Frequency response for nonisothermal adsorption in biporous pellets

Abstract

The frequency response for nonisothermal adsorption in a biporous pellet is analyzed theoretically, using a mathematical model which includes heat and mass transfer resistances in both micropores and macropores. It is confirmed that, when the heat effect is involved, the out-of-phase component may exhibit a bimodal form. Moreover, it is shown that when both macropore diffusion and micropore diffusion resistances are comparable, macropore diffusion behaves like a surface barrier and leads to an intersection of the in-phase and out-of-phase response functions. When either micropore diffusion or macropore diffusion alone is dominant, the frequency response is essentially the same and, therefore, provides no information concerning the nature of the controlling diffusional resistance. Experimental data for light linear paraffins-5A, reported by Yasuda (1991, J. phys. Chem. 95, 2486–2492), are reanalyzed by the present nonisothermal model. It turns out that the reported experimental response can be equally well represented by a nonisothermal model using several different combinations of mass transfer resistances. It, therefore, appears that the bimodal behavior of the experimental out-of-phase data is caused by the heat effect, thus contradicting the conclusion of Yasuda that there are two adspecies with different mobilities. With the reported data, it is, however, not possible to extract reliable values for the intracrystalline diffusion coefficient, and the nature of the controlling mass transfer resistances cannot be established with certainty.