Distillation eHANDBOOK
Distill Your Distillation Operations
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Effectively Break Azeotropes Homogenous and heterogeneous varieties require different methods
By Tom Schafer and Alan Erickson, P.E., Koch Modular Process Systems, LLC.
n azeotrope is a complex formed recovery often necessitates breaking by two or more chemicals and azeotropes when substances are required Ahas the same composition in the in a purer form, either downstream or in liquid and the vapor phase. This interest- other areas of the chemical process. Water ing property means that the compound removal may motivate others. Situational mixture distills without separating. Some and contextual factors also may impinge common well-known binary azeotropes are on process optimization. Several basic water (95.4%) and ethanol (4.6%) as well as types of processes to break azeotropes diethyl ether (33%) and halothane (66%), using distillation and real-world examples formerly used in anesthesia. However, as are described. sometimes happens in chemical processing, an azeotrope may be formed unexpectedly DISTILLATION AND DECANTATION or, even if recognized, may make down- The easiest azeotrope to break is a hetero- stream processes additionally challenging. geneous azeotrope. Unlike homogeneous azeotropes, heterogeneous azeotropes’ This article will discuss some methods used two compounds are not totally misci- to “break” azeotropes that may exist in ble. The combination of distillation and binary or ternary forms and their applica- decantation works with heterogeneous tions, with a focus on breaking azeotropes azeotropes whereby condensation of the using various distillation methods. Solvent two liquids results in two phases that then
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can be decanted. That is, each of the two However, at 95 psig, the mixture is closer phases can be fed to separate stripping to 88% THF. By operating two columns columns. Examples of mixtures typically at the different pressures, dry THF can be separated this way include the minimum retrieved from the bottom of the 95 psig boiling azeotrope of n-butanol and water column and the azeotrope emerging at the or dichloromethane (methylene chloride) top, recycled to the atmospheric column. and water. AZEOTROPIC DISTILLATION In the example of n-butanol and water, after This distillation process is used to break condensation, the water-rich phase goes to homogeneous azeotropes by introducing an a stripping column where butanol-free water additional component called an entrainer, comes out the bottom and the butanol-rich which forms a lower boiling decantable phase goes to another column where dry ternary azeotrope. The ethanol and water butanol comes out the bottom and the azeotrope (95.6:4.4) mentioned above is distillate of each column approaches the a classic example of this type of process azeotropic composition. Generally, chlori- whereby benzene (or the more commonly nated hydrocarbons and water or any of the seen cyclohexane) is used as the entrainer. aromatic or paraffinic solvents and water Similarly, an isopropanol and water azeo- can be separated that way. trope can be broken using di-isopropyl ether as the entrainer. The lower boiling PRESSURE SWING DISTILLATION azeotrope then is condensed and decants. When two substances are totally miscible, as in homogenous azeotropes, other factors can Typically, this process requires three sep- come into play. Pressure swing distillation is arate distillation columns. The first column effective when the azeotrope’s composition distills the alcohol to the azeotrope with is a reasonably strong function of pressure. water removed as the bottom product. The An azeotrope distilled at a low pressure second column is the azeotropic column, in subsequently is fed to a second distillation which the alcohol is removed at the bottom column operating at a higher pressure where and the ternary azeotrope is removed as the azeotropic composition is substantially the distillate product. This distillate then different. Acetonitrile and water and tetra- is decanted with entrainer-rich phase and hydrofuran (THF) and water are two typical returned to the azeotropic column and the azeotropes broken using this method. water-rich phase is sent to the third column to remove entrainer and alcohol from the At atmospheric pressure, the azeotrope water. Sometimes, columns one and three formed between THF and water is 95% THF. can be combined.
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In the case of ethanol and water, ethylene glycol may be used to change the behavior of the two substances and allow distillation to be effective.
Obviously, the use of specific entrainer between two liquid phases breaks azeo- substances will not be applicable to all tropes. Pyridine-water and THF-methanol industries. Introduction of a known toxin are examples of azeotropes that can be or carcinogen such as benzene, for exam- broken via LLE. ple, is a nonstarter for a pharmaceutical process. Newer membrane technologies — in which the membrane material is robust enough to EXTRACTIVE DISTILLATION withstand attack by the substances being In contrast to azeotropic distillation, in separated and the pore size can distin- extractive distillation an additional high guish the complexes formed by a mixture’s boiling component is introduced. The different components — also can break added component, which basically is a azeotropes. However, membranes also diluent, changes the original binary pair’s are subject to clogging. Over time, even behavior such that the azeotrope can trace amounts of solvents or particles, such be broken. Azeotropes of ethanol and as minute amounts of catalyst, can com- water or methanol and acetone can be promise the polymeric material used in broken this way. Azeotropes of refriger- membrane filtration. ants also are amenable to breaking using this method. DETERMINING THE BEST APPROACH In the case of ethanol and water, ethylene Breaking azeotropes or even recogniz- glycol may be used to change the behavior ing when they are present is not always of the two substances and allow distillation straightforward. Most undergraduate chem- to be effective. ical engineering curricula do not spend much time on nonideal separations in their ALTERNATE TECHNIQUES FOR mass transfer classes. Turning to companies BREAKING AZEOTROPES with expertise in mass transfer and separa- Liquid-liquid extraction (LLE) can be used tions is critical, particularly when there is a if the difference in the solubility of a solute suspicion that azeotropes may be in play.
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It takes a substantial amount of experience to develop and conceptualize the proper approach.
It takes a substantial amount of experience break the azeotrope, and pilot testing to develop and conceptualize the proper can uncover those trace components and approach. find solutions.
Pilot plant testing often is required to test Working with or around nonideal sepa- the actual feedstock and show that the rations, such as azeotropes, may require conceptual design first offered will meet multiple distillation columns and sev- required specifications. The drums of actual eral decades of chemical process design feedstock that customers send to com- experience. panies for pilot scale testing frequently contain minute amounts of other contami- TOM SCHAFER and ALAN ERICKSON, P.E., are vice nants, which differ from the ideal feedstock presidents at Koch Modular Process Systems, LLC. They upon which the concept design was devel- can be reached at [email protected] oped. This can affect the path chosen to and [email protected].
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