What are flooding, tower flooding, and tower overflows?

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In chemical plant operations, distillation towers are the most common and typical separation devices. Anyone involved in chemical production is familiar with distillation towers. However, the causes of common problems that arise during their operation, such as flooding, liquid level overflows, and other related issues, are not always clear. Moreover, there is a lack of sensitivity to changes in the tower’s parameters when such problems occur, which often delays the resolution of these issues and affects the plant’s production. Today, we will conduct a detailed analysis of the reasons behind the several issues mentioned above, and also provide examples to illustrate the changes in parameters and incorrect operations that occur when problems arise in production! First, let’s look at the flooding phenomenon that we are most familiar with. Image ► 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 divided 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 the flooding rate within a certain range to ensure its stable operation. At the onset of flooding, the pressure drop in the tower rises sharply, and its efficiency drops significantly. Subsequently, the operation of the tower was disrupted. ► What causes the flooding phenomenon? 1. The liquid in the downcomer flows back to the upper tray. Due to the resistance exerted by the tray on the rising 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. If the gas flow rate remains constant while the liquid flow rate increases, the resistance to the flow of liquid through the downcomer rises, and the liquid layer on the plate thickens, which increases the pressure difference between the upper and lower sides of the plate. All these factors cause the liquid level in the downcomer to rise, leading to flooding. 2. Excessive foam is carried up 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 more noticeable (the amount of liquid on the plate increases, along with the number and size of bubbles). The droplets carried away by the airflow through the thickened liquid layer increase 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. ► How many types are there of flooding phenomena? 1. The pressure difference between the bottom and top of the tower increases ； 2. The temperature difference between the bottom and top of the tower decreases ； 3. The liquid level in the tower top reflux tank decreases ； 4. The product output at the bottom of the tower decreases ； 5. The product quality at both the top and bottom of the tower is unsatisfactory. ► What method should be used for processing? 1. Rectify the bottom clearance of the downcomer plate ； 2. Reduce the amount of steam rising ； 3. Reduce the feed amount ； 4. Reduce steam volume and return flow. Note: Of the two reasons mentioned above that lead to flooding, excessive entrainment of foam is the more common one. The second common problem depicted in the image is tower flooding. During the distillation process, liquid gradually accumulates above a certain tray, eventually filling part of the tower section and obstructing the upward flow of gas. As a result, the mass and heat transfer between the gas and liquid phases cannot take place properly; this phenomenon is known as tower flooding. ► The phenomenon of tower flooding is a decrease in the temperature at the top of the tower ； The liquid level in the reflux tank is dropping ； The liquid level and pressure at the bottom of the tower increase. ► The reasons for tower flooding are as follows: 1. The settlement pipe is blocked, preventing the return fluid from flowing downward. At the start of operation, debris such as iron shavings and welding slag, the accumulation of corrosion products on the equipment during normal production, or the precipitation of solids from liquids, along with the self-polymerization of solutions, can all easily cause blockages in the downcomers. 2. The excessive amount of liquid overloads the downcomer. ► There are two methods for handling this: 1. Appropriately reduce the feed rate and return flow rate. 2. In the event of equipment failure, operations shall be halted for repair. The last common problem with images is tower flooding. During the normal operation of a distillation tower, the gas and liquid phase loads remain relatively stable. When both the gas and liquid phase loads are too high, the pressure drop across the tray increases, which leads to an increase in the liquid level in the downcomer ； As the liquid load increases, the liquid level height at the outlet weir rises. When the liquid fills the entire downcomer, the upper and lower trays become connected, distillation is completely disrupted, and flooding of the column occurs. ► The reasons for tower flooding are: any factor that leads to an excessive gas-liquid load inside the tower can cause flooding, such as the processing volume of crude oil, the low density of the feed material, the water content in the crude oil entering the tower, the amount of steam supplied at the tower bottom, excessively high feed temperature, or interruptions or uneven distribution of reflux. ► Phenomenon: When a tower surge occurs, the distillation efficiency inside the tower deteriorates, disrupting normal mass and heat transfer. As a result, the temperature and pressure at the top of the tower, as well as the temperature at the side product outlets and the reflux temperature, all increase. Meanwhile, the liquid level at the bottom of the tower drops suddenly, and the color of the distilled oil turns darker. ► The principle for handling this is to reduce the vapor-liquid load, that is, to decrease the reflux rate and the amount of heating steam at the bottom of the tower; if the processing volume is too high, the feed rate can be reduced. If necessary, feeding can be interrupted and the heating steam at the bottom of the tower can be turned off; once the temperatures of each tray return below normal levels, heating and feeding can be resumed. ► Analysis of the data shows that, based on the changes in the parameters of the stabilizer tower, a) the separation efficiency of the tower has declined, and the purity of the product at the tower bottom has decreased; as a result, even with an increased amount of steam supplied, the temperature of the sensitive plate remains below the values observed during normal operation ； b) With the top pressure remaining constant, increasing the reflux rate still results in a tray temperature below the normal value; this indicates that the purity of the product at the top of the tower decreases, and the separation efficiency worsens ； c) The temperature difference between the sensitive plate (the 3rd tray) and the 21st tray decreased significantly, indicating an increase in light components in the lower trays and an increase in heavy components in the upper trays. Since the liquid level at the bottom of the tower could still be controlled properly, it can be concluded that severe flooding has occurred in the tower. In the case of tower washing, the liquid level at the bottom of the tower will drop rapidly; this is a clear distinction between tower washing and flooding. ► What caused it? For a distillation column that has been designed and is operating properly, when surge or flooding occurs under conditions of little change in feed composition, the analysis should primarily focus on operational aspects. As can be seen from the comparative data on flooding in the stabilizer shown in the figure above, both the reflux rate of the stabilizer and the amount of heating steam at the bottom of the tower are higher than normal values; these are the most common operational factors that lead to flooding. The operators lack experience and do not have a thorough understanding of the operation of distillation columns. When the temperature of the sensitive plate is low, they increase the amount of heating steam at the bottom of the column; when the temperature at the top of the column is high, they increase the reflux rate. This back-and-forth process results in excessive amounts of heating steam and reflux, causing both the gas and liquid loads to far exceed the column’s design limits. As a consequence, flooding occurs, and the gas-liquid equilibrium within the column is disrupted. After flooding occurred in this stabilizer, the reflux rate and the amount of steam at the bottom of the tower were adjusted again, but after 16 hours the stabilizer still had not reached a normal equilibrium state. Finally, measures such as shutting off the heating steam, stopping feeding, and lowering the temperature were taken, and normal operation was restored after restarting.