Production Technology [Weekly Topic]: Production Q&A on flooding-related issues (2011-01-17~23)

In a distillation column or absorption tower, the gas and liquid phases flow in opposite directions. When the flow rates of both phases are low, the flow of one phase is not affected by the other. As the flow rates increase, some liquid droplets can be carried upward by the rising gas stream to the upper tray, resulting in mist entrainment. The liquid phase can also carry some bubbles into the downcomer; this entrainment phenomenon intensifies as the flow rates increase, and in severe cases it can lead to blockages in the flow channels and reverse flow of liquid within the column. This phenomenon is known as flooding. Foaming is the limit of counterflow between the gas and liquid phases. When flooding occurs, the pressure difference and liquid level in the tower fluctuate: when the tower pressure difference rises sharply, the liquid level at the bottom of the tower drops ； And when the pressure difference drops suddenly, the liquid level rises sharply ； At the same time, the pressure in the tower will also fluctuate ； The purity of the product will decrease. Measures to prevent flooding: (1) Control an appropriate liquid-to-gas ratio; (2) Control an appropriate load ； (3) Ensure the liquid is clean and does not foam easily ； (4) Ensure that the tray is clean and free of contaminants.

In counter-current gas-liquid reactors or mass transfer separation devices, 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 eventually overflows from the top of the tower, a phenomenon known as flooding. 　　The gas velocity or the velocity of the continuous phase at which flooding occurs is known as 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. 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: 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, where liquid starts to accumulate ； As it is operated, 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 ultimately 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.

Reply to 1# shmily327: Liquid flooding, also known as tower flooding, occurs when the gas phase in the tower rises from bottom to top across each tray due to pressure differences, while the liquid phase flows across each tray through the downcomers under the effect of gravity. If the liquid in the downcomer cannot flow downward properly, and it accumulates to the point where it overflows the top of the weir, causing the liquid between the two plates to merge into one mass, this phenomenon is known as flooding. It is an abnormal phenomenon in the operation of overflow trays, which severely reduces the efficiency of the trays, causes fluctuations in column pressure, and deteriorates the product separation performance. a. The liquid in the downcomer flows back to the upper tray. Since the trays exert resistance 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, where liquid starts to accumulate ； As it is operated, 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 ultimately 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.

This post was last edited by shmily327 on 2011-1-17 08:58. 1. Flooded column, also known as tower flooding, involves the gas phase rising from bottom to top across the plates due to pressure differences, while the liquid phase flows across the plates through the downcomers under the effect of gravity. If the liquid in the downcomer cannot flow downward properly, and it accumulates to the point where it overflows the top of the weir, causing the liquid between the two plates to merge into one mass, this phenomenon is known as flooding. It is an abnormal phenomenon in the operation of overflow trays, which severely reduces the efficiency of the trays, causes fluctuations in column pressure, and deteriorates the product separation performance. 2. Pan tower factors: (1) Backflow of liquid in the downcomer back to the upper tray ； 　　(2) Excess liquid foam is entrained to the upper tray. 3. Prevention methods: (1) Try to increase the cross-sectional area of the downcomer, but this will reduce the opening area of the tray plates. (2) Improve the tray structure to reduce the tray pressure drop. (3) Control the liquid return flow rate to be not too high.

In a distillation column or absorption tower, the gas and liquid phases flow in opposite directions. When the flow rates of both phases are low, the flow of one phase is not affected by the other. As the flow rates increase, some liquid droplets can be carried upward by the rising gas stream to the upper tray, resulting in mist entrainment. The liquid phase can also carry some bubbles into the downcomer; this entrainment phenomenon intensifies as the flow rates increase, and in severe cases it can lead to blockages in the flow channels and reverse flow of liquid within the column. This phenomenon is known as flooding. Foaming is the limit of counterflow between the gas and liquid phases. When flooding occurs, the pressure difference and liquid level in the tower fluctuate: when the tower pressure difference rises sharply, the liquid level at the bottom of the tower drops ； And when the pressure difference drops suddenly, the liquid level rises sharply ； At the same time, the pressure in the tower will also fluctuate ； The purity of the product will decrease. Measures to prevent flooding: Control an appropriate liquid-to-gas ratio ； Control an appropriate load ； Ensures the liquid remains clean and does not foam easily ； Ensure the tray is clean and free of contaminants.

An excessive flow rate in one of the gas-liquid phases causes an increase in the pressure drop across the upper and lower trays; as a result, the downcomer cannot function properly and the liquid backs up to the upper trays. Reason: 1. It is caused by the excessive vapor velocity at the rising point inside the tower, which exceeds the maximum allowable gas velocity. An excessive liquid load causes the liquid level in the overflow pipe to rise gradually, until the liquids on the upper and lower trays merge together, disrupting the normal operation of the tower. 3. Large adjustment range. Phenomenon: 1. When the load at the top of the tower is high, the liquid level in the top reflux tank is high, and the purity decreases. 2. When the liquid phase load is high, the liquid level at the bottom of the tower is high, the temperature drops, and more heat energy is required for reboiling. Prevention: 1. Pay attention to material balance, as well as gas-liquid phase balance and heat balance during operation. 2. Operate with gentle adjustments, fine-tuning.

In counter-current gas-liquid reactors or mass transfer separation devices, 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 eventually overflows from the top of the tower, a phenomenon known as 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: a. Backflow of liquid in the downcomer 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 it is operated, 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 ultimately the entire tower is filled with liquid.

Reply to 1# shmily327: In counter-current gas-liquid reactors or mass transfer separation devices, 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 eventually overflows from the top of the tower, a phenomenon known as flooding. 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, where liquid starts to accumulate ； As it is operated, 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 ultimately the entire tower is filled with liquid.

The excessive flow velocity of the gas in the lower layers of the column prevents the condensate from flowing downward, resulting in the formation of a liquid ring within the column. As the gas passes through this liquid ring, numerous bubbles are generated, which carry liquid to the upper layers of the column, leading to flooding. The main reason is that the gas flow velocity is too fast.

This post was last edited by slayerstar on 2011-1-18 at 15:10. Reply to 3# liwei3547: Strong currents caused by rising vapor prevent the liquid from flowing properly to the lower trays, which over time leads to flooding!