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Aggregation of silica particles in non-aqueous media
Fuel 90 (2011) 2592–2597
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Fuel journal homepage: www.elsevier.com/locate/fuel
Aggregation of silica particles in non-aqueous media Ying Jin a, Weikang Liu b, Qi Liu c, Anthony Yeung c,⇑ a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China c Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2V4 b
a r t i c l e
i n f o
Article history: Received 10 February 2010 Accepted 20 April 2011 Available online 1 May 2011 Keywords: Silica Hydrocarbon Non-aqueous colloids Paraffinic froth treatment
a b s t r a c t The aggregation of micron-sized silica particles in non-aqueous (i.e. hydrocarbon) media was examined on both the macroscopic and microscopic scales. The silica surfaces were either ‘‘clean’’ or ‘‘treated’’ (i.e. with irreversibly adsorbed materials from Athabasca bitumen); the hydrocarbons were mixtures of toluene and heptane at various ratios (to allow for different degrees of ‘‘aromaticity’’ in the solvent). On the macroscopic scale, gravity settling of the silica beads in non-aqueous media was monitored, and particle– particle interactions were characterized semi-empirically by the initial rates of sedimentation. On the microscopic scale, adhesive forces between individual glass spheres were directly measured using the microcantilever technique (again, in non-aqueous liquids). It was found that, for clean silica spheres, the settling rates of the suspensions were relatively insensitive to the interparticle adhesive forces. This is in contrast to the case for treated silica beads, where strong correlation was observed between the settling rate and particle–particle adhesion. These findings may have important relevance to the commercial ‘‘paraffinic froth treatment’’ process. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Canadian oil sands in northern Alberta represent the second largest proven reserve in the world (second only to that in Saudi Arabia); the hydrocarbon embedded in the sand is in the form of bitumen — a type of extra heavy crude oil [1,2]. Currently, bitumen is extracted from the oil sand via a water-based process that proceeds as follows: Oil sand ore is first mined using trucks and shovels; it is then ‘‘slurried’’ in warm water to enable its transport through pipelines. Under favorable conditions (i.e. with properlycontrolled temperature, water chemistry, mechanical agitation, etc.), bitumen fragments are liberated from the sand grains and become aerated (i.e. the fragments spontaneously spread onto air bubbles that are present in the slurry). The first stage of the separation process is achieved as the aerated bitumen is fed into large flotation vessels and collected at the top as an oil-rich froth [3,4]. Unfortunately, such a froth contains not just air and bitumen; it has also substantial amounts of unwanted water and fine solids that are entrained in the froth layer [5]. Removal of water and solids from the bitumen froth represents the next stage of separation (subsequent to flotation) known as ‘‘froth treatment.’’ Froth treatment begins with dilution of the bitumen froth with an organic solvent to create an oil-continuous medium that contains emulsified water and suspended solids (the oil phase in this case is ⇑ Corresponding author. E-mail address: [email protected] (A. Yeung). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.04.030
solvent-diluted bitumen). The reason for diluting the bitumen froth is to create a liquid hydrocarbon of lowered viscosity and density so that the impurities (i.e. the water droplets and fine solids) could, in principle, be eliminated by gravity settling and/or centrifugation [6,7]. For froth treatment processes that employ naphtha as diluting solvent, the results are less than ideal: even after multiple stages of centrifugation, the diluted bitumen product still contains 2–5 wt.% water (in the form of micron-sized droplets) and up to 1 wt.% fine solids [6,8,9]. In contrast, it was discovered serendipitously that if, instead of naphtha, a paraffnic solvent was used as diluent, all water drops and fine solids will aggregate and settle readily under gravity, leaving a supernatant (diluted bitumen) that is practically free of any impurity; this is the socalled ‘‘paraffinic froth treatment’’ or PFT process. Although much attention had been given to the PFT process [6,10,11], the underlying mechanism behind this method of separation remains not understood. It is not clear, for example, if the ‘‘floc networks’’ which result from asphaltene precipitation are responsible for entrapping and collecting all water droplets and fine solids in the hydrocarbon medium [12]. More subtle — but equally important — is whether the water droplets play an essential role in solids removal. (Stated differently: In the absence of emulsified water, is the PFT process capable of aggregating and removing suspended fine solids, or is water essential as a ‘‘collector’’ of the largely hydrophilic solids?) It is our goal to uncover the fundamental mechanism(s) behind the PFT process. As a first step, we will, in this study, examine a somewhat simplified system that by design
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2. Materials and methods 2.1. Basic materials Silica spheres (0.25 lm) were purchased from Fiber Optic Center Inc. (Massachusetts, USA). Two types of organic solvents were used in this study: toluene and n-heptane (both HPLC grade); these solvents were used without further purification. Bitumen samples (the so-called ‘‘DRU bottoms’’) were obtained from Syncrude Canada Ltd. This bitumen was the product of a naphtha-based froth treatment process and may contain trace amounts of solids (mostly clays). To eliminate these solids, the bitumen was diluted in toluene at a 1:1 mass ratio (to lower its viscosity) and centrifuged at 35,000g for 2 h. (Based on Stokes law, particles larger than ca. 20 nm in diameter would be removed [13].) After centrifuging and discarding the solids, toluene was allowed to evaporate from the supernatant until the original bitumen mass was recovered, leaving a sample of what was considered to be solids-freebitumen. 2.2. Preparation of silica beads The silica beads in this study were either ‘‘clean’’ or ‘‘treated’’ (i.e. bitumen-coated). Clean silica spheres were made by heating the samples at 650 °C for 6 h to vapourise any surface materials (the silica beads, as received, may have polymeric coatings on their surfaces). To prepare bitumen-treated silica, we began by dispersing 5 g of clean (i.e. heat-treated) spheres in a 100 mL toluene-diluted bitumen solution (at 1:1 toluene-to-bitumen mass ratio). The mixture was first sonicated for 5 min to break apart any aggregates; it was then stirred at 1000 rpm for 2 h using a standard magnetic stirrer to allow adsorption of bitumen materials (e.g. asphaltene molecules) onto the silica surfaces. The intention here was to create silica beads which mimic the indigenous particulates found in oil sand ores. Next, the silica beads were washed multiple times in toluene (i.e. repeated centrifugation, decantation of supernatant, and addition of fresh toluene) until the supernatant was clear. The solids were then recovered and dried under a fume hood. The resulting silica beads appeared as a slightly blackened powder owing to a layer of irreversibly adsorbed bitumen material on the sphere surfaces; any loose deposit on the silica would have been removed during the washing procedure.
gravity settling began — was defined as t = 0. The rate of sedimentation was quantified as follows: Small samples of the suspension were collected at a fixed location in the container at different times. In our experiments, samples of volume 0.5 mL were collected at a depth of 1 cm from the free surface (along the centerline of the container; see Fig. 1a). The solids contents in the samples were determined by ‘‘ashing,’’ i.e. by placing the 0.5 mL sample in a muffle furnace at 650 °C until all organic matters vapourised and only the inorganic materials remained. The mass of the inorganic material m (of order milligrams) was determined using a microbalance (Mettler Toledo, model MX5). As the silica spheres settled to the bottom of the jar, the solids contents in the collected samples would decrease; a typical plot of the solids mass m versus the settling time t is shown in Fig. 1b. In this study, we chose the initial slope of such a plot, denoted dm/dt, as a rough measure of the settling rate of the suspension. 2.4. Direct measurement of interparticle adhesion To complement the sedimentation (macro-scale) studies, we conducted also direct measurements of the adhesive forces between individual glass spheres (diameters of order 10 lm). Just as for the submicron silica particles in the jar settling tests, the glass spheres here were given the same surface treatments (i.e. clean and bitumen-treated) and were made to interact across the same hydrocarbon media (i.e. mixtures of toluene and n-heptane). The force measurements were made using micropipettes. This is a technique of studying micron-sized objects with the use of small glass capillaries. It was developed originally in the field of biological and biophysical sciences [14,15], and has since been adapted for applications in engineering science and oil sands research [16–18]. To measure the adhesive force between two glass spheres, the following adaptations to the micropipette were made: First, two borosilicate glass ‘‘spheres’’ were created by melting the tips of two micropipettes into rounded shapes; this was done by bringing the pipette tips close to a hot platinum wire. As seen in the photographs in Fig. 2, the resulting tips had radii of curvature of order 10 lm. To enable force detection, one of the micropipettes was bent into a ‘‘periscope’’ shape (by heating local regions of the pipette) so that it would deflect as a cantilever under axial load. This so-called ‘‘microcantilever technique’’ had been used in the past to directly measure interfacial tensions of emulsion drops [19], and
(a)
(b) solids mass m
involves neither asphaltene precipitation nor emulsified water. To be clear, the interaction between bitumen-coated silica particles in ‘‘pure’’ solvents (i.e. those that do not contain bitumen) will be studied. No water will be present in such a system. Moreover, without dissolved bitumen in the bulk liquid, asphaltene precipitation — and the formation of asphaltenic floc networks — is avoided entirely. The only bitumen that is present is in the form of an irreversibly adsorbed layer at the silica surface. We will, in this study, determine the influence of this adsorbed layer on particle–particle adhesion in non-aqueous environments.
2.3. Sedimentation of silica spheres in bitumen-free solvent This was a semi-empirical study of particle–particle interactions on the macroscopic scale. The sedimentation rate of a silica suspension (in hydrocarbon) was used as a qualitative indicator of interparticle adhesion. The hydrocarbons were mixtures of toluene and n-heptane at various ratios; this was a means of controlling the aromatic content of the solvent. Note that no bitumen was present in the bulk liquid. To begin, a suspension of a given solids mass fraction (typically 1–2 wt.%) was agitated vigorously through sonication. The time at which agitation ceased — and
initial slope dm/dt as measure of settling rate
1 cm
time t Fig. 1. Schematic of the sedimentation experiment. (a) Samples were taken from a fixed location — in this case, 1 cm from the free surface. The amount of solids in each sample was denoted m, and the settling time denoted t. (b) Typical plot of m versus t. The initial slope dm/dt was chosen as a measure of the settling rate.
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(a)
30 µm
had to be determined individually; the accuracy was typically ±5%.) Knowing the stiffness kc, the adhesive force between two ‘‘glass beads’’ was determined by the procedure shown in Fig. 2b. First, the tip of the straight pipette (on the left) was brought into contact with the tip of the cantilever pipette (on the right); the thick section of the cantilever (see Fig. 2a) was kept stationary throughout the experiment. Subsequent to making contact, the straight pipette was pulled back slowly. As seen in the microscope images (Fig. 2b), adhesion between the glass tips was strong enough to deflect the cantilever pipette, giving rise to a maximum deflection d before separation between the tips occurred. (As expected, the cantilever tip sprang back to its exact original position after separation.) The cantilever deflection d could easily be determined from the microscope images, and the corresponding adhesive force F was then obtained from the relation
F ¼ kc d As alluded to earlier, the pipette tips in this study were given the same surface treatments as the silica beads in sedimentation tests (i.e. either ‘‘clean’’ or ‘‘treated’’ surface). In particular, pipette tips subsequent to melting were considered ‘‘clean’’ (any impurity would be vapourised long before the glass melting temperature was reached), whereas ‘‘bitumen-treated’’ surfaces were prepared by immersing clean rounded tips into toluene-diluted bitumen (at 1:1 toluene-to-bitumen mass ratio) for 10–12 h, then rinsed repeatedly — followed by sonication — in toluene.
(b)
3. Results and discussion
30 µm Fig. 2. (a) A sketch of the microcantilever experiment for determining adhesive forces between two rounded pipette tips (which function as glass beads). The cantilever pipette on the right was kept stationary throughout; its only motion was its deflection as the pipette on the left was pulled back. (b) Actual microscope images of a force-measuring experiment. The cantilever deflection d provided a direct measure of the adhesive force.
also to quantify the strength of various aggregate structures [11,20]. In this study, a very similar approach was used to determine the adhesive force between individual glass spheres (i.e. the rounded tips) in non-aqueous media; a sketch of the experiment is shown in Fig. 2a. To be able to quantify forces, the effective stiffness of the cantilever must be known. This stiffness could be calculated straightforwardly from linear beam theory: With knowledge of the Young’s modulus of borosilicate glass (approximately 0.7 1011 Pa) and the detailed geometry of the ‘‘periscope pipette,’’ the cantilever stiffness kc was obtained from integration of the one-dimensional flexural equation for slender beams. A detailed account of this analysis, as well as its experimental verification, can be found in an earlier article [19]. In this study, the cantilever stiffness kc was approximately 20 nN/lm. (The kc of every microcantilever
1.6
settling rate dm/dt (mg/s)
δ
We begin by considering the sedimentation of silica spheres in bitumen-free solvents. Fig. 3 shows the settling rates (measured by dm/dt; see Fig. 1b) of clean and treated silica particles in different organic solvents — specifically, mixtures of n-heptane and toluene. (Every measurement in Figs. 3 and 4 was repeated at least three times.) It is important to first note that, in the limiting case of dilute, monodisperse, and non-aggregating silica, the settling rate dm/dt should in principle vanish — at least up to the moment when the clear liquid/suspension interface reaches the sampling point [21]. In our experiments, the lowest dm/dt values, which invariably corresponded to solvents with toluene contents of 70 vol.% or higher, were of order 104 mg/s. Such settling rates, if plotted in Fig. 3, would appear to be effectively zero. (Here, we associate these negligible settling rates to perfectly dispersed primary particles. The fact that dm/dt was not identically zero may be a result of a number of minor reasons, e.g. slight deviations of particles sizes from its mean.) It follows that any ‘‘non-zero’’ dm/dt in Fig. 3 must nec-
1.2 2 wt % untreated silica 2 wt % treated silica
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volume % toluene in solvent Fig. 3. Sedimentation of clean (i.e. untreated) and bitumen-treated silica in hydrocarbon solvents. The solvents were mixtures of n-heptane and toluene at various ratios.
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settling rate dm/dt (mg/s)
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(a) 2 wt % solids 1 wt % solids 0.5 wt % solids
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volume % toluene in solvent Fig. 4. Sedimentation of bitumen-treated silica at different mass concentrations. The curve for 2 wt.% solids is re-plotted from Fig. 3.
essarily be due to appreciable flocculation of the 0.25 lm silica. As the clean beads (solid squares in Fig. 3) settled much faster than bitumen-treated beads (open circles in the same figure), it is reasonable to infer that the aggregate sizes were bigger in the case of clean silica. Continuing further, we postulate that the particle– particle attraction between clean silica beads must be stronger than that between bitumen-treated beads: flocs that are formed under strong interparticle adhesion must be more robust and are therefore less likely to be disrupted (i.e. they will exist as larger aggregates under shear). Lastly, it is seen from Fig. 3 that, for both clean and bitumen-treated silica, the settling rates decreased with increasing solvent ‘‘aromaticity’’ (i.e. toluene content). As the settling rates are plotted on an absolute scale, one may further note that the dependence of dm/dt on solvent aromaticity was weak for the clean spheres (i.e. the values of dm/dt for pure heptane and pure toluene were not significantly different), whereas the functional dependence was far stronger in the case of treated beads (i.e. the ratio of dm/dt for pure heptane to dm/dt for pure toluene was very large — effectively ‘‘infinite’’). The sedimentation curve for treated silica (open circles in Fig. 3) is re-plotted in Fig. 4 on a magnified scale, along with settling data from experiments conducted at lower solids concentrations. As expected, the settling rates were slower for lower solids contents. This is not a surprising result, as the collision frequency for differential settling depends strongly on particle number density. What is intriguing, however, is that all three curves approached zero as the toluene content in the solvent was roughly 70 vol.% or higher, suggesting that no flocculation occurred at these higher aromatic contents; it follows that the interparticle attraction must be negligible in this regime. Fig. 5 presents visual verification of our earlier conjecture — that the rate of settling was directly related to the size of the particle aggregates. The microscope images are of suspensions of bitumen-treated silica, with 0.25 lm silica spheres as primary particles; the solids contents were 2 wt.% in all three cases. The silica spheres in Fig. 5a were suspended in 100% toluene. The solvent medium appeared to be completely devoid of particles, as the dispersed spheres were smaller than the wavelength of optical light and were therefore invisible. As the aromaticity of the solvent decreased (toluene contents of 50 vol.% in Fig. 5b, and 0% in Fig. 5c), it is clear that the primary particles began to manifest themselves as large aggregates, with the floc size appearing to increase monotonically with decreasing solvent aromaticity. In addition to settling experiments (i.e. macroscopic ‘‘jar tests’’), direct measurements of interparticle adhesion were also carried out on borosilicate glass spheres that were roughly 30 lm in diameter (see description in Materials and Methods section); this was to
(b)
(c)
20 µm Fig. 5. Microscope images of bitumen-treated silica in mixtures of toluene and nheptane at various ratios: (a) 100% toluene. (b) Equal volumes of toluene and heptane. (c) 100% heptane.
provide further insight into the mechanism behind the observed sedimentation phenomena. Fig. 6 shows simultaneous plots of the settling rate (solid squares; re-plotted from Fig. 3) and the corresponding adhesive forces (open triangles) for clean silica/glass surfaces. (Every adhesive force measurement in Figs. 6 and 7 was repeated at least six times using three cantilevers.) Note that the force measurements are reported here as ‘‘raw data’’; they were not scaled by the sphere radius (i.e. not presented as F/R), as is done in many AFM and surface force studies. (We felt there was no reason to report our data as F/R, as it was not established if the adhesive forces were reversible and short-ranged in nature. Also, as the sphere radii R were consistently around 30 lm, the unscaled force F was adequate as a parameter for comparative purposes.) As seen in Fig. 6, although the interparticle adhesive force
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1800 Settling rate of 2 wt % untreated solids Adhesive force of untreated solids
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adhesive force F nN
settling rate dm/dt mg/s
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volume % toluene in solvent Fig. 6. Settling rate (re-plotted from Fig. 3) and the corresponding interparticle adhesive forces for clean (i.e. untreated) solids. It is seen that the settling rate is not much affected by the magnitude of interparticle adhesion.
400 Settling rate of 2 wt % bitumen-treated solids Adhesive force of bitumen-treated solids
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adhesive force F nN
settling rate dm/dt (mg/s)
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volume % toluene in solvent Fig. 7. Settling rate (re-plotted from Fig. 3) and the corresponding interparticle adhesive forces for bitumen-treated solids. Here, strong correlation is seen between settling rate and interparticle adhesion.
varied significantly — from ca. 2000 nN in pure n-heptane to 170 nN in pure toluene — there was no apparent influence of this force on the settling rates (i.e. the parameter dm/dt did not change appreciably). This observation can be rationalized as follows: The weakest adhesive force of ca. 170 nN (at 100% toluene), although seemingly insignificant in Fig. 6, was quite sufficient to create aggregates that were strong enough to withstand the low shear associated with sedimentation. As such, the settling rates were not strongly correlated with the adhesive force. This is in sharp contrast with what is shown in Fig. 7, where the same two parameters, dm/dt and F, are plotted similarly for bitumen-treated surfaces. Here, a strong correlation is seen between the settling rate and adhesive force. (The visual comparison, as presented in Fig. 7, is legitimate since both vertical axes are shown on absolute scales.) The strong correlation suggests that the cohesive forces that hold together the silica aggregates are now comparable to the destructive forces (specifically, shear forces due to sedimentation) which act to tear apart the floc structures. More importantly, however, is the fact that the treated silica beads are realistic model particles which mimic the particulates in a real oil sand ore. The results from Fig. 7 may provide an important clue to the underlying mechanism behind the PFT process: As demonstrated here, particles with adsorbed bitumen material will adhere to each other in a paraffinic environment, resulting in appreciable rates of sedimentation; this was so despite the absence of asphaltene precipitation. This raises an intriguing question: Is the PFT process
predicated on the formation of asphaltenic floc networks (which act to entrap/collect all unwanted particulates), or will the unwanted particles homo-flocculate even in the absence of asphaltene precipitation? At this point, we are not in the position to discount asphaltene precipitation as an important mechanism of particle removal, but the strong interparticle attraction in aliphatic environments, as revealed in this study, must also be considered. Finally, we may speculate on the origin of the interparticle attraction in heptane, and also the absence of such forces in toluene. We propose that such forces are due fundamentally to van der Waals interaction. If the adsorbed layer of bituminous material on the silica/glass surface were ‘‘asphaltene-like,’’ it would extend outward — or ‘‘swell’’ — in toluene (a ‘‘good solvent’’), resulting in a steric barrier which stabilizes the particles. In contrast, when the same particles, with the same adsorbed layer, is immersed in an aliphatic liquid, the asphaltene-like material will shrivel and collapse onto the silica surface (to minimize its contact with the ‘‘bad solvent’’), thus allowing van der Waals attraction to come into effect at short range. This collapsed layer, however, has still a finite thickness and thus will attenuate the van der Waals attraction to some degree. This is consistent with the observation that, in pure heptane, the attractive force between clean glass surfaces was significantly stronger than that between bitumen-treated surfaces. Such a scenario, however, remains purely speculative at this point. 4. Conclusions The interaction between silica beads in non-aqueous environments was examined. The purpose of this study was to gain fundamental insight into the commercial ‘‘paraffinic froth treatment’’ process — a method of removing unwanted particulates from a diluted bitumen medium. The system employed in this study was rather simplified: the only bitumen that was present was in the form of an irreversibly adsorbed layer at the silica surface. It was shown that this adsorbed bitumen layer played a vital role in the colloidal interaction between dispersed particles: In a paraffinic environment, the particle–particle attraction was appreciable, leading to homo-flocculation and sedimentation of the silica beads. However, the same particles in an aromatic environment were colloidally stable, and the interparticle forces were undetectable using the microcantilever technique. Acknowledgements This research was conducted under the auspices of the COSI (Centre for Oil Sands Innovation) program. Y.J. would like to thank Dr. Feng Xin (Tianjin University) for his encouragement, and the China Scholarship Council (CSC) for providing the State Scholarship during her PhD studies. References [1] Anonymous. Worldwide look at reserves and production. Oil Gas J 2004;102(47):22–3. [2] Berkowitz N, Speight JG. Oil sands of Alberta. Fuel 1975;54(3):138–49. [3] Sparks BD, Kotlyar LS, O’Carroll JB, Chung KH. Athabasca oil sands: effect of organic coated solids on bitumen recovery and quality. J Pet Sci Eng 2003;39(3–4):417–30. [4] Liu JJ, Xu ZH, Masilyah J. Interaction forces in bitumen extraction from oil sands. J Colloid Interface Sci 2005;287(2):507–20. [5] Hepler LG, Smith RG. The Alberta oil sands: industrial procedures for extraction and some recent fundamental research. AOSTRA technical publication series No. 14. Edmonton (AB, Canada): Alberta Oil Sands Technology and Research Authority; 1994. [6] Long YC, Dabros T, Hamza H. Stability and settling characteristics of solventdiluted bitumen emulsions. Fuel 2002;81(15):1945–52. [7] Gray M, Xu ZH, Masliyah J. Physics in the oil sands of Alberta. Phys Today 2009;62(3):31–5.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Aggregation of silica particles in non-aqueous media Ying Jin a, Weikang Liu b, Qi Liu c, Anthony Yeung c,⇑ a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China c Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2V4 b
a r t i c l e
i n f o
Article history: Received 10 February 2010 Accepted 20 April 2011 Available online 1 May 2011 Keywords: Silica Hydrocarbon Non-aqueous colloids Paraffinic froth treatment
a b s t r a c t The aggregation of micron-sized silica particles in non-aqueous (i.e. hydrocarbon) media was examined on both the macroscopic and microscopic scales. The silica surfaces were either ‘‘clean’’ or ‘‘treated’’ (i.e. with irreversibly adsorbed materials from Athabasca bitumen); the hydrocarbons were mixtures of toluene and heptane at various ratios (to allow for different degrees of ‘‘aromaticity’’ in the solvent). On the macroscopic scale, gravity settling of the silica beads in non-aqueous media was monitored, and particle– particle interactions were characterized semi-empirically by the initial rates of sedimentation. On the microscopic scale, adhesive forces between individual glass spheres were directly measured using the microcantilever technique (again, in non-aqueous liquids). It was found that, for clean silica spheres, the settling rates of the suspensions were relatively insensitive to the interparticle adhesive forces. This is in contrast to the case for treated silica beads, where strong correlation was observed between the settling rate and particle–particle adhesion. These findings may have important relevance to the commercial ‘‘paraffinic froth treatment’’ process. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Canadian oil sands in northern Alberta represent the second largest proven reserve in the world (second only to that in Saudi Arabia); the hydrocarbon embedded in the sand is in the form of bitumen — a type of extra heavy crude oil [1,2]. Currently, bitumen is extracted from the oil sand via a water-based process that proceeds as follows: Oil sand ore is first mined using trucks and shovels; it is then ‘‘slurried’’ in warm water to enable its transport through pipelines. Under favorable conditions (i.e. with properlycontrolled temperature, water chemistry, mechanical agitation, etc.), bitumen fragments are liberated from the sand grains and become aerated (i.e. the fragments spontaneously spread onto air bubbles that are present in the slurry). The first stage of the separation process is achieved as the aerated bitumen is fed into large flotation vessels and collected at the top as an oil-rich froth [3,4]. Unfortunately, such a froth contains not just air and bitumen; it has also substantial amounts of unwanted water and fine solids that are entrained in the froth layer [5]. Removal of water and solids from the bitumen froth represents the next stage of separation (subsequent to flotation) known as ‘‘froth treatment.’’ Froth treatment begins with dilution of the bitumen froth with an organic solvent to create an oil-continuous medium that contains emulsified water and suspended solids (the oil phase in this case is ⇑ Corresponding author. E-mail address: [email protected] (A. Yeung). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.04.030
solvent-diluted bitumen). The reason for diluting the bitumen froth is to create a liquid hydrocarbon of lowered viscosity and density so that the impurities (i.e. the water droplets and fine solids) could, in principle, be eliminated by gravity settling and/or centrifugation [6,7]. For froth treatment processes that employ naphtha as diluting solvent, the results are less than ideal: even after multiple stages of centrifugation, the diluted bitumen product still contains 2–5 wt.% water (in the form of micron-sized droplets) and up to 1 wt.% fine solids [6,8,9]. In contrast, it was discovered serendipitously that if, instead of naphtha, a paraffnic solvent was used as diluent, all water drops and fine solids will aggregate and settle readily under gravity, leaving a supernatant (diluted bitumen) that is practically free of any impurity; this is the socalled ‘‘paraffinic froth treatment’’ or PFT process. Although much attention had been given to the PFT process [6,10,11], the underlying mechanism behind this method of separation remains not understood. It is not clear, for example, if the ‘‘floc networks’’ which result from asphaltene precipitation are responsible for entrapping and collecting all water droplets and fine solids in the hydrocarbon medium [12]. More subtle — but equally important — is whether the water droplets play an essential role in solids removal. (Stated differently: In the absence of emulsified water, is the PFT process capable of aggregating and removing suspended fine solids, or is water essential as a ‘‘collector’’ of the largely hydrophilic solids?) It is our goal to uncover the fundamental mechanism(s) behind the PFT process. As a first step, we will, in this study, examine a somewhat simplified system that by design
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2. Materials and methods 2.1. Basic materials Silica spheres (0.25 lm) were purchased from Fiber Optic Center Inc. (Massachusetts, USA). Two types of organic solvents were used in this study: toluene and n-heptane (both HPLC grade); these solvents were used without further purification. Bitumen samples (the so-called ‘‘DRU bottoms’’) were obtained from Syncrude Canada Ltd. This bitumen was the product of a naphtha-based froth treatment process and may contain trace amounts of solids (mostly clays). To eliminate these solids, the bitumen was diluted in toluene at a 1:1 mass ratio (to lower its viscosity) and centrifuged at 35,000g for 2 h. (Based on Stokes law, particles larger than ca. 20 nm in diameter would be removed [13].) After centrifuging and discarding the solids, toluene was allowed to evaporate from the supernatant until the original bitumen mass was recovered, leaving a sample of what was considered to be solids-freebitumen. 2.2. Preparation of silica beads The silica beads in this study were either ‘‘clean’’ or ‘‘treated’’ (i.e. bitumen-coated). Clean silica spheres were made by heating the samples at 650 °C for 6 h to vapourise any surface materials (the silica beads, as received, may have polymeric coatings on their surfaces). To prepare bitumen-treated silica, we began by dispersing 5 g of clean (i.e. heat-treated) spheres in a 100 mL toluene-diluted bitumen solution (at 1:1 toluene-to-bitumen mass ratio). The mixture was first sonicated for 5 min to break apart any aggregates; it was then stirred at 1000 rpm for 2 h using a standard magnetic stirrer to allow adsorption of bitumen materials (e.g. asphaltene molecules) onto the silica surfaces. The intention here was to create silica beads which mimic the indigenous particulates found in oil sand ores. Next, the silica beads were washed multiple times in toluene (i.e. repeated centrifugation, decantation of supernatant, and addition of fresh toluene) until the supernatant was clear. The solids were then recovered and dried under a fume hood. The resulting silica beads appeared as a slightly blackened powder owing to a layer of irreversibly adsorbed bitumen material on the sphere surfaces; any loose deposit on the silica would have been removed during the washing procedure.
gravity settling began — was defined as t = 0. The rate of sedimentation was quantified as follows: Small samples of the suspension were collected at a fixed location in the container at different times. In our experiments, samples of volume 0.5 mL were collected at a depth of 1 cm from the free surface (along the centerline of the container; see Fig. 1a). The solids contents in the samples were determined by ‘‘ashing,’’ i.e. by placing the 0.5 mL sample in a muffle furnace at 650 °C until all organic matters vapourised and only the inorganic materials remained. The mass of the inorganic material m (of order milligrams) was determined using a microbalance (Mettler Toledo, model MX5). As the silica spheres settled to the bottom of the jar, the solids contents in the collected samples would decrease; a typical plot of the solids mass m versus the settling time t is shown in Fig. 1b. In this study, we chose the initial slope of such a plot, denoted dm/dt, as a rough measure of the settling rate of the suspension. 2.4. Direct measurement of interparticle adhesion To complement the sedimentation (macro-scale) studies, we conducted also direct measurements of the adhesive forces between individual glass spheres (diameters of order 10 lm). Just as for the submicron silica particles in the jar settling tests, the glass spheres here were given the same surface treatments (i.e. clean and bitumen-treated) and were made to interact across the same hydrocarbon media (i.e. mixtures of toluene and n-heptane). The force measurements were made using micropipettes. This is a technique of studying micron-sized objects with the use of small glass capillaries. It was developed originally in the field of biological and biophysical sciences [14,15], and has since been adapted for applications in engineering science and oil sands research [16–18]. To measure the adhesive force between two glass spheres, the following adaptations to the micropipette were made: First, two borosilicate glass ‘‘spheres’’ were created by melting the tips of two micropipettes into rounded shapes; this was done by bringing the pipette tips close to a hot platinum wire. As seen in the photographs in Fig. 2, the resulting tips had radii of curvature of order 10 lm. To enable force detection, one of the micropipettes was bent into a ‘‘periscope’’ shape (by heating local regions of the pipette) so that it would deflect as a cantilever under axial load. This so-called ‘‘microcantilever technique’’ had been used in the past to directly measure interfacial tensions of emulsion drops [19], and
(a)
(b) solids mass m
involves neither asphaltene precipitation nor emulsified water. To be clear, the interaction between bitumen-coated silica particles in ‘‘pure’’ solvents (i.e. those that do not contain bitumen) will be studied. No water will be present in such a system. Moreover, without dissolved bitumen in the bulk liquid, asphaltene precipitation — and the formation of asphaltenic floc networks — is avoided entirely. The only bitumen that is present is in the form of an irreversibly adsorbed layer at the silica surface. We will, in this study, determine the influence of this adsorbed layer on particle–particle adhesion in non-aqueous environments.
2.3. Sedimentation of silica spheres in bitumen-free solvent This was a semi-empirical study of particle–particle interactions on the macroscopic scale. The sedimentation rate of a silica suspension (in hydrocarbon) was used as a qualitative indicator of interparticle adhesion. The hydrocarbons were mixtures of toluene and n-heptane at various ratios; this was a means of controlling the aromatic content of the solvent. Note that no bitumen was present in the bulk liquid. To begin, a suspension of a given solids mass fraction (typically 1–2 wt.%) was agitated vigorously through sonication. The time at which agitation ceased — and
initial slope dm/dt as measure of settling rate
1 cm
time t Fig. 1. Schematic of the sedimentation experiment. (a) Samples were taken from a fixed location — in this case, 1 cm from the free surface. The amount of solids in each sample was denoted m, and the settling time denoted t. (b) Typical plot of m versus t. The initial slope dm/dt was chosen as a measure of the settling rate.
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(a)
30 µm
had to be determined individually; the accuracy was typically ±5%.) Knowing the stiffness kc, the adhesive force between two ‘‘glass beads’’ was determined by the procedure shown in Fig. 2b. First, the tip of the straight pipette (on the left) was brought into contact with the tip of the cantilever pipette (on the right); the thick section of the cantilever (see Fig. 2a) was kept stationary throughout the experiment. Subsequent to making contact, the straight pipette was pulled back slowly. As seen in the microscope images (Fig. 2b), adhesion between the glass tips was strong enough to deflect the cantilever pipette, giving rise to a maximum deflection d before separation between the tips occurred. (As expected, the cantilever tip sprang back to its exact original position after separation.) The cantilever deflection d could easily be determined from the microscope images, and the corresponding adhesive force F was then obtained from the relation
F ¼ kc d As alluded to earlier, the pipette tips in this study were given the same surface treatments as the silica beads in sedimentation tests (i.e. either ‘‘clean’’ or ‘‘treated’’ surface). In particular, pipette tips subsequent to melting were considered ‘‘clean’’ (any impurity would be vapourised long before the glass melting temperature was reached), whereas ‘‘bitumen-treated’’ surfaces were prepared by immersing clean rounded tips into toluene-diluted bitumen (at 1:1 toluene-to-bitumen mass ratio) for 10–12 h, then rinsed repeatedly — followed by sonication — in toluene.
(b)
3. Results and discussion
30 µm Fig. 2. (a) A sketch of the microcantilever experiment for determining adhesive forces between two rounded pipette tips (which function as glass beads). The cantilever pipette on the right was kept stationary throughout; its only motion was its deflection as the pipette on the left was pulled back. (b) Actual microscope images of a force-measuring experiment. The cantilever deflection d provided a direct measure of the adhesive force.
also to quantify the strength of various aggregate structures [11,20]. In this study, a very similar approach was used to determine the adhesive force between individual glass spheres (i.e. the rounded tips) in non-aqueous media; a sketch of the experiment is shown in Fig. 2a. To be able to quantify forces, the effective stiffness of the cantilever must be known. This stiffness could be calculated straightforwardly from linear beam theory: With knowledge of the Young’s modulus of borosilicate glass (approximately 0.7 1011 Pa) and the detailed geometry of the ‘‘periscope pipette,’’ the cantilever stiffness kc was obtained from integration of the one-dimensional flexural equation for slender beams. A detailed account of this analysis, as well as its experimental verification, can be found in an earlier article [19]. In this study, the cantilever stiffness kc was approximately 20 nN/lm. (The kc of every microcantilever
1.6
settling rate dm/dt (mg/s)
δ
We begin by considering the sedimentation of silica spheres in bitumen-free solvents. Fig. 3 shows the settling rates (measured by dm/dt; see Fig. 1b) of clean and treated silica particles in different organic solvents — specifically, mixtures of n-heptane and toluene. (Every measurement in Figs. 3 and 4 was repeated at least three times.) It is important to first note that, in the limiting case of dilute, monodisperse, and non-aggregating silica, the settling rate dm/dt should in principle vanish — at least up to the moment when the clear liquid/suspension interface reaches the sampling point [21]. In our experiments, the lowest dm/dt values, which invariably corresponded to solvents with toluene contents of 70 vol.% or higher, were of order 104 mg/s. Such settling rates, if plotted in Fig. 3, would appear to be effectively zero. (Here, we associate these negligible settling rates to perfectly dispersed primary particles. The fact that dm/dt was not identically zero may be a result of a number of minor reasons, e.g. slight deviations of particles sizes from its mean.) It follows that any ‘‘non-zero’’ dm/dt in Fig. 3 must nec-
1.2 2 wt % untreated silica 2 wt % treated silica
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0.4
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volume % toluene in solvent Fig. 3. Sedimentation of clean (i.e. untreated) and bitumen-treated silica in hydrocarbon solvents. The solvents were mixtures of n-heptane and toluene at various ratios.
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settling rate dm/dt (mg/s)
0.16
(a) 2 wt % solids 1 wt % solids 0.5 wt % solids
0.12
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volume % toluene in solvent Fig. 4. Sedimentation of bitumen-treated silica at different mass concentrations. The curve for 2 wt.% solids is re-plotted from Fig. 3.
essarily be due to appreciable flocculation of the 0.25 lm silica. As the clean beads (solid squares in Fig. 3) settled much faster than bitumen-treated beads (open circles in the same figure), it is reasonable to infer that the aggregate sizes were bigger in the case of clean silica. Continuing further, we postulate that the particle– particle attraction between clean silica beads must be stronger than that between bitumen-treated beads: flocs that are formed under strong interparticle adhesion must be more robust and are therefore less likely to be disrupted (i.e. they will exist as larger aggregates under shear). Lastly, it is seen from Fig. 3 that, for both clean and bitumen-treated silica, the settling rates decreased with increasing solvent ‘‘aromaticity’’ (i.e. toluene content). As the settling rates are plotted on an absolute scale, one may further note that the dependence of dm/dt on solvent aromaticity was weak for the clean spheres (i.e. the values of dm/dt for pure heptane and pure toluene were not significantly different), whereas the functional dependence was far stronger in the case of treated beads (i.e. the ratio of dm/dt for pure heptane to dm/dt for pure toluene was very large — effectively ‘‘infinite’’). The sedimentation curve for treated silica (open circles in Fig. 3) is re-plotted in Fig. 4 on a magnified scale, along with settling data from experiments conducted at lower solids concentrations. As expected, the settling rates were slower for lower solids contents. This is not a surprising result, as the collision frequency for differential settling depends strongly on particle number density. What is intriguing, however, is that all three curves approached zero as the toluene content in the solvent was roughly 70 vol.% or higher, suggesting that no flocculation occurred at these higher aromatic contents; it follows that the interparticle attraction must be negligible in this regime. Fig. 5 presents visual verification of our earlier conjecture — that the rate of settling was directly related to the size of the particle aggregates. The microscope images are of suspensions of bitumen-treated silica, with 0.25 lm silica spheres as primary particles; the solids contents were 2 wt.% in all three cases. The silica spheres in Fig. 5a were suspended in 100% toluene. The solvent medium appeared to be completely devoid of particles, as the dispersed spheres were smaller than the wavelength of optical light and were therefore invisible. As the aromaticity of the solvent decreased (toluene contents of 50 vol.% in Fig. 5b, and 0% in Fig. 5c), it is clear that the primary particles began to manifest themselves as large aggregates, with the floc size appearing to increase monotonically with decreasing solvent aromaticity. In addition to settling experiments (i.e. macroscopic ‘‘jar tests’’), direct measurements of interparticle adhesion were also carried out on borosilicate glass spheres that were roughly 30 lm in diameter (see description in Materials and Methods section); this was to
(b)
(c)
20 µm Fig. 5. Microscope images of bitumen-treated silica in mixtures of toluene and nheptane at various ratios: (a) 100% toluene. (b) Equal volumes of toluene and heptane. (c) 100% heptane.
provide further insight into the mechanism behind the observed sedimentation phenomena. Fig. 6 shows simultaneous plots of the settling rate (solid squares; re-plotted from Fig. 3) and the corresponding adhesive forces (open triangles) for clean silica/glass surfaces. (Every adhesive force measurement in Figs. 6 and 7 was repeated at least six times using three cantilevers.) Note that the force measurements are reported here as ‘‘raw data’’; they were not scaled by the sphere radius (i.e. not presented as F/R), as is done in many AFM and surface force studies. (We felt there was no reason to report our data as F/R, as it was not established if the adhesive forces were reversible and short-ranged in nature. Also, as the sphere radii R were consistently around 30 lm, the unscaled force F was adequate as a parameter for comparative purposes.) As seen in Fig. 6, although the interparticle adhesive force
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1800 Settling rate of 2 wt % untreated solids Adhesive force of untreated solids
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adhesive force F nN
settling rate dm/dt mg/s
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volume % toluene in solvent Fig. 6. Settling rate (re-plotted from Fig. 3) and the corresponding interparticle adhesive forces for clean (i.e. untreated) solids. It is seen that the settling rate is not much affected by the magnitude of interparticle adhesion.
400 Settling rate of 2 wt % bitumen-treated solids Adhesive force of bitumen-treated solids
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adhesive force F nN
settling rate dm/dt (mg/s)
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volume % toluene in solvent Fig. 7. Settling rate (re-plotted from Fig. 3) and the corresponding interparticle adhesive forces for bitumen-treated solids. Here, strong correlation is seen between settling rate and interparticle adhesion.
varied significantly — from ca. 2000 nN in pure n-heptane to 170 nN in pure toluene — there was no apparent influence of this force on the settling rates (i.e. the parameter dm/dt did not change appreciably). This observation can be rationalized as follows: The weakest adhesive force of ca. 170 nN (at 100% toluene), although seemingly insignificant in Fig. 6, was quite sufficient to create aggregates that were strong enough to withstand the low shear associated with sedimentation. As such, the settling rates were not strongly correlated with the adhesive force. This is in sharp contrast with what is shown in Fig. 7, where the same two parameters, dm/dt and F, are plotted similarly for bitumen-treated surfaces. Here, a strong correlation is seen between the settling rate and adhesive force. (The visual comparison, as presented in Fig. 7, is legitimate since both vertical axes are shown on absolute scales.) The strong correlation suggests that the cohesive forces that hold together the silica aggregates are now comparable to the destructive forces (specifically, shear forces due to sedimentation) which act to tear apart the floc structures. More importantly, however, is the fact that the treated silica beads are realistic model particles which mimic the particulates in a real oil sand ore. The results from Fig. 7 may provide an important clue to the underlying mechanism behind the PFT process: As demonstrated here, particles with adsorbed bitumen material will adhere to each other in a paraffinic environment, resulting in appreciable rates of sedimentation; this was so despite the absence of asphaltene precipitation. This raises an intriguing question: Is the PFT process
predicated on the formation of asphaltenic floc networks (which act to entrap/collect all unwanted particulates), or will the unwanted particles homo-flocculate even in the absence of asphaltene precipitation? At this point, we are not in the position to discount asphaltene precipitation as an important mechanism of particle removal, but the strong interparticle attraction in aliphatic environments, as revealed in this study, must also be considered. Finally, we may speculate on the origin of the interparticle attraction in heptane, and also the absence of such forces in toluene. We propose that such forces are due fundamentally to van der Waals interaction. If the adsorbed layer of bituminous material on the silica/glass surface were ‘‘asphaltene-like,’’ it would extend outward — or ‘‘swell’’ — in toluene (a ‘‘good solvent’’), resulting in a steric barrier which stabilizes the particles. In contrast, when the same particles, with the same adsorbed layer, is immersed in an aliphatic liquid, the asphaltene-like material will shrivel and collapse onto the silica surface (to minimize its contact with the ‘‘bad solvent’’), thus allowing van der Waals attraction to come into effect at short range. This collapsed layer, however, has still a finite thickness and thus will attenuate the van der Waals attraction to some degree. This is consistent with the observation that, in pure heptane, the attractive force between clean glass surfaces was significantly stronger than that between bitumen-treated surfaces. Such a scenario, however, remains purely speculative at this point. 4. Conclusions The interaction between silica beads in non-aqueous environments was examined. The purpose of this study was to gain fundamental insight into the commercial ‘‘paraffinic froth treatment’’ process — a method of removing unwanted particulates from a diluted bitumen medium. The system employed in this study was rather simplified: the only bitumen that was present was in the form of an irreversibly adsorbed layer at the silica surface. It was shown that this adsorbed bitumen layer played a vital role in the colloidal interaction between dispersed particles: In a paraffinic environment, the particle–particle attraction was appreciable, leading to homo-flocculation and sedimentation of the silica beads. However, the same particles in an aromatic environment were colloidally stable, and the interparticle forces were undetectable using the microcantilever technique. Acknowledgements This research was conducted under the auspices of the COSI (Centre for Oil Sands Innovation) program. Y.J. would like to thank Dr. Feng Xin (Tianjin University) for his encouragement, and the China Scholarship Council (CSC) for providing the State Scholarship during her PhD studies. References [1] Anonymous. Worldwide look at reserves and production. Oil Gas J 2004;102(47):22–3. [2] Berkowitz N, Speight JG. Oil sands of Alberta. Fuel 1975;54(3):138–49. [3] Sparks BD, Kotlyar LS, O’Carroll JB, Chung KH. Athabasca oil sands: effect of organic coated solids on bitumen recovery and quality. J Pet Sci Eng 2003;39(3–4):417–30. [4] Liu JJ, Xu ZH, Masilyah J. Interaction forces in bitumen extraction from oil sands. J Colloid Interface Sci 2005;287(2):507–20. [5] Hepler LG, Smith RG. The Alberta oil sands: industrial procedures for extraction and some recent fundamental research. AOSTRA technical publication series No. 14. Edmonton (AB, Canada): Alberta Oil Sands Technology and Research Authority; 1994. [6] Long YC, Dabros T, Hamza H. Stability and settling characteristics of solventdiluted bitumen emulsions. Fuel 2002;81(15):1945–52. [7] Gray M, Xu ZH, Masliyah J. Physics in the oil sands of Alberta. Phys Today 2009;62(3):31–5.
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