Design And Experimental Test Of A Small Scale BOG Re-liquefaction Systemin Skid-mounted Package

As an important way of NG storage, transportation and commerce, LNG has attracted much attention in recent years. Under atmosphere pressure, LNG is liquid at the temperature of-162℃. Due to inevitable heat-transfer process, cryogenic LNG in tank evaporates and produces BOG(Boiled Off Gas) continually, which enhances the pressure in the LNG tank. To prevent the pressure in the tank from exceeding the safety range, BOG has to be released in time, which is a waste of energy. The rising pressure will enhance the BOR(Boiled Off Rate) of the LNG tank, and the released BOG will bring damages to the atmosphere and risks to the surroundings.BOG generated in LNG ships and LNG receiving terminals is relatively large in amount, adopting BOG re-liquefaction installations based on Nitrogen expansion refrigeration cycle or MRC refrigeration cycle to recycle BOG is feasible and economical. However, for small and medium scale LNG tanksonshore, such as LNG filling stations and LNG tank vehicles, the amount of BOG generated is much smaller, using conventional BOG re-liquefaction installations to recycle BOG is complex in process, miscellaneous in equipment, expensive in establishment and poor in economy.In this paper, a small scale BOG re-liquefaction system in skid-mounted package driven by high power coaxial PTC(Pulse Tube Cryocooler) is proposed and introduced. The installation is appropriate for small and medium size LNG tanks, which has advantages of simple process, compact structure and good economy.Firstly, re-liquefaction ability of BOG and nitrogen, pipe pressure drop and economic performance of the installation are calculated analyzed. It has been shown from the calculation results that the BOG re-liquefaction ability is 113~188kg/d and the N2 re-liquefaction ability is 101~117kg/d, the pressure drop in LNG pipe is 2658.1Pa and the pressure drop in LN2 pipe is 2606.2Pa, the cost recovery time of the installation is 1.4~2.9 years.Then, process of the BOG re-liquefaction system is designed in detail, experimental setup is completed, nitrogen liquefaction experiment and liquid nitrogen back flow experiment are conducted to the primary experimental setup, temperature of the PTC cold head, electrical power temperature in the liquefaction tank and net cooling capacity at different working pressures and working voltages are tested and analyzed, and “circulation without pump” of the installation is verified. Based on primary experimental results, several optimizations are conducted to the experimental setup and then experimental tests are completed on the optimized inatallation. It has been shownfrom the experimental results that the net cooling capacity is promoted from 268W@80K to 438W@80K after optimization.Lastly, the small scale BOG re-liquefaction system is moved to the LNG filling station and field experiments are conducted. It has been shown from the field experimental results that the installation can operate with good stability, reliability and safety, the BOG re-liquefaction ability of the system is as high as 106kg/d with a net cooling capacity of at least 550W@120K.