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Impurity Poisoning of Ni-Catalyst Pellets
B. Delmon and G.F. Frornent (Eds.) Catalyst Deactivation 1994 Studies in Surface Scicnce and Catalysis, Vol. 88 0 1994 Elsevicr Science B.V. All rights reserved.
623
IMPURITY POISONING OF Ni-CATALYST PELLETS D. RuSiC and S. ZrnEevic Faculty of Chemical Engineering & Technology, University of Zagreb, Croatia The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/A1203 in benzene hydrogenation. The kinetic experiments, activity tests, and poisoning experiments were mesured in a series of differential reactor experiments at atmospheric total pressure at temperatures 403, 427 and 448 K. It was found that for impurity poisoning concentration of benzene and thiophene decreases from a maximum value at the surface to the centre. This is reflected in the residual activity profile which has a maximum at the centre of the pellet. It also was found that there is satisfactory degree of correlation between calculated effectiveness factor and obtained with experimental observation.
1. INTRODUCTION In many industrial important gas-solid catalytic reactions the activity of the catalyst decreases with time on stream. Therefore, quantitative study of the activity-time relation is important in seeking the optimum design and operation of the reactor. The first step in treating the problem in an analysis of the behaviour of a single catalyst pellet. Since the most industrially important catalyst are porous pellets the reactant must be transported to and within particles before they can he converted. Transfer resistances strongly affect the rate of the main reaction and it is possible to assume that the rate of deactivation will also depend on the extent of mass transfer resistance. The effect of intraparticle diffusion on a catalytic process with accompanying catalyst deactivation has been presented in a number of papers. Most of them have been summarized in the monography by Hughes [ 11 and the review papers by Butt [2] and Morbidelli et al. [3]. For systems involving chemical poisoning it is generally necessary to solve a set of coupled partial differential equations describing the reactant and product concentrations and activity profile withinh catalyst particle, which are linked to a set of material balance relationships describing the change in concentration within the bulk phase of the reactor. If the poisoning reaction can only be represented by concentration independent rate equation it can easily he solved [4,5].
624
The purpose of the present paper is an experimental and modeling study of the effects of thiophene poisoning on the behaviour of diffusionally influenced Ni/A1203 pellet during benzene hydrogenation.
2. EXPERIMENTAL METHOD The kinetic experiments, activity tests, and poisoning experiments were carried out in a gas-flow isothermal fixed bed reactor [6] at the benzene partial pressure of 7.55 kPa; hydrogen partial pressure 99.82 kPa; thiophene partial pressure 0.032 kPa and the reaction temperatures 403, 427 and 448 K. The size of the commercial cylindrical catalyst pellet was 5x5mm (21%Ni on alumina, supplied by BASF). The nickel oxide containing precursor was activated by reduction with hydrogen at 743 K for 10 hr.
3. RESULTS AND DISCUSSION The main reaction i.e. benzene hydrogenation occuring inside porous Ni-catalyst pellets is accompanied by poisoning reaction in which the thiophene presented in the feed stream reacts irteversibly with the catalytic active sites. An analysis was made assuming isothermal behaviour [ 7 ] , the same effective diffusivity for reactant and poison and that the steady-state continuity equation represents a good approximation at all times [8,9]. Under these conditions the mass balances for benzene, thiophene and catalyst activity are d2C I d C D , l + -2 - (1- a,)dkC;IC: = 0 dr2 r dr d2Cp I d C + -2 - k,(l- u , ) ~ C , = 0 dr2 r dr
Dp-
3 = k,(l-
ap)dCp dt with initial and boundary conditions q = l(fresh) or q(r)(deactivated) Ci = Cio(r); i = A,P (dCi /dr)o = (dq/dr), = 0
The orthogonal collocation polynomial approximation using a single parameter trial function was employed to solve equations (l)-(3). In addition to the solution for time concentration and activity profiles, effectiveness factors representing the combined effect of mass transfer resistance and poisoning in terms of pellet surface conditions were computed according to
625
1
(1 -ap)dkCEC:o jY:d(r*nl) V
(4)
Parameters employed in the simulation are given in Table 1. Table 1. Parameters for Benzene Hydrogenation and Thiophene Poisoning Kinetics. ~~
~
~~
~
~
~
~
2
9
0"
T
105 k
m
403 427 448
6.53 7.29 7.9s
0.33 0.3 1 0.30
n
0.24 0.35 0.4 1
1 .o
1.o
0.8
0.8
0.6
-
4
0.4
lo5 k
d
6.24 5.41 5.01
0.91 0.75 0.76
0.6 0.4
0.2
0.2
0.0 0.0 0.2 0.4
~
Deactivation
Hydrogenation
0.6 dR
0.8
1.0
.
.
.
.
.
.
0.0 0.2 0.4 0.6 0.8 1.0 dR
0.0 1.0
0.5
0.0 dR
0.5
1.0
Figure 1. Variation of benzene concentration (A), thiophene concentration (B) and residual activity (C) within catalyst particle during poisoning. Typical results of the simulation are shown in Fig.1. It compares the change of benzene, thiophene and residual activity profiles inside the pellet with time due to thiophene poisoning for an intermediate value of diffusion resistances ($*= 5.10, ($= 3.70). The profiles of benzene and thiophene have, as expected, a maximum at the surface. The corresponding activity profiles will therefore show a minimum at the surface and the maximum at the pellet centre. With increasing time, the benzene and thiophene molecules are spread along the whole catalyst pellet and a nearly flat concentration appears when the time on stream is about 100 min. This is reflected in the residual activity profile.
626
1 0.80 - j 0.40
E 0.10
0.08
0.04
0.04 T-427 K
$A-
T-448 K
2.63
0.1 0.2
0.61.02.0
0
6.010.0
0.01 0.1 0.2
0.61.0 2.0
6.010.0
0
0.01 0.1 0.2
0.61.0 2.0
6.010.0
0
Figure 2 . Time-dependent variation of effectiveness factor. The calculated time-dependent effectiveness factor as a function of Thiele modulus along with experimental observation is shown in Fig. 2 . The solid lines represent the computed effectiveness factor using EQ.4 and the broken line represents experimental values. When compared with experimental data, we see that the mathematical model represents the actual behaviour quite well. Referring again to Fig. 2 one can see that the effect of mass transport on the rate of benzene hydrogenation is somewhat higher then predicted by model specially at shorter poisoning time. The reason for such behaviour can be found in the inaccuracy involved in the measurements and in the contribution of experimental uncertainity in parameter determination.
4. CONCLUSION The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/A1203 in benzene hydrogenation. It was found that for impurity poisoning concentration of benzene and thiophene decrease from a maximum value at the surface to the centre. This is reflected in the residual activity profile which has a maximum at the centre of the pellet. It was also found that there is a satisfactory degree of correlation between calculated effectiveness factor and obtained with experimental observation.
5. NOTATION CA CB Cp d
benzene concentration (mol dm-3) hydrogen concentration (mol dm-3) thiophene concentration (mol dm-3) deactivation order
627
DA
DP k kd
m
n 9
effective diffusion coefficient for benzene (cm2 s-1) effective diffusion coefficient for thiophene (cm2 s-l) constant of reaction rate (mol kg-' s-l) constant of deactivation rate (mol kg-' s-l) rate order for hydrogen rate order for benzene concentration of poison on catalyst, go maximum concentration corresponding to
t T
complete deactivation (mol kg-I) radial coordinate (cm) radius of catalyst pellet (cm) time (s) temperature (K)
@A
effectiveness factor Thiele modulus for benzene hydrogenation
@B
Thiele modulus for catalyst deactivation
0
dimensionless time
r
R
aP
=9bo
REFERENCES 1. R. Hughes, Deactivation of Catalyst, Academic Press, London, 1989. 2. J. B. Butt, Catalyst Deactivation and Regeneration, in Catalyst: Science and Technology, Vol. VI, Academie-Verlag-Berlin, 1985. 3. M. Morbidelli, P. Forzetti, G. Buzzi-Ferraris and S. Carra, Influence of Transport Phenomena on the behaviour of Single Particle, La Chimica el Industria, 1981. 4. S. Kirshnaswamy and J . R. Kittrell, A. I. Ch. E. J., 27 (1981) 120. 5 . D. RuSid and S. ZrnEevid, J . Chem. Tech. Biotech., 57 (1993) 217. 6. S. ZmEeviC, Z. Gomzi and E. Kotur, Ind. Eng. Chem. Res., 29 (1990) 774. 7. S. ZmCevic', Chem. Eng. Sci., 39 (1984) 1245. 8. C.E. Megiris and J. B. Butt, Ind. Eng. Chem. Res., 29 (1990) 1065. 9. R. Christoph and M. Baerns, Ber. Bunsenges. Phys. Chem., 90 (1986) 981.
623
IMPURITY POISONING OF Ni-CATALYST PELLETS D. RuSiC and S. ZrnEevic Faculty of Chemical Engineering & Technology, University of Zagreb, Croatia The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/A1203 in benzene hydrogenation. The kinetic experiments, activity tests, and poisoning experiments were mesured in a series of differential reactor experiments at atmospheric total pressure at temperatures 403, 427 and 448 K. It was found that for impurity poisoning concentration of benzene and thiophene decreases from a maximum value at the surface to the centre. This is reflected in the residual activity profile which has a maximum at the centre of the pellet. It also was found that there is satisfactory degree of correlation between calculated effectiveness factor and obtained with experimental observation.
1. INTRODUCTION In many industrial important gas-solid catalytic reactions the activity of the catalyst decreases with time on stream. Therefore, quantitative study of the activity-time relation is important in seeking the optimum design and operation of the reactor. The first step in treating the problem in an analysis of the behaviour of a single catalyst pellet. Since the most industrially important catalyst are porous pellets the reactant must be transported to and within particles before they can he converted. Transfer resistances strongly affect the rate of the main reaction and it is possible to assume that the rate of deactivation will also depend on the extent of mass transfer resistance. The effect of intraparticle diffusion on a catalytic process with accompanying catalyst deactivation has been presented in a number of papers. Most of them have been summarized in the monography by Hughes [ 11 and the review papers by Butt [2] and Morbidelli et al. [3]. For systems involving chemical poisoning it is generally necessary to solve a set of coupled partial differential equations describing the reactant and product concentrations and activity profile withinh catalyst particle, which are linked to a set of material balance relationships describing the change in concentration within the bulk phase of the reactor. If the poisoning reaction can only be represented by concentration independent rate equation it can easily he solved [4,5].
624
The purpose of the present paper is an experimental and modeling study of the effects of thiophene poisoning on the behaviour of diffusionally influenced Ni/A1203 pellet during benzene hydrogenation.
2. EXPERIMENTAL METHOD The kinetic experiments, activity tests, and poisoning experiments were carried out in a gas-flow isothermal fixed bed reactor [6] at the benzene partial pressure of 7.55 kPa; hydrogen partial pressure 99.82 kPa; thiophene partial pressure 0.032 kPa and the reaction temperatures 403, 427 and 448 K. The size of the commercial cylindrical catalyst pellet was 5x5mm (21%Ni on alumina, supplied by BASF). The nickel oxide containing precursor was activated by reduction with hydrogen at 743 K for 10 hr.
3. RESULTS AND DISCUSSION The main reaction i.e. benzene hydrogenation occuring inside porous Ni-catalyst pellets is accompanied by poisoning reaction in which the thiophene presented in the feed stream reacts irteversibly with the catalytic active sites. An analysis was made assuming isothermal behaviour [ 7 ] , the same effective diffusivity for reactant and poison and that the steady-state continuity equation represents a good approximation at all times [8,9]. Under these conditions the mass balances for benzene, thiophene and catalyst activity are d2C I d C D , l + -2 - (1- a,)dkC;IC: = 0 dr2 r dr d2Cp I d C + -2 - k,(l- u , ) ~ C , = 0 dr2 r dr
Dp-
3 = k,(l-
ap)dCp dt with initial and boundary conditions q = l(fresh) or q(r)(deactivated) Ci = Cio(r); i = A,P (dCi /dr)o = (dq/dr), = 0
The orthogonal collocation polynomial approximation using a single parameter trial function was employed to solve equations (l)-(3). In addition to the solution for time concentration and activity profiles, effectiveness factors representing the combined effect of mass transfer resistance and poisoning in terms of pellet surface conditions were computed according to
625
1
(1 -ap)dkCEC:o jY:d(r*nl) V
(4)
Parameters employed in the simulation are given in Table 1. Table 1. Parameters for Benzene Hydrogenation and Thiophene Poisoning Kinetics. ~~
~
~~
~
~
~
~
2
9
0"
T
105 k
m
403 427 448
6.53 7.29 7.9s
0.33 0.3 1 0.30
n
0.24 0.35 0.4 1
1 .o
1.o
0.8
0.8
0.6
-
4
0.4
lo5 k
d
6.24 5.41 5.01
0.91 0.75 0.76
0.6 0.4
0.2
0.2
0.0 0.0 0.2 0.4
~
Deactivation
Hydrogenation
0.6 dR
0.8
1.0
.
.
.
.
.
.
0.0 0.2 0.4 0.6 0.8 1.0 dR
0.0 1.0
0.5
0.0 dR
0.5
1.0
Figure 1. Variation of benzene concentration (A), thiophene concentration (B) and residual activity (C) within catalyst particle during poisoning. Typical results of the simulation are shown in Fig.1. It compares the change of benzene, thiophene and residual activity profiles inside the pellet with time due to thiophene poisoning for an intermediate value of diffusion resistances ($*= 5.10, ($= 3.70). The profiles of benzene and thiophene have, as expected, a maximum at the surface. The corresponding activity profiles will therefore show a minimum at the surface and the maximum at the pellet centre. With increasing time, the benzene and thiophene molecules are spread along the whole catalyst pellet and a nearly flat concentration appears when the time on stream is about 100 min. This is reflected in the residual activity profile.
626
1 0.80 - j 0.40
E 0.10
0.08
0.04
0.04 T-427 K
$A-
T-448 K
2.63
0.1 0.2
0.61.02.0
0
6.010.0
0.01 0.1 0.2
0.61.0 2.0
6.010.0
0
0.01 0.1 0.2
0.61.0 2.0
6.010.0
0
Figure 2 . Time-dependent variation of effectiveness factor. The calculated time-dependent effectiveness factor as a function of Thiele modulus along with experimental observation is shown in Fig. 2 . The solid lines represent the computed effectiveness factor using EQ.4 and the broken line represents experimental values. When compared with experimental data, we see that the mathematical model represents the actual behaviour quite well. Referring again to Fig. 2 one can see that the effect of mass transport on the rate of benzene hydrogenation is somewhat higher then predicted by model specially at shorter poisoning time. The reason for such behaviour can be found in the inaccuracy involved in the measurements and in the contribution of experimental uncertainity in parameter determination.
4. CONCLUSION The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/A1203 in benzene hydrogenation. It was found that for impurity poisoning concentration of benzene and thiophene decrease from a maximum value at the surface to the centre. This is reflected in the residual activity profile which has a maximum at the centre of the pellet. It was also found that there is a satisfactory degree of correlation between calculated effectiveness factor and obtained with experimental observation.
5. NOTATION CA CB Cp d
benzene concentration (mol dm-3) hydrogen concentration (mol dm-3) thiophene concentration (mol dm-3) deactivation order
627
DA
DP k kd
m
n 9
effective diffusion coefficient for benzene (cm2 s-1) effective diffusion coefficient for thiophene (cm2 s-l) constant of reaction rate (mol kg-' s-l) constant of deactivation rate (mol kg-' s-l) rate order for hydrogen rate order for benzene concentration of poison on catalyst, go maximum concentration corresponding to
t T
complete deactivation (mol kg-I) radial coordinate (cm) radius of catalyst pellet (cm) time (s) temperature (K)
@A
effectiveness factor Thiele modulus for benzene hydrogenation
@B
Thiele modulus for catalyst deactivation
0
dimensionless time
r
R
aP
=9bo
REFERENCES 1. R. Hughes, Deactivation of Catalyst, Academic Press, London, 1989. 2. J. B. Butt, Catalyst Deactivation and Regeneration, in Catalyst: Science and Technology, Vol. VI, Academie-Verlag-Berlin, 1985. 3. M. Morbidelli, P. Forzetti, G. Buzzi-Ferraris and S. Carra, Influence of Transport Phenomena on the behaviour of Single Particle, La Chimica el Industria, 1981. 4. S. Kirshnaswamy and J . R. Kittrell, A. I. Ch. E. J., 27 (1981) 120. 5 . D. RuSid and S. ZrnEevid, J . Chem. Tech. Biotech., 57 (1993) 217. 6. S. ZmEeviC, Z. Gomzi and E. Kotur, Ind. Eng. Chem. Res., 29 (1990) 774. 7. S. ZmCevic', Chem. Eng. Sci., 39 (1984) 1245. 8. C.E. Megiris and J. B. Butt, Ind. Eng. Chem. Res., 29 (1990) 1065. 9. R. Christoph and M. Baerns, Ber. Bunsenges. Phys. Chem., 90 (1986) 981.