CO2预冷双氮膨胀天然气液化工艺的海上适应性分析

优选出的CO₂预冷双氮膨胀制冷液化工艺提高了液化效率，增大了天然气液化处理能力，但其海上作业适应性还有待考察。为此，通过流程模拟和火用分析，对CO₂预冷、丙烷预冷和混合冷剂双氮膨胀制冷液化工艺流程进行了对比，并从热力学角度出发，分析了CO₂预冷双氮膨胀制冷液化工艺对原料气物性（温度、压力、组成）、流程操作参数（CO₂节流后的温度）以及CO₂纯度的敏感性，对其海上适应性做出了评价。结论认为：该工艺可适用于海况恶劣的环境，其对原料气温度、压力和组成变化不敏感，适合于中到大规模的天然气液化生产。最后，为保证流程的安全、高效运行，提出了该工艺应用中需注意的3个问题：①压缩机水冷器温度应低于31.1 ℃；②CO₂预冷温度应超过-53 ℃；③CO₂杂质含量应控制在1%以内。

Offshore adaptability of the dual nitrogen expander process with CO2 pre-cooling

The dual nitrogen expander process with CO2 pre-cooling can improve both the efficiency and the dealing capacity of natural gas liquefaction,but its offshore adaptability is still under investigation.In view of this,through the process simulation and exergy analysis,a comparative study is conducted of the dual nitrogen expander process respectively with CO2 pre-cooling,propane pre-cooling,and a mixed refrigerant.Then,in terms of thermodynamics,an analysis is made of what sensitivity the dual nitrogen expander process with CO2 pre-cooling has to the physical properties of feed gas including temperature,pressure and composition,the operating parameters of the process like the temperature of CO2 after throttled flow,and the CO2 purity.Finally,after the analysis of the offshore adaptability,such process is considered to be adaptable for the adverse offshore environment,not to be sensitive to any change in temperature,pressure and composition of feed gas,thus to be good for middle-or large-scale LNG production.In addition,in order to ensure the safety and high efficiency of the process,this paper highlights 3 points in the operation:(1) the temperature of the water cooler in the compressor should be below 31.1 ℃;(2) the CO2 pre-cooling temperature should be over-53 ℃;(3) the content of impurities in CO2 should be controlled within 1%.