Process Safety and Environmental Protection 122 (2019) 1–12 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep Comparison of pressure-swing distillation and heterogeneous azeotropic distillation for recovering benzene and isopropanol from wastewater Yue Cui, Xiaojing Shi, Chao Guang, Zhishan Zhang ∗ , Chao Wang, Chen Wang College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, China a r t i c l e i n f o Article history: Received 2 October 2018 Received in revised form 18 November 2018 Accepted 20 November 2018 Available online 22 November 2018 Keywords: Pressure-swing distillation Heterogeneous azeotropic distillation Wastewater Benzene Isopropanol Heat-intergration a b s t r a c t Two methods of pressure-swing distillation (PSD) and heterogeneous azeotropic distillation (HAD) are investigated to recover benzene and isopropanol from wastewater. This ternary mixture can form two homogenous azeotropes (binary) and two heterogeneous azeotropes (binary and ternary), which all are sensitive to pressure. The proposed configurations are rigorously simulated and optimized based on the minimum total annual cost (TAC) via the sequential iterative procedure, and meanwhile the partial heat integration is taken into account respectively. The results show that both configurations of PSD are superior to the HAD configuration combining with PSD from the perspectives of economic and energy. © 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction In the isopropanol (IPA) related industries, large amounts of wastewater containing benzene and IPA are produced. Both IPA and benzene are widely used as good organic solvents in many fields. Therefore, it is attractive and necessary to separate and reuse them from the wastewater in order to protect the environment and conserve resources. However, this mixture can form multiple complex azeotropes (heterogeneous and homogenous, binary and ternary), the conventional methods cannot effectively accomplish the separation of this mixture. There have been some methods for separating the azeotropic mixtures, such as pressure-swing distillation (PSD) (Cao et al., 2017; Fissore et al., 2006; Kiran and Jana, 2015; Knapp and Doherty, 1992; Lee et al., 2011; Li and Xu, 2017; Liang et al., 2017; Lladosa et al., 2011; Luyben, 2012, 2018; Modla et al., 2010; Wang et al., 2015; Zhu et al., 2016), azeotropic distillation (Arifin and Chien, 2007; Chaniago et al., 2016; Le et al., 2015; Tabari and Ahmad, 2015; Zhao et al., 2017), extractive distillation (Modla, 2013; Wang et al., ∗ Corresponding author. E-mail address: tjzza@163.com (Z. Zhang). 2018a,b; Wang et al., 2018d), and membrane separation (Ahmad et al., 2018; Mukherjee et al., 2003; Xia et al., 2011). Pressure-swing distillation is a kind of prevail method taking advantage of the feature that the azeotrope composition has a great shift with the change of column pressure. It can avoid the potential problems of adding third-party components. Recently, this approach is gradually applied to the separation of the mixtures containing multiple azeotropes. Zhu et al. (2016) proposed a process for separation of acetonitrile/methanol/benzene ternary azeotrope by the PSD, and compared four different separation configurations. Luyben (2017) studied the process of separating ternary mixture acetonitrile/methanol/benzene from different pressure-swing distillation sequences. Aurangzeb and Jana (2018) explored the separation of ternary mixtures of acetonitrile/methanol/benzene via pressure-swing dividing-wall column. Of course, partial and full heat-integration technologies are often coupled with PSD processes due to its economic attractiveness (Chen et al., 2018; Luyben, 2014; Qasim et al., 2015; Zhang et al., 2017, 2016). Azeotropic distillation sometimes is an attractive special separation method as well, especially heterogeneous azeotropic distillation (HAD). Homogeneous azeotropic distillation needs add an entrainer that can generate a new minimum-boiling azeotrope with the original one or more components, which can be separated as the distillate (Nava and Krishna, 2004). HAD is adapted to the https://doi.org/10.1016/j.psep.2018.11.017 0957-5820/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 2 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Nomenclature AC AR CSCinst H HAD ID LP M&S MP NF NR NT P PSD QC QR RR TAC TCinst UC UR T heat transfer area of condenser [m2 ] heat transfer area of reboiler [m2 ] installed cost of column shell [$] column height [m] heterogeneous azeotropic distillation inner diameter [m] low pressure Marshall & Swift Equipment Cost Index middle pressure feed location recycle location number of stages operation pressure pressure-swing distillation condenser duty [kW] reboiler duty [kW] reflux ratio total annual cost [$/y] installed cost of trays [$] heat transfer coefficient of condenser [kW/(K·m2 )] heat transfer coefficient of reboiler [kW/(K·m2 )] temperature difference [K] separation of the mixtures containing minimum-boiling heterogeneous azeotropes (Wang and Huang, 2012). In general, it is much easier than the separation of homogeneous azeotropes because the split of two liquid phases in a decanter can be used to facilitate the separation. The purpose of this article is to recover benzene and IPA from wastewater. Three separation configurations are proposed based on the non-ideal features of the water/benzene/IPA system, including two PSD sequences and one combined sequence of HAD and PSD. With the goal of minimum total annual cost (TAC), the optimal parameters are determined by the global sequential iterative search method. Furthermore, the partial heat-integration technology is applied to the optimal configuration for enhancing energy efficiency. Finally, the preferred solution for the wastewater treatment is confirmed through the comparisons of economic and energysaving. 2. Design basis Table 1 NRTL model parameters for the water/benzene/IPA system. Component i Component j water benzene water IPA benzene IPA Aij Aji Bij Bji Cij 140.0874 45.1905 −5954.31 591.3676 0.2 6.8284 −1.3115 −1483.46 426.3978 0.3 −0.7484 −0.1279 713.3123 148.1214 0.3 Fig. 1. The ternary diagram of the water/benzene/IPA system at 1 atm. The non-ideal features of the water/benzene/IPA system at atmospheric pressure are illustrated in Fig. 1, which can be applied to the feasibility analysis for the developed separations solutions. There are three distillation regions divided by distillation boundaries and one heterogeneous region in the ternary diagram. In each region, the component removed from the bottom of the column can be indicated along the residue curves from the unstable node toward the stable node. Note that these characteristics are also sensitive to the pressure variation. Therefore, two methods are alternative for this separation problem, including pressure-swing distillation with different sequences and heterogeneous azeotropic distillation. 2.1. Problem statement 2.3. Economic optimization and CO2 emission An industrial separation process is simulated with following data: the handled feed is an effluent from a pharmaceutical company consisting of 80 mol% water, 12 mol% benzene and 8 mol% IPA with the flowrate of 100 kmol/h. The purity of the recovered products is 99.99 mol% benzene and 99.99 mol% IPA, and the purified water with the composition of 99.999 mol% is discharged. In this work, the Aspen Plus v10.0 software is applied to the steady date simulation and the optimization results validation. 2.2. Non-ideality of system In the simulated study, the thermodynamic property method is fairly important for the creditability of simulation results. The non-random two liquid (NRTL) model with built-in binary interaction parameters (Table 1) is used to describe the phase equilibrium behaviors of the system in this work, because of the agreement between the predicted values and experiment data from NIST database. In order to make the process assessed from aspects of economy and environment impact, the minimum total annual cost (TAC) is used as the economic optimization objective as well as CO2 emission is calculated. Table 2 provides the basis of economics, the sizing relationships and parameters (Zhu et al., 2015). The major pieces of equipment including column vessels and plates, reboilers and condensers are concerned. Other items with small costs such as tubes, pumps, and valves are ignored in the preliminary design stage. The operating costs mainly consist of the consumption of steam and cooling water. The sequential iterative optimization method in this study is used to obtain the optimal parameters of three distillation columns, as shown in Fig. 2. During the optimization, the convergent condition is that the relative error of TAC as the objective function is within 1%. The constraints are the purity requirements of three products maintained by adjusting the corresponding bottom flowrates. In addition, the initial reflux ratios are assumed to be 1 and a step size of ±0.1 is used to update them. When reach- Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 3 ing near the minimum TAC, a smaller step size of ±0.01 is used to determine an optimum reflux ratio accurately. CO2 emission can be calculated for a given amount of fuel burnt through the expression given in the literature (Gadalla et al., 2006; Wang et al., 2018c). A detailed calculation procedure is shown as follows: [CO2] emission = Amt fuel NHV C × 100 ˛ (1) Where ␣ is the molar mass ratio of CO2 and C, 3.67; NHV is the net heating value; and C% is the carbon content of a specific fuel. Coal was taken as the fuel in this study, for which values for NHV and C% are 22,000 kJ/kg and 0.865 kg/kg. Amtfuel is the amount of fuel burnt (kW) and depends on the heat duty (Qseq) according to Eq. (2), Amt fuel = Qseq × hseq − 419 × seq T − T F 0 TF − TS (2) Where seq (kJ/kg) is the latent heat of utility steam; hseq (kJ/kg) is the enthalpy of utility steam; TF (K) is the flame temperature of the boiler flue gases; TS (K) is the stack temperature, and T0 (K) is the ambient temperature. 3. Process design and optimization 3.1. PSD sequence (water-benzene-IPA) Fig. 3(a) gives the flowsheet of the triple-column PSD separation sequence with the bottom product order of water-benzene-IPA. The distillates of these columns are close to the composition of ternary azeotropes at the individual column pressure. Fig. 3(b) illustrates the change of distillation regions and boundaries with the variation of pressure and overall mass-balance lines for the whole separation process. The straight line in the region II, B1-F-D3-D1 denotes the split of the column C-1 at 0.7 atm. The straight line in region III, B2D1-D2 denotes the split of the column C-2 at 6.5 atm. The straight line in region I, B3-D2-D3 denotes the split of the column C-3 at 5 atm. In order to achieve the minimum TAC of this process, twelve deign variables need to be optimized including the number of stages (NT1 , NT2 and NT3 ), reflux ratios (RR1, RR2 and RR3), feed locations (NR , NF1 , NF2 , and NF3 ) of three columns, and operating pressures (P2 and P3 ) of the columns C-2 and C-3. The bottom flowrates (B1, B2 and B3) of three columns are adjusted to meet the purity of the bottom products, respectively. The operating pressure of the column C-1 (P1 ) is fixed at 0.7 atm rather than higher pressure because it can reduce the energy consumption and also the cooling water still can be used for the high enough reflux temperature (330.70 K). Fig. 4 shows the relationships of TAC and each optimization variable listed. As shown in Fig. 5, the optimal partial heat-integrated PSD sequence (water-benzene-IPA) can reduce great energy consumption (1108.15 kW) by installing a condenser/reboiler with the transfer heat temperature difference (32.68 K) between the columns C-1 and C-2. Most of the overhead vapor of the column C-2 still needs the cooling water (303.15 K) to condense. 3.2. PSD sequence (water-IPA-benzene) Fig. 2. The sequential iterative optimization procedure. Fig. 6 (a) gives the flowsheet of the triple-column PSD separation sequence with the bottom product order of water-IPA-benzene. The distillates of these columns are close to the composition of ternary azeotropes at the individual column pressure. Fig. 6(b) illustrates the change of distillation regions and boundaries with the variation of pressure and overall mass-balance lines for the whole 4 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Table 2 Basis of economic evaluation. Parameters Formulas or data Units Condenser: Heat transfer coefficient (UC ) Heat transfer area (AC ) Capital cost 0.852 QC /(UC ×T) (M&S/280) ×3489.996×AC 0.65 kW/(K·m2 ) m2 $ Reboiler: Heat transfer coefficient (UR ) Heat transfer area (AR ) Capital cost 0.568 QR /(UR ×T) (M&S/280)×3489.996×AR 0.65 kW/(K·m2 ) m2 $ Column vessel: H TCinst CSCinst Capital cost Energy cost: 1.2 × 0.6096×(NT -2) (M&S/280)×262.556×ID1.55 ×H (M&S/280)×5485.171×ID1.066 ×H0.802 TCinst +CSCins m $ $ $ LP steam (433 K) MP steam (457 K) Cooling water M&S Payback period TAC 7.72 8.22 0.354 1431.7 3 (Total capital cost/payback period) +Annual energy cost $/GJ $/GJ $/GJ — y $/y Fig. 3. The optimal flowsheet with detailed information (a) and overall material balance lines (b) for PSD sequence (water-benzene-IPA). Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Fig. 4. The relationships of TAC and each optimization variable for PSD sequence (water-benzene-IPA): (a) RR1, (b) RR2, (c) RR3, (d) NT1 , (e) NT2 , (f) NT3 , (g) P2 and (h) P3 . 5 6 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Fig. 5. The optimal partial heat-integrated PSD sequence (water-benzene-IPA). Fig. 6. The optimal flowsheet with detailed information (a) and overall material balance lines (b) for PSD sequence (water-IPA-benzene). Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 7 Fig. 7. The relationships of TAC and each optimization variable for PSD sequence (water-IPA-benzene): (a) RR1, (b) RR2, (c) RR3, (d) NT1 , (e) NT2 , (f) NT3 , (g) P2 and (h) P3 . process. The material line of the column C-1 still is the straight line in the region II, B1-F-D3-D1. The straight line in region I, B2-D1-D2 denotes the split of the column C-2 at 3 atm. The straight line in region III, B3-D2-D3 denotes the split of the column C-3 at 7 atm. The optimization variables for this sequence (water-IPAbenzene) are identical with that of the sequence (water-benzeneIPA). Here no more detailed description. Fig. 7 shows the relationships of TAC and each optimization variable. 8 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Fig. 8. The optimal partial heat-integrated PSD sequence (water-IPA-benzene). As shown in Fig. 8, the optimal partial heat-integrated PSD sequence (water-IPA-benzene) can reduce great energy consumption (1040.86 kW) by installing a condenser/reboiler with the transfer heat temperature difference (36.20 K) between the columns C-1 and C-3. Most of the overhead vapor of the column C-3 still needs the cooling water (303.15 K) to condense. and organic phase. The combination of the column C-3 and the decanter is the typical HAD arrangement, which is used to remove the benzene product. The atmospheric column C-1 and the high pressure column C-2 constitute the conventional PSD configuration, which is used to achieve the separation of the water/IPA azeotrope. Fig. 9(b) illustrates the distillation regions and boundaries at the different pressures as well as heterogeneous region and overall mass-balance lines for the whole process. The straight line in the region II, B1-F1-D1 denotes the split of the column C-1 at 1 atm. The straight line in region I, B2-D1-D2 denotes the split of the column C-2 at 5 atm. The straight line in region III, B3-F3-D3 denotes the split of the column C-3 at 1 atm. The straight line, D3- 3.3. HAD and PSD combined sequence Fig. 9 (a) gives the flowsheet of the HAD and PSD combined sequence with three columns and a decanter. The whole separation process begins with the decanter to obtain the aqueous phase Table 3 Summary of detailed optimization parameters of three separation configurations proposed. Parameters Pressure (atm) NT NR NF ID RR Condenser duty(MW) Reboiler duty(MW) Capital cost (105 $) Energy cost (105 $/y) TAC(105 $/y) Total TAC(105 $/y) [CO2 ]Emiss (103 kg/h) Total [CO2 ]Emiss (103 kg/h) PSD (water-benzene-IPA) PSD(water-IPA-benzene) HAD and PSD C-1 C-2 C-3 C-1 C-2 C-3 C-1 C-2 C-3 0.7 12 2 3 0.76 0.04 1.25 1.11 6.75 2.59 4.84 15.60 0.89 3.86 6.5 28 – 19 0.90 0.4 1.25 1.66 8.55 4.05 6.90 5 34 – 8 0.56 0.01 0.85 0.80 6.01 1.86 3.86 3 31 – 6 0.78 0.01 1.06 1.32 7.60 3.22 5.76 7 15 – 7 0.79 0.3 1.13 1.32 7.05 3.06 5.41 1 17 – 10 0.73 0.3 0.59 0.68 4.89 1.58 3.21 1.64 1.06 1.06 1 35 – 31 1.28 1.2 3.51 3.90 15.77 9.04 14.29 32.19 3.14 6.72 5 49 – 16 1.18 1.6 3.42 3.77 16.25 9.27 14.69 1.33 0.7 8 2 4 0.69 0.01 1.30 1.04 6.49 2.44 4.61 15.78 0.84 2.96 3.03 0.55 Table 4 The head-to-head comparison of three partial heat-integrated separation configurations. Parameters PSD (water-benzene-IPA) PSD(water-IPA-benzene) HAD and PSD Condenser duty(MW) Reboiler duty(MW) Auxiliary condenser(MW) Heat integration duty(MW) Capital cost (105 $) Energy cost (105 $/y) Total TAC(105 $/y) Energy cost saving (%) Capital cost saving (%) TAC saving (%) [CO2 ]Emiss (103 kg/h) 3.35 3.57 0.14 1.11 18.52 5.93 12.10 30.24 13.09 22.44 1.97 3.49 3.68 0.09 1.04 18.37 6.31 12.44 27.64 13.10 21.17 2.12 7.52 8.35 2.73 0.68 35.90 18.30 30.27 7.99 2.74 5.96 6.17 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 9 Fig. 9. The optimal flowsheet with detailed information (a) and overall material balance lines (b) for HAD and PSD combined sequence. FM-D2 denotes the mixing process of the distillates D3 and D2. The tie line in the heterogeneous region, F1-F0-F3 denotes the split of the decanter at 308.15 K. As for the optimization of this process, there are ten optimization variables including the number of stages (NT1 , NT2 and NT3 ), reflux ratios (RR1, RR2 and RR3), feed locations (NF1 , NF2 and NF3 ) of three columns, and the operating pressure (P3 ) of the columns C-3. The bottom flowrates (B1, B2 and B3) of three columns are adjusted to meet the purity of the bottom product, respectively. The operating pressures of the columns C-1 and C-3 (P1 and P3 ) are both fixed at 1 atm. Fig. 10 shows the relationships of TAC and each optimization variable. Fig. 11 shows the HAD and PSD combined sequence with the partial heat-integration between the columns C-2 and C-3. The reduction of energy consumption equals to the heat duty (682.51 kW) of the condenser/reboiler with the transfer heat temperature difference (38.38 K). Most of the overhead vapor of the column C-2 still needs the cooling water (303.15 K) to condense. 10 Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 Fig. 10. The effect of optimization variables on TAC for HAD and PSD combined sequence: (a) RR1, (b) RR2, (c) RR3, (d) NT1 , (e) NT2 , (f) NT3 and (g) P2 . The detailed optimization parameters of the above-proposed three configurations are summarized in Table 3. Table 4 shows a head-to-head comparison of three partial heat-integrated separation configurations. Y. Cui et al. / Process Safety and Environmental Protection 122 (2019) 1–12 11 Fig. 11. Partial heat-integration process for HAD and PSD combined sequence. Both PSD configurations are superior to the HAD and PSD combined sequence in terms of TAC and energy consumptions, and are acceptable alternatives because of approximately the same TAC. With regards to heat integration, the PSD (water-benzene-IPA) sequence with partial heat integration can achieve more energy saving and CO2 emission than other two schemes. Note that only the partial heat integration with the heat transfer temperature difference greater than 25 K has been accomplished in this work. 4. Conclusions This article addressed the recovery of organic solvents benzene and IPA from wastewater via three separation configurations without adding any extra solvent, including the PSD (waterbenzene-IPA), the PSD (water- IPA-benzene) and the HAD and PSD combined sequence. The optimal operating conditions and the minimum TAC were determined by using the sequential iterative procedure. Furthermore, the partial heat-integration was carried out for improving energy efficiency and reducing CO2 emission. These studies revealed that the PSD process with the product order water-benzene-IPA can high effectively solve the treatment of wastewater containing benzene and IPA organic solvents. References Ahmad, T., Guria, C., Mandal, A., 2018. 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