Molecular Simulation Study on Adsorption and Diffusion Behaivor of Ethanol/Water Molecules in NaA Zeolite Crystal

摘要

Molecular simulations have been performed on adsorption euqilibria and diffusivities of pour and mixed-gases of ethanol/water in micropores of an NaA zeolite crystal by using the muVT-ensemble orientational-bias Monte Carlo and the NVT-ensemble equilibrium molecular dynamics technieques.Th etemperature and the total presure were set at 378 K and 0.1j MPa respectively for comparison with the dta peratue and the total pressure were set at 378 K and 0.1 MPa respectively for comparison with the data of vapor pemeaton of a correspondign system.Adsorption equilibrium selectivit of water ot ethanol was found to increase with an increasing ethanol composition in the gas phase,which was qualitatively in agreement with the experimental permselectivity,though the latter was three orders of mangnitude higher than the first.The diffusivity of water in the zeolite was found to be in teh region pf 3.9-7.9 X 10~(-10) m~2/s while the ethanol molecules were confined in the alopha-cage and did not go through the window during a simulation period of 3 ns,which indicated that the diffusivity of ethanol might be much smallejr than that of water.Therefore,it is concluded tha tthe high permselectivity fo water through an NaA zeolite membrane is ascribed to the cooperaton of two factors:the high adsorption selectivity and the high diffusivity of water.It is also noted that the selecitve adsorption of water may be important for preventing the blockage by ethanol molecules at the entrance and iside ofj the membrane.