ANALYSIS OF A ZEOLITE MEMBRANE PROCESS

Abstract

Zeolite membranes offer an interesting possibility of a truly continuous adsorption process for gas separation. Successful synthesis procedures for zeolite membrane have been reported only recently. Acquisition of permeation data on synthesized zeolite membranes is important for practical implementation of zeolite membranes for gas separation. Mathematical models, which can describe and predict the zeolite membrane performance would be of immense use in designing specific gas separation processes. Development of a comprehensive model to describe single component and binary permeation behavior of gases in a tubular zeolite membrane is the focus of this study. A transient model was initially developed for a trace single adsorbate system in order to understand the interaction effects of equilibrium and kinetic properties. The effect of defect in the membrane on the performance was studied. Additional considerations such as non-linearity of the adsorption isotherm, extension to a non- trace system and extension to a binary mixture were gradually included and their effects were studied, but these studies were restricted to steady state operation. A constant difusivity model based on Fickian formulation of diffusivity and a variable diffusivity model based on chemical potential theory were ultimately developed for a binary mixture. A parametric study was conducted on one and two-dimensional models, to analyze the effect of lumen resistance due to radial molecular diffusion. The range of operating conditions and the range of adsorbate systems where a one-dimensional model would be sufficient to suitably model a separation process and the domain where it would fail were also apparent from the study. The proposed model for binary separation predicted a decrease in binary selectivity of n-hexane and n-octane mixture in a silicalite membrane compared to their ideal selectivity (ratio of single component permeabilities). However, the agreement with the experimental results was more qualitative than quantitative. The selectivity reversal seen experimentally was also predicted by the model but at a much higher operating temperature. It was found that small variation in kinetic and equilibrium parameters might significantly affect the performance prediction by the model.