Thermodynamic and Molecular Simulation of Pure and Mixed Gas Sorption in Polymeric Membranes

The characterization of polymeric membranes for gas separation is often performed with pure-gas tests, which are poor predictors of the performance at multicomponent conditions. In this work, the Non-Equilibrium Lattice Fluid (NELF) model was applied to study mixed-gas sorption in traditional glassy polymers employed for CO2/CH4 separation, such as Cellulose Acetates, and innovative ones, such as polyimides (HAB-6FDA), Thermally Rearranged (TR) Polymers and Polymers of Intrinsic Microporosity (PIMs). The model results were validated against experimental data. Strong nonidealities are observed, due to competitive sorption and penetrant induced swelling, that radically modify the gas transport at multicomponent conditions compared to pure-gas cases. These effects were correctly predicted by the model, as well as temperature, pressure and concentration effects. The Dual Mode Sorption (DMS) model was tested for the same systems and a sensitivity analysis of its parameterization procedure revealed great uncertainty associated to its predictions of multicomponent sorption. A new measurement protocol was developed for the determination of sorption isotherms for gas mixtures with an arbitrary number of components. Mixed-gas sorption of binary C2H6/CO2 and C2H6/CH4 mixtures and of ternary C2H6/CO2/CH4 mixtures in PIM-1 was measured with this technique, finding strong competitive effects related to the presence of ethane. Predictions of the NELF model for binary and ternary sorption, performed using only pure-gas parameters as input, were in good agreement with the experimental data. Predictive Molecular Dynamics simulations were carried out to investigate the effect of CO2 up to high concentration on several properties of a polymeric material. A systematic evaluation of thermodynamic and structural properties, local dynamics, gas solubility and diffusivity yielded good agreement with the experimental data and meaningful trends with respect to temperature, gas concentration and polymer molecular weight were obtained, thus confirming the possibility to investigate the properties of materials at the molecular level with great accuracy.