Process Intensification of Sulfuric Acid Alkylation Using a

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Process Intensification of Sulfuric Acid Alkylation Using a Microstructured Chemical System Liantang Li, Jisong Zhang, Chencan Du, and Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: To realize process intensification of clean fuel production is becoming a hot topic for sustainable development. This work presents a new intensification technology of sulfuric acid alkylation of isobutane and butene using a microstructured chemical system. A microstructured chemical system has been designed and constructed. The reaction performance was determined, and the effect of acid concentration, phase ratio, reaction temperature, and ratio of isobutane to butene (I/O) were investigated carefully. The results show that the microsturctured chemical system could obviously improve the quality of alkylates with much smaller size reactor and much shorter reaction time for better transport performances. The conversion of olefin could be as high as 100% within 30 s. The selectivity of C8 was 71% with a research octane number (RON) 94.4 under the 94 wt % H2SO4. No obvious changes of H2SO4 concentration was observed after several cycles, indicating that the acid consumption could be effectively reduced in this microreaction process.

1. INTRODUCTION Alkylates are ideal blending components for gasoline, owing to advantages of no olefins, aromatics, or sulfuric, low vapor pressure, as well as high octane number.1,2 Strong acid catalysts can be used in alkylation such as sulfuric acid (H2SO4), hydrofluoric acid (HF), solid catalyst, and ionic liquids.3−8 Solid catalysts are considered as green catalysts and have been applied in alkylation study since in the 1960’s.9−11 Solid catalysts show good catalyst activities and high selectivity to main products.12 However, industrial application of solid catalysts is limited for their intrinsic defects.13 In addition to solid catalysts, ionic liquids are considered as a promising catalyst in alkylation.13,14 A variety of ionic liquids have been studied in alkylation since 1994.15,16 Though ionic liquids are considered as safe and environmentally friendly catalysts, problems still exist in the industrial application of ionic liquids, such as high price and the equipment corrosion of Cl−. At present, commercial catalysts for alkylation are mainly H2SO4 and HF.2 Compared with HF, H2SO4 is more acceptable in industry for the HF is easy to volatilize and harmful to environment.13 So H2SO4 is the mainly accepted catalyst in the current new alkylation plants. As a result, it is still worthwhile to improve the quality of alkylate products as well as to reduce the consumption of H2SO4.1,2 Hence, considerable effort should be directed toward intensifying the process. Some results reveal that alkylation is a chain process taking place though carbocation intermediates and several main steps occur during the alkylation.17−22 First, butene is protonated by H2SO4 as a tert-butyl carbon ion. Subsequently, tert-butyl carbon ion reacts with another butene to form trimethylpentane carbon ion (TMP+) or dimethylhexane carbon ion (DMH+). And then TMP+ or DMH+ undergoes hydrogen © XXXX American Chemical Society

exchange with isobutane and produces the main products (TMP and DMH). The alkylation of isobutane and butene is a fast reaction. In addition to the main reactions, the proposed side reactions such as polymerize of olefin, TMP+, or DMH+ reaction with butene to form C12+, fragmentation of large isoalkyl cations, and oxidation reaction also occur during the alkylation, which are dependent upon the reaction conditions. An important characteristic of this reaction process is the low isobutane solubility in H2SO4.23,24 However, the reaction occurs under the condition that isobutane gets into acid or to the interface of the acid and the hydrocarbon.25 As mentioned previously, the reaction is a fast reaction, so the reaction is controlled by the mass transfer of isobutane to the acid. Sprow et al. conducted several works on interfacial area in sulfuric acid alkylation and revealed that large interfacial area results in the higher alkylates quality. High interfacial area of two phases caused by good dispersion conditions could reduce side reactions and improve the quality of alkylates.25,26 Besides, the overall reactions are highly exothermic24,27 [enthalpy (ΔH) of this reaction is −90.7 kJ/mol], and activation energy (46 kJ/ mol) of alkylation obtained by thermodynamic calculation with transition state is smaller than side reactions. Thus, temperature is a significant factor influencing the reaction, and high heattransfer rates are important to maintain the reaction temperature to reduce the side reactions.21,27 There are several industry alkylation processes including the STRATCO process, the Exxon-Mobil process, and the Received: Revised: Accepted: Published: A

January 3, 2018 February 8, 2018 February 12, 2018 February 12, 2018 DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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

CDAlkySM process.3 The remarkable characteristic of the STRATCO process is the reactor,28 which is a horizontal eccentric stirred reactor, designed to eliminate dead zone and intensify mixing of two phases. The Exxon-Mobil process focuses on distributed feeding to increase the molar ratio of isobutane and butene by using the cascade-stirred reactors.29 Unlike the STRATCO process and the Exxon-Mobil process using the continuous stirred tank reactors, the CDAlkySM process introduces a cylindrical reactor with special packing to enhance the mixing of two phases at low reaction temperature.30 Among these processes, the consumption of H2SO4 could be as high as 70−100 kg per ton of alkylates with long reaction time (space velocity of 0.25 to 0.5 h−1) and high energy consumption. Thus, new reactors or strategies are still required to overcome the drawbacks of H2SO4 alkylation. Microstructure systems show great advantages in the mass transfer and heat transfer processes for its smaller droplet sizes and higher surface-to-volume ratio.31−35 The mass and heat transfer coefficients are nearly 10−100 times higher than those in conventional reactors.36,37 Microstructured chemical systems have been applied in several reactions to improve conversion and selectivity.38,39 As a result, microstructured chemical systems might be used to intensify the alkylation process, including mixing performance of two phases, mass and heat transfer, so as to improve the alkylates quality and reduce acid consumption. In this work, a microstructured chemical system has been designed to intensify H2SO4 alkylation of isobutane and butene. The objective of this work is to enhance the dispersion of two phases, mass and heat transfer, so as to decrease the side reactions and improve the quality of alkylates. Several factors influencing this reaction are investigated, including H2SO4 concentration, phase ratio, reaction temperature, and ratio of isobutane to butene. To the best of our knowledge, the work is the first time to report the intensification of alkylation with a microreaction system. This

study may provide a new reference to improve the quality of alkylates and overcome the drawbacks of H2SO4 alkylation.

2. EXPERIMENTAL SECTION 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 without any further purification. 2.2. Alkylation Apparatus and Process. All experiments were performed with H2SO4 and isobutane/2-butene mixed liquefied gas as the continuous and dispersed phases, respectively. The apparatus employed herein is shown in Figure 1. H2SO4 and the hydrocarbon feed (stored in a piston storage tank before) were delivered through two feeding pipes into a microreactor by two metering pumps (Beijing Satellite Co., Ltd.). The microreactor40,41 was a membrane dispersion reactor with average pore diameter 5 μm. The reactor was assembled with the order of Figure 1a, with the aid of crew holes in the corner of the models. The main structure of the reactor was the membrane and the dispersion channel, and they were sandwiched between 316L stainless steel (SST) models. The dispersed phase (isobutane and butene) passed through the membrane (made of SST and with average pore diameter of 5 μm) and dispersed into the continuous phase in the dispersion channel. The dispersion channel was 28 mm long, 0.6 mm wide, and 0.6 mm in depth. The reactor was connected by a certain length of delay loop to control the reaction time. The inner diameter of the delay loop was 2 mm, and external dimeter was 3 mm. A back pressure valve was connected directly downstream to the delay loop to control the reaction pressure (0.4 MPa). The feeding pipes, reactors, and delay loops were installed in a water bath to control the reaction temperature. B

DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 2.3. Analysis. A gas-chromatography/mass spectrometry (GC/MS) was used to identify products of reaction. All kinds of C5, C6, C7, and C8 hydrocarbons were identified, except for 2,2,5-trimethylhexane, and other heavier hydrocarbons were not identified. Agilent gas chromatography was used to detect the amount of alkylates. The chromatographic column was HPPONA capillary column (30 m × 0.25 mm × 0.25 μm). 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 calculated formula is shown as follows: Si =

Wi Wtotal

On the basis of the presented relationship of mass transfer coefficient and diffusion coefficient, the mass transfer coefficient (kc) in traditional stirring reactors could be predicted as the value of 5.1 × 10−4 m/s.24,42 Accordingly, Hatta number (Ha =

kLδ kc

) of >8000 could be calculated, indicating that

alkylation is a mass transfer controlled reaction. Thus, the enhancement of mass transfer can have a remarkable effect on the intensification of alkylation. In accordance with the research of Wang et al.34 in a liquid/liquid microdispersion process, the mass transfer coefficient of the H2SO4/hydrocarbon microdispersion system was calculated with the value of 0.18 m/s, which is much larger than that in traditional reactors. 3.2. Conversion of Butene. A series of experiments were carried out to determine the butene conversion at different reaction times with reaction temperatures of 0, 5, 10, and 15 °C, respectively. Figure 3 shows the results, indicating that the

(1)

where i presents each species in products. Among these products, these components are divided into three groups, including C5−C7, C8, and C9+. 2.4. Reliability of Experiments. Repeatability experiments were conducted under the following conditions: flow rate of continuous phase (99 wt % H2SO4) 12 mL/min, dispersed phase 2 mL/min, temperature 8 °C, isobutane/2-butene ratio 8, and reaction time 8 s. The experimental results in Table 1 show that the relative standard deviations of the three groups are acceptable. The relative standard deviations of C5−7, C8, and C9+ are 6.72%, 0.38%, and 4.90%, respectively. Table 1. Repeatability of Three Experiments at the Same Conditions selectivity (%)

components

1

2

3

average

standard deviation (%)

C5−C7 C8 C9+

23.99 51.66 24.35

20.89 51.99 27.12

20.84 52.14 27.02

21.91 51.93 26.16

1.47 0.20 1.28

relative standard deviation (%) 6.72 0.38 4.90

Figure 3. Butene conversion in different reaction time. (Fc = 8 mL/ min, Fd = 1 mL/min, and I/O = 8).

alkylation in the microstructured chemical system could be completed in less than 30 s. Increasing reaction temperature results in higher reaction rate. No butene in the final product is detected within 18 s at 15 °C and within 30 s at 0 °C, which is much shorter than that in the industrial reactors (20−30 min at 3 to 10 °C).18 As shown in Figure 3, because alkylation is a fast reaction and mass transfer is enhanced in the microstructured chemical system, the butene concentration in the microreaction process decreases sharply within 10 s. 3.3. Alkylation Reaction Performance in Microstructured Chemical System. Several influencing factors are important in alkylation and could affect the quality of alkylates, including acid concentration, mixing, reaction temperature, and isobutane/butene molar ratio. These factors were also investigated in the microstructured chemical system.

3. RESULTS AND DISCUSSION 3.1. Dispersion and Mass Transfer Performance of H2SO4/Hydrocarbon System in the Microstructured Chemical System. Dispersion plays a significant role in the reaction process. The dispersion performance of the H2SO4/ hydrocarbon system (H2SO4 as the continuous phase) in the microstructured chemical system was recorded by a microscope (BX51, Olympus) and a high-speed COMS camera (i-SPEED TR, Olympus), and the results at different flow rates are shown in Figure 2. As indicated, small droplet size of about 24.6 μm was obtained in the microstructured chemical system (ignore the coalescence of droplets in the process of observation).

Figure 2. Dispersion conditions of H2SO4/n-hexane system in the microstructured chemical systems. (a) Fc = 8 mL/min, Fd = 1 mL/min; (b) Fc = 10 mL/min, Fd = 1 mL/min; and (c) Fc = 12 mL/min, Fd = 1 mL/min. C

DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 3.3.1. Effect of H2SO4 Concentration on Reaction Performances. The H2SO4 concentration is an important factor to alkylation for the effect on protonation rate of olefin as well as the hydride transfer rate. 43 Varieties of H2SO4 concentrations (91−99 wt %) were studied. As shown in Figure 4, an increase of the selectivity to C8 is observed with

larger specific surface area for much higher transfer performances, which is helpful to enhance the main reaction so as to increase the selectivity to C8. The results are also consistent with the previous results.25,26 Flow rate of the dispersed phase rate just has little effect on droplet size. The droplet size increased a little in a higher dispersed phase rate as the continuous phase flow rate was constant, thus there was only a small increase in C8 selectivity in the lower dispersed phase flow rate. In order to ensure better dispersion performance, the flow rates of continuous phase and dispersed phase are 8 mL/ min and 1 mL/min, respectively, in the subsequent experiments. 3.3.3. Effect of Reaction Temperature. Except for dispersion performance of two phases, temperature is a key parameter to the alkylation.46 Several experiments were conducted in a range of 4−20 °C. As shown in Figure 6,

Figure 4. Selectivity of alkylates with different H2SO4 concentrations. (Fc = 8 mL/min, Fd = 1 mL/min, T = 8 °C, I/O = 8, and t = 8.37 s).

increasing H2SO4 concentration from 91 to 94 wt %. A further increase in the H2SO4 concentration results in lower C8 selectivity. Thus, the optimal concentration is about 94 wt %, which is in accordance with the previous results.43 The selectivity to C8 is only 42% when the H2SO4 concentration is 91 wt %. The reason for this observation is that the polymerization reaction of olefin is easier to occur under lower H2SO4 concentration.44 However, higher H2SO4 concentration is also unfavorable to the alkylation, due to the easier fragmentation of large isoalkyl cations. In accordance with the results, the H2SO4 concentration of 94 wt % is chosen in further studies. 3.3.2. Effect of Dispersed Phase Flow Rate. Dispersion is a critical factor expected to influence the quality of alkylates. Different dispersed phase flow rates were studied by setting the continuous phase flow rate constant at 8 mL/min. The results are shown in Figure 5, indicating that the dispersed phase flow rate has an effect on the quality of alkylates. The selectivity to C8 decreases from 71% to 65% as the dispersed phase rate rises from 1 to 6 mL/min. In accordance with the previous studies,45 lower dispersed phase rate results in smaller droplet sizes and

Figure 6. Selectivity of alkylates at different reaction temperatures. (H2SO4 concentrations = 94 wt %, Fc = 8 mL/min, Fd = 1 mL/min, T = 8 °C, I/O = 8, and t = 8.37 s).

lower temperature is beneficial for improving the selectivity to C8. A decrease to C8 selectivity is observed with the increase of reaction temperature. The selectivity to C8 is 77% when the reaction temperature is 4 °C, while the selectivity of C8 reduces to only 60% for a reaction temperature of 20 °C. Side reactions, such as polymerization of olefin, occur at higher temperature due to the higher activation energy. Moreover, oxidation reaction of H2SO4 is promoted at high temperature and causes formation of tars and the evolution of sulfur dioxide, which are unfavorable to the alkylation. Thus, low temperature could minimize side reactions effectively and improve the quality of alkylates. 3.3.4. Effect of I/O Molar Ratio. I/O molar ratio could also substantially affect the quality of alkylates. Different I/O molar ratios have been studied in the microstructured chemical system. Figure 7 shows the selectivity as a function of I/O molar ratio. The experiments reveal that increasing I/O molar ratio to 150:1 could improve the selectivity of C8 to 83%, while selectivity to C8 is only 71% as the I/O molar ratio reduces to 8:1. The I/O molar ratio could be as high as 300 to 1000 in industry for the recycle of H2SO4 and isobutane; the polymerization of olefin could be inhibited in high isobutane concentration to a certain extent, which is essential to promote alkylation reaction and obtain high RON. Besides, TMP+ would likely react with isobutane to form TMP rather than form C12+ with butene under high I/O molar ratio conditions. 3.4. Recycling Experiments of H2SO4 in Microstructured Chemical System. According to the butene conversion

Figure 5. Selectivity of alkylates with different dispersed flow rates. (H2SO4 concentrations = 94 wt %, Fc = 8 mL/min, T = 8 °C, I/O = 8, and t = 8.37 s). D

DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 7. Selectivity of alkylates with I/O molar ratios. (H2SO4 concentrations = 94 wt %, Fc = 8 mL/min, Fd = 1 mL/min, T = 8 °C, and t = 8.37 s).

Figure 9. Concentration of H2SO4 after several cycles. (Initial H2SO4 concentrations = 94 wt %, Fc = 8 mL/min, Fd = 1 mL/min, T = 8 °C, and t = 2.8 min).

results, the microstructured chemical system allows much shorter reaction time of alkylation of isobutane and butene. In addition to the reaction time, the quality of alkylates is the most critical for alkylation. The experiments were carried out in both the microstructured chemical system and the batch reactor as shown in Figure 8. It can be found that the continuous flow reaction in the microstructured chemical system could provide obvious advantages than batch reactors in alkylates quality as well as reaction time. The selectivity of C8 was only 40% in the batch reactor (T = 8 °C, stirring speed = 1000 rpm, H/C = 1.5, isobutane/2-butene = 8:1, and t = 12.5 min), while the selectivity could be 71% in the microstructured chemical system (T = 8 °C, Fc = 8 mL/min, Fd = 1 mL/min, isobutane/2-butene = 8:1, and t = 8.37 s). The possible reason is the relatively poor mixing and lower transfer performances in the batch reactor. In the microstructured chemical system, the reaction is conducted in a continuous flow mode. Small droplets can be generated in a short time, and very high specific surface area (2.8 × 104 m2/ m3) can be provided, even for the high viscosity and density reaction system. Thus, the microstructured chemical system provides a promising method to intensify alkylation. The microstructured chemical system shows obvious advantages in alkylation. The selectivity to C8 is much higher, and the reaction time is very short in the microstructured chemical system, resulting in an enhanced main reaction and inhibited side reactions. As mentioned previously, side reactions are the main reasons to cause the high acid consumption. So, the acid consumption should be reduced in the microsystem. To prove that, H2SO4 recycle experiments were conducted. Recycling experiments were carried out by reusing H2SO4 several times, and the H2SO4 concentration was determined after every cycle. As shown in Figure 9, the decrease in concentration of H2SO4 was very minor after several cycles.

The H2SO4 concentration was reduced by nearly 0.1% for each cycle. Although H2SO4 has been reused six times, the concentration of H2SO4 could be as high as 93.4%. Thus, H2SO4 exhibits excellent reusable performance in the microreaction process.

4. CONCLUSION H2SO4 alkylation process of isobutane and butene has been intensified using a microstructured chemical system. Compared with batch reactors, the reaction could be completed in a short time with high C8 selectivity in the microstructured chemical systems, due to the small droplet size and excellent mass transfer performance in mirostructured chemical systems. The C8 selectivity could be as high as 71% with the initial I/O molar ratio of 8 in the microstructured chemical systems. Lower dispersed phase flow, low temperature, and high I/O molar ratio result in high selectivity of C8. Increasing the I/O molar ratio to 150 will improve the selectivity of C8 to 83%. No obvious changes of H2SO4 concentration after several cycles, indicating that the acid consumption could be effectively reduced in the microstructure chemical systems. In general, microstructured chemical systems have the potential to be applied in alkylation so as to improve alkylates quality and reduce acid consumption. In the future, we will do more work on the kinetics and scaling-up the reaction in the microsystem.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-62783870. E-mail: [email protected]. ORCID

Guangsheng Luo: 0000-0002-0498-0224

Figure 8. Comparison of two reaction processes. (a) Reaction in a stirring batch reactor. (b) Continuous flow reaction process in a microstructured chemical system. E

DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation of China (Grants 91334201 and U1463208).

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DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.8b00040 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX