Why does the liquid level in tank V1 (methanol-water separation tank) rise after the low-temperature methanol washing system is operated at reduced capacity?

Does the amount of methanol carried to the absorption tower with the process gas also decrease accordingly?

I haven’t encountered this issue before; I’m looking forward to a good solution.

Is it because, after the load is reduced, the gas temperature drops, resulting in more CO2 dissolving; this causes the liquid to expand and thus the liquid level rises?

After the system’s load is reduced, the amount of methanol sprayed generally remains unchanged； The pressure in the system generally decreases somewhat. The driving force for this methanol to enter the water separation tower is provided by the pressure of the process gas; when this driving force is reduced, the liquid level rises.

In my opinion, the main reason for the increase in the liquid level in the separation tank is that, after reducing the load, the temperature of the process gas entering the separation tank drops, which leads to an increased amount of methanol condensing and thus raising the liquid level in the tank

Is it because the amount of methanol sprayed is controlled too tightly? Otherwise, how could the liquid level rise? During our actual operations, such a situation does not occur when the load is reduced. Please let us know once you have found the answer

I watched the process of adjusting the load yesterday; first, the system pressure decreased； When the system pressure decreases, the amount of recycled lean methanol decreases, while the methanol sprayed remains unchanged ； The liquid level in the T101 tower bottom is decreasing ； The V101 level rose sharply, then dropped a bit, and remained normal. When the pressure plummets, it’s almost simultaneously when V101 soars. I think the load has decreased, and less water is being introduced into the system; therefore, the amount of water does not affect the level in V101. The amount of methanol used for spraying remains unchanged, so spraying methanol also does not affect the level in V101 ； Considering the relative relationship between the system pressure drop and the surge in liquid level, it is likely that the pressure used to deliver liquid to V101 has decreased, resulting in less liquid being delivered and thus an increase in the liquid level ； When the level control valve operates, the liquid level drops sharply again, and then it settles at a stable level!

What was said on the second floor about less liquid being carried away does make some sense. But regarding what was mentioned for #8 – that the driving force for transportation is insufficient – I don’t think its impact is very significant. In the situations we encountered, when the liquid level rose, the level control valve was opened to its full extent, yet the liquid level continued to rise; therefore it was necessary to use a bypass valve for regulation. When the bypass valve was opened slightly and some liquid started flowing through it, the liquid level in tank 01 immediately began to drop, but at the same time the pressure in the separation tank of the downstream system rose immediately. However, when the level control valve was opened further, the pressure in the downstream system did not increase. So I believe it’s still a hardware issue; I suspect that the self-regulating valve is severely clogged. Once the self-regulating valve is opened to a certain degree, opening it any further has no effect because part of it is blocked. Therefore, when the load is reduced, as mentioned by #2, when the amount of liquid carried away decreases, the liquid level increases significantly.

Let’s discuss! :Lol, the rising liquid level is definitely because more fluid is entering ； 2. They go out less. For 1: The amount of methanol sprayed remains unchanged ； The water content resulting from the system’s reduced load can be decreased【taking 70% as an example, the water amount is reduced by 12 kmol/h】. Assuming that methanol experiences a 1% increase in volume expansion due to the increased amount of solute, the final value is 30.75 kmol/h. For 2: First, as my mentor said, the amount of methanol and water fed into T1 should be reduced. [Based on our 70% level, if we use 70% of the original amount, then 2.71 kmol/h more will need to be separated out.] The original amounts fed in were 39.94 of water and 55 of methanol ； Water at 1.81 for entering T1, methanol at 7.22 ； Water at 39.41, methanol at 47.38 to enter T5. Unit: kmol/h. 【Raw data】 Let’s take a look at the amount of liquid that is fed into V101 per hour: 55 + 30.75 = 85.75. 【Assuming the capacity to send to T5 remains unchanged】Amount sent per hour: (39.41+47.38)+6.32=93.11. My calculations might be incorrect; feel free to discuss if you’re interested. This post was last edited by z880347730 on 2009-3-15 13:06.]

No, when the system load decreases or increases, the amount of methanol used for spraying generally remains unchanged. I don’t think it’s because the amount of methanol sprayed is too high, as you mentioned!

In my opinion, 1 should have nothing to do with the amount of methanol sprayed; if it were caused by a decrease in gas flow, the amount of methanol reaching T1 would decrease, but not by such a significant amount. As for 2, it is likely related to the power used for transportation – since the pressure drops significantly after the load is reduced, the same valve setting is no longer sufficient to deliver the methanol, so the valve needs to be opened wider in order to maintain the liquid level