Study On Mechanism Of Mass Transport Of Fixed-Carrier Membrane For CO₂ Separation And Simulation And Optimization Of The Membrane Process

Because of the reversible chemical interaction between CO₂ and the carrier, the CO₂ transport in the fixed carrier membrane is facilitated remarkably. Hence, the fixed carrier membrane has excellent separation performance and great application potential. Currently, the mass transport mechanism of the fixed carrier membrane is still not quite clear and the application study of the fixed carrier membrane is needed. In this paper, the mechanism of the facilitated transport of the fixed carrier membrane is further studied. The application process of the fixed carrier membrane is simulated and optimized. The results can offer guidance for the development and application of the fixed carrier membrane.The characteristics of the facilitated transport process of the fixed carrier membrane were investigated experimentally, taking PEI-PVA/PS and PAAm-PAM/PS composite membranes as examples. The experimental data offered foundation for the research of the mechanism of the fixed carrier membrane.The microscopic mechanism of the facilitated transport of the fixed carrier membrane was analyzed, and an equilibrium region theory was proposed according to which novel mathematical model was founded. Analysis was carried out by using the novel model. The nonlinear distribution of the transport resistance and the relationship between the permeance of fixed-carrier membrane and the membrane thickness were obtained. The apparent activation energies for the CO₂ diffusion and collision between the CO₂ molecular and the active site in the fixed-carrier membrane were obtained by fitting the experiment data. The calculation results were in good agreement with the literature.A mathematical model reflecting the effect of the permeate side pressure was derived from the macroscopic model founded by Dr. Yi of our research group. The new model was used to analyze the effect of the permeate side pressure. Results showed that under different permeate pressures, the permeance was different. High permeance could be obtained by increasing the permeate side pressure. Therefore, the operating model of the combination of feed compression and permeate vacuum pumping was suggested. The calculation results of the cocurrent flow model, the countercurrent flow model and the cross flow model were analysed for the processes of the CO₂ removal from the natural gas and the CO₂ capture from the post-combustion gas. The cross-flow model proved simple and suitable for the two processes and was thus used in the analysis in this paper. The application potential of the fixed carrier membrane in the two processes was investigated, and the separation performance of the two stage system with recycle was compared with that of the single stage system. Results showed that the separation target could be achieved more easily by using the fixed carrier membrane than using the current commercial membranes; the separation performance of the two stage system with recycle is much better than that of the single stage system. The operating pressures of the two stage system was optimized and the CO₂ separation cost decreased remarkably. The cost of the optimized two stage system was competitive with the amine absorption method and the application potential of the fixed carrier membrane in the field of the CO₂ capture from the post-combustion gas was verified.Parametric analysis was carried out for the two stage system with recycle. Results indicated that for the membrane with high selectivity the membrane area could be reduced by increasing the slow gas (the gas component with low permeance) permeance. For the membrane with low selectivity, as the selectivity increases the recycle flowrate decreases and membrane area requirement decreases. The permeate gas with higher CO₂ concentration, which is beneficial to the further treatment of the permeate gas rich in CO₂ (for example, geo-sequestration), can be obtained by adopting membrane with high selectivity and low operating pressure ratio.