Studies On The Preparation And Performance Of Fixed Carrier Composite Membrane For Removal Of CO₂ From CH₄

Separation and removal of CO₂ using membrane technology is one of the most important subjects about energy and environment. 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, three kinds of CO₂ fixed carrier membrane materials were prepared successfully, they are poly(N-vinyl-γ-sodium aminobutyrate), poly(N-vinyl-γ-sodium aminobutyrate-co-sodium acrylate) and carboxymethyl chitosan. The composite membranes were developed respectively with the above three kinds of materials as active layers and polysulfone, polyethersulfone, polyacrylonitrile ultra-filtration membranes as the supports. FTIR(Fourier Transform Infrared Spectroscopy), XRD(X-ray diffractions), XPS(X-ray Photoelectron Spectroscopy), 1H NMR(Nuclear Magnetic Resonance), elemental analysis, 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 all of the materials contain secondary amine or primary amine and carboxyl groups and the active layers of the composite membranes are dense and smooth. The permselectivities of different composite membranes were measured with pure CO₂ and CH4 gas as well as binary mixture of CO₂ and CH4. The effects of many kinds of factors such as "coupling effects", material and molecule weight cut-off of the support membranes, the impurities in the dope solution, cross-linker, cross-linking time and carrier content were discussed in detail. The encouraging results were achieved. PVSA/PS,VSA-SA/PS and CM-CS/PES composite membranes present excellent comprehensive performances which are better than that of other fixed carrier membranes reported in literatures. For example, at 5.1cmHg of CO₂ pressure,PVSA/PS composite membrane displays a CO₂ permeation rate of 1.47(10-4 cm3 (STP)cm-2 s-1 cmHg-1 and CO₂/CH4 ideal separation factor of 155.9, at 5cmHg of pressure, VSA-SA/PS composite membrane displays a permeation rate of8.0(10-5cm3 (STP)cm-2 s-1 cmHg-1 and ideal separation factor of 406.2, at 5.3cmHg of pressure, CM-CS/PES composite membrane displays a permeation rate of 5.79(10-5cm3 (STP)cm-2 s-1 cmHg-1 and ideal separation factor of 84.6. The main reason is the introduction of the carrier, amine group and carboxyl group which can react reversibly with carbon dioxide in the membranes. The transport mechanism of CO₂ in PVSA/PS composite membrane was explored. In dry membrane CO₂ was transported in two ways, dissolved CO₂ and the complex (RNH(CH2)3COO-·CO₂)of CO₂ and carriers. While 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 and FT-Roman spectrum.The stability of three kinds of membranes performance is good. The membranes can keep good separation factor and CO₂ permeation rate for one week at the feed gas pressure 57cmHg.