Two-Directional Mass Transfer Between Phases

It follows that for the flow rate d/i of the key component in the height element dz, the mass balance is 

Integrating with respect to dz gives the height Z of the column for mass and heat 

If G and kg are not constant along Z, this must be taken into account during the integration. dy is the change of concentration in the upflow phase, in the height element dz, effected by the driving force y — y.  

The integral of Eq. (1-205) is, according to Chilton and Colburn [1.105] the Number of Transfer Units NTU). 

NTUgg gives a number of transfer units describing how often the driving force y -y is needed in the overall interval ya yu and the concentration differences between the entry and exit (z = 0 and z = Z) of the column. The overall mass transfer resistance is considered which is expressed by the index og (overall gas) and is related to the upflowing phase. 

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