Process Design And Experimental Study Of Gas Liquefaction System Based On Stirling Cryocooler With Large Cooling Capacity

With the strategic target of peak carbon dioxide emissions and carbon neutrality proposed,the consumption ratio of liquefied natural gas and other clean energy is increasing continuously.LNG produces boil-off gas during transport and storage,which will cause environmental pollution and energy loss if is directly emitted to environment.At present,there are mature liquefaction methods for BOG recovery of large LNG carriers or onshore natural gas stations.But only few studies have focused on BOG recovery on a small or medium scale.This paper summarized the numerical and experimental studies of BOG recovery,which show that the liquefaction method of using Stirling cryocooler with large cooling capacity has advantages of less equipment,more compact structure,lower energy consumption and etc.Meanwhile,it can satisfy the need of small and medium-sized LNG stations.So this paper carried out a systematic research on the liquefaction process which uses Stirling cryocooler as the key unit.The main content is as follows:(1)Taking nitrogen as the working medium,we studied on cooling and condensation mechanism of vapor in the cold head of the Stirling cryocooler which has good heat transfer performance by numerical simulation.A scheme of gas liquefaction system based on a large cooling capacity Stirling cryocooler was proposed,which includes a regenerator to recover the cold energy of unliquefied gas at the outlet of the cryocooler and pre-cool the incoming gas.(2)HYSYS simulation was used to study the influence of operating parameters such as inlet pressure,flow rate and temperature on system parameters such as liquefaction rate and the requirement for heat transfer performance of the regenerator,so as to guide its’ selection.It was also used to study the start-stop response of the gas liquefaction system,and it was found that the system can reach stable operation state quickly.The fluctuation of pressure,flow rate and temperature of each stream in the liquefaction system was studied when the inlet pressure or flow rate changes.(3)A test bench of nitrogen liquefaction was built.The experimental results show that the increase of liquefaction rate by adding heat regenerator into the process is obvious.What’s more,the larger the inlet flow rate,the better the heat transfer performance of the cold head.And also,the maximum liquefaction quantity is higher.But when the inlet flow rate reaches a critical value,the output cooling capacity of the cryocooler is basically unchanged.And at this point,the liquefaction quantity can remain unchanged by the pre-cooling effect of the regenerator.So this critical flow rate is the optimal inlet flow rate,which can keep both the cooling capacity and the liquefaction quantity maximized.Also,in order to remove the liquid retention in cold head and increase the cooling capacity of the cryocooler,the structure of the cold head was optimized.(4)The gas liquefaction system was applied to BOG re-liquefaction,and the improved design and calculation of the process were carried out.The calculation results show that the BOG liquefaction quantity can reach 12kg/h,and the specific energy consumption is 0.942 k Wh /kg.It proves that this gas liquefaction system is environmentally friendly and economy,so it has broad application prospects.