1. Introduction

In general, the matter that makes up the Earth and everything living on it has a heterogeneous nature in relation to the different states (solid, liquid and gas). In the case of liquid solutions, the development of separation technologies is associated with the chemical industry [1, 2], which is a driving force behind it. Any transformation process of matter requires preliminary and/or subsequent steps, which change the composition of the solutions involved. One objective may be to purify the products generated in a reactor, for example, by removing the presence of inert compounds, contaminants, by-products or excess reagents, and even to produce solutions with specific compositions, different to those obtained in the reactor. So, many processes in the chemical industry are essentially combinations of physical separation processes that do not require a chemical reaction.

There are a variety of separation processes known to date [3], but the performance of distillation [4] make it the most important and most used operation in the chemical industry. However, not all solutions can be separated into their simple components by classical multi-step distillation (rectification) techniques. Because of this, advanced methods aimed at resolving the limitations of distillation in relation to specific problems have been developed. Two of the most important complications in the correction of solutions are: (1) the presence of azeotropes and, (2) the proximity of the boiling points of the components in the dissolution (close boiling point). Similar strategies are followed to separate solutions with either of these scenarios, and the main differences between them lie in certain technical details of the design. Some examples are recorded in Table 1. Most of the procedures described try to modify the system either by changing the operating conditions (such as in pressure-swing-distillation), or by adding an extractant (entrainer), although the latter can present some reactivity with some of the components present. Other techniques combine the rectification with other operations based on different physical principles, such as pervaporation or liquid-liquid extraction.

engineering operations, and more specifically those cited above (E-M-S), actions that the authors have pursued in recent works [20, 21]. Initially, the phenomenon of azeotropy is considered, supported by the basic thermodynamic formulation, in an attempt to understand its origin and sensitivity to changes in the system conditions. The experimental methods available to measure azeotropic points are exposed, their strengths and limitations are discussed, and reference is made to tools [18, 19] to determine data quality. Regarding the modeling, different strategies are used to characterize VLE diagrams and to estimate the presence of azeotropes, with a critical analysis to predict the appearance of singular points. Finally, the information compiled is used in several examples to design azeotropic separation processes, taking into consideration different conditions

Enhanced distillation Particular cases Mixtures Entrainer Azeotropic distillation Minimum boiling point Not found Not found pressure-swing THF + water

Liquid-liquid extraction Ethanol + water

Pervaporation Ethanol + water

Extractive distillation With volatile solvent Not found Not found

Table 1. Advanced distillation techniques with industrial examples and details of the entrainer used.

Boundaries bending Hydrochloric acid + water

Methyl ethanoate + methanol

Non-available Non-available Non-available 141

http://dx.doi.org/10.5772/intechopen.75786

Sulfuric acid Sulfuric acid

Benzene, toluene Self-entraining Self-entraining Benzene

Cellulose acetate membrane

Tert-butylbenzene Ethylene glycol

Methyl ethanoate + isobutanol o-xylene + ionic liquid

Water + ethanol

Azeotropy: A Limiting Factor in Separation Operations in Chemical Engineering - Analysis, Experimental…

Nitric acid + water

Butanol + water Hydrocarbon + water Pyridine + water

Toluene + heptane

With heavy solvent Isoprene + pentane Furfural, acetonitrile With salt Ethanol + water Acetate-based salts

> m-xylene + p-xylene Ethanol + water

2. Azeotropy: description of the phenomenon and thermodynamic

Etymologically, the term "azeotrope," coined by the chemists J. Wade and R.W. Merriman [22], comes from the Greek combination of three words "a" (without), "zein" (boiling) and "trope" (change), in other words, to boil without change, referring to a solution for which the variables (p,T,x) remain unchanged, which is the main characteristic of this phenomenon. These authors described the phenomenon of azeotropy when studying the VLE of the

of ester and alkane solutions.

Reactive distillation Reactive entrainer without catalyzer

promoted

Reactive entrainer catalyzed-

representation

Although important from a practical perspective the problems posed by a "close boiling point" (CBP) do not require complicated theorization. This phenomenon tends to occur in dissolutions involving chemically similar compounds (of the same chemical nature), with a behavior close to ideality. Azeotropy, however, is a complex phenomenon with different modes of presentation for which the complexity increases exponentially with the number of compounds in solution. Many authors have attempted to write about azeotropy [5, 6], while others have focused on making experimental measurements with different solutions and/or compiling the results [7, 8]. However, the current literature is still scarce. It is especially important to clarify the physical causes of the azeotropes, influenced by the situation and their repercussions on process design (with the sequence: experimentation, E-modeling, M-simulation, S), particularly from a practical perspective.

For many years, our research group has conducted experiments on azeotropic systems (see [9–12]), mainly on solutions containing esters, alkanols and alkanes. Experimental developments have also been proposed to determine vapor-liquid equilibria (VLE) [13, 14], and theoretical approaches to model experimental thermodynamic data [14–17] and to assess their quality [18, 19]. In this chapter, the azeotropy is studied from different perspectives which governs the design of some Azeotropy: A Limiting Factor in Separation Operations in Chemical Engineering - Analysis, Experimental… http://dx.doi.org/10.5772/intechopen.75786 141


Table 1. Advanced distillation techniques with industrial examples and details of the entrainer used.

1. Introduction

processes that do not require a chemical reaction.

140 Laboratory Unit Operations and Experimental Methods in Chemical Engineering

larly from a practical perspective.

In general, the matter that makes up the Earth and everything living on it has a heterogeneous nature in relation to the different states (solid, liquid and gas). In the case of liquid solutions, the development of separation technologies is associated with the chemical industry [1, 2], which is a driving force behind it. Any transformation process of matter requires preliminary and/or subsequent steps, which change the composition of the solutions involved. One objective may be to purify the products generated in a reactor, for example, by removing the presence of inert compounds, contaminants, by-products or excess reagents, and even to produce solutions with specific compositions, different to those obtained in the reactor. So, many processes in the chemical industry are essentially combinations of physical separation

There are a variety of separation processes known to date [3], but the performance of distillation [4] make it the most important and most used operation in the chemical industry. However, not all solutions can be separated into their simple components by classical multi-step distillation (rectification) techniques. Because of this, advanced methods aimed at resolving the limitations of distillation in relation to specific problems have been developed. Two of the most important complications in the correction of solutions are: (1) the presence of azeotropes and, (2) the proximity of the boiling points of the components in the dissolution (close boiling point). Similar strategies are followed to separate solutions with either of these scenarios, and the main differences between them lie in certain technical details of the design. Some examples are recorded in Table 1. Most of the procedures described try to modify the system either by changing the operating conditions (such as in pressure-swing-distillation), or by adding an extractant (entrainer), although the latter can present some reactivity with some of the components present. Other techniques combine the rectification with other operations based on

different physical principles, such as pervaporation or liquid-liquid extraction.

Although important from a practical perspective the problems posed by a "close boiling point" (CBP) do not require complicated theorization. This phenomenon tends to occur in dissolutions involving chemically similar compounds (of the same chemical nature), with a behavior close to ideality. Azeotropy, however, is a complex phenomenon with different modes of presentation for which the complexity increases exponentially with the number of compounds in solution. Many authors have attempted to write about azeotropy [5, 6], while others have focused on making experimental measurements with different solutions and/or compiling the results [7, 8]. However, the current literature is still scarce. It is especially important to clarify the physical causes of the azeotropes, influenced by the situation and their repercussions on process design (with the sequence: experimentation, E-modeling, M-simulation, S), particu-

For many years, our research group has conducted experiments on azeotropic systems (see [9–12]), mainly on solutions containing esters, alkanols and alkanes. Experimental developments have also been proposed to determine vapor-liquid equilibria (VLE) [13, 14], and theoretical approaches to model experimental thermodynamic data [14–17] and to assess their quality [18, 19]. In this chapter, the azeotropy is studied from different perspectives which governs the design of some engineering operations, and more specifically those cited above (E-M-S), actions that the authors have pursued in recent works [20, 21]. Initially, the phenomenon of azeotropy is considered, supported by the basic thermodynamic formulation, in an attempt to understand its origin and sensitivity to changes in the system conditions. The experimental methods available to measure azeotropic points are exposed, their strengths and limitations are discussed, and reference is made to tools [18, 19] to determine data quality. Regarding the modeling, different strategies are used to characterize VLE diagrams and to estimate the presence of azeotropes, with a critical analysis to predict the appearance of singular points. Finally, the information compiled is used in several examples to design azeotropic separation processes, taking into consideration different conditions of ester and alkane solutions.
