Energy & Fuels 2009, 23, 481–486
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Demulsification of Asphaltenes and Resins Stabilized Emulsions via the Freeze/Thaw Method Xiaogang Yang, Wei Tan,* and Yu Bu School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed July 28, 2008. ReVised Manuscript ReceiVed October 6, 2008
The role of asphaltenes and resins in stabilizing water-in-crude oil emulsions was investigated by measuring the boundary tension, the zeta potential of the dispersive drops, and the dehydrating ratio of the model emulsions, which were demulsified via the freeze/thaw method. The model emulsion was prepared with dodecane and deionized water, and the water concentration was 30 v/v%. Three thawing types were used in the demulsifying experiments: air (room temperature), a 40 °C water bath, and microwave irradiation. The dehydrating ratio is improved significantly with microwave irradiation, and it can be more than 90 v/v% for the emulsion used in this experiment.
1. Introduction Crude oil containing water is harmful to the transportation, refinery, and quality of the products, so demulsification is one of the key steps in various sectors of the oil industry. It is wellknown that the stability of water-in-crude oil emulsions depends mainly on a rigid protective film that inhibits the water droplets from separating from the emulsion.1,2 This rigid interfacial film is believed to be composed of many natural emulsifying agents, such as asphaltenes, resinous substances, fine solids, oil soluble organic acids, and other finely divided materials.3-6 Among them, the asphaltenes and resinous substance play the most important role in the stabilization of the w/o emulsions. Asphaltenes are soluble in toluene but insoluble in alkanes, typically n-heptane or npentane, and resins are soluble in both aliphatic and aromatic solvents. Both asphaltenes and resins have the potential to accumulate on the water/oil interface because they contain some hydrophilic functional groups and consequently are surface-active. Demulsification is the process of oil and water separation from emulsions. Two principal approaches of demulsification are chemical and physical methods. The chemical method is the use of a proper demulsifier, and typical physical treatment techniques include electrical, microwave irradiation, ultrasonic, or a mechanical method such as centrifugation. The addition * To whom correspondence should be addressed. E-mail: tjuwtan@ yahoo.com.cn. (1) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. Water-in-hydrocarbon emulsions stabilized by asphaltenes at low concentrations. J. Colloid Interface Sci. 2000, 228, 52. (2) Yang, X. L.; Verruto, V. J.; Kilpatrick, P. K. Dynamic asphalteneresin exchange at the oil/water interface: time-dependent w/o emulsion stability for asphaltene/resin model oils. Energy Fuels 2007, 21, 1343. (3) Yan, Z. L.; Elliott, J. A.; Masliyah, J. H. Roles of various bitumen components in the stability of water-in-diluted-bitumen emulsions. J. Colloid Interface Sci. 1999, 220, 329. (4) Graham, B. F.; May, E. F.; Trengove, R. D. Emulsion inhibiting components in crude oils. Energy Fuels 2008, 22, 1093. (5) Hannisdal, A.; Ese, M. H.; Hemmingsen, P.; et al. Particle-stabilized emulsions: effect of heavy crude oil components pre-adsorbed onto stabilizing solids. Colloids Surf., A 2006, 276, 45. (6) Dicharry, C.; Arla, D.; Sinquin, A.; et al. Stability of water/crude oil emulsions based on interfacial dilatational rheology. J. Colloid Interface Sci. 2006, 297, 785.
of the demulsifier and electrical techniques are the most popular methods, but they have many disadvantages: power-wasting, a large addition of the demulsifier, bad water quality, and environmental pollution. Freeze/thaw treatment has been widely investigated for phase separation of emulsions in many papers. Chen7 studied the effect of freezing-thawing on the stability of crude oil-in-water emulsions and found that freezing-thawing could increase droplet size and lower the oil concentration in the emulsion. Truong and Phillips8 thought the breakage of the emulsion increased with both salinity and oil concentration in the freeze/ thaw cycles. Ganguly9 revealed that the nature of the emulsifier and the concentration of the salt could strongly influence emulsion characteristics (primarily freezing behavior) and the surfactant-electrolyte interactions in the water-in-oil (w/o) emulsions. Jean10,11 reported for the first time the feasibility of employing the freeze/thaw method to separate oil from an oily sludge through examining the freezing time and the required cost. He thought the freeze/thaw treatment could significantly improve the sludge’s deliquorability and markedly reduce the bound liquor concentration. Chen and He12,13 showed that the treatment of freeze/thaw can destroy a complex and tight w/o emulsion that was generated from the pretreatment step of the used lubricating oil rerefinery and hardly demulsified by (7) Chen, E. C. Influence of freezing-thawing on the stability of crude oil-in-water emulsions. J. Can. Pet. Technol. 1975, 14, 38. (8) Truong, L.; Phillips, C. R. Freezing of oil-water and oil-saline emulsions. EnViron. Sci. Technol. 1976, 10, 482. (9) Ganguly, S.; Mohan, V. K.; Bhasu, V. C.; et al. Surfactantelectrolyte interactions in concentrated water-in-oil emulsions: FT-IR spectroscopic and low-temperature differential scanning calorimetric studies. Colloids Surf. 1992, 65, 243. (10) Jean, D. S.; Lee, D. J.; Wu, J. C. S. Separation of oil from oily sludge by freezing and thawing. Water Res. 1999, 33, 1756. (11) Jean, D. S.; Lee, D. J. Expression deliquoring of oily sludge from a petroleum refinery plant. Waste Manage. 1999, 19, 349. (12) He, G. H.; Chen, G. H. Lubricating oil sludge and its emulsification. Drying Technol. 2002, 20, 1009. (13) Chen, G. H.; He, G. H. Separation of water and oil from waterin-oil emulsion by freeze/thaw method. Sep. Purif. Technol. 2003, 31, 83.
10.1021/ef800600v CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
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conventional demulsification techniques. Hindmarsh14,15 used nuclear magnetic resonance (NMR) techniques to monitor the freezing behavior of suspended drops and provided both rapid real-time dispersion and spatially resolved velocity measurements within suspended drops undergoing freezing. Vladana16 used the freeze/thaw method and microwave radiation to demulsify the emulsion samples prepared by mixing the metalworking-oil, FESOL 09, produced by FAM, Krusevac, Serbia, and deionized water. Microwave radiation could enhance heating of emulsion and improve the efficiency of oil removal. Lin et al.17 successfully used the freeze/thaw treatment to break the w/o emulsions with loosely packed droplets that were produced from the oils generally adopted as membrane phase in the emulsion liquid membrane process. However, knowledge about the demulsifying mechanism of freeze/thaw treatment is still limited. The purpose of the present study is to examine the effectiveness of the asphaltenes and resins substances extracted from the crude oil emulsion to act as emulsifiers in water-in-oil emulsions and to investigate the influence of thawing types on the dehydrating ratio of emulsions. This study forms part of a larger research project to isolate, identify, and characterize functional ingredients present in the crude oil emulsions. It is very meaningful for the demulsification in the area where the temperature is above the freezing point in the daytime and below freezing during the night. 2. Experimental Section 2.1. Materials. The crude oil used as a source of asphaltenes and resins substances throughout this research was supplied by the Dagang Oil Field in China. The oil used for the preparation of model emulsions was dodecane, and the water was deionized. All general chemicals used in this study were of analytical grade. 2.2. Extraction of Asphaltenes and Resins. The crude oil and n-pentane (ratio 1:5) were agitated homogeneously at room temperature, and the mixture was then centrifuged (LG10-2.4A, Jingli Centrifugal Equipment Co., Ltd., Beijing, China) at 2400 rpm for 10 min to separate the asphaltenes. The extraction of the resin fraction can be divided as follows: 2 g of silanol per milliliter crude oil was added to the supernatant from the asphaltenes precipitation. The mixture was stirred homogeneously, centrifuged for 3 min at 1000 rpm, and the supernatant was removed. The coated particles were then mixed with benzene, stirred, and centrifuged for 2 min at 1000 rpm. Decantation and filtration were done several times until the supernatant was nearly colorless after centrifugation. The desorption process was repeated using the same procedure with a blend of 7% methanol and methylene dichloride. The resins could be separated from the solvent/resin mixture after removal of solvent at 90 °C in a rotary evaporator, and this last step was processed under a nitrogen blanket to ensure that all of the solvent was removed.18 2.3. Model Emulsions Preparation. The model emulsions, which were investigated in this Article, were prepared by mixing (14) Hindmarsh, J. P.; Hollingsworth, K. G.; Wilson, D. I.; et al. An NMR study of the freezing of emulsion-containing drops. J. Colloid Interface Sci. 2004, 275, 165. (15) Hindmarsh, J. P.; Sederman, A. J.; Gladden, L. F.; et al. Rapid measurement of dispersion and velocity in freezing drops using magnetic resonance methods. Exp. Fluids 2005, 38, 750. (16) Vladana, R.; Dejan, S. Separation of water-in-oil emulsions by freeze/thaw method and microwave radiation. Sep. Purif. Technol. 2006, 49, 192. (17) Lin, C.; He, G. H.; Li, X. C.; et al. Freeze/thaw induced demulsification of water-in-oil emulsions with loosely packed droplets. Sep. Purif. Technol. 2007, 56, 175. (18) Ese, M. H.; Yang, X.; Sjoblom, J. Film forming properties of asphaltenes and resins. A comparative Langmuir-Blodgett study of crude oils from North Sea, European continent and Venezuela. Colloid Polym. Sci. 1998, 276, 800.
Yang et al. dodecane (analytical grade) and the deionized water, and its water concentration was 30 v/v%. The concentration of asphaltenes varied from 3 to 9 g/L, and the concentration of resins varied from 0.47 to 3.3 g/L. The preparation procedure for the asphaltenes or resins stabilized emulsion samples was the same: dispersing the desired amount of asphaltenes or resins into 7 mL of dodecane in a plastic test tube, and then adding 3 mL of deionized water. The phases were then mixed 300 times by a hand shaking motion at a frequency of 2 s-1. The experiments were repeated three times, and the results were a mean value of each result obtained, for all of the procedures and steps. 2.4. Surface Tension and Zeta Potential Measurements. Surface tension measurements were conducted on the prepared model emulsions using a digital tensiometer (JC2000A tensiometer, Zhongchen Digital Equipment Co., Ltd., Shanghai, China) with the mechanism of pendant-drop method. The electrical charge (zeta potential) of oil droplets in the emulsions was determined using a particle electrophoresis instrument (JS94G+, Zhongchen Digital Equipment Co., Ltd., Shanghai, China). The zeta potential was determined by measuring the direction and velocity of droplet movement in the applied electric field. Emulsions were diluted to a droplet concentration of approximately 1.0 v/v% using the deionized water to avoid multiple scattering effects. The diluted emulsions were mixed thoroughly and then injected into the measurement chamber of the instrument. The zeta potential of each individual sample was calculated from the average of five measurements on the diluted emulsion, and the results were reported as the mean and standard deviation. 2.5. Procedures of Demulsification Experiments. The demulsification experiments were performed by the use of a refrigerator, at the chosen temperatures (-25 °C) for 20 h before being defreezed in air (20 °C), a water bath (40 °C), or microwave oven (700 W, 2450 MHz). The thawed emulsion was place in air, and the volume of water depositing to the bottom could be read from the scale on the special graduated cylinder. While being defreezed with microwave irradiation, each sample (10 mL) was placed in the same position in the microwave oven for some seconds until totally thawed, and followed by gravity separation (without radiation of microwave). The dehydrating ratio (D) can be calculated from the following equation:
D,% )
Vs,mL × 100% Vo,mL
(1)
where D is the dehydrating ratio, %; Vs represents the volume of separated water, mL; and Vo represents the original volume of water, mL. The light transmittances of the separated water (LTSW) were measured with a UV spectrophotometer (UV-9200, Ruili Analytical Instrument Co., Beijing, China).
3. Results and Discussion 3.1. Infrared Characterization of Asphaltenes and Resins Substance. Both asphaltenes and resins have some hydrophilic functional groups and consequently are surfaceactive, so they are liable to accumulate on the water/oil interface. Photoacoustic spectroscopy-Fourier transform infrared (PASFTIR) spectra of asphaltenes and resins substances were obtained using a PAS-FTIR spectroscope (FTS3000MX, BIORAD, U.S.). The instrument was operated using the following parameters: aperture, 150; interferometer mirror velocity, 0.1582 cm/s; 256 scans per run; resolution, 4 cm-1; and data spacing, 0.964 cm-1. Samples for photoacoustic analysis underwent twostage purging with helium to remove CO2. The total purging time was about 30-60 min. The IR spectra of the asphaltenes and resins substances were presented in Figure 1a,b. The results obtained show that the concentration of the aromatic ring and carbonyl group in the resins is much larger than that in the
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Figure 1. Photoacoustic spectroscopy-Fourier transform infrared spectra of asphaltenes and resins substances.
asphaltenes through the comparison of the group peaks of methyl and methane (3120-2750 cm-1), carbonyl group (1750-1650 cm-1), and CdC bond on the aromatic ring (1650-1550 cm-1). 3.2. Influence of Asphaltenes or Resins Concentration on the Surface Tension and Zeta Potential of the Model Emulsions. The emulsion types of samples are explored by the dilution method in this Article. The type of model emulsions was water-in-oil (w/o). The type of emulsion changed to oilin-water (o/w) for the diluted emulsion, which was used in the measurement of zeta potential. The surface tensions at the air-emulsion interface and the zeta potentials at the oil drops surface were investigated next. The asphaltenes and resins have many hetero atoms and functional groups, so they have high polarity and surfaceactivity. The asphaltenes and resins are liable to accumulate on the water/oil interface, and their adsorption characteristics can influence the strength of the interfacial film largely, and subsequently impact the stability of the emulsion. Charge repulsion is another factor contributing to the stability of emulsion. The drops are hard to coalesce when the charge repulsion is larger than the van der Waals force. The electric double layer’s structures of the internal drops are directed influenced by the interfacial characteristics of the asphaltenes and resins. The zeta potential is the main parameter to characterize the surface charge of the disperse drop; its value is concerned with the structure of the electric double layer. The increasing thickness of the electric double layer can raise the zeta potential, and subsequently the charge repulsion and the mechanical stability of emulsion. Figures 2 and 3 show the influence of asphaltenes or resins concentration on the surface tension and zeta potential of the model emulsions. The results indicate that the surface tensions decline with the increase of the asphaltenes or resins concentration. At the same concentration (3 g/L), the surface tension of the resins emulsion was lower than that of the asphaltenes emulsion, suggesting that the resins are more surface-active and more liable to adsorb at the air-water interface and minimize the surface tension. This result suits the analysis on the photoacoustic spectroscopyFourier transform infrared spectra of asphaltenes and resins substances. The curve of the surface tension changes to flat with the increase of the asphaltenes concentration because the asphaltene molecular association results in the interfacial active groups rearranging at the surface. The charges of the disperse phase drops are generated through three methods: ionization, adsorption, and friction. The zeta potential of the internal oil droplets in the asphaltenes and resinsstabilized emulsion (o/w) is negative because some anionic
Figure 2. The influence of asphaltenes concentration on the surface tension and zeta potential.
Figure 3. The influence of resins concentration on the surface tension and zeta potential.
active material of the asphaltenes and resins adsorbed at the oil-water interface. Negative ions can also be generated after the ionization of some weak-acid active functional groups. On
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Figure 4. Crystallization behavior of model emulsions.
the other hand, oil drops can take the negative charge for a lower electric coefficient during the friction process. The zeta potential at the surface of the internal oil drops rises with the increase of asphaltenes and resins concentration. The reason is there are more polar active groups, which are included in the nature surfactants, adsorbing at the oil-water interface during the concentration increasing. At the same concentration (3 g/L), the oil drops zeta potentials of the asphaltenes and resins stabilized emulsions almost the same, although there are more active materials in the resins. This may be because the surface charges of the oil drops are not mainly produced by the ionization of active materials, but by the friction. 3.3. Fixing the Freezing Temperature in the Freeze/ Thaw Demulsification Experiments. The freezing temperature is an important factor that can influence the emulsification efficiency19 dehydrating ratio in the freeze/thaw demulsifying process. A differential scanning calorimeter (DSC 204 F1, Netzsch Instrument, Inc., Germany) was used to measure the solidification temperature of a model emulsion with certain concentration (9 g/L), which had the lowest surface tension and the highest zeta potential. About 10 mg samples were weighed and sealed in aluminum pans, and then placed inside the DSC and cooled at a fixed rate of 2.0 °C/min. The onset temperature of the exothermic peak was recorded as the solidification temperature. The crystallization behavior of the model emulsion is shown in Figure 4. The results indicate that the heat release peaks of water and oil phases overlap, because their solidification temperatures are very close. The peak value of temperature is -17.8 °C, and the crystal process finished at -21.5 °C. So both the water and the oil phases can be frozen at the chosen temperature (-25 °C) in the freeze/thaw demulsifying experiments. 3.4. Influence of Nature Emulsifiers on the Demulsifying Effect. The influence of the asphaltenes and resins concentration on the dehydrating ratio and the light transmittances of the separated water was studied next. The demulsification experiments were performed at the chosen temperatures (-25 °C) for 20 h and thawed in air (20 °C). The light transmittances of the separated water (LTSW) were measured with a UV spectrophotometer. The results are shown in Figures 5 and 6. The results indicate that the dehydrating ratios decrease with the rise of the asphaltenes or the resins concentration. The coalescence between water droplets can be inhibited by many functions of the emulsifier. Asphaltene can form quite strong (19) Hirai, T.; Hariguchi, S.; Komasawa, I. Biomimetic synthesis of calcium carbonate particles in a pseudovesicular double emulsion. Langmuir 1997, 13, 6650.
Figure 5. Influence of asphaltenes concentration on the demulsifying effect.
Figure 6. Influence of resins concentration on the demulsifying effect.
and viscoelastic films at the oil-water interface. The more asphaltenes or resins are adsorbed at the crude oil-water interface, the more they give rise to the strength and viscoelasticity of film. The spatial steric hindrance generated by the oleophilic group of the nature emulsifier can keep a distance between two water drops. The ionization of the nature emulsifier
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Table 1. Comparison of the Thawing Time Needed among the Varied Thawing Types thawing types
microwave irradiation
air (20 °C)
water bath (40 °C)
thawing time needed, min
4
80
7
can form the positive and negative ions, which easily adsorb at the water and oil side of the interfacial film, respectively. An electrostatic field forms during this process, and it can stop the drain of the film. The dehydrating ratio of the resins emulsion is much lower than that of the asphaltenes emulsion when their concentrations are the same (3 g/L). Resins have much more interfacial activity than do the asphaltenes and are liable to accumulate at the water-oil interfacial film. On the other hand, the high concentration of the asphaltenes can result in their aggregation. Stable emulsions are formed only when the asphaltenes in the oil phase flocculate slightly. Less stable emulsions will result when the asphaltenes either are too flocculated or exist in a molecularly dispersed state.20 The general changing tendency of the separated water’s light transmittances is a reduction with increase of asphaltenes or resins concentration because their disperse fraction in water arises at the same time. The LTSW of the resins stabilized emulsion is lower than that of asphaltenes stabilized emulsion. The reason may be that there are more active groups in the resins and they are easy to disperse in the water. Two inverse dispersing types of the asphaltenes in the water are simultaneous. The increase of the asphaltenes concentration can promote its dispersion scale in the water. Meanwhile, the high concentration asphaltenes are liable to aggregate, and the residual of the asphaltenes in the water decreased. That the LTSW rose slightly when the asphaltenes concentration changed from 5 to 7 g/L due to the aggregation of the asphaltenes plays an important role when its concentration is 7 g/L. 3.5. Influence of Thawing Method on the Demulsifying Effect. The thawing types and velocity are important factors that can influence the dehydrating ratio of the emulsion. Three different thawing types consisting of the air (room temperature), a 40 °C water bath, and microwave irradiation were explored next. The comparison of the thawing time needed among the varied thawing types is shown in Table 1. The thawing velocity with the microwave irradiation is the quickest, while defreezing in the air is the slowest. The dehydrating ratios varied in the freeze-thaw demulsifying experiments when thawing methods were different. The results are shown in Figure 7. The dehydrating ratio declines with the increase of the asphaltenes whatever the type of thawing was adopted. More and more asphaltenes adsorb at the air-water interfacial film with ascending asphaltene concentration. This variation can enhance the strength of the film and subsequently the emulsion stability. The highest dehydrating ratio, more than 90 v/v%, is reached when microwave irradiation is applied, while the dehydrating ratio of water bath is lower than thawing in air. The key process during the demulsification is the coalescence of the adjacent drops. The coalescence process includes two steps: the liquid medium is expelled from the space between two adjacent drops, and the liquid interfacial film of oil and water becomes thin; and the interfacial film is broken. The probable demulsifying mechanism of the freeze/thaw method (20) Tambe, D. E.; Sharma, M. M. Factors controlling the stability of colloid-stabilized emulsions I. An experimental investigation. J. Colloid Interface Sci. 1993, 157, 244.
Figure 7. Influence of thawing types on the dehydrating ratio.
is shown in Figure 8. Although the dispersive drops in the emulsion flocculate together, they are not coalescent and keep the shape during the inhibition action of the interfacial film. During the freezing process, the flocculent drops form a crystal whose rough surface can break the film. On the other hand, the crystal can pierce the film into the adjacent drops. Thus, two crystals of drops connect to each other and form “partial coalescence”. The drops keep the shape at this time. During the process of thawing, the crystals of “partial coalescence” change to crystals of genuine coalescence and form a larger drop that can be separated by gravity force. Microwave is an electromagnetic wave that has a frequency range from 300 MHz to 300 GHz. When freezing emulsion is thawed with microwave radiation, two kinds of effects take place simultaneously.21-23 The principal effect is the heating effect, which can increase the temperature of the emulsions and consequently lead to reduction of viscosity and coalescence. The heating mode by microwave is much different from the conventional thermal heating, as the former relies on the molecular interaction with the electromagnetic field, rather than the convection, conduction, and radiation of heat from the surface of the materials. According to the conclusions of Klaila,24 Wolf,25 and Fang et al.,26 molecular rotations that were induced by microwave radiation could destroy the electric double layers at the interface between oil and water molecules, followed by the reduction of the zeta potential, which suspends water droplets in an emulsion. Without the support of the zeta potential, water (oil) molecules can move up and down freely and enable the droplets to collide with each other and cause the coalescence. This function is considered to play a role in addition to the primary heating effect in the acceleration of microwave demulsification. (21) Xia, L. X.; Lu, S. W.; Cao, G. Y. Stability and demulsification of emulsions stabilized by asphaltenes or resins. J. Colloid Interface Sci. 2004, 271, 504. (22) Jiang, H. Y.; Lu, Q. L. Study on transportation technology of dehydrate and viscosity reduction for high pour point and viscous crude oil with microwave radiation technique. Oil Gas Storage Transport. 2004, 23, 34. (23) Han, H. J. Treatment of oil-containing wastewater by micro cell filter bed process. EnViron. Prot. Chem. Ind. 2000, 20, 19. (24) Klaika, W. J. Method and apparatus for controlling fluency of high viscosity hydrocarbon fluids. U.S. Patent 4,067,683, January 10, 1978. (25) Wolf, N. O. Use of microwave radiation in separating emulsions and dispersions of hydrocarbons and water. U.S. Patent 4,582,629, April 15, 1986. (26) Fang, C. S.; Lai, P. M. C. Microwave heating and separation of water-in-oil emulsions. J. MicrowaVe Power Electromagnetic Energy 1995, 30, 46.
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Figure 8. The probable demulsifying mechanism of the freeze/thaw method.
that can decrease the residual quantity of the asphaltenes in the water. The ruleless change of the LTSW may be the result of the different molecular aggregation level at the variant asphaltenes concentration. When the asphaltenes concentration is 7 g/L, the LTSW appears a turning point with the thawing methods of air and microwave irradiation. This may be because the aggregation of the asphaltenes plays an important role at this time. The turning point changes to 9 g/L when a water bath is used to thaw the emulsion. This may be because the emulsions are unceasingly placed in the water bath after being thawed. The temperature of the water bath (40 °C) is larger than that of the air (20 °C). This may induce the dissolution of the asphaltenes into the water and increase its concentration. The aggregation of the asphaltenes does not act obviously until its concentration reaches 9 g/L. 4. Conclusion
Figure 9. Influence of thawing types on the light transmittance of the separated water.
The thawing rate in the air (20 °C) is slow and can remove more water. The reason is that the time allowing the surfactant to form new hydrophobic micelles in the oil phase and to migrate away from the water-oil interface is longer. If the thawing rate is fast, less surfactant can move from the interface, and less hydrophobic micelles that take oil with them can form.16 On the other hand, the slow thawing rate can enhance the recrystallizing probability of the small crystals in the water drop. The small crystals growing larger can easily break the oil-water interfacial film. The LTSW can also be influenced by the thawing types, as shown in Figure 9. The results indicate that the general changing tendency of the separated water’s light transmittances is reduction with increase of asphaltenes concentration. The LTSW of the emulsion, which was thawed with microwave irradiation, is lower than in the other two thawing methods. The reason may be that the high frequency electromagnetic field, which was generated by the microwave irradiation, can make the polar molecule in the asphaltenes rotate and disengage from the interfacial film. The microwave heating can also enhance the solution of the asphaltenes in the water at the same time. The increase of the asphaltenes concentration can promote its dissolution in the water, and the possibility of aggregate
The emulsion stability is affected largely by the concentration of the asphaltenes or resins. Adsorption at the droplet interface of these nature emulsifiers can form a film with a certain mechanical strength, which prevents the droplets from coalescing. The surface tensions decline with the increase of the asphaltenes or resins concentration. The resins are more surfaceactive and more liable to adsorb at the air-water interface and minimize the surface tension. The zeta potential of the internal oil droplets in the asphaltenes and resins stabilized emulsion (o/w) is negative, and it rises with the increase of asphaltenes and resins concentration. Although there are more active materials in the resins, the oil drops the zeta potentials of the asphaltenes and resins stabilized emulsions almost the same at identical concentrations (3 g/L). The dehydrating ratios decrease with the rise of the asphaltenes or the resins concentration. The dehydrating ratios decrease with the rise of the asphaltenes or the resins concentration in the freezing and air thawing experiments. The dehydrating ratio of the resins emulsion is much lower than that of the asphaltenes emulsion when their concentrations are the same (3 g/L). The thawing types and velocity can influence the dehydrating ratio of the emulsion obviously. The thawing rate in the air (20 °C) is slower and can remove more water than in the water bath (40 °C). Thawing with microwave irradiation yields the highest dehydrating ratio (above 90 v/v%), considering the thawing procedure used in our experimental setup. EF800600V
