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Published: November 16, 2011

r 2011 American Chemical Society 1075 dx.doi.org/10.1021/ef201545q| Energy Fuels 2012, 26, 10751088

ARTICLE

pubs.acs.org/EF

LiquidLiquid Phase Equilibria in Asphaltene + Polystyrene + TolueneMixtures at 293 KM. Khammar and John M. Shaw*

Chemical and Materials Engineering, University of Alberta, Edmonton, T6G2 V4, Canada

ABSTRACT: The phase behavior of hydrocarbon mixtures where one of the constituents self-aggregates is a subject of signicantindustrial and academic interest. Here, a nonintrusive acoustic phased-array technique operated in pulse echo mode is used toinvestigate the phase behavior of asphaltenes, a well-known self-aggregating species, in mixtures with polystyrene and toluene at 293 Kand atmospheric pressure. This mixture exhibits liquidliquid phase behavior where both liquids are opaque to visible light, are ofuniform composition, and are stable over broad ranges of composition. One phase is asphaltene rich and the other phase ispolystyrene rich. Varying the polystyrene mean molar mass had little impact on the liquid to liquidliquid phase boundaries.

Liquidliquid critical points were identi

ed and phase compositions were con

rmed for axed global composition using theUVvisible spectrophotometry and mass balance equations. This is the rst report of liquidliquid phase behavior for such

mixtures. Depletion occulation is hypothesized to be the mechanism causing phase separation in this ternary mixture.

1. INTRODUCTION

Asphaltenes are dened as the fraction of crude oil insoluble inalkanes and soluble in benzene and toluene on the basis ofltration experiments (ASTM D4055). However, in toluene,asphaltenes aggregate to form sterically stabilized colloidalparticles.15 The size of these particles has been the subject ofseveral investigations. For example, Barre et al.6 measured the

radius of gyration of asphaltenes colloidal particles in a 3 vol %toluene solution using small-angle X-ray scattering (SAXS).They found that the asphaltene aggregates fall in the size range6.3 to 16 nm. In heavy oils (15 to 20 wt % asphaltenes7)nanoltration experiments showed that asphaltene nanoaggre-gates form large aggregates up to 50100 nm in both Athabascabitumen and Maya crude oil.8 Espinat et al.9 measured the size ofasphaltenes in toluene using small-angle X-ray scattering (SAXS),small-angle neutron scattering (SANS), and dynamic lightscattering to investigate the eect of temperature and pressureon the size of asphaltene aggregates in toluene. They found thatthe size of asphaltene aggregates decreased with temperature,while pressure did not appear to have a signicant eect on size.The interaction forces between asphaltene colloidal particles intoluene are dominated by repulsion. For example, Wang et al.10

measured steric repulsive forces between asphaltene-coatedsurfaces in toluene.

If asphaltene-containing mixtures are treated as colloidalsolutions, conventional separation methods include ultracentri-fugation, variation of pressure, temperature, solvent evaporation,and addition of antisolvent.11,12 Alternatives include polymeraddition.12 In a recent work, Lima et al.13 focused on the impactof adsorbing polymers, polycardanol and sulfonated polystyrene,on asphaltene solutions. Polycardanol polymers were added toasphaltene in toluene solutions (60 mg/L) and sulfonatedpolystyrene was added to asphaltene in toluene and acetonesolution. Asphaltene + polymer solutions were left for 24 h and

then centrifuged at 3000 rpm for 30 min. The e

ect of poly-mer addition was estimated by measuring the concentration of

asphaltenes remaining dispersed using UVvisible spectropho-tometry. At low polymer concentrations (


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1076 dx.doi.org/10.1021/ef201545q |Energy Fuels 2012, 26, 10751088

Energy & Fuels ARTICLE

be used to control the attractive forces between the colloidalparticles and to manipulate their phase behavior. The phasebehavior of mixtures of colloids and nonadsorbing polymers hasbeen the subject of a substantial number of experimental andtheoretical investigations.20 For example, Ramkrishnan et al.21

measured the phase behavior for silica particles, with a radius (a)of 50 nm, coated with 1-octadecanol + polystyrene + tolueneternary mixtures. They observed uidgel, uidcrystal, anduiduid transitions as the ratio of polymer radius of gyration(Rg) to colloidal particle radius (Rg/a) increased from 0.026 to1.395. Hennequin et al.22 measured the phase behavior of silicaparticles sterically stabilized with 1-octadecanol in toluene +

polystyrene for Rg/a = 4.1 and 5.2. They found that, after nearly1 h of homogenization and over a range of compositions, the

mixtures separated into two stable liquid phases: one colloid-richand one colloid-poor. The method developed by Bodnar et al.23

was then used to estimate the composition of the phases inequilibrium.

In this work, the behavior of Maya pentane asphaltene +toluene + polystyrene mixtures is investigated at asphalteneconcentrations greater than 5.0 vol %, at 293 K. At theseconcentrations, asphaltenes are expected to form stericallystabilized colloidal particles, and liquidliquid phase behavioris anticipated, as long as the asphaltene particles fall in anappropriate size range relative to the polystyrene moleculesfor the depletion occulation phase separation mechanismto occur.

2. EXPERIMENTAL METHODOLOGY

2.1. Chemicals and Solution Preparation. Toluene 99% waspurchased from Fisher Scientific. Pentane asphaltenes wereprepared from Maya crude oil according to ASTM standardD4055.24,25 Asphaltene in toluene mixtures were prepared by

adding toluene to vials containing asphaltenes. They were mixedwith a vortex mixer and hand shaken for at least 45 min until they

Figure 1. Experimental variation of mixture composition in the phasediagram.

Figure 2. Speed of sound proles in asphaltene + toluene mixtures at293 K and 1.01 bar. Asphaltene volume fraction is a parameter.

Figure 3. Speed of sound proles in toluene + polystyrene mixtures at293 K and 1.01 bar. Polymer volume fraction is a parameter. (a) Mw=393 400 g/mol, (b) Mw= 700 000 g/mol.


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appeared homogeneous and liquid like. Polystyrene with twodifferent average molecular weights (Mw): Mw= 393 400 g/mol(Rg25.6 nm) and Mw= 700 000 g/mol (Rg36.1 nm)

26werepurchasedfrom Aldrich. The volume fractions of asphaltenes andpolystyrene were calculated assuming an asphaltene density of1.17 g/cm34 and using the density of polystyrene specified by thesupplier, 1.047 g/cm3.

2.2. Acoustic Apparatus and Measurement Methodology.A detailed description of the acoustic apparatus and the measure-ment methodology is available elsewhere.27,28 Key points aresummarized here. Phase boundaries are detected on the basis of

two independent measurements, namely speed of sound differ-ences between phases, and spikes in acoustic wave attenuation,

that arise at liquidliquid interfaces. Uniformity of compositionswithin phases is evaluated from speed of sound profiles. Com-position gradients are readily detected as are composition dif-ferences between phases. Both sets of measurements are ob-tained from reflected waveforms measured with a phased arrayacoustic probe attached to the walls of a PolyBenzImidazolecell. Measurements were performed at 113 elevations (300 mapart) simultaneously. Acoustic wave attenuation is less accu-rate than speed of sound for interface detection but plays animportant supporting role in cases where the volume of a phaseis too small to obtain a speed of sound measurement within the

phase. The temperature of the cell interior was controlled towithin (0.1 K by circulating thermostatted fluid through tubesin the cell.

Asphaltene + polystyrene + toluene mixtures were prepared inthe cell by introducing a xed mass of an asphaltene + toluene +polystyrene mixture to which prepared mixtures of polystyrene intoluene and toluene were added using syringes. This permittedroughly orthogonal movements in the phase diagrams as shownin Figure 1. The contents were stirred for 5 min before the stirrerwas removed. The acoustic measurements were started afterremoving the stirrer to capture the acoustic properties just aftermixing at time (t) equals zero min, and were then obtained every5 min for at least the rst hour after homogenization. The rate ofthe measurements was then decreased.

2.3. Speed of Sound in Toluene + Asphaltenes andToluene + Polystyrene Binary Mixtures. Figure 2 shows the

Figure 4. Experimental speed of sound data at 293 K and 1.01 bar forthe binary mixtures: (a) asphaltene () + toluene, (b) polystyrene ()(Mw = 393 400 g/mol) + toluene, (c) polystyrene () (Mw = 700000

g/mol) + toluene.

Figure 5. Phase behavior observations for asphaltene + toluene +polystyrene mixtures on trajectories p, q and r, v, ae, and additionalmixtures for polystyrene molar masses (a)Mw= 393 400 g/mol,(b)Mw=700 000 g/mol. Liquidliquid phase behavior is denoted by crosses andliquid-phase behavior is denoted by triangles.


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variation of the speed of sound with elevation for five asphaltene +toluene mixtures. Each array of measurements was obtained at

least 1 h after introducing the mixtures into the cell. For 23.4 vol %asphaltene in toluene, the measurement was performed 17 h

later. Speed of sound is independent of elevation. Thus ateach composition, the asphaltene + toluene mixtures are stable

and macroscopically homogeneous. Results for polystyrene +toluene mixtures are comparable, as shown in Figure 3a and b

Table 1. Experimental Data for Mixtures (ae) Asphaltene + Polystyrene (Mw

= 393 400 g/mol) + Toluene

global

composition

speed of

sound (m/s)

asphaltene rich phase polymer-rich phase dierenceinterface elevation

(mm) Hinterfacevol. fraction

of upper phase Rtime for phase

separation (min)nal time

(min)phase

behavior

mix a 0.217 0.0058 1352.7 1352.7 1129 LL

mix b 0.142 0.0305 1348.5 1345.2 3.3 11.0 0.55 38 1413 LL

mix c 0.119 0.0383 1347.1 1343.8 3.3 9.2 0.68 24 156 LL

mix d 0.107 0.0419 1346.2 1343.0 3.2 8.6 0.73 33 427 LL

mix e 0.102 0.0438 1344.7 1341.8 2.9 8.0 0.76 40 1614 0000LL

Table 2. Experimental Data for a Mixture of Asphaltene + Polystyrene (Mw

= 393 400 g/mol) + Toluene (Composition, Speed ofSound Per Phase, LiquidLiquid Interface Elevation, and Volume Fraction of the Upper Phase)

global

composition

speed of sound

(m/s)

asphaltene-rich

phase

polymer-rich

phase dierencemix

elevation interface

(mm) Hint

vol. frac. of

upper phase R

time for phase

separation (min)

nal time

(min)

phase

behavior

0.069 0.0544 1342.4 1342.3 0.94 46 904 LL

0.098 0.0450 1346.0 1342.7 3.31345.0 4.1 0.83 37 374 LL

0.093 0.0429 1344.7 1341.6 3.11343.5 3.8 0.85 32 136 LL

0.089 0.0410 1343.4 1340.6 2.81342.3 3.2 0.88 35 129 LL

0.085 0.0392 1342.6 1339.7 2.91341.3 2.6 0.91 36 126 LL

0.078 0.0361 1342.0 1339.0 3.01340.7 2.0 0.93 41 222 LL

0.077 0.0354 1342.6 1338.7 3.91339.7 1.7 0.95 37 144 LL

0.073 0.0335 1338.0 1339.0 0.96 47 1082 LL

0.070 0.0323 1338.5 1338.6 0.96 48 1182 LL

0.196 0.0123 1353.6 1349.4 4.21354.2 14.3 0.25 50 306 LL0.185 0.0115 1351.3 1348.4 2.91351.5 15.2 0.25 43 144 LL

0.167 0.0105 1348.6 1345.9 2.71348.8 16.1 0.28 41 120 LL

0.153 0.0095 1346.8 1344.6 2.2 17.0 0.30 41 187 LL

0.146 0.0091 1344.9 1342.8 2.1 17.6 0.31 46 127 LL

0.128 0.0080 1342.3 688 L

0.118 0.0074 1341.2 1334 L

0.109 0.0068 1339.7 1705 L

0.148 0.0284 1349.7 1346.1 3.6 11.6 0.43 39 785 LL

0.142 0.0272 1347.9 1345.0 2.9 12.2 0.43 41 133 LL

0.134 0.0258 1346.1 1343.5 2.6 12.2 0.46 39 170 LL

0.127 0.0245 1345.0 1342.7 2.3 12.2 0.49 37 203 LL

0.118 0.0227 1343.9 1342.0 1.9 12.0 0.53 32 126 LL

0.113 0.0217 1342.5 1341.2 1.3 10.5 0.61 49 125 LL

0.108 0.0208 1341.4 1211 L

0.098 0.0188 1339.9 1643 L

0.168 0.0204 1349.8 1346.9 2.9 14.6 0.33 41 677 LL

0.139 0.0169 1344.7 1342.9 1.8 17.0 0.35 36 108 LL

0.118 0.0142 1341.4 1340.9 0.5 18.8 0.39 37 1400 LL

0.172 0.0254 1352.7 1348.6 4.1 17.9 0.36 62 2030 LL

0.154 0.0340 1353.1 1349.0 4.1 17.3 0.45 75 1104 LL

0.142 0.0398 1353.8 1349.6 4.2 16.4 0.52 89 8918 LL


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for Mw= 393 400 g/mol and Mw= 700 000 g/mol polystyrene,respectively. Measurements were performed following 2224h of equilibration. The small gradient in the speed of sound

values with elevation, evident for mixtures of 7.7 vol% poly-styrene in toluene, may reflect residual thermal or composi-

tion gradients. Averaged speed of sound values as functionsof composition for the binaries are presented in Figure 4.Both polymers and the asphaltenes have comparable impacts

on the speed of sound of mixtures vis-a-vis toluene at thesame volume fraction. Thus, detailed phase compositions

Table 3. Experimental Data for a Mixture of Asphaltene + Polystyrene (Mw

= 700 000 g/mol) + Toluene (Composition, Speed ofSound Per Phase, LiquidLiquid Interface Elevation, and Volume Fraction of the Upper Phase)

global composition

speed of

sound (m/s)

asphaltene

rich

polymer

rich dierence

elevation

(mm) Hint

vol. frac. of

upper phase R

time for phase

separation (min)

nal time

(min)

phase

behavior

0.075 0.0528 1339.0 0 1440 L

0.095 0.0462 1342.0 1339.4 2.6 2.3 0.90 59 394 LL

0.108 0.0419 1343.2 1340.7 2.5 4.4 0.83 50 169 LL

0.104 0.0405 1343.4 1340.6 2.8 4.1 0.85 45 688 LL

0.098 0.0384 1341.1 1338.8 2.3 3.5 0.88 49 182 LL

0.094 0.0368 1339.9 1337.9 2.0 3.2 0.89 45 404 LL

0.090 0.0351 1338.6 1337.1 1.5 2.6 0.92 44 133 LL

0.087 0.0341 1338.6 1336.7 1.9 2.0 0.94 43 137 LL

0.089 0.0323 1339.8 1336.7 3.1 1.4 0.96 41 604 LL

0.081 0.0315 1338.0 0.96 53 1571 LL

0.199 0.0115 1353.7 1350.3 3.4 15.2 0.25 61 124 LL

0.188 0.0109 1351.3 1348.2 3.1 15.8 0.26 60 309 LL

0.181 0.0105 1349.7 1347.0 2.7 16.4 0.26 50 127 LL

0.171 0.0099 1348.3 1345.9 2.4 17.0 0.28 50 128 LL

0.163 0.0094 1346.8 1344.7 2.1 17.6 0.29 52 128 LL

0.154 0.0089 1345.7 1343.8 1.9 17.9 0.32 53 129 LL

0.148 0.0086 1344.8 1343.1 1.7 18.3 0.32 55 127 LL

0.141 0.0081 1344.0 1342.5 1.5 18.8 0.34 59 123 LL

0.135 0.0078 1342.7 1341.7 1.0 19.5 0.34 72 845 LL

0.127 0.0074 1340.0 3053 L

0.119 0.0069 1338.3 2695 L

0.156 0.0258 1346.8 1343.6 3.2 11.3 0.46 50 425 LL

0.156 0.0258 1347.2 1344.0 3.2 11.3 0.45 50 942 LL0.152 0.0250 1345.0 1342.1 2.9 11.6 0.46 50 204 LL

0.135 0.0222 1342.4 1340.0 2.4 12.2 0.49 40 334 LL

0.130 0.0214 1341.5 1339.3 2.2 12.0 0.52 27 167 LL

0.121 0.0200 1341.0 1339.1 1.9 11.7 0.56 24 740 LL

0.112 0.0185 1339.8 1338.2 1.6 10.4 0.64 19 158 LL

0.107 0.0176 1338.6 1337.6 1.0 8.3 0.73 33 127 LL

0.104 0.0171 1338.4 1337.7 0.7 5.3 0.83 95 1069 LL

0.099 0.0163 1337.4 1331 L

0.094 0.0155 1336.6 727 L

Table 4. Phase Volume and Speed of Sound Measurement Repeatability for Mixture d

global composition speed of sound (m/s)

trial

asphaltene-rich

phase

polymer-rich

phase dierence

elevation interface

(mm) Hint

vol. fract. of

upper phase R

time for phase

separation (min)

nal time

(min)

1 0.107 0.042 1346.2 1343.0 3.2 8.6 0.73 33 427

2 1346.3 1343.1 3.2 8.3 0.74 35 142

3 1346.9 1343.5 3.4 8.3 0.74 34 685

4 1345.6 1342.5 3.1 8.3 0.74 30 345


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for asphaltene + polystyrene + toluene ternary mixtures arenot readily discriminated on the basis of speed of sound,measurements themselves, or models fit to binary data, atechnique limitation also noted elsewhere.29

2.4. UV-visible Spectrometry Measurements. A Variant

Cary 50 Scan UVvisible spectrophotometer was used to mea-sure the concentration of asphaltenes during only one liquid

liquid equilibrium experiment, as these measurements aredestructive. The specified instrument tolerance for absorbancemeasurement (Abs) is (0.005 Abs at 1 Abs. Samples were in-troduced in a 10-mm-path quartz cell. Absorbance spectrafor toluene, polystyrene (7.8 vol%) + toluene, and apparent

absorbance for nine asphaltene + toluene mixtures (rangingfrom 50 to 1100 mg/L) were used to calibrate composition

Figure 6. Speed of sound proles (i), acoustic wave attenuation spectra dierence (ii), and attenuation dierence at 7.9 MHz (iii), at the equilibrationtimes given in Table 1, and for global asphaltene + toluene + polystyrene (Mw= 393 400 g/mol) compositions along trajectory ae shown in Figure 5a.


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measurements. Neither toluene nor polystyrene + toluene mix-tures absorb significantly in the wavelength range 500800nm.Their absorbance is 0.03 Abs. Maya asphaltenes also do notcontain chromophores that sorb significantly in the range500900 nm.30Apparent absorbance in this range is attributedsolely to scattering by asphaltene particles. The apparent ab-orbance of asphaltenes is linearly proportional to composition

at low concentrations and an apparent absorbance of 1.7 Abswas attained at 1090 mg/L for scans at 700 nm.

3. RESULTS AND DISCUSSION

The phase behavior of asphaltene + toluene + polystyrenemixtures was explored by adding mixtures of toluene + poly-styrene to mixtures of asphaltenes + toluene + polystyrene(trajectories ae, and w) and by adding toluene to mixtures ofasphaltenes + toluene + polystyrene (trajectories p, q, r, v).Theseobservations are summarized in Figure 5. The bulk of the

observations fell in the liquidliquid region of the phasediagram. Global compositions based on the volume fractions

Figure 7. (i) Evolution of the liquidliquid, liquidair interfaces, (ii) speed of sound: (triangle) upper phase, (circle) lower phase, and (iii) speed ofsound dierence between the phases with time for global compositions (be) shown in Figure 5a: (I) composition (b), (II) composition (c), (III)composition (d), (IV) composition (e).


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of asphaltenes () and polystyrene (), speeds of sound in eachphase, liquidliquid interface elevation (Hint), upper-phasevolume fraction (R), and equilibration times for Maya asphal-tenes + toluene + polystyrene (Mw = 393400 and 700000g/mol) are presented in Tables 1, 2, and 3. Measurementrepeatability is illustrated in Table 4. These data are exploredin detail below.

3.1. Phase Behavior of Polystyrene (MW = 393 400 g/mol) +Toluene + Asphaltenes. Aliquots of polystyrene, = 0.077,in toluene were added to an asphaltene, = 0.234, in toluenemixture to construct the trajectory that follows the line segmentae in Figure 5a. Experimental data for these mixtures atequilibrium are summarized in Table 1. Speeds of sound andattenuation profile measurements are shown in Figure 6. Mixture

a remained homogeneous and there was no evidence of phaseseparation even after 19 h. As more of the polystyrene + toluene

mixture was added, an upper liquid phase and a lower liquidphase each with uniform compositions, separated by an interface,appeared. From the sequence of experiments, the upper liquidphase is richer in polymer and the lower liquid phase is richer inasphaltenes. The liquidliquid interface elevation and the airliquid interface elevation were detected most accurately asbreaks in the speed of sound measurements at equilibrium;Figure 6i, ae. Sound wave attenuation difference mapsFigure 6ii, ae and sound wave attenuation difference valuesat a specific frequency (7.9 MHz) Figure 6iii, ae providesecondary and less precise measures of interface location. At-tenuation differences are computed by subtracting the attenua-tion just after mixing from attenuations arising at another time, inthis case, the equilibration time.

These mixtures reach equilibrium within 40 min and nofurther change in interface elevation, speed of sound in eachphase, or speed of sounddierence arisesas shown in Figure 7 forpoints be along trajectory ae. The elevation of the liquid

liquid interface, and the speed of sound di

erence betweenthe separated phases are stable with time within 0.3 mm and0.4 m/s, respectively. These variations are comparable to therepeatability of individual phase separation measurements. Forexample, four sequential equilibration trials were performed withmixture d. The results are summarized in Table 4. For each trial,the mixtures were stirred for 5 min. The phases separated orreseparated within 35 min. Liquidliquid interface elevation wasrepeatable to within 0.3 mm. Consequently, the volume fractionof the upper phase, R, was repeatable to within 0.01. The speed ofsound dierence values between phases at equilibrium wererepeatable to within 0.3 m/s even though the speed of sounddrifted by as much as 1.5 m/s over the time frame of theexperiments. Based on the speed of sound of toluene, such a

drift can be accounted for with a temperature drift in the cellof 0.3 K. The liquidliquid separation is time invariant andrepeatable.

The variation of the elevation of the liquidliquid interface,Hinterface, the volume fraction of the upper phase, R, and thespeed of sound dierence between the phases in mixturesbe at equilibrium are shown in Figure 8ac. As the polysty-rene volume fraction increases along the trajectory ae theliquidliquid interface elevation decreases (Figure 8a), and thevolume fraction, R, of the upper-phase increases (Figure 8 b),while the speed of sound dierence between the phases isinvariant within experimental error (Figure 8c).

3.1.1. Identification of the LiquidLiquid Critical Behaviorfor Asphaltene + Polystyrene (Mw = 393 400 g/mol) + Toluene

Mixtures. Phase behavior measurements were made along tra-jectories p, q, and r as shown in Figure 5a. These measurementswere performed by adding small aliquots of toluene to threemixtures of asphaltene + polystyrene + toluene with differentvolume fraction ratios of polystyrene to asphaltenes. If sufficienttoluene is added, the trajectories intersect the liquidliquid tosingle-liquid phase boundary. In addition to providing pointsdefining the boundary, the location of the liquidliquid criticalpoint can be inferred from the phase volume and speed of sounddifference data and their trends with composition shown inFigure 9. As a liquidliquid critical point is approached, thecompositions of the asphaltene-rich and the polymer-rich phasesapproach one another. Consequently, the upper-phase volumefraction approaches 0.5 and the speed of sound difference

approaches 0. If the global composition is marginally richer inone component, the phase volumes diverge from 0.5 as the

Figure 8. Liquidliquid interface elevation (a), volume fraction of theupper phase (b), and speed of sound dierence between the coexistingphases (c) for global compositions be shown in Figure 5a.


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boundary is approached. The gradient and direction of thedivergence provide directional and proximity information relatedto the location of the critical point. For trajectory p, the liquidliquidinterfaceelevationdecreases from 4.1 to1.7mm, as the upper-

phase volume fraction increases from 0.83 to 0.95. As the phaseboundary approached the base of the cell (Hinterface < 1.2 mm),

acoustic wave attenuation was used to detect the presence of theinterface because thespeedof sound values in the asphaltene-richphase could not be measured accurately. Trajectory p appro-aches the two-phase to single-phase boundary withthe polymer-

rich phase near a point ( 0.07, 0.03) remote from theliquidliquid critical point. For trajectory q, the volume fraction

Figure 9. Elevation of the liquidliquid interface (i), volume fraction of the upper phase (ii), and speed of sound di erence between the coexistingphases (iii) for mixtures of asphaltenes + polystyrene (Mw=393 400 g/mol) + toluene. (I) trajectory p, (II) trajectory q, (III) trajectory r.

Figure10. Phase separation results fora mixture with global composition ( =14.72vol%, =2.88vol%):(a)speedofsoundprole,(b) acoustic waveattenuation spectrum dierence, and (c) ultrasound attenuation dierence at 7.9 MHz.


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of the upper phase increases from 0.43 to 0.61 and the speed ofsound difference between the separated phases decreases from3.6 to 1.3 m/s. Trajectory q crosses the phase boundary into thepolymer-rich phase near a point ( 0.110, 0.021). Fortrajectory r, the upper-phase volume fraction increases from 0.25to 0.31 and the speed of sound difference decreases from 4.2 to2.1 m/s. Trajectory r crosses the phase boundary into theasphaltene-rich phase near a point (0.137, 0.0086). Thusthe liquidliquid critical point falls between the intersection oftrajectories q and r with the phase boundary.

3.1.2. Phase Composition (Tie Line) Measurements in the

Liquidliquid Region. A mixture of asphaltene + polystyrene(Mw = 393 400 g/mol) + toluene ( = 0.147, = 0.029) with acomposition close to that of mixture 1 on trajectory q ( = 0.148, = 0.028) was prepared. Phase separation was completed within52 min, and the mixture was equilibrated for 167 min. Theequilibrium profiles for speed of sound and sound wave attenua-tion are shown in Figure 10. The volume fraction of the polymer-rich phase (R) is 0.42. This value compares well with thatmeasured for mixture 1 on trajectory q, where R= 0.43. Samplesfrom both liquid phases were extracted from the cell and aliquotsof 0.4 mL of each phase were diluted in 100 mL of toluene priorto UV analysis. The concentration of asphaltene in the dilutedsolutions extracted from the lower and upper phases were 938.7and 346.4 mg/L, respectively. This corresponds to asphaltene

volume fractions ofI = 0.201( 0.006andII = 0.074( 0.003 inthe asphaltene-rich and polymer-rich phases, respectively. From

the asphaltene material balance equation, the global volumefraction of asphaltene can be calculated as

R II 1 R I 0:147 ( 0:007 1

This value is in close agreement with the global volume fractionof asphaltenes in the mixture = 0.147 ( 0.006.

The volume fraction of polystyrene was determined indirectlyand by dierence following evaporation of toluene from samplesplaced in oven for 10 h at 90 C then in a vacuum oven at 80 C for6 h. The mass of polystyrene is determined by subtracting the

mass of asphaltenes, determined directly as noted above, fromthe residual mass. Consequently, the error is large at smallvolume fractions. The volume fraction of polystyrene in theasphaltene-rich and polymer-rich phases are I = 0.000 ( 0.007and II = 0.060 ( 0.003, respectively. The global polystyrenevolume fraction in the mixture can be calculated from thepolystyrene material balance

j RjII 1 RjI 0:025 ( 0:005 2

This value falls within the error of the global volume fraction ofpolystyrene (0.029 ( 0.003) in the mixture.

3.2. Phase Behavior of Polystyrene (MW= 700 000 g/mol) +Toluene + Asphaltenes Mixtures. The phase behavior results

obtained with this mixture are both qualitatively and quantita-tively similar to those obtained with the lower molar mass

Figure 11. Elevation of the liquidliquid interface (i), volume fraction of theupper phase (ii), speed of sound dierence between the coexisting phases(iii) for mixtures of asphaltenes + polystyrene (Mw= 700 000 g/mol) + toluene. (I) Trajectory p, (II) trajectory q, (III) trajectory r.


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polymer and are presented in summary form here. Phasebehavior measurements were performed along trajectories p, q,and r as shown in Figure 5band liquidliquid interface elevation,the volume fraction of the upper phase and the speed of sounddifference values are summarized in Figure 11. For trajectory p,the position of the liquidliquid interface decreases from 4.4 to1.4 mm, as the volume fraction of the polymer-rich upper phaseincreases from 0.83 to 0.96. As the liquidliquid interfaceapproaches the base of the cell, acoustic wave attenuation wasused to detect the interface. Trajectory p clearly approaches thetwo-phase to single-phase boundary with the polymer-rich phase

near the point (

0.08,

0.03) that is remote from theliquidliquid critical point. For trajectory q, the volume fractionof the upper phase diverges from 0.46 and rises to 0.83 as theliquidliquid to liquid boundary is approached while the speedof sound difference between the phases decreases from 3.2 to 0.7m/s. Trajectory q crosses the phase boundary into the polymer-rich phase near point ( 0.102, 0.0167). For trajectory r,the volume fraction of the upper phase increases from 0.25 to0.34 and the speed of sound difference decreases from 3.4 to 1.0m/s. Trajectory r crosses the phase boundary into the asphal-tene-rich phase near point ( 0.131, 0.0076). Again, theliquidliquid critical point falls between the intersection oftrajectories q and r.

3.3. Phase Diagrams for Polystyrene + Toluene + Asphal-

tene Mixtures. Phase diagrams for the two polystyrene +toluene + asphaltene ternaries are presented in Figure 12a

and b along with estimated critical points. The two-phaseregion occupies much of both phase diagrams. The polymer-rich and asphaltene-rich phases are found close to theirrespective composition axes. As polystyrene does not possess

polar groups that can associate with asphaltenes, asphaltene +polystyrene + toluene ternaries appear to be analogous tomixtures of sterically stabilized colloidal particles + polystyrene + toluene mixtures, where phase separation is driven bydepletion flocculation. The phase diagrams in Figure 12a andb possess characterist ics similar to phase diagrams for well-defined sterically stabilized colloidal particles + non-adsorbingpolymer + solvent mixtures.22,23 Unlike these mixtures, though,asphaltene colloidal particles are not monodispersed and theirmean size may also depend on global and phase compositions.

3.4. Phase Separation Kinetics. Phase separation times forasphaltene + toluene + polystyrene mixtures range from 30 to50 min following mixing. These times are slow compared totypical hydrocarbon liquidliquid phase separation times. Theevolution of the separation process for one mixture, asphaltene( = 0.148) + polystyrene (Mw= 39 3 400 g/mol, = 0.028) +toluene, shown in Figure 13 is illustrative. This mixturecorresponds to the first composition on trajectory q inFigure 5a and it is remote from the liquidliquid criticalpoint. The horizontal dashed lines in Figure 13 denote theliquidliquid and liquidair interfaces arising at equilibrium.At t= 0 min, the mixture is macroscopically homogeneous. Thespeed of sound profiles and the attenuation profile are feature-less. With increasing time, the speed of sound decreases in theupper part of the cell and increases in the lower part of the cell.Composition gradients in the liquid phase become evidentwithin 14 min (Figure 13c). High attenuation regions, related to

the formation of a liquidliquid interface, arise at 14 min andmove downward from the upper part of the cell and upwardfrom the lower part of the cell (Figure 13c, ii). They coalsce andform the liquidliquid interface at 35 min (Figures 13cg).No further change is observed in the profiles after 39 min(Figure 13h). This phase separation time is consistent with thevalues reported in Tables 14 in this work and with prior workon the time reported for completion of phase separation formixtures of colloidal particles + non-adsorbing polymers +solvents. For example, Hennequin et al.22 reported that afterhomogenizing mixtures of silica colloidal particles + polysty-rene + toluene, a fuzzy interface first formed at the bottom of thecuvette, moved upward, and became sharper as phase separa-

tion proceeded. All samples investigated reached equilibrium1 h after homogenization. They reported that the separatedphases were clearly distinguishable and the interface was sharp.Ramakrishnan et al.21 reported that for mixtures of colloidalsilica particles + polystyrene + toluene, a meniscus separatingtwo fluid phases appeared within minutes of mixing. The lowerphase was richer in colloidal particles than the upper phase andboth phases flowed easily. Aarts et al.31 reported that formixtures of fluorescent PMMA nanoparticles + polystyrene incis/trans decalin, phase separation was complete within 15 minat intermediate polymer concentrations and took up to fewhours at high polymer concentrations. In the future we planto evaluate options for normalizing the dynamic acoustic

profile data to extract interfacial zone thickness and interfacialcomposition.32

Figure 12. Phase diagrams for mixtures of asphaltenes + toluene +polystyrene: (a) polystyreneMw= 393 400g/mol,(b) polystyreneMw=700 000 g/mol. Two-phase region: (crosses), single-phase region:(triangles), phase compositions from UV measurements (small dots),

estimated liquidliquid critical points (large dots).


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Figure 13. Continued


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4. CONCLUSIONS

Repreatable and reversible liquidliquid phase behavior wasobserved for asphaltene + polystyrene + toluene mixtures over abroadrangeof compositionsat 293K and atmosphericpressure.Fromspeed of sound and UV spectroscopy measurements, one of thephaseswasfound to be asphaltene-rich andtheother waspolystyrene-rich.Liquidliquidcriticalpointswere also identiedalong theliquidliquid to liquid-phase boundary. Remote from the critical point, theasphaltene-saturated polymer-rich phase could comprise a volumefraction of asphaltenes as large as 0.06. By contrast, the polymervolume fraction in the asphaltene-rich phase remote from the criticalpoint was less than 0.01. The observed phase behavior conformswiththat of sterically stabilized colloidal particles (asphaltenes) + a

nonadsorbing polymer (polystyrene) + a good solvent (toluene).Depletion occulation is the phase separation mechanism associatedwith such mixtures. This is the rst report of liquidliquid phasebehavior arising in asphaltene + polystyrene + toluene mixtures andthe results are expected to open new lines of inquiry related toproperty prediction, property measurement, and process develop-ment linked to asphaltene behavior in solvents and in live oils.

AUTHOR INFORMATION

Corresponding Author*Tel.: +1-780-492-8236. E-mail: [email protected].

ACKNOWLEDGMENT

We aknowledge and thank Dr. Brent Zeller from EclipseScientic for assistance and support related to the conguration

and setup of the Focus LT acoustic equipment, ProfessorsMurray Gray and Gregory Dechaine from the University ofAlberta for suggesting and helping with the UV spectophoto-metry measurements, and Dr. Carlos Lira-Galena at the MexicanPetroleum Institute for the Maya crude oil sample. This researchwas supported by the Alberta Innovates Energy and Environ-ment Solutions, ConocoPhillips Inc., Imperial Oil Resources,Halliburton, Kellogg Brown and Root, NEXEN Inc., ShellCanada, Total, the Virtual Materials Group, and the NaturalScience and Engineering Research Council of Canada (NSERC).

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