What is flooding?

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1. What is flooding? In a distillation column, when the liquid phase accumulates beyond the available space for various reasons, this phenomenon is known as flooding. Foaming can be classified into downcomer foaming, mist entrainment foaming, etc. Liquid flooding in the downcomer refers to the accumulation of the liquid phase in the downcomer up to the upper tray. Foam entrainment flooding occurs when the gas flow velocity in the open spaces on the tray reaches a certain level, causing the liquid phase on the tray to be carried upward with the rising gas to the upper tray. The operating condition at which flooding occurs is called the flooding point. When designing a distillation column, it is necessary to control and maintain the flooding rate within a certain range to ensure its stable operation. At the onset of flooding, the pressure drop in the tower increases sharply, and its efficiency drops significantly. Subsequently, the operation of the tower was disrupted. 2. What causes the flooding phenomenon? Based on the different causes of flooding, it can be divided into two types of flooding: flooding caused by excessive mist entrainment and flooding in the downcomer. (1) The liquid in the downcomer flows back to the upper tray. Since the trays exert resistance on the upward-moving airflow, the pressure above the lower tray is higher than that above the upper tray. Only when the static head corresponding to the height of the foam in the downcomer is sufficient to overcome this pressure difference can the liquid flow downward. When the liquid flow rate remains constant while the gas flow rate increases, the pressure difference between the lower and upper plates also increases, causing the liquid level in the downcomer to rise. If the gas flow rate increases to such an extent that the liquid in the downcomer rises to the top of the weir, the liquid in the tube can no longer flow downward; instead, it begins to flow back upward toward the upper tray, where liquid starts to accumulate ； As operations proceed, liquid is continuously fed in from outside the tower, eventually filling the entire tower with liquid. This results in flooding. When the gas flow rate remains constant but the liquid flow rate increases, the resistance to liquid flow through the downcomer rises; additionally, the thickening of the liquid layer on the tray increases the pressure difference between the top and bottom of the tray. Both effects cause the liquid level in the downcomer to rise, ultimately leading to flooding. (2) Excess foam carried to the upper plate: The foam carried by the airflow to the upper plate can cause the liquid layer on the plate to thicken. Once it increases to a certain level, this thickening becomes noticeable (the amount of liquid on the plate increases, along with the number and size of bubbles). The mist carried away by the airflow through the thickened liquid layer increases further. This excessive entrainment of liquid foam reduces the distance between the top of the foam layer and the bottom of the upper plate. The entrainment of liquid foam continues to increase, with large droplets easily being ejected onto the upper plate; foam can also rise to that plate, until eventually the entire tower is filled with liquid. 3. How many types are there of flooding phenomena? The pressure difference between the bottom and top of the tower increases ； The temperature difference between the bottom and top of the tower decreases ； The liquid level in the tower top reflux tank has decreased ； The product output at the bottom of the tower has decreased ； The product quality at both the top and bottom of the tower is unsatisfactory. 4. What method should be used for processing? Adjust the gap at the bottom of the liquid dropping plate ； Reduce the amount of rising steam ； Reduce feed amount ； Reduce steam volume and return flow. Note: Of the two reasons mentioned above that lead to flooding, excessive entrainment of liquid foam is the more common one.