Mass Transfer Mechanism And Modelization For The Process Of GAS Mixture Membrane Separation

With the development of industry, more and more CO2and volatile organic compounds (VOCs) which were generated in the industry have been exhausted into the environment. This emission not only causes lots of environmental pollution but also wastes much energy. As a result, how to recover CO2and VOCs from exhaust gas becomes more and more important. With the virtue of simple operation and equipment, low energy consumption and no secondary pollution, membrane separation technology has been attracted more and more attention. However, the modelization of these processes does not develop so fast as the industrization of these processes. In the last twenty years, the studies were mainly about the process of gas permeating though glassy membrane or measuring the permeability of non-condensable gas in rubber membrane.Moreover, the study about permeation properties of membrane focused on binary non-condensable gas mixture.According to the shortage of previous studies, PDMS membrane that has the most opportunity for industrization was chosen in this study. Firstly, the thermodynamic problem of describing the equilibrium of multi-component gas mixture desloving in membrane was solved and the way of calculating activity parameters was obtained. Based on this, the mechanism of gas mixture permeating through membrane and the modelization were proceeded. Afer that, the mass transfer mechanism was used to analyze the separatin properties of spiral wound membrane.The content of our paper includes follows.1. The permeation properties of N2, CH4and CO2through poly dimethylsiloxane (PDMS) membrane at different pressure and temperature were measured by time-lag way. After that, the activity coefficients of these gases in PDMS membrane were obtained by the definition of solubility and gas-solid equilibrium equations from their solidities. At last, the interaction parameters between these gases and PDMS membrane were gotten by the way of non-linear fitting.2. According to the properties of condensable gases (saturated pressure at ambient temperature lower than1atm and high membrane swelling effect), a new method which composites time-lag way and sorption isotherms way was proposed. With this way, the diffusivity and solubility of C5-C7alkanes in PDMS membrane were determined, and the interaction parameter for UNIQUAC model was obtained by previous technique.3. By analyzing the process of gas dissolving into membrane and diffusion in membrane, FICK-HENRY (F-H), FICK-UNIQUAC (F-U) and MS-UNIQUAC (MS-U) combination models were used to describing the process of gas/vapor permeation through PDMS membrane. By compared the differenent results of these models used in different gas mixtures, the difference and reason of three models were analyzed. After that, the application scope of three models was obtained.4. N-S Eqs and F-H combination were used to describe the vapor permeatin through spiral wound membrane which is most probable to reealiz industrization. With this model, the influence of concentration polarization and structure parameters was investigated. The models for spiral wound membrane module based on different solution-diffusion models (F-H and MS-U) were compared. In the end, the separation results under different operating conditions was also analyzed by the model based on MS-U combinations.The results indicates that, for noncondensable gas, the permeability and solubility of different components at certain temperature and pressure ranks as their critical temperature; while the difference between their diffusivity is small. Their permeability and diffusivity increases with temperature and almost have no change with pressure. Their solubility decreases with temperature and have no change with pressure. For condensable gas, the permeability and solubility of different components at certain temperature and pressure ranks as their critical temperature; their diffusivity ranks reverse as their critical temperature. Their permeability and solubility increases with pressure and declines with temperature. Their diffusivity increases with temperature and almost does not change with pressure. The predicted results of three model combinations show that, for non-condensable gas mixture, all of three model combinations can give a good prediction; while, for condensable gas mixture, only MS-U model can predict exactly. The predicted results for spiral wound membrane module indicate that the structure of spiral wound membrane module almost have no influences on separation and the concentration polarization is not obvious. For condensable gas mixture, only MS-U model can be used to predict the separation results.