EN Thermodynamic analysis of the ethylene reliquefaction system on LEG carriers when replacing throttle devices with ejectors

Abstract

LEG carriers’ energy efficiency can be increased by improving the boil-off gas (BOG) reliquefaction installation processes in its components. In this study, it is proposed to replace the conventional reliquefaction process (CRP) of a LEG carrier “ANTIKITIRA” actual installation equipped throttle devices with an ejector reliquefaction process (ERP) having two-phase ejectors as expansion devices to increase the energy efficiency of the installation. Expansion devices EV2E and EV2R were replaced with two-phase ejectors in the proposed system. The ejector efficiencies are ensured by additional components (separators and the precooler in the bottoming stage of the cascade). Separators maintain a constant pressure at the compressor inlets, and the precooler reduces the load on the condenser-evaporator. An ejector model has been developed to analyze the proposed system. The design pressure at the ejector nozzle outlet is determined based on multivariate calculations. Energy and exergy analyses of ERP and CRP systems were carried out. According to the exergy analysis results, each component's influence on the energy efficiency of the CRP and ERP systems is estimated. The exergy losses in each ERP component are lower than the corresponding values in the CRP components. The exergy losses in the condenser and precooler of the CRP system are lower than in the ERP system. The total power consumption is the same in both cycles, and the cooling capacity of the ERP cycle has increased by 29.1 kW. Replacement of throttle devices led to a decrease in absolute exergy loss from 9.75 to 4.95 kW. The energy efficiency of the ERP cycle increased by 24%, and the exergy efficiency increased by 16%. The most significant exergy losses in both cycles are observed during compression in a two-stage cargo compressor of the bottoming stage of the cascade (29.0-34.1%). It is concluded that the proposed ERP cycle meets IMO requirements for vessel energy efficiency but requires additional capital investments