Mass transfer in vacuum sorption pumping of pure gases on molecular sieves

The major objective of this work was to develop a mathematical model to quantitatively describe the hydrodynamic and diffusive mass transfer processes involved in the vacuum sorption pumping of single-component gases on cylindrical sorbent beds. A set of equations describing these processes in terms of the pressure above and through the bed and the solid loading was derived from theory, and a finite difference solution of the set of partial differential equations was developed. The model shows hydrodynamic mass transfer through the bed interstices to be characterized by an interstitial mass transfer coefficient, κ, that is a function of only the channel geometry and the temperature and molecular weight of the flowing gas.

Sorption pumping experiments were conducted using nitrogen and carbon dioxide on Davison 4A zeolite molecular sieves. For N₂, the theoretical model predicted well both the pressure above and total pressure drop across the bed for times up to saturation of the sieves. The model also described accurately the pressure profiles obtained with CO₂ but failed to fit the pressure drop data.

Estimates of κ obtained from N₂ runs using two different sorbent particle sizes indicated that κ is proportional to the particle radius; this supports the assertion that κ is a function of channel geometry. From κ for one gas, it should be possible to estimate κ for another gas moving through the same bed. The κ's predicted theoretically for CO2 from those for N2 did not successfully model the experimental data.