Research On Energy-saving Optimization Of BOG Reliquefaction Technology Without External Transmission At LNG Satellite Statio

The number of small receiving stations,satellite stations and small peak shaving stations for liquefied natural gas(LNG)in China is increasing day by day.The heat exchange between LNG storage tanks and the external atmospheric environment will produce too much boil-off gas(BOG).For the conventional BOG recovery process,stable natural gas transmission channels are needed.The development of small skidmounted BOG reliquefaction recovery process has important practical significance for BOG zero emission recovery in China.Based on the classical liquefaction process,four different small skid-mounted BOG re-liquefaction processes were developed to promote the practical application of the re-liquefaction process.The main contents are as follows :(1)Select the classic nitrogen expansion liquefaction process to build a small skid-mounted preheating BOG nitrogen parallel expansion liquefaction process.Taking the unit energy consumption of the liquefaction process as a single objective function,the key parameters of the process are optimized by genetic algorithm(GA),and the optimal design parameters of the process are obtained.The liquefying process and heat transfer analysis were carried out.The results show that the unit energy consumption of the liquefaction process is reduced by 8.7 % after optimization by GA algorithm.The newly designed small skid-mounted preheating BOG nitrogen parallel expansion liquefaction process has better overall performance and saves 7.67 %energy compared with the traditional liquefaction process.(2)A small skid-mounted preheated BOG single mixed refrigerant liquefaction process was constructed based on the PRICO process.Taking the unit energy consumption of the liquefaction process as a single objective function,the GA algorithm is used to optimize the key parameters of the process,and the optimal design parameters of the process are obtained.The process before and after optimization was compared and analyzed by combining exergy analysis and heat transfer analysis.The process adaptability of the developed small skid-mounted preheating BOG single mixed refrigerant cycle process and nitrogen expansion liquefaction process was analyzed and compared.The results show that the energy consumption of the liquefaction process is reduced by 4.56 % after optimization by GA algorithm.The heat transfer interval between hot and cold fluids in the optimized heat exchanger is smaller,the total heat transfer in the process is reduced by 4.24 %,and the total exergy loss is reduced by 6.23 %.(3)In order to further improve the thermal efficiency of the re-liquefaction process,a small skid-mounted preheating BOG double mixed refrigerant cycle liquefaction process was constructed based on the single mixed refrigerant liquefaction process.Taking the unit energy consumption of the liquefaction process as a single objective function,the key parameters of the process are optimized by GA algorithm,and the optimal design parameters of the process are obtained.Combined with exergy analysis and heat transfer analysis,the process before and after optimization is compared and analyzed.The results show that the energy consumption of the liquefaction process is reduced by 7.08 % after optimization by GA algorithm,and the exergy efficiency of the process is increased to 0.1992.The total heat transfer of the optimized process is reduced by 4.80 %,and the total area of the heat exchanger is reduced to 195.23 m2.The total exergy loss after optimization is 11.00 % lower than that before optimization.(4)In order to solve the problems of difficult mixing ratio of mixed refrigerants and difficult replenishment of loss refrigerants,a small skid-mounted liquefaction process without external mixed refrigerants was constructed.In this process,BOG is used as a mixed refrigerant,and BOG is liquefied after pressurized condensation and expansion.In this study,the feasibility of BOG produced by satellite stations with different gas sources as a mixed refrigerant was analyzed,and the process parameters were optimized based on GA algorithm.By comparing with single mixed refrigerant cycle(SMR)and double mixed refrigerant cycle(DMR),it is found that BOG as a mixed refrigerant process(BMR)has strong recyclability and is a new type of BOG reliquefaction solution.(5)Four types of small skid-mounted re-liquefaction processes were compared in terms of economy.Simple bare module calculation method and empirical formula are used to calculate the equipment selection of the re-liquefaction process,and the equipment investment cost and annual operation cost of each type of re-liquefaction process are obtained.At the same time,the operating costs of the reliquefaction process in different years were compared and analyzed.After economic analysis,considering the long service life of the re-liquefaction process,the DMR reliquefaction process is preferred;considering the re-liquefaction process with a shorter service life,the SMR re-liquefaction process is preferred;considering the convenience of actual management and operation,nitrogen expansion reliquefaction process is selected.The BMR re-liquefaction process can be considered without considering refrigerant storage and short-term operation.