Characterization of Fine Solids in Athabasca Bitumen Froth before and

Jan 26, 2016 - Department of Chemical and Materials Engineering, University of Alberta, ... Consequently, hydrothermal treatment of Athabasca bitumen ...
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Characterization of Fine Solids in Athabasca Bitumen Froth before and after Hydrothermal Treatment Jun Zhao, Qi Liu, and Murray R. Gray*,† Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada ABSTRACT: The warm-water extraction of mined oil sands recovers the bitumen content as a froth, with significant amounts of emulsified water and fine mineral solids. Direct thermal processing of this bitumen−water mixture, giving hydrothermal conditions, could reduce the number of process steps. In this study, the mineralogy and wettability of fine solids in Athabasca bitumen froth before and after hydrothermal treatment at 392 °C for 30 min at 15 MPa were investigated. The clay minerals, mainly kaolinite and illite, were unchanged, but siderite (FeCO3) and pyrite (FeS2) were converted to pyrrhotite (Fe(1−x)S) after the treatment. This conversion can be advantageous during direct hydrothermal treatment of bitumen froth because it fixes organic sulfur. The initial fine solids in the bitumen froth possessed a wide range of wettability, but they turned uniformly more oil-wet after the hydrothermal treatment. Consequently, the fine solids mostly stayed in the oil phase after the hydrothermal treatment and did not act as stabilizers for water-in-oil emulsions. The filterability of the fine solids in the bitumen froth was also significantly increased. Consequently, hydrothermal treatment of Athabasca bitumen froth could be used to destabilize water-inoil emulsions and facilitate fine solids removal by filtration. influenced by mineral type,7 surrounding water,6 the cations on the surface of the mineral fines,8,9 and temperature.10,11 The fine solids in the Athabasca bitumen froth are mainly minerals with particle sizes smaller than 44 μm. These are predominantly the clay minerals kaolinite and illite. Under hydrothermal reaction conditions, these clays would not dehydroxylate.12−14 The fine solids in the bitumen froth also contain significant, but varying, amounts of nonclay minerals, including quartz, dolomite (CaMg(CO3)2), calcite (CaCO3), anatase, and rutile (TiO2), which would be stable during the hydrothermal reactions.15 However, there are often significant amounts of pyrite (FeS2) and siderite (FeCO3). Siderite can decompose to form iron oxide and carbon dioxide. Dhupe and Gokarn16 showed that siderite decomposed into different iron oxides under hydrothermal conditions, and in the presence of hydrogen sulfide, the iron oxides were converted to pyrrhotite (Fe(1−x)S, where x = 0−0.17). Ko et al.17 and Chadha et al.18 suggested that sulfur from hydrogen sulfide or from bitumen could enrich the sulfur content in the formed pyrrhotite. Despite the numerous studies on the mineralogy of fine solids in the Athabasca oil sands, as well as on the mechanism of organic−mineral interaction, no studies have examined the behavior of the fine solids after hydrothermal treatments. In this paper, the mineralogy and surface properties of fine solids in the bitumen froth and their interactions with hydrocarbons before and after a mild hydrothermal treatment at 392 °C were investigated, to test the hypothesis that the hydrothermal treatment could change the wettability of the fine solids, which, in turn, changes their ability to stabilize water-in-oil emulsions in the Athabasca bitumen froth.

1. INTRODUCTION The warm-water extraction of the Athabasca oil sands floats the bitumen as a froth, which contains about 60 wt % bitumen, 30 wt % water, 10 wt % fine mineral solids.1 After deaerating, the froth gives a water-in-oil emulsion stabilized by fine solids with intermediate contact angles. The addition of a paraffinic solvent such as hexane, to precipitate about 8% of the bitumen precipitates as asphaltenes, binds together the emulsified water and fine solids and enables their removal by gravity settling. The resulting bitumen is clean enough for pipeline transport and subsequent upgrading, but the loss in yield is significant. When naphtha is used as a solvent, the viscosity is reduced so that a portion of the water and fine solids can be removed by centrifugation or in inclined-plate settlers. In this case, the yield is high, but the bitumen product contains ca. 2% water and 0.5% fine solids. This material has too much water, dissolved salts, and fine mineral particles to allow pipeline transport or subsequent processing by hydroconversion. When using both paraffinic and naphtha solvents in the bitumen froth cleaning, a portion of the solvent is released into tailings ponds with the removed solids and water. An alternative to the current approach of froth treatment, followed by upgrading, is to process froth directly, through hydrothermal treatment. A hydrothermal reaction condition involves water at elevated temperature (∼400 °C). At high pressures, the water phase becomes supercritical and can dissolve the lighter components of the bitumen.2,3 Our focus is on lower pressures, up to ∼13 MPa, to enable operation at reasonable cost. We hypothesize that, during a mild hydrothermal reaction, the wettability of the emulsion-stabilizing fine solids in the bitumen froth will be altered so that the water-inoil emulsion can be destabilized, facilitating their removal. The polar sites of minerals likely interact with the polar groups from bitumen, especially asphaltenes.4,5 The asphaltene adsorption on mineral surfaces was reported to be an irreversible process,6 © XXXX American Chemical Society

Received: October 3, 2015 Revised: January 23, 2016

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2.7. Quantitative X-ray Diffraction Analysis (QXRD). Quantitative X-ray diffraction analysis (QXRD) was carried out to measure the mineralogical composition of the fine solids using the RockJock method.19 In each measurement, 1 g of the fine solids sample was mixed with 0.250 g of corundum internal standard. The mixed samples were ground in a McCrone micronizing mill for 5 min with 4 mL of ethanol. The ground samples were dried and shaken for 10 min in a plastic vial with three plastic balls and a small quantity of vertrel. Then, the samples were sieved, sidepacked into a holder to ensure random sample orientation.19 A Rigaku Ultima IV diffractometer (40 kV, 44 mA) with Cu Kα radiation and a graphite monochromator was employed to generate X-rays to scan the sample from 5° to 65° (2θ). The obtained XRD patterns were analyzed by JADE to identify the minerals; then, RockJock 11 was used to quantitatively determine the mineral composition in wt %. 2.8. X-ray Photoelectron Spectroscopic Analysis (XPS). Pellets for X-ray photoelectron spectroscopic analysis were prepared following the same procedure as in contact angle measurements. The analysis was performed using a Kratos Axis 165 spectrometer with monochromatic Al Kα radiation. Survey scan spectra were obtained to detect all the elements possibly present, and high-resolution scan spectra were then obtained for more detailed information about carbon, sulfur, iron, aluminum, and silicon. Calculations associated with relative elemental concentration through XPS analysis were based on survey scan spectra. 2.9. Water−Oil Partitioning of Fine Solids. For these measurements, the procedures of Yan20 and Dorobantu et al.21 were followed. The emulsifying ability of the fine solids was tested by mixing 0.2 g of the fine solids (unreacted fine solids, or fine solids reacted with bitumen and water at 392 °C) with 8 mL of water and 32 mL of toluene in a Teflon test tube. Thus, the solid concentration was 5 kg/m3, and the water volume fraction was 0.2. The test tubes were vortexed by IKA Vortex 3 at speed setting 6 for 5 min. Then, the mixtures were approximately equally split and transferred to two graduated cylinders. The cylinders were sealed and left standing for 24 h. Photos of the emulsion were taken, and the volumes of emulsions were compared to the results of Yan.20 To fractionate the fine solids into different wettability fractions, 1 g of the unreacted fine solids or the fine solids reacted with bitumen at 392 °C was placed in a Teflon separation funnel together with 50 mL of water and 50 mL of toluene. The mixture was agitated thoroughly and left to phase separate. The toluene layer, aqueous layer, and rag layer were separated. The separated layers were dried, and the fine solids in each layer were analyzed. 2.10. Filtration Test. The filterability of the fine solids from the bitumen froth is important to the oil sands industry. A lab scale filter was set up to test the filterability change of the fine solids after the hydrothermal reaction. A vacuum pump was used to generate vacuum at about −85 kPa (pressure could vary during the filtration). Two grams of the bitumen froth was hydrothermally reacted in the microreactor at 392 °C. Two identical batches (i.e., 4 g of bitumen froth) were partially diluted and transferred into a beaker. Four grams of each of the reacted and unreacted bitumen froth was diluted to 95 mL with toluene. Four grams of Athabasca bitumen was also diluted into 95 mL for reference purposes. Thus, there were three samples in total for the filterability test. In order to filter the samples, the filter assembly was set up and the vacuum pump was turned on. Then, the filter paper was rinsed with toluene. Once minimum pressure was reached, one 95 mL sample was quickly stirred and poured into the glass funnel, and the fine solids in the beaker were rinsed off by 5 mL of toluene. The total time required for toluene to pass through the filter paper was recorded as the filtration time. The filtration time for the three samples was compared. 2.11. Simulated Distillation Analysis. In order to determine the conversion of the heavy fractions in the froth during hydrothermal treatment, dry Athabasca bitumen from Syncrude Canada Ltd. was reacted in the same reactor for the same time and temperature, but without water or fine solids. This approach avoided having to remove water and solids from the reacted bitumen prior to simulated distillation analysis. This bitumen contained 55% vacuum residue (524

2. MATERIALS AND METHODS 2.1. Fine Solids Extracted from Bitumen Froth. The sample of Athabasca bitumen froth was obtained from a pilot plant at Devon, Alberta, courtesy of Imperial Oil Ltd. Subsamples were taken after thorough mixing of the entire sample. The well mixed bitumen froth sample contained approximately 10% water based on Dean−Stark extraction, and 5% fine solids from centrifuging the dilute bitumen. To determine the fine solids, and to recover this material for further experiments, the bitumen froth was diluted with toluene at a 1:1 vol ratio and centrifuged in a Beckman Coulter centrifuge at 30 000 relative centrifugal forces (RCF) for 30 min. After the supernatant was siphoned out, the sediments were mixed with toluene and centrifuged again. The procedure was repeated 4 times when the supernatant was clear. The final sediments were dried and used as the fine solids from the bitumen froth in the experiments in the present study. 2.2. Athabasca Bitumen. Athabasca bitumen provided by Imperial Oil was used to react with the extracted fines in the experiment. The sample was processed through a pilot plant at Devon, Alberta, using hexane as the solvent in paraffinic froth treatment. As a result, it had negligible mineral content and water and about half of the C5 asphaltene content of the raw bitumen (9 wt %), with 45 wt % vacuum residue (524 °C+ fraction) and 4.5 wt % sulfur. 2.3. Hydrothermal Reactions in Microreactors. Stainless steel batch tube reactors with a 15 mL volume (microreactor) were used in the experiments. The reactor was made from Swagelok tubes and fittings. To carry out the hydrothermal reaction, fine solids, bitumen, and water were loaded into the microreactor. The amount of fine solids and bitumen was 1 g each. These conditions were selected to provide samples of solids for subsequent characterization, using much higher solid:bitumen ratios than in actual froth. The total amount of water (deionized) added to the reactor was kept at about 0.5 g, giving a total pressure of 15 MPa at the reaction temperature based on calculations with the Peng−Robinson equation of state (VMGSim, Calgary, AB). The reactor was then pressurized with nitrogen to 2.7 MPa and placed in a hot sand bath to heat the reactor contents to the desired temperature of 392 °C. (The reaction temperature was varied between 350 and 430 °C, but 390 °C was found to be the optimum temperature with respect to bitumen froth cleaning.) It took about 2 min for the reactor to reach the target temperature. The reactor was left in the sand bath for a total of 30 min, then removed and cooled under blowing air for 5 min to quench the reactions. After cooling to room temperature, reactor contents were washed with toluene on a vacuum filter. The collected fine solids (filter cake) were vacuum-dried at 80 °C for 24 h. 2.4. Contact Angle Measurements. Contact angle measurements were carried out to assess the wettability of the fine solids using a FTA200 Drop Shape Analyzer. About 100 mg of the fine solids was pressed into a pellet, 1 cm in diameter, by using an ICL 12 TON E-Z Press pellet presser. The pressure was about 34 MPa (5000 psi). One drop of deionized water was placed on the surface of the pellet, and microphotographs of the water drop were taken about 1 sec after the drop completely contacted with the pellet. The contact angle was determined from the shape of the water drop. For each condition, 3−4 pellets were measured to ensure accuracy and repeatability. 2.5. Elemental Analysis. Elemental analysis was carried out by using a VARIOMICRO elemental analyzer (Elemental Analysis, Hanau, Germany). About 10 mg of different fine solids samples was combusted at 1200 °C. The carbon, nitrogen, hydrogen, and sulfur contents of the samples were measured. 2.6. SEM and EDX Analyses. Fine solids extracted from the bitumen froth before and after the hydrothermal reaction were mounted on carbon shielded stubs and coated with carbon. The morphology of the fine solids was analyzed by a JEOL JAMP 9500F scanning electron microscope (SEM). SEM images were obtained at an accelerating voltage of 10 keV with a working distance of 24 mm and 5000× magnification. The same mounted samples from SEM analysis were transferred to a Tescan Vega energy-dispersive X-ray spectrometer to determine the elemental composition of the sample. B

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Energy & Fuels °C+ fraction) by simulated distillation. Simulated distillation of the bitumen before and after reaction was used to determine the conversion of the vacuum residue fraction. A VARIAN 450-GC gas chromatograph with a flame ionization detector was used to determine the simulated distillation curve (SimDist) for both reacted and unreacted bitumen (water and solvent free base). The measurement was based on ASTM 5307 for boiling range distribution of high residue content crude petroleum. For calibration, Polywax 655 (SUPELCO Analytical 4-8477) and ASTM D5307 Crude Oil Quantitative Standard (SUPELCO Analytical 4-8179, C10−C44) were used. The conversion of the bitumen after the reaction was calculated based on the wt % of vacuum residue (524 °C+ fraction) measured from the distillation curve of the bitumen before and after reaction:

Table 1. Mineralogical Composition of the Fine Solids Obtained from Quantitative XRD Analysis mineral nonclay minerals anatase magnetite quartz pyrite pyrrhotite rutile siderite subtotal clay minerals illite kaolinite subtotal total

Conversion = 100% − Unreacted 524+ , wt % − (100% − Reacted 524+ , wt %) 100% − Unreacted 524+ , wt %

(1)

3. RESULTS AND DISCUSSION 3.1. Mineralogy. Figure 1 shows the XRD patterns of the fine solids from the bitumen froth sample with or without the

extracted unreacted fine solids, wt %

after reaction with bitumen at 392 °C for 30 min, wt %

1 0 5 6 0 3 19 34

1 2 4 0 20 3 1 31

17 49 66 100

18 51 68 100

hydrothermal conditions to form hydrogen sulfide. This partial hydrogenation would change the crystalline structure of pyrite to pyrrhotite. Because of the decomposition of siderite, inorganic carbon was released, which affects the total carbon content of the fine solids. Other than pyrite and siderite, the rest of the nonclay minerals were stable during the reaction. 3.2. Morphology of Particles. Figure 2 shows the SEM images of fine solids before and after the hydrothermal reaction. Both images show clear crystalline and layered structures of mineral particles. Organic materials could not be identified from the images. Therefore, the majority of the organic materials were present as thin films. On the basis of the SEM images, it was concluded that there were no significant changes to the morphology of the particles after the hydrothermal reaction. 3.3. Wettability. After hydrothermal reaction at 392 °C, the contact angle of the fine solids increased from 50 ± 6° to 78 ± 7°. This result indicates that hydrocarbon materials may have accumulated on the surface of the fine solids, which modified their wettability, or they were deposited more uniformly. The increase in hydrophobicity of the reacted fine solids was also observed during contact angle measurement: water droplets were observed to penetrate the pellets of unreacted fine solids in about 2 min, whereas they could not penetrate to the reacted fine solids after 30 min. In a separate “hydrothermal” reaction, the fine solids from the bitumen froth were reacted with Athabasca bitumen without adding water. The contact angle of the reacted fine solids was 88 ± 4°, which was higher than the contact angle of the fine solids (78 ± 7°) when water was added during the hydrothermal reaction. The results indicated that the hydrothermal reaction conditions did indeed affect the wettability of mineral particles more than bitumen and heat alone. 3.4. Filterability Test. The filterability of the fine solids was significantly improved after the hydrothermal reaction. Experiments on actual froth were used to assess filterability due to the importance of both the solids and the aqueous phase in the behavior of this material. The diluted unreacted bitumen froth plugged the filter paper very quickly. The filtration stopped after 30 min after only a very small amount of liquid passed through the filter paper. In contrast, the filtration of the 100 mL solution sample was complete in about 6 min after the bitumen froth was subjected to the hydrothermal treatment at 392 °C

Figure 1. XRD patterns of fine solids from the bitumen froth with or without the hydrothermal treatment. The hydrothermal reaction temperature was 390 °C, and the reaction time is indicated on the XRD pattern.

hydrothermal treatment. As can be seen, kaolinite and illite were the main clay minerals, which did not change before and after the hydrothermal treatment. However, the unreacted sample contained siderite and pyrite, and after the hydrothermal reaction, these minerals disappeared and were replaced with significant diffraction peaks for pyrrhotite. Quantitative XRD analysis was carried out, and the results are shown in Table 1. As can be seen, about two-thirds of the solids are clay minerals (kaolinite and illite), and one-third are nonclay minerals. The fine solids contained very high iron content resulting from about 20% siderite and 5% pyrite. Anatase and rutile were also detected in the fine solids. After the hydrothermal reaction, the clay minerals remained at about two-thirds of the total solid mass. However, virtually all siderite and pyrite were indeed converted to pyrrhotite. According to Ko et al.,17 siderite likely reacted with hydrogen sulfide, which would be generated from bitumen if the temperature was over 350 °C. In our tests, almost all siderite was reacted. Sulfur from pyrite could react with hydrogen (possibly with cracked hydrogen from bitumen) under the C

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Figure 2. SEM images of fine solids before (left) and after (right) hydrothermal reaction. The hydrothermal reaction was carried out at 392 °C for 30 min.

and they tended to stay in the toluene phase. After 24 h standing (settling), a thicker layer of fine solids accumulated at the oil/water interface on the oil side. Some light colored fine solid particles were observed at the bottom of the graduated cylinder with unreacted fine solids, but no such solid particles were observed in the case with reacted fine solids. The fine solids that stay in the aqueous phase should be highly hydrophilic. The absence of these fine solids from the aqueous phase after hydrothermal treatment indicated that the hydrothermal reaction in the presence of bitumen rendered the fine solids more hydrophobic. In addition, these hydrophobic fine solid particles settled much faster than the unreacted fine solids. Before reaction, the toluene phase stayed muddy, showing high concentrations of fine dispersed particles after 24 h standing. After reaction, most of the reacted fine solids settled in the toluene phase so that the toluene phase became clear. This observation implied that the effective particle size of the fine solids increased as a result of the hydrothermal reaction, due to either a sintering mechanism during reaction or flocculation after reaction. Jin et al.22 recently reported the settling behaviors of fine silica beads (0.25 μm diameter) in toluene or heptane, and their mixtures. The silica beads were coated with Athabasca bitumen. They observed that the bitumen-coated silica beads remain dispersed in toluene but form large aggregates and settled quickly in heptane. Our observed dispersion of the untreated bitumen froth fine solids in toluene was consistent with Jin et al.’s results as the fine solids were also coated with bitumen. However, our hydrothermal treated bitumen froth fine solids settled relatively quickly in toluene, which differed from Jin et al,’s results. Unfortunately Jin et al.22 did not carry out a detailed study on the surface coating of the bitumen on the silica beads nor the hydrophobicity of the coated beads. In order to better understand the observation, we investigated the changes in the fine solid surface properties before and after the hydrothermal treatment. 3.6. Characterization of the Fine Solids Collected from Different Phases. The data of Table 2 show the distribution of fine solids to the toluene and aqueous phases and their carbon content, obtained from the partitioning tests. The unreacted fine solids that were well dispersed in the toluene phase had very high carbon content, but the amount of the fine solids was small compared to the amount of fine solids that went to the rag layer. The fine solids in the rag layer had lower carbon content. Some very light colored fine particles could be separated from the aqueous phase, and the carbon

for 30 min. As a comparsion, when the Athabasca bitumen obtained from the paraffinic froth treatment process (theoretically solids and asphaltene free) was diluted and filtered, it only took 40 s for the solution to pass though the filter paper. 3.5. Water−Oil Partitioning of Fine Solids. Figure 3 shows a schematic of the distribution of fine solids in the

Figure 3. Distribution of fine solids in the oil and aqueous phases: unreacted fine solids (left), reacted fine solids (right).

organic (toluene) and aqueous phases before and after the hydrothermal treatment. The mixture contained 0.2 volume fraction of water. After vigorous shaking and 24 h standing, the toluene and water phases separate together with a rag layer. The rag layers were located on the aqueous side of the interface. This result was in contradiction to the observations from Yan20 and Dorobantu et al.,21 who reported that particles with contact angles of about 60° tended to form emulsions that were more than 40% in volume, and very few solid particles would stay in the oil phase. The reason for the contradiction could be that their particles had more uniformly distributed hydrophobicity, whereas the sample in this study had more varied wettability. The volume of rag layer in the case of unreacted fine solids was approximately 70% of the aqueous phase (about 15% of the total volume of water + toluene). However, the volume of the rag layer in the case of reacted fine solids was only about 20% of the aqueous phase. The fine solids had contact angles around 80° after the hydrothermal reaction, D

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Energy & Fuels Table 2. Distribution of Fine Solids in the Different Phases and Their Carbon Content sample unreacted fine solids

fine solids reacted at 392 °C

distribution, % total toluene layer rag layer aqueous layer total toluene layer rag layer aqueous layer

100 25.0 65.0 10.0 100 75.0 25.0 0

Table 3. Carbon Contents and Contact Angles of Two Samples Measured by a CNHS Elemental Analyzer and XPS

carbon content, wt % 18.0 27.6 16.7 4.1 15.4 16.8 14.9

± ± ± ± ± ± ±

analysis methods carbon content, wt % (elemental analysis) XPS, (C, wt %)/(Al + Si + Fe, wt %) apparent contact angle, deg

0.1 0.1 2.0 0.1 0.3 0.4 0.4

unreacted fine solids

reacted fine solids

18.0 ± 0.1

15.4 ± 0.3

0.94 50 ± 6

2.60 78 ± 7

surface concentration of carbon measured by XPS and the contact angle increased. This implied that organic material was coated on the fine solids surface during the reaction. In both cases, the XPS signals of the mineral lattice ions were significant in all cases, indicating that the minerals were only partly covered by the organic materials. This result was consistent with Wang et al.,25 who observed that, even when the kaolinite surface was saturated with asphaltenes, XPS analysis always showed signals of Al and Si, which indicated that kaolinite was not fully covered by asphaltenes. The data of Table 3 are consistent with a restructuring or redistribution of the surface organic coating, given lower mass of surface organic coating but probably more uniform surface coverage. On the basis of the quantitative XRD results, 1.66 wt % of the 18 wt % carbon in the unreacted fine solids was contributed by carbonate. When this amount of carbon content was deducted from the initial carbon content in unreacted fine solids, there was still about 16.3% of (organic) carbon left, which is slightly more than the measured carbon content after the hydrothermal reaction. Therefore, the carbon content decrease after the hydrothermal treatment was partly caused by the decomposition of carbonate, but also partly by the dissolution or removal of the organic material originally coated on the surfaces of the fine solids. 3.8. Sulfur Contents. The data of Table 4 show the sulfur content in the fine solids before and after the hydrothermal

content of those particles was much lower than that of the fine solids from other layers. After the hydrothermal treatment, most fine solids stayed in the toluene phase but their carbon content was lower than that of the unreacted fine solids that partitioned to the toluene phase. The carbon content of the fine solids in the rag layer was lower than that of the fine solids from the toluene layer, but the difference in the carbon content was not as large as that of the unreacted fine solids. XRD analysis was carried out on these fine solids to determine their mineralogical compositions. As the quantity of these fine solids samples was limited, the XRD analysis was only semiquantitative based on the intensities of the peaks. For the unreacted fine solids, siderite and pyrite were slightly more concentrated in the toluene phase than in the rag layer. A high content of siderite can contribute to the carbon content in the fine solids from the toluene phase. However, the carbon content of the fine solids in different phases was so different that siderite concentration was certainly not the only cause. The carbon content of the fine solids in the toluene phase significantly decreased after the fine solids were subjected to the hydrothermal treatment, and yet most of the fine solids stayed in the toluene phase. There was still a difference in the carbon content between the fine solids in the toluene phase and those in the rag layer, but the difference was much smaller. The general observation was that the hydrothermal treatment caused the carbon contents of the fine solids in the different phases to be closer to each other, so that they did not show the high disparity observed for the unreacted fine solids. A general observation from the literature is that the presence of adsorbed asphaltenes increases the contact angle of fine solids relative to the clean surface, as observed for silica, clays, and other minerals,22−25 and that the contact angle increases with organic content. Such experiments give much lower carbon content than that observed in the data of Table 2, for example, only 4−4.5 wt % on kaolin,23,25 implying very different surface deposits in the oil sands fine solids. Consequently, this literature is not helpful in explaining the wrong-way behavior in the present study, where hydrothermal treatment gave an increase in contact angle from 50 ± 6° to 78 ± 7° as the carbon content dropped from 18.0 to 15.4 wt %. 3.7. Bulk versus Surface Carbon Contents. The data of Table 3 show the carbon contents of two samples measured by a CNHS elemental analyzer and by XPS, and the apparent contact angles. The reported XPS result is the relative atomic concentration of carbon on the sample surface, to a depth of 7− 10 nm. The carbon content measured by the CNHS elemental analyzer showed an opposite trend to the measured contact angles and XPS analysis. The hydrothermal reaction decreased the total carbon content of the samples, however, but both the

Table 4. Sulfur Content in Fine Solids by Different Analysis Techniques analyze methods

untreated fines

reacted fines

elemental analysis, wt % XPS, (S, wt %)/(Al + Si + Fe, wt %) EDX, (S, wt %)/(Al + Si + Fe, wt %)

4.63 ± 0.31 0.033 0.088 ± 0.01

5.20 ± 0.37 0.12 0.12 ± 0.03

reactions via different analyzing techniques. In this table, the sulfur contents were all reported as relative concentrations. The EDX and CNHS elemental analysis measured the bulk sulfur content of the samples, and the calculated results were close to each other. Different from carbon content analysis, the sulfur content showed an increasing trend after the hydrothermal reaction in all cases. Sulfur content increased by about 17% after the hydrothermal treatment at 392 °C for 30 min based on CNHS elemental analysis, and by 50% based on EDX analysis. However, from XPS analysis, the relative surface sulfur concentrations increased by almost 300% after the reaction. On the basis of the mineralogical analysis, the formation of pyrrhotite could contribute to the increase in the sulfur content. The sulfur content in the unreacted fine solids was 3.21%, and it increased to 7.11% after the hydrothermal reaction with bitumen at 392 °C for 30 min. The sulfur content after the reaction was mostly contributed by pyrrhotite, which implied that the organic sulfur originally associated with the bitumen and the fine solids prior to the reaction participated in the E

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Figure 4. Graphical summary of the behavior of extracted fine solids from bitumen froth before and after hydrothermal treatment.

reactions to form pyrrhotite. The formation of pyrrhotite from siderite and pyrite could help reduce the organic sulfur content by 0.21 wt % if the fines content is 10% in the bitumen froth after the reaction. These results agreed with the results reported by Sankey et al.26 that minerals tended to adsorb sulfur, leading to cleaner bitumen products. 3.9. Conversion of Heavy Bitumen Components. From prior studies, the presence of water and fine solids would have little impact on the conversion of the vacuum residue fraction.27−30 Fine solids delay the onset of coke formation under more severe conditions than in the present study. The conversion of a vacuum residue fraction (boiling over 524 °C) of dry Athabasca bitumen was 8% after reaction at the same temperature and time (392 °C for 30 min). From this result, we infer that any Athabasca bitumen-derived feed would give similar low conversion under the hydrothermal reaction conditions of this study. A conversion of only 8% can be considered low, typical of visbreaking-type processes,31 and no coke was detected after the reaction. Consequently, these results indicate that hydrothermal conditions can dramatically alter the behavior of the fine solids without severe cracking of the bitumen. 3.10. Overall Effect of the Hydrothermal Reactions. A qualitative graphic summary of the overall effect to the fine solids in the bitumen froth after a hydrothermal treatment is shown in Figure 4. If the bitumen froth is directly diluted with toluene or other compatible solvents after applying sufficient agitation, an emulsion will be formed. Small water droplets are dispersed in the diluted bitumen froth, as shown on the left side of Figure 4. Many fine particles stay at the water−toluene interface. Most of the fine solids at the interface are biwettable. Those fine solid particles and asphaltene aggregates (if any) act as steric barriers to prevent water droplets from coalescing, thereby stabilizing the water-in-oil emulsion. Fines can also be observed in the oil phase and within the water droplets. This observation indicates that the contact angle of unreacted fine solids in the bitumen froth was an average of subpopulations of fine solids with very different wettability. This proposition is further proved by the tests on fractionating the fines, which gave very different carbon contents (Table 2). The contact angle of the fine solids increased significantly after the hydrothermal reaction, and they were no longer stably dispersed in toluene (Figure 3). As shown on the right side

of Figure 4, many emulsified water droplets coalesced into a bulk aqueous phase after the hydrothermal reaction (we have indeed observed that the bitumen froth separated into an oil layer (top) and a water layer (bottom) after hydrothermal reaction). Only a few fine solid particles remained at the water−toluene interface, while most of the fine particles went to the toluene phase because of the precipitation or adsorption of hydrocarbon on their surfaces during the hydrothermal treatment. Water-wet fine solids can no longer be observed in the aqueous phase. The wettability of the fine solids became more uniform after the hydrothermal treatment. The carbon content of the fine solids from different phases, as discussed in previous sections, also became more uniform after the hydrothermal treatment. This study did not investigate hydrothermal behavior for all possible compositions of bitumen froth, which encompass a range of mixtures of minerals, water chemistries, and solid:bitumen and water:bitumen ratios. Studies on a range of froth compositions are required prior to attempting to scale-up the present study for industrial application, to demonstrate the feasibility of filtering hydrothermal-treated froth.

4. CONCLUSIONS Fine solids in Athabasca bitumen froth samples were extracted by toluene and were subjected to a hydrothermal treatment with water and Athabasca bitumen at 392 °C. The amount of heavy organic coating on the solid surface was reduced by reaction, but coverage was more uniform, which significantly increased the hydrophobicity of the fine solids. In addition, the wide range of the surface properties of the initial particles with a high disparity of wettability was made uniformly hydrophobic. After hydrothermal treatment, most of the fine solids stayed in the oil phase so that the water-in-oil emulsion was less stabilized. The filterability of the bitumen froth was significantly improved partly due to the destabilization of the water-in-oil emulsions as a result of the hydrothermal treatment. Although siderite and pyrite in the extracted fine solids were converted to pyrrhotite, no significant changes to the clay minerals, kaolinite and illite, were observed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F

DOI: 10.1021/acs.energyfuels.5b02325 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Present Address

(29) Tanabe, K.; Gray, M. R. Energy Fuels 1997, 11, 1040−1043. (30) Sanaie, N.; Watkinson, A. P.; Bowen, B. D.; Smith, K. J. Fuel 2001, 80, 1111−1119. (31) Gray, M. R. Upgrading Oilsands Bitumen and Heavy Oil; University of Alberta Press: Edmonton, AB, Canada, 2015.

†

The Petroleum Institute, P.O. Box 2533, Abu Dhabi, UAE.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Institute for Oil Sands Innovation (IOSI) at the University of Alberta. We thank the staff from IOSI for their technical support.

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DOI: 10.1021/acs.energyfuels.5b02325 Energy Fuels XXXX, XXX, XXX−XXX