High-Temperature Catalytic Oxidative Conversion of Propane to

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Ind. Eng. Chem. Res. 2000, 39, 904-908

High-Temperature Catalytic Oxidative Conversion of Propane to Propylene and Ethylene Involving Coupling of Exothermic and Endothermic Reactions Vasant R. Choudhary,* Vilas H. Rane, and Amarjeet M. Rajput Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India

Coupling of the exothermic catalytic oxidative conversion and endothermic thermal cracking (noncatalytic) reactions of propane to propylene and ethylene over the SrO/La2O3/SA5205 catalyst in the presence of steam and limited oxygen was investigated at different process conditions (temperature, 700-850 °C; C3H8/O2 ratio in feed, 2.0-8.0; H2O/C3H8 ratio, 0.5-2.5; space velocity, 2000-15000 cm3 g-1 h-1). In the presence of steam and limited O2, the endothermic thermal cracking and exothermic oxidative conversion reactions occur simultaneously and there is no coke formation on the catalyst. Because of the direct coupling of exothermic and endothermic reactions, this process occurs in a most energy efficient and safe manner. The propane conversion, selectivity for propylene, and net heat of reaction (∆Hr) in the process are strongly influenced by the temperature and concentration of O2 relative to the propane in the feed. The C3H6/C2H4 product ratio is also strongly influenced by the temperature, C3H8/O2 feed ratio, and space velocity. The net heat of reaction can be controlled by manipulating the reaction temperature and C3H8/O2 ratio in the feed; the process exothermicity is reduced drastically with increasing the temperature and/or C3H8/O2 feed ratio. Introduction The demand for propylene in petrochemical industries has been increasing day by day. At present, propylene is produced along with ethylene (which is a main product) by the thermal cracking of ethane, an ethanepropane mixture, or naphtha in the presence of steam (Kniel et al., 1980; Zdonik, 1983). Because of the increasing demand for propylene, worldwide efforts have been made in the past decade to develop processes for producing propylene from propane. A number of studies on the catalytic oxidative dehydrogenation of propane to propylene at low temperatures (400-600 °C) using different catalysts (Sam et al., 1990; Smits et al., 1991; Burch and Crabb, 1993; Corma et al., 1993; Huff and Schmidt, 1994; Gao et al., 1994; Bharadwaj and Schmidt, 1995; Parmaliana et al., 1996; Watling et al., 1991; Lee et al., 1997; Pantazidis et al., 1998; Adesina et al., 1998; Buyevskaya and Baerns, 1998) have been reported. However, the low-temperature catalytic oxidative propane dehydrogenation process is difficult to put into practice because of its limitations, such as (i) high selectivity only at low conversion (e15%), (ii) a highly exothermic side reaction (complete combustion of propane), and (iii) a high concentration of oxygen in the feed (O2/ C3H8 g 0.5), which make the process highly hazardous. Whereas well-established hydrocarbon thermal-cracking processes are highly endothermic, consume large amounts of energy, and also involve extensive coke formation. Recently, Choudhary et al. (1998) investigated the conversion of propane by its simultaneous endothermic thermal-cracking and exothermic noncatalytic oxidative conversion to propylene and ethylene in the presence of limited oxygen at higher temperatures (635-800 °C). * To whom correspondence should be addressed. Fax: (+91)20-5893041. E-mail: [email protected].

This process is found to occur at a much lower temperature or contact time than that required for achieving the same conversion in the thermal-cracking process. Moreover, there was a coupling of exothermic and endothermic propane conversion reactions, making the process highly energy efficient with a drastic reduction in the external energy requirement and coke formation, and also it is safe to operate. It is interesting to know whether this process can be improved further by using a catalyst having high thermal/hydrothermal stability. Our earlier studies showed that the contact time required for achieving the same conversion in the oxycracking of ethane to ethylene (Choudhary et al., 1995, 1998; Mulla et al., 1998; Choudhary and Mulla, 1997a) and oxypyrolysis of natural gas to C2-C4 olefins (Choudhary et al., 1997; Choudhary and Mulla, 1997b), both involving coupling of the exothermic oxidative conversion and endothermic thermal-cracking reactions, is drastically reduced by using a catalyst (supported Srpromoted La2O3) in these processes. The present work was undertaken to invesitgate oxidative conversion of propane to propylene and ethylene in the presence of water and limited oxygen over a Sr-promoted La2O3 catalyst supported on a commercial low surface area macroporous catalyst carrier under the conditions such that both the exothermic oxidative conversion and endothermic thermal-cracking reactions occur simultaneously. Experimental Section The supported Sr-promoted La2O3 catalyst [SrO(17.3 wt %)/La2O3(17.9 wt %)/SA5205] used in this work is same as that reported earlier (Choudhary and Mulla, 1997a). The oxidative conversion of propane to propylene and ethylene was carried out at atmospheric pressure in a continuous quartz reactor packed with 4.0 g of catalyst

10.1021/ie990599f CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000

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Figure 1. Influence of temperature on the conversion and selectivity in the oxidative conversion of propane (C3H8/O2 ) 2.0, GHSV ) 3000 cm3 g-1 h-1, and H2O/C3H8 ) 0.5).

Figure 2. Influence of temperature on the conversion by thermal cracking (TC) and net heat of reaction (∆Hr) in the oxidative conversion of propane (C3H8/O2 ) 2.0, GHSV ) 3000 cm3 g-1 h-1, and H2O/C3H8 ) 0.5).

and having a very low dead volume. It was kept in a tubular electric furnace such that the reaction zone was in the constant temperature zone of the furnace. The feed was a mixture of propane, oxygen, and steam, diluted with helium. The concentration of He in the feed mixture was 80 vol %. The reaction was carried out at the following reaction conditions: temperature, 700-850

Figure 3. Influence of C3H8/O2 on the conversion and selectivity in the oxidative conversion of propane at 800 °C (GHSV ) 3000 cm3 g-1 h-1 and H2O/C3H8 ) 0.5).

Figure 4. Influence of C3H8/O2 on the conversion by thermal cracking (TC) and net heat of reaction (∆Hr) in the oxidative conversion of propane (temperature ) 800 °C, GHSV ) 3000 cm3 g-1 h-1, and H2O/C3H8 ) 0.5).

°C; C3H8/O2 ratio in the feed, 2.0-8.0; H2O/C3H8 ratio, 0.5-2.5; gas hourly space velocity (GHSV), 2000-15000 cm3 g-1 h-1. All the ratios of the feed components are mole ratios. The reaction temperature was measured by a Chromel-Alumel thermocouple located in the catalyst bed. The temperature gradient in the catalyst bed was very small (e5 °C). The reactor effluent gases, after water was removed by condensation at 0 °C, were

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the amount of free O2, CO, CO2, and H2O present in the feed and products was taken into consideration. The experimental runs with the error in C, H, and O mass balances less than 6% were considered; the runs with higher errors were discarded. Results and Discussion

Figure 5. Influence of H2O/C3H8 on the conversion and selectivity in the oxidative conversion of propane at 800 °C (C3H8/O2 ) 2.0 and GHSV ) 3000 cm3 g-1 h-1).

analyzed by an on-line gas chromatograph with a thermal conductivity detector (TCD) and flame ionization detector (FID) using Poropak-Q and Spherocarb columns. The water condensed from the products was measured quantitatively. The water formed in the reaction was obtained from the material balance. There was no formation of acetylene. The formation of O-containing products other than CO, CO2, and H2O was negligibly small (corresponding to less than 0.5% conversion of propane). For obtaining oxygen balance,

The results on the oxidative conversion of propane to propylene and ethylene and also to carbon oxides, methane, ethane, and C4+ hydrocarbons over the SrO/ La2O3/SA5205 catalyst in the presence of steam and limited oxygen are presented in Figures 1-6. The propane conversion process was carried out in the presence of limited oxygen such that both the endothermic thermal- (or noncatalytic) cracking and exothermic oxidative conversion reactions occur simultaneously. The feed (propane, steam, and oxygen) was diluted with helium to avoid the hot spot formation in the catalyst bed. The propane conversion process is influenced by the various process conditions (viz. temperature, C3H8/O2 and steam/ C3H8 ratios in the feed, and space velocity) as follows. Effect of Temperature. Results in Figures 1, 2, and 6 show that, with the increase in the temperature, the process performance is influenced as follows. As expected, the propane conversion is increased to a large extent. The net heat of the reactions (∆Hr) is increased (i.e., it becomes less negative), and hence the process exothermicity is decreased. The propane conversion by its thermal cracking is also increased. The decrease in the process exothermicity is due to the increase in the thermal cracking of propane (Figure 2). However, the influence of temperature on the product selectivity is small; the selectivity for ethylene and ethane is almost not affected but the propylene selectivity is decreased. The selectivity for COx and C4+

Figure 6. Influence of temperature, C3H8/O2 and H2O/C3H8 ratios, and space velocity on the C3H6/C2H4 product ratio in the oxidative conversion of propane.

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hydrocarbons is increased and the selectivity for methane is passed through a minimum with increasing the temperature. The C3H6/C2H4 product ratio is decreased with increasing the temperature (Figure 6). It is expected because of two reasons: (i) the formation of ethylene by the thermal cracking of ethane and higher hydrocarbons and (ii) since the reactivity of propylene is higher than that of ethylene, the former has greater conversion to combustion products. Effect of C3H8/O2 Ratio. The influence of the C3H8/ O2 ratio on the propane conversion is very strong; the conversion is decreased almost linearly with increasing the C3H8/O2 ratio (Figure 3). The propylene selectivity is increased markedly with increasing the C3H8/O2 ratio. The influence on the selectivity of other products is, however, small. Figures 3 and 4 show that not only the total conversion of propane but also the conversion of propane by thermal cracking alone is strongly influenced by the C3H8/O2 ratio. The propane conversion by thermal cracking is drastically reduced at the higher C3H8/O2 ratio, which is consistent with the observation made in the noncatalytic oxycracking of propane (Choudhary et al., 1998). The process exothermicity is decreased with increasing the C3H8/O2 ratio (Figure 4). This is because of a decrease in the rate of propane oxidative conversion as compared to that of propane thermal cracking at the higher C3H8/O2 ratio. It is interesting to note that there is a large increase in the C3H6/C2H4 product ratio when the C3H8/O2 ratio is increased. The observed lower propylene selectivity or C3H6/C2H4 ratio at the lower C3H8/O2 ratio is attributed mostly to the higher reactivity of propylene for its further oxidative conversion to the combustion products (viz. COx). Effect of Space Velocity. The influence of the space velocity on the product selectivity is found to be relatively very small. But the conversion of propane is decreased from 63% to 36% with increasing the space velocity from 2000 to 15000 cm3 g-1 h-1. The effect of space velocity on the C3H6/C2H4 product ratio is, however, strong. The C3H6/C2H4 ratio is increased markedly with increasing the space velocity (Figure 6). Effect of H2O/C3H8 Ratio. The results (Figure 5) show a small influence of the H2O/C3H8 ratio on the conversion and product selectivity. The conversion is decreased particularly at the higher H2O/C3H8 ratios and the selectivity for ethylene and propylene is increased with increasing the H2O/C3H8 ratio. The C3H6/ C2H4 product ratio is also not influenced very significantly by the H2O/C3H8 ratio (Figure 6). When this process was carried out in the absence of steam, a formation of tarlike products deposited on the cooler part of the reactor outlet was observed. However, when steam was added in the feed, no formation of tarlike products or coke deposition on the catalyst at the different process conditions was observed. Reactions Involved in the Process. A large number of free radical reactions (Falconer and Knox, 1959; Nguyen and Kung, 1991) are expected to occur in the homogeneous oxypyrolysis and thermal cracking of propane along with the heterogeneous catalytic oxidative conversion of propane in the presence of limited O2. The important heterogeneous (i.e., catalytic) and homogeneous (i.e., noncatalytic) propane conversion reactions are as follows:

Hetergeneous Reactions.

C3H8 + 0.5O2 f C3H6 + H2O + 28.3 kcal mol-1 (1) O2

C3H8 98 CO, CO2, CH4, and H2O (exo.)

(2)

The catalytic reaction is initiated on the catalyst surface most probably by the formation of propyl radicals from the surface reaction of propane with adsorbed or lattice oxygen, as follows:

C3H8 + [O] f C3H7 + [OH]

(3)

2[OH] + 0.5O2 f H2O + [O]

(4)

The propyl radicals desorb in the gas phase and undergo subsequent homogeneous reactions leading to the formation of propylene and other products. Homogeneous Reactions.

C3H8 f C2H4 + CH4 - 18.7 kcal mol-1

(5)

C3H8 f C3H6 + H2 - 30.9 kcal mol-1

(6)

C3H8 + 0.5O2 f C3H6 + H2O + 28.3 kcal mol-1 (7) C3H8 + 3.5O2 f 3CO + 4H2O + 286.4 kcal mol-1 (8) C3H8 + 5O2 f 3CO2 + 4H2O + 489.0 kcal mol-1 (9) The values of the heat of reaction corresponds to 700 °C. The free radical reactions involved in the propane conversion process are given elsewhere (Choudhary et al., 1998). Because the conversion of O2 in the overall process is not complete, the catalytic exothermic oxidative conversion reactions are expected to occur throughout the reactor simultaneously with the homogeneous endothermic and exothermic reactions (Choudhary et al., 1998). In this process, the endothermic thermal-cracking reactions of propane are expected to be predominant at higher temperatures, particularly at lower concentrations of O2. The exothermic oxidative conversion reactions become more important at higher concentrations of O2 (i.e., at lower C3H8/O2 ratios) and at lower temperatures. The observed decrease in the process exothermicity with increasing C3H8/O2 ratios (Figure 4) is consistent with this. Thus, the net heat of all the propane conversion reactions, which occur simultaneously in the process, can be controlled by manipulating the process conditions (viz. temperature and concentration of O2 relative to those of propane in the feed). Because of the use of steam and limited O2 in the feed and operation of the process at high temperature (700850 °C), it is possible to convert propane to ethylene and propylene at high conversion and selectivity in an energy-efficient manner, involving a coupling of the

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exothermic oxidative conversion and endothermic thermal-cracking reactions of propane and also without coke formation. Conclusions In the presence of steam and limited O2, propane can be converted to propylene and ethylene at high conversion and selectivity over a SrO/La2O3/SA5205 catalyst (at 700-850 °C) in a most energy efficient and safe manner requiring no external energy. No coke or tarlike product formation is observed when the process is operated in the presence of steam. The process involves direct coupling of simultaneously occurring endothermic thermal-cracking and exothermic oxidative conversion reactions. The process exothermicity can be controlled by manipulating the temperature and concentration of O2 relative to propane in the feed. In this process, the propylene/ethylene product ratio is strongly influenced by the temperature, C3H8/O2 feed ratio, and space velocity. The influence of the H2O/C3H8 feed ratio is, however, negligibly small. Literature Cited Adesina, A. A.; Cant, N. W.; Saberi-Moghaddam, A.; Szeto, C. H.; Trimm, D. L. Structrual Effects in Oxidative Dehydrogenation of Hydrocarbons over a Vanadia-Molybdena-Niobia Catalyst. J. Chemtech. Biotechnol. 1998, 72, 19. Bharadwaj, S. S.; Schmidt, L. D. Olefins by Catalytic Oxidation of Alkanes in Fludized Bed Reactors. J. Catal. 1995, 155, 403. Burch, R.; Crabb, M. Homogeneous and Heterogeneous Contribution to the Oxidative Dehydrogenation of Propane on Oxide Catalysts. Appl. Catal. 1993, 100, 111. Buyevskaya, O. V.; Baerns, M. Catalytic Selective Oxidation of Propane. Catal. Today 1998, 42, 315. Choudhary, V. R.; Mulla, S. A. R. Coupling of Exothermic and endothermic Reactions in Oxidative Conversion of Natural Gas into Ethylene/Lower Olefins over Diluted SrO/La2O3/SA5205 Catalyst. Ind. Eng. Chem. Res. 1997a, 36, 3520. Choudhary, V. R.; Mulla, S. A. R. Coupling of Endothermic Thermal Cracking with Exothermic Noncatalytic Oxidative Conversion of Ethane to Ethylene. AIChE J. 1997b, 43, 1545. Choudhary, V. R.; Uphade, B. S.; Mulla, S. A. R. Coupling of Endothermic Thermal Cracking with Exothermic Oxidative Dehydrogenation of Ethane to Ethylene Using Diluted Catalyst. Angew. Chem., Int. Ed. Engl. 1995, 34, 665. Choudhary, V. R.; Mulla, S. A. R.; Rajput, A. M. Noncatalytic Oxypyrolysis of Natural Gas to Lower Olefins in Most Energy Efficient and Safe Manner. Ind. Eng. Chem. Res. 1997, 36, 2075.

Choudhary, V. R.; Rane, V. H.; Rajput, A. M. Simultaneous Thermal Cracking and Oxidation of Propane to Propylene and Ethylene. AIChE J. 1998, 44, 2293. Corma, A.; Lopez Nieto, J. M.; Paredes, N. Influence of the Preparation Methods of V-Mg-O Catalysts on Their Catalytic Properties for the Oxidative Dehydrogenation of Propane. J. Catal. 1993, 144, 425. Falconer, J. W.; Knox, J. H. The High-Temperature Oxidation of Propane. Proc. R. Soc. 1959, A250, 493. Gao, X.; Ruiz, P.; Xin, Q.; Guo, X.; Delmon, B. Effects of Coexistence of Magnesium Vanadate Phases in the Oxidative Dehydrogenation of Propane to Propene. J. Catal. 1994, 148, 56. Huff, M.; Schmidt, L. D. Production of Olefins by Oxidative Dehydrogenation of Propane and Butane over Monolith at Short Contact Times. J. Catal. 1994, 149, 127. Kniel, L. O.; Winter, O.; Stork, K. Ethylene: Keystone to the Petrochemical Industry; Marcel Dekker: New York, 1980. Lee, K. H.; Yoon, Y. S.; Ueda, W.; Moro-oka, Y. An Evidence of Active Surface MoOx over MgMoO4 for the Catalytic Oxidative Dehydrogenation of Propane. Catal. Lett. 1997, 46, 267. Mulla, S. A. R.; Uphade, B. S.; Choudhary, V. R. Oxidative Dehydrogenation of Ethane to Ethylene over Sr-Promoted La2O3 Catalyst Supported on Low Surface Area Porous Catalyst Carrier. Stud. Surf. Sci. Catal. 1998, 113, 1023. Nguyen, K. T.; Kung, H. H. Analysis of the Surface-Enhanced Homogeneous Reactions during Oxidative Dehydrogenation of Propane on V-Mg-O Catalyst. Ind. Eng. Chem. Res. 1991, 30, 352. Pantazidis, A.; Bucholz, S. A.; Zanthoff, H. W.; Schuurman, Y.; Mirodatos, C. A TAP Reactor Invetigation of the Oxidative Dehydrogenation of Propane over V-Mg-O Catalyst. Catal. Lett. 1998, 40, 207. Parmaliana, A.; Sokolovskii, V.; Frusteri, F.; Miceli, D. Propane Oxidative Dehydrogenation on Silica Based Oxide Catalysts. Catal. Lett. 1996, 40, 105. Sam, D. S. H.; Soenen, V.; Volta, J. C. Oxidative Dehydrogenation of Propane over V-Mg-O Catalysts. J. Catal. 1990, 123, 417. Smits, R. H. H.; Seshan, K.; Ross, J. R. H. The Selective Oxidative Dehydrogenation of Propane over Niobium Pentoxide. J. Chem. Soc., Chem. Commun. 1991, 558. Watling, T. C.; Deo, G.; Seshan, K.; Wachs, I. E.; Lercher, J. A. Oxidative Dehydrogenation of Propane over Niobia Supported Vanadium Oxide Catalysts. Catal. Today 1991, 28, 139. Zdonik, S. B. Production and Economic in the Recovery of Ethylene, Other Olefins and Aromatic from Pyrolysis Systems. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983; p 377.

Received for review August 9, 1999 Revised manuscript received December 22, 1999 Accepted January 10, 2000 IE990599F