Oil Shale Retorting Kinetics - ACS Symposium Series - ACS Publications

7 Oil Shale Retorting Kinetics

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 31, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch007

P. H. WALLMAN, P. W. TAMM, and B. G. SPARS Chevron Research Company, 576 Standard Avenue, Richmond, CA 94802

Several aboveground oil shale retorting processes are characterized by rapid heating of the shale followed by pyrolysis of the kerogen at essentially isothermal conditions. The objective of this study is to investigate the retorting kinetics applicable to processes characterized both by rapid heating of relatively small particles and by rapid sweeping of the produced hydrocarbon vapors out of the retort. Rather surprisingly, accurate kinetics for these conditions are not available in the literature. Several previous investigators have taken an isothermal approach but have failed to eliminate significant heatup effects in the measured kinetics. The important investigation by Hubbard and Robinson (1) is in this category. Attempts were made to correct the Hubbard and Robinson data for the heatup effects by Braun and Rothman (2) and Johnson et al. (3). Allred (4) took new isothermal data with increased accuracy, but his results also suffered from interfering heat-transfer dynamics. Weitkamp and Gutberlet (5) used both isothermal and nonisothermal techniques but covered only low temperatures and presented no kinetic model. A frequent characteristic of past investigations is excessive complexity of the proposed kerogen pyrolysis models. The works of Fausett et al. (6) and Johnson et al. (3) belong in this category. A goal of the present investigation is to keep the model as simple as possible. One previous investigation that deserves special attention is that by the Lawrence Livermore Laboratory (LLL) described in Campbell et al. (7) and Campbell et al. (8). The LLL group determined retorting kinetics by both isothermal and

0097-6156/81/0163-0093$05.25/0 © 1981 American Chemical Society

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

OIL SHALE, TAR SANDS, AND RELATED MATERIALS


94

nonisothermal experiments with reasonable agreement between the two approaches. However, the LLL work was d i r e c t e d toward i n - s i t u r e t o r t i n g where heating rates are i n h e r e n t l y low. Low heating rates were found to decrease the o i l y i e l d below F i s c h e r Assay l e v e l s by i n c r e a s i n g coke formation. For small p a r t i c l e s , the detrimental e f f e c t of slow heating could be e l i m i n a t e d by sweeping the sample with an i n e r t gas implying that the coking was a s s o c i a t e d with holdup i n a l i q u i d s t a t e . Such coking i s not of importance i n the present i n v e s t i g a t i o n where the sample i s w e l l swept, and heatup r a t e s ar.e three orders of magnitude higher than t y p i c a l i n - s i t u r a t e s . The LLL k i n e t i c model p r e d i c t s that the maximum achievable o i l y i e l d i s that of F i s c h e r Assay and that the coke associated with F i s c h e r Assay i s s t o i c h i o m e t r i c a l l y r e l a t e d to the kerogen. T h i s assumption may be appropriate f o r i n - s i t u r e t o r t i n g , but i t i s not a p p l i c a b l e to the present c o n d i t i o n s where o i l y i e l d s higher than F i s c h e r Assay are measured. O i l y i e l d s as high as 110% of F i s c h e r Assay have been reported f o r high heating rates (Hinds, j [ ) . Therefore, another o b j e c t i v e of t h i s work i s to extend the k i n e t i c s of o i l production beyond the F i s c h e r Assay l i m i t . Experimental

Technique

A bench-scale f l u i d i z e d bed r e a c t o r shown i n Figure 1 was used to r e t o r t small samples o f o i l shale p a r t i c l e s . The g l a s s r e a c t o r held a bed of i n e r t s o l i d s such as glass beads or sand that was continuously f l u i d i z e d by a c o n t r o l l e d flow of helium or any other gas. A weighed sample of shale i n an amount no greater than 2% of the bed was dropped i n t o the preheated r e a c t o r , producing a n e g l i g i b l e drop i n bed temperature. Heat t r a n s f e r i n the f l u i d i z e d bed was very r a p i d , and the v o l a t i l e products were r a p i d l y swept out by the f l u i d i z i n g gas. The vapor residence time i n the r e a c t o r was t y p i c a l l y 3 seconds. A small sample stream was d i v e r t e d to a flame i o n i z a t i o n detector (FID i n Figure 1). The FID produced a s i g n a l p r o p o r t i o n a l to the concentration of t o t a l hydrocarbon. Heteroatom content of evolved products was assumed constant with time. Since the hydrocarbon concentration dropped to very low l e v e l s at the end of the r e t o r t i n g r e a c t i o n , the s e n s i t i v i t y of the detector had to be increased by at l e a s t a f a c t o r of ten as the r e t o r t i n g progressed. This increased s e n s i t i v i t y made i t p o s s i b l e to


WALLMAN ET A L .

Oil

Shale

Retorting

Kinetics


7.


95


96


record the f u l l product-evolution curve i n c l u d i n g the long " t a i l " which contains information on the k i n e t i c s at high conv e r s i o n l e v e l s . Attempts were made to use the FID f o r quantit a t i v e determination of v o l a t i l e hydrocarbon y i e l d s , but the r e s u l t s were of i n s u f f i c i e n t accuracy. The area under the curve d i d , however, give an approximate y i e l d which was used as an experimental check. O i l and gas y i e l d s were obtained from another branch of the apparatus shown i n Figure 1. The o i l was condensed i n a c o l d trap, and the gases were c o l l e c t e d i n a gas c y l i n d e r by l i q u i d displacement. The amount of o i l was determined g r a v i m e t r i c a l l y , and the amount of hydrocarbon gas was determined from the t o t a l volume of gas c o l l e c t e d and the gas composit i o n . The o i l was recovered by CS2 e x t r a c t i o n and subjected to GC a n a l y s i s , standardized against n - p a r a f f i n s . F i n a l l y , to c l o s e the hydrocarbon balance, the e n t i r e bed c o n s i s t i n g of i n e r t p a r t i c l e s and r e t o r t e d shale was recovered and i t s hydrocarbon content determined by burning o f f the organic matter and measuring the amount of oxygen consumed. The o i l c o l l e c t i o n trap shown i n Figure 1 proved to be a c r i t i c a l part of the apparatus. The product o i l tended to form a s t a b l e aerosol making i t d i f f i c u l t to c o l l e c t . T h i s problem was overcome by a trap design where the condensation occurred under a steep thermal g r a d i e n t . The i n s i d e w a l l of the cold trap was kept at 300°F while the opposite w a l l was i n contact with a bath at 5°F. A thermally induced outward r a d i a l flow promoted f i l m condensation on the c o l d w a l l . Interestingly, t h i s design eliminated aerosol formation when using helium as the f l u i d i z i n g gas; but with heavier gases such as argon, n i t r o g e n , and even methane, a e r o s o l formation s t i l l occurred. The cause of t h i s e f f e c t was not i n v e s t i g a t e d , but i t could be r e l a t e d to d i f f e r e n c e s i n c o n d u c t i v i t y between the gases. The s e l e c t e d bath temperature of 5°F proved p r a c t i c a l because no butanes condensed, and only a small p o r t i o n of l i g h t o i l (C5-C7) was l o s t to the gas. This l i g h t o i l was accounted f o r by use of the gas a n a l y s i s . Another area of experimental d i f f i c u l t y was gas analys i s . At low temperatures r e q u i r i n g long r e a c t i o n times, l a r g e amounts of helium were necessary; and the hydrocarbon products were i n very low c o n c e n t r a t i o n . This d i f f i c u l t y was overcome by r e c y c l i n g the gas back i n t o the f l u i d i z e d bed and thereby allowing the hydrocarbon c o n c e n t r a t i o n to b u i l d up. Some o i l vapor was undoubtedly r e c y c l e d i n c r e a s i n g the p o s s i b i l i t y of thermal c r a c k i n g .


7.

WALLMAN

ET AL.

Oil

Shale

Retorting

Kinetics


The shale samples used i n t h i s work were obtained by screening from a s i n g l e bulk sample of Colorado o i l shale ( A n v i l Points Mine, courtesy of Development Engineering Incorporated and the U.S. Department of Energy). The F i s c h e r Assay o i l y i e l d was 10.5 wt % based on f r e s h shale (27.5 gallons/ton) f o r the l a r g e r p a r t i c l e s and somewhat lower f o r the f i n e r s i z e cuts, f o r example, 10.15 wt % f o r 100 um particles. Yield

Results

Experiments were conducted to determine the e f f e c t of o i l shale p a r t i c l e s i z e on product y i e l d s at 930°F. The y i e l d s obtained f o r p a r t i c l e s of s i x d i f f e r e n t s i z e s are compared with F i s c h e r Assay y i e l d s i n Figure 2. I t i s apparent that o i l y i e l d s higher than F i s c h e r Assay are obtained f o r small p a r t i c l e s ; whereas large p a r t i c l e s produce F i s c h e r Assay y i e l d . The incremental o i l produced from small p a r t i c l e s i s balanced by a decreased coke make while the gas make remains constant. The o i l y i e l d appears to have a l i m i t at about 110 wt % F i s c h e r Assay, but t h i s may be e n t i r e l y due to the l i m i t e d range of p a r t i c l e s i z e s i n v e s t i g a t e d . I t i s p o s s i b l e that the o i l y i e l d would increase f u r t h e r f o r , say, 10 \m or 1 um p a r t i c l e s . However, p a r t i c l e s of t h i s s i z e could not be studied i n the apparatus of t h i s work. Not only do smaller p a r t i c l e s produce more o i l , but there i s a l s o a change i n the o i l composition. The c o n c e n t r a t i o n of 29+ P s i n the product o i l i s shown i n Figure 3. Increased o i l y i e l d s are accompanied by increased heavy ends. Hence, the c o n c l u s i o n i s that the incremental o i l obtained from small p a r t i c l e s i s of higher molecular weight. The e f f e c t of r e t o r t i n g temperature on the y i e l d s obtained from 0.4 mm p a r t i c l e s and the accompanying change i n o i l compos i t i o n are shown i n Figures 4 and 5. The important f i n d i n g s here are that coke y i e l d i s unaffected by r e t o r t i n g temperature and that o i l y i e l d i s increased due to decreased gas make at the lower temperatures. This second f i n d i n g suggests decreased c r a c k i n g since Figure 5 shows that a l i g h t e r o i l product i s obtained at the higher temperatures. I t w i l l a l s o be noted that the data set shown i n Figure 4 has some " e x t r a " cracking i n comparison with the data of Figure 2. This i s due to the f a c t that the r e s u l t s of Figure 4 were obtained i n the r e c y c l e gas mode where r e c y c l i n g of a small p o r t i o n of the o i l C

c o m

o n e n t


97

OIL SHALE, TAR SANDS, AND RELATED

98

mm

•

9

-*

Total Hydrocarbon


ir" C

Particle-size

iio

Oil

10 mm

Particle Figure 2.

+ 5

1 mm

100 /im

Size

effect on hydrocarbon yields at 930°F: (• ); Fischer assay retort ( ).

fluidized-bed

r

106 102-

4

10 Wt % C

Figure 3.

Correlation

MATERIALS

+ 2

9

12

_L 14

J

16

in Oil

between oil yield and oil

composition


retort

WALLMAN

Oil Shale

E TAL.

Retorting

Kinetics

—

Total Hydrxarbon

C 0il Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 31, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch007

5

800

850

900

Retorting Figure 4.

Temperature

950

+

1000

T e m p . , °F

effect on hydrocarbon assay (

yields for OA-mm ).

particles:

25

20

N \

o +

*

o

+

C

15

O ^

10

C C 5

0

J L

800

900 Retorting

Figure 5.

Oil composition

8

Temp.,

J

1000 °F

as a function of retorting

temperature


Fischer


100

occurred. In general, the gas make was found to be very s e n s i t i v e to equipment c o n d i t i o n s such as the temperature of the product l i n e leading to the condenser.


Kinetic

Results

The k i n e t i c complement to the y i e l d r e s u l t s discussed above was obtained from the FID response curve. I n t e g r a t i o n of t h i s curve gave the f r a c t i o n a l conversion. F i g u r e 6 shows the r e s u l t s of the p a r t i c l e - s i z e e f f e c t experiments p l o t t e d as the logarithm of the f r a c t i o n unconverted hydrocarbon versus time. I t appears that the r e s u l t s can be described by a p a i r of f i r s t - o r d e r processes since the curves can be approximated by two s t r a i g h t - l i n e segments. By comparing the slopes of the two segments, the rates of the two processes are found to d i f f e r by a f a c t o r of ten. This i s an important f i n d i n g with consequences f o r the p y r o l y s i s model to be proposed i n a subsequent s e c t i o n . The small d i f f e r e n c e s between the i n i t i a l segments of the k i n e t i c curves i s due to d i f f e r e n c e s i n heatup time f o r the d i f f e r e n t p a r t i c l e s i z e s . However, heatup time i s r e l a t i v e l y unimportant even f o r the 3 mm p a r t i c l e s because the s t r a i g h t l i n e segment extrapolates to only 15 seconds on the time axis. T h i s "experimental" heatup time i s about what one would c a l c u l a t e using a heat t r a n s f e r c o e f f i c i e n t of 100 Btu/hr f t ^ °F. An important feature of the r e s u l t s shown i n Figure 6 i s that the slope of the l a t t e r segment of the curve changes f o r p a r t i c l e s of d i f f e r e n t s i z e . The process corresponding to t h i s segment appears to be slower f o r the small p a r t i c l e s than f o r the large ones. T h i s unexpected c h a r a c t e r i s t i c i s at f i r s t surprising. I t i s , however, a consequence of the d i f f e r e n t y i e l d s f o r d i f f e r e n t p a r t i c l e s i z e s shown i n F i g u r e 2. The y i e l d d i f f e r e n c e s do not enter the k i n e t i c r e s u l t s of Figure 6 because the ordinate i s normalized by the t o t a l hydrocarbon evolved ( t h i s type of p l o t i s required f o r determination of the rate constants). The k i n e t i c and y i e l d data are combined i n Figure 7 f o r the 0.4 mm and 3 mm p a r t i c l e s . This f i g u r e shows that the hydrocarbon e v o l u t i o n i s e s s e n t i a l l y independent of p a r t i c l e s i z e up to 100% F i s h e r Asay o i l y i e l d . At t h i s l e v e l the o i l production stops f o r large p a r t i c l e s , whereas i t continues f o r small p a r t i c l e s at a reduced r a t e .



WALLMAN

E T AL.

Figure 6.

Oil Shale

Particle-size

Retorting

101

Kinetics

effect on retorting kinetics at

930°F



102


0

0

l

2

3

4

5

6

7

Retorting Figure 7.

Oil production

8 Time,

9

10

11

12

13

Min.

kinetics at

930°F


14

15

7.

WALLMAN

Oil

ET AL.

Shale

Retorting

103

Kinetics

The e f f e c t of temperature on the r e t o r t i n g k i n e t i c s i s shown i n Figure 8 f o r the 1 ram p a r t i c l e s . Both processes respond to temperature but the fast one more so than the slow one.


Table I Rate Expressions f o r Hydrocarbon Production from Kerogen (C I n i t i a l Kerogen Content) Q

L i g h t Hydrocarbon Production: f

k

Rate = i " i ' C

*e"

k

t 1

1

Amount - f " i ' C * ( l - e"" ^) 0

Primary Heavy O i l Production: Rate = f

# 2

,

k

2

C

# 0

k

e~( 2

+ k

c

) t

k

Amount = f ^ - r — C *[1 - e~( 2 ^ k. + k o 2 c I n t r a p a r t i c l e Coke Production:

+

k

9

t

c) ]

1

f

Rate = f 2 * c

# k

Amount = f * f . ^ c k 9

C

c* o

# e

""

( k 2

*

k

c

)

J

t

(k

^ - r — C *[1 - e"" 2 + k o I c L

+

k

c

) t :

] J

0

Kerogen P y r o l y s i s Model The combined k i n e t i c and y i e l d data can be c o r r e l a t e d with the p y r o l y s i s model shown i n Figure 9. Here kerogen decomposes i n t o a " l i g h t " hydrocarbon product and a heavy intermediate product, "bitumen." The l i g h t product i s l a r g e l y a vapor a t r e t o r t i n g conditions and i s , t h e r e f o r e , produced r a p i d l y without s i g n i f i c a n t secondary r e a c t i o n s . The bitumen, on the other hand, i s of high b o i l i n g range and remains i n the p a r t i c l e f o r s i g n i f i c a n t periods of time. I t becomes subject to two competing processes: (1) heavy o i l production and (2) i n t r a p a r t i c l e (liquid-phase) coking. The r e l e a s e d heavy o i l i s f u r t h e r




Reaction Figure 8.

Temperature

Time, M i n .

effect on retorting kinetics for 1-mm

particles


WALLMAN ET A L .

Oil

Shale

Retorting

Kinetics


7.


105


106


subjected to thermal cracking i n the vapor phase surrounding the p a r t i c l e s . F i r s t - o r d e r rate expressions are proposed i n Table I f o r the three p r i n c i p a l steps: l i g h t hydrocarbon production (equals kerogen decomposition), primary heavy o i l production, and coking. The model accounts f o r the dramatic change i n o i l product i o n rate which i s observed. The f a s t i n i t i a l r a t e i s governed by the rate constant kj_ ( f i r s t order i n kerogen c o n t e n t ) . The l a t t e r slow rate i s governed by the sum of the two bitumen r e a c t i o n s , which are assumed f i r s t order i n the i n t r a p a r t i c l e bitumen content and have rate constants k and k . At the temperatures of i n t e r e s t , k i s much greater than k + k so that the f i r s t step of the r e a c t i o n goes v i r t u a l l y to complet i o n before there i s any a p p r e c i a b l e conversion of the bitumen. 2

Q

1

2

c

Table I I L i g h t Hydrocarbon F r a c t i o n , f ^

Particle S i z e , mm

Total Light VolatileHydrocarbon Hydrocarbon Y i e l d , % of Yield, Total Volatile % of Kerogen Hydrocarbon

3 2 1 0.4

68.8 70.0 71.1 74.4

L i g h t Hydrocarbon Y i e l d , % of Kerogen 61.9 60.9 61.9 61.8 =61.6

90 87 87 83 Avg

The r e l a t i v e y i e l d s of the kerogen decomposition products are expressed as the f r a c t i o n s f ^ , f , and f i n F i g u r e 9. The (H 0, CO, C0 ) f r a c t i o n was set equal to that of F i s c h e r Assay because the amount of water i n the l i q u i d product could not be e a s i l y determined i n the y i e l d experiments. The l i g h t hydrocarbon f r a c t i o n , f ^ , was determined by a combination of the y i e l d and the k i n e t i c data. The l i g h t hydrocarbon y i e l d s as 2

2

w

2



7.

WALLMAN

ET AL.

Oil

Shale

Retorting

107

Kinetics

f r a c t i o n s of t o t a l v o l a t i l e hydrocarbon were obtained by e x t r a p o l a t i n g the slow r e a c t i o n segments of Figure 6 to zero time and reading the f r a c t i o n s o f f the o r d i n a t e . The r e s u l t s are shown i n Table I I . The values of f ^ obtained f o r the four part i c l e s i z e s are s u f f i c i e n t l y constant to j u s t i f y an average value of 61.6%. The l i g h t hydrocarbon y i e l d f r a c t i o n f-^ i s a l s o seen to be approximately independent of temperature from F i g u r e 8 where a l l the slow reactor segments extrapolate back to approximately the same point on the o r d i n a t e , namely 87% l i g h t hydrocarbon. The bitumen f r a c t i o n f i n F i g u r e 9 i s obtained by d i f f e r e n c e and equals 24%. Hence, f i s constant with both p a r t i c l e s i z e and temperature at l e a s t i n the range of 900-1000°F. F i n a l l y , a r a t i o between coke and gas i n the coking r e a c t i o n of 80:20 was set on the assumption that the gas y i e l d at 800°F i n Figure 4 i s the r e s u l t of coking alone. Campbell et a l . (8) used e s s e n t i a l l y the same coke-gas r a t i o f o r a s i m i l a r r e a c t i o n i n t h e i r r e a c t i o n sequence. In a d d i t i o n to the coking r e a c t i o n , vapor-phase c r a c k i n g of the heavy o i l released from the p a r t i c l e i s a source of gas. Cracking of the l i g h t hydrocarbon f r a c t i o n i s a l s o poss i b l e ; but because i t occurs to a l e s s e r extent, i t has been assumed to be zero. The k i n e t i c s of the cracking r e a c t i o n l i e o u t s i d e the scope of the present i n v e s t i g a t i o n , but t h i s important r e a c t i o n has been studied by Burnham and T a y l o r (10). A t h i r d source of gas i s the i n i t i a l decomposition of kerogen itself. However, the c o n t r i b u t i o n of each gas-generation step cannot be determined i n the present i n v e s t i g a t i o n because of the i n a b i l i t y of the FID detector to d i s t i n g u i s h between gas and o i l . The kerogen decomposition rate constant, k^, and the bitumen disappearance r a t e constant ( k + k ) are obtained d i r e c t l y as the slopes of the two s t r a i g h t - l i n e segments of Figures 6 and 8 (and s i m i l a r graphs f o r the other p a r t i c l e s i z e s ) . Figure 10 shows the temperature dependence of these r a t e constants. I t i s also seen that k^ i s independent of p a r t i c l e s i z e . This implies that there i s no s i g n i f i c a n t r e s i s t a n c e to the transport of l i g h t hydrocarbons from the i n t e r i o r of the p a r t i c l e into the bulk of the c a r r i e r gas. This c o n d i t i o n i s a consequence of the high vapor pressure of the l i g h t hydrocarbon f r a c t i o n . Because of the r a p i d transport out of the p a r t i c l e , t h i s f r a c t i o n has no p o s s i b i l i t y to coke. The bitumen, on the other hand, i s viewed as a high b o i l i n g l i q u i d which can undergo i n t r a p a r t i c l e coking. The 2

2

2

c




108

Figure 10.

Pyrolysis

rate constants


MATERIALS

WALLMAN

7.

Oil

ET AL.

Shale

Retorting

109

Kinetics

p a r t i c l e size dependence of ( k + k ) i n F i g u r e 10 suggests that a d i f f u s i o n a l r e s i s t a n c e may come into play i n the heavy o i l production step. A l s o , the a c t i v a t i o n energy f o r the reactions governing bitumen disappearance i s only 22.6 kcal/mole as compared to 43.6 kcal/mole f o r the kerogen decomposition reaction. In order to determine k and k i n d i v i d u a l l y , the k i n e t i c a l l y determined values f o r the sum ( k + k ) must be used i n consort with the expression f o r the coke y i e l d 0.8 f *k /(k +k ). The c a l c u l a t e d values of the r a t i o k / ( k + k ) together with the k i n e t i c a l l y obtained values of ( k + k ) are given i n the Appendix. The r e s u l t i n g k and k values show some i n t e r e s t i n g c h a r a c t e r i s t i c s : k i s independent of p a r t i c l e s i z e whereas k i s p r o p o r t i o n a l to p a r t i c l e s i z e . Both have the same temperature dependence because the coke y i e l d i s constant with temperature. The temperature dependence and the p a r t i c l e s i z e dependence of k are shown e x p l i c i t l y i n Table I I I . k i s seen to dominate over k even f o r the 0.4 mm p a r t i c l e s . For the 3 mm p a r t i c l e s , k i s i n s i g n i f i c a n t i n r e l a t i o n to k implying complete coking of the bitumen. 2

Q

2

Q


2

Q

#

2

c

c

2

2

2

c

c

c

2

c

2

c

Q

c

2

2

Q

Table I I I Rate Constants Kerogen k

x

1

(Min." )

Decomposition:

= 5.78-10

12

exp (- *3.6

kcal/mole

}

R *T Heavy O i l Production: i i o.m5 ,22.6 k^2 = 1.8 10 exp ( J

kcal/mole

2

N

)

R*T

Coking: , . 22.6 kcal/mole k = A *exp (R*T A

c

where

e

c

A

N

)

P a r t i c l e Size,

Q

isTo

5

9*10 5-10 3*10

5

5

5

mm

3 2 1 0.4



110


Discussion As part of t h i s i n v e s t i g a t i o n , kerogen p y r o l y s i s models d i f f e r e n t from the one proposed here were considered. One such model of t h e o r e t i c a l appeal i s s i m i l a r i n s t r u c t u r e to the one given i n Figure 9 but with a pure d i f f u s i o n process f o r the heavy o i l production. However, t h i s a l t e r n a t i v e model i s incompatible with some experimental f i n d i n g s : It predicts lower coke concentrations on the surface of the p a r t i c l e than i n the i n t e r i o r , whereas microprobe r e s u l t s i n d i c a t e a uniform coke d i s t r i b u t i o n . Further, t h i s d i f f u s i o n model p r e d i c t s zero coke y i e l d f o r i n f i n i t e l y small p a r t i c l e s , whereas the l i m i t e d amount of data a v a i l a b l e f o r small p a r t i c l e s i z e s suggest a l e v e l i n g - o f f of the coke y i e l d below a p a r t i c l e s i z e of 0.4 mm. The approach to F i s c h e r Assay y i e l d s t r u c t u r e with i n c r e a s i n g p a r t i c l e s i z e i s accounted f o r i n the proposed model by complete coking of the bitumen f r a c t i o n . The model p r e d i c t s a coke y i e l d of 19% of the kerogen when the bitumen i s comp l e t e l y coked, w e l l w i t h i n F i s c h e r Assay range. Therefore, f l u i d - b e d and F i s c h e r Assay r e t o r t i n g give d i f f e r e n t y i e l d s f o r small p a r t i c l e s only. The i n t e r p r e t a t i o n of t h i s i s that i n a F i s c h e r Assay r e t o r t small p a r t i c l e s produce the same amount of coke as large p a r t i c l e s because there i s no sweep gas to f a c i l i t a t e o i l removal from the small p a r t i c l e s . The f l u i d bed r e t o r t i n g experiments have shown that a d d i t i o n a l o i l can indeed be produced from small p a r t i c l e s . The proposed model can be compared with both the model of A l l r e d (4) and that of Campbell et a l . A l l r e d ' s model does not have the feature of competing p a r a l l e l r e a c t i o n s that i s e s s e n t i a l to the p y r o l y s i s model proposed here. I t does, however, have the intermediate product bitumen which reaches a maximum l e v e l almost i d e n t i c a l to the one i n t h i s work. A l l r e d p o s t u l a t e s that a l l kerogen decomposes i n t o bitumen, whereas bitumen i n the present work i s the remainder of the kerogen a f t e r the l i g h t hydrocarbon f r a c t i o n has been s t r i p p e d o f f . There are some i n t e r e s t i n g s i m i l a r i t i e s and c o n t r a s t s between the present model and the Lawrence Livermore Laboratory (LLL) model of Campbell et a l . ( 8 ) The a c t i v a t i o n energy of the i n i t i a l decomposition i s s i m i l a r i n both models, 48-54 kcal/mole i n the case of LLL and 44 kcal/mole here. Bitumen i s treated merely as an intermediate i n the kerogen decomposition by LLL; whereas, here i t i s one of s e v e r a l decomp o s i t i o n products. Coking steps are included i n both models,



7.

WALLMAN

ET AL.

Oil Shale

Retorting

Kinetics

111

but the m a t e r i a l involved i s d i f f e r e n t . The coking k i n e t i c s accounted f o r by LLL only apply to the l i g h t hydrocarbon of the present model, and t h i s coking r e a c t i o n does not occur here because of the high heating rate and the sweep gas. The coking considered i n the present model involves the intermediate bitumen product and the coking r a t e depends on p a r t i c l e s i z e . Small p a r t i c l e s produce l e s s coking and; consequently, o i l y i e l d s higher than F i s c h e r Assay. Both A l l r e d (4) and Weithamp and Gutberlet (5) observed the slow o i l production regime. C a l c u l a t i n g a r a t e constant f o r t h i s slow production regime at 856°F from the r e s u l t s of the l a t t e r i n v e s t i g a t o r s gives a value of 0.12 rain.~l i d e n t i c a l to ( k + k ) of t h i s work. A p r a c t i c a l i m p l i c a t i o n of the r e s u l t s of t h i s work i s that F i s c h e r Assay y i e l d i s probably a reasonable upper l i m i t f o r any r e t o r t i n g process. T h i s work has shown that a very small p a r t i c l e s i z e increases o i l y i e l d and decreases coke y i e l d , but long r e a c t i o n times are necessary. A l s o , low coke y i e l d s may not be d e s i r a b l e from o v e r a l l heat balance c o n s i d e r a t i o n s i f the coke i s to be used as an energy source f o r the process. Lowering the temperature a l s o increases o i l y i e l d but at the expense of the gas y i e l d and with the requirement of long r e a c t i o n times. T h i s work has added to the understanding of the very complex phenomena o c c u r r i n g during o i l shale r e t o r t i n g . The simple kerogen p y r o l y s i s model w i l l be u s e f u l i n modeling product y i e l d s from r e t o r t i n g processes handling small s i z e part i c l e s at high r e t o r t i n g r a t e s . 2

c

Abstract An isothermal fluid-bed reactor was used to retort small samples of 0.1-3mmoil shale particles. The rate of volatile hydrocarbon evolution was measured by a flame ionization detector. Yields of oil, gas, and residual coke were determined and the boiling range of the product oil and the composition of the gas were measured. Higher oil yields and lower coke yields are obtained for smaller particles. Retorting temperature in the range 8001000°F does not affect coke yield but has an effect on the relative yield of oil and gas. The experimental data are correlated by a simple empirical pyrolysis model where kerogen decomposes by a first-order reaction into a light hydrocarbon product and a heavy intermediate product, "bitumen." Subsequently, the bitumen is subject to two competing processes:


112


MATERIALS


(1) heavy oil production and (2) intraparticle (liquid-phase) coking. Both of these processes are modeled as first-order reactions. The produced oil is also subject to thermal cracking in the vapor phase surrounding the particles. The rate constants for the disappearance of bitumen by reactions (1) and (2) are an order of magnitude smaller than the kerogen decomposition rate constant which ranges from 1 to 10 min. over a temperature range of 900-1000°F. Literature Cited 1. Hubbard, A. B.; Robinson, W. E. "A Thermal Decomposition Study of Colorado Oil Shale," Report of Investigation 4744, Bureau of Mines, Washington, D.C., 1950. 2. Braun, R. L.; Rothman, A. J. Fuel, 1975, 54, 129. 3. Johnson, W. F.; Walton, D. K.; Keller, H. H.; Couch, E. J. Quarterly of the Colorado School of Mines, 1975, 70 (3), 237. 4. Allred, V. D. Chem. Eng. Progr., 1966 62, (8), 55. 5. Weitkamp, A. W.; Gutberlet, L. C. Ind. Eng. Chem. Process Des. Develop., 1970, 9 (3), 386. 6. Fausett, D. W.; George, J. H.; Carpenter, H. C. "SecondOrder Effects in the Kinetics of Oil Shale Pyrolysis," Report of Investigation 7889, Bureau of Mines, Washington, D.C., 1974. 7. Campbell, J. H.; Koskinas, G. H.; Stout, N. D. "The Kinetics of Decomposition of Colorado Oil Shale: I. Oil Generation," Report UCRL-52089 Prepared for the U.S. Energy Research and Development Administration, Lawrence Livermore Laboratory, June 1976. 8. Campbell, J. H.; Koskinas, G. H.; Coburn, T. T.; Stout, N. D. "Oil Shale Retorting: The Effects of Particle Size and Heating Rate on Oil Evolution and Intraparticle Oil Degradation," Report UCRL-52256 Prepared for the U.S. Energy Research and Development Administration, Lawrence Livermore Laboratory, April 1977.


7.

WALLMAN

Oil

ET AL.

Shale

Retorting

113

Kinetics

9. Hinds, G. P., "Effect of Heating Rate on the Retorting of Oil Shale," Paper Presented at the 71st Annual Meeting of the AIChE, Miami Beach, Florida, November 1978.


10. Burnham, A. K.; Taylor, J. R. "Shale Oil Cracking. 1. Kinetics," Report UCID-18284 Prepared for the U.S. Energy Research and Development Administration, Lawrence Livermore Laboratory, October 1979. AJPJ>J:NDJ.X HEAVY OIL PRODUCTION AND COKING RATE CONSTANTS, k

1

AND k (RATE CONSTANTS IN MIN." , COKE YIELDS IN WT % KEROGEN)

2

Q

Particle S i z e , mm

900° F k k Coke Y i e l d k /(k + c> c k 2

+

c

c

2

k

k

930° F

950°F

0.887 16.8 0.875 0.776 0.111

1.04 16.8 0.875 0.908 0.130

0.442 15.6 0.813 0.359 0.083

0.545 15.6 0.813 0.443 0.102

0.292 14.5 0.755 0.220 0.072

0.367 14.5 0.755 0.277 0.090

0.197 11.2 0.583 0.115 0.082

0.296 11.2 0.583 0.173 0.123

980°F

1000°F

0.378 14.5 0.755 0.285 0.093

0.587 14.5 0.755 0.443 0.144

2

k

+

k

2 c Coke Y i e l d k /(k c

2

+ c> k

k

c k

2

k

+ k

2

Coke Y i e l d k /(k + c> c k c

2

k

k

0.186 14.5 0.755 0.140 0.046

2

0.4

k

+

k

2 c Coke Y i e l d k /(k + k > c

2

c

k

c k

2

RECEIVED

January 19, 1981.


Oil Shale Retorting Kinetics - ACS Symposium Series - ACS Publications

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