Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 1

Jan 10, 2007 - The size of asphaltene solubility fractions has been studied by Groenzin et al.,18 and the results showed that the size of the solubili...
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Energy & Fuels 2007, 21, 1030-1037

Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 1. Interfacial Tension and Solvent Properties Martin Fossen,*,† Harald Kallevik,‡ Kenneth D. Knudsen,§ and Johan Sjo¨blom† Ugelstad Laboratory, Department of Chemical Engineering, Norwegian UniVersity of Science and Technology (NTNU), Trondheim, Norway, Statoil ASA, R&D Center, Trondheim, Norway, and Institute for Energy Technology, Kjeller, Norway ReceiVed July 5, 2006. ReVised Manuscript ReceiVed NoVember 23, 2006

Asphaltenes were precipitated into two fractions using a two-step precipitation procedure. The first fraction was obtained by mixing 3:1volumes of n-pentane/crude oil followed by filtration. In the following step the second fraction was precipitated out from the filtrate using 18:1 volumes of n-pentane/crude oil. Whole asphaltenes were precipitated using 40:1 pentane-to-crude oil ratio. Three crude oils were used and the asphaltene fractions obtained were characterized with regard to onset of precipitation, interfacial tension and radius of gyration. The second fraction was more soluble at increasing heptane-to-toluene ratios, more interfacially active and showed different organization properties at the interface between oil and water. Small angle neutron scattering showed that the second fraction formed aggregates with lower radius of gyration. The results show that asphaltenes consist of fractions with different solvent properties and indicates that asphaltenes should be looked at as more than one solubility class.

1. Introduction Asphaltenes are possibly the most studied and yet least understood material in the petroleum industry.1 As a definition, petroleum asphaltenes is the fraction of the crude oil that precipitates upon addition of n-alkanes at a solvent to oil ratio of 40:1.2 Furthermore, asphaltenes are monomeric polycyclic aromatic hydrocarbons containing heteroatoms such as N, O, and S and metal compounds such as V, Ni, and Fe.3,4 The petroleum asphaltenes constitute a potential risk for precipitation during oil recovery, leading to problems with organic deposition and water-in-crude oil formation.5-8 Asphaltenes that are obtained from dead crude oils in the laboratory may not be the same as asphaltenes precipitated under oil recovery conditions. This has in fact been shown recently by Klein et al.,9 who compared asphaltenes obtained down hole by pressure drop with * Corresponding author. E-mail: [email protected]. Phone: +47 73594149. † Ugelstad Laboratory. ‡ Statoil ASA. § Institute for Energy Technology. (1) Yen, T. F.; Chilingarian, G. V. Asphaltenes and Asphalts, 2; Elsevier Science B.V.: Amsterdam, 2000. (2) Speight, J. G. Petroleum asphaltenes. Part 1. Asphaltenes, resins and the structure of petroleum. Oil Gas Sci. Technol. 2004, 59, 467-477. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (4) Mullins, O. C.; Y. Sheu, E.; Hammami, A.; Marshall, A. G. Asphaltenes, HeaVy Oils, and Petroleomics; Springer Science+Business Media, LLC: New York, 2007. (5) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Solvent entrainment in and flocculation of asphaltenic aggregates probed by smallangle neutron scattering. Langmuir 2006, 22, 4487-4497. (6) Aquino-Olivos, M. A.; Andersen, S. I.; Lira-Galeana, C. Comparisons between asphaltenes from the dead and live-oil samples of the same crude oils. Pet. Sci. Technol. 2003, 21, 1017-1041. (7) Mullins, O. C. Molecular structure and aggregation of asphaltenes and petroleomics. SPE Int. 2005, SPE 95801, 1-10. (8) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants. Langmuir 2005, 21, 2728-2736.

heptane-precipitated asphaltenes. Nevertheless, asphaltenes obtained from precipitation by solvents are often the only source available for extensive characterizations. Furthermore, the detailed knowledge about asphaltene precipitation, besides that they tend to precipitate upon pressure reduction and at the addition of light n-alkanes, is still not fully understood. Parameters that can be relevant to the precipitation of asphaltenes are polarity, aromaticity, molecular weight, structure, solvent power, kinetics, temperature, and pressure.2 One standard procedure used to obtain asphaltenes is the dilution of crude oils to a 40:1 ratio with n-alkane (n-pentane or n-heptane predominantly). The reason for the excess n-alkane addition is to obtain consistent asphaltene fractions and avoid coprecipitation of resins.10,11 Although it makes sense that one should standardize the asphaltene precipitation procedures, the standard asphaltenes obtained with excess (40:1) n-alkane to crude oil ratio may as well mask important differences within this whole asphaltene fraction. The fractionation of asphaltenes based on the solubility and insolubility in different solvents has been treated by several authors.3,6,12-16 The reason for this interest in further fractionation of asphaltenes is the assumption that asphaltenes consist (9) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20, 1965-1972. (10) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadeh, K. Sensitivity of asphaltene properties to separation techniques. Energy Fuels 2002, 16, 462-469. (11) Goual, L.; Firoozabadi, A. Measuring asphaltenes and resins, and dipole moment in petroleum fluids. AIChE J. 2002, 48, 2646-2663. (12) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, S. Studies on asphaltenes through analysis of polar fractions. Ind. Eng. Chem. Res. 1999, 38, 964-972. (13) Acevedo, S.; Borges, B.; Quintero, F.; Piscitelly, V.; Gutierrez, L. B. Asphaltenes and other natural surfactants from cerro negro crude oil. Stepwise adsorption at the water/toluene interface: film formation and hydrophobic effects. Energy Fuels 2005, 19, 1948-1953.

10.1021/ef060311g CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007

Asphaltenes Precipitated by Two-Step Procedure

of several subfractions, which can be distinguished by proper separation methods. Often, the aim is to divide the whole asphaltene fraction into a range of subfractions with different properties, such as polarity.12 Asphaltenes can be fractionated by a variety of techniques using mixed solvents such as benzene/ pentane or toluene/pentane to get asphaltenes with more or less heteroatom or aromatic content.3 Spiecker et al.17 separated asphaltenes from four different crude oils using mixtures of heptane and toluene. They further determined the amount of soluble and precipitated fractions. The former was found to be more soluble than both the whole and the precipitated fractions when the different fractions were introduced into various heptane/toluene mixtures. They suggested that by removing the fraction soluble in the heptane/toluene mixture they decreased the kinetics of dissolution and the ultimate solubility of the precipitate fraction. The size of asphaltene solubility fractions has been studied by Groenzin et al.,18 and the results showed that the size of the solubility fractions increase with decreasing solubility. The works sited above use asphaltenes that prior to the fractionation have been precipitated using a standard procedure (40:1 n-alkane to crude oil or the like). In the present work the main focus was to compare asphaltenes that were precipitated stepwise in which the only difference would be the solubility in the crude oil at addition of n-pentane. This was done by adding a small amount (3:1) of n-pentane to crude oil, followed by filtration to obtain the asphaltenes precipitated at that dilution level. The second step is a continued precipitation by further addition of n-pentane (18:1) followed by a new filtration. By choosing this method the aim was to compare solvent properties of the less soluble (first fraction) with the more soluble (second fraction) asphaltenes. The whole (40:1 n-pentane-to-crude oil) asphaltenes were also precipitated for comparison. The asphaltenes were tested with regard to their solvent properties using interfacial tension with the pendant drop technique and precipitation onset by near-infrared (NIR) detection. Small angle neutron scattering (SANS) was performed on the first and second fractions. The pre-assumption was that the two solubility fractions and the whole asphaltenes precipitated from the crude oils would behave differently with regard to the solvent properties tested by the abovementioned techniques. 2. Materials and Methods 2.1. Materials. One crude oil from West Africa (WA) and two from the North Sea (NS-A and NS-B) were used in these experiments. The determined amount of asphaltenes and their numbering used in previous publications19-21 for the three oils are presented in Table 1. (14) Hu, Y. F.; Guo, T. M. Effect of temperature and molecular weight of n-alkane precipitants on asphaltene precipitation. Fluid Phase Equilib. 2001, 192, 13-25. (15) Trejo, F.; Centeno, G.; Ancheyta, J. Precipitation, fractionation and characterization of asphaltenes from heavy and light crude oils. Fuel 2004, 83, 2169-2175. (16) Wattana, P.; Fogler, H. S.; Yen, A.; Garcia, M. D.; Carbognani, L. Characterization of polarity-based asphaltene subfractions. Energy Fuels 2005, 19, 101-110. (17) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. Aggregation and solubility behaviour of asphaltenes and their subfractions. J. Colloid Interface Sci. 2003, 267, 178-193. (18) Groenzin, H.; Mullins, O. C. Molecular size of asphaltene solubility fractions. Energy Fuels 2003, 17, 498-503. (19) Fossen, M.; Hemmingsen, P. V.; Hannisdal, A.; Sjoblom, J. Solubility parameters based on IR and NIR spectra: I. Correlation to polar solutes and binary systems. J. Dispersion Sci. Technol. 2005, 26, 227241. (20) Hannisdal, A.; Hemmingsen, P. V.; Sjo¨blom, J. Group-type analysis of heavy crude oils using vibrational spectroscopy in combination with multivariate analysis. Ind. Eng. Chem. Res. 2005, 44, 1349-1357.

Energy & Fuels, Vol. 21, No. 2, 2007 1031 Table 1. Numbering and Total Asphaltene Content (wt %) in Previous Studies20 for the Three Crude Oils crude oil

WA

NS-A

NS-B

numbering asphaltene content (wt %)

7 1.7

9 1.2

10 0.8

n-Pentane used as a precipitant was analytical reagent from Rhone/Poulenc. n-Heptane used in the precipitation onset tests were HPLC grade from Merck. Water used in the IFT measurements was pH7 buffered water from Fluka added 3.5 wt % NaCl p.a. from Merck, and toluene was HPLC grade from Merck. In the SANS studies, the solvent was toluol-d8 99.8 atom % D from Dr. Glaser AG Basel and toluene-d8 99.6 atom % D from Isotech Inc. All experiments were performed at room temperature (23 °C) unless otherwise is mentioned. 2.2. Precipitation of the Asphaltenes. As a first test, the crude oil and n-pentane were mixed in a 1:1 volume ratio and left for 24 h on a shaker. The solution was filtrated using vacuum through a 0.45 µm Millipore filter. This ratio did not produce any detectable precipitation of asphaltenes upon filtration. The n-pentane ratio was increased to 3:1 volumes of n-pentane/crude oil, which led to detectable precipitation of asphaltenes after filtration. This fraction was named the first fraction. The second fraction was obtained by adding n-pentane to the filtrated oil/pentane mixture from the first precipitation in a total solvent-to-crude oil ratio of 18:1. This mixture was left on a shaker for another 24 h before filtration. The filtration procedure was performed by adding small volumes of n-pentane which was flushed through the filter cake until the liquid flowing through was colorless. The asphaltene fractions were dried and stored under N2 atmosphere. The whole asphaltenes were obtained from 40:1 n-pentane-to-crude oil solutions and stirred, filtrated, and dried using the same procedure as for the first and second fractions. 2.3. Onset of Precipitation in n-Heptane/Toluene Mixtures Using FT-NIR at 1600 nm. The precipitated asphaltene fractions were dissolved in toluene before n-heptane was added. Heptane was chosen in order to reduce errors due to vaporization when using pentane. The concentration of asphaltenes with regard to solvent was 2 mg/mL, and the n-heptane/toluene ratios were 0/100, 20/80, 50/50, 60/40, 70/30, 80/20, 90/10, and 100/0. The samples were sonicated for 5 min and left on a shaker for 24 h. Onset of precipitation was detected using near-infrared light at 1600 nm and looking at the baseline elevation due to scattering by particles. The instrument used for recording the NIR spectra was the multi-purpose analyzer (MPA) from Bruker Optics. 2.4. Interfacial Tension Studies. A pendant drop method was used to determine the interfacial tension (IFT) of toluene (containing dissolved asphaltenes) toward water, as a function of time. The aim was to compare the influence the asphaltenes had on the interfacial tension. The IFT studies were performed on a CAM 200 from KSV Instruments. The instrumental setup consisted of a syringe containing the oil phase and a water bath with a 3.5 wt % NaCl, pH 7 buffered water. The syringe was used to push the oil phase into the water phase, resulting in an oil droplet hanging upward from a needle in the water phase. The syringe is closed with a valve to prevent the droplet from changing volume. A camera is used to record pictures of the droplet. From the shape of the droplet and the densities of the oil and water, the interfacial tension can be calculated by solving the Laplace equation describing mechanical equilibrium under capillary and gravity forces.22 The oil phase was 100 mg/L asphaltenes dissolved in toluene. The first 1000 pictures were recorded with an interval of 1 s, and the next 740 pictures were recorded each minute (i.e., the duration of the pendant drop experiments was approximately 12.5 h). The results (21) Hemmingsen, P. V.; Silset, A.; Hannisdal, A.; Sjo¨blom, J. Emulsions of heavy crude oils. I: Influence of viscosity, temperature, and dilution. J. Dispersion Sci. Technol. 2005, 26, 615-627. (22) Jeribi, M.; Almir-Assad, B.; Langevin, D.; He´naut, I.; Argillier, J. F. Adsorption kinetics of asphaltenes at liquid interfaces. J. Colloid Interface Sci. 2002, 256, 268-272.

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Fossen et al.

Table 2. Percent Weight of Asphaltenes Obtained from the Whole and Two-Step Precipitation Procedures

oil

wt % first fraction (3:1)

wt % second fraction (18:1)

sum wt % first and second fractions

wt % whole fraction (40:1)

WA NS-A NS-B

0.8 1.0 0.4

0.9 0.8 0.5

1.7 1.8 0.9

2.0 1.8 1.6

of these experiments can be presented as a plot of the interfacial tension versus time. The interfacial tension of the toluene toward the water phase was also determined. 2.5. Small Angle Neutron Scattering (SANS). The first and second fractions of asphaltenes were analyzed on the SANS installation at the IFE reactor at Kjeller, Norway. The wavelength was set by the aid of a selector (Dornier), using a high fwhm for the transmitted beam (∆λ/λ ) 20%) and maximized flux on the sample. The neutron detector was a 128 × 128 pixels, 59 cm active diameter, 3He-filled RISØ-type detector, which is mounted on rails inside an evacuated detector chamber. The temperature at which the experiments were performed was 23 °C. The investigated scattering vector q-range was defined by the neutron wavelengths λ ) 10.2 and 5.1 Å and the sample-to-detector distances D ) 3.4 and 1.0 m, covering the experimental q-range 8‚10-3 e q e 0.3 Å-1. The scattering vector q is given by q)

θ 4π sin λ 2

()

(1)

where the scattering angle is 2θ. The samples were investigated in 2 mm Starna quartz cells using 1.0 wt % asphaltenes in deuterated toluene. Toluene-d8 was used as the solvent in order to minimize incoherent scattering and to maximize the scattering contrast between the asphaltenes and the solvent (scattering length density, Ftoluene-d8 ) 5.20‚1010 cm-2). Standard reductions of the scattering data, including transmission corrections, were conducted by incorporating data collected from empty cell, beam without cell, and blocked-beam background in order to obtain the coherent macroscopic scattering cross section dΣ/dΩ (q) of the system and thus intensities on an absolute scale (cm-1). Scattering intensity versus scattering angle, I(q) vs q, curves were fit to the Guinier approximation (eq 2): I(q) ) I0 exp(-q2RG2/3)

(2)

where I0 is the scattering intensity extrapolated to q ) 0, and RG is the radius of gyration.5 The Guinier approximation was applied by fitting a straight line through a plot of ln (I) versus q2 in the range of 0.02 Å < q < 0.08 Å.

3. Results 3.1. Asphaltene Precipitation. The amount of precipitated asphaltenes was compared with the amount obtained by using hexane in connection with the SARA analysis of the same oils (Table 2).20 In the present study the sum of the weight percents for the first and second fractions are comparable with the previous results for the WA and the NS-B asphaltenes but is larger for the NS-A asphaltenes. The amount of whole asphaltenes precipitated using n-pentane is a little higher for the WA and the NS-A asphaltenes as compared to the amount obtained with hexane. For the NS-B asphaltenes the amount precipitated was doubled using pentane as a precipitant as compared to hexane. 3.2. Onset of Precipitation. Onset of precipitation is plotted as the absorbance at 1600 nm versus the heptane/toluene (H/T) ratio (Figure 1). The first fraction of asphaltenes all has onset points at an H/T ratio of about 50/50. Asphaltenes from NS-A second fraction have an onset point at about 80 vol % n-heptane,

Figure 1. Onset of precipitation at increasing H/T ratio for the first fraction (solid-drawn), second fraction (stippled), and whole fraction (dotted) of the WA, NS-A, and NS-B asphaltenes. For all three crude oils the second fraction is more soluble than both the first and the whole fractions at increasing heptane-to-toluene (H/T) ratio.

while the WA and NS-B second fractions have their onset points at about 70 vol %. The whole asphaltenes have onset points at the same H/T ratio as the first fractions, except for NS-B asphaltenes, which has an onset at higher H/T ratio than the first fraction but still lower than the whole fraction. 3.3. Interfacial Tension. The determined interfacial tension (IFT) for pure toluene toward the water phase was 38 mN/m. When asphaltenes are dissolved in toluene the interfacial tension between toluene and water will in most cases be lowered as asphaltenes adsorb at the interface.13,22-27 IFT of the whole, (23) Sheu, E. Y.; Storm, D. A.; Shields, M. B. Adsorption kinetics of asphaltenes toluene/acid solution interface. Fuel 1995, 74, 1475-1479. (24) Bauget, F.; Langevin, D.; Lenormand, R. Dynamic surface properties of asphaltenes and resins at the oil-air interface. J. Colloid Interface Sci. 2001, 239, 501-508. (25) Poteau, S.; Argillier, J.-F.; Langevin, D.; Pincet, F.; Perez, E. Influence of pH on stability and dynamic properties of asphaltenes and other amphiphilic molecules at the oil-water interface. Energy Fuels 2005, 19, 1337-1341. (26) Silva Ramos, A. C. d.; Haraguchi, L.; Notrispe, F. R.; Loh, W.; Mohamed, R. S. Interfacial and colloidal behaviour of asphaltenes obtained from Brazilian crude oils. J. Pet. Sci. Eng. 2001, 32, 201-216.

Asphaltenes Precipitated by Two-Step Procedure

Energy & Fuels, Vol. 21, No. 2, 2007 1033 Table 3. Parameters Obtained after Fit of the Biexponential Decay Function to the Interfacial Tension Decay Curves

Figure 2. Interfacial tension versus time for the whole (dotted), the first (solid-drawn), and the second (stippled) fractions of the WA, NS-A and NS-B asphaltenes. In all three cases the second fraction reduces the interfacial tension between toluene and water considerably more than the first fraction. The whole fraction resulted in an interfacial tension that was between the first and second fraction.

first, and second fraction of the asphaltenes dissolved in toluene was measured as a function of time (Figure 2). After 12 h the second fractions reduce the interfacial tension to a much greater extent than both the first fraction and the whole fractions do. At the end of the measurement time, the interfacial tension between toluene and water is reduced by 15-20 mN/m for the second fraction, 2-10 mN/m for the first fraction, and about 5 mN/m for the whole fraction as compared with the initial value at the start of the measurements. The shapes of the decay curves were also different when the second fraction was compared with the first and the whole fraction for the three systems. The experimental data were fit to an exponential decay curve to derive the decay constants. A biexponential (four-parameter) decay equation (eq 3) fitted well to the experimental the decay curves:

y ) a exp-bx + c exp-dx

(3)

In eq 3, y is the interfacial tension as a function of time (x). The sum of the constants a and c, known as the initial

asphaltene fraction

a

b

c

d

R2

WA first WA second WA whole NS-A first NS-A second NS-A whole NS-B first NS-B second NS-B whole

4.00E+00 1.67E+01 3.96E+00 1.72E+01 6.23E+00 1.75E+00 3.47E+00 1.47E+01 3.45E+00

4.08E-04 1.93E-04 8.96E-04 6.87E-07 1.84E-04 1.14E-03 1.11E-03 1.54E-04 1.84E-03

3.17E+01 1.84E+01 2.96E+01 1.87E+01 2.85E+01 3.11E+01 2.89E+01 2.07E+01 2.56E+01

1.79E-06 1.63E-06 4.39E-06 6.87E-07 9.15E-06 3.60E-06 2.98E-06 5.29E-06 4.54E-06

0.99 0.99 0.99 0.99 0.99 0.99 0.98 1.00 0.99

population, is the value of the interfacial tension at time 0. The decay constants b and d describe the rate at which the interfacial tension decays. The fast decay is described by decay constant b while the slower decay is described by decay constant d. The curve fitting was performed using SigmaPlot 9.01 (Systat Software, Inc.), and the results of the population constants and the decay constants together with the R2 is shown in Table 3. The shape of the curves will then be described by eq 3 using the constants in Table 3. The contribution from the two terms in the equation was plotted (Figure 3) as a percent contribution to the decay (% decay contribution). 3.4. Small Angle Neutron Scattering. The scattering intensity versus the scattering angle, I(q) versus q for 1 w/w % of the asphaltene fractions from WA, NS-A, and NS-B in deuterated toluene were plotted on a logarithmic scale (Figure 4). The absolute intensities are not strong due to the relatively low concentration of asphaltenes. The absolute values at low q are in comparison with what Gawrys et al. found for 1 wt % asphaltenes in 90:10 d-toluene:d-methanol.28 There is a difference between the first and second fraction of the WA asphaltenes and also for the NS-B fractions at high q values, which means that the samples should be structurally distinguishable by SANS. The breakpoint in the range from 0.04 to 0.08 Å-1 indicates a characteristic particle size, although some larger particles seem to be present since the curves do not flatten out completely at low q. Guinier approximation in the intermediate q range by regression of ln(I) to q2 was used to extract the radius of gyration (RG), disregarding the particle shape. The RG for the samples are presented in Table 4. 4. Discussion 4.1. Asphaltene Precipitation. The choice of precipitant is an important factor when working with asphaltenes. It is wellknown that pentane will precipitate a larger amount of the crude oil than hexane and heptane.3 Furthermore, the asphaltenes precipitated using pentane may be different than those precipitated with lower or higher hydrocarbons and also temperature and pressure may affect the type of asphaltenes precipitated.9 It is usually acknowledged that the use of pentane could coprecipitate more resins than for example heptane, and also using lower proportions of precipitant could co-precipitate resins. The asphaltenes precipitated with a low proportion of n-pentane (3:1 and 18:1) could then contain more resins than whole asphaltenes precipitated with heptane. The choice of the ratio of 3:1 is explained in section 2.2. Furthermore, the use of 18:1 pentane(27) Szabo´, G. H.-.; Masliyah, J. H.; Elliott, J. A. W.; Yarranton, H. W.; Czarnecki, J. Adsorption isotherms of associating asphaltenes at oil/ water interfaces based on the dependence of interfacial tension on solvent activity. J. Colloid Interface Sci. 2005, 283, 5-17. (28) Gawrys, K. L.; Kilpatrick, P. K. Asphaltenic aggregates are polydisperse oblate cylinders. J. Colloid Interface Sci. 2005, 288, 325334.

1034 Energy & Fuels, Vol. 21, No. 2, 2007

Figure 3. Percent decay contribution of the two terms of the exponential decay function for the whole fraction (dotted), first fraction (solid-drawn), and second fraction (stippled) of the asphaltenes. The contributions are calculated (eq 3) using the parameters in Table 3.

to-crude oil was a choice based on the experience from the literature3 that most of the asphaltenes will precipitate at a n-pentane-to-crude oil ratio of 20:1 and that it was the properties of the less soluble fraction as compared to the more soluble that was the main interest of this study. The amount of asphaltenes precipitated in the present work is higher for the whole asphaltenes as compared to the amount precipitated using hexane.20 Furthermore the total amount precipitated by the twostep procedure is also higher for the NS-A asphaltenes but approximately the same for the WA and the NS-B as compared to the hexane (40:1) precipitated asphaltenes. From the amount precipitated, it is recognized that more asphaltenes have been precipitated in this study as compared to the more standard hexane precipitation used in previous SARA fractionations. The procedure used in the present work may have precipitated more resins (or at least some lighter compounds) than what the standard procedure would do. To confront this issue, caution was made to wash the asphaltenes in pentane after precipitation to dissolve pentane-soluble constituents. The asphaltene fractions should therefore be a result of insolubility at the different solvent conditions. Furthermore, the fractions obtained with the procedure in the present work may contain but also miss constitu-

Fossen et al.

Figure 4. SANS measurements, I(q) vs q. Filled markers represents the first fraction while open markers represents the second fraction. Table 4. Calculated RG from the SANS Measurements Using the Guinier Approximationa asphaltene fraction

RG (Å)

WA first WA second NS-A first NS-A second NS-B first NS-B second

30 25 ND ND 26 21

a The uncertainty of the instrument is (1 Å. ND (not determined) means that we were not able to deduce the RG for the NS-A fraction.

ents that would be part of the asphaltene fraction obtained with a more standard procedure. Still the results from the precipitation onset and interfacial tension measurements clearly show that there are corresponding trends in the results between the asphaltene solubility fractions from the three different crude oils. 4.2. Onset of Precipitation. The first fraction (3:1) precipitates at a lower H/T ratio meaning they are less soluble at increasing n-heptane addition. This is consistent with the fact that these were the asphaltenes that became insoluble in the 3:1 n-pentane/crude oil ratio and thus aggregated and precipitated out from the oil phase. The whole fraction should contain both solubility fractions that are represented by the first and

Asphaltenes Precipitated by Two-Step Procedure

the second fraction. Since the first fraction is less soluble, it should determine the onset of precipitation for the whole asphaltenes. This is indicated by the results (Figure 1) showing that the whole fraction has an onset of precipitation at the same H/T ratio as the first fraction for the WA and NS-B asphaltenes. For the NS-A whole fraction the onset is at 70% n-heptane while the first fraction has an onset at 60%. This may indicate some kind of interaction between the first fraction and the more soluble asphaltenes in which they interact in a way that increases the solubility. One argument for that explanation is the low absorbance obtained for the NS-A whole at the H/T ratios of 90% and 100%, which in this case, may be an indication of relatively smaller aggregates in the solution as compared to the ones obtained from the first and second fractions of NS-A asphaltenes. Generally, light scattering can be used to determine the size of colloid particles such as asphaltenes.29,30 Furthermore, the differences obtained in the apparent absorbance due to scattering may be a result of both the size of the aggregates and the density of particles in the solutions, but the deduction of the size of aggregates from near-infrared light measurements was not performed in the present work. For the WA first fraction, the same explanation for the low absorbance at 100% heptane is not valid. The reason for the reduction in the absorbance for the first fraction of WA is simply that the aggregates formed were so large that they fell to the bottom of the sample container during the measurement. This resulted in a reduced scattering effect and an apparently lower absorbance, even though the aggregates are bigger. The present results are comparable with those of Spiecker et al.17 where asphaltenes were fractionated into soluble and insoluble fractions depending on the solubility in mixtures of heptane and toluene. They found that when introducing the different subfractions into various heptane/toluene mixtures, the soluble fraction (which in our work would be the second fraction) was more soluble than both the whole and the precipitate fractions. The difference in the phase transition point, from soluble to insoluble, between the first and the second fractions must be due to some physical or chemical properties. Solubility of asphaltenes can be governed by aromaticity, size, polarity, and the chain length and structure of substituted alkyls. In order to determine which of these parameters and to what extent each parameter contributes to the difference in solubility, additional experiments must be performed. Experiments that can contribute to answer these questions may include proton and carbon nuclear magnetic resonance (NMR) spectroscopy, molecular weight analysis, and elemental analysis of the fractions. 4.3. Interfacial Tension. The ability of asphaltenes to reduce the interfacial tension (IFT) between toluene and water, both as a function of concentration and in dynamic studies where the reduction in surface tension as a function of time is monitored, has been reported.13,22-27,31,32 Molecular weight has a significant effect on the surface tension. An increase in molecular weight increases the surface tension both for normal and cyclic alkanes and for aromatic compounds.3 The results of the present study shows that the second fraction has, after 10 000 s, reduced the interfacial tension between toluene and the water phase to a greater extent as compared to the first and (29) Zimm, B. H.; Dandliker, W. B. Theory of light scattering and refractive index of solutions of large colloidal particles. J. Phys. Chem. 1954, 58, 644-648. (30) Mullins, O. C. Asphaltenes in crude-oil-absorbers and or scatterers in the near-infrared region. Anal. Chem. 1990, 62, 508-514. (31) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. Interfacial properties of asphaltenes. Fuel 1992, 71, 1277-1281. (32) Xu, Y. M. Dynamic interfacial-tension between bitumen and aqueous sodium-hydroxide solutions. Energy Fuels 1995, 9, 148-154.

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the whole fractions, and considerably more than both the first and the whole fractions after 12 h. Important parameters that influence the interfacial activity are the molecular size and the amphiphilic nature of molecules present in the fractions. For a molecule to exhibit amphiphilic properties it must contain both polar (water soluble) and nonpolar (oil soluble) constituents. The polarity of asphaltenes is a result of the functional groups containing heteroatoms (N, S, and O) while the oil-soluble part is determined by the aliphatic and also the aromatic constituents. Based on the results from the IFT measurements, the second fraction may be more amphiphilic and of lower molecular weight. If the polarity and amphiphilic properties are of main importance to the interfacial activity, the second fraction may contain heteroatoms and functional groups that increase the polarity of the polar part, and it may also have longer chain lengths for the substituted alkyl groups relative to the aromatic portion of the asphaltenes. If the size of the molecules, represented by the molecular weight, is the main cause for the different interfacial activities, then the first fraction (the less soluble) should consist of molecules with higher molecular weight. That the less soluble fraction will be of higher molecular weight has in fact been reported by Groenzin et al.18 using fluorescence depolarization. Furthermore, it has been reported that longer alkyl groups are assumed to help stabilize the asphaltenes in solution due to steric repulsion.33 The second fraction could then be assumed to have greater stability upon addition of n-alkane due to a lower molecular weight and relatively longer alkyl groups. One factor, which cannot be confirmed in the present work, is that a higher degree of aromaticity usually is correlated with a higher amount of nitrogen.3 Elemental analysis and molecular weight measurements of the fractions must be performed in order to give an extended interpretation of the results. The initial value of the interfacial tension measurements varies with more than 5 mN/m between the samples. The whole fraction has the lowest initial value, and the second fraction has the highest. The reason for this may be due to the different contributions from the two different mechanisms discussed below. For the whole fraction (which consists of both the first and the second solubility fractions), this may result in a higher initial interfacial activity as compared to the first and second fractions. Furthermore, there is a clear difference in the shape of the decay curve of the second fractions as compared to the first and whole fractions (Figure 2). The first and the whole fractions show a fast decay of the interfacial tension in the first 1000 s after which the decay curve flattens out to a more linear shape. The second fraction has also a rapid (but not as much as for the first and whole fractions) decrease, but the reduction in the IFT continues for a much longer period of time. This indicates that, for the second fraction, the kinetics is more time dependent. Intuitively, for the whole fraction, both the total reduction of the interfacial tension and the kinetics illustrated by the time dependent decay curves should be an average of the contributions from the first and second fractions. For the IFT at the end of the experiments this seems to be valid, but for the kinetics the results give proof to the contrary. To explain this, one must try to understand the mechanisms involved when introducing a droplet of toluene containing dissolved asphaltenes into the water phase. The two major mechanisms that are time dependent are the diffusion of molecules to the w/o interface and reorganization and exchange of molecules at the interface.23 (33) Chang, C. L.; Fogler, H. S. Stabilization of asphaltenes in aliphatic solvents using alkylbenzene-derived amphiphiles .1. Effect of the chemicalstructure of amphiphiles on asphaltene stabilization. Langmuir 1994, 10, 1749-1757.

1036 Energy & Fuels, Vol. 21, No. 2, 2007

The two proposed mechanisms (diffusion and reorganization) can explain the good fit, represented by R2 (Table 3), obtained using the biexponential decay function (eq 3). In the early stage, as the droplet is introduced to the water phase, the diffusion of asphaltenes from the solution to the interface is the main mechanism, which is relatively fast. The IFT of the first period of time is then determined by the molecules with the highest diffusion coefficient and can be attributed to parameter b in the equation for exponential decay (eq 3). As seen from Table 3, the parameter b is smaller for the second fraction of WA and NS-B as compared to the first and whole fractions. For the NS-A this parameter is smallest for the first fraction due to the fact that there is practically no initial fast decay (or the initial decay is so fast that it could not be recognized in the experiment), in fact the decay is more or less linear as can be seen from Figure 3. On the other hand, the second fraction of NS-A has a lower decay constant than the whole fraction, which represents the slower initial decay. From the decay constants (parameter b) and the percent decay contribution (Figure 3), it can be assumed that the diffusion of molecules to the interface is faster for the first and the whole fractions. Furthermore, it would be logical if it was the same type of asphaltene molecules that are responsible for the fast initial decay for the first and the whole fractions since these molecules seem not to be present in the second fraction. The shape of the decay curves for the first and the whole fractions are so similar as compared to the decay curve for the second fractions that one must assume a greater similarity of the molecules responsible for the fast initial decay for the first and the whole fractions. From the discussion above one can state that the initial kinetics determining the interfacial activity of the whole fraction is affected more by the first (less soluble) than the second (more soluble) fraction of asphaltenes. The second of the two major mechanisms responsible for the shape of the decay curve in the IFT versus time plots accounts for organization, aggregation, and packing of the asphaltenes at the interface and also maybe exchange of more interfacial active, slower diffusing asphaltenes with less active but faster diffusing asphaltenes. The whole fraction gives a reduction of the interfacial tension after 12 h, which is an average of the values for the first and the second fractions for all three crude oils. This shows that the second fraction probably plays a part on the overall reduction of the interfacial tension with its larger value for the decay constant d in the second term of the decay equation. Furthermore, the larger decay constant in the second term for the second fraction indicates that the reorganization process is a more active continuous process than for the first and whole fractions of the asphaltenes. The contribution from the two proposed mechanisms is shown in Figure 3 as the percent decay contribution of the two terms. The first term decreases its contribution whereas the second term increases its contribution as a function of time. These results show that the first and the whole fractions have a faster but shorter decay period than the second fraction. For WA and NS-B asphaltenes, the first and the whole fractions have a shorter decay time before they reache “equilibrium”, although the word equilibrium is not correct since the interfacial tension is still decreasing, although slowly. The second fractions, on the other hand, which is more interfacial active, has a longer decay period than the first term and contributes to the interfacial tension for a longer period of time before “equilibrium” is reached. For the NS-A first fraction, the first and second terms of the decay function contribute equally due to the apparently linear decay of the IFT. The second fraction and whole fraction shows similar contributions from the two terms in the decay equation as the second and whole

Fossen et al.

fractions, respectively, of WA and NS-B. The physical meaning of the good fit to the biexponential decay function is probably due to the two mechanisms, diffusion to the interface and organization at the interface, discussed above. The diffusion seemd to be most important for the decay of the interfacial activity for the less soluble (and less interfacial active) fractions while the organization process was more important for the more soluble (and more interfacial active) asphaltenes. Moreover, both diffusion and reorganization should be accounted for when studying adsorption at interfaces. To sum up, the interfacial tension as a function of time is different for asphaltene fractions from the same crude oil. A similar precipitation procedure using four steps of 3:1, 10:1, 15:1, and 20:1 respectively showed that one of the middle solubility fractions gave the largest reduction of the interfacial tension at end of the experiment time.34 Those results strengthen the indications that asphaltenes precipitated from a crude oil with regard to their solubility can hold very different properties and that those properties are not necessarily straight forward to predict. 4.4. SANS Measurements. SANS has been used to elucidate the aggregation sizes of asphaltenes based on assumed particle morphologies in solvents like toluene,35 decalin and 1-methylnaphthalene,36 quinoline,37 and toluene-methanol28 and in mixtures of solvents and non-solvents like toluene-heptane mixtures17 and also at elevated temperatures37 and pressures.38 Mason and Lin39 also showed that it was possible to study asphaltene particles in naturally occurring crude oils without using deuterated solvents. The monodisperse and polydisperse form factors typically applied to asphaltenic aggregates include spheres, cylinders, and ellipsoids. Gawrys et al. found that the model which best described the asphaltene aggregate structure was a flat polydisperse disk model.28 In the present work, the relative size of the aggregates was of interest rather than the form or solubility behavior; therefore, the Guinier approximation was used to calculate the radius of gyration (RG) for the fractions. RG was calculated for the first and second fractions of asphaltenes. The size of the aggregates formed (Table 4) was larger for the first fraction as compared to the second fraction for all three systems tested. The RG values calculated from the SANS data were somewhat lower as compared with values reported elsewhere5,38,40 for asphaltenes in toluene. which depending on the model used are in the range of 43 to 69 Å. Values lower than these have been reported for asphaltenes in solvents other than toluene.5,37 The plot of the I(q) versus q (Figure 4) is very different for the WA and NS-B asphaltene fractions as compared with the NS-A fractions. At low q values the NS-A asphaltenes have a scattering intensity which is 10 times higher than the WA and NS-B fractions, indicating that very large aggregates (34) Fossen, M.; Kallevik, H.; Jakobsson, J.; Sjoblom, J. A new procedure for direct precipitation and fractionation of asphaltenes from crude oil. J. Dispersion Sci. Technol. 2007, 28, 1-4. (35) Roux, J. N.; Broseta, D.; Deme, B. SANS study of asphaltene aggregation: concentration and solvent quality effects. Langmuir 2001, 17, 5085-5092. (36) Sirota, E. B. Physical structure of asphaltenes. Energy Fuels 2005, 19, 1290-1296. (37) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; et al. Aggregates structure analysis of petroleum asphaltenes with smallangle neutron scattering. Energy Fuels 2003, 17, 127-134. (38) Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Effects of temperature and pressure on asphaltenes agglomeration in toluene. A light, X-ray, and neutron scattering investigation. Energy Fuels 2004, 18, 1243-1249. (39) Mason, T. G.; Lin, M. Y. Asphaltene nanoparticle aggregation in mixtures of incompatible crude oils. Phys. ReV. E 2003, 67. (40) Fenistein, D.; Barre, L.; Broseta, D.; Espinat, D.; Livet, A.; et al. Viscosimetric and neutron scattering study of asphaltene aggregates in mixed toluene/heptane solvents. Langmuir 1998, 14, 1013-1020.

Asphaltenes Precipitated by Two-Step Procedure

are formed. There is no flattening out of the signal at low q values, meaning that a significant amount of the particles present in the NS-A system were larger than the measurement limit, which is around 700 Å. The very large aggregates for the NS-A can be explained by the existence of a critical aggregation concentration CAC (also referred to as critical micelle concentration, cmc)41 of asphaltenes in toluene. Since the concentration used is relatively high (1 g/L), it is not surprising that some asphaltenes from certain crude oils may be above the CAC at this concentration. One might think that this could be related to the precipitation onset experiments, and that asphaltenes that has a low CAC in toluene should precipitate at a lower H/T ratio. This may not be the case since there are different mechanisms, not studied here, that influence the aggregation processes as described in Yudin et al.41 Although the SANS data for the NS-A fractions did not give a plateau from which the RG could be calculated, the first fraction scatters more than the second fraction at low q values. This indicates that the first fraction forms aggregates that are larger than those present in the second fraction, showing that the results are at least qualitatively comparable with the WA and NS-B asphaltenes. As discussed above, the molecular weight as well as size may have a impact on the interfacial tension. Although the concentration in the SANS measurements were 10 times those of the IFT measurements, the asphaltene clusters or aggregates may affect the interfacial activity. If the size of the molecules or aggregates is supposed to be of importance to the behavior of the asphaltenes and thus the interfacial activity, then there is a consistency in results from the IFT measurements. The second fraction, which forms smaller aggregates, also results in the highest reduction of the interfacial tension between toluene and water. On the other hand, the differences in the RG values between the fractions are not very large and there may still not be comparable differences in the molecular weight of the asphaltene molecules with the calculated RG values. Therefore, no conclusions were still drawn if the molecular size affected the interfacial tension results. 5. Conclusions Asphaltenes were obtained using a two-step precipitation procedure on three different crude oils. The procedure consisted of a first precipitation using 3:1 volumes of n-pentane-to-crude oil followed by filtration and further addition of n-pentane at an 18:1 ratio to precipitate the second fraction. Whole asphaltenes were precipitated using n-pentane with a volume ratio of 40:1 to the crude oils. The asphaltenes were compared with regard to precipitation onset in H/T mixtures, interfacial tension of the asphaltenes dissolved in toluene toward a pH 7 buffered (41) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Markhashov, E. L.; Agayan, V. A.; et al. Crossover kinetics of asphaltene aggregation in hydrocarbon solutions. Phys. A (Amsterdam) 1998, 251, 235-244.

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water phase containing 3.5 wt % NaCl, and SANS measurements to determine the radius of gyration for the aggregates formed in toluene. The amount precipitated were larger as compared to earlier precipitations on the same crude oils using hexane (40:1). The second fraction, which does not contain the most insoluble asphaltenes, was more soluble at increasing H/T ratios than both the first and the whole fractions. This showed that the solubility behavior of the asphaltenes precipitated from the crude oils could be reproduced in a model system consisting of heptane and toluene. The second fraction reduced the interfacial tension between the toluene and the water phases to a greater extent than both the first and the whole fractions did. Moreover, the interfacial tension studies revealed different shapes of the decay curves of the second fractions as compared to the first and the whole fractions, which had similar shapes of the decay curves. It was assumed that the most insoluble asphaltenes determined the initial stage of the adsorption process for the whole fraction. The adsorption process includes both diffusion to the interface and organization of molecules at the interface. Furthermore, it was shown that the second fraction contributed to the overall reduction of the interfacial tension for the whole fraction, measured after 12 h. The interfacial tension did not reach complete equilibrium even after 12 h. The decay curves were fit to a biexponential decay function, and the percent contribution of the two terms were plotted. It was showen that the first fraction has a faster and shorter decay period than the second fraction of the precipitated asphaltenes. The good fit by the biexponential decay function could be due to the time dependence of the decrease in the interfacial tension represented by diffusion of molecules to the interface and organization at the interface. The SANS measurements were performed to determine aggregate sizes by calculating the RG for the two fractions from each oil. For the WA and NS-B the calculated radius of gyration from the scattering intensities versus the scattering angle showed that the first fraction formed larger aggregates than the second fraction. This was also the case for the NS-A asphaltene fractions, although the sizes could not be determined since there was no flattening out of the signal at low q values, meaning that a significant amount of the particles present in the NS-A system were larger than the measurement limit, which was around 700 Å. Acknowledgment. The authors are grateful for the financial contributions from the following partners: Statoil R&D Centre and the members of the Joint Industrial Project on “Particle-Stabilized Emulsions/Heavy Crude Oils” including Statoil ASA, Vetco Aibel AS, Shell Global Solutions, Aker Kvaerner, BP, Champion Technologies, Chevron Texaco, Norsk Hydro, Petrobras, Total, ENI Technology, and Maersk Oil&Gas. EF060311G