Article pubs.acs.org/EF
Molecular Origins of Crude Oil Interfacial Activity. Part 4: Oil−Water Interface Elasticity and Crude Oil Asphaltene Films Ramesh Varadaraj* and Cornelius Brons ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801, United States ABSTRACT: The ability of crude oil asphaltenes to impart elastic properties to an oil−water interface was investigated. Asphaltenes separated from Tulare, Hamaca, Cold Lake, Hoosier, Celtic, and Talco crude oils were used. These asphaltenes spanned a range of chemical compositions and molecular structures. An oscillatory tenisometer was used to measure the elastic modulus at the oil−water interface. The asphaltene molecular structure strongly influences the magnitude of elasticity imparted to the oil−water interface. Asphaltenes with high nitrogen and nickel contents possess the optimum molecular structure to adsorb and aggregate at the oil−water interface and form elastic films. Asphaltene films formed at the oil−water interface were isolated and examined by cross-polarized light microscopy. These films exhibited strong birefringence, indicating ordered packing of asphaltene molecules in the film. Asphaltene films that impart high elasticity to the oil−water interface stabilize water-in-oil emulsions. For asphaltene-stabilized emulsions, the oil−water interface elastic modulus correlates with emulsion stability. We find that the oil−water interface elastic modulus is a better indicator of emulsion stability compared to interfacial tension reduction.
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INTRODUCTION It has been recognized that crude oils with high concentrations of n-heptane-insoluble asphaltenes and naphthenic acids are difficult to produce and refine. During crude oil production, it is typical for produced water to emulsify into the crude oil, creating water-in-oil emulsions. Dewatering of the crude oils requires demulsification of the produced water-in-oil emulsion and is a process employed at a production facility to reduce the water content of crude oil prior to shipment to market. Prior to refining, crude oils are subject to a process called desalting. The desalting process involves the addition of water to crude oil, forming a water-in-crude oil emulsion and, thereafter, breaking the crude oil emulsion to produce desalted crude oil and salty water. A fundamental understanding of the interfacial properties of crude oil asphaltenes and naphthenic acids is key to finding cost-effective approaches for demulsifying crude oil emulsions. Several studies related to the interfacial activity of crude oil asphaltenes in hydrocarbon solvents have been reported.1−7 In an earlier paper, we reported the fundamental interfacial properties at an oil−water interface of n-heptane-insoluble asphaltenes derived from five heavy crude oils.8 A model oil, 60:20:20 n-tridecane/toluene/cyclohexane, was used. The five asphaltenes, regardless of their source and chemical composition, exhibited similar interfacial properties, such as critical aggregation concentration and effectiveness to reduce interfacial tension at the oil−water interface. The lack of dependence between these interfacial properties and asphaltene chemical composition lead us to investigate a relatively less explored oil−water interfacial property, namely, interface elasticity. The first objective of our study was to explore whether the asphaltene molecular structure was related to interface elasticity. The second objective was to explore whether one can further relate the asphaltene molecular structure to stability of asphaltene-stabilized water-in-oil emulsions. © 2012 American Chemical Society
Dicharry and co-workers have reported that the rheological properties of oil−water interfaces formed with a crude oil and its distilled fractions diluted in cyclohexane are strongly dependent upon the nature of solvent used for dilution, the oil concentration, the asphaltene and resin concentrations, and the resin/asphaltene ratio.9 They attribute the increased interface rheological properties to physical cross-links between the asphaltenes [two-dimensional (2D) gels] adsorbed at the oil−water interface. Their study suggests that such 2D gels stabilize water-in-oil emulsions. Fan et al. have studied the interfacial shear rheology of asphaltenes at oil−water interfaces and its relation to emulsion stability.10 They find that the asphaltene concentration and aromaticity of the oil are key factors influencing the interface rheological properties. They too observe a strong correlation between emulsion stability and interface rheology. Hannisdal et al. have investigated 30 crude oils and reported on the viscoelastic properties of crude oil components at oil−water interfaces.11,12 Their objective was to understand the properties contributing to the overall stability of crude oil emulsions. They report that water-in-oil emulsion stability seems to be determined by the aggregation state of the asphaltenes in bulk and the reduced sedimentation rate of the dispersed water droplets in concentrated systems. While these studies have vastly improved our understanding of the influence of crude oil asphaltenes on oil−water interface rheology and emulsion stability, a three-way relation between crude oil asphaltene molecular structure, oil−water interface elasticity, and emulsion stability has not yet emerged. The current study was focused to address the asphaltene molecular structure aspect. We believe that including the molecular structure aspect to the understanding is critical and provides Received: May 15, 2012 Revised: September 19, 2012 Published: November 19, 2012 7164
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prepared emulsion was a water-in-oil emulsion with 10−20 μm diameter water droplets dispersed in the oil phase, as determined by optical microscopy. A total of 100 mL of the prepared emulsion was allowed to stand at 22 °C for 5 h in a graduated cylinder. After 5 h, the amount of water that separated out of the emulsion was recorded and percent water retained in the emulsion was calculated. The percent water retained in the emulsion is a measure of emulsion stability.9,10 The higher the percent water retained, the higher the emulsion stability.
fundamental insight into the molecular origins of crude oil interfacial activity.
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EXPERIMENTAL SECTION
Materials. Six crude oils were obtained from Tulare, Hamaca, Hoosier, Cold Lake, Celtic, and Talco oil fields. Asphaltenes were separated from each of the crude oils by the n-heptane deasphalting method using a n-heptane solvent/crude oil ratio of 10:1. For tensiometry experiments, 50:50 by volume of n-hexadecane/toluene solvent mixture was used as the hydrocarbon oil. A 0.5 wt % NaCl solution free of any surface-active impurities and exhibiting an air− water interfacial tension of 72 mN/m was used as the water phase. Chemical Characterization. For asphaltene molecular weight determination, field desorption mass spectrometry (FDMS) was conducted on a VG-ZAB mass spectrophotometer. About 10 mg of the nonvolatile samples were dissolved in 10 mL of toluene. The solution (0.1 μL) was directly deposited onto the FD emitter using a syringe. Molecules were ionized by an intense electric field applied between carbon dendrimers on a thin filament and the source electrode. Electrons were removed from the analyte molecule via a process known as the quantum tunneling effect. The emitter was heated with a ramping current from 0 to 65 mA to assist the desorption of the molecules. The extraction electrodes were heated with 1.2 A current (corresponding to about 225 °C) to avoid analyte condensation. This process generates intact molecular ions with minimal fragmentation. The sulfur and total nitrogen contents of the asphaltenes were determined by AMI method 1086 and AMS method 88-003, respectively. The basic nitrogen content was determined by titration with perchloric acid.9 Vanadium and nickel contents were determined by AMI method 894, and the C/H ratio was determined by elemental analyses. The viscosity of each crude oil was determined at 25 °C using a Haake viscometer operating in a cup and cone mode. Oil−Water Interfacial Tension and Interface Elasticity. An automated oscillatory drop interfacial tensiometer (IT Concept TRACKER model) was used to determine oil−water interfacial tension (γ) and oil−water interface elasticity modulus (E). Interfacial tension and elasticity measurements were made in the pendant drop mode. The interface elasticity modulus measurement involves creating a droplet of oil in water of a given surface area, dilating the droplet to increase the surface area, i.e., stretching the oil−water interface, and measuring the change in force or tension required to create the added area.13−15 The instrument software calculates the oil−water interfacial tension (γ) and interfacial area (A) every second. The oil−water interface elasticity modulus (E) for each cycle of dilation and contraction is determined from the slope of interfacial tension (γ) versus surface area (A) plot. oil−water interface elasticity modulus
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RESULTS AND DISCUSSION The six crude oils obtained from Tulare, Hamaca, Hoosier, Cold Lake, Celtic, and Talco oil fields were well-head samples Table 1. Chemical Composition of Crude Oil Asphaltenes
property and composition crude oil viscosity at 25 °C (cP) asphaltenes (%) C/H MW (amu) Mw/Mn sulfur (%) total nitrogen (%) basic nitrogen (ppm) vanadium (ppm) nickel (ppm)
Tulare
Hamaca
Hoosier
Cold Lake
Celtic
Talco
type I
type II
type II
type II
type II
type III
1310
375603
1965
17690
3685
195
2.6 8.7 1676 1.49 1.73 2.35
16.3 8.5 1455 1.33 5.02 1.54
7.4 9.7 1734 1.29 5.65 1.06
21.2 8.5 1687 1.23 6.82 1.01
11.2 8.6 1749 1.07 7.22 0.78
9.1 8.8 2029 1.45 6.55 0.55
5163
2767
1919
1677
1448
1242
621
1510
621
640
571
72.3
434
332
232
248
273
57.4
Figure 1. Proposed molecular structure for type I asphaltenes.
E = d(γ )/d(ln A)
In our experiment, the oil phase comprised 0.1 wt % asphaltene solution in 50:50 (v/v) n-hexadecane/toluene solvent mixture. The water phase was 0.5 wt % NaCl solution. All experiments were conducted at 22 °C and ambient pressure. Water (25 mL) was placed in the observation cell, and a 50 μL pendant oil drop was made using a 500 μL microsyringe. The pendant oil drop was equilibrated for 5 min. Then, the program was initiated to dilate and contract the oil drop by 5 μL (10% volume) to produce a sinusoidal pattern of variation in the surface area. The instrument was programmed to perform 8 cycles of drop dilation and contraction, rest for 120 s, and then repeat the 8 cycle drop dilation and contraction. The interface elasticity modulus was measured every 15 min for 5 h. The reported elastic modulus at 5 h represents a steady-state value. Emulsion Stability. A water-in-oil emulsion was made by mixing 20 mL of 0.5 wt % NaCl solution with 80 mL of oil. The oil phase was a 50:50 (v/v) mixture of n-hexadecane and toluene containing 0.1 wt % solubilized asphaltene. A total of 20 mL of 0.5 wt % NaCl solution was added to 80 mL of oil phase in 10, 2 mL aliquots and mixed using a coaxial cylinder mixer (Silverson model L4RT-A) at 500 rpm. The
obtained directly from the production facility well and represented “untreated crude oils”. These crude oils had between 2 and 5 wt % produced water. The crude oils were not treated with any chemicals, such as demuslifiers, biocides, or corrosion inhibitors, thereby ensuring an investigation of the true interfacial activity of the asphaltenes without interference from added surface-active chemicals. Each crude oil was first centrifuged at temperatures between 30 and 60 °C to separate the crude oil from produced water. Prior to n-heptane deasphalting, it was determined that each crude oil had less than 0.1 wt % water, thereby providing a dry crude oil that was nearly free from surface-active impurities originating from produced water. In fact, the produced water that separated from the crude oils did show surface activity. An air−water surface tension between 55 and 60 mN/m was observed, confirming our concern of surface-active impurity contamination from produced water. 7165
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Figure 2. Proposed molecular structure for type II asphaltenes.
Figure 3. Proposed molecular structure for type III asphaltenes.
Table 2. Interfacial Properties at the Oil−Water Interface for Crude Oil Asphaltenes interfacial property γ (mN/m) E at t = 5 min (mN/m) E at t = 5 h (mN/m) emulsion stability (%)
Tulare
Hamaca
Hoosier
Cold Lake
Celtic
Talco
21 5.7
21 11.9
19 2.4
23 6.6
26 2.6
22 1.3
38
31
24
22
13
5
70
59
41
40
21
10
Figure 5. Oil−water interface elastic modulus versus sulfur content of asphaltenes.
Figure 6. Oil−water interface elastic modulus versus vanadium content of asphaltenes.
Chemical Composition and Molecular Structure. Table 1 lists the viscosity of each dry crude oil, the percent of nheptane-insoluble asphaltenes present in each crude oil, and the chemical composition information for the respective n-heptaneinsoluble asphaltenes. The suite of crude oils range in viscosity from 375 603 cP (Hamaca) to 195 cP (Talco), with the asphaltene molecular weight varying from 1455 amu (Hamaca) to 2029 amu (Talco). Further, the asphaltene content varies from 21.2 wt % in Cold Lake crude oil to 2.6 wt % in Tulare crude oil. We see no possible manner to group the asphaltenes
Figure 4. Interfacial tension versus emulsion stability.
Thus, the rigorous experimental methods that we adopted provided us with n-heptane-insoluble asphaltenes that best represented the native asphaltenes in the crude oil free from extraneous surface-active contaminants. 7166
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Figure 7. Oil−water interface elastic modulus versus nickel content of asphaltenes.
Figure 10. Oil−water interface elastic modulus versus emulsion stability.
hydrophilic moieties. We observe that all asphaltenes contain varying amounts of nitrogen, sulfur, vanadium, and nickel. On the basis of the sulfur, nitrogen, and metals content, we broadly classify the suite of six asphaltenes into three types as follows: type I with low sulfur (2 wt %), and high nickel (>400 ppm) contents, type II with high sulfur (>5 wt %), medium total nitrogen (∼1 wt %), and medium nickel (200−400 ppm) contents, and type III with high sulfur, low total nitrogen (∼0.5 wt %), and low nickel ( type II > type III, indicating a dependence between asphaltene chemical structure/composition and oil−water interface elasticity. To quantify the dependence of interface elasticity upon chemical composition, we plotted the measured interfacial elastic modulus versus sulfur, vanadium, nickel, basic nitrogen, and total nitrogen contents of the asphaltenes. These plots are shown in Figures 5−9, respectively. We observe that interface elasticity imparted by the asphaltene film to the oil−water interface does not exhibit a dependence upon the sulfur or vanadium content, whereas it does on total nitrogen, basic nitrogen, and nickel contents. These observations can be explained on the basis of the molecular structure of the asphaltenes. As seen from the proposed molecular structures (Figures 1−3), nitrogen can be present in the asphaltene molecules in pyrrolic or pyridinic functional groups. The pyridinic nitrogen is basic and expected to be more interfacially active than the pyrrolic nitrogen. Type I
Figure 12. Micrographs of isolated Tulare asphaltene film.
heptane-insoluble crude oil asphaltenes can be described as moderately surface active at the oil−water interface. Further, the extent of interfacial tension reduction does not correlate with asphaltene types. For example, Tulare (type I), Hamaca (type II), and Talco (type III) asphaltenes all reduce the interfacial tension by 12−13 mN/m. This observation suggests that either there is no significant difference in the intrinsic 7168
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ACKNOWLEDGMENTS The authors thank Kuangnan Qian for the mass spectrometry experiments and analyses, the Analytical Sciences Division of Corporate Strategic Research for the AMI, AMS, and ASTM tests, and Mike Siskin for his review, suggestions, and permission to use the asphaltene molecular structure figures.
asphaltenes with high nitrogen and pyridinic moieties are rendered more interfacially active and form strong elastic films at the oil−water interface. In contrast, type III asphaltenes with no pyridinic nitrogen are less surface-active and form weak elastic films at the oil−water interface. Type II asphaltenes fall in between these two extremes and form films of moderate strength. Further, the observed dependence of elastic modulus upon nickel content suggests that nickel porphyrin moieties, not shown in the molecular structures, also contribute to interfacial activity. The presence of sulfur-containing functional moieties (aromatic or cycloparaffinic) has no effect on interfacial activity. Such sulfur-containing moieties, although considered as polar functional groups, are known to be interfacally inactive at the oil−water interface. Next, we examined the correlation between emulsion stability and interface elastic modulus. Figure 10 is a plot of emulsion stability versus the measured interfacial elastic modulus. An excellent correlation is observed with R2 = 0.99. Emulsion stability increases with an increase in oil−water interface elastic modulus. This observation is in agreement with related studies reported in the literature.9−12 This can be explained on the basis of adsorption and aggregation of asphaltene molecules at the oil−water interface and forming elastic films.9 Water droplet size growth because of Ostwald ripening, a key mechanism for emulsion destabilization, is hindered because water molecules cannot easily diffuse across the oil−water interface.17 From Figures 1 and 10, it is clear that oil−water interface elastic modulus is a much better interfacial property indicator for stability of asphaltene-stabilized water-inoil emulsions than interfacial tension reduction. Finally, we isolated and characterized the asphaltene films formed at the oil droplet−water interface of our oscillatory tensiometer experiment. In a typical experiment, the asphaltene film was isolated as follows. After 5 h of equilibration, the pendant drop was slowly and carefully contracted. A series of pictures shown in Figure 11 set forth the observation that we made for a Tulare asphaltene-containing oil droplet. At the end of the contraction, a small balloon-like structure formed at the tip of the needle. This was carefully deposited onto a glass slide by positioning the glass slide right at the droplet−water interface. The slide was slowly withdrawn from the cell, airdried, and examined under a cross-polarized light microscope. Figure 12 is a micrograph of the isolated Tulare asphaltene film. Strong birefringence is observed, indicating ordered packing of asphaltene molecules in the film. Identical asphaltene films were obtained from Hamaca, Cold Lake, Celtic, and Hoosier asphaltenes. For Talco asphaltenes, we were unable to isolate the film because they were too brittle to form in the contraction experiment. In conclusion, this study has provided yet another step in understanding the molecular origins of crude oil interfacial activity. A correlation between the asphaltene molecular structure, oil−water interface elastic modulus, and emulsion stability has emerged. The salient finding is that asphaltenes with high nitrogen and nickel contents possess the optimum molecular structure to adsorb and aggregate at the oil−water interface, form elastic films, and stabilize water-in-oil emulsions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 7169
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