Asphaltenes Precipitation from Crude Oil and ... - ACS Publications

2544

Energy & Fuels 2006, 20, 2544-2551

Asphaltenes Precipitation from Crude Oil and Hydrocarbon Media Gaspar Gonza´lez,*,† Marcia A. Sousa,‡ and Elizabete F. Lucas‡ Petrobras Research Center/CENPES, Quadra 7, Cidade UniVersita´ ria, Ilha do Funda˜ o, Rio de Janeiro, RJ, CEP 21949-900, Brazil, and Instituto de Macromole´ culas-IMA/UFRJ, Cidade UniVersita´ ria, Ilha do Funda˜ o, Rio de Janeiro, Brazil ReceiVed May 17, 2006. ReVised Manuscript ReceiVed September 13, 2006

The asphaltenes precipitation onset and the precipitation yield were determined for four different crude oil samples. The results confirm that crude oil samples presenting low precipitation onsets effectively present instability and depositional problems in field operations, whereas those with a high onset are stable and do not present separation or precipitation in production operations. It was also confirmed that, as reported in the literature, the onset increased and the amount of solids separated decreased with the molecular weight of the alkane used to induce the precipitation. The same methodology was extended to asphaltenes, resins, and asphaltenes/resins mixtures dissolved in toluene using n-heptane and the ethyl alcohol as nonsolvents to evaluate the influence of the resins fraction on asphaltenes stabilization. The results analyzed using the Hildebrand solubility parameter point out for asphaltenes and resins being a continuum or family of complex molecules with a variation in molecular weight and polarity rather than two fractions containing chemically different compounds. This observation seems to be corroborated by the FTIR results that show similar spectra for asphaltenes separated using different solvents and for the resins fraction.

Introduction Asphaltenes have been under investigation for more than half of a century.1,2 Most of the efforts have been oriented to establish an accurate definition of the asphaltenes nature by focusing on the chemical composition and structure of the molecular species present in this fraction3 or the features of their self-aggregation4,5 and the equilibrium between asphaltenes and other components of petroleum crude oil.6,7 The stability of this fraction using solvent fractionation procedures has also been reported. The results indicate that asphaltenes solubility depends, basically, upon the polarity and molecular weight of the species present in this fraction as previously reported,8,9 as well as the aromaticity of the solvent media.10 In regard to the asphaltenes macrostructure, the formation of micelar aggregates has been reported.11 The molecular weight of these aggregates ranges from 1000 to several 100 000, and * To whom correspondence should be addressed. Telephone: 55-21-3865-6898. Fax: 55-21-3865-6796. E-mail: gaspargonzalez@ petrobras.com.br. † CENPES. ‡ IMA/UFRJ. (1) Mack, C. J. Phys. Chem. 1932, 36, 2901. (2) Preckshot, G. W.; DeLisle, N. G.; Cotel, C. E.; Katz, D. L. Trans. AIME 1943, 151, 188. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker, Inc.: New York, 1980. (4) Nelensteyn, J. F. J. Inst. Pet. Technol. 1924, 10, 311. (5) Pfeiffer, J. Ph. The Properties of Asphaltic Bitumens; Elsevier, Inc.: Amsterdam, The Netherlands, 1950. (6) Yen, T. F. Energy Sources 1974, 1, 447. (7) Dickie, J. P.; Yen T. F. Anal. Chem. 1967, 39, 1847. (8) Long, R. B. The Chemistry of Asphaltenes. In AdVancs in Chemistry; Bunger, J. W., Li, N., Eds.; American Chemical Society: Washington, DC, 1981; Series No. 195. (9) Waller, P. R.; Williams, A.; Bartle, K. D. Fuel 1989, 68, 520. (10) Carbagnani, L.; Olea, M. Pet. Sci. Technol. 1999, 17, 165. (11) Sheu, E. Y.; Storm, D. AsphaltenessFundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter 1.

such an extensive range of asphaltenes size distribution has been interpreted as evidence that asphaltenes may be dissolved and/ or partly suspended/peptized in the crude oil. While the first hypothesis corresponds to a reversible thermodynamic situation, representing a lyophilic colloid, the latter relates to a more complex lyophobic colloidal dispersion.12 In this latter representation, asphaltenes are considered solid particles, intrinsically insoluble in the solvent medium, surrounded by resins that act as peptizing agents to maintain the asphaltenes under the colloidal dispersion form within the crude oil. Despite the large number of experimental work available on the subject, it still seems necessary to gather some further information on the necessary conditions to maintain the crude oil polar fractions dissolved or dispersed in the solvent media. In this paper, studies on the precipitation of asphaltenes from four different samples of crude oil using different alkanes as precipitating agents and the effect of temperature on this process are presented. In complementary studies, the precipitation of asphaltenes from toluene solution under different experimental conditions is also described. Using this information, some remarks are presented in relation to asphaltenes solubility in crude oil or toluene solutions and on the interaction between asphaltenes and resins. Materials and Methods Four crude oil samples were considered in the present work: three of them were obtained from offshore production fields from Brazil and named samples A, B, and C, while the other one, named sample D, was obtained from Mexico. Asphaltenes and resins were separated from sample A. n-Pentane, n-hexane, n-heptane, n-octane, n-decane, n-dodecane, and toluene were purchased from Vetec Quı´mica Ltda (Rio de Janeiro, Brazil). Absolute ethyl alcohol was purchased from ISOFAR Ltda (Rio de Janeiro, Brazil). All of these materials were used as received. The main properties of the various solvents used in the present work are summarized in Table 1.13 (12) Kruit, H. R. Colloid Science; Elsevier, Inc.: New York, 1949.

10.1021/ef060220j CCC: $33.50 © 2006 American Chemical Society Published on Web 10/20/2006

Asphaltenes Precipitation

Energy & Fuels, Vol. 20, No. 6, 2006 2545

Table 1. Density, Solubility Parameter, and Molar Volume for the Solvents Used in This Work13 solvent

density (g/cm3)

solubility parameter (MPa)1/2

molar volume (cm3/mol)

n-pentane n-hexane n-heptane n-octane n-decane n-dodecane toluene ethyl alcohol

0.6262 0.6606 0.6838 0.7025 0.7300 0.7487 0.8669 0.7893

14.4 14.8 15.2 15.5 15.8 16.0 18.2 26

116 132 148 164 196 229 107

The crude oil samples were characterized in terms of some physical and chemical properties that were determined using the American Society for Testing and Materials (ASTM)14 or Universal Oil Products (UOP)15 standard procedures. The API degree and viscosity of the crude oil samples were measured using the ASTM D 4052 and ASTM D 445 procedures, respectively. The nitrogen content was quantified by the ASTM D 4629 and UOP 269 methods. The separation and quantification methods for asphaltenes and resins were based on a modification of the standard IP 14316 and the standard ASTM D 2007,14 respectively. The determination of the asphaltenes precipitation onset was made by spectrometric analysis using a UV-vis Shimadzu, 3101 PC spectrometer, at a wavelength of 850 nm. This wavelength was selected because the overall petroleum absorbance is not excessively high and decreases linearly with crude oil (or asphaltenes solutions) dilution. In a typical test, the absorbance of asphaltenes solutions in toluene or crude oil containing different amounts of nonsolvent (n-alkane or ethyl alcohol) was measured using 0.1 cm optical path quartz cells after equilibrating the mixture for at least 4 h after the addition of the flocculant. Asphaltenes and resins were characterized by Fourier transform infrared spectroscopy (FTIR). The infrared spectra were recorded using a Perkin-Elmer spectrophotometer, model 1720-X, as film on KBr windows. The solubility of asphaltenes in n-heptane/toluene mixtures, at several temperatures, was measured according to the following procedure: Aliquots of n-heptane were added to a solution of asphaltenes in toluene containing 90 g/L of asphaltenes, to reach n-heptane/toluene ratios of 1.0, 1.5, 2.0, 3.0, ..., to 14.0. The mixture remained resting for at least 1 h and, afterward, was filtered using a vacuum filtering, Milli-Q system fitted with an oil-permeable 0.47 µm filter. The solid material contained in the filter was washed with small amounts of n-heptane, dried in an oven to constant weight, and weighed. For the tests carried out at conditions different than room temperatures, with crude oil or asphaltenes solutions, a glass apparatus was built that permitted to complete the precipitation, filtration, and washing steps inside a temperature-controlled laboratory drying oven. The asphaltenes solution and n-heptane, both preheated at the selected temperature, were mixed together, and this mixture was kept in the oven for at least 1 h. After this period, the flask content was poured into the previously heated vacuum filtration apparatus. The retained material was washed with nheptane and dried in the oven to a constant weight.

Results Crude Oil Characterization. Table 2 shows some physical and chemical properties of the three samples (A, B, and C) that were obtained from offshore production fields from Brazil. The samples present different physical and chemical properties, (13) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983; pp 32. (14) www.astm.org. (15) www.uop.com. (16) Standards for Petroleum and Its Products, Standard No. IP 143/57, Institute of Petroleum, London, U.K.

Table 2. Physical and Chemical Properties of Crude Oil Samples A, B, and C sample

A

B

C

API degree relative density, 20/4 °C viscosity at 30 °C cSt pour point (°C) asphaltenes (wt %) resins (wt %) paraffins (wt %) N (wt %) S (wt %) metals (mg/kg) Ni V

19.2 0.934 357.7 -27 2.3 20-25 1.25 0.39 0.71

28.9 0.8786 22.4 -15 2.2 1.7 0.29 0.52

39.6 0.823 4.4 -36 0.4 8.0 3.16 268 0.08

18 26

9.0 19.0

1.0 5.0

including asphaltenes and resins content. Detailed characterization data for the Mexican crude oil (sample D) were not available, but it is known that asphaltenes depositional problems have been detected for this sample in field operations. Precipitation of Asphaltenes from Crude Oil. Figure 1 shows the absorbance as a function of the dilution with n-heptane for the crude oil samples. The diagrams represent typical tests carried out to assess the onset of asphaltenes precipitation. In the first branch of the curves, the absorbance is reduced by the addition of n-heptane because of dilution. At a certain n-heptane/crude oil ratio, the absorbance of the mixture starts to increase as the result of the appearance of asphaltenes particles in the medium that scatter part of the incident radiation. The reduction in the absorbance for subsequent additions of n-heptane, observed in the third branch of the curves, corresponds again to dilution and sedimentation of the asphaltenes particles. The ratio between the milliliters of solvents and the mass of oil at the points of minimum absorbance represents the asphaltenes precipitation onset. The onset values obtained for the four crude oils considered in Figure 1 are shown in Table 3, where the results are also presented in terms of the solvent mass per mass of oil. Table 3 also present the solubility

Figure 1. Absorbance against dilution plots used to determine the asphaltenes precipitation onset for the Brazilian (A, B, and C) and Mexican (D) crude oils. Table 3. Precipitation Onset Obtained for the Crude Oils Considered in Figure 1 and the Corresponding Solubility Parameter Calculated for Each Sample onset crude oil

(mL of n-C7/ g of oil)

(g of n-C7/ g of oil)

oil solubility parameter (MPa)1/2

A B C D

2.8 1.1 0.7 0.9

1.91 0.75 0.48 0.62

19.3 17.6 17.3 17.5

2546 Energy & Fuels, Vol. 20, No. 6, 2006

Gonza´ lez et al.

Table 4. Asphaltenes Precipitation Onset, Maximum Precipitation Yield, and Solubility Parameter at the Precipitation Onset and at 10/1 Dilution for Crude Oil A Using Different n-Alkanes alkane

pentane

hexane

heptane

octane

decane

dodecane

onset (mL of alkane/g of oil) (g of alkane/g of oil) asphaltenes precipitation yield (mg/g of oil) δ (MPa)1/2 at the onset δ (MPa)1/2 at 10/1 dilution

2 1.25 62 16.6 14.8

2 1.32 33 16.7 15.2

2.8 1.91 31 16.6 15.6

3.5 2.46

3.7 2.70 27 16.8 16.1

3.75 2.81 21 16.9 16.3

parameters calculated for each oil sample calculated using eq 2 and 16.6 (MPa)1/2 for the solubility parameter at the precipitation onset,17 as detailed in the Discussion. The precipitation yield and the onset strongly depend upon the nonsolvent used to induce the precipitation, as shown in Figure 2 and Table 4, respectively. Table 4 summarizes the results for the precipitation onset and the precipitation yield, obtained for crude oil A using different aliphatic hydrocarbons, and the respective solubility parameters calculated at the onset and at a dilution of 10 g of nonsolvent/g of crude oil. For low dilution ratios, no precipitation was observed (Figure 2). At a particular dilution, characteristic for each solvent, the precipitation onset is reached and asphaltenes particles start to separate from the crude oil. Above the onset, the amount of solids separated increases with the increase of the dilution ratio, and finally, at higher dilutions, a plateau is attained and no further precipitation is obtained by increasing the nonsolvent concentration. The graphs presented in Figure 2 do not clearly show the precipitation onset for each nonsolvent, and for this reason, the values presented in Table 4 were determined using plots similar to those shown in Figure 1. The precipitation onset increased with the increase of the alkane molecular weight. This increase however is slightly less significant when the results are expressed in terms of the mass of nonsolvent per mass of oil. Similar results were obtained by Hu and Guo for a Chinese crude oil from Shengly Oilfield.18 Wang and Buckley19 and more recently Wiehe et al.20 have reported results that indicate that the volume of n-alkane at asphaltenes precipitation onset goes through a maximum at a carbon number of 8-10. Such behavior was not observed in this work and was not further investigated. The precipitation yield decreases with the increase of the molecular weight of the alkane. This reduction is relatively large when n-hexane is used instead of n-pentane as a precipitating solvent, but the substitution of hexane by heptane or higher molecular-weight alkanes results in less significant reductions. The effect of temperature on the precipitation of asphaltenes for various dilutions of crude oil was examined for n-decane

Figure 2. Precipitation of asphaltenes from crude oil A by different alkanes at various dilution ratios ([, C5; b, C6; 2, C7; 9, C10; ], C12).

16.6 15.8

and n-dodecane. The results presented in Figure 3 indicate that the amount of solids separated was higher at 20 °C than at 50 °C, for both precipitant agents, indicating that the solubility of the asphaltenes particles is higher at higher temperatures. Similar results have been reported for various heavy oils and bitumens using pentane or heptane as precipitating agents.21 Characterization of Asphaltenes and Resin. Figure 4 presents the IR spectra obtained for asphaltenes separated from crude oil A using n-pentane or n-heptane as precipitating agents. The spectrum obtained for resins is shown in Figure 5. Both asphaltenes samples and the resins fraction present very similar spectra. Precipitation of Asphaltenes from Solutions. The precipitation onset for a 5 g/L asphaltenes/toluene solution using

Figure 3. Amount of asphaltenes precipitated from crude oil by n-decane and n-dodecane at 20 and 50 °C.

Figure 4. FTIR spectra of asphaltenes separated from crude oil A using (a) n-pentane and (b) n-heptane.


Figure 5. FTIR spectrum of resins separated from crude oil A.


Figure 7. Effect of resins on the asphaltenes precipitation onset using n-heptane as precipitating agents. Asphaltenes concentration ) 5 g/L. Resins concentrations (g/L) ) (a) 5, (b) 10, (c) 15, and (d) 25. Table 5. Solubility of Asphaltenes in Several n-Heptane/Toluene Mixtures for 50, 20, and 0 °C temperature (°C) asphaltenes solubility (mg/mL)

Figure 6. Absorbance against dilution plot used to determine the asphaltenes precipitation onset from asphaltenes solutions in toluene (5 g/L) using as a precipitation agent: (a) n-heptane and (b) ethyl alcohol.

n-heptane and ethyl alcohol as precipitating agents is shown in parts a and b of Figure 6, respectively. The onsets of asphaltenes precipitation for these two nonsolvents were obtained at the dilution ratios of 1.4 mL of n-C7/mL of toluene solution and 0.40 mL of ethyl alcohol/mL of toluene solution. Titration tests carried out to up to 9% asphaltenes with n-heptane and from 1 to 30 g/L of asphaltenes for ethyl alcohol conduced to similar values for the precipitation onset, indicating that for both nonsolvents the onset was independent of the asphaltenes concentration,22 being determined only by a particular solvent (17) Souza, M. A.; Oliveira, G. E.; Lucas, E. F.; Gonza´lez, G. Prog. Colloid Polym. Sci. 2004, 128, 283. (18) Hu, Y. F.; Guo, T. M. Fluid Phase Equilib. 2001, 192, 13. (19) Wang, J. X.; Buckley, J. S. SPE Pap. 2001, 64994. (20) Wiehe, L. A.; Yarranton, K.; Akbarzadeh, K.; Rahini, P.; Teclemarian, A. The Maximumin Volume with Carbon Number of N-Paraffins at the Onset of Asphaltenes Presipitation. In Proceedings of the 5th International Conference on Phase Behavior and Fouling, Banff, Canada, June 13-17, 2004. (21) Akbarzadeh, K.; Alboudwarej, H.; Svrcek, S. Y.; Yarranton, H. W. Fluid Phase Equilib. 2005, 232, 159. (22) Souza, M. A. M.Sc. Dissertation, Instituto de Macromolećulas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, March 2000, http://www.ima.ufrj.br/catalog/frcatal1.htm. (23) Praunitz, J. M. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice Hall, Inc.: Englewood Cliffs, NJ, 1969; pp 295-297.

n-C7 volume fraction

50

20

0

0.60 0.67 0.71 0.78 0.83 0.89

34.5 24.1 19.5 13.3 9.1 5.8

30.3 22.6 18.8 11.3 4.2 2.4

22.5 13.3 8.3 5.7 3.6 2.5

condition, characterized in the polymers solutions theory by the Flory interaction parameter and the Hildebrand solubility parameter.23 Table 5 shows the effect of temperature on the precipitation of asphaltenes from toluene solutions induced by n-heptane. In this case, the results are presented as the maximum solubility of asphaltenes in several n-heptane/toluene mixtures for three temperatures. For an n-heptane volumetric fraction of 0.60, asphaltenes solubility was reduced from 34.5 mg/mL at 50 °C to 22.5 mg/mL at 0 °C. In other words, this temperature drop causes the precipitation of 12 mg of asphaltenes/mL of solution. The precipitated amount did not change very much when the temperature was reduced from 50 to 20 °C but increased significantly from 20 to 0 °C. The minimum solubility for the system was observed when the proportion of n-heptane is maximal and, in this case, the influence of temperature is the less important. Effect of Resins on the Precipitation of Asphaltenes. The effect of resins on the asphaltenes precipitation onset using n-heptane as a precipitating agent is presented in Figure 7 and Table 6. The molar ratios reported in this table were calculated using the previously reported values of 1200 and 750 Da for the number average molecular weight of asphaltenes and resins, extracted from crude oil A and dissolved in toluene.21,24 These data are in good agreement with results reported previously by (24) Gonza´lez, G.; Travalloni-Louvisse, A. M.; Neves, G. B. M.; Saraiva, S.; Lucas, E. F.; Sousa, M. A. Energy Fuels 2003, 17, 879.


Gonza´ lez et al.

Figure 8. Effect of resins on the asphaltenes precipitation onset using ethyl alcohol as precipitating agents. Asphaltenes concentration ) 5 g/L. Resins concentrations (g/L) ) (a) 5, (b) 10, (c) 15, and (d) 25. Table 6. Effect of Resins on the Precipitation Onset of Asphaltenes in Toluene (Initial Asphaltenes Concentration ) 5 g/L) resin concentration (g/L)

0

5

10

15

25

resins/asphaltenes, molar ratioa precipitation onset (mL of n-heptane/mL of toluene)

0 1.4

1.6 1.3

3.3 1.4

4.8 1.7

8.0 2.2

a

On the basis of the M h n values reported in the literature.20

Travalloni and Freire for these samples.25 The onset was modified only when resins are at a concentration notably higher than asphaltenes. As in the case of pure asphaltenes, the absorbance presented a moderate increase after the minima and remained roughly in this value for higher dilutions (Figure 7), indicating that the particles do not sediment rapidly after being formed. When similar experiments were assayed using ethyl alcohol as a precipitating agent, the onset was not modified even at very high resins concentrations, as shown in Figure 8. Moreover, the shape of the absorption versus dilution curves, compared to that obtained for asphaltenes or asphaltenes and resins in the presence of n-heptane, was substantially modified. The absorption for dilutions higher than the onset presented a

Figure 9. Precipitation onset of resins induced by the addition of ethyl alcohol. Resins concentrations (g/L) ) (a) 10 and (b) 15.

large increase, followed by a sudden drop also observed in the case of the precipitation of asphaltenes by ethanol (Figure 6b). To find out whether these results were caused by the destabilization of resins, the precipitation onset of this fraction dissolved in toluene was measured, using ethyl alcohol as a nonsolvent. The results are presented in Figure 9. Although the absorption



of resins solution is very low at this wavelength, it is clear that, at a dilution around 0.7 mL of ethanol/mL of toluene solution, a sharp increase in the absorption is observed. As in the case of asphaltenes, this increase may be ascribed to the formation of solid particles that scatter part of the incident radiation and the subsequent decrease to the sedimentation of the particles. Discussion Asphaltenes Precipitation Onset. Accumulated results26 indicate that crude oil stable in relation to asphaltenes precipitation present a precipitation onset higher than 2 mL of C7/g of crude oil, while the ones with precipitation onset values smaller than 1 mL of C7/g of oil would present a tendency of asphaltenes toward deposition. The precipitation onset obtained for sample A (2.8 mL of n-C7/g of oil, Table 3) may be considered high, and it may be inferred that this oil is stable, meaning that, despite its relatively high content of asphaltenes (Table 2), no depositional problems may be expected. This inference, confirmed in field operations, may be interpreted as evidence for the existence of suitable physicochemical conditions for the asphaltenes stabilization in this oil. For crude oil B, the precipitation onset obtained was 1.1 mL of n-C7/g of oil and the formation of organic deposits was detected for some wells in the initial stages of the field development. For the sample of Mexican oil (sample D) that presents confirmed depositional problems in field operations, the precipitation onset obtained was 0.9 mL of n-C7/g of oil. For crude oil C, precipitation onset values of 0.7 mL of n-C7/g of oil and lower26 were obtained. Notwithstanding the rather low contents of asphaltenes in this sample (0.4%, Table 2), very serious problems related to asphaltenes precipitation and deposition have been faced in production operations and continuous injection of asphaltenes precipitation inhibitors has been necessary to maintain adequate production levels. Asphaltenes flocculation and the subsequent depositional problems have often been associated with this type of light crude oil;27 however, recently reported results indicate that heavy oils, presenting asphaltenes concentrations above 1%, may also be unstable.28 The results confirm that the precipitation onset is a simple and very useful parameter to preliminarily evaluate the possibilities of asphaltenes precipitation in crude oil. More elaborated depositional tests have been developed29 using live oil (i.e., samples preserving the original dissolved gas) and pressure and temperature conditions equivalent to those subsisting in the reservoir. These, however, are extremely expensive and timeconsuming procedures. Moreover, results reported by Buckley et al.30 indicate that asphaltenes precipitation caused by npentane or n-heptane and instability because of depressurization reflect, both, the asphaltenes phase behavior and that, for model systems, the precipitation onset because of depressurization may be correctly predicted using ambient conditions titration onset data. (25) Travalloni, A. M.; Freire, N. In Proceedings of the First International Symposium on Colloid Chemistry in Oil Production: Asphaltenes and Wax Deposition, IBP, Rio de Janeiro, Brazil, November 26-29, 1995; pp 252. (26) Marques, L. C. C.; Gonza´lez, G.; Monteiro, J. B. A. SPE Pap. 2004, 91019-MS. (27) Marques, L. C. C.; Machado, A. L. C.; Garcia, R. L. P.; Soldan, A. L.; Campagnolo, E. A. World Oil 1997, 125. (28) Garcıá, M. del C.; Henrı´quez, M. Asphaltenes Depositions and Control in a Venezuelan North Monages Oil Field. SPE Paper 80262. Presented at the SPE International Symposium on Oil Field Chemistry, Houston, TX, February 5-7, 2003. (29) Hammami, A.; Raines, M. A. SPE J. 1999, 4, 9. (30) Wang, J. X.; Buckley, J. S. SPE Pap. 2001, 64994.

In a previous publication,17 the precipitation onset of asphaltenes separated from various different sources and dissolved in different solvent media was studied. The results permitted us to conclude that, for various different conditions, the asphaltenes precipitation onset occurred when the solubility parameter of the solvent mixtures attained values around 16.6 (MPa)1/2. This information may be used to calculate the solvent capacity of the different crude oils and to interpret the changes observed in the precipitation onset when alkanes with different molecular weight are used as the precipitation solvent. The effective Hildebrand solubility parameter for a macromolecules solution in a binary mixture of solvents is normally expressed as the sum of the solubility parameter of each solvent by its respective volume fraction in the mixture. This approximation is valid only when the macromolecules are in a very low concentration, compared to the solvents.31 In the forthcoming analysis, the solvent medium of the crude oil and the added alkanes will be considered the solvents and the asphaltenes fraction will be considered the dissolved macromolecules. Considering that in the case of crude oils the accurate measurement of volumes is difficult, it is more convenient for these systems to refer these parameters to the weight fractions, defined as32 i m w ) 1 - jw ) i m + jm

i

(1)

where w represents the weight fraction for each solvent and m represents its respective mass. Using this definition, the solubility parameter of the mixture at the precipitation onset, oδ, becomes o

δ ) cδcw + aδaw

(2)

where the superscripts c and a refer to crude oil and the alkane, respectively. Because the precipitation of asphaltenes occurs at a solubility parameter17 of 16.6 (MPa)1/2, the application of eq 2 to the results reported in Figure 1 conduces to the solubility parameter of the crude oils reported in Table 3. The reported solubility parameter for asphaltenes31 is around 19 (MPa)1/2, which means that crude oil A represents a very good solvent for this fraction. For the other samples, the solubility parameters are too low to maintain stable asphaltenes solutions and a small addition of a nonsolvent will result in phase separation. This is particularly the case of crude oil C for which onset values lower than 0.4 mL of C7/g of oil have been reported using near-infrared spectroscopy to detect the precipitation concentration.26 This result may be interpreted as indicating that asphaltenes precipitation in unstable oils may be ascribed to the poor solubility power of oil rather than to any particular property of the asphaltenes fraction. Equation 2 may also be applied to account for the effect of alkane chain length on the precipitation of asphaltenes from crude oil presented in Figure 2 and Table 4, assuming the value of 19.3 (MPa)1/2 for the solubility parameter of crude oil A. The results indicate that, at the asphaltenes precipitation onset, the solubility parameter of the solvent media is very similar for the various alkanes considered and lies between 16.6 and 16.9 (MPa)1/2. Similar results were obtained when eq 2 was (31) Barton, A. F. M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990; pp 437-438. (32) Hildebrand, J. H., Scott, R. L. The Solubility of Nonelectrolytes; Reinhold Publishing Corp.: New York, 1950.


applied to the results reported for the crude oil from Shengly Oilfield.18 For higher dilution, the crude oil/alkane mixture presents different solubility parameter values for the different alkanes. In Table 4, the values calculated at a dilution of 10 g of alkane/g of oil are presented. The data indicate that the difference between the solubility parameter at the onset and the corresponding value at this dilution is 1.8 (MPa)1/2 for n-pentane and decreases with the increase of the alkane molecular weight. Hence, the precipitation yield for pentane, at a 10/1 dilution, corresponds to the fraction of components that become unstable and precipitate in the range of 16.6 to 14.8 (MPa)1/2 for the solubility parameter. Higher molecular-weight alkane presents a larger solubility parameter, and therefore, the difference between δ at the onset and δ at a 10/1 dilution is smaller, resulting in a smaller precipitation yield. These considerations may explain the differences in the amount of asphaltenes precipitated by different alkanes presented in Table 4 and results previously reported by various authors for crude oil samples18 or distillation residues.33 Characterization of Asphaltenes and Resins. The spectra for asphaltenes and resins presented in Figures 4 and 5 are very similar. The spectrum obtained for the asphaltenes separated from an oilfield residue obtained from crude oil C also presented a very similar shape. The bands and peaks observed are similar to those described for three asphaltenes fractions separated from samples of Mexican crude oils34 and to the spectrum reported by Yen and Erdman in a detailed study to characterize Baxkerville asphaltenes.35 The observed bands are as follows: a broad band corresponding to O-H bonds at 3300 cm-1 (A), sharp peaks for C-H axial stretching at 2923 and 2852 cm-1 (B), CdO and CdC bonds for aromatic rings at 1612 cm-1 (C), peaks at 1457 and 1376 cm-1 (D) corresponding to the CH3 asymmetric and symmetric stretching, a peak at 1032 cm-1 (E) characteristic of the sulfoxide group (C2SdO), bands next to 870 and 800 cm-1 (F) related to C-H out-of-plane deformation, and a signal around 750 cm-1 (G) characteristic of the vibration of four hydrogen atoms adjacent to an aromatic ring. According to Jada and Ait Chaou,36 the band at 3300 cm-1 corresponds to phenolic OH bonds and the small peak at 17001800 cm-1 is related to the carbonyl groups CdO bonds of carboxylic acids. It is interesting to note that groups containing nitrogen are not detected in the spectra presented in Figures 4 and 5 nor in other data presented in the literature for asphaltenes or resins, most likely, because of the fact that the bands corresponding to this group should appear in the regions corresponding to the CH2 (2967 cm-1), CdC (1600 cm-1), and aromatic C-H (∼800 cm-1). The results confirm the complex structure of the asphaltenes fraction, which contains several types of functional groups and confirms literature data, indicating that asphaltenes and resins present very similar chemical structures. The resins are typically composed of polar end groups, which also contain heteroatoms, as well as long nonpolar paraffinic chains. The main difference between resins and asphaltenes is that resins are soluble in n-pentane, while asphaltenes are not. In general, resins are less aromatic, less polar, and present a lower molecular weight than asphaltenes. Precipitation of Asphaltenes and Resins from Solution. Asphaltenes solutions may be considered as a mixture containing (33) Hong, E.; Wilkinson, P. Fuel 2004, 83, 1881. (34) Parra-Barraza, H.; Hernańdez-Montiel, D.; Lizardi, J.; Hernańdez, J.; Herrera, U. R.; Valdez, A. M. Fuel 2003, 82, 869. (35) Yen, T. F.; Erdman, J. G. Prepr.-Am. Chem. Soc., DiV. Pet. Chem. 1962, 7, 5. (36) Jada, A.; Ait Chaou, A. J. Pet. Sci. Eng. 2003, 39, 287.

Gonza´ lez et al.

various components whose solubility parameter presents a distribution around the value corresponding to the solvent, in the case of toluene, 18.2 (MPa)1/2. The addition of heptane reduces the solution solubility parameter, and as shown in Figure 6, for a dilution corresponding to a δ value of 16.6 (MPa)1/2, the more polar components become insoluble and precipitate. When ethyl alcohol is used as precipitating agent, the less polar components precipitate at a dilution corresponding to δ ) 20.3 (MPa)1/2. These values may be considered the lower and upper solubility parameter limits for the asphaltenes fraction. As previously mentioned,31 the reported solubility parameter for asphaltenes is around 19 (MPa)1/2, which is intermediate between the two values mentioned above, showing a good correlation with our results. In a very early paper, Nellensteyn37 attempted the precipitation of an asphalt solution in benzene or carbon disulfide and observed that benzin, an oil distillate with a boiling point between 40 and 60 °C and the same surface tension of hexane, and ethanol produced precipitates that he denominated “hard asphalt” and “soft asphalt”, respectively. According to the author, both precipitates corresponded to chemically different compounds. As in our case, these two components correspond to a subfraction of the asphaltenes fraction. As shown in Table 6, resins modify the asphaltenes precipitation onset induced by n-heptane only when their concentration is notably higher than asphaltenes. For instance, a shift from 1.4 (obtained in the absence of resins) to 1.7 mL of C7/mL of toluene, corresponding to a reduction of 0.15 units in the solubility parameter, was observed for a resins/asphaltenes molar ratio of 4.8. Adsorption data reported previously indicated that this asphaltenes sample presents around three sites for the binding of resins.24 From this information, it may be inferred that an aggregate containing up to three resins molecules per molecule of asphaltenes has, in fact, its solubility in n-heptane slightly increased in relation to the original n-heptane-insoluble asphaltenes. When ethyl alcohol was used as a nonsolvent, the addition of resins up to a concentration of 25 g/L did not change the asphaltenes precipitation onset (Figure 8). In this case, the precipitated fraction should correspond to the components whose polarity must be very close to the resins fraction polarity; therefore, no polarity alteration should be expected. An increase in the onset, in this case, would mean the formation of more polar aggregates, while a reduction would correspond to aggregates less polar than the less polar components of the asphaltenes fraction. These results are coherent with the previous considerations regarding asphaltenes/resins precipitated by nheptane. The other observation in relation to Figure 8 was that the absorbance at a dilution higher than the onset was considerably increased. For the precipitation of asphaltenes with ethyl alcohol, for instance, the maximum absorption reached was less than 0.5 units (curve b in Figure 7), whereas the corresponding values in the presence of 15 and 25 g/L of resins were 1.0 and 1.3, respectively. These figures may indicate that the number of scattering particles in the medium continued to increase beyond the asphaltenes precipitation onset, and this fact was ascribed to the formation of resins particles. Figure 9 that shows that the precipitation onset for resins was around 0.7 mL of ethyl alcohol/mL of toluene, seems to confirm this hypothesis. It is interesting to note that the onset for resins is slightly different than the value obtained for asphaltenes. Despite this difference, the absorbance-dilution curves for the asphaltenes/ resins mixture showed the same profile than that observed for (37) Nellensteyn, J. F. J. Inst. Pet. Technol. 1928, 14, 134.


the pure fractions, presenting one single onset value that provides evidence for similar macromolecular species. The results for asphaltenes, resins, and the asphaltenes/resins mixture precipitation onset point out the formation of relatively small, less polar asphaltenes/resins aggregates, presenting a higher solubility in aliphatic media requiring, for that reason, a lower solubility parameter to precipitate. The model describing adsorbed resins molecules stabilizing intrinsically insoluble asphaltenes particles, as in the case of dispersions or emulsions, extensively used to model asphaltenes depositional problems,38 apparently does not describe the structure of these aggregates accurately. Furthermore, it seems reasonable to consider asphaltenes and resins as a continuum or family of complex molecules with a variation in molecular weight and polarity, as stated by Long8 for asphaltenes, rather than two fractions containing chemically different compounds. This observation seems to be corroborated by the FTIR results that show similar spectra for asphaltenes separated using different solvents and for the resins fraction. Effect of Temperature on the Solubility of Asphaltenes. From the results presented in Figure 3 and Table 5, it is clear that asphaltenes follow the same solubility pattern in crude oil or toluene; i.e., their solubility rises with an increase of the temperature. Andersen,39 working with crude oils from Boscan and Kuwait, also observed that the amount of asphaltic material precipitated by n-heptane increased as the precipitation temperature was decreased. The results reported by Fuhr40 and Hu18 also confirm this point. For liquid-liquid equilibrium, this behavior is typical of systems that present an upper critical solution temperature,41 for which the solute-solvent interactions and the association leading to the phase separation are dispersion forces as in the case of paraffin and waxes. These observations, however, must not be generalized or extended to all types of crude oils or to other pressure and temperature conditions. There are cases reported in the literature41-46 in which asphaltenes precipitation may be caused or enhanced by temperature. This precipitation may be ascribed to a disruption of the polar (38) Leontaritis, K. J. SPE Paper 18892. In SPE Production Operation Symposium, Oklahoma, March 13-14, 1989. (39) Andersen, S. I. Fuel Sci. Technol. 1994, 12, 51. (40) Fuhr, B. J.; Cathrea, C.; Coates, L., Kalra, H.; Majeed, A. I. Fuel 1991, 70, 1293. (41) Prausnitz, J. M.; Ru¨diger, N.; Lichtenthaler, R. N.; Azevedo, G. E. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice Hall, Inc.: Englewood Cliffs, NJ, 1999; pp 276. (42) Leontaritis, K. J., Mansouri, A. G. J. Pet. Sci. Eng. 1988, 1, 229. (43) Kabir, C. S.; Jamaluddin, A. K. M. SPE Pap. 1999, 53155. (44) Almehaideb, R. A. J. Pet. Sci. Eng. 2004, 42, 157. (45) Caldas, J. N.; Thomas, M.; Behar, E.; Rajagopal, K. Influence of Solvents and the Asphaltenes Content on the Flocculation Threshold of Crude Oil and Asphaltene Solutions. In Proceedings of the First International Symposium on Colloid Chemistry in Oil Production: Asphaltenes and Wax Deposition, IBP, Rio de Janeiro, Brazil, 1995; pp 1. (46) Cimino, R.; Correra, S.; del Bianco, A.; Lockhart T. P. Solubility and phase behavior of asphaltenes in hydrocarbon media. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1955; pp 97-130.


interactions that maintain the asphaltenes supramolecular structures. If this disruption exposes to the nonpolar solvent large molecules containing polar groups, this process may result in phase separation. These phenomena are not considered by depositional models that describe the asphaltenes-asphaltenes interaction merely as dispersion forces. Conclusions The precipitation onset is a simple and very useful parameter to preliminarily evaluate the possibilities of asphaltenes precipitation problems. Crude oils presenting low precipitation onsets effectively present instability and depositional problems in field operations, whereas those with a high onset are stable and do not present separation or precipitation in production operations. The precipitation onset for asphaltenes strongly depends upon the properties of the nonsolvent used to induce the separation process. For n-alkanes, the onset increases and the maximum amount of precipitate decreases as the molecular weight of the nonsolvent increases. These results confirm previous works45 and illustrate the fact that asphaltenes may be split into subasphaltenic fractions using adequate solvents.46 It is interesting, however, to note that the solubility parameter of the solvent media at the asphaltenes precipitation onset is very similar for the various alkanes considered and, for crude oil A, lies between 16.6 and 16.9 (MPa)1/2. Also, as higher molecular-weight alkanes present a larger solubility parameter, the difference between δ at the onset and δ at higher dilution is smaller, resulting in a smaller precipitation yield. Asphaltenes separated from crude oil A using n-pentane or n-heptane as precipitating agents and the resins fraction present a very similar IR spectra. The band and peaks observed are similar to those described by Yen and Erdman35 in a detailed study to characterize the IR spectrum of Baxkerville asphaltenes and to those reported recently for three asphaltenes fractions separated from a sample of Mexican crude oil. The results of the precipitation onset for asphaltenes, resins, and the asphaltenes/resins mixture dissolved in toluene point out the formation of relatively small, less polar asphaltenes/ resins aggregates presenting a higher solubility in aliphatic solvents. Furthermore, it seems reasonable to consider asphaltenes and resins as a continuum or family of complex molecules with a variation in molecular weight and polarity, as stated by Long8 for asphaltenes, rather than two fractions containing chemically different compounds. Asphaltenes follow the same solubility pattern in crude oil or toluene; i.e., their solubility rises with an increase of the temperature. However, there are cases reported in the literature in which asphaltenes precipitation may be caused or enhanced by temperature. Acknowledgment. We thank ANP/FINEP/CTPETRO, CNPq, CAPES, FAPERJ, and Petrobras Research Center. EF060220J

Asphaltenes Precipitation from Crude Oil and ... - ACS Publications

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