Solubility Modeling of Asphaltenes in Organic Solvents - American

Energy & Fuels 1997, 11, 615-622

615

Solubility Modeling of Asphaltenes in Organic Solvents K. D. Mannistu, H. W. Yarranton, and J. H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2G6 Received October 29, 1996. Revised Manuscript Received February 4, 1997X

The solubilities of Athabasca asphaltenes in a variety of organic solvents are measured and compared with predictions using a solid-liquid equilibrium calculation based on ScatchardHildebrand solubility theory with a Flory-Huggins entropy of mixing. The necessary input for the calculation is the feed composition and the component molar volumes and solubility parameters. Previously determined molar mass distribution and property correlations are used to calculate the necessary asphaltene input. The solvent properties are obtained from published data. Both single solubility parameter and three-component solubility parameter models successfully predict asphaltene solubility in a variety of nonpolar and slightly polar organic solvents, including normal and branched alkanes, aromatics, dichloromethane, Decalin, and 1-hexene. The single-component solubility parameter model cannot accurately predict asphaltene solubility in highly polar solvents, and qualitative predictions, at best, are obtained from the three-component model.

Introduction Asphaltenes are the fraction of a crude oil that is soluble in toluene and insoluble in an alkane, typically pentane or heptane. The solubility of asphaltenes is of interest to the oil industry because asphaltenes can precipitate during oil production, reducing flow rates and fouling equipment. However, because asphaltenes are a solubility class rather than a pure compound, they are difficult to characterize, and asphaltene solubility has proven difficult to predict. There are two approaches to modeling asphaltene solubility. In the first approach, asphaltene micelles are assumed to be stabilized by resins and precipitation to be caused by the flocculation of the micelles. In the second approach, asphaltenes are assumed to exist as free molecules that precipitate in a conventional manner. In both cases, Scatchard-Hildebrand solubility theory with a Flory-Huggins entropy of mixing term forms the heart of the model.1 There is some evidence that the micelle model is appropriate for some asphaltenes, e.g., Boscan asphaltenes.2 However, the research study presented here deals with Athabasca asphaltenes at relatively low concentrations, below 2 wt %. In the case of Athabasca asphaltenes, there is no evidence of micellization at asphaltene concentrations below 18 kg/ m3 (2 wt %) in solutions of toluene and hexane.3 Therefore, the Athabasca asphaltenes are modeled as nonassociated molecules. When the solubility theory for nonassociated asphaltenes was first developed, the asphaltenes were treated as a uniform material.4 With this assumption, good

predictions of the asphaltene precipitation point were obtained but predictions of the amount of precipitated material were poor. Recently, more accurate results were obtained when the asphaltenes were treated as a multicomponent mixture with a variety of physical properties.5,6 However, because the physical properties of the asphaltenes are difficult to measure, they are generally approximated using the equations of state.7,8 Hence, the usefulness of the theory is limited by the accuracy of the physical property estimates. Yarranton and Masliyah3 measured the molar mass distribution of Athabasca asphaltenes and developed correlations for the physical properties required for the solubility calculation: molar volume and the solubility parameter. By use of the correlations and ScatchardHildebrand solubility theory with a Flory-Huggins entropy of mixing term, the precipitation point and the amount of precipitated asphaltenes in solutions of toluene and hexane were successfully predicted at ambient conditions. Studies on the precipitation of asphaltenes from bitumen in different solvents indicate that it should be possible to generalize a solubility model based on the Scatchard-Hildebrand theory to any solvent system provided there is no micellization.9 However, the prediction method had yet to be tested with solvents other than toluene and hexane. In this article, the solubility of Athabasca asphaltenes is measured in 15 solvent systems. The systems include branched, halogenated, and n-alkanes, cyclic alkanes, alcohols, ketones, and various aromatics. Predictions based on the Scatchard-Hildebrand theory including the Flory-Huggins entropy of mixing term and Yarran-

Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Cimino, R. S.; Correra, S.; Sacomani, E.; Spa E.; Carniani, C. Thermodynamic Modeling for Prediction of Asphaltene Deposition in Live Oils. SPE International Symposium Oilfield Chemistry, San Antonio, TX, February 1995. (2) Kowalewski, I.; Vandenbrouke, M.; Huc, A. Y. Energy Fuels 1996, 10, 97. (3) Yarranton, H. W.; Masliyah, J. H. AIChE J. 1996, 12, 3533. (4) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. SPEJ, Soc. Pet. Eng. J. 1984, 24, 283.

(5) Kawanaka, S.; Park, S. J.; Mansoori, G. A. SPE Reservoir Eng. 1991, 5, 185. (6) Ferworn, K. A.; Svercek, W. Y. Thermodynamic and Kinetic Studies of Asphaltene Deposition from Bitumens. Proceedings of the Canadian Chemical Engineering Conference; Calgary, October 1994. (7) Thomas, F. B.; Bennion, D. B.; Bennion, D. W.; Hunter, B. E. J. Can. Pet. Technol. 1992, 31 (1), 22. (8) Rassamdana, H. B.; Dabir, B.; Nematy, M.; Farhani, M.; Sahimi, M. AIChE J. 1996, 42, 10. (9) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149.

X

S0887-0624(96)00187-9 CCC: $14.00

© 1997 American Chemical Society

616 Energy & Fuels, Vol. 11, No. 3, 1997

Mannistu et al.

ton and Masliyah’s correlations are compared with the experimental results for each system. Predictions using single-component and Hansen three-component solubility parameters are examined. Experimental Method The asphaltenes used in the experiments were extracted from Syncrude coker feed Athabasca bitumen (bitumen that has been treated to remove sand and water and is ready for upgrading). These asphaltenes contain “solids” that make up 6.3 wt % of the asphaltenes.3 The “solids”, which include fine clays, ash, and some adsorbed hydrocarbons, are insoluble in toluene. The “solids” form part of the first material to precipitate and hence are always part of the insoluble portion of the asphaltenes-solids mixture. For all the results presented here, the mass ratios of insoluble asphaltenes to total asphaltenes are reported on a “solids”-free basis. Asphaltenes were extracted from the bitumen with a 40:1 volume ratio of heptane to bitumen. The mixture was stirred for 4 h and left to settle overnight. The supernatant liquid was removed and the remaining precipitate was further diluted with heptane at a 4:1 heptane:bitumen volume ratio, stirred for 1 h, and left to settle for 4 h. The final mixture was filtered, and the remaining asphaltenes dried at ambient conditions until there was no further change in mass. The asphaltenes recovered with this method make up 14.5% of the original bitumen. The solubility of asphaltenes in different solvent systems was determined in two ways: by solubility and by precipitation. The solubility method involved adding the asphaltenes to a premixed known ratio of a good solvent and a precipitator (poor solvent) where, for example, toluene is a solvent and hexane is a precipitator. The solution was stirred in a sonic mixer for 10 min, left overnight, and centrifuged at 3400 rpm for 5 min. The supernatant was poured off and the undissolved asphaltenes-solids subfraction dried until there was no further change in mass. The precipitation method was identical except that the asphaltenes were dissolved in the solvent before the precipitator was added. All of the reagents were of at least 98% purity. Toluene, heptane, octane, isopentane, isooctane, dichloromethane, and Decalin were purchased from Fisher Scientific. 1-Hexene and tert-butylbenzene were purchased from Aldrich Chemicals. Cyclohexane, pentane, and decane were purchased from Caledon. Nitrobenzene, hexane, methanol, and acetone were purchased from BDH Chemicals.

As discussed previously, the asphaltenes are assumed to be a mixture of nonassociated molecules. The precipitation of the asphaltenes can then be predicted with a multicomponent equilibrium calculation. To perform the equilibrium calculation, the following data are required: feed composition and equilibrium ratios for each component. To determine the equilibrium ratios, the molar volume and solubility parameter for each component are needed. To obtain the feed composition, the molar mass distribution of the asphaltenes must be measured. The Athabasca asphaltenes precipitated with the method described previously were found3 to range in molar mass from 2000 to 8500 g/mol with the following molar mass distribution

dfi ) constant(1 + R exp{-βMi}) dMi

∫MM

H

i

( )

dfi dMi ) 1 - fi ) dMi MH - Mi - R(exp{-βMH} - exp{-βMi}) MH - ML - R(exp{-βMH} - exp{-βML})

(1)

where Fi is the mass frequency of the ith asphaltene

(2)

where ML ) 2000 g/mol and MH ) 8500 g/mol. Solubility theory predicts that the highest molar mass asphaltene molecules precipitate first, and therefore, 1 - fi ) finsol, where finsol is the mass ratio of the insoluble subfraction of asphaltenes to total asphaltenes. For a mixture of high molar mass molecules such as asphaltene, the expression for the equilibrium ratio (Ki ) xis/xil , where xis and xil are the solid and liquid-phase mole fractions of component i, respectively) is as follows:3

{

Ki ) exp 1 -

()

vil vil vil (δ - δm)2 + ln + vm vm RT i

}

(3)

Here, R is the universal gas constant (J mol-1 K-1), T is temperature (K), and vil and vm are the liquid-phase molar volumes (cm3/mol) of component i and the solvent, respectively. δi and δm are the solubility parameters (MPa1/2) for the same respective constituents. The solvent mixture properties, vm and δm, are calculated using published values for the properties of the mixture components. The solubility parameter of a mixture is the volume average of the individual component solubility parameters. For the asphaltene properties, Yarranton and Masliyah3 developed the following correlations:

vil )

1000Mi 0.017Mi + 1080

δi ) [A(0.017Mi + 1080)]1/2

Theory

Fi )

component, fi is the cumulative mass frequency up to the ith asphaltene component, Mi is the molar mass (g/ mol) of the ith asphaltene component, and R and β have values of 50.63 and 0.001 65 mol/g, respectively. The integrated form of eq 1 that is used to solve for the mass ratio of the insoluble subfraction to total asphaltenes is given by

(4) (5)

where A is the change in the heat of vaporization of asphaltene with a change in molar mass. A value of 0.367 kJ/g for A gives the best prediction of asphaltene solubility in toluene/hexane solutions. The prediction is shown in Figure 1b. The solubility parameter is the square root of the cohesive energy density of component i and represents the total molecular interaction energy, which includes dispersion, polar, and hydrogen-bonding interaction energies. A single-component solubility parameter is expected to work well when dispersion forces dominate but may not accurately represent polar solvents where polar and hydrogen-bonding forces approach the dispersion forces in significance. Two-component solubility parameters accounting for the dispersion and polar interaction energies have been evaluated previously for asphaltenes.10 However, a three-component (Hansen) solubility parameter accounting for the different contributions of each interaction energy was developed (10) Wiehe, I. A. Fuel Sci. Technol. Int. 1996, 14 (1, 2), 289.

Modeling of Asphaltenes

Energy & Fuels, Vol. 11, No. 3, 1997 617

Figure 1. Solubility of asphaltenes in solutions of toluene and n-alkanes: (a) pentane (b) hexane; (c) heptane; (d) octane; (e) decane; (f) predictions for n-alkanes; (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model.

earlier,11 and there is extensive published data available for the three-component solubility parameters of many solvents. To determine Ki using Hansen parameters, eq 3 is modified as follows: (11) Barton, A. M. F. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983.

{

Ki ) exp 1 -

()

vil vil vil [(δ - δdm)2 + + ln + vm vm RT di

}

b[(δpi - δpm)2 + (δhi - δhm)2]]

(6)

where δd, δp, and δh are the dispersion, polar, and hydrogen-bonding solubility parameters, respectively. The variable b is a weighting factor, a property of


Mannistu et al.

Table 1. Physical Properties of Solvents

solvent

molar massa (g/mol)

densitya (kg/m3)

δ

pentane hexane heptane octane decane isopentane isooctane cyclohexane Decalinc benzene toluene ethylbenzene tert-butylbenzened naphthalene nitrobenzene dichloromethane 1-hexene methanol acetone

72.2 86.2 100.2 114.2 142.3 72.2 114.2 84.2 138.3 78.1 92.1 106.2 134.2 128.2 123.1 84.9 84.2 32.0 58.1

626 655 684 699 730 620 688 779 883 877 867 867 867 1025 1204 1327 673 791 790

14.3 14.9 15.3 15.5 15.8 13.8 14.0 16.8 18.4 18.8 18.25 17.8 17.4 20.3 21.7 19.8 15.0 29.6 20.0

solubility parameters (Mpa)1/2 b δd δp δh 14.3 14.9 15.3 15.5 15.8 13.7 14.3 16.8 18.4 18.4 18.0 17.8 17.4 19.2 20.0 18.2 15.1 15.1 15.5

0 0 0 0 0 0 0 0 0 0 1.4 0.6 0 2.0 8.6 6.3 1.9 12.3 10.4

0 0 0 0 0 0 0 0 0 2.0 2.0 1.4 1.0 3.9 4.1 6.1 0 22.3 7.0

δt 14.3 14.9 15.3 15.5 15.8 13.7 14.3 16.8 18.4 18.6 18.2 17.8 17.4 20.3 22.2 20.3 15.2 29.6 20.0

a Source: ref 15. b Source: ref 11. c Density and solubility parameters for Decalin are a volume average assuming that Decalin is 50 vol % in the cis configuration and 50 vol % in the trans configuration. d The three-component solubility parameter was estimated from the single-component value in ref 9 and from the Hansen values for benzene and ethylbenzene in ref 11.

component i, with a recommended value of 0.25.11 The Hansen parameters represent the dimensions of a solubility “sphere” for a given component. The expression in eq 6 containing the squared differences of the solubility parameters is the distance between the solvent solubility “sphere” and the solubility “sphere” of asphaltene component i. The closer the two spheres, the more likely the asphaltene is to be dissolved. Although eq 6 is less theoretically justified than eq 3, the predictions from eq 6 were shown to be more accurate for more polar solvents than those from eq 3.11 When dispersion forces dominate, i.e., for nonpolar solvents, eq 6 reduces to eq 3 and the predictions are expected to coincide. To use the three-component parameters, correlations for each component of the asphaltene solubility parameter must be developed. To approximate the asphaltene solubility parameters, it is assumed that all three are related to molar mass in the same manner as the single-component parameter given in eq 6.

δij ) (Aj(0.017Mi + 1080))1/2 j ) d, p, k

(7)

Hence, the respective values of A can be related as follows:

ψ1 ) ψ2 )

() ( ) () ( ) Ap δpi ) Ad δdi

2

Ah δhi ) Ap δpi

2

(8a)

(8b)

A value of 4 is assumed for ψ2 because this is the ratio for the known compounds most similar to asphaltenes: ethylbenzene and naphthalene. Values of 0.355 kJ/g and .01 for Ad and ψ1, respectively, give the best prediction for the toluene/hexane system. The prediction is shown in Figure 1b. The corresponding values of Ap and Ah are 0.003 55 and 0.0142 kJ/g, respectively. At the average asphaltene molar mass of 3920 g/mol, the three components of the asphaltene solubility parameter are 20.2, 2.0, and 4.0 MPa1/2 for δd, δp, and

δh respectively, with a total solubility parameter, δt, of 20.7 MPa1/2. These values are comparable to those of naphthalene given in Table 1. The total solubility parameter is in good agreement with published values for asphaltene of about 20.12 Results and Discussion Solubility calculations are made using a multicomponent equilibrium calculation. The asphaltene feed composition is determined from eq 2. Equilibrium ratios are calculated from eq 3 for the single-component solubility parameter model (δ1 model) and from eq 6 for the three-component solubility parameter model (δ3 model). Asphaltene molar volumes are estimated using eq 4, and the single- and three-component solubility parameters are estimated from eqs 5 and 7, respectively. Solvent molar volumes and solubility parameters are calculated from the constituent properties given in Table 1. The solvents chosen to test the models were selected to provide a broad range of molar masses, functional groups, and solubility parameters. In each case, a poor solvent, or precipitator, is paired with a good solvent and experiments and predictions performed for the entire composition range of solvents. Paired solvents allow a more rigorous test of the model because the exact points of full solubility and zero solubility can be located. Since the model was tuned to the toluene/ hexane system, all poor solvents are paired with the good solvent, toluene, and all good solvents are paired with hexane. In all cases, the asphaltene concentration is 8.8 kg/m3. Asphaltene concentration was shown previously3 to have little effect on finsol. Each group of solvents is discussed in detail below. n-Alkane Homologous Series. Predictions and experimental results for toluene with each of pentane, hexane, heptane, octane, and decane are presented in parts a-e of Figure 1, respectively. In all cases the predicted fractional precipitation is within 0.05 of the (12) Andersen, S. I. Fuel Sci. Technol. Int. 1992, 10 (10), 1743.


experimental data and the predicted precipitation point is within 0.10 of the experimental value. As expected for nonpolar solvents, both the single- and threecomponent solubility parameter predictions are comparable. Figure 1f shows the δ3 model prediction curves for the five systems. The predicted curves for heptane, octane, and decane are nearly identical even though, as shown in Table 1, the solubility parameter and molar mass both increase along the homologous series. Increasing the solvent molar mass tends to increase the fraction of insoluble material, i.e., shifts the solubility curve to the right. However, increasing the solvent solubility parameter has the opposite effect. The model correctly accounts for the compensating effects. Andersen13 found that the amount of asphaltene precipitate is greater when the asphaltenes are dissolved from solid form (solubility method) than when they are precipitated from a fully solubilized condition (precipitation method). In all our experiments, the same hysteresis is observed, although the difference in fractional precipitation is small, usually less than 0.05. The hysteresis suggests that there is an extra step in the mechanics of solubilizing or precipitating the asphaltenes that is not yet accounted for in the theory. In parts a-e of Figure 1, it can be seen that near the precipitation point the predictions of finsol are nearly linearly related to the volume fraction of toluene while the experimental insoluble mass fractions tail toward higher values of the toluene volume fraction. The tailing may occur because some asphaltenes, which would otherwise be soluble, adsorb on the “solids” and are captured in the insoluble material. Another possibility is that the asphaltene solubility parameter is not related solely to molar mass. At each molar mass there may be a distribution of solubility parameters. Such a distribution could easily give rise to the tailing near the precipitation point seen in Figure 1. At this time, no attempt is made to predict the tailing effect. Instead, the solubility models are tested on a variety of solvent systems to determine if the models could be generalized beyond the toluene/hexane system. The measure of the quality of the predictions is taken to be the difference between predicted and experimental fractions of insoluble asphaltenes. Branched Alkanes. Results for isopentane and isooctane each with toluene are given in parts a and b of Figure 2. For the isopentane/toluene system, the procedure was altered because isopentane has a high vapor pressure (79.3 kPa) and the evaporation of the mixture overnight was significant. Therefore, the mixture was left to come to equilibrium for 1 h instead of overnight. Since isooctane has a low vapor pressure and takes a long time to evaporate, the precipitated asphaltenes were filtered instead of centrifuged. For both isopentane and isooctane, the predictions overestimate the fraction of insoluble asphaltenes. However, the predicted fractional precipitation is still reasonably close to the experimental data. The polar and hydrogen-bonding components of the branched alkane solubility parameters are zero; hence, the δ1 and δ3 model predictions should be nearly identical for the branched alkane/toluene systems. The (13) Andersen, S. I.; Stenby, E. H. Hysteresis in Asphaltene Precipitation and Redissolution. Proceedings of the Canadian Chemical Engineering Conference, Calgary, October 1994.


Figure 2. Solubility of asphaltenes in solutions of toluene and branched alkanes: (a) isopentane; (b) isooctane; (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model.

predictions from each model for the isopentane/toluene system are similar. However, the predictions for the isooctane/toluene system are not the same because the isooctane solubility parameters for the δ1 and δ3 models are different, as shown in Table 1. The isooctane solubility parameters are different because they are taken from different data sources. Hence, the disagreement in the two models’ predictions reflects the data sources rather than the effect of using three-component versus single-component solubility parameters. Aromatics. Results for nitrobenzene and tert-butylbenzene each with hexane are shown in parts a and b of Figure 3. Nitrobenzene and tert-butylbenzene both have low vapor pressures, so again, the precipitated asphaltenes were filtered rather than centrifuged. For the tert-butylbenzene/hexane system, the solvents are relatively nonpolar and both the δ1 and δ3 models predict the fractional precipitation quite accurately. However, nitrobenzene is quite polar as is reflected in the difference between the δ1 and δ3 model predictions for the nitrobenzene/hexane system. As expected, the δ3 model is more accurate and predicts the fractional precipitation to within 0.05. Since the dispersion forces dominate in the nitrobenzene/hexane mixtures, the δ1 model still predicts the data reasonably well.


Figure 3. Solubility of asphaltenes in solutions aromatics and hexane: (a) nitrobenzene; (b) tert-butylbenzene; (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model.

Cyclic Compounds. Results for Decalin and cyclohexane each with hexane are presented in parts a and b of Figure 4, respectively. Decalin has a low vapor pressure, so the precipitated asphaltenes were filtered rather than centrifuged. Decalin, C10H18, is structurally equivalent to two cyclohexane rings attached along one side. For the Decalin/hexane system, both models predict the experimental data well, only slightly underestimating the fractional precipitation. However, the quality of the predictions is much poorer for the cyclohexane/hexane system. Both models predict complete solubility of asphaltenes in a mixture of 75% cyclohexane and 25% hexane. In fact, 30% of the asphaltenes are insoluble in pure cyclohexane. Predicted fractional precipitation deviates from experimental data quite significantly. To test that the experimental data represent the equilibrium condition, a solubility experiment was conducted with a 5 day interval between mixing and centrifuging instead of the usual 1 day interval. There was no difference in the results. As a further check, the solubility experiments were repeated at a lower concentration of asphaltenes, 4.4 kg/m3. As shown in Figure 4b, there is little difference from the results at 8.8 kg/m3 of asphaltenes. Hence, the experimental data appear to be valid and the discrepancy must arise from

Mannistu et al.

Figure 4. Solubility of asphaltenes in solutions of cyclic compounds and hexane: (a) Decalin; (b) cyclohexane; (b) precipitation method, CA ) 8.8 kg/m3; (O) solubility method, CA ) 8.8 kg/m3; (9) precipitation method, CA ) 4.4 kg/m3; (0) solubility method, CA ) 4.4 kg/m3; (-) δ1 model; (- - -) δ3 model.

the solubility models. The reason for the model failure may lie in the cyclic nature of the solvent. There is some evidence that the entropy of mixing between cyclic and linear molecules does not follow the form used in the Flory-Huggins term.14 Unfortunately, this hypothesis does not explain why the predictions are successful for Decalin, also a cyclic compound. However, the hypothesis cannot be tested further until the entropy of mixing between cyclic and linear molecules is better understood. In any case, a modified version of the theory is necessary to deal with cyclic compounds. Dichloromethane, 1-Hexene. Dichloromethane and 1-hexene are of interest because they have chemical properties quite different from those of the solvents to which the solubility models were tuned, toluene and hexane. Therefore, the two solvents are good test cases for the generality of the models. Results for dichloromethane with hexane are shown in Figure 5. The results for toluene with 1-hexene are presented in (14) Xu, R.; Leonard, J.; Bui, V. T. Comparative Behaviors between Cyclic and Linear Oligomers in Solution. Proceedings of the Canadian Chemical Engineering Conference, Kingston, Ontario, September 1996. (15) Weast, R. C. CRC Handbook of Chemistry and Physics, 59th ed.; CRC Press: Boca Raton, FL, 19779.



Figure 5. Solubility of asphaltenes in solutions of dichloromethane and hexane: (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model.

Figure 7. Solubility of asphaltenes in solutions of toluene and polar solvents: (a) acetone; (b) methanol; (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model. Figure 6. Solubility of asphaltenes in solutions of toluene and 1-hexene: (b) precipitation method; (O) solubility method; (-) δ1 model; (- - -) δ3 model.

Figure 6. For the dichloromethane/hexane system, both models slightly underestimate the fractional precipitation. As expected for the slightly polar solvent, the δ3 model is more accurate . For the toluene/1-hexene system, both models predict the fractional precipitation equally well. In all cases, the predictions are quite accurate. Polar Solvents. Polar solvents are expected to be the most difficult to model and the solvents for which the δ3 model is more appropriate. Results for acetone and methanol each with toluene are shown in parts a and b of Figure 7. The δ1 model predicts complete solubility of asphaltene in acetone, and therefore, there is no δ1 prediction curve on Figure 7a. As expected, the δ1 model is unsuccessful in predicting solubility in polar solvents. The δ3 model is superior but provides, at best, qualitative predictions. Conclusions The solubility of Athabasca asphaltenes in a variety of organic solvents is determined using a solid-liquid equilibrium calculation based on the Scatchard-Hildebrand solubility theory with a Flory-Huggins entropy

of mixing. The necessary input for the calculation is the feed composition, the molar volumes, and solubility parameters of the components. Previously determined molar mass distribution and property correlations are used to calculate the necessary asphaltene input. The solvent properties are obtained from published data. Both the single solubility parameter and three-component solubility parameter models are tested. Both the δ1 and δ3 models successfully predicted the solubility of asphaltene in a variety of nonpolar and slightly polar solvents, including normal alkanes, branched alkanes, aromatics, dichloromethane, 1-hexene, and Decalin. The models were unable to accurately predict asphaltene solubility in cyclohexane/hexane mixtures. The model failure may arise because the entropy of mixing between cyclic and linear molecules does not conform to the Flory-Huggins term. Once the mixing of cyclic and linear molecules is better understood, it may be possible to adapt the solubility theory to cyclic compounds. The δ3 model predicted the solubility of asphaltenes in slightly polar compounds better than the δ1 model. For highly polar solvents, the δ1 model failed while the δ3 model predicted the experimental data, at best, qualitatively. Hence, the three-component solubility parameter improves the predictions but is not adequate to describe the solubility of asphaltene in highly polar


solvents. Nonetheless, the solid-liquid equilibrium method is suitable for predicting the solubility of asphaltenes in a broad range of nonpolar and slightly polar solvents. Acknowledgment. We thank Syncrude Canada Ltd. and the Natural Sciences and Engineering Research Council of Canada. We are also indebted to Mr. Hisham Hussein for his assistance. Nomenclature A C f fi Fi K M R T

change in asphaltene enthalpy of vaporization per change in molar mass (kJ/mol) concentration (kg/m3) mass ratio of asphaltene subfraction to total asphaltene fraction cumulative mass frequency up to the ith asphaltene component mass frequency of the ith asphaltene component equilibrium ratio molar mass (g/mol) universal gas constant (J mol-1 K-1) temperature (K)

Mannistu et al. v x R, β δ ψ1 ψ2

molar volume (cm3/mol) mole fraction linear fit parameters for molar mass distribution solubility parameter (MPa1/2) ratio of Ap to Ad ratio of Ah to Ap

Subscripts A d h H i insol L m p

asphaltene dispersion component hydrogen-bonding component largest asphaltene molecule ith asphaltene component insoluble smallest asphaltene molecule solvent mixture polar component

Superscripts l s

liquid phase solid phase EF9601879

Solubility Modeling of Asphaltenes in Organic Solvents - American

Recommend Documents