PORE AND SURFACE-DIFFUSION IN MULTICOMPONENT ADSORPTION AND LIQUID-CHROMATOGRAPHY SYSTEMS

A generalized parallel pore and surface diffusion model for multicompo nent adsorption and liquid chromatography is formulated and solved num erically. Analytical solution for first- and second-order central mome nts for a pulse on a plateau input is used as benchmarks for the numer ical solutions. Theoretical predictions are compared with experimental data for two systems: ion-exchange of strontium, sodium, and calcium in a zeolite and competitive adsorption of two organics on activated c arbon. In a linear isotherm region of single-component systems, both s urface and pore diffusion cause symmetric spreading in breakthrough cu rves. In a highly nonlinear isotherm region, however, surface diffusio n causes pronounced tailing in breakthrough curves; the larger the ste p change in concentration, the more pronounced tailing, in contrast to relatively symmetric breakthroughs due to pore diffusion. If only a s ingle diffusion mechanism is assumed in analyzing the data of parallel diffusion systems, a concentration-dependent apparent surface diffusi vity or pore diffusivity results; for a convex isotherm, the apparent surface diffusivity increases, whereas the apparent pore diffusivity d ecreases with increasing concentration. For a multicomponent nonlinear system, elution order can change if pore diffusion dominates for a lo w-affinity solute, whereas surface diffusion dominates for a high-affi nity solute.