Recent Advances in Nonaqueous Extraction of Bitumen from Mineable

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Review

Recent Advances in Non-Aqueous Extraction of Bitumen from Mineable Oil Sands: A Review Feng Lin, Stanislav R. Stoyanov, and Yuming Xu Org. Process Res. Dev., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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Organic Process Research & Development

Recent Advances in Non-Aqueous Extraction of Bitumen from Mineable Oil Sands: A Review Feng Lin*, Stanislav R. Stoyanov, and Yuming Xu Natural Resources Canada, CanmetENERGY - Devon, One Oil Patch Drive, Devon, Alberta, Canada, T9G 1A8

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ACS Paragon Plus Environment


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GRAPHIC TABLE OF CONTENTS

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ABSTRACT Investigations of non-aqueous extraction (NAE) of bitumen from minable oil sands have been extensively revisited in the past decade as an alternative to Clark hot water extraction (CHWE). Significant advances have been achieved in understanding NAE processes at bench and pilot scales, although many questions remain regarding the commercialization of NAE. This critical review summarizes recent research findings and progress on fundamental and practical aspects associated with novel extraction processes, focusing on technological method, solvent selection, recovery of solvent, and removal of fines. The review also identifies opportunities and challenges for bitumen recovery from oil sands using non-aqueous processes.

Keywords: Organic process, solvent, oil sands, bitumen recovery, non-aqueous extraction

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INTRODUCTION Oil sand is a heterogeneous mixture of about 80 wt% to 90 wt% mineral solids including

sand and clay fines, 4 wt% to 18 wt% bitumen, and water.1,2 Oil sand deposits constitute an important part of global unconventional oil resources―in the long run, processing of oil sand bitumen is vital to meet world demand for petroleum and petrochemical products. Oil sand reserves are located around the globe, mostly in North and Latin America, and are estimated to be comparable in volume with the world’s total conventional crude oil.3,4 Depending the content of connate water and the nature of mineral solids, oil sand deposits are often classified as waterwetted (e.g. Canadian oil sands) and oil-wetted (e.g. United States or Indonesian oil sands), while many of anywhere-else deposits are inadequately characterized and their mineral surface properties are ill-determined. Oil sand bitumen is an extra-heavy, viscous crude whose components can be categorized according to their solubility in an alkane solvent (typically pentane, hexane, or heptane) into maltenes (soluble) and asphaltenes (insoluble, and the highest-molecular-weight fraction).2 Bitumen is found in a semisolid state with a viscosity usually greater than 10 Pa·s and a density of around 103 kg/m3 at deposit conditions, and is embedded in the sand matrix.5 Specialized technologies are required to separate bitumen from oil sand i.e. to extract the bitumen, in contrast to the recovery of conventional crude oil which can be pumped directly from the ground. The methods used to extract bitumen from oil sand are largely dependent on the depth of the deposit,6,7 but the scope of this review will be limited to surface-mined oil sands. For deposits less than ~75 m below ground, the ore is surface-mined, added to hot water, and transported to extraction plants where the bitumen is floated from the warm-water slurry.1 The Clark hot water extraction (CHWE) process,7 patented by Dr. Karl Clark, was the first process used for the commercial extraction of bitumen. The CHWE process is still in use by all companies involved in processing of mined Canadian oil sands, although some operating parameters have been modified. In the basic process, mined oil sand is crushed and mixed with heated water in a mechanical agitation tumbler to form a slurry. Sometimes a process aid is added. The resulting aqueous slurry is pumped down a hydro-transport pipeline in which slurry conditioning is 4


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initialized. During conditioning, if all conditions are favorable, bitumen is liberated from the sand grains as droplets, which coalesce and then engulf air bubbles.8 The conditioned slurry is then pumped into the primary separation vessel (PSV), a flotation-like separation vessel in which the aerated bitumen floats to the top to be collected as bitumen froth, while the material left in the bottom of the vessels is discharged as tailings into a pond. Actual operations incorporate more complex flow schemes to maximize bitumen recovery. For an economic incentive, minimum scale of operation, at least 0.1 million barrels of bitumen per day produced, is a necessity to all the current oil sands mining. Bitumen froth from the PSV typically far from pure,8 so further treatment of the froth is required to remove the remaining solids and water. During froth treatment, a solvent (i.e. diluent) is added to reduce froth viscosity, and then mechanical means are used to remove the water and solids from the now-diluted bitumen. Depending on the type of solvent used, froth treatment involves one of two distinct processes: naphthenic froth treatment (NFT) or paraffinic froth treatment (PFT).9 In both cases, the solvent is subsequently recovered from the bitumen in a solvent recovery unit and recycled. The oil sand tailings discharged from separation units are a muddy, aqueous, alkaline suspension of solids, unrecovered bitumen, soluble organics, and inorganic salts.10 For information regarding the specific subjects of water-based extraction process, please consult the relevant critical reviews.1,2,9-14 There are many drawbacks and challenges associated with the CHWE process: •

High thermal energy use and high emissions of greenhouse gases (GHG). Hot water is used in the CHWE process to make the oil sands slurry: about 2.5 m3 of water for each barrel of bitumen extracted. Heating water to high temperatures requires enormous quantities of thermal energy. It is sometimes possible to use electricity generated by steam from the combustion of waste coke or the reuse of waste heat from the upgraders on site. On average, oil sands mining gives rise to about 0.044 metric tons of carbon dioxide - equivalents (MT CO2-e) per barrel of bitumen produced. So, for example, Canadian mine production of about 365 million barrels15of bitumen in 2014 translates into a release of about 16 million MT CO2-e during that year.16 5



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•

Large amounts of fresh water are used in the water-based process. In aqueous extraction the water trapped in the beaches and fluid fine tailings of the tailings ponds has to be made up with the import of fresh water. About 3−4 barrels of make-up water are needed to produce each barrel of synthetic crude bitumen.15 Water withdrawal from the rivers, particularly during low flow conditions, and the resulting changes in the ecosystem could jeopardize aquatic life. Consequently, regulations have been established to limit water withdrawal from the Athabasca River, with the volume that can be withdrawn being dependent on flow in the river.

•

Increased oil sands extraction has resulted in rapid expansion of tailings ponds. Tailings ponds are costly to build and an environmental liability. The clay particles in the tailings from the CHWE process are fully dispersed, forming stable so-called fluid fine tailings (FFT) that remain in a fluid state indefinitely unless treated. Approximately a billion cubic metres of FFT have accumulated to date. 2,13 It is anticipated that the volume of FFT could be 2 billion m3 by 2030 if there is no significant improvement in tailings management practices.13,17 In addition to disrupting the landscape, oil sand tailings contain inorganic salts and dissolved organics. Tailings water is acutely and chronically toxic primarily because of the soluble organics. Volatilization of compounds from tailings outfalls and biodegradation of hydrocarbons by resident microbiota also lead to significant air-borne emissions of pollutes such as volatile organic compounds (VOCs), CO2, and CH4 from the tailings ponds.18

•

Difficulty in achieving desirable bitumen recovery from low-grade oil sands and from oilwetted deposits. Bitumen recovery using the CHWE process can exceed 90% for most water-wetted ores. However, for low-grade and water-wetted ores, which usually contain a relatively low bitumen content and large proportion of fines, bitumen recovery can be as low as 60% or less, with poor froth quality.2 With the eventual depletion of high-grade ores, it is inevitable that blends of medium- and low-grade ores will be processed. In addition, the use of the CHWE process in processing oil-wetted ores such as Utah or Indonesian oil sands is considered impractical and often results in poor recoveries.19-24

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With anticipated increases in bitumen production in coming years, the above-mentioned environmental implications and climate change challenges associated with the CHWE process will become more critical. Therefore, there is a need to develop environmentally sustainable and economically robust processes for bitumen production from mineable oil sands that reduce emissions and water use. Alternative extraction technologies have been studied at bench and pilot scales. Most of these technologies are designed to partially or completely replace water with organic solvent(s) in the extraction stage of the mining operation. These approaches are broadly known as nonaqueous extraction (NAE). NAE is not a new concept — research on NAE methods dates back over four decades, but still no NAE technology has been implemented beyond the pilot scale. In an effort to address the environmental impacts of tailings ponds and water consumption associated with the CHWE process, research into NAE technologies has been revisited in recent years. However, technological innovation in this field is gradual: until an alternative is in commercial service, oil sands mining operators and researchers in academia and government continue to search for technological breakthroughs, big or small, that can reduce the environmental impacts of oil sand mining operations. This paper is a review of NAE technologies from mineable oil sands deports and the associated fundamental aspects ─ solvent selection, solvent recovery, and fines removal, with emphasis on recent findings. Our objective is also to identify opportunities and gaps in NAE research and technology development. In this review, the term “non-aqueous extraction” refers to non-CHWE technologies from mineable oil sands that can address the above-mentioned drawbacks and challenges in the current commercial CHWE process. To be specifically considered as a viable replacement for aqueous extraction, the NAE technologies must have much lower energy or water usage, a smaller GHG emission and environmental footprint, and eventual success to eliminate the aqueous tailings accumulations, while maintaining high bitumen recovery including, ideally, high recovery from low-quality oil sand ores. Because of these constraints, those alternative technologies for shallow and mined deposits that involve large amount of water usage or operating at very high temperature or high pressure, and in situ technologies for deeper deposits, will be excluded from the scope of this review. 7



2. TECHNOLOGICAL METHODS 2.1. SOLVENT-ALONE EXTRACTION 2.1.1. General Backgrounds. In solvent-alone extraction (SAE), water is completely replaced by an organic solvent or solvent mixture. Despite differences in specific operating parameters in the various SAE methods, the fundamental principles are similar. A typical SAE process is illustrated in Figure 1. Solvent Mined Ore Bitumen Solution Supernatants Crush Primary Separation Solvent

Solvent Sediments

2nd/Multi Separation

Fines Removal

Wet Gangue

Digestion

Supernatants

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Solvent Recovery Bitumen Product

Solvent Solvent Wet Gangue Recovery

Gangue

Figure 1. Schematic flow diagram of a typical SAE process. SAE comprises the following steps: •

Solvent digestion of oil sand. In the first step, mined oil sand is crushed and blended with an organic solvent, recycled or fresh, to allow for ore digestion and the dissolution of bitumen from the ore into the solvent.

•

Separation and washing. After digestion, the mixture is fed into primary, secondary, or multistage separation vessels where a certain degree of solid-liquid separation occurs.

•

Removal of fines in diluted bitumen. Supernatant (i.e. product) from the separation vessels usually contains fine particles, with the level of purity depending on the physical handling and the type and dosage of solvent in the separation step. The fines are removed via chemical or mechanical means.

•

Solvent recovery from spent sands and fines (wet gangue). The sediment (so-called ‘wet gangue’) from the bottom of separation vessels, consisting of reject sands and fines 8


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contaminated with solvent-diluted bitumen, are dried or undergo steam stripping to recover the solvent, which can be reused in the blending and digestion steps. The underflow of this step is the tailings or ‘gangue’. •

Solvent recovery from bitumen solution. This step is accomplished in a solvent recovery unit in which the solvent in the solids-free diluted bitumen is recovered via a distillation process and also recycled upstream. The extracted bitumen product is then sent downstream for further upgrading into synthetic crude oil. SAE is one of the major NAE processes and has been investigated since the mid-1960s.25-

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Early tests of SAE technologies were conducted mainly in the United States, where oil sand

deposits were thought to consist of oil-wetted minerals.27-29 For example, Graham et al. reported a solvent extraction method, in which ore from a Utah open pit mine was crushed and bitumen was dissolved in a slurry with n-heptane in a dissolution tank, followed by extraction and separation cascades comprising hydrocyclones, centrifuges and filters.28 However, the extent of solvent losses in spent gangue and environmental impacts was not addressed in this and most other early studies of SAE processes. 2.1.2. Recent Academia Studies. The Institute of Oil Sands Innovation (IOSI) at the University of Alberta is active in researching SAE technologies. Researchers at IOSI recently applied two mixing steps in extracting different oil sand ores to achieve desired high bitumen recovery at ambient conditions.30-34 In the first step, a well-sealed bottle containing solvent and ore sample was tumbled end to end in a rotary mixer, after which the slurry was poured into a graduated cylinder for gravitational settling. The supernatant was slowly siphoned off as the “first product” and the sediments were extracted with the solvent by the same method. After two stages of tailings washing using a sieve, total bitumen recovery was 95% or higher and insensitive to the grade of ore or type of solvent (for most solvents studied). At optimal conditions, a bitumen product containing less than 2.9 wt% fines was produced.30,32 Solvent recovery was achieved by drying the wet tailings, and the amount of residual solvent in the dry tailings was measured by thermal desorption gas chromatography; for optimal solvents such as cyclohexane the solvent losses after drying were well below current levels of solvent loss in industrial operations.32 9



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Employing multifactor orthogonal methods, Li et al. reinvestigated the relative significance of each operating parameter in determining bitumen recovery and product quality using heptol.35 They found that solvent-to-ore (S/O) ratio and stirring rate were the most significant factors affecting bitumen recovery, compared to the insignificant impacts of contact time and temperature. The asphaltenes content in bitumen product was more sensitive to temperature and contact time. For single-stage extraction, the residual bitumen content in spent sands was far higher than the acceptable level for tailings; multistage solvent extraction was suggested as an efficient means of minimizing the residual bitumen.35 Similarly, single- and multi-stage, counter-current solvent extractions of bitumen from Indonesian oil sands and from Xinjiang oil sands in China were reported.24,36,37 The results revealed that lumps should be broken into sufficiently small sizes to ensure effective recovery. Bitumen recoveries for both two- and three-stage counter-current processes were higher than 93.5%, and the two-stage process was the most effective.35 However, it should be noted that the bench-scale extraction methods proposed in refs. 24, 35-37 require very high S/O values at ambient and elevated operating temperatures and are considered relatively poor processes. In contrast, Wu and Dabros described a novel protocol for extraction of bitumen from oil sand ores using light hydrocarbon solvents at much lower S/O values.38 In their protocol, centrifugation and filtration at ambient temperature and pressure were applied for a digested oil sand−solvent mixture to separate the solids from the diluted bitumen. For different ore grades, bitumen recovery rate and product quality were shown comparable to those achievable by the CHWE process. About 90% bitumen recovery can be obtained by singlestage centrifugal filtration extraction using S/O of about 0.5. If two-stage or three-stage extraction is used, the first-stage S/O can be as low as 0.1 to achieve very high bitumen recovery. The recovery of solvent remaining in the wet filtration cake was achieved by vacuum evaporation under ambient temperature. The content of residual solvent in dried tailings was determined by gas chromatography in combination with o-xylene extraction. It was demonstrated that cyclopentane and n-pentane were easier to recover than toluene. After evaporation, solvent losses in dried tailings were limited to about 250 ppm or less of extracted tailings.38 The results from a multistage centrifugal filtration solvent alone extraction (CF-SAE) process38 have proved 10


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very promising at laboratory scale. Scale-up development of this CF-SAE process to a pilot scale with continuous flow is under way at the CanmetENERGY research centre in Devon, Alberta. 2.1.3. Recent Industrial Patents. In the past decade, oil companies such as Shell and Imperial Oil have also been developing SAE technologies, and much of their research can be found in the US Patent and Trademark Office databases. Table 1 provides a summary of some recent patents of SAE technologies. In all patents listed in Table 1, at least two-stage separation vessels were used to extract bitumen-enriched phase from the slurry of solvent and oil sand ore. The methods used to effect phase separation varies in the patents, and includes gravitational settling, centrifugation, filtration, use of a light solvent that can precipitate a portion of the asphaltenes, and combinations of these.39-52 Table 1. Summary of Recent Solvent-Alone Extraction (SAE) Patents. Patent No.

Issue Date

20120240824 20130068664 20130068665 20130034105 20130299393 20130220890 20140083332

09/27/2012 03/21/2013 03/21/2013 08/29/2013 12/19/2013 08/29/2013 03/27/2014

20140166543

01/19/2014

8663462

03/04/2014

8728306

05/20/2014

8771502

07/08/2014

20140216985

08/07/2014

20140231312

08/21/2014

8968556

03/03/2015

Description ─ Similar separation methods in these 7 patents:39-45 Two staged filtrations using a paraffin. ─ Slightly different foci of these patents: • 1st patent aims at producing dry tailings, albeit the high content of asphaltenes.39 • 2nd to 5th patents focus on improving filtration efficiency via different means.40,41,43,44 • 6th patent focuses on reducing filtration time by increasing solvent/bitumen ratio (S/B).42 • 7th patent describes process flow diagram with several separation and recycling steps.45 ─ This patent claims a unique solvent mixture with low viscosity, boiling point and toxicity. 46 ─ The ore/solvent slurry is separated preferably by gravitational settling. ─ Fines contained in bitumen and solvent stream are removed by centrifugation or filtration. ─ Wet tailings are provided to a solvent stripper to recover the residual solvent.46 ─ The patent uses two sequential solvents: an aromatic (1st) and a polar (2nd).47 ─ Extraction involves ore dissolved in 1st solvent at 40°C with mild agitation. ─ Wet gangue is treated with 2nd solvent, e.g. methanol to displace 1st solvent and then dried. ─ Separation of the two solvents is achieved by distillation or by adding water.47 ─ This patent uses non-aromatic solvent, causing rejection of asphaltenes in the wet gangue.48 ─ The solvent/bitumen stream in subsequent filtration is recycled in the extraction process. ─ The thicken slurry prevents segregation and the formation of fines-rich supernatant.48 ─ Ore is blended with a C3 to C9 paraffin in a rotary breaker. 49 ─ The amount of added solvent is controlled to generate thick and non-segregating slurry. ─ Settling unit(s) are installed after the breaker to ensure thick slurry is fed to filtration unit. ─ Solvent vapors strip off solvent spent in the sands prior to the passage of sands for drying. ─ This process produces bitumen with a low fines content and readily-disposed gangue.49 ─ A combination of rotary breaker, settling and filtration is used for phase separation.50 ─ Wet gangue is passed through pressure reduction vessels, eliminating the sequent drying. ─ Bitumen product with low solids content is generated, along with dry tailings.50 ─ This patent describes improved blending of a solvent with ore in two rotating devices.51 ─ First screen removes oversized lumps; second unit for solvent addition and size reduction. ─ The size-reduced streams are combined or treated separately for filtration and separation.51 ─ Ore is contacted counter-currently with a heated light aromatic in 1st separator. 52 ─ The first-stage tailings are then contacted with light aliphatic in 2nd separator. ─ The separation steps are performed as a continuous, batch, or semi-batch process. ─ Solvent recovery in wet tailings is accomplished by a drying system.52

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2.2. SOLVENT EXTRACTION SPHERICAL AGGLOMERATION Solvent extraction spherical agglomeration (SESA) refers to a technology scheme that combines additions of solvent and a small amount of water at ambient conditions to separate bitumen from mineable oil sand. Figure 2 shows the flowsheet of a typical SESA process. As this figure shows, the basic steps of SESA include separation and solvent recovery steps. With the advantages of achieving high recoveries for all grades of ore and producing dry and wellconsolidated tailings, the SESA process is comparable to an SAE process as described in Section 2.1. This may be why SESA is often classified as an SAE technology in the literature.

Solvent

Solvent

Tumbler

Distillator

Bitumen Solution

Water

Crushed Ore

Thickener

Bitumen Product

Solvent

Belt Filter

Stripper

Solvent

Sediment

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Wet Gangue

Diluted bitumen

Water

Stripping Gangue Drum

Figure 2. Schematic representation of a SESA process. The distinguishing feature of SESA technology is that, upon contact of the oil sand ore with organic solvent, small amounts of water (acting as an immiscible bridging agent) are added. The water is added to trigger the generation of spherical agglomerates, allowing the bitumen to be extracted and the production of agglomerates of fine solids. The agglomerated solids are efficiently separated from the solvent-bitumen fluid by a series of liquid-solid separation steps (tumbler, thickener, agglomerate washing on belt filter, and filtration) at ambient conditions.53-57 The second distinctive feature of a SESA technology is that the entrained fines in the bitumen-enriched solvent are minimized so that the fines removal step, often required in an SAE process, can be eliminated. The recovery of trapped solvent from micro-agglomerates was 12


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understood to be easier.57 The amount of residual solvent in dried tailings would be controlled to 0.1 wt% or less.55 Various approaches that combine solvent with water during the extraction have been reported.58-65 They differ significantly from one another, in the context of operating procedures, type and dosage of solvent, operating temperature, and addition of process aids. In contrast to the SESA, other solvent-and-water extraction processes, such as aqueous/non-aqueous hybrid bitumen extraction, typically consume large amount of water.58-65 Tailings ponds remain a major liability to the implementation of those technologies,66 and the further discussions on them are beyond the scope of this review. 2.3. IONIC LIQUID ASSISTED SOLVENT EXTRACTION Recent work at lab and pilot scales has shown that bitumen can be cleanly and easily separated from “water-wet” and “oil-wet” oil sands, regardless the nature and grade of ore, using ionic liquid assisted solvent extraction (ILASE) at ambient temperature.67-71 This technology was initially developed by Painter and his coworkers at Pennsylvania State University. As illustrated in Figure 3, the process involves first the formation of a three-phase system obtained by simply mixing the ore with an ionic liquid (IL) and an organic solvent, followed by the standard solidliquid and liquid-liquid separations.67-74 The phases in the extracted mixture are distinct: a layer of sand and clays on the bottom, an IL layer in the middle, and an organic layer of bitumen/solvent on the top, which could be readily removed by decantation or other means. Using an IL, it was reported that bitumen was essentially detached from the sand and clay fines, with no fines in the organic phase detectable by infrared spectroscopy, and relatively clean sands and minerals in the tails. Water was not used in the extraction stage, but relatively small amounts were utilized to remove entrained IL from the sand and clays. It was easy to separate water from the IL via evaporation due to the negligible vapor pressure of an IL; both the IL and water could be recycled to the relevant stage of the process, leaving no significant tailings water. The initial proof concept of this ILASE process was conducted using a glass vial or a centrifuge tube on a magnetic stirrer and sequential laboratory separation as shown in Figure 3a.67,72 The scale-up work was developed using the improvised system of mixing tank and centrifuges shown in Figure 3b.70 Briefly, the oil sand sample, IL, and solvent were agitated in a 13



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tank to form a slurry. The slurry was then hydro-transported through a decanting centrifuge, in which “dry” solids, in particular the coarse sand, were expelled to underflow. The amount of residual IL left in the sand by the decanting centrifuge in one such separation depended on the speed of centrifugation and the passing rate of slurry.70 After washing with water, the extracted sands were relatively “clean,” with IL and hydrocarbon contents below the detection limit of infrared spectroscopy. Supernatants from the decanting centrifuge were then pumped into a highspeed centrifuge to remove the entrained fines and IL from the diluted bitumen. Two stages of centrifugation were required to separate residues that formed a rag layer and thereafter the twicecentrifuged bitumen product contained only trace amounts of contaminants.70

Figure 3. (a) Laboratory-scale and (b) pilot-scale flow charts of an ILASE solvent extraction process. Reprinted from refs. 67, 70, 72. Copyrights of 2010, 2011, 2015 American Chemical Society. The strong performance of the ILASE technology is mainly attributed to the strong separating function of an IL, together with the viscosity-reducing ability of an organic solvent. An IL is an organic salt with an unusually low melting point; it can be designed using different combinations of a cation and an anion, leading to the capability to achieve task-specific solvent properties.73,75-78 The most attractive benefit of ILs is their non-volatility and non-flammability at ambient conditions,79 compared with the VOCs. Additionally an IL does not pose a photochemical risk to the atmosphere. ILs are easily recyclable because they can be effectively 14


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separated from less-volatile hydrocarbons by centrifugation owing to the significant density difference, and also from water and VOCs via distillation. However, one of the challenges with using ILs is the very limited availability of toxicity data.76,80 No single IL has its toxicity of completely understood. The use of ILs in non-aqueous bitumen extraction has been limited to imidazolium tetrafluoroborate−based and deep ChCl/U eutectic solvents.67-70,72,81 The IL used serves as a “separating fluid” and builds a “phase barrier” between the sediment and the hydrocarbon phase.82-84 Garcia et al. declared that imidazolium ILs with short side chains were completely immiscible with nonpolar solvents/hydrocarbons, a factor driving the phase separation, while the organic solvent acted to reduce the viscosity of the bitumen and sharpen the phase boundaries between the phase-separated components, thus facilitating separation.82 On the other hand, the separation of bitumen from oil sand depends on the balance of interactions in the complex system. From atomic force microscopy (AFM) it was reported that the adhesive force and energy dissipation between bitumen and silica were much smaller (as low as one tenth) in an IL medium than in aqueous solution.70,72-74 Contact angle and surface tension measurements also indicated that the energy of separation of bitumen from silica was significantly less in an IL than in an aqueous solution as the interaction between the IL and the silica surface was favorable.74,85 The less bitumen-silica adhesion, the earlier and more complete the separation between them. 2.4. SWITCHABLE HYDROPHILICITY SOLVENT EXTRACTION Switchable hydrophilicity solvent extraction (SHSE) process was first tested in 2010 and patented by Dr. Philip Jessop and coworkers at Queen’s University. This technology is displayed schematically in Figure 4. It was proposed that the process start with a conventional extraction of oil sand ore with a switchable-hydrophilicity solvent (SHS) in its organic soluble (OS-SHS) form; CyNMe2 is an example of such a solvent.86-88 The mixture is separated by filtration or decantation into a liquid stream composed of solvent and bitumen, and a wet-solids stream of sand, clay, and residual bitumen and solvent. The liquid stream is treated with carbonated water (i.e. water and CO2) to expel the SHS by converting it to the water-soluble (WS-SHS) form and causing effective phase separation. Bitumen product is then separated with no need for distillation, and the SHS and water can be easily separated from one another by removing CO2, 15



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as the SHS switches back into its organic form and is recycled to the process.86-88 The wet solids stream is rinsed repeatedly by the OS-SHS to increase the recovery of bitumen and then washed with normal or carbonated water to remove residual solvent remaining in the sand.86-88 It was demonstrated that SHS, water, and CO2 can be recycled to a number of process units, and that the efficiency of the SHS is not noticeably affected, even after multiple reuses.88 However, the recovery of residual solvent and bitumen from wet solids appears to be non-trivial. In addition, the laboratory results suggest that the SHSE process can be expected to achieve bitumen recovery of over 98% for average or above-average mined ores and approximately 95% for lowgrade ores.88 The resulting solids waste stream was dry, free-flowing, and contaminated with only 0.4 wt % of bitumen and as little as 102 ppm of the solvent.88

Figure 4. Schematic of a switchable hydrophilicity solvent extraction process. As an example, CyNMe2 is a switchable-hydrophilicity solvent in its hydrophobic form, with the hydrophilic counterpart of [CyNMe2H][HCO3] upon the protonation in the presence of carbon dioxide (CO2). Reproduced with permission from ref. 88. Copyright 2012 Canadian Science Publishing or its licensors. 16


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The principle of the SHSE process is dependent on manipulating the solubility property of the SHS solvent such that the solvent switches back and forth between completely organicsoluble (OS) and water-soluble (WS) forms by a simple triggering.89-92 Amidine, secondary amine, and tertiary amine SHSs86-96 have been reported, whose dissolvability can be changed between the two forms in the absence or the presence of CO2 in the system. This reversible route permits effective separation and recycling of the SHS via the control of hydrophilicity, thus supplanting energy-intensive distillations and facilitating the substitution of VOCs in extraction processes.86-89,95,97 It was also established that the SHS solvent in the OS form is more effective at dissolving bitumen from oil sand than conventional organic solvents such as toluene, and without significant precipitation of asphaltenes.88 A general review of switchable solvents was provided by Jessop et al.97 In 2016, Sui et al. announced a similar method: a switchable-hydrophilicity solvent (tertiary amine) in its water-soluble forms (WS-SHS) was utilized to assist an organic solvent extraction of bitumen from a mined ore to increase the recovery rate and reduce the amount of solids entrained and residual solvent in the bitumen product.93 Their study suggested that the improved processability was mainly due to the adsorption of protonated cation ions of the WSSHS and the formation of ion pairs on bitumen and solid surfaces, resulting in an increase in the hydrophilicity of solids, a reduction in bitumen-silica interactions, and consequently more efficient detachment of bitumen from solids.93 Several technologies for simultaneous extraction and upgrading (SEU) have been recently attempted to take advantage of opportunities to integrate the two processes. The SEU technologies mainly include supercritical fluid extraction,98-107 solvent deasphalting,108-112 and pyrolysis,113-119 which are usually operated at critical or above-critical conditions involving high temperature or high pressure in order to selectively extract light fractions of bitumen or even break down large hydrocarbon molecules into smaller counterparts. The end product of SEU is an upgraded hydrocarbon liquid low in metals, nitrogen, and sulfur and ready for use as a feedstock for refining.98-119 However, because of the high energy intensity requirement, the relatively low extraction yield, and the extreme difficulty of material handling of oil sands, the SEU method that is not economically viable on a large scale is not described here in details. 17



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3. SELECTION OF SOLVENT 3.1. SOLVENT PROPERTIES The CHWE process uses hot water to reduce the viscosity of bitumen, thus liberating it from the sand grains trapping it. By contrast, non-aqueous extraction (NAE) processes use solvent to solubilize the bitumen from the sand grains trapping it. A solvent is characterized by familiar physical properties such as boiling point, density, and viscosity. Another important physical property of a solvent is the “solubility parameter” or solvation power. The solubility parameter characterizes the overall dissolving capability of a solvent and measures the intermiscibility of liquid materials. Solvent extraction of bitumen can be thermodynamically viewed as the mixing/dissolving process of solvent and bitumen components. The solubility parameter was first described by Hildebrand and Scott,120,121 providing a theoretical description of the relationships between solubility, heat of evaporation (cohesive energy density), and the enthalpy (heat) of mixing. The solubility parameter (SP) is defined as the square root of the cohesive energy density, the heat of vaporization divided by the molar volume (see eq. 1).120,121 If dissolution of a solute in a solvent (e.g. bitumen in the NAE), or the mixing of two liquids to form a single-phase solution occurs, the mixing enthalpy should be sufficiently small. = / =

∆ /

(1)

where is the Hildebrand (or total) solubility parameter, the cohesive energy density, ∆ the heat of vaporization, the gas constant, the absolute temperature, the molar volume. As indicated in eq. 2, once the difference between their SPs is small, solvent and solute are miscible with each other easily. Otherwise, their miscibility is limited. ∆ =

! "!

− $

where ∆ is the mixing enthalpy, the molar volume of the solution, and

(2) !

and

are

the volume fractions of the solvent and solute, respectively, and ! and are the solubility parameters of the solvent and solute, respectively. The use of the single-component Hildebrand solubility parameter is, however, impractical for systems with specific molecular interactions other than dispersion force. Hansen described the cohesive energy as the sum of three interactions: dispersion, polarity, and hydrogen bonding. 18


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Accordingly he proposed splitting the Hildebrand into three components: dispersion (% ), polar (& ), and hydrogen-bonding (' ), the so-called Hansen solubility parameter (HSP), by the relation in eq. 3.122-124 = % + & + '

(3)

Properties of common single NAE solvents are listed in Table 2.124,125 For a solvent blend, it is possible to calculate its solubility parameter based on the data for single components and the volume fractions using eq. 4.124-126 = where

,

+

,

(4)

is the volume fraction of each solvent in the blend, the subscripts 1 and 2 represent

components 1 and 2, respectively, and the subscript - represents T, D, P, or H. Table 2. Properties of Selected Single Organic Solvents. Solvent Normal alkanes butane pentane hexane heptane octane nonane Cyclic alkanes cyclopentane cyclohexane cycloheptane Aromatics benzene toluene ethylbenzene o-xylene Ketones acetone methyl ethyl ketone Alcohols methanol ethanol Others ethyl acetate chloroform

T/ (oC)

0 (kg/m3)

1 (mPa·s)

-0.5 36.1 68.7 98.5 125.5 151.0

gas 621 660 684 703 718

gas 0.24 0.31 0.42 0.54 0.71

49.0 80.7 118.4

745 778 811

80.5 110.6 136.2 144.4

% (MPa1/2)

& (MPa1/2)

' (MPa1/2)

14.1 14.5 14.9 15.3 15.5 15.7

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.44 0.97 0.98

16.4 16.8 17.2

0.0 0.0 0.0

1.8 0.2 0.1

876 862 866 880

0.60 0.59 0.68 0.81

18.4 18.0 17.8 17.8

0.0 1.4 0.6 1.0

2.0 2.0 1.4 3.1

56.5 79.6

790 805

0.32 0.43

15.5 16.0

10.4 9.0

7.0 5.1

64.7 78.4

792 789

0.59 1.14

14.7 15.8

12.3 8.8

22.3 19.4

77.2 61.2

901 1498

0.45 0.58

15.8 17.8

5.3 3.1

7.5 5.7

2 : boiling point temperature. 0: density at 20oC and 1 atm. 1: viscosity at 20oC and 1 atm. % , & and ' are the three components: dispersion, polar and hydrogen bonding, respectively, of Hansen solubility parameters.

In three-component HSP, the solubility between a solvent (subscript s in eq. 5) and a solute (subscript a) is quantitatively evaluated using the modified distance 3 .122-126 19



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/

3 = 446%,! − %, 7 + 6&,! − &, 7 + 6',! − ', 7 8

(5)

where RA is the normal distance by the transformation of the % with a weighting factor 4 as a practical tool to convert the spheroid plots of % with the other two components to a spherical one. Multiplication by 4 in the dispersion component was introduced empirically by Hansen to weigh the relative significance of each component. The results of the three-component HSP can be plotted in a three-dimensional (3D) graph. The 3D HSP sphere method is often utilized indirectly to determine the solubility of a particular substance in a complex mixture. In complex mixtures, it is impossible to measure or calculate the solubility parameter directly. The basic approach in the HSP sphere method is to perform solubility trials of samples of as many solvents as possible of known solubilities in a broad range. Good solvents are located within a particular region forming a sphere, while poor solvents are at the exterior of the sphere. The HSP of the desired substance is at the center of the sphere, with the radius of the sphere as the interaction radius 9 .122-126 9 is also the maximum HSP difference that still allows the sample to be dissolved in the solvent. The ratio 3 /9 is also defined as the relative energy difference, abbreviated as RED.122-126 Based on this, RED = 0 means that the solubility of solvent is the same as that of the sample, RED = 1 indicates that the HSP of solvent is on the surface of the sphere, RED > 1 suggests that the solvent is not a good solvent for the sample. 3.2. SOLUBILITY OF BITUMEN IN SOLVENT Rahimian et al., using a set of 76 liquid solvents, attempted to predict the solubility of the bitumen using a one-component Hildebrand parameter of the solvents.127 It was claimed that all good solvents for bitumen possessed a solubility parameter between 15.3 and 23 MPa/ ; however, not all liquids in that range are good bitumen solvents. Moreover, Redelius showed that the Hildebrand parameter could not give the complete picture of the solubility properties of bitumen via Heithaus titration, taking only dispersive forces into consideration.128 Painter et al. constructed a guide to the solubility of asphaltenes in a wide range of solvents. The critical value of the solubility parameter difference between an asphaltene and solvent was dependent on the polarity and on the ability of a solvent to “self-associate”.129 Other interactions like polar 20


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hydrogen bonding, are important and should be taken into account in systems of bitumen and its fractions.130-137 The use of the HSP has been reported to predict more accurately the solubility and stability behaviors of bitumen and asphaltenes than one-component Hildebrand parameter. Redelius et al. performed turbidimetric titrations in the 3D Hansen sphere using three titrants, each representing three-component interactions.138 Varying concentrations of bitumen, the boundary for complete solubility of bitumen could be estimated at all concentrations. It was suggested that the stability of bitumen was determined by the mutual solubility of all the bitumen fractions. This result pointed to the existence of a limited HSP stability space, depending the type and concentration of solvent. Once the solubility limit is exceeded due to a change of chemical environment, there is a risk of structural failure as indicated by formation of a precipitate. Using a set of 48 solvents with known solubility parameters, the authors then reported mean HSP values for Venezuelan bitumen and its fractions.138 Acevedo et al. fractionated Hamaca asphaltenes into fractions A1 (low solubility in toluene) and A2 (toluene soluble) using a pnitrophenol method.139-142 They used a similar sphere method to calculate the SP components of resins, asphaltenes, and fractions A1 and A2. The affinity between asphaltenes and resins (between A1 and A2) was high, as indicated by the low RED values, suggesting miscibility.139 Fossen and coworkers showed the potential of utilizing well-correlated infrared (IR) and nearinfrared (NIR) spectra with Hansen solubility parameters.143 IR spectra of oil fractions such as resins and asphaltenes are obviously grouped based on their relative contents, predicting solubility parameters for heavy oil and its fractions. Mutelet et al. utilized flocculation threshold experiments and inverse gas chromatography to confirm the validity of the HSP approach to asphaltene flocculation phenomena.144 Similar values of asphaltene solubility parameters from the two methods were demonstrated, despite the difference in asphaltene state. Sato et al. applied a dynamic light scattering method to monitor the particle size distributions of asphaltenes in various solvents and determined the solubilities of asphaltenes.145 It was reported that asphaltenes extracted from bitumen produced in different geographical regions had slightly different three-component HSP values that reflected the hydrogen over carbon (H/C) ratio, oxygen content, and average molecular weight of asphaltenes. 21



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The HSP values of bitumen and its fractions from selected sources are presented in Table 3.138,139,145 As can be seen, the solubility spheres of the asphaltene and maltene fractions overlap significantly. The difference in the HSPs between asphaltenes and maltenes at the same site is not sufficiently large to achieve insolubility between the two fractions, indicating that the asphaltenes are in fact dissolved in the maltenes128-130, 138, 139, 145-148 rather than dispersed.149 If plotted as a function of H/C ratio, solubility parameters of bitumen components are shown in Figure 5.129 Interestingly, there appears to be a continuous increase of the solubility parameter with deceasing H/C from saturates, aromatics, and resins to asphaltenes.128-145,150-155 Depending on the sample composition, solubility parameters of saturates and crude oils are clustered near 17 MPa/ and those of aromatics and resins near 19 MPa/ . Maltenes have solubility parameters ranging from 18 to 20 MPa/ . Most of the asphaltenes from bitumen and crude oils have average solubility parameters in the range 20−24 MPa/ . Table 3. Solubility Parameters (in MP1/2) for Bitumen Fractions. Reprinted from refs. 138,139,145. Copyrights of 2004, 2010, 2014 American Chemical Society. Sample δ% δ& δ' 9 Venezuelan bitumen 18.4 3.9 3.6 5.76 asphaltenes 19.6 3.4 4.4 5.3 maltenes 17.7 5.8 2.5 6.7 Canadian asphaltenes 19.1 4.2 4.4 6.1 Vacuum residue produced in the Middle East asphaltenes 19.4 3.4 4.2 4.4 Hamaca oil asphaltenes 19.5 4.7 4.9 7.3 asphaltenes A2 19.6 5.8 4.4 7.9 asphaltenes A1 20.9 5.6 6.8 7.8 resins 18.6 3.6 3.2 9.7 are the solubility parameters, with lower subscripts D, P and H representing the dispersion, polar, hydrogen bonding interactions, respectively. 9 is the radius of solubility sphere.

22


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Figure 5. Solubility parameters for asphalts and bitumen fractions plotted as a function of the H/C ratio of the material. Adapted from ref. 129. Copyright 2015 American Chemical Society. 3.3. SOLVENT SELECTION CRITERIA In an NAE process, the solvent dissolves bitumen from oil sand lumps; solvent choice can exert a considerable impact on bitumen extraction efficiency and product quality,25-28,34-38,156162

on the solvent removal rate and residual solvent content in the extraction gangue,26,34,38-52 as

well as on the cost. Selection of the optimal solvent, however, is very challenging because one must deal with a number of criteria/constraints that are often contradictory: extraction performance (recovery efficiency, product quality), environment, health and safety (EHS, toxicity, biodegradability, recyclability, boiling point, volatility, and so on), and economic cost (e.g. cost of raw material, ease of solvent reuse). A holistic approach in selecting the solvent is thus imperative (see Figure 6).163-168

23



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Holistic zone

(Recovery : Quality)

Extraction

EHS

Economy

(Environmental : Health/Safety)

(Capital : Operating cost)

Figure 6. The “triple-E” holistic zone of solvent selection algorithms that satisfies extraction performance, environmental, health and safety (EHS), and economic cost criteria. 3.3.1. Extraction Performance. Extraction performance criteria for solvent selection include two indicators ─ bitumen recovery efficiency and product quality. Aromatic compounds are regarded as good solvents for extracting bitumen. Bitumen recovery increases with increasing solvent aromaticity and the relative extraction coefficient is reduced with increasing boiling temperature of the solvent. Cyclic alkanes have similar effects as aromatic compounds on extraction yield and give recoveries more-or-less unrelated to those from other non-aromatic hydrocarbon solvents. Solvent blends, mostly composed of an aromatic or a cycloalkane, appear to be a suitable medium, with appropriate physical properties (e.g. with respect to solubility, viscosity, and boiling point) and better extraction performance than its neat solvents. It should be pointed out, however, that these correlations are not necessarily independent of one another. For example aromaticity, cyclicity and boiling point may be interdependent.156 For hydrocarbons having the same number of carbon atoms, often an aromatic solvent possesses the highest boiling temperature, followed by cycloalkane and alkane (see Table 2). Nikakhtari et al. screened thirteen light hydrocarbon solvents or solvent blends and demonstrated that the ranking of solvents on the basis of bitumen recovery was aromatics, cyclic alkanes, and heptols > isoprene > limonene.32 Leung and Phillips studied the impact of benzene, toluene, Gulfsol 2329, Gulfsol 3139, and kerosene solvents; higher mass transfer coefficient and minor quantities of bitumen remaining in the sand gangue were obtained via application of a solvent with higher 24


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aromaticity or lower boiling point.26 Wang et al. reported that bitumen recovery from Xinjiang oil sands was affected dramatically, not only by the solubility range of the solvent but also by the proportional distribution of three-component HSP.37 Also, the extracted fuel oil, but not the solvent, was solely responsible for extracting the asphaltenes from bitumen solely.37,157 Wu and Dabros investigated the solvent extraction capabilities of toluene, cyclopentane, n-pentane, and mixtures of n-pentane and cyclopentane at low solvent/bitumen (S/B) ratio.38 It was suggested that cyclopentane gave higher bitumen recovery rates than toluene, while n-pentene and the mixture had a slightly lower ability than toluene. Funk et al. recommended the use of a lowboiling-point solvent to efficiently dissolve bitumen, allowing the use of a very low S/B and fluid bed drying.29 U.S. Patent 8455405 discussed a blend of a polar and a non-polar solvent (PNP), neither of which was individually a good solvent for bitumen extraction.158 The polar component of the PNP could be a compound comprising a non-terminal carbonyl group, such as acetone or ketone, while the non-polar component was an aliphatic alkane such as pentane or heptane. The optimal PNP solvent had a Hansen hydrogen bonding parameter of 1.4 to 2.8 MPa/ ─ very close to that of toluene, but its viscosity and boiling point were much lower than those of toluene. When mixed in optimal proportions, the PNP solvent penetrated the oil sand matrix 2 to 3 times faster and produced more bitumen per unit time than toluene or xylene, making it an excellent solvent for extracting bitumen.158 U.S. patent 20140166543 described the utilization of a solvent blend containing more than 75 mol% dimethyl sulfide. The co-solvent was chosen from among aromatic or aliphatic solvents.159 The advantages of this solvent blend were that it provided a low viscosity and better bitumen-dissolving ability than toluene or xylene due to the relatively low boiling point of this blend, thus enabling high bitumen yield and rapid settling of solid fines.159 On the basis of bitumen product quality, which increases as the fines concentration is reduced, it is suggested that contamination of the bitumen product with fine solids is very sensitive to the solvent properties, indicating that selection of a suitable solvent or solvent blend for a particular separation method could control the behavior of these solids without compromising bitumen recovery.

25



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In gravitational separations, extraction of oil sand bitumen with an aliphatic solvent or analogue such as pentane, hexane, or heptane, often produces relatively clean product with low solids content in bitumen.25-28,53-57,109-112,160-162 Depending on its carbon number and S/B, an aliphatic solvent has controlled ability to precipitate out partially or completely the heaviest fraction of bitumen, the asphaltenes.128-155 Asphaltenes readily form floc networks, entrapping and capturing unwanted particulates and heavy metals, so asphaltenes and fines are separated from the extracted bitumen together.9 For example, Zahabi et al. showed that the removal of solid particles in oil started effectively at the onset of minimal asphaltene precipitation.161 The rapid solids flocculation was attributed to the adsorbed asphaltenes. Jin et al. reported a similar outcome, that the settling rate of bitumen-treated silica particles in heptol was substantially reduced from high heptane to high toluene content, and reached zero when toluene-in-heptol was about 70 vol% or higher.162 The authors reasoned that, despite asphaltene adsorption being a dominant mechanism, there was strong inter-particle attraction in an aliphatic condition, leading to the sedimentation of silica particles. Deliberate dropout of asphaltenes is also thought to be beneficial to upgrading and refining operations. Significant losses of asphaltenic hydrocarbons, however, is a trade-off in terms of extraction yield,109-112 as the asphaltene content in bitumen is positively correlated to bitumen recovery.37,157 In a filtration-like separation, the contrary result is observed that the solids percentage in the recovered bitumen decreases steeply with increasing solubility parameter (SP) of a solvent in the range less than about 16.5 MPa/ and then levels off when the SP increases to 19 MPa/ . This implies that a solvent with higher aliphatic content may not give less fines contamination in the product,32-34 possibly due to the fundamental difference between the passage of solids in a filter cake and the flocculation of solids in gravitational sedimentation. Nikakhtari et al. disclosed that, compared to aromatics and cycloalkanes, heptane-rich blends produced a much higher solids content in the recovered bitumen in an NAE process involving sieving.32 Using the same NAE method,32 Pal et al. made up solvent mixtures of cycloalkane and alkanes at a final SP of 16.45 to 16.65 MPa/ , resulting in minimal solids carryover to the bitumen product as well as high bitumen recovery of over 95%.34

26


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3.3.2. Environmental, Health, and Safety. The early integration of environmental, health and safety (EHS) factors into the selection of a solvent is crucial to the success of an NAE process. It is important to understand that EHS issues are considered ahead of time and that the idea of developing a process and subsequently making it green is to be avoided.164,165 The EHS considerations are driven not only by legislation and evolving attitudes towards environmental issues, but also by the health of operating personnel and the safety of operation in the long term. Effective solvent utilization, recycling, and reuse without purification are essential to minimize the environmental concerns. In a common NAE process, distillation and steam stripping are considered for solvent recovery from bitumen dilution and tailings sand desolventization, respectively. Given identical processes, solvent with lower boiling point (i.e. higher volatility), generally speaking, produces higher solvent recovery rate and the less residual solvent in the dried tailings.25-28, 34, 38-52 It is difficult, however, to control the loss of very volatile solvents such as pentane to the surroundings during extraction. A delicate balance between volatility during extraction and removal from the tailings with minimal energy expenditure should be considered.25 In addition to volatility, the toxicity, flammability, and reactivity of a prospective solvent must be well understood. It is a good practice to eliminate undesirably toxic solvents and reduce the use of highly flammable solvents rather than to add high-cost confinement safety systems to handle the hazards in the plant.167 Single or mixed solvents to the NAE process are practically inert; those solvents reactive to bitumen or other substances or leading to reactive or toxic intermediates and side products in the system should be avoided. It is also necessary to comprehend the inherent energy in the system, potential channels for escape of the solvent, and means of controlling the system safely.166,167 Recently, several chemical companies and institutions, including Pfizer, Astra Zeneca, Glaxo SmithKline, Sanofi, CHEM21 consortium, and the American Chemical Society, have published general-purpose solvent selection guides.164,165, 168-176 These guides mainly focus on the solvent EHS profiles for which the industrial and regulatory constraints were implemented. Conventional and less conventional solvents were ranked into four categories: preferred (recommended), usable (substitution advisable), hazardous (substitution requested), and highly 27



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hazardous (banned). A limitation of these guides is that they scarcely regard the chief purpose of a solvent ─ supporting and facilitating extraction, separation, or other specific process. It is better to provide a comprehensive approach to solvent selection because the data based on the EHS standards alone appears too restrictive for effective process screening.164,165 3.3.3. Economic Cost. Cost is of primary concern in the potential commercialization of an NAE process. For any solvent used in NAE, it is important to understand that, besides the purchase cost of the solvent, different capital and operational costs are incurred.166 Solvent selection plays a vital role in the development of process flow sheets and the choice of process equipment and facilities, leading to distinct capital expenses. The operating cost is determined not only by solvent price and extraction yield, but also by solvent recyclability. A significant portion of the total operating cost is attributable to separating and recovering solvent from the process and from wet sand. A solvent with a higher unit cost may be acceptable if it can be recycled/reused easily and completely, making it economically competitive. As none of the NAE processes have nowadays been implemented beyond the pilot scale, the further comments on economic cost of the NAE process parameters are out of scope of this review. 3.4. NEOTERIC SOLVENTS Until recently, the large majority of solvents used in NAE processes have been hazardous conventional organic solvents (COSs). Due to concern over health and safety challenges such as low-boiling VOCs from petroleum feedstocks,177 alternative solvents are being tested in oil sands extraction research. Amongst these alternatives ─ the so-called neoteric solvents ─ are ionic liquids (ILs), and switchable hydrophilicity solvents (SHSs), as discussed in Sections 2.3 and 2.4. It is interesting to briefly compare these two types of neoteric solvents against each of the abovementioned key criteria (see Figure 7).

28


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Neoteric Solvents

Ionic Liquid (IL)

Examples

Solubility Recyclability

Switchable Hydrophilicity Solvent (SHS)

Imidazolium tetrafluoroborate Imidazolium trifluoromethanesulfonate Choline chloride/urea Highly polar and immiscible with bitumen Organic co-solvent needed Easy separation Problematic if high purity is required

Cyclohexyldimethylamine Tunable in dissolving bitumen No distillation needed Readily form biphases by switching property

Health/safety Low vapor Zero vapor Limited EHS data available; some reported toxic Unknown toxicity Moderately expensive

Most are very expensive Some lower-cost ILs available

Cost

Figure 7. Neoteric solvents that have been used in non-aqueous bitumen extraction processes. 4. SOLVENT RECOVERY Solvent recovery from “wet” gangue/tailings is one of the most distinguishing features of an NAE process. Tailings should be “dry” of organic solvents to prevent releases to the environment. Additionally, solvent recovery is economically beneficial since both conventional and neoteric solvents are costly components of the NAE process. The amount of solvent lost in the dry tailings, therefore, must be held to minimum values ─ e.g. far lower than 4 volumes lost per 1,000 volumes of bitumen produced, which is set by the Alberta Energy Regulator for the collected solvent loss. Considering this significant challenge, methods of solvent recovery from the extracted tailings and the essential parameters affecting solvent recovery are reviewed in this section. 4.1. Solvent Recovery Methods 4.1.1. Steam and gas stripping. In this case stripping is a distillation-like method in which solvent contaminants are separated from tailings solids by a vapor stream – water steam or hot inert gas. Since the volatility of organic solvents is dominantly determined by the operating temperature and pressure, high stripping temperatures in stripping operations facilitate solvent 29



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removal. For example, Gable et al. proposed the recovery of aliphatic and aliphatic-aromatic extraction solvents using two-stage steam stripping.178 The first stage was achieved by passing steam into a solvent-saturating sand bed until just prior to steam breakthrough at the bottom of the filter bed; in the second stage, the partially steamed tailings were then passed into a rotary kiln dryer where solvent was stripped off by the stream of steam in countercurrent contact.178 Mehlberg described steam stripping of the absorbed solvent from solids in a vertical packed-bed column containing two separate stripping zones.179 Wu and Bhattacharya described a process using superheated steam to recover solvent. For efficient heat exchange, superheated steam went through a cycle of compression, condensation, decompression, re-vaporization, and superheating before being recycled for successive runs of drying spent oil sand solids.180 Nitrogen and more volatile gases were also reported as stripping agents to recover solvent entrained in wet gangue after solvent-alone extractions.50,181 In the SESA process, the washed solids containing entrained solvent and small amounts of bitumen were passed into stripping drums where solvent was recycled to a belt filter and the dried agglomerates exited the process as dry tailings.54,57 Stripping processes are selective and often produce a high solvent recoveries, but they suffer from high energy consumption and are time consuming. 4.1.2. Fluidized bed drying. This is a particularly effective and time-saving method in

which hot air or gas is injected in an upward direction at a velocity greater than the downward settling rate of solids such that the wet solids particles are blown up in the air stream (referred to as “fluidized”) and dried. In the process of Funk et al., low-boiling extraction solvents such as heptane and toluene are recovered up to 99% using fluidized bed drying with heated vapors to remove both unbound solvent and water followed by heated inert gas treatment.29 Kift et al. disclosed a drying process of wet tailings in fluidized bed columns that utilizes very hot gas at high flow rate. A disperser is used to break up large agglomerates of tailings particles before they are fed into the columns.182,183 Fluidized bed drying allows more efficient and even heat and mass exchange and is adaptable to handle a large solids throughput in the shortest residence time, but it also requires intensive energy input.

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4.1.3. Surfactant solution washing. This is a cost-effective remediation process

whereby organic contaminants including (residual solvent and bitumen) are recovered by washing solvent-containing tailings with surfactant/water solution. The amount of water use in the washing process is negligible compared with that in CHWE-based operations and does not cause a tailings pond issue. This washing technique is based on the principle that the entrained solvent/bitumen components are mobilized into the aqueous solution by means of displacement, dispersion, or emulsification. The type and concentration of remediating solution are critical parameters to process effectiveness; some promising surfactants proposed for use with extracted oil sand gangue desolventizing are alcohol ethoxylates, sodium dodecyl sulfate, and naphthenic acids.184-187 An adequate surfactant for washing should possess the ability to be sufficiently adsorbed at the oil/water interface, significantly reducing oil-water interfacial tension, weakening the capillary forces that hold back bitumen-solvent residues in the sand pores, and enhancing the wettability of solids surfaces in favor of hydrocarbon liberation from the surface.186,187 The formation of bicontinuous microemulsion that leads to greatly increased consumption of surfactant should be avoided.187 4.1.4. Assistance of second solvent. This refers to the separation of first-extraction

solvent from the first solvent-wet tailings achieved by the addition of second solvent. The advantage of this two-solvent approach is that the second solvent is much lighter, more volatile, and easier to strip from the wet tailings.27,47,52,188-191 Energy required to recover the second solvent may be minimal. The second solvent may be miscible or immiscible with the first solvent. A light hydrocarbon solvent, such as propane, butane, pentane, or liquefied petroleum gas are often suggested as a second solvent to dislodge the first extraction solvent entrained in the oil sand tailings.27,52,188,189 Light polar solvent including a low-molecular-weight alcohol, ketone, or ether is also an alternative as a second solvent to displace the first solvent from the first solvent-wet tailings.47,190,191 Drying of tailings wet with a low-boiling- point second solvent can be performed via one of the above-mentioned methods or their combination, e.g. in a fluid bed, where, in the first step, the fluidizing gas is the heated solvent vapor; in the second step, heated inert gas is employed to remove the bound solvent.191 31



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4.2. Parameters Affecting Solvent Recovery

It is believed that solvent recovery/drying from the extracted wet tailings without water addition during extraction typically involves two stages: a first-stage of fast drying at a linear rate until solvent mass content decreases to less than 0.1 relative to initial solvent content, followed by a slow drying process in the second stage that is governed by the further release of small amounts of residual solvent, along with the evaporation of connate water.32,192 The key underlying parameters that affect solvent drying are summarized below. 4.2.1. Role of Solvent Vapor Pressure. Vapor pressure (or boiling point) of a solvent

has a dominant effect on the first stage of solvent recovery/drying from wet tailings. High solvent vapor pressure (or low boing point) is beneficial for the initial solvent drying. The linear slope in the first stage of solvent drying suggests that it is governed by the constant-rate evaporation of solvent.32,192 Solvent evaporation is well correlated to its vapor pressure. Wu and Dabros evaluated four different solvents with respect to the initial drying rate and the residual solvent content.38 It was indicated clearly that the lightest n-pentane was the easiest solvent to remove, followed by cyclopentane, hexanes, and the heaviest, toluene, at an identical temperature in their study. Nikakhtari et al. also investigated aromatics, cycloalkanes, biologically derived solvents, and their mixtures of different vapor pressures.32 The initial drying rates of different solvents were linearly proportional to the vapor pressures of the solvents. In addition, the equilibration time for the first-stage drying and the residual solvent concentrations after drying decreased linearly as the vapor pressures of the solvents increased.32 In another study of the removal of cyclohexane solvent from tailings, Nikakhtari et al. reported that the drying rates of cyclohexane and water were increased by 97% and 100%, respectively, when temperature increased from 24 °C to 60 °C, since the vapor pressure of a given solvent was larger for the higher temperature.192 According to studies on the drying of two miscible liquid phases in a solid matrix, the residual solvent content was determined by the initial solvent concentrations and the gas-liquid equilibrium conditions, suggesting the solvent with the higher vapor pressure was favored to obtain drier gangue.193-195 4.2.2. Role of Asphaltenes/Bitumen Residues. Despite having little impact on the first-

stage recovery rate, the residual bitumen/asphaltenes are believed to be detrimental to the second 32


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stage of solvent recovery. Higher residual bitumen content in the tailings reduces solvent desorption. The second drying stage (see ref. 32) is slow, mainly due to the diffusion of residual solvent out of the porous sediment. It was shown that, in the second drying stage, the residual solvent (e.g. cyclohexane) content was relatively independent of drying temperature, suggesting that the residual solvent was likely trapped in the matrixes in the gangue.192 The residual bitumen or asphaltenes may delay the release of residual solvent during the second drying phase, based on slow diffusion rates of a solvent in bitumen fractions. In studying cyclohexane vapor sorption/desorption from asphaltenes, Noorjahan et al. found that the rate of solvent desorption in asphaltenes was much lower than solvent sorption rate and did not reach equilibrium over a month.196 The authors fitted the sorption/desorption data very well to the Weibull relaxation model,197 suggesting the strong possibility that asphaltene film undergoes structural relaxation during sorption and that the lower desorption rate can be attributed to solvent molecules diffusing through a more compact medium during desorption than during sorption.196 It is important to note that the presence of solvent causes relaxation of the asphaltenes, implying that solvent is entrapped in the asphaltenes either by absorption or by a combination of adsorption and relaxation into the asphaltene aggregates and stabilization. Noorjahan et al. also showed several order-of-magnitude smaller diffusion coefficients of the solvent in asphaltenes than the values in bitumen,196 implying that the solvents that reject heavy bitumen components, such as aliphatic solvents, could be more strongly entrained in the asphaltene-rich tailings and cause enhanced solvent retention. Tan et al. reported a more remarkable solvent sorption capacity on bitumen- or asphasltene-coated fine solids than on the virgin fine solids. High bitumen residue content produced decreased sorption and desorption rate due to solvent partitioning into the organic component.198 In addition to the residual bitumen, the interaction between the potential solvent and bitumen should be considered. Given the same amount of residual bitumen, weak interaction of solvent and bitumen is more favorable for achieving higher sorption and desorption rate. However, the effect of solvent-bitumen interaction on overall solvent recovery is complex. On one hand, weak solvent-bitumen interactions give rapid solvent release in the first stage, and cause high overall desorption rate if one does not consider the effect of residual bitumen. On the other hand, weak solvent-bitumen interactions often lead to solvent deasphalting and high 33



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residual bitumen or asphaltenes, which would hold larger amounts of solvent beyond the initial desorption stage and into the long-term, slow solvent release stage.196,198 Thus, on basis of solvent recovery, engineering design of a solvent should aim to find a balance between minimizing residual bitumen content and favoring weak solvent-bitumen interaction. 4.2.3.

Role of Clay Fines. The solid particles in the gangue after solvent extraction are

mainly coarse quartz sands and fine clays including kaolinite, illite, and traces of illite–smectite and chlorite.31 Although fines are a minority constituent, they have high surface-to-volume ratios and retain organic materials (e.g. residual bitumen), making solvent recovery problematic. A couple of studies observed that solvent vapor had higher sorption rate and quantities on bare kaolinite than on bare quartz sand, due the larger specific surface area of kaolinite compared to quartz.198-200 Osacky et al. stated that the non-swelling clays retained 20% to 58% of solventinsolvable organic materials of the total content, mainly on the basal planes and edges of clays in the form of patches.201 The amount of organic materials retained after extraction and solvent removal from the tailings correlated well with specific surface area of clays. Also, the increase of cation exchange capacity and layer charge density was reported to increase both the clay-organic interactions and the amount of organics retained. The results suggested a role for clays in enhancing solvent retention by trapping solvent in the adsorbed bitumen components and a correlation between solids surface characteristics and the amount of organics retained.198-201 4.2.4. Role of Water. Depending on the ore source and geology, oil sands contain

between

Recent Advances in Nonaqueous Extraction of Bitumen from Mineable

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