Critical Properties of the Reacting Mixture in the Alkylation of Benzene

Nov 4, 2003 - To ensure that the reaction was carried out under supercritical conditions near the critical point, the reaction conditions should be tu...
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Ind. Eng. Chem. Res. 2003, 42, 6531-6535

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Critical Properties of the Reacting Mixture in the Alkylation of Benzene with Propene Guofu Wang, Zhangfeng Qin, Jianguo Liu, Zhen Tian, Xianglin Hou, and Jianguo Wang* State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, People’s Republic of China

The critical properties of both binary and ternary mixtures (benzene + propene, benzene + cumene, cumene + propene, and benzene + propene + cumene) involved in the alkylation of benzene with propene were measured by using a high-pressure view cell with visual observation. Moreover, the critical properties of the ternary mixture were determined with the compositions representing the reaction proceeding extent of alkylation, with the initial mole ratios of benzene to propene being 3, 4, 5, and 6, respectively. Such information is essential for the alkylation of benzene with propene under supercritical conditions, which could be an interesting option for increasing throughput and prolonging the catalyst lifetime compared with current processes in the gas or liquid phase. The results showed that the critical properties of the reacting media change with the reaction proceeding extent (propene conversion) as well as the initial ratio of benzene to propene. To ensure that the reaction was carried out under supercritical conditions near the critical point, the reaction conditions should be tuned up according to the critical properties of the reacting media along the reaction course. 1. Introduction Cumene is an important intermediate product because of its elegant conversion into phenol and acetone. It is manufactured exclusively by alkylation of benzene with propene in either a liquid or gas phase; catalysts of Friedel-Crafts systems or proton donors are usually employed. An excess of benzene is normally used to minimize the facile further alkylation of cumene.1 Recent process developments concern the use of zeolite catalysts, which could be an alternative for a more environmentally friendly and noncorrosive process without the release of acidic catalyst components. Several zeolites including ZSM-5, Y, mordenite, and β were reported to be good catalysts for this reaction.2 However, the catalyst deactivation due to the carbonaceous deposition at a low mole ratio of benzene to propene and a high space velocity is still an essential problem to resolve. Supercritical fluid (SCF) technology is one of the new technologies that have been developing quickly since the 1980s in the chemical industry. Performing heterogeneously catalyzed reactions under supercritical conditions rather than in a gas or liquid phase could be an interesting option for increasing throughput and prolonging the catalyst lifetime.3-5 Alkylation under supercritical conditions is chosen to enhance the reaction rate, improve the selectivity, slow the catalyst deactivation, and make the process more environmentally * To whom correspondence should be addressed. Tel.: +86351-4046092. Fax: +86-351-4041153. E-mail: iccjgw@sxicc. ac.cn.

benign. In our previous work, the alkylation with a benzene/propene mole ratio of 5 on a series of β-zeolites in different phases was inspected.6 The highest yield of cumene and the stability of the catalysts were observed when the reaction was carried out under supercritical conditions near the critical point, which may be ascribed to the effective dissolution and diffusion of the coke precursors deposited on the zeolites. The liquidlike densities and enhanced transport properties of SCFs are exploited to extract coke precursors in situ, thereby extending the catalyst activity. Moreover, the operation in the regions near the critical point was most desirable for the effective removal of the coke precursors. However, the different phases in the previous work were categorized according to the vapor-liquid equilibria of the binary system of benzene + propene, which is actually improper because the composition and related critical properties of the reacting mixture change always along the reaction course. To capitalize on the unique characteristics of the reacting medium under supercritical conditions, it is essential to be cognizant of the critical properties of the reacting mixture along with the reaction course. However, the reaction conditions are usually determined by the critical properties of the initial reacting mixture and sometimes even by the critical properties of one pure substance in the reactants. Although a few of the references were concerning the high-temperature and high-pressure vapor-liquid equilibria of benzene, ethane, propene, ethylbenzene, and cumene,7-9 the critical properties of the reacting mixture for the benzene alkylation with propene available in the literature are quite limited. Such data will be certainly useful to

10.1021/ie030480d CCC: $25.00 © 2003 American Chemical Society Published on Web 11/04/2003

6532 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003

determine the phase behavior of the reacting mixture and the operation parameters for the alkylation of benzene with propene under supercritical conditions. The critical properties of mixtures may be determined from the disappearance of a meniscus on slow heating through the critical point or the reappearance of a meniscus on slow cooling,10 and it is customary to note that the reappearance of the meniscus is usually a sharper phenomenon than the disappearance. A constant-volume view cell was useful to observe such changes.8 In this work, the critical properties of both binary and ternary mixtures (benzene + propene, benzene + cumene, cumene + propene, and benzene + propene + cumene) involved in the alkylation of benzene with propene were measured by using a high-pressure view cell with visual observation. Moreover, the critical properties of the ternary mixture were determined with the compositions representing the reaction proceeding extent of alkylation, with the initial mole ratio of benzene to propene being 3, 4, 5, and 6, respectively. The determination of operation conditions for the alkylation of benzene with propene under supercritical conditions was then discussed. 2. Experimental Section Benzene (>99.5%) from Tianjin Tianda Chemical Factory and cumene (>99.5%) from Shanghai Chemical Reagent Corp. were further purified by distillation. Propene (>99.95%) from Beijing Analytical Instrument Corp. was used without further treatment. The purity of all compounds was determined by gas chromatography and was proven to be higher than 99.5 mol %. A constant-volume view cell similar to that in the literature was used in this work.8 The apparatus consisted of a high-pressure view cell of 18.00 cm3, a temperature-controlled air bath, and a pressure sensor. The glass windows are attached to the front and back of the cell to permit full visibility of all of the contents in the cell. The temperature and pressure were controlled to within (0.1 °C and (0.01 MPa, respectively. Before each measurement, the view cell was first evacuated with a vacuum pump. A known mass of benzene or the mixture of benzene + cumene was then charged into the cell. The propene was pressurized into the cell through a sampling tube. The amount of the mixture in the cell was controlled in such a way that the density of the mixture (the mass of the mixture divided by the cell volume) should be close to or slightly higher than its critical density, which was a priori unknown. After that, the view cell was heated with stirring. With an increase of the temperature, the pressure increased and the gas-liquid interphase became flat and faint and eventually vanished at the critical point. After a uniform phase of the SCF in the cell was formed, the temperature of the air thermostat was decreased gradually. At temperatures close to the critical point, the strong red-glow critical opalescence could be observed. With a decrease of the temperature to the critical value, the fluid color went from colorless to yellow, to red-yellow, and to black. Then the meniscus reappeared in the middle of the view cell, and two phases were formed. The temperature and pressure readings in the temperature decreasing mode were made at the moment when complete darkness was observed. To make a successful measurement, attention should be paid to ensure that the density of the fluid in

Table 1. Critical Properties of Pure Substances Tc/°C

Fc/g‚cm-3

Pc/MPa

substance

this work

ref 11

this work

ref 11

this work

ref 11

benzene cumene propene

289.0 358.0 91.7

289.01 358.00 91.61

4.89 3.30 4.62

4.898 3.209 4.613

0.310 0.321 0.261

0.3017 0.2810 0.2325

Table 2. Critical Properties of the Binary Mixtures of Benzene + Propene, Benzene + Cumene, and Cumene + Propene Pc/ Fc / MPa g‚cm-3

Tc/ °C

0 0.1459 0.1722 0.2022 0.2507 0.2564 0.3242 0.3739

289.0 282.8 278.8 278.5 271.6 269.3 258.3 251.1

Benzene (1) + Propene (2) 4.89 0.310 0.4826 235.6 5.16 0.333 0.5765 209.2 5.32 0.347 0.6719 192.4 5.34 0.326 0.7550 172.1 5.56 0.329 0.8252 149.6 5.65 0.432 0.9287 118.5 5.96 0.356 1 91.7 6.15 0.341

6.47 6.74 6.72 6.52 6.24 5.29 4.62

0.323 0.344 0.356 0.273 0.304 0.297 0.261

0 0.216 0.395 0.427 0.612

358.2 341.4 326.7 319.3 273.7

Cumene (1) + Propene (2) 3.30 0.321 0.697 247.8 4.28 0.331 0.800 217.6 5.07 0.324 0.895 161.7 5.44 0.330 1 91.7 7.20 0.332

7.77 8.04 7.29 4.62

0.330 0.328 0.310 0.261

0 0.2173 0.3989 0.6666 0.7496

358.2 349.2 341.2 323.6 316.8

Cumene (1) + Benzene (2) 3.30 0.321 0.7994 311.5 3.63 0.332 0.8328 309.1 3.93 0.330 0.8573 304.8 4.39 0.325 1 289.0 4.54 0.316

4.64 4.68 4.78 4.89

0.325 0.323 0.330 0.310

x2

Tc/ °C

Pc/ F c/ MPa g‚cm-3

x2

the cell is close to or slightly higher than its critical density. Good stirring/shaking is necessary to keep the contents in the cell uniform. For each measurement, the temperature-increasing and -decreasing processes were repeated at least three times, and an average of the temperature and pressure readings was taken as the values reported here. The accuracies of the critical temperature, critical pressure, and mole fraction were estimated within (0.3 K, (0.03 MPa, and (0.003, respectively. 3. Results and Discussion 3.1. Pure Substances. To check the reliability of the technique and the apparatus, the critical properties of pure benzene, cumene, and propene were measured. As shown in Table 1, the agreement between the critical temperature and pressure measured in this work and those in the literature was satisfactory.11 The difference between our measured density and the literature values indicated that the critical temperature and pressure could be obtained within the required uncertainty using a relatively wide range of densities of the mixture provided it was higher than the critical density. The measured critical density is the fluid density at which the critical properties were obtained and therefore is not very accurate (usually higher than the real value); nevertheless, it was not addressed in the current work. 3.2. Binary Mixtures. The critical properties of the binary mixtures of benzene + propene, benzene + cumene, and cumene + propene were listed in Table 2. The critical temperature of the binary mixture of benzene + propene decreases monotonically with the content of propene, while the critical pressure passes through a maximum at a mole fraction of propene of

Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6533

Figure 1. Critical properties of the binary mixture of benzene (1) + propene (2): O, critical temperatures; 4, critical pressures.

Figure 4. Critical point loci of the binary and ternary mixtures in a P-T projection: 4, benzene + propene; O, cumene + propene; 0, benzene + cumene; ], benzene + propene + cumene, with a mole ratio of benzene to cumene being 6; dotted lines, the vapor pressure of each pure component below the critical temperature.11

Figure 2. Critical properties of the binary mixture of cumene (1) + propene (2): O, critical temperatures; 4, critical pressures. Figure 5. Critical properties of the ternary mixture of benzene (1) + propene (2) + cumene (3) at x1/x3 ) 6: O, critical temperatures; 4, critical pressures. Table 3. Critical Properties of the Ternary Mixture of Benzene (1) + Propene (2) + Cumene (3) at x1/x3 ) 6

Figure 3. Critical properties of the binary mixture of cumene (1) + benzene (2): O, critical temperatures; 4, critical pressures.

about 0.62 (Figure 1). The critical lines of the binary mixture of cumene + propene are similar to those of benzene + propene, and the maximum critical pressure occurs at a mole fraction of propene of about 0.80 (Figure 2). By contrast, both the critical temperature and pressure of the binary mixture of benzene + cumene change monotonically with the cumene content between those of the two pure components (Figure 3), which may indicate that the mixture of benzene + cumene is closer to an ideal solution than the other two binary mixtures. The critical point loci of the three binary mixtures in the P-T projection were also shown in Figure 4, which may indicate that all of these three binary mixtures belong to the type I fluid phase behavior according to the classification of van Konynenburg and Scott.12 For the alkylation at a high mole ratio of benzene to propene, the binary mixture of benzene + propene represents the initial reacting mixture, while the final product will be a binary mixture of benzene + cumene when a complete conversion is realized and no side reaction was performed. The critical properties of the initial and final reacting mixtures are strongly dependent on the ratio of benzene to propene for the alkylation. 3.3. Ternary Mixtures with a Fixed Ratio of Benzene/Cumene. The critical properties of the ternary mixture with a fixed mole ratio of benzene to cumene

x1

x2

x3

(x1 + x3)/ (x2 + x3)

Tc/°C

Pc/MPa

Fc/g‚cm-3

0.857 0.771 0.751 0.709 0.670 0.604 0.513 0.477 0.456 0.316 0.251 0.187 0.117 0.047 0

0 0.101 0.124 0.173 0.218 0.295 0.402 0.443 0.468 0.632 0.707 0.782 0.864 0.945 1

0.143 0.128 0.125 0.118 0.112 0.101 0.086 0.080 0.076 0.053 0.042 0.031 0.019 0.008 0

7.00 3.92 3.51 2.85 2.37 1.78 1.23 1.07 0.98 0.54 0.39 0.27 0.15 0.06 0

304.8 297.1 295.0 291.5 289.7 278.7 268.9 257.2 252.7 222.2 192.6 165.6 146.1 112.6 91.7

4.78 5.05 5.15 5.27 5.34 5.77 6.07 6.39 6.50 6.93 7.02 6.75 6.30 5.29 4.62

0.330 0.389 0.352 0.381 0.354 0.336 0.320 0.327 0.324 0.328 0.298 0.324 0.278 0.244 0.261

of 6 were shown in Table 3 and Figure 5. Just like the binary mixtures containing propene, the critical temperature decreases with the mole fraction of propene in the system, while the critical pressure shows a maximum at a mole fraction of propene of about 0.70. The critical point locus of in the P-T projection was also given in Figure 4, which was placed in the midst of those for the two binary mixtures containing propene. This may suggest that the ternary mixture with a fixed ratio of benzene to cumene can be handled by assuming the benzene + cumene as an imaginary substance. 3.4. Critical Properties of the Reacting Mixture along the Reaction Course in the Alkylation of Benzene with Propene. The alkylation of benzene with propene is normally operated at a certain mole ratio of benzene to propene, and therefore the ratio of benzene + cumene to propene + cumene during the reaction will remain the same as the initial ratio of benzene to propene. Thereby, the critical properties of

6534 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 4. Critical Properties of the Reacting Mixture along the Reaction Course of Benzene Alkylation with Propene reaction extent (%) 0 20.2 39.4 59.0 79.9 100 Figure 6. Critical temperature and pressure of the reacting mixture along the reaction course, with the initial mole ratio of benzene to propene being 3: O, critical temperatures; 4, critical pressures.

Figure 7. Critical temperature and pressure of the reacting mixture along the reaction course, with the initial mole ratio of benzene to propene being 4: O, critical temperatures; 4, critical pressures.

the ternary mixture were measured with the concentration representing the reaction proceeding extent of alkylation, with the initial mole ratio of benzene to propene being 3, 4, 5, and 6, respectively (Table 4). The critical temperatures and pressures of the reacting mixture along the reaction course were also shown in Figures 6-9. The critical properties of the reacting mixture change with the reaction proceeding extent (propene conversion) as well as the initial ratio of benzene to propene; when the reaction proceeds to higher propene conversion, the critical temperature shifts to a higher value and the critical pressure to a lower one. The span of critical properties decreases with an increase of the initial ratio of benzene to propene. It was reported that the highest yield of cumene and stability of the catalysts were obtained when the alkylation was carried out under supercritical conditions near the critical point,6 and this work shows that the critical properties change with the reaction course. Therefore, to ensure that the reaction was carried out under supercritical conditions near the critical point, the reaction conditions should be tuned up according to the critical properties of reacting media along the reaction course. The reaction conditions will be a compromise of both initial reactants and final products when the reaction is carried out at fixed temperature and pressure. However, it will be preferred when the process can be operated in a series of successive reactors with reduced pressures and increased temperatures. This may be easily achieved because the alkylation is exothermic and the pressure may be reduced by a catalyst bed with proper resistance. It should be noted that side reactions such as polymerization, diakylation, and transalkylation may occur along with the alkylation, and the reacting mixture contains certain diisopropylbenzene and other side

mole fraction of the reactant mixture benzene propene cumene Tc/°C Pc/MPa

Initial Mole Ratio of Benzene to Propene ) 3 0.749 0.251 0 271.5 0.739 0.208 0.053 285.6 0.718 0.171 0.111 295.3 0.703 0.122 0.176 304.4 0.688 0.063 0.250 314.6 0.667 0 0.333 323.5

5.56 5.30 5.13 4.95 4.65 4.39

0 19.7 38.6 59.7 78.4 100

Initial Mole Ratio of Benzene to Propene ) 4 0.798 0.202 0 278.4 0.790 0.168 0.042 285.9 0.779 0.134 0.086 293.9 0.773 0.090 0.136 302.4 0.761 0.049 0.190 309.4 0.749 0 0.251 316.7 Initial Mole Ratio of Benzene to Propene ) 5 0.828 0.172 0 278.7 0.825 0.140 0.034 286.7 0.815 0.113 0.071 292.0 0.815 0.075 0.111 298.6 0.804 0.042 0.153 305.8 0.800 0 0.200 311.4

5.32 5.24 5.09 4.92 4.78 4.64

0 19.5 39.2 60.2 80.1 100

Initial Mole Ratio of Benzene to Propene ) 6 0.854 0.146 0 282.7 0.851 0.120 0.029 287.9 0.847 0.093 0.060 293.9 0.843 0.062 0.094 299.5 0.839 0.032 0.129 304.0 0.833 0 0.167 309.0

5.16 5.09 4.98 4.89 4.80 4.68

0 20.0 39.0 60.1 79.6 100

5.34 5.24 5.08 4.89 4.74 4.54

Figure 8. Critical temperature and pressure of the reacting mixture along the reaction course, with the initial mole ratio of benzene to propene being 5: O, critical temperatures; 4, critical pressures.

Figure 9. Critical temperature and pressure of the reacting mixture along the reaction course, with the initial mole ratio of benzene to propene being 6: O, critical temperatures; 4, critical pressures.

products as well as benzene, propene, and cumene. The critical properties of the actual reacting mixture of benzene alkylation with propene should then be determined while taking into account the side reactions.

Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6535

4. Conclusions

Literature Cited

The critical properties of both binary and ternary mixtures (benzene + propene, benzene + cumene, cumene + propene, and benzene + propene + cumene) involved in the alkylation of benzene with propene were measured by using a high-pressure view cell with visual observation. Moreover, the critical properties of the ternary mixture were determined with the compositions representing reaction proceeding extent of alkylation, with the initial mole ratio of benzene to propene being 3, 4, 5, and 6, respectively. The results showed that the critical properties of the reacting mixture of benzene alkylation with propene change with the reaction proceeding extent (propene conversion) as well as the initial ratio of benzene to propene. When the reaction proceeds to higher propene conversion, the critical temperature shifts to a higher value and the critical pressure to a lower one. The span of critical properties decreases with an increase of the initial ratio of benzene to propene. To ensure that the reaction was carried out under supercritical conditions near the critical point, the reaction conditions should be tuned up according to the critical properties of the reacting mixture along the reaction course, which may be a compromise of both initial reactants and final products when the reaction is carried out at fixed temperature and pressure.

(1) Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 3rd completely revised ed.; Lindley, C. R., translator; VCH: Weinheim, Germany, 1997; pp 342-343. (2) Siffert, S.; Gaillard, L.; Su, B.-L. Alkylation of benzene by propene on a series of beta zeolites: towards a better understanding of the mechanisms. J. Mol. Catal. A 2000, 153, 267-279. (3) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reaction at supercritical conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723-1778. (4) Baiker, A. Supercritical fluids in heterogeneous catalysis. Chem. Rev. 1999, 99, 453-473. (5) Subramaniam, B.; McHugh, M. A. Reactions in supercritical fluidssA Review. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 1-12. (6) Meng, X. Y.; Qin, Z. F.; Zhang, Y.; Dong, M.; Wang, J. G. Alkylation of benzene with propene on Beta zeolites under supercritical conditions. Catal. Lett. 2002, 83, 265-269. (7) Al-Ghamdi, A. M.; Kabadi, V. N. High-temperature VLE for the benzene-ethylbenzene system. J. Chem. Eng. Data 2001, 46, 1330-1332. (8) Liu, T.; Fu, J. Y.; Wang, K.; Gao, Y.; Yuan, W. K. Critical properties of ethylene + benzene + ethylbenzene. J. Chem. Eng. Data 2001, 46, 1319-1323. (9) Guo, J. Z.; Liu, T.; Dai, Y. C.; Yuan, W. K. Vapor-liquid equilibria of benzene and propylene under elevated temperature and pressure. J. Chem. Eng. Data 2001, 46, 668-670. (10) Hicks, C. P.; Young, C. L. The gas-liquid critical properties of binary mixtures. Chem. Rev. 1975, 75, 119-175. (11) Yaws, C. L. Chemical Properties Handbook; McGraw-Hill Book Co.: Beijing, 1999. (12) van Konynenburg, P. H.; Scott, R. L. Critical lines and phase equilibria in binary van der Waals mixtures. Philos. Trans. R. Soc. London, Ser. A 1980, 298, 495.

Acknowledgment The authors are grateful for the financial support of the State Key Fundamental Research Project of China.

Received for review June 6, 2003 Revised manuscript received September 19, 2003 Accepted September 25, 2003 IE030480D