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Coalescence behaviour of water droplets in water‐oil interface under pulsatile electric fields
Coalescence behaviour of water droplets in water-oil interface under pulsatile electric fields M. Hosseini PII: DOI: Reference:
S1004-9541(16)30265-8 doi: 10.1016/j.cjche.2016.04.007 CJCHE 509
To appear in: Received date: Revised date: Accepted date:
24 September 2015 18 January 2016 1 March 2016
Please cite this article as: M. Hosseini, Coalescence behaviour of water droplets in wateroil interface under pulsatile electric fields, (2016), doi: 10.1016/j.cjche.2016.04.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Separation Science and Engineering
Coalescence behaviour of water droplets in water-oil interface
PT
under pulsatile electric fields
RI
M. Hosseini
NU SC
Faculty of chemical engineering, Babol Noshirvani University of Technology, Babol, Iran
Article history: Received 24 September 2015 Received in revised form 18 January 2016
ED
Accepted 1 March 2016 Available online xxxx
PT
Abstract
MA
Corresponding author. E-mail address: [email protected]
CE
In this research, the deformation of water droplets in sunflower oil-interface under
AC
pulsatile electric field was studied experimentally. Three types of coalescence were observed: (i) complete coalescence, (ii) incomplete coalescence and (iii) nocoalescence. The first type is desirable because of leaving no secondary droplets. The second type produced secondary droplets which was caused by necking process, due to extreme elongation of droplets (mostly small droplets), was undesirable; because the small droplets were more difficult to coalesce and remove. The no-coalescence was caused by very fast coalescence and extensive pushing of droplet into the continuous phase. In this work the process was operated 1
ACCEPTED MANUSCRIPT with the utilization of a batch cylindrical separator with high voltage system. The lower part of the cylinder was filled with the aqueous phase and its top part was
PT
filled with sunflower oil to form an interface between the two phases. The effects
RI
of electric field strength, frequency, and waveform types were investigated. It was
NU SC
found that, the ramp-ac waveform was the best waveform, avoiding the production of secondary droplets and in this case the frequency also played an important role.
MA
Keywords: Drop-interface coalescence, Drops deformation, Non-uniform electric
ED
field, Di-electrophoresis, pulsatile electric field.
1 Introduction
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The formation of an emulsion is either desirable or undesirable in many chemical
CE
industries. Therefore the coalescence phenomenon for separation (oily wastewater
AC
treatment) on one hand and the stability of the emulsion for example in food industry on the other hand are very important factors. However it is necessary to study the coalescence behaviour of droplets and it is very important to investigate the stability of an emulsion under electric field. During crude oil production both oil and water phases are co-produced [1]. Nowadays, there are many techniques for enhancing the separation of water-in-oil emulsions, such as the addition of chemical demulsifier, gravity or centrifugal settling, heat treatment and electrostatic demulsification [2]. Electric field is 2
ACCEPTED MANUSCRIPT extensively used in the chemical, petroleum and biochemical industries. In a nonuniform electric field (DEP=di-electrophorese), the polarized particles experience
PT
a force causing them to move to those regions that have high or low electric field
RI
[3]. Because of economical reason and particularly because of environmental
NU SC
aspects, the related technique is very suitable and because of the simplicity of the apparatus and process for demulsification, the electrical way would be one of the best way of destroying o/w emulsions. The separation of emulsion with non-
MA
uniform electric field (DEP) has been studied and the effects of temperature, time, voltage and different types of electrical current were considered (4, 5).
ED
In this study, drop-interface coalescence (water drop in oil/water-interface) under
PT
non-uniform electric field with different condition of waveform types, different voltages, and different frequencies have been considered. In this kind of
CE
demulsifier, one electrode is settled inside the cylindrical emulsion container and
AC
the other electrode is copper formic cylinder jacket (Fig. 1). By passing electrical current through the two electrodes, the field is created in a radial fashion towards the electrode (3, 6).
If two water droplets located in insulating oil are subjected to an external electric field E0, a dipole moment is induced at the droplets center. The magnitude of the dipole moment, p, depends on the electric field intensity, the drop size and permittivity [7]. 2. Mechanisms of non-uniform electric field on drop interface coalescence 3
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When a water droplet is suspended in a non-uniform electrical field, the interaction
PT
of the applied field and induced dipole generates a force on the particle. This force termed di-electrophoresis (8).
RI
According to the equation (1), DEP force magnitude (Fdep) for cylindrical
Fdep
d 3 c d c E 2 2 ( d 2 c ) R ln 2 ( Dj / Dc )
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container has been estimated as the following formula:
(1)
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According to the Eq. (1), the amount of Fdep depends on the following parameters: 1. The ratio of the copper jacket diameter, Dj, to the Perspex cylinder diameter
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Dc (emulsion container) (Fig. 1). Eq. (1) shows that the force of DEP will
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be weaker when the above-mentioned ratio is increased, so a higher electric
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field strength is required. 2. Dielectric constant, c, which in continuous phase depends on the kind of
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emulsion.
3. The smaller the droplets diameters (d) are, the lower the DEP force required. 4. When the distance (R) between the center of the water drop and the central electrode increased, the effect of electrical field intensity from the central electrode to farther droplets would be decreased.
3. Objectives of the present work 4
ACCEPTED MANUSCRIPT In general, three types of drop coalescence may occur: complete coalescence, incomplete coalescence, and no-coalescence. The incomplete coalescence, because
PT
of producing smaller droplets (secondary droplets), which becomes more difficult
RI
to be removed, is undesirable.
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The no-coalescence process is caused by very fast and energetic coalescence between water droplets and interface. In this case, the water droplets push intensively to continuous phase and lead to an extreme droplets elongation.
MA
When two water droplets approach to each other, a film of oil phase is formed between them. In this work the investigation on the formation of droplets during
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the electro-coalescence of the primary droplets are reported.
4 Experimental methodology
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A schematic diagram of the system used in this study was shown in Fig 1. The
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non- uniform electric field apparatus involves a Perspex cylinder with a copper jacket. The diameter and the height of Perspex cylinder were 2 cm and 34 cm, respectively, and the diameter and height of copper jacket were 2.2 cm and 32 cm, respectively. In the front and back of the jacket, a small hole was made to facilitate visualization of the coalescence phenomenon and the diameter of this hole was 1.9 cm approximately. A high voltage was attached to the middle electrode (brass electrode), and the lower one (cooper jacket) was earthed. To produce high voltages, a positive polarity high-voltage direct current power 5
ACCEPTED MANUSCRIPT amplifier (TREK20/20C) was used. A high-speed digital video camera (Photon FAST CAMS A5), equipped with a micro lens (NAVITAR12_ Zoom Lens), was
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used to observe the phenomena within the batch cylinder with a framing rate of
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20000 fps (frame per second). It was focused on the area limited to coalescing
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drop-interface. The lower part of the electro-coalescence cylinder was filled with the aqueous phase and its top part was filled with sunflower oil to form an interface between the two phases and proceed with the drop–interface coalescence
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experiments. The top of this cylinder has a very small hole to allow a needle to be inserted to introduce the water drops by a syringe falling in oil to create a drop–
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interface coalescence. In the experiments conducted, sunflower oil was used
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(prepared from Morrisons Ltd, UK). A halogen cold lamp (veritas, constellation 60, 6650K) was used for lighting. The intensity of the lighting was accurately
CE
adjusted to facilitate focusing. De-ionized water was used as the dispersed phase.
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The properties of the liquids used are presented in Table 1. The diameter of the water droplets were in the range between 576 and 1196 μm, which were measured by Image-Pro software using the images obtained by Photron Fast cam SA5. Four waveforms types were used to study the effect of them on the secondary droplets formation. The experiments were performed approximately at a temperature of 23 °C.
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ACCEPTED MANUSCRIPT Table. 1 shows the properties of the liquid used in the experiments.
De-ionized
Conductivity /μS·m-1
Viscosity /mPa·s
5.49
1.00
7.62 10 5
46.5
1000
Dielectric constant 80
922
4.9
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oil
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water Sunflower
Density/kg·m-3
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Liquids
Fig.1. Schematic diagram of the non-uniform electrical separation instrument
5. Results and Discussions In this study, first, the effect of four different waveform types (ramp-ac, puls-ac, sinusoidal and squer-ac) on the behaviour of coalescence using various voltages and frequencies was investigated. The coalescence processes of water droplets at 7
ACCEPTED MANUSCRIPT an oil/water interface were recorded at 20000 fps (frame per second) and were shown in the sequence of images and movies. When the droplet was allowed to
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settle on the interface, the high strength electric field between the droplet and
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interface caused to clamp between them. There are two phenomena dominate: (i)
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pumping of droplet into its bulk phase (due to surface tention) and (ii) the necking process (the droplet push into continuous phase leading to increasing the droplet size). Whether a secondary droplet is formed depends on the process which is
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dominating [9]. In this area, the following major results were found: (a) complete coalscence: It can be seen from Fig. 2 and movie 1(see supplementary
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materials) that droplet was pumped into its bulk phase and no necking was occured
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leading to a complete coalescence because the pumping process was faster than the necking process. The droplet experiences a repelling Columbic force by the
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adjacent electrode, this force is not sufficiently strong (in the case of complete
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coalescence) to help the occurrence of necking process [9]. (b) incomplete coalescence: As shown in Fig. 3(movie 2,see supplementary materials), the necking process was faster than the pumping process and finally led to an incomplete coalescence. The necking process and the pumping process were constanly in competition with each other. (c) no coalescence: The no-coalescence was caused by a very fast coalescence and an extensive pushing of droplet into the continuous phase. The predominance of each of these processes depends on some parameters and will be discussed here. 8
ACCEPTED MANUSCRIPT The effect of operational parameters such as electric field strength, waveform types and frequency on behaviour of coalescence process was investigated. No
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secondary droplet for drop-interface coalescence was observed using amplitude
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less than 90 V·mm-1 for all of four mentioned waveform types. The effect of
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frequency on the behaviour of the coalescence was negligible using electric field strength lower than 90 V·mm-1. The application of high electric field up to 150 V·mm-1 caused secondary droplets in all the mentioned waveform types, except in
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ramp-ac waveform. Secondary droplets were produced even by using ramp waveform by applying low frequency between 1Hz and 50 Hz and amplitude up to
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150 V·mm-1. However, no secondary droplet was observed by applying ramp
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waveform and frequency between 60 and 70 Hz and using the same amplitude, namely 150 V·mm-1 (Fig. 2 and supplementary video 1). Therefore, the effect of
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frequency was significant in higher amplitude magnitudes.
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In electric field strengths upper than 150 V·mm-1, the video camera showed that the complete coalescence could only be made with ramp-ac waveform types. (producing no secondary droplets ) (Fig. 1).
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PT
ACCEPTED MANUSCRIPT
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Fig. 2 shows water droplet coalescence in oil-water interface under non-uniform electric
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field in ramp-ac waveform type by applying amplitude and frequency of 150 V·mm-1 and
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60 Hz, respectively.
A complete coalescence is demonstrated by Fig. 2 by using a ramp waveform because there were not observed any secondary droplets. There was pumping process dominated.
5. 1. Ramp-ac waveform in comparison to the sinusoidal waveform As it was seen in Fig. 3, under the sinusoidal waveform, a drop-interface coalescence by applying amplitude of 150 V·mm-1 and frequency of 60 Hz was an incomplete coalescence; because necking process was dominated and some 10
ACCEPTED MANUSCRIPT secondary droplets were produced at the end of coalescence. Therefore the effect of waveform type was very significant and the advantage of the ramp waveform in
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comparison to the sinusoidal waveform type was very clear (Fig. 2). The
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waveform type was the key factor that played major role avoiding the production
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of secondary droplets.
Fig. 3 demonstrates an incomplete drop-interface coalescence, sinusoidal waveform, 150 V·mm-1, 60 Hz.
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ACCEPTED MANUSCRIPT 5. 2. Ramp-ac waveform in comparison to the pulse-ac waveform Fig. 4 showed a drop-interface coalescence using pulse-ac and applying amplitude
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of 150 V·mm-1 and frequency of 60 Hz in comparison to the ramp waveform (Fig.
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2). The application of pulse-ac and using amplitude of 150 V·mm-1 and frequency
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of 60 Hz led to a situation even close to the no-coalescence because the rupture between the droplet and the interface was very strong, leading to increase the primary droplet size. In this case, the droplet was pushed strongly into continuous
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phase and finally some bigger secondary droplets were detached from primary droplet.
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However, using ramp-ac waveform with the same circumstances led to a complete
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coalescence (Fig. 2).
Fig. 4 shows water droplets coalescence in oil-water interface. Here the pattern is no coalescence under non-uniform electric field by using pulse-ac waveform type. The amplitude and frequency were 150 V·mm-1 and 60 Hz, respectively.
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5. 3. Ramp-ac waveform compare to the square-ac waveform
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The Fig. 5 showed a drop-interface coalescence using square-ac waveform and
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applying amplitude of 150 V·mm-1 and frequency of 60 Hz in comparison to the
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ramp-ac waveform (Fig. 2), led to a situation close to the pulse-ac waveform and no-coalescence because the rupture between the droplet and the interface was very strong. In this case, like the pulse waveform, the droplet was pushed strongly into
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continuous phase and finally some bigger secondary droplets were detached from
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primary droplet too.
13
ACCEPTED MANUSCRIPT Fig. 5 shows water droplet coalescence in oil-water interface as pattern of no- coalescence under non-uniform electric field by using square-ac waveform type. The amplitude and
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frequency were 150 V·mm-1 and 60Hz respectively.
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5. 4. The effect of the frequency on coalescence behaviour
The frequency played an important role in amplitude magnitudes higher than 90 V·mm-1. Despite of using similar waveform (ramp-ac) and the similar amplitude
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amount (150 V·mm-1), applying different frequencies caused to different results. Lower frequency magnitudes led to an incomplete coalescence due to produce
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some secondary droplets (necking process was dominated).
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As it was observed, under the ramp-ac waveform, a drop-interface coalescence by
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applying amplitude of 150 V·mm-1 was an incomplete coalescence because some
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secondary droplets were produced at the end of coalescence due to low frequency (20 Hz) (Fig. 6 in comparison to Fig. 1). It led to an incomplete coalescence due to occurrence of necking process. It means that, the necking process was faster than the pumping process.
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PT
ACCEPTED MANUSCRIPT
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Fig. 6 shows water drop coalescence in oil-water interface under non-uniform electric
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Hz, respectively.
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field using ramp-ac waveform type using amplitude and frequency of 150 V·mm-1 and 20
5. 5. Using high amplitude up to 250 V·mm-1 Fig 7 shows, how two drops react with interface and how two drops react to each other and with interface simultaneously in comparison to single drop (see supplementary movie 6). The Fig. 7 shows a pattern of no-coalescence of two water droplets coalescence in oil-water interface using pulse-ac waveform type by applying the amplitude and frequency of 250 V·mm-1 and 60 Hz, respectively. This figure could be considered as a pattern for the no-coalescence situation which has 15
ACCEPTED MANUSCRIPT been caused by elongation of water droplets in oil-water interface. The rupture between the droplets and the interface was very strong, leading to increment of the
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primary droplets sizes. In this case, the droplets were pushed strongly into
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continuous phase and therefore some bigger secondary droplets were detached and
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finally led to a no-coalescence.
Fig. 7 shows water drop coalescence in oil-water interface under non-uniform electric field using pule-ac waveform type by applying amplitude and frequency of 250 V·mm-1 and 60 Hz, respectively.
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ACCEPTED MANUSCRIPT Table 2 shows the range of the electric field in which a complete coalescence for all four different waveform types is predictable.In this work, the pulse waveform
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was done reacting by applying lower amplitude than the other waveform types but the range of the amplitude for a complete coalescence was short namely between
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20 V·mm-1 and 90 V·mm-1. If an o/w emulsion is placed in a non-uniform electric field (DEP), the higher the amplitude is, the more particles would coalesce to each
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other. Therefore, the ramp-ac waveform could be more suitable for demulsification of water in oil emulsion due to wider range of electric field strength.
different waveform types
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Table 2 The range of the electric field in which a complete coalescence for all four Min. amplitude /V·mm-1
Waveform type
Max. amplitude /V·mm-1
34
Sinus
28
110
Square
25
95
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Ramp
22
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6. Conclusions
90
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Pulse
150
Different frequencies and different amplitudes were experimented and finally, it was found out that the magnitudes of 60 Hz and 150 V·mm-1 were the optimum frequency and amplitude, respectively. Amplitude over 150 V·mm-1 occurs mostly the non-coalescence and lower amplitude than 150 V·mm-1 achieved a weaker electric field and it could not be suitable for demulsification of water in oil emulsion due to low electric field strength. When the droplet was allowed to settle on the interface, the high strength electric field between the droplet and interface 17
ACCEPTED MANUSCRIPT caused to clamp between them. There are two phenomena dominate: (i) pumping of droplet into its bulk phase (due to surface tention) and (ii) the necking process
PT
(the droplet push into continuous phase leading to increasing the droplet size).The
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necking process and the pumping process were constantly in competition with each
NU SC
other. If the pumping process (due to surface tension, the droplet will push into its bulk phase) is faster than the necking process, it will lead to a complete coalescence and conversely will lead to an incomplete coalescence (necking
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process act faster).In the no-coalescence process, the rupture between the droplet and the interface was very strong, leading to increase the primary droplet size. In
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this case, the droplet was pushed strongly into continuous phase and finally some
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bigger secondary droplets were detached from primary droplet. The effect of frequency could be negligible in amplitudes lesser than 90 V·mm-1. In amplitude
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magnitude higher than 90 V·mm-1, the frequency had a significant effect on
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coalescence process. Ramp-ac waveform could be applied to electric field strength up to 150 V·mm-1 without observing any secondary droplets. The application of high amplitudes up to 250 V·mm-1 led to extremely elongation of droplets. Consequently, larger secondary droplets were detached from the primary drop while coalescing. The waveform type was the key factor that played a major role in avoiding secondary droplets. It was found that the ramp-ac waveform was the best waveform and the magnitudes of 60 Hz and 150 V·mm-1 were the optimum frequency and amplitude, respectively. 18
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Nomenclature the Perspex cylinder diameter which contains the sample, mm
Dj
the copper-jacket diameter, mm
d
droplet diameter, mm
Ec
field intensity in continuous phase, V·mm-1
Ed
field intensity in disperse phase, V·mm-1
R
the distance between droplet and the central electrode, mm
t
time, ms
c
conductivity in continuous phase, s·m-1
d
conductivity in disperse phase, s·m-1
εc
dielectric constant in continuous phase
εd
dielectric constant in disperse phase
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PT
Dc
Acknowledgements The author would like to thank for providing research facilities and for the helpful comments of a large number of members of Institute of Particle Science and Engineering, University of Leeds, UK. Thanks are especially due to Professor Mojtaba Ghadiri for helpful comments and support. The author is grateful for the generous help of Dr. Vincenzino Vicqua, Dr. Donghai Yang, Dr. Ali Hassanpour. This research work was financially supported by Babol Noshirvani University of Technology, Iran. Appendix 19
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The video files related to this article can be found in the Supplementary Files.
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References
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[1] Berg, G., Lars E. Lundgaard, L. E., Nicolas Abi-Chebel, N., 2010. Electrically
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stressed water drops in oil, Chemical Engineering and Processing: Process Intensification. 49, 1229-1240.
[2] Guo, C., He, L., 2014. Coalescence behaviour of two large water-drops in
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viscous oil under a DC electric field, Journal of Electrostatics.72, 470-476. [3]. Flavio, H., Morales, F., Duart, J., Samitier-Marti, J., 2008. Bacterial handling
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under the influence of non-uniform electric fields: dielectrophoretic and electrohydrodynamic effects, Anais Annals of the Brazilian Academy of Sciences.
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80, 627-638.
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[4] Hosseini, M., Shahavi, M. H., 2012. Electrostatic Enhancement of Coalescence
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of Oil Droplets (in Nanometer Scale) in Water Emulsion, Chinese Journal of Chemical Engineering, 20, (4) 654—658. [5] Alinezhad, K., Hosseini, M., Mowagharnegad, K., Salehi, M., 2010. Exprimental and modeling approach to separation of water in crude oil emulsion under non-uniform electrical field, Korean j. chem. eng. 27, 198-205. [6] Draxler, J., 1992. Fluessige Membranen fuer die Abwasserreinigung, (dbvVerlag Graz). [7] Mohammadi, M., Shahhosseini, S., Bayat, M., 2014. Electrocoalescence of 20
ACCEPTED MANUSCRIPT binary water droplets falling in oil: Experimental study, Chemical Engineering research and Design. 92, 2694-2704.
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[8] Hughes, M., Morgan, H., 1998. Dielectrophoretic trapping of single
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sub-micrometre scale bioparticles J. Phys. D: Appl. Phys. 31, 2205–2210
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[9] Mousavichoubeh, M., Ghadiri, M., Shariaty-Niassarb, M., 2011.
Electro-
coalescence of an aqueous droplet at an oil–water interface, Chemical Engineering
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CE
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and Processing. 50, 338–344.
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ACCEPTED MANUSCRIPT
GRAPHICAL ABSTRACT
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Coalescence behaviour of water droplet in water-oil interface under pulsatile
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CE
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electric fields
22
ACCEPTED MANUSCRIPT HIGHLIGHTS • The coalescence behavior of water droplet in water-oil interface under pulsatile
PT
electric fields was investigated.
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• When the droplet was allowed to settle on the interface, the high strength electric
NU SC
field between the droplet and interface caused to clamp between them and led to coalesce.
• The effect of four waveforms, pulse, sinus, square and ramp has been
MA
investigated.
• The ramp waveform is the best prevention of producing secondary droplets in
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comparison to the other mentioned waveform types.
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• The frequency played an important role by applying amplitude higher than 90
AC
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V·mm-1.
23
S1004-9541(16)30265-8 doi: 10.1016/j.cjche.2016.04.007 CJCHE 509
To appear in: Received date: Revised date: Accepted date:
24 September 2015 18 January 2016 1 March 2016
Please cite this article as: M. Hosseini, Coalescence behaviour of water droplets in wateroil interface under pulsatile electric fields, (2016), doi: 10.1016/j.cjche.2016.04.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Separation Science and Engineering
Coalescence behaviour of water droplets in water-oil interface
PT
under pulsatile electric fields
RI
M. Hosseini
NU SC
Faculty of chemical engineering, Babol Noshirvani University of Technology, Babol, Iran
Article history: Received 24 September 2015 Received in revised form 18 January 2016
ED
Accepted 1 March 2016 Available online xxxx
PT
Abstract
MA
Corresponding author. E-mail address: [email protected]
CE
In this research, the deformation of water droplets in sunflower oil-interface under
AC
pulsatile electric field was studied experimentally. Three types of coalescence were observed: (i) complete coalescence, (ii) incomplete coalescence and (iii) nocoalescence. The first type is desirable because of leaving no secondary droplets. The second type produced secondary droplets which was caused by necking process, due to extreme elongation of droplets (mostly small droplets), was undesirable; because the small droplets were more difficult to coalesce and remove. The no-coalescence was caused by very fast coalescence and extensive pushing of droplet into the continuous phase. In this work the process was operated 1
ACCEPTED MANUSCRIPT with the utilization of a batch cylindrical separator with high voltage system. The lower part of the cylinder was filled with the aqueous phase and its top part was
PT
filled with sunflower oil to form an interface between the two phases. The effects
RI
of electric field strength, frequency, and waveform types were investigated. It was
NU SC
found that, the ramp-ac waveform was the best waveform, avoiding the production of secondary droplets and in this case the frequency also played an important role.
MA
Keywords: Drop-interface coalescence, Drops deformation, Non-uniform electric
ED
field, Di-electrophoresis, pulsatile electric field.
1 Introduction
PT
The formation of an emulsion is either desirable or undesirable in many chemical
CE
industries. Therefore the coalescence phenomenon for separation (oily wastewater
AC
treatment) on one hand and the stability of the emulsion for example in food industry on the other hand are very important factors. However it is necessary to study the coalescence behaviour of droplets and it is very important to investigate the stability of an emulsion under electric field. During crude oil production both oil and water phases are co-produced [1]. Nowadays, there are many techniques for enhancing the separation of water-in-oil emulsions, such as the addition of chemical demulsifier, gravity or centrifugal settling, heat treatment and electrostatic demulsification [2]. Electric field is 2
ACCEPTED MANUSCRIPT extensively used in the chemical, petroleum and biochemical industries. In a nonuniform electric field (DEP=di-electrophorese), the polarized particles experience
PT
a force causing them to move to those regions that have high or low electric field
RI
[3]. Because of economical reason and particularly because of environmental
NU SC
aspects, the related technique is very suitable and because of the simplicity of the apparatus and process for demulsification, the electrical way would be one of the best way of destroying o/w emulsions. The separation of emulsion with non-
MA
uniform electric field (DEP) has been studied and the effects of temperature, time, voltage and different types of electrical current were considered (4, 5).
ED
In this study, drop-interface coalescence (water drop in oil/water-interface) under
PT
non-uniform electric field with different condition of waveform types, different voltages, and different frequencies have been considered. In this kind of
CE
demulsifier, one electrode is settled inside the cylindrical emulsion container and
AC
the other electrode is copper formic cylinder jacket (Fig. 1). By passing electrical current through the two electrodes, the field is created in a radial fashion towards the electrode (3, 6).
If two water droplets located in insulating oil are subjected to an external electric field E0, a dipole moment is induced at the droplets center. The magnitude of the dipole moment, p, depends on the electric field intensity, the drop size and permittivity [7]. 2. Mechanisms of non-uniform electric field on drop interface coalescence 3
ACCEPTED MANUSCRIPT
When a water droplet is suspended in a non-uniform electrical field, the interaction
PT
of the applied field and induced dipole generates a force on the particle. This force termed di-electrophoresis (8).
RI
According to the equation (1), DEP force magnitude (Fdep) for cylindrical
Fdep
d 3 c d c E 2 2 ( d 2 c ) R ln 2 ( Dj / Dc )
NU SC
container has been estimated as the following formula:
(1)
MA
According to the Eq. (1), the amount of Fdep depends on the following parameters: 1. The ratio of the copper jacket diameter, Dj, to the Perspex cylinder diameter
ED
Dc (emulsion container) (Fig. 1). Eq. (1) shows that the force of DEP will
PT
be weaker when the above-mentioned ratio is increased, so a higher electric
CE
field strength is required. 2. Dielectric constant, c, which in continuous phase depends on the kind of
AC
emulsion.
3. The smaller the droplets diameters (d) are, the lower the DEP force required. 4. When the distance (R) between the center of the water drop and the central electrode increased, the effect of electrical field intensity from the central electrode to farther droplets would be decreased.
3. Objectives of the present work 4
ACCEPTED MANUSCRIPT In general, three types of drop coalescence may occur: complete coalescence, incomplete coalescence, and no-coalescence. The incomplete coalescence, because
PT
of producing smaller droplets (secondary droplets), which becomes more difficult
RI
to be removed, is undesirable.
NU SC
The no-coalescence process is caused by very fast and energetic coalescence between water droplets and interface. In this case, the water droplets push intensively to continuous phase and lead to an extreme droplets elongation.
MA
When two water droplets approach to each other, a film of oil phase is formed between them. In this work the investigation on the formation of droplets during
PT
ED
the electro-coalescence of the primary droplets are reported.
4 Experimental methodology
CE
A schematic diagram of the system used in this study was shown in Fig 1. The
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non- uniform electric field apparatus involves a Perspex cylinder with a copper jacket. The diameter and the height of Perspex cylinder were 2 cm and 34 cm, respectively, and the diameter and height of copper jacket were 2.2 cm and 32 cm, respectively. In the front and back of the jacket, a small hole was made to facilitate visualization of the coalescence phenomenon and the diameter of this hole was 1.9 cm approximately. A high voltage was attached to the middle electrode (brass electrode), and the lower one (cooper jacket) was earthed. To produce high voltages, a positive polarity high-voltage direct current power 5
ACCEPTED MANUSCRIPT amplifier (TREK20/20C) was used. A high-speed digital video camera (Photon FAST CAMS A5), equipped with a micro lens (NAVITAR12_ Zoom Lens), was
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used to observe the phenomena within the batch cylinder with a framing rate of
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20000 fps (frame per second). It was focused on the area limited to coalescing
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drop-interface. The lower part of the electro-coalescence cylinder was filled with the aqueous phase and its top part was filled with sunflower oil to form an interface between the two phases and proceed with the drop–interface coalescence
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experiments. The top of this cylinder has a very small hole to allow a needle to be inserted to introduce the water drops by a syringe falling in oil to create a drop–
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interface coalescence. In the experiments conducted, sunflower oil was used
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(prepared from Morrisons Ltd, UK). A halogen cold lamp (veritas, constellation 60, 6650K) was used for lighting. The intensity of the lighting was accurately
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adjusted to facilitate focusing. De-ionized water was used as the dispersed phase.
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The properties of the liquids used are presented in Table 1. The diameter of the water droplets were in the range between 576 and 1196 μm, which were measured by Image-Pro software using the images obtained by Photron Fast cam SA5. Four waveforms types were used to study the effect of them on the secondary droplets formation. The experiments were performed approximately at a temperature of 23 °C.
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De-ionized
Conductivity /μS·m-1
Viscosity /mPa·s
5.49
1.00
7.62 10 5
46.5
1000
Dielectric constant 80
922
4.9
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oil
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water Sunflower
Density/kg·m-3
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Liquids
Fig.1. Schematic diagram of the non-uniform electrical separation instrument
5. Results and Discussions In this study, first, the effect of four different waveform types (ramp-ac, puls-ac, sinusoidal and squer-ac) on the behaviour of coalescence using various voltages and frequencies was investigated. The coalescence processes of water droplets at 7
ACCEPTED MANUSCRIPT an oil/water interface were recorded at 20000 fps (frame per second) and were shown in the sequence of images and movies. When the droplet was allowed to
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settle on the interface, the high strength electric field between the droplet and
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interface caused to clamp between them. There are two phenomena dominate: (i)
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pumping of droplet into its bulk phase (due to surface tention) and (ii) the necking process (the droplet push into continuous phase leading to increasing the droplet size). Whether a secondary droplet is formed depends on the process which is
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dominating [9]. In this area, the following major results were found: (a) complete coalscence: It can be seen from Fig. 2 and movie 1(see supplementary
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materials) that droplet was pumped into its bulk phase and no necking was occured
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leading to a complete coalescence because the pumping process was faster than the necking process. The droplet experiences a repelling Columbic force by the
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adjacent electrode, this force is not sufficiently strong (in the case of complete
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coalescence) to help the occurrence of necking process [9]. (b) incomplete coalescence: As shown in Fig. 3(movie 2,see supplementary materials), the necking process was faster than the pumping process and finally led to an incomplete coalescence. The necking process and the pumping process were constanly in competition with each other. (c) no coalescence: The no-coalescence was caused by a very fast coalescence and an extensive pushing of droplet into the continuous phase. The predominance of each of these processes depends on some parameters and will be discussed here. 8
ACCEPTED MANUSCRIPT The effect of operational parameters such as electric field strength, waveform types and frequency on behaviour of coalescence process was investigated. No
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secondary droplet for drop-interface coalescence was observed using amplitude
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less than 90 V·mm-1 for all of four mentioned waveform types. The effect of
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frequency on the behaviour of the coalescence was negligible using electric field strength lower than 90 V·mm-1. The application of high electric field up to 150 V·mm-1 caused secondary droplets in all the mentioned waveform types, except in
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ramp-ac waveform. Secondary droplets were produced even by using ramp waveform by applying low frequency between 1Hz and 50 Hz and amplitude up to
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150 V·mm-1. However, no secondary droplet was observed by applying ramp
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waveform and frequency between 60 and 70 Hz and using the same amplitude, namely 150 V·mm-1 (Fig. 2 and supplementary video 1). Therefore, the effect of
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frequency was significant in higher amplitude magnitudes.
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In electric field strengths upper than 150 V·mm-1, the video camera showed that the complete coalescence could only be made with ramp-ac waveform types. (producing no secondary droplets ) (Fig. 1).
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Fig. 2 shows water droplet coalescence in oil-water interface under non-uniform electric
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field in ramp-ac waveform type by applying amplitude and frequency of 150 V·mm-1 and
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60 Hz, respectively.
A complete coalescence is demonstrated by Fig. 2 by using a ramp waveform because there were not observed any secondary droplets. There was pumping process dominated.
5. 1. Ramp-ac waveform in comparison to the sinusoidal waveform As it was seen in Fig. 3, under the sinusoidal waveform, a drop-interface coalescence by applying amplitude of 150 V·mm-1 and frequency of 60 Hz was an incomplete coalescence; because necking process was dominated and some 10
ACCEPTED MANUSCRIPT secondary droplets were produced at the end of coalescence. Therefore the effect of waveform type was very significant and the advantage of the ramp waveform in
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comparison to the sinusoidal waveform type was very clear (Fig. 2). The
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waveform type was the key factor that played major role avoiding the production
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of secondary droplets.
Fig. 3 demonstrates an incomplete drop-interface coalescence, sinusoidal waveform, 150 V·mm-1, 60 Hz.
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ACCEPTED MANUSCRIPT 5. 2. Ramp-ac waveform in comparison to the pulse-ac waveform Fig. 4 showed a drop-interface coalescence using pulse-ac and applying amplitude
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of 150 V·mm-1 and frequency of 60 Hz in comparison to the ramp waveform (Fig.
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2). The application of pulse-ac and using amplitude of 150 V·mm-1 and frequency
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of 60 Hz led to a situation even close to the no-coalescence because the rupture between the droplet and the interface was very strong, leading to increase the primary droplet size. In this case, the droplet was pushed strongly into continuous
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phase and finally some bigger secondary droplets were detached from primary droplet.
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However, using ramp-ac waveform with the same circumstances led to a complete
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coalescence (Fig. 2).
Fig. 4 shows water droplets coalescence in oil-water interface. Here the pattern is no coalescence under non-uniform electric field by using pulse-ac waveform type. The amplitude and frequency were 150 V·mm-1 and 60 Hz, respectively.
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5. 3. Ramp-ac waveform compare to the square-ac waveform
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The Fig. 5 showed a drop-interface coalescence using square-ac waveform and
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applying amplitude of 150 V·mm-1 and frequency of 60 Hz in comparison to the
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ramp-ac waveform (Fig. 2), led to a situation close to the pulse-ac waveform and no-coalescence because the rupture between the droplet and the interface was very strong. In this case, like the pulse waveform, the droplet was pushed strongly into
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continuous phase and finally some bigger secondary droplets were detached from
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primary droplet too.
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ACCEPTED MANUSCRIPT Fig. 5 shows water droplet coalescence in oil-water interface as pattern of no- coalescence under non-uniform electric field by using square-ac waveform type. The amplitude and
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frequency were 150 V·mm-1 and 60Hz respectively.
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5. 4. The effect of the frequency on coalescence behaviour
The frequency played an important role in amplitude magnitudes higher than 90 V·mm-1. Despite of using similar waveform (ramp-ac) and the similar amplitude
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amount (150 V·mm-1), applying different frequencies caused to different results. Lower frequency magnitudes led to an incomplete coalescence due to produce
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some secondary droplets (necking process was dominated).
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As it was observed, under the ramp-ac waveform, a drop-interface coalescence by
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applying amplitude of 150 V·mm-1 was an incomplete coalescence because some
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secondary droplets were produced at the end of coalescence due to low frequency (20 Hz) (Fig. 6 in comparison to Fig. 1). It led to an incomplete coalescence due to occurrence of necking process. It means that, the necking process was faster than the pumping process.
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Fig. 6 shows water drop coalescence in oil-water interface under non-uniform electric
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Hz, respectively.
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field using ramp-ac waveform type using amplitude and frequency of 150 V·mm-1 and 20
5. 5. Using high amplitude up to 250 V·mm-1 Fig 7 shows, how two drops react with interface and how two drops react to each other and with interface simultaneously in comparison to single drop (see supplementary movie 6). The Fig. 7 shows a pattern of no-coalescence of two water droplets coalescence in oil-water interface using pulse-ac waveform type by applying the amplitude and frequency of 250 V·mm-1 and 60 Hz, respectively. This figure could be considered as a pattern for the no-coalescence situation which has 15
ACCEPTED MANUSCRIPT been caused by elongation of water droplets in oil-water interface. The rupture between the droplets and the interface was very strong, leading to increment of the
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primary droplets sizes. In this case, the droplets were pushed strongly into
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continuous phase and therefore some bigger secondary droplets were detached and
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finally led to a no-coalescence.
Fig. 7 shows water drop coalescence in oil-water interface under non-uniform electric field using pule-ac waveform type by applying amplitude and frequency of 250 V·mm-1 and 60 Hz, respectively.
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ACCEPTED MANUSCRIPT Table 2 shows the range of the electric field in which a complete coalescence for all four different waveform types is predictable.In this work, the pulse waveform
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was done reacting by applying lower amplitude than the other waveform types but the range of the amplitude for a complete coalescence was short namely between
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20 V·mm-1 and 90 V·mm-1. If an o/w emulsion is placed in a non-uniform electric field (DEP), the higher the amplitude is, the more particles would coalesce to each
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other. Therefore, the ramp-ac waveform could be more suitable for demulsification of water in oil emulsion due to wider range of electric field strength.
different waveform types
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Table 2 The range of the electric field in which a complete coalescence for all four Min. amplitude /V·mm-1
Waveform type
Max. amplitude /V·mm-1
34
Sinus
28
110
Square
25
95
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Ramp
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6. Conclusions
90
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Pulse
150
Different frequencies and different amplitudes were experimented and finally, it was found out that the magnitudes of 60 Hz and 150 V·mm-1 were the optimum frequency and amplitude, respectively. Amplitude over 150 V·mm-1 occurs mostly the non-coalescence and lower amplitude than 150 V·mm-1 achieved a weaker electric field and it could not be suitable for demulsification of water in oil emulsion due to low electric field strength. When the droplet was allowed to settle on the interface, the high strength electric field between the droplet and interface 17
ACCEPTED MANUSCRIPT caused to clamp between them. There are two phenomena dominate: (i) pumping of droplet into its bulk phase (due to surface tention) and (ii) the necking process
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(the droplet push into continuous phase leading to increasing the droplet size).The
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necking process and the pumping process were constantly in competition with each
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other. If the pumping process (due to surface tension, the droplet will push into its bulk phase) is faster than the necking process, it will lead to a complete coalescence and conversely will lead to an incomplete coalescence (necking
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process act faster).In the no-coalescence process, the rupture between the droplet and the interface was very strong, leading to increase the primary droplet size. In
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this case, the droplet was pushed strongly into continuous phase and finally some
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bigger secondary droplets were detached from primary droplet. The effect of frequency could be negligible in amplitudes lesser than 90 V·mm-1. In amplitude
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magnitude higher than 90 V·mm-1, the frequency had a significant effect on
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coalescence process. Ramp-ac waveform could be applied to electric field strength up to 150 V·mm-1 without observing any secondary droplets. The application of high amplitudes up to 250 V·mm-1 led to extremely elongation of droplets. Consequently, larger secondary droplets were detached from the primary drop while coalescing. The waveform type was the key factor that played a major role in avoiding secondary droplets. It was found that the ramp-ac waveform was the best waveform and the magnitudes of 60 Hz and 150 V·mm-1 were the optimum frequency and amplitude, respectively. 18
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Nomenclature the Perspex cylinder diameter which contains the sample, mm
Dj
the copper-jacket diameter, mm
d
droplet diameter, mm
Ec
field intensity in continuous phase, V·mm-1
Ed
field intensity in disperse phase, V·mm-1
R
the distance between droplet and the central electrode, mm
t
time, ms
c
conductivity in continuous phase, s·m-1
d
conductivity in disperse phase, s·m-1
εc
dielectric constant in continuous phase
εd
dielectric constant in disperse phase
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Dc
Acknowledgements The author would like to thank for providing research facilities and for the helpful comments of a large number of members of Institute of Particle Science and Engineering, University of Leeds, UK. Thanks are especially due to Professor Mojtaba Ghadiri for helpful comments and support. The author is grateful for the generous help of Dr. Vincenzino Vicqua, Dr. Donghai Yang, Dr. Ali Hassanpour. This research work was financially supported by Babol Noshirvani University of Technology, Iran. Appendix 19
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The video files related to this article can be found in the Supplementary Files.
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References
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[1] Berg, G., Lars E. Lundgaard, L. E., Nicolas Abi-Chebel, N., 2010. Electrically
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stressed water drops in oil, Chemical Engineering and Processing: Process Intensification. 49, 1229-1240.
[2] Guo, C., He, L., 2014. Coalescence behaviour of two large water-drops in
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viscous oil under a DC electric field, Journal of Electrostatics.72, 470-476. [3]. Flavio, H., Morales, F., Duart, J., Samitier-Marti, J., 2008. Bacterial handling
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under the influence of non-uniform electric fields: dielectrophoretic and electrohydrodynamic effects, Anais Annals of the Brazilian Academy of Sciences.
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80, 627-638.
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[4] Hosseini, M., Shahavi, M. H., 2012. Electrostatic Enhancement of Coalescence
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of Oil Droplets (in Nanometer Scale) in Water Emulsion, Chinese Journal of Chemical Engineering, 20, (4) 654—658. [5] Alinezhad, K., Hosseini, M., Mowagharnegad, K., Salehi, M., 2010. Exprimental and modeling approach to separation of water in crude oil emulsion under non-uniform electrical field, Korean j. chem. eng. 27, 198-205. [6] Draxler, J., 1992. Fluessige Membranen fuer die Abwasserreinigung, (dbvVerlag Graz). [7] Mohammadi, M., Shahhosseini, S., Bayat, M., 2014. Electrocoalescence of 20
ACCEPTED MANUSCRIPT binary water droplets falling in oil: Experimental study, Chemical Engineering research and Design. 92, 2694-2704.
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[8] Hughes, M., Morgan, H., 1998. Dielectrophoretic trapping of single
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sub-micrometre scale bioparticles J. Phys. D: Appl. Phys. 31, 2205–2210
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[9] Mousavichoubeh, M., Ghadiri, M., Shariaty-Niassarb, M., 2011.
Electro-
coalescence of an aqueous droplet at an oil–water interface, Chemical Engineering
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and Processing. 50, 338–344.
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GRAPHICAL ABSTRACT
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Coalescence behaviour of water droplet in water-oil interface under pulsatile
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electric fields
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ACCEPTED MANUSCRIPT HIGHLIGHTS • The coalescence behavior of water droplet in water-oil interface under pulsatile
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electric fields was investigated.
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• When the droplet was allowed to settle on the interface, the high strength electric
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field between the droplet and interface caused to clamp between them and led to coalesce.
• The effect of four waveforms, pulse, sinus, square and ramp has been
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investigated.
• The ramp waveform is the best prevention of producing secondary droplets in
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comparison to the other mentioned waveform types.
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• The frequency played an important role by applying amplitude higher than 90
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V·mm-1.
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