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Catalyst Deactivation by Poisoning and Pore Plugging in Petroleum Processing
545
B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 0 1980 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
CATALYST DEACTIVATION BY POISONING AND PORE PLUGGING IN PETROLEUM PROCESSING S . T . SIE
Koninklijke/Shell-Laboratori~,Amsterdam (Shell Research B.V. )
, Badhuisweg 3,
1031 CM Amsterdam, (The Netherlands)
ABSTRACT A review is given of aspects of catalyst poisoning in major petroleum refining processes. Processes considered include hydrogen manufacture, polymerization of olefins, isomerization of light paraffins, reforming of naphtha, catalytic cracking, hydrotreating and hydrocracking. Particular attention is given to the hydroprocessing of residual feedstocks, where catalyst deactivation by poisoning and poreplugging represents a considerable, though not unsurmountable problem.
INTRODUCTION Catalytic processing is a keystone of today's petroleum refining. The catalytic processes applied in the petroleum industry are generally large-scale, continuous processes operating without interruption for long intervals. This is made possible by employing rugged catalysts and by adequate safeguarding against catalyst deterioration.
An unscheduled temporary shutdown of a plant necessitated by deterioration of catalyst performance (e.g. l o s s of activity, selectivity, permeability to flow, fluidizability, etc.) generally represents a significant economic penalty in view of the large capital tied up in the plant, and in the case of a noble metal catalyst, in the catalyst itself. Deactivation of the catalyst, which may occur by physical transformations (such as sintering), coking or poisoning, is therefore to be minimized as much as is feasible. The present paper reviews some aspects of catalyst poisoning, defined as the deactivation caused by strong, often irreversible adsorption of feed contaminants in major refinery processes employing heterogeneous catalysts. These include the processing of gases, distillates and residual feedstocks. Particular attention will be given to hydroprocessing of residues, since with this type of feedstock the problem of catalyst deactivation is much more inherent in the use of this feedstock than in the processing relatively clean gases or distillates.
546 SOME G E N E W FEATURES OF CATALYST POISONING IN INDUSTRIAL PROCESSES
The phenomenon of catalyst poisoning in continuous-flow processes as are usually applied in the petroleum industry is much more complex than in laboratory experiments designed to study the adsorption of poisons on the catalyst and its effect on activity or selectivity. With petroleum feedstocks a wide spectrum of contaminants is usually present, rather than just a single species. Thus, although it is quite common to refer to poisoning by "sulfur", "nitrogen" or "metals", one usually deals with an often unknown distribution of compounds which may have individual adsorption and poisoning characteristics (refs. 1 , 2). Unless the process conditions are sufficiently severe to convert these compounds to a common single species such as H2S, NH etc., it is an oversimplication to ignore the 3 individuality of the poisoning compounds. Fortunately, in many of the processes discussed hereafter this simplification appears to be permissible. The strong and fast adsorption of a poison may give rise to concentration gradients on a micro-scale as well as on a macro-scale. In the case of pore diffusion limitation (ref. 3 ) there will be a concentration profile across a catalyst particle which changes with time (shrinking active core). In a fixed catalyst bed there may likewise be a deactivation front which gradually moves in the flow direction. It is therefore not easy to relate overall process performance with the adsorption behaviour of a poison. The situation is further complicated by the usual practice of adiabatic operation (resulting in axial temperature profiles) and temperatureprogramming as a measure to offset catalyst deactivation in the course of time. A number of authors have theoretically analysed the phenomenon of catalyst fouling in industrial reactors and attempted to describe it mathematically, e.g., Chou et al. (ref.
4), Szepe and Levenspiel (ref. 5 ) , Lee and Crowe (ref. 6j , Butt
and Rohan (ref. 7 ) , Sadana and Doraiswamy (ref. 8) and Weekman (ref. 9). More recent work includes that of Schlosser (ref.
lo),
and Krishnaswamy amd Kittrell
(ref. ll), who describe the effect of temperature programming for constant conversion as applied in hydrocracking or reforming. Beeckman and Froment (ref. 12) considered deactivation by active-site coverage and pore blockage. Although they primarily considered coking as the cause of catalyst deactivation, there is an analogy with catalyst poisoning and pore plugging by metals as occur in processing of residual oils (see below). CATALYST POISONING IN PROCESSING OF GASES AND PETROLEUM DISTILLATES Hydrogen manufacture The manufacture of hydrogen as a supplement to by-product hydrogen from catalytic reforming of naphthas plays an increasingly important r81e in petroleum refining. The usual manufacturing route involves production of CO and H2 by either steam reforming of light hydrocarbons (methane in particular) or by non-catalytic partial oxidation of heavy oil, followed by CO shift and removal of residual CO by
547
rr,ethanation. (See Fig. 1 )
CH4 H20
-
STEAM REFORMING
-
H.T. L.T. SHIFT I SHIFT
CH4
+ H20
-
-
METHANATION
-
H2
CO + 3H2
CATALYST: Ni CO-SHI FT:
CO + H20
-C
H2 + CO2
H.T. CATALYST: Fe3O4 ( L.T.
,I
+ Cr203)
Cu/ZnO
METHANATION: CO
+ 3H2
CHq
+ H20
CATALYST: Ni
Fig. 1. Catalytic steps involved in hydrogen manufacture from methane Steam reforming. The most prominent poisons for the usual nickel-type catalyst are sulfur and halogens. Another poison of practical significance is arsenic, which may originate from CO2-removal process utilizing arsenous oxide as catalyst (e.g. the Giammarco-Vetrocoke process). The poisoning effect of sulfur appears to be reversible: a sulfur-poisoned catalyst may recover its full activity when operated without sulfur in the feed or a sulfur concentration below a certain threshold level. This level generally increases with operating temperature. For instance, for an ICI catalyst a sulfur tolerance has been reported ranging from 1 ppm at 800 OC to 25 ppm at 900
OC
(ref. 13). According to Rostrup-Nielsen (ref. I h ) ,
the reversible adsorption of sulfur can be described by a Langmuir isotherm with the H S/H ratio as concentration term. On this basis Christiansen and Anderson 2 2 (ref. 1 5 ) have developed a mathematical model to calculate transient sulfur profiles and sulfur break through in steam reformers.
548
Chlorine has an effect comparable to that of sulfur. Approximately the same concentration limits apply, and the effect is also reversible (ref. 13). Unlike sulfur and chlorine poisoning, arsenic poisoning is not reversible. Therefore, there is in principle no fixed maximum level for steady operation. In experiments carried out at Imperial Chemical Industries (ref. 13) the effect o f arsenic became noticeable when the As 0 content of the catalyst exceeded 50 ppm. 2 3 CO-shift. The high-temperature CO-shift catalyst used to convert the bulk of ..................................
CO at 320-500
is usually Fe30h stabilized by addition o f Cr 0 This catalyst 2 3' is fairly tolerant of sulfur: H S or COS in concentrations below about 200 ppm 2 have no effect on activity (ref. 13). Operation with much more s u l f u r appears to OC
be even possible, albeit at a reduced activity level. This is because FeS itself is a catalyst for the water gas shift reaction, being about half as active as Fe 0
3 4'
Consequently, high-temperature shift catalysts do not normally suffer from sulfur poisoning in practice, in particular not when they are preceded by the much more sensitive steam reforming catalysts. Large quantities of halogens would deactivate high-temperature shift catalysts, but these are not encountered during normal plant operation. The low-temperature CO-shift catalyst, which operates at lower temperatures .................................
(180-280 O C ) , where thermodynamics permit a higher conversion of CO, is generally composed of copper and zinc oxide. This catalyst is sensitive to sulfur and halogens, the most important poisons in practice. Both poisons react almost irreversibly
with the catalyst and therefore collect at the front end of the bed, thus
protecting the downstream portion of catalyst. Table 1 presents some data by Campbell (ref. 16), which may serve to illustrate this feature and to indicate concentration levels of interest. With respect to the poisoning mechanism, Young and Clarke (ref. 17) conclude that poisoning by sulfur is probably a surface effect and does not involve the formation of bulk cuprous sulfide. The latter would require an H S concentration of 3 ppm at about 230 OC, whereas deactivation 2
in a commercial plant was observed at much lower sulfur levels. The effect of chlorine is ascribed to the formation of low-melting zinc chloride, which may induce growth of zinc oxide crystallites. Klier (ref. 18), however, attributed chloride poisoning of a similar catalyst used in methanol synthesis to chemisorption on the Cu'
centres, which are assumed to be the sites for CO hydrogenation.
The more recently developed sulfur resistant CO-shift catalysts for rnediumand low-temperature duty are sulfided Co/Mo or Ni/Mo systems, often promoted with alkali metals. These mixed sulfides are obviously not poisoned by sulfur. Since they are akin to the hydrotreating catalysts (see below), they should be similarly rugged as far as poisoning is concerned.
549
TABLE 1 Effect of sulfur and chlorine poisoning of a low-temperature CO-shift catalyst in commercial operation (after Campbell (ref. 16) Position of catalyst in bed
TOP
Bottom
Activity relative to that at bottom of bed,
%
so3, %w
c1, %w
1.4
0.08
0.7 0.4
0.04
15
0.01 0.01
72
0.01
100
0.2 0.2
15
75
Methanation. Removal of residual CO in H2 is usually carried out by methanation over nickel catalysts. A s with steam reforming catalysts, sulfur is among the most notorious poisons. However, in the hydrogen-manufacturing context it is unlikely that sulfur poisoning is a serious practical problem since the Cu/ZnO low-temperature shift catalyst itself is poisoned by sulfur and a good sulfilr catcher. Halides also poison nickel methanation catalysts, but this does not normally pose a problem, for the same reason. The situation is different when a sulfur-resistant CO-shift catalyst is applied (which may even require operation with some sulfur to maintain the sulfidic state of the catalyst) or when only part of the gas i s passed through the shift section (as proposed in some substitute natural gas manufacturing schemes). As compared with the nickel catalyst in steam reforming, the catalyst in methanation is operated at a much lower temperature (e.g.
300-400
OC
versus 800-900 OC) and sulfur
is therefore more strongly bound in the methanation case. Serious deactivation already occurs when the sulfur content of a 30 % Ni catalyst exceeds about 0.1-0.2
%w
(ref. 13). The r81e of the H2S concentration in the reactant stream (more specifically the H2S/H2 ratio) is illustrated by some results of pilot plant methanation tests at Sasol, South Africa (ref. 19). At 0.05 ppm H S a supported Ni catalyst showed a 2
slow deactivation at the inlet portion of the bed, but no further deactivation farther into the bed even after 50 days on stream. On the other hand, 3 ppm H2S caused rapid and progressive poisoning, which was already appreciable after only six days of operation. Bartholomew et al. (ref. 2 0 , 21) also observed significant activity losses upon exposure of a nickel catalyst for one or a few days to a reaction mixture containing 1-10 ppm H2S. The strong bonding of sulfur under methanation conditions gives rise to a relatively sharp sulfur front in a plugflow-type reactor. The occurrence of such fronts has been demonstrated by Richardson (ref. 22). As with steam reforming catalysts, arsenic is a poison for methanation catalysts.
0.5 %w As on catalyst may result in loss of about half of the activity (ref. 13).
550
i2oigmerization of C&b
olefins
Polymerization of propene and butenes to produce oligomers suitable as gasoline component is carried
OUL
wit% acidic catalysts. The most widely used catalyst is
phosphoric acid on a silica support, e.g., kieselguhr in the process developed by Universal Oil Products Co (UOF) or granular quartz in the process of Standard Oil of California. 3asic components such as ammonia or amines are obvious poisons for the acidic catalyst and it is customary to remove them from the feed by water washing (ref. 23). Concentrations of nitrogen far below stoichiometric amounts, e.g., 0.1-0.2 %w on catalyst are sufficient to cause unsatisfactory performance in commercial units (ref. 24). This implies that the nitrogen content of the feed must be below 50 ppm in order to permit attainment of a satisfactory catalyst life (say, 200 gal. polymer/lb catalyst (ref. 24)). Isomerization of light paraffins Isomerization of n-butane to isobutane (desired as alkylation component) and isornerization of pentanes and hexanes (to improve the octane number of "tops") are also acid-catalysed reactions. However, in most modern isomerization processes a bifunctional catalyst is used under hydroprocessing conditions. In such a catalyst the acid function is combined with a hydrogenatioq/dehydrogenation function provided by a (noble) metal. The latter may serve to initiate the formation of olefins, which are converted into carbonium ions upon protonation. In the case of strong acids, which are capable of generating carbonium ions by abstraction of hydride ions from the paraffins, the function of the metal may be to maintain a suitable c a r b o n i m i o n c o n c e n t r a t i o n by establishing paraffin/olefin equilibrium under an appropriate hydrogen pressure. Thus, undesired disproportionation reactions leading to polymers and coke are kept under control (refs. 25, 26). Among the processes applying such dual-function catalysts are the ones developed by UOP (Butamer, Penex), British Petroleum (BP) and Shell (Hysomer). Catalyst poisons in these processes may affect either the noble metal function or the acid
function. Sulfur is the most prominent poison for the metal, but with a suitably chosen metal some sulfur can be tolerated. For instance, the Hysomer process can operate with a permanent sulfur level of 10 ppm, and temporary concentrations of 20 pprn (ref. 26) or even 100 ppm (ref. 2 6 ~ )are not harmful. Sulfur compounds are reportedtolowerthe activity of the UOP catalyst,but their effect istransient (ref. 27).Desulfurization of thefeedismentionedas apartoftheBPprocess (ref. 28). The acid function will be poisoned by basic impurities such as nitrogen compounds. Aromatics and water, being more basic than paraffins, in principle also act as poison. However, in the presence of hydrogen and an active noble metal small amounts of benzene are rendered harmless by hydrogenation in situ to cyclohexane, as occurs in the Hysomer and Penex processes. Alternatively, hydrogenation over a prebed can be applied (ref. 28).
551 With aluminium c h l o r i d e or c h l o r i n a t e d alumina as t h e a c i d i c component, water may a l s o reduce a c t i v i t y by s t r i p p i n g o f f H C I . A feed d r i e r i s t h e r e f o r e incorpo-
r a t e d i r ~~ i i eUOF p r o c e s s ( r e f . 2?), whereas t h e SF p r c c e s s not o n l y u s e s a dric-i feed but f e a t u r e s c o n t i n w u s a d d i t i o n of c h l o r i d e t o t h e feed a s make up ( r e f s . 30,
3 1 ) . S h e i l ' s Hysomer p r o c e s s uses a v e r y rugged z e o l i t e - b a s e d c a t a l y s t and o p e r a t e s without problems w i t h w a t e r c o n t e n t s i n t h e f e e d up t o t h e s a t u r a t i o n l e v e l a t ambient t e m p e r a t u r e ( r e f . 26~). C a t a l y t i c reforming o f naphtha C a t a l y t i c r e f o r m i n g of naphtha ( f r a c t i o n o f t h e crude o i l b o i l i n g i n t h e gaso l i n e r a n g e ) , which i s t h e major o c t a n e upgrading p r o c e s s i n t h e petroleum i n d u s t r y ,
i s c a r r i e d o u t o v e r a b i f u n c t i o n a l c a t a l y s t , which g e n e r a l l y c o n s i s t s o f platinum s u p p o r t e d on h a l o g e n a t e d alumina. I n modern b i - or m u l t i - m e t a l l i c c a t a l y s t s p l a t i n u m
i s combined w i t h o t h e r e l e m e n t s , such as Re, Ge, Ir o r Sn. Apart from g r a d u a l d e a c t i v a t i o n by d e p o s i t i o n o f carbon (which i s t a k e n c a r e o f i n t h e p r o c e s s by p e r i o d i c r e g e n e r a t i o n o f t h e c a t a l y s t ) t h e c a t a l y s t may be d e a c t i v a t e d by p o i s o n s a c t i n g on t h e p l a t i n u m o r t h e a c i d f u n c t i o n . The p l a t i n u m f u n c t i o n , which c a t a l y s e s t h e dehydrogenation of naphthenes and d e h y d r o c y c l i z a t i o n of p a r a f f i n s t o a r o m a t i c s ,
is impaired by t o o high c o n c e n t r a t i o n s o f s u l f u r . A t t h e h i g h t e m p e r a t u r e s a p p l i e d and under t h e hydrogen p r e s s u r e s u l f u r compounds l o s e t h e i r i d e n t i t y as t h e y a r e c o n v e r t e d i n t o H S ( r e f . 32), which b u i l d s up t o some e x t e n t i n t h e r e c y c l e g a s 2
system. H S i s a r e v e r s i b l e p o i s o n f o r p l a t i n u m and i t s c o n c e n t r a t i o n must g e n e r a l l y 2 be l i m i t e d t o l e s s t h a n 100, e . g . , 10 ppm, f o r t h e "monometallic" r e f o r m i n g c a t a l y s t s ( r e f . 3 2 ) . Whereas t h e s e o l d e r t y p e s o f c a t a l y s t a r e t h u s f a i r l y s u l f u r t o l e r a n t and even r e q u i r e t r a c e s o f s u l f u r t o a v o i d e x c e s s i v e m e t a l - c a t a l y s e d c r a c k i n g t h e more modern b i m e t a l l i c c a t a l y s t s a r e more s e n s i t i v e . Although s u l f u r s p e , c i f i c a t i o n depends on t h e f o r m u l a t i o n , t h e y a r e g e n e r a l l y o p e r a t e d a t s u l f u r l e v e l s o f one o r a few ppm a t most ( r e f s . 33, 36, 3 7 ) . To meet t h e s u l f u r s p e c i f i c a t i o n , t h e f e e d i s d e s u l f u r i z e d by h y d r o t r e a t i n g i n commercial p r a c t i c e . The m e t a l f u n c t i o n can be more permanently damaged by m e t a l s such as a r s e n i c and l e a d ( o r i g i n a t i n g , f o r i n s t a n c e , from t e t r a e t h y l e a d c o n t a m i n a t i o n ) , which a r e among t h e c l a s s i c a l p o i s o n s f o r platinum ( r e f s . 34, 3 5 ). However, t h e h y d r o t r e a t i n g p r o c e s s upstream p r o v i d e s e f f e c t i v e p r o t e c t i o n , s i n c e t h e s e metals a r e g e n e r a l l y caught by t h e h y d r o t r e a t i n g c a t a l y s t ( r e f . 3 6 ) . A s t o t h e s e r i o u s n e s s o f a r s e n i c p o i s o n i n g , H e t t i n g e r e t a l . ( r e f . 38) mention t h a t 0.01-0.02
%w As on c a t a l y s t h a s
l i t t l e e f f e c t on dehydrogenation and d e h y d r o c y c l i z a t i o n , b u t 0.5 %w c a u s e s a p p r e c i a ble deactivation. The a c i d f u n c t i o n i s poisoned by ammonia formed from o r g a n i c n i t r o g e n compounds when p r e s e n t i n t h e f e e d . I n commercial p r a c t i c e , n i t r o g e n compounds are removed from t h e f e e d c o n c u r e n t l y w i t h s u l f u r by h y d r o t r e a t i n g . N i t r o g e n l e v e l s below
2 ppm a r e u s u a l l y sought ( r e f . 3 2 ) . Water i s a f e e d contaminant which may a l s o a f f e c t t h e a c i d f u n c t i o n by s t r i p p i n g o f f t h e halogen from t h e c a t a l y s t , t h u s
552 u p s e t t i n g t h e b a l a n c e between t h e a c i d and m e t a l f u n c t i o n s . To cope w i t h t r a c e
amounts of w a t e r or oxygen compounds i n t h e f e e d , c h l o r i d e is c u s t o m a r i l y dosed t o t h e f e e d t o m a i n t a i n a c o n s t a n t l e v e l o f c h l o r i d e on t h e c a t a l y s t .
Catalytic cracking C a t a l y t i c c r a c k i n g o f gas o i l s or f l a s h e d d i s t i l l a t e t o produce i n t . a l . gaso l i n e u s e s an a c i d i c c a t a l y s t , v i z . amorpnous s i l i c a - a l u m i n a or t h e more r e c e n t l y developed c r y s t a l l i n e alumino s i l i c a t e s ( z e o l i t e s ) i n a m a t r i x o f s i l i c a , s i l i c a alumina, or c l a y ( r e f . 39). AreviewofthedevelopmentofcrackingcatalystshasbeengivenbyOblad(ref. 39).
A r e c e n t l y p u b l i s h e d m o n o g r a p h b y V e n u t o a n d H a b i b ( r e f . 39A) c o v e r s s e v e r a l a s p e c t s
o f f l u i d c a t a l y t i c c r a c k i n g , i n c l u d i n g f e e d s t o c k , c a t a l y s t , p r o c e s s hardware a n d p r o duct a s p e c t s . Apart from t h e normally a c c e p t e d d e a c t i v a t i o n by s u r f a c e a r e a d e c l i n e and coking, d e a c t i v a t i o n by p o i s o n i n g may o c c u r , a l t h o u g h t h e s e c a t a l y s t s a r e c o m p a r a t i v e l y r e s i s t a n t t o p o i s o n i n g , b e i n g q u i t e i n s e n s i t i v e t o many s u b s t a n c e s t h a t p o i s o n o t h e r c a t a l y s t s . S u l f u r , f o r i n s t a n c e , h a s l i t t l e e f f e c t on t h e p r e s e n t s y n t h e t i c c a t a l y s t s , a l t h o u g h it may a f f e c t t h e o l d e r n a t u r a l - c l a y c a t a l y s t s , persumably by f o r m a t i o n o f i r o n s u l f i d e from i r o n i n t h e s t r u c t u r e ( r e f s . 40, 4 1 ) . Oxygen compounds ( C O , p h e n o l s ) have l i t t l e e f f e c t e i t h e r . However, t h e c r a c k i n g r e a c t i o n i s i n h i b i t e d by b a s e s such as n i t r o g e n compounds or a l k a l i . The d e a c t i v a t i o n by n i t r o g e n compounds i s s e v e r e , b u t may be undone by b u r n i n g i n a i r i n t h e u s u a l f a s h i o n f o r r e g e n e r a t i o n ( r e f . 4 2 ) . The e f f e c t i v i t y as p o i s o n v a r i e s w i t h d i f f e r e n t n i t r o g e n compounds as i s demonstrated by t h e c l a s s i c a l d a t a o f Voge e t a l . ( r e f . 4 3 ) i n T a b l e 2.
TABLE 2
*) I n h i b i t i o n o f c r a c k i n g o f d e c a l i n by n i t r o g e n compounds R e s u l t s from c r a c k i n g d e c a l i n c o n t a i n i n g 0.11 % N i n v a r i o u s forms. S i l i c a - z i r c o n i a alumina c a t a l y s t ; atmospheric p r e s s u r e ; flow r a t e , 13.7 m o l e s / l . / h r . ; 500 OC, one hour p e r i o d . I n h i b i t i o n = p e r c e n t a g e r e d u c t i o n o f t o t a l c o n v e r s i o n .
Compound Added
None Ammonia Methylamine Diamylamine Dicyclohexylamine Pyridine Indole a-Naphthylamine Quinoline Acridine
Conversion,
Inhibition,
%
%
41.9
-
42.0 42.0 42.3 28.0 26.8 25.1 21.8
48
8.5
80
8.2
81
* ) Data from Voge e t . a l . ( r e f . 43)
0 0
0 33 36
40
553 More permanent reduction o f a c i d a c t i v i t y occurs by d e p o s i t i o n of sodium. Amounts of t h e order of 1 %w on c a t a l y s t can give r i s e to appreciable e f f e c t s ( r e f .
44).
Loss of a c t i v i t y not only occurs by suppression of a c i d i t y , but a l s o by a decrease
of surface a r e a ( r e f . b 4 A ) . Sodium on t h e c a t a l y s t is known t o decrease t h e s t a b i l i t y of t h e c a t a l y s t t e x t u r e i n thermal o r hydrothermal treatments ( r e f .
44B).
A permanent d e t e r i o r a t i o n of c a t a l y s t performance occurs when metals ( n i c k e l , vanadium and i r o n ) a r e deposited on t h e c a t a l y s t . Although t h e s e metals may a f f e c t a c t i v i t y by covering a c t i v e cracking s i t e s , t h e i r e f f e c t on s e l e c t i v i t y i s more important. For t h i s reason, t h e t e r n " s e l e c t i v i t y poison" has been proposed
(ref. 4 5 ) . Accumulation o f these metals on t h e c a t a l y s t gives r i s e t o t h e development of dehydrogenation a c t i v i t y l e a d i n g t o increased formation of gases and coke a t t h e expense of g a s o l i n e y i e l d . Table 3 shows t h e r e l a t i v e e f f e c t i v e n e s s of n i c k e l and vanadium i n t h i s r e s p e c t . I t can be i n f e r r e d from t h e d a t a t h a t n i c k e l i s about four times a s harmful a s vanadium. TABLE 3 R e l a t i v e e f f e c t i v e n e s s of n i c k e l and vanadium contamination i n c a t a l y t i c cracking ( d a t a by Cimbalo e t . a l . ( r e f . 4 6 )
Corivtision i n licroactivity test, I Carbon, %w Hydrogen, %w
Uncontaminated catalyst
Catalyst with 265 ppm N i
Catalyst with 830 ppm v
77.5
73.5 5.62 0.76
74.2 5.48
4.05 0.20
0.64
The e f f e c t of metals deposition i s g e n e r a l l y kept within l i m i t by increased c a t a l y s t make-up r a t e ( r e f . 46). A l t e r n a t i v e ways t o counteract t h e e f f e c t of met a l s i s t o p a s s i v a t e them, e.g. by a d d i t i o n of antimony ( r e f .
47) o r
t o remove
them by e x t r a c t i o n ( r e f . 48). S t i l l another way t o cope with heavy n e t a l s i n t h e feedstock i s t o use s p e c i a l c a t a l y s t s with enhanced metal t o l e r a n c e , which a r e o f f e r e d by s e v e r a l c a t a l y s t manufacturers. The way i n which metal t o l e r a n c e has been obtained i s g e n e r a l l y p r o p r i e t a r y know-how. However, it i s w e l l known ( r e f . 48B) t h a t u s u a l l y only p a r t of t h e deposited metals a r e e f f e c t i v e i n promoting t h e und e s i r e d r e a c t i o n , t h e remaining p a r t being i n a c t i v e because of s i l i c a t e formation
or encapsulation i n t h e c a t a l y s t matrix. This i n e f f e c t i v e f r a c t i o n might be increased by adapting t h e c a t a l y s t formulation. Agents similar t o t h o s e used for p a s s i v a t i n g t h e metals may a l s o be incorporated during manufacture of t h e c a t a l y s t . The above-mentioned poisoning e f f e c t s p e r t a i n t o t h e cracking f u n c t i o n per s e . A r e c e n t t r e n d i n c a t a l y t i c cracking i s t o i n c o r p o r a t e a l s o o t h e r c a t a l y t i c f u n c t i o n s i n t h e c a t a l y s t , i n p a r t i c u l a r t o promote t h e burning of CO t o C02 during regenerat i o n ( r e f s . 48c, 4811). These CO burn promoters, which c o n s i s t , f o r i n s t a n c e , of
554
minute quantities of platinum-group metals, may be incorporated in the manufactured catalyst, or they may be added as a solid or liquid additive during operation. Poisoning characteristics of these promoters will be different according to their nature. Hydrotreatin& Hydrotreating of distillate fractions to remove sulfur and/or nitrogen and to hydrogenate unsaturated compounds is generally carried out over sulfided Co/Mo or Ni/Mo on alumina. These catalysts are among the most rugged ones applied in industrial processes, being relatively insensitive to usual catalyst poisons. As their duty implies they remain active in the presence of H2S and NH3, although high concentrations of these compounds may suppress their activity. Apart from poisoning by trace metals (V, Ni), which is much more a problem in processing of residues than in hydrotreating of distillates (see below), deactivation by silicon (originating for instance from feed contamination with antifoam additives) is one of the relatively rare instances of hydrotreating catalyst poisoning (refs. 36, 49). As mentioned before, a hydrotreating catalyst bed may act as a guard against poisoning of a downstream reformer by arsenic and lead, apparently without being much affected itself. A hydrotreating catalyst containing an average of 1.2 %w As was found to have 70 % and 90 % of fresh activity for nitrogen and sulfur removal, respectively, after regeneration (ref. 49). Because of its ruggedness, it is not unusual for this type of catalyst to operate for years without replacement. In a survey mentioned by Roeder (ref. 50) catalyst lives betweed 175 and 1000 bbl/lb were found for 60 industrial units. Hydrocracking Hydrocracking of petroleum fractions to produce lighter distillates (e.g. gasoline from gas oil or flashed distillate) is generally carried out over a dualfunction catalyst. The functions serve to (a) crack higher-molecular-weight hydrocarbons and (b) activate molecular hydrogen and hydrogenate unsaturated compounds present in the feed or formed during the cracking process. The acidic cracking function is usually provided by an amorphous silica-alumina (similar to the catalyst in cat. cracking), a crystalline aluminosilicate (zeolite), or a halogenated alumina carrier. The major hydrogenation components are Pt, Pd, Ni, Co, Mo, W o r combinations of the latter four metals.
A survey of catalysts for hydrocracking has been given by Vlugter and Van 't Spijker (ref. 5 1 )
. Scott and Patterson
(ref. 5 2 ) have considered commercial
processes and Scott and Bridge (ref. 53) have described the technology and catalytic requirements for various applications. A more recent literature review has been given by Choudhary and Saraf (ref.
54).
555 The acid function is sensitive to poisoning by nitrogen bases. When the cracking duty demanded is rather severe, such as in the production of gasoline from neavier distillates, it is customary to pretreat the raw feed to remove nitrogen to a sufficiently low level. Thus the hydrocracking operation is carried out in two stages, the first stage being a hydrodenitrogenation stage and the second one performing the main hydrocracking duty. With amorphous silica-alumina-based second-stage catalysts, nitrogen should be removed as completely as possible (say, below 5 ppm (ref. 5 l ) ) , and ammonia is scrubbed from the first-stage effluent. Cracking catalysts based on crystalline aluminosilicates can stand some nitrogen and may actually prefer a certain nitrogen concentration for stable operation (ref. 5 1 ) . Their better nitrogen tolerance can be exploited in what is called series-flow operation: the first-stage effluent is passed through the succeeding hydrocracker without intermediate separation. The sensitivity of the hydrogenation function is, of course, dependent upon the choice of the metal. As a very high hydrogenation activity is not necessary
or even desirable, metal sulfides such as sulfided palladium or mixed sulfides of the nickel-tungsten type can be employed. This type of hydrogenation component is, of course, quite tolerant to sulfur and will even have to be operated with some sulfur to maintain the sulfidic state. The first-stage denitrogenation catalysts are generally of a kind similar to the previously discussed hydrotreating catalysts. However, in view of the more demanding denitrogenation the Ni/Mo-type sulfides are often based on an acidic support, e.g., alumina carriers containing silicon or fluorine (ref. 51 ) . These are of course quite resistant to sulfur. It has even been found that high H2S pressures are beneficial for the hydrodenitrogenation of pyridine (ref. 55).
As to poisoning of first-stage catalysts with other feed contaminants, the situation is rather similar to that discussed under hydrotreating. The presence of V and Ni in the feed gives rise to deposition of these metals on the catalyst, which may be especially serious in the case of residual feedstocks (see below). HYDROPROCESSING OF PETROLEUM RESIDUES Hydroprocessing of the heavy end of the crude oil has gained quite some importance in the last decade. In Japan, in particular, a number of residue hydrodesulfurizers are operating to produce low-sulfur fuel oil. In the processes employed the catalyst plays a key r81e, as it has to operate with a feedstock in which catalyst poisons are present, not as an incidental trace contaminant, but as a natural constituent. Particular problems are posed by the presence of metals (V, Ni and Fe) and polyaromatie, heteroatom-rich materials called asphaltenes, whieh are characterized by insolubility in solvents like n-pentane or n-heptane. Table
4
gives some compositional data of typical residual feedstocks from Middle East crudes, showing concentrations of vanadium (the main metal contaminant) between
556 50 and 100 ppmw (ref.
56). Residual feedstocks from some Californian and, in
particular, Venezuelan crudes are much richer in metals, vanadium contents being sometimes as high as about :000 ppmw. TABLE
4
Properties of some residual feedstocks from Middle East crudes*
Density, d 70/4 Sulfur, Vanadium, Nickel, Sodium, C 7 asphaltenes, Viscosity at 210 OF, Ramsb. carbon, Breakdown, %w (TBP) Fr. < 165 OC 165 - 250 OC 250 - 350 OC > 350 OC
%w ppmw ppmw ppmw %w cSt %w
Kuwait long residue
Iranian Heavy long residue
0.9336 4.21 53 12.7
0.9354 2.73 145 33.6 7.2 3.6 63 10.0
6.5 111.9 13.1
-
-
7.7 2.7
53.7 9.8
-
0.05
2.60 97.40
2.0
97.95 ~~~
Arabian Heavy long residue
Kuwait short residue
0.9574 4.31 88 26.9 25
0.14 4.35 95.51
0.987 5.20 100
29.3 9.0 6.9 1810 18.7 -
0.16 99.84 ~~~
*Data from Van Ginneken et. al. (ref. 56) Rale of asphaltenes and metals in catalyst deactivation As has been mentioned before in this paper, the phenomenon of catalyst poisoning in an industrial plug-flow-type reactor is a complex one. The state ofthe catalyst in the bed varies with its location and changes in the course of the run as a poisoning front traverses through the bed. Consequently, it is very difficult to deduce the catalyst deactivation mechanistics from the observed overall decline of process performance. For this reason, studies on catalyst deactivation in hydroprocessing of residues at the Shell Laboratory in Amsterdam have been carried out not only in pilot plants operated in a fashion similar to that followed in commercial practice, but also in fixed-bed bench-scale units featuring recycle of the major part of the liquid product (ref.
57). The latter units simulate a
continuous stirred-tank reactor (CSTR) characterized by uniformity of the catalyst throughout the reactor at any given time. Thus, the observed performance decline can be associated with the activity decline of each single catalyst particle. Figure 2 shows a typical activity decline thus determined in processing of a residual feed (ref. 57). After a rapid initial decline of desulfurization activity, a more gradual decline takes place between the points TSSD (time of start of slow decline) and TSm ( t h e of start of accelerated decline). It will be clear that the useful catalyst life (UCL) is determined mainly by the slow decline, rather than by the fast initial decline of activity.
557
1510 -
86-
42-
UCL
V REMOVAL RATE (ARBITRARY UNITS)
101
0
I
I000
I
I
2000
3000 CAT. AGE, kg/kg
Fig. 2 . Typical d e a c t i v a t i o n behaviour of a c a t a l y s t observed i n hydrodesulfurizat i o n of a r e s i d u a l feedstock i n a simulated CSTR.
From Fig. 3 it can be deduced t h a t carbon laydown i s not t h e cause of t h e slow d e c l i n e and t h e r e f o r e n o t t h e determining f a c t o r f o r UCL. The f i g u r e suggests t h a t carbon b u i l d s up r a p i d l y on t h e c a t a l y s t i n t h e very i n i t i a l p e r i o d of t h e run, but it soon reaches an equilibrium l e v e l on t h e c a t a l y s t . I n t h e l a t t e r s t a g e , t h e r a t e s of carbon d e p o s i t i o n and carbon removal by hydrogenation a r e apparently i n balance. Since t h e l a t t e r r a t e w i l l depend upon t h e hydrogen p r e s s u r e t h e s t e a d y - s t a t e carbon l e v e l w i l l a l s o depend on hydrogen p r e s s u r e . Figure
4
shows
an approximately i n v e r s e r e l a t i o n s h i p between t h e s e two parameters. While carbon d e p o s i t i o n may play a r z l e i n t h e f a s t i n i t i a l a c t i v i t y d e c l i n e , t h e more important slow d e c l i n e must be a s s o c i a t e d with a feed impurity o t h e r than a s p h a l t e n i c carbon, which impurity must be present i n a smaller c o n c e n t r a t i o n , i n view of t h e time s c a l e involved. The obvious candidates a r e t h e metals, i n p a r t i c u l a r vanadium and n i c k e l .
558
30
-
20
/A //
lI
:i/ I / 10
-
1
0
n
-
0
, /
V ----
I/
7
n
v
--_____ v
CAT. A, MIDDLE EAST RESIDUE w A
-
CAT.& MIDDLE EAST RESIDUE
CAT. B, CARIBBEAN RESIDUE
/
/
0 0
I I000
I
I
I
2000
3000
4000
1
5000 CAT AGE, kg/kg
Fig. 3. Carbon on c a t a l y s t a s a f u n c t i o n of c a t a l y s t age i n hydrodesulfurization of r e s i d u a l feedstocks i n a simulated CSTR.
559
30
25
CARBON, A R B I T R A R Y UNITS
-
/
20
-
/””
1 150-
1
1
I
10
15
20
i, ARBITRARY
UNITS
pH2
Fig.
4. Steady-state
l e v e l o f carbon on c a t a l y s t a s a f u n c t i o n of hydrogen part i a l pressure i n residue hydrodesulfurization.
Fig. 5 shows t h a t t h e s e keep accumulating on t h e c a t a l y s t , i n c o n t r a s t t o carbon. These f i n d i n g s a r e i n l i n e with observations by o t h e r s . For i n s t a n c e , Egi and Matsumoto ( r e f .
58)
examined t h e c a t a l y s t i n a f i x e d bed a f t e r 50 and 1000 hours
processing of a r e s i d u e and found approximately t h e same carbon c o n t e n t s a f t e r t h e s e two on-stream p e r i o d s . Moreover, t h e carbonaceous d e p o s i t s were not predominantly present on t h e i n l e t s i d e of t h e bed, but f a i r l y evenly d i s t r i b u t e d over t h e bed. A pronounced f o u l i n g p r o f i l e with highest concentrations on t h e i n l e t s i d e w a s observed f o r vanadium and n i c k e l , and t h e concentrations a f t e r 1000 hours on stream were an o r d e r of magnitude higher than a f t e r 50 hours.
560
+ Ni
25
DEPOSITS, . A R B I T R A R Y U N I T S
,’ CAT. ,/
8, CARRIBEAN RESIDUE
20
, 15
10
5
c 0
I000
2000
3000
4000 CAT. AGE, kg/kg
Fig. 5. Metals on catalyst as a function of catalyst age in hydrodesulfurization of residues in a simulated CSTR. Nature and catalytic rale of metal deposits Elementary analysis of used catalysts indicates that vanadium and nickel are present as sulfides, most probably as V2S3 and Ni3S2. Recent NMR and EPR studies by Silbernagel (ref. 5 9 ) indicate that at higher metal loadings V2S3 is the dominant form of deposited vanadium. Nickel and vanadium in petroleum are generally associated with the heavier constituents of the crude oil, hence their collection in the residual fractions. They are associated in particular with the asphaltenes and resins and present in metalloporphyrin chelate structures as well as non-porphyrinic complexes (ref. 60). The asphaltenic and resinous species have relatively high molecular weights (mol. weights between 1000 and 100 000 have been found by Drushel (ref. 6 1 ) ) and also relatively large molecular dimensions. Asphaltenes are generally visualized as discs composed of sheets of condensed aromatic rings with thicknesses of the order of 2 nm (refs. 62, 63).
56 1
Since the latter dimension is of‘ the same order of magnitude as the pore sizes in usual catalysts, it is plausible that asphaltene molecules do riot readily diffuse into the catalyst. Consequently their conversion, which accompanies or precedes the deposition of metals, is pore-diffusion limited. This is clearly evident from electronmicroprobe analysis of used catalysts, which shows an enrichment of vanadium and nickel in the superficial region of a catalyst particle. Fig. 6 shows examples of such intraparticle metal profiles. Similar profiles have
VANADIUM CONCENTRATION (APPROXIMATE)
I
2
______-___
J I
I
1
I
,L
----------
+
I
0 8 0.6 0.4 0.2 0
I
I
0.2 0.4 0.6 0.8
4
3
I_L+LL 0.8 0.6 0.4 0.2 0
0.2 0.4 0.6 0.8
0.6 0.4 0.2 0
0.2 0.4 0.6
DISTANCE FROM CENTRE OF PARTICLE, m m Fig.
6. Electron microprobe V-Ka scans of specimens of catalysts used in
hydro-
processing of residues. Accessibility of catalyst to asphaltenes decreases from 1 to 4.
562
been reported by Dautzenberg et. al. (ref. 571, Audibert and Duhaut (ref. 641, Todo et. al. (ref. 65), and Inoguchi et. al. (ref.
66). Radford and Rigg (ref. 67)
also reported that on the used catalyst the higher concentration of the metals was always found near the outer surface, while the inner volume of the catalyst still had only a low concentration of metals. The time scale and the amounts of deposited metals involved (cf. Figures 2 and
5) make it unlikely that the mechanism causing the gradual decline in activity
-
(between TSSD and TSm in Fig. 2) is a poisoning ---- ---- in the usual sense, although pore-mouth poisoning may play a role in the initial fast decline. The gradual plugging process, activity decline determining UCL is more likely a pore-mouth ------------------viz., a gradual decrease in accessibility of the catalyst particles as a result
of the deposition of massive amounts of metal sulfides in the peripherial zones. The steady increase in diffusional resistance to the transport of reactant molecules to the catalytic surface is manifested by a gradual decline in conversion rate until the pore openings are almost fully blocked, when an accelerated decline sets in. The reaction that leads to metal removal from the feedstock is apparently a surface-catalysed one which is not poisoned by the deposition of metal sulfides. This suggests that the metal sulfides themselves can act as catalyst. Thus, the metal-removal reaction may be considered to be an autocatalytic one. Evidence for this is given by Fig. 7, which shows that bare carriers may gradually acquire catalytic activity for removing metals from a residual feedstock, when exposed to this feedstock under hydroprocessing conditions.
VANADIUM R E M O V A L R A T E , A R B I T R A R Y U N I T S
40
I-
20
-
00
Fig.
I000
2000
3000
4 000 5000 CAT AGE, kg. kg-+
7. Development of activity for vanadium removal in hydroprocessing of a Middle East residue over bare carrier materials.
563 D e s c r i p t i o n of c a t a l y s t d e a c t i v a t i o n by a pore-mouth p l u g g i n g model The above m e c h a n i s t i c p i c t u r e of c a t a l y s t d e a c t i v a t i o n i n r e s i d u e p r o c e s s i n g forms t h e b a s i s o f a s i m p l e two-','arameter
model developed a t t h e S h e l l Laboratory
a t Amsterdam, which d e s c r i b e s t h e g r a d u a l a c t i v i t y d e c l i n e by pore-plugging
( r e f . 57)
T h i s model allowed t h e c o r r e l a t i o n of t h e r e s u l t s o f a l a r g e number o f e x p e r i m e n t s c a r r i e d o u t w i t h d i f f e r e n t c a t a l y s t s and f e e d s t o c k s under v a r i o u s c o n d i t i o n s . By way of example, F i g . 8 shows t h a t d i f f e r e n t c a t a l y s t s o p e r a t e d a t v a r y i n g p r e s s u r e
I .4
I.2
I .c
0.8
0.6
0.4
0.2
0 0
0.2
0.4
0.6
0.8
1.0
e
F i g . 8. R e l a t i o n between c a t a l y s t e f f e c t i v e n e s s f o r d e s u l f u r i z a t i o n (17) and r e l a t i v e c a t a l y s t age ( e ) . Runs a t d i f f e r e n t p r e s s u r e s w i t h d i f f e r e n t c a t a l y s t s (Dautzenberg e t a l . (ref. 57))
564
levels have an identical deactivation behaviour when results are normalized according to the model (ref. 57). Fig. 9 shows that the model can satisfactorily predict catalyst lives, once the pertinent activity decline parameters have been established. Another model of catalyst deactivation in residue desulfurization has been proposed by Newson (ref. 68). His model also considers a pore-plugging mechanism in combination with coking effects. Practical implications of the effect of metals in residual feedstocks Although the presence of metals is a formidable problem in catalytic processing of residues, the development of suitable metal-tolerant catalysts has diminished
0
I000
2000
3000
4000
EXPERIMENTAL UCL, kg/kg
Fig. 9. Comparison of calculated and experimentally determined catalyst lives in hydrodesulfurization of residues.
565 t h e problem t o an acceptable proportion f o r r e s i d u e s of low t o moderate metal cont e n t s , i . e . below some 100 ppmw. Commercial processes now a v a i l a b l e achieve run l e n g t h s between s i x months and a year b e f o r e c a t a l y s t replacement. A number of t h e s e processes a r e mentioned i n reviews given by, f o r i n s t a n c e , Nelson ( r e f .
69),
Ebel ( r e f . T O ) , Lawrance ( r e f . 71) and Aalund ( r e f . 7 2 ) . With feedstocks of higher metals c o n t e n t , c a t a l y s t replacement c o s t s w i l l become an unduly l a r g e f a c t o r i n t h e o v e r a l l processing c o s t s ( r e f . 73) while run l e n g t h s of fixed-bed r e a c t o r s would become t o o s h o r t t o be p r a c t i c a b l e . Apart from t h e obvious p o s s i b i l i t y t o f r a c t i o n a t e t h e f e e d , e . g . , by solvent e x t r a c t i o n , i n a metals-rich and metals-poor f r a c t i o n and only process t h e l a t t e r , t h e r e i s a l s o t h e option t o remove most of t h e m e t a l s c a t a l y t i c a l l y . For i n s t a n c e , t h e ebullated-bed d e s u l f u r i z a t i o n r e a c t o r a p p l i e d i n t h e H-oil process of Hydrocarbon Research I n c . and C i t i e s Service Co. can be preceded by another ebullated-bed r e a c t o r with an inexpensive n a t u r a l m a t e r i a l a s d e m e t a l l i z a t i o n c a t a l y s t ( r e f .
74).
The ebullated-bed p r i n c i p l e permits continuous replacement of t h i s m a t e r i a l . I n a process developed by S h e l l ( r e f .
561, a
s p e c i a l demetallization c a t a l y s t i s
a p p l i e d which f e a t u r e s a high d e m e t a l l i z a t i o n a c t i v i t y and a high metals uptake capacity. Replacement of t h e c a t a l y s t without i n t e r r u p t i n g t h e o p e r a t i o n i s made p o s s i b l e by a moving-bed-type r e a c t o r ( c a l l e d "bunker flow" r e a c t o r ) operated i n a t r i c k l e mode ( r e f . 5 6 ) .
CONCLUDING REMARKS
A s follows from t h e foregoing d i s c u s s i o n , t h e problem of c a t a l y s t poisoning has been reduced i n most petroleum r e f i n i n g processes t o a proportion which allows l a r g e - s c a l e continuous operation f o r a t l e a s t s e v e r a l thousands of hours, even with d i f f i c u l t feedstocks such as r e s i d u a l ones. This has r e s u l t e d from c o n t i n u a l improvement, over s e v e r a l decades, of processing knowhow. More i n s i g h t i n t o t h e n a t u r e and t h e e f f e c t of c a t a l y s t poisons, b e t t e r a n a l y t i c a l techniques for det e c t i n g t h e poisons, improved ways t o remove them, novel r e a c t o r systems, and - l a s t but not l e a s t - b e t t e r c a t a l y s t s with enhanced t o l e r a n c e of poisons, have a l l contributed i n a t t a i n i n g t h e present s i t u a t i o n . While t h i s i s i n g e n e r a l a g r a t i f y i n g s t a t e of a f f a i r s , it does not mean t h a t t h e r e would be no i n c e n t i v e f o r seeking f u r t h e r improvements such a s developing novel and improved c a t a l y s t s or r e a c t o r systems. A s o i l becomes scarce and i t s p r i c e i n c r e a s e s , it becomes more and more important t o achieve maximum y i e l d t o d e s i r e d products i n o i l r e f i n i n g . C a t a l y s t poisoning r e s u l t i n g in. s e l e c t i v i t y l o s s e s , e i t h e r by d i r e c t e f f e c t of poisons on s e l e c t i v i t y or by t h e temperature i n c r e a s e s necessary t o compensate f o r d e a c t i v a t i o n , w i l l become l e s s and l e s s acceptable. The e s c a l a t i o n i n t h e c o s t of r e a l i z a t i o n of i n d u s t r i a l p l a n t s and high i n t e r e s t r a t e s make e f f i c i e n t u t i l i z a t i o n of i n s t a l l e d equipment mandatory. Down-time because of c a t a l y s t d e a c t i v a t i o n w i l l become i n c r e a s i n g l y o b j e c t i o n a b l e .
566
Finally, the advent of l e s s conventional and generally more contaminated feedstocks such as heavy crude oils, oils from tar sands and shale and-last but not least-coal liquids, calls for significant improvements in processing technGlGgy and represents a great challenge to research and development in catalysis and engineering.
567 REFERENCES i E.B. Maxted and H.C. Evans, J. Chem. Soc., (1937 603, 1004. 2 E.B. Maxted, Adv. Catalysis, 3(1951) 129. 3 A. Wheeler, Adv. Catalysis, 3(1951) 250. 4 A. Chou, W.H. Ray and R. Aris, Trans. Inst. Chem 5 S. Szepe and 0. Levenspiel, Chem. Eng. Sci., 23( 6 S. Lee and C.M. Crowe, Chem. Eng. Sci., 25( 1970) 743. 7 J.B. Butt and D.M. Rohan, Chem. Eng. Sci., 23(1968) b89. 8 A. Sadana and L.K. Doraiswamy, J. Catalysis, 23(197i) 147. 9 V.W. Weekman Jr., Ind. Eng. Chem. Process Des. Develop., 7( 1968) 90; 8( 1969) 385. 10 G. Schlosser, Int. Chem. Eng., 17, no.1 (Jan. 1977) 41. 1 1 S. Krishnaswamy and J.R. Kittrell, Ind. Eng. Chem. Process Des. Develop., 18(1979) 399. 12 J.W. Beeckman and G.F. Froment, Ind. Eng. Chem. Fundam., 18(1979) 245. 13 Imperial Chemical Industries Ltd., "Catalyst Handbook", Wolfe Scientific Books, London 1970. 14 J.R. Rostrup-Nielsen, in "Steam Reforming Catalysts", Danish Technical Press, Copenhagen, 1973. 15 L.J. Cristiansen and S.L. Anderson, Chem. Eng. Sci., 35(1980) 314. 16 J.S. Campbell, Ind. Eng. Chem. Process Des. Develop., 9(1970) 588. 17 P.W. Young and C.B. Clark, Chem. Eng. Progress, 69(1973) 69. 18 K. Klier, "Metal-support interactions and deactivation in copper-based catalysts", paper presented at the conference on catalyst deactivation and poisoning, Berkeley, California, May 24-26, 1978. 19 J.A. Cusumano, R.A. Dalla Betta and R.B. Levy, "Catalysis in coal conversion", Academic Press, New York 1978, p.224. 20 C.H. Bartholomew, C.D. Weatherbee and G.A. Jarvi, J. Catalysis, 60(1979) 257. 21 R.W. Fowler Jr. and C.H. Bartholomew, Ind. Eng. Chem. Prod. Res. Develop., 18(1979) 339. 22 J.T. Richardson, J. Catalysis, 21(1971) 130. 23 G.E. Langlois and J.E. Walkley, Petrolem Refiner 31, no.8 (Aug. 1952) 79. 24 J.F. McMahon, C. Bednars and E. Salomon, in "Advances in Petroleum Chemistry and Refining", Vol. 7, Ed. J.J. McKetta Jr., Interscience Publ. Inc., New York, 1963, p.304. 25 W.C. van Zyll-Langhout, Proc. 9th World Petroleum Congress, b(1975) 197. 26 H.W. Kouwenhoven and W.C. van Zyll-Langhout, Chem. Eng. Progress, 67(1971) 65. 26A J.J. Collins, M. Symoniak, and W.C.J. Quik, "Total Isomerization ProcessWhy go half way?" paper presented at the Nat. Petr. Ref. Ass. Annual Meeting, Mar. 19-21, 1978, San Antonio, Texas. 27 H.S. Bloch, G.R. Donaldson and V. Haensel, "Hydroisomerization of light paraffin hydrocarbons", paper presented at the Am. Chem. SOC. Div. of Petroleum Chemistry meeting, Boston, April 5-10, 1959. 28 T.C. O'May, Proc. 6th World Petroleum Congress, 3( 1963) 17. 29 R.A. Erickson and G.F. Asselin, Chem. Eng. Progress 61, no.3 (March 1965) 53. 30 C.W. Cartmight and R.J. Stock, Oil and Gas J., Sept. 18, 1978, p.141 31 B.W. Burbidge, P. Watson and A.H. Richardson, "BP's isomerization process takes on a new look", paper presented at the Nat. Petr. Ref. Ass. annual meeting, Mar. 23-25, 1975, San Antonio, Texas. 32 F.G. Ciapetta, R.M. Dobres and R.W. Baker, in "Catalysis", Vol VI, Ed. P.H. Fhmett, Reinhold Publ. Corp., New York, 1958, p.641. 33 L.J. Daniels Jr., P. Sperling and A.G. Rouquier, Oil and Gas J., May 8, 1972, p.78. 34 E.B. Maxted, J. Chem. SOC., 117(1920) 1501. 35 E.B. Maxted, J. Chem. Soc., 119(1921) 225. 36 Nat. Petr. Ref. Ass., 1972 Question and Answer sessions on refinining technology. The Petroleum Publ. Co., Tulsa, Oklahoma, pp. 18-19. 37 Nat. Petr. Ref. Ass., Question and Answer session 1970, Oil and Gas J., Mar. 15 1971, p.67; Hydrocarbon Processing, Feb. 1971, p.93.
568
References (continued) 38 W.P. Hettinger Jr., C.D. Keith, J.L. Gring and J.W. Teter, "Hydroforming reactions and the effect of certain catalyst properties and poisons", paper presented at the Am. Chem. SOC., Div. Petroleum Chemistry meeting, New York, Sept. 11-17, 1954. 39 A.G. Oblad, Oil and Gas J., Mar. 27, 1972, p. 84. 39A P.B. Venuto and E.T. Habib Jr., "Fluid Catalytic Cracking with zeolite catalysts", Karcel Dekker Inc., New York and Basel, 1979. 40 C.H. Riesz and F.L. Morris, "Catalysts for Petroleum Refining", in "Advances in Petroleum Chemistry and Refining, Vol.IV, Ed. J.J. Mc.Ketta Jr., Interscience Publ. Inc., New York 1961, p.287. 41 G.A. Mills, Ind. Eng. Chem., 42(1950) 182. 42 H.H. Voge, "Catalytic cracking", in "Catalysis", Vol. IV, Ed. P.H. Emmett, Reinhold Publ. Corp., New York, 1951, p.422. 43 H.H. Voge, G.M. Good and B.S. Greensfelder, Proc. 3rd World Petroleum Congress, 4( 1951) p. 124. 44 Nat. Petr. Ref. Ass. 1978 Question and Answer Session, Hydrocarbon Processing, Mar. 1979, p.154. 44A Nat. Petr. Ref. Ass., Question and Answer Session 1975, Oil and Gas J., Mar. 22 1976, p. 82; Ibid., 1977, Oil and Gas J., Apr. 10, 1978, p. 62. 44B B.W.S. Letzsch, R.E. Ritter, and D.E.W. Vaughan, Oil and Gas J., Jan. 26 1976, p. 130. 45 W.B. Innes, "Carriers, promoters, accelerators, poisons, inhibitors", in "Catalysis", Vol. I, Ed. P.H. &ornett, Reinhold Publ. Corp., New York, 1954, p.302. 46 R.N. Cimbalo, R.L. Foster and J.S. Wachtel, Oil and Gas J., May 15, 1972, p.112. 47 G.H. Dale and D.L. McKay, Hydrocarbon Processing, Sept. 1977, p.97. and Gas J., Mar. 28 1977, p.43. 48 R.R. Edison, J.O. Siemssen and G.P. Masologites, Hydrocarbon Processing, May 1976, p.133. Dec. 20 1976, p.54. 48B G.P. Masologites and L.H. Beckberger, Oil and Gas J., 11 (47) 1973, p.49. 48C L. Rheaume, R. Ritter, J.J. Blazek and J.A. Montgomery, Oil and Gas J., May 17 1976, p.103; Ibid., May 24, 1976, p.66. 48D F.D. Hartzell and A.W. Chester, Oil and Gas J., Apr. 16 1979, p.83. 49 Nat. Petr. Ref. Ass. Question and Answer Session, Oil and Gas J., May 3, 1971, p. 102. 50 Nat. Petr. Ref. Ass. Question and Answer Session, Oil and Gas J., April 24, 1972, p.79. 51 J.C. Vlugter and P. van 't Spyker, Proc. 8th World Petroleum Congress, 4(1971)
159. 52 J.W. Scott and N.J. Patterson, Proc. 7th World Petroleum Congress, 4(1967) 97. 53 J.W. Scott and A.G. Bridge, Adv. Chem. Ser., 103(1971) 113. 54 N. Choudhary and D.W. Saraf, Ind. Eng. Chem., Prod. Res. Develop., 14, no.2
(1975) 74. 55 F. Goudriaan, H. Gierman and J.C. Vlugter, J. Inst. Petroleum 59(1973) 40. 56 A.J.J. van Ginneken, M.M. van Kessel, K.M.A. Pronk and G. Renstrom, Oil and Gas J., April 28, 1975, p.59.
57 F.M. Dautzenberg, J. van Klinken, K.M.A. Pronk, S.T. Sie and J-B. Wyffels,
56
59 60
61 62
"Catalyst deactivationthrough pore-mouth plugging during residue desulfurization", paper presented at the 5th Int. Symposium on reactor engineering, Houston, Mar. 13-15, 1978. K. Egi and K. Matsumoto, Chem. Econ. and Eng. Review, Nov. 1970, p.7. B.G. Silbernagel, J. Catalysis, 56(1979) 315. T.F. Yen, "The role of trace metals in petroleum", Ann Arbor Science Publ. Inc., Ann Arbor, Michigan, 1975. H.V. Drushel, "Analytical characterization of residua and hydrotreated products", paper presented at the Am. Chem. SOC., Div. Petroleum Chemistry meeting, New York, Aug. 27-Sept. 1, 1972. J.P. Dickie and T.F. Yen, Anal. Chem. 39( 1967) 1847.
569
References (continued) 63 T.F. Yen, "Present status of the structure of petroleum heavy ends and its significance to various technical applications", paper presented at the Am. Chem. Soc., Div. Petroleum Chemistry meeting, New York, Aug.27-Sept. 1, 1972. 64 F. Audibert and P. Duhaut, Rev. Inst. FranS. Petrole 25, n0.5 (May 1970) 613. 65 N. Todo, T. Kabe, K. Ogawa, M. Kurita, T. Sato, K. Shimada, Y. Kuriki, T. Oshima, T. Takematsu and Y. Kotera, Kogyo. Kagaku Zasshi, 74, no.k(l971) 563. 66 M. Inoguchi, S. Sakurada, Y. Satomi, K. Inaba, H. Kagaya, K. Tate, T. Mitzutori, R. Nishiyama, T. Nagai and S. Onishi, Bull. Japan Petroleum Inst., 14 no.2 (Nov. 1972) 153. 67 H.D. Radford and R.G. Rigg, Hydrocarbon Processing, Nov. 1970, p.187 68 E. Newsor., Ind. Ecg. Chem. Process Des. Develop. 14, no.l(l975) 27. 89 W.L. Nelson, Si;lphur, lOO(1972) 55. 70 R.H. Ebel, "Recent advances in fuel desulfurization technology", paper presented at the Am. Chem. Soc., Div. Petroleum Chemistry meeting, New York, Aug. 27Sept. 1, 1972. 71 P.A. Lawrance, Chemie Ing. Technik 45(1973) 1141; ErdoelU. Kohle 26(1973) 447. 72 L. Aalund, Oil and Gas J., Sept 11, 1972, p.79. 73 W.L. Nelson, Oil and Gas J., May 2, 1977, p . 247; Nov. 19, 1973, p.54. 74 K.E. Hastings, L.C. James and W.R. Mounce, Oil and Gas J., June 30, 1975, p. 122.
B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 0 1980 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
CATALYST DEACTIVATION BY POISONING AND PORE PLUGGING IN PETROLEUM PROCESSING S . T . SIE
Koninklijke/Shell-Laboratori~,Amsterdam (Shell Research B.V. )
, Badhuisweg 3,
1031 CM Amsterdam, (The Netherlands)
ABSTRACT A review is given of aspects of catalyst poisoning in major petroleum refining processes. Processes considered include hydrogen manufacture, polymerization of olefins, isomerization of light paraffins, reforming of naphtha, catalytic cracking, hydrotreating and hydrocracking. Particular attention is given to the hydroprocessing of residual feedstocks, where catalyst deactivation by poisoning and poreplugging represents a considerable, though not unsurmountable problem.
INTRODUCTION Catalytic processing is a keystone of today's petroleum refining. The catalytic processes applied in the petroleum industry are generally large-scale, continuous processes operating without interruption for long intervals. This is made possible by employing rugged catalysts and by adequate safeguarding against catalyst deterioration.
An unscheduled temporary shutdown of a plant necessitated by deterioration of catalyst performance (e.g. l o s s of activity, selectivity, permeability to flow, fluidizability, etc.) generally represents a significant economic penalty in view of the large capital tied up in the plant, and in the case of a noble metal catalyst, in the catalyst itself. Deactivation of the catalyst, which may occur by physical transformations (such as sintering), coking or poisoning, is therefore to be minimized as much as is feasible. The present paper reviews some aspects of catalyst poisoning, defined as the deactivation caused by strong, often irreversible adsorption of feed contaminants in major refinery processes employing heterogeneous catalysts. These include the processing of gases, distillates and residual feedstocks. Particular attention will be given to hydroprocessing of residues, since with this type of feedstock the problem of catalyst deactivation is much more inherent in the use of this feedstock than in the processing relatively clean gases or distillates.
546 SOME G E N E W FEATURES OF CATALYST POISONING IN INDUSTRIAL PROCESSES
The phenomenon of catalyst poisoning in continuous-flow processes as are usually applied in the petroleum industry is much more complex than in laboratory experiments designed to study the adsorption of poisons on the catalyst and its effect on activity or selectivity. With petroleum feedstocks a wide spectrum of contaminants is usually present, rather than just a single species. Thus, although it is quite common to refer to poisoning by "sulfur", "nitrogen" or "metals", one usually deals with an often unknown distribution of compounds which may have individual adsorption and poisoning characteristics (refs. 1 , 2). Unless the process conditions are sufficiently severe to convert these compounds to a common single species such as H2S, NH etc., it is an oversimplication to ignore the 3 individuality of the poisoning compounds. Fortunately, in many of the processes discussed hereafter this simplification appears to be permissible. The strong and fast adsorption of a poison may give rise to concentration gradients on a micro-scale as well as on a macro-scale. In the case of pore diffusion limitation (ref. 3 ) there will be a concentration profile across a catalyst particle which changes with time (shrinking active core). In a fixed catalyst bed there may likewise be a deactivation front which gradually moves in the flow direction. It is therefore not easy to relate overall process performance with the adsorption behaviour of a poison. The situation is further complicated by the usual practice of adiabatic operation (resulting in axial temperature profiles) and temperatureprogramming as a measure to offset catalyst deactivation in the course of time. A number of authors have theoretically analysed the phenomenon of catalyst fouling in industrial reactors and attempted to describe it mathematically, e.g., Chou et al. (ref.
4), Szepe and Levenspiel (ref. 5 ) , Lee and Crowe (ref. 6j , Butt
and Rohan (ref. 7 ) , Sadana and Doraiswamy (ref. 8) and Weekman (ref. 9). More recent work includes that of Schlosser (ref.
lo),
and Krishnaswamy amd Kittrell
(ref. ll), who describe the effect of temperature programming for constant conversion as applied in hydrocracking or reforming. Beeckman and Froment (ref. 12) considered deactivation by active-site coverage and pore blockage. Although they primarily considered coking as the cause of catalyst deactivation, there is an analogy with catalyst poisoning and pore plugging by metals as occur in processing of residual oils (see below). CATALYST POISONING IN PROCESSING OF GASES AND PETROLEUM DISTILLATES Hydrogen manufacture The manufacture of hydrogen as a supplement to by-product hydrogen from catalytic reforming of naphthas plays an increasingly important r81e in petroleum refining. The usual manufacturing route involves production of CO and H2 by either steam reforming of light hydrocarbons (methane in particular) or by non-catalytic partial oxidation of heavy oil, followed by CO shift and removal of residual CO by
547
rr,ethanation. (See Fig. 1 )
CH4 H20
-
STEAM REFORMING
-
H.T. L.T. SHIFT I SHIFT
CH4
+ H20
-
-
METHANATION
-
H2
CO + 3H2
CATALYST: Ni CO-SHI FT:
CO + H20
-C
H2 + CO2
H.T. CATALYST: Fe3O4 ( L.T.
,I
+ Cr203)
Cu/ZnO
METHANATION: CO
+ 3H2
CHq
+ H20
CATALYST: Ni
Fig. 1. Catalytic steps involved in hydrogen manufacture from methane Steam reforming. The most prominent poisons for the usual nickel-type catalyst are sulfur and halogens. Another poison of practical significance is arsenic, which may originate from CO2-removal process utilizing arsenous oxide as catalyst (e.g. the Giammarco-Vetrocoke process). The poisoning effect of sulfur appears to be reversible: a sulfur-poisoned catalyst may recover its full activity when operated without sulfur in the feed or a sulfur concentration below a certain threshold level. This level generally increases with operating temperature. For instance, for an ICI catalyst a sulfur tolerance has been reported ranging from 1 ppm at 800 OC to 25 ppm at 900
OC
(ref. 13). According to Rostrup-Nielsen (ref. I h ) ,
the reversible adsorption of sulfur can be described by a Langmuir isotherm with the H S/H ratio as concentration term. On this basis Christiansen and Anderson 2 2 (ref. 1 5 ) have developed a mathematical model to calculate transient sulfur profiles and sulfur break through in steam reformers.
548
Chlorine has an effect comparable to that of sulfur. Approximately the same concentration limits apply, and the effect is also reversible (ref. 13). Unlike sulfur and chlorine poisoning, arsenic poisoning is not reversible. Therefore, there is in principle no fixed maximum level for steady operation. In experiments carried out at Imperial Chemical Industries (ref. 13) the effect o f arsenic became noticeable when the As 0 content of the catalyst exceeded 50 ppm. 2 3 CO-shift. The high-temperature CO-shift catalyst used to convert the bulk of ..................................
CO at 320-500
is usually Fe30h stabilized by addition o f Cr 0 This catalyst 2 3' is fairly tolerant of sulfur: H S or COS in concentrations below about 200 ppm 2 have no effect on activity (ref. 13). Operation with much more s u l f u r appears to OC
be even possible, albeit at a reduced activity level. This is because FeS itself is a catalyst for the water gas shift reaction, being about half as active as Fe 0
3 4'
Consequently, high-temperature shift catalysts do not normally suffer from sulfur poisoning in practice, in particular not when they are preceded by the much more sensitive steam reforming catalysts. Large quantities of halogens would deactivate high-temperature shift catalysts, but these are not encountered during normal plant operation. The low-temperature CO-shift catalyst, which operates at lower temperatures .................................
(180-280 O C ) , where thermodynamics permit a higher conversion of CO, is generally composed of copper and zinc oxide. This catalyst is sensitive to sulfur and halogens, the most important poisons in practice. Both poisons react almost irreversibly
with the catalyst and therefore collect at the front end of the bed, thus
protecting the downstream portion of catalyst. Table 1 presents some data by Campbell (ref. 16), which may serve to illustrate this feature and to indicate concentration levels of interest. With respect to the poisoning mechanism, Young and Clarke (ref. 17) conclude that poisoning by sulfur is probably a surface effect and does not involve the formation of bulk cuprous sulfide. The latter would require an H S concentration of 3 ppm at about 230 OC, whereas deactivation 2
in a commercial plant was observed at much lower sulfur levels. The effect of chlorine is ascribed to the formation of low-melting zinc chloride, which may induce growth of zinc oxide crystallites. Klier (ref. 18), however, attributed chloride poisoning of a similar catalyst used in methanol synthesis to chemisorption on the Cu'
centres, which are assumed to be the sites for CO hydrogenation.
The more recently developed sulfur resistant CO-shift catalysts for rnediumand low-temperature duty are sulfided Co/Mo or Ni/Mo systems, often promoted with alkali metals. These mixed sulfides are obviously not poisoned by sulfur. Since they are akin to the hydrotreating catalysts (see below), they should be similarly rugged as far as poisoning is concerned.
549
TABLE 1 Effect of sulfur and chlorine poisoning of a low-temperature CO-shift catalyst in commercial operation (after Campbell (ref. 16) Position of catalyst in bed
TOP
Bottom
Activity relative to that at bottom of bed,
%
so3, %w
c1, %w
1.4
0.08
0.7 0.4
0.04
15
0.01 0.01
72
0.01
100
0.2 0.2
15
75
Methanation. Removal of residual CO in H2 is usually carried out by methanation over nickel catalysts. A s with steam reforming catalysts, sulfur is among the most notorious poisons. However, in the hydrogen-manufacturing context it is unlikely that sulfur poisoning is a serious practical problem since the Cu/ZnO low-temperature shift catalyst itself is poisoned by sulfur and a good sulfilr catcher. Halides also poison nickel methanation catalysts, but this does not normally pose a problem, for the same reason. The situation is different when a sulfur-resistant CO-shift catalyst is applied (which may even require operation with some sulfur to maintain the sulfidic state of the catalyst) or when only part of the gas i s passed through the shift section (as proposed in some substitute natural gas manufacturing schemes). As compared with the nickel catalyst in steam reforming, the catalyst in methanation is operated at a much lower temperature (e.g.
300-400
OC
versus 800-900 OC) and sulfur
is therefore more strongly bound in the methanation case. Serious deactivation already occurs when the sulfur content of a 30 % Ni catalyst exceeds about 0.1-0.2
%w
(ref. 13). The r81e of the H2S concentration in the reactant stream (more specifically the H2S/H2 ratio) is illustrated by some results of pilot plant methanation tests at Sasol, South Africa (ref. 19). At 0.05 ppm H S a supported Ni catalyst showed a 2
slow deactivation at the inlet portion of the bed, but no further deactivation farther into the bed even after 50 days on stream. On the other hand, 3 ppm H2S caused rapid and progressive poisoning, which was already appreciable after only six days of operation. Bartholomew et al. (ref. 2 0 , 21) also observed significant activity losses upon exposure of a nickel catalyst for one or a few days to a reaction mixture containing 1-10 ppm H2S. The strong bonding of sulfur under methanation conditions gives rise to a relatively sharp sulfur front in a plugflow-type reactor. The occurrence of such fronts has been demonstrated by Richardson (ref. 22). As with steam reforming catalysts, arsenic is a poison for methanation catalysts.
0.5 %w As on catalyst may result in loss of about half of the activity (ref. 13).
550
i2oigmerization of C&b
olefins
Polymerization of propene and butenes to produce oligomers suitable as gasoline component is carried
OUL
wit% acidic catalysts. The most widely used catalyst is
phosphoric acid on a silica support, e.g., kieselguhr in the process developed by Universal Oil Products Co (UOF) or granular quartz in the process of Standard Oil of California. 3asic components such as ammonia or amines are obvious poisons for the acidic catalyst and it is customary to remove them from the feed by water washing (ref. 23). Concentrations of nitrogen far below stoichiometric amounts, e.g., 0.1-0.2 %w on catalyst are sufficient to cause unsatisfactory performance in commercial units (ref. 24). This implies that the nitrogen content of the feed must be below 50 ppm in order to permit attainment of a satisfactory catalyst life (say, 200 gal. polymer/lb catalyst (ref. 24)). Isomerization of light paraffins Isomerization of n-butane to isobutane (desired as alkylation component) and isornerization of pentanes and hexanes (to improve the octane number of "tops") are also acid-catalysed reactions. However, in most modern isomerization processes a bifunctional catalyst is used under hydroprocessing conditions. In such a catalyst the acid function is combined with a hydrogenatioq/dehydrogenation function provided by a (noble) metal. The latter may serve to initiate the formation of olefins, which are converted into carbonium ions upon protonation. In the case of strong acids, which are capable of generating carbonium ions by abstraction of hydride ions from the paraffins, the function of the metal may be to maintain a suitable c a r b o n i m i o n c o n c e n t r a t i o n by establishing paraffin/olefin equilibrium under an appropriate hydrogen pressure. Thus, undesired disproportionation reactions leading to polymers and coke are kept under control (refs. 25, 26). Among the processes applying such dual-function catalysts are the ones developed by UOP (Butamer, Penex), British Petroleum (BP) and Shell (Hysomer). Catalyst poisons in these processes may affect either the noble metal function or the acid
function. Sulfur is the most prominent poison for the metal, but with a suitably chosen metal some sulfur can be tolerated. For instance, the Hysomer process can operate with a permanent sulfur level of 10 ppm, and temporary concentrations of 20 pprn (ref. 26) or even 100 ppm (ref. 2 6 ~ )are not harmful. Sulfur compounds are reportedtolowerthe activity of the UOP catalyst,but their effect istransient (ref. 27).Desulfurization of thefeedismentionedas apartoftheBPprocess (ref. 28). The acid function will be poisoned by basic impurities such as nitrogen compounds. Aromatics and water, being more basic than paraffins, in principle also act as poison. However, in the presence of hydrogen and an active noble metal small amounts of benzene are rendered harmless by hydrogenation in situ to cyclohexane, as occurs in the Hysomer and Penex processes. Alternatively, hydrogenation over a prebed can be applied (ref. 28).
551 With aluminium c h l o r i d e or c h l o r i n a t e d alumina as t h e a c i d i c component, water may a l s o reduce a c t i v i t y by s t r i p p i n g o f f H C I . A feed d r i e r i s t h e r e f o r e incorpo-
r a t e d i r ~~ i i eUOF p r o c e s s ( r e f . 2?), whereas t h e SF p r c c e s s not o n l y u s e s a dric-i feed but f e a t u r e s c o n t i n w u s a d d i t i o n of c h l o r i d e t o t h e feed a s make up ( r e f s . 30,
3 1 ) . S h e i l ' s Hysomer p r o c e s s uses a v e r y rugged z e o l i t e - b a s e d c a t a l y s t and o p e r a t e s without problems w i t h w a t e r c o n t e n t s i n t h e f e e d up t o t h e s a t u r a t i o n l e v e l a t ambient t e m p e r a t u r e ( r e f . 26~). C a t a l y t i c reforming o f naphtha C a t a l y t i c r e f o r m i n g of naphtha ( f r a c t i o n o f t h e crude o i l b o i l i n g i n t h e gaso l i n e r a n g e ) , which i s t h e major o c t a n e upgrading p r o c e s s i n t h e petroleum i n d u s t r y ,
i s c a r r i e d o u t o v e r a b i f u n c t i o n a l c a t a l y s t , which g e n e r a l l y c o n s i s t s o f platinum s u p p o r t e d on h a l o g e n a t e d alumina. I n modern b i - or m u l t i - m e t a l l i c c a t a l y s t s p l a t i n u m
i s combined w i t h o t h e r e l e m e n t s , such as Re, Ge, Ir o r Sn. Apart from g r a d u a l d e a c t i v a t i o n by d e p o s i t i o n o f carbon (which i s t a k e n c a r e o f i n t h e p r o c e s s by p e r i o d i c r e g e n e r a t i o n o f t h e c a t a l y s t ) t h e c a t a l y s t may be d e a c t i v a t e d by p o i s o n s a c t i n g on t h e p l a t i n u m o r t h e a c i d f u n c t i o n . The p l a t i n u m f u n c t i o n , which c a t a l y s e s t h e dehydrogenation of naphthenes and d e h y d r o c y c l i z a t i o n of p a r a f f i n s t o a r o m a t i c s ,
is impaired by t o o high c o n c e n t r a t i o n s o f s u l f u r . A t t h e h i g h t e m p e r a t u r e s a p p l i e d and under t h e hydrogen p r e s s u r e s u l f u r compounds l o s e t h e i r i d e n t i t y as t h e y a r e c o n v e r t e d i n t o H S ( r e f . 32), which b u i l d s up t o some e x t e n t i n t h e r e c y c l e g a s 2
system. H S i s a r e v e r s i b l e p o i s o n f o r p l a t i n u m and i t s c o n c e n t r a t i o n must g e n e r a l l y 2 be l i m i t e d t o l e s s t h a n 100, e . g . , 10 ppm, f o r t h e "monometallic" r e f o r m i n g c a t a l y s t s ( r e f . 3 2 ) . Whereas t h e s e o l d e r t y p e s o f c a t a l y s t a r e t h u s f a i r l y s u l f u r t o l e r a n t and even r e q u i r e t r a c e s o f s u l f u r t o a v o i d e x c e s s i v e m e t a l - c a t a l y s e d c r a c k i n g t h e more modern b i m e t a l l i c c a t a l y s t s a r e more s e n s i t i v e . Although s u l f u r s p e , c i f i c a t i o n depends on t h e f o r m u l a t i o n , t h e y a r e g e n e r a l l y o p e r a t e d a t s u l f u r l e v e l s o f one o r a few ppm a t most ( r e f s . 33, 36, 3 7 ) . To meet t h e s u l f u r s p e c i f i c a t i o n , t h e f e e d i s d e s u l f u r i z e d by h y d r o t r e a t i n g i n commercial p r a c t i c e . The m e t a l f u n c t i o n can be more permanently damaged by m e t a l s such as a r s e n i c and l e a d ( o r i g i n a t i n g , f o r i n s t a n c e , from t e t r a e t h y l e a d c o n t a m i n a t i o n ) , which a r e among t h e c l a s s i c a l p o i s o n s f o r platinum ( r e f s . 34, 3 5 ). However, t h e h y d r o t r e a t i n g p r o c e s s upstream p r o v i d e s e f f e c t i v e p r o t e c t i o n , s i n c e t h e s e metals a r e g e n e r a l l y caught by t h e h y d r o t r e a t i n g c a t a l y s t ( r e f . 3 6 ) . A s t o t h e s e r i o u s n e s s o f a r s e n i c p o i s o n i n g , H e t t i n g e r e t a l . ( r e f . 38) mention t h a t 0.01-0.02
%w As on c a t a l y s t h a s
l i t t l e e f f e c t on dehydrogenation and d e h y d r o c y c l i z a t i o n , b u t 0.5 %w c a u s e s a p p r e c i a ble deactivation. The a c i d f u n c t i o n i s poisoned by ammonia formed from o r g a n i c n i t r o g e n compounds when p r e s e n t i n t h e f e e d . I n commercial p r a c t i c e , n i t r o g e n compounds are removed from t h e f e e d c o n c u r e n t l y w i t h s u l f u r by h y d r o t r e a t i n g . N i t r o g e n l e v e l s below
2 ppm a r e u s u a l l y sought ( r e f . 3 2 ) . Water i s a f e e d contaminant which may a l s o a f f e c t t h e a c i d f u n c t i o n by s t r i p p i n g o f f t h e halogen from t h e c a t a l y s t , t h u s
552 u p s e t t i n g t h e b a l a n c e between t h e a c i d and m e t a l f u n c t i o n s . To cope w i t h t r a c e
amounts of w a t e r or oxygen compounds i n t h e f e e d , c h l o r i d e is c u s t o m a r i l y dosed t o t h e f e e d t o m a i n t a i n a c o n s t a n t l e v e l o f c h l o r i d e on t h e c a t a l y s t .
Catalytic cracking C a t a l y t i c c r a c k i n g o f gas o i l s or f l a s h e d d i s t i l l a t e t o produce i n t . a l . gaso l i n e u s e s an a c i d i c c a t a l y s t , v i z . amorpnous s i l i c a - a l u m i n a or t h e more r e c e n t l y developed c r y s t a l l i n e alumino s i l i c a t e s ( z e o l i t e s ) i n a m a t r i x o f s i l i c a , s i l i c a alumina, or c l a y ( r e f . 39). AreviewofthedevelopmentofcrackingcatalystshasbeengivenbyOblad(ref. 39).
A r e c e n t l y p u b l i s h e d m o n o g r a p h b y V e n u t o a n d H a b i b ( r e f . 39A) c o v e r s s e v e r a l a s p e c t s
o f f l u i d c a t a l y t i c c r a c k i n g , i n c l u d i n g f e e d s t o c k , c a t a l y s t , p r o c e s s hardware a n d p r o duct a s p e c t s . Apart from t h e normally a c c e p t e d d e a c t i v a t i o n by s u r f a c e a r e a d e c l i n e and coking, d e a c t i v a t i o n by p o i s o n i n g may o c c u r , a l t h o u g h t h e s e c a t a l y s t s a r e c o m p a r a t i v e l y r e s i s t a n t t o p o i s o n i n g , b e i n g q u i t e i n s e n s i t i v e t o many s u b s t a n c e s t h a t p o i s o n o t h e r c a t a l y s t s . S u l f u r , f o r i n s t a n c e , h a s l i t t l e e f f e c t on t h e p r e s e n t s y n t h e t i c c a t a l y s t s , a l t h o u g h it may a f f e c t t h e o l d e r n a t u r a l - c l a y c a t a l y s t s , persumably by f o r m a t i o n o f i r o n s u l f i d e from i r o n i n t h e s t r u c t u r e ( r e f s . 40, 4 1 ) . Oxygen compounds ( C O , p h e n o l s ) have l i t t l e e f f e c t e i t h e r . However, t h e c r a c k i n g r e a c t i o n i s i n h i b i t e d by b a s e s such as n i t r o g e n compounds or a l k a l i . The d e a c t i v a t i o n by n i t r o g e n compounds i s s e v e r e , b u t may be undone by b u r n i n g i n a i r i n t h e u s u a l f a s h i o n f o r r e g e n e r a t i o n ( r e f . 4 2 ) . The e f f e c t i v i t y as p o i s o n v a r i e s w i t h d i f f e r e n t n i t r o g e n compounds as i s demonstrated by t h e c l a s s i c a l d a t a o f Voge e t a l . ( r e f . 4 3 ) i n T a b l e 2.
TABLE 2
*) I n h i b i t i o n o f c r a c k i n g o f d e c a l i n by n i t r o g e n compounds R e s u l t s from c r a c k i n g d e c a l i n c o n t a i n i n g 0.11 % N i n v a r i o u s forms. S i l i c a - z i r c o n i a alumina c a t a l y s t ; atmospheric p r e s s u r e ; flow r a t e , 13.7 m o l e s / l . / h r . ; 500 OC, one hour p e r i o d . I n h i b i t i o n = p e r c e n t a g e r e d u c t i o n o f t o t a l c o n v e r s i o n .
Compound Added
None Ammonia Methylamine Diamylamine Dicyclohexylamine Pyridine Indole a-Naphthylamine Quinoline Acridine
Conversion,
Inhibition,
%
%
41.9
-
42.0 42.0 42.3 28.0 26.8 25.1 21.8
48
8.5
80
8.2
81
* ) Data from Voge e t . a l . ( r e f . 43)
0 0
0 33 36
40
553 More permanent reduction o f a c i d a c t i v i t y occurs by d e p o s i t i o n of sodium. Amounts of t h e order of 1 %w on c a t a l y s t can give r i s e to appreciable e f f e c t s ( r e f .
44).
Loss of a c t i v i t y not only occurs by suppression of a c i d i t y , but a l s o by a decrease
of surface a r e a ( r e f . b 4 A ) . Sodium on t h e c a t a l y s t is known t o decrease t h e s t a b i l i t y of t h e c a t a l y s t t e x t u r e i n thermal o r hydrothermal treatments ( r e f .
44B).
A permanent d e t e r i o r a t i o n of c a t a l y s t performance occurs when metals ( n i c k e l , vanadium and i r o n ) a r e deposited on t h e c a t a l y s t . Although t h e s e metals may a f f e c t a c t i v i t y by covering a c t i v e cracking s i t e s , t h e i r e f f e c t on s e l e c t i v i t y i s more important. For t h i s reason, t h e t e r n " s e l e c t i v i t y poison" has been proposed
(ref. 4 5 ) . Accumulation o f these metals on t h e c a t a l y s t gives r i s e t o t h e development of dehydrogenation a c t i v i t y l e a d i n g t o increased formation of gases and coke a t t h e expense of g a s o l i n e y i e l d . Table 3 shows t h e r e l a t i v e e f f e c t i v e n e s s of n i c k e l and vanadium i n t h i s r e s p e c t . I t can be i n f e r r e d from t h e d a t a t h a t n i c k e l i s about four times a s harmful a s vanadium. TABLE 3 R e l a t i v e e f f e c t i v e n e s s of n i c k e l and vanadium contamination i n c a t a l y t i c cracking ( d a t a by Cimbalo e t . a l . ( r e f . 4 6 )
Corivtision i n licroactivity test, I Carbon, %w Hydrogen, %w
Uncontaminated catalyst
Catalyst with 265 ppm N i
Catalyst with 830 ppm v
77.5
73.5 5.62 0.76
74.2 5.48
4.05 0.20
0.64
The e f f e c t of metals deposition i s g e n e r a l l y kept within l i m i t by increased c a t a l y s t make-up r a t e ( r e f . 46). A l t e r n a t i v e ways t o counteract t h e e f f e c t of met a l s i s t o p a s s i v a t e them, e.g. by a d d i t i o n of antimony ( r e f .
47) o r
t o remove
them by e x t r a c t i o n ( r e f . 48). S t i l l another way t o cope with heavy n e t a l s i n t h e feedstock i s t o use s p e c i a l c a t a l y s t s with enhanced metal t o l e r a n c e , which a r e o f f e r e d by s e v e r a l c a t a l y s t manufacturers. The way i n which metal t o l e r a n c e has been obtained i s g e n e r a l l y p r o p r i e t a r y know-how. However, it i s w e l l known ( r e f . 48B) t h a t u s u a l l y only p a r t of t h e deposited metals a r e e f f e c t i v e i n promoting t h e und e s i r e d r e a c t i o n , t h e remaining p a r t being i n a c t i v e because of s i l i c a t e formation
or encapsulation i n t h e c a t a l y s t matrix. This i n e f f e c t i v e f r a c t i o n might be increased by adapting t h e c a t a l y s t formulation. Agents similar t o t h o s e used for p a s s i v a t i n g t h e metals may a l s o be incorporated during manufacture of t h e c a t a l y s t . The above-mentioned poisoning e f f e c t s p e r t a i n t o t h e cracking f u n c t i o n per s e . A r e c e n t t r e n d i n c a t a l y t i c cracking i s t o i n c o r p o r a t e a l s o o t h e r c a t a l y t i c f u n c t i o n s i n t h e c a t a l y s t , i n p a r t i c u l a r t o promote t h e burning of CO t o C02 during regenerat i o n ( r e f s . 48c, 4811). These CO burn promoters, which c o n s i s t , f o r i n s t a n c e , of
554
minute quantities of platinum-group metals, may be incorporated in the manufactured catalyst, or they may be added as a solid or liquid additive during operation. Poisoning characteristics of these promoters will be different according to their nature. Hydrotreatin& Hydrotreating of distillate fractions to remove sulfur and/or nitrogen and to hydrogenate unsaturated compounds is generally carried out over sulfided Co/Mo or Ni/Mo on alumina. These catalysts are among the most rugged ones applied in industrial processes, being relatively insensitive to usual catalyst poisons. As their duty implies they remain active in the presence of H2S and NH3, although high concentrations of these compounds may suppress their activity. Apart from poisoning by trace metals (V, Ni), which is much more a problem in processing of residues than in hydrotreating of distillates (see below), deactivation by silicon (originating for instance from feed contamination with antifoam additives) is one of the relatively rare instances of hydrotreating catalyst poisoning (refs. 36, 49). As mentioned before, a hydrotreating catalyst bed may act as a guard against poisoning of a downstream reformer by arsenic and lead, apparently without being much affected itself. A hydrotreating catalyst containing an average of 1.2 %w As was found to have 70 % and 90 % of fresh activity for nitrogen and sulfur removal, respectively, after regeneration (ref. 49). Because of its ruggedness, it is not unusual for this type of catalyst to operate for years without replacement. In a survey mentioned by Roeder (ref. 50) catalyst lives betweed 175 and 1000 bbl/lb were found for 60 industrial units. Hydrocracking Hydrocracking of petroleum fractions to produce lighter distillates (e.g. gasoline from gas oil or flashed distillate) is generally carried out over a dualfunction catalyst. The functions serve to (a) crack higher-molecular-weight hydrocarbons and (b) activate molecular hydrogen and hydrogenate unsaturated compounds present in the feed or formed during the cracking process. The acidic cracking function is usually provided by an amorphous silica-alumina (similar to the catalyst in cat. cracking), a crystalline aluminosilicate (zeolite), or a halogenated alumina carrier. The major hydrogenation components are Pt, Pd, Ni, Co, Mo, W o r combinations of the latter four metals.
A survey of catalysts for hydrocracking has been given by Vlugter and Van 't Spijker (ref. 5 1 )
. Scott and Patterson
(ref. 5 2 ) have considered commercial
processes and Scott and Bridge (ref. 53) have described the technology and catalytic requirements for various applications. A more recent literature review has been given by Choudhary and Saraf (ref.
54).
555 The acid function is sensitive to poisoning by nitrogen bases. When the cracking duty demanded is rather severe, such as in the production of gasoline from neavier distillates, it is customary to pretreat the raw feed to remove nitrogen to a sufficiently low level. Thus the hydrocracking operation is carried out in two stages, the first stage being a hydrodenitrogenation stage and the second one performing the main hydrocracking duty. With amorphous silica-alumina-based second-stage catalysts, nitrogen should be removed as completely as possible (say, below 5 ppm (ref. 5 l ) ) , and ammonia is scrubbed from the first-stage effluent. Cracking catalysts based on crystalline aluminosilicates can stand some nitrogen and may actually prefer a certain nitrogen concentration for stable operation (ref. 5 1 ) . Their better nitrogen tolerance can be exploited in what is called series-flow operation: the first-stage effluent is passed through the succeeding hydrocracker without intermediate separation. The sensitivity of the hydrogenation function is, of course, dependent upon the choice of the metal. As a very high hydrogenation activity is not necessary
or even desirable, metal sulfides such as sulfided palladium or mixed sulfides of the nickel-tungsten type can be employed. This type of hydrogenation component is, of course, quite tolerant to sulfur and will even have to be operated with some sulfur to maintain the sulfidic state. The first-stage denitrogenation catalysts are generally of a kind similar to the previously discussed hydrotreating catalysts. However, in view of the more demanding denitrogenation the Ni/Mo-type sulfides are often based on an acidic support, e.g., alumina carriers containing silicon or fluorine (ref. 51 ) . These are of course quite resistant to sulfur. It has even been found that high H2S pressures are beneficial for the hydrodenitrogenation of pyridine (ref. 55).
As to poisoning of first-stage catalysts with other feed contaminants, the situation is rather similar to that discussed under hydrotreating. The presence of V and Ni in the feed gives rise to deposition of these metals on the catalyst, which may be especially serious in the case of residual feedstocks (see below). HYDROPROCESSING OF PETROLEUM RESIDUES Hydroprocessing of the heavy end of the crude oil has gained quite some importance in the last decade. In Japan, in particular, a number of residue hydrodesulfurizers are operating to produce low-sulfur fuel oil. In the processes employed the catalyst plays a key r81e, as it has to operate with a feedstock in which catalyst poisons are present, not as an incidental trace contaminant, but as a natural constituent. Particular problems are posed by the presence of metals (V, Ni and Fe) and polyaromatie, heteroatom-rich materials called asphaltenes, whieh are characterized by insolubility in solvents like n-pentane or n-heptane. Table
4
gives some compositional data of typical residual feedstocks from Middle East crudes, showing concentrations of vanadium (the main metal contaminant) between
556 50 and 100 ppmw (ref.
56). Residual feedstocks from some Californian and, in
particular, Venezuelan crudes are much richer in metals, vanadium contents being sometimes as high as about :000 ppmw. TABLE
4
Properties of some residual feedstocks from Middle East crudes*
Density, d 70/4 Sulfur, Vanadium, Nickel, Sodium, C 7 asphaltenes, Viscosity at 210 OF, Ramsb. carbon, Breakdown, %w (TBP) Fr. < 165 OC 165 - 250 OC 250 - 350 OC > 350 OC
%w ppmw ppmw ppmw %w cSt %w
Kuwait long residue
Iranian Heavy long residue
0.9336 4.21 53 12.7
0.9354 2.73 145 33.6 7.2 3.6 63 10.0
6.5 111.9 13.1
-
-
7.7 2.7
53.7 9.8
-
0.05
2.60 97.40
2.0
97.95 ~~~
Arabian Heavy long residue
Kuwait short residue
0.9574 4.31 88 26.9 25
0.14 4.35 95.51
0.987 5.20 100
29.3 9.0 6.9 1810 18.7 -
0.16 99.84 ~~~
*Data from Van Ginneken et. al. (ref. 56) Rale of asphaltenes and metals in catalyst deactivation As has been mentioned before in this paper, the phenomenon of catalyst poisoning in an industrial plug-flow-type reactor is a complex one. The state ofthe catalyst in the bed varies with its location and changes in the course of the run as a poisoning front traverses through the bed. Consequently, it is very difficult to deduce the catalyst deactivation mechanistics from the observed overall decline of process performance. For this reason, studies on catalyst deactivation in hydroprocessing of residues at the Shell Laboratory in Amsterdam have been carried out not only in pilot plants operated in a fashion similar to that followed in commercial practice, but also in fixed-bed bench-scale units featuring recycle of the major part of the liquid product (ref.
57). The latter units simulate a
continuous stirred-tank reactor (CSTR) characterized by uniformity of the catalyst throughout the reactor at any given time. Thus, the observed performance decline can be associated with the activity decline of each single catalyst particle. Figure 2 shows a typical activity decline thus determined in processing of a residual feed (ref. 57). After a rapid initial decline of desulfurization activity, a more gradual decline takes place between the points TSSD (time of start of slow decline) and TSm ( t h e of start of accelerated decline). It will be clear that the useful catalyst life (UCL) is determined mainly by the slow decline, rather than by the fast initial decline of activity.
557
1510 -
86-
42-
UCL
V REMOVAL RATE (ARBITRARY UNITS)
101
0
I
I000
I
I
2000
3000 CAT. AGE, kg/kg
Fig. 2 . Typical d e a c t i v a t i o n behaviour of a c a t a l y s t observed i n hydrodesulfurizat i o n of a r e s i d u a l feedstock i n a simulated CSTR.
From Fig. 3 it can be deduced t h a t carbon laydown i s not t h e cause of t h e slow d e c l i n e and t h e r e f o r e n o t t h e determining f a c t o r f o r UCL. The f i g u r e suggests t h a t carbon b u i l d s up r a p i d l y on t h e c a t a l y s t i n t h e very i n i t i a l p e r i o d of t h e run, but it soon reaches an equilibrium l e v e l on t h e c a t a l y s t . I n t h e l a t t e r s t a g e , t h e r a t e s of carbon d e p o s i t i o n and carbon removal by hydrogenation a r e apparently i n balance. Since t h e l a t t e r r a t e w i l l depend upon t h e hydrogen p r e s s u r e t h e s t e a d y - s t a t e carbon l e v e l w i l l a l s o depend on hydrogen p r e s s u r e . Figure
4
shows
an approximately i n v e r s e r e l a t i o n s h i p between t h e s e two parameters. While carbon d e p o s i t i o n may play a r z l e i n t h e f a s t i n i t i a l a c t i v i t y d e c l i n e , t h e more important slow d e c l i n e must be a s s o c i a t e d with a feed impurity o t h e r than a s p h a l t e n i c carbon, which impurity must be present i n a smaller c o n c e n t r a t i o n , i n view of t h e time s c a l e involved. The obvious candidates a r e t h e metals, i n p a r t i c u l a r vanadium and n i c k e l .
558
30
-
20
/A //
lI
:i/ I / 10
-
1
0
n
-
0
, /
V ----
I/
7
n
v
--_____ v
CAT. A, MIDDLE EAST RESIDUE w A
-
CAT.& MIDDLE EAST RESIDUE
CAT. B, CARIBBEAN RESIDUE
/
/
0 0
I I000
I
I
I
2000
3000
4000
1
5000 CAT AGE, kg/kg
Fig. 3. Carbon on c a t a l y s t a s a f u n c t i o n of c a t a l y s t age i n hydrodesulfurization of r e s i d u a l feedstocks i n a simulated CSTR.
559
30
25
CARBON, A R B I T R A R Y UNITS
-
/
20
-
/””
1 150-
1
1
I
10
15
20
i, ARBITRARY
UNITS
pH2
Fig.
4. Steady-state
l e v e l o f carbon on c a t a l y s t a s a f u n c t i o n of hydrogen part i a l pressure i n residue hydrodesulfurization.
Fig. 5 shows t h a t t h e s e keep accumulating on t h e c a t a l y s t , i n c o n t r a s t t o carbon. These f i n d i n g s a r e i n l i n e with observations by o t h e r s . For i n s t a n c e , Egi and Matsumoto ( r e f .
58)
examined t h e c a t a l y s t i n a f i x e d bed a f t e r 50 and 1000 hours
processing of a r e s i d u e and found approximately t h e same carbon c o n t e n t s a f t e r t h e s e two on-stream p e r i o d s . Moreover, t h e carbonaceous d e p o s i t s were not predominantly present on t h e i n l e t s i d e of t h e bed, but f a i r l y evenly d i s t r i b u t e d over t h e bed. A pronounced f o u l i n g p r o f i l e with highest concentrations on t h e i n l e t s i d e w a s observed f o r vanadium and n i c k e l , and t h e concentrations a f t e r 1000 hours on stream were an o r d e r of magnitude higher than a f t e r 50 hours.
560
+ Ni
25
DEPOSITS, . A R B I T R A R Y U N I T S
,’ CAT. ,/
8, CARRIBEAN RESIDUE
20
, 15
10
5
c 0
I000
2000
3000
4000 CAT. AGE, kg/kg
Fig. 5. Metals on catalyst as a function of catalyst age in hydrodesulfurization of residues in a simulated CSTR. Nature and catalytic rale of metal deposits Elementary analysis of used catalysts indicates that vanadium and nickel are present as sulfides, most probably as V2S3 and Ni3S2. Recent NMR and EPR studies by Silbernagel (ref. 5 9 ) indicate that at higher metal loadings V2S3 is the dominant form of deposited vanadium. Nickel and vanadium in petroleum are generally associated with the heavier constituents of the crude oil, hence their collection in the residual fractions. They are associated in particular with the asphaltenes and resins and present in metalloporphyrin chelate structures as well as non-porphyrinic complexes (ref. 60). The asphaltenic and resinous species have relatively high molecular weights (mol. weights between 1000 and 100 000 have been found by Drushel (ref. 6 1 ) ) and also relatively large molecular dimensions. Asphaltenes are generally visualized as discs composed of sheets of condensed aromatic rings with thicknesses of the order of 2 nm (refs. 62, 63).
56 1
Since the latter dimension is of‘ the same order of magnitude as the pore sizes in usual catalysts, it is plausible that asphaltene molecules do riot readily diffuse into the catalyst. Consequently their conversion, which accompanies or precedes the deposition of metals, is pore-diffusion limited. This is clearly evident from electronmicroprobe analysis of used catalysts, which shows an enrichment of vanadium and nickel in the superficial region of a catalyst particle. Fig. 6 shows examples of such intraparticle metal profiles. Similar profiles have
VANADIUM CONCENTRATION (APPROXIMATE)
I
2
______-___
J I
I
1
I
,L
----------
+
I
0 8 0.6 0.4 0.2 0
I
I
0.2 0.4 0.6 0.8
4
3
I_L+LL 0.8 0.6 0.4 0.2 0
0.2 0.4 0.6 0.8
0.6 0.4 0.2 0
0.2 0.4 0.6
DISTANCE FROM CENTRE OF PARTICLE, m m Fig.
6. Electron microprobe V-Ka scans of specimens of catalysts used in
hydro-
processing of residues. Accessibility of catalyst to asphaltenes decreases from 1 to 4.
562
been reported by Dautzenberg et. al. (ref. 571, Audibert and Duhaut (ref. 641, Todo et. al. (ref. 65), and Inoguchi et. al. (ref.
66). Radford and Rigg (ref. 67)
also reported that on the used catalyst the higher concentration of the metals was always found near the outer surface, while the inner volume of the catalyst still had only a low concentration of metals. The time scale and the amounts of deposited metals involved (cf. Figures 2 and
5) make it unlikely that the mechanism causing the gradual decline in activity
-
(between TSSD and TSm in Fig. 2) is a poisoning ---- ---- in the usual sense, although pore-mouth poisoning may play a role in the initial fast decline. The gradual plugging process, activity decline determining UCL is more likely a pore-mouth ------------------viz., a gradual decrease in accessibility of the catalyst particles as a result
of the deposition of massive amounts of metal sulfides in the peripherial zones. The steady increase in diffusional resistance to the transport of reactant molecules to the catalytic surface is manifested by a gradual decline in conversion rate until the pore openings are almost fully blocked, when an accelerated decline sets in. The reaction that leads to metal removal from the feedstock is apparently a surface-catalysed one which is not poisoned by the deposition of metal sulfides. This suggests that the metal sulfides themselves can act as catalyst. Thus, the metal-removal reaction may be considered to be an autocatalytic one. Evidence for this is given by Fig. 7, which shows that bare carriers may gradually acquire catalytic activity for removing metals from a residual feedstock, when exposed to this feedstock under hydroprocessing conditions.
VANADIUM R E M O V A L R A T E , A R B I T R A R Y U N I T S
40
I-
20
-
00
Fig.
I000
2000
3000
4 000 5000 CAT AGE, kg. kg-+
7. Development of activity for vanadium removal in hydroprocessing of a Middle East residue over bare carrier materials.
563 D e s c r i p t i o n of c a t a l y s t d e a c t i v a t i o n by a pore-mouth p l u g g i n g model The above m e c h a n i s t i c p i c t u r e of c a t a l y s t d e a c t i v a t i o n i n r e s i d u e p r o c e s s i n g forms t h e b a s i s o f a s i m p l e two-','arameter
model developed a t t h e S h e l l Laboratory
a t Amsterdam, which d e s c r i b e s t h e g r a d u a l a c t i v i t y d e c l i n e by pore-plugging
( r e f . 57)
T h i s model allowed t h e c o r r e l a t i o n of t h e r e s u l t s o f a l a r g e number o f e x p e r i m e n t s c a r r i e d o u t w i t h d i f f e r e n t c a t a l y s t s and f e e d s t o c k s under v a r i o u s c o n d i t i o n s . By way of example, F i g . 8 shows t h a t d i f f e r e n t c a t a l y s t s o p e r a t e d a t v a r y i n g p r e s s u r e
I .4
I.2
I .c
0.8
0.6
0.4
0.2
0 0
0.2
0.4
0.6
0.8
1.0
e
F i g . 8. R e l a t i o n between c a t a l y s t e f f e c t i v e n e s s f o r d e s u l f u r i z a t i o n (17) and r e l a t i v e c a t a l y s t age ( e ) . Runs a t d i f f e r e n t p r e s s u r e s w i t h d i f f e r e n t c a t a l y s t s (Dautzenberg e t a l . (ref. 57))
564
levels have an identical deactivation behaviour when results are normalized according to the model (ref. 57). Fig. 9 shows that the model can satisfactorily predict catalyst lives, once the pertinent activity decline parameters have been established. Another model of catalyst deactivation in residue desulfurization has been proposed by Newson (ref. 68). His model also considers a pore-plugging mechanism in combination with coking effects. Practical implications of the effect of metals in residual feedstocks Although the presence of metals is a formidable problem in catalytic processing of residues, the development of suitable metal-tolerant catalysts has diminished
0
I000
2000
3000
4000
EXPERIMENTAL UCL, kg/kg
Fig. 9. Comparison of calculated and experimentally determined catalyst lives in hydrodesulfurization of residues.
565 t h e problem t o an acceptable proportion f o r r e s i d u e s of low t o moderate metal cont e n t s , i . e . below some 100 ppmw. Commercial processes now a v a i l a b l e achieve run l e n g t h s between s i x months and a year b e f o r e c a t a l y s t replacement. A number of t h e s e processes a r e mentioned i n reviews given by, f o r i n s t a n c e , Nelson ( r e f .
69),
Ebel ( r e f . T O ) , Lawrance ( r e f . 71) and Aalund ( r e f . 7 2 ) . With feedstocks of higher metals c o n t e n t , c a t a l y s t replacement c o s t s w i l l become an unduly l a r g e f a c t o r i n t h e o v e r a l l processing c o s t s ( r e f . 73) while run l e n g t h s of fixed-bed r e a c t o r s would become t o o s h o r t t o be p r a c t i c a b l e . Apart from t h e obvious p o s s i b i l i t y t o f r a c t i o n a t e t h e f e e d , e . g . , by solvent e x t r a c t i o n , i n a metals-rich and metals-poor f r a c t i o n and only process t h e l a t t e r , t h e r e i s a l s o t h e option t o remove most of t h e m e t a l s c a t a l y t i c a l l y . For i n s t a n c e , t h e ebullated-bed d e s u l f u r i z a t i o n r e a c t o r a p p l i e d i n t h e H-oil process of Hydrocarbon Research I n c . and C i t i e s Service Co. can be preceded by another ebullated-bed r e a c t o r with an inexpensive n a t u r a l m a t e r i a l a s d e m e t a l l i z a t i o n c a t a l y s t ( r e f .
74).
The ebullated-bed p r i n c i p l e permits continuous replacement of t h i s m a t e r i a l . I n a process developed by S h e l l ( r e f .
561, a
s p e c i a l demetallization c a t a l y s t i s
a p p l i e d which f e a t u r e s a high d e m e t a l l i z a t i o n a c t i v i t y and a high metals uptake capacity. Replacement of t h e c a t a l y s t without i n t e r r u p t i n g t h e o p e r a t i o n i s made p o s s i b l e by a moving-bed-type r e a c t o r ( c a l l e d "bunker flow" r e a c t o r ) operated i n a t r i c k l e mode ( r e f . 5 6 ) .
CONCLUDING REMARKS
A s follows from t h e foregoing d i s c u s s i o n , t h e problem of c a t a l y s t poisoning has been reduced i n most petroleum r e f i n i n g processes t o a proportion which allows l a r g e - s c a l e continuous operation f o r a t l e a s t s e v e r a l thousands of hours, even with d i f f i c u l t feedstocks such as r e s i d u a l ones. This has r e s u l t e d from c o n t i n u a l improvement, over s e v e r a l decades, of processing knowhow. More i n s i g h t i n t o t h e n a t u r e and t h e e f f e c t of c a t a l y s t poisons, b e t t e r a n a l y t i c a l techniques for det e c t i n g t h e poisons, improved ways t o remove them, novel r e a c t o r systems, and - l a s t but not l e a s t - b e t t e r c a t a l y s t s with enhanced t o l e r a n c e of poisons, have a l l contributed i n a t t a i n i n g t h e present s i t u a t i o n . While t h i s i s i n g e n e r a l a g r a t i f y i n g s t a t e of a f f a i r s , it does not mean t h a t t h e r e would be no i n c e n t i v e f o r seeking f u r t h e r improvements such a s developing novel and improved c a t a l y s t s or r e a c t o r systems. A s o i l becomes scarce and i t s p r i c e i n c r e a s e s , it becomes more and more important t o achieve maximum y i e l d t o d e s i r e d products i n o i l r e f i n i n g . C a t a l y s t poisoning r e s u l t i n g in. s e l e c t i v i t y l o s s e s , e i t h e r by d i r e c t e f f e c t of poisons on s e l e c t i v i t y or by t h e temperature i n c r e a s e s necessary t o compensate f o r d e a c t i v a t i o n , w i l l become l e s s and l e s s acceptable. The e s c a l a t i o n i n t h e c o s t of r e a l i z a t i o n of i n d u s t r i a l p l a n t s and high i n t e r e s t r a t e s make e f f i c i e n t u t i l i z a t i o n of i n s t a l l e d equipment mandatory. Down-time because of c a t a l y s t d e a c t i v a t i o n w i l l become i n c r e a s i n g l y o b j e c t i o n a b l e .
566
Finally, the advent of l e s s conventional and generally more contaminated feedstocks such as heavy crude oils, oils from tar sands and shale and-last but not least-coal liquids, calls for significant improvements in processing technGlGgy and represents a great challenge to research and development in catalysis and engineering.
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References (continued) 38 W.P. Hettinger Jr., C.D. Keith, J.L. Gring and J.W. Teter, "Hydroforming reactions and the effect of certain catalyst properties and poisons", paper presented at the Am. Chem. SOC., Div. Petroleum Chemistry meeting, New York, Sept. 11-17, 1954. 39 A.G. Oblad, Oil and Gas J., Mar. 27, 1972, p. 84. 39A P.B. Venuto and E.T. Habib Jr., "Fluid Catalytic Cracking with zeolite catalysts", Karcel Dekker Inc., New York and Basel, 1979. 40 C.H. Riesz and F.L. Morris, "Catalysts for Petroleum Refining", in "Advances in Petroleum Chemistry and Refining, Vol.IV, Ed. J.J. Mc.Ketta Jr., Interscience Publ. Inc., New York 1961, p.287. 41 G.A. Mills, Ind. Eng. Chem., 42(1950) 182. 42 H.H. Voge, "Catalytic cracking", in "Catalysis", Vol. IV, Ed. P.H. Emmett, Reinhold Publ. Corp., New York, 1951, p.422. 43 H.H. Voge, G.M. Good and B.S. Greensfelder, Proc. 3rd World Petroleum Congress, 4( 1951) p. 124. 44 Nat. Petr. Ref. Ass. 1978 Question and Answer Session, Hydrocarbon Processing, Mar. 1979, p.154. 44A Nat. Petr. Ref. Ass., Question and Answer Session 1975, Oil and Gas J., Mar. 22 1976, p. 82; Ibid., 1977, Oil and Gas J., Apr. 10, 1978, p. 62. 44B B.W.S. Letzsch, R.E. Ritter, and D.E.W. Vaughan, Oil and Gas J., Jan. 26 1976, p. 130. 45 W.B. Innes, "Carriers, promoters, accelerators, poisons, inhibitors", in "Catalysis", Vol. I, Ed. P.H. &ornett, Reinhold Publ. Corp., New York, 1954, p.302. 46 R.N. Cimbalo, R.L. Foster and J.S. Wachtel, Oil and Gas J., May 15, 1972, p.112. 47 G.H. Dale and D.L. McKay, Hydrocarbon Processing, Sept. 1977, p.97. and Gas J., Mar. 28 1977, p.43. 48 R.R. Edison, J.O. Siemssen and G.P. Masologites, Hydrocarbon Processing, May 1976, p.133. Dec. 20 1976, p.54. 48B G.P. Masologites and L.H. Beckberger, Oil and Gas J., 11 (47) 1973, p.49. 48C L. Rheaume, R. Ritter, J.J. Blazek and J.A. Montgomery, Oil and Gas J., May 17 1976, p.103; Ibid., May 24, 1976, p.66. 48D F.D. Hartzell and A.W. Chester, Oil and Gas J., Apr. 16 1979, p.83. 49 Nat. Petr. Ref. Ass. Question and Answer Session, Oil and Gas J., May 3, 1971, p. 102. 50 Nat. Petr. Ref. Ass. Question and Answer Session, Oil and Gas J., April 24, 1972, p.79. 51 J.C. Vlugter and P. van 't Spyker, Proc. 8th World Petroleum Congress, 4(1971)
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57 F.M. Dautzenberg, J. van Klinken, K.M.A. Pronk, S.T. Sie and J-B. Wyffels,
56
59 60
61 62
"Catalyst deactivationthrough pore-mouth plugging during residue desulfurization", paper presented at the 5th Int. Symposium on reactor engineering, Houston, Mar. 13-15, 1978. K. Egi and K. Matsumoto, Chem. Econ. and Eng. Review, Nov. 1970, p.7. B.G. Silbernagel, J. Catalysis, 56(1979) 315. T.F. Yen, "The role of trace metals in petroleum", Ann Arbor Science Publ. Inc., Ann Arbor, Michigan, 1975. H.V. Drushel, "Analytical characterization of residua and hydrotreated products", paper presented at the Am. Chem. SOC., Div. Petroleum Chemistry meeting, New York, Aug. 27-Sept. 1, 1972. J.P. Dickie and T.F. Yen, Anal. Chem. 39( 1967) 1847.
569
References (continued) 63 T.F. Yen, "Present status of the structure of petroleum heavy ends and its significance to various technical applications", paper presented at the Am. Chem. Soc., Div. Petroleum Chemistry meeting, New York, Aug.27-Sept. 1, 1972. 64 F. Audibert and P. Duhaut, Rev. Inst. FranS. Petrole 25, n0.5 (May 1970) 613. 65 N. Todo, T. Kabe, K. Ogawa, M. Kurita, T. Sato, K. Shimada, Y. Kuriki, T. Oshima, T. Takematsu and Y. Kotera, Kogyo. Kagaku Zasshi, 74, no.k(l971) 563. 66 M. Inoguchi, S. Sakurada, Y. Satomi, K. Inaba, H. Kagaya, K. Tate, T. Mitzutori, R. Nishiyama, T. Nagai and S. Onishi, Bull. Japan Petroleum Inst., 14 no.2 (Nov. 1972) 153. 67 H.D. Radford and R.G. Rigg, Hydrocarbon Processing, Nov. 1970, p.187 68 E. Newsor., Ind. Ecg. Chem. Process Des. Develop. 14, no.l(l975) 27. 89 W.L. Nelson, Si;lphur, lOO(1972) 55. 70 R.H. Ebel, "Recent advances in fuel desulfurization technology", paper presented at the Am. Chem. Soc., Div. Petroleum Chemistry meeting, New York, Aug. 27Sept. 1, 1972. 71 P.A. Lawrance, Chemie Ing. Technik 45(1973) 1141; ErdoelU. Kohle 26(1973) 447. 72 L. Aalund, Oil and Gas J., Sept 11, 1972, p.79. 73 W.L. Nelson, Oil and Gas J., May 2, 1977, p . 247; Nov. 19, 1973, p.54. 74 K.E. Hastings, L.C. James and W.R. Mounce, Oil and Gas J., June 30, 1975, p. 122.