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Ind. Eng. Chem. Res. 2000, 39, 2592-2600
Predictive Heat Model for Australian Oil Shale Drying and Retorting Adam J. Berkovich,† John H. Levy,‡ Brent R. Young,*,§ and S. James Schmidt| Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, Sydney, Australia, Division of Energy Technology, Commonwealth Scientific and Industrial Research Organisation, Lucas Heights Science and Technology Centre, Menai, Australia, Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada, and Southern Pacific Petroleum (Development) Pty Ltd., Brisbane, Queensland, Australia
The exploitation of Australia’s oil shale reserves has been the subject of much research over several decades. The extraction of oil from shale is usually a thermal process and therefore oil shale processing is dominated by heat-transfer and thermal reactions. To fully develop any oil shale processing technology, knowledge of an oil shale’s thermodynamic properties is desirable. This paper presents a novel approach for the determination of an oil shale’s thermodynamic properties and subsequent development of a predictive heat model. This approach involves experimental determination of the thermodynamic properties of an oil shale’s individual components, in particular, the unique kerogen and clay minerals in Australian oil shale. The experimental data was then used to develop a predictive heat model for Australian oil shale based upon its kerogen and mineral, composition, and concentrations. The predictive heat model allows investigation of heat capacity, enthalpy, and total enthalpy for Australian oil shale during the process temperatures of drying and retorting. Introduction Australia has a vast reserve of oil shale deposited throughout its northeastern regions as well as smaller deposits located in its southeast. The current estimate of Australia’s in situ shale oil reserve is approximately 23 × 109 barrels, equivalent to 20 times Australia’s current crude oil reserves.1 In recent years there has been renewed interest in an Australian oil shale industry. In Australia, the proposed processing technique is the AOSTRA-Taciuk continuous flow retort system, which is based on rotary kiln technology.2 The processor consists of four compartments: drying/preheating of feed oil shale, retorting of dried oil shale, combustion of retorted oil shale, and heat recovery from combusted oil shale. Oil shale is fed into the drying/preheating stage of the processor where it is heated to ≈250 °C. At the preheat temperature, residual surface moisture as well as any water associated with the crystal structure of relevant minerals in the oil shale are liberated. The preheated oil shale (at 250 °C) then passes into the retort stage where it is mixed with recycled combusted oil shale (at 750 °C). The mixing of the preheated and combusted oil shale results in a retort temperature of ≈500 °C. In the retort, kerogen is pyrolyzed to predomi* To whom correspondence should be addressed. Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4. Phone: (403) 220-8751. Fax: (403) 282-3945. E-mail:
[email protected]. † University of Technology, Sydney. ‡ Commonwealth Scientific and Industrial Research Organization. § University of Calgary. | Southern Pacific Petroleum (Development) Pty Ltd.
nantly hydrocarbon vapor that is collected and transferred to the oil recovery section of the process. The organic pyrolysis reactions are accompanied by a number of other thermal reactions due to the mineral components in the oil shale. These reactions include the partial decomposition of pyrite, the dehydroxylation of the clay minerals, and the decomposition of some carbonate minerals.3 The retorted oil shale is then transferred to the combustion stage where the residual carbon from the oil shale is burnt at approximately 750 °C. The combusted oil shale is split into two streams. One stream is recycled through the retort to provide the necessary heat for retorting. The other stream is fed to a cooling zone where it transfers heat to the oil shale being fed into the preheat/drying stage. The continued development of the AOSTRA-Taciuk processor for oil shale processing requires appropriate process models. In a process dominated by heat-transfer and thermal reactions, knowledge of oil shale thermodynamic properties is important. The published examination of oil shale thermal characteristics related to processing dates back some 80 years.4,5 This work examined the reaction heats and heating requirements of whole Colorado oil shale and found that reaction heats and heating requirements were dependent on the oil shale composition. Colorado oil shale has also been the subject of much work on its thermal characteristics using differential scanning calorimetry and thermogravimetry.6-10 Comparisons between pyrolysis heats of Colorado and Devonian oil shale have also been made.7 The pyrolysis heats for Colorado oil shale were found to be lower because of the differences in mineralogy between Colorado and Devonian oil shales, in particular, the influence of pyrite found in the Colorado shales. Other work has shown a correlation between total measured enthalpy and oil
10.1021/ie990942g CCC: $19.00 © 2000 American Chemical Society Published on Web 06/13/2000
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shale grade; that is, the higher the oil yield for the shale, the higher the total measured enthalpy.8-10 The application of thermal analysis to coal and oil shale has been reviewed.11 Thermogravimetry and differential scanning calorimetry have been shown to be useful tools in determining the oil yield potential, reaction heats, and effect of minerals in different oil shales.12-15 Drop calorimetry has also been used to measure the heat of retorting for Green River oil shale up to 773 K.16,17 Thermal analysis has also been coupled with spectroscopy to investigate the retorting behavior of carbonaceous oil shales from the Nagoorin and Condor deposits in Australia.21 The thermal properties of Eastern and Green River oil shales from the United States have also been determined using a combination of thermodynamic data from the literature and the average concentrations of oil shale components, to sum the heat effects for an average shale.18,19 The heat capacities of kerogen and kerogen char were estimated using heat capacity data of model compounds, such as anthracene, polymeric organic materials, and graphite, and adjusting the heat capacities according to the hydrogen and carbon content.18,19 Australian oil shales have a more complex mineralogy than U.S. oil shales. For example, Australian oil shales from Stuart/Rundle are a mixture of kerogen, clays, carbonate minerals, silica minerals, and pyrite. The thermal decomposition, pyrolysis, and dehydroxylation of these oil shale components overlap in the temperature range from 673 to 823 K (400-550 °C).27,3 As a result, the overall mass loss, enthalpy, and heat capacity change for oil shale is a combination of all components. Previous experimental work in oil shale thermal characterization discussed above has treated oil shale as a single material. Consequently, thermal characterization measurements were made on whole oil shale and the data then mathematically treated (usually simultaneous linear regression) to determine the contributions of the individual components. This method is usually satisfactory for oil shales that have a simple mineralogy, such as the Green River oil shales, which have a carbonate mineral base and only one dominant clay mineral. However, this method cannot be applied satisfactorily to Australian oil shales, in this case Stuart oil shales, as the diverse mineralogy of Stuart oil shale makes measurement of thermodynamic properties difficult. Each component in the oil shale has a different concentration, different heat capacity, and different reaction heats, and the temperature range of thermal reactions overlap each other.28 This paper reports on a new experimental approach to oil shale thermal characterization, which involves treating oil shale as a mixture of different components that have individual thermal characteristics. In particular, the unique components of Australian oil shale, kerogen, smectite, kaolinite, and illite, are of the most interest. By chemical and physical separation of these unique components from whole oil shale, the thermodynamics of these individual components can be determined experimentally. The other more common mineral components found in Stuart oil shale, such as calcite, quartz, siderite, and feldspar, have well-known thermodynamic properties. For these components, thermodynamic data were obtained from databases in the literature.29-50 This study also employed the use of modulated
differential scanning calorimetry (DSC)51 for thermodynamic measurements. Modulated DSC is a relatively new calorimetric technique that is capable of quantitatively measuring heat capacity and enthalpy changes in a single experiment. This particular feature is unique to this technique and has important consequences for oil shale analysis. Siroquant52,53 is a software package for quantitative analysis of minerals by X-ray diffraction (XRD). It contains a specialized package for the analysis of clays.54 Siroquant has previously been applied successfully to Australian oil shales55 and was further developed in this study to determine mineral concentrations in oil shales, kerogens, and isolated clay mixtures. The thermal, spectroscopic, and literature data of the individual components were collated to develop a predictive heat model of Stuart oil shale. The model allows prediction of the thermodynamic properties of an oil shale based on its mineral and organic composition. The model can be directly applied or incorporated into oil shale process models and can be used to investigate the sensitivity of thermodynamic properties related to an oil shale’s composition. The predictive heat model was designed to be interactive, allowing the user to select any temperature range in the drying and retorting process temperature range. The model can be used in the process design, optimization, and control of industrial oil shale drying, retorting, and combustion plant. In process design, reliable estimates of the heat capacities and reaction enthalpies of raw, dried, and pyrolyzed oil shales are required for the determination of heat balances and the subsequent mechanical design of process equipment. The model can be directly incorporated into process models for process optimization by using the model to investigate the sensitivity of a process related to an oil shale’s composition. In a similar manner the model is useful in process control as it may be used to fine-tune the process operating conditions based on periodic laboratory oil shale composition measurements. Experimental Section Modulated differential scanning calorimetric (MDSC) analyses were performed on a TA Instruments MDSC 2920 instrument, with a working temperature range of 148 to 998 K. Samples were analyzed in aluminum pans with aluminum lids in an atmosphere of dry nitrogen flowing at 120 mL min-1. Nominal sample mass was 5 mg, which was measured to 1 µg with a microbalance. Samples were equilibrated at 273 K for 5 min and then heated at 5 K min-1 to 873 K. The modulation amplitude was (1 K with a period of 60 s. Temperature and enthalpy were calibrated using In, Sn, and Zn standards. Heat capacity was calibrated with a 99.97% sapphire standard. A TA Instruments SDT 2960 instrument was used for thermogravimetry. The samples were analyzed in open platinum pans at a heating rate of 5 K min-1 from ambient to 873 K in an atmosphere of dry nitrogen flowing at 120 mL min-1. Two types of oil shale samples were selected for this study, one set for kerogen isolation and the other set for clay mixture isolations. Samples with a high kerogen concentration were selected for kerogen isolation. Both brown and carbonaceous kerogens were isolated. The high kerogen concentration required minimum chemical treatment, as those minerals not affected by the chemi-
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Table 1. Combined Results for Siroquant and Chemical Analyses for Isolated Stuart Kerogens component concentration (% of kerogen) pyrite anatase miscellaneous minerals organic matter
K1
brown K2 K3
K4
4.7 5.9 3.4 7.9 0.7 0.4 0.7 0.3 0.8 0.7 1.6 0.7 93.8 93.0 94.3 91.1
carbonaceous K5 6.6 0.4 0.6 92.4
cal treatment would be concentrated to the least extent. The clay minerals in Stuart oil shale usually exist as mixtures of kaolinite, smectite, and illite in different ratios. The three clay types, kaolinite, smectite, and illite, in Stuart oil shale cannot be separated from each other. Therefore, a range of oil shales was selected for clay isolation to ensure a greater diversity in the composition for adequate characterization of the individual clays. Kerogen was isolated from oil shale using a HF/BF3 maceration technique56 where carbonate-type minerals are destroyed with hydrofluoric acid and boron trifluoride, generated from the reaction of hydrofluoric and boric acids. Pyrolyzed samples of kerogen and oil shale were also prepared by heating samples to 520 °C in an atmosphere of flowing nitrogen and then quenching the sample at room temperature while the nitrogen atmosphere was maintained. The clay minerals were isolated from oil shale using buffered acetic acid and hydrogen peroxide.57 Acetic acid destroyed carbonate minerals and hydrogen peroxide destroyed the kerogen. The result was a mixture of clay minerals and silica-type minerals such as quartz and feldspar. These heavier silica minerals were separated from the lighter clay mixture by centrifugation. The clay mixtures were comprised of kaolinite, smectite, and illite, in varying ratios. XRD patterns were collected using a Siemens Kristalloflex X-ray generator equipped with two powder cameras with Bragg-Brentano geometry. A Philips PW2276/20 X-ray tube was used at a power of 30 mA and 45 kV to produce cobalt X-rays. Samples were mounted in aluminum sample holders of dimensions 44mm long (in the plane of the X-rays), 12.5-mm wide, and 1.7-mm deep. XRD patterns were collected from 3.00° to 90.00°, at intervals of 0.04° 2θ using a count time of 10 s/interval. Results and Discussion Initially, 66 oil shale samples from the Stuart deposit were acquired and their XRD patterns measured. The patterns were analyzed using Siroquant to determine mineral concentrations in the oil shales. Samples to be included in the study were selected using these data. Five samples were selected for kerogen isolation and 16 samples were selected for clay mineral isolation. The purity of kerogen and clay minerals isolated from Stuart oil shale was assessed using XRD and chemical analyses. Siroquant analysis of the XRD patterns determined the concentrations of any mineral inclusions in the isolated materials. XRD patterns of isolated brown and carbonaceous kerogen showed peaks arising from anatase and pyrite: minerals which are inert to the chemical treatments used to isolate the kerogen. The combined results of the Siroquant and chemical analyses are shown in Table 1, while Table 2 presents Siroquant analysis results for pyrolyzed Stuart kerogens. These concentrations were subsequently used to
Table 2. Siroquant Analysis Results for Pyrolyzed Kerogens (Including Organic Matter) sample
PK1
PK5
pyrite (% of pyrolyzed kerogen) pyrrhotite (% of pyrolyzed kerogen) anatase (% of pyrolyzed kerogen) miscellaneous minerals (% of pyrolyzed kerogen) organic matter (% of pyrolyzed kerogen)
10.0 9.6 3.0 6.7 70.7
11.4 7.3 1.3 3.3 76.7
Table 3. Siroquant Results for Clay Minerals Isolated from Stuart Oil Shale sample
smectite kaolinite illite quartz albite cristobalite (%) (%) (%) (%) (%) (%)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16
31.0 7.5 11.6 0.4 43.2 41.3 48.9 38.7 30.8 10.9 73.7 32.4 23.6 37.7 42.6 34.6
19.9 18.9 38.2 11.7 14.3 17.4 10.1 21.7 22.0 28.9 10.6 15.1 20.2 21.3 21.9 17.7
28.1 25.8 20.1 20.7 9.0 8.8 28.7 11.6 13.7 21.7 12.5 24.6 15.5 9.0 8.8 9.8
12.0 41.4 17.8 58.6 25.7 20.7 8.9 14.9 25.0 22.6 2.0 20.1 31.0 25.8 21.1 31.6
2.9 5.7 4.0 8.6 7.4 9.4 2.4 5.8 5.8 4.4 1.2 5.2 6.4 5.8 5.2 6.3
6.0 0.6 8.3 0.0 0.4 2.5 1.0 7.3 2.8 11.5 0 2.5 3.3 0.5 0.5 0.0
Table 4. Thermogravimetric Mass Loss at 773 K for Isolated Kerogens kerogen sample
initial mass (mg)
final mass (mg)
mass loss (%)
K1 K2 K3 K4 K5
10.21 10.42 10.49 19.74 19.13
2.22 2.30 2.10 5.23 5.99
78.2 78.0 80.0 73.5 68.7
Table 5. Dehydroxylation Mass Losses for Clays Isolated from Stuart Oil Shale sample
dehydroxylation mass loss (%)
sample
dehydroxylation mass loss (%)
C1 C2 C3 C4 C5 C6
10.9 14.2 3.0 8.6 5.4 5.5
C7 C8 C9 C10 C11 C12
17.1 15.9 17.6 15.6 13.4 19.4
correct measured thermodynamic data on isolated Stuart kerogen and pyrolyzed kerogen. The X-ray patterns of the isolated clays also showed low concentration of included minerals: quartz, cristobalite, and albite. These silica-based minerals were unaffected by the chemical treatments and could not be completely separated from the clays by centrifugation. However, the concentrations of these minerals are relatively low and corrections for their effects could be made in subsequent analyses. Siroquant results for the isolated clays also show a diverse range of clay concentrations in each sample (Table 3). Results of thermogravimetric analyses performed on the isolated kerogen and clay samples are summarized in Tables 4 and 5. The raw heat capacity of pyrolyzed brown and carbonaceous isolated kerogen was measured using modulated DSC over the temperature range 273-773 K (0500 °C). The heat capacity of pure pyrolyzed kerogen (PK) was calculated by subtracting the heat capacities of the mineral matter components from the raw heat
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2595 Table 6. Heat Capacity Equation Coefficients (J g-1 K-1) for Pyrolyzed Kerogens sample coefficients
PK1
PK5
a b c d e R2 std. dev.
2.503 -1.781 × 10-2 6.690 × 10-5 -9.100 × 10-8 4.355 × 10-11 1.0000 2.792 × 10-10
-7.490 × 10-2 7.535 × 10-3 -1.889 × 10-5 3.035 × 10-8 -1.906 × 10-11 0.99877 0.008
capacity data. Heat capacity data for R-alumina were used for the miscellaneous minerals, which in any case represent only a small proportion of the material. These corrected data were fitted to a fourth-order polynomial equation of the type Cp ) a + bT + cT2 + dT3 + eT4. The coefficients are given in Table 6. The heat capacity of kerogen isolated from Stuart oil shale was measured using modulated DSC. However, at temperatures above 623 K, pyrolysis of the kerogen with the accompanying loss of mass began to influence the heat capacity data and a different approach had to be used. The heat capacity of isolated kerogens was measured up to 673 K (the beginning of pyrolysis). These data were corrected for moisture present in the samples, measured by TG/DTA, so that the heat capacity determinations were reported on a dry basis. The measured heat capacities were also corrected for mineral inclusions (Table 1). The adjusted data were fitted to the fourth-order polynomial Cp ) a + bT + cT2 + dT3 + eT4 to give the coefficients in Table 7. The temperature range used for fitting was 273-623 K because the onset of pyrolysis began to influence the results in the temperature region above 623 K (350 °C). The initial attempts to measure heat capacities of kerogens with the modulated DSC resulted in very low values of heat capacity data after pyrolysis. These values were often negative and quite different from the values for the heat capacity of the pyrolyzed kerogen as measured above, after allowance was made for the change in mass. Further investigation showed that the modulated DSC heat flow curve base line after pyrolysis was wrongly positioned because the instrument could not cope with the large mass losses of 69%-80% that occur during kerogen pyrolysis.28 A possible solution was to dilute the kerogens with R-alumina and measure the heat capacities of the mixtures, such that the alumina would help retain sufficient material in the sample pan during and after pyrolysis to give good coupling of the sample with the pan and hence the thermocouple sensor. This was attempted, but because of the loss of pyrolysis products, it was not possible to encapsulate the samples properly, resulting in heat capacity data that were still inconsistent with pyrolyzed kerogen data. The following method was employed to obtain consistent data. As stated previously, accurate heat capac-
Figure 1. Heat capacity curves for Stuart kerogen K1 (solid) and K5 (dash) between 273 and 773 K.
ity data had been measured for the kerogen up to the temperature prior to pyrolysis beginning at about 623 K. Heat capacity data for pyrolyzed kerogen have also been accurately measured across the full temperature range, to 773 K. It was reasonable to assume that the change in heat capacity between kerogen and pyrolyzed kerogen would be proportional to the change in mass of the kerogen. This change was therefore modeled using the thermogravimetric data obtained under the same heating conditions for the same sample (Table 4). Thus, the heat capacity value of kerogen at 623 K was taken as the initial value. The heat capacity value for the same kerogen pyrolyzed at 773 K (in J gpyrolyzed kerogen-1) was converted to J gkerogen-1 on the basis of mass loss observed by thermogravimetry (Table 4), corrected for the included minerals and taken as the final value. The difference between the initial and final values was then multiplied by the fraction of mass loss occurring at 1 K intervals between 623 and 773 K, and each value was subtracted from the initial value to construct the model curve. These data were poorly fitted with a fourth-order polynomial equation so an eighth-order polynomial equation was used over this temperature range to give the coefficients as shown in Table 8. The full heat capacity curves for a kerogen sample K1 (brown) and K5 (carbonaceous) are shown in Figure 1. Initial attempts to measure the enthalpy of pyrolysis for isolated Stuart kerogens using modulated DSC also gave unexpectedly low values, as found for the heat capacity measurements. These low results were attributed to the very large mass loss in the sample during pyrolysis and to the very endothermic nature of the pyrolysis reaction. To solve this problem, the kerogens were diluted with alumina before the enthalpies of pyrolysis were measured. Alumina is inert over this temperature range and
Table 7. Heat Capacity Equation Coefficients (J g-1 K-1) from 273 to 623 K for Isolated Kerogens kerogen sample coefficients
K1
K2
K3
K4
K5
a b c d e R2 std. dev.
-5.185 5.781 × 10-2 -1.809 × 10-4 2.632 × 10-7 -1.423 × 10-10 0.99418 0.027
-2.674 2.356 × 10-2 -3.816 × 10-5 2.950 × 10-8 -1.008 × 10-11 0.99875 0.014
-7.623 7.334 × 10-2 -2.346 × 10-4 3.361 × 10-7 -1.777 × 10-11 0.99858 0.012
-4.087 4.069 × 10-2 -1.245 × 10-4 1.753 × 10-7 -9.161 × 10-11 0.99875 0.011
-5.260 5.109 × 10-2 -1.408 × 10-4 1.887 × 10-7 -9.840 × 10-11 0.99779 0.017
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Table 8. Heat Capacity Equation Coefficients (J g-1 K-1) from 623 to 773 K for Isolated Kerogens sample coefficients
K1
K5
a b c d e f g h i R2 std. dev.
-6.765 × 105 5.360 × 103 -1.671 × 101 2.427 × 10-2 -1.149 × 10-5 -1.134 × 10-8 1.680 × 10-11 -6.762 × 10-15 5.326 × 10-19 0.9995 0.021
-4.948 × 105 4.002 × 103 -1.289 × 101 2.008 × 10-2 -1.270 × 10-5 -3.690 × 10-9 9.444 × 10-12 -4.007 × 10-15 2.599 × 10-19 0.9999 0.009
Table 9. Enthalpies of Pyrolysis of Stuart K1 Kerogen Diluted in r-Alumina concentration of kerogen (%)
∆H (J g-1)
30 50 60
27.20 121.8 179.0
concentration of kerogen (%)
∆H (J g-1)
70 80
188.1 267.0
Table 10. Regression Line Parameters for Stuart 65219K Kerogen in r-Alumina slope intercept R2 number of observations degrees of freedom
parameter
standard error
4.56 -108.0 0.974 5 3
0.41 16.6
provided bulk to the sample, thereby reducing the mass change occurring in the sample during the experiment while sample contact with the sensor was maintained. Diluted kerogen samples were prepared in concentrations of 30%, 50%, 60%, 70%, 80% w/w kerogen. Heat flow curves for the diluted samples were measured by modulated DSC. The pyrolysis peak from the heat flow curve was integrated to calculate enthalpy of each diluted kerogen sample and these data are presented in Table 9. A linear regression between measured enthalpy and kerogen concentration was performed and the regression parameters are shown in Table 10. From these coefficients, a raw enthalpy value for a kerogen concentration of 100% was calculated as 348.2 J g-1. This value must then be corrected for mineral inclusions in the kerogen (Table 1).
During pyrolysis, part of the pyrite is converted to pyrrhotite, assumed to be by reduction with hydrogen generated in the organic cracking process.18,60,61 The ∆H for this reaction was calculated as 290.4 J g-1, at the peak temperature of 748 K (475 °C), using Cp and ∆H data for pyrite and pyrrhotite. The analysis of the pyrolyzed kerogen (Siroquant data) shows that the reaction does not proceed to completion, being only 55.4% complete at 793 K (520 °C). When this same factor is used for kerogen from sample 65219K, ∆Hpyrite for 65219K was calculated as 160.9 J gpyrite analyzed-1. Then, when the concentration of pyrite in the kerogen is taken into account (5.9%), ∆Hpyrite for 65219K2 was 7.56 J gkerogen-1. In addition, the dilution effect of the pyrite, and for the anatase and miscellaneous minerals that were assumed inert during pyrolysis, also has to be accounted for. The result is a ∆H of 364 J g-1 for the enthalpy of pyrolysis of Stuart brown kerogen. Although heat capacities were measured on the clay mixtures isolated from Stuart oil shale, the heat capacities of the individual clays were not calculated from the experimental data. The errors associated with individual heat capacities (deconvoluted from heat capacities measured on the mixtures) were large compared to the differences among the heat capacities of the individual clays. This occurred because the constitutions of the various clays are all so similar: the chemical formulas of smectite, kaolinite, and illite all consist of aluminum, silica, and oxygen (that is, an aluminosilicate-base structure) with minor contributions by other substituents, such as potassium, calcium, or iron. Therefore, the value of the heat capacity of the clay is influenced more by the alumino-silicate-base structure (which is similar for all clays) rather than by the substituents. This was evident from the literature heat capacity data for smectite, kaolinite, and illite,36,45,49 where capacity values for these clays are similar. Consequently, heat capacity data from the literature were used for the individual clays. Heats of dehydroxylation were measured using modulated DSC for mixtures of smectite, kaolinite, and illite isolated from Stuart oil shale. These results are summarized in Table 11 which also shows the dehydroxylation enthalpies corrected for the mineral inclusion concentrations (Table 3) and the enthalpy change associated with the R to β transition of quartz. The normalized concentrations of smectite, kaolinite and
Table 11. Dehydroxylation Enthalpies for Clays Isolated from Stuart Oil Shale
sample
dehydroxylation peak temperature (K)
x1, normalized concentration smectite
x2, normalized concentration kaolinite
x3, normalized concentration illite
∆Hdehyrox’n (measured) (J g-1)
y, ∆Hdehyrox’n (corrected) (J g-1)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16
726.77 738.03 728.70 766.98 759.62 761.20 732.86 734.89 729.19 718.89 743.77 729.32 726.61 788.18 773.70 732.19
0.392 0.144 0.166 0.012 0.650 0.612 0.558 0.538 0.463 0.177 0.761 0.449 0.398 0.554 0.581 0.557
0.252 0.362 0.546 0.357 0.215 0.258 0.115 0.301 0.331 0.470 0.110 0.209 0.341 0.313 0.299 0.285
0.356 0.494 0.288 0.631 0.135 0.130 0.327 0.161 0.206 0.353 0.129 0.341 0.261 0.132 0.120 0.158
201.5 139.3 231.2 120.5 218.8 247.4 220.4 178.4 187.5 233.7 225.6 219.8 249.9 240.0 247.9 249.1
253.2 258.4 328.1 348.8 325.0 363.7 250.3 245.6 278.3 376.2 233.0 300.7 416.0 349.3 335.5 395.8
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2597 Table 12. Simultaneous Linear Regression Results for Isolated Clays constant std error of y R2 number of observations degrees of freedom x coefficients error of coefficients
0 56.4 0.2230 19 16 265.3 42.8
535.2 105.2
146.0 92.4
Table 13. Dehydroxylation Enthalpies for Smectite, Kaolinite, and Illite clay
∆Hdehydroxylation (J g-1)
error (J g-1)
% error (J g-1)
smectite kaolinite illite
265 535 146
(43 (105 (92
16 20 63
illite used for the regression analysis described below are also shown in Table 11. The dehydroxylation enthalpies for the individual clays, smectite, kaolinite, and illite, were calculated using a simultaneous linear regression analysis of the measured experimental data. The normalized concentrations of smectite, illite, and kaolinite were assigned to regression ranges x1, x2, and x3, respectively. The corrected enthalpies for each sample from each experiment were assigned the regression range, y. A simultaneous linear regression was performed to fit values of x1, x2, and x3 to y to calculate coefficients of x (the regression line was forced through the origin). The coefficients of x were equal to the dehydroxylation enthalpy for each individual clay. The results of the simultaneous linear regression analysis and subsequent dehydroxylation enthalpy values are shown in Tables 12 and 13. Inspection of the regression data shows relatively high errors for the fitted coefficients and subsequent dehydroxylation enthalpies. This is mirrored in the value of R2. There are a number of possible reasons for this. First, errors have been introduced by the difficulty in measuring the base line on the high-temperature side of the peak, resulting in low values for the illite, as it was possible that part of this peak was beyond the upper temperature limit of the calorimeter. Indeed, the enthalpy for illite has the highest relative error. Second, the error calculated for simultaneous linear regression analyses is dependent on the number of degrees of freedom in the data set. It is possible that if more samples were measured, there would be a reduction in the calculated error. Finally, it is quite probable that the individual clays within the clay mixtures are not identical, and this leads to significant variation in the enthalpies, which are reported in the errors. For example, it is difficult to believe that all the smectite clays have identical formulas given that the degree of substitution may vary within the deposit given that material was laid down at different geological times. Even kaolinite, which has a relatively uniform chemical formula, has been shown to exhibit different enthalpies of dehydroxylation dependent upon the crystallinity of the sample.62-66 Given these considerations, however, this is the first time that the dehydroxylation enthalpies have been measured for the clays in oil shales and represent a major step forward in our ability to be able to calculate the thermochemical properties of oil shales. The enthalpy associated with dehydration of oil shale was also determined using modulated DSC. An average value of 2196 J g-1 at a peak temperature of 110 °C for the loss of water was found for 12 oil shale samples.
Table 14. Sources of Error in the Predictive Heat Model error source
total error (%)
literature heat capacity data literature enthalpy data XRD/Siroquant MDSC heat capacity measurements MDSC enthalpy measurements MDSC temperature measurements polynomial fitting
8 5 3 1-3
