Peptizing effect of native heavy resin fraction on asphaltenes - Energy

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Peptizing effect of native heavy resin fraction on asphaltenes He Liu, Zhaoxian Liu, Aijun Guo, Kun Chen, Shengnan Sun, and Zongxian Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00208 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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Peptizing effect of native heavy resin fraction on asphaltenes He Liu†, Zhaoxian Liu†, Aijun Guo*†, Kun Chen†, Shengnan Sun†, Zongxian Wang† †

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering,

China University of Petroleum (East China), 66 Changjiang West Road, Qingdao Economic Development Zone, Qingdao Shandong 266580, China * Corresponding author: Tel: 086-0532-8698-0607, Fax: 086-0532-8698-1787, e-mail address: [email protected]. Abstract: C7-asphaltenes and heavy resin fraction (HRe, i.e. the fraction insoluble in n-pentane but soluble in n-heptane) were separated from two typical heavy residues. The role of HRe on asphaltene aggregation was mainly explored. Firstly, effect of HRe on the size of asphaltene aggregates in heptol solvents was studied by dynamic light scattering (DLS). The average aggregate size of asphaltenes upon the increase of resin concentration shows that HRe could inhibit asphaltene aggregation more efficiently than light resin fraction (LRe, i.e. the polar fraction from C5-maltenes by adsorption chromatography). However, an obvious size increase of asphaltene aggregates is observed when HRe exceeds some concentration. It can be tentatively attributed to the prior adsorption and penetration of resins in the less polar asphaltenes, followed by the re-aggregation of the remaining asphaltenes due to their more aromatic and polar nature. Then the thermodynamics of asphaltene phase behavior in solutions with the addition of resins were discussed in terms of the molar Gibbs free energy (∆g) based on Flory-Huggins polymer theory. HRe is more inclined to act as the dispersed phase and could effectively orient ∆g of asphaltene solutions. Moreover, 1

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adsorption isotherms of resins on asphaltene solids were obtained and the adsorption equilibrium constant (KAR) was then calculated. HRe presents a much higher KAR with asphaltenes than LRe. These results indicate that the more aromatic and polar resin fraction, HRe, could serve as a highly-efficient peptizing agent for asphaltene stabilization. Through X-ray diffraction (XRD) analysis, it is found that the aromatic stacking of asphaltenes can be effectively disrupted by peptization of HRe, thus leading to the increase of the layer distance between aromatic sheets (dm) and the average height of the stack of aromatic sheet (Lc) as well as the decrease of the average diameter of aromatic sheet (La). 1. Introduction With the depletion of conventional oil resources, heavy oil has attracted great attention in the global energy supplies.1 Heavy oil is generally characterized by a substantial amount of asphaltenes. Stability of asphaltenes is susceptible to the fluctuation of external surroundings and intrinsic molecular structures.2-4 Hence, asphaltenes are extremely prone to precipitate and pose detrimental hazards to exploitation, transportation and upgrading processes of heavy oil,5,6 such as well-head blocking, pipeline clogging, and coke deposition. As heavy oil gradually becomes much inferior, these problems relevant to asphaltene precipitation are even worse. Therefore, preliminary studies on the stability issue of asphaltenes would be beneficial to prevent asphaltene precipitation more efficiently and have always been hot spots in both upstream and downstream petroleum research fields. Despite the fact that asphaltenes have been focused on extensively for decades, the 2


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definition of asphaltenes still remains debatable.7 In terms of solubility class, asphaltenes are generally defined as the fraction of heavy oil that is insoluble in n-alkanes but soluble in benzene and toluene.8 The method to separate asphaltenes is still not standardized and remains subject to the preferences of the standards organizations of different countries.9 In general petroleum research, n-pentane and n-heptane are the widely-advocated solvents in laboratory to separate C5-asphaltenes and C7-asphaltenes, respectively. The asphaltene fraction composes the dispersed phase in colloidal system of heavy oil while the maltenes are the continuous phase. At the molecular level, asphaltenes mainly consist of condensed polyaromatic structures attached by alkyl side chains,10-12 with the inclusion of abundant heteroatoms (N, O, and S) and metals (mainly Ni and V).13-15 The asphaltene molecules have been proposed to be either “continent” or “archipelago” or both structure models coexist.16 These molecules could self-associate due to the intermolecular or intramolecular interactions (e.g. dipole-dipole interaction, charge-transfer interaction, hydrogen bonding).17 A variety of techniques, such as small-angle neutron scattering (SANS),18 small-angle X-ray scattering (SAXS),16,19-21 DLS,22-24 vapor pressure osmometry (VPO),25 nanofiltration,25 Rayleigh scattering25 and XRD10-12 have validated the existence of asphaltene aggregates in solutions and solids at different length scales.26,27 In contrast to asphaltenes, resins have fewer aromatic rings with longer alkyl chains and less abundant polar moieties (mainly carbonyl, amine, pyrrole, and indole derivatives). Traditionally, it is believed that the aggregation behavior of asphaltenes 3


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could be altered by the presence of resins to some extent. But the essential role of resins is controversial.9,28 Indeed, resins as well as the distinction between asphaltenes and resins are poorly identified.7,29 In addition to the polar fraction separated by adsorption through the formal SARA method,7 the fraction insoluble in n-pentane but soluble in n-heptane (i.e. C5-C7 fraction) separated by solubility has been taken as resins by some researchers.30 The viewpoints relevant to the role of resins in asphaltene aggregation could be colored by the definition and fractionation methodology, as asserted by Zhao et al.31,32 It is well-known that the resins sample collected from SARA fractionation, where adsorption process is inevitable, usually suffers the loss of the most polar molecules due to irreversible adsorption on the adsorbent. This phenomenon could be much worse for the separation of resins from C7-maltenes than that from C5-maltenes. In contrast, the loss can be avoided if the resins sample is prepared by solubility. Hence, this work utilizes the C7-asphaltenes as default asphaltenes, the C5-C7 fraction as heavy resin fraction (HRe), and the polar fraction from C5-maltenes by SARA as light resin fraction (LRe). A lot of studies on the influence of resins separated by adsorption on C7-asphaltene stability have been carried out.28,33-35 The UV-vis measurements showed that resins with amphiphilic nature and high surface activity could adsorb on asphaltene aggregates through charge-transfer or hydrogen bonding attractive forces.33,36 In analogy with the interaction mechanism between model amphiphiles and asphaltenes, a steric stabilization layer could be formed due to the repulsion caused by aliphatic side chains.37,38 Consequently, the produced asphaltene lyophilic colloids can be 4


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readily dispersed by the continuous phase. However, León et al.39,40 observed different adsorption isotherms of resins on asphaltenes from those of model amphiphiles. To explain this phenomenon, they proposed that the adsorbed resin layer could further penetrate into the bulk asphaltenes, break the microporous structures, and help them to diffuse in the continuous phase. The native resins with lower adsorbed amount but higher penetration power could exhibit a comparable effectiveness as asphaltene stabilizers with model amphiphiles.40 The penetration ability was correlated with the solubility of internal polyaromatic rings through a molecular thermodynamic approach, as studied by Rogel.41 Using SANS42,43 and DLS33 techniques, the decrease of asphaltene aggregate size and aggregation rate with the addition of resins was evidenced. Therefore, resins could reduce the molecular weight and increase the solubility of asphaltenes, which has been reported by Gawrys et al.42 These findings indicate that resins are effective at both dissolving the larger flocs into non-interacting aggregates and solubilizing the individual asphaltene aggregates.44 On the other hand, there are some observations supporting the incapability of resins to stabilize asphaltenes or even contribution to asphaltene destabilization. By isothermal microcalorimetry, Marques et al.36 found that resins and asphaltenes interacted only by weak van der Waals forces. This interaction could not prevent the flocculation and precipitation of asphaltenes. Goual and Firoozabadi 45 firstly reported the behavior that the mass of asphaltene precipitate increased with resins added. Pereira et al. 8 ascribed these contradictions to the self-interaction of resin molecules and put forward the repulsion/adhesion alternative model. In this model, the adsorbed 5


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layer of resins with strong self-interaction could promote the attraction and collision of asphaltenes. Nevertheless, there is no consistency for whether this resin fraction could aggregate or not. In regard to the resins separated by solubility (i.e. HRe), their role in asphaltene stability has been reported sporadically. From the SAXS measurements

[47]

, the

C7-maltenes showed a great q dependence and a high intensity, indicating the presence of large aggregates. Nevertheless, the C5-maltenes presented a weak q-dependent signal characteristic for a solvent behavior. Thereby, Eyssautier et al.48 proposed that C5-asphaltenes were best represented as dispersed phase. The aggregated structures present in C7-maltenes while absent in C5-maltenes have also been verified by freeze-fracture replication transmission electron microscopy.30 Accordingly, Li et al.17,30 pointed out that a part of polar and heavier resins could constitute the dispersed phase and contribute a lot to the formation of colloidal asphaltene-resin clusters. Many researchers have increasingly inclined to pay great attention to the mesoscale aggregate structure of C5-asphaltenes. Based on the detailed molecular weight and diffusion data of C7-asphaltenes and C5-asphaltenes, Yarranton et al.25 speculated that many of the additional components in HRe were still likely nonassociated. The asphaltenes might self-associate into a different size distribution upon the impact of these components. Zhao et al.31 have quantitatively evaluated the mass fraction of resins in C5-asphaltene-rich aggregates in nanofiltered oils at low temperatures. It further confirms that HRe could interact with asphaltenes and participate in the aggregation process. In spite of these ongoing studies addressing 6


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the importance of HRe on asphaltene aggregation, the essential effect of HRe on asphaltene stability still remains obscure and needs to be further clarified. This could also benefit the understanding of the distinction between resins and asphaltenes to some degree. In the present work, asphaltenes and two resin subfractions (HRe and LRe) were separated from heavy residues. The role of HRe on stability of asphaltenes was mainly explored. Effect of HRe on the average aggregate size of asphaltenes in heptol solvents was investigated using DLS. The thermodynamics of asphaltene phase behavior in solutions with the addition of resins were then discussed in terms of the molar Gibbs free energy based on Flory-Huggins polymer theory. Moreover, adsorption isotherms of resins on asphaltene solids were obtained and the adsorption equilibrium constant was calculated. Finally, the change of the macro-structures of asphaltenes caused by the peptizing effect of HRe was monitored by XRD analysis. 2. Experimental section 2.1 Sample preparation Vacuum residue with boiling point higher than 500 oC from Venezuelan heavy oil (VNVR) and that from Canadian oil sands bitumen (OSVR) were used as the feedstocks. Asphaltenes and resins were obtained from these residues by solvent extraction followed by alumina chromatographic separation. The scheme was shown in Figure 1. In brief, the residue was initially mixed with 40 mL n-pentane and 1 mL toluene/g of sample. The mixture was refluxed for 4 h, left for 24 h and filtered through a 1~3 µm filter paper. The filter cake was washed with n-pentane to remove 7


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the soluble materials. The obtained n-pentane insolubles were then extracted with n-heptane and toluene sequentially to recover heavy resins and asphaltenes, respectively. The n-pentane solubles were further fractionated into saturates (Sa), aromatics (Ar) and light resins on alumina chromatographic column with water content of 5 wt% eluted by sequential solvents with increasing polarity. The asphaltenes and resins samples were dried under vacuum at 60 oC for about 5 h and stored under nitrogen in order to prevent the oxidation. 2.2 Characterization of resin subfractions and asphaltenes Elemental composition was determined using a Vario EL III elemental analyzer from Elementar Company. 1H-NMR spectrum was recorded on Bruker av500 NMR spectrometer using deuterated chloroform as solvent and tetramethylsilane as internal standard. Based on the above analysis, the average molecular parameters of each structural unit for different fractions were calculated by the modified Brown-Ladner method.49,50 The detailed calculation process is described in Supporting Information. Main properties of these fractions are shown in Table 1. Fourier transform infrared spectrometry (FTIR) analysis was conducted on a Nicolet Nexus IR spectrometer from Thermo Nicolet Company. LRe was coated as film on KBr wafer while HRe and asphaltenes were finely ground with KBr powder and tableted. 2.3 Particle size analysis of asphaltenes The particle size analysis of asphaltenes was performed using DLS on a Malvern Zetasizer Nano zs at 25 oC. According to Rastegari et al.,51 asphaltene concentration in the narrow range of 0.05~0.1 mg/mL favors an accurate particle size measurement. 8


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Therefore, a series of solutions of 0.08 mg/mL asphaltenes in n-heptane/toluene (so-called “Heptol” mixture) with volume ratios of 60:40, 40:60, 20:80 and 0:100 were prepared. Subsequently, different amounts of resins were added into the asphaltene solution to ensure the resin concentration varying from 0.008 mg/mL to 0.072 mg/mL. All the solutions were completely mixed in volumetric flasks and left for 5~8 h to ensure adequate dissolution. The particle size distributions in these solutions were then monitored by DLS method. 2.4 Adsorption isotherms of resin subfractions on asphaltenes Measurement of the adsorption isotherms was referenced from previous reports.39,40 Specifically, a series of solutions of resin subfractions in n-heptane at concentrations ranging from 0.02 mg/mL to 1 mg/mL were prepared. A 10 mL aliquot was added to 10 mg asphaltenes in a flask and ultrasonically stirred for 12 h in a thermostatic bath at 25 oC. After reaching equilibrium, the slurry was centrifuged for 30 min at a speed of 8000 r/min. The absorbance of the supernatant at 400 nm was measured by UV-visible spectrometry on Varian Cary 50 spectrophotometer to evaluate the equilibrium concentration of resins. The amount of adsorbed resins is calculated from the depleted amount in the mass balance. 2.5 XRD analysis of asphaltenes Asphaltenes and HRe were ground to fine powders separately. Then a certain amount of black asphaltene powder and dark-brown resin powder was weighed accurately. The proportion of asphaltenes and HRe was the same as that in feed residue shown in Table 1. Finally, the asphaltene and resin powders were mixed and 9


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ground until the color was uniform to ensure the homogeneity of the mixtures. XRD patterns of asphaltenes, HRe, n-pentane insolubles and mechanical mixture of asphaltenes and HRe were recorded on a Panalytical X’Pert Pro MPD diffractometer, using Cu Kα radiation (λ=1.5418 Å). The operation temperature was 25 oC, the scanning angel 2θ was 0.017o. The XRD patterns were de-convoluted and analyzed as reported by previous researchers10-12 to obtain different crystallite parameters, including the layer distance between aromatic sheets (dm), the layer distance between aliphatic chains and naphthenic sheets (dγ), the average height of the stack of aromatic sheet (Lc) and the average diameter of aromatic sheet (La). 3. Results and discussion 3.1 Structural characterization of resin subfractions and asphaltenes Table 1 shows the elemental compositions and the average molecular parameters of each structural unit for different fractions from VNVR and OSVR. The H/C atomic ratio decreases with the solubility of fractions increasing, which correlates well with the increasing aromaticity (fA*) and aromatic ring (RA*).52 This indicates that the insoluble fraction is more abundant with aromatic structures than the soluble portion. Moreover, the fraction with lower solubility presents a much lower (HAU/CA)*, suggesting the inherent aromatic structures are more condensed. The strong π-π conjugation of these highly aromatic sheets could be greatly responsible for the aggregation behavior of the least soluble fraction (i.e. asphaltenes).17,25 Also, the value of fP* decreases when the fraction becomes heavier. It is generally believed that the paraffinic side chains attached to the aromatic moiety could hinder 10


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the asphaltene aggregation due to the steric repulsion.37 Since asphaltenes are relatively deficient in paraffinic structures, it can be inferred that the steric stabilization layer for asphaltene dispersion is not self-sufficient and could be supplied by resins to maintain the stability. Furthermore, each fraction of VNVR seems to be a little more aromatic and heavier than the corresponding fraction of OSVR. In addition, the heteroatom contents show that the heavier fraction tends to be more polar and functionalized. Figure 2 displays the FTIR spectra of different fractions from VNVR and OSVR. The peak at 3030 cm-1 is ascribed to the stretching vibration of phenyl C-H bonds and peaks at 874, 800, and 756 cm-1 represent the substitution to aromatic rings. The bands at 2800~2900 cm-1 and 1370~1465 cm-1 can be assigned to the stretching and bending modes of aliphatic C-H bonds, respectively. These evidences of aromatic and aliphatic structures are noticeable in spectra of all fractions. Compared with aromatics, resin subfractions and asphaltenes also show a broad band at 3100~3700 cm-1 assigned to O-H or N-H bond and a relatively weak band at 1000~1250 cm-1 attributed to C-O or C-N bond. This is resulted from the small amount of nitrogen and oxygen (< 0.3 wt%) in aromatics. In spectrum of VNLRe, the broad band corresponding to O-H or N-H bond shifts to lower wavenumber. Since VNLRe barely contains oxygen, this band should be assigned to N-H bond. OSLRe shows similar IR absorptions with OSHRe because of the oxygen content as high as 2.82 wt%. These polar functional groups can participate in hydrogen-bonding or van der Waals forces that partly contribute to aggregation of asphaltenes as well as 11


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adsorption of resins on asphaltene aggregates.52 The molecular characterization, particularly the FTIR spectra,29 indicates that HRe presents more similar chemical structures as asphaltenes than LRe. This is coherent with the view that asphaltenes and resins (especially HRe) should be considered as a continuum or family of complex molecules.29 3.2 Effect of resins on aggregate size of asphaltenes in solutions by DLS Figure 3 shows the particle size distribution of VNAs in heptol at different volume ratios. A unimodal size distribution can be observed for all asphaltene solutions, corroborating the size polydispersity of asphaltenes in hydrocarbons.23,25,53,54 In toluene, a large population of asphaltene particles falls in the 10~50 nm diameter range. The asphaltene particles slightly grow upon the addition of n-heptane. When the volume ratio of n-heptane reaches 50% and 60%, the diameter of asphaltene aggregates dramatically rises to 0.5~1.7 µm and 1~2.7 µm, respectively. The aggregate size of OSAs shows the same variation as that of VNAs. It appears that a threshold volume ratio of n-heptane exists for asphaltene aggregation, which can be confirmed by previous SANS54, ultra small angle X-ray scattering (USAXS)55 and photon correlation spectroscopy (PCS)56 measurements. Besides the aggregate size, the aggregation rate was also promoted in heptol with volume ratio of n-heptane over threshold value, as shown from the PCS result.56 Savvidis et al.55 argued that n-heptane as flocculant could decrease the solvent capacity of asphaltene surroundings thus increasing the intensity and range of attractive interactions between asphaltenes, resulting in the extensive emergence and high collision frequency of 12


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asphaltene particles.51,57 This would then affect the precipitation of asphaltenes. Consequently, there has been report as to the critical heptol concentration for the initial formation of asphaltene precipitates.52 The threshold value in different literatures is dependent on the origin and concentration of asphaltenes. To facilitate the study of the effect of resins on asphaltene aggregation, the aggregates in 60:40 heptol with larger diameters were selected as the target asphaltenes. Figure 4 shows the mean size of asphaltene aggregates as a function of the concentration of resin subfractions. The diameter of asphaltene aggregates obviously decreases with more HRe added in the initial stage. Based on the penetration model proposed by León et al.39, HRe can adsorb onto the surface of asphaltene aggregate and then penetrate into the macrostructure with porous and loose nature.58 The swelling behavior of asphaltenes has been verified by the cross-polarized and visible light microscopy59 and X-ray scattering measurements.60 This leads to the disintegration of large aggregates to produce low-diameter particles, as evidenced by SANS.43 However, in the second stage, the aggregate size increases abruptly with the presence of adequate HRe. Pereira et al.8 suggested that the resins with strong self-interaction could produce a thick and sticky adsorbed layer onto the surface of asphaltene aggregates and favor their growth. It seems as if the self-interaction of resins could be stronger than that of asphaltenes. However, due to the lower aromaticity and polarity of resins, the resin aggregation detected by SAXS was not as obvious as asphaltene aggregation.47 Meanwhile, this explanation appears to contradict the observation at low resin content. According to Acevedo et al.61, 13


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asphaltenes could be regarded as colloids with stacks of A1 as colloidal core while A2 as periphery. Therefore, A2 is the preferential structure that interacts with resins, which was well demonstrated by Pereira et al.8 It can be expected that the more A2 collapses due to the adsorption and penetration behavior of resins, the more A1 would be exposed to the surroundings, as illustrated in Figure 5. A1 exhibits stronger association forces because of the more polar and aromatic molecules.62 Once the amount of exposed A1 reaches the aforementioned threshold value, the aggregate size would notably increase. The excess resins could further act on the nascent asphaltene aggregate and decrease the overall particle sizes, as observed in the third stage. Since the nascent asphaltene aggregates are mainly consisted of the most polar and aromatic species, the effectiveness of resins on the nascent aggregates tends to be lower than that on the original ones.43 This possible procedure can also explain the phenomenon that the interaction between resins and asphaltenes was restricted at higher concentration of resins in the report of Aguiar et al.63 LRe shows a very different effect on the size of asphaltene aggregates from HRe. Overall, the size of asphaltene aggregates increases initially and then decreases with the increase of LRe concentration. On the one hand, this can only be partially rationalized by the penetration model.39 Compared with HRe, LRe is the less polar and aromatic resin fraction. A relatively weak interaction with asphaltenes can be predicted. The accumulation of adsorbed LRe layer makes the diameter of aggregates become somewhat larger. When the amount of adsorbed LRe is sufficient, a portion of this fraction could penetrate into asphaltene aggregates and results in the size decrease. 14


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The same results can be achieved with much lower concentration of HRe. It can be inferred that HRe presents stronger adsorption and penetration abilities than LRe. According to Rogel,41 this could be attributed to the more condensed polyaromatics and smaller aliphatic chains in HRe molecular structures. On the other hand, the asphaltene aggregation is greatly relevant to the solvent capacity of surrounding medium.34 Based on the study of Li et al.,30 LRe and part of HRe could serve as the continuous phase. Therefore, the asphaltene aggregation could be varied with the addition of resins by modifying the solvent capacity of surrounding medium.34 For comparison, effect of Ar that rarely peptizes asphaltenes on the size of asphaltene aggregates is simultaneously studied. Figure 5 clearly shows that Ar could increase the size of asphaltene aggregate on the whole. To preliminarily analyze the performance for dispersing asphaltenes, the ratio of fP* to fA* (fP*/fA*) for different fractions and heptol solvents is compared in Figure 6. It can be seen that the fP*/fA* of Ar is close to that of 60:40 heptol, indicating that Ar could possibly be inclined to act as precipitant and thus favor the aggregation of asphaltenes. LRe shows a ratio of fP*/fA* higher than 50:50 heptol. Hence, the presence of LRe in the continuous phase could also promote asphaltene aggregation. It further increases the size of asphaltene aggregates in addition to the adsorption performance. This is possibly the reason why a much larger asphaltene aggregate is seen with LRe added than that with Ar added at low concentration. Meanwhile, part of HRe with fP*/fA* of 0.80 in the continuous phase could serve as the dispersing solvent like 40:60 heptol and inhibits the asphaltene aggregation more efficiently. 15


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3.3 Thermodynamic analysis for the effect of resins on asphaltene stability in solutions With the addition of flocculant, the asphaltene molecules could initially associate with each other to form aggregates with various sizes. Interaction within large aggregates could result in the generation of floc.51 The further coalescence and growth of floc would finally induce the precipitation. Different thermodynamic models have been proposed to model the phase-behavior of asphaltene solutions.64 Resins could adhere on the surface of any asphaltene aggregate or floc to improve the diffusion of asphaltenes in continuous phase, and hence inhibit the precipitation. However, the role of resins in asphaltene aggregation remains to be explored from the perspective of thermodynamics. Traditionally, the phase behavior of asphaltenes in hydrocarbon solvents can be modeled based on Flory-Huggins polymer solution theory.65,66 The free energy for mixing of asphaltenes with solvents (∆a ) can be calculated as follows: ∆a = RTna lnφa + ns lnφs + χas Uφa φs

(1)

where n, φ and U are the number of moles, volume fraction and the number of repeating units, respectively. Subscriptions a and s are for asphaltenes and solvent phases, respectively. χas is the interaction parameter between asphaltenes and solvent. U and χas can be obtained as Eq. (2) and Eq. (3), respectively. U=U na + U ns χas =

vas RT

(2)

(δa - δs ) (3) 2

where, vas is the molar volume of the mixture that can be obtained as Eq. (4). 16


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δa and δs are solubility parameters of asphaltenes and solvents, respectively. vas =

1 (4) φa /va + φs /vs

In this work, Us is assumed to be unity and Ua can be calculated as the ratio of molar volume of asphaltenes (va ) to that of solvent (vs ). By combining the above equations, the molar Gibbs free energy of asphaltenes solution (∆ga ) can be expressed below: ∆ga =

φ ∆a =RT v a lnφa +φs lnφs +φa vas φs δa -δs 2 (5) a v s

The solubility parameter of asphaltenes δ could be estimated by the empirical formula proposed by Zuo et al.67 and Liang et al.,68 as displayed in Eq. (6) and Eq. (7): δa = 17.347×ρa +2.904 ρa =ρ25 ≈ ρ20 = 1.4673-0.0431×H (wt%) 4 4

(6) (7)

When it refers to the asphaltenes solutions in heptol in this study, ρa is taken as the average density of VNAs and OSAs (about 1.14 g/mL). vs and δ were obtained based on the volume fraction of toluene and n-heptane in heptol solvents: vs =

1 (8) φt /vt + φh /vh

δ s 2 = φt δ t 2 + φh δ h 2

(9)

where subscriptions t and h are for toluene and n-heptane, respectively. The phase behavior of asphaltenes in heptol is illustrated in Figure 7 from the view of the molar Gibbs free energy. A combination of factors, including the molar volume of asphaltenes and solubility parameters of solvents have significant influences on 17


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asphaltene stability. Asphaltenes that have larger molar volume can only maintain stable in solvents with higher solubility parameters. The curve (∆ga =0) represents the thermodynamic equilibrium between aggregation and solvation of asphaltenes. The area above the curve means that the solvation of asphaltenes prevails (∆ga 0). This is consistent with the presence of threshold volume ratio of n-heptane for the asphaltene aggregation observed by SAXS and DLS.56 As discussed in previous section, for 0.08 mg/mL asphaltene solutions, aggregation of asphaltenes arose in heptol with solubility parameter of 17.08 (J/mL)0.5. Meanwhile, the sharp growth of asphaltene aggregates occurred in heptol with solubility parameter of 16.47 (J/mL)0.5. It can be preliminarily deduced that the aggregation and solvation of asphaltenes are in equilibrium in heptol with volume ratio falling in the range of 60:40~40:60, corresponding to the red segment “AB”. On these bases, the molar volume of asphaltenes could probably vary from 1237 to 2240 mL/mol. Figure 7 also compares the phase behavior of asphaltenes at different concentrations within 10 mg/mL in solutions. It seems that the higher the concentration of asphaltenes, the stronger the tendency to aggregate in solvents with the same solubility parameter.69 In terms of the effect of resins on phase behavior of asphaltenes in hydrocarbon solvents, the variation of molar Gibbs free energy of the mixed asphaltene-resin solution (∆ga-r ) is also discussed. As widely accepted, asphaltenes and hydrocarbon solvents are the dispersed phase and continuous phase in the mixed solutions, respectively. Meanwhile, resins could serve as the transition phase at the interface 18


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between the two phases due to the structural composition of this continuum. Accordingly, the transition phase could be considered to be partitioned into dispersed phase and continuous phase.30 In this case, the molar volume and solubility parameter of the dispersed and continuous phases can be determined as follows: δar 2 = ar =

φa 2 + 2 + φa +

(10)

(11) / + / 2 + 2 2 = (12) + φs + rs = (13) / + / where subscriptions ar and rs stand for dispersed phase (asphaltenes and part of resins) and continuous phase (part of resins and solvents), respectively. Herein, the molar volumes of asphaltenes and resins are assumed to be 1500 and 900 mL/mol, respectively. The molar Gibbs free energy of asphaltene-resin solutions is then plotted with the volume fraction of resins, as shown in Figure 8. The partition ratio of resins into dispersed phase and continuous phase greatly affects the phase behavior of asphaltenes. When resins are more inclined to be present as the dispersed phase, the aggregation-solvation of asphaltenes can be readily changed with a small amount of resins added. This type of resins seems to act as a highly efficient agent for peptizing asphaltenes. Nevertheless, when resins are inclined to exhibit as the continuous phase, the effect of resins on the aggregation-solvation equilibrium of asphaltenes would be relatively weak. Combined with the above experimental analysis by DLS, it can be inferred that a much larger portion of HRe co-exist with asphaltenes in the dispersed 19


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phase due to their consistency in molecular structures and higher internal affinity.17 Therefore, HRe could orient ∆ga-r more effectively by modifying the composition of dispersed and continuous phases. The above discussions are well supported by the study of Aguiar et al.63 who stated that resins were able to stabilize asphaltenes by interacting strongly with them and modifying their solubility parameters. Driven by the interaction between asphaltenes and resins, a completely new system would be formed, as shown in Figure 5. In this system, part of asphaltenes (mainly A1) and a small amount of resins constitute the dispersed phase while part of asphaltenes (mainly A2) with the protection of resins and solvents can be regarded as the continuous phase. It can be suspected that the molar Gibbs free energy of this new mixing solution would subsequently be changed by the addition of more resins. This needs to be further studied in the future. 3.4 Adsorption equilibrium of resins on asphaltene solids To further investigate the interaction between asphaltenes and resins, the adsorption isotherms of resin subfractions on asphaltene solids were measured and shown in Figure 9. The isotherms of HRe display a continuous rise in the amount of adsorbed resins on asphaltenes. According to León et al.,39 this can be well interpreted using the penetration theory of the solute in the microporous structure of asphaltenes. Both the amount of HRe adsorbed onto asphaltenes and the increasing rate appear to be much higher than that of LRe. It further proves that HRe shows a higher ability for adsorption and penetration in asphaltenes than LRe. A plateau seems to appear in the isotherms of LRe at low concentrations. This might indicate the formation of 20


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multilayer of LRe on asphaltenes because of the low penetration ability.40,70 The multilayer of LRe would lead to the size increase of asphaltene aggregates shown in Figure 4. The equilibrium constants for adsorption of resin subfractions on asphaltenes (KAR) are then calculated based on the Langmuir adsorption isotherms:71

KAR [Re] (14) mAs KAR Re+1 [ReS ]

=

[ReT ]=[Re ]+[Re] S

(15)

where [As] is the asphaltene concentration, [ReS] is the equilibrium concentration of resin subfractions adsorbed on asphaltenes, [Re] is the concentration of resin subfractions in the continuous phase, [ReT] is the total concentration of resin subfractions, m is defined as the ideal concentration of resin subfractions for complete adsorption on asphaltenes. If m is assumed to be unity and the deviation factor ∆δ is introduced, the adsorption isotherm can be expressed as the following equation:

As 1 1 = + +∆δ [ReS ] KAR Re 1

(16)

The variation of [As]/[ReS] is then plotted as a function of 1/[Re], as shown in Figure 10. The good linear correlation indicates that the adsorption of resins on asphaltenes could be analyzed by the Langmuir adsorption model. The calculated KAR results are listed in Table 2. It shows that KAR for adsorption of OSHRe on OSAs is approximately twice as that for adsorption of OSLRe. The KAR for adsorption of VNHRe on VNAs can be 4.6 times as that for adsorption of VNLRe. The quantitative data confirm that HRe presents much stronger peptizing ability than LRe. Furthermore, KAR for VNHRe-VNAs seems to be higher than that for OSBHRe-OSBAs, suggesting that VNHRe is more efficient for maintaining 21


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asphaltene stability than OSHRe. 3.5 Change of the macro-structures of asphaltenes caused by peptizing effect of HRe by XRD analysis The change of the macro-structures of asphaltenes caused by peptizing effect of HRe was probed by XRD analysis. The XRD patterns of asphaltenes, HRe, n-pentane insolubles and the mechanical mixture of HRe and asphaltenes were obtained and shown in Figure 11. The profiles of asphaltenes are mainly characterized by the graphene band around 26o and the γ band around 19o, which can be assigned to the stacking of aromatic molecules and packing of saturated structures, respectively. The graphene band is used to estimate the layer distance between aromatic sheets (dm) and the average height of the stack of aromatic sheets (Lc) while the γ band is used to estimate the layer distance between aliphatic chains and naphthenic sheets (dγ).10-12 A relatively weak 10 band at about 43o can also be observed, which reveals the average diameter of aromatic sheet (La).72 In contrast, HRe shows no graphene band, indicating the aromatic stacking of resin molecules is relatively weak and beyond the detection limit of XRD since X-ray experiments are more sensitive to the aromatic-rich regions [44]. This is compatible with the suggestion of Lima et al.73 from conformation of resin dimers by computational study. The previous comparison of SAXS measurement47 for C5-maltenes and C7-maltenes showed that HRe could possess the self-association ability. Hence, the self-association of HRe could possibly be attributed to the hydrogen-bonding or van der Waals forces due to the incorporation of heteroatoms in molecules. 22


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The XRD patterns are then de-convoluted in order to calculate the crystallite parameters.72 The de-convoluted curves for mixture of HRe and asphaltenes are displayed in Figure 11 as an example. The crystallite parameters of different asphaltenes are listed in Table 3. The data in Table 3 show that dm and dγ for original asphaltenes from feed residues are around 3.500 Å and 4.380 Å, respectively. The two parameters present a very slight increase when HRe is mechanically mixed with asphaltenes. When asphaltenes are peptized with HRe (i.e. n-pentane insolubles), the layer distance between aromatic sheets as well as that between saturated sheets is further increased. This phenomenon could be further proved by transmission electron microscopy (TEM) that C5-asphaltenes present a larger mean interlayer spacing than C7-asphaltenes in the work of Zhang et al.74 Meanwhile, the value of Lc for asphaltenes peptized with HRe increases obviously. For OSAs, it has increased from 41.9 Å to 46.7 Å. It seems that asphaltenes are swollen in the presence of HRe, which could be attributed to the adsorption and penetration of HRe into asphaltene macro-molecules.40 Moreover, the average diameter of aromatic sheets (La) has correspondingly decreased. Hence, it can be concluded that the aromatic stacking of asphaltenes can be effectively disrupted by peptizing effect of HRe.74 Conclusions The role of HRe on asphaltene aggregation was explored in the present work. The aggregate size of asphaltenes upon the increase of resin concentration measured by DLS shows that HRe could inhibit asphaltene aggregation more efficiently than LRe. An obvious size increase of asphaltene aggregates is observed when HRe exceeds 23


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some concentration. It can be tentatively attributed to the prior adsorption and penetration of resins in the less polar asphaltenes, followed by the re-aggregation of the remaining asphaltenes due to their more aromatic and polar nature. The thermodynamics of asphaltene phase behavior based on Flory-Huggins polymer theory suggests that HRe is more inclined to act as the dispersed phase and could effectively orient ∆g of asphaltene solutions. HRe presents a much higher adsorption equilibrium constant with asphaltenes than LRe. These results indicate that the more aromatic and polar resin fraction, HRe, could serve as highly efficient peptizing agents for asphaltene stabilization. XRD analysis shows that the aromatic stacking of asphaltenes could be effectively disrupted by peptization of HRe, leading to the increase of the layer distance between aromatic sheets (dm) and the average height of the stack of aromatic sheet (Lc) as well as the decrease of the average diameter of aromatic sheet (La). Acknowledgement This work was supported by the project funded by the National Natural Science Foundation of China (21776313 and U1362101), the China Postdoctoral Science Foundation (2016M602219), the Provincial Natural Science Foundation of Shandong (ZR2017BB021), State Key Laboratory of Heavy Oil Processing (SLKZZ-2017003 and SLKZZ-2017011), the Qingdao Postdoctoral Applied Research Project (2016224), the Fundamental Research Funds for the Central Universities (Special Projects, 17CX05016), PetroChina Innovation Foundation (2017D-5007-0506), the Key Research and Development Plan of Shandong Province (2017GGX70108), and the 24


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China National Petroleum Corporation (PRIKY16066). The authors are grateful to master student Ummul-Khairi Ibrahim from Ghana for improving the English of this paper. References [1] Shah, A.; Fishwick, R.; Wood, J.; Leeke, G.; Rigby, S.; Greaves, M. Energy Environ. Sci. 2010, 3, 700-714. [2] Hajiakbari, N.; Teeraphapkul, P.; Fogler H S. Energy Fuels 2014, 28, 909-919. [3] Guzmán, R.; Ancheyta, J.; Trejo, F.; Rodríguez, S. Fuel 2017, 188, 530-543. [4] Rogel, E. Langmuir 2002, 18, 1928-1937. [5] Ghloum, E. F.; Al-Qahtani, M.; Al-Rashid A. J. Pet. Sci. Eng. 2010, 70, 99-106. [6] Derakhshesh, M.; Eaton, P.; Newman, B.; Hoff, A.; Mitlin, D.; Gray, M. R. Energy Fuels 2013, 27, 1856-1864. [7] Merinogarcia, D.; Shaw, J. M.; Carrier, H.; Yarranton, H.; Goual, L. Energy Fuels 2010, 24, 2175-2177. [8] Pereira, J. C.; López, I.; Salas, R.; Silva, F.; Fernández, C.; Urbina, C.; López, L. C. Energy Fuels 2007, 21, 1317-1321. [9] Speight J. G. The Chemistry and Technology of Petroleum (Third Edition, Revised and Expanded); Marcel Dekker, Inc., New York, 1999; pp 244-298. [10] Tanaka, R.; Sato, E.; Hunt, J. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118-1125. [11] Tanaka, R.; Hunt, J. E.; And, R. E. W.; Ehiyagarajan, P.; And, S. S.; Takanohashi, T. Energy Fuels 2003, 17, 127-134. 25


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[12] Kayukova, G. P.; Gubaidullin, A. T.; Petrov, S. M.; Romanov, G. V.; Petrukhina, N. N.; Vakhin, A. V. Energy Fuels 2016, 30, 773-783. [13] Speight, J. G.; Moschopedis, S. E. Fuel 1980, 59, 440-442. [14] Cataldo, F.; Garcíahernández, D. A.; Manchado, A. Mon. Not. R. Astron. Soc. 2013, 429, 3025-3039. [15] Cataldo, F.; Angelini, G.; Garcíahernández, D. A.; Manchado, A. Spectrochim. Acta, Part A 2013, 111, 68-79. [16] Eyssautier, J.; Levitz, P.; Espinat, D.; Jestin, J.; Gummel, J; Grillo, I.; Barré, L. J. Phys. Chem. B 2011, 115, 6827-6837. [17] Li, S.; Liu, C.; Que, G.; Liang, W.; Zhu, Y. Fuel 1997, 76, 1459-1463. [18] Sheu, E. Y. J. Phys.: Condens. Matter 2006, 18, S2485-S2498. [19] Pfeffer, H.; Dannel, F.; Walch-Liu, P.; Römheld, V. Fuel 2001, 80, 283-287. [20] Hurtado, J. L. A.; Chodakowski, M.; Long, B.; Shaw, J. M. Energy Fuels 2011, 25, 5100-5112. [21] Barré, L. J. Dispersion Sci. Technol. 2004, 25, 341-347. [22] Hashmi, S. M.; Firoozabadi, A. J. Phys. Chem. B 2010, 114, 15780-15788. [23] Eyssautier, J.; Frot, D.; Barré, L. Langmuir 2012, 28, 11997-12004. [24] Hashmi, S. M.; Quintiliano, L. A.; Firoozabadi, A. Langmuir 2010, 26, 8021-8029. [25] Yarranton, H. W.; Ortiz, D. P.; Barrera, D. M.; Baydak, E. N.; Barré, L.; Frot, D.; Eyssautier, J.; Zeng, H.; Xu, Z.; Dechaine, G.; Shaw, J. M.; McKenna, A. M.; Mapolelo, M. M.; Bohne, C.; Yang, Z.; Oake, J. Energy Fuels 2013, 27, 26


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5083-5106. [26] Maham, Y.; Chodakowski, M. G.; Zhang, X.; Shaw, J. M. Fluid Phase Equilib. 2005, 227, 177-182. [27] Barré, L.; Sébastien Simon, A.; Palermo, T. Langmuir 2008, 24, 3709-3717. [28] Sedghi, M.; Goual, L. Energy Fuels 2011, 24, 2275-2280. [29] González, G.; Sousa, M. A.; Lucas, E. F. Energy Fuels 2006, 20, 2544-2551. [30] Li, S.; Liu, C.; Que, G.; Liang, W.; Zhu, Y. Fuel 1996, 75, 1025-1029. [31] Zhao, B.; Becerra, M.; Shaw, J. M. Energy Fuels 2009, 23, 4431-4437. [32] Zhao, B.; Shaw, J. M. Energy Fuels 2007, 21, 2795-2804. [33] Anisimov, M. A.; Ganeeva, Y. M.; Gorodetskii, E. E.; Deshabo, V. A.; Kosov, V. I.; Kuryakov, V. N.; Yudin, D. I.; Yudin, I. K. Energy Fuels 2014, 28, 6200-6209. [34] Mclean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 196, 23-34. [35] Mclean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 189, 242-253. [36] Marques, L. C. C.; Pereira, J. O.; Bueno, A. D.; Marques, V. S.; Lucas, E. F.; Mansur, C. R. E.; Machado, A. L. C.; González, G. J. Brazil. Chem. Soc. 2012, 23, 1880-1888. [37] Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749-1757. [38] Correra, S.; Cimino, R.; Delbianco, A.; Lockhart, T. Solubility and Phase Behavior of Asphaltenes. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y.; Mullins, O. C., Ed.; Plenum Press, New York, 1996; pp 97-130. [39] León, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5106-5112. 27


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[40] León, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Espidel, J.; Acevedo, S. Energy Fuels 2001, 15, 1028-1032. [41] Rogel, E. Energy Fuels 2008, 22, 3922-3929. [42] Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21, 461-489. [43] Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9-27. [44] Bardon, C.; Barre, L.; Espinat, D.; Guille, V.; Min Hui Li; Lambard, J.; Ravey, J. C.; Rosenberg, E.; Zemb, T. Pet. Sci. Technol. 1996, 14, 203-242. [45] Goual, L.; Firoozabadi, A. AIChE J. 2004, 50, 470-479. [46] Mostowfi, F.; Indo, K.; Mullins, O. C.; Mcfarlane, R. Energy Fuels 2009, 23, 1194-1200. [47] Eyssautier, J.; Hénaut, I.; Levitz, P.; Espinat, D.; Barré, L. Energy Fuels 2012, 26, 2696-2704. [48] Eyssautier, J.; Espinat, D.; Gummel, J.; Levitz, P.; Becerra, M.; Shaw, J. M.; Barré, L. Energy Fuels 2012, 26, 2680-2687. [49] Wang, Q.; Jia, C.; Ge, J.; Guo, W. Energy Fuels 2016, 30, 2478-2491. [50] Zhang, J.; Tian, Y.; Qiao, Y.; Yang, C.; Shan, H. Energy Fuels 2017, 31, 8072-8086. [51] Rastegari, K.; Svrcek, W. Y.; Yarranton, H. W. Ind. Eng. Chem. Res. 2004, 43, 6861-6870. [52] Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 28


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[67] Zuo, J. Y.; Mullins, O. C.; Freed, D.; Dan, Z. J. Chem. Eng. Data 2010, 55, 2964-2969. [68] Liang, W. Heavy Oil Chemistry; Petroleum University Press, Dongying, China, 2000; pp 194-199. [69] Hoepfner, M. P.; Fogler, H. S. Langmuir 2013, 29, 15423-15432. [70] Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutiérrez, L.; Ortega, P. Fuel 1995, 74, 595-598. [71] Levan, M. D.; Vermeulen, T. J. Phys. Chem. 1981, 85, 3247-3250. [72] Alhumaidan, F. S.; Hauser, A.; Rana, M. S.; Lababidi, H. M. S.; Behbehani, M. Fuel 2015, 150, 558-564. [73] Lima, F. C. D. A.; Alvim, R. D. S.; Miranda, C. R. Energy Fuels 2017, 31, 11743-11754. [74] Zhang, L. L.; Yang, G. H.; Wang, J. Q.; Li, Y.; Li, L.; Yang, C. H. Fuel 2014, 128, 366-372. Supporting information. A complete description of calculation of H/C atomic ratio and the average molecular parameters by the modified Brown-Ladner method (Experimental), the 1H-NMR spectra of different fractions from VNVR (Figure S1), the assignments of different types of hydrogen in 1H-NMR spectra (Table S1), the equations for the modified Brown-Ladner method (Table S2). All illustrations

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Figure 1. Separation scheme of asphaltenes and resin subfractions from feed residues. (The aromatics, light resins, heavy resins and asphaltenes from VNVR are labeled as VNAr, VNLRe, VNHRe and VNAs, respectively. The corresponding fractions from OSVR are labeled as OSAr, OSLRe, OSHRe and OSAs, respectively.)

31


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Figure 2. FTIR spectra of different fractions from feed residues. (a) VNVR, (b) OSVR

33


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Figure 3. Particle size distribution for 0.08 mg/mL VNAs in heptol at different volume ratios.

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Figure 4. The mean size of asphaltene aggregates as a function of the concentration of 35


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resin subfractions and aromatics from feed residues. (a) VNVR, (b) OSVR.

Figure 5. The illustration for possible solvation-aggregation behavior of asphaltenes upon the effect of heavy resin fraction. (I) asphaltenes, (II) adsorption and penetration of heavy resins on asphaltenes, (III) formation of different resin-asphaltene macromolecules, (IV) re-aggregation of resin-asphaltene macromolecules with higher polarity and aromaticity.

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Figure 6. Comparison of the fP*/fA* for different fractions and heptol solvents.

Figure 7. Phase behavior of asphaltenes in different heptol solvents at different 37


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concentrations from the view of molar Gibbs free energy (Ca refers to the concentration of asphaltenes in solvents).

Figure 8. Plot of ∆ga-r of mixed asphaltene-resin solutions with the volume fraction of resins at different partition ratios of resins into dispersed phase and continuous phase.

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Energy & Fuels

Figure 9. The adsorption isotherms for resin subfractions on asphaltenes from feed 39


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

residues. (a) VNVR, (b) OSVR.

Figure 10. The plots of [As]/[ReS] as a function of 1/[Re] for adsorption of OSHRe on OSAs.

40


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Energy & Fuels

Figure 11. XRD patterns of asphaltenes, HRe, n-pentane insolubles and mixture of asphaltenes and HRe from VNVR.

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

Table 1. Chemical analysis results of fractions from VNVR and OSVR Sample

VNAr VNLRe VNHRe VNAs OSAr OSLRe OSHRe OSAs

Content/wt%

43.52

23.92

4.62

19.05

42.20

23.67

5.93

18.05

H/wt%

10.65

9.73

8.85

7.32

10.73

9.84

9.33

7.89

C/wt%

84.16

84.17

82.22

81.77

84.10

81.54

82.0

81.68

N/wt%

0.27

1.22

1.52

2.01

0.20

1.12

1.11

1.20

S/wt%

4.92

4.88

4.66

5.50

4.96

4.68

5.92

7.04

O a/wt%

0.00

0.00

2.75

3.40

0.01

2.82

1.64

2.10

H/C atomic ratio

1.52

1.39

1.29

1.08

1.53

1.45

1.37

1.16

f A* b

0.312

0.383

0.437

0.549

0.307

0.364

0.417

0.497

fN* c

0.253

0.184

0.204

0.192

0.231

0.238

0.254

0.201

fP* d

0.435

0.433

0.359

0.259

0.461

0.397

0.329

0.302

(HAU/CA)* e

0.748

0.601

0.583

0.414

0.737

0.687

0.663

0.481

RA* f

2.53

4.45

4.81

10.83

2.67

3.51

4.09

7.69

RN* g

2.46

2.78

2.86

4.25

2.38

2.63

2.81

3.65

Note: * refers to the parameter for each structural unit. Aromaticity.

c

Naphthenic carbon ratio.

d

a

Calculated by subtraction method.

Paraffinic carbon ratio.

aromatic rings. f Aromatic ring number. g Naphthenic ring number.

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e

b

Condensation degree of

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Energy & Fuels

Table 2. The equilibrium constants for adsorption of resin subfractions on asphaltenes Sample

m

KAR /mL/mg

∆

VNLRe-VNAs

1

0.457±0.043

1.333±1.344

VNHRe-VNAs

1

2.091±0.135

0.348±0.395

OSBLRe-OSBAs

1

0.534±0.066

1.379±1.581

OSBHRe-OSBAs

1

1.138±0.096

-1.744±0.761

Table 3. The crystallite parameters for asphaltenes, HRe, n-pentane insolubles and mixture of asphaltenes and HRe dm /Å

dγ /Å

Lc /Å

La /Å

fA

R2

Asphaltenes

3.507

4.384

40.3

14.9

0.17

0.9991

HRe

-

-

-

-

-

-

N-pentane insolubles

3.541

4.426

41.2

14.1

0.15

0.9994

Mixture of asphaltenes and HRe

3.511

4.388

39.0

14.8

0.15

0.9994

Asphaltenes

3.498

4.373

41.9

14.9

0.16

0.9996

HRe

-

-

-

-

-

-

N-pentane insolubles

3.507

4.383

46.7

14.6

0.13

0.9993

Mixture of asphaltenes and HRe

3.503

4.379

41.9

14.6

0.14

0.9992

Sample

VNVR

OSVR

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Peptizing effect of native heavy resin fraction on asphaltenes - Energy

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