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Heavy Crude Oils/Particle Stabilized Emulsions Iva Kralova , Johan Sjöblom, Gisle Øye, Sébastien Simon, Brian A. Grimes, Kristofer Paso Ugelstad Laboratory, Norwegian University of Sciences and Technology (NTNU), Department of Chemical Engineering, Sem Sælandsvei 4, N-7491 Trondheim, Norway abstract article info Available online 6 October 2011 Keywords: Electrocoalescence Gelation Naphthenic acids Particle-stabilized emulsions Phase diagrams Separation modeling Fluid characterization is a key technology for success in process design for crude oil mixtures in the future off- shore. In the present article modern methods have been developed and optimized for crude oil applications. The focus is on destabilization processes in w/o emulsions, such as creaming/sedimentation and occulation/ coalescence. In our work, the separation technology was based on improvement of current devices to promote coalescence of the emulsied systems. Stabilizing properties based on particles was given special attention. A variety of particles like silica nanoparticles (AEROSIL®), asphalthenes, wax (parafn) were used. The behavior of these particles and corresponding emulsion systems was determined by use of modern analytical equip- ment, such as SARA fractionation, NIR, electro-coalescers (determine critical electric eld), Langmuir technique, pedant drop technique, TG-QCM, AFM. © 2011 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Separation technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Crude oils and their emulsions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Crude oil components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Crude oil emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Particle stabilized emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Rheology of particle-stabilized emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Asphalthene stabilized emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Asphaltenes precipitated by multi-step precipitation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Polyaromatic surfactants as model compounds for asphaltenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Wax in crude oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Naphthenic acids and phase diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1. Langmuir lm properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Separation modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.1. Model description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2. Notes on the lm drainage model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3. Separation modeling results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction Heavy and extra heavy oils are strategic reserves but pose new challenges to the oil industry. One major problem is their high vis- cosity which makes them difcult to transport. Heavy crude oil components are often categorized into the group-type classes [1] of saturates, aromatics, resins, and asphaltenes (SARA). The resins are high-molecular-weight polar hydrocarbons, which are known to sta- bilize asphaltenes in petroleum uids. Resin fractions are easily obtained during high-performance liquid chromatography (HPLC) separation by polar solvent extraction. Asphaltenes are polyaromatic hydrocarbons and are often precipitated from crude oils using low molecular-weight alkane solvents, such as pentane or hexane. They are considered by most researchers [2,3] together with resins to be Advances in Colloid and Interface Science 169 (2011) 106127 Corresponding author. Tel.: + 47 73591605; fax: + 47 73594080. E-mail address: [email protected] (I. Kralova). 106 107 107 108 109 111 112 113 115 116 116 119 120 121 121 122 122 126 126 126 0001-8686/$ see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2011.09.001 Contents lists available at SciVerse ScienceDirect Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis
Transcript
  • Heavy Crude Oils/Particle Stabilized Emulsions

    . . .

    . . .3.2.1. Particle stabilized emulsio

    le-stabilsionspitatedmodel. . .e diagraperties. . .. . .

    Advances in Colloid and Interface Science 169 (2011) 106127

    Contents lists available at SciVerse ScienceDirect

    Advances in Colloid an

    .e4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    1. Introduction components are often categorized into the group-type classes [1] of

    126126126Heavy and extra heavy oils are strategicchallenges to the oil industry. One major prcosity which makes them difcult to tran

    Corresponding author. Tel.: +47 73591605; fax: +E-mail address: [email protected] (I. Kr

    0001-8686/$ see front matter 2011 Elsevier B.V. Alldoi:10.1016/j.cis.2011.09.001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .e model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .lts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    1211221223.7.2. Notes on the lm drainag3.7.3. Separation modeling resu3.7. Separation modeling . . .3.7.1. Model description3.2.2. Rheology of partic3.3. Asphalthene stabilized emu

    3.3.1. Asphaltenes preci3.4. Polyaromatic surfactants as3.5. Wax in crude oils . . . .3.6. Naphthenic acids and phas

    3.6.1. Langmuir lm prons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .lized emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .by multi-step precipitation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .compounds for asphaltenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    111112113115116116119120121reserves but pose newoblem is their high vis-sport. Heavy crude oil

    saturates, aromahigh-molecular-wbilize asphaltenobtained duringseparation by pohydrocarbons anmolecular-weighare considered b

    47 73594080.alova).

    rights reserved.. . . . . . . . . . . . . . . . . . . . . . . . . . .109

    3.1. Crude oil components . .3.2. Crude oil emulsions . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    . . . . . . . . . . . . . . . . . . . . . . . .1082. Separation technology . . . . . . . . . . . . . . . . . . . . . .3. Crude oils and their emulsions. . . . . . . . . . . . . . . . . . .Iva Kralova , Johan Sjblom, Gisle ye, Sbastien Simon, Brian A. Grimes, Kristofer PasoUgelstad Laboratory, Norwegian University of Sciences and Technology (NTNU), Department of Chemical Engineering, Sem Slandsvei 4, N-7491 Trondheim, Norway

    a b s t r a c ta r t i c l e i n f o

    Available online 6 October 2011

    Keywords:ElectrocoalescenceGelationNaphthenic acidsParticle-stabilized emulsionsPhase diagramsSeparation modeling

    Fluid characterization is a key technology for success in process design for crude oil mixtures in the future off-shore. In the present article modern methods have been developed and optimized for crude oil applications.The focus is on destabilization processes in w/o emulsions, such as creaming/sedimentation and occulation/coalescence. In our work, the separation technology was based on improvement of current devices to promotecoalescence of the emulsied systems. Stabilizing properties based on particles was given special attention. Avariety of particles like silica nanoparticles (AEROSIL), asphalthenes, wax (parafn) were used. The behaviorof these particles and corresponding emulsion systems was determined by use of modern analytical equip-ment, such as SARA fractionation, NIR, electro-coalescers (determine critical electric eld), Langmuir technique,pedant drop technique, TG-QCM, AFM.

    2011 Elsevier B.V. All rights reserved.

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    106107107j ourna l homepage: wwwd Interface Science

    l sev ie r .com/ locate /c istics, resins, and asphaltenes (SARA). The resins areeight polar hydrocarbons, which are known to sta-

    es in petroleum uids. Resin fractions are easilyhigh-performance liquid chromatography (HPLC)lar solvent extraction. Asphaltenes are polyaromaticd are often precipitated from crude oils using lowt alkane solvents, such as pentane or hexane. Theyy most researchers [2,3] together with resins to be

  • model system and the process parameters (ow rate, pressuredrop and water cut). It was observed that increased pressuredrops resulted in droplets with smaller mean diameter and amore narrow size distribution and an increasing the water cut ata certain pressure drop for a certain system gave an increase inthe mean diameter of the droplets.

    3. Crude oils and their emulsions

    The proper understanding of the mechanisms and the chemistrybehind the stability of crude oil based emulsions is an essentialstep in crude oil processing. The stability mechanism is, of course, re-lated to the composition of crude oil such as the nature and the con-centration of surface-active components present in oil, the physicalproperties of oil (viscosity, density for instance) and the interfacialrheology of the interface around water droplet which informs

    107I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127responsible for the high stability of water/in/crude oil emulsions,mainly because of their capacity to form a stable thick network atthe interface [4]. In addition, solid particles [5] originating from res-ervoir adsorbed at the water/oil interface may also contribute to thestability of the water droplets together with waxes [6] and naph-thenic acids [7]. The combination of these components creates acomplex picture of several contributing mechanisms to the stabilityof water-in-oil emulsions [3]. Co-production of brine and crude oiloften results in the formation of stable water-in-oil emulsionswhen turbulent mixing conditions are encountered during the trans-portation process. When the crude oil is processed from the wellhead to the manifold, there is usually a substantial pressure reduc-tion with a pressure gradient over chokes and valves where the mix-ing of oil and water can be intense. After this the well-stream isentering the separator where most of the water is separated fromthe crude. The nal treatment normally takes place in the electrocoa-lescer after which the level of water should be below 0.5%. The pro-cessing of the oil and water offers several possibilities to mix thephases and create an emulsion [3]. The life time of emulsions de-pends on the kind of stability mechanisms involved and compositionof interfacial material. Due to the important role of solid particlesduring formation of the interfacial lms, the presence and solubilitystate of waxes, asphaltenes, resins and naphthenates is a key factorinuencing emulsion stability and separation [8].

    In order to understand the complex nature behind water-in-oilemulsion stability under real process conditions, a thorough knowl-edge should involve the properties of the crude oil components,their association tendencies and accumulation at w/o interfaces,their solubilities and their association structures in water/oil systems.It is also our intention to connect these investigations with new ex-perimental techniques which were applied on crude oil systems pre-viously [4]. Such conventional techniques are SARA fractionation, NIR,electrocoalescers (determine critical electric eld), Langmuir tech-nique, pedant drop technique, TG-QCM, AFM.

    2. Separation technology

    The separation process normally involves a combination of me-chanical impact and chemistry to break w/o emulsions. Separationinlets and internals have been developed to maximize the resolu-tion of water at a reasonable residence time. The whole separationtechnology has been pushed forward by introducing electrostaticdevices facilitating the droplet growth and coalescence of the emul-sied systems. For gravity separators the settling and coalescenceprocesses, as well as the obstructions to coalescence describedabove are valid. If a background electric eld is present, as in indus-trial electrocoalescers, the kinematics of droplets and the coales-cence will be completely different. Several mechanisms which arepresent in gravimetrically-induced destabilization of emulsions areprobably still present, but their relative level of inuence may becompletely different due to forces induced by strong electrical elds[9]. The electrostatic effects arise from the very different propertiesof oil and water, water having dielectric permittivity and conductiv-ity values much higher than those of oil and polarization effects inwater droplets. Electrocoalescence is a process targeted to assist ap-proach, contact and fusion of water droplets emulsied in a contin-uous of low dielectric permittivity in order to increase their size,thus accelerating their settling velocity and the total separationtime. Various designs of these electrocoalescers have been intro-duced as the Vessel Internal Electrostatic Coalescer (VIEC) [10,11],the High Temperature VIEC (HTVIEC Fig. 1) and the upcomingLow Water Content Coalescer (LOWACC Fig. 2) [12].

    The patented Compact Electrostatic Coalescer (CEC) technolo-gy is a small lightweight ow-through system that greatly enhancesthe separation performance of existing downstream gravity separa-

    tion equipment. It enhances separation performance by coalescingemulsied water droplets entrained in the crude oil into much larg-er droplets that readily settle in a downstream separator. The coa-lescing action takes place very rapidly under turbulent owconditions, as the emulsion is subjected to an intense electrostaticeld inside the CEC unit. Fig. 3 presents typical CEC unit con-sisting of a series of concentric circular electrodes with a capacityof 130000 barrels per day [13,14]. The CEC is insensitive to vesselmotions, and is not prone to plugging by solids in the well uids.Voltages of up to several thousand volts can be applied to the elec-trodes. By doing so, an intense electrical eld is established in theannular channels, so that the coalescing process is much fasterthan what is normally achieved in conventional grid units. Waterdroplets merge several times within a matter of seconds and in-crease their size around ten times in the coalescing section. Thewater droplets and oil then enter a gravity separator for separation,but one with much reduced dimensions compared to a normal elec-trostatic coalescer. Ugelstad Laboratory has together with industrialpartners built up a Electrostatic Separation Unit (ESU) which is adown-scaled CEC, although with another ow prole (laminar)than the commercial CEC (turbulent ow), (Fig. 4) [15]. The lengthis one third of the commercial CEC and a ow channel which alsoshould resemble the CEC with respect to dimensions and electriceld strength (Fig. 5), but not to ow properties. Fossen et al. [16]constructed a ow loop where water and oil are pumped separatelyusing positive displacement pumps and then mixed in a tee beforeowing through a choke valve where shear may be induced inorder to form a dispersion which separates in the test separator.The water and oil then are led (separately) back to the feed separa-tor (Fig. 6). The droplet sizes were determined using DVM on dilut-ed solutions. The droplet sizes determined were from 2 to 90 mapproximately which was conrmed by theoretical calculations ofmaximum droplet sizes at the relevant pressure drops. The meandiameter droplet size was between 4 and 12 m depending on the

    Fig. 1. Single HTVIEC module (left) and arrangement of 6 modules in a large separator(right) [11].about the elasticity and viscosity and for instance the presence of a

  • Fig. 2. VIEC and LOWACC installed in a separator [12].

    108 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127skin for the specic case of petroleum crude oil at this interface. Inthis part we focused on our recent major results about characteriza-tion of chemical composition and physical properties of crude oil aswell as the interfacial rheology properties at the water/oil interface.By their nature, emulsions are physically unstable. The breakup ofemulsions consists of several processes, which are mostly coupledand result in different levels of instability, which could be dividedinto two parts: creaming/sedimentation and occulation/coalescence.

    Fig. 3. On the right the CEC unit with a capacity of 130000

    Fig. 4. A water-in-oil (w/o) emulsion is produced by shear over a choke valve and led to theelectric eld over the two spaced plates. The oil and water exits through a opening approx3.1. Crude oil components

    Although the SARA fractionation method is a rough sorting of thecrude oil constituents, it can provide an important classication ofcrude oils. The SARA fractionation method has found great utility incombination with high-performance liquid chromatography (HPLC).As an example of the SARA fractionation method we will discuss theprocedure from a study by Hannisdal and co-workers [17]. The ability

    barrels per day, on the left the CEC components [14].

    inlet of the lab scale ESU. The ESU will work as small separator even when there is noimately equal in area to the CEC [15].

  • of vibrational spectroscopy (IR and near-IR) to predict SARA compo-nents in heavy and particle rich crude oil will be discussed in the follow-ing section. In this study, 20 crude oil samples (Table 1) at ambienttemperature and pressure were received from exploration sites on theNorwegian Continental Shelf and sites located in Brazil, France, theSouth China Sea, the Atlantic Ocean, and the Gulf of Mexico. These oilswere quantitatively fractionated into saturates, aromatics, resins, andasphaltenes (SARA) by asphaltene precipitation in n-hexane and pre-parative HPLC. Here, the focus will be on the HPLC procedure. Two col-umns were used: one 21.2250 mm column packed with unbondedsilica 15 m and one 21.250 mm amino (10 m) column. Dichloro-methane (99.8%) and n-hexane (95%) were used as mobile phases.

    Twenty crude oils were analyzed especially for vibrational featuresin the NIR and IR region. Parallel experiments showed good reproduc-ibility for the spectroscopic measurements and the fractionation meth-od. NIR spectra were used in combination with partial least squares

    Thirty crude oils were diluted in 30 vol.% toluene and analyzedwith respect to the viscoelastic response to a sinusoidal modulationin the frequency range from 0.01 to 1 Hz [20]. Diluted and undilutedsystems were compared. Moreover, long-time dynamic interfacialtension experiments of static drops of undiluted crude oil in waterwere performed. As expected in the low frequency range (0.011 Hz), molecular exchange from bulk strongly affected the measureddilational parameters. For this reason the systems which exhibitedparticularly low magnitude of the dilational modulus were of theheaviest crude oils in the sample set, whereas the systems with great-est dilational modulus were among the lightest crude oils. The fre-quency dependence of the dilational modulus increased with itsmagnitude as expected for diffusion-controlled relaxation of solublelms. Overall, the undiluted crude oilwater interfaces had similar re-laxation characteristics as the diluted samples except for slightly re-duced magnitude of the dilational modulus.

    The same samples of crude oils were analyzed with respect to bulkand interfacial properties and the characteristics of their w/o emul-sions to investigate the relative level of inuence that individual pa-rameters have over the overall stability of w/o emulsions. Thestability of emulsions was investigated by the Ecritical cell [21]. Asexpected, a strong covariance between several physicochemical prop-erties was identied. The comparison of the experimental time for de-stabilization with the theoretical time of droplet approach is showedin Fig. 10 (left) for a water-in-heavy oil emulsion with droplet size of6 m. Given enough time, the water-in-heavy oil emulsions could be

    109I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127(PLS) regression to predict SARA components from the crude oil matrix.Regression models were built for each SARA component from NIR datato predict the amount of SARA components. NIR spectroscopy proved toperform well for the prediction of SARA components with predictionvariances (RMSEP) of 2.82 wt.% (S), 1.47 wt.% (A), 1.46 wt.% (R), and0.44 wt.% (asphaltenes). As an example the prediction vs. measuredamount of resins is presented graphically in Fig. 7. The resin and satu-rate fractions were also predicted in an excellent way from IR data.However adequate models for the aromatic and asphaltene contentswere not obtained. It was hypothesized that the similarity in the vibra-tional features from aromatic carbon of asphaltenes and aromaticscaused this. These models successfully tted the experimental datafrom NIR analyzes and showed good predictive ability for the crudeoil composition.

    Combination of spectroscopy and multivariable analysis could alsopredict crude oil emulsion stability. Indeed Aske and co-workers [18]studied correlation between emulsion stability asmeasured by the crit-ical electric eld method (Ecritical) and physicochemical properties asSARA composition determined by NIR spectroscopy, density and TANvalue of test matrix consist of 18 crude oils. Analysis of the signicanceof the regression coefcients of the model revealed that the NIR spec-tra, among some other factors, were closely correlated to the measuredemulsion stability. Based on this, an attempt was made to predictemulsion stability exclusively from NIR spectra (Fig. 8). NIR spectracontain information on both the aggregation state of asphaltenes andon chemical composition. This may explain the good predictivepower for the Ecritical values.

    3.2. Crude oil emulsions

    It was assumed that dilational relaxation properties (surfacerheology) of surface active components can be probably of greatimportance during droplet fragmentation and coalescence. The

    Fig. 5. The ow channel is formed by placing one coated steel plate and one bare alu-

    mina plate against each other spaced by a rubber gasket [15].oscillating pendant drop method was used to study diluted crudeoilwater interfaces and the effect of altering aromaticity of the dil-uent and the concentration of crude oil [19]. The storage E and lossE moduli determined by dilational rheology of one of the crudeoil/water systems is presented in Fig. 9. At a perturbation frequencyof 0.1 Hz, the equilibrium storage and loss moduli passed throughdistinct maxima as a function of bulk concentration. The apparentlylow viscoelasticity of the interfaces in systems with high bulk con-centration was at least partly caused by high diffusion ux of inter-facially active components from bulk. A direct relation between themeasured interfacial relaxation parameters and the overall emul-sion stability was not identied.

    Fig. 6. Flow scheme of the separator system with the main process units. Arrows indi-cate the ow directions designed by Fossen et al. to study oil/water separation [16].destabilized even at very low electric eld magnitude (0.4 kV/cm).

  • Table 1Experimental compositions of crude oils separated into SARA and water fractions [17].

    Crude oil no. Origin Saturates (wt.%) Aromatics (wt.%) Resins (wt.%) Asphaltenes (wt.%) Water (wt.%) Yield (wt.%)

    Mean Sdev

    Calibration set1 Brazil 33.5 46.5 18.8 1.2 0.1 0.8 100.82 Brazil 38.8 36.6 12.6 1.9 0.1 0.9 90.93 Brazil 30.5 43.2 20.9 5.0 0.3 0.2 99.94 China 36.0 30.8 22.1 3.1 0.1 9.6 101.55 Mexico 43.0 29.7 10.4 1.2 0.1 0.3 84.56 North Sea 26.5 38.8 20.5 2.8 0.1 12.6 101.37 West Africa 46.9 37.9 14.0 1.7 0.0 0.1 100.68 Brazil 33.6 38.1 16.8 12.9 0.2 0.1 101.79 North Sea 45.0 32.9 9.8 1.2 0.1 0.2 89.210 North Sea 45.7 38.6 11.5 0.8 0.1 0.1 96.611 North Sea 43.6 32.9 7.2 0.6 0.0 0.4 84.812 North Sea 33.0 42.8 11.6 3.9 0.0 0.5 91.813 China 32.0 32.5 28.5 4.3 0.2 1.2 98.614 West Africa 53.1 30.9 8.2 1.0 0.0 0.2 93.315 Brazil 33.4 38.6 18.2 4.6 0.2 0.1 94.816 Brazil 37.3 42.6 14.1 3.8 0.4 0.3 98.117 North Sea 41.5 33.3 7.0 0.2 0.0 0.1 82.018 Brazil 26.0 41.1 21.9 10.2 0.3 0.1 99.419 North Sea 44.3 26.3 8.1 0.2 0.0 0.5 79.4

    1.5

    8.16.06.98.91.9

    110 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127The expression for the characteristic time of droplet approach wasmodied for a constantly increasing eld magnitude (Eq. (1)):

    8 1=3 dE0 2=3 5=31

    1=31

    20 France 23.1 47.5 1

    Replicates5 Mexico 43.3 29.58 Brazil 34.9 36.2 111 North Sea 44.4 30.215 Brazil 34.8 41.2 118 Brazil 26.1 40.7 2theo 5 dt 6W

    where is viscosity and is permittivity of the continuous phase.w is water cut (in this case 0.3) and dE0/dt is eld rate. The primitivemodel in Eq. (1) describes to a certain extent, though a simpliedforce balance, the dielectrophoretic effect and the resulting dragforce on spherical water droplets. Even by assuming monodispersityof the emulsion, perfect spherical and non-charged droplets, no

    Fig. 7. Correlation between modeled and measured amount of resins from partial least squarand IR (right) region [17]. Stars represent samples that were used to calibrate the model, andThe solid line is a linear least-squares regression calculated from the calibration set, and the1.46 wt.% with 6 PCs.contribution from interfacial dynamics of surfactants, and insigni-cant contribution of thermal or gravitational forces, Eq. 1 can onlymodel droplets separated by a distance much greater than their radi-us. When droplets approach each other this primitive model does notaccount for the nal approach of droplets, that is, lm-thinning

    5.4 0.1 4.2 91.7

    1.2 0.1 0.3 82.412.3 0.2 0.1 99.50.6 0.0 0.4 82.54.6 0.2 0.1 99.6

    10.6 0.3 0.1 99.3forces, neither concentrated and polydisperse systems, captured thedifference between the 30 crude oils reasonably well also at othereld rates (Fig. 10). It was proposed that the destabilization of staticwater-in-heavy crude oil emulsions in an electric eld was predomi-nantly retarded by the viscosity of the oil phase [21]. When dropletsapproach each other in an inhomogeneous electric eld the eldmagnitude increases greatly. Strong dielectrophoretic forces disinte-grate the lms and result in coalescence.

    es calibration (stars) or prediction (circles) models of spectral features in the NIR (left)open circles indicate unknown samples that were predicted with full cross-validation.broken line is the corresponding regression calculated from validation samples. RMSEP,

  • It was investigated how different kinds of modications on crudeoils, like deasphalting, dilution, and alkaline washing, affect the emul-sion stability [22,23]. Generally, for the ve crude oils, the emulsion sta-bility decreases as the viscosity is decreased bydilution. However, as thecrude oils are diluted, the concentration of surface active compoundslike resins and asphaltenes is also decreasing, which will also affectthe emulsion stability. Likewise, the interfacial tension and interfacialelasticity will also change as a result of the dilution. The most interest-ing information from Fig. 11 is what happens to crude oils 3 and 4. Athigh viscosity their corresponding emulsions show high stability, butas we increase the dilution (decreasing the viscosity), the stabilitydrops, and levels off at a certain level, independent of the viscosity. Inthis region, where the E-critical value is fairly stable, the line up of thewater droplets cannot be the limiting step for the break-up of theemulsion.

    The E-critical also depends on water cut; increases at lowerwater cut, due to the increased distances the droplets must moveto form linear chains between the two electrodes [22]. The samestudy showed in general, that the emulsion stability decreaseswith increased dilution, because by the diluting the crude oils, weare also decreasing the concentration of surface-active compoundslike resins and asphaltenes, which will affect emulsion stability.However, some systems show regions where the emulsion stability

    The type of emulsion obtained (w/o or o/w) depends on both thecontact angle of particles and the water cut [28]. Emulsions preparedwith Aerosil R7200 particles at different volume fraction showed thatat 50% v/v or more of water cut, o/w emulsions were formed. This isconsistent with the fact that Aerosil R7200 is preferentially wettedby the aqueous phase rather than the oil phase. The catastrophicphase inversion (inversion of the type of emulsion by changing theoil water ratio) happens at water cut close to 40% v/v. For lowerwater cuts, w/o emulsions are formed. There is no catastrophicphase inversion for emulsions stabilized by Aerosil R972 particles:w/o emulsions are formed for water cuts lower than 60% which isconsistent with the hydrophobic character of Aerosil R972, whereasfor water cuts N60%, there is phase separation into a water phaseand a w/o emulsion, reminiscent of a synaeresis phenomenon.

    Stabilizing properties of four different types of commercially avail-able silica nanoparticles (Aerosil from Degussa, Table 2) were stud-ied. Two of these products are hydrophobic; one is extremelyhydrophilic, and one is expected to be wet to an intermediate extentby both oil and water. These dry particles have been modied withasphaltenes and resins and we have investigated the performance ofthese solids as stabilizers in model oil/water emulsion systems [29].Adsorption studies, using the QCM-D technique, have shown theadsorbed amount from resin solution is dramatically smaller thanwhen asphaltenes contribute to the adsorption. The stabilization ef-ciency of the particles was explained from a thorough characterization

    111I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127is independent of the dilution ratio or viscosity of the crude oil. Theemulsion stability also shows temperature dependence according toArrhenius law. Viscosity and emulsion stabilities for water-in-oilemulsions were measured for the broad crude oil matrix (27crude oils) and results were analyzed by multivariate analysis. Ingeneral, there is an increase of the emulsion stability as the viscos-ity increases. However, viscosity also correlates with SARA data ofthe crude oils. This makes it difcult to conclude whether the vis-cosity is the important stabilization factor or if it is the heaviestcomponents in the crude oil. For the high viscous crude oils, the an-swer is probably both [22].

    It was proved that deasphalting crude oils resulted in very unsta-ble emulsions [23]. Emulsion stability is also strongly affected by re-moving acidic compounds from the crude oils. The polar resins playa very important role in stabilizing the asphaltenes. By removingthe most polar resins we have disturbed the interaction pattern be-tween the resins and the asphaltenes, and the asphaltenes left inthe crude oil have a much higher propensity to stabilize emulsions.This is in accordance with other study by Spiecker [24].

    Fig. 8. Predicted vs. measured plot from the PLS regression of emulsion stability (Merit)based on the NIR spectra [18]. Stars represent samples that were used to calibrate the

    model. The solid line is a linear least-squares regression calculated from this calibration set.3.2.1. Particle stabilized emulsionsParticle-stabilized emulsions are an important topic in petroleum

    science. Indeed there are several types of particles present in the pe-troleum uids such as reservoir particles (silica, clays), mineral scales(CaCO3, BaCO3, SrSO4, CaSO4), corrosion products (FeS, oxides) and soon. When exposed to crude oil, inorganic particles may be modiedby the adsorption of heavy crude oil components like resins andasphaltenes [25,26]. Consequently, you can have formation of petro-leum emulsions stabilized by particles. It has even been shown thatemulsions containing inorganic particles can be more stable thanthose stabilized by asphaltenes only [27]. To mimic particle-stabilizedemulsions encountered in petroleum production, we have studiedstabilizing properties of four different types of commercially availablesilica nanoparticles (aerosil from Degussa, Table 2).

    Fig. 9.Near-equilibrium(2.5 h after preparation) of the storage E and loss Emoduli deter-mined by dilational rheology at a diluted crude oil/brine (3.5% NaCl solution) interface as afunction of the bulk crude oil concentration and the aromaticity of the solvent. The crudeoil was diluted to different extents in heptanetoluenemixtures as indicated in the legend(vol.%H). Themeasurementswere carried outwith a constant applied frequency (0.1 Hz).The characteristics of the crude oil are reported in Hannisdal et al. [19].of their surface properties, including spectroscopy, contact angle

  • measurements () and zeta potential measurements. The stabilizationefciency was greatly enhanced by adsorption of crude oil componentsonto very hydrophilic or very hydrophobic silica. Unmodied silica par-ticles Aerosil 200 (w/s/air=14) did not act as good stabilizers due totheir very hydrophilic character whereas silica particles coated withresins (w/s/air=73, Fig. 12, top) or asphaltenes (w/s/air=84, lower)gave emulsions of large droplets which were very stable to coalescence(induced by a high centrifugal eld). However, when the particles weresuspended in the water phase prior to emulsication, emulsion stabil-ity was signicantly reduced. The stability of the emulsions decreasedprogressively with increasing volume fraction of the disperse phase,in line with increased drop size. Droplet size distributions of stableemulsions revealed that the total interfacial area of a system was di-rectly determined by the amount of particles present. Thus, the total in-terface area remained constant when changing the volume fraction ofthe disperse phase. Such observation is consistent with a mixingmech-anism characterized by both droplet fragmentation and coalescenceprocesses.

    that the yield stress decreases when the mass fraction of particle in-creases (for o/w emulsions, see Fig. 14). G and G display the sametrends (they decreases with the adding of hydrophobic particles)-

    Fig. 11. Emulsion stability, measured by means of critical electric eld technique, as a

    n an electric d.c. eld. Destabilization of a water-in-heavy crude oil emulsion under the in-left). Destabilization of 30 water-in-crude oil emulsions at a eld rate of 0.004 kV/cm s1

    o is theoretical value of droplet approach. The experimental time is given by the measured

    112 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 1061273.2.2. Rheology of particle-stabilized emulsionsTwo different particles were used: Aerosil 7200 (hydrophilic) and

    Aerosil 972 (hydrophobic). There were dispersed in solvents: Aero-sil 7200 in a NaCl 3.5% solution and Aerosil 972 in decane. The par-ticle concentration was kept constant at 25 g/L. The rheologicalproperties of the emulsionswere determined by steady shear and oscil-latory shear measurements using parallel plate geometry (PhysicaMCR301 Rheometer) [30]. First, it was investigated the rheologicalproperties of emulsions stabilized by a single type of particles (eitherhydrophilic or hydrophobic) then by mixtures of these particles. Dueto experimental requirements (emulsions must not sediment/creamin the rheometer cell), only o/w emulsions stabilized by Aerosil 7200particles and w/o stabilized by Aerosil 200 were characterized. Fig. 13presents a typical ow and oscillation curve. This emulsion exhibits ayield stress, a shear-thinning behavior and thixotropy. This system pre-sents the following properties: G is higher than G (about ten times)and G and G are constant over the whole investigated frequencyrange. All these rheological features are specic for gels, according tothe phenomenological denition proposed by Almdal et al. [31]. Thisgel character is consistentwith the yield stress previously found and in-dicates that there is formation of a network of connected droplets. Theobserved shear thinning behavior could be due to breaking of interac-tions between droplets and thixotropy to time needed to reform inter-actions. We have also investigated the properties of w/o emulsionsstabilized by R972 particles. They have similar features as the o/w emul-sions stabilized by Aerosil R7200 particles that is, presence of a yieldstress and time-dependant viscosity (either thixotropy or rheopexy).

    Fig. 10. Time for destabilization compared to the theoretical time for droplet approach iuence of a constant background electric eld with magnitude from 0.4 to 4.0 kV/cm(where exp is characteristic time of destabilization of emulsions in the d.c eld and the

    CEF value and dE0/dt whereas the theoretical time is predicted according to Eq. 1 (right). [2Their viscosity also increases with the dispersed phase volume fraction.To conclude, rheological properties of w/o emulsions stabilized by R972particles are similar to those of o/w emulsions stabilized by R7200 par-ticles. It also seems that, in the investigated systems, rheological proper-ties of particle-stabilized emulsions are related to the rheologicalproperties of the dispersions of their particles.

    So far we have only considered rheology of emulsions stabilizedby one type of particles. In this part we have investigated the prop-erties of emulsions stabilized by mixtures of hydrophilic and hydro-phobic particles. Emulsions were prepared by keeping the totalconcentration of particles constant but varying the mass ratio be-tween hydrophobic particle (Aerosil 972) and hydrophilic parti-cles (Aerosil 7200). There are formation of o/w emulsions formass fractions of hydrophobic particles lower than 0.9 whereas w/o emulsions are formed at higher mass fractions (Particle concen-tration=25 g.L1 and water cut=50% v/v). This transition is calledtransitional phase inversion [32]. Emulsions prepared with mixturesof particles present the same features as emulsions stabilized by asingle type of particles. In particular their ow curves exhibit yieldstress, shear-thinning behavior and thixotropy. It was observed

    function of the viscosity of the crude oil, for ve different oils. The viscosity was changedby dilution of the different crude oils by a heptanetoluene mixture (70:30 vol.%) [22].1].

  • results not shown. Finally, the stability of o/w emulsions against co-alescence decreases when the mass fraction of hydrophobic particleincreases. Fig. 15 presents a sketch of our view of structures of o/w emulsions stabilized by only hydrophilic particles and mixturesof hydrophilic and hydrophobic particles. With only hydrophilicparticles (left), aggregated silica particles are adsorbed at the liq-uid/liquid interface. There exists a network of particles connectingthe droplets to each other. This network explains the rheologicalproperties of these emulsions. When hydrophobic particles areadded (right), as the concentration of hydrophilic particles, the con-nections between droplets are looser since the hydrophobic parti-cles are preferentially dispersed in the oil phase. That explainswhy the rheological properties of these emulsions are weaker thanwith only hydrophilic particles. However we do not know if hydro-phobic particles displace the hydrophilic particles from the interface(right bottom of Fig. 15) or not (right top of Fig. 15).

    3.3. Asphalthene stabilized emulsions

    Asphalthenes are typically dened as the fraction of petroleum in-soluble in n-alkanes (typically heptane, but also hexane or pentane),but soluble in toluene, i.e. it is a solubility class. The molecules arecomposed of small polyaromatic parts linked by aliphaltic or naphte-nic moieties. They contain the major part of the heteroatoms (Nitro-gen, Oxygen and Sulfur) and metal atoms (Nickel, Vanadium)present in a crude oil. Moreover these molecules can associate in so-lution (in crude oils or in model solvents) to form aggregates with a(weight) average molar mass which can vary between 10,000 and1,000,000 g.mol1 in model solvents (such as toluene), dependingon thermodynamic conditions such as solvent nature, temperatureor pressure. In the crude oil, presence of asphaltenes induces the for-mation of very peculiar colloidal suspensions. It is commonly knownthat the asphaltenes precipitates when the crude oil is treated witha light aliphatic hydrocarbon.

    NIR and MIR spectra were correlated to the Hildebrand and Hansensolubility parameters, using multivariate data analysis [33,34]. Modelswere built from NIR and MIR spectra of different solvents and solventmixtures. Table 3 shows the results from the correlation of solubility pa-rameters to NIR spectra. Furthermore, the solubility parameters of SARAfractions and crude oils were predicted using the models developed

    Table 2Chemistry and specication of silica particles [52].

    Product id. After treated with Specic surface area (BET) (m2/g) Tapped density (g/L) Primary particle size (nm)

    Aerosil 200 Hydrophilic 20025 50 12Aerosil 7200 Hydrophilic 3-Methacryl-oxypropyl-trimethoxysilane 15025 230Aerosil 202 Hydrophobic Polydimethyl-siloxane 10025 60 14Aerosil 972 Hydrophobic Dimethyl-dichlorosilane 11025 50 16

    113I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127Fig. 12. Particle-stabilized emulsions of silica (Aerosil 200) coated with resins (top)and asphaltenes (lower). The ordinate axes show the amount of disperse phase re-solved after 10 min centrifugation at 1580g. The used solvents are water: NaCl 3.5%solution and oil: heptane/toluene 70/30 v/v [29].1 10 100 1000

    sh

    ear s

    tress

    / Pa

    1

    10

    100

    Shear rate / s-11 10 100 1000

    vis

    cosi

    ty /

    Pa.s

    0,010,1

    110

    100

    .

    shear rate / s-1

    .

    , G'' (

    Pa)

    1000

    G'G''Frequency (Hertz)1010,1

    G'

    100

    Fig. 13. Flow and oscillation curve of o/w emulsions stabilised by Aerosil 7200 particles,water cut=50% v/v. Particle concentration=25 g.L1.

  • depressurized in steps, and the resulting NIR spectra were recordedat each pressure level and analyzed with multivariate analysis. Theasphaltene precipitation onset pressure was identied from increasedoptical density due to light scattering. The reversibility of the asphal-tene aggregation was studied for the crude oil and a model system byrepressurizing the systems stepwise to the original pressure. The sys-tems were then left to equilibrate for several hours. In this model sys-

    1000100101

    Sh

    ear s

    tress

    / Pa

    1

    10

    100

    m R972=0 o/w

    m R972=0.2 o/w

    m R972=0.4 o/w

    m R972=0.5 o/w

    m R972=1 w/o

    .

    Shear rate / s-1

    Fig. 14. Flow curve (lled symbols: up curve, empty symbols: down curve) of emul-

    Table 3Results from PLS modeling and predictions based on the correlation of solubility pa-rameters to unmodied NIR spectra.

    Qualityparameters

    Hildebrand(total)

    Dispersiveparameter

    Hydrogenbonding

    Polarcontribution

    Principalcomponents

    4 4 2 4

    Correlationvalidation

    0.92 0.88 0.95 0.95

    Correlationprediction

    0.92 0.53 0.91 0.78

    RMSEV (MPa1/2) 1.8 0.6 1.7 1.1RMSEP (MPa1/2) 2.6 1.1 3.5 3.8ExplainedY-variance (%)

    87 81 90 89

    ExplainedX-variance (%)

    80 84 67 82

    114 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127(Table 4). This study proved that IR and NIR spectra in general can becorrelated to Hansen solubility parameters. Furthermore, IR and NIRspectra can be used to distinguish between crude oils and crude oilcomponents.

    Aliphatic solvent conditions and pressure reductions will increaseasphaltene aggregation size. Under increasingly unfavorable solventcondition, asphaltenes aggregate and eventually precipitate as largeasphaltene occulates. The asphaltene precipitation onset gives im-

    sions stabilized by mixtures of Aerosil R7200 and R972 particles (v/v water=50% v/v, total particle concentration=25 g.L1) tted by the HerschelBulkley equation(solid line: up curve, dash line: down curve).portant information about the solubility of the asphaltenes in agiven hydrocarbon system. Aske and co-workers [35] have investigat-ed the asphaltene aggregation behavior from crude oils and modelsystems under high pressure (300 bar). The systems were then

    Hydrophilic particles Hydrophobic particles

    water oil

    Only hydrophilic particles

    Fig. 15. Sketch of structure of o/w emulsions stabilized by only hydrophilic partem the asphaltene aggregation was only partially reversible.However, the possibility that the aggregation of the model systemasphaltene is reversible if even more time is allowed for equilibrationcannot be excluded.

    In order to hinder asphaltene deposition, the petroleum industryinjects large volumes of chemicals into reservoirs and pipelines.These chemicals are supposed to imitate the indigenous resin frac-tion, by dispersing the asphaltenes in the hydrocarbon mixture as dis-cussed earlier in this chapter. In addition to the direct problemsconcerning asphaltene deposition in process equipment, the stabilityof water-in-oil emulsions will be strongly dependent on the asphal-tene aggregation size and solubility state [36,37]. The results showedthat additives, which are efcient in replacing hydrogen bonds inasphaltene aggregates, possess dispersive power and can serve as in-hibitors. Recently, in a study concerning the potential for hydrateplugging, crude oils washed with a strong alkaline solution (pH 14)showed much higher water-in-oil emulsion stability than the original

    water

    Mixture hydrophobic/hydrophilic oil

    or

    ticles (left) and mixtures of hydrophilic and hydrophobic particles (right).

  • crude oils [38]. The pH 14 wash extracts the most polar resins, typi-cally napthenic acids and phenols. Removing them should cause theasphaltenes to precipitate earlier when titrating the crude oil withan n-alkane.

    3.3.1. Asphaltenes precipitated by multi-step precipitation procedureAsphaltenes were precipitated into two fractions using a two-step

    precipitation [39] procedure and later extended to separate asphal-tenes in 4 fractions [40]. In the two/step precipitation procedure,the rst fraction was obtained by mixing 3:1 volumes of n-pentane/crude oil followed by ltration. In the following step the second frac-

    2, the 10:1 middle fraction exhibited the highest interfacial activity,measured after 12 h. The measured interfacial activity at the end ofthe experiments (equilibrium value) was plotted as a function ofthe n-pentane-to-crude oil ratio (Fig. 17). It was neither the leastnor the most soluble fractions, within the fractions studied, whichwere the most interfacially active compounds. This is in fact verywell in accordance with general surfactant chemistry. For a com-pound to be interfacially active, it has to have both water solubleand oil soluble parts. This principle might be transferred to these re-sults by suggesting that it is neither the least nor the most solublefraction that is the more interfacial active, but a middle fraction. Thecommon 40:1 fraction may mask important features of asphaltenesresulting in erroneous assumptions based on properties studies andthe structure elucidation of the asphaltenes. This has large technical

    115I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127tion was precipitated out from the ltrate using 18:1 volumes of n-pentane/crude oil. Whole asphaltenes were also precipitated using a40:1 n-pentane-to-crude oil ratio. The amount of each fraction wasdetermined for comparison with the whole precipitated asphaltenes.Three crude oils (named WA, NS-A and NS-B) were used and theasphaltene fractions obtained were characterized with regard toonset of precipitation, interfacial tension (o/w) and radius of gyration(RG) of the aggregates. It was proved that asphaltenes precipitated atthe 3:1 dilution ratio (rst fractions) had a faster initial reduction ofthe o/w, while the second fractions (18:1 dilution ratio) led to alower o/w over time. Whole asphaltenes had a fast initial reductionlike the rst fractions and reduced the value of the interfacial tensionmore, indicating that there were also compounds which will not in-uence the value of o/w after a longer period of time. The SANS mea-surements showed that the aggregates of the rst fractions werelarger than the aggregates of the second fractions (Table 5). Precipita-tion procedure used was suitable for fractionating asphaltenes by adirect stepwise precipitation from the crude oils without rst precip-itating the whole fraction. It was also shown that the solvent proper-ties of the two solubility fractions were quite different [39]. The rstfraction that precipitated upon addition of a small amount of n-pentane was less soluble in the precipitation onset experiments,formed larger aggregates and had a different (lower) interfacial activ-ity as compared to the second, more soluble fraction. The assump-tions made, based on the results, were that the rst fraction wouldcontain molecules with a higher molecular weight, and be morepolar and more aromatic. In next study, Fossen et al. [41] character-ized and quantied the functional groups, aromaticity, polarity andsize of the asphaltene solubility fractions precipitated before. Whatis important to keep in mind is that asphaltenes are mixtures of thou-sands of different, yet relatively similar compounds. It means thatvalues of these measurements will result in average values of all thecompounds in the asphaltene sample. It was shown that the relativeamounts of the polar heteroatoms S, N and O were slightly higher inthe rst (least soluble) fractions than in the second fractions andwhole asphaltenes. LDI-MS showed that the average molecularweight is higher for the rst fractions compared to the second frac-tions the whole asphaltenes. There were no indications from the re-sults that the alkyl chains substituted on the aromatic were longerfor the second fraction.

    The second fractions were also more substituted. All the parame-ters obtained are an average of the molecular mixture in the samples,thus there are no guarantee that the relative differences commented

    Table 4Results from prediction of the solubility parameters on the SARA fractions presented asthe lowest and highest predicted value.

    Fraction Hildebrand Dispersive Hydrogen Polar

    1/2 1/2 1/2 1/2

    [MPa] [MPa] [MPa] [MPa]

    Saturates 16.416.7 16.216.4 4.75.6 0.00.7Aromatics 16.716.9 16.416.6 4.95.4 0.71.0Resins 17.318.6 15.616.4 5.98.1 1.22.3Asphaltenes 18.019.1 15.015.8 7.610.3 1.23.2upon in this work are the ones responsible for the properties deter-mined in the previous work. For example comparing the aromaticityof the rst and second fractions one nds, according to the assump-tions, that for the WA and NS-B the less soluble fraction has a higheraromaticity, while for the NS-A the aromaticities are equal for the twosub-fractions [41]. The trends found that support the assumptions arethat the less soluble fractions which formed larger aggregates (SANS)were less interfacial active (pendant drop) were more aromatic, morepolar (in the aromatic core) and had a higher average molecularweight. The second fractions had alkyl groups that were probablymore branched and contained a somewhat larger portion of naphte-nic rings and had more of the hydroxyl and carboxylic groups onthe aliphatic parts which could explain the higher interfacial activity.

    There is probably no single reason for the precipitation of asphal-tene that can be explained by the molecular structure or weight. This,and the previous results, indicates that it is of great interest and im-portance to fractionate asphaltenes into less and more soluble asphal-tenes. Furthermore, it is not yet determined what the ultimatecharacter of the least soluble asphaltene fraction is, and if this fractionis the most harmful with regard to adsorbtion to surfaces and emul-sion stability. Fossen et al. [40] extended the two-step precipitationto a four-step precipitation procedure where it was shown that theinterfacial activity was not linearly dependent on the order of the sol-ubility fraction. The intention here was to further investigate whichpart of the second fraction was more interfacially active. The precipi-tation procedure was principally similar to procedure before, onlyhere with four dilution and ltration steps. Precipitation followedby inter-step ltration off of the precipitated material was performedstep-wise after an addition of 3:1, 10:1, 15:1 and 20:1 n-pentane tocrude oil, named fraction 1, 2, 3 and 4 respectively. These fractionswere analyzed with regard to precipitation onset in toluene/heptanemixtures and interfacial tension (Fig. 16). Two oils were tested inthis study, the NS-A and the NS-B crude oils. The results from the pre-cipitation onset experiments verify that the solubility of the fractionsis in the order of which they were precipitated.

    Fig. 16 shows a great difference in the interfacial tensions for thedifferent fractions from both crude oils. It is remarkable that fraction

    Table 5Calculated RG from the SANSmeasurements using the Guinier approximation, whereWA,NS-A and NS-B are names of crude oils. The uncertainty of the instrument is1 . N.D.(not determined) indicating the values were outside the detection limit for the instru-ment. This meant that these samples contained aggregates which were much larger(N700 ) than the other samples.

    Asphaltene fraction RG()

    WA rst 30WA second 25NS-A rst N.D.NS-A second N.D.NS-B rst 26NS0-B second 21consequences within crude oil production and processing.

  • 3.4. Polyaromatic surfactants as model compounds for asphaltenes

    Asphaltenes as a solubility class comprise a very broad distributionof chemical structures. Several studies have investigated the asphalteneaverage structure and physico-chemical properties to understand theirbehavior as a function of chemical structures [39,40]. Oneway to do thisis to divide the asphaltenes into several subfractions, but each subfrac-tion contains thousands of different compounds, and a never-endingapproach is to keep on dividing into numerous fractions to get morenarrow structure distributions. Nordgrd et al. [42] used a different ap-proach. They rst synthesized molecules with known structures andsmall structural variations, and correlate the physico-chemical proper-ties obtained directly to introduction of functional groups. Fig. 18shows these synthesized compounds as polyaromatic surfactants,with size andmolecular weights in the same region as the currently ac-cepted average molecular weight as asphaltenes. The molecular designis one xed part of themolecule and one part with varying hydrophilic-ity. Also, three of themolecules incorporated an acidic groupwhichwasexpected to increase the interfacial activity of such compounds. Table 6presents interfacial tensions measured with the pendant drop tech-nique ans shows that the acidic molecules were highly interfacially ac-tive at low concentrations in toluene towards a pH 9 aqueous solution.The absence of the acidic group however resulted in a total absence ofinterfacial activity at corresponding concentrations. Pressure-area iso-therms using the Langmuir technique (Fig. 19) showed that the acidiccompounds formed stable monolayer lms with high collapse pres-sures, and themoleculeswere arrangedwith the aromatic cores normalto the aqueous surface, yielding a sheet-like arrangement in the lmresulting in highly favorable aromatic interactions. The non-acidic com-pound did not show this arrangement and did not form a stable mono-layer lm. The studies showed that the presence of an acidic group insuch compounds were essential for their interfacial and lm properties,

    micrometer-size structure, and the nal large network gel. Crystalli-zation of parafns also depends on physical factors (cooling rate,shear force and so on). The length of the crystals is dependent onthe temperature and cooling rate. The crystal aggregation is alsovery sensitive to the shear rate. At static condition, individual disksform a colloidal network. At high shear rates, the aggregates becomemore spherical in shape and less polydisperse.

    Wax precipitation and deposition is a recurring challenge in trans-portation of crude oil, and increased knowledge about the behavior ofsuch systems is necessary. Microscopy, rheometry and DifferentialScanning Calorimetry (DSC) were used to follow the crystallizationof wax for two model systems. Chen et al. [44] investigated the owand viscoelastic behavior around the wax precipitation temperature,and the yield stress was determined both after dynamic and staticcooling. They proved that long crystals were formed during low cool-ing rate, resulting in the strongest gel structures. The gels wereformed at very low amounts of solid wax crystals (0.30.4%), andwax precipitation was promoted by increased wax content dissolved

    116 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127Fig. 16. Interfacial tensions of 100 mg/L solutions of asphaltene fractions from NS-Aand NS-B crude oils in toluene. Water phase is pH 7 buffered water containing

    3.5 wt.% NaCl [39].and the unusual interfacial activitywere attributed to the formation of astable lmwhich were stabilized by aromatic interactions due to an ar-rangement with the aromatic cores normal to the surface plane.

    3.5. Wax in crude oils

    The presence of long-chain saturated alkanes in crude oil can leadto severe problems associated with wax precipitation and depositionin petroleum transport pipelines and processing equipment. Indeed,the parafn deposition on the cold walls of pipelines will restrictthe ow, and in the worst case entirely plug the pipelines. Secondly,presence of solid waxes in continuous oil systems may give rise tothe difculty of prediction and evaluation of the ow propertieswhich is largely dependent on the waxy constituents of the oils.Waxes are typically classied molecular weight isoparafns and cy-cloparafns. Crude oils generally contain not only n-parafns butalso considerable amounts of isoparafns and cyclic compounds,which in fact constitute the largest fraction. Visintin et al. [43] foundthat n-parafn dissolved in organic solvents display a sharp transitionin gel strength at the pour point, whereas by addition of isoparafns,the buildup in gel strength as a function of temperature is much moregradual, because increasing isoparafn fraction facilitates the forma-tion of amorphous wax solids. At high temperatures, waxes are inthe molten state, and crude oils normally behave like Newtonian liq-uids. When the temperature drops below Wax Precipitation Temper-ature (WPT), solid wax crystals precipitate out of oils and form a gel.In the denition of wax crystallization at a more microscopic level,the gelation mechanisms involves three different processes: forma-tion of lamellar with thicknesses of ca. 1.53 nm, sheet-like crystals,

    Fig. 17. The plot shows the interfacial tension of each fraction at the end of the exper-iment time. The second fraction (second points on curves), that is the fraction precip-itated after the 3:1 fraction was taken out of the solution and when a 10:1 ratio ofpentane-to-crude oil was added, was the most interfacially active compound in theseexperiments [39].in the samples. Dynamic cooling conditions decreased the gel

  • halt

    117I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127strengths considerably as the imposed shear forces affect both crystalmorphology and crystalcrystal interactions. Fig. 20 shows the differ-ence between wax A and B is further underlined by looking at the vis-cosity at different amounts of precipitated wax. Wax A has steepincrease in viscosity from 0.25 to 0.35 vol.%, as wax precipitates andform a gel. Around 0.2 vol.%, only a small inection point is seen forwax B and the increase in viscosity is very small. Clearly, there are

    Fig. 18. Structures and molecular weights of four synthesized model compounds for aspbeen varied inserting different amines [42].no strong interactions between the wax crystals in this case, andthe network formation is not signicant when the wax amount isless than 1%. In this study it was also investigated the effects ofwater cut, asphaltene amount and wax amount, and the results inter-preted in terms of microstructure and aggregate state of wax crystalsand water droplets.

    Fig. 21 proved that the yield stress increases signicantly withmore wax in the oil phase and also as the water cut increased. Thevolume fraction of the dispersed phase is known to be a very impor-tant variable in emulsion rheology, and the relative viscosity of thesuspension with respect to the pure uid is often well described bythe Krieger and Dougherty equation. Wax content and water cut hasthe most pronounced effect upon the viscosity and yield stress ofthe systems. The asphaltene content also result in increased viscosityand yield stress values, but to a lesser degree than the previousvariables.

    A new analyticalmethodwas developed for investigatingwax depo-sition in petroleum systems in our group. The technique is based on aquartz crystal microbalance which has been modied to accommodate

    Table 6Interfacial tension of asphaltene model compounds at 50 M after 500 s between tolu-ene and pH 9.

    Compound (IFT)

    [mN/m]C5 Pe 1PAP 4TP 4BisA 36a temperature gradient in the direction normal to themeasuring surfaceaswell as a continuous ow of uid above the crystal. The instrument iscalled a Thermal Gradient Quartz Crystal Microbalance (TG-QCM), andprovides a very sensitive probe of the mass and physical properties ofmaterial which adheres to a deposition surface by recording changesin the resonance frequency and energy dissipation of a quartz crystalresonator. The TG-QCM method is demonstrated using the model

    enes. The right side of the aromatic core has been kept constant, while the left side haswaxy oil which consists of a rened parafnwax dissolved in dodecane.Wax deposition is performed on quartz crystals coated with gold, stain-less steel, and polyethylene. It is shown that rigid deposit formation andgel deposition are separate and distinct phenomena, both of which cancontribute to the formation of incipient wax deposits. Rigid depositlayers grow slowly, and their formation depends on the wettability ofthe solid substrate material. Gel deposits, on the other hand, form rap-idly on all surfaces at high wax concentrations, and contain a largeamount of occluded oil.

    A special uid chamber was designed and built with the capabilityto impose a temperature gradient in the uid regions above the

    Fig. 19. Pressure-area Langmuir isotherms of the model compounds at room tempera-ture. The probe was a Wilhelmy plate and the barrier speed was 5 mm/min. The sub-phase consisted of pure Milli-Q water at pH 5.8 [42].

  • 14 mm piezoelectric quartz crystal [45]. Fig. 22 shows a conceptualcartoon of the thermal gradient QCM chamber. The uid chamber ispositioned between a lower thermal block and an upper thermal

    block, such that an applied temperature difference between theupper and lower blocks results in a temperature gradient in the ver-tical direction. The uid chamber is constructed from Teon and hasan upright cylindrical geometry, with a height of 10 mm and a diam-eter of 20 mm. The measuring quartz crystal is positioned on thelower block, such that the coated side of the quartz crystal is in con-

    Fig. 20. The viscosity of wax A and B (20% in decane) at low fractions of precipitatedcrystals [44].

    Fluid Chamber

    Lower

    Quartz Crystal

    Upper

    Fluid Outlet

    Fluid Inlet

    Fig. 22. Conceptual cartoon of the TG-QCM chamber [45].

    118 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127Fig. 21. a) Yield stress at 20 C and 30 C for emulsions with different wax content. Thedotted lines represent the t to the power law equation. b) Yield stress at 20 C foremulsions with different water cuts [44].tact with the uid chamber. The lower block is in thermal communi-cation with the piezoelectric quartz crystal through a thermallyconducting crystal support body. The temperature of the lowerblock is regulated via software using a Peltier element which transfersthermal energy to a radiator heat sink cooled by a fan. The upperblock contains a heating element which is also in thermal communi-cation with the uid chamber, and is controlled manually from an ex-ternal temperature control unit. Fluid ow into the measurementchamber is driven by an external peristaltic pump.

    Before each experiment, the chamber and tubing are rinsed withlarge amounts of ethanol and/or toluene, followed by drying. Subse-quently, the measurement chamber is lled with dodecane ormodel uid, and specied temperature values are set in the upperand lower blocks. In each experiment, resonance frequencies and dis-sipation factors are monitored for the 1st (fundamental), 3rd, 5th,7th, 9th, and 11th harmonic overtones (Figs. 23 and 24). After stablebaselines are obtained, wax deposition is initiated on the quartz crys-tal surface by pumping the waxy model oil through the measurementchamber at a constant volumetric ow rate.

    Rigid incipient wax layers are sufciently thin to be probed by theacoustic shear wave propogating across the quartz crystal during os-cillation [46], and are evidenced by scalable reductions in resonancefrequencies measured at various harmonic overtones. Surface wetta-bility is shown to have a large effect on the formation of rigid waxlayers. Adsorbed water lms on stainless steel completely preventthe formation of wax deposits at low super-saturation conditions,due to interfacial hydrophobic forces. Gel deposits, on the other

    200

    ft (10

    -6 )

    3rd Overtone5th Overtone7th Overtone9th Overtone0

    100

    0 50 100 150 200Time (min)

    Dis

    sipa

    tion

    Shi 11th Overtone

    Fig. 23. Dissipation factor shifts measured during deposition of the 10 wt.% wax modeluid unto a stainless steel surface. The lower block temperature is 20 C, and the uidreservoir temperature is 40 C [45].

  • hand, form on all surfaces at high wax contents and contain a largeamount of occluded oil. The observed role of hydrophobic forces in re-pelling the formation of rigid wax layers has signicant implicationsfor the mechanism of incipient wax deposition in standard stainlesssteel pipelines. Therefore, incipient wax deposits must form by gela-tion instead of surface crystallization. Additionally, depending onthe shear conditions present, wax deposits may not always form onstainless steel surfaces at low under-cooling conditions, affordingpipeline operators greater latitude in avoiding wax deposit formation.

    The wax deposits can be monitored and visualized by AFM. Vari-ous modulation techniques can be used when AFM experiments arecarried out and measure the attracting/repulsive forces arisingwhen two bodies are close to each other. The force is measured byusing a cantilever with a certain spring constant (0.01 to100 Nm1) that is pulled towards the investigated surface in caseof attraction, or pushed up in case of repulsion from the investigatedsurface. Force spectroscopy, that is moving the cantilever in the z di-rection with no movement in the xy plane, is used for measuring dif-ferent surface mechanical properties. There are three main types of

    randomly clustered structure which closely resembles the substratepolyethylene material, which has possibly been modied by a thinlayer of deposited cyclic or branched parafns. The AFM-measuredwax layer thickness of 270 nm is in reasonable agreement with the225 nm average layer thickness derived from impedance analysismodeling of the TGQCM measurements performed during thedeposition.

    3.6. Naphthenic acids and phase diagrams

    It is well known that oil-continuous emulsions can be stabilized bymultiple layers of surfactant instead of only a monolayer [49]. In anequilibrium situation this corresponds to a sample location in athree-phase area where two solution phases (L1 and L2) are in equi-librium with a lamellar liquid crystalline phase (so-called D phase).This situation is of relevance in crude oil systems with high levels ofnaphthenic acids. In order to simulate the situation in high asphalte-nic crude we also combined the D-phase stabilization with asphaltene

    Fig. 25. Shaded topography image obtained using TappingModeTM for the dried waxlayer deposited from the 10% Sasolwax 5405 solution onto QCM crystal: a) gold-coated,b) polyethylene-coated. The depth of the observed indentation toward the bottom ofthe image is approximately 0.4 m.

    119I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127tip mode like contact, non-contact and tapping mode [47].The technique has also been used for investigating the effect of

    resins and asphaltene inhibitors on the precipitation of colloidalasphaltene [48]. In this study monolayers of asphaltenes and resinswere transferred onto mica substrates using the L-B technique, andthe topography analyzed by means of AFM. This work shows thestructural change in the monolayer when the composition of thelm was gradually changed from pure asphaltenes to pure resins. Pic-tures of pure asphaltene show a closed-packed layer of round disks orrod formed units. Addition of resins will change this rigid structuretowards a more open network with regions completely uncoveredby lm material. Pure resins build up a layer with an open structure,i.e., more like a fractal pattern. Fig. 25 illustrates the morphologicalimages of wax deposits and substrate materials which were obtainin TappingModeTM probes. Lamellar contours are clearly visible inthe image, and correspond to the growth steps of the wax crystalsformed during the deposition process. However, because the gellayer has likely collapsed during the drying period in which the dode-cane solvent evaporated, the imaged deposit morphology is unrepre-sentative of the initial gel structure. Similar images were obtained atvarious positions on the crystal, indicating homogenous surface cov-erage of the initial gel layer. In case of wax deposition on the polyeth-ylene-coated crystal it was observed two distinct recurring domains.The rst recurring domain is a clearly observed rigid wax layer,appearing as interlocking corrugated layers of solid wax, as depictedon the right hand side of gure. The second recurring domain, asdepicted on the left hand side of gure, is a less well dened

    -200

    -100

    0

    100

    0 50 100 150 200Time (min)

    delta

    F (H

    z)

    3rd Overtone5th Overtone7th Overtone9th Overtone11th Overtone

    Fig. 24. Normalized resonance frequency shifts measured during deposition of the 10wt.% wax model uid unto a stainless steel surface. The lower block temperature is

    20 C, and the uid reservoir temperature is 40 C [45]. particles. Lowmolecular weight naphthenic acids have relatively high

  • water solubility compared to other crude oil components. High mo-lecular weight naphthenic acids are interfacially active and willform metal salts at the oilwater interface [50,51]. This leads to pro-duction of acidic waste water, which represents a major environmen-tal problem. It has been shown that a lamellar liquid crystalline phase(LLC) might act as an efcient emulsier when present in a three-phase system The LLC phase will cover the emulsion droplets andlead to reduced interfacial mobility and bending ability. Presence ofother organized structures in a multiphase systemmight have the op-posite effect with regard to emulsion stability. In contrast to lamellarliquid crystals, microemulsions will reduce the emulsion stability.

    Knowledge about phase behavior in an oilsurfactantwater systemis of crucial importance when studying the mechanisms behind emul-sion formation and stability in such a system. Hager and coworkers[52], using model naphthenic acids, proved increasing pH decreasesthe lipophilicity of the acid species and may induce a transition fromw/o emulsion to an o/w emulsion. There were observed strong compe-tition between salinity effect and pH effect especially in case of systemswith anionic surfactant included fatty acids. Stable w/o emulsions werefound at a pH close to 8. All emulsions were shown to be stabilized by aliquid gel phase consisting of a lamellar structure.

    Many ternary systems based on a fatty acid (CnCOOH) and long-chain alcohol (CnOH) have been thoroughly investigated in past

    pacity was obviously reduced. Hager et al. [55] also investigated theeffect of the ratio between the undissociated (RCOOH) and the disso-ciated (RCOO~) acid. It was detected the solubilization limit of theacid around 10 wt.%. Increasing the concentration of the surfactantleads to formation of the hexagonal (E) phase. Addition of moreacid leads to a transition to a D phase region above 40 wt.% surfactant.It means that replacing the surfactant by the acid gave only normalmicellar and hexagonal structures. The poor phase behavior of thesystem can be explained in terms of a low ability of the acid to solu-bilisate into the different self-assembly structures.

    3.6.1. Langmuir lm propertiesA rigid interface on the emulsion droplets prevents coalescence

    while a highly compressible lm is more easily ruptured, leavingthe droplets free to coalesce. By means of the Langmuir technique,asphaltenes are found to build up close-packed rigid lms, whichgive rise to quite high surface pressures. Resin lms, on the otherhand, are considerably more compressible [56]. This may explainthe experimental observations showing that asphaltenes are able tostabilize crude oil based emulsions, while resins alone fail to do so.The more hydrophilic resin fraction starts to dominate the lm prop-erties due to the higher afnity towards the surface. Highly compress-ible resin lms alone will not stabilize a crude oil emulsion. Related tothis, demulsiers, which form lms of low rigidity and high com-

    120 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127years. However, there are only a few reports where the phase behaviorhas been studied in systems based on polar aromatic acids or alcohols.Horvath-Szabo and coworkers [53,54] have studied the phase equilib-ria in a sodium naphthanatewater system and sodium naphthanatewaterhydrocarbon systems. The inuence on phase diagrams ofmodel compounds for naphthenic acids and phenols when mixedwith water were studied based on the compounds are 5-phenylvalericacid, 5-phenylvalerate, 1-decanol, and 4-pentylphenol [55]. Fig. 26 il-lustrates an extended isotropic o/wmicellar (L1) phase exists up to ap-proximately 45 wt.% surfactant along the binary surfactantwater axis.In the dilute aqueous regime only monomer exists. Addition of morealcohol leads to a turbid lamellar liquid crystalline (D) phase, whichwas identied by microscopic investigation (Fig. 27 (a) and (b)). Thetypical texture of Maltese crosses and oily steaks is an indication ofthe lamellar structure. The reason for this transformation is that the al-cohol gives the aggregates a more hydrophobic character and reducesthe charge density of the polar surface layer, which will favor the for-mation of the lamellar bilayer structure. This D phase has a very highswelling capacity, i.e., it has a large extension towards the water

    Fig. 26. Partial phase diagram based on weight fraction of the ternary system based on

    5-phenylvalerate, 4-pentylphenol, and water at 25 C.apex. The swelling of a lamellar phase can be explained by a differencein chemical potential of pure water and water incorporated into the in-tervening layers of the lamellae. A lamellar phase in the producedwater of a crude oil may have a large impact of the elements that con-taminate the water stream. That the lamellar phase is formed at lowconcentrations is important information when treating productionwater. In the alcohol-rich comer of the phase diagram the reverse w/o micellar (L2) phase exists. Also a reverse hexagonal (F) structure isformed between the D and L2 phase. This phase is found at the surfac-tant content around 20 wt.%. Microscopic investigation identied theanisotropic phase on the basis of the characteristic optical fan-like tex-ture (Fig. 27 (c)). The two-phase region between the D and F phasewas also recorded by polarizing microscopy (Fig. 27 (d)).

    The phase behavior was found to depend on salinity of the system.The system in the presence of salt was shown to signicantly changethe character of the lamellar phase. Its region of existence was muchsmaller than that the system in the absence of salt, i.e., its swelling ca-

    Fig. 27. Light microscopymicrographs showing the (a)Maltese cross and (b) oily steak tex-tures of the lamellar liquid crystalline (D) phase based on5-phenylvalerate, 4-pentylphenol,and water at 25 C. The fan-like texture in (c) is an indication of the hexagonal liquid crys-talline (E or F) phase. The two-phase region (d) consisted of a lamellar and hexagonalphase; both Maltese cross and fan-like textures were observed, respectively.pressibility, should be most efcient [57]. When used as demulsiers,

  • the efciency depends on the ability of the chemicals to interact withand modify the lm built up by asphaltene particles. Addition ofdemulsiers of high molecular weight in the asphaltene lm gavethe isotherms (Fig. 28). In this gure, Chemical G is highly effectivewith respect to increased compressibility together with a reduced ri-gidity. The efciency depends not only on the direct inuence ofchemical additives within the lm, but also on the ability of demulsi-ers to reach the w/o interface in an emulsion (diffusion through theuid). This is a critical step regarding the effective concentration ofdemulsiers at the interface. The results obtained from the Langmuirinterfacial lm studies are important in explaining why certain che-micals are more effective as inhibitors than as demulsiers. Obviouslythe inhibitor/asphaltene interaction is so strong in the bulk oil phasethat the interfacial structures being gradually built up will no longerpossess properties required to stabilize w/o emulsions.

    Fig. 29 illustrates interfacial pressure isotherms of lms formed be-

    ship based on an appropriate lm drainage time, the natural formationof the dense packed layer is determined. This model represents a gen-eral approach to modeling batch gravity separation based on rstprinciples and provides the methodology to incorporate various ef-fects of the bulk uid properties, interfacial properties, properties ofsurfactants, and the cumulative squeezing force encountered in a sub-stantially thick layer of densely packed droplets.

    3.7.1. Model descriptionAs discussed above, the model is based on a population balance

    framework. This implies that the drop-size distribution is representedby a continuous density function that represents the average numberof droplets within an innitesimal droplet volume element and inn-itesimal axial position element. The consequence of this denition isthe ability to express the motion of droplets through real space bytheir sedimentation velocity while the coalescence process can berepresented as a birth-death process where the droplet volume is

    121I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127tween water containing different types of particles and an oil phase ofpure decane determined with the Langmuir technique. The oil phaseconsists of pure decane while 1 wt.% particles are added to the waterphase. One of the particle fractions (type I) forms a relatively rigid lmat the interface,while theother (type II) ismore hydrophilic and remainsdispersed in the aqueous phase. Studies of these particles ability to stabi-lize emulsions have shown that type I particles are highly efcient emul-siers, while type II particles will not form stable emulsion [58].

    3.7. Separation modeling

    The separation of liquidliquid dispersions in the form of crude-oil and water emulsions is a particularly crucial process in the petro-leum industry, and the development of a fundamental understandingand elucidation of this process is becoming more important as exist-ing oil elds mature and newer elds with increasingly heavier oilsare relied upon to meet global demands for energy. Curiously, de-spite the long history of large scale oilwater separation processes,modeling of the emulsion gravity separation process has been limit-ed [5968] and rarely elucidates the effect of the poly-dispersity ofthe emulsion drop-size distribution [6568]. Additionally, Grimes etal. [69] have demonstrated that surface active compounds can havenon-uniform local concentrations in a separating emulsion whichcan dynamically evolve with time; such a mechanism can have sig-nicant implications with respect to the chemical additives as wellas indigenous compounds of a crude oil, particularly in terms ofcompounds that stabilize emulsions or cause scaling and foulingwhich are prevalent in heavy crude oils. Consequently, there still isneed for an emulsion gravity separation model that properly ac-counts for the effect of the poly-dispersity of the emulsion, whileemploying physically meaningful, mechanistic models for sedimenta-tion, coalescence, and droplet collision that can be tied directly to thelocal concentrations of key interfacially active molecules.

    Fig. 28. A isotherms of mixed monolayers of asphaltenes and two different demul-sifers on pure water. The mixed monolayers are compared with the pure asphaltene

    and the pure resin lm.In the following paragraphs, a new, fundamental approach [70,71]for modeling the batch separation of emulsions in terms of the popu-lation balance equation [70,72,73] is discussed. Readers interested inthe detailed formulation of the model and development of the systemequations are directed to Grimes [70]. The formulation of this model[70] elucidates the methodology of incorporating the physical proper-ties of the bulk liquids as well as the physical properties of the phaseinterface by formulating the coalescence closure relationships in termsof coalescence times obtained from lm drainage models [7479] thatappropriately describe the specic emulsion system as well as colli-sion frequencies determined by the simultaneous incorporation of dif-ferential sedimentation and Brownian motion; it should be noted thatthe appropriate lm drainage model should be chosen to best describethe specic oilwater system, e.g., one must consider the nature of theadsorbed surfactant layer and determine if interfacial mobility is im-portant and identify the physical mechanisms that contribute to thedisjoining pressure while the physical properties of the uids dictatewhether signicant dimpling of the liquid lm occurs [7679], or ifthe liquid lm can be satisfactorily modeled with a parallel disk lmdrainage model [74,75]. This approach [70] can satisfactorily incorpo-rate both the physical properties of the bulk uids as well as the phys-ical properties of the phase interface to describe the binarycoalescence phenomena. Additionally, interfacial coalescence isaccounted for in terms of the coalescence time obtained from appro-priate lm drainage expressions and, critically, a method is presentedthat explicitly accounts for the deformation of the emulsion zone dueto the dynamic growth of the volume of the resolved dispersed phase.Furthermore, hydrodynamically hindered sedimentation is consideredand, when combined with an interfacial coalescence closure relation-

    Fig. 29. Interfacial pressure isotherms of lms formed between water containing differ-ent types of particles and an oil phase of pure decane.conserved upon each coalescence event [72]. Furthermore, measureable

  • oil B and the demulsier additive will be referred to as chemical 7.Model simulations are compared to experimental data for the separa-tion of crude oil B emulsions when the concentration of chemical 7 is10 ppm and 50 ppm. Table 7 lists the model parameters for the sepa-ration system having a concentration of chemical 7 at 10 ppm whileTable 8 lists the model parameters for the system having a concentra-tion of chemical 7 at 50 ppm.

    In Fig. 30, the initial volume and number density distribution ofthe droplets is given. The volume density distribution was obtaineddirectly from the NMR measurements and the number density wasconverted directly from the measured volume density by conserv-ing the fraction of the area under the curve at the mean droplet ra-dius and median droplet radius. The average droplet size by volumewas determined from the st moment of the volume density and is11.9 m. The characteristic radius, dened as the value of the drop-let radius where the integral of the initial volume density is 0.990,is 20.5 m. This distribution is composed of a fairly small initialdroplet size distribution with a small to medium poly dispersity rel-ative to the next case studied which is crude oil B with 50 ppm ofchemical 7.

    In Fig. 31 the iso-volume fraction proles of the dispersed phaseare presented for the separation of crude oil B with 10 ppm of chem-ical 7; the open symbols are the experimental data measured by NMRand the solid lines are the model prediction. The results in Fig. 31 in-dicate that the model prediction is very good for 1% water (the dis-persed phase is water) and 100% water. The worst model predictionis for the 10% water curve and, while the initial prediction for the de-velopment of the dense packed layer is very good, the simulation re-sults diverge from experiment as time progresses. One important tonote is that both experiment and theory indicate that a small popula-

    122 I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127quantities such as the local droplet volume fraction, local mean dropletvolume (radius), and standard deviation of the droplet distribution canbe obtained directly from the statistical moments of the droplet volumedensity function. Finally, the coalescence process can then be describedin terms of conformal integrals of the local droplet volume density func-tion that employ a collision and efciency kernel that describes theprobability of coalescence in terms of the bulk physical properties ofthe uids and interfacial forces that are derived from fundamental rstprinciples of physics.

    In summary, the model is formulated to include the following spe-cic attributes and physical mechanisms:

    Dynamic evolution of a non-uniform (poly-disperse) drop-sizedistributionHindered sedimentation and formation of a dense packed layerDroplet collisions based on simultaneous consideration of differ-ential sedimentation rates and Brownian motionFilm drainage based on density difference, continuous phase vis-cosity, interfacial tension, droplet radius, and interfacial forces(expressed in terms of London-van der Waals dispersive forces)Interfacial coalescence of drops with their homophase expressedin terms of the lm drainage equation described aboveTracking explicitly the moving interface of the separated dis-persed phase (homophase) through the employment of Eulerianboundary conditions

    3.7.2. Notes on the lm drainage modelA complex model for lm drainage based on the models devel-

    oped by Slattery and coworkers [7478] has been constructed andsolved during the course of this work. The numerical lm drainagemodel is used to provide a detailed picture of the lm drainage phe-nomenon based on the characteristic parameters of the systems stud-ied. That is, based on the dispersed and continuous phase densities,the continuous phase viscosity, the interfacial tension, the interfacialdilational and shear viscosities, and the interfacial forces involved,and appropriate model for the lm drainage time can be selected tobest represent the system. Factors to consider are, for example, thedeformation of the droplet interface, the effect of surfactant mobility,and the effect of the surface viscosity. The complete numerical lmdrainage model considers the following:

    London-van der Waals dispersion forces between interfaciallayersInterfacial stress deformation expressed in terms of interfacialtension as well as interfacial shear and dilational viscositiesAxisymmetric deformation of the interfacial lmInterfacial surfactant diffusion (GibbsMaragoni effect)

    The complete details of the model formulation will not be repeat-ed here; readers interested in the detailed formulation of the lmdrainage model can consult Ref. [78].

    Simulations of the numerical lm drainage model indicated thatfor a viscous crude oil employed in the results section below havingdrop radii less than 150 m, the interfacial lm could be adequatelyrepresented by a parallel disk lm drainage model [74]. The represen-tation of the interfacial forces purely in terms of the retardedHamaker constant is, strictly speaking, not entirely correct as the in-terfacial forces due to steric repulsions should also be considered[80,81]. However, in this case, the retarded Hamaker constant canbe considered to be a lumping of these two mechanisms and is re-ferred to as an interfacial force parameter. The separation of theseto interfacial force mechanisms will be addressed in later work.

    3.7.3. Separation modeling resultsSimulations of the model were applied to NMR data for the sepa-

    ration of a heavy crude oil with various concentrations of a chemical

    demulsier additive. The heavy crude oil will be referred to as crudetion of very small droplet eventually becomes isolated at the top ofthe separator since earlier binary coalescence thins out the popula-tions and decreases the collision rate of these droplets signicantly.Thus, the very small droplets b5 m remain in an effective suspendedstate since the high viscosity of the oil phase and small droplet sizes

    Table 7Model parameters for the separation of crude oil B with 10 ppm of chemical 7.

    Parameter description Symbol Value

    Initial water volume fraction 0 0.3767Absolute temperature T 306.15 K (33 C)Column height Hc 1.74 cmDispersed phase density d 995 kg/m3

    Continuous phase density c 927 kg/m3

    Continuous phase viscosity c 118 mPasEquilibrium interfacial tension 0 14 mN/mInterfacial force parameter B0 4.051036 N/m2 (est.)Fitting parametersCollision rate parameter kcr 1.0Coalescence efciency parameter kef 0.5Interfacial coalescence parameter kic 1.0

    Table 8Model parameters for the separation of crude oil B with 50 ppm of chemical 7.

    Parameter description Symbol Value

    Initial water volume fraction 0 0.3773Absolute temperature T 306.15 K (33 C)Column height Hc 1.68 cmDispersed phase density d 995 kg/m3

    Continuous phase density c 927 kg/m3

    Continuous phase viscosity c 118 mPasEquilibrium interfacial tension 0 10.8 mN/mInterfacial force parameter B0 1.651031 N/m2 (est.)Fitting parametersCollision rate parameter kcr 1.0Coalescence efciency parameter kef 0.5Interfacial coalescence parameter kic 1.0

  • Fig. 30. Initial drop volume and number density distribution of crude oil B with 10 ppm of chemical 7. The mean radius by volume is 11.9 m.

    123I. Kralova et al. / Advances in Colloid and Interface Science 169 (2011) 106127mean any further separation will only occur after extremely long pe-riods of time. This result may have important implication in terms ofaccessing processing strategies for viscous crude oils.

    In Fig. 32, the axial volume fraction proles (separation proles) atseveral different times are presented for the mode


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