Density Estimation of Asphaltenes Using Molecular ... - ACS Publications

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Energy & Fuels 2003, 17, 378-386

Density Estimation of Asphaltenes Using Molecular Dynamics Simulations E. Rogel* and L. Carbognani PDVSA-INTEVEP, P.O. Box 76343, Caracas 1070A, Venezuela Received September 4, 2002

Molecular dynamics simulations are carried out to estimate the density of average structures representative of various asphaltenes in an effort to evaluate the predictive capacities of this kind of structural models. Comparison between calculated and experimental values reveals that calculated densities are lower than experimental ones, but nevertheless calculated values show the correct tendency. This indicates that the chemical information provided by the average structures of asphaltenes is essentially correct and can be used to qualitatively estimate densities. On the basis of these results, the effect of different structural factors on calculated densities of asphaltenes was systematically studied. Large condensed aromatic rings and low hydrogen-tocarbon ratio are the main characteristics of the molecules which yield the highest densities. Also, it was found that a group-contribution method recently developed yields better density values than molecular dynamics simulations, although still lower than experimental values.

Introduction The addition of n-heptane or other alkanes to crude oils originates the selective precipitation of the highmolecular-weight, high-aromaticity, and high-polarity components.1 The precipitated material which is further soluble in aromatic solvents is known as asphaltenes and is a complex mixture of substances with similar solubility properties.2 Asphaltenes can also separate from crude oils as insoluble solid phases when pressure and temperature conditions vary.3,4 Asphaltenes are a well-known problem for the oil industry. Their tendency to form deposits on wells, tubing, piping, and also during refining processes causes heavy losses to the oil industry every year.5 Although the chemical composition of asphaltenes varies little for diverse crude oils (hydrogen/carbon ratio ≈ 1.15, aromaticity ≈ 0.50), these small variations induce important changes for asphaltene stability within crude oil.4-6 For instance, asphaltenes from unstable crude oils (crude oils with asphaltene precipitation problems during production operations) and from deposits show slightly higher aromaticities and lower hydrogen-to-carbon ratios than asphaltenes from stable crude oils.6-8 Therefore, it is clear that small changes * Present address: 15315 SW 78 Court, Miami, FL 33157. (1) Mullins, O. C. Anal. Chem. 1990, 62, 508. (2) Bestougeff, M. A.; Byramjee, R. J. In Asphaltenes and Asphalts. 1. Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 1994. (3) Peramanu, S.; Clarke, P. F.; Pruden, B. B. J. Pet. Sci. Eng. 1999, 23, 13. (4) Rassamdana, H.; Farhani, M.; Dabir, B.; Mozaffarian, M.; Sahimi, M. Energy Fuels 1999, 13, 176. (5) Thawer, R.; Nicols, D.; Dick, G. SPE Prod. Eng. 1990, November, 475. (6) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 351. (7) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes & Asphalts. 2. Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 2000.

in composition and chemical characteristics induce significant differences in stability behavior. For this reason, the study of asphaltene molecular structures is of great practical interest. However, the asphaltene fraction contains at least 105 different molecules9 which makes the isolation and even the identification of individual components impossible.10 As a consequence of their extraordinary complexity, the chemical characterization of asphaltenes and other heavy fractions is based on the identification of molecular types and structural groups. It has been found that asphaltenes are composed of polyaromatic condensed rings, aliphatic chains, and heteroatoms such as nitrogen, oxygen, sulfur, and various metals.11 The chemical information of asphaltenes obtained from diverse techniques is usually summarized in a condensed way using average molecular parameters. The values of these parameters indicate the structural types contained in the asphaltenes.10 Even more, using these parameters it is possible to build a molecular average structure that represents the main chemical characteristics of the studied asphaltene sample. Although the use of average structures to represent asphaltenes is a very common technique, very few attempts have been made to determine whether this type of models can predict the physical and chemical behavior of the fractions that they represent.11 In fact, many limitations have been attributed to the use of average structures.10,12,13 The value of a unique or even several structures to describe the wide variety of dif(8) Rogel, E.; Leon, O.; Espidel, J.; Gonzalez, J. SPE Prod. Facil. 2001, SPE72050. (9) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (10) Speight, J. G. Appl. Spectrosc. Rev. 1994, 29, 269. (11) Speight J. G. In Asphaltenes and Asphalts. 1. Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 1994. (12) Boduszynski, M. M. Liquid Fuels Technol. 1984, 2, 211. (13) Shenkin, P. S. Liquid Fuels Technol. 1984, 2, 233.

10.1021/ef020200r CCC: $25.00 © 2003 American Chemical Society Published on Web 02/22/2003

Density Estimation of Asphaltenes Using MD Simulations

ferent compounds that compose the asphaltenes is considered doubtful.12,13 Additionally, even though the functionality and polyaromatic ring systems experimentally determined are real, the connectivity or the combination of these parameters can be misleading.10 Despite the significant progress in asphaltene characterization during the last years, there are still some doubts about the structural characteristics of this fraction. In particular, the size of the aromatic nuclei and the asphaltene molecular weight are still open to question.10,14-18 The number of rings per aromatic nucleus reported10 using different techniques varies from 6 to 20, and even a value of 90 rings per aromatic nucleus has been reported.19 The average molecular weight of asphaltenes also varies widely depending on the technique used to determine them.14 This is because asphaltenes tend to self-assemble and such behavior depends on many factors such as solvent, temperature, and concentration. Based on this diverse experimental information, asphaltenes are represented by aromatic structures of widely different size. During the past few years, molecular dynamics and molecular mechanics simulations have been successfully used to describe the aggregation and solubility behavior of molecules that represent asphaltenes and resins.20-23 Rather recently, this approach has been extended to the study of the activity of amphiphiles as asphaltene stabilizers.24,25 These theoretical studies have provided useful information about the nature of the interaction in these complex systems. Besides information about the atomistic behavior of a system, the molecular simulations can also be used to determine thermodynamic properties such as density or solvation free energy.26 Volumetric calculations have been carried out for pure hydrocarbons27,28 and for natural gas.29 There are also volumetric studies for coals using average structures and molecular simulations30-32 and, rather recently, density values for average structures of asphaltenes from Arab Berri crude oil have been calculated using molecular dynamics simulations.33 It has been found (14) Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. Rev. Inst. Fr. Pet. 1985, 40, 51. (15) Strausz, O. P.; Mojeslsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (16) Su, Y.; Artok, L.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 1265. (17) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. (18) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (19) Favre, A.; Boulet, R. Rev. Inst. Fr. Pet. 1984, 39, 485. (20) Rogel, E. Colloids Surf., A 1995, 104, 85. (21) Murgich, J.; Rodrı´guez, J.; Aray, Y. Energy Fuels 1996, 10, 68. (22) Murgich, J.; Abanero, J.; Strausz, O. Energy Fuels 1999, 13, 278. (23) Rogel, E. Energy Fuels 2000, 14, 566. (24) Rogel, E.; Leon, O. Energy Fuels 2001, 15, 1077. (25) Rogel, E.; Contreras, E.; Leon, O. Pet. Sci. Technol., in press. (26) van Gunsteren, W. F.; Berendsen, H. J. C. Angew. Chem., Int. Ed. Engl. 1990, 29, 992. (27) Cuia, S. T.; Cummings, P. T.; Cochrana, H. D. Fluid Phase Equilib. 1997, 141, 45. (28) Neubauer, B.; Delhommelle, J.; Boutin, A.; Tavitian, B.; Fuchs, A. H. Fluid Phase Equilib. 1999, 155, 167. (29) Neubauer, B.; Tavitian, B.; Boutin, A.; Ungerer, P. Fluid Phase Equilib. 1999, 161, 45. (30) Nakamura, K.; Murata, S.; Masakatsu, N. Energy Fuels 1993, 7, 347. (31) Murata, S.; Nomura, M.; Nakamura, K.; Kumagai, H.; Sanada, Y. Energy Fuels 1993, 7, 469. (32) Dong, T.; Murata, S.; Miura, M.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 1123. (33) Diallo, M. S.; Cagin, T.; Faulon, J. L.; Goddard, W. A., III. In Asphaltenes & Asphalts. 2. Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 2000.

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that the densities of different solid products of crude oil correlate very well with the hydrogen-to-carbon ratio and also with solubility.34,35 Density can be easily determined in the lab and it is often used to calculate other thermophysical properties. Even more, the density is a physical property strongly related to the molecular topological characteristics and also to the molecular size of the molecules.36,37 Therefore, this property can be a useful tool for evaluating the predictive capacities of the average structure approach, including the comparison of different structural models such as those containing large and small polyaromatic rings. In the present work, molecular mechanics and molecular dynamics simulations were carried out to determine the density of average structures representative of various asphaltenes. The effect of different structural and compositional aspects such as connectivity, heteroatom content, and hydrogen-to-carbon ratio on the calculated densities is explored. This study also includes the comparison of the calculated densities using molecular simulations with experimental values and with calculated densities using a group-contribution model. The main objective of the present work is to evaluate the predictive capacities of the average structural models of asphaltenes using the density as a reference property and also to determine the effect of different structural factors on asphaltene density. Methods Simulation Details. Molecular mechanics and molecular dynamics simulations were carried out to determine the density of average structures representative of various asphaltenes. The molecular mechanics and molecular dynamics methods used are those included in the commercial programs InsightII, Discover 2.9, and Amorphous Cell of Molecular Simulations Inc.38 The force field cvff (consistent valence force field) was used in the calculations. This force field has been successfully used to describe the aggregation behavior of asphaltenes and resins as well as the activity of amphiphiles as asphaltene stabilizers in previous works.21-23 The charges in the atoms are assigned according to the database cvff and remain constant during the calculations. The average structures selected for this study are shown in Figure 1. These structures correspond to asphaltenes isolated from Venezuelan crude oils of different origin using the standard method IP-143. They were obtained based on elemental analysis and 1H NMR data according to a procedure previously described.8 Six of the structures (CN, MO29, MO21, MG27, BC6, BC5) correspond to asphaltenes from stable crude oils, four of them (VG3, FU1, BO7, CO2) to asphaltenes from unstable crude oils and finally two correspond to asphaltenes isolated from solid deposits removed from field facilities. One of the solids came from production tubing of a well located in western Venezuela (DTJ). The other solid is a bottom tank sediment formed in about 4 years in a tank located in eastern Venezuela (34) Laux, V. H.; Rahimian, I. Erdo¨ l Erdgas Kohle 2000, 116, 16. (35) Carbognani, L. Energy Fuels 2001, 15, 1013. (36) Zander, M. Fuel 1987, 66, 1459. (37) Satou, M.; Nakamura, T.; Hattori, H.; Chiba, T. Fuel 2000, 79, 1057. (38) Molecular Simulations, Inc., San Diego, 1994.

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Rogel and Carbognani

Figure 1. Average structures for the studied asphaltene samples.

(DTQ). Some of these structures have been previously reported.39 Besides, two structures obtained using experimental data from ruthenium ion catalyzed oxidation were used in the calculations. The first is the reported structure for Athabasca asphaltene,15 while the second corresponds to the proposed structure of Arab crude oil asphaltene.40 These structures are characterized by a lower number of rings per aromatic nuclei, when compared with the structures presented in Figure 1. (39) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (40) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287.

Additionally to the structures mentioned, different molecules that do not represent specifically any asphaltene fraction were employed to evaluate the methodology used and also the effect of different structural parameters on the calculated density values. The optimized average structures for each asphaltene were obtained using the following procedure: the potential energy of the constructed model was minimized, followed by the calculation of a constant temperature (NVT) molecular dynamics of 100 ps. The five conformations of lowest energy from the molecular dynamics calculation were selected, and then, the potential energy of each one was minimized again in order to lower the



Figure 2. Simplified scheme of the calculation of densities by molecular simulation.

energy even more. These five molecules were used to generate five amorphous structures. The initial density for these structures was estimated from a correlation between the density (F) and the hydrogen-to-carbon ratio (H/C) based on experimental data.34 This correlation is the following:

F ) 1.3447 H/C-0.5396 r2 ) 0.9666

(1)

The original experimental data used to obtain this correlation comprised crude oil related materials including asphaltenes from virgin and processed crude oils.34 The five amorphous structures obtained in the previous procedure were relaxed to minimize energy and the

relaxed structures were subject to simulated annealing (five repeated cycles of 5000 ps from 200 to 500 K and back) using NVT molecular dynamics. At the end of each annealing cycle, the structure was again minimized. This procedure was used to prevent metastable local high-energy minima. For the fully relaxed amorphous structures, isothermal-isobaric (NPT) molecular dynamics for 300 ps were run at 298 K. Each NPT molecular dynamics run started with an equilibration phase of 50 ps (that is, 50 000 molecular dynamics steps with time step ) 1 fs). The data (densities) was collected during the last 100 ps. A scheme of the calculation procedure is shown in Figure 2.

382



Table 1. Experimental Density and Other Properties of the Studied Asphaltene Samples asphaltene

origin

H/C

fa

density (g/cm3)

BC5 BC6 MG27 MO21 MO29 CN CO2 BO7 VG3 FU1 DTJ DTQ Coal A

stable crude oil stable crude oil stable crude oil stable crude oil stable crude oil stable crude oil unstable crude oil unstable crude oil unstable crude oil unstable crude oil deposit deposit coal

1.23 1.15 1.11 1.22 1.11 1.13 0.96 0.99 1.05 1.02 1.00 0.98 0.55

0.47 0.49 0.53 0.46 0.53 0.52 0.60 0.60 0.59 0.61 0.58 0.59 -

1.17 1.16 1.19 1.17 1.19 1.18 1.23 1.20 1.22 1.26 1.25 1.28 1.52

Figure 3. Comparison between experimental densities obtained in this work and results previously reported for similar samples.

Experimental Methods Density Measurements. The density of the asphaltenes shown in Figure 1 was measured using a standard glass pycnometer. n-Heptane was used as displacing fluid. The pycnometer has a capacity of 10 cm3. The procedure was as follows: 0.50 to 0.55 g of the sample are placed in the pycnometer. Then 9 to 9.5 cm3 of n-heptane are added, and trapped air bubbles are eliminated by ultrasonication. The pycnometer is filled with n-heptane and thermostated by inmersion in a water bath maintained at 25 °C. The stopper is pressed firmly in place, the pycnometer is removed, dried, and weighed. The asphaltene volume is calculated as the difference of the pycnometer volume and the n-heptane volume contained within it. The n-heptane volume is determined by knowing its mass and its density previously determined at the operating temperature. Asphaltene density is calculated as the ratio of the used mass and determined volume. The determinations were found to be accurate to the second decimal figure. Absolute deviations were (0.02 g/cm3. Aromaticity Determination. Carbon aromaticity was determined by 13C nuclear magnetic resonance. Spectra were determined with a Brucker ACP-400 spectrometer at a frequency of 100.614 MHz. Samples were dissolved in CD2Cl2, adding 0.2 M of CrIII acetylacetonate. An inverse gated decoupled technique was adopted for suppression of the Overhauser effect. Chemical shifts were referenced to tetramethylsilane. Elemental Analysis. Carbon and hydrogen contents were determined by a combustion method using a Leco instrument, model CHSN-932.

Results and Discussion Experimental Densities. Table 1 shows the experimental densities determined for the selected asphaltenes including a sample of Venezuelan coal used as reference. In Figure 3, the experimental values obtained in this work are compared to similar results obtained by different techniques for crude oil related materials34 and coals.41 This comparison indicates that the obtained densities are similar to values previously reported using more sophisticated techniques, although slightly lower than them. As previously mentioned, the selected asphaltene samples included asphaltenes extracted from stable crude oils, unstable crude oils, and deposits. Asphaltenes are defined as nC7-insoluble/toluene-soluble materials. Their isolation either from crude oils or from solid deposits has been previously reported.7 Since the (41) Strugala, A. Fuel 2000, 79, 743.

Figure 4. Experimental densities as a function of (a) H/C ratio, and (b) aromaticity (fa).

density of polyaromatics is closely related to the molecular topology,36 it is worth evaluating the relationship between density and structural characteristics of asphaltenes. Figure 4, parts a and b, show the experimental densities as a function of the H/C ratio and asphaltene aromaticity, respectively. In these figures, asphaltenes from stable crude oils are clearly separated from asphaltenes from unstable crudes and solid deposits. According to these results, asphaltenes from unstable crude oils and deposits exhibit higher densities, lower hydrogen-to-carbon ratios, and higher aromaticities than asphaltenes from stable crude oils. This kind of tendency has been previously reported for different samples.6-8



Figure 5. Comparison between calculated densities by molecular simulation and experimental densities.

Calculated Densities. The five independent amorphous configurations generated for each average structure shown in Figure 1 were used to estimate the densities according to the procedure previously described. All the densities reported in the present work were determined as an average of the set of five amorphous structures. Absolute deviations were around (0.02 g/cm3 for all the cases studied. Unfortunately, as can be seen in Figure 1, only a few of the average molecules studied included heteroatoms. In Figure 5, calculated densities are plotted as a function of the experimental densities for the studied samples with (a) assigned heteroatoms, and (b) no assigned heteroatoms. The solid line indicates where the points would lay if experiments and calculations yield the same results. The comparison between calculated and experimental densities indicates that calculated values are considerably lower than experimental values. However, despite the dispersion of results probably derived from the experimental uncertainty ((0.02 g/cm3), the calculated values corresponding to each set of results (assigned and not assigned heteroatoms) show the correct trend, i.e., they increase as the experimental values increase. In other words, the structures qualitatively predict the tendency of the density. The introduction of heteroatomic functionalities seems to improve the matching between determined and calculated values. This aspect will be discussed in detail in an ensuing section. Volumetric studies for coal have shown that the simulation techniques can be used to optimize the average models for coals.30-32 In fact, in these works it was found that slight structural variations can induce significant changes in the calculated densities. On the basis of these previous findings, a preliminary study based on structural changes of the ovalene molecule (Figure 6) was carried out. In this work, different structural modifications were made on ovalene molecules to determine their effect on the calculated density. Figure 6 shows the different molecules studied and the calculated density obtained for them using the procedure already described. In this figure, two tendencies can be identified. (1) The increase of the length of the alkyl appendages produces a decrease in the calculated densities (molecules A to E) with the exception of molecule C. (2) Different isomers such as molecules F and G or E and H can yield significant different densities. On the basis of this preliminary work, the effect of different structural factors on calculated densities of asphaltenes was systematically studied.

Figure 6. Effect of different structural modifications on calculated density by molecular simulations.

Effect of Hydrogen Deficiency. Two different models were used in this section: a structure representative of an amphoteric fraction reported earlier11 and a hypothetical asphaltene structure with similar hydrogen-to-carbon ratio (H/C) and molecular weight. Both structures are shown in Figure 7. The two molecules studied were changed in order to decrease their H/C. To this end, methylene and methyl groups were gradually eliminated from the structures and after each step, the density was calculated according to the procedure previously detailed. In Figure 8, the density of the structures generated in each case is plotted as a function of the H/C ratio. As expected, the density increases as the H/C ratio decreases. It is also interesting to observe that the density values obtained for the amphoteric molecule A are always lower than the densities calculated for the asphaltene model B. This reflects differences that are related to the topological characteristics of the molecules such as ring size, connectivity, and chain positions. In view of these findings, these characteristics were also explored using molecular simulations. Effect of the Size of the Polyaromatic Nuclei. The main difference between the molecules shown in Figure 7 is the size of their polyaromatic nuclei. In fact, H/C ratio, molecular weight, and heteroatom content are similar in both models. Then, it is possible to suppose that the differences in density can be at least partially attributed to differences in the polyaromatic ring sizes. The amphoteric model contains polyaromatic rings considerable smaller than the asphaltene model used.

384



Figure 9. Effect of the heteroatoms on calculated density.

Figure 7. Average structures representative of (A) amphoteric fraction, and (B) asphaltene.

Figure 8. Effect of the hydrogen deficiency on calculated density for molecules shown in Figure 7.

Two other structures with relatively small polyaromatic rings were studied. The first structure corresponds to the Athabasca asphaltene15 and the second to an asphaltene isolated from a mixture of Arabian crude oils.40 Both structures were obtained from ruthenium ion-catalyzed oxidation. The density values calculated using these structures were: 1.03 g/cm3 and 1.00 g/cm3 for Athabasca and Arabian Mixture asphaltene, respectively. As a reference, experimental density for Athabasca asphaltenes is around 1.16 g/cm3.42 On (42) Yarranton, H. W.; Masliyah, J. H. AICHE J. 1996, 41, 3533.

the other hand, density values for average structures with assigned heteroatoms from Table 1 vary from 1.06 to 1.17 g/cm3. These structures are composed of larger polyaromatic rings than structures for Athabasca and Arabian Mixture asphaltenes. The comparison between both sets of values indicates that the presence of larger polyaromatic nuclei increases the density of the structures. These results contribute another point of view into the open debate on the size and condensation of asphaltenes. Effect of the Connectivity. The average molecular structures of the Athabasca asphaltene15 and the asphaltene from Arabian mixture40 were used as samples. Both molecules were split in 10 and four smaller molecules, respectively. Then, the densities for both sets of molecules were calculated. This new calculation yields 1.00 g/cm3 for both set of molecules. These results indicated that, at least in these cases, there are not significant density differences considering the whole molecules (1.00-1.03 g/cm3) or the set of their fragments in the calculations (1.00 g/cm3). Effect of Heteroatoms. For some of the molecules studied, the density calculations were carried out using molecules with assigned heteroatoms and without assigned heteroatoms. In Figure 9, a comparison between both sets of values is shown. As expected, this comparison reveals that the introduction of heteroatoms in the molecular structure induces a slight increase in the calculated density. This effect can be attributed to the introduction of heavier atoms in the molecule and also to the increase in the molecular interactions because of the presence of heteroatomic functionalities. Of course, this last aspect is directly related to the type of functionality and its position inside the molecule. Comparison of Molecular Dynamics with a Group Contribution Method Used To Estimate Density of Polyaromatics. Rather recently, a new method for estimation of the molar volume of hydrocarbons including condensed polyaromatic hydrocarbons was developed.37 This method has been successfully used to describe the molar volumes of heavy hydrocarbons which behave as sticky liquids or crystalline solids at 298 K. It has been shown to be clearly superior to other similar methods in its estimations. Even though, according to the authors, this new group contribution method is applicable in the case of polyaromatic hydrocarbons with aromatic rings of a number up to seven,37 it can be used to estimate the density


Figure 10. Comparison between calculated densities using molecular simulations and a group additive method.

of the asphaltenes. The proposed method employs structural information to obtain the molar volume of the molecule as a sum of contributions from the individual structural groups that compose the molecule. According to this method, the molar volume of a given hydrocarbon, Vm, is expressed by

Vm ) 32.8 + 16.3Nt - 39.6Nar 19.6Nnr + 2.8Nac + 8.6Nai (2) where Nt is the total number of carbons, Nar is the number of aromatic rings, Nnr is the number of naphthenic rings, Nac is the number of aromatic conjunction atoms, and Nai is the number of aromatic inner carbons. In Figure 10, a comparison between the two sets of calculated densities, using molecular dynamics and the group contribution method, is shown. The solid line indicates where the points would lie if experiments and calculations yield the same results. As can be seen, the results obtained using the group contribution method are superior, although they are still lower than the experimental values. The superiority of the group contribution method over the molecular dynamics method can be attributed to the fact that group contribution methods are developed on the basis of a wide range of different molecules. Then, in some way, the obtained results reflect an average over a certain number of isomers that the method cannot distinguish. For instance, eq 2 cannot make any difference between molecules with the same aliphatic chains in different aromatic positions or between molecules with the same number of aliphatic carbons but different chain lengths. In contrast, different isomers can lead to different calculated densities when the molecular dynamics method is used, as shown in a previous section. Besides, in the present work only one average molecule was used for each calculation. This gives a natural advantage for a method such as the group contribution method, which averages over a significant number of different molecules. It is expected that the introduction of more molecules in the calculation would improve the calculated densities. For instance, an asphaltene sample can be represented by a mixture of different molecules. This could improve the calculated densities in two ways: first, partially eliminating the effect of considering


different isomers, and second, the presence of molecules of different sizes can improve their packing in the same volume. This last effect could play an important role for correction of the low calculated densities obtained in the present work using molecular dynamics. Further work exploring these hypotheses is now in progress. Another interesting aspect about Figure 10 is the fact that both sets of calculated values correlate quite well with a correlation coefficient of 0.848. This indicates that both molecular parameters and their graphic representation (average structures) supply information that is qualitatively correct. Therefore, this information could be used to estimate trends in density values as well as in other thermodynamic properties. At the present moment, it is clear from the results obtained that average structures cannot be used to calculate accurate values of properties for asphaltenes by means of molecular simulations. It is also clear that there should be an effort to (1) develop better structural representations of asphaltenes from the experimental point of view, and (2) improve the procedures used to calculate the properties of the average structures via molecular simulations. Even more, molecular dynamics simulations can be further developed as a tool to improve the average structure based in the testing of different isomers and the fitting of the calculated properties to the experimental ones. In this sense, density seems to be a good candidate to be fitted because it can be easily determined experimentally and it strongly depends on the structural characteristics of the molecule. It was also found that a group-contribution method gives better results than the molecular simulations that were performed. However, it is important to point out that any group contribution yields only one value of a particular property in some standard pressure-temperature conditions. In contrast, if an optimized procedure for the use of average structures using molecular simulations is developed, it could be possible to determine a wide variety of thermophysical properties under different conditions. The results obtained in this work are just a first step toward the development of a new methodology which will help to find a link between molecular structure and thermophysical properties of asphaltenes. Conclusions Asphaltenes from unstable crude oils and deposits exhibit higher densities, higher aromaticities, and lower hydrogen-to-carbon ratios than asphaltenes from stable crude oils. The results obtained have shown that the chemical information provided by the molecular parameters and average structures of asphaltenes is essentially correct and can be used to qualitatively estimate densities. The calculation of asphaltenes density using average structures by molecular dynamics simulations is subject to large errors but nevertheless gives values that are qualitatively correct and in the right range. Even more, this method can show the effect in the density of different structural modifications in the average structures of asphaltenes and, therefore, can be a useful tool to improve their condensed structural representation.

386


The average structures of asphaltenes which yields the highest densities are characterized by large condensed polyaromatic rings and low hydrogen-to-carbon ratios. The large differences found between experimental and calculated densities using molecular dynamics can be mainly attributed to the use of a unique molecule to


represent the asphaltene which can lead to poor packing and therefore, to lower densities. A group contribution calculation was observed to improve the matching between the experimentally determined and calculated density values. EF020200R

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