Abstract: In the process of transportation of large LNG transport ship,it will absorb the heat from the outside world,and make LNG to be the natural gas. In order to avoid the risk of pressure overload of the LNG transport ship,it is a preferred treatment method to re-liquefy the nature gas using re-liquefaction system. In this paper,a new type of nitrogen expansion process was simulated and the key parameters of the process were optimized. By comparing the optimized process and propane precooled mixed refrigerant refrigeration process,the paper get some conclusions. First,the unit energy consumption of LNG was regarded as the measurement index. 5 key parameters (the outlet temperature of BOG gas in the heat exchanger,the outlet pressure of the BOG first stage compressor,the nitrogen outlet temperature of the heat exchanger,the pressure of the expansion of the expander and the pressure distribution of the nitrogen compressor) were optimized to reduce the power consumption of the system. Second,compared with the propane precooling mixed refrigeration process,the nitrogen expansion process has slightly higher unit energy consumption,simpler simulation,less equipment,smaller area and more secure. Third,the selected nitrogen expansion refrigeration process was more suitable for re-liquefying on LNG transport ship than propane precooled mixed refrigerant flow.

Key words: nitrogen expansion refrigeration, offshore adaptability, simulation and optimization, process comparison, LNG, BOG

摘要： 大型液化天然气（LNG）运输船在运输过程中，会吸收外界热量，而使LNG受热气化为天然气。为避免压力超限LNG运输船发生危险，用蒸发天然气（BOG）再液化系统将天然气再液化成为一种优选处理方式。本文针对一种新型氮膨胀流程进行模拟，并进行流程中关键参数的优化。将优化后的流程与丙烷预冷混合冷剂制冷流程进行对比，结果表明：以产品LNG比功耗为衡量指标，对5个关键参数（换热器中BOG气体出口温度、BOG一级压缩机出口压力、换热器中氮气出口温度、膨胀机膨胀后压力及氮气压缩机的压力分配等）进行优化，降低了系统的比功耗；与丙烷预冷混合制冷流程比较，氮膨胀流程比功耗略高，流程简单，设备较少，更加安全；文中所选氮膨胀制冷流程比丙烷预冷混合冷剂流程更适合于LNG运输船上BOG再液化。

关键词: 氮膨胀制冷, 海上适应性分析, 模拟优化, 流程对比, 液化天然气, 蒸发天然气