Study on Feasibility of Reactive Distillation Process for the Direct

Article pubs.acs.org/IECR

Study on Feasibility of Reactive Distillation Process for the Direct Hydration of Cyclohexene to Cyclohexanol Using a Cosolvent Ting Qiu,* Chen-hui Kuang, Cheng-gang Li, Xiao-wu Zhang, and Xiao-da Wang College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China S Supporting Information *

ABSTRACT: A reactive distillation process is suggested for the production of cyclohexanol from cyclohexene and water using 1,4-dioxane as cosolvent and ion-exchange resin A-36 as catalyst. It not only can avoid the drawbacks of the conventional direct hydration of cyclohexene process, but also has more advantages than it, especially greatly improving conversion and reaction rate. On the basis of the fixed point analysis of residue curve maps, the feasibility of this novel process is analyzed and verified. The phase equilibrium and reaction kinetic experiments are carried out to correlate the parameters in activity coefficient model and kinetic model, respectively, which are necessary for calculating residue curve. ers2−4 studied the solvent effects of sulfolane on the hydration of cyclohexene by a strong acidic ion-exchange resin catalyst. In their studies, the hydration reaction was carried out in a cyclohexene-saturated water−sulfolane mixture. The solubility of cyclohexene was determined with different water−sulfolane mixtures, and the solvent effects of sulfolane on the kinetics and chemical equilibrium of cyclohexene hydration were investigated. Selecting HZSM-5 as catalyst, Shan et al.5 studied the solvent effects of ethylene glycol on the kinetic and chemical equilibrium of cyclohexene hydration. Both of these two solvents are not suitable for the reactive distillation process manufacturing cyclohexanol. The chemical structure of sulfolane is stable. However, it is the much higher boiling point of sulfolane than cyclohexene and water which made them difficult to contact with each other in the reactive distillation column that leads sulfolane to be an unfit cosolvent. In addition to the problem similar with sulfolane, ethylene glycol still suffers from the problem of reacting with cyclohexene easily caused by its two unstable hydroxyls. In short, neither sulfolane nor ethylene glycol can be selected as a cosolvent for the reactive distillation process producing cyclohexanol. According to our preliminary experiments and analysis, 1,4-dioxane can be selected as a suitable cosolvent for several reasons. First, the mutual solubility between cyclohexene and water is greatly improved for its special molecular configuration. How 1,4-dioxane does this will be analyzed in detail with the liquid−liquid equilibrium experiment results in the section below. Second, the 1,4-dioxane would not react with other substances for its stable cyclical structure in the reaction conditions. Third, the much lower boiling point of 1,4-dioxane than cyclohexanol makes them easily separated. Last, the small difference of boiling point between 1,4-dioxane and the reactants makes them possible to contact each other in the column, which is necessary for 1,4-dioxane to play its role.

1. INTRODUCTION Cyclohexanol is an important intermediate of polyamide products, widely used in the production of nylon and plasticizer. Conventionally, there are three commercial routes to produce cyclohexanol: the hydrogenation of phenol, the oxidation of cyclohexane, and the direct hydration of cyclohexene. Phenol hydrogenation route is the earliest method of these, but it mainly suffers from high phenol prices and heavy hydrogen consumption. The cyclohexane oxidation route is the frequently used way to produce cyclohexanol. This process suffers from several disadvantages, such as high energy consumption, low selectivity and conversion, and so on, among which the main one is the risk with respect to operational safety of the oxidizer. Thus, both of these two methods have fallen behind. At present, the cyclohexene hydration route which is more economical and safer has been widely studied. The most mature cyclohexene hydration route was developed by Asahi Chemical Industry Co., Ltd. However, it still suffers from very slow reaction rate and fairly low equilibrium conversion, which reduces its application value to a great extent. Taking into account that the cyclohexene hydration reaction is slightly exothermic and equilibrium limited, Steyer et al.1 suggested producing cyclohexanol by reactive distillation to improve the cyclohexene hydration route developed by Asahi Chemical Industry Co., Ltd. However, the process still has two drawbacks, low mutual solubility between cyclohexene and water and difficulty in loading the powdered catalyst into the column. In order to overcome these two drawbacks, a novel process, characterized by adding a cosolvent into the reactive system to improve the solubility of cyclohexene in water and loading the strong-acid ion-exchange resin catalyst with high activity into the reactive distillation column, is proposed in this paper. Compared with process proposed by Steyer et al.,1 our route is more likely to be industrialized. Before further research, it is evident that the proper cosolvent and catalyst should be selected. Previously, some scholars have studied the solvent effects on the cyclohexene hydration reaction. Panneman and Beenack© XXXX American Chemical Society

Received: November 15, 2012 Revised: April 28, 2013 Accepted: May 24, 2013

A

dx.doi.org/10.1021/ie303144k | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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correlating the liquid−liquid equilibrium data. All these parameters are necessary to calculate residue curve. At last, the feasibility of reactive distillation process for the direct hydration of cyclohexene to cyclohexanol using a cosolvent was analyzed by the residue curve maps.

Currently, HZSM-5 is the catalyst most widely used to catalyze cyclohexene hydration reaction in industry for its high activity to this reaction. However, once HZSM-5, a powdered catalyst with particle size less than 10 μm, is loaded in the reactive distillation column by catalyst strapping package technology which is widely used in reactive distillation industry now, there would be large pressure drop along the reactive distillation column and ineffectiveness of mass transfer in catalyst bags. The ion-exchange resin which has the advantage of high activity, stable structure and easy recovering is widely used in the reactive distillation column. According to the preliminary experiment, strong-acid cationic exchange resin A36 with high activity suitable for cyclohexene hydration reaction is selected as a catalyst in our experiment. The process flow raised in this article is shown in Figure 1. In this process, water and cyclohexene are injected into the

2. EXPERIMENTAL SECTION 2.1. Chemicals. All the chemicals used in this work and their related physical parameters are listed in Table 1. Table 1. Description of Chemicals chemical name

source

water 1,4-dioxane cyclohexene A-36

SINOPHARM SINOPHARM SINOPHARM Rohm and Haas

mass purity

analysis method

boiling point (°C)6

0.998 0.999 0.998

GC GC GC

100 101.3 82.9

Cyclohexene and 1,4-dioxane were commercially supplied by Sinopharm Group Co. Ltd. with purity greater than 0.995 in mass fraction. The purities of these materials were checked by gas chromatography. Deionized water was prepared in our laboratory. The catalyst A-36wet was purchased from Rohm and Haas with maximum service temperature as high as 453 K. The catalyst should be washed thoroughly with deionized water to remove the impurities before it can be used. After being dried in an oven at 353 K for 24 h, it was deposited in a dry place for future use. 2.2. Apparatus and Procedure. The liquid−liquid equilibrium experiment for ternary system of water (1) + 1,4dioxane (2) + cyclohexene (3) was carried out in a still of liquid−liquid equilibrium. Its detailed structure was the same as that already described.7 The still’s temperature was controlled by a 501 type thermostatic water-circulator bath and a DLSB-1 type cryostat. The accuracy of temperature control was ±0.1 K. The magnetic agitator was used to stir. During the course of experiment, the temperature control system was started first to keep the equilibrium still at constant temperature. Then the prepared mixture was introduced into equilibrium cell. After being stirred strongly for 4 h, the mixture was left to settle at least 10 h. Finally, the samples of the organic phase and aqueous phase were drawn out and analyzed respectively. The hydration reaction was performed in a stainless steel reactor of 500 mL equipped with agitation and temperature control device. In each run, a specified amount of catalyst, water, and 1,4-dioxane was first charged into the reactor. After being sealed up, the reactor was filled with nitrogen at the pressure of 1.0 MPa to ensure the reactants to keep liquid under the reaction temperature. A slow stirring speed of 100 rpm was set during the heating process to ensure uniform temperature distribution in the reactor. As the mixture was heated to reaction temperature, the cyclohexene was added into the reactor by a tranquil flow pump rapidly. The agitation speed was raised to the desired level at the same time, and the corresponding time was regarded as the initial time. After a fixed reaction time, stirring was stopped, and the reactor was cooled with cooling water. When the reactor was cooled to ambient temperature, pour out the mixture to a separating funnel. Then, sample from the organic and aqueous phases, respectively. The samples were centrifuged at high rotation speed to ensure that the solid catalyst was separated from the liquid before the contents of every component were analyzed.

Figure 1. Flowsheet of the reactive distillation column proposed in the present work.

reactive distillation column from the top and bottom of reactive section, respectively. Pure cylcohexanol is received at column bottom, and the mixture of unreacted cyclohexene, excess water, and inert 1,4-dioxane is collected at column top. After passing through a decanter, the mixture is separated into organic phase and aqueous phase. The organic phase, mainly containing cyclohexene and 1,4-dioxane, returns to the reactive distillation column from the bottom of the reaction section. The aqueous phase, mainly containing water and 1,4-dioxane, returns to the top. During the manufacturing process, only the reacted water and cyclohexene are added into the column continuously, while cosolvent 1,4-dioxane and excess water can be injected previously. The process presented in this paper is slightly similar to that of Steyer et al.1 However, using 1,4dioxane as cosolvent and ion-exchange resin A-36 as catalyst not only makes our research different from theirs, but also increases the possibility in industrialization. In this paper, the kinetic behavior of the hydration reaction was investigated using 1,4-dioxane as cosolvent and A-36 as catalyst. Also, the data of reactive kinetic experiment was used to correlate the parameters in homogeneous dynamics model. In order to replenish thermodynamic data for the reactive system, liquid−liquid equilibrium data for ternary water + 1,4dioxane + cyclohexene system were determined in liquid− liquid equilibrium stills. Also, the binary interaction parameters in NRTL activity coefficient model were obtained by B



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added into the mixture composed of cyclohexene and water, some of them dissolve into the organic phase, and the others dissolve into the aqueous phase. The existence of 1,4-dioxane in aqueous phase can lead to the increment of cyclohexene in this phase and extrude water into other phase correspondingly. As a result, the contents of cyclohexene in aqueous phase and water in organic phase both increase. Namely, the mutual solubility between cyclohexene and water increases. Therefore, it is a good idea to add 1,4-dioxane into the system to increase the contents of cyclohexene in aqueous phase. In this section, the equilibrium data was further processed to verify their reliability by Othmer−Tobias correlation as below:

For this liquid−liquid−solid three-phase reaction, it was difficult to continuously take representative samples from the reactor during the experiment and one time of kinetic experiment could get only a point in a kinetic curve. 2.3. Analysis. The 1,4-dioxane, cyclohexene, and cyclohexanol components in both the organic phase and the aqueous phase were determined using a Varian CP-3800 gas chromatograph (GC) equipped with a hydrogen flame ionization detector (FID). A 30 m capillary column (0.32 mm diameter with a 0. 5 μm film thickness) was used with a temperatureprogrammed analysis. The column temperature was first kept at 323 K for 1 min, then was heated at 8 K min−1 to 353 K and kept for 1 min, and then was heated at 20 K min−1 to 493 K. Injection mode was split flow, and split ratio was set at 50/1. The injector temperature and detector temperature were both set at 513 K. Nitrogen was used as carrier gas at 20 mL min−1. The injected volume was liquid sample of 0.2 μL. The internal standard method was used with methylbenzene as an internal standard. The water concentration was determined by the Metrohm 756 KF Coulometer.

ln(1 − x3oil) x3oil

xwater 1

=A+

B ln(1 − x1water) x1water

(1)

xoil 3

Here is mole fraction of water in water rich phase, is mole fraction of cyclohexene in cylcohexene phase, and A and B are the constants of the Othmer−Tobias correlation. The correlation constants A, B and correlation coefficient values R2 are listed in Figure 3. As can be seen in Figure 3, the experimental data are reliable.

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Liquid−Liquid Equilibrium. 3.1.1. Experimental Data. The liquid−liquid equilibrium data for ternary system of water (1) + 1,4-dioxane (2) + cyclohexene (3), which were determined at the atmospheric pressure and the temperature of 313.15 K, are listed in Table S1 (Supporting Information), and plotted in Figure 2. All compositions are expressed as mole fraction.

Figure 3. Othmer−Tobias plots of the water (1) + 1,4-dioxane (2) + cyclohexene (3) ternary system at 313 K and 1 atm.

3.1.2. Data Correlation. Only the interaction parameters of water (1) + 1,4-dioxane (2), water (1) + cyclohexene (3), and 1,4-dioxane (2) + cyclohexene (3) were obtained by fitting the experimental data for ternary water (1) + 1,4-dioxane (2) + cyclohexene (3) system. The interaction parameters of water (1) + cyclohexanol (4) and 1,4-dioxane (2) + cyclohexanol (4) can be obtained by fitting the liquid−liquid equilibrium data for ternary system of water (1) + 1,4-dioxane (2) + cyclohexanol (4) described in ref 8. The interaction parameter of cyclohexene (3) + cyclohexanol (4) was taken from ref 9. The measured experimental data were correlated with the NRTL model of Renon and Prausnitz.10 For the NRTL model, the nonrandomness parameters αij were taken from other references or set at fixed values. They are listed in Table 2. The NRTL model was fitted to experimental data using an iterative computer program, based on Particle Swarm Optimization developed by Kennedy and Eberhart.11,12 The objective function OF used in this case was

Figure 2. Phase diagram of LLE for ternary water (1) + 1,4-dioxane (2) + cyclohexene (3) system at 313 K and 1 atm.

As shown in Figure 2, the ternary system studied exhibits type-I behavior. The mutual solubility between cyclohexene and water increases obviously with the addition of 1,4-dioxane. These can be explained by the intermolecular force. Owing to the hexatomic ring structure of cyclohexene which is substantially different from water molecule, cyclohexene and water dissolve each other with difficulty. The addition of 1,4dioxane improves this situation. 1,4-Dioxane is not only similar to cyclohexene on the hexatomic ring structure but also can form hydrogen bond with water. So 1,4-dioxane and cyclohexene or water can dissolve each other. After 1,4-dioxane is C



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The errors between experimental values and the calculated ones and the RMSD% for the two ternary systems, which were shown in Table S2 (Supporting Information) and Table S3 (Supporting Information), demonstrate that the parameters in Table 2 are reliable. 3.2. Reaction Kinetic Experiment. The kinetic behaviors of cyclohexene hydration catalyzed by the A-36 were studied using 1,4-dioxane as cosolvent by determining the effects of stirring speed, catalyst particle size, reaction temperature, and catalyst loading on cyclohexene hydration reaction in our work. 3.2.1. Elimination of Mass-Transfer Resistance. For the determination of the intrinsic kinetics of cyclohexene hydration, both the internal and external mass transfer resistances should be eliminated. As shown in Figure 4, the conversion of cyclohexene is found to be evidently increased with raising the stirring speed from 100 to 300 rpm. However, there is no

Table 2. Interaction Parameters of NRTL Model for Quaternary Water (1) + 1,4-Dioxane (2) + Cyclohexene (3) + Cyclohexanol (4) System i−j

Δgij J mol‑1

Δgij J mol‑1

αij

1−2 1−3 1−4 2−3 2−4 3−4

3846.5 22 159.7 12 240.5 3425.0 −2110.3 3300.19

2253.9 11 746.0 −564.7 −2361.9 5637.4 157.39

0.300 007 0.243 199 0.269 049 0.300 00 0.200 00 0.793 629

⎛ K exp − K cal ⎞2 ij ij ⎟ /3N OF = ∑ ∑ ⎜⎜ exp ⎟ K ij ⎠ j=1 i=1 ⎝ N

3

(2)

where

K ijcal = γijoil /γijwater

(3)

K ijexp = xijoil /xijwater

(4)

where K, N, xi, and γi are distribution coefficient, number of tieline, mole fractions, and activity coefficient, respectively. The superscripts oil and water denote organic phase and aqueous phase, respectively. The fit interaction parameters are shown in Table 2. The parameters in Table 2 were used to carry out liquid− liquid equilibrium calculation to test their reliability by comparing the liquid−liquid equilibrium data with the predicted ones. The liquid−liquid equilibrium equations can be expressed as follows: xioilγioil − xiwaterγi water = 0

(5)

∑ xioil − 1 = 0

(6)

∑ xiwater − 1 = 0

(7)

Combined with the interaction parameters, liquid−liquid equilibrium data for the two ternary systems, water (1) + 1,4dioxane (2) + cyclohexanol (4) and water (1) + 1,4-dioxane (2) + cyclohexene (3), could be obtained by solving eqs 5−7. There were six unknowns for the ternary system under a defined pressure and temperature with only five equations, and it was impossible to solve the equations. Therefore, the contents of water in aqueous phase were provided as a known quantity. The equations were solved by an iterative computer program named simplex algorithms. The objective function OF′ used in this case was 3

OF′ =

∑

|xioilγioil − xiwaterγi water| 3

i=1

(8) Figure 4. (a) Effect of stirring speed on the conversion of cyclohexene, XCHE. Conditions: catalyst, ion-exchange resin A-36; cosolvent, 1,4dioxane; reaction temperature, 393.15 K; catalyst loading, 0.1 g/mL (aqueous phase volume); initial mole ratio of 1,4-dioxane to cyclohexene, 1:1; initial mole ratio of water to cyclohexene, 14:1. (b) Effect of particle size of catalyst on the conversion of cyclohexene, XCHE. Conditions: catalyst, ion-exchange resin A-36; cosolvent, 1,4dioxane; stirring speed, 500 rpm; reaction temperature, 393.15 K; catalyst loading, 0.1 g/mL (aqueous phase volume); initial mole ratio of 1,4-dioxane to cyclohexene, 1:1; initial mole ratio of water to cyclohexene, 14:1.

The reliability of these results was further validated calculating the corresponding root-mean-square deviation (RMSD%) values using the following equation N

RMSD% = 100

2

3

∑∑∑ k=1 j=1 i=1

exp cal 2 − xijk (xijk )

6N

(9)

where the subscript j denotes a different phase and N denotes the number of tie-lines. D



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appreciable increase in conversion when the stirring speed is increased to 500 rpm. In addition, the breakage phenomenon of the resin particles was also observed if the stirring speed was further improved in the experiment. Thus, the speed of 500 rpm sufficient to avoid external mass transfer resistances is regarded as appropriate stirring speed. As shown in Figure 4, no obvious variation of the cyclohexene conversion is observed, which suggests that the internal diffusion resistance could be ignored. 3.2.2. Effect of Reaction Temperature. The reaction temperature is an important factor for kinetic experiments. Activation energy of the cyclohexene hydration can be obtained by studying the effect of temperature on the hydration reaction. During the course of experiment, the behavior of cyclohexene hydration was determined over the temperature from 373.15 to 403.15 K. It can be seen from Figure 5 that the increment of Figure 6. Effect of catalyst loading on the conversion of cyclohexene, XCHE. Conditions: catalyst, ion-exchange resin A-36; cosolvent, 1,4dioxane; stirring speed, 500 rpm; reaction temperature, 393.15 K; initial mole ratio of 1,4-dioxane to cyclohexene, 1:1; initial mole ratio of water to cyclohexene, 14:1. The dots represent experiment results; the lines represent model results.

even though the cosolvent 1,4-dioxane is added. Consequently, it forms a three-phase reaction system by the two liquid phases and the solid catalyst. As the external surface of ion-exchange resin catalyst, where there is a sulfonic acid group, is hydrophilic, the catalysts are always surrounded by the aqueous phase during the reaction. As a result, it is regarded that the cyclohexene hydration reaction occurs in the aqueous phase. In the present work, cyclohexene hydration reaction was assumed to occur in the aqueous phase, and the liquid−solid reaction in the aqueous phase was regarded as a homogeneous reaction. The kinetics model can be defined as

Figure 5. Effect of reaction temperature on the conversion of cyclohexene, XCHE. Conditions: catalyst, ion-exchange resin A-36; cosolvent, 1,4-dioxane; stirring speed, 500 rpm; catalyst loading, 0.1 g/ mL (aqueous phase volume); initial mole ratio of 1,4-dioxane to cyclohexene, 1:1; initial mole ratio of water to cyclohexene, 14:1. The dots represent experiment results; the lines represent model results.

temperature is apparently favorable to accelerate the hydration reaction. Meanwhile, the cyclohexene conversion decreases from 0.1752 to 0.1541 as the reaction temperature increases from 383.15 to 403.15 K. This indicates that the cyclohexene hydration reaction is a reversible exothermic reaction. 3.2.3. Effect of Catalyst Loading. The effect of the catalyst loading on the hydration reaction was also studied in the experiment. In this work, catalyst loading is defined as the weight concentration of the catalyst to the volume of aqueous phase. The hydration reaction was studied for different catalyst loading (from 0.075 to 0.125 g/mL), and the results are shown in Figure 6. It was observed that the increment of catalyst loading leads to an increment of the reaction rate and a decrement of time to the reaction equilibration. The reason is that the higher the catalyst loading, the higher the total number of available actives sites for the cyclohexene hydration. From the experimental results, we can also obtain that there is no relationship between catalyst loading and the chemical equilibrium of cyclohexene hydration reaction, which is consistent with the theory of chemical equilibrium. 3.3. Kinetic Modeling. It is well-known that the mutual solubility between cyclohexene and water is extremely small,

water water water rCHOL = k f αCHE αH2O − k bαCHOL

(10)

⎛ E ⎞ k f = k f,0 exp⎜ − f ⎟ ⎝ RT ⎠

(11)

⎛ E ⎞ k b = k b,0 exp⎜ − b ⎟ ⎝ RT ⎠

(12)

The relationship between kf and kb is

Kα =

kf kb

(13)

Bring eq 13 to eq 10 water ⎞ ⎛ water water αCHOL α H 2O − rCHOL = k f ⎜αCHE ⎟ K ⎠ ⎝

(14)

where rCHOL is the formation rate of cyclohexanol per unit mass of the catalyst, kf is the reaction rate constant of forward reaction, kf,0 is the pre-exponential factor of forward reaction, Ef is the activation energy of forward reaction, kb is the reaction rate constant of reverse reaction, kb,0 is the pre-exponential factor of reverse reaction, Eb is the activation energy of reverse reaction. rCHOL can be calculated by rCHOL = E

NCHE0 ⎛ dXCHE ⎞ ⎜ ⎟ W ⎝ dt ⎠

(15)



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respectively, and the ones of backward reaction are 1.9836 × 107 mol/kg (cat)/h and 42.2246 kJ/mol, respectively. Bring the parameters back to the kinetic model to calculate the conversion of cyclohexene, and the predicted values were compared with the experimental values. The results are shown in Figures 5 and 6. It indicates that the predicted values and the experimental ones agree well.

where NCHE0 is the initial mole ratio of cyclohexene, W is the weight of catalyst, XCHE is the conversion of cyclohexene, and t is the reaction time. The equilibrium constant of cyclohexene hydration reaction can be calculated by ⎛ α water ⎞ ⎛ x water ⎞ ⎛ γ water ⎞ CHOL CHOL ⎜ ⎟ ⎟ ⎜ CHOL ⎟ Kα = ⎜ water water ⎟ = ⎜⎜ water water ⎟ ⎜ water water ⎟ ⎝ αCHE αH2O ⎠eq ⎝ xCHE x H2O ⎠eq ⎝ γCHE γH2O ⎠

4. RESIDUE CURVE MAPS (RCMS) 4.1. Modeling for Residue Curve. Today there is an increasing interest in the theoretical research and practical application of reactive distillation. As presented by Gadewar et al.,13 the design and development of reactive distillation involve four stages, the first of which deals with the analysis of feasibility, that is, the analysis of possible top and bottom products. Residue curve maps are usually used to predict the feasibility of simple distillation. Fien and Liu14 presented a comprehensive review on the synthesis and shortcut design of nonreactive separation processes based on RCMs. Barbosa and Doherty15 developed the reactive residue curve model to analyze the feasibility of reactive distillation processes with single chemical equilibrium reaction. Ung and Doherty16,17 extended this model to multiple chemical equilibrium reaction systems. Solokhin et al.18,19 developed the reactive residue curve model for reactive distillation processes of kinetic control. The Damköhler number Da used to show the extent of the reaction was introduced into the model. Since the liquid phase of reaction system was not always homogeneous, Qi et al.20 developed the reactive residue model for the reaction system with liquid phase spilitting and Damköhler number Da was also introduced into their model. These research results have been widely used, and there are too many to mention here. Due to the strong nonideality of reaction system containing cyclohexene and water, the liquid phase still split into two phases, even though cosolvent 1,4-dioxane was added. As a result, the reactive residue curve model taking into account the contribution of liquid phase spilitting built by Qi et al.20 was applied to analyze the feasibility of the process proposed in present work. The model equation can be written as

eq

(16)

where Kα is the equilibrium constant in terms of activity, and α is the activity. The activity coefficient of each component is calculated by the relationship of LLE. Generally, the equilibrium constant is determined by temperature only; the relation is

dln Kα ΔHr = dT RT 2

(17)

where ΔHr is the heat of reaction, and T is the absolute temperature. ΔHr can be assumed as a constant within a range of temperature, so the relationship between ln Kα and 1/T is linear in the rectangular coordinates. The relationship between the equilibrium constant ln Kα and 1/T is shown in Figure 7. With the regression analysis of least-squares, we obtain eq 18: ⎛ 1383.7 ⎞ Kα = exp⎜ − 5.4794⎟ ⎝ T ⎠

(18)

d zi k = (zi − yi ) + f (vi − vtotalzi)Da 9 dt k f,ref

(19)

where zi is the mole fraction of component i in liquid phase, yi is the mole fraction of component i in vapor phase, kf,ref is the constant of reaction rate at the reference temperature, which is generally the lowest boiling point of the system, vi is the stoichiometric coefficient of component i, and vtotal is the total mole change of reaction. Da is Damköhler number which is always regarded as a constant. No reaction takes place in the system at Da = 0, which is equal to a simple distillation process. The reaction reaches chemical equilibrium at Da → ∞, which is equal to a reactive distillation process of balance control. The reaction dose not reach chemical equilibrium at Da ∈ (0,∞), which is equal to a reactive distillation process of kinetic control. The dimensionless reaction rate is given by r 9 = CYOL kf (20)

Figure 7. Logarithm of the equilibrium constants, ln(Kα), as a function of temperature.

The activity of each component in aqueous phase was obtained by liquid−liquid equilibrium calculation according to the model and parameters in section 3.1. In this work, the experimental data obtained at different temperatures were used to correlate rate constant of forward reaction according eq 14 by the least-squares method first, and the rate constant of backward reaction was obtained incidentally according to eq 13. Then the pre-exponential factor and the activation energy of forward and backward reaction were calculated by the least-squares method according to eqs 11 and 12 with the forward and backward reaction rate constants obtained above, respectively. The values of preexponential factor and the activation energy of forward reaction are 8.2702 × 104 mol/kg (cat)/h and 30.740 kJ/mol,

where rCHOL is calculated according to eq 14. For the homogeneous region, the ideal gas-phase behavior can be assumed if the system pressure is not too high, and then yi is given by F


Industrial & Engineering Chemistry Research yi =

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ziγipisat

case, the unstable node 1 disappears, and there is no longer an unstable node in the system. Pure cyclohexanol is no longer a stable node because it is decomposed into water and cyclohexene, which leads stable node 1 to move out of the cyclohexanol corner, and turn into the mixture containing cyclohexene, water, and cyclohexanol instead. From their moving trend, it can be observed that stable node 1 and saddle point 3 are becoming close to each other. Besides, the saddle point of pure 1,4-dioxane changes into a stable node (marked as stable node 3), and the other singular points do not change with the variation of Da number. In general, at Da = 0.02, due to the appearance of the new stable node 1,4-dioxane, the RCM can be divided into three areas. In this case, most of the residue curves end at stable node 1, and only few of them end at stable node 2 and stable node 3. With the continuous increment of Da number, the area where residue curves end at stable node 1 is shrinking, and both the areas where residue curves end at stable node 2 and stable node 3 are expanding. The RCM with Da = 0.05 is shown in Figure 8c. In this case, stable node 1 and saddle point 3 are closer to each other. At Da ≈ 0.08, the RCM is divided into two areas again. Some of the residue curves end at stable node 2, and the others end at stable node 3, as shown in Figure 8d. In this case, the area where residue curves end at stable node 1 disappears, because stable node 1 and saddle point 3 meet and extinguish each other at the critical Damköhler number Da ≈ 0.08 on a special point called kinetic tangent pinch,22 which can be seen from the trajectories of stable node 1 and saddle point 3 in Figure 9. As shown in Figure 8, stable node 1 and saddle point 3 move only on the plane of cyclohexene + water + cyclohexanol, for 1,4-dioxane does not react with any components in the system. As shown in Figure 8e, there is little change of the singular points on RCM at Da > 0.08. The analysis of residual curve can be used to determine the feasibility of reactive distillation process manufacturing cyclohexanol. As cyclohexanol is the target product, stable node 1 where cyclohexanol enriches only existing at lower Da number requires special attention. In the case of lower Da number, reactive distillation technology can be applied to break the chemical equilibrium limit of cyclohexene hydration reaction. In particular, if Da exceeds its critical number of 0.08, stable node 1 disappears, which means cyclohexanol cannot be collected. In other words, the reactive distillation process proposed in the present paper is controlled by the reactive kinetics. Besides, pure cyclohexanol is not a stable node at Da ≠ 0 as shown in Figure 8b−e, but is a stable one at Da = 0. That is to say, a stripping section could be added under the reaction section to get the pure cyclohexanol at column bottom. In addition, a rectifying section can be added above the reaction section to prevent cyclohexanol mixing into the cycling streams at column top. According to the above analysis, the process shown in Figure 1 is reasonable and feasible.

(21)

P

where pisat is the saturated vapor pressure of component i calculated by Antoine equation whose parameters are shown in Table 3, P is the system pressure, γi is activity coefficient of component i calculated by NRTL model, and T is the bubble point calculated by normalization equation:

∑ yi = 1

(22)

Table 3. Antoine Equations for the Four Given Components21 lg(psat/mmHg) = A − (B/((T/°C) − C)) substance

A

B

C

water 1,4-dioxane cyclohexene cyclohexanol

7.966 81 7.006 30 6.872 40 6.255 30

1668.21 1288.50 1221.90 912.87

228.00 211.01 223.18 109.13

For the heterogeneous region, the ideal gas-phase behavior can also be assumed if the system pressure is not too high, and then yi is given by yi =

xioilγioilpisat

=

xiwaterγi waterpisat P

(23)

zi = (1 − β)xioil + βxiwater

(24)

P and zi is given by

where β is the relative liquid mole holdup of aqueous phase. The residue curve maps can be obtained by solving the ordinary differential equation eq 19 from the given initial value. Set the left of eq 19 be equal to 0, and then (zi − yi ) +

kf k f,ref

(vi − vtotalzi)Da 9 = 0

(25)

The singular points of residue curves were obtained by solving eq 25. The singular points include stable node, unstable node, and saddle point. Stable point is the end of residue curves, which is the possible column bottom product composition. Unstable point is the starting point of the residue curves, which is the possible column top product composition. And the saddle point is a point that residue curves approach infinitely but never pass. 4.2. Analysis of Residue Curve Maps. By solving eq 19, the RCMs were obtained at different Da numbers, as shown in Figure 8. The RCM with Da = 0 is shown in Figure 8a. In this case, the unstable node is the azeotropic point between water and cyclohexene (marked as unstable node 1), and the stable nodes are pure cyclohexanol and pure water (marked as stable node 1 and stable node 2, respectively). At Da = 0, the RCM can be divided into two areas. Most of the residue curves originate from unstable node 1 and end at stable node 1, and only few of them originate from unstable node 1 and end at stable node 2. Besides, pure cyclohexene, pure 1,4-dioxane, the azeotropic point between cyclohexanol and water, and the azeotropic point between 1,4-dioxane and water are saddle points (marked as saddle point 1, 2, 3, and 4, respectively). With increasing Da number, the RCM changed correspondingly. The RCM with Da = 0.02 is shown in Figure 8b. In this

5. CONCLUSIONS Conclusions are as follows: (1) On the basis of the liquid− liquid equilibria (LLE) experimental results of the ternary system of water + 1,4-dioxane + cyclohexene and the related references, the interaction parameters of the NRTL model for water +1,4-dioxane + cyclohexene + cyclohexanol were obtained in this work. (2) The reaction kinetics data of the cyclohexene hydration were obtained in the experiments which take 1,4-dioxane as the cosolvent and ion-exchange resin A-36 as the catalyst. On the basis of the model parameters G



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Figure 8. Residue curve maps for cyclohexene hydration reaction system using 1,4-dioxane as cosolvent.

determined by the kinetics data, the pseudohomogeneous kinetic model in terms of activity coefficient was established. (3) A novel process for the production of cyclohexanol by

means of reactive distillation using 1,4-dioxane as a cosolvent and ion-exchange resin A-36 as a catalyst is suggested and analyzed by RCMs. It is found that the process, which is H



Article

R idea gas constant T absolute temperature W mass of catalyst xiwater, xioil mole fraction of component i in raffinate and extract liquid phase, respectively XCHE conversion of cyclohexene yi mole fraction of component i in vapor phase zi total mole fraction of component i in liquid phase Greek Letters

αiwater = activity of component i in raffinate liquid phase β = relative liquid mole holdup of aqueous phase γi = activity coefficient of component i in raffinate liquid phase Subscripts

CHOL cyclohexanol ENE cyclohexene H2O water Superscripts

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Figure 9. Bifurcation behavior of stable node 1 and saddle point 3 with respect to Damköhler number for cyclohexanol reaction system. For some singular points, the corresponding value of Damköhler number is marked beside the mark.

ASSOCIATED CONTENT

S Supporting Information *

Additional tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Steyer, F.; Qi, Z.; Sundmacher, K. Synthesis of cyclohexanol by three-phase reactive distillation: influence of kinetics on phase equilibria. Chem. Eng. Sci. 2002, 57 (9), 1511−1520. (2) Panneman, H.; Beenackers, A. A. C. M. Solvent effects on the hydration of cyclohexene catalyzed by a strong acid ion-exchange resin. 1. Solubility of cyclohexene in aqueous sulfolane mixtures. Ind. Eng. Chem. Res. 1992, 31 (4), 1227−1231. (3) Panneman, H.; Beenackers, A. A. C. M. Solvent effects on the hydration of cyclohexene catalyzed by a strong acid ion-exchange resin. 2. Effect of sulfolane on the reaction kinetics. Ind. Eng. Chem. Res. 1992, 31 (6), 1425−1433. (4) Panneman, H.; Beenackers, A. A. C. M. Solvent effects on the hydration of cyclohexene catalyzed by a strong acid ion-exchange resin. 3. Effect of sulfolane on the equilibrium conversion. Ind. Eng. Chem. Res. 1992, 31 (6), 1433−1440. (5) Shan, X.; Cheng, Z.; Li, Y. Solvent effects on hydration of cyclohexene over H-ZSM-5 catalyst. J. Chem. Eng. Data 2011, 56 (12), 4310−4316. (6) Gmehling, J.; Menke, J.; Krafczyk, J.; Fischer, K. Azeotropic Data, 1st ed.; VCH: Weinheim, 1994. (7) Qiu, T.; Li, S.; Li, S.; Wu, Y. Liquid-liquid phase equilibria of ternary system of water/1,4-dioxane/dihydromyrcene. Fluid Phase Equilib. 2009, 280 (1), 84−87. (8) Qiu, T.; Wang, X..; Tian, H.; Huang, Z. Liquid-liquid equilibrium for system water+1,4-dioxane+cyclohexanol over the temperature range of (313.2 to 343.2) K. Fluid Phase Equilib. 2012, 324, 28−32. (9) Steyer, F.; Sundmacher, K. VLE and LLE data for the system cyclohexane + water + cyclohexene + cyclohexanol. J. Chem. Eng. Data 2004, 49 (6), 1675−1681. (10) Renon, H.; Prausnitz, J. M. Local positions in thermodynamic excess function for liquid mixtures. AIChE J. 1968, 14 (1), 135−144. (11) Kennedy, J. In The particle swarm: Social adaptation of knowledge. Proceedings of the IEEE International Conference on Evolutionary Computation, Indianapolis, USA, 1997, 303−308. (12) Eberhart, R. C.; Shi, Y. In Particles swarm optimization: Developments, applications and resources. Proceedings of the IEEE International Conference on Evolutionary Computation, Seoul, Korea, 2001, 81−86. (13) Gadewar, S. B.; Tao, L.; Malone, M. F.; Doherty, M. F. Process alternatives for coupling reaction and distillation. Chem. Eng. Res. Des. 2004, 82 (2), 140−147. (14) Fien, G. A. F.; Liu, Y. A. Heuristic synthesis and shortcut design of separation processes using residue curve maps: Review. Ind. Eng. Chem. Res. 1994, 33 (11), 2505−2522.

feasible at Da < 0.08, is a course of reaction distillation controlled by the reactive kinetics.

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oil organic phase water aqueous phase

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (21176049) and Natural Science Foundation of Fujian Province (2011J01038) for the financial support.

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NOMENCLATURE Da Damköhler number Ef, Eb forward and backward activation energy, respectively Δgij interaction parameter for NRTL activity model between i and j ΔHr reaction heat kf, kb forward and backward rate constant kf,ref forward and backward rate constant at reference temperature Kik the distribution coefficient of component i along a tie line k Kα chemical equilibrium constant N number of tie line NCHE,0 initial moles of cyclohexene psati saturated vapor pressure for component i P system pressure rCHOL reaction rate of cyclohexanol I



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(15) Barbosa, D.; Doherty, M. F. The simple distillation of homogeneous reactive mixtures. Chem. Eng. Sci. 1988, 43 (3), 541− 550. (16) Ung, S.; Doherty, M. F. Calculation of residue curve maps for mixture with multiple equilibrium chemical reactions. Ind. Eng. Chem. Res. 1995a, 34 (10), 3195−3202. (17) Ung, S.; Doherty, M. F. Theory of phase equilibria in multiple reaction systems. Chem. Eng. Sci. 1995b, 50 (20), 3201−3216. (18) Solokhin, A. V.; Blagov, S. A.; Serafimov, L. A.; Timofeev, V. S. Open evaporation processes accompied by chemical reaction in the liquid phase. Theor. Found. Chem. Eng. 1990a, 24 (6), 103−109. (19) Solokhin, A. V.; Blagov, S. A.; Serafimov, L. A.; Timofeev, V. S. Dynamical system for open evaporation process in presence of A ∼ B chemical reaction. Theor. Found. Chem. Eng. 1990b, 24 (3), 377−382. (20) Qi, Z.; Kolah, A.; Sundmacher, K. Residue curve maps for reactive distillation systems with liquid-phase splitting. Chem. Eng. Sci. 2002, 57 (1), 163−178. (21) Reid, C. R.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids; McGraw Hill: New York, 1987; Appendix. (22) Qi, Z.; Flockerzi, D. Singular points of reactive distillation systems. AIChE J. 2004, 50 (11), 2866−2876.

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Study on Feasibility of Reactive Distillation Process for the Direct

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