Entrainer-Enhanced Reactive Distillation for the Production of Butyl

Apr 16, 2014 - production of butyl acetate from butanol and acetic acid via an esterification ... butyl acetate (BuOAC) in a continuous reactive disti...
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Entrainer-Enhanced Reactive Distillation for the Production of Butyl Acetate Minjeong Cho, Sanghwan Jo, Gunhyung Kim, and Myungwan Han* Department of Chemical Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Korea ABSTRACT: A new reactive distillation (RD) process using a mass separation agent (extraneous entrainer) for the efficient production of butyl acetate from butanol and acetic acid via an esterification reaction was proposed. Until now, butyl acetate, a product of the esterification reaction, has been used as an internal entrainer in the production of butyl acetate. It has also been accepted that a configuration consisting of a prereactor followed by a reactive distillation column is most suitable for the production of butyl acetate. The use of an internal entrainer may shift the reaction equilibrium to be favorable to the reverse reaction in the reactive section, limiting the reaction yield. However, an extraneous entrainer, such as cyclohexane, can resolve the problem. It can be used to break the water−alcohol azeotrope and remove water more efficiently from the reactive section. In this study, we proposed a single reactive distillation configuration without a prereactor and the use of an extraneous entrainer (cyclohexane) and investigated the influence of various parameters, such as entrainer composition, reboiler duty, feed location, and number of reactive stages, on process performance. We demonstrated that the use of an extraneous entrainer instead of the internal entrainer, butyl acetate, can significantly improve the process efficiency and achieve great energy savings and capital cost reduction compared to the use of a prereactor followed by RD with butyl acetate as an entrainer.

1. INTRODUCTION n-Butyl acetate is an important solvent in the chemical industry and is typically produced by the esterification of acetic acid (HOAc) with n-butanol (BuOH). Recently, the synthesis of butyl acetate (BuOAC) in a continuous reactive distillation column with solid acid catalysts has been studied.1−9 Esterification is an equilibrium-controlled reaction, limited by the chemical equilibrium in the presence of water. In the case of equilibrium-controlled reactions such as esterification, the application of a reactive distillation (RD) system is known to be useful. The chemical equilibrium can then be shifted in the desired direction of the forward reactions through the continuous removal of products from the reactive section. However, butanol forms a minimum-boiling azeotrope with the product water so that butanol is removed with water at the top of the column. If the alcohol is not entirely consumed in the reactive section, reaction conversion is limited, and this process requires additional separation steps. The high boiling point of alcohol affects the temperature of the reactive section. If the temperature of the reactive section is closely related to the thermal stability of the catalyst, its activity and lifetime may be affected. A mass separation agent, usually known as an entrainer, can be used to solve this problem. The entrainer is utilized to improve the performance of RD by increasing the relative volatility of the products, thus facilitating separation; this is called an entrainer-enhanced RD (ERD) system. Enhanced water removal can facilitate the recycling of unreacted alcohol and improve the conversion of the reactant. Because the entrainer forms a minimum-boiling azeotrope with water, the temperature of the reactive section can be lower than the permissible limit for the catalyst. The use of reactive distillation columns using butyl acetate as an internal entrainer for the synthesis of butyl acetate has been studied.1−9 Janowsky et al.1 performed butyl acetate synthesis © 2014 American Chemical Society

experiments to study steady-state column performance at three different pressures over a range of 0.65−1.105 bar. The stripping section, which was filled with catalyst, acted as a reactive section, and the feed from a prereactor, containing a considerable amount of butyl acetate, was introduced at the top of the reactive section. At higher pressure, they observed a significant amount of 1-butene at the top of the column and the unwanted byproduct dibutyl ether (DBE) at the bottom. Hanika et al.2 studied two different catalytic distillation configurations: a single catalytic distillation column and a fixed-bed reactor followed by a catalytic distillation column. A mixture of acetic acid and butanol (in excess) was preheated and either fed into the catalytic zone of the column or into the prereactor. The output from the prereactor, containing an almost-equilibrium mixture of acetic acid, butanol, butyl acetate, and water, was preheated almost to its boiling point and fed into the catalytic zone of the RD column. Steinigeweg and Gmehling6 developed a configuration consisting of a prereactor and a heterogeneously catalyzed reactive distillation process for the production of n-butyl acetate. They studied the thermodynamic properties, reaction kinetics, and RD system through experiments and simulation and suggested that a prereactor followed by an RD column is the best process alternative. Gangadwala et al.7−9 studied conceptual process design for butyl acetate synthesis and reported that a configuration consisting of prereactor and a column with a reactive middle section and nonreactive stripping and rectifying sections is most suitable. In all these processes, butyl acetate, Received: Revised: Accepted: Published: 8095

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HOAc + n‐BuOH ↔ n‐BuOAc + H 2O

the reaction product, was used as an entrainer for water removal from the reactive section. Several studies using an entrainer in reactive distillation have been reported.10−16 Entrainer-based reactive distillation for the synthesis of fatty acid esters has been studied by Dimian et al.10 The authors investigated the effect of various entrainers with different polarities for the esterification of a fatty acid with isopropanol. Suman et al.12 studied entrainer-based reactive distillation (EBRD) for the esterification of ethylene glycol with acetic acid. The use of 1,2 dichloroethane as an entrainer results in the efficient removal of water formed in the reaction, leading to nearly complete conversion. Furthermore, the use of 1,2-dichloroethane as an entrainer controls the reaction temperature and prevents the catalyst from possible thermal degradation. Hu et al.15 and Zhang et al.16 proposed a RD column with a sidedraw and selected n-BuOAC as a mass separating agent to carry water from the middle of the column instead of from the top reflux of ethyl acetate or isopropyl acetate functioned as the entrainer; the authors reported that the proposed processes offered significant energy savings and capital cost savings compared to the conventional process. Wang et al.17 have studied an entrainer-enhanced reactive distillation process for the production of butyl cellosolve acetate. In this work, we proposed a novel entrainer-enhanced reactive distillation process for efficient n-butyl acetate production. In all previous studies, n-butyl acetate, the product of the esterification reaction, has been used as an entrainer for the removal of water generated during the reaction, and a prereactor followed by a reactive distillation column has been reported to be the most suitable setup for the synthesis of butyl acetate. We introduced reactive distillation using an extraneous entrainer for the improved removal of water from the reactive section and enrichment of the reactants in the reaction zone of the reactive distillation column. We showed that the performance of the proposed process was superior to the conventional process in terms of capital cost and energy consumption.

We chose a pseudohomogeneous model proposed by Steinigeweg and Gmehling6 to describe the esterification reaction heterogeneously catalyzed by ion-exchange resins. Esterification reactions are known to be reversible second-order reactions. The pseudohomogeneous model can be written as follows: rBuOAc =

The temperature dependency of the rate constant is expressed by Arrhenius’ Law: ⎛ −EA, i ⎞ ki = ki0exp⎜ ⎟ ⎝ RT ⎠

esterification hydrolysis

EA(kJ/mol)

1 −1

6.1084 × 104 9.8420 × 104

56.67 67.66

3. PROCESS DESIGN We have considered three possible reactive distillation configurations for the production of n-butyl acetate. Figure 1 shows the three different RDC configurations to be considered: (a) a reactive distillation column using butyl acetate as an entrainer; (b) a prereactor followed by reactive distillation column, and one decanter using butyl acetate as an entrainer; (c) a reactive distillation column using an extraneous entrainer (cyclohexane). All simulations were carried out with the steadystate model RADFRAC from the process simulator Aspen Plus. Stages are numbered from the top to the bottom. The RD column consists of 28 stages, and the reactive section is from stage 8 to 18. The feed flow rates for both reactants, HOAc, and n-BuOH, are 36 kmol/h, respectively. The configuration is based on Steinigeweg and Gmehling’s6 study. In the RD column with an entrainer, overhead vapor is condensed and then separated into aqueous and organic phases in a decanter. The aqueous phase containing mostly water is withdrawn as the distillate, while the organic phase involving mostly entrainer is refluxed to the column. There are three zones in the RD column. The rectification zone separates the reactants from the entrainer−water azeotrope, and the stripping section separates the reactants from the heavy product, butyl acetate. The three designs were assumed to have the same

Table 2. Composition and Temperature of the Azeotrope Involved in the System at Atmospheric Pressure components

water

water− BuOAc water− BuOH water− BuOAC− BuOH BuOH− BuOAc

0.7072

BuOH

BuOAc

azeotrope temp (K)

azeotrope

0.2928

334.05

heterogeneous

365.76

heterogeneous

0.7517

0.2483

0.6892

0.1013

0.2095

363.23

heterogeneous

0.7802

0.2198

390.13

homogeneous

(3)

The parameters (k01,k0−1,EA,1,EA,−1) are given in Table 1. The kinetic model (eq 2) assumes a linear relationship between the catalyst mass and the reaction rate. There are two minimum-boiling binary heterogeneous azeotropes (water and butanol, water and butyl acetate), one minimum-boiling homogeneous azeotrope (butanol and butyl acetate) and one minimum-boiling ternary heterogeneous azeotrope (water, butanol, and butyl acetate) in this system (Table 2). To account for the nonideal vapor−liquid equilibrium (VLE) or vapor−liquid−liquid equilibrium (VLLE), in this butyl acetate system, a UNIQUAC model is used for the activity coefficients. The UNIQUAC model binary interaction parameters, Uij (Table 3), were obtained from the DECHEMA vapor−liquid equilibrium data collection (Chemistry Data Series edited by J. Gmeling and U. Onken). The van der Waals properties r and q (Table 4) were taken from the Dortmund Data Bank, version 2002.19 Among these azeotropes, the water−BuOAc−BuOH azeotrope is a minimumboiling azeotrope, which means that BuOAc can act as an entrainer, removing water from the reaction zone.

Table 1. Kinetic Parameters of Esterification for the Pseudohomogeneous Kinetic Model3 k0i (mol/g·s)

1 1 dni = k1aHOAcaBuOH − k −1aBuOAcaH2O mcat vi dt (2)

2. CHEMICAL KINETIC AND THERMODYNAMIC MODEL n-Butyl acetate is produced by the esterification of acetic acid and n-butanol via the following reversible reaction:

i

(1)

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Table 3. Binary Interaction Parameters for UNIQUAC Equation (cal/mol)a U11 U12 U13 U14

= = = =

U21 U22 U23 U24

0.0 581.1471 (a) 527.9269 (b) 461.4747 (c)

= = = =

U31 = −343.593 (b) U32 = −131.7686 (d) U33 = 0.0 U34 = −298.4344 (f)

68.0083 (a) 0.0 148.2833 (d) 82.5336 (e)

U41 U42 U43 U44

= = = =

685.71 (c) 24.6386 (e) 712.2349 (f) 0.0

a

All interaction parameters, Uij, are taken from the Vapor−Liquid Equilibrium Data Collection of the DECHEMA Chemistry Data Series edited by J. Gmehling and U. Onken. The volume and page numbers are as follows: (a) Vol.1, Part 1b, p 254; (b) Vol. 1, Part 1, p 106; (c) Vol. 1, Part 1b, p 338; (d) Vol. 1, Part 2d, p 157; (e) Vol. 1, Part 2b, p 197; (f) Vol. 1, Part 5, p 147. Components: (1) water; (2) butanol; (3) acetic acid; (4) butyl acetate.

position and, hence, to lower conversions in these sections. The two reactants are fed into the prereactor, and the prereactor stream is fed into stage 8, which is the first stage in the reactive section. Chemical kinetics proposed by Steinigeweg and Gmehling6 were chosen for the prereactor. The diameter and length of the prereactor were 0.35 m and 5.52 m, respectively, and the temperature in the reactor was maintained at 348.15 K. The influence of important design factors on the reaction yield was investigated to develop guidelines for the design of the most effective entrainer-enhanced reactive distillation process. The major design parameters considered were the role of entrainer, reboiler duty, the number and location of feed positions, and the number of reactive stages. Total number of stages was chosen to be 28 because the column performance is insensitive to the number of stages if more than 27 stages are given. Role of Entrainer. In configurations (a) and (b), n-butyl acetate, which is a product of this reaction, is used as an entrainer to remove water in the reaction zone of the column. The presence of the product, n-butyl acetate, in the reaction zone has an unfavorable effect on the reaction yield. Figure 1c shows the proposed entrainer-enhanced reactive distillation [configuration (c)], which has only one reactive distillation column, using an extraneous entrainer without a prereactor. The main purpose of an extraneous entrainer in the esterification of butanol and acetic acid is to remove water from the reaction zone without shifting the reaction equilibrium caused by the presence of butyl acetate in the reaction section of the column. The extraneous entrainer removes water and lowers the concentration of butyl acetate in the reaction zone, leading to a high butyl acetate yield. The two products, butyl acetate and water, are formed in the reaction zone and move out of the reaction zone, while the reactants are concentrated in the reaction zone. The use of an extraneous entrainer that is more effective than butyl acetate could make the reactive

Table 4. Area and Volume Parameters for the UNIQUAC Equationa component

r

q

water butanol acetic acid butyl acetate

0.92 3.4543 2.2024 4.8274

1.4 3.052 2.072 4.196

a

van der Waals properties r and q (Table 1) were taken from the Dortmund Data Bank (DDB), version 2002, which was kindly provided by DDBST GmbH Oldenburg, Germany.

number of reacting stages and nonreacting stages and the same catalyst loading in RD column. The prereactor was designed to yield approximately 60% conversion of the reactants by adjusting the catalyst loading and length of reactor. We assumed that 500 kg of catalyst was loaded in the prereactor and that 100 kg of catalyst was loaded at each stage of the reactive distillation column. A reactive distillation column using an internal entrainer (butyl acetate) [configuration (a) and configuration (b)] for the production of butyl acetate has been investigated by several researchers.1−9 Configuration (a) was simulated as a base configuration for comparison with configuration (b) and configuration (c). The use of a prereactor in configuration (b) will improve the product yield compared to configuration (a) because the reactant is converted by approximately 60% in advance and the mixture from the prereactor is fed into the column. This application of a prereactor is a favorable process alternative when there are minor differences in the reactant boiling points. Steinigeweg and Gmehling6 concluded that the optimal feed location of the stream leaving the prereactor is above the first reactive stage. This results in an optimal distribution of the reactants in the reactive section. Feeding the prereactor stream into the column below the first reactive stage leads to a higher amount of water in the sections above the feed

Figure 1. Schematic diagram of three configurations for producing butyl acetate: (a) reactive distillation without extraneous entrainer and prereactor [configuration (a)]; (b) the conventional process consisting of prereactor and RD columns [configuration (b)]; (c) the proposed RD column with extraneous entrainer (cyclohexane) [configuration (c)]. 8097

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Figure 2. Ternary map: (a) water−BuOAc−BuOH, (b) water−cyclohexane−BuOH.

We have considered several entrainers as candidates for our system and have selected cyclohexane, which satisfies the conditions given by Dimian et al.10 Cyclohexane forms an azeotrope with water and butanol at a temperature 342.35 K, while butyl acetate forms an azeotrope with water and butanol at a temperature 363.15 K. The low temperature of the azeotropic mixture formed with cyclohexane indicates that the azeotropic mixture can be more easily separated from the reaction mixture than when butyl acetate is used as an entrainer. Figure 2 shows that water concentration at the azeotropic point formed with the extraneous entrainer, cyclohexane, is lower than that formed with butyl acetate. The molar ratio in the ternary azeotrope of n-butyl acetate and water is 1:3.4344, and the molar ratio in the ternary azeotrope of cyclohexane and H2O is about 1:0.4426. This difference means that more cyclohexane is necessary than n-butyl acetate for the same water removal. A ternary map of cyclohexane (Figure 2) also shows a larger immiscibility region between the entrainer and water, which is closer to both the water and the entrainer vertices, than butyl acetate. This means that the entrainer has low solubility in water; thus, there is low water content in the entrainer phase, while the recycled butyl acetate stream from the organic phase in the decanter also contains a larger amount of water. The high water concentration in the recycled butyl acetate stream contributes to a decreasing water removal rate from the reaction region when butyl acetate is used as an entrainer. All these effects contribute to the water removal ability from the reaction region of the RD column. However, essential improvement of process efficiency comes from the column composition profile suitable for better reaction yield which can be obtained by using the extraneous entrainer (cyclohexane). Figure 3 illustrates three different qualitative concentration profiles for configurations (a) to (c). In configurations (a) and (b), the concentration profiles start from n-butyl acetate at the bottom to water−BuOAc−BuOH azeotrope at the top of the column. The bottom product, butyl acetate, takes a role of an entrainer to remove water as an overhead product. It makes concentration of butyl acetate high at the top as well as at the bottom of the column so that butyl acetate is widely dispersed along the RD column. The high concentration of butyl acetate in the reaction zone, caused by the dispersion of butyl acetate, tends to shift the reaction equilibrium in the direction favorable to the reverse reaction. However, in configuration (c), cyclohexane takes a role of entrainer instead of butyl acetate. The concentration profile starts from n-butyl acetate at the bottom to water−cyclohexane

Figure 3. Three conceptual design configurations for the synthesis of BuOAc. (A) configuration (a); (B) configuration (b); (C) configuration (c); (■,T) top composition; (▲,B) bottom composition; (black dotted line) phase diagram; (→) column composition profile; () two phase splitting; (blue dashed line, W) water removal; (green dashed line, R) recycle.

Figure 4. Yield of butyl acetate vs reboiler duty (reactant mole ratio = 1).

distillation system more efficient. Dimian et al. (2004)10 provided the following entrainer selection guide. (1) The entrainer can form a minimum-boiler heterogeneous azeotrope with water. (2) The entrainer has low solubility in water, and the reciprocal solubility of water in entrainer is reduced. (3) The level of impurities in the final product is acceptable. 8098

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Figure 5. (a) Variation of butyl acetate yield with increasing and decreasing reboiler duty for configuration (c); (b) column composition profile at 647.1 kW (increasing reboiler duty); (c) column composition profile at 647.1 kW (decreasing reboiler duty).

azeotrope at the top of the column. Cyclohexane is concentrated at the top, and butyl acetate is concentrated at the bottom of the column. The resulting column composition profile, which makes the reactant concentrations high in the reactive zone, is more suitable for achieving better reaction yield. This explains the main reason why using an extraneous entrainer like cyclohexane is better than using an internal

entrainer, butyl acetate, in terms of reaction yield and energy consumption. Reboiler Duty. Figure 4 shows that the reaction yield increases with increasing reboiler duty. As the reboiler duty increases, the recycling rate of the entrainer also increases, leading to an increased water removal rate from the reactive section. This results in an increased reaction yield. Comparing the reaction yield between configuration (b) and configuration 8099

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configuration (c) attains a higher reaction yield than configuration (b), which results from the favorable concentration profile caused by the addition of extraneous entrainer. In general, using an extraneous entrainer (i.e., cyclohexane) allows the system to attain a higher reaction yield without a prereactor. Figure 5a shows reaction yield vs reboiler duty in configuration (c) when cyclohexane is used as an entrainer. The reaction yield increases to maximum and suddenly drops to low value with the increase in reboiler duty. The reaction yield follows a different path when the heat duty increases than when the heat duty decreases. This indicates the existence of multiple steady states. The multiplicity appears to be caused by the transition from a minimum-boiling azeotrope with the extraneous entrainer (cyclohexane) to a minimum-boiling azeotrope with butyl acetate. Parts b and c of Figure 5 show the composition profiles for two different steady states at the same reboiler duty. One is the steady state in which cyclohexane only acts as an entrainer as shown in Figure 5b and the other is the steady state in which butyl acetate as well as cyclohexane act as entrainer as shown in Figure 5c. The transition takes place as the following. As the reboiler duty increases, the column profile moves up, and the butyl acetate concentration at the top of the column increases, which makes cyclohexane retreat from the rectifying section to the decanter and is diluted with the increased butyl acetate in the decanter. After being diluted with butyl acetate, both cyclohexane and butyl acetate together take a role of the entrainer, degrading the entrainer efficiency. The transition from the steady state where cyclohexane acts as an entrainer to the steady state where butyl acetate also takes a role of entrainer makes a sudden drop in the reaction yield. Therefore, it is crucial to maintain an inventory of cyclohexane in the rectifying section in order to keep the system at the steady state wherein cyclohexane only acts as the entrainer. The sudden drop in the reactive distillation system was reported to be caused by interaction between the separation and reaction kinetics20 or nonlinearity in VLE with a reaction,21,22 leading to multiplicities. Kim and Han23 demonstrated that the dynamic behavior of the RD column including the sudden drop can be well explained by nonlinear wave propagation theory. A similar phenomenon can be observed in azeotropic distillation systems, such as an IPA dehydration column using the entrainer cyclohexane.18 Feed Tray Location. According to the normal boiling-point ranking, the reactants butanol and acetic acid are intermediate boilers, while the two products are the lightest (water) and heaviest (butyl acetate) components of the system. The two reactants have almost the same boiling point; thus, it is advantageous to feed the two reactants together. This leads to a high concentration of reactants in the reaction region. Split feeding separates the two reactants, resulting in a low concentration of the reactants in the reaction region. The configuration with a prereactor followed by RD [configuration (b)] was reported to have an optimum feed location above the first reactive stage because the feed contains a large amount of water and butyl acetate and feeding it into the reaction region decreases the reaction conversion.6 We also choose the first tray in the reactive section as a feed tray for prereactor effluent in configuration (b). The simulation results show that feeding the two reactants into the same stage at the middle of the reactive section in the proposed process [configuration (c)] is optimal because this feeding strategy yields a high concentration of both reactants in the reaction region, as shown in Figure 6.

Figure 6. Effect of feed stage for configuration (c): composition profiles (feed mole ratio = 1, reboiler duty = 646.3 kW); (a) feed stage 8; (b) feed stage 13.

Figure 7. Effect of the number of reaction stages on reaction yield (feed mole ratio = 1, reboiler duty = 704.96 kW [configuration (b)], 646.3 kW [configuration (c)]).

(c) with varying reboiler duties, the reaction yield of configuration (c) is higher than that for configuration (b) in the relevant range of reboiler duty, 620−670 kW. Additionally, 8100

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Number of Reactive Stages. Figure 7 shows the effect of the number of reactive stages on the reaction yield for configurations (b) and (c). Reaction yields in configurations (b) and (c) are shown to increase with the number of reactive stages. It can be seen that 10 reaction stages give a best performance in terms of the reaction yield for both configurations. It can be seen that the highest reaction yield in configuration (c) is better than that in configuration (b).

4. COMPARISON OF REACTIVE DISTILLATION WITH AND WITHOUT EXTRANEOUS ENTRAINER We compared the three configurations based on the optimized process design given in Table 5. The optimum values for Table 5. Optimized Parameters for the Three Configurations total stages reaction zone in RD feed stage feeding rate (kmol/h) catalyst loading per stage column pressure catalyst loading (prereactor) prereactor temperature

configuration (a) configuration (b)

configuration (c)

28 8−18

28 8−18

28 8−18

acetic acid 13; butanol 13 acetic acid 36; butanol 36 100 kg

8 acetic acid 36; butanol 36 100 kg

acetic acid 13; butanol 13 acetic acid 36; butanol 36 100 kg

1 atm N/A

1 atm 500 kg

1 atm N/A

N/A

348.15 K

N/A

various design parameters were determined on the basis of the analysis of the effect of each parameter on the process performance. Rigorous optimization techniques were not used for getting the optimum values. However, this provides a comparison result based on close-to-optimal designs between the three configurations. Figure 8 shows the operating conditions and simulation results of the three configurations. The capital cost consists of the costs of the prereactor, column, trays, and heat exchangers. The cost models and corresponding values are given in Appendix A. Figure 9 shows the column temperature and reaction rate profiles for reactive distillation with and without an extraneous entrainer. The entrainer forms a minimum-boiling heterogeneous azeotrope with water. Its heterogeneous azeotropic temperature determines the temperature difference between

Figure 9. Comparison of (a) column temperature and (b) reaction rate profiles between the three configurations.

the azeotrope and the reaction mixture in the rectifying and reactive sections. In general, formation of the azeotrope tends to lower the reaction temperature in the reactive section, thus avoiding catalyst degradation and suppressing side reactions that are activated by high temperature in that section. The larger the temperature difference, the better the separation that is achieved, meaning that the number of required stages in the rectifying section can be reduced. Thus, water in the reaction region can be more easily removed so that reaction conversion

Figure 8. Operating conditions and simulation results for (a) reactive distillation process without extraneous entrainer and prereactor [configuration (a)]; (b) conventional process consisting of prereactor and RD columns [configuration (b)]; (c) proposed RD column with extraneous entrainer [configuration (c)]. 8101

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Figure 11. Column composition profiles for configuration (b): (a) liquid; (b) vapor.

Figure 10. Column composition profile for configuration (a): (a) liquid; (b) vapor.

separates the azeotropic mixture from the reaction mixture than configurations (a) and (b) using butyl acetate as an entrainer. It is possible that low temperature in the reactive zone for configuration (c) may lead to low reaction rate. However, reaction rate profiles for the three configurations in Figure 9b indicate that configuration (c) reaches the required reaction conversion, and the low temperature effect in configuration (c) is overcome by higher reactant concentrations in the reaction zone. The same amount of catalyst is used in the RD column for the three configurations. Figure 10 presents the composition profile for configuration (a), showing that acetic acid is enriched in the rectifying section, instead of in the reaction section. The stripping section indicates that sufficient separations do not occur in the rectifying and stripping sections of the column. These lead to the lowest reaction yield and highest energy requirement for a given reaction yield among the three configurations. Figures 11 and 12 display the column composition profiles of vapor and liquid for the configurations (b) and (c), respectively. In configuration (b), a butanol, butyl acetate, and water azeotrope forms at the top of the column so that a large amount of butanol is outside of the reaction zone and a high concentration of butyl acetate is maintained in the reaction region, which contributes to a low reaction yield. This is confirmed by the

and yield can be substantially increased by using an extraneous entrainer (cyclohexane). Janovsky et al.,1 and Steinigeweg and Gmehling6 reported that side products such as 1-butene and dibutyl ether were generated at high temperature in the reaction zone. We considered the formation of unwanted side product dibutyl ether (DBE), which forms by n-BuOH dehydration. Gangadwala et al.8,9 and Singh et al.24 studied the side reaction in reactive distillation for n-butyl acetate. The reaction kinetics are expressed as the following: rDBE =

a′2BuOH 1 1 dni = KDBE mcat vi dt (1 + a′BuOH + a′DBE + a′H2O )2 (4)

The parameters were taken from Gangadwala et al.8 Consideration of the side reaction shows that the DBE flow rates at the bottom in configuration (a) and (b) are 0.075 kmol/h, 0.033 kmol/h, respectively, but DBE flow rate at the bottom in configuration (c) is 0.004 kmol/h. This result supports that an extraneous entrainer, which decreases the temperature in the reaction zone, suppresses the generation of side products. Configuration (a) shows the highest temperature, and configuration (c) shows the lowest temperature in the reaction region. This means that configuration (c) better 8102

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Table 7. Reaction Yield, Product Purity, Energy Consumption, and Capital Cost for the Three Configurations

configuration (a) configuration (b) configuration (c)

reaction yield

bottom purity

energy consumption (kW)

TAC ($/y)

0.966

0.966

805.4

85155.1

0.99

0.987

705.0

102298.5

0.99

0.985

646.3

81710.9

Table 8. Operating and Capital Costs for the Three Configurations configuration (a)

configuration (b)

configuration (c)

3032 21225 1236

20362 1164

344 213 26051

325 313 22165

47266 51 46297 93615

43386 71 36865 74323

Capital Cost reactor cost column cost distillation column tray cost heat exchanger cost reboiler condenser total steam cost cooling water cost catalyst cost total

21054 1222

341 182 22800 Operating Cost 46639 52 30865 77555

acetate, is propelled downward in the reaction region by the extraneous entrainer, which increases the concentration of both reactants and decreases the temperature in the reaction region. The reaction yield increases because the reactants are enriched, although the temperature is low in the reaction region. The vapor composition profile in the RD column using cyclohexane as an entrainer(Figure 11b) shows that the vapor phase at the top consists only of cyclohexane and water which indicates that good separation has been achieved in the rectifying section. Table 6 gives the composition results for each stream. In the prereactor followed by reactive distillation [configuration (b)], the organic phase contains acetic acid (3.8%), BuOH (16.1%), and water (18.4%), and the aqueous stream contains acetic acid (1.5%). The entrainer recycle stream in configuration (b) contains a large amount of reactants and water, which tends to distribute the reactants widely along the column, instead of leaving them concentrated in the reaction region, degrading the performance of the butyl acetate entrainer. In contrast,

Figure 12. Column composition profiles for configuration (c): (a) liquid; (b) vapor.

vapor composition profile in the RD column using butyl acetate as an entrainer (Figure 10b). One can see that large amounts of butyl acetate, butanol, and acetic acid are in the overhead vapor, indicating inefficient separation in the rectifying section. However, in configuration (c) using an extraneous entrainer, the entrainer−water azeotrope is realized at the top of the column, and water can be removed from the reaction region while maintaining a low concentration of butyl acetate and high concentration of butanol in the reaction region. These have a favorable effect on the reaction yield. The high boiler, butyl

Table 6. Compositions in the Organic Phase, Aqueous Phase, and Bottom of the RD Column for the Three Configurations configuration (a) organic

HOAc BuOH BuOAc water entrainer

aqueous

configuration (b) bottom

organic

37.3

35.7

36.0

19.4

0.093 0.240 0.431 0.236 0

0.037 5.7 × 10−03 9.2 × 10−04 0.956 0

3.9 × 10−08 0.034 0.966 2.9 × 10−16 0

0.038 0.161 0.618 0.184 0

aqueous

configuration (c)

bottom

Mole Flow [kmol/h] 36.2 36.1 Mole Fraction 0.015 3.6 × 10−07 0.003 0.013 0.001 0.987 0.981 1.8 × 10−15 0 0 8103

make-up

organic

aqueous

bottom

5.7 × 10−04

50.76

35.8

36.2

0 0 0 0 1

0.001 0.011 6.5 × 10−07 0.001 0.985

0.003 0.001 7.3 × 10−10 0.995 1.4 × 10−05

0.0062 0.0083 0.985 1.0 × 10−11 2.5 × 10−09

dx.doi.org/10.1021/ie403049z | Ind. Eng. Chem. Res. 2014, 53, 8095−8105

Industrial & Engineering Chemistry Research

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where QR (W) is the reboiler duty, the overall heat-transfer coefficient UR is assumed to be 788.45 W m−2 K−1, and the temperature driving force ΔTR (K) in the reboiler is assumed to be 20 K. (2) Condenser heat transfer area (AC)

configuration (c) shows that very little amount of water (0.1%), and very low concentrations of reactants (acetic acid 0.3%, BuOH 0.1%) are contained in the organic phase. This makes the reactants concentrated in the reaction zone, improving the reaction yield. Table 7 provides a comparison between the results from conventional processes and those from proposed novel process in terms of reaction yield, energy consumption, and total annual cost. The reaction yields of configurations (b) and (c) are set to be equal for fair comparison at the optimized design conditions. The proposed reactive distillation using cyclohexane as an entrainer [configuration (c)] gives lower energy consumption and lower total annual cost than the processes using butyl acetate as an entrainer [configuration (b)] for the same reaction yield. High capital cost of configuration (b) is mainly attributed to the presence of prereactor. The reaction yield of configuration (a) in Table 7 is the highest value which configuration (a) can attain, but its value is far below those of configurations (b) and (c). Table 8 lists operating and capital costs at the optimized conditions for the three configurations.

A C (m 2 ) =

LC(m) = 0.7315NT where NT is the total number of trays. The capital cost is calculated according to: (1) Column cost M&S (101.9DC1.066LC0.802 280

Column cost[$] =

(2.18 + FC))

where FC = FmFp = 3.67 and DC is calculated by the simulation. (2) Tray cost Tray cost[$] =

(M&S) (4.7DC1.55LC FC) 280

where FC = 4.5 and DC is calculated by the simulation. (3) Heat exchanger cost Heat exchanger cost[$] =

M&S 0.65 (A (2.29 + FC)) 280

where FC = (Fd + Fp)Fm = (1.35 + 0) × 3.75 for the reboiler and FC = (Fd + Fp)Fm = (1 + 0) × 3.75 for the condenser. (4) Reactor cost Reactor cost calculation follows the procedure of Douglas (1988):25

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APPENDIX A. TOTAL ANNUAL COST CALCULATION The total annual cost (TAC) is defined as25

M&S (101.9DC1.066LC0.802 280

Reactor cost[$] =

(2.18 + FC))

capital cost payback period

The unit is assumed to operate 8150 h/year, and the operating cost is calculated according to: (1) Steam cost

A capital pay-back period of 3 years is assumed, and a Marshall and Swift (M&S) index of 1108.1 (2002) is used in the calculation. The calculation of capital cost follows the procedure of Douglas (1988) and specific equations from Elliott and Luyben (1996),26 Chiang et al. (2002),27 and Tang et al. (2005).28 The equipment is sized as follows: (1) Reboiler heat transfer area (AR) AR (m2) =

UCΔTC

where QC (W) is the condenser duty, the overall heat-transfer coefficient UC is assumed to be 473. 07 W m−2 K−1, and the log mean temperature driving force ΔTC (K) depends on the dew points and bubble points for a total condenser. (3) Column length (LC)

5. CONCLUSIONS The synthesis of butyl acetate using RD with an extraneous entrainer was investigated in this study. All previous works have reported that the use of butyl acetate as an entrainer and a configuration consisting of a prereactor followed by reactive distillation are most suitable for butyl acetate synthesis. However, the use of the product as an entrainer has an unfavorable effect on reaction conversion because butyl acetate tends to shift the reaction equilibrium toward the reverse reaction. We proposed a reactive distillation system using an extraneous entrainer, cyclohexane, for the synthesis of butyl acetate. More efficient removal of water and butyl acetate formed in the reaction and almost complete conversion were achieved. Furthermore, this configuration controls the reaction temperature in the reactive section, which prevents the catalyst from thermal degradation and suppresses the formation of side products. The extraneous entrainer-based reactive distillation was successfully applied to the synthesis of butyl acetate. The proposed process was demonstrated to provide lower energy consumption and lower capital cost than other processes without an extraneous entrainer for getting the same reaction yield.

TAC = operating cost +

QC

Steam cost[$/year] =

⎛ Q R ⎞⎛ h ⎞ ⎜ ⎟⎜8150 ⎟ year ⎠ 453.59 kg/10 lb ⎝ λV ⎠⎝ $Cs/103 lb

3

where Cs is the saturated steam price and λV is the latent heat of the steam, which depends on the bottom temperature of the column. (We assumed Cs is $2.28 per 103 lb and λV is 2199.6 kJ/kg according to the RD column in Lin et al.29)

QR UR ΔTR

(2) Cooling water cost: 8104

dx.doi.org/10.1021/ie403049z | Ind. Eng. Chem. Res. 2014, 53, 8095−8105

Industrial & Engineering Chemistry Research

Article

3rd International Symposium on Multifunctional Reactors and the 18th Colloquia on Chemical Reaction Engineering, 2003; pp 191−194. M. Tech. Dissertation, Indian Institute of Technology: Bombay, India, 2002. (8) Gangadwala, J.; Mankar, S.; Mahajani, S. M.; Kienle, A.; Stein, E. Synthesis of Butyl Acetate in the Presence of Ion-Exchange Resins as Catalysts. Ind. Eng. Chem. Res. 2003, 42 (10), 2146−2155. (9) Gangadwala, J.; Kienle, A.; Stein, E.; Mahajani, S. Production of Butyl Acetate by Catalytic Distillation: Process Design Studies. Ind. Eng. Chem. Res. 2004, 43 (1), 136−143. (10) Dimian, A. C.; Omota, F.; Bliek, A. Entrainer-Enhanced Reactive Distillation. Chem. Eng. Process 2004, 43, 411−420. (11) Wang, S. J.; Wong, D. S. H. Design and Control of EntrainerAdded Reactive Distillation for Fatty Ester Production. Ind. Eng. Chem. Res. 2006, 45, 9042−9049. (12) Suman, T.; Srinivas, S.; Mahajani, S. M. Entrainer Based Reactive Distillation for Esterification of Ethylene Glycol with Acetic Acid. Ind. Eng. Chem. Res. 2009, 48, 9461−9470. (13) Hasabnis, A. C.; Mahajani, S. M. Entrainer-Based Reactive Distillation for Esterification of Glycerol with Acetic Acid. Ind. Eng. Chem. Res. 2010, 49, 9058−9067. (14) De Jong, M. C.; Zondervan, E.; Dimian, A. C.; de Haan, A. B. Entrainer Selection for the Synthesis of Fatty Acid Esters by EntrainerBased Reactive Distillation. Chem. Eng. Res. Des. 2010, 88, 34−44. (15) Hu, S.; Zhang, B. J.; Hou, X. Q.; Li, D. L.; Chen, Q. L. Design and simulation of an entrainer-enhanced ethyl acetate reactive distillation process. Chem. Eng. Process. 2011, 50, 1252−1265. (16) Zhang, B. J.; Yang, W. S.; Hu, S.; Liang, Y. Z.; Chen, Q. L. A reactive distillation process with a sidedraw stream to enhance the production of isopropyl acetate. Chem. Eng. Process. 2013, 70, 117− 130. (17) Wang, S. J.; Huang, H. P. Design of entrainer-enhanced reactive distillation for the synthesis of butyl cellosolve acetate. Chem. Eng. Process. 2011, 50, 709−717. (18) Chien, I.-L.; Zeng, K.-L.; Chao, H.-Y. Design and control of a complete heterogeneous azeotropic distillation column system. Ind. Eng. Chem. Res. 2004, 43, 2160−2174. (19) Dortmund Data Bank; DDBST GmbH: Oldenburg, Germany, 2002 (www.ddbst.de). (20) Kienle, A.; Marquardt, W. Nonlinear dynamics and control of reactive distillation processes. In Reactive Distillation Status and Future Directions; Sundmacher, K., Kienle, A., Eds.; Wiley-VCH: Weinheim, 2002; pp 241−281. (21) Singh, A.; Tiwari, A.; Bansal, V.; Gudi, R. D.; Mahajani, S. J. Recovery of acetic acid by reactive distillation: Parametric study and nonlinear dynamic effects. Ind. Eng. Chem. Res. 2007, 46, 9196−9204. (22) Singh, B. P.; Singh, R.; Kumar, M. V. P.; Kaistha, N. Steady-state analysis of reactive distillation using homotopy continuation. Chem. Eng. Res. Des. 2005, 83, 959−968. (23) Kim, B.; Han, M. Dynamics and control of reactive distillation under multiple steady states based on a nonlinear wave theory. Ind. Eng. Chem. Res. 2012, 51, 16393−16409. (24) Singh, Ajay; Hiwale, R.; Mahajani, S. M.; Gudi, R. D. Production of butyl acetate by catalytic distillation. Theoretical and experimental studies. Ind. Eng. Chem. Res. 2005, 44, 3042−3052. (25) Douglas, J. M. Conceptual Design of Chemical Process; McGrawHill: New York, 1988. (26) Elliott, T. R.; Luyben, W. L. Quantitative assessment of controllability during the design of a ternary system with two recycle streams. Ind. Eng. Chem. Res. 1996, 35, 3470. (27) Chiang, S. F.; Kuo, C. L.; Yu, C. C.; Wong, D. S. H. Design alternatives for the amyl acetate process: coupled reactor/column and reactive distillation. Ind. Eng. Chem. Res. 2002, 41, 3233. (28) Tang, Y. T.; Chen, Y. W.; Huang, H. P.; Yu, C. C.; Hung, S. B.; Lee, M. J. Design of reactive distillations for acetic acid esterification. AIChE J. 2005, 51 (6), 1683. (29) Lin, Y. D.; Chen, J. H.; Cheng, J. K.; Huang, H. P.; Yu, C. C. Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis. Chem. Eng. Sci. 2008, 63, 1668.

Cooling water cost[$/year] $0.03 ⎛ 0.001 m 3 ⎞⎛ Q C ⎞⎛ h ⎞ = ⎟⎜ ⎟⎜8150 ⎟ 3⎜ year ⎠ 3.785 m ⎝ kg ⎠⎝ 30 ⎠⎝

(3) Catalyst cost (assuming a catalyst life of 3 months): Catalyst cost[$/year] = mcat [kg] × ($7.7162/kg) × 4/year

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) through a grant from the Ministry of Environment (GT-11-C-01-250-0).

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NOMENCLATURE HOAc acetic acid BuOAc n-butyl acetate LLE liquid−liquid equilibrium VLE vapor−liquid equilibrium ai activity coefficient of component i rBuOAc rate of reaction for BuOAc formation(kmol/kg·s) rDBE rate of reaction for BuOAc formation(kmol/kg·s) ki kinetic constant k0i pre-exponential factor mcat mass of catalyst KDBE reaction rate constant for etherfication reaction R gas coefficient P pressure T temperature Ui,j UNIQUAC interaction parameter between components i and j

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REFERENCES

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