Kinetics of sII and Mixed sI/sII, Gas Hydrate Growth for a Methane/Propane Mixture Using Neutron Diffraction

Gas hydrates are solid solutions formed from water and enclathrated gas molecules. The hydrate types sI and sII are formed from pure methane or propane, while mixtures of these gases can form either type. In this study, eight experiments are presented of gas hydrates grown under semibatch conditions using D2O ice powder and a feed gas of methane (90%)–propane (10%). In situ neutron scattering was used to measure the phase fractions of sI and sII hydrates, ice, and liquid–water components during hydrate growth and dissociation. The composition of the free gas and the methane/propane cage occupancies were derived from the observed phase fractions using mass-balance equations and Gibbs energy calculations. In all cases, the sII hydrate was formed before, or concurrent with, the sI phase. After an initial period of hydrate growth from ice, the temperature was increased to release liquid water by melting the ice particles. Different reaction mechanisms were observed; two occurred while the ice was melting and were heat-transfer limited, and two occurred while the water was completely frozen or liquid and were mass-transfer limited. It is shown that the constantly changing gaseous-propane concentration is an important factor affecting sI growth and the stoichiometry and heterogeneity of the sII hydrates. It was observed that sI can grow more rapidly than sII during ice melting, despite the latter having a much higher driving force. A similar effect can be seen in other mixed hydrate studies using stirred batch reactors; the similarities and differences to our own results are discussed. Complex equilibration processes occur involving the homogenization and stoichiometric change of the sII hydrates. For mixed sI/sII hydrates, the equilibration also involves an interconversion between sI and sII. Within the energy sector, gas hydrates are of great importance, whether it is due to the safety and economic risks of gas-pipeline blockages or the possible benefits of gas hydrates as an energy resource, a method of carbon dioxide sequestration, or as a means for gas transport. In each case, this study offers important new information on the kinetics of gas hydrate growth and highlights the complications that must be considered when dealing with mixed gas and mixed hydrate types