Review and Comprehensive Analysis of Deaggregation and Separation Methods for As

Cai Xinheng; Long Jun; Dong Ming; Wang Wei; Hou Huandi; Tian Songbai

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: Asphaltenes generally exist in the form of molecular aggregates in crude oil or in petroleum residues, and asphaltene aggregates can usually cause serious problems to oil exploitation, transportation, and processing. Achieving deaggregation and separation of asphaltene aggregates is a premise and basis for molecular characterization and processing of heavy oils. Aiming at the intermolecular interactions in asphaltene molecular aggregates, it has proposed and summarized that aspahltene aggregates can be subject to deaggregation by means of five approaches, including solvent diluting,removing active sites, moderate heating, ultrasonication and on-line molecular collision. Moreover, asphaltenes can be further separated to narrow fractions for molecular-level research based on polarity difference, molecular size difference,acid-base properties, and reactivity difference.

Key words: asphaltene; molecular; aggregates; deaggregation; separation

1 Introduction

Asphaltenes are petroleum components that can satisfy some specific operational definition. Commonly, they are de fined as a solubility class that is insoluble in light paraffins such as n-pentane or n-heptane, while it is soluble in light aromatics such as benzene and toluene[1-2].Further being identi fied from chemical nature, asphaltenes are a complex mixture, which is composed of a massive number of molecules with different structures. The real molecular composition and structures of asphaltenes have always been a scienti fic challenge in petroleum chemistry,which is also the foundation for developing efficient heavy oil processing technologies in the petrochemical industry. However, as many literature sources[3-13]reported, asphaltenes generally exist in the form of molecular aggregates in crude oil, petroleum residues, or even solvent system. Besides, these asphaltene aggregates usually cause serious problems to oil exploitation,transportation, and processing. Therefore, achieving the deaggregation and separation of asphaltene aggregates is the premise and basis for their molecular characterization.Although it is still quite difficult to deaggregate and separate asphaltenes to an absolute molecular level, many researchers have made great progress and presented their ideas and findings on this issue. Based on these literature reports and our recent studies on asphaltene molecular aggregates model as well as analysis of their intermolecular interactions[14-15], methods and potential directions for deaggregation and separation of asphaltene aggregates were reviewed, categorized and comprehensively analyzed, so as to provide information on asphaltene research.

2 Deaggregation Methods for Asphaltene Aggregates

2.1 Deaggregation by solvent dilution

As summarized in the literature[14], intermolecular interactions such as the van der Waals force, the electrostatic force, and the Pauli repulsive force are inversely related to the distance between molecules.Therefore, intermolecular interactions in asphaltenes can be effectively weakened by solvent dilution; besides,an appropriate solvent can promote dissolution of asphaltenes as a dispersion medium according to the principle stating that similar chemicals have better mutual solubility. Yu, et al.[16]studied the molecular con figuration changes of model asphaltene molecules in solvents covering n-heptane, toluene, and pyridine by molecular dynamics simulation. As shown in Figure 1, it is found that both n-heptane and asphaltene molecules have the tendency of keeping away from each other by bending deformation, while toluene and pyridine molecules can be distributed in an approximately parallel form around the two sides of asphaltene molecule due to π-π interaction between aromatic rings from solvent molecules and asphaltene molecules, in addition, the bending angle of asphaltene molecule is decreased evidently. Judging from these observations, it is evident that asphaltene molecules are inclined to be detached from the bulk phase of inappropriate solvent and exhibit a strong tendency of aggregation, while good solvents such as toluene and pyridine can retain asphaltene molecules and effectively alleviate the aggregation of asphaltene molecules.Carauta, et al.[17]reported similar phenomena in modeling solvent effects on asphaltene dimers and found that in n-heptane solvent the distance between aromatic ring systems of two asphaltene molecules is about 3.5 Å (1 Å=10-10m), which is just the distance of layer spacing between aromatic sheets in the crystallite of asphaltene particle in the Yen model, and it shows that asphaltenes are in the form of molecular aggregates in this situation.However, in the presence of good solvent of toluene, the distance between aromatic ring systems of two asphaltene molecules goes above 5.0 Å, indicating that the asphaltene molecular dimer has actually been dissociated.

Lu, et al.[18]applied the molecular dynamics simulation to study the effect of deaggregation for asphaltene aggregates using six different solvents including nitrobenzene, quinoline, pyridine, 1-methylnaphthalene,dibromomethane, and benzene. It is demonstrated that the conformation of asphaltene aggregates changed significantly after 200 ps of molecular dynamics simulation in different solvents at 573 K. Especially in benzene and nitrobenzene, the layer structure of stacked aromatic sheets in the asphaltene aggregates was loosened obviously and sometimes even was completely broken up, indicating that dilution by appropriate solvents could overcome the intermolecular forces and would thereby achieve the effect of deaggregation. Andreatta,et al.[19]used the ultrasonic velocity testing method to study the asphaltene aggregation trend by virtue of the concentration change of asphaltene solution, with the results shown in Figure 2. It can be seen from Figure 2 that asphaltenes in toluene solution changed from a molecular state to a nanoaggregate state at a concentration of 147 mg/L, which was defined as the critical nanoaggregate concentration (i.e., CNAC). Based on the above analysis, it is believed that asphaltene aggregates can be deaggregated into molecular solution by the solvent diluting method and it deserves a further study.

Figure 1 Conformation of asphltene molecule in different solvents

Figure 2 Ultrasonic velocity of asphaltene in toluene versus concentration at 25 °C showing the CNAC at 147 mg/L

2.2 Deaggregation by removing active sites

As typi fied in the linear polymerization-analogous model or the supramolecular assembly model for aggregation of asphaltenes, some researchers hold the view that the groups containing sulfur, nitrogen, oxygen, or metals in asphaltene molecules are the main factors serving as active sites to promote aggregation of asphaltene molecules. Therefore, application of physical or chemical methods to remove these active sites should be one of the effective ways to achieve deaggregation of asphaltene aggregates. Adams, et al.[20]contributed a systematic review which summarized common active groups in asphaltene molecules and sorbents that can cause adsorption interaction of asphaltene aggregates,as depicted in Figure 3. When containing active groups such as carboxylic, sulfoxide, pyridinic, quinolyl,phenolic hydroxyl, and thiophenic or pyrrolyl functional groups, the asphaltene molecules can generate aggregates readily through these active sites, and moreover, they are prone to participate in physical or chemical adsorption interactions with sorbents in types of hydroxyl silanol,hydroxyl aluminol, metal alkoxides, metals, metal oxides,metal sul fides, active carbon, and polymers. Accordingly,the authors believe that the asphaltene molecules and aggregates which have active groups can be selectively removed from various types of petroleum through adsorption by sorbents.

Zimmer, et al.[21]achieved sequential extraction of heteroatom compounds from a toluene solution of asphaltenes by preferential adsorption with magnesium oxide (MgO) and nickel oxide (NiO) nanoparticles,leaving weakly adsorbed molecules in the toluene solution. Upon comparing the high resolution mass spectra and FT-IR spectra of the asphaltene solution before and after the sequential extraction with MgO and NiO, it is found that MgO particles mainly adsorb the carboxyl-containing asphaltene molecules and have a certain effect on phenolic and pyrolic molecules as well,while NiO particles mainly adsorb the pyridyl-containing asphaltene molecules. After sequentially removing these molecules with adsorption activity, the complexity and aggregation tendency of asphaltenes were reduced signi ficantly. Besides being removed through adsorption,the asphaltene molecules with active sites also can be removed by chemical reaction. Masson, et al[22-23]found that polyphosphoric acid (PPA) may react with pyrolic molecules such as indole to form PPA adducts, and can react with pyridine type molecules such as quinoline to form PPA salts. Meanwhile, these two types of nitrogencontaining functional groups are widely distributed in asphaltenes, and are highly involved in the formation of hydrogen bonds leading to aggregation of asphaltene molecules. Therefore, it is feasible to disrupt the hydrogen-bond network in asphaltenes by transformation of nitrogen compounds into PPA adducts through successive phosphorylation, and reduce the apparent molecular weight of asphaltenes aggregated through hydrogen bonds, which indicates that it can achieve some deaggregation effect. Wang, et al.[24]reported a new method for selective separation of sulfur compounds such as thiophenic and sul fidic types of species from petroleum by methylation/demethylation. In the methylation reaction, sul fidic and thiophenic sulfur compounds can be methylated to sulfonium salts by AgBF4and CH3I even at room temperature, and meanwhile the pyrolic nitrogen compounds are adapted to the reaction pathway as well. After methylation, the polar salts can then be separated by precipitation from liquid petroleum phase.At present, this approach has been applied to separate and analyze the sulfur compounds in diesel, vacuum gas oil, and crude oil, and whether it can be used to remove sulfur and nitrogen molecules highly involved in hydrogen-bonding aggregation from asphaltenes is worthy of further study.

Besides the above aspects, organometallic compounds are also an important component driving the asphaltene aggregation. For example, nickel porphyrin (Ni2+) or vanadium porphyrin (VO2+) can be axially coordinated with Lewis bases such as pyridine type molecules,resulting in the formation of metal coordination complexes. Therefore, in order to reduce the contribution of organometallic compounds for aggregation in asphaltenes, it might be advantageous to removal of metal active sites by using the demetallization method. As shown in Figure 4, the reaction between metalloporphyrins and methanesulfonic acid can effectively remove the metal center of the metalloporphyrin, thereby eliminating the coordination site and reducing the asphaltene aggregation.

Figure 3 Typical active functional groups on asphaltene surface and sorbent surface and adsorption interaction of an asphaltene aggregate at a sorbent surface through active sites

Figure 4 Demetalization reaction of petroporphyrin with methanesulfonic acid

2.3 Deaggregation by moderate heating

The reason why asphaltene aggregates can exist stably is that among asphaltene aggregates there are widespread intermolecular interactions, which make the asphaltene aggregate system attain an energetic optimum. Therefore,by taking into account from the viewpoint of energy,heating is supposed to be a possible way to overcome cohesive energy among asphaltene aggregates to achieve partial deaggregation. Takanohashi, et al.[25]calculated the energy-minimum conformation for asphaltene aggregates from a Khafji vacuum residue and studied the variation induced by heating for aggregates conformation using the molecular mechanics-molecular dynamics simulation.The results showed that the most stable aggregated conformations are those formed by some noncovalent interactions between the asphaltene molecules, and during molecular dynamics simulation the changes in distances between aromatic-aromatic clusters and distance of hydrogen bond in aggregates conformation are shown in Figure 5. It can be found from Figure 5 that after the temperature reached 523 K the intermolecular hydrogen bonding in asphaltene aggregates was weakened, while the π-π stacking was stable, and additionally the π-π stacking interaction still remained stable even when the temperature reached 673 K. However, at a high temperature of 673 K some bond-breaking reactions such as cracking of alkyl side chain or ring-opening reaction might occur. Through further research of these reactions,when asphaltene molecules were modified by removing aliphatic chains and polar functional groups, asphaltene aggregates can be deaggregated more sufficiently even at a lower temperature as compared to the original structure before modification. Similarly, visbreaking is a practical thermal process that is commonly used to reduce the viscosity of heavy oils or vacuum residues.During this process, one of the main physicochemical change is thought to be the deaggregation of asphaltene molecular aggregates which were transformed into a lesser aggregated degree in vacuum residues, leading to the reduction of viscosity.

2.4 Deaggregation by ultrasonication

Sonic wave is a common physical phenomenon that refers to the transmission of mechanical vibration state or energy of an object, and in terms of vibration frequency range the sonic waves above 20000 Hz (the upper limit of human hearing) are classi fied as ultrasonic waves. When ultrasonic wave is transmitted in a medium, the ultrasonic wave will cause physical and chemical changes to the medium, which can further generate the ultrasonic effect from a series of mechanical, thermal, electromagnetic and chemical aspects. Based on these acoustic characteristics,the ultrasonic effect has been widely used in medical science, military, industrial and agricultural development,and multiple fundamental research.

In the research field of heavy oil and asphaltenes, Yen, et al.[26-28]pioneered the application of ultrasonication as a tool in the study of asphlatenes. Moderate treatment by ultrasonication can cause damage to the layer structure of asphaltenes and reduce the size of asphaltene particles[29].Zhou, et al.[30]also reported that ultrasonication made the colloidal particles become smaller in the crude oil.Sakanishi, et al.[31]studied the depolymerization and demetallization effects on asphaltenes by ultrasonic irradiation in solvents of tetrahydrofuran (THF) or 1-methylnaphthalene (1-MN) in the presence of adsorbents. The results showed that the treatment by ultrasonication and adsorption caused the dissociation of the asphaltene aggregate structure, and converted the asphaltenes into soluble maltene fractions with little change of asphaltene molecular structure. Sun, et al.[32-33]investigated the effect of ultrasonic treatment on asphaltene structure before and after hydrotreating of vacuum residue, and found that the association number of structural units in asphaltene aggregates is decreased by ultrasonication, in addition, it also caused some ring-opening reaction and dealkylation reaction to asphaltene molecules, which modified asphaltene molecular structure, so they indicated that the structure and reactivity of asphaltenes in vacuum residue were improved for hydrotreating by ultrasonic treatment.

Figure 5 Changes in distances between aromatic-aromatic clusters and distance of hydrogen bond

2.5 Deaggregation by molecular collision

The deaggregation methods described above are all considered from the perspective of pretreatment and processing of asphaltene aggregates. Besides these methods, for analysis purpose there is an on-line deaggregation approach by analytical instrument such as the collision induced dissociation coupled with the electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (CID-ESI FT-ICR MS).Electrospray ionization (ESI) is a typical soft ionization technique that does not break the covalent bond, and even allows dimers or multimers to be detected. On this basis, collision with inert gas molecules using a moderate energy can induce the dissociation of dimers or multimers and help to achieve analysis of monomer composition from molecular level. Rodgers, et al.[34]analyzed the Canadian bitumen by a low-resolution linear ion trap mass spectrometer, and found that ESI mass spectrum obtained at a high analyte concentration of 10 mg/mL showed a clear bimodal distribution as depicted in Figure 6 (a), which indicated the multimer existing in sample formation. Then, a narrow mass range segment attributed to multimers was selected for MS2isolation, and the mass spectrum for that isolated segment is shown in Figure 6 (b). After the ions representing multimers in the selected mass window were isolated, these ions suffered a collision from inert gas molecules at a moderate energy,and produced the same monomer distribution in Figure 6 (c) as that in Figure 6 (a) at the low-mass segment. It demonstrated that the collision induced dissociation can be used for on-line deaggregation and characterization of asphaltene molecular multimers or aggregates, and it also suggested that asphaltene molecules were associated to form molecular aggregates through non-covalent bonding.

3 Methods for Separation of Asphaltenes

Asphaltenes are multi-scale complex mixtures composed of hydrocarbons and non-hydrocarbon compounds at molecular level and molecular aggregates of these compounds at supramolecular level. Even if the asphaltene aggregates can be subject to deaggregation by methods mentioned above, asphaltenes still contain a huge number of molecules with different structures and properties, and there is no doubt that it will not only pose a serious challenge to performance of catalysts and refining processes, but also can affect the peak capacity and resolution power in analytical chemistry,so it is quite necessary to separate asphaltenes into subfractions with a relatively lower complexity for further research.

Figure 6 ESI mass spectrum of a bitumen sample

3.1 Separation by polarity difference

The molecular types in petroleum include alkanes,cycloalkanes, aromatics, heteroatom compounds, and facultative compounds with more than one structural feature in the molecules. The difference in molecular structure determines the difference in polarity of petroleum molecules, and there is a particularly obvious difference in molecular polarity of vacuum residue and asphaltenes. Therefore, heavy oils and asphaltenes can be further separated based on the difference in molecular polarity. Chawla, et al.[35]from ExxonMobil developed a separation and quantitative technique for heavy petroleum fractions by high performance liquid chromatography(HPLC). In this technique, through using two separation columns combined with valves switching and gradient elution, heavy oils can be separated into seven classes of compounds including saturates, 1—4+ring-structured aromatics, sul fides and polars according to polarity sequence that can be quantitatively determined.By using this method, the separation performance and typical data are shown in Figure 7. It is reported that this method was currently adapted for petroleum samples with boiling points ranging from 550 °F to 1050 °F. However,it is well known that asphaltenes are the fractions which have the greatest molecular weight and polarity in heavy oils, so they determine the properties of low volatility and strong adsorption for asphaltenes, and accordingly limit the utilization of HPLC in separation of asphaltenes.

Wang, et al.[36]carried out some exploratory work in separating asphaltenes by the solid phase extraction(SPE) method. Upon considering that activated silica gel is commonly used as the stationary phase packing,not with standing its strong adsorption to asphaltenes would lead to problems of low recovery and weak representativeness for separating asphaltenes, they used a partly deactivated silica gel as the SPE column packing and optimized the combination of different polar solvents for gradient elution. In consequence, asphaltenes can be separated into six sub-fractions according to the polarity difference. Figure 8 shows the distribution of compound classes in F1-F6 fractions, and it can be found that the relative abundance of weak polarity compounds, such as aromatic hydrocarbons (HC class) and sulfur-containing aromatics, has a decreasing trend from F1 to F6 fractions, while the relative abundance of strong polarity compounds such as the nitrogen-containing aromatics and the poly-heteroatom aromatics would increase with an increasing polarity of sub-fractions. The results of polarity distribution have confirmed that asphaltenes can be further separated based on polarity difference by the relevant method and moreover it has a satisfactory recovery rate of more than 80 % when it is applied to the separation of several asphaltene samples.

Figure 7 HPLC chromatogram for separation of a petroleum distillate and quanti fication data of seven fractions

Figure 8 Compound class distribution of six fractions obtained from separation of asphaltene by solid-phase extraction

3.2 Separation by molecular size difference

Since most modern oil refining technologies are catalytic processes, they demand utilizing catalysts with appropriate pore size according to the molecular size of feedstock oil. For example, hydrogenation or catalytic cracking of heavy oils generally requires that catalysts for heavy oils should have some proportion of macro-pores to ensure that the macromolecules or molecular aggregates can have access to the catalyst channels and reduce the pore blockage and coke formation[37]. Therefore, the size distribution of asphaltene molecules and their molecular aggregates which are the largest component in heavy oils is of great signi ficance for heavy oil processing.

The gel permeation chromatography (GPC) or the size exclusion chromatography (SEC) is a powerful tool for researching asphaltenes based on separation by molecular size. Strausz, et al.[38-39]employed the GPC method to investigate the distribution of molecular weight and size of the Athabasca asphaltenes. It was observed that there was a trimodal distribution in the GPC spectrum of Athabasca asphaltene solution (with a mass fraction of ≤0.05%) as shown in Figure 9(a). In the spectrum,the peak III with a longer retention time and better separation effect represented the components with a size and weight mainly corresponding to the molecular level range, while the components represented by the peak I and peak II might be covalently bonded molecules with high molecular weight or weakly interacted molecular aggregates. Furthermore, according to the retention time region of each peak in Fig. 9(a), the whole asphaltenes were separated into three sub-fractions (as shown in Fig. 9(b) to (d)) by stepwise solvent elution, and the evolution trend of the three sub-fractions in solvent with elapsed time was examined as illustrated in Figure 9(e). Judging from the results, it was found that about 82% of the fraction I was converted to the components in the peak III range after 5 days of storage in a diluted CH2Cl2solution, and about 52% of the fraction II was also converted to the components in the peak III range after 14 days of storage in a diluted CH2Cl2solution,however, there was little change for fraction III after 14 days of storage in a diluted CH2Cl2solution. These phenomena indicated that the fraction III was mainly composed of covalent monomeric units, while the fractions I and II were mainly molecular aggregates.Based on the above discussion, it demonstrated that heavy oil processing actually confronted a poly-dispersed and multi-level complex mixture composed of molecules and molecular aggregates. Hall, et al.[40]used the analytical and preparative GPC to characterize the size distribution of solubles and asphaltenes in petroleum residue and investigated its effect on hydrotreating of residues. It was found that the molecular size of solubles was mostly smaller than the pore size of catalysts and consequently the soluble molecules could diffuse into the catalysts,while a substantial portion of asphaltenes could not enter the catalyst channels. Through these analyses,it is possible to provide an important information for researching the reaction network of residues and the deactivation mechanism of catalysts.

Figure 9 GPC chromatograms for separation of Athabasca asphaltene and changes in GPC pro files of fraction I-III with increase in days of storage

3.3 Separation by acid-base properties

In the supramolecular assembly model for asphaltene aggregation[41], the acid-base interaction is considered to be the strongest driving force for formation of asphaltene aggregates. Although in other models there are still different opinions on the weight contributed from acidbase interaction, at least it indicates the objective and ubiquitous existence of acidic species and bases in petroleum asphaltenes. Ion exchange chromatography is a special technique that can be used to research the acid-base composition of asphaltenes. By using this technique, not only the content of acidic, basic, neutral and amphoteric components in asphaltenes can be determined, but also these corresponding sub-fractions can be prepared for further analysis and reaction research.Selucky, et al.[39]applied the anion-cation exchange chromatography to separate the Athabasca asphaltenes according to the experimental procedure shown in Figure 10. It can be found from Fig. 10 that when separation was carried out firstly on the anion exchanger and then on the cation exchanger (anion-cation exchanger column sequence), the mass fraction of acids and bases was equal to 56.5% and 23.3%, respectively, however, the proportion of acids came down to 39.8% and that of the bases went up to 39.6% by reversing the ion exchanger sequence. It indicates that about 16% of the asphaltenes can interact with both of the anion exchanger and the cation exchanger, which means that the molecules in those 16% of the asphaltenes are amphoteric from the acid-base property. Thus, the Athabasca asphaltenes could be separated into several sub-fractions by acid-base property and its final composition was deduced to contain approximately 40% of acids and 23% of bases along with about 20% of neutral components.

3.4 Separation by reactivity difference

Since the asphaltene molecules contain aromatic rings in their structure, they can participate in electrophilic reactions such as sulfonation, nitration, and halogenation.In addition, when some asphaltene molecules have nonhydrocarbon functional groups, these groups can also take part in reactions such as silylation to hydroxyl groups or esteri fication to carboxyl groups contained in asphaltene molecules. Therefore, it is propitious to the development of separation methods based on the reactivity differences in asphaltenes. Acevedo, et al.[42-43]reported a fractionation method that used p-nitrophenol (PNP) to react with aromatic rings in asphaltene molecules to form the charge-transfer complexes, and they found it could generate two kinds of products, among which one was the toluene-insoluble (A1-PNP) and the other was the toluene-soluble (A2-PNP) charge-transfer complexes.After precipitation, filtration and extraction of the charge-transfer complexes, asphaltenes were separated into a low-toluene-soluble fraction A1 with a solubility of less than 100 mg/L and a toluene-soluble fraction A2 with a solubility of about 5 — 100 g/L. Through further analyses by LDMS and1H/13C-NMR for the sub-fractions obtained from the PNP reaction method[43], it was found that A1 fraction was mainly composed of continental or island type molecules with a single, rigid and flat polycyclic nucleus, while A2 fraction was mainly composed of archipelago molecules featured by alkyl-bridged multiple small cores with significant mobility in the overall structure. Sun, et al.[44]utilized the polymerization reaction between phenols and formaldehyde to separate a low-temperature coal tar. Since coal tar is particularly rich in phenols, they treated it with formaldehyde under alkaline conditions to generate phenolic polymers,which can enwrap other components covering especially polycyclic aromatic hydrocarbons in coal tar. After gradient separation with different solvents, oils and residues from the reaction products were characterized as complicated mixture mainly composed of free alkanes,glycolic acid, alkyl PAHs, and bridged PAHs.

Figure 10 Scheme for separation of Athabasca asphaltenes on ion exchangers and quanti fication of sub-fractions(A―acids; B―bases)

4 Conclusions

Asphaltenes generally exist in the form of molecular aggregates in crude oil or in petroleum residues, and it is quite necessary to achieve deaggregation and separation for asphaltene aggregates. Aiming at the intermolecular interactions in asphaltene molecular aggregates, it has been proposed and summarized that aspahltene aggregates can be subject to deaggregation by five approaches,including solvent diluting, removing active sites,moderate heating, ultrasonication, and on-line molecular collision. Moreover, asphaltenes can be further separated to narrow fractions for conducting the molecular-level research based on the polarity difference, the molecular size difference, the acid-base properties, and the reactivity difference.

The deaggregation and separation of asphaltene aggregates is the premise and basis for realizing molecular characterization of asphaltenes; in addition,the real asphaltene molecular information derived from deaggregation is the foundation for designing catalysts and developing technologies for processing the asphaltenes-enriched heavy oils. At present, although it is still far from sufficient to understand all the details of asphaltene aggregates, in future it is expected that through the combination of multiform deaggregation and separation tactics, joint efforts from analytical and chemical process research will greatly promote the elucidation of asphaltenes.