(422b) Novel LNG Boil-Off Gas Re-Liquefaction System Design Using Dual-Mixed-Refrigerant Cycle and an Ejector

During transportation of LNG using ships, the fuel is stored cryogenically at 111K and ambient pressure. Due to heat transfer from the environment to the tanks a significant amount of LNG (approximately 0.15% per day) evaporates, known as boil off gas (BOG). This may lead safety issues due the increase in tank pressure, or economic losses if the excess vented without being recovered. However, many methods have been considered, such as combustion, using it as fuel or venting to atmosphere but are undesirable either because of concerns over pollution, low efficiency or high operating cost.

Conceptually for the re-liquefaction system of the BOG is attractive, and have been a number of studies, there are two limitations. In most recent proposed re-liquefaction system, non-liquefied gas remains in the stream returned to the tank, with the surplus gas returned to the tank and still has low efficiency (approximately 41.3%). Thus it was proposed to combine two existing technologies, new dual mixed refrigerants cycle based on cascade cycles and a re-liquefaction system using ejector, and develop it by modifying design. In a novel process design that avoids the return of gas to the tank, while reducing the energy consumption of the compressor stage, to improve energy efficiency. A mathematical model is also introduced for more accurately calculating conditions at the ejector-outlet.

The results show, the power consumption of the compressor stage for BOG entering the system is reduced and performance indicators such as energy efficiency and coefficient of performance, related to operating cost, is much improved than earlier proposals. This study would be the basis of the development of the midstream industry in the aspect of energy consumption and efficiency of the LNG Boil-off gas re-liquefaction system.