604
Ind. Eng. Chem. Process Des. Dev. 1981, 20, 604-608
Kinetics of Benzene Oxidation over a V,O, Catalyst Goniil Tufan and Aydln Akgerman' Chemical Engineering Depafiment, Ege University, Izmir, Turkey
+
+
+
The gas-phase oxidation of benzene over a V205 $205 MOO, Sb203catalyst supported on a kieselguhr carrier is studied at the temperature range 344-393 C. The catalyst is a selective one to partial oxidation to maleic anhydride. The reaction proceeds according to a two-step redox mechanism and the reaction rate is -RB = KIKnyo,ye/KIye
+ K2yO2mol of benzene/g
of catalyst h
where K , = 8.96 X lo' exp(-19200/RT) and K2 = 8.89 exp(-10 750/RT).
Introduction
Scheme I
The purpose of this study was to determine the kinetics of benzene oxidation over a V205catalyst impregnated with other oxides. Partial oxidation of benzene yields maleic anhydride (MA). Benzene oxidation, according to Hammar (Sampson and Shooter, 1965) proceeds as shown in Scheme I. The most important catalytic component in benzene oxidation is Vz05. A catalyst for this purpose is usually modified by addition of MOO, (Allen, 1963; Vanhove and Blanhard 1975). It is claimed that VzO5 catalyst with 33-5076 MOO, is the most suitable one for selectivity to MA and that kieselguhr is a better carrier compared to silica gel, alumina, and CaSO, (Butler and Weston, 1963; Sampson and Shooter, 1965). Ciquier and Carfanton (1974), obtained good selectivity to MA with a catalyst composed of 10-20% SbzO3, &70% Vz05, and a total of 10-50% Pz05,MOO,, and COOover an inert carrier at the temperature range 360-380 "C. An Indian patent (Rafi, 1971) proposes a catalyst composed of 72% SiOz, 10% MOO,, 15% VzO5 and the carrier being washed with an acid, preferably HC1. Another catalyst containing V2O5/MoO3/PzO5/Na20or KzO to be used at 370 "C is also specified (Suzaki et al., 1976). Benzene oxidation to MA or CO and C02 is usually found to be first order with respect to benzene concentration. Mars and Van Krevelen claim that at high concentrations of benzene (6 mmHg or higher) the reaction rate is independent of benzene concentration (Sampson and Shooter, 1965). Ioffe and Lyubarski (1962) found that M) the reaction at low Ozconcentration (less than 4 X rate is second order in Oz whereas at high O2concentration (greater than 4 X lo-, M) the reaction rate is zero order in oxygen. Activation energies given in literature are also contradictory. Although the catalysts used are not the same, activation energies vary between 15 and 37 kcal/mol (Table I). In relation to activation energies, Butler and Weston (1963) studied benzene oxidation to MA over catalysts of different composition and found that as the content of Moo3 increases the activation energy decreases. As the content of GeOz increases the activation energy increases. The compositions of the catalysts prepared together with their surface areas are presented in Table 11. The results of the selectivity screening test are given in Table 111. Catalysts 11-14 are alumina based and since no significant *Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843. 0196-4305/81/1120-0604$01.25/0
-
r
A-0. I
TI
+s
'0
1
0
0
activity is noted, they'are not included in Tables I1 and 111. Catalyst no. 5 is chosen for this investigation since it is the most selective one. In this study, the catalyst used is composed of 74% kieselguhr, 15.5% Vz05,8% MOO,, 2% Sbz03, and 0.5% PzOP This catalyst was chosen after a selectivity study made on 14 different VzO5 based catalysts (Tufan and Akgerman, 1980). In selectivity studies and later in kinetic studies conversion to total oxidation products are determined by difference. Total concentration of C02in the exit stream is measured and from this COz formed by oxidation of benzene to MA is subtracted.
Experimental Section A. Catalyst Preparation. Catalyst no. 5 was prepared by reducing the appropriate amount of ammonium m-vanadate (NH4V0,) in warm water by oxalic acid (CzO4Hz) until the solution takes a blue color. To this solution the appropriate amounts of ammonium molybdate ((NH4)6Mo70N.4Hg0),sodium phosphate (Na3P04-12H20),and antimony oxide (SbzO3) dissolved in HC1 were added. After the solution was well mixed, kieselguhr carrier was added and the solution was evaporated to a thick paste over a water bath with continuous stirring. The paste was then dried in a 140 'C oven, ground to fine powder, and pelletized in an IR presser. The pellets, 1-2 mm thick and 13 mm in diameter, were divided into four pieces and activated at 400 "C. The surface area was determined by a Fischer sorptometer using the one-point BET technique and found to be 4.14 m2/g. Catalysts 1-6 were all prepared similarly. Catalysts 7-10 were prepared in a slightly different fashion. In catalyst 7, HC1 was used as the reducing agent instead of oxalic acid. In catalyst 8, only Vz05and M a 3 were used and in 10, only VzO, was used. In catalyst 9, silica gel was employed as carrier and Niz03was added as an active ingredient. 0 1981 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981 605
Table I. Reaction Order and Activation Energies for Benzene Oxidation (Sampson and Shooter, 1965)
Hammar reaction order with respect to benzene for oxidation to MA reaction order with respect to benzene for complete combustion reaction order with respect to 0, for oxidation to MA activation energy, kcal/mol temperature range "C oxidation to MA complete combustion
Stager
Mars and Van Krevelen Holsen
Ioffe
Butler and Weston
Hayashi and Hudgins Hauffe
1
1
0-1
1
1
1
1
1
1
1
0-1
1
1
1
1
1
1
0-1
1
0-2
0-1
1
325-450 19-20 19-20
380-440 37 37
350-400 27
-
375-400 28+ 4 28 + 4
450-530 15 15
R-Reyulatar FR-Fine R e g l a f o r C- Contact Thermometer R-Resistance Thermometer
Figure 1. Experimental setup.
B. Experimental Setup. A tubular reactor operating in the differential mode was employed in kinetics study. The experimental setup is given in Figure 1. The air stream, used for oxidation was metered by a U manometer after passing through a series of driers, and was fed to the reactor passing through a heated piping. Benzene was kept in a saturator located in a constant-temperature bath. Nitrogen was used as the hydrocarbon carrier and was dried like air and metered with a rotameter. The hydrocarbon stream from the saturator was fed to the reactor through a Pyrex tube that was kept over 100 "C in order to prevent benzene condensation. The reactor was a differential tubular reactor, 26 mm in diameter, made of Pyrex glass and contained 8 g of catalyst. The catalyst bed was located in the middle of the reactor and was approximately 30 mm in height. The catalyst bed was supported by glass wool and the reactor was filled with small rashing rings in order to achieve ideal plug flow conditions for the flow in the tube up to the catalyst bed. Although the bed diameter to bed height ratio was about 1.0 (not sufficient to conclude plug flow behavior), since a differential reaction mode is employed plug flow reaction was assumed throughout the analysis of the kinetic data. The reactor was located in an electrical oven and heated by radiation. The temperature in the reactor was recorded by calibrated resistance thermometers located 2 cm apart. There were three resistance thermometers located l x m apart at the top, in the middle, and at the bottom of the catalyst bed. and the oven temperature was controlled by the one located in the middle. Throughout the experiments a constant temperature profile over the reactor bed length was maintained. The product stream from the reactor passed through three cold traps connected in series. The first cold trap was kept in ice, the secone one in a salt-ice mixture, and the third one in acetone + dry ice. After passing through the cold traps, the solid and liquid phase components in the product stream were trapped and the stream passed through a Beckman IR gas analyzer for C02 determination. The products collected in all three
-
300-400 18+ 3 182 3
traps were dissolved in acetone and analyzed by gas chromatography. The exit from the IR gas analyzer was vented to atmosphere. Since the exit stream was vented a relatively short distance after the bed, the pressure drop was neglected and the reactor was taken at atmospheric pressure. Knowing the inlet composition to the reactor and measuring the amount of C 0 2 and the product distribution of the trapped components, we can calculate the reaction rate and product composition by using an iterative procedure (Tufan and Akgerman, 1980). In analysis, the product distribution of solid and liquid phase products dissolved in a common solvent, acetone, and concentration of one gas phase product, COP, was measured. Since air was used very much in excess and conversion was little, the amount of oxygen used cannot be accurately determined by direct measurement. Therefore the amount of oxygen used was calculated. In the iterative process, first it was assumed that all of the C02 formed comes from complete oxidation of benzene. From this, by use of the reaction C & 3+ 7.502 6C02 + 3H2O
-
the amount of benzene reacted (in moles) can be determined. Knowing the amount fed to the reactor, we calculate the amount of unreacted benzene in the products. From the product distribution, taking benzene as the basis, we can calculate the amounts of MA and BQ. The C02 produced together with MA and BQ can be determined from reactions C& + 4.502 C4H203 + 2CO2 + 2H20 C & 3+ 1.502 C6H402 + H20
--
This amount of C02 subtracted from the COPmeasured gives the C02 produced by the combustion reaction only. Using this iterative procedure, we can calculate the amounts of produds (in moles) and 0% By use of standard procedures (Smith, 1970) it was shown that both external and internal diffusion do not influence the reaction rate. External diffusion effects are calculated by use of the generalized j , factor and it was found that (C,- C,)/Cb = lo*, so it was assumed that the Cb C,. To calculate the internal diffusion effects, the generalized Thiele modulus based on the reaction rate was determined. The effective diffusivity was estimated by use of a pore size of 5000 A for kieselguhr employing the procedure given by Butler and Weston (1963) based on average pore volume, density of catalyst, and surface area. The generalized and Thiele modulus was found to be of the order hence the effectiveness factor was assumed to be unity. Since the catalyst particles used were prepared by pressing disks 13 mm in diameter and 2 mm thick and dividing those to four or more pieces, considering the size of par-
606
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981
ala@ Y Y Y
Y Y Y . I . I .e e e e
0 0
t0
U
a
%I I I
1
'U
2
E
."0 Y
.e
u a
a
u a a
.-M
Ind. Eng. Chem.
ticles internal diffusion effects were not expected. An experimental run took 8-10 h and no catalyst deactivation is noted. In fact, the same catalyst particles were used in experiments at the same and different conditions with no apparent deactivation. No effort is spent to determine the deactivation time on stream. In oxidation reactions, catalyst deactivation is mainly due to temperature gradients within the catalyst particle due to highly exothermic reaction conditions. Therefore pores get blocked by melting V206at around 700 OC. In the experiments, since low conversions were employed and the maximum temperature was around 400 "C,no deactivation at short periods was expected. If deactivation had occurred there would have been a drop in the amount of COPwhich was continuously monitored.
Results and Discussion Most catalytic oxidation reactions proceed by a two-step redox mechanism. Especially in oxidation over a V206 catalyst this mechanism is usually proposed (Ioffe and Lyubarski, 1962; Mann and Dosi, 1973; Mathur and Viswanath, 1974; Murthy and Rajamani, 1974; Subramaniam and Murthy, 1972, 1974, 1974). With that in mind a two-step redox mechanism is proposed for this reaction as well. According to this mechanism, the first step is the adsorption of hydrocarbon on the catalyst surface and its reaction with the oxygen atom of the catalyst to yield the products and the reduced catalyst, and the second step is the oxidation of the reduced catalyst by oxygen. Therefore 4
C6H6
+ Cat.,,d. cat.,,d.
kl
+ 02
+
Process Des. Dev., Vol. 20, No. 4, 1981
607
Table IV. Experimental Results reaction mole rate, rate, fraction mol of mL/s at benzene conv of benzene/ teomp, 25 "C, in feed benzene, g of cat. C 1 atm x lo2 % h x lo4 flow
expt no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
344 344 344 344 354 354 354 363 363 363 363 380 380 380 380 393 393 393 393
12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.6 12.7
0.966 1.497 1.913 2.394 1.004 1.538 2.004 1.000 1.351 1.703 2.499 0.960 1.431 1.924 2.378 1.009 1.497 1.943 2.266
3.98 3.28 2.78 2.53 5.90 4.33 3.90 7.17 5.58 4.63 3.78 7.15 5.97 4.46 4.61 10.38 8.51 1.03 6.20
0.972 1.289 1.442 1.677 1.494 1.743 2.106 1.735 1.888 2.026 2.567 1.848 2.385 2.450 3.027 2.653 3.358 3.648 3.911
180
160
1 LO
products + Cat.,,d.
k2
120
Cat.,,d.
Since cat.,,+ cat.,d. = cut. = m = constant, employing the steady-state approximation
100
eo
GO
and solving LO
20
The reaction rate can be given as
(Ye) I
~ / ( Y o ~ ) I ~
0 0
002
004
006
008
010
012
01L
Figure 2. Determination of the rate constant.
or rearranging
1 1 +KgBg2 K o a ~
-1- --
-RB
(3)
where K1 = mkl and K 2 = mk2 But the reaction rate can also be given as (4)
If a = p = 0.1 then the rate expression can be written as
Since the reactor used is a differential reactor (constant temperature and limited conversion) instead of the actual hydrocarbon and oxygen concentrations logarithmic mean
concentration of each based on the entrance and exit concentrations can be used and taken as constant, hence
Since - R B = F,/ W, rearranging the above equation would yield
1 (YB)lm +-K1 K2 b O 2 ) I m
-b B-) h - -1 -RB
(7)
If in this equation (yB)h/(y,+)lm is plotted VS. bB)h/(-RB), a straight line would be obtamed with the slope being 1/KZ and the intercept l / K 1 . In order for this mechanism to be true, both the slope and the intercept should be positive and, since K 1 = mkl, should also satisfy the Arrhenius equation.
608
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981
Table V. Values of K , and K,
temp, "C
K,
344 354 363 380 393
0.01303 0.02137 0.02481 0,03154 0.04437
K2 0.00142 0.00162 0.00189 0.00202 0.00283
1 In K i
0
&ld LEO
152
1.56
i.54
1.58
1.60
182
Figure 3. Arrhenius plot for K1.
-5.8
1
K2
-6D
-6.2
-6.4
-6.6 L50
1.52
1.54
156
1.58
160
162
Figure 4. Arrhenius plot for Kz.
Experimental results are presented in Table IV and Figure 2 shows this plot for each temperature and K1 and K z values obtained by a least-squares fit is given in Table V. It can be seen that the model fits the experimental data fairly well. The arrhenius plots for K1 and K z are presented in Figures 3 and 4. Between 344 and 393 "C K1 = 8.96 X lo4 exp(-19200/RT)
K2 = 8.89 exp(-l0750/RT)
Since K z
