Adsorption in non-isobaric fixed beds

Abstract

The paper examines the effect of finite pressure gradients on the dispersion of a pulse of an adsorbable gaseous species as it flows through a fixed bed of adsorbing spherical particles.It is shown that, even for significant deviations from isobaric conditions within the bed, there is an asymptotic linear relationship between the first moment of the response (the mean retention time) and the reciprocal of the exit flow rate, a characteristic of the isobaric situation. However, unlike the isobaric case, extrapolation of the asymptotic linearity to infinite flow rate leads to a non-zero first moment at that point. Furthermore, extrapolation to zero first moment corresponds to a hypothetical flow rate which is dependent only on the parameters of the packed bed and the viscosity of the carrier gas and, specifically, is independent of the nature of the adsorbed species. The theory has been confirmed by a series of chromatographic adsorption experiments under non-isobaric conditions.Subsequently, the same experimental data have been employed in conjunction with the theory to obtain adsorption equilibrium constants for four normal alkanes (propane, butane, pentane, and hexane) on alumina in the temperature range 50–200°C with an uncertainty of ±3%.