【Weekly Topic】Production Technology: What is flooding and what are its causes? 2011.05.23~05.29

Reply 1# sun-rock: In distillation operations, the liquid on the lower trays rises to the upper trays, disrupting the normal operation of the tower; this phenomenon is known as flooding. The cause of flooding is mainly due to the steam rising inside the tower at an excessive speed, exceeding the maximum allowable speed. Additionally, in distillation operations, it is common to encounter a situation where the liquid load is too high, causing the liquid level in the overflow pipe to rise and resulting in the liquids on the upper and lower trays mixing together, which disrupts the normal operation of the tower; this is also a form of flooding. Both of these phenomena belong to flooding, but their causes are different. When the gas velocity increases to a certain level, foam entrainment occurs.

There are several abnormal operating conditions in distillation columns: flooding, liquid leakage, dry plate, and foam entrainment. All of these conditions are related to the gas-liquid phase loads. Flooding, also known as liquid overload, occurs when the gas or liquid flow rate increases to the point where the liquid level in the downcomer rises to its top. As a result, the liquid on the trays cannot flow downward properly, causing the space between two trays to become filled with liquid. This disrupts normal operation and makes it difficult to maintain control of the process. In essence, as the steam flow increases, the resistance across the tray increases, which means the pressure difference between the upper and lower sides of the tray rises, causing the liquid level in the downcomer to increase. As the liquid flow rate increases, the resistance to the flow of the liquid rises, causing the liquid level in the downcomer to increase, eventually leading to tower flooding. In practical operations, when the viscosity of the material is relatively high, the phenomenon of foam entrainment is quite common; the extent of this issue varies, and it reduces the efficiency of distillation. It is not easy to address this problem. However, liquid flooding – that is, the tower being filled with liquid – constitutes an emergency situation. Solutions include: reducing the processing volume and lowering the temperature. If possible, remove the tower removal system and carry out a full reflux operation, which may allow for a faster recovery.

For a tower of fixed diameter, the cross-section through which both gas and liquid phases can flow freely is limited. If the flow rate of either one of them increases to a certain level, the liquid in the downcomer cannot flow downward smoothly ； When the liquid in the tube rises to the top of the overflow weir on the upper plate, it will spill over onto that upper plate, causing abnormal accumulation of liquid; ultimately, this can result in the space between the two plates being filled with foam. This phenomenon is known as flooding, also referred to as tower flooding. 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. The main factors that contribute to flooding are as follows: 1. Backflow of liquid in the downcomer back to the upper tray. Due to the resistance exerted by the trays on the upward-flowing gas, 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, causing liquid to accumulate on that tray ； 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. Excess 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; under normal conditions, however, this increase is not significant. At a certain liquid flow rate, if the gas flow rate is increased to a certain level, the thickening of the liquid layer becomes significant (more liquid accumulates on the plate, and more bubbles form that are also larger in size). 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. Among the causes of flooding, excessive liquid foam entrainment is a common one. There are many other factors as well, mainly the properties of the filler, the physical properties of the liquid, and the liquid-to-gas ratio. Treatment: Reduce the feed load and operate at normal return flow rates. Stop feeding and withdrawing from the tower, and operate in full reflux until the product meets the specifications.

This post was last edited by cfzh56 on 2011-5-23 00:10. Liquid flooding occurs when the liquid from the lower trays is carried up to the upper layers, affecting the normal heat and mass transfer between gas and liquid within the tower. The formation reasons are related to the gas phase load and liquid phase load, as well as the opening ratio of the tray plates, the overflow weir, and the flow area of the downcomer

An excessive liquid load, an excessive vapor load, or too small a downcomer area result in an increased pressure drop across the tray. This leads to an increase in the liquid level inside the downcomer and to an elevation of the foam layer on the tray until it comes into contact with the liquid flow between the trays, thereby severely reducing the efficiency of the tray and impairing the quality of product separation.

Flooding: When the drainage capacity of the downcomer is insufficient, liquid continues to be added. This causes the liquid level in the downcomer to rise above the overflow weir of the upper tray. As a result, the drainage from the upper tray is impaired, leading to an accumulation of liquid on that tray until flooding occurs throughout the column. Phenomenon: When flooding occurs, the pressure drop of the gas through the tray increases sharply; a large amount of liquid is carried away with the gas exiting the tower, the quality of the product at the top of the tower deteriorates rapidly, and normal operation is disrupted. Reason: (1) Excessive gas flow rate or excessive liquid flow rate. (2) Excess liquid entrained in the gas increases the drainage load on the downcomers; if the lower end of the downcomer in a certain tray becomes blocked, it can cause flooding in the section of the tower above that tray. Treatment: (1) Flooding caused by blockage can be resolved by cleaning the tower and pre-filtering the tower liquids. (2). Flooding caused by excessive liquid entrainment can be resolved through load control. ＜2＞. Mist entrainment limit: When gas passes through the liquid layer on the tray and reaches the liquid surface, the bubbles burst and the gas shoots upward. As the gas moves upward, it carries some of the liquid in the form of thin films with it; these small droplets vary in size. As they rise, they collide with each other, causing them to grow larger. The larger droplets rise to a certain height before settling back down before reaching the upper trays, while the smaller droplets are carried upward by the flowing gas to the upper trays. This phenomenon is known as mist entrainment. 　Harm of mist entrainment: (1) It causes liquid backmixing, reducing the mass transfer efficiency. (2) Increasing the load on the downcomer raises the liquid layer thickness on the tray, thereby increasing the resistance to gas flow through the tray; as a result, the amount of liquid entrained with the gas increases, and in severe cases this can lead to flooding.

Reply 1# sun-rock: The upward steam flow rate inside the middle section of the distillation column is too high; it exceeds the maximum allowable value and rises to a certain level. The liquid is prevented from flowing downward by the gas, so it accumulates more and more. It can even overflow from the top of the tower, a phenomenon known as flooding. The cause of flooding is an excessively high velocity of the rising steam flow.

Inside the distillation tower, the liquid flows downward layer by layer along the tray through the overflow weir, where it comes into contact with the hotter vapor on the tray, resulting in heat transfer as well as partial evaporation and partial condensation. If the liquid on the tray has difficulty flowing down through the overflow hopper, causing the liquid level in the overflow hopper to rise higher and higher until it reaches the same level as the liquid level on the tray and the liquid can no longer flow downward, this condition is known as \"liquid suspension\" or \"liquid flooding\". As shown in Figure 56, when the liquid level in the overflow hopper exceeds 50% of the hopper’s height, it is considered that mild flooding has begun. During normal flow of gas and liquid in a distillation column, since the vapor flows from bottom to top, it must overcome the resistance of the tray plates. Therefore, the pressure at the upper part (P2) is lower than the pressure at the lower part (P1). Its pressure difference (P1-P2) reflects the magnitude of the resistance per tray. The flow of liquid from top to bottom is from a region of lower pressure to a region of higher pressure; therefore, the liquid level in the overflow hopper must be sufficiently higher than the liquid level on the tray for the liquid to flow out. At the same time, as the liquid flows through the overflow hopper, it must also overcome the resistance at the outlet. Therefore, the liquid level in the overflow hopper will only remain stable and allow the liquid to flow smoothly when the pressure generated by the liquid column is sufficient to overcome the pressure difference between the upper and lower parts of the tray as well as the resistance of the overflow hopper. When the plate resistance increases, resulting in an increase in the pressure difference between the upper and lower sides of the plate, or when the resistance in the overflow funnel increases, the existing liquid level within the overflow funnel is no longer sufficient to overcome this pressure difference and resistance; as a result, the liquid cannot flow downward for a while. When the liquid level in the overflow hopper rises to a certain height, a new equilibrium is reached. When the plate resistance or the overflow tray resistance is too high, it will cause the liquid level in the overflow tray to rise further, until it reaches the same level as the liquid level on the previous plate; accordingly, the liquid on the plate also rises. When the liquid on the tray rises to a level that the rising vapor can no longer support, it flows down through the sieve holes. If the cause of flooding is not eliminated, the above process will repeat itself. Thus, when flooding occurs inside the tower, both the resistance and the liquid level will fluctuate significantly. It also disrupts the distillation process inside the tower; as a result, the purity of the product often fails to meet the required standards, and there are large fluctuations, making it impossible to maintain normal production. During operation, every effort should be made to avoid flooding, and it should be addressed promptly.

The cause of flooding is an excessive vapor load on the tray. Essentially, the liquid from the previous tray does not have enough time to flow down through the downcomer and the gap between the trays; as a result, the liquid accumulates on that tray. Eventually, the liquid also fills the tray above it, and at that point, flooding occurs. After flooding occurs, the pressure at the bottom of the tray increases, making it more difficult for the fluid to flow downward; if the heating is reduced, a full-liquid level will appear at the bottom of the tower. Measures to prevent flooding are: (1) operate at the specified feed rate ； (2) Operate according to the specified return flow rate.

Inside the distillation tower, the liquid flows downward layer by layer along the tray through the overflow weir, where it comes into contact with the hotter vapor on the tray, resulting in heat transfer as well as partial evaporation and partial condensation. When the liquid on the tray has difficulty flowing down through the overflow trough, causing the liquid level inside the trough to rise continuously until it becomes level with the liquid level on the tray, and no further flow occurs, this phenomenon is called “flooding”. When the liquid level in the overflow hopper exceeds 50% of the hopper’s height, it is considered that mild flooding has begun. Reasons for flooding: 1. Liquid flowing back from the downcomer to the upper tray. Due to the resistance exerted by the trays 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 decreases, 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, causing liquid to accumulate on that tray ； Furthermore, as liquid is continuously fed into the tower during operation, it eventually fills the entire tower, resulting 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. Excess 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; under normal conditions, however, this increase is not significant. At a certain liquid flow rate, if the gas flow rate is increased to a certain level, the thickness of the liquid layer increases significantly (more liquid accumulates on the plate, and more bubbles form); the droplets carried away by the gas stream as it passes through this thicker liquid layer also 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. Among the causes of flooding, excessive liquid foam entrainment is a common one.