Characterization of Precipitation, Gelation, Yield Stress, and Morphology

Mar 19, 2009 - in yield stress when they were highly aggregated. Oscillatory testing showed that in such cases asphaltene-asphaltene interactions cont...
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Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology Jack F. Tinsley,*,† Justin P. Jahnke,‡ Heather D. Dettman,§ and Robert K. Prud’home*,| Department of Chemical Engineering, Princeton UniVersity, Princeton, New Jersey 08544 and National Centre for Upgrading Technology, 1 Oil Patch DriVe, Suite A202, DeVon, Alberta, Canada T9G 1A8 ReceiVed August 3, 2008. ReVised Manuscript ReceiVed January 31, 2009

We examine the effects of asphaltenes upon the crystallization behavior of a model waxy oil. Yield stress measurements on the model waxy oils with asphaltenes isolated from Shengli crude oil showed that both the relative amount of wax to asphaltenes and the aggregation state of the asphaltenes affected the crystallization properties of the wax. At very low asphaltene concentrations and high wax concentrations, the yield stress of the waxy gel is not significantly affected. At higher asphaltene concentrations, the asphaltenes significantly degraded the microscopic structure of the wax network and drastically reduced the yield stress. There is a threshold ratio of ∼100 paraffin/asphaltene molecules for such behavior. Asphaltenes produced large decreases in yield stress when they were highly aggregated. Oscillatory testing showed that in such cases asphaltene-asphaltene interactions contributed to the gel strength, in addition to the wax platelet interactions. Asphaltenes increased the wax precipitation temperature at high concentrations when large aggregates were present. However, at lower concentrations where the asphaltenes were less aggregated they suppressed precipitation. The aliphatic nature of the Shengli asphaltenes is an important determinant of the observed decrease in precipitation temperature and yield stress.

Introduction Wax precipitation causes several problems in crude oil recovery. Higher viscosity increases pumping requirements, deposition on pipes and equipment restricts flow, and gelation of wax during periods of temporary shut-down require enough pumping capacity to exceed the yield stress of the waxy gel.1 Saturated hydrocarbon waxes are the highest molecular weight nonpolar fraction of crude oil and can be classified as branched (iso-paraffins), cyclic, or linear (normal or n-paraffins), the latter of which dominate crystallization properties.2,3 Colder temperatures induce crystallization of n-paraffins, usually into orthorhombic crystals.4 When a sufficient amount of crystals has precipitated, they overlap with each other and form a gel with solid-like properties.5 Due to their large aspect ratios, only small volume fractions are needed for gelation; gelation has been observed with as little as 0.5% wax.6 To combat wax crystal* To whom correspondence should be addressed. E-mail: prudhomm@ princeton.edu (R.K.P.), [email protected] (J.F.T.). † The contribution of this author was performed while at Princeton University. Current address: BASF SE, GVC/F s J 500, 67065 Ludwigshafen, Germany. ‡ The contribution of this author was performed while at Princeton University. Current address: Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106-5080. § National Centre for Upgrading Technology. | Princeton University. (1) Misra, S.; Baruah, S.; Singh, K. SPE Prod. Facil. 1995, 10 (1), 50– 54. (2) Garcia, M. D.; Orea, M.; Carbognani, L.; Urbina, A. Pet. Sci. Technol. 2001, 19 (1-2), 189–196. (3) Garcia, M. D.; Urbina, A. Pet. Sci. Technol. 2003, 21 (5-6), 863– 878. (4) Dirand, M.; Bouroukba, M.; Chevallier, V.; Petitjean, D. J. Chem. Eng. Data 2002 2002, 47, 115–143. (5) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16 (2), 352–355. (6) Paso, K.; Senra, M.; Yi, Y.; Sastry, A. M.; Fogler, H. S. Ind. Eng. Chem. Res. 2005, 44 (18), 7242–7254.

lization, semicrystalline polymers have often been employed that interfere with the wax crystallization and aggregation processes. Such polymers usually lead to crystals that are smaller and/or deformed in shape and can reduce yield stress, gelation temperatures, and even the wax precipitation temperature.7-9 The interactions of wax with asphaltenes has been a subject of study during the past decade.10-18 Asphaltenes themselves are defined as a solubility class: the fraction of crude oil that precipitates in light n-alkanes (e.g., heptane) but dissolves in toluene. Chemically this class of compounds contains condensed aromatic rings, heteroatoms (such as N, S, O), often heavy metals (such as Ni, Fe, V), and aliphatic chains.19 Under most conditions asphaltenes do not exist as separate molecules but as colloidal aggregates. Above concentrations of ∼0.01 wt % (7) Ashbaugh, H. S.; Radulescu, A.; Prud’homme, R. K.; Schwahn, D.; Richter, D.; Fetters, L. J. Macromolecules 2002, 35, 7044–7053. (8) Pedersen, K. S.; Ronningsen, H. P. Energy Fuels 2003, 17 (2), 321– 328. (9) Tinsley, J. F.; Prud’homme, R. K.; Guo, X. H.; Adamson, D. H.; Callahan, S.; Amin, D.; Shao, S.; Kriegel, R. M.; Saini, R. Energy Fuels 2007, 21 (3), 1301–1308. (10) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13 (2), 351–358. (11) Chanda, D.; Sarmah, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam, B.; Das, H. C. Fuel 1998, 77 (11), 1163–1167. (12) Garcia, M. D. Energy Fuels 2000, 14 (5), 1043–1048. (13) Garcia, M. D.; Carbognani, L. Energy Fuels 2001, 15 (5), 1021– 1027. (14) Kriz, P.; Andersen, S. I. Energy Fuels 2005, 19 (3), 948–953. (15) Oh, K.; Deo, M. D. Energy Fuels 2002, 16 (3), 694–699. (16) Oliveira, G. E.; Mansur, C. R. E.; Lucas, E. F. J. Dispersion Sci. Technol. 2007, 28 (3), 349–356. (17) Venkatesan, R.; Ostlund, J. A.; Chawla, H.; Wattana, P.; Nyden, M.; Fogler, H. S. Energy Fuels 2003, 17 (6), 1630–1640. (18) Yang, X. L.; Kilpatrick, P. Energy Fuels 2005, 19 (4), 1360–1375. (19) Sjoblom, J.; Aske, N.; Harald Auflem, I.; Brandal, O.; Erik Havre, T.; Saether, O.; Westvik, A.; Eng Johnsen, E.; Kallevik, H. AdV. Colloid Interface Sci. 2003, 100-102, 399–473.

10.1021/ef800636f CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009

Waxy Gels with Asphaltenes

they form particles a few nanometers in size,20,21 consisting of ∼8 molecules.20 It has been proposed that the aromatic regions form the core of such particles and the aliphatic chains are on the periphery to interact with the surrounding oil.20,22 At higher concentrations larger clusters of these nanoaggregates form, which are more weakly bonded and can be broken up at higher temperatures.17,20,23,24 A handful of studies have examined the effects of asphaltenes upon wax crystallization properties. Studies by Venkatesan et al.,17 Kriz and Andersen,14 and Oliviera et al.16 used a model oil consisting of asphaltenes, a waxy oil, and an aromatic solvent, the latter of which provided solubility for the asphaltenes. Such a system allowed the independent control of asphaltene concentration. Other studies have used dead crude oils, comparing the behavior of the original oil to that of the oil after the asphaltenes have been removed.11,12 Since the asphaltenes must be removed by solvent precipitation, this requires reconstitution of the oil, which has led to problems when reintroducing the asphaltenes due to asphaltene flocculation.12 These latter studies have provided information about the effect of asphaltenes but are limited in their ability to control the amount and thus to quantify the role of asphaltene concentration. These studies have shown that asphaltenes generally decrease the strength of waxy gels. Decreases were observed in gelation temperature,11,16,17 indicating that the asphaltenes act like natural pour point depressants.25 The presence of asphaltenes also decreases the yield stress. For example, Chanda et al.11 removed asphaltenes from Indian crude oils and saw an increase in yield stress. Using selective addition of asphaltenes to a model oil, Venkatesan et al.17 observed a monotonic decrease in yield stress with asphaltene addition. Microscopy showed that addition of asphaltenes changed the morphology from rod-like crystals to less-ordered globular structures. Kriz and Andersen14 also observed a decrease in yield stress at higher asphaltene concentrations. However, the yield stress increased at low concentrations (0.01 wt %). The authors proposed that the peak in yield stress corresponded to the point of maximum asphaltene dispersion. However, no complimentary information on the asphaltene or wax structure was provided (such as microscopy). Studies have also shown different effects by asphaltenes on the wax precipitation temperature. Kriz and Andersen14 observed that addition of asphaltenes increased the wax appearance temperature (WAT), again with a maximum at low asphaltene concentration. In the study by Garcia,12 flocculated asphaltenes in a reconstituted oil increased the WAT. Microscopy indicated that the wax crystals grew from these asphaltene particles, which thus served as nucleation sites. However, Oliviera et al.16 observed no significant change in the WAT. The different behavior reported may be due to the varied chemical nature of the asphaltenes studied. Venkatesan et al. tested asphaltenes that were fractionated by polarity and found that the least polar, most aliphatic fraction depressed the gelation (20) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy Fuels 2007, 21 (5), 2785–2794. (21) Storm, D. A.; Sheu, E. Y.; Detar, M. M. Fuel 1993, 72 (7), 977– 981. (22) Carbognani, L.; Rogel, E. Pet. Sci. Technol. 2003, 21 (3-4), 537– 556. (23) Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Energy Fuels 2004, 18 (5), 1243–1249. (24) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 267, 178–193. (25) Ronningsen, H. P.; Bjorndal, B.; Hansen, A. B.; Pedersen, W. B. Energy Fuels 1991, 5 (6), 895–908.

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Figure 1. Composition of multicomponent wax as determined by gas chromatography.

temperature to the largest extent, whereas the most polar fraction had little effect.17 However, as Kriz and Andersen concluded, the state of asphaltene aggregation may also affect the wax crystallization behavior. Asphaltene dispersion was also identified as an important factor by Chanda et al.11 based on measurements using solvents that strongly influenced asphaltene aggregation (e.g., benzene or hexane). Addition of such solvents to the original oil reduced viscosity significantly, whereas their addition had little effect in oils in which the asphaltenes and resins had been removed. The aims of this study were two-fold. One aim was to examine the effects of both asphaltene concentration and asphaltene aggregation on wax crystallization behavior. The second goal was to characterize the effects of asphaltenes on a model waxy oil in order to provide a baseline for tests with wax control polymers, which have been presented separately.26 This study measured three properties related to different stages in the crystallization process, namely, precipitation temperature, gelation temperature, and yield stress. In addition, the morphology of the wax crystals and asphaltene aggregates were examined by optical microscopy, providing insight into the gelation and yield stress results. Oscillatory rheological measurements were also employed. These measured the dynamic moduli before yielding and thus provided additional structural information about the waxy gel. Experimental Section Model Oil. Wax. The wax was a blend of two waxes from Aldrich: 55 wt % Aldrich No. 327204 with a melting point from 53 to 57 °C and 45 wt % Aldrich number 411663 with a minimum melting point of 65 °C. It provided a multicomponent system whose continuous carbon-number distribution ranged from C20 to C47 as shown in Figure 1. It contained about 85% n-paraffins, with the remainder being branched and cyclic paraffins, and thus provided a system whose interactions would be dominated by its crystalline character. Asphaltenes. The asphaltenes from Shengli Oil (China) were used for this study. This crude oil was waxy with a significant content of asphaltenes (38% wax and about 3% heptane-insoluble asphaltenes). The ratio of H/C was 1.41, indicating the content of saturated carbon. The asphaltenes were isolated from the vacuum residue of the Shengli crude by precipitation with n-heptane: 40 mL of heptane and 1 g of vacuum residue. The heptane was HPLC grade (99.4%) from Fisher Scientific, and the vacuum residue was provide by the National Center for Upgrading Technology (Devon, Canada). Heptane was added to the vacuum residue, sonicated for 45 min, and let stand overnight. The next day the mixture was sonicated for 15 min, filtered through a medium porosity fritted glass filter (10-15 µm), and dried in a vacuum oven. Because long-term (26) Tinsley, J. F.; Jahnke, J. P.; Dettman, H. D.; Prudhomme, R. K. Waxy Gels with Asphaltenes 2: Use of Wax Control Polymers. submitted.

2058 Energy & Fuels, Vol. 23, 2009 exposure to oxygen or light may degrade the asphaltenes,27 the asphaltenes were stored under argon and in the dark. SolVent. A combination of Norpar12 (ExxonMobil) and 1-methylnaphthalene (97%, Fisher Scientific) was used as a solvent. Norpar12 is a mixture of normal alkanes from C10 (decane) through C14 (tetradecane). 1-Methylnaphthalene (97%, Fisher Scientific) was used at 22 wt % to provided an aromatic solvent for the asphaltenes. This concentration was chosen to match the fraction of aromatic carbon in the original crude oil, as determined by 13C NMR.28 Both solvents had high boiling points (at least 185 °C) that enabled high temperatures to be used without significant solvent loss. Model Oil Preparation. The wax was dissolved in Norpar12. Separately, the asphaltenes were added to the 1-methylnapthalene and shaken to disperse them. The appropriate amount of asphaltene solution was then added to the wax solution and heated to ∼90 °C before use. Precipitation Temperature. The onset of wax precipitation was measured via differential scanning calorimetry (DSC) using a Pyris 1 differential scanning calorimeter from Perkin-Elmer (Norwalk, CT). Enthalpies and temperatures were calibrated with indium and cyclohexane standards. Stainless steel sample pans were used with O-rings that provided hermetic seals. Samples were initially heated to 90 °C for 3 min, and a 2 °C/min cooling rate was employed over the range of temperatures where precipitation occurred. The onset of precipitation was recorded as the temperature at which the first increase from the baseline occurred. Rheological Tests. All rheological tests described below were performed on a MCR501 rheometer by Anton-Paar Physica (Graz, Germany) that was equipped with a parallel plate geometry. The temperature of the lower surface was controlled by a Peltier plate, and a hood with a Peltier heating device was used to heat the parallel plate and limit evaporation of the solvent. The liquid samples were accommodated with a simple dish/collar assembly provided by the manufacturer that mounted on top of the lower Peltier plate. A 50 mm parallel plate with a 1 mm gap was employed. To prevent shear slip a serrated parallel plate was employed, and the surface of the sample dish was roughened by use of a wire brush. Solutions to be loaded on the rheometer were first heated to 90 °C to completely dissolve all wax and/or polymer. For each sample, approximately 4 mL of liquid was loaded on the rheometer, which had been preheated to 90 °C for at least 5 min. Once the parallel plate was lowered, no air was passed through the hood to minimize solvent loss, and the sample was held at 90 °C for at least 5 min to equilibrate before beginning the test. Gelation Temperature. After the initial holding time at 90 °C, the sample was cooled rapidly to 40 or 35 °C. The temperature was chosen to ensure that the sample remained above the wax precipitation temperature. After allowing 30 s for the temperature to stabilize, the temperature was decreased at 0.5 °C/min while imposing a 0.05 Pa oscillatory stress at 2 rad/s. The onset of gelation was indicated by the increase of the elastic and viscous moduli (G′ and G′′) from low levels, characteristic of a liquid to higher levels associated with a gel. Frequently, the gelation temperature is taken as the temperature at which G′ crosses above G′′.29 However, for these very low viscosity fluid samples the crossover is not easily identified. Because G′ and G′′ were both roughly 10-3 Pa in the liquid state and because G′ ultimately reached values of ∼103 Pa, the gelation point was taken as the temperature at which G′ ) 1 Pa. Yield Stress (Unidirectional and Oscillatory). Yielding behavior of the gelled waxy solutions was characterized by unidirectional stress ramps and oscillatory stress ramps. For both measurements, the sample was cooled to 0 °C and was held at this temperature for 20 min, after which time a stress ramp was initiated. The cooling (27) Strausz, O. P.; Lown, E. M., Chemistry of Alberta Oil Sands, Bitumens and HeaVy Oils; Alberta Energy Research Institute: Calgary, 2003. (28) Japanwala, S.; Chung, K. H.; Dettman, H. D.; Gray, M. R. Energy Fuels 2002, 16, 477–484. (29) Venkatesan, R.; Singh, P.; Fogler, H. S. SPE J. 2002, 7 (4), 349– 352.

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Figure 2. Typical results from a yield stress test.

rate imposed was the cooling limit of the Peltier device in the rheometer, which was consistent through all tests. For the range of precipitation and gelation temperatures of the waxy solutions used in these studies the instantaneous rate was between 10 °C/min (at higher temperatures, ∼30 °C) and 5 °C/min (at lower temperatures, ∼5 °C). For unidirectional stress ramps, the stress was increased stepwise logarithmically, with 20 stress levels per decade and 8 s per data point. The initial stress was usually 0.05 Pa, and the test finished after the sample yielded. The rheometer was programmed to stop the test when a high rotation speed was reached. Three trials were performed with each sample, reheating each sample to 90 °C for 7 min between trials to completely redissolve the samples. For oscillatory stress ramps, the stress ramp was imposed with an oscillatory motion 10 rad/s frequency, using 10 points per decade and 8 s per data point. The initial stress was usually 0.5 Pa. In a logarithmic ramp an equal amount of time is spent at each stress level, but the steps between each level are spaced logarithmically. As a result, the stress loading rate increases in proportion to the imposed stress. This is significant because of the creeping nature of the waxy gels. Slower stress loading rates lead to lower observed yield stresses and vice versa. However, linear stress ramps were experimentally impractical, because fast rates were not sensitive to very low yield stresses, and the slower stress rates required too much time for samples with high yield stresses. The logarithmic stress ramp thus provided a reasonable method to probe the wide experimental range. In addition, the faster stress loading at higher stresses is partly offset by the fact that longer times are spent at lower stresses, providing more time for creep. Figure 2 shows the results from a typical unidirectional yielding test. Initially, the low applied stress was below the yield stress, and the resulting viscosity was essentially infinite. As the stress increased further, the gel crept for a period of time and then catastrophically failed.30-32 The stress at failure was recorded as the yield stress. Microscopy. Microscopic examination of waxy samples was performed on one of two microscopes, both of which were used in transmission mode and had cross polarization optics. The first was an Axioplan 2 from Carl Zeiss (Oberkochen, Germany). Objectives and numerical apertures (NA) were as follows: 10× (0.30NA), 20× (0.40NA), and 40× (0.60NA). The 40× had color correction for the coverslip. Images were captured with a Carl Zeiss Axiocam HRc camera using Carl Zeiss Axiovision 3.1 software. Samples viewed with this microscope were cooled to 0 °C and then imaged. Samples were prepared as follows: The original wax solution was heated to dissolve all wax and then loaded into a preheated rectangular glass capillary (4 mm × 0.4 mm), the ends of which were then sealed with 5 min epoxy. The sample was heated to dissolve the wax (over 60 °C) and then chilled in ice for 10 min to crystallize the wax. Capillaries were removed from the ice, taped to a microscope stage, and imaged. Also used was an Eclipse TE300 inverted microscope manufactured by Nikon (Tokyo, Japan). Objectives and numerical apertures (30) Ashbaugh, H. S.; Fetters, L. J.; Adamson, D. H.; Prud’homme, R. K. J. Rheol. 2002, 46 (4), 763–776. (31) Chang, C.; Boger, D. V.; Nguyen, Q. D. Ind. Eng. Chem. Res. 1998, 37 (4), 1551–1559. (32) Wardhaugh, L. T.; Borger, D. V. J. Rheol. 1991, 35, 1121–1156.

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Figure 3. Effect of asphaltenes upon precipitation temperature: (a) varying wax content and (b) varying asphaltene content.

Figure 4. Effect of asphaltenes upon gelation temperature: (a) varying wax content and (b) varying asphaltene content.

Figure 5. Effect of asphaltenes upon yield stress: (a) varying wax content and (b) varying asphaltene content. Before testing, samples were gelled for 20 min at 0 °C.

(NA) were as follows: 4× (0.10NA), 10× (0.25NA), 20× (0.40NA), and 40× (0.60NA). The 40× objective was shorter for extra long working distances and had color correction for the coverslip. Images were captured with a Qimaging Retiga 1350B camera using IPLab v4.04 software, both of which were supplied by BioVision Technologies (Exton, Pennsylvania). This second microscope allowed the use of a home-built cooling stage for some samples. The stage consisted of copper block roughly 3 cm × 6 cm × 0.7 cm with a channel for cooling water on three sides. A slot in the center permitted insertion of a microscope slide. Typically, a standard microscope slide was scored and cut to length so as to fit almost entirely in the slot. Cooling water was supplied by a temperature bath. A flat thermocouple placed inside the stage monitored the actual temperature of the stage.

Results Precipitation Temperature. The effect of asphaltenes on the precipitation temperatures of model waxy oils is shown in Figure 3. Figure 3a shows the effect of 0.1 wt % asphaltenes at different wax concentrations, and Figure 3b examines the effect of asphaltene concentration for 8 and 10 wt % wax. Addition of 0.1 wt % asphaltenes depressed the precipitation for all the wax concentrations tested (Figure 3a). This depression was between 1.2 and 2.5 °C for 3-10 wt % wax. At 1 wt % wax the depression was higher (5.4 °C). Repeated measurements with 8 and 1 wt % wax (with no asphaltenes) showed standard deviations of 0.5 and 0.7 °C, respectively. Figure 3b shows a clear decrease in precipitation temperature at 0.1 and 0.2 wt % asphaltenes, but no definitive trend in precipitation temperature

as function of asphaltene concentration is clear given the above experimental uncertainties. Gelation Temperature. The effect of asphaltenes upon gelation temperature is shown in Figure 4. As with precipitation temperature, data are shown for both (a) the effect of 0.1 wt % asphaltenes at different wax concentrations and (b) the effect of varying asphaltene concentration. For a set of three gelation tests with 8 wt % wax + 0.2% asphaltenes the standard deviation was 0.6 °C. Single repeats from other concentrations had similar variability. Comparison of samples with asphaltenes and those without shows that the asphaltenes depress the gelation temperature. However, no clear trend is visible between 0.05 and 0.2% asphaltenes. Yield Stress. Effects of asphaltenes upon yield stress are shown in Figure 5, where both (a) varying wax concentrations and (b) varying asphaltene concentrations were tested. Figure 5a shows that the effect of adding 0.1 wt % asphaltenes depended on the wax concentration. At low wax concentration the yield stress was greatly reduced: 84-fold and 140-fold for 3 and 5 wt % wax, respectively. However, the effect was much less at higher wax concentrations, with reductions of 67 and 22% for 8 and 10 wt % wax, respectively. The yield stress of solutions without asphaltenes was found to depend exponentially on the wax concentration, with the exponent being ∼2.5, in accord with results of Venkatesan et al.33 Figure 5b shows the dependence of yield stress for the 8 and 10 wt % wax solutions on the asphaltene concentration. At higher asphaltene concentrations the yield stress was greatly

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Figure 6. Effect of asphaltenes upon oscillatory yield stress for 8 and 10% wax. Asphaltene concentrations were 0 (]), 0.05 ([), 0.1 (•), and 0.2 wt % (2). Before testing, samples were gelled for 20 min at 0 °C. Samples with two-plateau behavior had initial failure at ∼0.1% strain as indicated by the dotted circles.

reduced. At asphaltene concentrations above 0.2 wt % the sample did not gel but precipitated and settled to the bottom of the sample vial. The asphaltenes thus acted like wax control polymers.7 Since the experiments produced error bars of ( 15% we draw no conclusion about the apparent increase in yield stress for 0.05 wt % asphaltenes. The yield stress is not significantly reduced until asphaltene concentrations reach 0.1 wt %. Oscillatory Yield Stress. To further investigate the structure of the gels, the 8 and 10 wt % wax solutions were subjected to oscillatory stress ramps. The results are shown in Figure 6. To understand this graph, consider the data for 8 wt % wax with 0 wt % asphaltenes (Figure 6sleft side, open symbols). As the stress upon the sample increased, a fairly constant elastic modulus (G′) was observed. When the stress was increased to a sufficiently high level, creep was observed (G′ decreased) until the sample finally yielded (indicated by the catastrophic drop in G′ to low levels). Increasing the asphaltene concentration led to lower plateau values and lower yield stresses. Comparison of the 8 wt % wax samples to those containing 10 wt % wax shows that the effect of the asphaltenes was more pronounced at the lower wax concentration, particularly for 0.05 and 0.1 wt % asphaltenes. The trends in yielding are similar whether measured by unidirectional shear or by oscillatory shear. In addition, Figure 6 shows that two plateaus in G′ are observed at higher asphaltene concentrations. For 8 wt % wax these occur at 0.1 and 0.2 wt % asphaltenes. For 10 wt % wax this is observed at 0.2 wt % asphaltenes. These two distinct elastic regions indicate the existence of two types of network connectivity, one of which is more brittle and breaks down at lower stresses. The second transition is more elastic, although it has weaker connectivity. For samples with two-plateau behavior the critical strain of the first transition is ∼0.1%, whereas the final yielding behavior is at ∼10% critical strain. Although more discussion will be provided later, asphalteneasphaltene interactions are likely responsible for this behavior. It is also noted that G′′ also shows a two-plateau behavior. Some of the yield stresses measured in oscillation were up to 50% smaller than those observed in unidirectional testing. Additional tests showed that the yield stress in oscillation was further reduced about 40% by extending the measurement time of each stress level from 8 to 60 s (see Figure 7). Such behavior is attributed to fatigue caused by cyclic deformation in oscillation. The stress loading rate in the oscillatory tests was also different than in the unidirectional test. Both tests had logarithmic stress ramps, but the oscillatory test had 10 points per decade, not 20. However, this would actually reduce the time for creep in the oscillatory test, which would lead to higher (33) Venkatesan, R.; Nagarajan, N. R.; Paso, K.; Yi, Y. B.; Sastry, A. M.; Fogler, H. S. Chem. Eng. Sci. 2005, 60 (13), 3587–3598.

Figure 7. Effect of testing time in oscillatory measurements for 8 wt % wax solutions. Open symbols correspond to data taken using 8 s per data point and filled symbols are data obtained using 60 s per data point. Results are shown for samples with asphaltenes (triangles) and with 0.1 wt % asphaltenes (circles). Samples were gelled for 20 min at 0 °C. A logarithmic stress ramp was used with 10 points per decade.

measured yield stress,31,34 the opposite of what was observed in oscillation. Thus, the effect of the stress loading rate is minor compared to fatigue from cyclical deformation. The creeping, time-dependent nature of the yielding of waxy gels has been documented by others.31,34,35 Microscopy. Figure 8 shows microscopic images for gelled 8 wt % wax solutions with various asphaltene concentrations. These samples were cooled to 0 °C and viewed on the Zeiss microscope. Without asphaltenes, the crystals are rod-like structures about 50 µm long that appear to be composed of bundles of fibers. Addition of 0.05% asphaltenes slightly degrades this structure. Higher asphaltene concentrations have a greater effect. The rod-like structures are less apparent, and cross polarization shows less contrast, reflecting a loss of crystalline content. Interestingly, no asphaltene particles were visible in the gelled samples. To verify this, an 8 wt % wax solution with 0.1 wt % asphaltenes was heated to dissolve the wax and then imaged at 35 °C (above the precipitation temperature). Figure 9 shows the asphaltene particles in a range of sizes from 1-3 µm (the limit of resolution) to 20-30 µm. All the particles larger than a few microns were clusters of smaller particles. In contrast, in Figure 8, no asphaltene particles with sizes ∼10 µm are visible when the wax is crystallized. This will addressed in the Discussion section. Yield Stress and Asphaltene Aggregation. To investigate the role of asphaltene dispersion, additional tests were performed with less 1-methylnaphthalene (1-MN) in order to reduce the aromatic character of the oil. As shown in Figure 10, reducing the 1-MN concentration from 22 to 5 wt % produced larger (34) Ronningsen, H. P. J. Pet. Sci. Eng. 1992, 7, 177–213. (35) Visintin, R. F. G.; Lapasin, R.; Vignati, E.; D’Antona, P.; Lockhart, T. P. Langmuir 2005, 21 (14), 6240–6249.

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Figure 8. Micrographs from gelled 8% wax solutions. Asphaltene concentrations as follows: (a) 0, (b) 0.05, (c) 0.1, and (d) 0.2 wt %. The left side contains images without cross-polarization and those with 90° cross-polarization are on the left. Black splotches are defects/dirt on the glass surface, not asphaltene aggregates in the bulk wax sample (b, d).

asphaltene aggregates in solution at temperatures above the wax precipitation temperature. Yield stress tests showed that addition of asphaltenes in solutions with 5 wt % 1-MN produced larger yield stress reductions than in those with 22 wt % 1-MN (Figure 11). This effect was most pronounced at 0.05 wt % asphaltenes. Precipitation Temperature and Asphaltene Aggregation. Asphaltene concentration and aggregation was found to affect the precipitation temperature as well. Addition of 3 wt % asphaltenes, the concentration found in the original Shengli oil, increased the DSC precipitation temperature of the 8 wt % wax solution to 35 °C (versus 29.8 °C with no asphaltenes). At this

concentration, microscopy showed that there were asphaltene aggregates greater than 50 µm above the precipitation temperature, as shown in Figure 12. Wax crystallization proceeded from these aggregates as shown at 25.0 and 7.3 °C (Figure 12). DCS showed that precipitation started at 35.0 °C, with a large peak that extended below 0 °C. This large peak was due to wax precipitation, as confirmed by further DSC tests with 3% asphaltenes and no wax, where such a peak was absent. Thus, the aggregates shown at 35.3 °C were asphaltenes with possibly a small amount of wax, whereas the growth of the aggregates at 25.0 and 7.3 °C is attributed to wax precipitation. It is

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Figure 9. Micrograph of asphaltene aggregates in solutions with 8 wt % wax and 0.1 wt % asphaltenes. Samples were heated to dissolve wax, and images were taken at 35 °C (above the wax precipitation temperature). The Nikon microscope and home-built heating stage were used to acquire this image.

interesting to note that the morphology of the wax is similar to that of the asphaltenes. Discussion To summarize the experimental results, we have observed the following: (1) The addition of asphaltenes lowered both the precipitation temperature and the gelation temperature. (Figures 3 and 4) (2) For a constant wax concentration, higher concentrations of asphaltenes decreased the yield stress and degraded the crystal structure. (Figures 5b and 8) (3) For the 8 and 10 wt % wax solutions with higher asphaltene additions (0.1-0.2 wt % in this case), yielding occurred in two distinct regimes: one stronger but more brittle; the second weaker but more elastic. (Figure 6) (4) Asphaltene aggregates that were visible above the wax precipitation temperature were not apparent among the wax crystals of gelled samples. (c.f., Figures 8 and 9) (5) Increasing the asphaltene aggregation by reduction of the content of aromatic solvent caused greater reductions in yield stress. (Figures 10 and 11) (6) Large asphaltene aggregates formed at high concentrations served as wax nucleating sites and raised the wax precipitation temperature. (Figure 12) Gelation and Precipitation Temperatures. The reduction in gelation temperature shows that the asphaltenes interfere with the formation of volume-spanning wax networks and thus function as natural pour point depressants. Such results agree with previous studies11,16,17 and will not be discussed further. As mentioned in the introduction, previous studies with the selective addition of asphaltenes to waxy oils have shown contradictory behavior with respect to precipitation temperature. Oliveira et al.16 observed 1-2 °C decreases in precipitation temperatures measured by viscosity, with experimental uncertainties of (1 °C. In two other studies the addition of asphaltenes increased the wax appearance temperature (WAT), that is, the temperature at which wax precipitation was first observed microscopically.12,14 The reductions in precipitation temperature observed in our study are not to be attributed to insensitivity of DSC. Indeed, microscopic observation is more sensitive to the onset of precipitation than DSC. Using microscopy we observed the WAT of the 8 wt % wax solution with 0.1 wt % asphaltenes at 29.3 °C, versus 28.6 °C using DSC. However, both values are below the DSC precipitation temperature of the wax solution with no asphaltenes, 29.8 °C. Instead, differences in the effect of asphaltene addition are most likely due to the chemical nature of the asphaltenes. As

Tinsley et al.

mentioned earlier, Venkatesan et al. found that the least polar, most aliphatic fraction of their asphaltenes depressed the gelation temperature to the largest extent, much like a wax control additive.17 This is likely the case for the Shengli asphaltenes used in our study. They were very aliphatic, having an H/C ratio of 1.41. Although H/C ratios for asphaltenes can be found as high as 1.3818 and as low as 1.05,10 the value of 1.15 ( 0.5 has been reported as typical.36 The asphaltenes used in this study may be likened to polymer additives that lower precipitation temperatures, as seen with the maleic anhydride copolymers containing appended alkyl side-chains (MAC polymers)26 and as seen in other studies with poly(alkylacrylates).37,38 However, at much higher concentrations (3 wt % asphaltenes) the Shegli asphaltenes increased the precipitation temperature, and the large asphaltene aggregates served as nucleation sites. As seen in Figure 12, wax growth started at the asphaltene particles and grew outward. Similar behavior has been microscopically observed by Garcia.12 Thus, at high concentrations, where the asphaltenes were poorly dispersed, they served as nucleation sites. At low concentrations, when the asphaltenes are well dispersed, they acted more like polymer additives that can suppress nucleation and/or crystal growth. Ratio of Wax to Asphaltenes. In yield stress tests the relative amount of wax to asphaltenes is important. As described in the Results section, addition of 0.1 wt % asphaltenes had a large effect on 3 and 5 wt % wax solutions but a much smaller effect for 8 and 10 wt % wax solutions (Figure 5a). For a given wax concentration, increased addition of asphaltenes increasingly interfered with the wax network, as shown by both yield stress and microscopy. Addition of 0.05 wt % asphaltenes had little effect on the crystal structure or the average yield stress of an 8 wt % wax solution (Figure 5b and Figure 8). However, 0.1 wt % asphaltenes produced a 67% reduction in yield stress and a degraded crystal structure. For 0.2 wt % asphaltenes the effect was even greater. These results suggest that, for a given wax concentration, there is a threshold asphaltene concentration beyond which the network begins to be significantly degraded. The molar ratios of n-paraffin to asphaltene were estimated using the carbon number distribution of the wax and assuming an asphaltene molecular weight of 800 g/mol. This molecular weight was based on experimentally determined molecular weights for different asphaltenes using various analytical methods.39 The estimated n-paraffin/asphaltene ratios are shown in Table 1. Values in bold represent solutions where addition of asphaltenes reduced the yield stress 3-fold or more. At a ratio of ∼100 n-paraffins/asphaltene molecule there appears to be a transition to significant degradation of the wax network. It is interesting to compare this with the effect of MAC Et22 on the 8 wt % wax solution with no asphaltenes (see the companion article to this one on the use of polymers).26 Addition of 0.1 wt % of MAC Et22, about 200 n-paraffins per alkyl tail on the polymer, reduced the yield stress 3 orders of magnitude. Disappearance of Asphaltenes. The asphaltene particles observed in Figure 9 were not observed in samples with gelled wax (Figure 8). The same result was found in waxy deposits formed on a cold surface using this wax/asphaltene system in (36) Rivas, O. R. Vision Tecnol. 1995, 2 (2), 4–17. (37) Ding, X. Z.; Qi, G. R.; Yang, S. L. Polymer 1999, 40 (14), 4139– 4142. (38) Wang, K. S.; Wu, C. H.; Creek, J. L.; Shuler, P. J.; Tang, Y. C. Pet. Sci. Technol. 2003, 21 (3-4), 359–368. (39) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Energy Fuels 2008, 22 (3), 1765–1773.

Waxy Gels with Asphaltenes

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Figure 10. Effect of 1-methylnaphthalene content on asphaltene aggregation. Both solutions contained 8 wt % wax and 0.1 wt % asphaltenes. Samples were heated to dissolve wax, and micrographs were subsequently taken at 35 °C (above the wax precipitation temperature). The Nikon microscope and home-built heating stage were used to acquire this image.

Figure 11. Effect of 1-methylnaphthalene (1-MN) content on yield stress. Samples were gelled for 20 min at 0 °C.

flow-loop deposition tests.40 The explanation is similar to the one used in that case. First, the asphaltenes are incorporated into the wax crystals and not phase separated in the liquid phase. This is supported by the altered crystal morphology and by previous NMR self-diffusion studies by Venkatesan et al., which indicated that asphaltenes were locked into the solid phase of waxy gels, not the entrapped liquid.17 Second, the absence of large asphaltene aggregates in Figure 8 resulted from either insufficient contrast in optical microscopy or from incorporation of the nanometer-sized asphaltene subunits into or onto the growing wax crystals. With respect to the latter, a model for such disappearance of large asphaltenes aggregates may be found in the disappearance of solid catalyst particles during the polymerization of polypropylene.41 Polymerization occurs at multiple catalyst sites within a catalyst pellet. As the polymer grows, the pellet is forced apart and broken into fragments that expand away from each other. Such growth has been called the “expanding universe model”.41 Asphaltenes may behave this way, because they have a distribution of sizes that extends down to the nanometer range17,20,24,42 as well as open structures characterized by fractal dimensions of ∼2.42-44 Thus, a large amount of surface area is accessible to interact with crystallizing wax, and wax may grow at or on these asphaltene subunits. Further studies would be important to prove or disprove such a mechanism. Modification of Yield Behavior. The yielding behavior seen in Figure 6 with two elastic plateaus has not been reported before (40) Tinsley, J. F. The Effects of Polymers and Asphaltenes upon Wax Gelation and Deposition; Ph.D. dissertation; Princeton University: Princeton, 2008;Ch. 8. (41) Albizzati, E.; Cecchin, G.; Chadwick, J. C.; Collina, G.; Giannini, U.; Morini, G.; Noristi, L. Catalysts and Polymerizations. In The Polypropylene Handbook: Polymerization, Characterization, Properties, Processing, Applications; Edward, P., Moore, J., Eds.; Hanser/Gardner: New York, 1996; Ch. 2. (42) Fenistein, D.; Barre, L. Fuel 2001, 80 (2), 283–287. (43) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Langmuir 2006, 22 (10), 4487–4497. (44) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21 (3-4), 461–489.

in wax/asphaltene systems. Waxy crude oils typically have a yielding process that involves an initial elastic response, creeping behavior, and finally viscous flow after yield.31 However, twostage yielding similar to that observed here has been reported in recent studies on mixed colloidal clay systems where the major component is a relatively large flat sheet and the second component is a smaller colloidal sphere that attractively interacts with the sheets. Ten Brinke et al.45 prepared 2.5 wt % solutions of lath-shaped, positively charged Hectorite clay (288 nm × 63 nm × 6 nm) with 12 nm diameter negatively charged silica particles at Hectorite/silica ratios of 10:1, and they observed two plateaus in both oscillatory strain sweeps and unidirectional stress ramps (i.e., flow curves). The existence of the plateau was attributed to a transition from a slow creep regime to one with fast creep, where the transition showed a stress-dependent delay time. Baird and Walz46 performed yield stress tests on plate-like kaolinite (5 µm) with differently sized silica nanospheres (7-22nm) and observed two distinct stages in the yielding behavior. We propose that in our systems the wax crystals act as the larger sheet-like objects and that the asphaltene clusters are acting as the smaller bridging objects. For the studies on colloidal systems, the addition of the smaller particle increased the network strength by adding additional connecting points between larger sheets. Ten Brinke et al. observed increases in elastic modulus over 10-fold and increases in yield stress greater than 3-fold.45 For the kaolinite particles studied by Baird and Walz, addition of nanosized spheres transformed the mixture from one that settled to a gel that could be sliced.46 Similarly, Kriz and Anderson observed increases in yield stress when very low concentration of asphaltenes (0.01 wt %) were added to their model waxy oil.14 However, the behavior of our wax/asphaltene system is more complex than the inorganic colloidal systems in that the asphaltene clusters not only act as additional junction points, but also interfere with wax crystallization and reduce the sizes of the wax sheets. The first effect (additional junction points) increases the modulus, whereas the second effect (degrading wax crystal structure) decreases the modulus. Thus, increasing the strength of the waxy gel through bridging is in competition with the modification of the wax crystal size and shape by the asphaltenes. This explains why Kriz and Andersen observed increases in yield stress at low asphaltene additions but decreases in yield stress at higher asphaltene concentrations.14 We observe similar reductions in yield stress and modulus with increasing asphaltene concentration (Figures 5b and 6). (45) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2008, 4, 337–348. (46) Baird, J. C.; Walz, J. Y. J. Colloid Interfacial Sci. 2007, 306, 411– 420.

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Figure 12. Micrographs of 8 wt % wax with 3 wt % asphaltenes near the precipitation temperature (35.3 °C) and significantly below the precipitation temperature (25.0 and 7.3 °C). As confirmed by DSC, little or no wax is precipitated at 35.3 °C, and asphaltene particles are visible. At the lower temperatures wax precipitates and grows from the asphaltene particles. The Nikon microscope and home-built heating stage were used to acquire these images. Table 1. Molar Ratio of n-Paraffins to Asphaltenes in Model Oils Used for Testinga

interpreted as an increased surface concentration of aliphatic groups on asphaltene particles.

wax concentration (wt %) asphaltene conc. (wt %)

3

5

8

10

Conclusions

0.05 0.1 0.2

64

107

342 171 86

428 214 107

This study reveals two important considerations for the effect of asphaltenes upon crystallization of waxy oils: first, the relative amount of wax with respect to asphaltenes and, second, the aggregation state of the asphaltenes. At low asphaltene concentrations and relatively high wax concentrations, asphaltenes do not have a significant effect on the yield stress of the waxy gel. However, at higher asphaltene concentrations, the asphaltenes behave as wax crystal-modifying additives, significantly decreasing the yield stress and degrading the crystal morphology. There appears to be a threshold ratio of wax/asphaltenes for such behavior. The aliphatic nature of the Shengli asphaltenes used in these tests enables them to interact substantially with the wax crystal structure. When well dispersed at low concentration they reduce the wax precipitation temperature, acting like nucleation inhibitors. However, when large asphaltene aggregates were present at higher concentrations, they increased the wax precipitation temperature and served as nucleation sites for wax growth. Increased asphaltene aggregation (due to a change in solvent quality) also induced larger yield stress reductions than when the asphaltenes were well dispersed. Oscillatory testing indicated that asphaltene-asphaltene interactions contribute to the gel strength, weakened as it may be. Experimental evidence from this and the study by Kriz et al.14 demonstrate the phenomena, and the studies on model colloidal systems of mixed shapes45-47 demonstrate the mechanism whereby this occurs. The interaction of the aliphatic asphaltene with the wax crystal set up a competition between the asphaltenes acting as wax crystal modifiers that decrease gel strength and as bridging agents that increase the network strength.

a

Molecular weight of asphaltenes is assumed to be 800 g/mol.

The two yielding transitions are also explained by the results on the colloidal gels. Ten Brinke et al. found that the critical strain for yielding decreased when the nanospheres, which adsorbed on the larger Hectorite sheets, were added. The critical strain for disrupting this set of contacts was set by the dimension of the smaller particle.45 In our oscillatory tests, the critical strain for the first plateau with asphaltene clusters was ∼0.1%, and the complete failure of the waxy gel occurred at a critical strain of ∼10%. Thus, the critical strain at ∼0.1% likely reflects the limit of connectivity provided by the asphaltenes. The lower modulus and lower ultimate yield strength of these samples reflects the degrading effect of the asphaltenes upon the wax crystals themselves. Solvent Quality. The presence of larger asphaltene aggregates induced by reducing the 1-MN content of the wax solution led to reduction in the yield stress (Figures 10 and 11). This effect was most pronounced at 0.05 wt % asphaltenes. From the standpoint of mixed colloidal systems, part of this effect may be that larger asphaltene clusters are less efficient at reinforcing the primary network, as shown in systems with nanospheres and larger kaolinite discs.47 Also, the poorer solubility of the asphaltenes may facilitate their wax crystal-modifying properties. For example, comparison of two maleic anhydride-based copolymers showed the less soluble polymer (i.e., that precipitated at a higher temperature) was more effective at reducing yield stresses.26 It has been proposed that asphaltenes form aggregates in which the aromatic regions form the core and the aliphatic chains are on the periphery to interact with the surrounding oil.20,22 The poor solvent conditions may increase this segregation and enhance the ability of the aliphatic portions of the asphaltenes to interact with the wax. This may be (47) Baird, J. C.; Walz, J. Y. J. Colloid Interfacial Sci. 2006, 297, 161– 169.

Acknowledgment. J.F.T. and R.K.P. thank the following: Halliburton for their generous support of this project; Professor Ilhan A. Aksay (Princeton University) for access to the Zeiss microscope; and Dr. Paul Beales for his assistance with microscopy and for providing the cooling stage. We also thank Professor Jan Vermant for valuable insight into the two-plateau modulus observations. EF800636F