Guide to Packing Height Specification

In developing our major design expression (Equation 5.33), it was assumed that the overall mass transfer coefficient and the gas side coefficient were the 

In writing the energy balance (Equation 5.17) we slipped another fast one by the reader by assuming h to be the same as ho, the heat transfer coefficient from the liquid film-air film interface to the air. If a significant resistance is associated with the liquid film, h is better expressed by the overall heat transfer coefficient (U), which consists of both he andhL. Note that hL is the convection coefficient from the liquid film to the interface [7], When appreciable liquid film resistance exists, our energy balance should be written as 

We can also write an expression for the material balance with interfacial values  

By applying the Lewis number prediction (hG/kCs =1), where k is the gas side mass transfer coefficient, and using Equation 5.29, we get 

The number of diffusion units is computed from the relation /dT/(Hs -H). It is determined from process conditions imposed on the tower alone, not by the tower s performance. Htu is the only parameter determined experimentally. 

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