Equilibrium and Diffusion in Molecular Sieves

Abstract

Two topics in equilibrium and one in diffusion in zeolites are examined. The sorption of CO2 in 5A and 13X zeolites is detailed. Thirteen studies from the literature and one unreported study are examined for saturation loading, qmax, Henry's constants, KH, isosteric heat of adsorption, -ΔHq, and isotherm model for the new data. The saturation loadings, qmax, are greater than those derived from theoretical calculations of density based on the Modified Rackett equation combined with the relevant crystallographic data for the zeolite when the reduced temperature is greater than 0.9. This implies that the molar volume of the adsorbate is less than that in the equivalent liquid phase. Henry's constant, KH, values for both zeolites are calculated. The isosteric heat of adsorption on 13X zeolite is less than the calorimetric value; a hypothesis is advanced for this phenomenon. Fits of the multisite Langmuir isotherm of n-alkanes on 5A zeolite are examined. The values of qmax and hence θ are predetermined using a recent paper. Some of the new fits were successful but many were not. Best fits were achieved for C2 and C4. The study of nonisothermal sorption of water on 5A zeolite that has been measured on an Intelligent Gravimetric Analyzer (IGA) is modeled. This data shows that for a partial pressure step change of zero to 1 mbar at 20˚C, a temperature rise of 17˚C arises causing the adsorption to be nonisothermal. This rise in temperature raises the diffusion coefficient causing the mass uptake to be a two resistance model rather than a one resistance model. The mass uptake and temperature are modeled.