Theoretical Foundations of Chemical Engineering, Vol. 39, No. 1, 2005, pp. 16–23. Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 1, 2005, pp. 19–26. Original Russian Text Copyright © 2005 by Ivanova, Timoshenko, Timofeev.
Synthesis of Flowsheets for Extractive Distillation of Azeotropic Mixtures L. V. Ivanova, A. V. Timoshenko, and V. S. Timofeev Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571 Russia E-mail:
[email protected] Received April 23, 2004
Abstract—An algorithm for synthesizing extractive distillation flowsheets for separation of complex ternary azeotropic mixtures with different vapor–liquid equilibrium patterns is proposed.
Separation of end fractions of given compositions from azeotropic mixtures is complicated by thermodynamic and topological constraints, which can be surmounted using the principle of redistribution of concentration fields between separation regions [1–3]. This principle can be implemented using homogeneous and heterogeneous (involving extraction, adsorption, absorption, and chemical and other separation techniques) separation systems. In homogeneous systems, distillation can be designed so that, at one of the steps, an azeotropic fraction is isolated, which is then separated by special methods. If this fraction contains two or more components, one can use extractive distillation or separation of the azeotropic mixture at different pressures. If the fraction contains more than two components, separation can be performed by methods using the curvature of the separating manifold [3, 4]. Previously [2, 5], a general approach to synthesizing flowsheets for distillation of multicomponent nonideal (including azeotropic) mixtures was put forward, which is based on the concept of a distillation region. Later [6–8], some approaches to synthesizing flowsheets for distillation of multicomponent mixtures containing a (‡) A
single binary azeotrope were proposed, which use separation of an azeotropic mixture at different pressures. Note that, in practice, thermodynamic and topological constraints on separation of end fractions of given compositions are most often surmounted by using extractive distillation. However, conventional extractive distillation methods are highly energy-intensive. Therefore, it is topical to develop and use optimal energy-saving technologies. This problem is solved in several steps: structural optimization of a flowsheet, optimization of the operating parameters of distillation columns, design optimization of flowsheet elements. Since distillation is irreversible, its thermodynamic efficiency depends on the process path—a set of flowsheets or distillation trajectories. To date, approaches to synthesizing extractive distillation flowsheets have been developed obviously insufficiently. This question was considered only in few works. For example, a number of systems of different structures for separation of multicomponent azeotropic mixtures were proposed [9]. Certain technological solutions for distillation of an (b)
B
A
F
F S S
(c) A B C F
(d) A
(f)
B A B
F
F S
B
(e)
S C
S
C
F
(j) A
F
S C B
A
C
B
S
Fig. 1. Flowsheets for separation of (a, b) ternary and (c–g) quaternary zeotropic mixtures: A, B, C are components of a mixture; S is a low-volatile extractant; F is feed (a, b) ABS and (c–g) ABCS. 0040-5795/05/3901-0016 © 2005 MAIK “Nauka /Interperiodica”
SYNTHESIS OF FLOWSHEETS FOR EXTRACTIVE DISTILLATION A
B
C
A
F
17
B
C
S Fn S S
Fig. 2. Representation of the distillation flowsheet in Fig. 1c as the orgraph L. The nodes represent columns (empty circles) and fractions (filled circles).
(‡) A
Fn
Fig. 3. Graph M obtained by splitting the node F of the graph L.
(b) B
C
A
(‡) B
C
A
Fn S
(b) A
B
Fn
Fn
S
S B
S Fig. 4. Graph U obtained by merging two nodes S (representing fractions) of the graph M.
Fig. 5. Graphs corresponding to flowsheets for extractive distillation of an A–B binary azeotropic mixture.
ethanol–water mixture were presented [10]. However, thermodynamic and topological constraints on the compositions of the products were not always taken into account and the proposed solutions are inoperable. Thus, structural optimization of extractive distillation currently remains virtually beyond the scope of consideration. Solving this problem is expediently begun from developing general algorithms for synthesizing flowsheets of this class. Within such a problem formulation, one can analyze not only distillation systems involving columns from which the products are withdrawn from two points, but also more complex structures with partially coupled heat and material flows. However, early in our consideration, we restrict ourselves to synthesizing a flowsheet for distillation of ternary azeotropic mixtures in systems of two-point withdrawal columns. By now, extractive distillation flowsheets have been classified. All the possible variants of separation can be divided into two large classes [9]. The first class includes flowsheets in which an extractant is used as far upstream as the first column of a distillation system and, consequently, thermodynamic and topological constraints on the compositions of the product fractions are lifted. In flowsheets of the second class, the initial multicomponent mixture is predistilled until a fraction of azeotrope-forming components is isolated, which is then separated by extractive distillation. Taken together, the two classes encompass all the possible variants of separation. Flowsheets of the first class for distillation of mixtures with low dimensionality of the
concentration space can be used as elements of separation of mixtures in the second class of flowsheets for mixtures of higher dimensionality. Let us consider an algorithm for synthesizing flowsheets of the first class. Introduction of an extractant lifts the thermodynamic and topological constraints on the compositions of the product flows; therefore, as prototypes for synthesis, we use sets of flowsheets for distillation of zeotropic mixtures in simple two-section columns. Let the object of separation be an (n + 1)component mixture consisting of an n-component initial azeotropic mixture and an extractant. Let us assume that the extractant is the least volatile component of the mixture. Figure 1 presents the flowsheets for distillation of ternary and quaternary zeotropic mixtures. For further analysis, we represent them as oriented graphs in which nodes characterize columns and fractions, and edges describe flow relationships between them. Let the nodes that characterize the fractions be labeled with the corresponding letters. For example, the flowsheet in Fig. 1c is represented by the graph in Fig. 2. The graph in Fig. 2 describes the separation sequence. The next step of the synthesis is to split the flow F of the (n + 1)component mixture into two individual flows: the n-component mixture Fn and the extractant S. Formally, if we start from the graph L in Fig. 2, we will obtain the graph M in Fig. 3. This structure already almost fully represents the extractive distillation flowsheet. It is only necessary to create a cycle for the extractant S by merg-
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IVANOVA et al. (a) A
(b) B
C
A
(c) S
C
S
(d) A
B
S
(e) C
A
A
B F
F
F
S
B
(f) A
C
(j) B
C
A
B
(h) S
C
S
S
(i) A
C
F
F
B
S
(g) C
A
A B
F
F
F
S
B
(k) A
C
(l) B
C
Fn
A
C
Fn S
B
(m) S
S
B
Fn B
S
(n) A
S
(o) C
A
A B C
Fn
Fn C
C
F
F
S
B
Fig. 6. Synthesis of graphs corresponding to extractive distillation flowsheets.
ing two nodes (characterizing the fractions) labeled with S. We will obtain the graph U in Fig. 4. Note that, depending on the vapor–liquid equilibrium pattern, more than one edge may leave the node S (the extractant may also be necessary for separating the mixture in the second column of the system) (Fig. 4b). The extractant S is assumed to ensure separation of the entire n-component mixture into pure components. Since the extractive distillation system has a closed (without taking into account losses) cycle for the extractant, one of the conditions of operability of flowsheets synthesized using the proposed algorithm is the presence of a cycle in the orgraph. By a cycle, we mean the possibility of passing through a number of graph nodes and returning to the starting node with allowance for the orientation of edges. A cycle must include at least two nodes representing columns. A second condition of operability of the proposed flowsheets is the introduction of the extractant into the column in which an azeotropic pair of components is separated. Let us consider the simplest example, namely, separation of an A–B binary azeotropic mixture. Obviously, for the given type of flowsheets, the extractant must be introduced into the first separation column; therefore,
Fig. 5 presents only such variants. It is seen that the first operability condition is satisfied only by the graph in Fig. 5a. The structure of this graph fully corresponds to the classical flowsheet for extractive distillation of a binary azeotropic mixture with a low-volatile extractant. The example considered is trivial. Therefore, let us present our algorithm and illustrate it by the example of separation of a ternary azeotropic mixture. (1) Choose a vapor–liquid equilibrium pattern according to Serafimov’s classification (Table 1) [11]. As an example, we consider a pattern of type 3.1.1 t1a with direct orientation of distillation lines. (2) Synthesize flowsheets for distillation of a quaternary zeotropic mixture (Figs. 1c–1g) and represent them as orgraphs of the L type (Figs. 6a–6e). (3) In flowsheets of the set obtained (Figs. 6a–6e), select and label nodes to which an edge from the node S (representing the extraction column) should be directed. For the selected type of extractive distillation flowsheets, these nodes describe the columns that are the first in the separation sequence (filled circles).
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Table 1. Types of phase diagrams of ternary azeotropic mixtures with direct and reverse orientation of distillation lines Diagram type
Direct
Reverse
A
C
Diagram type
Direct
Reverse
A
3.1.0 t1a
3.2.1 t3a B
C
B
A
A
B
C
C A
3.1.0 t1b
C
3.2.1 t3b B
C
B
C
A
B
A
C
B
C
3.1.0 t2
A A
3.3.0 t1a B
A
B
A
C
B
C
A
B
A
3.1.1 t1a
C A
3.3.0 t1b B
C
B
C
A
B
A
C
B
C
3.1.1 t1b
C A
3.3.0 t2 B
A
B
C
B
A
A
B
A
3.2.0 t1
C A
3.3.1 t1b B
C
B
A
C
C
B
A
3.2.0 t2a
C A
3.3.1 t1c B
C
B
A
A
B
C
C
B
A
3.2.0 t2b
C A
3.3.1 t2 B
C
B
A
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B
C
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IVANOVA et al.
Table 1. (Contd.) Diagram type
Direct
Reverse
A
C
Diagram type
3.2.0 t2c
Direct
Reverse
A
A
3.3.1 t3a B
C
B
A
C
B
A
C
B
C
A
3.2.1 t1
A
3.3.1 t3b B
A
B
C
A
B
C
C
B
C
A
3.2.1 t2a
A
3.3.1 t4 B
C
B
A
A
B
C
B
C
C
3.2.1 t2b B
C
B
A
(4) Split the node F into nodes Fn and S and remove the node S incident to the incoming edge. This operation is equivalent to replacement of the node F by Fn. (5) Connect the node S to the labeled node (representing the extraction column) with an oriented edge leaving the node S and check the obtained structures (Figs. 6k–6o) for operability. Of five graphs obtained, only two (Figs. 6k, 6l) meet the operability conditions. They correspond to quite trivial flowsheets in Fig. 7. Analyzing the graphs in Fig. 6, one can reveal a certain set of flowsheets whose operability depends only on the phase portraits of the corresponding distillation systems. These are the graphs in Figs. 6k, 6l, and 6o. In
A
B
C
A
ABC
B
ABC S
S
C
Fig. 7. Operable flowsheets for the vapor–liquid equilibrium pattern of type 3.1.1 t1a.
the corresponding flowsheets, there is extractant recirculation, and their applicability is determined by the vapor–liquid equilibrium pattern. Therefore, when analyzing structures for phase diagrams of different types, we further consider only these variants. In the flowsheets corresponding to the graphs in Figs. 6m and 6n, there is no extractant recirculation, and these flowsheets do not meet the operability conditions. Let us consider an algorithm for synthesizing extractive distillation flowsheets of the second class. In this case, the first step is the distillation of the initial mixture; therefore, for all the flowsheets of this class, the extractant is used at any separation step other than the first. Below are the main steps of the algorithm. (1) Determine the type of the phase diagram. As an example, we will consider the same vapor–liquid equilibrium pattern of type 3.1.1 t1a with direct orientation of distillation lines. (2) Synthesize flowsheets for distillation of a quaternary zeotropic mixture. When determining the sequence of separation of components, it is necessary to consider the possibility of distilling the initial mixture into an azeotropic constituent and a zeotropic constituent. It is seen that the component C can also be isolated without using extractive distillation. Of the five
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SYNTHESIS OF FLOWSHEETS FOR EXTRACTIVE DISTILLATION (‡)
(b)
A
F1
(c)
F1n
B
21
A
B
(d)
A
F1
B A
B
ABC F
F
F
S C
C
S
S S
C
S
C
Fig. 8. (a–c) Graphs and (d) flowsheet for extractive distillation.
possible separation variants proposed, only one flowsheet (Fig. 8) meets this condition. (3) Determine and label graph nodes to which an edge from the node S (representing the extraction column) should be directed. For the selected type of extractive distillation flowsheets, these nodes describe the columns where azeotrope-forming components are separated. (4) Design extractant recirculation. In this case, add a node corresponding to the introduction of the extractant into the column of separation of an azeotropic pair of components. In Fig. 8b, such a node is labeled by S. (5) Check the obtained structures for operability. Among extractive distillation flowsheets of the second class, some variants of process design can also be a priori discarded. These are the flowsheets in Figs. 1d, 1f, and 1g. In the flowsheets in Figs. 1d and 1f, with allowance for the separation of the zeotropic constituent of the mixture in the first column, the extractant is withdrawn upstream of the point of separation of the azeotropic pair of components. To implement the flowsheet in Fig. 1g, in the case of precise distillation, the initial mixture must contain at least four components. Thus, using the algorithm proposed, for separation of the ternary mixture described by the vapor–liquid equilibrium pattern of type 3.1.1 t1a with direct orientation of distillation lines, we synthesized three operable extractive distillation flowsheets (Fig. 7, Fig. 8d).
Two of them belong to the first class of flowsheets, in which azeotrope-forming components are separated in the first column and extractive distillation is used starting with the first column; the third flowsheet belongs to the second class of flowsheets, in which the initial mixture is predistilled. Let us consider the same vapor–liquid equilibrium pattern of type 3.1.1 t1a but with reverse orientation of distillation lines. We will not dwell on algorithm steps in detail but will consider its results. It is seen that, in this case, the algorithm leads to other operable flowsheets (Fig. 9). The flowsheet in Fig. 9a belongs to the first class of extractive distillation flowsheets and has quite an unconventional structure, and the flowsheet in Fig. 9b belongs to the second class of flowsheets, in which the zeotropic constituent is preseparated. Thus, the selection of flowsheets for separation of azeotropic mixtures is a complex problem, which involves the investigation of the vapor–liquid equilibrium pattern and the analysis of all the possible variants of flowsheets. Therefore, we further consider the possibility of applying some flowsheets or others by the example of known types of phase diagrams of ternary (‡) A
(b) B
C
F
(‡) ë
A
B
B
S
B
ë
S
B
C
A
S
B
F S
Fig. 9. Flowsheets for the vapor–liquid equilibrium pattern of type 3.1.1 t1a with reverse orientation of distillation lines.
B
(e)
F
S
C
C
(d) A
A F
S
ABC
ABC
A F
(b) A
(c)
C
S
Fig. 10. Flowsheets of the (a–c) first and (d, e) second class of extractive distillation flowsheets.
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IVANOVA et al. A
B
C
A
B
C
ABC
ABC
S
S
S
Fig. 11. Separation flowsheets for complex vapor–liquid equilibrium patterns.
mixtures. Earlier, we determined the flowsheets that can be implemented, depending on the type of phase diagram of a mixture being separated (Fig. 10). The results of analyzing these variants are presented in Table 2. In addition, there are two types of phase diagrams for which separation can also be performed withTable 2. Operability of flowsheets in Figs. 10a–10e for various types of phase diagrams with direct/reverse orientation of distillation lines when, in the first column, (I) an extractant is used and (II) the initial mixture is distilled I
II
Diagram type
10a
10b
10c
10d
10e
3.1.0 t1a 3.1.0 t1b 3.1.0 t2 3.1.1 t1a 3.1.1 t1b 3.1.1 t2 3.2.0 t1 3.2.0 t2a 3.2.0 t2b 3.2.0 t2c 3.2.1 t1 3.2.1 t2a 3.2.1 t2b 3.2.1 t3a 3.2.1 t3b 3.3.0 t1a 3.3.0 t1b 3.3.0 t2 3.3.1 t1a 3.3.1 t1b 3.3.1 t1c 3.3.1 t2 3.3.1 t3a 3.3.1 t3b 3.3.1 t4
+/+ +/+ +/+ +/+ +/+ +/+ + +/+ +/+ +/+ +/+ +/+ +/+ + +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+
+/– +/+ +/– +/– +/+ +/– + +/– +/+ +/+ –/+ +/– +/– + +/+ –/– –/– –/– –/– –/– –/– –/– –/– –/– –/–
–/+ +/+ –/+ –/+ +/+ –/+ – –/+ –/– –/– +/– –/+ –/+ – –/– –/– –/– –/– –/– –/– –/– –/– –/– –/– –/–
–/+ –/+ –/+ –/+ +/– –/– – –/+ –/+ –/+ –/– –/+ –/+ – –/– –/+ –/– –/+ –/+ –/– –/– –/– –/– –/– –/–
+/– +/– +/– +/– –/+ –/– – +/– +/– +/– –/– +/– +/– – +/– +/– –/– +/– +/– –/– –/– –/– –/– +/– –/–
out using an extractant. These are types 3.1.2 t1b and 3.1.1 t1b. Within the concentration simplexes corresponding to these types, a single bundle of distillation lines develops. This makes possible the separation of the pure components A, B, and C in a sequence of two distillation columns. Note that, in each particular case, it is necessary to carefully study the applicability of a separation variant. For example, for a number of vapor–liquid equilibrium patterns (3.2.0 t2b, 3.2.0 t2c, 3.2.1 t2b, 3.2.1 t3b, 3.3.0 t2, 3.3.1 t3b), flowsheets of the second class are implementable only when the initial feed composition belongs to a certain distillation region of the concentration simplex. For example, for the system characterized by the vapor–liquid equilibrium pattern of type 3.2.0 t2b with direct orientation of distillation lines, distillation of the initial mixture (isolation of the component C) is possible if the initial feed composition belongs to the region AzÄÇ–B–C–AzÄë and the composition of the distillate of this column corresponds to the segment B–AzÄÇ. As noted above, the operability of a flowsheet is determined by the presence of a cycle in the corresponding orgraph. The direction of the edge representing the extractant recirculation is determined by the vapor–liquid equilibrium pattern of the initial mixture. The extractant must be directed only to the column (columns) in which the extractant is necessary for separation of the mixture. In the case of a complex vapor– liquid equilibrium pattern (3.1.1 t2, 3.2.1 t1, 3.2.1 t2b, 3.3.0 t1a, 3.3.0 t1b, 3.3.0 t2, and also all the subtypes of 3.3.1), the extractant must be introduced simultaneously into two columns of the distillation system. Therefore, such process design should always be compared with two consecutive extractive distillation systems for revealing the most efficient solution (Fig. 11). Thus, we have developed a universal algorithm for synthesizing flowsheets for extractive distillation of multicomponent azeotropic mixtures, which is based on the use of flowsheets for distillation of zeotropic mixtures in simple two-section columns as prototypes of extractive distillation flowsheets. This algorithm should be applied at the step of structural optimization of distillation flowsheets for reducing the energy consumption for separation. It was noted that the operable flowsheets thus obtained should be subjected to further
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analysis and discrimination for applicability to a particular separation problem. 6.
ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 04-03-32987. 7.
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