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 {A figure is presented}, where - 1 ≤ λ ≤ + 1. For diffusion in zeolite, a theory is given for predicting λ, which requires equilibrium adsorption information of the gas mixture. For Langmuirian gases following ideal adsorbed solution behavior, i.e. with no lateral interactions, λ = + 1 for co-diffusion and λ = -1 for counter-diffusion. Since the Fickian diffusivities (D_tii 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 co-diffusion and to reduce both fluxes in most cases of counter-diffusion.