Liquid nitrogen is produced using a single tower; the purity of the nitrogen is unstable and cannot be maintained at a constant level
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My setup is a single-tower system for producing liquid nitrogen. The air is cooled to 30% humidity and fed into the distillation tower (0.5 MPa); nitrogen condenses at the top of the tower, and liquid nitrogen is extracted; no nitrogen-free product is produced. The bottom liquid, after being throttled, enters the condenser at the top of the tower (0.3 MPa) and then expands. During the cooling and liquid accumulation process, the liquid nitrogen valve was not opened; the material in the distillation tower moved in and out only once, meaning the distillation tower was in a full reflux mode. During the purification stage (by closing the throttle valve), the purity of nitrogen gradually increases, but the fluctuations are quite significant, and it is difficult to achieve a level below 10 ppm O2. Due to an accidental event, the throttle valves were closed completely, causing the pressure in the distillation tower to rise, the air flow rate to drop sharply, and the purity of nitrogen to fall to 99%. After the throttle valve was opened, the system gradually stabilized, and the purity quickly dropped below 3 ppm O2 with no fluctuations; at that point, the liquid nitrogen valve was opened to produce the product. Preliminary analysis suggests that the failure to remove liquid nitrogen for an extended period may have caused the distillation column to enter a state of flooding or leakage, resulting in instability in the column and significant fluctuations in purity. How can these problems be solved? Should the liquid nitrogen valve be opened once there is a liquid level in the distillation tower? ? I’m seeking help from everyone
Be mindful of the down-tower resistance. Furthermore, even if the purity of liquid nitrogen is satisfactory, there is still a decline in the readings provided by the analyzer; it is estimated that the purity remains acceptable only when the online analyzer is in use, as the sensor of the analyzer gets contaminated when the purity of liquid nitrogen is low.
If liquid flooding is suspected, pay attention to the fluctuations in tower pressure and tower resistance. Also, pay attention to the method used for displacement during purification – incomplete displacement may be the reason why purity does not decrease as expected. Additionally, monitor the material balance, such as the amount of air used and the nitrogen extraction rate; an adequate supply of cooling capacity is also a very important factor~~
I tested it again today, trying every means to reduce the purity to 4 ppm of O2, but it still fluctuated around 6 ppm. Eventually, by closing the V2 liquid nitrogen throttle valve and then opening it slightly, the purity stabilized. I really can’t figure it out. The unit is not equipped with a tower pressure difference gauge, so this parameter cannot be viewed. Everyone, please analyze the possible reasons and discuss how to avoid them. Currently, the V2 throttle valve controls the liquid level, tower pressure, and air volume simultaneously, indirectly controlling purity; as a result, it is difficult to control.
Is the Air Liquide throttle a manual valve? It’s very difficult to control the opening degree~~ It might be advisable to replace it with another control valve~~
It is possible to check the liquid-air throttle valve positioner as well as extend the purging time of the analyzer; if there is no significant change in column resistance, it is not a process-related issue
Once the distillation conditions are established, liquid nitrogen should be produced; any product that is not up to standard in the initial stage is discharged directly, with a reduced amount being produced.