Liquefied natural gas – liquid accumulation in the cooler
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Our cryogenic box is equipped with a three-stage plate-fin heat exchanger, a dehydrogenation tower, and a distillation tower. Due to an operational error, about 2 tons of isopentane were added to the system at once. As a result, the pressure difference downstream of the refrigerant vapor throttle valve has increased; the temperature of the refrigerant exiting the cryogenic box has dropped to around -6 degrees Celsius, while the temperatures at all other points have risen. The current drawn by the compressor is also very high, and production volume has decreased. It appears that there is liquid accumulation in the cryogenic box. What should be done? Have other professionals encountered similar situations?
I’ve just started in this field and am waiting for experts to provide answers
Well, it’s indeed fluid accumulation in the cold box. 2 tons of isopentane! Just thinking about it gives me chills... Fluid accumulation is accompanied by an increase in the pressure difference at the bottom of the cold box and in the reheat pipelines; initially, the system pressure decreases slowly, but then, as efforts are made to compensate for this, the system pressure rises, and naturally, the motor current increases as well. At the same time, the cooling capacity decreases and the product temperature rises; therefore, it is necessary to reduce the natural gas output. However, this leads to a decrease in the heat load, resulting in a negative value for the temperature of the refrigerant exiting the cold box…… The situation in your plant is serious – there’s no point in trying to fix it; shut down the system to allow it to warm up again before restarting it. But! ! Be sure to pay close attention to this: the temperature must be fully restored to normal levels, and it’s essential to ensure that the cold box does not have a negative temperature. This is very important, as driving with a negative temperature in the cold box can easily lead to the formation of additional liquid buildup! ! Remember! The Lesson of Tears······
Our factory has a high-pressure storage tank from which both gas and liquid phases can be extracted. If you have such a setup, you can try to extract the liquid phase by gradually reducing the size of the liquid-phase throttle valve, thereby decreasing the liquid-phase circulation; at the same time, increase the size of the gas-phase throttle valve to slowly carry the liquid phase to the compressor and then extract it from there
The reformed components in the refrigerant system are significantly high; the natural gas system does not require rewarming treatment. Liquid is gradually collected from the separator at the end of the refrigerant compressor, thereby reducing the amount of reformed components entering the system
The reformed components in the refrigerant system are significantly high; the natural gas system does not require rewarming treatment. Liquid is gradually collected from the separator at the end of the refrigerant compressor, thereby reducing the amount of reformed components entering the system
If there is an excess of isopentane, it is best to compress it and then cool it down forcibly for separation, so as to recover some of it – otherwise it can be dangerous. Firstly, an excess of isopentane poses a risk to the equipment, as it can lead to liquid accumulation; the current level may also increase. Additionally, liquid can accumulate in the heat exchangers. As you have observed, if too much isopentane enters the pre-cooling area, it won’t have time to vaporize and will flow downward to the deep-cooling section, causing the isopentane to enter the deep-cooling heat exchanger. A large amount of isopentane in such a heat exchanger will severely affect heat exchange, resulting in the LNG product temperature not dropping low enough. Since isopentane has a low boiling point, it will never evaporate at the extremely low temperatures in the deep-cooling section. To solve this problem, it is necessary to first cool the isopentane in the cold box by reducing the JT valve to a slight open position, as well as reducing the high-pressure refrigerant valve to a slight open position. The refrigerant compressor should be set to reverse flow (anti-surge mode), while natural gas flow should be increased. Don’t worry about waste at this stage; if there is a recovery line connected to the cold box, use it to prevent waste and help evaporate the isopentane inside the cold box heat exchanger. If necessary, force cooling at the compressor outlet to lower the temperature as much as possible, followed by recovery. If you’re unsure how to do this, feel free to send me a private message!
It’s okay. 1. Reduce the load; time will smooth things out. Just turn down the throttle valve of the liquid-phase refrigerant; that will do. Rebalance the system – it’s a minor issue. 2. Release a small amount of the coolant into the coolant collection tank while it is still being produced, and at the same time add additional coolant to re-establish the proper coolant ratio.
Liquid collection reduces production; however, the current drawn by the compressor fluctuates significantly during this process. I’m not sure how many compression stages are used in your chiller compressors. If there is an inter-stage separation tank, set its liquid level higher so that more isopentane can be separated out. Also, I’m not sure whether you have the high-pressure liquid return tank mentioned by a previous user – by directing a portion of the liquid phase at high pressure into this tank, it’s possible to replenish some of the ethylene and propylene