Unstable operation of the distillation tower with side-line discharge
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We have a plate distillation tower with off-spec product discharge on one side, and it has recently experienced operational instability during operation; I would like to ask for advice on how to resolve this issue. The situation is as follows: The column uses superheated low-pressure steam (around 200 degrees) to heat the reboiler. When the amount of steam supplied increases, the liquid level in the column bottom drops rapidly and the temperature there rises; when the amount of steam supplied is reduced, the liquid level in the column bottom rises quickly, to such an extent that it seems impossible to control it. We have tried very precise adjustments in an attempt to find a point at which the steam supply and the liquid level in the column bottom can remain stable, but this has not been possible. Apart from operational issues, I wonder whether it might be related to the structure of the column or to the side-stream extraction. If there is indeed a connection, I’m not sure what causes the instability in the liquid level and temperature at the column bottom Looking forward to everyone's advice, thank you!
The temperature difference between the bottom of the tower and the heat source (superheated steam) should not be too large, as a large difference can lead to operational instability. Did you also use steam as a heat source before?
Why does a large temperature difference cause unstable operation?
A large temperature difference means a high driving force for heat transfer. When this driving force is high, even slight changes in the steam flow can cause a significant amount of vaporization of the liquid in the tower bottom, resulting in unstable operation.
My own opinion, for your reference! Is the density of your material highly affected by temperature? The measurement provided by the level transmitter is related to the density of the material, which is why both temperature and level levels become unstable! You can consider it from this perspective!
Is it because there is too much light component and the latent heat is low, that the amount of steam has a significant impact on the liquid level?
It is related to the components of the material; additionally, it may be connected to unstable feed control as well as unstable extraction and reflux rates. By addressing these aspects, the temperature of the sensitive plate used for side-stream extraction is particularly important – it needs to be well controlled in order to maintain system stability
1. The low liquid flow rate in the tower is not compatible with the reboiler. 2. The superheated steam is inappropriate; it turns into low-pressure saturated steam after depressurization. 3. Changes in the liquid level and temperature at the bottom of the tower have no relation to the side-stream extraction; as long as the calculations for the tower are correct, there should be no problems
I think it’s likely related to an improper structural design of the tower base, with little to do with operation. Please send over a schematic diagram of the tower base so I can help analyze it