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Energy consumption in pervaporation, conventional and hybrid processes to separate toluene and i-octane
Chemical Engineering & Processing: Process Intensification 128 (2018) 46–52
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Energy consumption in pervaporation, conventional and hybrid processes to separate toluene and i-octane
T
⁎
Ali Khazaeia, Vahid Mohebbia, , Reza M. Behbahania, S.A Ahmad Ramazanib a b
Department of Gas Engineering, Petroleum University of Technology, Ahwaz, Iran Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy Pervaporation Hybrid process Extractive distillation
Chemical industries need to employ new process designs due to environmental policies and energy optimization because of the global energy challenge. Pervaporation has been introduced as a promising alternative for conventional processes such as distillation, known as energy intensive process, in chemical plants. In this work, the energy consumption of different processes for separation of toluene and i-octane (representatives of aromatics and aliphatic mixtures) has been evaluated, based on our previous laboratory pervaporation experiments using Polyvinyl alcohol/Graphene oxide mixed matrix membranes. Accordingly, hybrid distillation-pervaporation and cascade pervaporation systems have been compared with distillation and extractive distillation (conventional process). The results indicated that although the energy demand of hybrid system is much less than distillation, but it is not comparative with extractive distillation. However, when purification of aliphatics is required, cascade pervaporation system is more effective than extractive distillation and the energy demand for purification of i-octane is 56.1% lower in cascade pervaporation process. The results can assist engineers to design novel and low energy consumer process plants for removing aromatics from aliphatic mixtures.
1. Introduction
compared with the conventional processes [2]. Thus far, several studies have been done to compare new and hybrid membrane systems with conventional processes. Lutze and Gorak [2] reviewed researches associated with membrane-assisted distillation processes, especially application of pervaporation. Naidu and Malik [1] optimized a hybrid distillation–pervaporation process with different structures using MINLP method. Hömmerich and Rautenbach [6] studied on different process configurations integrating pervaporation and vapor permeation into Huels process for production of MTBE using Aspen Plus software. Pervaporation-based hybrid processes combining pervaporation, distillation and chemical reactors were investigated by Lipnizki et al. [7]. They also evaluated the economic characteristics of hybrid processes at industrial scale. O’Brien et al. [8] designed a pervaporation process for recovery of ethanol from fermentation broths based on experimental data. Van Hoof et al. [3] made an economic analysis on processes for dehydration of isopropanol/water by comparing azeotropic distillation with hybrid systems consisting of distillation and pervaporation. Vier [9] studied on the economical potential of a combined distillation/pervaporation process for production of dimethylcarbonate (DMC). Moreover, some researches have proposed new processes focusing on low energy consumption. Nagy et al. [10] studied on energy
Separation processes are one of the largest energy consumers in chemical industries, for instance, the energy demand of distillation and related processes are approximately 40% of total energy consumption of a chemical plant [1,2]. Because of high energy cost, global competition and environmental impacts of conventional separation processes, novel and low energy consumer processes are required as alternatives [2]. In addition, in the case of close boiling point or azeotropic mixtures separation, simple distillation column is ineffective and consequently alternatives such as extractive or azeotropic distillation must be applied. Meanwhile, both mentioned processes are energy-intensive and also have unfavorable effects associated with using solvents [3]. Pervaporation has been introduced as an optimistic options for conventional energy intensive separation techniques. Pervaporation could be used for a wide range of separation processes, including close boiling point and azeotropic mixtures with lower capital and utility costs. Additionally, it could be more economical, safe and ecofriendly [4]. Integration of distillation with pervaporation named “hybrid process” system is a relatively new process presented in recent years [5]. The hybrid system has process and economical advantages such as low energy consumption, compact and modular design and more flexibility,
⁎
Corresponding author. E-mail address: [email protected] (V. Mohebbi).
https://doi.org/10.1016/j.cep.2018.04.009 Received 25 January 2018; Received in revised form 17 March 2018; Accepted 9 April 2018 Available online 12 April 2018 0255-2701/ © 2018 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 128 (2018) 46–52
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polynomial (permeation flux) equations to employ in the simulation. Accordingly, the equations were obtained as follows:
consumption of pervaporation process for production of biofuel. They revealed that by employing standalone pervaporation process at very high separation coefficient, a fuel-grade quality of biofuel could be achieved. Furthermore, hybrid processes have lower energy consumption than distillation. Kreis and Górak [5] investigated on energy saving of hybrid membrane processes for separation of a non-ideal ternary mixture of acetone, isopropanol and water. Pribic et al. [11] compared energy consumption of hybrid processes for separation of alcohol – aqueous mixtures with conventional two column pressure swing distillation (PSD). Maus et al. [12] applied subroutine simulation packages developed by Sulzer Chemtech in order to design a hybrid process for separation of isopropanol, bio-ethanol and tetrahy-drofurane. They revealed that energy saving of more than 50% could be attained in comparison with conventional processes. Huang et al. [13] and Vane et al. [14] proposed new hybrid processes containing distillation and vapor permeation to improve the energy consumption. Although the energy consumption of pervaporation and hybrid systems for chemical process have been widely reviewed, there is no detailed analysis comparing pervaporation-based processes with conventional processes for separation of aromatic and aliphatic mixtures. Such a study could be highly practical for developing new processes in petrochemical industries. In this work, energy consumption of pervaporation-based and conventional processes for separation of toluene and i-octane (representatives of aromatic/aliphatic mixtures) have been analyzed. Accordingly, cascade pervaporation and hybrid pervaporation/distillation processes are compared with distillation and extractive distillation as a conventional process for separation of aromatic and aliphatics mixtures. Simulation of pervaporation process is based on our previous experimental study [15]. It is explained that how pervaporation could be a promising alternative to conventional separation processes.
Selectivity = −0.193 φ + 12.96
(1)
Normalized permeation flux (g μm/h cm ) = 701.17φ − 186.76φ + 175.02 (2) 2
2
where φ is volume fraction of toluene in the feed. Eqs. (1) and (2) are used for simulation in hybrid and cascade pervaporation processes. 3. Simulation and results As revealed, four different processes have been considered in this study, including distillation tower, extractive distillation, cascade pervaporation and hybrid distillation-pervaporation process. These processes can potentially be used at industrial scale for separation of aromatic/aliphatic mixtures. The mass flowrate of the feed was considered as 60 t/hr, with temperature of 90 °C and pressure of 2 bar, for all processes. Composition of the feed was also considered as 50 wt. % of toluene and i-octane. The simulations of all processes was conducted by Aspen Hysys V7.3 software using NRTL equation as the activity coefficient model and total energy consumption of different processes were calculated. For better assessment, two different levels of purification were considered; high level (99%) and low level (95%) purification. However, in cascade pervaporation system, due to selectivity fall in high toluene content, it was impossible to reach mentioned purifications for both toluene and i-octane. Therefore, only purification of toluene (aromatic removal) was studied. Four different utilities were considered for the processes; medium pressure (20 bar) saturated steam for heating reboilers, cooling water (35 °C inlet, 45 °C outlet), liquid nitrogen (saturated at 3 bar), and power. Pressure drop of the heat exchangers was assumed to be 0.5 bar. The following items were considered for total energy of the processes: Duty of reboiler, duty of condenser, power for pump, power for vacuum, duty of pervaporation chilling, duty of pervaporation preheat.
2. Experimental background The experimental procedure of pervaporation experiments is presented in our previous work [15]. The mixed matrix membrane contains polyvinyl alcohol (PVA) with hydrophilic structure (selective toward aromatics and low swelling in organic mixtures) and graphene oxide (GO), which can create S and π bonds with aromatics and increase selectivity. The experiments were performed at temperature of 330 K and under vacuum pressure of 0.15 torr. The best result was obtained from non-crosslinked PVA membrane containing 0.5 wt. % of GO. However, PVA/GO membranes employed for this study have a moderate separation performance compared with other works [15]. The dependency of selectivity and normalized permeation flux (permeation flux for specific membrane thickness) to concentration of toluene (Fig. 1), is consistent with the literature[16]. As indicated, increasing toluene content in the feed leads to increasing permeation flux and decreasing selectivity due to swelling of the membranes. This curves can be modeled with a linear (selectivity) and a 2-degree
3.1. Extractive distillation The industrial process for separation of aromatic/aliphatic mixtures is extractive distillation using special solvents such as n-formyl morpholine (NFM) and furfural. For simulation of this process, actual industrial process data from one of the petrochemical plants as state-ofthe-art process were employed. Fig. 2 shows the process and related operating data. As indicated, there are two steps for separation of toluene and i-octane; firstly, the solvent (NFM) extracts aromatic component and purified aliphatic compound is going away from top of the extraction tower. The rich solvent is directed to stripping section in which NFM and toluene is separated under vacuum pressure of 0.4 bar. A portion of the required duty for the reboiler of extraction tower can be provided from lean solvent (bottom of stripper). Because the amount of solvent is recycled completely to the extraction tower, the make-up solvent is negligible.
Fig. 1. a) Selectivity and b) normalized permeation flux versus volume fraction of toluene. 47
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Fig. 2. Extractive distillation process for separation of toluene and i-octane.
Liquid-liquid extractor was employed in software for simulation. Binary coefficients of NRTL model, especially for NFM/toluene and NFM/ioctane, in Aspen Hysys were completed using experimental data [17,18]. By varying reflux ratio of the towers, purification of the extraction and stripping section were changed to reach the required purifications. The other parameters such as number of trays, pressures feed and solvent tray were fixed at the optimum values originated from industrial data. Tables 1 and 2 present the results of simulation and utility consumption for extractive distillation, respectively. As indicated, the total duty to achieve 99% and 95% purification of toluene and i-octane is found to be 17.63 MW and 15.86 MW, respectively. It can be stated that almost total energy demand is attributed to condenser and reboiler of extraction and stripping tower. Additionally, using second reboiler for the extraction section can save 1.3 MW of energy and using vacuum minimizes energy demand of stripping section.
Table 2 Utility consumption for extractive distillation. Utility
99% Extractive distillation
95% Extractive distillation
MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
18,164 654,853 0 402
16,471 580,671 0 400
of toluene and i-octane are close, the distillation tower is not economical for separation in a chemical plant, meanwhile, the simulation data are useful for comparison. Simulation was conducted using distillation column subflowsheet in the software. The effect of number of trays on separation efficiency and purification of toluene and i-octane must be investigated. For this purpose, the bottom flow rate (toluene) and the top product (i-octane) were considered equal, number of trays changed from 10 to 70 and concentration of top and bottom product were recorded (Fig. 3). The reflux
3.2. Distillation Tower The simplest process for separation of toluene and i-octane is a single distillation tower. Because mentioned before, the boiling points Table 1 Results of extractive distillation for purification of toluene and i-octane.
Number of tray Top pressure (bar) Bottom pressure (bar) Feed tray Solvent tray Condenser Duty (MW) Reboiler duty (MW) Pump duty (MW) Vacuum duty (MW) Total duty (MW)
99% purification
95% purification
Extraction
Stripping
Extraction
Stripping
60 1.6 1.9 50 10 3.57 5.08 0.02 0.38 17.63
20 0.4 0.4 5 – 4.11 4.47
60 1.6 1.9 50 10 3.04 4.30 0.02 0.38 15.86
20 0.4 0.4 5 – 3.77 4.36
Fig. 3. Effect of number of tray on separation efficiency of distillation tower for toluene/i-octane. 48
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chiller and associated duty required for heat transfer was calculated. It should be noted that liquid nitrogen is recirculated and not consumed in the process. The temperature of chilling was considered as −55 °C to condense toluene/i-octane vapor at vacuum pressure of 0.001 bar. Component splitter in the software was applied for simulation of pervaporation stages. Fig. 6 indicates the process of cascade pervaporation with 2 stages. To obtain optimum number of pervaporation stages, different stages with different rates of permeate and retentate were simulated. Fewer stages require higher permeate and retentate rates and therefore higher membrane area and energy for condensation is needed. It should be noted that selectivity and permeation flux of the membrane for each stages are calculated from Eqs. (1) and (2), accordingly, composition and flowrate of permeate and retentate are calculated. The membrane area is a dependent parameter calculated from permeation flux needed for each stage. Tables 3 and 4 presents the results of 1 to 4 stages cascade pervaporation process and corresponding utility consumption, for purification of i-octane over than 99%. As shown, there is no remarkable difference in total energy consumption, total membrane area and flow rate of products, however, 2-stages process is more economical. Total demand energy for cascade pervaporation is 7.73 MW which is associated to chilling in pervaporation stages. Pump and vacuum power also have a small portion of demand energy. Retentate of the process is 20,880 kg/h purified i-octane and mass content of toluene in permeate is 77%. Also, total membrane area required for 60 t/h feed is 1727 m2. This would be practical using parallel cylindrical membrane modules. As indicated, energy consumption in cascade membrane system is much less than that of other processes.
Fig. 4. Concentration of top and bottom products versus reflux ratio in a distillation tower for separation of toluene and i-octane.
ratio was also fixed at 5 and the feed tray was considered at middle of the tower. As indicated, the changes of purifications are sharper when 10–30 trays are used and slighter when higher than 50 trays is applied. This illustrates, the difference of mass fractions is less than 1% from 60 to 70 trays. Accordingly, 60 trays was considered for tower in distillation and hybrid system. Figs. 4 and 5 present the effect of reflux ratio on duties of condenser and reboiler and concentration of toluene and i-octane. As indicated, to reach high purification, it is required to enhance reflux ratio and accordingly duty of both reboiler and condenser. Reflux ratio of 30 with total duty of 141 MW is required to reach purification of 95%, which is a huge energy demand. This shows that application of distillation tower for separation of toluene and i-octane is ineffective and higher values of purification (more than 90%) is difficult to reach due to extraordinary energy demand of condenser and reboiler. Furthermore, sizing calculations showed that minimum diameter required for 95% purification is 8.6 m, which is impracticable.
3.4. Hybrid distillation pervaporation As indicated, selectivity of the mixed matrix membranes increases in low and decreases in high concentration of toluene. Consequently, pervaporation in hybrid system could only be applied for low toluene concentration stream, e.g. top of the distillation tower. On this basis, hybrid process consisting distillation and pervaporation was designed for separation of toluene and i-octane. In this process, the purified toluene is derived from bottom and a mixture of toluene and i-octane is obtained from top of the distillation tower. The top product is routed to the pervaporation section consisting two stages. Retentate of the first stage is routed to the second stage and retentate of the second stage is the purified i-octane. Due to moderate selectivity of the membranes, permeate stream has relatively high content of i-octane and could not be mixed with toluene product, therefore, permeate from two stages are mixed and recycled to the tower. Chilling of the membrane stages are the same as cascade process. Fig. 7 presents the hybrid process: The hybrid process was simulated using Aspen Hysys V7.3 software with NRTL fluid package and component splitter for pervaporation. Tables 5 and 6 present the results of hybrid process for two levels of purification (95% and 99%) and Table 7 shows the utility consumption of process. As indicated, energy consumption of the hybrid process is 42 and 35 MW for purification level of 99% and 95%. The main portion of energy is attributed to the distillation. The duties of hybrid process system are much more than extractive distillation and the process is not competitive in terms of energy consumption. Meanwhile, it has some other advantages such as no need to solvent and being eco-friendly.
3.3. Cascade pervaporation process The other method which could be applied for separation of toluene and i-octane is cascade pervaporation process. The main difference between cascade pervaporation and other studied processes (distillation, extractive distillation and hybrid distillation/pervaporation) is that in cascade pervaporation, only one of the components (i-octane) can be purified. As revealed, separation factor of a feed containing high amounts of toluene is low and membrane is ineffective. Therefore, the retentate (rich i-octane) of each stages was routed to the next stage and permeate (rich from toluene) of all stages were mixed together. In pervaporation process, a chiller is required to liquefy the permeate and a pump is required to pressurize. Liquid nitrogen was considered as
4. Discussion The results show that the separation of toluene and i-octane as a close boiling point system is very difficult by distillation. The duty required for achieving 95% purification of 60 t/hr feed with composition of 50 wt. % toluene is 141 MW (with reflux ratio of 30), which is not economical and higher purification is not feasible, in terms of condenser and reboiler duty, sizing and construction of the tower. The demand of energy for extractive distillation using NFM is 15.86 MW
Fig. 5. Duties of condenser and Reboiler versus reflux ratio in a distillation tower for separation of toluene and i-octane. 49
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Fig. 6. The process of two-stage cascade pervaporation for separation of toluene and i-octane.
mixture has no energy saving in comparison with conventional extractive distillation system. However, cascade membrane system is highly effective where purification of an aliphatic mixture is required. This could be a suitable alternative in refineries and petrochemical plants for removing aromatics from fuels and purification of aliphatics. Increasing selectivity of the membrane leads reducing energy consumption of the hybrid process and also purification toluene in cascade pervaporation. Therefore, the two pervaporation-based processes can potentially be more effective than liquid extraction. Accordingly, ideal selectivity of the membrane was calculated and the related results have been illustrated in Fig. 9. As indicated, ideal selectivity of the membrane for hybrid system to reduce energy consumption lower than extractive distillation, is 180 and 170 for 99% and 95% purification, respectively. Moreover, the ideal selectivity for the cascade system to reach 99% and 95% purification for both toluene and i-octane is 38.5 and 35.5. The ideal selectivities have been reported in some previous experimental studies [19–22]. It can be concluded that for purification of such close boiling point system, distillation has no advantage and any hybrid system including distillation could not be effective. The best system for such process is cascade pervaporation. Using ideal selectivities, total energy consumption for purification of both component is about 8 MW (depending on permeation flux), 50% lower than extractive distillation. The size of membrane area depends on permeation flux; high permeation flux results in low membrane area and low capital investment.
Table 3 Results of 1–4 stages cascade pervaporation process for purification of i-octane over than 99%. Cascade pervaporation Characteristics
1-stage
2-stages
3-stages
4-stages
Membrane Surface (m2) Mass flow of retentate (kg/h) Mass flow of permeate (kg/h) Weigh fraction of toluene in permeate Total duty (MW)
1552.94 20100 39900 0.752
1727.07 20,880 39120 0.767
1773.99 20244 39755 0.755
1873.16 20120 39879.6 0.752
7.84
7.73
7.82
7.84
Table 4 Utility consumption for cascade pervaporation process. Utility MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
0 0 71,378 4103
and 17.63 MW, for 95% and 99% purification, respectively. In hybrid system consisting of one-stage distillation tower and two-stage pervaporation process, energy demand for hybrid process is higher than extractive distillation, e.g. 34.63 MW and 41.77 MW, for 95% and 99% purification. In hybrid and cascade pervaporation systems, the membrane could only be used in the streams with low content of toluene. For cascade membrane process, 1–4 stages with different rate and membrane area were evaluated and results showed that there is no significant difference in total membrane area, energy consumption and composition of products. Owing to reduce selectivity of membranes in high content of toluene, the maximum purification of toluene in this system is approximately 77%, but i-octane could reach to higher than 99% level of purification. The energy consumption of cascade pervaporation system is lower than distillation-based processes. Fig. 8 present energy consumption of different processes. The results show that applying hybrid system for separation of aromatic/aliphatic
5. Conclusion In this study, energy consumption of different processes for separation of toluene and i-octane (as representative of aromatic and aliphatic) is compared in detail. The conventional process for separation of such close boiling point mixtures is extractive distillation. Accordingly, extractive distillation data from one of the petrochemical plant was used for simulation. This process was compared with distillation tower, hybrid membrane and cascade pervaporation process. The pervaporation simulation was based on experimental data using linear (selectivity) and a 2-degree polynomial (permeation flux) 50
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Fig. 7. The process of hybrid distillation-pervaporation for separation of toluene and i-octane.
Table 5 Results of hybrid distillation-pervaporation process for purification of toluene and i-octane.
Number of tray Top pressure (bar) Bottom pressure (bar) Feed tray Recycle tray Condenser Duty (MW) Reboiler duty (MW) Pervaporation chilling and preheating duty (MW) Pump duty (MW) Vacuum duty (MW) Total duty (MW)
99% purification
95% purification
60 1.3 1.7 50 10 19.68 20.91 0.402
60 1.6 1.9 50 10 16.08 17.31 0.408
0.005 0.785 41.77
0.005 0.823 34.63
Fig. 8. Comparison of energy consumption for different processes.
Table 6 Results of pervaporation stages in hybrid process for purification of toluene and i-octane.
Weight fraction of toluene in feed Permeate flow rate (kg/h) Membrane Surface (m2) Vacuum duty (MW) Chilling Duty (MW) Total duty (MW)
99% purification
95% purification
Stage 1
Stage 2
stage 1
stage 2
0.128 5688.5 349.84 0.57 0.28 0.85
0.042 2313.4 137.38 0.22 0.12 0.34
0.170 4801.9 293.65 0.49 0.23 0.72
0.097 3433.0 209.98 0.34 0.17 0.51
Table 7 Utility consumption for hybrid process.
Fig. 9. Ideal selectivities for hybrid and cascade process.
Utility
99% hybrid system
95% hybrid system
MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
39,771 1,788,482 7,913 790
32,924 1,481,690 8,029 828
equations. According to the experimental background, selectivity of the membrane is high in low content aromatic mixtures and vice versa. By using PVA/GO membranes with moderate selectivity, the results show that when purification of both components is required, extractive distillation is the most economical process, excluding environmental issues related to toxic solvents. Moreover, cascade pervaporation process 51
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could be extremely effective when purification of an aliphatic is required (aromatic removal). In this case, the energy consumption of cascade pervaporation process is 56.1% lower than that of extractive distillation. This could be very attractive for removing aromatics from gasoline according to the new environmental regulations as well as purification of aliphatics in petrochemical plants. The hybrid process is more economical than extractive distillation, if the ideal selectivity is 180 and 170 and also ideal selectivity for purification of both component in cascade process is 38.5 and 35.5, for 99% and 95% purification, respectively. This shows that distillation and hybrid process have no advantages for separation of close boiling point systems. In this condition, the most suitable process is cascade pervaporation. In addition to low energy consumption, the process has other remarkable advantages: non using solvents and being eco-friendly, compact, modular and flexible design. Gradually, the membrane seems to close industrial standards for separation of aromatic/aliphatic mixtures as an alternative to the conventional processes.
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Energy consumption in pervaporation, conventional and hybrid processes to separate toluene and i-octane
T
⁎
Ali Khazaeia, Vahid Mohebbia, , Reza M. Behbahania, S.A Ahmad Ramazanib a b
Department of Gas Engineering, Petroleum University of Technology, Ahwaz, Iran Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy Pervaporation Hybrid process Extractive distillation
Chemical industries need to employ new process designs due to environmental policies and energy optimization because of the global energy challenge. Pervaporation has been introduced as a promising alternative for conventional processes such as distillation, known as energy intensive process, in chemical plants. In this work, the energy consumption of different processes for separation of toluene and i-octane (representatives of aromatics and aliphatic mixtures) has been evaluated, based on our previous laboratory pervaporation experiments using Polyvinyl alcohol/Graphene oxide mixed matrix membranes. Accordingly, hybrid distillation-pervaporation and cascade pervaporation systems have been compared with distillation and extractive distillation (conventional process). The results indicated that although the energy demand of hybrid system is much less than distillation, but it is not comparative with extractive distillation. However, when purification of aliphatics is required, cascade pervaporation system is more effective than extractive distillation and the energy demand for purification of i-octane is 56.1% lower in cascade pervaporation process. The results can assist engineers to design novel and low energy consumer process plants for removing aromatics from aliphatic mixtures.
1. Introduction
compared with the conventional processes [2]. Thus far, several studies have been done to compare new and hybrid membrane systems with conventional processes. Lutze and Gorak [2] reviewed researches associated with membrane-assisted distillation processes, especially application of pervaporation. Naidu and Malik [1] optimized a hybrid distillation–pervaporation process with different structures using MINLP method. Hömmerich and Rautenbach [6] studied on different process configurations integrating pervaporation and vapor permeation into Huels process for production of MTBE using Aspen Plus software. Pervaporation-based hybrid processes combining pervaporation, distillation and chemical reactors were investigated by Lipnizki et al. [7]. They also evaluated the economic characteristics of hybrid processes at industrial scale. O’Brien et al. [8] designed a pervaporation process for recovery of ethanol from fermentation broths based on experimental data. Van Hoof et al. [3] made an economic analysis on processes for dehydration of isopropanol/water by comparing azeotropic distillation with hybrid systems consisting of distillation and pervaporation. Vier [9] studied on the economical potential of a combined distillation/pervaporation process for production of dimethylcarbonate (DMC). Moreover, some researches have proposed new processes focusing on low energy consumption. Nagy et al. [10] studied on energy
Separation processes are one of the largest energy consumers in chemical industries, for instance, the energy demand of distillation and related processes are approximately 40% of total energy consumption of a chemical plant [1,2]. Because of high energy cost, global competition and environmental impacts of conventional separation processes, novel and low energy consumer processes are required as alternatives [2]. In addition, in the case of close boiling point or azeotropic mixtures separation, simple distillation column is ineffective and consequently alternatives such as extractive or azeotropic distillation must be applied. Meanwhile, both mentioned processes are energy-intensive and also have unfavorable effects associated with using solvents [3]. Pervaporation has been introduced as an optimistic options for conventional energy intensive separation techniques. Pervaporation could be used for a wide range of separation processes, including close boiling point and azeotropic mixtures with lower capital and utility costs. Additionally, it could be more economical, safe and ecofriendly [4]. Integration of distillation with pervaporation named “hybrid process” system is a relatively new process presented in recent years [5]. The hybrid system has process and economical advantages such as low energy consumption, compact and modular design and more flexibility,
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Corresponding author. E-mail address: [email protected] (V. Mohebbi).
https://doi.org/10.1016/j.cep.2018.04.009 Received 25 January 2018; Received in revised form 17 March 2018; Accepted 9 April 2018 Available online 12 April 2018 0255-2701/ © 2018 Elsevier B.V. All rights reserved.
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polynomial (permeation flux) equations to employ in the simulation. Accordingly, the equations were obtained as follows:
consumption of pervaporation process for production of biofuel. They revealed that by employing standalone pervaporation process at very high separation coefficient, a fuel-grade quality of biofuel could be achieved. Furthermore, hybrid processes have lower energy consumption than distillation. Kreis and Górak [5] investigated on energy saving of hybrid membrane processes for separation of a non-ideal ternary mixture of acetone, isopropanol and water. Pribic et al. [11] compared energy consumption of hybrid processes for separation of alcohol – aqueous mixtures with conventional two column pressure swing distillation (PSD). Maus et al. [12] applied subroutine simulation packages developed by Sulzer Chemtech in order to design a hybrid process for separation of isopropanol, bio-ethanol and tetrahy-drofurane. They revealed that energy saving of more than 50% could be attained in comparison with conventional processes. Huang et al. [13] and Vane et al. [14] proposed new hybrid processes containing distillation and vapor permeation to improve the energy consumption. Although the energy consumption of pervaporation and hybrid systems for chemical process have been widely reviewed, there is no detailed analysis comparing pervaporation-based processes with conventional processes for separation of aromatic and aliphatic mixtures. Such a study could be highly practical for developing new processes in petrochemical industries. In this work, energy consumption of pervaporation-based and conventional processes for separation of toluene and i-octane (representatives of aromatic/aliphatic mixtures) have been analyzed. Accordingly, cascade pervaporation and hybrid pervaporation/distillation processes are compared with distillation and extractive distillation as a conventional process for separation of aromatic and aliphatics mixtures. Simulation of pervaporation process is based on our previous experimental study [15]. It is explained that how pervaporation could be a promising alternative to conventional separation processes.
Selectivity = −0.193 φ + 12.96
(1)
Normalized permeation flux (g μm/h cm ) = 701.17φ − 186.76φ + 175.02 (2) 2
2
where φ is volume fraction of toluene in the feed. Eqs. (1) and (2) are used for simulation in hybrid and cascade pervaporation processes. 3. Simulation and results As revealed, four different processes have been considered in this study, including distillation tower, extractive distillation, cascade pervaporation and hybrid distillation-pervaporation process. These processes can potentially be used at industrial scale for separation of aromatic/aliphatic mixtures. The mass flowrate of the feed was considered as 60 t/hr, with temperature of 90 °C and pressure of 2 bar, for all processes. Composition of the feed was also considered as 50 wt. % of toluene and i-octane. The simulations of all processes was conducted by Aspen Hysys V7.3 software using NRTL equation as the activity coefficient model and total energy consumption of different processes were calculated. For better assessment, two different levels of purification were considered; high level (99%) and low level (95%) purification. However, in cascade pervaporation system, due to selectivity fall in high toluene content, it was impossible to reach mentioned purifications for both toluene and i-octane. Therefore, only purification of toluene (aromatic removal) was studied. Four different utilities were considered for the processes; medium pressure (20 bar) saturated steam for heating reboilers, cooling water (35 °C inlet, 45 °C outlet), liquid nitrogen (saturated at 3 bar), and power. Pressure drop of the heat exchangers was assumed to be 0.5 bar. The following items were considered for total energy of the processes: Duty of reboiler, duty of condenser, power for pump, power for vacuum, duty of pervaporation chilling, duty of pervaporation preheat.
2. Experimental background The experimental procedure of pervaporation experiments is presented in our previous work [15]. The mixed matrix membrane contains polyvinyl alcohol (PVA) with hydrophilic structure (selective toward aromatics and low swelling in organic mixtures) and graphene oxide (GO), which can create S and π bonds with aromatics and increase selectivity. The experiments were performed at temperature of 330 K and under vacuum pressure of 0.15 torr. The best result was obtained from non-crosslinked PVA membrane containing 0.5 wt. % of GO. However, PVA/GO membranes employed for this study have a moderate separation performance compared with other works [15]. The dependency of selectivity and normalized permeation flux (permeation flux for specific membrane thickness) to concentration of toluene (Fig. 1), is consistent with the literature[16]. As indicated, increasing toluene content in the feed leads to increasing permeation flux and decreasing selectivity due to swelling of the membranes. This curves can be modeled with a linear (selectivity) and a 2-degree
3.1. Extractive distillation The industrial process for separation of aromatic/aliphatic mixtures is extractive distillation using special solvents such as n-formyl morpholine (NFM) and furfural. For simulation of this process, actual industrial process data from one of the petrochemical plants as state-ofthe-art process were employed. Fig. 2 shows the process and related operating data. As indicated, there are two steps for separation of toluene and i-octane; firstly, the solvent (NFM) extracts aromatic component and purified aliphatic compound is going away from top of the extraction tower. The rich solvent is directed to stripping section in which NFM and toluene is separated under vacuum pressure of 0.4 bar. A portion of the required duty for the reboiler of extraction tower can be provided from lean solvent (bottom of stripper). Because the amount of solvent is recycled completely to the extraction tower, the make-up solvent is negligible.
Fig. 1. a) Selectivity and b) normalized permeation flux versus volume fraction of toluene. 47
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Fig. 2. Extractive distillation process for separation of toluene and i-octane.
Liquid-liquid extractor was employed in software for simulation. Binary coefficients of NRTL model, especially for NFM/toluene and NFM/ioctane, in Aspen Hysys were completed using experimental data [17,18]. By varying reflux ratio of the towers, purification of the extraction and stripping section were changed to reach the required purifications. The other parameters such as number of trays, pressures feed and solvent tray were fixed at the optimum values originated from industrial data. Tables 1 and 2 present the results of simulation and utility consumption for extractive distillation, respectively. As indicated, the total duty to achieve 99% and 95% purification of toluene and i-octane is found to be 17.63 MW and 15.86 MW, respectively. It can be stated that almost total energy demand is attributed to condenser and reboiler of extraction and stripping tower. Additionally, using second reboiler for the extraction section can save 1.3 MW of energy and using vacuum minimizes energy demand of stripping section.
Table 2 Utility consumption for extractive distillation. Utility
99% Extractive distillation
95% Extractive distillation
MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
18,164 654,853 0 402
16,471 580,671 0 400
of toluene and i-octane are close, the distillation tower is not economical for separation in a chemical plant, meanwhile, the simulation data are useful for comparison. Simulation was conducted using distillation column subflowsheet in the software. The effect of number of trays on separation efficiency and purification of toluene and i-octane must be investigated. For this purpose, the bottom flow rate (toluene) and the top product (i-octane) were considered equal, number of trays changed from 10 to 70 and concentration of top and bottom product were recorded (Fig. 3). The reflux
3.2. Distillation Tower The simplest process for separation of toluene and i-octane is a single distillation tower. Because mentioned before, the boiling points Table 1 Results of extractive distillation for purification of toluene and i-octane.
Number of tray Top pressure (bar) Bottom pressure (bar) Feed tray Solvent tray Condenser Duty (MW) Reboiler duty (MW) Pump duty (MW) Vacuum duty (MW) Total duty (MW)
99% purification
95% purification
Extraction
Stripping
Extraction
Stripping
60 1.6 1.9 50 10 3.57 5.08 0.02 0.38 17.63
20 0.4 0.4 5 – 4.11 4.47
60 1.6 1.9 50 10 3.04 4.30 0.02 0.38 15.86
20 0.4 0.4 5 – 3.77 4.36
Fig. 3. Effect of number of tray on separation efficiency of distillation tower for toluene/i-octane. 48
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chiller and associated duty required for heat transfer was calculated. It should be noted that liquid nitrogen is recirculated and not consumed in the process. The temperature of chilling was considered as −55 °C to condense toluene/i-octane vapor at vacuum pressure of 0.001 bar. Component splitter in the software was applied for simulation of pervaporation stages. Fig. 6 indicates the process of cascade pervaporation with 2 stages. To obtain optimum number of pervaporation stages, different stages with different rates of permeate and retentate were simulated. Fewer stages require higher permeate and retentate rates and therefore higher membrane area and energy for condensation is needed. It should be noted that selectivity and permeation flux of the membrane for each stages are calculated from Eqs. (1) and (2), accordingly, composition and flowrate of permeate and retentate are calculated. The membrane area is a dependent parameter calculated from permeation flux needed for each stage. Tables 3 and 4 presents the results of 1 to 4 stages cascade pervaporation process and corresponding utility consumption, for purification of i-octane over than 99%. As shown, there is no remarkable difference in total energy consumption, total membrane area and flow rate of products, however, 2-stages process is more economical. Total demand energy for cascade pervaporation is 7.73 MW which is associated to chilling in pervaporation stages. Pump and vacuum power also have a small portion of demand energy. Retentate of the process is 20,880 kg/h purified i-octane and mass content of toluene in permeate is 77%. Also, total membrane area required for 60 t/h feed is 1727 m2. This would be practical using parallel cylindrical membrane modules. As indicated, energy consumption in cascade membrane system is much less than that of other processes.
Fig. 4. Concentration of top and bottom products versus reflux ratio in a distillation tower for separation of toluene and i-octane.
ratio was also fixed at 5 and the feed tray was considered at middle of the tower. As indicated, the changes of purifications are sharper when 10–30 trays are used and slighter when higher than 50 trays is applied. This illustrates, the difference of mass fractions is less than 1% from 60 to 70 trays. Accordingly, 60 trays was considered for tower in distillation and hybrid system. Figs. 4 and 5 present the effect of reflux ratio on duties of condenser and reboiler and concentration of toluene and i-octane. As indicated, to reach high purification, it is required to enhance reflux ratio and accordingly duty of both reboiler and condenser. Reflux ratio of 30 with total duty of 141 MW is required to reach purification of 95%, which is a huge energy demand. This shows that application of distillation tower for separation of toluene and i-octane is ineffective and higher values of purification (more than 90%) is difficult to reach due to extraordinary energy demand of condenser and reboiler. Furthermore, sizing calculations showed that minimum diameter required for 95% purification is 8.6 m, which is impracticable.
3.4. Hybrid distillation pervaporation As indicated, selectivity of the mixed matrix membranes increases in low and decreases in high concentration of toluene. Consequently, pervaporation in hybrid system could only be applied for low toluene concentration stream, e.g. top of the distillation tower. On this basis, hybrid process consisting distillation and pervaporation was designed for separation of toluene and i-octane. In this process, the purified toluene is derived from bottom and a mixture of toluene and i-octane is obtained from top of the distillation tower. The top product is routed to the pervaporation section consisting two stages. Retentate of the first stage is routed to the second stage and retentate of the second stage is the purified i-octane. Due to moderate selectivity of the membranes, permeate stream has relatively high content of i-octane and could not be mixed with toluene product, therefore, permeate from two stages are mixed and recycled to the tower. Chilling of the membrane stages are the same as cascade process. Fig. 7 presents the hybrid process: The hybrid process was simulated using Aspen Hysys V7.3 software with NRTL fluid package and component splitter for pervaporation. Tables 5 and 6 present the results of hybrid process for two levels of purification (95% and 99%) and Table 7 shows the utility consumption of process. As indicated, energy consumption of the hybrid process is 42 and 35 MW for purification level of 99% and 95%. The main portion of energy is attributed to the distillation. The duties of hybrid process system are much more than extractive distillation and the process is not competitive in terms of energy consumption. Meanwhile, it has some other advantages such as no need to solvent and being eco-friendly.
3.3. Cascade pervaporation process The other method which could be applied for separation of toluene and i-octane is cascade pervaporation process. The main difference between cascade pervaporation and other studied processes (distillation, extractive distillation and hybrid distillation/pervaporation) is that in cascade pervaporation, only one of the components (i-octane) can be purified. As revealed, separation factor of a feed containing high amounts of toluene is low and membrane is ineffective. Therefore, the retentate (rich i-octane) of each stages was routed to the next stage and permeate (rich from toluene) of all stages were mixed together. In pervaporation process, a chiller is required to liquefy the permeate and a pump is required to pressurize. Liquid nitrogen was considered as
4. Discussion The results show that the separation of toluene and i-octane as a close boiling point system is very difficult by distillation. The duty required for achieving 95% purification of 60 t/hr feed with composition of 50 wt. % toluene is 141 MW (with reflux ratio of 30), which is not economical and higher purification is not feasible, in terms of condenser and reboiler duty, sizing and construction of the tower. The demand of energy for extractive distillation using NFM is 15.86 MW
Fig. 5. Duties of condenser and Reboiler versus reflux ratio in a distillation tower for separation of toluene and i-octane. 49
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Fig. 6. The process of two-stage cascade pervaporation for separation of toluene and i-octane.
mixture has no energy saving in comparison with conventional extractive distillation system. However, cascade membrane system is highly effective where purification of an aliphatic mixture is required. This could be a suitable alternative in refineries and petrochemical plants for removing aromatics from fuels and purification of aliphatics. Increasing selectivity of the membrane leads reducing energy consumption of the hybrid process and also purification toluene in cascade pervaporation. Therefore, the two pervaporation-based processes can potentially be more effective than liquid extraction. Accordingly, ideal selectivity of the membrane was calculated and the related results have been illustrated in Fig. 9. As indicated, ideal selectivity of the membrane for hybrid system to reduce energy consumption lower than extractive distillation, is 180 and 170 for 99% and 95% purification, respectively. Moreover, the ideal selectivity for the cascade system to reach 99% and 95% purification for both toluene and i-octane is 38.5 and 35.5. The ideal selectivities have been reported in some previous experimental studies [19–22]. It can be concluded that for purification of such close boiling point system, distillation has no advantage and any hybrid system including distillation could not be effective. The best system for such process is cascade pervaporation. Using ideal selectivities, total energy consumption for purification of both component is about 8 MW (depending on permeation flux), 50% lower than extractive distillation. The size of membrane area depends on permeation flux; high permeation flux results in low membrane area and low capital investment.
Table 3 Results of 1–4 stages cascade pervaporation process for purification of i-octane over than 99%. Cascade pervaporation Characteristics
1-stage
2-stages
3-stages
4-stages
Membrane Surface (m2) Mass flow of retentate (kg/h) Mass flow of permeate (kg/h) Weigh fraction of toluene in permeate Total duty (MW)
1552.94 20100 39900 0.752
1727.07 20,880 39120 0.767
1773.99 20244 39755 0.755
1873.16 20120 39879.6 0.752
7.84
7.73
7.82
7.84
Table 4 Utility consumption for cascade pervaporation process. Utility MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
0 0 71,378 4103
and 17.63 MW, for 95% and 99% purification, respectively. In hybrid system consisting of one-stage distillation tower and two-stage pervaporation process, energy demand for hybrid process is higher than extractive distillation, e.g. 34.63 MW and 41.77 MW, for 95% and 99% purification. In hybrid and cascade pervaporation systems, the membrane could only be used in the streams with low content of toluene. For cascade membrane process, 1–4 stages with different rate and membrane area were evaluated and results showed that there is no significant difference in total membrane area, energy consumption and composition of products. Owing to reduce selectivity of membranes in high content of toluene, the maximum purification of toluene in this system is approximately 77%, but i-octane could reach to higher than 99% level of purification. The energy consumption of cascade pervaporation system is lower than distillation-based processes. Fig. 8 present energy consumption of different processes. The results show that applying hybrid system for separation of aromatic/aliphatic
5. Conclusion In this study, energy consumption of different processes for separation of toluene and i-octane (as representative of aromatic and aliphatic) is compared in detail. The conventional process for separation of such close boiling point mixtures is extractive distillation. Accordingly, extractive distillation data from one of the petrochemical plant was used for simulation. This process was compared with distillation tower, hybrid membrane and cascade pervaporation process. The pervaporation simulation was based on experimental data using linear (selectivity) and a 2-degree polynomial (permeation flux) 50
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Fig. 7. The process of hybrid distillation-pervaporation for separation of toluene and i-octane.
Table 5 Results of hybrid distillation-pervaporation process for purification of toluene and i-octane.
Number of tray Top pressure (bar) Bottom pressure (bar) Feed tray Recycle tray Condenser Duty (MW) Reboiler duty (MW) Pervaporation chilling and preheating duty (MW) Pump duty (MW) Vacuum duty (MW) Total duty (MW)
99% purification
95% purification
60 1.3 1.7 50 10 19.68 20.91 0.402
60 1.6 1.9 50 10 16.08 17.31 0.408
0.005 0.785 41.77
0.005 0.823 34.63
Fig. 8. Comparison of energy consumption for different processes.
Table 6 Results of pervaporation stages in hybrid process for purification of toluene and i-octane.
Weight fraction of toluene in feed Permeate flow rate (kg/h) Membrane Surface (m2) Vacuum duty (MW) Chilling Duty (MW) Total duty (MW)
99% purification
95% purification
Stage 1
Stage 2
stage 1
stage 2
0.128 5688.5 349.84 0.57 0.28 0.85
0.042 2313.4 137.38 0.22 0.12 0.34
0.170 4801.9 293.65 0.49 0.23 0.72
0.097 3433.0 209.98 0.34 0.17 0.51
Table 7 Utility consumption for hybrid process.
Fig. 9. Ideal selectivities for hybrid and cascade process.
Utility
99% hybrid system
95% hybrid system
MP Steam (kg/h) Cooling water (kg/h) Liquid nitrogen (kg/h) Power (kw)
39,771 1,788,482 7,913 790
32,924 1,481,690 8,029 828
equations. According to the experimental background, selectivity of the membrane is high in low content aromatic mixtures and vice versa. By using PVA/GO membranes with moderate selectivity, the results show that when purification of both components is required, extractive distillation is the most economical process, excluding environmental issues related to toxic solvents. Moreover, cascade pervaporation process 51
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could be extremely effective when purification of an aliphatic is required (aromatic removal). In this case, the energy consumption of cascade pervaporation process is 56.1% lower than that of extractive distillation. This could be very attractive for removing aromatics from gasoline according to the new environmental regulations as well as purification of aliphatics in petrochemical plants. The hybrid process is more economical than extractive distillation, if the ideal selectivity is 180 and 170 and also ideal selectivity for purification of both component in cascade process is 38.5 and 35.5, for 99% and 95% purification, respectively. This shows that distillation and hybrid process have no advantages for separation of close boiling point systems. In this condition, the most suitable process is cascade pervaporation. In addition to low energy consumption, the process has other remarkable advantages: non using solvents and being eco-friendly, compact, modular and flexible design. Gradually, the membrane seems to close industrial standards for separation of aromatic/aliphatic mixtures as an alternative to the conventional processes.
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