Effects of cyclopentane on CO₂ hydrate formation and dissociation as a co-guest molecule for desalination

Highlights: • CP decreases CO₂ hydrate phase equilibrium pressure by forming CO₂-CP hydrates. • The increase of CP can't decrease hydrates phase equilibrium pressure unlimitedly. • Higher CP concentration lowers CO₂ hydrate gas uptake. • The optimal CP molar ratio is 0.01 based on hydrate phase equilibrium and gas uptake. - Abstract: Cyclopentane (CP) is considered to be a potential co-guest molecule in carbon dioxide (CO₂) hydrate-based desalination. The experimental thermodynamic data of CO₂-CP hydrates were measured for a salt solution, where CP was chosen as a hydrate promoter. Seven experimental cases (62 cycles) were studied with different molar ratios of CP/water (0, 0.0025, 0.005, 0.0075, 0.01, 0.02, and 0.03). Hydrate phase equilibrium data were generated using an isochoric method, and the hydrate saturations were calculated based on gas uptake. The results indicated that the increase in CP concentration significantly decreased the CO₂ hydrate equilibrium pressure to a certain limit; the hydrate saturation also decreased during this process. Also, it was determined that CP encouraged the formation of s-II double CO₂-CP hydrates, which are different from s-I simple CO₂ hydrate. The CO₂-CP guest provides a strengthened stability and moderate hydrate phase equilibrium conditions for hydrate-based desalination. The recommended optimal molar ratio of CP is 0.01 when the increase in equilibrium was more than 10 K, and the decrease in hydrate saturation was less than 2%.