Thermodynamic analysis of extraction processes for the utilization of LNG cold energy

Thermodynamic analysis of the extraction processes from a constant pressure LNG (liquefied natural gas) vessel was performed in this study. LNG was assumed to be a binary mixture of 90% methane and 10% ethane by mole fraction (83:17 by mass fraction). The changes in the thermodynamic properties and the amount of utilizable cold energy were predicted during the extraction processes. Both vapour and liquid extraction processes were investigated using a computer model. During vapour extraction, the temperature of the LNG in the vessel increases dramatically, and the extracted vapour composition of methane decreases rapidly near the end of the extraction process. Utilizable cold energy has a maximum at a residual mass ratio of about 0.2. It was found that the temperature gradient due to the vapour composition change had a major effect on the behaviour of the cold energy during the vapour extraction process at a constant pressure. During the liquid extraction process, the changes in the thermodynamic properties and utilizable cold energy are negligible. When the pressure of the vessel increases, the total cold energy which can be utilized from LNG decreases.