520
Energy & Fuels 1987, 1, 520-525
Kerogen Decomposition Kinetics of Selected Green River and Eastern U.S. Oil Shales from Thermal Solution Experiments Don R. Leavitt, A. Lamont Tyler,* and A. Scott Kafesjiant Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112 Received June 16, 1987. Revised Manuscript Received August 17, 1987
Thermal solution experiments have been conducted to examine the kinetics of the first step in the decomposition of kerogen for two oil shales-a Utah (Green River) oil shale and a Kentucky (Sunbury) oil shale. The global rate of kerogen decomposition in the Green River oil shale can be described by two parallel first-order lumped reactions. A t temperatures above 350 "C, the dominant lumped reaction has an activation energy of 45.7 kcal/mol; below 350 OC the dominant lumped reaction has an activation energy of 20.8 kcal/mol. Over 95% of the parent kerogen can be removed by thermal solution. For the Sunbury oil shale, only 75% to 80% of the parent kerogen can be removed from the inorganic matrix by thermal solution. The decomposition of this kerogen is best described by a series of reactions that represent increasingly slower steps in the solution attack on somewhat unreactive structures in this kerogen. A lumped-reaction model with an activation energy of 50.4 kcal/mol for the rate-determining step is presented. The lumped-reaction models predict the results of both linear heating and isothermal decomposition experiments.
Introduction In the thermal solution of kerogen, soluble decomposition products are removed from the oil shale matrix into a solvent-usually present in excess. This provides a favorable method for studying the reaction kinetics of the first steps in the decomposition of kerogen, i.e. the steps in the conversion of insoluble kerogen to a soluble bitumen intermediate. Several early investigators have reported work in which the organic portion of oil shale was recovered by thermal More recently, the investigation of the tehrmal solution of oil shale has branched into many areas to examine the effects of processing parameters, as well as dissolution mechanisms and kinetic^."^ Over 95% of the total organic material can be recovered by thermal solution of Green River oil shales. This occurs at considerably lower temperatures than those required for "dry" pyrolytic oil production and is consistent with the postulate that the first step in the decomposition of kerogen occurs at relatively low temperatures.2 When the reactions are quenched rapidly, analyses of the organic residue in the spent shale provide information that can be used to determine the decomposition kinetics of kerogen converting to the first soluble intermediates. In the work discussed here, thermal solution was used to study kerogen-to-bitumen decomposition kinetics of a Green River oil shale from an outcrop in Hell's Hole Canyon, Uintah County, UT, and a Sunbury oil shale from Montgomery County, KY. Table I summarizes the physical and chemical characteristics of these two oil shales. The oil shale of the Green River Formation is characterized by a relatively small fraction of aromatic carbon in the parent kerogen. This oil shale is unique among the world's large oil shale deposits in the large fraction of the kerogen that is converted to oil when subjected to conventional or "dry" pyrolytic decomposition.1° Eastern U.S. oil shales such as Sunbury oil shale from the Mississippian Formation contain a much larger fraction of aromatic carbon, however, and are more characteristic Current affiliation: Hercules, Inc., Magna, UT.
0887-0624/87/2501-0520$01.50/0
Table I. Summary of Properties of Raw Oil Shales Used in Thermal Solution Tests particle size, pm total organic wt % wt. % aromatic carbon of total organic carbon
Green River 580-2380 25.7 27
Sunbury 580-1300 21.8 42
organic composition c, wt 70 H, wt % N, wt % 0 S," wt % aromatic C, wt % bitumen, wt %
81.4 10.6 2.2 5.8 22.0 8.9
70.8 9.4 2.3 17.5 29.7 1.3
Fisher assay oil, w t 70 water, wt % coke, wt % gas + loss: w t % ash, wt %
16.4 0.8 4.9 4.4 73.5
5.9 2.6 10.3 3.0 78.2
+
Determined by difference.
of oil shales worldwide. Simple thermal decomposition of these oil shales results in less oil and much more carbonaceous residue, or char, per unit of kerogen.'l (1) Dulhunty, J. A. Proc. Linn. SOC.N.S. W. 1942,67, 238-248. (2) Robinson, W. E.; Cummins, J. J. J. Chem. Eng. Data 1960, 5, 74-80. (3) Jensen, H. B.; Barnet, W. I.; Murphy, W. I. R. Bull.-US., Bur. Mines 1933, No. 533. (4) Wheelwright, S. M. B.S. Thesis, University of Utah, 1978. (5) Baldwin, R. M.; Briley, R. A.; Baughman, G. L.; Minden, C. S. Proceedings 1983 Eastern Oil Shale Symposium; Institute of Mining and Minerals Research Lexington, KY, 1984; pp 333-336. (6) Baldwin, R. M.; Frank, W. L.; Baughman, G. L.; Minden, C. S. Oil Shale Symp. Proc. 1983, 16th, 388-393. (7) Cronauer, D. C.; Solash, J.; Danner, D. A.; Galya, L. G. Proceedings,1983 Eastern Oil Shale Symposium; Institute of Mining and Minerals Research Lexington, KY, 1984; pp 17-23. (8)McKay, J. F.; Chong, S. Proceedings, 1983 Eastern Oil Shale Symposium; Institute of Mining and Minerals Research: Lexington, KY, 1984; pp 235-239. (9) Patzer, J. F., 11; Moon, W. G. Chem. Eng. Sci. 1986, 41, 1005. (10) Miknis, F. P.; Maciel, G. E. Oil Shale Symp. Proc. 1981, I4th, 270-281.
0 1987 American Chemical Society
Kerogen Decomposition Kinetics
Energy & Fuels, Vol. 1, No. 6,1987 521
The success of thermal solution in removing over 95% of the organic material in the Green River oil shale, including approximately 95% of the aromatic carbon, prompted interest in using this technique to study the recovery of the organic fraction from the more highly aromatic Sunbury oil shale. Kinetic studies using the thermal solution technique were conducted by established nonisothermal procedures at constant heating rates using 1,2,3,4-tetrahydronaphthalene(tetralin) as the solvent. The thermal solution yields and the rate parameters of the kerogen-to-bitumen decomposition model are presented for the two oil shales studied in this work. Experimental Apparatus and Procedures The principal experimental apparatus used in conducting the thermal solution tests consisted of an electrically heated, 1-L stainless-steel stirred autoclave. The temperature inside the autoclave was controlled by using a programmable controller with a heating rate range of 0.1-9.9 OC/min. The entire apparatus is described in detail by Kafesjian.12 Prior to the thermal solution tests, oil shale samples were crushed, screened, and riffled to insure homogeneity. In each test a dried, 70 g sample of oil shale was placed in the autoclave, along with a premeasured volume of tetralin (500 mL in the testa with Green River oil shale and 250 mL in the tests with Sunbury oil shale). After the autoclave was sealed, the vapor space was purged with nitrogen. The autoclave was heated at constant rates of 0.20 OC/min to 0.55 OC/min with continuous agitation of the shale and solvent. The low heating rates were intended to insure that reaction kinetics rather than mass transport would be rate controlling. When the desired maximum temperature was reached, the reaction was quenched and the test terminated by rapidly cooling the autoclave and its contents-typically a t a rate of 30 OC/min. The spent shale was recovered from the reactor and cleaned by using exhaustive Soxhlet extraction with cyclohexane (for the spent Green River oil shale) or toluene (for the spent Sunbury oil shale). The spent shale solids were then dried in a vacuum oven at 100 "C. Portions of the cleaned spent shale were prepared for three separate analyses. The first of these was a low-temperature ash analysis (LTA) to determine percent organic residue in the spent shale. Samples were heated in the presence of flowing oxygen at 450-480 "C to oxidize the organic residue. The low ashing temperatures prevented significant decomposition of the inorganic carbonates present in the Green River oil shale. Analyses for carbon, hydrogen, and nitrogen were performed on samples of the spent shale to determine the elemental composition of the organic residue. The C-H-N analyses for the Green River spent shale samples were conducted a t 450 " C to suppress carbonate decomposition;the analyses for the Sunbury spent shale samples were conducted at 950 OC. Additionally, spent shale from selected tests were analyzed by using solid-state 13CNMR spectroscopy with cross-polarization and magic angle spinning to determine the fraction of aromatic carbon in the organic residue. All of the above analyses were also conducted on samples of the untreated parent oil shales used in this study.
Data Reduction and Representation The extent of kerogen-to-bitumen conversion for each thermal solution test was characterized by fractional organic yield, Y. This was defined as the weight fraction of the original organic material that was solubilized and removed from the oil shale:
y=- mr - ma m1
(1)
See the Glossary for the definition of terms. In practice, the yield was determined by using the organic residue fractions in the raw and spent oil shales, f, (11) Miknii, F. P.; Szeverenyi, N. W.; Maciel, G. E. Fuel 1982,61,341. (12)Kafesjian, A. S. Ph.D. Thesis, University of Utah, 1983.
and f,, respectively, determined by low-temperature-ashing analyses:
The yield data were analyzed to determine kinetic parameters by using established integral, differential, and difference-differential methods. Properly applied, each method results in a linear plot of modified data from which the kinetic parameters can be determined graphically. Assumed mechanisms and/or reaction orders are required in using these methods of analysis, since they are based on specific differential rate equations describing those mechanisms. Although differential rate equations are intrinsically time-dependent, the use of a linear heating rate allows these equations to be transformed to a temperature-dependent form that can be analyzed by the above methods. The differential, or Arrhenius, method employs the temperature-dependent rate expression directly. This method requires numerical approximations of the derivative dY/dT, which generally magnify the uncertainty in the data. In using this method to analyze a first-order reaction, calculated values of -In [(AY/AT)/(l- Y)] are plotted against reciprocal absolute temperature. If Arrhenius rate behavior exists, the activation energy, E , is determined from the slope, and the preexponential factor, A , from the intercept as follows
E = RS
(3)
A = fie-'
(4)
where S is the slope, I is the intercept, and fi is the constant heating rate, dT/dt. The integral method of Coats and Redfern13is based on integration of the temperature-dependent rate equation and requires the numerical approximation to the integral of the Arrhenius expression: sTre-EIRTd T = (R/E)T 2 e-EIRT
(5)
However, in this method the yield data are used directly without requiring numerical differentiation or other manipulations that tend to magnify errors. For a simple first-order reaction, values of -In [-(ln (1- Y))/T2] are plotted vs. 1/ T by which the value of E is determined from eq 3 (as in the differential method) and the value of A from eq 6. A = (fiE/R)e-'
(6)
The difference-differential method developed by Freeman and Carroll was used to analyze the reaction order, as well as to provide another way to estimate the activation energy.14 However, the compounded numerical differentiation inherent in this method makes it especially sensitive to experimental error. The linearized expression upon which the plots are based is A[ln (AY/AT)I E A[ln (1 - Y)] T2 = - nT2 (7) AT AT R The resulting slope of the plot gives the reaction order, n , directly and the intercept gives the activation energy. The use of the quantity 1 - Y in all three of the above methods assumes that the parent kerogen is ultimately completely depleted by the postulated reactions.
+
(13)Coats, A. W.; Redfern, J. P. Nature (London) 1964,201,68-69. (14)Freeman, E. S.;Carroll, B. J. Phys. Chem. 1958,62,394-397.
Leauitt et al.
522 Energy & Fuels, Vol. 1, No. 6,1987
18
. .
0.0 0.8
>
5!
16
'
0.7
$
17
15
t
0.5 o.6
14
W
C
13 12
140
1.45
150
1.55
1.60
1.65
170
175
1iT x 1000, K" 180
300
320
340
360
380
400
420
Peak Run Temperature, "C
Figure 1. Fractional yield versus peak run temperature for thermal solution tests using Green River oil shale.
0,
a
; .-c
80
70
0
g 6
in Figure 2. Within experimental error, the residue composition, including aromatic carbon, is observed to remain constant over the entire test range. Low values for percent carbon at the highest temperatures are probably the result of measurement errors due to very small fractions of organic residue. A constant organic residue composition suggests a single insoluble "lumped" constituent decomposing to soluble products. Apparent differences between the compositions from the spent shale and those of the raw kerogen are probably caused by the presence of the 9% natural bitumen in the raw oil shale. The Green River oil shale test results were initially analyzed based on a simple first-order decomposition as described by the rate equation d ( 1 - Y) - -1 -A -- k ( l - Y) = -e-E/RT(l - Y) (8) dT P P Integral and differential methods were then applied as previously discussed to evaluate the parameters E and A . The resulting plots reveal two apparently linear, or nearly linear, regions of different slope that intersect at about 1/T = 0.0016 K-l, or T = 350 "C. This implies that rather than a single simple decomposition scheme, two parallel firstorder reactions are involved: bitumen T < 350 OC
I
leo oo
al
p:
Figure 3. Comparison of differential and integral analysis methods applied to the Green River oil shale yield data.
60
Aromatic Carbon 50 4
40
B
280
Hydrogen Nitrogen
300
320
340
360
380
400
420
Peak Run Temperature, "C
Figure 2. Elemental composition of organic residue in Green River spent shale.
In addition to the use of the linearized analysis methods to determine the kinetic parameters, nonlinear leastsquares techniques were used to estimate the parameters directly from the integrated differential rate expressions. The sums of squares of the differences between experimentally determined yields and predicted yields from the integrated rate equations were minimized by using simplex function minimi~ation'~ or Gauss-Newton methods. Integration of the differential rate equations was accomplished either by function approximation to obtain an explicit expression for the yield, or by numerical integration of the appropriate rate equations. Rate Analysis of Green River Oil Shale Tests A series of 18 thermal solution tests were performed with the Green River oil shale using a heating rate of 0.20 "C/min. Figure 1 shows the fractional yield for these testa plotted as a function of the temperature at which the reaction was quenched. Yields from approximately 0.09 a t 285 OC to 0.95 a t 410 "C were observed. The fractional yields obtained between 285 and 300 "C closely correspond to the 0.089 value for the fraction of naturally occurring bitumen determined by exhaustive extraction of raw oil shale in cyclohexane a t 80 "C. Thus these data indicate no significant formation of pyrobitumens up to about 300 "C using the 0.20 "C/min heating rate. Elemental analyses and solid-state 13CNMR analyses of the spent Green River oil shale provide some insight concerning the kerogen conversion mechanism. Percent elemental composition of the organic residue in the spent shale is plotted as a function of maximum run temperature (15) Nelder, J. A.; Mead,
R. Comput. J. 1965, 7,308-313.
kerogen
Y
(9) bitumen
T
>
350
OC
Evaluation of the two activation energies associated with the low-temperature and high-temperature regions initially yielded different results by the differential and integral methods. When the yield data were modified to exclude the naturally occurring bitumen, better agreement was observed-particularly at the lower temperatures where the contribution of the natural bitumen to the calculation is most significant. The differential method is insensitive to the effect of the natural bitumen. The results with data modified to exclude the natural bitumen are presented in Figure 3. The decomposition reaction was checked to verify the first-order rate assumption a t high temperatures by using a difference-differential analysis. The reaction order determined was 0.95; the assumption of a first-order decomposition was considered valid. The magnification of errors in data that is inherent with this method made it impossible to perform such a check a t the low conversions associated with the low temperatures. The parallel decomposition model is described by the temperature-dependent rate equation
Kerogen Decomposition Kinetics
Energy & Fuels, Vol. 1, No. 6,1987 523
1.o
>
P
.-5
= =
20.8 kcal/molo 2 . 2 ~ 1 04 /mln.
0.6
0.5
-
0.4
-
0
ue.
El AI
0.7
0.3
o'z 0.1 0.0
--e E
0
-
0
1e 1
280
300
U
360
340
380
400
0
e o
0.2
= = 320
: :I
I
420
260
280
300
Peak Run Temperature, "C
from which an analytical model for predicted yield was derived by separating the variables and integrating. The predicted yield expressed as
$)
- A2E2 Fp(
340
360
380
400
420
440
Peak Run Temperature, "C
Figure 4. Comparison of experimental yield data with the predicted yields from the parallel decomposition model.
YP = 1 - exP[ -=P( AlEl
320
$)I
(11)
(where p ( z ) represents a series expansion approximation to the exponential integral) was used in a simplex function minimization to evaluate El, E,, Al, and A* The activation energies calculated by this method were 20.8 kcal/mol for the low-temperature region and 45.7 kcal/mol for the high-temperature region. These values are shown in Figure 4. The magnitude of the two activation energies (i.e. "averagedn activation energies of many simultaneous reactions) indicate that the decomposition of Green River oil shale kerogen involves the breaking of weak chemical or physical bonds a t low temperatures and much stronger chemical bonds a t higher temperatures. Patzer and Moon investigated the kerogen decomposition kinetics of a Green River oil shale in an FCC decant They reported an activation energy of 49.2 kcal/mol for the conversion of kerogen to a heavy oil in the range 394-462 "C. The rate-controlling step a t temperatures below 350 "C involves cleavage of weak chemical and/or physical adsorption bonds, as suggested by Butler and Barker.16 The noticeable curvature of the data in Figure 3 observed a t the low temperatures in the differential analysis may reflect any of several possible influences. These include the adsorption/desorption of thermal bitumens, mass transport effects on the rate, or simply the result of the method of data reduction, which involved data smoothing for numerical differentiation. The results from the integral method do not show this curvature. Figure 4 shows the predicted fractional yield as a solid curve with the experimental data points also shown. The good agreement between predicted and experimental yields shows that the competitive parallel-reaction mechanism is adequate in describing the kerogen-to-bitumen decomposition step for the Green River oil shale. Rate Analysis of Sunbury Oil Shale Tests Two sets of thermal solution tests comprising 37 total runs were conducted with the Sunbury oil shale at heating rates of 0.33 and 0.55 "C/min. Yields from these two sets are plotted versus maximum run temperature in Figure 5. In both sets of data the yields range from about 0.03 (16)Butler, E. B.; Barker, C. Geochim.Cosmochim.Acta 1986, 50, 2281.
Figure 5. Fractional yield versus peak run temperature for thermal solution testa using Sunbury oil shale. 100 I
al
I10
00
E
80
2
70
0
A
+
260
Total C a h n Aromatic Carbon Oxygen + Sulfur Hydrogen
280
300
320
340
360
380
400
420
440
Peak Run Temperature, "C
Figure 6. Elemental composition of organic residue in Sunbury spent shale from tests at 0.55 OC/min.
at the low-temperature end to about 0.75 a t the hightemperature end, at which value the yields appear to level off with increasing temperature. At the low temperatures (270-310 "C) the fractional yield increases slowly to values of 0.07-0.08 and then increases more rapidly with increasing temperatures. Since only a very small amount of natural bitumen was detected in the Sunbury oil shale (corresponding to a fractional yield of 0.013) this observation of a significantly larger yield at low temperatures suggests some distinct type of decomposition product is removed. As with the Green River oil shale tests, elemental and solid-state 13CNMR analyses of the spent shale from the 0.55 "C/min test series were used to examine compositional changes in spent shale organic residue. Figure 6 shows the elemental composition of the organic residue as a function of the peak run temperature. These data reveal distinct changes in the elemental composition of the spent shale organic residue as a function of run temperature. Two regions are observed on either side of approximately 395 "C. The residue above this temperature was significantly higher in aromatic carbon and nitrogen and lower in hydrogen than the residue below 395 "C. Additionally, the residue composition a t the lower temperatures shows a significant depletion of hydrogen, oxygen, and sulfur from the parent kerogen. These composition data are consistent with the observed formation of water droplets inside the autoclave and the strong odor of H2S detected upon opening the autoclave after each run. Taken together, the yield and composition data suggest a series decomposition scheme for the kerogen-to-bitumen conversion in the Sunbury oil shale. The most favorable model considered involves the sequential formation of two additional insoluble "lumped" constituents:
524 Energy & Fuels, Vol. 1, No. 6, 1987
kerogen
1
Leauitt et al.
2
+
intermediate HzO, H2S, and HC's carbon residue + bitumen (12)
17
r
16
,
13
. . .
12
.
11
'
15
This model can be described mathematically by using three differential equations to account for the three insoluble constituents:
14
I
0 ,
Figure 7. Comparison of differential and integral analysis methods applied to the Sunbury oil shale yield data.
The coefficients pK1and pc represent the stoichiometric fractions of the intermediate and the carbon residue obtainable from the parent kerogen. Since no attempt was made to distinguish any separate constituents in the spent shale organic residue, it was necessary to express the rate equations in terms of the total organic residue measured, bT. This is accomplished by adding together the three rate equations, since -d@T = - + -d@KO d@K1 d@c (16) dt dt dt dt By assuming, as observed, that the kerogen-to-intermediate conversion (eq 13) has essentially gone to completion before the second step in the mechanism begins, all terms involving @KOmay be dropped in connection with the data above 330 "C. With the use of mass balance and stoichiometric relationships to express @K1 in terms of @T, a differential rate equation based on @T may then be obtained:
+-
eq 17 may be written in terms of observed yield and rearranged to the form
where the rate constant, k l , has been expressed in an Arrhenius form and dt has been expressed as (1IP)dT. Evaluation of E and A was again accomplished by applying the differential and integral analysis methods used with the Green River oil shale test data where 1- Y was replaced by Ym, - Y. The value YK1, the yield equivalent of the stoichiometric conversion of kerogen to the intermediate, was used as the lower limit of integration. Values of 0.07 and 0.75 were used for YK1 and Ym, respectively. The resulting differential and integral analysis plots for the Sunbury oil shale tests are shown in Figure 7, along with the lines representing the linear regressions for these data. In order to check the assumed first-order reaction rate, the difference-differential method was employed, and linear regression of the 0.33 "C/min data yielded values for n and E of 0.99 and 47.6 kcal/mol, respectively. Regression analysis of the 0.55 "C/min data resulted in values of 1.1 for n and 38.0 kcal/mol for E, confirming the assumption of first-order behavior. Insufficient test data were obtained for characterizing the part of the model described by eq 13.
o
Experimental dala for 0.55'CImin. lest5 Piediaed yields
> 0.7
.-
^.
".. E
L
0.3 LL
I
41 .O kcallmole
A. 1.~x10'21min.
0.2 0.1
0.0 260
'
280
-
300
320
340
360
380
400
420
440
Peak Run Temperature, "C
Figure 8. Comparison of experimental yield data with the predicted yields for thermal solution of Sunbury oil shale.
Nonlinear parameter analysis of the unmodified yield data resulted in estimates of the rate parameters that were somewhat different from those obtained by the linearized analysis methods. Figure 8 lists the parameter estimates from the nonlinear analyses and compares the resulting predicted yields with the experimental yields at both heating rates. Differences between the parameter estimates of the linearized methods and those of the nonlinear method are probably due to the weighting inherent in the linearized methods, as well as the effects of nonideal temperature histories. The lower activation energy at the higher heating rate suggests that the mass transport rates, or other factors, may have had an important influence on that set of data. Since no tests with the Sunbury oil shale were conducted a t heating rates lower than 0.33 "C/min, the reliability of the values of the kinetic parameters was checked by using the parameters to predict the experimental yields from isothermal extraction tests with tetralin. The complete temperature histories of those isothermal tests, including the heatup time, were used in predicting the fractional yields, which ranged from 0.07 to 0.74. On the basis of four isothermal tests at temperatures from 300 to 375 "C, the average deviation between predicted fractional yield and the experimental yield was 0.007 for the 0.33 "C/min parameters and 0.033 for the 0.55 "C/min parameters.17 The ability of the model using the kinetic parameters from the 0.33 "C/min test data to predict accurately the yields from isothermal tests verifies their reliability in representing the pyrolysis kinetics in the thermal solution of Sunbury oil shale, The less accurate predictions with the parameters from the 0.55 "C/min test data support the possibility that (17) Leavitt, D. R. Ph.D. Thesis, University of Utah, 1987.
Energy & Fuels 1987,1, 525-529 some additional phenomena, such as mass-transfer limitations, had affected those data. The two-step reaction model is admittedly a simplified representation of the actual mechanism, which includes a myriad of species and reactions. Nevertheless, the observed agreement between experimental yields and yields predicted from the model demonstrate the utility of the model in describing the decomposition of the parent kerogen to bitumen products. Examination of some of the data suggests the justification of a still more complicated model involving an additional "lumped" intermediate. However, insufficient data are available to determine, with confidence, the parameters required for the additional sequential step. Conclusions Over 95% of the Green River kerogen is converted to soluble bitumen by parallel, competing reactions-one that is rate controlling above 350 "C and another that is rate controlling below 350 "C. Approximately 75% of the parent kerogen in the Sunbury oil shale can be removed by thermal solution in tetralin. The Sunbury kerogen decomposition is best described by a two-step series decomposition model. The implication of the thermal decomposition being described by a series of reactions for the Sunbury oil shale and by parallel reactions for the Green River oil shale is consistent with the known reactivity of the molecular structures of the two oil shales. Aromatic eastern US. oil shales, such as Sunbury oil shale, have a coal-like structure with large fractions of condensed aromatic, polycyclic, and heterocyclic rings.18 These cyclic portions of the kerogen molecule are known to form chars upon pyrolysis and form the residues upon thermal solution.1° A sequential-reaction model represents the increasingly sluggish steps that occur as the tetralin solvent attacks the more and more unreactive structures that remain in the (18)Miknis, F. P.;Smith, J. W.Oil Shale Symp. h o c . 1982, 15th, 50-62.
525
shale matrix after the reactive bonds are cleaved to form soluble molecular fragments. The final residue at 430 "C, comprising approximately 20% of the kerogen for the Sunbury oil shale samples, is over 60% aromatic and contains the most unreactive portions of the kerogen. In the Green River oil shale, virtually all of the kerogen (over 95%) is reactive with tetralin at 400 "C. Almost no organic structures are so unreactive that they cannot be removed from the shale matrix as soluble fragments. Thus a sequential reaction model should not be needed to describe the bitumen formation. In fact, for a given temperature region, the bitumen formation is described by a single step. Glossary A Arrhenius pre-exponential factor min-' E Activation energy, cal/mol, kcal/mol fraction or organic material, dimensionless f K kerogen reaction rate constant, min-' k mass of organic material, g m n reaction order, dimensionless series approximation of exponential integral P(Z) gas constant, cal/(mol K) R T temperature K, "C t time, min Y yield, dimensionless yield at stoichiometric conversion Y* heating rate (dT/dt), "C/min P weight fraction, dimensionless 4 stoichiometric constant, dimensionless Y
Subscripts carbonaceous residue unreacted Sunbury kerogen KO first intermediate Sunbury kerogen K1 predicted P raw oil shale r spent or partially spent oil shale S T total unreacted species 0 1 first reaction step, first intermediate second reaction step, second intermediate 2
C
Isolation and Identification of Carboxylic Acid Salts as Major Components of Green River Oil Shale J. F. McKay* and M. S. Blanche Western Research Institute, Laramie, Wyoming 82071 Received June 15, 1987. Revised Manuscript Received September 8, 1987 Carboxylic acid salts were isolated from Green River shale by treating the shale in an autoclave with methanol and water at 400 "C (for 5 min to 1 h). Approximately 22 w t % of the organic matter in the shale was isolated in the form of carboxylic acid salts when the shale was treated for about 18 min. The carboxylic acid salts were characterized by spectroscopic techniques such as mass, infrared, and atomic absorption spectrometry and were shown to be a complex mixture of n-alkanecarboxylic acids, branched and cyclic aliphatic carboxylic acids, heteroatom-containing carboxylic acids, and dicarboxylic acids. The authors suggest that carboxylic acid salts are indigenous to the shale and can be isolated rapidly by solvent extraction at elevated temperatures. With treatment times of about 45 min or more the salts are converted to free carboxylic acids. Introduction Knowledge of chemical composition is important for assessing the commercial potential of shale resources as well as for the design of new or more efficient oil recovery 0887-0624/87/2501-0525$01.50/0
processes. It is especially useful for predicting product distributions from recovery processes and predicting potential environmental problems associated with recovery processes. The magnitude of the domestic shale resource 0 1987 American Chemical Society
