Ind. Eng. Chem. Res. 2005, 44, 5585-5592
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Adsorption of Asphaltenes on Metals Hussein Alboudwarej,† David Pole, William Y. Svrcek, and Harvey W. Yarranton* Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada, T2N 1N4
The adsorption of Athabasca and Cold Lake C7-asphaltenes on stainless steel (304L), iron, and aluminum powders was measured using UV-vis spectrophotometry. The effects of resins, temperature, and n-heptane-to-toluene ratio were also investigated. In all cases, Langmuir (type I) isotherms were observed, indicating that asphaltenes saturated the available surface area for adsorption. The saturation adsorptions of the asphaltenes on metals (0.25-2.7 mg/m2) were of the same order of magnitude as adsorption of asphaltenes on minerals. The saturation adsorptions were less than the monolayer surface coverage observed on water-in-hydrocarbon emulsion interfaces, indicating that there are a limited number of adsorption sites on the metals. Higher molar saturation adsorptions were observed for resins and low molar mass asphaltenes, suggesting that adsorption was limited by the morphology of the metal surface. In general, higher mass saturation adsorptions were observed when asphaltenes self-associated to greater extents and consequently larger molecules adsorbed on the surface. Introduction Heavy oil and bitumen production and processing often involve dilution of the oil to reduce its density and viscosity. In several current and potential applications including condensate dilution of heavy oil for transport, paraffinic oil sands froth treatment, and the VAPEX method of heavy oil recovery, there is potential for asphaltenes to precipitate and deposit. To avoid or mitigate asphaltene deposition problems, it is necessary to understand the mechanism of asphaltene deposition on surfaces. The mechanism of asphaltene deposition has not been well established as yet; however, there are likely several steps in the deposition process including: (1) precipitation of asphaltenes upon a change in the operating conditions (temperature, pressure, composition), (2) flocculation of precipitated asphaltene particles, and (3) adhesion of the flocculated asphaltenes to the reservoir rock or production and processing equipment surfaces. Asphaltenes can also adsorb directly on a surface before they precipitate. Direct adsorption is therefore a potential source of deposition but may also be a factor in the adhesion of the flocculated asphaltenes that likely form the bulk of any deposited material. The adsorption of asphaltenes on minerals has been extensively studied because asphaltene adsorption on the formation rock may alter the wettability of the rock and reduce oil production from the reservoir.1-5 In general, asphaltene adsorption on minerals follows a Langmuir (type I) behavior, indicating that the asphaltenes saturate the solid surface available for adsorption.6 However, asphaltenes may also form multilayers on minerals7 depending on the type of asphaltenes and the relaxation time of asphaltene macromolecules on the surface.8,9 The extent of asphaltene adsorption on various minerals such as kaolin, alumina, dolomite, calcite, Berea sandstone, etc., ranges from approximately 1 to * To whom correspondence should be addressed. Tel.: +1 (403) 220-2659. Fax: +1 (403) 284-4852. E-mail: hyarrant@ ucalgary.ca. † Current address: Oilphase-DBR, Schlumberger Canada Ltd., 9450 17th Ave., Edmonton, Alberta, Canada, T6N 1M9.
4 mg/m2. The adsorption isotherm is also affected by the solvent type10 and the aggregation state of the asphaltenes.11 There have been very few studies on adsorption of asphaltenes on metals. Ekholm et al.12 examined the adsorption of asphaltenes and resins onto a gold surface with a quartz crystal microbalance. In this study, an AC voltage was applied to a piezoelectric quartz crystal sandwiched between two gold electrodes, and the change in resonance frequency was monitored as a measure of adsorbed asphaltenes or resins. The extent of asphaltene and resin adsorption on the hydrophilic surface of gold reached 8 and 2 mg/m2, respectively, at concentrations up to 10 000 ppm. The reported adsorption is double that of asphaltenes and resins on minerals. The extent of adsorption might have been enhanced due to electrodeposition effects. The purpose of the present study is to investigate the direct adsorption of asphaltenes on metal surfaces. The adsorption is measured in systems of asphaltenes, toluene, n-heptane, and powdered metals (stainless steel (304L), iron, and aluminum). The amount of adsorption is determined from the change in asphaltene concentration once the metal is added to the solution. Powdered metals were selected because they have a relatively large surface area per unit mass. The effects of temperature, solvent, and resins content are also considered. Experimental Section Chemicals and Materials. Regent grade n-heptane, n-pentane, toluene, and acetone (provided by Vopak Inc.) were used for the extraction of asphaltenes and resins. Attapulgus clay (used for the extraction of resins) was obtained from Engelhard Corp., NJ. Spectrophotometric grade toluene and n-heptane (Omnisolv) were used for the adsorption experiments. Both the toluene and the n-heptane were dehydrated using a molecular sieve (Fisher Scientific Inc.). Stainless steel (304L) powder was provided by OMG Americas. The aluminum and iron powders were provided by Alfa Aesar. Specific surface areas and particle sizes of the powdered metals
10.1021/ie048948f CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 Table 2. Density and Molar Mass of Athabasca Asphaltenes and Cold Lake Asphaltenes and Resins sample
density (kg/m3)a
molar mass (g/mol)b
Athabasca asphaltenes (SW) Cold Lake asphaltenes (SW) Cold Lake resins
1192 1190 1019
9100 10400 930
a Measured with an Anton Paar DMA 46 density meter.14,15 Measured with vapor pressure osmometry in toluene at 50 °C and 10 kg/m3 asphaltene concentration.14
b
Figure 1. SEM microphotograph of powdered metals: (a) stainless steel (304L), (b) iron, (c) aluminum. Table 1. Surface Area and Particle Size of Stainless Steel (304L), Iron, and Aluminum Powder metal powder
specific area (m2/g)
average particle size (µm)
stainless steel (304L) iron aluminum
0.15 0.43 2.37
-∆G Al. The calculations were based on a methodology developed by Askvik and Fotland,21 using contact angle measurements between the asphaltene substrate and water, glycerol, and diiodomethane. The predicted trends are consistent with the results of Figures 5 and 6. The reason for such a trend is not clearly understood, but the adsorption of asphaltenes is influenced by the surface charge of the asphaltenes18 as well as the properties of the metal surface such as the number or nature of the adsorption sites on the metal surfaces. The presence of elements such as chromium, nickel, silicon, and carbon and heteroatoms such as sulfur and phosphorus in the structure of stainless steel may have helped better electrostatic bonding between asphaltenes and stainless steel surface. The presence of metal oxides may have also contributed to the difference between the surfaces. Another possibility is that relatively large molecules such as asphaltenes may not have access to the entire available powder surface area due to different surface morphology. In all cases, the adsorption of Cold Lake C7-asphaltenes (SW) is approximately 35% higher than that of Athabasca asphaltenes. A possible explanation is the difference in molar mass of the two types of asphaltenes. The molar mass of associated Cold Lake asphaltenes at concentrations higher than 5 kg/m3 is lower than that of Athabasca asphaltenes at the same concentrations.13
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Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 Table 6. Effect of Resins Content on Saturation Adsorption and Equilibrium Constant of Cold Lake C7-Asphaltenes on Stainless Steel (304L) Powder at 22 °C sample
As (mg/m2)
Ks (m3/kg)
Soxhlet-washed unwashed SW asphaltenes + resinsa resins
2.7 1.2 1.16 0.7
105 120 140 128
a
1:2 mass ratio of asphaltenes to resins.
Table 7. Mass and Molar Saturation Adsorption of Asphaltenes and Resins on Stainless Steel (304L) Powder sample
Figure 7. Effect of resin content on adsorption of Cold Lake C7asphaltenes on stainless steel (304L) powder at 22 °C (lines are fitted Langmuir isotherms).
However, at lower concentrations such as those used in the adsorption tests, the molar masses of Cold Lake and Athabasca C7-asphaltenes are similar. The different adsorptions of the two asphaltenes could also be attributed to small differences in the structure of some or all of the asphaltenes or to differences in the extent and nature of functional groups and aromatic moieties in the asphaltenes structure. Available characteristic data on Athabasca and Cold Lake asphaltenes do not suggest a significant difference in properties.22 However, small compositional and structural differences could change the amount of adsorption particularly on heterogeneous surfaces. Due to a very small amount of adsorbed asphaltenes on metal surfaces, no attempts were made to recover adsorbed asphaltenes for further analysis. However, a recent high-resolution mass spectroscopy analysis of asphaltenes deposited on a metal surface shows that the deposited material has a higher content of heteroatoms than the source asphaltenes.23 Studies of water-inhydrocarbon emulsions prepared from different asphaltene fractions confirm this observation and indicate that the surface active asphaltens are drawn from throughout the asphaltene molar mass distribution.24,25 It is possible that the Cold Lake asphaltenes contain slightly more higher heteroatom content species with higher surface activity than the Athabasca asphaltenes. Effect of Resins. Figure 7 shows the adsorption isotherms of unwashed and Soxhlet-washed Cold Lake C7-asphaltenes on stainless steel (304L) at 22 °C. Recall that Soxhlet washing removes most of the trapped and/ or coprecipitated resins and therefore the Soxhletwashed asphaltenes are a less “resinous” asphaltene fraction. The adsorption isotherms of resins and a mixture of resins and Soxhlet-washed asphaltenes (1:2 wt/wt asphaltenes: resins, all from Cold Lake bitumen) are also shown in Figure 7. All of the adsorption isotherms are again Langmuir (Type I) isotherms. The saturation adsorptions and the equilibrium constants are also shown in Table 6. The mass saturation adsorption of the unwashed asphaltenes is approximately half that of the Soxhletwashed asphaltenes, and the mass saturation adsorption of resins is half that of the unwashed asphaltenes. The trend of decreasing mass saturation adsorption with increasing resin content is consistent with the results of Ekholm et al.12 Mixtures of asphaltenes and resins
Athabasca asphaltenes (SW) (23 °C) Athabasca asphaltenes (SW) (60 °C) Cold Lake asphaltenes (SW) Cold Lake asphaltene (UW) Cold Lake asphaltene (SW) + resinsa Cold Lake resins a
As (mass) (mg/m2)
molar mass (g/mol)
As (mol) (mol/m2 × 106)
2.25
5400
0.42
2.15
4900
0.44
2.7
5000
0.54
1.2
2800
0.43
1.16
1400
0.83
0.7
960
0.73
1:2 mass ratio of asphaltenes to resins.
also had lower mass saturation adsorption than asphaltenes only. A possible explanation is that resins have lower molar mass than asphaltenes and also that the addition of resins reduces the extent of asphaltene self-association and consequently the average asphaltene associated molar mass. Hence, for a fixed number of adsorption sites, the saturation adsorption is expected to decrease on a mass basis but remain constant on a molar basis. The molar saturation adsorptions from the data in Figure 7 are reported in Table 7. The Soxhletwashed and unwashed asphaltenes have the same molar saturation adsorption within experimental error. The difference in mass saturation adsorption arises simply because Soxhlet-washed asphaltenes are larger molecules. However, the Cold Lake resins have approximately 35% greater molar saturation adsorption than the Cold Lake asphaltenes. The mixture of Soxhletwashed asphaltenes and resins also had a higher saturation adsorption. The difference in saturation adsorption suggests that adsorption is limited by the surface morphology; that is, the relatively large asphaltene molecules appear to access less of the surface than the smaller resins. The addition of resins to the Soxhletwashed asphaltenes appears to decrease the association of the asphaltenes, and the relatively smaller associated species could also access more of the surface. Effect of Temperature. The effect of temperature on the adsorption isotherm of Athabasca C7-asphaltenes (SW) on stainless steel (304L) is presented in Figure 8. The maximum asphaltene adsorption and the equilibrium constants are given in Table 8. Although all three adsorption isotherms are within the range of the data scatter, the saturated asphaltene adsorption at 60 °C appears to be smaller than the saturated adsorption at room temperature. The decrease in adsorption with increased temperature is likely due to a decrease in the size of the asphaltene aggregates. It has been shown that at higher temperatures the asphaltene-associated molar mass decreases.16 Note that the molar saturation adsorptions are consistent when the change in molar mass is accounted for, as was shown in Table 7. The
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is almost double that in pure toluene. Asphaltene selfassociation may increase in a poor solvent (higher H/T ratio) and results in larger molecules on the surface and higher mass saturation adsorption. Also, a poor solvent may drive asphaltenes to adsorb on weaker or less accessible sites that were not adsorbed on in a better solvent. Conclusions
Figure 8. Effect of temperature on adsorption of Athabasca C7asphaltenes (SW) on stainless steel (304L) powder (lines are fitted Langmuir isotherms).
Figure 9. Effect of the n-heptane-to-toluene ratio on adsorption of Cold Lake C7-asphaltenes on stainless steel (304L) powder at 22 °C. Table 8. Effect of Temperature on Saturation Adsorption and Equilibrium Constant of Athabasca C7-Asphaltenes (SW) on Stainless Steel (304L) Powder temp (°C)
As (mg/m2)
Ks (m3/kg)
22 40 60
2.25 2.35 2.15
117 110 120
equilibrium constant does not appear to change with temperature over the small range of temperature considered. Effect of n-Heptane-to-Toluene Ratio. The effect of the medium on asphaltene adsorption is addressed through the n-heptane-to-toluene (H/T) ratio because toluene is a good solvent for asphaltenes, but the addition of n-heptane creates a poor solvent. Adsorption may increase in poor solvents. The effect of the medium was investigated using stainless steel (304L) powder at 22 °C. Cold Lake C7-asphaltenes (UW) were dissolved in toluene, and n-heptane was added until the mixtures of n-heptane and toluene had volume ratios of 0/100, 25/75, 35/65, and 45/55 (all at an overall concentration of 3 kg/m3). Note that the onset of precipitation for these asphaltenes occurs approximately at a 50/50 volume ratio of n-heptane to toluene. The results of the adsorption measurements are presented in Figure 9. As the H/T ratio increases, asphaltene adsorption also increases. The adsorption near the onset of precipitation
The adsorption of Athabasca and Cold Lake C7asphaltenes on stainless steel (304L), iron, and aluminum powders was measured using UV-vis spectrophotometry. The effects of resins, temperature, and n-heptane-to-toluene ratio were also investigated. Adsorption reached steady state in less than 24 h even at low asphaltene concentrations. In all cases, Langmuir (type I) isotherms were observed, indicating that asphaltenes saturated the available surface area for adsorption. The saturation adsorptions were less than the monolayer surface coverage observed on water-in-hydrocarbon emulsion interfaces, indicating that there are a limited number of adsorption sites on the metals. Higher molar saturation adsorptions were observed for resins and low molar mass asphaltenes, suggesting that adsorption was limited by the morphology of the metal surface; that is, larger molecules could not access the entire surface. In all cases, the maximum amount of adsorption and the equilibrium constants for the adsorption were similar to the corresponding reported values for minerals. The amount of adsorption decreased from stainless steel to iron to aluminum. For all of the metals, Cold Lake asphaltenes adsorbed on average 35% more than Athabasca asphaltenes. The saturation adsorptions ranged from 0.2 to 2.7 mg/m2. This small amount of adsorption is not expected to be a significant direct contribution to asphaltene deposition. Nonetheless, asphaltene adsorption may be a significant step in asphaltene deposition because precipitated asphaltene particles likely adhere to the adsorbed asphaltene layer rather than directly onto the metal surface. In general, higher mass saturation adsorptions were observed when asphaltenes self-associated to greater extents, and consequently larger molecules adsorbed on the surface. The addition of resins reduces mass saturation adsorption either because resins reduce the extent of asphaltene association or because the lower molar mass resins displace asphaltenes from the surface. Increased temperature appeared to reduce asphaltene mass saturation adsorption, although scatter in the data obscured the trend. Increased temperature reduces asphaltene self-association. Asphaltene adsorption increased with increasing n-heptane content in the solvent. Possibly, asphaltene self-association increases or adsorption is driven to a greater extent in a poor solvent. The results presented here are based on the adsorption of redissolved asphaltenes on powdered metals. How far can they be extended to asphaltene adsorption from crude oils onto pipeline surfaces? First, while different metals and possibly different surface morphologies lead to different adsorptions, all of the reported adsorptions are less than monolayer coverage. Hence, as long as asphaltene adsorption is site limited, the direct adsorption per specific surface area of the pipeline is expected to be in the order of 10 mg/m2 or less. Second, asphaltenes may have a different self-association structure in the crude oil. However, most experimental data suggest that asphaltenes are less self-associated in
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crude oils in which they are dissolved than they are in solutions of toluene and heptane.14 Hence, the mass of adsorbed asphaltenes is expected to be less from a crude oil. In general, asphaltene adsorption in crude oil pipelines is expected to be smaller in magnitude than the results presented here. Possible exceptions are situations where asphaltenes are precipitating, the pipe surface is hot, or other materials are depositing as well. Acknowledgment We would like to thank Mr. Dan Tillman from the Department of Civil Engineering at the University of Calgary for performing SEM scans. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Energy Research Institute (AERI), and Oilphase-DBR is appreciated. We also thank Syncrude Canada Ltd. and Imperial Oil Ltd. for supplying bitumen samples. Literature Cited (1) Collins, S. H.; Melrose, J. C. Adsorption of Asphaltenes and Water on Reservoir Rock Minerals, SPE International Symposium on Oilfield and Geothermal Chemistry, SPE # 11800, Denver, Colorado, June 1-3, 1983. (2) Piro, G.; Canonico, L. B.; Galbariggi, G.; Bertero, L.; Carniani, C. Asphaltene Adsorption onto Formation Rock: An Approach to Asphaltene Formation Damage Prevention. SPE Prod. Facil. 1996, August, 156-160. (3) Crocker, M. E.; Marchin, L. M. Wettability and Adsorption Characteristics of Crude Oil Asphaltene and Polar Fractions, SPE/ DOE Fifth Symposium on Enhanced Oil Recovery, SPE # 14885, Tulsa, OK, April 20-23, 1986. (4) Yan, J.; Plancher, H.; Morrow, N. R. Wettability Changes Induced by Adsorption of Asphaltenes. SPE Prod. Facil. 1997, 12, 239-266. (5) Buckley, J. S.; Liu, Y.; Monsterleet, S. Mechanism of Wetting Alteration by Crude Oils. SPE J. 1998, 3, 54-61. (6) Dubey, S. T.; Waxman, M. H. Asphaltene Adsorption and Desorption from Mineral Surfaces. SPE Reservoir Eng. 1991, 389395. (7) Acevedo, S.; Castillo, J.; Fernandez, A.; Goncalves, S.; Ranaudo, M. A. A Study of Multilayer Adsorption of Asphaltenes on Glass Surfaces by Photothermal Surface Deformation. Relation of this Adsorption to Aggregate Formation in Solution. Energy Fuels 1998, 12, 386-390. (8) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutierrez, L.; Ortega, P. Adsorption of Asphaltenes and Resins on Organic and Inorganic Substrates and Their Correlation with Precipitation Problems in Production Well Tubing. Fuel 1995, 74, 595-598. (9) Acevedo, S.; Ranaudo, M. A.; Garcia, C.; Castillo, J.; Fernandez, A.; Caetano, M.; Goncalves, S. Importance of Asphaltene Aggregation in Solution in Determining the Adsorption of This Sample on Mineral Surfaces. Colloids Surf., A 2000, 166, 145152.
(10) Akhlag, M. S.; Gotze, P.; Kessel, D.; Dornow, W. Adsorption of Crude Oil Colloids on Glass Plate: Measurement of Contact Angles and the Factors Influencing Glass Surface Properties. Colloids Surf., A 1997, 126, 25-32. (11) Pernyeszi, T.; Patzko, A.; Berkesi, O.; Dekany, I. Asphaltene Adsorption on Clays and Crude Oil Reservoir Rocks. Colloids Surf., A 1998, 137, 373-384. (12) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjo¨blem, J.; Kornfeldt, A. A Quartz Crystal Microbalance Study of the Adsorption of Asphaltenes and Resins onto a Hydrophilic Surface. J. Colloid Interface Sci. 2002, 247, 342-350. (13) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzedeh, K. Sensitivity of Asphaltene Properties to Extraction Techniques. Energy Fuels 2002, 16, 462-469. (14) Alboudwarej, H.; Akbarzedeh, K.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W. A Regular Solution Model for Asphaltene Precipitation from Bitumens and Solvents. AIChE J. 2003, 49, 2948-2956. (15) Akbarzadeh, K.; Ayatollahi, Sh.; Moshfeghian, M.; Alboudwarej, H.; Yarranton, H. W. Estimation of SARA Fraction Properties with the SRK EOS. J. Can. Pet. Technol. 2004, 43, 31-39. (16) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Investigation of Asphaltene Association with Vapor Pressure Osmometry and Interfacial Tension Measurements. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (17) Alboudwarej, H.; Jakher, R. K.; Svrcek, W. Y.; Yarranton, H. W. Spectrophotometric Measurement of Asphaltene Concentration. Pet. Sci. Technol. 2004, 22, 647-664. (18) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and Adsorption Properties of Asphaltenes. Colloids Surf., A 1995, 94, 253-265. (19) Sztukowski, D. M.; Jafari, M.; Alboudwarej, H.; Yarranton, H. W. Asphaltene Self-Association and Water-in-Hydrocarbon Emulsions. J. Colloid Interface Sci. 2003, 265, 179-186. (20) Xie, K.; Karan, K. Kinetics and Thermodynamics of Asphaltene Adsorption on Metal Surfaces: A Preliminary Study. Energy Fuels, in press. (21) Askvik, K. M.; Fotland, P. Experimental Determination of Hamaker Constants for Asphaltenes and Crude Oils, 4th International Conference on Petroleum Phase Behavior and Fouling, Torondheim, Norway, June 23-26, 2003. (22) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Decker: New York, 1999. (23) Kharrat, A. M. Oilphase-DBR, Personal Communications, 2005. (24) Yang, X.; Hamza, H.; Czarnecki, J. Investigation of Subfractions of Athabasca Asphaltenes and Their Role in Emulsion Stability. Energy Fuels 2004, 18, 770-777. (25) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. Water-inHydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations. J. Colloid Interface Sci. 2000, 228, 52-63.
Received for review October 29, 2004 Revised manuscript received May 5, 2005 Accepted May 17, 2005 IE048948F
