An approach to optimal design of pressure-swing distillation for separating azeotropic ternary mixtures

An approach to optimal design of pressure-swing distillation for separating azeotropic ternary mixtures

Mario R. Eden, Marianthi Ierapetritou and Gavin P. Towler (Editors) Proceedings of the 13th International Symposium on Process Systems Engineering – P...

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Mario R. Eden, Marianthi Ierapetritou and Gavin P. Towler (Editors) Proceedings of the 13th International Symposium on Process Systems Engineering – PSE 2018 July 1-5, 2018, San Diego, California, USA © 2018 Elsevier B.V. All rights reserved. https://doi.org/10.1016/B978-0-444-64241-7.50057-4

An approach to optimal design of pressure-swing distillation for separating azeotropic ternary mixtures Xu Huanga, Yiqing Luoa, Xigang Yuana,b*, a

School of Chemical Engineering and Technology, Tianjin University, 300072, Tianjin, China b State Key Laboratory of Chemical Engineering, Tianjin University, 300072, Tianjin, China [email protected]

Abstract The separation of ternary mixture with azeotropes by distillation into three high-purity products is difficult due to the existence of distillation boundaries. However, combinations of pressure swing distillation (PSD) configurations and recycle streams can provide alternatives for feasible pathways to the separation of the mixtures. In this work, a two-step approach is proposed to the optimal design of pressure swing distillation system to separate azeotropic ternary mixtures. An example of separating a ternary mixture with azeotropes of C2H5OH/C4H8O-01/C4H8O-02 is used for illustrating and validating the proposed method. Keywords: ternary mixture; binary azeotrope; PSD; optimization;

1. Introduction Several methods in industry are used to separate mixtures with azeotropic, including extractive distillation, azeotropic distillation, pressure swing distillation and relatively new membrane separation and so on (Luyben, 2012 and Soto, M, 2011, Xia et al. 2012). Application of extractive distillation or azeotropic distillation in industry is relatively mature. PSD is first proposed by Lewis (Lewis,1928), however, the pressure swing distillation from then has been mainly applied for separations of binary azeotropes where the composition of azeotrope is sensitive to pressure change, such as 2-butanone (MEK)-cyclohexane. Studying the separation of binary mixture (ethanol-water) separation, Knapp (Knapp,1992) mentioned that when the two components are in different distillation zones and the azeotropes are insensitive to pressure changes, a distillation boundary line, one end of which is sensitive to the pressure change can be formed by addition of a new component. However, such studies have been mainly focused in on binary azeotropic mixtures (Li et al. 2014, Li and Xu, 2017), few workers have addressed PSD for azeotrope containing ternary mixture separations (Yang and Gao, 2010). In this paper, a approach to the conceptual design of PSD for separating a kind of azeotropic ternary mixture is proposed. If binary minimum-boiling azeotrope(s) is/are formed in a ternary mixture, and the composition of a binary azeotrope is sensitive to pressure, a three-column PSD process combined with two recycle streams can be designed by the method proposed in the following sections. The separation process of C2H5OH/C4H8O-01/C4H8O-02 ternary mixture will be used as an example to illustrate the method.

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2. Generation of PSD flowsheet 2.1 Residual curve analysis The residual curve map (RCM) at 101.3 kPa of C2H5OH/C4H8O-01/C4H8O-02 mixture is shown in figure 1. Three binary minimum azeotropes by each two of the components result in two distillation boundary lines. Three pure components to be separated are divided into three different distillation regions (as shown in Figure 1). As shown in figure 2, the composition of the C2H5OH/C4H8O-01 azeotrope is pressure sensitive. And thus, if the feed composition of the ternary mixture falls in Zone 2, it seems that a right pressure could be found so that the feed composition is on the C-D line, as shown in figure 3, and a distillation column, say column T1, can be used to separate the ternary mixture into the product of C4H8O-02 at the bottom of T1 and the azeotrope of the other two components at the top. However, if the position of feed locates in the remaining two zones, no matter how the pressure changes, no a feasible distillation exists to separate the ternary mixture into a pure component and a binary azeotrope mixture, as shown by the D-E and B-F lines. So, our design method is valid only when the feed composition locates in Zone 2.

. Fig. 1 RCM of the ternary mixture at 101.3 kPa

(a) (b) Fig. 2 RCMs of the mixture at different pressures: (a) 600 kPa; (b) 50 kPa

Figure 3 also implies that azeotrope composition at the opposite side of the triangle should be sensitive enough to pressure so that a C-D line can be found for a proper feed composition of the ternary mixture. Inspired by figure 3, we find that the top product can be introduced into a column, let say T2 that operates in a lower pressure so that the bottom product can be pure component B and at the top we can get an azeotrope with richer component A. This azeotrope can then be introduced to a column T3 with higher pressure to produce in the bottom the pure A and the top the azeotrope richer with B, which can be recycled to column T2. As a result, the ternary mixture is separated into three pure components by the three-column distillation system.

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Fig. 3 Possible distillation separations for three kinds of feed compositions

2.2. Synthesis of three-column PSD process Suppose that the feed flowrate is 10 kmol·h-1 with a temperature of 25 °C and a pressure of 101.3 kPa. The composition of feed and the product requirements are given in Table 1. It is found in our calculation that the cost of the distillation by column T1 depends on its pressure, and 600 kPa is found having the lowest cost by calculating its TAC and comparing with other four cases having different pressures of T1 column. The corresponding residue curve combination chart is shown in Figure 4. In this case, C2H5OH product (B2) should be added to the feed so that FEED moves to FEED1 that locates on the B1-D1 line as shown in Figure 4. The process chart of the three-column PSD obtained from Figure 4 is shown in Figure 5. The operating pressures for the three columns are 600 kPa, 50 kPa and 101.3 kPa, respectively. The feed stream needs to be pressurized to 600 kPa before mixed with part of C2H5OH product from T2. Table 1 The composition of raw materials and product specifications Components

Composition /%(mol)

Product specifications /%(mol)

C2H5OH

33

99

C4H8O-01

20

99

C4H8O-02

47

99

In this case, the corresponding distillation boundaries become the blue curves in Figure 4. The mixed feed stream FEED1 passes through T1 column, C4H8O-02 product is obtained at the bottom, i.e. B1 in Figure 4, and the C2H5OH/C4H8O-01 azeotrope (D1) is obtained at the top. After decompression to 50kpa, the stream of composition of D1 enters T2, the distillation boundaries after decompression becomes green line with in Figure 4. And D1 and C2H5OH product are in the same distillation zone, after separation in T2, the C2H5OH product is obtained at the bottom, i.e. B2 in Figure 4; Part of the product recycles back to form the feed to T1, and the rest is recovered as C2H5OH product. The top product of T2 of composition of D2 is introduced into T3 and separated at pressure 101.3 kPa into C2H5OH/C4H8O-01 azeotrope of composition D3 at the top of T3 and C4H8O-01 product at the bottom, and now, the corresponding distillation boundaries are red lines; then the D3 stream is depressurized to 50 kPa and then is recycled to column T2, to form the feed composition of FEED2 to column T2. Finally, the ternary mixture is effectively separated by three-column PSD process.

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Fig. 4 Distillation boundaries at different operating pressures

Fig. 5 PSD configuration for the ternary mixture separation

2.3 Selection of column pressures The position of the original feed in the residue curve is changed by varying pressure of T1column and recycle flow rates of C2H5OH product, as shown in Figure 4 and then column T1 can separate C4H8O-02 product and C2H5OH/C4H8O-01 azeotrope. Figure 6 gives the corresponding distillation boundaries at 600 and 700 kPa respectively. And thus, the corresponding composition of the feed to T1 is FEED1 and FEED1*, respectively. In this way, six different pressures (P1=570, 600, 700, 800, 900 and 1000 kPa) for column T1 are tested to find the best one. It is found that when P1 is 600 kPa, the corresponding temperature of T1’ bottom, 419K, attended the maximum temperature that can be provided by the medium-pressure steam used as the hot utility. To use less expensive cooling water as cold utility, P2 (pressure of T2) takes 50 kPa; the corresponding temperature of C2H5OH/C4H8O-01 azeotrope is 326 K. P3 (pressure of T3) is 101.3 kPa, meeting the conditions that the difference between azeotropic compositions at the top of T2 and T3 is greater than 5mol%.

3. Optimization results and discussion In this paper, the total annual cost (TAC) as a criterion is estimated using the method suggested by Douglas (Douglas, 1988). In this section, column T1 at 600 kPa is optimized as an example to illustrate the optimization. 3.1 Optimization of the flow rate of RECYC2 Figure 7 gives the changes in purity of products at the top and bottom of column T1 with the recycle stream. It shows that when the flowrate of RECYC2 is 2.2 kmol•h-1, the purity of C4H8O-02 product at the bottom of T1 reaches the maximum value.

Design of pressure-swing distillation for separating azeotropic ternary mixtures

Fig. 6 RCMs of C2H5OH/C4H8O-01/C4H8O02 at 600kpa and 700kpa

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Fig. 7 The dependence of the T1 column product purities on the flow rate of RECYC2.

3.2 Optimization of T1 column Figure 8 shows the relationship between the number of stages (NS1) in T1 and the reflux ratio (RR1). The value corresponding to where the number of column plates varies most significantly with the slope of reflux ratio curve is selected as the optimum number of theoretical stages, and the corresponding reflux ratio is taken as the optimum. As shown in Figure 8, NS1 is 97 and the corresponding RR1 is 28.6. As shown in Figure 9, the feed stage position (FS1) of T1 is 45.

Fig. 8 Relationship of NS1and RR1 in T1column

Fig. 9 The dependence of the reboiler heat duty on feed stage of T1 column

3.3 Optimization of columns T2 and T3 A method of sequential iterative optimization for T2 and T3 column is adopted in this paper. This method is widely used in many studies of optimization of PSD. Results of five case studies at different pressures of T1 are collected in Table 2. The heat duty of T1 is much greater than the sum of the heat duties of the other two columns, therefore, the difficulty in separating the mixture lies in the separation of C4H8O-02. It is found that when P1 is 600 kPa, the total annual cost (TAC) is 1025.2×103/$·y-1, the lowest cost value, 6.27% lower than that when P1 is 800 kPa. Table 2 Result of case studies at different pressures of column T1 Variables

Case1

Case2

Case3

Case4

Case5

P1/kPa

600

700

800

900

1000

NS1/ NS2/ NS3

97/45/50

102/43/50

95/55/67

104/54/60

95/55/55

FS1 FSD1/FSRECYC1 FS3

45 19/13 25

79 30/27 39

71 24/19 26

80 22/15 24

70 32/23 35

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RR1/ RR2/ RR3

28.6/1/1.5

25.6/1.7/1.5

21.2/1.2/2

18.7/1.2/1.5

15.5/1.2/1.5

FRECYC1/kmolg gh-1

2.2

3.3

4.5

6.0

8.8

QREB1/kW

2002.1

2052.5

1944.6

1989. 8

2089.7

QREB2+QREB3/kW

650.9

641.2

660.2

629.4

576.0

TACh103/$gy-1

1025.2

1094.0

1089.5

1102.7

1104.4

4. Conclusions Using the residue curves under different pressures, a method to synthesis a PSD system separating a kind of ternary mixtures with azeotropes is proposed. Separation of mixture C2H5OH/C4H8O-01/C4H8O-02 is used to illustrate the method. If the feed composition falls in a distillation area containing C4H8O-02 product, separation of three components can be achieved. A three-column PSD process with two recycle streams has been proposed to have achieved effective separation of the mixture. Furthermore, the operating pressure of T1 not only determines the economy of the column itself, but also determines the flowrate of the RECYC2, thereby affecting the economy of the overall process. By choosing five cases with different pressures of T1, this paper has studied the influence of P1 on the economy, with the optimized calculation results shown in Table 2.

Acknowledgements Support by National Natural Science Foundation of China (Grant 91434204) is acknowledged.

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