Deqing Liang

Molecular dynamics simulations for the growth of CH4-CO2 mixed hydrate

Molecular dynamics simulations are performed to study the growth mechanism of CH4-CO2 mixed hydrate in xCO2 = 75%, xCO2 = 50%, and xCO2 = 25% systems at T = 250 K, 255 K and 260 K, respectively. Our simulation results show that the growth rate of CH4-CO2 mixed hydrate increases as the CO2 concentration in the initial solution phase increases and the temperature decreases. Via hydrate formation, the composition of CO2 in hydrate phase is higher than that in initial solution phase and the encaging capacity of CO2 in hydrates increases with the decrease in temperature. By analysis of the cage occupancy ratio of CH4 molecules and CO2 molecules in large cages to small cages, we find that CO2 molecules are preferably encaged into the large cages of the hydrate crystal as compared with CH4 molecules. Interestingly, CH4 molecules and CO2 molecules frequently replace with each other in some particular cage sites adjacent to hydrate/solution interface during the crystal growth process. These two species of guest molecules eventually act to stabilize the newly formed hydrates, with CO2 molecules occupying large cages and CH4 molecules occupying small cages in hydrate.

Molecular dynamics simulations of carbon dioxide hydrate growth in electrolyte solutions of NaCl and MgCl2

Molecular dynamics simulations are performed to study the growth of carbon dioxide (CO2) hydrate in electrolyte solutions of NaCl and MgCl2. The kinetic behaviour of the hydrate growth is examined in terms of cage content, density profile, and mobility of ions and water molecules, and how these properties are influenced by added NaCl and MgCl2. Our simulation results show that both NaCl and MgCl2 inhibit the CO2 hydrate growth. With a same mole concentration or ion density, MgCl2 exhibits stronger inhibition on the growth of CO2 hydrate than NaCl does. The growth rate of the CO2 hydrate in NaCl and MgCl2 solutions decreases slightly with increasing pressure. During the simulations, the Na+, Mg2+, and Cl− ions are mostly excluded by the growing interface front. We find that these ions decrease the mobility of their surrounding water molecules, and thus reduce the opportunity for these water molecules to form cage-like clusters toward hydrate formation. We also note that during the growth processes, several 51263 cages appear at the hydrate/solution interface, although they are finally transformed to tetrakaidecahedral (51262) cages. Structural defects consisting of one water molecule trapped in a cage with its hydrogen atoms being attracted by two Cl− ions have also been observed.