Thermodynamic and kinetic effects in gas hydrate and semi-clathrate systems of CH4–C2H6–H2s gas mixtures with quaternary ammonium salt promoters

Gas hydrate formation is a promising approach for the separation of natural gas components due to differences in hydrate stability of individual gases. This study investigates the thermodynamic equilibrium, kinetics, and dissociation enthalpy of gas hydrates formed from a CH 4 (92.73 mol.%)–C 2 H 6 (5.46 mol.%)–H 2 S (1.80 mol.%) mixture in pure water and in aqueous solutions of quaternary ammonium salts tetra-n-butylammonium Fluoride (TBAF) and tetra-n-butylammonium nitrate (TBANO 3 ) at various concentrations. The novelty of the work lies in the simultaneous analysis of thermodynamic equilibria, formation kinetics, and energetic properties for the gas system, revealing the coexistence and sequential dissociation of structure I (sI) gas hydrates and quaternary ammonium salts-based semiclathrates. Experimental dissociation conditions, obtained using a rocking cell apparatus, demonstrate that TBAF acts as a strong thermodynamic promoter, significantly shifting equilibria to higher temperatures. In contrast, TBANO 3 exhibits a complex, concentration- and pressure-dependent behavior, which corrects its role as a simple promoter and is attributed to the formation of different semiclathrate structures (TBANO 3 ·26H 2 O and TBANO 3 ·32H 2 O). Kinetic parameters, induction times, and gas uptake reveal distinct mechanisms of promotion and inhibition. Dissociation enthalpies (ΔH d ) were determined for these complex systems. The values for salt-containing systems (e.g., 153–263 kJ·mol⁻¹ for TBAF) substantially exceed that for pure water (66 kJ·mol⁻¹), confirming the enhanced stability of the mixed hydrate/semiclathrate phases. The appearance of two inflection points on dissociation curves, coupled with distinct ΔH d values for these points, is evidenced by structures coexistence. These findings demonstrate the potential of special hydrate systems as functional materials for selective ethane separation from natural gas.