Molecular Weight and Specific Gravity Distributions for Athabasca and

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Ind. Eng. Chem. Res. 1999, 38, 3121-3130

3121

Molecular Weight and Specific Gravity Distributions for Athabasca and Cold Lake Bitumens and Their Saturate, Aromatic, Resin, and Asphaltene Fractions Subodhsen Peramanu* and Barry B. Pruden The University of Calgary, Calgary, Alberta, Canada T2N 1N4

Parviz Rahimi NCUT, CANMET, Natural Resources Canada, Devon, Alberta, Canada T0C 1E0

Molecular weight and specific gravity distribution data are required for characterizing oils containing complex mixtures, and this characterization information is very essential for the computation of thermodynamic properties and phase equilibria. The accuracy of these computations will be enhanced if molecular weight and specific gravity data of fractions containing similar groups/structures or common solubility properties are used. This is because the critical properties normally correlate better for a single fraction than for the whole oil. Data in this work is relevant to the phase equilibrium calculations and predictions of asphaltene precipitation from representative Canadian bitumens. Athabasca and Cold Lake bitumen samples were used and divided into asphaltene and deasphalted oil fractions by adding 40 volumes of n-heptane. The deasphalted oils were divided into saturate, aromatic, and resin fractions using a modified ASTM D2007 procedure. The average molecular weights of these SARA fractions were measured using vapor pressure osmometry (VPO), and the molecular weight distributions of the SARA fractions were measured using gel permeation chromatography (GPC) calibrated with polystyrene standards. Results were verified using VPO measurements, and the correction factors for the GPC distributions were calculated. The specific gravities of saturate and aromatic fractions were measured using an Anton-Paar densitometer, and resin fraction values were obtained using a water pycnometer and those of asphaltenes using a helium pycnometer. Specific gravity distributions were computed using the measured data and the correlation reported in the literature. Introduction To determine thermodynamic properties and phase equilibria of oils containing mixtures of various compounds, it is necessary to divide oil into pseudocomponents. Critical properties (PC and TC) and acentric factors (ω) of the pseudocomponents are then computed. Because these properties cannot be measured directly for oils, it is common practice to predict them with empirical correlations using boiling point curves or molecular weight distributions. The specific gravity distribution data are also needed in these correlations, and the current practice is to assume an average value of specific gravity for the whole distribution. Although measuring boiling point curves (ASTM D-2887) as opposed to molecular weight distribution is preferred, this method is not feasible for heavy oils and bitumens containing high-boiling fractions. When the distillation temperature exceeds 400 °C, cracking prevails, leading to erroneous results. Vacuum distillation can bring the equivalent boiling temperature to 540 °C, but for the feedstocks studied, there is still about 50% weight of materials that remain in the residue form. Accordingly, for heavy oils and bitumens found in tar sand deposits, molecular weights are measured instead of boiling points. * To whom correspondence should be addressed. E-mail: [email protected].

Tar sand deposits are widely distributed throughout the world in countries such as Africa, Canada, Europe, Venezuela, USSR, and USA.1 The Alberta (Canada) tar sand reserves such as Athabasca, Cold Lake, Peace River, Wabasca, Grosmont, Lloydminster, and Suffield deposits are well-known relative to other reserves in the world because commercial operations have been in place for over 20 years.2 Alberta’s total in-place bitumen reserves amount to 267 billion m3 (1.7 trillion barrels), and these are one of the largest hydrocarbon resources in the world. Bitumen reserves in the Athabasca area are about 57% and in Cold Lake about 13% of Alberta’s total reserves. It is very hard to establish a truly standard method for asphaltene precipitation (separation) because asphaltenes are not only a complex chemical fraction but also a complex physical fraction that is extremely difficult to define. Nonetheless, researchers3 compared the performance of various techniques in an attempt to reach a method that could be generally acceptable. Asphaltenes are recognized as brown to dark-brown powdery materials at room temperature and are normally precipitated using n-pentane or n-heptane solvents. The yield and the quality of asphaltenes depend not only on the type of solvent but also on the volume of the solvent per unit volume of sample. It was identified that using low carbon number solvents yields more and shinier asphaltenes than using high carbon number solvents. An increase in the amount of solvent

10.1021/ie9806850 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999

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increases the asphaltene yield up to a maximum at about 40 vol of solvent/vol of sample. There is a preference for pentane if the deasphalted oil (maltene fraction) is to be further subdivided by adsorption chromatography and adsorbent holdup is to be minimal. However, heptane separation is preferred if a more stable asphaltene is desired.3 To ensure efficient separation of asphaltenes, normally more than 30 vol of paraffin solvent and standing times in the range of 8-20 h are used. Temperature and pressure are also standardized to about 25 °C and 1 atm, respectively. Speight et al.3 compared analytical procedures for asphaltene precipitation such as ASTM D893, ASTM D2006, ASTM D2007, ASTM D3279, IP 143/57, IP 143/ 77, and Syncrude methods. Among these techniques, ASTM D2007 involves the method for fractionating deasphalted oil into saturates, aromatics, and resins by using clay-gel adsorption chromatography. Although clay-gel chromatography (ASTM D2007) is more laborious and time-consuming than high-performance liquid chromatograpy (HPLC),4 it has been extensively used because the procedure is standardized and the equipment is inexpensive. In clay-gel adsorption chromatography, the resins get adsorbed on attapulgus clay, aromatics get adsorbed on silica gel, and saturates elute directly. The adsorbed fractions are then removed using appropriate solvents. In HPLC methods paraffins elute first and are normally detected by a refractive index detector.5,6 The aromatics elute next, and the adsorbed resins elute after the column is backflushed. Aromatics and resins are detected by a UV absorbance detector. There are several methods for molecular weight determination.7,8 One group yields absolute molecular weights (“absolute” methods) without the use of any standard, and a second group requires calibration with a material of known molecular weight (“relative” methods). Molecular weight methods are also classified into those that give an average value and those that provide a complete distribution. In the category of absolute methods, membrane osmometry, cryoscopy, eulliometry, and light scattering measure the average molecular weight while equilibrium ultracentrifuge measures the molecular weight distribution. In the category of relative methods, viscosity and vapor pressure osmometry (VPO) measure the average molecular weight and gel permeation chromatography (GPC) measures the molecular weight distribution. Among these methods, VPO9-13 and GPC4,13-15 have been extensively used because relative methods requiring calibration are generally easier and faster than absolute methods. Although the VPO method is reasonably accurate for saturate, aromatic, and resin fractions, the accuracy of the molecular weights of asphaltene fractions is a debatable issue because the data vary considerably.9 The measured asphaltene molecular weight depends not only on the nature of the solvent and the temperature but also on the solute concentration. Measured molecular weights normally increase with an increase in the solute concentration for solvents with high dielectric constants. Moschopedis et al.9 noted that in a nonpolar solvent such as benzene or toluene asphaltenes tend to self-associate to form aggregates and thereby display higher molecular weights. It has also been identified that this asphaltene self-association decreases with an increase in temperature. The presence of aggregates can be detected by small-angle X-ray scattering16,17 or smallangle neutron scattering.17,18 Moschopedis et al.9 pro-

posed a highly polar solvent, nitrobenzene, and temperatures in the range of 100-150 °C to produce low molecular weights that are compatible with those expected on the basis of structural determinations by proton magnetic resonance spectroscopy. It was identified later by Champagne et al.14 that the hygroscopic nature of nitrobenzene causes considerable variations in readings because of humidity changes. Chung et al.10 devised a method for determining a concentration range in which linear extrapolation for zero concentration is possible, by performing molecular weight measurements for benzil-chloroform solutions and 14 other systems. Wiehe11 obtained the most consistent results for molecular weights of petroleum fractions using o-dichlorobenzene at a temperature of 130 °C. It was found that at 130 °C (theta temperature) there is little change in the apparent molecular weight with concentration whereas at 70 °C the molecular weight increases linearly with an increase in concentration. The asphaltene molecular weights obtained using toluene were higher and those obtained using nitrobenzene were lower, when compared to the results obtained using o-dichlorobenzene. Toluene gave higher molecular weights because of asphaltene aggregation. Observation of asphaltenes in a nitrobenzene solvent with an optical microscope at 600× magnification revealed that the solvent does not completely dissolve the asphaltenes.11 Therefore, the low molecular weight measurements obtained with nitrobenzene could result from the higher molecular weight fraction not being in solution. Although GPC is an attractive technique for determining molar-average molecular weight distribution of petroleum fractions, it is important to realize that petroleum contains constituents of a wide range of polarities and types, and each particular type interacts with the gel surface to a different degree. The strength of the interaction increases with increasing polarity of the constituents and with decreasing polarity of the solvent. Therefore, it must be recognized that the lack of realistic standards of known number-average molecular weight distribution and of chemical nature similar to that of the constituents of petroleum for calibration purposes is an important issue. The mobile effluent normally being used in a GPC method is tetrahydrofuran (THF), and the calibration standard is polystyrene.4,13-15 Although it is recommended to use oil standards instead of polystyrenes,19 the choice of compounds becomes very difficult and elution times of those compounds do not correlate well with molecular weights.15 Champagne et al.14 identified that different groups of equal molecular weight will have different retention times. Therefore, a preparative method was proposed where the fractions are collected during the run and the molecular weights of the fractions are measured using VPO. According to Rodgers et al.,15 the refractive index method of calculating weight fractions is not exact because the refractive index normally increases with an increase in molecular weight and aromaticity. Because the GPC elution volume depends not only on molecular weight but also on molecular structure, they developed a correlation which uses the GPC elution volume, hydrogen-to-carbon ratio, and hydrogen distribution (R, β, γ) determined by NMR. However, this method is very expensive and complex because it is necessary to have a sufficient quantity of subfractions from GPC to perform the analyses on the hydrogen-to-carbon ratio and hydrogen distributions.

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3123 Table 1. Type of Analyses and the Laboratory Facilities Used analysis

place

SARA fractionation and VPO measurements specific gravity measurements C, H, N, and O analysis GPC measurements and S analysis viscosity and 13C NMR analysis

National Center for Upgrading Technology, Natural Resources Canada, Devon, AB Core Laboratories Canada Ltd., Calgary Canadian Microanalytical Service Ltd., Delta Alberta Sulfur Research, University of Calgary, Calgary Analytical Chemistry Laboratory, University of Calgary, Calgary

Because of large uncertainties in the published molecular weights measured using VPO and GPC methods, it was necessary to evaluate these methods for fractions containing components with similar structures. It may be realized that using groups of similar structure and common solubility these measuring techniques would be more accurate. Therefore, in this study, bitumens were first divided into saturate, aromatic, resin, and asphaltene (SARA) fractions. The molecular weights of these fractions obtained using VPO were compared with those from GPC, and the required correction factors for GPC were calculated. Specific gravity distributions were calculated using the measured average specific gravities and the correlation by Kokal and Sayegh.20 The SARA fractionation, density, viscosity, elemental analysis, 13C NMR, molecular weight, and specific gravity measurements were carried out at the laboratory facilities as listed in Table 1. SARA Fractionation and Elemental Analysis SARA fractionation was carried out on the bitumens where the bitumens were first deasphalted with a 40:1 n-heptane-to-sample ratio. For this, approximately 10 g of material was added to 400 mL of n-heptane in a flask. The flask was then placed in an ultrasonic bath for 45 min and left to settle overnight. The mixture was then placed in an ultrasonic bath again for 30 min, and asphaltenes were filtered using a medium-porosity (1015 µm) fritted glass disk. The asphaltenes were mixed with 50 mL of n-heptane, and sonicating and filtering were repeated to ensure the complete removal of maltenes. Asphaltenes were then dried at 45 °C under vacuum for 3 h. After filtration, the maltenes were recovered by evaporating the solvent using a rotary evaporator and then placed in a vacuum oven at 45 °C for 3 h. The asphaltenes and maltenes were allowed to cool in a desiccator to room temperature and weighed. Clay-gel adsorption chromatography based on a modified ASTM-D2007M procedure was used for the separation of maltenes into saturate, aromatic, and resin fractions. The adsorption column consisted of two identical glass sections assembled vertically. The upper adsorption column had 150 g of freshly activated attapulgus clay, and the lower column had 200 g of activated silica gel plus 50 g of attapulgus clay on top of the gel. A piece of glass wool was placed over the top surface of the clay in the upper column to prevent agitation of the clay while charging the eluent solvents. To improve the wetting and solvent flow characteristics, about 25 mL of n-pentane was added to the top of the clay portion of the assembled column and allowed to percolate into clay. The maltene fraction was dissolved in 300 mL of pentane and charged to the column. Resins were adsorbed onto attapulgus clay while the aromatics were adsorbed onto silica gel. The remaining saturates eluted directly and were collected in a flask. A solvent mixture of pentane and toluene (50:50) in the amount of 1560 mL was added to the column to remove any aromatics present in the attapulgus clay section, and the eluted

Table 2. Properties and SARA Fractionation Results for Athabasca and Cold Lake Bitumens Athabasca

Cold Lake

8.05 323

10.71 65

saturates (wt %) aromatics (wt %) resins (wt %) asphaltenes (wt %)

17.27 39.70 25.75 17.28

20.74 39.20 24.81 15.25

carbon (wt %) hydrogen (wt %) sulfur (wt %) oxygen (wt %) nitrogen (wt %) residue (wt %)

83.34 10.26 4.64 1.08 0.53 0.15

83.62 10.50 4.56 0.86 0.45 0.01

API gravity viscosity at 24 °C (Pa‚s)

solution containing some aromatic fraction was collected in a flask. The silica gel column was carefully detached from the clay-gel column, and the balance of the aromatic fraction still adsorbed on the gel was removed by refluxing the silica gel column. Refluxing was carried out using 200 mL of toluene for 2 h at a rate of 10 mL/ min. To collect the resin fraction, a solvent mixture of toluene and acetone (50:50) in the amount of 500 mL was charged slowly to the top clay column and the effluent was collected. The saturate, aromatic, and resin fractions were recovered by evaporating the solvents using rotary evaporators and then placed in a vacuum oven at 45 °C until there was no change in their weights with time. The bitumens were also analyzed for C, H, N, O, and S contents. For C, H, and N measurement the sample was combusted in oxygen. Resulting carbon dioxide, water, and nitrogen were measured by a thermal conductivity gas chromatography unit (Carlo Erba 1104) and compared directly with known standards, acetanilide and nitroaniline. Oxygen was analyzed in a similar manner, but the gas formed was carbon monoxide, which was measured by thermal conductivity. For sulfur measurements, after combustion, the resulting sulfur dioxide was measured by a UV fluorescence technique (Antek 7000). Table 2 lists SARA analysis results and C, H, N, S, and O compositions for Athabasca and Cold Lake bitumens. Athabasca bitumen was found to be heavier and contained more resins and asphaltenes. Also, Athabasca bitumen had more heteroatoms than Cold Lake bitumen. Molecular Weight Measurements Using VPO VPO was used to measure the overall molecular weight of bitumens and their SARA fractions, while GPC was used to measure the corresponding molecular weight distributions. For VPO measurements the relation of molecular weight with voltage difference (∆E) and solute concentration (C2) is derived in Appendix A and is given by

(

)

∆E 1 )K + A2C2 C2 M2

(1)

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fraction). Bitumen molecular weights calculated using the SARA weight fractions and their VPO molecular weights were 560 g/mol for Athabasca and 533 g/mol for Cold Lake, which are very close (difference < 3%) to their direct values listed in Table 3. Molecular Weight Distribution Measurement Using GPC

Figure 1. VPO plot for Athabasca and Cold Lake bitumens and their asphaltene fractions. Table 3. Molar-Average Molecular Weights of Athabasca and Cold Lake Bitumens and Their SARA Fractions Using VPO bitumen saturates aromatics resins asphaltenes

Athabasca

Cold Lake

557

550

381 408 947 2005

378 424 825 1599

The molecular weights were measured using a Jupiter VPO (model 833) with o-dichlorobenzene (M ) 147.004) solvent at 120 °C. To determine the proportionality constant (K), the apparatus was calibrated with a solution of o-dichlorobenzene prepared with benzil solute (C6H5COCOC6H5; M ) 210.234), a molecular weight standard. The proportionality constant was calculated by extrapolating a standard (∆E/C2 versus C2) plot to zero concentration. Using this K value and the plot for the unknown samples, the molecular weights of the unknown samples were obtained. The sample concentrations were in the range of 1-4 g/L, and the VPO response was in a rectilinear region such that extrapolation to zero concentration was straightforward. The condition for the rectilinear relationship is given by Chung et al.10

ηN2 e N1

(2)

where the concentration parameter, η, was assumed to be 15, which is the value for the toluene solvent. Figure 1 shows the VPO plots obtained for Athabasca and Cold Lake bitumens and their asphaltene fractions. They are clearly linear in the range which was investigated. Table 3 gives molecular weights calculated for the two bitumens and their SARA fractions. The molecular weight of Athabasca bitumen was found to be slightly higher than that of Cold Lake. Molecular weights of saturate and aromatic fractions of these bitumens were quite close, whereas molecular weights of resin and asphaltene fractions of Athabasca bitumen were significantly higher than those of Cold Lake (15% for the resin fraction and 35% for the asphaltene

GPC is a technique that sorts molecules according to molecular size and shape.21 The column is filled with a highly cross-linked, porous packing, and the sample is carried through the column by a mobile effluent. The size classification takes place by repeated exchange of the solute molecules between the mobile phase and the stagnant liquid phase within the pores of the packing. The pore size range of the packing determines the molecular size separation characteristics of the column. Depending on the tendency of molecules to favor the stagnant phase, they migrate through the column at different velocities and elute from the column at different times. Smaller molecules favor the stagnant phase, migrate slowly, and elute from the column last, whereas larger molecules migrate fast and elute earlier. The distributed sample exiting the column is identified with a refractive index (RI) or ultraviolet (UV) detector that presents the concentration response against retention time or volume. To convert the elution profiles to molecular weight distributions, raw data are transformed by a calibration curve developed from the elution times of standard compounds through the column. The molecular weight distributions of bitumens and their SARA fractions were measured using Waters GPC equipment. The equipment consisted of three Waters ultrastyragel columns connected in series, a Waters 410 RI detector, and a Waters 510 HPLC pump. Each column was 30 cm long and had 7.8 mm inside diameter. For saturates, aromatics, and resins the columns were packed with ultrastyragel of pore sizes 1000, 500, and 100 Å, respectively. For asphaltenes the columns were packed with 10 000, 1000, and 500 Å ultrastyragel. The mobile phase used was tetrahydrofuran (THF), and the runs were carried out at 22 °C. Flow enters the column containing the largest pore size packing and leaves the column containing the smallest pore size packing. Calibration was performed using 8-10 polystyrene standards covering the molecular weight range from 106 to 90 000, where ethylbenzene was used as the lowest polystyrene standard. The flow rate of THF for both calibration and actual runs was 1 mL/min, and the sample injection volume was 50 µL with a concentration of 6 g/L in THF. GPC results for Athabasca bitumen are given in Figure 2 and those for SARA fractions of Athabasca bitumen in Figure 3. The experiments were repeated for bitumen, resin, and asphaltene fractions, the results of which are shown by broken lines in the figures. The experimental measurements are fairly reproducible. From Figure 3 it can be observed that the peaks become broader in the order of saturate, aromatic, resin, and asphaltene fractions. The asphaltene fraction was found to contain compounds of molecular weights ranging from 100 to more than 100 000. Weight-average molecular weights were calculated from Figures 2 and 3 and are given in Table 4. Molar molecular weight distributions were obtained from weight distribution data (Figures 2 and 3) and are given in Figures 4 and 5 for Athabasca bitumen and its SARA fractions, respectively. Molar-

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3125

Figure 2. GPC molecular weight distribution with weight percent for Athabasca bitumen.

Figure 4. GPC molecular weight distribution with mole percent for Athabasca bitumen.

Figure 3. GPC molecular weight distribution with weight percent for Athabasca bitumen SARA fractions.

Figure 5. GPC molecular weight distribution with mole percent for Athabasca bitumen SARA fractions.

Table 4. Weight-Average Molecular Weights of Athabasca and Cold Lake Bitumens and Their SARA Fractions Using GPC

Table 5. Molar-Average Molecular Weights of Athabasca and Cold Lake Bitumens and Their SARA Fractions Using GPC

Athabasca

Cold Lake

bitumen

2079

1774

bitumen

saturates aromatics resins asphaltenes

458 529 2069 9564

487 591 1977 7045

saturates aromatics resins asphaltenes

average molecular weights were calculated from Figures 4 and 5 and are given in Table 5. Figures 6 and 7 represent the GPC data for Cold Lake bitumen and its SARA fractions, respectively. Again, the molecular weight distributions become broader in the order of saturate, aromatic, resin, and asphaltenes

Athabasca

Cold Lake

529

541

332 310 830 1801

334 324 810 1504

fraction. While both Athabasca and Cold Lake bitumens start at the same molecular weight, the distribution for Athabasca bitumen was more tapered than that for Cold Lake. The calculated weight-average molecular weights for Cold Lake from the distributions are given in Table 4. Molar distributions of the molecular weights were

3126 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999

Figure 6. GPC molecular weight distribution with weight percent for Cold Lake bitumen.

Figure 8. GPC molecular weight distribution with mole percent for Cold Lake bitumen.

Figure 7. GPC molecular weight distribution with weight percent for Cold Lake bitumen SARA fractions.

Figure 9. GPC molecular weight distribution with mole percent for Cold Lake bitumen SARA fractions.

obtained using the weight distribution data and are given in Figures 8 and 9 for Cold Lake bitumen and its SARA fractions, respectively. Molar-average molecular weights were calculated from these distributions and are given in Table 5. Unlike VPO results, the GPC results show that the molecular weight of Athabasca bitumen is slightly lower than that of Cold Lake bitumen. Also, the aromatic fraction molecular weights for these bitumens are lower than their respective saturate fraction molecular weights. The molecular weights of saturate, aromatic, and resin fractions of the two bitumens are quite close, whereas the molecular weight of the asphaltene fraction of Athabasca bitumen is significantly higher (17%) than that of Cold Lake bitumen.

Bitumen distributions were calculated using the SARA distributions and the SARA weight fractions. The SARA average distributions for Athabasca are given in Figure 2 for weight percent collected and in Figure 4 for mole percent collected, and the corresponding plots for Cold Lake bitumen are given by Figures 6 and 8. The molecular weights calculated using these SARAaverage distributions were 458 g/mol for Athabasca bitumen and 461 g/mol for Cold Lake bitumen. These values were about 13% lower for Athabasca and 15% lower for Cold Lake bitumen when compared to their direct values. Another method was used to compare the whole bitumen and their SARA fractions where the bitumen-average molecular weight was computed using the GPC-average molecular weights of SARA fractions

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3127 Table 6. Calculated Correction Factors for GPC Results on the Basis of VPO Average Molecular Weights for Athabasca and Cold Lake Bitumens and Their SARA Fractions Athabasca FCorre

Cold Lake FCorre

bitumen

1.05

1.02

saturates aromatics resins asphaltenes

1.15 1.31 1.14 1.11

1.13 1.31 1.02 1.06

and SARA weight fractions. From this method, the molecular weights were determined to be 453 g/mol for Athabasca bitumen and 447 g/mol for Cold Lake bitumen. These values are about 14% lower for Athabasca and 17% lower for Cold Lake when compared to their direct values. Considering the possibility that GPC measurement accuracy is better for fractions, whole bitumens may be better represented by SARA-average distributions than the distributions obtained directly with whole bitumens. When Tables 3 and 5 are compared, it can be seen that GPC results exhibit lower molecular weights than those obtained from VPO. The low values are attributed to polystyrene standards used for calibration. It was identified by Champagne et al.14 that calibration with polystyrene standards would underpredict the molecular weights of compounds containing nonfused and fused polyaromatics. This effect will accentuate if there are any heteroatoms attached to the molecules. Because the GPC was calibrated with polystyrene standards, relatively lower molecular weights for aromatic, resin, and asphaltene fractions which contained nonfused and fused polyaromatic compounds resulted. The molecular weights for saturate fractions were also low because the fractions contained naphthenic and aromatic compounds. The VPO molecular weights were considered more accurate than GPC molecular weights. Therefore, the GPC results were multiplied by a correction factor (FCorre) such that the average molecular weight calculated was equal to the VPO results. Calculated correction factors given in Table 6 range from 1.02 to as high as 1.31. The values were lower for resin and asphaltene fractions of Cold Lake bitumen than those of Athabasca bitumen, which is possibly due to fewer heteroatoms present in Cold Lake resin and asphaltene fractions. Specific Gravity Distribution Calculations The densities of Athabasca and Cold Lake bitumens and their SARA fractions were measured using different techniques. The densities of the whole bitumens, saturate, and aromatic fractions were measured by ASTM D-5002 using a digital densiometer (Anton Paar DMA4500). The resin densities were measured by ASTM D-70 using a water partial pycnometer. The water partial-pycnometer method was not successful for asphaltenes because it resulted in very low densities, attributed to the hydrophobic nature of asphaltenes resulting in higher volumes when mixed with water. Therefore, a modified ASTM D-70 was adopted for asphaltenes where the sample volume was measured by displacing helium and not water. The measured densities and specific gravities of Athabasca and Cold Lake bitumens and their SARA fractions are listed in Table 7. For the calculation of specific gravities, the density of water at 25 °C was 997.1 kg/m3. When bitumen densities were calculated using the SARA

Table 7. Density and Specific Gravity Values for Athabasca and Cold Lake Bitumens and Their SARA Fractions at 25 °C Athabasca

Cold Lake

density (kg/m3)

SG

density (kg/m3)

SG

bitumen

1011.5

1.014

992.5

0.995

saturates aromatics resins asphaltenes

882.7 994.9 1033.7 1200.0

0.885 0.998 1.037 1.203

871.1 991.6 1019.1 1180.0

0.874 0.994 1.022 1.183

Table 8. Calculated and Measured Values of Average Carbon Number and Aromaticities for Athabasca Bitumen and Its SARA Fractions calculated values Cn bitumen

% Far

40.21 34.53

saturates 27.64 aromatics 29.57 resins 68.07 asphaltenes 143.64

17.72 43.68 30.47 32.54 (42.56)

measured values

%F h ar C (wt %) 40.35

83.34

18.47 46.02 35.42 52.62

80.36 84.55 79.11 82.26

Cn

% Far

38.65 33.42 25.49 28.72 62.37 137.32

16.39 45.80 29.23 47.56

fraction densities and their weight percent, the Athabasca bitumen density was 1012.4 kg/m3 and the Cold Lake bitumen density was 993.9 kg/m3. These values are barely 0.1% higher for the Athabasca bitumen and 0.14% higher for the Cold Lake bitumen than their direct values. Because subfractions obtained from GPC were not measured for densities, it was necessary to predict the specific gravity distributions using the available data of molecular weight distributions and the fraction densities. Therefore, a correlation given by Kokal and Sayegh20 was used which relates specific gravities to carbon number (Cn) and aromaticity fraction (Far):

SG ) 0.846 exp(0.6807Far) + 0.0003Cn 1 (3) 0.72Cn exp(-3.26Far) + 1.2 The carbon number can be calculated from molecular weights using the approximation developed by Whitson:22

Cn )

1 (M + 6) 14

(4)

The values for carbon number (Cn) and aromaticity (Far) for Athabasca and Cold Lake bitumens and their SARA fractions are reported by Suzuki et al.23 with their detailed survey of 13C NMR spectra. However, those values could not be used because the bitumens may not be from the exact region and depth, and their SARA fractionation method was not identical with the one used in this study. To calculate the specific gravity distribution, it was assumed that all of the subfractions (pseudocomponents) have the same aromaticity (Far) because it varies negligibly from one subfraction to other.20 The approach was to adjust the value of Far such that the calculated specific gravity distribution gives an average value that is equal to the measured value. For these calculations the corrected GPC molecular weight distributions were used. The average carbon number values and aromaticity values (Far) are given in Table 8 for Athabasca bitumen and its fractions and in Table 9 for Cold Lake bitumen and its fractions. To verify the assumption of

3128 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 9. Calculated and Measured Values of Average Carbon Number and Aromaticities for Cold Lake Bitumen and Its SARA Fractions calculated values Cn bitumen

% Far

39.71 31.77

saturates 27.43 aromatics 30.71 resins 59.36 asphaltenes 114.64

15.09 40.99 30.48 37.87 (46.58)

measured values

%F h ar C (wt %) 36.39

83.62

15.96 43.44 34.61 52.70

82.55 83.00 80.45 82.29

Cn

% Far

38.29 32.75 25.98 29.30 55.26 109.55

15.04 38.00 28.06 42.47

constant aromaticity (Far) for a given distribution, the molar-average molecular weights of each fraction were directly used to predict the experimental specific gravity value by adjusting the average aromaticity fraction h ar are given in value (F h ar). The calculated values of F Tables 8 and 9 for Athabasca and Cold Lake bitumens and their fractions, respectively. If the value of Far is close to F h ar it indicates that the assumption of constant aromaticity for a given distribution is valid. The results show that except for asphaltene fractions this assumption is fairly valid for all other fractions and whole bitumens. For asphaltene fractions the assumption of constant aromaticity resulted in aromaticity values being lower than values for aromatic fractions. Because the assumption of constant aromaticity was not valid for the asphaltene fraction, it was necessary to fit an equation for aromaticity value as a function of carbon numbers. Cyr et al.,24 in their GPC studies on Athabasca bitumen, have indicated that aromaticity decreases with carbon number and closely follows a logarithmic function. Therefore, the specific gravity distributions for asphaltene fractions were computed by assuming logarithmic functions for aromaticity while the function coefficients were adjusted until the average specific gravity of the distributions equaled their corresponding measured values. The logarithmic function obtained for Athabasca bitumen is given by

Far ) -0.0717 ln(Cn) + 0.73

Figure 10. Calculated specific gravity distribution with weight percent for Athabasca bitumen.

(5)

and that for Cold Lake bitumen is given by

Far ) -0.0647 ln(Cn) + 0.73

(6)

Equations 5 and 6 indicate that for a given carbon number the aromaticity values for Athabasca bitumen are lower than those for Cold Lake bitumen, and the values become closer at lower carbon numbers. The aromaticity values were distributed in the ranges of 0.12-0.54 for Athabasca asphaltenes and 0.16-0.58 for Cold Lake asphaltenes. The average values of percent aromaticity obtained from the molar distribution of aromaticity are 42.56% for Athabasca asphaltenes and 46.58% for Cold Lake asphaltenes, and these values are reported inside the closed brackets in Tables 8 and 9, respectively. The calculated specific gravity distributions for Athabasca bitumen and its SARA fractions are given in Figures 10 and 11, respectively. Similar results for Cold Lake bitumen and its SARA fractions are given in Figures 12 and 13. Specific gravity distributions were reasonable for whole bitumens and their fractions except for asphaltene fractions, where the values were as high as 3.2 at the highest molecular weight region. These high values for asphaltene fractions may be due to the decreased effectiveness of the specific gravity correlation (eq 3) for solid compounds. Nevertheless, these values

Figure 11. Calculated specific gravity distribution with weight percent for Athabasca bitumen SARA fractions.

are important for thermodynamic computations as there are no experimental data available for the specific gravity distribution of asphaltene fractions. To check the accuracy of the computed carbon numbers using eq 4, the values were compared with the measured carbon numbers which were obtained from the elemental carbon analysis and the VPO molecular weight data. Measured values of carbon content and carbon numbers for Athabasca bitumen and its fractions are given in Table 8, and those for Cold Lake bitumen and its fractions are given in Table 9. The results indicate that calculated values of carbon numbers are very close to the measured values. To check the accuracy of computed aromaticities, these values were measured for each SARA fraction using 13C NMR (Varian XL200).

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3129

Figure 12. Calculated specific gravity distribution with weight percent for Cold Lake bitumen.

lower than the VPO molecular weights, which might be attributed to polystyrene standards used for GPC calibration. GPC resulted in lower molecular weight values for aromatic fractions than for saturate fractions because the effect of a polystyrene calibration standard was larger for aromatic fractions than for saturate fractions. The molecular weights obtained from VPO were presumed to be more accurate than those obtained from GPC. Hence, the GPC molecular weights were corrected using the average molecular weights obtained from VPO. The molecular weights for whole bitumen calculated using the SARA fraction weight percents and VPO molecular weights differ by less than 3% for both Athabasca and Cold Lake bitumens, and the densities calculated differ by less than 0.15% when compared to their direct values. The specific gravity distributions were obtained using the measured average specific gravities and the literature correlations. The assumption of constant aromaticity for the prediction of specific gravity was fairly valid for whole bitumens, saturate fractions, aromatic fractions, and resin fractions but was not for asphaltene fractions. The specific gravity values obtained for asphaltene fractions using aromaticity distributions were of high value at high molecular weight regions. This is possibly due to reduced accuracy of the literature correlation for solid-phase specific gravity predictions. Because of the nonavailability of any other proven correlations, the computed specific gravities were reasonably accurate because measured and calculated values of carbon numbers and aromaticities for whole bitumens and their fractions were found to agree very well. Acknowledgment The authors are grateful to Olga Gafonova and Sara Salmon for their assistance in SARA fractionation and Dr. Harvey Yarranton for his valuable comments. Nomenclature

Figure 13. Calculated specific gravity distribution with weight percent for Cold Lake bitumen SARA fractions.

The experiments were performed with a 5 mm tube using a CH2Cl2 solvent. The scan count used was 3616, and the delay between each scan was 15 s. The measured aromaticities for Athabasca and Cold Lake bitumens and their SARA fractions are given in Tables 8 and 9, respectively. The results indicate that the computed values are in good agreement with the measured values. Conclusions The molecular weights from VPO were increased in the order of saturate, aromatic, resin, and asphaltene fractions. The GPC molecular weights were found to be

A2 ) coefficient A3 ) coefficient Far ) aromaticity fraction F h ar ) aromaticity fraction from average molecular weight FCorre ) correction factor C2 ) concentration of solute (g L-1) Cn ) carbon number ∆Hlv ) heat of vaporization (J) K ) proportionality constant M ) molecular weight (g mol-1 or Da) M1 ) molecular weight of solvent (g mol-1 or Da) M2 ) molecular weight of solute (g mol-1 or Da) Mapp ) apparent molecular weight of solute (g mol-1 or Da) N1 ) number of moles of solvent (mol) N2 ) number of moles of solute (mol) Psat ) vapor (saturation) pressure (Pa) ∆Psat ) vapor (saturation) pressure lowering (Pa) R ) universal gas constant (Pa m3 mol-1 K-1) T ) temperature (K) ∆V ) change in voltage (µV)

3130 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 ∆Vlv ) change in molar volume from vapor to liquid (m3 mol-1) X2 ) mole fraction of the solute

Appendix A VPO is based on a principle where the magnitude of the vapor pressure decrease with the addition of solute can be related to temperature by the Clapeyron equation:

dPsat ∆Hlv ) dT T∆Vlv

(A1)

For vaporization at low pressures, it can be assumed that the vapor phase is an ideal gas and that the molar volume of the liquid is negligible when compared to the molar volume of the vapor:

∆Hlv dPsat ) dT RT2/Psat

(A2)

For small changes in pressure, dPsat can be replaced by ∆Psat, and for small changes in temperature, dT can be replaced by ∆T. For a sufficiently dilute solution the vapor pressure lowering can be related to concentration using Raoult’s law and is given by

N2 ∆Psat ) PsatX2 ) Psat N1 + N2

(A3)

where X2 is the mole fraction of the solute in the solution and N1 and N2 represent the number of moles of solvent and solute, respectively. For very small N2, N1 + N2 ≈ N1, and eq A2 becomes

∆T )

RT2 M1 C2 ∆Hlv 1000 M2

(A4)

where C2 is the solute concentration in g/kg and M1 and M2 are the molecular weights of solvent and solute, respectively. Because the voltage change, ∆E observed, is proportional to the temperature difference ∆T, eq A4 can be written as

C2 ∆E ) K M2

(A5)

where C2 can be expressed in g/kg or g/L, and K is the proportionality constant which also includes heat lost by radiation, conduction, and convection. For finite solute concentrations when the assumption of ideal solution is not valid, the relation is given by

(

)

1 1 ∆E )K )K + A2C2 + A3C22 + ... C2 Mapp M2

(A6)

For most solutions it is sufficient to include only the first power of concentration in eq A6, so that experimental data can be fit to a straight line for extrapolation to a zero concentration:

(

)

1 1 ∆E )K )K + A2C2 C2 Mapp M2

(A7)

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Received for review October 21, 1998 Revised manuscript received April 27, 1999 Accepted April 30, 1999 IE9806850