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PART 1
FUNDAMENTALS AND TOOLS
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CHAPTER 1
INTRODUCTION
An azeotrope is a mixture of chemical components that has identical compositions of the liquid and vapor phases in equilibrium with each other. This azeotropic phenomenon occurs because of molecular interactions between different chemical components. If the components contain similar elemental molecules and structure, the molecular interaction is very slight and azeotropes do not occur. Mixtures of hydrocarbons (propane, butane, pentane, etc.) are important examples. These mixtures have phase equilibrium behavior that is close to ideal and often have relative volatilities that are almost constant. However, if the components are dissimilar, the molecules can exhibit either repulsion or attraction. Repulsion increases the effective vapor pressures of the components at a given temperature and can produce minimum-boiling azeotropes. In terms of liquid-phase activity coefficients, discussed in detail in Chapter 2, repulsion produces activity coefficients greater than unity. If the repulsion is large enough, the repulsive forces can lead to the formation of heterogeneous minimum-boiling azeotropes (two liquid phases). The azeotrope boils at a temperature that is lower than the boiling point of the lighter component.
The methanolâwater mixture is an example of a system that exhibits modest nonideality. The OH end of the methanol molecule is similar to the OH end of the water molecule, but the hydrocarbon CH3 end of the methanol molecule is different from water. So the system exhibits a modest amount of nonideality, which can be represented by the change in relative volatility as liquid composition changes.
If we add another CH2 group and move to the ethanolâwater system, there is more repulsion because the CH3âCH2 end of the ethanol molecule is quite different from the OH end of me water molecule. The system exhibits more nonideality and a minimum-boiling azeotrope occurs. This azeotrope is homogeneous (only one liquid phase in equilibrium with a vapor phase). If we add two more CH2 groups and move to the n-butanolâwater system, the repulsion is even more extreme. The result is the formation of a heterogeneous minimum-boiling azeotrope with two liquid phases in equilibrium with a vapor phase.
In other chemical systems, the molecules can attract instead of repulse. The results can be the formation of maximum-boiling azeotropes because the molecular attraction reduces the effective vapor pressures of the components. Examples include nitric acid-water, acetoneâchloroform, formic acidâwater, and n,n-dimethyl acetamideâacetic acid.
Chapter 2 is devoted to a detailed discussion of the vaporâliquidâliquid phase equilibrium of azeotropic systems. In this chapter we provide some historical perspective of the field, discuss several typical and important applications, and provide some journal references in the area of control of azeotropic distillation systems.
Also included is an example of one of the important complexities in trying to deal with azeotropic distillation systems. These systems are highly nonlinear and exhibit the phenomenon of multiple steady states.
1.1 HISTORY
The existence of azeotropes has been known for many years, probably as long as man has made ethanol. Designing chemical engineering processes to separate azeotropes has been dealt with in some of the earliest chemical engineering books. The first chemical engineering textbook used by the senior author was the 1937 Principles of Chemical Engineering,1 authored by three of the very early workers in the field of chemical engineering. On page 526 of that pioneering book, the xy curve for ethanolâwater was presented. Subsequent pages showed curves for typical homogeneous minimum and maximum-boiling azeotropes.
The earliest book that concentrated on distillation as a unit operation Elements of Fractional Distillation,2 was first published in 1922 and was into its fourth edition by 1950. Chapter 10 of this work discussed extractive and azeotropic distillation. In an era before computers, the authors presented how plate-to-plate component and phase equilibrium equations can be solved manually (mechanical calculator or slide rule) to design several extractive distillation systems. One interesting example was the maximum-boiling nitric acidâwater system using sulfuric acid as the heavy entrainer (solvent). They also presented a detailed design of azeotropic distillation of ethanolâwater using benzene as the light entrainer. Ternary diagrams and composition profiles were presented. The labor-intensive calculations probably took a hard-working graduate student many days or weeks to complete. We are fortunate to now have the tools to perform these calculations in seconds.
The earliest book that concentrated exclusively on extractive and azeotropic distillation was published in 1964 by E. J. Hoffman.3 Ternary diagrams for many azeotropic systems were discussed, including the concepts of residue curves, and alternative flowsheet configurations were presented.
One of the pioneers in developing a method for separating azeotropic mixtures was Donald F. Othmer, who worked for Eastman Kodak before moving to Brooklyn Polytechnic University. Othmerâs many papers and patents in the 1930s and 1940s were major contributions to the field. His early paper,4 discussed the use of azeotropic distillation to facilitate the separation of acetic acid and water (which do not form an azeotrope but which exhibit a severe pinch in the high-water region) by the addition of a light entrainer (ethylene dichloride). Othmer developed these process designs without the aid of modern tools.
The advent of computer technology greatly enhanced our ability to explore azeotropic systems in a quantitative manner. Two of the early workers in developing and applying modern computational tools were Mic...
