- Email: [email protected]
An experimental study on the motion of water droplets in oil under ultrasonic irradiation
Ultrasonics Sonochemistry 28 (2016) 110–117
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
An experimental study on the motion of water droplets in oil under ultrasonic irradiation Xiaoming Luo a,⇑, Limin He a, Hongping Wang b, Haipeng Yan a, Yahua Qin c a
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, PR China CNOOC Refinery QingDao Engineering Co., Ltd., Qingdao 266100, PR China c Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6151, Australia b
a r t i c l e
i n f o
Article history: Received 10 May 2015 Received in revised form 3 July 2015 Accepted 3 July 2015 Available online 4 July 2015 Keywords: Ultrasonic irradiation Water droplet in oil Displacement effect Oscillatory motion Critical parameters
a b s t r a c t The motion of a single water droplet in oil under ultrasonic irradiation is investigated with high-speed photography in this paper. First, we described the trajectory of water droplet in oil under ultrasonic irradiation. Results indicate that in acoustic field the motion of water droplet subjected to intermittent positive and negative ultrasonic pressure shows obvious quasi-sinusoidal oscillation. Afterwards, the influence of major parameters on the motion characteristics of water droplet was studied, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, and droplet diameter, etc. It is found that when the acoustic intensity and frequency are 4.89 W cm2 and 20 kHz respectively, which are the critical conditions, the droplet varying from 250 to 300 lm in lower viscous oil has the largest oscillation amplitude and highest oscillation frequency. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The natural emulsifiers in crude oil and chemicals artificially injected during the oil exploitation adsorb at the oil–water interface and form a strong viscoelastic interfacial film, resulting in kinetic barriers to coalescence of dispersed droplets and invalidation of conventional gravity separation [1]. At present, the general methods of demulsification include heat treatment, chemical demulsification, electrostatic coalescence, ultrasonic demulsification and microbial demulsification, etc. Among these, ultrasonic irradiation is quite simple and effective, which impels water droplets to move directionally, aggregate, then collide and coalesce under the mechanical oscillation and thermal effect of the ultrasonic wave and consequently enhances the separation efficiency [2]. With the help of external energy which drives dispersed droplets in the emulsion to move, the efficiency of collision and coalescence can be promoted. In recent years, electrostatic coalescence technology develops rapidly and breakthroughs have been made in both microcosmic mechanism and industrial application. Under electric field, polarization effect makes the two ends of water droplet carry positive and negative charges respectively, which induces dipole force between the neighboring droplets ⇑ Corresponding author. E-mail address: [email protected] (X. Luo). http://dx.doi.org/10.1016/j.ultsonch.2015.07.004 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
and accelerates the drainage of liquid film, promoting the efficiency of coalescence consequently. Meanwhile, electrophoresis, oscillatory deformation and dielectrophoresis will increase the collision frequency between the droplets [3–6]. Nevertheless, when the electric field strength is excessively high and the field stress exceeds the critical value, the interface becomes unstable and dispersion occurs [7–11]. Therefore, the dynamic behaviors of water droplets in oil subjected to electric field are mainly affected by electric field parameters and physical properties of water droplets. About ultrasonic demulsification, currently most researches focus on the evaluation of the demulsification performance. Singh [12] found that ultrasonic had better demulsification effect on certain stable emulsions while the chemical demulsification did not have influence on them. Kotayusov et al. [13] theoretically deduced that the optimum frequency for droplets cohesion was higher than 10 kHz. Check et al. [14] evaluated the effects of ultrasonic power, irradiation time, and temperature on water removal from crude oil and provided optimal parameters theoretically and experimentally. Nii et al. [15] investigated the mechanism of demulsification under ultrasonic irradiation by observing the formation and lifting process of emulsion flocculates. Sun et al. [16,17] found that the cavitation effect of ultrasonic could lead to oil–water emulsification. Thus they suggested acoustic intensity should be controlled under cavitation threshold and the optimum demulsification frequency varied from 21 to 41 kHz. Ye et al. [18,19] systematically investigated the effect of acoustic intensity,
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
ultrasonic frequency, irradiation time and temperature on demulsification. Meanwhile, they indicated that acoustic intensity was more important than other factors. As to the microcosmic mechanism, current researches are mainly about the characteristics of cavitation bubbles under ultrasonic field, while few works reported the mechanism of ultrasonic demulsification [20–27]. However, the physical properties between oil–water system and gas–liquid system are significantly different. Therefore, it is necessary to explore the behavior of droplets in oil under the influence of ultrasonic field, which provides the theoretical foundation for the development of ultrasonic demulsification technology. In this paper, the motion characteristics of an isolated water droplet in oil under ultrasonic irradiation are studied carefully with the help of high-speed photography. And the effects of ultrasonic parameters and physical properties on droplet motion characteristics are discussed in detail.
2. Theoretical background There are two main forces when droplets are exposed to the ultrasonic field: one is the primary acoustic force which helps to agglomerate the droplets at the pressure nodes or antinodes; the other is the secondary acoustic force which will be an attractive force when two drops have identical compressibility. In this paper, our main purpose is to explore the law of motion of one isolated droplet under ultrasonic irradiation. Accordingly, we mainly focused on the primary acoustic force. It is widely accepted that in the standing wave acoustic field, the smaller droplet would be subjected to a time-averaged force in the direction parallel to the propagation of the sound field
111
known as the primary acoustic force, which is given for a one-dimensional field as [28]:
F 1;ac ¼ 4pa3 jEac F sinð2jxÞ
ð1Þ
where a is the droplet radius, j is the wave number of the acoustic field, Eac is the energy density of the acoustic field, x is the distance between the droplet and a pressure antinode of the standing wave and F is the acoustic contrast factor which is crucial in determining the direction of droplet motion. F is given by [29]:
F¼
qr þ ð2=3Þðqr 1Þ 1 ð1 þ 2qr Þ ð3r2 qr Þ
ð2Þ
with qr being the ratio of the droplet density to the continuous phase density and r being the ratio of the speed of sound through the drop phase to that through the continuous phase. It could be inferred from Eqs. (1) and (2) that the droplet motion will be significantly influenced by the size of droplet and the frequency and intensity of the acoustic field, which will be discussed in detail in this paper. In addition, drag force opposes the relative motion of a droplet through the continuous phase. Therefore, the viscosity is another main factor which is to be discussed in this paper as well (see Fig. 1). 3. Experiment 3.1. Experimental apparatus The experimental apparatus, as shown in Fig. 2, are composed of an ultrasonic generator, a transparent test cell, a high-speed digital video camera system, an LED light source, and a data
Fig. 1. Schematic of displacement effect for water droplets in oil [16]. (a) The initial distribution of water droplets. (b) The motion of droplet under ultrasonic irradiation.
Fig. 2. Schematic of microscopic test system of droplet under ultrasonic irradiation.
112
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
Fig. 3. Displacement of water droplet in No. 1 white oil under ultrasonic irradiation. The ultrasonic frequency is 20 kHz, the intensity is 4.89 W cm2. The diameter of water droplet is around 300 lm.
acquisition system. The power of ultrasonic generator varies from 0 to 540 W, corresponding to the ultrasonic intensity range of 0– 6.26 W cm2. In general, there is no cavitation effect within this intensity range. For the ultrasonic vibrator employed in this work, three frequencies are available (20, 28 and 40 kHz) and acoustic intensity maintains constant when the frequency changes. To realize the idea, the acoustic intensity in different frequencies was tested by an acoustometer and was adjusted to an identical value by changing the input power. The test cell is made of Perspex and the ultrasonic vibrator is attached to one side of the test cell. Ultrasonic waves generated by the vibrator propagate forward, hit the other side of the test cell, and then reflect. Two waves superpose together to yield a horizontal standing wave field in the test cell. The high-speed digital video camera (NAC Hotshot 1280) is equipped with a lens (Mitutoyo 5 objective), which can accurately capture the motion of micron-sized droplets. An LED is used for lighting without changing the oil temperature in the cell. 3.2. Experimental medium White oil (obtained from YanChang Petrochemical Product Co., Ltd., Beijing) of different viscosities is used as the continuous phase. No.1 white oil’s viscosity at 20 °C is 1410 mPa s and the viscosity of No. 2 white oil is 791 mPa s. The distilled water is employed to produce dispersed droplets in the range of 100– 400 lm. Surfactant (SDBS, Aladdin Industrial Inc.) is added to change the oil–water interfacial tension in the range of 8.31– 43.90 mN m1. 3.3. Experimental procedure The experimental temperature is 20 °C. Before experiment, oil is initially transferred to the test cell. Then a single micron-sized droplet is injected into the oil phase by micropipettor (Brand GmbH & CO.KG, Germany). After that, ultrasonic field is applied and the high-speed camera is triggered to capture the motion of the droplet at the same time. Fig. 3 shows the displacement of water droplet in oil under ultrasonic irradiation. The origin of coordinate system is positioned at the center of the test cell. The directions of x-axis and y-axis are shown in Fig. 2. The motion is characterized by the central position of water droplet. Through the image processing program, the center of the droplet can be obtained in time.
Fig. 4. Displacement of water droplet with different diameters (D) in No. 1 white oil along x and y directions without ultrasonic irradiation. (a) Displacement in x direction. (b) Displacement in y direction.
4. Results and discussion In the absence of ultrasonic irradiation, the water droplet suspended in oil would move randomly resulting from its collision with the quick atoms or molecules in the liquid, which is called Brownian motion. According to the displacement in x direction
(Fig. 4(a)), it can be seen that the water droplet oscillates irregularly around its equilibrium position and the amplitude of oscillation increases with the decrease of droplet diameter. When droplet diameter is about 100 lm, the maximum amplitude is around 7.5 lm. Additionally, according to the displacement in y direction
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
113
Fig. 5. Displacement of water droplet with different diameters (D) in No. 1 white oil under ultrasonic irradiation. (a) Displacement in x direction. (b) Displacement in y direction.
Fig. 6. Displacement of water droplet in No. 1 white oil under ultrasonic irradiation of different intensities (I). (a) Displacement in x direction. (b) Displacement in y direction.
(Fig. 4(b)), it is obvious that larger droplet subjected to gravity moves downward vertically with smaller amplitude. Displacement of droplets with different diameters under ultrasonic irradiation is shown in Fig. 5. Compared with the random motion of water droplet without ultrasonic irradiation (Fig. 4), water droplet moves directionally due to intermittent positive and negative ultrasonic pressure. Thus, water droplet has obvious quasi-sinusoidal oscillation along x and y directions. The oscillation amplitude is several times as the value obtained in the absence of ultrasonic irradiation and it increases apparently with the increment of droplet diameter. For example, when the droplet diameter is about 400 lm, the oscillation amplitude has a sixfold increase. For the temperature does not change much by using the LED, the increase of amplitude mainly results from the ultrasonic irradiation rather than the Brownian motion. The change of the horizontal and vertical components of central position could be fitted by:
where t is the time, x and y are the horizontal and vertical component of central position respectively, Ax and Ay are the amplitude, tcx and tcy are fitting parameters, and x = 1/(2fw) with fw being the oscillation frequency of water droplet motion. To investigate the effects of major factors, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension and droplet diameter, on the motion characteristics represented by the parameters of amplitude Ax and Ay as well as the oscillation frequency of water droplet fw under ultrasonic irradiation, the following studies are carried out.
x ¼ x0 þ Ax sin y ¼ y0 þ Ay sin
hp
x hp
x
i ðt tcx Þ
ð3aÞ
i ðt t cy Þ
ð3bÞ
4.1. Effect of acoustic intensity Fig. 6 shows the change of central position of droplet under ultrasonic irradiation of different intensities. It can be seen that the oscillation amplitude of motion strongly depends on the intensity. By fitting the displacement curves of water droplet with time, the oscillation amplitude and frequency of motion can be obtained (see Fig. 7). Because the drift force rises with the increase of acoustic intensity, the oscillation amplitude of motion gradually approaches to its peak at 4.89 W cm2 (see Fig. 7(a)). For higher intensity, the
114
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
Fig. 7. Oscillation amplitude and frequency of motion under ultrasonic irradiation of different intensities (I). (a) Oscillation amplitude. (b) Oscillation frequency.
acoustic energy would cause a steady oil flow called acoustic streaming which could reduce the amplitude of motion as well as the slip velocity between oil and water. According to the previous mesoscopic-scale experiments performed by Check and Mowla [14] and Ye et al. [19], there exists an optimum input power, i.e. the acoustic intensity in this paper, under which the removal efficiencies of the desalting/dehydration process were the highest. These results could be explained by our micro-scale experimental results showed in Fig. 7(a). Under a certain acoustic intensity, droplet has the largest amplitude of the oscillatory motion, meaning a largest kinetic energy might be reached, which is beneficial to the collision of droplets, the drainage of oil film and the rupture of interface. As a result, the efficiency of dehydration approaches to its peak under this condition. However, it should be noted that they both pointed out this optimum input power is equal to the cavitation threshold. In our research, since there was no cavitation effect within our experiments, it could be inferred that the optimum acoustic intensity is lower than the cavitation threshold. Therefore, to achieve the optimum input power or acoustic intensity, more detailed researches should be performed in the next step. Fig. 7(b) shows that under low-intensity ultrasonic irradiation the oscillation frequency is relatively high (about 8.5 Hz), while
Fig. 8. Motion of water droplet under ultrasonic irradiation of different frequencies (f). (a) Displacement in x and y directions. (b) Oscillation amplitude versus acoustic intensity under different frequencies.
the oscillatory amplitude is small (just 4 lm). With the increase of acoustic intensity, the oscillation frequency of water droplet rapidly decreases and finally approaches to zero. As is discussed in the above paragraph, maybe it is reasonable to assume that at I = 4.89 W cm2, there would be a relative high efficiency of dehydration. Nevertheless, corresponding to this intensity, the oscillation frequency of water droplet is modest. It illustrates that the higher or lower frequency of oscillatory motion of droplet would not yield an optimum coalescence of water droplets in oil emulsions. For the lower acoustic intensity and higher frequency of oscillatory motion of droplet, there would be not enough energy and contact time for droplet to collide and coalesce; for higher acoustic intensity, as mentioned above, acoustic streaming would happen and the droplets may migrate at almost the identical velocity, resulting in the decreased probability of collision. 4.2. Effect of ultrasonic frequency Fig. 8 shows the influence of ultrasonic frequency on the motion of water droplet. From Fig. 8(a), it can be seen that when the ultrasonic frequency is 20 kHz, the motion shows apparent quasi-sinusoidal oscillation. With the increase of frequency, the
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
115
Fig. 9. Oscillation amplitude and frequency of motion with different diameters (D) under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
Fig. 10. Oscillation amplitude and frequency of motion with different viscosities (g) of oil under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
oscillation becomes weakened. The amplitude Ax is obtained by fitting curves using Eq. (3) and is plotted with the acoustic intensity in Fig. 8(b). From Fig. 8(b), it is obvious that under different ultrasonic frequencies, with the rise of acoustic intensity, the oscillation amplitude Ax first increases to its peak value, and then drops. When the ultrasonic frequency is 20 kHz, the oscillation amplitude significantly changes with acoustic intensity. However with the increase of ultrasonic frequency, Ax decreases obviously. It is probably because for the lower frequency ultrasonic field, the interval time between stretching and compressing is longer. Therefore, it might be inferred that the optimum ultrasonic frequency is 20 kHz under the experimental conditions. Pangu and Feke [20] indicated that the rate of coalescence at lower frequency is significantly higher than that at higher frequency at earlier time when the droplets are small. They attributed the comparatively rapid initial coalescence to the larger rate of volume cleared by coalescence of particular droplet pairs caused by the higher wavelength at lower frequency. Maybe our research shows another reason. At lower frequency, the amplitude of the oscillatory motion of droplet is relative high, which is helpful to the collision and drainage.
4.3. Effect of droplet diameter Fig. 9 shows the oscillation amplitude and frequency of motion with different diameters of droplets under ultrasonic irradiation. From Fig. 9(a), it is found that Ax and Ay are independent of diameters, Ax 18 lm and Ay 60 lm when f = 20 kHz, I = 4.89 W cm2. With the increase of droplet diameter, the oscillation frequency increases first and then drops (see Fig. 9(b)). Oscillation frequency is relatively high, up to 3.5 Hz, with the droplet diameters ranging from 250 to 300 lm. The effect of droplet diameter on the vibration frequency could be explained by the vibration theory. As the motion of water droplet shows obvious quasi-sinusoidal oscillation, maybe we could regard the water drop as a damped oscillator. For the droplets with the diameter smaller than the critical one (corresponding to the highest frequency of oscillatory motion), with increasing diameter the primary acoustic force would gradually increase and become more important than the drag force (as the primary acoustic force F1,ac a3 while the drag force Fd a2). Consequently, the frequency of oscillatory motion would increase. However, for the droplets with the diameter larger than
116
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
4.5. Effect of interfacial tension Fig. 11 shows the variation of oscillation amplitude and the frequency of water droplet with different interfacial tensions under ultrasonic irradiation. The oscillation amplitude is almost independent of the interfacial tension. Ax is around 15–20 lm and Ay is around 50–60 lm. With the decrease of interfacial tension, the oscillation frequency first increases and then decreases. For example, when the interfacial tension varies in the range of 7.40– 43.90 mN m1, the oscillation frequency is relatively high, reaching 3–3.5 Hz; when the concentration of surfactant reaches to its critical micelle concentration, the interfacial tension falls to 2.94 mN m1 and the oscillation frequency of motion decreases to 2 Hz. For smaller interfacial tension, the interfacial film intensity is lower, and the water droplet experiences more drastic oscillation. When the interfacial tension decreases to a certain value, the system energy significantly declines and the water droplet reaches a steady state. In addition, the surfactant is absorbed at the oil–water interface, weakening the oscillation motion. 5. Conclusions
Fig. 11. Oscillation amplitude and frequency of motion with different interfacial tensions (r) under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
the critical one, the inertia of drops would increase with increasing diameter. Accordingly, the motion of droplets would be difficult to change by the primary acoustic force, so the frequency of oscillatory motion would decrease.
4.4. Effect of oil viscosity The influence of oil phase viscosity on the motion of water droplet is shown in Fig. 10. It can be seen that both the oscillation amplitude and frequency increase with the decrease of oil viscosity. When the oil viscosity is the same, with the increase of droplet diameter, the oscillation amplitude almost remains constant while the oscillation frequency first increases to its peak value, and then decreases. As shown in Fig. 10(b), with the viscosity of the oil phase decreasing, the frequency of the oscillatory motion of droplet is slightly increasing. It could also be explained by the damped oscillator. The damping due to the continuous phase would decrease with the reducing viscosity of oil phase, and the frequency fw would increase accordingly.
Under ultrasonic irradiation, the motion of a single water droplet in oil is investigated with the help of high-speed photography. Results obtained in the study indicate that the motion of droplet shows obvious quasi-sinusoidal oscillation due to the intermittent positive and negative ultrasonic pressure, which drives water droplet to move directionally, aggregate, subsequently collide and coalesce, consequently enhancing the separation efficiency. The effects of acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, droplet diameter and other parameters on the droplet motion characteristics are systematically analyzed. It was found that at the critical acoustic intensity of 4.89 W cm2, the oscillation amplitude of droplet increases at first and then drops with the increase of acoustic intensity. When the ultrasonic frequency reaches to 20 kHz and the droplet diameter varies from 250 to 300 lm, the motion of water droplets in lower viscosity oil has the largest oscillation amplitude and highest oscillation frequency. Acknowledgements The work is financially supported by the National Natural Science Foundation of China (Grant No. 51274233), Shandong Province Natural Science Foundation (Grant No. ZR2014EEM045) and the Fundamental Research Funds for the Central Universities. References [1] K. Kumar, A.D. Nikolov, D.T. Wasan, Mechanisms of stabilization of water-incrude oil emulsions, Ind. Eng. Chem. Res. 40 (2001) 3009–3014. [2] A. Fakhru’l-Razi, A. Pendashteh, L.C. Abdullah, D.R. Biak, S.S. Madaeni, Z.Z. Abidin, Review of technologies for oil and gas produced water treatment, J. Hazard. Mater. 170 (2009) 530–551. [3] T.Y. Chen, R.A. Mohammed, A.I. Bailey, P.F. Luckham, S.E. Taylor, Dewatering of crude oil emulsions: emulsion resolution by the application of an electric field, Colloids Surf. A 83 (1994) 273–284. [4] J.S. Eow, M. Ghadiri, Electrostatic enhancement of coalescence of water droplets in oil: a review of the technology, Chem. Eng. J. 85 (2002) 357–368. [5] G. Supeene, C.R. Koch, S. Bhattacharjee, Deformation of a droplet in an electric field: nonlinear transient response in perfect and leaky dielectric media, J. Colloid Interface Sci. 318 (2008) 463–476. [6] J.S. Eow, M. Ghadiri, The behaviour of a liquid–liquid interface and dropinterface coalescence under the influence of an electric field, Colloids Surf. A 215 (2003) 101–123. [7] J. Ha, S. Yang, Deformation and breakup of Newtonian and non-Newtonian conducting drops in an electric field, J. Fluid Mech. 405 (2000) 131–156. [8] J.S. Eow, M. Ghadiri, A. Sharif, Deformation and break-up of aqueous drops in dielectric liquids in high electric fields, J. Electrostat. 51 (2001) 463–469. [9] J.S. Eow, M. Ghadiri, Motion, deformation and break-up of aqueous drops in oils under high electric field strengths, Chem. Eng. Process. 42 (2003) 259–272.
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117 [10] J.S. Eow, M. Ghadiri, A. Sharif, Experimental studies of deformation and break-up of aqueous drops in high electric fields, Colloids Surf. A 225 (2003) 193–210. [11] N. Dubash, A.J. Mestel, Breakup behavior of a conducting drop suspended in a viscous fluid subject to an electric field, Phys. Fluids 19 (2007) 072101. [12] B.P. Singh, B.P. Pandey, Ultrasonication for breaking water in oil emulsions, Proc. Indian Nat. Sci. Acad. 58 (1992) 181–194. [13] A.N. Kotyusov, B.E. Nemtsov, Induced coagulation of small particles under the action of sound, Acta Acust. United Ac. 82 (1996) 459–463. [14] G.R. Check, D. Mowla, Theoretical and experimental investigation of desalting and dehydration of crude oil by assistance of ultrasonic irradiation, Ultrason. Sonochem. 20 (2013) 378–385. [15] S. Nii, S. Kikumoto, H. Tokuyama, Quantitative approach to ultrasonic emulsion separation, Ultrason. Sonochem. 16 (2009) 145–149. [16] B. Sun, D. Yan, W. Qiao, The study of demulsification with ultrasonic irradiation on oil emulsion, Acta Acustica 24 (1999) 327–331. [17] B. Sun, W. Qiao, J. Fu, Demulsifying of oil-in-water emulsion of tertiary oil recovery, Acta Petrolei Sin. 21 (2000) 97–101. [18] G. Ye, X. Lu, P. Han, X. Shen, Desalting and dewatering of crude oil in ultrasonic standing wave field, J. Petrol. Sci. Eng. 70 (2010) 140–144. [19] G. Ye, X. Lü, F. Peng, P. Han, X. Shen, Pretreatment of crude oil by ultrasonicelectric united desalting and dewatering, Chinese J. Chem. Eng. 16 (2008) 564– 569. [20] G.D. Pangu, D.L. Feke, Kinetics of ultrasonically induced coalescence within oil/ water emulsions: modeling and experimental studies, Chem. Eng. Sci. 64 (2009) 1445–1454.
117
[21] C. Browne, R.F. Tabor, D.Y.C. Chan, R.R. Dagastine, M. Ashokkumar, F. Grieser, Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions, Langmuir 27 (2011) 12025–12032. [22] A. Brotchie, F. Grieser, M. Ashokkumar, Characterization of acoustic cavitation bubbles in different sound fields, J. Phys. Chem. B 114 (2010) 11010–11016. [23] J. Lee, M. Ashokkumar, K. Yasui, T. Tuziuti, T. Kozuka, A. Towata, Y. Lida, Development and optimization of acoustic bubble structures at high frequencies, Ultrason. Sonochem. 18 (2011) 92–98. [24] M. Ashokkumar, J. Lee, S. Kentish, F. Grieser, Bubbles in an acoustic field: an overview, Ultrason. Sonochem. 14 (2007) 470–475. [25] J. Lee, S.E. Kentish, M. Ashokkumar, The effect of surface-active solutes on bubble coalescence in the presence of ultrasound, J. Phys. Chem. B 109 (2005) 5095–5099. [26] M. Ashokkumar, The characterization of acoustic cavitation bubbles–an overview, Ultrason. Sonochem. 18 (2011) 864–872. _ On the concentration of aerosol particles by means of drift forces in a [27] H. Czyz, standing wave field, Acta Acust. United Ac. 70 (1990) 23–28. [28] G.D. Pangu, D.L. Feke, Acoustically aided separation of oil droplets from aqueous emulsions, Chem. Eng. Sci. 59 (2004) 3183–3193. [29] G.D. Pangu, D.L. Feke, Droplet transport and coalescence kinetics in emulsions subjected to acoustic fields, Ultrasonics 46 (2007) 289–302.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
An experimental study on the motion of water droplets in oil under ultrasonic irradiation Xiaoming Luo a,⇑, Limin He a, Hongping Wang b, Haipeng Yan a, Yahua Qin c a
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, PR China CNOOC Refinery QingDao Engineering Co., Ltd., Qingdao 266100, PR China c Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6151, Australia b
a r t i c l e
i n f o
Article history: Received 10 May 2015 Received in revised form 3 July 2015 Accepted 3 July 2015 Available online 4 July 2015 Keywords: Ultrasonic irradiation Water droplet in oil Displacement effect Oscillatory motion Critical parameters
a b s t r a c t The motion of a single water droplet in oil under ultrasonic irradiation is investigated with high-speed photography in this paper. First, we described the trajectory of water droplet in oil under ultrasonic irradiation. Results indicate that in acoustic field the motion of water droplet subjected to intermittent positive and negative ultrasonic pressure shows obvious quasi-sinusoidal oscillation. Afterwards, the influence of major parameters on the motion characteristics of water droplet was studied, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, and droplet diameter, etc. It is found that when the acoustic intensity and frequency are 4.89 W cm2 and 20 kHz respectively, which are the critical conditions, the droplet varying from 250 to 300 lm in lower viscous oil has the largest oscillation amplitude and highest oscillation frequency. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The natural emulsifiers in crude oil and chemicals artificially injected during the oil exploitation adsorb at the oil–water interface and form a strong viscoelastic interfacial film, resulting in kinetic barriers to coalescence of dispersed droplets and invalidation of conventional gravity separation [1]. At present, the general methods of demulsification include heat treatment, chemical demulsification, electrostatic coalescence, ultrasonic demulsification and microbial demulsification, etc. Among these, ultrasonic irradiation is quite simple and effective, which impels water droplets to move directionally, aggregate, then collide and coalesce under the mechanical oscillation and thermal effect of the ultrasonic wave and consequently enhances the separation efficiency [2]. With the help of external energy which drives dispersed droplets in the emulsion to move, the efficiency of collision and coalescence can be promoted. In recent years, electrostatic coalescence technology develops rapidly and breakthroughs have been made in both microcosmic mechanism and industrial application. Under electric field, polarization effect makes the two ends of water droplet carry positive and negative charges respectively, which induces dipole force between the neighboring droplets ⇑ Corresponding author. E-mail address: [email protected] (X. Luo). http://dx.doi.org/10.1016/j.ultsonch.2015.07.004 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
and accelerates the drainage of liquid film, promoting the efficiency of coalescence consequently. Meanwhile, electrophoresis, oscillatory deformation and dielectrophoresis will increase the collision frequency between the droplets [3–6]. Nevertheless, when the electric field strength is excessively high and the field stress exceeds the critical value, the interface becomes unstable and dispersion occurs [7–11]. Therefore, the dynamic behaviors of water droplets in oil subjected to electric field are mainly affected by electric field parameters and physical properties of water droplets. About ultrasonic demulsification, currently most researches focus on the evaluation of the demulsification performance. Singh [12] found that ultrasonic had better demulsification effect on certain stable emulsions while the chemical demulsification did not have influence on them. Kotayusov et al. [13] theoretically deduced that the optimum frequency for droplets cohesion was higher than 10 kHz. Check et al. [14] evaluated the effects of ultrasonic power, irradiation time, and temperature on water removal from crude oil and provided optimal parameters theoretically and experimentally. Nii et al. [15] investigated the mechanism of demulsification under ultrasonic irradiation by observing the formation and lifting process of emulsion flocculates. Sun et al. [16,17] found that the cavitation effect of ultrasonic could lead to oil–water emulsification. Thus they suggested acoustic intensity should be controlled under cavitation threshold and the optimum demulsification frequency varied from 21 to 41 kHz. Ye et al. [18,19] systematically investigated the effect of acoustic intensity,
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
ultrasonic frequency, irradiation time and temperature on demulsification. Meanwhile, they indicated that acoustic intensity was more important than other factors. As to the microcosmic mechanism, current researches are mainly about the characteristics of cavitation bubbles under ultrasonic field, while few works reported the mechanism of ultrasonic demulsification [20–27]. However, the physical properties between oil–water system and gas–liquid system are significantly different. Therefore, it is necessary to explore the behavior of droplets in oil under the influence of ultrasonic field, which provides the theoretical foundation for the development of ultrasonic demulsification technology. In this paper, the motion characteristics of an isolated water droplet in oil under ultrasonic irradiation are studied carefully with the help of high-speed photography. And the effects of ultrasonic parameters and physical properties on droplet motion characteristics are discussed in detail.
2. Theoretical background There are two main forces when droplets are exposed to the ultrasonic field: one is the primary acoustic force which helps to agglomerate the droplets at the pressure nodes or antinodes; the other is the secondary acoustic force which will be an attractive force when two drops have identical compressibility. In this paper, our main purpose is to explore the law of motion of one isolated droplet under ultrasonic irradiation. Accordingly, we mainly focused on the primary acoustic force. It is widely accepted that in the standing wave acoustic field, the smaller droplet would be subjected to a time-averaged force in the direction parallel to the propagation of the sound field
111
known as the primary acoustic force, which is given for a one-dimensional field as [28]:
F 1;ac ¼ 4pa3 jEac F sinð2jxÞ
ð1Þ
where a is the droplet radius, j is the wave number of the acoustic field, Eac is the energy density of the acoustic field, x is the distance between the droplet and a pressure antinode of the standing wave and F is the acoustic contrast factor which is crucial in determining the direction of droplet motion. F is given by [29]:
F¼
qr þ ð2=3Þðqr 1Þ 1 ð1 þ 2qr Þ ð3r2 qr Þ
ð2Þ
with qr being the ratio of the droplet density to the continuous phase density and r being the ratio of the speed of sound through the drop phase to that through the continuous phase. It could be inferred from Eqs. (1) and (2) that the droplet motion will be significantly influenced by the size of droplet and the frequency and intensity of the acoustic field, which will be discussed in detail in this paper. In addition, drag force opposes the relative motion of a droplet through the continuous phase. Therefore, the viscosity is another main factor which is to be discussed in this paper as well (see Fig. 1). 3. Experiment 3.1. Experimental apparatus The experimental apparatus, as shown in Fig. 2, are composed of an ultrasonic generator, a transparent test cell, a high-speed digital video camera system, an LED light source, and a data
Fig. 1. Schematic of displacement effect for water droplets in oil [16]. (a) The initial distribution of water droplets. (b) The motion of droplet under ultrasonic irradiation.
Fig. 2. Schematic of microscopic test system of droplet under ultrasonic irradiation.
112
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
Fig. 3. Displacement of water droplet in No. 1 white oil under ultrasonic irradiation. The ultrasonic frequency is 20 kHz, the intensity is 4.89 W cm2. The diameter of water droplet is around 300 lm.
acquisition system. The power of ultrasonic generator varies from 0 to 540 W, corresponding to the ultrasonic intensity range of 0– 6.26 W cm2. In general, there is no cavitation effect within this intensity range. For the ultrasonic vibrator employed in this work, three frequencies are available (20, 28 and 40 kHz) and acoustic intensity maintains constant when the frequency changes. To realize the idea, the acoustic intensity in different frequencies was tested by an acoustometer and was adjusted to an identical value by changing the input power. The test cell is made of Perspex and the ultrasonic vibrator is attached to one side of the test cell. Ultrasonic waves generated by the vibrator propagate forward, hit the other side of the test cell, and then reflect. Two waves superpose together to yield a horizontal standing wave field in the test cell. The high-speed digital video camera (NAC Hotshot 1280) is equipped with a lens (Mitutoyo 5 objective), which can accurately capture the motion of micron-sized droplets. An LED is used for lighting without changing the oil temperature in the cell. 3.2. Experimental medium White oil (obtained from YanChang Petrochemical Product Co., Ltd., Beijing) of different viscosities is used as the continuous phase. No.1 white oil’s viscosity at 20 °C is 1410 mPa s and the viscosity of No. 2 white oil is 791 mPa s. The distilled water is employed to produce dispersed droplets in the range of 100– 400 lm. Surfactant (SDBS, Aladdin Industrial Inc.) is added to change the oil–water interfacial tension in the range of 8.31– 43.90 mN m1. 3.3. Experimental procedure The experimental temperature is 20 °C. Before experiment, oil is initially transferred to the test cell. Then a single micron-sized droplet is injected into the oil phase by micropipettor (Brand GmbH & CO.KG, Germany). After that, ultrasonic field is applied and the high-speed camera is triggered to capture the motion of the droplet at the same time. Fig. 3 shows the displacement of water droplet in oil under ultrasonic irradiation. The origin of coordinate system is positioned at the center of the test cell. The directions of x-axis and y-axis are shown in Fig. 2. The motion is characterized by the central position of water droplet. Through the image processing program, the center of the droplet can be obtained in time.
Fig. 4. Displacement of water droplet with different diameters (D) in No. 1 white oil along x and y directions without ultrasonic irradiation. (a) Displacement in x direction. (b) Displacement in y direction.
4. Results and discussion In the absence of ultrasonic irradiation, the water droplet suspended in oil would move randomly resulting from its collision with the quick atoms or molecules in the liquid, which is called Brownian motion. According to the displacement in x direction
(Fig. 4(a)), it can be seen that the water droplet oscillates irregularly around its equilibrium position and the amplitude of oscillation increases with the decrease of droplet diameter. When droplet diameter is about 100 lm, the maximum amplitude is around 7.5 lm. Additionally, according to the displacement in y direction
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
113
Fig. 5. Displacement of water droplet with different diameters (D) in No. 1 white oil under ultrasonic irradiation. (a) Displacement in x direction. (b) Displacement in y direction.
Fig. 6. Displacement of water droplet in No. 1 white oil under ultrasonic irradiation of different intensities (I). (a) Displacement in x direction. (b) Displacement in y direction.
(Fig. 4(b)), it is obvious that larger droplet subjected to gravity moves downward vertically with smaller amplitude. Displacement of droplets with different diameters under ultrasonic irradiation is shown in Fig. 5. Compared with the random motion of water droplet without ultrasonic irradiation (Fig. 4), water droplet moves directionally due to intermittent positive and negative ultrasonic pressure. Thus, water droplet has obvious quasi-sinusoidal oscillation along x and y directions. The oscillation amplitude is several times as the value obtained in the absence of ultrasonic irradiation and it increases apparently with the increment of droplet diameter. For example, when the droplet diameter is about 400 lm, the oscillation amplitude has a sixfold increase. For the temperature does not change much by using the LED, the increase of amplitude mainly results from the ultrasonic irradiation rather than the Brownian motion. The change of the horizontal and vertical components of central position could be fitted by:
where t is the time, x and y are the horizontal and vertical component of central position respectively, Ax and Ay are the amplitude, tcx and tcy are fitting parameters, and x = 1/(2fw) with fw being the oscillation frequency of water droplet motion. To investigate the effects of major factors, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension and droplet diameter, on the motion characteristics represented by the parameters of amplitude Ax and Ay as well as the oscillation frequency of water droplet fw under ultrasonic irradiation, the following studies are carried out.
x ¼ x0 þ Ax sin y ¼ y0 þ Ay sin
hp
x hp
x
i ðt tcx Þ
ð3aÞ
i ðt t cy Þ
ð3bÞ
4.1. Effect of acoustic intensity Fig. 6 shows the change of central position of droplet under ultrasonic irradiation of different intensities. It can be seen that the oscillation amplitude of motion strongly depends on the intensity. By fitting the displacement curves of water droplet with time, the oscillation amplitude and frequency of motion can be obtained (see Fig. 7). Because the drift force rises with the increase of acoustic intensity, the oscillation amplitude of motion gradually approaches to its peak at 4.89 W cm2 (see Fig. 7(a)). For higher intensity, the
114
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
Fig. 7. Oscillation amplitude and frequency of motion under ultrasonic irradiation of different intensities (I). (a) Oscillation amplitude. (b) Oscillation frequency.
acoustic energy would cause a steady oil flow called acoustic streaming which could reduce the amplitude of motion as well as the slip velocity between oil and water. According to the previous mesoscopic-scale experiments performed by Check and Mowla [14] and Ye et al. [19], there exists an optimum input power, i.e. the acoustic intensity in this paper, under which the removal efficiencies of the desalting/dehydration process were the highest. These results could be explained by our micro-scale experimental results showed in Fig. 7(a). Under a certain acoustic intensity, droplet has the largest amplitude of the oscillatory motion, meaning a largest kinetic energy might be reached, which is beneficial to the collision of droplets, the drainage of oil film and the rupture of interface. As a result, the efficiency of dehydration approaches to its peak under this condition. However, it should be noted that they both pointed out this optimum input power is equal to the cavitation threshold. In our research, since there was no cavitation effect within our experiments, it could be inferred that the optimum acoustic intensity is lower than the cavitation threshold. Therefore, to achieve the optimum input power or acoustic intensity, more detailed researches should be performed in the next step. Fig. 7(b) shows that under low-intensity ultrasonic irradiation the oscillation frequency is relatively high (about 8.5 Hz), while
Fig. 8. Motion of water droplet under ultrasonic irradiation of different frequencies (f). (a) Displacement in x and y directions. (b) Oscillation amplitude versus acoustic intensity under different frequencies.
the oscillatory amplitude is small (just 4 lm). With the increase of acoustic intensity, the oscillation frequency of water droplet rapidly decreases and finally approaches to zero. As is discussed in the above paragraph, maybe it is reasonable to assume that at I = 4.89 W cm2, there would be a relative high efficiency of dehydration. Nevertheless, corresponding to this intensity, the oscillation frequency of water droplet is modest. It illustrates that the higher or lower frequency of oscillatory motion of droplet would not yield an optimum coalescence of water droplets in oil emulsions. For the lower acoustic intensity and higher frequency of oscillatory motion of droplet, there would be not enough energy and contact time for droplet to collide and coalesce; for higher acoustic intensity, as mentioned above, acoustic streaming would happen and the droplets may migrate at almost the identical velocity, resulting in the decreased probability of collision. 4.2. Effect of ultrasonic frequency Fig. 8 shows the influence of ultrasonic frequency on the motion of water droplet. From Fig. 8(a), it can be seen that when the ultrasonic frequency is 20 kHz, the motion shows apparent quasi-sinusoidal oscillation. With the increase of frequency, the
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
115
Fig. 9. Oscillation amplitude and frequency of motion with different diameters (D) under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
Fig. 10. Oscillation amplitude and frequency of motion with different viscosities (g) of oil under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
oscillation becomes weakened. The amplitude Ax is obtained by fitting curves using Eq. (3) and is plotted with the acoustic intensity in Fig. 8(b). From Fig. 8(b), it is obvious that under different ultrasonic frequencies, with the rise of acoustic intensity, the oscillation amplitude Ax first increases to its peak value, and then drops. When the ultrasonic frequency is 20 kHz, the oscillation amplitude significantly changes with acoustic intensity. However with the increase of ultrasonic frequency, Ax decreases obviously. It is probably because for the lower frequency ultrasonic field, the interval time between stretching and compressing is longer. Therefore, it might be inferred that the optimum ultrasonic frequency is 20 kHz under the experimental conditions. Pangu and Feke [20] indicated that the rate of coalescence at lower frequency is significantly higher than that at higher frequency at earlier time when the droplets are small. They attributed the comparatively rapid initial coalescence to the larger rate of volume cleared by coalescence of particular droplet pairs caused by the higher wavelength at lower frequency. Maybe our research shows another reason. At lower frequency, the amplitude of the oscillatory motion of droplet is relative high, which is helpful to the collision and drainage.
4.3. Effect of droplet diameter Fig. 9 shows the oscillation amplitude and frequency of motion with different diameters of droplets under ultrasonic irradiation. From Fig. 9(a), it is found that Ax and Ay are independent of diameters, Ax 18 lm and Ay 60 lm when f = 20 kHz, I = 4.89 W cm2. With the increase of droplet diameter, the oscillation frequency increases first and then drops (see Fig. 9(b)). Oscillation frequency is relatively high, up to 3.5 Hz, with the droplet diameters ranging from 250 to 300 lm. The effect of droplet diameter on the vibration frequency could be explained by the vibration theory. As the motion of water droplet shows obvious quasi-sinusoidal oscillation, maybe we could regard the water drop as a damped oscillator. For the droplets with the diameter smaller than the critical one (corresponding to the highest frequency of oscillatory motion), with increasing diameter the primary acoustic force would gradually increase and become more important than the drag force (as the primary acoustic force F1,ac a3 while the drag force Fd a2). Consequently, the frequency of oscillatory motion would increase. However, for the droplets with the diameter larger than
116
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117
4.5. Effect of interfacial tension Fig. 11 shows the variation of oscillation amplitude and the frequency of water droplet with different interfacial tensions under ultrasonic irradiation. The oscillation amplitude is almost independent of the interfacial tension. Ax is around 15–20 lm and Ay is around 50–60 lm. With the decrease of interfacial tension, the oscillation frequency first increases and then decreases. For example, when the interfacial tension varies in the range of 7.40– 43.90 mN m1, the oscillation frequency is relatively high, reaching 3–3.5 Hz; when the concentration of surfactant reaches to its critical micelle concentration, the interfacial tension falls to 2.94 mN m1 and the oscillation frequency of motion decreases to 2 Hz. For smaller interfacial tension, the interfacial film intensity is lower, and the water droplet experiences more drastic oscillation. When the interfacial tension decreases to a certain value, the system energy significantly declines and the water droplet reaches a steady state. In addition, the surfactant is absorbed at the oil–water interface, weakening the oscillation motion. 5. Conclusions
Fig. 11. Oscillation amplitude and frequency of motion with different interfacial tensions (r) under ultrasonic irradiation. (a) Oscillation amplitude. (b) Oscillation frequency.
the critical one, the inertia of drops would increase with increasing diameter. Accordingly, the motion of droplets would be difficult to change by the primary acoustic force, so the frequency of oscillatory motion would decrease.
4.4. Effect of oil viscosity The influence of oil phase viscosity on the motion of water droplet is shown in Fig. 10. It can be seen that both the oscillation amplitude and frequency increase with the decrease of oil viscosity. When the oil viscosity is the same, with the increase of droplet diameter, the oscillation amplitude almost remains constant while the oscillation frequency first increases to its peak value, and then decreases. As shown in Fig. 10(b), with the viscosity of the oil phase decreasing, the frequency of the oscillatory motion of droplet is slightly increasing. It could also be explained by the damped oscillator. The damping due to the continuous phase would decrease with the reducing viscosity of oil phase, and the frequency fw would increase accordingly.
Under ultrasonic irradiation, the motion of a single water droplet in oil is investigated with the help of high-speed photography. Results obtained in the study indicate that the motion of droplet shows obvious quasi-sinusoidal oscillation due to the intermittent positive and negative ultrasonic pressure, which drives water droplet to move directionally, aggregate, subsequently collide and coalesce, consequently enhancing the separation efficiency. The effects of acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, droplet diameter and other parameters on the droplet motion characteristics are systematically analyzed. It was found that at the critical acoustic intensity of 4.89 W cm2, the oscillation amplitude of droplet increases at first and then drops with the increase of acoustic intensity. When the ultrasonic frequency reaches to 20 kHz and the droplet diameter varies from 250 to 300 lm, the motion of water droplets in lower viscosity oil has the largest oscillation amplitude and highest oscillation frequency. Acknowledgements The work is financially supported by the National Natural Science Foundation of China (Grant No. 51274233), Shandong Province Natural Science Foundation (Grant No. ZR2014EEM045) and the Fundamental Research Funds for the Central Universities. References [1] K. Kumar, A.D. Nikolov, D.T. Wasan, Mechanisms of stabilization of water-incrude oil emulsions, Ind. Eng. Chem. Res. 40 (2001) 3009–3014. [2] A. Fakhru’l-Razi, A. Pendashteh, L.C. Abdullah, D.R. Biak, S.S. Madaeni, Z.Z. Abidin, Review of technologies for oil and gas produced water treatment, J. Hazard. Mater. 170 (2009) 530–551. [3] T.Y. Chen, R.A. Mohammed, A.I. Bailey, P.F. Luckham, S.E. Taylor, Dewatering of crude oil emulsions: emulsion resolution by the application of an electric field, Colloids Surf. A 83 (1994) 273–284. [4] J.S. Eow, M. Ghadiri, Electrostatic enhancement of coalescence of water droplets in oil: a review of the technology, Chem. Eng. J. 85 (2002) 357–368. [5] G. Supeene, C.R. Koch, S. Bhattacharjee, Deformation of a droplet in an electric field: nonlinear transient response in perfect and leaky dielectric media, J. Colloid Interface Sci. 318 (2008) 463–476. [6] J.S. Eow, M. Ghadiri, The behaviour of a liquid–liquid interface and dropinterface coalescence under the influence of an electric field, Colloids Surf. A 215 (2003) 101–123. [7] J. Ha, S. Yang, Deformation and breakup of Newtonian and non-Newtonian conducting drops in an electric field, J. Fluid Mech. 405 (2000) 131–156. [8] J.S. Eow, M. Ghadiri, A. Sharif, Deformation and break-up of aqueous drops in dielectric liquids in high electric fields, J. Electrostat. 51 (2001) 463–469. [9] J.S. Eow, M. Ghadiri, Motion, deformation and break-up of aqueous drops in oils under high electric field strengths, Chem. Eng. Process. 42 (2003) 259–272.
X. Luo et al. / Ultrasonics Sonochemistry 28 (2016) 110–117 [10] J.S. Eow, M. Ghadiri, A. Sharif, Experimental studies of deformation and break-up of aqueous drops in high electric fields, Colloids Surf. A 225 (2003) 193–210. [11] N. Dubash, A.J. Mestel, Breakup behavior of a conducting drop suspended in a viscous fluid subject to an electric field, Phys. Fluids 19 (2007) 072101. [12] B.P. Singh, B.P. Pandey, Ultrasonication for breaking water in oil emulsions, Proc. Indian Nat. Sci. Acad. 58 (1992) 181–194. [13] A.N. Kotyusov, B.E. Nemtsov, Induced coagulation of small particles under the action of sound, Acta Acust. United Ac. 82 (1996) 459–463. [14] G.R. Check, D. Mowla, Theoretical and experimental investigation of desalting and dehydration of crude oil by assistance of ultrasonic irradiation, Ultrason. Sonochem. 20 (2013) 378–385. [15] S. Nii, S. Kikumoto, H. Tokuyama, Quantitative approach to ultrasonic emulsion separation, Ultrason. Sonochem. 16 (2009) 145–149. [16] B. Sun, D. Yan, W. Qiao, The study of demulsification with ultrasonic irradiation on oil emulsion, Acta Acustica 24 (1999) 327–331. [17] B. Sun, W. Qiao, J. Fu, Demulsifying of oil-in-water emulsion of tertiary oil recovery, Acta Petrolei Sin. 21 (2000) 97–101. [18] G. Ye, X. Lu, P. Han, X. Shen, Desalting and dewatering of crude oil in ultrasonic standing wave field, J. Petrol. Sci. Eng. 70 (2010) 140–144. [19] G. Ye, X. Lü, F. Peng, P. Han, X. Shen, Pretreatment of crude oil by ultrasonicelectric united desalting and dewatering, Chinese J. Chem. Eng. 16 (2008) 564– 569. [20] G.D. Pangu, D.L. Feke, Kinetics of ultrasonically induced coalescence within oil/ water emulsions: modeling and experimental studies, Chem. Eng. Sci. 64 (2009) 1445–1454.
117
[21] C. Browne, R.F. Tabor, D.Y.C. Chan, R.R. Dagastine, M. Ashokkumar, F. Grieser, Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions, Langmuir 27 (2011) 12025–12032. [22] A. Brotchie, F. Grieser, M. Ashokkumar, Characterization of acoustic cavitation bubbles in different sound fields, J. Phys. Chem. B 114 (2010) 11010–11016. [23] J. Lee, M. Ashokkumar, K. Yasui, T. Tuziuti, T. Kozuka, A. Towata, Y. Lida, Development and optimization of acoustic bubble structures at high frequencies, Ultrason. Sonochem. 18 (2011) 92–98. [24] M. Ashokkumar, J. Lee, S. Kentish, F. Grieser, Bubbles in an acoustic field: an overview, Ultrason. Sonochem. 14 (2007) 470–475. [25] J. Lee, S.E. Kentish, M. Ashokkumar, The effect of surface-active solutes on bubble coalescence in the presence of ultrasound, J. Phys. Chem. B 109 (2005) 5095–5099. [26] M. Ashokkumar, The characterization of acoustic cavitation bubbles–an overview, Ultrason. Sonochem. 18 (2011) 864–872. _ On the concentration of aerosol particles by means of drift forces in a [27] H. Czyz, standing wave field, Acta Acust. United Ac. 70 (1990) 23–28. [28] G.D. Pangu, D.L. Feke, Acoustically aided separation of oil droplets from aqueous emulsions, Chem. Eng. Sci. 59 (2004) 3183–3193. [29] G.D. Pangu, D.L. Feke, Droplet transport and coalescence kinetics in emulsions subjected to acoustic fields, Ultrasonics 46 (2007) 289–302.