- Email: [email protected]
Oil–water de-emulsification using ultrasonic technology
Accepted Manuscript Oil-Water De-emulsification using Ultrasonic Technology Muhammad Mohsin, Mahmoud Meribout PII: DOI: Reference:
S1350-4177(14)00173-4 http://dx.doi.org/10.1016/j.ultsonch.2014.05.014 ULTSON 2610
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
12 September 2013 6 April 2014 19 May 2014
Please cite this article as: M. Mohsin, M. Meribout, Oil-Water De-emulsification using Ultrasonic Technology, Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.05.014
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.
Oil-Water De-emulsification using Ultrasonic Technology Abstract In this paper an oil-water de-emulsification process within large tanks using ultrasonic technology is presented. As the device would operate in hazardous areas, it should not consume an excessive amount of electrical power. Hence, the paper investigates the suitable oil-water concentrations (10 to 90 % concentrations in step of 10%) which would lead to the fastest separation while consuming the minimum amount of power. Extensive experiments which were conducted using a powerful 20 KHz ultrasonic sensor were indicative with good repeatability that the emulsion layer with less water content (i.e. 10 to 40 % water-cut) gets significantly faster separation. The experimental study was then validated through a set of finite element-based simulations for different ratios of oil water emulsions. This led to suggest a new feasible deemulsifying device which consists of a one dimensional array of ultrasonic sensors which are vertically distributed to emit ultrasonic waves in horizontal direction and in a time multiplexed manner.
1. Introduction In industrial applications, an emulsion can be defined as a dispersion of one immiscible liquid into another. Eliminating the emulsion layer is required in several industrial applications for either economical or environmental motivations. For instance, during crude oil transportation in oil fields, it has been demonstrated that water concentration of more than 2 % may lead to some severe corrosions in the pipeline network [1]. In other industries which involve various industrial
processes such as cutting, lubrication cooling, surface cleaning and corrosion prevention, deemulsification of various industrial wastes is important to protect the ground water and the environment in general [2]. These needs have attracted the attention of many researchers the works of whom have resulted in some commercially available tools. Electrochemical deemulsification, electrostatic coalescence of water droplets, and with a higher degree chemical surfactants based de-emulsification were among the most widely used techniques [3]. In this paper a de-emulsification process using ultrasonic technology is suggested. The same technique has already been widely used for solid-liquid, liquid-liquid, or solid-solid separation in different industrial applications. For instance, in [4], an ultrasound-assisted production of biodiesel fuel from vegetable (canola) oil was suggested. The corresponding experimental results revealed that the application of 1 KW ultrasonic radiations with a resonance frequency of around 20- KHz could transform 99% of triglycerides into fatty acid methyl esters within a reaction time of 50 minutes. In [5], the effect of low intensity ultrasonic waves on the separation of oil droplets from oil in water emulsions in three different types of porous media was addressed. An electronic controller was used to regulate the power factor close to unity to improve the efficiency of the system. Experiments with and without porous medium revealed that porous medium enhances more effectively the separation of oil and water phases. In contrast to this research, this paper mainly focuses on water-oil emulsions which may exist within tanks. Therefore, it does not consider the effect of the medium porosity in the separation process. In addition, one single ultrasonic sensor may not be power-efficient for large tanks since the propagation of ultrasonic energy throughout the emulsion depends on its oil-water concentration. Hence, focusing the ultrasonic energy on a desirable area within the tank constitutes one of the research contributions in this paper. In contrast to another similar work [6] which considers one fixed oil in water
concentration (i.e. 50% water-cut), this paper addresses the effect of constant frequency and power ultrasonic waves on different oil-water concentrations (i.e. speed-up the oil-water separation while consuming a minimum amount of electrical energy). This constitutes the main design driving factor for the de-emulsification device suggested in this paper since fast oil-water separation increases the oil production rate. Similarly, as the amount of electrical power available for the field instruments is restricted not to exceed a predefined value, especially in hazardous areas, the design need to consider an upper limit of the total power to be supplied to the ultrasonic device. In addition of being chemical free and environmental friendly, the device has the advantage of being easily transportable and commissioned and does not generate any kind of hazardous radiations. To our knowledge, none of the previous related works have addressed the effect of oil-water concentration on the efficiency of separation. Furthermore, no literature has been found presenting the simulation models and validating the experimental findings.
2. Overview of the Ultrasonic-based De-Emulsification Module The ultrasonic de-emulsification module used in this study is the YP-S83 system, from Hangzhou Success Ultrasonic Equipment Co [7]. It requires 220 VAC and can operate within a temperature of up to 800 0C to generate a pressure towards the emulsion of up to 35 MPa. Figure 1 shows its overall block diagram. It consists of two sub-modules: an electrical power amplifier and a mechanical power. The power amplifier which consists of a DC power amplifier and a high frequency inverter generates a powerful electrical signal (up to 3 KW power output) of around 20 KHz resonance frequency to be applied across the ultrasonic transducer. This would lead the piezoelectric sensor to vibrate at its resonance frequency. The induced pressure however
requires further mechanical amplification which is provided by the booster and the horn, which are preceded by a stack of several piezoelectric sensors.
Stack of Piezoelectric Transducers
/2 Booster
/2 Flange
Horn
Figure 1 Mechanical vibrator and amplification part of Ultrasonic setup
The amplification gain is controlled by the length of the booster and horn which is usually taken as equal to half of the ultrasonic wavelength ( /2). In addition, as shown in Figure 1, a flange is used to couple the amplified mechanical signal to the horn which in turn ensures an impedance matching with regard to the target fluid. The resonance frequency of the ultrasonic generator depends upon the thickness and diameter of the piezoelectric material. However, it is recommended that the thickness to diameter ratio would be equal to a critical value corresponding to the most intense longitudinal and radial ultrasonic vibrations [8]. Following the above discussions, it can be deduced that for an ultrasonic sensor of 20 KHz resonance frequency, the total length of the ultrasonic sensor would be equal to 60 cm if a power of around 2 kW is sought. Hence, with reference to Figure 1, the sensor can be immersed either in the vertical or horizontal positions. However, as the axial force is higher in the actual design, and in order to focus on a specific height within the tank, a horizontal positioning of the ultrasonic source is recommended. This can be quite practical with regard to the overall diameter of the oil tanks. Figure 2 shows a detailed block diagram of the ultrasonic source and its connection to the ultrasonic generator. To maintain both the frequency and output power of the ultrasonic energy to a target value, a PID closed loop controller is used (Figure 2). It rectifies any loss of
synchronism between the electrical and mechanical signals. This is achieved by continuous comparison of both the electrical and pre-amplified mechanical signals in phase and frequency. If an error is noticed between the two signals, it is fed back to the power amplifier which adjusts the electrical signal accordingly. The procedure is continuous and hence maintains the electrical and mechanical signals at the same phase, frequency and amplitude.
Signal Generator (Oscillator)
AC‐DC Rectifier
Set Frequency and output Power
Power Amplifier
High Frequency Inverter
Control Circuit
Frequency Comparator
Stack of Transducers
Booster & Horn Figure 2 Ultrasonic wave Generation block diagram
Figure 3 shows a photograph of the existing prototype. It consists of a laboratory scale glass tank of one meter height in which oil-water emulsion is filled. A sensing part which consists of five ultrasonic sensors of 200 KHz resonance frequency each is used to determine the water-cut profile.
Cooling Fan
Power signal cable Control cables Glass Tank W / O Emulsion
Sensing Part Adjustable ultrasonic source
Figure 3 Pictorial view of experimental setup
3. Experimental Results and Validation 3.1 Materials Selected and Methods Extensive laboratory experiments were conducted at small scale level using a glass beaker (of 100 ml in volume) to investigate the effect of ultrasonic waves on emulsion at macroscopic level. The maximum power generated by the ultrasonic sensor for CW mode was 35 W, measured calorimetrically. The surface of the transducer in direct contact with the solution was 19.625 cm2. Thus the maximum acoustic intensity is 1.78 W.cm-2. The immersed probe is made of titanium and is connected to the ultrasonic processor via a special 4-wires cable. The technological parameters of the ultrasonic device are detailed in [7].
The emulsions have been prepared within the glass beaker by considering different proportions of olive oil intermixed with pure water, under a steady state temperature of around 25 0C. In each experiment, a powerful magnetic stirrer was used to mix the emulsion uniformly for 15 minutes at 700 revolutions per minute. A square shaped ceramic based top plate (232 cm2) of the stirrer made it possible to mix the emulsion smoothly with the capability of mixing up to 2.5 liters of liquid. The behavior of the emulsions was then studied in normal conditions as well as with the application of ultrasonic irradiations. The water-oil concentrations considered in the study are 10 %, to 90% water in oil with a step increase of 10% water in oil for each sample. The laboratory scale experimental setup used in this study is shown in figure 4 below. The results obtained during the first few experiments were as per the theoretical expectations where the ultrasonic waves produce strong mechanical vibrations inside the emulsion to weaken the intermolecular forces present between water and oil phases. In addition, they also cause a small rise in temperature of emulsion strengthening the rate of water droplets coalescence and reducing the viscosity of the emulsion mixture which accelerates the rate of sedimentation.
Electrical Generator
Magnetic Mixer
Adjustable Ultrasonic source
Oil/ Water emulsion
Coaxial cable for ultrasonic source Figure 4 A pictorial view of the emulsion with magnetic mixer and ultrasonic generation setup
The increased rate of coalescence and sedimentation settles down the aqueous phase at the bottom of the beaker. In addition, it has been found that ultrasonic radiations have a significant impact on flocculation as compared to natural separation. Figure 5 shows the phenomena of flocculation and coalescence (in red circles) which were obtained in one of the experiments. In Figure 5 (a), the small red circle enclose two oil droplets which were flocculated, while the larger circle enclose many oil droplets present at the oil-water interface. Figure 5 (b) shows the coalescence of the same droplets shown in Figure 5 (a), in addition to the settling down of water after one minute. Hence, the experiment clearly shows the importance of the distance between the droplets towards the rate of coalescence.
(a)
(b)
Flocculation
Coalescence
Figure 5 Coalescence and Flocculation of oil droplets in 40% water in oil emulsion
Figures 6 and 7 show the results of two experiments (e.g. Study 1 and Study 2) obtained from different percentages of oil in water emulsions by keeping the total volume of the emulsion constant (e.g. total volume set to 800 ml). Ten (10) experiments were conducted for each oil/water concentration and a repeatability of 99.98 % was achieved. It has been noticed that ultrasonic waves cause faster de-emulsification of water in oil emulsions especially up to a level of 50-50% water-oil emulsion. On increasing the amount of water beyond 50%, the intermolecular forces between the water molecules become stronger and the emulsion settles down quickly in both natural and ultrasonic settlings. This makes difficult to differentiate between natural separation and ultrasonic separation of aqueous and oil phases. Sub plot in figure 6 shows the zoomed in view of the two studies for 50 to 90 % oil in water emulsions as it bars were too small to visualize on normal scale.
Figure 6 Time taken by different percentages of water in oil emulsification for natural settling of water and oil phases
Figure 7 Time taken by different percentages of water in oil emulsification for ultrasonic settling of water and oil phases
Figure 8 shows the results corresponding to natural and ultrasonic separations in case of 40% water in oil emulsion. It can be noticed that natural separation left water droplets in the aqueous
phase even after 15 minutes while the application of ultrasonic for around 1 minute left almost no oil droplet in aqueous phase and clear separation of the two phases could be achieved.
(a)
(b)
(c)
Figure 8 Comparison of natural and forced ultrasonic separation with the original emulsion sample before mixing: (a) Initial water and oil put together without mixing, (b) natural separation after 15 minutes of placement, (c) ultrasonic forced separation after 1 minute ultrasonic irradiation of the sample
Figure 9 summarizes the results of several experiments highlighting the reduction in settling time when applying ultrasonic waves for different concentration of water in oil emulsions (i.e. watercut up to 40%). Hence, ten (10) experiments were conducted for each oil/water concentration and a repeatability of 99.85 % was achieved. It is evident that the usage of ultrasonic waves was consistently effective since a reduction of settling time could reach up to 96% as compared to natural separation.
Figure 9 Reduction in settling time of the oil and water phases on application of ultrasonic waves as compared to natural settling
3.2 Numerical Simulations In order to validate the results obtained in the above experiments, a computerized finite element analysis-based simulation was conducted. The pressure acoustic module with a focus on acoustic structure interaction was numerically solved using the Helmholtz equation. Hence, the model considered in this paper did not consider the behavior in terms of fluid flow, of the flocculation, coalescence and sedimentation with interaction of ultrasonic effects. Nevertheless, it focuses merely on the pressure distribution and sound pressure level which also defines the rate of flocculation. This simplified model considers a three dimensional block as an oil phase having a volume of 1 x 0.5 x 0.1 m3. The left boundary of this oil block is defined as ultrasonic source (20 KHz resonance frequency) with spherical radiations while its right side has a 0.05 x 0.5 x 0.1 m3 aluminum block acting as a metallic reflector for ultrasonic waves. The initial pressure of the fluid domain and ultrasonic source were set to 0 Pa and 1Pa respectively. The radius of the water
droplets was set to 50 mm and an equidistant uniform distribution of the droplets was configured in this model. Three different scenarios were considered in the study: two droplets (i.e. water-cut of 5 %), four droplets (i.e. water-cut of 10 %), and six droplets (i.e. water cut of 20 %). Figure 10 shows the results corresponding to each of these three cases. It can be noticed that almost the same pressure distribution and sound pressure level are obtained in all the three cases. Therefore, in case of less number of droplets, the separation process is slower since the force being exerted on the droplets is uniformly distributed through the whole fluid domain and less surface area is exposed to a particular pressure force. This leads to less coagulating force and consequently slower coagulation and separation of water. Increasing the number of droplets increases the surface area exposed to same pressure force which leads to faster coagulation of water droplets. This result matches the results obtained in the experiments.
Spherical radiation source boundary
Aluminum block
( a)
(b)
Water droplets
(c) Figure 10 Simulation results with different no. of water droplets in continuous oil phase (a) two water droplets, (b) four water droplets, (c) eight water droplets
4. Conclusion In this paper, ultrasonic technology was investigated for removal of oil-water emulsions which may appear in large tanks. Computerized simulations as well as lab scale experiments were performed and the results are found to match each other. Different values of water-cut emulsions have been considered to assess its effect on the separation process. Hence, the usage of ultrasonic waves was justifiable since the corresponding separation rate for emulsions, of 10 to 40 % water cut, is found to be faster than the natural separation by an order of magnitude. This leads to suggest a new de-emulsifying device that would consists of a one dimensional array of ultrasonic sensors, vertical distributed, among which, only one sensor emits ultrasonic waves (because of the limited power allowed in hazardous areas). The active sensor corresponds to the height with water-cut ranging from 10 to 40%. Hence, an emulsion profiler needs to be deployed in the tank to provide in real-time the emulsion profile [9]. In addition of being safe and energy efficient, the suggested device features the advantage of not changing the physical properties of oil.
Reference [1]
N. Nadarajah, A. Singh, and O. P. Ward, "Evaluation of a mixed bacterial culture for deemulsification of water-in-petroleum oil emulsions," World Journal of Microbiology and Biotechnology, vol. 18, pp. 435-440, 2002.
[2]
K. Bensadok, S. Benammar, F. Lapicque, and G. Nezzal, "Electrocoagulation of cutting oil emulsions using aluminium plate electrodes," Elsevier Journal of hazardous materials, vol. 152, pp. 423-430, 2008.
[3]
P. Cañizares, F. Martínez, J. Lobato, and M. A. Rodrigo, "Break-up of oil-in-water emulsions by electrochemical techniques," Elsevier Journal of hazardous materials, vol. 145, pp. 233-240, 2007.
[4] L. Thanh, K. Okitsu, Y. Sadanaga, N. Takenaka, Y. Maeda, and K. Bandow, “Ultrasoundassisted production of biodiesel fuel from vegetable oils in a small scale circulation process", Bioresource Technology Journal, Elsevier, Volume 101, pp. 639-645, 2010. L. [5]
G. D. Pangu and D. L. Feke, "Acoustically aided separation of oil droplets from aqueous emulsions," Chemical engineering science, vol. 59, pp. 3183-3193, 2004.
[6]
A. M. Al-Naamany, M. Meribout, and K. A. Busaidi, "Design and implementation of a new nonradioactive-based machine for detecting oil–water interfaces in oil tanks," IEEE Transactions on Instrumentation and Measurement, vol. 56, pp. 1532-1536, 2007.
[7]
URL: http://hsuecl.en.china.cn/
[8]
S. Lin and F. Zhang, "Measurement of Ultrasonic Power and Electro-acoustic Efficiency of High Power Transducers," Journal of Ultrasonics, vol. 37, pp. 549-554, 2000.
[9]
M. Meribout, A. Al Naamany, and K. Al Busaidi, "An acoustic system for providing the two-phase liquid profile in oil field storage tanks," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 56, pp. 2241-2250, 2009.
Paper title: Oil-Water De-emulsification using Ultrasonic Technology
-
Author Names: Muhammad Mohsin & Mahmoud Meribout
-
Affiliation: Electrical Engineering Department, Petroleum Institute, Abu Dhabi, UAE
Corresponding Author: Mahmoud Meribout. Email: [email protected]
S1350-4177(14)00173-4 http://dx.doi.org/10.1016/j.ultsonch.2014.05.014 ULTSON 2610
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
12 September 2013 6 April 2014 19 May 2014
Please cite this article as: M. Mohsin, M. Meribout, Oil-Water De-emulsification using Ultrasonic Technology, Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.05.014
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.
Oil-Water De-emulsification using Ultrasonic Technology Abstract In this paper an oil-water de-emulsification process within large tanks using ultrasonic technology is presented. As the device would operate in hazardous areas, it should not consume an excessive amount of electrical power. Hence, the paper investigates the suitable oil-water concentrations (10 to 90 % concentrations in step of 10%) which would lead to the fastest separation while consuming the minimum amount of power. Extensive experiments which were conducted using a powerful 20 KHz ultrasonic sensor were indicative with good repeatability that the emulsion layer with less water content (i.e. 10 to 40 % water-cut) gets significantly faster separation. The experimental study was then validated through a set of finite element-based simulations for different ratios of oil water emulsions. This led to suggest a new feasible deemulsifying device which consists of a one dimensional array of ultrasonic sensors which are vertically distributed to emit ultrasonic waves in horizontal direction and in a time multiplexed manner.
1. Introduction In industrial applications, an emulsion can be defined as a dispersion of one immiscible liquid into another. Eliminating the emulsion layer is required in several industrial applications for either economical or environmental motivations. For instance, during crude oil transportation in oil fields, it has been demonstrated that water concentration of more than 2 % may lead to some severe corrosions in the pipeline network [1]. In other industries which involve various industrial
processes such as cutting, lubrication cooling, surface cleaning and corrosion prevention, deemulsification of various industrial wastes is important to protect the ground water and the environment in general [2]. These needs have attracted the attention of many researchers the works of whom have resulted in some commercially available tools. Electrochemical deemulsification, electrostatic coalescence of water droplets, and with a higher degree chemical surfactants based de-emulsification were among the most widely used techniques [3]. In this paper a de-emulsification process using ultrasonic technology is suggested. The same technique has already been widely used for solid-liquid, liquid-liquid, or solid-solid separation in different industrial applications. For instance, in [4], an ultrasound-assisted production of biodiesel fuel from vegetable (canola) oil was suggested. The corresponding experimental results revealed that the application of 1 KW ultrasonic radiations with a resonance frequency of around 20- KHz could transform 99% of triglycerides into fatty acid methyl esters within a reaction time of 50 minutes. In [5], the effect of low intensity ultrasonic waves on the separation of oil droplets from oil in water emulsions in three different types of porous media was addressed. An electronic controller was used to regulate the power factor close to unity to improve the efficiency of the system. Experiments with and without porous medium revealed that porous medium enhances more effectively the separation of oil and water phases. In contrast to this research, this paper mainly focuses on water-oil emulsions which may exist within tanks. Therefore, it does not consider the effect of the medium porosity in the separation process. In addition, one single ultrasonic sensor may not be power-efficient for large tanks since the propagation of ultrasonic energy throughout the emulsion depends on its oil-water concentration. Hence, focusing the ultrasonic energy on a desirable area within the tank constitutes one of the research contributions in this paper. In contrast to another similar work [6] which considers one fixed oil in water
concentration (i.e. 50% water-cut), this paper addresses the effect of constant frequency and power ultrasonic waves on different oil-water concentrations (i.e. speed-up the oil-water separation while consuming a minimum amount of electrical energy). This constitutes the main design driving factor for the de-emulsification device suggested in this paper since fast oil-water separation increases the oil production rate. Similarly, as the amount of electrical power available for the field instruments is restricted not to exceed a predefined value, especially in hazardous areas, the design need to consider an upper limit of the total power to be supplied to the ultrasonic device. In addition of being chemical free and environmental friendly, the device has the advantage of being easily transportable and commissioned and does not generate any kind of hazardous radiations. To our knowledge, none of the previous related works have addressed the effect of oil-water concentration on the efficiency of separation. Furthermore, no literature has been found presenting the simulation models and validating the experimental findings.
2. Overview of the Ultrasonic-based De-Emulsification Module The ultrasonic de-emulsification module used in this study is the YP-S83 system, from Hangzhou Success Ultrasonic Equipment Co [7]. It requires 220 VAC and can operate within a temperature of up to 800 0C to generate a pressure towards the emulsion of up to 35 MPa. Figure 1 shows its overall block diagram. It consists of two sub-modules: an electrical power amplifier and a mechanical power. The power amplifier which consists of a DC power amplifier and a high frequency inverter generates a powerful electrical signal (up to 3 KW power output) of around 20 KHz resonance frequency to be applied across the ultrasonic transducer. This would lead the piezoelectric sensor to vibrate at its resonance frequency. The induced pressure however
requires further mechanical amplification which is provided by the booster and the horn, which are preceded by a stack of several piezoelectric sensors.
Stack of Piezoelectric Transducers
/2 Booster
/2 Flange
Horn
Figure 1 Mechanical vibrator and amplification part of Ultrasonic setup
The amplification gain is controlled by the length of the booster and horn which is usually taken as equal to half of the ultrasonic wavelength ( /2). In addition, as shown in Figure 1, a flange is used to couple the amplified mechanical signal to the horn which in turn ensures an impedance matching with regard to the target fluid. The resonance frequency of the ultrasonic generator depends upon the thickness and diameter of the piezoelectric material. However, it is recommended that the thickness to diameter ratio would be equal to a critical value corresponding to the most intense longitudinal and radial ultrasonic vibrations [8]. Following the above discussions, it can be deduced that for an ultrasonic sensor of 20 KHz resonance frequency, the total length of the ultrasonic sensor would be equal to 60 cm if a power of around 2 kW is sought. Hence, with reference to Figure 1, the sensor can be immersed either in the vertical or horizontal positions. However, as the axial force is higher in the actual design, and in order to focus on a specific height within the tank, a horizontal positioning of the ultrasonic source is recommended. This can be quite practical with regard to the overall diameter of the oil tanks. Figure 2 shows a detailed block diagram of the ultrasonic source and its connection to the ultrasonic generator. To maintain both the frequency and output power of the ultrasonic energy to a target value, a PID closed loop controller is used (Figure 2). It rectifies any loss of
synchronism between the electrical and mechanical signals. This is achieved by continuous comparison of both the electrical and pre-amplified mechanical signals in phase and frequency. If an error is noticed between the two signals, it is fed back to the power amplifier which adjusts the electrical signal accordingly. The procedure is continuous and hence maintains the electrical and mechanical signals at the same phase, frequency and amplitude.
Signal Generator (Oscillator)
AC‐DC Rectifier
Set Frequency and output Power
Power Amplifier
High Frequency Inverter
Control Circuit
Frequency Comparator
Stack of Transducers
Booster & Horn Figure 2 Ultrasonic wave Generation block diagram
Figure 3 shows a photograph of the existing prototype. It consists of a laboratory scale glass tank of one meter height in which oil-water emulsion is filled. A sensing part which consists of five ultrasonic sensors of 200 KHz resonance frequency each is used to determine the water-cut profile.
Cooling Fan
Power signal cable Control cables Glass Tank W / O Emulsion
Sensing Part Adjustable ultrasonic source
Figure 3 Pictorial view of experimental setup
3. Experimental Results and Validation 3.1 Materials Selected and Methods Extensive laboratory experiments were conducted at small scale level using a glass beaker (of 100 ml in volume) to investigate the effect of ultrasonic waves on emulsion at macroscopic level. The maximum power generated by the ultrasonic sensor for CW mode was 35 W, measured calorimetrically. The surface of the transducer in direct contact with the solution was 19.625 cm2. Thus the maximum acoustic intensity is 1.78 W.cm-2. The immersed probe is made of titanium and is connected to the ultrasonic processor via a special 4-wires cable. The technological parameters of the ultrasonic device are detailed in [7].
The emulsions have been prepared within the glass beaker by considering different proportions of olive oil intermixed with pure water, under a steady state temperature of around 25 0C. In each experiment, a powerful magnetic stirrer was used to mix the emulsion uniformly for 15 minutes at 700 revolutions per minute. A square shaped ceramic based top plate (232 cm2) of the stirrer made it possible to mix the emulsion smoothly with the capability of mixing up to 2.5 liters of liquid. The behavior of the emulsions was then studied in normal conditions as well as with the application of ultrasonic irradiations. The water-oil concentrations considered in the study are 10 %, to 90% water in oil with a step increase of 10% water in oil for each sample. The laboratory scale experimental setup used in this study is shown in figure 4 below. The results obtained during the first few experiments were as per the theoretical expectations where the ultrasonic waves produce strong mechanical vibrations inside the emulsion to weaken the intermolecular forces present between water and oil phases. In addition, they also cause a small rise in temperature of emulsion strengthening the rate of water droplets coalescence and reducing the viscosity of the emulsion mixture which accelerates the rate of sedimentation.
Electrical Generator
Magnetic Mixer
Adjustable Ultrasonic source
Oil/ Water emulsion
Coaxial cable for ultrasonic source Figure 4 A pictorial view of the emulsion with magnetic mixer and ultrasonic generation setup
The increased rate of coalescence and sedimentation settles down the aqueous phase at the bottom of the beaker. In addition, it has been found that ultrasonic radiations have a significant impact on flocculation as compared to natural separation. Figure 5 shows the phenomena of flocculation and coalescence (in red circles) which were obtained in one of the experiments. In Figure 5 (a), the small red circle enclose two oil droplets which were flocculated, while the larger circle enclose many oil droplets present at the oil-water interface. Figure 5 (b) shows the coalescence of the same droplets shown in Figure 5 (a), in addition to the settling down of water after one minute. Hence, the experiment clearly shows the importance of the distance between the droplets towards the rate of coalescence.
(a)
(b)
Flocculation
Coalescence
Figure 5 Coalescence and Flocculation of oil droplets in 40% water in oil emulsion
Figures 6 and 7 show the results of two experiments (e.g. Study 1 and Study 2) obtained from different percentages of oil in water emulsions by keeping the total volume of the emulsion constant (e.g. total volume set to 800 ml). Ten (10) experiments were conducted for each oil/water concentration and a repeatability of 99.98 % was achieved. It has been noticed that ultrasonic waves cause faster de-emulsification of water in oil emulsions especially up to a level of 50-50% water-oil emulsion. On increasing the amount of water beyond 50%, the intermolecular forces between the water molecules become stronger and the emulsion settles down quickly in both natural and ultrasonic settlings. This makes difficult to differentiate between natural separation and ultrasonic separation of aqueous and oil phases. Sub plot in figure 6 shows the zoomed in view of the two studies for 50 to 90 % oil in water emulsions as it bars were too small to visualize on normal scale.
Figure 6 Time taken by different percentages of water in oil emulsification for natural settling of water and oil phases
Figure 7 Time taken by different percentages of water in oil emulsification for ultrasonic settling of water and oil phases
Figure 8 shows the results corresponding to natural and ultrasonic separations in case of 40% water in oil emulsion. It can be noticed that natural separation left water droplets in the aqueous
phase even after 15 minutes while the application of ultrasonic for around 1 minute left almost no oil droplet in aqueous phase and clear separation of the two phases could be achieved.
(a)
(b)
(c)
Figure 8 Comparison of natural and forced ultrasonic separation with the original emulsion sample before mixing: (a) Initial water and oil put together without mixing, (b) natural separation after 15 minutes of placement, (c) ultrasonic forced separation after 1 minute ultrasonic irradiation of the sample
Figure 9 summarizes the results of several experiments highlighting the reduction in settling time when applying ultrasonic waves for different concentration of water in oil emulsions (i.e. watercut up to 40%). Hence, ten (10) experiments were conducted for each oil/water concentration and a repeatability of 99.85 % was achieved. It is evident that the usage of ultrasonic waves was consistently effective since a reduction of settling time could reach up to 96% as compared to natural separation.
Figure 9 Reduction in settling time of the oil and water phases on application of ultrasonic waves as compared to natural settling
3.2 Numerical Simulations In order to validate the results obtained in the above experiments, a computerized finite element analysis-based simulation was conducted. The pressure acoustic module with a focus on acoustic structure interaction was numerically solved using the Helmholtz equation. Hence, the model considered in this paper did not consider the behavior in terms of fluid flow, of the flocculation, coalescence and sedimentation with interaction of ultrasonic effects. Nevertheless, it focuses merely on the pressure distribution and sound pressure level which also defines the rate of flocculation. This simplified model considers a three dimensional block as an oil phase having a volume of 1 x 0.5 x 0.1 m3. The left boundary of this oil block is defined as ultrasonic source (20 KHz resonance frequency) with spherical radiations while its right side has a 0.05 x 0.5 x 0.1 m3 aluminum block acting as a metallic reflector for ultrasonic waves. The initial pressure of the fluid domain and ultrasonic source were set to 0 Pa and 1Pa respectively. The radius of the water
droplets was set to 50 mm and an equidistant uniform distribution of the droplets was configured in this model. Three different scenarios were considered in the study: two droplets (i.e. water-cut of 5 %), four droplets (i.e. water-cut of 10 %), and six droplets (i.e. water cut of 20 %). Figure 10 shows the results corresponding to each of these three cases. It can be noticed that almost the same pressure distribution and sound pressure level are obtained in all the three cases. Therefore, in case of less number of droplets, the separation process is slower since the force being exerted on the droplets is uniformly distributed through the whole fluid domain and less surface area is exposed to a particular pressure force. This leads to less coagulating force and consequently slower coagulation and separation of water. Increasing the number of droplets increases the surface area exposed to same pressure force which leads to faster coagulation of water droplets. This result matches the results obtained in the experiments.
Spherical radiation source boundary
Aluminum block
( a)
(b)
Water droplets
(c) Figure 10 Simulation results with different no. of water droplets in continuous oil phase (a) two water droplets, (b) four water droplets, (c) eight water droplets
4. Conclusion In this paper, ultrasonic technology was investigated for removal of oil-water emulsions which may appear in large tanks. Computerized simulations as well as lab scale experiments were performed and the results are found to match each other. Different values of water-cut emulsions have been considered to assess its effect on the separation process. Hence, the usage of ultrasonic waves was justifiable since the corresponding separation rate for emulsions, of 10 to 40 % water cut, is found to be faster than the natural separation by an order of magnitude. This leads to suggest a new de-emulsifying device that would consists of a one dimensional array of ultrasonic sensors, vertical distributed, among which, only one sensor emits ultrasonic waves (because of the limited power allowed in hazardous areas). The active sensor corresponds to the height with water-cut ranging from 10 to 40%. Hence, an emulsion profiler needs to be deployed in the tank to provide in real-time the emulsion profile [9]. In addition of being safe and energy efficient, the suggested device features the advantage of not changing the physical properties of oil.
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Paper title: Oil-Water De-emulsification using Ultrasonic Technology
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Author Names: Muhammad Mohsin & Mahmoud Meribout
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Affiliation: Electrical Engineering Department, Petroleum Institute, Abu Dhabi, UAE
Corresponding Author: Mahmoud Meribout. Email: [email protected]