Absorption Title


Gas absorption can be carried out in a packed column. We will look at both column diameter and packed height in this Section.

The Figure below showed a typical gas-liquid flow in a packed column. Click here for more information of packings.

VL flow in packed column.

Determination of column diameter involves the analysis of pressure drop across the packed bed.


As for packed height, the design used in the early days was based on the HETP method. This was largely replaced by the Method of Transfer Units.


Comparison between number of theoretical trays, HETP and Method of Transfer Units

The Number of Transfer Units (NTU) and Height of Transfer Units (HTU) such as NOG, HOG should not be confused with the number of theoretical trays (N), and the height equivalent to theoretical plate (HETP) respectively.

When the operating line and equilibrium line are straight and parallel:

NTU = N ; and HTU = HETP

Otherwise, the NTU can be greater than or less than N as shown in the Figure below:

Relatioship between NTU and N

When the operating line is straight but not parallel, we have the following relationships:

where is the Absorption Factor.

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