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Deactivation of the high-temperature water-gas shift catalyst in nonisothermal conditions
Applied
Catalysis A: General,
Elsevier
Science
APCAT
A2294
Publishers
87 (1992)
185-203
185
B.V., Amsterdam
water-gas
Deactivation of the high-temperature catalyst in nonisothermal conditions
shift
Riitta L. Keiski Department
of Process Engineering,
University
of Oulu, SF-90570
Oulu (Finland)
and Tapio Salmi Department (Received
of Chemical 27 December
Engineering,
Abe Akademi,
1991, revised manuscript
SF-20500
received
Turku
(Finland)
25 April 1992)
Abstract The water-gas shift reaction over a commercial iron oxide/chromium oxide catalyst was investigated to determine the kinetics of the catalyst deactivation and the physical properties of the catalyst during the deactivation. A nonisothermal fixed bed reactor was used to collect data for the kinetics of catalyst deactivation. The laboratory scale reactor operated close to industrial practice at 575-723 K. The decline of the catalyst activity was found to be due to a sintering process. No carbon formation was detected according to scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The decay of the catalyst activity was quite rapid during the first 150 h of operation and less rapid between 150 and 600 h. The decay of the catalyst activity was linked to a decrease of the surface area and to an increase of the mean pore size of the catalyst. The kinetic and deactivation parameters were determined by nonlinear regression using the temperature and carbon monoxide concentration profiles in the catalyst bed. The level of catalyst activity as a function of the catalyst age was best described by a hyperbolic modelfor the frequency factor kb =A,/(1 +at)“whereA,=6.60*106 (dm3)‘.03 mol-0-03 kg-Is_‘, az0.042 h-’ and n = l/3. The time dependence of the surface area (S) of the catalyst was also described best by a hyperbolic model S=S,/ (l+ bt)” where S,=53.2 m’/g, b=8.70 hh’ and m=0.048. Calculation of the order of the sintering kinetics from the k& vs. S relationship showed that two different processes of the activity decay are involved: a fast initial activity decay with sintering order of 3 and a slow activity decay with a high order of sir&ring. Keywords: catalyst
deactivation,
iron oxide/chromium
oxide, kinetics,
water-gas
shift.
INTRODUCTION
The water-gas shift reaction CO+H,O=CO,+H,
(1)
Correspondence
to: Dr. T. Salmi,
Turku,
Tel. ( + 358-21)654427,
Finland.
0926-3373/92/$05.00
Department
0 1992 Elsevier
of Chemical
Engineering,
Abe Akademi,
fax. ( + 358-21)654479.
Science
Publishers
B.V.
All rights
reserved.
SF-20500
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
186
is a step in many industrial processes. The most common catalyst for this reaction is the iron oxide/chromium oxide catalyst that is used to enhance the water-gas shift reaction in ammonia synthesis, hydrogen production and town gas purification [ 1,2]. The activity of the iron oxide/chromium oxide catalyst decreases under process conditions. The causes of the decline in activity of the shift catalyst are: poisoning, coking and sintering [ 3-61. It has been shown by many workers that the most probable cause of the loss in activity is sintering. Sintering of the iron oxide/chromium oxide catalyst leads to lower surface area, reduction in porosity, increase in particle size and decrease of the number of pores smaller than 30 nm [ 71. The loss in activity, i.e. in surface area, is shown [8] to be a function of the reaction temperature and the age of the catalyst: l/S”l/St = k,t exp ( -E” /R,T), where S, is the initial surface area (m”/g), S is the surface area of the catalyst at time t, E” is the activation energy of sintering (kJ/mol) and n is the order of sintering. The calculated values of n and E” according to Chinchen et al. [ 91 are 4.7 and 196.5 kJ/mol, respectively. Hoogschagen and Zwietering found values of 6.2-7.3 and 272 kJ/mol, respectively [8]. Chandra et al. [lo-121 have found that the activity decreases rapidly during the initial period up to 100-150 h, the fall in activity is quite fast during 150400 h, less fast between 400 and 700 h and after 1000 h the activity remains rather constant. Chinchen et al. [9] have also found a fast initial decay and a slow decay in the catalyst activity and they have shown that the initial decay of activity is of much lower order (n= 1.3-3.0) than the slow decay of activity. These two stages of activity were also described by Singh and Saraf [ 131. These observations provide strong evidence for different processes controlling the loss in activity over different time periods. In the initial decay the low value of n has been attributed, according to Chinchen et al. [ 91, to particle growth by two simultaneous but different mechanisms, or to the rapid agglomeration of very small particles. The aim of the present work was to study the deactivation kinetics of the iron oxide/chromium oxide catalyst under conditions close to those experienced industrially and to investigate the physical reasons for this deactivation phenomenon. To determine the kinetics of catalyst deactivation and the cause of deactivation, experiments were performed to analyze the carbon monoxide conversions at different time intervals and to determine the physical properties of fresh and used catalysts, EXPERIMENTAL
PROCEDURE
Nitrogen physisorption at 77 K was used to determine the surface area, pore size distribution, average pore radius and total pore volume of the pores between l-40 nm for fresh and used catalysts. The mercury penetration method
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
187
TABLE 1 Experimental conditions in the four separate experimental sets to determine deactivation kinetics Experiment
To W)
Dry gas space velocity (h-l)
Number of samples
Samples taken between (h)
A B C D
575 600 625 650
795 1997 1997 1997
10 8 5 11
73-400 87-262 90-366 28-404
was used to obtain the pore size distribution of larger pores ( > 50 nm). The nature of the catalyst deactivation was studied by X-ray photoelectron spectroscopy (XPS ) and scanning electron microscopy (SEM). Experimental conditions close to industrial practice were used in the kinetic and deactivation studies. The nonisothermal packed bed reactor and the outline of measurements are presented in earlier publications [ 14-161. The reactor inlet temperature was varied between 575 and 675 K, the water-to-dry gas ratio was varied between 0.33 and 1.24 and the dry gas composition was typically 7-30% carbon monoxide, 3-20% carbon dioxide and 25-70% hydrogen. Temperature and carbon monoxide conversion values were measured from six axial positions in the reactor and inlet and product gas compositions were analyzed by a gas chromatograph. Cylindrical iron oxide/chromium oxide catalyst particles (CCE, C12,3.2 x 3.2 mm) were used from Catalysts and Chemicals Europe. The deactivation studies were made by repeating a set of experiments after certain time intervals. Four different sets were considered. The inlet temperature varied between 575 and 650 K and the dry gas space velocity was 795 or 1997 h-l. The experimental conditions are summarized in Table 1. The inlet dry gas composition was in all deactivation experiments: 13.6% carbon monoxide, 9.2% carbon dioxide, 54.4% hydrogen and 22.8% nitrogen. The water-to-dry gas ratio was 0.83 and the total pressure was 1.08 bar. The reaction conditions were changed using the inlet temperature, component concentration and water-to-dry gas ratio as variables. RESULTS AND DISCUSSION
Physical properties
of the catalyst
The pore size distributions for a fresh and two used catalysts (CCE, C12) measured by nitrogen condensation and mercury penetration methods are shown in Fig. 1. The pore size distribution is shifted towards larger pore radii
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203 -
30.00
25.00
20.00
15.00
10.00
5.00
ao-
0.00 0.1
0.01
1.00
Dp/w
Fig. 1. The pore size distribution for one fresh and two used iron oxide/chromium oxide catalysts using the nitrogen adsorption and mercury penetration methods: (m) a fresh catalyst, (0 ) a catalyst used in the process gas at 575-723 K for 430 h and (0 ) a catalyst used in a commercial plant for two to three years.
during the catalyst deactivation. The surface area of the catalyst decreases, the average pore radius increases, the total pore volume remains almost unchanged and the number of smaller pores decreases. It may be suggested that the decrease in the amount of smaller pores with the corresponding decrease in the catalytic activity means that the smaller pores ( < 30 nm ) contribute most to the shift reaction. If the pore size distribution of the catalyst (CCE, C12), which had been in industrial use over two years is compared to the pore size distributions of a fresh catalyst and a catalyst used under laboratory conditions, it is observed that the smaller pores in the industrial catalyst have almost completely disappeared and the typical pore sizes are between 90 and 200 nm. Thus, it appears that if an iron oxide/chromium oxide catalyst is used up to complete deactivation there will be no smaller pores present and all the pores will have a diameter larger than 50 nm, which compares with the pore size of magnetite [ 171. In laboratory use (e.g. t=430 h and 7’,,,= 675-723 K) the values of the surface area, total pore volume and average pore radius change as follows compared to a fresh catalyst: the surface area decreases from 53.2 to 35.5 m2/g, the total pore volume decreases from 0.136 to 0.104 cm3/g and the average pore
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
189
radius increases from 5.1 to 6.0 nm. These observations are in accordance with literature data [ 18,191. The estimated and experimental surface area vs. maximum reaction temperature is shown in Fig. 2. The behaviour of the catalyst surface area as a function of the maximum reaction temperature in the catalyst bed and the catalyst age is described by the empirical relationship: SBET=122.4-O.OOO&-O.l24T,,,
(21
The maximum reaction temperatures and catalyst ages for the catalysts used in the parameter estimation were: catalyst I T,,, = 873 K and t = 250 h, catalyst II T,,, = 872 K and t = 45 h, catalyst VII T,,, = 724 K and t = 263 h, catalyst VIII T,,, = 709 K and t = 125 h, and catalyst XII T,,, = 730 K and t = 610 h. The deactivation of the iron oxide/chromium oxide catalyst may also be due to a coke formation process. Therefore, some fresh and used catalysts, as indicated earlier, were analyzed by SEM and XPS. According to these analyses both fresh and used catalysts contained carbon species but according to the elemental analysis the amount of carbon had not increased on the catalyst surface during the reaction. Thus, carbon formation during the process seems to be negligible or very small and the main cause for the deactivation is sintering. Reaction kinetics The reaction kinetics are described by the power-law model [ 1,15,16]
5 10
I,
ho
650
7w
750
800
850
T/K
Fig. 2. The experimental and estimated surface area as a function of the maximum reaction temperature;catalystbedI(O),II(~),VII(+),VIII(O)andXII(*):t=300h(-_),t=1OOOh (---).
190
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
and KT is the equilibrium constant. The kinetic parameters of rate eqn. (3) were determined [ 14-161 using the carbon monoxide conversion and temperature profiles from the nonisothermal plug-flow reactor. The following parameters were obtained: wherep=
(CCO~H~ )/(c
co c H&T)
r=kb exp( -9600K/T)c~~4c~~~cc~~‘8(1-p)
(4)
Modelling of the catalyst deactivation by sintering The empirical k’=kb
rate constant
is given by
exp( -E/RGT)
(5)
The frequency factor kb is proportional to the concentration the catalyst surface, c, (in mol/kg catalyst):
of active sites on
k;, =k&,Y,
(6)
where a! is an empirical exponent. The concentration of active sites based on the catalyst surface area, c h, (mol/ m2 catalyst) is related to c, by &S=c,
(7)
where S is the specific surface area of the catalyst. eqn. (6) we obtain for the frequency factor:
After inserting
k;, =k,,c:S= For sintering, by dS dt=
eqn. (7) into (8)
an empirical
model for the loss in surface area [3 ] is described
_ k”Sfl
(9)
where k” is the rate constant of sintering. time dependence of the surface area: S=SO exp( -k”t)
forp=l
Integration
of eqn. (9) gives the (10)
and S= [S~-8+k”(p-l)t][1’(1-P)1 which are inserted
for/?#l
(11)
in eqn. (8):
kb=k,(ck)“Stexp(--“cut)
forp=l
(12)
and (13)
R.L. Keiski and T. Salmi /Appl.
191
Catal. A 87 (1992) 185-203
It should be noted that eqn. (11) is similar to the equation proposed by Hoogschagen and Zwietering [8]: p- 1 corresponds to n and k” (p- 1) corresponds to k,exp( -F/&T). If we take into account that in principle 12” is also temperature dependent, the frequency factor kb can be given by the following equations: exp[-kicrtexp(-E”/RoT)]
kb=k,(c~)“S~
for/?=1
(14)
and
“=
h,(ckJ”% [I+&’
forPZ3
(/&1)&f-‘t][“/(&l)l
exp(__E”/&J)
(15)
After introducing the following lumped parameters A,=lz,(c~)*S;
(16)
for/?=1
a,=k$a
(17)
ao=ki(p-l)S[-’
forP#l
(13)
and n=cX/(b-1)
(19)
the frequency factor kb can be expressed by the following two equations: kb=A,,
exp [ --a0 exp( -E”/RoT)t]
far/3=1
(20)
and
Izb= [l+a,
A0 exp( -E”/R,T)t]”
(21)
forPZ1
If the temperature dependence of k” is neglected, eqns. (20) and (21) are reduced to eqns. (22) and (23): k&=Ao
exp(--at)
forj3=1
(22)
and kb=A,J(l+at)”
for/?#l
(23)
wherea=k”cxfor/?=l anda=k”(p-l)S,P-’ forP#l. In principle the parameter a is also temperature dependent according to eqns. (20) and (21) and the determination of the parameters A,,, kz and E” should be carried out simultaneously using nonisothermal rate data and inserting the expressions (20) and (21) for the frequency factor in the equation of the rate constant (5 ) : k’=A,
exp[ - (E/RoT+aO
exp( -E”/RoT)t]
far/3=1
(24)
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
192
and
41 ew( -NW)
k’=[l+ao exp(--E”/RoT)t]n
forPZ1
(25)
Estimation of the kinetic parameters of the deactivation models Parameter estimation The kinetics of the catalyst deactivation were determined using the four separate experimental sets (Table 1) from the catalyst bed that contained pelletized FesOl-CrzO, catalyst particles (3.2 x 3.2 mm). The carbon monoxide conversion data obtained from each reactor outlet are shown in Fig. 3. In the
, r __-..-_-_
r
-_--m-1-,
The decrease of the catalyst activity during the reaction,ttot = 430h: (a) To=5% K and (c) T,,=625KandSV=1997h-‘, (d) To=65OK andS,~l997h~*;(~)L~0.2dm,(O)L=0.6dm,(+)L=l.Odm,(*)L=~.3dm~(~)L~l~6 dm; the reaction gas composition: 13.6% CO, 9.2% COZ, 54.4% Hz and 22.8% Nz, water/dry gas=0.83. Fig.
3.
,$=79sh-1, (b) T0=600KandS,=1997h-‘,
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
193
first stage the parameter values in eqn. (23)) where all parameters are temperature independent, were estimated. The estimation of the deactivation parameters was carried out using the following procedure. First, the power law equation (3)where E/RG,n,m,pwerefixedto E/RG=9600K,n=0.74,m=0.47 andp= -0.18 [15,16] [eqn. (4)] was used to determine the frequency factor kb from each experiment using the conversion versus reactor bed length data obtained [ 141. Then the kb values versus the catalyst age relationship were expressed with models (22) and (23) with n=2,1,1/2,1/3,1/4 and l/5 (the two parameter model). The parameter values in eqn. (23 ) were also estimated allowing n to be unknown a priori (the three parameter model). Parameter estimation using the values n = 1,2,3 [ eqn. (23) ] and n = CXI[ eqn. (22 ) ] was studied in detail in earlier work [ 141, where we observed that the best fit to deactivation data was provided by n=l when the cases n=1,2, .... co were considered. The model equation (23) was reparametrized to reduce the correlation between A, and a. The modified parameters were: P, = l/A 61’p3),Pz =cI/A~~'~~) and P, = n.The parameters PI, Pz and P3 were determined using the algebraic option of the nonlinear regression program REPROCHE [ 201. The parameter values obtained from each experiment (A, B, C and D) are presented in Table 2. The best fits according to the values of the mean residual square (MRS ) were obtained when the hyperbolic model [eqn. (23)) where n = l/3 or l/2] was used. For experiments A and B the mean residual square was at its least for the two parameter model with n = 1 and l/2, respectively. The three parameter model gave n values for these experiments as follows: n=0.97 for experimental set A and n=0.66 for experimental set B. For the experiments C and D the best fits were obtained with n=1/3 and n=1/4, respectively. The values of n obtained in the estimation of the three parameter models were n=0.28 for C and n=0.26 for D. The standard deviations of the parameters increased with decreasing n.This was due to the numerical value of kb predicted by the model [eqn. (23) 1, approaching unity when n ( =P3) approached zero. The experimental data and the estimated Izbprofiles for the two parameter models with n= &l/2,1/3 and l/4 can be seen in Figs. 4-5 for experimental sets A and D, respectively. The frequency factor k& was very similar in experimental sets B, C, and D. Therefore the parameters in eqn. (23) were also determined by combining the data from the experimental sets B, C and D. The parameter values obtained from this estimation are presented in Table 3. The values of A,,and a for the three parameter model and two parameter models calculated from the parameters I’,, P, and P, are also shown in Table 3. The best fit was obtained with the hyperbolic model, eqn. (23) with n= l/3. The parameter values obtained were A0=6.6-106 (dm3)‘.03/(mo10.03kgs) and a = 4.2. lo-’ h- ‘. The next best fit was obtained with the three parameter model. The parameter values obtained were n = 0.315, A, = 6.90. lo6 (dm3) ‘.03/
194 TABLE
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203 2
Estimation
of the parameters in the deactivation
Experiment
Ps=n
P,
A
0.969 2
B
0.239
1.023 0.765 1.02
0.378” 0.390 0.331
l/2
1.82.10-’ 5.O6*1O-9
0.662 2 1
0.0782 0.452 0.196
l/2 l/3
0.0292 0.0000559 2.49.10-7
0.281 2 1 112 l/3 l/4
D
MRS. 10’
113 l/4
l/4 C
Pp. lo3
0.517 0.251 0.0378
1
0.259 2 1 l/2 l/3 l/4 l/5
model { reparametrized
0.882 0.474 0.190
0.491 1.43
0.516 1.89
0.257”
0.631 0.378 0.169 0.0522
5.06
0.269 0.228 0.227 0.326 1.70
4.57.10-9 0.467
0.0763 1.24
0.852”
0.212 0.0372
0.442 0.276 0.128
2.38 1.54 0.850
0.0436
1.03
0.00347 5.49*10-‘”
2.83
8.16*10-@
0.0643
0.359”
0.447
1.82
0.193 0.0309
0.606 0.360
8.09 5.96
0.00342 1.76.10-lo
0.153 0.0538 0.0147
1.05*10-”
kb = A0 [ 1/ (1 + at)” I }
2.62 0.771 0.409 0.484
“Three parameter model; others are two parameter models with fixed P3 = n values.
(mo1°.03kgs) and a= 5.64. 1O-2 h-l. The mean residual squares for these two estimations were MRS = 0.0109 and 0.0113, respectively. The standard deviations of the parameters were, of course, larger with the three parameter model. The experimental data and the estimated & curves obtained for n=l, l/2, l/3, l/4 are shown in Fig. 6. Since the best value of II is less than 1, it can be concluded that sintering is of a higher order according to eqn. (19). Order of the sintering kinetics To determine the order of the sintering kinetics (j?), the parameter values in eqn. (8) b. = k, (CL )” and CL were assumed to be constant and were determined using the S versus t and kb versus t (Table 2) relationships. When the numerical values of a! and n are known the order of the sintering kinetics can be calculated from eqn. ( 19 ) .
R.L. Keiski and T. Salmi / Appl.
Catal.
A 87 (1992)
195
~~~-~03
Fig. 4. The experimental and estimated frequency factors (experiment A): (a) kb 4 l/ (PI +I’&), (c) kb=1/(P,+P,t)1’3and (d) kb=1/(P,+PSt)114. (b) k~=1/(Pl+Pzt)“2,
2.5 -
2
..I
0
50
100
150
200 Catalyst
d0
300
350
400
ageh
Fig. 5. The experimental and estimated frequency factors (experiment D): (a) k& = 1/ (P,+P,t), (c) kb=1/(P,+P2t)“3and (d) kb=1/(P,+P2t)“4. (b) k;=l/(P,+P,t)“‘,
The surface areas at different times are shown in Table 4. The relationship between the surface area (S) and time (t ) was tested with the model given by eqn. (11). For parameter estimation eqn. (11) was transformed using parameters b= k” (/I-l)S{-’ and m= l/(/3-1). The unknown model parameters are thus S,,, b and m. The results of the parameter estimation are shown in Table 5 and the experimental and estimated (three parameter model) surface areas are shown in Fig. 7. The best fit between S and t was obtained when the three parameter model (the unknown parameters SO,b and m) was used. The
11
0.315 1.0 l/2 l/3 114 115
0.00216 0.196 0.0314 0.00348 1.15.10-‘0 7.95.10-‘2
PI
101 3.0 6.2 14 Large Large
s(S)
0.122 0.589 0.349 0.145 0.0509 0.0146
P,*103
35 6.7 4.7 3.7 4.4 9.1
s(%)
1.13” 3.34 1.74 1.09 1.36 3.84
MRS+lO*
“Three parameter model; others are two parameter models with fixed value of P3 = n.
s(W)
P,=n
k~=A,[l/(l+~t)~~~~~] k;=A,[l/(l+at)] k~=Ao[l/(l+at)“2] k&=Ao[l/(l+at)“3] k;=Ao[l/(l+at)1’4] k;=Ao[l/(l+ut)1’5]
Model
Estimation of the parameters in the deactivation model {reparametrized kb =A, [ l/ (1 + at)“]}
TABLE 3
6.90 5.10 5.64 6.60 305.0 263.0
kg-‘~-~]
mo1-o.034
0.056 0.000301 0.0111 0.0418 0.442.106 0.184.10s
R,L, K&ki
2O
and T. Salmi / Appl. Catal. A &’ (1992) 185-203
50
100
150
200 catalyst
2.50
300
350
197
400
age/h
Fig. 6. The experimental and estimated frequency factors (parameter values from experiments B, C and D); (a) kb=A,[l/(l+at)], (b) k~=A,[1/(1+at)“2], (c) kb=Ao[1/(1+at)“31, (d) k&=A,[1/(1+at)“4];experimentalvalues (0) B, (+) C,and (*)D. TABLE 4 Experimentally determined catalyst surface areas Catalyst age(h)
S (m’/g)
Catalyst bed number
T,,.(K)
0 125 430 610 765
53.2 38.2 35.5 35.3 35.2
Fresh catalyst VIII, IX X XII XIII
667-715 675-722 675-730 673 (isothermal)
TABLE 5 Estimation of the parameters So, b and m in the S versus t relationship [ eqn. ( 11) ] b (h-l)
m
MRS
W/g)
No. of parameters
48.2 49.1 53.1 53.2
0.65.10-3 0.17*10W2 0.12 8.70
1 0.5 0.1 0.048
27 23 2.3 0.13
2 2 2 3
SO
parameter values obtained were So = 53.2 m’/g, b = 8.70 h- ’ and m = 0.048. The order of sintering kinetics (/I) can be determined from m= l/ (p- 1), giving /3x 22. The relationship between I& and S calculated from the estimated Izb versus t curve [Iz~=6.60-106/(1+4.2~10-2t)‘~3] and S versus t curve [S=53.2/
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
198
300
100
200
300
>_A_500
400
600
700
800
catalyst age/h
Fig. 7. The experimental (0 ) and estimated surface areas; three parameter model.
15.6 15.4?
15.2
-2 :
f
15i
14.6
“s4ss3.6
3.65---c;
----3.75
3.8
3.85
3.9
3.95
J 4
In s
Fig. 8. The experimental and estimated kb vs. S relationship: calculated from k& vs. t and S vs. t relationships (-), calculated k& vs. experimental S ( 0 ), experimental kb for experiment B vs. calculated S (0 ), experimental kb for experiment C vs. calculated S ( + ), and experimental kb for experiment D vs. calculated S ( X ).
(1 + 8.70t)0.048] is shown in Fig. 8. In the same figure the experimental values of S and Fzb are shown as follows: calculated kb values versus experimental values of S (Table 4, 5 experimental points), experimental values of kb for experiment B (Table 1,8 experimental points) versus calculated values of S, experimental values of kb for experiment C (Table 1, 5 experimental points) versus calculated values of S, and experimental values of k b for experiment D (Table 1,9 experimental points) versus calculated values of S. If cy is calculated using the experimental In kb vs. calculated In S values the following values are obtained: Q = 5.44,5.74 and 6.98 for experiments D, C and B, respectively. The order of the sintering kinetics j? [ eqn. (9) ] for the slow
199
R.L. Keiski and T. Salni 1 Appl. Catal. A 87 (1992) 185-203 TABLE 6 Estimation of the parameters in the deactivation model eqn. (21) Estimation In a,, (h-l)
s (%)
E”/Ra (K)
s (%)
A,.lO-@’
1 2 3 4
2.8 2.6 15.0 70.0
22 400 20 100 20 280 20 220
2.5 2.2 9.9 52
6.00 6.93 6.60 6.90
31.2 27.9 28.0 28.2
s
(So) 76 140
n
S
(W) 0.3 l/3 l/3 0.315 37
MRS No. of parameters estimated 0.017 0.011 0.011 0.012
2 2 3 4
n(*3)1.03 mol-0.03 kg-1 s-1.
catalyst age/II
Fig. 9. The experimental (experimental sets B, C and D) and estimated kb vs. t relationship; estimation 2 (-), 3 (---) and 4 (-*-) in Table 6.
activity decay is thus between 17-22. The corresponding values of a! and j? calculated using the calculated In kb vs. experimental In S values for the initial fast activity decay are cy= 1.84 and p= 6.5. The value of rzused in the calculations was n= l/3 (Table 3). If we calculate approximate values using experiments B, C, and D together for cyfor the initial fast activity decay and for the slow activity decay by using the estimated In Izb vs. In S relationship, we get (x=0.72 for the initial fast activity decay and LY= 6.3 for the slow activity decay. The calculated orders of the sintering kinetics are then /?=3.2 for the fast initial activity decay and p= 20 for the slow activity decay. The values of the order of sintering kinetics obtained here are in accordance with the literature values [8,9]. In the activity of the iron oxide/chromium oxide catalyst a fast activity decay with low order sintering kinetics (p= 2.66.5) and a slow activity decay with high order sintering kinetics (p= 17-22) were observed.
200
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
The high numerical value of the sintering order (p) is caused by the low value of m. Few experimental S versus t data available can cause an error in the value of m determined by eqn. ( 11) . The S versus t values between O-125 h could probably have increased the m-values and thus made the parameter estimation more reliable. Temperature dependence of thedeactivation parameters In the following stage the temperature dependence of the deactivation parameter (a) [ eqn. (23 ) ] was taken into account in the parameter estimation. The parameters in eqn. (21) were estimated stagewise by considering first A, and n to be constants while the values of a, and E” were determined (the two parameter model). The parameter values of the three (ao, E )I,A0 as unknown parameters) and four (a,,, E”, A,, n as unknown parameters) parameter models were estimated in the next stage. The data (kb vs. t) used in the parameter estimation were those containing the results from the sets B, C and D (in total 22 experiments). The results of the parameter estimation are presented in Table 6. The experimental and estimated Kb vs. t values are shown in Fig. 9. The best fit was obtained with the three parameter model with n= l/3. If n was also treated as an unknown parameter the value of the mean residual square increased from 0.0114 to 0.0119 and the standard deviations of the parameters increased. Thus, the best values for the deactivation model with temperature dependent parameter values [eqn. (21) ] were In &=2&O h-l, E” = 169 kJ/ mol, Ao=6.60*106 (dm3)1-03mol-0.03kg-1s-1 and n= l/3. The parameter values obtained for the four parameter model were In a, = 28.2 h-l, E” = 168 kJ/ mol, A0 = 6.90.lo6 (dm3)‘~03mol-0~03kg-1s-1 and n=0.315. If the order of the sintering kinetics is calculated from eqn. (19) using the values of n obtained (n = l/3 and n = 0.315) the order of the sintering kinetics /? becomes (a=0.72 and 6.27) 3.2 (n=1/3) and 3.3 (n=0.315) for the fast initial activity decay and 20 (n = l/3) and 21 (n = 0.315) for the slow activity decay. It can be concluded that when the temperature dependence of the deactivation parameter a is taken into account there is only a slig& change in the value of the deactivation parameter A0 and the values of n and p remain unchanged. The activation energy of sintering was found to be around 168 kJ/mol. CONCLUSIONS
The kinetics of the deactivation of the iron oxide/chromium oxide catalyst in the water-gas shift reaction was studied. The parameters of the hyperbolic models (23) and (25) of the catalyst deactivation were determined by applying the nonisothermal approach to packed-bed reactor modelling. The decay in the catalyst activity was linked to a decrease in the total surface area and an increase in the average pore radius. The pore size distribution was
R.L. Keiski and T. Salmi /Appl. Catal. A 87 (1992) 185-203
201
shifted towards large pore radii during deactivation (Fig. 1). For a fresh catalyst the total surface area and typical pore sizes were 53.2 m”/g and 2-40 nm, respectively, whereas the corresponding values for an almost totally deactivated catalyst were 15.9 m2/g and 90-200 nm (Fig. 1). The reduction of the catalyst activity was best described by a hyperbolic model for the frequency factor kb =A,/ (1 + at)” [eqn. (23) ] where A0=6.60*106 (dm3)‘.03 mol-0~03kg-‘s-1, ~~0.042 h-l and n=1/3 (Fig. 6). Calculation of the order of the sintering kinetics using the frequency factor versus surface area relationship showed two different stages in activity decay: the sintering order of the fast initial activity decay was 3.2 and that of the slow activity decay was of clearly higher order. ACKNOWLEDGEMENTS
The authors express their sincere thanks to Prof. John B. Butt at Northwestern University for the valuable discussions. R.L. Keiski expresses her warm thanks to The Academy of Finland for the financial support which made this work possible. Financial support from Tauno Tiinningin Siiatib is also gratefully acknowledged. NOMENCLATURE
Ao
parameter in the deactivation model, eqns. (22 )- (23)) h-l parameter in the deactivation model, eqns. (17)- (18)) h-’ parameter in the deactivation model, eqn. (16)) ( dm3) ‘.03
b
parameter in the deactivation model, h-‘, b= k” (/I- 1) S{O-~
bo
constant, bo= concentration concentration concentration
a a0
ci
CIn
tin DP E E” ko ko
kb kb’ k’ k” KT
mol-0.03
kg-l
s-’
k,(c&)” of component i, mol dme3 of active sites, mol kg-’ of active sites, mol m -’
catalyst pore diameter, pm activation energy, kJ mol-’ activation energy of sintering, kJ mol-’ frequency factor in the sintering equation given by Hoogschagen and Zwietering [ 81 proportionality factor, eqn. (6)) mol’-” g”-’ s-’ frequency factor, mol g-l s-l frequency factor, gp-’ (rn2)lmPs-’ rate constant, mol g-l s-’ rate constant of sintering, gs-’ ( m2) 1-P 5-l equilibrium constant
Keiski and
202 m LkS n n n P PI, PP, p3 ;1, s 5
S SO S BET t T TO T max V, ; P
eqn. parameter m - l/ (p- 1) mean residual square CO concentration exponent, eqn. (3) exponent in the sintering equation given by Hoogschagen Zwietering [ 81 parameter in deactivation model, eqn. (19) COz concentration exponent, eqn. (3 ) parameters in regression model reaction rate, mol gg’ s-’ gas constant, Ro=8.314 JK-’ mol-’ standard deviation percentage dry gas space velocity, h-l surface area, m2 g-l initial surface area, m2 g- ’ BET surface area, m2 g-’ time, s temperature, K reactor inlet temperature, K maximum reaction temperature, K volume of adsorbed gas, cm3 g-l empirical exponent, eqn. (6 ) order of sintering, eqn. (9) reversibility factor, eqns. (2)) (3 )
and
REFERENCES 1 2 3
D.S. Newsome, Catal. Rev. Sci. Eng., 21 (2) (1980) 275. M.V. Twigg, Catalyst Handbook, 2nd edn. Wolfe, London, 1989. J.B. Butt and E.E. Petersen, Activation, Deactivation, and Poisoning of Catalysts, Academic Press, San Diego, 1988. S.N. Hoque, D.S. Chabra and B. Sen, Indian J. Technol., 20 (1982) 493. J.J. Carberry and J.B. Butt, Catal. Rev. Sci. Eng., 10 (2) (1974) 221. B.W. Wojciechowski, Can. J. Chem. Eng., 46 (1968) 48. S.N. Hoque, D.K. Ghorai, NC. Mehta, B. Sen and N.B. Bhattacharya, Fert. Technol., 14 (4) (1977) 381.
J. Hoogschagen and P. Zwietering, J. Chem. Phys., 21 (1953) 2224. G.C. Chinchen, R.H. Lougan and M.S. Spencer, Appl. Catal., 12 (1984) 89. M. Chandra, S.S. Singh, B. Banerjee, A.K. Sinha and N.K. Sinha, Technol. (India), 9 (4) (1972) 358. 11 M. Chandra, S.S. Singh and S.R. Ghosal, J. Indian Chem. Eng., 6 (1972) 27. 12 M. Chandra, S.S. Singh, A.K. Sinha, N.K. Sinha, B. Banerjee and S.R. Ghosal, Technol. (India), 10 (1973) 208. 13 G.P.A. Singh and D.N. Saraf, Ind. Eng. Chem., Process Des. Dev., 16 (1977) 313. 14 R.L. Keiski, T. Salmi and V.J. Pohjola, Acta Chem. Stand., 44 (1990) 777. 8 9 10
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203 15
16
17 18 19 20
203
R.L. Keiski, T. Salmi and V.J. Pohjola, Chem. Eng. J., 48 (1992) 17. R.L. Keiski, The Water-Gas Shift Reaction in Nonisothermal Conditions over an Iron Oxide/Chromium Oxide Catalyst, Acta Ouluensis Universitatis, C 57, University of Oulu Printing Center, Oulu, 1991. A.M. Ray, B. Yadav, A.K. Roy, B. Sen andH.C. Roy, Indian J. Technol., 25 (2) (1987) 61. V.K. Puri, H. Mahapatra, R.M. Cursetji, NC. Ganguli and S.P. Sen, Technol. (India), 10 (1973) 224. CCE Cl2 Catalyst Manual, Catalysts and Chemical Europe, Brussels (1985). P. Vajda and P. Valko, REPROCHE, Regression Program for Chemical Engineers, Eurecha Manual, Budapest, 1985,36 pp.
Catalysis A: General,
Elsevier
Science
APCAT
A2294
Publishers
87 (1992)
185-203
185
B.V., Amsterdam
water-gas
Deactivation of the high-temperature catalyst in nonisothermal conditions
shift
Riitta L. Keiski Department
of Process Engineering,
University
of Oulu, SF-90570
Oulu (Finland)
and Tapio Salmi Department (Received
of Chemical 27 December
Engineering,
Abe Akademi,
1991, revised manuscript
SF-20500
received
Turku
(Finland)
25 April 1992)
Abstract The water-gas shift reaction over a commercial iron oxide/chromium oxide catalyst was investigated to determine the kinetics of the catalyst deactivation and the physical properties of the catalyst during the deactivation. A nonisothermal fixed bed reactor was used to collect data for the kinetics of catalyst deactivation. The laboratory scale reactor operated close to industrial practice at 575-723 K. The decline of the catalyst activity was found to be due to a sintering process. No carbon formation was detected according to scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The decay of the catalyst activity was quite rapid during the first 150 h of operation and less rapid between 150 and 600 h. The decay of the catalyst activity was linked to a decrease of the surface area and to an increase of the mean pore size of the catalyst. The kinetic and deactivation parameters were determined by nonlinear regression using the temperature and carbon monoxide concentration profiles in the catalyst bed. The level of catalyst activity as a function of the catalyst age was best described by a hyperbolic modelfor the frequency factor kb =A,/(1 +at)“whereA,=6.60*106 (dm3)‘.03 mol-0-03 kg-Is_‘, az0.042 h-’ and n = l/3. The time dependence of the surface area (S) of the catalyst was also described best by a hyperbolic model S=S,/ (l+ bt)” where S,=53.2 m’/g, b=8.70 hh’ and m=0.048. Calculation of the order of the sintering kinetics from the k& vs. S relationship showed that two different processes of the activity decay are involved: a fast initial activity decay with sintering order of 3 and a slow activity decay with a high order of sir&ring. Keywords: catalyst
deactivation,
iron oxide/chromium
oxide, kinetics,
water-gas
shift.
INTRODUCTION
The water-gas shift reaction CO+H,O=CO,+H,
(1)
Correspondence
to: Dr. T. Salmi,
Turku,
Tel. ( + 358-21)654427,
Finland.
0926-3373/92/$05.00
Department
0 1992 Elsevier
of Chemical
Engineering,
Abe Akademi,
fax. ( + 358-21)654479.
Science
Publishers
B.V.
All rights
reserved.
SF-20500
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
186
is a step in many industrial processes. The most common catalyst for this reaction is the iron oxide/chromium oxide catalyst that is used to enhance the water-gas shift reaction in ammonia synthesis, hydrogen production and town gas purification [ 1,2]. The activity of the iron oxide/chromium oxide catalyst decreases under process conditions. The causes of the decline in activity of the shift catalyst are: poisoning, coking and sintering [ 3-61. It has been shown by many workers that the most probable cause of the loss in activity is sintering. Sintering of the iron oxide/chromium oxide catalyst leads to lower surface area, reduction in porosity, increase in particle size and decrease of the number of pores smaller than 30 nm [ 71. The loss in activity, i.e. in surface area, is shown [8] to be a function of the reaction temperature and the age of the catalyst: l/S”l/St = k,t exp ( -E” /R,T), where S, is the initial surface area (m”/g), S is the surface area of the catalyst at time t, E” is the activation energy of sintering (kJ/mol) and n is the order of sintering. The calculated values of n and E” according to Chinchen et al. [ 91 are 4.7 and 196.5 kJ/mol, respectively. Hoogschagen and Zwietering found values of 6.2-7.3 and 272 kJ/mol, respectively [8]. Chandra et al. [lo-121 have found that the activity decreases rapidly during the initial period up to 100-150 h, the fall in activity is quite fast during 150400 h, less fast between 400 and 700 h and after 1000 h the activity remains rather constant. Chinchen et al. [9] have also found a fast initial decay and a slow decay in the catalyst activity and they have shown that the initial decay of activity is of much lower order (n= 1.3-3.0) than the slow decay of activity. These two stages of activity were also described by Singh and Saraf [ 131. These observations provide strong evidence for different processes controlling the loss in activity over different time periods. In the initial decay the low value of n has been attributed, according to Chinchen et al. [ 91, to particle growth by two simultaneous but different mechanisms, or to the rapid agglomeration of very small particles. The aim of the present work was to study the deactivation kinetics of the iron oxide/chromium oxide catalyst under conditions close to those experienced industrially and to investigate the physical reasons for this deactivation phenomenon. To determine the kinetics of catalyst deactivation and the cause of deactivation, experiments were performed to analyze the carbon monoxide conversions at different time intervals and to determine the physical properties of fresh and used catalysts, EXPERIMENTAL
PROCEDURE
Nitrogen physisorption at 77 K was used to determine the surface area, pore size distribution, average pore radius and total pore volume of the pores between l-40 nm for fresh and used catalysts. The mercury penetration method
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
187
TABLE 1 Experimental conditions in the four separate experimental sets to determine deactivation kinetics Experiment
To W)
Dry gas space velocity (h-l)
Number of samples
Samples taken between (h)
A B C D
575 600 625 650
795 1997 1997 1997
10 8 5 11
73-400 87-262 90-366 28-404
was used to obtain the pore size distribution of larger pores ( > 50 nm). The nature of the catalyst deactivation was studied by X-ray photoelectron spectroscopy (XPS ) and scanning electron microscopy (SEM). Experimental conditions close to industrial practice were used in the kinetic and deactivation studies. The nonisothermal packed bed reactor and the outline of measurements are presented in earlier publications [ 14-161. The reactor inlet temperature was varied between 575 and 675 K, the water-to-dry gas ratio was varied between 0.33 and 1.24 and the dry gas composition was typically 7-30% carbon monoxide, 3-20% carbon dioxide and 25-70% hydrogen. Temperature and carbon monoxide conversion values were measured from six axial positions in the reactor and inlet and product gas compositions were analyzed by a gas chromatograph. Cylindrical iron oxide/chromium oxide catalyst particles (CCE, C12,3.2 x 3.2 mm) were used from Catalysts and Chemicals Europe. The deactivation studies were made by repeating a set of experiments after certain time intervals. Four different sets were considered. The inlet temperature varied between 575 and 650 K and the dry gas space velocity was 795 or 1997 h-l. The experimental conditions are summarized in Table 1. The inlet dry gas composition was in all deactivation experiments: 13.6% carbon monoxide, 9.2% carbon dioxide, 54.4% hydrogen and 22.8% nitrogen. The water-to-dry gas ratio was 0.83 and the total pressure was 1.08 bar. The reaction conditions were changed using the inlet temperature, component concentration and water-to-dry gas ratio as variables. RESULTS AND DISCUSSION
Physical properties
of the catalyst
The pore size distributions for a fresh and two used catalysts (CCE, C12) measured by nitrogen condensation and mercury penetration methods are shown in Fig. 1. The pore size distribution is shifted towards larger pore radii
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203 -
30.00
25.00
20.00
15.00
10.00
5.00
ao-
0.00 0.1
0.01
1.00
Dp/w
Fig. 1. The pore size distribution for one fresh and two used iron oxide/chromium oxide catalysts using the nitrogen adsorption and mercury penetration methods: (m) a fresh catalyst, (0 ) a catalyst used in the process gas at 575-723 K for 430 h and (0 ) a catalyst used in a commercial plant for two to three years.
during the catalyst deactivation. The surface area of the catalyst decreases, the average pore radius increases, the total pore volume remains almost unchanged and the number of smaller pores decreases. It may be suggested that the decrease in the amount of smaller pores with the corresponding decrease in the catalytic activity means that the smaller pores ( < 30 nm ) contribute most to the shift reaction. If the pore size distribution of the catalyst (CCE, C12), which had been in industrial use over two years is compared to the pore size distributions of a fresh catalyst and a catalyst used under laboratory conditions, it is observed that the smaller pores in the industrial catalyst have almost completely disappeared and the typical pore sizes are between 90 and 200 nm. Thus, it appears that if an iron oxide/chromium oxide catalyst is used up to complete deactivation there will be no smaller pores present and all the pores will have a diameter larger than 50 nm, which compares with the pore size of magnetite [ 171. In laboratory use (e.g. t=430 h and 7’,,,= 675-723 K) the values of the surface area, total pore volume and average pore radius change as follows compared to a fresh catalyst: the surface area decreases from 53.2 to 35.5 m2/g, the total pore volume decreases from 0.136 to 0.104 cm3/g and the average pore
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
189
radius increases from 5.1 to 6.0 nm. These observations are in accordance with literature data [ 18,191. The estimated and experimental surface area vs. maximum reaction temperature is shown in Fig. 2. The behaviour of the catalyst surface area as a function of the maximum reaction temperature in the catalyst bed and the catalyst age is described by the empirical relationship: SBET=122.4-O.OOO&-O.l24T,,,
(21
The maximum reaction temperatures and catalyst ages for the catalysts used in the parameter estimation were: catalyst I T,,, = 873 K and t = 250 h, catalyst II T,,, = 872 K and t = 45 h, catalyst VII T,,, = 724 K and t = 263 h, catalyst VIII T,,, = 709 K and t = 125 h, and catalyst XII T,,, = 730 K and t = 610 h. The deactivation of the iron oxide/chromium oxide catalyst may also be due to a coke formation process. Therefore, some fresh and used catalysts, as indicated earlier, were analyzed by SEM and XPS. According to these analyses both fresh and used catalysts contained carbon species but according to the elemental analysis the amount of carbon had not increased on the catalyst surface during the reaction. Thus, carbon formation during the process seems to be negligible or very small and the main cause for the deactivation is sintering. Reaction kinetics The reaction kinetics are described by the power-law model [ 1,15,16]
5 10
I,
ho
650
7w
750
800
850
T/K
Fig. 2. The experimental and estimated surface area as a function of the maximum reaction temperature;catalystbedI(O),II(~),VII(+),VIII(O)andXII(*):t=300h(-_),t=1OOOh (---).
190
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
and KT is the equilibrium constant. The kinetic parameters of rate eqn. (3) were determined [ 14-161 using the carbon monoxide conversion and temperature profiles from the nonisothermal plug-flow reactor. The following parameters were obtained: wherep=
(CCO~H~ )/(c
co c H&T)
r=kb exp( -9600K/T)c~~4c~~~cc~~‘8(1-p)
(4)
Modelling of the catalyst deactivation by sintering The empirical k’=kb
rate constant
is given by
exp( -E/RGT)
(5)
The frequency factor kb is proportional to the concentration the catalyst surface, c, (in mol/kg catalyst):
of active sites on
k;, =k&,Y,
(6)
where a! is an empirical exponent. The concentration of active sites based on the catalyst surface area, c h, (mol/ m2 catalyst) is related to c, by &S=c,
(7)
where S is the specific surface area of the catalyst. eqn. (6) we obtain for the frequency factor:
After inserting
k;, =k,,c:S= For sintering, by dS dt=
eqn. (7) into (8)
an empirical
model for the loss in surface area [3 ] is described
_ k”Sfl
(9)
where k” is the rate constant of sintering. time dependence of the surface area: S=SO exp( -k”t)
forp=l
Integration
of eqn. (9) gives the (10)
and S= [S~-8+k”(p-l)t][1’(1-P)1 which are inserted
for/?#l
(11)
in eqn. (8):
kb=k,(ck)“Stexp(--“cut)
forp=l
(12)
and (13)
R.L. Keiski and T. Salmi /Appl.
191
Catal. A 87 (1992) 185-203
It should be noted that eqn. (11) is similar to the equation proposed by Hoogschagen and Zwietering [8]: p- 1 corresponds to n and k” (p- 1) corresponds to k,exp( -F/&T). If we take into account that in principle 12” is also temperature dependent, the frequency factor kb can be given by the following equations: exp[-kicrtexp(-E”/RoT)]
kb=k,(c~)“S~
for/?=1
(14)
and
“=
h,(ckJ”% [I+&’
forPZ3
(/&1)&f-‘t][“/(&l)l
exp(__E”/&J)
(15)
After introducing the following lumped parameters A,=lz,(c~)*S;
(16)
for/?=1
a,=k$a
(17)
ao=ki(p-l)S[-’
forP#l
(13)
and n=cX/(b-1)
(19)
the frequency factor kb can be expressed by the following two equations: kb=A,,
exp [ --a0 exp( -E”/RoT)t]
far/3=1
(20)
and
Izb= [l+a,
A0 exp( -E”/R,T)t]”
(21)
forPZ1
If the temperature dependence of k” is neglected, eqns. (20) and (21) are reduced to eqns. (22) and (23): k&=Ao
exp(--at)
forj3=1
(22)
and kb=A,J(l+at)”
for/?#l
(23)
wherea=k”cxfor/?=l anda=k”(p-l)S,P-’ forP#l. In principle the parameter a is also temperature dependent according to eqns. (20) and (21) and the determination of the parameters A,,, kz and E” should be carried out simultaneously using nonisothermal rate data and inserting the expressions (20) and (21) for the frequency factor in the equation of the rate constant (5 ) : k’=A,
exp[ - (E/RoT+aO
exp( -E”/RoT)t]
far/3=1
(24)
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
192
and
41 ew( -NW)
k’=[l+ao exp(--E”/RoT)t]n
forPZ1
(25)
Estimation of the kinetic parameters of the deactivation models Parameter estimation The kinetics of the catalyst deactivation were determined using the four separate experimental sets (Table 1) from the catalyst bed that contained pelletized FesOl-CrzO, catalyst particles (3.2 x 3.2 mm). The carbon monoxide conversion data obtained from each reactor outlet are shown in Fig. 3. In the
, r __-..-_-_
r
-_--m-1-,
The decrease of the catalyst activity during the reaction,ttot = 430h: (a) To=5% K and (c) T,,=625KandSV=1997h-‘, (d) To=65OK andS,~l997h~*;(~)L~0.2dm,(O)L=0.6dm,(+)L=l.Odm,(*)L=~.3dm~(~)L~l~6 dm; the reaction gas composition: 13.6% CO, 9.2% COZ, 54.4% Hz and 22.8% Nz, water/dry gas=0.83. Fig.
3.
,$=79sh-1, (b) T0=600KandS,=1997h-‘,
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
193
first stage the parameter values in eqn. (23)) where all parameters are temperature independent, were estimated. The estimation of the deactivation parameters was carried out using the following procedure. First, the power law equation (3)where E/RG,n,m,pwerefixedto E/RG=9600K,n=0.74,m=0.47 andp= -0.18 [15,16] [eqn. (4)] was used to determine the frequency factor kb from each experiment using the conversion versus reactor bed length data obtained [ 141. Then the kb values versus the catalyst age relationship were expressed with models (22) and (23) with n=2,1,1/2,1/3,1/4 and l/5 (the two parameter model). The parameter values in eqn. (23 ) were also estimated allowing n to be unknown a priori (the three parameter model). Parameter estimation using the values n = 1,2,3 [ eqn. (23) ] and n = CXI[ eqn. (22 ) ] was studied in detail in earlier work [ 141, where we observed that the best fit to deactivation data was provided by n=l when the cases n=1,2, .... co were considered. The model equation (23) was reparametrized to reduce the correlation between A, and a. The modified parameters were: P, = l/A 61’p3),Pz =cI/A~~'~~) and P, = n.The parameters PI, Pz and P3 were determined using the algebraic option of the nonlinear regression program REPROCHE [ 201. The parameter values obtained from each experiment (A, B, C and D) are presented in Table 2. The best fits according to the values of the mean residual square (MRS ) were obtained when the hyperbolic model [eqn. (23)) where n = l/3 or l/2] was used. For experiments A and B the mean residual square was at its least for the two parameter model with n = 1 and l/2, respectively. The three parameter model gave n values for these experiments as follows: n=0.97 for experimental set A and n=0.66 for experimental set B. For the experiments C and D the best fits were obtained with n=1/3 and n=1/4, respectively. The values of n obtained in the estimation of the three parameter models were n=0.28 for C and n=0.26 for D. The standard deviations of the parameters increased with decreasing n.This was due to the numerical value of kb predicted by the model [eqn. (23) 1, approaching unity when n ( =P3) approached zero. The experimental data and the estimated Izbprofiles for the two parameter models with n= &l/2,1/3 and l/4 can be seen in Figs. 4-5 for experimental sets A and D, respectively. The frequency factor k& was very similar in experimental sets B, C, and D. Therefore the parameters in eqn. (23) were also determined by combining the data from the experimental sets B, C and D. The parameter values obtained from this estimation are presented in Table 3. The values of A,,and a for the three parameter model and two parameter models calculated from the parameters I’,, P, and P, are also shown in Table 3. The best fit was obtained with the hyperbolic model, eqn. (23) with n= l/3. The parameter values obtained were A0=6.6-106 (dm3)‘.03/(mo10.03kgs) and a = 4.2. lo-’ h- ‘. The next best fit was obtained with the three parameter model. The parameter values obtained were n = 0.315, A, = 6.90. lo6 (dm3) ‘.03/
194 TABLE
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203 2
Estimation
of the parameters in the deactivation
Experiment
Ps=n
P,
A
0.969 2
B
0.239
1.023 0.765 1.02
0.378” 0.390 0.331
l/2
1.82.10-’ 5.O6*1O-9
0.662 2 1
0.0782 0.452 0.196
l/2 l/3
0.0292 0.0000559 2.49.10-7
0.281 2 1 112 l/3 l/4
D
MRS. 10’
113 l/4
l/4 C
Pp. lo3
0.517 0.251 0.0378
1
0.259 2 1 l/2 l/3 l/4 l/5
model { reparametrized
0.882 0.474 0.190
0.491 1.43
0.516 1.89
0.257”
0.631 0.378 0.169 0.0522
5.06
0.269 0.228 0.227 0.326 1.70
4.57.10-9 0.467
0.0763 1.24
0.852”
0.212 0.0372
0.442 0.276 0.128
2.38 1.54 0.850
0.0436
1.03
0.00347 5.49*10-‘”
2.83
8.16*10-@
0.0643
0.359”
0.447
1.82
0.193 0.0309
0.606 0.360
8.09 5.96
0.00342 1.76.10-lo
0.153 0.0538 0.0147
1.05*10-”
kb = A0 [ 1/ (1 + at)” I }
2.62 0.771 0.409 0.484
“Three parameter model; others are two parameter models with fixed P3 = n values.
(mo1°.03kgs) and a= 5.64. 1O-2 h-l. The mean residual squares for these two estimations were MRS = 0.0109 and 0.0113, respectively. The standard deviations of the parameters were, of course, larger with the three parameter model. The experimental data and the estimated & curves obtained for n=l, l/2, l/3, l/4 are shown in Fig. 6. Since the best value of II is less than 1, it can be concluded that sintering is of a higher order according to eqn. (19). Order of the sintering kinetics To determine the order of the sintering kinetics (j?), the parameter values in eqn. (8) b. = k, (CL )” and CL were assumed to be constant and were determined using the S versus t and kb versus t (Table 2) relationships. When the numerical values of a! and n are known the order of the sintering kinetics can be calculated from eqn. ( 19 ) .
R.L. Keiski and T. Salmi / Appl.
Catal.
A 87 (1992)
195
~~~-~03
Fig. 4. The experimental and estimated frequency factors (experiment A): (a) kb 4 l/ (PI +I’&), (c) kb=1/(P,+P,t)1’3and (d) kb=1/(P,+PSt)114. (b) k~=1/(Pl+Pzt)“2,
2.5 -
2
..I
0
50
100
150
200 Catalyst
d0
300
350
400
ageh
Fig. 5. The experimental and estimated frequency factors (experiment D): (a) k& = 1/ (P,+P,t), (c) kb=1/(P,+P2t)“3and (d) kb=1/(P,+P2t)“4. (b) k;=l/(P,+P,t)“‘,
The surface areas at different times are shown in Table 4. The relationship between the surface area (S) and time (t ) was tested with the model given by eqn. (11). For parameter estimation eqn. (11) was transformed using parameters b= k” (/I-l)S{-’ and m= l/(/3-1). The unknown model parameters are thus S,,, b and m. The results of the parameter estimation are shown in Table 5 and the experimental and estimated (three parameter model) surface areas are shown in Fig. 7. The best fit between S and t was obtained when the three parameter model (the unknown parameters SO,b and m) was used. The
11
0.315 1.0 l/2 l/3 114 115
0.00216 0.196 0.0314 0.00348 1.15.10-‘0 7.95.10-‘2
PI
101 3.0 6.2 14 Large Large
s(S)
0.122 0.589 0.349 0.145 0.0509 0.0146
P,*103
35 6.7 4.7 3.7 4.4 9.1
s(%)
1.13” 3.34 1.74 1.09 1.36 3.84
MRS+lO*
“Three parameter model; others are two parameter models with fixed value of P3 = n.
s(W)
P,=n
k~=A,[l/(l+~t)~~~~~] k;=A,[l/(l+at)] k~=Ao[l/(l+at)“2] k&=Ao[l/(l+at)“3] k;=Ao[l/(l+at)1’4] k;=Ao[l/(l+ut)1’5]
Model
Estimation of the parameters in the deactivation model {reparametrized kb =A, [ l/ (1 + at)“]}
TABLE 3
6.90 5.10 5.64 6.60 305.0 263.0
kg-‘~-~]
mo1-o.034
0.056 0.000301 0.0111 0.0418 0.442.106 0.184.10s
R,L, K&ki
2O
and T. Salmi / Appl. Catal. A &’ (1992) 185-203
50
100
150
200 catalyst
2.50
300
350
197
400
age/h
Fig. 6. The experimental and estimated frequency factors (parameter values from experiments B, C and D); (a) kb=A,[l/(l+at)], (b) k~=A,[1/(1+at)“2], (c) kb=Ao[1/(1+at)“31, (d) k&=A,[1/(1+at)“4];experimentalvalues (0) B, (+) C,and (*)D. TABLE 4 Experimentally determined catalyst surface areas Catalyst age(h)
S (m’/g)
Catalyst bed number
T,,.(K)
0 125 430 610 765
53.2 38.2 35.5 35.3 35.2
Fresh catalyst VIII, IX X XII XIII
667-715 675-722 675-730 673 (isothermal)
TABLE 5 Estimation of the parameters So, b and m in the S versus t relationship [ eqn. ( 11) ] b (h-l)
m
MRS
W/g)
No. of parameters
48.2 49.1 53.1 53.2
0.65.10-3 0.17*10W2 0.12 8.70
1 0.5 0.1 0.048
27 23 2.3 0.13
2 2 2 3
SO
parameter values obtained were So = 53.2 m’/g, b = 8.70 h- ’ and m = 0.048. The order of sintering kinetics (/I) can be determined from m= l/ (p- 1), giving /3x 22. The relationship between I& and S calculated from the estimated Izb versus t curve [Iz~=6.60-106/(1+4.2~10-2t)‘~3] and S versus t curve [S=53.2/
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
198
300
100
200
300
>_A_500
400
600
700
800
catalyst age/h
Fig. 7. The experimental (0 ) and estimated surface areas; three parameter model.
15.6 15.4?
15.2
-2 :
f
15i
14.6
“s4ss3.6
3.65---c;
----3.75
3.8
3.85
3.9
3.95
J 4
In s
Fig. 8. The experimental and estimated kb vs. S relationship: calculated from k& vs. t and S vs. t relationships (-), calculated k& vs. experimental S ( 0 ), experimental kb for experiment B vs. calculated S (0 ), experimental kb for experiment C vs. calculated S ( + ), and experimental kb for experiment D vs. calculated S ( X ).
(1 + 8.70t)0.048] is shown in Fig. 8. In the same figure the experimental values of S and Fzb are shown as follows: calculated kb values versus experimental values of S (Table 4, 5 experimental points), experimental values of kb for experiment B (Table 1,8 experimental points) versus calculated values of S, experimental values of kb for experiment C (Table 1, 5 experimental points) versus calculated values of S, and experimental values of k b for experiment D (Table 1,9 experimental points) versus calculated values of S. If cy is calculated using the experimental In kb vs. calculated In S values the following values are obtained: Q = 5.44,5.74 and 6.98 for experiments D, C and B, respectively. The order of the sintering kinetics j? [ eqn. (9) ] for the slow
199
R.L. Keiski and T. Salni 1 Appl. Catal. A 87 (1992) 185-203 TABLE 6 Estimation of the parameters in the deactivation model eqn. (21) Estimation In a,, (h-l)
s (%)
E”/Ra (K)
s (%)
A,.lO-@’
1 2 3 4
2.8 2.6 15.0 70.0
22 400 20 100 20 280 20 220
2.5 2.2 9.9 52
6.00 6.93 6.60 6.90
31.2 27.9 28.0 28.2
s
(So) 76 140
n
S
(W) 0.3 l/3 l/3 0.315 37
MRS No. of parameters estimated 0.017 0.011 0.011 0.012
2 2 3 4
n(*3)1.03 mol-0.03 kg-1 s-1.
catalyst age/II
Fig. 9. The experimental (experimental sets B, C and D) and estimated kb vs. t relationship; estimation 2 (-), 3 (---) and 4 (-*-) in Table 6.
activity decay is thus between 17-22. The corresponding values of a! and j? calculated using the calculated In kb vs. experimental In S values for the initial fast activity decay are cy= 1.84 and p= 6.5. The value of rzused in the calculations was n= l/3 (Table 3). If we calculate approximate values using experiments B, C, and D together for cyfor the initial fast activity decay and for the slow activity decay by using the estimated In Izb vs. In S relationship, we get (x=0.72 for the initial fast activity decay and LY= 6.3 for the slow activity decay. The calculated orders of the sintering kinetics are then /?=3.2 for the fast initial activity decay and p= 20 for the slow activity decay. The values of the order of sintering kinetics obtained here are in accordance with the literature values [8,9]. In the activity of the iron oxide/chromium oxide catalyst a fast activity decay with low order sintering kinetics (p= 2.66.5) and a slow activity decay with high order sintering kinetics (p= 17-22) were observed.
200
R.L. Keiski and T. Salmi / Appl. Catal. A 87 (1992) 185-203
The high numerical value of the sintering order (p) is caused by the low value of m. Few experimental S versus t data available can cause an error in the value of m determined by eqn. ( 11) . The S versus t values between O-125 h could probably have increased the m-values and thus made the parameter estimation more reliable. Temperature dependence of thedeactivation parameters In the following stage the temperature dependence of the deactivation parameter (a) [ eqn. (23 ) ] was taken into account in the parameter estimation. The parameters in eqn. (21) were estimated stagewise by considering first A, and n to be constants while the values of a, and E” were determined (the two parameter model). The parameter values of the three (ao, E )I,A0 as unknown parameters) and four (a,,, E”, A,, n as unknown parameters) parameter models were estimated in the next stage. The data (kb vs. t) used in the parameter estimation were those containing the results from the sets B, C and D (in total 22 experiments). The results of the parameter estimation are presented in Table 6. The experimental and estimated Kb vs. t values are shown in Fig. 9. The best fit was obtained with the three parameter model with n= l/3. If n was also treated as an unknown parameter the value of the mean residual square increased from 0.0114 to 0.0119 and the standard deviations of the parameters increased. Thus, the best values for the deactivation model with temperature dependent parameter values [eqn. (21) ] were In &=2&O h-l, E” = 169 kJ/ mol, Ao=6.60*106 (dm3)1-03mol-0.03kg-1s-1 and n= l/3. The parameter values obtained for the four parameter model were In a, = 28.2 h-l, E” = 168 kJ/ mol, A0 = 6.90.lo6 (dm3)‘~03mol-0~03kg-1s-1 and n=0.315. If the order of the sintering kinetics is calculated from eqn. (19) using the values of n obtained (n = l/3 and n = 0.315) the order of the sintering kinetics /? becomes (a=0.72 and 6.27) 3.2 (n=1/3) and 3.3 (n=0.315) for the fast initial activity decay and 20 (n = l/3) and 21 (n = 0.315) for the slow activity decay. It can be concluded that when the temperature dependence of the deactivation parameter a is taken into account there is only a slig& change in the value of the deactivation parameter A0 and the values of n and p remain unchanged. The activation energy of sintering was found to be around 168 kJ/mol. CONCLUSIONS
The kinetics of the deactivation of the iron oxide/chromium oxide catalyst in the water-gas shift reaction was studied. The parameters of the hyperbolic models (23) and (25) of the catalyst deactivation were determined by applying the nonisothermal approach to packed-bed reactor modelling. The decay in the catalyst activity was linked to a decrease in the total surface area and an increase in the average pore radius. The pore size distribution was
R.L. Keiski and T. Salmi /Appl. Catal. A 87 (1992) 185-203
201
shifted towards large pore radii during deactivation (Fig. 1). For a fresh catalyst the total surface area and typical pore sizes were 53.2 m”/g and 2-40 nm, respectively, whereas the corresponding values for an almost totally deactivated catalyst were 15.9 m2/g and 90-200 nm (Fig. 1). The reduction of the catalyst activity was best described by a hyperbolic model for the frequency factor kb =A,/ (1 + at)” [eqn. (23) ] where A0=6.60*106 (dm3)‘.03 mol-0~03kg-‘s-1, ~~0.042 h-l and n=1/3 (Fig. 6). Calculation of the order of the sintering kinetics using the frequency factor versus surface area relationship showed two different stages in activity decay: the sintering order of the fast initial activity decay was 3.2 and that of the slow activity decay was of clearly higher order. ACKNOWLEDGEMENTS
The authors express their sincere thanks to Prof. John B. Butt at Northwestern University for the valuable discussions. R.L. Keiski expresses her warm thanks to The Academy of Finland for the financial support which made this work possible. Financial support from Tauno Tiinningin Siiatib is also gratefully acknowledged. NOMENCLATURE
Ao
parameter in the deactivation model, eqns. (22 )- (23)) h-l parameter in the deactivation model, eqns. (17)- (18)) h-’ parameter in the deactivation model, eqn. (16)) ( dm3) ‘.03
b
parameter in the deactivation model, h-‘, b= k” (/I- 1) S{O-~
bo
constant, bo= concentration concentration concentration
a a0
ci
CIn
tin DP E E” ko ko
kb kb’ k’ k” KT
mol-0.03
kg-l
s-’
k,(c&)” of component i, mol dme3 of active sites, mol kg-’ of active sites, mol m -’
catalyst pore diameter, pm activation energy, kJ mol-’ activation energy of sintering, kJ mol-’ frequency factor in the sintering equation given by Hoogschagen and Zwietering [ 81 proportionality factor, eqn. (6)) mol’-” g”-’ s-’ frequency factor, mol g-l s-l frequency factor, gp-’ (rn2)lmPs-’ rate constant, mol g-l s-’ rate constant of sintering, gs-’ ( m2) 1-P 5-l equilibrium constant
Keiski and
202 m LkS n n n P PI, PP, p3 ;1, s 5
S SO S BET t T TO T max V, ; P
eqn. parameter m - l/ (p- 1) mean residual square CO concentration exponent, eqn. (3) exponent in the sintering equation given by Hoogschagen Zwietering [ 81 parameter in deactivation model, eqn. (19) COz concentration exponent, eqn. (3 ) parameters in regression model reaction rate, mol gg’ s-’ gas constant, Ro=8.314 JK-’ mol-’ standard deviation percentage dry gas space velocity, h-l surface area, m2 g-l initial surface area, m2 g- ’ BET surface area, m2 g-’ time, s temperature, K reactor inlet temperature, K maximum reaction temperature, K volume of adsorbed gas, cm3 g-l empirical exponent, eqn. (6 ) order of sintering, eqn. (9) reversibility factor, eqns. (2)) (3 )
and
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