Discussion on low-cost pre-cooling of low-temperature ethylene tanks
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Before the low-temperature ethylene tank is put into use for the first time, it needs to be purged with nitrogen; through pre-cooling and purge operations, the temperature inside the tank is finally brought down to around -100 degrees. This process consumes large amounts of nitrogen and ethylene gas. According to some studies, conventional pre-cooling processes require more than 100 tons of ethylene gas. What I would like to discuss with everyone is whether it is possible to utilize the ethylene/propylene cascade refrigeration system built into the cryogenic tank; during the tank cooling process, nitrogen could be used in place of ethylene gas. By initiating a cooling cycle in this way, it would be possible to achieve pre-cooling using only a small amount of nitrogen. But in practice, is the pre-cooling rate controllable? How to control it? How low can it get? Once it has cooled down to the optimal pre-cooling temperature allowed by the compressor, how can it be pre-cooled further? It requires a great deal of experience or theoretical support. I wonder if anyone has studied this issue. There aren’t many articles available on pre-cooling. As I understand it, one of the challenges with pre-cooling is the difficulty in controlling the cooling temperature of the incoming air, an issue that most people don’t pay much attention to.
Just pour it directly into liquid nitrogen, right? . . .
I have seen some articles suggesting the use of liquid nitrogen for pre-cooling ethylene storage tanks in order to save ethylene used during startup. Your approach is also good, but to use liquid nitrogen for pre-cooling the storage tank to temperatures of over -100 degrees in order to enable the direct feeding of ethylene at low temperatures (so as to avoid waste of ethylene during tank purging and pre-cooling), there are several difficulties that are hard to overcome, and the overall benefits are not necessarily significant: 1. Nitrogen is a non-condensable gas compared to ethylene; it is therefore impossible to utilize the evaporation, liquefaction, condensation, and recycling processes inherent in the ethylene system to achieve a cooling cycle ； 2. The temperature of liquid nitrogen at atmospheric pressure is much higher than that of ethylene; during the later stages of pre-cooling, it may be impossible to keep the temperature at the bottom of the tank under control, leading to temperatures that exceed the design limits of the tank and thus posing safety risks ； 3. The non-condensable gases such as nitrogen in the ethylene system ultimately need to be vented (through a flare system); otherwise, it will affect the stable operation of the system. If no action is taken at the initial stage to remove this nitrogen by using the gas in the storage tanks for displacement, more ethylene gas will be wasted later on (since nitrogen and ethylene have similar molecular weights, separation of the non-condensable gases accumulated in the system can only be achieved by taking advantage of the higher condensation temperature of ethylene, its ease of liquefaction, and the stratification that occurs due to temperature differences).
1. Nitrogen cannot establish a cold cycle. 2. Liquid nitrogen can be used for pre-cooling in the initial stage, followed by the use of ethylene; at that point, gas displacement takes place simultaneously to further reduce the temperature. 3. It is also possible to use liquid nitrogen for pre-cooling and then directly fill with ethylene – during the filling process with liquefied ethylene, nitrogen is gradually driven out