Molecular modelling approaches to phase equilibria in facilitated transport membranes
Facilitated transport membranes (FTM) are a promising new class of materials for CO2-selective gas separation, with potential for application to post-combustion carbon capture and sequestration. FTM separation takes advantage of the chemical reactions of CO2 to increase throughput and selectivity, with potential for reducing costs and improving purity in the resulting gas streams. However, the process performance of FTM separation relies on material properties, determined by a complex chain of dependencies including polymer composition, water absorption, reactivity, and temperature, with the underlying physical processes not being fully understood. The development of new FTM materials is in turn limited by a lack of quantitative models and poor understanding of the microscopic mechanisms of FTM separation. This motivates the development of new theory and molecular models for estimating process performance and thermodynamic behaviour in this important class of materials. Unlike simple fluids, FTM systems exhibit multiple complex features which have limited the application of existing model frameworks. In this work, three key aspects of facilitated transport membranes are examined in order to build a predictive model of thermodynamics and phase equilibrium processes in FTM systems: (i) Thermodynamics of polymer melts and vapour-liquid equilibrium in polymer solutions are approached by both molecular simulation and theoretical models, in order to characterise the behaviour of polyvinylamine, a prototypical FTM polymer. Simulation of oligomers based on a TraPPE-UA molecular model are found to produce good quality rho p T data for common polymers, and a hybrid simulation / equation of state approach is proposed for predicting polymer properties. By substituting experimental measurement with molecular simulation, equation of state models are obtained with PC-SAFT even where the real polymer is thermally unstable or experimentally inaccessible. (ii) The aqueous-organic electrolyte solution originating from amine-CO2 is considered based on theoretical and simulation approaches, with significant shortcomings found in current models. New molecular parameters are proposed for aliphatic ammonium chlorides and bicarbonate, optimized against experimental phase equilibrium data, allowing CO2 solubility in aqueous amine systems to be simulated at varying stages of absorption. (iii) The reactive aspects of CO2 absorption in aqueous amine solutions is examined in light of literature approaches. A new hybrid methodology is proposed, using infinite dilution experimental data in combination with molecular simulation utilizing the newly optimised molecular models. Together, advancements in the three aspects outlined above permit description of facilitated transport membranes by molecular simulation, and in turn parametrisation of equation of state models for reactive polyamine systems. The knowledge gained for polymer-solvent systems, electrolytes, and reactive gas absorption allow prediction of material properties and process performance of FTM polymers. Here, the quaternary system PVAm - H2O - CO2 - N2 is examined as a prototypical post-combustion FTM system, in order to chart the dependence of process performance on operating conditions. These advances inform future application of associating equations of state to amine-based facilitated transport membranes in particular, as well as the wider field of reactive electrolyte systems.