The methane content in the off-gases from the cryogenic distillation tower is high
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The off-gases from our cryogenic distillation tower have been substandard since it was put into operation in December. Even when the temperature of the NH-rich off-gases exiting the top separator reaches -170°C, the methane content in these off-gases remains above 20% (the required level is below 10%). Before that, the off-gases were of acceptable quality at temperatures below -160°C. Additionally, the pressure in the distillation tower is now slightly higher than before. There is another issue: the pressure in the LNG storage tank keeps dropping and cannot be maintained. What could be causing this?
The distillation tower has a high methane content; the high temperature at the bottom of the tower leads to significant methane evaporation. The condensation amount in the top condenser is low, resulting in less reflux and incomplete gas-liquid contact. This fails to suppress the pressure in the LNG storage tank due to the evaporated methane. One factor contributing to this is self-evaporation, and another is the nitrogen content in the feed liquid. The flash evaporation of nitrogen in LNG can increase the tank pressure. However, maintaining a low pressure ensures that the product meets quality standards and reduces losses. If it’s necessary to keep the tank pressure stable, reflux can be achieved by using pumps or loading arms, thereby increasing the amount of BOG returned to the tank. If there is a BOG compressor available, it can be used to extract less gas
What was said on the 2nd floor makes some sense; the original poster could also ask in the process engineering area – it’s possible that there are issues already at the process design stage
Yes, the composition of the feed gas we designed differs significantly from that in actual production; originally, it was planned to be 73% CH4 and 19% hydrogen, but in reality, methane makes up around 85% while hydrogen accounts for about 3%； To ensure that the exhaust emissions meet the standards, it is necessary to lower the temperature at the bottom of the tower and increase the cooling capacity of the condenser, right?
Well, we design it at 65%, but in practice it’s 75-80%. If the temperature at the bottom of the tower drops too much, the nitrogen content in the product can exceed the specified limits. For the condenser at the top of the tower, we use pressurized nitrogen (with the pressure adjusted slightly) to lower the temperature. If it’s possible to increase the cooling capacity of the condenser, then the temperature at the bottom of the tower can be increased slightly, which in turn increases the evaporation rate and the amount of reflux, thereby reducing the temperature across the entire distillation column. This results in a lower methane content in the output product. Alternatively, the temperature at the bottom of the tower can be reduced to decrease evaporation; however, this tends to increase the nitrogen content in the product at the bottom of the tower
Sure, lower the reboiler temperature
Reduce the reboiler temperature, lower the liquid level in the distillation column, reduce the temperature in the subcooling section, and then increase the nitrogen flow rate and pressure