Optimization Of Ethylene/ethane BOG Reliquefaction Process

Ethylene production capacity represents the scale and level of petrochemical industry in a country. As Chinese economy skyrockets, the demand for ethylene is also on the increase. To address issues related to China's finite ethylene resource and large quantities of demand, alternative seaborne ethylene from foreign countries has been proposed. The volume of liquefied ethylene is about 490 times less in volume than that of its gaseous equivalent. The volume of liquefied ethane is about 435 times less in volume than that of its gaseous equivalent. Accordingly, the shipbuilding industry is experiencing a boom in the demand for LEG (Liquefied Ethylene/Ethane Gas) vessels. A reliquefaction plant is usually installed to provide sufficient cooling capacity to maintain the pressure and temperature within a safe range. The BOG handling system plays an important role not only because it is economical determinant, but also it is the key to resolving hazards and malfunctions of the cargo tank.In this paper, the current situation of low-temperature liquefied gas vessels and the present research of BOG reliquefaction technology are introduced. In view of stratospheric ozone depletion and the low exergy efficiency of the conventional BOG (boil-off gas) reliquefaction plant, a new configuration of ethylene/ethane BOG refrigeration cycle with alternative refrigerant is presented. Mathematic models for all the components are established. The models consist of compressor, heat exchanger, throttle valve and mixer. The P-R and SRK-hexamer are regarded as the calculation model of the vapor-liquid equilibrium for ethylene, ethane, and refrigerants, and the LKP equation is used to calculate the enthalpy and entropy of workingfluid, by those equations simulation calculations of processes are resolved. The exergy analysis models of the cycle components are introduced, and the behavior of BOG reliquefaction is investigated by the exergy method. The performance characteristics of the conventional and optimized cycle are investigated and compared based on the exergetic efficiency. The results show that under the calculation conditions, the exergy efficiency of the optimized process is 31.05%, and it is 13.4% higher than that of the conventional process which is 27.38%. Exergy analysis obtained here will provide theoretical basis and technical support for the design and manufacture of BOG reliquefaction process in LEG vessels.