What Fraction of the Asphaltenes Stabilizes Water-in-Bitumen

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What Fraction of the Asphaltenes Stabilizes Water-In-Bitumen Emulsions? Jair Rocha, Elaine Baydak, and Harvey William Yarranton Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03532 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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What Fraction of the Asphaltenes Stabilizes Water-In-Bitumen Emulsions? J.A. Rocha1, E.N. Baydak1, H.W. Yarranton1* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada; * Corresponding author: Email: [email protected], Phone: 1-403-220-6529

ABSTRACT It is hypothesized that only a fraction of the asphaltenes act to stabilize emulsions and that this fraction consists of the most self-associated (least soluble) asphaltenes. To test the hypothesis, the effects of removing the least soluble versus the most interfacially adsorbed asphaltenes on emulsion stability, film properties, and mass surface coverage were compared. The least soluble asphaltenes were removed by precipitation from solutions of asphaltenes in heptane and toluene. The most adsorbed asphaltenes were removed by separating an asphaltene stabilized emulsion from its continuous phase. Brine-in-oil emulsions were prepared using organic phases of 10 g/L of the residual asphaltene fractions from the supernatant or continuous phase. The stability of the emulsions was assessed in terms of percentage of water resolved after repeated treatment cycles involving heating at 60°C and centrifugation at 3500 rpm. The three asphaltenes examined were extracted from a mined oil sand bitumen, a bitumen from a cyclic steam process, and a bitumen from a SAGD process.

Only some of the species in the asphaltenes were found to strongly stabilize emulsion and the size of this fraction ranged from 2 to >65% in the three samples of this study. The most adsorbed, highly stabilizing material tended to be concentrated in the least soluble fraction of the asphaltenes, consistent with the proposed hypothesis. The emulsion stability data were generally consistent with a previously observed threshold of 5 mg/m² asphaltene surface coverage for stable emulsions. Fractionating the asphaltenes eventually removed enough of the self-associated material that the surface coverage dropped below the threshold and unstable emulsions were observed.

Key Words:

oil sand bitumen, asphaltene fractionation, mass adsorption, water-in-oil

emulsion, emulsion stability, salinity, surface active asphaltenes.

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1. Introduction In heavy oil and bitumen processes, the most challenging emulsions to break are water-in-oil emulsions. These emulsions are typically stabilized from components within the asphaltene fraction which adsorb at the water-oil interface and form rigid interfacial films that prevent coalescence1–8.

Asphaltenes are a solubility class containing hundreds of thousands of

species9 and it is likely that only some of these species act to stabilize emulsions. In other words, the type and concentration of the stabilizers is not known and therefore the response to dilution or additives is challenging to predict.

The surface activity of asphaltenes likely arises from the presence of polar heteroatomic functional groups within the hydrocarbon skeleton of the asphaltene molecules10,11. Several authors have reported high heteroatom content in asphaltenes recovered from the water-oil interface7,12,13. Varadaraj et al.14 inferred that the nitrogen and oxygen bearing moieties are most likely to contribute to asphaltene surface activity. Czarnecki et al.7 observed that the interfacial material collected from the skin of water droplets emulsified in bitumen contained O3S2 class groups. Qiao et al.15,16 found that the asphaltenes most enriched with sulfoxides contributed most to interfacial film elasticity and emulsion stability. McKenna et al.17 detected and characterized the presence of vanadyl porphyrins in heavy oil bitumen. While the abundance of porphyrins in bitumen is low (3 wt% of the heteroatom content of pentane extracted asphaltenes), they are believed to be a major contributor to emulsion stabilization due to their high interfacial activity. Xu et al.18 and later Yang et al.10 found that only the most surface active fraction of the asphaltenes contributed to high emulsion stability. Yang et al.10 isolated the interfacial material from emulsified water droplets and prepared emulsions using the residual asphaltenes. They found that the most interfacially active fraction of the asphaltenes represented about 2 wt% of the whole asphaltene fraction and its removal significantly reduced emulsion stability. They also found that the interfacial material from emulsified water droplets contained higher molecular weight asphaltenes than the bulk asphaltenes. Jarvis et al.13 showed that emulsions prepared with the isolated material (highly interfacially active) were much more stable than those prepared with the non-interfacially active or remaining asphaltene fraction. Note, in this work, high surface activity indicates that the material preferentially adsorbed at the interface not that it significantly lowered interfacial tension. 2 ACS Paragon Plus Environment

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It is often presumed and there is some evidence to suggest that the asphaltenes that stabilize emulsions are not only the asphaltenes with the highest heteroatom and metal (Ni, V, and Fe) content but also the asphaltenes with the highest apparent molecular weight; that is, the most self-associated species. Asphaltenes with high apparent molecular weight are nanoaggregates; the higher the molecular weight, the greater the degree of self-association19,20,21. Until recently, the evidence linking asphaltene association to emulsion stability has been indirect. For example, it is well known that asphaltenes contribute to the stability of the emulsions because they tend to form: 1) rigid films that mechanically stabilize the emulsion; and/or 2) thick layers that sterically stabilize the emulsion7,22.23. The formation of rigid films is considered to be related to the ability of self-associating asphaltene species to cross-link and become irreversibly adsorbed at the water/oil interface. In addition, the size of the nanoaggregates contributes to the surface coverage and thickness of the film24,25. Alvarez et al.26 found that larger nanoaggregates preferentially adsorb at the interface confirming a link between asphaltene self-association and interfacial films. Recently, Rocha et al.27 examined model emulsions prepared from asphaltenes and observed that stable emulsions were formed when the apparent molecular weight of the interfacial material exceeded 7000 g/mol (equivalent to a mass surface coverage of 5 mg/m²). These asphaltenes formed rigid films at the water-oil interface. Assuming a density of 1150 kg/m³ for the asphaltenes, the hypothetical dry film thickness (only asphaltenes at the interface) was 4.3 nm. The actual wet film thickness would be much higher due to entrained solvent23.

To sum up, the literature suggests that only a small fraction of the asphaltenes act as stabilizers and that this fraction is enriched in heteroatomic functional and metal groups and has a relatively high apparent molecular weight at the interface; that is, it is more highly associated into nano-aggregates. Highly associated asphaltenes are typically the least soluble fraction of the asphaltenes21,28. The least soluble asphaltenes have also been shown to contain the highest heteroatom and metal contents

22,29

. Hence, it follows that relatively high

molecular weight, low solubility, nano-aggregates are required to stabilize water-in-oil emulsions and that these nano-aggregates are enriched in heteroatoms. The enrichment may occur because polar functional groups promote self-association and therefore the larger nanoaggregates that dominate the interface will have a high heteroatom content. Alternatively, the nano-aggregates containing these functional groups may adsorb more strongly at the interface. 3 ACS Paragon Plus Environment

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In either case, a strong correlation between poor solubility and interfacial adsorption is to be expected since both would be proportionate to nano-aggregate size and heteroatom content.

The objectives of this study are to: 1) determine the amount of stabilizers in asphaltenes from three different sources; 2) compare the effects of removing the least soluble versus the most adsorbed asphaltenes on emulsion stability, film properties, and mass surface coverage; 3) determine if the least soluble asphaltenes are the emulsion stabilizers. Drop size and interfacial tension are also considered because they may contribute to emulsion stability. Note, the term “most adsorbed” is used here instead of “most surface active” because the asphaltenes that irreversibly adsorb at water-oil interfaces are not necessarily those that most reduce interfacial tension.

The three source bitumens were obtained from an oil sands extraction process, a cyclic steam operation, and a steam assisted gravity drainage operation. The asphaltenes were fractionated to remove either the least soluble components (on average the most associated and highest molecular weight material) or the most adsorbed components. The least soluble asphaltenes were selectively precipitated in solutions of n-heptane and toluene. The asphaltenes that remained in solution were recovered and used to prepare fresh emulsions. The procedure was repeated on fresh feedstock with different ratios of heptane to toluene to obtain asphaltene fractions with different amounts of the least soluble material removed. The most adsorbed asphaltenes were removed by preparing emulsions and separating the settled emulsion from the continuous phase. The asphaltenes that remained in the continuous phase were recovered and used to prepare fresh emulsions. The procedure was repeated on fresh feedstock with different volume fractions of water to obtain asphaltene fractions with different amounts of the most adsorbed material removed. All emulsions were prepared with the same concentration of asphaltenes. Since asphaltene stabilized emulsions are sensitive to the presence of salt in the aqueous phase27, the measurements were carried out with both water and brine to better assess the amount of stabilizing material in each asphaltene.

2. Experimental Methods 2.1 Chemicals and Materials Three bitumen samples were used this study: an oil sand bitumen (OS) supplied by Suncor Energy Inc., a bitumen from a cyclic steam stimulation (CSS) process provided by Shell 4 ACS Paragon Plus Environment

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Canada Ltd., and a bitumen from a steam assisted gravity drainage (SAGD) process supplied by Suncor Energy Inc. The CSS and SAGD samples were recovered from the field prior to addition of any chemicals. The OS bitumen was blended with naphtha and then centrifuged to facilitate the water separation. The final product bitumen was collected in the field and sent to Maxxam Analytics Inc. for distillation to remove the naphtha.

The CSS sample contained emulsified water and was treated in the lab to remove most of this water. The sample was sonicated at room temperature for at least 7 days in order to separate the water from the oil matrix. It was then placed in a separatory funnel, left to settle for 5 days at 50°C, and the water was decanted. The residual water content was less than 3.5 wt% in all cases.

Asphaltenes were precipitated from the bitumen with n-heptane and designated as C7asphaltenes. Toluene insoluble material (inorganic solids and associated hydrocarbons), which precipitate with asphaltenes, were removed to obtain “solids-free” C7-asphaltenes. Details of the procedure are provided elsewhere28. The asphaltene and toluene insoluble (TI) contents of the bitumen samples are provided in Table 1. Asphaltene yields and TI contents are repeatable to ±0.7% and ±0.05%, respectively, based on a 95% confidence interval.

Toluene (purity >99.9%) and n-heptane (purity >99.9%) from EMD Millipore were purchased from VWR International and were used to prepare the organic phase for the model emulsions. Aqueous phases were prepared from reverse osmosis (RO) water provided by the University of Calgary. The salt used in this work was sodium chloride (NaCl).

Table 1. C7-asphaltene (“solids-free”) and asphaltene toluene insoluble (TI) contents of bitumen samples used in this study. Sample

C7-Asphaltene TI Content wt% wt%

OS

14.5

0.68

CSS

16.2

0.28

SAGD

18.3

0.07


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2.2 Asphaltene Fractionation Selective Precipitation of Least Soluble Asphaltenes Figure 1 shows the fractional precipitation for OS C7-asphaltenes in solutions of n-heptane and toluene. The onset of asphaltene precipitation occurred at approximately 48 wt% nheptane. Above the onset, the asphaltenes can be fractionated into an insoluble precipitate and the soluble material that remains in the supernatant. The fraction of each can be controlled by adjusting the ratio of n-heptane to toluene based on the fractional precipitation curve. A similar fractional precipitation curve was prepared for the CSS C7-asphaltenes. Only a small fraction of the SAGD asphaltenes was removed and the required ratio of n-heptane to toluene was determined by trial and error.

To perform a fractionation, 2 g of asphaltenes were placed in centrifuge tubes, dissolved in toluene, and sonicated for 20 minutes. Then n-heptane was added to achieve the target heptane-to-toluene ratio and the mixture was sonicated for an additional 30 minutes. The volumes of n-heptane and toluene were chosen to achieve an asphaltene concentration of 10 g/L. After sonication, the mixture was settled for 60 minutes and then centrifuged at 4000 rpm for 6 minutes. The precipitated asphaltenes in the tubes were dried and weighed to calculate the mass. The supernatant was decanted and the solvent evaporated in a vacuum oven at 60oC until the asphaltene mass no longer changed. The dried asphaltenes from the supernatant were used in the emulsion stability and film property experiments. The OS, CSS and SAGD asphaltene fractions precipitated at various n-heptane contents are shown in Table 2. Note, the fractionated asphaltenes from this method will be labeled as AP (asphaltene precipitation method) in figure legends.


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100 preliminary data

Asphaltene Precipitation. wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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fractionations

80

60

40 onset 20

0 40

50

60 70 80 90 Heptane Content, wt%

100

Figure 1. Fractional precipitation of asphaltenes from solutions of 10 g/L OS solids-free C7asphaltenes in mixtures of toluene and n-heptane at 21°C.

Table 2. Asphaltene fraction removed at various heptane-to-toluene ratios. OS CSS n-Heptane wt% Removed n-Heptane wt% Removed

50 54 60 75

5.0 10.2 21.7 64.3

44 49 60 70

5.3 13.8 37.2 57.8

SAGD n-Heptane wt% Removed

49 54 ---

1.8 8.5 ---

Selective Separation of Most Adsorbed Asphaltenes When an emulsion is prepared, the most surface active or the most irreversibly adsorbing components are expected to dominate the water-oil interface. They can be removed simply by separating the emulsified droplets from the continuous phase. The amount of asphaltenes removed can be controlled by changing the interfacial area at a fixed asphaltene concentration. The interfacial area depends primarily on the drop size distribution and the volume of water in the emulsion. The homogenizer used in this study gives similar drop size distributions over a wide range of water contents and therefore the interfacial area was controlled by adjusting the amount of emulsified water.


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The fractionations were performed with an organic phase consisting of 10 g/L C7-asphaltenes in 25/75 heptol (25 vol% n-heptane and 75 vol% toluene). Asphaltenes were first dissolved in toluene and sonicated for 20 minutes to ensure complete dissolution. Then n-heptane was added and the mixture was sonicated for an additional five minutes to ensure homogeneity between all the components. The aqueous phase was reverse osmosis water (no salt) added to make up the volume fractions shown in Table 3.

The emulsions were prepared with a CAT 520D homogenizer mixing at 18,000 rpm for 5 minutes. The aqueous phase was added dropwise to the organic phase while mixing. After mixing, the emulsion was settled for 1.5 hours by which time two phases were visible: a continuous organic phase (supernatant) and a concentrated or settled emulsion. The supernatant was decanted and the solvent was evaporated in a vacuum oven at 60°C to recover the less surface-active asphaltenes. The mass of asphaltenes on the interface was calculated from the change in concentration of the continuous phase before and after emulsification as follows:  C eq m I , A = m t 1 − A0 CA 

  

where mI,A is the mass of asphaltenes on the interface, mt is the total initial

(1) mass of

asphaltenes in the emulsion, CAeq is the asphaltene concentration at equilibrium (concentration in the organic phase after settling), and CA0 is the initial asphaltene concentration. The mass on the interface was used to determine the fraction of asphaltenes removed, Table 3. Note, the SAGD emulsions were too unstable to selectively remove the interfacially adsorbed asphaltenes. The fractionated asphaltenes from this method will be labeled as IM (interfacial material method) in figure legends.

The amount of adsorbed asphaltenes that could be removed by this method was limited by the volume of supernatant that could be collected above the settled emulsion layer. This volume decreased as the water content increased. Above 20 to 25 vol% water, there was too little free supernatant to recover sufficient asphaltenes. Hence, only 10 to 12 wt% of the asphaltenes could be removed using this method.


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Table 3. Water content of emulsion and weight percent asphaltenes removed. Water Content (Vol %) 8 14 20 25

OS wt% Removed 1.7 4.1 7.0 12.8

CSS wt% Removed 4.3 8.0 10.8 ----

2.3 Emulsion Tests Preparation of Model Emulsions Water-in-oil emulsions were prepared using 40 vol% aqueous phase and 60 vol% organic phase. The organic phase was prepared with 10 g/L of asphaltenes in 25/75 heptol using the same procedure described previously. In this case, the asphaltenes were either whole or fractionated asphaltenes. After settling for 1.5 hours, the supernatant was decanted, its volume measured, and a sample taken to determine the equilibrium asphaltene concentration and the mass surface coverage on interface. The settled emulsion was tested for emulsion stability and its drop size distribution was measured.

Stability Test Procedure Emulsion stability was measured in terms of water resolved after repeated treatment cycles of heating and centrifugation. The settled emulsion was placed into 12 cm³ graduated tubes, capped and centrifuged for 5 minutes at 3500 rpm in a Heraus Megafuge. Then the tubes were immersed in a water bath at 60°C for 2 hours and centrifuged again for five minutes. The heat-centrifuge cycles were repeated for a total of 10 hours (5 cycles). The volume of free water was recorded at the end of each cycle. In all cases, the amount of free water increased monotonically over time. The emulsion stability is therefore reported as the percentage of water resolved into free water after 10 hours. In general, the repeatabilities were ±10% for model emulsions in reverse osmosis (RO) water and 1.2% for emulsions in brine, and a 90% confidence interval based on a subset of data with 2 to 3 repeats each.

Emulsion Drop Size Distribution After 1.5 hours of settling, a drop of settled emulsion was placed onto a hanging-drop glass slide. A drop of 25/75 heptol or continuous phase was added to the slide to ensure better 9 ACS Paragon Plus Environment

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separation of individual water droplets for image analysis. Images were taken with a Carl Zeiss Axiovert S100 inverted microscope equipped with a video camera. Drop sizes of at least 500 droplets from different images and locations within each image were measured. The Sauter mean diameter, , was calculated with an expected error between 5 and 9% 30.

Asphaltene Mass Surface Coverage The surface coverage of asphaltenes on the water/oil interface, ΓA, is the mass on the interface (Eq. 2) divided by the surface area of the emulsion and is given by: ΓA =

m t d 32 6V w

 C eq  1 − A0 CA 

  

(2)

where d32 is the Sauter mean diameter and Vw is the total volume of water emulsified in the organic phase. The initial volume of water and asphaltene concentration are known controlled variables. The Sauter mean diameter was calculated from the drop size distribution. The equilibrium asphaltene concentration was calculated from gravimetric analysis as follows. The continuous phase or supernatant was decanted after 1.5 hours of settling and its volume measured. The solvent was completely evaporated and the mass remaining was calculated. The equilibrium asphaltene concentration is the residual mass divided by the volume of the decanted continuous phase or supernatant. The repeat abilities were 0.8 mg/m² for model emulsions, based on a 90% confidence interval

2.4 Interfacial Film Properties The interfacial tension was measured using an IT Concept (now Teclis) Tracker drop size analyzer (DSA) and Windrop Software. The apparatus and procedures are described in detail elsewhere5,30 and are summarized here. A droplet of organic phase was formed on the tip of a U-shaped needle placed in an optical glass cuvette containing the aqueous phase. The droplet was left in the solution for one hour. Images of the drop profile were captured with a CCD camera and used to calculate interfacial tension (IFT), drop surface area, and the drop volume over time.

Dynamic interfacial tensions were also measured and used to create surface pressure isotherms. Surface pressure is the difference between the interfacial tension of the pure solvent and the interfacial tension with an adsorbed surface active agent. Surface pressure isotherms describe the change in surface pressure while the film is under compression and are 10 ACS Paragon Plus Environment

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a plot of surface pressure versus film ratio (the ratio of the compressed surface area to the initial surface area of the droplet). The film ratio at which the film crumples is defined as the crumpling ratio (CR). To measure surface pressure, an organic phase droplet was prepared as described for the IFT measurement. The initial droplet volume was approximately 18 µL. The droplet was retracted stepwise into the capillary by reversing the direction of the drive motor of the Drop Shape Analyzer apparatus. After each step, the interface was allowed to stabilize for an interval of 30 seconds, and then the interfacial tension and droplet surface area were measured. In most cases, the experiment ended when the film crumpled upon further compression.

3. Results and Discussion First, the effect of removing the least soluble or most adsorbed asphaltene fraction on emulsion stabilization was determined and compared for OS, SAGD, and CSS asphaltenes. Then the following factors that may influence emulsion stability were examined: drop size distribution, interfacial tension, film properties, and mass surface coverage.

3.1 Emulsion Stability after Selective Asphaltene Fractionation Emulsion stability was evaluated for model systems of 10 g/L of fractionated asphaltenes dissolved in 25/75 heptol and with a salt content of the aqueous phases varying from 0 to 1 wt% at neutral pH. Figure 2 shows water resolved as a function of the asphaltene mass removed via asphaltene precipitation (open-symbols) and selective adsorption on the interface (closed-symbols) for OS (a), CSS (b), and SAGD (c) asphaltene samples. Zero wt% removal corresponds to emulsions prepared from the whole asphaltenes (no fractionation). As was previously observed, the OS asphaltenes are the most effective stabilizers and the SAGD asphaltenes are the least effective27. In all cases, the emulsions prepared with brine were more stable than those prepared with pure water. Rocha et al.27 attributed the increase in stability to the observed increase in asphaltene mass surface coverage at the emulsion interface when salt was present. They speculated that the increased counterion concentration from the added salt suppressed the surface charge, allowing the molecules at the interface to pack more closely together and provide a more effective steric barrier.

The OS emulsions prepared with brine were stable even after removing up to 64 wt% of asphaltenes, Figure 2a. In other words, the stabilizing material is distributed throughout the 11 ACS Paragon Plus Environment

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OS asphaltenes. The CSS emulsions became unstable once approximately 65% of the asphaltenes were removed, Figure 2b. The SAGD emulsions were relatively unstable to begin with and became completely unstable when approximately 2% of the asphaltenes were removed, Figure 2c. The small percentage of stabilizers in the SAGD asphaltenes is the most consistent with the literature10,11,13. For instance, Yang et al 10 showed that only 2 wt% of the asphaltene fraction was the major contributor to emulsion stability. However, this correspondence is likely coincidental since the fraction of asphaltenes capable of stabilizing emulsions clearly varies significantly with the source or possibly the recovery process of the bitumen. One implication from this observation is that emulsion stability is not expected to correlate well to the total asphaltene content of the oil.

100

100

(b) 80

80

60

Water Resolved, %

Water Resolved, %

0 wt% NaCl 0.1wt% NaCl 1wt% NaCl

40

20

60

40 0 wt% NaCl

20

0.1 wt% NaCl

(a)

1 wt% NaCl

0

0

0

20 40 60 80 Mass Removed, wt %

100

0

20

40

60

80

100

Mass Removed wt%

100

(c) 90

Water Resolved, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

70 0 wt% NaCl

60

0.1wt% NaCl 1wt% NaCl

50 0

20 40 60 80 Mass Removed, wt %

100

Figure 2. Effect of asphaltene fractionation on emulsion stability for (a) OS, (b) CSS, and (c) SAGD asphaltenes. Open symbols: least soluble asphaltenes removed; Closed symbols: most 12 ACS Paragon Plus Environment

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adsorbed asphaltenes removed. Organic phase: 10 g/L of asphaltenes in 25/75 heptol; aqueous phase: RO water and salt; 40 vol% aqueous phase. The reason for the wide variation in the amount of effective stabilizers in the different bitumen samples is not known. The oil sands bitumen is from a surface mine and has been biodegraded and possibly weathered (oxidized). The CSS and SAGD samples are from in situ recovery processes and may have experienced different degrees of biodegradation. Some surface active components from these oils may remain in the reservoir and/or the oils may have been altered by exposure to steam. Note that caution is advised when comparing results from different publications since the amount of stabilizers detected also depends on the emulsion treatment methods. For example, a severe treatment method that broke most of the emulsion would detect fewer stabilizers than a gentle method that broke only a small fraction of the same emulsion.

For the OS asphaltenes, almost identical trends were observed for emulsions prepared with the less soluble or the most adsorbed fraction removed. For CSS emulsions, the trends were similar except that removing the least soluble 10 wt% of the asphaltenes increased the stability of emulsions prepared with pure water. This unexpected effect is discussed later. The SAGD emulsions were too unstable for the effect of removing the most adsorbed asphaltenes to be assessed, and no comparison could made. Nonetheless, removing 2 wt% of the least soluble asphaltenes completely destabilized the emulsion suggesting that the stabilizers were concentrated in the least soluble fraction of the asphaltenes. Overall, there is a good correlation between the effect of removing the least soluble or the most adsorbed asphaltenes. Hence, the data support the hypothesis that, in general, the stabilizing components from the asphaltenes are also the most self-associated components.

3.2 Potential Factors in Emulsion Stabilization Drop Size Distribution before Emulsion Treatment Drop size dictates the collision diameter of the droplets and is related to the probability of coalescence. In general, smaller droplets are less likely to coalesce and therefore are associated with greater emulsion stability. For the emulsions prepared in this study, the micrometer-scale droplets generated by the homogenizer will coalesce until sufficient asphaltenes are trapped at the interface to prevent further coalescence 23,30. If these stabilized drop size distributions are different with different asphaltenes, the emulsion stability may also 13 ACS Paragon Plus Environment

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differ. The difference in stability could arise from the different collision diameter but also from a higher or lower compressiblity film on the interface that stopped the initial coalescence at a different point.

As discussed in Section 2.2, drop size distributions were gathered after a model emulsion had been settled for 1.5 hours, well after this initial period of rapid coalescence had occurred. Figure 3a shows that for OS model emulsions, the drop size distributions remain identical even after removing up to 64 wt% of the stabilizing asphaltenes. Similar results were observed for the CSS emulsions (not shown here). Note, the shape of the distributions were similar for CSS and OS asphaltenes Figure 3b, and no clear difference between removing the less soluble (AP) and most adsorbed (IM) asphaltenes was observed. For a substance that contains enough stabilizing material, such as the OS and CSS asphaltenes, the shape of the distribution is likely set by the homogenizer and only altered slightly by the limited coalescence that occurs in these emulsions before treatment.

60

60

50

2.8 AP wt% 7.1 AP wt% 23.0 AP wt% 64.0 AP wt%

OS IM 7.0 wt% OS AP 7.1 wt% CSS IM 8.1 wt% CSS AP 6.8 wt%

(b) 50 Number Frequency, %

(a)

Number Frequency, %

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40

30

20

40

30

20

10

10

0

0 0

5 10 Diameter (µ µm)

15

0

5

10 Diameter (µ µm)

15

20

Figure 3. Effect of asphaltene fractionation on droplet size distributions for emulsions prepared with: a) OS asphaltenes with least soluble asphaltenes removed (AP); b) OS and CSS asphaltenes with least soluble (AP) and most adsorbed asphaltenes (IM) removed. Organic phase: 10 g/L of asphaltenes in 25/75 heptol; aqueous phase: RO water and salt; 40 vol% aqueous phase. All distributions taken before emulsion stability testing.

Interfacial Tension


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Interfacial tension is the driving force for coalescence and it is possible that emulsion with higher IFT will be less stable than similarly stabilized emulsions with lower IFT31,32. Further, if the most surface active asphaltenes dominate the water-oil interface, then the asphaltenes with the most adsorbed component removed would have higher IFT. Figure 4 shows the IFT of model systems of 10 g/L fractionated asphaltenes of (a) OS (b) CSS. Surprisingly, there was little impact on IFT by either method of removal although the IFT was slightly higher for asphaltenes with the most adsorbed material removed. This observation supports the hypothesis that the most self-associated asphaltenes stabilize these emulsions, not necessarily the most surface active species. In other words, the species that can cross-link to form an irreversibly adsorbed film remain at the interface when it is compressed during the limited coalescence that occurs between drop formation and stabilization. These species likely have a range of surface activity and an average value is measured. This average value does not change significantly from the most to least adsorbed (or soluble) material. Consequently, there is no correlation between the IFT and emulsion stability.

27

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AP 26

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IFT, mN/m

IFT, mN/m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

24 23 22

24 23 22

21

21

(a)

(b)

20

20 0

5 10 15 20 Asphaltene Removed, wt%

25

0

5 10 15 20 Asphaltene Removed, wt%

25

Figure 4. Interfacial tension of 10 g/L of fractionated asphaltenes (a) OS, (b) CSS in 25/75 heptol versus RO water at 21°C. AP: asphaltene precipitated; IM: asphaltene adsorbed on the interface.

Surface Pressure Isotherms The surface active fraction of the asphaltenes is known to form a rigid interfacial films at the water oil interface10,11,13. Therefore, it is expected that removing the most adsorbed or least soluble asphaltenes would lead to more compressible films that are less capable of stabilizing 15 ACS Paragon Plus Environment

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Page 16 of 24

emulsions. Figure 5 shows surface pressure isotherms of interfacial films from solutions of 10 g/L of whole or fractionated OS asphaltenes in 25/75 heptol aged for 60 minutes before compression at 21°C. In all cases, the characteristic behavior of an irreversibly adsorbed film is observed. The compressibility of the film increases as it is compressed (decreased film ratio) until a crumpling ratio is reached. The crumpling ratio is the point at which the film buckles and, in Figure 5, is the data point at the lowest film ratio for each isotherm. Surprisingly, the surface pressure isotherms prepared from asphaltenes with the least soluble or the most adsorbed fraction removed are very similar to the original isotherm of the whole asphaltenes. The only differences are a slight shift to lower crumpling ratios and a small increase in the initial surface pressure (at a film ratio of unity) indicating a small decrease in IFT when a significant fraction of the asphaltenes are removed. The consistency of the film properties confirms that surface active asphaltenes are present throughout the OS asphaltenes. In addition, there is no correlation between film compressibility and emulsion stability.

For the CSS asphaltenes, Figure 6, the crumpling ratio also shifted to a lower value for films prepared from fractionated asphaltenes, most notably when the most adsorbed material was removed. It appears that the CSS asphaltenes contains a small quantity of a relatively highly surface active component that contributes to more rigid films. This material does not correspond to the least soluble fraction of the asphaltenes. Hence, the film properties and the emulsion stability are slightly different when the most adsorbed material is removed versus the least soluble material removed. Nonetheless, there is no correlation between film compressibility and emulsion stability.

At first glance, this observation appears to contradict the substantial body of literature that correlates emulsion stability to the film rheology, compressibility, and/or crumpling ratio. For example, a clear correlation has been established between film properties and emulsion stability when the solubility parameter of the medium changes1,5 or a surfactant is added31. In these cases both the film properties and emulsion stability changed. One possible source for the discrepancy is that the surface pressure isotherms in current study were measured at an asphaltene concentration of 10 g/L to match the initial concentration of the asphaltenes in the model emulsions. However, the equilibrium asphaltene concentration in the emulsions is lower depending on how much of the asphaltenes adsorb at the interface. It is possible, that the surface pressure isotherms change with concentration and that the change is different for 16 ACS Paragon Plus Environment

Page 17 of 24

the different fractions.

This difference could obscure the correlation to film properties.

Another possibility, as will be discussed later, is that a rigid, irreversibly adsorbed film is only one of several necessary conditions for emulsion stability.

35

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(a)

(b) 30

Surface Pressure, mNm


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OS Whole OS 3.0 wt% AP OS 19 wt% AP

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OS Whole OS 4.3 wt% IM OS 8.3 wt% IM

10 0

0.2

0.4

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0

0.2

Film Ratio

0.4

0.6

0.8

1

Film Ratio

Figure 5. Surface pressure isotherms for 10 g/L fractionated OS asphaltenes in 25/75 heptol versus RO water at 21°C after 60 minutes: (a) least soluble asphaltenes removed; (b) most adsorbed asphaltenes removed. Lines are a visual aid only. 35

35

(a)

(b) 30


30


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Energy & Fuels

25

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CSS Whole CSS 6.4 wt% AP CSS 18 wt% AP

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CSS Whole CSS 2.4 wt% IM CSS 6.2 wt% IM

10 0

0.2

0.4

0.6

0.8

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Film Ratio

0.4

0.6

0.8

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Film Ratio

Figure 6. Surface pressure isotherms for 10 g/L fractionated CSS asphaltenes in 25/75 heptol versus RO water at 21°C after 60 minutes: (a) least soluble asphaltenes removed; (b) most adsorbed asphaltenes removed. Lines are a visual aid only.


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Mass Surface Coverage As discussed previously, Rocha et al.27 found that, for the samples in their study, a mass surface coverage of 5 mg/m² was required for the stabilization of water-in-oil emulsions with asphaltenes at the interface. Now, the least soluble asphaltenes are known to consist of the most self-associated (highest apparent molecular weight) asphaltenes28 and therefore are likely to adsorb with a higher mass surface coverage. Hence, emulsions prepared with fractionated asphaltenes, where this material has been removed, may be less stable than those prepared with whole asphaltenes.

Figure 7 shows that, consistent with the observed emulsion stability, the mass surface coverage of emulsions prepared from whole OS and CSS asphaltenes in brine are notably higher than the surface coverage of the SAGD emulsions. Interestingly, the whole CSS asphaltenes have a much higher surface coverage in brine than any other sample and even removing a small fraction of the least soluble or most adsorbed asphaltenes reduces the surface coverage substantially. This observation is consistent with previous interpretation that the CSS asphaltenes contain a small amount of material with different film properties than the rest of the CSS asphaltenes.

Figure 7a shows that the emulsions prepared from highly fractionated OS asphaltenes have a lower surface coverage than the whole OS asphaltenes. The difference becomes noticeable at 20 wt% removed for emulsions with pure water and 65 wt% removed with brine. It appears that a substantial fraction of the self-associated material must be removed to affect the mass surface coverage. There appears to be little difference between removing the least soluble or the most adsorbed asphaltenes again consistent with the hypothesis that, in general, the stabilizing components from the asphaltenes are also the most self-associated components.

The same general trends were observed for the CSS asphaltenes; however, in this case, the mass surface coverage increased for asphaltenes with up to 15 wt% of the least soluble components removed, Figure 7b. It is possible that the small amount of more adsorbed material with high surface coverage is concentrated in the more soluble asphaltenes. Hence, removing the least soluble asphaltenes leaves a higher proportion of the high surface coverage material in the fractionated asphaltenes and therefore they have a higher average surface coverage and greater emulsion stability. 18 ACS Paragon Plus Environment

Page 19 of 24

10

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0 wt% NaCl

Asphaltene Mass on Interface, mg/m²


0.1 wt% NaCl 8

1 wt% NaCl

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Asphaltene Removed, wt%

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Energy & Fuels


1 wt% NaCl

6

4

2

(c) 0 0

20

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80


Figure 7. Effect of asphaltenes removed on mass surface coverage for model emulsions (a) OS, (b) CSS, (c) SAGD. Open symbols: less soluble asphaltenes removed; Closed symbols: most surface active asphaltenes removed. Organic phase: 10 g/L of asphaltenes in 25/75 heptol; aqueous phase: reverse osmosis water and salt; 40 vol% aqueous phase.

The data in Figures 2 and 7 suggest that there is a relationship between emulsion stability and mass surface coverage. Figure 8 shows the water resolved versus asphaltene mass surface coverage for the (a) OS, (b) CSS and (c) SAGD fractionated samples. Regardless of the method used to remove the most stabilizing material, there appears to be a minimum threshold for emulsion stability (here defined as less than 10% resolved water) at a mass surface 19 ACS Paragon Plus Environment

Energy & Fuels

coverage in of approximately 3.5 mg/m². This threshold for emulsion stability may be equivalent to the minimum interfacial thickness that must be overcome to destabilize the emulsion and is slightly lower than the 5 mg/m² reported in previous work for emulsions prepared with unfractionated asphaltenes27. If there were no solvent in the interfacial film, a mass surface coverage of 3.5 mg/m² would be equivalent to a dry film thickness of 3.0 nm. As expected, this value is lower than reported wet film thicknesses of approximately 10 nm inferred from scattering data 33 and 10 to 20 nm determined from surface force measurements 23,34

. Verruto and Kilpatrick

33

report solvent contents of 75 to 90% for asphaltene films.

Applying these values, a surface coverage of 3.5 mg/m² is equivalent to a wet film thickness in the order of 10 to 30 nm.

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0 wt% NaCl 0.1 wt% NaCl 1 wt% NaCl


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all fractions whole (Ref 8) 80

Water Resolved, %

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Water Resolved, %

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Figure 8. The correlation of emulsion stability to mass surface coverage: a) OS; b) CSS; c) SAGD; d) all samples plus data from Rocha et al.27. Open symbols: less soluble asphaltenes removed; Closed symbols: most adsorbed asphaltenes removed. Organic phase: 10 g/L of asphaltenes in 25/75 heptol; aqueous phase: reverse osmosis water and NaCl. Emulsions prepared with 40 vol% aqueous phase.

The threshold is not exact indicating that the mass surface coverage is not the only factor governing the stability of these emulsions. One possibility is that the thickness is the true threshold and that the thickness of the film is not linearly related to the mass surface coverage. For example, the nano-aggregates from different fractions may have different shapes or crosslink differently, leading to different lengths protruding into the oil phase even if the mass surface coverage is the same. Alternatively, the films formed from different fractions may have different solvent volume fractions leading to different thicknesses. Also, as noted previously, the film properties may be different at equilibrium concentrations in the emulsion versus the 10 g/L measurement concentration. Hence, undetected differences in the film properties may influence the stability data. Nonetheless, irreversibly adsorbed films were observed in all cases and, therefore, it appears that both rigid, irreversible films and a minimum mass surface coverage of approximately 3.5 mg/m² are required for asphaltenes to stabilize the emulsions. Note that the data collected in this study were from Western Canadian asphaltenes and the same threshold may not apply to asphaltenes from other sources.

CONCLUSIONS Only some of the species in the asphaltenes act to strongly stabilize emulsions, and the size of this fraction varies considerably in different asphaltenes (2 to >65% in the three samples of this study). The highly stabilizing material tended to be concentrated in the least soluble fraction of the asphaltenes.

In the OS asphaltenes, the least soluble asphaltenes were equivalent to the material found on the interface in terms of film properties, mass surface coverage, and emulsion stabilization. Removing the most adsorbed asphaltenes had little impact on IFT or film properties. These observations are consistent with the most self-associated asphaltenes acting as stabilizers, not necessarily the most surface active asphaltenes.


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Page 22 of 24

Similar behavior was observed for the CSS asphaltenes except that these asphaltenes also appeared to contain a small amount of more surface active material with a high mass surface coverage. The SAGD asphaltenes did not stabilize emulsions sufficiently to separate the most adsorbed components and therefore no comparison could be made.

The emulsion stability data were generally consistent with a threshold of approximately 3.5 mg/m² asphaltene surface coverage for stable emulsions. The threshold was not exact indicating that other factors also contribute to emulsion stability. Fractionating the asphaltenes eventually removed enough of the self-associated material that the surface coverage dropped below the threshold and unstable emulsions were observed.

ACKNOWLEDGEMENTS We are grateful to the sponsors of the NSERC Industrial Research Chair in Heavy Oil Properties and Processing: NSERC, Nexen Energy ULC, Petrobras, Schlumberger, Shell, Suncor Energy Inc., and Virtual Materials Group. We thank Suncor Energy Inc. for providing samples.

NOMENCLATURE CAeq

asphaltene equilibrium concentration (kg/m³)

CA0

initial asphaltene concentration (kg/m³)

d32

sauter mean diameter (m)

mt

total mass of asphaltenes in emulsion (kg)

mI , A

mass of asphaltene on interface (kg)

Vw

total volume of water phase (m³)

Greek Symbols

Γm,s

asphaltene mass surface coverage (kg/m²)

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Energy & Fuels

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What Fraction of the Asphaltenes Stabilizes Water-in-Bitumen

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