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Catalytic Hydrodemetalation of Heavy Oils
D.L. Trimm et al. (Editors), Catalysts in Petroleum Refining I989 0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
363
CATALYTIC HYDRODEMETALATION OF HEAVY OILS A. G. BRIDGE Chevron Research Company, 576 Standard Avenue, Richmond, California
ABSTRACT In the past two decades, residuum hydrotreating has become a very important refining process. Designed originally to reduce the sulfur in heavy fuel oils, it has also been used in a variety of schemes for converting heavy o i l s into more valuable transportation fuels. Chevron Research Company offers for 1 icense the Chevron Residuum Hydrotreating Process which contains the best features of the processes developed independently by the Chevron and Gulf research organizations before their two parent companies merged in 1985. Since residuum feedstocks contain organometallic impurities, studies of the hydrodemetalation reaction were instrumental in the successful development of the catalysts used in this process. This reaction has the unique feature that it is a heterogeneous catalytic rection in which the organometallic reactant leaves an easily identifiable deposit on the catalyst surface at the precise place where the reaction occurred. Chevron Research has developed catalysts and processes which remove the corrmon trace metals present in crude oil residua--nickel, vanadium, and iron. Demetalation kinetics have been measured over a variety o f catalysts in order to generate more effective catalysts. Spent catalysts have been analyzed by microprobe techniques to try to understand the long-range effects of metals Pore deposition on reactor pressure drop buildup and catalyst deactivation. diffusion theory has been used to help correlate the data and improve the scale-up operation. This paper reviews these developments and points out the importance of catalyst porosity, size, and shape on catalyst activity and life. Factors which can contribute to a pressure drop buildup in a fixed bed reactor are also covered.
INTRODUCTION The conversion of heavy residual oils into lighter fuel oils and transportation fuels is a major technological challenge for today's refiners. This has been brought about by (a) changes in crude oil availability, particularly a shift toward heavier crudes; (b) a decrease in demand for heavy fuel oil with an increase in demand for transportation fuels; and (c) a variety o f environmental considerations. Both here in Kuwait as in other countries direct residuum hydrotreating is being used as a first step in residuum conversion schemes (2). The Chevron Residuum Hydrotreating Process. developed originally to produce low sulfur fuel
364
oil from high sulfur crudes, has been combined with hydrocracking fluid catalytic cracking and coking to effectively convert residuum into more valuable lighter products. Another commercially proven residuum conversion scheme i s solvent deasphalting of the residuum followed by hydrocracking of the deasphalted oil (DAO). The Chevron Isocracking Process has been used in this service since 1967. These hydroprocesses employ porous solid Catalysts which must be resistant to the soluble nickel, vanadium, and iron compounds present in most residual oils. Hydrodemetalation of these compounds leaves a deposit of mixed metal sulfides on and in the catalyst. The buildup of these metal deposits is irreversible and largely determines catalyst life. The seriousness of this problem can be illustrated by considering the volume of deposit that can accumulate in a day. If a unit has to handle 30,000 barrels per day o f feedstock from which 100 ppm of vanadium contaminant is removed, then each day about 8 ft3 of vanadium sulfide will deposit in the reactor system. The success of the process development and design in residuum hydrotreating is dependent on an accurate prediction of where the metal sulfides deposit and what effect the deposits have on catalyst performance and system pressure drop. Early workers recognized that (a) the demetalation reaction rate was limited by the rate of diffusion of the reactant through the liquid-filled catalyst pores and (b) that the reaction was autocatalytic resulting in very high local concentrations of deposited metal sulfides. This was shown by electron microprobe analysis of spent catalysts (12, 23, 24, 25, 27, 28). Attempts to predict the effect of metals deposition on catalyst life have included correlation of the experimental data with the help of pore diffusion theory. These efforts have only been partially successful because of t h e complexity of the system and the unavailability of high quality experimental data. This paper reviews Chevron's early experience in this field. It includes a di scus si on on the factors which i nf 1 uence hydrodemetal at i on catalyst act i v i ty , catalyst fouling and pressure drop through the fixed bed reactor. EXPERIMENTAL METHODS Petroleum residua from different sources display a wide range in the amounts of organometallic impurities. Nickel, vanadium, and iron are most comnonly in the highest concentrations and, therefore, of greatest importance.
365
The org anome t a l l i c s a r e found i n t h e asphaltene and r e s i n f r a c t i o n s ( 1 7 , 22). They are present as very l a r g e molecules, a t l e a s t a p o r t i o n of which have been i d e n t i f i e d as p o r p h y r i n s . Residua from s everal d i f f e r e n t crude sources were T h e i r concent rat ions o f organoemployed i n t h e experiments t o be described. m e t a l l i c i m p u r i t i e s span a r e l a t i v e l y wide range as shown i n Table I . ~
Arabian Light
Arabian Heavy
57OoF+
650"F+
Gravity 'API
17.6
14.0
16.9
12.6
Sulfur, Wt %
3.0
4.3
2.5
5.6
Feedstock
650"F+
Iranian Heavy
Alaskan North Slope 650°F
Boscan (Venezuelan) Crude
~
1250
1900
3300
5400
29
83
136
1270
Nickel, ppm
8
25
4%
105
Iron, ppm
3
15
6
10
Asphaltenes, Wt %
4.3
10.4
5.5
15.7
Nitrogen, ppm Vanadium, ppm
Table I.
Properties of Residuum Feedstocks
The feedstocks shown i n Table I do n o t g e n e r a l l y c o n t a i n h i g h c onc ent rat io ns o f s o l u b l e i r o n . Naphthenic crude o i l s found i n C a l i f o r n i a a r e ex c ept io nal i n t h i s r e g a r d and even when t h e residuum f rom them i s deasphalted by s o l v e n t e x t r a c t i o n , measurable i r o n l e v e l s are s t i l l seen i n t h e DAO. For example, a 60% y i e l d o f DAO from a residuum which cont ained 120 pprn n i c k e l , 73 ppm vanadium, and 82 ppm i r o n , shows 21.7 ppm n i c k e l , 6.5 ppm vanadium, and 4.8 ppm iro n--a l e v e l which we w i l l show l a t e r i s q u i t e s i g n i f i c a n t . Most of t h e residuum hydroprocessing c a t a l y s t s which w i l l be discussed i n t h i s paper were aged i n small p i l o t u n i t s . Elaborat e precaut ions were t a k e n t o ensure t h a t t h e t e s t s d i d n o t s u f f e r from nonuniform l i q u i d d i s t r i b u t i o n c o m o n t o bench-scale u n i t s operated i n t h e t r i c k l e f l o w mode (29). The c a t a l y s t s employed i n t h l s study contained Group IVB and Group VIII metals and a low a c i d i t y m a t r i x . T h e i r pore s t r u c t u r e was unimodal and microporous. They were g e n e r a l l y c y l i n d r i c a l ext rudat e p a r t i c l e s w i t h L/D > 2, alt h ough some o f t h e work was c a r r i e d o u t on smaller s i z e d c a t a l y s t s produced by c rus hing l a r g e r extrudate.
366
Details of the microprobe techniques used in the study on catalyst aging are given elsewhere (27). CATALYST ACTIVITY
Published hydrodemetalation kinetic data have been correlated with rate expressions of both first order (3, 5) and second order (1, 4) in the metals concentration. It has been suggested (4, 26), that there may really be a multitude of first-order reactions with different reaction rates occurring simultaneously. The larger metal-containing molecules are generally less reactive than the smaller ones. Data obtained at low conversions could, therefore, give the impression of simple first-order behavior. and only at higher conversions would nonlinear effects become obvious. Chevron Research pilot plant demetalation kinetic data (4) for Arabian Heavy atmospheric residuum are shown in Figure 1. Second-order kinetics gives the simplest expression capable of describing the data. Experiments with different-sized catalysts were also carried out. (See Figure 2.) Here both desulfurization and demetalation rate constants (second order) are plotted versus temperature for both 1/16-in. catalyst and the same catalyst crushed t o 28-60 mesh. The desulfurization data show no significant particle size effect over the temperature range considered. The demetalation data, however, show a substantial pore diffusion limitation at all temperatures above 550°F. Both catalyst activity and activation energy are higher for the crushed catalyst. Residuum demetalation is a process which usually operates in the diffusioncontrolled region.
367 Temperature,
1.0h
800 L
L
O
10.0
(Ni+V) r Rate d Demetalation Expression e r
I
800
700
\
OF
I
500 I
,
Open -Desulfurization Closed -Demetalation 0-28-60 Mesh -1/16-ln. Extrudate
.. 0)
c
:
Gravity, "API 14.0 Sulfur, WT TO 4.3 Ni + V, pprn 107
K
0.0011
I
0.8
irr x Fig. 1. Desulfurization and Dernetalation K i n e t i c s
4
1.o
0.9 103, IPR
F i g . 2. Desulfurization and Demetalation K i n e t i c s E f f e c t s o f Temperature and P a r t i c l e Size
-
Chevron Research has investigated the e f f e c t o f c a t a l y s t pore s i z e and p a r t i c l e size on the hydrodemetalation o f Boscan crude o i l ( 3 ) .
The c a t a l y s t s
used were a l l nickel-molybdenum based and a l l had unimodal microporous pore s i z e distributions. They were each tested a t the same pressure l e v e l and the same d e s u l f u r i z a t i o n severity level. Product vanadium l e v e l s measured during t h e f i r s t three days o f operation were used t o characterize the c a t a l y s t ' s i n i t i a l act iv i t y
.
Because
vanadium
conversion
levels
were
low,
a first-order
kinetic
expression was found t o adequately describe the data. These r a t e constants were normalized per u n i t o f i n t e r n a l surface area and p l o t t e d versus temperature on Figure 3.
Based on a single p a i r o f runs, the a c t i v a t i o n energy seems t o be
about 15 kcal/mole.
Here the smaller pore diameter c a t a l y s t s Show l e s s a c t i v i t y
than the larger pored ones and clearcut p a r t i c l e s i z e effect's
can a l s o be
seen. Figure 4 shows the r a t e constants estimated from Figure 3 a t 640°F p l o t t e d versus the term Dp(pore dia)-lm5. This term contains t h e two important parameters which e f f e c t the Thiele modulus and which we have allowed t o vary i n t h i s work.
The p a r t i c l e size (Dp)
n a t u r a l l y enters the Thiele modulus t o the
f i r s t power but, as discussed e a r l i e r , the pore diameter i s a parameter which
368
influences the e f f e c t i v e d i f f u s i o n c o e f f i c i e n t i n an unknown and complex way. We have found t h a t by applying a -1.5 power t o the pore diameter, the s c a t t e r tn
Figure 4 is reduced and the points f i t a single curve. The points can now be forced t o f i t a curve based on the development of Thiele theory. This f i t t i n g technique i s s i m i l a r t o t h a t used
,130
9.4
Fig. 3.
9.2
9.0
8.8
*
A dp
8.6
Boscan Demetalation Kinetics
Fig. 4. S i m p l i f i e d Thiele P l o t For Demetalation o f Boscan Residuum a t 640°F
when one compares performance o f two c a t a l y s t s o f d i f f e r e n t sizes. The p o i n t where the curve intercepts the y-axis represents the maximum a c t i v i t y (per u n i t area) t h a t can be achieved a t t h i s temperature. The r a t i o o f a r a t e constant a t any p o i n t on the curve t o t h i s maximum value i s approximately equal t o t h e demetalation effectiveness f a c t o r f o r a c a t a l y s t w i t h the p a r t i c u l a r combination o f Dp and pore diameter which corresponds t o the point. Having correlated the data i n t h i s fashion, p r e d i c t i o n s can be made. Figure 5 shows the predicted a c t i v i t y versus pore diameter w i t h p a r t i c l e Size as a parameter. This p l o t , which assumes a c a t a l y s t pore volume o f 0.5 cc/g. shows t h a t the optimum pore diameter for c a t a l y s t a c t i v i t y varies w i t h the p a r t i c l e
369
diameter.
A small p a r t i c l e size,
small Pore diameter c a t a l y s t i s the most
The f a c t t h a t small p a r t i c l e size,
high i n t e r n a l surface area c a t a l y s t s are
active.
optimum
is
intuitively
obvious
for
a
pore
diffusion
limited
reaction.
Sometimes, however, one i s forced t o choose a larger p a r t i c l e s i z e based on other considerations (because of pressure drop considerations i n a f i x e d bed o r f l u i d i z i n g v e l o c i t y considerations i n a f l u i d bed). I n any case, there i s an optimum i n t e r n a l surface area and pore diameter f o r each c a t a l y s t size.
The
amount o f hydrogenation component i n the i n i t i a l c a t a l y s t i s also known t o be important ( 7 ) .
As Spry and Sawyer (26) have pointed out, each crude o i l w i l l
have a d i f f e r e n t optimum combination o f c a t a l y s t size and p o r o s i t y f o r maximum activity.
50
100
150
200
250
300
Fig. 5. Estimated E f f e c t of Catalyst S i z e and Pore Diameter on Boscan Demetalation Kinetics The above shows t h a t we have made some progress i n understanding the f a c t o r s a f f e c t i n g c a t a l y s t a c t i v i t y . I t i s q u i t e l i k e l y , however, t h a t the m s t a c t i v e catalyst w i l l not r e s u l t i n the longest c a t a l y s t l i f e and commercially, the l i f e i s o f t e n more s i g n i f i c a n t than a c t i v i t y . Chevron has looked a t shortt e r m and long-term metals deposition p r o f i l e s on c a t a l y s t s t o t r y t o OpthIiZe catalyst l i f e .
Much o f t h i s work was reported by Tam, Harnsberger, and Bridge
370
(27). It was carried out with a 1/16 in. extrudate catalyst except where otherwise stated. CATALYST LIFE
Several authors (5, 10. 13, 14, 15, 19, 20, 21) have attempted to elucidate the mechanisms by which hydroprocessing catalysts deactivate, and a number of others have formulated models, based on proposed mechanisms, for predicting catalysts deactivation at commercial operating conditions. Residuum hydroprocessing catalysts deactivate in a very characteristic fashion, as shown in a number of articles (4, 8, 15, 19, 27). The average catalyst temperature of a hydrotreater is normally raised to compensate for the deactivation and thereby hold a particular product specification constant. An example of a typical temperature-time curve is shown in Figure 6. Here normalized temperatures are shown instead of actual temperatures to account for the slight variations in feed rate and conversion which inevitably occur.
0.2
0.4
0.6
0.8
1.o
Reduced Time
Fig. 6. Typical Deactivation Curve Arabian Heavy Atmospheric Residuum Desulfur ization, Constant Sulfur The deactivation curve can be subdivided into three distinct parts. The initial period is characterized by a rapid, but continuously decreasing,
371 deactivation rate.
T h i s d e a c t i v a t i o n has o f t e n been blamed on coke d e p o s i t i n g
on t h e c a t a l y s t , a phenomenon known t o be M r e pronounced i n t h e e a r l y p a r t o f a run.
The f i n a l p e r i o d , o r u l t i m a t e d e a c t i v a t i o n , i s c h a r a c t e r i z e d by a r a p i d ,
continuously
increasing
deactivation
rate.
Following
(9), t h i s
Hiemenz
d e a c t i v a t i o n has comnonly been blamed on t h e c o n s t r i c t i o n o f t h e c a t a l y s t p o r e mouths by metal deposits.
These two p e r i o d s a r e separated by a p e r i o d i n which
t h e d e a c t i v a t i o n r a t e appears t o be n e a r l y constant. I n o r d e r t o g a i n a b e t t e r understanding o f c a t a l y s t d e a c t i v a t i o n ,
o f s h o r t p i l o t p l a n t t e s t s were made on Arabian Heavy residuum.
a number
I n each p a i r o f
t e s t s , o n l y one v a r i a b l e was changed, t h e o b j e c t being t o determine by t h e e l e c t r o n m i c r o p r o b l e how these v a r i a b l e s i n f l u e n c e d t h e d e p o s i t i o n of m e t a l s o n t o the catalyst.
SHORT-TERM EXPWIMENTS Examples o f t h e d e p o s i t i o n a l p r o f i l e s f o r n i c k e l , vanadium,
and i r o n a t
b o t h t h e i n l e t and o u t l e t o f t h e c a t a l y s t bed (27) a r e shown i n F i g u r e 7. i s found p r i m a r i l y o u t s i d e t h e c a t a l y s t p a r t i c l e as a t h i n scale. g e n e r a l l y t h e case. to
differences
organometallic concentration
in
molecules. inside
These d i f f e r e n c e s i n d e p o s i t i o n a l p a t t e r n s a r e
the
reactivities
Both
t h e edge
of
nickel the
c o n c e n t r a t i o n approaches t h e edge o f reactor.
This i s
N i c k e l g e n e r a l l y seems t o p e n e t r a t e t h e c a t a l y s t t o a
g r e a t e r e x t e n t t h a n vanadium. due
Iron
and/or
and
particle,
diffusivities
vanadium
display
of
the
a
maximum
but the point o f
maximum
t h e c a t a l y s t near t h e o u t l e t of
the
372
..
012-
. .
.
008-
004
. Vanadium
' Iron 0
0.016
0.008
-
2 1
8+--7-&
.
1
Reactor Inlet
'-,
Nickel
Vanadium
Reactor Outlet
'I0[
1.o
0.5
I
I ) '
0.6
0.4
0.2
I
0
Fractional Radius
Fig. 7. Typical Depositional Patterns f o r Nickel, Vanadium, and I r o n Reaction Temperature = 700°F Hydrogen P a r t i a l Pressure = 1825 p s i a The f a c t t h a t maximum concentrations are found i n s i d e the edge o f the part i c l e i s d i f f i c u l t t o explain. I t may be due t o hydrogen s u l f i d e being a reactant or i t may be due t o s p e c i f i c reaction intermediates being formed. It complicates data analysis,
since pore d i f f u s i o n theory coupled w i t h a simple
r e a c t i o n mechanism does not p r e d i c t an i n t e r n a l maximum. Despite t h i s , i t i s i n t e r e s t i n g t o compare the change i n the maximum deposit concentrations from reactor i n l e t t o reactor o u t l e t w i t h the change i n concentration o f metals i n t h e oil. during the t e s t i n which the p r o f i l e s shown i n Figure 7 were generated. the average vanadium removal was 58% and the average n i c k e l removal was 42%. The maximum deposit concentrations o f both metals decreased by approximating
80% from
reactor
inlet
to
outlet,
demetalation i s not a simple f i r s t - o r d e r reaction.
clearly
showing
that
The change i n t h e maximum
deposit concentrations i s close t o what one would p r e d i c t using second-order k i n e t i c s , assuming t h a t the concentrations o f lnetals i n the feed and product o i l apply t o the maxima a t the respective ends o f the reactor.
373
1.0
0.4 0.2 Fractional Radius
0.8
0.6
0
Fig. 8. The Effect of Reaction Temperature on Vanadium Deposition Hydrogen Partial Pressure = 1825 psia The effect of increasing the reaction temperature on the deposit profiles, shown in Figure 8 was to increase the concentration at the maximum and decrease the effectiveness factor. The effect of temperature on the intrinsic reaction rate has been previously shown to fit Arrheniusl law with an activation energy of 30 kcal/g-mol. Since temperature does not have much of an effect on the effective diffusivities of the reacting molecules. i4s effect on reactivity predominates and the changes in the concentration profiles are "directionally" those predicted by the theory. The effect o f changing hydrogen partial pressure was found to be similar (27). The effect of changing feed source on the depositional patterns was illustrated by the results for Arabian Heavy and Alaskan North Slope residua in figure 9. The vanadium levels in these two residua differ by about a factor of 2 and the maximum deposit concentrations at the top of the bed differ by nearly the same factor. Therefore, despite the fact that vanadium removal in the
374
i n t e g r a l reactor appears t o follow second-order k i n e t i c s f o r both residua, the i n t r i n s i c r e a c t i v i t i e s of the most r e a c t i v e organometallics i n each crude appear on a f i r s t - o r d e r basis, t o be similar. The e f f e c t o f decreasing the c a t a l y s t pore diameter, shown i n Figure 10, was t o concentrate the metals nearer the external surface o f the p a r t i c l e . A change i n c a t a l y s t pore diameter does affect t h e effective d i f f u s i v i t y o f t h e reacting molecules and the observed changes i n t h e metals p r o f i l e s are consistent w i t h the theory.
J
Atrn. Resiuum
c
. 5 0
o.16-
0
Ov = 0.33 Alaskan North Slope
Atrn. Residuum Ov = 0.48
E' *Small Pore. Bv = 0.15 .Large Pore, 8" = 0.20
:z
0.12
psh"
0.04
5
Fractional Radius Fig. 9. The E f f e c t o f Feed Source on Vanadium Deposition Reaction Temperature = 700°F Hydrogen P a r t i a l Pressure = 1825 p s i a
>
Fig. 10. Diameter Reaction Hydrogen
1.0
08
0.6
0.4
0.2
0
Fractional Radius
The E f f e c t o f Catalyst Pore on Vanadium Deposition Temperature = 700°F P a r t i a l Pressure = 1825 p s i a
375 A t the reactor i n l e t , the e f f e c t o f changing the c a t a l y s t p a r t i c l e size,
shown
in
Figure 11,
contaminated
by
is
feed
only
metals.
t o change the For
the
volume f r a c t i o n
catalysts
shown
of
catalyst Figure 11,
in
approximately 90% o f the volume of the 1/32-in. diameter c a t a l y s t i s contaminated by vanadium, while only about 50% o f the volume o f the 1/16-in. diameter c a t a l y s t i s s i m i l a r l y contaminated. A t the reactor o u t l e t , the s i t u a t i o n i s complicated by the f a c t t h a t the reactant molecules remaining i n the o i l are not the same f o r the two c a t a l y s t beds. small-diameter
catalyst
removes more metals per
This r e s u l t s because the unit
of
reactor
length.
Therefore, since the organometallics display a spectrum o f r e a c t i v i t i e s and/or d i f f u s i v i t i e s , the absolute penetration o f metals i n t o the c a t a l y s t p a r t i c l e s can be d i f f e r e n t a t the o u t l e t s o f the two c a t a l y s t beds.
o.20r... .
Ov (1/32-ln.) = 0.38
0.16 -
. 0
Ov (1/16-ln.) = 0.20
0
0
0.12-*
.
% 0
0.08-,
0.04-
.
0 0
b
1
0
G
\%\.
C
1/32-ln. Catalyst
1
1/16-ln. Catalyst
I
1
Fig. 11. The E f f e c t o f Catalyst P a r t i c l e Size on Vanadium Deposition. Reaction Temperature = 7OO0F, Hydrogen P a r t i a l Pressure = 1825 pSia LONG-TERn EXPWIHENTS
The change i n the depositional patterns o f n i c k e l and vanadium w i t h time was studied by i n t e r r u p t i n g the run shown I n Figure 6, a t the indicated points, t o recover small amounts o f catalyst. showed a number o f
The r e s u l t s o f the microprobe analyses
i n t e r e s t i n g features.
Vanadium showed a decrease I n
376 e f f e c t i v e n e s s f a c t o r from the i n l e t t o the O u t l e t o f the reactor, showed t h e reverse behavior. reactor
outlet
compounds.
appeared
to
but n i c k e l
The r e a c t i v i t y of the n i c k e l compounds a t the be about
the
same as t h a t o f the vanadium
Nickel seemed t o have about twice t h e d i f f u s i v i t y of vanadium.
In
general, the metals deposition r e s u l t s i n d i c a t e t h a t t h e p e n e t r a t i n g metals t e n d t o deposit i n much the Same place i n the c a t a l y s t throughout t h e run.
This
gives credence t o our analysis of the s h o r t p i l o t p l a n t runs r e p o r t e d e a r l i e r i n t h i s paper. Figure 12 shows t h e change w i t h time o f t h e concentration o f vanadium a t A simple t h e p o i n t of maximum buildup f o r t h r e e l e v e l s i n the c a t a l y s t bed. c a l c u l a t i o n on monolayer coverage of vanadium s u l f i d e , suggests t h a t a t t h e t o p o f the bed the maximum deposit represents 5-12 monolayers.
I f t h e d e p o s i t were
V3Sq and had t h e density of the bulk s u l f i d e , such a deposit would be 15-40
A in
For a c a t a l y s t w i t h a pore diameter i n the range 100-200 A, t y p i c a l o f many residuum hydroprocessing c a t a l y s t s (6. 16). such a deposit would reduce t h e depth.
diameter o f the pores s i g n i f i c a n t l y .
0.2c -I
0.4
0.6
0.8
1.o
Reduced Time
Fig. 12. Maximum Vanadium Deposit Concentration as a Function o f Reactor P o s i t i o n and Time
377 The physical obstruction o f the pore mouths would decrease the e f f e c t i v e d i f f u s i v i t y f o r the reactant molecules and, thereby, increase the Thiele modulus I f the desired reaction were already near t h e f o r the desired reaction. d i f f u s i o n l i m i t when the c a t a l y s t was fresh, i t might well be expected t o become d i f f u s i o n l i m i t e d when the c a t a l y s t was heavily laden w i t h metals.
I n this
case, temperature would have t o be raised a t an ever increasing r a t e t o maintain conversion.
Such
a
situation
is
typical
of
the
latter
stages
of
a
hydroprocessing run as i l l u s t r a t e d i n Figure 6. The e f f e c t o f t h i s pore mouth plugging on c a t a l y s t a c t i v i t y was measured q u a n t i t a t i v e l y i n another experiment. A c a t a l y s t bed which had reached a t y p i c a l end-of-run condition was divided i n t o s i x sections, and the a c t i v i t y was measured independently f o r each section. A dramatic a c t i v i t y p r o f i l e was found. The top one-third o f the bed was v i r t u a l l y dead, having l i t t l e more than one-third the a c t i v i t y o f t h e average bed and l e s s than one-sixth the a c t i v i t y o f the bottom o f the bed. The bottom one-third o f the bed, while s i g n i f i c a n t l y deactivated r e l a t i v e t o t h e f r e s h c a t a l y s t , was r e l a t i v e l y unaffected by pore plugging and s t i l l had s u f f i c i e n t a c t i v i t y t o be useful. Similar a c t i v i t y
profiles
have
been observed
comnercially
where
the
temperature r i s e across i n d i v i d u a l sections o f the c a t a l y s t bed gives a measure of c a t a l y s t a c t i v i t y (18). Pore plugging, therefore, occurs as a "wave" which, a f t e r an induction time, moves from the i n l e t o f the reactor toward the o u t l e t . PORE MOUTH PLUGGING The onset o f the pore-plugging wave and the r a p i d i t y w i t h which i t moves
through the bed are dependent on the d e t a i l s o f the c a t a l y s t pore structure, t h e d i s t r i b u t i o n o f metals w i t h i n the c a t a l y s t p a r t i c l e s , and the d i s t r i b u t i o n o f metals along the length o f the c a t a l y s t bed. The pore s t r u c t u r e d i r e c t l y determines the maximum local deposit which can be t o l e r a t e d before i n t r a p a r t i c l e transport
of
reactants
and products
i s adversely
affected.
The maximum
concentration o f deposit w i t h i n a c a t a l y s t p a r t i c l e a t a given time has already been shown t o be dependent on process and c a t a l y s t variables. The more Uniform the i n t r a p a r t i c l e d i s t r i b u t i o n , the lower the concentration a t the maximum w i l l be a f t e r a given time, and, therefore, the l a t e r the onset o f pore plugging w i l l occur.
The r a t e o f advance o f the pore plugging wave, on the other hand, i s
r e l a t e d t o the u n i f o r m i t y o f the i n t e r p a r t i c l e d i s t r i b u t i o n along the length of
378
The more uniform t h i s d i s t r i b u t i o n i s , t h e more r a p l d l y t h e wave transverse the reactor. This simple p r i n c i p l e i s i l l u s t r a t e d by t h e
t h e reactor.
will
f o l l o w i n g example.
-
820
P
e
3
c
E 0) n
E
1/164n. Catalyst 0
1/30-ln. Catalyst
800 -
t
760 780
I
0
1
0.2
I
0.4
Reduced Time =
0.6
0.8
J
1.o
t tEOR (1/30-ln. Catalyst)
f i g . 13. The E f f e c t o f Catalyst P a r t i c l e Size on Catalyst Deactivation I r a n i a n Heavy Atmospheric Residuum Desulfurization, Constant S u l f u r Two c a t a l y s t s having i d e n t i c a l properties, except f o r t h e i r p a r t i c l e size, were Used t o desulfurize I r a n i a n Heavy atmospheric residuum t o an equal extent
a t i d e n t i c a l processing conditions. Figure 13.
Their deactivation curves are compared i n
The onset o f pore plugging a t the top o f the c a t a l y s t bed would
occur a t e s s e n t i a l l y the same time i n these two t e s t s because t h e porous properties of the catalysts are the same, and the processing conditions are t h e same ( w i t h the exception o f the subsequent temperature program). However. t h e speed w i t h which the pore plugging wave moves through the bed i s very d l f f e r e n t ; and, therefore, the deactivation schedules are very d i f f e r e n t . Because a l a r g e r f r a c t i o n o f the c a t a l y s t volume i s accessible t o the depositing metals w i t h t h e small s i z e catalyst, more metal i s accommodated a t the top o f the bed; and t h e metal concentration p r o f i l e down the c a t a l y s t bed i s steepened.
A t the
decreased concentrations o f metal contaminants t o which the lower p a r t o f t h e
379
bed i s exposed,
more t i m e i s r e q u i r e d f o r
t h e maximum d e p o s i t t o r e a c h i t s
l i m i t i n g value, thereby slowing t h e r a t e of t r a v e l o f t h e p o r e p l u g g i n g wave.
INITIAL CATALYST DEACTIVATION The c a t a l y s t d e a c t i v a t i o n which occurs b e f o r e t h e onset o f p o r e mouth p l u g g i n g i s more d i f f i c u l t t o c h a r a c t e r i z e and t h e r e i s c o n t r o v e r s y r e g a r d i n g whether i t i s due t o coke or m e t a l s d e p o s i t i o n . I n t + e e a r l y stages o f a hydroprocessing run, a f r a c t i o n o f t h e c a t a l y s t ' s s u r f a c e area i s converted from i t s o r i g i n a l s t a t e t o a s u r f a c e composed o f mixed n i c k e l and vanadium s u l f i d e s .
While these s u l f i d e s do have c a t a l y t i c a c t i v i t y
f o r hydrogenolysi s, they a r e c o n s i d e r a b l y l e s s a c t i v e t h a n t h e f r e s h c a t a l y s t s used i n these s t u d i e s .
Under these c o n d i t i o n s , t h e c a t a l y s t temperature must be
r a i s e d a c c o r d i n g l y t o h o l d conversion constant.
This form o f " p a r t i a l surface
p o i s o n i n g " may be t h e major cause o f d e a c t i v a t i o n i n t h e e a r l y p a r t o f a run. As
i l l u s t r a t e d i n Figure 6,
the period o f i n i t i a l c a t a l y s t deactivation
is
c h a r a c t e r i z e d by a high, b u t d e c l i n i n g , d e a c t i v a t i o n r a t e which a s y m p t o t i c a l l y approaches a c o n s t a n t value a t a reduced t i m e o f a p p r o x i m a t e l y 0.25.
Such
behavior would be a reasonable consequence o f t h e proposed p a r t i a l s u r f a c e p o i s o n i n g mechanism i f m u l t i l a y e r s o f t h e contaminant d e p o s i t have t h e same c a t a l y t i c a c t i v i t y as t h e i n i t i a l monolayer. A
high level of
catalyst. the
coke does f o r m r a p i d l y
i n an o u t e r annulus o f t h e
However, i t d e c l i n e s s l o w l y as f e e d metals d e p o s i t and i s o f f s e t by
increase o f
coke
i n the
i n t e r i o r o f the catalyst.
Since t h e i n i t i a l
d e p o s i t i o n a l p a t t e r n o f coke p a r a l l e l s t h a t o f t h e metals, b o t h p r o b a b l y b e i n g due t o t h e presence o f
h i g h molecular weight species,
unequivocally assign r e s p o n s i b i l i t y f o r the
initial
contaminant.
can
However,
several
arguments
be
it i s difficult
to
deactivation t o e i t h e r offered
which
favor
o r g a n o m e t a l l i c s as t h e primary d e a c t i v a n t when t h e metals c o n t e n t o f t h e f e e d exceeds about 10 ppm.
The l e n g t h o f t h e i n i t i a l d e a c t i v a t i o n p e r i o d i s d i r e c t l y
r e l a t e d t o the concentration o f organometallics i n the feed but n o t t o t h e c o n c e n t r a t i o n o f coke p r e c u r s o r s (as measured, f o r example, by Conradson Carbon c o n t e n t ) i n t h e feed.
The p e r i o d o f a c c e l e r a t e d coke laydown i s s h o r t r e l a t i v e
t o t h e e n t i r e i n i t i a l d e a c t i v a t i o n period, complex changes throughout t h i s time.
and t h e d e p o s i t e d coke undergoes
On t h e o t h e r hand, t h e m e t a l d e p o s i t s
b u i l d up m o n o t o n i c a l l y , and t h e time r e q u i r e d t o achieve monolayer coverage
380 throughout the reactor i s comparable i n length t o the i n i t i a l d e a c t i v a t i o n p e r i od
.
REACTOR BED PLUGGING Catalyst l i f e can sometimes be d i c t a t e d by the reactor pressure drop increasing t o an excessive l e v e l . The most comnon instances o f t h i s are due t o the presence o f p a r t i c u l a t e material i n the feed stream. This problem can be avoided i f p a r t i c u l a r a t t e n t i o n i s given t o the crude o i l desalting and the feed f i l t e r i n g operations. Chevron has experienced pressure drop increases i n one of i t s u n i t s i n which the feedstock p a r t i c u l a t e material i s n e g l i g i b l e . This u n i t i s the No. 1 Isomax p l a n t i n Chevron U.S.A.'s Richmond Refinery. I t hydrocracks up t o 32,000 BPOO o f a 14"API C a l i f o r n i a deasphalted o i l containing 17 ppm n i c k e l , 7 ppm vanadium, and 9 ppm iron.
The product i s metal free.
The i r o n i s
thought t o e x i s t i n the form o f i r o n naphthenates. The c a t a l y s t l i f e o f t h i s u n i t i s l i m i t e d by pressure drop buildup due.. t o plugging o f the top beds ( 4 ) .
Chemical analysis o f the i n t e r s t i t i a l m a t e r i a l
found i n the f i r s t r u n i s shown i n Figure 14. The major plugging component was i r o n sulfide. The quantity o f i r o n found i n the deposit agreed very w e l l w i t h the quantity o f soluble i r o n present i n the feed during the course o f the run. For t h i s i n i t i a l operation the i r o n l e v e l i n the feed was between 4 and 5 ppm.
38 1
i .i
I\
Fraction of Top Bed
Fig. 14. Composition o f I n t e r s t i t i a l Deposit Found i n Top Bed of Richmond Isomax No. 1 Reactor Run No. 1 1966 Spent catalysts from t h i s u n i t show a buildup of i r o n s u l f i d e on t h e
-
external surface o f the p a r t i c l e s .
This indicates t h a t i r o n naphthenates r e a c t
w i t h hydrogen t o form i r o n s u l f i d e and t h a t the i r o n s u l f i d e CatalyZeS t h e
subsequent reaction.
The i r o n naphthenates are apparently very reactive.
The
r a t e of buildup o f the i r o n s u l f i d e layer i s m c h f a s t e r than the r a t e of pore mouth plugging by nickel and vanadium sulfides. and seems t o be unique t o C a l i f o r n i a heavy o i l s .
This i s an unusual S i t u a t i o n
CONCLUSION Hydrodemetalation i s an example o f a reaction whose r a t e i s c o n t r o l l e d by pore d i f f u s i o n .
Catalyst a c t i v i t y i s therefore influenced by c a t a l y s t p a r t i c l e
size and porosity. Catalyst l i f e i s a complicated f u n c t i o n o f these and other variables, including process conditions and feedstock properties. Microprobe analysis o f catalysts, used i n upgrading metal containing heavy o i l s , reveal
382
metal deposits which are consistent with, although somewhat more complex than, those expected from simple pore d i f f u s i o n theory.
C a t a l y s t d e a c t i v a t i o n appears
t o be the r e s u l t o f a t l e a s t two d i f f e r e n t mechanisms, both i n v o l v i n g metals deposition. These are surface poisoning, and pore mouth plugging. very complicated. temperatures
Process o p t i m i z a t i o n i s
because surface poisoning i s most serious a t low o p e r a t i n g
and pressures
whereas
pore mouth plugging
i s worst
at
high
temperatures and pressures. There i s a strong economic i n c e n t i v e t o develop improved c a t a l y s t s and processes f o r upgrading these heavy o i l s . capable
of
describing
the
complicated
I n order t o develop accurate models r e a c t i o n k i n e t i c s and
deactivation
phenomena, more experimental e f f o r t i s needed i n feedstock c h a r a c t e r i z a t i o n and the importance o f c a t a l y s t pore s i z e d i s t r i b u t i o n and operating c o n d i t i o n s on c a t a l y s t performance. S c i e n t i s t s a t Chevron Research,
some o f whom s t a r t e d t h e i r careers
G u l f ' s RHOS development e f f o r t are continuing work i n t h i s f l e l d . the
in
As a r e s u l t
Chevron Residuum Hydrotreating Process now uses a v a r i e t y o f
tailored
c a t a l y s t combinations a v a i l a b l e from a number o f d i f f e r e n t sources (11).
ACKNOWLEDGMENTS The author wishes t o acknowledge the c o n t r i b u t i o n s o f h i s f e l l o w workers a t Chevron Research, p a r t i c u l a r l y the work o f Paul W. Tamm i n t h e g a t h e r i n g and i n t e r p r e t a t i o n o f the microprobe work.
LITERATURE CITED Beuther, H.; Schmid, 8. K.; Section 3, Paper 20, S i x t h World Petroleum Congress, Frankfurt, Germany. June 1963. Bridge, A. G.; Gould, G. 0.; Berkman, J. F.; O i l Gas Journal, January 19. 1981, 85. Bridge, A. G.; Green, 0. C.; ' # D i f f u s i o n a l Considerations i n Residuum Hydrodemetalation," 178th National Meeting o f t h e American Chemical Society, Washington, D.C., September 9-14, 1979. Cash, 0. R.; Paper No. 14a, 74th Bridge, A. G.; Reed, E. M.; Tamm, P. W.; National AIChE Meeting, New Orleans, LA, March 1979. Dautzenberg, F. M.; Van Klinken, J.; Pronk, K. M. A.; Sie, S. T.; W i j f f e l s , J. B. ACS; Symp. Ser. 1978, 65. 254. "Chemistry o f C a t a l y t i c Gates, B. C.; Katzer, J. R.; Schuit, G. C. A.; Processes," McGraw-Hill: New York, 1979. Green, 0. C. and Broderick D. H.; "Chemical Engineering Progress," December 1981, 33. Henke, A. M. ; O i 1 Gas J. 1970, 68 (14), 97.
383 9 10 11 12 13 14 15 16 17
18 19 20
21 22 23 24
25 26 27 28 29
Hiemenz, W.; Discussion, S e c t i o n 3, Paper 20. S i x t h World Petroleum Congress, F r a n k f u r t , Germany, June 1963. Hughes, C. C.; Mann, R.; ACS Symp. Ser. 1978, 65, 201. Hung, C. W.; Howell, R. L.; Johnson, D. R.; CEP March 1986. 57. Inoguchi, M.; Kagaya, H.; Diago, K.; Sakurada, S.; Nagai, T.; Satomi, Y.; Inaba, K.; Tate, K.; Nishiyama, R.; Onishi, S . ; B u l l . Jpn. Pet. I n s t . 1971, 13 ( 2 ) , 153. Inoguchi, M.; Sakurada. 5.; Satomi, Y.; Inaba. K.; Kagaya, H.; Tate, K.; M i z u t o r i . T.; Nishiyama, R.; Nagai, T.; Onishi, S . ; B u l l . Jpn. Pet. I n s t . 1972. 1 4 - ( 2 ) , 153. Newson. E. J.: Ind. Ena. Chem. Process DeS. Dev. 1975. 14. 27. Nitta,.H.; Takatsuka, -T.; Kodama, S . ; Yokoyama, T.; - D e a c t i v a t i o n Model f o r Residual H y d r o d e s u l f u r i z a t i o n C a t a l y s t s , 86th N a t i o n a l AIChE Meeting. Houston, Texas, A p r i l 1979. Ohtsuka, T.; Catal. Rev. 1977. 16 ( 2 ) . 291. O x e n r e i t e r , M. F.; Frye, C. G.; Hockstra, G. 8.; Sroka, J. M.; F u e l O i l D e s u l f u r i z a t i o n Symposium, Japan Petroleum I n s t i t u t e , Tokyo, Japan, November 19, 1972. Satomi, Y.; Hisamitsu, T.; PD18 (4), N i n t h World Petroleum Ozaki, H.; Congress, Tokyo, Japan, 1975. Parkin, E. S.; Paraskos, J. S.; Frayer, J. A.; Use o f Analog Computer S i m u l a t i o n i n t h e Development o f a Commercial HDS Process, 7 4 t h N a t i o n a l AIChE Meeting, New Orleans, LA. March 1973. Prasher, 8. 0.; G a b r i e l , G. A.; Ma. Y. H.; Ind. Eng. Chem. Process Des. Dev. 1978, 17, 266. Rajagopolan, K.; Luss, 0.; Ind. Eng. Chem. Process Des. Dev. 1979, 18, 459. Reynolds, J. G., L i q u i d Fuels Technology 3 (1). 73-105 (1985). Sato, M.; Takayama, N.; K u r i t a , S.; Kwan, T.; Nippon Kagaku Zasshi 1971, 92, 834. S c o t t , J. W.; Bridge, A. G.; Christensen, R. I.; Gould, G. 0.; Fuel O i l D e s u l f u r i z a t i o n Symposium, Japan Petroleum I n s t i t u t e , Tokyo, Japan, March 1970. S c o t t , J . W.; Bridge, A. G.; Adv. Chem. Ser. 1971, No. 103 113. Spry, J. C.; Sawyer, W. H.; Paper No. 30C, S i x t h - E i g h t h Annual AIChE Meeting, Los Angeles. CA, 1975. I and EC Process Design Tam, P. W.; Harnsberger, H. F.; and Bridge, A. G.; and Development, 1981, 20, 262. Todo, N. e t al., Kogyo Kagaku Zasshi, 1971, 74 ( 4 ) 563. Weekman, V. W.; Chem. React. Eng., Proc. I nt . Symp. 4th. 1976, 615.
363
CATALYTIC HYDRODEMETALATION OF HEAVY OILS A. G. BRIDGE Chevron Research Company, 576 Standard Avenue, Richmond, California
ABSTRACT In the past two decades, residuum hydrotreating has become a very important refining process. Designed originally to reduce the sulfur in heavy fuel oils, it has also been used in a variety of schemes for converting heavy o i l s into more valuable transportation fuels. Chevron Research Company offers for 1 icense the Chevron Residuum Hydrotreating Process which contains the best features of the processes developed independently by the Chevron and Gulf research organizations before their two parent companies merged in 1985. Since residuum feedstocks contain organometallic impurities, studies of the hydrodemetalation reaction were instrumental in the successful development of the catalysts used in this process. This reaction has the unique feature that it is a heterogeneous catalytic rection in which the organometallic reactant leaves an easily identifiable deposit on the catalyst surface at the precise place where the reaction occurred. Chevron Research has developed catalysts and processes which remove the corrmon trace metals present in crude oil residua--nickel, vanadium, and iron. Demetalation kinetics have been measured over a variety o f catalysts in order to generate more effective catalysts. Spent catalysts have been analyzed by microprobe techniques to try to understand the long-range effects of metals Pore deposition on reactor pressure drop buildup and catalyst deactivation. diffusion theory has been used to help correlate the data and improve the scale-up operation. This paper reviews these developments and points out the importance of catalyst porosity, size, and shape on catalyst activity and life. Factors which can contribute to a pressure drop buildup in a fixed bed reactor are also covered.
INTRODUCTION The conversion of heavy residual oils into lighter fuel oils and transportation fuels is a major technological challenge for today's refiners. This has been brought about by (a) changes in crude oil availability, particularly a shift toward heavier crudes; (b) a decrease in demand for heavy fuel oil with an increase in demand for transportation fuels; and (c) a variety o f environmental considerations. Both here in Kuwait as in other countries direct residuum hydrotreating is being used as a first step in residuum conversion schemes (2). The Chevron Residuum Hydrotreating Process. developed originally to produce low sulfur fuel
364
oil from high sulfur crudes, has been combined with hydrocracking fluid catalytic cracking and coking to effectively convert residuum into more valuable lighter products. Another commercially proven residuum conversion scheme i s solvent deasphalting of the residuum followed by hydrocracking of the deasphalted oil (DAO). The Chevron Isocracking Process has been used in this service since 1967. These hydroprocesses employ porous solid Catalysts which must be resistant to the soluble nickel, vanadium, and iron compounds present in most residual oils. Hydrodemetalation of these compounds leaves a deposit of mixed metal sulfides on and in the catalyst. The buildup of these metal deposits is irreversible and largely determines catalyst life. The seriousness of this problem can be illustrated by considering the volume of deposit that can accumulate in a day. If a unit has to handle 30,000 barrels per day o f feedstock from which 100 ppm of vanadium contaminant is removed, then each day about 8 ft3 of vanadium sulfide will deposit in the reactor system. The success of the process development and design in residuum hydrotreating is dependent on an accurate prediction of where the metal sulfides deposit and what effect the deposits have on catalyst performance and system pressure drop. Early workers recognized that (a) the demetalation reaction rate was limited by the rate of diffusion of the reactant through the liquid-filled catalyst pores and (b) that the reaction was autocatalytic resulting in very high local concentrations of deposited metal sulfides. This was shown by electron microprobe analysis of spent catalysts (12, 23, 24, 25, 27, 28). Attempts to predict the effect of metals deposition on catalyst life have included correlation of the experimental data with the help of pore diffusion theory. These efforts have only been partially successful because of t h e complexity of the system and the unavailability of high quality experimental data. This paper reviews Chevron's early experience in this field. It includes a di scus si on on the factors which i nf 1 uence hydrodemetal at i on catalyst act i v i ty , catalyst fouling and pressure drop through the fixed bed reactor. EXPERIMENTAL METHODS Petroleum residua from different sources display a wide range in the amounts of organometallic impurities. Nickel, vanadium, and iron are most comnonly in the highest concentrations and, therefore, of greatest importance.
365
The org anome t a l l i c s a r e found i n t h e asphaltene and r e s i n f r a c t i o n s ( 1 7 , 22). They are present as very l a r g e molecules, a t l e a s t a p o r t i o n of which have been i d e n t i f i e d as p o r p h y r i n s . Residua from s everal d i f f e r e n t crude sources were T h e i r concent rat ions o f organoemployed i n t h e experiments t o be described. m e t a l l i c i m p u r i t i e s span a r e l a t i v e l y wide range as shown i n Table I . ~
Arabian Light
Arabian Heavy
57OoF+
650"F+
Gravity 'API
17.6
14.0
16.9
12.6
Sulfur, Wt %
3.0
4.3
2.5
5.6
Feedstock
650"F+
Iranian Heavy
Alaskan North Slope 650°F
Boscan (Venezuelan) Crude
~
1250
1900
3300
5400
29
83
136
1270
Nickel, ppm
8
25
4%
105
Iron, ppm
3
15
6
10
Asphaltenes, Wt %
4.3
10.4
5.5
15.7
Nitrogen, ppm Vanadium, ppm
Table I.
Properties of Residuum Feedstocks
The feedstocks shown i n Table I do n o t g e n e r a l l y c o n t a i n h i g h c onc ent rat io ns o f s o l u b l e i r o n . Naphthenic crude o i l s found i n C a l i f o r n i a a r e ex c ept io nal i n t h i s r e g a r d and even when t h e residuum f rom them i s deasphalted by s o l v e n t e x t r a c t i o n , measurable i r o n l e v e l s are s t i l l seen i n t h e DAO. For example, a 60% y i e l d o f DAO from a residuum which cont ained 120 pprn n i c k e l , 73 ppm vanadium, and 82 ppm i r o n , shows 21.7 ppm n i c k e l , 6.5 ppm vanadium, and 4.8 ppm iro n--a l e v e l which we w i l l show l a t e r i s q u i t e s i g n i f i c a n t . Most of t h e residuum hydroprocessing c a t a l y s t s which w i l l be discussed i n t h i s paper were aged i n small p i l o t u n i t s . Elaborat e precaut ions were t a k e n t o ensure t h a t t h e t e s t s d i d n o t s u f f e r from nonuniform l i q u i d d i s t r i b u t i o n c o m o n t o bench-scale u n i t s operated i n t h e t r i c k l e f l o w mode (29). The c a t a l y s t s employed i n t h l s study contained Group IVB and Group VIII metals and a low a c i d i t y m a t r i x . T h e i r pore s t r u c t u r e was unimodal and microporous. They were g e n e r a l l y c y l i n d r i c a l ext rudat e p a r t i c l e s w i t h L/D > 2, alt h ough some o f t h e work was c a r r i e d o u t on smaller s i z e d c a t a l y s t s produced by c rus hing l a r g e r extrudate.
366
Details of the microprobe techniques used in the study on catalyst aging are given elsewhere (27). CATALYST ACTIVITY
Published hydrodemetalation kinetic data have been correlated with rate expressions of both first order (3, 5) and second order (1, 4) in the metals concentration. It has been suggested (4, 26), that there may really be a multitude of first-order reactions with different reaction rates occurring simultaneously. The larger metal-containing molecules are generally less reactive than the smaller ones. Data obtained at low conversions could, therefore, give the impression of simple first-order behavior. and only at higher conversions would nonlinear effects become obvious. Chevron Research pilot plant demetalation kinetic data (4) for Arabian Heavy atmospheric residuum are shown in Figure 1. Second-order kinetics gives the simplest expression capable of describing the data. Experiments with different-sized catalysts were also carried out. (See Figure 2.) Here both desulfurization and demetalation rate constants (second order) are plotted versus temperature for both 1/16-in. catalyst and the same catalyst crushed t o 28-60 mesh. The desulfurization data show no significant particle size effect over the temperature range considered. The demetalation data, however, show a substantial pore diffusion limitation at all temperatures above 550°F. Both catalyst activity and activation energy are higher for the crushed catalyst. Residuum demetalation is a process which usually operates in the diffusioncontrolled region.
367 Temperature,
1.0h
800 L
L
O
10.0
(Ni+V) r Rate d Demetalation Expression e r
I
800
700
\
OF
I
500 I
,
Open -Desulfurization Closed -Demetalation 0-28-60 Mesh -1/16-ln. Extrudate
.. 0)
c
:
Gravity, "API 14.0 Sulfur, WT TO 4.3 Ni + V, pprn 107
K
0.0011
I
0.8
irr x Fig. 1. Desulfurization and Dernetalation K i n e t i c s
4
1.o
0.9 103, IPR
F i g . 2. Desulfurization and Demetalation K i n e t i c s E f f e c t s o f Temperature and P a r t i c l e Size
-
Chevron Research has investigated the e f f e c t o f c a t a l y s t pore s i z e and p a r t i c l e size on the hydrodemetalation o f Boscan crude o i l ( 3 ) .
The c a t a l y s t s
used were a l l nickel-molybdenum based and a l l had unimodal microporous pore s i z e distributions. They were each tested a t the same pressure l e v e l and the same d e s u l f u r i z a t i o n severity level. Product vanadium l e v e l s measured during t h e f i r s t three days o f operation were used t o characterize the c a t a l y s t ' s i n i t i a l act iv i t y
.
Because
vanadium
conversion
levels
were
low,
a first-order
kinetic
expression was found t o adequately describe the data. These r a t e constants were normalized per u n i t o f i n t e r n a l surface area and p l o t t e d versus temperature on Figure 3.
Based on a single p a i r o f runs, the a c t i v a t i o n energy seems t o be
about 15 kcal/mole.
Here the smaller pore diameter c a t a l y s t s Show l e s s a c t i v i t y
than the larger pored ones and clearcut p a r t i c l e s i z e effect's
can a l s o be
seen. Figure 4 shows the r a t e constants estimated from Figure 3 a t 640°F p l o t t e d versus the term Dp(pore dia)-lm5. This term contains t h e two important parameters which e f f e c t the Thiele modulus and which we have allowed t o vary i n t h i s work.
The p a r t i c l e size (Dp)
n a t u r a l l y enters the Thiele modulus t o the
f i r s t power but, as discussed e a r l i e r , the pore diameter i s a parameter which
368
influences the e f f e c t i v e d i f f u s i o n c o e f f i c i e n t i n an unknown and complex way. We have found t h a t by applying a -1.5 power t o the pore diameter, the s c a t t e r tn
Figure 4 is reduced and the points f i t a single curve. The points can now be forced t o f i t a curve based on the development of Thiele theory. This f i t t i n g technique i s s i m i l a r t o t h a t used
,130
9.4
Fig. 3.
9.2
9.0
8.8
*
A dp
8.6
Boscan Demetalation Kinetics
Fig. 4. S i m p l i f i e d Thiele P l o t For Demetalation o f Boscan Residuum a t 640°F
when one compares performance o f two c a t a l y s t s o f d i f f e r e n t sizes. The p o i n t where the curve intercepts the y-axis represents the maximum a c t i v i t y (per u n i t area) t h a t can be achieved a t t h i s temperature. The r a t i o o f a r a t e constant a t any p o i n t on the curve t o t h i s maximum value i s approximately equal t o t h e demetalation effectiveness f a c t o r f o r a c a t a l y s t w i t h the p a r t i c u l a r combination o f Dp and pore diameter which corresponds t o the point. Having correlated the data i n t h i s fashion, p r e d i c t i o n s can be made. Figure 5 shows the predicted a c t i v i t y versus pore diameter w i t h p a r t i c l e Size as a parameter. This p l o t , which assumes a c a t a l y s t pore volume o f 0.5 cc/g. shows t h a t the optimum pore diameter for c a t a l y s t a c t i v i t y varies w i t h the p a r t i c l e
369
diameter.
A small p a r t i c l e size,
small Pore diameter c a t a l y s t i s the most
The f a c t t h a t small p a r t i c l e size,
high i n t e r n a l surface area c a t a l y s t s are
active.
optimum
is
intuitively
obvious
for
a
pore
diffusion
limited
reaction.
Sometimes, however, one i s forced t o choose a larger p a r t i c l e s i z e based on other considerations (because of pressure drop considerations i n a f i x e d bed o r f l u i d i z i n g v e l o c i t y considerations i n a f l u i d bed). I n any case, there i s an optimum i n t e r n a l surface area and pore diameter f o r each c a t a l y s t size.
The
amount o f hydrogenation component i n the i n i t i a l c a t a l y s t i s also known t o be important ( 7 ) .
As Spry and Sawyer (26) have pointed out, each crude o i l w i l l
have a d i f f e r e n t optimum combination o f c a t a l y s t size and p o r o s i t y f o r maximum activity.
50
100
150
200
250
300
Fig. 5. Estimated E f f e c t of Catalyst S i z e and Pore Diameter on Boscan Demetalation Kinetics The above shows t h a t we have made some progress i n understanding the f a c t o r s a f f e c t i n g c a t a l y s t a c t i v i t y . I t i s q u i t e l i k e l y , however, t h a t the m s t a c t i v e catalyst w i l l not r e s u l t i n the longest c a t a l y s t l i f e and commercially, the l i f e i s o f t e n more s i g n i f i c a n t than a c t i v i t y . Chevron has looked a t shortt e r m and long-term metals deposition p r o f i l e s on c a t a l y s t s t o t r y t o OpthIiZe catalyst l i f e .
Much o f t h i s work was reported by Tam, Harnsberger, and Bridge
370
(27). It was carried out with a 1/16 in. extrudate catalyst except where otherwise stated. CATALYST LIFE
Several authors (5, 10. 13, 14, 15, 19, 20, 21) have attempted to elucidate the mechanisms by which hydroprocessing catalysts deactivate, and a number of others have formulated models, based on proposed mechanisms, for predicting catalysts deactivation at commercial operating conditions. Residuum hydroprocessing catalysts deactivate in a very characteristic fashion, as shown in a number of articles (4, 8, 15, 19, 27). The average catalyst temperature of a hydrotreater is normally raised to compensate for the deactivation and thereby hold a particular product specification constant. An example of a typical temperature-time curve is shown in Figure 6. Here normalized temperatures are shown instead of actual temperatures to account for the slight variations in feed rate and conversion which inevitably occur.
0.2
0.4
0.6
0.8
1.o
Reduced Time
Fig. 6. Typical Deactivation Curve Arabian Heavy Atmospheric Residuum Desulfur ization, Constant Sulfur The deactivation curve can be subdivided into three distinct parts. The initial period is characterized by a rapid, but continuously decreasing,
371 deactivation rate.
T h i s d e a c t i v a t i o n has o f t e n been blamed on coke d e p o s i t i n g
on t h e c a t a l y s t , a phenomenon known t o be M r e pronounced i n t h e e a r l y p a r t o f a run.
The f i n a l p e r i o d , o r u l t i m a t e d e a c t i v a t i o n , i s c h a r a c t e r i z e d by a r a p i d ,
continuously
increasing
deactivation
rate.
Following
(9), t h i s
Hiemenz
d e a c t i v a t i o n has comnonly been blamed on t h e c o n s t r i c t i o n o f t h e c a t a l y s t p o r e mouths by metal deposits.
These two p e r i o d s a r e separated by a p e r i o d i n which
t h e d e a c t i v a t i o n r a t e appears t o be n e a r l y constant. I n o r d e r t o g a i n a b e t t e r understanding o f c a t a l y s t d e a c t i v a t i o n ,
o f s h o r t p i l o t p l a n t t e s t s were made on Arabian Heavy residuum.
a number
I n each p a i r o f
t e s t s , o n l y one v a r i a b l e was changed, t h e o b j e c t being t o determine by t h e e l e c t r o n m i c r o p r o b l e how these v a r i a b l e s i n f l u e n c e d t h e d e p o s i t i o n of m e t a l s o n t o the catalyst.
SHORT-TERM EXPWIMENTS Examples o f t h e d e p o s i t i o n a l p r o f i l e s f o r n i c k e l , vanadium,
and i r o n a t
b o t h t h e i n l e t and o u t l e t o f t h e c a t a l y s t bed (27) a r e shown i n F i g u r e 7. i s found p r i m a r i l y o u t s i d e t h e c a t a l y s t p a r t i c l e as a t h i n scale. g e n e r a l l y t h e case. to
differences
organometallic concentration
in
molecules. inside
These d i f f e r e n c e s i n d e p o s i t i o n a l p a t t e r n s a r e
the
reactivities
Both
t h e edge
of
nickel the
c o n c e n t r a t i o n approaches t h e edge o f reactor.
This i s
N i c k e l g e n e r a l l y seems t o p e n e t r a t e t h e c a t a l y s t t o a
g r e a t e r e x t e n t t h a n vanadium. due
Iron
and/or
and
particle,
diffusivities
vanadium
display
of
the
a
maximum
but the point o f
maximum
t h e c a t a l y s t near t h e o u t l e t of
the
372
..
012-
. .
.
008-
004
. Vanadium
' Iron 0
0.016
0.008
-
2 1
8+--7-&
.
1
Reactor Inlet
'-,
Nickel
Vanadium
Reactor Outlet
'I0[
1.o
0.5
I
I ) '
0.6
0.4
0.2
I
0
Fractional Radius
Fig. 7. Typical Depositional Patterns f o r Nickel, Vanadium, and I r o n Reaction Temperature = 700°F Hydrogen P a r t i a l Pressure = 1825 p s i a The f a c t t h a t maximum concentrations are found i n s i d e the edge o f the part i c l e i s d i f f i c u l t t o explain. I t may be due t o hydrogen s u l f i d e being a reactant or i t may be due t o s p e c i f i c reaction intermediates being formed. It complicates data analysis,
since pore d i f f u s i o n theory coupled w i t h a simple
r e a c t i o n mechanism does not p r e d i c t an i n t e r n a l maximum. Despite t h i s , i t i s i n t e r e s t i n g t o compare the change i n the maximum deposit concentrations from reactor i n l e t t o reactor o u t l e t w i t h the change i n concentration o f metals i n t h e oil. during the t e s t i n which the p r o f i l e s shown i n Figure 7 were generated. the average vanadium removal was 58% and the average n i c k e l removal was 42%. The maximum deposit concentrations o f both metals decreased by approximating
80% from
reactor
inlet
to
outlet,
demetalation i s not a simple f i r s t - o r d e r reaction.
clearly
showing
that
The change i n t h e maximum
deposit concentrations i s close t o what one would p r e d i c t using second-order k i n e t i c s , assuming t h a t the concentrations o f lnetals i n the feed and product o i l apply t o the maxima a t the respective ends o f the reactor.
373
1.0
0.4 0.2 Fractional Radius
0.8
0.6
0
Fig. 8. The Effect of Reaction Temperature on Vanadium Deposition Hydrogen Partial Pressure = 1825 psia The effect of increasing the reaction temperature on the deposit profiles, shown in Figure 8 was to increase the concentration at the maximum and decrease the effectiveness factor. The effect of temperature on the intrinsic reaction rate has been previously shown to fit Arrheniusl law with an activation energy of 30 kcal/g-mol. Since temperature does not have much of an effect on the effective diffusivities of the reacting molecules. i4s effect on reactivity predominates and the changes in the concentration profiles are "directionally" those predicted by the theory. The effect o f changing hydrogen partial pressure was found to be similar (27). The effect of changing feed source on the depositional patterns was illustrated by the results for Arabian Heavy and Alaskan North Slope residua in figure 9. The vanadium levels in these two residua differ by about a factor of 2 and the maximum deposit concentrations at the top of the bed differ by nearly the same factor. Therefore, despite the fact that vanadium removal in the
374
i n t e g r a l reactor appears t o follow second-order k i n e t i c s f o r both residua, the i n t r i n s i c r e a c t i v i t i e s of the most r e a c t i v e organometallics i n each crude appear on a f i r s t - o r d e r basis, t o be similar. The e f f e c t o f decreasing the c a t a l y s t pore diameter, shown i n Figure 10, was t o concentrate the metals nearer the external surface o f the p a r t i c l e . A change i n c a t a l y s t pore diameter does affect t h e effective d i f f u s i v i t y o f t h e reacting molecules and the observed changes i n t h e metals p r o f i l e s are consistent w i t h the theory.
J
Atrn. Resiuum
c
. 5 0
o.16-
0
Ov = 0.33 Alaskan North Slope
Atrn. Residuum Ov = 0.48
E' *Small Pore. Bv = 0.15 .Large Pore, 8" = 0.20
:z
0.12
psh"
0.04
5
Fractional Radius Fig. 9. The E f f e c t o f Feed Source on Vanadium Deposition Reaction Temperature = 700°F Hydrogen P a r t i a l Pressure = 1825 p s i a
>
Fig. 10. Diameter Reaction Hydrogen
1.0
08
0.6
0.4
0.2
0
Fractional Radius
The E f f e c t o f Catalyst Pore on Vanadium Deposition Temperature = 700°F P a r t i a l Pressure = 1825 p s i a
375 A t the reactor i n l e t , the e f f e c t o f changing the c a t a l y s t p a r t i c l e size,
shown
in
Figure 11,
contaminated
by
is
feed
only
metals.
t o change the For
the
volume f r a c t i o n
catalysts
shown
of
catalyst Figure 11,
in
approximately 90% o f the volume of the 1/32-in. diameter c a t a l y s t i s contaminated by vanadium, while only about 50% o f the volume o f the 1/16-in. diameter c a t a l y s t i s s i m i l a r l y contaminated. A t the reactor o u t l e t , the s i t u a t i o n i s complicated by the f a c t t h a t the reactant molecules remaining i n the o i l are not the same f o r the two c a t a l y s t beds. small-diameter
catalyst
removes more metals per
This r e s u l t s because the unit
of
reactor
length.
Therefore, since the organometallics display a spectrum o f r e a c t i v i t i e s and/or d i f f u s i v i t i e s , the absolute penetration o f metals i n t o the c a t a l y s t p a r t i c l e s can be d i f f e r e n t a t the o u t l e t s o f the two c a t a l y s t beds.
o.20r... .
Ov (1/32-ln.) = 0.38
0.16 -
. 0
Ov (1/16-ln.) = 0.20
0
0
0.12-*
.
% 0
0.08-,
0.04-
.
0 0
b
1
0
G
\%\.
C
1/32-ln. Catalyst
1
1/16-ln. Catalyst
I
1
Fig. 11. The E f f e c t o f Catalyst P a r t i c l e Size on Vanadium Deposition. Reaction Temperature = 7OO0F, Hydrogen P a r t i a l Pressure = 1825 pSia LONG-TERn EXPWIHENTS
The change i n the depositional patterns o f n i c k e l and vanadium w i t h time was studied by i n t e r r u p t i n g the run shown I n Figure 6, a t the indicated points, t o recover small amounts o f catalyst. showed a number o f
The r e s u l t s o f the microprobe analyses
i n t e r e s t i n g features.
Vanadium showed a decrease I n
376 e f f e c t i v e n e s s f a c t o r from the i n l e t t o the O u t l e t o f the reactor, showed t h e reverse behavior. reactor
outlet
compounds.
appeared
to
but n i c k e l
The r e a c t i v i t y of the n i c k e l compounds a t the be about
the
same as t h a t o f the vanadium
Nickel seemed t o have about twice t h e d i f f u s i v i t y of vanadium.
In
general, the metals deposition r e s u l t s i n d i c a t e t h a t t h e p e n e t r a t i n g metals t e n d t o deposit i n much the Same place i n the c a t a l y s t throughout t h e run.
This
gives credence t o our analysis of the s h o r t p i l o t p l a n t runs r e p o r t e d e a r l i e r i n t h i s paper. Figure 12 shows t h e change w i t h time o f t h e concentration o f vanadium a t A simple t h e p o i n t of maximum buildup f o r t h r e e l e v e l s i n the c a t a l y s t bed. c a l c u l a t i o n on monolayer coverage of vanadium s u l f i d e , suggests t h a t a t t h e t o p o f the bed the maximum deposit represents 5-12 monolayers.
I f t h e d e p o s i t were
V3Sq and had t h e density of the bulk s u l f i d e , such a deposit would be 15-40
A in
For a c a t a l y s t w i t h a pore diameter i n the range 100-200 A, t y p i c a l o f many residuum hydroprocessing c a t a l y s t s (6. 16). such a deposit would reduce t h e depth.
diameter o f the pores s i g n i f i c a n t l y .
0.2c -I
0.4
0.6
0.8
1.o
Reduced Time
Fig. 12. Maximum Vanadium Deposit Concentration as a Function o f Reactor P o s i t i o n and Time
377 The physical obstruction o f the pore mouths would decrease the e f f e c t i v e d i f f u s i v i t y f o r the reactant molecules and, thereby, increase the Thiele modulus I f the desired reaction were already near t h e f o r the desired reaction. d i f f u s i o n l i m i t when the c a t a l y s t was fresh, i t might well be expected t o become d i f f u s i o n l i m i t e d when the c a t a l y s t was heavily laden w i t h metals.
I n this
case, temperature would have t o be raised a t an ever increasing r a t e t o maintain conversion.
Such
a
situation
is
typical
of
the
latter
stages
of
a
hydroprocessing run as i l l u s t r a t e d i n Figure 6. The e f f e c t o f t h i s pore mouth plugging on c a t a l y s t a c t i v i t y was measured q u a n t i t a t i v e l y i n another experiment. A c a t a l y s t bed which had reached a t y p i c a l end-of-run condition was divided i n t o s i x sections, and the a c t i v i t y was measured independently f o r each section. A dramatic a c t i v i t y p r o f i l e was found. The top one-third o f the bed was v i r t u a l l y dead, having l i t t l e more than one-third the a c t i v i t y o f t h e average bed and l e s s than one-sixth the a c t i v i t y o f the bottom o f the bed. The bottom one-third o f the bed, while s i g n i f i c a n t l y deactivated r e l a t i v e t o t h e f r e s h c a t a l y s t , was r e l a t i v e l y unaffected by pore plugging and s t i l l had s u f f i c i e n t a c t i v i t y t o be useful. Similar a c t i v i t y
profiles
have
been observed
comnercially
where
the
temperature r i s e across i n d i v i d u a l sections o f the c a t a l y s t bed gives a measure of c a t a l y s t a c t i v i t y (18). Pore plugging, therefore, occurs as a "wave" which, a f t e r an induction time, moves from the i n l e t o f the reactor toward the o u t l e t . PORE MOUTH PLUGGING The onset o f the pore-plugging wave and the r a p i d i t y w i t h which i t moves
through the bed are dependent on the d e t a i l s o f the c a t a l y s t pore structure, t h e d i s t r i b u t i o n o f metals w i t h i n the c a t a l y s t p a r t i c l e s , and the d i s t r i b u t i o n o f metals along the length o f the c a t a l y s t bed. The pore s t r u c t u r e d i r e c t l y determines the maximum local deposit which can be t o l e r a t e d before i n t r a p a r t i c l e transport
of
reactants
and products
i s adversely
affected.
The maximum
concentration o f deposit w i t h i n a c a t a l y s t p a r t i c l e a t a given time has already been shown t o be dependent on process and c a t a l y s t variables. The more Uniform the i n t r a p a r t i c l e d i s t r i b u t i o n , the lower the concentration a t the maximum w i l l be a f t e r a given time, and, therefore, the l a t e r the onset o f pore plugging w i l l occur.
The r a t e o f advance o f the pore plugging wave, on the other hand, i s
r e l a t e d t o the u n i f o r m i t y o f the i n t e r p a r t i c l e d i s t r i b u t i o n along the length of
378
The more uniform t h i s d i s t r i b u t i o n i s , t h e more r a p l d l y t h e wave transverse the reactor. This simple p r i n c i p l e i s i l l u s t r a t e d by t h e
t h e reactor.
will
f o l l o w i n g example.
-
820
P
e
3
c
E 0) n
E
1/164n. Catalyst 0
1/30-ln. Catalyst
800 -
t
760 780
I
0
1
0.2
I
0.4
Reduced Time =
0.6
0.8
J
1.o
t tEOR (1/30-ln. Catalyst)
f i g . 13. The E f f e c t o f Catalyst P a r t i c l e Size on Catalyst Deactivation I r a n i a n Heavy Atmospheric Residuum Desulfurization, Constant S u l f u r Two c a t a l y s t s having i d e n t i c a l properties, except f o r t h e i r p a r t i c l e size, were Used t o desulfurize I r a n i a n Heavy atmospheric residuum t o an equal extent
a t i d e n t i c a l processing conditions. Figure 13.
Their deactivation curves are compared i n
The onset o f pore plugging a t the top o f the c a t a l y s t bed would
occur a t e s s e n t i a l l y the same time i n these two t e s t s because t h e porous properties of the catalysts are the same, and the processing conditions are t h e same ( w i t h the exception o f the subsequent temperature program). However. t h e speed w i t h which the pore plugging wave moves through the bed i s very d l f f e r e n t ; and, therefore, the deactivation schedules are very d i f f e r e n t . Because a l a r g e r f r a c t i o n o f the c a t a l y s t volume i s accessible t o the depositing metals w i t h t h e small s i z e catalyst, more metal i s accommodated a t the top o f the bed; and t h e metal concentration p r o f i l e down the c a t a l y s t bed i s steepened.
A t the
decreased concentrations o f metal contaminants t o which the lower p a r t o f t h e
379
bed i s exposed,
more t i m e i s r e q u i r e d f o r
t h e maximum d e p o s i t t o r e a c h i t s
l i m i t i n g value, thereby slowing t h e r a t e of t r a v e l o f t h e p o r e p l u g g i n g wave.
INITIAL CATALYST DEACTIVATION The c a t a l y s t d e a c t i v a t i o n which occurs b e f o r e t h e onset o f p o r e mouth p l u g g i n g i s more d i f f i c u l t t o c h a r a c t e r i z e and t h e r e i s c o n t r o v e r s y r e g a r d i n g whether i t i s due t o coke or m e t a l s d e p o s i t i o n . I n t + e e a r l y stages o f a hydroprocessing run, a f r a c t i o n o f t h e c a t a l y s t ' s s u r f a c e area i s converted from i t s o r i g i n a l s t a t e t o a s u r f a c e composed o f mixed n i c k e l and vanadium s u l f i d e s .
While these s u l f i d e s do have c a t a l y t i c a c t i v i t y
f o r hydrogenolysi s, they a r e c o n s i d e r a b l y l e s s a c t i v e t h a n t h e f r e s h c a t a l y s t s used i n these s t u d i e s .
Under these c o n d i t i o n s , t h e c a t a l y s t temperature must be
r a i s e d a c c o r d i n g l y t o h o l d conversion constant.
This form o f " p a r t i a l surface
p o i s o n i n g " may be t h e major cause o f d e a c t i v a t i o n i n t h e e a r l y p a r t o f a run. As
i l l u s t r a t e d i n Figure 6,
the period o f i n i t i a l c a t a l y s t deactivation
is
c h a r a c t e r i z e d by a high, b u t d e c l i n i n g , d e a c t i v a t i o n r a t e which a s y m p t o t i c a l l y approaches a c o n s t a n t value a t a reduced t i m e o f a p p r o x i m a t e l y 0.25.
Such
behavior would be a reasonable consequence o f t h e proposed p a r t i a l s u r f a c e p o i s o n i n g mechanism i f m u l t i l a y e r s o f t h e contaminant d e p o s i t have t h e same c a t a l y t i c a c t i v i t y as t h e i n i t i a l monolayer. A
high level of
catalyst. the
coke does f o r m r a p i d l y
i n an o u t e r annulus o f t h e
However, i t d e c l i n e s s l o w l y as f e e d metals d e p o s i t and i s o f f s e t by
increase o f
coke
i n the
i n t e r i o r o f the catalyst.
Since t h e i n i t i a l
d e p o s i t i o n a l p a t t e r n o f coke p a r a l l e l s t h a t o f t h e metals, b o t h p r o b a b l y b e i n g due t o t h e presence o f
h i g h molecular weight species,
unequivocally assign r e s p o n s i b i l i t y f o r the
initial
contaminant.
can
However,
several
arguments
be
it i s difficult
to
deactivation t o e i t h e r offered
which
favor
o r g a n o m e t a l l i c s as t h e primary d e a c t i v a n t when t h e metals c o n t e n t o f t h e f e e d exceeds about 10 ppm.
The l e n g t h o f t h e i n i t i a l d e a c t i v a t i o n p e r i o d i s d i r e c t l y
r e l a t e d t o the concentration o f organometallics i n the feed but n o t t o t h e c o n c e n t r a t i o n o f coke p r e c u r s o r s (as measured, f o r example, by Conradson Carbon c o n t e n t ) i n t h e feed.
The p e r i o d o f a c c e l e r a t e d coke laydown i s s h o r t r e l a t i v e
t o t h e e n t i r e i n i t i a l d e a c t i v a t i o n period, complex changes throughout t h i s time.
and t h e d e p o s i t e d coke undergoes
On t h e o t h e r hand, t h e m e t a l d e p o s i t s
b u i l d up m o n o t o n i c a l l y , and t h e time r e q u i r e d t o achieve monolayer coverage
380 throughout the reactor i s comparable i n length t o the i n i t i a l d e a c t i v a t i o n p e r i od
.
REACTOR BED PLUGGING Catalyst l i f e can sometimes be d i c t a t e d by the reactor pressure drop increasing t o an excessive l e v e l . The most comnon instances o f t h i s are due t o the presence o f p a r t i c u l a t e material i n the feed stream. This problem can be avoided i f p a r t i c u l a r a t t e n t i o n i s given t o the crude o i l desalting and the feed f i l t e r i n g operations. Chevron has experienced pressure drop increases i n one of i t s u n i t s i n which the feedstock p a r t i c u l a t e material i s n e g l i g i b l e . This u n i t i s the No. 1 Isomax p l a n t i n Chevron U.S.A.'s Richmond Refinery. I t hydrocracks up t o 32,000 BPOO o f a 14"API C a l i f o r n i a deasphalted o i l containing 17 ppm n i c k e l , 7 ppm vanadium, and 9 ppm iron.
The product i s metal free.
The i r o n i s
thought t o e x i s t i n the form o f i r o n naphthenates. The c a t a l y s t l i f e o f t h i s u n i t i s l i m i t e d by pressure drop buildup due.. t o plugging o f the top beds ( 4 ) .
Chemical analysis o f the i n t e r s t i t i a l m a t e r i a l
found i n the f i r s t r u n i s shown i n Figure 14. The major plugging component was i r o n sulfide. The quantity o f i r o n found i n the deposit agreed very w e l l w i t h the quantity o f soluble i r o n present i n the feed during the course o f the run. For t h i s i n i t i a l operation the i r o n l e v e l i n the feed was between 4 and 5 ppm.
38 1
i .i
I\
Fraction of Top Bed
Fig. 14. Composition o f I n t e r s t i t i a l Deposit Found i n Top Bed of Richmond Isomax No. 1 Reactor Run No. 1 1966 Spent catalysts from t h i s u n i t show a buildup of i r o n s u l f i d e on t h e
-
external surface o f the p a r t i c l e s .
This indicates t h a t i r o n naphthenates r e a c t
w i t h hydrogen t o form i r o n s u l f i d e and t h a t the i r o n s u l f i d e CatalyZeS t h e
subsequent reaction.
The i r o n naphthenates are apparently very reactive.
The
r a t e of buildup o f the i r o n s u l f i d e layer i s m c h f a s t e r than the r a t e of pore mouth plugging by nickel and vanadium sulfides. and seems t o be unique t o C a l i f o r n i a heavy o i l s .
This i s an unusual S i t u a t i o n
CONCLUSION Hydrodemetalation i s an example o f a reaction whose r a t e i s c o n t r o l l e d by pore d i f f u s i o n .
Catalyst a c t i v i t y i s therefore influenced by c a t a l y s t p a r t i c l e
size and porosity. Catalyst l i f e i s a complicated f u n c t i o n o f these and other variables, including process conditions and feedstock properties. Microprobe analysis o f catalysts, used i n upgrading metal containing heavy o i l s , reveal
382
metal deposits which are consistent with, although somewhat more complex than, those expected from simple pore d i f f u s i o n theory.
C a t a l y s t d e a c t i v a t i o n appears
t o be the r e s u l t o f a t l e a s t two d i f f e r e n t mechanisms, both i n v o l v i n g metals deposition. These are surface poisoning, and pore mouth plugging. very complicated. temperatures
Process o p t i m i z a t i o n i s
because surface poisoning i s most serious a t low o p e r a t i n g
and pressures
whereas
pore mouth plugging
i s worst
at
high
temperatures and pressures. There i s a strong economic i n c e n t i v e t o develop improved c a t a l y s t s and processes f o r upgrading these heavy o i l s . capable
of
describing
the
complicated
I n order t o develop accurate models r e a c t i o n k i n e t i c s and
deactivation
phenomena, more experimental e f f o r t i s needed i n feedstock c h a r a c t e r i z a t i o n and the importance o f c a t a l y s t pore s i z e d i s t r i b u t i o n and operating c o n d i t i o n s on c a t a l y s t performance. S c i e n t i s t s a t Chevron Research,
some o f whom s t a r t e d t h e i r careers
G u l f ' s RHOS development e f f o r t are continuing work i n t h i s f l e l d . the
in
As a r e s u l t
Chevron Residuum Hydrotreating Process now uses a v a r i e t y o f
tailored
c a t a l y s t combinations a v a i l a b l e from a number o f d i f f e r e n t sources (11).
ACKNOWLEDGMENTS The author wishes t o acknowledge the c o n t r i b u t i o n s o f h i s f e l l o w workers a t Chevron Research, p a r t i c u l a r l y the work o f Paul W. Tamm i n t h e g a t h e r i n g and i n t e r p r e t a t i o n o f the microprobe work.
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