LNG boil-off gas reliquefaction by Brayton refrigeration system – Part 2: Improvements over basic configuration

Exergy analysis of reliquefaction system based on Reverse Brayton cycle (RBC), which is used to condense boil-off gas (BOG) generated in LNG carrier ships, was presented with its basic configuration in Part 1. In this paper, three modifications of the basic system are evaluated. One modification is based on intercooling and precooling of BOG before compressor suction. Others involve ambient compression of BOG. Effects of pressure ratio of RBC, mass flow rate of nitrogen, thermal sizes of heat exchangers and entry temperature of BOG to reliquefaction system on each configuration have been evaluated. Sensitivity analysis shows that 7.8% improvement of rational exergy efficiency is obtained for systems with ambient compression. When BOG enters as superheated gas, modified configurations could keep up their performance while basic system deteriorated. Higher adiabatic efficiencies of compressors and turbines improve the performance of the reliquefaction systems across the board without any additional weight or space. Design and operating parameters have been optimized with the aid of in-built optimiser tool of Aspen HYSYS® V8.6. It resulted in an improvement of rational exergy efficiency of configurations by 1%–3% and decrease of total UA by 5–7% compared to best values predicted by sensitivity analysis. •Evaluates and compares improved configurations for BOG reliquefaction.•Detrimental effects of higher BOG entry temperature effectively circumvented.•Nondimensionlization of parameters helps to apply results to any size of LNG ship.•Evaluated effects of higher adiabatic efficiency of compressor and turbine.•Optimises rational exergy efficiency with reasonable heat exchanger sizes.