【Daily Question】Chemical Engineering Principles 103: Plate Columns (July 8)

Liquid flow rate, tray structure, and liquid flow length； A sieve plate tower with a simple structure can be used, along with double or multiple overflow downcomers.

This post was last edited by Desert Fish on 2015-7-8 10:51. What are the reasons for the liquid level difference in plate-type Tata columns? How to prevent it? The contact condition between the gas and liquid phases on the tray is an important factor determining the hydrodynamics of two-phase flow as well as the laws of mass and heat transfer on the tray. When the liquid flow rate is constant, four different contact states can occur as the gas velocity increases. (1) Bubble contact regime: When the gas velocity is low, the gas passes through the liquid layer in the form of bubbles. Due to the small number of bubbles, the resulting gas-liquid mixture is primarily liquid-based; the surface area where the gas and liquid phases come into contact is small, resulting in very low mass transfer efficiency. (2) In the honeycomb contact state, as the gas velocity increases, the number of bubbles keeps rising. When the formation rate of bubbles is greater than their rising rate, the bubbles accumulate in the liquid layer. The bubbles collide with each other, forming various polyhedral large bubbles, while the surface consists of a gas-liquid mixture dominated by gas. Since the bubbles are not easy to burst and the surface is not renewed, this condition is unfavorable for heat and mass transfer. (3) Foam contact state: As the gas flow rate continues to increase, the number of bubbles rises sharply; these bubbles collide and burst continuously. At this point, most of the liquid on the plate exists in the form of a liquid film between the bubbles, resulting in dynamic foams with small diameters and intense turbulence. Only a thin layer of liquid can be seen on the plate. Due to the large surface area of the foam contact state, which is constantly being renewed, it provides favorable conditions for heat and mass transfer between the two phases, making it an excellent contact state. (4) Jet contact state: As the gas velocity continues to increase, the high kinetic energy of the gas sprays the liquid on the plate into droplets of varying sizes. The larger droplets fall back onto the plate due to gravity, while the smaller droplets are carried away by the gas, resulting in liquid mist entrainment. At this point, the gas on the tray is the continuous phase, while the liquid is the dispersed phase; the area for mass transfer between the two phases is the outer surface of the liquid droplets. As the droplets return to the tray and are dispersed again, this repeated formation and aggregation of droplets increases the mass transfer area; moreover, the surface is continuously renewed, which facilitates mass transfer and heat transfer, representing an excellent state of contact.

Liquid flow rate, tray structure, liquid flow length. Dual overflow trays are used.

The factors that cause a liquid level difference on the trays of a plate tower include liquid flow rate, tray structure, and liquid flow length. To prevent this, double or multiple downcomers can be used, as well as sieve tray towers with a simple structure.

Liquid flow rate, tray structure, and liquid flow length； A sieve plate tower with a simple structure can be used, along with double or multiple overflow downcomers.

Liquid flow rate, tray structure, and liquid flow length； A sieve plate tower with a simple structure can be used, along with double or multiple overflow downcomers.

Liquid flow rate, tray structure, and liquid flow length； A sieve plate tower with double or multiple downcomers, which has a simple structure, can be used.

The contact condition between the gas and liquid phases on the tray is an important factor determining the hydrodynamics of two-phase flow as well as the laws of mass and heat transfer on the tray. When the liquid flow rate is constant, four different contact states can occur as the gas velocity increases. (1) Bubble contact regime: When the gas velocity is low, the gas passes through the liquid layer in the form of bubbles. Due to the small number of bubbles, the resulting gas-liquid mixture is primarily liquid-based; the surface area where the gas and liquid phases come into contact is small, resulting in very low mass transfer efficiency. ! c4 [) O- C) [( l6 t2 U （2）As the gas velocity increases, the number of bubbles keeps rising in the honeycomb-like contact state. When the formation rate of bubbles is greater than their rising rate, the bubbles accumulate in the liquid layer. The bubbles collide with each other, forming various polyhedral large bubbles, while the surface consists of a gas-liquid mixture dominated by gas. Since the bubbles are not easy to burst and the surface is not renewed, this condition is unfavorable for heat and mass transfer. $ a% H/ @; B0 @. y8 s (3) Foam contact state: As the gas flow rate continues to increase, the number of bubbles rises sharply, and the bubbles keep colliding and bursting. At this point, most of the liquid on the plate exists in the form of a liquid film between the bubbles, resulting in dynamic bubbles with small diameters and intense turbulence; only a thin layer of liquid can be seen on the plate. Due to the large surface area of the foam contact state, which is constantly being renewed, it provides favorable conditions for heat and mass transfer between the two phases, making it an excellent contact state. (4) Jet contact state: As the gas velocity continues to increase, the high kinetic energy of the gas sprays the liquid on the plate into droplets of varying sizes. The larger droplets fall back onto the plate due to gravity, while the smaller droplets are carried away by the gas, resulting in liquid mist entrainment. At this point, the gas on the tray is the continuous phase, while the liquid is the dispersed phase; the area for mass transfer between the two phases is the outer surface of the liquid droplets. As the droplets return to the tray and are dispersed again, this repeated formation and aggregation of droplets increases the mass transfer area; moreover, the surface is continuously renewed, which facilitates mass transfer and heat transfer, representing an excellent state of contact.

The reasons for the liquid level difference on the trays of a plate tower include liquid flow rate, tray structure, and liquid flow length； A sieve plate tower with a simple structure can be used, along with double or multiple overflow downcomers.