Systematic kinetic studies on mixed gas hydrates by Raman spectroscopy and powder X-ray diffraction

This study presents results from systematic time-resolved experiments regarding the guest molecule geometry. The in situ observations of formation and dissociation processes of multicomponent hydrates were performed by means of Raman spectroscopy and a newly designed experimental setup including powder X-ray diffraction (PXRD) whose capabilities will be presented here in more detail. Both experimental setups allow investigating hydrate kinetics as a function of pressure, temperature, and feed gas composition. The unique feature of both setups is the continuous gas flow providing a constant composition of the gas phase during the whole experiment. This is crucial for the formation of mixed hydrates formed from feed gas mixtures that contain one or more components in low concentrations. The formation of structure II hydrates including C 3H 8, iso-C 4H 10, n-C 4H 10, or neo-C 5H 12 besides CH 4 was analysed according to a multi-step model. For the initial phase it turned out that hydrates grown from the gas mixture containing 2% n-C 4H 10 and 98% CH 4 have the highest formation rate at defined p, T conditions in comparison to other hydrates formed from gas mixtures containing about 2 vol% of the above mentioned hydrocarbons besides CH 4. But the reaction mechanisms for each hydrate system emerged to be different. Furthermore, Raman and time-resolved PXRD experiments were performed to study the formation of structure H hydrates with a low-concentrated large hydrocarbon guest molecule. In case of a gas mixture containing 1% iso-C 5H 12 and 99% CH 4 the formation of a simple structure I CH 4 hydrate was observed at first. Later on, structure H CH 4 + iso-C 5H 12 hydrate was formed resulting in a coexistence of both structures.