Kinetics Study of Sulfuric Acid Alkylation of Isobutane and Butene

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Kinetics, Catalysis, and Reaction Engineering

Kinetics Study of Sulfuric acid Alkylation of Isobutane and Butene Using a Microstructured Chemical System Liantang Li, Jisong Zhang, Chencan Du, Kai Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05924 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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Industrial & Engineering Chemistry Research

Kinetics Study of Sulfuric acid Alkylation of Isobutane and Butene Using a Microstructured Chemical System Liantang Li, Jisong Zhang, Chencan Du, Kai Wang, Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China Abstract The kinetics of sulfuric acid alkylation of isobutane and 2-butene was first determined using a microstructured chemical system, which shows obvious advantages for fast reactions. The concentration of key components, including trimethylpentane (TMP), dimethylhexane (DMH), light end (LE), and heavy end (HE), were obtained at different reaction temperatures. A kinetics model, contains kinetics parameters of both main and side reactions, was established to predict the alkylation process in the microstructured chemical system. The model fitted with the experimental data very well and further confirmed by the simulation results calculated with COMSOL software. Compared with the kinetics parameters determined in a traditional batch reactor, the model developed in this work is much reliable to describe the reaction, because much shorter reaction time, faster mass transfer rate, and precise control of reaction time have been reached. Keywords:

Alkylation;

Microreactor;

H2SO4;

Kinetics model

Corresponding author. Email: [email protected]. Tel.: +86-10-62783870.

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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

1. Introduction Alkylation of isobutane and butene is extensively studied because alkylates is an ideal component added in gasoline to reduce the contents of olefins, aromatics and sulfuric, as well as improve octane number1, 2. Alkylation occurs in the presence of strong acids, such as H2SO4, HF, solid catalysts, and ionic liquids3-10. Among these catalysts, H2SO4 and HF are the main commercial catalysts, and H2SO4 is considered as the major catalyst in new alkylation plants for the serious environmental effects of HF11. H2SO4 has been applied in alkylation of isobutane and butene since the 1930s and immense efforts have been devoted to intensify the process so as to improve the quality of alkylates as well as reduce the consumption of H2SO412-15. One major characteristic of this process is the low solubility of isobutane in H2SO416-18. H2SO4 alkylation is an interfacial reaction, meaning the reactants of isobutane and butene are required to contact at the two-phase interface or into H2SO4 to ensure the reaction17. Thus, the relationship between mass transfer and reaction kinetics is of great importance to find out effective methods to intensify the process. The mechanism and kinetics of reactions play essential roles in designing and optimizing the reaction process. A deep understanding of the mechanism and kinetics is highly required. Plenty of works focused on the mechanism of alkylation. As to the H2SO4 alkylation, the mechanism is more complex than HF alkylation, and different mechanisms have been proposed11,

19-22.

The classic carbonium ion mechanism is

extensively accepted in the process of the H2SO4 alkylation23-26. Regardless of different expressions, the published results indicate that the main reaction would undergo several main steps as following23, 26: 1) Butene is first protonated by H2SO4 and forms the carbon ion, which is immediately isomerized to tert-butyl cation ion (iC4+); C=4  H   iC+4 (1)

2) iC4+ reacts with another butene and leads to the formation of


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trimethylpentane carbon ion (TMP+) or dimethylhexane carbon ion (DMH+); iC+4  C=4  TMP + (2) iC+4  C=4  DMH + (3)

3) TMP+ or DMH+ finally exchanges hydrogen with a molecule of isobutane and produces TMP or DMH. At the same time, another i-C4+ is formed and it reenters into a new cycle; TMP +  iC4  iC4  TMP (4) DMH +  iC4  iC4  DMH

(5)

4) The reaction is finished along with the complete consumption of butene and tert-butyl cation ion loss of hydrogen ion. iC+4  iC=4  H + (6)

Simultaneously, side reactions occur due to the complexity of H2SO4 alkylation27-30. TMP+ or DMH+ would like to react with butene and produce C12+, a group of undesirable products (heavy ends (HE)). Moreover, the high carbon carbocation would produce a lower carbocation and olefin by β-scission, an important method to produce light ends (LE). The processes of the total reactions are expressed with equations as follows: + (7) TMP + (or DMH + )  C=4  C12 + C12  iC4  C12 (HE)  iC+4 (8) + C12  LE   LE + (9)

LE   iC4  LE  iC+4 (10)

On the basis of the above reaction mechanism, several studies have been focused on the reaction kinetics of alkylation of isobutane and butene31, 32. Lee et al.33 studied the kinetics of H2SO4 alkylation considering side reactions of butene polymerization, using the uniform hydrocarbon drops and ensuring a short contact time in an unstirred reactor, under the assumption that reaction occurs in the acid phase and regardless of the eventual diffusion of products. Their simplified model indicated that the kinetics



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parameter of butene polymerization is about 4 times larger than alkylation. In addition, Sun et al. studied the kinetics of H2SO4 alkylation in a batch reactor (stirred tank reactor)34, and they established kinetics models considering the key components (TMP, DMH, and HE). In their research, they hold the view that a balance of 1-butene, 2-butene, and iso-butene has been reached before the alkylation. Their results provide the guiding role in the kinetics study of the H2SO4 alkylation. However, in the opinion of Albright et al.20, the balance is difficult to reach before the reaction because the process of the protonation reaction is quite fast, on the basis of results that the ratio of TMP and DMH changes significantly from about 1 to 15 at different conditions. Thus, the formation of the DMH requires to be reconsidered. Besides, alkylation is a fast reaction and a short time is required to obtain the accurate kinetics parameters while it is difficult in a batch reactor. Furthermore, there was little research focused on the kinetics study of alkylation considering more side reactions at short reaction time, such as the formation of the HE and LE. Therefore, more accurate kinetics models need to be further developed. Microstructured chemical systems show obvious advantages in kinetics measurement because of their good dispersion and the precise control of reaction time, as well as reaction conditions, especially for the rapid reaction of a heterogeneous

system35-37.

Plenty

of

kinetics

studies

were

conducted

in

microstructured chemical systems, especially for a fast reaction controlled by mass transfer38-44. Zhang et al. obtained the reaction kinetics of dehydrochlorination of dichloropropanol in a microstructured chemical system, showing distinct values for reaction mechanism understanding45. Wang et al. conducted experiments on the kinetics study of aniline and benzoyl chloride reaction in a microstructured chemical system46. Their results indicated that the microstructured chemical system provided a useful approach to study the kinetics and the value obtained is more accurate than the batch reactors. An appropriate kinetics model describing the H2SO4 alkylation process is expected to provide a better understanding of the effects of the reaction parameters and optimize the operating conditions. In fact, the alkylation is controlled by the mass


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transfer, thus, eliminating the effect of mass transfer is quite necessary to obtain the kinetics parameters. Our previous study47 indicated that the droplet size of the dispersed phase in a microstructured chemical system is about 24.6 μm, much smaller than that in the batch reactor. The mass transfer coefficient is about 0.18 m/s, much larger than the value in the batch reactor. Hence, the microstructured chemical system is a useful tool for the kinetics study of H2SO4 alkylation. The objective of this work is to study the kinetics of H2SO4 alkylation in the microstructured chemical system. A group of trimethylpentane (TMP), dimethylhexane (DMH), light end (LE), and heavy end (HE) are considered as key components. Based on the obtained kinetics parameter, kinetics models including both main reactions and side reactions are established. In order to verify the kinetics models, a comparison is made between the results obtained from the COMSOL software and the established kinetics model. In addition, the relationship of mass transfer and reaction also obtained with the COMSOL software.

2. Experimental

2.1 Materials and chemicals

H2SO4 (A.R 95%-98%) was purchased from Beijing Chemical Plant. Both isobutane and 2-butene were purchased from ZhaoGe Gas Plant. All chemicals were used as received without any further purification.

2.2 Equipment and experimental process

Similar to the previous study, H2SO4 and isobutane/2-butene mixed liquefied gas were used as the continuous and dispersed phases, respectively. The total setup is shown in Figure 1. H2SO4 and hydrocarbon feed were delivered into the membrane dispersion microreactor (with the average of diameter 5 μm) by metering pumps (Beijing Satellite Co., Ltd.), respectively. The details of the reactor have been



presented in our previous work 44. The delay loop (an inner diameter of 2 mm and an external diameter of 3 mm) with different length was connected to the reactor. Another microreactor was connected to the delay loop to introduce water to quench the reaction. A back pressure regulator was directly connected after the second microreactor to control the reaction pressure. All of the feeding pipes, reactors, and delay loops were installed in a water bath (with ethanol as the refrigerant).

Figure 1. The schematic overview of the experimental setup. (a) Detailed structures of the membrane dispersion reactor. (b) Appearance of the membrane dispersion reactor. (c) The schematic overview of the setup of the microstructured chemical system.

2.3 Analysis

The products of the reaction were identified by gas chromatography-mass spectrometry (GC-MS). All kinds of C5, C6, C7, and C8 hydrocarbons were identified, and except for 2,2,5-trimethylhexane, other heavier hydrocarbons were


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not identified. An Agilent gas chromatography was used to detect the amount of alkylates. The chromatographic column was HP-PONA capillary column (30 m × 0.25 mm × 0.25 μm). The injector and the detector temperature were set to 250 oC and 280 oC, respectively. The temperature program was as follows: firstly, initial temperature was 60 oC and held for 2 min, then increased to 120 oC at a speed of 5 oC

/min, increased to 180 oC at a speed of 10 oC /min, and finally at 200 oC for 20

min. All the products were calculated with an area normalization method for the correction factors are close to 1.0 with the standard substance benzene. The calculation formula is shown as follows: 𝑊𝑖

𝑆𝑖 = 𝑊𝑡𝑜𝑡𝑎𝑙

(11)

where i presents each species in products. Among these products, these components are divided into four groups, including C5-C7 (LE), TMP, DMH, and C9+ (HE).

2.4 Reliability of experiments

The repeatability of the experiments was carried out and the experimental conditions were as follows: the flow rate of the continuous phase 8 mL/min and dispersed phase 1 mL/min, reaction temperature 12 C, the molar ratio of isobutane and 2-butene 8, reaction time 12 s, and reaction pressure 0.5 MPa. The experimental results are listed in Table 1, showing that the relative standard deviations of the four groups are acceptable. The relative standard deviations of LE, TMP, DMH, and HE are 8.64%, 7.92%, 9.04%, and 2.01%, respectively.

Table 1. Repeatability of three experiments at the same conditions

Content (mol/mol) ×102

Components

TMP

1

2

3

Average

1.07

0.88

0.92

0.96

Standard

Relative standard

deviation (%)

deviation (%)

0.08

8.64



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DMH

3.42

2.81

3.10

3.11

0.02

7.92

HE

0.80

0.64

0.72

0.72

0.07

9.04

LE

0.30

0.28

0.29

0.29

0.01

2.01

3. Results and discussion

In our previous work, we have studied the mass transfer and intensified this system in the microstructured chemical system47,

48.

The high mass transfer coefficient of

0.18 m/s was obtained using the model of Wang et al.49. However, the mass transfer coefficient calculated using butene conversion data was 0.09 m/s at 0 °C when the flow rates of the continuous and dispersed phases were 8 mL/min and 1 mL/min. Hence, the conclusion was made that the total reaction is controlled by the reaction rate in the microstructured chemical system. Thus, in this work, the same flow rates of the two phases were chosen to study the kinetics of alkylation.

3.1 Effect of reaction time on the key components

Reaction time is a key factor for reactions, and the contents of the products vary with the reaction time. The butene conversion at different reaction temperatures with reaction time is shown as Figure 2 (a). Increasing reaction time results in the increase of butene conversion. When reaction time is 32 s, the butene conversion reaches nearly 100% at each reaction temperature. The contents of the key components with reaction time increase from 6 s to 32 s at a varying temperature of 0 to 15 °C as shown in Figures 2 (b) to 2 (e). As this figure indicates, the content of each group is increased with increasing reaction time. When reaction temperature is 0 °C, the content of TMP is 0.0015 mol/mol at 6 s, and then increases to 0.0063 mol/mol at 30 s. The same increasing trend exits at 5 °C and 10 °C. Upon increasing reaction temperature to 15 °C, the content of TMP increases from 0.0059 mol/mol to 0.0141 mol/mol when reaction time increases from 6 s to 22 s, and then the content changes a


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little with the reaction time increased from 22 s to 32 s. The TMP content is the largest at the low reaction temperature, such as 0 °C, 5 °C, and 10 °C. On the contrary, the HE content became the largest at 15 °C, which is in good consistent with the conclusion that the reaction requires occurring at low reaction temperature.

Figure 2. The butene conversion and experimental data of key component contents at different reaction times (a) Butene conversion (b) 0 °C; (c) 5 °C; (d) 10 °C; (e) 15 °C (Fc=8 mL/min, Fd=1 mL/min, I/O=8)



3.2 Kinetics models

Based on the published work, the processes of H2SO4 alkylates are complex. The detailed kinetics models were established by Sun et al.34 and kinetics parameters were published. We compared the calculated results using the kinetics models with our experimental results, as shown in Figure 3. The models cannot predict our experimental results obtained in the microstructured chemical system. The important reason for this phenomenon including the longer determination time in batch reactors than microreactors, as well as the long mass transfer time, is required in the batch reactor. Hence, a kinetics model for alkylation based on microreactors is necessary.

Figure 3. Comparison of calculated data by Sun’s models with experimental results

According to the complex formula, fitting the experiments and the kinetics parameter of each step using the nonlinear least-squares fitting method with Matlab is difficult and inaccurate. Thus, the process can be simplified in order to fit the kinetics parameter. We considered the final formation of the key component, and the simplified formula is shown as follows: k1 C=4  iC4   TMP (12) k2 C=4  iC4   DMH (13)


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k3 2C=4  iC4   HE (14) k4 2C=4  iC4   2 LE (15)

According to the simplified formula, the kinetics models are formulated as follows:

dc1  k1c1c2  k2 c1c2  k3c1c2 2  k4 c1c2 2 (16) dt dc2  k1c1c2  k2 c1c2  2k3c1c2 2  2k4 c1c2 2 (17) dt dc3  k1c1c2 (18) dt dc4  k2 c1c2 (19) dt dc5  k3c1c2 2 (20) dt dc6  2k4 c1c2 2 (21) dt The species are represented with the following numbers: 1, isobutane; 2, butene; 3, TMP; 4, DMH; 5, HE; 6, LE. According to the simplified formula, the experimental data are fitted in the Matlab with a nonlinear function. The calculated data are also shown in Figure 4. As indicated, the calculated values fit with the experimental data very well. According to the fitting results, the values and confidence intervals of k1~k4 at different reaction temperatures are listed in Tables 2. The results indicate that the values of k1~k4 increase with the increase of reaction temperature. The value of k1 is 0.0033 mol·mol-1·s-1 at 0 °C and increases to 0.0145 mol·mol-1·s-1 at 15 °C. Compared the values of k1~k4 at the same reaction temperature, the value of k3 is the largest. The reason for this situation is that the polymerization of butene is much easier than alkylation, as reported by Lee et al33. Compared our kinetics parameter with the published work by Sun et al.34, the kinetics parameter of TMP formation in our experiments is about 0.0051 mol·mol-1·s-1 at 3 °C, approximately 100 times higher



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than the published value (about 0.0001 mol·mol-1·s-1), showing the microstructured chemical system could provide more accurate kinetics for fast chemical reactions. The effect of mass transfer could reduce the value of the kinetics parameter, thus causing the value obtained is smaller. Moreover, the reaction time is well controlled in microstructured chemical systems, especially for the fast reaction, which is difficult for the batch reactor.

Figure 4. The experimental data and calculated data of key component contents at different reaction times (a) 0 °C; (b) 5 °C; (c) 10 °C; (d) 15 °C (Fc=8 mL/min, Fd=1 mL/min, I/O=8)

Table 2. Values and confidence intervals of k at different temperatures

Values of k

Temperature (°C)

k1

k2


k3

k4

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(mol·mol-1·s-1)

(mol·mol-1·s-1)

(mol2·mol-2·s-1) (mol2·mol-2·s-1)

0.0033

0.0008

0.0191

0.0119

[0.0033~0.0034]

[0.0008~0.0009]

[0.0183~0.0200]

[0.0115~0.0123]

0.0069

0.0017

0.0491

0.0153

[0.0069~0.0069]

[0.0017~0.0018]

[0.0485~0.0496]

[0.0150~0.0155]

0.0112

0.0034

0.1104

0.0404

[0.0111~0.0113]

[0.0033~0.0035]

[0.1088~0.1119]

[0.0397~0.0410]

0.0145

0.0056

0.4158

0.1246

[0.0142~0.0147]

[0.0053~0.0058]

[0.4071~0.4246]

[0.1217~0.1275]

0

5

10

15

According to the obtained values of k at the different reaction temperatures, we did the fitting of -lnk with 1/RT as shown in Figure 5. This figure indicates a good linear relationship between -lnk and 1/RT. The activation energy of each reaction was obtained using the results in Figure 5, and the results are presented in Table 3. According to the results in Table 3, the activation energy of TMP formation is the smallest with the value of about 64 kJ/mol, which is quite close to the calculated results. However, the activation energy of DMH formation is about 85 kJ/mol, which is larger than that TMP formation. This is the reason why the value of TMP/DMH is larger at the low reaction temperature. In contrast, the activation energy of HE formation is the largest (the value of 131 kJ/mol), indicating that HE formation requires the higher energy and it occurs easily at higher reaction temperature.



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Figure 5. The plot of –lnk with 1/RT for different reactions in the kinetics model.

Table 3. The activation energies and pre-exponential factors of different reactions

Reactions

Activation Energies

Pre-exponential Factors

(kJ/mol)

(mol·mol-1·s-1)

k1 C=4  iC4   TMP

64.7

8.64×109

k2 C=4  iC4   DMH

85.6

1.97×1013

k3 2C=4  iC4   HE

131.5

2.37×1023

k4 2C=4  iC4   2 LE

104.6

8.99×1017

In order to further verify the reliability of the results, a simulation combining the kinetics parameters and mass transfer was performed using COMSOL software.

3.3 Simulation results with COMSOL


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According to the obtained kinetics of H2SO4 alkylation in the microstructured chemical system, we also studied the relationship between the mass transfer rate and reaction process using COMSOL software. The kinetics values, diffusion coefficients, and droplet size have been applied in the COMSOL software. The mass transfer equations used were as follows:

ci     Di ci   u ci  Ri t

(22)

N i   Di ci  uci

(23)

where i presents the components of the system, including isobutane, butene, TMP, DMH, HE, and LE; c is the concentration of components, D is the diffusion coefficient, u is the mass mean velocity, R is the reaction rate, N is the mass transfer flux. The contents of different key components were obtained at different reaction temperatures, and the results of TMP and DMH are shown in Figures 6 and 7. When the reaction time is about 6 s, the calculated content of TMP is about 0.0016 mol/mol, 0.0032 mol/mol, 0.0052 mol/mol, and 0.0058 mol/mol at 0 °C, 5 °C, 10 °C, and 15 °C, respectively. Compared these values with the experimental data (0.0015 mol/mol, 0.0037 mol/mol, 0.0054 mol/mol, and 0.0059 mol/mol), the simulation results are in good consistent with the experimental results (see Table 4). Similar to the TMP content, the obtained contents of DMH using COMSOL software fit well with the experimental data. The simulated content is about 0.0004 mol/mol, 0.0010 mol/mol, 0.0019 mol/mol, and 0.0022 mol/mol, and the experimental data is 0.0005 mol/mol, 0.0009 mol/mol, 0.0017 mol/mol, and 0.0022 mol/mol, respectively. The results further prove that the obtained kinetics parameter is reliable in the microstructured chemical system and it can predict the content of key components at different reaction time.

Table 4. Comparison of TMP and DMH content between experiments and calculation Temperature (°C)

TMP (mol/mol) Experiment

Calculation

DMH (mol/mol) Relative

Experiment


Calculation

Relative


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error (%)

error (%)

0

0.0015

0.0016

6.67

0.0005

0.0004

20.00

5

0.0037

0.0032

13.51

0.0009

0.0010

11.11

10

0.0054

0.0052

3.70

0.0017

0.0019

11.76

15

0.0059

0.0058

1.69

0.0022

0.0022

0

Figure 6. The TMP content calculated using COMSOL software at different reaction temperatures (D=2.4×10-8 m2/s; r=0.025 mm) (a) 0 °C; (b) 5 °C; (c) 10 °C; (d) 15 °C

Figure 7. The DMH content calculated using COMSOL software at different reaction temperatures (D=2.4×10-8 m2/s; r=0.025 mm) (a) 0 °C; (b) 5 °C; (c) 10 °C; (d) 15 °C


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On the basis of verifying the reliability of the results, we further explored the relationship between reaction and mass transfer using COMSOL software. In our previous opinion, the reaction is controlled by the intrinsic reaction in the microstructured chemical system at the chosen flow rates. In order to confirm this, we also studied the relationship between mass transfer and reaction with COMSOL software to prove the controlling step of this process in the microstructured chemical system. We hold the view that when the reaction is controlled by mass transfer, the increasing of mass transfer coefficient could increase the reaction rate, and the content of component changes obviously at the same reaction time. Firstly, we increased the diffusion coefficient of the process to 1000 times and 1000000 times, as shown in Figures 8(b) and (c). Compared the butene content obtained from the data in Figure 8(a), there is no obvious changes has been observed. The results indicate that the mass transfer coefficient is large enough, and it is supported by the conclusion that the process is controlled by the reaction, as the result of the small droplet size in the microstructured chemical system.

Figure 8. The butene content calculated using COMSOL software at different diffusion coefficient (T=0°C; r=0.025 mm) (a) D=2.4×10-8 m2/s; (b) D=2.4×10-5 m2/s; (c) D=2.4×10-2 m2/s

On the other hand, we increased the droplet size to 1.6 mm and simulated the process, the results of butene content is shown in Figure 9(a). According to Figure



9(a), the process is controlled by the mass transfer of butene at this situation for the obvious concentration gradient. We also increased the diffusion coefficient to 10 times and 100 times, as the results shown in Figures 9(b) and (c). Compared Figure 9(b) with 9(a), it is clearly observed that the concentration gradient is weakened and the mass transfer process is intensified. Further increasing the diffusion coefficient to 100 times, the results shown in Figure 9(c) implying that the concentration gradient disappears and the mass transfer is eliminated, which are similar to Figure 8. The simulated results of reaction occur at different droplet sizes demonstrate that the droplet size is a significant factor for the determination of reaction kinetics parameter. The small droplet is necessary to eliminate the effect of mass transfer and obtain more accurate kinetics results. The microstructured chemical system is an important tool to obtain the small droplet and eliminate the mass transfer effect, in which the more accurate results can be determined.

Figure 9. The butene content calculated using COMSOL software at different diffusion coefficient (T=0 °C; r=0.8 mm) (a) D=2.4×10-8 m2/s; (b) D=2.4×10-7 m2/s; (c) D=2.4×10-6 m2/s

4. Conclusions

The kinetics study of H2SO4 alkylation was conducted in a microstructured chemical system at short reaction time. The kinetics parameters of different reactions


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were obtained using the nonlinear function in Matlab and the fitting results are in good agreement with the experimental data. The kinetics parameter of HE formation is the largest because of the easy polymerization of butene. The activation energy of the formation of TMP is the smallest, while the HE is the largest, indicating that the reaction should be performed in low reaction. The simulation results of COMSOL using the obtained kinetics data are in good agreement with experimental results, showing that the established kinetics models were believable and the process is controlled by the reaction for the small droplet size in the microstructured chemical system. On the contrary, the process can be controlled by the mass transfer at large droplet size. Compared with the kinetics results in a batch reactor, the reaction rate constant determined in the microstructured chemical system is much larger for the small droplet size, and the model can describe the reaction better at short reaction time, as a result of precise control of reaction time. The established kinetics model provides guidance for the intensification of H2SO4 alkylation and optimization of reaction conditions.

Acknowledgements

We gratefully acknowledge the support from the National Natural Science Foundation of China (Nos.91334201, U1463208).

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For Table of Contents Only


Kinetics Study of Sulfuric Acid Alkylation of Isobutane and Butene

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