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The gas solubility in polymeric membranes affects the separation performance, particularly in the case of CO2 capture processes. Solubility and solubility-selectivity in membranes of multicomponent mixtures can deviate rather markedly from the corresponding pure gas values, due to swelling and competition phenomena, and require dedicated time-consuming measurements. Many experiments can be avoided by using a suitable thermodynamic tool, such as an Equation of State (EoS) model, to represent the gases sorption in the membrane. Such models require, for parameterization, knowledge of the polymer behavior above the glass transition Tg, which is a limit for membrane modeling, because the most attractive polymers for gas separation are rigid matrices characterized by very high Tg values, difficult to reach experimentally. In this work, we study the sorption of CO2/CH4 mixtures in a high-Tg polyimide membrane (Matrimid®) using a bottom-up approach. Pressure-volume-temperature data for Matrimid® above Tg are generated using NPT Molecular Dynamics simulations: the results are regressed to find Matrimid® parameters for the PC-SAFT Equation of State. Finally, the Non Equilibrium PC-SAFT macroscopic model (NE-PC-SAFT) is used to calculate CO2 and CH4 solubility and solubility-selectivity as a function of gas mixture pressure, composition and temperature. The approach is tested successfully over many experimental pure gas and vapor sorption data in Matrimid®. Mixed gas calculations predict a marked competition, which affects more methane than CO2 sorption, and results in a higher-than-ideal value of solubility-selectivity. Combined with the fact that experimental mixed gas permeability-selectivity is lower than the ideal value, such results indicate that the diffusivity of CH4 in Matrimid is significantly enhanced in presence of CO2, causing a decrease of diffusivity-selectivity. The gas solubility in polymeric membranes affects the separation performance, particularly in the case of CO2 capture processes. Solubility and solubility-selectivity in membranes of multicomponent mixtures can deviate rather markedly from the corresponding pure gas values, due to swelling and competition phenomena, and require dedicated time-consuming measurements. Many experiments can be avoided by using a suitable thermodynamic tool, such as an Equation of State (EoS) model, to represent the gases sorption in the membrane. Such models require, for parameterization, knowledge of the polymer behavior above the glass transition Tg which is a limit for membrane modeling, because the most attractive polymers for gas separation are rigid matrices characterized by very high Tg values, difficult to reach experimentally. In this work, we study the sorption of CO2/CH4 mixtures in a high-Tg polyimide membrane (Matrimid®) using a bottom-up approach. Pressure-volume-temperature data for Matrimid® above Tg are generated using NPT Molecular Dynamics simulations: the results are regressed to find Matrimid® parameters for the PC-SAFT Equation of State. Finally, the Non Equilibrium PC-SAFT macroscopic model (NE-PC-SAFT) is used to calculate CO2 and CH4 solubility and solubility-selectivity as a function of gas mixture pressure, composition and temperature. The approach is tested successfully over many experimental pure gas and vapor sorption data in Matrimid®. Mixed gas calculations predict a marked competition, which affects more methane than CO2 sorption, and results in a higher-than-ideal value of solubility-selectivity. Combined with the fact that experimental mixed gas permeability-selectivity is lower than the ideal value, such results indicate that the diffusivity of CH4 in Matrimid® is significantly enhanced in presence of CO2, causing a decrease of diffusivity-selectivity.

A model is introduced here for volume relaxation in glassy polymers, induced by sorption of low molecular weight species. The model relies on description of thermodynamic properties of glassy mixtures obtained through NET-GP model and it considers simple linear viscoelastic laws to interpret the stress–strain relation for the polymeric material. Results for the case of sorption kinetics in constant fugacity processes, obtained from the model, are compared with experimental values for vapour sorption in glassy polymeric microspheres. Satisfactory representations were obtained both for instant solute content measured in experimental run and for mass uptake registered in ‘relaxation stage’ of sorption processes. Viscosity data obtained from fitting of sorption experimental results are finally discussed in terms of temperature dependence and compared with values obtained form purely mechanical volume relaxation experiments.

In this work, an atomistic approach is employed to simulate pvT data of polymers at high temperature (in the rubbery region), by using a molecular dynamics tool. Data are then regressed with the desired EoS to obtain the polymer characteristic parameters, which are used to evaluate the gas solubility in the polymer with NET-GP model.

A new approach for modeling sorption and swelling of glassy polymer matrix with carbon dioxide at high pressure has been developed. The ability of the model to predict gas solubility at different operative conditions, with only one adjustable parameter, was investigated through the comparison with experimental data in a wide pressure range.

The non-equilibrium thermodynamic approach to describing properties of glassy polymers, earlier applied in the so-called NELF model using Lattice Fluid Theory, is generalized to the use of arbitrary equations of state. Application to Statistical Associating Fluid Theory (SAFT) is discussed in detail and use of this approach to predict opr correlate gas solubility in glassy polymers is presented for the case of low-pressure sorption. Prediction procedures rely on volumetric data for the pure polymer and gaseous species, while for data correlation a temperature independent binary interaction coefficient is used as an adjustable parameter. Results from model calculations are compared with literature data for gas solubility in polycarbonate and polysulfone. Analysis of solubility coefficient variations above and below the glass transition temperature indicates thet the non-equilibrium approach is the proper procedure to extend an EoS description of thermodynamic properties to the glassy state.