Ind. Eng. Chem. Res. 2003, 42, 1329-1333
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Reaction Coupling in the New Processes for Producing Styrene from Ethylbenzene Zhangfeng Qin, Jianguo Liu, Ailing Sun, 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 possibility of coupling the ethylbenzene dehydrogenation with water-gas shift, CO2 methanation, and nitrobenzene hydrogenation has been investigated thermodynamically. The chemical equilibria of these reactions have been calculated on the basis of the Soave-RedlichKwong equation of state, and the effects of the feed composition, temperature, and pressure upon ethylbenzene equilibrium conversion have been studied. It was found that the equilibrium conversion could be greatly enhanced by the reaction coupling, especially with nitrobenzene hydrogenation. When coupling with water-gas shift, the ethylbenzene equilibrium conversion can be elevated to 82.4% from 25.2% for the single ethylbenzene dehydrogenation at 550 °C. When coupling with nitrobenzene hydrogenation, the ethylbenzene equilibrium conversion can reach 98.5% at 400 °C, compared with the conversion of 3.5% at the same temperature for the single ethylbenzene dehydrogenation. The primary experiments on a series of catalysts also proved that the reaction coupling is an effective measure to improve the ethylbenzene dehydrogenation, although much more work is still necessary to develop proper catalysts for the coupling reactions. 1. Introduction Styrene (ST) is one of the most important basic chemicals and is mainly used as the monomer of synthetic polymers. It is commercially produced by the dehydrogenation of ethylbenzene (EB) on iron oxide catalysts at 600-700 °C, just below the temperature where thermal cracking becomes significant.1 Because of its high endothermic character, a large amount of superheated steam is used to supply heat, lower the partial pressure of the reactant, and avoid the formation of carbon deposits. However, much of the latent heat of steam is lost in the gas-liquid separator instead of recovering. Thus, it is highly desirable to develop new processes and catalysts that can lower the reaction temperature and water/EB ratio. Reaction coupling is an effective approach to improve the equilibrium conversion in the dehydrogenation reactions.2 Several investigations on reaction coupling were carried out in the dehydrogenation of EB in the presence of carbon dioxide instead of steam,3-12 which combined the EB dehydrogenation together with the reverse water-gas shift reaction. It is estimated that the energy required for producing per ton ST in the coupling process is 1.5-1.9 × 108 cal, compared with 1.5 × 109 cal in the current commercial process.6,8 In addition, CO2 was found to be able to suppress the catalyst deactivation. Therefore, the new process could be an energy-saving and environmentally friendly one that has been attracting more and more attention. Because the commercial Fe-Cr-K catalyst does not work effectively in such coupling systems, some new catalysts were reported.3-12 Among these works, Badstube et al.9 investigated the catalytic behavior of iron supported on an activated carbon (AC) in the EB dehydrogenation * Corresponding author. Tel.: +86-351-4046092. Fax: +86351-4041153. E-mail:
[email protected].
coupled with the reverse water-gas shift, and high conversion and selectivity to ST were observed at 550 °C. However, the reverse water-gas shift is an endothermic reaction. For the reaction coupling, the provision of the necessary process heat by a concurrent exothermic reaction is favorable to drive the EB dehydrogenation. Thus, the oxidative dehydrogenation of EB at lower reaction temperatures has attracted considerable attention,13-16 in which oxygen was used as the hydrogen acceptor, yielding water as a byproduct. More recently, a process involving the combination of the endothermic dehydrogenation of EB to ST with the strongly exothermic hydrogenation of nitrobenzene (NB) to aniline (AN) has been investigated. Bautista and coworkers1 studied the gas-phase catalytic hydrogentransfer reaction between EB and NB, to produce ST and AN, at 360-460 °C on different catalysts. It was found that the EB conversion under oxidative conditions was always considerably higher than that in nonoxidative conditions. Moreover, NB played an important role as a hydrogen acceptor, not only shifting the EB dehydrogenation equilibrium but also restricting the secondary reactions by lowering the level of available hydrogen, especially when supported metals were used as catalysts. All of these works proved that the reaction behavior could be improved greatly by coupling dehydrogenation with hydrogenation, endothermic reaction with exothermic reaction, and reductive reaction with oxidative reaction. However, the intrinsic factors of such reaction coupling have not been investigated in detail as yet. Therefore, it is difficult to select the proper coupling reaction to estimate the benefits of coupling and to optimize the operating conditions such as reaction temperature and reactant composition. In the present paper, the possibility of reaction coupling of EB dehydrogenation with reverse water-gas shift, CO2 metha-
10.1021/ie020762y CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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nation, and NB hydrogenation has been investigated thermodynamically. The chemical equilibria of the reactions have been calculated on the basis of the SoaveRedlich-Kwong (SRK) equation of state, and the effects of the feed composition, temperature, and pressure upon EB equilibrium conversion have been studied. On the basis of the calculation, the feasibility and benefit of the coupling were clarified. The primary experiments on a series of catalysts also proved that the reaction coupling is an effective measure to improve the EB dehydrogenation, although much more work is still necessary to develop proper catalysts for the coupling reactions. 2. Theory The dehydrogenation of EB to ST can be expressed by
Figure 1. Dehydrogenation of EB to ST: equilibrium conversion at different temperatures and pressures.
C6H5-C2H5 (EB) ) C6H5-C2H3 (ST) + H2 ∆H25 °C0 ) +117.6 kJ mol-1
(1)
The possible coupling reactions considered include the reverse water-gas shift
CO2 + H2 ) CO + H2O ∆H25 °C0 ) +41.2 kJ mol-1
(2-1)
the CO2 methanation
Figure 2. Coupling of EB dehydrogenation with a reverse watergas shift reaction: effects of the feed composition (in mole ratio) and temperature on the equilibrium conversion of EB at 0.1 MPa.
CO2 + 4H2 ) CH4 + 2H2O ∆H25 °C0 ) -165.0 kJ mol-1
(2-2)
and the NB hydrogenation
C6H5-NO2 (NB) + 3H2 ) C6H5-NH2 (AN) + 2H2O ∆H25 °C0 ) -464.3 kJ mol-1
(2-3)
The equilibrium constants of these reactions are defined as follows:
K10 ) KP,1Kφ,1(P0)-1 )
( )( )( ) xSTxH2 xEB
K210 ) KP,21Kφ,21 ) K220 ) KP,22Kφ,22(P0)2 )
(
eq
φEB
( )( xCOxH2O xCO2xH2
)(
eq
-1
P0 P
)
φCOφH2O
eq
φCO2φH2
xCO2xH24
φCH4φH2O2
eq
φCO2φH24
xANxH2O2 xNBxH2
P0 P
2
(4-2)
eq
3
eq
3. Results and Discussion
eq
φANφH2O2
3
(3)
(4-1)
)( ) ( )( )
xCH4xH2O2
K230 ) KP,23Kφ,23P0 )
φSTφH2
φNBφH2
SRK equation of state.19 The equilibrium conversion can then be gained by solving eqs 3 and 4 simultaneously with the Newton-Raphson method. To examine the effects of the reaction coupling and operating conditions, the dehydrogenation of EB coupled with water-gas shift, CO2 methanation, and NB hydrogenation at temperatures of 300-700 °C and pressures of 0.01-1.0 MPa were examined along with the single dehydrogenation and dehydrogenation with N2 dilution. The mole ratios of EB to N2 or CO2 were from 1:1 to 1:10, while the mole ratios of EB to NB were from 6:1 to 2:1.
P0 P
eq
(4-3)
Here φi is the fugacity coefficient and xi the mole fraction of component i. As described elsewhere,17 the value of K0 can be calculated from the heats of formation, entropies, and heat capacities of each component18 and Kφ from the
3.1. Single Dehydrogenation of EB to ST. The dehydrogenation of EB to ST is an endothermic and volume-increasing reaction; high temperature and low pressure are favored for the conversion of EB. As shown in Figure 1, the equilibrium EB conversions at 500 °C are only 13.8% and 41.9% under 0.1 and 0.01 MPa, respectively. When the temperature is increased to 600 °C, the conversions can be elevated to 37.8% and 81.1% respectively under 0.1 and 0.01 MPa. 3.2. Coupling with Water-Gas Shift. Figure 2 gives the equilibrium conversion of EB in the presence of N2 or in the coupled system with water-gas shift at different temperatures and different mole ratios of EB to CO2 under 0.1 MPa. It can be seen that the conversion is quite low in the case of pure EB dehydrogenation even at high temperatures, for example, 69.7% at 690 °C. The conversion can be increased obviously by introducing nitrogen into the reaction system because of the decrease of the partial pressures of all of the components in the system, just like the effect of lowering the reaction
Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1331
Figure 3. Coupling of EB dehydrogenation with CO2 methanation: effects of the feed composition (in mole ratio) and temperature on the equilibrium conversion of EB at 0.1 MPa.
pressure shown in Figure 1. This is one of the reasons for using large amounts of steam in the commercial process. However, this enhancement by dilution is limited; for example, the conversion of EB is increased from 25.2% to 57.8% at 550 °C in the presence of nitrogen (mole ratio EB:N2 ) 1:10). To enhance the conversion of EB or reduce the operating temperature considerably, it is necessary to eliminate the produced hydrogen in situ by coupling with hydrogen-consuming reactions. It can be seen that the conversion is improved greatly by coupling with reverse water-gas shift. Moreover, the conversion increases with an increase in the ratio of CO2 to EB. The conversions are 63.9% and 82.4% at 500 and 550 °C, respectively, in the case of the mole ratio of CO2/EB being 10. The margin between the lines of dilution and coupling suggests the superiority of the coupling over dilution, which makes it possible to improve the equilibrium conversion of EB at a certain temperature or reduce the operating temperature at the same ST yield. It should be mentioned that the supply of necessary process heat to drive the EB dehydrogenation from outside is necessary because the reverse water-gas shift is a weak endothermic reaction (∆H550 °C0 ) 35.8 kJ mol-1). 3.3. Coupling with CO2 Methanation. Figure 3 shows the equilibrium conversion of EB when the dehydrogenation is coupled with CO2 methanation at different temperatures for different ratios of CO2 to EB under 0.1 MPa. It can be seen that the conversion can also be improved by introducing CO2 into the system when a proper catalyst is available for the methanation. The conversions are 54.5% and 70.2% at 500 and 550 °C, respectively, in case of the mole ratio of CO2/EB being 10. The difference between coupling and dilution is not so large, which means that the improvement is mainly due to the dilution effect of CO2. However, the coupling with CO2 methanation is favored energetically because the methanation is an exothermic reaction (∆H550 °C0 ) -186.3 kJ mol-1). 3.4. Coupling with Hydrogenation of NB. In the case of coupling with NB hydrogenation, the equilibrium conversions of EB at different temperatures for different ratios of EB to NB under 0.1 MPa are shown in Figure 4. Stoichiometrically, the molar ratio of NB to EB for the coupling reaction is 1:3. At this ratio, the conversion of EB can reach as high as 98.5% even at 400 °C, while the conversions at the same temperature are only 3.5% and 11.1% for the pure dehydrogenation and dehydrogenation with N2 dilution (EB:N2 ) 1:10), respectively.
Figure 4. Coupling of EB dehydrogenation with NB hydrogenation: effects of the feed composition (in mole ratio) and temperature on the equilibrium conversion of EB at 0.1 MPa.
Figure 5. Comparison of the coupling of EB dehydrogenation with different reactions at 0.1 MPa. The feed composition (in mole ratio) is EB:NB ) 3:1 for NB hydrogenation, EB:CO2 ) 1:10 for reverse water-gas shift, and EB:N2 ) 1:10 for N2 dilution.
Although the equilibrium conversion of EB increases with an increase of the temperature and the NB/EB mole ratio in the reactants mixture, the improvement of conversion is no longer evident when the reaction temperature is higher than 400 °C and the NB/EB mole ratio is higher than 1:3. At this condition, the conversion of NB is the same as that of EB. Therefore, the reactant mixture of EB and NB at a molar ratio of 3 can be almost quantitatively transformed to ST and AN at temperatures higher than 400 °C when a proper catalyst is available. Moreover, in contrast to the EB dehydrogenation, the hydrogenation of NB is a strongly exothermic reaction (∆H400 °C0 ) -473.5 kJ mol-1). The necessary process heat to drive the EB dehydrogenation can be provided by introducing the exothermic NB hydrogenation reaction concurrently, which makes the coupling reaction a weakly exothermic process (∆H400 °C0 ) -106.4 kJ mol-1). Thus, the reaction of coupling with NB hydrogenation is also an energetically favored one. 3.5. Comparison of Different Coupling Systems. The comparison of the coupling of EB dehydrogenation with the reverse water-gas shift, CO2 methanation, and NB hydrogenation is shown in Figure 5. The equilibrium conversion of EB can be improved in all of these three cases. Among them, coupling with water-gas shift is better than that with CO2 methanation, and EB can be quantitatively transformed into ST at 400 °C in the case of coupling with NB hydrogenation. On the other hand, both EB dehydrogenation and reverse water-gas shift are endothermic, while CO2 methanation and NB hydrogenation are exothermic. Therefore, the coupling
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Table 1. Experimental Results for Different Reaction Systems coupling
no coupling
water-gas shift
NB dehydrogenation
feed (by mole) reaction temperature (°C) catalysts (mmol/g of carbon) F/W (mL of EB/g of catalyst‚h) EB equilibrium conversion (%) EB actual conversion (%) ST actual selectivity (%)
N2:EB ) 10:1 550 Fe(3)Li(0.3)/AC 2.0 57.8 24.0% 83.2
CO2:EB ) 10:1 550 V(0.87)La(0.05)/AC 2.0 82.4 64.0% 96.8
NB:EB ) 1:3 400 Pt(0.001)/AC 1.5 98.4 33.8% 99.2
with CO2 methanation, especially with NB hydrogenation, is favored energetically for the EB dehydrogenation. In practice, operating a reaction under proper pressure can greatly reduce the size of the reactor and increase the production ability. As a consequence, the conversions of EB in the coupling with NB hydrogenation or reverse water-gas shift at different temperatures under different pressures are shown in Figure 6. It can be found that the EB conversion decreases with an increase of the pressure in both cases because the dehydrogenation is a volume-increasing reaction. However, in the case of NB hydrogenation, the conversions are close to 100% for the pressures up to 1.0 MPa when temperature is higher than 450 °C, indicating that the coupling reaction can be operated under proper pressures higher than 0.1 MPa to reduce the reactor size and increase the production ability. 3.6. Primary Experimental Results of Catalytic Tests. Thermodynamically, coupling the EB dehydrogenation with water-gas shift and NB hydrogenation can improve the equilibrium conversion or lower the reaction temperature. However, to realize the effects of reaction coupling, the proper catalysts are necessary and kinetic problems must be considered. Fortunately, a lot of research work has shown that there were a series of potential catalysts for the coupling reactions.1,3-12 Moreover, Table 1 gives our primary experimental results for different reaction systems. The reaction temperature was chosen according to the calculations so that the EB equilibrium conversion can reach a practical level (>80%) while the reaction temperature is still low enough to prevent the products from further decomposition. With the alkali, alkaline-earth, or rare-earth metal-promoted vanadium/AC catalysts, high EB conversion (50-60%) and selectivity to ST (9598%) were observed at 550 °C when coupling with water-gas shift. The EB conversion of 33.8% and ST selectivity of 99.2% were obtained over Pt(0.02 wt %)/ AC at 400 °C when coupling with NB hydrogenation.
The actual EB conversion with the coupling was much higher than that of the pure dehydrogenation in the inert atmosphere, and the ST selectivity could also be improved by the coupling. However, there exist some problems with current catalysts. The life of the ACsupported catalyst for the reaction coupling is still too short (about 10 h) for the practical process and, moreover, it deactivates more seriously at higher temperature. Although a lot of work is still necessary to improve the catalyst activity and alleviate the deactivation, such results suggested that the reaction coupling is a potential measure to improve the EB dehydrogenation. 4. Conclusions The possibility and benefit of reaction coupling of EB dehydrogenation with reverse water-gas shift, CO2 methanation, and hydrogenation of NB were investigated thermodynamically. The chemical equilibrium of the reactions was calculated on the basis of the SRK equation of state, and the effects of the feed composition, temperature, and pressure on EB equilibrium conversion were studied. It was found that the equilibrium conversion could be greatly enhanced by the reaction coupling, especially with NB hydrogenation. When coupling with water-gas shift, the EB equilibrium conversion can be elevated to 82.4% from 25.2% for the single EB dehydrogenation at 550 °C. When coupling with NB hydrogenation, the EB equilibrium conversion can reach 98.5% at 400 °C, compared with the conversion of 3.5% at the same temperature for the single EB dehydrogenation. The primary experiments on a series of catalysts also proved that the reaction coupling is an effective measure to improve the EB dehydrogenation, although much more work is still necessary to develop proper catalysts and to consider kinetic problems in the development of such new processes. Acknowledgment The authors are grateful for the financial support of the National Fundamental Research Project. Nomenclature K0 ) equilibrium constant Kp ) equilibrium constant based on partial pressure Kφ ) fugacity coefficient ratio P ) pressure [MPa] T ) temperature [K] xi ) mole fraction of component i ∆HT0 ) standard enthalpy change of reaction at temperature T φi ) fugacity coefficient of i
Figure 6. Coupling of EB dehydrogenation with NB hydrogenation or reverse water-gas shift reaction: effect of pressure on the equilibrium conversion of EB. The feed composition (in mole ratio) is EB:NB ) 3:1 for NB hydrogenation and EB:CO2 ) 1:10 for reverse water-gas shift reaction.
Superscript 0 ) classical Subscripts AN ) aniline EB ) ethylbenzene
Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1333 eq ) at equilibrium NB ) nitrobenzene 1 ) ethylbenzene dehydrogenation reaction (1) 21 ) water-gas shift reaction (2-1) 22 ) CO2 methanation reaction (2-2) 23 ) nitrobenzene hydrogenation reaction (2-3)
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Received for review September 24, 2002 Revised manuscript received January 16, 2003 Accepted January 29, 2003 IE020762Y
