Methane-ethane and methane-propane hydrate formation and decomposition on water droplets

Gas hydrate formation and decomposition on water droplets using an 89.4% methane—10.6% ethane mixture, and a 90.1% methane—9.9% propane mixture were carried out in a new apparatus suitable for morphology studies. As expected the induction time was found to be much shorter when the water had hydrate memory. All droplets nucleated simultaneously and the droplet size and shape had no noticeable effect on induction time and macroscopic crystal growth morphology for hydrates from the methane-ethane mixture. However, the surface of the hydrate crystals from methane-propane had a “hairy-like” appearance which changed to a smooth surface over time. Moreover, the smaller droplets during hydrate reformation showed an extensive hydrate growth and looked like snow-flakes. Sequential pictures generated by time-lapse videos showed that the time required for hydrate to cover the water droplet surface ranged from 10 to 23 s and was shorter when there was gas-phase agitation (mixing). The growth is postulated to occur in two stages. The first stage lasts about 10-23 s and growth takes place laterally. Growth takes place at the hydrate/gas and the hydrate/water interfaces during the second stage. The implication of the findings for process design of hydrate formation vessels is also discussed.