Rigorous modelling and multi-objective optimization for hybrid absorption-adsorption separation process of CH₄/C₂H₆ using ZIF-8/iso-hexadecane slurry

Efficient separation of CH₄ and C₂H₆ remains a critical challenge in the recovery of light hydrocarbons from natural gas. So we developed a zeolite imidazolate framework-8 (ZIF-8)/iso-hexadecane slurry for CH₄/C₂H₆ separation, and conducted solubility measurements for pure components and equilibrium separation experiments for gas mixtures. A rigorous absorption-adsorption coupled phase equilibrium model and a single equilibrium stage mathematical model were established to describe the phase equilibrium performance of gas-slurry system. The results indicated that the model exhibits high predictive accuracy for the solubility of CH₄ and C₂H₆ in the slurry, with an average relative error of less than 3.053%. Furthermore, the whole process of absorption-adsorption-desorption was designed, and the mathematical model and multi-objective optimization framework were established. The effects of operating pressure and gas-slurry ratio on product purity, recovery ratio, and total unit energy consumption were investigated. The results showed that the C₂H₆ purity in the feed gas is enriched from 22 mol/mol to 94.69 mol/mol, the C₂H₆ recovery ratio reaches 95.21%, and the total unit energy consumption is 0.4558 kW·h·Nm⁻³ (feed gas). In addition, the effects of pressure, temperature, and ZIF-8 solid content on slurry separation performance were analyzed using the established equilibrium-stage model. The results showed that the introduction of ZIF-8 enhances the separation performance of the iso-hexadecane solvent for mixed gases. Compared to the pure solvent, the separation factor increased by an average of 72.5%, which theoretically proved the advantages of slurry separation process.