Computer simulations of the dynamics of multicomponent ion exchange and adsorption in fixed beds—gradient-directed moving finite element method

Abstract

In most fixed-bed separation processes, interference (competition of solutes for sorbent sites) coupled with mass transfer effects results in complex concentration waves. A general rate equation model was developed to simulate the column dynamics of multicomponent adsorption and ion exchange. This model takes into account axial dispersion, film and intraparticle diffusion, size exclusion and interference effects for systems with nonlinear isotherms or variable separation factors. The model equations were solved with a new method of orthogonal collocation on gradient-directed moving finite elements. Both the boundaries and the number of finite elements were adjusted to focus the collocation points in the region of steep concentration gradients. The computation time for simulating a step change was reduced to about 20% of that of orthogonal collocation on fixed finite elements. Column dynamics of step input and elution processes were simulated with the rate equation model. The predicted effluent histories are shown to be in close agreement with the data on protein elution. This model is important for studying interference phenomena coupled with mass transfer effects, especially for systems with a large number of components, small feed pulses, small interparticle diffusivities and large sorbent particles.