In vacuum distillation, the temperature at the bottom of the tower is high, and the liquid level at the bottom of the tower continues to rise
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Our unit uses vacuum distillation, with the reboiler at the bottom of the tower being heated by a siphon reboiler. According to our material specifications, it should be easy to evaporate at a reboiler temperature of around 110 degrees. In actual production, the following problems occurred: the pressure at the bottom of the tower remained unchanged while the temperature continued to rise, reaching around 150 degrees. With all other process parameters remaining constant, the temperature at the top of the tower did not improve, even after reducing the reflux rate. Additionally, the liquid level at the bottom of the tower increased slowly. I suspect there is a problem with the reboiler’s circulation – it may not be circulating properly, and as a result only a partial heating of the material takes place. Someone with experience in this area, please help solve it!
Possible causes: 1. There are issues with the installation; the heat exchanger does not circulate, resulting in localized overheating. 2. The pressure at the outlet of the heat exchanger is too high, indicating pipeline resistance or blockage.
The vacuum gauge might be faulty; otherwise, the deviation wouldn’t be this large
The possible reasons are as follows: (1) The area of the heat exchanger is insufficient (or the flow rate of the cooling circulating water or liquid is too low, or the temperature is too high), which results in a reduced heat exchange efficiency. As a result, a large amount of gas-liquid mixture fails to condense properly and enters the vacuum system directly, ultimately leading to a decrease in vacuum level and an increase in the system temperature. This accelerates the vaporization of the liquid inside the tank, manifested as a rapid rise in the liquid level within the tank； (2) Obstructions in the system piping lead to excessively high local temperatures ；
It could also be due to steam leakage; I’ve encountered this issue before, where the liquid level kept rising. Also, it’s possible that the materials in the bottom of the tower are high-boiling substances that cannot be vaporized.
There is a problem with the tower bottom liquid level; this situation occurs quite often. If the liquid level exceeds the vapor inlet of the siphon reboiler, it becomes difficult to establish a circulation; this is manifested by an increase in the temperature of the reactor, but with very low vaporization volume.
An excessively high liquid level in the tower bottom can cause circulation to be hindered, leading to high local temperatures; meanwhile, the amount of vaporization is low, so the temperature inside the tower actually decreases. If possible, lower the liquid level in the tower bottom; by re-establishing circulation in the reboiler, the temperature at the tower bottom will decrease, and the temperatures inside the tower will also return to normal.
Is there resistance between the top of the tower and the bottom, which prevents an increase in the vaporization rate at the bottom of the tower?
Possible causes: 1. Equipment-related issues: improper installation of the reboiler, leaks in the reboiler, etc； II. Process-related reasons: 1. Changes in the composition of the feed – with less or no high-molecular-weight components present – result in an inability to maintain circulation in the reboiler; the temperature of 150 degrees actually refers to the temperature of the gas phase ； 2. The feed rate is too low, below the minimum load of the distillation tower ； Solutions: If it is due to a problem with the equipment, then it should be disassembled for inspection or reinstalled. If it is a process-related issue, in the first case, the proportion of high-molecular-weight components in the feed material should be increased; alternatively, the heat source of the reboiler can be turned off first, with continuous feeding maintained. Once the reboiler is filled with material, the heat source can be slowly turned on – very slowly, and this depends on the skill level of the operator. The second case is simpler; increasing the feed rate is generally above 60% of the design value.
What you said is too vague; it’s difficult to determine what the cause is. Generally speaking, an increase in temperature while the pressure remains constant is caused by changes in the composition, with an increase in heavier components. As the amount of Reconstituted components increases, the reboiler’s circulation capacity decreases, the evaporation rate drops, and the liquid level rises. When the liquid level rises to such an extent that it covers the vaporization port, the reboiler loses its ability to circulate, until no more evaporation takes place. The solution is to periodically discharge high-boiling substances in order to reduce the heavy components in the bottom of the tower.
Personally, I think it’s due to an insufficient installation height of the reboiler or an inadequate heat exchange area. Additionally, there may be too much resistance in the system, or some blockage somewhere; it would be best to install a vacuum gauge at the bottom of the tower as well, to help identify the problem