Regulation of air separation load and balance with bottom column pressure

Firstly, based on the situation you described, the V11 valve is a liquid nitrogen return valve. Setting aside your question for now, I would like to explain how the pressure in the lower tower is determined. The steps are as follows: 1. Determine the resistance at the point where the contaminated nitrogen is drawn off, by considering the frictional losses along a certain flow path (usually the path for contaminated nitrogen). For example, the contaminated nitrogen is drawn off from the top of the upper tower, then sent to a heat exchanger for reheating, followed by being sent to a molecular sieve for heating and regeneration, before finally being released. Thus, we have: an exhaust silencer + electric heater (or steam) + molecular sieve + plate heat exchanger + internal pipes + external pipe resistance; from this, the pressure at the top of the upper tower can be determined. 2. The surface pressure of the liquid oxygen in the main cooling system is obtained by adding the values of the aforementioned resistances to the resistance associated with the upper tower (whether it’s a packed tower or a sieve tray tower). 3. The pressure at the bottom of the liquid oxygen is obtained by adding the surface pressure of the liquid oxygen to the pressure generated due to its height. 4. The average temperature of the liquid oxygen is calculated using values 2 and 3. 5. By considering the temperature difference, assuming it to be 1.5 k, the temperature at the top of the lower tower can be determined, and thus its pressure as well. 6. The pressure at the bottom of the lower tower (usually the pressure at the air inlet) is equal to the pressure at the top of the lower tower plus the resistance within that tower. 7. The discharge pressure of the air compressor is determined by adding the resistance in the air flow path to the pressure at the bottom of the lower tower. When operating at reduced capacity, it is necessary to take into account the relationship between flow rate and resistance; for example, at 75% of full capacity, the discharge pressure of the air compressor must be reduced accordingly to obtain a new pressure value for the lower tower. The significant impacts of V11 opening are as follows: it affects the heat load on the main cooler, as well as the reflux ratio in the lower column. By adjusting the opening of V11 to ensure that the pressure in the lower column changes as required, the aforementioned theoretical method only serves as a guide for the operation of actual plants. Actual load reduction operation is achieved by lowering the liquid oxygen level and adjusting the liquid level to reduce the heat exchange area of the main cooler, thereby decreasing the pressure in the lower column. A larger V11 opening is not necessarily better. It will affect the purity of liquid oxygen in the lower column, thereby worsening the distillation process in the upper column. Finally, the opening degree is determined based on the liquid nitrogen purity and liquid air oxygen purity at levels below V11. There is one thing you need to understand: whether it is by adjusting the temperature difference of the main cooler to change the pressure in the lower column, or by adjusting the pressure in the lower column to change the temperature difference. Reflection leads to gains

The simplest method of adjustment: adjusting the load on the device and reducing the amount of air (by closing the guide vanes) effectively reduces the load; similarly, reducing the amount of pure liquid nitrogen sent to the upper tower (by closing the throttle valve for the pure liquid nitrogen going to the upper tower) will do the trick, and this has nothing to do with V11. Reducing the flow of pure liquid nitrogen to the upper column throttle valve increases the flow of reflux liquid in the lower column, thereby reducing the amount of air entering it. This results in a lower load on the lower column (lower pressure) and less liquid nitrogen produced. To ensure proper distillation in the lower column, it is necessary to reduce the amount of pure liquid nitrogen sent to the upper column. This is necessary to ensure low-load production. Also, when reducing the load, it is necessary to appropriately control the amount of expansion. (For reference)

The last edit to this post was made by home1984 on 2009-5-31 at 11:32. salivacat: I don’t quite agree with your view. By reducing the height of the liquid oxygen, what is decreased is the hydrostatic column of liquid oxygen; this in turn lowers the temperature of the liquid oxygen at the bottom, resulting in a smaller temperature difference in the main cooler, and thus a lower pressure in the lower tower. However, in typical designs, the adjustment range for the main cooling fluid level is quite narrow. Considering the requirements of full-submersion operation, it is unsafe to rely on adjusting the main cooling fluid level in order to control the load. Furthermore, V11 is fully open during normal driving, and there is no such thing as saying that \"the greater the V11 opening, the better.\" Please note that some air separation units do not have V11 valves. Agree to CHANGBAISHI’s adjustment method. Please also see http://bbs.hcbbs.com/viewthread.php?tid=465644&page=1#pid2534713. Zhouyou – answer while asking questions?

Our unit also has a V11 valve, but when it is operating normally, this valve remains fully open and does not play any role in regulation. The pressure and resistance in the lower tower are controlled by the liquid nitrogen throttle valve and the contaminated liquid nitrogen throttle valve. There is no liquid product; the purity of the liquid oxygen is relatively low, around 34%. The level of liquid in the main cooler generally isn’t adjusted either when the load is increased or decreased, and it operates at a constant level. After reading what the senior technicians above said, I wonder if there might be an issue with our operations

Imported air separation units from abroad generally do not have V11 valves; to adjust the load, one simply reduces the amount of air supplied. The principle is as follows: by reducing the amount of air, the pressure in the lower column decreases, while the pressure in the upper column remains unchanged. This inevitably results in a smaller temperature difference in the main cooler – due to the lower pressure in the lower column – thereby reducing the load on the main cooler. This is the principle of reducing temperature differences, lowering load, and reducing pressure. But the pressure at the bottom of the tower will decrease. In China, regulation is also achieved using valve V11; by closing this valve, the backflow of liquid nitrogen in the main cooler is reduced, and part of the area in the nitrogen gas condensation channel gets submerged by liquid. This reduces the heat exchange area on the nitrogen gas side of the main cooler, thereby lowering its load. As a result, the pressure in the lower tower increases, preventing enough air from entering, which in turn leads to a decrease in the air volume supplied by the air compressor. This is the principle behind reducing the load caused by a decrease in heat exchange area; by using this method, it is not necessary to lower the pressure in the lower tower. The above are the functions and differences of v11.

In principle, V11 should be fully open. Reducing the load by lowering V11 is equivalent to throwing the company’s profits down the drain, which is heartbreaking. Adjusting V11 should only be considered when the pressure at the lower tower is too low and it affects production.

There are many ways to adjust the conditions for 5# zlzyp168; some of them are commonly used, but in extreme situations there are also extraordinary methods that are practical and effective.

2# salivacat First of all, thank you for your help! But I’m wondering whether it’s the main cooling temperature difference that should be adjusted to change the pressure in the lower column, or rather the pressure in the lower column that should be adjusted to change the temperature difference. \"Thought leads to gains,\" yet there are still doubts; there is an interaction between the two. The pressure difference can change the temperature difference (a higher pressure in the lower tower results in a larger temperature difference, leading to better heat exchange). A temperature difference can also affect the pressure difference (as the temperature difference increases, the cooling load increases, more liquid is produced, and the distillation capacity of the lower column increases, which leads to a decrease in the pressure in that column). However, I find this contradictory: if the pressure in the upper column is reduced, the temperature difference for cooling increases, which in turn leads to a decrease in the pressure in the lower column ； This is what we expect. However, increasing the pressure in the lower tower increases the temperature difference of the main coolant, but it also raises the back pressure of the air compressor, leading to increased energy consumption. Therefore, we choose the former; in our understanding, it is the temperature difference that should primarily affect pressure. I’m thinking about this in this way; please give me some advice!

2# salivacat Thank you; it’s great information. However, I’d like to know how temperature can be calculated using the liquid oxygen level in the tower and the pressure at the bottom of the tower. Thank you, and I’ll reward you generously if this question is answered.

The v11 valve is designed to ensure the distillation conditions in the lower column. Generally speaking, the more reflux liquid flows at the bottom, the more air will be drawn in, but there will be insufficient liquid in the upper tower, which affects the purity of the various products. Generally, no action is required for v11; it is sufficient to maintain the set opening while driving. This is true if the load on the air compressor decreases too much. v11 should be adjusted appropriately to ensure the liquid-air purity and resistance in the lower tower. To adjust the air separation load, simply close the compressor guide vanes and the product discharge valve in order to reduce the load on the argon system. Unless the high-load limit is adjusted. Otherwise, there is no need to adjust v11.