ancheyta

Extraction and Characterization of Asphaltenes fromDifferent Crude Oils and Solvents

J. Ancheyta,*,†,‡ G. Centeno,† F. Trejo,†,§ G. Marroquın,† J. A. Garcıa,†E. Tenorio,† and A. Torres†

Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Mexico 07730 D.F., Mexico,Instituto Politecnico Nacional, ESIQIE, Mexico 07738 D. F., Mexico, and

Facultad de Quımica, UNAM, Ciudad Universitaria, Mexico 04510 D.F., Mexico

Received December 21, 2001

Three crude oils were employed for precipitation of asphaltenes using two solvents, n-pentaneand n-heptane. Crude oils were analyzed by API gravity, elemental composition and metalscontent. Asphaltenes were characterized by VPO molecular weight, liquid State 1H and 13C NMR,elemental composition, and metals content. Spectra were divided in three and two different regionsfor 1H and 13C NMR, respectively, to determine the most important structural parameters ofasphaltenes. To avoid errors when determining asphaltene content and characterization, a solvent-to-oil (S/O) ratio of 60:1 was used. This optimal ratio was defined after conducting variousexperiments with different values of S/O in the range of 5:1 to 100:1. It was found that solventtype has a very important influence in composition of asphaltenes, which were also very differentfor the three crude oils studied. Aromaticity of asphaltenes was higher when n-heptane wasemployed.

Introduction

Asphaltenes are the heaviest and most complexfraction of crude oil. It consists of condensed polynucleararomatics and contains small amounts of heteroatoms(S, N, and O), and traces of nickel and vanadium.Speight1 has defined asphaltenes as the brown andblack powdery material produced by treatment ofpetroleum, petroleum residua, or bituminous materialswith a low-boiling liquid hydrocarbon, e.g., pentane orheptane; being soluble in benzene (and other aromaticssolvents), carbon disulfide, and chloroform (or otherchlorinated hydrocarbon solvents).

Asphaltenes are the main cause of problems associ-ated with catalytic processing of petroleum residua,since they adversely affect the overall rate of hydrode-sulfurization, act as coke precursors, contribute tocatalyst deactivation, etc.2

Studies of petroleum asphaltenes have rapidly in-creased during the past years because of the increasingproduction of heavier crude oils. This can be clearly seenin Figure 1, which illustrates the number of paperspublished in scientific journals from year 1950 untilNovember 2001. Due to this growing volume of heavycrude oils refiners face drastic changes in petroleum feedproperties, which will affect all refinery conversionprocesses. For this reason, our research group has beenextensively studying how to improve the quality of

heavy oils by catalytic hydrotreating in one3,4 and tworeaction stages.5

As it is well-known asphaltenes are the primary causeof catalysts deactivation during hydrotreating of heavyoils, in which oil and resin fractions are converted veryfast to smaller fragments compared to asphaltenesfraction. These complex molecules are precipitated onthe catalyst surface and block the pore mouth.6

Hydrotreating catalysts used for heavy oils are dif-ferent than those employed for lighter fractions, sincethe formers are operated comparatively at higher reac-

* Author to whom correspondence should be addressed. FAX(+52-5) 333-8429. E-mail: [email protected].

† Instituto Mexicano del Petroleo.‡ Instituto Politecnico Nacional.§ Ciudad Universitaria.(1) Speight, J. G. Handbook of Petroleum Analysis; John Wiley &

Sons: New York, 2001.(2) Calemma, V.; Rausa, R.; D’Antona, P.; Montanari, R. Energy

Fuels 2001, 12, 422-428.

(3) Ancheyta, J.; Betancourt, G.; Marroquın, G.; Perez, A.; Maity,S. K.; Cortes, M. T.; del Rıo, R. Energy Fuels 2001, 15, 120-127.

(4) Ancheyta, J.; Maity, S. K.; Betancourt, G.; Centeno, G.; Rayo,P.; Gomez, M. T. Appl. Catal., A 2001, 216, 195-208.

(5) Ancheyta, J.; Betancourt, G.; Marroquın, G.; Centeno, G.; Cas-taneda, L. C.; Alonso, F.; Munoz, J. A. D.; Gomez, M. T.; Rayo, P. Appl.Catal., A 2002, 233, 159-170.

(6) Trimm, D. L. Deactivation, Regeneration and Disposal of Hy-droprocessing Catalysts. In Catalysts in Petroleum Refining 1989;Trimm, D. L., Ed.; Elsevier: Amsterdam; Vol. 41.

Figure 1. Number of publications about asphaltenes in theperiod 1950-2001.

1121Energy & Fuels 2002, 16, 1121-1127

10.1021/ef010300h CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 08/14/2002

tion severity. In the case of heavy crude oils, the designof hydrotreating catalyst is a challenging problembecause of the presence of asphaltenes which deactivatethe catalyst at a faster rate.

Therefore, to develop suitable catalysts for heavycrude oil hydrotreating, studies of characterizing as-phaltenes are very important. Much emphasis has beenput on characterization in terms of chemical structureand elemental analysis. The techniques frequently usedto characterize asphaltenes are IR, NMR, ESR, massspectroscopy, X-ray, electron microscopy, VPO, GPC,among others.

The use of a specific technique depends mainly on theobjective of the research. For instance, Wong and Yen7

used ESR to examine the ability of microwave power to

dissociate petroleum asphaltene macrostructure. Leonet al.8 employed elemental analysis, VPO, and NMR toexplore the possible relations among the structuralparameters of the asphaltenes, their self-aggregation,and the instability behavior of crude oils. Joshi et al.9studied the characteristics of the asphaltene precipitateutilizing optical scattering techniques. Shirokoff et al.10

reported the structural characterization of four SaudiArabian crude-oil-derived asphaltenes by X-ray diffrac-

(7) Wong, G. K.; Yen, T. F. J. Pet. Sci. Eng. 2000, 28, 55-64.(8) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000,

14, 6-10.(9) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden,

J. Energy Fuels 2001, 15, 976-986.(10) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997,

11, 561-565.

Table 1. Summary of Recent Works about Characterization of Asphaltenesa

author technique objective of the research

Wong and Yen7 ESR To examine the ability of microwave power todissociate petroleum asphaltenes macrostructures

Leon et al.8 ST, EC, VPO, 13C NMR To study aspects related to the asphaltenedeposition and composition of crude oils

Joshi et al.9 OST To study the characteristics of the asphalteneprecipitate

Shirokoff et al.10 XRD To report the structural characterization ofSaudi Arabia asphaltenes

Sharma et al.11 HRTEM To test the stacking and stacking disruptionBuenrostro et al.12 FD, FES, 13C NMR, IR To explore the aspects of asphaltene

chemical structureAlboudwarej et al.13 VPO To examine the effect of different degrees of

washing on the asphaltene propertiesGutierrez et al.14 VPO, TLC To isolate asphaltene fractions and study the

solubility in tolueneYarranton et al.15 VPO, IT To study the molar mass and solvent-water

interfacial tensions of asphaltenesGuiliano et al.16 GC/MS, EC, FTIR To investigate the potential of supercritical

fluid extraction for crude oil asphaltenesGroenzin and Mullins17 FD To survey the molecular size of a broad range

of asphaltenesFuhr et al.18 TGC To examine the analytical separation of

asphaltenes, wax, and soluble oil fractionsDomin et al.19 SEC, PDMS, VPO, LDMS, MALDI To examine and compare the molecular-weight

distributionsGroenzin and Mullins20 FD To analyze FD rates of very dilute solutions

of asphaltenesCarbognani et al.21 SEC, IR, 13C NMR To study the precipitation of asphaltenes during

oil production and storageArtok et al.22 Py/GC/MS, GPC, 1H/13C NMR,

MALDI TOFTo elucidate the distribution of the aliphatic

carbonsPeramanu et al.23 VPO, GPC, EC, 13C NMR To analyze the molecular weight of asphaltenes

and their compositionTojima et al.24 VPO, 1H NMR To apply a new asphaltene fractionation method

using a binary solvent systemMiller et al.25 VPO, SEC, EC, SANS, 13C NMR,

LDMS, HRMSTo separate and characterize Mayan asphaltene

in two fractionsMcLean and Kilpatrick26 FTIR, 13C NMR To determine and compare the composition of

asphaltenes of four different crude typesAndersen27 FTIR, EC, HPLC-SEC To investigate whether asphaltenes could be

extracted directly from the crude oil by a polarsolvent mixture

Andersen28-29 1H/13C NMR, FS, HPLC-SEC,VPO, EC

To investigate effects of the temperature onprecipitation of asphaltenes

Fuhr et al.30 VPO, 13C NMR To study some properties of asphaltenes precipitated asa function of the solvent and temperature

Adarme et al.31 VPO, 13C NMR To study how metals are removed from asphaltenesand study thermal and catalytic effects

a OST: Optical scattering technique. FD: Fluorescence depolarization. FES: Fluorescence emission spectroscopy. 13C NMR: 13-CarbonNuclear magnetic resonance. IR: Infrared. ST: Surface tension. EC: Elemental composition. VPO: Vapor pressure osmometry. GPC:Gel permeation chromatography. IT: Interfacial tension. ESR: Electron spin resonance. GC/MS: Gas chromatography/mass spectrometry.FTIR: Fourier transformed infrared. SEC: Size-exclusion chromatography. HTGC: High-temperature gas chromatography. XRD: X-raydiffraction. py/GC/MS: Pyrolysis gas chromatography/mass spectrometry. MALDI TOF: Matrix-assisted laser desorption/ionization time-of-flight. PDMS: Plasma desorption mass spectrometry. LDMS: Laser desorption mass spectrometry. HRTEM: High-resolutiontransmission electron microscopy. SANS: Small-angle neutron scattering. HRMS: High-resolution mass spectrometry. TLC: Thin-layerchromatography. FS: Fluorescence spectroscopy. HPLC: High-performance liquid chromatography.

1122 Energy & Fuels, Vol. 16, No. 5, 2002 Ancheyta et al.

tion, NMR, and HP-GPC. Table 1 summarizes some ofthe recent reports about asphaltenes characterizationwhere techniques and objectives of the research arehighlighted.7-31

During our studies of hydrotreating process to im-prove the quality of heavy crude oils, such as Maya(21.3°API), our main goal is to transform this crude intolight crude oils, such as Isthmus (33.1°API) or, evenbetter, into an Olmeca crude oil (38.7°API). To do thisrequires an appropriate catalytic system. The mainconcept for developing a suitable catalyst for thispurpose depends on the particle size, pore size, and itsdistribution in order to achieve optimum heteroatomsremoval as well as high metal tolerance. Here is whereasphaltenes characterization becomes very importantsince these complex molecules are precipitated on thecatalyst surface during hydrotreating and block the poremouth. They are also coke precursors and deactivate theHDT catalysts.

As was mentioned previously, Maya heavy crude oilis being used in our HDT experiments, and we wantthe hydrotreated crude oils to have properties similarto those lights crude oils (Isthmus or Olmeca). There-fore, it is mandatory to know details about asphaltenestructure and composition of these crudes in order tohave more information for catalyst design purposes. Forthis reason, we extract and characterize the asphaltenesfraction of Maya, Isthmus, and Olmeca crude oils, whichhave very different properties and asphaltene contents.

Experimental Section

Feedstocks. Maya, Isthmus, and Olmeca crude oils wereemployed for asphaltenes extraction. Main properties of thesethree crudes are shown in Table 2. It can be seen that Maya

crude is the heaviest with more content of contaminants andOlmeca is the lightest.

Extraction of Asphaltenes. The asphaltenes fraction wasisolated by addition of an excess of solvent to each of the threecrude oils used in this study. Various tests with differentsolvent-to-oil (S/O) ratios were performed in order to determinethe appropriate ratio for asphaltene precipitation.

Figure 2 illustrates the results obtained for precipitation ofasphaltenes in Maya crude oil. Two solvents were used forthese tests: (1) a high-pentane content refinery stream(composition in mol %: 40.9% n-C5, 30.5% i-C5, 3.4% C3-C4,and 25.9% C6-C7) and (2) n-pentane. For these tests we useda refinery stream because we were trying to substituten-pentane solvent by another cheaper solvent. However,asphaltene content was quite different with both solvents, andwe finally decided to use the common solvents. Anyhow, allthese experimental information was useful for deciding theoptimal S/O ratio.

Results showed in Figure 2 clearly indicate that asphaltenescontent remains constant after an S/O ratio of 40:1 for bothsolvents. Extraction with refinery solvent exhibited higherasphaltenes content than n-pentane, which is mainly due tothe content of isopentane in the former.32 To avoid errors inthe determination of the amount of asphaltene fraction andin its characterization, we decided to use a solvent-to-oil ratioof 60:1.

Figure 2 also shows results of repetition of some values ofasphaltene content with refinery solvent. The maximumabsolute difference observed was 0.1 wt %. This indicates thatrepeatability of extraction method is very good.

Once the S/O ratio was defined, asphaltenes were extractedfor other crude oils and solvent following the proceduredescribed in ASTM D-3279 method.

Briefly, a sample of crude oil (7.5 g) was mixed with 450mL of solvent in a 500 mL round-bottom flask in order to have

(11) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. EnergyFuels 2002, 16, 490-496.

(12) Buenrostro, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C.Energy Fuels 2001, 15, 972-978.

(13) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.;Akbarzadeh, K. Energy Fuels 2002, 16, 462-469.

(14) Gutierrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo. S.Energy Fuels 2001, 15, 624-628

(15) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem.Res. 2000, 39, 2916-2924.

(16) Guiliano, M.; Boukir, A.; Doumenq, P.; Mille, G.; Crampon, C.;Badens, E.; Charbit, G. Energy Fuels 2000, 14, 89-94.

(17) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677-384.(18) Fuhr, B. J.; Holloway, L. R.; Hammami. A. Energy Fuels 1999,

13, 336-339.(19) Domin, M.; Herod, A.; Kandiyoti, R.; Larsen, J. W.; Lazaro,

M-J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552-557.(20) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103,

11237-11245.(21) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13,

3, pp 351-358.(22) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nombra,

M. Energy Fuels 1999, 13, 287-296.(23) Peramanu, S.; Pruden, B.; Rahimi, P. Ind. Eng. Chem. Res.

1999, 38, 3121-3130.(24) Tojima, M.; Suhara, S.; Imamura, M.; Furuta, A. Catal. Today

1998, 43, 347-351.(25) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.;

Hunt, J. E. Energy Fuels 1998, 12, 1290-1298.(26) McLean, J. D.; Kilpatrick, P. L. Energy Fuels 1997, 11, 570-

585.(27) Andersen, S. I. Pet. Sci. Technol. 1997, 15, 185-198.(28) Andersen, S. I. Fuel Sci. Technol. Int. 1995, 13, 579-604.(29) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12, 51-74.(30) Fuhr, B. J.; Cathrea, C.; Jcoates, L.; Kalra, H.; Majeed, A. I.

Fuel 1991, 70, 1293-1297.(31) Adarme, R.; Sughrve, F. I.; Jonson, M. M.; Kidd, D. R.; Phillips,

M. D.; Shaw, J. E. Symposium on Resid Upgrading; AmericanChemical Society: Washington, DC, 1990, 614-618. (32) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149-152.

Table 2. Properties of Crude Oils

Maya Isthmus Olmeca

API gravity 21.3 33.1 38.7elemental analysis (wt %)

carbon 83.96 85.40 85.91hydrogen 11.80 12.68 12.80oxygen 0.35 0.33 0.23nitrogen 0.32 0.14 0.07sulfur 3.57 1.45 0.99H/C atomic ratio 1.687 1.782 1.788

metals (wppm)nickel 53.4 10.2 1.6vanadium 298.1 52.7 8.0

asphaltenes (wt %)in nC5 14.10 3.63 1.05in nC7 11.32 3.34 0.75

Figure 2. Effect of solvent-to-oil ratio on asphaltenes extrac-tion. (O) Refinery solvent and (b) n-C5. (/) Repetitions withrefinery solvent.

Asphaltenes from Crude Oils and Solvents Energy & Fuels, Vol. 16, No. 5, 2002 1123

an S/O ratio of 60 g/mL. The resulting solution was stirredgently for 20 min, heated to reflux (92 °C), and then allowedto settle for 1 h at room temperature. Afterward, the suspen-sion was filtered on a 1.5 µm pore size microfiber filter(Whatman GF/A). The precipitate was washed with threeportions of n-heptane of about 10 mL each until the solventwash was colorless.

Asphaltenes from Maya crude oil required up to 50 mL ofn-heptane during washing. This washing was used to drainout the precipitate coating the flask wall and to thoroughlyrinse the solid collected on the filter. Finally, the filteredsample was dried at 107 °C during 20 min and weighted fordetermining asphaltenes content.

Taking into account that precipitated asphaltenes canabsorb certain amount of maltenes and/or resin, before ana-lytical characterization we have checked it these componentsare still present. To do this some of extracted asphaltenes weredissolved in toluene at about 100 °C, and after allowing themixture to stand for 4 h, a total dissolution was observed.Additionally, asphaltenes were reprecipitated with n-heptanefollowing the above-described procedure and high percentagesof this material were found (about 98%). During the washingstep, n-heptane was also very light in color. All this indicatesthat the original asphaltene sample had very little contamina-tion of other materials.

Various extractions were needed to have enough amount ofsample for characterization, especially with Isthmus andOlmeca crude oils due to their small content of asphaltenes.

Analysis of Asphaltenes. The following methods wereemployed to analyze both crude oils and asphaltenes proper-ties:

(1) C, H, and N contents were obtained by combustion at950 °C on an ELEMENTAR VARIO EL model analyzer.

(2) Sulfur content was determined by combustion at 1350°C using a LECO SC-444 model analyzer.

(3) The measurements for oxygen contents were obtainedby difference then they are less accurate than the others.

(4) Nickel and vanadium contents were obtained by atomicabsorption. Solid samples were first heated at high tempera-ture (550 °C) to eliminate possible organic material. They werethen digested in a mixture of HCl/HNO3 with heating untiltotal dissolution. Finally, samples were filtered and analyzedin a Perkin-Elmer 5000 model spectrophotometer.

Additionally the following methods were used for character-izing the asphaltene fraction:

(1) Molecular weight was evaluated by vapor pressureosmometry (VPO) in a Corona Wescan 232A Model equipmentin toluene at 25 °C.

(2) Liquid State 1H and 13C NMR were carried out in a JEOLEclipse 300 model spectrometer operating at 1H resonancefrequency of 300 MHz and 13C resonance frequency of 75 MHz.

1H NMR spectra were obtained using a deuterated chloro-form (CDCl3) solution with a flip angle of 45°, tube diameterof 5 mm, and spectral width of 12 ppm while 13C NMR spectrawere evaluated by applying an inverse-gated decouplingtechnique to suppress NOE effect with a flip angle of 75 ° andspectral width of 220 ppm. Chromium acetylacetonate (Cr-(acac)3 0.1 M in the final solution) was added to ensurecomplete nuclear magnetic moment relaxation between pulses.In addition, tetramethylsilane (TMS) was employed as aninternal reference. These conditions are necessary to havequantitative 13C NMR signals.

The measurements were performed for 30000 scans. For 1HNMR, the spectrum was divided into three regions (0.5-2,2-4, and 6-9 ppm) and 13C NMR spectrum have been dividedonly into two different integration domains (10-60 and 110-160 ppm). The assignments of chemical shift range for 1H and13C NMR spectra were made according to the data presentedin Table 3.

Structural parameters for asphaltenes were determinedwith the data of nuclear magnetic resonance, molecular weight,and elemental analyses.

Result and Discussion

Crude Oil Properties. Asphaltenes contents againstAPI gravity of crude oils are presented in Figure 3. Lessasphaltenes precipitation was obtained with n-heptanecompare to n-pentane. This behavior is well documentedin the literature.33

Sulfur content and other heteroatoms content are veryclose related to asphaltene concentration in crude oil(Figure 4), which are lower in Olmeca crude and higherin Maya crude. The vanadium-to-nickel ratio is almostthe same as that for the three crude oils (5.1-5.6).

It is also observed in Table 2 that the hydrogen/carbon(H/C) atomic ratio is highest in Olmeca crude, whichimplies that it is more paraffinic than the others.

Analytical Data of Asphaltenes. The elementalanalysis (C, H, O, N, and S) and metals content (Ni andV) of asphaltenes obtained from Maya, Isthmus, andOlmeca crude oils with n-pentane and h-heptane arepresented in Table 4. The asphaltene composition ofthese three crude oils shown in this table covers a widerange. Particularly, metals (Ni+V) vary between 580and 1480 wppm while sulfur content is between 3.2 and8.5 wt %.

From left to right and for each solvent, carbon contentincreases while hydrogen, sulfur, and metals contentsdecrease as the crude oil becomes lighter. Nitrogen

(33) Speight, J. G. The Chemistry and Technology of Petroleum;Marcel Dekker, Inc.: New York, 1999.

Table 3. Assignments of 1H and 13C Chemical Shifts in NMR

chemical shift range(ppm from TMS) assignment

1H aliphatic NMR 0.5-2 â+γ hydrogen-to-aromatic ring (Hâ+γ)2-4 R hydrogen-to-aromatic ring (HR)

1H aromatic NMR 6-9 aromatic hydrogen (Har)13C NMR 10-60 aliphatic carbon (Cal)

110-160 aromatic carbon (Car)

Figure 3. Asphaltenes versus API Gravity of crude oils. (O)n-C7 and (b) n-C5.


content did not change when n-pentane was used, butit decreased when n-heptane was employed as solvent.Oxygen as calculated by difference, although not a veryaccurate measure is also lower in asphaltenes from lightcrude oil. The H/C atomic ratio and other elements-to-carbon ratio varied in the same way as the correspond-ing element was changed.

n-Heptane solvent yields a product that is differentfrom the n-pentane solvent. For example, the H/Catomic ratio of asphaltenes obtained with n-C7 solventis lower than that of the n-C5 precipitating medium.This indicates a higher degree of aromaticity in then-heptane precipitate. N/C, O/C, and S/C atomic ratioswere higher in the n-C7 precipitate, indicating higherproportions of these heteroatoms in this material.33

Differences in some heteroatom content betweenn-pentane and n-heptane solvents are smaller in Olmecacrude compared to that among other crude oils, whichis mainly due to Olmeca crude’s smaller asphaltene andheteroatom contents.

With asphaltene and sulfur contents in each crudeoil and sulfur content in the corresponding asphaltenefraction, a mass balance procedure has been performedto determine how much sulfur in each crude oil ispresent in asphaltenes fraction (asphaltenic sulfur). Wealso performed this mass balance procedure for otherelements, and the final results are summarized in Table5. Only the results for n-pentane are presented in thistable because asphaltene extraction is more completewith this solvent, and hence, calculations gave higher

percentages of asphaltenic elements than those obtainedwhen n-heptane was used.

It is observed that the three crude oils exhibited verydifferent percentages of asphaltenic heteroatoms. As-phaltenic sulfur, nitrogen, and oxygen are higher inMaya crude and lower in Olmeca crude, which corre-sponds to the behavior observed with these elementsin the whole crude oils. Asphaltenic nickel follows thesame trend with very similar percentages in Isthmusand Olmeca crudes. However, percentage of asphaltenicvanadium was higher in Olmeca crude compared to theothers. This may be attributed to its very low asphal-tenes content (∼1 wt %), in which vanadium surelyconcentrates.

Structural Properties of Asphaltenes. 13C NMRspectra of asphaltenes obtained with n-pentane asprecipitating medium are presented in Figure 5. Thedifferences in aliphatic (Cal: 10-60 ppm) and aromatic(Car: 110-160 ppm) carbons for asphaltenes of the threecrude oils are clearly distinguished. Similar differences

Table 4. Elemental Analysis and Metal Content of Asphaltenes

Maya Isthmus Olmeca

solvent: n-C5 n-C7 n-C5 n-C7 n-C5 n-C7

elemental analysis (wt %)carbon 81.23 81.62 83.90 83.99 86.94 87.16hydrogen 8.11 7.26 8.00 7.30 7.91 7.38oxygen 0.97 1.02 0.71 0.79 0.62 0.64nitrogen 1.32 1.46 1.33 1.35 1.33 1.34sulfur 8.25 8.46 6.06 6.48 3.20 3.48

atomic ratiosH/C 1.198 1.067 1.144 1.043 1.092 1.016O/C 0.009 0.009 0.006 0.007 0.005 0.006N/C 0.014 0.015 0.013 0.014 0.013 0.013S/C 0.038 0.039 0.027 0.029 0.014 0.015

metals (wppm)nickel 268.7 320.2 155.4 180.4 81.9 157.7vanadium 1216.6 1509.2 710.3 746.6 501.0 703.8

Figure 4. Asphaltenes in n-heptane versus heteroatomscontents of crude oils (O) metals (Ni + V) in wppm × 102, (b)nitrogen in wt % × 10-1, and (0) sulfur in wt %.

Table 5. Percentages of Asphaltenic Heteroatoms inEach Crude Oil with n-Pentane Solvent

Maya Isthmus Olmeca

sulfur 32.6 15.2 3.4nitrogen 58.2 34.5 20.0oxygen 39.1 7.8 2.8nickel 70.9 55.6 55.1vanadium 57.5 49.0 65.7ni+v 59.6 50.0 64.0

Figure 5. NMR spectra of asphaltenes using n-pentanesolvent. (A) Olmeca, (B) Isthmus, and (C) Maya.


are observed with the other solvent and 1H NMRspectra.

The main molecular parameters from NMR spectraare the aromatic carbon fraction or better known asaromaticity factor (fa), average number of carbons peralkyl side chain (n), percent of substitution of aromaticrings (As), and the aromatic ring number (Ra), whichare evaluated as follow:34,35

where Car are the total aromatic carbons, Cal the totalaliphatic carbons, Csub the alkyl-substituted aromaticcarbons, and Cp the peripheral aromatic carbons (Cp )Cus + Csub, Cus representing the unsubstituted aromaticcarbons).

The molecular parameters evaluated with eqs 1-4and VPO molecular weights of the three different originasphaltenes using n-pentane and n-heptane solvents arelisted in Table 6.

Asphaltenes precipitated with n-heptane showedhigher MW than those obtained with n-pentane, whichis mainly due to the solvent power: the ability of thesolvent to dissolve asphaltenes. For instance, asphaltenecontent in n-heptane for Maya crude oil were lower thanthat in n-pentane (11.32 and 14.10 wt %, respectively):both asphaltenes are different in composition as wasshown with elemental analyses. In the case of n-heptane, the asphaltene fraction is heavier than thatobtained with n-pentane implying that n-pentane pre-cipitates fractions with lighter asphaltenes.

As consequence of the higher MW exhibited by as-phaltenes precipitated with n-heptane solvent, othermolecular parameters are also different than thoseobtained with n-pentane. In general, fa, n, As, and Rawere also higher when n-heptane was employed. Somedata do not followed this behavior may be due toexperimental error.

Number of aromatic rings of asphaltenes in n-C7almost doubles those in n-C5, and they decreased as thecrude oil is lighter. Aromaticity factors were also higher

with n-heptane. With these results it is also confirmedthat n-heptane gives a more aromatic asphaltenicfraction than n-pentane.

Aromaticity factor was higher in Olmeca crude andlower in Maya crude. In addition, light crude oil hashigher percentage of asphaltenic vanadium as wasshown by elemental analyses. This implies that thealmost insignificant amount of asphaltenes in lightcrudes is more refractory than heavy ones.

Application of Results of Asphaltenes Charac-terization. To understand the mechanisms of catalystdeactivation by coke during HDT of heavy crude oils itis important to study the changes of asphaltene struc-ture and composition. First, it is necessary to know theasphaltene properties of the HDT feedstock, i.e., Mayacrude oil, and second, characterizing the asphaltenesobtained from light crudes, i.e., Isthmus or Olmeca, isalso necessary. Finally, asphaltenes from hydrotreatedcrude oils should be analyzed. All this information canbe employed to make changes in the properties of theHDT catalyst, mainly in pore size distribution.

In the case of the present study we have extractedasphaltenes from a hydrotreated crude oil with n-heptane, which was obtained at the following operatingconditions: 70 kg/cm2 pressure, 5000 ft3/bbl H2-to-oilratio, 0.5 h-1 LHSV, and reaction temperature of 420°C. Properties of this sample together with those of thehydrotreated crude oil are presented in Table 7.

It can be observed that hydrotreated product hasalmost the same sulfur content in asphaltenes and morethan twice the asphaltene content than Isthmus crudeoil. With respect to asphaltenes, it is seen that the H/Cmolar ratio of hydrotreated crude is higher than thatof the Maya crude oil indicating, that asphaltenes aremore aromatic in the former, which is also verified withNRM results.

fa and n of asphaltenes from Isthmus and hydrotreat-ed crudes are equal; however, Ra and MW are consider-ably lower in HDT crude oil asphaltenes, which indi-cates that the latter has a lesser number of aromaticrings.

Metals content increased in asphaltenes of HDT crudeoils which is due to the localization of these heteroatomsinside the asphaltene molecule, and because of this, theyremain almost unchanged after HDT, and hence theyare concentrated in asphaltenes.

Conclusions

Asphaltenes obtained from three crude oils have beenprecipitated with two solvents (n-pentane and n-hep-tane) and characterized using common techniques.

(34) Seki, H.; Kumata, F. Energy Fuels 2000, 14, 980-985.(35) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L.

O. Energy Fuels 1995, 9, 225-230.

Table 6. Structural Parameters for Average Molecule ofAsphaltenes

Maya Isthmus Olmeca

solvent: n-C5 n-C7 n-C5 n-C7 n-C5 n-C7

MW 3680 5190 2603 3375 1707 2663fa 0.47 0.52 0.59 0.57 0.61 0.62n 7.4 6.8 4.8 5.0 4.1 5.5Ra 35 62 34 45 24 40As 35.6 38.9 37.9 41.0 39.1 32.9

fa )Car

Car + Cal(1)

n )Cal

Csub(2)

As ) 100(percent substituted aromatic carbon

percent nonbridge aromatic carbon ) (3)

Ra )Car - Cp

2- 1 (4)

Table 7. Properties of Hydrotreated Crude Oil and of ItsAsphaltenes

hydrotreatedcrude

asphaltenesin n-C7

H/C molar ratio 1.230 1.097Ni, wppm 144 378V, wppm 36.9 1893sulfur, wt % 2.0 6.47asphaltenes content, wt % 6.5MW 1513fa 0.58n 5.05Ra 17.3As 35.6


Solvent type has a very important influence incomposition of asphaltenes. Molecular weight of as-phaltenes precipitated with n-heptane showed highervalues than those obtained with n-pentane, which wasattributed to solvent power.

Number of aromatic rings and aromaticity factorswere higher with n-heptane compared to n-pentane,

which indicates a higher degree of aromaticity inasphaltenes obtained with the former solvent.

Acknowledgment. The authors thank InstitutoMexicano del Petroleo for its financial support. F. Trejoalso thanks CONACyT for financial support.EF010300H


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