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【原创】MAPS中towhee模块做的例子 已有2人参与
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In this application the MAPS Towhee [1] plug-in was used to calculate the relative adsorption of N2 and CO2 in an LSX-Li zeolite. This type of simulation is important for the gas separation process, where companies would like to use zeolites to separate mixtures of gases to produce pure gas components. Variation in the type of zeolite, as well as varying cation content or the type of cation can have a significant impact on the selective adsorption of different gases and hence on the efficiency of the separation process. The framework structure of LSX for this study was taken from a zeolite database. Then the cation locator script within MAPS was employed to replace a number of silicon atoms by aluminum atoms and to place Li cations at the appropriate places in the framework.  Finally the accessible free volume was calculated, and areas not accessible by diffusion were blocked by hard spheres for the following Towhee simulation. The image below shows the free volume in the zeolite, the non accessible free volume is characterized by small spherical volume areas. Atomistic structures of Nitrogen and Carbon Dioxide were also constructed and used for the following Grand Canonical Monte Carlo simulation with the Towhee Monte Carlo simulation package integrated into MAPS. Simulation parameters were taken from literature [2] [3] [4] and simulation setup and analysis were done within MAPS. Both adsorbent molecules were considered rigid, for N2 a three point charge model was used in order to appropriately represent the quadruple moment of the N2 molecule. Simulation Grand Canonical Monte Carlo (GCMC) with the Towhee plug-in was performed in order to calculate the absorption isotherms at 293 K. The fugacity and hence the chemical potential for each N2 and CO2 was calculated at the desired pressures by using PCSAFT equations of state with the SciTherm plug-in within MAPS. Results： The following graphs represent the results of the GCMC Towhee calculations:   As can be clearly seen, CO2 is adsorbed at lower pressures in the zeolite compared to N2, likely due to its more polar character. The upper loading limit is also higher for CO2 compared to N2. This is well in agreement with experiment. For absolute numbers, the simulation results rank higher than the experimental loading curves. It has been shown in a different set of simulations however, that by keeping the cations flexible throughout the simulations absolute agreement between experimental and simulated results can be significantly improved. Figure 4 shows the final result of the Towhee runs at 6.30 bar.  References: 1. M.G. Martin, J. I. Siepmann, J. Phys. Chem. B, 103, 4908 – 4917, 1999. 2. K. Watanabe, N. Austin and M. R. Stapleton, Molecular Simulation, 15, 197, 1995.3. 3. A.J. Richards, K.Watanabe, N.Austin and M.R.Stapleton, Journal of Porous Materials, 2, 43, 1995. 4. J. G. Harris, K. H. Yung, Journal Chemical Physics, 99(31), 12021, 1995. [ Last edited by ghcacj on 2010-12-17 at 12:33 ] |
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