Effect of material surface on the formation and dissociation of gas hydrate in restricted space between two parallel substrates

Abstract

Visual observations of the formation and dissociation of gas hydrate are conducted when reacting CH4-C2H6 gas with a water droplet in restricted space between a silicon wafer and quartz crystal surface, to study the surface effect of materials on hydrate formation and decomposition. It is found that hydrate first appears at the gas–water-quartz triple-phase contact line at 2.95 MPa and 2.2 °C and then grows preferentially around the quartz surface which is more hydrophilic, while hydrate dissociates earlier at the silicon surface which is less hydrophilic either by heating (beginning at 2.90 MPa and 9.2 °C) or depressurization (beginning at 1.03 MPa and −0.4 °C). A three-dimensional model, based on the classical nucleation theory, is established to understand the nucleation mechanism of gas hydrate at a triple-phase contact line. The results obtained indicate that triple-phase contact line is with higher driving force for hydrate nucleation and the hydrate nucleation at the triple-phase contact line on quartz has a lower free energy barrier and smaller critical size of nucleus compared with that on silicon wafer. Furthermore, molecular dynamics simulation is applied to compare the effects of two surfaces with different wettability on methane hydrate dissociation. It is found that methane bubbles will form on the less hydrophilic surface upon hydrate dissociating, which promotes the decomposition of the nearby hydrate. According to the results discussed above, the property of material surface can play a significant role on hydrate formation and dissociation, e.g., hydrate forming preferentially at a more hydrophilic surface while dissociating earlier at a more hydrophobic surface in restricted space.