Abstract
Direct simulation results for stationary gas transport through pure silica zeolite membranes (MFI, LTA and DDR types) are presented using a hybrid, non-equilibrium molecular dynamics simulation methodology introduced recently. The intermolecular potential models for the investigated CH$_{4}$ and H$_{2}$ gases were taken from literature. For different zeolites, the same atomic (Si and O) interaction parameters were used, and the membranes were constructed according to their real (MFI, LTA, or DDR) crystal structures. A realistic nature of the applied potential parameters was tested by performing equilibrium adsorption simulations and by comparing the calculated results with the data of experimental adsorption isotherms. The results of transport simulations carried out at 25$^0$C and 125$^0$C, and at 2.5, 5 or 10 bar clearly show that the permeation selectivities of CH$_{4}$ are higher than the corresponding permeability ratios of pure components, and significantly differ from the equilibrium selectivities in mixture adsorptions. We experienced a transport selectivity in favor of CH$_{4}$ in only one case. A large discrepancy between different types of selectivity data can be attributed to dissimilar mobilities of the components in a membrane, their dependence on the loading of a membrane, and the unlike adsorption preferences of the gas molecules.
Highlights
Zeolites are made up of silicon, aluminum, and oxygen atoms linked together so that they form structurally well-defined pores
We started from the DCV-GCMD approach [5] containing two equilibrium control cells distinguished by unequal intensive thermodynamic parameters on the two sides of the membrane, where random particle insertion/deletion moves are applied to maintain bulk phase properties in the control cells and to uphold the desired driving force through the membrane
We present results for the adsorption and transport of gases on allsilica MFI, LTA and DDR zeolites at T = 298.15 and 398.15 K, and at p = 250 and 500 kPa
Summary
Zeolites are made up of silicon, aluminum, and oxygen atoms linked together so that they form structurally well-defined pores. The atomistic simulation results can explain or, in some cases, substitute the experimental results Nowadays, these simulations play an important role in the description of processes that occur in crystalline adsorbents and membranes [5,6,7,8,9,10,11,12,13]. We developed a simple atomistic simulation method for membrane transport that can maintain a driving force without significantly disturbing the previously-developed steady-state flux [19], while it properly mimics the common experimental situation in gas permeations measurements, where pressure is the main control parameter. Direct simulation results for steady-state gas transport through some relevant pure silica zeolite membranes are reported using our novel hybrid MD simulation method. We briefly outline the applied transport simulation technique, and specifications of the performed simulations and results for the separation of the technologically important CH4-H2 mixture (that might be relevant in the development of engine fuels with high hydrogen content) will be presented in more detail
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