Energy & Fuels 1996, 10, 873-882
873
The Influence of Pressure on the Thermal Cracking of Oil Ronald J. Hill,*,†,‡ Yongchun Tang,‡ Isaac R. Kaplan,† and Peter D. Jenden‡ Department of Earth and Space Sciences, University of California, Los Angeles, California 90024, and Chevron Petroleum Technology Company, La Habra, California 90633 Received July 18, 1995. Revised Manuscript Received March 28, 1996X
The influence of pressure on gas, liquid, and solid products of thermal cracking of a C9+ fraction of a saturate-rich Devonian oil from the Western Canada Basin has been investigated. Confined pyrolysis was performed in sealed gold tubes at 350, 380, and 400 °C and pressures ranging from 90 to 2000 bar for 72 h. At the temperatures investigated, the effect of pressure on oil cracking and product generation is small. Rates of early hydrocarbon gas generation (350 and 380 °C, 72 h) decrease with increasing pressure by 9-15% in the 90-210 bar range and by 7% for gas generation (400 °C, 72 h) in the 90-345 bar range. Gas generation rates then steadily increase 10-15% to a maximum at 690 bar for all temperatures. From 690 to 2000 bar, the rates of gas generation steadily decrease by 5-17%. Activation volume values were estimated to be ∆Vq ) 47 cm3/mol in the 90-210 bar range, ∆Vq ) -14 cm3/mol in the 345-690 bar range and ∆Vq ) 5 cm3/mol in the 690-2000 bar range. Extrapolation of results to geologic conditions shows that the pressure effects on oil cracking are larger under geologic conditions than laboratory pyrolysis conditions but still secondary to temperature. The effect of pressure on gas generation rates is also reflected in methane carbon isotopes, which show nearly 2‰ fractionation with increasing pressure to 1380 bar. Ethane and propane showed almost no detectable fractionation with pressure. At 350 and 380 °C, C8+ n-alkane yields generally increase as pressure increases from 90 to 690 bar and decrease as pressure increases from 690 to 2000 bar, parallel to that observed for the gases. At 400 °C, however, the n-alkane yields are highest at 345 and 2000 bar, where gas yields are lowest. This suggests that n-alkanes are generated, in part, from heavier molecules at 350 and 380 °C, and at 400 °C, n-alkanes are cracked more rapidly than they are formed to produce gas.
Introduction Oil generation from kerogen and the subsequent cracking of oil to natural gas is generally regarded as a temperature-controlled process with pressure being secondary in importance.1 Laboratory experiments addressing the influence of pressure on model compound, kerogen, and oil cracking have provided apparently conflicting results. Many investigators performing confined pyrolysis (products confined with reactants during pyrolysis as opposed to being swept away by an inert gas after generation) in stainless steel bombs and gold tubes with limited pressure ranges report suppression of coal, kerogen, and model compound maturation with increasing pressure.2-11 Other investigators using limited pressure ranges report little or no effect of †
University of California. Chevron Petroleum Technology Co. * Corresponding author. Present address: Exxon Production Research Co., P.O. Box 2189, Houston, TX 77252-2189. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1984; 699 pp. (2) Hesp, W.; Rigby, D. Erdoel Kohle, Erdgas, Petrochem. 1973, 26, 70-76. (3) McTavish, R. A. Nature 1978, 271, 648-650. (4) Cecil, B.; Stanton, R.; Allshouse, S.; Cohen, A. Ninth International Congress of Carboniferous Stratigraphy and Geology; University of Illinois, Urbana, Illinois, May 19-23, 1979. (5) Horvarth, Z. A. Acta Geol. Hung. 1980, 26, 137-148. (6) Sajgo, C.; McEvoy, J.; Wolff, G. A.; Horvarth, Z. A. Adv. Org. Geochem. 1986, 10, 331-337. (7) Price, L. C.; Wenger, L. M. Adv. Org. Geochem. 1992, 19, 141160 ‡
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pressure on organic maturation,12-17 whereas others report increases in the rate of organic maturation at low pressures and suppression of maturation rates at higher pressures.18-21 Michel et al.22-24 addressed the nature of the pressurizing medium on kerogen pressure pyrolysis results. (8) Blanc, P.; Connan, J. Energy Fuels 1992, 6, 666-677. (9) Freund, H.; Clouse, J. A.; Otten, G. A. Energy Fuels 1993, 7, 1088-1094. (10) Perlovsky, L. I.; Vinkovetsky, Y. A. Boll. Geofis. Teor. Appl. 1989, 122, 87-90. (11) Domine, F. Energy Fuels 1989, 3, 89-96. (12) McNab, J. G.; Smith, P. V.; Betts, R. L. Geochim. Cosmochim. Acta 1952, 44, 2556-2563. (13) Henderson, J. H.; Weber, L. J. Can. Pet. Technol. 1965, OctDec, 209-212. (14) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985, 8, 275-292. (15) Monthioux, M.; Landais, P.; Durand, B. Org. Geochem. 1986, 10, 299-311. (16) Ungerer, P.; Behar, F.; Villalba, M.; Heum, O. R.; Audibert, A. Org. Geochem. 1988, 13, 857-868. (17) Jackson, K. J.; Burnham, A. K.; Braun, R. L.; Knauss, K. G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 1614-1620. (18) Fabuss, B. M.; Smith, J. O.; Satterfield, C. N. Thermal cracking of pure saturated compounds. In Advances in Petroleum Chemistry and Refining; McKetta, J. J., Kobe, K. A., Eds.; Interscience: New York, 1964; pp 157-201. (19) Doue, F.; Guiochon, G. J. J. Chim. Phys. 1968, 65, 395-409. (20) Braun, R. L.; Burnham, A. K. Energy Fuels 1990, 4, 132-146. (21) Hill, R. J.; Jenden, P. D.; Tang, Y. C.; Teerman, S.; Kaplan, I. R. The influence of pressure on the pyrolysis of coal. In Reevaluation of Vitrinite Reflectance as a Maturity Parameter; Mukhopadhyay, P. K., Dow, W. G., Eds.; ACS Books: Washington, DC, 1994; Chapter 11, pp 161-193. (22) Michels, R.; Landais, P.; Philp, R. P.; Torkelson, B. E. Energy Fuels 1994, 8, 741-754.
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They found limited pressure influence on organic maturation during anhydrous and hydrous gold tube pyrolysis, but significant retardation of organic maturation during hydrous pyrolysis in stainless steel bombs. Michel et al.22-24 concluded that pressure pyrolysis results depend on the nature of the pressurizing medium and experimental setup. The apparent conflict in conclusions from pressure studies on organic maturation are due, in part, to the different techniques used to evaluate the effect of pressure on organic cracking. More important, perhaps, is the pressure medium, the range of pressures chosen in these studies, and the number of different pressures at which data were obtained. To determine the magnitude of enhancement or suppression of oil cracking rates with increasing pressure, we performed anhydrous, gold tube pyrolysis experiments using a Devonian oil from the Western Canada Basin of North America. Changes in gas products, residual oil products, and solid pyrobitumen products with increasing pressure were monitored. We have estimated activation volumes for methane generation and extrapolated the pressure effect to geologic conditions. Methods and Materials Starting Material. The starting material for the gold tube pyrolysis experiments was a C9+ topped, 35.0° API gravity, Devonian oil collected from a partially dolomitized pinnacle reef from the West Pembina area of the Western Canada Basin in Alberta, Canada. Bulk composition, determined after topping the oil at 35 °C for 2.5 h under vacuum is 63.74% saturates, 22.93% aromatics, 6.59% resins, and 6.74% asphaltenes. The topped whole oil stable carbon isotope ratio is -29.47‰. All pyrolysis was done from a single, topped Nisku oil sample to eliminate the effect of sample variability on results. Pyrolysis. Approximately 50 mg of oil was loaded with a syringe into a cleaned, 50 mm × 5 mm gold tubes. The gold tubes were then flushed with argon, to eliminate oxygen in the tube and sealed under an Ar atmosphere by arc welding. The gold tubes were placed in a Rene Ni-alloy vessel, connected to a pressurized water line, and heated in a battery of resistance furnaces at different temperatures and pressures. Pressure was maintained near the pressure of interest while the bombs were heated, as described in Hill et al.21 Experiments were conducted for 72 h at 350, 380, and 400 °C (( 2.5 °C) and pressures ranging from 90 to 2000 bar (( 5 bar). The experimental times and temperatures were chosen to approximate low to significant amounts of oil cracking, based on the work of Lewan.25 Product Analysis. Product analyses included determination of the non-hydrocarbon (He, H2, Ar + O2, N2, CO2, and CO) and hydrocarbon gases (C1-C5), hydrocarbon liquids, and residual solids. The liquid nitrogen noncondensable gases (He, H2, Ar + O2, N2, CH4, and CO) were collected on a vacuum line using a toepler pump and analyzed directly on a United Technologies gas chromatograph. The liquid nitrogen condensable gases (C2-C5, and CO2) were frozen into a glass tube and analyzed on a Hewlett-Packard 5880 gas chromatograph. Three gas standards of known composition were used to check the vacuum line and cryogenic gas collection procedures for compositional accuracy. The error bars in the gas yield figures (23) Michels, R.; Landais, P.; Philp, R. P.; Torkelson, B. E. Energy Fuels 1994, 9, 204-215. (24) Michels, R.; Landais, P.; Torkelson, B. E.; Philp, R. P. Geochim. Cosmochim. Acta 1995, 59, 1589-1604. (25) Lewan, M. D. Philos. Trans. R. Soc. London 1985, 315, 124134.
Hill et al. are determined from these reproducibility experiments. The accuracy is (6% for methane and hydrogen, (4% for carbon dioxide, (3% for ethane, propane, the butanes, and the pentanes, and (18% for hexane. The reproducibility of multiple gold tube experiments run at the same temperature and pressure was found to be (5% for methane and (6% for ethane, propane, the butanes, the pentanes, and CO2. The gold tube extracts were analyzed by on column injection using a Hewlett-Packard 5880 gas chromatograph (GC). GC run reproducibility was (9%. Pyrobitumen weight was found by filtration and weighing with reproducibility of (6%. Percent oil cracked (% oil cracked ) initial oil wt in gold tube - (gas weight + pyrobitumen weight)) has a reproducibility of (6%. The details of the methods and instrumentation used in product analysis are described in Hill et al.21 Methane, ethane, and propane δ 13C isotope ratios were measured at each pressure. The gases were separated and combusted using a Finnigan Mat gas chromatography combustion system and measured on a Finnigan Mat 251 isotope ratio mass spectrometer.
Results The gas yield, normal alkane yield, and pyrobitumen yield data for confined pyrolysis of the Devonian oil pyrolyzed at 350, 380, and 400 °C for 72 h are summarized in Tables 1, 2, and 3, respectively. As each set of experiments were run at constant time and temperature, measured changes in compound yield reflect changes in the rate of generation or destruction of the compound with increasing pressure. Yields are reported as milliliters (STP) per gram of unpyrolyzed oil loaded into the gold tube (mL/g) for the gases and as milligrams of product per gram of unpyrolyzed oil loaded into the gold tube for the n-alkanes (mg/g). Gas Products. Although the pressure effect on oil cracking is small, three trends are observed in C1-C5 yields with increasing pressure (Figures 1 and 2). At 350 and 380 °C there is a slight decrease in C1-C5 gas yields from 90 to 210 bar. Gas yields then steadily increase as pressure increases from 210 to 690 bar and then steadily decrease as pressure increases from 690 to 2000 bar. At 350 °C, isobutane and normal butane and isopentane and normal pentane data show more scatter than data from higher temperatures. This may reflect the small volumes of gas generated at 350 °C and the greater difficulty in analyzing these samples. At 400 °C, C1-C5 gas yields decrease as pressure increases from 90 to 483 bar. A steady increase in C1-C5 gas yields is then observed as pressure increases from 483 to 690 bar and is followed by a steady decrease in C1C5 gas yield as pressure increases from 690 to 2000 bar. Our results are similar to what Fabuss et al.18 found in their review of the literature, which suggested that, at pressures above 300 bar, gas generation reaction rates will steadily decrease with increasing pressure. Hydrogen and propene yields are summarized in Figure 3 for experiments at 350, 380, and 400 °C, respectively. Hydrogen and propene yields are low at all temperatures, although some general trends are observed. Hydrogen and propene yields are both highest at 90 bar and decrease as pressure increases to 2000 bar although, at 380 °C, propene yields increase at 1380 bar before decreasing again at 2000 bar. This may reflect reaction between hydrogen and alkenes or retardation of generation of the products with increasing pressure.
Influence of Pressure on Thermal Cracking of Oil
Energy & Fuels, Vol. 10, No. 4, 1996 875
Table 1. Gas Yield, n-Alkane Yield, and Pyrobitumen Yield for Oil Pyrolyzed at 350 °C and Various Pressures sample no. pressure (bar) N2 (mL/g) O2 + Ar (mL/g) H2 (mL/g) CO (mL/g) CO2 (mL/g) CH4 (mL/g) C2 (mL/g) C3 (mL/g) C3-ene (mL/g) iC4 (mL/g) nC4 (mL/g) iC5 (mL/g) nC5 (mL/g) C6 (mL/g) wetnessa % oil cracked n-C16b n-C20b n-C24b n-C32b pyrobitumenb a
229
241
232
233
243
244
234
237
245
246
247
248
239
bigoray oil
90 1.09 5.16 0.27
