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Thermal oscillations during the catalytic hydrogenation of nitrobenzene
237
Journal of Molecular Catalysis, 54 (1989) 237 - 242
THERMAL OSCILLATIONS DURING THE CATALYTIC HYDROGENATION OF NITROBENZENE L. PETROV, CH. VLADOV, A. ELIYAS, N. KIRKOV, K. TENCHEV, CH. BONEV, D. FILKOVA and L. PRAHOV Institute of Kinetics and Catalysis, Bulgarian Academy (Received October 24,1988;
of Sciences, Sofia 1040 (Bulgaria)
accepted November 15,1988)
Summary Three commercial Kieselguhr-supported copper, nickel and coppernickel catalysts were investigated under severe conditions in the hydrogenation of nitrobenzene to aniline. Self-sustained oscillations of the catalyst bed temperature in the 438 - 593 K range were observed after a partial deactivation of the studied samples. The oscillation mode was changed upon external heating. The experimental results show that the oscillations are due to mass transfer retardation effects and the nature of the chemical processes. An explanation of the observed phenomenon is proposed, according to which a hot zone is formed in the front layer of the catalyst bed. The zone cycles up and down along the catalyst bed.
Introduction
Today almost all the world output of aniline (An) is manufactured via gas phase catalytic hydrogenation of nitrobenzene (NB). The most widely used catalysts contain copper or nickel, or both, as active components [ 11. Together with An, a series of reaction products have also been detected, namely, benzene, ammonia, cyclohexane, cyclohexylamine, dicyclohexylamine, diphenyl, methane and several other compounds [l]. Recently, the steady-state kinetics of NB hydrogenation over copper and nickel catalysts [ 2, 31 and the deactivation kinetics on a copper catalyst [ 41 have been studied under conditions which are very close to those used in industry. The aim of the present work is to investigate the behaviour of the reaction system under severe conditions of performance which are applied to evaluate the life of industrial catalyst samples [ 5, 61.
Experimental
Three Kieselguhr-supported catalysts were used in this study: (i) a nickel catalyst (Harshaw 3250 T l/8 in) with BET area of 150 m2 g-‘; 0304-5102/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
238
(ii) a copper catalyst supplied from Poland; (iii) a copper-nickel catalyst (M-19, Neftochim Bulgaria), the last two having a BET area of 180 m* g-l. Reduction of the catalysts was conducted in a flow of hydrogen under the following conditions (i) nickel samples - drying at 393 K for 1 h and reduction at 523 and 693 K for 1 and 2 h, respectively; (ii) copper and copper-nickel samples - drying at 393 K for 1 h and reduction at 523 K for 3 h. After reduction, the temperature of the catalyst bed was lowered to 473 K in a flow of hydrogen. Then NB was fed into the reactor for 2 h at LHSV of 2.4 h-i. The LHSV of NB was subsequently increased to 4.9 h-l, kept constant for 2 h, and finally reached the value of 8.6 h-i. For aII the runs, the feed molar ratio between hydrogen and NB gases was 15:l. This ratio was a five-fold excess of hydrogen relative to the stoichiometrically required ratio (3:l) for the reaction of NB hydrogenation to An. The study was carried out at atmospheric pressure in a fixed bed flow reactor loaded with 1 g of catalyst. Industrial pellets were crushed and the fraction of 0, 4 - 0, 8 mm was selected. The temperature of the catalyst bed was kept constant within *l K. It was measured by a chromel-alumel thermocouple and a digital millivoltmeter with an accuracy of kO.1 K. NB of 99.9% purity was introduced into the reactor by a Gilson model 302 pump, the error of the set flow rate did not exceed 1%. Hydrogen flow was monitored by a mechanical differential regulator with an accuracy of 3%. H, was purified from oxygen traces by a palladium Deoxo unit. Prior to the reactor inlet, the NB was passed through a preheater at 493 K. Analysis of the converted mixture was performed on a Perkin-Elmer Sigma 300 gas chromatograph equipped with a flame ionization detector. A 2 m column, filled with 10% Apieson L on Chromaton N-AW-DMCS treated with 3% potassium hydroxide, was used to separate the components of the converted mixture. The following conditions were applied: column temperature 473 K, vaporizer and detector temperature 503 K. Argon was used as a carrier gas at the rate of 30 ml mini.
Results and discussion According to the method proposed earlier [ 5,6] for evaluating catalyst durability, the study was carried out under severe conditions, i.e. high LHSV of NB and low values of the H,/NB ratio. It should be noted that under the commonly applied conditions in industry, the space velocity is -0.2 - 0.5 h-’ and the H’/NB ratio varies from 2O:l to 5O:l [ 11. At a nitrobenzene LHSV of 8.6 h-l, the reaction system passed over to autothermal operating conditions. As a result, the catalysts were deactivated to’a certain extent for -2 h and the temperature of the catalyst bed started to oscillate. The shape of the self-sustained oscillations was of the saw-like type (Figs. 1 - 3). A round minimum and a sharp maximum were observed. The oscillation mode was significantly changed upon external heating,
239
573.
553.
533-
513.
493
473
t
I
.
.
.
.
.
20
40
60
60
100
. 12'Zmin
Fig. 1. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Harshaw 3250 T l/8 inch nickel catalyst. TK 573'
553'
533.
/@&I/
513'
493.
473. I
c
20
40
60
60
100
120
ICmin
Fig. 2. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Polish copper catalyst.
becoming rather irregular than ordered (Fig. 4). The oscillation period varied from 13 to 31 and from 12 to 26 min with the nickel and copper catalysts, respectively. The oscillation amplitude with the nickel catalyst was 55 - 120 K, with a minimum around 438 - 458 K and a maximum at 513 - 563 K. The copper catalysts showed an amplitude AT” = 85 - 110 K, with a minimum around 483 - 498 K and a maximum at 583 - 593 K. The amount of unconverted NB at the reactor outlet varied between 0.05 and 0.46%, i.e. the degree of NB conversion was between 99.54 and 99.95%. Aniline was the major product of NB hydrogenation; its concentration in the converted mixture varied from 91.66 to 99.43%.
423
323
20
co
60
00
too
120 tmin
Fig. 3. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Bulgarian M-19 copper-nickel catalyst.
TK 573 I 553.
533.
513.
20
40
60
00
100
120 tmin
Fig. 4. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An upon external heating with either of the above catalysts.
At the ~mperatu~ of the observed oscillations (438 - 593 K), some of the An was further converted. Gas chroma~~aphic data indicated, besides An and NB, the presence of benzene (0.23 - 7.97%), cyclohexane (0.94 2.53%) and ammonia. A few other compounds were also detected in the converted mixture which were not identified. According to [l], possible byproducts are cyclohexylamine, p-phenylenediamine, diphenyl, diphenylamine, p-nitroaniline, methane, carbazole, etc. At the maximum oscillation temperatures (up to 593 K) the catalytic activity was high, however the selectivity for An diminished as a result of the increase in the share of side reactions. The thermal effects of the desired reaction, as well as those of some side reactions, at 298 K are as follows: C6H,N02 + 3H, __+ C&IsNH, + 2H,O + 4728 kJ mol-’
(1)
241
C&I,NH, + H, &H6 + 3H, -
C!,H, + NH, + 37.7 kJ mole1
(2)
C&II2 + 205 kJ mol-’
(3)
C&II2 + 6H, __* 6CH, + 573 kJ mol-’
(4)
2C&,+
(5)
C12H10+ H, - 3.3 kJ mol-’
The heats of reactions (1 -5) were calculated on the basis of the standard heats of formation of the corresponding compounds [7]. Further, we evaluated the temperature gradient, AT,, in the catalyst grain for the M-19 samples employing the formula:
QGh
AT,=
\
(6)
b?ff
where Q (kJ mol-‘) is the heat of reaction (l), C,, is the feed NB concentration, Deff is the effective diffusion coefficient of NB in the catalyst, and heff is the thermal conductivity coefficient of the catalyst. For Co = 2.797 X 10m6 mol cmw3, Deff = 2.63 cm2 h-l at 600 K, and bff = 0.84 X 10m2kJ cm-’ h-l K-l we obtained the value of 0.4 K. Also, the temperature difference, AT,, between the surface of the catalyst grain and the gas phase was calculated according to the formula: AT, =
Qrd2eff 4(1-
e)Nu heff
(7)
where r is the rate of reaction (l), deff is the effective diameter of the grain, Nu is the Nusselt criterion for the flow of the H2 + NB mixture, and e is the porosity of the catalyst. For F = 0.089 mol h-l cmm3, deff = 0.53 mm, Nu = 2.06 and e = 0.2, we obtained the value of 2.1 K which indicates that this temperature difference is negligibly small. These results show that the self-sustained thermal oscillations are not due to difficulties in heat transfer, but rather due to the mass transfer and the nature of the processes occurring in that system. The value of the Weisz criterion [8] is: = 8.5
(8)
This means that the reaction takes place in the diffusion region. The reaction depth in the grain, I& is @ven by the expression: H
=
l/2
( ) DeffC0 F
(9)
A value of 0.09 mm was obtained for H, which indicates that the reaction proceeds from 30 to 50% of the radius into the depth of the catalyst grain.
242
Under the severe conditions of catalyst performance, when the oscillations were observed, the reaction picture becomes very complicated. Together with the basic reaction, a great number of undesired parallel and consecutive parasitic reactions also proceeded. Since the reaction occurred in the diffusion region, a strong influence was also exerted by the mass-transfer retardation effects. The temperature changes, as well as the variation in composition of the reaction mixture; caused alterations in the reaction rate. A possible explanation for the appearance of the oscillations is as follows. Reaction (1) starts off in the front layer of the catalyst bed. As a result of the liberated reaction heat in that part of the bed, a hot zone is formed. Further, the rise in temperature of the front layer causes an increase in the reaction rate. The selectivity for An is decreased because of the increased share of the side reactions [2 - 51. Gradually the hot zone moves downwards along the catalyst bed. The temperature of the front layer is lowered because the reaction products (An, benzene, cyclohexylamine, cyclohexane, diphenyl, etc.) do not succeed in diffusion into the gas phase, and thus the rates of reactions (1- 5) are decreased. Because of the partial blocking of the active sites by the reaction products, part of the NB is not converted in the front zone but is hydrogenated in the next layers of the catalyst bed. After a certain period of time, the hot zone reaches the end of the catalyst bed. During this period the reaction products in the front layer diffuse into the gas phase and thus the active sites become accessible for the reaction again. Reaction (1) takes place again in the front zone. These conditions are consistent with the so-called ‘travelling wave’ [9]. When the hot zone is located in the front layers of the bed, part of the aniline is further converted into benzene, cyclohexane, cyclohexylamine, etc.
References 1 Yu. T. Nikolaev and A. M. Yakubson, Aniline, Khimiya, Moscow, 1984 (in Russian). 2 L. Petrov, N. Kirkov and D. Shopov, Kinet. K&al., 26 (1985) 897. 3 L. Petrov, N. Kirkov, K. Tenchev and D. Shopov, Chem. Znd., (1985) 443 (in Bulgarian). 4 L. Petrov, N. Kirkovand K. Kumbilieva, in D. Shopov, A. Andreev, A. Palazov and L. Petrov (eds.), Proc. 6th Znt. Symp. Heterogeneous Catal., Sofia, 1987, Part 1, Bulgarian Acad. Sci., Sofia, 1987, p. 111. 5 Bulgarian Pat. 74945117, (1986) to L. Petrov, Ch. Vladov, Ch. Bonev, N. Neshev, L. Prahov, M. Vassileva, D. Filkova and S. Dancheva. 6 L. Petrov, Ch. Vladov, Ch. Bonev, N. Neshev, L. Prahov, M. Vassileva, D. Filkova and S. Dancheva, Chem. Znd., (1985) 316 (in Bulgarian). 7 B. P. Nikolskii (ed.), Chemist’s Handbook, Vol. 1, Goskhimizdat, Moscow, 1962, p. 855 (in Russian). 8 P. B. Weisz, 2. Phys. Chem., 2 (1957) 1. 9 Yu. Sh. Matros, Catalytic Processes under Nonstationary Conditions, Nauka, Novosibirsk, 1987, p. 78 (in Russian).
Journal of Molecular Catalysis, 54 (1989) 237 - 242
THERMAL OSCILLATIONS DURING THE CATALYTIC HYDROGENATION OF NITROBENZENE L. PETROV, CH. VLADOV, A. ELIYAS, N. KIRKOV, K. TENCHEV, CH. BONEV, D. FILKOVA and L. PRAHOV Institute of Kinetics and Catalysis, Bulgarian Academy (Received October 24,1988;
of Sciences, Sofia 1040 (Bulgaria)
accepted November 15,1988)
Summary Three commercial Kieselguhr-supported copper, nickel and coppernickel catalysts were investigated under severe conditions in the hydrogenation of nitrobenzene to aniline. Self-sustained oscillations of the catalyst bed temperature in the 438 - 593 K range were observed after a partial deactivation of the studied samples. The oscillation mode was changed upon external heating. The experimental results show that the oscillations are due to mass transfer retardation effects and the nature of the chemical processes. An explanation of the observed phenomenon is proposed, according to which a hot zone is formed in the front layer of the catalyst bed. The zone cycles up and down along the catalyst bed.
Introduction
Today almost all the world output of aniline (An) is manufactured via gas phase catalytic hydrogenation of nitrobenzene (NB). The most widely used catalysts contain copper or nickel, or both, as active components [ 11. Together with An, a series of reaction products have also been detected, namely, benzene, ammonia, cyclohexane, cyclohexylamine, dicyclohexylamine, diphenyl, methane and several other compounds [l]. Recently, the steady-state kinetics of NB hydrogenation over copper and nickel catalysts [ 2, 31 and the deactivation kinetics on a copper catalyst [ 41 have been studied under conditions which are very close to those used in industry. The aim of the present work is to investigate the behaviour of the reaction system under severe conditions of performance which are applied to evaluate the life of industrial catalyst samples [ 5, 61.
Experimental
Three Kieselguhr-supported catalysts were used in this study: (i) a nickel catalyst (Harshaw 3250 T l/8 in) with BET area of 150 m2 g-‘; 0304-5102/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
238
(ii) a copper catalyst supplied from Poland; (iii) a copper-nickel catalyst (M-19, Neftochim Bulgaria), the last two having a BET area of 180 m* g-l. Reduction of the catalysts was conducted in a flow of hydrogen under the following conditions (i) nickel samples - drying at 393 K for 1 h and reduction at 523 and 693 K for 1 and 2 h, respectively; (ii) copper and copper-nickel samples - drying at 393 K for 1 h and reduction at 523 K for 3 h. After reduction, the temperature of the catalyst bed was lowered to 473 K in a flow of hydrogen. Then NB was fed into the reactor for 2 h at LHSV of 2.4 h-i. The LHSV of NB was subsequently increased to 4.9 h-l, kept constant for 2 h, and finally reached the value of 8.6 h-i. For aII the runs, the feed molar ratio between hydrogen and NB gases was 15:l. This ratio was a five-fold excess of hydrogen relative to the stoichiometrically required ratio (3:l) for the reaction of NB hydrogenation to An. The study was carried out at atmospheric pressure in a fixed bed flow reactor loaded with 1 g of catalyst. Industrial pellets were crushed and the fraction of 0, 4 - 0, 8 mm was selected. The temperature of the catalyst bed was kept constant within *l K. It was measured by a chromel-alumel thermocouple and a digital millivoltmeter with an accuracy of kO.1 K. NB of 99.9% purity was introduced into the reactor by a Gilson model 302 pump, the error of the set flow rate did not exceed 1%. Hydrogen flow was monitored by a mechanical differential regulator with an accuracy of 3%. H, was purified from oxygen traces by a palladium Deoxo unit. Prior to the reactor inlet, the NB was passed through a preheater at 493 K. Analysis of the converted mixture was performed on a Perkin-Elmer Sigma 300 gas chromatograph equipped with a flame ionization detector. A 2 m column, filled with 10% Apieson L on Chromaton N-AW-DMCS treated with 3% potassium hydroxide, was used to separate the components of the converted mixture. The following conditions were applied: column temperature 473 K, vaporizer and detector temperature 503 K. Argon was used as a carrier gas at the rate of 30 ml mini.
Results and discussion According to the method proposed earlier [ 5,6] for evaluating catalyst durability, the study was carried out under severe conditions, i.e. high LHSV of NB and low values of the H,/NB ratio. It should be noted that under the commonly applied conditions in industry, the space velocity is -0.2 - 0.5 h-’ and the H’/NB ratio varies from 2O:l to 5O:l [ 11. At a nitrobenzene LHSV of 8.6 h-l, the reaction system passed over to autothermal operating conditions. As a result, the catalysts were deactivated to’a certain extent for -2 h and the temperature of the catalyst bed started to oscillate. The shape of the self-sustained oscillations was of the saw-like type (Figs. 1 - 3). A round minimum and a sharp maximum were observed. The oscillation mode was significantly changed upon external heating,
239
573.
553.
533-
513.
493
473
t
I
.
.
.
.
.
20
40
60
60
100
. 12'Zmin
Fig. 1. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Harshaw 3250 T l/8 inch nickel catalyst. TK 573'
553'
533.
/@&I/
513'
493.
473. I
c
20
40
60
60
100
120
ICmin
Fig. 2. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Polish copper catalyst.
becoming rather irregular than ordered (Fig. 4). The oscillation period varied from 13 to 31 and from 12 to 26 min with the nickel and copper catalysts, respectively. The oscillation amplitude with the nickel catalyst was 55 - 120 K, with a minimum around 438 - 458 K and a maximum at 513 - 563 K. The copper catalysts showed an amplitude AT” = 85 - 110 K, with a minimum around 483 - 498 K and a maximum at 583 - 593 K. The amount of unconverted NB at the reactor outlet varied between 0.05 and 0.46%, i.e. the degree of NB conversion was between 99.54 and 99.95%. Aniline was the major product of NB hydrogenation; its concentration in the converted mixture varied from 91.66 to 99.43%.
423
323
20
co
60
00
too
120 tmin
Fig. 3. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An. Bulgarian M-19 copper-nickel catalyst.
TK 573 I 553.
533.
513.
20
40
60
00
100
120 tmin
Fig. 4. Shape of the self-sustained thermal oscillations during catalytic hydrogenation of NB to An upon external heating with either of the above catalysts.
At the ~mperatu~ of the observed oscillations (438 - 593 K), some of the An was further converted. Gas chroma~~aphic data indicated, besides An and NB, the presence of benzene (0.23 - 7.97%), cyclohexane (0.94 2.53%) and ammonia. A few other compounds were also detected in the converted mixture which were not identified. According to [l], possible byproducts are cyclohexylamine, p-phenylenediamine, diphenyl, diphenylamine, p-nitroaniline, methane, carbazole, etc. At the maximum oscillation temperatures (up to 593 K) the catalytic activity was high, however the selectivity for An diminished as a result of the increase in the share of side reactions. The thermal effects of the desired reaction, as well as those of some side reactions, at 298 K are as follows: C6H,N02 + 3H, __+ C&IsNH, + 2H,O + 4728 kJ mol-’
(1)
241
C&I,NH, + H, &H6 + 3H, -
C!,H, + NH, + 37.7 kJ mole1
(2)
C&II2 + 205 kJ mol-’
(3)
C&II2 + 6H, __* 6CH, + 573 kJ mol-’
(4)
2C&,+
(5)
C12H10+ H, - 3.3 kJ mol-’
The heats of reactions (1 -5) were calculated on the basis of the standard heats of formation of the corresponding compounds [7]. Further, we evaluated the temperature gradient, AT,, in the catalyst grain for the M-19 samples employing the formula:
QGh
AT,=
\
(6)
b?ff
where Q (kJ mol-‘) is the heat of reaction (l), C,, is the feed NB concentration, Deff is the effective diffusion coefficient of NB in the catalyst, and heff is the thermal conductivity coefficient of the catalyst. For Co = 2.797 X 10m6 mol cmw3, Deff = 2.63 cm2 h-l at 600 K, and bff = 0.84 X 10m2kJ cm-’ h-l K-l we obtained the value of 0.4 K. Also, the temperature difference, AT,, between the surface of the catalyst grain and the gas phase was calculated according to the formula: AT, =
Qrd2eff 4(1-
e)Nu heff
(7)
where r is the rate of reaction (l), deff is the effective diameter of the grain, Nu is the Nusselt criterion for the flow of the H2 + NB mixture, and e is the porosity of the catalyst. For F = 0.089 mol h-l cmm3, deff = 0.53 mm, Nu = 2.06 and e = 0.2, we obtained the value of 2.1 K which indicates that this temperature difference is negligibly small. These results show that the self-sustained thermal oscillations are not due to difficulties in heat transfer, but rather due to the mass transfer and the nature of the processes occurring in that system. The value of the Weisz criterion [8] is: = 8.5
(8)
This means that the reaction takes place in the diffusion region. The reaction depth in the grain, I& is @ven by the expression: H
=
l/2
( ) DeffC0 F
(9)
A value of 0.09 mm was obtained for H, which indicates that the reaction proceeds from 30 to 50% of the radius into the depth of the catalyst grain.
242
Under the severe conditions of catalyst performance, when the oscillations were observed, the reaction picture becomes very complicated. Together with the basic reaction, a great number of undesired parallel and consecutive parasitic reactions also proceeded. Since the reaction occurred in the diffusion region, a strong influence was also exerted by the mass-transfer retardation effects. The temperature changes, as well as the variation in composition of the reaction mixture; caused alterations in the reaction rate. A possible explanation for the appearance of the oscillations is as follows. Reaction (1) starts off in the front layer of the catalyst bed. As a result of the liberated reaction heat in that part of the bed, a hot zone is formed. Further, the rise in temperature of the front layer causes an increase in the reaction rate. The selectivity for An is decreased because of the increased share of the side reactions [2 - 51. Gradually the hot zone moves downwards along the catalyst bed. The temperature of the front layer is lowered because the reaction products (An, benzene, cyclohexylamine, cyclohexane, diphenyl, etc.) do not succeed in diffusion into the gas phase, and thus the rates of reactions (1- 5) are decreased. Because of the partial blocking of the active sites by the reaction products, part of the NB is not converted in the front zone but is hydrogenated in the next layers of the catalyst bed. After a certain period of time, the hot zone reaches the end of the catalyst bed. During this period the reaction products in the front layer diffuse into the gas phase and thus the active sites become accessible for the reaction again. Reaction (1) takes place again in the front zone. These conditions are consistent with the so-called ‘travelling wave’ [9]. When the hot zone is located in the front layers of the bed, part of the aniline is further converted into benzene, cyclohexane, cyclohexylamine, etc.
References 1 Yu. T. Nikolaev and A. M. Yakubson, Aniline, Khimiya, Moscow, 1984 (in Russian). 2 L. Petrov, N. Kirkov and D. Shopov, Kinet. K&al., 26 (1985) 897. 3 L. Petrov, N. Kirkov, K. Tenchev and D. Shopov, Chem. Znd., (1985) 443 (in Bulgarian). 4 L. Petrov, N. Kirkovand K. Kumbilieva, in D. Shopov, A. Andreev, A. Palazov and L. Petrov (eds.), Proc. 6th Znt. Symp. Heterogeneous Catal., Sofia, 1987, Part 1, Bulgarian Acad. Sci., Sofia, 1987, p. 111. 5 Bulgarian Pat. 74945117, (1986) to L. Petrov, Ch. Vladov, Ch. Bonev, N. Neshev, L. Prahov, M. Vassileva, D. Filkova and S. Dancheva. 6 L. Petrov, Ch. Vladov, Ch. Bonev, N. Neshev, L. Prahov, M. Vassileva, D. Filkova and S. Dancheva, Chem. Znd., (1985) 316 (in Bulgarian). 7 B. P. Nikolskii (ed.), Chemist’s Handbook, Vol. 1, Goskhimizdat, Moscow, 1962, p. 855 (in Russian). 8 P. B. Weisz, 2. Phys. Chem., 2 (1957) 1. 9 Yu. Sh. Matros, Catalytic Processes under Nonstationary Conditions, Nauka, Novosibirsk, 1987, p. 78 (in Russian).