Energy Fuels 2010, 24, 2281–2286 Published on Web 10/27/2009
: DOI:10.1021/ef900934t
Influence of Hydrocarbon Distribution in Crude Oil and Residues on Asphaltene Stability† Javier Dufour,*,‡ Jose A. Calles,‡ Javier Marugan,‡ Ra ul Gimenez-Aguirre,‡ Jose Luis Pe~ na,‡,§ and § Daniel Merino-Garcı´ a ‡ Department of Chemical and Environmental Technology, Escuela Superior de Ciencias Experimentales y Tecnologı´a (ESCET), Universidad Rey Juan Carlos, C/ Tulip an s/n, 28933 M ostoles, Madrid, Spain, and §Technology Center Repsol, Crtra. A-5 Km 18, 28931 M ostoles, Madrid, Spain
Received August 27, 2009. Revised Manuscript Received October 9, 2009
Flow assurance problems caused by asphaltene deposition are related to the solvent quality of the surrounding medium rather than the amount of asphaltenes. Traditionally, solvent quality has been focused on resins, which are the most similar fraction to asphaltenes, in terms of aromaticity and polarity. Still, no answer is available to the question whether resins (defined as the soluble fraction in n-heptane but insoluble in n-pentane) act as co-solvents or have a more critical interaction with asphaltenes. This work deals with the study of asphaltenes stability in two South American crude oils and their distilled residues (190þ, 370þ, and 565þ). The total amount of asphaltenes for each sample has been determined by the IP143 standard method. When asphaltene percentage is normalized to whole crude oil using the true boling point (TBP) data from distillation following the ASTM D-2812 standard, a shift to a greater asphaltene mass is observed when light cuts are removed. Stability of asphaltenes and aggregation kinetics have been assessed by focused-beam laser reflectance measurements (FBRMs). Differences between each crude oil and its residues are observed. FBRM evidence has been confirmed by transmittance analysis using a Turbiscan device. Resins (defined as C5-insoluble and C7-soluble) and an aromatic compound [R-methylnaphthalene (RMN)] have been doped into the original crude, to assess their effect on stability. Results have shown that the C5-C7 fraction increases asphaltene stability to a similar extent as a diaromatic molecule, such as RMN. All of this evidence may indicate that the effect of C5-C7 resins and diaromatic addition and the removal of light ends have a similar impact on asphaltene stability, supporting the belief that the solvent quality as a whole rules asphaltene behavior.
If one uses saturates, aromatics, resins, and asphaltenes (SARA) terminology, solvent quality is related to the relative amount of saturates, aromatics, and resins; saturates worsen the capacity of keeping asphaltene nanoaggregates stable, while aromatics and resins improve the quality of the medium. There is a significant controversy on the role of resins on asphaltene stability, as recently reviewed by Goual.6 Resins represent the transition between the nonpolar medium (saturates and aromatics) and the polar asphaltenes. Historically, the depiction of the asphaltene state in crude oil included a layer of resins surrounding asphaltene colloids, such as in the Pfeiffer and Saal model.7 Several thermodynamic models have been derived from this kind of interaction between asphaltenes and resins.8,9 Even if significant concerns10,11 have been raised against a depiction such as that proposed by Pfeffier and Saal, there are many references in the literature that
Introduction The asphaltene fraction is traditionally defined as a solubility class, because it comprises molecules that are soluble in toluene and insoluble in a light n-alkane (such as heptane).1,2 This definition, apart from creating significant confusion in the literature,3 leads to the fact that asphaltene problems in upstream operations are not correlated with the amount of asphaltenes. In fact, it is more common to observe asphaltene plugging with light undersaturated oils4 rather than with heavy aromatic crudes with high asphaltene concentrations (Boscan, Venezuela).5 This indicates that the quality of the surrounding medium is more relevant for asphaltene phase equilibrium than the actual amount of molecules that would precipitate upon heptane addition. † Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed: Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, 28933 M ostoles, Madrid, Spain. Telephone: þ34914888138. Fax: þ34-914887068. E-mail:
[email protected]. (1) Institute of Petroleum (IP). IP 143/90. Standards for petroleum and its products. IP, London, U.K., 1985. (2) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999. (3) http://www.ualberta.ca/dept/chemeng/asphaltenes. (4) Thawer, R.; Nicoll, D. C. A.; Dick, G. SPE Tech. Pap. 18473; SPE Production Engineering, 1990; pp 475-480. (5) de Boer, R. B.; Leerlooyer, K.; Eigner, M. R. P.; van Bergen, A. R. D. SPE Tech. Pap. 24987; SPE Production and Facilities, 1995; p 55.
r 2009 American Chemical Society
(6) Goual, L. Role of resins on asphaltene stability. Proceedings of the 10th Petroleum Phase Behaviour and Fouling Conference, Rio de Janeiro, Brazil, June 14-18, 2009. (7) Pfeiffer, J. P.; Saal, R. N. Phys. Chem. 1940, 44, 139–149. (8) Leontaritis, K. J.; Mansoori, G. A. SPE Tech. Pap. 16258; SPE International Symposium on Oilfield Chemistry, San Antonio, TX, Feb 4-6, 1987. (9) Victorov, A. I.; Smirnov, N. A. Ind. Eng. Chem. Res. 1998, 37, 3242–3251. (10) Correra, S.; Donaggio, F. SPE Tech. Pap. 58724; Proceedings of the 2000 SPE International Symposium on Formation Damage, Lafayette, LA, Feb 23-24, 2000. (11) Mostowfi, F.; Indo, K.; Mullins, O. C.; McFarlane, R. Energy Fuels 2009, 23, 1194–1200.
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: DOI:10.1021/ef900934t
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demonstrate the interaction between resin and asphaltene fractions in model solvents12,13 and also inside crude oil.14 Still, no answer is available to the question about how critical this interaction is. That is to say, it is unclear whether resins act as co-solvents, increasing the solvent quality similar to aromatics, or play a critical role in the formation and stability of asphaltene nanoaggregates. To complicate matters more, research on this issue is enormously impeded by the fact that no unified definition is currently used to separate resins. For instance, Shaw and co-workers have shown that nanoaggregates in Maya oil and Athabasca Bitumen are mainly formed by molecules that fit the C5-insoluble definition. The role of resins in nanoaggregate formation is clearly biased by the criteria used to delimit asphaltene and resin fractions. In fact, it has been proposed to merge resins and asphaltenes into one single fraction (associating material). This view is more coherent with the idea of having a continuum of molecules in crude oil. This work explores the effect of different families of compounds on asphaltene stability and destabilization kinetics. Onset and the asphaltenes aggregation kinetics near the onset were determined by focused-beam laser reflectance measurements (FBRMs). Moreover, stability is also assessed by means of the separability number (SN), obtained in Turbiscan experiments. The effect of the loss of light ends (associated to mainly saturate compounds) is addressed by analyzing the stability behavior of crude oil residues (190þ, 370þ, and 565þ). On the other hand, resins (defined as C5-insoluble and C7-soluble) and aromatics [represented by a diaromatic compound, R-methyl-naphtalene (RMN)] are evaluated by doping the crude with increasing amounts of these compounds.
Table 1. Characteristics of Crude Oils Used in This Study crude oil A C a
°API
source
21 34
South America South America
classificationa naphthenic paraffinic
Determined through KUOP measurements.
the resin definition to a fraction of the amount of resins obtained by the ASTM standard (D-2007).15 The idea was to assess the influence of the molecules that are more similar to asphaltenes, which are believed to be comprised of the fraction between C5 and C7. Laser-Reflectance Measurements. A FBRM S400A LASENTEC manufactured by Mettler-Toledo (Columbus, OH) was employed. 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 duration of the signal of the backscattered light from these particles renders a characteristic measurement of the particle geometry, i.e., a chord. The number of recorded chords is reported as counts.16,17 FBRM experiments with crude oils and residues were performed at 60 °C in a close system. Working at this temperature is necessary to avoid wax particle interferences, to reduce residue viscosity, and to enhance mixing with heptane. In a typical FBRM experiment, crude oil was introduced in a glass beaker and heated to the test temperature. The stirring rate was maintained at 400 rpm. Titration was performed using a dosing pump until the onset n-heptane/oil ratio (Ronset) was reached. Volumetric steps of 0.1 mL of n-heptane/g of oil were used. Each injection was carried out after a stabilization time of 20 min. The chord length distributions (CLDs) were recorded every 10 s in 120 channels of sizes up to 30 μm linear scale. Following the process exposed in previous works,16,17 the CLDs were transformed into particle size distributions (PSDs), with the model for spherical particles developed by Hukkanen and Braatz.18 Aggregation kinetic studies were carried out with a 10% excess of n-heptane with respect to the onset value. This criterion was taken to increase kinetics with respect to the slow rates observed at the onset. When the aggregation kinetics was expressed as a percentage of the onset volume, it was considered that all samples were subjected to the same supersaturation. Figure 1 shows an illustration of the aggregation rate at the onset and at a greater supersaturation (10% in volume of heptane). Turbiscan Measurements. Stability of a crude oil sample is determined by determining the SN, following ASTM standard D-706118 with a Turbiscan (TMA 2000 heavy fuel, formulaction). In these experiments, excess heptane is added to a mixture of crude oil and toluene. All asphaltenes are forced to precipitate, and the particle size is assumed to be related to asphaltene stability. Sedimentation is monitored for 15 min from the bottom of the test tube to the top, by means of transmittance scans every minute (wavelength of 850 nm). Unstable oils form large asphaltene particles that sediment quickly, while stable oils form small aggregates that show small changes in transmittance. SN is defined as the standard deviation of the average transmittance. Values lower than 5 indicate high oil stability, while values greater than 10
Experimental Section Samples. Two crude oils and their 190þ, 370þ, and 565þ distillation residues were studied. Table 1 shows the density, source, and nature of the different oils tested. Crude oil A has shown to be stable from the point of view of asphaltene deposition in upstream operations, whereas crude oil C showed deposition problems. Reagents. n-Heptane reagent-grade (density, 0.68 g/cm3; 99% pure) was used as received. Toluene reagent-grade ACS ISO (density, 0.87 g/cm3; 99.9% pure) was used as received. R-Methyl-naphthalene (tagged as RMN in this work, 97%) was used as received. All of the reagents have been supplied by Scharlab S.L. Asphaltene Separation. The amount of asphaltenes of crude oils and their residues were obtained by means of the IP-143 standard method.1 In this procedure, an excess of n-heptane is added to precipitate asphaltenes and the insolubles are separated by filtration. Once solids are isolated, they are first washed with heptane and then toluene at their respective boiling points in a Sohxlet extractor. Evaporation of toluene permits the quantification and isolation of asphaltene solids. Resin Separation. Resins are defined in this work as the family of compounds that are soluble in heptane and insoluble in n-pentane. The authors are aware of the fact that this limits (12) Merino-Garcia, D.; Andersen, S. I. Langmuir 2004, 20 (11), 4559– 4565. (13) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19 (1 and 2), 1. (14) Moschopedis, S. E.; Speight, J. Fuel 1976, 55 (3), 187–192. (15) American Society for Testing and Materials (ASTM). ASTM D-2007/03. Standard test method for characteristic groups in rubber extender and processing oils and other petroleum-derived oils by the clay-gel absorption chromatographic method. ASTM, West Conshohocken, PA, 2003.
(16) Calles, J. A.; Dufour, J.; Marugan, J.; Pe~ na, J. L.; Gimenez-Aguirre, R.; Merino-Garcı´ a, D. Energy Fuels 2008, 22, 763–769. (17) Marugan, J.; Calles, J. A.; Dufour, J.; Gimenez-Aguirre, R.; Pe~ na, J. L.; Merino-Garcı´ a, D. Energy Fuels 2009, 23, 1155–1161. (18) Hukkanen, E. J.; Braatz, R. D. Sens. Actuators 2003, 96, 451–459. (19) American Society for Testing and Materials (ASTM). ASTM D-7061/04. Standard test method for measuring n-heptane induced phase separation of asphaltene-containing heavy fuel oils as separability number by an optical scanning device. ASTM, West Conshohocken, PA, 2004.
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Figure 1. Aggregation kinetics at the onset and at a 10% supersaturation with crude A.
Figure 2. Illustration of a Turbiscan experiment to obtain the SN.
percentage of each sample to total crude oil to determine the influence of the light ends on asphaltene content. Table 2 shows the percentage of weight of each residue with respect to crude oil, the IP-143 content of asphaltenes, and the normalized amount. Results show that crude oil A has similar asphaltene content as crude oil C. The amount of IP-143 asphaltenes rises significantly as more light compounds are removed from crude A. On the other hand, the effect in crude C is minimal once asphaltene content has been normalized. Figure 3 shows the influence of the surrounding medium on the amount of asphaltenes that come out of the solution. Crude A yields more asphaltenes when light ends are
indicate that the asphaltenes are unstable. Figure 2 displays transmittance signal as a function of the probe length, recorded after 1 min (beginning of test) and at the end of a 15 min experiment.
Results and Discussion Asphaltene Content. The total amount of asphaltenes in crude oils and their residues was determined through the IP143 procedure. From true boiling point (TBP) curves of crude oils obtained in a controlled vacuum distillation following the ASTM D-2892 standard, the relative weight of residue fractions to crude oil weight was calculated. This value has allowed for normalization of the IP-143 asphaltene 2283
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Table 2. Yield to Crude Oil for Residues, Asphaltene Content Determined by IP-143, and Normalized to Crude Oil Asphaltenes crude oil/ residues A A 190þ A 370þ A 565þ C C 190þ C 370þ
weight/crude IP-143 weight (%) asphaltenes (%) 100 89 70 33 100 73 36
normalized to crude asphaltenes (%)
1.2 1.5 2.5 5.5 1.0 1.5 3.0
1.2 1.4 1.7 1.8 1.0 1.1 1.1
Figure 4. Normalized kinetics for samples studied at onset þ 10%: (a) crude A and their residues and (b) crude B and the 370þ residue.
Figure 3. Shifting of the normalized asphaltene percent when light cuts are removed. Table 3. Asphaltene Onset of Studied Samples Determined by FBRM Ronset (mLn-C7/gcrude oil)
A C
crude oil
190þ
370þ
565þ
2.6 0.2
3.9 0.2
3.9 0.7
3.9
removed; however, this behavior is not reproduced when a more paraffinic crude, such as crude C, is used. Asphaltene Onset by FBRM. The laser reflectance technique was used (following the procedure exposed in the Experimental Section) to determine the asphaltene onset at 60 °C for each sample. Table 3 shows the results obtained. For crude oil A, a shifting to a higher value of onset is observed when light ends are eliminated; once lighter compounds are removed, asphaltene stability determined by onset studies is held, although heavier compounds have been taken out. For crude C, the displacement to higher values of Ronset, related to more stable asphaltenes, occurs when larger quantities of lighter compounds are eliminated from crude oil (65%). The different nature of crudes (crude A is naphthenic, and crude C is paraffinic) can explain this behavior. Moreover, crude C is more unstable that crude A, because the onset is lower in all cases, despite the initial similar content. Asphaltene Aggregation Kinetics by FBRM. A kinetic study was carried out to determine asphaltene aggregation behavior. The criterion of 10% excess heptane was modified for onset values below 1 mL/g, because 10% was not a significant increase if the onset was around 0.2 mL/g. In these cases, one addition of 0.1 mL/g was made to achieve the desired n-heptane excess, i.e., 10% excess. Results for both crudes are shown in panels a and b of Figure 4. The number of counts in the kinetic study is normalized for all
Figure 5. CLD study at different times for residue 190þ of crude A.
Figure 6. CLD study at 330 min for crude A and its residues.
samples to the maximum value measured in the interval showed in the figure. For crude A, it is observed that the kinetics are slower as light ends are removed, leading to greater times until final particle sizes are reached; the slow 2284
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: DOI:10.1021/ef900934t
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PSDs in crude oil and residues can be compared at a fixed time, to address the effect of the composition of the surrounding medium. Figure 6 shows the results of this analysis for crude A and its residues. At this time, no size shifting is observed in residues. The amount of particles increases because the concentration of asphaltenes increases from the crude to the residues. With the information gathered from Figures 4-6, one can conclude that the removal of light ends slows down the precipitation of asphaltenes but yields particles of similar size. Determination of SN. The TURBISCAN stability is calculated for all samples and expressed as SN. Asphaltenes with higher sedimentation rates (greater SN) are related to more unstable crudes. Panels a and b of Figure 7 show the results for crude A and C, respectively. For crude oils A and C, the transmittance varies less with time as more light compounds are removed from the crude, confirming that stability of asphaltenes in residues is increased with respect to crude oil. To analyze the contribution of other compounds to solvent quality, Turbiscan experiments were carried out with doped crude oil samples. First, C5-C7 resins were added in concentrations ranging from 5 to 20%. To assess the magnitude of stabilization by resins, equivalent experiments were carried out with a diaromatic compound (RMN). Panels a-d of Figure 8 show the evolution of transmittance with time for all experiments with crudes A and C. Qualitatively, one can
kinetic growth indicates that asphaltenes are more stable in residues than in crude oil. Crude C does not show any trend. With respect to average size distribution, minimal differences have been observed in all samples as a function of time. The number of particles increases, but the distributions are centered on the same values, as displayed in Figure 5 (190þ residue of crude A).
Table 4. SN in Turbiscan Experiments Doping the Crudes with C5-C7 Resins or RMN crude A
Figure 7. Results of stability experiments by ASTM D-7061: (a) crude A and (b) crude C.
crude C
% addition
SNRMN
SNC5-C7
% addition
SNRMN
SNC5-C7
0 5 10 20
3.6 1.0 1.1 1.1
3.6 2.6 2.3 2.2
0 5 10 20
12.0 11.6 9.9 8.5
12.0 9.9 9.9
Figure 8. Results of stability experiments by ASTM D-7061: (a) crude A þ C5-C7 fraction, (b) crude A þ RMN, (c) crude C þ C5-C7 fraction, and (d) crude C þ RMN.
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see that the addition of these substances decreases the variation in transmittance, i.e., increasing the stability of the crude. The SNs are displayed in Table 4. Larger amounts of the C5-C7 fraction or RMN reduce SN, indicating that, in both cases, asphaltene stability is increased. Crude A is very stable (SN lower than 5), and the effect of doping substances is lower. Crude C is unstable, and the effect is greater, with RMN being a better product than C5-C7 resins. Nevertheless, the range of variation is very similar for both kinds of substances. Coming back to the topic of the role of resins in asphaltene stability, these experiments have shown that there is a measurable effect of the C5-C7 fraction (typically considered inside the resin fraction) on asphaltene stability but the magnitude of this effect is not as significant as one would expect if it was a critical factor. This analysis has included both a stable and unstable crude oil. Moreover, the effect is of a comparable magnitude with respect to the addition of a diaromatic and also the removal of light ends. Thus, these tests support the belief that the solvent quality as a whole rules asphaltene behavior.
Conclusions With the information gathered from FBRM experiments, one can conclude that the removal of light ends increases the amount of C7 needed to reach the onset, slows down the precipitation of asphaltenes, but yields particles of similar size. Turbiscan results have confirmed that light-end removal from crude oil leads to the stabilization of asphaltenes. Resins (defined as C5-insoluble and C7-soluble) and an aromatic compound (RMN) have been doped in the original crude, to assess their effect on stability. Results have shown that the C5-C7 fraction increases asphaltene stability to a similar extent as a diaromatic molecule, such as RMN. All of this evidence indicate that the effect of C5-C7 resins and diaromatic addition and the removal of light ends have a similar impact on asphaltene stability, supporting the belief that the solvent quality as a whole rules asphaltene behavior. Acknowledgment. Thanks are due to Fernando Dodero Navalpotro and Rut Rodriguez Rey for their help with some of the experiments. The authors also wish to acknowledge the financial support of Repsol.
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