Potential Theory of Adsorption for Associating Mixtures: Possibilities and Limitations

Abstract

The applicability of the multicomponent potential theory of adsorption (MPTA) for the prediction of the adsorption equilibria of several associating binary mixtures on different industrial adsorbents was investigated. In the MPTA, the adsorbates are considered to be distributed fluids subject to an external potential field emitted by the adsorbent. In this work, the theory was extended to include the cubic-plus-association (CPA) equation of state (EoS) for the description of the fluid–fluid interactions of associating mixtures. The Dubinin–Radushkevich–Astakhov (DRA) potential function was utilized to describe the solid–fluid interactions. The potential was extended to include adsorbate–absorbent-specific capacities rather than an adsorbent-specific capacity. Correlations of pure-component isotherms were generally found to be excellent with individual capacities, although adsorption on silicas at different temperatures still poses a challenge. The quality of the correlations was usually found to be independent of the applied EoS. Predictions for binary mixtures indicate that MPTA + SRK is superior when adsorption occurs on nonpolar or slightly polar adsorbents, whereas MPTA + CPA performs better for polar adsorbents or when the binary mixtures contain only associating compounds. Predictions were typically improved by about 3% when individual capacities were employed, but improvements in some cases were as large as 45%. When individual capacities and the best-performing EoS were used, average absolute deviations of the selectivity were as low as 7–12%. Predictions of the selectivity were generally found to be superior to predictions of the adsorbed amounts. The sensitivity of the model was also tested, and it was concluded that the predictions are very sensitive to the adsorption energies.