Quantification of the mass-transfer coefficient of the external surface of zeolite crystals by molecular dynamics simulations and analytical treatment

Analytical treatment of the problem of molecular transport through the external surface of zeolite crystals was applied to develop an expression for the mass-transfer coefficient of the external crystal surface (α) under the conditions of sorption equilibrium. The underlying model used in the derivation of this expression takes into account the possibility of strong adsorption of sorbate molecules on the external crystal surface. The new expression allows estimating the value of α if the following quantities are known: (i) the sorbate concentrations on both sides of the external crystal surface and, (ii) the probability for a sorbate molecule located on this surface to enter the zeolite lattice. The validity of the expression for α was confirmed by molecular dynamics simulations of the tracer exchange of methane molecules between the membrane of a silicious A-type zeolite and the surrounding gas phase.