Relationship between downcomer area, leakage, and flooding
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I am currently working on a graduation project related to sieve plate towers, and I have encountered issues related to operational flexibility and the area of the downcomers. Given the liquid flow rate within the tower, can it be understood that as the area of the downcomers increases, the opening area of the sieve plates decreases? This, in turn, leads to an increase in the gas flow velocity. As a result, the operational flexibility of the tower (gas velocity/velocity at the point of liquid leakage) improves. However, as the gas velocity increases, the pressure drop across the sieve plates rises sharply. To ensure smooth flow of the liquid over the sieve plates, the liquid level in the downcomers must rise in order to overcome this increased pressure drop, which increases the risk of flooding There is another question I would like to ask everyone: how can we understand that the greater the liquid flow rate, the lower the corresponding flooding gas velocity?
As the gas velocity increases, the area of the downcomer needs to be increased to prevent flooding in the downcomer
Correct. The flooding gas velocity refers to the gas velocity at which flooding occurs. There are mainly two types of flooding phenomena: one is flooding caused by liquid foam entrainment, and the other is flooding in the downcomer. From what the OP seems to mean, it refers to the first situation. In practice, flooding often occurs due to the first situation as well.
Hehe... The first case is like this: as the liquid flow rate increases, the liquid layer thickness on the plate increases, and mist entrainment becomes more severe. Is the situation with liquid overflow in the downcomer the same as well? ? When we draw the tray performance diagram, don’t we take into account liquid overflow from the downcomer? And is the flooding line slope negative? The main cause of flooding in the downcomer is an excessive pressure drop across the tray. If flooding occurs in the downcomer as well, and the greater the liquid flow rate, the lower the flooding velocity, then how should we take into account the pressure drop across the tray caused by these two factors? On the one hand, as the gas velocity decreases, the resistance of the dry plate drops sharply; on the other hand, as the thickness of the liquid layer on the plate increases, the resistance to gas passing through this liquid layer rises. How should these two factors be taken into account in practical design?
For a given tower diameter, the width of the downcomer is fixed depending on the plate spacing, and much work has already been done by previous researchers.
Weeping refers to the excessive accumulation of the liquid phase inside the tower. The first is foam entrainment flooding, and the second is downcomer flooding. Foam entrainment flooding: The liquid flow rate is high, and the liquid phase is dispersed on the tray in the form of foam; as the gas flow velocity increases, the height of the foam increases. When the distance between the trays is small, the foam of the gas-liquid phase approaches the upper tray. As this surface layer gets closer to the upper tray, entrainment increases rapidly, leading to an accumulation of liquid phase on the upper tray. There are mainly two types of liquid flooding in downcomers: 1. Downcomer backmixing flooding: The gas-entrained liquid flows back into the downcomer due to reasons such as the pressure drop across the tray, the height of the foam layer on the tray, and the frictional resistance at the inlet of the downcomer. As the liquid flow rate increases, all these factors also increase; when the gas flow rate increases, the tray pressure drop also increases. When the gas-liquid mixture flows back into the downcomer over the distance between trays, the liquid phase accumulates on the upper tray, causing backmixing of the liquid in the downcomer. 2. Downcomer blockage and flooding: As the liquid flow rate increases, the flow velocity of the gas-liquid mixture in the downcomer also increases. Beyond a certain limit, the frictional resistance in the downcomer and at the inlet becomes too high, preventing the mixed foam liquid phase from being transported to the next tray; as a result, a buildup of liquid phase occurs on the upper tray. As mentioned above, the causes of flooding can be attributed to two factors: one is flooding due to entrainment caused by an excessive gas phase load, which is primarily determined by the gas velocity. The second is flooding in the downcomer caused by an excessive liquid phase load, which is primarily determined by the highest clear liquid layer on the tray and represents the upper limit for the liquid phase load. I’m not quite sure what the original poster means by the statement that the greater the liquid flow rate, the lower the corresponding flooding gas velocity. If the gas velocity remains constant, the higher the liquid flow rate, the lower the flooding gas velocity should be.