I n d . E n g . C h e m . Res. 1990, 29, 520-521
520
Metal Deposition in Hydrotreating Catalysts. 2. Comparison with Experiment Carmo J. Pereira,* J. W. Beeckman, W.-C. Cheng, and W. Suarez W . R. Grace & Company, Research Division, 7379 Route 32, Columbia, Maryland 21044
Intraparticle metals deposition profiles in hydrotreating catalyst pellets play a n important role in determining t h e demetalation rate, t h e metals storage capacity, and t h e combined desulfurization-demetalation performance of the catalyst. Metals capacity factors for catalyst pellets having varying shapes and size and a range of pore properties, obtained from experimentally determined metals profiles of isothermally aged catalyst pellets, were found to be in good agreement with the calculated metals capacity factors using an earlier derived mathematical model. Hydrotreating catalysts of varying shapes and sizes and having a range of pore properties have been prepared and aged at constant temperature on a Lloydminster vacuum resid feedstock. Metals profiles of the aged catalysts have been determined by using electron microprobe analysis. This information has been used to obtain the metal capacity factor, OM, values for each catalyst. Feed properties, catalyst properties, and operating conditions have also been used to calculate OM for each catalyst using a previously developed mathematical model (Pereira, 1990).
Experimental Section Ten catalysts were examined in this study. Each catalyst contained approximately 13 wt 5% MOO, and 3 wt % COO. Pore properties of these catalysts were obtained by using a Micromeritics Autopore 9200 mercury porosimeter. A range of pore properties was examined: micropore volumes varied between 0.432 and 0.551 cm3/g, macropore volumes between 0.030 and 0.421 cm3/g, micropore radii between 37 and 87 A, and volume-averaged macropore radii between 1500 and 5000 A. The micropore radius was calculated as 2 X micropore volume/BET surface area. Three pellet shapes were examined: cylindrical extrudates, spheres, and Minilith-shaped pellets. The Minilith catalyst, developed in our laboratory, is in the shape of a wheel containing four spokes of outer diameter 2.54 mm. The wall thickness of both the rim and the spokes is around 0.4 mm (Pereira et al., 1988). The diffusion path length, L, for each pellet is defined as the ratio of the pellet volume to its external surface area. L is the half-thickness of a flat slab, half the radius of an infinite cylinder, and onethird the radius of a sphere. L varied between 0.020 and 0.064 cm in our study. Approximately 10 pellets of each catalyst were placed at the inlet of a fixed bed reactor having an inside diameter of 1 in. The reactor was operated in an upflow model to ensure complete wetting of the catalyst pellets and minimize liquid channeling. Lloydminster vacuum resid feed was used in all the experiments. Feed properties are shown in Table I. Reactor pressure was 2000 psig pressure, and the hydrogen circulation rate was 4000 scf/barrel. The temperature of the pellets was monitored during the course of the run. The catalysts were presulfided and aged a t reaction conditions for around 13 days. After the completion of a run, the pellets were unloaded from the reactor, and nickel and vanadium deposit profiles were determined with a Cameca Camebax electron microprobe. Metals profiles were used to calculate eM.Catalyst properties, operating conditions, and the OM value for vanadium are reported in Table 11. eMvalues varied between 0.17 and 0.86. 0888-5885/90/2629-0520$02.50/0
Table I. ProDerties of Llovdminster Vacuum Resid .4PI gravity 7.4 sulfur, wt % 4.5 vanadium, wppm 151 nickel, wppm 71 iron, wppm 5.0 total nitrogen, wt % 0.5 Conradson carbon, wt 70 16.7 21.3 pentane insolubles, wt % ASTM D1160 distillation, O F IBP (initial bp) 534 5% 720 10% 835 890 20% 30% 965 1042 40%
Results and Discussion First-order demetalation kinetics for Lloydminster vacuum resid were verified by separate experiments in an ebullating bed reactor. Typical nickel and vanadium profiles for catalysts C and G having large and small values of eM, respectively, are shown in Figure 1. At the conditions of this study, the nickel penetration profiles are similar to vanadium. For purposes of simplicity, the two metals are lumped together. Metals sulfide deposites on each of the catalyst pellets examined at the end of the run were estimated to be over 10 wt 70.Takeuchi et al. (1985) have shown that, after an accumulation of 10 wt 70vanadium on catalyst, a bare alumina support having little initial activity has essentially the same demetalation activity as a catalyst having the same pore structure; i.e., activity is primarily due to the activity of metal sulfide containing deposits. Even though the catalyst pellets are each exposed to 222 ppm metals in feed for only around 13-15 days, the experimentally determined OM’S are assumed to be near their line-out values. Catalyst properties are shown in Table 11. Macropore diameters are around 20 times the diameter of the micropores. Mercury porosimetry measurements of the spent catalysts show that the macropore volume does not decrease appreciably during the course of the run. The tortuosity, T , is taken to have a value of 2. Very little information on the size distribution of metals-bearing molecules a t reaction conditions is available in the literature. An average feed molecule size of 20 A was used in this study. The density of mixed metal sulfide deposits of 4.0 g/cm3 was used in our calculations. Shimura et al. (1986) have reported that the mean diameter for vanadium-bearing molecules in Boscan crude is 25 8, and have estimated the mixed metal sulfide deposit density at 4.26 g/cm3. The bulk diffusion coefficient for metal0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990 521 T a b l e 11. Catalyst Properties a n d Operating D a t a ~~
~
catalyst
A
B
micropore vol ( Vmo) macropore vol ( VMo) micropore radius (rm0) macropore radius (rMo) pellet density (p,) BET surface area diffusion path length ( L ) shapea
0.543 0.179 47 2500 0.969 232 0.064 C
0.473 0.247 39 2500 0.965 242 0.040 C
D E Catalyst Properties 0.457 0.511 0.510 0.030 0.081 0.157 37 74 87 1500 1.258 1.144 1.059 248 138 117 0.040 0.040 0.020 C C C
F
G
H
I
J
0.489 0.421 49 5000 0.836 200 0.053
0.432 0.402 76 5000 0.883 152 0.028
0.551 0.221 76 1500 0.936 146 0.027
S
S
S
0.492 0.197 69 1500 1.023 143 0.022 M
0.519 0.153 61 2000 1.021 170 0.022 M
average temp, O F time on stream, days eM value observed predicted
777 13.0
753 15.4
Operating Conditions 772 788 791 11.2 11.2 14.4
772 11.2
753 15.4
786 13.3
753 15.4
790 11.2
0.17 0.20
0.48 0.43
0.23 0.20
0.38 0.36
0.78 0.81
0.58 0.58
0.86 0.80
0.47 0.54
C
0.33 0.33
0.66 0.68
"C = cylinder, S = sphere, M = Minilith.
I
O B
c
/.
0.4: 0.2
1
1 I
t
0
0.1
0.2
0.3
0.4
0.6
Obrerved
0.8
0.7
0.8
0.8
1
GM
Figure 2. Comparison of observed versus calculated
OM
values.
is good agreement between model calculated and observed O M values.
Nomenclature L = diffusion path length, cm = tortuosity = metals distribution factor Registry No. MOO,, 1313-27-5; COO, 1307-96-6; nickel, 7440-02-0; vanadium, 7440-62-2. T
@M
Literature Cited Figure 1. Typical nickel and vanadium deposition profiles. Top: catalyst G. Bottom: catalyst C. (- - -) Nickel; (-) vanadium.
bearing molecules is taken to be 4.37 X lo6 cm2/s (Chantong and Massoth, 1983). As seen in Table 11, the average temperature of the catalysts varied between 750 and 791 O F . An activation energy of 48 kcal/mol was used to normalize for temperature. Activation energies of similar magnitude have been reported in the literature (Hung et al., 1986). The preexponential was adjusted so as to obtain a best fit between the observed and calculated O M values. The rate constant was estimated to be 1.58 x cm/s a t 762 O F . Kobayashi et al. (1987) have determined that the initial vanadium removal rate constant is 5.76 X lo4 cm/s at 752 O F for a Khafji feed. By use of the parameters discussed above and the properties in Table 11, the previously described model of Pereira (1990) is used to calculate O M values for each catalyst pellet. As shown in Table I1 and Figure 2, there
Chantong, A.; Massoth, F. E. Restrictive Diffusion in Aluminas. AIChE J . 1983, 29, 725. Hung, C.; Howell, R. L.; Johnson, D. R. Hydrodemetallation Catalysts. Chem. Eng. Prog. 1986, 3, 57. Kobayashi, S.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Inoue, K.; Shimizu, Y.; Egi, K. Kinetic Study on the Hydrotreating of Heavy Oil. 1. Effect of Catalyst Pellet Size in Relation to Pore Size. Ind. Eng. Chem. Res. 1987, 26, 2241. Pereira, C. J. Metal Deposition in Hydrotreating Catalysts: A Regular Perturbation Solution Approach. 1. Theory. Ind. Eng. Chem. Res. 1990, preceding paper in this issue. Pereira, C. J.; Cheng, W.-C.; Beeckman, J. W.; Suarez, W. Performance of the Minilith-A Shaped Hydrodemetallation Catalyst. Appl. Catal. 1988, 42, 47. Shimura, M.; Shiroto, Y.; Takeuchi, C. Effect of Catalyst Pore Structure on the Hydrotreating of Heavy Oil. Ind. Eng. Chem. Fundam. 1986, 25, 330. Takeuchi, C.; Asaoka, S.; Nakata, S.-I.; Shiroto, Y. Characteristics of Residue Hydrodemetailation Catalysts. Prepr. Pup.-Am. Chem. Soc., Diu. Pet. Chem. 1985,30, 96.
Received for review May 3, 1989 Revised manuscript received November 6 , 1989 Accepted December 5, 1989
