New Intensified Distillation Systems for Quaternary Petlyuk Configuration Ben-Guang Rong,a* Massimiliano Errico,b Juan Gabriel Segovia-Hernandezc aUniversity of Southern Denmark, Niels Bohrs Alle 1, 5230 Odense,Denmark. bUniversità degli Studi di Cagliari, 09123 Cagliari, Italy. cUniversidad de Guanajuato, Campus Guanajuato, 36050 Guanajuato, Mexico. bgr@kbm.sdu.dk

Abstract While dividing-wall column for ternary Petlyuk configuration has received wide applications in industries, the dividing-wall column for quaternary Petlyuk configuration has not reached the same applications due to its complexity in both design and control. In this work, the synthesis on the alternative intensified distillation systems than Petlyuk configuration for four-component distillation was investigated. First, the simple column configuration for the separation sequence of Petlyuk configuration is presented. Then four strategies are introduced to change the structure of the simple column configuration in terms of both condensers/reboilers and column sections. These strategies can be used to synthesize the alternative intensified configurations from the simple column configuration of the separation sequence. The objective of this work is to give the synthesis method to generate the new intensified distillation systems for quaternary Petlyuk configuration. In total, five new alternative intensified systems are obtained each with only two columns. The alternative intensified systems are amenable to be easier in both design and control due to the simplicity in the structures. The synthesis method together with the new generated alternative configurations are presented in this work. The numerical design and optimization, as well as dynamics and control of these new alternative configurations are underway. Keywords: Petlyuk configuration, intensified distillation system, synthesis and design, systematic method, energy saving

1. Introduction Synthesis of new intensified distillation systems with significant reductions on energy and capital costs is ever being an important research topic for both academics and industries. This is because distillation is the widely used separation technology not only for fossil-based, but also for renewable-based products manufacturing. Among the feasible configurations for multicomponent distillation, the Petlyuk configuration (Petlyuk et al., 1965) has been proved to have the minimum energy requirement than other possible configurations. This is due to that the Petlyuk configuration has the lowest thermodynamic irreversibility with the nonsharp splits and thermal couplings. For an N-component mixture, the Petlyuk configuration needs N-1 columns with one condenser and one reboiler in the product column. The implementation of Petlyuk configuration for ternary distillations has achieved remarkable success in the form of dividing-wall column (DWC). A single shell column with one dividing wall (DWC) can perform a ternary separation with three pure products. Such DWC columns have been successfully used in many industrial separations with remarkable savings on both energy and capital costs (30 % up to 50 %).

Jiří Jaromír Klemeš, Petar Sabev Varbanov and Peng Yen Liew (Editors) Proceedings of the 24th European Symposium on Computer Aided Process Engineering – ESCAPE 24 June 15-18, 2014, Budapest, Hungary. Copyright © 2014 Elsevier B.V. All rights reserved.

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Motivated by the ternary Petlyuk column, the Petlyuk configuration has ever been the preferred configuration for four or more component mixtures (Olujic et al., 2012). However, examining the separation sequence of the Petlyuk configuration for four or more component mixtures, it is known that it has the maximum number of individual splits due to the nonsharp splits. Even though the final configuration has only N-1 columns and only one condenser and one reboiler for an N-component mixture, the maximum number of thermal couplings makes the system much more complex for both design and control. When coming to the dividing-wall column for Petlyuk configuration for four or more component mixtures, it is even more difficult for the design and control of a single shell equipment to implement the multiple dividing walls. This is the main reason that the Petlyuk configuration so far is not well implemented in industries for four or more component distillations compared to ternary mixtures. The objective of this work is to study the synthesis method to generate the new intensified distillation systems for quaternary Petlyuk configuration. The alternative intensified systems are aiming at being easier in both design and control in terms of system’s structure. First, the synthesis method is investigated and formulated, then the new generated alternative configurations for quaternary distillation are presented.

2. Petlyuk configuration for quaternary distillation Petlyuk et al. (1965) summarized four features for the fully thermally coupled configuration for an N-component distillation. 1) the total number of sections required for separating an N-component mixture is equal to N(N-1), instead of 2(N-1) in the conventional scheme; 2) it is sufficient to have only one condenser and one reboiler independent of the number of components to be separated; 3) the key components in each column are the two components with the extreme volatilities; 4) N products of a given purity are obtained in the product column. In an earlier work to synthesize the heat-integrated configurations for Petlyuk arrangements, we illustrated that the Petlyuk configuration was generated from the unique nonsharp separation sequence in which all of the mixtures with three or more components are separated by the symmetric sloppy splits (Rong et al., 2006). Such unique separation sequence was called fully sloppy separation sequence. For quaternary mixtures, the fully sloppy sequence is: ABCD→ABC→BCD→A/B→B/C→C/D. The quaternary Petlyuk configuration is presented in Figure 1(a).

(a) (b) Figure 1. (a) Quaternary Petlyuk configuration; (b) Dividing-wall-column of quaternary Petlyuk configuration.

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For an N-component mixture, the total number of thermal couplings in the Petlyuk arrangement is (N-2)(N+1)/2. There are N-2 thermal couplings associated with the submixtures involving the most volatile component, which are located at the top ends of the columns. Similarly, there are N-2 thermal couplings associated with the submixtures involving the least volatile component, which are located at the bottom ends of the columns. There are (N-2)(N-3)/2 thermal couplings associated with the submixtures composed of only the middle components, which are located at the intermediate locations in the columns. The two ends of the product column are connected with the only condenser and the only reboiler. The commonly studied dividing-wall-column for quaternary Petlyuk configuration is presented in Figure 1(b) (Christiansen et al., 1997).

3. A method to derive the intensified alternatives for Petlyuk configuration Starting from the fully sloppy separation sequence with the intended individual splits, it is clear that we need different strategies to deal with the condensers and reboilers, as well as the individual simple columns to achieve a distillation configuration with less number of columns and heat exchangers. To fully explore the possibility to generate the new configurations, we have found that the simple column configuration (SCC) of the sloppy separation sequence is the best representation. This is because that the maximum structural flexibility is kept in the SCC configuration, from which different mechanisms can be explored to change its structure, and from which different intensified distillation systems can be generated. Figure 2 presents the simple column configuration (SCC) for the fully sloppy separation sequence of a quaternary mixture. To synthesize the new intensified distillation systems with fewer columns and heat exchangers, we have introduced four strategies which are aiming at changing the structure of the SCC configuration, at the same time, also aiming at reducing the number of columns and heat exchangers. Figure 2. The simple column configuration (SCC) for the fully sloppy separation sequence of a quaternary mixture.

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Strategy 1: Heat-Integration strategy to combine the individual columns. This strategy is used to combine two columns by heat integration between a condenser and a reboiler involving only intermediate components. This will reduce the number of columns than the SCC configuration. Strategy 2: Thermal coupling strategy to eliminate a condenser or a reboiler. This strategy is used to eliminate a condenser or a reboiler which is associated with a mixture of binary or more components. This will reduce the number of heat exchangers than the SCC configuration. Strategy 3: Rearrangement of column sections strategy to generate thermodynamically equivalent structures. This strategy is used to recombine the column sections in a thermally coupled configuration through movement of the movable column sections. This will generate the thermodynamically equivalent structures which have different columns than the original thermally coupled configuration. Strategy 4: Elimination of the single-section-side columns strategy to produce the intensified distillation systems. For a thermally coupled configuration, there are thermodynamically equivalent structures in which there are single-section-side columns. This strategy is used to eliminate the single-section-side columns which will generate the intensified distillation systems with fewer columns. In the following, for the simple column configuration shown in Figure 2, we will illustrate that systematic use of the four strategies will presents a new method to synthesize all of the possible intensified distillation systems with fewer columns and heat exchangers.

4. Systematic synthesis of the new intensified distillation systems for quaternary Petlyuk configuration The original quaternary Petlyuk configuration shown in Figure 1(a) has three columns, in which five two-way thermal couplings communicate between the columns. It is difficult for practical implementation due to the complexity in design, control and operation. Here, we present the synthesis of new intensified distillation systems which have fewer columns than that of Petlyuk configuration. The synthesis procedure is starting from the SCC configuration shown in Figure 2, then systematically applying the above four strategies to reduce the number of columns and the number of heat exchangers. The key is to generate the thermodynamically equivalent structures in which there are single-section-side columns. A single-section-side column is either serving as transporting an intermediate mixture or as purifying a final product. In certain cases, they can be eliminated to obtain the intensified distillation systems with fewer columns. In total, five distinct new alternative intensified systems are obtained which are shown in parts (a)-(e) of Figure 3. It is seen that each of the new intensified distillation systems has only two columns. Generation of alternative Figure 3(a): Starting from the SCC in Figure 2, we can use four steps to generate the Figure 3(a). Step 1: combining the columns co-producing the middle products B and C through strategy 1. Step 2: eliminating condenser ABC and reboiler BCD through strategy 2. Step 3: removing the movable sections 3 and 6 to

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rearrange the column sections through strategy 3. Step 4: eliminating the single-section-side columns 4 and 5 through strategy 4 to generate the Figure 3(a). Generation of alternative Figure 3(b): Starting from the SCC in Figure 2, we can use four steps to generate the Figure 3(b). Step 1: combining the two columns co-producing the middle product B through strategy 1. Step 2: eliminating condenser ABC, reboiler BCD and reboiler CD through strategy 2. Step 3: removing the movable sections 3, 6and 12 to rearrange the column sections through strategy 3. Step 4: eliminating the single-section-side columns 4, 5 and 11 through strategy 4 to generate the Figure 3(b). Generation of alternative Figure 3(d): Starting from the SCC in Figure 2, we can use four steps to generate the Figure 3(d). Step 1: combining the two columns co-producing the middle product B through strategy 1. Step 2: eliminating condenser ABC, condenser BC and reboiler CD through strategy 2. Step 3: removing the movable sections 3, 12 and 7+8+9+13 togetherto rearrange the column sections through strategy 3. Step 4: eliminating the single-section-side columns 4, 10 and 11 through strategy 4 to generate the Figure 3(d). Similarly, alternatives Figure 3(c) and Figure 3(e) can be generated by applying the above four strategies in a systematic manner. Furthermore, the thermally coupled schemes of the intensified systems can be generated by replacing the condensers and reboilers associated with mixtures of binary or more components. For example, Figure 3(f) presents the thermally coupled scheme of the intensified system of Figure 3(a).

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(d) (e) (f) Figure 3. The intensified distillation systems for quaternary Petlyuk configuration

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5. Comparison of the intensified distillation systems with the Petlyuk configuration The intensified distillation systems shown as in Figure 3 have distinct features than the Petlyuk configuration shown as in Figure 1(a). First, instead of three columns, two columns are used in the intensified systems. Second, to facilitate controllability and operability, each column in an intensified system has employed an overhead condenser and a bottom reboiler. Therefore, there are four heat exchangers in the intensified systems. However, the number of heat exchangers can be further reduced by introducing thermal couplings to replace the condensers or reboiler associated with mixtures. For example, Figure 3(f) is obtained from Figure 3(a) by introducing thermal couplings to replace the condenser AB and reboiler CD. As a consequence, the intensified system can also have only one condenser and one reboiler as in the Petlyuk configuration. Also, in Figure 3, the intermediate submixtures are transporting in one-way between the columns, this will also make the control and operation easier than the two-way communications. Finally, in the intensified distillation systems, the products can be obtained from one column as in parts (a), (d) and (e), or obtained from two columns as in parts (b) and (c) in Figure 3. Furthermore, an extra intermediate product B can be obtained in Figures 3(c) and 3(e), and an extra product C can be obtained in Figures 3(b) and 3(d).

6. Conclusions The Petlyuk configuration is proved to have the minimum energy requirement than other possible configurations for a multicomponent distillation. However, the Petlyuk configuration with the theoretical minimum energy consumption for four or more component mixture is very difficult for practical application due to its complexity. The reason is that the fully thermally coupled Petlyuk configuration has too many thermal couplings between columns, which is difficult for control and operation. On the other hand, its single shell dividing-wall-column needs to implement multiple dividing walls in one column which is even more difficult for equipment design, control and operation. In this paper, the intensified alternative distillation systems for quaternary Petlyuk configuration are presented. The intensified distillation systems use fewer columns than the Petlyuk configuration. Moreover, the fully thermally coupled schemes of the intensified alternatives have less number of two-way thermal couplings between columns than the Petlyuk configuration. These distinct features make the intensified distillation systems not only attractive in terms of economics, but also amenable to systems’ design, control and operation. The method for quaternary Petlyuk configuration presented in this work is applicable to generate the intensified distillation systems for Petlyuk configuration with a feed mixture of any number of components.

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