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Anal. Chem. 1981, 53, 121-122
Characterization of Oil Shales by Differential Scanning Calorimetry Sir: The need to develop alternative energy sources to petroleum has underlined the importance of oil shale as an economically viable substitute. From an energy viewpoint, the most important constituent of oil shales is the organic matter which comprises of both bitumen and kerogen ( I ) . The amount of indigenous organic matter in a shale deposit determines the economic value of that particular geological formation. The Fischer assay (2) has been the traditional method for estimating the potential oil yield of oil shale deposits. This method, however, is quite time consuming. Therefore, alternative analytical techniques have been proposed in recent years for the characterization of oil shales. These include laser pyrolysis-gas chromatography (3, 4), microwave dielectric spectroscopy (5), thermal chromatography (6),and nuclear magnetic resonance (NMR) techniques (7-9). In this paper, we present a differential scanning calorimetry (DSC) technique for the characterization of oil shales. The linear correlation that is observed between the enthalpy of decomposition of the organic matter (AH)and oil yields for Green River oil shalea suggests that thistechnique might be useful as a rapid screening tool for resource evaluation of an oil shale deposit. EXPERIMENTAL SECTION Oil shale blocks, from which the test samples were prepared, originated from the US.Department of Energy Mine at Anvil Point near Rifle, CO. The massive blocks were cored to suitable dimensions (nominally 2.5 cm diameter and 1cm thick). Specific gravity measurements were then carried out on these cylindrical cores to determine the organic content (IO). Selected samples were also assayed at Laramie Energy Technology Center, WY, by the pulsed NMR technique described elsewhere (7). Oil yields obtained by the two methods on identical corea were in agreement with the limits of experimental error (vide infra). The cored shale samples were crushed to particles which passed through 100-mesh sieves. Ten to fifteen milligram batches of these crushed shale particles were then subjected to enthalpy measurements in the DSC assembly. A sample of kerogen concentrate was obtained from Laramie Energy Technology Center. DSC measurements were carried out on a DuPont 990 thermal analysis system fitted with the DSC accessory module. All measurements were carried out in a flowing atmosphere of prepurified N2. A heating rate of 10 "C/min was employed for the enthalpy determinations. The fusion endotherm of zinc (mp 419.4 "C, AHf = 27.05 cal/g) was used as the calibration standard. All measurements were carried out in duplicate. RESULTS AND DISCUSSION Figure 1shows a representative DSC thermogram obtained for oil shale. The endothermic peak in the temperature range 250-450 "C corresponds to the thermal decomposition of oil shale kerogen (1). The area under this peak (i.e., the measured enthalpy value) should be directly proportional to the amount of kerogen in the shale in the absence of minerals which are thermally active in this temperature range (vide infra). Figure 2 illustrates the correlation between the measured enthalpy values and the corresponding oil yields (OY) for Green River oil shales. Representative mean deviations in the AH values are shown as error bars. We estimate the precision of an oil yield determination as f8 L/metric ton. A least-squares fit of the experimental data yields an equation of the form
AH = -0.90
+ 0.25(0Y)
with a correlation coefficient of 0.99.
(1)
Heating Rate : IO'C /min Atmosphere; Flawing N, Sample Mass: 1 3 . 3 m g
f
0.5
I
0
300 Ternperolure
5. 3
400
,OC
Figure 1. Representative DSC thermogram on Green Rhw oil shale (oil yield: 316 L/metric ton). The sample was heated in an open aluminum pan with an Mentlcal empty pan as refe'ence.
z 04 0 40
160 160
280 400 011Yield, l i t e r s / metric tan
520
640
Figure 2. Correlation of the enthalpy of decomposition, AH, with oil yields for Green River oil shales. The sample with 011 yield corresponding to 572 Llmetric ton is a kerogen concentrate.
The present data and eq 1 are strictly valid only for oil shales of the Green River formation. Compositional heterogeneity of the organic matter and variations in the mineral matrix from one depositional area to the other require that calibration curves similar to those shown in Figure 2 be established for each shale formation. It is also pertinent to note that the present shale samples contained no minerals that decompose below 500 "C. The presence of minerals such as nahcolite, analcite, dawsonite, and pyrite would bring about additional thermal effects which have to be corrected for in the analysis of the organic matter (1). The linear correlation that is observed in the present study between decomposition enthalpies and oil yields is indicative of the potentiality of the DSC technique as an assay tool for rapid screening of oil shales. By use of this technique, a sample can be assayed in about 1 h. More importantly, this technique offers the advantage of being relatively simple and also does not involve sophisticated measurement procedures. The technique is also readily amenable to automation and thus suited for routine applications, unlike some of the assay methods previously reported in the literature for characterizing oil shales (3-9).
LITERATURE CITED (1) Rajeshwar, K.;Nottenburg, R. N.; DuBow, J. B. J . Meter. Sci. 1070, 74. 2025. (2) Stanfield, K. E.;Frost, I. C. Rep. Invest.-U.S., Bur. Mines 1046. No. 3977. (3) Biscar, J. P. Anal. Chem. 1071, 43, 982.
0003-2700/81/0353-0121$01.00/00 1980 American Chemical Society
Anal. Chem. 1981, 53, 122-124
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(4) Hanson, R. L.; Vanderborgh, N. E.; Brookins, D. G. Anal. Chem. 1975, 47, 335. (5) Judzis, A., Jr. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 1978. (6) Reed, P. R., Jr.; Warren, P. L. 0.Cob. Sch. Mines 1974, 69, 221. (7) Miknis, F. P.; Decora, A. W.; Cook, G. L. R e p . Invest.-U.S. Bur. Mines 1976, No. 7984. (8) Resing, H. A.; Garroway, A. N.; Hazlett, R. N. Fuel '978, 57, 450. (9) Vitorovic, D.; Vucelic, D.; Gasic, M. J.; Juranic, N.; Macura, S. Org. Geochem. 1979, 1, 89. (10) Smith, J. W. Report of Investigations, LERC/RI-76/6; Laramle Energy Technology Center: Laramie WY, 1976.
Department of Electrical Engineering Colorado State University Fort Collins. Colorado 80523
RECEIVED for review April 11, 1980. Accepted October 10, 1980. This research received generous financial support from the U S . Department of Energy and the Laramie Energy Technology Center. Reference to a trade name or product does not imply endorsement by the authors or by the US. Department of Energy.
'Present address: Paraho Oil Shale Corp. Grand Junction, CO 81501.
Krishnan Rajeshwar* Donald B. Jones' Joel B. DuBow
AIDS FOR ANALYTICAL CHEMISTS Purification of Cylinder Gases Used in Solvent Evaporation for Trace Analysis T. J. Nestrick' and L. L. Lamparski Analytical Laboratories, Dow Chemical U.S.A., Midland, Michgn 48640
Solvent-removal steps to effect residue concentration are commonly encountered in trace analytical procedures. When the compounds being determined are reasonably nonvolatile, the analyst is afforded some latitude in the method by which these concentrations may be accomplished. Two common approaches are the use of a Kuderna-Danish evaporative concentrator or a flowing stream of clean gas. The Kuderna-Danish system involves a classical fractional distillation, requires residue transfer to appropriate equipment, and is the more applicable system when dealing with larger solvent volumes (1-3). A flowing stream of clean gas can be advantageous in those cases where small volumes of volatile solvents (
