Ind. Eng. Chem. Process Des. Dev.
rap = intrinsic rate of coke formation
T = absolute temperature Greek Symbols /3 = catalyst loading
variance of experimental error lower limit on uE2 = upper limit on uE2 f:: = variance of the estimate of the rate parameter Registry No. Carbon, 7440-44-0; molybdenum, 1439-98-7; nickel, 7440-02-0. Literature Cited u E ~ ~ =
uEJ2 =
Appleby, W. 0.; Gibson, J. W.; Good, 0. M. Ind. Eng. Chem. Process Des. Dev. 1882. 1 , 102. Beuther, H.; Lareon, 0. A.; Perrotta, A. J. “Proceedlngs, Internatlonai Symposium on Catalyst Deacthratlon”; Delmon, 8.; Froment, 0. F., Ed.; Elsevier Scientific Publlshlng Company: New York, 1980, p 271. Brooks, D. G.; Guln. J. A,; Curtis, C. W.; Tarrer, A. R. Paper presented at 74th Annual AIChE Meeting, New Orleans, LA, Nov 1981. Coreila. J.; Asua. J. M. Ind. Eng. Chem. Process Des. Dev. 1882, 21, 55. Curtis. C. W.: Guln. J. A.: Tarrer. A. R.: Huana, W. J. Fuel Proc. Techno/. 1883, 7, 277. Furlmsky, E . Ind. Eng. Chem. Prod. Res. Dev. 1870, 17, 329. Gertenbach, D. D.;Baldwln. R. M.; Bain, R. L. Ind. Eng. Chem. Process Des. Dev., 1882, 21, 490. Khang, S. J.; Levenspiel, 0. Ind. Eng. Chem. Fundam. 1873, 12, 185.
1983,22, 653-659
853
Mltcheii, T. 0. CoalProcess. Techno/. 1880, 6 , 28. Mohan, G.; Sllla, H. Ind. Eng. Chem. Process Des. Dev. 1881, 20, 349. Nace, D. M.; Voltz, S. E.; Weekman, V. W. I n d . Eng. Chem. Process D e s . Dev. 1871. 10, 530. OcamDo. A.; Schrodt, J. J.; Kovach, S. M. Ind. Eng. Chem. Prod. Res. Dev.1878, 17, 58. O’Leaty, J. R.; Rappe, G. C. Chem. Eng. Prog. 1881, 7 7 , 67. Pyun, C. W. J. Chem. Educ. 1871, 48, 194. Ruderhausen, C. 0.; Watson, C. C. Chem. Eng. Sci. 1854, 3 , 110. Seinfeld, J. H.; Gavalas, 0. R. AIChE J. 1870, 16, 644. Seinfeld, J. H.; Lapldus, L. “Mathematlcal Methods in Chemical Engineering”; Prentice Hall: Englewood Cilffs, NJ, 1974; Voi. 111, p 387. Shalabi, M. A.; Baldwln, R. M.; Baln, R. L.; Gary, J. H.; Golden, J. D. Ind. Eng. Chem. ProcessDes. Dev. 1870. 18, 474. Sie, S. T. “Proceedlngs, International Symposlum on Catalyst Deactivation”; Deimon, B.; Froment, G. F., Ed., Elsevler Sclentiflc Publlshlng Company: New York, 1980 p 557. Stiegei, 0. J.; Poiinski, L. M.; Tischer, R. E. I n d . Eng. Chem. Process D e s . Dev. 1882, 21, 477. Weekman, V. W. Ind. Eng. Chem. Process Des. Dev. 1888, 7 , 90.
Received for review July 6 , 1982 Revised manuscript received January 31, 1983 Accepted February 24, 1983
The authors are grateful to the U.S. Department of Energy under Contract No. DEFG2280PC30209,and to the Cities Service Research and Development Company for support of this work.
Effect of Catalyst Properties and Operating Conditions on Hydroprocessing High Metals Feeds Jose M. Pazos, Jose C. Gonzalez, and Armando J. Salazar-Gulllen Intevep S A . . Centro de Investlgacion y Desatrollo de Petroleos de Venezuela S.A., Apdo. 76343, Caracas 1070, Venezuela
+
Catalytic hydroprocessing of high metals heavy oils, containing over 480 ppm Ni V, was carried out in trickle bed pilot units. The analyses of the used catalysts (coke, metals content, and vanadium distribution) were correlated with the deactivation runs. The deactivation by coke is very much dependent on the catalyst physical properties (mean pore diameter), rather than on the chemical properties, and on the nature of the feed. As metals removal is a diffusion-controlled reaction, catalysts and operating conditions that increase the Thiele modulus, e.g., high activii and small pore catalysts, high hydrogen pressures and temperatures, show a stronger deactivation by feed metals. In this case, most of the vanadium was deposited in the outer edge of the catalyst particle. Unconventional vanadium proflles along the reactor length were obtained under certain conditions. Based on these data, a kinetic model was proposed which considers that demetalllzation is a complex reaction that occurs through a series of consecutive and parallel reactions.
Introduction The petroleum industry is faced with the problem of processing heavier feeds. Great research effort is being dedicated to the upgrading of petroleum resids and heavy oils to more valuable products, such as gasoline and distillates. Huge reserves of heavy oils are present in Canada and Venezuela. Heavy oil feeds are characterized by having a high concentration of impurities, such as sulfur, nitrogen, nickel, and vanadium, and a low hydrogen to carbon ratio. Catalytic hydroprocessing is being considered by several oil companies as the first upgrading step to remove the impurities to certain levels so that the product can be efficiently processed in downstream conversion units (coking, catalytic cracking, hydrocraking). Some conversion can also be obtained in the hydroprocessing units, depending on the operating conditions, catalyst type, etc. Useful information in the development of technology for hydroprocessing heavy feeds has been obtained from the existing 0196-4305/83/1122-0653$01.50/0
commercial processes of resid desulfurization. The high metal concentration and coking tendency of heavy oils reduce considerably the life of hydroprocessing catalysts. These catalysts show, in general, a period of rapid deactivation, followed by a more gradual activity decline, and finally an accelerated aging (Sie, 1980). The period of initial deactivation has been traditionally attributed to coke deposition on the catalyst (Beuther and Schmid, 1963; Chang and Silvestri, 1976). Tamm et al. (1981) proposed that the initial deactivation could be caused by the partial surface poisoning by the feed metals. The fact that most of the coke is deposited during the first 24 h of operation, while the initial deactivation lasts longer, may support this theory (Pazos et al., 1981; Gajardo et al., 1982). Catalyst life is mainly determined by the intermediate deactivation, generally attributed to gradual pore plugging by the feed metals (Montagna et al., 1975; Sie, 1980). The final deactivation is caused by complete physical ob0 1983 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983
Table I. Properties of the Feedstocks feed Iranian heavy North Slope atm. resid vac. resid API gravity 15.6 "C/15.6 "C specific gravity 15.6 "C/15.6 "C sulfur, wt % nickel, wt ppm vanadium, wt ppm nitrogen, wt ppm Conradson carbon, w t % C,-asphaltenes, wt % a
14.7 0.9679 2.76 50 176 3960 10.2 3.3
Lagomedio atm. resid
8.2 1.0129 2.1 4.4 9.0 6150 17.5 4.6
18.6 0.9426 1.9 21 181 2100 7.2
_-
Job0 crude
Bachaquero crude
BoscsCn crudea
9.3 1.0050 2.75 86 334 4000 19.4 11.3
13.0 0.9792 2.6 53 430 3775 10.3 9.9
13.6 0.9752 4.1 70 790 4500 9.5 8.3
Diluted with 34 wt % of vacuum gas oil.
Table 11. Properties of the Catalysts catalyst
MOO,, wt % c o o , wt % NiO, wt % nominal diameter, mm pore volume, surface area, mz/g mean diameter at 50% pore volume, A a
Across two lobes.
A
B
C
D
E
F
15.5 5.5 0 1.5 0.66 285 63
19.0 4.4 0 1.5 0.44 188 104
12.2 4.2 0 1.5= 0.80 269 118
15.0 0 3.5 1.5 1.07 300 72
10.2 2.3 0 1.5 0.55 163 104
6.7 2.0 0 1.5a 0.80 269 118
io4
Bimodal catalyst.
struction of the pores (Tamm et al., 1981). Moreover, as the catalyst temperature is raised to maintain a constant conversion in the commercial operation, the increasing coke deposition at the critical higher temperatures can make an important contribution to the final aging at the end of the cycle. The design of catalysts with low coking tendencies and, at the same time, high capacity to accumulate metals is a major need to hydroprocessing heavy feeds. Montagna et al. (1975) and Riley (1978) stated that the coking tendency is controlled by the intrinsic surface properties, rather than by the pore size distribution of the catalyst. On the other hand, Beuther et al. (1980) and Nakamura et al. (1980) observed that for residual oils the coke deposit increases with the pore diameter of the catalyst. However, catalysts with high pore diameters are required to allow metal containing molecules to penetrate into the catalyst structure (Richardson and Alley, 1975; Riley, 1978) and to obtain meaningful catalyst lives (Nakamura et al., 1980; Krasuk, 1980). It has been reported (Hensley and Quick, 1980) that catalysts with high hydrogenation activity exhibit higher deactivation rates by feed metals, because the reaction becomes more diffusion controlled. Operating conditions must be selected accordingly to diminish both deactivation by coke and metals. It is well known that high temperatures increase the coke deposition on the catalyst (Beuther and Schmid, 1963; Inoguchi et al., 1971), while increasing hydrogen pressure reduces the coke concentration (Sie, 1980). However, high hydrogen pressures can decrease catalyst life, because metals are deposited towards the edge of the catalyst, accelerating the pore plugging mechanism (Tamm et al., 1981; Nielsen et al., 1981). The same effect occurs at high reaction temperatures (Tamm et al., 1981). The published literature shows that catalysts and operating conditions that reduce deactivation by coke may not be the most suitable to the hydroprocessing of metal containing feeds. Furthermore, most of the published deactivation studies have been carried out with oil resids of less than 200 ppm Ni + V, and, generally, less than 100 PPm.
This paper presents pilot plant hydroprocessing runs carried out with high metal feeds of over 480 ppm Ni V. It is important to know if the deactivation mechanisms already proposed are also valid for these kinds of feeds. Catalysts of different physical and chemical properties, as well as different operating conditions, were used. The analyses of the spent catalysts were correlated with the deactivation runs, the final aim being to provide some useful criteria for selecting the best operating conditions and catalysts when hydroprocessing high metal feeds.
+
Experimental Section Table I shows the main properties of several international heavy feeds. Venezuelan feeds always show the highest concentration of metals, particularly Ni + V, even resids from conventional crudes such as Lagomedio. The feeds used in thiswork were Bachaquero crude and Bmc6n crude diluted with 34 w t % of vacuum gas oil. Both feeds have similar API gravities, Conradson Carbon and asphaltenes contents, but different sulfur and Ni + V concentrations. Several catalysts of different active metal concentration and pore size distribution were considered. Table I1 shows the most significant properties of the catalysts. Catalysts B and E have the same mean pore diameter at 50% pore volume, but they have been impregnated on different y-alumina supports. Catalysts C and F have been prepared on the same support. The pore diameter at 50% pore volume is determined from the actual pore size distribution of the catalyst by reading the pore diameter corresponding to 50% of the pore volume. The experiments were carried out in a fixed bed downflow pilot unit. The oil feed and hydrogen were premixed before entering the reador. The light gases were separated from the liquid product in a high-pressure separator. The liquid, after being stripped with nitrogen, was analyzed. The catalyst was diluted with carborundum, nominal size 28-48 mesh, in a ratio of 3:2 (yol) diluent/catalyst to avoid flowdynamic problems (Garcia and Pazos, 1982). Before each run was started, the catalyst was sulfided with an H,S/H, mixture (1:lO N vol/vol) at atmospheric pressure,
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983 655 Table 111. Average Coke and Vanadium Concentration on Used Catalysts with Bachaquero Feed catalyst
B
A 1.0 13.6
relative coke, wt %" vanadium, w t %
1.9 1.2 12.8 14.8 Based on the fresh catalyst.
Based on t h e alumina support.
D
E
2.3 6.9
1.0 19.0
C
F 1.4 2.7
1.9 8.7
HDV 0
-
/-
-20
i
o
CLIlL"S1
HOS
A
Y 0 1
02
oa
0 4
03
mEiirivf
06
07
08
09
0
IO
riii
Figure 1. Temperature increase require to maintain a constant vanadium in the product for Bachaquero feedstock. Operatin conditions: hydrogen pressure = lo00 psig; LHSV = 2 vol vol" h-
P.
from 180 "C up to 350 "C at 50 OC/h (1 h at 350 "C). All the runs were carried out at hydrogen pressures between 1000 and 1700 psig, and at a constant space velocity of 2 vol vol-' h-' in an isothermal reactor. The runs were made either at a constant temperature or a t three temperature levels. The activity results are expressed by the relative rate constant, when the experiments were made at a constant reaction temperature, or by the temperature increase required to keep a constant product specification, in the case of runs carried out at three temperature levels. Liquid samples were analyzed for sulfur (ASTM D1552) and vanadium (neutron activation analysis). The metal removal was followed by analyzing vanadium, as this is the largest metals component in the feeds studied (Table I). The used catalysts were separated into four to six layers along the reactor length, washed with xylene to extract the oil remaining in the catalyst pores, and analyzed for carbon (Leco WR12 apparatus) and vanadium (atomic absorption). Selected catalyst particles were also examined in a scanning electron microscope, provided with an X-ray system of dispersive energy, for vanadium distribution across the particle. Results Effect of Catalyst Properties on Deactivation. Several aging runs were carried out with catalysts of different pore size distributions and concentrations of active metals in order to study their effect on the decay of hydrodesulfurization (HDS) and vanadium removal (HDV) activities. The feedstock used was Bachaquero crude. The operating conditions selected were rather severe (hydrogen pressure of 1000 psig and high temperature) so that accelerated depositions of coke and metals occurred on the catalyst. The duration of the runs was adjusted to obtain significant levels of metal deposits on the catalyst, usually over 15 wt 9o Ni + V, based on the fresh catalyst. Figure 1shows the deactivation curves for catalysts A, B, C, and D for the HDV reaction. The experiments were performed at exactly the same operating conditions for all the catalysts. Strong initial deactivations can be observed for all catalysts, in particular for the larger pore C and D. Catalyst life is probably limited to this initial period from commercial considerations; however, the duration of the
3
10
20
I5
25
x)
V-T h,1'2 Figure 2. Relationship between initial deactivation and coke deposition for catalyst F and Bachaquero feedstock. Operating conditions: hydrogen pressure = lo00 psig; LHSV = 2 vol vol-' h-l; reactor temperature = 420 OC. loo
T
-
CATlLYST B CATALYST E
f z
+
0
.
- 20 0
01
02
03
04
03
RELlTlVF
OS
01
011
09
TIME
Figure 3. Temperature increase required to keep a constant sulfur in the product for Bachaquero feedstock. Operating conditions: hydrogen pressure = 1000 psig; LHSV = 2 vol vel-' h-l.
run for catalyst C was extended to observe the deactivation profiles at higher metal depositions on the catalyst. The initial deactivation is followed by a long period of smoother deactivation, especially for catalyst C. The smaller pore catalysts A and B show a second strong deactivation, when over 15 wt % vanadium was accumulated on the catalyst. Figure 2 correlates the HDS and HDV relative activities with the coke content of the catalyst (Voorhies, 1945). The experiment was made with catalyst F and Bachaquero feed, at a constant hydrogen pressure of 1000 psig and a constant temperature of 420 "C. The good correlation obtained for both HDS and HDV activities suggests that the initial deactivation seems to be due to the coke laydown on the catalyst, although more experimental data are needed. The average amount of vanadium, deposited after 200 h on stream, was as high as 8.7 wt %, based on the fresh catalyst. Figure 3 compares the HDS deactivation of catalysts B and E. Similar behavior was obtained for the HDV reaction. These two catalysts are characterized by having the same mean pore diameter (104 A) but different concentrations of active metals. Although the high metals catalyst B exhibits a higher initial activity as expected, it
050
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983 CATALYST D
I:o.s8
t
IO,
; $
z0, 0
/,
,
, 011
0 2
FRACTIOUAL
06
08
1
m,
061
1
Y
$
041
1
m]
10
I
L
RADIUS
Figure 4. Distribution of vanadium on used catalyst A (d, = 63 A) and catalyst D (dp = 70 8, and lo' A); d, is the mean pore diameter at 50% of pore volume.
0
2
6
4
8
10
12
ib
I6
4
~YLRIGL
ciriiisr
v i i 4 0 i u ~ox F R E S H
22
20
IO
!.t%i
Figure 6. Effect of hydrogen pressure on catalyst deactivation at 415 O C (Boscan feedstock, catalyst E).
CATALYST E
/ I:0.82
11;:
HIGH PRESSURE
L1700~~~~l
0-032
LW PRESSURELI~WPSIQJ 0.057
Y
r Y i -
I
0
02
Q4
06
08
10
FRACTIONAL R I D l U S
02
0
Figure 5. Distribution of vanadium on used catalyst B (23.4% COO + MoOJ and catalyst E (12.5% COO + Moo3).
shows a higher deactivation rate than catalyst E. Table I11 shows the relative average coke and vanadium content of the catalysts used in the deactivation runs (Figures 1 and 3) and catalyst F (Figure 2) after 45 and 200 h on stream. It can be seen that the amount of coke laydown increases with the mean pore diameter of the catalyst. Also, catalysts with higher concentrations of hydrogenating metals have no reduction in the coke deposition (compare catalysts B vs. E, and catalyst C vs. F). Representative samples of the catalysts were also analyzed by scanning electron microscopy to study the vanadium distribution along the cross section of the catalyst. The extent of the use of the catalyst surface area is well characterized by the parameter 4, defined by T a " et al. (1981) as
i l V ( r ) r dr (1)
where V(r)is the output voltage proportional to the radial vanadium concentration, Vm, is the maximum voltage along the cross section, and r is the fractional radius. Figures 4 and 5 show the radial vanadium distribution for catalysts A (dp = 63 A) and D (bimodal d, = 70 and lo4 A), and for catalysts B (23.4% COO + Moo3) and E (12.5% COO+ Moo3),respectively, after being used in the deactivation runs showed in Figures 1 and 3. The vanadium concentration is defined for each catalyst as the ratio of the radial concentration to the maximum concentration. The poor vanadium distribution and the low 4 parameter for the small pore diameter catalyst A (Figure 4) and for the high active metal loading catalyst B (Figure 5), suggest a diffusion-controlled reaction. It can also be observed that the maximum vanadium concentration for these catalysts is not in the outer edge, as would be expected for simple pore diffusion controlled reactions. Influence of Operating Variables on Deactivation. The effect of the two main process variables, hydrogen pressure and reactor temperature, on catalyst deactivation was studied by using a diluted Boscln crude. Catalyst E was chosen for these experiments. Figure 6 shows the variation of the relative vanadium rate constant with the
I O
08
04
06
FRACTIONAL
RADIUS
Figure 7. Distribution of vanadium on catalyst E, used with Boscan feedstock at different reaction pressures. 10,
:
I
5
061
0
1.415'C
A
T:395'C
u1
;
0 4 -
Y
02
I
i
0
2
4
6
8
10
12
l V E R 4 G E VLNAOIUM OW FRESH C L T l L Y S T
14
16
18
lwI%l
Figure 8. Effect of reaction temperature on catalyst deactivation at 1700 psig hydrogen pressure (Boscan feedstock, catalyst E).
vanadium accumulated on the catalyst, at hydrogen pressures of 1200 and 1700 psig, at a constant temperature of 415 "C. Similar behavior is observed for the first 8 w t % vanadium deposited on the catalyst, although a stronger deactivation was expected at the more severe condition (1200 psig) from the results obtained with the Bachaquero feed. The run at higher hydrogen pressure shows a very strong deactivation after ca. 10 wt % vanadium was deposited on the catalyst, while the run at 1200 psig exhibits a smoother deactivation and a much longer catalyst life. Figure 7 shows the vanadium distribution for catalyst E used in the deactivation run (Figure 6). At high pressures, the vanadium is readily deposited on the outer edge of the catalyst (4 = 0.32); at low pressures, the metalbearing molecules are able to penetrate more into the catalyst particle and a more homogeneous distribution is obtained (4 = 0.57). Figure 8 shows the effect of reactor temperature on HDS relative activity at 395 and 415 "C, at a constant hydrogen pressure of 1700 psig. Similar behavior was observed for HDV conversion. Although the activity is higher at 415 "C, the catalyst shows very strong deactivation and is practically dead after 17.5 wt YO vanadium has accumulated on it. The run at 395 "C has lower activity but a much longer catalyst life. Figure 9 shows the vanadium distribution for the used catalysts. It can be observed that the vanadium molecules are able to diffuse better into the
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983 657
0.2
0
0.4 FRACTIOIAL
0.6
0.8
1.0
RAOIUS
Figure 9. Distribution of vanadium on catalyst E, used with Boecan feedstock at different reactions temperatures. 1
2b
0 VI c
. 2
02
08
06
Ob
10
FRACTION OF R E A C T O R L E N G T H
A
Figure 11. Effect of operating conditions on vanadium profiles for Boscan feedstock and catalyst E.
16
VI z
12 0
f
5 >
I
0
02
Ob
06
08
k-7
1
ZOO
1
FRACTION OF R E A C T O R LENGTH 4 >
Figure 10. Effect of the active metal content on the catalyst on vanadium profile for Bachaquero feedstock (catalyst B has 23.4% COO + MOO, and catalyst E 12.5% COO + MOO,.
u I
= VI
Y
U z
Table IV. Coke and Vanadium Deposition o n Catalyst E for Different Feeds feed operating conditions hours o n stream temperature, "C LHSV, vol v01-l h-' H,/feed, N m3/m3 hydrogen pressure, psig average carbon, wt %a average vanadium, wt % a Based o n the support.
Bachaquero
b
.
6
2 2
Boscdn
170 200 395 395 2 2 800 800 1200 1100 1500 14.7 12.5 9.3 5.8 20.5 17.5 Based o n the fresh catalyst.
0
02
04
06
06
i 10
F R A C T I O N OF R E A C T O R L E N G T H
Figure 12. Effect of hours on stream (HOS) on vanadium profiles for Bachaquero feed and catalyst F.
281
internal surface of the catalyst at the lower reaction temperature (4 = 0.39 at 395 OC) than at 415 O C (4 = 0.32). Table IV compares the coke and vanadium concentration on catalyst E used with Bachaquero and Bosch feeds at similar operating conditions. Bachaquero feed always gives a higher coke deposition on the catalyst and a lower vanadium concentration. Also,it can be observed that coke laydown is significantly reduced at higher hydrogen pressures. Vanadium Profiles along the Reactor. In catalytic hydrodemetalization the metals removed from the feedstock are deposited along the length of the catalyst bed. The vanadium profiles obtained with different catalysts, operating conditions, and feedstocks have been studied with particular interest. Figure 10 shows the vanadium distribution along the reactor length for catalysts B and E, after being used to hydrotreat Bachaquero crude (see Figure 3). These catalysts have similar mean pore diameters but different active metal concentrations (see Table 11). It is interesting to observe that, while the high intrinsic activity catalyst B exhibits the typical decreasing vanadium profile (Sie, 1980),catalyst E shows a rather unconventional distribution, where the vanadium concentration increases along the reactor. Figure 11 shows the vanadium profile for catalyst E, after being used with Bosch feed at high ac-
nos
I
\
d\
I A \
..v 1 0
I I 02 FRACTION
0.6 OS OF R E A C T O R L E N G T H
0b
10
Figure 13. Effect of the feedstock on vanadium profiles for catalyst
E.
tivity conditions (1700psig and 415 "C)and at low activity conditions (1200 psig and 395 " C ) . A maximum in the vanadium concentration is observed around 0.4 fractional reactor length for the low activity condition. The conventional decreasing profile is observed for the high activity condition. Figure 12 shows that the maximum in the vanadium concentration observed in the fiit part of the reactor, after 45 h on stream and ca. 3 wt % vanadium on the fresh
658
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983
catalyst, is displaced toward the reactor outlet after 200 h on stream. Figure 13 shows that vanadium profies along the reactor could change dramatically when using different feeds, for the same catalyst and similar operating conditions.
Discussion Deactivation by Coke. The initial deactivation observed for HDS and HDV reactions, under the severe operating conditions used in this work, is attributed to the coke deposition on the catalyst. At these conditions, the coke concentration is mainly controlled by the catalyst pore structure rather than by the amount of active metals or by the chemical properties of the surface. Thus, it can be said that a great part of coke is formed from the adsorption of highly polyaromatic molecules, which are degraded to coke. These big molecules are allowed to enter into the larger pore catalysts and, in consequence, the period of initial deactivation is longer for macroporous catalysts, as shown in Figure 1. The nature of the feedstock is an important parameter in the coke formation on the catalyst. Bachaquero feed always produces a much higher coke concentration on catalyst than Bosch (see Table IV). It is interesting to note that these two feeds have similar asphaltene and Conradson carbon content (Table I). Thus, physical properties of the feed, such as asphaltenes and Conradson carbon, are not recommended for predicting the deactivation of hydroprocessing catalysts by coke. Deactivation by Metals. It is reasonable to think that metal concentrations, as high as 8 wt % (see previous section), deposited on the catalyst during the initial period of deactivation, should have some poisoning effect on the active sites of the original (Tamm et al. 1981). Whether the metals stay adsorbed on the active sites of the catalyst (poison effect) or migrate to the free alumina support (continuous regeneration of active sites) is not clear. It must also be considered that Ni + V deposits exhibit some activity (Sie, 1980). In any case, the possible effect of the metal deposits on the initial deactivation seems to be overlapped by coke deactivation for catalysts used to process heavy feeds, at least under the operating conditions used in this work. The characteristic deactivation of hydroprocessing catalysts by feed metals is the consecutive plugging of catalyst pores. This is due to the low diffusion rate of the metal bearing molecules, as compared with their chemical reaction rate. In the particular case of metals from heavy oil feeds, this fact seems to be a consequence of the big volume more than the high reactivity of the metal containing molecules. Therefore, catalysts and operating conditions should be selected for each particular feed in accordance with the kinetic criteria, based on the ratio between the chemical reaction rate and the diffusion rate or the Thiele modulus. Larger pore catalysts show higher diffusion rates, and metal deposits are homogeneously distributed; on the other hand, metals concentrate in the edge of the particle for smaller pore catalysts (Figure 3). In consequence, the deactivation by metals pore plugging is lower for larger pore catalysts (Figure 1). Catalysts with high intrinsic activity (high active metal concentration) increase the ratio of chemical reaction to diffusion rate; hence, metals are readily deposited on the outer edge of the catalyst (Figure 5 ) , causing a rapid deactivation (Figure 3). In addition, catalysts used at operating conditions which increase the Thiele modulus, such as high temperatures and high pressures, also result in rapid deactivation rates (Figures 6, 7, 8, and 9).
Bachaquero feed shows a lower vanadium reactivity than Bosch, at similar operating conditions (Table IV). According to Dean and Whitehead (1963), 14% of the metals in Bachaquero crude exists as porphyrins, while the concentration of metal porphyrins for a Bosch crude is 25%. This suggests that the metals, which are present in welldefined structures, e.g., porphyrin type molecules, are more reactive than nonporphyrin metals, in agreement with the results reported by Silbernagel and Riley (1980). From the theory, it is expected that metals present in highly reactive molecules, should give a poor metal distribution on the catalyst (higher values of the Thiele modulus). In fact, an even vanadium distribution was obtained for Bachaquero (Figure 5, catalyst E), while metals from Bosciin were deposited mainly in the edge of the catalyst particle (Figure 7). Comments on the Mechanism of Vanadium Removal. The nonconventional vanadium profiles along the reactor obtained in this work (Figures 10 to 13) cannot be easily interpreted by the kinetic theory of first- or second-order reactions. Also, the fact that the maximum in the vanadium concentration is not in the outer edge of the catalyst particle for small pore catalysts, corroborates that HDV is not a simple pore diffusion controlled reaction. Tamm et al. (1981) proposed that HDM could be limited by H2S concentration at the outer edge of the particle, especially at the top of the reactor. I t was assumed that H2S was necessary for the HDM reaction, and it comes from the HDS reaction. Therefore, the maximum vanadium rate connot occur in the outer particle edge, according to these authors. However, this hypothesis cannot explain the increasing vanadium profiles along the reactor. H$ concentrations along the reactor length were evaluated for these experiments. These are all in excess of that required to form metal sulfides and similar for conditions which show decreasing vanadium profiles to those that exhibit increasing vanadium concentrations. Because of the broad range and complexity of the metal molecules present in the oil, it seems reasonable to think that HDM cannot be expressed as a simple reaction. Rankel (1981) in a study of the reactions of metalloporphyrins, detected hydrogenated intermediate porphyrins. This suggests that HDM could be expressed by a consecutive reaction mechanism. In the case of a plug flow reactor, the vanadium deposits along the catalytic bed are proportional to the concentration of the intermediate species. From the theory of consecutive reactions, the position of the maximum concentration of the intermediate, and hence the vanadium deposits, depends on the rate constants of the steps involved (Levenspiel, 1972): the maximum vanadium concentration is near the top of the reactor under conditions that increase the reaction rate, but the vanadium deposits would increase along the catalytic bed under conditions of low reactivity. Figures 10 to 13 seem to support this hypothesis: the use of active catalysts (Figure lo), operating conditions that enhance the HDV (Figure 11)and reactive feedstocks (Figure 13) move the maximum vanadium concentration toward the top of the bed. In the same way, at the start of run conditions when the catalyst is very active, the maximum vanadium deposition occurs near the top (Figure 12). Accordmg to the hypothesis of consecutive reactions, the concentration of vanadium deposits at the top of the reactor must be nil. Although for practical reasons it was very difficult to analyze the vanadium concentration at the very top of the reactor, the results obtained seem to indicate that there are vanadium deposits at the top of the reactor. This could be explained by assuming that certain
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983 659
Table V. Relationship between the Rate Constants of the Demetalization Model Figure 11 Figure 13 Figure 10 Figure 12‘ severe mild Bachaquero Boscdn exptl ref cat. B cat. E conditions conditions 45 HOS 200 HOS feed feed ~
k,lk, kJk3
36 0.6
35 1.9
0.8 0.08
330 4
170 3.2
37 1.6
35 1.9
0.9 0.01
HOS: hours on stream.
vanadium species can react directly to deposit on the catalyst. Thus, HDV can be expressed by a series of consecutive and parallel reactions “1
The concentration of vanadium deposits (V,) is expressed by the following equations.
(
V,j = V* K 3 exp(-(Kl
+ K3)X/LHSV) +
exp(-K2X / LHSV))
)
(2)
where
v* =
V,~LHSV~,10-4 Pc
(3)
The individual ki constants were estimated from the different experimental results shown in Figures 10 to 13 by using a nonlinear regression technique. Table V shows the relationship between the k values of the proposed model for the experimental results reported in this work. The high values obtained for k l / k 3 ,in nearly all the cases, indicate that most of the vanadium reacts through the formation of the intermediate VI. However, the contribution of the direct route becomes mare important when the ratio k l / k 3 and/or k 2 / k 3 is lower than unity. The unconventional vanadium profiles occur when both k l / k3 and k 2 / k 3are higher than unity. The lines of Figures 10 to 13 show the vanadium profiles along the reactor, as predicted by eq 2. There is a close agreement between these curves and the experimental points. However, there is need for further experiments to prove the proposed mechanism of the catalytic vanadium removal. Conclusions Catalytic hydroprocessing of high metals feeds, containing over 480 ppm Ni + V, is a feasible process, but operating conditions and catalyst physical and chemical properties must be carefully selected for each particular feed in order to reduce deactivation by coke and metals. Coke deactivation is dependent on the catalyst physical properties (mean pore diameter), rather than the chemical properties, and on the nature of the feed. Catalysts and operating conditions, which favor a diffusion controlled reaction, such as small pore and high activity catalysts, high operating temperatures, and pressures, exhibit stronger deactivations by feed metals. It is proposed that the HDM reaction occurs through a series of consecutive and parallel reactions, the con-
trolling step being a function of the particular operating conditions, catalyst, and feedstock. Acknowledgment Permission to publish this paper has been given by Petrdleos de Venezuela S.A. The authors acknowledge the contribution of the Analytical Department of Intevep S.A. Nomenclature ki = kinetic constant, h-’ LHSV = liquid hourly space velocity, vol vol-I h-’ r = fractional particle radius t = time on stream, h Vo = vanadium concentration in the crude oil, ppm V* = parameter of the eq 2 (see eq 3) V , = vanadium deposited on the fresh catalyst, wt % V,, = output voltage proportional to the maximum vanadium concentration, V V(r) = output voltage proportional to the radial vanadium concentration, V X = fractional reactor length Greek Letters = catalyst bulk density, g/cm3 pL = oil density, g/cm3 4 = vanadium distribution parameter (see eq 1) Registry No. Ni,7440-02-0;V,7440-62-2; C,7440-44-0; Mo, 7439-98-7; GO,7440-48-4. pc
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Received for review August 2,1982 Accepted February 7, 1983
