Influence of Solvent Nature on Extractive Distillation of the Benzene

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Influence of Solvent Nature on Extractive Distillation of the Benzene Hydrogenation Products Raphael Soeiro Suppino* and Antonio José Gomez Cobo Laboratory for Development of Catalytic Processes, Department of Chemical Systems Engineering, School of Chemical Engineering, University of Campinas, Cidade Universitária “Zeferino Vaz”, 13083-852, Campinas, São Paulo Brazil S Supporting Information *

ABSTRACT: The present work aims to study the influence of the solvents monoethanolamine (MEA), N-methyl-2-pyrrolidone (NMP), and monoethylene glycol (MEG) on the subsequent extractive distillation of a benzene−cyclohexene−cyclohexane mixture. Process simulations were carried out using the Aspen Plus software, considering a mixture composition obtained from previous catalytic tests for benzene hydrogenation in the liquid phase. The adopted production of cyclohexene, the main product of interest, was based on the capacity of a large chemical company in São Paulo state (Brazil). The results show that MEA is efficient only to separate benzene, whereas with the use of NMP or MEG all components are separated with high purity. Monoethylene glycol is a more advantageous solvent for this extractive distillation since its use leads to more practicable operating conditions and lower energy consumption, besides being a less expensive and biodegradable compound.

1. INTRODUCTION The partial hydrogenation of benzene is a reaction of environmental and industrial interest. Besides being an important alternative for benzene removal, the intermediate product of the consecutive reaction benzene → cyclohexene → cyclohexane can be used for the production of polyamides, such as nylon.1,2 To increase the yield of cyclohexene, the use of suitable catalysts and reaction media is necessary because the thermodynamics is strongly favorable for the undesired total hydrogenation of benzene, leading to the formation of cyclohexane.3 Ruthenium based catalysts,4,5 prepared from chlorinated precursors and supported on low specific surface area solids,6 have been increasingly applied for this reaction, since higher yields of intermediate product are achieved. In liquid reaction medium, water and hydrophilic catalysts have been widely used to inhibit the hydrogenation of the formed cyclohexene.7 The main role of the water is to withdraw the cyclohexene from the catalyst surface, reducing its undesired hydrogenation. Spinace and Vaz8 studied the addition of methanol, ethanol, glycerol, and monoethylene glycol, among others, to the reaction medium of benzene hydrogenation on Ru/SiO2 catalyst. The highest selectivity of cyclohexene was obtained for the addition of monoethylene glycol. The authors suggested that this result was due to the absence of hydrophobic groups in the solvent molecule. Such a characteristic favors the interaction between the hydroxyl groups in the solvent molecules and the water surrounding the catalyst particles. Recently, Suppino et al.9 have conducted studies regarding the benzene hydrogenation with a 5 wt % Ru/Al2O3 catalyst, prepared by wet impregnation from the RuCl3·xH2O precursor. The reactions were conducted in a “slurry” Parr reactor at 100 °C and under a H2 pressure of 5 MPa, employing different solvents in the reaction medium. At the beginning of every © 2014 American Chemical Society

reaction, besides the solvent, a catalyst mass of 300 mg, 25 mL of benzene, 30 mL of distilled water, and 5 mL of n-heptane (internal standard for gas chromatographic analysis) were introduced into the reactor. In order to evaluate the effects of the amount of solvent added to the medium, Suppino et al.9 conducted exploratory catalytic tests, using the amounts of 500 ppm, 9 mL, and 30 mL for each solvent. For the tests, the amount of solvent plus water was kept constant at 30 mL (50% of the total volume of the reaction medium). The higher yields of cyclohexene were obtained for the following quantities of solvent: 9 mL for monoethylene glycol (MEG) and N-methyl-2-pyrrolidone (NMP), and 500 ppm for monoethanolamine (MEA). As presented in Figure 1, the yield of cyclohexene follows the order MEA > MEG > NMP > without solvent. According to Suryawanshi and Mahajani,10 with the agreement of Fan et al.,11 amines and alcohols act by weakening the chemical bonding between cyclohexene and Ru. This would facilitate the cyclohexene desorption, thus decreasing its undesired hydrogenation. It is evident, therefore, that the addition of solvents to the reaction medium of benzene hydrogenation can increase the yield of cyclohexene, but does not eliminate the formation of large amounts of cyclohexane. Thus, a mixture is obtained containing benzene, cyclohexene, cyclohexane, and the solvent added to the medium which should be adequately separated for marketing or recycling of the compounds. Such a separation is among the most difficult processes in the chemical industry. It is particularly challenging since it involves substances whose boiling points are very close to one another for all ranges of composition of the reaction mixture. Received: Revised: Accepted: Published: 16397

April 28, 2014 September 25, 2014 September 30, 2014 October 13, 2014 dx.doi.org/10.1021/ie501753n | Ind. Eng. Chem. Res. 2014, 53, 16397−16405

Industrial & Engineering Chemistry Research

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change in the relative volatility of the system components. In extractive distillation columns, the solvent, which must have a boiling point higher than that of the original mixture components, is added on top of the extraction column. In this kind of distillation, unlike in azeotropic distillation, the separation agent is added in order to reduce the volatility of one component of the system. The component that becomes more volatile is obtained at the column top. In the bottom stream, one obtains the other components and the separating agent, whose recovery takes place in a conventional distillation column.14 Müller and Eisenlohr15 presented a set of ideal physicochemical properties for the selection of solvents to be used in extractive distillation. According to the authors, the solvent must have low viscosity, high density, and a boiling point above the one of the mixture to be separated. Finally, it should be thermally and chemically stable at its boiling point. Many methods to select solvent candidates for extractive distillation have been studied.16,17 Most of these methods are based on computer-aided molecular design, in which the required properties of a solvent are specified, and its structure is then calculated through the use of group contribution models.18 Pretel et al.17 employed a group contribution molecular design method to select potential solvents for extractive distillation, using a set of submolecular groups (UNIFAC) for the synthesis of molecular structures with desired solvent properties. The work of Pretel et al.17 required a few restrictions imposed for solvent screening. For instance, to select solvents capable of efficiently separating mixtures of toluene and methylcyclohexane, those restrictions included a relative volatility of at least 3 and a difference between the boiling points of the solvent and the toluene of at least 50 °C. As a result, Pretel et al.17 recommend N-methylpyrrolidone as a solvent suitable for separating toluene from methylcyclohexane. According to Garcia Villaluenga and Tabe-Mohammadi,14 extractive distillation is widely used in the chemical industry to separate mixtures of low relative volatility, such as the benzene−cyclohexane system. For this particular case, the authors recommend furfural as the separating agent. With this agent, high purity cyclohexane is obtained at the top of the extractive distillation column. The mixture furfural−benzene in the bottoms stream is fed to a second column, where benzene is recovered and furfural returns to the extractive column. The authors report that a disadvantage of this process is the elevated energy consumption for obtaining high purity cyclohexane. Ishikawa et al.19 developed a process for extractive distillation of benzene−cyclohexene−cyclohexane mixtures, employing nitrogenous compounds as separation agents. According to the authors, such compounds are preferably 1,3-dimethyl-2imidazolidinone or N-methyl-2-pyrrolidone, which should be used in the presence of water for better separation efficiency. The amount of solvent should be greater than or equal to the quantity of mixture to be separated. Simulations of the extractive distillation process with Nmethyl-2-pyrrolidone were conducted by Ishikawa et al.,19 using the group contribution model UNIFAC and Aspen software. These simulations were performed for a single extraction column with separation of the cyclohexane−cyclohexene mixture, considered by the authors as the most difficult in the case of the benzene−cyclohexene−cyclohexane system. For the simulations, an equimolar cyclohexene−cyclohexane mixture was considered, fed to the extractive column along

Figure 1. Effects of the solvents on the yield of cyclohexene. Adapted with permission from ref 9. Copyright 2013 Elsevier.

According to Berg,12 benzene boils at 80.1 °C, cyclohexene boils at 83.2 °C, and cyclohexane boils at 80.8 °C. The relative volatility between benzene and cyclohexene is 1.10, whereas that between benzene and cyclohexane is 1.02. The increase of relative volatility between the components of the system considerably reduces the number of stages required for the separation. As an example, the author considers the case of a relative volatility of 1.02, to which 470 theoretical stages are required to achieve a separation with 99% purity. However, for a relative volatility of 1.5, the number of theoretical stages required is around 20. Thus, the best way to achieve an efficient separation is to increase the relative volatilities of the components, which can be accomplished by adding a solvent to the mixture. Then, depending on the interaction between the solvent and the mixture, separation can be done by either azeotropic or extractive distillation.12 In azeotropic distillation, the solvent forms an azeotrope with one or more system components. The presence of azeotropes on the stages of a rectification column alters the relative volatility in a direction to make the separation on each stage greater and thus require fewer stages to either effect the same separation or make possible a greater degree of separation with the same number of stages. The solvent is added to the column feed, being recovered at the column top together with the most volatile component of the system. The azeotropic mixture is then separated by cooling, followed by phase separation or extraction with other solvents.12 The solvent used to form the azeotrope can be recovered and reused in its entirety in azeotropic distillation.13 According to Garcia Villaluenga and Tabe-Mohammadi,14 azeotropic distillation is economically recommended only when a small amount of nonaromatic compounds is present. For the separation of benzene−cyclohexane mixtures, the authors state that azeotropic distillation is recommended only when the concentration of benzene is greater than 90%, in which case acetone is used as solvent. Alcohols (ethanol or 2-propanol), esters (ethyl acetate or methyl acetate) and acetone are the main solvents for the azeotropic distillation of the benzene−cyclohexene−cyclohexane system.12 On the other hand, in extractive distillation a solvent (or separating agent) is added to the original mixture, providing a 16398

dx.doi.org/10.1021/ie501753n | Ind. Eng. Chem. Res. 2014, 53, 16397−16405


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Figure 2. Vapor−liquid equilibrium curves of the pairs (A) benzene (1)−cyclohexane (2) (experimental data from Rolemberg21) and (B) benzene (1)−N-methyl-pyrrolidone (2) (experimental data from Gupta et al.22).

with a solvent flow 5 times greater than that of the mixture. An operating pressure of 0.10 MPa was adopted for the column containing 18 theoretical stages. The feed mixture was made on the 14th stage, from the top, and solvent was introduced on the fourth stage. The authors concluded that nitrogenous solvents, such as N-methyl-2-pyrrolidone, are efficient for the separation of the studied mixture. They are thermally and chemically stable under the process conditions, modifying the relative volatility of the system components, which leads to a high efficiency separation for the cyclohexane. Müller and Eisenlohr15 reported that the use of water, together with the extracting solvent, increases its separation efficiency. The water assists in the distillation of higher aromatics, forming azeotropes with such compounds, which decrease the boiling temperature of the column bottom stream, resulting in lower energy demand. Vega et al.20 developed a methodology for the selection of suitable solvents for the extractive distillation of benzene− cyclohexene−cyclohexane mixtures. According to the authors, the solvent must have a high boiling point in order to reduce the relative volatility of the system components. According to them, various solvents have been tested to separate that mixture, such as sulfones, morpholines, amides, esters, and pyrrolidones. In their study, the authors examined the separation efficiencies of the solvents N,N-dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide, phenyl acetate, and dimethyl malonate, employing nonstationary gas chromatography. The results revealed that the solvents N,Ndimethylacetamide and N-methyl-2-pyrrolidone were the most effective. The separation of the cyclohexane−cyclohexene mixture was the most difficult, although these compounds have the greatest difference between their boiling temperatures (2.24 °C). Garcia Villaluenga and Tabe-Mohammadi14 proposed a new separation process by membranes, known as pervaporation. In this separation, membranes selective to desired components are employed and permeability governs the process. In pervaporation, the components of a liquid stream permeate a membrane and evaporate at different rates. The fact that the components undergo a phase change makes pervaporation a single process in the membrane field. The driving force of this process is the difference between the chemical activities of the components in the feed and on the permeate. The separation occurs because of

the difference in solubility and diffusion of the components in the membrane. The authors attribute to the pervaporation the great advantage of being a process independent of relative volatility and, thus, independent of vapor−liquid equilibrium limitations. In this context, the present work aims to study the extractive distillation of benzene−cyclohexane−cyclohexene mixtures with the use of monoethanolamine or N-methyl-2-pyrrolidone or monoethylene glycol, which was already added to the reaction medium as additives for the partial hydrogenation of benzene. Thus, it is intended to suggest an integrated process with the use of suitable solvents, both for the partial hydrogenation of benzene and for the separation of the reaction products.

2. MATERIALS AND METHODS The simulations performed in the present study were based on experimental results obtained by Suppino et al.9 for the hydrogenation of benzene in the liquid phase with Ru/Al2O3 catalysts. The compositions adopted for the feed streams of the extractive distillation process are based on the benzene conversions and cyclohexene yields presented in Table S1 (Supporting Information). The extractive distillation of the benzene−cyclohexene− cyclohexane mixture was simulated using Aspen Plus software. For each pair of components, a thermodynamic model was selected among the models NRTL, UNIQUAC, and UNIFAC to calculate the activity coefficient of the components in the liquid phase. The models were selected based on a methodology of the Aspen Plus software, considering the operating conditions of the process. For all pairs of components and the pairs component−solvent, the curves of vapor−liquid equilibrium were constructed by the analysis tool of the Aspen Plus software, considering the selected models. The curves of vapor−liquid equilibrium were compared to experimental data available in the literature,21,22 in order to establish the thermodynamic model that better described the system. Figure 2 illustrates the experimental points and curves obtained of the vapor−liquid equilibrium for the pairs benzene−cyclohexane (Figure 2A) and benzene−N-methyl-2pyrrolidone (Figure 2B). The curves provided by the NRTL and UNIQUAC models practically overlap themselves in both cases and are closer to 16399



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Table 1. Total Mass Flow and Compositions of the Feed Streams of the Distillation Unit mass fraction (wt %) solvent

mass flow (103 kg/h)

benzene

cyclohexane

cyclohexene

solvent

water

MEA NMP MEG

97.6 256 196

17.1 14.4 9.40

15.7 16.4 20.4

6.65 2.50 3.30

0.050 15.7 16.7

60.5 51.0 50.2

Figure 3. Process of extractive distillation with MEA.

of work. To obtain these values, it was assumed that this industry works 330 days/year operating 24 h/day. The total mass flow and compositions of the feed streams in the distillation units are given in Table 1. It is noteworthy that, for each solvent, there is a value of total mass flow and of feed stream composition, which is due to the catalytic performance in the presence of the solvent (Table S1, Supporting Information). However, the cyclohexene mass flow at the entrance of the distillation unit is the same for all the studied solvents (6.5 × 103 kg/h); the obtained results are directly related to this value.

the experimental data than the curve predicted by the UNIFAC model. Few experimental data regarding the vapor−liquid equilibrium of the components of the benzene−cyclohexene− cyclohexane mixture as well as the solvents used in this work were found in the literature.21−23 Thus, the choice of a suitable thermodynamic model was restricted by the availability of experimental data, constituting a limitation of the approach. On the basis of the data presented in Figure 2, the UNIQUAC model was chosen to represent the system in this study. A stage efficiency of 100% was considered and the pressure drop through all equipments of the separation processes was disregarded in simulations. Such considerations were made so that one can observe the separation of the components without the influence of these factors. They are important in real industrial processes, but would be somewhat inaccurate in the simulation due to the lack of an industrial baseline. Furthermore, the distillation conditions were optimized regarding temperature, feed positions, and number of theoretical stages in order to maximize the desired separation and to minimize the heat load required by the reboilers. In all studied cases, the first step consisted of lowering the pressure of the output current of the reactor (5 MPa) to atmospheric pressure (0.1 MPa), allowing safer operation of the columns and reducing significantly operational costs. This operation was conducted through a turbine (standard efficiency of 84%), and the recovered energy (825 kW) can be directed to another process step. All equipment after the turbine operate at atmospheric pressure. The values estimated for the annual production of cyclohexene were based on the productive capacity of a large chemical company in São Paulo state (Brazil), which is around 52 × 103 tons/year, or 6.5 × 103 kg/h for a year with 8 × 103 h

3. RESULTS AND DISCUSSION 3.1. Separation with MEA. A schematic representation of the separation process with the MEA solvent is shown in Figure 3, where “ns” refers to the number of theoretical stages and “R” refers to the mass reflux ratio of the column. In this process, the feed (stream 1) is first cooled and submitted to a phase separation on a vessel (S-1), in order to separate the water and some of the MEA (stream 3) from the other compounds. Water is recovered at the top of the dehydration column (DH-1), and MEA is collected at its bottom. These streams may be recycled to the reaction unit. Stream 2, which contains mostly products of the reaction, undergoes extractive distillation on column E-1, which is also fed with pure MEA (stream 4). Benzene is obtained at the top of column E-1 (stream 6) with the aid of a separation vessel fed with pure water. At the bottom of column E-1, a mixture of cyclohexane, cyclohexene, and MEA is obtained and then redirected to a conventional distillation column (D-1) in order to remove the MEA present in this mixture (stream 9). This removal is necessary to prevent a negative impact on the 16400



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Table 2. Conditions of the Main Streams of the Extractive Distillation Process with MEA stream condition temp (°C) total mass flow (kg/h) mass flow (kg/h) benzene cyclohexane cyclohexene water MEA

1

4

6

9

10

12

13

14

100 9.8 × 104

80 1.7 × 104

30 1.7 × 104

169 1.7 × 104

75 7.0 × 106

169 7.0 × 106

82 5.7 × 103

169 1.1 × 103

1.5 × 104 9.1 × 102

90.7 41.7 5.6 × 103

1.7 × 104

1.7 × 104 22.0 4.10 10.4 22.5

7.0 × 106

7.0 × 10−3

1.7 × 1.5 × 6.5 × 5.9 × 48.7

104 104 103 104

1.0 × 10−3

1.7 × 104

Table 3. Heat Duties for the Extractive Distillation Process with MEA heat duty (MW) cooler/condenser

reboiler

C-1 C-2 DH-1 E-1 E-2 D-1 D-2 total

6.3 0.4 117 7.7 4.0 8.8 2.4 147

120 9.4 611 9.5 2.4 753

1.1 × 103

column E-2. The total energy consumption for this process, including the duties of coolers, condensers, and reboilers shown in Table 3, was estimated at 900 MW, mostly due to the MEA stream. The inefficiency of MEA for the separation of the mixture cyclohexene−cyclohexane is probably due to the low solubility of both components in this solvent, and therefore an integration of this separation process with the reaction unit does not seem to be advantageous. 3.2. Separation with NMP. A schematic representation of the separation process with the NMP solvent is shown in Figure 4. Similarly to what occurs in the process with MEA, water is first separated from the other compounds on a dehydration unit (S-1). Water is recovered at the top of the dehydration column DH-1, and NMP is collected at its bottom. Then, stream 2 is fed to the extractive distillation column E-1 along with NMP and water19 (stream 4). In this case, cyclohexane is obtained at the column top (stream 6) after being condensed with an external heat exchanger, as recommended by Ishikawa et al.19 The remaining mixture at the bottom of this column is led to a conventional distillation unit (D-1) in order to remove the NMP from this stream. The top stream of column D-1 is then fed to a separation vessel (S-2) with the purpose of separating the water (stream 7) from the other organic compounds (stream 9) which is then led to a second extractive distillation unit (E-2). In this column a large amount of NMP is required to perform the separation of the benzene−cyclohexene mixture. A mixture of benzene and NMP is recovered at the top of this column, being separated via conventional distillation on the D1 unit. In its turn, the bottoms of the column E-2 (stream 13) is fed to the extractive distillation column E-3, together with water (stream 14) that promotes this separation. A cyclohexene− water mixture is obtained on the top of this column and separated on a vessel (S-3). Cyclohexene is obtained with elevated purity (stream 15), while the water is recycled to the column E-3 as external reflux. The conditions obtained for the main process streams are given in Table 4, whereas the heat duties required for this separation are presented in Table 5. The conditions of all streams of this process are presented in Table S3 (Supporting Information). The results show that NMP presents a high efficiency to separate cyclohexane from the mixture present in the reactor. At the top of the extraction column E-1 (Figure 4), stream 5 contains 98.7 wt % of the total quantity of cyclohexane in the process. This result is in agreement with what was observed by Ishikawa et al.,19 as well as by Vega et al.20 The use of NMP, in comparison to the process with MEA, presents the advantage of enabling the recovery of all

performance of column E-2, which is therefore fed with a stream containing mainly cyclohexane and cyclohexene (stream 8). A large amount of MEA (stream 10) is required in this column in order to promote the separation. At the bottom of column E-2, a large flow of a mixture of cyclohexane and MEA (stream 12) is obtained, which is difficult to separate due to the amount of the MEA solvent. At the top a mixture of cyclohexene and MEA (stream 11) is collected. Then, stream 11 is sent to a conventional distillation column (D-2), where cyclohexene is obtained with relatively high purity (stream 13) and the remaining MEA is recovered (stream 14). The conditions of the main streams of the extractive distillation process with MEA are given in Table 2, whereas the heat duties required for this separation are reported in Table 3. The conditions of all streams of this process are presented in Table S2 (Supporting Information).

equipment

7.0 × 106

The addition of MEA to the reaction medium led to the higher maximum yield of cyclohexene (16%), as presented in Figure 1. As a separation agent, this solvent shows high efficiency for the separation and recovery of the benzene not converted in the reaction. The benzene is obtained in high purity (99.6 wt %) in the extractive distillation column E-1, which allows its recycle to the reactor. In contrast, for the separation of the mixture cyclohexene− cyclohexane, MEA does not present an elevated separation efficiency, leading to a cyclohexene loss of about 900 kg/h shown on the process stream 12. In this case, it is worth noting that, in order to obtain a cyclohexene purity above 97 wt %, it was necessary to add an industrially unfeasible amount of MEA (7.0 × 106 kg/h) to column E-2, which made it impossible to obtain pure cyclohexane. Furthermore, the large supply of MEA increases significantly the energy consumption. This causes an economic disadvantage, due to the high costs of the necessary equipment to accommodate this flow as well as of the utilities to supply the thermal load for the reboiler on the extractive 16401



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Figure 4. Process of extractive distillation with NMP.

Table 4. Conditions of the Main Streams of the Extractive Distillation Process with NMP stream condition

1

4

6

8

10

11

12

15

16

temp (°C) total mass flow (kg/h) mass flow (kg/h) benzene cyclohexane cyclohexene water NMP

100 2.6 × 105

28 1.6 × 105

30 4.2 × 104

204 1.6 × 105

50 5.0 × 106

72 3.7 × 104

204 2.3 × 103

30 6.4 × 103

204 5.0 × 106

5.0 × 10 1.6 × 105

4.1 × 102 4.2 × 104 4.67 6.16 13.0

5.0 × 106

3.6 × 104 4.7 × 102 21.7 28.6 7.0 × 10−3

3.7 4.2 6.4 1.3 4.0

× × × × ×

104 104 103 105 104

3

1.6 × 105

heat duty (MW) cooler/condenser

C-1 C-2 C-3 C-4 DH-1 E-1 E-2 E-3 D-1 D-2 total

16 54 0.91 86 374 30 33 17 611

2.3 × 103

6.4 × 103 2.10 0.43

0.43 5.0 × 106

required as a coextracting agent in order to improve the separation efficiency. These findings are corroborated by the results of Vega et al.,20 who observed a high efficiency of NMP for the separation of the mixture cyclohexane−cyclohexene. However, the separation efficiency of this extracting agent was drastically reduced for the mixture benzene−cyclohexene. The authors concluded that the selectivity of NMP is much higher for cyclohexane than for the other components of the system. According to the authors, the ability of the solvent to solubilize the system components (solvency) is drastically changed by the increase of temperature and depends on the nature of the organic compound. The very high mass amount of NMP required for this process (5.16 × 106 kg/h) involves a great operational cost. Indeed, the energy consumption of this process was estimated to be 1676 MW, almost twice the energy consumption of the process with MEA. Furthermore, this solvent high flow makes it necessary to insert a stream of water to promote the separation between NMP and cyclohexene, leading to additional costs of energy and materials. 3.3. Separation with MEG. A schematic representation of the separation process with the MEG solvent is shown in Figure 5. This process has been integrated with respect to its streams in order to optimize the separation operations and minimize solvent consumption as well as component loss. This process was integrated due to the ability of MEG in the separation of all components and its recovery with high purity, as detailed hereafter. In Figure 5, “H” represents heating equipment, while “C” stands for cooling units.

Table 5. Heat Duties for the Extractive Distillation Process with NMP equipment

46.1

reboiler

379 52 451 88 40 55 1065

components with high purity (over 98 wt %), allowing recovery also of the added solvent (Table 4). However, the use of NMP requires a high flow rate of extraction solvent, increasing the process expenses, especially regarding power requirements and manufacturing operation. The results of Ishikawa et al.19 were obtained for a low flow feed with cyclohexane and cyclohexene in equimolar amounts, in the absence of ternary mixtures containing benzene. The consideration of the presence of benzene in the system is very important, mainly because of the difficulty of separating the mixture benzene−cyclohexene using NMP. In fact, to obtain high purity of cyclohexene, the flow of NMP must be very elevated (5.0 × 106 kg/h). Furthermore, the addition of water is 16402



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Figure 5. Integrated extractive distillation process with MEG.

Table 6. Conditions of the Main Streams of the Extractive Distillation Process with MEG stream condition temp (°C) total mass flow (kg/h) mass flow (kg/h) benzene cyclohexane cyclohexene water MEG

1

4

8

11

13

14

15

100 196 × 103

197 4.9 × 103

77 40 × 103

35 18 × 103

82 6.5 × 103

30.0 98 × 103

197 32 × 103

18 × 103 19.1 1.02 13.3 5.70

0.057 1.09 6.5 × 103 7.0 × 10−3

134 0.66 4.67 98 × 103 0.025

0.040

4.9 × 103

36.88 40 × 103 0.50 2.93 95.5

18 × 103 40 × 103 6.5 × 103 98 × 103 33 × 103

The process with MEG is also similar to the previously described processes. The organic compounds are separated in the extractive distillation column E-1 by MEG addition (stream 4). The bottom stream of E-1 (stream 5) contains a cyclohexene−MEG mixture that is easily separated on a conventional distillation column D-1. A cyclohexane−benzene−MEG mixture is collected at the top of column E-1, separated on a vessel (S-2) in order to feed the next extractive distillation unit E-2 with mostly cyclohexane and benzene (stream 6). By addition of MEG to this column (stream 7), cyclohexane is obtained at the top and a benzene−MEG mixture is obtained at the bottom. In sequence, this mixture is fed to the distillation unit E-3, in which a mixture of water and MEG is added (stream 10) to promote the separation. Thus, at the column top a water−benzene mixture is recovered and is separated on vessel S-4, resulting in high purity benzene (stream 11). The bottom stream of E-3 still contains an amount of cyclohexene that can be separated via conventional distillation. Therefore, stream 12 is fed to column D-2 to ensure maximum recovery of cyclohexene. This top stream is driven to column D-1, in which high purity cyclohexene is obtained (stream 13). Finally, streams 14 and 15 contain, respectively, water and MEG recovered in the process. Given their purity, these streams may be recycled to the reaction unit. The conditions obtained for the main process streams are given in Table 6, and the heat duties required for this separation are listed in Table 7. The conditions of all streams of

32 × 103

this process are presented in Table S4 (Supporting Information). MEG is a solvent used in the extractive distillation of different mixtures, such as those found in the production of anhydrous alcohol. This is due to its thermal and chemical stability, as well as the low volatility of this compound, which Table 7. Heat Duties for the Extractive Distillation Process with MEG heat duty (MW)

16403

equipment

cooler/condenser

C-1 C-2 C-3 C-4 C-5 C-6 H-1 DH-1 E-1 E-2 E-3 D-1 D-2 total

11 1.6 1.6 0.23 0.23 1.5 205 26 15 11 2.7 1.4 277

heater/reboiler

8.2 209 19 11 17 2.9 1.7 269



Article

N-Methyl-2-pyrrolidone, the solvent less selective for obtaining cyclohexene, makes the process very costly from operational and energetic viewpoints. In turn, monoethylene glycol is a selective solvent for obtaining cyclohexene, as well as an efficient agent for the studied extractive distillation. Its use leads to the highest recovery of pure components, with practicable operating conditions and relatively low energy consumption, besides being a less expensive and biodegradable compound. Since monoethanolamine was the best additive to obtain high cyclohexene yields and monoethylene glycol was the best extractive agent for the separation of the reaction products, future studies may consider the integration between the reaction and separation units with the use of both of these compounds.

are suitable for the application. Then, again, the addition of MEG to the reaction medium of benzene hydrogenation led to a significant increase in the yield of cyclohexene (Figure 1). Therefore, the use of this solvent as a separation agent appears to be interesting from the industrial viewpoint. As can be seen in Figure 5, the separation process involves a little more equipment and a few more unit operations in comparison to the processes with MEA and NMP. This fact is due to the integration of the streams, which requires more investment in material and heat transfer, though leading to an almost complete recovery of all components of the system with purity above 99 wt %. Such a recovery enables the marketing of both reaction products (cyclohexene and cyclohexane) and the reactant (benzene) can be recycled to the reaction. One can also observe from Table 6 that all MEG added to extractive distillation process was recovered with high purity and recycled to the process, feeding the columns that need this solvent. Therefore, the loss of MEG was drastically decreased. The comparison of the consumption of solvents shows that, besides the impracticable flow rate required (7.02 × 106 kg/h), MEA does not efficiently separate the products. NMP is efficient for the component separation, but its required flow rate (5.16 × 106 kg/h) is equally industrially unfeasible. In turn, MEG leads to the highest recovery of components, with a purity degree of almost 100 wt %. This result is observed for a practicable solvent flow rate (about 36 × 103 kg/h), although it is still high, without the need of a high amount of water and with a relatively low energy consumption. Indeed, the total energy consumption of this process was estimated to be 546 MW, which is nearly 60% of the energy consumed in the process with MEA and 35% of the energy required in the process with NMP. The high separation efficiency of MEG can be related to a higher solubility of the components in this solvent. In general, the efficiency of the separation agent in an extractive distillation is directly related to the solubility of the components therein, with the more soluble component being carried by the solvent to the bottoms stream of the column.14 Since cyclohexene is the first component carried by MEG, while benzene is entrained in a second operation, the sequence of solubility in MEG may be as follows: cyclohexene > benzene > cyclohexane. It is noteworthy that solubilities of the components in the studied solvents were not found in the literature. Therefore, one can observe that MEG, besides being an interesting additive to the reaction medium, is also a remarkable solvent for the separation of benzene−cyclohexene−cyclohexane mixtures. Moreover, in a comparison made with the costs of analytical grade solvents obtained from Merck and Acros for the reaction study,9 it was observed that the cost of MEG is half the cost of MEA and 5 times less expensive than NMP. Hence, the use of MEG as an extractive agent for this separation may also constitute an economical and environmental advantage due to its low cost and biodegradability, whereas MEA and NMP are more toxic and nonbiodegradable, as stated on their material safety data sheets.

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ASSOCIATED CONTENT

* Supporting Information S

Previously published reaction results used in order to estimate the composition of the feed streams as well as the conditions of all streams of the extractive distillation processes studied in this article. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +55 19 3521-3894. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the “Coordination for the Improvement of High Level Personnel” (CAPES) for financial support and the research scholarship granted to R.S.S.

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REFERENCES

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4. CONCLUSIONS Although the addition of monoethanolamine to the reaction medium for benzene hydrogenation leads to higher yields of the desired product (cyclohexene), this solvent is not an efficient agent for the separation of benzene−cyclohexene− cyclohexane mixtures by extractive distillation. 16404



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Influence of Solvent Nature on Extractive Distillation of the Benzene

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