Synthesis of Methacrylic Anhydride by Batch Reactive Distillation

Oct 14, 2014 - School of Chemical Engineering, Fuzhou University, Fuzhou 350108, ... Catalysts preparation, characterization, and reaction kinetics, p...
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Synthesis of Methacrylic Anhydride by Batch Reactive Distillation: Reaction Kinetics and Process Hongxing Wang, Xiangwei Bu, Zhixian Huang,* Jinbei Yang, and Ting Qiu School of Chemical Engineering, Fuzhou University, Fuzhou 350108, Fujian China ABSTRACT: The reaction kinetic of the synthesis of methacrylic anhydride was investigated with a batch stirred-tank reactor at atmospheric pressure using methacrylic acid and acetic anhydride as raw materials. The reactions were conducted at several temperatures between 343.15 and 363.15 K and at various starting reactant compositions. A homogeneous reversible reaction of the second order was described. A batch reactive distillation column was employed to obtain methacrylic anhydride. The influence of feed molar ratio, reflux ratio, and entrainer on the reaction was investigated, and a suitable operating parameter was obtained. Under these conditions, the conversion of methacrylic acid was 99.95%, and the yield of methacrylic anhydride was 86.99%.

1. INTRODUCTION Methacrylic anhydride (MA2O), a colorless transparent liquid, can be used as a reagent in the synthesis of methacrylic amides, such as dimethyl aminopropyl, (meth)acrylamide, or methacrylates and methacrylic ester. MA2O can also be used as a raw material for the synthesis of special fine chemicals.1 To date, MA2O has acquired increasing attention in the polymerization industry. It is possible to prepare an anhydride by reacting acetic anhydride with the acid corresponding to the desired anhydride. Dupont et al. reported a method in which MA2O was formed in a batch reactor with methacrylic acid (MAA) and acetic anhydride (Ac2O) as reactants, and next, a distillation column is used to purify MA2O.1 However, one of the challenging aspects of the method stems from the fact that MAA is unstable and has a tendency to form polymers. Therefore, an effective polymerization inhibitor is necessary to ensure the safe and efficient operation of manufacturing plants.2 A comprehensive mathematical model for the inhibition of acrylic acid polymerization was developed by Rujun.3,4 Using a polymerization inhibitor, processes for the synthesis of MA2O were developed by Dupont,1,5 Hurtel,6 and Murata.7 However, the implementation of this process comes up against polymerization problems. In addition, the amount of anhydride produced is limited by the size of the reactor and thus by the amount of reagents loaded into this reactor. Similar to transesterification, transanhydrization is a typical equilibrium-limited reaction. The yield for the transanhydrization reaction is strongly limited by the equilibrium conversion. Therefore, the combination of reaction and separation process into a single unit, such as a reactive distillation (RD) column, may be used. The RD process has been widely accepted in many chemical processes, such as etherification, esterification, and transesterifications.8−11 Reactive distillation offers several advantages such as overcoming of thermodynamic limitations and increasing reaction yield and selectivity, which can lower raw material consumption and energy consumption and then reduce total costs. Although reactive distillation seems to be a great idea, its application is fairly restricted. The RD process requires not only a match between the temperature favorable for reaction and separation but also a reasonably large specific reaction rate. If the © 2014 American Chemical Society

reactions are very slow, the required tray holdups and number of reactive trays would be too large to be economically feasible in a distillation column.12 In contrast to etherification, esterification, and transesterifications, only little information about transanhydrization reactions can be found in the literature. For the optimization of an industrial process, the reaction kinetics should be welldetermined because the design of a large-scale reactor should be based on the rate equations.13−15 An appropriate rate equation should be based on the true mechanism, including the elementary steps of the primary and side reactions.16,17 However, there were no operation, kinetics and mechanisms available for the synthesis of methacrylic anhydride in the open literature. Thereby, in this work, the reaction kinetic of the synthesis of MA2O was investigated, and the synthesis of MA2O in a batch reactive distillation column was studied.

2. MATERIALS AND METHODS 2.1. Materials. The chemicals used for the reaction kinetic experiments were of analytical grade (>99.8 wt %). The chemicals were dried over molecular sieve prior to use. For the batch reactive distillation experiments, the chemicals were of reaction grade (>99 wt %) and were used without further purification. All of the chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and the purity of the chemicals was verified by gas chromatography. 2.2. Analysis. The analysis was carried out in a gas chromatograph (Agilent 7890A), equipped with an FID and electronic pressure control. The capillary column was DB-1701 (30 m × 0.53 mm × 1 μm) with nitrogen as the carrier gas and toluene as the internal standard substances. The FID temperature and gasification temperature were 533.15 and 503.15 K, respectively. The initial temperature of the oven was 323.15 K Received: Revised: Accepted: Published: 17317

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and held for 1 min, and then the oven temperature was increased to 503.15 at 10 K/min.

Table 1. Initial Molar Ratios (θ) and Temperatures of the Reaction Kinetic Experiments

3. REACTION KINETICS 3.1. Experiments. The kinetic experiments were conducted in a three-neck flask (500 mL). The setup was equipped with a heater (DF-101S, including a temperature controller and stirrer speed controller) manufactured by YuHua Company (Gongyi, China). The temperature inside the reactor was controlled within the accuracy of ±0.1 K. A reflux condenser was installed to avoid any loss of volatile components. Liquid samples of 1 mL were taken using a syringe and were weighed (accuracy of the balance ±0.0001 g). To make the reaction occur in the liquid phase and eliminate polymerization, the reaction temperature was kept at 343.15 to 363.15 K. To ensure the starting point of the kinetic experiments precisely, the following procedure was used. First, a three-neck flask was filled with MAA and polymerization inhibitor (0.3 wt %). Then, the reactant was heated to the desired temperature. At the same time, Ac2O was heated to the same temperature in another flask and was fed into the three-neck flask quickly. This time was considered the starting point of the reaction. During one experiment, between 15 and 20 samples were taken. The samples were cooled rapidly to 277.15 K by an ice-bath to avoid any further reaction and were analyzed by gas chromatography. Various experiments were performed starting from different compositions, and the effects of the temperature and the molar ratio of the reactants on the reaction were studied. The kinetic experiments lasted for 6 to 8 h. The initial molar ratio of MAA to Ac2O and the temperature of reaction kinetic experiments are shown in Table 1. During the kinetic reaction experiments, no solid polymer was observed. Therefore, polymerization can be neglected through reasonable control. The molar yields calculated from the analytical results for replicate experiments agreed to within a few percent (±1.5%). 3.2. Reaction Mechanism. The reaction of MAA to MA2O consists of two cascade reversible reactions, reaction 1 and 2, with an intermediate, methacrylic-acid acetic anhydride (AMAOAc).

experiment

temperatures (K)

molar ratio (θ = molMAA:molAc2O)

run 1 run 2 run 3 run 4 run 5 run 6 run 7 run 8

343.15 348.15 353.15 358.15 363.15 353.15 353.15 353.15

2.0:1.0 2.0:1.0 2.0:1.0 2.0:1.0 2.0:1.0 2.0:0.75 2.0:1.25 2.0:1.5

Based on the characteristics of the reaction, a possible reaction mechanism was deduced.

3.3. Elimination of the Polymerization Effect. MAA is easily polymerized because the molecular structure of MAA contains a carbon−carbon double bond, which can undergo radical-initiated addition reactions. The polymerization can be catalyzed by heat, light, or peroxides. To maintain the analysis precision and minimize the polymerization effect, therefore, the amount of the mixture of polymerization inhibitor was investigated. Many types of polymerization inhibitors can be used to prevent the polymerization of MAA. Our previous studies showed that 2,4-ditert-butyl cresol (6BX), 2,6-tert-butyl6-para-cresol (BHT), and trioctyl phosphate (TOP) were effective polymerization inhibitors, and the optimal mass ratio was 1:1.67:1.33 (6BX:BHT:TOP).18 In these experiments, the mixture of polymerization inhibitors was added into the binary mixture of HAc + MAA. Next, the mixture was fed into the three-neck flask and heated to 363.15 K.

Up to now, there has been no available reaction mechanism for the synthesis of methacrylic anhydride in the open literature. 17318

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Liquid sample of about 1 mL was taken using a syringe every 2 h, and the concentration of MAA was analyzed by gas chromatography. The degree of polymerization was calculated from eq 5. ⎛ C ⎞ degree of polymerization = ⎜1 − t ⎟ × 100% C0 ⎠ ⎝

(5)

where C0 is the molar concentration of MAA at the initial time and Ct is the molar concentration of MAA measured at different times. The effect of the dosage of the polymerization inhibitor on the polymerization of MAA is displayed in Figure 1. The results

Figure 1. Effect dosage of the polymerization inhibitor on the polymerization of MAA. Figure 2. Effect of temperature on the reaction at θ = 2.0:1.0.

demonstrate that, when the dosage of polymerization inhibitor is 0.3 wt %, the effect of MAA polymerization can be ignored. In the subsequent experiments, therefore, the mixtures of polymerization inhibitors are added with 0.3 wt %. 3.4. Effect of Temperature. The equilibrium constant, Keq, is the ratio of the equilibrium concentrations of products over the equilibrium concentrations of reactants each raised to the power of their stoichiometric coefficients. Equilibrium constant does not depend on the initial concentrations of reactants and products but does depend on temperature. To investigate the effect of the reaction temperature, the transanhydrization reactions were performed in the temperature range from 343.15 to 363.15 K. Typical results are shown in Figure 2a. It can be observed that the reaction rate increases substantially with the increasing temperature. However, as the temperature increases, the equilibrium concentration and the highest concentration of AMAOAc decrease, which indicates that reaction 1 is an exothermic reaction. However, the different effect of the reaction temperature on the equilibrium conversion of MA2O demonstrates that reaction 2 is an endothermic reaction. From the equilibrium concentration of the reaction products, the equilibrium constants of the reactions in this work were calculated. Because of the absence of the physical properties of these substances, especially AMAOAc, all of the activity coefficients were set to 1, yielding the following mathematical formulations for the molar-based equilibrium constants, Keq,1 and Keq,2. Keq,1 =

E k1 + CE C HAc = AMAOAc E E k1 − CMAA CAc2O

(6)

Keq,2 =

k 2+ CE CE = E MA2O HAc E k 2− CAMAOAcCMAA

(7)

reaction rate constants of reaction 1, respectively; k2+ and k2‑ are the forward and backward rate reaction constants of reaction 2, respectively; CEi (i = MA2O, HAc, MAA, Ac2O, and AMAOAc) represents the molar concentrations of MA2O, HAc, MAA, Ac2O, and AMAOAc at chemical equilibrium, respectively. The molar-based equilibrium constants can be calculated from the experiment runs 1−5, as shown in Table 2. The temperature dependence of the chemical equilibrium constant can be correlated using the integral form of the van’t Hoff equation, eq 8. Therefore, the standard enthalpy of the reaction, ΔH0, can be determined through a linear least-squares regression of the experimental data to the van’t Hoff equation. ΔH 0 (8) dT RT 2 where R is the gas constant, T is the absolute temperature, and ΔH0 is standard enthalpy of the reaction. Reaction enthalpies were obtained from eq 8 by plotting ln Keq against 1/T (as shown in Figure 3). The positive or negative value of enthalpy indicates that the reaction is endothermic or exothermic in nature. The linear plot shows that the first reaction, reaction 1, is an dln Keq

where Keq,1 and Keq,2 are chemical equilibrium constant of reactions 1 and 2; k1+ and k1‑ are the forward and backward

=

Figure 3. Relationship between ln Keq and 1/T. 17319

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Table 2. Kinetic Parameters for the Homogeneous Consecutive Kinetic Model temp/K

343.15

348.15

353.15

358.15

363.15

Keq1 Keq2 k1+ × 104(L·mol−1·min−1) k1‑ × 104(L·mol−1·min−1) k2+ × 104(L·mol−1·min−1) k2‑ × 104(L·mol−1·min−1)

1.47 0.99 49.51 33.70 5.78 5.80

1.40 1.07 66.47 47.40 8.43 7.90

1.38 1.11 119.47 86.53 14.42 13.01

1.33 1.16 177.38 128.48 19.99 17.29

1.29 1.21 221.02 170.95 25.06 20.74

exothermic reaction with ΔH0 = −6.29 kJ/mol, and the second reaction, reaction 2, is an endothermic reaction with ΔH0 = 9.62 kJ/mol. 3.5. Effect of the Initial Reactant Molar Ratio. To investigate the effect of the initial molar ratio of MAA to Ac2O on the transanhydrization reaction, the initial molar ratios MAA to Ac2O were changed from 2.0:0.75 to 2.0:1.50, whereas the other operating variables were kept at constant values. Figure 4 shows

∫0

C MA2O

dCMA2O

= k 2+

∫0

t

⎛ C C ⎞ ⎜⎜CAMAOAcCMAA − MA2O HAc ⎟⎟ dt Keq2 ⎝ ⎠

(12)

Both sides of eq 9 are replaced with two variables, X1 and Y1, as shown in eq 13 and 14. Y1 =

X1 =

∫0

t

∫0

⎛ C C ⎞ ⎜⎜CMAACAc2O − AMAOAc HAc ⎟⎟ dt Keq1 ⎝ ⎠

CAc2O

dCAc2O

(13)

(14)

In the same way, X2 and Y2 are used in eq 12, and then eqs 15 and 16 are obtained. Y2 =

X2 =

Figure 4. Effect of the initial molar ratio on the reaction at T = 353.15 K.

r2 =

dCAc2O = k1 +CMAACAc2O − k1 −CAMAOAcC HAc dt

(9)

(10)

∫0

dCAc2O = k1 +

∫0

C MA2O

dCMA2O

(15)

(16)

and 6, the forward reaction rate constant of reactions 1 and 2, k1+ and k2+, were found. According to eqs 6 and 7, the reverse reaction rate constants, k1‑ and k2‑, were calculated, as shown in Table 2. The temperature dependency of the rate constant is expressed by the Arrhenius law: ⎛ −E ⎞ ki(T ) = ki0 exp⎜ a ⎟ ⎝ RT ⎠

The integrated form of eq 9 and 10 can be written as t

⎛ C C ⎞ ⎜⎜CAMAOAcCMAA − MA2O HAc ⎟⎟ dt Keq2 ⎝ ⎠

Figure 5. Relationship between X1 and Y1.

dCMA2O = k 2 +CAMAOAcCMAA − k 2 −CMA2OC HAc dt

CAc2O

∫0

t

By applying eqs 13−16 to the experimental data, Figures 5 and 6 were obtained. From the slopes of the lines given in Figures 5

the effect of the initial molar ratio on the conversion of MAA. With the change in the initial molar ratio of MAA to Ac2O from 2.0:0.75 to 2.0:1.50, the conversion of MAA increased from 41 to 60%. This increase is due to the higher Ac2O molar concentration, which, in turn, shifted the reaction equilibrium toward the product side. So the equilibrium conversion of MAA can be effectively enhanced by using an excess of Ac2O. 3.6. Kinetic Modeling. The experimental results are needed to develop a kinetic model, which can be integrated into a reactive distillation model to simulate the process of the synthesis of methacrylic anhydride in a reactive distillation column. From the reaction mechanism, Ac2O reacting with MAA can be regarded as a reversible reaction of the second-order. Because of the absence of the physical properties of these substances, the molar concentrations are used instead of activities. Thus, the rates of reactions 1 and 2, r1 and r2, can be written as follows: r1 = −

∫0

⎛ C C ⎞ ⎜⎜CMAACAc2O − AMAOAc HAc ⎟⎟ dt Keq1 ⎝ ⎠

(17)

k0i

where represents the reaction pre-exponential factor and Ea represents the activation energy. According to eq 17, the relationship between ln(k) and 1/T is presented in Figure 7, and

(11) 17320

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(K = Keq1/Keq2) almost remained the same (K ≈ 1.5). Therefore, we can deduce that the reaction is under thermodynamic control. For a reversible chemical reaction, if one of products can be removed simultaneously during the course of the reaction, the reaction can be shifted to the forward direction, and the conversion can be enhanced. Thus, the chemical equilibrium constraint on conversion can be overcome, and high conversions can be achieved, even in cases with small chemical equilibrium constants.12 In this reaction system, the byproduct HAc has the lowest boiling point among the five components and is easily removed, so reactive distillation can be used to increase the reaction conversion. However, it can be deduced from Figure 2 that the reaction requires a long time to reach the desired balance. For example, when the reaction temperature is 363.15 K, it takes 100 min to reach chemical equilibrium. Therefore, the reaction is very slow. Before the reaction reaches the chemical equilibrium, the intermediate AMAOAc has a maximal concentration peak (as shown in Figure 1), indicating that the reaction rate of reaction 1 is faster than that of reaction 2. The intermediate AMAOAc is not simply removed because its boiling point is close to that of MA2O. Therefore, AMAOAc must be almost completely reacted in reaction 2, which requires a long reaction time. Batch reactive distillation (BRD) is a batch distillation system wherein the reaction takes place in a reboiler. A large holdup in the reboiler ensures that it has enough hydraulic retention time for the reaction. Therefore, batch reactive distillation is suitable for the synthesis of MA2O. 4.2. Experimental Setup. BRD experiments were performed in a laboratory-scale glass column with an inner diameter of 25 mm and a total height of 1500 mm, as shown in Figure 9. The glass column was packed with θ ring stainless steel packing (2 × 2 mm). To describe the BRD column separation efficiency quantitatively, the number of theoretical stages of the BRD column was estimated using the Fenske method by separating a mixture of ethanol and isopropyl alcohol.19 The results indicated that the separation efficiency of the column was equivalent to 20 theoretical stages. A three-necked flask (500 mL) was used as a reboiler and was immersed in an oil bath. The column was equipped with a reflux splitter located directly below a total condenser. The column was wrapped with glass wool to obtain nearadiabatic operating conditions. Temperature sensors (Pt 100) were provided at the top and bottom of the column. A vacuum pump was connected to the condenser and a vacuum gauge was used to measure the pressure inside the column. The pressure was controlled by adjusting the opening of the vent valve on the buffer. 4.3. Reactive Distillation Column Operation. MAA and Ac2O were mixed and introduced into the flask, and a small amount of polymerization inhibitor was also added into the flask. Some of the polymerization inhibitor dissolved in acetic acid was introduced at the top of the distillation column from the reflux location. Thus, any risk of polymerization in the flask and in the column is avoided. To facilitate the operation of drawing off the HAc, and hence, the formation of MA2O, the initial operating pressure was set to 10 kPa and was subsequently adjusted depending on the temperature of the bottom (T < 363.15 K). The distillate and bottom compositions were measured by gas chromatography. 4.4. Batch Reactive Distillation Results and Discussion. The following parameters were analyzed: the influence of the reflux ratio, the molar feed ratio of the reactants, and the use of entrainer. The primary objective of these experiments was to investigate the feasibility of the synthesis of methacrylic anhydride in a batch reactive distillation column. These experiments

Figure 6. Relationship between X2 and Y2.

Figure 7. Arrhenius diagram of the rate constants.

Table 3. Kinetic Parameters for the Homogeneous Kinetic Model reaction R1 R2

parameter

k0 (L·mol−1·min−1)

Ea (kJ·mol−1)

k1+ k1‑ k2+ k2‑

1.71 × 10 8.37 × 1010 5.81 × 108 2.00 × 107

82.42 88.04 78.78 69.16

10

Figure 8. Effect of the reaction temperature on the mole concentration of MA2O as a function of reaction time at θ = 2.1:1.0.

Ea and k0i of reactions 1 and 2 are obtained by linear fit (as shown in Table 3). Figure 8 shows a comparison between the experimental and calculated MA2O molar concentration. The kinetic model shows a good agreement between the experimental data and predictions for the MA2O molar concentration, which can provide important insights into the mechanism of the transanhydrization reaction.

4. BATCH REACTIVE DISTILLATION EXPERIMENTS 4.1. Feasibility Analysis. When the initial molar ratios MAA to Ac2O were changed from 2.0:0.75 to 2.0:1.50, the value of K 17321

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Figure 10. Bottom product composition profile.

Figure 9. Setups of the batch reactive distillation experiment. 1. Oil bath, 2. heat conduction oil, 3. rotor, 4. three-necked flask, 5. mercury thermometer, 6. column, 7. packing, 8. thermocouple, 9. condenser, 10. vacuum gauge, 11. controller of reflux ratio, 12. valve, 13. receptor, 14. buffer, 15. vacuum pump, and 16. inlet valve.

Figure 11. Temperature and pressure at different operation times.

4.4.1. Effect of the reflux ratio. In the methacrylic anhydride synthesis system, HAc is not only a side product but a more volatile component. Because Ac2O is a reactant and may be entrained off the column by HAc, selecting an appropriate reflux ratio is necessary to prevent the loss of Ac2O. The operating conditions of the three experiments E1−E3 are listed in Table 5,

provide consistent and reliable process data that can be used for further extensive model validation. In this work, seven batch reactive distillation experiments were successfully performed. An overview of the operating conditions is summarized in Table 4. Figure 10 presents the bottom composition in the BRD, and Figure 11 displays the temperature and pressure profile. From Figure 10, it can be seen that the temperature at the bottom (Tbottom) is not greater than 363 K, which was achieved by decreasing the operating pressure gradually, and these temperatures matched the temperatures used in the kinetic study.

Table 5. Influence of the Reflux Ratio on the Column Performancea

Table 4. Experimental Details for the Batch Reactive Distillation Experiment run

reflux ratio

molar feed ratio MMAA/MAc2O

E1 E2 E3 E4 E5 E6 E7

5:1 5:3 5:5 5:5; 5:3a 5:5; 5:3a 5:5; 5:3a 5:5; 5:3a

2.0:1.0 2.0:1.0 2.0:1.0 2.0:1.0 2.0:1.2 2.4:1.0 2.0:1.2

run

reflux ratio

operating time/h

ηMAA%

YMA2O%

SMA2O%

E1 E2 E3

5:1 5:3 5:5

8.83 5.58 4.75

94.85 95.06 91.13

85.64 86.84 82.37

90.29 91.35 90.39

Note: η, conversion; S, selectivity; Y, yield. η = (moles of unreacted MAA in the product)/(moles of MAA in the feed) × 100%. S = (moles of MA2O in the product)/(moles of MA2O in the product + moles of AMAOAc in the product) × 100%. Y = ηS. a

entrainer

and the effect of the reflux ratio on the HAc purity during the distillation is presented in Figure 12. It can be seen from Figure 12 that the purity of HAc during the distillation is close to 100 wt % under three different reflux ratios, and its possible the reason is that HAc is the lightest substance in the reaction system and is easily separated. The results given in Table 6 demonstrate that the reflux ratio has little effect on the conversion of MAA, the yield, and selectivity of

toluene

5:5; 5:3 means that the reflux is set to 5:5 for the first 3 h, and then it is changed to 5:3 until the end of the experiment. a

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not suitable to use excessive MAA. On the other hand, if Ac2O is excessive, the separated AMAOAc can be reused as the reactant in the reaction 2. Therefore, a suitable molar feed ratio is MMAA:MAc2O= 2.0:1.2. 4.4.3. Effect of Entrainer. Because HAc is removed as distillate, the amount of generated HAc will decrease gradually; the concentration of HAc in the reboiler will become very low at the last of the BRD experiment, which increases the difficulty of the separation of HAc from the reaction system. Moreover, to obtain more MA2O, it is necessary to remove HAc from the reaction system in time to keep reaction 2 moving forward. Toluene and HAc can form a binary azeotrope, which contains HAc 28.1 wt % at 101.325 kPa. Hence, toluene as entrainer was fed in the reboiler by a peristaltic pump at 0.6 mL/min during the last 2 h of experiment E7. Table 7 shows the influence of Table 7. Influence of Entrainment YMA2O, %

run E5 E7

Figure 12. Influence of the reflux ratio on HAc purity in distillate.

not use entrainment use entrainment

85.99 88.12

entrainer on the yield of MA2O. Using toluene as entrainer can improve the yield from 85.99% to 88.12% under the same operating conditions. However, using an entrainer requires additional separation equipment to recover toluene. Therefore, the choosing whether to use an entrainer, one needs to consider the operating costs and investment costs.

MA2O; however, it has a significant impact on the operating time. In the batch reactive distillation, the amount of HAc produced by the reaction is certain. Maintaining a constant evaporation, the increase of the reflux ratio means that the flow rate of the distillation decreases. Taking into account the possible polymerization of MAA, long operating time may have a negative impact on the reaction. Therefore, the reflux ratio is set to 5:5 in the first 3 h, and then it is changed to 5:3 until the end of the reaction. 4.4.2. Effect of the Molar Feed Ratio. The effect of the molar feed ratio is studied, and the results are shown in Table 6. As shown in Table 6, the molar feed ratio has a significant effect on the BRD column performance. The reaction converting MAA to MA2O is a consecutive reaction involving the transanhydride of Ac2O with MAA to the intermediate AMAOAc and the byproduct HAc as the first step, and the second step is the transanhydride of AMAOAc with MAA to the product MA2O. As discussed in section 3, the first step, reaction 1, is a fast reaction compared with the second step, reaction 2. When Ac2O is excessive, a large amount of MAA reacts first with Ac2O to generate AMAOAc in reaction 1, causing the molar ratio of MAA to AMAOAc to be lower than the stoichiometry of reaction 2. The result of experiment E5 is that the conversion of MAA is very high, 99.95%, but the concentration of the intermediate AMAOAc is higher at the bottom of the tower. In contrast, when MAA is excessive, the molar ratio of MAA to AMAOAc is greater than the stoichiometry of reaction 2. As a result, the selection of MA2O is higher, 99.02%, but the conversion of the MAA is lower. From Table 6, it can be deduced that, when the reactant is excessive, separation equipment is required to remove impurities to obtain the desired MA2O. Because the price of MAA is highe2r than that of Ac2O and MAA is easily polymerized, it is

5. CONCLUSIONS The synthesis of methacrylic anhydride is a consecutive equilibrium-limited reaction. The reaction kinetics, chemical equilibrium, and reaction mechanism were discussed for the synthesis of methacrylic anhydride, and the pre-exponential factors and activation energies of reactions 1 and 2 were obtained by fitting the experimental data. The feasibility of the transanhydrization of MA2O in two cascade reversible reactions among five components with an intermediate AMAOAc in a batch reactive distillation procedure was discussed. MA2O has been prepared in a batch reactive distillation column, and the influences of the feed composition, reflux ratio, and entrainer on the reaction were investigated. The appropriate operating conditions for the synthesis of MA2O were determined, including reflux ratios 5:5 and 5:3 and feed molar ratio MMMA/MAc2O = 2.0:1.2. Under these operation conditions, the conversion of MAA is 99.95%, and the yield of MA2O is 86.99%. If toluene is used as an entrainer, the yield can reach 88.13%.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Table 6. Influence of the Molar Feed Ratioa

a

run

molar feed ratio MMAA/MAc2O

xAc2O, %

xMAA, %

xMAAOAc, %

xMA2O, %

SMA2O, %

ηMAA, %

YMA2O, %

E4 E5 E6

2.0:1.0 2.0:1.2 2.4:1.0

1.13 5.49 0.02

8.14 0.15 1.35

11.53 20.05 1.35

79.10 74.18 82.40

95.88 86.38 99.02

92.59 99.95 84.78

83.14 85.99 84.24

Note: xi (i = Ac2O, MAA, MAAOAc, and MA2O) denotes the weight fraction. 17323

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an alcohol mixture containing ethanol and n-butanol. Comput. Chem. Eng. 2013, 57, 63−77. (12) Luyben W. L., Yu C. C. Reactive distillation design and control; Wiley: Hoboken, NJ, 2009. (13) Fuchigami, Y. Hydrolysis of Methyl Acetate in Distillation Column Packed with Reactive Packing of Ion Exchange Resin. J. Chem. Eng. Jpn. 1990, 23, 354−359. (14) Podrebarac, G. G.; Ng, F. T. T.; Rempe, G. L. The production of diacetone with catalytic distillation: Part I: Catalytic distillation experiments. Chem. Eng. Sci. 1998, 53, 1067−1075. (15) Robert, R.; Tapio, S.; Antti, V.; Heikki, H.; Juha, L.; Anna, S.; Esko, T. Development of a kinetic model for the esterification of acetic acid with methanol in the presence of a homogeneous acid catalyst. Chem. Eng. Sci. 1997, 52, 3369−3381. (16) Mclafferty, F. W. Interpretation of Mass Spectra, 3rd ed.; University Science Books: Millvalley, CA, 1980. (17) Son, K. I.; Moon, J. C.; Park, K. Y. Activity difference between the internal and external sulfonic groups of macroreticular resin catalyst in Isobutylene hydration. Ind. Eng. Chem. Res. 1988, 27, 41−45. (18) Yan C. L. Synthesis of methacrylic anhydride via reaction distillation, Master thesis, Fuzhou University, 2011. (19) Asteria, N. G.; Josedel Carmen, Z. L.; Luis Enrique, V. B.; Jose Antonio, R. U. Design of batch distillation columns using short-cut method at constant reflux. J. Eng. 2013, 1−14.

ACKNOWLEDGMENTS We acknowledge the financial support for this work from the National Natural Science Foundation of China (Grants 21306025 and 21176049) and the Natural Science of Fujian Province (Grant 2012J01040).

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NOMENCLATURE η = conversion [%] k = reaction rate constant [L·mol−1·min−1] k0 = pre-exponential factor [L·mol−1·min−1] x = weight fraction [-] ΔH0 = standard enthalpy [J·mol−1] C = molar concentration [mol·L−1] Ea = activation energy [kJ·mol−1] Keq = chemical equilibrium constant [-] M = molar ratio [-] R = gas constant [J·mol−1·K−1] S = selectivity [%] t = time [min] T = temperature [K] Y = yield [%]

Subscripts

1,2 = reaction 1, reaction 2 +, − = the forward and backward reaction Abbreviations

6BX = 2,4-ditert-butyl cresol Ac2O = acetic anhydride BHT = butylated hydroxy toluene HAc = acetic acid MA2O = methacrylic anhydride MAA = methacrylic acid AMAOAc = methacrylic-acid acetic anhydride TOP = trioctyl phosphate

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REFERENCES

(1) Dupont, B.; Paul, J. M. Process for preparing (Meth) acrylic anhydrides, U.S. Patent, 7074957 B2, 2006. (2) Jaroslav, M.; Renaud, N.; Kishore, K. K.; Stanley, O. F.; Michael, D. C.; Richard, S. H.; Matyjaszewski, K. Efficient polymerization inhibition systems for acrylic acid distillation: New liquid-phase inhibitors. Ind. Eng. Chem. Res. 2012, 51, 3910−3915. (3) Rafikov, E. A.; Goldfein, M. D.; Stepukhovich, A. D. The kinetics of the inhibited methacrylic acid polymerization in bulk and in solution. Polym. Sci. 1981, 23 (1), 77−83. (4) Li, R. J.; Schork, J. F. Modeling of the inhibition mechanism of acrylic acid polymerization. Ind. Eng. Chem. Res. 2006, 45, 3001−3008. (5) Hurtel, P.; Laurent, D.; Rondini, J. Process for the synthesis of (Meth) acrylic anhydrides, U.S. Patent, 4857239, 1989. (6) Dupont, B.; Paul, J. M. Process for preparing methacrylic anhydrides, Chinese Patent, CN1396149A, 2003. (7) Murata, N.; Tamura, K.; Yoshhida, Y.; Ohkita, M. Process for producing (Meth) acrylic anhydride and process for producing (Meth) acrylic ester, U.S. Patent, 6642394B2, 2003. (8) Lin, Y. D.; Chen, J. H.; Cheng, J. K.; Huang, H. P.; Yu, C. C. Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis. Chem. Eng. Sci. 2008, 63, 1668−1682. (9) Lai, I.-K.; Liu, Y. C.; Yu, C. C.; Lee, M. J.; Huang, H. P. Production of high-purity ethyl acetate using reactive distillation: Experimental and start-up procedure. Chem. Eng. Sci. 2008, 62, 878−898. (10) Al-Arfaj, M. A.; Luyben, W. L. Design and control of an olefin metathesis reactive distillation column. Chem. Eng. Sci. 2002, 57, 715− 733. (11) Wu, Y. C.; Lee, H. Y.; Tsai, C. Y.; Huang, H. P.; Chien, I.-L. Design and control of a reactive-distillation process for esterification of 17324

dx.doi.org/10.1021/ie501607v | Ind. Eng. Chem. Res. 2014, 53, 17317−17324