THE KINETICS OF MOLECULAR SIEVE ACTION. SORPTION OF NITROGEN–METHANE MIXTURES BY LINDE MOLECULAR SIEVE 4A

Abstract

This system illustrates "partial molecular sieve action". Methane has a higher affinity for the sorbent and hence is preferentially sorbed at equilibrium, while nitrogen diffuses through the crystal more rapidly and thus is preferentially taken up during the early stages of sorption. Measurements were made with the pure gases and with mixtures at 0 °and −79.4 °C. The sorption isotherms fit Langmuir equations and the isosteric heats of sorption are essentially independent of concentration. The sorption rates for the pure gases may be characterized by diffusion coefficients, D, calculated in the usual manner assuming the flux of diffusion to be proportional to the concentration gradient. The resultant values for D increase with increasing sorbate concentration. Diffusion of a mixture may be formally characterized by D's for each component. While that for methane is approximately the same as for methane alone, D for nitrogen in mixtures is much larger than for the pure gas and also varies with composition. This, as well as the existence of a temporary maximum in the amount of nitrogen sorbed, may be explained by considering the driving force for diffusion to be the gradient in chemical potential rather than in concentration.