Evaluation of excess and absolute equilibrium adsorption isotherms using the Ono-Kondo model: Effects of uncertainty propagation on calculated results

Adsorption uses porous solids (such as zeolites or activated carbon) to selectively capture CO₂. Equilibrium adsorption isotherms are crucial for assessing how well these materials remove CO₂ from gas mixtures such as natural gas. In this work, the excess adsorption capacity of zeolite Y pellets for CO₂ and CH₄ was determined using a gravimetric analyzer at 20°C and pressures ranging from 0 to 47 bar. To accurately model adsorption and thermodynamic analysis, absolute adsorption isotherms are required. Excess isotherms can be converted into absolute isotherms if the adsorbate density is known. The adsorbate density cannot be measured directly, but it can be computed using models such as the Ono-Kondo (OK). Thus, experimental excess isotherm data were used to adjust the OK model parameters, which were then used to convert excess adsorption into absolute adsorption. When carrying out such a calculation, the uncertainties associated with experimental data and model parameters must be propagated to the computed property, which was done using a Monte Carlo Simulation (MCS)-based probabilistic technique. A total of 10,000 MCS trials were performed, and the standard uncertainty of the absolute adsorption was obtained from the empirical probability distribution. For CH₄, the excess and absolute adsorption results were within data uncertainties, indicating that not all adsorbent sites were occupied by the adsorbate and that the two properties can be considered equivalent. For CO₂, given the calculated uncertainties, the absolute adsorption at higher pressures exceeded the excess adsorption, indicating saturation of the adsorbent sites and trapping of part of the adsorbate within the pores. Therefore, excess and absolute adsorption cannot be considered equivalent.