Ind. Eng. Chem. Res. 1992,31, 155-163 k , = rate constant for catalytic pyrolysis, m3/((kgof catalyst) 8)
kt = rate constant for thermal pyrolysis, s-l
L = reactor length, m Le = equivalent reactor length, m L1, Lz = reactor lengths (measured from the inlet) until the end of the first thermal pyrolysis and catalytic zone, respectively, m Lie, Lze = equivalent reactor lengths corresponding to L, and Lz, m mN = mass flow rate of naphtha, kg/s Qo = inlet volumetric flow rate (naphtha + steam), m3/s R = gas constant, kJ/(kmol K) S = cross-sectional area for flow in the catalytic zone, m2 Sa = flow area of annulus in the thermal zone, m2 T = reactor temperature, K TE = reference temperature, K V = reactor volume, m3 VE= equivalent reactor volume, m3 W = mass of catalyst, kg XA1,XA2,XAP= naphtha conversion at L1,Lz, and L, respectively z = axial coordinate, m Greek Letters 6 = dilution ratio, kg of steam/(kg of naphtha) 6’ = molar dilution ratio, kmol of steam/(kmol of naphtha) t = expansion factor CB = void fraction of catalyst bed pp = catalyst pellet density, kg/m3 v = moles of product formed per mole of T = space time, s
naphtha cracked
Registry No. A114Ca12033, 12005-57-1; CH,,74-82-8; C2H4, 74-85-1; C3H6,115-07-1.
Literature Cited Bajus, M.; Vesel’y, V.; Leclercq, P. A.; Rijks, J. A. Steam Cracking of Hydrocarbons. 3. Straight-Run Naphtha. Ind. Eng. Chem. Prod. Res. Deu. 1980, 19, 556-563.
155
Egiazarov, Yu. G.; Cherches, B. Kh.; Krut’ko, N. P.; Paushkin, Ya. M. Reactions of Petroleum Fractions and Hydrocarbons on an Indium Oxide Catalyst at High Temperatures. Neftekhimiya 1978,18, 237-243. Ellig, D. L.; Lal, C. K.; Mead, D. W.; Longwell, J. P.; Peters, W. A. Pyrolysis of Volatile Aromatic Hydrocarbons and n-Heptane over Calcium Oxide and Quartz. Ind. Eng. Chem. Process Des. Deu. 1985,24, 1080-1087. Fischer, F.; Roatrupp-Nielson, J.; Wrisberg, J. Unsaturated Hydrocarbons by Catalytic Vapor cracking. German Patent 2,340,904, 1974. Hirato, M.; Yoshioka, S. Simulation of the pyrolysis of naphtha, kerosene and gas oil with a tubular reactor. Int. Chem. Eng. 1973, 13,347-355. Hougen, 0. A.; Watson, K. M. Chemical Process Principles; Wiley: New York, 1947; Part 111, p 884. Kikuchi, K.; Tomita, T.; Sakamoto, T.; Ishida, T. A New Catalytic Cracking Process. Chem. Eng. Prog. 1985,81 (6), 54-58. Kolombos, A. J.; McNice, D.; Wood, D. C. Olefins by Cracking a Petroleum Distillate. German Patent 2,641,455, 1977a. Kolombos, A. J.; McNice, D.; Wood, D. C. Olefins. German Patent 2,640,278, 1977b. Kumar, P.; Kunzru, D. Modeling of Naphtha Pyrolysis. Ind. Eng. Chem. Process Des. Dev. 1985,24, 774-782. Lemonidou, A. A.; Vasalos, I. A. Preparation and Evaluation of Catalysts for the Production of Ethylene via Steam Cracking. Effect of Operating Conditions on the Performance of 12Ca07A1203Catalyst. Appl. Catal. 1989,54, 119-138. Levenspiel, 0. Chemical Reaction Engineering; Wiley: New Delhi, 1972; p 110. Sahu, D.; Kunzru, D. Effect of Benzene and Thiophene on Rate of Coke Formation during Naphtha Pyrolysis. Can. J. Chem. Eng. 1988,66,808-815. Tomita, T.; Noda, M.; Yamaguchi, Y.; Uwano, K. I. Catalysts and Methods of Making and Using Them. British Patent 1,478,899, 1977. van Damme, P. S.; Froment, G. F.; Balthasar, W. B., Scaling Up of Naphtha Cracking Coils. Ind. Eng. Chem. Process Des. Deu. 1981, 20,366-376. Received for reuiew March 21, 1991 Revised manuscript received July 29, 1991 Accepted August 23, 1991
Kinetics of the Poisoning by Thiophene of Supported Nickel Catalysts Alicia Aguinaga, Mario Montes,* Jose C. de la Cal, and Jos6 M. Asua Grupo de Ingenie& Quimica, Departamento de Quimica Aplicada, Facultad de Ciencias Quimicas, Universidad del Pais Vasco, Apdo. 1072, 20080 San Sebastibn, Spain
The kinetics of deactivation of a series of nickel catalysts by thiophene during benzene hydrogenation has been studied. The amount of sulfur retained by the catalyst for different amounts of thiophene fed into the reactor was measured, and the effect of the retained sulfur on both catalyst activity and metallic surface area was investigated. On the basis of these results, a kinetic model for the process was developed and ita parameters were estimated. The model includes a relationship between the deactivation rate constant and both the physicochemical characteristics of the catalyst and the total amount of catalyst in the reactor. A good agreement between experimental results and model prediction was achieved.
Introduction Sulfur from impurities or selectivity modifiers is a common poison of metallic catalysts, and therefore, extensive work has been devoted in the literature to both the chemistry and the kinetics of deactivation of metallic catalysts by sulfur (Barbier, 1982; Butt, 1984; Butt and Petersen, 1988; Figuereido, 1981; Hegedus and McCabe, 1984, Hughes, 1984; Oudar and Wise, 1987; Petersen and Bell, 1987). Experiments carried out at high temperature showed that the deactivating effects of different sulfur
molecules were similar to that of H2S (Barbier, 1982). This suggests that the sulfur molecules were hydrogenolyzed on the metallic catalyst producing H2S which was the actual sulfiding agent. Under deactivating conditions, the rate of the main reaction depends on both reaction conditions and catalyst activity. A substantial part of the Catalyst sulfur poisoning kinetic studies (Bartholomew and Bourman, 1985; Bartholomew and Katzer, 1980; del Angel et al., 1982; Erekson and Bartholomew, 1983; Fowler and Bartholomew, 1982;
oaaa-5a85/92/ 2631-0155~03.00/0 0 1992 American Chemical Society
156 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
Table I. Physicochemical Properties of the Prepared Catalysts sample % Ni
SBET, m2P T N ~O ,C S N i , m2 gNT1
f dNit TOa
a
nm
Si-I 9.62 82 500 1.0 1.0 5 X lo2 0.2
300 8.0 1.0 68 1.1
500 134 0.94 3.0 2.9
Si-PD 11.31 122 400 143 0.86 2.6 2.2
(Si-PD),,.. 15.5 280 500 129 0.95 3.2
300 41.0 0.44 4.6 1.0
SiAl-I 9.38 125 500 36.0 1.0 15.0 3.6
500 90.5 1.0 4.8 3.2
SiAI-PD 8.34 202 400 118 0.81 3.2 2.2
300 24.9 0.46 8.0 0.8
AI-I 9.91 205 500 45.0 0.63 6.0 1.5
AI-PD 9.82 238 500 71.2 0.72 4.4 2.6
Initial toxicity (atoms of Ni (molecule of thiophene)-').
(3)
In the present work, the kinetics of deactivation of a series of nickel catalysts by thiophene during the benzene hydrogenation has been studied. The amount of sulfur retained by the catalyst for different amounts of thiophene fed into the reactor was measured and the effect of the retained sulfur on both catalyst activity and metallic surface area of the catalyst investigated to determine (i) if the catalyst surface was ideal, (ii) the behavior of the poison (selective or nonselective), and (iii) the type of reactor with respect the poison (differential or integral). On the basis of these results, a kinetic model for the process was developed and its parameters were estimated. The model includes a relationship between the deactivation rate constant, used in chemical engineering oriented studies to evaluate the sensitivity of the catalyst to the poison, and the initial toxicity which is the parameter used in the catalysis-orientedworks for this purpose. This gave the relationship between the deactivationrate constant and the physicochemical characteristics of the catalyst.
where -rA is the reaction rate measured at a given time under some reaction conditions, (-r.& is the reaction rate corresponding to the fresh catalyst at the same reaction conditions, a is the catalyst activity, and $(Ci,T) is the deactivation function that depends on both concentration of the poisoning species in the catalytic bed, Ci,and temperature, 2'. Often, eq 3 is simplified by making Ciequal to the concentration of the poisoning species in the feed (Bartholomew and Bourman, 1985; Bartholomew and Katzer, 1980; del Angel et al., 1982; Erekson and Bartholomew, 1983; Fowler and Bartholomew, 1982; Seoane et al., 1989; Zrneevic and Gomzi, 1983; Zrneevic and Rusic, 1988; Zrncevic et al., 1990). This simplification was made based on the assumption that a differential reactor was used. This assumption was checked routinely considering the conversion reached for the main reaction at the exit of the reactor. However, often, no measurement of the conversion of the deactivating (sulfur) molecules was made. Taking into account the great affinity of the nickel for the sulfur molecules, the assumption that a differential reactor for the main reaction is also differential with respect to the poison is open to discussion. Thus, Zrneevic and coworkers (1990), for the deactivation by thiophene of a nickel catalyst during the benzene hydrogenation, found that the concentration of thiophene at the exit of the reactor varied from zero at the beginning of the deactivation process to a constant value when the catalyst was completely deactivated. On the other hand, the separability of the catalyst activity term in the kinetic equation should not be taken for granted. Thus, Lynch and Eming (1989) have shown that the separability is only strictly guaranteed for a very restricted class of mechanisms, and Onal (1981), for the benzene hydrogenation poisoned by thiophene, fitted their experimental data using a nonseparable model in which the adsorption constant for benzene changed during deactivation.
Experimental Section Catalysts. Catalysts were prepared by both incipient wetness and precipitation-deposition on silica, alumina, and silica-alumina (particle size 100-200 pm). Their preparation is fully described elsewhere (Aguinaga et al., 1991). Prior to catalytic testing and some characterizations, the samples were reduced in situ, at either 300,400, or 500 "C, in a 150 cm3/min stream of hydrogen for 16 h (heating rate 8 "C/min). Catalysts prepared by these techniques will be referred to as follows: Ni/Si02 prepared by precipitation-deposition, Si-PD; Ni/Si02 prepared by incipient wetness, Si-I; Ni/Si02-A1203 prepared by precipitation-deposition, SiAl-PD; Ni/Si02-A1203prepared by incipient wetness, SiAl-I; Ni/A1203 prepared by precipitation-deposition, Al-PD; Ni/A1203prepared by incipient wetness, Al-I. The reduction temperature is indicated after the sample reference [e.g., (Al-PD)500, (SiAl-I)300, ...]. The first part of the work, in which the influence of sulfur on the activity and the metallic surface area was studied, was carried out with a different catalyst. This catalyst, referred to as (Si-PD),,, was prepared on silica (Kali Chemie AF 125) as (Si-PD) but with a higher nickel content. Characterization. After the catalysts were dissolved in hydrofluoric acid, nickel content ( 7% Ni) was measured by titration with EDTA-murexide. Total surface areas (SBET) of the catalysts were determined by nitrogen adsorption at 77 K using the BET method in a volumetric apparatus. Metallic surface areas (SNJwere measured by selective chemisorption of hydrogen at room temperature. The degree of reduction (f) was determined after hydrogen chemisorption, by measuring the uptake of pure oxygen at 430 "C (Bartholomew and Farrauto, 1976). Further details about the experimental methods and the expresions used to calculate nickel particle size can be found elsewhere (Aguinaga et al., 1991). Table I presents the nickel content, total surface area (Ssm),metallic surface area (SNJ, degree
Seoane et al., 1989; Zrneevic and Gomzi, 1983; Zmeevic and Rusic, 1988; Zrneevic et al., 1990) assumed that the effect of the reaction conditions and the catalyst activity can be separated as follows: -rA = f(reaction conditions) g(cata1yst activity) (1) This model implicitly assumes an ideal catalyst surface composed by uniform sites without lateral interactions. This means that the chemisorption of both reactant molecules and poison molecules on the free surface of the catalyst is not affected by the poisoning of the rest of the surface. In addition, the chemisorption heat is independent of the surface coverage. If this is the case, the poison will be classified as nonselective and the overall kinetics governed by the following equations (Corella and Asua, 1982): - r A = (-rA)& (2) da/dt = -\t(Ci,T) Aa)
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 157 Table 11. Influence of the Amount of Sulfur Retained by the Catalyst on the Activity and Metallic Surface Area of the (Si-PD),, Catalyst at Different Amounts of Thiophene Fed into the Reactor molecules of UE of atoms of -rA, mol of Th/atom of Sjmg of Sfatom of Bz/(EN~ h) Ni,, cat Ni,", 0 129 8.3 0 0 6.6 0.085 1.5 0.088 103 5.8 0.192 3.66 0.21 90 4.20 0.44 7.9 0.47 51 0.48 0.62 8.1 0.43 4.7 0.72 7.2 60 2.4 0.72 18.4 0.997 12.12 0.12 0 1.710 15.58 0.93
of reduction (f),and mean size of nickel particles (dNJ of the different catalysts prepared. Catalytic Reactions. Reactions were carried out in a differential fixed-bed reactor at 200 "C and atmospheric pressure. The benzene (Merck P.A.) was first treated with Raney nickel under reflux for 3 h and distilled over dry sodium (Merck). Benzene treated in this way was used in an attempt to check the extension of the deactivation in the absence of thiophene, and no deactivation was found for a 6-h-long catalytic tests. In order to prepare the sulfurized feedstock mixture, 5 ppm of thiophene was added to the purified benzene. The mixture was stored in two thermostated gas saturators. After reduction of the catalyst, the reactor was cooled to the reaction temperature in a flow of argon. The reaction mixture was obtained by passing a flow of hydrogen throughout the saturators at temperatures between 6 and 32 "C. The benzene and thiophene partial pressures were calculated assuming that the diluted thiophene in benzene solution (5 ppm) behaved as an ideal solution. In order to calculate the saturation prwures at the saturator temperature, Antoine's equations were used (Reid et al., 1988). Pure hydrogen was used throughout the work. Conversions in benzene less than 10% (differential reactor with respect to benzene) were obtained adjusting the weight of catalyst (1O-lOOO mg)and the weight hourly space velocity (5-1000 h-l). The conversion was measured by on-line gas chromatographic analysis, using a Hewlett-Packard 5710 A gas chromatograph equipped with a TC detector. The conditions for the analysis were as follows: column Chemipack C18, 80/100 6 ft, 1/8-in. stainless steel; injector temperature 150 "C, oven temperature 80 "C; detector temperature 150 "C; carrier gas hydrogen, 36 cm3/min. Catalyst Sulfur Content. In order to study the effect of the amount of sulfur retained by the catalyst on both activity and metallic surface area, the sulfur content of the catalyst should be measured. Because the accuracy of these measurements is related to the amount of sulfur content of the catalyst, a high nickel content catalyst (Si-PD)15was used in these experiments and the amount of thiophene in the benzene was increased up to 0.4%. Several hydrogenation reactions were carried out varying the total time on stream of the catalyst. The rest of the experimental conditions were kept constant (P = 1atm, T = 200 "C,Q = 600 cm3/min, PBz = 45.5 Torr). The conversion was determined by on-line gas chromatographic analysis. At the end of the reaction, the catalyst was cooled in an argon stream to room temperature. Then, it was passivated in an argon stream containing 1 mol % 02. Sulfur content of the samples was determinated with an ANTEK 701C fluorescence sulfur system. The sulfur content of the catalysts given as micrograms of sulfur per milligram of catalyst (ug of S / m g of cat) and as the number of sulfur atoms per nickel atom in the fresh catalyst surface
2 1
0
I
2
4
6
8
1
0
1
2
1
4
molec Th / at Ni
Figure 1. Number of sulfur atoms retained per nickel atom on the fresh catalyst surface as a function of the total amount of thiophene molecules fed into the reactor per atom of nickel on the freah catalyst surface.
(atoms of S/atom of N$J, as presented in Table II. Also, this table includes the metallic surface area (m2/gNi)of these partially deactivated catalysts after a standard reduction at 500 "C. The hydrogenation activity of these catalyst could not be determined while the catalysts were being deactivated because in these experiments integral reactors with respect to the benzene were used. Therefore, a fraction of the passivated catalyst was reactivated and the reaction rate for hydrogenation measured in an independent experiment carried out in a differential reactor. These data are also included in Table 11. Results and Discussion Influence of the Amount of Sulfur Retained by the Catalyst on the Nickel Catalyst Properties. Figure 1 presents the number of sulfur atoms retained per atom of nickel on the catalyst surface against the total amount of thiophene molecules fed into the reactor per atom of nickel on the catalyst surface. Note that, for a constant feed rate, the total amount of thiophene is proportional to the time on stream. It can be seen that at the beginning of the process the slope of this curve is equal to 1. This means that the sulfur was completely retained by the catalyst. Later, the slope decreases indicating that the sulfur was not completely retained by the catalyst. Figure 1can be used as a guide to estimate if a given reactor is differential with respect the thiophene because the slope of the sulfur retained versus thiophene fed curve gives the conversion of thiophene. Taking a 10% conversion as the upper limit for a differential reactor, Figure 1shows that the assumption for differential reactor can be safely made when the total number of molecules of thiophene fed into the reactor per atom of nickel on the catalyst surface is greater than 5 (slope of the curve less than 0.1). Otherwise, the reactor is integral with respect to the thiophene and this has to be taken into account in the kinetic analysis. It should be pointed out that this conclusion can be only strictly applied to catalyst (Si-PD)15 under this reaction conditions. Nevertheless, it gives a reference for the other nickel catalysts. The three points marked with an arrow in Figure 1 correspond to the moment in which the hydrogenation activity of the catalyst vanished. Each point came from a different experiment, showing an excellent reproductibility. The point with the highest sulfur content corresponds to a time on stream 4 times longer than that at which the hydrogenation activity of the catalyst disappeared. This means that the sulfurization of the catalyst continued after its complete deactivation for hydrogenation. A t 200 "C, poisoning is produced by H2S which
158 Ind. Eng. Chem. Res., Vol. 31, No. 1,1992
1
0.8
.-ti
ti B u"
0
0.8
0
i
0.6
0.6 0
%
$#
i x
0
Y
0.4 0.2
0.2 0
0
10
20
30
40
50
-
SNi 60
00
,
0
0.2
0.4
0.6
4
0.8
1
1.2
1.4
at Slat Ni
t
Figure 2. Time evolution of the conversions of benzene (*) and thiophene (0).
Figure 4. Metallic surface area fraction (SNi) as a function of the number of sulfur atoms retained per nickel atom on the fresh catalyst surface (atoms of S/atom of Ni).
' I m
rnm 0.4
m 0.2
o
0.2
0.6
0.4
0.8
1
SNi Figure 3. Benzene hydrogenaGon activity ( a )versus metallic surface area fraction (SNi).
results from thiophene hydrogenolysis (Bourne et al., 1964). Therefore, this result suggests that the catalyst keeps some hydrogenolysis activity after its complete deactivation for hydrogenation. In order to confirm this result, an experiment was carried out under the same experimental conditions but using a feed richer in thiophene (5.6%). Both the hydrogenation activity and the hydrogenolysis activity of the catalyst were determined by on-line gas chromatographic analysis of benzene, cyclohexane, thiophene, and butane. Figure 2 presents the time evolution of the conversions of benzene and thiophene. It can be seen that the catalyst was active in hydrogenolysis after loosing its hydrogenation activity. This result is in agreement with the well-known hydrogenolysis activity of the nickel sulfide, which is used in several industrial hydrodesulfurization catalysts. Figure 3 presents the relationship between the metallic surface area fraction (SNi) and the hydrogenation activity (a). The metallic surface area fraction was defined as the ratio between the metallic surface area of the deactivated catalyst and that of the fresh catalyst. It can be seen that the catalyst activity in hydrogenation was equal to the fraction of sites that remained unsulfided. Figure 4 presents the effect of the amount of sulfur retained by the catalyst on the metallic surface area fraction. It can be seen that a linear relationship was obtained showing that the extent of sulfurization of the core of the metallic particles was negligible until the surface of these particles was completely sulfurized. Figures 3 and 4 show that, while the catalyst was active in hydrogenation, a given amount of sulfur retained on the catalyst produced the same decrease of the catalyst activity as that of metallic surface area fraction. In addition, this decrease was in-
Table 111. Total Amount of Thiophene Molecules Fed into the Reactor per Atom of Nickel on the Fresh Catalyst Surface at the End of the Kinetics Experiments samde molecules of Th/atom of Ni.,," (Si-PD)500 0.62 (Si-PD)400 0.38 (Si-PD)300 1.26 0.64 (SiA1-PD)500 0.56 (SiAl-PD)400 2.62 (SiAl-PD)300 0.17 (Al-PD)500 (Al-I)500 1.25 0.43 (SiAl-I)500 0.49 (Si-I)500 0.82 (Si-I)300
dependent of the fraction of the catalyst that remained unsuKded. This is proof that the catalyst surface behaved ideally for both the main reaction and poisoning process and that the thiophene is a nonselective poison. Therefore, a separable kinetic equation is suitable for the kinetic analysis. Kinetic Analysis. Figures 5 and 6 present the time evolution of the reaction rate for benzene hydrogenation [mol/(g of nickel-h)] for the catalysts presented in Table I. Each figure includes the results obtained at different partial pressures of benzene. It can be seen that deactivation was more severe as the partial pressures increased. Note that, because thiophene has almost the same vapor pressure as benzene, thiophene partial pressures in the feed were proportional to those of benzene. Therefore, the greater the benzene partial pressure, the greater the thiophene content of the reaction mixture and, consequently, the greater the deactivation observed. From Figures 5 and 6,the time evolution of the catalyst activity can be calculated as the ratio of the reaction rate at a given time and the reaction rate of the fresh catalyst given by the intercept of the reaction rate plot. These results are presented in Figures 7 and 8. Table 111 presents the total amount of thiophene molecules fed into the reactor per atom of nickel at the end of the experiment. These data correspond, for each catalyst, to the experiment carried out using the largest thiophene feed rate. It can be seen that these values are lower than 5 (upper reference value for an integral reactor) and, hence, these reactions were carried out in integral reactors with respect to thiophene. Often, the initial toxicity of the catalyst (70) is used to evaluate the sensitivity of the catalyst to a given position. 70 is defined as the number of active sites (atoms of Ni at the catalyst surface) that are deactivated by the first molecule of thiophene entering the reactor (Barbier, 1985).
~
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 159 A PB= 36.8 mmHg (pn=0.00013 mmHg)
+
+
PB= 86.7 mmHg (pTheO.00033 mmHg)
PB= 82.7 mmHg (pn=0.00032 mmHg)
o PB=130.3 mmHg CP,=0.00051 mmHg) 6
-i6
(Si-PD) 400 1
0
100
I
200
‘0 300 0
0 PB=124.7 mmHg (P?h=0.00018 mmHg)
0
I
I
100
200
300
PB=130.3 mmHg 0?,,,=O.a0051 mmHg)
00
t (min) t (min) Figure 5. Time evolution of the reaction rate for the hvdroaenation of benzene, poisoned by thiophene, at different partial pressures. Points, - exierimental results; solid line, model predictions.
The inverse of the initial toxicity, Le., the number of atoms of sulfur required to deactivate one atom of surface nickel, is called initial thioresistance. Taking into account that the catalysts were uniform, that the poison was nonselective, and that there was a linear relationship between the activity and the fraction of catalytic surface that remained unchanged, T~ can be estimated from the slope at the intercept of the plots of the activity versus total amount of thiophene per nickel atom on the surface of the fresh catalyst fed into the reactor. An example of these plots is given in Figure 9. The values of the initial toxicity ( T ~ atoms , of Ni/molecule of thiophene) calculated for the different catalysts are given in Table I. It should be pointed out that the values found for a given catalyst a t different partial pressures of thiophene in the feed are almost the same. The values in Table I1 are averaged ones. Note that the calculation of T~ involves the assumption that the first molecules of thiophene entering the reactor were completely retained by the catalyst. This assumption is in agreement with the results showed in Figure 1. As shown previously, a separable kinetic equation is suitable for this system. The separable kinetic model
involves the choice of a kinetic equation for the reaction rate corresponding to the fresh catalyst plus a deactivation equation. The kinetic expression for the benzene hydrogenation derived by Prasad et al. (1983) is used in this work for the reaction rate corresponding to the fresh catalyst. This equation summarizes the main mechanisms usually accepted for this reaction (Chou and Vanice, 1987; Dalmai-Imelik and Massardier, 1976; Prasad et al., 1983; Van Barnevel and Ponec, 1974; Van Meerten and Coenen, 1977; Van Meerten et al., 1976, 1977).
The deactivation rates can be expressed as follows: da rate of deactivation of active sites X no. of active sites in the fresh catalyst turnover no. of the active sites being deactivated (5) turnover no. in the fresh catalyst
160 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
-i
A pB= 38.8 mmHg (Pm=0.00015 mmHg) -k pB= 80.8 mmHg (Pm=0.00031 mmHg) 0 PB=130.3 mmHg (pm=0.00051 mmHg)
6 -
5
0
PB= 61.8 mmHg (P,,,=O.o0024 mmHg) PB= 95.2 mmHg (P,=O.00037 mmHg)
g- 34 P
h
2
L;
' 1 I
1
I
300
100
200
I
Od
200
100
t (min)
31
I A PB= 39.9 mmHg (P,=0.00015
mmHg)
4- PB= 86.7 mmHg (€'.,,,4.00033 mmHg)
-
3 t (min)
0
2
PB=1U.7 mmHg (P.,,,=O.o0048 mmHg)
m
4- PB= 82.7 mmHg (Pn=0.00032 mmHg)
-.
0.6
z,-
0.4
.-
8
0
PB=124.7 mmHg (p,=O.OLWS mmHg)
h
L: ' 0.2
(Si-I)300 I
0.0
I
Figure 6. Time evolution of the reaction rate for the hydrogenation of benzene, poisoned by thiophene, at difterent partial pressures. Points, experimental results; solid line, model predictions.
In addition, the turnover number of the active sites that were being deactivated at any time should be equal to that of the fresh catalyst because, as shown previously, the catalytic sites were uniform. Therefore, eq 5 reduces to
--da dt
-
rate of deactivation of active sites number of sites in the fresh catalyst
(6)
The rate of deactivation of active sites is given by: rate of deactivation of active sites = (rate of retention of sulfur molecules by nickel atoms of the catalyst surface)TO(7) The rate of retention of sulfur molecules by the catalyst while the catalyst is active in hydrogenation can be estimated from Figure 1taking into account that, as can be seen in Figure 4, the sulfurizationof the core of the metallic particles was negligible until the surface or the particles was completely sulfurized. Figure 1 shows that the rate of retention of sulfur molecules by nickel atoms on the catalyst surface is a maximum for the fresh catalyst and decreased as the extent of the sulfurization of the surface of the metallic particles increased. The simplest kinetic equation for this type of process is as follows: d(atoms of S/atom of Ni) dt
FT
= -SNi NO
work 1 + (KHPH)'I2 N 1,the overall kinetics is governed by the following equations:
(8)
where FT is the molar feed rate of thiophene and No is the number of nickel atoms in the fresh catalyst surface. In addition, Figure 3 shows that SNi = a and hence, eq 6 becomes (9) Taking into account that under the conditions used in this
where kd is the deactivation rate constant given by k d = - T-O accessible Ni atoms deactivated NO molecule of thiophene total number of accessible Ni atoms - accessible metal atoms deactivated total number of accessible Ni atoms molecule of thiophene (12) Therefore fraction of accessible Ni atoms deactivated kd = (13) molecule of thiophene It is important to emphasize the differences between r o and kd because both are used to evaluate the resistance of catalyst to poisoning. T~ is the absolute number of accessible metallic atoms deactivated by a molecule of thiophene. On the other hand, kd is the fraction of the total accessible metallic atoms in the bed deactivated by a molecule of thiophene. In other words, T~ is a microscopic characteristic of the catalyst that indicates the absolute thioresistance of the nickel in a particular catalyst and kd is a macroscopic characteristic of the catalytic bed that indicates its relative thioresistance and, therefore, it depends on the type of catalyst, the amount of sample used in the experiment ( W), the nickel content ('70Ni), and the metallic surface area:
-
-
/
/
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 161 1 .o 38.8 mmHg 82.7 mmHg
0.8
..-
0.6
Y
Y E-
2
%++
0.4 0.2
+++
0.4
-
0.2 (Si-PD) 500
*
1 .o
A
+0
0.8
.-+E-, c1
. 0.6 -
.I Y
2
0.4
0.2
h
+++ +++++
I
I
200
-
0.4
-
Y
0.2
b A
.
A A ~ ~ A AA A
++
10.0; 300
'AAA~
+++++++++
9 +
I
1
200
300
A pB= 38.8 mmHg
+ PB= 82.7 mmHg
AA
o0++
0.4
-
-
(SiAI-PD)400
where the area occupied by surface nickel atom was assumed to be 6.33 X m2 (Coenen and Linsen, 1970). It has to be pointed out that, for experiments carried out using the same amount of a given catalyst and constant feed flow rate, eq 9 reduces to
This can explain the good agreement between experimental results and model predictions achieved by ZrnCevic et al. (1990). These authors found that the concentration of thiophene at the exit of the reactor varied from zero at the beginning of the process to a constant value when the catalyst was completely deactivated. This means that, at least at the beginning of the process, the reactor was integral with respect to the thiophene. However, this result was not taken into account in the kinetic analysis and they proposed a deactivation kinetic equation proportional to the thiophene partial pressure at the entry of the reactor. In addition, this partial pressure was assumed constant throughout the reactor, because the reador was differential for the main reaction. The estimation of the parameters of the kinetic equation
0.0
++t
I
+AA
-
0.2 I
%mw
A
o.6
+++
'++
100
0 PB=124.7 mmHg
'AA
++
A
+*
-0%
b A
I
'AAA
(SiAI-PD) 500
A pB= 38.8 mmHg +PB=82.7mmHg 0.8
-
0.0 *
0 PB=130.3 mmHg
AA
000++++
1.0
++++
AAA AA
00'
*
+"A
+
O+ O+
0.2
100
.- +>AA +
A pB= 36.8mmHg
+
OoOo 0
I
300
+ PB= 86.7 mmHg
A '
6++ o +
oco
0.0 L 0
0.6
8
I
200
AA
f
++
(Si-PD) 300
E-
I 0* t
I
100
0
-
Y
2
++ +++
-
1.0
..-
P 1124.7 mmHg
o++
0 "0 0 00
(Si-PD) 400
A
-
Oa8
1
0.0 I 3 0
I
I
0.0
AAAA
AA
oo ++++ AAuAAA 0 ++
oo
+++
00
OOO
(SiAI-PD)300
+++++
+
oosoo
I
can be carried out by analyzing separately the kinetics of the main reaction and the deactivation kinetics (Corella and Asua, 1982). However, in order to determine the kinetics of the main reaction, the values of the reaction rates for the fresh catalysts should be obtained. These values can be obtained by the intercept of the time evolution of the reaction rate. Nevertheless, in this work, the parameters of the kinetic equation for the main reaction were estimated together with those of the deactivation reaction using the whole reaction rate versus time curve. All of the measured points in the three experiments carried out for each catalyst were used to estimate the rate constants of the kinetics equations corresponding to these catalysts. Parameter estimation was carried out by means of an algorithm for parameter estimation in ordinary differential equations proposed by Hwang and Seinfield (1972). The algorithm is based on an extended GaussNewton method. Table IV presents the values of K2,K3,and k d estimated by means of the proposed algorithm. Catalyst (Si-I)500 presented almost no time decay of the activity, and hence, k d was not estimated for this catalyst. Figures 5 and 6 present a comparison between the experimental values of
162 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 1
n.
1.03
i
A PB= 38.8mmHg
e~
+ PB= 80.8 mmHg "
A
L
~
~
PB=130.3 mmHg
0
-
0.8 -
I
+ PB= 61.8 mmHg oft+
AAAAAA
++++
' 0
0.4 -
0.2
0 PB= 95.2 mmHg
O+ AAAA
0.6 -
AAaM..
A PB= 38.9 mmHg
$A
++++ OO00 +++
00
-
OOoo
AA
++
(AI-I) 500 0.0
1
200
t (min)
3c
1 .o
A PB= 39.9mmHg PB= 86.7mmHg
.PAAA
I
100
+
0.8
0.6 0.4 0.2 0.0
'
0
I
I
(Si I)300 I
I
I
I
100
200
300
0.0
A PB= 38.8mmHg 82.7 mmHg 0 PB=124.7 mmHg
+ P,= I
I
3 Kl t(min) Figure 8. Time evolution of the catalyst activity at different partial pressures. See Figure 6 for thiophene partial pressures. t (min)
1
-
h
0.6
0
0.4 0.2 0 0
100
200
Table IV. Estimated Values of the Kinetic Parameters K, Kz 10"kd TO 10'8(~~/iV~) (Si-PD)BOO 257 34 1.0 2.9 1.0 (SLPD)400 826 108 0.9 2.2 0.9 (Si-PD)300 355 44 1.0 1.0 0.9 (SiAl-PD)500 255 32 1.3 3.2 1.2 (SiAl-PD)400 636 79 1.1 2.2 1.2 (SiAl-PD)300 289 40 1.2 0.8 1.3 (A1-PD)500 474 75 1.2 2.6 1.4 ( Al-I) 500 1.3 359 74 1.4 1.5 (SiAl-I)500 58 15 1.4 3.6 1.4 (Si-I)300 1.1 0.5 416 84 0.5
0.8
.=c .-
0
0.2 0.4 0.6 0.8 molecule Th I atom Ni surface
Figure 9. Normalized catalyst activity as a function of the amount of thiophene fed into the reactor for the catalyst (Si-PD)500,at three different partial pressures. See Figure 5 for thiophene partial pressures.
the reaction rates and the predictions of eqs 10 and 11 using the parameters given in Table IV. It can be seen that a good fitting of experimental data was achieved. Table It can be IV also presents the values of the ratio T~/N,,, seen that good agreement was found between the estimated values of the deactivation constant and the measured values of the ratio T,,/N,,, The dependence of kd on the physicochemical characteristics of the catalysts (dNi, f , SNJ is concealed by the dependence of kd on the extensive properties of the catalyst as the nickel content and the weight of the catalyst bed. On the other hand, the initial toxicity of the catalyst depends on the characteristics of the catalyst. Thus, Aguinaga et al. (19911, using the same series of catalysts as in this work, found that r0 decreased as the mean size of the nickel particles increased for catalysts with weak interaction between the nickel and the support (impregnated samples on silica and silica-alumina). On the other hand, when the nickel support interaction is important (Si-PD,
SiAl-PD, Al-I, and Al-PD), it was found that as the degree of reduction increased.
T~
increased
Conclusions In the foregoing, the kinetics of deactivation of a series of nickel catalysts by thiophene during the benzene hydrogenation has been studied. Measurements of the amount of sulfur retained by the catalyst for different amounts of thiophene fed into the reactor showed a high nickel affinity for the sulfur, and hence, the reactor was integral with respect to the poison. In addition, a linear relationship was obtained between the amount of sulfur retained by the catalyst, the loss of hydrogenation activity, and the loss of metallic surface area. This means that the catalysts behaved ideally for both the main reaction and the poisoning process and that the thiophene was a nonselective poison. Therefore, separable kinetic equations are suitable for the kinetic analysis. On the basis of these findings, a kinetic model for the process was developed. The model includes a relationship between the deactivation rate constant and both the physicochemical characteristics of the catalyst and extensive properties such as the nickel content and the weight of catalyst. The parameters of the kinetic equation for the main reaction were estimated together with those of the deactivation equation using all the measured data for each catalyst. A good
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 163 agreement between experimental results and model predictions was achieved. Acknowledgment The scholarship support for A.A. by the Ministerio de Educaci6n y Ciencia (Programme FPI),and the financial support by the Universidad del Pais Vasco (Grant 215.06-46/86)are gratefully appreciated. Nomenclature a = normalized activity
Ci = concentration of poisoning species i (mol of i m-3) dNi= nickel particle size (nm) f = degree of reduction FTh= molar feed rate of thiophene (molecules of thiophene h-l) kd = deactivation rate constant (molecule of thiophene-') (eq
11) k'= deactivation rate constant given in eq 15 (m3mol-' h-') K H = hydrogen adsorption equilibrium constant K 1= constant given in eq 4 that groups together rate and adsorption equilibrium constants (atm-l) K 2 = constant given in eq 4 K3 = constant given in eq 10 (mol of benzene gNc' h-' atm13/2) %Ni = nickel content (%) No = number of nickel atoms in the fresh catalyst surface PBz= partial pressure of benzene (atm) PH = partial pressure of hydrogen (atm) -rA = reaction rate (mol of benzene gN?' h-') (-rA)o = reaction rate of the fresh catalyst (mol of benzene gN;' h-') % S = sulfur content (% ) SBET = total surface area (m2g-') SNi= metallic surface area (m2gN[') SNi = metallic surface area fraction [SNi = SNi/(SNi)O] T = temperature (K) T,, = initial toxicity (atoms of Ni/molecule of thiophene) W = weight of catalyst (8) Registry No. Ni, 7440-02-0;C&, 71-43-2;thiophene, 110-02-1.
Literature Cited Aguinaga, A.; Montes, M.; Asua, J. M. Effect of the Preparation on the Initial Toxicity of Supported Nickel Catalysts Deactivated by Thiophene. J. Chem. Technol. Biotechnol. 1991,in press. Barbier, J. Effect of Sulfur on Catalytic Properties of Metals. In Metal-Support and Metal-Additive Effectsin Catalysts; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; pp 293-306. Barbier, J. Effect of Poison on the Activity and Selectivity of Metallic Catalysts. In Deactivation and Poisoning of Catalyst; Oudar, J.; Wise, H., Eds.; M. Dekker: New York, 1985; pp 109-150. Barthomolew, C. H.; Farrauto,l R. J. Chemistry of Nickel-Alumina Catalysts. J. Catal. 1976,45,41-53. Bartholomew, C. H.; Katzer, J. R. Sulfur Poisoning of Nickel in CO Hydrogenation. In Catalyst Deactivation.; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1980; pp 375-396. Bartholomew, C. H.; Bourman, R. M. Sulfur Poisoning of Cobalt and Iron Fischer-Tropsch Catalysts. Appl. Catal. 1985,15, 59-67. Bourne, K. J.; Holmes, P. D.; Pitkethcy, R. C. The Selectivity of Sulphided Nickel Catalysts. In Proceedings of the Third Znternational Congress on Catalysis; North-Holland: Amsterdam, 1964;pp 1400-1415. Butt, J. B. Catalyst Deactivation and Regeneration. In Catalysis. Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984;Vol. VI, p 1. Butt, J. B.; Petersen, E. E. Activation, Deactivation and Poisoning of Catalysts; Academic Press: London, 1988. Chou, P.; Vanice, M. A. Benzene Hydrogenation over Supported and Unsupported Palladium. 11. Reaction Model. J. CUtUl. 1987,107, 140-153. Coenen, J. W.E.; Linsen, B. G. Structure and Activity of SilicaSupported Nickel Catalysts. In Physical and Chemical Aspects
of Adsorbents and Catalysts; Linsen, B. G., Ed.; Academic Press: New York, 1970; pp 471-527. Corella, J.; Asua, J. M. Kinetic Equations of Mechanistic Type with Nonseparable Variables for Catalyst Deactivation by Coke. Models and Data Analysis Methods. Znd. Eng. Chem. Process Des. Dev. 1982,21,55-61. Dalmai-Imelik, G.; Maasardier, J. Catalytic Activity of Single Crystal Faces: Ethylene and Benzene Hydrogenation on (lll),(110)and (loo), Faces of Ni Single Crystals. In Proceedings of the Sixth International Congress on Catalysis; Royal Society of Chemistry: London, 1976;90-100. Del Angel, G.; Coq, R.; Figueras, F.; Fuentes, S.; GBmez, R. Kinetics of the Deactivation by Thiophene of Supported Rhodium Catalysts. In Actus del ' 8 Simposio Zberoamericano de Catdlisis;La Mbida, CSIC Madrid, 1982;pp 180-188. Erekson, E. J.; Bartholomew, C. H. Sulfur Poisoning of Nickel Methanation Catalysts. 11. Effects of HzS Concentration, CO and H 2 0 Partial Pressures and Temperature on Deactivation Rates. Appl. Catal. 1983,5,323-336. Figueiredo, J. L., Ed. In Progress in Catalyst Deactivation; Martinus Nijhoff: The Hague, 1981. Fowler, R. W.; Bartholomew, C. H. Activity Adsorption, and Sulfur Tolerance Studies of Fluidized Bed Methanation Catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1982,18,339-347. Hegedus, L. L.; McCabe, R. W. Catalysts Poisoning; Dekker: New York, 1984. Hughes, R. Deactivation of Catalysts; Academic Press: New York, 1984. Hwang, M.; Seinfeld, J. H. A New Algorithm for the Estimation of Parameters in Ordinary Differential Equations. AZChE J. 1972, 18,90-93. Lynch, D. T.; Eming, G. On the Separability of Catalyst Activity and Kinetic Behavior. Chem. Eng. Sci. 1989,44,1275-1280. Onal, I. Kinetic Separability and Structure Sensitivity of Poisoning Process. Ph.D. Dissertation, Northwestern University, 1981, University Microfilms International. Oudar, J., Wise, H., Eds. Deactivation and Poisoning of Catalysts; Dekker: New York, 1987. Petersen, E. E., Bell, A. T., Eds. Catalyst Deactivation; Dekker: New York, 1987, Prasad, K. H. V.; Prasad, K. B. S.; Mallikarfunen, M. M.; Veidyeswaran, R. Self Poisoning and Rate Multiplicity in Hydrogenation of Benzene. J. Catal. 1983,84,6573. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1988. Seoane, X. L.; Arcoya, A.; Gonz&lez, J. A.; Travieso, N. Hydrogenation of Ethylbenzene over a Nickel/Mordenite Catalyst. Catalytic Decay by Thiophene Poisoning. Znd. Eng. Chem. Res. 1989,28, 260-264. Van Barnevel, W. A. A.; Ponec, V. On Some Problems of the Hydrogenation of Benzene on Nickel and Nickel-Copper Alloys. Recl. Trav. Chim. 1974,93,243-246. Van Meerten, R. 2.C.; Coenen, J. W. E. Gas Phase Benzene Hydrogenation on a Nickel-Silica Catalyst. VI. Rate-Equation and Curve Fitting. J. Catal. 1977,46,1-12. Van Meerten, R. Z. C.; Verhaak, A. C. M.; Coenen, J. W. E. Gas Phase Benzene Hydrogenation on a Nickel-Silica Catalyst. 11. Gravimetric Experiments of Benzene, Cyclohexene and Cyclohexane Adsorption and Benzene Hydrogenation. J. Catal. 1976, 44,217-225. Van Meerten, R. Z. C.; De Grauf, T. F. M.; Coenen, J. W. E. Gas Phase Benzene Hydrogenation in a Nickel-Silica Catalyst. 111. Low-Field Magnetization Measurements on Hydrogen, Benzene, Cyclohexene and Cyclohexane Adsorption and Benzene Hydro. 46, 1-12. genation. J. C a t a ~1977, Zrnpevic, S.; Gomzi, Z. Catalyst Poisoning in the Benzene Hydrogenation. Chem. Eng. Sci. 1983,38,1351-1355. Zrnqevic, S.;Rusic, C. Verification of the Kinetic Model for Benzene Hydrogenation by Poisoning Experiment. Chem. Eng. Sci. 1988, 43,763-767. Zrnpevic, S.; Gomzi, Z.; Kotur, E. Thiophene Poisoning of Ni-SiOzAlz03 in Benzene Hydrogenation. Deactivation Kinetics. Ind. Eng. Chem. Res. 1990,29,774-777.
Received for review March 6 , 1991 Revised manuscript received July 11, 1991 Accepted September 9, 1991
