Correlation between overflow weir height, liquid drop plate gap, and tower pressure difference
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We use a distillation column with a diameter of 2400 MM, equipped with 20 layers of floating valve trays. The weir height is 25, and the gap between the liquid distribution plates and the trays is 19. During operation, carbon buildup occurred in the liquid collection chambers of the lower layers of trays, blocking these gaps and preventing the column from functioning properly; regular cleaning is required, which significantly affects work efficiency. A few days ago, we changed the height of the weir on the following three layers of trays to 40, and adjusted the gap between the liquid distribution plates to 32; no changes were made to the other trays. After driving it again, a more serious problem arose: the tower pressure difference increased from 10 KPA to 20 KPA, while the heat supplied was only half of what it used to be; as a result, it was impossible to operate it, so it had to be shut down once more for modifications in order to restore it to its original state. I would appreciate some advice from fellow sailors! Thanks!
This problem may be related to an issue with the system; if you want to resolve it fundamentally, it’s best to start by changing the internal components of the tower and replacing them with newer types of components. You have increased the weir height by too much; as a result, the pressure drop will increase significantly, and energy consumption will also rise. Could we increase the steam volume a bit? !
Even after adjusting the weir height and bottom gap of the 3-layer floating valve tray, the pressure drop doesn’t increase by 10 KPA; at most it increases by 1–2 KPA. . . There must be other problems as well. Also, does this tower have a high liquid load? Does your 25 meter weir height refer to the overflow weir?
It can be said that the height of the weir and the gap at its base, in combination with its length, are used to control the flow rate. By increasing the height of the weir, you are essentially raising the liquid level in the tray, which in turn leads to an increase in pressure drop； By increasing the clearance gap, the area at the outlet expands, the flow velocity decreases, and naturally, the mass transfer efficiency also changes. You haven’t specified your material system and operating conditions here, so it’s difficult to provide you with appropriate reference values.
The height of the overflow weir cannot be increased; otherwise, the liquid level on the tray rises, the resistance increases, and consequently the pressure drop also increases. If the bottom gap is increased, it is only possible to raise the height of the outlet weir.
What the two gentlemen upstairs said is correct, but there is one issue: even if the height of the overflow weir is increased from 25 to 40, this will increase the liquid layer height by at most 20 mm. Such an increase in head pressure certainly won’t double the pressure drop across the tray! This question is very strange. . .
Thank you all, dear sea friends; the problem has been resolved. It wasn’t due to the tray; after inspection, it turned out that a valve on the liquid phase reflux pipe was broken. The pointer indicated that the valve was in the open position, but upon examination it was found that the valve was only slightly open, which caused both the tower pressure and the system pressure to rise
I have a question: why does the backflow cause such a significant increase in pressure drop? Is there a relationship between backflow and pressure drop?