Further Molecular Insights into Formation of Tetra-n-Butyl Bromide Semi-Clathrate Hydrates

Abstract

In this research, melting temperature and the growth process of tetra-n-butyl phosphonium bromide (TBPB) and tetra-n-butyl ammonium bromide (TBAB) hydrates with memory effect phenomenon at the microscopic level are investigated. All the simulation runs are carried out using optimized potentials for liquid simulations-all-atom (OPLS-AA) force field under NPT conditions (a constant number of atoms, pressure, and temperature). In addition, at a constant pressure of 1 MPa, the effectiveness of different thermostat algorithms (including Berendsen, Nose−Hoover, and velocity-rescale) and effect of temperature (250−350 K) are assessed in the formation of TBPB and TBAB semiclathrate hydrates. The melting temperatures of 280 and 283 K (at P = 0.1 MPa) correspond to the TBPB and TBAB semiclathrate hydrates, respectively, by following the peaks in the Z-density profile. Instantaneous temperature and potential energy results during simulation runs indicate that the Berendsen thermostat is very efficient for relaxing a system to the target temperature and damping the temperature oscillations. A reasonable match is observed between the results of the current research and previous studies available in the literature. According to the simulation results, growth of TBAB and TBPB semiclathrate hydrates is observed at 250 K over the simulation runs, and most of the molecules form a semiclathrate gas hydrate structure. At a higher temperature, the balance of hydrogen bond between water molecules in the semiclathrate hydrate network becomes weak. Thus, the TBPB and TBAB hydrates are completely decomposed. This research offers promising details about the growth of semiclathrate hydrates through the memory effect phenomenon, which can be used for better management of gas hydrate production operations.