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Continuous synthesis of methyl ethyl ketone peroxide in a microreaction system with concentrated hydrogen peroxide
Journal of Hazardous Materials 181 (2010) 1024–1030
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Continuous synthesis of methyl ethyl ketone peroxide in a microreaction system with concentrated hydrogen peroxide Jing Zhang, Wei Wu, Gang Qian, Xing-Gui Zhou ∗ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 5 April 2010 Accepted 26 May 2010 Available online 31 May 2010 Keywords: Microreactor Peroxidation Safety Methyl ethyl ketone Hydrogen peroxide
a b s t r a c t Methyl ethyl ketone peroxide (MEKPO) is widely used in polymer industry. It is highly sensitive to heat, friction, shock, flame or other sources of ignition, causing risks in production, storage and transportation. In this article, MEKPO is synthesized at a high throughput with concentrated hydrogen peroxide in a microreactor for on-site and on-demand production. The influences of acid concentration, residence time, feeding rate and ratio, and reaction temperature on the yield and the mass fractions of residual methyl ethyl ketone (MEK) and active oxygen of the product are systematically investigated. Under optimized condition, the reaction is completed in a few seconds, and the product contains less than 2 wt% residual MEK and has a mass active oxygen fraction higher than 22 wt%, which meets the standard for industrial application. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Methyl ethyl ketone peroxide (MEKPO) is an organic peroxide widely used as catalyst for acrylic resins, as hardening agent for glassfiber-reinforced plastics, or as curing agent for unsaturated polyester resins. MEKPO is highly explosive and the synthesizing process of MEKPO is highly exothermic. When MEK is mixed with an acidified hydrogen peroxide solution, a series of reactions will occur [1], as shown in Fig. 1. In this article, these reactions are denoted as R0 , R1 , . . ., and R7 and the peroxides are respectively denoted as P0 , P1 , . . ., and P7 . Peroxide P0 is regarded as the precursor of all other linear peroxides. Several ring peroxides may be formed while the most abundant ring is the one with three peroxide groups. The concentrations of different peroxides in the final product depend on the reaction conditions. Since separating the peroxides is difficult and determining the composition of the mixture is expensive, the final product is usually characterized by the mass fraction of active oxygen and the flash point. Currently, MEKPO is manufactured by adding acidified hydrogen peroxide slowly into MEK in a vigorously stirred vessel so to control the rate of reaction and heat release. The time for reaction time is usually long, so do for separation and further dilution
∗ Corresponding author at: 369 Campus box, East China University of Science and Technology, Shanghai 200237, PR China. Tel.: +86 21 64253509; fax: +86 21 64253528. E-mail address: [email protected] (X.-G. Zhou). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.05.117
treatment with a solvent, e.g. dimethyl-o-phthalate. Consequently its producing efficiency is comparably low. Moreover, MEKPO is highly sensitive to friction, shock, flame or other sources of ignition, leading to potential risks in its storage and transportation [2,3]. For these reasons, although many strict measures were issued and followed for safe production, storage and handling of MEKPO, there were still a number of accidents reported in recent years, causing tremendous losses in people’s lives and properties [4]. Microreactors have distinct advantages for highly exothermic reactions because of their large surfaces for heat transfer [5–8]. Moreover, because of their small holdups, chemical synthesis with unstable chemicals as intermediates or products will be relatively safe in microreactors. Stable and continuous production is easy to realize by microreactor technology and the production will become less labor-intensive. Moreover, all the micro-units can be integrated into a compact and mobile device. For MEKPO synthesis in a microreactor, several advantages can be identified. First, safe and continuous production can be realized, and on-site and on-demand production becomes possible. This eliminates the risk in synthesis, storage and transportation. Second, peroxidation of MEKPO can be completed in a short time because of the intensified heat and mass transfer, which enhances significantly the space-time yield. Finally, the throughput of the production can be easily increased by numbering up the process units. In our previous study, peroxidization of MEK was carried out in microchannel reactor. Because of the relatively high reaction temperature and the low hydrogen peroxide concentration (30 wt%), the active oxygen content in the product was lower than industrial requirement, and the yield was also low. In this article, peroxi-
J. Zhang et al. / Journal of Hazardous Materials 181 (2010) 1024–1030
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Fig. 1. A possible reaction mechanism of peroxidation of MEK.
dation of MEK was carried out with a more concentrated H2 O2 (60 wt%) solution and at lower temperatures. Unconverted MEK in the product was measured to indicate the conversion of MEK and the stability of the product. The influence of operating conditions on the product yield, MEK conversion and activated oxygen fraction in the product was systematically investigated. Under optimized conditions, the product met the standard for industrial use.
2. Experimental The microreactor was a T-mixer (shown in Fig. 2) [9], followed by a stainless steel capillary coil for peroxidation. Compared with the one used in our previous study [10], the T-mixer used in this study had only two, instead of four, opposed inlet channels and a smaller outlet channel, both of which helped to improve the mixing performance under the same volumetric total feeding rate. Moreover, this microreactor was more compact and was fabricated by precise machining. The microchannels for the two inlet streams had a rectangular cross section which was 300 m wide and 100 m deep, while the outlet channel was round and 300 m in diameter.
Fig. 2. Configuration of the T-mixer: (1) inlet and outlet structure; (2) mixing plate; (3) cover plate; (4) fixing bolt.
Fig. 3 shows the configuration of the microreaction system for MEK peroxidation. Concentrated hydrogen peroxide acidified by phosphoric acid was brought into contact with MEK in the Tmicromixer. To keep the reaction temperature at a certain level, the microreactor system, which included the stainless steel capillary
Fig. 3. Configuration of the microreaction system for MEK peroxidation.
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Table 1 Mass active oxygen fractions and theoretical yields of different peroxides. Peroxides
Active oxygen fraction (wt%)
Yield
P0 P1 P2 P3 P4 P5 P6 P7
15.09 26.23 22.86 21.48 20.73 20.25 19.93 18.18
1.47 1.69 1.46 1.38 1.34 1.32 1.30 1.22
coils (1 mm i.d.) for pre-cooling of MEK and H2 O2 , the micromixer, and the stainless steel capillary coil (1 mm i.d.) for the reaction, was completely immersed in a water bath. The feeding ratio of MEK and H2 O2 was adjusted by two constant-flow pumps. In the effluent the MEKPO appeared as the supernatant liquid after phase separation, while the phosphoric acid and unconverted hydrogen peroxide were in the water phase. To ensure the safety of the operation, only under steady-state conditions was the effluent collected for analysis; otherwise the effluent was mixed with caustic soda to kill the MEKPO. Because of the similar physical properties and instability of MEKPO, it is still very difficult to determine the concentrations of MEKPO by spectrometry method. As universally practiced, only the mass fraction of active oxygen in the product was determined. The method to determine the active oxygen fraction in the product employed in this study was the same as used in our previous article [10]. The mass yield of the product was calculated as the weight of the harvested MEKPO divided by the weight of fed MEK, which was slightly lower than the theoretical value because a very small amount of MEKPO still remained in the water phase owing to the solubility equilibrium. MEK was more soluble in MEKPO than in water. In the effluent, because the MEK concentration in the MEKPO and water was at equilibrium, the mass fraction of residual MEK in the MEKPO provided an indication of the MEK conversion. Moreover, it was an important index of the stability of the product because MEK had a low boiling and flash point. In this article, the mass fraction of residual MEK in the MEKPO was analyzed by gas chromatography with n-hexane as internal standard substance and n-butanol as solvent. It was found that under normal operating conditions when a considerable MEKPO was formed, the amount of MEK in the water phase was very small, indicating the mass fraction of residual MEK in the MEKPO was indeed an good indication of the MEK conversion. 3. Results and discussion From the stoichiometry of MEK peroxidation, the theoretical active oxygen fraction and the yield for MEKPO with different molecular structures were calculated and listed in Table 1. As the precursor for the peroxidation, peroxide P0 was unstable and was less possible to exist in the final product. Peroxides P1 –P7 had the descending theoretical active oxygen fractions and mass yields and the final product would have an active oxygen fraction from 18.3 to 26.2 wt% and a mass yield from 1.22 to 1.69 if other residuals, mainly MEK, were excluded. From the series-parallel reaction pathways, one could expect that if the MEK was in large excess, more big peroxides would be formed, leading to a smaller active oxygen fraction and a smaller mass yield. Besides, the unconverted MEK remained in the product would further decreased the active oxygen fraction. If H2 O2 was in large excess, peroxide P1 would appear in the product in a considerable fraction and would increase the active oxygen fraction and yield of the product. The residual MEK in the product would be low at high H2 O2 concentration, but more unconverted H2 O2 would go to the wastes. The composition
of the product would also change with temperature, as a result of the changed reaction equilibrium and kinetics. In addition to the complex series-parallel reaction pathways and product distribution, mixing and phase separation during reaction also played a key role in determining the yield and property of the final product. All this rendered theoretical prediction of the reaction performance and product property impossible. 3.1. Influence of acid concentration In the peroxidation of MEK, phosphoric acid served as a catalyst by providing the carbon atom on the carbonyl group with a positive charge to attack H2 O2 to form MEKPO. To investigate the effect of acid concentration on the reaction rate, the H2 O2 solution was premixed with different amounts of phosphoric acid. The peroxidation reaction was carried out at 3 ◦ C with a total flow rate of 27.5 ml/min, corresponding to a residence time 27.2 s, and a H2 O2 to MEK molar ratio of 3:1 by changing the volume ratio of acidified H2 O2 solution to MEK. The MEK and H2 O2 concentration in the mixed stream would correspondingly be decreased when more phosphoric acid was added. Fig. 4 shows the results of MEK peroxidation at different phosphoric acid fractions. When the phosphoric acid fraction in the mixture was below a certain amount (i.e. 3 wt%), the effluent was homogenous and no phase separation occurred. As a result no product could be harvested. This was an indication of the negligible amount of MEKPO formed in the reactor. When the phosphoric acid mass fraction was increased to 4.4 wt%, the peroxidation was still very slow and was not completed in the capillary coil, as manifested by the warm effluent. Nevertheless, a significant amount of MEKOP was formed at a yield of 1.23 and with an active oxygen fraction of 16.8 wt%. The low active oxygen fraction was due to the large amount residual MEK in the product, and the low yield was due to increased fraction of big peroxides. However, when the phosphoric acid fraction was higher than 5.8 wt%, the active oxygen fraction and yield became constant at about 22 wt% and 1.5, even the MEK and H2 O2 concentration in the mixed stream was decreased. Because MEK was miscible with H2 O2 solution, and the residence time was long enough (as will be discussed below), the small changes in the mixing degree owing to the different volume ratio of acidified H2 O2 solution to MEK, from 1.62 to 2.67 for the phosphoric acid fraction from 4.4 to 35 wt%, would not change the reaction behavior in the microreactor. The constant active oxygen fraction and yield at a phosphoric acid fraction higher than 5.8 wt% was attributed to the fast and complete reaction which reached equilibrium in the reactor. It is noted that the large amount residual MEK in the product obtained at a low phosphoric acid fraction was due to not only the slow reaction, but also the high MEK concentration in the mixed stream, which increased the equilibrium MEK concentration. The phosphoric acid concentrations in the MEKPO product and waste water were determined by titration with NaOH solution and phenolphthalein indicator. The MEKPO product had a very small phosphoric acid concentration of 0.9 wt%, while in the waste water it was 46.7 wt% when a phosphoric acid fraction of 35 wt% was used for the peroxidation. When the phosphoric acid fraction of 16.1 wt% was used in the mixture, the phosphoric acid concentrations in the product and waste water were 0.7 and 24.8 wt%. This indicates that only a small fraction of fed phosphoric acid went into the product. This small phosphoric acid fraction in the product will not affect the performance of MEKPO in most applications. On the other hand, it is useful to guarantee the stability of MEKPO. 3.2. Influence of residence time Experiments were carried out at the same bath temperature of 3 ◦ C, H2 O2 to MEK molar ratio of 3:1, phosphoric acid fraction
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Fig. 4. Effect of phosphoric dosage on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); temperature 3 ◦ C; residence time 27.2 s.
of 35 wt%, and total flow rate of 27.5 ml/min, but with different lengths of stainless steel capillary coil connected to the outlet of the mixer to investigate the influence of residence time on the results of reaction. Fig. 5 shows that the peroxidation was fast enough and complete in the reactor within a residence time of 3.4 s, correspond-
ing to a 2 m long stainless steel capillary coil at the outlet of the reactor. When the residence time was very small, e.g. 0.17 s, the product had a low active oxygen fraction, a large amount residual MEK, and a small yield. The stainless steel capillary coil served both as a reactor and a heat exchanger. When the tube was too short, the
Fig. 5. Effect of residence time on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); phosphoric acid fraction 35 wt%; temperature 3 ◦ C.
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Fig. 6. Effect of feeding rate on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Molar ratio 2.5:1 (H2 O2 :MEK); phosphoric acid fraction 33.9 wt%; temperature 3 ◦ C.
temperature of the reactive mixture would be significantly higher than the bath temperature. This would change the reaction equilibrium and the results would be equivalent to those obtained at elevated bath temperature, as will be seen below. 3.3. Influence of feeding rate To investigate the influences of mixing degree on the results of peroxidation, a long stainless capillary coil (16 m) at the outlet of the reactor was used to guarantee complete reaction after the feeds were mixed. The experiments were carried out at 3 ◦ C, with the same H2 O2 to MEK molar ratio of 3:1, and phosphoric acid fraction of 33.9 wt%, but with different total inlet flow rates from 12.8 to 28.8 ml/min. The results are shown in Fig. 6. At a total feeding rate of 12.8 ml/min, the reaction was not complete because of poor mixing. The product was harvested at a small yield and the active oxygen fraction was small as a result of increased amount of residual MEK. Increasing the feed rate would increase the degree of mixing and therefore increase the conversion of MEK. Consequently, the residual MEK in the production was decreased and the mass active oxygen fraction in the product increased. 3.4. Influence of molar ratio of reactants The effect of molar ratio of H2 O2 to MEK was investigated by changing the flow rate ratio of acidified H2 O2 to MEK. The experiments were carried out at 3 ◦ C, with a total feeding rate of 28.8 ml/min corresponding to a residence time of 26.2 s. At a H2 O2 to MEK molar ratio of one, the phosphoric acid fraction in the mixture was 26 wt%. When the molar ratio of H2 O2 to MEK was increased, the acid concentration increased accordingly because acid was added in advance in the H2 O2 solution. However, as shown above, the phosphoric acid concentration had little influence on the reaction when it was higher than 15 wt%. Fig. 7 indicates the residual MEK dropped rapidly, while the active oxygen fraction and the yield increased remarkably when
H2 O2 concentration was increased. This is clear because the rate of MEK conversion was increased at higher H2 O2 concentration. However, when the molar ratio was increased further beyond 3, the yield on the contrary decreased with increasing molar ratio. This was because H2 O2 stream had introduced more water and therefore dissolved more MEKPO product. Moreover, at higher H2 O2 concentrations, peroxide P1 that was more soluble in water would be in larger fraction in the product, which increased the yield loss. The reaction routes for MEK peroxidation shown in Fig. 1 indicate that H2 O2 was consumed by two competitive reactions, i.e. R0 and R1 . Overdosing of H2 O2 would reduce not only the residual MEK concentration but also the MEKPO precursor (P0 ) in the product stream. Moreover, P0 was consumed by a series of competitive reactions, forming linear peroxides P1 , P2 , P3 , and so on. If the reactions R2 , R3 , R4 , R5 , and R6 had similar rate constants, peroxides P1 –P6 would present in the product stream with descending concentrations. This explains why the mass active oxygen fraction was close to that of peroxide P2 when H2 O2 was in large excess. The H2 O2 usage was calculated from the amount of H2 O2 that converted to MEKPO, which was calculated by the yield and the active oxygen fraction of the product, over the amount of fed H2 O2 . The results are shown in Fig. 7d. At a molar ratio of H2 O2 to MEK of one, 89 wt% of the fed H2 O2 was converted to MEKPO, while at a practical molar ratio of two which produced high quality MEKPO, 74 wt% of the fed H2 O2 was utilized. A blank test with only H2 O2 and phosphoric acid in the feed revealed that self-decomposition of H2 O2 in the stainless steel mixer and capillary coil was negligible. Therefore the lower than 100% usage of H2 O2 was due to the equilibrium limitation. Because H2 O2 was hardly soluble in MEKPO, just like phosphoric acid, most of the unconverted H2 O2 would remain in the water phase. 3.5. Influence of temperature Fig. 8 shows the effects of reaction temperature on the results of reaction at a total feeding rate of 27.5 ml/min, a H2 O2 to MEK
J. Zhang et al. / Journal of Hazardous Materials 181 (2010) 1024–1030
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Fig. 7. Effect of molar ratio of reactants on residual MEK fraction (a), active oxygen fraction (b), yield (c), and H2 O2 usage (d). Total feeding rate 28.8 ml/min; phosphoric acid fraction >26 wt%; temperature 3 ◦ C; residence time 26.2 s.
molar ratio of 3:1, a phosphoric acid fraction of 35 wt% and a residence time of 27.2 s. The peroxidation reactions were exothermic and reversible. Increasing the temperature would decrease the equilibrium conversion, therefore decrease the difference in the concentrations of peroxides, which was equivalent to low-
ering the concentration of small peroxides and increasing the concentration of big peroxides. As a result, the yield of and active oxygen mass fraction in the product became small. When the temperature was too low, the reaction rate would be slow and the reaction might not be complete, resulting in an increasing
Fig. 8. Effect of temperature on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); phosphoric acid fraction 35 wt%; residence time 27.2 s.
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amount of MEK in the product and a slightly decreased yield of the product. 4. Conclusions Peroxidization of methyl ethyl ketone was continuously carried out in a compact microreactor, the core of which was a T-mixer with rectangular inlet channels. The influences of acid concentration, residence time, feeding rate and ratio, and reaction temperature on the MEK residual, active oxygen fraction, and yield of the product were systematically investigated. The peroxidation was fast enough when the phosphoric acid concentration in the mixture was 5.8 wt% and the reaction was completed in a few seconds at a temperature of 3 ◦ C when the two feeds were well mixed. Increasing further the acid concentration did not make any difference in the results of the reaction, while the temperature should be kept neither too high nor too low. Too high a temperature would decrease the equilibrium conversion and thus decrease the oxygen content and yield, while too low a temperature would result in an incomplete conversion of MEK. Increasing the feeding rate would intensify the mixing and thus increasing the conversion of MEK. High molar ratio of H2 O2 to MEK led to a high conversion of MEK, but overdosing H2 O2 would, in addition to produce more waste, decrease the mass yield because more water was introduced by H2 O2 and dissolved more MEKPO. The commercial MEKPO product has typically a mass oxygen content of 11.0–11.5 wt% and a residual MEK of 7 wt%. Under optimized operation condition, the MEK residual in the crude MEKPO was less than 2 wt%. After dilution by dimethyl-o-phthalate, as practiced in industry, the residual MEK in the final product would be even smaller, much smaller than the MEK residual of 7 wt% in the commercial product. Therefore the MEKPO produced in the microreaction system would be safer in transportation and storage. The throughput of the final diluted product on the microreaction system with only one mixer and a capillary coil was about 1.3 kg/h, corresponding to about 10 metric tons/year, which would satisfy
most of the end-users of MEKPO. While the microreaction technology helped to increase the production efficiency, by decreasing remarkably the reactor volume, and the safety, by minimizing or eliminating the risks in production, transportation and storage, the concentrated hydrogen peroxide solution for the synthesis helped to increase the yield and quality of the product, i.e. higher yield, higher oxygen fraction and lower MEK residual. Acknowledgements This work is financially supported by 863 Project of Ministry of Science and Technology of China (No. 2007AA030206), Natural Science Foundation of China (No. 20476026), and the Creative Team Development Project of Ministry of Education of China (IRT0721). References [1] N.A. Milas, A. Golubovic, Studies in organic peroxides. XXV. Preparation, separation and identification of peroxides derived from methyl ethyl ketone and hydrogen peroxide, J. Am. Chem. Res. 81 (1959) 5824–5826. [2] Y.W. Wang, C.M. Shu, Y.S. Duh, C.S. Kao, Thermal runaway hazards of cumene hydroperoxide with contaminants, Ind. Eng. Chem. Res. 40 (2001) 1125–1132. [3] P.Y. Yeh, C.M. Shu, Y.S. Duh, Thermal hazard analysis of methyl ethyl ketone peroxide, Ind. Eng. Chem. Res. 42 (2003) 1–5. [4] Z.M. Fu, X.R. Li, H. Koseki, Y.S. Mok, Evaluation on thermal hazard of methyl ethyl ketone peroxide by using adiabatic method, Loss Prev. Process Ind. 16 (2003) 389–393. [5] J. Yoshida, A. Nagaki, T. Iwasaki, S. Suga, Enhancement of chemical selectivity by Microreactors, Chem. Eng. Technol. 28 (2005) 259–266. [6] D.M. Roberge, L. Ducry, N. Bieler, P. Cretton, B. Zimmermann, Microreactor technology: a revolution for the fine chemical and pharmaceutical industries, Chem. Eng. Technol. 8 (2005) 318–323. [7] P.D.I. Fletcher, S.J. Haswell, E.P. Villar, B.H. Warrington, P. Watts, S.Y.F. Wong, X.L. Zhang, Microreactors: principles and applications in organic synthesis, Tetrahedron 58 (2002) 4735–4757. [8] J.C. Charpentier, Process intensification by miniaturization, Chem. Eng. Technol. 28 (2005) 255–258. [9] H. Nagasawa, N. Aoki, K. Mae, Design of a new micromixer for instant mixing based on the collision of micro segments, Chem. Eng. Technol. 28 (2005) 324–330. [10] W. Wu, G. Qian, X.G. Zhou, W.K. Yuan, Peroxidization of methyl ethyl ketone in a microchannel reactor, Chem. Eng. Sci. 62 (2007) 5127–5172.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Continuous synthesis of methyl ethyl ketone peroxide in a microreaction system with concentrated hydrogen peroxide Jing Zhang, Wei Wu, Gang Qian, Xing-Gui Zhou ∗ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 5 April 2010 Accepted 26 May 2010 Available online 31 May 2010 Keywords: Microreactor Peroxidation Safety Methyl ethyl ketone Hydrogen peroxide
a b s t r a c t Methyl ethyl ketone peroxide (MEKPO) is widely used in polymer industry. It is highly sensitive to heat, friction, shock, flame or other sources of ignition, causing risks in production, storage and transportation. In this article, MEKPO is synthesized at a high throughput with concentrated hydrogen peroxide in a microreactor for on-site and on-demand production. The influences of acid concentration, residence time, feeding rate and ratio, and reaction temperature on the yield and the mass fractions of residual methyl ethyl ketone (MEK) and active oxygen of the product are systematically investigated. Under optimized condition, the reaction is completed in a few seconds, and the product contains less than 2 wt% residual MEK and has a mass active oxygen fraction higher than 22 wt%, which meets the standard for industrial application. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Methyl ethyl ketone peroxide (MEKPO) is an organic peroxide widely used as catalyst for acrylic resins, as hardening agent for glassfiber-reinforced plastics, or as curing agent for unsaturated polyester resins. MEKPO is highly explosive and the synthesizing process of MEKPO is highly exothermic. When MEK is mixed with an acidified hydrogen peroxide solution, a series of reactions will occur [1], as shown in Fig. 1. In this article, these reactions are denoted as R0 , R1 , . . ., and R7 and the peroxides are respectively denoted as P0 , P1 , . . ., and P7 . Peroxide P0 is regarded as the precursor of all other linear peroxides. Several ring peroxides may be formed while the most abundant ring is the one with three peroxide groups. The concentrations of different peroxides in the final product depend on the reaction conditions. Since separating the peroxides is difficult and determining the composition of the mixture is expensive, the final product is usually characterized by the mass fraction of active oxygen and the flash point. Currently, MEKPO is manufactured by adding acidified hydrogen peroxide slowly into MEK in a vigorously stirred vessel so to control the rate of reaction and heat release. The time for reaction time is usually long, so do for separation and further dilution
∗ Corresponding author at: 369 Campus box, East China University of Science and Technology, Shanghai 200237, PR China. Tel.: +86 21 64253509; fax: +86 21 64253528. E-mail address: [email protected] (X.-G. Zhou). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.05.117
treatment with a solvent, e.g. dimethyl-o-phthalate. Consequently its producing efficiency is comparably low. Moreover, MEKPO is highly sensitive to friction, shock, flame or other sources of ignition, leading to potential risks in its storage and transportation [2,3]. For these reasons, although many strict measures were issued and followed for safe production, storage and handling of MEKPO, there were still a number of accidents reported in recent years, causing tremendous losses in people’s lives and properties [4]. Microreactors have distinct advantages for highly exothermic reactions because of their large surfaces for heat transfer [5–8]. Moreover, because of their small holdups, chemical synthesis with unstable chemicals as intermediates or products will be relatively safe in microreactors. Stable and continuous production is easy to realize by microreactor technology and the production will become less labor-intensive. Moreover, all the micro-units can be integrated into a compact and mobile device. For MEKPO synthesis in a microreactor, several advantages can be identified. First, safe and continuous production can be realized, and on-site and on-demand production becomes possible. This eliminates the risk in synthesis, storage and transportation. Second, peroxidation of MEKPO can be completed in a short time because of the intensified heat and mass transfer, which enhances significantly the space-time yield. Finally, the throughput of the production can be easily increased by numbering up the process units. In our previous study, peroxidization of MEK was carried out in microchannel reactor. Because of the relatively high reaction temperature and the low hydrogen peroxide concentration (30 wt%), the active oxygen content in the product was lower than industrial requirement, and the yield was also low. In this article, peroxi-
J. Zhang et al. / Journal of Hazardous Materials 181 (2010) 1024–1030
1025
Fig. 1. A possible reaction mechanism of peroxidation of MEK.
dation of MEK was carried out with a more concentrated H2 O2 (60 wt%) solution and at lower temperatures. Unconverted MEK in the product was measured to indicate the conversion of MEK and the stability of the product. The influence of operating conditions on the product yield, MEK conversion and activated oxygen fraction in the product was systematically investigated. Under optimized conditions, the product met the standard for industrial use.
2. Experimental The microreactor was a T-mixer (shown in Fig. 2) [9], followed by a stainless steel capillary coil for peroxidation. Compared with the one used in our previous study [10], the T-mixer used in this study had only two, instead of four, opposed inlet channels and a smaller outlet channel, both of which helped to improve the mixing performance under the same volumetric total feeding rate. Moreover, this microreactor was more compact and was fabricated by precise machining. The microchannels for the two inlet streams had a rectangular cross section which was 300 m wide and 100 m deep, while the outlet channel was round and 300 m in diameter.
Fig. 2. Configuration of the T-mixer: (1) inlet and outlet structure; (2) mixing plate; (3) cover plate; (4) fixing bolt.
Fig. 3 shows the configuration of the microreaction system for MEK peroxidation. Concentrated hydrogen peroxide acidified by phosphoric acid was brought into contact with MEK in the Tmicromixer. To keep the reaction temperature at a certain level, the microreactor system, which included the stainless steel capillary
Fig. 3. Configuration of the microreaction system for MEK peroxidation.
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Table 1 Mass active oxygen fractions and theoretical yields of different peroxides. Peroxides
Active oxygen fraction (wt%)
Yield
P0 P1 P2 P3 P4 P5 P6 P7
15.09 26.23 22.86 21.48 20.73 20.25 19.93 18.18
1.47 1.69 1.46 1.38 1.34 1.32 1.30 1.22
coils (1 mm i.d.) for pre-cooling of MEK and H2 O2 , the micromixer, and the stainless steel capillary coil (1 mm i.d.) for the reaction, was completely immersed in a water bath. The feeding ratio of MEK and H2 O2 was adjusted by two constant-flow pumps. In the effluent the MEKPO appeared as the supernatant liquid after phase separation, while the phosphoric acid and unconverted hydrogen peroxide were in the water phase. To ensure the safety of the operation, only under steady-state conditions was the effluent collected for analysis; otherwise the effluent was mixed with caustic soda to kill the MEKPO. Because of the similar physical properties and instability of MEKPO, it is still very difficult to determine the concentrations of MEKPO by spectrometry method. As universally practiced, only the mass fraction of active oxygen in the product was determined. The method to determine the active oxygen fraction in the product employed in this study was the same as used in our previous article [10]. The mass yield of the product was calculated as the weight of the harvested MEKPO divided by the weight of fed MEK, which was slightly lower than the theoretical value because a very small amount of MEKPO still remained in the water phase owing to the solubility equilibrium. MEK was more soluble in MEKPO than in water. In the effluent, because the MEK concentration in the MEKPO and water was at equilibrium, the mass fraction of residual MEK in the MEKPO provided an indication of the MEK conversion. Moreover, it was an important index of the stability of the product because MEK had a low boiling and flash point. In this article, the mass fraction of residual MEK in the MEKPO was analyzed by gas chromatography with n-hexane as internal standard substance and n-butanol as solvent. It was found that under normal operating conditions when a considerable MEKPO was formed, the amount of MEK in the water phase was very small, indicating the mass fraction of residual MEK in the MEKPO was indeed an good indication of the MEK conversion. 3. Results and discussion From the stoichiometry of MEK peroxidation, the theoretical active oxygen fraction and the yield for MEKPO with different molecular structures were calculated and listed in Table 1. As the precursor for the peroxidation, peroxide P0 was unstable and was less possible to exist in the final product. Peroxides P1 –P7 had the descending theoretical active oxygen fractions and mass yields and the final product would have an active oxygen fraction from 18.3 to 26.2 wt% and a mass yield from 1.22 to 1.69 if other residuals, mainly MEK, were excluded. From the series-parallel reaction pathways, one could expect that if the MEK was in large excess, more big peroxides would be formed, leading to a smaller active oxygen fraction and a smaller mass yield. Besides, the unconverted MEK remained in the product would further decreased the active oxygen fraction. If H2 O2 was in large excess, peroxide P1 would appear in the product in a considerable fraction and would increase the active oxygen fraction and yield of the product. The residual MEK in the product would be low at high H2 O2 concentration, but more unconverted H2 O2 would go to the wastes. The composition
of the product would also change with temperature, as a result of the changed reaction equilibrium and kinetics. In addition to the complex series-parallel reaction pathways and product distribution, mixing and phase separation during reaction also played a key role in determining the yield and property of the final product. All this rendered theoretical prediction of the reaction performance and product property impossible. 3.1. Influence of acid concentration In the peroxidation of MEK, phosphoric acid served as a catalyst by providing the carbon atom on the carbonyl group with a positive charge to attack H2 O2 to form MEKPO. To investigate the effect of acid concentration on the reaction rate, the H2 O2 solution was premixed with different amounts of phosphoric acid. The peroxidation reaction was carried out at 3 ◦ C with a total flow rate of 27.5 ml/min, corresponding to a residence time 27.2 s, and a H2 O2 to MEK molar ratio of 3:1 by changing the volume ratio of acidified H2 O2 solution to MEK. The MEK and H2 O2 concentration in the mixed stream would correspondingly be decreased when more phosphoric acid was added. Fig. 4 shows the results of MEK peroxidation at different phosphoric acid fractions. When the phosphoric acid fraction in the mixture was below a certain amount (i.e. 3 wt%), the effluent was homogenous and no phase separation occurred. As a result no product could be harvested. This was an indication of the negligible amount of MEKPO formed in the reactor. When the phosphoric acid mass fraction was increased to 4.4 wt%, the peroxidation was still very slow and was not completed in the capillary coil, as manifested by the warm effluent. Nevertheless, a significant amount of MEKOP was formed at a yield of 1.23 and with an active oxygen fraction of 16.8 wt%. The low active oxygen fraction was due to the large amount residual MEK in the product, and the low yield was due to increased fraction of big peroxides. However, when the phosphoric acid fraction was higher than 5.8 wt%, the active oxygen fraction and yield became constant at about 22 wt% and 1.5, even the MEK and H2 O2 concentration in the mixed stream was decreased. Because MEK was miscible with H2 O2 solution, and the residence time was long enough (as will be discussed below), the small changes in the mixing degree owing to the different volume ratio of acidified H2 O2 solution to MEK, from 1.62 to 2.67 for the phosphoric acid fraction from 4.4 to 35 wt%, would not change the reaction behavior in the microreactor. The constant active oxygen fraction and yield at a phosphoric acid fraction higher than 5.8 wt% was attributed to the fast and complete reaction which reached equilibrium in the reactor. It is noted that the large amount residual MEK in the product obtained at a low phosphoric acid fraction was due to not only the slow reaction, but also the high MEK concentration in the mixed stream, which increased the equilibrium MEK concentration. The phosphoric acid concentrations in the MEKPO product and waste water were determined by titration with NaOH solution and phenolphthalein indicator. The MEKPO product had a very small phosphoric acid concentration of 0.9 wt%, while in the waste water it was 46.7 wt% when a phosphoric acid fraction of 35 wt% was used for the peroxidation. When the phosphoric acid fraction of 16.1 wt% was used in the mixture, the phosphoric acid concentrations in the product and waste water were 0.7 and 24.8 wt%. This indicates that only a small fraction of fed phosphoric acid went into the product. This small phosphoric acid fraction in the product will not affect the performance of MEKPO in most applications. On the other hand, it is useful to guarantee the stability of MEKPO. 3.2. Influence of residence time Experiments were carried out at the same bath temperature of 3 ◦ C, H2 O2 to MEK molar ratio of 3:1, phosphoric acid fraction
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Fig. 4. Effect of phosphoric dosage on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); temperature 3 ◦ C; residence time 27.2 s.
of 35 wt%, and total flow rate of 27.5 ml/min, but with different lengths of stainless steel capillary coil connected to the outlet of the mixer to investigate the influence of residence time on the results of reaction. Fig. 5 shows that the peroxidation was fast enough and complete in the reactor within a residence time of 3.4 s, correspond-
ing to a 2 m long stainless steel capillary coil at the outlet of the reactor. When the residence time was very small, e.g. 0.17 s, the product had a low active oxygen fraction, a large amount residual MEK, and a small yield. The stainless steel capillary coil served both as a reactor and a heat exchanger. When the tube was too short, the
Fig. 5. Effect of residence time on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); phosphoric acid fraction 35 wt%; temperature 3 ◦ C.
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Fig. 6. Effect of feeding rate on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Molar ratio 2.5:1 (H2 O2 :MEK); phosphoric acid fraction 33.9 wt%; temperature 3 ◦ C.
temperature of the reactive mixture would be significantly higher than the bath temperature. This would change the reaction equilibrium and the results would be equivalent to those obtained at elevated bath temperature, as will be seen below. 3.3. Influence of feeding rate To investigate the influences of mixing degree on the results of peroxidation, a long stainless capillary coil (16 m) at the outlet of the reactor was used to guarantee complete reaction after the feeds were mixed. The experiments were carried out at 3 ◦ C, with the same H2 O2 to MEK molar ratio of 3:1, and phosphoric acid fraction of 33.9 wt%, but with different total inlet flow rates from 12.8 to 28.8 ml/min. The results are shown in Fig. 6. At a total feeding rate of 12.8 ml/min, the reaction was not complete because of poor mixing. The product was harvested at a small yield and the active oxygen fraction was small as a result of increased amount of residual MEK. Increasing the feed rate would increase the degree of mixing and therefore increase the conversion of MEK. Consequently, the residual MEK in the production was decreased and the mass active oxygen fraction in the product increased. 3.4. Influence of molar ratio of reactants The effect of molar ratio of H2 O2 to MEK was investigated by changing the flow rate ratio of acidified H2 O2 to MEK. The experiments were carried out at 3 ◦ C, with a total feeding rate of 28.8 ml/min corresponding to a residence time of 26.2 s. At a H2 O2 to MEK molar ratio of one, the phosphoric acid fraction in the mixture was 26 wt%. When the molar ratio of H2 O2 to MEK was increased, the acid concentration increased accordingly because acid was added in advance in the H2 O2 solution. However, as shown above, the phosphoric acid concentration had little influence on the reaction when it was higher than 15 wt%. Fig. 7 indicates the residual MEK dropped rapidly, while the active oxygen fraction and the yield increased remarkably when
H2 O2 concentration was increased. This is clear because the rate of MEK conversion was increased at higher H2 O2 concentration. However, when the molar ratio was increased further beyond 3, the yield on the contrary decreased with increasing molar ratio. This was because H2 O2 stream had introduced more water and therefore dissolved more MEKPO product. Moreover, at higher H2 O2 concentrations, peroxide P1 that was more soluble in water would be in larger fraction in the product, which increased the yield loss. The reaction routes for MEK peroxidation shown in Fig. 1 indicate that H2 O2 was consumed by two competitive reactions, i.e. R0 and R1 . Overdosing of H2 O2 would reduce not only the residual MEK concentration but also the MEKPO precursor (P0 ) in the product stream. Moreover, P0 was consumed by a series of competitive reactions, forming linear peroxides P1 , P2 , P3 , and so on. If the reactions R2 , R3 , R4 , R5 , and R6 had similar rate constants, peroxides P1 –P6 would present in the product stream with descending concentrations. This explains why the mass active oxygen fraction was close to that of peroxide P2 when H2 O2 was in large excess. The H2 O2 usage was calculated from the amount of H2 O2 that converted to MEKPO, which was calculated by the yield and the active oxygen fraction of the product, over the amount of fed H2 O2 . The results are shown in Fig. 7d. At a molar ratio of H2 O2 to MEK of one, 89 wt% of the fed H2 O2 was converted to MEKPO, while at a practical molar ratio of two which produced high quality MEKPO, 74 wt% of the fed H2 O2 was utilized. A blank test with only H2 O2 and phosphoric acid in the feed revealed that self-decomposition of H2 O2 in the stainless steel mixer and capillary coil was negligible. Therefore the lower than 100% usage of H2 O2 was due to the equilibrium limitation. Because H2 O2 was hardly soluble in MEKPO, just like phosphoric acid, most of the unconverted H2 O2 would remain in the water phase. 3.5. Influence of temperature Fig. 8 shows the effects of reaction temperature on the results of reaction at a total feeding rate of 27.5 ml/min, a H2 O2 to MEK
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Fig. 7. Effect of molar ratio of reactants on residual MEK fraction (a), active oxygen fraction (b), yield (c), and H2 O2 usage (d). Total feeding rate 28.8 ml/min; phosphoric acid fraction >26 wt%; temperature 3 ◦ C; residence time 26.2 s.
molar ratio of 3:1, a phosphoric acid fraction of 35 wt% and a residence time of 27.2 s. The peroxidation reactions were exothermic and reversible. Increasing the temperature would decrease the equilibrium conversion, therefore decrease the difference in the concentrations of peroxides, which was equivalent to low-
ering the concentration of small peroxides and increasing the concentration of big peroxides. As a result, the yield of and active oxygen mass fraction in the product became small. When the temperature was too low, the reaction rate would be slow and the reaction might not be complete, resulting in an increasing
Fig. 8. Effect of temperature on residual MEK fraction (a), active oxygen fraction (b), and yield (c). Total feeding rate 27.5 ml/min; molar ratio 3:1 (H2 O2 :MEK); phosphoric acid fraction 35 wt%; residence time 27.2 s.
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amount of MEK in the product and a slightly decreased yield of the product. 4. Conclusions Peroxidization of methyl ethyl ketone was continuously carried out in a compact microreactor, the core of which was a T-mixer with rectangular inlet channels. The influences of acid concentration, residence time, feeding rate and ratio, and reaction temperature on the MEK residual, active oxygen fraction, and yield of the product were systematically investigated. The peroxidation was fast enough when the phosphoric acid concentration in the mixture was 5.8 wt% and the reaction was completed in a few seconds at a temperature of 3 ◦ C when the two feeds were well mixed. Increasing further the acid concentration did not make any difference in the results of the reaction, while the temperature should be kept neither too high nor too low. Too high a temperature would decrease the equilibrium conversion and thus decrease the oxygen content and yield, while too low a temperature would result in an incomplete conversion of MEK. Increasing the feeding rate would intensify the mixing and thus increasing the conversion of MEK. High molar ratio of H2 O2 to MEK led to a high conversion of MEK, but overdosing H2 O2 would, in addition to produce more waste, decrease the mass yield because more water was introduced by H2 O2 and dissolved more MEKPO. The commercial MEKPO product has typically a mass oxygen content of 11.0–11.5 wt% and a residual MEK of 7 wt%. Under optimized operation condition, the MEK residual in the crude MEKPO was less than 2 wt%. After dilution by dimethyl-o-phthalate, as practiced in industry, the residual MEK in the final product would be even smaller, much smaller than the MEK residual of 7 wt% in the commercial product. Therefore the MEKPO produced in the microreaction system would be safer in transportation and storage. The throughput of the final diluted product on the microreaction system with only one mixer and a capillary coil was about 1.3 kg/h, corresponding to about 10 metric tons/year, which would satisfy
most of the end-users of MEKPO. While the microreaction technology helped to increase the production efficiency, by decreasing remarkably the reactor volume, and the safety, by minimizing or eliminating the risks in production, transportation and storage, the concentrated hydrogen peroxide solution for the synthesis helped to increase the yield and quality of the product, i.e. higher yield, higher oxygen fraction and lower MEK residual. Acknowledgements This work is financially supported by 863 Project of Ministry of Science and Technology of China (No. 2007AA030206), Natural Science Foundation of China (No. 20476026), and the Creative Team Development Project of Ministry of Education of China (IRT0721). References [1] N.A. Milas, A. Golubovic, Studies in organic peroxides. XXV. Preparation, separation and identification of peroxides derived from methyl ethyl ketone and hydrogen peroxide, J. Am. Chem. Res. 81 (1959) 5824–5826. [2] Y.W. Wang, C.M. Shu, Y.S. Duh, C.S. Kao, Thermal runaway hazards of cumene hydroperoxide with contaminants, Ind. Eng. Chem. Res. 40 (2001) 1125–1132. [3] P.Y. Yeh, C.M. Shu, Y.S. Duh, Thermal hazard analysis of methyl ethyl ketone peroxide, Ind. Eng. Chem. Res. 42 (2003) 1–5. [4] Z.M. Fu, X.R. Li, H. Koseki, Y.S. Mok, Evaluation on thermal hazard of methyl ethyl ketone peroxide by using adiabatic method, Loss Prev. Process Ind. 16 (2003) 389–393. [5] J. Yoshida, A. Nagaki, T. Iwasaki, S. Suga, Enhancement of chemical selectivity by Microreactors, Chem. Eng. Technol. 28 (2005) 259–266. [6] D.M. Roberge, L. Ducry, N. Bieler, P. Cretton, B. Zimmermann, Microreactor technology: a revolution for the fine chemical and pharmaceutical industries, Chem. Eng. Technol. 8 (2005) 318–323. [7] P.D.I. Fletcher, S.J. Haswell, E.P. Villar, B.H. Warrington, P. Watts, S.Y.F. Wong, X.L. Zhang, Microreactors: principles and applications in organic synthesis, Tetrahedron 58 (2002) 4735–4757. [8] J.C. Charpentier, Process intensification by miniaturization, Chem. Eng. Technol. 28 (2005) 255–258. [9] H. Nagasawa, N. Aoki, K. Mae, Design of a new micromixer for instant mixing based on the collision of micro segments, Chem. Eng. Technol. 28 (2005) 324–330. [10] W. Wu, G. Qian, X.G. Zhou, W.K. Yuan, Peroxidization of methyl ethyl ketone in a microchannel reactor, Chem. Eng. Sci. 62 (2007) 5127–5172.