of copolymer only 1.82cm3(CO2)/cm3 (dry membrane). Moreover, Sorption mechanism of CO2 and CH4 pure gases in the homogeneous membrane was studied. The carbon dioxide sorption behavior was described by the dual-mode sorption model and methane sorption behavior was described by the Henry-mode sorption model. The membranes selectively absorb carbon dioxide preferentially. The high capacity and high selectivity reversible CO2 sorption of membrane is all-important for the membrane presenting facilitated transport effect and high permeability as well as high selectivity.The permselectivities of different composite membranes were measured with pure CO2 and CH4 gas. The effects of many kinds of factors such as ratio of monomer, material and molecule weight cut-off of the support membranes, system temperature, cross-linking, thickness of separation layer and carrier content were discussed in detail. The encouraging results were achieved.Poly(AM-MA)/PS, poly( DM-AN )/PTFE and poly (DM-AA)/PS composite membranes present excellent comprehensive performances. For example, poly (AM-MA)/PS membrane displays a CO2 permeability of 5xlO"12 cm3(STP)/cm2.s.Pa, and a CH4 permeability of 2xlO¹3cm3(STP)/cm2.s.Pa. at 25°C,1140Pa of gas pressure;poly (DM-AN) membrane displays a CO2 permeation rate of 3.53xlO"12cm3(STP) /cm2.s.Paand CO2/CH4 ideal selectivity of 104.5 at 25°C,1140Pa of gas pressure, poly (DM-AA) /PS membrane displays a CO2 permeation rate of 6.12xlO'7cm3(STP) /cm2.s.Pa and CO2/CH4 selectivity of 253.1 at 26°C,1140Pa of gas pressure, poly (DM-AA) /PS composite membranes are better than that of other fixed carrier membranes reported in literatures, the main reason is the introduction of the carrier, tertiary amine group and carboxyl group which can react reversibly with carbon dioxide in the membranes. The transport mechanism of CO2 in poly (DM-AN) membrane was explored. In humidified membranes, part of complex was transformed into small and easy to move ion HCO3". The above mechanism was further characterized by FTIR using attenuated total reflectance techniques.Facilitated transport mechanism of CO2 in poly(DM-AA) membrane was discussed. CO2 Permeation flux is made up of two parts—facilitated transport and common diffusion permeation. The tertiary amine and carboxylate can form HCO3" with CO2 in the membrane at low CO2 concentration, CO2 transport in membrane is predominant by facilitated transport till CO2 concentration reaches fixed carrier saturation.CO2 Permeation flux is predominant by common diffusion and the flux increases slowly with the CO2 pressure increasing.