Hosseini-Dastgerdi Et Al-2015-Asia-Pacific Journal of Chemical Engineering

Review

A comprehensive study on mechanism of formation andtechniques to diagnose asphaltene structure; molecularand aggregates: a review

Z. Hosseini-Dastgerdi,1,2 S.A.R. Tabatabaei-Nejad,1,2* E. Khodapanah1,2 and E. Sahraei1,2

1Sahand Oil and Gas Research Institute (SOGRI), Sahand University of Technology, Tabriz, Iran2Chemical Engineering Department, Sahand University of Technology, Tabriz, Iran

Received 24 February 2014; Revised 14 June 2014; Accepted 30 June 2014

ABSTRACT: As the formation of asphaltene aggregates causes its deposition in various processes, exact determination ofthe molecular and aggregate structure and mechanism of aggregate formation is very important in order to prevent asphaltenedeposit formation. The aim of this paper is to review the molecular and aggregate structure of asphaltene using varioustechniques. The other purpose is to specify and compare different mechanisms of aggregate formation, suggested byresearchers. These mechanisms are proposed by researchers according to the molecular characteristics of asphaltene andthe structure of aggregates. Researchers have considered four general models for the formation mechanism of asphalteneaggregates: the micellar/colloidal, polymeric, solubility, and modied Yen models. In comparison with other models, themodied Yen model shows more consistency with new studies, which determine the island molecular structure and smallnano-aggregates for asphaltene. Experimental studies show that the dominant mechanism of aggregate formation dependson asphaltene architecture, intermolecular forces, and the solubility denition of asphaltene. Depending on the asphaltenesource, solvent, thermodynamic conditions, and various techniques such as near-infrared spectroscopy, nuclear magneticresonance, small-angle neutron scattering, and small-angle X-ray scattering, different structural information on the shape(spheres, discoid, oblate, membrane-like, prolate cylinders, and mass fractal aggregates) and size of aggregates has beenreported. 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

KEYWORDS: asphaltene; structure; mechanism; aggregate; review

INTRODUCTION

Nowadays, because of the abundant production of lightoil, a sharp decline has been seen in the availability ofthese reserves, and many of these reservoirs have beendrained. For this reason, the oil industry has focused onthe use of heavier oils and offshore elds. Although theproduction and processing of heavy crude oil require theapplication of diluents and temperature increase to reducethe viscosity, the addition of diluents may result inasphaltene precipitation and deposition in reservoirs.[15]

Any changes in pressure, temperature, and compo-sition of the uids can cause the asphaltene moleculesto associate and form aggregates that may separateand form a new phase.[6] Various techniques have beenapplied to study the effect of different parameters onasphaltene aggregation and precipitation. Nielsenet al. studied the effect of pressure and temperatureon asphaltene particle size for six different crude oils

with the asphaltene concentration of 0.616.7wt%.The results indicate that the mean asphaltene particlesize increases by decreasing pressure and decreasesslightly by increasing temperature.[7] Studies haveshown that asphaltene aggregation increases bypressure reduction up to the bubble point at which themaximum amounts of asphaltene precipitation occur.[6]

By pressure reduction below the bubble point, the sizeof aggregates and the amount of precipitated asphaltenedecrease.[6] Natural depletion experiments werecarried out by Moradi et al. using three differentcrude oils with asphaltene content of 0.99, 8.7, and13.75wt%.[8] Results show the asphaltene aggregationand precipitation by pressure reduction for crude oilswith high-asphaltene content. It should be mentionedthat precipitation does not show any relationship withthe asphaltene content of the oil. For example, a crudeoil from Venezuela with 17.2wt% asphaltenes didnot show production problems, whereas the HassiMessaoud eld in Algeria had numerous problems withonly 0.15wt% asphaltenes.[9] According to the studies,by increasing the asphaltene concentration in toluenesolutions, the size of aggregates and the amount of

*Correspondence to: S.A.R. Tabatabaei-Nejad, Sahand ResearchInstitute of Oil and Gas, Sahand University of Technology, Tabriz,Iran. E-mail: [email protected]

2014 Curtin University of Technology and John Wiley & Sons, Ltd.Curtin University is a trademark of Curtin University of Technology

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2015; 10: 114Published online 25 July 2014 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/apj.1836

precipitated asphaltene increase. When the concen-tration increases to about 100mg/L, the moleculesform nano-aggregates. At higher concentration levels,greater than 5000mg/L, nano-aggregates form clustersand precipitate.[6]

Different processes of production and enhanced oilrecovery such as natural depletion, gas injection, andacidizing treatments can cause asphaltene destabili-zation. The destabilized asphaltene deposit on thesurface of reservoir rocks, transportation facilities,tubing, wellheads, and ow lines results in permea-bility reduction followed by reservoir productiondecrease, equipment deactivation, and increase inoperational expenses.[16]

Because of the natural tendency of asphaltene toaggregation, precipitation, and deposition on differentsurfaces, many studies have been conducted for thestructural analysis of the asphaltene molecules andaggregates and their formation mechanism. Anextensive research on different studies shows that thedetermination of a proper mechanism of asphalteneaggregate formation from the asphaltene molecule isan important stage in asphaltene precipitation process.According to the studies, the structure of asphaltenemolecules and aggregates plays an important role inspecifying the asphaltene aggregation mechanism.With the advancements in science and technology,several techniques have been used to study thestructure of the asphaltene molecule and aggregate.Each technique conrms other results in some aspects.This study is intended to provide an effective and

comprehensive overview of the structure of asphaltenemolecules and aggregates in different experimentalconditions using various techniques and the differentformation mechanisms of asphaltene aggregates,suggested by researchers. The description of therelationship between the aggregate and molecularstructure and aggregation mechanisms has also beenattempted in this study.

ASPHALTENE MOLECULAR STRUCTURE

Asphaltene is the most polar fraction of crude oil,which mainly includes compact poly-aromatic ringswith long or short aliphatic chains, different functionalgroups, and heteroatom.[10,11] There is poly-dispersityin the chemical composition of asphaltene withhydrogen-to-carbon atom ratio in the range of 11.3.The aforementioned results are obtained using nuclearmagnetic resonance (NMR) and Fourier transforminfrared spectroscopy, which give information onfunctional groups and the aromatic and alkyl carbonfractions. It has been proposed using these techniquesthat the aromatic clusters in the asphaltene moleculeare connected by some heteroatoms and aliphatic

linkages.[1214] X-ray Raman spectroscopy is anothertechnique that has been used for the determination ofthe asphaltene aromatic carbon fraction by spectrumadjustment of asphaltene and materials with differentchemical structures.[15,16] Different concentrations ofpolar heteroatoms (nitrogen, sulfur, and oxygen) in theasphaltene molecule have been detected using X-rayphotoelectron spectroscopy (XPS) and X-ray absorptionnear edge structure spectroscopy (XANES).[1719]

The presence of carboxylic-, carbonyl-, pyrrolic-, andpyridinic-type functional groups in the molecularstructure of asphaltene has large impact on asphalteneinteraction with different molecules.[1721] These groupsare able to donate or accept protons intermolecularlyand intramolecularly.[22]

Determination of the number, size, and geometry ofpoly-aromatic rings in the asphaltene molecule is achallenging task and is of continuing interest to theoil industry. So far, several methods have beendeveloped to answer this question. During the lastdecades using different techniques, the number ofaromatic rings in the asphaltene fused ring system hasbeen determined among 2 and 20.[2329] However, theaverage number of fused rings in the asphaltenemoiety, which has been determined by X-rayscattering,[23,25] optical absorption,[30,31] uorescenceemission spectroscopy,[32,33] and uorescence depolari-zation (FD),[34] is less than ten. The results of theresearch by Ralston et al. have shown that themaximum uorescence emission for asphaltene occursat 450 nm, which corresponds to four to ten aromaticrings in the molecular structure.[32] According todirect imaging with scanning tunneling microscopy(STM) studies, six to seven fused aromatic rings havebeen considered for the asphaltene aromaticmoiety.[35] Using this technique, the molecular sizeof about 1 nm has been found for asphaltene. Theresults of high-resolution transmission electronmicroscopy (HRTEM) have been conrmed by theresults obtained from STM.[36] It is necessary to notethat the STM method only measures the aromaticportion of the molecule. For asphaltene molecule, thearomatic portion includes about 40% of the total sizeof molecule.[35]

The molecular size of asphaltene can be speciedusing uorescence correlation spectroscopy (FCS) andFD technique.[3742] The results of rotational correlationtime of the asphaltene molecule in FD technique indicatethat the molecular diameter of asphaltene is in the rangeof 12 nm.[41] In FCS, knowing the translation diffusioncoefcient of molecules, the hydrodynamic radius,which is an estimate of its molecular radius, has beenmeasured 1 nm.[37]

Another key issue of asphaltene molecular structureis the architecture. Depending on the type of linkagebetween poly-aromatic hydrocarbons and alkyl chains,two different architectures have been considered for the

Z. HOSSEINI-DASTGERDI ET AL. Asia-Pacic Journal of Chemical Engineering2

2014 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2015; 10: 114DOI: 10.1002/apj

asphaltene molecule: the island and archipelagoarchitectures. In the island model, the fused aromaticrings of asphaltene are similar to palms, and thesurrounding alkyl groups are like ngers. In thearchipelago model, the fused aromatic rings areconnected via saturated bridges. Consequently, accor-ding to the island model, there is one aromatic moietyper asphaltene molecule, whereas in the archipelagomodel, there is more than one aromatic moiety perasphaltene molecule.[43,44] Different techniques havebeen developed to study the asphaltene moleculararchitecture. Whereas the archipelago molecular archi-tecture has been conrmed using bulk decompositiontechniques such as pyrolysis,[21,45,46] Ruthenium Ion-Catalyzed Oxidation (RICO) analyses,[47] and uorescencespectroscopy,[48] the island architecture of asphaltenemolecule has been revealed using techniques such asFD,[40,42,4951] Taylor Dispersion (TD), [52] MolecularOrbital Calculations (MO),[53] and FCS.[3739]

Two-step laser desorption ionization massspectrometry (L2MS) is another technique that hasbeen used to determine the asphaltene moleculararchitecture. In this method, the molecular frag-mentation of different archipelago and island com-ponents has shown the instability of archipelagocompounds.[44] The resistance of asphaltene anddifferent island components to fragmentationrepresents the dominance of the island moleculararchitecture for asphaltene.[44] Laser-induced acousticdesorption mass spectrometry is a decompositionmethod. The island architecture has been conrmedfor asphaltene molecule using laser-induced acousticdesorption technique. According to the results,asphaltene and island compounds tend to have smallmass loss by decomposition, whereas archipelagocompounds show large mass loss.[54]

It has been suggested by some of the studies that thearchitecture of asphaltene molecules is in a continuumof island and archipelago types. Durand et al. andOliviera et al. studied the self-diffusion coefcientof asphaltene from different origins in toluene usingthe diffusion-ordered spectroscopy (DOSY) NMRtechnique.[55,56] According to the results of Durandet al., the chemical interaction in the solution anddiffusion property of asphaltene aggregates dependson the repartition of archipelago and island structureasphaltenes.[55] Oliviera et al. revealed that thepredominant molecular architecture of asphaltenecontributes to different aggregation properties.[56]

Acevedo et al. separated asphaltene into two solubilitygroups, using para-nitrophenol. Each group showed adifferent solubility in toluene. They proposed that thevariation in asphaltene solubility in toluene is due tothe molecular architecture difference of each solubilitygroup.[57,58] Gutierrez et al. concluded that asphalteneis a mixture of different components with differentstructures and solubilities in aromatic solvents.[58] It

has been revealed by these studies that the dominantarchitecture of asphaltene molecule strongly dependson the asphaltene origin and the solvent used forasphaltene precipitation from oil. Whereas theasphaltene structure and architecture have been studiedextensively, there are still disagreements on the exactchemical structure of asphaltene. As it is discussed inthe following section, the architecture and differentfunctional groups of the asphaltene molecule have alarge impact on asphaltene aggregation mechanism.According to the continental model, asphalteneaggregates contain four to six molecules with interaction between their parallel aromatic cores.[59]

The aggregation of archipelago-type asphaltenemolecules is a complex procedure in comparison withcontinental-type asphaltene molecules. Because of thelower aromaticity of archipelago-structured asphaltene,other intermolecular forces such as hydrogen boundinggain importance. Planar aggregates are formed as theresults of lower staking with lateral intermolecularbonding for archipelago-structured molecules.[59]

ASPHALTENE AGGREGATION MODELS

Nellensteyn for the rst time suggested the concept ofasphaltene aggregation in oil; later on, his theory wasdeveloped and expanded by Pfeiffer and Saal.[60,61]

According to their theory, dispersion of asphalteneaggregates in oil is due to the presence of resins andother aromatic components. The primary studies ofaggregate structure led to a great advance in manystudies on the mechanism of asphaltene aggregateformation. On the basis of the model proposed byYen in 1967, the asphaltene molecular structure playsan important role on the aggregation mechanism. Inthis model, a hierarchical picture of asphaltenes hasbeen provided.[62] According to the X-ray diffractionstudies by Yen et al., interaction between thearomatic cores of the asphaltene molecule constructsthe bases of the aggregate formation mechanism.[62]

The presence of heteroatoms in the molecular structureof asphaltene links the aggregates by hydrogenbonding. Nitrogen, in pyrrolic, carbonyl, and pyridinicfunctional groups, acts as an acceptor of hydrogenbond.[1722] The role of resins on the solubility of theasphaltene aggregates has been conrmed by a largenumber of studies.[6366] According to these studies,the presence of the resin molecule increases thesolubility of the aggregates and keeps them smaller inthe solution. This is because of the smaller aromaticcore of the resin molecules, in comparison with thatof the asphaltene molecules, which makes the resinmolecules soluble in aromatic solvents. The existenceof polar functional groups in the resin moleculedisturbs the electron donoracceptor interaction, whichin turn makes the asphaltene aggregates smaller.[66]

Asia-Pacic Journal of Chemical Engineering A STUDY ON MECHANISM OF FORMATION AND STRUCTURE OF ASPHALTENE 3


From the results obtained from the research onasphaltene molecular structure, it has been concludedthat the most important intermolecular forces attributedto the asphaltene aggregate formation include the interaction between aromatic sheets, hydrogen bondbetween functional groups, content of alkyl groups,heteroatom types, and electron transfer betweenmolecules.[6668] Gray et al. proposed that in additionto the stacking and hydrogen bounding, Brnstedacidbase interactions, metal coordination complexes,and interactions between cylcoalkyl and alkyl groupsare determinative for asphaltene aggregation.[69]

On the basis of different studies in last decades, fourgeneral mechanisms have been suggested for theasphaltene aggregate formation. These mechanismsinclude the colloidal/micellar, polymeric, solubility,and modied Yen models.The intermolecular forces and the architecture of

asphaltene are the important factors that can determinethe mechanism of aggregate formation. The asphaltenemolecules with the island architecture and highlycondensed aromatic ring system may form asphalteneaggregates through the bonds of aromaticcores.[62] This mechanism is likely to happen incolloidal and modied Yen models. If the asphaltenemolecules have an archipelago structure, they canconnect to each other through different interactions: stacking, hydrogen bonding, and van der Waalsand acid-based interactions.[70] This mechanism mayhappen in linear polymerization and micellization.

Colloidal/micelle model

The rst theory presented for the physical structure ofasphaltene aggregates suggests that asphaltenes canform micelles or colloids, depending on the polarityof oil medium and the presence of other componentsin the oil. The asphaltene molecules tend to self-assemble and form micelles in the shape of discs,spheres, or cylinders because of hydrophilic andhydrophobic interactions in aromatic and differentpolar solvents.[61,7173] The aliphatic ring and poly-aromatic core in the asphaltene molecules act as solvent-loving and solvent-hating parts, respectively.[61,7173]

According to the experimental studies, micelle formationhappens as the asphaltene concentration exceeds athreshold known as critical micelle concentration(CMC) in aromatic solvents.[27,7479] A higher increase

in the asphaltene concentration leads to theformation of bigger micelles and large aggregatesthat may separate and form a new phase. Figures 1and 2 show the formation of asphaltene micelles inan aromatic solvent. Figure 2 indicates that byincreasing the asphaltene concentration in thesolution, the number of micelles increases, whichleads to the formation of macro-aggregates ofmicelles.[72] Different techniques have been appliedto study the nano-structured micelle formation andthe CMC detection, including calorimetry,[77,78]

surface tension,[27,78] and ultrasound compressibilitymeasurements.[79] Different studies have publisheda huge range of values for CMC. According to thesurface tension measurements, at concentrationslower than the critical concentration of micelleformation, asphaltene is molecularly dissolved intoluene, but at higher concentrations, the micellesare formed.[27] Andersen and Birdi used the calorimetrictitration technique for analysis of asphaltene micelleformation in toluene-normal heptane solution andconcluded that increasing the amount of normal heptaneleads to a decrease in the critical concentration of micelleformation. They mentioned that the relative amount ofdecrease in the critical concentration depends on theasphaltene origin.[78]

Priyanto et al. investigated the micelle formationof asphaltene in polar solvents at different con-centrations at nanoscale. In this study, the criticalconcentration of micelle formation and the coacer-vation concentration have been determined usingviscosity measurements.[72]

Friberg proposed that the asphaltene micellization inhydrocarbon is of inverse kind, which is different from

Figure 1. Asphaltene micelle formation in the presence of polar (aromatic) solvents.[72,73]

Figure 2. Asphaltene aggregate formation from micelleswhen asphaltene concentration increases in polar (aromatic)solvents.[72,73]



normal micellization in aqueous systems. In normalmicellization, the intermolecular forces in the solventplay an important role in the formation of micelles. Inthis mechanism, the micelle growth is limited by therepulsion between polar groups. In inverse micellizationof asphaltene, the interior strong intermolecular forces ofthe aromatic cores (polar groups) dominate themicellization of asphaltene. According to the denitionof the inverse micellization of asphaltene, there is notany specic mechanism that would reduce theintermolecular interactions of aromatic cores.[80] Itmeans that in asphaltene inverse micelle, there is nolimitation for aggregate growth, contrary to the experi-mental results. Thus, micelle formation mechanism ofasphaltene cannot entirely represent the asphalteneaggregation, and a complete aggregation mechanismfor asphaltene association is needed.Another proposed model for asphaltene aggregation

mechanism is the steric-colloidal model, which hasbeen the most common model for asphaltene aggregateformation mechanism since 1940.[72,78,8184] Accor-ding to this model, the suspended colloidal aggregatesof asphaltene with the surfaces covered by resinmolecules are dispersed in the oil phase. The adsorbedresin molecules prevent the formation of largeraggregates.[66,72] The stability of asphaltene isdependent on the molecular interactions of theasphaltene and resin constituents (aromatic andsaturated fraction).[66,72] Changes in temperature,pressure, or composition of the solution can cause thedesorption of resin molecules from the asphaltene

aggregates, which leads to the formation of largeraggregates. Priyanto et al. proposed that dependingon the presence of other components in the oil(parafns, resins, and aromatics), asphaltene aggre-gates can be steric-colloidal or micellar.[72] In somestudies, it has been shown that asphaltenes appear inthe micellar form in aromatic solvents such as tolueneand methyl naphthalene and in colloidal form inparafnic solvents.[72,85] Figure 3 shows that in thepresence of extra amounts of resins, asphaltenemolecules form steric-colloidal aggregates. In thismodel, the ability of resins to limit the asphalteneaggregation growth and solvation of asphaltene is dueto the small aromatic core and polarity of the resinmolecules. Resin molecules tend to disturb the interaction between aromatic sheets and the polar andhydrogen bonding between asphaltene molecules.[66]

Since 2009, different studies have rejected the key roleof resin molecules in the solvation and stability ofasphaltene aggregates. The asphaltene molecules havebeen shown to form nano-aggregates with limited sizewithout resins in aromatic solvents and live oils.[8688]

In addition, asphaltene nano-aggregates in live crudeoil have shown very little asphalteneresininteraction.[89] Researchers propose that resin mole-cules may be adsorbed on the asphaltene aggregatesurface, but they cannot stabilize the asphalteneaggregation.[88] They explained the role of resins inthe asphaltene aggregate formation mechanism byspecifying the denition of asphaltene and resin.Because asphaltene and resin molecules have similar

Figure 3. Colloidal aggregate formation from asphaltene molecules with archipelagoarchitecture in the presence of resins.[66]



chemical characteristics and they can form aggregatesin different solvents, the distinction between theirdenitions is based on the solubility group, not thechemical classication.[88] Thus, depending on thesolvent agent used for separating asphaltene and resinfrom the oil, the nature of asphalteneresin interactioncould be different. From this point of view, thecolloidal model is encountered with limitation toexplain the asphaltene aggregation mechanism.[63]

Polymeric/macromolecular model

In this model, the asphaltene aggregation occurs similarto polymerization.[9092]

Hirschberg and Hermans[93] and Agrawala andYarranton[70] proposed a linear polymerization mecha-nism for asphaltene aggregation. They suggested thatthe polymerization process of asphaltene terminatesbecause of the interaction of asphaltene with thecomponents such as resins. Resins disturb thepolymerization process by linking to the interactingasphaltene molecules. According to this model,asphaltene molecules are considered as polymericmolecules, which have several active sites (heteroatomsand aromatic cores) and are able to make aggregates withsimilar molecules. Therefore, asphaltene molecules actas propagators in a polymerization-like associationreaction. In this model, resin molecules have only oneactive site and interact with only one molecule.Therefore, they act as terminators in polymerization-likeassociation reactions.Agrawala and Yarranton[70] described the effect of

temperature and the solvent type on asphaltenepolymerization mechanism. According to the experi-mental results, the number and intensity of the activesites in asphaltene monomers depend on the solventtype and temperature. For example, in a polarsolvent-like nitrobenzene (a good solvent), the asphal-tene aggregation decreases because the asphaltenesolvent interaction intensity is more than theasphalteneasphaltene interaction intensity, so the strongsites make asphaltenesolvent bonds and weak sitesmake asphalteneasphaltene bonds. By temperatureincrease the number of sites with the ability to makebonds and the tendency of asphaltene molecules foraggregation decrease. This issue is shown in Fig. 4(a).Figure 4(b) and 4(c) show that in low-polar solvents(poor solvents) and at low temperatures, linear orrandom aggregation takes place.Long et al. in 2007 used single molecular force

spectroscopy to study the asphaltene structure. Thistechnique has been used for tension measurementsof biological and polymeric macromolecules. In thismethod, the response of a single aggregate ofasphaltene to an external force was studied using forcecurve measurements. The force curves conrmed thelinear polymerization of asphaltene aggregates.[94]

Despite the fact that some investigators proposed apolymerization mechanism for asphaltene aggregation,in most of experimental studies, the asphaltenes havebeen shown to be nano-aggregates with spherical,cylindrical, or disc-like shapes, contrary to what isdescribed in the polymerization mechanism.

Solubility model

This model was presented by Acevedo et al.[57,58,95]

They fractionated the asphaltene into two solubilitygroups in polar solvents such as para-nitrophenol.Fraction A1 showed low solubility, and Fraction A2showed high solubility in polar solvents. Theyproposed that the large difference in the solubility ofasphaltene is due to the island-type structure for A1and archipelago-type structure for A2 molecules. TheA1-type molecules can interact effectively, via the interaction between aromatic sheets forming aggregatesthat have low solubility in solvent. The A2-typemolecules have high solubility in solvent. Theymentioned that in polar solvents, the asphalteneaggregates are composed of A1-type molecules in thecore of aggregates surrounded by A2-type moleculesand solvent media. It seems that in this model, theA2-type molecules restrict the A1-type molecular

Figure 4. Asphaltene aggregation in linear polymerizationmechanism.[70]



association in different solvents. These colloids consistof an insoluble core surrounded by soluble molecules.The association of these colloids causes the formationof fractal structured aggregates. The solubility modelconsiders the effect of two different architectures ofasphaltene on aggregation. However, in this model,the exact mechanism of aggregate formation indifferent solvents is poorly dened; therefore, thesolubility model deserves a closer look.

The modied Yen model

Although according to colloidal and polymeric models,the presence of other components such as resins isnecessary for the stability of asphaltene aggregates. Inthe modied Yen model, there is no need for othercomponents to stabilize the asphaltene aggregates.[86]

The modied Yen model, which was presented byMullins et al. (2010), is based on the Yen model thatsuggests a progressive model for asphalteneaggregation and species the asphaltene structure atdifferent length scales in crude oil and solvents.[62,86]

Figure 5 shows a graphical representation of thismodel. As it can be seen in this gure, the asphaltenemolecule has an island structure that can form nano-aggregates with only six molecules per nanoparticle.[96]

In this model, the fundamental force for aggregationcomes from the attraction between large aromatic coresof the asphaltene molecules with island architecture.The external surface of nano-aggregates is controlledby alkyl substituents. Steric repulsion of alkylsubstituents limits the molecular association. Theinterior attraction forces and the exterior repulsionforces of asphaltene molecules lead to the formationof nano-aggregates with low number of molecules pernanoparticle.[86] These nano-aggregates then formasphaltene clusters. The clusters are not much biggerthan nano-aggregates. According to the modied Yenmodel, nano-aggregates join each other with a lowerbinding energy than the inter-nano-aggregate energybond.[86] These nano-aggregates are formed whenasphaltene is added to toluene, and there is no need

for resin molecules to stabilize asphaltene nano-aggregation. Mullins found that in spite of theexistence of resin molecules in the asphaltene nano-aggregate structure, the resin content may not be enoughto act as a surfactant neither in aromatic and parafnicsolvents nor in live and dead oil.[88] Various techniquessuch as centrifugation, nanoltration, and calorimetryhave been used to conrm this statement.[64,87,97]

The major difference between the colloidal, polymericmodels and the modied Yen model is the role of resinsas a stabilizing factor for the asphaltene aggregation. Asdiscussed previously, the dominant mechanism ofaggregate formation depends on the asphaltene solubilitydenition for the system under study.The modied Yen model is consistent with several

studies and explains exactly the formation of aggregatesfrom the asphaltene molecules.[98107] This model isestablished on the basis of the island architecture of theasphaltene molecule. Although a large number of studiesconrm the island architecture of asphaltene,[3740,42,4954]

some of the recent studies using NMR technique revealthe fact that asphaltene is a combination of the island andarchipelago architectures.[5558]

THE SIZE AND STRUCTURE OF ASPHALTENEAGGREGATES

Up to now, different techniques have been used toanalyze the formation of the asphaltene aggregates,diagnose critical points, and characterize the structureof aggregates. In addition to X-ray diffraction, small-angle X-ray scattering (SAXS), small-angle neutronscattering (SANS), NMR, near-infrared spectroscopy(NIR), dynamic light scattering nanoltration; anddifferent microscopic methods such as scanning electronmicroscopy (SEM), tunneling electron microscopy,confocal microscopy and atomic force microscopy(AFM) have been used by many researchers. Thecolloidal behavior of asphaltene and the structure of theasphaltene aggregates dependent on the parameters,including the asphaltene source, solvent type,

Figure 5. Aggregate formation mechanism based on the modied Yen model, frommolecules to clusters.[96] This gure is available in colour online at www.apjChemEng.com.



thermodynamic conditions, and techniques used toanalyze asphaltene aggregates, have been conrmedusing various studies. The size and structure ofaggregates are directly related to the aggregationmechanism and molecular structure of asphaltene.

SAXS and SANS techniques

The SANS and SAXS are two suitable and powerfulmethods to analyze the size, shape, and molecularweight of colloid structure of asphaltene. Using thedistribution curve of scattering intensity and dataanalysis, researchers compute the asphaltene aggregatecharacteristics such as particle gyrating radius. In manystudies, these methods have been used to investigatethe effect of temperature, pressure, asphaltene source,and solvent type on the asphaltene aggregatestructure.[98] The essential difference between SAXSand SANS techniques is that SANS is based on thenucleus density measurements and SAXS is based onelectron density measurements. By applying SANSmeasurements in asphaltene solutions, there is noenough contrast for a neutron to differentiate betweenthe asphaltene and solvent. Preparing a solution ofasphaltene in deuterium increases the contrast forSANS measurements. By the application of SANSmethod, it is possible to measure the whole size ofthe asphaltene aggregate. On the other hand, usingSAXS technique, only the aromatic core of asphalteneis measured.[98]

Determination of the size and morphology of theaggregates has been the subject of many studies. Sofar, by applying the Schultz distribution function,Guinniur approximation, and Beaucage function,different morphologies including the monodisperseand polydisperse spheres, disc-like particles, prolateellipsoids, oblate cylinder, vesicles, and fractalshape have been suggested for the asphalteneaggregates.[98,3,101114] However, some studies revealthat a simple geometric model cannot represent theaggregate structure of asphaltene. Different structuralcharacteristics of asphaltene may be the reason for thisconclusion.[115]

Between the years 2000 and 2012, different studiesof the size of aggregates have been reported withinthe range of 2110 using SANS and SAXStechniques. Although molecular size does not changevery much, aggregate size can change because of thevariety of operating conditions.[116119,103,110] Thenanometric size and colloidal structure of asphalteneaggregates obtained from the SAXS and SANSexperimental analysis are consistent with the modiedYen model and island-type molecular structure. Indifferent studies, the effects of temperature, solventtype, and pressure have been investigated.[110,120123]

The effect of temperature on the size of aggregatewas studied by different researchers. According to the

results, increasing the temperature leads to a decreasein the size of the aggregates.[110,122,124]

Roux et al. studied the structure of asphaltene intoluene solution as a function of temperature (from 73to 8 C) and the asphaltene concentration (volumefractions ranging from 0.3% to 10%) using SANStechnique in 2001. Asphaltenes have been found toform nanometric aggregates whose average masses andradii of gyration increased as temperature decreased. Inthe diluted regime, where the concentration of asphalteneis lower than 3-4%, the gyration radius of asphaltenenano-aggregates and their average masses do notchange.[122] At temperatures 8 and 20 C, SANSmeasurements have shown the formation of micron-sizeaggregates.[122]

Tanaka et al. in 2003 have studied the effect oftemperature on the asphaltene aggregate formation inthree different oil samples using SANS measurements.All samples of asphaltene except the Maya sampleexhibited a prolate ellipsoid structure in deuterateddecalin, 1-methyl naphthalene, or quinolone solventsat 25 C. They found that by increasing the temperatureto 350 C, the size of the particles decreases, and theirstructure deforms to a spherical shape with the size of25. The Maya asphaltene in the decalin solventshowed a fractal shape, which remains unchanged byincreasing temperature.[110]

In another study in 2004, Tanaka et al. studied theXRD and SAXS scattering curves of three differentsamples of asphaltene at the temperatures of 30, 150,and 300 C. By calculating the size and fractal featureof aggregates and the number and the distance betweenaromatic sheets, they developed a general model forasphaltene aggregate formation.[124] This modelcorresponds to the modied Yen model. According tothis model, three types of aggregates are consideredfor asphaltene:

(1) Core aggregates with the size of 20 that are madeby the association of asphaltene molecules.

(2) Medium aggregates that are formed by theinteraction of core aggregates with oil, maltene,media, or solvent. The size of medium aggregatesis in the range of 50500.

(3) Fractal aggregates that are formed by the assemblingof medium aggregates. The formation of theseaggregates does not depend on the type of themedium. The asphaltene aggregate formation diagramaccording to this model is shown in Fig. 6.[124]

Many researchers studied the fractal structure ofasphaltene aggregates using the SAXS and SANStechniques and found that asphaltene solution inheptane and crude oil makes fractal-like aggregates.The results show that the fractal dimension dependson the source of asphaltene and the experimentalconditions. The mean fractal dimension obtained for



asphaltene aggregates can vary in the range between1.5 and 3.[111,104,109,116,118,119,122]

Recently, in 2013, Hoepfner et al. proposed amechanism for the asphaltene precipitation on the basisof the fractal feature of aggregates using SAXStechnique. They found that asphaltenes in crude oilare composed of soluble and insoluble aggregates withfractal structures. The fractal dimension of insolubleclusters has been higher than that of the solubleclusters.[125]

In other studies, various solvents have been used forstudying the effect of the solvent type on the asphalteneaggregate size. It has been shown that in high-polarsolvents such as pyridine and tetra-hydrofuran, the sizeof particles is three to four times smaller than insolvents with low polarization such as benzene.[82]

Asphaltenes are insoluble in nonpolar solvents suchas normal alkanes. The researchers found that anychange in the solubility and aggregate size ofasphaltene relates to its molecular structure. Spieckeret al. observed that the insoluble fraction of asphaltenein a mixture of heptane and toluene has the higher ratioof carbon to hydrogen, whereas the soluble fraction hasthe higher content of heteroatoms.[126]

Different studies have used SAXS and SANStechniques to study the effect of resins on the size andstructure of asphaltene aggregates. In research studiesconducted by Bardon et al. in 1996,[114] Espinat et al.in 2004,[123] and Gawrys et al. in 2006,[103] it has beenobserved that an increase of the resin-to-asphalteneratio causes a decrease in the scattering intensity atlow values of applied intensity, which indicates theformation of small nanometric aggregates with a highfractal dimension.

Carnahan et al. in 1993 studied the effect of pressureon the size of asphaltene aggregates in near criticalsolution using SAXS technique. Mixtures of 0.6volume fraction crude oil in n-pentane at a temperatureof 110 C and pressure range between 25 and 400 barshave been used in this study.[127] The results indicatedthat the lower the pressure, the higher the size ofaggregates. This is in contrast with the results observedby Espinat et al., who used the solution of asphalteneaggregates in toluene.[123]

NMR technique

Measuring the asphaltene diffusion coefcient insolutions is another technique to study the structureand morphology of asphaltene aggregates. There aremany techniques to measure the diffusion coefcient,such as pulsed eld gradient H NMR. Using thistechnique, the time interval between the two gradientpulses applied in the pulse sequence is computed. Thedisplacement that is made by the Brownian motion ofa molecule is related to molecular self-diffusioncoefcient.[128]

Norinaga et al. used this technique to studyasphaltene aggregate formation in 2001. They analyzeddifferent mixtures of asphaltene in pyridine byasphaltene diffusion coefcient measurements usingsignal amplitude changes according to the gradientamplitude.[128] In the same year, Ostlund et al. studiedthe asphaltene diffusion coefcient at differentconcentrations of deuterated toluene. They estimatedthe asphaltene poly-dispersity by tting the signalattenuation to the normal distribution of diffusioncoefcients. According to the results, as the

Figure 6. Hypothetical representation of asphaltene aggregations by Tanaka et al.[124]

This gure is available in colour online at www.apjChemEng.com.



concentration of asphaltene increases, the diffusioncoefcient of the solution also increases, which impliesthat the asphaltene particle size increases. The ttedmodel of diffusion coefcient has revealed a disc-likestructure for asphaltene aggregates that is consistentwith the island molecular structure.[129] In somestudies, the effect of normal alkanes on asphalteneaggregation has been investigated using NMRtechnique. Normal alkenes have been shown to causeaggregate size and poly-dispersity to increase.[130]

Sjoblom et al. applied pulsed eld gradient-SE NMRto study the effect of various additives, such asnaphthenic acid, on asphaltene aggregation. Theadditives reduce the aggregate size by asphaltene disso-lution in solvent.[131]

Lisitza et al. in 2009 used DOSY NMR method tomeasure the diffusion coefcients of asphaltene at itsdifferent concentrations in deuterated toluenesolution.[132] Assuming a spherical shape of asphalteneaggregate, they used the EinsteinStoke equation andcalculated the radius of asphaltene to be 1.2 nm, atlow asphaltene concentrations. They observed that asasphaltene concentration increases, the diameter ofaggregates increases threefold.[132] In the studiesperformed by Durand et al. in 2010 and Oliveiraet al. in 2013, the effect of molecular structure ofasphaltene on the aggregation properties has beendetermined using the DOSY NMR technique and self-diffusion coefcient measurements.[55,56] Two forms ofaggregates (nano-aggregates andmicro-aggregates) havebeen detected for two different asphaltene types. It wassuggested that one type of asphaltenes with the islandmolecular architecture can form aggregates via stacking interactions between aromatic cores, in a dilutedsolution. However, the other type of asphaltenes with thearchipelago architecture can form aggregates at a high-asphaltene concentration by hydrogen bonding.[55,56]

The modied Yen model is inconsistent with the studiesof Durand et al. and Oliviera et al.. According to themodied Yen model, the island structure of theasphaltene molecule is the primary fundamentfor asphaltene aggregation. According to this model, ifasphaltene molecules had an archipelago structure, theywould form gel because of multiple binding sites in asingle molecule of asphaltene, or they would formporous micro-structure aggregates entraining a largeamount of solvent.[66,96]

NIR technique

The NIR is a technique developed to determine theonset of asphaltene formation and the size ofaggregates in an asphaltene solution. Using thistechnique, it is possible to analyze the aggregateformation behavior and investigate the effect ofdifferent factors such as pressure and solvent type onthe aggregation process.[133135] Although identifying

very small nanometric particles using the NIR techniqueis not possible, it can be used to identify large particlesand study the effect of different parameters on theaggregation behavior with a good accuracy. This methodalso has the capability of identifying asphalteneaggregation at high pressures.[133,134]

The NIR spectroscopy in the visible spectral range,inscribes optical transmission inside a sample as afunction of wavelength. Incident light intensity, I0,decreases by absorption or light scattering to Iintensity. The optical density is expressed by thefollowing equation:

OD log 1I0

(1)

Where, OD, I0, and I are the optical density, incidentlight intensity, and exiting light intensity, respectively.[136]

Thus, by asphaltene aggregation, the optical densitychanges because of visible light scattering in thesample. Joshi et al. in 2001 applied NIR spectroscopyto characterize the asphaltene aggregation from liveoil by pressure reduction. They presented the formationof asphaltene large aggregates with the size increasingfrom 300 nm to 3.2m by pressure reduction of crudeoil.[133] The so-called large aggregates are probablythe insoluble clusters at different pressures that consistof small nano-aggregate association.In 2002, Aske et al. have compared the asphaltene

aggregate formation in live oil and synthetic mixtureof asphaltene in toluene and pentane by decreasingpressure using the NIR technique. According to theresults, the measured optical density of live oil hasshown less increase compared with the measuredoptical density of the synthetic mixture. It has beenshown that the amount of aggregate formation in liveoil is less than synthetic mixture.[134]

In 2004, Yen et al. studied the effect of solvents onasphaltene aggregate formation using NIR technique.According to the results, asphaltene aggregationdepends on solvent aromaticity. The higher thearomaticity of solvents, the higher the amount ofsolvent required for aggregation.[135]

Other techniques

Besides the aforementioned methods, which are usedby many researchers to study the aggregation andstructure of asphaltene, some techniques such ascentrifuging, ltering, or light scattering are also usedto study the structure of aggregates. Indo et al. in2009 calculated the asphaltene particle size in live oilat reservoir pressure and temperature using thecentrifuging method. In this method, the centrifugationof oil samples induces gravitational gradient that istted to the Boltzmann distribution using the



Archimedes buoyancy. The only comparative parameterin data tting, i.e. aggregate size, was calculated to be2 nm.[89] These small entities perhaps consist of island-type molecules.Light scattering and laser particle analyzer

techniques are two other methods to study asphalteneaggregation in different experimental conditions. Theresults of light scattering studies represent thatdepending on the asphaltene concentration, the size ofaggregates changes in the range of nanometers.[137]

The effect of pressure and temperature on asphalteneaggregate size was studied using the laser particleanalyzer technique. The results showed a mean particlesize in the range of hundreds of micrometers forasphaltene aggregates.[7]

Different studies used the ltration method toanalysis asphaltene aggregate size. In the study byZhao and Shaw in 2007, ceramic membranes were usedto characterize the distribution of the asphalteneparticle size at high temperatures. The results showedthat nano-aggregates smaller than 20 nm and clusterslarger than 200 nm coexist in oil at reservoirpressure.[138] Ching et al. in 2010 studied the size ofasphaltene particles by the ltration of crude oil usingGORE-TEX membranes with nominal pore sizes assmall as 30 nm. They revealed that the asphalteneaggregates are smaller than the normal pore size ofthe membrane, i.e. 30 nm.[139] Tabatabaei-nejad et al.studied the size of asphaltene aggregates collected fromcrude oil during CO2 gas injection on Whatman lter

paper with the mean pore size of 2m. The size ofparticles was estimated to be 100 nm, using SEMmethod.[140]

The SEM, tunneling electron microscopy, HRTEManalysis methods are used to characterize theasphaltene aggregates on surfaces. The effects ofdifferent parameters such as oil solvent type andtemperature on the precipitated asphaltenes from oilhave been studied using these techniques by differentresearchers.[141145] In the SEM studies, two differentmorphologies including compact structure and porousstructure have been observed.[142144] The porousstructure of precipitates may be the result of thearchipelago-type molecular structure of asphaltene, asconcluded by Spiecker et al.[66] Different HRTEMimages revealed that asphaltene is organized bynanometric aggregates.[141,145,146]

Asphaltenes on surface are usually analyzed usingAFM. The results of AFM analysis showed that theprecipitated asphaltene lms consist of spherical ordiscoid nanoparticles.[147,148] The small size of nano-aggregates shown in these studies is in accord withthe modied Yen model. The large aggregates in thenanoscale and microscale, which are detected bydifferent techniques, are probably formed by theassociation of nano-aggregates at different asphalteneinstability conditions.Table 1 represents a summary of the basic results on

the asphaltene size and shape (molecular andaggregate) via different techniques.

Table 1. A summary of the results on asphaltene characterization by different techniques.

Topics Techniques of the analysis Reported values References

Morphology of aggregates SANS, SAXS Sphere, disc, ellipsoid,cylinder, fractal

[98],[104], [3], [101114],[116], [118,119], [122],[125]

HNMR Disc, sphere [129], [132]TEM, HRTEM Sphere, fullerene

structure[141], [145,146]

SEM Sheet, sphere, compact,and porous structure

[140], [142144]

AFM Sphere, discoid [147,148]Size of aggregates SANS, SAXS 2500A0 [115],[116119], [103], [105],

[110], [123],[124]HNMR 1040A0 [132]NIR 300 nm to 3.2m [133]Centrifugation 2 nm [89]Nanoltration Smaller than 30 nm and

larger than 200 nm[138], [139]

SEM, TEM, AFM 30550 nm [140,141], [148]Size of asphaltene molecule FD, FCS 12 nm [37], [41,42]Number of aromatic rings inasphaltene molecule

Optical absorption, FD, FCS,NMR, STM, HRTEM

Less than ten [23], [25], [3036]

Molecular architecture ofasphaltene molecule

RICO, uorescencespectroscopy, pyrolysis

Archipelago [21], [4548]

FD, TD, MO, FCS, L2MS,LIAD

Island [3740], [42], [44], [4953]

DOSY NMR Both archipelago andisland structures

[55,56]



A precise investigation on the asphaltene experimentalstudies shows that the experimental conditions havelarge impact on the results of asphaltene characterization.Different parameters such as temperature, pressure,dilution medium, injected uid, and the source ofasphaltene show different results on the size and shapeof the aggregates.[110,120130] The reported structures(size and shape) of the asphaltene aggregates have beendifferent from study to study. For example, analysisof SANS and SAXS data for the determination ofasphaltene structure in different deuterated solventshave shown several structures: sphere, cylinder, thindisc, and ellipsoid. The diverse set of structuralcharacteristics of asphaltene is a result of differentsource of asphaltene, different solvents used forthe dilution of asphaltene solution, and differentmethods to study asphaltene aggregation at differentexperimental conditions.[98,3,101114,149]

CONCLUSIONS

Different studies conducted by researchers represent thattechnical methods used in experimental studies providevaluable information on the molecular structure,formation mechanism, morphology, and size ofasphaltene aggregates. The results show the following:

(1) The asphaltene molecular structure is determined tobe in the form of poly-aromatic rings and aliphaticchains with different heteroatom functional groups,using various techniques.

(2) The asphaltene molecular architecture dependson the type of oil and solvent used to separatethe asphaltene. So far, two different modelsmonomeric and archipelagohave been suggestedfor the molecular architecture of asphaltene.

(3) On the basis of the studies conducted, themolecular architecture, different fundamentalforces (e.g. the interaction between aromaticsheets, hydrogen bonds, heteroatom type, andelectron transfer), and the solubility denition ofasphaltene are the most important factors in theasphaltene aggregate formation mechanism.

(4) A comprehensive research on different studiessuggests four mechanisms for the aggregation ofasphaltene: the micellar/colloidal, polymeric,solubility, and modied Yen models. Althougheach model is consistent with some experimentalresults, all the experimental observations cannotbe explained by a particular model because ofconstraints in the model denition. In comparisonwith other models, the modied Yen model showsmore consistency with new experimental results.This model provides a complete denition forasphaltene nano and cluster aggregation fromasphaltene molecules with the island structure.

(5) Depending on the asphaltene source, asphalteneconcentration, solvent type used in an asphaltenemixture, thermodynamic conditions, and technicalmethod used for asphaltene analysis, the size ofaggregates changes in the range between severalnanometers to several micrometers, and themorphology of aggregates is in the form ofspherical, discoid, membrane-like oblate, prolatecylinders, and mass fractal aggregates.

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