Energy & Fuels 1997, 11, 945-950
945
Determination of Volatile Hydrocarbons in Coals and Shales Using Supercritical Fluid Extraction and Chromatography Wenbao Li,† Iulia M. Lazar, Yanjian J. Wan, Steven J. Butala, Yufeng Shen, Abdul Malik,‡ and Milton L. Lee* Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700 Received October 9, 1996. Revised Manuscript Received June 25, 1997X
Conventional analytical techniques, such as headspace gas chromatography and Soxhlet extraction, can provide compositional information for the gaseous (C1-5) and heavy (C15+) hydrocarbon constituents, respectively. The volatile (C6-14) hydrocarbons, if present, usually go undetected because of volatility fractionation and loss. In this study, supercritical CO2 was used to extract the C6-C14 volatile hydrocarbons from pulverized coal samples. Capillary column gas chromatography/mass spectrometry was used to identify the mixture components, and packed capillary column supercritical fluid chromatography was used to separate and quantify the aliphatic and aromatic hydrocarbon class fractions. It was found that the compositions of the light hydrocarbon fractions included several homologous series of normal and branched aliphatic hydrocarbons, cyclic and aromatic hydrocarbons, and alkyl-substituted benzenes and naphthalenes; the concentrations of these volatile hydrocarbons ranged between 0.01 and 0.2 wt % of the bulk material for different coal and shale samples.
Introduction The extraction of coals with supercritical fluids (SFE), also referred to as supercritical gas extraction, has been applied for a variety of reasons during the past 20 years.1,2 The primary reasons include the production of liquid fuels from coal, the elucidation of coal structure and mechanisms of coalification, and the selective removal of sulfur from coal. When SFE has been applied to coal, usually an organic solvent under supercritical conditions was used. The process is analogous to both solvent extraction and distillation; supercritical gas extraction is usually carried out at 350-450 °C and at a pressure of 10-20 MPa.2 Therefore, supercritical gas extraction generally extracts heavy materials as well as volatile components, similar to an organic solvent, and results in possible thermal degradation of the coal macromolecular network.3 SFE with CO2 offers several advantages over conventional supercritical gas extraction and solvent extraction. Supercritical CO2 has higher diffusivity and lower viscosity compared to liquid solvents, which should result in improved mass transfer properties during extraction. The solvent strength of supercritical CO2 is dependent on its temperature and pressure, which can be easily manipulated to extract certain classes of compounds. Carbon dioxide is relatively nonreactive, * Author to whom correspondence should be addressed. † Current address: Haskell Laboratory, DuPont Central Research and Development, P.O. Box 50, Newark, DE 19714. ‡ Current address: Department of Chemistry, University of South Florida, Tampa, FL 33620. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Kershaw, J. R. J. Supercrit. Fluids 1989, 2, 35-45. (2) Olcay, A. In New Trends in Coal Science; Yu¨ru¨m, Y., Ed. Kluwer Academic Publishers: New York, 1988; pp 401-415. (3) Chang, H. C. K. Ph.D. Dissertation, Brigham Young University, 1989.
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nonpolar, nontoxic, available in purified form, and has a low critical temperature. These properties make supercritical CO2 an ideal vehicle for extraction of nonpolar hydrocarbons and allow the extraction to be performed at relatively low temperatures to avoid any possible thermal degradation. Supercritical CO2 has been used successfully to extract polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and aliphatic hydrocarbons from different matrices.4-7 The object of this study was to develop a method for extracting and analyzing relatively light hydrocarbons (C6-14) from coal which would provide for (a) quantitative trapping of the volatile extracts and (b) rapid grouptype analysis of the collected fractions. Conventional analytical techniques such as headspace gas chromatography and solvent extraction can only provide information for the gaseous (C1-5) and heavy hydrocarbon (C15+) constituents.8,9 The C6-14 fraction usually goes undetected because of volatility loss. In order to address this problem, a method for using supercritical CO2 to extract the C6-14 hydrocarbons from coal, followed by group-type separation of the extracts with packed capillary column supercritical fluid chromatography (SFC), was developed in this study. Twenty different (4) Hawthorne, S. B.; Krieger, M. S.; Miller, D. Anal. Chem. 1989, 61, 736-740. (5) Cross, R. F.; Ezzell, J. L.; Porter, N. L.; Richter, B. E. Am. Lab. 1994, Aug, 12-17. (6) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem. 1993, 65, 2549-2551. (7) Brooks, M. W.; Uden, P. C. J. Chromatogr. 1993, 637, 175-179. (8) Rao, B. R. Determination of the Maximum Emissions from Storage Tanks for Heavy Fuel Oil. In Applied Headspace Gas Chromatography; Kolb, B., Ed., Heydon and Sons, Ltd.: New York, 1980; Chapter 7. (9) Chang, H-C. K.; Bartle, K. D.; Markides, K. E.; Lee, M. L. Structural Comparison of Low Molecular-Weight Extractable Compounds in Different Rank Coals using Capillary Column Gas Chromatography In Advances in Coal Spectroscopy; Meuzelar, H., Ed., Plenum Press: New York, 1991.
© 1997 American Chemical Society
946 Energy & Fuels, Vol. 11, No. 5, 1997
Li et al.
Table 1. Hydrocarbon Group-Type Quantitation of Low Molecular Weight Hydrocarbons (C6-C14) in Selected United States Coals and Interbedded Shale Kerogensa,b sample
location (county, state)
water (%)
aliphatic (%)
aromatic (%)
Beulah-Zap coalc Wyodak-Anderson coalc Blind Canyon coalc Basal Fruitlandd Basal Fruitland shale kerogend Intermediate Fruitland coale Intermediate Fruitland shale kerogene Basal Fruitland coalc Basal Fruitland shale kerogenc Basal Fruitland coalf Basal Fruitland shale kerogenf Illinois No. 6 coalc Pittsburgh No. 8 coalc Lewiston-Stockton coalc Upper Freeport coalc Pottsville coalg Pottsville coalh Pottsville coali Pottsville shale kerogeni Pocahontas No. 3 coalc
Mercer, ND Campbell, WY Emery, UT San Juan, NM San Juan, NM La Plata, CO La Plata, CO La Plata, CO La Plata, CO La Plata, CO La Plata, CO St. Clair, IL Greene, PA Logan, WV Indiana, PA Jefferson, AL Jefferson, AL Jefferson, AL Jefferson, AL Buchanan, VA
28.5 21.6 3.3 0.5 14.8 0.0 k 0.4 k k k 4.5 0.4 0.3 0.3 0.1 0.2 0.1 0.6 k
0.01 0.04 0.20 0.13 0.07 0.05 0.12 0.05 0.11 0.08 0.14 0.03 0.15 0.04 0.07 0.03 0.04 0.03 0.04 0.05
j 0.06 0.08 0.04 0.02 j 0.03 0.01 0.03 0.01 0.02 0.01 0.09 0.01 j j j j 0.01 0.08
a Based on wt % of total coal or shale. b Determined from supercritical fluid chromatographic peak area measurements. Values contain minor contributions from hydrocarbons larger than C15. c Premium coal sample from Argonne National Laboratory. d Hamilton No. 3 well, San Juan Basin (30 T32N R10W). e Valencia Canyon Southern Ute No. 32-1 well, San Juan Basin (32 T33N R11W). f Southern Ute Tribal H-1 well, San Juan Basin (18 T32N R10W). g Pratt, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). h Mary Lee, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). i Black Creek, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). j Means not detected or less than 0.01%. k Information unavailable.
Figure 1. Schematic diagram of the supercritical fluid extraction system.
US coals and shale samples covering a broad range of geologic age were analyzed using this new method. Experimental Section Supercritical Fluid Extraction. SFE with neat CO2 (Scott Specialty Gases, Plumsteadville, PA) was performed offline using a Lee Scientific Model 501 SFC system (Dionex, Sunnyvale, CA). A 55 mm × 9.2 mm i.d. stainless steel extraction cell rated to 34.5 MPa and containing a volume of 3.5 mL (Dionex, Salt Lake Technical Center, Salt Lake City, UT) was used for all extractions within a pressure range of 10.1-35.5 MPa. The collection vials were home-built from 1/8 in. stainless steel tubing, with dimensions of 250 mm × 3.0 mm i.d. A simplified schematic diagram of the SFE system is shown in Figure 1. In the case that there was not enough coal sample to fill the extraction cell, silanized glass beads (25 µm diameter; Supelco, Bellefonte, PA) was used to take up the remaining cell void volume. In order to further minimize any dead volume, a 200 µm i.d. fused silica capillary (Polymicro Technologies, Phoenix, AZ) was used after the extraction cell to conduct the extraction fluid to the collection vial. A 15 µm i.d. fused silica capillary (Polymicro Technologies) was used as a flow restrictor. Spectral grade carbon disulfide (EM Science, Gibbstown, NJ), maintained at -5 °C with a cooling system, was used for collection of extracts.
Supercritical Fluid Chromatography. The SFC apparatus used for group-type separation was the same as is described in ref 10. Timed-split injection with 1.2 s valve-open time was used for sample introduction. Several fused silica capillary columns (98 cm × 200 µm i.d., Polymicro Technologies) packed with 10 µm (60 Å) silica particles (Keystone Scientific, Bellefonte, PA) were used to prepare the analytical columns. Each column was prepared using a CO2 slurry packing method.11 A 35 cm × 10 µm i.d. deactivated fused silica capillary was used as a flow restrictor for the SFC. Group separations were conducted at a constant temperature of 45 °C and constant pressure of 25.3 MPa. Gas Chromatography/Mass Spectrometry. Individual components in the extracts were identified using an HP5890 gas chromatograph equipped with an HP5970 mass selective detector (Hewlett-Packard, Wilmington, DE). A 25 m × 200 µm i.d. fused silica open tubular column coated with SE-S4 (df ) 0.25 µm) was used as the separation column. High-purity helium (99.99%) was used as carrier gas. The column temperature was programmed from 40 to 300 °C at 2.5 °C/min after an initial 10 min isothermal period. Peak assignments were made by comparison of sample component retention times, elution patterns, and mass spectra with those of standard compounds. Standard Chemicals and Materials. All chemical standards (99%) were purchased from Aldrich (Milwaukee, WI) and used as received. They include n-alkanes from C7 to C25, methylcyclohexane, cis-1,2-dimethylcyclohexane, cis- and trans1,4-dimethylcyclohexane, 1,4-dimethyl-1-cyclohexene, propylcyclohexane, ethylcyclopentane, cycloheptatriene, bicylo[2.2.1]hepta-2,5-diene, 1-octene, toluene, 2-ethyltoluene, mesitylene, sec-butylbenzene, 1,2,3,4-tetrahydronaphthalene, 2-methylnaphthalene, phenanthrene, biphenyl, p-terphenyl, acenaphthylene, fluorene, and 2-ethyltoluene. Sources of the coal and shale samples are listed in Table 1. Analytical Procedure. Before performing an extraction, the extraction cell was washed with a series of organic solvents (tetrahydrofuran, methylene chloride, and carbon disulfide) and then dried at 120 °C in an oven. This procedure was (10) Li, W.; Malik, A.; Lee, M. L.; Jones, B. A.; Porter, N. L.; Richter, B. E. Anal. Chem. 1995, 67, 647-654. (11) Malik, A.; Li, W.; Lee, M. L. J. Microcol. Sep. 1993, 5, 361369.
Volatile Hydrocarbons in Coals and Shales
Energy & Fuels, Vol. 11, No. 5, 1997 947
Table 2. General Composition of the Supercritical CO2 Extracts of Coal and Shale Samples peak no.
compounds
peak no.
compounds
O 0 1 2 3 4 5 6 7 8 9 10 11 12 13
n-alkane branched alkane C1-cyclohexane C2-cyclopentane C3-cyclopentane toluene C2-cyclohexane C4-cyclopentane C3-cyclohexane C2-benzene C3-cyclohexane C3-benzene C4-cyclohexane C4-benzene C5-benzene
14 15 16 17 18 19 20 21 22 23 24 25 26 27
C5-cyclohexane naphthalene C6-benzene C1-naphthalene biphenyl C2-naphthalene C1-biphenyl C3-naphthalene C2-biphenyl C4-naphthalene C5-naphthalene C6-naphthalene pristane phytane
utilized to remove polar, intermediate, and nonpolar residues. The cell was filled with a pulverized sample (200 mesh) and then installed in the extraction system. The mass of coal extracted was determined by mass difference (i.e., subtracting the mass of the empty cell and the mass of the glass beads from the total mass to give the mass of coal extracted). The extraction was conducted dynamically at 120 °C and 20.3 MPa for 2 h. A 0.75-1.0 mL volume of carbon disulfide was preloaded into the collection vial and placed in a cooling bath. After the system was depressurized, the extract was transferred to a 2 mL vial and analyzed by GC/MS and SFC. Compounds identified by GC/MS are listed in Table 2. The SFC group-type analysis was conducted as described in ref 10, and quantitation was based on standard calibration. Standard solutions (concentrations ranging from 5 µg mL-1 to 10 mg mL-1) containing equal amounts of n-alkanes from C6 to C14 were prepared in carbon disulfide and used for calibrating the SFC for aliphatic hydrocarbons; standard solutions of 2-ethyltoluene were used for calibration for aromatic hydrocarbons. For the aliphatic hydrocarbon quantitation, it was found necessary to correct the peak area as the CS2 solvent coeluted with the aliphatic fraction. This was accomplished by measuring the pure CS2 peak area 10 times and then subtracting the average value from the aliphaticCS2 peak.
Results and Discussion It is well-known that coal is a porous material12 containing macropores (diameters greater than 40 nm), mesopores (diameters between 2 and 40 nm), and micropores (diameters less than 2 nm). Walker et al.13 reported that CO2 can be easily taken up by the micropores, which constitute most of the surface area. Hence, based on kinetic considerations only, CO2 should be an ideal solvent for the extraction of coal matrices. The extraction efficiency and selectivity obviously depend on the CO2 solvating power which is somewhat dependent on CO2 density, temperature, and extraction time. The extraction conditions were selected to maximize the extraction of the C6-14 hydrocarbons, and at the same time, to minimize the extraction of higher molecular weight hydrocarbons and polar compounds. The selection of the extraction temperature is of primary importance, since temperature considerably affects the supercritical fluid density. Increasing the temperature increases the solute solubility and volatility (12) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. The Structure and Reaction Processes of Coal; Plenum Press: New York, 1994; p 157. (13) Walker, P. L.; Verma, S. K.; Rivera-Utrilla, J.; Davis, A. Fuel 1988, 67, 1615-1623.
Figure 2. Determination of time for supercritical fluid extraction completeness.
Figure 3. Determination of collection efficiency after supercritical fluid extraction.
and also increases the possibilities of degradation and extraction of heavy hydrocarbons. Experiments were conducted at 120, 150, 200, and 300 °C for a Blind Canyon coal. It was found that by simply increasing the temperature, the content of lower molecular weight hydrocarbons did not change significantly (0.9990. Peak areas were used for quantitation, and different types of hydrocarbons were assumed to have equal detector responses. Twenty different coal and shale samples were extracted using supercritical CO2. These extracts were then separated into aliphatic and aromatic groups using packed capillary column SFC. Each result is the average of three repeated injections. The estimated standard error for the three repeat injections was (10%. As can be seen from these data, the contents of low molecular weight aliphatic hydrocarbons in the coal and shale samples ranged between 0.02 and 0.20 wt % of the bulk source rock material. The contents of aromatic hydrocarbons ranged from 0.01 to 0.1%. The C6-14 hydrocarbons in the shale samples have a narrower range and greater abundance than those in the coal samples. Acknowledgment. This work was funded by the Gas Research Institute, Contract Number 5091-260-2239. EF960176F
