Effect of Thermal Treatment on Acidic Organic Species from

Dec 12, 2008 - Liquid products from HVGOs treated under an inert N2 atmosphere at temperatures of 300, 325, 350, and 400 °C were each characterized b...
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Energy & Fuels 2009, 23, 314–319

Effect of Thermal Treatment on Acidic Organic Species from Athabasca Bitumen Heavy Vacuum Gas Oil, Analyzed by Negative-Ion Electrospray Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry Donald F. Smith,†,‡ Ryan P. Rodgers,*,†,‡ Parviz Rahimi,§ Alem Teclemariam,§ and Alan G. Marshall*,†,‡ Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State UniVersity, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310-4005, Department of Chemistry and Biochemistry, Florida State UniVersity, Tallahassee, Florida 32306, and National Centre for Upgrading Technology, 1 Oil Patch DriVe, DeVon, Alberta T9G 1A8, Canada ReceiVed July 14, 2008. ReVised Manuscript ReceiVed October 10, 2008

We examine suspected molecular transformations of thermally treated Athabasca bitumen heavy vacuum gas oil (HVGO) by ultrahigh-resolution negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Liquid products from HVGOs treated under an inert N2 atmosphere at temperatures of 300, 325, 350, and 400 °C were each characterized by class (heteroatom content), type (double-bond equivalents ) number of rings plus double bonds to carbon), and carbon number distribution. In addition, the inert N2 sweep gas of the autoclave was collected, condensed, and analyzed. The total acid number (TAN) of the HVGO liquid products decreases with an increasing treatment temperature (from 4.13 at 300 °C to 1.46 at 400 °C), indicative of potential carboxylic acid decomposition. The highly abundant O2 class contains species with DBE ) 3; however, no compositional changes occur with increased treatment temperature. A bimodal DBE distribution is observed for the S1O2 class, suggesting two possible stable core structures. Only low relative abundance classes show slight changes with thermal treatment. Condensed nitrogen sweep gas obtained at 350 and 400 °C contains highly abundant O2 species with DBE of 3 and but at lower carbon number. Similarly, the condenser product S1O2 classes display the same bimodal DBE distributions as the HVGO liquid products but with lower carbon number (∼18-27 for condenser versus ∼25-35 for the liquid products). The similarity of the O2 speciation in the HVGO liquid products after thermal treatment combined with the detailed analysis of the condenser products suggests that the gross decrease in total acid number (TAN) at higher temperature is due to global (class, DBE, and carbon number indiscriminant) decomposition of the naphthenic acids as well as a small contribution from the loss of the lower boiling, lower carbon number acids by simple distillation.

Introduction The oil sand deposits in Alberta, Canada, the second largest reserve of recoverable crude oil, are estimated at 175 billion barrels.1 The low American Petroleum Institute (API) gravity (7-15°) and high heteroatom content (nitrogen, sulfur, and oxygen, NSO) of recovered bitumen creates transportation and refining challenges for these feedstocks. In particular, the organic acids, also known as naphthenic acids, are of considerable interest because of their role in refinery corrosion (so-called naphthenic acid corrosion).2,3 Feedstocks with a total acid number (TAN, mg of KOH needed to neutralize 1 g of crude * To whom correspondence should be addressed. Telephone: +1-850644-2398. E-mail: [email protected] (R.P.R.); Telephone: +1-850644-0529. Fax: +1-850-644-1366. E-mail: [email protected] (A.G.M.). † National High Magnetic Field Laboratory, Florida State University. ‡ Department of Chemistry and Biochemistry, Florida State University. § National Centre for Upgrading Technology. (1) Berkowitz, N.; Speight, J. G. Oil sand of Alberta. Fuel 1975, 54 (3), 138–149. (2) Turnbull, A.; Slavcheva, E.; Shone, B. Factors controlling naphthenic acid corrosion. Corrosion 1998, 54 (11), 922–930. (3) Slavcheva, E.; Shone, B.; Turnbull, A. Review of naphthenic acid corrosion in oil refining. Br. Corros. J. 1999, 34 (2), 125–131.

oil) > 0.5 and side streams with TAN > 1.5 are considered unsuitable for processing in typical refineries. However, industry experience shows that, although TAN may be a good indicator of whether a crude oil may be corrosive, it is not necessarily useful to determine the severity of expected corrosion. Moreover, Athabasca bitumen has a TAN of ∼3.2-5.5, well above the established limit, but reports on refinery corrosion because of naphthenic acids in oil sand processing are limited. Naphthenic acid corrosion commonly occurs in atmospheric distillation towers, in which temperatures are between 220 and 400 °C.3,4 However, there is little information on the thermal stability of naphthenic acids at those temperatures. Corrosive feeds in that distillation range contain low-molecular-weight (m/z 160-350), low double-bond equivalents (DBEs or rings plus double bonds) organic acids4,5 based on negative-ion fast-atom bombardment mass spectrometry. In addition, petroleum acids (4) Laredo, G. C.; Lopez, C. R.; Alvarez, R. E.; Cano, J. L. Naphthenic acids, total acid number and sulfur content profile characterization in Isthmus and Maya crude oils. Fuel 2004, 83 (11-12), 1689–1695. (5) Laredo, G. C.; Lopez, C. R.; Alvarez, R. E.; Castillo, J. J.; Cano, J. L. Identification of naphthenic acids and other corrosivity-related characteristics in crude oil and vacuum gas oils from a Mexican refinery. Energy Fuels 2004, 18, 1687–1694.

10.1021/ef8005564 CCC: $40.75  2009 American Chemical Society Published on Web 12/12/2008

Thermal Treatment on Acidic Organic Species

as a whole have been characterized by a number of methods, including Fourier transform infrared (FTIR) spectroscopy,6-9 13C nuclear magnetic resonance (NMR),7-9 two-dimensional gas chromatography,10 gas chromatography-mass spectrometry,9,11-13 and liquid-secondary ion mass spectrometry.14 Various ionizationsourceshavealsobeenused,suchasfastatombombardment,4,5,15 chemicalionization,14,16 atmosphericpressurechemicalionization,7,14 and electrospray ionization (ESI).7,14,17-25 In the late 1990s, Blum, Olmstead, and co-workers pointed out that naphthenic acids can be thermally decomposed to yield a lower TAN (6) Yu, S. K. T.; Green, J. B. Determination of total hydroxyls and carboxyls in petroleum and syncrudes after chemical derivatization by infrared spectroscopy. Anal. Chem. 1989, 61 (11), 1260–1280. (7) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Tandem mass spectrometric characterization of commercial naphthenic acids and a maya crude oil. Energy Fuels 2002, 16, 1178–1185. (8) Tomczyk, N. A.; Winans, R. E.; Shinn, J. H.; Robinson, R. C. On the nature and origin of acidic species in petroleum. 1. Detailed acid type distribution in a california crude oil. Energy Fuels 2001, 15 (6), 1498– 1504. (9) Seifert, W. K.; Teeter, R. M. Identification of polycyclic aromatic and heterocyclic crude oil carboxylic acids. Anal. Chem. 1970, 42 (7), 750– 758. (10) Hao, C.; Headley, J. V.; Peru, K. M.; Frank, R.; Yang, P.; Solomon, K. R. Characterization and pattern recognition of oil-sand naphthenic acids using comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry. J. Chromatogr., A 2005, 1067, 277–284. (11) St. John, W. P.; Rughania, J.; Green, S. A.; McGinnisa, G. D. Analysis and characterization of naphthenic acids by gas chromatographyelectron impact mass spectrometry of tert-butyldimethylsilyl derivatives. J. Chromatogr., A 1998, 807, 241–251. (12) Holowenko, F. M.; MacKinnonb, M. D.; Fedorak, P. M. Characterization of naphthenic acids in oil sands wastewaters by gas chromatographymass spectrometry. Water Res. 2002, 36, 2843–2855. (13) Clemente, J. S.; Prasad, N. G. N.; MacKinnon, M. D.; Fedorak, P. M. A statistical comparison of naphthenic acids characterized by gas chromatography-mass spectrometry. Chemosphere 2003, 50, 1265–1274. (14) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Enegry Fuels 2000, 14 (1), 217–223. (15) Fan, T. Characterization of naphthenic acids in petroleum by fast atom bombardment mass spectrometry. Enegry Fuels 1991, 5, 371–375. (16) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Determination of naphthenic acids in California crudes and refinery wastewaters by fluoride ion chemical ionization mass spectrometry. Anal. Chem. 1988, 60 (13), 1318–1323. (17) Hsu, C. S.; Liang, Z.; Campana, J. E. Hydrocarbon characterization by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry. Anal. Chem. 1994, 66 (6), 850–855. (18) Zhan, D. L.; Fenn, J. B. Electrospray mass spectrometry of fossil fuels. Int. J. Mass Spectrom. 2000, 194 (2-3), 197–208. (19) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and identification of elemental compositions for more than 3000 crude acids in heavy petroleum by negative-ion microelectrospray high-field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15 (6), 1505–1511. (20) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. K. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2002, 33 (7), 743– 759. (21) Stanford, L. A.; Kim, S.; Klein, G. C.; Smith, D. F.; Rodgers, R. P.; Marshall, A. G. Identification of water-soluble heavy crude oil organicacids, bases, and neutrals by electrospray ionization and field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry. EnViron. Sci. Technol. 2007, 41 (8), 2696–2702. (22) Stanford, L. A.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Characterization of compositional changes in vacuum gas oil distillation cuts by electrospray ionization Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. Energy Fuels 2006, 20 (4), 1664–1673. (23) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A.; Taylor, S. Detailed elemental compositions of emulsion interfacial material versus parent oil for nine geographically distinct light, medium, and heavy crude oils, detected by negative- and positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2007, 21 (2), 973–981. (24) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry for complex mixture analysis. Anal. Chem. 2006, 78 (16), 5906–5912.

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product and reduce viscosity by what they termed “heat soakinduced naphthenic acid decomposition”.26,27 However, the class, type, and carbon number reactivity of the naphthenic acids were not addressed. Here, we report the results of thermal treatment of Athabasca bitumen heavy vacuum gas oil (HVGO, 350-525 °C) to determine the thermal stability of bitumen HVGO acids. Thermal treatment was performed between temperatures of 300 and 400 °C, and the liquid products and condensed sweep gas products were analyzed by various bulk methods as well as by negative-ion ESI Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Bulk TAN decreases markedly at higher treatment temperature (see Table 1). The ultrahigh mass resolving power (m/∆m50% > 350 000, in which ∆m50% is the mass spectral peak full width at half-maximum peak height) and high mass accuracy (better than 500 ppb) of FT-ICR MS allow for the assignment of a unique elemental composition to each peak in the mass spectrum.28-30 Thus, the elemental composition assignment allows for the organization of compounds by class (heteroatom content), type (DBEs, the number of rings and/or double bonds), and carbon number as a function of the thermal treatment temperature. ESI FT-ICR MS identifies those polar organic acids that persist after thermal treatment and can thus track those that may be responsible for the decrease in TAN at higher treatment temperatures. Experimental Methods Sample Description and Bulk Measurements. HVGO was obtained by American Society for Testing and Materials (ASTM) D1160 distillation of Athabasca bitumen, approximately 30 wt % with a boiling range of 350-525 °C.31 The TAN was measured by the ASTM D-664 method.32 Water content was determined by Karl Fischer titration. Bulk molecular weight was determined by vapor pressure osmometry in o-dichlorobenzene at 130 °C. Thermal Treatment. HVGO thermal treatment experiments were carried out in a 1 L stirred tank autoclave at reaction temperatures of 300, 325, 350, and 400 °C at 100 psi for a 60 min residence period as previously documented.26,27 A continuous flow of nitrogen at 30 mL/min was used to remove any water from decarboxylation reactions and to prevent reverse reactions. The liquid products were collected from the autoclave for analysis. The light ends were captured in a 100 mL condenser (0 °C) placed in an ice bath. The liquid products (autoclave) and light ends (condenser) were analyzed by FT-ICR MS. Gaseous products were collected in a 1 L inert gas capture bag and analyzed by gas chromatography. The experiments were performed at the National (25) Smith, D. F.; Schaub, T. M.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Self-association of organic acids in petroleum and Canadian bitumen characterized by low- and high-resolution mass spectrometry. Energy Fuels 2007, 21 (3), 1309–1316. (26) Blum, S. C.; Olmstead, W. N. Viscosity reduction by heat soakinduced naphthenic acid decomposition in hydrocarbon oils. U.S. Patent 5,976,360, April 24, 1997. (27) Blum, S. C.; Olmstead, W. N.; Bearden, R., Jr. Thermal decomposition of naphthenic acids. U.S. Patent 5,820,750, Feb 9, 1996. (28) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Fourier transform ion cyclotron resonance mass spectrometry: A primer. Mass Spectrom. ReV. 1998, 17 (1), 1–35. (29) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53–59. (30) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A–27A. (31) American Society for Testing and Materials (ASTM). ASTM Standard D1160-03. In Standard Test Method for Distillation of Petroleum Products at Reduced Pressure; ASTM International: West Conshohocken, PA, 2003. (32) American Society for Testing and Materials (ASTM). ASTM Standard D664-07. In Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration; ASTM International: West Conshohocken, PA, 2003.

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Table 1. Bulk Properties of HVGO Feed, Treatment Liquid Products, and Condenser Products liquid products elemental analysis (wt %) carbon hydrogen nitrogen sulfur oxygen H/C ratio ((0.2) TAN (mg of KOH/g of oil) molecular weight (VPO, g/mol) liquid recovery (wt %) condenser recovery (wt %) water (KF, wt %) total liquid recovery (wt %) gaseous products (wt %) a

condenser products

HVGO feed

300 °C

325 °C

350 °C

400 °C

85.33 10.44 0.14 3.48 0.83 1.47 4.32 345

84.72 11.20 0.21 3.40 0.93 1.59 4.13 333 99.07 0.097 0.048 99.21 0.221

84.53 11.42 0.22 3.51 0.94 1.62 3.89 338 99.49 0.122 0.040 99.65 a

84.75 11.40 0.21 3.56 0.92 1.61 3.58 337 99.40 0.970 0.034 100.4 a

84.91 11.36 0.24 3.32 0.83 1.61 1.46 343 96.38 6.32 0.035 102.7 a

350 °C

400 °C

84.20 12.60 0.02 2.74 0.94 1.80 3.93 422

83.8 13.20 0.01 2.23 0.91 1.60 5.76 723

0.13

0.40

Negligible gaseous products recovered.

Centre for Upgrading Technology (Devon, Alberta, Canada) in either autoclave or continuous units placed in safe modules designed to limit exposure to the operators. Sample Preparation for ESI FT-ICR MS. Bitumen HVGOs were dissolved in 50:50 (v/v) toluene/methanol to a concentration of 1 mg/mL. Ammonium hydroxide was added (10 µL to every 1 mL of sample solution) to ensure efficient ionization (deprotonation) for negative-ion electrospray analysis. All solvents were HPLCgrade (Fisher Scientific, Pittsburgh, PA). Instrumentation. Samples were analyzed with a custom-built FT-ICR mass spectrometer operated in a 220 mm horizontal roomtemperature passively shielded 9.4 T magnet (Oxford Corp., Oxford Mead, U.K.).33,34 Negative ions were generated by infusing sample solution to a microelectrospray source35 (50 µm i.d. fused silica needle) at a flow rate of 400 nL/min by a syringe pump. A Predator data station handles data acquisition and data processing.33,36 ESI FT-ICR MS experimental parameters for crude oil and bitumen analysis have been described in more detail elsewhere.19-23,25 Mass Calibration and Data Analysis for FT-ICR MS. FTICR mass spectra were internally calibrated37,38 with respect to a high abundance homologous series of ions, each containing two oxygen atoms. As previously reported, all ions were singly charged.19 Ion masses (250-1000 Da) from mass spectral peaks with relative abundance greater than 6 times the standard deviation of the baseline noise were exported to a spreadsheet. Measured masses were converted to the Kendrick mass scale39 and sorted by (33) Senko, M. W.; Canterbury, J. D.; Guan, S.; Marshall, A. G. A highperformance modular data system for FT-ICR mass spectrometry. Rapid Commun. Mass Spectrom. 1996, 10, 1839–1844. (34) Hendrickson, C. L.; Quinn, J. P.; Emmett, M. R.; Marshall, A. G. Quadrupole mass filtered external accumulation for Fourier transform ion cyclotron resonance mass spectrometry. In the 48th American Society for Mass Spectrometry Annual Conference on Mass Spectrometry and Allied Topics, Long Beach, CA, 2000; MPB 083. (35) Emmett, M. R.; White, F. M.; Hendrickson, C. L.; Shi, S. D. H.; Marshall, A. G. Application of micro-electrospray liquid chromatography techniques to FT-ICR MS to enable high-sensitivity biological analysis. J. Am. Soc. Mass Spectrom. 1998, 9 (4), 333–340. (36) Blakney, G. T.; van der Rest, G.; Johnson, J. R.; Freitas, M. A.; Drader, J. J.; Shi, S. D.-H.; Hendrickson, C. L.; Kelleher, N. L.; Marshall, A. G. Further improvements to the MIDAS data station for FT-ICR mass spectrometry, In Proceedings of the 49th American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics, Chicago, IL, May 2001; WPM265. (37) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Space charge effects in Fourier transform mass spectrometry. Mass calibration. Anal. Chem. 1984, 56, 2744–2748. (38) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Comparison and interconversion of the two most common frequency-to-mass calibration functions for Fourier transform ion cyclotron resonance mass spectrometry. Int. J. Mass Spectrom. 2000, 195/196, 591– 598. (39) Kendrick, E. A mass scale based on CH2 ) 14.0000 for high resolution mass spectrometry of organic compounds. Anal. Chem. 1963, 35 (13), 2146–2154.

Kendrick mass defect values to facilitate identification of homologous series and peak assignments as previously described.40

Results and Discussion Bulk Properties of HVGO Feed and Treatment Products. Properties of the HVGO feed, thermal treatment liquid products, and condenser products are shown in Table 1. The elemental analyses are very similar over the treatment temperature range, with a slight decrease in oxygen content at 400 °C. The condenser products have a higher wt % of hydrogen and, thus, a higher H/C ratio, suggesting that these species are less aromatic than the treatment liquid products. The TAN for the liquid products decreases slowly up to 350 °C, where at 400 °C, it drops dramatically from 3.58 to 1.46. The condenser products have high TANs, an indication that, at elevated temperature, acids are carried from the reactor to the condenser by the nitrogen sweep gas. The molecular weights [determined by vapor pressure osmometry (VPO)] of the HVGO feed and liquid products are all very similar, between 330 and 345 g/mol. However, the condenser products exhibit higher VPO molecular weights, especially the 400 °C condenser sample, with a molecular weight greater than twice that of the HVGO. The condenser products display anomalous molecular weights because of solution-phase aggregation of acidic species in the chosen VPO solvent.25 Good material balances were obtained for all reactions, with very few gaseous products formed. The major liquid products had little water, but as expected, the condenser products contained more water. Simulated distillation of the HVGO feed as well as the major thermal treatment liquid products are shown in Figure 1. There is little change in the boiling point distribution until the 400 °C treatment temperature, at which lower boiling products are observed. The similarity of the bulk analyses prompted the analyses of the samples by ultrahigh-resolution ESI FT-ICR MS to assess changes in the detailed chemical composition of polar acidic species after thermal treatment. Negative-Ion ESI FT-ICR MS. Figure 2 shows a broadband negative-ion ESI FT-ICR mass spectrum of the HVGO feed. The inset illustrates the need for ultrahigh resolution across the entire mass range (300 > m/z > 1000); isobaric compounds, such as those with elemental compositions differing by C3 versus S1H4 (m2 - m1 ) 3.4 mDa) require a mass resolving power (40) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Kendrick mass defect spectroscopy: A compact visual analysis for ultrahigh-resolution broadband mass spectra. Anal. Chem. 2001, 73, 4676–4681.

Thermal Treatment on Acidic Organic Species

Figure 1. Percentage (by weight) lost as a function of the temperature for the major liquid products collected from the autoclave for thermally treated Athabasca bitumen HVGO. Low-boiling species are produced in greater abundance at the 400 °C treatment temperature.

Figure 2. Broadband negative-ion ESI 9.4 T FT-ICR mass spectrum of Athabasca bitumen HVGO. The mass-scale-expanded inset highlights a mass difference of 3.4 mDa that is resolved throughout the entire mass range, ensuring accurate elemental composition assignment.

Figure 3. Heteroatom class distribution (heteroatom content) for Athabasca bitumen HVGO feed and its thermal treatment liquid products derived from negative-ion ESI FT-ICR mass spectra. Inset shows the N1O2, S1O4, and O4 classes in more detail.

greater than m/∆m50% ) 147 000 to resolve signals of equal magnitude at m/z 500. All thermal treatment liquid products display the same mass range (m/z 300-700) (data not shown). The class analysis (heteroatom content) for the HVGO feed and thermally treated liquid products is shown in Figure 3. The O2 class, presumably naphthenic acids, is the most abundant in

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Figure 4. Color-coded isoabundance contours for plots of DBEs versus carbon number for the O2 class derived from negative-ion ESI FTICR MS in HVGO (top left) and its liquid products collected at 300 °C (top center), 325 °C (top right), 350 °C (bottom left), and 400 °C (bottom right).

each sample, followed by the S1O2 class. The HVGO has the highest relative abundance of O2 species, and the thermally treated liquid products show approximately the same relative abundance over the treatment temperature range. The S1O2 class shows similar relative abundance for the HVGO and 300, 325, and 350 °C liquid products and shows a decrease upon heating to 400 °C. The inset in Figure 3 shows the (low-abundance) N1O2, S1O4, and O4 class analyses in more detail. The relative abundances of those classes all decrease slightly with an increase in the treatment temperature. Although the low relative abundance of those classes makes confirmation of small relative changes difficult, their magnitudes suggest they are not responsible for the large decrease in TAN upon thermal treatment. Isoabundance contours for plots of DBEs versus carbon number for the O2 class are shown in Figure 4. The predominant species in all of the samples have DBE ) 3 [non-aromatic, consistent with a general formula of R(CH2)n ) COOH, in which R is predicted to be two cyclopentane or cyclohexane rings],16 with DBE values ranging from ∼1 to 7 and little change in the carbon number distribution. The O2 speciation of the liquid products shows little alteration during thermal treatment. Thus, the dramatic reduction in TAN at higher thermal treatment temperatures cannot result from class-, type-, or carbon-numberspecific alteration or decomposition of the O2 class. Figure 5 shows isoabundance contours for plots of DBEs versus carbon number for the S1O2 class. The carbon number and DBE distributions are similar for the HVGO feed and the liquid products, except for a slight increase in carbon number and DBE at the highest treatment temperature (400 °C). The bimodal DBE distribution, with maxima at DBE ) 4-5 and 7-8 and a common carbon number of ∼30, suggests two stable core structures. Thiophene and benzothiophene cores with a carboxylic acid group (S1O2 class) are consistent with DBE values of 4 and 7. The HVGO feed, with highest TAN, shows higher abundance of the lower DBE core structure, whereas the more aromatic core structure is more abundant with increased thermal treatment temperatures. Thus, thermal treatment appears to remove lower DBE species rather than generate new species by chemical cracking reactions. The inert sweep gas from the thermal reactor for the 350 and 400 °C treatment temperatures was condensed and analyzed. The high TAN values for the condenser products (see Table 1) suggested a high level of naphthenic acids. The heteroatom class

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Figure 5. Plots are the same as in Figure 4, but for the S1O2 class.

Figure 6. Heteroatom class distribution (heteroatom content) for Athabasca bitumen HVGO thermal treatment liquid products and the corresponding condenser products for the 350 and 400 °C treatment temperatures, derived from negative-ion ESI FT-ICR mass spectra.

graphs for the condenser products at the 350 and 400 °C treatment temperatures are found in Figure 6 along with the corresponding HVGO liquid products at the same treatment temperature. Overall, O2 class species are even more dominant in the condenser products than in the HVGO liquid products, and all samples contain a small amount of S1O2 species. The 400 °C condenser product contains somewhat more multiheteroatom species (S1O3, O3, and O4) than the 350 °C condenser product (although all at relatively low abundance), consistent with expected heteroatom distillation trends.22,41,42 The higher abundance of acids found in the condenser products is consistent with but cannot be solely responsible for (see Table 1) the decrease in TAN of the liquid products with an increase in treatment temperature. Specifically, the TAN of the recovered liquids accounts for only 82% (350 °C) and 33% (400 °C) of the initial acids present in the HVGO. The inclusion of the recovered condenser products raises the total recoveries to only 83% (350 °C) and 41% (400 °C). Furthermore, the naphthenic (41) Fu, J.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Nonpolar compositional analysis of vacuum gas oil distillation fractions by electron ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (2), 661–667. (42) Smith, D. F.; Schaub, T. M.; Rodgers, R. P.; Rahimi, P.; Marshall, A. G. In Characterization of acidic species from Athabasca Canadian bitumen by negative ion ESI FT-ICR MS. Oilsands 2006, Edmonton, Alberta, Canada, Feb 22-24, 2006.

Smith et al.

Figure 7. Plots are the same as in Figure 4, but for the O2 class from the 350 °C condenser product (top left), 350 °C liquid product (top right), 400 °C condenser product (bottom left), and 400 °C liquid product (bottom right).

acid liquid recovery for the two lower temperatures is 95% (300 °C) and 90% (325 °C), with no significant amount of recovered condenser products, suggesting that naphthenic acid loss occurs in the absence of significant distillation losses. Thus, global (class-, DBE-, and carbon-number-independent) decarboxylation must be responsible for the gross TAN decline at all treatment temperatures but is especially pronounced at higher treatment temperatures. Figure 7 shows isoabundance contours for plots of DBE versus carbon number for the O2 class of the condenser products from the 350 and 400 °C treatment temperatures and their corresponding HVGO liquid products at the same treatment temperature. The 350 and 400 °C condenser products exhibit similar DBE range. Species with DBE ) 3 are still prominent. However, the carbon number drops from ∼33 in the HVGO liquid products to ∼22 for the condenser products, suggestive of simple distillation of lower boiling acid species. For reference, 1H-indene-4-propanoic acid,43 a cycloparaffinic carboxylic acid (C22H44O2, DBE ) 3), has a predicted boiling point of 365.9 ( 10 °C,44 roughly the treatment temperature range at which low carbon number acids are observed in the condenser products. Thus, distillation rather than thermal cracking of the native carboxylic acids accounts for a small (less than 10%) portion of the decrease in TAN at the higher treatment temperatures. Figure 8 shows plots of DBEs versus carbon number for the S1O2 class. Here, the 400 °C condenser product extends to higher DBEs (12) than the 350 °C sample (DBE ) 8). The condenser products favor the less aromatic S1O2 core structure (DBE ) 4-5), again suggesting that those species are distilled from the liquid products. The low carbon number species observed in the condenser products are likely lower boiling-point species that distill from solution and are carried into the condenser by the N2 sweep gas. The loss of these specific species (O2 and S1O2) appears to contribute to a greater extent to the decrease in TAN at higher thermal treatment temperature as reflected in the increased TAN and yield of condensed products. However, (43) Boto, A.; Hernandez, R.; Suarez, E.; Betancor, C.; Rodriguez, M. S. Tandem carbon-radical peroxidation-addition to carbonyl groups reaction. A new synthesis of steroidal β-peroxy lactones. J. Org. Chem. 1995, 60 (25), 8209–8217. (44) CAS Registry Number 171366-97-5. 2007 ed.

Thermal Treatment on Acidic Organic Species

Figure 8. Plots are the same as in Figure 7, but for the S1O2 class.

the distillation contribution to the TAN reduction is small compared to the decarboxylation losses. Conclusions Ultrahigh-resolution FT-ICR MS is well-suited to reveal compositional changes for thermally treated petroleum samples. In the HVGO samples, TAN decreases with increasing thermal treatment temperatures; however, ESI FT-ICR MS reveals no

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change in the naphthenic acid (O2 class) composition of the liquid products. A slight shift to a higher aromatic core structure is observed for the S1O2 class at higher treatment temperature, similar to distillation trends rather than thermal conversion. Changes in relative abundance for multi-heteroatom-containing classes, such as N1O2, S1O4, and O4, are small and not likely to account for the large decrease in TAN with increased treatment temperature. ESI FT-ICR MS identifies high relative abundance, low carbon number O2 species in the high TAN condensed sweep gas products. These low carbon number species are likely lower boiling acids that distill out of solution and are swept into the condenser at higher treatment temperatures. The present results suggest that the simple distillation loss of these species partially contributes to the reduction in TAN. However, the weight of the recovered condenser products cannot account for the gross change in TAN. The prominent loss, due to decarboxylation reactions, is responsible for the large changes in TAN as the reaction temperature increases and is not class-, type-, or carbon-number-dependent. Acknowledgment. This work was supported by the National Science Foundation (NSF) Division of Materials Research through DMR-0654118 and the State of Florida. The National Centre for Upgrading Technology (NCUT) work was supported by the Canadian Program for Energy Research and Development (PERD). EF8005564