Phase equilibrium of semiclathrate hydrates of methane in aqueous solutions of tetra-n-butyl ammonium bromide (TBAB) and TBAB–NaCl

• Phase equilibria of semiclathrate hydrate of methane in TBAB are investigated.
• This study clarifies the discrepancy of published data in the literature and their reliability.
• Two distinct equilibrium points for semiclathrate hydrates within pure hydrate stability zone are emphasized.
• One closely corresponds to the pure methane hydrate phase stability curve.
• Phase equilibria of semiclathrate hydrate of methane in TBAB + NaCl are investigated.

Phase equilibria of semiclathrate hydrates are important for their successful engineering applications due to more favorable process conditions compared to classical gas hydrate systems. Though sufficient information on the phase equilibria of semiclathrate hydrates of methane (CH4) in tetra-n-butyl ammonium bromide (TBAB) seems to be available, there are pronounced disagreements on the phase equilibrium data, particularly for 0.05 and 0.20 mass fraction (w) of TBAB. In this work, experimental studies are carried out to generate the equilibrium pressure (P) and temperature (T) for hydrates and semiclathrate hydrates of CH4 in an aqueous solution containing wTBAB = 0.05 and 0.20 at P and T range of 1.02–13.73 MPa and 281.63–294.54 K, respectively. This study tries to clarify the discrepancy of published data in the literature and their reliability. Additionally, we present interesting insights into the phase behavior of semiclathrate hydrate of methane in TBAB based on the formation and dissociation curves observed in the experiments. It is observed that there existed two equilibrium points during the dissociation of semiclathrate hydrates of methane in TBAB; one closely corresponds to the pure methane hydrate phase stability curve and the second one to the semiclathrate hydrate system of methane. In addition phase equilibrium data is generated for the quaternary system of CH4 + TBAB + H2O + NaCl for wNaCl = 0.03 and 0.10 and wTBAB = 0.05 and 0.20 in an aqueous solution at a P and T range of 1.65–20.71 MPa and 281.19–296.38 K, respectively. This is not yet available in the open literature. It is observed that NaCl inhibits the semiclathrate hydrate formation of CH4 in TBAB for wNaCl = 0.03 and 0.10 in wTBAB = 0.20 in the system. However, a promotion effect is observed for wNaCl = 0.03 in wTBAB = 0.05. This study calls for more detailed investigations on the effect of salts on semiclathrate hydrate systems, which may find potential use in engineering applications.