Capillary condensation: Limitations of the multicomponent potential theory of adsorption (MPTA)

Molecular simulation and classical density functional theory (DFT) are known to reproduce the main effects of confinement on transport and phase equilibrium properties of fluids adsorbed in micro and mesopores. Never-theless, their complex mathematical formulations and high computational power demanded by the calculations are usually considered barriers for their extensive use in industrial applications. The multicomponent potential theory of adsorption (MPTA) has emerged as an important alternative to overcome these obstacles, capable of matching single and mixture adsorption data with relatively good accuracy. In this work, we evaluate the prediction of adsorption isotherms of selected single components - carbon dioxide, methane, and propane - on carbon slit pores, employing an alternative version of MPTA, coupled with a global phase stability test. The adsorption is framed as a phase equilibrium problem between bulk and confined phases, for which the total volume, the total amount, and the temperature are fixed and known, beforehand. A Helmholtz energy-based minimization method is proposed to compute the local density profiles and the adsorbed amount. The Peng- Robinson equation of state is employed to account for the interactions among the fluid molecules. Interactions between the fluid and the adsorbent walls are modeled with the Steele 10-4-3 and the Dubinin-Radushkevich-Astakhov adsorption potentials. While the proposed methodology could accurately correlate experimental data at temperatures above the critical temperature, meaningful deviations are observed when subcritical conditions are considered. Generally, the MPTA coupled with the Steele potential fails to reproduce the qualitative trend of the experimental isotherms, whereas the DRA potential provides good agreement, for some of the systems evaluated. For systems that exhibit capillary condensation, the model is incapable of reproducing the sudden jump in the density of the adsorbed phase. By applying a modified version of the Peng-Robinson equation of state extended to fluids confined in slit pores, we show that, under such conditions, a more rigorous approach is needed, possibly within a statistical mechanical framework.

Automated summary

In this work, the adsorption isotherms of carbon dioxide, methane, and propane on carbon slit pores are predicted using an alternative version of MPTA coupled with a global phase stability test. The results show that while the proposed methodology accurately correlates experimental data at temperatures above the critical temperature, significant deviations are observed when subcritical conditions are considered. Generally, the MPTA coupled with the Steele potential fails to reproduce the qualitative trend of the experimental isotherms, whereas the DRA potential provides good agreement, for some of the systems evaluated.