C h a p t e r 12 Influence on
of
Metal
C o m p l e x e s in F o s s i l
Industrial
Fuels
Operations
Jan F. Branthaver
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Western Research Institute, Box 3395, University Station, Laramie, WY 82070
Metal complexes are incorporated in the organic matter of most fossil fuels, sometimes in substantial concentration. These compounds significantly affect the processing of the materials in which they occur. Metal complexes decompose during hydrodesulfurization and catalytic cracking operations, resulting in metal deposition on catalysts. These metal deposits enhance undesirable side-reactions and cause reductions in catalyst activity and lifetime. As heavy crudes and other materials that usually are of high metal content replace conventional crudes as refinery feedstocks, it will be necessary to find ways to at least partially nullify the deleterious effects of metal complexes. Processes based on metals removal and passivation, hydrogen addition, carbon rejection, and improved catalyst design are actively under investigation. While metal complexes in fossil fuels usually have adverse impacts on processing, product properties, and pose environmental problems, some fossil fuels may be sources of metals in the future.
As a c o n s e q u e n c e o f t h e i r g e o c h e m i c a l h i s t o r y , m o s t c r u d e o i l s , t a r sands, c o a l s , and o i l s h a l e s c o n t a i n measurable amounts o f m e t a l complexes. These complexes a r e n o t m e r e l y p h y s i c a l l y a s s o c i a t e d , as a r e d i s p e r s e d m i n e r a l p a r t i c l e s , b u t a r e p a r t o f t h e o r g a n i c phases o f f o s s i l fuels. Concentrations of organically chelated m e t a l s i n t h e a b o v e f o s s i l f u e l s r a n g e f r o m t h e ppm l e v e l t o o v e r 1000 ppm. M e t a l s f o u n d a s c o m p l e x e s i n f o s s i l f u e l s a r e t r a n s i t i o n m e t a l s , m o s t l y those o f t h e f i r s t l o n g row o f t h e p e r i o d i c table. Among t h e b e s t known m e t a l c o m p l e x e s t h a t o c c u r i n f o s s i l f u e l s a r e m e t a l l o p o r p h y r i n s , which a r e d e r i v e d l a r g e l y from t h e c h l o r i n s o f t h e v e g e t a b l e m a t t e r t h a t was t h e s o u r c e m a t e r i a l f o r f o s s i l f u e l s .
0097-6156/87/0344-0188$06.00/0 © 1987 American Chemical Society
Filby and Branthaver; Metal Complexes in Fossil Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
12.
BRANTHAVER
Influence
of Metal Complexes
on Industrial
Operations
189
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M e t a l Complexes i n P e t r o l e u m In t h e m a j o r i t y o f c r u d e o i l s , v a n a d i u m a n d n i c k e l a r e t h e two metals which o c c u r a s c h e l a t e s i n g r e a t e r t h a n t r a c e (>10 ppm) a m o u n t s (J_). Some h i g h - s u l f u r crudes have t o t a l vanadium and n i c k e l c o n t e n t s e x c e e d i n g 1000 ppm ( 2 ) . S u b s t a n t i a l amounts o f i r o n h a v e b e e n d e t e c t e d i n some c r u d e s , b u t i t i s n o t c e r t a i n t o w h a t e x t e n t i r o n c o m p o u n d s a r e n a t i v e o r become i n c o r p o r a t e d i n t o c r u d e s as a r e s u l t o f h a n d l i n g . Other metals occur i n i n o r g a n i c s a l t s i n b r i n e s a s s o c i a t e d w i t h crude o i l s , but these s a l t s a r e not t r u e components o f p e t r o l e u m . I t i s k n o w n t h a t some f r a c t i o n o f t h e m e t a l c o m p l e x e s f o u n d i n petroleum a r e p o r p h y r i n complexes o f such d i v a l e n t i o n s as n i c k e l and v a n a d y l . The s t r u c t u r e s o f t h e p o r p h y r i n s f o u n d i n p e t r o l e u m h a v e b e e n s t u d i e d i n some d e t a i l ( 3-4 ; F i l b y a n d V a n B e r k e l , this volume). I n most c r u d e s , p o r p h y r i n s c o n s i s t o f homologous s e r i e s of d e s o x o p h y l l o e r y t h r o e t i o (DPEP), e t i o , and rhodo types (Figure 1). The n a t u r e o f n o n - p o r p h y r i n m e t a l c h e l a t e s i n p e t r o l e u m i s still a matter o f some c o n t r o v e r s y . T h i s i s because no such compounds have been i s o l a t e d and c h a r a c t e r i z e d w i t h t h e r i g o r t h a t has been the case with metalloporphyrins. Indeed, some investigators q u e s t i o n whether non-porphyrin metal chelates are present i n petroleum. O r i g i n a l l y , the existence of non-porphyrin complexes i n petroleum was i n f e r r e d from the fact that metal c o n t e n t s o f many c r u d e s a r e o b s e r v e d t o e x c e e d p o r p h y r i n c o n t e n t s , as measured by U V - v i s i b l e s p e c t r o m e t r y . Because the h e a v i e r crudes which contain relatively large amounts o f m e t a l s a r e c o l l o i d a l systems i n v o l v i n g d i s p e r s e d m o l e c u l a r a g g r e g a t i o n s , i t has been argued that UV-visible s p e c t r o m e t r y u n d e r s t a t e s t h e amounts o f porphyrins i n such systems (5) , a n d t h a t t h e e x i s t e n c e o f n o n porphyrinic species i s only apparent. Other i n v e s t i g a t o r s (6-12) have unearthed a v a r i e t y o f e v i d e n c e t o i n d i c a t e t h a t a s u b s t a n t i a l f r a c t i o n o f metals i n p e t r o l e u m a r e complexed w i t h l i g a n d s t h a t a r e not c l a s s i c a l porphyrins. I t has been suggested that thenonporphyrin coordination sites are part of large aromatic sheets ( 13). T h e r e i s some e v i d e n c e t h a t n o n - p o r p h y r i n m e t a l c o m p l e x e s o f p e t r o l e u m may n o t i n v o l v e c o o r d i n a t i o n w i t h f o u r n i t r o g e n atoms (14-15 ; F i s h , e t a l . ; t h i s volume). The n a t u r e o f t h e l i g a n d s c h e l a t e d w i t h m e t a l s o t h e r than vanadium and n i c k e l i s n o t w e l l known, a l t h o u g h t h e e x i s t e n c e o f i r o n p o r p h y r i n s i n a heavy c r u d e has been r e p o r t e d ( 1 6 ) . M e t a l complexes o f vanadium and n i c k e l a r e c o n c e n t r a t e d i n asphaltene and r e s i n fractions of petroleum. A s p h a l t e n e s and r e s i n s c o n s t i t u t e t h e most p o l a r and a r o m a t i c p o r t i o n o f a c r u d e . A s p h a l t e n e s a r e p r e c i p i t a t e d as b l a c k s o l i d s by t r e a t i n g a c r u d e w i t h l a r g e volumes o f an a l k a n e s o l v e n t , u s u a l l y n-pentane o r nheptane. Materials soluble i n the alkane solvent are referred t o as m a l t e n e s or petrolenes. The r e s i n fraction i s o b t a i n e d by chromatographic treatment of maltenes. The m o s t p o l a r c o m p o n e n t s of the maltenes become c o n c e n t r a t e d i n t h e r e s i n s . Therefore r e s i d u a l f r a c t i o n s from d i s t i l l a t i o n p r o c e s s e s w i l l c o n t a i n almost a l l metals o r i g i n a l l y present i n a crude. Metalloporphyrins are s l i g h t l y v o l a t i l e a t h i g h e r t e m p e r a t u r e s , a n d t h e r e f o r e t h e y may b e present i n higher d i s t i l l a t i o n fractions.
Filby and Branthaver; Metal Complexes in Fossil Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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190
M E T A L C O M P L E X E S IN FOSSIL FUELS
Figure
1.
a. b. c. d.
S t r u c t u r e s o f P o r p h y r i a s and a C h l o r i n . M = M e t a l ( N i , V = 0, e t c . ) Meta11ated E t i o p o r p h y r i n Metallated Deoxophylloerythroetioporphyrin M e t a l l a t e d Rhodoporphyrin Metallated Chlorin
(DPEP)
Filby and Branthaver; Metal Complexes in Fossil Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
12.
BRANTHAVER
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2017 | http://pubs.acs.org Publication Date: July 6, 1987 | doi: 10.1021/bk-1987-0344.ch012
Refining
Influence
of Metal Complexes
on Industrial
Operations
191
o f Heavy Crudes
The energy c r i s e s of the 1970's w i t h the accompanying i n c r e a s e s i n the p r i c e of petroleum r e s u l t e d i n the p r o c e s s i n g of g r e a t e r amounts of heavy crudes by refineries. Heavy crudes are c h a r a c t e r i z e d by low API g r a v i t y (
