PREDICTION OF CROSS-TERM COEFFICIENTS IN BINARY DIFFUSION - DIFFUSION IN ZEOLITE

A theoretical framework is derived for predicting the cross-term phenomenological coefficients, L(ij), from the main-term coefficients, L(ii) and L(jj), resulting in L(ij) = lambda square-root L(ii)L(jj), where -1 less-than-or-equal-to lambda less-than-or-equal-to +1. For diffusion in zeolite, a theory is given for predicting-lambda, which requires equilibrium adsorption information of the gas mixture. For Langmuirian gases following ideal adsorbed solution behavior, i.e. with no lateral interactions, lambda = +1 for co-diffusion and lambda = -1 for counter-diffusion. Since the Fickian diffusivities (D(ii) and D(ij)) can be related to the L coefficients (based on certain assumptions), it is possible to use the theory to predict all multicomponent Fickian diffusivities from the single-component Fickian diffusivities. Results are also reported for thirteen series of binary diffusion experiments of CO2/C2H6 in 4A zeolite using the differential adsorption bed technique. Reasonable agreements are obtained between the experimental data and theoretical predictions. For both counter- and co-diffusion, the multicomponent effects are most pronounced for the fast-diffusing component. The multicomponent effects are to enhance both fluxes in codiffusion and to reduce both fluxes in most cases of counter-diffusion.