Knowledge about flooding

The gas phase load is high, resulting in a large pressure difference between the top and bottom of the tower; this causes the tower temperature to invert. Solution: Reduce the amount of reflux and steam used at the tower bottom

Reply 1# nsc1981: In a counter-current gas-liquid reactor or mass transfer separation device, the gas flows from bottom to top. When the flow rate of the gas increases to a certain value, the liquid is prevented from flowing downward by the gas; it accumulates more and more until it overflows from the top of the tower, a phenomenon known as flooding. 　　The gas velocity or continuous phase velocity at which flooding occurs is called the flooding velocity ； This operating condition is known as the flooding point. When designing equipment, it is necessary to ensure that its operation does not lead to flooding. Foaming in tower equipment and its causes: At the onset of foaming, the pressure drop in the tower increases sharply, and its efficiency drops significantly. Subsequently, the operation of the tower was disrupted. The main factors that contribute to flooding are as follows: a. Backflow of liquid in the downcomer back to the upper tray. Due to the resistance exerted by the tray on the rising gas flow, 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. 　　b. 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; 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 and grow larger). 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. 　　Of the two reasons mentioned above that lead to flooding, excessive entrainment of liquid foam is the more common one.

Foaming leads to backmixing of the material, which is detrimental to mass transfer. Foaming includes entrainment foaming and overflow foaming; the former occurs due to an excessive gas flow rate and a long residence time of the liquid phase on the plate, while the latter results from limitations in the downcomer flow rate, primarily due to high gas-liquid flow rates

The formation of flooding is related to the flow rates of the gas and liquid phases. For a certain liquid flow rate, an excessively high gas velocity can cause flooding ； On the contrary, for a certain gas flow rate, an excessive liquid volume can also lead to flooding.

　In a counter-current gas-liquid reactor or mass transfer separation device, the gas flows from bottom to top. When the flow rate of the gas increases to a certain value, the liquid is prevented from flowing downward by the gas; it accumulates more and more until it overflows from the top of the tower, a phenomenon known as flooding. 　　The gas velocity or continuous phase velocity at which flooding occurs is called the flooding velocity ； This operating condition is known as the flooding point. When designing equipment, it is necessary to ensure that its operation does not lead to flooding. 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 limit, 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; eventually, 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 increases sharply, and its efficiency drops significantly. Subsequently, the operation of the tower was disrupted. The main factors that contribute to flooding are as follows: a. Backflow of liquid in the downcomer back to the upper tray. Due to the resistance exerted by the tray on the rising gas flow, 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. 　　b. 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; 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 and grow larger). 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. 　　Of the two reasons mentioned above that lead to flooding, excessive entrainment of liquid foam is the more common one.

Let me answer your question; this is my personal opinion. To put it simply: 1. In a distillation tower, the liquid level is such that the gas pressure generated within the tower is just enough to support the liquid flowing back into the tower, meaning that there is a pressure balance between the rising gas and the falling liquid inside the tower. 2. (1) It may be that the system’s load adjustments involve large fluctuations in magnitude. (2). Check what types of materials are present in your tower; it’s possible that excessive light components were introduced from your raw materials, and it’s also possible that the material at the bottom of the tower tends to foam at high temperatures. 3. Consequences of liquid overflow: (1.) The pressure difference inside the tower increases; in the case of a pressurized high-pressure distillation tower, this can lead to vibrations and oscillations in the tower. (2). The liquid at the bottom of the tower will rise, and the refluxing liquid will not be able to flow downward, resulting in the evaporation of the liquid phase at the tower bottom and causing the heater to overheat. (3) It will result in the product obtained at the bottom of the tower being substandard. Additional note: Liquid splashing generally only occurs in pressure-driven distillation; it does not happen in low-pressure distillation. Handling overflow (based on personal work experience): Reduce steam flow, lower the reflux rate, decrease the pressure difference – breaking the pressure balance will do the trick. Once regurgitation occurs, parameters should be adjusted promptly. Produce qualified products as soon as possible.

Well, pay attention to the pressure when there is liquid overflow, and address it promptly. to avoid greater losses

It is mainly a problem of equilibrium between the gas and liquid phases； Whether it is gas or liquid, once it exceeds its equilibrium point ； Then flooding will occur.