Separation of CH4/N2 Using ZIF-8 Porous Slurry: Phase Equilibrium Experiments and Hybrid Absorption–Adsorption Model

Abstract CH4/N2 separation is essential for upgrading low-concentration natural gas but remains challenging because of their similar molecular sizes and low polarity. Here, we develop a rigorous hybrid absorption–adsorption phase equilibrium model for ZIF-8 slurries in iso-hexadecane (C16), methylcyclohexane (MCH), and ethylene glycol/water (EG/H2O). Solvent absorption was described using the Peng–Robinson equation of state or the UNIFAC model, while ZIF-8 adsorption used Langmuir isotherms with a component-specific osmotic pressure correction for the interfacial permeation barrier in the polar slurry. The model predictions were in good agreement with the experimental data, with mean relative errors below 10%. At 283.15 K and 2.0 MPa, the slurries increased CH4 uptake by 81–208% and CH4/N2 separation factors from 0.71–2.05 to 2.30–2.82. The differential barrier enabled the polar slurry to surpass pristine ZIF-8 selectivity at low pressure, providing a thermodynamic basis for multistage and full process modeling of continuous CH4/N2 upgrading with ZIF-8 slurries.