Ind. Eng. Chem. Process Des. Dev. 1904, 23,335-337
335
Kinetics of Liquid-Phase Reduction of 2,4-Dimethylnitrobenzene to 2,4-Dimethylaniline by Hydrogen with Pd/C as Catalyst Oemer M. Kut, Thomas Buehlmann, Franr Mayer, and Guenther Gut' Department of Chemical Engineering and Industrial Chemistry. Swlss Federal Institute of Technology (ETH), CH-8092 Zurich, Swkerknd
The kinetics of the liquid-phase hydrogenation of 2,4dimethylnitrobenzene to 2,4dimethylaniline was studied in the absence of solvent with 5 % Pd/C as a catalyst in the temperature and pressure ranges of 40 to 80 "C and 6 to 100 bar, respectively. The intrinsic kinetics can be described by a Langmuir-Hinshelwood model with noncompetitive adsorption of organic species and hydrogen on the catalyst surface. The extent of internal and external mass transfer effects was estimated and found not to appreciably Influence the overall kinetics.
Introduction
Various aromatic amines are industrially produced by the catalytic reduction of the corresponding nitroaromatics in the liquid phase. Such amines are of interest as intermediates in the production of fine chemicals and pharmaceuticals. Data concerning the hydrogenation kinetics of several nitroaromatics with different catalysts and solvents have been published. Thus Yao and Emmett (1959) found that with colloidal palladium as a catalyst the reaction order with respect to nitro compound depends on the nature of the solvent, and that the hydrogenation rate increases proportionally to the hydrogen partial pressure up to 1 bar. Acres and Cooper (1972) studied the reduction of dinitrotoluene by using various palIadium/charcoal catalysta with special emphasis on mass transfer effects. The rate of hydrogenation was found to be independent of the nitro compound concentration up to high conversion. In the pressure range of 1 to 6 bar, the rate was first order in hydrogen pressure and tended to zero order at higher pressure levels, where the data showed a large scattering. The observed apparent activation energy E, was found to be 44 kJ/mol in the kinetic regime. A similar pressure dependence of reaction rate was also observed by Sokols'kii et al. (1977) in the hydrogenation of nitrobenzene on palladium black. The hydrogenation of nitrobenzoic acid on Pd/C in water also showed zero-order dependence with respect to the nitro compound and first order in hydrogen concentration up to 5 bar; the activation energy was found to be 36 kJ/mol (Andersson, 1982). In the hydrogenation of o-nitroaniline in methanol with Pd/C at 69 "C and 7.5 to 20 bar, Kalantri and Chandalla (1982) found that the reaction rate was independent of the hydrogen pressure; the activation energy was 54.4 kJ/mol. Until now only data concerning the hydrogenation of nitroaromatics in solution have been published. Since kinetic data without solvent effects, which may be used to design a continuous reactor, are not available, the objective of this study was to evaluate such intrinsic kinetic data over an appropriate range of temperature and pressure, respectively. E x p e r i m e n t a l Section
Technical grades of 2,4-dimethylnitrobenzene (over 99.6% purity, Schweiz. Sprengstoff-FabrikAG, Dottikon, Switzerland) and hydrogen (over 99.999% purity) were used without further treatment. A fine powdered commercial palladium catalyst (5% Pd/C, (d&= 4.0 pm, < 15.2 pm, Engelhard Co.) was employed. 0196-4305/84/ 1123-0335$01.50/0
The experiments were performed in a 6.3 cm (id.) 500-mL stainless steel autoclave without baffles, equipped with a magnetic driven 6-bladed hollow shaft turbine type agitator (Autoclave Engineering) and an external heating/cooling system. In each run 250 g of 2,4-dimethylnitrobenzene was hydrogenated under constant temperature and pressure. The speed of agitation was maintained over 2000 rpm. During the course of the reaction, samples were withdrawn at appropriate intervals and analyzed with a flame ionization gas chromatograph (Pye Unicam), equipped with 2.1-m glass column, packed with OV 225 on Chromosorb at 140 "C. 2,4-Dimethylaniline was the only product detected in appreciable amounts. Various samples were also analyzed by thin-layer chromatography on silica gel with benzene as solvent, but no other components could be found. Results a n d Discussion M o d e l i n g of t h e Reaction. The kinetic runs were
performed in the temperature and pressure ranges of 40 to 80 "C and 6 to 100 bar, respectively. To avoid runaway conditions, the catalyst loading was chosen as 0.1 to 0.4 w/w %o. In Figure 1 some typical conversion plots are presented. It can be seen that under constant hydrogen pressure the reaction shows zero-order behavior up to high conversions. The slight tailing of the curves at high conversions can be explained by the competitive adsorption of the product on the catalyst surface. In most of the experimental runs, an induction period was observed. Similar observations were made with the slurry-phase hydrogenation of nitrobenzene by using 3% Pd/SiOPA1,0, (Jana et al., 1978). This induction period may be due to time taken to establish a steady-state among the different consecutivesteps of the reaction. It may also be attributed to the partial oxidation of the catalyst in storage. However, experiments with catalyst samples pretreated with hydrogen at 60 "C and 40 bar overnight also showed an induction period. The attempts to correlate the length of this induction period with the main operational parameters such as temperature, pressure, or amount of catalyst were unsuccessful. Beside this, at low pressure the conversion plots showed a slight curvature, which may be caused by the decreasing hydrogen solubility in the system with increasing conversion: solubility at 25 "C and 1 bar in nitrobenzene cHi = 1.52 x mol/L; in aniline c H ~= 1.17 X mol/L (Just, 1901). In Figure 2 the reaction rates are plotted against the amount of catalyst adjusted for a pressure of 31 bar. As expected from absorption rate 0 1984 American Chemical Society
336
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
Table I. Kinetic Model Parametem a t 7 0 "C k = 3.35 x lo-) molIL s wlw oleo K H I H = 0.082 bar-' E. = 40.7 kJ/mol mk0 = 0.03 Wlw oleo Q = 0.02
CA lcno
Table 11. Mans % 3 n S D O r t Limitations
T,
"C t lmini
Figure 1. Conversion/time curves at 80 OC and 31 bar on Pd/C for different amounts of catalyst. ra/cao ~ m i n - ' ~ 0.015
0 010
TI: 80°C
1
mK,
XA =
I
200
LOO
mK I 103 w/w %o I
Figure 2. Hydrogenation rate as a function of catalyst loading. Experimental data adjusted with model equation to 31 bar. i-hJ/mK lmol/L min
w/w %o J t
LO
20
CH~
0.81 0.93 0.88 0.89 0.94 0.96
~I)H~
0.89 0.98 0.92 0.93 0.97 0.99
')Hi
CAK 1 cALa
')Ai
0.96 0.99 0.98 0.98 0.99 1.00
0.95 0.92 0.98 0.96 0.96 0.94
0.97 0.94 0.99 0.99 0.99 1.00
0.99.
where k = ko exp(-E,/RT) and Q = In this rate equation, m~~ stands for the amount of poisoned catalyst. The numerical results of the nonlinear regression calculations based on this model are given in Table I. By use of an Arrhenius type expression, the apparent temperature dependence of the term (KH/H)was found to be very small (lo-' kJ/mol) and statistically insignificant. For further calculations, this term was assumed to be temperature independent. The relative adsorptivity Q has a small numerical value, indicating that the nitro compound is much stronger adsorbed on the surface than the corresponding amine. It was assumed that Q is also independent of the temperature. Mass Transfer Effects. In order to check whether the given model describes the intrinsic hydrogenation kinetics, the extent of the mass transfer effects under the experimental conditions was estimated in the usual manner (Satterfield, 1970; Zwicky and Gut, 1978; Gut and Buehlmann, 1981). The absorption characteristics of the hydrogenation apparatus were formerly studied with phenol as a substrate (Buehlmann, 1982; Buehlmann et al., 1982). It was found that for agitation rates above 1500 rpm the volumetric absorption coefficient kHLa has a numerical value of at least 1.0 s-l. To estimate the saturation concentration of hydrogen cHiin the substrate and product, data measured by Just (1901)for nitrobenzene and aniline at 25 "C and 1bar were extrapolated to the appropriate temperature and pressure conditions, with a heat of solution of 10 kJ/mol and assuming that Henry's law is valid in the whole pressure range. Some typical results of the mass transfer calculations are given in Table 11. It is seen from Table I1 (column 5) that at high temperature and low pressure conditions there is a small hydrogen concentration gradient (cm/cHi) at the gas-liquid interphase, and that consequently the external effectiveness factor for hydrogen ?He is diminished. Although the amount of catalyst used is very small (mK= 0.1-0.4w/w Ym),there is no hydrogen gradient to be expected at the liquid-solid interphase. This is caused by the very small mean particle diameter (dp = 4 X lo* m) of the catalyst employed for the hydrogenation. The external mass transfer of the nitro compound to the catalyst (Table 11, column 7) can also be neglected because of the high concentrations used. A small decrease of the internal effectiveness factor for the nitro compound 7 A i
rs::i
6r
0
CHLI
atoms on which hydrogen dissociates (Carberry, 1976; Gut, 1982)
n U
2
80 80 60 60 60 60
P,
bar wlw Q / o ~ 10 0.32 31 0.20 6 0.36 10 0.36 16 0.28 31 0.28
LO
60
80
r00 120 PH [bar]
Figure 3. Hydrogenation rate as a function of hydrogen pressure. Curves calculated with model equation.
calculations, the rates increase linearly with increasing catalyst loading, but the observed data revealed that the substrate incorporated a small amount of an impurity, which inactivates a certain part of the catalyst employed (Gut et al., 1975). The data of Figure 3 indicate that the reaction order with respect to hydrogen is a function of pressure. At low pressure conditions it is first order, and in the high-pressure region the rate approaches zero order. This confirms the earlier observation of Sokol'skaya (1975). In our case the observed pressure dependence of the reaction rate implies that in the kinetically controlled regime no useful purpose will be served in practice if pressures higher than that of 40 bar are used at 40 to 60 "C. All these observations can be described by using a Langmuir-Hinshelwood type kinetic model, assuming noncompetitive adsorption of organic species and hydrogen (Zwicky and Gut, 1978). The model equation (1)is based on the assumption that although the catalyst's surface is completely covered by chemisorbed organic compounds, there are still enough sites available for the adsorption of the much smaller hydrogen molecules. Furthermore, it is assumed that an active site consista of two adjacent metal
Ind. Eng. Chem. Process Des. Dev. 1084, 23,337-341
(column 8) is to be expected at conversion levels higher than 97% only for the fastest reactions measured. During the reaction the decrease of the hydrogen diffusion coefficient DHand saturation concentration cHiwith changing liquid composition does not affect the extent of mass transfer effects on global kinetics significantly. From mass transfer calculations it may be concluded that most of the experiments were'not influenced by mass transfer effects. Only the runs at higher temperatures and low pressures may be affected to some extent by absorption limitations. However, the apparent activation energies at different pressure levels do not show a tendency to decrease with decreasing pressure. Nomenclature
ci = concentration of component i, mol/L cHi = saturation concentration of hydrogen, mol/L Di = diffusivity of component i, m2/s d = particle diameter, m $ = activation energy, kJ/mol H = Henry's constant, (L bar)/mol Ki = adsorption constant of component i, L/mol k = rate constant, mol/(L s w/w o/w) kHLa = volumetric absorption coefficient, s-l m K = amount of catlayst used, w/w k mKO= amount of catalyst poisoned, w/w L p H = hydrogen pressure, bar Q = KB/KA = relative adsorptivity R = gas constant, J/(mol K) -PA = reaction rate, mol/(L s)
XA
337
= conversion of component A
vie = external effectiveness factor for component i vii
= internal effectiveness factor for component i
Subscripts A = 2,4-dimethylnitrobenzene B = 2,4-dimethylaniline H = hydrogen K = catalyst L = liquid phase Registry No. 2,4-Dimethylnitrobenzene, 89-87-2; 2,4-dimethylaniline, 95-68-1;Pd, 7440-05-3. Literature Cited Acres, G. J. K.; Cooper, B. J. J . Appl. Chem. Biotechnol. 1972, 22, 769. Andersson, B. AIChE J . 1982, 2 8 , 333. Buehlmann, Th. PhD. Thesis No. 7115, ETH Zurich, 1982. Buehlmann, Th.; Gut, G.; Kut. 0. M. Chlmle 1982, 36, 469. Carberry, J. J. "Chemical and Catalytic Reaction Engineering";McGraw-HIII: New York, 1976; p 386. Gut, G.; Meier, R. U., Zwicky, J. J.; Kut. 0. M. Chimle 1975, 2 9 , 295. Gut, G.; Buehlmann, Th. Chlmle 1981, 35, 64. Gut, G. Swiss Chem. 1982, 4 / 3 a , 17. Jana, D.; Malti, M. M.; Avasthl, 8. N.; Pallt, S . K. "Proceedings, 4th National Symposium on Catalysis, 1970", 1980; p 140. Just, G. 2.Phys. Chem. 1901, 37, 342. Kalantrl, P. B.; Chandalla, S. B. Ind. Eng. Chem. Process Des. Dev. 1982, 2 1 , 108. Satterfield, C. N. "Mass Transfer In Heterogeneous Catalysis"; Cambridge University Press: Cambridge, MA, 1970. Sokol'skaya. A. M. Russ. J . Phys. Chem. 1975, 49, 246. Sokol'skii, d. v.; Omarkulov, T. 0.; Dzharikbaev, T. K. DIM. Akad. Nauk SSSR 1977, 232, 1359. Yao, H. C.; Emmett, P. H. J . Am. Chem. SOC. 1959, 81, 4125. Zwlcky, J. J.; Gut, 0. Chem. Eng. Sci. 1978, 33, 1363.
Receiued for reuiew December 27, 1982 Accepted June 10, 1983
T = temperature, K, "C t = time s, min
Lateral Mixing of Solids in Batch Gas-Solids Fluidized Beds Yan-fu Shlt and L. T. Fan' Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506
The lateral dispersion coefficients of particles, D,, were measured in a rectangular gas-solids fluidized bed and
the effects of various factors on D, were studied. Based on the results, an expression has been derived to correlate D, as a function of the particle characteristics, properties of the fluidizing medium, and operating conditions.
Table I. Properties of Solid Particles
Introduction
Lateral mixing of solids in gas-solids fluidized beds influences the performance of physical and chemical processes carried out in them, e.g., thermal decomposition and drying of particulate matter and combustion of coal (see, e.g., Fan et al., 1979; Fan and Chang, 1981; Chang et al., 1982). According to Grace (1981), knowledge of such lateral or radial mixing can be more important than that of axial mixing in assessing the performance of a gas-solids fluidized-bed processing unit. Thus, the subject of lateral mixing of solids in fluidized beds has attracted the interest of a substantial number of researchers (Brotz, 1956 Gabor, 1964; Mori and Nakamura, 1965; Highley and Merrick, 'On leave from Chengdu University of Science and Technology,
Chengdu, China.
material silica gel sand
d,
mm 1.125 0.491
Umf I
PS,
g/cm3 2.61 2.62
Emf
0.387 0.446
cmls 61.71 20
1971; Hirama et al., 19751, and some experimentally measured lateral dispersion coefficients have been reported; however, the available data are far from sufficient. On the basis of the so-called bubble model, Kunii and Levenspiel (1969) have proposed an equation for predicting the lateral dispersion coefficient, D,,, but it appears that the equation has not been thoroughly validated experimentally (Hirama et al., 1975). The purposes of this work were to determine experimentally the lateral dispersion coefficientsof particles, D,,, in a batch gas-solids fluidized bed, to interpret the data
0196-4305/84/1 l23-0337$Q1.50/0 0 1984 American
Chemical Society
