Effect of Precipitating Conditions on Asphaltene Properties and

May 26, 2015 - Intermolecular aggregation for these fractions was determined to follow the same order. .... In order to assess the properties of these...
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Effect of Precipitating Conditions on Asphaltene Properties and Aggregation Lante Carbognani Ortega,†,* Estrella Rogel,‡ Janie Vien,‡ Cesar Ovalles,‡ Hector Guzman,† Francisco Lopez-Linares,‡ and Pedro Pereira-Almao† †

Schulich School of Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada Petroleum and Materials Characterization Unit, Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94801, United States

Energy Fuels 2015.29:3664-3674. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/25/19. For personal use only.

‡

ABSTRACT: Asphaltene, resin, and maltene physical isolation procedures involving different alkane precipitants and solvent/ sample ratios were applied in this work to Athabasca bitumen vacuum residue. Samples were characterized by solubility profiling, size exclusion chromatography, fluorescence spectroscopy, X-ray photoelectron spectroscopy and density−viscosity analyses. Isolated fractions were found to display systematic property changes. Thus, it was found that denser, more polar, higher molecular weight (MW), more viscous, red-shifted fluorescence materials were sequentially ranked as follows: solvent extracted asphaltenes, C7 (unwashed) asphaltenes, C5 (unwashed) asphaltenes, resins, and maltenes. Intermolecular aggregation for these fractions was determined to follow the same order. Decreasing contents of resins in the same order were found to increase aggregation phenomena. This work further reports on aspects of possible practical interest, i.e., the liquid nature of asphaltenes at 300 °C and the possible existence of oxidative reactions affecting fraction isolation that follow standard methods which do not contemplate inert atmospheres. Preliminary assessment of chemical functionalities within isolated fractions highlighted the possible enrichment of pyrrolic compounds within resins and oxygen functionalities in asphaltenes.

1. INTRODUCTION The term “asphaltenes” describes one hydrocarbon group-type operationally isolated from petroleums and derived fractions via solubility fractionation, using as criterion the combined alkane insolubility and solubility in aromatics. The group comprises the most polar and higher molecular weight (MW) compounds, as indicated by Long.1 One among their most dramatic features is the severe viscosity increases caused by their presence,2 which is derived from their inherent ability to generate intermolecular aggregates among them, with other hydrocarbon fractions and with solids like reservoir minerals and oil facilities building materials. Apart from paving applications and other specialty uses (thermal-sonic insulation and carbon fibers) for which they are useful components, asphaltenes are generally undesirable materials since they precipitate during oil operations due to changes in temperature, pressure, and exposure to solid surfaces, thus contributing to plugging of reservoirs and solid deposition within tanks, pipelines, distillation towers, and upgrading units.3 Systematic studies on the variables affecting asphaltenes precipitation were carried out in the 1970−1980s by Speight and co-workers and the alkane chemical nature, dilution ratio, precipitation/filtration temperatures, and precipitation contact time effects summarized in a well-known monograph published by Speight.4 The complex interaction of the former parameters is believed to be a key cause for the existence of a wide set of standard asphaltene precipitation methods. From an analytical point of view, n-heptane (n-C7) precipitation is more reproducible,5 thus methods like ASTM D3279,6 D4124,7 and D6560,8 IP-143,9 DIN 51−595,10 and AFNOR NF T60− 11511 have been proposed and standardized choosing this precipitant. Precipitation with n-pentane (n-C5) resembles © 2015 American Chemical Society

more industrial deasphalting operations carried out with C4− C6 condensates, thus giving origin to corresponding standard methods like ASTM D893 (conceived for lubricant oils),12 ASTM D2007 (considers n-C5 deasphalting for carrying out hydrocarbon saturates, aromatics, resins, asphaltenes −SARAseparation),13 GOST 11858-56 (Russian standard)14 and AFNOR NF T60-115 (French method standardized for nC5/n-C7 asphaltene determination). Different conditions of precipitation, such as temperature, precipitant alkane, contact time, filtration temperature, solvent/sample ratios, filter pore size, and coprecipitated resin back extraction (or not) with the precipitant solvent, are set up within the former methodologies, thus making it very difficult or even impossible to compare results from different sources. The former aspects have been taken into account by other authors aiming to identify the best trade off for asphaltene isolation/characterization.15 One aspect not contemplated in the cited standard isolation techniques is atmosphere inertness; one article showed that the presence of air (oxygen) can have very important effects on both the yield and the determined asphaltene properties.16 Asphaltene precipitation/dissolution kinetics have not been widely studied; however, it is a topic addressed by Fogler and co-workers, indicating that high temperatures speed up precipitation kinetics17 and that higher contents of high polar asphaltenes lower dissolution kinetics.18 Authors studying the properties of precipitated asphaltenes have described that variations in their properties arise from compositional changes that follow regular patterns, namely, that Received: March 23, 2015 Revised: May 25, 2015 Published: May 26, 2015 3664

DOI: 10.1021/acs.energyfuels.5b00597 Energy Fuels 2015, 29, 3664−3674

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Energy & Fuels

Among feasible ways under study for asphaltene valorization, oxidative reactions35 and/or nanocatalytic asphaltene hydroprocessing36 are underway, thus requiring substantial amounts of asphaltene feedstocks to carry out the studies. n-C5 asphaltenes were physically precipitated from an Athabascabitumen vacuum residue (VR) at a preparative laboratory scale, using 4 L of solvent and 0.3 kg of sample. In order to assess the properties of these solids, comparisons were made with asphaltenes, resins, and maltenes isolated from the said VR (at a lower scale), using varying solvent ratios, different solvent precipitants, and including (or not) resin solvent back extraction. In the open literature, resins are defined based on three criteria: (I) solvent back extraction of precipitated solids using the same precipitant alkane, (II) adsorption of polar material from deasphalted oil (i.e., resins) over polar sorbents, and (III) solvent extraction of solids precipitated with one small MW alkane, using a larger MW alkane for extraction (which is the criterion selected in the present work for the C5−C7 studied resins). It is the objective of the present article to study the properties of asphaltenes, resins, and oils derived from the cited separation experiments; results indicate how varying solvent nature, solvent/sample ratios, and solvent washing affect asphaltene properties in a systematic way. Higher MW solvents, high solvent/sample ratios, and precipitated solid washing were found to increase density, MW, and viscosity while decreasing solubility properties and aggregation onsets. Preliminary results show resins to be enriched in pyrrolic functionalities, compared to those of asphaltene fractions. The presence of a complex array of oxygenated fractions in both asphaltenes and resins suggested air oxidation influences during sample isolation procedures.

lower H/C and higher N/C atomic ratios, higher MWs, and aromaticities are determined when asphaltene yields decreased.19−21 One author devoted sustained efforts to this topic during the 1990s (S. I. Andersen), and three of his findings are herein considered of particular importance: (I) asphaltenes precipitated with different solvents/different temperatures have similar properties when the precipitate yields are similar;22 (II) temperature effects are not totally understood; in general, higher temperatures led to lower yields of more aromatic precipitates; however, yield maxima were described within the 4−98 °C range, explained by coprecipitation effects maximizing around 25 °C, then dissolution brought by higher temperatures;22−24 and (III) hysteresis was determined in sequential precipitation−redissolution experiments, indicating that molecular rearrangements affected the complex polydispersed solids.25,26 Solid asphaltene separation into discrete fractions has been addressed by several techniques, among them solvent fractionation, a method requiring simple operations, with the benefit of providing an ample set of narrow fractions easily isolated and characterized in one single run. The basis for the procedure is asphaltene dissolution in a good solvent, then solid precipitation is induced by adding increasing amounts of the precipitant, removing solids by filtration during each precipitant addition step. Fogler and co-workers selected the highly volatile (convenient) solvent pair n-C5/dichloromethane (DCM) for asphaltene fractionation,18,27,28 and other authors used mixtures of n-C7/toluene (tol.),29,30 n-C5/toluene,21 acetone/toluene,29 and MeOH/toluene31 to achieve asphaltene fractionation. Interesting findings from the former set of articles ensue: (I) in experiments using the pairs n-C7/tol., n-C5/tol., and n-C5/ DCM, property changes were determined to follow a regular pattern, the initially precipitated fractions with low solvent polarity being more aromatic; (II) experiments carried out with acetone/tol. provided isolated fractions with varying properties, depending on separations modulated by complex interactions of aromatic/aliphatic ratios and fractions polarity; (III) partitions using MeOH/tol. mixtures rich in MeOH provided small amounts of high MW, high polarity, and low H/C atomic ratio asphaltenes, followed by materials with opposite properties recovered with increasing toluene contents; and (IV) nC7/tol. mixtures applied to highly paraffinic oils provided highly paraffinic materials at the beginning (waxy hydrocarbons), which were recovered together with inorganic solids (metal oxides and salts). The preceding results obtained via solvent fractionation put in evidence that many alternatives are available for asphaltene subfractionation; however, the variability of results makes it difficult to compare experiments. This situation is made even worse if other published fractionation alternatives like supercritical fluid extraction fractionation (SFEF) and size exclusion chromatography (SEC) are considered; in one example using SFEF with an n-C5 solvent, results showed regular changes in extracted fraction properties, and except for the first soluble material, all of the fractions were shown to be able to aggregate, especially the nonextractable high MW end cut.32 SEC preparative separation of asphaltenes showed that the eluates of higher apparent MW were the most aggregative fractions, possessing the higher contents of heteroatoms and lower H/C ratios.33,34 The Canadian province of Alberta possesses one of the most abundant bitumen reserves worldwide; thus, a surplus of asphaltenes from upgrading processes is a fact affecting the province from environmental and economic points of view.

2. EXPERIMENTAL SECTION 2.1. Samples Isolation. Athabasca 510 °C+ VR was produced following standard procedures (ASTM D289237 and ASTM D523638), from a bitumen sample produced via steam assisted gravity drainage (SAGD). n-C5 asphaltenes were isolated from the VR by slowly pouring 300 g of sample fluidized at 100 °C into a 5 L roundbottomed flask which contained 4 L of solvent, while stirring was continuously applied. The mixture was refluxed during 30 min, then left to reach ambient temperature and filtered (Whatman #2 filter paper). The retained precipitate was washed with n-C5 until the filtrate was pale orange colored, in order to free the solid from the sticky resinous materials covering its surface, which hamper recovery from the filter and transfer for further testing. Solvent from filtrates was removed in a rotary-evaporator, then the maltene phase was dried inside a vacuum oven (2 days at 80 °C, 130 mmHg); filtered solids were brought to dryness in the same way as that described for maltenes (vacuum oven). Resins were isolated taking aliquots from the raw filtered solids, then extracting in a Soxhlet using n-heptane until no color was detectable in the thimble zone; the filtrate in this case contained the back-extracted resins that were coprecipitated with asphaltenes during their isolation. This set of experiments produced the following samples: maltenes (ATVR C5M 13/1), asphaltenes (ATVR C5 A 13/1), extracted asphaltenes (ATVR C5 A 13/1 extr.C7), and resins (ATVR C5 R13/1 extr.C7). Asphaltene precipitation with n-C7 and higher solvent/VR ratios were carried out in a 1 L flask, using 30 g of VR and 400 mL of n-heptane (ratio ∼13/1) or 20 g of VR and 800 mL of n-pentane (ratio ∼40/1). Filtered solids from the n-heptane experiment were brought to dryness, and resins were isolated taking aliquots from the raw filtered solids, then extracting in a Soxhlet using n-heptane until no color was detectable in the thimble zone. This procedure yielded the following fractions: maltenes (ATVR C7M 13/1), asphaltenes (ATVR C7 A 13/ 1), extracted asphaltenes (ATVR C7 A 13/1 extr.C7), and resins (ATVR C7 R13/1 extr.C7). Similarly, the pentane experiments yielded 3665

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Energy & Fuels Table 1. Characterization of the Studied Samples sample type asphaltenes

extracted asphaltenes resins

maltenes

sample IDa ATVRC5 A 40/1 ATVRC5 A 13/1 ATVRC7 A 13/1 ATVR C5 A 13/1 extr.C7 ATVR C7 13/1 extr.C7 ATVR C5 R 13/1 (extr.C7) ATVR C7 R 13/1 (extr.C7) ATVR C5M 13/1 ATVR C5M 40/1 ATVR C7M 13/1

%wt (VR basis)

density (g/ cm3)

C (% wt)

H (% wt)

N (% wt)

ΔPSb

Tavb (min)

Mnc (a.m.u.)

Mwc (a.m.u.)

37.1 36.2 24.0 28.7 18.4 7.7

1.16010 1.14123 1.17881 1.17546 1.18638 1.08005

80.50 80.91 80.86 80.70 80.32 81.14

8.07 8.29 7.92 8.02 7.83 9.42

1.24 1.17 1.29 1.28 1.31 0.69

1.69 1.63 1.59 1.56 1.44 0.26

14.77 14.55 15.23 14.83 15.18 13.64

832 794 792 835 894 678

989 901 941 938 1040 707

5.5

1.10188

82.14

8.67

0.82

0.30

13.63

678

713

63.1 62.9 75.9

1.02696 1.02212 1.03024

82.50 82.58 82.35

10.45 10.40 10.28

0.46 0.50 0.55

N.A. N.A. N.A.

N.A. N.A. N.A.

647 651 676

665 675 704

a

Keys for samples ID: ATVR, Athabasca vacuum residue; C5, C7, precipitant alkane; 13/1 or 40/1, solvent/sample ratio; A,R,M, asphaltenes, resins, maltenes; extr.C7, sample back extracted with nC7; N.A., not applicable. bDelta solubility profile and average elution time (see ref 40). cNumber and weight MWs (by SEC). μL. On the basis of the ELSD detector, a curve was generated that is related to the solubility properties of the asphaltenes and can be quantified to reflect the tendency of the sample toward asphaltenes precipitation. Two main variables were calculated using this procedure: Tav, the average time of elution of the asphaltenes in the sample, which is related to the average solubility parameter of the asphaltenes, and ΔPS, which reflects the tendency of asphaltenes toward precipitation and is measured as the difference in time between the maximum of the first peak and 75% of the distribution. The larger the ΔPS, the more prone toward precipitation are the asphaltenes39,40 2.5. Fluorescence Spectrophotometry. Fluorescence spectrophotometry was used to evaluate the critical nanoaggregate concentration (c.n.a.c). This is the concentration at which a measurable change in behavior is observed due to a substantial formation of aggregates. Asphaltene solutions in toluene with concentrations in the range 5 to 180 ppm were used. Fluorescence spectra of the asphaltene solutions were recorded on a Fluorescence Spectrometer Hitachi Model F-4500 with a 150 W xenon Lamp as the excitation source. In this instrument, the fluorescence is measured at a 90° angle relative to the excitation light. Emission and excitation slits were set at 5 nm. The scanning speed was set constant (1200 nm/ min). An excitation wavelength of 310 nm was used for the solutions, and the emission spectra were recorded for wavelengths spanning the 200 to 900 nm range. The measurements were carried out at room temperature in a quartz cell of 2 mm path length. 2.6. Elemental Analysis. C, H, and N elemental analyses were determined by a standard combustion method, using a Carlo Erba model 1108 analyzer. 2.7. Viscosimetry. Viscosity determinations were performed with a Brookfield model DV-II+PRO system, which provides dynamic viscosity values. Quick coupling of spindles is achieved with a EZ-lock device. The system was provided with a thermosel oven, able to operate within the ambient to 300 °C range. Disposable aluminum sample cells were used. Brookfield # 21 and 28 spindles were used. The system was operated inside a drybox provided with inert atmosphere (nitrogen gas). 2.8. X-ray Photoelectron Spectroscopy (XPS). XPS for selected samples was carried out with a PHI VersaProbe 5000 spectrometer. The spectra were taken using monochromatic Al source (1486.6 eV) at 50 W and beam diameter of 200.0 μm with a take off angle of 90°. The samples were pressed on a double sided tape, and spectra were taken with double neutralization. The binding energies were reported relative to C 1s at 284.8 eV. The sample sputtering protocol involved 10 min of Argon sputtering at 45°, 2 kV, 1.5 μA 2 × 2 (less than 10.5 nm/min); calibration was performed with a wafer SiO2/Si having a SiO2 layer of 100 nm.

the following fractions: maltenes (ATVR C5M 40/1) and asphaltenes (ATVR C5 A 40/1). In this last case, resins were not extracted. Relative percent standard deviation (% RSD) for large scale asphaltene precipitation (15 replicas) indicated values lower than 5% and recoveries in the 99−100% range. For small scale (2−3 replicas), relative errors were found within the same ranges. 2.2. Sample Densities. Sample densities were determined with an Anton-Paar densitometer model DSA-5000M. Several (3−4) toluene solutions of the samples within the concentration range from 0.5 to 1.5% wt/vol. were prepared and their densities determined. Expression 1 provided sample density calculated values, where ideal mixing is implied. ρasph =

masph (1/ρsolv )(msolv + masph) − (1/ρsolv )msolv

(1)

where ρ, density; m, mass; asph, asphaltene; and solv, solvent. 2.3. Molecular Weight Distributions (SEC). Molecular weights were determined by size exclusion chromatography (SEC) using a 30 cm × 7.5 mm PLgel “mixed E” column (Agilent Technologies), recommended for oligomers and polymers up to 25 000 g/mol. Solutions of the fractions (10 ppm) were prepared in methylene chloride to determine molecular weights. For aggregation studies, toluene solutions were prepared using concentrations from 5 to 180 ppm. The solutions were eluted with a 90/10 methylene chloride/ methanol blend at a flow rate of 1.0 mL/min. The temperature was kept constant at 25 °C. An HPLC Agilent model 1100 liquid chromatograph provided with an evaporative light scattering detector Alltech 2000 was used. The molecular weights were calculated based on a calibration that uses porphyrins, dyes, and polyaromatics as standards. Details have been published elsewhere.39 2.4. Solubility Profile. Solubility profile tests were carried out as described before40 to evaluate the effect of exhaustive extractions on solubility distributions. Solutions of the samples in methylene chloride (0.1 wt %) were prepared and injected into a column packed with an inert material using n-heptane as the mobile phase. This solvent induces the precipitation of asphaltenes and, as a consequence, their retention in the column. The first eluted fraction from the column is the maltene, which is soluble in n-heptane. After all of this fraction has eluted, the mobile phase is changed gradually from pure n-heptane to 90/10 methylene chloride/methanol and then to 100% methanol. Asphaltenes are quantified using an evaporative light scattering detector (ELSD). The HPLC system consisted of a HP Series 1100 chromatograph and an Alltech ELSD 2000 detector. The flow rates used were kept constant during the entire experiments. The volumetric flow of the mobile phase was 4.0 mL/min and 3.5 L/min of nitrogen was used for the nebulizing gas of the ELSD. The analysis took 35 min to be completed. Duplicates were run for each sample to ensure repeatable results. The injected volume in all the experiments was 80 3666

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3. RESULTS 3.1. Sample Properties and Evidence of Cohesive Forces as Indicated by Determined Densities. Table 1 presents the set of determined properties for the studied samples. The keys for identifying samples are presented within the second column from the table, believed to be selfexplanatory; however, the keys for resins are deemed worthy of explanation in more detail. Resins in this work are defined as the materials that are back extracted from C7 and C5 precipitated asphaltenes, in the present case using nC7 as extracting solvent in all cases, thus indicating that nC7-nC7 and nC5-nC7 resins are studied in the present work. Within the present section, three observed trends will be discussed, leaving others for upcoming sections. First, H contents were observed to follow the expected trends for the studied samples, i.e., asphaltenes were found depleted in this element, while resins and maltenes increased in their contents. N contents also followed expected trends, being exactly the opposite to those discussed for H. The third aspect pertains to determined densities. A plot of determined densities as a function of H/C atomic ratio is presented in Figure 1. A clear systematic trend is

Figure 2. Determined MWs as a function of density (ρ).

the findings included in the figure, more dense samples were found in those displaying higher Mn, which is a further indication of larger intermolecular interactions that correlate with higher boiling points, as presented elsewhere.42 From the findings discussed in the present section, it can be deduced that the large scale n-C5 precipitated asphaltenes in a ratio 13/1 display lower density, higher H/C ratio, and lower N content (see Table 1). However, their corresponding maltenes are not that different compared with the other two maltene samples (see Table 1), these being deemed important results when upgrading schemes are applied to maltene phases, like catalytic steam cracking following catalytic upgrading technology (CUT) processing.43 3.2. Solubility Profile. Solubility profile either via theoretical or experimental approaches39,40,44 is a powerful characterization technique for oil fractions. It is particularly useful for differentiating virgin versus thermally affected asphaltenes, the latter produced during refining upgrading or naturally derived from reservoirs maturation. Virgin asphaltenes are enriched in low solubility parameter components, while thermally treated ones show the appearance of higher solubility parameter components. In severe thermal treated cases, more than one high solubility signal can appear. This method is also useful for revealing solubility differences of virgin asphaltenes that are precipitated/extracted in different ways, as presented on Figure 3. Several techniques have been described for data

Figure 1. Determined densities of samples as a function of H/C atomic ratios.

observed with the results determined: the samples with lower H/C values, i.e., those with higher aromaticities, were those displaying the higher densities. Sample densities are first indicators for the diverse cohesive forces affecting molecules. C7 asphaltenes, particularly those isolated with larger solvent/ sample ratios, showed these forces the most (Figure 1). A clear decreasing trend is observed from C7 to C5 asphaltenes, then resins and finally the oily (maltene) phases (Figure 1). Soxhlet extraction removed coprecipitated resins, leading to larger intermolecular interactions among the extracted asphaltenes fractions. These later correspond to the largest densities and lowest H/C ratios. It has been reported that solubility parameters are linearly related to densities for hydrocarbons.41 This implies that densities for hydrocarbons are directly related to cohesive energy, and therefore, their increase is a direct consequence of larger intermolecular interactions. Determined MWs provided further evidence of the cohesive forces affecting molecules compounding the studied samples. Figure 2 presents a correlation determined between Mn (MW determined via SEC) and density for the studied samples. From

Figure 3. Solubility profiles for the studied asphaltene samples. SP: solubility parameter. 3667

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MW preseparated fractions toward high MW entities that eluted at higher apparent MWs being reported before.33 Figure 4 presents SEC chromatograms for one asphaltene and one resin from the set of studied materials, determined as a

handling, i.e, two have been particularly useful to describe mathematically the solubility distributions: ΔPS and Tav.40 Tav is the average time of elution of the asphaltenes in the sample and reflects the average solubility parameter of the asphaltenes. ΔPS reflects the tendency of asphaltenes toward precipitation. Both values are included in Table 1 for the asphaltenes and resins studied. In principle, maltenes cannot be analyzed using nC7 as the first solvent for the elution sequence; combined use of alkanes with different MWs might allow one to differentiate among resins; however, this possibility has not been investigated so far. Results plotted in Figure 3 showed that selective extraction of the more soluble molecules from the asphaltenes can be observed with the solubility profiles, noticing that the first eluted peak (so-called easy to dissolve asphaltenes containing low solubility parameter components)40 becomes smaller as the extraction progresses either by increasing the solvent/sample ratio or by Soxhlet extraction. Results indicate that the solubility profile provides the expected systematic changes, meaning that nC7 asphaltenes are enriched in high solubility parameter components compared to that of nC5 solids. Similarly, extracted solids are enriched in high solubility parameter components, compared to the raw precipitated (unwashed) asphaltenes. The discussed results further suggest that the large scale precipitated ATVR C5 A 13/1 is a “better” sample under solubility premises (i.e., enriched in resins), compared to the asphaltenes isolated using the higher solvent ratio of 40/1, which is around the ratio recommended in the standard procedures.8,9 This tendency is also reflected in Tav values that increase as the extraction proceeds. Tav values can be converted to solubility parameter values, and their increase implies a decrease in the solubility of the asphaltenes.40 All of the findings from this section suggest that the presence of coprecipitated resins is deemed as a cause of the improved solubility for the associated asphaltenes. 3.3. Molecular Aggregation. The preceding sections addressed topics on density, MW, and solubility for the studied samples, illustrating how more dense and higher MW materials (having larger molecular interactions) were those showing lower solubility, as determined via the solubility profile technique. Lesser contents of coprecipitated resins caused by using different precipitant solvents, different solvent/sample ratios, or by back extraction of these compounds were deemed responsible for such behavior. Thus, it can be expected that these same samples, particularly asphaltenes depleted in resins (“core asphaltenes”), will show higher aggregation tendencies, a topic that will be addressed in the present section based on SEC and fluorescence spectrophotometry (FL) results. SEC is a mature liquid chromatography technique45 that separates analytes based on their percolation through packed beds with different pore size distributions, based on the molecules’ solution hydrodynamic radii. The former indicates that molecules can be covered with peripheral interacting solvent molecules or that they can interact among them creating aggregates whose gyration radii are larger than those of the individual molecules, thus eluting at shorter retention times. Oil fractions have been analyzed via SEC since the technique was developed four decades ago, putting into evidence at these early stages that nonsize effects like adsorption, can affect the elution order of sample analytes.46 Asphaltene aggregation phenomena have been studied relying on SEC literature,23,25,26,31,33,34,46,47 with reaggregation phenomena of low

Figure 4. SEC chromatograms for selected asphaltenes and resins as a function of solution concentration (ppm wt./vol.).

function of sample solution concentrations. The results show that the asphaltene sample is able to aggregate with increasing concentrations; however, the resin sample does not show the same tendency under the concentration range explored in these experiments. The former behavior was also observed for the other resin studied in this work. Asphaltenes proved more complex to understand, as a comparison carried out for the studied asphaltenes shows (Figure 5). Results indicate that three asphaltenes aggregated under the set up experimental conditions, producing a high MW elution mode appearing at about 4.5−7 min elution time. The other two samples did not show this behavior, which is deemed unexpected since for the nC7 asphaltenes and for the extracted nC5 asphaltenes, resin contents should be low, thus helping aggregation phenomena. One of the asphaltenes, the already extracted (i.e., resins depleted) ATVR C7 A 13/1 extr.C7 asphaltene, which is expected to be “core enriched” the most, was the one showing the stronger aggregation tendency, demonstrating the expected tendency. The preceding indicates that several inconsistencies were observed in the discussed results obtained via SEC; at present, no reasons have been identified for these; it might be that higher concentrations are necessary to observe aggregates in the two cases where aggregation was not detected. The SEC 3668

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Figure 5. SEC chromatograms for the studied asphaltenes. Sample solution concentrations: 1000 ppm wt./vol.

results discussed grossly indicated that many, however not all, aggregation tendencies for the studied asphaltenes and resins agreed with what other authors have published previously.23,25,26,31,33,46,47 Deviations are believed to arise from nonsize effects,46 sample concentration affecting aggregation onsets, and effects derived from the DCM-MeOH solvent, which is no t co mmonly select ed for SEC elution.23,25,26,31,33,34,46,47 For many years, fluorescence spectrophotometry (FL) has been used as a technique in the study of asphaltene aggregation phenomena.19,22,24,29,48 FL emission spectra have found particular application for estimating the size distributions of fused polyaromatic rings present in asphaltenes,49,50 and FL depolarization studies have been carried out to estimate the MW of asphaltenes.51,52 FL spectra of aromatic moieties are fundamentally related to the aromatic fluorophores’ sizes and conjugation; the appearance of new bands, shifts in emission wavelengths, and FL intensity have been correlated with changes surrounding these fluorophores, like the bathochromic red shifts caused by intermolecular aggregation.53−55 One problem with asphaltenes is their color, i.e., their concentrated solutions are black, which causes Beer−Lambert law deviations, the so-called “inner filter effect” in FL spectrophotometry.55,56 To overcome this problem, front face instead of 90° spectrometer cells geometry have been used. FL spectra of the studied samples in the present work were determined as a function of their concentration within the 5− 1000 ppm wt./vol. range. No corrections were made since the observed aggregation phenomena occurred under low concentrations (167 eV) as well as in the resin, again 3671

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Figure 14. N1S XPS spectra and deconvoluted signals for ATVR C7 A 13/1 (asphaltenes) before and after sputtering and for ATVR C7 R 13/1 extr.C7 (resins). Signal assignments are from Kumar et al.64 Figure 13. S2P XPS spectra and deconvoluted signals for ATVR C7 A 13/1 (asphaltenes) before and after sputtering and for ATVR C7 R 13/1 extr.C7 (resins). Signal assignments are taken from Kelemen et al.66 and XPS reference pages.67 Sulfoxide assignments are from Sexton et al.68

surface, compared to C-oxygenated species (see preceding discussion). The resin sample proved to be much simpler compared to the asphaltene, i.e., less oxygenated forms were detected in the N1S spectrum, suggesting that N forms present in asphaltenes seem to be more prone to oxidation reactions. Again, the existence of oxygen functionalities in asphaltenes seems to be a leading factor for their precipitation, as discussed before regarding C−O functionalities. Polarity as derived from the presence of oxygen functions is one of the two most important properties for asphaltene definition as proposed a long time ago by Long.1 This preliminary section on XPS analysis was deemed worthy of inclusion in the present article for three reasons. (I) The data suggest that pyrrolic compounds are preferentially included in the resin, compared to the analyzed asphaltenes. II) C, S and N XPS for all analyzed cases suggested the existence of oxidative processes believed to be related to routine isolation and manipulation procedures which do not contemplate an inert atmosphere. (III) Observed increased contents of oxygen functionalities in asphaltenes compared to that in resins support the long time proposed definition for asphaltenes.1 Oil fraction oxidation during routine isolation has been discussed before;16,71 the existence of oxidized forms in Athabasca asphaltenes has also been published based on XPS analysis.72 The larger aggregation tendency for solvent extracted

suggesting the existence of oxidative processes during fraction isolation and manipulation. It seems that the most important sulfur functions correspond to sulphidic forms at about 163.5 ± 1 eV (blue traces) and thiophenics at about 165 ± 1 eV (green traces, slightly above to what was reported in the open literature66,67), which are the main nonpolar S compounds reported for Athabasca bitumen and Athabasca nC5 asphaltene.69 Pyridinic (∼398 eV), pyrrolic (∼400 eV), and oxidized Nforms (∼402−403 eV) have been assigned in XPS studies of organic compounds.64,70 Figure 14 presents the results determined in the present work. Again, the presence of oxidized N-forms tells us about concerns of oxidation occurring during fractions isolation and manipulation. The more significant signal for non-oxidized N-functions corresponds to pyrrolics in all the studied cases, particularly for the resin sample. Sputtering seemed to remove some oxygenated materials from the surface of the asphaltenes, suggesting that N-oxygen compounds are more abundant over the sample’s 3672

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Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: New York, 2000; Chapter 13 in Asphaltenes and Asphalts, 2; pp 335−362. (4) Speight, J. G. The Chemistry and Technology of Petroleum, 4th, ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2006. (5) Mendez, A.; Lima, E. Third Round Robin on Heavy Crudes and Bitumen, Statistical Analysis on Cerro Negro Crude: Analytical Data, Final Report; United Nations Institute for Training and Research, Center for Heavy Crudes and Tar Sands (UNITAR); INTEVEP. S.A., Research and Technological Support Center of Petroleos de Venezuela: S.A Caracas, Venezuela, 1993. (6) ASTM D3279. Standard Test Method for n-Heptane Insolubles; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2011. (7) ASTM D4124. Standard Test Method for Separation of Asphalt into Four Fractions; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2011. (8) ASTM D6560. Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2011. (9) IP143. Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products; IP: International Petroleum Test Methods; Institute of Petroleum: London, U.K. (10) DIN 51-595. Determination of Asphaltene Content of Petroleum Products by Precipitation in Heptane; Deutsches Institut fur Normung E.V. (11) AFNOR NF T60-115. Petroleum Products. Determination of Asphaltene Contents (Heptane Insolubles). (12) ASTM D893. Standard Test Method for Insolubles In Used Lubricating Oils; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2011. (13) ASTM D2007. Standard Test Method for Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum−Derived Oils by the Clay−Gel Absorption Chromatographic Method; American Society for Testing and Materials (ASTM): West Conshohocken, PA, 2011. (14) GOST 11858. Determination of Asphaltenes. http://en. petrotech.ru/en/standards/gost/1/GOST%2011858/. (15) Calles, J. A.; Dufour, J.; Marugan, J.; Pena, J. L.; GimenezAguirre, R.; Merino-Garcia, D. Energy Fuels 2008, 22, 763−769. (16) 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), 671−699. (17) Maqbool, T.; Srikiratiwong, P.; Fogler, H. S. Energy Fuels 2011, 25, 694−700. (18) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14, 25−30. (19) Andersen, S. I.; Keul, A.; Stenby, E. Pet. Sci. Technol. 1997, 15 (7&8), 611−645. (20) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (21) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J.; Yang, M.-G.; Jones, D. Energy Fuels 2003, 17, 498−503. (22) Andersen, S. I.; Birdi, K. S. Fuel Sci. Technol. Int. 1990, 8 (6), 593−615. (23) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (1), 51−74. (24) Andersen, S. I. Fuel Sci. Technol. Int. 1995, 13 (5), 579−604. (25) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (11&12), 1551− 1577. (26) Andersen, S. I.; Stenby, E. H. Fuel Sci. Technol. Int. 1996, 14 (1&2), 261−287. (27) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, S. Ind. Eng. Chem. Res. 1999, 38, 964−972. (28) Wattana, P.; Fogler, H. S.; Yen, A.; Garcia, M. C.; Carbognani, L. Energy Fuels 2005, 19, 101−110. (29) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martinez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732−741. (30) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Energy Fuels 2006, 705−714. (31) Andersen, S. I. Pet. Sci. Technol. 1997, 15 (1&2), 185−198.

asphaltenes discussed in the preceding paragraphs might in part derive from their extended oxidation occurring during additional exposure to air. Since most standard asphaltene isolation methods routinely practiced for many decades do not contemplate inert atmospheres,6−13 the present results attempt to make a point in this regard.

4. CONCLUSIONS Asphaltene, resin, and maltene physical isolation procedures involving different alkane precipitants and solvent/sample ratio provided fractions displaying systematic changes in properties and aggregation tendencies. Thus, it was found that denser, more polar, higher MW, more viscous, red-shifted fluorescence materials were ranked as follows: nC7 extracted asphaltenes, C7 raw asphaltenes, C5 raw asphaltenes, resins, and maltenes. Different analytical techniques supported the cited order, including density, solubility parameter profiling, fluorescence spectroscopy, and viscosity; however, SEC used for the same purpose, failed to provide a sound support, deviations observed by SEC not being explained at present. Intermolecular aggregation for the ranked samples was determined to follow the same order presented above, ascribing many of the aggregation phenomena to the presence/absence of resins. This work further reports aspects of possible practical interest, i.e., the liquid nature of asphaltenes at 300 °C and the possible existence of oxidative reactions that affect fractions during isolation and manipulation procedures according to standard methods which do not contemplate inert atmospheres. Preliminary assessment of chemical functionalities within isolated fractions showed possible enrichment of pyrrolics within resins and oxygen functionalities in asphaltenes. The latter finding agrees with the long time accepted definition of asphaltenes, i.e., materials having high MW/high polarity or both and in the present case, those having higher contents of oxygen functionalities (“high polarity”).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from CHEVRON-ETC, NSERC/Nexen/AIEES Industrial Research Chair in Catalysis for Bitumen Upgrading, Canada Foundation for Innovation (CFI), the Institute for Sustainable Energy, Environment and Economy (ISEE), and Schulich School of Engineering is acknowledged. Chevron ETC is thanked for permission to publish this work. Japan Company for Oil Sands (Jacos) is acknowledged for providing the Athabasca bitumen sample. Large scale asphaltene isolation provided by Lina Diaz and Linda Roa-Fuentes is greatly appreciated.

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