Deactivation Kinetic Parameters of Nickel Sulfur Poisoning during

was carried out at initial conversions of below 6% within a bed diluted with glass beads to achieve ... poisoning on several metals are given by Butt ...
0 downloads 0 Views 72KB Size
Download PDF
5420

Ind. Eng. Chem. Res. 2002, 41, 5420-5426

Deactivation Kinetic Parameters of Nickel Sulfur Poisoning during Benzene Hydrogenation Luismar M. Porto* and John B. Butt Department of Chemical Engineering, Northwestern University, 2137 Sheridan Road, Evanston, Illinois 60201

Intrinsic deactivation kinetic parameters of a commercial nickel/Kieselguhr catalyst (Ni-0104T) were determined by poisoning of the benzene hydrogenation reaction with 1-propanethiol from 68 to 120 °C. Mercaptan concentrations of 75, 79, 115, and 290 ppm were used. The reaction was carried out at initial conversions of below 6% within a bed diluted with glass beads to achieve better temperature control. A procedure to measure the deactivation kinetic parameters based on the cyclohexane molar concentration is described in detail. Poisoning values of the preexponential factor and activation energy were found to be 1.45 × 10-4 (Pa‚s)-1 and 3.87 kJ‚mol-1, respectively, and were comparable with integral reactor data and parameters previously found for thiophene poisoning. Introduction The benzene hydrogenation reaction is an important industrial process1 and has been used as a model reaction for a long time because of its particular features in a heterogeneous catalytic system.2 Poisoning by sulfur is certainly one of the major causes of concern in fixed-bed reactor operation packed with nickel, platinum, and palladium.3 In particular, nickel catalyst supported on Kieselguhr is commercially used in the reduced state in vapor- and liquid-phase hydrogenation reactions such as in the production of cyclohexane and hardening of vegetable oils. Reviews covering sulfur poisoning on several metals are given by Butt and Billimoria,4 Bartholomew et al.,5 Hegedus and McCabe,6 Oudar and Wise,7 Bartholomew,8 Butt and Petersen,2 and Barbier et al.9 Some questions that are still to be answered and that pose some challenging research problems are related to the mechanism of sulfur adsorption on metals such as nickel, iron, and platinum and the stoichiometry of the surface and bulk sulfur structures involved. Here we show that kinetic determinations must consider these effects that may otherwise mislead the real meaning of the deactivation rate constant and poisoning activation energy. Industrial nickel catalyst is usually supplied in a reduced and stabilized form that makes the catalyst nonpyrophoric. The stabilization process involves the adsorption of carbon dioxide to avoid further oxidation of the catalyst. The degree of reduction is an important factor and may affect the thioresistance behavior even at room temperature.10 Although very important from the design and operation standpoints, deactivation kinetic parameters are barely found in the literature, probably because of difficulties usually associated with dealing with sulfur, usually at the ppm level, and irreversible processes. In this work we report data for individual thiophene and 1-propanethiol poisoning and call attention to a methodology that makes use of a * To whom correspondence should be addressed. Fax: +5548-3319687. E-mail: [email protected]. Current address: Departamento de Engenharia Quı´mica e Engenharia de Alimentos, Universidade Federal de Santa Catarina, Caixa Postal 476, CEP 88040-900 Floriano´polis, Santa Catarina, Brazil.

simple kinetic expression and measurements to evaluate poisoning kinetics under feasible laboratory conditions. Modeling the Deactivation Benzene Hydrogenation Kinetics and Mole Balance. The hydrogenation of benzene over a nickel catalyst can be very selective and can be taken as irreversible in the temperature range of 60-200 °C, at atmospheric pressure. The reaction

benzene + H2(excess) f cyclohexane with excess hydrogen (>98% v/v) is very sensitive to sulfur compound poisons present in the feed even at very small concentrations. For an isothermal differential reactor, the mass balance for benzene may be written as

-

dxB FcMg ) (-rB) dt Fg

(1)

dxB Mg ) (-rB) dτ Fg

(2)

or

with

xB ) xB°

for τ ) 0

(3)

where τ ≡ tFc/ is the space time. The benzene hydrogenation rate over nickel usually follows an Eley-Rideal mechanism11 and can be expressed by

(-rB) )

kKBP2xBxH 1 + KBPxB

(4)

We consider here a limiting case: when hydrogen is in great excess, changes in xH can be safely neglected; for low temperatures and relatively high benzene concentrations, the rate expression may be reduced to a pseudo-first-order reaction:

10.1021/ie020127s CCC: $22.00 © 2002 American Chemical Society Published on Web 09/27/2002

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5421

(-rB) = kPxH°

By differentiating eq 14 with respect to time, we have

(5)

Therefore, the mass balance for an isothermal differential reactor can be simplified to

I.C.: xB ) xB°

Mg dxB ) kPxH° dτ Fg

at τ ) 0

-

Integrating with respect to τ,

Mg ∆τ Fg

c4 ≡ c2c3 ) kdPxP°

(7)

Mg ∆τ(-rB°) Fg

-ln(xC/c3) ) c2t (8)

where xC is the molar fraction of cyclohexane, assuming that

xC ) xB° - xB

c1 ≡ kPxH°

Mg Fg

(10)

Activity and Arrhenius Deactivation Parameters, kd° and Ed. It has been observed experimentally12 that the rate of deactivation of Ni/Kieselguhr catalysts, rd, by sulfur poisons such as H2S, CS2, (C2H5)S, and C4H4S, is proportional to the partial pressure of the poison, PxP, and the unpoisoned fraction of active sites, or activity, a

rd )

da ) -kdPxPa dt

(11)

For a differential reactor, the fraction of poison does not change significantly and xP may be considered approximately constant (xP = xP°). After integration considering a ) 1 at t ) 0,

a ) exp(-c2t), with c2 ≡ kdPxP°

-

dxB Mg (-rB°)a ) dτ Fg

I.C.: xB ) xB°, a ) 1, at τ ) 0

(13)

Integration over τ leads to

xB° - xB ) xC ) c3a

(14)

where

c3 ≡

Mg ∆τ(-rB°) Fg

xC ) xC°

(15)

at t ) 0

(19)

Accordingly, from eq 16,

∫0texp(-c2t) dt ) c3[1 - exp(-c2t)]

xC° - xC ) c4

(20)

or

[

-ln 1 -

]

1 (x ° - xC) ) c2t c3 C

(21)

At the initial condition, we can write the mass balance equations as

|

dxC dτ

τ)0

)-

|

dxB dτ

τ)0

=

Mg (-rB°) Fg

(22)

which, by integration, gives

xC° ) xB° - xB )

Mg (-rB°)∆τ ) c3 Fg

(23)

Thus, eq 23 can be reduced to eq 18, and the latter can be rewritten as

(12)

Poisoning Mole Balance: Differential Reactor. When poison is present in the feed and deactivation takes place, the benzene mass balance has to be modified to include the activity a, so that

(18)

The same result is obtained when considering extrapolation to the initial condition

(9)

and c1 is a constant for a given temperature, pressure, and hydrogen partial pressure, defined as

(17)

Using the integral method of kinetic analysis,13 eq 16 can be integrated by inserting the time-dependent function a (eq 12) and integrating from time t to ∞, at which xC is equal to zero (the catalyst is assumed to be completely deactivated). After rearrangement,

or

xC ) c1∆τ

(16)

where

(6)

xB° - xB ) kPxH°

dxC dxB ) ) c4a dt dt

-ln(xC/xC°) ) c2t

(24)

-ln[xC(t)/xC°] ) kd(T) PxP°t

(25)

or

Therefore, from the slope of -ln(xC/xC°) versus t, kd can be calculated using eq 25 and, from the plot of -ln kd versus 1/T, we can obtain the values of Ed and kd°, the activation energy and the preexponential factor for the deactivation reaction, respectively. The described method to evaluate the kinetic deactivation parameters was used by Weng14 to determine the values of Ed and kd° for thiophene poisoning and was used in the present work to determine the deactivation rate parameters for 1-propanethiol poisoning. Notice that this method allows us to quantify the kinetic deactivation parameters without measuring the exit concentration of poison. This is an advantage over other methods (e.g., see work by Bartholomew),15 where the

5422

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002

exit concentration of poison, usually very low values, needs to be measured directly. Poison Adsorption Capacity from the Mole Balance: Integral Reactor. The poison adsorption capacity of the Ni/Kieselguhr catalyst, MT, expressed as the number of moles of thiophene per mass of catalyst, was found to be the most important parameter in the socalled MTv model (thiophene poisoning), an isothermal in cycles fixed-bed reactor model that considers an increase in the adsorption capacity of the catalyst as the temperature of the experiment is increased to make up for some previous isothermal deactivation.16-18 Surprisingly, the adsorption capacity increases with increasing temperature, against the general trend observed by the adsorption equilibrium behavior of most adsorbents. By recognition that changes in MT will affect the relative adsorption “activity” when the temperature is increased, the MT model successfully resembles the deactivation pattern observed for several deactivation isotherms. The value of MT may be determined by the JIS method,8 condensing the effluent of a bed packed with 20 g of catalyst and measuring the concentration of thiophene with a spectrophotometer. The JIS method allows the determination of MT for isothermal and nonisothermal reactors. However, because the adsorption capacity might be a function of temperature, the use of another method, where determinations are done at constant temperature, would be more appropriate. If we assume, by simplification, that the benzene hydrogenation reaction obeys a first-order reaction rate expression and that the poisoning reaction rate is given by eq 11, the plug-flow reactor (PFR) isothermal model leads to an analytical solution that permits the evaluation of MT, from the poison breakthrough curve. Under deactivation conditions, the dimensionless isothermal PFR model can be given by the following set of equations:

∂x ) -k1xa ∂ξ

(26)

∂y ) -k2ya ∂ξ

(27)

∂a ) -k3ya ∂θ

(28)

y(0,θ) ) 1, for θ g 0 (29)

where the auxiliary independent variables and dimensionless groups are defined as follows:

z ξ ≡ , θ ≡ kdPxP°t L

k1 ≡

v ≡ k3θ

(31)

∂y ∂a ) ) -ya ∂u ∂v

(32)

Therefore,

that is, the Bohart-Adams wave equation, whose solution is given by Butt and Petersen2

y)

ek3θ ek2ξ + ek3θ - 1

(33)

ek2ξ + ek3θ - 1

(34)

and

a)

k2ξ

e

The corresponding solution for the benzene dimensionless concentration, x, is obtained by substituting the above expression in the benzene mass balance:

(

)

k1ek2ξ ∂x x )- kξ ∂ξ e 2 + ek3θ - 1

(35)

which after rearrangement and integration, for x ) 1 at ξ ) 0, leads to

x ) [e-k2θ(ek2ξ - 1) + 1]k1/k2

(36)

The value of MT can then be computed from the breakthrough curve for the poison at the reactor exit. Then, at z ) L,

y(1,θ) )

ek3θ e + ek3θ - 1 k2

(37)

Rearranging eq 37 to a linear form, we have

[

]

1 - 1 ) -θ + ln(ek2 - 1) y(1,θ)

(38)

Therefore, if the poison concentration is known at the entrance and measured at the exit as a function of time, the value of MT for a given temperature can be calculated from the plot of ln[1/y(1,θ) - 1] versus θ

x(0,θ) ) 1, for θ g 0

x≡

u ≡ k2ξ

ln

with boundary and initial conditions given by

a(ξ,0) ) 1, for 0 e ξ e 1

By a convenient change in the independent variables, we can define

xB xP , y≡ xB ° xP°

LFcMgkPxB° LFcMgMTkdPxP° , k2 ≡ , k3 ≡ 1 u s Fg u s Fg (30)

MT(T0) )

usFg ln(eb + 1) LFcMgkd(T0)PxP°

(39)

Because all of the necessary parameters are usually readily available, the poison adsorption capacity can be evaluated from the definition of k2 and the intercept b ) ln(ek2 - 1). This is valid for a constant temperature T0. A set of isothermal experiments would allow the determination of MT as a function of temperature. Experimental Section Catalyst Characterization and Pretreatment. Catalyst tablets (Harshaw Ni-0104T) were crashed and sieved to the range of 80-100 mesh to be used in the kinetic experiments. Other catalyst properties that

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5423

might be of interest are nickel load ) 58%, BrunauerEmmett-Teller surface area ) 150 m2/gcat, metal surface area ) 24 m2/gcat, average crystallite size ) 85 Å, and average pore radius ) 37 Å. The catalyst pretreatment was similar to the one used elsewhere19 in pilot-plant poisoning experiments. Nitrogen flow was set to 1000 cm3(STP)‚min-1, and the reactor was warmed to 50 °C, where it stayed for 1 h while purging the system. The gas flow was then set to 200 cm3(STP)‚min-1, and the temperature was ramped up to 120 °C at 1 °C‚min-1 and held for 3 h. At a rate of 3 °C‚min-1, the temperature was increased from 120 to 160 °C, after which H2 at 200 cm3(STP)‚min-1 replaced N2. Pure hydrogen flow was fed for 5 h, and then the reactor was cooled to the reaction temperature. The temperature program was set by a customized computer program. Differential Glass Reactor. A 1.5 cm i.d. glass reactor, operated with conversions below 6%, was used for the deactivation kinetic experiments. Good gas flow control was achieved by using a mass flow controller, model 5850E, for 500 cm3(STP)‚min-1, supplied by Brooks Instruments. The reactor was heated by a custom-made electric furnace. Two thermocouples were used: one was placed inside a glass well in the center of the reactor, in contact with the diluted catalyst load. This was used as a monitoring thermocouple to determine the reactor internal temperature; the other, placed outside the reactor, was used to control the heat supplied to the furnace, through the temperature controller. Reactants and Poisons. Ultrahigh-purity-grade gases were used for catalyst pretreatment, reaction, and gas chromatography (GC). Nitrogen gas was used for purging of the reactor system and in the catalyst pretreatment. Hydrogen was used for pretreatment, reactions, and GC. Helium was used as a GC carrier gas. Air was used in GC for the flame ionization detector (FID). Gases were all supplied by Bennett Co. and Great Lakes Airgas, Inc. Benzene, “Baker analyzed” grade, containing less than 1 ppm thiophene and less than 2 ppm total sulfur, was supplied by J. T. Baker Chemical Co. and was not submitted to any further pretreatment. Thiophene (purity > 99.99%) was supplied by Eastman Kodak Co. 1-Propanethiol, a linear saturated mercaptan, 99% purity, was supplied by Aldrich Co. All poisons were used as received, without any additional pretreatment. Data Aquisition. A computer program was written to take advantage of the data acquisition system installed. The sampling time was set to 3 min, and a dedicated integrator was connected to improve data accuracy. The procedure used is described below. Starting from eq 25, derived earlier, the following steps were followed: (1) After the reaction mixture containing pure benzene and hydrogen was introduced to the glass reactor, the conversion was checked by GC, using a FID. Hydrogen flow and temperature were adjusted to a level such that the initial benzene conversion would be less than 6%, a criterion used to satisfy differential operation. (2) After steady state had been reached, usually after 30 min of operation at constant conditions, the mixture containing the mercaptan (otherwise continually flowing through a bypass) was introduced into the reactor and the deactivation thus started. The initial cyclohexane

Figure 1. Typical deactivation plot shown as dimensionless conversion as a function of time (data for run D3; Table 2). Table 1. Operating Conditions for the Evaluation of the Intrinsic Kinetic Deactivation Parameters for 1-Propanethiol Poisoning (Data from the Differential Reactor) v˘ deactivation mcat T P (mg) (cm3‚min-1) (°C) (kPa) run D1 D2 D3 D4

301.1 78.1 64.1 32.2

547 245 200 785

68 84 104 120

102.6 102.5 105.3 101.7

xH°

xB°

0.966 0.962 0.912 0.976

0.034 0.038 0.088 0.024

xP° X (ppm) (%) 115 75 290 79

5.1 1.6 3.9 4.9

Table 2. Intrinsic Kinetic Parameters for 1-Propanethiol and Thiophene Deactivation (Data from the Differential Reactor) poison

preexponential factor kd° (Pa‚s)-1 × 104

activation energy Ed (kJ‚mol-1)

1-propanethiol thiophenea

1.45 1.80

3.87 4.53

a

The values for thiophene were determined by Weng.14

concentration, xC°, was taken as the concentration of the steady-state operation at time zero. The cyclohexane mole fraction at regular intervals was calculated from the integrator data. A logarithmic plot of xC as a function of poisoning time, according to eq 24, gives the value of c2 and was used to determine the value of kd for the particular temperature of the run. A typical deactivation plot is shown in Figure 1; operating conditions are given in Table 1. (3) Steps 1 and 2 were repeated for other catalyst loads, at different temperatures (see Table 1 for the conditions used). (4) The preexponential factor, kd°, and the activation energy of deactivation, Ed, were determined by the Arrhenius plot. The intrinsic kinetic deactivation parameter values determined are shown in Table 2. For comparison, Table 2 also gives the values for thiophene poisoning, as determined by Weng.14 Integral Reactor. Experiments to determine the poison adsorption capacity were carried out in an integral tubular stainless steel fixed-bed reactor, 1.64 cm i.d. and 50 cm in length, equipped with six independent heating zones to improve temperature control. Design details are given by Porto19 elsewhere. A series of isothermal deactivation “cycles” were performed usually under three isothermal conditions, as shown in Figure 2. Results and Discussion Intrinsic Kinetic Parameters. For the determination of the kinetic parameters, we used a differential

5424

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 Table 3. Thiophene Adsorption Capacity of the Ni/ Kieselguhr Catalyst as a Function of Temperature (Data from the Integral Reactor) MT,thiophene (mol‚gcat-1) × 104 run

Tfirst (°C)

poisoning time (min)

from eq 39

theoretical value, from total S fed

S/Nis

T0 T1 T2

68 85 100

141 79 42

3.98 4.50 4.95

3.81 4.31 4.58

0.62 0.70 0.74

Table 4. 1-Propanethiol Adsorption Capacity of the Ni/Kieselguhr Catalyst (Data from the Integral Reactor) Figure 2. Integral reactor operation scheme. Tfirst, Tsecond, and Tthird represent the three deactivation isothermal temperatures or cycles. Tfirst < Tsecond < Tthird. From Tfirst to Tsecond and from Tsecond to Tthird, the reactor is operated under pure hydrogen, fed from a bypass line.

glass reactor. Then, applying the method described earlier, we determined the intrinsic kinetic deactivation parameters, kd° and Ed, for the hydrogenation of benzene under 1-propanethiol poisoning. Table 1 shows the operating conditions used to obtain the poisoning parameters. It is not surprising that the activation energy for 1-propanethiol poisoning is very small and comparable to what was previously found by Weng14 for deactivation of the Ni/Kieselguhr catalyst when poisoned by thiophene. Also, the lower value of the preexponential factor, when comparing to thiophene, is in agreement with what was observed for higher conversion operation (integral fixed-bed reactor). It is important to observe, however, that these values, particularly the activation energies, are subjected to large experimental errors. The low Ed values for both cases of thiophene and mercaptan poisonings are responsible for two general kinetic characteristics of sulfur poisoning: (a) The deactivation rate is essentially temperature-independent and (b) adsorption rates are large even at relatively low temperatures. Poisoning Adsorption Capacity. (i) Experiments with Thiophene Poisoning. Table 3 shows the results obtained from integral fixed-bed experiments19 and compares them to values calculated by eq 39. Good agreement between these values was achieved, with the differences within the experimental errors. Based on these values, the calculated S/Nis ratios for the deactivation isotherms of runs T0, T1, and T2 are 0.62, 0.70, and 0.74, respectively. Considering the active nickel surface (Nis) as the reduced nickel surface provided by the manufacturer (24 m2‚gcat-1) and an average of 1.55 × 1015 Ni atoms‚cm-2, we have

24 m2‚gcat-1 × (100)2 cm2‚m-2 × 1.55 × 1015 Ni atoms‚cm-2 ) 3.72 × 1020 Ni atoms‚gcat-1 A S/Nis ratio of 1:1 corresponds to 6.18 × 10-4 molthiophene‚gcat-1. Megiris20 suggested a linear relationship for the temperature dependence of the thiophene adsorption capacity, given by

MT,thiophene ) 0.030 116 × 10-4T (°C) + 1.935829 × 10-4 mol‚gcat-1 (40)

run

Tfirst (°C)

xP (ppm)

poisoning time (min)

MT,1-propanethiol (mol‚gcat-1) × 104

S/Nis

P1 P2 P3 P4 P5 P6

70 70 70 70 68 68

133 686 405 283 256 256

525 54 70 129 123 210

14.30 7.35 5.71 7.01 6.69 11.40

2.31 1.19 0.92 1.13 1.08 1.84

between 65 and 170 °C (3.893 × 10-4-7.056 × 10-4 mol‚gcat-1) and a constant (maximum) value of 7.056 × 10-4 mol‚gcat-1 between 170 and 200 °C. Thiophene poisoning isothermal experiments at varying space velocities (runs T0-T2; Table 3) showed that a guard-bed effect, that is, the existence of a period where the activity does not decay significantly, keeping the benzene conversion at a relatively constant level, should not be attributed to a simple function of temperature. Varying the space velocity was necessary to achieve high conversions at initial high temperature. The first deactivation isotherm of run T2, at Tfirst ) 100 °C, still showed a linear pattern, as opposed to a second isotherm of run T1, at Tsecond ) 91 °C, taken after the thermal reactivation promoted by the temperature of the next deactivation “cycle”, which developed a concave behavior (see Figure 2). The linear decay of the first isotherms persisted for the fresh catalyst, independently of the initial temperature (68, 85, or 100 °C of runs T0, T1, and T2, respectively). “Constant” conversion conditions, therefore, seem to be dependent on the surface coverage, that is, the extent of poisoning to which the catalyst had been previously subjected. (ii) Experiments with 1-Propanethiol Poisoning. It is apparent from the data shown in Table 2 that 1-propanethiol is a weaker poison when compared to thiophene. Also, in integral fixed-bed experiments, while thiophene would poison the bed in a typical three-cycle experiment in about 6 h, the mercaptan required a 6-fold longer period for similar operating conditions. Slower deactivation of the 1-propanethiol compared to thiophene is in agreement with what is found previously in our laboratory. The most surprising feature of the mercaptan deactivation, however, was the formation of the same plateau type of behavior as that found for thiophene. According to a mechanism proposed by Lyubarskii et al.,21 one would not expect any significant steric effect when dealing with a saturated linear mercaptan. The dual mode of adsorption (perpendicular and coplanar to the surface) has no strong support here because unsaturated π-bond adsorption is not possible. However, the plateau for 1-propanethiol was even more pronounced. Although the variations in MT for 1-propanethiol and S/Nis values (number of sulfur atoms per surface nickel atom) in Table 4, based on the extrapolation of the first isotherms for runs P1-P6, are big, it is

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5425

clear that, assuming fast and irreversible adsorption, the catalyst poison capacity is larger for mercaptan than for thiophene (compare Tables 3 and 4). It is difficult to explain these differences in terms of site blocking only, and the high values of S/Nis suggest that, in the case of poison with 1-propanethiol, nickel bulk sulfide formation is involved. These values, therefore, reflect not only adsorption but also adsorption plus three-dimensional sulfide formation. Bulk sulfiding was also confirmed by magnetic moment measurements made in a superconducting quantum interference device (SQUID).22 Conclusions We have developed a useful approach to determine the kinetic parameters for 1-propanethiol and thiophene poisons for the benzene hydrogenation reaction on a nickel/Kieselguhr catalyst. The simplicity of the analysis and experimental proceduressonly cyclohexane/benzene chromatographic peaks are requiredsmakes it a suitable model platform for nickel catalyst thioresistance evaluation. The combination of differential and integral reaction data used allowed the evaluation of the poison capacity of the catalyst. The isothermal in cycles mode of operation shows that integral reactor behavior is very complex and predictable only for a fresh catalyst operated at constant temperature (first deactivation isotherm). Catalytic beds that had undergone previous deactivation and are subjected to higher temperatures that allow thermal activity recovery (second cycle on) show sulfur resistance that we have associated with a regime of bulk sulfiding as determined by catalyst adsorption capacity and S/Nis calculations. These results confirm previously SQUID magnetic measurements carried out to determine the extension of nickel sulfide formation. Therefore, kinetic parameters such as the ones reported in this work should be used, having in mind that partially poisoned catalysts may show different deactivation kinetic behavior when compared to fresh catalysts. Acknowledgment This work was supported in part by the National Science Foundation. L.M.P. acknowledges the financial support from The National Research Council of Brazil (CNPq) and The Federal University of Santa Catarina (UFSC), Floriano´polis, Brazil. Notation a ) catalyst relative activity for benzene hydrogenation or normalized activity ) (-rB)/(-rB°) ) Ns/Ns° b ) intercept of the straight line given by eq 38 Ed ) activation energy for poisoning, kJ‚mol-1 k ) benzene hydrogenation rate constant, mol‚(gcat‚Pa‚s)-1 kd ) poisoning rate constant, (Pa‚s)-1 kd° ) preexponential factor for poisoning, (Pa‚s)-1 KB ) adsorption constant for benzene, Pa-1 L ) reactor length, m mcat ) mass of the catalyst, mg Mg ) molecular weight of gas or vapor, g‚mol-1 MT ) catalyst adsorption capacity for sulfur poison, molpoison‚gcat-1 MTv ) symbol used to represent the thiophene increasing adsorption capacity of the catalyst Ns ) available number of active sites, at time t Ns° ) initial number of active sites, at time t ) 0

P ) total pressure, Pa R ) universal gas constant, 8.314 kJ‚mol-1‚K-1 ) 1.987 cal‚mol-1‚K-1 or 82.057 cm3‚atm‚mol-1‚K-1 (-rB) ) rate of benzene hydrogenation, mol‚gcat-1‚s-1 (-rB°) ) rate of benzene hydrogenation for the fresh catalyst, mol‚gcat-1‚s-1 rd ) rate of the deactivating reaction, s-1 t ) time under deactivation, s; also clock time, s T ) temperature, °C or K T0 ) temperature of the deactivation isotherm, °C or K u ) auxiliary axial position variable, according to eq 31 us ) superficial gas velocity, m‚s-1 v ) auxiliary time variable, according to eq 31 v˘ ) volumetric flow rate, cm3‚min-1 x ) relative benzene mole fraction, according to eq 30 X ) benzene conversion xB ) mole fraction of benzene xB° ) inlet mole fraction of benzene xC ) mole fraction of cyclohexane xC° ) inlet mole fraction of cyclohexane xH ) mole fraction of hydrogen xH° ) inlet mole fraction of hydrogen xT ) mole fraction of thiophene xT° ) inlet mole fraction of thiophene xP ) mole fraction of 1-propanethiol or poison xP° ) inlet mole fraction of 1-propanethiol or poison y ) relative poison mole fraction, according to eq 30 z ) reactor axial position or length variable, m Greek Symbols  ) bed porosity or void fraction Fc ) bulk density of catalyst, gcat‚m-3 Fg ) gas- or vapor-phase density, g‚m-3 θ ) dimensionless time, according to eq 30 τ ) space time, defined as τ ≡ tFc/, gcat‚s‚m-3 ξ ) dimensionless axial position, according to eq 30 Subscripts B ) benzene C ) cyclohexane c, cat ) catalyst g ) gas or vapor phase P ) poison or 1-propanethiol (n-propylmercaptan) s ) surface T ) thiophene

Literature Cited (1) Li, H. X.; Xu, Y. P. Liquid-Phase Benzene Hydrogenation to Cyclohexane over Modified Ni-P Amorphous Catalysts. Mater. Lett. 2001, 51 (2), 101. (2) Butt, J. B.; Petersen, E. E. Activation, Deactivation, and Poisoning of Catalysts; Academic: San Diego, CA, 1988. (3) Bartholomew, C. W. Mechanisms of Catalyst Deactivation. Appl. Catal. A 2001, 212 (1-2), 17. (4) Butt, J. B.; Billimoria, R. M. Catalyst Deactivation. ACS Symp. Ser. 1978, 72, 288. (5) Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Sulfur Poisoning of Metals. Adv. Catal. 1982, 31, 135. (6) Hegedus, L. L.; McCabe, R. W. Catayst Poisoning; Chemical Industries Series 17; Marcel Dekker: New York, 1984. (7) Oudar, J., Wise, H., Eds. Deactivation and Poisoning of Catalysts; Chemical Industries Series 20; Marcel Dekker: New York, 1985. (8) Bartholomew, C. W. Mechanism of Nickel Catalyst Poisoning. Catalyst Deactivation. In Catalyst Deactivation; Delmon, B., Froment, G. F., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, The Netherlands, 1987. (9) Barbier, J.; Lamy-Pitara, E.; Mare´cot, P.; Boiteaux, J. P.; Cosyns, J.; Verna, F. Role of Sulfur in Catalytic Hydrogenation Reactions. Adv. Catal. 1990, 37, 279.

5426

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002

(10) Dı´az, A.; Gandia, L. M.; Odriozola, J. A.; Montes, M. Influence of the Nickel Reduction Degree on the Toxicity of H2S and Thiophene over a Ni/SiO2 Catalyst. J. Catal. 1996, 162 (2), 349. (11) Kehoe, J. P. G.; Butt, J. B. Kinetics of Benzene Hydrogenation by Supported Nickel at Low Temperature. J. Appl. Chem. Biotechnol. 1972, 22, 23. (12) Richardson, J. T. Sulfiding of Nickel Catalyst Beds. J. Catal. 1971, 21, 130. (13) Butt, J. B. Reaction Kinetics and Reactor Design, 2nd ed.; Marcel Dekker: New York, 2000. (14) Weng, H.-S. Deactivation Effects in Fixed Bed Reactors. Ph.D. Thesis, Northwestern University, Evanston, IL, 1974. (15) Bartholomew, C. W. Carbon Deposition in Steam Reforming and Methanation. Catal. Rev. Sci. Eng. 1982, 24 (1), 67. (16) Megiris, C. E.; Butt, J. B. Effects of Poisoning on the Dynamics of Fixed Bed Reactors: Cyclic and TemperatureIncreased Operation Policies. Chem. Eng. Sci. 1988, 43 (8), 2239. (17) Megiris, C. E.; Butt, J. B. Effects of Poisoning on the Dynamics of Fixed Bed Reactors. 1. Isothermal in a Cyclic Policy of Operation. Ind. Eng. Chem. Res. 1990, 29, 1065.

(18) Megiris, C. E.; Butt, J. B. Effects of Poisoning on the Dynamics of Fixed Bed Reactors. 2. Constant Conversion Policy of Operation. Ind. Eng. Chem. Res. 1990, 29, 1072. (19) Porto, L. M. Co-Deactivation of a Fixed Bed Reactor. Poisoning of a Nickel/Kieselguhr Catalyst by Thiophene and 1-Propanethiol. Ph.D. Thesis, Northwestern University, Evanston, IL, 1993. (20) Megiris, C. E. Effects of Poisoning on the Dynamics of Fixed Bed Reactors: Temperature-Increased Reactor Operation. Ph.D. Thesis, Northwestern University, Evanston, IL, 1987. (21) Lyubarskii, G. D.; Avdeeva, L. B.; Kul’kova, N. V. Study of Thiophene Poisoning of Nickel Catalysts. Kinet. Catal. 1962, 3 (1), 102. (22) Porto, L. M.; Butt, J. B. Magnetic Moment Measurements of a Poisoned Nickel Catalyst. Latin Am. Appl. Res. 1999, 29 (1), 55.

Received for review February 11, 2002 Revised manuscript received August 6, 2002 Accepted August 6, 2002 IE020127S