Out of the following gas-liquid contacting devices, for a given set of operating conditions, gas pressure drop is the least in __________ tower.

ANS:A - wetted wall

A wetted wall column, also known as a wetted wall scrubber or wet scrubber, is a type of gas-liquid contacting device used for the removal of pollutants or contaminants from a gas stream. It consists of a vertical column with a wetted wall surface along its interior. Here's how it typically works:

1. Gas Stream Entry: The contaminated gas stream enters the bottom of the wetted wall column.
2. Wetted Wall: Inside the column, there is a surface that is kept wet, either by a liquid flowing down it or by spray nozzles that continuously wet the surface. This wetted surface can be made of various materials, such as ceramic or metal.
3. Gas-Liquid Contact: As the gas stream moves upward through the column, it comes into contact with the wetted wall surface. The contaminants in the gas stream transfer from the gas phase to the liquid phase due to absorption or adsorption onto the wetted surface. The liquid film on the wall captures the pollutants.
4. Clean Gas Exit: After passing through the wetted wall section, the gas stream exits the top of the column, now with reduced pollutant concentration.
5. Liquid Drainage: The liquid containing the captured contaminants collects at the bottom of the column and is typically drained out for further treatment or disposal.

Wetted wall columns are often used for the removal of soluble gases, such as sulfur dioxide (SO2), hydrogen sulfide (H2S), ammonia (NH3), and volatile organic compounds (VOCs), from industrial gas streams. They are particularly effective for applications where the gas stream contains high concentrations of pollutants or where high removal efficiency is required. One of the advantages of wetted wall scrubbers is their simplicity and ease of operation. However, they may have limitations in terms of handling high gas flow rates and may require regular maintenance to ensure proper wetting of the wall surface and efficient pollutant removal.