GAS-MIXTURE ADSORPTION ISOTHERMS AND DIFFUSIVITIES IN MOLECULAR SIEVES BY PERTURBATION CHROMATOGRAPHY

A critical analysis of the perturbation chromatography techniques for determining gas adsorption equilibria and intraparticle diffusivities in molecular sieves is presented.; Based on mathematical tractability use of the concentration-pulse method for determining gas adsorption equilibria for mixtures containing three or more components is impractical. The tracer-pulse method, on the other hand, is readily applicable to multicomponent systems. For binary systems, concentration-pulse retention volume data can in some cases be used in conjunction with the Van der Vlist-Van der Meijden method of data reduction to yield reliable gas-mixture adsorption isotherms. Limitations on this procedure are discussed. For binary systems where one component is not adsorbed, the concentration-pulse method gives reliable pure-component isotherms. The tracer-pulse method can be applied to all pressures, even those lower than atmospheric by using helium-dilution. The combined method of the concentration- and tracer-pulse techniques is an extremely valuable method from both economic and practical points of view.; A mathematical model has been developed to describe the chromatographic behavior of a tracer pulse. The moment method was used to extract the intraparticle diffusivities from the experimental data. The adsorption equilibrium constants and diffusivities of ethane and ethylene in 13X molecular sieves were obtained at 118 pKa over a temperature range of 298 to 373 K by the tracer-pulse method. Micropore tracer-diffusivities for C(,2)H(,4) and C(,2)H(,6) range between 3 x 10('-9) and 5 x 10('-8) cm('2)/sec. The activation energies for micropore diffusion are approximately 4.0 for C(,2)H(,4) and 5.5 kcal/mole for C(,2)H(,6). The micropore diffusion is an activated diffusion process. The macropore tracer-diffusivities for C(,2)H(,4) and C(,2)H(,6) are 5.9 x 10('-2) to 7.3 x 10('-2) cm('2)/sec depending on temperature. The diffusion in the macropores is in the ordinary diffusion region instead of the Knudsen region. The adsorption kinetics in small particles were found to be controlled by the micropore diffusion process, but in larger particles by both the macro- and micropore diffusion processes, particularly at high temperature.