Pre-cooling + mixed refrigeration for LNG liquefaction

If the temperature of the cold box is lower than normal during operation, it may indicate a lack of methane in the refrigerant. Since methane has a high boiling point, a lack of methane leads to a decrease in the system’s overall cooling capacity, thereby reducing the temperature of the cryogenic tank. .

This causes the operating temperature to be higher, resulting in an increased cooling temperature

A preliminary assessment is made based on the compressor outlet pressure and the temperatures of various layers in the cryogenic tank (experience is required); component analysis is possible if the necessary equipment is available.

1. Understand the refrigeration principle of mixed refrigerants – In the process of pre-cooling and mixed-refrigeration liquefaction of LNG, the various components of the mixed refrigerant play different roles. Methane contributes significantly to cooling in the low-temperature range throughout the refrigeration process. - The mixed refrigerant removes heat by absorbing it through evaporation inside the cryogenic tank, thereby liquefying natural gas. Different refrigerant components evaporate at different temperature stages, thereby producing a cooling effect. 2. Observe the temperature changes in the cryogenic tank and compare them with normal conditions – under normal circumstances, the temperature inside the cryogenic tank gradually decreases as the mixed refrigerant performs its cooling function, until it reaches the temperature required for LNG liquefaction. When there is sufficient methane in the refrigerant, the cryogenic tank can maintain a stable low temperature in the range of around -160°C to -170°C, ensuring the effective liquefaction of LNG. - If the cryogenic tank fails to achieve a sufficient temperature drop, or the temperature drops slowly, in the low-temperature range where methane is supposed to play the main role in cooling, it may be due to a lack of methane in the refrigerant. For example, it becomes difficult to further lower the temperature of the cryogenic tank to around -120°C, whereas under normal conditions this temperature should be able to drop even lower, close to the LNG liquefaction temperature (around -162°C). This is likely due to a shortage of methane. Because methane has a low boiling point (around -161.5°C), it plays a crucial role in cooling during the low-temperature phase of LNG liquefaction; without it, the cooling efficiency of the cryogenic tank in low-temperature ranges is significantly reduced.