Phase equilibrium and morphology characteristics of hydrates formed by tetra-n-butyl ammonium chloride and tetra-n-butyl phosphonium chloride with and without CO2

• Phase equilibrium conditions of TBAC and TBPC semiclathrate hydrates are measured.
• Phase equilibrium conditions of CO2 + TBAC and CO2 + TBPC double hydrates are measured.
• Morphologies of TBAC and TBPC hydrate crystals with and without CO2 are observed.

In this study, experimental investigations on hydrate equilibrium conditions of hydrates formed by tetra-n-butyl ammonium chloride (TBAC: C16H36ClN) and tetra-n-butyl phosphonium chloride (TBPC: C16H36ClP) with and without CO2 are carried out using isochoric experimental method at mass fractions w = 0.05, 0.10, 0.20, 0.35 and 0.50. The equilibrium data of CO2 + TBAC and CO2 + TBPC hydrates are measured at pressures of 0.6 to 4.1 MPa and at temperatures from 279.0 to 292.0 K. Meanwhile, the equilibrium data of TBAC and TBPC hydrates without CO2 are measured at atmospheric pressure and at temperatures from 279.0 to 286.0 K. Moreover, the visualization is conducted to characterize the morphologies of hydrate crystals forming at subcoolings of 4.0 to 6.0 K, and the results are discussed in comparison with those of tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl phosphonium bromide (TBPB) hydrate crystals forming with and without CO2 presented in our previous papers.