Settling Properties of Aggregates in Paraffinic Froth Treatment

Article pubs.acs.org/EF

Settling Properties of Aggregates in Paraffinic Froth Treatment Jan Zawala,*,†,‡ Tadeusz Dabros,† and Hassan A. Hamza† †

CanmetENERGY, Natural Resources Canada, Devon, Alberta, Canada Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland

‡

ABSTRACT: This paper presents results of studies on structural parameters and settling rates of aggregates formed during bench- and pilot-scale paraffinic froth treatment (PFT) at about 80 °C, during extraction of bitumen from the oil sands. The structures of individual aggregates were investigated using light microscopy, and their composition was estimated on the basis of image analysis. In addition, the settling velocities and dimensions of the aggregates were measured. It was found that the energy input during mixing of solvent and froth (proportional to mixing speed and duration) during PFT influenced the composition and settling velocity of the aggregates. Greater energy input resulted in higher concentrations of mineral particles (DS) in the aggregates. In bench-scale experiments, the concentration of minerals in the aggregates increased from 10 wt % to about 65 wt % with increasing energy input. The corresponding settling velocities of the aggregates increased from about 50 mm/min to above 700 mm/min. In pilot-scale tests, the average mineral content of aggregate samples was around 70 wt %, while the settling velocity ranged from 600 to 900 mm/min. It was found that the aggregates formed at higher temperature had relatively dense, nonporous structures consisting mainly of minerals and precipitated asphaltenes, and they settled rapidly. The average density of aggregates formed during pilot-scale tests, estimated on the basis of settling tests, was reaching 2000 kg/m3.

1. INTRODUCTION Typical oil sand is a mixture of coarse sand grains, fine mineral and clay particles, formation water, electrolytes, and bitumen. Bitumen recovery (separation of bitumen from the matrix) is commonly carried out through hot water extraction and froth treatment.1 Bitumen froth is an intermediate material generated during oil sand processing and typically contains about 60 wt % bitumen, 30 wt % water, and 10 wt % solids. To meet bitumen specifications for pipeline transport and upgrading, the water and solids need to be removed. To facilitate separation of these contaminants during froth treatment, the froth is diluted with light hydrocarbons in order to reduce the viscosity of the oil phase and increase the density difference between oil and water phases. During paraffinic froth treatment (PFT) paraffinic solvent is mixed with bitumen froth at the required S/B (solvent to bitumen ratio by mass), which results in the formation of aggregates of emulsified water droplets (WD), dispersed mineral solids (DS), and precipitated asphaltenes (PA).2−4 The aggregated contaminants are easily removed in conventional settlers, which require less energy than other techniques, such as centrifugation. In contrast to the earlier approach to froth treatment using naphtha solvent, PFT significantly reduces contaminant levels (water, mineral particles) in the bitumen product. In addition, removal of some of the asphaltenes reduces the viscosity of the extracted bitumen and can be regarded as partial upgrading of the bitumen. The degree of asphaltene removal depends on S/B, the type of paraffinic solvent, and temperature. The influence of mixing temperature on the structural and settling characteristics of WD/DS/PA aggregates was studied by Long et al.2,3 In their work, the mixing energy of solvent and froth was relatively small and was held constant for all experimental series. They reported that the WD/DS/PA aggregates exhibit zone settling in solvent-diluted bitumen, © 2012 American Chemical Society

including zones of clean oil, hindered settling, and consolidation, and that the type of paraffinic solvent used and the mixing temperature have significant effects on the settling rate of the aggregates formed. The increase in settling velocity of WD/DS/PA aggregates measured at ambient temperature conditions indicated that mixing temperature affected the size of aggregates. The average diameter of aggregates increased almost 2-fold when the temperature during mixing was increased from 30 to 75 °C. The average porosity of WD/ DS/PA aggregates was found to be 0.40−0.75, depending on the S/B and type of solvent used.3 Moreover, the average aggregate density varied within the range 870−1000 kg/m3. The highest measured settling velocity of the WD/DS/PA aggregates was 97 mm/min. The settling velocities observed by Long et al.,2,3 however, were almost order of magnitude lower than values typically observed during PFT carried out in pilot plant tests at about 80 °C. This indicates that, although temperature is an important parameter affecting the settling characteristics of aggregates, it is not the only one. This paper presents results from systematic studies on the influence of mixing energy on the structural and settling properties of aggregates over a much broader range of energy inputs. Samples of diluted froth collected during bench- and pilot-scale PFT experiments were investigated using light microscopy and settling tests in order to determine the reasons for the significantly higher settling velocities of aggregates at elevated temperature.

2. EXPERIMENTAL SECTION 2.1. Materials. Samples of diluted froth obtained during pilot-scale PFT were collected directly from the settler. Similarly, homogenized Received: May 22, 2012 Revised: August 3, 2012 Published: August 20, 2012 5775

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bench-scale tests, due to the different solvent composition used, δdilbit was slightly larger and equal to 15.8 (MPa)1/2. The calculated volume fractions of the mixtures of toluene/n-pentane or toluene/n-heptane solvents having δ = δdilbit are presented in Table 1. To extract

samples of typical bitumen froth from Athabasca oil sands were used in bench-scale PFT runs. Reagent-grade n-pentane (EMD Chemicals Inc.) and n-heptane and toluene (Fischer Scientific) were used as diluents. 2.2. Equipment Setup and Procedure. 2.2.1. Bench-Scale PFT. The 120 g of bitumen froth, consisting of 56% of bitumen, 34% water, and 10% minerals, and 122.7 g of n-pentane solvent were combined in an autoclave (Parr 4843, Parr Instruments Co.) cell and heated slowly to 80 °C. The mixing of froth and solvent was carried out in a 0.0625m diameter (T) cylindrical tank (autoclave cell) with four baffles. The height of the cell was H = 3T, while the clearance C = 0.5T. During heating, the froth and solvent were gently mixed (150−200 rpm) using two coupled disk-style six-blade turbine impellers of diameter D = 0.5T. After the froth and solvent mixture reached 80 ± 2 °C the mixing speed was increased to the desired level (700−1500 rpm) and kept constant for a chosen period of time (2−60 min). The pressure inside the autoclave was maintained at about 0.6 MPa using pressurized nitrogen supplied from an external tank. When the desired experimental conditions were reached, about 50 mL sample of diluted froth was collected into a 250-mL Teflon bottle containing about 50−80 mL diluent prepared according to the procedure elaborated in section 2.2.3. Samples of diluted froth were collected via a cooling coil immersed in a water−ice bath. The transfer of the froth from the autoclave to the Teflon bottles was driven by the pressure difference between the autoclave and the ambient atmosphere. 2.2.2. Settling Tests Inside the Autoclave Cell. During bench-scale PFT experiments, each run was repeated after sample collection. In the repeat runs, instead of sampling, video recordings were made of the freely settling aggregates through the glass window of the autoclave cell. This “in situ” recording of the aggregates passing through the diluted bitumen near the cell window was carried out using a Canon EOS 60D digital camera equipped with a macro lens. 2.2.3. Sample Preparation. The samples of diluted bitumen froth collected during typical PFT processing were completely opaque suspensions of aggregates in diluted bitumen. To view the composition of the aggregates and determine their settling velocities, a sample dilution procedure was applied. Due to the different solubilities of asphaltenes in aromatic and paraffinic solvents, original pentane solvent could not be used for this purpose. To preserve the aggregate structure, a new method of sample dilution was employed whereby the diluent prepared was a mixture of aromatic and paraffinic solvents (toluene combined with suitable proportions of C5 or C7). These proportions were calculated on the basis of the solubility parameter δ of the original diluted froth sample. According to the concept of solubility parameter of a liquid, originally proposed by Hildebrand5 and Scatchard,6 the δ of diluted bitumen (δdilbit) can be calculated as

Table 1. Volume Fractions of Solvents Calculated for Preparation of Diluent with δ = δdilbit pilot-scale PFT bench-scale PFT

ϕC5

ϕtoluene

ϕC7

ϕtoluene

0.70 0.66

0.30 0.34

0.84 0.76

0.16 0.24

aggregates from the opaque diluted bitumen sample collected during a bench or pilot test, a small amount of original suspension was collected in a plastic pipet. The pipet was hold in a vertical position for several seconds to allow the aggregates settle to the pipet tip. Then, these aggregates were released in a new Teflon bottle with fresh diluent prepared according to the procedure described. This approach allows the transfer mainly of aggregates, with small amount of concentrated solution of diluted bitumen. If needed, this procedure of dilution was repeated twice. 2.2.4. Settling Tests Using Square Glass Column at Ambient Conditions. The settling velocities of the resulting individual aggregates were measured at ambient conditions using an experimental setup consisting of (i) a square glass column (130 mm in height and 40 × 40 mm in cross section), (ii) fiber-optic illumination, and (iii) Motic2000 or Canon EOS 60D digital cameras for image acquisition. Pictures of settling aggregates were taken about 10−20 mm above the bottom of the column, long after the terminal (constant) velocity was reached. The column was filled with prepared diluent and covered by an aluminum foil cap to prevent solvent evaporation. The aggregates were transferred to the column through a small hole in the middle of the aluminum foil cap using glass or plastic pipettes. Special care was taken to release only one up to few aggregates from the pipet. Figure 1A presents a series of recorded frames in which different positions of

n

δdilbit =

∑ ϕδi i i=1

(1)

where ϕi is a volume fraction of the diluted bitumen components and δi are their solubility numbers. The δi of each component can be calculated as ⎛ ΔE ⎞1/2 ⎛ ΔHi − RT ⎞1/2 δi = ⎜ i ⎟ = ⎜ ⎟ Vi ⎝ Vi ⎠ ⎝ ⎠

Figure 1. Images of aggregates taken during settling tests: (A) individual aggregate in square glass column, (B) aggregates falling in glass cylinder (pilot-scale sample).

(2)

where ΔEi (J·mol−1) is the specific cohesive energy of the liquid, that is, the energy required to separate the molecules present in 1 cm3, hence, to vaporize it, ΔHi (J·mol−1) is the enthalpy of vaporization of the liquid, Vi (m3·mol−1) is the molar volume of the liquid, R (J·K−1·mol−1) is the universal gas constant, and T (K) is temperature. The average composition of partially deasphalted bitumen was taken as 9% asphaltenes and 91% maltenes. The S/B was equal to 1.8. The thermodynamic data for diluted bitumen components as well as the solubility parameters of maltenes (δm) and asphaltenes (δa) obtained from the literature7−10 are δm = 19.1 (MPa)1/2, while δa is in the range 19−21 (MPa)1/2. In our case δa = 21 (MPa)1/2 was used. It was found, on the basis of eq 1, that the solubility parameter of samples of diluted bitumen taken during pilot-scale PFT was δdilbit = 15.6 (MPa)1/2. For

a settling aggregate can be seen. The pictures recorded were analyzed using Sigma Scan Pro 5.0 and/or ImageJ 1.45s software. The settling velocity of an individual aggregate (u) was calculated as

u=

ΔL Δt

(3)

where ΔL = ((xi+2 − xi+1) + (yi+2 − yi+1) ) , (xi+2 − xi+1) and (yi+2 − yi+1) are the coordinates of the subsequent position of the aggregate, and Δt is the interval of photo acquisition. The settling velocity was calculated as the average from 5 to 15 individual values determined. 2

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The aggregate equivalent diameter (deq, the diameter of a sphere having identical area) was calculated as

deq =

seen, the shapes and dimensions of aggregates varied, but their composition appears to be generally similar. With darkfield illumination applied, the mineral particles are visible as green areas or shining bright spots, while dark (black) areas indicate the presence of organic phase, most probably precipitated asphaltenes. As can be seen, the aggregates consist of agglomerated and well-packed mineral particles. Moreover, the aggregates are nonporous, and no water droplets can be seen in their structure. 3.2. Composition of AggregatesBench-Scale Tests. Figure 3 presents the micrographs of aggregates formed during bench-scale PFT experiments where mixing speed was kept constant (700 rpm), with mixing duration gradually increasing from 2 to 12 min. As can be seen, although the diameters of the aggregates are comparable, their compositions depended strongly on mixing duration. A similar trend was observed for experiments where mixing time was constant while mixing speed was changed gradually from 700 to 1500 rpm. Clearly, the content of DS making up the aggregates increases with mixing speed and duration. It should be emphasized here that a small amount of water, in the form of droplets incorporated into the aggregate structure, was noticed for the aggregates formed when the mixing time was 2 and 5 min. These aggregates had a more porous structure, with small cavities filled by the solvent. However, for longer mixing times (>5 min) and higher mixing speeds (>700 rpm), no water droplets were observed in the aggregate’s structure. The absence of water in the structure of aggregates was a consequence of rapid water droplet separation observed during the experiments. Whenever the mixing was turned off, large water droplets appeared almost immediately at the bottom of the autoclave cell and were observed to coalesce very quickly, which indicates that under experimental conditions the water-in-oil emulsion was unstable. 3.3. Aggregate Mineral Content vs Mixing Conditions. To determine the influence of mixing speed and duration on aggregate composition, the mineral content of aggregates was estimated for both bench- and pilot-scale samples. Figure 4 illustrates the procedure used for mineral content approximation. The color threshold of the aggregate micrograph was adjusted to delineate the individual components. Based on visual identification, a suitable threshold value was chosen to accurately estimate the differences between areas occupied by mineral particles and the rest of the aggregate (white and black areas in Figure 4B). Then, the area not occupied by solid particles (black) was measured automatically by the software. The content of DS particles was calculated from the difference between the total viewed area of the aggregate and the sum of DS areas measured. To estimate the DS content and express it in terms of weight percentage, it was assumed, based on visual (microscopic) observations of structures of the aggregates (see sections 3.1 and 3.2) presented in Figures 2 and 3, that the aggregates consisted only of aggregated mineral particles (DS) bonded together by small agglomerates of precipitated asphaltene (PA). Moreover, it was assumed that the ratio of DS calculated on the basis of image analysis represents the DS volume fraction of the whole DS/PA three-dimensional aggregate. The densities of the solid particles and precipitated asphaltenes were taken as 2650 kg/m3 and 1250 kg/m3, respectively,1,11,12 and the aggregate diameter was calculated using eq 4. To obtain a deeper insight into the relation between the parameters influencing the density and composition of

A ag π

(4)

where Aag is average area of the falling aggregates determined by image analysis. 2.2.5. Settling Tests Using Glass Cylinder at Ambient Conditions. The settling velocities of the aggregates formed during pilot-scale PFT were measured in a 100-mL glass cylinder. The cylinder was filled with 97−99 mL of diluent. About 1−3 mL of fresh sample of diluted froth was added to the cylinder directly from a settler sampling coil and videos of the settling aggregates were recorded using the Canon EOS 60D digital camera. Figure 1B presents the example of a single frame extracted from one of the videos recorded. As seen in this case, the aggregate suspension was relatively concentrated. The settling velocity and size distribution of the aggregates were calculated on the basis of eqs 3 and 4. 2.2.6. Light Microscopy. Photos of individual aggregates from the samples collected during PFT tests were taken in the covered cell using a light microscope (Zeiss Axiovert 100A microscope with an XCite 120Q Lumen Dynamics lamp). To investigate aggregate composition, darkfield illumination from above was applied. Additionally, the sample was slightly illuminated from below to intensify the background contrast. The Canon EOS 60D digital camera was coupled to the microscope by means of a suitable adapter and used for image acquisition. To increase the depth of field of the final image, a series of micrographs (25−80) were taken of each aggregate, with the focal point set at evenly spaced intervals into the depth of field. Then, the images in the series were transferred to a PC and stitched together using Adobe Photoshop CS5 software. 2.2.7. Image Analysis. The DS (minerals) content in the aggregates was determined on the basis of image analysis. The series of aggregate micrographs registered at similar illumination conditions was processed using ImageJ 1.45s software. The procedure for estimating DS content was based on adjustment of the color threshold, which, under the illumination conditions applied, was different for different aggregate components. The amount of DS in the aggregate structure was estimated from the difference between the areas occupied by mineral particles and the total area of the aggregate in view. The DS contents of aggregates formed under a given condition were calculated as averages from 10 individual images processed.

3. RESULTS AND DISCUSSION 3.1. Composition of AggregatesPilot-Scale Tests. Micrographs of typical aggregates extracted from samples collected during pilot-scale PFT are presented in Figure 2. As

Figure 2. Micrographs of aggregates from samples collected during pilot-scale PFT. 5777

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Figure 3. Images of aggregates formed during bench-scale PFT mixing at 700 rpm for 2−12 min.

Figure 4. Aggregate (A) observed under microscope, (B) after color threshold adjustment, and (C) after procedure of area calculations.

Figure 5 presents estimated values of the weight percentage of DS in the aggregates formed during bench-scale PFT as a

aggregates, the results of image analysis (wt % DS) were compared with mixing energy input calculated theoretically for both bench- and pilot-scale tests. In the case of bench-scale PFT, where impeller mixer was used, the energy input per unit mass (E) during mixing is given by

E=

Ptmix m

(5)

where m is the mass of froth−solvent mixture, tmix is mixing duration, and P is the power dissipated in the fluid by the impeller. P can be expressed as13 P = fr ρN3D5

(6)

where f r is a factor (power number) depending on the impeller type and geometry, N is the rotation speed of the impeller (in rpm), and D is a impeller diameter. The value of f r = 2 was taken from the literature14 for an impeller of similar type and geometry. During the pilot-scale tests, static mixers were used to combine solvent and bitumen froth. In this case, the pressure drop (Δp) across a static mixer is a measure of the mixing energy.15 The power required to overcome the pressure drop across the static mixer is given by P = Q v Δp

Figure 5. Mineral solids content in aggregates as a function of mixing energy during bench-scale tests.

function of mixing energy. The quantitative results confirmed the qualitative observationshigher mixing energy during PFT results in aggregates with higher concentrations of mineral particles. For the range of E studied, the DS content varies from about 7 ± 2 wt % to 64 ± 6 wt %. For aggregates obtained from several series of pilot-scale PFT tests, the average DS content was as high as 69 ± 5 wt %, which is an upper limit expected for aggregates without water, based on mineral content of the bitumen froth and amount of asphaltenes precipitated during the process.

(7)

where Qv is the volumetric flow rate of the solvent−froth mixture. The mixing energy can be calculated as E=

Q v Δp Qm

(8)

where Qm is the mass flow rate of the mixture. 5778

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3.4. Settling Velocities of Aggregates. Comparison of the densities of the DS and PA aggregate components suggests that increasing the DS content should increase aggregate density. Figure 6 presents theoretically calculated values of

Figure 7. Average settling velocities of aggregates observed in the autoclave cell (bench-scale tests).

mixture, the amount of mixing energy per unit mass of solvent−froth mixture (eqs 5−6) during the experiment reported by Long et al.3 was about 0.3 kJ/kg, in good agreement with the data presented in Figure 7. A similar graph for aggregates from samples collected during pilot-scale PFT is presented in Figure 8. Generally, it can be

Figure 6. Density of mineral/asphaltenes aggregate (DS/PA) as a function of mineral content.

overall DS/PA aggregate density plotted as a function of DS content. The graph confirms that the density of DS/PA aggregates increases with DS content. According to the general expression describing the balance between gravitation, buoyancy, and drag forces acting on a settling solid sphere, the settling velocity can be calculated as

u=

4gdΔρ 3C Dρl

(9)

where g is the gravitational constant, d is the sphere diameter, Δρ is the density difference between the solid (ρs) and liquid (ρl) phases, and CD is a drag coefficient. For creeping flow conditions, that is, when Re ≪ 1, CD is equal to 24/Re and eq 9 can be rewritten as the well-known Stokes formula: u=

Δρgd 2 18μ

(10)

It can be seen from eq 10 that under steady-state conditions the settling rate depends mainly on the particle size and the density difference between the solid and liquid phases. Therefore, according to the data presented in Figure 6 we can expect that the settling velocity of aggregates should be positively correlated with DS content. The average settling velocities of aggregates determined by direct observation in the autoclave cell are presented in Figure 7 as a function of mixing energy. The graph shows that, indeed, when mixing energy was increased during PFT processing, DS content was higher in the aggregates formed (see also Figure 5) and a significant increase in settling velocity was observed. It is worth adding here that the maximum settling velocity reported by Long et al.,3 97 mm/min, was the settling velocity of aggregates formed during an experiment in which froth and solvent were mixed for 15 min at 100 °C using an impeller geometry similar to that used in the present work and a rotation speed of 600 rpm. Assuming similar masses of solvent−froth

Figure 8. Average settling velocities of aggregates determined in glass cylinders for pilot-scale samples (line is added to guide the eye).

observed that the range of mixing energies in this case is significantly narrower than in the bench-scale experiments. Nevertheless, the tendency observed for bench-scale tests (Figure 7) can also be observed here. Greater mixing energies are associated with higher settling velocities of aggregates. Of course, the response of settling velocity to increased mixing energy is much weaker in this case, as expected in view of the relatively large standard deviations of the calculated average velocities. The aggregates formed during pilot-scale PFT, where the volumetric flow rate of solvent-plus-froth processed is in the order of several liters per minute, will always be comparatively 5779

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different experimental conditions could be readily distinguished. Again, the settling velocities of the aggregates formed when more mixing energy was added to the system (1500 rpm, 60 min) were significantly higher. The size distributions presented in Figure 10 indicate that this significant difference

inhomogeneous. Moreover, collection of representative samples of aggregates during the pilot-scale tests was extremely difficult. Nevertheless, the tendency for increased aggregate settling velocity with increasing mixing energy can be seen in Figure 8 and was confirmed by the Student’s t-test. According to the ttest performed, the difference between the average settling velocity of aggregates formed for E ranging between 100 and 150 J/kg and the settling velocity measured for about 380 J/kg mixing energy is statistically significant at the significance level α = 0.05. It is also seen in Figure 8 that for pilot-scale PFT processing the measured settling velocities of aggregates ranged between 500 and 900 mm/min. It needs to be emphasized here that these values were obtained during experiments under ambient conditions. During actual PFT processing, aggregate settling rates should be higher due to lower diluted bitumen viscosity and higher density difference between the phases. Taking into account only the effect of liquid viscosity reduction due to the temperature increase of approximately 50 °C, it can be approximated from eq 10 that the settling velocities of aggregates during PFT processing should be higher by a factor of 1.3, which is reduced, however, due to hindered settling conditions. The settling velocities and sizes of individual aggregates measured in the square glass column are plotted graphically in Figure 9. The aggregates were collected during bench-scale

Figure 10. Size distributions of the aggregates collected during benchscale tests under two different mixing conditions: (A) 700 rpm, 6 min, (B) 1500 rpm, 60 min.

cannot be explained by variations in aggregate diameter since the average diameters of the two aggregate populations investigated are comparable, 110−120 μm. The observed aggregate behavior can be attributed to increased aggregate average density, resulting from increased probability of formation of more compact aggregate with higher DS concentration (Figure 5). Similar dependence for individual aggregates formed during pilot-scale PFT is illustrated in Figure 11. Additionally, the theoretical curve representing settling velocity of aggregates as a function of diameter is presented there. The theoretical curve was calculated from eq 9, where CD was estimated according to the empirical relation given by Jimenez and Madsen.16 They reported that, for quartz spheres of 0.06 mm < d < ∼1 mm in water, the CD can be approximated by the following expression, valid for Re ranging from 0.2 to 127: 1 C D = (A + A2 + 16B /Re )2 (11) 3

Figure 9. Settling velocities of individual aggregates taken from samples collected during bench-scale tests, determined in square glass column for two different mixing conditions: black dots −700 rpm, 6 min, white dots −1500 rpm, 60 min.

tests for two different mixing conditions represented by the first and last points of dependency in Figure 5. There is considerable scatter of aggregate velocities within each group of data points in Figure 9. For example, for mixing at 1500 rpm for 60 min (last point of dependency in Figure 5) and aggregates around 100 μm in diameter, the measured settling velocities range from about 1200 to about 2200 mm/min. For mixing at 700 rpm for 6 min (first point of dependency in Figure 5) and the identical aggregate size, the ranges of settling velocities are comparable, ranging from about 200 to about 1000 mm/min. These results suggest that the composition of aggregates formed during pilotscale PFT varied. Nevertheless, the average settling velocities measured for the two populations of aggregates formed under

where A and B are the fitting parameters of the function fitted to empirical data, equal to 0.79 and 4.61, respectively. In our case, the Re of settling aggregates varied between about 0.3 and 15. The density of the solid phase (ρs), (nonporous DS/PA aggregate) was calculated on the basis of DS content estimated by image analysis. For DS content of 69 wt % the DS/PA 5780

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speed and duration, which are both proportional to the overall mixing energy input during PFT. The aggregates observed were practically free of water owing to the elevated temperature conditions, which facilitated coalescence of water droplets (WD) and separation of free water. We found that, by increasing the mixing energy, the DS content of resulting aggregates can be increased from a few percent up to about 70% by weight without significant changes in average aggregate size. The increase in concentration of DS in DS/PA aggregates increases their average density to values as high as around 2000 kg/m3 in pilot processing. As a result, the settling velocity of aggregates increases dramatically. In the case of bench-scale tests, where the DS content of aggregates ranged from a few percent up to 65 wt %, the aggregate settling rate increased from about 50 mm/min to above 700 mm/min, respectively. For samples collected during pilot-scale PFT, where a much narrower range of mixing energies was investigated, settling velocities varied within the range 600−900 mm/min. The results obtained indicate that the significant increase in settling rates of aggregates formed during PFT at elevated temperature compared to a low-temperature process is a consequence of increased probability of formation of aggregate of higher densities resulting from enhanced aggregation of minerals and rejection of water.

Figure 11. Settling velocities and diameters of individual aggregates: points, experimental values determined in square-glass column for the samples collected during pilot-scale PFT, line, theoretical curve calculated for DS/PA with 69 wt % of DS.

aggregate density is 1980 kg/m3. As seen in Figure 11, despite some scatter, the theoretically calculated dependence correlates well with experimental results. This indicates that the average density of aggregates formed during pilot tests can be taken as approximately 2000 kg/m3. The size distribution of aggregates formed during pilot-scale PFT is presented in Figure 12. The average diameter of the aggregates formed was about 70−80 μm, less than the average diameter of aggregates formed during bench-scale tests.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to express their gratitude to Canadian government’s Panel of Energy Research and Development (PERD) for financial support of this project.

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REFERENCES

(1) Masliyah, J. H.; Czarnecki, J.; Xu, Z. Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands, Vol. I: Theoretical Basis; Kingsley Knowledge Publishing: Alberta, Canada, 2011. (2) Long, Y.; Dabros, T.; Hamza, H. Fuel 2002, 81, 1945−1952. (3) Long, Y.; Dabros, T.; Hamza. Fuel 2004, 83, 823−832. (4) Shelfantook, W. E. Can. J. Chem. Eng. 2004, 82, 704−709. (5) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-Electrolytes; Reinhold: New York, 1959. (6) Scatchard, G. Chem. Rev. 1949, 44 (1), 7−35. (7) Koenhen, D. M.; Smolders, C. A. J. Appl. Polym. Sci. 1975, 19, 1163−1179. (8) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1985. (9) Mannistu, K. D.; Yarranton, H. W.; Masliyah, J. H. Energy Fuels 1997, 11, 615−622. (10) Aray, Y.; Hernandez-Bravo, R.; Parra, J. G.; Rodríguez, J.; Coll, D. S. J. Phys. Chem. A 2011, 115, 11495−11507. (11) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J.X.; Gill, B. S. Petroleum Sci. Technol. 1998, 16, 251−285. (12) Luo, P.; Gu, Y. Fuel 2007, 86, 1069−1078. (13) Chapple, D.; Kresta, S. M.; Wall, A.; Afacan, A. Trans. Inst. Chem. Eng. 2002, 80A, 364−372. (14) Perry, R. H.; Chilton, C. H. Chemical Engineer’s Handbook, 5th ed.; The McGraw-Hill Companies, Inc.: New York, 1973. (15) Loung, T. C. T. Procesni a Zapracovatelska Technika 2002, 1−9. (16) Jimenez, J. A.; Madsen, O. S. J. Waterw. Port Coastal Ocean Div., Am. Soc. Civil Eng. 2003, 129, 70−79.

Figure 12. Size distributions of the aggregates collected during pilotscale PFT.

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CONCLUSIONS The results of bench-scale tests and data obtained for pilot-scale PFT show that the mixing energy input during PFT at elevated temperature (about 80 °C) profoundly affects the structural and settling properties of aggregates formed. Aggregates formed during the tests consisted mainly of minerals and precipitated asphaltenes (DS/PA) in concentrations that depend on mixing 5781

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