Journal of Colloid and Interface Science241,469–478 (2001)doi:10.1006/jcis.2001.7731, available online at http://www.idealibrary.com on

The Stabilization of Water-in-Hydrocarbon Emulsionsby Asphaltenes and Resins

Olga V. Gafonova and Harvey W. Yarranton1

Department of Chemical and Petroleum Engineering, University of Calgary, Alberta, Canada T2N 1N4

Received February 13, 2001; accepted May 25, 2001; published online July 30, 2001

The role of asphaltenes and resins in stabilizing water-in-crudeoil emulsions was investigated by measuring the interfacial compo-sition and stability of model emulsions composed of water with mix-tures of toluene, heptane, asphaltenes, resins, and native solids. Theinterfacial composition (mass surface coverage) was determinedfrom a combination of emulsion surface area measurements andconcentration measurements of both the continuous and the emul-sion phases. The emulsion surface area was calculated from dropsize distributions measured with optical microscopy. The concen-trations were found from gravimetric analysis. The stability of themodel emulsions was assessed from the amount of water resolvedafter heating and periodic centrifugation of the emulsions. Asphal-tene surface coverage was found to increase with an increase inthe asphaltene bulk concentration until a limiting surface coveragewas achieved. Surprisingly, while asphaltenes always tend to sta-bilize these emulsions, the stability of the emulsions decreased asasphaltene surface coverage increased. This change in stability wasattributed to a change in the asphaltene configuration on the inter-face. The addition of a good solvent was found to reduce both theamount of adsorbed asphaltenes and the emulsion stability. The ad-dition of resins always destabilized model emulsions. It appears thatresins act as a good solvent for the asphaltenes and, at sufficientlyhigh concentrations, are able to replace asphaltenes on the inter-face. Naturally occurring solids that coprecipitate with asphalteneshad little or no effect on asphaltene adsorption but dramatically in-creased emulsion stability. The results suggest that the combinationof asphaltenes and native solids causes the most stable emulsions.C© 2001 Academic Press

Key Words: asphaltenes; resins; solids; emulsions; water-in-oil;stability.

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INTRODUCTION

The formation of water-in-crude oil emulsions is a problethat can arise during the recovery, treatment, and transption of crude oil. In conventional and heavy oil productioemulsions occur when crude oil and produced water or injesteam mix in the reservoir, well bore, or surface facilities.refineries, water-in-oil emulsions are deliberately created

1 To whom correspondence should be addressed.

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46

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then broken to “wash out” contaminants that are present inproduced water emulsified in the crude oil (1). In oil sands prcessing, very stable water-in-oil emulsions are formed durithe hot water extraction process used to recover bitumen frthe oil sands (2). In most cases, the emulsions are undesirbecause they have higher volume and viscosity than the croil and lead to increased capital and operating costs. They malso carry contaminants such as chlorides through to a dowstream process leading to corrosion problems (1, 3, 4). Whmost water-in-crude oil emulsions can be broken with heat achemical additives, there are many cases where the convtional treatment fails. To devise more effective treatments fthese emulsions, it is necessary to understand how they arebilized.

It is well known that the stability of water-in-crude oil emulsions depends mainly on a rigid protective film encapsulatithe water droplets (5–9). This rigid interfacial film is believed bmany researchers to be composed predominantly of asphalteresins, and/or fine solids (3, 6–8). Asphaltenes and resins arefined as solubility classes of petroleum. Asphaltenes are soluin toluene but insoluble in alkanes, typicallyn-heptane orn-pentane. Asphaltenes are large polyaromatic hydrocarbonsconsist of condensed aromatic rings, aliphatic side chains,various heteroatom groups. Resins are soluble in both aliphand aromatic solvents but can be separated from other soluity fractions with liquid chromatography. Resin molecules astructurally similar to asphaltenes but have lower molar mahigher hydrogen/carbon ratio, and lower heteroatom conteAsphaltenes and resins both have a largely hydrophobic hydcarbon structure containing some hydrophilic functional grouand consequently are surface-active (6, 7, 10, 11). Hence, basphaltenes and resins have the potential to accumulate onwater/oil interface.

The form in which asphaltenes and resins adsorb on the emsion interface depends on the form at which they exist in solutioHowever, the behavior of asphaltenes and resins in crude ostill poorly understood. Asphaltenes are known to self-associin crude oil and form colloids or micelles (10–13). Resinare known to strongly inhibit asphaltene association (14, 1The resin molecules may adsorb on the asphaltene aggreg(10, 11) or may simply act as a good solvent. Therefor

9 0021-9797/01 $35.00Copyright C© 2001 by Academic Press

All rights of reproduction in any form reserved.

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470 GAFONOVA AN

asphaltenes and resins may adsorb on the emulsion inteas independent molecules or as different types of aggregat

Eleyet al.(16) proposed that asphaltenes stabilize water-inemulsions if they are near or above the point of incipient floclation; that is, they may be solid particles. Others have suggethat asphaltene colloids are responsible for stable emuls(6, 7, 17). The asphaltenes may collect on the interfacthe form of fine solid particles or asphaltene–resin colloids12). However, Yarrantonet al. (5) showed that, at low asphatene concentrations (<0.2 wt%), asphaltenes appear to stalize emulsions as a molecular monolayer on the water/oilterface. Furthermore, the examination of the interfacial fiformed by means of the Langmuir–Blodgett technique (8)the thin liquid film pressure balance technique (9) indicatedasphaltenes adsorb on the interface in molecular rather thanloidal form.

It is of interest to determine the interfacial compositionhigher asphaltene concentrations that are closer to realistic coil compositions. It is also necessary to consider the effecresins as they may influence asphaltene aggregation, adsoon the interface, and emulsion stability (7–9, 18). In this wothe interfacial composition is measured at asphaltene contrations up to 40 kg/m3 (approximately 4 wt%). The interfaciacomposition is also measured at different solvent compositand various asphaltene/resin ratios. Finally, the stability ofemulsions is measured and related to interfacial compositio

While the main goal of this study is to investigate the roleasphaltenes and resins in the stabilization of emulsions, theof native solids is also considered. Fine solids, such as sand,silica, and organic particles present in crude oil are also abstabilize water-in-crude oil emulsions (3, 19). In many caspolar surface-active materials present in crude oil, particulasphaltenes, adsorb on the solids, rendering them surface aThe solids can then adsorb on the emulsion interface and scally stabilize crude oil emulsions (20). It has been shown twhen asphaltenes are extracted from bitumen by precipitathem with a solvent, solids also co-precipitate (21). Since soincrease emulsion stability, their presence in the asphaltenetion may overshadow the stabilization capacity of asphalteand resins in emulsions. Solids may also interfere with thesorption of asphaltenes and/or resins on the emulsion interTherefore, the role of native solids in emulsion stabilizationalso examined by comparing the interfacial compositionstability of emulsions prepared with and without solids.

EXPERIMENTAL METHOD

To isolate the role of each component, the experiments wconducted on model emulsions consisting of deionized waasphaltenes, resins, native solids, and toluene/heptane mixin different ratios. The mixture of toluene and heptane isferred to as “heptol.” The composition of the heptol is indicaas the volume percent of toluene indicated in brackets. Foample, heptol containing 75 vol% toluene and 25 vol% hept

is denoted as “heptol (75).”

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The effect of native solids was not tested directly; rather emsions were prepared from asphaltenes together with theirprecipitated solids or from asphaltenes that had been treateremove the native solids. The mixture of asphaltenes and sois referred to as “asphaltenes,” and the asphaltenes that hbeen treated to remove solids are referred to as “solids-freephaltenes,” unless otherwise noted.

Materials

Reagent-graden-heptane andn-pentane were obtained fromPhillips Chemical Company. The 99.9% purity toluene was pchased from VWR. Distilled water used for emulsions was suplied by the University of Calgary water plant.

The asphaltenes and resins used in these experiments wertracted from two bitumens: Athabasca oil sands bitumen (cokfeed bitumen) and Cold Lake bitumen (recovered by steamjection from an underground reservoir). Both bitumens had betreated in the field to remove most of the sand and water and wready for upgrading.

Asphaltenes were precipitated from the bitumens with a 40volume ratio ofn-heptane to bitumen as described previous(22). Resins were extracted from the bitumen by using a modifiASTM-D2007M SARA fractionation procedure (22). SARA ian acronym for four crude oil solubility classes: saturates, amatics, resins, and asphaltenes. In SARA fractionation, resare extracted from deasphalted oil using clay-gel adsorptchromatography. Note that the deasphalted oil used for reextraction was obtained by precipitation of asphaltenes wn-pentanenot n-heptane. Saturate and aromatics can alsoextracted with this method; however, these fractions wererequired for this work. The SARA analysis of the Athabasand Cold Lake bitumens used in this work are given in Table

Removal of Native Solids from Asphaltenes

Native solids were removed from the asphaltenes precipitafrom the bitumens by using two different methods: a filtratiomethod and a precipitation method. The filtration methodvolves two steps. In the first step, the asphaltene solids weresolved in toluene at a volume ratio of 100 : 1 toluene : asphalteThe mixture was sonicated for 20 min to ensure that all of tasphaltenes were dissolved and then centrifuged at 3500

TABLE 1SARA Fractions and Solids Content of Athabasca

and Cold Lake Bitumens

Fraction Athabasca (mass %) Cold Lake (mass

Saturates 16.3 18.3Aromatics 39.8 39.1Resins 26.8 25.8Asphaltenes (pentane-extracted) 17.1 16.8Asphaltenes (heptane-extracted) 13.8 12.2Coarse Solidsa 7.5 2.4

a Mass percentage of heptane-extracted asphaltenes.

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(1260g) for 5 min. The supernatant liquid was decanted frthe settled solids. To obtain asphaltenes with only “coarse” soremoved, the solvent was evaporated from the supernatantil dry asphaltenes with fine solids remained. The size ofcoarse solids removed in the centrifuge can be estimatedStoke’s law assuming unhindered settling, spherical particand a solid density of 2000–2200 kg/m3. The analysis indicatethat this method can remove solids of higher than 0.8µm diam-eter. This value is probably too low because particles with sa small diameter may redisperse through convection or Brnian motion. Hence, asphaltenes with “coarse” solids remocontain solids with a diameter less than approximately 1µm.

To obtain “solids-free” asphaltenes, the supernatant liqfrom Step 1 was filtered under vacuum through a 0.5µm Metri-grad Glass fiber filter (Pall Corporation). The filtrate was plain a vacuum oven at 45◦C and left until the change in masof asphaltenes was less than 0.1% over a 24-h period. Tasphaltenes may still contain solids with a diameter less0.5µm, although smaller diameter particles may be removewell if they become trapped within the filter cake.

The filtration method is too slow to practically treat signicant quantities of asphaltenes. Therefore, a second methodon precipitation was tested. The native solids appear toprecipitate with the first asphaltenes to come out of solu(5, 23). Hence, most or perhaps all of the solids can be remby precipitating a small fraction of the asphaltenes. Here a 55heptane to toluene volume ratio was used. At this ratio, accing to asphaltene solubility data (21), approximately 2 wt%Cold Lake and Athabasca asphaltenes precipitates as well asolids.

Initially, asphaltene solids were dissolved in toluene at a ccentration of 1 kg/m3. The mixture was sonicated for 20 minensure that all of the asphaltenes were dissolved and thencentrifuged at 3500 rpm (1260g) for 5 min. The supernatanliquid was recovered and evaporated until only dry asphalteremained. This method is suitable for the extraction of significquantities of solids-free asphaltenes. However, it is not cerif all of the solids are removed with this method. It should abe noted that a small fraction of the asphaltenes is removedthe solids. Both factors could influence emulsion stability.will be discussed later, emulsions prepared from precipitattreated asphaltenes were compared with those preparedfiltration-treated asphaltenes and nearly identical stabilityobserved.

Emulsion Preparation and Drop Size Distribution

To prepare an emulsion, a known amount of asphaltenesresins was dissolved in toluene. The solution was stirred wsonic mixer for 20 min. Heptane was added after asphalteneresins were completely dissolved in the toluene. The mixwas again sonicated for five minutes to ensure homogenThe prepared oil phase was then processed with a CAT-5homogenizer with a 17-mm flat rotor generator configuratio

17,000 rpm. During mixing a given amount of dispersed wa

RBON EMULSIONS 471

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phase was added dropwise to the system. After all the waterbeen added, the mixing continued for five minutes at the saspeed.All the emulsions were prepared with 40 vol% water.

All of the emulsion experiments (drop size distribution anaysis, gravimetric analysis, and stability tests) were performon settled emulsions, i.e., emulsions that were allowed to stafor 1.5 h after preparation. In general, the emulsions settled aseparated into an emulsion layer on the bottom and a clear ctinuous phase on top within a few minutes. A standing time1.5 h was chosen to ensure that the smallest droplets had scient time to settle. There was no evidence of a change in fwater content or drop size distribution over this period.

Emulsion samples were placed in a hanging drop slide adiluted with continuous phase to disperse the droplets for imaanalysis. Drop size distributions of the settled emulsion phawere analyzed using aCarl Zeiss Axiovert S100inverted mi-croscope equipped with a video camera. Images from thecroscope were captured on a computer, and the drop sizetributions were then measured usingImage Proimage analysissoftware. In the present work 400 drops were used to obtdrop size distributions, giving an expected error of 5–10% acording to Dixonet al. (24). The drop size distribution is usedto calculate the Sauter mean diameter,d32, defined as

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where fi is the number frequency of droplets with diameterdi ,Vw is the volume of water in the emulsion, andAw is the surfacearea of the emulsion (the total surface area of the water drople

Determination of Interfacial Composition

The asphaltenes and resin surface coverage, that is, the mof asphaltenes and/or resins per area of the water/oil interfacan emulsion, can be calculated from the emulsion Sauter mdiameter and a gravimetric analysis of the continuous phasesimplify the explanation, the mass of asphaltenes will be usas an example.

The asphaltene mass surface coverage,0A, is simply the totalmass of the asphaltenes on the interface,mI , divided by thesurface area of the emulsion,Aw:

0A = mI

AW= mId32

6Vw. [2]

Now since asphaltenes are only soluble in the continuousdrocarbon phase,mI can be determined from the change in continuous phase asphaltene concentration upon emulsification

mI = mt

(1− Ceq

A

C0A

), [3]

wheremt is the total mass of asphaltenes in the emulsion,C0A

is the initial asphaltenes concentration in the continuous ph

terprior to emulsification, andCeq

A is the equilibrium asphaltene

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472 GAFONOVA AN

concentration in the continuous phase after emulsification. Bare measurable quantities. Substituting Eq. 3 into Eq. 2 giveexpression for asphaltene surface coverage in terms of meable quantities;

0A = mtd32

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The initial mass and concentration of asphaltenes and the wvolume are experimentally controlled variables. The measment of the Sauter mean diameter was discussed previoTo measure the equilibrium concentration of asphaltenesemulsification, the continuous phase of a settled emulsiondecanted and its volume measured. It was then placed inrotary evaporator, allowing the toluene/heptane solvent to evrate until only asphaltenes were left in the flask. The asphaltwere dried under nitrogen in a vacuum oven at 60◦C, after whichtheir weight was determined gravimetrically. The balanceaccurate to±0.002 g. The concentration of asphaltenes incontinuous phase is simply the weight of the asphaltenes divby the measured volume of supernatant. From repeated meaments, the mass surface coverage of asphaltenes on the intwas accurate to±0.0012 g/m2 (a 95% confidence interval).

When emulsions were prepared from a mixture of asphalteand resins, it was necessary to determine the mass and theof these fractions on the emulsion interface. The total mass oasphaltene–resin mixture remaining in the continuous phaster emulsification was determined gravimetrically as descriabove. Then the dry asphaltene–resin mixture was dissolv30 cm3 of n-heptane to reprecipitate the asphaltenes. Thelution was sonicated for 1 h and allowed to settle overnighAfter settling it was sonicated again for 15 min and centrifugfor 10 min at 3500 rpm. The supernatant containing the rfraction was decanted, set aside until most of the heptaneevaporated, and then placed into the vacuum oven at 45◦C un-til complete drying had occurred. The mass of the dried rewas determined gravimetrically. The centrifuge tube containthe precipitated asphaltenes was also placed in the vacuumand the mass of dried asphaltenes was determined. Therial balance of the asphaltene–resin mixture was checked.results where the material balance closed to within 10% arported. The mass of asphaltenes and resins on the interfaccalculated as previously described for the asphaltenes.

One problem with this method is that the presence of remay affect the solubility of the asphaltenes. In other words,all of the asphaltenes may precipitate in heptane if resins arepresent. Therefore, the method was applied to control samwith known amounts of asphaltenes and resins. The methodasphaltene masses accurate to within 17%.

Stability Tests

The stability of a given emulsion was assessed by measuthe water separated from the emulsion as a function of t

Samples of the settled emulsion phase (after 1.5 h of room t

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FIG. 1. Resolved water versus time for water-in-heptol (50) emulsions sbilized by Athabasca asphaltene/resin mixtures.

perature settling) were transferred into 15-cm3 graduated cen-trifuge tubes. The tubes were closed with caps to preventvent evaporation, centrifuged for 5 min at 4000 rpm (1650g),and placed in a constant temperature water bath at 60◦C. After2 h the emulsions were centrifuged for 5 min and the amoof resolved water was measured. The centrifuge tubes wereturned to the heating bath for another 2 h and then centrifugedbefore measuring the amount of resolved water. This procedwas repeated until all of the water was separated. The hing/centrifuging time applied to an emulsion sample is referrto as the destabilization time (τdes). The amount of resolvedwater was reported as the percent of total water volume ctained in the given emulsion sample. Examples of destabilition over time of model water-in-heptol emulsions are shownFig. 1. The reported percentages of resolved water are accura±11 vol%. The relative stability of all of the emulsions was asessed by comparing the amount of resolved water at a gdestabilization time.

RESULTS AND DISCUSSION

Before investigating the role of asphaltenes and resins,necessary to consider other factors that may influence intecial composition and emulsion stability, that is, drop size athe presence of native solids. Larger droplets tend to coalemore readily than small droplets. Therefore, drop size mustaken into account when comparing the relative stability of dferent emulsions. The mean diameter of a series of typical moemulsions is plotted versus asphaltene concentration in FigThe diameter decreases dramatically as asphaltene concetion increases to 4 kg/m3. Above 4 kg/m3, the mean drop size

em-is invariant at a value of approximately 8µm. The diameter

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FIG. 2. Effect of asphaltene concentration on the Sauter mean diamewater-in-heptol (50) emulsions stabilized by asphaltenes.

is invariant at high asphaltene concentrations because thphaltenes quickly adsorb on the water/oil interface and stlize the droplets before coalescence can occur. Hence, 8µm isthe mean drop size created by the homogenizer. At low asptene concentrations, there are insufficient asphaltenes to stathe droplets and coalescence occurs until monolayer coveraachieved (5). All of the emulsions presented here have a simrelationship between mean diameter and asphaltene concetion unless otherwise noted.

As noted previously, native solids are known to increase emsion stability. To assess the effect of solids, emulsions werepared from untreated asphaltenes, asphaltenes with coarseremoved by centrifugation, asphaltenes with fine solids remoby filtration, and asphaltenes with solids removed by preciption. All of these emulsions were prepared with 5 kg/m3 of as-phaltene solids in heptol (50). The stability of these emulsiis compared in Table 2. The removal of coarse solids haseffect on emulsion stability. However, the removal of fine sol

dramatically increases the amount of resolved water; that is, it

ter

adsorption.

TABLE 2Effect of Removing Solids on Water Resolved from Emulsions

Water-in-hydrocarbon emulsions Water-in-bitumen emulsions (Ref. 3)

Organic phase (heptol (50) and Resolved water Resolved waasphaltene-solids) (vol%) Organic phase (vol%)

Asphaltenes with solids 5.6 Bitumen (with solids) 0Asphaltenes with coarse solids removed 9.6 Bitumen with coarse solids removed (8µm filtered) 0

(> 1µm centrifuged)Asphaltenes with fine solids removed 48 Bitumen with fine solids removed (0.22µm filtered) 20

(0.5µm filter)Solids-free asphaltenes (percipitated) 49

RBON EMULSIONS 473

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FIG. 3. Adsorption isotherms of Athabasca asphaltenes and solids-freephaltenes in water-in-heptol (50) emulsions.

reduces stability. These results are consistent with Yanet al.’s (3)measurements for diluted bitumen also given in Table 2. Csequently, the experiments conducted with asphaltenes werpeated with “solids-free” asphaltenes whenever possible. Anote that the precipitation method gave the same results astering. Given its greater efficiency, the precipitation method wused for all of the other results presented here.

Asphaltene Adsorption Isotherms

Interfacial composition and structure can be illustratthrough adsorption isotherms, that is, plots of asphaltemass surface coverage,0A, versus the equilibrium asphaltenconcentration in the bulk phase,Ceq

A . The asphaltene adsorptioisotherm for model water-in-heptol (50) emulsions is showin Fig. 3. Isotherms for both asphaltenes and solids-fasphaltenes are shown, and they are within 10% of each oindicating that solids do not significantly affect asphalte

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474 GAFONOVA AN

As discussed above, at low asphaltene concentrations (C◦A <4 kg/m3 or Ceq

A < 2 kg/m3), the asphaltene surface coverais constant at a surface coverage of 4 mg/m2. Above Ceq

A <

2 kg/m3, the asphaltenes are essentially adsorbing onto ainterfacial area (constant Sauter mean diameter) and the sucoverage follows theform of a Langmuir adsorption isothermSurface coverage increases until aCeq

A of approximately40 kg/m3 and then plateaus, reaching a limiting value ofproximately 11 mg/m2. The asphaltene adsorption isothermin good agreement with asphaltene surface coverages meaby the Langmuir–Blodgett film technique. The LB surface covages range from 6 to 8 mg/m2 at initial asphaltene concentrationfrom 1 to 8 kg/m3 (8). Eseet al. (8) also observed an increain the surface coverage with an increase in the bulk asphaconcentration.

The asphaltene adsorption isotherm has the appearancLangmuir adsorption isotherm but it isnota Langmuir isothermbecause all of the adsorption is at or above monolayer coveThe surface coverage of 4 mg/m2 observed at low concentrationappears to be a monolayer coverage (5). When an emulsicreated with interfaces partially covered with asphaltenesemulsion coalesces until this coverage is achieved. As wellestimated thickness of the interfacial layer is consistent wimonlayer of asphaltenes. There are several possible exptions for the increase in the asphaltene surface coverage aa monolayer value:

1. At higher bulk phase concentration, asphaltene molecundergo a configuration change. At low concentrations, tspread out on the interface in a planar form, and at high ccentrations, they align into a more compressed “vertical” cfiguration allowing more compact packing of molecules oninterface, as shown in Fig. 4. This leads to a higher surface coage. The observed plateau at highCeq

A indicates that asphalteneachieve the maximum packing on the interface.

2. Larger asphaltene molecules adsorb preferentially. Atconcentrations, there are insufficient large molecules to satthe interface. At higher concentrations, the interface becosaturated with large molecules and the mass surface coveplateaus.

3. The asphaltenes adsorb as multilayers on the interfaceplateau might arise because the adsorption of a second laenergetically favorable, but the adsorption of additional layis less favorable as the separation distance between the inteand the adsorbed molecules increases.

As will be seen, observations on emulsion stability suggthat the increase in surface coverage is likely due to a conmation change rather than to multilayer formation or selecadsorption.

Effect of Asphaltene Adsorption and Solidson Emulsion Stability

The stability of different emulsions can be compared at

given destabilization time (τdes). The data given below is re-

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FIG. 4. Possible configurations of asphaltene molecules on the emulsinterface.

ported at a destabilization time of 16 h for more stable emusions and 6 h for less stable emulsions. These times were fouto provide data that best illustrated the trends in emulsion stabity discussed below. However, the same trends were detectaat all times prior to complete destabilization.

A plot of resolved water versus the concentration of aphaltenes in the water-in-heptol (50) emulsion is given in Fig.As concentration increases up to 5–10 kg/m3, the emulsions

FIG. 5. Correlation between Sauter mean diameter and emulsion stab

(heptol (50),τdes = 16 h).

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WATER-IN-HYDROC

FIG. 6. Comparison of the stability of emulsions stabilized by asphaltsolids or solids-free asphaltenes (heptol (50),τdes= 6 h).

become more stable. This increase in stability correlatessonably well with the decrease in mean diameter also showFig. 5. Above 10 kg/m3, the amount of resolved water (stabity) does not change significantly. However, these asphaltcontain solids, which may enhance emulsion stability.

The resolved water for emulsions stabilized with solifree asphaltenes are compared with asphaltene solids stabemulsions in Fig. 6. As discussed previously, the removasolids significantly decreases emulsion stability. The remof solids also reveals an unexpected trend. For the solidsemulsions, as asphaltene concentrationincreasesabove 5 kg/m3,emulsion stabilitydecreases. Clearly, the solids mask this behavior in the asphaltene solids stabilized emulsions. Butdoes stability decrease as the concentration of the asphalan emulsion stabilizer, increases?

This unexpected behavior contradicts the argument thaaccumulation of asphaltenes on the interface leads to the creof a stronger, thicker interfacial barrier that increases emulstability. Hence, the reduction in stability argues against themation of asphaltene multilayers on the interface or the sation of the interface with larger molecules. One remaining inpretation is that there is a change in the asphaltene configuron the interface.

At low asphaltene concentrations and consequentlysurface coverage, molecules may be spread out on the inteIn this position, the molecules may have many points of attament on the interface, as shown in Fig. 4a. At high asphalconcentrations, the molecules may adsorb in a more “vertalignment on the interface so that they become attached at oonly a few points per molecule. Lateral interaction betweenasphaltenes may preserve this configuration even if multipltachments are energetically favorable. In the vertical alignmmuch more material can be attached to a given surface are

shown in Fig. 4b. When molecules are attached to the interf

ARBON EMULSIONS 475

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at only one point, they are more extended into the continuophase and may be more mobile than when attached at sevpoints. This more mobile interface may present less of a barto coalescence. When molecules are attached at several sthe interface may be more rigid and the emulsion more stab

Effect of Solvent Composition

The effect of solvent composition on the interfacial adsorptiof asphaltenes and emulsion stability is shown in Fig. 7. The tsolvents compared are heptol (50) and heptol (75). In the besolvent (heptol (75)), the asphaltene adsorption is approxima20% less for the whole range of measured concentrations. Ttrend is consistent with Eseet al.’s (1998) Langmuir–Blodgettfilm results. The stability is also less (more resolved water)the heptol (75) system over the entire measured range ofphaltenes concentrations.

Figure 8 compares the Sauter mean diameter of the emulsfrom Fig. 7. AtCeq

A less than 5–10 kg/m3, the mean diameter ofthe water-in-heptol (75) emulsion is higher than in the emulsimade with heptol (50). Above thisCeq

A , the mean diameters ofboth emulsions are almost identical. Once again, at low asptene concentrations, emulsion stability appears to be goverby emulsion drop sizes and the larger droplets formed in hep(75) lead to less stable emulsions.

At higher asphaltene concentrations, the reduction in staity of emulsions containing 75 vol% toluene may be relatto the reduction in asphaltene adsorption on the emulsionterface. As shown in Fig. 4c, “good” solvent molecules tento solvate (surround) the asphaltenes to a higher degree.solvated molecules are more loosely packed on the interfathus decreasing the effective surface coverage. Furthermora better solvent, the strength of asphaltene–solvent interact

FIG. 7. Effect of solvent composition on the asphaltene solids adsorpt

aceisotherm and emulsion stability (τdes= 16 h).

D

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476 GAFONOVA AN

FIG. 8. Effect of solvent composition on the Sauter mean diameter.

approaches that of the asphaltene–asphaltene interacTherefore, in a better solvent, asphaltenes may be more mothe interface less rigid, and the emulsion less stable.

Emulsion stability for a broad range of toluene/heptane cpositions is shown in Fig. 9. First consider toluene contegreater than 50%. As expected, when the amount of heptancreases and the solvent becomes poorer, the emulsion staincreases. However, at a toluene content of approximatelythe trend reverses. This reversal corresponds to the precipitof asphaltenes. As demonstrated by Yarrantonet al.(5), the pre-

FIG. 9. Effect of solvent composition and the presence of solids on em

sion stability (C0

A = 5 kg/m2; τdes= 6 h).

YARRANTON

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cipitated asphaltenes do not participate in stabilizing emulsioand as more asphaltenes precipitate, fewer asphaltenes areable to stabilize the emulsions. Hence below a toluene conte45%, the addition of heptane reduces emulsion stability. Simresults were obtained by McLean and Kilpatrick (7). Emulsiostabilized by solids-free asphaltenes exhibit trends that aresame as those shown in Fig. 9. Note that once the asphaltbegin to precipitate the solids are removed as well and bothphaltene solids and solids-free asphaltene stabilized emulshave identical stability.

Effect of Resins

Emulsions containing resins are quite unstable. Emulsiprepared with resins and no asphaltenes broke completely ucentrifuging (within minutes of preparation). In the case of emsions containing mixtures of resins and asphaltenes, theemulsions that did not coalesce during the measurementriod contained a 1 : 3 resin/asphaltene (R : A) mass ratiosolved in heptol (50). In emulsions with higher R : A ratios ain all solids-free emulsions, drop rupture was observed durmicroscopy measurements. Nonetheless, gravimetric meaments were performed for these emulsions keeping in mindthe measured interfacial compositions for these emulsionsrough estimates.

Figure 10 shows the adsorption isotherms of asphaltenes (solids) in emulsions containing 1 : 3 and 1 : 1 R : A ratio versCeq

A in a range from about 5 to 30 kg/m3. The adsorption isothermof pure asphaltene solids is also shown for comparison. Thedition of resins decreases the amount of asphaltenes adsorbthe interface. The decrease in asphaltene adsorption is relatsmall at low resin concentrations or low resin/asphaltenes raFurthermore, when the interfacial composition was determin

FIG. 10. Adsorption isotherms of asphaltene/resin mixtures in water-

heptol (50) emulsions.

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WATER-IN-HYDROC

no resins were found on the emulsion interface at resin contrations below approximately 20 kg/m3. It appears that, at lowresin concentrations, the resins merely act as a good solvethe asphaltenes.

At a 1 : 1 R : A ratio, the resins completely replace thephaltenes on the interface once the resin concentration rea20 kg/m3. It appears that the total resin: concentration is a msignificant factor than the resin-asphaltene ratio. This suggthat, at sufficiently high concentration, resins may act morecompetitive surfactants than as a mediator of asphaltene adtion. In any case, they appear to replace the asphaltenes wfilm of resins that is likely more mobile and less likely to prevecoalescence.

The resolved water in emulsions containing 1 : 3, 1 : 2, 1and 2 : 1 R : A ratios in the continuous phase are comparedresin-free emulsions in Fig. 11. Here, all of the asphaltenestain solids. Stability tests were performed for the same seemulsions with solids-free emulsions as shown in Fig. 12. Nthat Figs. 11 and 12 are both plotted versus initial asphalconcentration rather than equilibrium asphaltene concentrabecause many of the resin-containing emulsions were toostable to measure an accurate equilibrium concentration.

The emulsions containing solids are more stable as expeHowever, consistent patterns are observed in emulsions prepwith and without solids. As asphaltene concentration increato approximately 5 kg/m3, emulsion stability increases. Abov5 kg/m3, the stability of the emulsions decreases. The iniincrease in stability is caused by a decrease in the mean diamas discussed previously. The subsequent decrease in stablikely caused by resins solvating the asphaltenes and, atresin concentration, replacing asphaltenes on the interface

The stability of emulsions at all the R : A ratios is much lethan in emulsions stabilized only by asphaltenes for the wh

FIG. 11. Effect of resins on emulsion stability (heptol (50),τdes= 6 h).ions

RBON EMULSIONS 477

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FIG. 12. Effect of resins and solids removal on emulsion stability (he(50),τdes= 4 h).

range of measured concentrations. Furthermore, the emubecome less stable as the R : A ratio increases, especially aresin concentrations. Resins appear to act as demulsifierscases.

Comparison of Athabasca and Cold Lake Asphaltenes

To determine if the observations made on Athabascaphaltenes extend to asphaltenes from other sources, somperiments were repeated with Cold Lake asphaltenes. Thsorption isotherms of Cold Lake and Athabasca asphaltenecompared in Fig. 13. The Cold Lake asphaltenes adsorb in l

FIG. 13. Comparison of adsorption isotherms and stability of emuls

containing Athabasca or Cold Lake asphaltenes (heptol (50),τdes= 6 h).

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478 GAFONOVA AN

amounts than the Athabasca asphaltenes at the same asphconcentration. However, the shapes of the adsorption isotheare identical. The resolved water for emulsions stabilized byAthabasca and Cold Lake solids-free asphaltenes are alsopared in Fig. 13. The stability of the two emulsions is identicThe similarity in stability curves and in adsorption isothermindicates that Cold Lake and Athabasca asphaltenes exhibisame general behavior.

CONCLUSIONS

Fine solids, asphaltenes, and resins all play a significrole in the stabilization of water-in-hydrocarbon emulsions. Aphaltenes and native solids act as emulsion stabilizers wresins destabilize emulsions. The asphaltenes appear to aas a monolayer with interfacial configurations (or packing dsities) that depend on the asphaltene concentration. The higpacking density occurred at high concentrations but led tostable emulsions. The most likely explanation is that therefewer points of attachments to the interface per moleculehigher packing density and, hence, the interface is more fland the emulsions less stable.

Emulsion stability increased in poorer solvents unlessphaltenes precipitated. In poor solvents, the asphaltenes aredifficult to displace from the interface and consequentlyemulsions are more stable. Upon precipitation, emulsionbility decreased as more asphaltenes precipitated. Precipiasphaltenes do not participate in stabilizing emulsions. Henceffect, precipitation removes stabilizing asphaltenes. Resinsinterfacially active but do not stabilize emulsions. In all casresin addition decreased emulsion stability. Resins are a gsolvent for asphaltenes and also, at high concentrations,replace asphaltenes on the interface.

Naturally occurring solids were found to have no effectasphaltene adsorption but significantly increased emulsionbility. The increased stability was attributed to solids less th1µm in diameter. These solids were found to precipitate withphaltenes. Hence, they may obscure the stabilization effect ophaltenes in emulsion studies. The presence of solids shoulways be accounted for when investigating asphaltene-stabilemulsions.

The above results suggest that appropriate surfactant orvent addition can be an effective treatment for water-in-cruoil emulsions stabilized by asphaltenes. However, native somay be the key factor in “problem” emulsions that persist afconventional treatment methods. Future research may betargeted toward the role of solids.

ACKNOWLEDGMENTS

We thank Imperial Oil Ltd. and NSERC for financial support. We are agrateful to Ms. D. Sztukowski for assistance with the experimental work

Dr. B. McGarvey for helpful advice.

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NOMENCLATURE

Aw surface area of an emulsion (m2)C0

A initial concentration of asphaltenes in the continuousphase before emulsification (kg/m3)

CeqA concentration of surface-active asphaltenes in th

continuous phase after emulsification (kg/m3)di diameter ofith water droplet (m)d32 Sauter mean diameter (m)fi number frequency of water droplets of diameterdi

mI mass of asphaltenes on the interface (mg)mt mass of asphaltenes in the emulsion (mg)Vw total volume of water phase (m3)0A asphaltene mass surface coverage on the wate

hydrocarbon interface (mg/m2)τdes destabilization time (h)φw volume fraction of water in emulsion

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