Separation of a highly nonideal mixture for solvent recovery

Table of contents

Acknowledgements

Abstract

List of figures

List of tables

1 Formulation of the problem

2 Introduction
2.1 Azeotropism
2.1.1 Binary azeotropes
2.1.1.1 Minimum boiling azeotrope
2.1.1.2 Maximum boiling azeotrope
2.1.1.3 Heterogeneous azeotrope
2.1.2 Ternary azeotropes
2.1.2.1 Residue curve maps
2.1.2.2 Distillation lines
2.2 Breaking azeotropes through distillation
2.2.1 Processes without additional entrainer
2.2.1.1 Pressure sensitive distillation
2.2.1.2 Heterogeneous azeotropic distillation
2.2.2 Processes with additional entrainer
2.2.2.1 Azeotropic distillation
2.2.2.1.1 Forming a homogeneous azeotrope
2.2.2.1.2 Forming a heterogeneous azeotrope
2.2.2.2 Extractive distillation
2.2.2.3 Reactive distillation
2.2.2.4 Salted distillation

3 Investigation of the given quaternary solvent mixture
3.1 Existing azeotropes
3.2 Residue curves of the ternary mixtures

4 Simulation
4.1 Separation into two ternary mixtures
4.2 Ethyl acetate cycle
4.3 Isopropyl acetate cycle
4.4 Summary of the Simulation

5 Experimental Investigation of the First Column
5.1 Evaluation of the theoretical number of plates
5.1.1 Laboratory column packings
5.1.2 Theoretical number of stages
5.2 Gas chromatography and Karl Fischer titration
5.2.1 Calibration of the gas chromatograph
5.2.2 Karl Fischer titration
5.3 Test arrangement
5.4 Test Results from Laboratory Column
5.5 Comparison of measurement and simulation

6 Summary

7 Nomenclature and Units

8 References

Appendix A: Benzene carbon tetrachloride table

Appendix B: Equipment used for measurement

Acknowledgements

First of all, I would like to thank my supervisor, Dr. Péter Mizsey for his assistance, ideas, and encouragement throughout my time in Budapest. Without his guidance this diploma work would not have been possible.

I am grateful to Prof. Dr. Zsolt Fonyó, the chairman of the Department of Chemical Engineering, for his help and encouragement.

I am especially grateful to o.Univ.Prof.Dipl.-Ing.Dr.techn.Dr.h.c. R. Marr, the chairman of the Institute of Chemical Engineering and Environmental Technology at the Technical University of Graz, for his support to make it possible for me to write this diploma work.

I would also like to thank the Department of Chemical Engineering at the Budapest University of Technology and Economics for providing the support, resources and opportunity for me to perform this work.

I would like to thank Prof. József Manczinger and Mrs. Györgyi Panyiné for their help and advises in the laboratory and Dr. Endre Rév for his help and advises in theoretical issues of this work.

Finally, I am very grateful to the ERASUMUS Program for the financial support of my stay in Hungary.

And to my sweetheart, dear Mariann, providing me with motivation and confidence trough my time at the university.

Finally, I want to thank my parents and my brothers, for their support and understanding throughout my education and my life. Thank you for encouraging me to think differently and for teaching me how to accomplish my goals.

Abstract

The separation of complex nonideal mixtures is a common problem in the process industries. The solvent recovery is an important task for chemical engineers to minimize burden upon the environment due to exhaustive use of solvents. The recovery of the individual components is complicated by the highly nonideal features of these mixtures. The separation of such highly nonideal mixtures can be limited by the presence of azeotropes, which can create distillation boundaries. These distillation boundaries are forming distillation regions which are difficult to overcome with the standard rectification. Distillation systems for these highly nonideal azeotropic mixtures are particularly difficult to design and to operate in an efficient way.

In printing companies often four component mixtures of ethanol, ethyl acetate, isopropyl acetate, and water arise as waste. A separation scheme of multicomponent azeotropic distillation is developed and successfully used for a highly nonideal quaternary mixture. The composition of the mixture in mass percent is ethanol 30%, water 20%, ethyl acetate 25% and isopropyl acetate with 20%. The rest of the mixture (5%) consists of n-propane, isopropane, cyclohexane, and etoxypropane. For the further investigation just the quaternary mixture is examined. Generally, every component should be recovered as pure as possible from the mixture.

In the mixture namely five binary and two ternary azeotropes are formed by the components. Based on the synthesis procedure proposed by Rev et al. (1994) and Mizsey et al. (1997) a new separation technology is developed followed up the vapor-liquid-liquid equilibrium behavior of the mixture. They have recommended a general framework for designing feasible schemes of multicomponent azeotropic distillation. This procedure recommends to study in detail the vapor-liquid-liquid equilibrium data to explore immiscibility regions, azeotropic points, and separatrices for ternary and quaternary regions. On the behalf of the VLLE data the set of feasible separation structures is explored. This procedure is followed and a new separation structure is developed and tested experimentally.

First, the quaternary mixture is separated into two ternary mixtures by distillation. The two ternary mixtures containing ethyl acetate, ethanol, water and isopropyl acetate, ethanol, water, respectively. Due to the analogous behavior of the two ternary mixtures similar separation cycles can be designed. The two ternary mixtures are separated by extraction followed by azeotropic distillations. Finally, the components are recovered with a purity of higher then 95%. The key point of the separation technology is the first column, in which the split of the quaternary mixture into two ternary mixtures takes place. This unit operation is tested in a laboratory size column containing structured packing. Steady-state continuous distillation is carried out for 5 hours controlling the column according to the typical top temperature. The distillate and bottom products are analyzed and compared with the simulation data. The comparison of the simulated and experimental data shows a good agreement which proves the accuracy of the VLLE method and the simulated results.

The good agreement of the simulated and measured results assures the successful applicability of the new solvent recovery technology. It can be concluded that the new separation sequence for this highly nonideal mixture can be used for this purpose.

Abstract

Die Trennung nicht idealer Gemische ist ein allgemeines Problem in der Chemischen Industrie. Besonders die Aufarbeitung von Lösungsmittelabfällen spielt eine bedeutende Rolle um den Schutz der Umwelt zu gewährleisten. Ein Lösungsmittelrecycling wird erschwert durch die Komponentenvielfalt sowie den damit verbundenen Azeotropen in diesen Gemischen und schränkt die Rektifikation als mögliche Trennvariante ein. Das Gleichgewichtsverhalten von ternären Gemischen lässt sich durch den Verlauf von Destillationslinien beschreiben. Die Existenz von Azeotropen führt zur Ausbildung von Senken und Bergen in der Siedetemperaturfläche der Gemische, wobei Destillationsgrenzen entstehen, welche das Gemisch in Destillationsgebiete unterteilen. Je nach Anzahl und Charakter der Azeotrope kann ein Gemisch mehrere Destillationsgrenzen ausbilden welche durch Rektifikation nicht oder nur durch geeignete Maßnahmen zu überwinden sind.

Ziel der Arbeit war die Untersuchungen eines nicht idealen Lösungsmittelgemisches welches in einer Druckerei anfällt. Dieses Gemisch in Gewichtsprozent besteht aus 20% Wasser, 30% Ethanol, 25% Ethylacetat und 20% Isopropylacetat. Die restlichen 5% bestehen aus n-Propan, Isopropan, Cyclohexan und Etoxypropan. Für die weiter Untersuchung wurde nur das Vierkomponentengemisch herangezogen. Grundsätzlich soll jede Komponente so rein wie möglich gewonnen werden. Die Simulation wurde durch den Einsatz von Simulationspogrammen (Hysys, Aspen Plus) realisiert. Basierend auf einem Syntheseverfahren, vorgeschlagen von Rev et al. (1994) und Mizsey et al. (1997), wurde eine Verfahrensweise zu Trennung des Gemisches entwickelt. Die empfohlene Vorgehensweise befürwortet die ausgiebige Untersuchung des Gemisches, wobei alle azeotropen Zusammensetzungen, heterogenen Bereiche, Destillationslinien oder Rückstandskurven und Destillationsgrenzen ermittelt werden. Aufgrund dieser Erkenntnisse wird eine geeignete Trennsequenz entwickelt. Im Falle des quartären Gemisches wurden eine Trennung in zwei ternäre Gemische in einer Rektifikationskolonne erreicht. In dieser Kolonne werden die zwei Acetate getrennt, wobei im Kopf der Kolonne ein Wasser, Ethanol, Ethylacetat Gemisch und im Sumpf ein Wasser, Ethanol, Isopropylacetat Gemisch abgezogen wird. Aufgrund des ähnlichen Verhaltens und der fast gleichartigen Destillationsdiagramme wurden analoge Überlegungen für die weitere Trennung der ternären Ströme angestellt. Im Folgenden wird mittels eines Extraktors die systemeigene Mischungslücke ausgenutzt und Ethanol aus dem Gemisch entfernt. Durch eine nachgeschaltete azeotrope Destillation werden jeweils aus der wässrigen und organischen Phase die Reinkomponenten gewonnen. Die Trennung wird in zwei Kreisläufen durchgeführt, wobei in jeder Kolonne das jeweilige ternäre Azeotrope anfällt und als Kopfprodukt recycelt wird. Für einen eventuellen Wiedereinsatz wurden Reinheiten mit mindestens 97% der jeweiligen Komponente erreicht, wobei Wasser nahezu rein das System wieder verlässt.

Der Schlüsselpunkt dieser Lösemittelaufbereitung ist die Trennung in der ersten Kolonne. Um diesen Trennschritt und das Systemverhalten des Gemisches zu belegen, wurde anhand einer experimentellen Untersuchung die Realisierbarkeit bestätigt. Dazu wurde in einer Laboratoriumskolonne, gefüllt mit strukturierten Packungen, das quartäre Gemisch in zwei ternäre Gemische getrennt. Die Trennung konnte erfolgreich realisiert werden und die nötigen Reinheiten erzielt werden. Mit mehr als 5 Stunden wurde eine ausreichende Kontinuität erreicht und die Konstanz der Messung belegt.

Die im Rahmen der experimentellen Untersuchung erhaltenen Ergebnisse bestätigen die Realisierbarkeit des Trennverfahrens. Die Übereinstimmung der simulierten Daten mit den experimentellen Ergebnissen bestätigt die Korrektheit der Beschreibung des Phasengleichgewichtes durch die verwendeten Simulationsprogramme.

List of figures

Figure 1 Txy-plot and xy-plot of ethyl acetate and isopropyl acetate

Figure 2 Txy-plot and xy-plot of the system water and ethanol

Figure 3 Txy-plot and xy-plot of the system acetone and chloroform

Figure 4 Txy-plot and xy-plot of the system water and isopropyl acetate

Figure 5 Residue curve map for ethyl-, isopropyl-, and n-propyl acetate

Figure 6 Residue curve map for water, ethanol and benzene

Figure 7 Difference between residue line and distillation line [17]

Figure 8 Pressure-swing distillation [6]

Figure 9 Heterogeneous azeotropic distillation [6]

Figure 10 Separation sequence for a homogeneous azeotrope

Figure 11 Aniline, water and hydrazine ternary diagram

Figure 12 Heterogeneous distillation with entrainer [9]

Figure 13 Ternary diagram for water, ethanol and benzene [9]

Figure 14 Extractive distillation

Figure 15 Residue curve map for isopropyl acetate, ethanol and ethyl acetate

Figure 16 Residue curve map for water, isopropyl acetate and ethyl acetate

Figure 17 Residue curve map for water, ethanol and ethyl acetate

Figure 18 Residue curve map for water, ethanol and isopropyl acetate

Figure 19 Quaternary mixture

Figure 20 Flowsheet of the total process

Figure 21 Ethyl acetate cycle

Figure 22 Isopropyl acetate cycle

Figure 23 SULZER laboratory packing

Figure 24 Batch distillation arrangement

Figure 25 Theoretical stages vs. F-factor

Figure 26 Gas chromatograph plot

Figure 27 Calibration curves

Figure 28 Calibration curves

Figure 29 Test arrangement

Figure 30 Composition over time

Figure 31 Composition over time

List of tables

Table 1 Physical properties of the pure components [5]

Table 2 Binary azeotropes

Table 3 Ternary azeotropes

Table 4 Unit operation settings

Table 5 Stream table for the first column

Table 6 Stream data for the ethyl acetate cycle

Table 7 Stream data for the isopropyl acetate cycle

Table 8 Results of the simulation

Table 9 Determination of the F-factor

Table 10 Definite compositions

Table 11 Distillate results

Table 12 Bottom results

Table 13 Global mass balance

Table 14 Comparison of measurement and simulation results

Table 15 Benzene carbon tetrachloride evaluation table [12]

1 Formulation of the problem

The resulting solvent mixture in a printing office should be recovered. The composition of the mixture in mass fraction is:

illustration not visible in this excerpt

The rest (5%) consists of n-propane, isopropane, cyclohexane, etoxypropane.

In general fluctuations around +/- 5% are possible.

For a possible reuse of the different solvents, the following purities should be recovered there should be at least a purity of:

- at least 90% ethanol with maximum 4% water and maximum 6% acetates
- at least 90% ethyl acetate with maximum 5% isopropyl acetate and maximum 5% ethanol
- at least 90% isopropyl acetate with maximum 5% ethyl acetate and maximum 5% ethanol

Generally, every component should be recovered as pure as possible from the mixture.

Realization of the diploma work:

- First of all the quaternary mixture should be investigated for possible azeotropes and special behavior. With the help of a process simulation program this mixture should be simulated, suggestions for a successful separation should be developed and an appropriate solution should be designed.
- After the simulation the predicted solution should be experimental confirmed. For this a laboratory plant should be built and the necessary measurements performed.

2 Introduction

The separation of complex nonideal mixtures is a common problem in the process industries. Due to the fact that the separation system with capital and operating expenses of up to 70%, the production costs are strongly dependent on the process scheme chosen [1]. Thus, it is important to find the appropriate separation scheme. On one hand the steady improvement in the field of individual separation processes and on the other hand the combinations of the applied processes are important for the costs of separating a mixture.

The rectification is a widely used and investigated separation method and should be the preferred separation technology to deal with in this diploma work. But under certain circumstances, other unit operations are also competitive and should be taken into consideration, for instance liquid/liquid extraction, adsorption and addition of salt to the mixture. In our case, we give priority to the development of a system with mainly rectification units.

The separation of such highly nonideal mixtures can be limited and complicated by the presence of azeotropes, which can create distillation boundaries. These distillation boundaries are forming distillation regions. Distillation systems for these highly nonideal azeotropic mixtures are particularly difficult to design and to operate in an efficient way. In order to time pressure and resource limitations, the final design of processes, splitting such highly nonideal mixtures, is often based on prior knowledge and experience of existing designs of similar separation problems, instead of investigating a large number of design alternatives.

When designing or evaluating any separation process, the representation of the phase equilibrium is fundamental to the accuracy of the simulation. The existence of binary and ternary azeotropes should be predicted and the classification of mixtures as homogeneous or heterogeneous should be carried out. Also the shape of the phase diagram and/or the predicted liquid-liquid-vapor phase equilibrium for simulation continuous distillation column operations should be developed. This is achieved, even for highly nonideal azeotropic mixtures, by generating residue curves and distillation line maps. These maps provide valuable insights into the physical behavior of mixtures, including the presence of azeotropes and the resulting separation limiting lines.

Most of the separation processes for azeotropic mixtures require additional agents, solvents and entrainers for azeotropic and extractive rectification and extraction. In these cases, the needed recycle of material is used for two principle tasks [17]:

- To minimize the needed amount of additional agent through circulation behavior
- To avoid redundant separation processes, if in some apparatus appear incomplete separation

For some separation processes it is not necessary to provide the additional agent in a pure form, so the process variants are increasing. At the same time as well it is getting more difficult, to discover this additional variants, because of the necessary iterations during simulation and the unknown mass streams and composition. In the literature, there is no corresponding approach for a synthesis of such complex process structures. Thus, the mixture should be investigated exhaustively not to overlook some possible interesting alternatives. The literature for syntheses of complex separation processes consists essentially of applications for binary mixtures.

Some points should be preconceived before get into the detail of developing a separation process:

- Identifying the presence of azeotropes to provide insights into the complexity of the separation problem
- Analyzing the mixture behavior to locate distillation boundaries before design
- If necessary, selecting the appropriate entrainer to facilitate component separation in a mixture
- Quickly studying alternative separation schemes to determine the optimal design of both continuous and batch distillation processes
- Identifying opportunities for heat integration of the distillation system to minimize energy usage

The next pages will give an overview about the types of azeotropes, their behavior and ways to separate them. A general overview can be obtained from [4], [5] and [6]. A good overview about the manners of homogeneous azeotropic distillation is given by [2]. The main features about heterogeneous azeotropic distillation are discussed in [3].

2.1 Azeotropism

The mode of separation processes is achieved by forming two or more coexisting phases which difference in temperature, pressure, composition and/or phase state. In distillation processes, we take advantage of the different composition in equilibrium state of the vapor and the one or more liquid phases. To bring the vapor and liquid phases into intimate contact, random or structured packings and plates or trays are used in columns.

2.1.1 Binary azeotropes

For processes with nearly ideal behavior, based on equilibrium between gas and liquid phases and we can express the VLE according to Raoult’s and Dalton’s law (1).

illustration not visible in this excerpt (1)

In this case the activity coefficient is equal to unity. In most of the industrial applications where the system pressure is the atmospheric and when the effect of the system pressure on the liquid phase, given by the Poynting factor, is very small, we can express the vapor liquid equilibrium as

illustration not visible in this excerpt (2)

Thus, only by knowing the activity coefficient and the vapor pressures of the different species this simple equation is valid.

During distillation according to the partial vaporization of a liquid mixture or the partial condensation of a vapor mixture, the vapor phase becomes enriched in the more volatile components while the liquid phase is depleted of these components. In some cases, there are just small differences in the composition of the vapor and the liquid phases and a high number of partial vaporizations and partial condensations are needed to achieve the desired separation. In case of an azeotrope, the vapor and the liquid phase have the same composition. Thus,

illustration not visible in this excerpt for all i = 0,1…s (3)

is the simplest way to define an azeotropic behavior. The existence of a binary azeotrope can also be detected by the relative volatility defined as

illustration not visible in this excerpt (4)

For an ideal vapor phase (assumption of low pressure) and unity Pointing factor we can simplify again and write

illustration not visible in this excerpt (5)

In case of an azeotrope the relative volatility is equal to unity, hence equation (5) can be written to

illustration not visible in this excerpt (6)

That means that the relation between the two activity coefficients is the same as the relation between the two vapor pressures of the pure components. Hence it appears that the azeotropic composition can be strongly temperature dependent, which is sometimes used for separation.

illustration not visible in this excerpt

Figure 1 Txy-plot and xy-plot of ethyl acetate and isopropyl acetate

In most binary systems, one of the components is more volatile than the other over the entire composition range. Figure 1 shows the Txy- and xy-plot of ethyl acetate and isopropyl acetate, where ethyl acetate is more volatile than isopropyl acetate. According to Horsley [7] the tendency of a mixture of two components to form an azeotrope is mainly dependent on two factors:

- The difference in boiling points (vapor pressure)
- Deviation from the ideal system (activity coefficients)

In case of components with similar boiling points, there is just a relatively small deviation from the ideal behavior necessary to cause an azeotropic behavior. Mixtures of components whose boiling points differ by more than about 30°C generally do not exhibit azeotropes even if large deviations from Raoult’s law are present [5].

The basis for understanding the behavior of azeotrope formation is the idea that liquid properties are related to the degree of bonding between molecules. The hydrogen bond is the most important of the bonds and serves as the most important criterion. It depends on the formation and detachment of hydrogen bounds.

In nearly ideal systems, the activity coefficients are around unity like in the binary system ethyl acetate - isopropyl acetate (Figure 1). In this case the interaction forces between the constituents are more or less the same. With growing activity coefficients, the deviation from the ideal system gets bigger and bigger and the system is forming a minimum boiling azeotrope (Figure 2). Here, the tendency to form hydrogen bonds between the different components is not given. Sometimes the activity coefficients are so big, that both components are immiscible in a certain region (Figure 4). But the deviation can also be negative (the activity coefficients are smaller than one) and the system is forming a maximum boiling azeotrope (Figure 3). In this case there exists a much more strong interaction force between the different constituents as between molecules from the same species. If the azeotrope is a maximum or minimum boiling azeotrope, therefore depends on the deviation from Raoult’s law. If the azeotrope forms a single phase, it is called homogeneous. If a multiple liquid phase appears, the azeotrope is called heterogeneous.

2.1.1.1 Minimum boiling azeotrope

The deviation from Raoult’s law is in this case always positive and results in a maximum vapor pressure [6]. This is corresponding with a minimum boiling point and we can apply following rules

illustration not visible in this excerpt and illustration not visible in this excerpt (7)

Subscript 1 denotes the component with the higher pure component vapor pressure and subscript 2 refers to the lower pure component vapor pressure.

illustration not visible in this excerpt

Figure 2 Txy-plot and xy-plot of the system water and ethanol

An example of minimum boiling azeotrope formation is shown in Figure 2. The Txy- and xy-plot of a water ethanol mixture shows clearly the forming of a minimum boiling azeotrope around 10 mol percent of water at 1.013 bar. Such homogeneous azeotropes are mostly completely miscible at all temperatures, when the species have close boiling points and rather small liquid phase nonidealities.

2.1.1.2 Maximum boiling azeotrope

The deviation from Raoult’s law is in this case always negative and results in a minimum vapor pressure. This is corresponding with a maximum boiling point and we can apply again the following rules [6]

illustration not visible in this excerpt and illustration not visible in this excerpt (8)

Subscript 1 denotes the component with the higher pure component vapor pressure and subscript 2 refers to the lower pure component vapor pressure. To form two liquid phases it is generally required, that the activity coefficients are strongly positive. That’s, why a maximum boiling azeotrope is never heterogeneous. An example of maximum boiling azeotrope formation is shown in Figure 3 for the acetone chloroform system at 1.013 bar. This system has a maximum-boiling azeotrope at 34 mole percent of acetone which occurs at 64,5°C.

illustration not visible in this excerpt

Figure 3 Txy-plot and xy-plot of the system acetone and chloroform

2.1.1.3 Heterogeneous azeotrope

An example of heterogeneous azeotrope formation is shown in Figure 4 for the water isopropyl acetate system at 1.013 bar.

illustration not visible in this excerpt

Figure 4 Txy-plot and xy-plot of the system water and isopropyl acetate

At a liquid compositions between 0 and 15 mole percent water the liquid phase is homogeneous. Phase splitting into two separate liquid phases (one with 3 mole percent water and the other with nearly 100 percent water) occurs for any overall liquid composition between 15 and 100 mole percent water. A minimum boiling heterogeneous azeotrope occurs at 76°C when the vapor composition and the overall composition of the two liquid phases are 41 mole percent water.

The fact that immiscibility occurs does not guarantee that the azeotrope will be heterogeneous. The azeotropic temperature is sometimes outside the rang of temperatures at which a system exhibits two liquid phases and/or the azeotropic composition may not fall within the composition range of the two liquid phase region [5].

2.1.2 Ternary azeotropes

Azeotropes are important not only as thermodynamic barriers for separation using distillation but also as thermodynamic possibilities to separate mixtures which present a lower boiling azeotrope. In the relevant literature, the descriptions of separation processes are widely investigated by representation of the mixture in ternary diagrams. These diagrams are triangles, where the composition is shown on the sides of the triangle and just two components can be independently chosen from the three constituents.

Two widely used tools for the description of azeotropic distillation are the residue curves [8] and the distillation lines [16]. Composition profiles of packed columns operating at infinite reflux coincide with residue curves. Composition profiles of tray columns operating at infinite reflux coincide with distillation lines. Distillation lines do not generally coincide with residue curves. In both cases the complete amount of vapor that is leaving the first stage is condensed and reenters the column in the liquid aggregate state. If there is no withdraw of product in the bottom and the top of the column, the compositions of the rising vapor and the compositions of the liquid above are the same. On the assumption of thermo dynamical equilibrium between the different stages, the concentration profile of a column operating at infinite reflux can be calculated according to series of dew points and boiling points, depending o the starting point in the column.

2.1.2.1 Residue curve maps

A residue curve is the phase plane for the liquid composition in an isobaric (or isothermal) open evaporation. The equations describing the process are analogous to the equations describing a batch distillation and were discussed by Doherty and Perkins [8]. In this case, the composition profiles of packed columns operating at infinite reflux coincide with residue curves.

A residue curve map has several characteristics:

- The residue curve always points into the direction of increasing temperature, therefore the liquid composition is changing into the direction of increasing temperature
- Pure components and azeotropes define fixed points in the map
- Azeotropes define boundaries in the composition space
- Boundaries define distillation regions that cannot be crossed by a simple stage-by-stage distillation
- Residue curves are equivalent to the composition profile of a distillation tower at fixed pressure and infinite reflux
- Oscillation are not arising

Residue curves are governed by the set of differential equations [8]

illustration not visible in this excerpt i=1,…,s-1 (9)

where i is the component index, s is the number of pure components in the mixture, yi (xi) are the mole fractions of component i in the vapor (liquid) phase, and x is the dimensionless nonlinear time-scale. To characterize the VLE behavior, it is necessary to draw different residual curves into a triangle diagram. A distillation region is a subset of the composition simplex, in which all residue curves originate from a locally lowest-boiling pure component or azeotrope and travel toward a locally highest boiling one. The curves that separate different distillation regions are called residue curve boundaries [2].

For every distillation region, we can define nodes:

Low-boiling node The most volatile composition on the boundary (pure component or a minimum-boiling azeotrope)

High-boiling node The least volatile composition on the boundary (pure component or a maximum-boiling azeotrope)

Intermediate-boiling saddle All other pure components and/or azeotropes (no residue curve originates or ends there)

There are several VLE methods to handle mixtures which contain polar substances. This complex predictive equations involve binary-interaction parameters for each pair of components in the mixture. Popular expressions are Margules, NRTL, Wilson and UNIQUAC. If there is no appropriate data available, it is also possible to use group contribution methods like UNIFAC for estimations. The property package used for all calculations in this diploma work is UNIQUAC for the liquid phase and Ideal Gas for the vapor phase. For example, the residue curve map of an almost ideal system of ethyl acetate, isopropyl acetate and n-propyl acetate at 1.013 bar is shown in Figure 5. Just by looking at the residue curve map, it can be seen that there are no distillation boundaries and azeotropes: pure ethyl acetate is an unstable node, pure n-propyl acetate is a saddle node and pure isopropyl acetate is a stable node. The residue curves show the path traced in the composition space by the liquid in the batch distillation still. The arrows point towards the direction of increasing temperature. From the almost ideal system of ethyl acetate, isopropyl acetate and n-propyl acetate it is evident, that no possible feed mixture is limited by a distillation boundary.

illustration not visible in this excerpt

Figure 5 Residue curve map for ethyl-, isopropyl-, and n-propyl acetate

Systems with more azeotropes will, in general, result in more complex residue curve maps. Doherty and Perkins [8] have pointed out that the distillation boundaries or separatrices can be crossed by simple continuous distillation only if all residue curves diverge from it.

Bossen [9] gives three theorems which are more precise, how to decide, if a distillation boundary can be crossed or not:

- Two residue curves on the convex side of a separatrix cannot intersect, and therefore, it is not possible to cross the separatrix from the convex side
- All curved separatrices can be crossed from the concave side of the separatrix, if it is possible to obtain a liquid composition profile close to the separatrix and within the concave region.
- A true distillation boundary must always be a straight line.

An example of a residual curve map with a ternary and three binary azeotropes is shown in Figure 6. The investigated well known mixture consists of water, ethanol, and benzene.

illustration not visible in this excerpt

Figure 6 Residue curve map for water, ethanol and benzene

The ternary azeotrope is the low boiling unstable node in this system. From this point, every residue curve starts to the three endpoints. The three stable nodes, represented by pure water, pure ethanol and pure benzene, are the highest boiling points in their distillation regions. The three binary azeotropes are the intermediate boiling saddle points. Figure 6 shows also the heterogeneous liquid boiling envelope of the respective mixture at 40°C. Liquid compositions located inside the heterogeneous liquid boiling envelope will split into two liquid phases whose compositions will lie on the envelope. A straight line which connects two liquid phases in equilibrium is called tie line. Based on this, the ternary azeotrope, which lies in the heterogeneous region, will split into an aqueous and an organic phase. The presence of a ternary azeotrope in the residue curve maps causes the formation of three distillation regions. Some parts of two of the distillation regions are lying in the heterogeneous region.

The appearance of the distillation boundaries gives rise to three different structures in the composition space and feed mixtures in one region cannot give products in a different region by simple distillation in general.

2.1.2.2 Distillation lines

The other widely used tool for the description and understanding of azeotropic distillation are the distillation lines [16].

illustration not visible in this excerpt

Figure 7 Difference between residue line and distillation line [17]

The distillation lines are trajectories which are defined by the different compositions of vapor and liquid on the stages. Figure 7 shows this behavior and also makes clear the difference of residue lines and distillation lines [17]. Distillation lines do not generally coincide with residue curves, but are quit similar to them. Point a marks an arbitrary liquid composition x. The according composition that is in equilibrium is signed by y* is a point of the distillation line through x and must lie on the tangent of the residue curve.

For this reason the course of the residue lines and distillation lines are very different for mixtures with big differences in boiling temperatures and in regions with strong curvature. But still this difference is negligible for practical use.

Finally, using binary data can be hazardous, and ternary data are required either from the literature or calculated from some correlation’s. A ternary diagram is very useful in interpreting results of calculations and in considering possible distillation schemes.

2.2 Breaking azeotropes through distillation

Distillation is a familiar and robust unit operation, the required equipment already exists and is highly developed. Binary and ternary distillation column design is an interactive process. As the feed, the distillate and the bottoms compositions is partially specified, the composition space can be visually examined, the feasible separation schemes evaluated, intelligent design decisions and optimal column sequences determined. The presence e of an azeotrope, however, severely limits the use of standard distillation, because the vapor and the liquid phase have the same composition. But still there are ways to separate an azeotropic mixture through rectification. The following explanations refer to binary systems and should give an overall view to the possibilities of separating azeotropic mixtures.

2.2.1 Processes without additional entrainer

These processes realize the separation by taking advantage of specific properties to overcome the distillation boundaries. These processes are pressure-sensitive distillation and heterogeneous azeotropic distillation.

2.2.1.1 Pressure sensitive distillation

This separation is particularly challenging if the azeotrope is homogeneous, so if the azeotrope does not split after condensation. Some azeotropes are pressure sensitive and in some cases, this results in a significant change in the azeotropic composition. The principle of this rectification is based on the changing relative volatility, which is reduced to the dependency of the changing activity coefficient and vapor pressure. The requirements of making use of the pressure sensitivity is, that the vapor pressure gradients are sufficient different and this difference is not compensated by the change of activity coefficients. The pressure sensitivity is favored by a big difference in boiling temperature and under certain circumstances, it is possible, that the azeotrope disappears. If the separation is realized with a dual-column process with different pressures, the process is called pressure-swing distillation.

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Figure 8 Pressure-swing distillation [6]

Figure 8 shows the principle arrangement for a pressure swinging distillation. In case of a minimum azeotrope the two pure constituents can be yield in the bottom of the two columns. The distillate of the low pressure column (p1) will be the feed of the high pressure column. This is achieved with a pump. The composition of the feed for the high pressure column (p2) lies above the azeotropic composition, as it can bee seen from the xy-diagram in Figure 8. In each case the distillate has more or less the azeotropic composition. Due to the fact that in most cases the xy-diagrams show just small curvature, a lot of stages and a high reflux ratio are needed to achieve the sufficient separation. Unfortunately, azeotropic data frequently is not available at non-atmospheric pressures and generating such information is expensive. So, process designers do not routinely consider design schemes with variation of pressures.

2.2.1.2 Heterogeneous azeotropic distillation

In systems with strong positive deviation from Raoult’s law, there is not only a minimum boiling azeotrope, but also a heterogeneous region. The activity coefficients are so big, that both components are immiscible in a certain region and two phases appear. This characteristic of the system can be used for separation if the azeotrope is lying inside this immiscible region after condensation or cooling.

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Figure 9 Heterogeneous azeotropic distillation [6]

For such a binary separation sequence without adding an entrainer, there are just two columns and one decanter at the same pressure necessary. Such an arrangement is shown in Figure 9. Each distillate flow has the azeotropic composition and is mixed in the decanter. In the decanter the liquid is splitting into two phases and is recycled to the respective column. Because of the strong positive deviation from Raoult’s law, there is just a minimum boiling azeotrope possible. The two pure components are obtained in the bottom of the respective column and marked with S and L.

2.2.2 Processes with additional entrainer

These processes realize the separation by taking advantage of specific properties by adding a third suitable component to overcome the distillation boundaries and facilitate the separation of the system by distillation.

These processes are azeotropic distillation, extractive distillation, reactive distillation and salted distillation. These types of operations are more complex and expensive than ordinary distillations because a foreign compound is added which can be special and/or expensive.

2.2.2.1 Azeotropic distillation

In a typical azeotropic distillation process, the added component (or entrainer) forms an azeotrope with the component to be taken out. The entrainer may already be present in the feed mixture (self entraining mixture) or may be an added agent. The entrainer modifies the activity coefficients of the components and must have a volatility comparable to that of the feed mixture. If the entrainer has a volatility near that of the feed, it is mixed with it to form a single stream. If the volatility is below that of the feed, the entrainer should be added above the feed. The added entrainer forms a minimum boiling azeotrope with one or more feed components and distills overhead. This minimum boiling azeotrope can be a homogenous or heterogeneous one.

2.2.2.1.1 Forming a homogeneous azeotrope

The addition of a third component alters the relative volatility of the two azeotropic constituents without liquid-liquid phase separation. This means, that the system do not exhibit a two liquid phase behavior or the liquid phase behavior cannot be exploited in the separation sequence. For this behavior, additional separation sequences must be carried out. There are possibilities to recover the entrainer by washing out with an additional constituent, using an azeotrope former, extraction with a selective solvent, absorption, or membrane treatment. In some cases, unusual features can appear during homogeneous azeotropic distillation, like decreasing separation by increasing the reflux or increasing the number of plates [2].

A possible sequence to separate a mixture with a homogeneous azeotrope is shown in Figure 10. It is a separation of aniline from water using hydrazine as an entrainer [20]. Aniline and water are forming an minimum azeotrope.

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Figure 10 Separation sequence for a homogeneous azeotrope

The added component (hydrazine) in this case forms an additional azeotrope with water. The azeotropic column C1 is adjusted in this way, that the whole amount of aniline is recovered from the mixture as a bottom product B1. Point D1 contains the remaining amount of water and hydrazine.

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Figure 11 Aniline, water and hydrazine ternary diagram

Water and hydrazine form a maximum boiling azeotrope. Thus, in column C2 the remaining water hydrazine mixture is separated into pure water D2 and the binary azeotrope R1, which is recycled back to column C1. The maximum boiling azeotrope and pure aniline form a distillation boundary. In this case it is important, that the feed mixture is not lying in the wrong distillation region, because then no satisfactory separation can be obtained.

2.2.2.1.2 Forming a heterogeneous azeotrope

The addition of a third component alters the relative volatility of the two azeotropic constituents with inducing liquid-liquid phase separation. In this case, the separation turn out to be easier, when the formed azeotrope after condensation and if necessary cooling splits into two liquid phases which are lying in different distillation regions. In this case, a simple decanter or sometimes a multistage extractor can be used to overcome the distillation regions. Since liquid-liquid tie lines are unaffected by distillation boundaries, this is a powerful mechanism for crossing distillation boundaries.

An example for azeotropic distillation with exploitation of the heterogeneous phase splitting is the separation of water and ethanol with benzene as an entrainer.

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Figure 12 Heterogeneous distillation with entrainer [9]

The flow sheet of this well known application is shown in Figure 12. B1 is to be assumed to be a pure solute water and point D1 is determined from the intersection of the lines through B1-D and F1-L. F1 lies below the binary azeotropic point. Crossing of the distillation boundary may occur in both columns. For the recovery column, crossing occurs when D1 is in region II or III, since D is in region I. For the azeotropic column, crossing occurs when F0 is in region I, since B is in region II. Since crossing is not possible in the case of a convex separatrix, the location of F0 in region I in the case of a convex separatrix represent an infeasible design.

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Figure 13 Ternary diagram for water, ethanol and benzene [9]

The entrainer selection for azeotropic distillation is more difficult than choosing a solvent for extractive distillation, because one of the feed components must be part of the azeotrope. Essential features for a fitting entrainer:

- Should form an azeotrope
- Soluble in all components of the feed at all temperatures and conditions within the tower
- Entrainer should give a residue curve with specific distillation regions and node temperatures
- Forming of an azeotrope, which lies in a liquid-liquid immiscibility region

2.2.2.2 Extractive distillation

Usually, this separation requires two distillation columns, an extractive column and a solvent recovery column. In the extractive column, since the solvent is chosen to be nonvolatile, remains at a relatively high concentration in the liquid phase throughout the column. The relatively nonvolatile added component (or solvent) is added to the azeotropic feed mixture to alter the volatility’s of the key components without the formation of any additional azeotropes. The solvent is added at or near the top of the column and above the feed stage and is usually a heavy boiler, so that the light boiler is recovered as a pure distillate in the first column. The solvent recovery column separates the second azeotropic constituent, the intermediate boiler that is recovered as a pure distillate, from the solvent. The solvent is then recycled to the first column.

Essential features for a fitting solvent:

- Solvent must affect the liquid phase behavior of the key components differently
- Solvent must be a higher boiling component than the key components
- Relatively nonvolatile
- No forming of additional azeotropes

An example of extractive distillation is the separation of water and ethanol with ethylene glycol as a solvent which is shown in Figure 14.

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Figure 14 Extractive distillation

In this extracting distillation sequence, ethylene glycol is added at the top of the extractive column and is present throughout the whole column. D1 is the distillate of the first column and consist of more or less pure ethanol. The bottom of C1 consists of water and ethylene glycol which is sent to C2, the recovery column. In this column the solvent ethylene glycol is recovered and D2 represents the pure water. In general a high concentration of solvent is necessary to alter the activity coefficients of the components being separated.

2.2.2.3 Reactive distillation

During reactive distillation, chemical reaction and distillate separation are carried out simultaneously in one column. Reactive distillation may be advantageous when [5]:

- The reaction must be carried out with a large excess of one or more of the reactants
- The reaction can be driven to completion by removal of one or more of the products as they are formed
- The product recovery scheme is complicated or is made infeasible by azeotrope formation.

Reactive distillation can provide a means of breaking azeotropes by altering or eliminating the condition of azeotrope formation in the reaction zone through the combined effects of vaporization-condensation and consumption-production of the species in the mixture. Alternatively, a reaction may be used to convert the species into components that are more easily distilled.

2.2.2.4 Salted distillation

In this case, the addition of a nonvolatile, soluble ionic salt effects the separation and modifies the liquid phase behavior. For example the separation of a binary mixture, the salt is fed at the top of the column by dissolving it in the reflux stream. The salt is essentially nonvolatile, remains in the liquid phase on each tray, and alters the relative volatility throughout the whole column. The bottoms product is recovered from the salt by evaporating or drying and the salt is recycled. The use of a salting agent presents a number of problems, such as difficulty of transporting, slow mixing or dissolution rate of salt, limits to solubility in the feed components and potential for corrosion.

3 Investigation of the given quaternary solvent mixture

One important aspect of modeling is the quality of the physical properties and interaction parameter being used. Usually, estimation methods for vapor-liquid equilibrium calculations are compared against binary data. This fundamental problem has been analyzed by several authors and will not be reiterated on its importance. From the point of view in this paper, a reasonably adequate thermodynamic model is available to model the mixture. ASPEN recommends the activity coefficient property methods WILSON, NRTL, UNIQUAC and their variances for solving azeotropic separation problems. In our case, the property package used is UNIQUAC for the liquid phase and Ideal Gas for the vapor phase. In order to avoid blind search for the feasible structure as well as to become able to explain why a separation process works, a preliminary exploration of the immiscibility regions and the simple distillation region boundaries is necessary. A similar problem was investigated by Mizsey [18] and also experimental tested by [19]. In this work a four step synthesis is suggested to determine all necessary information about the mixture and its behavior.

The following steps should be considered:

- Exploration of the immiscibility regions
- Exploration of the azeotropic points, the 1-separatrices and the ternary simple distillation regions
- Exploration of the 2-separatrices and the ternary simple distillation regions
- Exploration of the set o feasible separation structures

This procedure can provide a means of creating and evaluating designs alternatives quickly with a minimum experimental and computational effort. Another way is to use the so called expert systems. Expert systems are computer programs which are used to solve special problems in a restricted scopes of duties. The use of an expert system is recommendable when a complex, well defined problem should be solved, an expert can not be consulted or to give the experts a release.

We start the examination of the system by investigating the pure components. In Table 1 the essential physical properties of the four main components are given.

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