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Properties of Asphaltenes Precipitated with Different n-Alkanes. A Study To Assess the Most Representative Species for Modeling† José A. Calles,*,‡ Javier Dufour,‡ Javier Marugán,‡ José Luis Peña,‡,§ Raúl Giménez-Aguirre,‡ and Daniel Merino-García§ URJC-Repsol-YPF Flow Assurance Laboratory, Department of Chemical and EnVironmental Technology, ESCET, UniVersidad Rey Juan Carlos, C/ Tulipán s/n, 28933 Móstoles, Madrid, Spain, and Centro Tecnológico Repsol-YPF, Carretera N-V Km 18, 28931 Móstoles, Madrid, Spain ReceiVed July 13, 2007. ReVised Manuscript ReceiVed September 21, 2007
This work deals with the study of asphaltene precipitation from a +190 °C residue at different temperatures, using several n-alkanes and n-alkane/oil ratios. Experiments were carried out at the boiling point of the solvent (IP-143 standard) and at other temperatures in order to elucidate if asphaltene yields are conditioned by the nature of the solvent and/or by the boiling temperature of the alkanes. The results show that lower asphaltene yields were obtained when using solvents with higher molecular weights at the same temperature, but also when using higher temperatures with the same solvent. Consequently, both effects that simultaneously influence asphaltene precipitation could be conveniently separated. Preliminary characterization of the asphaltenes obtained at different conditions has been carried out by means of NMR spectroscopy and elemental analysis. Further characterization has focused on the particle size distributions obtained during kinetic tests of asphaltene flocculation by a laser reflectance technique (focused-beam reflectance measurement, FBRM): significant differences have been obtained, all related to the n-alkane and ratio used in the flocculation.
Introduction The production of crude oil from a reservoir is by no means an easy task. One of the greatest challenges operators may face during the life of a field is the handling of solids. These solids range from inorganic material, such as salts and sand, to organic material that comes from the crude oil (waxes, asphaltenes, hydrates, and naphtenates). Apart from sand, the rest of the solids usually precipitate due to changes in composition (commingling of fluids), temperature, or pressure. It is therefore of paramount importance to proceed to the determination of the solid–liquid equilibrium lines in the phase diagram of a crude oil. In this way, whenever possible, the production can be kept outside the regions where solids are formed. If this is unavoidable, the proper measures to avoid deposition have to be applied. These measures may be mechanical (periodic removal of deposits1), chemical (addition of inhibitors2 or solvents3 to retard or inhibit flocculation), or thermal (insulation and active heating4). Of all solids that can precipitate out of petroleum, asphaltenes are probably the ones with the worst ratio between effort and † Presented at the 8th International Conference on Petroleum Phase Behavior and Fouling. * Corresponding author. Tel.: +34-914887378. Fax: +34-914887068. E-mail:
[email protected]. ‡ Universidad Rey Juan Carlos. § Centro Tecnológico Repsol-YPF. (1) Esmaeilzadeh, F.; Mowla, D.; Asemani M. In Proceedings of the 2006 SPE Annual technical Meeting; SPE (Society of Petroleum Engineers): Richardson, TX, 2006; SPE Paper 102049. (2) Bello, O.; Fasesan, S.; Teodoriu, C.; Reinicke, K. Pet. Sci. Technol. 2006, 24 (2), 195–206. (3) Voloshin A. I.; Ragulin, V. V.; Telin, A. G. Addition of Solvents. In Proceedings of the SPE International Symposium on Oilfield Chemistry; SPE (Society of Petroleum Engineers): Richardson, TX, 2005; SPE Paper 93128. (4) Sarmento, R.; Ribbe, G.; Azevedo, L. Heat Trans. Eng. 2004, 25 (7), 2–12.
knowledge. The fact that a website was created in 2004 to discuss the standardization of asphaltenes5 and recently published works6 shows how far the scientific community is from wrapping up the description of the behavior of asphaltenes. As a first approach, asphaltenes can be described as the fraction of crude oil that comprises the most polar components of crude oil, as they concentrate the majority of the heteroatoms (N, O, and S)7 and the metals (mainly Fe, Ni, and V).8 Asphaltenes combine this polarity with a high molecular weight and also a high aromaticity (carbon to hydrogen ratio around 1.0 or 1.2).1 They are a nuisance, not only by their possible segregation as solids, which may cause formation damage and problems such as the reduction in flow rates and pipe blocking, but also indirectly, by the stabilization of emulsions.9 Asphaltenes usually come out of solution during depressurization:10,11 as the crude oil is produced, its pressure decreases, and this leads to an increase in molar volume. This decreases the solvent power of the crude oil toward asphaltenes, reaching a point where they are no longer stable in solution and asphaltenes start to flocculate and precipitate. Once the pressure drops below the bubble point, gas is evolved, the solvent power of the remaining fluid is increased, and asphaltenes are mostly solubilized again. Therefore, (5) http://www.ualberta.ca/dept/chemeng/asphaltenes. (6) Creek, J. L. Energy Fuels 2005, 19 (4), 1212–1224. (7) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999. (8) Yen T. F. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; p 6. (9) Stark, J. L.; Asomaning, S. Energy Fuels 2005, 19 (4), 1342–1345. (10) Jamaluddin, A. K. M.; Nighswander, J.; Joshi, N. In Proceedings SPE Annual Technical Conference and Exhibition; SPE (Society of Petroleum Engineers): Richardson, TX, 2001; SPE Paper 71546. (11) Thou, S.; Ruthammer, G.; Potsch, K. In Proceedings 13th SPE European Petroleum Conference; SPE (Society of Petroleum Engineers): Richardson, TX, 2002; SPE Paper 78321.
10.1021/ef700404p CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2007
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most asphaltene problems are observed in the areas where pressures are near the bubble pressure. Giving the difficulties in obtaining live oil samples and working with pressure, asphaltenes are usually obtained in the laboratory by a different method. Starting from stabilized stock tank oil (STO), a bad solvent for asphaltenes (i.e., n-alkane) is added in excess to simulate the decrease in solvent power of the crude oil and to flocculate asphaltenes. Laboratory asphaltenes are loosely defined by the IP-143 standard12 as a solubility class: The fraction contains the material that is insoluble in n-heptane and soluble in toluene, at their respective boiling points at atmospheric pressure. The characterization of this fraction is hampered by the limitations of most analytical techniques, as they have to deal with substances that tend to aggregate in most solvents,13 do not possess a significant crystallinity,14 are difficult to vaporize, have a wide melting range,15 and tend to adsorb in all packing material. Asphaltene characterization is further complicated by the fact that hardly anybody presents results where asphaltenes are obtained following the standard: Many research groups refer to a “modified IP-143”, in which, for instance, asphaltene precipitation is not carried out at the boiling point of the n-alkane16 or the washing is not carried out with solvent reflux.13 As pointed out by Alboudwarej et al.,17 the washing of asphaltenes is a critical stage, and doing it in different ways leads to significantly different asphaltene properties. There are also other standards,18 but the main characteristics of the method in IP-143 are kept. The absence of an agreement on this topic greatly hinders the development of asphaltene science. It has to be remembered that the focus on asphaltenes comes from the need to assess the potential asphaltene deposition in reservoir, piping, or facilities during the whole life of a crude oil field. Asphaltene characterization is needed to obtain the properties of the asphaltene pseudocomponents used in the thermodynamic model that provides the asphaltene phase envelope. After the discussion presented above, several questions come to mind: - Is it possible to utilize property values available in the literature, even if they have been obtained with a different procedure to separate asphaltenes? - Can the average characterization of laboratory asphaltenes be useful to describe pressure-drop asphaltenes? Can all asphaltenes be considered the same? Is it convenient to base the precipitation modeling in one single pseudocompound? The first question refers to the importance of the standard used to obtain asphaltenes. The influence of the n-alkane used has been already extensively reported,19,20 as well as the effect (12) Asphaltene (Heptane Insolubles) in Petroleum Products, IP 143/ 90. Standards for Petroleum and Its Products; Institute of Petroleum: London, 1985; p 143.1. (13) Merino-Garcia, D. Calorimetric investigation of asphaltene selfassociation and interaction with resins. Ph.D. Thesis, Technical University of Denmark, Department of Chemical Engineering: Lyngby, Denmark, 2004. (14) Andersen, S. I.; Jensen, J. O.; Speight, J. G. Energy Fuels 2005, 19, 2371–2377. (15) Gray, M. R.; Assenheimer, G.; Boddez, L.; McCaffrey, W. C. Energy Fuels 2004, 18, 1419–1423. (16) Andersen, S. I.; Lira-Galeana, C.; Stenby, E. H. Pet. Sci. Technol. 2001, 19 (3 and 4), 457–467. (17) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadeh, K. Energy Fuels 2002, 16 (2), 462–469. (18) ASTM D-6560. Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products; ASTM: West Conshohocken, PA, 2000. (19) Stachowiak, C.; Viguié, J. R.; Grolier, J. P. E.; Rogalski, M. Langmuir 2005, 21 (11), 4824–4829. (20) Gonzalez, G.; Sousa, M. A.; Lucas, E. F. Energy Fuels 2006, 20 (6), 2544–2551.
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of temperature,21,22 but the fact that asphaltene separation with different solvents is carried out at different temperatures (the boiling point of the selected n-alkane) has been given less importance. What has more influence on the characteristics of the separated asphaltenes? Is it the n-alkane used or the fact that each separation is done at the boiling point of the solvent? Does temperature have a significant impact on asphaltene average properties or only on the total yield? This has important implications on the use of properties of asphaltenes obtained by other research groups. Regarding the differences between laboratory and pressuredrop asphaltenes, the main question is to see whether asphaltenes keep their average properties during the sequence of precipitation with an n-alkane or not. The idea is to assume that, given that in both cases the causes of separation are a decrease in solvent quality, the first asphaltenes that come out of solution are equal in both cases. When using an n-alkane, the fact that an excess of solvent is used to separate large amount of solids may affect the average properties; if this is the case, average properties obtained in the laboratory may not be realistic for the asphaltene pseudocomponents that are used to model the behavior with pressure. Regarding the n-alkane/oil ratio, the question that arises is whether the first asphaltenes separated are equal to the last or not. This has important implications for the modeling, as a positive answer to this question would imply that one average “pseudocompound” changing concentration between two phases would be enough to model asphaltene separation. Most of the commercial software packages available to simulate asphaltene deposition use a single pseudocompound approach to define asphaltenes. In case the answer is negative, a distribution of molecules would be needed to describe the process of asphaltene separation, as different asphaltene molecules would be separating while precipitation is progressing. To answer these questions, this article analyzes the properties of laboratory asphaltenes separated in different ways; the factors studied are the temperature, the n-alkane used for precipitation, and the n-alkane/oil ratio. Apart from the use of the traditional elemental analysis23,24 and NMR,25 this article explores the particle size distributions after the flocculation of asphaltenes, which is a very important issue that has not been given enough attention so far26,27 and has mainly been studied in model systems of asphaltenes/toluene/heptane.28–30 In this work, asphaltene aggregation kinetics in n-alkane/crude oil is reported in terms of particle size distribution. The evolution of size with time has given more insight on the impact of using different (21) Andersen, S. I.; Birdi, K. S. Fuel Sci. Tech. Int. 1990, 8 (6), 593– 615. (22) Correra, S.; Donaggio, F.; Capuano, F.; Onorati, N. In Proceedings 4th Petroleum Phase BehaVior and Fouling Conference; Sjöblom, J., Ed.; Trondheim, Norway, 2003. (23) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Energy Fuels 2006, 20 (2), 705–714. (24) Buenrostro-González, E.; Andersen, S. I.; García-Martínez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16 (3), 732–741. (25) Seidl, P. R.; Chrisman, E. C. A. N.; Carvalho, C. C. V.; Leal, K. Z.; Menezes, S. M. C. de. J. Disp. Sci. Tech. 2004, 25 (3), 349353. (26) Jamaluddin, A. K. M.; Nighswander, J.; Joshi, N.; Calder, D. In Proceedings SPE Asia Pacific Oil and Gas Conference and Exhibition; SPE (Society of Petroleum Engineers): Richardson, TX, 2002; SPE Paper 77936. (27) Ibrahim, H. H.; Idem, R. O. Energy Fuels 2004, 18 (4), 1038– 1048. (28) Rastegari, K.; Svrcek, W. Y.; Yarranton, H. W. Ind. Eng. Chem. Res. 2004, 43 (21), 6861–6870. (29) Permsukarome, P.; Chang, C.; Fogler, H. S. Ind. Eng. Chem. Res. 1997, 36 (9), 3960–3967. (30) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Ind. Eng. Chem. Res. 2005, 44 (1), 75–84.
Properties of Asphaltenes with Different Alkanes
procedures for asphaltene separation. To the best of the authors’ knowledge, this is the first time that a FBRM technique has been reported as being applied to asphaltenes. As described below, this powerful technique allows a detailed analysis of the evolution of particle size with time in the range 0.5–1000 µm. Experimental Section Chemicals and Materials. A 190+ residue of a South American crude oil (approximately 31 °API) has been used. This oil is characterized by a high content of asphaltenes. Moreover, this oil is unstable from the point of view of asphaltene precipitation. The reason to use the residue instead of the complete crude oil is to avoid problems of light-end losses while working at high temperatures. Additionally, the use of a residue has allowed a greater amount of asphaltenes to be obtained for characterization. n-Pentane reagent grade (density 0.63 g/ cm3) was obtained from Scharlau and was 99% pure. n-Hexane HPLC grade (density 0.66 g/ cm3) was obtained from Scharlau and was 99% pure. n-Heptane (density 0.68 g/ cm3) was obtained from SDS and was 99% pure. Toluene reagent ACS grade (density 0.86 g/ cm3) was obtained from Riedelde Haën and was 99.9% pure. Asphaltene Separation. The asphaltene separation was carried out according to the IP-143 standard.12 When the temperature effect was studied, asphaltene deposition was performed in a thermostatic bath where the flasks were placed and magnetically stirred for 75 min. The rest of the stages were the same as in IP-143. Elemental Analysis. Measurements of elemental HCNS of asphaltene samples were carried out in a Vario EL III Elementar Analysensysteme GmbH. A 3 mg portion is combusted at 1150 °C with an oxygen flow of 65 mL/min. Temperature reduction is set to 850 °C. NMR. 1H NMR spectra were recorded on a Varian Mercury Plus operating at 400 MHz. The samples were dissolved in deuterated chloroform with 1% tetramethylsilane as internal standard. This technique provides the hydrogen distribution in the molecules. It is able to distinguish between aromatic hydrogen (Har) and aliphatic hydrogen in the different positions in a branch (HR, Hβ, and Hγ). According to Seidl et al.,25 the displacement intervals applied to identify the different hydrogens are as follows: 0.5–1 for Hγ, 1.0–2.0 for Hβ, 2.0–4.0 for HR, and 6.0–9.0 for Har. Particle Size Distribution. In order to assess the particle size distribution, a focused-beam reflectance measurement system FBRM M500LF LASENTEC manufactured by Mettler Toledo (USA) was used. The FBRM instrument operates by scanning a highly focused laser beam at a fixed speed across particles in suspension. As the solution is stirred, particles move along with the fluid. When a particle crosses the area where the beam is focused, it reflects part of the light. The measurement of the duration of the backscattered light from these particles renders a characteristic measurement of the particle geometry, i.e. a chord. More details of the application of this technique in flocculation processes can be found elsewhere.31 Samples were tested by placing the required amount of oil and solvent to achieve the desired n-alkane/oil ratio (R) using 100 mL of total solution. At zero time, the n-alkane is added and the chord length distributions are recorded every 2 s in 90 channels of sizes from 1 to 1000 µm following a geometric progression of ratio 101/30 ∼ 1.0798. The raw outcome of the FBRM is the chord length distribution (CLD), which is influenced not only by the particle size but also by its shape. To estimate the actual particle size distribution (PSD), the inversion of a model that gives the probability of every chord length as function of the particle size is required. This model is called PSD-CLD and allows the estimation of the CLD corresponding to a given PSD. In order to do so, a shape factor is needed. In this work a CLD-PSD model reported by Hukkanen and Braatz32 (31) Blanco, A.; Fuente, E.; Negro, C.; Tijero, J. Can. J. Chem. Eng. 2002, 80, 734–740. (32) Hukkanen, E. J.; Braatz, R. D. Sens. Actuators 2003, 96, 451–459.
Energy & Fuels, Vol. 22, No. 2, 2008 765 Table 1. Asphaltene Separated Weight Percent Obtained with R ) 10 mL n-alkane/g sample T ) 25 °C
T ) 36.5 °C
T ) 50 °C
n-alkane
% asp
StD
% asp
StD
% asp
StD
n-C5 n-C6 n-C7
3.89 3.28 2.73
0.15 0.04 0.05
3.85 3.24 2.70
0.41 0.06 0.27
3.21 2.29
0.15 0.10
has been applied by assuming a spherical shape of asphaltene aggregates. According to this model, the transformation of the particle sizes vector, f, to the chord length counts vector, c, can be represented by the following linear equation: c ) Pf (1) Where, P is an upper-triangular matrix defined as follows for midpoint interval distributions:
The jth column of this matrix represents the distribution of chords of the particles in the jth size interval, Dj. Obviously, the probability of producing chords longer than their size is null, and this is the reason why Mij ) 0 for i > j. The inverse matrix P–1 allows estimating of the PSD associated to every experiment. Due to the noise of the response, a five-point smoothing function has been applied in the transformation. For comparison purposes, the distributions have been normalized to the oil concentration, as it varies from test to test. The analysis of these PSDs has been done by calculating the moments of the curves. The nth moment of the distribution is defined as follows:
∫
Mn )
∞
0
Dnf(D) dD
(3)
The zero-moment, M0, represents the integrated area of the distribution, which is proportional to the number concentration of particles in the system. The number mean particle size (Dav,n) is defined by:
∫ Df(D) dD M ) ) ∫ f(D) dD M ∞
Dav,n
0
1
∞
(4)
0
0
Whereas, the size-averaged mean particle (Dav,D) size is defined as:
∫ D f(D) dD M ) ) ∫ Df(D) dD M ∞
Dav,D
2
0
2
∞
(5)
1
0
The ratio between both parameters is usually called the polydispersity (PD) and gives an idea of the width of the distribution: PD )
Dav,D M2M0 ) Dav,n M1
(6)
Results and Discussion The results of asphaltene separation as a function of temperature, ratio (R), and n-alkane are summarized in Table 1 and Figures 1 and 2. They are presented as percentages with respect
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Figure 1. Asphaltene precipitation as function of n-alkane and temperature (R ) 30 mL/g).
Figure 2. Asphaltene precipitation as function of temperature (R ) 30 mL/g). Table 2. HCNS Analysis of Asphaltene Samples in Weight Percent %N exp conditions n-C5, n-C6, n-C7, n-C5, n-C6, n-C7, n-C7, n-C7,
25 °C, R ) 10 mL/g 25 °C, R ) 10 mL/g 25 °C, R ) 10 mL/g 25 °C, R ) 30 mL/g 25 °C, R ) 30 mL/g 25 °C, R ) 30 mL/g 36.5 °C, R ) 30 mL/g 50 °C, R ) 30 mL/g
%C
%S
av
StD
av
StD
av
StD
2.386 2.461 2.469 2.360 2.454 2.464 2.481 2.487
0.020 0.040 0.001 0.008 0.019 0.038 0.015 0.009
86.788 85.655 86.563 86.409 86.368 86.259 86.537 86.619
0.835 0.970 0.33 0.034 0.023 0.211 0.137 0.08
2.110 2.080 2.096 2.077 2.131 2.106 2.194 2.180
0.026 0.09 0.032 0.011 0.043 0.078 0.112 0.021
to the sample, including their standard deviation (StD) after three repetitions of each sample. Except for 25 and 50 °C, tests have been performed at the boiling point of the n-alkanes (36.5, 69, and 98.4 °C for n-C5, n-C6, and n-C7, respectively) for R ) 30 mL/g (Figure 2). No data are available for n-alkanes above their normal boiling points. As expected, the heavier the n-alkane, the lower the percentage of precipitated asphaltenes (Figure 1). This holds for both R values used. As shown in Figure 2 and Table 1, the amount of asphaltenes precipitated tends to decrease with temperature for all n-alkanes, confirming the results obtained by Andersen.21 This decrease is more pronounced for R ) 30 mL/g (Figure 2).
Figure 3. Differences observed in aromaticity when separating asphaltenes at 25 °C.
These observations confirm the fact that the differences in yield obtained with the n-alkanes are not only due to the n-alkane but also to the separation temperature. In addition to this, HCNS analyses (Table 2) show slight variations in heteroatom content (N and S) from sample to sample: regarding nitrogen, there is not an observable relationship with R, but C5asphaltenes have significantly less nitrogen than the others. Sulfur content seems to be unaffected by the method of separation. The main differences in elemental analyses regard the aromaticity of the asphaltenes. As expected, a lighter n-alkane yields less aromatic asphaltenes, as more material is being precipitated. Regarding the ratio of n-alkane, more aromatic asphaltenes (lower H/C) are obtained at greater ratios in the separation with n-heptane, but it remains basically unaltered for the other n-alkanes (Figure 3 and Table 2). Notice that the values of the H/C ratio are very similar, and differences between these values are very small. The standard deviation showed in Figure 3 indicates that elemental analysis measures are reproducible, except for separation with n-heptane and R ) 30 mL/g. Figure 4 displays the 1H NMR spectra of asphaltenes precipitated with different n-alkanes at constant temperature and n-alkane/oil ratio. The analysis of the spectra has been carried out by integrating the areas corresponding to the different kinds of protons according to the chemical shift ranges defined by Seidl et al.25 As it can be seen, the degree of aromaticity of the asphaltenes increases with the molecular weight of the n-alkane. It is also in agreement with the elemental analyses reported in Figure 3. Assuming that the solids precipitated with n-C5 include the solids precipitated with n-C7, this shows that n-pentane precipitates some components with larger aliphatic branches that remain soluble in n-heptane. This is in agreement with previously reported results, which indicate that C5-asphaltenes contain more resinlike material than C7-asphaltenes.13 To provide more insight on the different characteristics of the isolated asphaltenes, aggregation was investigated in a kinetic study. As mentioned above, the FBRM provides the chord length distribution (CLD) of the precipitated material. The particle size distribution (PSD) is obtained by means of eqs 1 and 2. Figure 5 represents the original CLDs obtained for one of the precipitations with n-C7 at different times and the associated PSDs. It is observed that the number of chords increases with time, with a shift to longer lengths. The corresponding PSDs show that, for a certain CLD, the associated
Properties of Asphaltenes with Different Alkanes
Energy & Fuels, Vol. 22, No. 2, 2008 767
Figure 4. 1H NMR spectra of asphaltenes precipitated with different n-alkanes obtained with R ) 30 mL/g at 25 °C.
Figure 5. Transformation of a chord size distribution into a particle size distribution. Tests were with n-C7 and R ) 30 mL/g at 25 °C. Figure 7. Comparison of particle size distributions obtained with n-hexane and three n-alkane/oil ratios at 25 °C. Table 3. Differences in Particle Size Distribution Tests at 25 °C: Effect of R and n-Alkane exp conditions n-C5, n-C5, n-C5, n-C6, n-C6, n-C6, n-C7, n-C7, n-C7, c
Figure 6. Evolution of particle size distribution in flocculation tests with several n-alkanes. Tests were with R ) 10 mL/g at 25 °C.
PSD loses symmetry and shifts to longer sizes. However, the tendency with time is not modified: particles grow and greater sizes are observed at longer times. The particle size distributions of precipitated asphaltenes have been analyzed as a function of the n-alkane (Figure 6) and the n-alkane/oil ratio (Figure 7). Note that all tests have been carried out at constant ambient temperature. The particle distributions were equilibrated after 10 min, so the analysis that follows has been carried out by comparing the PSDs obtained at that time.
R R R R R R R R R
) ) ) ) ) ) ) ) )
3 mL/g 10 mL/g 30 mL/g 3 mL/g 10 mL/g 30 mL/g 3 mL/g 10 mL/g 30 mL/g
M0a
Dav,nb
Dav,Dc
PDd
3303 12739 31869 2414 9394 24609 1511 6793 17651
17.9 44.0 77.9 14.7 22.3 47.8 10.8 16.1 32.8
25.4 77.9 129.7 20.2 32.6 75.7 14.2 21.8 48.0
1.42 1.77 1.67 1.37 1.47 1.59 1.31 1.35 1.47
a Integrated area of the distribution. b Number mean particle size. Size-averaged mean particle. d Polydispersity.
Figure 6 shows the influence of the n-alkane on the PSD: a clear shift to lower particles size can be observed when increasing the molecular weight of the solvent. This is obviously related to the amount of asphaltenes that precipitate: greater yields are associated to greater particle sizes. Accordingly, experiments carried out with greater ratios of n-alkanes lead to PSDs shifted to greater sizes (Figure 7). Table 3 summarizes the values of size parameters after 10 min for the different experiments of precipitation. As expected, (33) Rastegari, K.; Svrcek, W. Y.; Yarranton, H. W. Ind. Eng. Chem. Res. 2004, 43 (21), 6861–6870.
768 Energy & Fuels, Vol. 22, No. 2, 2008
Figure 8. Comparison of the number concentration of particles at 25 °C as a function of time for all the tests.
the results show that the maximum amount of particles and the greatest diameters are obtained when using higher n-alkane/oil ratios and solvents with lower molecular weight. In addition to this, FBRM is also a very useful technique for the analysis of the precipitation kinetics. First of all, it has to be said that all experiments equilibrate very fast: equilibrium in particle size distribution is reached in less than 10 min (Figure 8). To compare the order of magnitude, Rastegari et al.33 have reported longer times (more than 100 min) of equilibration in tests with heptane/toluene mixtures. The explanation for this difference may be found in the fact that asphaltenes probably are more associated in the liquid state in crude oil than in toluene prior to the addition of n-alkane (Figure 8). Figure 8 displays the evolution of M0 with time for the different experiments. In all cases, a fast increase is observed, reaching an equilibrium value in more than 2 min. Kinetic rates are observed to be faster for low-Mw n-alkanes than for highMw n-alkanes, for the three ratios under study: this could be explained by saying that, as more material is precipitated, more nucleation sites are created and the number of particles grows at a greater rate than when n-C7 is used. The same phenomenon is observed for lower R values, although it is difficult to observe this in Figure 8: lower amounts of n-alkane lead to slower kinetics due to the presence of less nucleation sites. On the other hand, the evolution of Dav,n and Dav,D is slightly different, as displayed in Figures 9 and 10, respectively. In both
Calles et al.
cases, an almost constant value is observed for both parameters when using an n-alkane/oil ratio of 3, but a significant decrease of both variables is observed for higher ratios. This phenomenon could be explained by the shear exerted by the stirrer. After an initial quick growth of asphaltene particles, as given by M0, the shear starts to break some of the flocs, leading to lower values of Dav,n and Dav,D. On the other hand, the growth in tests with n-C7 is slower, so the maximum observed for C5 is hardly noticeable. In any case, M0 is observed to remain constant during the variation in average diameter because the changes deal with structural changes, not with the precipitation of new material. The polydispersity of the distributions tends to follow the same trend as M0 (the only exception is R ) 10 mL/g with n-hexane): PD is greater when more particles are created (due to a greater R or a different n-alkane being used). What can be derived from these experiments? First of all, it has been shown that part of the differences in asphaltene yields when using different n-alkanes is due to the fact that the separation is carried out at a different temperature. As expected, relevant differences in aromaticity are observed when varying the n-alkane used for the precipitation. With respect to temperature and ratio, the elemental analyses show some variations in properties, especially in aromaticity, but the differences may not be significant. In connection with this, no conclusive results have been obtained to assess whether asphaltenes precipitated with a low R are equivalent to the ones precipitated with a greater R. However, this work raises doubts about the convenience of using structural data from other research groups that have separated asphaltenes by a different standard than the original IP-143. The question that remains unanswered is which procedure is more realistic, in the quest to separate asphaltenes in the laboratory that are representative of pressure-drop asphaltenes. Consequently, this will have to be addressed in the future, by comparison with asphaltenes separated by pressure drop. Regarding the particle size distributions, the results follow the trends expected in terms of ratio and n-alkane. One additional aspect deserves a comment: a great difference in aggregate size (Figure 7) is observed between tests with R ) 10 and 30 mL/g. Nevertheless, the yields of asphaltenes separated by the standard are similar, with a change from 3.28 to 3.44 wt % in the case of n-hexane when R rises from 10 to 30 mL/g. Similar tends are observed as well for C5- and C7-asphaltenes. It is improbable to think that there are such great changes in density, so one can
Figure 9. Evolution with time of the number-averaged particle diameter at 25 °C for all the tests carried out.
Properties of Asphaltenes with Different Alkanes
Energy & Fuels, Vol. 22, No. 2, 2008 769
Figure 10. Evolution with time of the size-averaged particle diameter at 25 °C for all the tests carried out.
positively say that the asphaltenes precipitated during FBRM experiments yield greater amounts than standard laboratory asphaltenes. The difference is most probably attributable to the washing of standard asphaltenes, which is not carried out during a FBRM test. The washing seems to decrease the differences in yield between different conditions. As stated above, the objective of this study is to provide an asphaltene characterization that can be used to model pressuredrop asphaltenes. The use of an n-alkane tries to mimic the decrease in solvent quality caused by pressure drop. As reported by Rodgers et al.,34 mass spectroscopy results show that the average properties of pressure-drop asphaltenes are quite different (less aromatic) than C7-asphaltenes. A small decrease in aromaticity has also been observed in tests with n-heptane, when changing from R ) 10 to 30 mL/g (see Figure 3). Most probably, the first asphaltenes that come out of solution with n-heptane are more similar to pressure-drop asphaltenes, and the differences are increased as more n-heptane is added to the crude oil.
data of asphaltene structural properties for asphaltene modeling. Significant differences in yield and properties have been found in asphaltenes precipitated with different solvents and experimental conditions. These differences suggest the presence of different asphaltene species, making it more difficult to compare results obtained by other laboratories, unless experimental methodologies are identical.
Conclusions
Acknowledgment. Thanks are due to M. Suarez and L. de la Vega for their help with some of the experiments and Dr. C. Negro and Dr. E. de la Fuente (Universidad Complutense de Madrid) for their kindly help performing the FBMR measurements.The authors also wish to acknowledge the financial support of Repsol-YPF.
This experimental work is directed to assess the most suitable method for asphaltene separation, in order to provide reliable (34) Rodgers; R. Presented at the IFP Rencontre Scientifique Molecular Structure of Heavy Oils and Coal Liquefaction Products, Lyon, France, 2007.
A laser reflectance technique focused-beam reflectance measurement (FBRM) has been applied to study particle size distribution in kinetic experiments. This technique offers great possibilities for the study of asphaltene aggregation kinetics in crude oil/n-alkane mixtures. The observed differences in yield between the kinetic studies and the standard asphaltene separation increase the belief that more effort has to be put on improving the laboratory standard, so that the yielded asphaltenes more closely resemble the asphaltenes that separate in the field.
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