The simultaneous adsorption of carbon dioxide and water vapour by fixed beds of molecular sieves

Abstract

In an isothermal fixed bed absorption system for constant inlet flowrate of inert gas and concentrations of two adsorbates the more strongly adsorbed component will displace the weaker one and effect its breakthrough characteristics. A small scale technique was used to study this phenomenon for the adsorption of water vapour and carbon dioxide on 4A type synthetic zeolites with helium as the carrier gas. Single adsorbate equilibria and rate data determined with the binary mixtures were then used to calculate the results for the ternary one. Partial differential equations were derived to describe the mass transfer of each adsorbate based on pore diffusion in the adsorbent as a major rate controlling mechanism. With a simplified binary Langmuir equilibrium to describe the effect of water on the adsorption of carbon dioxide, and assuming that the effective pore diffusion coefficient of one adsorbate was not affected by the presence of the other, accurate predictions of the concentration transient were obtained. Four zones are formed in the bed with the ternary mixture resulting from the desorption of carbon dioxide by water, the concentrations of the former rising above the inlet value. An enhanced rate of adsorbed phase transport of water was found, additional to that expected from macropore diffusion, suggesting that surface or capillary flow of the adsorbate occurred.