Translated Abstract
In recent years, the demand for ethylene has risen sharply in our country. A large amount of ethylene is imported to make up the difference between supply and demand. Meanwhile, it is necessary to decrease the ethylene raw material consumption by technical innovation of chemical industry. Liquid ethylene vessel is an important tool for long-distance importation of ethylene. With the growth of ethylene trade, developing liquid ethylene vessel has attracted intensive concern in the field of chemical raw materials commercial transport. The tank pressure is built up due to the boiling off the liquid ethylene in the long-distance transportation. The boil off gas (BOG) must be processed properly to stabilize the safe pressure of the liquid tank. The BOG will be exhausted into the environment directly in conventional process method, which causes not only huge economic losses, but also seriously pollution of the environment. Therefore, it is necessary to develop a more efficient and reasonable process to solve this problem.
The author summarizes the development status of ethylene industry and liquefied gas vessels as well as the research status of BOG re-liquefaction technology both at home and abroad. At present, there are three kinds of the re-liquefaction plants: direct, indirect and cascade reliquefaction. The cascade re-liquefaction cycle is generally used to re-liquefy the liquid with a boiling point below 198 K. Several types of ethylene re-liquefaction plants have been reported using different refrigerants with different system parameters. It is difficult to compare the performance of these systems fairly. The purpose of this study is to scientifically compare the re-liquefaction performance and energy consumption characteristics of the different BOG re-liquefaction systems, and put forward some improving advices.
In this thesis, five kinds of refrigerants were analyzed based on both the environmental impact and thermal properties, and R290 was selected as the alternative refrigerant of R22 in the BOG re-liquefaction system. Four BOG re-liquefaction processes for liquid ethylene vessels were selected from the published literatures as the research objects. These four potential processes have been compared in terms of coefficient of performance (COP) and exergy efficiency based on the same working fluid and reference parameters. 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 analysed. Based on the above analysis, the parameters of the processes were optimized individually with the aim to minimize the system power consumption. The results showed that a cascade process denoted as Case-I is superior to other three processes in terms of system COP and exergy efficiency. The optimal COP of Case-I is about 0.767, and the maximum exergy efficiency is 60.9%. This work could provide some reference for selection of the BOG re-liquefaction processes for LEG vessels.
Moreover, an ejector enhanced re-liquefaction process (EERP) for LEG vessels is proposed to improve the performance of the conventional re-liquefaction process (CRP). In Aspen HYSYS, an ejector model was developed based on energy, mass conservation and momentum equations to analyse the performance of the proposed cycle system. The effects of the propane evaporating pressure, intermediate pressures of refrigeration and liquefaction cycles on the performances of both the existing and improved processes were studied. The results show that the entrainment ratio and boost ratio are two key factors that affect the power consumption saved by the improved process. In the EERP, Ejector2 has a more significant effect on EERP＇s energy savings compared with Ejector1. Under the baseline conditions, the optimal and of CRP are 400 kPa and 450 kPa, while the optimal of EERP is 500 kPa, and the optimal is 600 kPa, respectively. Under the optimal operating conditions, the total exergy loss of the proposed EERP is reduced by 8.4% compared with CRP. Re-liquefying 3000 of ethylene BOG, the power of 19 kW can be saved by the proposed EERP.
Translated Keyword
[BOG re-liquefaction process, Ejector, Liquid ethylene vessels, Performance improvement]
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