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Kinetics of catalyst coking in the hydrogenation of nitrobenzene to aniline—Investigations in an isothermal catalytic wall reactor
Catalyst Deactivation 1999 B. Delmon and G.F. Froment (Editors) o 1999 Elsevier Science B.V. All rights reserved.
97
Kinetics of Catalyst Coking in the Hydrogenation of Nitrobenzene to Aniline - Investigations in an Isothermal Catalytic Wall Reactor E. Klemm, B. Amon, H. Redlingsh6fer, G. Emig Lehrstuhl fiir Technische Chemie I, University of Erlangen-Nuremberg, Egerlandstr. 3, 91058 Erlangen, Germany
Abstract A Catalytic Wall Reactor operated under isothermal conditions was used to study both the steady state kinetics as well as the deactivation kinetics of the vapour phase hydrogenation of nitrobenzene to aniline using a palladium catalyst supported on a-alumina. The activity of the catalyst declined with time on stream due to coke formed from nitrobenzene parallel to the main reaction. Axial coke profiles were measured by total carbon analysis of subsequent wall segments. By fitting the measured coke profils at different reaction conditions a coke formation kinetic was determined. In order to get a sufficient fit it had to be differentiated between coke on the active site and coke on the support. The coke content on the active site was correlated with the activity function of both the main reaction as well as the coke formation reaction. Concerning the deactivation of the main reaction it was found that the mechanism of the main reaction changes with proceeding coking. 1. I N T R O D U C T I O N Deactivation by coking is often described empirically by the Voorhies correlation [1], which describes coke formation only depending on the square root of time. This approach does not consider the dependence of coke formation on temperature and on the partial pressures of the species of the reaction system. Thus, this kinetic approach is not suitable for predicting and optimizing the operating conditions of the reactor by simulation. Froment and Bischoff were the first to relate these factors quantitatively with the rate of coking [2]. This requires the measurement of the coke formation at well defined partial pressures and temperatures. As a very effective device a gradient free recycle electrobalance can be used [3]. In the case of an integral reactor coke profiles have to be recorded which can be realized by discharging the catalyst section by section for coke analysis [4]. Another possibility is the use of a Catalytic Wall Reactor (CWR) in which the catalyst is coated on the inner wall of the tube and the catalyst is scratched off the wall sectionally and the corresponding coke profile can be determined. Compared to the usual integral fixed bed reactor the
98 CWR has the additional advantage to guarantee isothermal conditions at the catalyst and along the axial coordinate of the reactor. Especially in the case of strongly exothermic reactions like the hydrogenation of nitrobenzene (AHR = -443 kJ/mol) it is difficult to ensure isothermal conditions in a fixed bed reactor. The transport of mass between the gas bulk inside the CWR and the catalyst on the wall can be improved by an inert bed. Therefore, the kinetic measurements are not limited by mass transfer in the film.
2. E X P E R I M E N T A L 2.1. E x p e r i m e n t a l Set-up The experiments were performed in a fully automated experimental set-up. The gases hydrogen and nitrogen for balancing the total flux and the liquids nitrobenzene and aniline were dosed by thermal mass flow controllers (Bronkhorst Hi-Tec). Gaseous and liquid reactants were mixed and the liquids were vaporised in a commercial evaporater (CEM; Bronkhorst Hi-Tec). The bypass line allowed controlling the adjusted flow rates and starting the reaction at a certain point in time by switching two three-way-valves. A small part of the product stream was analysed online by a gaschromatograph (Chrompack CP9001) equipped with a HP-5 fused silica column and a flame ionisation detector. Reactant A
Reactant B
2.2 A s s e m b l y of the catalytic wall r e a c t o r
and preparation of the catalyst The CWR (see Fig. 1) consists of several reactor segments with varying length and an inner diameter of 10mm. The segments possess screw threads on both ends so that they can be interlinked by a nut and sealed by graphite gaskets. The individual tube segments are manufactured out of V4A-steel. The inner wall of the reactor segments was coated with catalyst by means of a suspension process [5]. To ensure the isothermal operation of the CWR the temperatures very close to the catalyst surface were measured by thermocouples positioned in radial holes in the wall of the reactor. The temperature in the gas bulk (axial direction) was determined by a movable thermocouple in the centre of the tube.
Tube segment
Heating
d e v ~ . . . . ~.,.,i
i~a~
ial 1 E
couple
Thermocouple . . . . .
Tube
i, ~'t~b,. I
Insulation
~
Product
Figure 1: Catalytic Wall Reactor
99
2.3 Experimental conditions All experiments were carried out at atmospheric pressure. The temperature was varied between 275~ and 425~ the inlet partial pressure of nitrobenzene between 1.7 kPa and 6.8 kPa. The molar ratio of hydrogen to nitrobenzene was adjusted between 3:1 and 24:1. The influence of aniline was investigated at partial pressures of 1.7 kPa, 3.4 kPa and 6.8 kPa. Tube segments with a catalyst coating between 2 cm and 12 cm in length were used for the experiments. The catalyst weight varied between 0.15 g and 1.8 g. To activate the catalyst, 10 vol.-% hydrogen in nitrogen was fed to the reactor at reaction temperature, before the hydrogenation reaction was started. The reaction was stopped after 1 h, 2 h, 5 h and 10 h, respectively. To remove sorbed components from the catalyst, the reactor was flushed with nitrogen for 3 h at reaction temperature.
2.4 Determination of the coke profiles along the reactor After every experiment the reactor was cooled down and the reactor segments were dismantled. The coked catalyst was scratched off the tubes in intervals of i cm and the carbon content of each sample was determined by TC analysis (CMat 550; StrShlein Instruments). The TC was calibrated with a mixture of CaO/CaCOa containing I wt% carbon.
3. RESULTS To exclude the possibility of reactions at the free metal surface of the reactor, at the inert bed or at the catalyst support (A1208), experiments were performed under reaction conditions without the active catalyst component Pd. The conversion of nitrobenzene was always less than 1% and is therefore negligible in the hydrogenation experiments. On the other hand coke formation was already observed to a considerable amount on the support. Figure 2 depicts the coke profiles on the support with and without palladium measured at the same reaction conditions. Comparing the coke content at the reactor inlet it can be concluded that at the chosen reaction
1,6 1,41,21,00,8 0,6i ,' i i i 0,4 0,0 1,0 2,0 3,0 4,0 5,0 6,0 .,.
I
. . . . . .
,__,
i . . . . .
,
,
,
i
i
~
a
. . . . .
~
!
i
I
i
!
i
i l l i
I
i i i i i i i i 1 I
. . . .
a
,
,
i
. . . . .
.
. . . . .
i
i
i i i i
I
l
i i i i
I
l i l l
I
z [cm]
Figure 2. Coke profiles of the support with (~) and without palladium (=) at pN~,o= 3,38 k P a , pH2,0-- 20,28 kPa and T = 425 ~
100
conditions approximately 70 % of the coke is formed on the support. The high contribution of the coke deposited on the support was also found in TPO measurements of the coked catalyst samples which showed two peaks belonging to coke on the active sites (C-a) and coke on the support (C-s). It can be clearly seen that in the case of the active catalyst the coke content is decreasing towards the reactor outlet which can be explained by the formation of coke from nitrobenzene parallel to the main reaction. As expected from t h a t conclusion adding the product aniline to the reactants has no influence to the coke formation. 3.1 S t e a d y - s t a t e k i n e t i c s The initial conversions and the initial reaction rates at different reaction conditions were obtained by extrapolating the conversion curves to time t = 0 min. It could be shown that hydrogenation of nitrobenzene on the fresh catalyst follows a Langmuir-Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step [5]: k . K ~ .Km . p ~ . p ~ to= _0.5 ~2 (l+Km3"Pm3 +Kin'Pro)
3.2 D e a c t i v a t i o n k i n e t i c s For the determination of the unsteady-state 1.0 kinetics the same reaction conditions were chosen as for the experiments described in Section3.1. The only difference was the ~° o.s length of the coating. In the experiments presented in this section the length of the 0.4 catalyst coating was 8 cm to resolve the coke profile. 0.2 '' . . . . . . . ~ .... ~ .... Figure 3 depicts the axial profiles of the coke 0.0 2.0 4.0 6.0 8.0 z [cm] content depending on time at a reaction temperature of 275°C. The partial pressure Figure 3. Time development of the coke of nitrobenzene was p~,o--3.4 k Pa and t h a t profile ( l h .; 2h A; 5h .; 1Oh .; 20h x) of hydrogen pH2,o=20.4 kPa.
101
1.5
0.7
!!
1 . 2 0.9 0.6
" - ............ .
0.6 0.5
0.2 0.2
0.3 0.0
2.0
4.0
6.0
z [cm]
Figure 4. Temperature dependence of coke profiles after 10h (275~ =; 325~ e; 375~ A; 425~ , )
' 0.0
2.0
, 4.0 z [cm]
6.0
8.0
Figure 5. Coke profiles at different hydrogen partial pressures after 10h (pH2,o= 10.1 kPa A; pro,o= 20.2 kPa ,; pH2,o= 40.4 kPa . )
Figure 4 shows the coke profiles at different reaction temperatures. At temperatures higher than approximately 325~ the coke content increases noticeably. In Figure 5 the coke profiles at different hydrogen partial pressures are shown. It can be seen that hydrogen counteracts the deactivation by coking. The higher the hydrogen content in the feed the lower the coke content on the catalyst. By fitting the measured coke profiles and the corresponding nitrobenzene conversions in the time domain the deactivation kinetics can be determined. For modelling this unsteady state problem the accumulation term and the timedependend hydrogenation rate must be considered in the mass balance of the reactor. Thus the following partial differential equation must be solved: C~NB _ ~t
c3(u " P ~ ) _ R . T . p . ru (t ) ~z
Indicating the coke content on the active sites with wc.~, the mass balance for the pseudo-species coke on the active sites can be written as: (~42C
~t
a
-
=rc_~(t )
The coke formation on the support is assumed not to be deactivated and its rate can be formulated as follows: ~hCC-s -- r
c3t
-
c- s, o
102
By the total carbon analysis it can not be distinguished between coke on active sites and coke on the support. Only the total carbon content wc is measured and thus the following equation has to be taken into account: W C = WC_ a + Wc_ s
Following the approach of Szepe and Levenspiel [6] the time dependend reaction rate r(t) is separated into the time independend initial rate ro and the activity function a(t): r(t) = ro 9a(t)
The initial hydrogenation rate rH,o can be written as shown in section 3.1. The initial coking rates rc.a,o and rc-~,o both depend on temperature and the partial pressures of nitrobenzene and hydrogen. A power law concerning partial pressure with a rate constant obeying Arrhenius law proved to be an appropriate rate equation for both coking rates. It was assumed that the orders ml and m2 in nitrobenzene and hydrogen partial pressure are equal in both cases:
rc_~.o = kc_,, p ~ P~
rc-~.o = kc-~ P~CBm2 PH2
With the assumption that only coke on the active sites is responsible for the decline in activity of the main reaction and the coke formation, aj can be expressed in terms of the coke content on active sites: a i =f(Wc_~)
By assuming different functions for the activity term, exponential equation indicated the most promising result:
the
following
a / = exp(-kde~Wc_,,) However, only the coke profiles could be fitted satisfactorily whereas fitting the conversion-time curves of nitrobenzene a distinct lack of model adequacy was observed. Thus, the model had to be modified to obtain a better fit to the experimental data. Only by dividing the exponential expression into three segments a sufficient description was achieved: an = exp(-kd~l., wc_a) ;.for
wc_a <_0.2%
103 a n = exp(-kae~2,nwc_a)
;,for
a H = exp(--kdea3,nWc__a)
;for
0.2 % < Wc_ . < 0.4 % >0.4%
Wc_ ~
The necessity of this rather uncommon approach points out that the coke formation and especially its influence on the main reaction are rather complex. The division into three segments allows the speculation t h a t the mechanism of the main reaction changes with the coke content and with reaction time, respectively. The determined kinetic constants of the unsteady state model are all significant as can be seen from Table 1:
Parameter
Estimated Value .
.
Standard deviation
.
.
.
%
koa,0 [kPa(m'ml)s"l]
242.5
6.0
,,,
Exc.a [kJ/mol]
3 7.1
1.1
kos,0 [kPa("aml)sl]
1.83" 10.3
7.63" 10.4
EA,c-, [kJ/mol]
50.7 .
.
.
.
.
.
.
2.1
.
ml [-]
1.42
m2 [-]
0.45
.
.
.
kdeal,H ['] .
.
.
.
kao,Z,H[-]
.
.
.
.
.
.
.
.
.
.
.
.
0.02 13.2 .
327
.
.
461 .
.
0.04 .
.
.
.
.
.
2.8
,,,
ka=,3,H[-]
574
22.2
ka~,,c-, [-]
4325
68.2
Table 1. Kinetic constants of the unsteady state model
The parity plots of calculated and experimental data are depicted in Figure 6.a (nitrobenzene) and Figure 6.b (coke). The deviations are for the most part within 20 % and in the case of coke formation there is statistical scattering whereas in the case of the nitrobenzene partial pressures there are still trends in the deviations.
104
a)
b) 4.0
2.0
.
1.5
3.0 ,-...,
'.
............ ~...... + 2 0 % ..... ~'... : ....... .:,/ -
..~ :o,
0. .i
9~ d
2.0
- -//
1.0
:. !
_ /
-..
El.
1.0
0.5
0.0
"
~
..............................
0.0
0.0
1,0
2.0
PNB,e~[kea]
3,0
4.0
0.0
0.5
1.0
1.5
~.0
Wc,exp [%]
Fig. 6. Unsteady state kinetics: parity plots of calculated and experimental data. a) nitrobenzene, b) coke 5. CONCLUSIONS The experiments showed that a Catalytic Wall Reactor is an appropriate tool to perform both steady-state and unsteady-state kinetic measurements for highly exothermic reactions under isothermal conditions. By separating the initial reaction rate from the deactivation rate it was found that the hydrogenation of nitrobenzene follows a Langmuir-Hinshelwood approach. Descending axial coke profiles measured by total carbon analysis identified nitrobenzene as coke precursor whereas high partial pressures of hydrogen suppressed the formation of coke. Aniline had no influence neither on the main reaction nor on the deactivation by coking. TPO measurements of the coked catalyst showed two peaks belonging to coke on the active sites and coke on the support. The adjustment of calculated and measured coke profiles confirmed the TPO measurements. 6. REFERENCES
[1] [2] [3] [4] [5]
A. Voorhies, Ind. Eng. Chem. 37, 318 (1945). G.F. Froment, K.B. Bischoff, Chem. Eng. Sci. 17, 105 (1962). H.C. Beirnaert, R. Vermeulen, G.F. Froment, Stud.Surf.Sci.Catal. 88, 97 (1994). K. Liu, S.C. Fung, T.C. Ho, D.S. Rumschitzki, Ind. Eng. Chem. Res. 36, 3264 (1997) B. Amon, E. Klemm, G. Emig, in: Reaction Kinetics and the Development of Catalytic Processes, Proceedings of the Int. Symp., Brugge, Belgium, 1999, in press. [6] S. Szepe and O. Levenspiel, Catalyst deactivation, in Chemical Reaction Engineering, Pergamon Press, Oxford,1971.
97
Kinetics of Catalyst Coking in the Hydrogenation of Nitrobenzene to Aniline - Investigations in an Isothermal Catalytic Wall Reactor E. Klemm, B. Amon, H. Redlingsh6fer, G. Emig Lehrstuhl fiir Technische Chemie I, University of Erlangen-Nuremberg, Egerlandstr. 3, 91058 Erlangen, Germany
Abstract A Catalytic Wall Reactor operated under isothermal conditions was used to study both the steady state kinetics as well as the deactivation kinetics of the vapour phase hydrogenation of nitrobenzene to aniline using a palladium catalyst supported on a-alumina. The activity of the catalyst declined with time on stream due to coke formed from nitrobenzene parallel to the main reaction. Axial coke profiles were measured by total carbon analysis of subsequent wall segments. By fitting the measured coke profils at different reaction conditions a coke formation kinetic was determined. In order to get a sufficient fit it had to be differentiated between coke on the active site and coke on the support. The coke content on the active site was correlated with the activity function of both the main reaction as well as the coke formation reaction. Concerning the deactivation of the main reaction it was found that the mechanism of the main reaction changes with proceeding coking. 1. I N T R O D U C T I O N Deactivation by coking is often described empirically by the Voorhies correlation [1], which describes coke formation only depending on the square root of time. This approach does not consider the dependence of coke formation on temperature and on the partial pressures of the species of the reaction system. Thus, this kinetic approach is not suitable for predicting and optimizing the operating conditions of the reactor by simulation. Froment and Bischoff were the first to relate these factors quantitatively with the rate of coking [2]. This requires the measurement of the coke formation at well defined partial pressures and temperatures. As a very effective device a gradient free recycle electrobalance can be used [3]. In the case of an integral reactor coke profiles have to be recorded which can be realized by discharging the catalyst section by section for coke analysis [4]. Another possibility is the use of a Catalytic Wall Reactor (CWR) in which the catalyst is coated on the inner wall of the tube and the catalyst is scratched off the wall sectionally and the corresponding coke profile can be determined. Compared to the usual integral fixed bed reactor the
98 CWR has the additional advantage to guarantee isothermal conditions at the catalyst and along the axial coordinate of the reactor. Especially in the case of strongly exothermic reactions like the hydrogenation of nitrobenzene (AHR = -443 kJ/mol) it is difficult to ensure isothermal conditions in a fixed bed reactor. The transport of mass between the gas bulk inside the CWR and the catalyst on the wall can be improved by an inert bed. Therefore, the kinetic measurements are not limited by mass transfer in the film.
2. E X P E R I M E N T A L 2.1. E x p e r i m e n t a l Set-up The experiments were performed in a fully automated experimental set-up. The gases hydrogen and nitrogen for balancing the total flux and the liquids nitrobenzene and aniline were dosed by thermal mass flow controllers (Bronkhorst Hi-Tec). Gaseous and liquid reactants were mixed and the liquids were vaporised in a commercial evaporater (CEM; Bronkhorst Hi-Tec). The bypass line allowed controlling the adjusted flow rates and starting the reaction at a certain point in time by switching two three-way-valves. A small part of the product stream was analysed online by a gaschromatograph (Chrompack CP9001) equipped with a HP-5 fused silica column and a flame ionisation detector. Reactant A
Reactant B
2.2 A s s e m b l y of the catalytic wall r e a c t o r
and preparation of the catalyst The CWR (see Fig. 1) consists of several reactor segments with varying length and an inner diameter of 10mm. The segments possess screw threads on both ends so that they can be interlinked by a nut and sealed by graphite gaskets. The individual tube segments are manufactured out of V4A-steel. The inner wall of the reactor segments was coated with catalyst by means of a suspension process [5]. To ensure the isothermal operation of the CWR the temperatures very close to the catalyst surface were measured by thermocouples positioned in radial holes in the wall of the reactor. The temperature in the gas bulk (axial direction) was determined by a movable thermocouple in the centre of the tube.
Tube segment
Heating
d e v ~ . . . . ~.,.,i
i~a~
ial 1 E
couple
Thermocouple . . . . .
Tube
i, ~'t~b,. I
Insulation
~
Product
Figure 1: Catalytic Wall Reactor
99
2.3 Experimental conditions All experiments were carried out at atmospheric pressure. The temperature was varied between 275~ and 425~ the inlet partial pressure of nitrobenzene between 1.7 kPa and 6.8 kPa. The molar ratio of hydrogen to nitrobenzene was adjusted between 3:1 and 24:1. The influence of aniline was investigated at partial pressures of 1.7 kPa, 3.4 kPa and 6.8 kPa. Tube segments with a catalyst coating between 2 cm and 12 cm in length were used for the experiments. The catalyst weight varied between 0.15 g and 1.8 g. To activate the catalyst, 10 vol.-% hydrogen in nitrogen was fed to the reactor at reaction temperature, before the hydrogenation reaction was started. The reaction was stopped after 1 h, 2 h, 5 h and 10 h, respectively. To remove sorbed components from the catalyst, the reactor was flushed with nitrogen for 3 h at reaction temperature.
2.4 Determination of the coke profiles along the reactor After every experiment the reactor was cooled down and the reactor segments were dismantled. The coked catalyst was scratched off the tubes in intervals of i cm and the carbon content of each sample was determined by TC analysis (CMat 550; StrShlein Instruments). The TC was calibrated with a mixture of CaO/CaCOa containing I wt% carbon.
3. RESULTS To exclude the possibility of reactions at the free metal surface of the reactor, at the inert bed or at the catalyst support (A1208), experiments were performed under reaction conditions without the active catalyst component Pd. The conversion of nitrobenzene was always less than 1% and is therefore negligible in the hydrogenation experiments. On the other hand coke formation was already observed to a considerable amount on the support. Figure 2 depicts the coke profiles on the support with and without palladium measured at the same reaction conditions. Comparing the coke content at the reactor inlet it can be concluded that at the chosen reaction
1,6 1,41,21,00,8 0,6i ,' i i i 0,4 0,0 1,0 2,0 3,0 4,0 5,0 6,0 .,.
I
. . . . . .
,__,
i . . . . .
,
,
,
i
i
~
a
. . . . .
~
!
i
I
i
!
i
i l l i
I
i i i i i i i i 1 I
. . . .
a
,
,
i
. . . . .
.
. . . . .
i
i
i i i i
I
l
i i i i
I
l i l l
I
z [cm]
Figure 2. Coke profiles of the support with (~) and without palladium (=) at pN~,o= 3,38 k P a , pH2,0-- 20,28 kPa and T = 425 ~
100
conditions approximately 70 % of the coke is formed on the support. The high contribution of the coke deposited on the support was also found in TPO measurements of the coked catalyst samples which showed two peaks belonging to coke on the active sites (C-a) and coke on the support (C-s). It can be clearly seen that in the case of the active catalyst the coke content is decreasing towards the reactor outlet which can be explained by the formation of coke from nitrobenzene parallel to the main reaction. As expected from t h a t conclusion adding the product aniline to the reactants has no influence to the coke formation. 3.1 S t e a d y - s t a t e k i n e t i c s The initial conversions and the initial reaction rates at different reaction conditions were obtained by extrapolating the conversion curves to time t = 0 min. It could be shown that hydrogenation of nitrobenzene on the fresh catalyst follows a Langmuir-Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step [5]: k . K ~ .Km . p ~ . p ~ to= _0.5 ~2 (l+Km3"Pm3 +Kin'Pro)
3.2 D e a c t i v a t i o n k i n e t i c s For the determination of the unsteady-state 1.0 kinetics the same reaction conditions were chosen as for the experiments described in Section3.1. The only difference was the ~° o.s length of the coating. In the experiments presented in this section the length of the 0.4 catalyst coating was 8 cm to resolve the coke profile. 0.2 '' . . . . . . . ~ .... ~ .... Figure 3 depicts the axial profiles of the coke 0.0 2.0 4.0 6.0 8.0 z [cm] content depending on time at a reaction temperature of 275°C. The partial pressure Figure 3. Time development of the coke of nitrobenzene was p~,o--3.4 k Pa and t h a t profile ( l h .; 2h A; 5h .; 1Oh .; 20h x) of hydrogen pH2,o=20.4 kPa.
101
1.5
0.7
!!
1 . 2 0.9 0.6
" - ............ .
0.6 0.5
0.2 0.2
0.3 0.0
2.0
4.0
6.0
z [cm]
Figure 4. Temperature dependence of coke profiles after 10h (275~ =; 325~ e; 375~ A; 425~ , )
' 0.0
2.0
, 4.0 z [cm]
6.0
8.0
Figure 5. Coke profiles at different hydrogen partial pressures after 10h (pH2,o= 10.1 kPa A; pro,o= 20.2 kPa ,; pH2,o= 40.4 kPa . )
Figure 4 shows the coke profiles at different reaction temperatures. At temperatures higher than approximately 325~ the coke content increases noticeably. In Figure 5 the coke profiles at different hydrogen partial pressures are shown. It can be seen that hydrogen counteracts the deactivation by coking. The higher the hydrogen content in the feed the lower the coke content on the catalyst. By fitting the measured coke profiles and the corresponding nitrobenzene conversions in the time domain the deactivation kinetics can be determined. For modelling this unsteady state problem the accumulation term and the timedependend hydrogenation rate must be considered in the mass balance of the reactor. Thus the following partial differential equation must be solved: C~NB _ ~t
c3(u " P ~ ) _ R . T . p . ru (t ) ~z
Indicating the coke content on the active sites with wc.~, the mass balance for the pseudo-species coke on the active sites can be written as: (~42C
~t
a
-
=rc_~(t )
The coke formation on the support is assumed not to be deactivated and its rate can be formulated as follows: ~hCC-s -- r
c3t
-
c- s, o
102
By the total carbon analysis it can not be distinguished between coke on active sites and coke on the support. Only the total carbon content wc is measured and thus the following equation has to be taken into account: W C = WC_ a + Wc_ s
Following the approach of Szepe and Levenspiel [6] the time dependend reaction rate r(t) is separated into the time independend initial rate ro and the activity function a(t): r(t) = ro 9a(t)
The initial hydrogenation rate rH,o can be written as shown in section 3.1. The initial coking rates rc.a,o and rc-~,o both depend on temperature and the partial pressures of nitrobenzene and hydrogen. A power law concerning partial pressure with a rate constant obeying Arrhenius law proved to be an appropriate rate equation for both coking rates. It was assumed that the orders ml and m2 in nitrobenzene and hydrogen partial pressure are equal in both cases:
rc_~.o = kc_,, p ~ P~
rc-~.o = kc-~ P~CBm2 PH2
With the assumption that only coke on the active sites is responsible for the decline in activity of the main reaction and the coke formation, aj can be expressed in terms of the coke content on active sites: a i =f(Wc_~)
By assuming different functions for the activity term, exponential equation indicated the most promising result:
the
following
a / = exp(-kde~Wc_,,) However, only the coke profiles could be fitted satisfactorily whereas fitting the conversion-time curves of nitrobenzene a distinct lack of model adequacy was observed. Thus, the model had to be modified to obtain a better fit to the experimental data. Only by dividing the exponential expression into three segments a sufficient description was achieved: an = exp(-kd~l., wc_a) ;.for
wc_a <_0.2%
103 a n = exp(-kae~2,nwc_a)
;,for
a H = exp(--kdea3,nWc__a)
;for
0.2 % < Wc_ . < 0.4 % >0.4%
Wc_ ~
The necessity of this rather uncommon approach points out that the coke formation and especially its influence on the main reaction are rather complex. The division into three segments allows the speculation t h a t the mechanism of the main reaction changes with the coke content and with reaction time, respectively. The determined kinetic constants of the unsteady state model are all significant as can be seen from Table 1:
Parameter
Estimated Value .
.
Standard deviation
.
.
.
%
koa,0 [kPa(m'ml)s"l]
242.5
6.0
,,,
Exc.a [kJ/mol]
3 7.1
1.1
kos,0 [kPa("aml)sl]
1.83" 10.3
7.63" 10.4
EA,c-, [kJ/mol]
50.7 .
.
.
.
.
.
.
2.1
.
ml [-]
1.42
m2 [-]
0.45
.
.
.
kdeal,H ['] .
.
.
.
kao,Z,H[-]
.
.
.
.
.
.
.
.
.
.
.
.
0.02 13.2 .
327
.
.
461 .
.
0.04 .
.
.
.
.
.
2.8
,,,
ka=,3,H[-]
574
22.2
ka~,,c-, [-]
4325
68.2
Table 1. Kinetic constants of the unsteady state model
The parity plots of calculated and experimental data are depicted in Figure 6.a (nitrobenzene) and Figure 6.b (coke). The deviations are for the most part within 20 % and in the case of coke formation there is statistical scattering whereas in the case of the nitrobenzene partial pressures there are still trends in the deviations.
104
a)
b) 4.0
2.0
.
1.5
3.0 ,-...,
'.
............ ~...... + 2 0 % ..... ~'... : ....... .:,/ -
..~ :o,
0. .i
9~ d
2.0
- -//
1.0
:. !
_ /
-..
El.
1.0
0.5
0.0
"
~
..............................
0.0
0.0
1,0
2.0
PNB,e~[kea]
3,0
4.0
0.0
0.5
1.0
1.5
~.0
Wc,exp [%]
Fig. 6. Unsteady state kinetics: parity plots of calculated and experimental data. a) nitrobenzene, b) coke 5. CONCLUSIONS The experiments showed that a Catalytic Wall Reactor is an appropriate tool to perform both steady-state and unsteady-state kinetic measurements for highly exothermic reactions under isothermal conditions. By separating the initial reaction rate from the deactivation rate it was found that the hydrogenation of nitrobenzene follows a Langmuir-Hinshelwood approach. Descending axial coke profiles measured by total carbon analysis identified nitrobenzene as coke precursor whereas high partial pressures of hydrogen suppressed the formation of coke. Aniline had no influence neither on the main reaction nor on the deactivation by coking. TPO measurements of the coked catalyst showed two peaks belonging to coke on the active sites and coke on the support. The adjustment of calculated and measured coke profiles confirmed the TPO measurements. 6. REFERENCES
[1] [2] [3] [4] [5]
A. Voorhies, Ind. Eng. Chem. 37, 318 (1945). G.F. Froment, K.B. Bischoff, Chem. Eng. Sci. 17, 105 (1962). H.C. Beirnaert, R. Vermeulen, G.F. Froment, Stud.Surf.Sci.Catal. 88, 97 (1994). K. Liu, S.C. Fung, T.C. Ho, D.S. Rumschitzki, Ind. Eng. Chem. Res. 36, 3264 (1997) B. Amon, E. Klemm, G. Emig, in: Reaction Kinetics and the Development of Catalytic Processes, Proceedings of the Int. Symp., Brugge, Belgium, 1999, in press. [6] S. Szepe and O. Levenspiel, Catalyst deactivation, in Chemical Reaction Engineering, Pergamon Press, Oxford,1971.