On the heat effect in measurements of sorption kinetics by the frequency response method

Abstract

The effect of heat transfer on the frequency response of sorption kinetics is theoretically investigated using a detailed model incorporating a single diffusion resistance, a surface barrier and heat transfer resistance at the adsorbent surface and at the chamber wall. The steady-state periodic solutions of this model are obtained analytically. Furthermore, a moment analysis is undertaken to establish the time constants of the different transfer mechanisms, which can be quantitatively compared so as to identify the dominant mechanism(s). It is shown that frequency response data can be considerably altered by the heat effect and in certain situations the response assumes a bimodal form. Under the isothermal assumption, this second peak can be erroneously attributed to an additional mass transfer process. Moreover, it is demonstrated that the frequency response of pressure and temperature are independent of variation in the applied volume amplitude. It is also confirmed by the present work that the frequency response is very sensitive to the nature of the transfer mechanisms, making it theoretically feasible to discriminate between surface barrier control and intraparticle diffusion.