EXACT SOLUTION AND LINEAR DRIVING FORCE APPROXIMATION FOR CYCLIC MASS-TRANSFER IN A BIDISPERSE SORBENT

A purely periodic analytic solution for cyclic mass transfer in an adsorbent pellet with a bidisperse pore structure is obtained. The solution is used to define the effective rate coefficient, K, for the linear driving force (LDF) approximation for cyclic adsorption and desorption. The results obtained differ from those previously derived using an equivalent solid diffusion model for the pellet with a single diffusional time constant, approaching this limit only when macropore diffusion is dominant. In fact, for rapid cycling the mass transfer rate cannot be expressed in the terms of a simple linear combination of the diffusional time constants for the bidisperse pellet independent of cycle time. Thus, in this case, the exact solution must be used to arrive at an appropriate definition of the K-values. With these K-values the LDF approximation is found to provide a reasonable representation of mass transfer rates in cyclic adsorption, which can be used for the simulation of sorptive separations.