Two-Step Reactive Distillation Process for Cyclohexanol Production

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Ind. Eng. Chem. Res. 2009, 48, 9534–9545

Two-Step Reactive Distillation Process for Cyclohexanol Production from Cyclohexene Amit Katariya,† Hannsjo¨rg Freund,† and Kai Sundmacher*,†,‡ Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany, and Process Systems Engineering, Otto-Von-Guericke UniVersity Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany

The present work deals with a novel and attractive alternative process route for the production of cyclohexanol by indirect hydration of cyclohexene using reactive distillation. The proposed two-step process comprises the esterification of cyclohexene with formic acid followed by the hydrolysis of the formed ester. The principle feasibility of this new process has recently been proved by means of residue curve analysis [Steyer et al. Ind. Eng. Chem. Res. 2008, 47, 9581-9587]. In the present contribution, detailed equilibrium stage simulations are performed to identify the optimum design and operating conditions for the coupled reactive distillation column scheme. The simulations reveal that it is possible to achieve almost complete conversion of cyclohexene to cyclohexanol with a comparatively low amount of catalyst. To the best of our knowledge, this is the first time that such an outstanding performance of a process for the production of cyclohexanol is reported. Introduction Cyclohexanol, a bulk chemical needed as an intermediate for nylon production,1 is a high volume chemical with its production exceeding 1 million pounds annually only in the U.S. The conventional processes for the production of cyclohexanol are currently based on the oxidation of cyclohexane, the hydrogenation of phenol, or the direct hydration of cyclohexene. The most commonly used method, the gas phase oxidation of cyclohexane to cyclohexanol, implies severe safety risks associated with the operation due to formation of explosive mixtures when mixing cyclohexane with air for oxidation. It also has a fairly low selectivity, even at very low conversions, and is very energy consuming because of large external recycle streams. Phenol hydration mainly suffers from high phenol prices as compared to benzene and again has the drawback of large hydrogen consumption. An alternative process has been developed by Asahi Chemical Co.,2 where a zeolite catalyst of the HZSM5-type is used. The process involves the liquid-phase synthesis of cyclohexanol by hydration of cyclohexene using acidic catalysts. This is already an improved approach for the cyclohexanol production; however, due to strongly limited mutual solubilities of cyclohexene and water, two liquid phases are formed. This leads to a kinetic limitation of the reaction, and thus, the use of large amounts of catalyst is unavoidable. High reaction rates can only be obtained when the mutual solubility of the reactants is improved and/or the wettability of the heterogeneous catalyst with both the organic and the aqueous phase can be adjusted individually. As a promising alternative we propose a reactive distillation process where cyclohexene is hydrated indirectly to cyclohexanol. Since the direct hydration of cyclohexene is very slow, we use formic acid as a reactive entrainer. In the first step the respective ester is being produced. Then, the addition of water leads to the hydrolysis of the ester, and finally, the desired product cyclohexanol is obtained. There is a significant improvement in the overall rate of the reaction for cyclohexanol * To whom correspondence should be addressed. Tel.: +49-3916110-350. E-mail: [email protected]. † Max Planck Institute for Dynamics of Complex Technical Systems. ‡ Otto-von-Guericke University Magdeburg.

formation due to the addition of formic acid as a reactive entrainer, implying an enhancement of the process performance even at moderate catalyst loadings. The aspect of limited mutual solubilities of the components is also important for this process; however, its influence on the process performance is by far not as limiting as for the direct hydration process. In our earlier contribution,3 we have confirmed the feasibility of the process by means of residue curve maps (RCM) using experimentally approved thermodynamic and kinetic data.4,5 In the present contribution, the results from the feasibility analysis carried out earlier are used as a basis for identification of optimum design and operating conditions for the coupled cyclohexanol production process. Simulations were carried out using a rigorous equilibrium stage model with consideration of phase stability analysis and splitting calculations and chemical reaction in one of the liquid phases. The special feature of the process presented here is that theoretically complete conversion of cyclohexene to cyclohexanol can be obtained. To the best of our knowledge, no other process with such a performance has been reported so far. Due to the presence of the coupled reactions, the interaction of reaction and distillative separation, and the liquid-liquid phase splitting, the task of modeling this process is very complex. The challenge is involved in identifying a suitable column configuration and appropriate design and operating parameters. Reactive Distillation Process Process Description. Cyclohexene can be produced by partial hydrogenation of benzene. However, the reaction involved is a series reaction in which benzene is first hydrogenated to cyclohexene which is easily further hydrogenated to cyclohexane. Various experimental studies on the partial hydrogenation of benzene to cyclohexene using different catalyst systems are reported in the literature.6-10 Most of these studies report that selectivities toward cyclohexene of up to 90% can be achieved but only at the costs of very low conversions of benzene. An exception is the work by Nagahara et al.,8 who observed, for 90% conversion of benzene, a selectivity toward cyclohexene of 75%. A ruthenium based metallic catalyst which consisted of zink compounds has been utilized. The presence of zink salts

10.1021/ie801649v CCC: $40.75  2009 American Chemical Society Published on Web 04/03/2009

Ind. Eng. Chem. Res., Vol. 48, No. 21, 2009

Figure 1. Reaction triangle involved in the production of cyclohexanol by indirect hydration of cyclohexene using formic acid as a reactive entrainer.

of strong acids have remarkably improved the selectivity. In 1990, Asahi Chemical Co. commercialized the process for production of cyclohexanol from benzene in Japan.1 The process involves three steps: In a first step benzene is selectively hydrogenated to cyclohexene. Then, in the second step cyclohexene is separated from the unreacted benzene and the overhydrogenated product cyclohexane. As benzene, cyclohexene, and cyclohexane are close boiling components, a series of azeotropic distillation columns are used to obtain pure cyclohexene from this mixture. After this upstream part of the process, in the third step cyclohexene is hydrated to cyclohexanol. In our studies we focus on the hydration step of the process. We first considered a pure cyclohexene feed in our analysis of the indirect hydration process for cyclohexanol production. With regard to the upstream hydrogenation part of the process, it is reasonable to consider a cyclohexene feed with impurities, such as cyclohexane and benzene. The use of cyclohexene feed mixed with inerts such as unreacted benzene and overhydrogenated cyclohexane to the reactive distillation unit leads to a better separation of inerts. Otherwise separation of inerts would require separate distillation units as well as solvents for the azeotropic separation. Hence an analysis of the column with cyclohexane impurities in the feed mixture and their influence on the proposed process scheme is also presented. Figure 1 shows the triangle of reactions, taking place in the cyclohexanol production by indirect hydration of cyclohexene using formic acid as reactive entrainer. Both of the reaction steps involved in the production of cyclohexanol, i.e. the esterification and the ester hydrolysis, are moderately fast and mildly exothermic. Also, there is a significant difference in the volatilities of the species; the difference in the boiling points of cyclohexene and formic acid cyclohexyl ester is more than 80 °C. In addition, the conditions for the reaction and distillation are matching. This makes the esterification of cyclohexene using formic acid and the subsequent hydrolysis of the ester a potential candidate for the use of reactive distillation. Similar conclusions have been obtained with the help of feasibility analysis using residue curve maps (RCMs).3 The process flow scheme for the preparation of cyclohexanol from cyclohexene is shown in Figure 2. Here the reactions are carried out in a series of two reactive distillation columns. In the first column the ester formation takes place, followed by the subsequent hydrolysis of the formed ester in the second column to give cyclohexanol as a bottom product. Formic acid can be recovered and recycled to the process. It turned out3 that only the upper part of both columns should be realized as reactive zones. In the first column, a nonreactive lower part is necessary in order to avoid a back-splitting of the formed cyclohexyl ester to formic acid and cyclohexene. In the case of

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the second column, residue curve map (RCM) analysis revealed that pure cyclohexanol can be obtained without any nonreactive stripping section. However, the presence of a nonreactive stripping section accelerates the cyclohexanol formation reaction due to the simultaneous separation of cyclohexanol from the reactive section. This also leads to a reduced height of the reactive section. Hence, the lower part of the second column can be designed as a nonreactive zone for economical reasons. With the results of the RCM analysis as a starting point, a simulation based approach is utilized to arrive at a theoretically optimum process performance, and optimum design and operating parameters are identified. Kinetic and Thermodynamic Data. There are three reactions taking place in the proposed production process of cyclohexanol, viz. Reaction 1 cyclohexene + formic acid T cyclohexyl formate Reaction 2 cyclohexyl formate + water T cyclohexanol + formic acid Reaction 3 cyclohexene + water T cyclohexanol In our earlier work,5 we have studied the detailed kinetics of all the three reactions using commercial ion-exchange resin catalyst (Amberlyst 15). The kinetic investigations were carried out in the two liquid phase region due to the strong immiscibility between the components involved. It was observed that a significant decomposition of formic acid to carbon monoxide and water takes place at temperatures higher than about 60 °C. To avoid any formation of carbon monoxide, the reaction temperature in the experiments is kept below 60 °C. Equation 1 is a generalized Langmuir-Hinshelwood type rate expression to describe the kinetics of all the three reactions:

[

R ) mcatkfhet

Kads,AKads,B [1 +

∑K

ads,iai]

i

2

(

+ nFAkfhom

∏a

]

reactants

×

-

∏a

products

Keq

)

(1)

In this equation, mcat denotes the catalyst amount used, kfhet and kfhom are the forward rate constants (Arrhenius type) of the heterogeneous and homogeneous reactions, respectively. Kads,i are the adsorption equilibrium constants for species i taking part in a particular reaction, and the notation A and B is given for the reactants adsorbed on the catalyst surface for the corresponding reaction. Keq is the temperature-dependent chemical equilibrium constant, the ai is the liquid phase activity of the chemical species, and nFA is the molar amount of formic acid which acts as a homogeneous catalyst. The reaction is also taking place homogeneously due to the presence of formic acid in the reaction mixture. Table 1 contains the parameters needed for the calculation of the reaction rate. The presence of water and formic acid makes the system highly nonideal. There is a significant miscibility gap between various components involved. Figure 3a and b illustrates the limited mutual solubilities of the components. Hence, the NRTL model has been used for calculating the liquid phase activity coefficients. In our earlier work, we have carried out detailed experiments and published the thermodynamic data for all the components in question.4 Model Description. As a first step toward the design and optimization of the reactive distillation process, detailed simulations of the coupled columns, as shown in Figure 2, have been carried out and are presented here. The considered design case is a cyclohexanol production process for a cyclohexene feed of

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Figure 2. Flow scheme for the proposed two-step reactive distillation process. Table 1. Kinetic Parameters for Reaction Rate Calculations5 reaction

hom kf,0 [1/s]

EAhom [J/mol]

esterification ester hydrolysis cyclohexene hydration

1.7089 × 1011 7.2738 × 105

95467 52287

het kf,0 [mol/(kgcat s)]

EAhet [J/mol]

4.5701 × 1025 1.2148 × 1016 7.7083 × 1012

114395 100240 93687

substance

0 ∆fHliquid [J/mol]

S0 [J/(mol K)]

CP [J/(mol K)]

Kads

cyclohexene cyclohexanol water FCE formic acid

-37820 -351831 -285830 -487129 -425379

216.33 203.87 69.95 275.50 129.00

148.83 213.59 75.38 219.50 99.84

0.056839 0.77324 19.989 3.6770 7.7290 × 10-7

F ) 10-3 m3/h (pilot scale). The assumptions used for the modeling of the process are the following: 1. Constant molar holdup of liquid and negligible vapor holdup (although we restrict our analysis to steady state, the constant holdup assumption is required due to the presence of the homogeneous reaction) 2. Equilibrium stage model 3. Liquid phase reactions only 4. Ideal vapor phase and nonideal liquid phase, the mixing behavior of which is described by the NRTL activity coefficient model 5. Adiabatic and constant pressure operation of each column. Due to the presence of polar components such as water and formic acid, liquid-liquid phase splitting is expected to occur within the reactive distillation columns. The experimentally validated thermodynamic parameters (NRTL and vapor pressure) are taken from previous studies.4 Due to the observed decomposition of formic acid at high temperatures, the column is operated at below atmospheric pressure. The pressure was set such that the temperature in the reactive section does not exceed 60 °C. The equilibrium stage model has been formulated based on the MESH (Mass-Equilibrium-Summation-Enthalpy) equations. The details about the model equations and the solution methods can be found elsewhere.11 The model has been implemented in the simulation environment DIVA.12 DIVA uses the equation oriented approach for solving all the differential and algebraic equations simultaneously. DIVA provides a built-in package for continuation and stability analysis of the differential-algebraic

equation (DAE) systems. An external subroutine has been incorporated for analyzing the stability of the liquid phase on each stage as well as for performing the liquid-liquid equilibrium calculation. For the details of the phase splitting calculations and the implementation of the algorithm within the column simulations, one can refer to the work of Bausa and Marquardt,13 Steyer et al.,14 and Gangadwala et al.15 Results and Discussion Esterification Column with Pure Cyclohexene Feed. The base case for the esterification column was identified with the help of exploratory simulation studies and is given in Table 2. The 15 theoretical stages include the condenser, the reboiler, and 6 reactive stages [1 + 6(reactive) + 7 + 1]. Cyclohexene is fed at the bottom of the reactive section (stage 7), and the formic acid feed is sent in a stoichiometric amount at the top of the reactive section (stage 2). As the feed is at stoichiometric conditions and both are pure feeds, the total vapor from the top of the column is condensed and sent back to the column. The reflux point is placed at the top of the column (stage 2). The design objective is the complete conversion of cyclohexene. As there is no water present in the esterification column, only reaction 1 (i.e., the esterification) takes place. Various simulations have been carried out to arrive at optimum performance. The results of the analysis are presented in Figures 4-6. Figure 4 shows the effect of the change in the number of reactive stages and the reboiler duty on the conversion of cyclohexene in the esterification column. It can be seen from the diagram that for any number of reactive stages it is possible to achieve 100% conversion of cyclohexene. The reason for the 100% conversion is the fact that cyclohexene is the lightest boiler in the system and is kept in the upper column part at total recycle, and the column has only one outlet which is the bottom product stream. The increase in the reboiler duty causes unreacted cyclohexene and formic acid to move up in the column. Total recirculation from the condenser to the top of the column leads to a high residence time of the two reactants in the reactive section. From the analysis of the simulation results depicted in Figure 4, we have chosen the column design with 7 reactive stages, because for this configuration 100% conversion of cyclohexene is obtained at the smallest possible reboiler load.


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Figure 3. Liquid-liquid phase equilibrium diagram illustrating the limited mutual solubilities of the components. Table 2. Base Case Configuration for the Esterification Column Number of Theoretical Stages total reactive section stripping section

15 6 7

Operating Conditions reflux ratio reboiler load operating pressure catalyst amount

total reflux 110.0 W 0.1 bar 2.0 kg

Feed Location and Condition cyclohexene feed formic acid feed

2.6746 × 10-3 mol/s, (294 K) on stage 7 2.6746 × 10-3 mol/s, (310 K) on stage 2

Further simulations were carried out by fixing the value of seven reactive stages and varying the cyclohexene feed location. The results are presented in Figure 5. It has been observed that, as the cyclohexene feed point is moved up in the column, a better performance of the column is achieved at a lower reboiler load. This is because, for the column with total reflux, locating both feed points at the top provides mole ratios of the reactants close to the equimolar condition at each reactive stage. Moreover, due to the distillation effect, the formed ester is simultaneously separated from the reactive

Figure 5. Sensitivity of the cyclohexene conversion with regard to the cyclohexene feed location and the reboiler heat load (base case configuration (Table 2) with seven reactive stages).

Figure 6. Effect of the reboiler heat load and the catalyst amount on the cyclohexene conversion (base case configuration (Table 2) with seven reactive stages and feed of cyclohexene on the third stage).

Figure 4. Effect of the number of reactive stages and the reboiler heat load on the conversion of cyclohexene (base case configuration (Table 2)).

zone. This will suppress the reverse reaction of ester to cyclohexene and formic acid and will thus enhance the ester formation. In further simulations, presented in Figure 6, the cyclohexene feed location is fixed at the third stage with seven

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Figure 7. Column composition (mole fractions) and temperature profiles for the optimized column configuration (Table 4).

reactive stages in the column. All other parameters are given in Table 2. The sensitivity of the cyclohexene conversion with respect to catalyst loading at four different levels of the reboiler load was investigated. To select the optimum parameters in this case, it is necessary to carry out detailed energy and catalyst cost analysis. Here, a reboiler load of 130 W and a catalyst amount of 2 kg is selected as the most reasonable choice. It is also possible to improve the performance of the column at reduced reboiler loads or reduced catalyst amounts. This can be achieved by increasing the pressure, which leads to a temperature increase in the column, which in turn increases the rates of chemical reactions. But still one has to restrict to a pressure value such that the decomposition of formic acid is avoided, i.e., the pressure is set such that the temperature in the reactive section does not exceed 60 °C. In conclusion, the final optimized esterification column consists of in total 15 theoretical stages including condenser and reboiler. The reactants, pure cyclohexene and formic acid, are fed in a stoichiometric ratio on stage numbers two and three of the column, respectively. A decanter is provided at the top of the column to separate unreacted cyclohexene and formic acid, which form a heterogeneous azeotrope. The upper section of the column is the reactive section, consisting of seven theoretical stages. The esterification column is operated at total reflux whereby achieving complete conversion of cyclohexene to formic acid cyclohexyl ester. The ester is then fed into the second column of the process scheme, where ester hydrolysis is performed. The final column profiles for the optimal configuration of the esterification column are depicted in Figure 7. As indicated by the phase fraction plot in Figure 7, phase splitting takes place only in the upper part of the reactive section. Also, it can be clearly observed that a nearly equimolar ratio of the reactants is obtained at the top of the column which is desirable for a reasonable reaction rate. Esterification Column with Cyclohexene/Cyclohexane Feed. In the presence of cyclohexane as an inert in the feed mixture, residue curve map analysis reveals that the heterogeneous azeotrope of cyclohexane and formic acid will be the expected top product.3 The presence of the inert cyclohexane decreases the overall mole fraction of the high boiling ester on each stage. In particular, the lower boiling cyclohexane will preferably be accumulated in the upper reactive section of the column. This effect leads to a reduction of temperatures in

Table 3. Base Case Configuration for the Esterification Column with Consideration of Cyclohexane in the Feed Number of Theoretical Stages total reactive section stripping section

20 12 6

Operating Conditions reflux ratio

total reflux of polar phase (at stage 2) partial reflux of organic phase 120.0 W 0.1 bar 2.0 kg

reboiler load operating pressure catalyst amount

Feed Location and Condition cyclohexene feed mixture formic acid feed

3.20952 × 10-3 mol/s, (294 K) on stage 12 2.67460 × 10-3 mol/s, (310 K) on stage 2

comparison to the temperature values at pure cyclohexene feed. This implies that there is a restriction with regard to the temperature being achieved in the column at a fixed pressure. To achieve complete conversion of cyclohexene, it is necessary to maintain a certain value of the Damko¨hler number in the reactive section. The Damko¨hler number can be modified either by increasing the catalyst amount or by varying the pressure which governs the bubble point temperature of the mixture. This implies that there will be significant modifications in the design and operation of the esterification column when dealing with a feed that contains an additional inert component besides the reactant cyclohexene. As the heterogeneous azeotrope of cyclohexane and formic acid is the feasible top product, the base case column configuration as given in Table 3 is considered for the simulation based design. The RCM analysis with cyclohexene and cyclohexane in the feed mixture performed in our previous work3 revealed the need for a longer reactive section. Therefore, the base case for a mixed feed has to be changed accordingly. In addition, we are now removing a part of the distillate as a top product; hence, there is no total recirculation at the top as in the case of a pure cyclohexene feed. Therefore, the cyclohexene/cyclohexane feed is sent at the bottom of the reactive section to provide a longer residence time and to avoid short-circuiting of the reactant through the distillate. In the following subsections, we first present the analysis for the cyclohexene feed mixture with 20 mol


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Table 4. Comparison of the Base Case and Optimized Column Configuration Number of Theoretical Stages 0% cyclohexane

total reactive stripping

20% cyclohexane

30% cyclohexane

40% cyclohexane

base

optimum

base

optimum

base

optimum

base

optimum

15 6 7

15 7 6

20 12 6

20 15 3

20 12 6

20 15 3

20 12 6

20 15 3

Operating Conditions reflux ratio organic phase polar phase reboiler load [W] pressure [bar] catalyst amount [kg]

total total 110

85%

89%

80%

total 130

0.1 2.0

120 0.1

130 0.2 2.0

83%

80% total 140

total 133 0.1

0.247 2.0

0.1

0.248 2.0

Feed Location and Condition cyclohexene feed mixture [mol/s] temperature [K] feed location formic acid [mol/s] temperature [K] feed location

2.67460 × 10-3 294 7 3 2.67460 × 10-3 310 2

% inert cyclohexane in it. Then, the comparison is also presented for cases with 30 and 40 mol % of inert material. 20% Cyclohexane in the Cyclohexene Feed Mixture. The base case configuration features in total 20 theoretical stages including the condenser, the reboiler, and 12 reactive stages [1 + 12(reactive) + 6 + 1]. The cyclohexene feed mixture is sent to the bottom of the reactive section (stage 12), and pure formic acid feed is introduced in a stoichiometric amount to cyclohexene at the top of the reactive section (stage 2). The vapor from the top of the column is condensed and sent partially back to the column. The condenser contains a heterogeneous mixture of cyclohexene, cyclohexane, and formic acid. The organic phase of this mixture, which is rich in cyclohexene and cyclohexane, is partially recycled to the bottom of the reactive section. The second liquid phase, being rich in formic acid, is completely sent back to the top of the reactive section. We refer to this formic acid rich phase as the polar phase. As a starting value for the simulation, 85% of the organic phase from the decanter is recycled back to the column. Simulations were carried out under variation of the reboiler load, the number of reactive stages, the organic phase reflux

3.20952 × 10-3 294 12 2.67460 × 10-3 310 2

3.47698 × 10-3 294 12 2.67460 × 10-3 310 2

3.74444 × 10-3 294 12 2.67460 × 10-3 310 2

rate, and the reflux position at the column. These are the crucial parameters for optimal process performance. The remaining parameters such as the two feed locations, the polar phase reflux position, the catalyst amount, and the column pressure are kept constant at values and/or positions that appeared to be optimal from practical considerations. For example, the complete polar phase from the condenser has to be recycled to the top of the column, as it is rich in formic acid. Initially, simulations were carried out by varying the number of reactive stages, keeping the total number of stages constant. The maximum conversion is obtainable with 16 reactive stages. Figure 8a and b show the effect of the reboiler heat load, the organic phase reflux amount, and the reflux position on the cyclohexene conversion for the base case column configuration (Table 3), but here with 16 reactive stages. When increasing the reboiler load, the conversion first increases, reaches a maximum, and then decreases with a further increase in the reboiler load. This decrease in the conversion is the result of an increase in the vapor flow rate, which reduces the extent of the liquid phase reaction. It can be seen that the maximum conversion of cyclohexene that can be achieved in these cases

Figure 8. Diagram showing the effect of the reboiler heat load, the organic phase reflux amount, and the position of the reflux on cyclohexene conversion (base case column configuration (Table 3) with 16 reactive stages).

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Figure 9. Diagram showing the effect of the reboiler heat load and the number of reactive stages on the cyclohexene conversion (base case column configuration (Table 3) with 88% organic reflux on the ninth stage).

Figure 10. Diagram showing the effect of the reboiler heat load and the organic phase reflux position on the cyclohexene conversion (base case column configuration (Table 3) with 16 reactive stages, 88% organic reflux, and a very high amount of catalyst (9 kg)).

does not exceed 86.0%. Also, by changing the number of reactive stages, the maximum conversion could not be increased above 86.0% (Figure 9). This is due to the fact that for a fixed amount of the catalyst and a fixed pressure inside the column,

there is a restriction with regard to the maximum Damko¨hler number that can be achieved in the reactive section of the column. This fact can be verified by changing the Damko¨hler number in the reactive section. There are three ways to change the Damko¨hler number: the first one is to increase the amount of the catalyst, the second possibility is to increase the pressure of the column, and the third way is to reduce the feed flow rates in order to increase the residence time. Figure 10 illustrates that up to 99% conversion of cyclohexene can be achieved if one can increase the Damko¨hler number correspondingly in the reactive section. As there is a restriction with regard to the maximum amount of catalyst that can be installed in the column, we vary the pressure of the column. However, we have to keep in mind that the maximum temperature in the reactive section of the column should not exceed 60 °C, since above this temperature the decomposition of formic acid in the presence of acid catalyst has been observed experimentally.5 Figure 11a-c shows the influence of the column pressure on cyclohexene conversion for a fixed amount of organic reflux sent at various reflux positions and for different numbers of reactive stages. It can be seen that, with an increase of the number of reactive stages, the conversion attains a maximum for the column with 15 reactive stages; a further increase in the number of reactive stages leads to a decrease of conversion. This happens due to the increasing occurrence of the reverse reaction of the formed ester in the lower part of the reactive section. Multiple operating points have also been predicted (see Figure 11a) for the column with 14 reactive stages, which has importance when studying the dynamics of the system. The diagrams also show that as we increase the number of reactive stages, the maximum conversion is obtained at lower pressures. Hence for the further simulations, we selected the column configuration with 15 and 16 reactive stages to find the optimum values for the reboiler load, the reflux amount, and the reflux position. In Figure 12a, the fixed amount of organic reflux (i.e., 88%) was positioned at the seventh stage of the column, which has 15 reactive stages. Then, the effect of the reboiler load and the column pressure was investigated. A similar analysis was also carried out for the column with 16 reactive stages and a reflux position on stage 8 (Figure 12b). In both cases, more than 98% conversion of cyclohexene can be achieved. Also, for the column with 16 reactive stages, the maximum is obtained at lower pressure as compared to the column with 15 reactive stages.

Figure 11. Bifurcation diagrams for esterification column: Effect of column pressure, number of reactive stages, and position of 88% organic phase reflux (see Table 3 for the base case configuration).


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Figure 12. Bifurcation diagrams for esterification column: effect of column pressure and reboiler load.

Figure 13. Bifurcation diagrams for esterification column: effect of column pressure, amount of organic phase reflux, and the reflux position (base case column as given in Table 3; 15 reactive stages, 130 W heat load).

Figure 14. Optimal column composition and temperature profiles [optimized column configuration as given in Table 4].

To identify a column configuration that allows for a further improvement of the conversion of cyclohexene, simulations were carried out for the column with 15 reactive stages to optimize the organic phase reflux amount and the position. Figure 13a

represent the results of the analysis of the column performance by varying the organic reflux amount at fixed location (ninth stage). The maximum conversion is obtained with 89% organic reflux, which is then used to verify its location as shown in Figure 13b. The remainder of the organic phase, which is taken as a distillate from the column, is >99% pure cyclohexane which is an inert contained in the feed mixture coming from the upstream process. The final optimum column configuration and the corresponding design and operating parameters can be found in Table 4, which involves in total 20 theoretical stages [1 + 15(reactive) + 3 + 1] including condenser and reboiler. Formic acid is fed at the top (stage 2) of the column. The mixture cyclohexene/ cyclohexane is fed at stage 12. From the total condenser, the polar phase, rich in formic acid (more than 99%), is recycled to the top (stage 2) of the column and 89% of the organic phase (mixture of cyclohexene/cyclohexane) is recycled to stage 9. The column is operated at 0.2 bar pressure, and 130 W heat is supplied to the reboiler. Figure 14 shows the optimum column profiles, for which 99.20% cyclohexene conversion is achieved. The bottom product of the column is the formic acid cyclohexyl ester (FCE) with greater than 99.5% purity.

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Figure 15. Esterification column (30% cyclohexane in feed mixture): (a) effect of the organic phase reflux amount on cyclohexene conversion at 130 W heat supply to the reboiler; (b) effect of the reboiler heat load on cyclohexene conversion for 83% organic reflux (base case column configuration as in Table 3 with 15 reactive stages, organic phase reflux on stage 9). Table 5. Base Case Configuration for the Ester Hydrolysis Column Number of Theoretical Stages

total reactive section stripping section

base case

optimum case

20 12 6

20 12 6

Operating Conditions reflux distillate reboiler load operating pressure catalyst amount

total organic phase returned at stage 2 total polar phase as a distillate 120.0 W 502.1 W 0.1bar 0.1bar 2.0 kg 2.0 kg Feed Location and Condition

ester feed ester feed location water feed water feed location

2.6748 × 10-3 mol/s (363.15 K) stage 2 stage 4 2.6748 × 10-3 mol/s, (319.15 K) stage12 stage12

In addition to the simulation studies presented above, similar simulations were carried out assuming a cyclohexene feed mixture with 30% and 40% cyclohexane. The formic acid is fed in a stoichiometric amount with regard to cyclohexene in the feed mixture. As the amount of inert in the feed is increased, it is necessary to increase the distillate rate. As an example for 30% inert in the feed mixture, Figure 15a gives the optimum value of the amount of reflux required to obtain nearly 99% conversion of cyclohexene, which turns out to be 83% of organic reflux. Similarly, the optimum value of the reboiler heat load can be identified from the simulations reported in Figure 15b. The optimal point turned out to be at a reboiler load of 133 W and a column operating pressure of 0.247 bar, where the maximum conversion of 98.78% is obtained. A further increase in the reboiler load does not significantly improve the conversion; in fact, there is a reduction in the conversion level. This happens due to an increase in the vapor flow rate, which reduces the extent of the liquid phase reaction. The detailed comparison of the base cases and the optimized column configurations is given in Table 4. Summary of Esterification Column Simulations. As compared to the design of the column with pure cyclohexene, a longer reactive section is required if the feed contains significant proportion of cyclohexane. At 0.1 bar pressure, the presence of

cyclohexane in the feed puts a restriction on the maximum conversion obtained to 86%. This situation can be improved by increasing the Damko¨hler number on each stage as well as the number of theoretical stages in the reactive section. For fixed feed conditions and fixed amount of catalyst installed in the reactive section of the column, the only way to increase the Damko¨hler number is to increase the pressure. Although the desired conversion of cyclohexene to FCE is achieved at higher pressures, the temperature level in the reactive section does not exceed 45 °C, which ensures that the decomposition of formic acid is suppressed. Ester Hydrolysis Column. The feed streams for the second column are the pure ester, irrespective of the feed used for the esterification column, and the water. It has been observed that along with the hydrolysis reaction, the back-splitting of the ester to formic acid and cyclohexene is unavoidable. This implies that all the three reactions, as specified in Figure 1, are taking place in the hydrolysis column, which makes the design of the column a challenging task. A decanter has to be provided for the hydrolysis column to recycle the cyclohexene rich organic phase. This internalizes the cyclohexene and the selectivity toward the desired product cyclohexanol can be improved. A similar simulation-based design approach as presented for the esterification column has also been carried out for the hydrolysis column in order to find an optimum column design and optimal operating conditions. The base case configuration of the ester hydrolysis column is given in Table 5. A column with in total 20 theoretical stages including condenser, reboiler, and 12 reactive stages has been considered. The formic acid cyclohexyl ester (FCE) is fed at the top of the reactive section (stage 2), and water is fed at the bottom of the reactive section (stage 12). The reactants are fed in a stoichiometric quantity according to the hydrolysis reaction. The vapor from the top of the column is condensed and sent partially back to the column. The condenser contains a heterogeneous mixture of cyclohexene, formic acid, and water. The organic phase of the mixture, rich in cyclohexene, is recycled completely to the top of the column. The polar phase, rich in formic acid, is withdrawn as a distillate and can be recycled by using it as a feed for the esterification column. Here, the polar phase can also be referred to as aqueous phase, as it might be the water containing phase


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Figure 16. Bifurcation diagram for ester hydrolysis column: effect of ester feed position and reboiler load (base case column configuration as in Table 5).

Figure 17. Bifurcation diagram for ester hydrolysis column: effect of water feed position and reboiler load (base case column configuration as in Table 5 with ester feed at stage 4).

Figure 18. Bifurcation diagram for ester hydrolysis column: effect of the number of reactive stages and reboiler load (base case column configuration as in Table 5 with ester feed at stage 4 and water feed at stage 12).

if unreacted water is present. The ester hydrolysis column is operated at 0.1 bar pressure. Various simulations were carried out by changing the feed position, reboiler load, height of the

reactive section, etc., to arrive at an optimum column configuration and optimum operating parameter values. The design objective for the ester hydrolysis column is to maximize the

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Figure 19. Steady state composition and temperature profiles for optimized ester hydrolysis column (optimum column configuration as given in Table 5, R1 esterification; R2 ester hydrolysis reaction; R3 direct hydration).

conversion of FCE and the selectivity toward cyclohexanol simultaneously (i.e to maximize the cyclohexanol yield). In Figure 16a and b, the conversion of FCE and the purity of cyclohexanol (i.e., the mole fraction) in the bottom product have been analyzed by varying the reboiler heat load and the ester feed position. For the ester feed at stage 4, the maximum yield of cyclohexanol (>99%) is obtained. Therefore, for the analysis presented in Figure 17a and b, the FCE feed position is fixed at stage 4, and the water feed position is varied with the reboiler load to analyze the yield of cyclohexanol from the column. The optimum is obtained when water is fed on stage 12 (as in the base case). After the identification of the optimum feed positions, the sensitivity of the column performance with respect to the number of reactive stages in the column was analyzed. Figure 18 shows that with 12 reactive stages in the column and for a reboiler duty of about 500 W, greater than 99% yield of cyclohexanol in the bottom is observed. Although in all cases the desired performance is achieved, as can be seen in Figures 16-18, the operating range with regard to the reboiler load is quite narrow. The reason for this complex system behavior is the interaction of multiple competing chemical reactions and the reactive separation process along with the presence of liquid phase splitting in the column, showing multiple operating points. The highly nonlinear nature of the curves in Figures 16-18, showing multiple operating points, illustrates the complexity of the process under investigation. Finally, the simulated optimum column temperature and composition profiles are presented in Figure 19; the optimum design and operating parameters are given in Table 5. It can be seen from the diagram that, in the upper part of the reactive section, from stages 2 to 7, the ester formation reaction (reaction 1) is dominating. In the lower part of the reactive section, from stages 8 to 13, the cyclohexanol formation by ester hydrolysis (reaction 2) is dominating. In this section along with cyclohexanol formation, the splitting of ester back to cyclohexene and formic acid is taking place. However, this is compensated by the ester formation taking place in the upper section. The direct hydration of cyclohexene (reaction 3) is also occurring simultaneously; but, the rate of this reaction is negligibly small. In contrast to the esterification column, where only one reaction (reaction 2) is taking place, in the ester hydrolysis column all reactions feature significant reaction rates. This makes the operation of the second column very complex.

Figure 20. Steady state column composition profiles for the coupled column configuration (optimum column configurations are given in Tables 4 and 5).

Coupled Reactive Distillation Process. In case of the coupled column scheme, as shown in Figure 2, pure cyclohexene is fed slightly below the top of the esterification column (similar conclusions can be made for the coupled column scheme with cyclohexene/cyclohexane feed mixture as one can obtain pure ester in any case). The formic acid has to be fed to the esterification column only during start-up, and then formic acid from the hydrolysis column can be totally recycled to the first column with a small makeup stream of formic acid. The distillate stream from the hydrolysis column is the polar phase stream of the two liquid-phase mixture formed in the decanter. This stream is rich in formic acid, but it also has cyclohexene and water in small amounts. It is important to know that by fixing the temperature of the decanter of the hydrolysis column, the molar fractions of the components involved can be fixed. For the success of the coupled process, it is essential to minimize the water going to the esterification column with the help of a proper control mechanism since the presence of large amounts of water will seriously affect the yield of the ester obtained in the esterification column. Figure 20 illustrates the steady state composition profiles for the coupled column scheme. It is possible to continue with the design parameters, same as those being discussed for the individual columns in the previous sections, with small modifications of the operating parameters. In case of the esterification column, heat supplied to the reboiler has to be increased due to the presence of small amount of water in the formic acid feed stream and the formation of cyclohexanol due to this water. The bottom product from the esterification column consists of more than 96% of ester and remaining cyclohexanol. The presence of cyclohexanol, in the feed to the hydrolysis column, does not have any influence on the design and operating parameters of this column. Hence, it is possible to achieve complete conversion of cyclohexene to cyclohexanol using a coupled reactive distillation scheme. As another advantage, we expect that a significantly lower amount of catalyst is required when compared to the direct hydration process for the production of cyclohexanol from cyclohexene. This is due to the fact that the rate of the direct hydration reaction is very low. With the presented steady state simulations, it is straightforward to couple the columns using the formic acid recycle stream. However, the dynamic operation of such a coupled scheme is a challenging task. Slight variations in the operating conditions may change the composition of the formic acid recycle stream, and this can severely affect the dynamic


operation of the esterification column. To achieve (and maintain) optimum process performance, a reliable model based control scheme is a prerequisite. Therefore, a detailed study of the dynamics of the coupled process with respect to perturbations of operating parameters has to be carried out in the future. Conclusions In this work, a novel process for the production of cyclohexanol is proposed and investigated by means of rigorous simulations. The process scheme consists of two coupled reactive distillation columns and involves the esterification of cyclohexene with formic acid, followed by hydrolysis of the ester. For the simulation, an equilibrium stage model is formulated and solved, using the kinetic and thermodynamic data which were thoroughly identified in previous experiments.5,4 Due to the limited mutual solubilities of the reactants, phase stability analysis and phase splitting calculations (VLLE) at each stage in the column are performed. The presence of the coupled reactions, the interaction of reaction and distillation, and the presence of liquid-liquid phase instability all make the modeling of the system a demanding task. The simulations of the twostep reactive distillation process are carried out in order to identify the optimal design and operating parameters. The results show that using the proposed novel process scheme, almost complete conversion of cyclohexene to cyclohexanol can be achieved with moderate amounts of catalyst. For the esterification column with a pure cyclohexene feed and formic acid in a stoichiometric amount, it is possible to achieve complete conversion of cyclohexene to the respective ester if sufficient heat is supplied to the reboiler. From the overall process point of view (i.e., with regard to the upstream hydrogenation of benzene), it seems appropriate to additionally consider the case of a cyclohexene/cyclohexane feed mixture for the esterification column. Although there are significant differences in the design and operating conditions, complete conversion of cyclohexene to the ester can still be achieved since cyclohexane can be regarded as an inert component that can easily be separated in the first column. Irrespective of the feed composition, a product stream with an ester purity of almost greater than 99.20% can be achieved. This stream is then fed to the second column where the ester hydrolysis to cyclohexanol and formic acid is realized. In this column, along with the ester hydrolysis reaction, a back-splitting of the ester to cyclohexene and formic acid takes place. The competing reactions along with the distillative separation increases the complexity of the system which makes the design and operation of this column more difficult. Due to multiple operating points, the operation of this column has to be restricted to a relatively narrow operating window where a yield of >99% of cyclohexanol can be realized. To the best of our knowledge, such a performancesa process with nearly 100% conversion of cyclohexene to cyclohexanolshas

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thus far not been reported in the literature concerning cyclohexanol production. This fact clearly demonstrates the attractiveness of the proposed coupled reactive distillation process scheme. The next step in the process development is to perform an experimental validation of the simulation studies. Such experiments are currently being carried out by our group in a pilot plant. Literature Cited (1) Musser, M. T. Cyclohexanol and Cyclohexanone. In Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; VCH, Weinheim: New York, 2003; Vol. 10, pp 279-289. (2) Mitsui, O.; Fukuoka, Y. Process for Producing Cyclic Alcohols. United States Patent 4,588,846, 1986. (3) Steyer, F.; Freund, H.; Sundmacher, K. A Novel Reactive Distillation Process for the Indirect Hydration of Cyclohexene to Cyclohexanol Using Formic Acid a Reactive Entrainer. Ind. Eng. Chem. Res. 2008, 47, 9581– 9587. (4) Steyer, F.; Sundmacher, K. VLE and LLE Data Set for the System Cyclohexane + Cyclohexene + Water + Cyclohexanol + Formic Acid + Formic Acid Cyclohexyl Ester. J. Chem. Eng. Data 2005, 50, 1277–1282. (5) Steyer, F.; Sundmacher, K. Cyclohexanol Production via Esterification of Cyclohexene with Formic Acid and Subsequent Hydration of the Ester: Reaction Kinetics. Ind. Eng. Chem. Res. 2007, 46, 1099–1104. (6) Hu, S.; Chen, Y. Partial Hydrogenation of Benzene to Cyclohexene on Ruthenium Catalyst Supported on La2O3-ZnO Binary Oxides. Ind. Eng. Chem. Res. 1997, 36, 5153–5159. (7) Suryawanshi, P. T.; Mahajani, V. V. Liquid Phase Hydrogenation of Benzene to Cyclohexene Using Ruthenium-Based Heterogeneous Catalyst. J. Chem. Technol. Biot. 1997, 69, 154–160. (8) Nagahara, H.; Ono, M.; Konishi, M.; Fukuoka, Y. Partial Hydrogenation of Benzene to Cyclohexene. Appl. Surf. Sci. 1997, 121/122, 448– 451. (9) Hu, S.; Chen, Y. Liquid Phase Hydrogenation of Benzene to Cyclohexene on Ruthenium Catalyst Supported on Zink Oxide-Based Binary Oxides. J. Chem. Technol. Biot. 2001, 76, 954–958. (10) Ning, J.; Xu, J.; Liu, J.; Lu, F. Selective Hydrogenation of Benzene to Cyclohexene Over Colloidal Ruthenium Catalyst Stabilized by Silica. Catal. Lett. 2006, 109, 175–180. (11) Katariya, A. M.; Moudgalya, K. M.; Mahajani, S. M. Nonlinear Dynamic Effects in Reactive Distillation for Synthesis of TAME. Ind. Eng. Chem. Res. 2006, 45, 4233–4242. (12) Mangold, M.; Kienle, A.; Mohl, K. D.; Gilles, E. D. Nonlinear Computation in DIVA–Methods and Applications. Chem. Eng. Sci. 2000, 55, 441–454. (13) Bausa, J.; Marquardt, W. Quick and Reliable Phase Stability Test in VLLE Flash Calculations by Homotopy Continuation. Comput. Chem. Eng. 2000, 24, 2447–2456. (14) Steyer, F.; Flockerzi, D.; Sundmacher, K. Equilibrium and RateBased Approachs for Liquid-Liquid Phase Splitting Calculations. Comput. Chem. Eng. 2005, 30, 277–284. (15) Gangadwala, J.; Radulescu, G.; Kienle, A.; Steyer, F.; Sundmacher, K. New Processes for Recovery of Acetic Acid from Waste Water. CHISA ′06, Prague, August 27-31, 2006; paper H4.4.

ReceiVed for reView October 30, 2008 ReVised manuscript receiVed March 9, 2009 Accepted March 11, 2009 IE801649V

Two-Step Reactive Distillation Process for Cyclohexanol Production

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