Solubility Parameters of Bitumen-Derived Narrow Vacuum Resid

Energy Fuels , 2009, 23 (1), pp 386–391 ... Narrow fractions of Athabasca vacuum topped bitumen (VTB) were prepared by supercritical fluid extractio...
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Energy & Fuels 2009, 23, 386–391

Solubility Parameters of Bitumen-Derived Narrow Vacuum Resid Fractions Xingyi Wang, Zhiming Xu, Suoqi Zhao,* Chunming Xu, and Keng H. Chung State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Changping, Beijing 102249, People’s Republic of China ReceiVed August 24, 2008. ReVised Manuscript ReceiVed October 16, 2008

Narrow fractions of Athabasca vacuum topped bitumen (VTB) were prepared by supercritical fluid extraction and fractionation (SFEF) and characterized. Phase equilibria of various narrow fractions in the propane system were determined using a high-pressure PVT unit, operating at 30-50 °C, 4-6 MPa, and solvent/oil (S/O) ratio of 1.5 to 5 (wt/wt). Solubility parameters were determined using the activity coefficient equation from Scathard-Hildebrand’s regular solution theory. The value of the solubility parameter of the VTB fraction was obtained by extrapolation of the Gaussian function at the S/O ratio approaching zero. It was found that the solubility parameter of the VTB fraction was not constant, varying from 14 to 16 MPa1/2. The solubility parameter exhibited a Gaussian distribution as a function of the S/O ratio, with a maximum at the S/O ratio of 3.25, and a parabolic function of pressure, with a maximum at 5 MPa. It decreased with temperature. As the VTB fraction becomes heavier, the solubility parameter of the VTB fraction and the maximum value of the solubility parameter increased. The solubility parameter of the VTB end-cut (primarily C5-insoluble asphaltenes) was obtained using a modified titration method by varying the composition of a pentane/toluene blend solvent. The correlation of the solubility parameter of the VTB fraction was derived as a function of various key feedstock properties. The measured solubility parameters of narrow VTB fractions were compared to predictions from five published models. The results showed that the published models overestimate solubility parameters of the VTB fractions. In some cases, the trend of predictions was inconsistent.

Introduction The solubility parameter (δ) is used in petroleum applications to correlate and predict the phase stability of petroleum during transportation and processing. It has been used to model asphaltene flocculation and deposition in petroleum production and pipeline transportation,1-4 separation of asphaltenes from bulk oil in the solvent deasphalting process,5 and phase splitting, leading to coke formation in petroleum processing operations, such coking and fouling.6,7 A number of solubility parameter models for hydrocarbons, crude oil, heavy oil, bitumen, and asphaltenes have been proposed with various degrees of complexity: one-dimensional,1,8-10 two-dimensional,11,12 and three-dimensional.13,14 However, they do not adequately describe * To whom correspondence should be addressed. Telephone: +86-1089733743. E-mail: [email protected]. (1) Yang, Z.; Ma, C. F.; Lin, X. S.; Yang, J. T.; Guo, T. M. Fluid Phase Equilib. 1999, 157, 143. (2) Pazuki, G. R.; Nikookar, M. Fuel 2006, 85, 1083. (3) Browarzik, D.; Laux, H.; Rahimian, I. Fluid Phase Equilib. 1999, 154, 285. (4) Akbarzadeh, K.; Alboudwarej, H.; Svcek, W. Y.; Yarranton, H. W. Fluid Phase Equilib. 2005, 232, 159. (5) Shui, H.; Shen, B.; Gao, J. Fuel 1998, 77 (8), 885. (6) E, H.; Watkinson, P. Fuel 2004, 83, 881. (7) Schabron, J. F.; Pauli, A. T.; Rovani, J. F., Jr. Fuel 2001, 80, 529. (8) Small, P. A. J. Appl. Chem. 1953, 3, 71. (9) Riazi, M. R.; Al-Sahhaf, T. A. Fluid Phase Equilib. 1996, 117, 217. (10) Laux, H.; Rahimian, I.; Butz, T. Fuel Process. Technol. 1997, 53, 69. (11) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (12) Wiehe, I. A. Fuel Sci. Technol. Int. 1996, 14 (1 and 2), 289. (13) Laux, H.; Rahimian, I.; Butz, T. Fuel Process. Technol. 2000, 67, 79. (14) Redelius, P. G. Fuel 2000, 79, 27.

complex petroleum systems. The solubility of petroleum components is mainly governed by nonpolar field forces.11,12 While the solubility parameter of asphaltene systems has been investigated extensively, there is no adequate model for heavy oil or bitumen. This is due to the lack of phase equilibrium data for resid fractions. This accounts for as much as 50% of bitumen. Most of the research has considered the resid as a pseudo-compound. Heavy oil fractions are soluble in liquid hydrocarbons. Therefore, it is difficult to determine the latent heat of heavy oil fractions for the solubility parameter calculations, because of their high boiling temperatures over 520 °C. Solubility parameter models have been derived for solubility class species of resid: saturates, aromatics, resins, and asphaltenes (SARA), by correlating them with bulk properties of the SARA species. In summary, proposed solubility parameter models that have been developed for heavy oil and bitumen have limited practical applications because none of the commercial processes are operated in the regimes similar to these pseudo-component systems. When phase separation occurs in a real oil system, the oil in each phase is a mixture of hydrocarbons with varying molecular sizes and complexities. In this paper, the solubility parameters of Athabasca vacuum topped bitumen (VTB) fractions were determined and correlated to properties of the fractions. The experimental data were compared to the results from various model predictions. Experimental Section Athabasca VTB was obtained from the commercial Syncrude oilsands plant in Fort McMurray, Alberta, Canada. The VTB was subjected to supercritical fluid extraction and fractionation (SFEF)

10.1021/ef800697f CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

Solubility Parameters of the VTB Fraction

Figure 1. Density and molecular weight of VTB subfactions.

Energy & Fuels, Vol. 23, 2009 387

Figure 3. SARA compositions of VTB subfractions.

Figure 2. Viscosity and H/C atomic ratio of VTB subfractions.

using n-pentane as the solvent to prepare multiple narrow fractions. The details and procedure of SFEF have been described elsewhere.15,16 In this work, the VTB sample was fractionated into 12 narrow extractable fractions, in about 50 g increments and an end-cut (nonextractable fraction consisting primarily of asphaltenes). The molecular weight of the extractable fraction was measured using the vapor pressure osmosis meter (VPO, KNAUER K-7000). Each of the extractable narrow VTB fractions was mixed with various amounts of propane in a PVT unit. The details of the PVT experiment have been described elsewhere.17 The phase compositions of narrow VTB fraction-propane mixture at various temperatures and pressures were determined. The phase compositions were used to compute the solubility parameters. For the nonextractable end-cut, an improved Schabron and Speight’s titration method18,19 was used to determine its solubility parameter.

Results and Discussion Feedstock Characterization. Figure 1 shows that the density of VTB fraction increased as the fractions became heavier. However, the molecular weight of the VTB fraction increased slightly for the extractable fractions, except for the end-cut, which was 4 times larger than those of extractable fractions. Figure 2 shows that the viscosity of the VTB fraction increased substantially as the fractions became heavier. The hydrogen/ carbon ratio of the VTB fraction decreased as the fractions became heavier. This indicates that the heavier fractions were more deficient in hydrogen. Figure 3 shows that the SARA components were unevenly distributed in VTB fractions. The amount of saturates was concentrated in the light fractions and decreased as the fractions became heavier. The amount of (15) Yang, G.; Wang, R. A. J. Pet. Sci. Eng. 1999, 22, 47. (16) Chung, K. H.; Xu, C. M.; Hu, Y. X.; Wang, R. A. Oil Gas J. 1997, 95 (3), 66. (17) Zhao, S.; Wang, R.; Lin, S. Pet. Sci. Technol. 2006, 24, 285. (18) Schabron, J. F.; Speight, J. G. Pet. Sci. Technol. 1998, 16 (3-4), 361. (19) Andersen, S. I.; Speight, J. G. J. Pet. Sci. Eng. 1999, 22, 53.

Figure 4. Phase equilibria of VTB fraction 4-propane mixture as a function of the S/O ratio at 4 MPa and 50 °C: (a) light-phase volume fraction, (b) oil content of the light phase, (c) densities of both light and heavy phases.

aromatics was relatively constant for all of the extractable fractions; only the end-cut had a much lower concentration of aromatics. The amount of resins increased as the fractions became heavier. The end-cut consisted mostly of asphaltenes, with a small amount of aromatics and resins. All of the extractable fractions were asphaltenes and ash-free.

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Figure 5. Solubility parameter of VTB fraction 4 as a function of the S/O ratio.

Figure 7. Solubility parameter of VTB fraction 4 as a function of the temperature.

The solubility parameter of propane can be estimated using the Bradford and Thodos equations:21 δ1 - δC ) 15.24(1 - Tr)0.446

Figure 6. Solubility parameter of VTB fraction 4 as a function of the pressure at the S/O ratio of 2.5 wt/wt and 50 °C.

Solubility Parameters. Each of the extractable VTB fractions was subjected to a PVT phase equilibrium study, in which propane was used as the solvent. The equilibrium data of VTB fraction 4 were arbitrarily selected for computing its solubility parameter. The phase density and composition of heavy and light phases were obtained by varying temperature from 30 to 50 °C, pressure from 4 to 6 MPa, and solvent/oil (S/O) ratio from 1.5 to 5 wt/wt. Figure 4a shows that the volume fraction of the light phase increased with the S/O ratio and leveled off at the S/O ratio of 3.25. Figure 4b shows that the oil content in the heavy phase reached a maximum at the S/O ratio of 3.25. The oil content in the light phase gradually decreased. Figure 4c shows that the density of the heavy phase decreased slightly, whereas that of the light phase increased slightly. The trend of density difference between the heavy and light phases is similar to that of the heavy-phase density. The solubility parameter of the VTB fraction, δ, can be obtained using the Scatchard-Hildebrand regular solution theory,20 RT ln γ1 ) V1Φ22(δ1 - δ2)2

(1)

RT ln γ2 ) V2Φ12(δ2 - δ1)2

(2)

(xiγi)L ) (xiγi)H

(3)

where γ is the activity coefficient, x is the molar fraction of species, V is the molar volume (kg/mol), and Φ is the volume fraction of species. The subscripts 1 and 2 denote propane and the VTB fraction, respectively, and L and H denote light and heavy phases, respectively. (20) Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed.; Prentice Hall: New York, 1985; Chapter 7.

(4)

where δ1 is the solubility parameter of liquid propane, δC is the solubility parameter of propane (4.83 MPa1/2) at critical conditions, and Tr is the reduced temperature. Figure 5 shows that the solubility parameter of VTB fraction 4 as a function of the S/O ratio at 50 °C and 4 MPa. At low and high S/O ratios, the solubility parameter of the VTB fraction was constant at 14 MPa1/2. It reached a maximum value of 16 MPa1/2 at the S/O ratio of 3.25 wt/wt. The data in Figure 5 can be fitted with a Gaussian distribution expression - 3.075 ( ( S/O0.482 ))

δ ) 14.0 + 2.16724 exp -

2

(5)

Results similar to Figure 5 were obtained for four other VTB fractions. This shows that the value of the solubility parameter of the VTB fraction is not constant and depends on the S/O ratio. It is likely that at low and high S/O ratios, at which the oil-solvent interaction is relatively less significant, the solubility parameter for the VTB fraction exhibits its “normal” value. However, it shows a higher solubility parameter value at a critical S/O ratio, at which the colloidal interactions between resins and other oil components are breaking up. Figure 6 shows that the solubility parameter of VTB fraction 4 as a function of the pressure at 50 °C and the S/O ratio of 1.5 wt/wt. The data show that the solubility parameter increased from 14 MPa1/2 at 4 MPa to a maximum of 16 MPa1/2 at 5 MPa and then decreased to 14 MPa1/2 at 6 MPa. Figure 7 shows the solubility parameter of VTB fraction 4 as a function of the temperature at 4 MPa and the S/O ratio of 1.5 wt/wt. As the temperature increased, the solubility parameter decreased slightly by 0.5 MPa1/2 from 30 to 50 °C. However, the solubility parameter difference (∆δ) between the solvent and oil increased; the oil became less soluble in the solvent as the temperature increased. Similar PVT experiments were carried out on four other selected extractable VTB fractions. The normal and maximum solubility parameters of these VTB fractions are summarized in Table 1. The solubility parameter of the nonextractable end-cut (primarily asphaltenes) was determined using the modified Schabron and Speight’s asphaltene precipitation procedure.18,19 Mixing an asphaltene-toluene solution with a paraffinic solvent that has a lower solubility parameter value causes asphaltene precipitation. The onset of asphaltene precipitation was detected (21) Bradford, M. L.; Thodos, G. Can. J. Chem. Eng. 1966, 44, 345.

Solubility Parameters of the VTB Fraction

Energy & Fuels, Vol. 23, 2009 389

Figure 8. Solubility parameter difference and solubility of the end-cut as a function of the toluene/(toluene + pentane) ratio.

Figure 9. Solubility parameter of the end-cut as a function of the toluene/(toluene + pentane) ratio.

by measuring the solubility of the end-cut in various toluenepentane blends. The solubility parameter of the end-cut can be obtained using the followed equation:18,19 ln aa ) ln xa + Ma/RTFa[Φs2(δs - δa)2]

(6)

The Scatchard-Hildebrand theory for tiny solid precipitation, the activity of asphaltenes (aa), is 1. Hence, eq 6 can be expressed as ln xa ) -(Ma/RTFa)(δs - δa)2

(7)

where M is the average molecular weight, R is the idea gas constant, T is the temperature, and F is the density at 20 °C. The subscripts a and s denote the asphaltenes and solvent, respectively. Using the Schabron and Speight’s procedure,18,19 pentane was added to the asphaltene-toluene solution with the toluene/ (tolune + pentane) ratio varied from 0.1 to 0.7. The concentrations of asphaltenes in the toluene-pentane blends were determined and are shown in Figure 8. The results indicate that the amount of dissolved asphaltenes was linearly correlated with the toluene/(toluene + pentane) ratio. Therefore, by extrapolating the linear correlation at 100% dissolved asphaltenes, the onset toluene/(toluene + pentane) ratio of 0.783 was obtained, at which asphaltenes were completely dissolved.

Figure 10. Comparison of solubility parameters of VTB subfractions to those from various model predictions.

Figure 11. Average and maximum solubility parameters of VTB subfractions from various feedstocks.

The solubility parameter of asphaltenes as a function of the toluene/(toluene + pentane) ratio can be obtained using eq 7 by substituting the values of solubility parameters of pentane and toluene of 14.5 and 18.3 MPa1/2, respectively,22 and the molar fraction of dissolved asphaltenes. Figure 8 shows that the solubility parameter difference between the asphaltenes and toluene-pentane solvent decreased as the toluene/(toluene + pentane) ratio increased and leveled off to 1.3 MPa1/2 at the toluene/(toluene + pentane) ratio of 0.5. The solubility parameter of asphaltene as a function of the toluene/(toluene + pentane) ratio is shown in Figure 9. From Figure 9, the maximum and minimum solubility parameters are 18.7 and 16.5 MPa1/2, respectively. Hence, the average solubility parameter is 17.6 MPa1/2. Many one-dimensional solubility parameter models have been proposed for the crude oil system, especially the asphaltenes. In the work, the model predictions are compared to the experimental data on heavy bitumen fractions and VTB fractions. The published models can be divided into three groups. The first group has only molecular weight (M) as a variable, such as Riazi and Al-Sahhaf9 δ ) 5.30789 + 0.48649 ln M

(8)

Table 1. Properties and Solubility Parameter of Athabasca Bitumen VTB and Its SFEF Fractions sample

cut yield (wt %)

cumulative yield (wt %)

density at 20 °C (g/cm3)

M (g/mol)

H/C

saturates

feed cut 2 cut 4 cut 6 cut 8 cut 10 end-cut

100 6.1 5.8 5.3 5.2 5.5 33.3

100 11.8 23.1 34 44.5 55.8 100

1.0596 0.9767 0.9913 1.0084 1.0281 1.0523 1.0857

1147 622 687 748 858 1033 8061

1.43 1.63 1.6 1.54 1.38 1.38 1.25

7.8 18.15 11.01 5.07 1.59 0.0 0.0

SARA (wt %) aromatics resins 41.52 63.69 67.71 63.32 63.41 50.12 5.8

32.6 17.19 21.28 31.41 34.99 49.88 18.84

C7 asphaltenes

δ (MPa1/2)

δmax (MPa1/2)

18.09 0.0 0.0 0.0 0.0 0.0 75.86

14.6 14.5 14.8 15.5 16.8 17.6

16.6 16.7 16.8 17.5 18.8 18.7

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Table 2. Properties and Solubility Parameters of SFEF End-Cuts from Various Origins SARA (wt %) SFEF residue

yield (wt %)

M (g/mol)

H/C

density 20 °C (g/cm3)

saturates

aromatics

resins

C7 asphaltenes

δ (MPa1/2)

Athabasca VTB Saudi Light Saudi Middle Iran Light Iran Heavy Daqing Huabei Dagang Liaohe Gudao

33.3 19.8 33.4 19.5 23.0 12.2 14.2 24.5 22.0 26.1

8061 2952 2882 3964 4254 2458 4421 7636 4124 5549

1.25 1.16 1.28 1.19 1.15 1.38 1.28 1.43 1.28 1.36

1.0857 1.1105 1.0340 1.0600 1.0620 1.0456 1.0934 1.0678 1.0893 1.0664

0 0.2 0.8 0.5 0.3 0.2 0.1 0.0 0.2 0.2

5.8 11.1 7.3 16.6 11.9 7.4 3.8 5.8 6.8 3.2

18.84 34.9 62.2 43.6 22.7 92.1 82.9 82.9 59.9 62.4

75.86 53.9 29.7 49.0 65.0 0.3 13.2 8.6 33.2 34.2

19.1 18.6 17.5 18.5 18.8 16.9 18.3 18.1 18.2 18.0

and Chung et al.’s model for C7+ fractions23 δ ) 6.743 + 0.938 ln M - 0.0395(ln M)2 - 13.039/ln M (9) The second group is models with chemical composition or molecular structure information of feedstock as variables. van Krevelen’s model24 contains H/C, O/C, N/C, and S/C atomic ratios, aromatic factor (fa), and density

∑ (N /C)F i

δ)

i

i

) VM/carbon 68.5H/C + 300O/C + 115N/C + 225S/C + 66.5fa (10) MC/F

where MC ) 12.00 + 1.008H/C + 16.00O/C + 32.00S/C

(11)

Laux et al.’s model10 includes the hydrogen-deficiency factor ZRa. A hydrocarbon molecule can be expressed as CnH2n + 2 - ZRa

(

δ ) 16.55 + 0.00464 +

)

3.2285 (ZRa - 2) + ∆δHe (12) nc

where ∆δHe ) F

yi

∑F A

(13)

i

i

i

∆δHe is the heteroatom correction, F is the increment value, y is the mass fraction, and A is the molar mass of heteroatoms of species i. The third group is empirical models with molecular weight, density, and boiling point as variables, such as the Yang and Guo’s model1

( MF )

δ ) 0.500765Tb0.982382

0.482472

(14)

where Tb is boiling temperature in K. For heavy oil fractions, the boiling temperature can be estimated using the following equation:25,26 Tb ) 85.66F0.2081M0.3547

(15)

The solubility parameters for heavy bitumen fractions, including SFEF cuts and end-cut from VTB were correlated (22) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (23) Chung, F.; Sarathi, P.; Jones, R. Topical Report Number 498, National Institute for Petroleum and Energy Research, Bartlesville, OK, 1991; pp 1-43. (24) van Krevelen, D. W. Fuel 1965, 44, 229. (25) Shi, T. P.; Xu, Z. M.; Cheng, M.; Hu, Y. X.; Wang, R. A. Energy Fuels 1999, 13 (4), 871. (26) Zhao, S.; Xu, Z.; Xu, C.; Chung, K. H.; Wang, R. A. Fuel 2005, 84, 635.

with three key feedstock parameters: molecular weight, density, and H/C atomic ratio F ( H/C ) F ) 19.92( H/C )

δav ) 16.14M0.0166 δmax

0.4393

0.3788

(16) (17)

where δav is the average solubility parameter at 25 °C at which the effect of the solvent on oil molecule interaction is negligible and δmax is the maximum solubility parameter at 25 °C at which the solvent effect on oil molecule interaction is significant enough that the oil molecules appear to have little affinity for each other. Equations 16 and 17 show that the solubility parameter is strongly dependent upon molecular structure characteristics, such as density and H/C atomic ratio. Figure 10 provides a comparison of the model predictions to the experimental data. Most of the published models overestimate solubility parameters of the extractable VTB fractions. The trend of van Klevlen’s model24 and Yang and Guo model1 are inconsistent with that of experimental data. Riazi and AlSahhaf’s model9 predictions are close to the values of δmax. The solubility parameter of the end-cut predicted by van Klevlen’s model24 is in agreement with the experimental value. Yang and Guo’s model1 predictions are in-line with the values of δmax for the lighter extractable VTB fractions. The inconsistent and decreased trend of Yang and Guo’s model for predicting the solubility parameters of the heavier VTB fractions is likely due to its strong dependency upon boiling temperature, especially the end-cut, which is a solid. Figure 11 compares the solubility parameters of various vacuum resid fractions calculated using the feedstock characterization data of Zhao et al.26 The solubility parameters of extractable VTB fractions are higher than those of vacuum resid fractions of Iranian Heavy, Arabian Light, and Daqing. Table 2 shows a comparison of the solubility parameters of end-cuts from various feedstocks. Generally, the solubility parameters of the end-cuts are comparable, because they are pentaneinsoluble asphaltenes, which consists primarily of heptane (C7)insoluble asphaltenes and resins. The solubility parameter of the VTB end-cut is the highest, which has the highest amount of C7 asphaltenes and accounts for 33.3 wt % of VTB. The VTB end-cut can easily precipitate in the presence of paraffinic solvents but is difficult to process. The solubility parameter of the Daqing end-cut is the lowest, which has the lowest amount of C7 asphaltenes. Conclusions Solubility parameter of narrow resid fraction increased as the fraction became heavier. The range of variation of solubility parameters for Athabasca bitumen resid fractions was 2 MPa1/ 2. The solubility parameter of the narrow resid fraction was not

Solubility Parameters of the VTB Fraction

constant; it varied with the solvent/oil ratio because of the solvent-oil interaction. The maximum solubility parameter was obtained at a critical solvent/oil ratio. A solubility parameter model was developed for narrow vacuum resid fractions by correlating with the density, H/C atomic ratio, and molecular weight of each fraction. Model predictions were in good agreement with measured solubility parameters of various narrow vacuum resid fractions over a wide range of molecular weights. The measured solubility parameters of narrow vacuum resid fractions were compared to predictions from five published

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models. The results showed that the published models either overestimate or are inconsistent in predicting the solubility parameters of narrow vacuum resid fractions. Acknowledgment. This work is supported by the National Key Basic Research Development Program of China (973 Program, 2004CB217801 and 2004CB217803), the National Science Fund for Distinguished Young Scholars (20525621), and the Program of Introducing Talents of Discipline to Universities (B07010). EF800697F