Oxidation Evidenced during Hydrocarbon Preparative Fraction

Energy & Fuels 2006, 20, 1137-1144

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Oxidation Evidenced during Hydrocarbon Preparative Fraction Isolation. Intrinsic Stability Aspects Affecting Crude Oils Lante Carbognani* Consultant, Caracas, Venezuela. Present address: AICISE, Chemical and Petroleum Engineering Department, Calgary UniVersity, 2500 UniVersity DriVe N. W., Calgary, Alberta, T2N 1N4, Canada

Eduardo Buenrostro-Gonzalez Branch of Molecular Engineering Research, Mexican Institute of Petroleum, Eje Central Lazaro Cardenas152, Col. San Bartolo Atepehuacan C. P. 07730, Mexico D. F., Mexico ReceiVed NoVember 16, 2005. ReVised Manuscript ReceiVed March 23, 2006

Despite the fact that published evidence on hydrocarbon oxidation is abundant, reports linking preparative isolation procedures to HC oxidation in the open literature are scarce. This paper presents some findings on HC oxidation during isolation of asphaltenes and resins, emphasizing the importance for improved variable control leading to sample degradation. Variables such as sample handling, storage conditions, air and light influence with respect to crude oils, and vacuum residua studies are addressed. Case studies were selected from Venezuela’s upstream and downstream historical records and from crude oil production operations in Mexico. Some evidence linking oxygen compound abundance with crude oil intrinsic stability is reviewed.

Introduction Crude oil has been cited as an excellent example of the socalled very complex fluids.1 As mentioned within the field of hydrocarbon separation and characterization, compositional knowledge of such fluids often follows a well-known strategy that Caesar immortalized: the principle of “Divide, then Conquer”.2 During the past five decades, many hydrocarbon (HC) separation schemes have been described as starting steps for further characterization.3-5 A widely adopted separation methodology came to be known during the 1970s as the SARA method, for the hydrocarbon group-types isolated, that is, saturates, aromatics, resins, and asphaltenes. Resins, sometimes referred as “polars”, are compounds bearing chemical functionalities due to the presence of heteroatoms, mainly N, S, and O. The most complex of these four hydrocarbon groups is the asphaltene fraction, which is defined in solubility terms. It is comprised of those compounds displaying the highest molecular weights, the highest polarities, or both.6 According to the definition, asphaltene types are not soluble in alkane solvents but are soluble in aromatic solvents.6 Once the asphaltene group is removed from the original sample by precipitation with one selected alkane, the remainder of the sample, known as the * Corresponding author. Tel: (403)2109735. Fax: (403)2103973. E-mail: [email protected]. (1) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (2) Boduszynski, M. M.; Rechsteiner, C. E.; Carlson, R. M. Proc. 7th UNITAR Int. Conf. HeaVy Crude Tar Sand., Vol. 2, 1998; Paper 168, pp 1579-1587. (3) Altgelt, K. H.; Gouw, T. H. Chromatography in Petroleum Analysis; Chromatographic Series; Marcel Dekker: New York, 1979; Vol. 11. (4) Lundanes, E.; Greibrokk, T. JHRC&CC 1994, 17, 197. (5) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of HeaVy Petroleum Fractions; Marcel Dekker: New York, 1994. (6) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999.

deasphalted material or maltene phase, is generally separated by chromatographic means into SAR HC group-types. Polar adsorbents are generally chosen for SAR group-type chromatographic separations of maltene phases. Derivatized phases are very often reported in the open literature, since they allow better recoveries of isolated fractions.5,7-9 Use of underivatized alumina or silica substrates was not recommended particularly for heavy oils or vacuum residua, since these samples left noticeable amounts of irreversible adsorbed components after the separation process was terminated. Many facts on crude oil HC oxidation have been reported in the open literature. (1) Detailed mechanistic studies on hydrocarbon oxidation have been addressed.10,11 (2) Intermolecular association due to oxygenates has been described.12-15 (3) Closed correlations have been established among precursor HCs and their oxidized derivatives.16 (4) Oxygen compounds and oxidation processes reportedly affect asphalt physical properties.17,18 (5) Resins are known to be readily oxidized under mild conditions.18-20 However, one aspect poorly addressed in public (7) Lundanes, E.; Greibrokk, T. J. Liq. Chromatogr. 1984, 19, 443. (8) Carbognani, L.; Izquierdo, A. J. Chromatogr. 1989, 484, 399. (9) Robbins, W. K. J. Chromatogr. Sci. 1998. 36 (9), 457. (10) Petersen, J. C. Pet. Sci. Technol. 1998, 16 (9&10), 1023. (11) Herrington, P. R. Pet. Sci. Technol. 1998, 16 (9&10), 1061. (12) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976 55 (7), 184. (13) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55 (7), 187. (14) Ignasiak, T.; Strausz, O. P.; Montgomery, D. S. Fuel 1977, 56 (10), 359. (15) Speight, J. G.; Moschopedis, S. E. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 1981, 26 (4), 907. (16) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumens and HeaVy Oils; Alberta Energy Research Institute, Calgary, Canada, 2003. (17) Boduszynski, M. M.; McKay, J. F.; Latham, D. R. Chapter in Asphalt PaVing Technology 1980, Proc. AAPT 1980, 49, 123-143. (18) Scarsella, M.; Mastrofini, D.; Barre, L.; Espinat, D.; Fenistein, D. Energy Fuels 1999, 13 (3), 739.

10.1021/ef050379g CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006

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Table 1. Properties of Mexican Crude Oils crude

intrinsic stabilitya

saturates (wt %)

aromatics (wt %)

resins (wt %)

asphaltenes (wt %)

°API

PC KU

unstable stable

41.7 18.5

34.2 31.8

21.8 37.8

2.3 11.9

36 19

a

Unstable samples show organic deposition during production operations.

reports pertaining to SARA separations is the oxidative degradation of isolated groups. This paper has two objectives: (1) to present findings on HC oxidation during fractions isolation, emphasizing the importance of improved variable control leading to decreased sample degradation; and (2) to discuss certain aspects of published oxidation mechanisms, emphasizing the observed correlations between oxygen compounds abundance and crude oils intrinsic stability. Experimental Section Studied Oils and Vacuum Residua. Venezuelan and Mexican crude oils and residua were studied in this work. The Venezuelan crudes were sampled from four regions: (1) Maracaibo Lake basin, (2) Southern Barinas-Apure basin, (3) Orinoco Heavy Crude Oil Belt, and (4) Eastern Monagas-Anzoategui basin. Properties of Venezuela’s vacuum residua 21-23 and crude oils24-26 have been previously reported. The Mexican crudes labeled as Puerto Ceiba (PC) and KuMaloob-Zaap Mayan (KU) were gathered from the southern production region of Tabasco state and from the offshore region of the Gulf of Mexico, respectively. Some properties of these oils are given in Table 1. Two other crude oils gathered from wells drilled within the same production regions were selected for oxidation/stability correlation studies. Their properties will be described within the Discussion section. Multidimensional Preparative SARA Separation. A multidimensional HPLC separation methodology was developed for the quantitative recovery of HC SAR group-types from vacuum residua. Asphaltenes were precipitated from residua samples by treatment with an excess of n-heptane (40:1 wt/vol). SAR group-types were separated from maltenes with a combination of nitrile-derivatized silica plus nude silica HPLC columns in series. The nitrile column acts as a resin trap. The silica column acts as an aromatics trap. Saturates are unretained in the system, being eluted with the initial mobile phase (cyclopentane). Trapped hydrocarbons are released by, respectively, backflushing the traps with chloroform/methanol (polars from nitrile column) and pure methylene/chloride and/or pure chloroform (aromatics from silica columns). Details on this methodology have been published.8 One important aspect related to this methodology is the sample dissolution process for its injection into the HPLC system. The general procedure consists of weighing the sample inside a vessel (19) Boukir, A.; Aries, E.; Guiliano, M.; Asia, L.; Doumenq, P.; Mille, G. Chemosphere 2001, 43, 279. (20) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19 (1&2), 1. (21) Carbognani, L.; Garcı´a, C.; Izquierdo, A.; DiMarco, M. P.; Pe´rez, C.; Rengel, A.; Sa´nchez, V. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 1987, 32, 406. (22) Izquierdo, A.; Carbognani, L.; Leo´n, V.; Parisi, A. Prepr. Pap.s Am. Chem. Soc., DiV. Pet. Chem. 1988, 33 (2), 292. (23) Carbognani, L.; Espidel, J.; Carbognani, N.; Albujas, L.; Rosquete, M.; Parra, L.; Mota, J.; Espidel, A.; Querales, N. Pet. Sci. Technol. 2000, 18 (5-6), 671. (24) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13 (2), 351. (25) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes, Asphalts, 2. DeVelopments in Petroleum Science, 40; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, Lausanne, New York, Oxford, Shannon, Tokyo, 2000; Chapter 13, pp 335-362. (26) Carbognani, L.; Contreras, E.; Guimerans, R.; Leo´n, O.; Flores, E.; Moya, S. SPE 64993. Proc. 2001 Int. Symp. Oilfield Chem., Houston, TX, 2001.

provided with airtight closure, adding cyclopentane, and allowing the sample to dissolve in the closed vessel for approximately 1218 h before transferring it into a volumetric flask. Closed vessels were commonly left inside laboratory cabinets, not exposed to sunlight. The dissolution of solid or very viscous residua is facilitated without operator intervention and avoids the use of excessive amounts of solvent. Asphaltene Isolation Details. Sample aliquots were poured into round-bottom flasks. n-Heptane in a ratio of 40 mL/g sample was added and the mixture was refluxed for 45 min after solvent boiling started. The flask contents were left standing until laboratory temperature was reached (25 °C, about 4 h required). Precipitated solids were filtered (Whatman paper #41). The filtered solids were placed inside a Soxhlet extractor thimble and were extracted with n-heptane until no color was visually observed in the extractor (about half a day required). The extracted solids were recovered with methylene chloride. Paper fibers and mineral fines were removed from the extract solution by filtering again with Whatman #41 paper. Until this step, no provision had been taken for protecting the samples from atmospheric influence. Solvents were then removed with a rotary evaporator provided with a nitrogen blanket and a vacuum system. At the end of the distillation process, benzene or toluene was added to aid in water removal by azeotropic distillation. The solids were brought to constant weight inside a vacuum oven maintained at 80 °C and 130 mmHg (about 2 days required). Particle Size Effects on Asphaltene Precipitation from Vacuum Residua; Characterization of Asphaltene Isolated Fractions. Aliquots from a solid >550 °C vacuum residua from one Orinoco’s Oil Belt Heavy crude (identified as CN550+) were ground with an agate mortar. Three ranges of particles sizes were separated by using standard ASTM sieves. The mesh ranges will be described in the ensuing discussion. n-Heptane asphaltenes were isolated as described in the preceding section, employing both the unground resid and the sieved fractions as substrates. Isolated asphaltenes were characterized by oxygen elemental analysis (Leco CHNS-932 analyzer) and infrared spectrophotometry (IR). IR spectra were carried out on Nicolet 20SXB FTIR equipment. KBr pellets were prepared for acquisition of IR spectra, containing about 3-3.5 wt % of sample. More experimental details are available.23 Preparative Size Exclusion Chromatographic Separation (SEC) of Resins and Asphaltenes. A preparative SEC system was recently described for the isolation of “size-separated” subfractions from isolated resins and asphaltenes.27 Large styrene-divinylbenzene commercial columns were selected for this application. Resins and asphaltenes were separated from Venezuelan crudes which were produced from reservoirs located in Maracaibo Lake, Orinoco Oil Belt, and Northern Monagas-Anzoategui basin. Studies carried out with the later are particularly important due to the dramatic propensity of these oils to plug production tubing and generate organic sediments within storage tanks.24-26 Details on characterization of some of the fractions have been published.28 Resin Isolation from Mexican Crudes with Derivatized HPLC Stationary Phases. A preparative HPLC method that uses nheptane, methylene chloride, and chloroform as mobile phases and one amino-modified silica stationary phase for preparative HPLC was developed for the separation of resins from crude oils. Details of the method are available.29 A fraction of the isolated resins from both of the Mexican crudes was analyzed the day after they were separated from the crude oils. Hereafter, these fractions will be referred to as “fresh resins”. The remaining portion of resins was stored for a period of 24 months and preserved in different ways. One aliquot of resins separated from PC oil was kept dry in a transparent beaker covered by (27) Carbognani, L. Pet. Sci. Technol. 2003, 21 (11-12), 1685. (28) Carbognani, L.; Espidel, J. Pet. Sci. Technol. 2003, 21 (11-12), 1705. (29) Islas-Flores, C. A.; Buenrostro-Gonzalez, E.; Lira-Galeana, C. Energy Fuels 2005, 19, 2080.

Oxidation in Hydrocarbon PreparatiVe Fraction Isolation aluminum foil, whereas a different portion was dissolved in toluene and stored in a sealed amber glass vial. In the case of KU oil, the resins were dissolved in methylene chloride and stored inside a sealed amber glass vial. Characterization of Mexican Resins. The original fresh and stored resins were characterized by IR using a Nicolet 710 FTIR spectrophotometer and operated with a setting of 32 scans acquired at a resolution of 4 cm-1. Samples for IR spectra were prepared following a sample film-spreading technique from sample solutions. Laboratory Oxidation Methodology for Mexican Crude Oils. Two crude oils selected from the Mexican production regions described formerly were chosen for this part of the study. One Maya (18° API) labeled as HO (heavy oil) and one Puerto Ceiba (33° API) labeled as LO (light oil) were studied. Light ends from these oils were carried away by bubbling nitrogen for 4 h, maintaining the samples at 70 °C. Removal of light ends allows proper closure of mass balances after preparative group-type separations are carried out. A 50-mL quantity of the topped oil and 450 mL of toluene were placed inside a 1-L round-bottom flask provided with two openings. One reflux condenser was attached to one of the openings. Through the second opening, compressed dry air was passed into the solution through a 2-µm stainless steel filter acting as sparger. The oxidation was carried out for a period of 8 h, setting the temperature of the system at 102 °C (refluxing toluene) with a heating mantle. At the end of the oxidation process, toluene was removed in a rotaryevaporator set at 85 °C and 258 mmHg. The oxidized crude oil samples were labeled OX and the unoxidized as REF. SARA analysis was carried out for both sets of samples. Asphaltenes were separated and quantified by treatment with n-heptane in a ratio 40:1 vol/wt. SAR group-types were determined for the maltene fractions by preparative HPLC separation. Details of HPLC separation have been published.29 Determination of Precipitation Onsets. Onsets of asphaltene precipitation for OX/REF crude oil samples were determined by spectrophotometry. A UV-vis Perkin-Elmer λ35 spectrometer set at 780 nm was used for the analysis. Viscosity and strong absorbing properties of HO samples (OX and REF) were lowered by diluting them with toluene at a dilution ratio 1:1 by volume. The absorbance of solutions containing n-heptane/crude oil at different ratios was measured 24 h after addition of the alkane precipitant. The precipitation onset was identified as the dilution value (expressed as mL n-heptane/mL crude oil) for which a minimum in the absorbance versus dilution curve was observed.

Results and Discussion Sunlight Effects on Isolation of Resins from Venezuelan Vacuum Residua. A multidimensional HPLC system provided with one derivatized nitrile-silica column was assembled for HC group-type preparative SAR separations.8 The combination of one nitrile column with nude silica columns plus the appropriate solvent mixtures and chromatographic events allows the isolation of SAR groups in multigram quantities for further characterization. Continuous usage of the columns (1 year) and quantitative fraction recoveries were some of the major benefits derived from this separation methodology. Many advantages of derivatized HPLC stationary phases have been discussed by others as well.7,9 During the development of characterization studies carried out with Venezuelan vacuum residua upgraded in hydrocracking processes,21,22 unusual degradative properties were observed in some cases. HPLC separation of some maltene samples was carried out by preparing the solution during the same workday, making successive injections and separations of the freshly prepared sample solutions. In these cases, the prepared cyclopentane solutions were inadvertently exposed to sunlight. Some of the samples were observed to maintain the light-brown to dark-brown color originally displayed when the solution was

Energy & Fuels, Vol. 20, No. 3, 2006 1139

Figure 1. Amount of resins isolated from Venezuelan vacuum residua. Maltene solutions were stored with/without exposure to sunlight. Hydrocarbons SAR group-types were separated by HPLC.8 Notation in resid identification refers to cut temperature (°C).

prepared. However, others were observed to darken when exposed to sunlight for approximately 4 h. The results presented in Figure 1 illustrate the HC group-type distributions found during SAR preparative isolation for both protected and irradiated sample solutions. It can be observed that two of the analyzed residua (Me510+ and CN540+) displayed about the same amount of resins independently of sunlight irradiation. However, the sample Gu510+, which visually became black after a short exposition to sunlight, showed a dramatic change with respect to the resins contents. In this case, resins increased about 46%, increasing from about 32 to 47 wt % after an irradiation period of roughly 4 h. These effects were surprising, since they were made obvious only by serendipity. However, photochemical oxidation reactions of chemical functionalities such as those present in oil polar components have been evidenced before.30 Also, environmentally promoted HC photooxidation has recently been studied for spilled oils.19 Photooxidation is then a possibility that demands closer attention during HC isolation processes for maintaining sample integrity. Attempting to unveil clues on what is happening, some properties of the studied residua were determined. These are presented in Table 2. Selected feedstocks containing these residues were processed within Venezuela’s refineries. Those containing Gu510+ were reported to be “refractory” feeds, causing more fouling inside the upgrading units and requiring harsh operational conditions in order to reach the same conversion levels displayed by other feeds, mostly containing Orinoco’s heavy oil residua such as CN540+ and Me510+. As supported during the ensuing discussion, the data displayed in Table 2 permit identification of possible causes for the refractory nature of the Gu feed. However, no conclusive explanations were provided with these results for the diverse photooxidative behaviors observed. Different levels of sulfur are believed to be one of the causes for diverse upgradeability of the feeds. The C-S bond is weaker than C-C bonds, particularly if the sulfur-bearing functionalities are not aromatic (i.e., thiophenic).16 Orinoco’s heavy crudes resemble Athabasca bitumens in many regards. The Canadian bitumens have been shown to possess significant amounts of sulfidic sulfur.16 If this also holds true for the majority of (30) Strausz, O. P.; Gunning, H. E. Chapter in The Chemistry of Sulphides; Trobolsky, A. V., Ed.; Interscience Publishers (John Wiley & Sons), New York, 1968; p 23.

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Carbognani and Buenrostro-Gonzalez

Table 2. Properties of Venezuelan Vacuum Residua resida

Me510+ CN540+ Gu510+

whole resid

nC7 asphaltene fraction

processabilityb

oil matur.c

V (ppm)

Ni (ppm)

S (wt %)

wt %

pericond. param.d

H/C

high high medium

44 48 51

531 1038 62

120 200 190

3.69 4.50 1.22

20.0 18.8 16.3

1.63 1.15 1.98

1.13 1.11 1.05

a Notation refers to cut-points temperatures (°C); Me and CN samples are Orinoco’s extraheavy oils; Gu came from Llanos-Apure basin. b Field (refinery’s) relative rating in hydrocracking processes. c Metalloporphyrin maturity parameter, Porphyrin DPEP/Etio ratio.40 (Values near 100: immature oil; near 0: mature oil.) d Aromatics pericondensation parameter determined by 1H NMR giving a rank for the abundance of junction carbon atoms within polyaromatic molecules (the higher the value, the more pericondensed, see refs 21, 22).

Orinoco’s crudes, as reportedly is the case for at least one (Cerro Negro31), then it is easy to explain that their upgradeability will be larger compared to the feed possessing less sulfur (Gu). The aromatic pericondensation index, higher for the Gu feed as shown in Table 2, is likely important from an upgrading point of view, since it will be more difficult to access the increased number of carbon atoms located in the inner aromatic junctions of aromatic moieties.32 Another property believed to be of great consequence is the lower hydrogen content of the asphaltene fraction showed by the Gu residue. H/C atomic ratios have been shown to correlate with coking tendencies in a dramatic way.33 Reduced hydrogen content always leads to increased coke production. The fouling caused by the Gu residue can then be ascribed in part to its hydrogen deficiency. One interesting property of the Gu feed is its reversed measured metal ratio. Most of the Venezuelan crudes typically show V/Ni ratios of about 5-10:1, which is typical of their saline-marine depositional environments. This is not the case for Gu, suggesting another type of organic matter or depositional conditions giving origin to Gu crudes. In closing this section of the discussion, it is to be said that at this point, all the discussed aspects provided some clues for the diverse upgradeability of the studied vacuum residua, but no reasons for their diverse photooxidative behaviors. One point deserving further research is the diverse geochemical origin of the studied samples. Air Influence on Asphaltenes Isolation from Venezuelan Vacuum Residua. Factors influencing solvent precipitation of asphaltenes have long been studied.6,23,34 However, when solid or very viscous vacuum residua are the starting materials, representative sampling is an issue. Embrittlement of the residue through freezing in liquid nitrogen before fracturing is one suggested procedure, not widely followed due to the problems caused by water vapor precipitation over the cold surface. Sample homogeneity is a problem for which the addition of a certain proportion of a suitable solvent is recommended for sample predissolution and homogenization. A greater ratio of alkane precipitant is suggested for balancing the solvent power, with varying results depending on the procedure.23,35-37 Grinding is another published technique evaluated for improving the diffusion and extraction of trapped maltenes inside solid particles.23 Results from this approach will be discussed, (31) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1986, 9 (6), 357. (32) Izquierdo, A.; Carbognani, L.; Leo´n, V.; Parisi, A. Fuel Sci. Technol. Int. 1989, 7 (5-6), 561. (33) Wiehe, I. A. Energy Fuels 1994, 8 (3), 536. (34) Speight, J. G.; Long, R. B.; Trowbridge, T. D. Fuel 1984, 63 (5), 616. (35) Tsuji, H.; Yamazoe, S.; Ohno, Y.; Fukuy, Y. Report on APD200A Equipment; Cosmo Ventures: Tokyo, Japan, 1987. (36) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (12&12), 1551. (37) Andersen, S. I.; Keul, A.; Stenby, E. Pet. Sci. Technol. 1997, 15 (7&8), 611.

Figure 2. Particle size effect during the isolation of n-heptane asphaltenes from a Venezuelan Orinoco Oil Belt vacuum residue (CN550+). Plotted points represent the average of a minimum of three analysis replicas. Bars included indicate the spanned ranges for analysis replicas. Atmosphere inertness was not controlled during the precipitation and filtration steps. “Coarse” refers to ungrounded particles.

focusing on the oxidative properties evidenced when diverse surface areas are chosen for the experiments. A brittle solid was selected for these experiments, the >550 °C vacuum residue from Cerro Negro Heavy Petroleum (Orinoco’s Oil Belt). Results presented in Figure 2 demonstrate that the amount of isolated asphaltenes increased from 23.3 to about 25 wt %, when small resid particles (30-100 mesh) were selected instead of coarse resid particles for the alkane precipitation. These results were initially surprising, since with smaller resid particles (i.e., more surface area) less maltene occlusion was expected and consequently a smaller percentage of precipitates was also expected. However, the results were the antithesis as exemplified in Figure 2. Characterization of isolated asphaltenes provided an explanation for these findings. Oxygen contents were observed to be greater for asphaltenes isolated from small residua particles compared to asphaltenes precipitated from coarse particles. Evidence of this is presented in Figure 3. The differential IR spectrum shows that asphaltenes separated from the smaller particles, enriched in oxygen content, displayed greater absorbance bands in the carbonyl (1661 cm-1) and sulfoxide (1033 cm-1) regions. These paramount findings correlate precisely with hydrocarbon oxidation mechanistic studies.10,11 Preceding findings point toward the unquestionable importance of achieving strict and optimal control of laboratory procedures. The precipitation, filtration, and Soxhlet extraction steps, as routinely practiced in many laboratories, do not take into consideration inert atmospheres. This omission equally applies to routine standard methodologies.38,39 During isolation (38) Asphaltene (Heptane insolubles) in Petroleum Products. Standards for Petroleum and Its Products, Standard N° IP 143/90; Institute of Petroleum: London, 1985. (39) ASTM D-3279. Standard Test Method for Heptane Insolubles; American Society for Testing and Materials, 1990.

Oxidation in Hydrocarbon PreparatiVe Fraction Isolation

Figure 3. Differential plot of the IR spectrum for asphaltenes isolated from small particles (80-100 mesh) minus the IR spectrum for asphaltenes isolated from coarse particles (ungrounded) (size effects formerly presented on Figure 2). CN550+ vacuum residue was the source of asphaltenes in both cases. Elemental oxygen contents were, respectively, 1.070 ( 0.003 and 1.025 ( 0.002 wt %.

steps, air influence can be deduced from the results presented in Figure 2. These findings suggest that asphaltene isolation methodologies should include mandatory procedures for avoidance of air influence. Is Oxidation Related to Crude Oil Intrinsic Stability? Recent studies on physicochemical properties of oils and their SEC isolated fractions focused on possible differences responsible for the intrinsic stability displayed by oil samples in field operations.27,28 Previous investigations revealed that samples containing highly aromatic polar fractions coexisting with highly paraffinic maltene phases showed decreased stability.24,25 There are also preliminary indications that these facts are in some way related to the thermal maturity of oils, those more mature being prone to cause more precipitates during production, transportation, and upgrading.40 Preliminary experimental results suggest that polar resin and asphaltene fractions isolated from unstable oils are enriched in oxygenates.28 Examples are presented in Figure 4, supporting this assertion. The most significant oxygenates absorption bands observed in the presented IR spectra are those corresponding to sulfoxides (ca. 1030 cm-1), carbonyl types (1650-1680 cm-1), carbonyl bands from carboxylic acids (1700-1720 cm-1), and C-O-C types which were observed in fewer cases, likely related to alcohols (1270-1320 cm-1). Three claims can be made in examining the results presented in Figure 4. (1) Resins in all cases showed the greatest contribution of polar bands compared to asphaltene fractions. This fact can be rationalized by considering that the spectral contribution of a functional group within smaller molecules should produce a corresponding stronger band in their IR spectrum. (2) Typical oxygenated bands (carbonyl and sulfoxides) are not observed in the asphaltenes derived from stable crudes and are barely visible in asphaltene samples derived from the studied unstable oils. (3) The low-molecular-weight fractions of both unstable and stable oils reveal the presence of oxygenated bands in the IR spectra. However, in the case of unstable crudes, these bands are dramatically intensified. The intention of presenting the above facts is to attempt to find answers to a couple of questions: (1) Are oxidation products in some way related to the undesirable characteristics of unstable oils? (2) Are oxygen species generated within the reservoir or are they created during the analysis process? (40) Carbognani, L. Preliminary Insights on the Crude Oil Maturation Effects on Asphaltene Stability and Processing. Pet. Sci. Technol., accepted for publication.

Energy & Fuels, Vol. 20, No. 3, 2006 1141

Figure 4. IR spectra for resins and asphaltenes isolated from Venezuelan crudes and vacuum residua. Sample identification includes cut temperatures (°C) for studied vacuum residua. Asphaltenes were precipitated with n-heptane. Resins were preparatively isolated by HPLC.8 Low-molecular-weight asphaltene fractions were preparatively isolated from whole asphaltenes by SEC. 27

Aerobic hydrocarbon oxidation is generally accepted to occur in shallow reservoirs (less than 1-km depth, temperatures e 60 °C). A recent article using advanced synchrotron radiation techniques reported that the formation of solid deposits mostly occurs in the shallow horizons of a 1-km deep well.41 With this tool, a fast analysis of properly preserved samples showed large compositional changes taking place from deep layers to upper layers. Abundant sulfides present in deep layers disappear in shallow horizons, giving rise to sulfoxides and sulfone species. These transformations correlated with the appearance of solid deposits, where aggregation of the oxidized forms with clays and minerals was observed to occur. Other authors have previously described these interactions of oxygen compounds with mineral matter, being the cause of solid precipitation and emulsion stabilization.42-46 These published results provide an answer to one of the questionssoxygen compounds are indeed related to crude oil stability and deposits formation. The results suggest that aerobic oxidation is acting in shallow wells such as the one described by Chouparova and co-workers. However, in the case of Venezuela’s unstable crudes, whose polar fractions are enriched in oxygenates (Crude S20 and C3 from Figure 4), an oxidative mechanism like the one described by Chouparova et al.41 cannot be ruled out but is very difficult to support, since these samples originated at very deep sediments (5-6-km depth). Without knowing the burial history it is not possible to assume an aerobic oxidative generation of the oxygenates, which further survived the burial process. Recent evidence has been published that reveals that anaerobic bio-oxidation can also take place in deep reservoirs.47 (41) Chouparova, E.; Lanzirotti, A.; Feng, H.; Jones, K. W.; Marinkovic, N.; Whitson, C.; Philp, P. Energy Fuels 2004, 18, 1199. (42) Ignasiak, T.; Kotlyar, L.; Sanman, N.; Montgomery, D. S.; Strausz, O. P. Fuel 1983, 63 (3), 363. (43) Mikula, R. J.; Axelson, D. E.; Sheeran, D. Fuel Sci. Technol. Int. 1993, 11 (12), 1695. (44) Kotlyar, L. S.; Sparks, B. D.; Kodama, H.; Gratham-Bellew, P. Energy Fuels 1988, 2, 589. (45) Kotlyar, R. S.; Ripmeester, J. A.; Sparks, B. D.; Woods, J. R. Fuel 1988, 67 (11), 1529. (46) Axelson, D. E.; Mikula, R. J.; Potoczny, Z. M. Fuel Sci. Technol Int. 1989, 7 (5&6), 569. (47) Aitken, C. M.; Jones, D. M.; Larter, S. R. Nature 2004, 431, 291.

1142 Energy & Fuels, Vol. 20, No. 3, 2006

Carboxylic metabolites from the anaerobic oxidation of naphthalene were detected in many of the studied set of 77 crudes. Origin of the studied samples was diverse (marine or lacustrine), citing reservoirs with maximum thermal gradients reaching 85 °C. This is a new field of hydrocarbon research, most interesting for further studies. However, at this point there is no certainty that a mechanism like this one is operating in deep crudes, such as S20 and C3 included in Figure 4. Reservoir temperatures of layers giving origin to these Venezuelan samples reportedly are in the vicinity of 150 °C. On the basis of no additional information, it is therefore difficult to believe in the anaerobic bio-oxidation mechanism for the observed oxygenated species in the deep S20 and C3 Venezuelan oils. The lack of clear evidence supporting the aerobic or anaerobic mechanisms for generation of oxygen compounds in very deep Venezuelan crude oils opens the door to the possibility of other causes, one being the general subject covered within this paper. The cause proposed is hydrocarbon oxidation during handling, storage, shipping, and analysis of the samples. Conducted queries showed that no provisions were taken by field operators for preventing the damage of the samples before arriving at the analysis laboratory. Also, no prevention was taken during asphaltene precipitation, since no indications were included within the standard methodology for using a precautionary inert atmosphere. Asphaltene oxidation of samples stored under presumed inert nitrogen atmospheres has been recently reported. This phenomenon is believed to be a cause of intermolecular hydrogen-bonding, aggregation, and decreased solubility for these complex HC mixtures.48 Consequently, lack of proper precautionary actions has, with high probability, caused oxidation in samples C20 and C3, after these crude oils were carried to surface facilities. This possibility suggests the importance of exerting careful control of transportation, handling, and analysis protocols for guaranteeing sample integrity. However, another issue demands further attention. It is the fact that independently of the origin of the oxygenates, these species seem to contribute in greater amounts to those samples clearly defined as unstable by field operators. This is the case of the studied C20 and C3 samples. All these findings agree with the previously reported correlation between oxygen compounds abundance, crude oil instability, and tendency to solid deposition.24-25,42-46 Storage Influence on Resins Isolated from Mexican Crude Oils. Figure 5 presents the IR spectra for the different resin fractions from Puerto Ceiba crude oil: (a) fresh, (b) stored dry under air and light exposure, and (c) stored in toluene without exposure to sunlight or ambient air. Both stored resin fractions, those stored under air and sunlight exposure (b) and those stored in a closed amber vial (c), suffered from oxidation processes that can be deduced by the appearance of one band at 1270 cm-1, corresponding to C-O-C types and the increase of a band in the carboxylic region (1700 cm-1). However, the spectrum of resins stored under air and sunlight exposure shows a higher contribution to the carboxyl band. This discovery suggests that storage under seemingly protected conditions from air and sunlight partially prevented further extended oxidation processes for fraction c. However, at this point it is clear that to ensure oxygen absence in the closed vial, additional degassing procedures are necessary. Solvent degassing and sample handling under a truly inert atmosphere (drybox) are mandatory in these instances. This was not done in the experiments here discussed, and the appearance of oxidation bands in fraction c is ascribed to the nonexhaustive sample handling. The band (48) Carbognani, L. Prepr.Pap.sAm. Chem. Soc., DiV. Pet. Chem. 2004, 49 (3), 268.

Carbognani and Buenrostro-Gonzalez

Figure 5. IR spectra of Mexican PC resins: (a) freshly isolated, (b) stored dry for 24 months, exposed to light and air, and (c) stored for 24 months in toluene solution maintained in one amber closed vial.

Figure 6. IR spectra of Mexican KU resins: (a) freshly isolated, (b) stored for 24 months in methylene chloride solution maintained in one amber closed vial.

appearing at 740 cm-1 in the spectra for both stored resins is a feature caused by toluene remaining during sample preparation for IR spectra acquisition. Regarding KU resins whose IR spectra are presented in Figure 6, effects derived from storage can be observed by the increase of the carboxyl band at 1710 cm-1. The remaining oxygenate

Oxidation in Hydrocarbon PreparatiVe Fraction Isolation

Energy & Fuels, Vol. 20, No. 3, 2006 1143

Table 3. Hydrocarbon Group-type SARA Distributions for Oxidized and Unoxidized Mexican Crude Oilsa LO (Puerto Ceiba light oil)

HO (Mayan heavy oil)

fractionb

REF

OX

REF

OX

saturates aromatics resins asphaltenes

48.5 26.8 22.1 2.5

49.4 26.4 21.3 2.9

24.0 28.8 30.8 16.4

23.7 19.2 36.1 21.0

a Results presented in wt %. b Determined by n-heptane deasphalting and HPLC maltene SAR separation;29 REF/OX: respectively, unoxidized/ oxidized samples.

bands (C-O-C at 1270 cm-1 and SdO at 1030 cm-1) were not observed to increase for stored KU resins. As formerly discussed, a partially protected environment provided by amber closed vials can be credited for avoidance of extended oxidation of this sample, allowing only the appearance of additional carboxylic acid species. However, other contributing factors can be also present, as will be discussed. Despite the fact that PC resins stored under closed conditions showed the appearance of diverse oxygen compounds (carboxylics, C-O-C types, and sulfoxides), this was not the case for KU resins stored under similar conditions (only carboxylics were observed). Then, the multiplicity of oxygen functionalities generated for PC resins can be attributed to causes other than storage. Considering that the PC crude is an unstable sample from the production point of view (Table 1), its large oxidation tendency can be correlated with its intrinsic stability. This brings again the proposal that there exists a correlation between oxidative tendencies of crudes and their intrinsic instability. This has been already highlighted in the preceding discussion focused on Venezuelan samples. The existence of two sets of crude oils, namely, stable and unstable, has been introduced in the past.25 Recently, grouping into two families of a very large set of crude oils from different regions worldwide has been presented.49 Understanding the possible correlations between stability aspects and properties for these two broad oil families seems an interesting area for further investigation. Preliminary Findings on the Stability of Oxidized “Stable” and “Unstable” Crude Oil Samples. To get preliminary data supporting the discussed correlation among oxygen species with the intrinsic stability of crude oils, one Mexican “stable” (HO) and one Mexican “unstable” oil (LO) were submitted to an oxidative process. Compositional changes for unoxidized samples (REF) were compared to those of oxidized aliquots from the same crude oils (OX). These compositional comparisons were established in terms of SARA hydrocarbon group-type distributions. SARA analyses are presented in Table 3. As can be seen, composition of the light oil (LO) was not affected very much by the oxidative process. On the other hand, the heavy oil (HO) dramatically increased its resin and asphaltene contents, with the balance being the disappearance of aromatic compounds. The stability of unoxidized/oxidized samples was determined by titration experiments with a precipitant n-alkane. This approach ranks crude oil stabilities according to the amount of precipitant needed for initiating asphaltenes aggregation. The more intrinsically unstable a sample is, the less precipitant is required. A recent article discusses several aspects about these types of titration experiments.50 n-Heptane was selected as the precipitant, and the results are presented in Figure 7. For these experiments, absorbance initially decreases due to dilution with n-heptane. At a certain point, absorbance starts to increase due (49) Evdokimov, I. N. Fuel 2005, 84, 13. (50) Rogel, E.; Leon, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J.; Zambrano, A. Energy Fuels 2003, 17, 1583.

Figure 7. Precipitation onsets for unoxidized/oxidized Mexican crude oil samples. n-Heptane was used as precipitant alkane. UV-vis absorbance detection was carried out (780 nm). HO/LO: heavy oil/ light oil gathered, respectively, from Mayan/Puerto Ceiba Mexican oilfields. For carrying out these experiments, HO samples (OX and REF) were diluted with toluene at a dilution ratio 1:1 vol.

to solid asphaltene particles, scattering part of the visible light radiation. The minima observed within the curves are considered to be the onset asphaltene precipitation points. At the end of the titration experiment, a decrease in absorbance is again observed, this time caused by sedimentation of asphaltene particles. As can be clearly deduced from the results in Figure 7, after oxidation both crude oil samples were destabilized. These findings agreed with the starting premise that oxidation is related to crude oil stability. However, part of the hypothesis was not confirmed with these preliminary experiments, because both “stable” and “unstable” oils became more unstable after the oxidative process. It is presumed that this happened because the set oxidation conditions appeared to be very severe for the stable heavy oil, as deduced for its dramatic compositional change (Table 3). The results presented in Figure 7 also provided supporting evidence that the crude oil considered “unstable” from an operational point of view indeed proved to be more unstable both before and after oxidation when compared with the samples from the “stable” oil. Less precipitant was always required for the unstable samples. The decreased amount of precipitating alkane for the unstable oxidized crude was also large in magnitude, being 0.15 mL n-heptane/mL oil compared to 0.1 mL n-heptane/mL oil for the stable crude oil. Conclusions Crude oils are complex mixtures of organic compounds, prone to oxidative aerobic or anaerobic processes. This paper presents evidence that oxidation processes additionally take place during sampling, storage, transportation, and analysis of samples, suggesting the importance of exerting careful control of these operations for preserving the integrity of samples. Contribution of oxygenates is more important for samples clearly identified as unstable and prone to solid deposition within field facilities. Understanding the origin of oxygenates demands further investigation, since the presence of oxygenates in oils correlates with their observed intrinsic stabilities and solid deposition tendencies.

1144 Energy & Fuels, Vol. 20, No. 3, 2006 Acknowledgment. Helpful discussions held with Drs. A. Izquierdo, J. Espidel, O. Leon, and the late F. Cassani are acknowledged by one of the authors (L.C.). Funding provided by PDVSA during the 1990s made possible getting the data presented for Venezuelan samples. IMP is acknowledged for funding (Labo-

Carbognani and Buenrostro-Gonzalez ratorio de Termodinamica PVT, Project I.00348) and permission to publish the results for Mexican crudes. Dr. F. Lopez-Linares and John Thompson from AICISE are credited for the critical review and improvement of the original manuscript. EF050379G