| Thin film composite (TFC) membranes exhibit high, selective fluxes for gas and vapor permeation, making them viable for a wide range of applications. The high fluxes may also increase the importance of the resistance of the porous support structure depending on the application and process conditions. A comprehensive modeling approach for TFC membranes is introduced, which considers boundary layer resistances near the membrane surface, solution-diffusion through the coating, and the influence of the porous sublayer.; Permeation through the support structure is described by the dusty gas model (DGM). The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary, continuum diffusion. To apply the DGM, the porous support structure has to be characterized by three morphological parameters, which can be determined from single gas and binary gas mixture permeation experiments. The DGM shows an excellent fit to experimental data of TCE/nitrogen permeation through uncoated membrane supports. The membrane supports exhibit different resistances towards the various transport mechanisms that occur, and the resistances vary considerably with process conditions.; Flat sheet and hollow fiber membranes with silicone rubber coatings were tested and compared for the separation of volatile organic compounds (VOCs) from gas streams. The overall model for the composite membrane describes experimental data on TCE/nitrogen separation using a sweep gas on the permeate side very well. Model calculations were also performed for vacuum assisted permeation. Since silicone rubber is extremely permeable to TCE, the main resistance towards TCE permeation is the porous support. It is shown that changes in the support morphology can greatly enhance the performance of the composite membranes.; The proposed model is able to describe the composite membrane performance and to identify optimum process conditions for a given membrane. It can also be used to aid in the development of membrane structures for enhanced performance. |
