Optimization and comparison of boiling-off gas re-liquefaction processes for liquid ethylene vessels

Liquid Ethylene Gas (LEG), which is stored at - 103.7 °C and 0.1 MPa, is usually transported by vessels with a vacuum insulated tank over oceans and lakes, or along rivers. A part of LEG would be vaporized into Boil-Off Gas (BOG) due to the inevitable heat leakage. To guarantee the vessels safety, an on board re-liquefaction plant is widely equipped to condense the BOG and return it to the tank. Improving the energy efficiency of the re-liquefaction system has attracted more and more attention. In this paper, three potential BOG re-liquefaction processes have been analyzed based on a simulation model of Aspen HYSYS V7.2. The effects of the ethylene storage pressure, propane condensing temperature, propane evaporating pressure, intermediate pressures of refrigeration and liquefaction cycles on the system performances were analyzed. Employing the optimizer of Aspen HYSYS, the optimal design conditions of the researched processes were determined. A cascade process which is composed of two subsystems, a two-stage compression refrigeration cycle using propane as working fluid and another two-stage compression ethylene liquefaction cycle, was proved to be superior to other processes. This work could provide some reference for choosing the BOG re-liquefaction processes for LEG vessels.