MRC mixed refrigeration, compressor inlet pressure fluctuations
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Our plant utilizes a five-component MRC mixed refrigeration process involving methane, nitrogen, ethylene, isopentane, and propane! After the unit increased its natural gas processing capacity, the pressure at the compressor inlet fluctuated, and as a result, the flow rates of the gas phase and liquid phase entering the cold box also fluctuated! The measure taken was to reduce the liquid phase circulation flow rate; then, after the compressor ran for some time, the pressure returned to normal! It’s happened several times; the exact cause cannot be determined, and all are just guesses (such as an excessive liquid phase circulation volume, incomplete vaporization of the liquid phase refrigerant, or deviations in the refrigerant composition). There is no clear direction to follow – I need help!
You can check the pressure differences at various sections of the cold box’s plate-fin heat exchangers caused by the refrigerant (we have pressure difference gauges; I wonder if your process involves such equipment as well). A large pressure difference may lead to blockages in certain pipe sections. Additionally, a high volume of liquid phase can result in incomplete vaporization; this can be observed from the backflow temperature
We don’t have a differential pressure gauge! ! The temperature of the refrigerant in the gas phase exiting the lower plate-fin section is -33°C; after mixing with the liquid-phase refrigerant, the temperature of the refrigerant entering the upper plate-fin section of the cold box is -36°C. The temperature of the refrigerant exiting the cold box is also normal! It is usually between 20-25 degrees. Is this consideration correct: there is little vapor refrigerant in the cold box, and simply increasing the liquid-phase circulation volume leads to incomplete vaporization!
There is a problem with the refrigerant ratio: too little light component and too much heavy component. The rotation of the light component is not sufficient to drive the heavy component, which leads to fluctuations in inlet pressure. Concentrate the liquid.
Each time the liquid phase circulation volume is reduced, the compressor inlet pressure gradually stabilizes! With fewer light components, can it drive the circulation of heavier components? Causes incomplete vaporization?
This is due to an insufficient proportion of refrigerant. To increase the amount of natural gas intake, more refrigerant is required. From what you’ve said, it seems there isn’t enough heavy refrigerant, leading to an imbalance between gas and liquid in the compressor. In summary, there’s a shortage of heavy refrigerant, which increases the compressor’s load
It’s probably because there isn’t enough light component.
There are many reasons for this. If there are too many light components, the inlet pressure will rise and the current level will be high; if there are too many heavy components, the outlet pressure will be lower. However, if you notice that the inlet pressure is low despite adding a lot of material, it indicates that there are few light components. Additionally, it’s important to consider the circulation volume at the inlet and outlet of the cold box. If there’s no issue with the refrigerant ratio but fluctuations still occur, it’s likely related to the cold box itself – you can purge the cold box, both the liquid phase and the gas phase, as well as the main pipes