Which of the following gas-liquid contacting devices incurs the least pressure drop for a particular duty ?

ANS:C - Wetted wall tower

A wetted wall tower, also known as a packed column with a wetted wall, is a type of gas-liquid contacting device used in chemical engineering and process industries for various separation and absorption processes. In a wetted wall tower, the column wall is kept wetted with the liquid phase, while the gas phase flows upward through the column. The liquid phase is continuously distributed over the column wall, forming a thin film. As the gas phase flows upward, it comes into contact with this thin film of liquid on the column wall. Mass transfer occurs between the gas and liquid phases through the interface of the liquid film. The wetted wall tower is often packed with inert packing material to increase the surface area available for gas-liquid contact and enhance mass transfer efficiency. The packing material helps to maintain the thin film of liquid on the column wall by promoting even distribution of the liquid and preventing it from draining down the column too quickly. This type of column design offers several advantages:

1. High Mass Transfer Efficiency: The intimate contact between the gas and liquid phases on the wetted column wall promotes efficient mass transfer, making it suitable for processes such as absorption, stripping, and distillation.
2. Low Pressure Drop: Compared to some other gas-liquid contacting devices, wetted wall towers typically incur relatively low pressure drops, especially when operated under optimal conditions.
3. Compact Design: Wetted wall towers can be designed with relatively compact footprints, making them suitable for installations where space is limited.
4. Flexibility: Wetted wall towers can handle a wide range of gas and liquid flow rates and are often adaptable to different process conditions and requirements.

However, there are also some limitations to consider, such as potential fouling of the column wall due to deposition of solids or heavy components from the liquid phase, and the requirement for proper design and operation to ensure uniform wetting of the column wall and efficient mass transfer.