Why does the absorption liquid in a packed tower show a wall-favoring tendency as it passes through the packing?
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The absorbing liquid descending in a packed tower should, in theory, move downward in a uniform and vertical manner as it convects with the gas; in practice, however, it moves diagonally outward gradually. This results in insufficient mixing between the gas and liquid, leading to low efficiency in the intermediate section; why does this phenomenon occur?
Could it be that the filler isn’t filled evenly, resulting in bridging?
When filling the packing, it’s not done in a staggered manner; it’s filled in sequence. Are you using corrugated regular packing?
It’s simple; this is the tower wall phenomenon in packed towers. At first, the distribution is quite uniform, but after descending a certain distance, due to the larger gaps near the tower walls, the solution gradually concentrates towards those walls. To overcome this phenomenon, when the packing layer is high, a liquid redistributor is added in the middle to ensure as even a distribution of liquid as possible within the packed tower.
One is related to whether the filler is filled evenly, and the other is related to the distribution and redistribution of the solution.
As the liquid flows downward through the packing layer, it tends to concentrate toward the tower walls, resulting in an increasing flow rate of liquid near those walls; this phenomenon is known as wall flow； The wall flow effect causes uneven distribution of the gas-liquid phases within the packing layer, thereby reducing the mass transfer efficiency; this is a common phenomenon in packed towers. Therefore, when the packing layer is high, it needs to be segmented, with a redistribution device installed in between. The liquid redistribution device consists of a liquid collector and a liquid redistributor. The liquid that flows down from the upper packing is collected by the liquid collector and then sent to the liquid redistributor, where it is redistributed before being sprayed onto the lower packing. When installing the packing, it is necessary to ensure it is level and that the plates are tightly packed together; for towers with high requirements, it is possible to consider adding anti-wall-flow rings and similar elements.
Reply to 6# chinazwr: Your argument is basically the same as that of 116. It makes sense for me to think that way as well. The relatively large gaps around the outside indeed indicate that it is a channel prone to flow. Furthermore, since the wall surface section is under compression, it is not easy for the filler material to return to that area on one side only. It’s not clear whether this factor exists or not. Another idea is that as gas flows, the middle part moves forward while the air mass moves in a rotating motion; as a result, even when gas flows in a straight line, it can also move sideways. Additionally, due to the viscosity of liquids, when they come into contact with large surfaces, wall flow is formed, just as mentioned. There are a few possibilities as well, all of which are just ideas; they have no basis and it’s not clear whether they are correct or not.
The main cause of wall flow is still the uneven distribution of gas and liquid. Adding a distributor is the conventional approach to solving the problem. Place a distributor at an appropriate distance, one that does not affect the air velocity and liquid flow rate, while effectively addressing the wall flow issue.
All packed towers exhibit wall flow, as the solution flowing along the wall encounters the least gas resistance and thus follows the shortest path; this phenomenon is unrelated to the structure or the way the packing is arranged. To reduce wall flow, packed towers are typically filled in sections, with liquid redistributors placed between each section to cause the solution that flows along the walls to return back onto the packing. Please remember that when a liquid and a gas come into contact in reverse order, they will definitely take the path with the least resistance and the shortest distance.