Application of ZLC technique for a comprehensive study of hydrocarbons’ kinetics in carbon molecular sieves and zeolites

Abstract

The mass transfer of hydrocarbon gases was studied in a commercial CMS and zeolite 5A (as a reference adsorbent). The zero length column (ZLC) technique was applied to determine the mass transfer resistances to gas sorbates. To confirm the ZLC results, a comprehensive study involving uptake and kinetic behavior of the constituent gas components was also carried out using the gravimetric technique. The comparison between ZLC and gravimetric results established the strong dominance of surface barrier for both adsorption and desorption of sorbates in CMS. As a result of surface barrier resistance in the CMS, the adsorption kinetics is extremely slow. The ZLC method required a considerably shorter measurement time compared to the gravimetric method, e.g., less than 14% of the time required for the gravimetric method, which proved the advantage of the ZLC as a fast screening technique to rank a large number of adsorbents particularly those with large surface barrier effects. To apply the ZLC technique for measuring the kinetics in CMS adsorbent, surface resistance model and modified form of ZLC model (combined surface and diffusion resistances model) were applied. Since the surface resistance is the main barrier to the sorbate molecules diffusing out from the CMS pores, the application of combined model resulted in a considerable inaccuracy in the kinetic parameter’s estimations. This was primarily due to the existence of low experimental value of the L' criterion, representing the ratio of diffusion to surface resistance, e.g., <0.33, thus contradicting the model criteria required by the ZLC method, i.e., L′ > 0.33. Consequently, only surface barrier was considered using the ZLC model for the CMS kinetic parameter estimates. According to the experimental results, the combined model is suitable for ZLC analysis of adsorbents with minor to moderate surface resistance. In these adsorbents, the increase in L', e.g., higher than 0.33, indicates greater contribution of diffusion resistance compared to surface barrier. As a result, the zeolite 5A, which has a minor surface resistance effect (L' ≫ 0.33), was applied to verify the consistency of the combined model in predicting the kinetics of hydrocarbon gases.