Analysis of thermal effects in adsorption rate measurements

Abstract

The simultaneous differential heat and mass balance equations describing sorption under non-isothermal conditions are solved, subject to certain simplifying assumptions. The solution provides a theoretical model which can be used to determine the significance of thermal effects in a transient sorption rate measurement and to analyse the uptake curves obtained when diffusion is too fast for the isothermal approximation to be valid. Experimental uptake curves for selected systems are analysed. The rapid uptake of butane in small commercial Linde 13X crystals is essentially controlled by heat transfer whereas in the A zeolite thermal effects are of only minor significance. The diffusion of 2,2,4-trimethylpentane in large (24–39 µm) crystals of 13X is an intermediate case in which both heat transfer and intracrystalline diffusion control the rate. The extent to which the intrusion of thermal effects can explain the anomalous dependence of apparent diffusivity on crystal size and the discrepancy between sorption and n.m.r. diffusivities is briefly considered.