Effect of reduced pressure on oil shale retorting. 1. Kinetics of oil

Robert L. Braun , Alan K. Burnham , John G. Reynolds , and Jack E. Clarkson ... Edward M. Arnett , B. Chawla , K. Amarnath , and L. G. Whitesell , Jr...
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Ind. Eng. Chem. Process Des. Dev. 1085, 2 4 , 265-270

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Effect of Reduced Pressure on Oil Shale Retorting. 1. Kinetics of Oil Generation Hong Yong Sohn' and Hyun S. Yangt Depatfments of Fuels Englneering and of Metallurgy and Metallurgical Englneering, University of Utah, Salt Lake City, Utah 84 112

The rate of oil generation in oil shale retorting under reduced pressure was determined by a nonisothermaitechnique. First-order kinetics adequately approximated the overall retorting rate of Colorado oil shale regardless of the type and pressure of the sweep gas. The activation energy remained relatively constant with pressure for each sweep gas, while the preexponentiai factor increased as pressure decreased. The rate expressions for retorting with nitrogen and carbon dioxide were substantially the same, while retorting with water vapor produced a higher rate constant than that with nitrogen or carbon dioxide. The kinetics expression for oil generation remained relatively unchanged for different grades of oil shale containing organic matter of similar compositions.

Introduction The complex nature of kerogen and its decomposition reaction have complicated the interpretation of experimental data and led to various postulates for the decomposition mechanism. Several previous investigators have taken an isothermal approach to determine the decomposition kinetics but often failed to eliminate significant heatup effects in the measured kinetics. Hubbard and Robinson (1950) determined the kinetics of oil generation under isothermal conditions using a mechanism consisting of two fmt-order steps for the decomposition of kerogen to bitumen which subsequently decomposes to oil, gas, and carbon residue. Attempts were made to correct the Hubbard and Robinson data for the heatup effects by Braun and Rothman (1975) and Johnson et al. (1975). Braun and Rothman examined the data and showed that the kinetics of oil generation could be explained more adequately by including a thermal induction period in the data analysis to account for the nonisothermal heating effect in the experiment. Campbell et al. (1976) determined the retorting kinetics by both isothermal and nonisothermal experiments with reasonable agreement between the two approaches. They showed that an overall first-order reaction rate could adequately approximate the decomposition behavior of Colorado oil shale. Shih and Sohn (1980) used a nonisothermal technique with various heating rates for the determination of overall first-order intrinsic kinetics of oil generation from Colorado oil shale. They observed that an overall fmt-order kinetics obtained with various heating rates represented the decomposition of organic matter in oil shale quite satisfactorily. Nobel et al. (1981) and Burnham and Singleton (1982) measured the oil yield as a function of time or temperature at an elevated pressure and showed that the rate of oil generation was retarded by pressure. The purpose of this study was to measure the intrinsic kinetics of oil generation in the retorting of various oil shales under reduced pressure. The shale samples used were Colorado, Michigan, and Australian oil shales. The particle size of the oil shale sample was -8 to +48 mesh. Nitrogen, carbon dioxide, and water vapor were used as sweep gases. A flow rate of about 60 cm3/min corrected

* Departments of Fuels Engineering and of Metallurgy and Metallurgical Engineering. Department of Fuels Engineering. 0196-4305/85/1124-0265$01.50/0

to 25 "C and 0.86 atm was used for retorting in nitrogen and carbon dioxide. About 0.1 g/min of water was used for water vapor retorting. These flow rates were high enough for the swift removal of oil vapor and mist produced in the retort and low enough for the efficient condensation at the collecting system.

Mat hematical Procedures A number of previous authors (Friedman, 1965; Reich and Stivala, 1978; Shih and S o h , 1980; Suzuki et al., 1980) introduced mathematical procedures for determining kinetics parameters from nonisothermal data and discussed their advantages and disadvantages. In this study, two of these methods were used for the determination of the kinetics of oil generation. In the first step, Friedman's procedure was used to determine reaction order, activation energy, and preexponential factor. Using the value of reaction order obtained by this method, the integral method used by Shih and S o h was employed to simplify the kinetics of a firstor second-order reaction kinetics. In this study, the following well-known rate expression is used

where x is the extent of conversion of organic matter, A is the preexponential factor for the rate constant, E is the activation energy, n is the reaction order, t is time, T is temperature, and R is the universal gas constant. With x equal to w/wo, in which w is the weight of oil evolved up to time t and w o is the total weight of oil evolved during the process, eq 1can be rewritten with the replacement of dx/dt by AxlAt as follows l n ( LWOg )At =lnA+nln Several values of w/wo can be selected at equal intervals of time from each heating rate, from which the values of (l/wo)(Aw/At) and 1 / T can be determined. From the plots of In [(l/wo)(Aw/At)]vs. 1/T at different values of w/wo, the values of - E / R from the slopes and In [A(1w / w ~ )from ~ ] the intercepts are obtained. Thus the average value of the activation energy is obtained. Substituting experimental parameters and the values of activation energy into eq 2 and plotting In [A(1 - W / W ~ ) ~ ] vs. In (1- w/wo), a straight line is obtained with a slope @ 1985 American

Chemical Society

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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985 WATER SOURCE 0

RESERVOIR WATIR F l o w

H E A T I N G TAPE

WATER

VAPOR

TE R

Figure 1. Schematic diagram of oil shale retorting apparatus.

n and intercept In A. The calculated reaction orders were close to either 1 or 2; either an overall f i t - or second-order kinetics was assumed and used with an integral method. For these two cases, experimental data obtained under a linear temperature increase should follow the following equations, respectively. For a first-order reaction -h In ( 1 - w / w o ) A E -In 1 - =In--(3) In RP E RT For a second-order reaction

(

y)

10

1

'

"

r

'

"

,

'

,

,

,

6

'

-

* * a

0.0351 *K/s

0

0.0583

A

0.0717

0 8-

O

A

o A 'A

* *

'

A

O

A 0

0

*

where h is the heating rate. The values of E and A associated with the overall firstor second-order rate expression can be obtained by repeated application of least-square fits of eq 3 or 4 to the experimental data as follows: by using an approximate E in the left-hand side of equation as a linear function of 1/T and obtaining -E/R from the slope and A / E from the intercept. The value of E obtained is used in the left-hand side and successivelya more accurate value of E is obtained until no improvement in the value of E takes place. Experimental Section A schematic diagram of the apparatus used for oil shale retorting is given in Figure 1. The oil shale sample was crushed and screened to -8 to +48 mesh, and the shale particles were heated to between 104 and 110 "C for 20 min to remove the free water contained in raw shale. A sample of about 70 g was charged in the retort vessel. With the sample in the vessel, the desired flow rate of sweep gas and retort pressure were maintained using a vacuum pump, pressure regulators, and metering valves. Metering valves connected in series were used for controlling the gas flow rate precisely and for maintaining a constant mass flow rate of sweep gas from a high-pressure gas cylinder. The pressure was determined from the two U-tube manometer readings which showed the pressure difference between the inlet and outlet of the retort vessel and the surroundings. The sample was subjected to a linear temperature increase using a temperature controller, and the temperature of the sample was recorded on a temperature recorder. A three-way valve was connected to the outlet tube so that the amount of oil can be measured at various time intervals. Oil was collected in three 15-mL graduated centrifuge tubes immersed in ice-salt, dry-ice, and iceaalt baths and connected in series. The oil trap which was packed with fine stainless steel wires was designed to catch all the vapor that has passed without being condensed in the preceding tubes. Upon completion of the experiment,

*

A O

CA

t

eo

e g o 0

A

Temperature

OK

Figure 3. Conversion vs. temperature under 0.86 atm of nitrogen at different heating rates.

the collected oil and water product was heated at 40 O C for several minutes to increase its fluidity and centrifuged for about 15 min in a graduated centrifuge tube. For water vapor retorting, a simple water vapor generating apparatus was used, as shown in Figure 2. Results Kinetics Parameters for Retorting with Nitrogen. The cumulative weight of oil collected under 0.86 atm as a function of temperature is shown in Figure 3. The instantaneous rate of oil generation is plotted in Figure 4. In this figure, the rate curves were obtained by plotting the average rate AxlAt against the average temperature for the period in which the weight was collected, and then drawing smooth curves (solid lines) through the points. The value At indicates the time interval between two temperatures at which oil was measured. Nine values of w / w o were selected, ranging from 0.1 to 0.9 at equal intervals. The values of ( l / w o ) ( A w / A t )and T were determined for each w / w o value of each heating ] 1 / T according to eq rate. Plots of In [ ( l / w o ) ( A w / A t )vs. 2 are shown for different values of w / w oin Figure 5. The slope of each line gives the value of -E/R, and the intercept is equal to In [ A ( 1 - w/wo)"]. The values of E and In [A(1 - w/wo)"]are plotted as functions of w / w o in Figure 6. The average value of activation energy E is 49470 (cal/g-mol). The calculated values of In [ A ( 1 - w / w ~ ) ~ ] are plotted as a function of In (1 - w/wo)in Figure 7 , where a reasonably good straight line is obtained over the entire range of conversion indicating that the reaction order remains quite constant. Based on the curve, the reaction

Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985 267 Table I. Kinetics Parameters for Retorting with Nitrogen Friedman's procedure atm n E , cal/a-mol A, s-l 1.08 49 470 1.262 X 1013 0.86 1.115 X lo'* 0.5 1.12 48 020 0.1 1.20 48 720 2.335 X 10l2 1.09 47 100 6.853 X 10l1 0.05

first-order int. method E,cal/g-mol A , s-l 46 460 3.177 X 10" 46 308 3.039 X 10" 46 300 3.371 X 10" 46810 5.254 X 1Ol1

n

1.0 1.0 1.0

1.0

30

t

1

28 26

55

4o

I

t I

0 ' 1

a

620

a

660

8

.

I

1

740

700

# I

Figure 4. Instantaneous rate of oil generation under 0.86 atm of nitrogen at different heating rates. 675

I

I

I

I

0.4

I

I

0.8

0.6

I

]

I

050

I

I

Figure 6. Kinetics parameters as functions of conversion.

OK

700

I

I

0.2

W/W.

Temperature,'K

725

I

780

1-I

* I

27

-2.4

I

-2.0

I

-1.6

I

-1.2

I

0.8

I

-0.4

1

I n 1 i- "/w0 ) Figure 7. Determination of preexponential factor and reaction order. 0.0351%/s 0

A

0.1

0.0583 0.0717 I 1.4

I

I

I

1.5

1000/T

I K-'

)

Figure 5. Friedman's procedure for determining kinetics parameters using experimental data under 0.86 atm of nitrogen with different heating rates.

order and preexponential factor are determined to be 1.08 and 1.262 X 1013 s-l, respectively. Since the reaction order is close to unity it would be advantageous to simplify the kinetics to f i b o r d e r kinetics. Figure 8 represents the least-squares fit of eq 3 to all of the experimental data. The overall values of E and A

obtained are 46460 cal/g-mol and 3.177 X 10" s-l, respectively. The experimental data obtained under flowing nitrogen at 0.86 atm and curves calculated using the overall firstorder kinetics parameters are shown in Figure 9 for comparison. The determination of rate expression under other pressures followed the same procedure. The kinetics parameters determined by the two methods for various pressures are summarized in Table I. As seen in the table, the retorting pressure has little effect on the activation energy in the first-order kinetics, whereas the preexponential factor changed considerably with pressure. The rate constant for retorting oil shale under zero pressure represents the intrinsic rate constant for the decomposition of organic matter since all of the oil evolved during retorting would be in the vapor phase and thus

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Table 11. Kinetics Parameters for Retorting with Carbon Dioxide Friedman's procedure atm n E. cal/a-mol A, s-l 7.615 X 10" 0.86 1.13 47 500 0.1 1.08 47 820 1.097 X lo1* 0.05 1.21 47 620 1.126 X 10''

1.0 1.0

Table 111. Kinetics Parameters for Retorting with Water Vapor Friedman's procedure atm n E, cal/g-mol A, s-l 0.86 1.05 45 210 2.765 X 10" 0.5 0.1 1.12 45 130 3.344 x 10" 0.05 0.94 44 680 2.148 X 10" -14

I

I

first-order int. method E. cal/a-mol A, s? 46 460 3.137 X 10" 6.177 X 10" 47 100 46 530 4.384 X 10"

n 1.0

first-order int. method E, cal/g-mol A, 5-l 44 730 1.852 X 10" 2.352 X 10" 45 010 44 500 1.843 X 10" 45 020 2.917 X 10"

n 1.0

1.0 1.0 1.0

I 0

0.80.0351 'KIs 0

0.0351°K/s 0.0583

e 0.0717

0.6-

0.0583

;

A 0.0717

VI L

0.4

C

e

.

0

0

0.2 -

T e m p e r a t u r e , 'K

Figure 9. Comparison between experimental data obtained at nitrogen pressure of 0.86 atm and curves calculated using the overall first-order kinetics parameters.

c

-1

IOOO/T(K-')

Figure 8. Determination of the overall values of kinetics parameters according to the integral method.

recovered as soon as formed without being degraded into coke and gas. Using the results of kinetics parameters in Table I, the values of preexponential factor were recalculated at each pressure with an activation energy of 46460 cal/g-mol. A least-squares method with a quadratic equation was used for determining the best-fit values of preexponential factor as a function of pressure. Figure 10 shows the extrapolation using the values of recalculated preexponential factor, and the rate expression at zero pressure was determined to be knitrogen= 4.486

X

10l1exp(-46460/RT) s-'

(5)

6 4.5

h

.

0

L

nitrogenin

J

1

carbon d i o x i d e i r l

3.0

0

0.2

0.4

0.6

0.8

.o

Pressure, atm

The kinetic expression in eq 5 can be considered as the equation for the intrinsic kinetics of the decomposition of the organic matter in Colorado oil shale in the presence of nitrogen. The kinetics of oil recovery at other pressures depend on the degradation of produced oil. Detailed mathematical analysis of the effect of pressure and heating rate, incorporating oil degradation, on the rate and yield of oil generation is given in a subsequent paper (Yang and Sohn, 1985). Kinetics Parameters for Retorting with Carbon Dioxide. The kinetics parameters calculated from the two methods using the experimental data for retorting 36.6 f

Figure 10. Determination of preexponential factor at zero pressure by extrapolation.

0.6 gallton of Colorado oil shale are summarized in Table 11. As observed in the retorting with nitrogen, the activation energy remains relatively constant, but the preexponential factor varies with pressure. To determine the rate constant at zero pressure, the activation energy 46460 cal/g-mol was used to fiid the preexponential fador at each pressure and extrapolated to zero pressure as shown in Figure 10. A final kinetic expression for the decomposition of organic

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Table IV. Kinetics Parameters for Retorting 27.0 f 0.5 gal/ton of Colorado Oil Shale with Nitrogen Friedman's procedure first-order int. method atm n E , cal/g-mol A, n E , cal/g-mol A , s-l 48 750 1.941 X 1OI2 1.0 46 870 0.86 1.23 4.232 X 10" 47 630 9.331 X 10" 1.0 47 010 0.1 1.14 5.474 x 10" 46 810 5.752 X 10" 1.0 46 300 0.05 1.15 3.612 X 10" Table V. Kinetics Parameters for Retorting Various Oil Shales at a Nitrogen Pressure of 0.86 atm Friedman's procedure integral method source of sample n E, cal/g-mol A , s-l n E, cal/g-mol A , s-l Australian 1.22 54 810 2.840 X 1014 1.0 48 940 2.686 X 10l2 Michigan 2.0 60 950 7.900 X 10l6 Alpena 1.91 59 870 3.181 X 1OI6 Port Huron 2.11 66 040 8.187 x 1017 2.0 63 420 1.090 x 1017

matter in Colorado oil shale in the presence of carbon dioxide was calculated to be essentially the same as shown in eq 5. Kinetics Parameters for Retorting with Water Vapor. The kinetics parameters calculated from the two methods using the experimental data for 36.6 f 0.6 gal/ton Colorado oil shale are summarized in Table 111. It is also seen that the activation energy remains relatively constant with pressure and is quite close to the value determined in the presence of nitrogen or carbon dioxide, whereas the preexponential factor changes considerably. Using the activation energy 46460 cal/g-mol as used above, the preexponential fador was recalculated at each pressure which provided best-fit to the experimental result. The extrapolation using the recalculated values of preexponential factors is shown in Figure 10, and the kinetics expression at zero pressure is calculated to be kwatervapor =

8.263 X 10" exp(-46460/RT) s-1

OK 750

700

125

675

650

625

10-21

r 0.05 a t m

__

0.86

(6)

Figure 11 shows the comparison of rate constants for retorting with nitrogen and water vapor, which indicates that the water vapor retorting produces a higher rate constant in the pressure range studied. For instance, the rate constants for retorting Colorado oil shale at 700 K and 1.21 X and 2.35 X s-l 0.1 atm are 1.16 X under nitrogen, carbon dioxide and water vapor, respectively. The rate constants for retorting with nitrogen and carbon dioxide are substantially the same at any pressure ranging from 0.05 to 0.86 atm. As mentioned by several authors (Campbell and Taylor, 1978; Allred, 1979), the use of water vapor as a reactive gas in the retorting of oil shale lowers the retorting temperature, which reduces the amount of degradation. Kinetics Parameters for Retorting Different Grades of Oil Shale with Nitrogen. Two different grades of oil shale (36.6 f 0.6 and 27.0 f 0.5 gal/ton) which came from the Anvil Points Mine in Colorado were used to determine whether the shale grade had any effect on kinetics under reduced pressures. Shown in Table IV are the kinetics parameters for the retorting of 27.0 f 0.5 gal/ton oil shale. The kinetics parameters for 36.6 f 0.6 gal/ton oil shale have been shown in Table I. As can be seen in Table IV, the activation energy remains relatively constant with pressure, whereas the preexponential factor changes considerably. Figure 1 2 shows the comparison of rate constants obtained from the two different grades of oil shale and indicates that the effect of reduced pressure on kinetics of oil generation is similar for both samples. Kinetics Parameters for Retorting Oil Shales Obtained from Different Geographical Deposits. Three other shales used in this study were an Australian and two Michigan oil shales. The Michigan oil shales came from

1.5

1.4

1000/T,

1.6

OK-'

Figure 11. Comparison of rate constants for retorting with nitrogen and water vapor.

the Antrim Basin, near Alpena, and Port Huron. The reaction order for retorting Michigan oil shale was determined to be 2, whereas Australian oil shale produced approximately unity. The kinetics parameters obtained at 0.86 atm for those samples are summarized in Table V. The Michigan oil shale which has a higher fraction of aromatic carbons than the Green River oil shale produced an overall second-order kinetics with a higher activation energy. However, the Australian oil shale produced a first-order kinetics and a higher rate constant at 0.86 atm than the Colorado oil shale. Other investigators have studied the pyrolysis rates of oil shales from various sources. Herrell and Arnold (1976) reported that the decomposition of shale from the chattanooga black shale formation had a reaction order of 3.7 and an activation energy of 57 100 cal/g-mol. Snow et al. (1979) determined that the eastern shales pyrolyzed substantially faster than the Colorado shales above 350 "C under dielectric heating. Mickelson and Rostam-Abadi (1980) investigated the pyrolysis of Michigan Antrim oil shale by a weight-loss technique. They observed that the decomposition reaction was of second order with an activation energy of 60 280 cal/g-mol. These values are in good agreement with the results of the present study despite the fact that our measurements were specifically on

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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985 O K

750

700

725

675

650

plotted as a function of pressure and extrapolated to zero pressure. The rate constant for retorting oil shale at zero pressure represents the intrinsic rate constant for the decomposition of organic matter due to the reason explained in the text. The recommended first-order rate constant for retorting Colorado oil shale from the Anvil Points Mine in the presence of nitrogen or carbon dioxide was determined to be

k = 4.486 X 10" exp(-46460/RT)

s-I

and that in the presence of water vapor is

k = 8.236 X 10'l exp(-46460/RT)

s-'

Furthermore, the kinetics expression for oil generation remained relatively unchanged for different grades of oil shale containing organic matters of similar composition. Acknowledgment The authors express their thanks to Dr. I. C. Lee and Dr. D. M. Bodily for their technical assistance and discussions during the course of this study. Registry No. Nitrogen, 1121-31-9;carbon dioxide, 124-38-9; water, 7132-18-5. 1.5

1.4

1000/T,

OK-'

Figure 12. Comparison of rate constants for retorting two grades of oil shale under reduced pressures.

the kinetics of oil generation rather than total weight loss including gaseous products. Nuttall et al. (1983) and Ekstrom et al. (1983) determined the pyrolysis kinetics of oil shales from various sources of the world. Direct comparison of their results with the present results is not readily possible because they did not separately determine the reaction order and also used rate expressions based on multiple reactions. Conclusion The rate constant increased with decreased pressure, and the first-order kinetics adequately approximated the overall retorting rate for Colorado oil shale regardless of the type and pressure of sweep gas. The Michigan oil shales produced second-order kinetics, whereas the Australian oil shale showed a first-order dependence. As observed from the experimental results of kinetics parameters, the activation energy remained relatively constant in the pressure range studied, whereas the preexponential factor changed significantly with pressure. The retorting with water vapor produced a higher rate constant at all the pressures studied compared with retorting with nitrogen or carbon dioxide. To determine the true intrinsic kinetics of the decomposition of organic matter in Colorado oil shale, the preexponential factors measured at various pressures were

Literature Cited Alired, V. D. "Proceedings, 12th 011 Shale Symposlum"; Colorado School of Mines Press: Golden, CO, 1979; p 241. Braun, R. L.; Rothman, A. J. Fmi 1075, 54, 129. Burnham, A. K.; Singleton, M. F. ACS Symp. Ser. 230, 1983, 335. Campbell, J. H.; Kosklnas, 0. J.; Stout, N. D. Fuel 1978, 57, 372. Campbell. J. H.; Taylor, J. R. Lawrence Livermore National Laboratory, Rept. UCID-17770, Livermore, CA, 1978. Ekstrom, A,; Hurst. H. J.; Randall, C. H. ACS Symp. Ser. 230 1983, 317. Friedman, H. L. J. Po/ym. Sci. 1085, C 6 , 183. Herrell, A. Y.; Arnold, C., Jr. Thermhim. Acta 1978, 17, 165. Hubbard, A. B.; Robinson, W. E. U . S . Bur. Mines, Rept. Invest. 4744, 1950. Johnson, W. F.; Walton, D. K.; Keller, H. H.; Couch, E. J. Coio. Sch. Mines 0.1975, 70(3), 237. Mickelson, R. W.; Rostam-Abadi, M. "Proceedings. 15th Intersociety Energy Conversion Engineering Conference"; Seattle, WA, Aug 18-22, 1980; Paper 809057, p 291. Noble, R. D.; Harris, H. G.; Tucker, W. F. Fuel 1981, 60, 561. Nuttall, H. E.; Guo, T.-M.; Schrader. S.;Thakur, D. S. ACS Symp. . . Ser. 230 1083, 269. Reich, L.; Stlvala, S. S. Thermochlm. Acta 1978, 24, 9. Shih, S A . ; Sohn, H. Y. Ind. Eng. Chem. R o c . Des. Dev. 1980, 19, 420. 11 Snow, R. H.; Bridges, J. E.; Goyal, S. K.; Taflove, A. "Proceedings, 12th 0 Shale Symposiums"; Colorado, School of Mines Press: Golden, CO. 1979: D 283. Suzukl, M:; Chihara, K.; Smith, J. M. J. Chem. Eng. Jpn. 1980, 13(3), 249. Yang, H. S.; Sohn, H. Y. Ind. Eng. Chem. Proc. Des. Dev. 1985, accompanying article (third) in this Issue.

Received for review August 1, 1983 Revised manuscript received April 19, 1984 Accepted May 21, 1984 This work was supported in part by the U S . Department of Energy under Contract No. DE-AS3-78 ET 13095, a Research Grant from the University of Utah Research Committee, and the University of Utah College of Mines and Mineral Industries Mineral Leasing Fund.