An ejector-enhanced re-liquefaction process (EERP) for liquid ethylene vessels

For liquid ethylene gas (LEG) vessels, developing high-efficient re-liquefaction plant to handle the continuously yielded boil-off gas (BOG) could significantly improve its energy efficiency. In this study, an ejector enhanced re-liquefaction process (EERP) for LEG vessels is proposed to improve the performance of the conventional re-liquefaction process (CRP) with a cascaded two-stage compression refrigeration cycle. In Aspen HYSYS, an ejector model was developed to analyse the performance of the proposed cycle system. The effects of the evaporating pressure of upper cycle p_{R0 '} and the intermediate pressures of upper and bottom cycles (p_R3 and p_E3) on the performances of both the CRP and EERP were studied. The results show that better performances can be reached with lower p_{R0 '} for the CRP and EERP. In order to achieve the best performances, the optimal p_R3 and p_E3 are, respectively, lifted to 510 kPa and 630 kPa by the EERP. When the proposed cycle system with the EERP is employed to re-liquefy 3000 kg/h of ethylene BOG, its coefficient of performance (COP) and exergy efficiency can be improved by 2.95–5.31% and 2.70–4.86% over that of the cycle system with the CRP. Correspondingly, the EERP could reduce the total power consumption of 15.7–27.9 kW compared to the CRP.