Essential design criteria for safe and efficient operation of an LNG boil-off gas reliquefier under deteriorated performance of system components

Abstract

Reverse Brayton cycle (RBC) working with nitrogen is used to reliquefy the boil-off gas (BOG) generated in LNG storage tanks. The reliquefier consumes relatively low power if the heat exchanger surface area and the operating parameters are optimally chosen. However, these parameters depend on the isentropic efficiencies of the compressors and turbine that show degradation over years of operation. This paper attempts to find a way to maintain efficient and safe operation of the system even with deteriorated efficiencies of compressor and turbine. The analysis shows that while deterioration of compression system can be made up by applications of variable frequency drive and inlet guide vanes keeping the reliquefaction quantity unaltered, the deterioration of turbine efficiency demands that the pressure ratio of the compressor system should be higher. If appropriate flexibility is incorporated right at the design stage, a reliquefier can be re-adjusted to mitigate the system deterioration. If the isentropic efficiency turbine deteriorates from 90% to 75%, the pressure ratio of nitrogen compressor has to be increased from 3.7 to 4.7 to mitigate increased power input to the reliquefier by as much as 310 kW for a ship carrying 266,000 m3 of LNG.