Vacuum Sorption Pumping Studies of Argon and Oxygen on 4A Molecular Sieves

Abstract

Cryosorption pumping is a method of evacuating enclosed volumes by adsorbing gas on a deep bed of sorbent, such as Davison 4A zeolite, at cryogenic temperatures. Modeling the dynamic behavior of these systems for air pumping requires information on two major constituents of air: oxygen and argon. Particle size variation was the major variable in determining the mechanism of the process. The model comprises a fluid-phase mass balance representing the dynamics of gas in the bed and a spherical, one-dimensional diffusion equation describing adsorption in pellets of 4A zeolite. The present model calculates effective pore diffusivity taking into account Knudsen diffusion, ordinary diffusion, and Poiseuille flow. The primary diffusional resistance appears to occur in the macropores formed by the pelletizing clay binder, rather than in the bed interstices or microporous zeolite crystals.