Zero methane loss in reliquefaction of boil-off gas in liquefied natural gas carrier ships by using packed bed distillation in reverse Brayton system

Abstract

Reverse Brayton refrigeration cycle with nitrogen as working fluid is widely used in reliquefaction of boil-off gas generated in LNG tanks onboard. These reliquefiers allow venting of some vapour that has methane in it. This entails a loss of energy and release of greenhouse gas into the environment. As the boil-off gas contains low-boiling temperature nitrogen, its complete reliquefaction is possible only by decreasing the pressure at the turbine outlet that makes the reliquefaction system bulkier. In this paper, two proposed modifications have succeeded in achieving zero methane loss without any reduction of turbine outlet pressure. In one modification, the vent gas is recycled and completely reliquefied with the boil-off gas. In other modification, a distillation column purifies the vent gas to less than a part per million of methane in it and near-pure nitrogen is fed back to the system. Rational exergy efficiency of vent-recycled and distillation column systems are higher than the basic system by 14.9% and 8.7% respectively albeit with some increase in mass flow rate of nitrogen compressor and the size of heat exchangers. The period of return on investment reduces by one month and increases by six months in vent-recycled and column system respectively.