In this research thermodynamic approach was used to evaluate the possibility of gas hydrate formation. First considering the output gas composition from three of the Isfahan Province City Gas Station and equilibrium hydrate formation diagram was plotted using HydraFlash software and possibility of gas hydrate formation in Kashan, Semirom, and Baharestan City Gas Stations was studied. Gas hydrate formation is possible in output gas from Baharestan City Gas Station and in a higher degree in output gas from Semirom and Kashan City Gas Station in case of temperature drop in cold seasons of the year. Hydrate dissociation in pipelines was simulated using computational fluid dynamic and using hydrate dissociation kinetic model. Heat transfer between hydrate and gas flow and pipe wall and simultaneous effect of heat transfer and mass transfer was performed by Fluent Software. Output gas composition and temperature, pressure and real geometry of the City Gas Station was used in this simulation. The simulation of dissociation of gas hydrates in a 6-inch pipe with length of 0.5 meters was performed. In order to investigate shape effect of formed gas hydrates balls with 0.1, 0.2, 0.3, 0.4, 0.5, 1, and 1.5 cm radius and ring hydrates connected pipe wall with a volume equal with hydrate balls with 0.5, 1, and 1.5 cm radius were considered. Output gas temperature from city gas stations is 283.15 K which in this research in order to investigate the effect of temperature drop on gas hydrate dissociation simulations were performed at 3 different temperatures of 275.15, 281.15, and 283.15 K. It was concluded that gas hydrate dissociation rate in case of formed hydrate on pipe wall is more than that of gas hydrate in the shape of sphere particles due to larger surface for heat and mass transfer. The hydrate dissociation rate decreases due to temperature drop and so hydrate dissociation time increases. The maximum of hydrate dissociation time for hydrate ball with 1.5 cm radius and the minimum of hydrate dissociation time for hydrate ball with 0.1 cm radius were respectively obtained equal to 1567.7 and 86.2 seconds. It can be concluded that hydrate dissociation occurs isothermal conditions and dissociation is controlled by mass transfer. Key Words : Gas hydrate, city gas station, hydrate formation, hydrate dissociation, kinetic model
