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cyclohexane
Energy 186 (2019) 115756
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Investigation of an energy-saving double-thermally coupled extractive distillation for separating ternary system benzene/toluene/ cyclohexane Ao Yang a, Yang Su a, I-Lung Chien b, Saimeng Jin a, Chenglei Yan a, Shun'an Wei a, Weifeng Shen a, * a b
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, PR China Department of Chemical Engineering, National Taiwan University, Taipei, 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2019 Received in revised form 17 June 2019 Accepted 14 July 2019 Available online 18 July 2019
An intensified scheme for the separation of ternary azeotropic system is explored to reduce the energy consumption and recycle important organic solvents. Herein, a novel double-thermally coupled ternary extractive distillation (DTCTED) for separating azeotropic system benzene/toluene/cyclohexane (denoted as B/T/CH) is proposed to achieve energy-saving and emissions reduction. Thermodynamic feasible insights of the B/T/CH using dimethyl formamide as entrainer are firstly analyzed via residue curve maps to find separation constraints. Following that, the proposed intensified scheme is optimized via the inhouse multi-objective genetic algorithm software while using total annual cost and CO2 emissions as objective functions. The results show that the total annual cost and CO2 emissions of the proposed intensified DTCTED scheme are significantly reduced by 18.60% and 20.22% compared with the existing single-thermally coupled ternary extractive distillation process. Furthermore, exergy loss and relative volatility are introduced to explore the essence of energy-saving in the proposed DTCTED scheme. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Energy-saving Residue curve maps Ternary mixtures Extractive distillation Thermally coupled
1. Introduction The separation of azeotropic or close-boiling mixture is necessary for the recovery of valuable solvents from waste liquid from the perspective of sustainable development [1e3]. Extractive distillation (ED) as an effective distillation technique is frequently used to separate azeotropic or close-boiling mixtures by adding a third component (denoted as entrainer) to improve the relative volatility [4e10]. Doherty and Malone [4] reported the conceptual design approach of the ED sequence for separating binary azeotropic mixtures. Furthermore, a systematic method to design of the ED for the separation of azeotrope system using heavy entrainers is proposed by Shen et al. [5]. Following that, the conceptual design for the separation of non-ideal mixture using light entrainers is explored [6]. Alternative configurations for separating binary azeotropic mixtures methanol/dimethyl carbonate are studied [7]. To separate ternary azeotropic mixtures, some kinds of separation sequences are presented by Wang et al. [8]. A novel design method
* Corresponding author. E-mail address: [email protected] (W. Shen). https://doi.org/10.1016/j.energy.2019.07.086 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
for separating acetone/chloroform mixtures via the ED is reported [9]. An energy-saving triple-column extractive distillation process for separating ternary azeotropic mixture with multi-azeotrope are reported by Yang et al. [10]. Recently, development of the ED has been presented by two review papers [11,12]. However, the high quality energy input is required in the reboiler to execute the separation task since the ED has a low thermodynamic efficiency [13]. In addition, the increasing awareness of the energy crisis has made process intensification a promising trend for energy-saving strategies in the chemical and petroleum industries. Hence, an intensified separation technique partial thermally coupled ED (extractive dividing wall column, EDWC) with a significantly advantage than that of the conventional ED is proposed for separating non-ideal azeotrope mixtures [14e22]. An intensified EDWC scheme is proposed to obtain the high-purity of bioethanol [14]. The EDWC scheme for the separation of methylal/methanol binary mixtures with dimethyl formamide (DMF) as an entrainer is further reported [15], and they demonstrated that the intensified scheme had an 11.60% of total annual cost (TAC) savings compared with that of the conventional ED. Chien's group [16] reported the EWDC configuration for
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Nomenclature B CI CH CTCED C% DMF DTCTED E ED EDWC El EI Ex FE FV2 H hseq ID MOGA NF1
benzene capital investment [ 103 $] cyclohexane conventional triple-column ternary extractive distillation carbon content dimethyl formamide double-thermally coupled ternary extractive distillation entrainer extractive distillation extractive dividing wall column exergy loss [kW] energy investment [ 103 $] exergy [kW] flow rate of entrainer [kmol/h] flow rate of sidestream V2 [kmol/h] enthalpy [kW] enthalpy of the hot utility [kJ] internal diameter of column [m] multi-objective genetic algorithm feed locations of fresh feed
separating mixtures of acetone/methanol and reported a slight reduction in TAC (i.e., 1.79%) through the intensified sequence compared with the double-column ED sequence. Sun et al. [17] conducted the design of EWDC for the separation of benzene/ cyclohexane system, and they displayed that the optimized EWDC can save 22.00% in the reboiler duty, and meanwhile, energy investment and TAC are decreased by 1.80% and 4.80%, respectively. Ethanol dehydration is investigated through the EDWC scheme by Tututi-Avila [18], and they demonstrated that TAC of 12.42% could be reduced. The investigation of EDWC for separating ethyl acetate/ isopropyl alcohol system is reported by Zhang et al. [19]. In one of our recent studies [22], we proposed a novel decanter assisted EDWC for the separation of ternary mixture with multi-azeotrope and we demonstrated that the EDWC has a 15.14% reduction than that of the ED with an additional decanter. According to above review, the intensified EDWC separation configurations can substantially reduce the capital cost and energy cost in the separation of non-ideal azeotropic system. However, the energy input with high quality is needed for the reboiler of EDWC to achieve the separation task making the effect of energy-saving less remarkable [20]. All the energy-saving of EDWC which is attributed to the capital investment is reduced because the extractive distillation and entrainer recovery columns are integrated into a single shell. On the contrary, the reduction of operating cost (also denoted as energy cost) is tiny (or no) in EDWC schemes [16]. Consequently, some alternative thermally coupled extractive distillation sequences should be proposed to solve and overcome this problem. Following that, the column is operated at low pressure in the single-thermally coupled ternary extractive distillation (STCTED) to perform the separation task with low pressure steam to avoid utilizing high pressure energy inputs [23], which is based on the exploration of partially thermally coupled schemes to separate ternary azeotropic mixtures with different types of ternary diagram by Timoshenko et al. [24]. Moreover, two STCTED schemes for separating ternary tetrahydrofuran/ethanol/ water azeotropic mixture with low energy consumptions, CO2 emissions and TAC are explored by Zhao et al. [25]. However, it is
NV2 NHV QC Qfuel Qseq QR R1 RCMs S STCTED
withdraw stages of sidestream V2 net heating value [kJ/kg] heat duty of condenser [kW] heat requirement of the fuel [kJ] energy consumption of the design process [kJ] heat duty of reboiler [kW] reflux ratio residue curve maps entropy [kJ/kg.K] single-thermally coupled ternary extractive distillation T toluene T0 temperature of the reference [K] TAC total annual cost [ 103 $] TF flame temperature [K] Ti (i¼A, B, C, and E) boiling point temperature of component i (i ¼ A, B, C, and E) TS stack temperature [K] V2 vapor stream to column C2 V3 vapor stream to column C3 VLE vapor-liquid equilibrium a molar mass ratio of CO2 and C ai/j relative volatility of components i and j lseq latent heat [kJ/kg]
tough to achieve the optimization of the thermally coupled processes because of coupling interaction between key variables. As such, multi-objective approaches are proposed to optimize those complex systems [26e31]. A novel EDWC is designed using the multi-objective optimization technique with economic and environmental indicators as objective function [26]. Alcocer-García et al. [27] investigated a multi-objective genetic algorithm (MOGA) for the optimization of intensified processes to purify levulinic acid. Benzene, toluene, and cyclohexane (B/T/CH) could be used as important solvents in the chemical and electronics industries. In addition, T and CH could be employed as clean and sustainable working fluids in the organic Rankine cycle systems as reported by Yagli et al. [32]. Hence, the separation of these systems has significant application promising. The separation of this system is difficult and complex because it forms multiazeotropes [24]. In summary, valuable insights on conceptual design and multiobjective optimization on double-thermally coupled ternary extractive distillation (DTCTED) have not yet been reported. Hereby, a novel DTCTED configuration for the separation of azeotropic mixtures B/T/CH employing DMF as the entrainer is proposed in this study to achieve the performance of energy conservation and emission reduction. First, the best separation configuration is screened based on the thermodynamic principles, before the thermodynamic feasibility insight in the ED process (e.g., isovolatility line and residue curve maps) is carried out for the separation of system B/T/CH utilizing the entrainer DMF. Following that, Pareto-optimal front with key design parameters of the proposed process is obtained by using the in-house MOGA based on the minimum TAC and CO2 emissions. Eventually, exergy loss and relative volatility are introduced to explore the essence of the energy-saving and emission reduction. 2. Existing processes for separating B/T/CH system 2.1. Conventional triple-column extractive distillation Fig. 1 represents an existing CTCED for separating B/T/CH system
A. Yang et al. / Energy 186 (2019) 115756
using entrainer DMF [23]. C1 is the extractive distillation column with 80 stages; C2 represents the conventional distillation column with 41 stages; C3 denotes the regeneration column with 21 stages. Three columns are operated at 0.07, 0.11 and 0.012 MPa, respectively. The fresh feed is azeotropic mixtures of B, T and CH (i.e., 100 kmol/h with 30 mol% B, 30 mol% CH and 40 mol% T) feeding at 40th stage, and the entrainer stream is fed at 20th stage. The high purity product cyclohexane (99.00 mol%) is obtained at the overhead of column C1. Feed locations of C2 and C3 are 20th and 15th, respectively. The purity products of B and T (99.00 mol% and 99.22 mol%) are attained at the overhead of C2 and C3, respectively. 2.2. Single-thermally coupled ternary extractive distillation To reduce the reboiler duty and TAC of the CTCED process, an existing improved STCTED process for separating B/T/CH azeotropic system is proposed by Luyben [23] and demonstrated in Fig. 2 where only two reboilers are needed compared to three reboilers in the existing CTCED scheme in Fig. 1. Total theoretical trays of C1C3 are 80, 20 and 21, respectively and 0.08, 0.04 and 0.012 MPa are determined for three columns. Stage of the withdraw vapor stream is 60 while the feed locations of entrainer and fresh feed are 20th and 40th, respectively. The second column C2 is combined into the first column C1, which is applied for cyclohexane production with 99.00 mol %. The third column is used to recover entrainer. The purity of DMF reaches no less than 99.95 mol % at the bottom stream of column C3. Then, the high purity DMF and make up entrainer is mixed to the column C1. 3. Methodology In this contribution, we propose a systematic procedure for the proposed DTCTED scheme including screening the best thermally coupled configuration, conceptual design, proposing an energysaving process, MOGA optimization and exploration of energysaving as illustrated in Fig. 3. Several alternative DTCTED sequences are firstly explored based on thermodynamic and chemical engineering principles to determine the best suitable sequence to
3
separate of B/T/CH azeotrope system. Next, the separation feasibility of extractive distillation is analyzed via the residue curve maps (i.e., RCMs). After that, optimization of the proposed design is performed by using the MOGA with TAC and CO2 emissions as objective function to obtain the optimal parameters (e.g., feed locations and reflux ratio). Finally, exergy loss and relative volatility are introduced to explore the essence of the energy-saving in the proposed scheme. 3.1. Determination of the best thermally coupled configuration For the separation of ternary azeotrope system, Timoshenko et al. [24] reported four available STCTED separation sequences (i.e., single-thermally coupled configurations IeIV) with the first column C1 being extractive distillation column. Four separation sequences in Fig. 4aed are determined within the following assumptions: the boiling temperature of three components A, B, and C is assumed as TA < TB < TC; a heavy entrainer E is determined (i.e., TE > TC > TB > TA). The withdraw stages of sidestream is located below the extraction section in the column C1 (Fig. 4a). The component B is obtained in the distillate of the column C2. The column C3 is regeneration column while the high purity entrainer is achieved at the bottom stream and then recycled back to column C1. Similarly observation can be attained from Fig. 4bed. Fig. 5 represents two double-thermally coupled configurations IeII with one column and two rectifiers [24]. The four configurations with single-thermally coupling of Fig. 4aed correspond to the two separation sequences with double-thermally coupled of Fig. 5a and b, respectively. Two vapor sidestreams are withdrawn from the stages below extractive section in the column C1 as illustrated in Fig. 5a. Three products (i.e., A, B, and C) are obtained in the distillate stream of columns C1, C2, and C3, respectively. Similarly observation can be got for the sidestream in Fig. 5b. 3.2. Thermodynamic insights via RCMs The class 1.0-1a of Serafimov's classification represents binary azeotropic system (i.e., A and B) using a high boiling-point solvent
Fig. 1. Conventional triple-column extractive distillation for separating ternary system B/T/CH.
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Fig. 2. Single-thermally coupled ternary extractive distillation for the separation of ternary system B/T/CH.
Fig. 3. Proposed optimal design approach for the double-thermal coupled ternary extractive distillation.
(denoted as E) [33,34]. The general feasibility criterion for the separation of non-ideal mixtures incorporating with the RCMs and isovolatility lines is proposed in our previous studies [35,36] as in Fig. 6. The isovolatility curve (i.e., aAB ¼ 1) divides the ternary diagram into two regions, BAE in the upper region and ABE (i.e., feasibility region) in the lower region. Composition of the distillate stream is a binary azeotropic mixture and then moves along with the line of aAB ¼ 1 reaching the point of xP when the entrainer gradually enters the column. The A-B azeotropic could be broke and the desired product A could be obtained when the flow rate of entrainer achieves a minimum requirement. Finally, the flow rate of entrainer in the ED scheme is optimized in the optimization process.
3.3. Process optimization based on the MOGA approach To the best of our knowledge, it is tough to achieve the optimization of the thermally coupled design (see Fig. 5a) via the sequence quadratic program approach because of coupling interaction between key variables. Therefore, the in-house MOGA software is used to optimize the proposed intensified DTCTED scheme. The optimization process is carried out with TAC and CO2 emissions as objective functions by varying four discrete variables (e.g., feed locations) and seven continuous variables (e.g., reflux ratio) under specified purities to attain the optimal design parameters. To ensure a fair comparison, the feed conditions, product specifications, and total theoretical trays of C1-C3 are kept consistent with those in the existing STCTED process. Optimization process of the proposed thermally coupled
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Fig. 4. Flowsheets of four single-thermally ternary coupled configurations I-IV
Fig. 5. Flowsheets of two double-thermally coupled ternary extractive distillation configurations I-II
configuration using MOGA (improved NSGA-II model) is illustrated in Fig. 7 including two sub-algorithms, optimization module and simulation module. The optimization module is implemented via the in-house MOGA software while the simulation module is carried out with Aspen Plus.
main design parameters (e.g., withdraw stages of sidestream, reflux ratio, feed locations and solvent flow rates).
3.3.1. Objective functions In this contribution, TAC and CO2 emissions (see Eq. (1)) are introduced as objective functions to assess economics and environmental impact of the proposed process and it is sensitive to the
f
min TAC min CO2 emissions
(1)
The economic indicator TAC [37] involves energy and capital investments, which is calculated as follow,
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Fig. 6. Extractive distillation scheme and thermodynamic features of class 1.0-1a at 1 atm.
Fig. 7. Flowsheet of the multi-objective genetic algorithm optimization approach.
TAC ¼
CI þ EI payback period
(2)
where the CI represents capital investment of reboiler, condenser, cooler, column trays and shells; the EI illustrates energy investment of the cooling water and steam. Moreover, the CI of pipes, valves and pumps is frequently ignored because it is much smaller compared with the CI of columns and heat exchangers. Detailed
parameters (e.g., equipment sizing) could be referred in our previous work [2,38,39]. CO2 emissions could be introduced as an economy index to assess the environment impact and sustainability of the investigated processes [40e47]. However, CO2 emission in the chemical engineering process is a complex problem because the energy consumption of the reboiler (i.e., steam) could be produced from heavy fuel oil, nature gas and coal [48]. In order to simplify the
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Fig. 8. Best selected double-thermally coupled ternary extractive distillation configuration with one extractive distillation and two rectifiers for the studied system in this work.
calculation, a simple model for the computation of CO2 emissions for the distillation system is proposed by Gadalla et al. [49], which is illustrated as follow,
ðCO2 Þemissions ¼ ð
Qfuel C% Þa Þð 100 NHV
(3)
where a ¼ 3.67 is the molar mass ratio of CO2 and C, the net heating value is denoted as NHV (39771 kJ/kg), and the carbon content C% is 86.5 kg/kg. In addition, the heat requirement of the fuel (Qfuel) could be calculated as follows,
Qfuel ¼
Qseq
lseq
ðhseq e419Þ ð
TF eT0 Þ TF eTS
(4)
where latent heat in kJ/kg and enthalpy in kJ of the hot utility are expressed as lseq and hseq; Qseq in kJ illustrates the energy consumption of the design process. Furthermore, 298.15K is determined as reference temperature T0, TF ¼ 2073.15K is the flame temperature, TS ¼ 433.15K is the stack temperature. 3.3.2. Constraints The design specifications (i.e., products purities) are defined in Eqs. (5)e(8).
locations (i.e., NF1 and NF2) and withdraw stages of sidestream V2 and V3 (NV2 and NV3) are defined in Eqs. (9)e(12). Besides, lower and upper bounds of nine continuous variables of reflux ratio (R1), three distillate rates (D1, D2 and D3), the vapor flow rate of sidestream (V2 and V3), and entrainer flow rate (FE) are defined in Eqs. (13)e(19). max Nmin F1 NF1 N F1
(9a,b)
max Nmin FE NFE N FE
(10a,b)
max Nmin V2 NV2 N V2
(11a,b)
max Nmin V3 NV3 N V3
(12a,b)
R1 Rmax Rmin 1 1
(13a,b)
max F min V2 FV2 F V2
(14a,b)
max F min V3 FV3 F V3
(15a,b)
xCH D1 99:00 mol%
(5)
FE F max F min E E
(16a,b)
xBD2 99:00 mol%
(6)
D1 Dmax Dmin 1 1
(17a,b)
xTD3 99:40 mol%
(7)
D2 Dmax Dmin 2 2
(18a,b)
xDMF B1 99:95 mol%
(8)
D3 Dmax Dmin 3 3
(19a,b)
where, the desired purities of toluene and benzene in the distillate of C2 and C3 are denoted as xBD2 ¼ 99.00 mol% and xTD3 ¼ 99.40 mol %, respectively; xCH D1 represents the purity of cyclohexane in the distillate of extractive distillation column C1 and the expected cyclohexane purity is 99.00 mol%; xDMF B1 ¼ 99.95 mol% is the specified purity of bottom stream (i.e., DMF) in the column C1. 3.3.3. Variable bounds Lower and upper bounds of four discrete variables of feed
3.4. Exergy and relative volatility evaluations The essence of the energy-saving and emission reduction could be explored by energy efficiency [50e52] and relative volatility [35]. The exergy loss (El) of a given system is defined as Eq. (20), which shows the difference between total input and output of exergy [53].
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El ¼
X
Exinput
X
Exoutput
Srcm,E. The lower region of ternary diagram (i.e., CH-B-DMF) is a feasible region because it satisfies two conditions of the general feasibility criterion under infinite reflux: CH is connected to DMF via a residue curve following decreasing temperature direction from DMF to CH (condition 1) and CH is the most volatile one in this region (condition 2). Following that, fresh feed and entrainer is mixed as a mixture at M1 point. CH (cyclohexane) and DMF/B mixtures will be obtained at the distillate and bottom, respectively, based on the lever rules.
(20)
Ex is illustrated as exergy in Eq. (21), which could be computed through the enthalpy (H) and entropy (S) [54e56]:
Ex ¼ ðH H0 Þ T0 ,ðS S0 Þ
(21)
where (HH0) and (SS0) represent enthalpy and entropy difference between the system and the reference state and temperature of the reference is denoted as T0 (298.15 K). The ratio of the distribution coefficient of components i and j is defined as the relative volatility in Eq. (22) [3].
.
.
aij ¼ ðyi =xi Þ ðyj xj Þ
4.3. Results of optimization The key design variables are eventually obtained via the inhouse MOGA software using TAC and CO2 emissions as objective functions. In the optimization process, the proposed global optimization method, MOGA, is carried out and the calculation is implemented on the personal computer with Intel Core i5-2450M [email protected], 6 GB memory. Parameters of the MOGA used in the optimization process are listed in Table A1. The lower and upper bounds of the design parameters are summarized in Table A2 while those parameters could be obtained via the sensitivity analysis. In addition, calculation of the total optimization process takes about 132 h while Pareto-optimal front for the proposed DTCED is illustrated in Fig. 10. Pareto-optimal front of the proposed DTCED scheme for a population of 300 and generation of 190 is shown in Fig. 10 while the Pareto front is obtained after 190 generations and the vector of decision variables do not produce any meaningful improvement. Pareto fronts between CO2 emissions and TAC of 190 generations are demonstrated in Fig. 11. Three solutions of interest studied designs (sol 1-the most environmentally protective design, sol 2-the design considered optimal and sol 3-the most expensive design) are listed in Table 3. The sol 1 is the most environmentally protective design with 994.158 103 $ of TAC and 1122.909 kg/h of CO2 emissions and sol 3 is the most expensive design with 994.174 103 $ of TAC and 1122.879 kg/h of CO2 emissions indicating that environment and economy are mutually restrictive relations. Following the suggestion of Alcocer-García et al. [27], sol 2 is the design considered optimal. Optimal flowsheet of the proposed intensified DTCTED configuration for separating ternary mixtures B/CH/T is illustrated in Fig. 12. The total numbers of stages of C1-C3 (i.e., NT1 ¼ 80, NT2 ¼ 20 and NT3 ¼ 20) are kept consistent with those in the existing improved process STCTED to ensure a fair comparison. The optimized withdraw location of vapor streams V2 and V3 are 53rd and 68th stages, respectively. Besides, fresh feed and entrainer are fed at 44th and 24th stages in the middle and top sections of C1, respectively. The optimized R1 and flow rates of entrainer (FE) of the column C1 are 3.4865 and 112.6072 kmol/h, respectively. Furthermore, an entrainer makeup (i.e., 0.3560 kmol/h) is added to compensate the solvent losses along with three product streams.
(22)
4. Computational results and discussion 4.1. Best thermally coupled configuration For the existing STCTED process, the entrainer and fresh feed (i.e., B/CH/T mixtures) are fed at the top and middle section of the column C1, whilst the withdraw stages of the sidestream is located at the bottom of the extractive section (see Fig. 2). This separation sequence belongs to the single-thermally coupled I as illustrated in Fig. 4a. In addition, the operating pressure of the extractive distillation column must greater than other two columns to avoid using compressor. 'Finally, double-thermally coupled I configuration of Fig. 5a (i.e., Fig. 8) is determined as the best configuration to separate the B/CH/T mixtures using DMF as the entrainer. 4.2. Thermodynamic insights and RCMs Following the suggestions of Luyben [23], the NRTL model is determined to calculate the VLE of B/T/CH utilizing an entrainer DMF. Meanwhile, the built-in binary interaction parameters of the NRTL model are given in Table 1. Topologic features of B/T/CH/DMF mixture are summarized in Table 2. The boiling points of components B, T, CH and DMF are 80.13, 110.68, 80.78 and 151.77 C, respectively. In addition, there is a B/CH azeotrope (xB ¼ 55.01 mol%), a CH/DMF azeotrope (xCH ¼ 99.93 mol%), and a T/CH azeotrope (i.e., xT ¼ 99.58 mol%). From Table 2, the azeotrope point B/CH is an unstable node because it has a minimal boiling point among these components; likewise, another two azeotrope points along pure component with B, CH and T behave as saddle nodes. Meanwhile, the solvent DMF is a stable node. Herein, a useful tool, Distillation Synthesis, in the Aspen Plus, is used to explore the thermodynamic feasibility of ternary system. Fig. 9 illustrates the RCMs for B/CH/DMF system. As demonstrated in Fig. 9, the RCMs are changed from point USrcm,azeo,CH-B to the Table 1 Binary interaction parameters of NRTL for B/T/CH using DMF as an entrainer. Component i
B
B
CH
B
CH
T
Component j
CH
T
T
DMF
DMF
DMF
Temperature units
aij aji bij bji cij
0.0000 0.0000 182.7550 43.3406 0.3000
C
C
2.8852 2.1911 1123.9500 863.7310 0.3000
C
0.1776 0.3062 102.2200 171.9450 0.3000
C
2.1557 2.1489 300.1090 544.7530 0.3000
C
0.0000 0.0000 639.6760 253.5430 0.3000
C
9.2521 2.5549 2440.0700 542.7610 0.3000
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Table 2 Topologic and thermodynamic features of B/T/CH system. Pressure
Components
Sngular points
Temperature ( C)
Composition
1 atm
B T CH DMF BeCH CHeDMF TeCH
saddle stable node saddle stable node unstable node saddle saddle
80.13 100.68 80.78 151.77 77.54 80.78 110.68
e e e e xB ¼ 0.5501 xCH ¼ 0.9993 xT ¼ 0.9958
Azeotrope at 1 atm
Fig. 9. Ternary diagram analysis with component balance lines for extractive column C1. Fig. 11. Pareto front between CO2 emissions and TAC for the proposed DTCTED scheme.
Table 3 Design parameters and performance indexes for the DTCTED schemes.
Column C1
Column C2
Column C3
Product purities
Fig. 10. Pareto-optimal front of the proposed DTCED scheme for a population of 300 and generation of 190.
The obtained optimal vapor flow rates of sidestream V2 and V3 are 53.3589 and 53.4346 kmol/h, respectively. Simultaneously, the obtained optimized distillate rates D1, D2 and D3, of three columns are 30.0322, 30.1151 and 40.2067 kmol/h, respectively. Fig. 13 illustrates the liquid composition and temperature profiles in columns C1-C3. The CH composition (marked as red circle) increases drastically from 1st stage to 24th stage due to the azeotrope breaking and high purity of CH is achieved by the extractive section (Fig. 13a). The 99.00 mol % of CH is eventually obtained at 1st stage. The B and T compositions significantly change at 53rd and 68th stages because vapor flow rate of sidestream are removed at these trays in column C1. The temperature changes in the feed
Parameters
Sol 1
Sol 2
Sol 3
NT1 NF1 NFE D1 (kmol/h) FE(kmol/h) R1 NT2 NV2 D2 (kmol/h) V2 (kmol/h) NT3 NV3 V3 (kmol/h) D3 (kmol/h) Cyclohexane (mol%) Benzene (mol%) Toluene (mol%) DMF (mol%) TAC (103 $) CO2 emissions (kg/h)
80 44 24 30.0322 112.6071 3.4865 20 53 30.1153 53.3607 20 68 53.4361 40.2082 99.00 99.00 99.40 99.95 994.158 1122.909
80 44 24 30.0322 112.6072 3.4865 20 53 30.1151 53.3589 20 68 53.4346 40.2067 99.00 99.00 99.40 99.95 994.169 1122.883
80 44 24 30.0322 112.6072 3.4865 20 53 30.1147 53.3589 20 68 53.4342 40.2065 99.00 99.00 99.40 99.95 994.174 1122.879
stages are not obvious indicating that the exergy loss is relatively small (see Fig. 13b). The above analysis demonstrates that the column C1 is well optimized. Similarly observation can be made for the temperature and liquid composition profiles in columns C2 and C3 from parts cef of Fig. 13. 4.4. Comparative economic and CO2 emission evaluations In this work, the processes of existing conventional triplecolumn extractive distillation (CTCED), existing improved singlethermally coupled ternary extractive distillation (STCTED), and
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Fig. 12. Optimal double-thermally coupled ternary extractive distillation (i.e., sol 2) for separating azeotropic system B/T/CH.
Fig. 13. Liquid composition and temperature profiles in (a,b) column C1,(c,d) column C2 and (e,f) column C3.
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the proposed alternative double-thermally coupled ternary extractive distillation (DTCTED) are compared by introducing TAC and CO2 emissions. The existing improved process STCTED is taken as a base case in this work to evaluate the economic and environment impacts of proposed processes. The TAC and CO2 emissions of all separation sequences are calculated and the corresponding computational results of economics and environment are summarized in Table 4 and 5.0. The proposed DTCTED can achieve 18.60% of TAC savings compared with the existing improved process STCTED. This significant reduction mainly contributes to the combination of C2 and C3 with C1 improving the energy efficiency, reducing the total reboiler duty and saving capital investment. The comparisons among two proposed alternative configurations considering CO2 emissions are shown in Table 5. The CO2 emissions are reduced by 20.22% in case of intensified DTCTED scheme as compared to existing STCTED scheme for separating ternary azeptropic system B/CH/T. To explore the essence of energy saving, the relative volatility of B vs CH (aB/CH) of STCTED and DTCTED processes through the extractive distillation is illustrated in Fig. 14. While the aCH/B in extraction section of DTCTED is higher than that in extraction section of STCTED process contributing enough separation driving force. Furthermore, in stripping section, the aCH/B in the intensified scheme is also higher than that in the existing STCTED configuration. As such, the separation of B/CH/T ternary matures in the DTCTED is much easier than that of the existing improved process STCTED resulting in much more reduction in energy consumption of the proposed scheme. The exergy loss profiles for the proposed intensified DTCTED scheme and the existing STCTED process are illustrated in Fig. 15. The exergy loss profiles for both ED processes demonstrate that high exergy loss (see large peaks in Fig. 15) often happens at feed or withdraw stages. For example, a high exergy loss in the extractive column C1 of the intensified DTCTED configuration (see Fig. 15a) takes place at 24th, 44th, 53rd and 68th stages where fresh feed, entrainer, vapor sidestreams V2 and V3 are respectively fed to (or withdrawn from) the extractive column C1. The total exergy losses in STCTED and DTCTED processes are summarized in Table 6. Total exergy loss of the proposed intensified DTCTED scheme can reduce 9.73% than the existing STCTED process indicating that the intensified scheme is more reasonable. In summary, the proposed intensified DTCTED scheme have not only higher energy
11
Table 5 Results of the CO2 emissions of three configurations.
QR1 (MW) QR2 (MW) QR3 (MW) Qtotal (MW) CO2 emissions (kg/h) CO2 emissions saving (%)
CTCED [23]
STCTED [23]
Proposed DTCTED
1.731 1.820 0.6785 4.2295 1639.070 ¡16.45
2.789 / 0.843 3.6320 1407.520 0.00
2.8744 / / 2.8980 1122.883 20.22
Fig. 14. Relative volatility of B vs CH in the extractive distillation column through the existing STCTED and the proposed DTCTED processes.
efficiency but also superior than other processes from the view of economic and environmental aspects. 5. Conclusions An intensified double-thermally coupled ternary extractive distillation (DTCTED) scheme for the separation of benzene/ toluene/cyclohexane homogeneous ternary system with three azeotropes is explored to achieve energy-saving and emission-
Table 4 Comparison of optimal design between the existing CTCED and STCTED processes and the proposed DTCTED process.
C1
C2
C3
parameters
CTCED [23]
STCTED [23]
Proposed DTCTED
QR1 (MW) QC1 (MW) ID (m) NT1 QR2 (MW) QC2 (MW) ID (m) NT2 QR3 (MW) QC3 (MW) ID (m) NT3
1.731 1.251 1.190 80 1.820 1.608 1.080 41 0.6785 1.137 1.764 21
2.789 1.124 1.570 80 / 0.837 1.010 20 0.843 1.126 1.720 21
2.898 1.138 1.567 80 / 0.467 0.809 20 / 0.671 1.282 20
Qcooler (MW) Qtotal,reb (MW) Energy saving (%) Total capital cost (103 $/y) Total operating cost (103 $) TAC(103 $) TAC saving (%)
0.199 4.2295 16.45 2388.200 531.30 1327.400 ¡8.68
0.199 3.6320 0.00 2274.600 463.200 1221.40 0.00
0.517 2.8980 20.21 2009.403 324.369 994.169 18.60
12
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Fig. 15. Exergy loss profiles in (a) column C1, (b) column C2 and (c) column C3 of the existing STCTED and the proposed DTCTED processes.
Acknowledgments
Table 6 Results of the exergy loss of three configurations.
Total exergy loss (kW) Exergy loss saving (%)
CTCED
STCTED
DTCTED
349.89 2.80
360.00 0.00
324.97 9.73
reduction in this research. The optimal Pareto front of the proposed DTCTED process is obtained by using the multi-objective genetic algorithm. The results demonstrated that the CO2 emissions and total annual cost of the intensified DTCTED scheme are significantly reduced by 20.22% and 18.60%, compared with the existing optimized single-thermally coupled extractive distillation process. From the observation of exergy loss and relative volatility, the essence of energy-saving of the proposed DTCED scheme is enhancing the relative volatility of B vs CH (aB/CH) and reducing exergy loss of the proposed scheme. The proposed intensified configuration and systematic method are also applicable for the separation of other ternary homogeneous multi-component system with two (or more) azeotropes based on the proposed energy-saving double-thermal coupled extractive distillation such as tetrahydrofuran/methanol/water, acetonitrile/methanol/benzene, ethyl acetate/ethanol/water, and tetrahydrofuran/ethanol/water mixtures. It may be also applicable to separate heterogeneous mixtures such as methanol/toluene/ water and ethanol/toluene/water system. Of note is that the proposed energy-saving scheme and method may not be extended to the reaction systems.
We acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 21606026, 21878028); the Fundamental Research Funds for the Central Universities (Nos. 2019CDQYHG021, 2019CDXYHG0012); the Chongqing Innovation Support Program for Returned Overseas Chinese Scholars (No. CX2018048). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.07.086. References [1] Yang A, Shi T, Sun S, Wei SA, Shen W, Ren J. Dynamic controllability investigation of an energy-saving double side-stream ternary extractive distillation process. Separ Purif Technol 2019;225:41e53. https://doi.org/10.1016/ j.seppur.2019.05.063. [2] Shi T, Yang A, Jin S, Shen W, Wei SA, Ren J. Comparative optimal design and control of two alternative approaches for separating heterogeneous mixtures isopropyl alcohol-isopropyl acetate-water with four azeotropes. Separ Purif Technol 2019;225:1e17. https://doi.org/10.1016/j.seppur.2019.05.061. [3] Liang S, Cao Y, Liu X, Li X, Zhao Y, Wang Y, et al. Insight into pressure-swing distillation from azeotropic phenomenon to dynamic control. Chem Eng Res Des 2017;117:318e35. https://doi.org/10.1016/j.cherd.2016.10.040. [4] Doherty MF, Malone MF. Conceptual design of distillation systems. New York: McGraw-Hill; 2001. [5] Shen W, Dong L, Wei SA, Li J, Benyounes H, You X, et al. Systematic design of an extractive distillation for maximum-boiling azeotropes with heavy entrainers. AIChE J 2015;61(11):3898e910. https://doi.org/10.1002/aic.14908. [6] Shen WF, Benyounes H, Dong LC, Wei SA, Li J, Gerbaud V. Conceptual design of
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Investigation of an energy-saving double-thermally coupled extractive distillation for separating ternary system benzene/toluene/ cyclohexane Ao Yang a, Yang Su a, I-Lung Chien b, Saimeng Jin a, Chenglei Yan a, Shun'an Wei a, Weifeng Shen a, * a b
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, PR China Department of Chemical Engineering, National Taiwan University, Taipei, 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2019 Received in revised form 17 June 2019 Accepted 14 July 2019 Available online 18 July 2019
An intensified scheme for the separation of ternary azeotropic system is explored to reduce the energy consumption and recycle important organic solvents. Herein, a novel double-thermally coupled ternary extractive distillation (DTCTED) for separating azeotropic system benzene/toluene/cyclohexane (denoted as B/T/CH) is proposed to achieve energy-saving and emissions reduction. Thermodynamic feasible insights of the B/T/CH using dimethyl formamide as entrainer are firstly analyzed via residue curve maps to find separation constraints. Following that, the proposed intensified scheme is optimized via the inhouse multi-objective genetic algorithm software while using total annual cost and CO2 emissions as objective functions. The results show that the total annual cost and CO2 emissions of the proposed intensified DTCTED scheme are significantly reduced by 18.60% and 20.22% compared with the existing single-thermally coupled ternary extractive distillation process. Furthermore, exergy loss and relative volatility are introduced to explore the essence of energy-saving in the proposed DTCTED scheme. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Energy-saving Residue curve maps Ternary mixtures Extractive distillation Thermally coupled
1. Introduction The separation of azeotropic or close-boiling mixture is necessary for the recovery of valuable solvents from waste liquid from the perspective of sustainable development [1e3]. Extractive distillation (ED) as an effective distillation technique is frequently used to separate azeotropic or close-boiling mixtures by adding a third component (denoted as entrainer) to improve the relative volatility [4e10]. Doherty and Malone [4] reported the conceptual design approach of the ED sequence for separating binary azeotropic mixtures. Furthermore, a systematic method to design of the ED for the separation of azeotrope system using heavy entrainers is proposed by Shen et al. [5]. Following that, the conceptual design for the separation of non-ideal mixture using light entrainers is explored [6]. Alternative configurations for separating binary azeotropic mixtures methanol/dimethyl carbonate are studied [7]. To separate ternary azeotropic mixtures, some kinds of separation sequences are presented by Wang et al. [8]. A novel design method
* Corresponding author. E-mail address: [email protected] (W. Shen). https://doi.org/10.1016/j.energy.2019.07.086 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
for separating acetone/chloroform mixtures via the ED is reported [9]. An energy-saving triple-column extractive distillation process for separating ternary azeotropic mixture with multi-azeotrope are reported by Yang et al. [10]. Recently, development of the ED has been presented by two review papers [11,12]. However, the high quality energy input is required in the reboiler to execute the separation task since the ED has a low thermodynamic efficiency [13]. In addition, the increasing awareness of the energy crisis has made process intensification a promising trend for energy-saving strategies in the chemical and petroleum industries. Hence, an intensified separation technique partial thermally coupled ED (extractive dividing wall column, EDWC) with a significantly advantage than that of the conventional ED is proposed for separating non-ideal azeotrope mixtures [14e22]. An intensified EDWC scheme is proposed to obtain the high-purity of bioethanol [14]. The EDWC scheme for the separation of methylal/methanol binary mixtures with dimethyl formamide (DMF) as an entrainer is further reported [15], and they demonstrated that the intensified scheme had an 11.60% of total annual cost (TAC) savings compared with that of the conventional ED. Chien's group [16] reported the EWDC configuration for
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Nomenclature B CI CH CTCED C% DMF DTCTED E ED EDWC El EI Ex FE FV2 H hseq ID MOGA NF1
benzene capital investment [ 103 $] cyclohexane conventional triple-column ternary extractive distillation carbon content dimethyl formamide double-thermally coupled ternary extractive distillation entrainer extractive distillation extractive dividing wall column exergy loss [kW] energy investment [ 103 $] exergy [kW] flow rate of entrainer [kmol/h] flow rate of sidestream V2 [kmol/h] enthalpy [kW] enthalpy of the hot utility [kJ] internal diameter of column [m] multi-objective genetic algorithm feed locations of fresh feed
separating mixtures of acetone/methanol and reported a slight reduction in TAC (i.e., 1.79%) through the intensified sequence compared with the double-column ED sequence. Sun et al. [17] conducted the design of EWDC for the separation of benzene/ cyclohexane system, and they displayed that the optimized EWDC can save 22.00% in the reboiler duty, and meanwhile, energy investment and TAC are decreased by 1.80% and 4.80%, respectively. Ethanol dehydration is investigated through the EDWC scheme by Tututi-Avila [18], and they demonstrated that TAC of 12.42% could be reduced. The investigation of EDWC for separating ethyl acetate/ isopropyl alcohol system is reported by Zhang et al. [19]. In one of our recent studies [22], we proposed a novel decanter assisted EDWC for the separation of ternary mixture with multi-azeotrope and we demonstrated that the EDWC has a 15.14% reduction than that of the ED with an additional decanter. According to above review, the intensified EDWC separation configurations can substantially reduce the capital cost and energy cost in the separation of non-ideal azeotropic system. However, the energy input with high quality is needed for the reboiler of EDWC to achieve the separation task making the effect of energy-saving less remarkable [20]. All the energy-saving of EDWC which is attributed to the capital investment is reduced because the extractive distillation and entrainer recovery columns are integrated into a single shell. On the contrary, the reduction of operating cost (also denoted as energy cost) is tiny (or no) in EDWC schemes [16]. Consequently, some alternative thermally coupled extractive distillation sequences should be proposed to solve and overcome this problem. Following that, the column is operated at low pressure in the single-thermally coupled ternary extractive distillation (STCTED) to perform the separation task with low pressure steam to avoid utilizing high pressure energy inputs [23], which is based on the exploration of partially thermally coupled schemes to separate ternary azeotropic mixtures with different types of ternary diagram by Timoshenko et al. [24]. Moreover, two STCTED schemes for separating ternary tetrahydrofuran/ethanol/ water azeotropic mixture with low energy consumptions, CO2 emissions and TAC are explored by Zhao et al. [25]. However, it is
NV2 NHV QC Qfuel Qseq QR R1 RCMs S STCTED
withdraw stages of sidestream V2 net heating value [kJ/kg] heat duty of condenser [kW] heat requirement of the fuel [kJ] energy consumption of the design process [kJ] heat duty of reboiler [kW] reflux ratio residue curve maps entropy [kJ/kg.K] single-thermally coupled ternary extractive distillation T toluene T0 temperature of the reference [K] TAC total annual cost [ 103 $] TF flame temperature [K] Ti (i¼A, B, C, and E) boiling point temperature of component i (i ¼ A, B, C, and E) TS stack temperature [K] V2 vapor stream to column C2 V3 vapor stream to column C3 VLE vapor-liquid equilibrium a molar mass ratio of CO2 and C ai/j relative volatility of components i and j lseq latent heat [kJ/kg]
tough to achieve the optimization of the thermally coupled processes because of coupling interaction between key variables. As such, multi-objective approaches are proposed to optimize those complex systems [26e31]. A novel EDWC is designed using the multi-objective optimization technique with economic and environmental indicators as objective function [26]. Alcocer-García et al. [27] investigated a multi-objective genetic algorithm (MOGA) for the optimization of intensified processes to purify levulinic acid. Benzene, toluene, and cyclohexane (B/T/CH) could be used as important solvents in the chemical and electronics industries. In addition, T and CH could be employed as clean and sustainable working fluids in the organic Rankine cycle systems as reported by Yagli et al. [32]. Hence, the separation of these systems has significant application promising. The separation of this system is difficult and complex because it forms multiazeotropes [24]. In summary, valuable insights on conceptual design and multiobjective optimization on double-thermally coupled ternary extractive distillation (DTCTED) have not yet been reported. Hereby, a novel DTCTED configuration for the separation of azeotropic mixtures B/T/CH employing DMF as the entrainer is proposed in this study to achieve the performance of energy conservation and emission reduction. First, the best separation configuration is screened based on the thermodynamic principles, before the thermodynamic feasibility insight in the ED process (e.g., isovolatility line and residue curve maps) is carried out for the separation of system B/T/CH utilizing the entrainer DMF. Following that, Pareto-optimal front with key design parameters of the proposed process is obtained by using the in-house MOGA based on the minimum TAC and CO2 emissions. Eventually, exergy loss and relative volatility are introduced to explore the essence of the energy-saving and emission reduction. 2. Existing processes for separating B/T/CH system 2.1. Conventional triple-column extractive distillation Fig. 1 represents an existing CTCED for separating B/T/CH system
A. Yang et al. / Energy 186 (2019) 115756
using entrainer DMF [23]. C1 is the extractive distillation column with 80 stages; C2 represents the conventional distillation column with 41 stages; C3 denotes the regeneration column with 21 stages. Three columns are operated at 0.07, 0.11 and 0.012 MPa, respectively. The fresh feed is azeotropic mixtures of B, T and CH (i.e., 100 kmol/h with 30 mol% B, 30 mol% CH and 40 mol% T) feeding at 40th stage, and the entrainer stream is fed at 20th stage. The high purity product cyclohexane (99.00 mol%) is obtained at the overhead of column C1. Feed locations of C2 and C3 are 20th and 15th, respectively. The purity products of B and T (99.00 mol% and 99.22 mol%) are attained at the overhead of C2 and C3, respectively. 2.2. Single-thermally coupled ternary extractive distillation To reduce the reboiler duty and TAC of the CTCED process, an existing improved STCTED process for separating B/T/CH azeotropic system is proposed by Luyben [23] and demonstrated in Fig. 2 where only two reboilers are needed compared to three reboilers in the existing CTCED scheme in Fig. 1. Total theoretical trays of C1C3 are 80, 20 and 21, respectively and 0.08, 0.04 and 0.012 MPa are determined for three columns. Stage of the withdraw vapor stream is 60 while the feed locations of entrainer and fresh feed are 20th and 40th, respectively. The second column C2 is combined into the first column C1, which is applied for cyclohexane production with 99.00 mol %. The third column is used to recover entrainer. The purity of DMF reaches no less than 99.95 mol % at the bottom stream of column C3. Then, the high purity DMF and make up entrainer is mixed to the column C1. 3. Methodology In this contribution, we propose a systematic procedure for the proposed DTCTED scheme including screening the best thermally coupled configuration, conceptual design, proposing an energysaving process, MOGA optimization and exploration of energysaving as illustrated in Fig. 3. Several alternative DTCTED sequences are firstly explored based on thermodynamic and chemical engineering principles to determine the best suitable sequence to
3
separate of B/T/CH azeotrope system. Next, the separation feasibility of extractive distillation is analyzed via the residue curve maps (i.e., RCMs). After that, optimization of the proposed design is performed by using the MOGA with TAC and CO2 emissions as objective function to obtain the optimal parameters (e.g., feed locations and reflux ratio). Finally, exergy loss and relative volatility are introduced to explore the essence of the energy-saving in the proposed scheme. 3.1. Determination of the best thermally coupled configuration For the separation of ternary azeotrope system, Timoshenko et al. [24] reported four available STCTED separation sequences (i.e., single-thermally coupled configurations IeIV) with the first column C1 being extractive distillation column. Four separation sequences in Fig. 4aed are determined within the following assumptions: the boiling temperature of three components A, B, and C is assumed as TA < TB < TC; a heavy entrainer E is determined (i.e., TE > TC > TB > TA). The withdraw stages of sidestream is located below the extraction section in the column C1 (Fig. 4a). The component B is obtained in the distillate of the column C2. The column C3 is regeneration column while the high purity entrainer is achieved at the bottom stream and then recycled back to column C1. Similarly observation can be attained from Fig. 4bed. Fig. 5 represents two double-thermally coupled configurations IeII with one column and two rectifiers [24]. The four configurations with single-thermally coupling of Fig. 4aed correspond to the two separation sequences with double-thermally coupled of Fig. 5a and b, respectively. Two vapor sidestreams are withdrawn from the stages below extractive section in the column C1 as illustrated in Fig. 5a. Three products (i.e., A, B, and C) are obtained in the distillate stream of columns C1, C2, and C3, respectively. Similarly observation can be got for the sidestream in Fig. 5b. 3.2. Thermodynamic insights via RCMs The class 1.0-1a of Serafimov's classification represents binary azeotropic system (i.e., A and B) using a high boiling-point solvent
Fig. 1. Conventional triple-column extractive distillation for separating ternary system B/T/CH.
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Fig. 2. Single-thermally coupled ternary extractive distillation for the separation of ternary system B/T/CH.
Fig. 3. Proposed optimal design approach for the double-thermal coupled ternary extractive distillation.
(denoted as E) [33,34]. The general feasibility criterion for the separation of non-ideal mixtures incorporating with the RCMs and isovolatility lines is proposed in our previous studies [35,36] as in Fig. 6. The isovolatility curve (i.e., aAB ¼ 1) divides the ternary diagram into two regions, BAE in the upper region and ABE (i.e., feasibility region) in the lower region. Composition of the distillate stream is a binary azeotropic mixture and then moves along with the line of aAB ¼ 1 reaching the point of xP when the entrainer gradually enters the column. The A-B azeotropic could be broke and the desired product A could be obtained when the flow rate of entrainer achieves a minimum requirement. Finally, the flow rate of entrainer in the ED scheme is optimized in the optimization process.
3.3. Process optimization based on the MOGA approach To the best of our knowledge, it is tough to achieve the optimization of the thermally coupled design (see Fig. 5a) via the sequence quadratic program approach because of coupling interaction between key variables. Therefore, the in-house MOGA software is used to optimize the proposed intensified DTCTED scheme. The optimization process is carried out with TAC and CO2 emissions as objective functions by varying four discrete variables (e.g., feed locations) and seven continuous variables (e.g., reflux ratio) under specified purities to attain the optimal design parameters. To ensure a fair comparison, the feed conditions, product specifications, and total theoretical trays of C1-C3 are kept consistent with those in the existing STCTED process. Optimization process of the proposed thermally coupled
A. Yang et al. / Energy 186 (2019) 115756
5
Fig. 4. Flowsheets of four single-thermally ternary coupled configurations I-IV
Fig. 5. Flowsheets of two double-thermally coupled ternary extractive distillation configurations I-II
configuration using MOGA (improved NSGA-II model) is illustrated in Fig. 7 including two sub-algorithms, optimization module and simulation module. The optimization module is implemented via the in-house MOGA software while the simulation module is carried out with Aspen Plus.
main design parameters (e.g., withdraw stages of sidestream, reflux ratio, feed locations and solvent flow rates).
3.3.1. Objective functions In this contribution, TAC and CO2 emissions (see Eq. (1)) are introduced as objective functions to assess economics and environmental impact of the proposed process and it is sensitive to the
f
min TAC min CO2 emissions
(1)
The economic indicator TAC [37] involves energy and capital investments, which is calculated as follow,
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Fig. 6. Extractive distillation scheme and thermodynamic features of class 1.0-1a at 1 atm.
Fig. 7. Flowsheet of the multi-objective genetic algorithm optimization approach.
TAC ¼
CI þ EI payback period
(2)
where the CI represents capital investment of reboiler, condenser, cooler, column trays and shells; the EI illustrates energy investment of the cooling water and steam. Moreover, the CI of pipes, valves and pumps is frequently ignored because it is much smaller compared with the CI of columns and heat exchangers. Detailed
parameters (e.g., equipment sizing) could be referred in our previous work [2,38,39]. CO2 emissions could be introduced as an economy index to assess the environment impact and sustainability of the investigated processes [40e47]. However, CO2 emission in the chemical engineering process is a complex problem because the energy consumption of the reboiler (i.e., steam) could be produced from heavy fuel oil, nature gas and coal [48]. In order to simplify the
A. Yang et al. / Energy 186 (2019) 115756
7
Fig. 8. Best selected double-thermally coupled ternary extractive distillation configuration with one extractive distillation and two rectifiers for the studied system in this work.
calculation, a simple model for the computation of CO2 emissions for the distillation system is proposed by Gadalla et al. [49], which is illustrated as follow,
ðCO2 Þemissions ¼ ð
Qfuel C% Þa Þð 100 NHV
(3)
where a ¼ 3.67 is the molar mass ratio of CO2 and C, the net heating value is denoted as NHV (39771 kJ/kg), and the carbon content C% is 86.5 kg/kg. In addition, the heat requirement of the fuel (Qfuel) could be calculated as follows,
Qfuel ¼
Qseq
lseq
ðhseq e419Þ ð
TF eT0 Þ TF eTS
(4)
where latent heat in kJ/kg and enthalpy in kJ of the hot utility are expressed as lseq and hseq; Qseq in kJ illustrates the energy consumption of the design process. Furthermore, 298.15K is determined as reference temperature T0, TF ¼ 2073.15K is the flame temperature, TS ¼ 433.15K is the stack temperature. 3.3.2. Constraints The design specifications (i.e., products purities) are defined in Eqs. (5)e(8).
locations (i.e., NF1 and NF2) and withdraw stages of sidestream V2 and V3 (NV2 and NV3) are defined in Eqs. (9)e(12). Besides, lower and upper bounds of nine continuous variables of reflux ratio (R1), three distillate rates (D1, D2 and D3), the vapor flow rate of sidestream (V2 and V3), and entrainer flow rate (FE) are defined in Eqs. (13)e(19). max Nmin F1 NF1 N F1
(9a,b)
max Nmin FE NFE N FE
(10a,b)
max Nmin V2 NV2 N V2
(11a,b)
max Nmin V3 NV3 N V3
(12a,b)
R1 Rmax Rmin 1 1
(13a,b)
max F min V2 FV2 F V2
(14a,b)
max F min V3 FV3 F V3
(15a,b)
xCH D1 99:00 mol%
(5)
FE F max F min E E
(16a,b)
xBD2 99:00 mol%
(6)
D1 Dmax Dmin 1 1
(17a,b)
xTD3 99:40 mol%
(7)
D2 Dmax Dmin 2 2
(18a,b)
xDMF B1 99:95 mol%
(8)
D3 Dmax Dmin 3 3
(19a,b)
where, the desired purities of toluene and benzene in the distillate of C2 and C3 are denoted as xBD2 ¼ 99.00 mol% and xTD3 ¼ 99.40 mol %, respectively; xCH D1 represents the purity of cyclohexane in the distillate of extractive distillation column C1 and the expected cyclohexane purity is 99.00 mol%; xDMF B1 ¼ 99.95 mol% is the specified purity of bottom stream (i.e., DMF) in the column C1. 3.3.3. Variable bounds Lower and upper bounds of four discrete variables of feed
3.4. Exergy and relative volatility evaluations The essence of the energy-saving and emission reduction could be explored by energy efficiency [50e52] and relative volatility [35]. The exergy loss (El) of a given system is defined as Eq. (20), which shows the difference between total input and output of exergy [53].
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A. Yang et al. / Energy 186 (2019) 115756
El ¼
X
Exinput
X
Exoutput
Srcm,E. The lower region of ternary diagram (i.e., CH-B-DMF) is a feasible region because it satisfies two conditions of the general feasibility criterion under infinite reflux: CH is connected to DMF via a residue curve following decreasing temperature direction from DMF to CH (condition 1) and CH is the most volatile one in this region (condition 2). Following that, fresh feed and entrainer is mixed as a mixture at M1 point. CH (cyclohexane) and DMF/B mixtures will be obtained at the distillate and bottom, respectively, based on the lever rules.
(20)
Ex is illustrated as exergy in Eq. (21), which could be computed through the enthalpy (H) and entropy (S) [54e56]:
Ex ¼ ðH H0 Þ T0 ,ðS S0 Þ
(21)
where (HH0) and (SS0) represent enthalpy and entropy difference between the system and the reference state and temperature of the reference is denoted as T0 (298.15 K). The ratio of the distribution coefficient of components i and j is defined as the relative volatility in Eq. (22) [3].
.
.
aij ¼ ðyi =xi Þ ðyj xj Þ
4.3. Results of optimization The key design variables are eventually obtained via the inhouse MOGA software using TAC and CO2 emissions as objective functions. In the optimization process, the proposed global optimization method, MOGA, is carried out and the calculation is implemented on the personal computer with Intel Core i5-2450M [email protected], 6 GB memory. Parameters of the MOGA used in the optimization process are listed in Table A1. The lower and upper bounds of the design parameters are summarized in Table A2 while those parameters could be obtained via the sensitivity analysis. In addition, calculation of the total optimization process takes about 132 h while Pareto-optimal front for the proposed DTCED is illustrated in Fig. 10. Pareto-optimal front of the proposed DTCED scheme for a population of 300 and generation of 190 is shown in Fig. 10 while the Pareto front is obtained after 190 generations and the vector of decision variables do not produce any meaningful improvement. Pareto fronts between CO2 emissions and TAC of 190 generations are demonstrated in Fig. 11. Three solutions of interest studied designs (sol 1-the most environmentally protective design, sol 2-the design considered optimal and sol 3-the most expensive design) are listed in Table 3. The sol 1 is the most environmentally protective design with 994.158 103 $ of TAC and 1122.909 kg/h of CO2 emissions and sol 3 is the most expensive design with 994.174 103 $ of TAC and 1122.879 kg/h of CO2 emissions indicating that environment and economy are mutually restrictive relations. Following the suggestion of Alcocer-García et al. [27], sol 2 is the design considered optimal. Optimal flowsheet of the proposed intensified DTCTED configuration for separating ternary mixtures B/CH/T is illustrated in Fig. 12. The total numbers of stages of C1-C3 (i.e., NT1 ¼ 80, NT2 ¼ 20 and NT3 ¼ 20) are kept consistent with those in the existing improved process STCTED to ensure a fair comparison. The optimized withdraw location of vapor streams V2 and V3 are 53rd and 68th stages, respectively. Besides, fresh feed and entrainer are fed at 44th and 24th stages in the middle and top sections of C1, respectively. The optimized R1 and flow rates of entrainer (FE) of the column C1 are 3.4865 and 112.6072 kmol/h, respectively. Furthermore, an entrainer makeup (i.e., 0.3560 kmol/h) is added to compensate the solvent losses along with three product streams.
(22)
4. Computational results and discussion 4.1. Best thermally coupled configuration For the existing STCTED process, the entrainer and fresh feed (i.e., B/CH/T mixtures) are fed at the top and middle section of the column C1, whilst the withdraw stages of the sidestream is located at the bottom of the extractive section (see Fig. 2). This separation sequence belongs to the single-thermally coupled I as illustrated in Fig. 4a. In addition, the operating pressure of the extractive distillation column must greater than other two columns to avoid using compressor. 'Finally, double-thermally coupled I configuration of Fig. 5a (i.e., Fig. 8) is determined as the best configuration to separate the B/CH/T mixtures using DMF as the entrainer. 4.2. Thermodynamic insights and RCMs Following the suggestions of Luyben [23], the NRTL model is determined to calculate the VLE of B/T/CH utilizing an entrainer DMF. Meanwhile, the built-in binary interaction parameters of the NRTL model are given in Table 1. Topologic features of B/T/CH/DMF mixture are summarized in Table 2. The boiling points of components B, T, CH and DMF are 80.13, 110.68, 80.78 and 151.77 C, respectively. In addition, there is a B/CH azeotrope (xB ¼ 55.01 mol%), a CH/DMF azeotrope (xCH ¼ 99.93 mol%), and a T/CH azeotrope (i.e., xT ¼ 99.58 mol%). From Table 2, the azeotrope point B/CH is an unstable node because it has a minimal boiling point among these components; likewise, another two azeotrope points along pure component with B, CH and T behave as saddle nodes. Meanwhile, the solvent DMF is a stable node. Herein, a useful tool, Distillation Synthesis, in the Aspen Plus, is used to explore the thermodynamic feasibility of ternary system. Fig. 9 illustrates the RCMs for B/CH/DMF system. As demonstrated in Fig. 9, the RCMs are changed from point USrcm,azeo,CH-B to the Table 1 Binary interaction parameters of NRTL for B/T/CH using DMF as an entrainer. Component i
B
B
CH
B
CH
T
Component j
CH
T
T
DMF
DMF
DMF
Temperature units
aij aji bij bji cij
0.0000 0.0000 182.7550 43.3406 0.3000
C
C
2.8852 2.1911 1123.9500 863.7310 0.3000
C
0.1776 0.3062 102.2200 171.9450 0.3000
C
2.1557 2.1489 300.1090 544.7530 0.3000
C
0.0000 0.0000 639.6760 253.5430 0.3000
C
9.2521 2.5549 2440.0700 542.7610 0.3000
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Table 2 Topologic and thermodynamic features of B/T/CH system. Pressure
Components
Sngular points
Temperature ( C)
Composition
1 atm
B T CH DMF BeCH CHeDMF TeCH
saddle stable node saddle stable node unstable node saddle saddle
80.13 100.68 80.78 151.77 77.54 80.78 110.68
e e e e xB ¼ 0.5501 xCH ¼ 0.9993 xT ¼ 0.9958
Azeotrope at 1 atm
Fig. 9. Ternary diagram analysis with component balance lines for extractive column C1. Fig. 11. Pareto front between CO2 emissions and TAC for the proposed DTCTED scheme.
Table 3 Design parameters and performance indexes for the DTCTED schemes.
Column C1
Column C2
Column C3
Product purities
Fig. 10. Pareto-optimal front of the proposed DTCED scheme for a population of 300 and generation of 190.
The obtained optimal vapor flow rates of sidestream V2 and V3 are 53.3589 and 53.4346 kmol/h, respectively. Simultaneously, the obtained optimized distillate rates D1, D2 and D3, of three columns are 30.0322, 30.1151 and 40.2067 kmol/h, respectively. Fig. 13 illustrates the liquid composition and temperature profiles in columns C1-C3. The CH composition (marked as red circle) increases drastically from 1st stage to 24th stage due to the azeotrope breaking and high purity of CH is achieved by the extractive section (Fig. 13a). The 99.00 mol % of CH is eventually obtained at 1st stage. The B and T compositions significantly change at 53rd and 68th stages because vapor flow rate of sidestream are removed at these trays in column C1. The temperature changes in the feed
Parameters
Sol 1
Sol 2
Sol 3
NT1 NF1 NFE D1 (kmol/h) FE(kmol/h) R1 NT2 NV2 D2 (kmol/h) V2 (kmol/h) NT3 NV3 V3 (kmol/h) D3 (kmol/h) Cyclohexane (mol%) Benzene (mol%) Toluene (mol%) DMF (mol%) TAC (103 $) CO2 emissions (kg/h)
80 44 24 30.0322 112.6071 3.4865 20 53 30.1153 53.3607 20 68 53.4361 40.2082 99.00 99.00 99.40 99.95 994.158 1122.909
80 44 24 30.0322 112.6072 3.4865 20 53 30.1151 53.3589 20 68 53.4346 40.2067 99.00 99.00 99.40 99.95 994.169 1122.883
80 44 24 30.0322 112.6072 3.4865 20 53 30.1147 53.3589 20 68 53.4342 40.2065 99.00 99.00 99.40 99.95 994.174 1122.879
stages are not obvious indicating that the exergy loss is relatively small (see Fig. 13b). The above analysis demonstrates that the column C1 is well optimized. Similarly observation can be made for the temperature and liquid composition profiles in columns C2 and C3 from parts cef of Fig. 13. 4.4. Comparative economic and CO2 emission evaluations In this work, the processes of existing conventional triplecolumn extractive distillation (CTCED), existing improved singlethermally coupled ternary extractive distillation (STCTED), and
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Fig. 12. Optimal double-thermally coupled ternary extractive distillation (i.e., sol 2) for separating azeotropic system B/T/CH.
Fig. 13. Liquid composition and temperature profiles in (a,b) column C1,(c,d) column C2 and (e,f) column C3.
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the proposed alternative double-thermally coupled ternary extractive distillation (DTCTED) are compared by introducing TAC and CO2 emissions. The existing improved process STCTED is taken as a base case in this work to evaluate the economic and environment impacts of proposed processes. The TAC and CO2 emissions of all separation sequences are calculated and the corresponding computational results of economics and environment are summarized in Table 4 and 5.0. The proposed DTCTED can achieve 18.60% of TAC savings compared with the existing improved process STCTED. This significant reduction mainly contributes to the combination of C2 and C3 with C1 improving the energy efficiency, reducing the total reboiler duty and saving capital investment. The comparisons among two proposed alternative configurations considering CO2 emissions are shown in Table 5. The CO2 emissions are reduced by 20.22% in case of intensified DTCTED scheme as compared to existing STCTED scheme for separating ternary azeptropic system B/CH/T. To explore the essence of energy saving, the relative volatility of B vs CH (aB/CH) of STCTED and DTCTED processes through the extractive distillation is illustrated in Fig. 14. While the aCH/B in extraction section of DTCTED is higher than that in extraction section of STCTED process contributing enough separation driving force. Furthermore, in stripping section, the aCH/B in the intensified scheme is also higher than that in the existing STCTED configuration. As such, the separation of B/CH/T ternary matures in the DTCTED is much easier than that of the existing improved process STCTED resulting in much more reduction in energy consumption of the proposed scheme. The exergy loss profiles for the proposed intensified DTCTED scheme and the existing STCTED process are illustrated in Fig. 15. The exergy loss profiles for both ED processes demonstrate that high exergy loss (see large peaks in Fig. 15) often happens at feed or withdraw stages. For example, a high exergy loss in the extractive column C1 of the intensified DTCTED configuration (see Fig. 15a) takes place at 24th, 44th, 53rd and 68th stages where fresh feed, entrainer, vapor sidestreams V2 and V3 are respectively fed to (or withdrawn from) the extractive column C1. The total exergy losses in STCTED and DTCTED processes are summarized in Table 6. Total exergy loss of the proposed intensified DTCTED scheme can reduce 9.73% than the existing STCTED process indicating that the intensified scheme is more reasonable. In summary, the proposed intensified DTCTED scheme have not only higher energy
11
Table 5 Results of the CO2 emissions of three configurations.
QR1 (MW) QR2 (MW) QR3 (MW) Qtotal (MW) CO2 emissions (kg/h) CO2 emissions saving (%)
CTCED [23]
STCTED [23]
Proposed DTCTED
1.731 1.820 0.6785 4.2295 1639.070 ¡16.45
2.789 / 0.843 3.6320 1407.520 0.00
2.8744 / / 2.8980 1122.883 20.22
Fig. 14. Relative volatility of B vs CH in the extractive distillation column through the existing STCTED and the proposed DTCTED processes.
efficiency but also superior than other processes from the view of economic and environmental aspects. 5. Conclusions An intensified double-thermally coupled ternary extractive distillation (DTCTED) scheme for the separation of benzene/ toluene/cyclohexane homogeneous ternary system with three azeotropes is explored to achieve energy-saving and emission-
Table 4 Comparison of optimal design between the existing CTCED and STCTED processes and the proposed DTCTED process.
C1
C2
C3
parameters
CTCED [23]
STCTED [23]
Proposed DTCTED
QR1 (MW) QC1 (MW) ID (m) NT1 QR2 (MW) QC2 (MW) ID (m) NT2 QR3 (MW) QC3 (MW) ID (m) NT3
1.731 1.251 1.190 80 1.820 1.608 1.080 41 0.6785 1.137 1.764 21
2.789 1.124 1.570 80 / 0.837 1.010 20 0.843 1.126 1.720 21
2.898 1.138 1.567 80 / 0.467 0.809 20 / 0.671 1.282 20
Qcooler (MW) Qtotal,reb (MW) Energy saving (%) Total capital cost (103 $/y) Total operating cost (103 $) TAC(103 $) TAC saving (%)
0.199 4.2295 16.45 2388.200 531.30 1327.400 ¡8.68
0.199 3.6320 0.00 2274.600 463.200 1221.40 0.00
0.517 2.8980 20.21 2009.403 324.369 994.169 18.60
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Fig. 15. Exergy loss profiles in (a) column C1, (b) column C2 and (c) column C3 of the existing STCTED and the proposed DTCTED processes.
Acknowledgments
Table 6 Results of the exergy loss of three configurations.
Total exergy loss (kW) Exergy loss saving (%)
CTCED
STCTED
DTCTED
349.89 2.80
360.00 0.00
324.97 9.73
reduction in this research. The optimal Pareto front of the proposed DTCTED process is obtained by using the multi-objective genetic algorithm. The results demonstrated that the CO2 emissions and total annual cost of the intensified DTCTED scheme are significantly reduced by 20.22% and 18.60%, compared with the existing optimized single-thermally coupled extractive distillation process. From the observation of exergy loss and relative volatility, the essence of energy-saving of the proposed DTCED scheme is enhancing the relative volatility of B vs CH (aB/CH) and reducing exergy loss of the proposed scheme. The proposed intensified configuration and systematic method are also applicable for the separation of other ternary homogeneous multi-component system with two (or more) azeotropes based on the proposed energy-saving double-thermal coupled extractive distillation such as tetrahydrofuran/methanol/water, acetonitrile/methanol/benzene, ethyl acetate/ethanol/water, and tetrahydrofuran/ethanol/water mixtures. It may be also applicable to separate heterogeneous mixtures such as methanol/toluene/ water and ethanol/toluene/water system. Of note is that the proposed energy-saving scheme and method may not be extended to the reaction systems.
We acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 21606026, 21878028); the Fundamental Research Funds for the Central Universities (Nos. 2019CDQYHG021, 2019CDXYHG0012); the Chongqing Innovation Support Program for Returned Overseas Chinese Scholars (No. CX2018048). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.07.086. References [1] Yang A, Shi T, Sun S, Wei SA, Shen W, Ren J. Dynamic controllability investigation of an energy-saving double side-stream ternary extractive distillation process. Separ Purif Technol 2019;225:41e53. https://doi.org/10.1016/ j.seppur.2019.05.063. [2] Shi T, Yang A, Jin S, Shen W, Wei SA, Ren J. Comparative optimal design and control of two alternative approaches for separating heterogeneous mixtures isopropyl alcohol-isopropyl acetate-water with four azeotropes. Separ Purif Technol 2019;225:1e17. https://doi.org/10.1016/j.seppur.2019.05.061. [3] Liang S, Cao Y, Liu X, Li X, Zhao Y, Wang Y, et al. Insight into pressure-swing distillation from azeotropic phenomenon to dynamic control. Chem Eng Res Des 2017;117:318e35. https://doi.org/10.1016/j.cherd.2016.10.040. [4] Doherty MF, Malone MF. Conceptual design of distillation systems. New York: McGraw-Hill; 2001. [5] Shen W, Dong L, Wei SA, Li J, Benyounes H, You X, et al. Systematic design of an extractive distillation for maximum-boiling azeotropes with heavy entrainers. AIChE J 2015;61(11):3898e910. https://doi.org/10.1002/aic.14908. [6] Shen WF, Benyounes H, Dong LC, Wei SA, Li J, Gerbaud V. Conceptual design of
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