Ind. Eng. Chem. Res. 2004, 43, 4105-4111
4105
Alkylation of Benzene with Ethylene in a Packed Reactive Distillation Column Zhiwen Qi* and Ruisheng Zhang UNILAB Research Center of Chemical Reaction Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
The alkylation of benzene with ethylene suffers from heavy deactivation of the catalyst. However, this problem can be solved by reactive distillation, as demonstrated through experiments guided by modeling work. The simulations predict a very low liquid composition of ethylene in most of the catalyst bed. The experiments explore a low feasible molar feed ratio of benzene to ethylene, i.e., 1.5-2.0, at which ethylene can be fully converted, and the selectivity toward the desired product is higher than 99.7%. After 408 h of use, the deactivation of the catalyst is slight except for the upper part to a lesser extent. Therefore, a long life for the catalyst can be expected. One additional advantage is that the byproducts toluene and xylene are not detected in this work, which is favorable for the downstream separation process. Introduction Ethyl benzene (EB) is an important intermediate for styrene synthesis. In industry, EB is mainly manufactured by the alkylation of benzene with ethylene via two methods, i.e., the gas-phase method and the liquidphase method. The recently developed gas-phase method is the Mobil-Badger technology1 over the catalyst molecular ZSM-5. The gas-phase method employs moderate pressure (1.0-20.8 MPa) and high temperature (573-773 K), which leads to higher energy consumption and strict requirements for the apparatus. Moreover, this method suffers from several disadvantages. For example, more byproducts are produced, especially toluene at about 1000-2000 ppm, which is much higher than the levels required by the downstream processes; the selectivity toward ethyl benzene is low, and the deactivation of the catalyst is so serious that it requires periodic regeneration. The traditional liquid-phase methods are performed over catalyst AlCl3. The Unocal-Lummus-UOP technology2 over a solid acid catalyst containing Y-zeolite is the most developed one that can improve the defects of the gas-phase method. The temperature is lower (250 K at 1.8 MPa), ethylene has very strong potential to concentrate in the reactive section. However, the fast reaction of ethylene with benzene consumes much of the ethylene as it is being concentrated along the reactive zone toward the top of column. Moreover, because benzene is lighter than the alkylated benzenes (difference of boiling points > 75 K at 1.8 MPa), the latter will be continuously removed toward the bottom of the column during the course of the reaction through the simultaneous distillation. As a result, the liquid phase in the reactive zone contains mainly benzene (molar fraction of about 0.9), and the local ratio of benzene to ethylene is higher than 50, which is much higher than the feed ratio. Consequently, the further consecutive polyalkylation and transalkylation reactions can be greatly depressed such that the concentrations of the polyalkylated benzenes can be maintained at a very low level, which is favorable not only for a high selectivity toward the desired product but also for suppressing the deactivation of the catalyst. In addition, by reactive distillation, the heat released by the reaction is continuously removed by liquid-phase evaporation during the reaction process, which results in a much better temperature profile of the reactor for control, as shown in Figure 2b.
4108 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 3. Influence of the molar flow rate ratio of the reflux to the benzene feed (rf) on the liquid composition of ethylene. P ) 1.8 MPa, B/E ) 3.9.
Figure 5. Experimental packed reactive distillation column for alkylation of benzene with ethylene. Table 2. Comparison of Simulated and Experimental Results: Temperature Profile (K) Figure 4. Influence of the feed B/E ratio on the liquid composition of ethylene. P ) 1.8 MPa, rf ) 10.
Figure 3 illustrates the influence of the molar flow rate ratio of the reflux to the benzene feed, rf, on the ethylene composition profile. In general, a higher rf value is favorable for achieving a lower liquid composition of ethylene in the reactive section. Because more benzene is refluxed inside the column at higher rf, much of the concentrated ethylene can be consumed. Consequently, the composition of ethylene in the reactive zone does not change dramatically with respect to rf. This feature implies that the column can be operated at a relatively low rf value, e.g., 10 in the later Experiment section. The influence of the feed B/E ratio is predicted in Figure 4. At relatively high feed B/E ratios (2-6), decreasing the B/E ratio essentially does not change the ethylene profile. However, at low values of B/E (1.5) could be applied. In the next section, an experiment is
1.7 2.1 3.9 5.7
stripping section bottom
reactive sections
B/E (mol/mol) simu exp simu exp simu exp simu exp
T1
T2
T3
T4
T5
T6
T7
196.4 204.7 202.8 208.0 206.9 209.7 210.7 214.2
211.4 216.0 213.2 216.0 215.4 216.2 214.8 216.1
214.8 217.1 214.6 216.2 216.3 216.4 215.2 216.3
217.4 218.2 215.5 217.3 216.9 216.4 216.1 216.4
219.8 218.6 216.4 217.6 217.5 216.6 216.9 216.4
221.5 219.1 219.6 218.2 218.3 217.1 217.5 216.9
252.3 251.8 230.9 229.4 225.6 225.1 219.2 218.6
Table 3. Comparison of Simulated and Experimental Results: Composition of Bottom (mol %) B/E (mol/mol) 1.7 2.1 3.9 5.7
benzene simu exp
ethyl benzene simu exp
48.59 55.82 76.71 83.27
48.23 41.85 21.25 15.81
47.34 54.98 75.98 82.99
48.88 43.24 22.61 16.37
diethyl benzene simu exp 3.17 2.33 2.04 0.93
3.08 1.52 1.25 0.58
organized to explore the influence of the feed B/E ratio on the product quality and on the deactivation of the catalyst. Experiment The packed reactive distillation column for benzene alkylation is illustrated in Figure 5. The column includes two parts: the reactive section at the upper part and the nonreactive section at the lower part. The outer diameter of the column is 50 mm. The catalyst, i.e., 48 g of cylinder-type Y-zeolite offered by the Institute of Petroleum Processing Shanghai, is averagely distributed in four segments of high effective packing16 with
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4109 Table 4. Experimental Results: Product Compositiona B/E (mol/mol)
B
EB
IPB
1.7 2.1 3.9 5.7
47.34 54.98 75.98 82.99
48.88 43.24 22.61 16.37
0.006 -
composition in bottom (mol %) MPB DEB DMEB TEB
4EB
EBP
TMA
selectivity (%)
0.020 0.005 0.003 0.002
0.203 0.062 0.045 0.009
0.011 0.010 0.009 0.009
0.017 0.003 -
99.5 99.8 99.8 99.9
3.08 1.52 1.25 0.58
0.034 0.009 -
0.412 0.163 0.105 0.046
a IPB ) isopropyl benzene, MPB ) methyl propyl benzene, DMPB ) dimethyl propyl benzene, 4EB ) tetraethyl benzene, EBP ) 3-ethylebp biphenyl, TMA ) R,R,γ-trimethyl anthracene.
a total height of 1200 mm. In the nonreactive section, φ-type stainless Cannon packing (φ ) 3 mm, height ) 400 mm) is installed. Benzene (supplied by Shanghai Feida Chemicals and purified before application to guarantee a purity higher than 99.95 wt %) is pumped into the column at the top of the catalyst part after being preheated. Ethylene (supplied by Shanghai Institute of Chemical Technology with a purity higher than 99.95 wt %) is introduced at the bottom of the reactive section. Both the gas and the liquid flow rates are measured through mass flow gauges (Brooks 5950 TR). Two in-house-manufactured ring-type distributors are applied for good distribution of the feeds in the catalytic packing. It is worth noting that the difference of the boiling temperature between ethylene and other components is very large, e.g., 183.5 K at the normal atmosphere and 251.2 K at the investigated pressure compared with benzene. Therefore, very slight change of the ethylene composition in the liquid phase will leads to an obvious variation of the boiling point of the liquid mixture. In this work, the concentration of ethylene inside the column is not measured because of its strong evaporation and the high system pressure. Instead, the compositions, especially ethylene, in the liquid phase are estimated by simulation compared with the measured temperature. For this purpose, several thermocouples are installed at important positions of the column (T1T7 in Figure 5). The column, including the condenser and reboiler, is covered with heat-insulating material. Before the reaction experiments, the overall heat effectiveness of the column (i.e., the ratio of the heat-exchange duty of the condenser to that of the reboiler) is measured, which is about 0.84 at the investigated conditions. During the experiment, the flow rate of the cooling agent for the condenser is determined to maintain the molar flow rate ratio rf at about 10. The system pressure is measured and controlled through a pressure controller with the aid of an electric heater wrapping the reboiler. The samples of bottom product are analyzed by gas chromatograph and mass spectrograph GC-MS (TurboMass, PE). Because diethyl benzene and triethyl benzene can be transalkylated to ethyl benzene,15 EB, DEB, and TEB together are taken as the desired product. The selectivity is defined as
β)
xEB + xDEB + xTEB 1 - xB
(14)
In the experiments, the feasibility of prolonging the catalyst life and product quality at different feed B/E ratios is studied. The extent of catalyst deactivation is characterized by the content of carbon in the catalyst. The experiments are carried out at different feed ratios of benzene to ethylene (B/E ) 5.7 f 3.9 f 2.1 f 1.7) and at a constant pressure of 1.8 MPa. Before the
reaction experiment, nitrogen is first introduced to exhaust the air inside the column. Then, 800 cm3 of benzene is pumped into the column. During the heating to reach the given system pressure, the inert gas (nitrogen) is expelled by adjusting the valve over the condenser. After the system reaches steady state for 2 h, benzene is fed into the column at the specified flow rate and ethylene is regulated to the column at an amplitude of 0.8 cm3/s until the assigned flow rate is reached. The product is vented from the reboiler and is cooled. No any distillate is withdrawn. The experiments are operated continuously with a total time of 408 h. During the whole experiment, the flow rate of benzene is fixed at 1.55 × 10-3 mol/s, and the flow rate of ethylene is regulated correspondingly. For each of the first three runs (B/E ) 5.7, 3.9, 2.1), it takes about 8-12 h to reach a steady state. After operating at steady state for 12 h, the flow rate of ethylene is regulated for the next run. For the last run (i.e., B/E ) 1.7), the operating time is 330 h. Experimental results and discussions Experimental Results. By using GC-MS, the experimental products are analyzed, and the structures of the compounds are determined. Before the installation of the catalyst into the packing, a preliminary experiment is carried out by introducing benzene and ethylene into the column. No product can be detected. For B/E ) 1.7-5.7 and P ) 1.8 MPa, the experimental results are shown in Table 4. Ethylene is fully converted, and the selectivity toward EB, DEB, and TEB is higher than 99.5%. The principal alkylated benzene product is EB. In addition, there are other polyalkylated products including DEB, TEB, tetraethyl benzene, and 3-ethyl diphenyl. At lower feed B/E ratios, the composition of byproducts increases, and even more byproducts are produced. For example, at B/E ) 1.7, dimethyl ethyl benzene, isopropyl benzene, and the multi-ring substance R,R,γ-trimethyl anthracene are found, the latter two of which are not detected at B/E ) 2.1. In other words, when the feed B/E ratio is too low, more highboiling alkylated benzene compounds are formed, which is harmful to the catalyst. It is worth noting that the byproducts toluene and xylene, normally produced in other reactors, cannot be detected in this work. Deactivation of the Catalyst. One of most important features of the alkylation of benzene with ethylene is the deactivation of the catalyst, which leads to practical problems in industry. After running for 408 h in reactive distillation, the apparent color of most catalyst (80%, at the lower part of the reactive section) changes slightly compared to that of fresh catalyst. Only a minor part (20%, upper part of the reactive section) shows some color change. As studied by Gao,10 the extent of catalyst deactivation for this reaction system can be characterized principally by the content of carbon
4110 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 5. Relationship between Catalyst Activity and Carbon Content10 carbon content (wt %)
catalyst activity
5.60 9.63 10.43
0.99 0.91 0.83
Table 6. Element Analysis of Carbon in Catalyst reaction conditionsa fresh catalyst under benzene at 523.15 K and 6.5 MPa for 8 h this work, about 80% catalyst for 408 h this work, about 20% catalyst for 408 h supercritical reaction for 60 h liquid reaction for 14 h under ethylene at 523.15 K and 6.5 MPa for 10 min vapor reaction for 5 h a
C (wt %) ref 0.00 1.29 1.77 5.63 6.81 10.37 14.40 15.90
10 10 10 10 10
The catalysts for all experiments are the same.
Figure 6. Carbon content of the catalyst along the catalyst bed.
inside the catalyst. The relationship between the catalyst activity and the carbon content is given in Table 5. The catalyst samples at different positions are analyzed and compared with other kinds of reaction process, as shown in Table 6. The profile of carbon content in the reactive section is illustrated in Figure 6. As can be seen from Table 6, the activity of the catalyst is not particularly sensitive to benzene (carbon content of 1.29 wt % at pure benzene for 8 h) but is very sensitive to ethylene (carbon content of 14.40 wt % at pure ethylene for only 10 min). Compared to other reaction processes, the extent of the catalyst deactivation in reactive distillation is slight. For most of the catalyst, after 408 h, the carbon content is only slightly higher than the case under pure benzene. For the most deactivated catalyst in this work, the mean carbon content is only 5.63 wt % and the activity is about 99 wt %, which are much better than the results obtained with other reaction processes, especially the liquidphase-based reactor using the same catalyst. The expected life of the catalyst in reactive distillation can be longer than 1 year according to the deactivation rate. Discussion. From the above experimental and simulation results, the technology of reactive distillation provides several benefits for the alkylation of benzene with ethylene. First, the feasible feed ratio of benzene to ethylene is lower than that required by the liquidphase-based reactor (normally 4-10), and ethylene is fully converted. The benefit from this low feed B/E ratio can reduce the subsequent separation and recovery
problems and can provide great savings in investment and operating costs. Second, because the boiling points of xylene and ethyl benzene are close (2.16-8.22 K of normal boiling points), they are difficult to separate to obtain high-purity ethyl benzene through traditional distillation. In the reactive distillation process, xylene and toluene are not produced, which can improve the product quality and simplify the downstream separation processes. As mentioned in the Introduction, the main problem in the alkylation of benzene with ethylene in the liquid phase is the serious deactivation of the catalyst, the mechanism of which was presented by Gao.10 Because of the high liquid composition of ethylene, coke is formed by the polymerization of ethylene to high-boiling components (so-called precoking) and by the alkylation of ethyl benzene to polyalkylated benzene, further leading to a multi-ring substance. The former dominates the formation of coke, especially when the molar fraction of ethylene is higher than 0.1 in the liquid phase. The coke covers the active center on the catalyst surface and blocks the micropores inside the zeolite, which leads to catalyst deactivation. Therefore, the key to prolonging the catalyst life is to reduce the liquid composition of ethylene. In other words, an approach is strongly needed to keep a higher ratio of benzene to ethylene in the local liquid phase at a low feed B/E ratio. In the reactive distillation process, the composition profiles in the liquid phase are determined not only by the properties of components but also by the reaction kinetics. In addition, feasible operating conditions are helpful. As mentioned in the Process Simulation section, under the experimental conditions tested, the joint effect of concentrating ethylene toward the top of the column and consuming ethylene in the reactive section results in a low liquid composition of ethylene. The local molar ratio of benzene to ethylene on the catalyst surface is much higher than the feed ratio. Meanwhile, because the alkylated products are the higher-boiling components, they can be continuously removed from the reactive zone as soon as they are formed. The high local B/E ratio and low composition of alkylated benzenes in the reactive zone can significantly slow the formation of coke and hence prolong the life of the catalyst. The important advantage of reactive distillation is that the composition profiles in the liquid phase are not strongly dependent on the feed ratio. Therefore, the feed B/E ratio could be relatively lower. However, one should realize that at too low of a feed B/E ratio, e.g., close to the stoichiometric coefficient ratio of 1:1, the liquid composition of ethylene will increase dramatically. As a result, more byproducts will be formed, and the deactivation of the catalyst will become serious. Taking into account the deactivation of the catalyst and product quality, the recommended feed B/E ratio is about 1.52.0. Conclusions The alkylation of benzene with ethylene in a packed reactive distillation column is carried out by the aid of mathematical simulations. The influence of the molar feed ratio of benzene to ethylene is investigated. The deactivation of the catalyst is analyzed and compared to that observed in other reaction processes. The experiments demonstrate the feasibility of alkylation of benzene with ethylene in reactive distillation at low feed ratios of benzene to ethylene. The recom-
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4111
mended feed B/E ratio is 1.5-2.0. Ethylene can be fully converted, and the selectivity toward the desired product can be higher than 99.5%. No toluene and xylene are detected in this work. After a long running time, the catalyst deactivation is slight, and the expected life of the catalyst is much longer than in other reaction processes. Nomenclature a ) specific vapor-liquid interfacial area, m2/m3 Ac ) column cross section, m2 B/E ) molar ratio of feed flow rates of benzene to ethylene, mol/mol c ) molar density, mol/m3 C ) concentration, mol/m3 e ) energy flux through the vapor-liquid interface, kJ/ m2‚s H, h ) enthalpies for the vapor and liquid phases, respectively, kJ/mol K ) chemical equilibrium constant Ki ) distribution coefficient of species i ki ) mass transfer coefficient of species i in the vapor phase, m/s L, V ) flow rates for the liquid and vapor phases, respectively, mol/s ni ) molar flux of species i, mol/m2‚s NC ) number of components NR ) number of reactions P ) pressure, MPa r ) reaction rate, mol/s‚m3 rf ) molar flow rate ratio of the reflux to the benzene feed, mol/mol T ) temperature, K x, y ) molar compositions in the liquid and vapor phases, respectively z ) column coordinate, m vi,k ) stoichiometric coefficients of species i in reaction k Superscripts and Subscripts F, f ) feed I ) vapor-liquid interface i ) species i j ) stage j k ) reaction k L, V ) liquid and vapor phases, respectively Abbreviations DEB ) diethyl benzene
EB ) ethyl benzene TEB ) triethyl benzene
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Received for review December 1, 2003 Revised manuscript received March 26, 2004 Accepted May 20, 2004 IE0342816
