Gas Hydrate Formation Kinetics in Semi-Batch Flow Reactor Equipped with Static Mixer

Abstract

Gas hydrate is an ice-like solid and a kind of inclusion compounds of which the cage-like structure formed by hydrogen-bonded water molecules can include various kinds of guest gas molecules. In general, gas hydrates are formed with “host” water and “guest” gas molecules under lower temperature and higher pressure conditions, but sometimes large differences in the hydrate formation conditions are observed among guest gases. In such cases, if gas hydrate is formed with such a gaseous mixture, it can be anticipated that the component of which the hydrate formation condition is milder (that is, higher temperature and lower pressure conditions relatively) could be enriched in the hydrate phase. Effective gas separation, or higher selectivity, can be achieved for gas mixtures with larger differences in the hydrate formation conditions. On the other hand, multi-component gas hydrates are formed under higher pressure and lower temperature conditions in which any component of gaseous mixture can change to hydrate. Several applications have been proposed in environmental and energy fields by using the inclusion abilities in the framework of gas hydrates; natural gas transport (Gudmundsson & Borrehaug, 1996), gas storage (Lee et al., 2005), and gas separation (Kang & Lee, 2000) and so on, and thus many investigations for gas hydrate formation, especially thermodynamics and gas hydrate formation kinetics, have been carried out in batch systems. The solid hydrate can be dissociated to recover a product gas. The selectivity and production rate are key factors in determining the performance of hydrate-based applications. Although the selectivity is limited by the thermodynamic equilibrium of the hydrate phase and the feed vapour phase (Nagata et al., 2009), the production rate is dependant on the hydrate formation rate and the system design. Gas hydrate-based applications would require an efficient formation or production process of gas hydrates, and the elucidation of the formation mechanism of gas hydrates. Gas hydrate formation is similar to crystallization from liquid mixture, and gas-liquid system changes to liquid-solid or gas-solid systems. In general, it is known gas hydrate forms on gas-liquid interface, and thus the gas-liquid interfacial area, the driving force, and kinetic constant can affect hydrate formation. Therefore, an efficient way to increase these factors is necessary for continuously forming gas hydrate solid in gas-liquid system. For example, several efficient processes to increase the interfacial area for gas hydrate formation have been demonstrated, including a spray (Fukumoto et al., 2001) or jet reactor (Szymcek et al., 2008; Warzinski et al., 2008), and a bubble column (Luo et al, 2007; Hashemi et al., 2009)