A modelling study of vacuum sorption characteristics of CO₂₊ on 4A zeolite molecular sieve

A model that describes the isothermal adsorption of carbon dioxide (CO₂) and of nitrogen (N₂) on 4A zeolite molecular sieves under cryogenic conditions is presented. The model is comprised of a fluid phase mass balance that represents the dynamics of gas in the bed and the diffusion equation in one dimension that represents adsorption in the solid. Cubic zeolite 4A crystals are assumed to be spherical. The concentration dependence of the diffusivity of the sorbate in both the gas and solid phases is considered. Numerical solution of the parabolic partial differential model equations is accomplished by using orthogonal collocation in conjunction with an ordinary differential equation integrator that is suitable for stiff equations.

Langmuir's adsorption isotherm is used to represent equilibrium concentrations at the gas-sorbent interface. The primary diffusional resistance is assumed to occur in the microporous zeolite crystals, and not in the bed interstices or the macropores formed by the clay binder used to pelletize the crystals. This is in contrast to the assumption of macropore resistance in the precursory experimental and theoretical work of Crabb et al. (1986) at the Oak Ridge National Laboratory.

Experimental results obtained by Crabb et al. are used for comparisons with the theoretical results described herein. Good agreement with experimental results for CO₂ cryosorption was obtained by increasing the Langmuir saturation constant by 20% over the value estimated by Crabb and Perona (1985) and by specifying an effective value for the mean pore radius of gas flow channels in the bed. The degree of agreement between theoretical and experimental results for N₂ cryosorption supports an earlier conclusion reached by Crabb et al., that the controlling diffusional resistance in N₂ cryosorption is in the bed interstices and micropores.