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Chapter 3

Equivalent Distillation: A Path to a Better Understanding of Asphaltene Characteristics and Behavior Estrella Rogel,*,1 Michael Roye,2 Janie Vien,2 and Matthias Witt3 1Chevron Energy Technology Company, Richmond, California 94801, United States 2Chevron Oronite Company LLC, Richmond, California 94801, United States 3Bruker Daltonik GmbH, 28359 Bremen, Germany *E-mail: [email protected].

Sequential elution fractionation was used to evaluate property patterns in asphaltenes and their relationship to their behavior. Using the data, we developed a series of correlations that link chemical composition with solubility and thermal behavior. Additionally, an effort was made to identify the essential chemical/molecular characteristics that make petroleum materials prone to asphaltene precipitation. It was found that high hydrogen deficiencies and uneven solubility fraction distributions are the main contributors to asphaltene instability.

Introduction Solvent fractionation of petroleum and coal-related materials has been used for more than 70 years to separate nondistillable fractions (1). Usually, solvent fractionation requires several days of work using time-consuming Soxhlet extractions, repeated washing, etc. A faster and more robust method to perform solvent fractionation is the one developed by Boduszynski et al. (1) and originally applied to the study of solvent-refined coal. Later, this same approach was successfully used to produce non-distillable petroleum fractions and allowed a detailed characterization of them (2, 3). This method is called “sequential elution fractionation” and involves the precipitation-dissolution of the fractions inside an inert column and avoids the complications inherent in traditional techniques. © 2018 American Chemical Society

Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Using this method, Boduszynski demonstrated that non-distillable fractions followed the same patterns than distillation cuts and that solubility could be used to perform an equivalent distillation for non-distillable materials (4). The realization of this continuity of patterns was followed by the introduction of the atmospheric equivalent boiling point (AEBP) (2, 5) that extends the boiling range of petroleum even to non-volatile fractions and permits the description of an entire crude oil in uniform terms (6). As a corollary of this relationship between volatility and solubility, the less soluble fractions will have the highest boiling points and, at the same time, the largest complexity, in terms of both the variety and the structure of their molecules (6). This high complexity at high boiling point renders many analytical techniques unsuitable for analyzing these complex fractions. The analytical limitations gave rise to the concept of the “average molecule (7–10)”. It was assumed that a whole group of molecules, mainly asphaltenes, could be represented by just one molecule. This average molecule has the average properties of the whole set of molecules, (i.e., elemental content (H, C, N, S), aromaticity, molecular weight, etc. ) and, therefore, it should also represent the average physical properties of the set, such as density and volatility. This approach has been shown to be arbitrary (11) and is limited due to the variety of molecular weight and volatility of the various chemical classes of compounds comprising these complex mixtures (12). Fractionation using solubility helps to overcome the limitations from the use of an average structure or average properties to describe heavy fractions, in particular, asphaltenes. Asphaltenes are fascinating materials that have eluded comprehension for several decades. In the petroleum industry, they are well known as troublemakers that can affect different stages of the production, transportation, and processing of crude oils. Significant resources have been used in trying to understand the origin of these problems. Based on phenomenon observation, experimental testing, and thermodynamic modeling, it was possible to understand how changes in temperature, pressure, or chemical modifications affect asphaltene behavior. However, finding the link between chemical characteristics of asphaltenes and their solubility behavior has proved to be an elusive goal. The separation of asphaltenes into fractions has two great advantages: it reduces the complexity of the material to study, and it provides a distribution of properties instead of just averages. This strategy has been applied successfully to improve asphaltene characterization (13–18) and has shown regular variations in properties for the different fractions, as expected based on the continuity model (6). In general, the less soluble fractions have shown decreased H/C molar ratios and increased aromaticity and heteroatom content. One downside is that these separation methodologies produce fractions with significant overlaps (15–18). Despite this disadvantage, asphaltene fractionation can provide significant insight into the distribution of properties on asphaltenes. This extensive characterization, together with the practical knowledge of deposition, can shed light on this phenomenon. This chapter discusses the characteristics of a series of solubility fractions extracted from different sources (materials with and without asphaltene deposition as well as deposits) (16, 17) that have been analyzed using a variety of different 52 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

analytical techniques. The emphasis is on those aspects of the property distributions that are related to asphaltene precipitation behavior. This chapter also explores the link between chemical composition, physical properties, and stability based on the concept of equivalent distillation.

Asphaltene Fraction Distribution The sequential elution fractionation technique is based on the principle of increasing the solvent power of the solvent used to dissolve the sample. This type of fractionation establishes a scale of solubility for the obtained fractions. Based on the cohesion parameter approach (19), it is expected that the solubility parameter of the fractions increases from the first eluted fraction (SF-1) to the last one (SF-6). The increase in the solubility parameter is related to the increase of the cohesion or intermolecular forces among the molecules that compose the fraction. At the same time, an increase in these forces produces an increase in boiling point. In this chapter, we analyze the results obtained using the technique suggested by Boduszynski et al. (1) carried out with current instrumentation. An Accelerated Solvent Extractor (ASE), Dionex 300 was used in the extractions. In this new version, the sample was deposited on 60 mesh non-porous PTFE particles. After drying at room temperature under nitrogen, the PTFE-supported sample was packed into a cylindrical 35 x 2.83 cm stainless steel cell. The sample was extracted with a sequence of solvents: heptane, 15/85 CH2Cl2/n-heptane, 30/70 CH2Cl2/n-heptane, 100% CH2Cl2 and 90/10 CH2Cl2/methanol at room temperature. This procedure generated four fractions: SF-1 (maltenes) and SF-2 to SF-5 (asphaltene fractions). Finally, the cell was extracted three times with 90/10CH2Cl2/MeOH at 120°C for 15 minutes to produce the last fraction (SF-6). Usually, this last fraction SF-6 represented less than 0.5 wt.% of the sample. This separation is compatible with the classical definition of maltenes as the soluble material in heptane and asphaltenes as the insoluble one. After the extractions, solvents were evaporated on a hot plate under a nitrogen atmosphere. The mass balances indicated average recoveries of 90 to 99 %. In general, recoveries were better for processed materials. The composition of the heptane insoluble fractions or asphaltenes in terms of the fractions show significant differences depending on the origin and nature of the samples (16, 17). Figure 1 shows that the fractionation depends on the original composition of the material. HCOVR, HCOAR1, and HCOAR2 are virgin materials, while OA is an oxidized asphalt, VBR is a visbroken residue, and HP is a hydroprocessed sample. The deposit was collected from a submersible pump (16). In Figure 1, the average solubility parameter of the fractions increases from SF-2 to SF-6. SF-5 and SF-6 represent less than 10 % of the asphaltenes except for HP. Based on these distributions, it is difficult to establish a correlation between asphaltene incompatibility (HCOAR2, VBR, and HP are self-incompatible) and fraction content. Regarding the deposit shown in Figure 1, it is enriched in fraction SF-4 that has a higher solubility parameter than the other two dominant fractions in the samples (SF-2 and SF-3). 53 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 2 shows that elemental composition of similar fractions varied widely. In this figure, the H/C Molar Ratio of fractions SF-1 to SF-4 for three samples is presented. There is a decrease in the hydrogen to carbon ratio (H/C) from SF-1 to SF-4 as well as an increase in nitrogen content for the heptane insoluble fractions SF-2 to SF-4. Comparison of the H/C ratios indicates that the hydrogen deficiency is larger for the fractions coming from processed materials and the deposit than for those extracted from the virgin sample (16, 17). This is consistent with claims in the literature indicating that high hydrogen deficiency in asphaltenes correlates with precipitation behavior.

Figure 1. Composition of samples. (Fraction SF-1 is excluded from the representation). Data from references (16) and (17).

Relationship between Hydrogen Deficiency and Other Properties Density, Solubility Behavior, and Polyaromatic Core Sizes Experimental determination of density for the different fractions shown in Figure 1 indicated a correlation between density and hydrogen deficiency. Figure 3 shows this correlation (r2=0.97). In this plot, density increases as the hydrogen content in the fractions decreases (17). This correlation is valid for any heptane insoluble fraction of the crude oils as it can predict the density of asphaltenes obtained by IP-143. This is a consequence of the increase in aromaticity of the fractions as reflected by the hydrogen deficiency. However, this correlation also 54 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

seems to indicate that the main intramolecular interactions among asphaltene molecules are non-polar. In other words, hydrogen bonding and interactions generated by the presence of heteroatomic functionalities are minor factors in determining the cohesive nature of the asphaltenes.

Figure 2. Distribution of the fractions for HCVO, HCOAR1, and Deposit. H/C molar ratio and nitrogen contents. Data from references (16) and (17). 55 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 3. Density as a function of the hydrogen content for SF fractions. Data from reference (17).

In fact, solubility parameters calculated by using the third-rule that relates density with solubility parameter for hydrocarbon molecules (20) correlate rather well with values determined using the solubility profile test (17) as shown in Figure 4 (r2=0.92). In this test (21), n-heptane insolubles are gradually re-dissolved by continuously increasing the solvent power from heptane to 90/10 methylene chloride/methanol blend. This procedure dissolves from the “easy to dissolve” material (low solubility parameter) to the “hard to dissolve” (high solubility parameter). The eluted material is monitored using an Evaporative Light Scattering Detector (ELSD). The timescale can be converted to a solubility parameter scale because the solubility parameter of a mixture is given by the volumetric average of the components, as shown in this equation (19).

where δ is the solubility parameter of the blend, φi is the volume fraction, and δi is the solubility parameter of the components. Figure 5 shows a comparison of the solubility parameter distributions for fractions SF-4 of the samples examined in Figure 4. In this figure, there is a significant shift of the observed peaks to the right as SF-4 fractions became more and more deficient in hydrogen. In an insert in Figure 5, average elution times are plotted as a function of hydrogen content for these fractions indicating a correlation between both parameters (r2=0.95). In fact, a direct relationship between the solubility parameter of petroleum-related materials and hydrogen content has been reported previously (22). 56 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 4. Comparison between Average Solubility Parameter calculated based on Solubility Profile Test and using Third-Rule. Data from references (16) and (17).

Figure 5. Comparison of the solubility profile of SF-4 fractions and the correlation between the Average Elution Time and Solubility Parameter. Data from references (16) and (17). 57 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Considering that asphaltenes are composed mainly by aromatic molecules as shown by Atomic Force Microscopy (23), the decreases in hydrogen content can be attributed primarily to two factors: the reduction in the number of aliphatic chains and the increase in the size of the polyaromatic cores. It is well known that the incorporation of aliphatic chains in big polyaromatic molecules increases their solubility, i.e., hexabenzocoronene (24, 25). The increase in solubility depends on the number and volume of the chains (26, 27). Shorter chains or fewer chains per molecule decrease solubility in big polyaromatic molecules and induce different aggregation modes (28, 29) On the other hand, it has been shown that increasing the surface area of the polyaromatic core leads to a more pronounced stacking propensity and reduces solubility (26, 30) The increase in the size of the polyaromatic cores increases the magnitude of attraction between cores as it increases the polarizability of them (31–33). This behavior is a clear indication that the interactions in molecules containing big polyaromatic cores are mainly the result of the high polarizability of de-localized electrons (33) and the increase in size produces a decrease in the hydrogen content. It is important to mention that polyaromatic cores containing more than six polyaromatic rings have been identified in asphaltenes from diverse sources using atomic force microscopy (AFM) (23) so the presence of large polyaromatic rings is a fundamental fact that it is linked to the low solubility of this fraction. Fluorescence measurements provide indirect evidence of the increase in the size of polyaromatic cores as the hydrogen deficiency increases. Figure 6 shows the increase in wavelength maximum of the fluorescence spectra for fractions SF-1 to SF-4 measured at concentrations of 5 ppm for several samples. This increase is related to the increase in the size of the polyaromatic rings. Studies of the fluorescence spectra of polycyclic aromatic hydrocarbons containing from 3 to 24 rings revealed the increase in the fluorescence wavelength as the size of these compounds increases (34, 35). Additionally, it was found that the number and the substitution pattern of n-alkyl chains in large polycyclic aromatic hydrocarbons did not have any influence upon their electronic properties (36). Then, it might stand to reason to assume that the changes in the maximum wavelength are due to an average increase in the size of the polyaromatic cores as the fractions became less soluble. Also, the increase in the maximum wavelength for all the studied fractions correlated roughly to the hydrogen deficiency as shown in Figure 6 (r2=0.66). Aggregation Behavior The aggregation behavior of several fractions was analyzed using size exclusion chromatography (SEC). This chromatography technique assumes that the elution volume of a compound is related to its molecular size or another size-related parameter. Since its beginning, this technique been used extensively for characterization of asphalts, asphaltenes, crude oils and their fractions (37–43). However, there are some doubts about the application of SEC to petroleum samples based on the possible adsorption of some components as well as in data that shows that the elution of polyaromatic components in some solvents do not 58 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

proceed in order of decreasing molecular weight (44, 45). Also, problems with the appropriate standards for calibration were brought up earlier in the application of this technique to hydrocarbons (44, 46).

Figure 6. Maximum Fluorescence Wavelength of SF fractions plotted as a function of the H/C molar ratio. Data from references (16) and (17). Nevertheless, SEC has provided valuable insights into the characteristics and interactions of heavy fractions. For instance, the analysis of asphalt fractions separated based on their fluorescence characteristics (non-fluorescing and fluorescing) showed significant differences regarding size between both fractions. The larger sizes observed in the non-fluorescing fraction was attributed as substantial evidence for intermolecular association (42). Concentration effects in the analysis of petroleum asphaltenes indicate increasing aggregation as the concentration of asphaltenes increases (43). In contrast, maltenes did not show aggregation behavior during SEC experiments (47). In the SEC study, the elution patterns of several fractions were analyzed as a function of the concentration (from 5 to 180 ppm) using a 30 cm x 0.10 cm Mixed E column. The solutions were eluted with a 90/10 methylene chloride/methanol blend at a flow rate of 1.0 mL/min. Signals were followed using an ELSD Alltech 2000. Average molecular weights were calculated based on a calibration that uses porphyrins, dyes, and polyaromatics as standards. Figure 7 shows an example of how the chromatograms changed as a function of the concentration for the different fractions obtained from HVCO. In this plot, as the concentration increases, there is an increase in the material eluted from the column at shorter times indicating an increase in size. The shift at longer times becomes more significant from fraction SF-1 to SF-4 as expected considering the increase in solubility parameter. Figure 8 shows how the number average molecular weight calculated for each fraction varies with concentration. The 59 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

molecular weight increases as concentration increases for all the fractions except SF-1, the so-called “maltene” fraction. This same behavior was found for all the SF-1 fractions studied using SEC in agreement with a previous study (47).

Figure 7. Chromatograms for fractions of HVCO as a function of concentration (5 ppm to 125 ppm)

Figure 8. Number Average Molecular Weights of the HVCO fractions calculated from SEC measurements. 60 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Another important relationship between aggregation and elemental composition is shown in Figure 9. For the fractions studied using SEC, it was found that the weight-average molecular weights calculated from the chromatograms at each concentration have a power correlation to the hydrogen content of the fraction. In the range between 5 ppm to 125 ppm, the correlation becomes better with a r2 that increases from 0.73 to 0.92 as the concentration increases. Number-average molecular weights also show a certain degree of correlation, but it is not as good as the correlations found for the weight-average molecular weight. The correlations shown in Figure 9 for concentrations of 125 ppm and 30 ppm are just another indication that the principal driver for asphaltene solution behavior is related to hydrogen deficiency.

Figure 9. Molecular Weight for SF fractions as a function of the hydrogen content.

Boiling Point At first sight, the concept of boiling point for n-heptane insoluble materials might seem as lacking practical use, as they have boiling points that are impossible to reach with current technology without thermal decomposition. In fact, simulated distillations of SF-2 to SF-4 showed that the distillable material in these fractions varies between 4 wt. % and 30 wt. % (17). However, based on the equivalence principle between distillation separations and sequential elution fractionation, it is possible to correlate properties of the fractions such as density and molecular weight with mid-atmospheric equivalent boiling points (Mid-AEBP) (6). Mid-AEBP were calculated for several sequential elution 61 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

fractions based on the fitting of Weibull distributions to simulated distillations (48). The values obtained were within the range found using the Altgelt and Boduszynski correlation for Boscan SF fractions (6). Figure 10 shows a plot of the Mid-AEBP estimated for the fractions as a function of the hydrogen content. In this plot, processed and non-processed samples seem to follow different correlations concerning hydrogen content. Processed samples (VBR and HP) show comparatively lower mid-AEBP at lower hydrogen content. This behavior might be related to the effect of cracking reactions that eliminate aliphatic chains and, therefore, decrease the molecular weight. When chains are eliminated, the fractions become more insoluble, but at the same time as the molecule reduces its size, the dispersion forces that held the fluid or material together decrease as these forces depend on the molecular volume. In fact, one of the fractions from the hydroprocessed sample (HP) was partially insoluble in toluene and/or CS2.

Figure 10. Mid-AEBP of the fractions as a function of their hydrogen content. Some of the data was taken from reference (17).

Coking Behavior It has been reported that the phase behavior of asphaltenes during catalytic and thermal upgrading is closely related to coke formation (49). The solubility of asphaltenes in the fluid is critical in the generation of coke. If asphaltene solubility decreases, aggregation begins and a new phase would separate. This new phase leads to coke formation (50). In fact, the primary mechanism in crude oil heat exchanger fouling comprises two steps: asphaltene precipitation due to incompatibility with crude oil, and subsequent carbonization or coke formation (22, 51). In the case of residues, the colloidal stability is related to the coke formation characteristics. The worse the stability of the residue, the more likely the coke is to form (52). 62 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 11 shows the amount of coke produced by the fractions. The coke amount was evaluated using a TGA apparatus as the residue obtained at 900°C under a nitrogen atmosphere. In Figure 11, the amount of coke for all the studied fractions is plotted as a function of the hydrogen content. As expected, the amount of coke decreases as the hydrogen content increases in the fractions.

Figure 11. Coke formation as a function of the hydrogen content of the SF fractions. Data from reference (17).

Continuity and Overlapping of Fractions In previous sections, it was demonstrated that average properties of the SF fractions change predictably based on their hydrogen content. However, the examination of the properties has still been carried out based on average measurements. Obviously, fractionation has the significant advantage of producing a distribution of properties, instead of unique values. In this section, analysis of the fractions is carried out based on two techniques that can produce distributions: solubility profile and ultrahigh resolution mass spectrometry. The solubility profile was already introduced in a previous section (see Figure 5) and allows the evaluation of solubility characteristics of the fractions as the material eluting as a function of the time is related to the solvent power of the fluid used during the elution. The solubility profile method can be used to quantify the stability of petroleum-related materials based on the distribution of the peaks (21). The quantification is based on the principle that the solubility parameters of two substances differ enough that they are immiscible. Therefore, the substances can be mutually solubilized by the addition of a third component (53). It has been shown that this method can be used to evaluate the presence of high solubility parameter asphaltenes and the lack of intermediate material that helps in the peptization of the former molecules (21, 54). 63 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 12. Comparison of the solubility profiles of fractions SF-2 to SF-4 for virgin (HVCO) and processed materials (VBR and HP). Data from reference (17). Figure 12 shows a comparison of the solubility profile of fractions from three different samples (HVCO, VBR, and HP). HVCO is a virgin sample, while the VBR and HP are a visbroken residue and a hydroprocessed product. These plots reflect the continuity of the fractions regarding solubility characteristics. In the plots, the distributions for each fraction have been scaled up to represent their respective content in the n-heptane insoluble material. The envelope in each plot represents the sum of the three distributions, and it would be considered as the solubility profile of the asphaltenes. This figure exemplifies the main differences in the distribution of solubility for samples of different origins. In processed samples, the first asphaltene fraction (SF-2) is less abundant than in the virgin material, and the third fraction (SF-4) is clearly moved to the right. Both characteristics are consistent with lower asphaltene stability in the processed materials (21): a low 64 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

first peak that acts as an intermediate material between maltenes and the rest of the n-heptane insoluble and a predominant third peak displaced to longer times and, therefore, eluted at higher solvent power. Notice the wider gap between the SF-2 and SF-4 in the processed samples and its absence in the virgin one. This wider gap has been associated with the instability of the samples regarding asphaltene precipitation (21, 54, 55). In Figure 13, corresponding to the deposit, the fractions SF-1 to SF-3 have almost completely disappeared, and the deposit is enriched in SF-4 that it is also clearly displaced to the right of the chromatogram. Another interesting aspect revealed in Figure 12, and 13 is the significant overlapping of the fractions. This overlapping of areas of the fractions in these plots is substantial and represents an important overlapping concerning solubility.

Figure 13. Solubility profile of fractions SF-2 to SF-4 extracted from a deposit. Data from reference (16).

Some of the fractions were also studied using ultra-high resolution mass spectrometry (56, 57). This technique has been successfully applied in providing detailed information about crude oils (58, 59), shale oils (60), asphaltenes (61, 62), and asphaltene deposits (63–65). among other materials. It has also been used to validate the compositional continuum of petroleum components model (66), first developed by Boduszynski (2). Among the different possible ionization techniques, atmospheric pressure photoionization (APPI) can efficiently ionize polycyclic aromatic compounds whether they show basic, acidic or neutral characteristics (67) and this was the technique used in the study of the fractions (56, 57). Figure 14 shows a comparison of the compositional space for HVCO fractions. These fractions were analyzed by APPI positive-ion-mode coupled to Fourier transform ion cyclotron resonance mass spectrometry (56). In Figure 14, 65 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

DBE is the double bond- equivalence that represents the number of rings plus the number of double bonds in a given molecular formula and was calculated using the following equation (68):

For the elemental formula CcHhNnOoSs. The C number represents the number of carbons for each species.

Figure 14. DBE versus carbon number for fractions SF-2 to SF-4 (HVCO) using APPI positive ion-mode. Data from reference (56). 66 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 15 shows how the distributions from SF-2 to SF-4 shift towards larger DBEs while the size of the molecules does not seem to increase significantly. These shifts point out to an average increase in the size of the polyaromatic cores of the molecules in agreement with the discussions of previous sections. The plots in Figure 15 also reveal a significant overlapping among the fractions like the one observed using the solubility profile method. In fact, calculations regarding the number of species indicate that between 60 to 80 % of the species identified in each fraction are also present in other fractions. Similar percentages can be calculated for fractions extracted from deposits (57).

Figure 15. Distribution of the fractions as a function of a) carbon number and b) DBE. Fractions SF-2 to SF-4 (HVCO). Data from reference (56). 67 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Concluding Remarks More than 30 years ago, Boduszynski demonstrated that non-distillable fractions followed the same patterns than distillation cuts and that solubility could be used to perform an equivalent distillation for non-distillable materials. The use of solubility fractionation to expand the characterization of non-distillable materials has been useful in the characterization of asphaltenes. Sequential elution fractionation has led to a self-consistent picture of the asphaltene properties. It has been shown that regular patterns in solubility and aggregation behavior in asphaltenes are linked to hydrogen deficiency. There is also substantial evidence that the decrease in hydrogen content observed for all the asphaltene fractions is related to the increase in the size of polyaromatic cores. Asphaltene instability is correlated to the presence of high solubility parameter components and to the uneven solubility distribution that indicates the lack of intermediate components that help in the peptization of the high solubility parameter molecules.

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