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M o l e c u l a r M e c h a n i s m of Oil S h a l e P y r o l y s i s Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch015
in N i t r o g e n a n d H y d r o g e n A t m o s p h e r e s F. HERSHKOWITZ, W. N. OLMSTEAD, R. P. RHODES, and K. D. ROSE Exxon Research and Engineering Company, Corporate Research-Science Laboratories, Linden, NJ 07036
This paper describes the changes in carbon functionality that occur during the pyrolysis and hydropyrolysis of Colorado oil shale. This paper is different from earlier work in that characterization of shale and products is combined with highly mass-balanced reactions to allow a mechanistic discussion of the role of functionalities in the generation of oil during shale pyrolysis. We identify some important factors in maximizing the conversion of kerogen to oil. Colorado oil shale was pyrolyzed under conditions of slow heatup (6°C/min) and short gas residence times (2-4 sec) in a nitrogen or hydrogen atmosphere at 2600 kPa. Product characterization was by elemental analysis, GC (gas), and NMR (solid & liquid). The aliphatic portion of the shale either cracks to give oil and gas or aromatizes to give aromatics in the oil or spent shale. There is an 80% increase in aromatic carbon during pyrolysis. The aromatic portion of the kerogen either cracks to give oil or ends up in the spent shale. Mineral carbonates, rather than organic functionalities, are the source of most of the CO . Hydrogen is effective at inhibiting the reactions which lead to aromatization and formation of residual carbon. Molecular hydrogen in the system also reduces carbonates to methane and water. 2
Increased knowledge of the molecular transformations which occur during oil shale pyrolysis (retorting) is essential for maximizing the yield and quality of products from this vast source of hydrocarbons. Compared to other sources such as petroleum and coal, there is l i t t l e known about the molecular structure of the insoluble organic material (kerogen) in oil shale. There is 0097-6156/83/0230-0301$06.00/0 © 1983 American Chemical Society In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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even l e s s known about the molecular transformations that occur during p y r o l y s i s of o i l shale to give gaseous, l i q u i d and s o l i d products. P y r o l y s i s in a hydrogen atmosphere gives enhanced l i q u i d y i e l d s , but l i t t l e i s known about the chemical o r i g i n of t h i s e f f e c t Q_,_2_). This paper describes a study designed to f i l l these gaps in our knowledge of the molecular mechanisms of oil shale p y r o l y s i s in nitrogen and hydrogen atmospheres. There i s l i t t l e l i t e r a t u r e on the chemical mechanisms of p y r o l y s i s of o i l shale to give gaseous, l i q u i d and s o l i d products. One reason f o r t h i s i s that workers in that f i e l d have e i t h e r studied the k i n e t i c s of p y r o l y s i s and have not f u l l y analyzed the products, or they have analyzed the products (esp e c i a l l y the o i l ) without f u l l y material balancing the s t a r t i n g material and products. A second reason i s the lack of knowledge about the molecular s t r u c t u r e of the s t a r t i n g m a t e r i a l , the shale organic kerogen. One important parameter, the aromaticity of shale kerogen, has been q u a n t i f i e d using recent advances i n s o l i d s t a t e C-NMR (nuclear magnetic resonance). This technique has placed the carbon aromaticity value at about 2 0 % f o r Colorado o i l shale and has led to some e l u c i d a t i o n of the p y r o l y s i s mechanism (3^_5). There i s a strong c o r r e l a t i o n between shale Fischer Assay o i l y i e l d and the quantity of a l i p h a t i c carbon i n the kerogen. There i s a l s o a nearly q u a n t i t a t i v e equivalence of aromatic carbon in the spent shale to aromatic carbon in the o r i g i n a l shale under Fischer assay c o n d i t i o n s . The conclusion i s that the aromatic carbons in the shale are l a r g e l y i n e r t towards thermal processing and remain in the spent shale as r e s i d u a l carbon. An admitted l i m i t a t i o n of the model ( 4 J i s the f a i l u r e to account f o r aromaticity of the o i l product which i s t y p i c a l l y 2 5 % (6_,_7_). It w i l l be seen in t h i s paper that an important reaction during o i l shale p y r o l y s i s i s the transformation of a l i p h a t i c carbon i n the s t a r t i n g shale t o aromatic carbon i n the products. The k i n e t i c s of shale p y r o l y s i s have been studied extensively. In the l a t e 1 9 4 0 ' s , Hubbard and Robinson ( 8 J studied the conversion of shale kerogen (the i n s o l u b l e organic matter) to bitumen (soluble organic matter), o i l ( v o l a t i l e organic matter) and gas. T h e i r comprehensive data set f o r conversions versus temperature and time has been the subject of kinetic analyses by subsequent t h e o r i s t s ( 9 , 1 0 ) . These k i n e t i c analyses present a shale p y r o l y s i s model as f o l l o w s : 1J
Kerogen
k* kp -------> Bitumen -------> Oil (+ gas + coke) (+ gas + coke)
Although considerable experimental refinements have been made (11), t h i s model has remained as a phenomenologically sound representation of r e t o r t i n g k i n e t i c s . As discussed above, a
In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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15.
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Oil Shale Pyrolysis in N and H 2
2
303
more s c i e n t i f i c understanding of r e t o r t i n g k i n e t i c s requires an understanding of the chemical changes that occur during p y r o l y sis. The f o l l o w i n g s t u d i e s are of note i n t h i s regard. The study of shale and product aromaticity mentioned above led to the proposal of the mechanism in Figure 1. Although no attempt was made to quantify the reactions in t h i s f i g u r e , i t presents a s i g n i f i c a n t increase in our understanding of shale pyrolysis. In other s t u d i e s , a k i n e t i c a n a l y s i s was combined with d e t a i l e d c h a r a c t e r i z a t i o n of the gaseous products of pyr o l y s i s (12-14). This has r e s u l t e d i n a good understanding of the mechanisms of gas f o r m a t i o n : the e v o l u t i o n of Hp and CH4 involves processes that were i n t e r p r e t e d as a "primary pyrolysis of the kerogen to generate o i l , and a higher temperature ( > 5 0 0 ° C ) "secondary" p y r o l y s i s of the carbonaceous r e s i d u e . Recent shale hydropyrolysis research i s l i m i t e d (in the open l i t e r a t u r e ) to a program at the I n s t i t u t e of Gas Technology (1_,2_). Product c h a r a c t e r i z a t i o n and mass balancing were combined to develop some understanding of the reactions of Colorado oil shale h y d r o p y r o l y s i s . Kerogen was converted predominantly in the 425 to 525°C temperature range at pressures up to 3450 kPa. In constant heating rate experiments, organic carbon conversions exceeding 90% were achieved by 5 5 0 - 6 0 0 ° C before the i n o r g a n i c carbon s i g n i f i c a n t l y decomposed, Above 6 0 0 ° C , f u r t h e r increases in organic carbon conversion were achieved only with l a r g e simultaneous conversions of carbonate minerals. As the temperature approached 7 0 0 ° C , carbonate mineral conversion approached 100%. The organic carbon and carbon oxide balances i n d i c a t e d that some carbon oxides (from mineral decomposition) were being converted t o methane by hydrogen i n the reactor. We have retorted Colorado o i l shale under nitrogen and hydrogen atmospheres in one reactor under the same c o n d i t i o n s . C h a r a c t e r i z a t i o n of the gaseous, l i q u i d and s o l i d products t o gether with good material and elemental balances has enabled an increased understanding of what reactions occur during r e t o r t ing. Comparison of r e s u l t s in a hydrogen vs. nitrogen atmosphere has e l u c i d a t e d the important pathways which are a f f e c t e d by hydrogen, l e a d i n g to enhanced o i l y i e l d s . Experimental Oil shale (33 gal/ton by F i s c h e r assay) from the Piceance Rasin (Green River Formation, Colony mine) i n Colorado was crushed and sieved to -16/+60 mesh. Retorting was c a r r i e d out in a f i x e d bed reactor at a pressure of 2.6 MPa (380 psig) with a slow ( 6 ° C / m i n . ) heat up to 600°C followed by 10 min. at 6 0 0 ° C Short gas residence times were used to minimize secondary o i l degradation reactions. Rases were analyzed by GC using a C a r l e 157A r e f i n e r y gas a n a l y z e r . Liquids were c o l l e c t e d in a cyclone, and the water was separated from the o i l by d i s t i l l a t i o n .
In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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GEOCHEMISTRY AND CHEMISTRY OF OIL SHALES
Figure 1. A proposed reaction modelfor oil shale pyrolysis. (Reproduced with permission from Ref 4. Copyright 1981, Colorado School of Mines.)
In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
15.
HERSHKOWITZ ET AL.
Oil Shale Pyrolysis in N and H 2
2
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Carbon and hydrogen contents were determined by combustion a n a l y s i s , nitrogen by the Mettler method, s u l f u r by the Leco method and carbonate carbon by measuring the CO2 evolved on a c i d treatment. Hydrogen and carbon NMR spectra were obtained using Variajn EM-360 and Varian XL-100 spectrometers, respectively. For Z measurements on chloroform-dj solutions, sample and instrument c o n d i t i o n s were chosen which c o n s i s t e n t l y provide quantitative carbon r a t i o s , i n c l u d i n g the use of a chromium paramagnetic r e l a x a t i o n reagent and gated proton decoupling f o r NOE suppression. Solid-state C NMR spectra were obtained on a JE0L-FX600S spectrometer using high-power d i p o l a r deoupling, HC c r o s s - p o l a r i z a t i o n and magic-angle sample s p i n n i n g . A twom i l l i s e c o n d CP contact time and two second r e p e t i t i o n delay were used throughout. Kel-F rotors f i l l e d with 400 mg of sample were rotated at about 2200 Hz. Carbon aromaticity values on s o l i d samples were determined by measuring the f r a c t i o n of t o t a l i n t e grated i n t e n s i t y which appeared between 200 and 90 ppm ( r e l a t i v e to externally referenced tetramethylsilane). This chemical s h i f t region a l s o includes carboxyl carbons. The carboxyl content of the shale used in t h i s work i s 8
accurate a n a l y s i s ;
H
H
"13 ° 6 . 5
H
H
hydropyrolysis.
H i n spent shale too low f o r
Total
Ci η 4.9
2
co
5
Ci η 0.3
Corg
2 6
CO
C1-C4 Gases
Water
6 4
Corg
Oil
2
Corg 7 C i n
1 0 3
1 8
Cin
2 5
Cin 2.8
Cin 4.1
Cin
"Ζ.
a
N
1 > 3
2.0
N
H
a
2 2 3
N
3 > 3
s
1 > 2
S
1 A
0.2
S
Shale
53 ° 2 6 . 5
131
?
"39
H
H
H
Of Colorado Oil
see t e x t f o r d i s c u s s i o n of hydrogen i n c o r
Corg
n
8 4
8
Corg
Corg
Corg
CorgjooCin^H^^gS^o
Molar Balances from Retorting
Shale:
Spent Shale
Starting
Table V.
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Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch015
.
Oil Shale Pyrolysis in N and H
HERSHKOWITZ ET AL.
}
2
CVC (5.2) 4
CO + ^(5.4)
C
ALIPH
< ) 76
C
ALIPH
I
Gas
< ) 51
Oil
0 C
AROM(
2 4
OLEF
) C
( >
AROM
