Distillation | Azeotropic Distillation AA Kiss, AkzoNobel, Deventer, The Netherlands ã 2013 Elsevier Inc. All rights reserved.
Introduction Working Principle Residue Curve Map Entrainer Selection Process Design and Simulation Industrial Applications Alcohol Dehydration Acetic Acid Dehydration Production of Esters Concluding Remarks References
Glossary Azeotrope Constant boiling mixture that does not change in composition by distillation, and generally it has a boiling point higher or lower than any of its pure constituents. Distillation boundary It is a one-dimensional curve that restricts how far a distillation can proceed – the composition space on a residue curve map (RCM) being consequently split into distillation regions. Entrainer A chemical species that acts as a mass separating agent (MSA) by forming a light binary azeotrope with only one of the components of the original mixture. K-value Ratio of the concentration of a given component in the vapor phase to its concentration in the liquid phase when the V–L phases are in equilibrium. Pinch point A tangent pinch point is graphically shown as a dip or inflection on the xy diagram, being the
1 1 2 3 5 5 6 6 7 7 8
point where the rectifying line is tangent to the equilibrium curve. Relative volatility Defined as the ratio of the K-values of two components, it is a measure of the ease with which the two components can be separated by distillation. Residue curve Describes the change of the composition of the liquid phase of a chemical mixture during continuous evaporation at vapor–liquid equilibrium condition (open distillation). Residue curves map (RCM) A collection of multiple residue curves for a single system. Tie line Is the line on the ternary diagram that connects the two-liquid phases in equilibrium. Vapor pressure It is the pressure at which a liquid and its vapor are in phase equilibrium, at a given temperature.
Introduction Distillation is the main process used for the separation of liquid mixtures, and is based on differences in the boiling points or relative volatility of the constituent components.1,2 However, the separation of azeotropic mixtures or close-boiling components is not feasible in a single conventional distillation column.3 In such cases, the addition of another component (entrainer, solvent, or mass separating agent, MSA) is used to alter the relative volatility of the components to be separated.4,5 This entrainer is selected to assist the separation by taking advantage of the nonideal behavior. Examples of separation processes that make use of entrainers are liquid–liquid extraction, azeotropic distillation (AD), and extractive distillation (ED). AD takes advantage of azeotropes that form naturally between many chemicals, and it involves the formation of an azeotrope or the use of an existing one, to effect a desired separation.6,7 Note that an azeotropic point occurs when the compositions of the coexisting liquid and vapor phases are the same.8 This work focuses only on AD, addressing its working principle, residue curve maps (RCMs), ternary diagrams, solvent selection, process design options, as well as various industrial applications.
Working Principle AD is typically carried out by adding other light chemicals to generate a new, lower-boiling azeotrope that is heterogeneous – thus producing two, immiscible liquid phases that can be easily separated in a decanter. In an AD process, the light entrainer is thus evaporated and recycled – this being in contrast to ED, which makes use of a high-boiling solvent (not forming azeotropes) that does not have to be evaporated.
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering
Distillation | Azeotropic Distillation
Figure 1 T–xy diagrams for a homogeneous (left) and heterogeneous azeotrope (right).
An azeotrope can be homogeneous (one-liquid phase) or heterogeneous (two-liquid phases), as illustrated in Figure 1.6 A heterogeneous azeotrope can be conveniently separated using a decanter coupled with one or more distillation columns, which exploits both vapor-liquid equilibrium (VLE) and liquid–liquid equilibrium (LLE) driving forces.7,9 A homogeneous azeotrope that is pressure sensitive (i.e., composition changes with pressure) can be separated using pressure-swing distillation (PSD) in two distillation columns operating at different pressures, with an appropriate recycle strategy to achieve the desired separation. When the azeotropic composition is not very sensitive to pressure, then PSD involves large recycle flow rates (resulting in an uneconomical process) or it might not be feasible at all. In such cases, a minimum-boiling azeotrope can be formed by the introduction of a specific MSA (entrainer) to an existing azeotropic mixture or close-boiling mixture (a relative volatility close to unity could indicate the presence of a tangent pinch point, as shown in Figure 2) that is difficult to separate by conventional distillation. In practice, homogeneous AD is carried out in a sequence using a liquid-liquid extraction (LLX) column to separate the overhead product from the entrainer (Figure 3, left), while heterogeneous AD needs only one distillation column and a decanter taking advantage of the liquid–liquid split (Figure 3, right) (Lee and Wytcherley, 2000). When an entrainer is added in order to perform a heterogeneous AD, the flowsheet consists of two distillation columns and a decanter (shown in Figure 4, left). The two columns can be also conveniently integrated into a dividing-wall column (DWC) – as the one illustrated in Figure 4 (right), also known as a split shell column with common overhead section and divided bottoms section.10 It is worthwhile noting that DWC is already a proven process intensification technology in distillation, allowing much lower investment and operating costs (25–40%) while also reducing the equipment and carbon footprint.11
Residue Curve Map RCMs are typically applied to test the consistency of azeotropic data, predict the order and content of cuts in batch distillation, check if a given mixture is separable, identify entrainers, predict attainable compositions, make qualitative prediction of composition profile shape, identify the limiting separation achievable by distillation, and synthesize hybrid separation sequences.6,11,12 A RCM is a collection of liquid residue curves originating from different initial compositions (showing how the composition of the liquid residue changes over time). In a RCM, all the residue curves originate at the light (lowest boiling) pure component in a region, move toward the intermediate-boiling component, and end at the heavy (highest boiling) pure component in the same region. The lowest temperature nodes are termed as unstable nodes, as all trajectories leave from them; while the highest temperature points in the region are termed stable nodes, as all trajectories ultimately reach them. The point that the trajectories approach from one direction and end in a different direction (as always is the point of intermediate-boiling component) are termed saddle points (S). Residue curves that divide the composition space into different distillation regions are called distillation boundaries. The characteristic points are especially important in classifying the azeotropic mixtures.13,14 Figure 5 shows a selection of feasible RCMs for ternary heterogeneous mixtures – many others being possible.6 Although the RCM-based design is very powerful and can be used to find novel separation sequences and identify novel entrainers, several limitations still exist. The available tools for computing and sketching RCMs still require either a good property model or an extensive azeotrope database.7,8 Moreover, the interpretation of RCMs and the synthesis of feasible separation sequences require specific knowledge. However, the current property and process simulators drastically reduce these hurdles and make the RCM-based design easier accessible.
Distillation | Azeotropic Distillation
Figure 2 T–xy and xy diagrams for the binary mixture acetone–water – a pinch point is present at high acetone concentrations making the separation by distillation difficult.
Figure 3 Typical homogeneous (left) and heterogeneous (right) azeotropic distillation systems.
Figure 4 Azeotropic distillation with entrainer, in a two-column setup (left) and combined in a dividing-wall column (right).
Entrainer Selection The most used way to separate homogeneous azeotropes is by adding an extra component (entrainer or MSA) to facilitate the separation. Entrainers are also used to enable the separation of non-azeotropic mixtures where this is uneconomical (e.g., due to the presence of tangent pinch points, as in the case of water and acetone) or not feasible due to process constraints (e.g., maximum operating temperature). An entrainer helps the separation by selectively altering the relative volatilities of the components, thereby effectively breaking the azeotrope.7 Entrainer selection is a critical step in the synthesis and conceptual design of AD processes, since
Distillation | Azeotropic Distillation
Figure 5 Residue curve maps for various azeotropic systems involving homogeneous and/or heterogeneous azeotropes.
the choice of entrainer determines the separation sequence and the overall process economics. The desirable properties for an entrainer suitable for AD can be summarized as follows4:
• • • • • • •
Heterogeneous azeotrope formation with one component, for ease of entrainer recovery Low latent heat of vaporization (important due to complete evaporation of the entrainer) Thermally and chemically stable (to prevent degradation or undesired reactions) Low viscosity to provide high tray efficiencies Low freezing point to allow ease of handling and storage Nontoxic and noncorrosive substance Commercially available and preferably inexpensive
A systematic approach for entrainer selection begins by plotting the RCM and LLE diagram using rigorous process simulators, or by sketching them from the azeotropic temperature and approximating composition and solubility data using available methodologies.15 The actual screening of entrainers and the determination of the separating sequence for a binary mixture, involves the following steps7:
Compile a list of candidate entrainers, including: components that are already present in the process (e.g., reactants) or on the plant site (to avoid introduction of new chemicals), water (as it forms heterogeneous azeotropes with many components), entrainers used for similar components, and other commonly available chemicals.7,8 Plot the RCM for each candidate entrainer (e.g., ternary mixture of A, B, and entrainer). Use a detailed property model if available, otherwise estimate the missing properties using UNIFAC or similar predictive methods (but check the agreement with available azeotrope data). If no property model is available at all, the RCM can be sketched from available azeotropic temperature, composition, and solubility data.15 If no property model and no azeotrope information are available, the data can be estimated using predictive methods or by an educated guess that must be experimentally validated. Using the RCM, determine if the candidate entrainer is feasible for separation: for example, both components lie in the same distillation region, or the entrainer introduces a liquid-liquid tie-line that crosses the distillation boundary, dividing components into different distillation regions.
Distillation | Azeotropic Distillation
• • •
Synthesize all the separation sequences for each entrainer, using the RCM structure and the mass balance – thus determining the connections of distillation columns and decanters. Identify the entrainer feasibility conditions for the most promising candidate entrainers, if their feasibility had been determined from either azeotropic data or estimated using group contribution techniques. Verify experimentally these conditions, and further develop or update the detailed property model for the mixture, based on the experimental data. Design, simulate, and optimize the separation sequences using a rigorous process simulator (e.g., Aspen Plus, ChemCAD, ChemSep, gPROMS, HYSYS, Pro/II, ProSim, or UniSim).
Process Design and Simulation Considering the nonidealities, phase splitting, distillation boundaries, and the possible existence of multiple steady states in such systems, the AD columns can be difficult to design, simulate, and operate. A systematic approach to this problem is therefore advisable15:
• • • • • •
After the entrainer selection, plot the RCM with the liquid phase envelope superimposed, and the binodal plot (showing the liquid split) at the anticipated condenser conditions. Make a conceptual process design using the RCM and considering that the distillate and bottoms product must lie on the same residue curve, and also that the feed, distillate, and bottoms product must be collinear (e.g., all lie on the same straight material balance line). Distillation boundaries in homogeneous mixtures cannot be crossed by residue curves. Therefore, in order to isolate two pure components which lie in different distillation regions, two feed compositions (one from each of the two regions) and two distillation columns are needed. All feed to the AD column (reflux, makeup, and process feed) should be entered near the top. The composition of the vapor leaving the top tray must be near or at the ternary azeotrope. The vapor composition in equilibrium with a (liquid) point on a residue curve must lie on the tangent to that curve. This tangency condition relates each residue curve with its associated vapor boil-off curve – in hetero-region there is only one vapor boil-off curve, the vapor line. Thus, the vapor composition in equilibrium with a point on a hetero residue curve lies at the intersection of the tangent to the residue curve and the vapor line. The liquid boiling envelope is the projection of the heterogeneous liquid boiling surface onto the composition base plane. This is critical because the top tray equilibrium liquid composition must lie outside the liquid boiling envelope, so it is clearly not enough to lie outside the condenser binodal plot heterogeneous envelope. The column should be operated such that only one liquid phase exists on the stages, but the condenser must be operated in the heterogeneous region. Consequently, the vapor coming from the top tray must be in equilibrium with a single liquid phase, but it must condense to two-liquid phases. This can be achieved by careful manipulation of the condenser operating conditions (e.g., by lowering the temperature in order to enlarge the two-liquid phase region, sufficiently enough to include the distillate composition) and/or by mixing the organic and aqueous phases from the decanter in the reflux (e.g., by adding water to the decanter in order to shift the overall composition into the two-liquid phase region). A feasible AD requires that entrainers and top tray vapor compositions are selected, such as to generate liquid–liquid tie lines that spread across at least one distillation boundary. This jump effect can be practically exploited, and each distillation column is provided with a feed composition in the required distillation regions: for example, organic phase refluxed to the dehydration column, while decanted aqueous phase goes to the entrainer recovery tower. The composition of the two-liquid phases must be taken from tie lines on a binodal plot calculated at condenser (not top tray) conditions. The relative proportions of the two-liquid phases in the condenser, is given by the lever rule: F ¼ (ye,i xaq,i)/ (ye,i xorg,i). Many heterogeneous AD systems show a very high sensitivity to small changes in the operating conditions or requirements, such as: small changes in the reflux ratio or bottoms products purity specs can significantly impact the column temperature profile and/or entrainer requirements; minor losses of decanter interface can cause the entire column sequence to malfunction; and slight fluctuations in pressure can move the overhead composition outside the heterogeneous region, thus causing column failure. Within certain ranges, heterogeneous AD systems exhibit multiple steady states, which in close proximity can cause erratic behavior in the column and difficult control. Therefore, it is recommended to perform a sensitivity analysis in order to select a suitable operating point.
Industrial Applications Table 1 summarizes the industrial and potential AD applications, while the next part focuses on the most important industrial applications.4,9,11 Note that the main advantage of AD is the ability to separate chemicals that cannot be separated by conventional distillation in a feasible manner (e.g., systems containing azeotropes or pinch points), and therefore improving the economics by
Distillation | Azeotropic Distillation
Selected applications of azeotropic distillation
Entrainer (mass separating agent)
Acetic acid recovery or purification Terephthalate acid solvent recovery
Acetic acid/water Acetic acid/water
Preparation of high-purity esters
Water/esters (to change equilibrium)
THF purification Acetone purification Purification of 1,1,1,2tetrafluoroethane (refrigerant) Recovery of perchloroethylene (dry cleaning solvent) Solvent recovery from tire manufacturing Dehydration of alcohols
THF/water azeotrope Acetone/water 1,1,1,2-Tetrafluoroethane/hydrogen fluoride and/or 1-chloro-2,2-difluoroethylene Perchloroethylene and residue
Ethyl acetate, butyl acetate, isopropyl acetate Ethyl acetate, butyl acetate, isobutyl acetate, p-xylene Alcohols, p-xylene, n-heptane, hydrocarbons, methyl cyclopentane n-Pentane Toluene, benzene Components present in system
(Bio)Ethanol dehydration Production of L-aspartylL-phenylalanine methyl ester C9 separation Acetonitrile production Recovery of hydrocarbons
Ethanol/water n-Propanol/water Isopropanol/water Ethanol/water azeotrope
Diisopropyl ether (DIPE), iso-octane, benzene
Acetic acid/toluene azeotrope 1,3,5-Trimethylbenzene/1-methy1-2-ethylbenzene Acetonitrile/water Octene/oxygenated species
Benzene, cyclohexane, n-pentane, hexane, n-heptane, iso-octane Water Ethylene glycol monomethylether Hexylamine, butyl acetate Light-boiling binary entrainer (ethanol and water)
Source: Lee, F. M.; Wytcherley, R. W. Azeotropic Distillation. In: Encyclopedia of Separation Science; 2000; pp 990–995; Li, J.; Lei, Z.; Ding, Z.; Li, C.; Chen, B. Azeotropic Distillation: A Review of Mathematical Models. Sep. Purif. Rev. 2005, 34, 87–129; Kiss, A. A. Advanced Distillation Technologies – Design, Control and Applications; Wiley: New York, 2013.
saving energy and increasing recovery. However, the drawbacks of AD are the larger diameter column (required to allow an increased vapor volume due to adding an azeotropic agent), and the more complex control as compared to simple distillation.
Alcohol Dehydration The use of AD technology for alcohol dehydration has the longest history within the industry, and it is still used today. As ethanol is a hydroxyl compound (like water) and yet an organic chemical, it should exhibit analogies and also form azeotropes with both water and organic compounds. When benzene is used as an azeotropic agent, the conceptual process design of AD can be sketched on a ternary diagram showing the liquid split envelope combined with a RCM, as shown in Figure 6 along with the process flowsheet.11 The near azeotropic feed stream (F) is fed to the first distillation column, together with the recycle streams D2 and Dc1 – their mix residing in the triangle F-D2-Dc1 on the RCM. The introduction of benzene as a heterogeneous azeotropic entrainer in the dehydration column C1 produces the ternary azeotrope ethanol/benzene/water that boils at 64.93 C and it can be easily separated from ethanol (b.p. 78.43 C). The products of the first distillation column are ethanol as the bottoms (B1) and the azeotropic distillate (D1) – which is fed to a decanter separating the organic and aqueous phases according to the liquid split tie lines. The heterogeneous azeotrope forms two-liquid phases: a benzene-rich phase (Dc1) fed back to the first column (C1), and an aqueous phase (Dc2) containing nearly equimolar proportions of ethanol and water. The aqueous phase stream is fed to a second distillation column (C2) that produces water as bottom product (B2) and the binary azeotrope ethanol/water as distillate product (D2) which is recycled back to the first column (C1). The classic sequence of two columns and a decanter can also be implemented practically as an azeotropic DWC system, as shown in Figure 4, right.11 The A-DWC alternative consists of a single shell, two reboilers, and only one condenser. Consequently, two bottom products are collected: ethanol on the feed side and water on the opposite side. The azeotropic top stream is fed to a decanter from which the organic phase is recycled back to the feed side, while the aqueous phase is returned to the other side of the A-DWC column. When using n-pentane as entrainer in an A-DWC, it is possible to have 20% energy savings as compared to a conventional AD system involving two columns.11,16
Acetic Acid Dehydration There are many industrial processes where acetic acid must be dehydrated, in order to be used as high-purity raw material, like for instance in the production of terephthalic acid (TA). The production process includes two main sections: oxidation (where p-xylene
Distillation | Azeotropic Distillation
Figure 6 RCM of the ternary system ethanol–water–benzene (left), and flowsheet of the azeotropic distillation for ethanol dehydration using benzene as entrainer (right).
is catalytically oxidized to produce crude TA) and purification to PTA (purified TA). Acetic acid – present in the oxidation reactor as a solvent, but also beneficial to the reaction itself – must be separated from the water produced by oxidation. The recovery and recycling of the acetic acid solvent is critical to the efficient and economical operation of a TA plant, as any acid losses count against the process economics as increased make-up solvent or increased wastewater treatment costs. Water and acetic acid exhibit a pinch point at high water concentrations (similar to the one shown in Figure 2) making it very difficult to recover the pure acid. A conventional acetic acid recovery unit in a PTA process consists of two absorbers (low and high pressure) and an acid dehydration column – which is similar to Figure 3 (right). An AD column is preferred, as the separation of acetic acid and water by conventional distillation requires tall columns of 70–80 trays.4 A common azeotropic agent used is n-butyl acetate, which exhibits limited miscibility with water and forms a heterogeneous azeotrope (b.p. 90.23 C). n-Butyl acetate is added in sufficient quantities to form an azeotrope with all the water being fed to the dehydration column. The binary azeotrope with water can then be distilled as an overhead stream, leaving pure acetic acid as bottoms product. The heterogeneous azeotrope forms two phases upon condensation: an organic layer containing almost pure n-butyl acetate (saturated with water) and an aqueous layer phase containing almost pure water (saturated with n-butyl acetate). The organic phase is recycled back to the dehydration column, while the aqueous phase is fed to a stripping column where water is removed as the bottom product, and the relatively small amount of entrainer is recovered as azeotropic overhead and recycled back to the dehydration column. Due to the lower heat of vaporization of the azeotrope compared with that of water alone, AD can save 34% of the energy used in conventional distillation. Since AD results in a cleaner separation, the amount of acetic acid lost in the aqueous discharge can be reduced by almost 40%.4 Note that a similar AD process is also used in the production of acetic acid – by methanol carbonylation (over 90% of all new acetic acid capacity worldwide), butane or naphtha catalytic liquid-phase oxidation and acetaldehyde oxidation – or acetic acid derivates.4
Production of Esters Esters are produced by the reversible esterification reaction of alcohols with carboxylic acids. The yield is usually limited by the chemical equilibrium (and not by the reaction rate) hence higher conversions could be obtained by removing at least one of the products – as done so effectively in reactive distillation processes.11 When water is removed during the reaction, the equilibrium is shifted (pulled rather than being pushed) to produce more ester product. High-purity esters can be produced using AD to remove water/alcohol from the esters using aromatic and aliphatic hydrocarbons as entrainers.4 For example, esters (e.g., isobutyl acetate, n-butyl acetate, and isoamyl acetate) could be purified using an entrainer such as nheptane, methyl cyclopentane or various other hydrocarbons to simultaneously remove water and alcohol. The reaction products are fed to an AD column where the entrainer is used to remove alcohol and water as overhead products. The final purification of the ester product is carried out in a distillation column with only a few stages, while the entrainer is recovered using a single step liquid– liquid extraction (LLX) column with water as extractant. Moreover, AD is also useful in the purification of esters produced using an enzymatic route, such as isopropyl myristate prepared from myristic acid and iso-propanol using (immobilized) lipase.4
Concluding Remarks For over a century, AD technology played a key role in the separation and purification of many industrial chemicals. AD processes benefits from distinct advantages, such as energy savings, increased recovery, and ability to separate mixtures hindered by close boiling points, pinch points, and azeotropes. However, one has to consider the larger column diameter required by the increased vapor flow rate (due to the addition of an entrainer) and the more difficult control as AD systems can exhibit complex dynamic behavior, multiple steady states, and parametric sensitivity. However, for the coming decades, AD will certainly remain a viable alternative and a standard tool for simplifying difficult separations found in industry.
Distillation | Azeotropic Distillation
References 1. Resetarits, M. R.; Lockett, M. J. Distillation. In: Encyclopedia of Physical Science and Technology, 3rd ed.; 2003; pp 547–559. http://dx.doi.org/10.1016/B0-12-227410-5/ 00182-4; http://www.sciencedirect.com/science/article/pii/B0-12-227410-5.00182-4. 2. Petlyuk, F. B. Distillation Theory and Its Application to Optimal Design of Separation. Cambridge University Press: Cambridge, 2004. 3. Fair, J. R. Distillation. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2000. 4. Lee, F. M.; Wytcherley, R. W. Azeotropic Distillation. In Encyclopedia of Separation Science; 2000; pp 990–995. http://dx.doi.org/10.1016/B0-12-226770-2/00681-5; http:// www.sciencedirect.com/science/article/pii/B0122267702006815. 5. Luyben, W. L.; Chien, I.-L. Design and Control of Distillation Systems for Separating Azeotropes. Wiley-AIChE: Hoboken, NJ, 2010. 6. Doherty, M. F.; Knapp, J. P. Distillation, Azeotropic and Extractive. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2004. 7. Julka, V.; Chiplunkar, M.; O’Young, L. Selecting Entrainers for Azeotropic Distillation. Chem. Eng. Prog. 2009, 47–53. 8. Gmehling, J.; Menke, J.; Krafczyk, J.; Fischer, K. Azeotropic Data. Wiley-VCH: Weinheim, 2004. 9. Li, J.; Lei, Z.; Ding, Z.; Li, C.; Chen, B. Azeotropic Distillation: A Review of Mathematical Models. Sep. Purif. Rev. 2005, 34, 87–129. 10. Yildirim, O.; Kiss, A. A.; Kenig, E. Y. Dividing Wall Columns in Chemical Process Industry: A Review on Current Activities. Sep. Purif. Technol. 2011, 80, 403–417. 11. Kiss, A. A. Advanced Distillation Technologies – Design, Control and Applications. Wiley: New York, 2013. 12. Beneke, D.; Peters, M.; Glasser, D.; Hildebrandt, D. Understanding Distillation Using Column Profile Maps. Wiley: New York, 2012. 13. Stichlmair, J. G.; Fair, J. R. Distillation – Principles and Practice; Wiley-VCH: New York, 1998. 14. Kiva, V. N.; Hilmen, E. K.; Skogestad, S. Azeotropic Phase Equilibrium Diagrams: A Survey. Chem. Eng. Sci. 2003, 58, 1903–1953. 15. Doherty, M. F.; Malone, M. F. Conceptual Design of Distillation Systems. McGraw-Hill: New York, 2001. 16. Kiss, A. A.; Suszwalak, D. J.-P. C. Enhanced Bioethanol Dehydration by Extractive and Azeotropic Distillation in Dividing-Wall Columns. Sep. Purif. Technol. 2012, 86, 70–78.