Relation between HTU and HETP

As noted in Chapter 5, the performance of a particular packing is often expressed in terms of a height equivalent to a theoretical plate (HETP) and is so reported by packing manufacturers. Performance data in terms of a HTU, or a volumetric mass transfer coefficient Ka, have a sounder theoretical basis but are relatively rare (see Table 5.6). A link between HTU and HETP can, however, be easily established for the case of a linear equilibrium relation. To show this, we start with the definitions of HTU and HETP and simply divide the two expressions. We obtain 

The ratio of number of plates to number of transfer units is obtained from the Kremser Equation 7.15b and Equation 8.4d. There results 

Because the typical operating range for the extraction factor is E = 1.2 to 2, we find 

At the threshold value E = 1, HTU and HETP have the same value. Thus, for the linear case at least, the HTU is typically 10 to 30% lower than the HETP. 

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