Comparing Asphaltene Adsorption with Model Heavy Molecules

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Energy & Fuels 2007, 21, 234-241

Comparing Asphaltene Adsorption with Model Heavy Molecules over Macroporous Solid Surfaces Manuel F. Gonza´lez, Clementina Sosa Stull, Francisco Lo´pez-Linares, and Pedro Pereira-Almao* Schulich School of Engineering, Department of Chemical and Petroleum Engineering, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4 ReceiVed May 1, 2006. ReVised Manuscript ReceiVed October 16, 2006

The adsorption behavior of Athabasca bitumen C7 asphaltene (AB-C7-A) over a macroporous silica-alumina, kaolin, at room temperature is described by comparison with three model molecules such as Xylenol Orange (amine modified, XYO), Violanthrone-78 (VO-78), and Violanthrone-79 (VO-79). The results show that adsorption uptake at equilibrium of these model molecules is independent up to a certain initial concentration limit on the structure, the nature, and the total content of heteroatoms present. The final uptake and the dynamics of the adsorption seem to be more determined by the nature and specific content of heteroatoms, which allow them to interact stronger and faster with the macroporous solid surface.

Introduction Asphaltene precipitation is an important problem faced at different levels in petroleum processing, where it has an economic impact associated to oil production. They can precipitate following changes in operational conditions (temperature, pressure, and composition) resulting in precipitation, flocculation, and adhesion of the asphaltene to the reservoir rock or production equipment. Also, they can adsorb on different surfaces prior to precipitation, which is a potential source of bulk of any deposited material. Different authors have studied the adsorption of asphaltene on minerals.1-9 The driving force for research in that area is the impact of asphaltene adsorption in the well and the induced wettability changes over the rocks causing the detriment of oil productivity from the reservoir.4,8,10-14 Those studies have been performed on various minerals with * To whom correspondence should be addressed. Tel.: +1 (403) 2204799 Fax: +1 (403) 284-4852. E-mail: [email protected]. (1) Clementz, D. M. Interactions of Petroleum Heavy Ends with Montmorillonite. Clays Clay Miner. 1976, 312. (2) Collins, S. H.; Melrose, J. C. Adsorption of Asphaltene and Water on Reservoir Rock Minerals. Presented at SPE International Symposium on Oilfield and Geothermal Chemistry, Denver, CO, June 1-3, 1983; SPE #11800. (3) Dean, K. R; McAtee, J. M., Jr. Asphaltene Adsorption on Clay. Appl. Clay Sci. 1986, 313-319. (4) 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, Tulsa, OK, April 20-23, 1986; SPE #14885. (5) Dubey, S. T.; Waxman, M. H. Asphaltene Adsorption and Desorption from Mineral Surfaces. SPE ReserVoir Eng. 1991, 389-395. (6) 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. (7) Pernyeszi, T.; Patzko, A.; Berkesi, O.; Dekany, I. Asphaltene Adsorption on Clays and Crude Oil Reservoir Rocks. Colloids Surf., A 1998, 137, 373-384. (8) Gonza´lez, G.; Moreira, M. B. C. The Wettability of Mineral Surfaces Containing Adsorbed Asphaltene. Colloids Surf. 1991, 58, 293-302. (9) Gonza´lez, G.; Moreira, M. B. C. Adsorption of Asphaltene and Resins on Various Minerals. Asphaltenes and Asphalts; Yen, T. F., Ed.; Elsevier: New York, 1994; p 219. (10) Yan, J.; Plancher, H.; Morrow, N. R. Wettability Changes Induced by Adsorption of Asphaltene. SPE Prod. Facil. 1997, 12, 239-266.

an adsorption range from ∼1-2 mg/m2.3,5,8 In general, Langmuir-Type I isotherms are reported, meaning that asphaltene molecules cover the solid surface available for adsorption.5 Recent studies show that they may form multilayers on minerals depending of the type of asphaltene and their contact time with the surface.15-17 Commonly, the asphaltene is defined as a fraction of a crude oil insoluble in n-heptane and soluble in toluene, so they are not a chemical family per se. They contain a large heterogeneity of chemical functionalities, polynuclear aromatics, and they are the heaviest components of crude oils with the highest heteroatoms contents.18 Because of the complexity of the asphaltene structures, the studies of their adsorption behavior have had some limitations. Very recently, Yarranton19 reported that, depending on the precipitation method, different asphaltene models can be (11) Crocker, M. E.; Marchin, L. M. Wettability and Adsorption Characteristics of Crude-Oil Asphaltene and Polar Fractions. J. Pet. Technol. 1988, 470-474. (12) Buckley, J. S.; Liu, Y.; Monsterleet, S. Mechanism of Wetting Alteration by Crude Oils. SPE J. 1998, 3, 54-61. (13) Al-Maamari, R. S. H.; Buckley, J. S. Asphaltene Precipitation and Alteration of Wetting: Can Wettability Change during Oil Production? Presented at the 2000 SPE/DOE IOR Symposium, Tulsa, OK, Apr 3-5, 2000; Paper SPE 59292. (14) Amroun, H.; Tiab, D. Alteration of Reservoir Wettability Due to Asphaltene Deposition in Rhourd-Nouss Sud Est Field, Algeria. Presented at the 2001 RMPTC, Keystone, May 21-23, 2001; Paper SPE 71060. (15) Acevedo, S.; Castillo, J.; Fernandez, A.; Goncalves, S.; Ranaudo, M. A. A Study of Multilayer Adsorption of Asphaltene on Glass Surfaces by Photothermal Surface Deformation. Relation of this Adsorption to Aggregate Formation in Solution. Energy Fuels 1998, 12, 386-390. (16) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutierrez, L.; Ortega, P. Adsorption of Asphaltene and Resins on Organic and Inorganic Substrates and Their Correlation with Precipitation Problems in Production Well Tubing. Fuel 1995, 74, 595-598. (17) 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, 145-152. (18) Speight, J. G. Petroleum Chemistry and Refining; Taylor & Francis: Washington, DC, 1998. (19) Yarranton H. W. Issues in Characterizing Asphaltene and other Heavy Fraction Components. Presented at The 6th International Conference on Petroleum Phase Behavior and Fouling, Amsterdam, The Netherlands, June 19-23, 2005.

10.1021/ef060196+ CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006

Comparing Asphaltene Adsorption with Model HeaVy Molecules

Energy & Fuels, Vol. 21, No. 1, 2007 235

produced. Another aspect regarding asphaltene structure is related with the nature of the crude oil used. Thus, different approximations of their molecular structure can be proposed.20 In this sense, Gray and co-workers have synthesized and used a set of well-defined molecules such as pyrene and hexabenzocoronene derivatives to study the associative properties of asphaltene.21,22 They studied the self-association in diluted solution in o-dichlorobenzene and found that pyrene and dipyrenyl decane did not give significant association in diluted solution while the presence of polar functional groups, such as ketones and hydroxyls, gave stronger association of pyrene derivatives, resulting in dimer formation in the range of 75 and 100 °C. More recently, the same authors reported the synthesis and characterization of C6- and C9-hexasubstituted hexabenzocoronenes (HBCs), using them for characterization of the associative properties of C6-HBC. In particular, these studies showed that C6-HBC tends to form dimers in diluted solutions. The addition of o-dichlorobenzene reduced but did not eliminate self-association. Moreover, experimental studies at high temperature showed that C6-HBC tends to self-associate up to 400 °C, and computational results supported these observations, indicating the self-association of C6-HBC through the favorable interplay of alkyl-alkyl and δ-δ stacking interactions.23 Extending that line of reasoning, the use of model compounds that show similar solubility behaviors to those of asphaltene, such as being soluble in toluene and insoluble in n-heptane, and that contain some of the structural features reported for them could be useful. This would constitute a good and fast approximation to study their adsorptive properties as well as a contribution to both understanding what triggers their deposition and their adsorptive capabilities for selective polar asphaltene separation uses. In this paper in specific, the use of a set of structurally known heavy model molecules helps to minimize the difficulties associated to the studies of asphaltene adsorption due to their complex structure. These model molecules may include specific properties and contain chemical functionalities similar to the ones present on some averaged well-characterized asphaltenic fractions, such as molecular weight, aromaticity, naphtenicity levels, and number and nature of functional groups. To explore this alternative, in this article we report on the use of three model molecules which have a molecular weight between 700 and 2500 g/mol and contain some functional groups that have been identified for Athabasca bitumen C7asphaltene.24 Two of those model molecules, Violanthrone 78 and 79 (VO-78 and VO-79, respectively) represent the asphaltene of the continental type, which consists of one rather large aromatic region per molecule plus side chains. Those molecules attempt to resume the views of different authors.24-26 In the

other hand, amine modified Xylenol Orange tetrasodic salt (XYO) is used to resemble the “archipelago” molecular model of asphaltene.27-31 The objective is to progressively assess differences and similarities by comparison of model molecules adsorption with the asphaltene adsorption process on macroporous kaolin prepared to minimize physical constraints for the access of both asphaltene and model molecules. The level of adsorption is determined from the change of concentration of the model molecules and/or asphaltene once the solid is added to the solution. The solid was prepared by achieving a large macroporosity in order to minimize physical constraints for the access of the molecules to the surface of the adsorbent.

(20) Carbognani, L. Facts and Some Unknown Aspects of Asphaltene and Heavy Hydrocarbon Fractions. Presented at Workshop, University of Calgary, Alberta, Canada, Nov 2004. (21) Akbarzadeh, K.; Bressler, D. C.; Wang, J.; Gawrys, K. L.; Gray, M. R.; Kilpartrick, P. K.; Yarranton, H. W. Association Behavior of Pyrene Compounds as Models for Asphaltene. Energy Fuels 2005, 19, 1268-1271. (22) Rakotondradany, F.; Fenniri, H.; Rahimi, P.; Gawrys, K.; Kilpartrick, P. K.; Gray, M. R. Synthesis and Properties of Model Compounds for Bitumens Residue. Presented at The 6th International Conference on Petroleum Phase Behaviour and Fouling, Amsterdam, The Netherlands, June 19-23, 2005. (23) Rakotondradany, F.; Fenniri, H.; Rahimi, P.; Gawrys, K.; Kilpartrick, P. K.; Gray, M. R. Hexabenzocoronene Model Compounds for Asphaltene Fractions: Synthesis & Characterization. Energy Fuels 2006, 20, 24392447. (24) Groenzin, H.; Mullins, O. C. Molecular Size and Structure of Asphaltene from Various Sources. Energy Fuels 2000, 14, 677 (25) Zhao, S.; Kotlyar, L. S.; Woods, J. R.; Sparks, B. D.; Hardacre, K.; Chung, K. H. Molecular transformation of Athabasca bitumen end-cuts during coking and hydrocracking. Fuel 2001, 80, 1155-1163.

Experimental Section Materials. Toluene (spectrophotometric grade, Sigma-Aldrich) was used for the adsorption experiments. The model molecules selected were Violanthrone-78, Violanthrone-79 (95%, both from Sigma-Aldrich), and Xylenol Orange tretrasodium salt (OmniPur-EDM), named all the time in this work as VO-78, VO-79, and XYO, respectively, and with molecular weights in the range of 700-2500 g/mol. The molecular structures of those model molecules are displayed in Figure 1. The Xylenol Orange (XYO), initially water soluble, was transformed to be organic soluble by adapting a procedure reported in the literature.32 Triooctylamine (95%, SigmaAldrich), concentrated hydrochloric acid, and anhydrous magnesium sulfate (reagent grade, Sigma-Aldrich) were used for the Xylenol Orange modification. C7-Asphaltene from Athabasca bitumen was obtained by n-heptane precipitation and then redissolved in toluene without further washing in the precipitant alkane.33-35 Their average molecular weight was 4000 g/mol, as estimated by vapor-phase osmometry (VPO) reported elsewhere.27 These unwashed asphaltenes contain resins, compounds that are distributed between the maltene-solvent phase and the precipitated solid. The amount of these coprecipitated resins reportedly varied within the range 10-27% w/w according to different authors.27,34 Experiments carried out with this macroporous kaolin, using a resins fraction isolated from this bitumen27 dissolved in toluene, reveal that faster initial adsorption in comparison with the ABC7-A fraction is observed, particularly at the first 120 min. Furthermore, another experiment using a core AB-C7-A fraction, which does not contain resins according to the literature,27 (26) Rogel, E.; Carbognani, L. Density Estimation of Asphaltene Using Molecular Dynamics Simulations. Energy Fuels 2003, 17, 378 (27) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumens and HeaVy Oils; Alberta Energy Research Institute: Calgary, AB, Canada, 2003; pp 459-662. (28) Murgich, J.; Abanero, J. A.; Strausz, O. P. Molecular Recognition in Aggregates Formed by Asphaltene and Resin Molecules from the Athabasca Oil Sand. Energy Fuels 1999, 13, 278-286. (29) Gray, R. M. Consistency of Asphaltene Chemical Structures with Pyrolysis and Coking Behaviour. Energy Fuels 2003, 17, 1566 (30) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Quantitative Molecular Representation and Sequential Optimization of Athabasca Asphaltene. Energy Fuels 2004, 18, 1377-1384. (31) Aguilera-Mercado, B.; Herdes, C.; Murgich, J.; Muller, E. A. Mesoscopic Simulation of Aggregation of Asphaltene and Resin Molecules in Crude Oils. Energy Fuels 2006, 20, 327 (32) Herrmann, W. A.; Kohlpaintner, C. W. Water-Soluble Ligands, Metal Complexes, and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis. Angew. Chem., Int. Ed. Engl. 1993, 32, 1524-1544. (33) 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. (34) 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. (35) Some procedures used for asphaltene precipitation are as follows: UOP 614-68, ASTM D4124, IP 143, Syncrude Analytical Method 5.1.

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Figure 1. Model molecules used in the present work.

indicates that the adsorption behavior is sensibly modified by the presence of resins. Experiments regarding the effect of resins content in the asphaltene fraction on the overall adsorption behavior over this solid are in progress, and the results will be published elsewhere. Adsorbent. The adsorbent characteristics were designed for the solid to have an important macropore’s proportion with an average diameter > 50 nm in order to allow the penetration throughout these pores of the aforementioned molecules and to accommodate them in the macroporous surface of the solid adsorbent. After the preparation of the absorbent, their characterization was performed (pore volume, surface area, pore diameter, etc.) by the Brunauer-Emmett-Teller (BET) method using a CHEMBET-3000 equipment from Quantachrome Instruments and mercury porosimetry. Each sample was dried at 150 °C prior to adsorption experiments and stored in desiccators for further use. Spectrophotometric Measurements. The amount of adsorbate in toluene solution adsorbed on the macroporous solid was determined from the change in concentration of solution before and after being in contact with the macroporous solid. The initial concentration was known from the mass of adsorbate and the volume of toluene used in each sample preparation. The final concentration was measured with a Cary 4E spectrophotometer from Varian Canada, Inc. The absorbance (ABS) measurements were converted to concentrations from a calibration curve using Beer-Lambert’s Law. Spectrophotometric measurements are typically conducted at maximum absorbance wavelengthsthe maximum wavelength determination was conducted following a reported procedure.36 Maximum absorbances were λmax ) 281 nm for AB-C7-A, λmax ) 577 nm for VO-78, λmax ) 633 nm for VO-79, and λmax ) 440 nm for XYO. Wavelength shifts were not observed during UV-vis monitoring of the adsorption process, suggesting that adsorption is not selective. This feature has been observed previously.37

Adsorption Isotherm Experiments. It has been reported that asphaltene adsorption on different solid matrixes reaches equilibrium between 24 and 48 h.16,17 Some authors reported a period of 96 h.9,38 Recently, Alboudwarej et al.39 reported for asphaltene adsorption over metal surfaces that 72 h was enough to reach the steady-state condition. In our experiments, a period of 48 h was sufficient time to reach equilibrium for all model molecules and the Athabasca bitumen C7 asphaltene fraction tested. The adsorption isotherms were determined by measuring the reduction in concentration of the adsorbate in toluene solutions after being in contact with macroporous kaolin. The experimental procedure was as follows: toluene solution (10 mL) of the corresponding adsorbate was transferred to a cylindrical screw-cap glass vial. The adsorbent (1 g) was placed in each vial, closed, and secured with Parafilm. The vials were placed in the fume hood at room temperature for 48 h. Adsorption Kinetics. The kinetics experiments were carried out by continuously measuring the change of absorbance for the solution of either model molecule or asphaltene in contact with the macroporous solid. The instrument used for these experiments was a Cary 4E spectrophotometer from Varian, with dual beam. This procedure was necessary in order to perform direct light absorption measurements of those solutions. Typically, the procedure was as follows: toluene solution (3 mL) of the corresponding adsorbate was placed in a screw-cap cuvette (spectrosil 3.5 m, rectangular cell quartz, open-top cap, light path ) 10 mm, from Sigma-Aldrich) and the initial absorbance (A0) was measured; 0.3 g of the macroporous solid was added, the system was closed, and the cuvette cell was placed in the spectrophotometer; and every 15 min a spectra was recorded for a period of 12 h, in a static mode at room temperature. The kinetics plots were obtained by transforming the absorbance A(t) in relative absorbance RA(t) (in order to make it independent of the initial concentration) as a function

(36) Marczewski, A.; Szymula, M. Adsorption of Asphaltene from Toluene on Mineral Surface. Colloids Surf., A 2002, 208, 259-266. (37) Alboudwarej, H.; Jakher, R. K.; Svrcek, W. Y.; Yarranton, H. W. Spectrophotometric Measurement of Asphaltene Concentration. Pet. Sci. Technol. 2004, 22, 647-664.

(38) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and Adsorption Properties of Asphaltene. Colloids Surf., A 1995, 94, 253265. (39) Alboudwarej, H.; Pole, D.; Svrcek, W.; Yarranton, H. W. Adsorption of Asphaltene on Metals. Ind. Eng. Chem. Res. 2005, 44, 5585-5592.



of time. RA(t) was transformed at solution concentration Cs(t) (mg/L) using eq 1,

Cs(t) ) RA(t)C0

(1)

where RA0 and C0 are the initial relative absorbance and concentration, respectively. The amount of model molecule and asphaltene adsorbed at any time Ca(t) per gram of adsorbent (mg/g) was calculated using eq 2,

Ca(t) ) C0 - Cs(t)V/m

(2)

where V is the solution volume (L) and m is the mass (g) of the macroporous kaolin.

Figure 2. Adsorption behavior of model molecules dissolved in toluene on a macroporous kaolin (295 K).

Results and Discussion The adsorption of asphaltene on different solid matrixes has been studied by different authors.1-10 Normally, the asphaltene fractions employed are previously isolated from crude oil using a precipitation procedure and then redissolved in toluene prior to the adsorption studies.33,34,39 An important disadvantage is in regards to the large amount of solvent and the long time required for performing the precipitation. To overcome this timeconsuming and cumbersome procedure, well-known pure compounds that contain selected chemical structures can be advantageously used to accelerate the development of adsorbents for asphaltene. Functional groups that can provide hydrogen bonding to the surface, polar groups, and a structural backbone of polycyclic aromatic hydrocarbons plus alkyl side chains that are present in proposed asphaltene structures27 are all contemplated within the selected model molecules. Condensed (continental)25-27 or open (Archipelago)28-31 molecular representations are attempted to be covered by the selected compounds. This study focuses on Athabasca bitumen C7-asphaltene. Degradative characterization studies have demonstrated that it contains polyaromatic rings (i.e., coronene, benzo(a)pyrene, anthracene, tryphenylene, and phenanthrene); polar groups, such as ketone, ester, ether, and carboxylic acid; amines; and alkyl side chains attached to aromatic carbons (C2-C9 low molecular weight fraction; C13-C23 high molecular weight fraction).27 Considering the previous information, three model molecules were selected: two of those containing polyaromatic rings (dibenzanthrones), ketone, ester, ether groups with alkyl side chains of C8 and C18, respectively, VO-78 and VO-79, which try to cover the condensed or continental type model.25-27 With those molecules, the objective is to evaluate the effect of the polyaromatic and oxygen groups in the adsorption process and compare it with the asphaltene adsorption behavior. The third molecule, originally a water-soluble dye, was chemically modified in order to make it soluble in toluene (XYO). This molecule contains nonfused aromatic rings (phenyl) and different polar groups such as hydroxyl, ketone, carboxylate, sulfonic acid, and alkyl amine with the aim to simulate a kind of opened or archipelago model.28-31 This molecule has the peculiarity that it contains a high wt % of heteroatoms in comparison with the previous ones. With this molecule, the effect of the type and amount of heteroatoms on the adsorption would be evaluated. Finally, a sample of Athabasca bitumen C7-asphaltene (ABC7-A) was obtained in the lab33-35 in order to determine its adsorption behavior on the macroporous kaolin in comparison with the adsorption behavior of the mentioned model molecules.

Adsorption Experiments The adsorption studies were performed over macroporous kaolin at 22 °C. Several solutions at different concentrations from 1 to 30 000 mg/L for asphaltene and 1-10 000 mg/L for the model molecules were used. The initial and final concentrations were determined via UV/vis. One aspect that has to be considered regards the potential chemical difference between adsorbed and bulk asphaltene that may affect the determination of the bulk concentration by UV absorbance. In this sense, Alboudwarej et al.37 found during spectrophotometric measurement of different asphaltene fractions that the concentration of asphaltene in solution shows that a difference between the calibration constants of the soluble vs the whole asphaltene is in the order of 8%. The difference in the results can be interpreted in terms of the average molar mass of the associated asphaltene in the toluene solutions used to determine the calibration constants. At low concentrations, the asphaltene selfassociates to a limited extent and there is little difference in the molar masses of each sample. At higher concentrations, asphaltene associates to higher average molar masses and the difference between the asphaltene from each phase is more pronounced. The original calibration plots may be used with reasonable accuracy if the expected change in the molar mass of asphaltene in solution is small.37 Considering that we are working at low concentration (∼60 ppm), molar mass changes are small; therefore, the data obtained with our UV/vis equipment can be handled with good reliability. In Figure 2, the uptake at equilibrium ((mg of adsorbed molecule)/(g of adsorbent)) for the model molecules and asphaltene versus the initial concentration (mg/L) at 295 K is displayed. The molecular weight of the model molecules as well as of the AB-C7-A27 is included in the legend. As can be seen in Figure 2, for low initial concentration (up to 2500 mg/L), VO-78, XYO, and ABC7-A have the same mass level of adsorption at equilibrium, suggesting that, at such a concentration, there is no limitation to the adsorption uptake associated to the size of the molecules. The VO-79 showed lower adsorption behavior. At initial concentrations over 2500 mg/L, only XYO and ABC7-A continue adsorbing over the macroporous kaolin, and at initial concentrations above 5000 mg/L, AB-C7-A does not adsorb anymore. The previous results indicate variable adsorption behavior for the model molecules used and for AB-C7-A. Both VO-78 and VO-79 show lower uptake compared to Athabasca bitumen C7 asphaltene, even though those molecules have a lower molecular weight. As is observed, VO-78 showed a higher uptake than VO-79 despite of its bigger size (molar volume calculated: VO-


Gonza´ lez et al.

Figure 3. Adsorption isotherms of model molecules dissolved in toluene solution on a macroporous kaolin (295 K). Table 1. Molecule Uptake and Surface Coverage over the Macroporous Kaolin at 295 K molecule

uptake (mg/g)

molecules ((1023)/m2)

XYO VO-78 VO-79 AB-C7-A

88.0 24.0 12.0 48.0

18.2 11.8 8.4 6.0

79 ) 585.9 ( 3.0 cm3 vs VO-78 ) 910.3 ( 3.0 cm3). Meanwhile, XYO keeps adsorbing until C0 ) 10 000 mg/L, despite its bigger molecular size. At this stage, it can be said that the uptake at equilibrium of the model molecules over this macroporous kaolin is not governed by molecular size. Even the largest molecular weight (MW) adsorbate AB-C7-A asphaltene is able to enter the 50 nm kaolin pores. The adsorption is probably related to the nature of functional groups present on such molecules, an aspect that will be discussed later on. Figure 3 shows the adsorption isotherms for AB-C7-A asphaltene redissolved in toluene and the model molecules dissolved in toluene at ambient temperature. The equilibrium uptake (mg/g) of the molecules vs the solution concentration at the equilibrium (mg/L) is presented. According to Figure 3, all the adsorption isotherms show a Langmuir Type I behavior, and the amount of adsorbed model molecule and AB-C7-A at equilibrium is indicated in mg/m2. A type I isotherm suggests that the surface of the macroporous kaolin is covered with adsorbate, leading to a monolayer. At ambient temperature, the maximum adsorption was observed for XYO (∼88 mg/g), while for asphaltene, VO-78 and VO-79, the adsorption reached approximately 48 mg/g, 22 mg/g, and 11 mg/g, respectively. For AB-C7-A asphaltene, the uptake of 48 mg/g value is in the range of previous reports for asphaltene over poorly crystallized kaolin (48.6 mg/g).3 With the isotherm curves obtained for each molecule, the surface coverage is calculated and the results are presented in Table 1. From Table 1, it is observed that VO-79 and AB-C7-A showed close surface coverage of the solid. It may be an indication that this model molecule and the asphaltene have a similar type of accommodation on the active sites of the kaolin surface (silica-framework-alumina-framework). VO-78 and XYO have 2× and 3× more coverage of the kaolin surface with respect to the AB-C7-A, suggesting that both molecules interact with the solid surface either by accommodating more molecules per site or by forming a stacking of interlayer energy within the magnitude of the monolayer formation; otherwise, a Langmuir isotherm type would not be maintained.

Figure 4. Kinetics of the adsorption of model molecules and ABC7-A in toluene solution on macroporous kaolin (295 K).

The different adsorption uptake and surface coverage displayed for the model molecules as well as for AB-C7-A is an indication that their molecular structures impact their adsorption behavior over the studied solid. If we analyze carefully the chemical structure of all model molecules, it is found that VO78 and VO-79 contains the same aromatic framework and the difference between them regards the number of oxygen atoms. VO-79 contains four oxygen atoms (two ketone and two ether groups; 8.98 wt % O), while VO-78 contains six oxygen atoms (two ketone and two ester groups; 9.4 wt % O). The results above suggest that the higher the oxygen atoms content in the molecule, the higher is the adsorption in the macroporous solid. Regarding the XYO, this molecule contains the largest amount of heteroatoms (13 O, 7 N, and 1 S atoms: 8.57 wt %, 4.04 wt %, and 1.32 wt %, respectively) as well as the higher molecular weight of the model molecules tested. However, the larger uptake displayed suggests that these heteroatoms may be responsible for the strong interaction with the active sites of the adsorbent surface in the same way that it was reported by Kokal et al. for C5 asphaltene from a Sullield (South Eastern Alberta) crude oil.38 The AB-C7-A is the largest MW studied adsorbate (about 4 000-6 000 MW reportedly40) and contains heteroatoms in different proportions: nitrogen (1.1 wt %), oxygen (1.3 wt %), and sulfur (8.2 wt %).27 It seems that these asphaltenes have an intermediate type of heteroatom sufficiently exposed to the active sites of the adsorbent, which leads to the observed uptake. Kinetics of the Adsorption The adsorption kinetics were determined for each model molecule and the AB-C7-A. In all experiments, the absorbance A(t) was converted in relative absorbance RA(t) ((absorbance at time t)/(initial absorbance)) vs time (min) in order to make it independent of the initial concentration. Typical errors in the absorbance scale do not exceed 10% relative. The results are displayed in Figure 4, where the MW and the chemical formula of the different model molecules kinetically evaluated are included. At the initial concentration of 60 mg/L, it is observed that all model molecules have faster initial adsorption rates compared with the Athabasca asphaltene. It is now evident that the rate of adsorption strongly depends on specificities of the molecules (40) Agrawala, M.; Yarranton, H. W. Asphaltene Association Model Analogous to Linear Polimerization. Ind. Eng. Chem. Res. 2001, 40, 46644672.



Table 2. Apparent First-Order Rate Constant for the Adsorption of Model Molecules and AB-C7-A in Toluene Solution over Macroporous Kaolin

Table 3. Effect of the Heteroatoms Content on the Kinetics of the Adsorption of Model Molecules and AB-C7-A in Toluene Solution on Macroporous Kaolin (295 K)

a

molecule

K × 103 a (min-1)

XYO VO-78 VO-79 AB-C7-A

10.3 ( 0.4 2.4 ( 0.6 2.1 ( 0.6 1.8 ( 0.4

R2 b 0.996 0.980 0.987 0.995

At 295 K. b Correlation coefficient for first-order fitting.

being adsorbed. Starting from Figure 4 and considering the kinetics analysis reported previously for precipitated asphaltene from Furrial crude oil,15 it was possible to calculate the kinetics constant, fitting those values for a first-order reaction, and the results obtained are shown in Table 2. As can be seen therein, the adsorption of XYO over this solid is 5× faster than that of AB-C7-A, while those of VO-78 and VO-79 are 1.2× faster than that of asphaltene. The fact that XYO adsorbs faster than the other model molecules suggest that a different parameter other than molecular weight controls the kinetics of adsorption. A good fitting to the proposed order kinetics was also observed for every studied molecule and for asphaltene. An analysis of the kinetics data targeting a mechanistic adsorption model implies assessment of the relevance of diffusion to the outer surface of the solid, diffusion along the interface, and diffusion into the pores, which can extensively complicate the interpretation of the data.41 Assuming that the particle size and the pore size of our solid help to avoid internal diffusion control of the adsorption process, we can only consider relevant the external diffusion for the purpose of our study. Let’s assume [C]b, [C]I, and [C]s are the sample concentrations corresponding to model molecules in the bulk, interface, and surface, respectively. Then k1, k_1, and k2 would be rate constants corresponding to diffusion from the bulk to the outer surface, for the reverse of this process and for adsorption. Then, the following expression would apply for the adsorption K1

K2

C(b) {\ } C(i) 98 C(s) K -1

(3)

Applying the steady-state approach for C(i), the following equation is obtained for the rate of adsorption:

Rate: k2k1[C(b)]/(k-1 + k2)

(4)

When k2 . k1, it is obtained

(5)

Rate: k1[C(b)]

(6)

If external diffusion were not relevant under the conditions we used, the initial rate expression would be similar, but instead of k1, eq 6 would contain k2 instead. Whether external diffusion or the chemical process controls the kinetics, it does not change the qualitative interpretation reflected in Figure 5, in the sense that MW has a minor role in determining the adsorptive properties of asphaltene and model molecules on the surface of macroporous kaolin. Figure 5 shows that adsorption at the initial stage does not depend on the molecular weight. For instance, XYO has a MW intermediate between AB-C7-A and the VOs, nevertheless displaying the highest adsorption rate. The Violanthrones, with (41) Giles, H. C. Anionic Surfactant. Physical Chemistry of Surfactant Action. In Surfactant Science Series, Vol. 11; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; Chapter 4.

model molecule

total heteroatom content (wt %)

rate × 106 (mg L-1 min-1)

AB-C7-A VO-79 VO-78 XYO

11.4 8.9 9.3 13.9

0.121 0.140 0.146 9.476

a MW difference of >400 g/mol, present similar initial adsorption rates. A plausible explanation for this behavior may be related to the type and specific content of heteroatoms rather than the molecular weight. The model molecules used in this work allow determining the influence of N, S, and O on the adsorption. An interesting correlation was found by plotting the heteroatoms content in the model molecules and AB-C7-A in weight percentage27 versus the adsorption rate. In Table 3, those results are presented. As is presented in Table 3, for the model molecules, an increase of the rate of the adsorption with the total heteroatoms content is observed. For VO-78 and VO-79, the total heteroatoms content varies 0.4 wt % and the adsorption rate changed 0.06 × 106 mg L-1 min-1. Moreover, a paramount variation is observed when VO-78 is compared with XYO; the total heteroatoms content varies 4.6 wt %, while the adsorption rate increases more that 600×. It seems that increasing the total heteroatoms content increases the adsorption rate. Nevertheless, when XYO is compared with the AB-C7-A, the total heteroatoms content between them varies only 2.5 wt % but the initial adsorption rates change dramatically. These observations indicate that not only is the total heteroatoms content governing the adsorption but also the type of heteroatoms present on these molecules contributes. In order to get an explanation for this finding, Figure 6 presents the content of each heteroatom within the studied samples with their corresponding adsorption rate. As observed therein, the initial rate of adsorption does not correlate with oxygen content. This is evident when moving from asphaltene (O ) 2.8 wt %) to VO-78 (O ) 8.98 wt %), VO-79 (O ) 9.44 wt %), and XYO (O ) 8.57%). Furthermore, when nitrogen is incorporated in XYO (N ) 4.1 wt %) and oxygen is maintained in the same range (O: 8.5 wt %), a paramount effect on the initial adsorption rate is observed. It seems that the presence of nitrogen (amino) in the molecule increases the initial rate of adsorption in comparison with the oxygen contained in the molecules. Geochemical studies reveal that nitrogen compounds present high affinity toward clay mineral surfaces,42,43 which is not surprising given the characteristic surface acidity of aluminosilicates (clays) and the basic character of many nitrogen compounds. Regarding the presence of sulfur, AB-C7-A has the highest content of all samples tested (∼8 wt %). Nevertheless, sulfur does not seem to help the rate of adsorption of asphaltene when compared with the model molecules being tested. The lack of polarity provided by sulfur compounds is a well-known issue that complicates the chromatographic separation between aromatic compounds and sulfur heterocyclics.44 (42) Hill, H. J.; Milburn, J. D. SPE Rep. Ser. 2003, 55, 31-38. (43) Li, M.; Larter, S. R.; Frolow, Y. B. Adsorptive Interaction between Nitrogen Compounds and Organic and/or Mineral Phases in Subsurface Rocks. J. High Resolut. Chromatogr. 1994, 17, 230-236.


Gonza´ lez et al.

Figure 5. Adsorption rate vs molecular weight of model molecules and AB-C7-A in toluene solution on macroporous kaolin (295 K).

Figure 6. Effect of the type of heteroatoms content on the kinetic of the adsorption of model molecules and AB-C7-A in toluene solution on macroporous kaolin (295 K).

Conclusions The present study shows that the adsorption uptake on macroporous kaolin of Athabasca bitumen C7-asphaltene and that of selected model molecules is independent of their molecular size. The total uptake might depend more on molecular specificities such as structure and heteroatom content and type. With the kinetics of the adsorption of those molecules (44) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions. Marcel Dekker: New York, Basel, Hong Kong, 1994.

being found to be of first order, it seems to be driven by the nature and specific content of heteroatoms present on these molecules. Furthermore, the results indicate that Violanthrone79, a molecule that resembles more an asphaltene molecular model of the “continental” type, saturates the surface of kaolin at a similar molecular extent than the reportedly open asphaltene used in this study. On the contrary, model molecules of the archipelago type adsorbed much more abundantly and strongly than the asphaltene tested; all of this suggests that the hydrocarbon skeleton of these asphaltenes plays a relatively minor role in the adsorption of asphaltene with respect to the



heteroatom nature and content in these molecules for the adsorption on macroporous kaolin. Oxygen and particularly nitrogen present in the structure of the adsorbate molecules were found to be the heteroatoms determining the adsorption dynamics and extent over this solid. This work shows that it is possible to model the adsorptive properties of the Athabasca bitumen precipitated asphaltene via successive approximations both in molecular size and chemical functionalities using structurally known heavy molecules. The overall results of this paper suggest that a model molecule which may resemble the adsorption behavior of asphaltene should have approximate similarities in the proportion and nature of heteroatoms instead of similar molecular size or similar structural hydrocarbon frame. Future papers from the authors will expand on research being performed in that direction.

Acknowledgment. The authors acknowledge Mr. Lante Carbognani for his numerous and useful discussions that increased the quality of this article. Also we acknowledge the financial support provided by several institutions for the creation of a research group on topics related to the content of this paper. In particular, this work is being supported by the Alberta Ingenuity Centre for Insitu Energy (AICISE) of the University of Calgary, Alberta Ingenuity Fund through the Ingenuity Scholar support provided to P.P.-A.. Finally, we acknowledge the financial support in the form of discovery and strategic research grants respectively provided by the National Science and Engineering Research Committee of Canada (NSERC) and the Alberta Energy Research Institute (AERI) of the province of Alberta, Canada. EF060196+

Comparing Asphaltene Adsorption with Model Heavy Molecules

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