Ind. Eng. Chem. Res. 2001, 40, 3133-3138
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Kinetics of Isopropylation of Benzene over HBeta Catalyst U. Sridevi, B. K. B. Rao, and Narayan C. Pradhan* Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721 302, India
S. S. Tambe,† C. V. Satyanarayana,‡ and B. S. Rao‡ Chemical Engineering and Catalysis Divisions, National Chemical Laboratory, Pune 411 008, India
The kinetics of benzene alkylation with isopropyl alcohol has been studied in the vapor phase over the H form of Beta zeolite as the catalyst. On the basis of the product distribution pattern obtained over this large-pore zeolite, a reaction mechanism has been proposed. From this reaction network, a simple stoichiometric model has been derived and fitted to the kinetic data. The deactivation kinetics has also been investigated in a selected low-temperature range. The activation energies of all reactions in the network have been determined. The activation energy for the deactivation reaction in the chosen temperature range is found to be lower than that for the main reaction. Introduction Alkylation reactions are widely used in industry for the production of several commercially important alkyl aromatics. Cumene is one such alkyl aromatic produced by the isopropylation of benzene. In recent years, the cumene production capacity throughout the world has increased significantly. This is because more than 90% of the world’s phenol demand is met by cumene, which is still the chief starting material for phenol production. Several zeolite-based catalytic processes have been developed recently for the production of cumene by the alkylation of benzene with propylene.1-8 Some of these processes have also been commercialized. In recent years, much interest has been generated by the largepore zeolite Beta. It has been observed that no significant formation of n-propyl benzene occurs over largepore zeolites, whereas n-propyl benzene constitutes a significant fraction of the product formed on medium pore zeolites. Beta is a high silica, crystalline aluminosilicate that has some unique features. It is the only large-pore zeolite having chiral pore intersections. The pore structure of Beta consists of 12-membered rings interconnected by cages formed by the intersections of the channels. The dimensions of the pore opening in the linear channels are 5.7 Å × 7.5 Å. The tortuous channel system consists of intersections of the two linear channels with approximate dimensions of 5.6 Å × 6.5 Å. Zeolite Beta has a pore volume of about 0.2 cm3/g. Several investigators have studied the reaction of benzene with isopropyl alcohol/propylene for cumene synthesis. The effects of various process parameters in benzene isopropylation on Beta zeolite have been studied and the performance of the catalyst has been compared with that of other catalysts.9-11 Cumene synthesis on various modifications of Beta zeolite has also been reported in the literature.11-14 Parikh et al.15 studied the kinetics of cumene synthesis over ferri* Author to whom all correspondence should be addressed. E-mail:
[email protected]. † Chemical Engineering Division. ‡ Catalysis Division.
silicate of MFI. Their report confirmed that significant formation of the highly undesirable n-propyl benzene occurs on this catalyst. Recently, Corma et al.16 reported liquid-phase benzene alkylation with propylene over zeolites MCM-22, Beta, and ZSM-5. They performed a kinetic study of this reaction on MCM-22 and concluded that it follows an Eley-Rideal mechanism in which benzene can compete for the active sites but its coverage is much lower than that of propylene. There is, however, very little information in the literature on the kinetics of the cumene synthesis reaction with isopropyl alcohol as the alkylating agent. It was, therefore, thought desirable to investigate the kinetics of this commercially important reaction over Beta zeolite, which does not favor the formation of n-propyl benzene. Another objective of this study was to develop a kinetic model based on the product distribution that considers secondary reactions such as the dehydration of alcohol and the formation and isomerization of DIPB. A study of the deactivation kinetics for the benzene isopropylation reaction at low temperatures was also considered to be highly relevant in the present context. Experimental Section Materials. Beta catalyst with a size of 1.5 mm (extrudates with 20% binder) in its active protonated form with a Si/Al ratio of 15 was obtained from M/s UCIL, India, and used for the reaction. For the studies involving mass transfer effects, Beta zeolite powder having the same Si/Al ratio was obtained from the pilot plant of National Chemical Laboratory, Pune, India; it was pelletized with 20% binder (the same as with extrudate), crushed, and sieved for the desired particle size. The benzene and isopropyl alcohol used in this study were of “Analytical Reagent” grade. Benzene was obtained from E Merck (India) Ltd. and isopropyl alcohol from Qualigens Fine Chemicals, both of Mumbai, India. Procedure. The vapor-phase isopropylation of benzene was carried out in a fixed-bed, down-flow glass reactor at atmospheric pressure with a condenser in the downstream. Feed containing benzene and isopropyl alcohol in the desired proportions was fed by a positive displacement pump. Reactant vapors were carried to the
10.1021/ie000929s CCC: $20.00 © 2001 American Chemical Society Published on Web 06/15/2001
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Ind. Eng. Chem. Res., Vol. 40, No. 14, 2001 Table 1. Comparison of Activity of Fresh and Regenerated Catalystsa composition (mol %) component
fresh catalyst
regenerated catalystb
IPA aliphatics benzene cumene m-DIPB p-DIPB
11.25 0.71 87.88 0.13 0.05 0.05
11.19 0.72 87.90 0.13 0.05 0.04
a Conditions: temperature, 388 K; benzene-to-isopropyl alcohol mole ratio, 8:1, W/FAo ) 61.80 kg h/kg mol. b After 14 successive regenerations.
Table 2. Effect of Temperature on Catalytic Activity of HBetaa performance parameter Figure 1. Time on stream behavior of HBeta for benzene isopropylation reaction. Conditions: temperature, 473 K; benzeneto-IPA mole ratio, 8:1; WHSV, 3 h-1
catalyst bed using nitrogen as the carrier gas. The condensed reactor effluent stream was analyzed in a capillary column fitted to a Schimadzu 15A gas chromatograph unit. In a typical run during the catalyst activity tests and kinetic study, 2 g of catalyst was charged into the reactor (15 mm i.d.). The upper preheating zone and the section below the catalyst bed were filled with inert beads. The reactor was operated under isothermal conditions, and the mass balance was on the order of 99.7%. The term “conversion” used in the study is defined as isopropyl alcohol conversion (mol %) ) moles of isopropyl alcohol consumed per unit time × 100 moles of isopropyl alcohol fed per unit time conversion to product P (mol %) ) moles of P formed per unit time × 100 moles of isopropyl alcohol fed per unit time
The cumene selectivity is defined as cumene selectivity (mol %) ) moles of cumene formed per unit time moles of total aromatics (excluding benzene) formed per unit time × 100
Results and Discussion Catalyst Stability. The stability of HBeta was tested for the isopropylation of benzene with isopropyl alcohol at 473 K and atmospheric pressure for about 60 h time on stream. The isopropyl alcohol conversion remained constant during this period, as shown in Figure 1. It can also be seen from this figure that the conversion of isopropyl alcohol to cumene, as well as the cumene selectivity, remained more or less constant for the whole period studied. These results were also tested for reproducibility and found to be consistent. Moreover, the catalyst activity could be restored fully even after successive regenerations. It can be seen from Table 1 that the product distribution remains almost unchanged even after 14 regenerations with no noticeable change in the isopropyl alcohol conversion. During experimentation, the catalyst loaded in the reactor was regenerated in situ by calcining at 500 °C in a stream of air for
378
383
temperature (K) 388 393 403
458
conversion of IPA 9.6 13.88 21.04 31.26 73.44 100 (mol %) conversion to cumene 1.51 1.41 1.89 6.67 28.16 31.59 (mol %) cumene selectivity 76.24 62.13 55.73 73.95 81.61 67.82 (mol %) a Conditions: benzene-to-isopropyl alcohol mole ratio, 8:1; WHSV, 6.7 h-1.
12 h. Physical inspection revealed the color change of coked catalyst from black to white. The regenerated catalyst was reused for subsequent reactions. In the lower temperature range (378-388 K), however, catalyst deactivation was observed because of the conversion of isopropyl alcohol into coke precursors causing pore blockage. Hence, the deactivation function was considered in this range of temperatures, and the deactivation rate was computed as discussed in a later section. Effect of Temperature. Reactions were carried out at different temperatures but with same feed composition and space velocity. Both the catalyst activity and the product composition were found to change with changing reaction temperature. The influence of temperature on the catalytic performance is shown in Table 2. It is evident from this table that the activity of HBeta (in terms of conversion of isopropyl alcohol) increases with increasing temperature from 378 to 458 K. However, the selectivity to cumene did not show a particular trend and was found to be highest at 403 K and decreased at higher temperature (458 K) because of the formation of more-substituted aromatics. Effect of Mole Ratio. The effect of the benzene-toisopropyl alcohol mole ratio on the overall conversion of isopropyl alcohol and on the conversion of isopropyl alcohol to cumene was studied in the range of mole ratios from 1:1 to 10:1. Figure 2 shows that the conversion of isopropyl alcohol increases with an increase in this ratio. The conversion of isopropyl alcohol to cumene was found to be low, as the temperature of reaction favored conversion of isopropyl alcohol to coke (as mentioned earlier). The cumene selectivity, however, was found to be high and to vary from 27 to 75% as the feed composition varied from 1 to 10 mol of benzene/ mol of isopropyl alcohol. This conforms with the findings of Pradhan et al.9 Mass Transfer Considerations. For any kinetic study, it is important that the mass transfer resistances be negligible during the reaction. To estimate the external diffusional effects, experiments were carried
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Figure 2. Effect of benzene-to-isopropyl alcohol (IPA) mole ratio on IPA conversion. Conditions: temperature, 383 K; WHSV, 6.7 h-1 Table 3. Effect of External Mass Transfer Resistance on Conversion of Isopropanola IPA conversion (mol %) b c
W/FAo [kg h/(kg mol)} 57.76 72.87 100.73
7.57 10.63 13.74
7.52 10.50 13.72
a Conditions: temperature, 383 K; benzene-to-isopropyl alcohol mole ratio, 8:1; catalyst size, 1.5 mm. b With 0.002 kg of catalyst. c With 0.004 kg of catalyst
Table 4. Effect of Intraparticle Diffusion on Conversion of Isopropanola particle size (mm) 0.42 1.50
conversion (%) of IPA with W/FAo [kg h/(kg mol)] of 57.76 72.87 100.73 7.62 7.57
10.80 10.63
13.87 13.74
a Conditions: temperature, 383 K; benzene-to-isopropyl alcohol mole ratio, 8:1.
out at constant space time (W/FAo) and catalyst size, but with varying feed rates. The results are shown in Table 3, which indicates the absence of external diffusional resistance in the liquid feed range studied. Experiments were also conducted to test the intraparticle diffusional limitations by varying the catalyst particle size, while keeping W/FAo constant. The experimental data obtained are presented in Table 4. The results show that there was no change in isopropyl alcohol (IPA) conversion with catalyst size indicating negligible intraparticle mass transfer resistance in the particle size range studied. The particle sizes employed in the kinetic study were within the intraparticle diffusion-free range. In zeolite-catalyzed reactions, two types of diffusion processes are involved: (a) micropore diffusion inside the zeolite crystal and (b) macropore diffusion between the catalyst pellets. The above experiments for mass transfer resistances confirm only the absence of diffusion in the macropores. The resistance due to micropores could not be evaluated, as it requires a modification of the synthesis conditions of the zeolite that affect the micropore size of the crystals, which would subsequently affect the diffusional characteristics. As the channel
Figure 3. Effect of space time on isopropyl alcohol conversion.
dimensions of Beta zeolite are comparable to the kinetic diameters of benzene, isopropyl alcohol, cumene, and diisopropyl benzenes, micropore diffusional effects cannot be ruled out. Hence, the kinetic parameters presented here include these diffusional effects, if any. Kinetic Model Kinetic runs were taken at three different temperatures, namely, 378, 383, and 388 K. At each temperature, the space time, W/FAo, was varied by changing the liquid feed rate. In all of these runs, the mole ratio of benzene to isopropyl alcohol was kept at 8, and the ratio of liquid hydrocarbon feed to carrier gas nitrogen was maintained at 0.5. The variation of isopropyl alcohol conversion with W/FAo is shown in Figure 3. It is evident from this figure that the isopropyl alcohol conversion increases with space time at all three temperatures. In accordance with the product distribution, the system can be described with the following reactions: Main reaction Benzene alkylation
k1
B + IPA 98 C + D
(1)
Secondary reactions (i) Cumene alkylation
k2
C + IPA 98 p-DIPB + D (2) k3 , K
(ii) p-DIPB isomerization p-DIPB S m-DIPB (iii) IPA dehydration
k4
2IPA 98 DIPE + D
(3) (4)
The ortho isomer of diisopropyl benzene could not be detected in the product and, hence, does not feature in the system of reactions. Also, cumene cracking has been excluded because it is negligible at the temperatures studied. In the kinetic model developed here, cumene alkylation to p-diisopropyl benzene (p-DIPB) has been considered to be an irreversible first-order reactions because of the small concentration of p-DIPB seen in the product distribution at various space times, as shown in Table 5. Thus, the equilibrium of this reaction has not been reached yet. A similar approach has been used by other investigators.16,17 As can be seen from Table 5, the ratio of m-DIPB/p-DIPB for large contact times
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Table 5. Variation of Product Distribution with Space Timea
Table 6. Variation of Kinetic Parameters with Temperature
W/FAo [kg h/(kg mol)] 100.73 72.87 63.42
product distribution (mol %) IPA aliphatics benzene cumene m-DIPB p-DIPB
9.40 0.92 89.28 0.23 0.13 0.05
9.90 0.75 89.01 0.21 0.09 0.05
10.26 0.79 88.72 0.14 0.05 0.04
a Conditions: temperature, 388 K; benzene-to-isopropyl alcohol mole ratio, 8:1.
are 2.6 and 1.8. The equilibrium ratio is usually reported as 2.0.18 This indicates that, at large contact times, the isomerization of p-DIPB to m-DIPB is reversible, as shown above. The rate of formation of different components can, therefore, be expressed as follows:
Rate of formation of cumene rC ) dXC/dτ ) k1 pApB
temperature (K) kinetic constants k1 [kg mol/(kg h atm2)] k2 [kg mol/(kg h atm2)] k3 [kg mol/(kg h atm)] K k4 [kg mol/(kg h atm2)] kd (h-1)
378
383
388
0.083 4.25 0.37 1.00054 0.002 0.105
0.142 5.75 0.425 1.00053 0.003 0.134
0.218 8.50 0.436 1.00052 0.005 0.148
Table 7. Apparent Activation Energies and Pre-exponential Factors for Different Reactions reaction benzene alkylation cumene alkylation to p-DIPB p-DIPB isomerization to m-DIPB IPA dehydration to ether deactivation reaction
Ea ko (kJ/mol) (in consistent units) 117.74 84.79 20.06 111.62 44.32
1.60 × 1015 2.22 × 1012 2.22 × 102 5.29 × 1012 1.40 × 105
(5)
Rate of formation of p-diisopropyl benzene rpDIPB ) dXp/dτ ) k2pCpA - k3(pp - pm/K)
(6)
Rate of formation of m-diisopropyl benzene rmDIPB ) dXm/dτ ) k3(pp - pm/K)
(7)
For the rate of disappearance of isopropyl alcohol several models, including the Langmuir-HinshelwoodHougen-Watson and Eley-Riedel models, were attempted to fit the kinetic data. The model with the following rate expression for disappearance of isopropyl alcohol was found to fit the data significantly better than the other models.
-rA ) dXA/dτ ) k1pApB + k2pApC + k4pA2
(8)
The partial pressures in the above equations are related to the fractional conversions and the total pressure P as indicated by the following expressions. The fractional conversions relate to the amounts of different products present at an isopropyl alcohol conversion of XA.
pA ) (1 - XA)P/27
(9)
pB ) (8 - XB)P/27
(10)
pC ) (XC)P/27
(11)
pp ) (Xp)P/27
(12)
pm ) (Xm)P/27
(13)
A nonlinear regression algorithm19 was used for parameter estimation, and the kinetic constants obtained at three different temperatures are presented in Table 6. The optimum values of various constants were estimated by minimizing the objective function n
φ)
[(Xpred)i - (Xexptl)i]2 ∑ i)1
(14)
where the index i indicates the key species considered (benzene, isopropyl alcohol, or cumene) and Xpred and Xexptl are the predicted and experimental fractional
Figure 4. Experimental versus predicted conversions of isopropyl alcohol.
conversions, respectively. Figure 4 shows the validity of the fit of the proposed model. It is evident from this figure that the predicted conversions agree well with the experimental ones. Values of the preexponential factors and apparent activation energies obtained by the Arrhenius relationship are reported in Table 7. The activation energies of the benzene and cumene alkylation reactions, as well as the isopropyl alcohol dehydration and diisopropyl benzene isomerization reactions, compare well with the values of similar reactions on zeolites obtained by other investigators.20 Gentry and Rudham21 reported an activation energy of 109 kJ/mol in the temperature range of 383-419 K and 110 kJ/ mol in the temperature range of 384-425 K for the dehydration of isopropyl alcohol on X zeolites. Similar observations were made by Yue and Olaofe22 on 13X and mordenite and by Rudham and Stockwell23 on Y zeolite for isopropyl alcohol dehydration. At low temperatures such as those chosen in our study, the reaction presumably occurs in the kinetic regime where micropore diffusional effects are at a minimum. Hence, the apparent activation energies are expectedly high in view of the known high intrinsic activation energy of the order of 146 kJ/mol for alkylation reactions. The values lower than this observed in
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5; the slopes of the lines represent the deactivation constant kd at the temperature under consideration. The values of kd are shown along with the kinetic constants of the main reaction in Table 6. The apparent activation energy and preexponential constant have been calculated and are shown in Table 7. The activation energy for the deactivation reaction is found to be much lower than that for the main reaction. This is because the coke formed in the lower temperature range is almost completely olefinic in nature9 and hence is formed faster so that the catalyst is more susceptible to pore blockage because of this coking. Conclusions
Figure 5. Time on stream behavior of HBeta during deactivation.
Table 7 are due to the higher heats of adsorption of aromatics on zeolites. Kinetics and Deactivation Zeolites are known to be deactivated by the coking phenomenon wherein the active sites are covered and the pores of the zeolite are blocked by coke. In the present study, no significant deactivation was observed at higher temperatures, even after 60 h time on stream, as shown in Figure 1. However, at the temperatures chosen for our kinetic study to obtain low conversions of isopropyl alcohol, the catalyst was observed to become deactivated with time on stream. Hence, it is necessary to look into the rate of deactivation in this range of temperatures. Moreover, it was confirmed by X-ray diffraction of the catalyst after the time-on-stream runs that there were no structural changes in the catalyst indicating its good thermal stability. The deactivation phenomenon can be represented by
(-rA)t ) (-rA)0a
(15)
where the activity a is defined as the ratio of the rate of reaction at any time t to that at time t ) 0. The reaction scheme for deactivation can be expressed as a parallel-type mechanism represented by
isopropyl alcohol + benzene f propyl benzenes (16) isopropyl alcohol + benzene f coke
(17)
For a differential plug-flow reactor, the input concentration is nearly the same as the output concentration. Moreover, the product concentration also remains constant for a specific feed rate. The parallel deactivation, therefore, reduces to independent deactivation, and we have the performance equation for a constant flow of feed as given by Levenspiel24
ln[ln(CA0/CA)] ) ln(kτ′) - kdt
(18)
where k represents the rate constant of the main reaction and τ′ is the weight time. To study the kinetics of deactivation, time-on-stream data were recorded at three different temperatures and concentrations at a fixed contact time. A plot of ln[ln(CA0/ CA)] vs t at the three temperatures is shown in Figure
The H form of Beta zeolite showed a good activity and selectivity for cumene synthesis by the alkylation of benzene with isopropyl alcohol. Significantly, no formation of n-propyl benzene was observed in the reaction products. This highly undesirable byproduct was known to form in the medium-pore zeolites. A time-on-stream study of the catalyst at temperatures of 473 K and above showed that the catalyst was quiet stable. The catalyst activity could be restored to the original level even after several regenerations. However, in the lower temperature range, deactivation occurred through the formation of olefinic coke, leading to pore blockage of the catalyst. The activation energy of this deactivation reaction was observed to be lower than that of the main reaction. A simple stoichiometric model was found to fit the data significantly better than the other models tested. Acknowledgment One of the authors, U. Sridevi, is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for a Senior Research Fellowship during the tenure of this work. Nomenclature A ) isopropyl alcohol B ) benzene C ) cumene CA0 ) concentration of isopropyl alcohol in the feed, kg mol/ m3 CA ) concentration of isopropyl alcohol, kg mol/m3 D ) water DIPB ) diisopropyl benzene DIPE ) diisopropyl ether Ea ) apparent activation energy, kJ/mol FAo ) feed flow rate of isopropyl alcohol, kg mol/h IPA ) isopropyl alcohol k1, k2, k4 ) kinetic constants, kg mol/(kg h atm2) k3 ) forward rate constant of reaction 3, kg mol/(kg h atm2) kd ) deactivation reaction rate constant, h-1 ko ) frequency factor, kg mol/(kg h atm2) K ) equilibrium constant for reaction 3 pi ) partial pressure of species i, atm r ) rate of reaction, kg mol/(kg of cat h) t ) time, h T ) reaction temperature, K W ) catalyst weight, kg WHSV ) weight hourly space velocity, h-1 W/FAo ) space time, kg h/(kg mol) referred to the isopropyl alcohol feed rate Xi ) fractional conversion of species i
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Subscripts d ) deactivation exptl ) experimental pred ) predicted m ) meta p ) para Greek Letters φ ) objective function defined by eq 14 τ ) space time, kg h/(kg mol) τ′ ) weight time, kg h/m3(CAoW/FAo)
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Received for review October 30, 2000 Revised manuscript received March 27, 2001 Accepted April 11, 2001 IE000929S
