Mass transfer analysis packed column distillation

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. 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

Size the Distillation Column. This includes the hydraulic analysis to establish operating ranges, pressure drop, and mass transfer efficiency. The result is a set of dimensions, including column diameter and height, number of actual trays (or height of packed bed), details of internal devices, and profiles of temperature and pressure. 

How the analysis of distillation proceeds further depends on the internals of the distillation column equipment. In almost all laboratory columns and in many modern commercial columns, these internals are random or structured packing. The packing in small columns is often the same as the random packing used for absorption. The packing in commercial columns is more commonly structured because it permits higher flows at modest pressure drops. In both cases, the analysis uses a mass balance on one phase. This mass balance depends on rates of mass transfer summarized as an overall mass transfer coefficient. The analysis then is a close parallel to that used for absorption. When the distillation involves a dilute solution, the column size can often be estimated analytically. When the distillation involves a concentrated solution, the estimation of column size is similar but requires numerical integration. 

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