1050
Ind. Eng. Chem. Res. 1990,29, 1050-1057
Gillespie, P. C.; Wilson, G. M. Gas Processors Association RR-48, Tulsa, OK, April 1982. Holmes, H. F.; Mesmer, R. E. Thermodynamics of Sodium Chloride Solutions at High Temperatures. J. Solution Chem. 1986,15,495. Liu, C.; Lindsay, W. T., Jr. Thermodynamics of Sodium Chloride Solutions at High Temperatures. J. Solution Chem. 1972, I , 45. Long, F. A.; McDevit, W. F. Activity Coefficients of Nonelectrolyte Solutes in Aqueous Salt Solutions. Chem. Rev. 1952, 51, 119. Malinin, S. D. Thermodvnamics of the H,O-CO,- Svstem. Geokhimiya.1974, I O , 1523. Malinin, S. D.; Kurovskaya, N. A. Solubility of COz in Chloride Solutions at Elevated Temperatures and COz Pressures. Geokhimiya 1975,4, 547. Malinin, S. D.; Savelyeva, N. I. The Solubility of COz in NaCl and CaCl, Solutions a t 25. 50 and 75 OC Under Elevated CO, Pressure; Geokhimiya 1972,6,643. McKay, A. C. Activities and Activity Coefficients in Ternary Systems. Trans. Faraday SOC.1953,49, 237. O'Sullivan, T. D.; Smith, N. 0. The Solubility and Partial Molar Volume of Nitrogen and Methane in Water and in Aqueous Sodium Chloride from 50 "C to 100 O C and 100 to 600 Atm. J. Phys. Chem. 1970, 74, 1460. Pabalan, R. T.; Pitzer, K. S. Heat Capacity and Other Thermodynamic Properties of NazSO,(Aq) in Hydrothermal Solutions and the Solubilities of Sodium Sulfate Minerals in the System Na-
Cl-S04-OH-H,0 to 300 "C. Geochim. Cosmochim. Acta 1988,52, 2393. Phutela, R. C.; Pitzer, K. S. Densities and Apparent Molar Volumes of Aqueous Magnesium Sulfate and Sodium Sulfate to 473 K and 100 bar. J . Chem. Eng. Data 1986, 31, 320. Pitzer, K. S. Thermodynamics of Electrolytes. I. Theoretical Basis and General Equations. J . Phys. Chem. 1973, 77, 268. Prausnitz, J. M.; Lichtenthaler, R. N.; Gomes de Azevedo, E. Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ 1986; p 380. Rogers, P. S. Z.; Pitzer, K. S. High-Temperature Thermodynamic Properties of Aqueous Sodium Sulfate Solutions. J.Phys. Chem. 1981,85, 2886. Rogers, P. S. Z.; Pitzer, K. S. Volumetric Properties of Aqueous Sodium Chloride Solutions. J. Phys. Chem. Ref. Data 1982,II, 1.
Setschenow, J. Uber die Konstitution der Salzlosungen auf Grund ihres Verhaltens zu Kohlensiire. 2.Phys. Chem. 1889, 4, 117. Takenouchi, S.; Kennedy, G. C. The Solubility of Carbon Dioxide in NaCl Solutions at High Temperatures and Pressures. Am. J . Sci. 1965. 263, 445.
Receiued for review June 8, 1989 Revised manuscript received November 3, 1989 Accepted November 22, 1989
Effect of Catalyst Poisoning on Adsorption and Surface Reaction Rates in Liquid-Phase Hydrogenation Luis A. Arrua,?Ben J. McCoy, a n d J. M. Smith* Department of Chemical Engineering, University of California, Davis, California 95616
Hydrogenation of a-methylstyrene with Pd/AlZO3(0.05 w t W )catalyst was studied as a function of poisoning with thiophene. The data were obtained by using a hydrogen-gas, pulse-response technique in a well-mixed, three-phase slurry reactor containing styrene and catalyst particles and operating a t 298 K and atmospheric pressure. Such dynamic data in contrast to steady-state measurements are sufficient to determine both adsorption and surface rate constants. The temporal moments of the response curves were interpreted with a model involving poisoned and unpoisoned sites with hydrogen absorption occurring on both kinds of sites. The overall rate constant (per kilogram of catalyst) decreased sharply with the first additions of poison. However, thiophene, even in large amounts, did not nullify more than 78% of the initial activity. The rate constant per unit mass of catalyst for adsorption decreased with poisoning, while the rate constant for the surface reaction did not change appreciably. Neither adsorption nor surface reaction appears to control the rate of the overall process, evec a t different levels of poisoning. Analysis by gas chromatography for thiophene in the slurry liquid showed that thiophene adsorption was not irreversible, even at very low concentrations. Reactions between Hz and thiophene and formation of palladium hydride were not detected. Poisoning by adsorption of impurities on active sites is important in many catalytic processes, particularly for low-temperature hydrogenations. Hugh Hulburt, as a leader in developing commercial chemical processes, was concerned with catalyst deactivation, and we are pleased to dedicate to him the research described here. Recently, there have been several fundamental catalyst and surface science studies of poisoning caused by adsorption of sulfur impurities on metal catalysts. Summaries are given by Hegedus and McCabe (1980), Oudar (19801, Bartholomew et al. (1982), and Barbier (1985). Most of the work has focused on poisoning of nickel catalysts by H2S. Information about poisoning of catalysts used in liquid-phase hydrogenation processes at moderate pressure and low temperature is meager. Gut et al. (1975) On leave from INTEQUI (CONICET-UNSL), San Luis, Argentina, under a fellowship from Consejo Nacional d e Investigaciones Cientificas y Tecnicas d e la Republica Argentina.
and Zwicky and Gut (1978) investigated the influence of poisoning on the performance of a suspended palladium catalysts for the liquid-phase hydrogenation of o-cresol. They fit experimental activities by a Langmuir-Hinshelwood adsorption model, modified to describe the effect of poisoning. No interaction was found between poison (othiocresol) and substrate on the remaining active catalyst surface. Holah et al. (1979) studied the liquid-phase hydrogenation of l-alkenes over nickel boride catalyst with different poisons including phosphines, n-butanethiol, and thiophene. A good approach to the general problem of poisoning of hydrogenation catalysts is described by Boitiaux et al. (1987). The objective of this research is to investigate how a poison (thiophene) affects adsorption and surface rate constants and the overall hydrogenation rate in a threephase slurry reactor. The reaction chosen was the liquid-phase hydrogenation of a-methylstyreneon Pd/Al203 catalysts at atmospheric pressure and temperatures of
0888-5885/90/2629-1050$02.50/0 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 1051 Table I. Properties of AllOI Catalyst Support (Chen et al., 1987) 165 X 108 surface area (N, adsorption), m2/kg pore vol, m3/kg 0.33 X 3.07 X 103 solid density, kg/mg 60 X av pore radius, m pP, kg/m3 1.53 x 103 R, m 6.27 X lo* P 0.503
Figure 1. Schematic drawing of apparatus, showing (1) Hzcylinder,
(2) Nzcylinder, (3) pressure regulator, (4) shut-off valve, (5) flow controller, (6) needle valve, (7) rotameter, (8) three-way valve, (9) six-way valve, (10) sample loop, (11)Deoxo unit, (12) dryer, (13) Oxiclear gas purifier, (14) dispersion tube, (15) impeller, (16) baffle, (17) constant-temperature bath, (18) flow splitter, (19) cold trap, (20) thermal conductivity cell, (21) heated zone, (22) soap bubble meter, (23) mercury manometer.
298-323 K. This irreversible reaction produces only cumene as a product and is usually regarded as first order in hydrogen and zero order in the a-methylstyrene when carried out in pure a-methylstyrene at low pressures and temperatures (Babcock et al., 1957; Satterfield et al., 1968; Germain et al., 1974). A dynamic-response method was used to determine separate values for the rate constants for adsorption and surface reaction as a function of the extent of poisoning. In this method, pulses of hydrogen gas are bubbled through a well-agitated slurry of catalyst particles in liquid styrene, and the hydrogen response is analyzed in the gas effluent. Using this method, Chen et al. (1986a,b, 1987) for the same system studied the effects of catalyst reduction temperature, palladium content, and carbon disulfide poisoning on individual adsorption and reaction rates. It may be noted that steady-state data are insufficient to obtain separate values for adsorption and surface reaction rate constants. Since hydrogenations are highly exothermic, reaction experiments at steady state must be analyzed carefully (Cerveny et al., 1983). This thermal problem is avoided by using the pulse response technique in a gas-liquid system.
Experimental Section The experimental equipment used in this work was basically the same as described by Chen et al. (1986a, 1987). Figure 1 is a diagram of the apparatus. The slurry reactor is a cylindrical Pyrex vessel equipped with an eight-bladed impeller and eight stationary baffles. Details of the dimensions and locations of the baffles, impeller, and sparger are given by Furusawa and Smith (1973). The total volume (including the reactor, lines, and sample loop) between the six-way valve and the tee (where the gas flow was split before the detector) was measured by means of pulse experiments without catalyst in the reactor and with a-methylstyrene. The results obtained were corrected for the effect of dissolution of hydrogen in the solvent, using Henry’s law constant of this system at the temperature of m3, the experiments. The total volume was 1.244 X which is in good agreement with the value obtained by geometric measurements. The reactor was immersed in a constant-temperature bath. Cumene for adsorption runs, a-methylstyrene for reaction runs, and thiophene for poisoning experiments were technical grade from Kodak. Purified palladium chloride and reagent ACS hydrochloric acid were from Fisher Scientific Co.
The catalyst support was y-alumina (T-126, Girdler Chemical Co.), whose physical properties are shown in Table I. Alumina particles were crushed and sieved; the fraction between 100 and 150 mesh (average mesh opening m) was used for the catalyst. A single batch of 0.125 X of 0.5 kg of catalyst with 0.05 w t % Pd was prepared by using the incipient-wetness, impregnation method. Samples of 0.1 kg were soaked for 12 h at 333 K in 0.1 X m3 of a solution containing 4.69 X loe2mol/m3 of PdC12 in 4.5% HC1. Then each 0.1-kg sample was dried at 393 K for at least 24 h. The 0.1-kg samples were reduced in flowing hydrogen at 553 K for 110 h and cooled to room temperature in hydrogen. Finally, the five 0.1-kg samples were mixed to provide the single 0.5-kg batch, which was stored in a desiccator under a purified nitrogen atmosphere. The small particles, long soaking time, and low concentration helped ensure a near uniform intraparticle distribution of palladium. After the catalyst was added to the reactor, the activity (as measured by the conversion of hydrogen) was stabilized by introducing at least 50 pulses of hydrogen using nitrogen as the carrier gas. This was done to eliminate temporary fluctuations on the catalyst surface. Constant activity with time and with the number of hydrogen pulses was checked before beginning the experiments. The response curves were monitored by a thermal conductivity cell (Varian GC 1400). Continuous voltages from the detector cell were converted to a digital signal, amplified with a Keithley System 570 data-acquisition workstation, and stored in an IBM PC. After completion of reaction runs with fresh catalyst at 298,311, and 323 K, poisoning experiments were carried out at 298 K. Various volumes of 0.5 and 5 vol % thiophene dissolved in a-methylstyrene were added to the slurry. The resulting initial concentrations of thiophene in the styrene in the slurry were very low, (0.29-30) X mol/m3. The solution containing the poison was stirred for 4 h before making reaction runs and before taking liquid samples for analysis of thiophene. Since cumene is the only product of reaction detected at our experimental conditions, cumene was used as a solvent to determine (without reaction) the adsorption equilibrium constants of hydrogen on fresh and poisoned catalysts at 298 K. This information is necessary for analysis of the reaction data. For the adsorption experiments, the poison was added from 0.5 and 5 vol % solutions of thiophene in cumene. A summary of operating conditions for the reaction and adsorption experiments is given in Table 11. In addition, Table I1 gives the equivalent volumes of pure thiophene added to the slurries to give the various poison levels for the reaction runs. Thiophene in the liquid phase of the slurry was analyzed by using a Perkin-Elmer 8500 gas chromatograph with flame ionization detector and n-heptane as an internal standard.
Theory Experimental values of the zero, first-reduced, and second central moments of the response curve were cal-
1052 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
rates, provided the Henry’s law constant for hydrogen in the liquid is known. The second central moment for this kind of reversible adsorption is given by the expression (Niiyama and Smith, 1976)
Table 11. Summary of Owrating Conditions total vol of the system 1.244 X m3 9.0 x lo4 m3 vol of injection loop reactor pressure impeller speed
atmospheric 950 rpm
(A) Reaction Runs fresh catalyst temp 298, 311, 323 K wt of catalyst 0.0125 kg gas flow rate range (1 atm, 298 K) (0.8-5.3) X 10” m3/s VL of a-methylstyrene 1.076 X m3 with poison temp 298 K w t of catalyst 0.025 kg gas flow rate range (1 atm, 298 K) (0.8-3.9) X 10” m3/s 1.072 X 10” m3 VL of a-methylstyrene equiv vol of pure thiophene added 0, 0.25, 1, 2, 4, 6, 9, 17.5, 26 X lo4 m3 to the slurry conversion range of H2 1843% range of Thiele modulus 0.40-1.05 range of effectiveness factor 0.94-0.99
(B)Adsorption Runs temp 298 K wt of catalyst 0.025 kg gas flow rate range (1 atm, 298 K) (0.8-3.8) vol of cumene 1.136 X
X
10” m3/5 m3
culated by numerical integration of the digital data stored in the computer. The equations for these experimental moments are
(3)
The zero moment is a measure of the conversion, pI is an average retention time of the response curve, and pz,c is a measure of the spread (variance) of the response curve. The derivation of theoretical zero and first temporal moments expressions for the concentration of reactant in effluent gas for a three-phase, well-stirred, isothermal slurry reactor has been presented by Ahn et al. (1986) and Chen et al. (1986a). The mathematical model for deriving these expressions accounts for mass transfer from the gas bubble to the liquid and from the liquid to the particle, for intraparticle diffusion and for the surface processes of adsorption and reaction. The conservation equation for these surface processes of hydrogen on a nonpoisoned surface is dn/dt = Nk(ci - n/NK) - k,n
(4)
where k , and Nk are the surface reaction and adsorption rate constants, and NK is the equilibrium adsorption constant per unit mass. The method of analysis was to use the experimental moments with the theoretical equations to evaluate the rate and equilibrium parameters. Analysis of Adsorption Data. In the absence of reaction ( k , = o), the dimensionless zero moment (m,) is unity so that the theoretical expression for the reduced first moment becomes (Niiyama and Smith, 1976) ~1
= Vz(1
+ m,NK)/HQ +
T~
(
m$JK H
+1
J
)[
1 + exp(-KL) 1 - exp(-KL) ](vL/Q)2
(6)
where (7)
These moment equations and also those for the reaction case (eq 10,12, and 14) are derived by solving the dynamic mass balance differential equations for Hz in the Laplace domain for the gas phase, the liquid phase, and the particles. Equations 4,8, and 9 are for the particles and reflect the assumption of first-order reversible adsorption and irreversible surface reaction. In principle, eq 6 can be used to obtain the adsorption rate constant, Nk, from second central moments of the response curve for experiments without reaction since NK is known from the first moment analysis (eq 5). This is difficult in practice since experimental second moments are usually of uncertain accuracy. Analysis of Reaction Data. For partially poisoned catalyst, the mass balance of hydrogen can be described by means of two equations (Chen et al., 1986a) accounting for first-order reversible adsorption on the poisoned (p) sites, dn,/dt = N,k,(ci - n,/Nd(J and for reversible adsorption and irreversible first-order reaction on the unpoisoned (a) sites, dna/dt = Naka(ci- na/NaKa)- krna
(9)
With these mass balances, the expression for the zero moment is
--1
-
1 - mn 1-
)I”
Bi Bi + 4 coth 4 - 1
+
+ 1 (10)
Equation 10 shows that plotting 1/(1- m ),, vs Q / VL should give a straight line whose intercept is related to the gasto-liquid mass-transfer coefficient, and the slope is a function of the Thiele modulus defined by the following equation,
(5)
where T ~ the , residence time of the gas, is small enough (-0.5 s) to be neglected. From eq 5, NK can be obtained with experiments without reaction at different gas flow
Hence, the overall intrinsic rate constant per mass of catalyst (k,,Na) can be determined from the slope of the zero moment plots.
Ind. Eng. Chem. Res., Vol. 29, No. 6,1990 1053 Table 111. Solubility and Mass-Transfer Parameters 298 K 311 K 323 K
H," H,b 1O2k,; m/s 108D,,'m2/s
12.6 15.2 0.128 1.26
10.5 13.9 0.141 1.38
9.16 12.5 0.156 1.49
OSatterfield et al. (1968). *Herskowitz et al. (1979). 'Ahn et al. (1986).
O
t/ V
0
I
200
I
400
1
1
I
I
600
800
1000
1200
VL/Q, s
Figure 2. First moments of the response curve for Hz adsorption on fresh and poisoned catalysts in cumene at 298 K.
The first moment equation corrected by the residence time in the dead volume, from Ahn et al. (1986),takes the form
\
I
'j[ I ! & 1+
PP
-
+
2
(12)
From eq 12, a straight line should result when p?(Q/ VL) is plotted vs mo(l + 1/KL - l/md(L)2. The quantity v B ( 1 - Q) can be neglected because the gas holdup, VB, is small. The slope of this line can be used to calculate 5,
E = NpKp + [Kfla(Naka)2/(N&a+ kfl&a)'I (13) Knowing E, eq 13 provides a relationship between k, and k f i , . The intercept of the line is a measure of the ratio Vo/ VL, which may be compared with that estimated from the geometry of the apparatus. With eq 11 and 13, separate values of k, and N,k, can be evaluated, provided N S P and N,K, are known.
Results and Discussion Adsorption Experiments. To calculate k , and N,k, separately from the reaction experiments, the adsorption equilibrium constant for unpoisoned sites, N a n ,must be known. This is accomplished by using the adsorption-incumene data and eq 5, modified to account for adsorption on both poisoned and unpoisoned sites. The modified expression, including the residence time in the dead volumes, is PI' = V L [ + ~ m,(NaKa + N d ( , ) l / H Q + (VO/Q)
(14)
This equation can be used to evaluate the total adsorption N a n + NpKp or N,Ka alone when the data are for fresh catalyst. All the zero moments for the reversible adsorption experiments were the same (unity) whether measured at the gas inlet or outlet. This confirms the absence of reaction of hydrogen with cumene solvent and Pd/A1203 catalyst and confirms the reversibility of adsorption in cumene.
Table IV. Adsorption Equilibrium and Rate Constants at 298 K 103 x 1o9vw0, 103(kd~,), 1 0 3 ( k m , (K./N,), m3 0.0 0.25 1.00 2.00 4.00 6.00 9.00 17.50 26.00
m3/(kg.s) m3/(kg.s) 102k,, s-l 7.23 4.05 1.50 1.24 4.06 5.95 1.05 4.46 4.20 2.65 4.43 0.73 0.57 2.39 4.22 0.50 2.56 3.99 1.44 4.16 0.33 0.33 1.44 4.16 1.44 4.16 0.33
m /kg 46.7 38.6 32.7 22.7 17.7 15.6 10.3 10.3 10.3
k,lk,lK, 0.262 0.263 0.308 0.379 0.313 0.242 0.296 0.296 0.296
Also there was no change in zero moment for successive pulses, again suggesting no reaction with the catalyst (that is, no palladium hydride formation). Probably this is because the concentration of hydrogen in the liquid in contact with the catalyst is very low due to the low solubility and gas-liquid mass-transfer resistance. First-reduced moments, corrected by subtracting the residence time between tee and detector, are plotted in Figure 2 for fresh catalyst and three levels of poisoning (0, 1, 9, and 26 X m3 of pure thiophene). The catalyst loading was 0.025 kg, and the volume of cumene ( VL) was 1.136 X lo4 m3. The dead volume (Vo= 0.1 X 103m3) was estimated by subtracting the volume of liquid cumene and catalyst particles from the total volume. The plot shows no significant dependence of the slopes on the extent of poisoning. Hence, the total adsorption equilibrium constant (N&, + Nd(,) is the same regardless of the amount of poison. By regression analysis of all data, the total equilibrium constant calculated from eq 14 is (NaKa),= (NaKa+ Ndc,)lp= 38.7 X m3/g (15) where the subscripts f and lp denote, respectively, fresh catalyst and different levels of poison. Also the concentration of unpoisoned sites plus poisoned sites ( N , + N p ) is constant. The constant values of N , + N p along with eq 15 mean that adsorption occurs to the same extent on both kinds of sites. The same result was found for carbon disulfide poisoning for Pd/A1203 (Chen et al., 1986a,b). These articles reported an activation energy of 16.7 kJ/ mol, suggesting a weakly activated nondissociative adsorption. Madon et al. (1978) studied the catalytic hydrogenation of cyclohexene on supported platinum and refer to the nondissociated, adsorbed hydrogen molecule as being in a physically adsorbed state. The total adsorption equilibrium constant in cumene given by eq 15 needs to be corrected to its value in amethylstyrene by the expression (NaKa + Nd(p)styrene = (NaKa + N&p)cumene(Hs/Hc) (16) Henry's law constants Hc and H,were obtained from Satterfield et al. (1968) and Herskowitz et al. (1979) and are listed in Table 111. Table IV,next-blast column, gives the corrected N,K,. For example, the value for fresh catalyst (46.7) is given in the first row of this column of Table IV.
1054 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 8
v
296 311
0
323
0
936 651 443
6
2
0
0.004
0.002
0.006 I
I
Q / V L , s-1
0
Figure 3. Zero momenta for reaction runs with fresh catalyst.
I
I
I
0 001
0 002
I
I
0 003
I 0 004
Q/v,, s - 1 Figure 5. Zero moments for reaction runs with fresh and poisoned catalysts a t 298 K. 16
m
5
14
o k,N,
m x
1
x?
. m m Y 'x
x
io3
9
im3/ kgvs)
i
12
10
A
k a N a x 10' (m3/ kg.s\
-i
08
06 04
0.00305
0.00315
1/T,
0.00325
0 00335
K-'
Figure 4. Temperature dependence of the overall rate constant, k,, for fresh catalyst.
Second momenta for the adsorption data were calculated from eq 3 and corrected by subtracting the contribution in the dead volume. The values of dispersion in the dead volume were experimentally measured for a nearly insoluble system, nitrogen gas bubbling through a saturated sodium chloride solution (H= 384), with the same dead volume that was used for adsorption measurements (V, = 0.1 X 10-3m3). The results showed that the dead volume contribution ranged from 16% to 37% of the total second moment. Such corrected second central momenta were not accurate enough to determine the adsorption rate constant by plotting gz,,(Q/ V,) vs V,/Q, according to eq 6. Reaction Experiments. The reaction runs with fresh catalyst were carried out at 298,311, and 323 K. The zero moment values are plotted as a function of the gas flow rate in Figure 3. The intercepts of the straight lines are a function of the gas-to-liquid mass-transfer coefficient, and the slopes are related to the overall rate constant according to eq 10. From these data, it is possible to evaluate the Thiele-type modulus (4) and hence kJV, via eq 11, if we know the values of k , and De. Because the particles are very small, liquid-to-particle and intraparticle mass transfer have a negligible effect on the observed data. Hence, highly accurate values of k, and De are not required in these. The nature of the bracketed term in eq 10 is such that large changes in either De or k, cause very little change in the function of kJVa and k&aNa in brackets in eq 11. The effective diffusivity 1.26 X lo-* m3/s at 298 K was obtained from the results of Herskowitz et al. (1979) for the same reaction and catalyst and k , was obtained from the Furusawa and Smith (1973) correlation. Effectiveness factors were from 0.94 to 0.99, and k, was 0.128 cm/s. Sensitivity analysis showed that a 60% change in either
z o ,
02 0 0
4
8
12
16
20
24
29
Volume of Thiophene Added x 109 (m3)
Figure 6. Reaction rate constants for fresh and poisoned catalysts.
De or k , resulted in a change in k a a of but 2-3%. The coefficients used in the calculations are given in Table 111. The values of kJVa for fresh catalyst so calculated are displayed in an Arrhenius plot in Figure 4. The activation energy is 18.8 kJ/mol, which is lower than the values 28.8 kJ/mol given by Germain et al. (1974),33.4 kJ/mol given by Ahn et al. (1986), and 28.0 and 30.0 kJ/mol given by Chen et al. (1986a, 1987). It is interesting to note that the overall rate constants for our catalyst were several times greater than the values found by the previously mentioned workers, indicating a much more active catalyst. Zero moment results at 298 K for fresh catalyst and different levels of poisoning are illustrated in Figure 5. The slopes of the lines increased with poison level at the lower poison concentrations, corresponding to a decrease in conversion and in Nak,. At the three highest levels, corresponding to additions of 9.0, 17.5, and 26.0 X lo4 m3 of pure thiophene, the slope did not change. Hence, these three sets of data were fitted with only one line. The intercepts were nearly the same, as they should be since the intercept is a measure of mass transfer from gas bubble to liquid. The values of kJVa as a function of poison level, plotted in Figure 6, decrease sharply with the first additions of poison. However, the catalyst retains approximately 22% of its original activity, regardless of the amount of poison in the slurry. This tolerance indicates that some sites remain active even with high concentrations of thiophene in the liquid phase. Holah et al. (1979) found a similar effect for thiophene on nickel boride or Raney nickel catalyst, in studies of the liquid-phase hydrogenation of 1-alkenesat ambient temperature. Bourne
Ind. Eng. Chem. Res., Vol. 29, No. 6,1990 1055 0.28
c
J
/
? 0 X
0 7
5
0.12
t 0
0
v X
0.2
04
06
0.0
2.0 9.0;17.5; 26.0
08
10
mo [ 1 + ( 1 / K ~ )- ( l / m o K ~ ) I 2
Figure 7. First momenta for reaction runs with fresh and poisoned catalysts at 298 K.
et al. (1965) have pointed out that, for a nickel catalysts, about 19% of the sites cannot be poisoned by thiophene adsorbed in a coplanar manner because geometric requirements restrict the access of thiophene to a portion of the catalyst, but do not restrict the access of reactants. Perhaps this is reasonable for reactions carried out at relatively low temperatures and pressures, where the surface mobility of adsorbed poison is very low and the poison does not change its molecular structure. There are alternate explanations, e.g., reversible poisoning or nonuniform distribution of the thiophene within the alumina particles. The first moment results for fresh and poisoned catalysts (0.0, 2.0, 9.0, 17.5 and 26.0 X m3 of pure thiophene added) a t 298 K are shown in Figure 7. According to eq 12 for the first-reduced moment, the intercepts are equal to the ratio Vo/VL. The experimental values for the intercepts are between 0.14 and 0.163, which agree acceptably well with the value of Vo/VL = 0.153 obtained from the preliminary measure of dead volume without catalyst in the reactor. The slopes of the regression lines in Figure 7 provide the second relationship between N,k, and k , given by eq 13. Individual rate constants k,N, and k , can be evaluated provided we can estimate the values of N,K, and NpKp separately. Chen et al. (1986a) found for the same reaction system that N,k, decreased linearly with the amount of irreversible poison CS2. Hence, it was logical to assume that N,k, was proportional to the number of unpoisoned sites and that the activity per site (k,,) was constant. With thiophene, the decrease was not linear (Figure 6). However, it is consistent with our data again to assume that the overall rate constant per site (k,) is constant. If there were strong nonuniformity in the distribution of poison, it is possible that k , could actually vary with the poison level while the observed data would suggest a false but constant k,. However, this is a less likely situation. Assuming k , to be constant, we can estimate N,K, for different levels of poison (denoted by subscript lp) from the relationship (17) (NaKa)lp = ( N S J f [ (N&Jlp/(Nako)d To do this, one uses the values of the overall rate constants determined from Figure 5 to calculate the term within brackets. Also, KJVp for the poisoned sites can be estimated from W&p)lp (NaKa)d1- (NakJ~p/(Naho)fl (18) Now, N&, and k, can be evaluated from the slopes in Figure 7 and the N,k, values by using eq 12 and 13. The
0
4
8
12
16
20
24
28
Volume of Poison Added to Slurry lo9 (m3 of Pure Thiophene)
Liquid x
Figure 8. Thiophene detected in the liquid phase as a function of the thiophene volume added to the slurry.
results for these rate constants, as well as the ratio k,/ k , / K , for all poison levels, are plotted in Figure 6. The surface rate constant, k,, does not show significant change with poisoning, while N,k, decreases. The ratio k,/k,/K, is a measure of the relative rates of surface reaction and desorption ( k , / k d ) . The values of this ratio for fresh and poisoned catalyst are between 0.24 and 0.38, indicating that the overall reaction is not controlled by adsorption or surface reaction, regardless of the poison level. This conclusion is in agreement with previous works (Ahn et al., 1986; Chen et al., 1986b, 1987). AU these values as well as N,k, and N&, for different levels of poisoning are given in Table IV. With the assumption that k, is independent of the poison level, the decrease in N,K, in Table IV as poison is increased is entirely due to the decrease in number of unpoisoned sites. Additional approximate information was obtained by gas chromatographic analysis of the liquid phase from the slurry reactor. After the first additon of only 0.25 X lo* m3 of thiophene to the slurry, the poison was detected in the solution. This means that, even for a very small amount of thiophene in the system, the adsorption was not irreversible. Thus, an amount of poison less than that initially added to the reactor is able to deactivate the catalyst by about 17% (Figure 6). The measurements of thiophene in the liquid phase could not be made accurately enough to obtain by difference the adsorbed poison, especially at low concentrations (see Figure 8). Hence, the adsorption isotherm of the poison could not be evaluated. However, the data in Figure 8 at higher concentrations suggest that the volume of adsorbed thiophene, that is, the difference between the poison added initially and detected in the slurry liquid, is about constant and of the order 1 X m3 of pure thiophene or 4 X lo-* m3/kg of catalyst. This low adsorbed concentration corresponds to an estimated maximum (assuming 100% dispersion) coverage of less than a monolayer and is consistent with the retention of some hydrogenation activity even at high poison levels. Conclusion With a dynamic response technique, the adsorption equilibrium constant and the adsorption and surface reaction rate constants were estimated for hydrogenation of a-methylstyrene with Pd/A1203catalyst as a function of poisoning with thiophene. Hydrogen adsorption experiments with nonreactive cumene as the slurry liquid demonstrated that adsorption occurred to the same extent on both poisoned and unpoisoned sites. The overall rate constant per kilogram of catalyst decreased strongly with
1056 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
the first additions of poison. However, relatively large amounts of thiophene did not completely deactivate the palladium catalyst; a constant activity equal to 22% of that of fresh catalyst remained. The residual activity may be due to restricted access of thiophene, but not of hydrogen, to a part of the catalyst surface, although other explanations are possible. The adsorption rate constant per unit of mass of catalyst decreased with poisoning, while the rate constant per site for the surface reaction did not show significant change. This suggests that adsorbed poison does not affect appreciably the remaining active sites. Such evidence implies that poisoning of this Pd /A1203catalyst with thiophene, a t room temperature and atmospheric pressure, is due more to geometric than electronic effects. The values of the rate constant indicate that neither adsorption nor surface reaction rate controls the overall process a t any poison level.
Acknowledgment A grant from CONICET of Republica Argentina made possible Dr. L. A. Arrua’s stay a t UC Davis; this is gratefully acknowledged. The authors appreciate the financial assistance of Chevron Research Corporation.
Nomenclature U B = surface area of gas bubbles per unit volume of bubbleand particle-free liquid, m2/m3 a, = surface area of catalyst particles per unit volume of bubble- and particle-free liquid, m2/m3 Bi = Biot number, Rk,/D, C, = hydrogen concentration in the gas phase leaving the reactor, mol/m3 C, = hydrogen concentration in the feed pulse, m01/m3 CL = hydrogen concentration in the bulk liquid, mol/m3 ci = hydrogen concentration in the liquid-filled pores of the catalyst, mol/m3 D, = effective diffusivity in the liquid-filled pores of the catalyst, m2/s H = Henry’s law constant, (C,/C,) uil H,= Henry’s law constant for the shbility of hydrogen in cumene H,= Henry’s law constant for the solubility of hydrogen in styrene k = adsorption rate constant, m3/(site.s) k , = adsorption rate constant on unpoisoned sites, m3/(site.s) kd = desorption rate constant on unpoisoned sites, s-l; this intrinsic constant is a combination of k , and K , kL = gas bubble-to-liquid mass-transfer coefficient, m/s k , = intrinsic overall reaction rate constant, m3/(site.s) k , = adsorption rate constant on the poisoned sites, m3/(site.s) k , = surface reaction rate constant on unpoisoned sites, s-l k, = liquid-to-particle mass-transfer coefficient, m/s K = adsorption equilibrium constant for hydrogen, m3/site K , = adsorption equilibrium constant for hydrogen on unpoisoned sites, m3/site K p = adsorption equilibrium constant for hydrogen on poisoned sites, m3/site KL = dimensionless quantity, eq 7 L = depth of slurry, m mo = zero moment, eq 1 m, = mass of catalyst particles per unit volume of bubble- and particle-free liquid, kg/m3 n = concentration of hydrogen adsorbed, mol/ kg n, = concentration of hydrogen adsorbed on unpoisoned sites, mollkg np = concentration of hydrogen adsorbed on poisoned sites, mol/kg N = site concentration, sites/kg of catalyst N , = unpoisoned site concentration, siteslkg of catalyst
2=
= poisoned site concentration, sites/kg of catalyst
volumetric gas flow rate, m3/s R = radius of the catalyst particle, m t = time, s U B = vertical component of the bubble velocity, m/s VB = bubble volume per unit volume of bubble- and particle-free liquid Vo = dead volume with no gas flow, including gas space over slurry, in sample loop and in tubing, m3 V , = volume of liquid in slurry, m3 VtMo = equivalent volume of pure thiophene added to the slurry, m3 Greek Symbols
fl = porosity of catalyst particles = quantity defined in eq 13, m3/kg K~ = reduced first moment, s fila = reduced first moment, including dead volume, s p2,, = second central moment, corrected for dispersion in dead volume, s2 p, = density of catalyst particle, kg/m3 rg = residence time of gas in slurry, VBVL/Q, s 4 = Thiele-type modulus, eq 11 Subscripts f = fresh catalyst lp = level of poison Registry No. Pd, 7440-053;a-methylstyrene,98-83-9cumene, 98-82-8;thiophene, 110-02-1.
Literature Cited Ahn, B. J.; Smith, J. M.; McCoy, B. J. Dynamic Hydrogenation Studies in a Catalytic Slurry Reactor. AZChE J. 1986, 32, 566-574. Babcock, B. D.; Mejdell, G. T.; Hougen, 0. A. Catalyzed Gas-Liquid Reactions in Trickling-Bed Reactors. MChE J. 1957,3,36€-373. Barbier, J. Effect of Poisons on the Activity and Selectivity of Metallic Catalysts. In Deactivation and Poisoning of Catalysts; Oudar, J., Wise, H., Eds.; Marcel Dekker: New York, 1985. Bartholomew, C. H.; Agrawal, P. K; Katzer, J. R. Sulfur Poisoning of Metals. Adv. Catal. 1982,31,135-242. Boitiaux, J. P.; Cosyns, J.; Verna, F. Poisoning of Hydrogenation Catalysts. How to Cope with this General Problem. In Catalyst Deactiuation; Delmon, B., Froment, G. F., Eds. Elsevier: Amsterdam, 1987. Bourne, K. H.; Holmes, P. D.; Pitkethly, R. C. The Selectivity of Sulfided Nickel Catalyst. Proc. Intern. Congr. Catalysis, 3rd, Amsterdam 1964,North-Holland: Amsterdam, 1965;Vol. 2,pp 1400-1411. Cerveny, L.; Herd, V.; Ruzicka, V. Hydrogenation Autoclave as a Reactor with Imperfect External Mass and Heat TransferMathematical Modeling. Chem. Tech. (Leipzig) 1983, 35, 579-583. Chen, S.Y.;McCoy, B. J.; Smith, J. M. Dynamic Studies of Catalyst Poisoning: Effect of Adsorption and Surface Reaction Rates for Hydrogenation of @-Methyl Styrene by Pd/A1203. AZChE J. 1986a,32,2056-2066. Chen, S. Y.; Smith, J. M.; McCoy, B. J. Dynamic Response and Poisoning Studies of a Liquid-Solid Catalytic Reaction: Effects of Metal Content on Adsorption and Surface Reaction Rates. J. Catal. 1986b, 102,365-376. Chen, S.Y.;Smith, J. M.; McCoy, B. J. Effect of Hydrogenation Catalyst Activity on Adsorption and Surface Reaction Rates. Chem. Eng. Sci. 1987,42,293-306. Furusawa, T.; Smith, J. M. Effectiveness Factor and Mass Transfer in Trickel-Bed Reactors. AZChE J. 1979,25,272-283. Furusawa, T.; Smith, J. M. Fluid-Particle and Intraparticle Mass Transfer Rates in Slurries. Ind. Eng. Chem. Fundam. 1973,12, 197-203. Germain, A. H.; Lefebvre, A. G.; L’Hommer, G. A. Catalytic Trickle-Bed Reactor. Adv. Chem. Ser. 1974,133,164-180. Gut, G.; Meier, R. U.; Zwicky, J. J.; Kut, 0. M. Activity, Selectivity and Catalyst Poisoning in the Liquid Phase Hydrogenation of Phenols on Palladium. Chimia 1975,29,295-299. Hegedus, L. L.; McCabe, R. W. Catalyst Poisoning. In Catalyst Deactivation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1980.
Ind. Eng. Chem. Res. 1990,29, 1057-1064 Herskowitz, M.; Carbonell, R. G.; Smith, J. M. Effectiveness Factor and Mass Transfer in Trickle-Bed Reactors. AIChE J. 1979,25, 272-283. Holah, D. G.; Hoodless, I. M.; Hughes, A. N.; Sedor, L. J. Kinetics of Liquid-Phase Hydrogenation of 1-Alkenes over a Partially Hydrogenated Nickel Boride and the Effects of Catalyst Poisons upon These Hydrocarbons. J. Catal. 1979,60, 148-155. Madon, R.J.; O’Connell, J.; Boudart, M. Catalytic Hydrogenation of Cyclohexane. 11: Liquid Phase Reaction on Supported Platinum in a Gradientless Slurry Reactor. AZChE J. 1978, 24, 904-91 1. Niiyama, H.; Smith, J. M. Adsorption of Nitric Oxide in Aqueous Slurries of Activated Carbon: Transport Rates by Moment
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Analysis of Dynamic Data. AZChE J. 1976,22, 961-970. Oudar, J. Sulfur Adsorption and Poisoning of Metallic Catalyst. Catal. Rev. Sci. Eng. 1980,22, 171-195. Satterfield, C. N; Ma, Y. H.; Sherwood, T. K. The Effectiveness Factor in a Liquid-Filled Porous Catalyst. Znst. Chem. Eng., Symp. Ser. 1968,28, 22-29. Zwich, J. J.; Gut, G. Kinetics, Poisoning and Mass Transfer Effecta in Liquid-Phase Hydrogenations of Phenolic Compounds over a Palladium Catalyst. Chem. Eng. Sci. 1978, 33, 1363, 1369.
Received for review May 3, 1989 Revised manuscript received September 6 , 1989 Accepted September 25, 1989
Mechanistic Study of Chemical Reaction Systems John Happel* Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027
Peter H. Sellers The Rockefeller University, 1230 York Avenue, New York, New York 10021
Masood Otarod Department of Mathematicslcomputer Science, University of Scranton, Scranton, Pennsylvania 18510
This paper presents a method for characterizing complex chemical systems often encountered in studies of homogeneous, heterogeneous catalytic, and enzyme reactions. From a list of chosen elementary steps, i t is shown how all possible mechanisms and corresponding overall chemical equations can be enumerated. This furnishes a valuable tool for the further consideration of desired reactions and elucidation of their kinetics. Illustrative examples are given, including problems of current interest. A computer program is available to implement the procedures required.
In the study of a chemical reaction, the first step is often the proposal of appropriate mechanisms to show how elementary steps may be combined to produce observed overall reactions. We have shown that for heterogeneous catalytic systems (Happel and Sellers, 1982,1983;Happel, 1986)it is possible to determine a unique set of mechanisms corresponding to a system specified by an initial choice of elementary reactions, where reactants are specified as being either intermediates or terminal species. These mechanisms, which we have termed direct, demonstrate the various ways that the stoichiometric equations involving only terminal species can break down into elementary reactions. Each such direct mechanism is irreducible in the sense that it cannot be separated into submechanisms, each of which produces the same overall reaction. What we call a direct mechanism is a formalization of what is usually called simply a mechanism in the chemical literature without an explicit definition being given. A further simplification, useful for development of rate equations based on appropriate mechanisms, is the extension of the concept of directness to the enumeration of stoichiometricequations corresponding to given systems. Such direct overall reactions avoid repetition of species among reactions, a concept that is also used by chemists and engineers without formal identification. The concept of directness as applied to both mechanisms and reactions has now been developed in detail (Happel and Sellers, 1989). In a chemical system involving terminal species, intermediate species, and elementary reactions, we can ask, fmt, what are all the possible mechanisms and, second, what are all the corresponding overall reactions. The way we answer these questions is to list only the
mechanisms that are not separable into submechanisms for the given system and to list only the overall reactions that are not separable into subreactions of the system. Directness is often system dependent for overall reactions. In some circumstances, one may assume that “all possible molecular transformations can occur among a set of species” (Aris and Mah, 1963; Amundson, 1966). In this case, the atom-by-species matrix gives a valid way to determine the maximum number of linearly independent overall reactions in a system. For such a system, the definition of direct reactions requires that, if any one of the components in a given reaction is deleted, a reaction among the remaining components no longer exists. Such reactions have been termed single (Petho and Kumer, 1985). For systems in which all possible transformations do not occur, the number of direct overall reactions will be fewer. In all cases, the sets generated are finite, permitting a logical scheme for selection of appropriate kinetic relationships among terminal species. For simplicity, it is assumed that a single direct mechanism will characterize a system. This is not always the case. For example, it is sometimes possible for a homogeneous reaction mechanism to exist in parallel with that promoted by a heterogeneous catalyst. It is possible to develop the criteria for combining direct mechanisms such that certain combinations will also be cycle-free (Sellers, 1984). The situation is different for direct overall reactions. The initial step in the calculation procedure for a chosen system generates a single mechanism and a corresponding set of overall reactions which, though linearly independent, may not all be direct. The number of such overall reactions obtained is, however, unique for the given system. From
0888-5885/90/2629-1057$02.50JO 0 1990 American Chemical Society
