Chemistry of Shale Oil Cracking - ACS Symposium Series (ACS

4 Chemistry of Shale Oil Cracking A. K. BURNHAM

Downloaded by NORTH CAROLINA STATE UNIV on October 18, 2012 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch004

Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550

Oil shale contains organic material consisting mostly kerogen (a solid polymer) and a small amount of bitumen (a soluble, high-molecular-weight material) (1). Most currently proposed methods for recovering the energy from oil shale involve the pyrolysis of kerogen (and bitumen) to shale o i l at temperatures of about 400 to 550°C. Depending on the processing conditions, part of this oil may be degraded into less desirable products: coke and gas (2-11). Previous work here at Lawrence Livermore National Laboratory (LLNL) developed a quantitative kinetic scheme for the degradation of liquid o i l to mostly solid products (coking) at temperatures below 450°C (9,12). We now describe a kinetic scheme for the degradation of vapor-phase o i l into mostly gaseous products (cracking) at temperatures above 500°C. Shale o i l cracking can be significant in an indirect-heat retort in which the o i l shale is pyrolyzed by contact with hot solids or hot oxygen-free gas. To minimize the shale residence time in surface processes, the retorting temperature is frequently maintained above 500°C. The residence time of the shale o i l vapor in the reactor may be long enough that a significant amount of thermal cracking may occur, especially in a hot-solids retort to which no sweep gas is added. Significant shale o i l cracking can also take place in an in-situ retort in which thermal cracking may occur both inside large shale blocks and in the gas stream. If the thermal gradient within a block is large, o i l produced near the center of the block can crack (mostly to form gas) as it travels to the hotter block surface. More important, the o i l emerging from the block enters a gas stream that may be 200°C hotter than the block surface (13) . Because this gas stream may also contain oxygen, high-temperature oil-yield loss in the gas stream may take place by both combustion and associated cracking. In this work, we report kinetics for the thermal cracking of shale o i l over shale. The data are most appropriate for thermal cracking inside large blocks during in-situ processing and in the TOSCO-II and Lurgi processes, where relatively low temperatures 0097-6156/81/0163-0039$05.50/0 © 1981 American Chemical Society

In Oil Shale, Tar Sands, and Related Materials; Stauffer, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


40

OIL

SHALE,

T A R SANDS,

A N DRELATED

MATERIALS

(500 to 600°C), r e l a t i v e l y long residence times ( s e v e r a l seconds), and n e a r l y autogenous c o n d i t i o n s p r e v a i l . We p r e v i o u s l y gave a more d e t a i l e d report of our k i n e t i c measurements (JA) . We a l s o demonstrate the e f f e c t o f thermal c r a c k i n g on shale o i l composition and present r e s u l t s of C, H, and N a n a l y s i s , c a p i l l a r y - c o l u m n gas chromatography (GC), and GC/MS. We compare the compositions of shale o i l samples produced under l a b o r a t o r y c o n d i t i o n s with those of samples produced i n l a r g e - s c a l e experiments. A more d e t a i l e d i n v e s t i g a t i o n of o i l p r o p e r t i e s i n c l u d i n g IR and 13c NMR spectra (_15) and a r e s u l t i n g d i a g n o s t i c method based on o i l composition (16) have been reported e a r l i e r . Experimental Figure 1 shows the apparatus used i n the c r a c k i n g experiments. T h i s assay apparatus i s a m o d i f i c a t i o n of the LLNL modified F i s c h e r assay apparatus described p r e v i o u s l y (17). I t i s used f o r a complete mass- and carbon-balanced assay under various heating schedules. For the c r a c k i n g experiments, a second furnace and reactor were added. Both r e a c t o r s were made of Type 304 s t a i n l e s s s t e e l . A 165-ym s t a i n l e s s s t e e l f r i t (6.3 mm high by 32 mm i n diam) allowed gases but not shale to pass through the bottom of the r e a c t o r s . Raw shale samples were taken from a 92-litre/Mg (22-gal/ton) master batch (17) o f Mahogany Zone o i l shale mined from the Department o f Energy f a c i l i t y at A n v i l P o i n t s , Colorado. The raw shale had been ground to pass a 20-mesh screen (< 841 pm) and then s p i n - r i f f l e d to obtain 95-g a l i q u o t s . The shale contained 9.9% organic carbon (12.2% kerogen), 22.2% acid-evolved C 0 (48.3% c a l c i t e and dolomite), and the remainder mostly quartz and s i l i c a t e s . A l l percentages are c a l c u l a t e d on a weight b a s i s . Organic carbon i s determined by the d i f f e r e n c e between t o t a l carbon and carbon from acid-evolved CO2. Raw shale contained i n the top furnace and r e a c t o r was r e t o r t e d at a l i n e a r heating r a t e . Gases and vapors evolved during r e t o r t i n g passed through the second reactor at 504 to 610°C where the o i l was thermally cracked. Temperatures were measured at the center of the bottom r e a c t o r by a s t a i n l e s s - s t e e l - s h e a t h e d thermocouple (Type K). Temperature v a r i a t i o n across the r e a c t o r was less than 3°C. To simulate c o n d i t i o n s i n s i d e a shale block, the bottom reactor contained pieces o f shale. We used burnt shale (mostly s i l i c a t e s and MgO) i n most o f the experiments because i t i s thermally s t a b l e above 500°C. In two experiments, we used r e t o r t e d shale (2.7% organic carbon, 24.4% acid-evolved CO2) and no shale, r e s p e c t i v e l y . The r a t e o f gas e v o l u t i o n was monitored by a pressure transducer i n the c o l l e c t i o n b o t t l e . The r a t e of gas e v o l u t i o n peaked sharply during the kerogen p y r o l y s i s at about 460°C. To minimize d i f f e r e n c e s i n residence times caused by the 2



4.

BURNHAM

Chemistry

of

Shale

Oil

41

Cracking

3

A r g o n , 3 t o 10 c m / m i n

Heated t o 5 0 0 ° C

(

at 6 and 1 2 ° C / m i n

j

Isothermal, 504to610°C

) (

0

O i l generated

II u

cracked ,

,

c

r

a

c

k

e

d

Gas collection system

Ice baths

Figure 1.

Experimental

apparatus

used in the gas-phase cracking shale

of shale oil over


42

OIL

SHALE,

TAR

SANDS, A N D R E L A T E D

MATERIALS

time-dependent gas e v o l u t i o n r a t e , we purged the r e a c t o r with a slow sweep of argon (3 to 10 cm^/min). (The argon c o n s t i t u t e d 10 to 20% of the c o l l e c t e d gases.) The average residence time was v a r i e d by changing both the h e a t i n g rate and the volume of the second r e a c t o r . Products were c o l l e c t e d and weighed to determine a mass balance. Except f o r two experiments, i n which the volume of gases exceeded the capacity of the gas c o l l e c t i o n system and the l a s t p o r t i o n was vented, the mass balance ranged from 95 to 98% (_14). We measured C, H, N, and acid-evolved C0 content for a l l r e t o r t e d shales and f o r some burnt shales from the c r a c k i n g experiments. O i l s were analyzed f o r C, H, and N. Gases were analyzed by gas chromatography (thermal c o n d u c t i v i t y detector f o r H2, CO, CO2, N , and CH4; flame i o n i z a t i o n detector f o r hydrocarbons) and by mass spectrometry. The analyses permitted an organic carbon balance to be c a l c u l a t e d for the four experiments i n which the shale i n the bottom reactor was analyzed; values from 100 to 105% were obtained (14). Further measurements were made on the o i l samples. In a d d i t i o n to the samples prepared on the above apparatus, other 011 samples were obtained from the LLNL 6-ton r e t o r t , the Laramie Energy Technology Center (LETC) 150-ton r e t o r t , the 1972 TOSCO-II semi-works operation, O c c i d e n t a l O i l Shale's modified i n - s i t u experiment No. 6, and LETC s Rock Springs No. 9 true i n - s i t u experiment. Spectroscopic techniques used were c a p i l l a r y - c o l u m n GC/MS, IR, and 13c NMR spectroscopy. These studies have been reported i n d e t a i l p r e v i o u s l y (15). Only c a p i l l a r y - c o l u m n gas chromatography r e s u l t s are reported i n d e t a i l here. A Hewlett-Packard Model 5880 chromatograph with a f l a m e - i o n i z a t i o n detector (FID) was used f o l l o w i n g a p r e v i o u s l y described procedure (16) . Samples were made by d i s s o l v i n g about 0.5 ml of neat shale o i l in 2 ml of CS . A Quadrex f u s e d - s i l i c a column (0.23 mm i . d . by 50 m) coated with SP2100 (methyl s i l i c o n e o i l ) was used. The temperature was programmed from 60°C to 275°C at the rate of 4°C/min and held at 275°C f o r 30 min.


2

2

1

2

Result s K i n e t i c Measurements. The r e s u l t s of the shale o i l cracking experiments are summarized i n Table I . O i l y i e l d s are reported as a percentage of the LLNL assay r e s u l t on both a condensed-oil b a s i s and a C5+ b a s i s . To conduct the k i n e t i c a n a l y s i s , an e f f e c t i v e residence time had to be determined. I t was assumed for s i m p l i c i t y that the gas-and-oil e v o l u t i o n p r o f i l e could be approximated by a square pulse. The average residence time was c a l c u l a t e d by m u l t i p l y i n g the void volume of the bottom r e a c t o r by the time i n t e r v a l over which three-fourths of the products were evolved and then d i v i d i n g by the t o t a l volume of gases and vapors at the c r a c k i n g temperatures (14). The void volume was


4.

BURNHAM

Table I:


Experiment number

111 113 115 119 121 123 125 127

Chemistry

of

Shale

Oil

Cracking

43

Conditions for p r e p a r a t i o n of l a b o r a t o r y o i l samples.

Temperature of bottom reactor (°C)

508 610 505 558 504 585 585 610

Residence t ime (s)

7.0 3.8 10.4 4.9 9.3 2.5 2.7 2.0

Shale i n bottom reactor

Burnt Burnt Spent Burnt Burnt Burnt Empty Burnt

Oil yield (wt% of assay) Condensed C5+

95 55 90 81 91 77 87 68

96 59 91 83 92 82 89 74

determined by s u b t r a c t i n g the volume of the burnt or r e t o r t e d shale from the volume of the empty r e a c t o r . (The former value was c a l c u l a t e d by d i v i d i n g the weight of shale by i t s d e n s i t y , which was determined by mercury porosimetry). T h i s method of determining the residence time was checked f o r Experiment 115 by using a more complicated method. A time-dependent gas and o i l - v a p o r e v o l u t i o n r a t e was estimated from oil-and-gas e v o l u t i o n r a t e measurements (12,18). Residence times c a l c u l a t e d from t h i s r a t e ranged from 80 s at 350°C to about 6 s at 450°C. A weighted average of t h i s residence-time d i s t r i b u t i o n gave an average residence time of 11 s, which was i n s u r p r i s i n g l y good agreement with the value of 10.4 s determined by the simple method. However, the more complicated method i s a l s o approximate because the c a l c u l a t i o n of the residence time does not allow the extent of c r a c k i n g during the experiment (and hence instantaneous product volume) to depend on the instantaneous residence time. For t h i s reason, we have used the simpler method to estimate residence time. T h i s introduces a systematic u n c e r t a i n t y into the k i n e t i c parameters. In our previous report (_14), we determined a global r a t e constant at each temperature on the b a s i s of the y i e l d of condensed o i l . The r e s u l t i n g four r a t e constants were then f i t t e d to an Arrhenius expression. In the present r e p o r t , we use a s l i g h t l y d i f f e r e n t technique. The t y p i c a l f i r s t - o r d e r r a t e expression,

dt

yAe

-B/T

can be rearranged to give


(1)

44

OIL

In

= In k

=

SHALE,

T A R SANDS,

AND RELATED

In A - B/T ,

MATERIALS

(2)


where y i s the y i e l d of shale o i l and A and B are Arrhenius parameters. In e f f e c t , a f i r s t - o r d e r r a t e constant i s determined from each experiment, and a r a t e expression can be determined from a t y p i c a l Arrhenius p l o t . The r e s u l t i n g p l o t i s shown i n F i g u r e 2 f o r the y i e l d of C5+ o i l . T h i s gives a r a t e expression o f k

1

f

( s " ) = 4.8 x 10

8

exp (-19340/T)

(3)

T h i s r a t e constant has a s l i g h t l y higher a c t i v a t i o n energy and i s 1.2 times less at 550°C than the r a t e constant reported p r e v i o u s l y (14) f o r the y i e l d of condensed o i l . Given the systematic u n c e r t a i n t y i n the residence time, the d i f f e r e n c e s are not s i g n i f i c a n t . Two a d d i t i o n a l cautions should be mentioned concerning the use of Equation ( 3 ) . Experiment 125 (no shale i n the bottom r e a c t o r ) was used i n n e i t h e r k i n e t i c a n a l y s i s because the conversion was s u b s t a n t i a l l y lower than expected on the b a s i s o f the other experiments. T h i s discrepancy may have r e s u l t e d from e i t h e r a c a t a l y t i c e f f e c t of the shale or a h e a t - t r a n s f e r l i m i t a t i o n . In a d d i t i o n , Dickson and Yesavage (_19) found that there i s a 60 to 70% conversion l i m i t f o r shale o i l c r a c k i n g . T h i s r e s u l t s from the presence and a d d i t i o n a l formation of aromatics, which are r e s i s t a n t to c r a c k i n g . T h i s implies that our expression w i l l f a i l at high conversions. Table I I gives the product d i s t r i b u t i o n f o r thermal c r a c k i n g o f shale o i l . We defined o i l as the sum of condensed o i l and C5-C9 hydrocarbons i n the gas. The amount of each gaseous product was determined from the slope of the curve p l o t t i n g gas production versus cracking l o s s (conversion) (14). The amount of coke produced was determined by d i f f e r e n c e , but i t agreed w e l l with the measured value f o r the few experiments i n which carbon was analyzed i n the shale from the bottom r e a c t o r . The alkene/alkane r a t i o s i n the gas depended more s t r o n g l y on the c r a c k i n g temperature than on the extent of conversion. This t o p i c i s discussed i n greater d e t a i l i n another paper published in these proceedings (20). O i l P r o p e r t i e s of Laboratory Samples. The p r o p e r t i e s of the l i q u i d a l s o change during the conversion o f a hydrocarbon l i q u i d to gas and s o l i d . S p e c i f i c a l l y , the H/C r a t i o decreases and the concentration of aromatic molecules increases g r e a t l y . Elemental and spectroscopic analyses confirmed these trends f o r the shale o i l s produced i n our experiments. The H/C r a t i o and percentage n i t r o g e n are p l o t t e d i n Figure 3 as a f u n c t i o n of conversion to gas and coke. I t i s


4.

BURNHAM

Chemistry

of

Shale

Oil

45

Cracking

Temperature, ° C


600

I 1.1

550

l

I

Arrhenius

I

1.2 1000/T,

Figure 2.

500

I 1.3

k"

1

plot of shale oil cracking data from which the rate in Equation 3 was determined

O i l c r a c k i n g loss, %

expression

O i l cracking loss, %

Figure 3. The effect of oil cracking on the H/C atomic ratio and nitrogen content of the shale oil. The data points indicate cracking over burnt shale (O), retorted shale (%), and in an empty reactor ([J). The H/C ratio is probably a function of both cracking temperature and loss. Aromatic nitrogen compounds are concentrated selectively by cracking.


OIL SHALE,

46 Table

Compound

H CO CH c

Products

II.

a

184 22 151 155 85 68

4


2

4 Coke C

a

C+ 5

from s h a l e

Volume (cm3/g t

2

TAR SANDS, AND RELATED

STP)

oil

cracking.

Weight

MATERIALS

3

(g/g)

0.02 0.03 0.11 0.20 0.16 0.17 0.31

= oil.

evident that the H/C r a t i o decreases as c r a c k i n g i n c r e a s e s . Of f u r t h e r i n t e r e s t i s the increase i n n i t r o g e n content as c r a c k i n g i n c r e a s e s . T h i s increase occurs because the n i t r o g e n in shale o i l i s contained i n aromatic molecules O ) , which are r e s i s t a n t to c r a c k i n g ( i . e . , thermodynamically more s t a b l e ) . As the alkanes and alkenes are p a r t i a l l y converted to gases, the n i t r o g e n compounds become s e l e c t i v e l y concentrated. Therefore, a 50% c r a c k i n g conversion r e s u l t s i n a doubling of the n i t r o g e n content. The trends reported here for c r a c k i n g are the d i r e c t opposite of those observed by Stout et a l . (8) for o i l coking (Figure 4). O i l coking i s caused by liquid-phase polymerization and condensation r e a c t i o n s . I t i s most important at low temperatures and slow h e a t i n g r a t e s — c o n d i t i o n s under which residence times i n the l i q u i d phase are g r e a t e s t . Nitrogen content i n the o i l i s reduced and the H/C r a t i o i s increased by o i l coking because the aromatic n i t r o g e n compounds are apparently the most s u s c e p t i b l e to coking r e a c t i o n s . Lower temperatures a l s o favor alkane rather than alkene formation i n the o i l , as demonstrated elsewhere for ethane and ethene (200 • In Figures 5a and 5b, we compare the FID chromatogram of o i l produced under F i s c h e r assay c o n d i t i o n s with o i l that has undergone extensive thermal c r a c k i n g at 610°C. S p e c i f i c aromatic compounds formed i n shale o i l by thermal c r a c k i n g were i d e n t i f i e d by IR and c a p i l l a r y column GC/MS (15) ; a l k y 1 - s u b s t i t u t e d aromatics are e s p e c i a l l y p r e v a l e n t . Because of t h e i r usefulness as i n d i c a t o r s i n combustion r e t o r t s (16) , we show three 1-alkene/n-alkane r a t i o s and the naphthalene/(C;Q + C i ) r a t i o , r e s p e c t i v e l y , as a f u n c t i o n of o i l - y i e l d loss by c r a c k i n g ( F i g u r e s 6 and 7). (In t h i s case, C]_]_ i s the sum of n-undecane and 1-undecene and C^ i s the sum of n-dodecane and 1-dodecene.) Alkene/alkane r a t i o s are shown for Cg, C i , and Ci8 because these regions of the chromatograms appeared to be 2

2

2



BURNHAM

Chemistry

of

Shale

Oil

Cracking

Figure 4. The effect of oil coking on the H/C atomic ratio and nitrogen content of the shale oil. Coking reduces the alkene and aromatic nitrogen content of the oil.

American Chemical Society Library 1155 16th St. N. w. In Oil Shale, Tar Sands, 0. and C. Related Materials; Stauffer, H.; Washington, 20030 ACS Symposium Series; American Chemical Society: Washington, DC, 1981.



S1VIH3JLVJM Q31V13H QNV 'SCINVS tfVl '31VHS 1IO

In Oil Shale, Tar Sands, and Related Materials; Stauffer, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 41% o i l cracking loss

Figure 5. A comparison of the FID chromatograms of (a) shale oil produced under Fischer assay conditions and (b) shale oil that has undergone extensive thermal cracking (41% conversion to gas and coke). The proportion of aromatic hydrocarbons in the cracked oil has increased dramatically.

b)


OIL SHALE, TAR SANDS, AND RELATED

50


i

1

1

MATERIALS

r

O i l cracking loss, %

Figure 6. Effect of the extent of cracking (condensed-oil basis) on three 1-alkene/ n-alkane ratios. The ratios were determined by capillary column chromatography with an FID detector. C (%); C (O); C O8

0

10

20

12

30

18

40

Oil crackingtoss,% Figure 7. Effect of the extent of cracking (condensed-oil basis) on a naphthalene/ (Cii + C ), where C and C are the sums of the respective n-alkanes and 1alkenes. Comparing these results with those in Figure 6 shows that cracking to 30% conversion produces primarily alkenes and that further cracking produces primarily aromatic compounds. 12

u

12



4.

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Chemistry

of Shale

Oil

Cracking

51

p a r t i c u l a r l y free o f i n t e r f e r i n g compounds. For Cg, the 1-alkene/n-alkane r a t i o increases with conversion. For longer chains, the r a t i o becomes constant or even decreases at the highest conversion. For comparison, the ethene/ethane r a t i o seemed to c o r r e l a t e b e t t e r with c r a c k i n g temperature than with the extent of conversion ( c r a c k i n g l o s s ) . We a l s o made two q u a l i t a t i v e observations on o i l q u a l i t y . F i r s t , the v i s c o s i t y of the o i l appeared to decrease with c r a c k i n g . A small amount of c r a c k i n g to reduce v i s c o s i t y ( v i s - b r e a k i n g ) i s a common i n d u s t r i a l process (21). Second, the o i l s with 19% or more cracking loss d i d not s o l i d i f y on c o o l i n g to -15°C. T h i s might be expected since the pour point i s dominated by long-chain alkane components (wax), which are the most s u s c e p t i b l e to c r a c k i n g r e a c t i o n s (21). P i l o t and F i e l d Retort O i l Samples. The data we have presented to t h i s point are for o i l c r a c k i n g at r e l a t i v e l y low temperatures (500 to 610°C) and long residence times (2 to 11 seconds) under an e s s e n t i a l l y autogenous atmosphere. These c o n d i t i o n s e x i s t i n at l e a s t two aspects of o i l shale r e t o r t i n g : 1) a h o t - s o l i d s r e t o r t such as TOSCO-II or L u r g i , and 2) the i n t e r i o r of large blocks i n an i n - s i t u r e t o r t . In the TOSCO-II process, ceramic b a l l s heated to 600°C are mixed with raw shale to heat i t to about 500°C (22). L o c a l hot spots or long residence times can cause shale o i l c r a c k i n g . For large b l o c k s i n an i n - s i t u r e t o r t , the temperature of the i n t e r i o r t y p i c a l l y lags that o f the surface by 200°C. O i l generated i n the i n t e r i o r can be cracked as i t migrates to the hot block surface. However, we demonstrate below that high-temperature c r a c k i n g i n the gas stream i s more important i n combustion r e t o r t s . We f i r s t consider the o i l from the 1972 operation o f the TOSCO-II semi-works. F i g u r e 8 shows the FID chromatogram o f t h i s oil. In comparison to F i s c h e r assay o i l , s i g n i f i c a n t l y higher concentrations o f aromatics are evident. We determined 1-alkene/n-alkane and n a p h t h a l e n e / ( C ^ + C12) r a t i o s from the FID chromatogram. We obtained C g C\2, and C18 r a t i o s o f 1.36, 1.22, 1.05, and a naphthalene/(C^^ + C12) r a t i o of 0.047. These r a t i o s a l s o i n d i c a t e a y i e l d o f from 75 to 85% on a condensed-oil b a s i s and 80 to 85% on a C5+ b a s i s . In c o n t r a s t , TOSCO r e p o r t s a 93% y i e l d f o r i t s 1972 run (22). A probable source o f t h i s discrepancy i s the d i f f e r e n c e i n p y r o l y s i s temperature. There i s nothing i n the mechanism described i n our i n t r o d u c t i o n that r e q u i r e s absence of o i l coking in a F i s c h e r assay (12°C/min). In f a c t , i t has been demonstrated that y i e l d s greater than 100% of F i s c h e r assay ( i . e . , l e s s coking than i n F i s c h e r assay) might be obtained under very f a s t h e a t i n g r a t e s and higher p y r o l y s i s temperatures (10, 23). However, our experiments were conducted so that the maximum p o s s i b l e y i e l d (no cracking) would be 100% o f F i s c h e r assay. T h i s i s not n e c e s s a r i l y true i n the TOSCO-II process. Therefore, f u r t h e r 5


In Oil Shale, Tar Sands, and Related Materials; Stauffer, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. "16

^M/W

Figure 8. FID chromatogram of TOSCO-II oil from the 1972 semiworks operation. Thel-alkene/ n-alkane ratios and aromatic hydrocarbon content are high compared with those of the Fischer assay oil fsee Figure 5a).

Retention time-

-15


"19

1

> r

2

W

> H

a

m r > H w

> a

> o

C/J

> w H >

a

g P

to

4.

BURNHAM

Chemistry

of Shale

Oil

Cracking

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experiments are required to develop a q u a n t i t a t i v e method to determine o i l y i e l d from o i l composition f o r h o t - s o l i d s r e t o r t s . We next compare the compositions o f our cracked shale o i l s with those from combustion r e t o r t s . We show i n F i g u r e 9 the FID chromatogram of shale o i l from Rock Springs No. 9, one of LETC s true i n - s i t u experiments. Several d i f f e r e n c e s are evident between the chromatogram of t h i s sample and those shown i n Figure 5. O i l s produced by combustion r e t o r t i n g u s u a l l y have much lower C^-Cg content than those produced i n r e t o r t i n g experiments with no sweep gas. A corresponding increase i s observed i n the C5-C9 content of the offgas from combustion r e t o r t s compared to the gas c o l l e c t e d from the laboratory experiments. I t should be noted that some of the l i g h t ends of the laboratory-produced samples evaporated during h a n d l i n g . The low 1-alkene/n-alkane r a t i o s i n d i c a t e that more than 20% of the o i l generated was converted to coke because o f the low r e t o r t i n g temperature. The high concentration of naphthalenes i n d i c a t e s that high-temperature thermal cracking occurred to part of the generated o i l . However, t h i s thermal c r a c k i n g occurred i n such a way that e s s e n t i a l l y no 1-alkenes were formed. As discussed below, t h i s i s c h a r a c t e r i s t i c of o i l burning i n a combustion r e t o r t . The c a p i l l a r y GC/MS was quite h e l p f u l i n e s t a b l i s h i n g the d i f f e r e n c e between o i l c r a c k i n g i n our laboratory experiments and that associated with o i l burning i n a combustion r e t o r t (15). Naphthalene/2-methylnaphthalene r a t i o s were determined from the r e l a t i v e 128- and 142-m/e peak heights i n s p e c i f i c - i o n - c u r r e n t chromatograms from the GC/MS when the concentrations were too low to be measured a c c u r a t e l y from the FID chromatogram. To convert the ion r a t i o s to weight r a t i o s , we compared the ion r a t i o s to area r a t i o s from the FID chromatogram o f Samples 113, Oxy No. 6, and Rock Springs No. 9. In Table I I I we l i s t the naphthalene/2-methylnaphthalene weight r a t i o s determined f o r these and other samples. We a l s o l i s t some previous r e s u l t s obtained by Dinneen (24). One trend that stands out from Dinneen's data and from Experiments 113 and 127 i s that the naphthalene/2-methy1naphthalene r a t i o depends strongly on the temperature at which o i l c r a c k i n g occurs and only weakly on the amount of c r a c k i n g . T h i s apparently occurs because the a c t i v a t i o n energy for d e a l k y l a t i o n of aromatics i s higher than for aromatic formation. Even at very high conversions, t h i s r a t i o i n o i l s cracked near or below 600°C i s not d r a m a t i c a l l y d i f f e r e n t than that i n assay o i l — e v e n though the amount o f naphthalene has increased t e n f o l d . The naphthalene/2-methylnaphthalene r a t i o i n o i l s from combustion r e t o r t s i n which a s i g n i f i c a n t amount o f o i l burning has occurred i s s u b s t a n t i a l l y higher than the r a t i o i n assay oil. T h i s i n d i c a t e s that most cracking i n i n - s i t u r e t o r t s occurs at high temperatures associated with combustion. Preferential o x i d a t i o n of a l k y l aromatics may a l s o c o n t r i b u t e . A t these high


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F/gwre 9. Chromatogram of shale oil from Rock Springs No. 9, a true in situ experiment. The alkene/alkane ratios are very low (coking) and the naphthalene content are very high (combustion and associated cracking). The naphthalene/methylnaphthalene ratios are high compared with the cracked shale oil in Figure 5b.

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Chemistry of Shale Oil Cracking - ACS Symposium Series (ACS

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