Dynamics study of a closed diffusion cell by using a frequency response method: single resonator

Abstract

The frequency response (FR) of a closed diffusion cell with one resonator is analysed theoretically using non-isothermal Fickian diffusion models for both homogeneous and bi-porous particles. It is shown that with this diffusion cell configuration, the pressure response in the modulating reservoir has many significant characteristics: a larger phase difference with respect to the volume perturbation, a lower resonating frequency and most importantly a much higher sensitivity to the nature of diffusion mechanisms when dealing with biporous pellets. This high sensitivity makes it practically feasible to determine which diffusional resistance is rate-limiting without experimenting on pellets of different sizes. The pressure response of the second reservoir exhibits a large phase delay with respect to the volume modulation and also a high sensitivity to the diffusion mechanism. Finally, the pressure difference between the two reservoirs has a significantly larger phase angle than the pressure response in a batch adsorber FR system. More importantly, it is practically independent of the heat effect and has two distinct zones (only affected by macropore diffusion at lower frequencies) making it an ideal monitored response for the study of mass transfer.