330
Ind. Eng. Chem. Fundam. 1986, 25, 330-337
K, = dissociation constant for the protonated base K , = equilibrium constant for reaction 7 K 2 = equilibrium constant for reaction 8 K , = water dissociation constant m = molarity of chemical base, g-mol/L Nco2,N H ~=Sabsorption rate of COPand HzS, g-mol/(cm2s) Pco2,PH = partial pressures of COz and HzS, atm &02’ #H,S = equilibrium vapor pressures of C02and HzS, atm S T = thermodynamic selectivity (eq 1) ST,max = highest value of S T S K = selectivity for a coupled absorber-regenerator plant YCO YHs = fractional saturations of base B with respect to C6, and H2S CYHS, ace, = total contents of HzS and COz in the solution (both in physically dissolved and chemically combined forms), g-mol/cm3 /3 = number slightly larger than unity Registry No. H2S, 7783-06-4; CO,, 124-38-9; methyldiethanolamine, 105-59-9.
Literature Cited Astarlta, G.; Savage, D. W. Chem. Eng. Sci. 198Oa, 35, 1959. Savage, D. W. Chem. Eng. Sci. 1980b, 35, 1755. Astarlta, 0.; Blauwhoff, P. M. M.;Versteeg, C. F.; Van Swalj, W. P. M. Chem. fng. Sci. 1984~1,39, 207. Blauwhoff, P. M. M.; Kampuis, 6.; Van Swalj, W. P. M.; Westerterp. K. R., personal communication, 1983. Cornelissen, A. E. Trans. Inst. Chem. Eng. 1980, 58, 242. Danckwerts, P. V. Gas-LlquM Reactions; McGraw-Hill: New York, 1970. Donaldson, T. L.; Nguyen, Y. N. Ind. Eng. Chem. Fundam. 1980, 19, 260. Frazier, H. H.; Kohl. A. L. Ind. fng. Chem. 1950, 42, 2288. Goar. B. G. Oil Gas J. 1980, 78(18), 239. Hiklta, H.; Asai, S.;Ishkawa, H.; Honda, M. Chem. fng. J . 1977, 13, 7. Johnson, R. R.; Say, G. R. Presented at the 3rd International Conference on Control of Sulfur and Other Gaseous Emissions, University of Salford, England, April 1979. Kohl, A. L.; Blohm, C. L. Pet. fng. 1950, 22, C-37. Olander, D. R. AIChf J. 1980, 6, 233. Ouwerkerk, C. Hydrocarbon Process. 1978, 5 7 , 4, 89. Savage, D. W.; Astarlta, G.; Joshi, S . Chem. fng. Sci. 1080, 35, 1513.
Received for review February 13, 1984 Revised manuscript received February 4, 1985 Accepted July 11, 1985
Effect of Catalyst Pore Structure on Hydrotreating of Heavy Oil Mltsunorl Shbnura, Yoshlml Shlroto, and Chlrato Takeuchl Research Department, Research and Development Center, Chiycda Chemical Engineering and Construction Co., Ltd., 3- 13, Moriya-cho, Kanaga wa-ku, Yokohama 22 1, Japan
In the catalytic hydrotreating of heavy oil, it is necessary to crack the asphaltenes in the oil. However, since the asphaltenes are extremely large molecules, which consist of highly condensed heterocyclic and aromatic rings to which sulfur, nitrogen, oxygen, and metals (mainly vanadium and nickel) are bound, it is anticipated that their diffusion into catalyst pores will be difficult and that they will be main sources for coke and metal depositions on the catalyst surface. Therefore, to design the catalysts for hydrotreatlng of heavy oil, it is important to understand the behavior of such a large molecule In the catalyst pores. I n this paper, the diffusMties of standard polystyrenes and asphaltenes in several hydrotreating catalysts having the various pore slzes were measuredby use of solid-liquid chromatography techniques, and the effect of the ratio of diffusing molecular diameter to the pore diameter was investigated on the effective diffusion coefficient. Furthermore, hydrotreatlng tests were performed for these catalysts with Boscan crude, which contains a large amount of metals and asphaltenes. As a result, a quantitative relation between the catalyst pore structure and the reactivities of asphaltene cracking and demetalation was obtained.
1. Introduction In the catalytic hydrotreating of heavy oils, as in hydrodesulfurization, it is known that asphaltenes in the oils have a detrimental effect on the activity and life of a catalyst and that their presence a t large concentrations makes treatment extremely difficult. Since asphaltenes are large molecules which consist of highly condensed heterocyclic and aromatic rings to which sulfur, nitrogen, oxygen, and metals (mainly vanadium and nickel) are bound, it is anticipated that their diffusion into catalyst pores will be difficult and that they will be main sources for coke d e p i t a on the catalyst surface. Most of the metal compounds in the oil are concentrated in asphaltenes, so hydrodemetalation of these compounds leaves deposits of mixed metal sulfides in the catalyst pores. The accumulation of these metal deposits governs the decline of the catalyst activity and determines catalyst life. In selecting the catalysts for hydrotreating of heavy oils, it is important to understand the diffusion behavior of such large molecules in the catalyst pores and the effect of changes in the catalyst pore structure, caused by metal
deposits, on the catalyst activity and life. Numerous studies have been made for diffusion in fine porous materials. For example, Renkin (1954) and Anderson and Quinn (1974) proposed theoretical correlations representing the effective diffusion coefficients as functions of pore diameter and molecular diameter. Satterfield et al. (1973) measured the effective diffusion coefficients of paraffinic and aromatic hydrocarbons in silica-alumina catalyst having a pore diameter of 32 A and proposed an experimental correlation. Likewise, Prasher and Ma (1977) measured the effective diffusion coefficients of hydrocarbons in alumina having a pore diameter of 57 or 66 A and proposed a similar empirical formula. For hindered diffusion of asphaltenes, Baltus and Anderson (1983) showed a very strong pore size effect on the diffusion coefficient in the range 70-500-A pore radius. For hydrotreating, several authors have attempted to elucidate the mechanisms by which catalysts deactivate and a number of others have formulated models based on proposed mechanisms for predicting catalyst deactivation at commercial operating conditions. Newson (1975) de@ 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25,
3.a
0--A 1.0 .
:0-0 =E
4
A-E
7 0.5
O
al VI
PEAK 1
No. 3, 1986 331
A
9
P 5 0.1 u
Y
X
g 0.05
o.o\
I
3
I
0.5 1-MD/PD
I
l
l
1
1
[-1
Figure 1. Relation between De and 1 - MD/PD.
veloped a model predicting the life of demetalation catalysts assuming uniform pore plugging. Hughes and Mann (1978) presented a model for computing the activity of plugged catalysts, neglecting the diffusional limitations and assuming the deposit to be in the form of a wedge. For the first time, Spry and Sawyer (1975) examined the influence of pore size distribution on initial demetalation activity, accounting for the influence of pore sizes on the diffusivity. Rajagopalan and Luss (1979) developed simple algebraic relations that predicted pore size, which yielded the optimal initial activity and optimal lifetime activity for demetalation. These models examined primarily the activities of demetalation and desulfurization for the catalysts. There has been, however, no study reported about the change of the activity of asphaltene cracking taking account of the change in catalyst pore structure. The purpose of this work is to develop a model for predicting the effect of the catalyst pore structure on hydrotreating heavy oils which accounts for the restricted diffusion of the large reactant species in the pores and the change of the catalyst pore structure due to metal sulfides and coke deposits. 2. Diffusion of Large Molecules within Porous
Materials As previously stated, a number of studies exist concerning diffusion of molecules in liquid-filled pores of porous materials. Though Renkin's equation was derived for a rigid-sphere model, many experimental results (Beck and Schultz, 1972; Stein, 1967) were explained well by it. For the diffusion of large molecules, Cannell and Rondelez (1980) observed that Renkin's equation gave an excellent description of experimental data for the diffusion of dilute polystyrenes through cylindrical pores. We also have measured the effective diffusion coefficients, De, of standard polystyrenes having various molecular weights in porous materials by using solid-liquid chromatography techniques. Crushed y-alumina was used as a packing material with particle sizes ranging from 60 to 100 mesh. Median pore diameters (MPD) were measured by a mercury porosimeter are five different sizes: 108 (A), 145 (B), 223 (C), 215 (D), and 610 8, (E). The experimental apparatus and procedure are described in the Appendix. In order to obtain a quantitative relationship between the effective diffusion coefficient, De, the molecular diameter of polystyrenes, MD, and the pore diameter of porous materials, PD, the values of De, obtained by this experiment, were correlated against the values of 1- MD/PD, as shown in Figure 1. It is noted that MD is twice the value of the root mean square of gyration calculated from the equation reported by 0 t h and Desreux
RELATIVE RESIDENCE
TIME
Figure 2. Response curves of Khafji vacuum residue. Table I. Properties of Boscan Crude specific gravity ( d 3 viscosity (at 50 " C ) asphaltenes conradson carbon residue sulfur vanadium nickel
1.004 4950 CP 11.9 wt % 16.3 w t % 5.52 wt % 1190 wt ppm 112 wt ppm
(1954). PD is the median pore diameter, MPD, corresponding to 50% of the total pore volume. In this correlation, the porosity and tortuosity factors of the porous materials were assumed to be equal. It shows that the effective diffusion coefficient is proportional to the fourth power of 1- MD/PD, which coincides with a theoretical formula due to Renkin (1954). Considering the differences of the molecular shapes, it is not clear that the diffusion phenomena of asphaltenes and metal-containing compounds in heavy oils are similar to those of polystyrenes. However, Spry and Sawyer (1975) successfully applied Renkin's equation (1954) for diffusion phenomena of the large molecules in heavy oil and found that the phenomena were represented by his fourth-power correlation. Diffusion measurements were also made for a Khafji vacuum residue. Figure 2 shows the pulse response curves of Khafji vacuum residue obtained for four porous materials, A, B, C, and D. As shown in Figure 2, the response curves having two peaks were obtained for the materials having a pore diameter smaller than 250 A. Peak 1,shown in Figure 2, was found in the vicinity of the relative residence time of 1.0. Therefore, the peak may have been due to molecules which have a small effective diffusion coefficient and can hardly diffuse into porous materials. There are also appreciable heavier fractions containing large amounts of asphaltenes in residue. Therefore, a high resistance to diffusion of asphaltenes would be expected when the pore diameters are less than about 200 A. 3. Effect of Catalyst Pore S t r u c t u r e on Catalytic Conversion of Large Molecules The catalytic hydrotreating tests were performed for Boscan crude, by using six catalysts having different median pore diameters. The feedstock contains a large amount of metals and asphaltenes. Based on these results, the following items were investigated quantitatively for vanadium removal and asphaltene cracking reaction by modeling the catalyst pore structure and the catalytic reaction: (1)reaction order on catalyst surface, (2) effect of catalyst pore structures on the catalyst activity, and (3) effect of catalyst pore structures on the catalyst life. 3.1. Experimental Section. a. Feedstock. A feedstock used for the hydrotreating tests is Boscan crude produced
332 Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
FEED PUMP HOT SEPAR4TM
Figure 3. Simplified flow diagram of test unit. Table 11. Catalyst Physical Properties surface median pore area, catalyst diameter, A m2/g 1 2 3 4 5 6
128 169 264 300 360 610
175 217 183 210 113 83
--.I CI,
pore volume, cm3/g
V
0.56 0.86 1.12 1.25 0.98 0.89
in Venezuela; its properties are shown in Table I. b. Test Unit and Operating Conditions. The tests were carried out by a high-temperature, high-pressure, flow-type unit. A simplified flow diagram is shown in Figure 3. The feed oil and hydrogen passed by the once-through upflow mode through a fixed-bed reactor (2.54-cm i.d. X 560-mm length), and the catalyst bed was maintained isothermally. The conditions of the hydrotreating tests were as follows: temperature, 405 "C; pressure, 1990 ft lb/in.2; H2/oil, 1000 (standard L/L); LHSV, 1.0 h-l. The LHSV is the ratio of the volume rate of liquid feed to the volume of catalyst. The catalyst volume in the reactor is 90 cm3. c. Catalysts. The catalysts, especially prepared for the tests, were all of the cobalt-molybdenum type, supported on a carrier of an alumina. The sizes and shapes of these catalysts were the same, cylinders 0.8 mm in diameter and about 3 mm in length. The representative physical properties are shown in Table 11. 3.2. Mathematical Model for Catalytic Conversion of Large Molecules. The models were developed for the catalyst pore structure and reactions in a catalyst particle and also for the catalyst deactivation by considering the change of pore structure caused by foulant deposits in the particles. a. Catalyst Pore Structure. For a catalyst having an arbitrary pore size distribution, the pore structure is assumed to be represented by a set of nonintersecting cylindrical pores of variable radii, as illustrated in Figure 4. The total pore volume is divided into n partial pore volumes, each having an average pore diameter, PDF The porosity is assumed to be uniform within the catalyst. For the catalyst with pore diameters between PD1 and PD,, the partial pore volume is defined by PVj = SAjPDj/4 (1)
PORE
DIAMETER 18)
Figure 4. Pore volume distributions.
where SA. is the internal surface area owing to pores at diameter 6Dj. The total pore volume and surface area are obtained from summation of partial pore volume and the surface area of each partial pore, respectively. n
n
PV = CPVj = CSAjPDj/4 j=l
j=1
n
n
SA = CSAj = C4PVj/PDj j=l
j=l
(3)
b. Reaction Rate in a Single Catalyst Particle. In the development of a rate equation, the catalyst particle was assumed to be spherical, although the catalyst actually used in the reaction was of a cylindrical shape. The particle size, L,, is defined as the ratio of geometric volume to the geometric outer surface area, and the equivalent spherical radius of the catalyst, R, is R = 3L,
(4)
Assuming that the surface reaction rate is proportional to the nth power of the reactant concentration, material balances of reactants in a single catalyst particle are written as
where
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
I
Boundary conditions are Cj(R,z,t) = C(z,t); dCj(O,Z,t)/dr = 0
i-7 Reacla n t
The distribution of reactant concentration in a partial pore is obtained by solving eq 5 under the boundary conditions. According to the theory of Thiele (1939), the effectiveness factor of the catalyst is defined as the ratio of the actual reaction rate into a catalyst to the reaction rate without resistance to diffusion. Therefore, the effectiveness factor of a partial pore with a diameter PDj can be expressed by 4~ xRkiSAj(r,z,t)Pcat.[Cj(rizit)1"r2dr E,j(z,t) =
(9) (4/3).lrR3kisAj(r,z,0)~cat.[C(z,t)l"
By using this effectiveness factor, we obtain the reaction rate in a partial pore with a diameter PDj given by Rj(z,t) = kiSAj(r,z,O)Pcat.[C(zit)lnE,j(zit)
(10)
Therefore, the reaction rate is a single catalyst particle is obtained from summation of the rates in partial pores as n
Rcat.(zit) = CRj(z,t) j=1
(11)
c. Overall Reaction Rate in the Catalyst Bed. For a thin layer of the catalyst bed at distance z from the inlet, where the change of the reactant concentration can be neglected, the reaction rate is expressed by Rbed(Z,t) Rcat.(zit)(Pbed/Pcat.) (12)
A material balance of the reactant in this layer is F dC(z,t) = Rbed(Z,t) dz (13) By integrating eq 13 along the axis of the catalyst bed, one can obtain the reactant concentration at the outlet of the catalyst bed as
d. Catalyst Deactivation. In general, residuum hydrotreating catalysts are deactivated by foulant deposits which are composed mainly of metal sulfides and coke. This is because these deposits cause a loss in catalyst surface activity and a decrease of the surface area and pore diameter. Therefore, the accumulated amount of these deposits is assumed to be the most important factor determining the catalyst deactivation. From the elemental analysis of the spent catalyst used for the hydrotreating tests, it was confirmed that these deposits consisted mainly of vanadium, nickel, sulfur, and carbon. The contents of vanadium and sulfur were much higher than those of the other two elements. So, after further analysis by X-ray diffraction (Asoaka et al., 1983),the main constituent was identified as vanadium sulfide, V3S4. This is quite similar to the form of mixed metal sulfides in a spent residuum HDS catalyst, as reported by Tamm et al. (1979). Consequently, the density of the mixed metal sulfides in these deposits was estimated to be 4.26 g/cm3 of V3S4. On the other hand, for deposits of coke, Inoguchi et al. (1971) reported that in residuum hydroprocessing the level of coke rose quickly in the initial stages of the run and then remained constant throughout the rest of the run. Based on this information, the change of the catalyst surface state due to these deposits is assumed to take place as follows. In the early stages of the hydrotreating run, a rapid decline
f /
333
Vanadium Sulfide and Coke Deposits ////"','"
1 -
"
' "' '
I r=R
r=rl
r=O
Figure 5. Distribution of deposits in a partial pore.
of catalyst activity occurs because the surface is converted from its original state to a surface contaminated with metal sulfides and coke. While it is considered that a catalyst surface loaded with these compounds is catalytically active for hydrogenolysis, the surface activity is less than that of the original catalyst but is scarcely changed with a further increase in the amount of deposits. The catalyst deactivation model was developed taking into account the change of the pore structure by deposits, as follows. The distribution of deposits in a partial pore having a diameter PDj at any time T = t is schematically described in Figure 5. Hereafter, the volume element of a volume 4 p ~ r ? A rat a distance of ri from the center of the catalyst will be considered. A volume loss in a partial pore, due to vanadium sulfide and coke deposits, is defined by OTj(ri,z,t)
=
Ocj
+ OMj(ri,Z,t)
(15)
Ocj is the volume loss in the pore due to coke deposit after the early stage of the run. The value of Ocj, obtained from the analysis of the spent catalysts used by the hydrotreating tests, was in the range of about 0.09-0.14. The density of coke was assumed as 1.0 g/cm3, as Newson (1975) reported. OMj(ri,z,t)is a volume loss in the partial pore due to vanadium sulfide deposits. For a finite increment of time from t to t At, the loss in the partial pore volume due to vanadium sulfide deposits can be expressed by
+
Therefore, OTj(ri,z,t+At) is given by OTj(ri,z,t+At) = Bcj + OMj(ri,z,t)+ OMj(ri,z,At)(17) By using OTj, we calculated the partial surface area and pore diameter of a volume element at r = ri and T = t + At as SAj(ri,z,t+At) = SAj(ri,z,O)[l - BTj(ri,z,t+At)]'/2
(18)
PDj(ri,z,t+At) = PDj(ri,z,O)[l - OTj(ri,z,t+At)]1/2 (19) In summary, the change of the catalyst pore structure due to deposits is expressed by eq 18 and 19 and the deactivation of a catalyst can be quantitatively estimated by numerical calculations using eq 5-14, taking into account changes of the catalyst pores by eq 18 and 19. 4. Results and Discussion Typical test results are shown in Figure 6. These deactivation curves can be divided into three distinct parts. In the initial period, a rapid decline of catalyst activity is observed due to metal sulfides and carbon deposits. On the other hand, in the final periods a somewhat accelerated decline due to constriction of the catalyst pore mouth by metal sulfide deposits is also observed. These two stages are separated by intermediate periods in which the deactivation rate appears to be nearly constant. Since the
334
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986 t l
0.71
1
Vonodium Removal
0.61 zI.5
t
0.5
I-
O
gl$
/ I
1
I
J
1
.'
0.7
Aspholtene
I
I
100
200
PROCESS
300
Y
Crocking-
0.61
0
I
I
I
400
500
TIME
0 35, I
I
I 600
(HRI
MEDIAN PORE DIAMETER
Figure 6. Typical results of hydrotreating test.
(AI
Figure 8. Effect of catalyst pore structure on initial catalyst activities.
0.3
Vanadium Removal
Asahaltene Crackina
Table 111. Values of MD and De Obtained from Hydrotreating Test of Boscan Crude MD, 8, De, cm2/s reaction vanadium removal 25 (1.6-3.2) X asphaltene cracking 40 (0.5-1.7) X
I
.PI I
1
20
10 1 / LHSV
30
(hrl
Figure 7. Determination of intrinsic reaction order for vanadium removal and asphaltene cracking.
initial deactivation is due to the change in the surface state of the catalyst by coke and vanadium sulfide deposits, the quantitative analysis for this phenomenon is difficult. In the present study, therefore, the initial activity was defined by extrapolation from the activity of the intermediate period as shown in Figure 6. a. Determination of Surface Reaction Order for Vanadium Removal and Asphaltene Cracking. The data previously published on hydrodemetalation rates have been correlated with rate expressions both first order (Dautzenberg et al., 1978) and second order (Beuther and Schmid, 1963)in the metals concentration. However, very few studies have been made of the surface reaction order for vahadium removal and asphaltane cracking. Generally, the surface reaction order can be directly obtained from experiments with a differential reador. But, also, the surface reaction order can be determined by using an integral reactor in which case only the space velocity has been changed, while all other conditions, such as feed oil, catalyst, reactor temperature, etc., have been kept constant. In this study, an integral reactor was used and the tests were carried out by using a catalyst having a mean pore diameter (MPD) of 264 A at LHSV = 0.3, 0.5, 1.0, and 1.5 h-*.
The ratio of concentrations of vanadium and asphdtenes between product and feed during the initial period of the run was correlated against the reciprocal of LHSV, as shown in Figure 7. The lines in Figure 7 were calculated with the intrinsic reaction order, n,in eq 6 chosen as first and second, respectively. The data for vanadium removal fit the first-order assumption well; those for asphaltene cracking fit the second-order assumption. Hereafter, these surface orders are used for the vanadium removal and asphaltene cracking reactions. b. Effect of Catalyst Pore Structure on Initial Catalyst Activities. The initial catalyst activities for vanadium removal and asphaltene cracking obtained from these tests were correlated against the median pore diameters of catalysts, as shown in Figure 8. Catalyst activities were represented by the values calculated by the following formula for vanadium removal and asphaltene cracking.
C,f
k, = LHSV In CV,P
(20)
The units of k, and k, are h-' and g-mol-' h-l, respectively. The lines in Figure 8 were obtained by the optimization of ki,a,and MD in eq 6 and 7 to fit the test data best by the method of Beveridge and Schechter (1970). The values of the reactant molecular diameters and the range of the effective diffusion coefficients of the reactants within the various cata€ystare summarized in Table 111. It is obvious from Figure 8 that the catalyst pore diameter has a significant effect on activity and an optimum pore diameter exists in the vicinity of 200 A for both reactions of vanadium removal and asphaltene cracking at initial activities. The optimum pore diameter for vanadium removal is somewhat smaller than that for asphaltene cracking. Also note that the average molecular size of vanadium-containing compounds is calculated to be
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
.
Catalyst 2 IM P D = 169 A 1
+ Catalyst 3 I M P D z 2 6 4 A ) Catalyst 5 IM P D = 3601 1
-5
3 0 *
Vanadium Removal
x
2-1
kJ
2 3 + O
23
G2
Proms Time'300nR
eM=o 221
e# -0 310
::350
F a aa t
q7, en = o
Process T i m e = K Q m
K3X
25
3
@
Process Time'100m 6r = O 113
0 250
10-
U
8
3
Process T i m e - O m
335
PORE VOLUME
05-
E22
k Asphaltene -
in Pore volume due to Metal sulfides Deposition
LOSS
CrackingSOLVENT TANK
H
\*\* *
a
ew
Figure 11. Typical depositional patterns for vanadium sulfides obtained from deactivation model.
0 3
+
Melol Suitides Deposition
PRESSURE GAG E
-4 U
SAMPLE LlOUlO
6- PORT SAMPLING VALVE
I
I
DIFFERENTIAL REFRACTOMETER
u
SIPHON COUNTER
I
\
14-J -
Figure 12. Experimental apparatus. 5.0 I
0
01
02
03
1
1
I
04
05
06
LOSS I N C A T A L Y S T P O R E V O L U M E D U E TO M E T A L S U L F I D E S D E P O S I T I O N Kosp
Cololysl Activily of Aspholiene Crocking
kv
Cotolysl Aclivily of Vanodium R c m w o l
Figure 10. Change of catalyst activities due to vanadium sulfides deposition. k, is the catalyst activity of asphaltene cracking, and k, is the catalyst activity of vanadium removal.
smaller than that of asphaltenes. The initial activities of catalysts with very large or very small pores are low since in the former case the intrinsic activity, proportional to the surface area, is low while in the latter case the diffusional resistance is very high. c. Effect of Catalyst Pore Structure on Catalyst Life. The changes in the catalyst activity by vanadium sulfide deposits are shown in Figure 9 for catalysts having MPD of 169, 264, and 360 A. The solid lines were calculated from the deactivation model. Agreement with data seems good. The rapid decrease in activity of the catalyst having a MPD of 169 A is due to its low effectiveness factor for vanadium removal and the rapid deposits of metal sulfides, which cause plugging near the pore mouth. The catalyst having a MDP of 360 A has a high effectiveness factor for vanadium removal; the pattern of vanadium sulfides deposits is substantially uniform in its pore size, and the decrease of catalyst activity proceeds slowly.
As a calculation example using the deactivation model, Figure 10 shows the expected change of catalyst activity due to vanadium sulfide deposits for a catalyst having the physical structure of Figure 11. The decrease of catalyst activities for two reactions becomes rapid when a loss in pore volume due to vanadium sulfides exceeds 0.3. The computed patterns of vanadium sulfide deposits in various pores are shown in Figure 11. It is clearly observed in Figure 11that the slopes of the deposit curves for these compounds near the pore mouths are steep in small pores, for which the contraction of the pore mouths is rapid. Consequently, pores having smaller inlets are not utilized effectively. 5. Conclusion 1. Adequate pore size of a carrier must be selected for sufficient diffusion of large molecules such as asphaltenes, present in heavy residue. 2. Surface reactions are first-order for vanadium removal and second-order for asphaltene cracking. 3. Optimum catalyst pore diameters exist for vanadium removal and the asphaltene cracking reaction, respectively. 4. By modeling the change of the catalyst pore structure due to vanadium sulfides and coke deposits, one can quantitatively represent the mechanism of catalyst deactivation.
Appendix Experimental Apparatus and Procedure. A diagram of the apparatus is shown in Figure 12. The glass column (5-mm i.d. X 530-mm length) was filled with porous materials. Toluene, used as a solvent, was supplied into the column by a metering pump at a flow rate of 1cm3/min as measured by the siphon counter. A loop of stainless steel tubing provided on a six-port sampling valve was fed with 0.01 cm3 of sample, and the valve was switched for a pulse input. The concentration of the sample solute at the column outlet was detected by a differential refractometer, and a response curve was obtained on a recorder.
336
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
Table IV. Properties of Khafji Vacuum Residue specific gravity (di5) 1.023 sulfur 4.59 wt % 9.67 wt 70 asphaltenes 147 w t ppm vanadium nickel 49 w t ppm
I
at
at
€,
\
z 0 + a
Ez
De=0.0881107cmz/sw
I
2-
I
W
I
u z
I I
8
v)
0
Figure 13. Comparison of observed response curves for standard polystyrene and calculated ones. 5
n
M : Molecular Weight.
4
z 0
IU
E 3 w u z V 0
2 v)
v)
W -I
z
E 1 W
zn
-€,
ax
OBSERVED
--- CALCULATED
M=WOO
The axial dispersion coefficient in the column was measured by using nonporous inert a-alumina packing having the same particle size as the porous materials. Since a Peclet number of 91 was obtained, it w&sconcluded that the interparticle flow pattern in the column was a plug flow. Samples and Packing Materials. Test solutions were 2 w t % of five kinds of the standard polystyrene having average molecular weights of 600,2200,4000,lO OOO, and 20 OOO and a 2 wt % solution of Khafji vacuum residue in toluene. The properties of the Khafji vacuum residue are shown in Table IV. To avoid adsorption of solute during the experiments, the porous materials were soaked in a 20 w t 90toluene solution of Khafji vacuum residue at room temperature for 1day, then washed with toluene, and dried beforehand. During this treatment, little change was observed in the median pore diameters of the packing materials. I t was also confirmed that after the treatment no further adsorption of sample solute could occur on the surface of the packing, since the same response curves were obtained from several pulse inputs of the same sample into the columns. Mathematical Model for Diffusion. The diffusion of the large molecules in the column is expressed by eq A1 and A2 based on the following assumptions: (1)The flow pattern in the column is pluglike. (2) No sample solute is adsorbed on the surface of porous materials. (3) The mass transfer of the sample solute within the porous materials can be expressed by diffusion. In the column 1 aq ac, = ac, -+ -
ii.I
3-
(-41) '0
where
0.5
1.0
1.5
2.0
2.5
,
.O
RELATIVE RESIDENCE TIME
q = -$"C,r2 3
R,3
dr
0
Boundary conditions are
C,=O
att=O
aC,/ax = 0 at x = L C, = Cc,o at t = to,x = 0 In a porous material of spherical shape
Figure 14. Response curves of standard polystyrenes.
experimentally and those calculated numerically, as shown in Figure 13. Response Curves of Standard Polystyrenes. Figure 14 shows pulse response curves of the standard polystyrenes for the porous material having a median pore diameter of 145 A (B). The response curve for the standard polystyrene with a molecular weight of 20 OOO has a peak value at a relative residence time of 1.0. This implies that this polystyrene did not diffuse into the pores of the packing material. All the other standard polystyrenes, having smaller molecular weights, have relative residence times which increase with a decrease in molecular weight.
Boundary conditions are C,=O
att=O
aC,/ar = 0 at r = 0 C, = C,
at r = Rp
The pulse response curves of solute at the outlet of the column were calculated by solving these equations under the boundary conditions, by a finite difference method. The effective diffusion coefficients of the solutes were determined by comparing the response curves obtained
Nomenclature constant in eq 7 reactant concentration outside catalyst particle C(z,t) Ca,f asphaltenes content in feed C*,P asphaltenes content in product solute concentration outside porous material cc reactant Concentration in partial pore solute concentration in porous material vanadium content in feed C,f Cvj(r,z,t) vanadium concentration in partial pore CV,P vanadium content in product a
2(r9z*t)
Ind. Eng. Chem. Fundam. 1988, 25,337-343
effective diffusion coefficient of solute effective diffusion coefficient of reactant in partial pore effectiveness factor of partial pore linear velocity in catalyst bed pore area fraction, defined by eq 8 catalyst activity for asphaltene cracking intrinsic reaction rate constant per unit surface area catalyst activity for vanadium removal length of packed bed liquid hourly space velocity molecular weight of standard polystyrene molecular diameter of reactant reaction order pore diameter total pore volume per unit weight of catalyst partial pore volume per unit weight of catalyst average concentration of solute within porous material radius of catalyst reaction rate in catalyst bed reaction rate in a single catalyst particle reaction rate in partial pore reaction rate defined by eq 6 radius of porous material radial distance total surface area per unit weight of catalyst partial surface area per unit weight of catalyst time linear velocity in packed bed axial position in packed bed axial position in catalyst bed total porosity void fraction of column partial porosity volume loss in partial pore due to coke deposit volume loss in partial pore due to vanadium sulfide deposits
337
OTj(r,z,t) volume loss in partial pore due to vanadium sulfide and coke deposits bulk density Pcat. catalyst particle density Pvs density of vanadium sulfide Registry No. V, 7440-62-2; Co, 7440-48-4; Mo, 7439-98-1; polystyrene, 9003-53-6. L i t e r a t u r e Cited Anderson, J. L.; Quinn, J. A. Bbchem. Blophys. Acta 1974, 255, 273. Asaoka. S.; Nakata, S.; Shiroto, Y.; Takeuchi. C. Ind. Eng. Chem. process Des. Dev. 1983, 2 2 , 242. Baltus, R. E.; Anderson, J. L. Chem. Eng. Sci. 1983, 38, 1959. Beck, E. M.; Schultz, J. S. Blochem. Blophys. Acta 1972, 255, 273. Beuther. J.; Schmid, B. K. I n proceedings of the 6th W o M Petroleum Congress, Frankfurt, June 7963; Verein zur Ftirderung des 6.Welt-ErdaCKongress: Hanburg; section 3,paper 20. p 297. Beverkige, 0. S. 0.; Schechter, R. S. Optimizatbn: Theory and fractkx; McGraw-HiII: New York, 1970;p 51. Cannell, D. S.;Rondeier, F. Macromolecules 1980. 13, 1599. Deutzenberg, F. M.; Van Kllnken, J.; Pronk, K. M. A.; Sie. S. T.; Wijffeis, J. E. ACS Symp. Ser. 1978, No. 65, 254. Hughes, C. C.; Mann. R. ACS Symp. Ser. 1978, No. 65, 201. Inoguchl. M.; Kagaya, H.; Daigo, K.; Sakwada, S.; Satoml, Y.; Inaba, K.; Tate, K.; Nishiyama, R.; Onlshl, S.; Nagai, T. BUN. Jpn. Pet. Inst. 1971, 13, 153. Newson, E. J. Ind. Chem. RocessDes. Dev. 1975, 14, 27. Oth, J.; Desreux, V. Bull. Soc. Chim. Be&. 1954, 63, 285. Prasher, B. D.; Ma. Y. H. AIChE J . 1977, 23, 303. Rajagopalan, K.; Luss, D. Ind. Chem. process Des. Dev. 1979, 78. 459. Renkin, E. M. J . e n . fhysiol. 1954, 38, 225. Salterfield, C. N.; Colton, C. K.; Pitcher, W. H. A I C M J . 1973, 79, 628. Spry, J. C.; Sawyer, W. H. Abstracts of Papers, 68th AIChE Annual Meeting, Los Angeles, CA; AIChE: New York, 1975;paper 30c. Stein, W. D. The Movement of Mokcuks Across Cell Membranes ; Academic: New York, 1967;p 112. Tamm, P. W.; Hernsberger, H. F.; Brldge. A. 0. Abstracts of Papers, 72nd AIChE Annual Meeting, San Francisco, CA; AIChE: New York, 1979; paper 23b. Thiele, E. W. Ind. Eng. Chem. 1939, 37, 916.
Received for review March 15, 1984 Revised manuscript received May 31, 1985 Accepted July 22, 1985
Silicated Aluminas Prepared from Tetraethoxysilane: Catalysts for Skeletal Isomerization of Butenes BJorrn P. Nllsen, Julia H. Onuferko, and Bruce C. Gates" Center for Cata&t/c Sclence and Technology, Department of Chemical Englneerlng, Unlverslty of Delaware, Newark. Delaware 79716
Catalysts were prepared by the condensation reaction of tetraethoxysilane with the surface of alumina and characterized by X-ray photoelectron spectroscopy to determine surface compositions and by infrared spectroscopy with adsorbed pyridine to determine surface acidity. Silicon-containing groups formed on the alumina surface at less than monolayer coverage, inducing strong acidity; both Brernsted and Lewis acid groups were detected. The catalytic activity for skeletal isomerization of 1-butene was determined in flow reactor experiments at atmospheric pressure and 450-525 'C. The reaction is firstorder In the reactant; the activity increased with increasing surface acidity, Le., with increasing Si content of the surface and apparently with increasing Br~nstedacidity. The surface is comparable in structure and catalytic activity to that of silica-alumina, and the catalytic sites are suggested to be Si-OH groups positioned next to coordinatively unsaturated AI3+ sites.
Introduction
Supported catalysts with simple, stable structures can be prepared by reaction of organometallic compounds with surface functional groups of metal oxides (Yermakov, 1983). There are excellent prospects for tailoring of catalytic properties of these materials by appropriate com0196-4313/86/1025-0337$01.50/0
binations of the precursor metal complex and the support. Industrial catalysts in this class include silica-supported complexes derived from chromocene, used for ethylene polymerization (Karol et al., 1978);related polymerization catalysts have been prepared from complexes of Cr, Zr,and Ti (Yermakov e t al., 1981). There are numerous other 0 1986 American Chemical Society
