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Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns
Accepted Manuscript Title: Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns Author: Elena Anokhina Andrey Timoshenko PII: DOI: Reference:
S0263-8762(15)00069-6 http://dx.doi.org/doi:10.1016/j.cherd.2015.03.006 CHERD 1804
To appear in: Received date: Revised date: Accepted date:
20-10-2014 3-3-2015 5-3-2015
Please cite this article as: Anokhina, E., Timoshenko, A.,Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Research Highlights
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We studied seven different azeotropic mixture separations by extractive distillation. Partially thermally coupled columns in extractive distillation were investigated. Energy efficiency compared to conventional flowsheets has been identified. The evaluation criterion was proposed of the complex column effectiveness in extractive distillation.
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*Manuscript
Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns Elena Anokhina, Andrey Timoshenko* [email protected]
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Lomonosov Moscow University of Fine Chemical Technology, Moscow, Russia
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Abstract The aim of this work is the statement of the reasons for different energy effectiveness of extractive distillation systems with the partially coupled heat and material flows (PCEDS) for various mixtures and entrainers. The estimation of the effectiveness was performed for PCEDS application for the separation of seven binary mixtures with different initial compositions and various entrainers. The criterion of the summary power consumption in the reboilers of the columns under the optimal operating parameters was used. It has been concluded on the basis of the obtained results that PCEDS effectiveness depends on the value of the reflux ratio in the entrainer recovery column of the conventional extractive distillation flowsheet. If the reflux ratio in that apparatus has a small value, then the energy consumption decrease due to the use of PCEDS will be insignificant. The criterion for the evaluation of the PCEDS energy efficiency for binary mixtures separation has been formulated as follows: PCEDS is useful if the reflux ratio at the entrainer recovery column has a value equal to or greater than 1.
Introduction
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Keywords Extractive distillation, partially thermally coupled columns, energy saving
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Distillation is the most widely used method to separate different industrial mixtures into pure components. This method unfortunately is very energy intensive and requires a lot of heat. The problem of distillation energy consumption reduction may be solved by the use of contemporary mass-exchange devices and by using a thermodynamically reversible process (Olujic et al., 2009). The implementation of this approach was first proposed by Wright (1949) and then developed in the theoretical and practical aspects by Petlyuk. The Petlyuk fully thermally coupled (FTC) arrangement from the thermodynamic point of view is characterized by minimum entropy production. This does not indicate the high energy efficiency. However, it is high enough. For example, it is proved that the minimum energy consumption for an ideal multicomponent mixture is always obtained for the fully thermally coupled configuration (Halvorsen et al., 2003). A considerable number of studies have been devoted to the investigation of possible alternative configurations of the Petlyuk column (Agrawal et al., 1998, 1999, 2000; Caballero et al., 2003; Hernandez et al., 2006; Rong et al., 2006a, 2006b). The realization of Petlyuk’s ideas actively began in 1985 when BASF built the first industrial dividing wall column (DWC). Now there are 1
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more than 100 DWCs around the world, which enable savings of both operating and investment costs (Kaibel et al., 1987, 2004; Kolbe et al. 2004). Another way to increase the thermodynamic efficiency of distillation is to use partially thermally coupled (PTC) systems. Common approaches to the synthesis of technological schemes of this class have been proposed by Timoshenko et al. (2001). Several authors have developed other approaches to the synthesis of distillation flowsheets with partially thermally coupled flows (Rong et al. 2003, 2004; Errico et al., 2009; Caballero et al., 2014). Distillation PTC systems may also be designed as dividing wall columns. The result of intensive studies and the beginning of the industrial applications of the FTC and PTC to separate non-azeotropic mixtures led to the development of research for extractive distillation. Extractive distillation (ED) is used in the industry for the separation of mixtures with similar relative volatilities and azeotropes, for example, the separation of hydrocarbons with close boiling points (Liao et al., 2001; Lei et al., 2002; Wentink et al., 2007), the recovery of aromas or fragrances (Berg, 1983; Pollien et al., 1998; Chaintreau, 2001; Chen et al., 2003; Ghaee et al., 2008), the separation of aqueous alcohol solutions (Meirelles et al., 1992; Pinto et al., 2000; Zhigang et al., 2002; Arifin and Chien, 2008; Li G. and Bai P, 2012), the separation of ether and alcohol mixtures (Muñoz et al., 2006; Wang et al., 2010), and the separation of methylal and methanol mixtures (Wang et al., 2012). The conventional extractive distillation (CED) flowsheet for the separation of binary mixtures (Figure 1a) includes an extractive column (EC) and an entrainer recovery column (RC). Mixture A–B and entrainer E are fed into the extractive column. The presence of the entrainer alters the relative volatility between components A and B, causing A to move to the top and B to move toward the bottom of the EC. The extractive column bottom’s stream is fed into the recovery column to produce almost pure B component as the distillate and almost pure entrainer as the bottom product. Despite the fact that extractive distillation is considered one of the most efficient methods of azeotropic mixture separation, it is characterized, like conventional distillation by high specific energy consumption. Thus, power consumption reduction of ED is an urgent task of chemical technology.
Figure 1. Conventional ED flowsheet (a) and ED systems with partially coupled heat and material flows (b), A,B – components of the initial feed, E– entrainer, MC – main column, SS – side section, EC – extractive distillation column, RC – entrainer recovery column, 1–5 – column’s sections Recently, ED systems with partially coupled heat and material flows (PCEDS) have been applied for saving energy (Timoshenko et al., 2003; Hernández, 2008; Gutiérrez-Guerra et al., 2009; Wang et al., 2010). PCEDS is a single complex column that consists of a main column (MC) and a refining side section (SS) for the cases of binary mixture separation by ED with a heavy boiling entrainer (Figure 1b). In Figure
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1, heat exchangers, such as condensers, reboilers and refrigerators for the entrainer, are not shown. ED systems with partially coupled heat and material flows may be arranged as dividing wall column (Kiss et al., 2012; Xia et al., 2012). The power consumption reduction by PCEDS differs considerably for the various mixtures up to ~30% (Timoshenko et al., 2003) and as little as ~ 4.7% (Wang et al., 2010). Until now, it was unclear why in some cases the application of PCEDS significantly reduced the power consumption and in other cases it had almost no effect. Therefore, we conducted a systematic study to investigate the energy consumption reduction dependence by PCEDS on various factors of the process.
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Design of ED systems with partially coupled heat and material flows Previously, we developed an algorithm for the synthesis of PCEDS flowsheets (Timoshenko et al., 2005; Timoshenko et al., 2007). The flowsheet in Figure 1a can be represented in accordance with this algorithm as a graph of Figure 2a. Vertices in the graph correspond to the intersections separating sections of the columns and edges to the flows of vapour and liquid within the column and the streams linking the columns. The merger of the two vertices connected by oriented edge BE of the graph (Figure 2a) gives a graph (Figure 2b) that corresponds to a complex column with a side rectifier (Figure 1b). Note that the algorithm can be applied to the synthesis of PCED’s flowsheets for multicomponent azeotropic mixture separation.
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Figure 2. Graphs of flowsheets (see Figure 1) preimage (a) and images (b, c) at the ED with hard boiling entrainer; — feed (input), — output (exit) with removing heat, — output (exit) with heat input, — vertex without properties of input or output PCEDS for binary mixtures separation was obtained by Gutiérrez-Guerra et al., 2009, by similar transformation but without the use of graph theory. Problem statement The aim of this work is to present the reasons for different degrees of energy effectiveness of PCEDS for various mixture separations with different entrainers. As the energy efficiency criterion ( DQ ), the relative reduction of energy consumption in PCEDS the reboiler of PCEDS ( Qreb ) in comparison with the CED reboilers total energy CED PCEDS Qreb Qreb 100% . CED Qreb We estimated the DQ of the PCEDS application for the separation of seven binary mixtures with various entrainers. Extractive distillation systems were selected for different azeotropic mixtures with various entrainers as the objects of study: dimethyl
CED consumption ( Qreb ) was used: DQ
3
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formamide (DMFA), water, ethylene glycol (EG), butyl propionate (BP), dimethyl sulfoxide (DMSO), and propylene glycol (PG) (Table 1). Table 1. Some characteristics of the separated azeotropic mixtures at 101.3 kPa pressure.
V VI VII
Methyl acetate (1) – methanol (2) Ethyl acetate (1) – ethanol (2) Methyl acetate (1) – chloroform (2)
54.0 min 71.8 min 67.7 max
81.3 69.0 25.58
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Acetone
Methanol
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IV
DMFA, water EG
Allyl acetate Allyl alcohol
BP
DMFA EG
DMSO PG DMFA, EG, DMSO
Distillate of RC or SS
Acetone
Isobutyl alcohol Isobutyl acetate Methyl acetate Ethyl acetate
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III
DMFA
Distillate of EC or MC
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II
Entrainer
M
I
tAZ, C; x1AZ , min/max mass % boiling point Az. Acetone (1) – 64.5 22.0 Chloroform (2) max Acetone (1) – 55.6 86.3 methanol (2) min Allyl alcohol (1) – 95.1 62.9 allyl acetate (2) min Isobutyl alcohol (1) 107.4 78.3 – isobutyl acetate min (2) Binary mixtures
Methyl acetate
Isobutyl acetate Isobutyl alcohol Methanol Ethanol
Chloroform
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No mixture
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For mixtures IV, V, and VI, the quantities and compositions of the feed, as well as the product qualities, were set according to the data of Muñoz R., et al. 2006, Heinz E., 1994, Wang S.-J., et al. 2010, respectively. In all cases, the initial feed had a boiling point temperature. The characteristics of the initial feed and product flow compositions are fixed by calculations and are presented in Table 2. Aspen HYSYS was used to simulate the ED. The vapour-liquid equilibrium of all investigated mixtures except mixture IV were calculated using the nonrandom two-liquid (NRTL) equation with the parameters presented in Table 3. The UNIQUAC activity model was used for the vapour-liquid equilibrium calculation of mixture IV with parameters published by Muñoz R., et al. 2006 (see Table 4). In all cases, the vapour phase was assumed to be ideal. Results and discussion Optimization of the conventional ED flowsheet We used the criterion of the summary power consumption in the reboilers of the CED columns ( Qreb ) to identify the optimal operating parameters. The design variables to be optimized in the conventional ED flowsheets include the total number of theoretical plates (Ntotal) (except the ED of mixture IV with BP), the entrainer temperature (tE), the entrainer-to-feed ratio (E:F), and the entrainer and feed tray locations (NE and NF). The operating variables include the reflux ratios and reboiler duties of both columns. The reflux ratios and reboiler duties were varied to satisfy the distillate and bottom product specifications, respectively.
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Table 2. Concentrations of the initial feed and products flows and VLE models
I with DMFA
100
x1F , Pressu Entrain Distilla Distilla Bottom Type of VLE mass %
22.0
re, kPa
101.3
er feed, XE mass % 99.90
te of te of product model, EC or RC or of RC or parameters MC, XE references MC, SS, mass mass mass % % % NRTL, 99.50 99.90 99.90
86.3
101.3
99.90
99.50 99.50
100
101.3
99.90
99.50 99.50
99.90
100
86.3 20.0 20.0
101.3
99.90
99.50 99.50
99.90
IV with BP
1500
41.0
101.3
99.60
97.60 99.60
99.60
IV with DMFA
1500
41.0
20
99.60
V with EG
12200 77.9
VI with DMSO 11279 77.8 11279 77.8
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99.60 97.60
99.60
99.90
99.50 99.99
99.90
30
99.90
99.50 99.99
99.90
101.3
99.90
99.50 99.99
99.90
101.3
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VI with PG
99.90
M
II with water Case 1 Case 2 III with EG
VII with DMFA 1000 25.58
101.3
99.90
99.50 99.90
99.90
VII with EG
1000 25.58
101.3
99.90
99.50 99.90
99.90
1000 25.58
25
99.90
99.50 99.90
99.90
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VII with DMSO
Anokhina E.A. et al., 2008 NRTL, Anokhina E.A. et al., 2009 NRTL, Anokhina E.A. et al., 2011
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100
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II with DMFA
Feed, kg/h
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Mixture
NRTL, Anokhina E.A., et al., 2006 UNIQUAC, Muñoz R., et al. 2006 UNIQUAC, Muñoz R., et al. 2006 NRTL, Rudakov D.G., et al, 2013 NRTL, Rudakov D.G., et al, 2013 NRTL, Rudakov D.G., et al, 2013 NRTL, Anokhina E.A. et al., 2013 NRTL, Anokhina E.A. et al., 2013 NRTL, Anokhina E.A. et al., 2013
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Table 3. NRTL binary interaction parameters Component I
Component J
binary interaction parameters, K A(I,J)
A(J,I)
B(I,J)
B(J,I)
α
Chloroform
0.000
0.000
-194,170
-10,010
0.300
Acetone
DMFA
0.000
0.000
55.743
20.700
1.696
Chloroform
DMFA
0.000
0.000
-172.370 -148.630
0.300
Acetone
Methanol
0.000
0.000
92.945
112.039
0.308
Acetone
Water
0.000
0.000
317.554
602.558
0.534
Methanol
Water
0.736
0.511
-360.692
199.854
0.244
Methanol
DMFA
0.000
0.000
-380.050
292.370
0.300
Allyl alcohol
Allyl acetate
0.000
0.000
294.100
-77,200
0.370
Allyl alcohol
EG
0.000
0.000
169.580
95,950
0.300
Allyl acetate
EG
0.000
0.000
415.250
541,660
0.200
Methyl acetate
Methanol
-0.282
-0.359
299.176
251.233
0.117
Methyl acetate
EG
0.000
Methanol
EG
0.000
Ethyl acetate
Ethanol
0.000
Ethyl acetate
DMSO
0.000
Ethanol
DMSO
Ethyl acetate
PG
Ethanol
PG
Methyl acetate
0.000
43.937
265.466
0.321
0.000
-54.958
24.654
0.298
0.000
154.208
162.349
0.299
0.000
285.156
120.035
0.297
0.000
0.000
150.976
-409.379
0.300
0.000
0.000
896.337
-107.416
0.202
0.000
0.000
217.896
-41.049
1.042
Chloroform
0.000
0.000
-334.148
163.414
0.305
Methyl acetate
DMFA
0.000
0.000
152.902
-88.479
0.800
Chloroform
EG
0.000
0.000
-68.977
92.884
0.300
Methyl acetate
DMSO
0.000
0.000
276.363
51.969
0.309
Chloroform
DMSO
0.000
0.000
825.439
-312.080
1.114
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Acetone
Table 4. UNIQUAC binary interaction parameters according to the data of Muñoz R., et al. 2006 Component I
Component J
binary interaction parameters, kcal/kmol A(I,J)
A(J,I)
B(I,J)
B(J,I)
Isobutyl alcohol
Isobutyl acetate
116.170
361.900
-0.530
-0.450
Isobutyl alcohol
BP
35.346
49.224
0.000
0.000
Isobutyl acetate
BP
131.014
-128.309
0.000
0.000
Isobutyl alcohol
DMFA
56.603
-155.920
0.000
0.000
Isobutyl acetate
DMFA
551.320
-281.210
0.000
0.000
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At the first stage, we determined the optimal value of the total number of theoretical plates for the extractive column and entrainer recovery column at a fixed ratio E:F and a fixed entrainer temperature. The energy consumption in the column reboilers (Qreb.) was the selection criterion. The optimal initial feed tray locations were identified for each column with assigned Ntotal. For extractive distillation columns, the optimal entrainer feed plates were also determined. Figure 3 shows the dependence of the energy consumption at the reboilers of EC and RC on the number of plates for the ED of a methyl acetate – chloroform mixture with EG as the entrainer. The energy consumption in the reboiler of the extractive column increased slightly (< 1%) as the number of plates decreased from 38 to 35. With a further reduction in the number of plates, the Qreb increase became more visible. The change is 9.4% when reducing the number of plates from 35 to 31. Therefore, during all further calculations, we took the number of theoretical plates equal to 35 for the extractive column for the methyl acetate – chloroform mixture separation with EG as the entrainer. Similarly, the energy consumption in the reboiler of the entrainer recovery column increased slightly (< 1%) as the number of plates decreased from 20 to 12. With a further reduction in the number of plates, the increase in Qreb becomes more apparent. It increases by 2.2% from 12 to 10 plates and by 31% from 10 to 7 plates. Therefore, during all further calculations, we set the number of theoretical plates in the RC for this case equal to 12.
Figure 3. The dependence of the energy consumption in the reboilers of the extractive column (a) and recovery column (b) on the number of plates for the ED of the methyl acetate – chloroform mixture with EG. E: F = 5:1, tE= 60 C We performed a similar analysis for the EC and RC of all other investigated mixture extractive distillations, except mixture IV with BP. The number of plates in this case was set in accordance with the work of Muñoz R. et al., 2006. The number of theoretical plates in the columns is shown in Table 6. These values can be considered optimal only to a certain extent because they are found at a fixed
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temperature and entrainer flowrate, excluding the price of the cooling water and without entrainer flow heat recovery. At the second stage, we investigated the dependence of the energy consumption in the reboiler of the EC on the entrainer temperature at a fixed ratio E:F by the previously identified total number of theoretical plates, as described above. The optimal initial feed and entrainer feed tray locations were determined at each temperature. Figure 4 shows the dependence of the energy consumption in the reboiler and the reflux ratio of the extractive column on the EG temperature for the extractive distillation of the methyl acetate – chloroform mixture. The energy consumption in the reboiler of the EC decreases significantly (~ 16%) upon increasing the EG temperature from 60 to 80 C. The reflux ratio in this case increases slightly from 0.1 to 0.3. The Qreb decrease becomes less apparent with a further EG temperature increase: an EG temperature increase from 80 to 90 C decreases Qreb by 6.0%, an increase from 90 to 100 C decreases Qreb by 3.0%, and an increase from 100 to 110 C decreases Qreb by 1.4%. The reflux ratio increases substantially at temperatures above 90°C: to 1.8 at 100°C and to 2.9 at 110°C. Thus, an entrainer feed temperature of 90С was used. In this case, we do not take into account the amount of heat that can be achieved by entrainer cooling. Considering this factor, the temperature may vary. Similar analyses were performed for the ED of all other investigated mixtures. The values of tE found as a result of this analysis are presented in Table 6.
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Figure 4. The dependence of the EC energy consumption in the reboiler (1) and reflux ratio (2) on the EG temperature for the extractive distillation of the methyl acetate – chloroform mixture under E:F=5:1. At the third stage, we investigated the dependence of the energy consumption in the reboiler of the EC on the entrainer flowrate for several values of NE/NF. Figure 5 shows the dependence of the energy consumption in the reboiler of the extractive column on the EG flowrate by the extractive distillation of the methyl acetate – chloroform mixture for NE/NF= 4/14.
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Figure 5. The dependence of the energy consumption at the reboiler of the extractive column on the EG flowrate for the ED of the methyl acetate – chloroform mixture for NE/NF= 4/14 and EG temperature 90 °С
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The energy consumption in the reboiler of the ED column has a minimum at an EG flowrate of 4600 kg/h. The methyl acetate concentration is lower than 99.5% in the extractive column distillate at an EG flowrate less than 4250 kg/h. Thus, 4250 kg/h is the minimal value of the entrainer flowrate at which the given product quality is possible to obtain in the EC distillate. The values of the optimal and minimal entrainer flowrate for other NE / NF are presented in Table 5.
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Table 5. Optimal and minimal EG flowrates for the ED of the methyl acetate – chloroform mixture for several NE/NF at an EG temperature of 90 0С Qreb, kW entrainer flowrate, kg/h NE/NF optimal minimal optimal minimal flowrate flowrate 3\14 4650 4250 328.5 356.4 3\15 4750 4350 329.1 355.9 3\16 4900 4450 333.8 379.7 4\14 4600 4250 329.5 360.0 4\15 4750 4350 330.1 357.8 4\16 4850 4500 334.6 358.2 5\14 4600 4300 334.0 352.9 5\15 4700 4350 334.0 365.7 5\16 4850 4600 338.1 362.3
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At the fourth stage, we investigated the dependence of the entrainer recovery column feed tray location on the entrainer flowrate at certain intervals of values. The minimum value was chosen for all values of "minimal entrainer flowrate", and we took it as a lower variation interval boundary. For example, the value is 4250 kg/h in the case of Table 5. The maximum value was chosen among all of the values of the "optimal entrainer flowrate ". For example, the value is 4900 kg/h in the case of Table 5. A value somewhat larger than the maximum value of the "optimal entrainer flowrate” was established as the upper interval of the variation boundary. Furthermore, calculations of the flowsheets were performed, and the optimal feed tray locations in the recovery columns were determined for several values of EG flowrate from the performed interval. For example, the entrainer flowrate variation interval was 4250–5000 kg/h in the case of the methyl acetate – chloroform mixture extractive distillation with EG. Similar studies were conducted for the extractive distillation flowsheets of mixtures I-VII.
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The calculations showed that the optimum feed tray locations in the entrainer recovery columns at selected entrainer flowrate intervals are practically unchanged. This is due to two facts. The first is that in the RC’s feed, the concentration of an entrainer is significantly greater than the concentration of the initial binary mixture component. Thus, the concentration ratio varies only slightly with the varying flowrate of the entrainer. From another point of view, this may be because the entrainer flowrate changes in relatively narrow interval and does not significantly affect the concentration ratio in the feed of the RC. In the case of the ED of the methyl acetate – chloroform mixture with EG, the optimal feed plate number of the entrainer recovery column is 5. The optimization procedure of the conventional ED flowsheet is summarized below: 1) Guess the entrainer flowrate. 2) Guess the optimal feed tray location in recovery column, as previously found for the given flow rate of entrainer, as described above. 3) Guess the entrainer feed tray location in the extractive column (NE). 4) Guess the initial feed tray location in the extractive column ( NEC F ). 5) Change the reboiler duty and reflux ratio in the extractive column until the product specifications are achieved. EC 6) Go back to step 4, guess a new value of NEC F , repeat step 5. Change NF until CED is minimized. Qreb 7) Go back to step 3, guess a new value of NE, repeat steps 4–6. Change NE until CED is minimized. Qreb 8) Go back to step 1, guess a new value of the entrainer flowrate, repeat steps 2–7. CED Change the entrainer flowrate until Qreb is minimized. The results are presented for conventional ED flowsheet optimization for all investigated mixtures in Table 6.
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Optimization of ED systems with partially coupled heat and material flows The design variables to be optimized in the PCEDS include the entrainer temperature (tE), entrainer-to-feed ratio (E:F), the entrainer, the feed and the side section outlet tray locations (NE, NF, NS), and the value of flowrate directed to the side section (FS). The operating variables include the reflux ratios of the main column (RMC) and side section (RSS) and the reboiler duty of the main column. The reflux ratios are manipulated to satisfy the main column and side section distillate product specifications. The reboiler duty is manipulated to satisfy the specification of entrainer purity. The total number of theoretical plates for the PSEDS is set to the same value as for the conventional ED flowsheet. As mentioned above, the synthesis algorithm of the PCEDS’s number of plates of the main column (NMC) is obtained by summing the number of plates of the extractive column of the conventional ED flowsheet and the stripper section of the recovery column under the optimal design. The number of plates of the side section (NSS) is equal to the number of plates of the rectifier section of the conventional ED flowsheet recovery column under the optimal design. At the first stage, we investigated the dependence of the energy consumption at the reboiler of the main column on the entrainer temperature at a fixed ratio E:F and the value of the flowrate directed to the side section. The optimal initial mixture and entrainer feed tray locations were determined for each value of temperature. Figure 6 shows the dependence of the reboiler energy consumption and reflux ratio of the
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main column on the EG temperature in the case of the methyl acetate - chloroform mixture ED.
RC EC RC EC RC EC RC EC RC EC RC EC RC
VI with DMSO VI with PG VII with DMFA VII with EG
Ac
VII with DMSO
22 22 35 10 35 14 35 14 24 9 50 30 60
4 /10 –/6 4/18 –/5 17/27 –/9 12/23 –/9 5/15 –/5 10/35 –/15 14/46
3.50:1 – 4.90:1 – 1.20:1 – 1:1 – 3.60:1 – 1.87:1 – 1.76:1
2.20 3.60 3.00 2.80 3.30 6.10 11.2 2.00 1.40 1.10 5.09 4.07 6.50
28.10 27.10 66.10 19.70 56.00 30.70 36.06 76.50 38.20 16.40 651.4 399.3 639.6
20 50 10 30 10 50 10 30 15 35 12 22 9
–/7 3/33 –/6 4/17 –/6 4/33 –/6 4/12 –/5 3/14 –/5 3/11 –/4
60 – 70 – 70 – 70 – 100 – 120 – 101.0 6 – 40 – 40 – 40 – 80 – 90 – 30 –
– 2.87:1 – 0.78:1 – 2.97:1 – 3.45:1 – 4.50:1 – 0.9:1 –
4.40 1.04 0.18 0.66 0.19 0.95 0.40 2.60 5.00 2.00 0.80 1.04 0.12
586.6 4763 2751 1925 957.6 4487 2233 246.8 327.1 330.0 238.6 89.80 92.70
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V with EG
Reflux ratio (R)
Qreb, kW
CED , Qreb
kW
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EC RC EC RC EC RC EC RC EC RC EC RC EC
IV with DMFA
Entrain er-tofeed ratio (E:F)
cr
I with DMFA
IV with BP
Entrai ner tempe rature tE, C
us
Ntotal
M
Extractive (EC) or recovery (RC) column
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Mixture and entrainer
II with DMFA II with water Case 1 II with water Case 2 III with EG
Entrainer / feed trays location, NE/NF
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Table 6. Energy consumption of classical ED flowsheets for different mixture separations under the optimal parameters
55.20 85.80 86.60 113.1 54.6 1050 1226 7513 2882 6719 573.9 568.6 182.5
The relationships in Figure 6 are similar to the relationships in Figure 4. The energy consumption decreases in the boiler of the PCEDS‘s main column significantly (~ 9.7%) with increasing EG temperature from 60 to 80 C. The reflux ratio in this case increases slightly from 0.1 to 0.2. The Qreb decrease becomes less visible with a further increase of the EG temperature; when the EG temperature increases from 80 to 90 C, it decreases by 3.0%, when the temperature increases from 90 to 100 C, it decreases by 2.9%, and when the temperature increases from 100 to 120 C, it decreases by 1.0%. The reflux ratio significantly increases at EG temperatures of more than 90 °C: to 1.7 at 100 °C and to 3.9 at 120 °C. The optimal EG feed temperature of the PCEDS’s main column was 90°C, for the same reasons as for the conventional ED flowsheet. Similar analyses were
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performed for the other investigated mixtures by PCEDS separation. The values of tE found as a result of this analysis are presented in Table 7. In all cases, the relationships of the reboiler duty and the reflux ratio of the main PSEDS’s column on the entrainer temperature were similar to the relationships of the reboiler duty and the reflux ratio of the conventional extractive column.
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Figure 6. The dependence of the energy consumption at the reboiler (1) and reflux ratio (2) of MC on the EG temperature in the case of the methyl acetate - chloroform mixture ED under E:F=4.5:1 and FS=1000 kg/h.
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The optimization procedure of PCEDS is summarized below: 1) Guess the entrainer flowrate. 2) Guess the value of the flowrate directed to the side section (FS). 3) Guess the entrainer feed tray location in the main column (NE). 4) Guess the initial feed tray location in the main column (NF). 5) Guess the tray location of the outlet to the side section (NS). 6) Change the reboiler duty and reflux ratios in the extractive column and side section until the product specifications are achieved. PCEDS 7) Go back to step 5, guess a new value of NS, repeat step 6. Change NS until Qreb is minimized. 8) Go back to step 4, guess a new value of NF, repeat steps 5–7. Change NF until PCEDS Qreb is minimized. 9) Go back to step 3, guess a new value of NE, repeat steps 4–8. Change NE until PCEDS Qreb is minimized. 10) Go back to step 2, guess a new value of FS, repeat steps 3–9. Change FS until PCEDS Qreb is minimized. 11) Go back to step 1, guess a new value of the entrainer flowrate, repeat steps 2– PCEDS 10. Change the entrainer flowrate until Qreb is minimized. The results are presented for PCEDS’s optimization for all investigated mixtures in Table 7.
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Table 7. Energy consumption of PCEDSs under the optimal parameters Mixture
Main col. num ber of trays NMC
Side Entrainer/ Entrainer Entraine Flow Reflu Reflux Q PCEDS , reb col. feed /side temperatu r-to-feed to x ratio ratio kW numb section ratio side main side re, tE, C er of outlet trays (E:F) secti col. col. MC trays location on, R RSS SS N NE/NF/NS FS, kg/h
34
10
4 /11/24
60
3.50:1
120
1.90 1.20
II with DMFA II with water Case 1 II with water Case 2 III with EG
40 40
5 9
4/18/36 18/26/35
70 70
4.80:1 1.50:1
19 33
3.00 0.30 3.10 2.40
72.60 69.60
40
9
12/22/34
70
1.20:1
175
10.6 1.60
100.4
28
5
5/15/25
100
3.50:1
23
1.40 0.30
50.80
IV with BP
65
15
10/35/52
120
1.90:1 2500 4.68 1.86
839.4
IV with DMFA
73
7
15/43/58
101.06
1.97:1 1400 6.50 1.00
860.5
V with EG
54
6
3/33/51
40
VI with DMSO 34
6
4/16/30
40
VI with PG VII with DMFA VII with EG VII with DMSO
54 40
6 5
4/33/51 4/11/29
40 80
42 27
5 4
3/15/38 3/11/23
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2.91:1 2700 0.95 0.04 7407.3
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90 30
40.90
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I with DMFA
0.82:1 2530 0.64 0.05
2812
2.93:1 2640 0.96 0.11 3.67:1 1150 2.67 1.18
6541 389.6
4.18:1 0.80:1
501.9 180.7
770 750
1.62 0.22 1.30 0.06
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We conducted a comparison of the total energy consumption at the reboilers of the conventional ED flowsheets and the PCEDS flowsheets for separation of these mixtures. Table 8 and Figure 7 show the data of the values of the energy consumption reduction by applying PCEDS systems compared to classical ED flowsheets. The difference in the energy effectiveness can be due to the different types of azeotropes, different entrainers, different processes at the PCEDS and CED extractive sections and above it and some effects at other sections of flowsheets. Mixtures I and VII have azeotropes with a high boiling point, and the others have azeotropes with a minimum boiling point. An explicit dependence on the type of the azeotrope is not observed. There is some dependence on the type of entrainer, e.g., PCEDSs with DMF in all cases have relatively high efficiency, but the relationship is not precise. The efficiency criterion DQ values are different; for cases with DMF, they are nearly double (16.0–32.1%), and for cases with EG, they are more than 8 times (1.4–11.7%).
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Table 8. Reduction of the energy consumption by using PCEDSs Recovery column reflux ratio (RRC) Mixture 3.60 I with DMFA 2.80 II with DMFA
DQ, % 25.9 16.0
6.10 2.00 1.10
19.6 11.2 7.10
IV with BP
4.07
20.1
IV with DMFA
4.40
V with EG
0.18
VI with DMSO
0.19
VI with PG VII with DMFA VII with EG VII with DMSO
0.40
1.40
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2.40 2.70 32.1 11.7 1.00
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II with water, Case 1 II with water, Case 2 III with EG
Figure 7. Reflux ratio in the entrainer recovery columns (RRC) and reduction of the energy consumption by using PCEDS for different mixture separations
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We analysed the profiles of the temperature and component concentrations in the two columns of the conventional ED flowsheet and the main column and side rectifier column of the PCEDS. Figures 8 and 9 are examples of the profiles of the temperature and component concentrations at the columns of the ED methyl acetate - chloroform mixture with EG.
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Figure 8. Temperature profiles of the extractive column (a) and recovery column (c) of the conventional ED flowsheet and the main column (b) and side section (d) of the PCEDS for the ED methyl acetate – chloroform mixture with EG under optimal parameters.
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Figure 9. Liquid composition profiles for ED methyl acetate – chloroform mixture separation with EG under optimal parameters. Extractive column (a) and recovery column (c) of the conventional ED flowsheet and main column (b) and side section (d) of the PCEDS. The temperature and the concentration profiles are identical in sections 1-3 of the conventional extractive distillation column and the main column of the PCEDS (Figures 1, 8 and 9). The temperature and concentration profiles for sections 4 and 5 are different for the conventional extractive distillation and PCEDS. The process of the heavy boiling component and entrainer separation mainly exists in these parts of the scheme. It is characterized by the reflux ratios in the entrainer recovery column and side rectifier. The difference in these parts of the flowsheets is that the stream directed to the side rectifier has a higher concentration of chloroform than the recovery column’s feed. The increasing chloroform concentration leads to a temperature reduction on the plates of the side section compared with the section 4 recovery column. Simultaneously, the chloroform concentration growth leads to a decreasing side section reflux ratio in comparison to the reflux ratio of the entrainer recovery column and thus to a reduction of power consumption.
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Conclusions We concluded that the PCEDS effectiveness depends on the value of the reflux ratio in the entrainer recovery column (RRC) of the conventional ED flowsheet. If the reflux ratio in the apparatus has a small value, then the energy consumption decrease because using PCEDS will be insignificant. It is evident that the reduction in the energy consumption is not directly proportional to reducing RRC. However, a significant reduction in energy consumption at a level equal to or greater than 10% is observed at an RRC value close or equal to 1. This is confirmed by the data from other authors; see the Table 9. The data (Wang et al., 2010) are somewhat different from ours (case VI) because they are for separations at another pressure.
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Table 9. The data from literary sources for the reduced energy consumption by PCEDSs Recovery column reflux Mixture DQ, % Data ratio RRC references 0.41 4.7 Wang et al., Ethyl acetate – ethanol with 2010 DMSO at 101.3 kPa 1.15 about 10.0 Kiss et al., Ethanol– water with EG 2012 Methylal – methanol with 0.595 8.3 Xia et al., DMFA 2012
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Thus, the criterion for the evaluation of the PCEDS energy efficiency for binary mixtures separation can be formulated as follows: PCEDS is useful if the reflux ratio at the entrainer recovery column has value equal to or greater than 1.
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This work was conducted according to the state task of the Russian Ministry of Education and Science # 10.99.2014/K.
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NOMENCLATURE
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energy efficiency criterion, % entrainer flowrate, kg/h feed flowrate, kg/h flowrate directed to the side section, kg/h number of tray locations main column number of trays side column number of trays total number of theoretical plates energy consumption, kW reflux ratio temperature, °C mass concentration of liquid, %
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DQ E F FS N NMC NSS Ntotal Q R t X
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Subscripts E entrainer F feed reb reboiler S side section
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Superscripts CED conventional extractive distillation EC extractive column F feed MC main column PCEDS extractive distillation system with the partially coupled heat and material flows RC recovery column SS side section
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S0263-8762(15)00069-6 http://dx.doi.org/doi:10.1016/j.cherd.2015.03.006 CHERD 1804
To appear in: Received date: Revised date: Accepted date:
20-10-2014 3-3-2015 5-3-2015
Please cite this article as: Anokhina, E., Timoshenko, A.,Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Research Highlights
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We studied seven different azeotropic mixture separations by extractive distillation. Partially thermally coupled columns in extractive distillation were investigated. Energy efficiency compared to conventional flowsheets has been identified. The evaluation criterion was proposed of the complex column effectiveness in extractive distillation.
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*Manuscript
Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns Elena Anokhina, Andrey Timoshenko* [email protected]
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Lomonosov Moscow University of Fine Chemical Technology, Moscow, Russia
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Abstract The aim of this work is the statement of the reasons for different energy effectiveness of extractive distillation systems with the partially coupled heat and material flows (PCEDS) for various mixtures and entrainers. The estimation of the effectiveness was performed for PCEDS application for the separation of seven binary mixtures with different initial compositions and various entrainers. The criterion of the summary power consumption in the reboilers of the columns under the optimal operating parameters was used. It has been concluded on the basis of the obtained results that PCEDS effectiveness depends on the value of the reflux ratio in the entrainer recovery column of the conventional extractive distillation flowsheet. If the reflux ratio in that apparatus has a small value, then the energy consumption decrease due to the use of PCEDS will be insignificant. The criterion for the evaluation of the PCEDS energy efficiency for binary mixtures separation has been formulated as follows: PCEDS is useful if the reflux ratio at the entrainer recovery column has a value equal to or greater than 1.
Introduction
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Keywords Extractive distillation, partially thermally coupled columns, energy saving
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Distillation is the most widely used method to separate different industrial mixtures into pure components. This method unfortunately is very energy intensive and requires a lot of heat. The problem of distillation energy consumption reduction may be solved by the use of contemporary mass-exchange devices and by using a thermodynamically reversible process (Olujic et al., 2009). The implementation of this approach was first proposed by Wright (1949) and then developed in the theoretical and practical aspects by Petlyuk. The Petlyuk fully thermally coupled (FTC) arrangement from the thermodynamic point of view is characterized by minimum entropy production. This does not indicate the high energy efficiency. However, it is high enough. For example, it is proved that the minimum energy consumption for an ideal multicomponent mixture is always obtained for the fully thermally coupled configuration (Halvorsen et al., 2003). A considerable number of studies have been devoted to the investigation of possible alternative configurations of the Petlyuk column (Agrawal et al., 1998, 1999, 2000; Caballero et al., 2003; Hernandez et al., 2006; Rong et al., 2006a, 2006b). The realization of Petlyuk’s ideas actively began in 1985 when BASF built the first industrial dividing wall column (DWC). Now there are 1
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more than 100 DWCs around the world, which enable savings of both operating and investment costs (Kaibel et al., 1987, 2004; Kolbe et al. 2004). Another way to increase the thermodynamic efficiency of distillation is to use partially thermally coupled (PTC) systems. Common approaches to the synthesis of technological schemes of this class have been proposed by Timoshenko et al. (2001). Several authors have developed other approaches to the synthesis of distillation flowsheets with partially thermally coupled flows (Rong et al. 2003, 2004; Errico et al., 2009; Caballero et al., 2014). Distillation PTC systems may also be designed as dividing wall columns. The result of intensive studies and the beginning of the industrial applications of the FTC and PTC to separate non-azeotropic mixtures led to the development of research for extractive distillation. Extractive distillation (ED) is used in the industry for the separation of mixtures with similar relative volatilities and azeotropes, for example, the separation of hydrocarbons with close boiling points (Liao et al., 2001; Lei et al., 2002; Wentink et al., 2007), the recovery of aromas or fragrances (Berg, 1983; Pollien et al., 1998; Chaintreau, 2001; Chen et al., 2003; Ghaee et al., 2008), the separation of aqueous alcohol solutions (Meirelles et al., 1992; Pinto et al., 2000; Zhigang et al., 2002; Arifin and Chien, 2008; Li G. and Bai P, 2012), the separation of ether and alcohol mixtures (Muñoz et al., 2006; Wang et al., 2010), and the separation of methylal and methanol mixtures (Wang et al., 2012). The conventional extractive distillation (CED) flowsheet for the separation of binary mixtures (Figure 1a) includes an extractive column (EC) and an entrainer recovery column (RC). Mixture A–B and entrainer E are fed into the extractive column. The presence of the entrainer alters the relative volatility between components A and B, causing A to move to the top and B to move toward the bottom of the EC. The extractive column bottom’s stream is fed into the recovery column to produce almost pure B component as the distillate and almost pure entrainer as the bottom product. Despite the fact that extractive distillation is considered one of the most efficient methods of azeotropic mixture separation, it is characterized, like conventional distillation by high specific energy consumption. Thus, power consumption reduction of ED is an urgent task of chemical technology.
Figure 1. Conventional ED flowsheet (a) and ED systems with partially coupled heat and material flows (b), A,B – components of the initial feed, E– entrainer, MC – main column, SS – side section, EC – extractive distillation column, RC – entrainer recovery column, 1–5 – column’s sections Recently, ED systems with partially coupled heat and material flows (PCEDS) have been applied for saving energy (Timoshenko et al., 2003; Hernández, 2008; Gutiérrez-Guerra et al., 2009; Wang et al., 2010). PCEDS is a single complex column that consists of a main column (MC) and a refining side section (SS) for the cases of binary mixture separation by ED with a heavy boiling entrainer (Figure 1b). In Figure
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1, heat exchangers, such as condensers, reboilers and refrigerators for the entrainer, are not shown. ED systems with partially coupled heat and material flows may be arranged as dividing wall column (Kiss et al., 2012; Xia et al., 2012). The power consumption reduction by PCEDS differs considerably for the various mixtures up to ~30% (Timoshenko et al., 2003) and as little as ~ 4.7% (Wang et al., 2010). Until now, it was unclear why in some cases the application of PCEDS significantly reduced the power consumption and in other cases it had almost no effect. Therefore, we conducted a systematic study to investigate the energy consumption reduction dependence by PCEDS on various factors of the process.
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Design of ED systems with partially coupled heat and material flows Previously, we developed an algorithm for the synthesis of PCEDS flowsheets (Timoshenko et al., 2005; Timoshenko et al., 2007). The flowsheet in Figure 1a can be represented in accordance with this algorithm as a graph of Figure 2a. Vertices in the graph correspond to the intersections separating sections of the columns and edges to the flows of vapour and liquid within the column and the streams linking the columns. The merger of the two vertices connected by oriented edge BE of the graph (Figure 2a) gives a graph (Figure 2b) that corresponds to a complex column with a side rectifier (Figure 1b). Note that the algorithm can be applied to the synthesis of PCED’s flowsheets for multicomponent azeotropic mixture separation.
Ac
Figure 2. Graphs of flowsheets (see Figure 1) preimage (a) and images (b, c) at the ED with hard boiling entrainer; — feed (input), — output (exit) with removing heat, — output (exit) with heat input, — vertex without properties of input or output PCEDS for binary mixtures separation was obtained by Gutiérrez-Guerra et al., 2009, by similar transformation but without the use of graph theory. Problem statement The aim of this work is to present the reasons for different degrees of energy effectiveness of PCEDS for various mixture separations with different entrainers. As the energy efficiency criterion ( DQ ), the relative reduction of energy consumption in PCEDS the reboiler of PCEDS ( Qreb ) in comparison with the CED reboilers total energy CED PCEDS Qreb Qreb 100% . CED Qreb We estimated the DQ of the PCEDS application for the separation of seven binary mixtures with various entrainers. Extractive distillation systems were selected for different azeotropic mixtures with various entrainers as the objects of study: dimethyl
CED consumption ( Qreb ) was used: DQ
3
Page 4 of 23
formamide (DMFA), water, ethylene glycol (EG), butyl propionate (BP), dimethyl sulfoxide (DMSO), and propylene glycol (PG) (Table 1). Table 1. Some characteristics of the separated azeotropic mixtures at 101.3 kPa pressure.
V VI VII
Methyl acetate (1) – methanol (2) Ethyl acetate (1) – ethanol (2) Methyl acetate (1) – chloroform (2)
54.0 min 71.8 min 67.7 max
81.3 69.0 25.58
ip t Chloroform
Acetone
Methanol
cr
IV
DMFA, water EG
Allyl acetate Allyl alcohol
BP
DMFA EG
DMSO PG DMFA, EG, DMSO
Distillate of RC or SS
Acetone
Isobutyl alcohol Isobutyl acetate Methyl acetate Ethyl acetate
us
III
DMFA
Distillate of EC or MC
an
II
Entrainer
M
I
tAZ, C; x1AZ , min/max mass % boiling point Az. Acetone (1) – 64.5 22.0 Chloroform (2) max Acetone (1) – 55.6 86.3 methanol (2) min Allyl alcohol (1) – 95.1 62.9 allyl acetate (2) min Isobutyl alcohol (1) 107.4 78.3 – isobutyl acetate min (2) Binary mixtures
Methyl acetate
Isobutyl acetate Isobutyl alcohol Methanol Ethanol
Chloroform
ed
No mixture
Ac
ce pt
For mixtures IV, V, and VI, the quantities and compositions of the feed, as well as the product qualities, were set according to the data of Muñoz R., et al. 2006, Heinz E., 1994, Wang S.-J., et al. 2010, respectively. In all cases, the initial feed had a boiling point temperature. The characteristics of the initial feed and product flow compositions are fixed by calculations and are presented in Table 2. Aspen HYSYS was used to simulate the ED. The vapour-liquid equilibrium of all investigated mixtures except mixture IV were calculated using the nonrandom two-liquid (NRTL) equation with the parameters presented in Table 3. The UNIQUAC activity model was used for the vapour-liquid equilibrium calculation of mixture IV with parameters published by Muñoz R., et al. 2006 (see Table 4). In all cases, the vapour phase was assumed to be ideal. Results and discussion Optimization of the conventional ED flowsheet We used the criterion of the summary power consumption in the reboilers of the CED columns ( Qreb ) to identify the optimal operating parameters. The design variables to be optimized in the conventional ED flowsheets include the total number of theoretical plates (Ntotal) (except the ED of mixture IV with BP), the entrainer temperature (tE), the entrainer-to-feed ratio (E:F), and the entrainer and feed tray locations (NE and NF). The operating variables include the reflux ratios and reboiler duties of both columns. The reflux ratios and reboiler duties were varied to satisfy the distillate and bottom product specifications, respectively.
4
Page 5 of 23
Table 2. Concentrations of the initial feed and products flows and VLE models
I with DMFA
100
x1F , Pressu Entrain Distilla Distilla Bottom Type of VLE mass %
22.0
re, kPa
101.3
er feed, XE mass % 99.90
te of te of product model, EC or RC or of RC or parameters MC, XE references MC, SS, mass mass mass % % % NRTL, 99.50 99.90 99.90
86.3
101.3
99.90
99.50 99.50
100
101.3
99.90
99.50 99.50
99.90
100
86.3 20.0 20.0
101.3
99.90
99.50 99.50
99.90
IV with BP
1500
41.0
101.3
99.60
97.60 99.60
99.60
IV with DMFA
1500
41.0
20
99.60
V with EG
12200 77.9
VI with DMSO 11279 77.8 11279 77.8
us
an
99.60 97.60
99.60
99.90
99.50 99.99
99.90
30
99.90
99.50 99.99
99.90
101.3
99.90
99.50 99.99
99.90
101.3
ce pt
VI with PG
99.90
M
II with water Case 1 Case 2 III with EG
VII with DMFA 1000 25.58
101.3
99.90
99.50 99.90
99.90
VII with EG
1000 25.58
101.3
99.90
99.50 99.90
99.90
1000 25.58
25
99.90
99.50 99.90
99.90
Ac
VII with DMSO
Anokhina E.A. et al., 2008 NRTL, Anokhina E.A. et al., 2009 NRTL, Anokhina E.A. et al., 2011
cr
100
ed
II with DMFA
Feed, kg/h
ip t
Mixture
NRTL, Anokhina E.A., et al., 2006 UNIQUAC, Muñoz R., et al. 2006 UNIQUAC, Muñoz R., et al. 2006 NRTL, Rudakov D.G., et al, 2013 NRTL, Rudakov D.G., et al, 2013 NRTL, Rudakov D.G., et al, 2013 NRTL, Anokhina E.A. et al., 2013 NRTL, Anokhina E.A. et al., 2013 NRTL, Anokhina E.A. et al., 2013
5
Page 6 of 23
Table 3. NRTL binary interaction parameters Component I
Component J
binary interaction parameters, K A(I,J)
A(J,I)
B(I,J)
B(J,I)
α
Chloroform
0.000
0.000
-194,170
-10,010
0.300
Acetone
DMFA
0.000
0.000
55.743
20.700
1.696
Chloroform
DMFA
0.000
0.000
-172.370 -148.630
0.300
Acetone
Methanol
0.000
0.000
92.945
112.039
0.308
Acetone
Water
0.000
0.000
317.554
602.558
0.534
Methanol
Water
0.736
0.511
-360.692
199.854
0.244
Methanol
DMFA
0.000
0.000
-380.050
292.370
0.300
Allyl alcohol
Allyl acetate
0.000
0.000
294.100
-77,200
0.370
Allyl alcohol
EG
0.000
0.000
169.580
95,950
0.300
Allyl acetate
EG
0.000
0.000
415.250
541,660
0.200
Methyl acetate
Methanol
-0.282
-0.359
299.176
251.233
0.117
Methyl acetate
EG
0.000
Methanol
EG
0.000
Ethyl acetate
Ethanol
0.000
Ethyl acetate
DMSO
0.000
Ethanol
DMSO
Ethyl acetate
PG
Ethanol
PG
Methyl acetate
0.000
43.937
265.466
0.321
0.000
-54.958
24.654
0.298
0.000
154.208
162.349
0.299
0.000
285.156
120.035
0.297
0.000
0.000
150.976
-409.379
0.300
0.000
0.000
896.337
-107.416
0.202
0.000
0.000
217.896
-41.049
1.042
Chloroform
0.000
0.000
-334.148
163.414
0.305
Methyl acetate
DMFA
0.000
0.000
152.902
-88.479
0.800
Chloroform
EG
0.000
0.000
-68.977
92.884
0.300
Methyl acetate
DMSO
0.000
0.000
276.363
51.969
0.309
Chloroform
DMSO
0.000
0.000
825.439
-312.080
1.114
Ac
ce pt
ed
M
an
us
cr
ip t
Acetone
Table 4. UNIQUAC binary interaction parameters according to the data of Muñoz R., et al. 2006 Component I
Component J
binary interaction parameters, kcal/kmol A(I,J)
A(J,I)
B(I,J)
B(J,I)
Isobutyl alcohol
Isobutyl acetate
116.170
361.900
-0.530
-0.450
Isobutyl alcohol
BP
35.346
49.224
0.000
0.000
Isobutyl acetate
BP
131.014
-128.309
0.000
0.000
Isobutyl alcohol
DMFA
56.603
-155.920
0.000
0.000
Isobutyl acetate
DMFA
551.320
-281.210
0.000
0.000
6
Page 7 of 23
Ac
ce pt
ed
M
an
us
cr
ip t
At the first stage, we determined the optimal value of the total number of theoretical plates for the extractive column and entrainer recovery column at a fixed ratio E:F and a fixed entrainer temperature. The energy consumption in the column reboilers (Qreb.) was the selection criterion. The optimal initial feed tray locations were identified for each column with assigned Ntotal. For extractive distillation columns, the optimal entrainer feed plates were also determined. Figure 3 shows the dependence of the energy consumption at the reboilers of EC and RC on the number of plates for the ED of a methyl acetate – chloroform mixture with EG as the entrainer. The energy consumption in the reboiler of the extractive column increased slightly (< 1%) as the number of plates decreased from 38 to 35. With a further reduction in the number of plates, the Qreb increase became more visible. The change is 9.4% when reducing the number of plates from 35 to 31. Therefore, during all further calculations, we took the number of theoretical plates equal to 35 for the extractive column for the methyl acetate – chloroform mixture separation with EG as the entrainer. Similarly, the energy consumption in the reboiler of the entrainer recovery column increased slightly (< 1%) as the number of plates decreased from 20 to 12. With a further reduction in the number of plates, the increase in Qreb becomes more apparent. It increases by 2.2% from 12 to 10 plates and by 31% from 10 to 7 plates. Therefore, during all further calculations, we set the number of theoretical plates in the RC for this case equal to 12.
Figure 3. The dependence of the energy consumption in the reboilers of the extractive column (a) and recovery column (b) on the number of plates for the ED of the methyl acetate – chloroform mixture with EG. E: F = 5:1, tE= 60 C We performed a similar analysis for the EC and RC of all other investigated mixture extractive distillations, except mixture IV with BP. The number of plates in this case was set in accordance with the work of Muñoz R. et al., 2006. The number of theoretical plates in the columns is shown in Table 6. These values can be considered optimal only to a certain extent because they are found at a fixed
7
Page 8 of 23
ce pt
ed
M
an
us
cr
ip t
temperature and entrainer flowrate, excluding the price of the cooling water and without entrainer flow heat recovery. At the second stage, we investigated the dependence of the energy consumption in the reboiler of the EC on the entrainer temperature at a fixed ratio E:F by the previously identified total number of theoretical plates, as described above. The optimal initial feed and entrainer feed tray locations were determined at each temperature. Figure 4 shows the dependence of the energy consumption in the reboiler and the reflux ratio of the extractive column on the EG temperature for the extractive distillation of the methyl acetate – chloroform mixture. The energy consumption in the reboiler of the EC decreases significantly (~ 16%) upon increasing the EG temperature from 60 to 80 C. The reflux ratio in this case increases slightly from 0.1 to 0.3. The Qreb decrease becomes less apparent with a further EG temperature increase: an EG temperature increase from 80 to 90 C decreases Qreb by 6.0%, an increase from 90 to 100 C decreases Qreb by 3.0%, and an increase from 100 to 110 C decreases Qreb by 1.4%. The reflux ratio increases substantially at temperatures above 90°C: to 1.8 at 100°C and to 2.9 at 110°C. Thus, an entrainer feed temperature of 90С was used. In this case, we do not take into account the amount of heat that can be achieved by entrainer cooling. Considering this factor, the temperature may vary. Similar analyses were performed for the ED of all other investigated mixtures. The values of tE found as a result of this analysis are presented in Table 6.
Ac
Figure 4. The dependence of the EC energy consumption in the reboiler (1) and reflux ratio (2) on the EG temperature for the extractive distillation of the methyl acetate – chloroform mixture under E:F=5:1. At the third stage, we investigated the dependence of the energy consumption in the reboiler of the EC on the entrainer flowrate for several values of NE/NF. Figure 5 shows the dependence of the energy consumption in the reboiler of the extractive column on the EG flowrate by the extractive distillation of the methyl acetate – chloroform mixture for NE/NF= 4/14.
8
Page 9 of 23
ip t
cr
Figure 5. The dependence of the energy consumption at the reboiler of the extractive column on the EG flowrate for the ED of the methyl acetate – chloroform mixture for NE/NF= 4/14 and EG temperature 90 °С
an
us
The energy consumption in the reboiler of the ED column has a minimum at an EG flowrate of 4600 kg/h. The methyl acetate concentration is lower than 99.5% in the extractive column distillate at an EG flowrate less than 4250 kg/h. Thus, 4250 kg/h is the minimal value of the entrainer flowrate at which the given product quality is possible to obtain in the EC distillate. The values of the optimal and minimal entrainer flowrate for other NE / NF are presented in Table 5.
ce pt
ed
M
Table 5. Optimal and minimal EG flowrates for the ED of the methyl acetate – chloroform mixture for several NE/NF at an EG temperature of 90 0С Qreb, kW entrainer flowrate, kg/h NE/NF optimal minimal optimal minimal flowrate flowrate 3\14 4650 4250 328.5 356.4 3\15 4750 4350 329.1 355.9 3\16 4900 4450 333.8 379.7 4\14 4600 4250 329.5 360.0 4\15 4750 4350 330.1 357.8 4\16 4850 4500 334.6 358.2 5\14 4600 4300 334.0 352.9 5\15 4700 4350 334.0 365.7 5\16 4850 4600 338.1 362.3
Ac
At the fourth stage, we investigated the dependence of the entrainer recovery column feed tray location on the entrainer flowrate at certain intervals of values. The minimum value was chosen for all values of "minimal entrainer flowrate", and we took it as a lower variation interval boundary. For example, the value is 4250 kg/h in the case of Table 5. The maximum value was chosen among all of the values of the "optimal entrainer flowrate ". For example, the value is 4900 kg/h in the case of Table 5. A value somewhat larger than the maximum value of the "optimal entrainer flowrate” was established as the upper interval of the variation boundary. Furthermore, calculations of the flowsheets were performed, and the optimal feed tray locations in the recovery columns were determined for several values of EG flowrate from the performed interval. For example, the entrainer flowrate variation interval was 4250–5000 kg/h in the case of the methyl acetate – chloroform mixture extractive distillation with EG. Similar studies were conducted for the extractive distillation flowsheets of mixtures I-VII.
9
Page 10 of 23
ed
M
an
us
cr
ip t
The calculations showed that the optimum feed tray locations in the entrainer recovery columns at selected entrainer flowrate intervals are practically unchanged. This is due to two facts. The first is that in the RC’s feed, the concentration of an entrainer is significantly greater than the concentration of the initial binary mixture component. Thus, the concentration ratio varies only slightly with the varying flowrate of the entrainer. From another point of view, this may be because the entrainer flowrate changes in relatively narrow interval and does not significantly affect the concentration ratio in the feed of the RC. In the case of the ED of the methyl acetate – chloroform mixture with EG, the optimal feed plate number of the entrainer recovery column is 5. The optimization procedure of the conventional ED flowsheet is summarized below: 1) Guess the entrainer flowrate. 2) Guess the optimal feed tray location in recovery column, as previously found for the given flow rate of entrainer, as described above. 3) Guess the entrainer feed tray location in the extractive column (NE). 4) Guess the initial feed tray location in the extractive column ( NEC F ). 5) Change the reboiler duty and reflux ratio in the extractive column until the product specifications are achieved. EC 6) Go back to step 4, guess a new value of NEC F , repeat step 5. Change NF until CED is minimized. Qreb 7) Go back to step 3, guess a new value of NE, repeat steps 4–6. Change NE until CED is minimized. Qreb 8) Go back to step 1, guess a new value of the entrainer flowrate, repeat steps 2–7. CED Change the entrainer flowrate until Qreb is minimized. The results are presented for conventional ED flowsheet optimization for all investigated mixtures in Table 6.
Ac
ce pt
Optimization of ED systems with partially coupled heat and material flows The design variables to be optimized in the PCEDS include the entrainer temperature (tE), entrainer-to-feed ratio (E:F), the entrainer, the feed and the side section outlet tray locations (NE, NF, NS), and the value of flowrate directed to the side section (FS). The operating variables include the reflux ratios of the main column (RMC) and side section (RSS) and the reboiler duty of the main column. The reflux ratios are manipulated to satisfy the main column and side section distillate product specifications. The reboiler duty is manipulated to satisfy the specification of entrainer purity. The total number of theoretical plates for the PSEDS is set to the same value as for the conventional ED flowsheet. As mentioned above, the synthesis algorithm of the PCEDS’s number of plates of the main column (NMC) is obtained by summing the number of plates of the extractive column of the conventional ED flowsheet and the stripper section of the recovery column under the optimal design. The number of plates of the side section (NSS) is equal to the number of plates of the rectifier section of the conventional ED flowsheet recovery column under the optimal design. At the first stage, we investigated the dependence of the energy consumption at the reboiler of the main column on the entrainer temperature at a fixed ratio E:F and the value of the flowrate directed to the side section. The optimal initial mixture and entrainer feed tray locations were determined for each value of temperature. Figure 6 shows the dependence of the reboiler energy consumption and reflux ratio of the
10
Page 11 of 23
main column on the EG temperature in the case of the methyl acetate - chloroform mixture ED.
RC EC RC EC RC EC RC EC RC EC RC EC RC
VI with DMSO VI with PG VII with DMFA VII with EG
Ac
VII with DMSO
22 22 35 10 35 14 35 14 24 9 50 30 60
4 /10 –/6 4/18 –/5 17/27 –/9 12/23 –/9 5/15 –/5 10/35 –/15 14/46
3.50:1 – 4.90:1 – 1.20:1 – 1:1 – 3.60:1 – 1.87:1 – 1.76:1
2.20 3.60 3.00 2.80 3.30 6.10 11.2 2.00 1.40 1.10 5.09 4.07 6.50
28.10 27.10 66.10 19.70 56.00 30.70 36.06 76.50 38.20 16.40 651.4 399.3 639.6
20 50 10 30 10 50 10 30 15 35 12 22 9
–/7 3/33 –/6 4/17 –/6 4/33 –/6 4/12 –/5 3/14 –/5 3/11 –/4
60 – 70 – 70 – 70 – 100 – 120 – 101.0 6 – 40 – 40 – 40 – 80 – 90 – 30 –
– 2.87:1 – 0.78:1 – 2.97:1 – 3.45:1 – 4.50:1 – 0.9:1 –
4.40 1.04 0.18 0.66 0.19 0.95 0.40 2.60 5.00 2.00 0.80 1.04 0.12
586.6 4763 2751 1925 957.6 4487 2233 246.8 327.1 330.0 238.6 89.80 92.70
ce pt
V with EG
Reflux ratio (R)
Qreb, kW
CED , Qreb
kW
ip t
EC RC EC RC EC RC EC RC EC RC EC RC EC
IV with DMFA
Entrain er-tofeed ratio (E:F)
cr
I with DMFA
IV with BP
Entrai ner tempe rature tE, C
us
Ntotal
M
Extractive (EC) or recovery (RC) column
ed
Mixture and entrainer
II with DMFA II with water Case 1 II with water Case 2 III with EG
Entrainer / feed trays location, NE/NF
an
Table 6. Energy consumption of classical ED flowsheets for different mixture separations under the optimal parameters
55.20 85.80 86.60 113.1 54.6 1050 1226 7513 2882 6719 573.9 568.6 182.5
The relationships in Figure 6 are similar to the relationships in Figure 4. The energy consumption decreases in the boiler of the PCEDS‘s main column significantly (~ 9.7%) with increasing EG temperature from 60 to 80 C. The reflux ratio in this case increases slightly from 0.1 to 0.2. The Qreb decrease becomes less visible with a further increase of the EG temperature; when the EG temperature increases from 80 to 90 C, it decreases by 3.0%, when the temperature increases from 90 to 100 C, it decreases by 2.9%, and when the temperature increases from 100 to 120 C, it decreases by 1.0%. The reflux ratio significantly increases at EG temperatures of more than 90 °C: to 1.7 at 100 °C and to 3.9 at 120 °C. The optimal EG feed temperature of the PCEDS’s main column was 90°C, for the same reasons as for the conventional ED flowsheet. Similar analyses were
11
Page 12 of 23
us
cr
ip t
performed for the other investigated mixtures by PCEDS separation. The values of tE found as a result of this analysis are presented in Table 7. In all cases, the relationships of the reboiler duty and the reflux ratio of the main PSEDS’s column on the entrainer temperature were similar to the relationships of the reboiler duty and the reflux ratio of the conventional extractive column.
an
Figure 6. The dependence of the energy consumption at the reboiler (1) and reflux ratio (2) of MC on the EG temperature in the case of the methyl acetate - chloroform mixture ED under E:F=4.5:1 and FS=1000 kg/h.
Ac
ce pt
ed
M
The optimization procedure of PCEDS is summarized below: 1) Guess the entrainer flowrate. 2) Guess the value of the flowrate directed to the side section (FS). 3) Guess the entrainer feed tray location in the main column (NE). 4) Guess the initial feed tray location in the main column (NF). 5) Guess the tray location of the outlet to the side section (NS). 6) Change the reboiler duty and reflux ratios in the extractive column and side section until the product specifications are achieved. PCEDS 7) Go back to step 5, guess a new value of NS, repeat step 6. Change NS until Qreb is minimized. 8) Go back to step 4, guess a new value of NF, repeat steps 5–7. Change NF until PCEDS Qreb is minimized. 9) Go back to step 3, guess a new value of NE, repeat steps 4–8. Change NE until PCEDS Qreb is minimized. 10) Go back to step 2, guess a new value of FS, repeat steps 3–9. Change FS until PCEDS Qreb is minimized. 11) Go back to step 1, guess a new value of the entrainer flowrate, repeat steps 2– PCEDS 10. Change the entrainer flowrate until Qreb is minimized. The results are presented for PCEDS’s optimization for all investigated mixtures in Table 7.
12
Page 13 of 23
Table 7. Energy consumption of PCEDSs under the optimal parameters Mixture
Main col. num ber of trays NMC
Side Entrainer/ Entrainer Entraine Flow Reflu Reflux Q PCEDS , reb col. feed /side temperatu r-to-feed to x ratio ratio kW numb section ratio side main side re, tE, C er of outlet trays (E:F) secti col. col. MC trays location on, R RSS SS N NE/NF/NS FS, kg/h
34
10
4 /11/24
60
3.50:1
120
1.90 1.20
II with DMFA II with water Case 1 II with water Case 2 III with EG
40 40
5 9
4/18/36 18/26/35
70 70
4.80:1 1.50:1
19 33
3.00 0.30 3.10 2.40
72.60 69.60
40
9
12/22/34
70
1.20:1
175
10.6 1.60
100.4
28
5
5/15/25
100
3.50:1
23
1.40 0.30
50.80
IV with BP
65
15
10/35/52
120
1.90:1 2500 4.68 1.86
839.4
IV with DMFA
73
7
15/43/58
101.06
1.97:1 1400 6.50 1.00
860.5
V with EG
54
6
3/33/51
40
VI with DMSO 34
6
4/16/30
40
VI with PG VII with DMFA VII with EG VII with DMSO
54 40
6 5
4/33/51 4/11/29
40 80
42 27
5 4
3/15/38 3/11/23
cr
us
2.91:1 2700 0.95 0.04 7407.3
an
M
ed
90 30
40.90
ip t
I with DMFA
0.82:1 2530 0.64 0.05
2812
2.93:1 2640 0.96 0.11 3.67:1 1150 2.67 1.18
6541 389.6
4.18:1 0.80:1
501.9 180.7
770 750
1.62 0.22 1.30 0.06
Ac
ce pt
We conducted a comparison of the total energy consumption at the reboilers of the conventional ED flowsheets and the PCEDS flowsheets for separation of these mixtures. Table 8 and Figure 7 show the data of the values of the energy consumption reduction by applying PCEDS systems compared to classical ED flowsheets. The difference in the energy effectiveness can be due to the different types of azeotropes, different entrainers, different processes at the PCEDS and CED extractive sections and above it and some effects at other sections of flowsheets. Mixtures I and VII have azeotropes with a high boiling point, and the others have azeotropes with a minimum boiling point. An explicit dependence on the type of the azeotrope is not observed. There is some dependence on the type of entrainer, e.g., PCEDSs with DMF in all cases have relatively high efficiency, but the relationship is not precise. The efficiency criterion DQ values are different; for cases with DMF, they are nearly double (16.0–32.1%), and for cases with EG, they are more than 8 times (1.4–11.7%).
13
Page 14 of 23
Table 8. Reduction of the energy consumption by using PCEDSs Recovery column reflux ratio (RRC) Mixture 3.60 I with DMFA 2.80 II with DMFA
DQ, % 25.9 16.0
6.10 2.00 1.10
19.6 11.2 7.10
IV with BP
4.07
20.1
IV with DMFA
4.40
V with EG
0.18
VI with DMSO
0.19
VI with PG VII with DMFA VII with EG VII with DMSO
0.40
1.40
cr
2.40 2.70 32.1 11.7 1.00
ce pt
ed
M
an
0.12
29.8
us
5.00 0.80
ip t
II with water, Case 1 II with water, Case 2 III with EG
Figure 7. Reflux ratio in the entrainer recovery columns (RRC) and reduction of the energy consumption by using PCEDS for different mixture separations
Ac
We analysed the profiles of the temperature and component concentrations in the two columns of the conventional ED flowsheet and the main column and side rectifier column of the PCEDS. Figures 8 and 9 are examples of the profiles of the temperature and component concentrations at the columns of the ED methyl acetate - chloroform mixture with EG.
14
Page 15 of 23
ip t cr us an M ed ce pt
Ac
Figure 8. Temperature profiles of the extractive column (a) and recovery column (c) of the conventional ED flowsheet and the main column (b) and side section (d) of the PCEDS for the ED methyl acetate – chloroform mixture with EG under optimal parameters.
15
Page 16 of 23
ip t cr us an M ed ce pt
Ac
Figure 9. Liquid composition profiles for ED methyl acetate – chloroform mixture separation with EG under optimal parameters. Extractive column (a) and recovery column (c) of the conventional ED flowsheet and main column (b) and side section (d) of the PCEDS. The temperature and the concentration profiles are identical in sections 1-3 of the conventional extractive distillation column and the main column of the PCEDS (Figures 1, 8 and 9). The temperature and concentration profiles for sections 4 and 5 are different for the conventional extractive distillation and PCEDS. The process of the heavy boiling component and entrainer separation mainly exists in these parts of the scheme. It is characterized by the reflux ratios in the entrainer recovery column and side rectifier. The difference in these parts of the flowsheets is that the stream directed to the side rectifier has a higher concentration of chloroform than the recovery column’s feed. The increasing chloroform concentration leads to a temperature reduction on the plates of the side section compared with the section 4 recovery column. Simultaneously, the chloroform concentration growth leads to a decreasing side section reflux ratio in comparison to the reflux ratio of the entrainer recovery column and thus to a reduction of power consumption.
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Conclusions We concluded that the PCEDS effectiveness depends on the value of the reflux ratio in the entrainer recovery column (RRC) of the conventional ED flowsheet. If the reflux ratio in the apparatus has a small value, then the energy consumption decrease because using PCEDS will be insignificant. It is evident that the reduction in the energy consumption is not directly proportional to reducing RRC. However, a significant reduction in energy consumption at a level equal to or greater than 10% is observed at an RRC value close or equal to 1. This is confirmed by the data from other authors; see the Table 9. The data (Wang et al., 2010) are somewhat different from ours (case VI) because they are for separations at another pressure.
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Table 9. The data from literary sources for the reduced energy consumption by PCEDSs Recovery column reflux Mixture DQ, % Data ratio RRC references 0.41 4.7 Wang et al., Ethyl acetate – ethanol with 2010 DMSO at 101.3 kPa 1.15 about 10.0 Kiss et al., Ethanol– water with EG 2012 Methylal – methanol with 0.595 8.3 Xia et al., DMFA 2012
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Thus, the criterion for the evaluation of the PCEDS energy efficiency for binary mixtures separation can be formulated as follows: PCEDS is useful if the reflux ratio at the entrainer recovery column has value equal to or greater than 1.
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This work was conducted according to the state task of the Russian Ministry of Education and Science # 10.99.2014/K.
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NOMENCLATURE
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energy efficiency criterion, % entrainer flowrate, kg/h feed flowrate, kg/h flowrate directed to the side section, kg/h number of tray locations main column number of trays side column number of trays total number of theoretical plates energy consumption, kW reflux ratio temperature, °C mass concentration of liquid, %
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DQ E F FS N NMC NSS Ntotal Q R t X
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Subscripts E entrainer F feed reb reboiler S side section
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Superscripts CED conventional extractive distillation EC extractive column F feed MC main column PCEDS extractive distillation system with the partially coupled heat and material flows RC recovery column SS side section
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