Theoretical Foundations of Chemical Engineering, Vol. 39, No. 5, 2005, pp. 463–470. Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 5, 2005, pp. 491–498. Original Russian Text Copyright © 2005 by Timoshenko, Anokhina, Ivanova.
Extractive Distillation Systems Involving Complex Columns with Partially Coupled Heat and Material Flows A. V. Timoshenko, E. A. Anokhina, and L. V. Ivanova Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571 Russia e-mail:
[email protected] Received April 23, 2004
Abstract—A method based on the representation of flowsheets as graphs is proposed to synthesize flowsheets of extractive distillation of multicomponent azeotropic mixtures in complex columns with partially coupled heat and material flows. It is shown that the flowsheets constructed can provide a significant decrease in the energy consumption for separation because the process becomes structurally closer to thermodynamically reversible distillation.
Extractive distillation is one of the most popular methods for separating azeotropic mixtures. Extractive distillation is widely used in organic synthesis and in isolation of monomers. Over many years, the main attention in developing extractive distillation has been focused on the search for an efficient separating agent. Obviously, without this step, a technology is impossible to design. However, if a chosen extractant has lower volatility than the components of the mixture, the flowsheet of a distillation system is predetermined (Fig. 1a). Other solutions in the industry are virtually absent. Conventional extractive distillation systems are very energy-intensive. Systems with multiple withdrawals are known to significantly decrease the energy consumption for separation [1–11]. However, to date, such systems have been used only in distillation of zeotropic mixtures. At the same time, there are not any technical or thermodynamic constraints on the use of complex columns with multiple withdrawals and feeds and systems with coupled heat and material flows in extractive distillation. The first attempts to estimate the efficiency of complex columns in extractive distillation were made by Brito et al. [12]. However, their proposed variant of a column with side withdrawal does not ensure high product purity. To reach high product quality, which is characteristic of the organic synthesis industry and monomer synthesis, distillation systems with side stripping or rectifying sections should be used. Methods for synthesizing such energy-saving flowsheets for distillation of zeotropic mixtures were developed in detail in our previous works [8, 9, 13]. The efficiency of the proposed algorithm was confirmed in designing a number of technologies of distillation of zeotropic mixtures [6, 8–10, 14–18]. The purpose of this work is to extend this approach to extractive distillation flowsheet synthesis. The proposed algorithm [8, 9, 13] uses certain initial approxi-
mations as preimage flowsheets; as such a preimage, it is expedient to take the conventional process flowsheet (Fig. 1a). It is only necessary to exclude the operation of collapsing the oriented edge that explicates the extractant recycle. Under the assumption that the extractant is slightly volatile and provides only a change, rather than an inversion, of relative volatilities, the number of variants of distillation of a binary mixture is 1 (Fig. 1a). The operation of transformation u [8, 9, 13] can give a single extractive distillation system with coupled flows (Fig. 1b). This system is a complex column with a rectifying side section. The bottoms product—extractant—is cooled (the cooler is not shown) and returned to the upper part of the main column. Such a process design provides a significant decrease in the energy consumption for separation [19]. A deeper transformation of the separation system leads to a system involving a complex column with side withdrawal of the product B in the vapor phase below the feed section. However, such
S
(a) Ä
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AB
AB
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S S Fig. 1. Separation of a binary azeotropic mixture by extractive distillation with a slightly volatile extractant in (a) a conventional process, (b) a system with partially coupled heat and material flows, and (c) a complex column with side withdrawal. A and B are the components of the mixture in order of decreasing relative volatility, S is the separating agent (extractant), and the subscript V refers to the withdrawal in the vapor phase.
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Fig. 2. Separation of a binary azeotropic mixture by extractive distillation with a highly volatile extractant in (a) a conventional process and (b) a system with partially coupled heat and material flows. The notation is the same as in Fig. 1.
Fig. 3. Diagrams of vapor–liquid equilibrium of ternary mixtures with a single binary azeotrope according to Serafimov’s classification: (a, b) 3.1.0t1a, (c, d) 3.1.0t1b, and (e, f) 3.1.0t2.
a process design does not ensure high purity of the product B [12]. Thus, for extractive distillation of binary azeotropic mixtures, systems with the flowsheet shown in Fig. 1b are more suitable, which provide a significant decrease in the energy consumption for separation. Note that, for the flowsheets shown in Fig. 1, the reflux of the product A is efficient only when the separating agent and the reflux have like effects. If their effects are opposite, the product A should be isolated without reflux [20]. In designing a real process, this should be taken into account; however, we consider general approaches to synthesis; therefore, in all the figures, extractive distillation columns include the reflux of the highly volatile component. There is one more possibility of separating a binary azeotropic mixture by extractive distillation (Fig. 2a). This is used if the extractant is highly volatile. The corresponding flowsheet for a complex column with a stripping side section is presented in Fig. 2b. In this case, the withdrawal of the liquid into the side section is above the feed section. If the extractant is highly volatile, one can continue transformations (as above) and obtain a system with a complex column with side withdrawal. Further in this work, we consider only variants of the most widely used extractive distillation with a slightly volatile extractant. Flowsheets of extractive distillation of multicomponent mixtures often involve the preliminary isolation of a binary azeotropic mixture. Some approaches to determining the number of flowsheets for such a process design were proposed in our previous work [21], where flowsheets containing uniform separation systems were considered and represented as graphs whose nodes correspond to columns. Such an approach is applicable to the synthesis of extractive distillation flowsheets only for determining the number of variants of predistillation up to isolation of a mixture of azeotrope-forming
components. Since an extractive distillation system necessarily contains unlike columns, it is better to use synthesis algorithms based on the representation of a flowsheet as a graph with edges corresponding to column sections and coupling of sections [8, 9, 13]. Obviously, with an increase in the number of components in the feed and product fractions, the number of possible variants of distillation flowsheets increases. However, for azeotropic mixtures, the structure and number of flowsheets are affected by the thermodynamic–topological portrait of vapor–liquid equilibrium. Let us consider the simplest variants. There are six types of diagrams of vapor–liquid equilibrium of ternary mixtures with a single binary azeotrope, which are pairwise topologically isomorphic (Fig. 3). We apply the previously [8, 9, 13] proposed approach to the synthesis of a separation flowsheet with a vapor–liquid equilibrium portrait of the type shown in Fig. 3a and represent the possible distillation flowsheets as graphs (Fig. 4). A single operation of collapsing gives the graph of a system consisting of a simple column and an extractive distillation system with coupled flows. The distillation of a mixture with the vapor–liquid equilibrium portrait presented in Fig. 3b differs only by the order of isolation of components. The predistillation in the simple column yields the product A as the distillate, and the B–C mixture is separated by extractive distillation. The question of the possibility of combining the entire separation into a single system arises. This is likely to be possible if its elements corresponding to the extraction part of the column remain unchanged in the transformation. Such elements are indicated by dashed lines in Fig. 4. According to our previous works [8, 9, 13], we transform the graph presented in Fig. 4b by the operation of collapsing the oriented edge AB. The graph obtained is shown in Fig. 5. The column section corresponding to the node ABS necessarily contains the
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Fig. 4. Graphs of extractive distillation flowsheets of a mixture with the vapor–liquid equilibrium portrait shown in Fig. 3a in (a) a conventional process and (b) a system with partially coupled heat and material flows. The dashed lines indicate the extraction and higher sections of the column. The graph nodes represent (1) the property inlet, (2) the property heat supply, (3) the property heat removal, and (4) the absence of these properties.
extractant. Consequently, with the liquid flow, the extractant enters side column 1 and the product C. This is acceptable only if the relative volatility of the extractant is higher than that of the product C. However, in the variant under consideration, the extractant is slightly volatile. Therefore, the separation of the mixture with the vapor–liquid equilibrium portrait shown in Fig. 3a in a single complex column is unpractical because this would require the installation of an additional column for separating the C–S mixture. Another situation takes place if the mixture has the vapor–liquid equilibrium portrait presented in Fig. 3b. In this case, a number of transformations of graphs (Figs. 6a–6c) lead to a flowsheet with side column 1 for isolating the highly volatile component A (Fig. 6d). One can see that there are no obstacles to the implementation of such a process since, even if the slightly volatile extractant enters side column 1 with the vapor flow (Fig. 6), the extractant does not enter the product A. However, the question of the economic and energy efficiency of such a system still remains open, although one can assume a decrease in the energy consumption for separation because the process becomes structurally closer to thermodynamically reversible distillation. Let us consider the distillation of a mixture with the vapor–liquid equilibrium portrait shown in Fig. 3c. Without extractant added, in the first step of separation, one can isolate either the slightly volatile component C or the A–C azeotropic mixture. One can also implement a set of intermediate variants of separation [20]. In the isolation of the component C, the flowsheet involves no extractive distillation. In the isolation of the highly volatile product whose composition is close to that of the A–C azeotropic mixture, the bottoms product is either an A–B or a B–C mixture, depending on the feed composition.
–B
ABC C
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Fig. 5. (a) Graph and (b) flowsheet of extractive distillation of a mixture with the vapor–liquid equilibrium portrait shown in Fig. 3a: (1) side column, (2) main column, and (3) side section.
Formally, there can be two distillation flowsheets. First, they include systems of distillation of a binary azeotropic mixture, which have already been considered from the standpoint of applicability of complex columns with coupled heat and material flows. Second, each of the two flowsheets consists of four distillation columns and, hence, is less competitive in terms of capital costs than the other variants of separation. A similar situation also takes place in the separation of mixtures with the topologically isomorphic vapor–liquid equilibrium portrait (Fig. 3d). Let us analyze the distillation of a mixture with the vapor–liquid equilibrium portrait presented in Fig. 3e. In the first step, by simple distillation, one can isolate the product A and then separate the B–C azeotropic mixture. This approach gives the same set of variants (Fig. 6) as for Fig. 3b. Correspondingly, for the separation of a mixture with the vapor–liquid equilibrium portrait presented in Fig. 3f, we obtain separation systems that are characteristic of the separation of the mixture with the vapor–liquid equilibrium portrait shown in Fig. 3a. By and large, if a ternary mixture is initially distilled to form a pure component and a binary azeotropic mixture, then, for the vapor–liquid equilibrium portraits shown in Figs. 3b and 3e, there can be two variants of extractive distillation flowsheets: both in a single column with partially coupled heat and material flows and in its combination with a simple two-section column (Figs. 6b, 6c). The separation of the mixtures with the vapor–liquid equilibrium portraits presented in Figs. 3a and 3f is possible only in two-column systems. For the four types of mixtures, the transformation of the initial extractive distillation system into a single complex column is possible if the vapor–liquid equilibrium portrait at any feed compositions imposes no thermodynamic– topological constraints on the isolation of the highly
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Fig. 6. (a–c) Graphs and (d) flowsheet of extractive distillation of a mixture with the vapor–liquid equilibrium portrait shown in Fig. 3a. The notation is as in Fig. 5.
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volatile component from a mixture by simple distillation and the extractant is slightly volatile. In all the flowsheets considered, one of the components or a binary zeotropic mixture is isolated by simple distillation. Thus, the preimage flowsheets are distillation sequences involving extractive distillation systems. A fundamentally different approach to flowsheet synthesis is the use of the extractant at the first step. This allows one to avoid the thermodynamic–topological constraints of ordinary distillation. Such flowsheets were reviewed by Frolkova [22]. To the first group, Frolkova [22] assigned systems intended to isolate, in the first column, either the highly volatile component A and a B–C zeotropic mixture (Figs. 7a, 7b) or the indi-
vidual component A and a B–C zeotropic mixture that is further separated by the same (Fig. 7c) or another (Fig. 7d) extractant. The second group of flowsheets involves the division of the feed into fractions and will be considered below. Let us apply the proposed [8, 9, 13] algorithms to the flowsheets shown in Fig. 7. Sequential operations of collapsing the oriented edges in the graph (Fig. 8a) explicating the flowsheet presented in Fig. 7a yield a number of flowsheets. The first operation gives two flowsheets. The first of them involves the isolation of the component A as the distillate of the extractive distillation column. The isolation of the components B and
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Fig. 9. (a–d) Graphs and (e) flowsheet as images of the extractive distillation flowsheet in Fig. 7b.
C and the extractant recovery are performed in a complex column with a rectifying side section. The second flowsheet involves the extractive distillation in a system with coupled heat and material flows and the isolation of the component A from the distillate and the component B as the product of the rectifying side section. The bottoms product of this column is a mixture of the component C and the extractant and is then separated in a simple column. Operations of collapsing the oriented edges in the graphs corresponding to these flowsheets lead to a single graph (Fig. 8b) corresponding to a complex column with two feed sections and two rectifying sections (Fig. 8c). As before, the extraction part of the column remains unchanged in the transition from the preimage flowsheet to the image flowsheet. Thus, extractive distillation systems with the flowsheet shown in Fig. 7a can be transformed into two two-column systems and one single-column system.
Let us examine the flowsheet presented in Fig. 7b as a preimage. Depending on the order of operations of collapsing in the graph of the flowsheet, two different results are obtained. A single operation of collapsing the edge BCS of the graph of the preimage flowsheet (Fig. 9a) yields a graph of a two-column flowsheet (Fig. 9b). Further transformations of this graph are inexpedient since they lead to inoperative flowsheets. A single operation of collapsing the edge B–C gives a graph (Fig. 9c) that is also an image of a two-column system but with a different flowsheet. This graph can be transformed further since the oriented edge B–C–S is incident to the node corresponding to the extreme section of the column. The obtained graph (Fig. 9d) is an image of a complex column with coupled flows (Fig. 9e). Let us note certain common features of the flowsheets shown in Figs. 6d and 9e. Each of the flowsheets consists of a single complex column that is an extractive distillation system with partially coupled
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heat and material flows. In both cases, the component A is isolated as the distillate of the predistillation side column. Each of the variants has a side section and a side column. The flowsheets differ in the number of sections in the side and the main columns, the type of the side section, and the extractant recycle direction. The relative positions of the main column and the side apparatuses are also different. By and large, these distillation flowsheets, albeit somewhat similar, are fundamentally topologically different. The transformations of the graphs representing the flowsheets shown in Figs. 7c and 7d are presented in Figs. 10 and 11, respectively. One can see that the images of the flowsheets given in Fig. 7c can be both two- and single-column systems with two recycles. The
preimage flowsheet (Fig. 7d) uses two different extractants and is a sequence of two systems. Therefore, only the systems, rather than their coupling, can be transformed. A double operation of collapsing leads to a single variant, which involves no simple columns (Fig. 11b). Practically, this is a sequence of two extractive distillation systems with coupled flows (Fig. 11b). The second group of flowsheets (branched flowsheets) according to Frolkova’s classification [22] involves the division of the feed into fractions, on which thermodynamic constraints may be either imposed or not imposed. To this class of flowsheets, the above algorithms can also be applied to obtain certain sets of possible technological solutions. An example of such a flowsheet is given in Fig. 12a. It is undeniable
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distillation flowsheets can be transformed into a system with partially coupled heat and material flows. The economic efficiency of such solutions has already begun to be evaluated, and the first results are hopeful. For example, the energy consumption for separation of an acetone– chloroform mixture was reduced by 26.2% [19, 23].
(a) AB
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ë
ABC B
ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 04-03-32987.
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REFERENCES
–
–
B
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+
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ABC CS + S
NOTATION A, B, C—components of a mixture; S—separating agent (extractant); u—operation of transformation.
+
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ë –
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Fig. 12. (a) Preimage flowsheet, (b) its graph, and (c–e) the graphs of image flowsheets of extractive distillation with coupled flows.
that this flowsheet can be transformed by the operation of collapsing the edge C–S of the graph (Fig. 12b) to form a two-column system with coupled flows (Fig. 12c). The question of whether or not the operation of collapsing the edge A–B should be performed has to be discussed. If the reflux and the separating agent have like effects and, correspondingly, the reflux is favorable to extractive distillation, then such an operation is possible. This gives a system whose side section is above the extractant injection section (Fig. 12d). However, the serviceability of this flowsheet, as well as the flowsheet obtained from it by further transformations (Fig. 12e), should be confirmed by experiments or calculations. Thus, the general principles of synthesis of distillation flowsheets with partially coupled heat and material flows, which were previously [8, 9, 12, 13] developed for zeotropic mixtures, turned out to be quite universal. This allowed us to extend these principles to extractive distillation of azeotropic multicomponent mixtures and show that virtually any of the conventional extractive
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