HETP Prediction—Mass Transfer Models

Three approaches are commonly used for HETP prediction mass transfer models, rules of thumb, and data interpolation. These ap- 

The Bravo and Fair correlation (122). This correlation is based on the two-film model, that assumes resistance to mass transfer in both the vapor and liquid phases. The correlation treats the mass transfer coefficient independently from the interfadal area, and accounts for effects of partial wetting, The correlation is based on extensive commercial and pilot scale efficient measurements for first generation packings and Psill rings, and applies to random packings only. 

For calculating mass transfer coefficients, the Bravo and Fair correlation uses the relationship by Onda et al. (123). 

The transfer units for each phase are calculated from 

Some data relevant to the application of Bravo and Fair s correlation are tabulated in Table 9,1. Note that the values used in deriving this correlation are slightly different from those used by Billet and Schultes (Table 8.2). 

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Because there are only few variables that significantly affect randompacking HETP (Sec. 9.1.3), and due to the unreliability of even the best mass transfer model, rules of thumb for HETP successfully compete with mass transfer models. It has been the author s experience that rules of thumb for HETP prediction are more accurate and more reliable than mass transfer models. A similar conclusion was reached by Porter and Jenkins (127). 

The ideal model (Chapter 7) assiunes an infinite column efficiency. This makes the band profiles that it predicts unrealistically sharp, especially at low concentrations. This sharpness is explained by the fact that the ideal model propagates concentration discontinuities or shocks. For a hnear isotherm, the elution profile would be identical to the input profile, clearly an unacceptable conclusion. The effects of a nonideal column are significant in three parts of the band profile. The shock is replaced by a steep boimdary, the shock layer, whose thickness is related to the coefficients of the column HETP (axial dispersion and mass transfer resistance see Chapter 14). The top of the band profile is roimd, instead of being... 

In packed columns, it is conceptually incorrect to use the staged model even though it works if the correct height equivalent to a theoretical plate (HETP) is used. In this chapter we will develop a physically more realistic model for packed columns that is based on mass transfer between the phases. After developing the model for distillation, we will discuss mass transfer correlations that allow us to predict the required coefficients for common packings. Next, we will repeat the analysis for both dilute and concentrated absorbers and strippers and analyze cocurrent absorbers. A simple model for mass transfer on a stage will be developed for distillation, and the estimation of stage efficiency will be considered. After a mass transfer analysis of mixer-setder extractors. Section 16.8 and the appendix to Chapter 16 will develop the rate model for distillation. 

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