(122d) Using a Volumetric Apparatus to Distinguish between Diffusion and Surface Resistance Mass Transfer Kinetics in Commercial Adsorbents

When designing fast cycle or kinetically controlled pressure swing adsorption processes an understanding of the prevailing mechanism of mass transfer and the knowledge of the kinetic constants is an essential requirement. Professor Ruthven has made significant contributions in this area and in this paper we present a methodology that allows to distinguish clearly between internal diffusion and surface resistances when studying the kinetics of adsorption on commercial pellets.

For fast adsorption kinetics, the traditional method of converting the pressure signal of the dosing cell into an uptake curve and then plotting this vs the square root of time is not easily applicable because of the initial effect of the flow through the valve between the dosing and uptake cells. The new approach we propose is based on a semi-log plot of the reduced pressure in the dosing cell vs time, and provided that the volumetric experiment is carried out under isothermal and linear conditions, a simple inspection of this plot reveals clearly which process is prevailing. A detailed analysis of the models indicates also that the long-time asymptotic decay is very sensitive to the mass transport mechanism, because the equilibrium properties are fully defined by the initial and final state of the system.

The methodology will be demonstrated on commercial adsorbents used in air separation. To discuss both limiting behaviours experimental results will be presented on LiLSX beads which are macropore diffusion controlled and on CMS beads which exhibit primarily a surface resistance. The talk will conclude with a discussion of how to determine the kinetic constants when both processes contribute to the overall mass transport.