Esterification of Methacrylic Acid with Methanol: Process Optimization

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Kinetics, Catalysis, and Reaction Engineering

Esterification of Methacrylic Acid with Methanol: Process Optimization, Kinetic Modeling and Reactive Distillation Ran Ran, Jie Li, Gang Wang, Zengxi Li, and Chunshan Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03842 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Industrial & Engineering Chemistry Research

Esterification of Methacrylic Acid with Methanol: Process Optimization, Kinetic Modeling and Reactive Distillation Ran Ran a ,b, Jie Li b, c, Gang Wang a, b, Zengxi Li a, b *, Chunshan Li a, b, d* a

School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China

b

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green

Process and Engineering, Key Laboratory of Multiphase Complex Systems, Zhongke Langfang Institute of Process Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. c

School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China

d

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical

Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China Corresponding E-mail: [email protected] (prof. Z. X. Li) [email protected] (prof. C. S. Li) Abstract: The esterification of methacrylic acid with methanol catalyzed by NKC-9 resin (a cation exchange resin with sulfonate groups) was conducted in a stainless stirred batch reactor. The effects of catalyst loading, initial molar ratio of reactants, temperature and pressure on this esterification reaction were investigated. A kinetic model based on Langmuir-Hinshelwood mechanism was developed in which methacrylic acid and methanol were considered to be uniformly adsorbed on the surface of catalysts. Model parameters including reaction equilibrium constants, activation energy, enthalpy change, entropy change as well as rate constants were solved. Accuracy of the model was validated by means of both experimental proofs and standard deviation between the predicted data and experimental ones. Finally, the reactive distillation process was proposed for continuous synthesis and separation of methyl methacrylate. 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: esterification; methyl methacrylate; kinetic modeling; reactive distillation 1. INTRODUCTION Methyl methacrylate (MMA) is an important industrial chemical for fabrication of durable resin products, such as organic glass, shock resistant modifier of polyvinyl chloride and surface coating.1-3 Polymethyl methacrylate produced from MMA holds the characteristics of good transparency, impact resistance and excellent electrical performance. Copolymers formed from MMA and other vinyl monomers are used to manufacture transparent plastic, surface coating and adhesives.4,5 The demand for methyl methacrylate is still rising with high profit margins and great economic benefits.6 At the beginning of the 20th century, the production of MMA was firstly industrialized.7 At present, industrial production methods of MMA include acetone cyanohydrin strategy, isobutene/tertiary butanol route, ethylene/methyl propionate process and ethylene/propionaldehyde pathway. While the direct esterification of methacrylic acid (MAA) with methanol (MeOH) for synthesis of methyl methacrylate is the simplest method.8-14 Strong acids, such as sulfuric acid and hydrochloric acid, were traditionally used as catalysts for the esterification in homogeneous system.15,16 Ionic liquids which can reduce serious environmental problems to some extent were used as green catalysts for esterification in recent study of He.17 In order to overcome the disadvantages of homogeneous catalysts in difficult separation of components and insufficient environmental protection, heterogeneous catalysts including clays, zeolites, sulfated metal oxides and tetrabutyl titanate have been developed.18-22 In recent years, ion exchange resins with excellent reactivity and selectivity appeared in organic synthesis.23 Conventional ion exchange resins like Amberlyst 15,24 Indion 225H,25 PD206

26

and Purolite CT-17527 were reported to be

used in esterification. The typical features of the ion exchange resins with macro reticular and sulfonate groups which can not only provide catalytic active sites, but also promote selectivity of main product.28 The NKC-9 resin (a cation exchange resin with sulfonate groups) was adopted by Zheng et al. to the synthesis of 2


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polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane.29 Xu et al. used NKC-9 resin for transesterification of methyl acetate with n-butanol.30 Zuo et al. studied etherification of acetic acid with methanol catalyzed by the NKC-9 resin and indicated that the catalyst showed high catalytic activity and desirable stability.31 In a nutshell, NKC-9 resin has been employed frequently in the reactions of acid-base catalytic system. With the extensive studies of esterification, investigations are also focused on reaction kinetic. Chandane reported the esterification of propionic acid with benzyl alcohol through Pseudo-Homogeneous (PH) model to speculate the effect of reaction condition.32

Synthesis

of

monostearin

was

investigated

by

following

Langmuir-Hinshelwood (L-H) mechanism in the work of Karan.33 Jyoti developed the kinetic model according to Eley-Rideal (E-R) theory to in heterogeneous esterification of acrylic acid with ethanol.34 In current work, the esterification of MAA with MeOH catalyzed by NKC-9 resin under pressure was studied. Effects of catalyst loading, initial molar ratio of reactants, temperature, stirring speed, catalyst size and pressure on conversion of methacrylic acid were investigated. The parameters of the kinetic model were estimated to be reasonable with L-H model. Finally, reactive distillation process for this esterification was proposed and simulated which could improve the yield further by separating the product constantly and play an important role in industrial application. 2. EXPERIENTAL SECTION 2.1. Chemicals and catalyst MeOH (>99.5%) was purchased from Beijing Chemical Works. MAA (>99.0%) stabilized with 200 ppm hydroquinone monomethyl ether (MEHQ) was purchased from Aladdin company. Ethanol (>99.7%) was purchased from Tianjin Damao chemical reagent factory. Hydroquinone as polymerization inhibitor was purchased from Sinopharm Chemical Reagent Co. Ltd. Sodium chloride (NaCl, >99.5%) and 3



HCl (36-38%) were purchased from Beijing Chemical Works. Sodium hydrate (NaOH) was purchased from Xilong scientific incorporated company. All of the above reagents are analytical and can be used directly. The NKC-9 resin was purchased from Chemical Plant of Nankai University. This resin is functionalized with styrene-divinylbenzene and sulfonic groups. And the volume exchange capacity of dry resin is 4.7 mmol/g. The NKC-9 resin was washed with water and ethanol to remove impurities and then dried at 353.15 K overnight before use. D110 resin with carboxyl groups and D301 ion exchange resin with amino groups were purchased from Tianjin Bohong science and technology Co. Ltd. 2.2. Experimental apparatus. The reactions were carried out in a 100 mL stainless batch reactor, and its structure diagram was shown as Figure 1. Reaction temperature was controlled by the thermocouple along with a heating jacket. Reactants were added into polytef lining and stirred by a magneton in the kettle. There were inlet and outlet line for nitrogen and sampling port on the reactor. High-pressure nitrogen gas cylinder was another essential equipment needed in this work.

Figure 1. Schematic diagram of the reactor 1-5: valves, 6: reaction kettle, 7: heating jacket, 8: control panel, 9: thermocouple, 10: nitrogen cylinder, 11: pressure meter

2.3. Experimental procedure Temperatures between solution and outside jacket were calibrated to make sure 4


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the accurate reaction temperature. The weighted reagents are added into the kettle which is heated to the desired temperature. Nitrogen is filled in to ensure reaction pressure, under which both the reactants and the products can maintain liquid phase, and then timekeeping of the reaction is started. A little bit of liquid sample is withdrawn at regular intervals, while stirring is going on to ensure the compositions of whole liquid mixture are homogeneous enough. As shown in Figure 1, before collecting samples, valves 2 and 3 are opened, and valve 1 is closed. Valve 1 is opened and valve 3 is closed after sampling, then nitrogen is introduced to press the liquid sample that remained in pipeline back to the reactor. Product samples were analyzed by gas chromatography (GC). The experimental conditions were optimized after the catalysts screening experiments. In order to investigate the effect of internal diffusion, catalysts were sieved and divided into 10-20, 20-30, 30-40 and 40-50 meshes. Apart from a blank control experiment of 50 rpm, the stirring speed ranging from 300 to 500 rpm were studied to eliminate external diffusion. Experiment was carried out under reaction conditions of different catalyst loading, initial molar ratio of reactants, temperature and pressure. Kinetic experiments were proceeded at temperatures changing from 323.15 to 368.15 K with catalyst loading of 8 wt. % (based on reactants), initial MeOH/MAA molar ratio of 1.2 and reaction pressure of 0.3 MPa. The acid-base intensity of resin was determined by titration curve. 1.0 mol/L NaCl was used as solvent to prepare 0.1mol/L NaOH and HCl solution. Resin samples containing less than 2 mmol exchange groups were imported into the conical flasks. Series volumes of 0.1mol/L NaOH (for acidic ion exchange resin) or HCl (for alkaline ion exchange resin) solution were added into flasks, and then 1.0 mol/L NaCl was introduced to a certain volume. Shaking until pH was constant, and then pH of solution in each conical flask was measured by pH meter to make the titration curve. 2.4. Analytical methods and characterizations. Quantity analysis of product samples were performed on a gas chromatography (GC-Plus 2010, Shimadzu, Japan), equipped with Barrier Discharge Ionization Detector and a chromatographic capillary column of Rtx-Wax (30 m × 0.20 mm × 5



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0.25 μm). The temperature of vaporizing chamber and detector was set as 473.15 and 503.15 K respectively. Column temperature started from 303.15 to 503.15 K depending on temperature programmed method. Highly pure helium (>99.999%) was served as carrier gas. Conversion of MAA and yield of MMA was determined by the internal standard substance. This esterification catalyzed by ion exchange resins is thought to be without by-products approximately. The calculation method of conversion (Xi) and yield (yi) is shown as formulas (1) and (2), where ni is the molar number of component i, ni,t is the molar number of component i after t time. Xi =

yi =

ni,0 - ni,t ni,0

×100%

i = (MAA, MeOH)

ni,t nMAA,0

× 100%

i = (MMA, H2O)

(1)

(2)

Scanning electron microscopy (SEM) providing the surface morphology of the catalyst was operated on SU8020 (HITACHI), after pretreatment of plating gold on the surface of the catalyst on account of the insulating property of the resin NKC-9. Pore volume and size distribution of catalysts were measured with Brunauer Emmett - Teller (BET) aperture analyzer (ASAP 2460) after the samples were degassed at 403.15 K for 6 h. The Acid-base properties of ion exchange resins were determined by the titration curve, which was detected with pH meter (METTLER TOLEDO). The heat resistance of catalyst was characterized by TG-DTA analyzer (DTD-60H, Shimadzu, Japan). 3. RESULTS AND DISCUSSION 3.1. Kinetic experiments 3.1.1. Catalyst screening Ion exchange resin of NKC-9, D110 and D301 were used as catalysts. It is clearly shown in Figure 2, the conversion in the presence of NKC-9 resin was 80%, which is the highest among these resins. In addition, no by-products were discovered 6


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on GC-MS when NKC-9 was selected.

Conversion of MAA (%)

100 NKC-9 D110 D301

80 60 40 20 0

0

50

100 150 200 Time (min)

250

300

Figure 2. Catalytic activity evaluation of NKC-9, D110 and D301 (temperature, 368.15 K; catalyst loading, 10 wt. %; MeOH/MAA=1.2; pressure, 0.5 MPa; catalyst size, 40-50 meshes)

In order to explain the difference in activity, the primary acid-base properties of these catalysts were analyzed by titration curve. The measured curves were shown in Figure 3. A sharp increase appeared in the titration curve of NKC-9 resin, indicating that it is strong acid. However, it changes gently in curves of D110 and D301, showing weak acid and base property that caused by carboxyl groups and tertiary amino groups respectively. Strong acid exchange resins are more likely to provide the protons needed for esterification. 12

8

12

a

10

b

6 8 pH

pH

6

2

3

4

0

50

100 150 200 250 NaOH/catalyst (mg/g)

300

5 4

6

4

0

c

7

10

8 pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

10 20 30 40 NaOH/catalyst (mg/g)

50

2

0

10

20 30 40 HCl/catalyst (mg/g)

50

Figure 3. Titration curves of (a) NKC-9, (b) D110, (c) D301

Therefore, comprehensive consideration of catalytic activity and analysis results of acid-base property, NKC-9 resin was selected for further studies. 3.1.2. Diffusion elimination To study intrinsic kinetic, experiments were performed to eliminate the effects of diffusion. Stirring speeds varying from 50 to 500 rpm were adopted to exclude the external diffusion. As it can been seen from Figure 4, in a certain range, reaction rate was improved by increasing stirring speed. When the stirring speed is above 400 rpm, 7



it has no effect on the conversion of MAA, indicating the external diffusion could be neglected. Meanwhile, the NKC-9 resin catalyst was screened into 10-20, 20-30, 30-40 and 40-50 meshes to eliminate internal diffusion. Figure 5 shows that the resin size almost has no influence on the conversion of MAA, so the size effect of catalyst is weak enough to be ignored, which indicates that the internal mass transfer is not the step that can impact rates. Xu also found this phenomenon during esterification and considered the NKC-9 resin was consist of small particle and possessed large caves.30 Subsequent characterization results of SEM and BET proved this opinion.


100 80 60 50rpm 300rpm 400rpm 500rpm

40 20 0 0

100 200 300 400 500 600 Time (min)

Figure 4. Effect of the stirring speeds on the conversion of MAA (temperature, 368.15 K; catalyst loading, 10 wt. %; MeOH/MAA=1.2; pressure, 0.5 MPa; catalyst size, 40-50 meshes) 100 Conversion of MAA (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 10-20meshes 20-30meshes 30-40meshes 40-50meshes

40 20 0 0

100 200 300 400 500 600 Time (min)

Figure 5. Effect of the catalyst sizes on the conversion of MAA (temperature, 368.15 K; catalyst loading, 10 wt. %; MeOH/MAA=1.2; pressure, 0.5 MPa; stirring speed, 400rpm)

The surface morphology of NKC-9 resin was clearly illustrated by the SEM image with a magnification of 50000× as shown in Figure 6. Characterization result indicated that the catalyst consists of particles within 100 nm in diameter which matches the previous experimental phenomenon in one way. 8


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Figure 6. SEM image of NKC-9 catalyst with a magnification of 50000×.

The basic pore conditions of NKC-9 resin were determined by BET test. Figure 7 shows the adsorption and desorption curves of NKC-9. According to the pore size distribution curve in Figure 8, the pore size distribution of the catalyst ranges from 10 to 130 nm, which proved the previous conclusion in another way. Specific surface area, pore volume and average pore diameter of the catalyst calculated by BET and

60 50

3

Quantity Adsorbed (cm /g STP)

BJH models are 10.47 m2/g, 0.092 cm3/g and 47.23nm respectively.

40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 7. Adsorption and desorption curves of NKC-9 resin.

3

dV/dlog(D) Pore Volume (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.15

0.10

0.05

0.00

0

20

40 60 80 100 120 140 Pore Diameter (nm)

Figure 8. Pore size diameter distribution curve of NKC-9 resin. 9



3.1.3. Effect of catalyst loading amount The effect of catalyst loading on esterification was observed by experiments with a series of catalyst concentrations, including 10, 8 and 6 wt. % of the whole liquid mixture. As a comparison, a blank test without catalyst was carried out in order to reflect the performance of the catalyst. From Figure 9, it can be seen that the conversion of MAA was enhanced obviously in comparison to the blank test whose conversion rate was only 20% after 600 minutes. As the catalyst amount increased, the reaction rate was increased. As a whole, just little difference in conversion was observed between the catalyst loading of 8 and 10 wt.%, so 8 wt.% was chosen as the appropriate loading. 100 Conversion of MAA (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60

10 wt.% 8 wt.% 6 wt.% none

40 20 0 0

100 200 300 400 500 600 Time (min)

Figure 9. Effect of the catalyst loading on the conversion of MAA (temperature, 368.15 K; catalyst size, 20-30 meshes; MeOH/MAA, 1.2; pressure, 0.5 MPa; stirring speed, 400rpm)

3.1.4. Effect of MeOH / MAA initial molar ratio Another important factor for this esterification reaction was studied experimentally with different initial ratio of MeOH to MAA containing 0.8, 1.0, 1.2 and 1.4, while keeping other conditions similar. In Figure 10, a conclusion can be summarized that high initial ratio of MeOH to MAA would promote the conversion of MAA which conforms to the equilibrium theory.35 The conversion increased significantly with feeding molar ratio changing from 0.8 to 1.2. By contrast, the increase between 1.2 and 1.4 appears unconspicuous, which seems to be a limiting trend. Sanz accounted for the similar experimental phenomena with saturation of alcohol molecules on resin surface.36 Although increasing the concentration of MeOH was advantageous for conversion, it would lead to the burden of separation. Taking 10


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consideration of both equilibrium conversion and the energy consumption of the later separation, the intial ratio of MeOH to MAA was selected as 1.2. 100

Conversion of MAA(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 1：1.4 1：1.2 1：1 1：0.8

40 20 0

0

100

200

300

400

500

600

Time (min)

Figure 10. Effect of MAA / MeOH initial ratio (temperature, 368.15 K; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; pressure, 0.5 MPa; stirring speed, 400rpm)

3.1.5. Effect of reaction pressure The conversion of MAA is particularly affected by the reaction temperature that high temperature can promote conversion, nevertheless, it would give rise to the vaporization of methyl methacrylate, which leads to separation of MMA from the inhibitor and occurance of polymerization. To solve this problem, reaction pressure ranging from 0 to 1.0 MPa (gauge pressure) were applied to ensure the whole system remained in the state of liquid phase at high temperature. Figure 11 shows that pressure imported in the esterification indeed increase the yield, although the effect of magnitude of pressure on the liquid phase reaction is too unconspicuous to be taken into account. 0.3 MPa is considered as the suitable pressure, because it is convenient to control and can keep reaction mixture in liquid phase. 100 Yield of MMA (%)

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80 60 0MPa 0.1MPa 0.3MPa 0.5MPa 1.0MPa

40 20 0 0

100

200

300

400

500

600

Time (min)

Figure 11. Effect of reaction pressure (temperature, 368.15 K; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; MeOH/MAA, 1.2; stirring speed, 400rpm) 11



3.1.6. Effect of reaction temperature In addition to the effect of temperature on the reaction, other factors including diffusion, catalyst loading, initial MeOH/MAA molar ratio and reaction pressure have been explored. To study the effect of temperature, experiments were repeated at temperatures of 323.15, 338.15, 353.15 and 368.15 K with the optimized pressure of 0.3 MPa, catalyst size of 20-30 meshes, catalyst loading of 8 wt. %, initial MeOH/MAA molar ratio of 1.2, stirring speed of 400 rpm. Figure 12 indicates that the conversion of MAA changes regularly with temperature. Namely, within the limit of temperature range, the high temperature could promote reaction rate and equilibrium conversion, due to bonds breakage and formation are easier to carry out as a result of the high temperature. Temperature can not to be increased without limit because of the poor heat resistance of the resin catalyst and the insatiable double bond in MAA and MMA. 100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

323.15K 338.15K 353.15K 368.15K

80 60 40 20 0

0

50

100

150

200

250

300

Time (min)

Figure 12. Effect of reaction temperature (pressure, 0.3 MPa; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; MeOH/MAA, 1.2; stirring speed, 400rpm)

3.1.7. Study of reverse reaction. The esterification reaction is reversible in normal conditions, but the degree depends on reaction condition. Prior to study kinetic model, a certain identification of reverse reaction should be finished to determine the reaction mechanism. The MMA and H2O with initial molar ratio of 1.0 was adopted under the stirring speed of 400 rpm, catalyst size of 20-30 meshes, temperature of 368.15 K, catalyst loading of 8 wt. %. The conversion of MMA is only 6.6% after 300 minutes, which is so low compared with 80% of the forward reaction that it can be neglected. 12


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3.1.8. Stability of NKC-9 resin. NKC-9 resin shows good catalytic performance in previous experiments in the batch reactor. In order to determine that it meets the requirements of industrial application, the stability of the catalyst must be evaluated. The heat resistance was characterized by TG-DTA as shown in Figure 13. Below 350 K, weight loss appears in the TG curves accompanied by an endothermic peak in the DTA curves due to desorption of physically adsorbed water. Above 550 K, the second-stage weight loss can be observed result from the decomposition of NKC-9 resin. Thus, NKC-9 resin catalyst should be stable below 550 K. The durability was demonstrated by exploring its reusability. Experiments were proceeded at temperatures of 368.15 K with pressure of 0.3 MPa, catalyst size of 20-30 meshes, catalyst loading of 8 wt. %, initial MeOH/MAA molar ratio of 1.2, stirring speed of 400 rpm. The catalyst from the last experiment was filtered, washed, and dried over night before being used as a recycled catalyst for the next experiment. As shown in Figure 14, the catalytic performance of the catalyst is basically unchanged after five cycles and conversion of MAA stay around 80%. 120

200 TG DTA 160

100

60 350 K 40

120 550 K

80

20

40

0

0

-20 300

400

500 600 700 Temperature (K)

800

900

Figure 13. TG and DTA curves of NKC-9 resin

13


DTA (uV)

80 Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



100 Conversion of MAA (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0

1

2

3 4 Cycle times

5

Figure 14. Reusability of the NKC-9 resin catalyst (temperature,368.15; pressure, 0.3 MPa; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; MeOH/MAA, 1.2; stirring speed, 400rpm)

3.2. Kinetic modeling section 3.2.1. Establishment of kinetic modeling Compositions of this esterification system contain MAA, MeOH, MMA and H2O. To describe this esterification kinetic, the L-H model used in this work was found to provide satisfactory simulation results via verification. According to L-H model, both MAA and MeOH were uniformly adsorbed on the surface of catalyst, followed by reaction and desorption of product. The surface reaction is simplified into irreversible based on above experimental results. The L-H model was divided into three processes: adsorption, reaction and desorption, which can be expressed as follows. Adsorption:

MAA + S

k1

MAA*S

k-1

(Ⅰ)

k2

MeOH + S

MeOH*S

k-2

(Ⅱ)

Reaction:

MAA*S + MeOH*S

kr

MMA*S + H2O*S

(Ⅲ)

Desorption:

MMA*S

k-3 k3

MMA + S (Ⅳ)

14


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H2O*S

k-4 k4

H2O + S

(Ⅴ)

Surface reaction is often the rate-controlling step in esterification catalyzed by strong acid.35 Depending on above L-H mechanism, the kinetic equations based on concentrations can be described as following eqs. 3 to 5.

1

θv =

1 + Σ KiCi

(3)

KiCi

θi =

1 + Σ KiCi

(4)

r = rMMA= krθMAA θMeOH krKMAA KMeOH CMMACMeOH

=

=

(1 + KMAACMAA + KMeOHCMeOH + KMMA CMMA +KH2O CH2O)2 K CMAACMeOH (1 + KMAA CMAA + KMeOHCMeOH + KMMACMMA +KH2OCH2O)2

(5)

In the equation, K = krKMAAKMeOH, θv is the concentration of the uncovered active sites, θi is the concentration of the covered active sites, Ci is concentration of substance i, ki is the reaction rate constant of substance i, Ki is adsorption equilibrium constant of substance i, expressed as ki / k-i, i is represent MAA, MeOH, MMA and H2O. Model parameters were calculated by least square method expressed as eq. 6, which is the minimum sum of absolute relative error between the experimental and calculated data. Based on the established kinetic equations and experimental data at different temperatures shown in Figure 12, model parameters were solved through matlab software. Figure 15 offers the comparison between the simulated concentration and experimental values which intuitively shows the high fitting degree of the model. min

ER = sum

ncal,i-ni ni

(6)

15



6 Measureed (mol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

323.15K 338.15K 353.15K 368.15K

5 4 3 2 1 0

0

1

2 3 4 5 Modeled (mol/L)

6

Figure 15. Simulation results of kinetic model using experimental data. (pressure, 0.3 MPa; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; MeOH/MAA, 1.2; stirring speed, 400rpm)

Correctness of the model was validated by means of calculating the standard deviation (σ) according to eq.7.37 The standard deviation on the basis of concentration calculated by the L-H model (vcal,i) relative to the measured concentration (vmea,i) is shown in Table 1. The maximum standard deviation is just 2.92 %, which turns out that the selected model provides an accurate estimation to composition.

σ=

N

 i

(v

− v mea, i )

2

cal,i

(7)

N −1 Table 1. Standard deviation values of model

Temperature (K)

standard deviation (%)

323.15

0.67

338.15

1.31

353.15

1.94

368.15

2.92

On the other hand, experimental method was used to verify the feasibility of L-H model. The eq. 5 is simplified with the experimental data at the initial reaction stage in the context of that conversions were less than 15%. Because of the low conversion at this stage, the concentration of the product was assumed to be negligible and the reactants can be regarded as initial concentration. Simplified result of eq. 5 is shown as eq. 10, where r-1/2 and Ci-1 are linearly dependent on the certain conditions. Rate expression: 16


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r=

krKMAA KMeOH CMAACMeOH (1 + KMAACMAA + KMeOHCMeOH + KMMACMMA +KH2OCH2O)2

(8)

Simplified:

r=

krKMAA KMeOH CMAACMeOH (1 + KMAACMAA + KMeOHCMeOH )2

=

krKMAA KMeOH kCi2 (1 + KMAA KMeOHkCi)2

(9)

Reciprocal and square root:

1 + K MAA K MeOH kCi 1 = r K MAA K MeOH k r k Ci

=

K MAA K MeOH k 1 1 1 + = K' + K' ' Ci K MAA K MeOH k r k Ci K MAA K MeOH k r k

(10)

Where Ci is the lower concentration of the reactants and i refers to MAA or MeOH; the constant K´= (KMAAKMeOHkrk)-1/2 and K´´=K´KMAAKMeOHk. To determine whether the adsorption of MeOH conforms to eq. 10, experiments with different concentration ratio of MAA to MeOH including 1:0.7, 1:0.5, 1:0.3 and 1:0.1 were carried out, the results of which were shown in Figure 16. In the experimental time range, the concentration of MMA was proportional to the reaction time. In Figure 17, the treated data were in a linear relationship based on the eq. 10, which manifests that the adsorption of methanol on the surface of NKC-9 resin. With the same conditions, different concentration ratios of MAA to MeOH varying from 0.7:1 to 0.1:1 were introduced to determine the adsorption of MAA on catalyst surface. Figures 18 and 19 indicate that there is an ideal linear relationship between r-1/2 and CMAA-1. Combined with these two experimental results, the correctness and feasibility of the model were proved again.

17



Concentration of MMA (mol/L)

0.8

MAA:MeOH=1:0.7 MAA:MeOH=1:0.5 MAA:MeOH=1:0.3

0.6

MAA:MeOH=1:0.1

0.4 0.2 0.0

0

10

20 30 40 Time (min)

50

60

Figure 16. Effect of MeOH concentration on conversion rate (temperature, 368.15 K; pressure, 0.3 MPa; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; stirring speed, 400rpm) 22 20 18

14

r

-1/2

16

12 10 8

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -1 CMeOH

Figure 17. Relationship between r-1/2 and CMeOH-1. 1.0

Concentration of MMA (mol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MAA:MeOH=0.7:1 MAA:MeOH=0.5:1

0.8

MAA:MeOH=0.3:1 MAA:MeOH=0.1:1

0.6 0.4 0.2 0.0

0

10

20 30 40 Time (min)

50

60

Figure 18. Effect of MAA concentration on conversion rate (temperature, 368.15 K; pressure, 0.3 MPa; catalyst size, 20-30 meshes; catalyst loading, 8 wt. %; stirring speed, 400rpm)

18


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Page 19 of 30

14 13

-1/2

12 11

r

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 9 8 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 -1 CMAA Figure 19. Relationship between r-1/2 and CMAA-1.

3.2.2. Parameter calculation. The esterification of MAA with MeOH using NKC-9 resin as catalyst can be represented integrally as the following expression: O

O

H2C OH CH3

MeOH, NKC-9 -H2O

H2C

O

CH3

CH3

(Ⅵ)

Equilibrium constant (Ke) indicates the reaction degree when it reaches equilibrium at a certain temperature. Equilibrium constant of the esterification was obtained from equilibrium conversion (XMAA) and molar ratio of reactants (θ) and the simplified equation is as eq. 11. The calculated results of equilibrium constant values were listed in the Table 2.

Ke =

XMAA2 (1-XMAA)(θ-XMAA)

(11)

Table 2. Equilibrium constants at different temperatures

Temperature (K)

equilibrium conversion (%)

Ke

323.15

30.51

0.1497

338.15

47.38

0.5875

353.15

69.00

3.0120

368.15

79.00

7.2468

19



As shown in Figure 20, the data of ln(Ke) changed linearly with 1/T at experimental temperatures, which offered parameters to describe the effect of temperature on equilibrium constant and to calculate thermodynamic parameters by Van't Hoff equation (eq. 12). Effect of temperature on rate constants were reflected through Arrhenius equation (eq. 13). -△ Hi △ Si + lnKe = RT R (12)

 E   k r = Aexp  −  RT 

(13)

R = 8.314J  mol -1  K −1

(14)

2 1

ln (Ke)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

0 -1 -2 2.7

2.8

2.9

3.0

3.1

-1

1000/T (K ) Figure 20. Relationship between ln (Ke) and 1/T.

Where ΔHi is the enthalpy change and ΔSi is the entropy change of reaction, R is the gas constant, A is the pre-exponential factor, E is the activation energy. The estimated model parameters are shown in Table 3. Pre-exponential factor and activation energy were obtained by fitting eq. 13 based on rate constants. Activation energy of this esterification is 44.38 kJ/mol, which is consistent with the experimental phenomenon and the normal range. In addition, equilibrium constants at different temperatures were introduced to solve enthalpy change and entropy change, which were within reasonable range. The enthalpy change is greater than zero which confirms the endothermal property of esterification reactions. 20


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Values of the model parameters parameters

values

A

8.18×105

E(kJ / mol)

44.38

∆ H(kJ / mol)

87.70

∆ S(J / (mol·K))

255.68

As a result, the effect of temperature on rate constants were estimated and expressed in eq. 15. kr=8.18×105exp(-5337.98/T)

(15)

3.3. Reactive distillation A new catalytic reactive distillation technology was proposed and simulated to be used in continuous production of MMA, which mainly involved column design and process optimization. The EQ-EQ model approach was introduced to the column simulation which assumed that chemical equilibrium and phase equilibrium exist in each stage. Chemical equilibrium of reactive distillation was described by the equilibrium data obtained in the batch reactor in section 3.2. As shown in Figure 21, the reactive distillation column consists of three parts: rectifying section, reactive distillation section and stripping section. Rectifying section was designed to separate low-boiling azeotropic systems. Reaction section filled with NKC-9 catalyst is the core of the column where MMA is obtained through esterification of MeOH with MAA which was added from upper section of column. The main function of the stripping section where MeOH is fed is to separate high-boiling MMA.

21



Figure 21. Schematic diagram of reaction distillation column

Optimization of reactive distillation process was accomplished by the sensitivity analysis and process calculation on Aspen Plus. The reactive distillation column was operated at 368.15 K under pressure of 0.4 MPa. The optimal condition was determined by investigating the effects of theoretical stage number, distillate rate, feeding position and reaction stage on the reactive distillation column which can be seen from Figures 22, 23 and 24. The optimization results are shown in Table 4 and vapor-liquid composition distribution in the column are reflected in Figure 25.

Figure 22. Effect of theoretical stage number and distillate rate on the reactive distillation column

Figure 23. Effect of feeding position on the reactive distillation column. 22


Page 22 of 30

Page 23 of 30

Figure 24. Effect of reaction stage on the reactive distillation column. Table 4. Parameters and results of simulation and optimization Parameters

Optimal results

Number of theoretical plates

40

Distillate rate (kg/h)

151

MAA feeding position

5

MeOH feeding position

30

Reaction stage

9-28

MMA concentration in the bottom (%)

≥99.5


≥99.0

Mass Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Vapor phase composition H2O MAA MeOH MMA Liquid phase composition H2O MAA MeOH MMA

0

5

10

15 20 25 Stage Number

30

35

40

Figure 25. Vapor-liquid composition distribution in column.

4. CONCLUSIONS The esterification reaction of MAA with MeOH in the present of NKC-9 resin as catalyst was studied experimentally in a batch reactor. The optimized conditions of catalyst load of 0.8 wt.%, initial ratio of MeOH to MAA of 1.2, stirring speed of 400 23



rpm, and relative pressure of 0.3 MPa, were obtained in this work. The L-H model used in this work had high fitting degree and applicability to experimental data which were reflected on standard deviations within 0.0292. In addition, the consistency of the L-H model was proved experimentally. The significant model parameters including activation energy, entropy change and enthalpy change were estimated through kinetic study. In addition, a reactive distillation process was proposed for the continuous production of MMA with the purity of 99.5 % and conversion of 99.0 %. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge the National Key Projects for Fundamental Research and Development of China (2016YFB0601303), Key Research Program of Frontier Sciences, CAS, (Grant NO. QYZDB-SSW-SLH022), National Natural Science Foundation of China (NO. 21676270), Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant NO. 2017 -K08) and NSFC-Key Projects of Shanxi Coal Based Low Carbon Joint Foundation (NO. U1610222). NOMENCLATURE E-R Eley-Rideal L-H Langmuir-Hinshelwood PH pseudo-Homogeneous MAA methacrylic acid MeOH methanol MMA methyl methacrylate E activation energy, kJ·mol-1 Δ H standard enthalpy change for reaction, kJ·mol-1 Δ S standard entropy change for reaction, J·mol-1·K-1 kr reaction rate constant 24


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Esterification of Methacrylic Acid with Methanol: Process Optimization

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