Opening degree of the liquid nitrogen reflux valve in the lower column of the air separation tower
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During the hot start-up of the air separation unit, is the liquid nitrogen reflux valve in the lower column open or closed? Which method will shorten the startup time? Is it still basically the same?
Open the valve to 2/3 position first, and gradually increase it according to the load until it is fully open.
It needs to be distributed; during the cooling phase, it must be completely closed so that the nitrogen channels used for cooling can cool down properly. It also needs to be closed when liquid accumulates, as this helps to maintain an appropriate level of liquid oxygen. During the purification process, the valve should be gradually opened until the purity of the liquid air and liquid nitrogen in the lower tower meets the required standards. You should know why this is necessary.
The answer on the 4th floor is completely correct; this is how it should be done!
It was turned off at first; once Air Liquide arrived, adjustments were made based on the operating conditions
During the cooling phase, it must be turned off completely; it also needs to be turned off when liquid accumulates. When adjusting the purity, it should be gradually opened up!
Personal opinion: 1. Generally, domestic units are equipped with this return valve, while imported air separation units do not have such a valve. Even if it does exist, the valve’s sealing rating is very low. 2. This return valve should be open during the cooling phase and closed during the liquid accumulation phase; it should be gradually opened more as the purification process begins. 3. It’s fine even if it remains fully open throughout; simply, the liquid accumulation speed will be slower when it’s fully open, but the purification process will proceed more quickly. When fully open, the resistance to descending the tower increases gradually as liquid accumulates; the purity of liquid nitrogen remains satisfactory first, but due to the larger amount of air present when liquid accumulates, the heat loss is higher, resulting in a slower overall speed.
What was said on the fourth floor is correct; I agree.
I tend to agree with the view from the 8th floor. It should be on—off—on sequence.
From personal experience, the cooling stage should be set to full capacity so that the pipes can cool down evenly and quickly； Accumulation stage: It is fully closed, but this depends on the circumstances; if the liquid-air throttle valve is designed to be small, full closure may result in too low a temperature behind the expander, leading to the presence of liquid. There is another scenario: when starting up the system, we pay attention to the amount of oxygen produced within a certain number of hours, and then we adjust the purity of nitrogen. As a result, the liquid nitrogen throttle valve is opened before the accumulation process is complete; this way, oxygen is produced, but it takes a long time to adjust the nitrogen level and to determine the appropriate opening degree of this valve – that is, this valve needs to be opened during the purification phase. Also, once the operating conditions are stable, this valve can be opened as wide as possible while maintaining the purity of nitrogen. Therefore, I believe that if the completion of startup is defined as the production of qualified oxygen and nitrogen, then the total startup time remains the same regardless of whether this valve is operated before pre-cooling; otherwise, why wouldn’t foreign countries use such a valve? ； If the goal is to produce only qualified oxygen, then it’s what people refer to as on–off–on. :time: