The simulation of multicomponent ion exchange in fixed-bed columns

A mathematical model based on the Nernst-Planck equations has been developed to describe multicomponent ion exchange process. Digital computer programs have been written in FORTRAN IV to solve the equations of the model with the following features: - mass transfer coefficients are determined from fundamental properties of exchanging ions and ion exchange resins - allowing three exchanging ions for combined mass transfer resistance or any number for a single phase resistance - constant or variable self-diffusion coefficients - constant or variable separation factors - allowing partial column saturation or arbitrary column initial concentration profiles. To verify the equations of the model a comparison between experimental and predicted breakthrough curves has been made. This involved obtaining independent equilibrium data and measuring column effluent concentration histories. Equilibrium studies of K-H, Na-H and K-Na-H systems with Dowex 50W X8 at solution concentration of 0.1 N showed that variable separation factors must be used in the breakthrough curve predictions. Effluent concentrations and temperature from a 5 cm ID and 200 cm long fixed-bed ion exchange column, measured by ion selective electrodes and a LXS700 temperature sensor respectively, were recorded by a data logging system consisting of a Motorola MEK 6800D2 evaluation kit, an Orion 701A ionalyzer, a teletype and a 10-bit analog to digital converter. A comparison of computed results with experimental data has shown that the model developed can be used to accurately predict the breakthrough curve behaviour from a multicomponent fixed-bed ion exchange column