Development and analysis of a natural gas reliquefaction plant for small gas carriers

A small-scale reliquefaction plant for natural gas boil-off gas from gas tankers has been developed, analysed and experimentally tested in laboratory and in full-scale. The reliquefaction plant is installed onboard a 10 000 m3 gas carrier intended for transport of LNG between export and receiving terminals, initially in Scandinavia and in the Baltic Sea. Typical production capacity of a full-scale plant is 20 tonnes of LNG per day. Standard refrigeration components, easily available as off the shelf products, are used in the system design. Copper brazed plate heat exchangers are among the key components. The combination of using a lubricant injected screw compressor and a mixed component refrigerant contributes to achieve relatively high energy efficiency. The gas carrier is also designed to carry other gases such as ethylene, LPG and VCM, and is therefore equipped with a newly developed ordinary cascade refrigeration plant. When the vessel is in LNG service the capacity of the cascade plant is used for precooling of LNG boil-off as well as for precooling of the mixed component refrigerant. A theoretical analysis of the plant shows a specific power consumption of 0.49 kWh/kg LNG, corresponding to an exergy efficiency of 34%. The specific suction volume is 1.8 m3/kg LNG with the current mixed refrigerant composition. The loss analysis shows several possible improvement possibilities. Successful full-scale tests have been performed, verifying the liquefaction capacity and operation of the plant.