Computer Simulation of Osmosis and Reverse Osmosis in Structured Membranes.

摘要

We have carried out detailed studies to investigate the separation of two gas mixtures, N2/O2 (air) and CO2/N2 using a range of zeolite membranes (Faujasite, Chabazite and MFI). These two mixtures were selected because they are known to show contrasting separation characteristics in the membranes selected. The main goal of this work was first to establish the viability of studying such membrane based separations using molecular simulations. After validating the methods used, we used the simulations to understand at the molecular level the controlling mechanisms for these separations, with the ultimate goal of making these processes more efficient. Our results clearly indicated that molecular simulations can realistically model these separations, since they correctly predicted qualitatively all the observed trends reported experimentally, including those in which these mixtures showed contrasting behavior. We were also able to establish the diffusion mechanisms inside the membrane, including clearly determining what molecular mechanisms were ultimately responsible for the observed selectivities of the membrane. 2. We have examined ion-exchange in NaA zeolite membranes by carrying out molecular simulations in which solutions of LiCl, and CaCl2 were made to come in contact with a NaA zeolite ion exchange membrane. Experimental observations indicate that the ion-exchange process is relatively slow for LiCl solutions, but much more rapid for CaCl2 solutions. We were able to show using our simulations, that the exchange process is almost entirely energetically driven, rather than entropically driven. In addition the energetic driving force for Ca2+ ions was much larger than for Li+ ions, and that explained the observed differences in the ion-exchange rate. We were also able to validate the method by examining the hydration of the zeolites by water and comparing with available experimental results, and comparing the positions of the exchanged cations with known experimental data. Our simulations showed qualitative agreement with experimental results. We believe our results have shown that reliable screening studies for determining the suitability of a potential ion exchange process can be carried using molecular simulations.