Asphaltene Precipitation from Athabasca Bitumen Using an Aromatic

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Asphaltene Precipitation from Athabasca Bitumen Using an Aromatic Diluent: A Comparison to Standard n-Alkane Liquid Precipitants at Different Temperatures Ørjan Bjorøy,*,† Per Fotland,§ Eimund Gilje,∥ and Harald Høiland†,‡ †

Department of Chemistry, and ‡Centre for Integrated Petroleum Research (CIPR), University of Bergen, Allégaten 41, 5007, Bergen, Norway § Statoil Research and Development, 5020 Bergen, Norway ∥ Statoil Research and Development, 4035 Stavanger, Norway S Supporting Information *

ABSTRACT: Asphaltenes are precipitated from Athabasca bitumen upon the addition of liquid precipitants. The asphaltene onset and the amount of precipitated asphaltenes using different equivalents of added liquid precipitant are determined. The deasphalting power of an aromatic solvent with structural similarities to toluene, namely, α,α,α-trifluorotoluene (BTF), is investigated, and the results are compared to using n-pentane (n-C5), n-hexane (n-C6), and n-heptane (n-C7) as liquid precipitants. A correlation between maximum asphaltene precipitation and the surface tension of the pure liquid precipitants studied is observed. Increasing the temperature results in increasing the volume fraction of liquid precipitant at the asphaltene onset and a decrease in the amount of maximum asphaltene precipitation. Composition elemental analyses show a small increase in the relative amounts of sulfur atoms in asphaltenes with increasing precipitation temperatures. The appearances of asphaltenes precipitated at different temperatures are compared before and after thorough washing with excess liquid precipitant. Asphaltene precipitation experiments are time-consuming and have motivated us to introduce an empirical function to describe asphaltene yields between asphaltene onset and maximum asphaltene precipitation requiring less experimental data. Our results provide useful tools for future studies of gradient asphaltene precipitation; processes that are of current interest with the development of new(er) techniques for bitumen extraction.

1. INTRODUCTION When dealing with product analyses and related properties of bitumen, asphaltenes are typically the first bulk component isolated. Asphaltenes are treated as a solubility class or even as a complex matrix of numerous solubility classes, and the term is neither directed at nor confined to specific structural properties.1−4 Asphaltenes are most commonly defined as the material within crude oil that is insoluble in n-pentane or nheptane and soluble in benzene or, more preferably for safety reasons, toluene; cf. standardized methods for determination of asphaltenes in petroleum products.4 A broader definition would include all n-alkanes as suitable liquid precipitants. According to a more precise definition, asphaltenes are soluble in solvents or solvent mixtures with a surface tension greater than 25 mN m−1; incidentally, this include most aromatic solvents available, leading to a common misconception that asphaltenes are soluble in all aromatics.4,5 Other parameters, including the Hildebrand solubility parameter and total base numbers, seem to give good correlations to the asphaltene content.5,6 With respect to solubility parameters, it has been suggested that the three-component Hansen solubility parameter better predicts the behavior of asphaltenes in crude oil.7,8 Asphaltenes typically have a solubility parameter around 20 MPa1/2.7−10 In theory, when low-molecular-weight n-alkanes are added, the solubility power of the mixture is lowered. At the point of incipient asphaltene precipitation, the solubility power is decreased to a point at which the asphaltenes are destabilized and begin to © 2012 American Chemical Society

aggregate. This condition, with a certain volume fraction of nalkane added to the mixture, is also known as the flocculation point or (asphaltene) onset.9 It is suggested that the recent discovery of liquid crystals in precipitated asphaltenes may provide new insight into understanding the behavior of bitumen and asphaltenes.11 Nonetheless, discussions around the topic of asphaltenes often seem to raise more questions than answers. For example, asphaltenes may be regarded as solids dissolved within the crude oil or even as solids dispersed in the bulk oil.7,8,12,13 Asphaltenes are mainly considered to be organic deposits. Elemental composition analyses also reveal some sulfur atoms, slightly lesser amounts of oxygen and nitrogen atoms, and often trace amounts of mainly vanadium and nickel atoms.1,14−17 In addition, the asphaltene fraction is usually of higher molecular weight, (higher) polarity, and/or higher aromatic content compared to the remaining oil fractions.4 There is some disagreement regarding whether asphaltene precipitation is a reversible process (thermodynamic theory) or an irreversible process (colloidal theory). Luo and Gu18 have suggested that the ability Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 15, 2011 Revised: January 31, 2012 Published: February 9, 2012 2648

dx.doi.org/10.1021/ef201395x | Energy Fuels 2012, 26, 2648−2654

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2. EXPERIMENTAL SECTION

Aldrich. Fluid properties of liquid precipitants, including values of density and surface tension, were taken from the literature.29,33−35 One problem, when working with bitumen, is the transfer of the heavy oil, especially in small quantites. The high viscosity makes the oil less eligible for precise mass transfer, even when heated. In a typical procedure, a larger volume of bitumen was diluted, using the appropriate liquid precipitant, to a specific mole ratio of 1:4 (bitumen/liquid precipitant). Knowing the composition of bitumen and liquid precipitant in the mixture allows for a certain mass to be transferred to a glass centrifuge tube, equipped with a glass stopper. The sample was further diluted with liquid precipitant to the intended mass concentration. After thorough mixing, including the use of ultrasound, the samples were stored in the dark at room temperature overnight (16 h). During our initial investigation, we found that centrifugation and subsequent removal of the remaining oil mixture were a reasonable easy and clean method, giving reproducible results. The liquid and semi-solid phases was separated using a Sigma 6-10 laboratory centrifuge programmed to run for 10 min at a relative centrifugal force equivalent to 1446g. The excess liquid was mechanically removed by suction using a Pasteur pipet after centrifugation, and the precipitate was washed several times with the appropriate n-alkane until the asphaltene precipitate easily dispersed from the bottom of the sample tube and the supernatant appeared colorless. The asphaltene precipitate was dissolved in small amounts of DCM/methanol (93:7, v/v), and after evaporation of the solvent, the sample mass was measured until a constant mass of product was obtained. The asphaltene yields reported are an average of five samples; the range is the difference between the highest and lowest value. The sample preparation for BTF−oil mixtures was as described above; however, no separation of solid precipitate from the oil mixture was achieved when applying centrifugal forces on the oil−BTF mixtures. This is likely due to the higher density of these mixtures caused by the higher density of pure BTF (ρ = 1.2 g cm−3) compared to the n-alkanes studied (ρ ∼ 0.6−0.7 g cm−3).29,33 Thus, the precipitate when using BTF as the liquid precipitant had to be collected by filtration using Whatman GF/C 1.2 μm glass microfiber filters.6 The precipitate was further washed with the liquid precipitant until the running-through liquid appeared colorless. To ensure that results from the two different methods of collecting the precipitate, centrifugation and filtration, were comparable, samples prepared identically using a n-alkane were worked up in both ways. In addition, samples at both room temperature (22 °C) and 60 °C were analyzed to investigate if the choice of method was influenced by temperature effects. Similarity in the boiling point between BTF and n-heptane made the latter a suitable choice for these investigations. Apparent molecular weights were determined using a Löser type 20 automatic cryometer, measuring the freezing point depression of solutions compared to pure benzene. Composition elemental analyses (CHNS) were measured using a VarioEL III elemental analyzer. 2,5Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (Sigma-Aldrich) was used to verify analyzer accuracy and reproducibility.3 Onset was determined using the classical asphaltene spot test, placing a drop of the mixture onto a piece of filter paper. Solid asphaltenes, if present, remain at the center of the spot, while the liquid-phase oil mixture will gradually move outward from the initial drop. A more precise determination of the onset was accomplished by closer inspection of mixtures suspected near the onset, with part of the solution being placed between a glass slide and a coverslip and examined under the microscope. All glass equipment was preheated to the temperature of the solutions studied. The physical appearance of precipitated asphaltenes was examined with a light microscope (Heerbrugg Wild Makroskop M 420), equipped with a Jenoptik ProgRes C12Plus digital microscope camera.

A sample of Athabasca bitumen, produced by steam-assisted gravity drainage (SAGD), was provided by Statoil. The average molecular weight and density of the sample was measured to 524 atomic mass units (amu) and 0.990 g cm−3 (50 °C), respectively. The water content was given as less than 0.5 wt %. n-Alkanes (p.a. quality) were delivered by Merck. Anhydrous BTF was purchased from Sigma-

3. RESULTS AND DISCUSSION 3.1. Asphaltene Precipitation Yields. Athabasca bitumen does not flow at reservoir conditions without being heated or diluted. The addition of liquid hydrocarbons to this highly

to redissolve asphaltenes is dependent upon the period of time from formation. Left standing for several hours, the asphaltenes formed could not be dissolved in the heavy oil saturated with propane from which they had precipitated. Conductivity studies have shown that irreversibility is dependent upon sample history.19 The occurrence and amounts of asphaltenes may vary with changes in experimental conditions, such as solvent, pressure, contact time, temperature, and the ratio between the liquid precipitant and crude oil.17−28 Calles et al.21 have studied the temperature effect on asphaltene precipitation using npentane, n-hexane, and n-heptane, while Maqbool et al.22,23 studied the kinetics of asphaltene precipitation. Luo et al.24 have investigated the effect of using gaseous and (partially) liquefied propane as the precipitating agent, as compared to using n-pentane and n-heptane. Recently, the solvent deasphalting power of dimethyl ether, a promising alternative to diesel fuels, has been found to be comparable to that of npentane.25 Also, using a well-documented solvent mixture of nheptane and toluene (heptol), the solvent deasphalting power can be decreased with increasing amounts of toluene added to n-heptane.26,27 Several papers describe the solubility of nalkane-precipitated asphaltenes in different solvent and solvent mixes, including ethyl acetate, acetone, methyl isobutyl ketone, acetyl acetone, and carbon tetrachloride, in work performed by Seidl et al.28 Mitchell and Speight5 tested an extensive list of liquid precipitants for asphaltene precipitation using an Athabasca bitumen sample and found correlations between the quantities of asphaltenes precipitated and both the surface tension and Hildebrand solubility parameter of the liquid precipitants used in their study. One aim of our study was to investigate the deasphalting power of α,α,α-trifluorotoluene (BTF). Commonly known as benzotrifluoride or BTF, α,α,α-trifluorotoluene is an aromatic solvent used to replace dichloromethane (DCM) in some standard organic protecting group reactions, where a higher boiling point of the solvent is desirable.29 Nonetheless, there are significant differences in both the surface tension and Hildebrand solubility parameter of BTF and DCM, and although there is a structural relationship between toluene and BTF, the Hildebrand solubility parameter and surface tension value of toluene are more comparable to those of DCM. Toluene is indeed a good solvent for asphaltenes; cf. the definition of asphaltenes as being precipitated by the addition of n-alkanes and soluble in toluene.4 In fact, most aromatic compounds have a surface tension higher than 25 mN m−1 and give no asphaltene precipitation.5,7 In the comprehensive work of Mitchell and Speight,5 only two of the tested aromatic solvents, 1-phenylnonane and 1-phenyldecane, were found to give asphaltene precipitation and then in barely traceable amounts. Modeling asphaltene precipitation behavior is of current interest with the development of new(er) techniques for bitumen extraction, e.g., vapor extraction (VAPEX).30−32 Our results may provide useful tools for future studies of gradient asphaltene precipitation.

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Asphaltene precipitation yields are given as the weight ratio of precipitated asphaltenes/bitumen (av), where av = ma/mo, i.e., mass of asphaltenes experimentally determined (ma) divided by the mass of bitumen (mo). Traditionally, asphaltene precipitation yields are presented as a function of the volume ratio (vr) of liquid precipitant (Vp) to crude oil (Vo); vr = Vp/ Vo. We have chosen to present our data using the volume fraction (vp) of liquid precipitant; vp = Vp/(Vp + Vo), because this better illustrates the area with the most rapid increase in asphaltene precipitation. The different graphical methods in projecting the experimental data are shown in Figure 3, using

viscous oil may result in asphaltene precipitation or flocculation. Thorough washing of the precipitate with excess liquid precipitant gives solid dry asphaltenes, as shown in Figure 1.

Figure 1. (Left) Sample of Athabasca bitumen. (Right) Asphaltenes precipitated from bitumen after the addition of excess n-heptane.

Asphaltene precipitation is a function of the ratio of liquid precipitant/bitumen (vr). In our analyses, the total volume was held constant and the amount of bitumen varied. To validate that our results are not influenced by the amount of bitumen, the asphaltene yields when adding an excess of 40 volume parts of liquid precipitant were measured and found to increase in a linear relationship with the amount of bitumen analyzed (in the range of 0.3−2 g of bitumen). Thauerkorn et al.20 have previously shown a similar relationship using n-hexane as the liquid precipitant. The contact time between the liquid precipitant and bitumen may influence asphaltene yields.17,22,23 For samples with a high ratio of liquid precipitant/bitumen, the contact time was not found to influence the asphaltene yields, in accordance with observations made by Maqbool et al.22 However, we found that samples worked up during the same day as they were prepared gave varying results, while samples stored overnight did seem to give more reproducible yields (Figure 2). Values of the

Figure 3. Asphaltene yield as a function of both the volume ratio (vr) and volume fraction (vp) of BTF.

BTF as the liquid precipitant. Either way, the asphaltene yield rapidly increases from the onset until it levels out as the ratio between the liquid precipitant and oil is increased. A further increase in this ratio does not result in higher yields because the maximum amount of liquid-precipitant-specific asphaltenes has been precipitated from the crude oil. The asphaltene onset increased with an increasing carbon number of n-alkanes; i.e., a higher volume fraction of the liquid precipitant was necessary when using n-heptane (n-C7) compared to n-hexane (n-C6) or n-pentane (n-C5). The onset using BTF as the liquid precipitant was considerably higher than for the n-alkanes studied. Maximum asphaltene yields decrease in the following order: n-C5 > n-C6 > n-C7 > BTF. Thus, using BTF results in less asphaltene precipitation, and as the liquid precipitant, BTF has the ability to dilute more of the bitumen before reaching the asphaltene onset. Still, prominent asphaltene precipitation occurs with BTF added to bitumen. BTF is in a molecular sense quite similar to toluene, the standard solvent used for dissolving asphaltenes. The difference in the solubility power of BTF and toluene may, in part, be explained by the differences in the surface tension of the pure solvents, 23.4 and 28.5 mN m−1 (20 °C), respectively.34,35 These results support claims that asphaltenes are soluble in solvents having a surface tension greater than 25

Figure 2. Asphaltene yield as a function of the contact time, using npentane as the liquid precipitant and keeping the ratio between the liquid precipitant and bitumen constant (40:1, v/v).

asphaltene onset were usually found to be higher for samples examined a few hours after mixing than for samples stored 16 h (overnight). It is reported that asphaltene yields vary with contact time at moderate ratios of liquid precipitant/crude oil.22,23 All of our samples were studied using an invariable contact time of 16 h. 2650


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mN m−1 but stand as a reminder that asphaltenes are not necessarily soluble in all aromatic solvents. In Figure 4, both the

somewhere between cyclohexane and toluene along one dimension (dispersion forces). In the other dimension, the polar attributes of BTF would position it somewhere close to DCM. Unfortunately, complete use of the Hansen parameters would require experimental testing on a much larger number of solvents and liquid precipitants.7,8 However, in comparison to cyclohexane and toluene, it may seem that the liquid precipitant qualities of BTF cannot be explained solely by solubility parameters. Nikooyeh and Shaw38 warn in a recent paper against “the use of solubility parameter or other simple solution thermodynamic concepts to describe asphaltene + diluent mixture behaviour”. 3.2. Describing Asphaltene Precipitation. Experimental data are often time-consuming and difficult to obtain, and it is often necessary to reduce the amount of experimental data. Knowing the two extreme limits, the asphaltene onset and maximum amounts of precipitated asphaltenes, we suggest using the following empirical formula (eq 1) to describe asphaltene yields at different ratios of liquid precipitant and bitumen:

Figure 4. Maximum asphaltene yields at 22 °C as a function of the Hildebrand solubility parameter and surface tension of pure liquid precipitants.

Hildebrand solubility parameter and surface tension for pure liquid precipitants have been plotted against maximum asphaltene yields for the n-alkane series and BTF. Asphaltene yields using n-alkanes were consistent with values reported by Mitchell and Speight.5 They found that asphaltene yields can be linked to both Hildebrand solubility parameter and surface tension values. When comparing data, we found that a linear correlation was observed between maximum asphaltene yields and surface tension values of the pure liquid precipitant. The Hildebrand solubility parameter, defined as the square root of the cohesive energy density, has been shown to give a good correlation to asphaltene yields within the series of homologous liquid precipitants.5 However, expressing the yields as a function of the solubility parameter revealed that the results using BTF significantly differed from trends observed in data presented by Mitchell and Speight.5 The solubility parameter of cyclohexane is comparable to that of BTF. Nonetheless, the addition of cyclohexane to bitumen did not result in asphaltene precipitation. The threecomponent or three-dimensional Hansen solubility parameter considers the dispersive, polar, and hydrogen-bonding interactions of the solvent and solvent mixtures, which, when plotted in three dimensions, would form a solubility sphere. Using solubility parameters in Table 1, taken from Hansen Solubility Parameters: A User’s Handbook,36 and combining the polar and hydrogen-bonding interactions, forming a twodimensional solubility parameter, as suggested by Wiehe,37 it was possible to compare BTF with toluene, cyclohexane, and DCM, three solvents that did not give asphaltene precipitation when added to bitumen. Thus, BTF would be placed

av =

γa

δDb

δPb

δHb

n-pentane n-hexane n-heptane cyclohexane BTF toluene DCM

16.1 18.4 20.1 25.2 23.4 28.5 27.8

14.5 14.9 15.3 16.8 17.5 18.0 18.2

0 0 0 0 8.8 1.4 6.3

0 0 0 0.2 0 2.0 6.1

(1)

In eq 1, av is the weight fraction of precipitated asphaltenes relative to the amount of oil. The volume fraction of the liquid precipitant (vp) is limited to vc ≤ vp ≤ 1, where vc is the critical volume fraction of the liquid precipitant at the onset. The parameter k is a constant for each liquid precipitant; k = [(1 − vc) − am]/[am(1 − vc)s]. The maximum weight fraction of precipitated asphaltenes relative to the oil at infinite dilution (am) is, for practical reasons, defined as the weight fraction of asphaltenes when using a liquid precipitant/heavy oil ratio of 40:1 (v/v). The unknown parameter, s, is an empirical exponent to be determined by curve fitting. It follows then that, when vp = vc, i.e., at the onset with the beginning of asphaltene precipitation, av must be 0. Also, when vp is going toward 1, reaching maximum asphaltene precipitation, av is equal to am. Knowing maximum precipitation for a few liquid precipitants may allow for an estimation of the yields of others from the surface tension of the pure liquid precipitant (Figure 4). Once the precipitation curves are known, eq 1 contains only one unknown and av is not very sensitive to the value of s. The leastsquares method was used to find the value of s. This gave the following values of s; 1.5 (BTF), 1.5 (n-C7), 1.8 (n-C6), and 2.0 (n-C5). These results indicate an increase in the value of s with a decreasing carbon number, although the small differences in the values of s only show minor influences on the described asphaltene yields. The respective k values were as follows: 12.7 (BTF), 7.9 (n-C7), 8.7 (n-C6), and 8.4 (n-C5). Experimental values of asphaltene yields are shown in Figure 5, along with the calculated curves from eq 1. 3.3. Temperature Effects. The appearance and morphology of asphaltenes and co-precipitate are seemingly influenced by the temperature and liquid precipitant. The following observations were made on the original precipitate before it was repeatedly cleaned by washing with liquid precipitant: The precipitate appeared less viscid with an increasing carbon number of the n-alkane liquid precipitant. At 60 °C, the precipitated materials appeared powder-like and dry when both n-C7 and n-C6 were used as the liquid precipitant. At 60 °C, the experimental conditions exceed the boiling point of n-C5;

Table 1. Surface Tension Values and Hansen Solubility Parameters of Liquid Precipitants/Solvents34−36 liquid precipitant/solvent

(vp − vc) 1 + k(vp − vc)s

Surface tension, γ (mN m−1, at 20 °C); bsolubility parameters, δD (dispersion), δP (polarity), and δH (hydrogen bonding) (MPa1/2). a

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with both brown and shiny black surfaces; see Figure 7 for a comparison of the three types of n-C7 asphaltenes. The nhexane asphaltenes precipitated at 22 °C looked very much like the asphaltenes precipitated using n-C7 at equal temperatures. At 30 °C, the color changes to brown. Asphaltenes precipitated at 45 and 60 °C are also brown in color, but there is a distinct change in appearance as the surface becomes rougher as the experiment temperature increases. The n-pentane precipitates are all small in size compared to the asphaltenes precipitated using n-C6 and n-C7. The material precipitated at 22 °C is brown and powder-like. As the temperature increases, the color gradually changes toward black and the appearance becomes more glass-like. All BTF asphaltenens are similar in size to those from n-C5. The material is black and brittle, partly clusters. However, the material flocculating at 90 °C is brown and more powder-like. The effect of the temperature on the maximum asphaltene precipitation is best studied with n-heptane as the precipitating solvent. The yield of precipitated asphaltenes tends to decrease as the temperature increases (Figure 8). The effect is greater between 30 and 45 °C. A further decrease is observed between 45 and 60 °C before yields are stabilizing at 90 °C. Calles et al.21 observed a similar trend with a greater decrease in asphaltene precipitation between 25 and 36.5 °C. The parameter values (s and k) used in eq 1 were determined using n-heptane as the liquid precipitant at different temperatures. Between 22 and 90 °C, the value of s dropped from 1.5 to 1.3. The value of k appeared constant at 7.7 (±0.3). With the s and k values (eq 1) not differing much with the temperature and av not being very sensitive to the value of s, we find the parameters in eq 1 to be usable over a wide temperature range. The decrease in asphaltene yields using n-C5 and n-C6 follow the trends observed for n-C7, with the exception of the n-C5 study at 60 °C, giving higher yields of asphaltene precipitation. The experiments performed at 45 and 60 °C using n-C5 are above the boiling point. Increasing the internal pressure may thus account for the higher yields observed at 60 °C. A linear correlation was observed between maximum asphaltene yields at 22 °C and surface tension values of the pure liquid precipitant. Surface tension values are linearly decreasing with the temperature, and one would therefore expect an increase in the asphaltene precipitation at higher temperatures. When the temperature was increased, amounts of precipitated asphaltenes were lowered. The impact of temperature effects on the Hildebrand solubility parameter would also give expectations of higher asphaltene yields with an increasing

Figure 5. Asphaltene yields at different volume fractions of liquid precipitants studied. Experimental values as well as calculated curves from eq 1.

the precipitate from using n-C5 as the liquid precipitant was sticking to the surface of the glass tube. Obtaining solid asphaltenes from this latter precipitate was difficult; although looking through the microscope, the sticky material was seemingly dry and powder-like. The precipitate at 90 °C when using n-C7 as the liquid precipitant resembles the n-C5 precipitate at 60 °C. Comparisons of the initial precipitate at three different temperatures when using n-C7 as the liquid precipitant are shown in Figure 6. At room temperature, the nC7-precipitated material appeared partly to consist of a coprecipitated dense liquid phase. The materials precipitated at 60 and 90 °C both seem to be solid, although the material precipitated at 60 °C was dry and powder-like, while the material precipitated at 90 °C was sticking to itself and the walls of the glass tube. It appears that the differences seen in the two asphaltene fractions are due to changes in the surface energy of the different asphaltenes. Asphaltenes precipitated using nalkanes were separated from the liquid phase by centrifugation. This may lead to some compression of the material, although it is worth mentioning that n-C7 asphaltenes separated by filtration did not differ in appearance from n-C7 asphaltenes separated by centrifugation. BTF-precipitated asphaltenes were all separated by filtration. All precipitated asphaltenes appeared dry and brittle after proper washing with the correct liquid precipitant, although some variations in appearance were observed: Asphaltenes precipitated using n-heptane at 22, 30, and 45 °C appeared as large black and brittle particles with smooth, hard surfaces and mostly straight edges. At 60 °C, the color changes to brown and the surface seems rougher compared to asphaltenes isolated at lower temperatures. At 90 °C, the asphaltenes were smaller

Figure 6. Appearance of initial n-C7-precipitated asphaltenes, before repeatedly cleaning with excess n-C7: (A) precipitated at room temperature (22 °C), (B) precipitated at 60 °C, and (C) precipitated at 90 °C. 2652


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Figure 7. Appearance of dry n-C7-precipitated asphaltenes, after repeatedly cleaning with excess n-C7: (A) precipitated and treated at room temperature (22 °C), (B) precipitated at 60 °C, and (C) precipitated at 90 °C.

Composition elemental analyses of the precipitated dry asphaltenes indicate a stable ratio between hydrogen and carbon atoms of about 1.1−1.2, independent of temperature changes. Likewise, the nitrogen atom content was found to be stable at 1.3 nitrogen atoms per 100 carbon atoms. The relative quantities of sulfur atoms also seem stable and independent of the choice of liquid precipitant, approximately 3.8 sulfur atoms per 100 carbon atoms. Asphaltenes precipitated using nheptane and BTF were found to have higher relative amounts of sulfur atoms compared to n-pentane asphaltenes. A small increment in the sulfur content was observed with increasing temperatures (Figure 10). The results are consistent with

Figure 8. Maximum asphaltene precipitation yields when the ratio between the liquid precipitant and bitumen is 40:1 (v/v). The effect of different liquid precipitants and temperatures is studied.

temperature. Asphaltenes are groups of compounds that, at a given temperature and pressure, are not dissolvable in a mixture of oil and liquid precipitant. When considering changes in asphaltene yields with the temperature, one also has to take into account the physical changes in oil mixtures and precipitated material. It is likely that, within the asphaltene fraction, we find compounds having different solubilities in the oil mixtures at different temperatures and also that the degree of asphaltene self-association is influenced by the temperature. Increasing the temperature also reduces viscosity and affects precipitation kinetics in the liquid precipitant−oil mixtures.23 Asphaltene onset values were found to slightly increase as the temperature increased. The relative difference in onset between n-C6 and n-C7 between 22 and 60 °C appeared not to change when the temperature increased. The relative difference in the onset between n-alkanes and BTF was found to increase as the temperature increased (see Figure 9).

Figure 10. Relative number of sulfur atoms per 100 carbon atoms from asphaltenes precipitated at different temperatures and using different liquid precipitants. All asphaltenes were precipitated using a constant liquid precipitant/bitumen ratio of 40:1 (v/v).

general observations that the least soluble asphaltene fractions usually contain higher amounts of sulfur atoms.13,17

4. CONCLUSION Asphaltenes are usually well-soluble in most aromatic solvents. Nonetheless, this work has shown that significant amounts of asphaltenes can be precipitated when adding BTF to Athabasca bitumen. BTF shows interesting properties as the liquid precipitant used in asphaltene precipitation. The initial high density of pure BTF results in higher densities of the oil−BTF mixtures, leading to asphaltene flocculation rather than asphaltene precipitation. Also, higher onset values and lesser maximum precipitation yields were observed when using BTF compared to n-C5, n-C6, and n-C7. The asphaltene onset increases and the maximum precipitation yields decrease with an increasing carbon number within the n-alkane series. The effect of the temperature on maximum precipitation yields and asphaltene onset has been investigated,

Figure 9. Apparent changes in onset values for different liquid precipitants and at different temperatures. 2653


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and increased temperatures lead to a decrease in maximum precipitation yields and higher asphaltene onset values. Elemental composition analyses of the precipitated asphaltenes show that asphaltenes precipitated at higher temperatures have a higher content of sulfur atoms. A linear correlation was observed between maximum asphaltene yields and surface tension values of the pure liquid precipitant. The Hildebrand solubility parameter may be useful for predictions within a homologous series of liquid precipitants (e.g., n-alkanes) but could not successfully predict the precipitating qualities of BTF. These parameters were not useful in predicting asphaltene yields as a function of increasing temperatures. We have introduced an empirical formula to describe asphaltene yields at different ratios of liquid precipitant/ bitumen. Experimental data are often time-consuming and difficult to obtain. Future studies may benefit using this formula, reducing the amount of necessary experimental data.

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ASSOCIATED CONTENT

S Supporting Information *

Values of the asphaltene onset and amounts of asphaltenes precipitated using different equivalents of added liquid precipitant to bitumen and at different temperatures (Tables S1−S3) and results from composition elemental analyses (CHNS) and relative atomic ratios of H, N, and S relative to C of asphaltenes precipitated at different temperatures (Table S4 and S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-55-583-382. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financed by VISTA, a research cooperation between The Norwegian Academy of Science and Letters and Statoil. Parts of this research were carried out at Statoil Research Centre/Bergen Laboratory. Employees at the Bergen Laboratory are thanked for providing technical assistance. Helpful discussions with Principal Researcher Hans Kristian Hornnes and Senior Researcher Dr. Kristin Erstad (both with Statoil Research and Development) are highly appreciated. Compositional elemental analyses were performed by Chief Engineer Inger Johanne Fjellanger (Department of Chemistry, University of Bergen).

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REFERENCES

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Asphaltene Precipitation from Athabasca Bitumen Using an Aromatic

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