Modeling the selective attenuation of thin film composite membranes: Implementing the mass transfer perspective

Numerous experiments have demonstrated that the porous support layer limits the diffusion behavior and leads to a reduction in permeability of thin film composite (TFC) membranes. Although this geometric restriction has been described using empirical models, it is still rough and unable to describe the effect of support layer on selectivity, an accurate theoretical model to quantitatively calculate the permeation performance of TFC membranes is lacking. In this study, an equivalent theoretical model is proposed to describe the mass transfer process of TFC membrane, by properly handling the tangential diffusion at the interface between the selective layer and the support layer. Both the computational fluid dynamics (CFD) simulations performed in this work and experimental data in some literatures prove the validity of the equivalent model. Analysis of the theoretical model and CFD simulation results show that: Restriction factor Psi is independent of the solubility coefficients but dependent of the diffusion coefficients and support layers, which would alter the TFC membrane selectivity to the diffusing species with different diffusion coefficients. Tuning the structural parameters of the support layer is an effective way to bring the performance of TFC membranes closer to that of ideal free-standing membranes. Among them, High surface porosity of the support layer is the most critical factor affecting TFC membrane performance, both for permeability and selectivity.