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Experimental study on the effect of spatial distribution and action order of electric field and magnetic field on oil-water separation
Chemical Engineering & Processing: Process Intensification 145 (2019) 107658
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Experimental study on the effect of spatial distribution and action order of electric field and magnetic field on oil-water separation Kai Guoa,b,1, Yuling Lva, a b
⁎,1
T
, Limin Hea, Xiaoming Luoa, Jie Zhaoa
College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, 266580, PR China College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Spatial distribution Action order Separation Magnetic field Ion enrichment
Electromagnetic synergistic promotion of emulsion separation is a new oil-water separation technology under the background of interdisciplinary and multi-field coupling. However, the lack of research on the influence of characteristic parameters on separation efficiency limits the development of this technology. In this paper, the effect of the spatial distribution and the action order of electric field and magnetic field on oil-water separation was investigated by using the electromagnetic synergistic oil-water separation experimental system. The experimental results show that the spatial position and action order of electric field and magnetic field have a significant influence on the separation performance, and the separation efficiency is the highest when they are vertically distributed and synchronously operated. The theory of ion enrichment based on electromagnetic synergism was proposed. It is considered that the polarization-electromagnetic force and ion enrichment effect are the main reasons for the higher separation efficiency of emulsion treated by the electromagnetic synergistic field with vertical distribution and synchronization. Among them, the polarization-electromagnetic force affects the dynamic behavior of droplets in the direction perpendicular to the electric field, while the ion enrichment effect affects the distribution of droplets in the electric field and the forces inside and outside the liquid bridge.
1. Introduction The transmission friction, the secondary emulsification probability of crude oil and the corrosion probability of pipeline are increased by the water in crude oil [1–3]. Therefore, efficient oil-water separation and deep dehydration are of great significance to the treatment and transportation of crude oil. However, in the process of oil recovery, multi-chain chemical agents with complex polar groups such as surfactants and polymers promote the formation of a complex cross-linking structure at the oil-water interface [4–6], which makes the oil-water separation more difficult. The electrostatic treatment has become the most widely used method in many separation technologies due to its low energy consumption and high separation efficiency [7–12]. However, some experimental results show that when the electric field strength exceeds the critical field strength, the water droplets break up into smaller droplets due to electrical dispersion, that is, when the oil-water emulsion is treated by electrostatic technology, the separation efficiency has an upper limit [13–15]. Therefore, it is difficult to achieve deep
dehydration through single electrostatic treatment. With further research, synergistic technology based on electrostatic treatment has been proposed, such as heating-electrostatic technology, centrifugal-electrostatic technology, chemical-electrostatic technology, filtration-electrostatic technology, etc [11,16]. The electromagnetic cooperative processing technology (referred to as electromagnetic synergy) is produced under this background [17]. Although electromagnetic collaboration has not been applied in actual production, the effectiveness and efficiency of this technology have been proved by experimental results. Campos et al. treated the emulsion with an average particle size of 400 μm in an electromagnetic synergistic device with a voltage of 650 V and a magnetic field intensity of 0.455 ( ± 0.009 T), the particle size statistics showed that the average particle size of the dispersed phase under electromagnetic synergism was significantly larger than that under electric field [18]. The separation efficiency was tested under an external field, the results show that the treatment effect of the synergistic field of magnetic field and AC electric field is better than that of single AC electric field [19], the influence of magnetic field intensity on dehydration performance was explored, and
⁎
Corresponding author at: College of Pipeline and Civil Engineering, China University of Petroleum (East China), No. 66 Changjiang West Road, Qingdao, Shandong, 266580, PR China. E-mail address: [email protected] (Y. Lv). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cep.2019.107658 Received 8 July 2019; Received in revised form 20 August 2019; Accepted 9 September 2019 Available online 21 September 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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Nomenclature
Ca ε0 εc E r γ △Pd-b
φ Water content, % ηw Dehydration efficiency, % mw,untreated Total mass of water in emulsion, g mo,untreated Total mass of oil in emulsion, g mem,untreated Total mass of untreated emulsion, g mem,treated Total mass of treated emulsion, g mem,loss Mass loss in emulsion transfer, g Vw,treated Volume of separated water, mL ρw Density of water, g/cm3 δ Separation efficiency difference, % ε Performance enhancing ratio ηw, AC Dehydration efficiency in AC, % α contact angle, ° β critical contact angle, ° Db Length of liquid bridge, m Fe Electrostatic pressure, N Fc Capillary pressure, N
Electric capillary number Electrical constants, F·m−1 Relative permittivity, F·m−1 Electric field intensity, V/m Radius of a droplet, m Interfacial tension, N·m−1 Pressure difference
Subscripts o Oil w Water b Bridge d Droplet AC AC electric field em Emulsion loss Mass loss in emulsion transfer treated Treated by external fields untreated Untreated by external fields
field and magnetic field, lacking research on the effect of the spatial distribution and the action order of electric field and magnetic field. In this study, the effect of the spatial distribution and the action order of electric field and magnetic field on oil-water separation was investigated by using the electromagnetic synergistic oil-water separation experimental system. Based on the experimental results, the theory of ion enrichment is proposed, which is of great significance to improve the influence of electromagnetic field characteristic parameters on oil-
the optimal magnetic field intensity range in was [0.269, 0.355] T [20]. The mechanism of enhancing water droplet coalescence by electromagnetic synergy and the influence of electromagnetic field characteristic parameters on separation efficiency are the theoretical basis of strengthening coalescence device design and operation management. However, the application of electromagnetic synergistic enhanced coalescence technology in crude oil dewatering is mostly focused on the macro-evaluation of oil-water separation efficiency under the electric
Fig. 1. The sketch of the experimental setup.① Oscilloscope; ②Arbitrary waveform generator; ③ High voltage source;④ Intelligent DC power supply; ⑤ Electromagnet; ⑥ Sample chamber; ⑦ Bracket. I: Voltage monitor port; II: Current monitor port; III: Signal input; IV: High voltage output; V: Grounded. 2
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only 2%. C45+ is mainly composed of gum, asphaltene and polycyclic aromatic phenols with a mass fraction of 29.55%. The interfacial energy of droplets was lowed and the structure of the interfacial film was stabilized by the natural surfactants in crude oil. The strength of the interfacial film was enhanced the aggregation of droplets was hindered by C45 + . Therefore, the enhanced coalescence effect of external fields was hindered and the dehydration efficiency was reduced by C45+ and various impurities [21,23–25]. The total mass of each emulsion was 50 g. According to mass moisture content, water and oil were weighed by the electronic scale and transferred to the agitating tank. The emulsion was prepared by stirring the fixed time in a stirring emulsifier. The agitator is a fourblade impeller agitator. The length and diameter of the connecting rod are 180 mm and 5.5 mm respectively. The length, thickness and maximum surface height of the single blade are 25 mm, 1 mm and 3 mm respectively. The structure of the agitating tank is cylindrical. The diameter and height of the tank are 60 mm and 90 mm, respectively. The position of the agitator is fixed, the distance from the lowest point of the blade to the bottom of the agitating tank is 7 mm, and the distance from the highest point of the blade to the liquid surface is 10 mm. The temperature of the stirred medium is about 65 ℃.
water separation. 2. Experiments 2.1. Experimental test rig The experimental system and sample chamber used in this study are similar to the ones used by Guo et al. [19] and are depicted in Fig. 1. The experimental setup can be divided into 3 parts: an electric field system, a magnetic field system, and a sample chamber. The sample chamber was made with Perspex, and the electrodes were polished brass plates. The models of experimental devices are listed in Table 1. The dimensions of the electrode plate, electromagnet and sample chamber were measured. The geometric size of the electrode plate is 40*40*1 mm and the material is 304 stainless steel. The geometric size of the electromagnet is 1450*500*600 mm, the maximum diameter of the magnetic pole is 120 mm and the working pole is 100 mm, the spacing of magnetic poles is continuously adjustable in the range of 0–680 mm. The geometric size of the sample chamber is 40*40*40 mm and the material is plexiglass. Experimental steps: (1) the emulsion was prepared by a propeller agitator and the particle size distribution of untreated emulsion was tested by the particle size distribution evaluation system. (2) The prepared emulsion was transferred to the sample chamber and treated by external fields. (3) The treated emulsion was placed in a constant temperature water bath for thermal deposition after testing the particle size distribution. The volume of water separated during sedimentation was recorded to calculate the dehydration rate. The emulsion treated by external fields was transferred to the centrifugal tube, and then the centrifugal tube was placed in a constant temperature tank at 65 ℃ for thermal settlement. The initial time was recorded while the emulsion was put into the thermostat, then the sequence time and the volume of the separated water were recorded. The dimensions of the thermostat and tip-bottom centrifugal tube were measured. The diameter and height of the thermostat are 300 mm and 270 mm respectively. The type of centrifugal tube is 50 mL, which consists of a column and a cone. The volume and height of the column section are 35 mL and 50 mm respectively, and the volume and height of the cone section are 15 mL and 50 mm respectively.
2.3. Outfield parameters and particle size distribution of dispersed phase The separation performances of blank(BS), alternating current field (AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic field-alternating current vertical distribution synergetic field(DM&AC⊥) and electric field after magnetic field (DM@AC⊥) are compared. The frequency of the alternating current is 50 Hz. The electric field is perpendicular to the magnetic field when exploring the effect of the sequence of synergistic field action. Outfield parameters and control parameters are shown in Table 3. The application of electric field after magnetic field: The emulsion is placed in the sample, and the DC power supply supplied to the electromagnet is turned on for 10 s, then the DC power supply is turned off and the AC power supply supplied to the electrodes is switched on immediately. The electric field action time is 10 s. The particle size of the dispersed phase has a great influence on the speed and efficiency of oil-water separation [26]. When the properties of the emulsion are the same, the experimental results can be evaluated. The consistency of the properties of the emulsions to be treated by different fields is demonstrated by the particle size evaluation system. The particle size evaluation system was composed of a microscope (XSP-17CE, Shanghai Changfang Optical Instruments Co., Ltd.), an SLR camera (Nikon D7200) and a control computer. Photographs were processed with Image-Pro plus 6.0 to obtain droplets size, then the average particle size was calculated and the particle size distribution was fitted. The particle size of the dispersed phase was measured and counted before the emulsion was treated by external fields. The emulsion with 20% water content was prepared by agitating emulsifier at 500 r/min for 5 min, and the emulsion with 30% water content was prepared at 500 r/min for 8 min. The dispersed phase in the prepared emulsion was photographed and measured by the particle size distribution evaluation system. The results are shown in Fig. 3. In Fig. 3, pk is the probability function of particle size xk, and
2.2. Materials In this study, the crude oil (provided by Shengli Oilfield, Sinopec) and the water (the Institute of Chemical Engineering, China University of Petroleum (East China)) were used to prepare emulsions. The main physical properties of experimental materials are presented in Table 2. Wax molecules in crude oil are aligned in a magnetic field, thus the physical properties of the oil are changed [21,22]. Therefore, the wax content in the test medium is of great significance to the repetition of experimental results. It was measured by differential scanning calorimeter (DSC 821e, Metler-Toledo), and the value was 11%. The chemical compositions of crude oil were detected according to the simulated distillation method (ASTM D2887-08 & NB/SH/T 0558-2016). The test results are presented in Fig. 2. As shown in Fig. 2, the mass fraction of wax (C15–C18) is 10.78%, and the error between this value and the result by the DSC method is Table 1 The models of experimental devices. System
Device
Brand
Model
Function
Electric field system
Waveform generator Power amplifier Oscilloscope Intelligent DC power Electromagnet
Rigol Trek Tektronix Litian Litian
DG2041A 20/20C TDS2002B HCP20-130 PEM-5005H
Setting up signal Amplifying input signal Monitoring signal Power of electromagnet Generating magnetic field
Magnetic field system
3
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Table 2 Physical properties of oil and water. Water
Crude oil
Parameter Unit value Parameter Unit value Parameter unit value
Density ρ/kg·m−3 998 Density(65 ℃) ρ/g·cm−3 868 Condensation point ℃ 30
Dynamic viscosity μ/mPa·s 1.02 Colloid % 16.62 Wax precipitation ℃ 49
Surface tension γ/mN·m−1 72.41 Asphaltene % 1.58 Abnormal point ℃ 40
Conductivity σ/S·m−1 3.86×10−2 Conductivity σ/S·m−1 4.5×10−9 Dynamic viscosity(65 ℃) μ/mPa·s 67.5
Relative permittivity εr 78 Relative permittivity εr 2.44 Dynamic viscosity(70 ℃) μ/mPa·s 80
Notes: except for the notes in parentheses, the medium temperature is 25℃.
ηw =
ρw Vw,treated × 100% m w,untreated
(2)
The separation efficiency difference δ is defined as the D-value between the separation efficiency of the emulsion after an external field and that of the emulsion after AC field.
δ = ηw − ηw,AC
(3)
Separation performance improvement rate ε is defined as the ratio of separation efficiency under an external field to that under the AC field. ε can be calculated as follows.
ε= Fig. 2. The chemical compositions of crude oil.
2
δY =
2
2
2
⎛ ∂Y δx1 ⎞ + ⎛ ∂Y δx2 ⎞ + ⎛ ∂Y δx3 ⎞ +⋯+⎛ ∂Y δx n ⎞ ⎝ ∂x1 ⎠ ⎝ ∂x2 ⎠ ⎠ ⎝ ∂x n ⎝ ∂x3 ⎠ ⎜
⎟
⎜
⎟
⎜
⎟
⎜
⎟
(5)
According to Eq. (5), the uncertainty of water content φ, separation efficiency ηw, and separation performance-enhancing ratio ε are 2.91%, 4.42%, and 4.71%, respectively. Mass loss due to wall-sticking effect occurs when the prepared emulsion is transferred from the stirring tank to the sample chamber. It is necessary to test and verify the mass conservation performance of the experimental system in order to evaluate the effect of mass loss on the experimental results. The mass conservation equation is defined as follows.
me,untreated = me,treated + me,loss
2.4. Performance undices, uncertainty analysis and results validation
(6)
The mass loss during the transferring process of emulsions with the water content of 30% was measured, and the validation results are illustrated in Fig. 4. Almost all of the absolute differences in mass are less than 6%. The relative effect of mass loss on separation efficiency is 0.0638%, the rationality of the experimental system is demonstrated. 16 experiments were carried out to verify the mass conservation properties of the system. The total mass of the emulsion in the stirred tank was 50 g in each experiment. In Fig. 4, the Y-axis represents the mass of the emulsion poured out from the agitating tank. The red hollow square represents the results of the first to eighth experiments, which serial numbers are listed in the bottom abscissa. The blue hollow
Three indicators for characterizing oil-water separation performance are defined, the separation efficiency ηw, separation efficiency difference δ, and the separation performance-enhancing ratio ε. The water content of emulsion is defined as follows.
m w,untreated × 100% m w,untreated + mo,untreated
(4)
The uncertainties in this study were calculated according to the uncertainty propagation method [28].
cumulative probability F(x) is the sum of the probability of xk with X≤x [27]. The particle size distribution of the dispersed phase of the emulsion with 20% water content is shown in Fig. 3(a). The average particle size of the emulsion treated by five kinds of field is 86.76 μm, 86.30 μm, 87.70 μm, 84.73 μm and 84.37 μm, respectively. The probability function and cumulative probability distribution curve of the droplet size are basically the same. The particle size distribution of the dispersed phase of the emulsion with 30% water content is shown in Fig. 3(b). The same rule as Fig. 3(a) can be obtained from Fig. 3(b). The stirring time of the emulsion in Fig. 3(a) is 5 min, and that in Fig. 3(b) is 8 min. Compared with Fig. 3(a), the size of emulsion is smaller with longer stirring time.
φ=
ηw ηw,AC
(1)
The separation efficiency ηw is defined as the ratio of the quality of separated water to the total mass of water in untreated emulsions. Table 3 Control parameters of experimental conditions. External field
Spatial distribution ∥/⊥
Action order &/@
Type of electric field AC/DC
Voltage kV
Magnetic field intensity T
Action time s
BS AC DM&AC∥ DM&AC⊥ DM@AC⊥
– – ∥ ⊥ ⊥
– – & & @
– AC AC AC AC
– 10 10 10 10
– – 0.355 0.355 0.355
– 10 10 10 10
Notes: ∥-parallel; ⊥-vertical; &-synchronization; @-first magnetism, then electricity. 4
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Fig. 3. Particle size distribution of dispersed phase. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content.
pentagon represents the results of the ninth to sixteenth experiments, which serial numbers are listed in the top abscissa. 3. Results and discussion 3.1. Effect of spatial distribution of electric field and magnetic field on separation efficiency The effect of the spatial distribution of the electric field and magnetic field on separation efficiency is studied. Emulsions treated by blank(BS), alternating current field(AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic fieldalternating current vertical distribution synergetic field(DM&AC⊥) are deposited in a constant temperature tank. The oil-water separation efficiency of emulsions with 20% and 30% water content is shown in Fig. 5(a) and (b), respectively. As shown in Fig. 5, it can be seen that the separation efficiency of the emulsion under the magnetic field-AC parallel distribution synergistic field is basically the same as that of the emulsion under AC electric field, while the separation efficiency of the emulsion under the magnetic field-AC vertical distribution synergistic field is the highest. Therefore, when the spatial relationship between the magnetic field and AC electric field is vertical distribution, the electromagnetic synergistic field has the effect of promoting oil-water separation. According to the proposed theory of polarized electromagnetic force [19], when the magnetic field and electric field are vertically distributed, the droplets in the AC electric field are subjected to the polarized electromagnetic force, which belongs to the special ampere force of the current element. Therefore, when the magnetic field is
Fig. 4. Mass conservation of the experimental system. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
5
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field. For the electric field after the magnetic field, the emulsion is magnetized for a period of time and then treated by an electric field. Because the magnetization time is short and the medium temperature is higher than the wax precipitation point, the magnetization has no obvious effect on promoting oil-water separation [22].
3.3. Theoretical analysis The separation efficiency difference δ and the separation performance-enhancing ratio ε are used to characterize the performance of the synergistic field with AC electric field as a reference. The δ and ε of emulsion treated by alternating current field(AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic field-alternating current vertical distribution synergetic field (DM&AC⊥) and electric field after magnetic field(DM@AC⊥) are shown in Fig. 7. As can be seen in Fig. 7, the spatial position and action order of electric field and magnetic field have a great influence on the performance of oil-water separation. When the electric field and magnetic field are vertically distributed and the electric field and magnetic field are synchronized, the synergistic field can effectively promote oil-water separation. A theory of ion enrichment based on electromagnetic synergism is proposed. The analysis shows that when the vertical electric field and magnetic field work synchronously, oil-water separation is promoted through two functions: first, the polarization-electromagnetic force theory [19], the droplet is subjected to the electric field force and polarization electromagnetic force in the electromagnetic synergistic field,
Fig. 5. Effect of spatial distribution of electric field and magnetic field on separation efficiency. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content.
parallel to the electric field, there is no polarized electromagnetic force. According to this theory, when the magnetic field is parallel to the AC electric field, the oil-water separation performance is not different from that of the single AC electric field. When the magnetic field is vertical to the AC electric field, the oil-water separation performance is better than that of the single AC electric field. This conclusion is consistent with the experimental results. 3.2. Effect of action order of electric field and magnetic field on separation efficiency The effect of action order of electric field and magnetic field on separation efficiency is studied. Emulsions treated by blank(BS), alternating current field(AC), magnetic field-alternating current vertical distribution synergetic field(DM&AC⊥) and electric field after magnetic field(DM@AC⊥) are deposited in a constant temperature tank. The oilwater separation efficiency of emulsions with 20% and 30% water content is shown in Fig. 6(a) and (b), respectively. As shown in Fig. 6, it can be seen that the separation efficiency of the emulsion under the electric field after the magnetic field is basically the same as that of the emulsion under AC electric field, the separation efficiency of the emulsion by electromagnetic synchronization is the highest. Therefore, when the sequence of action between the magnetic field and AC electric field is synchronous synergistic, the electromagnetic synergistic field has the effect of promoting oil-water separation. The dispersed phase droplets are subjected to electric field force and polarized electromagnetic force in the synchronous electromagnetic
Fig. 6. Effect of action order of electric field and magnetic field on separation efficiency. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content. 6
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ΔPd-b = Pdroplet − P bridge ≈
and the polarization electromagnetic force makes the droplet oscillate and coalesce in the direction perpendicular to the electric field, thus improving the probability of droplet collision. Secondly, the ion enrichment theory. The theory of ion enrichment is shown in Fig. 8. As shown in Fig. 8(a), ions with different charges are distributed on both sides of the droplet under an electric field. As shown in Fig. 8(b), electrostatic attraction force Fg is generated between heterogeneous charge layers, which induces droplets to approach each other. As shown in Fig. 8(c), ions are enriched at both ends of the droplet. Droplets are close to each other under the action of electrostatic attraction force Fg, and a very small liquid bridge is formed. Under the action of the electric field, the exchange of positive and negative ions through the liquid bridge will lead to the Coulomb repulsion force Fr between the two droplets to prevent the two droplets from coalescing [29]. As shown in Fig. 8(d), when the electric field (E) and magnetic field (B) are distributed vertically and synchronously, the ions on the droplet wall in the direction of electric field are enriched by Lorentz force to the two ends of the droplet perpendicular to the electric field [30–32], resulting in fewer ions in the contact part of the droplet, thus reducing the ion transfer probability and the repulsion force caused by the same charge ions between droplets. Under the dual effects of polarization-electromagnetic force and ion enrichment, the separation efficiency of emulsion under the electromagnetic field with vertical distribution and synchronous operation is obviously higher than that under other external fields. As shown in Fig. 9(a), half of the contact angle of droplets in the critical state of coalescence and separation is defined as the critical contact angle β. The contact angle α and liquid bridge diameter Db during coalescence are shown in Fig. 9(b). Droplets are polarized in an electric field, and charges with opposite polarity are distributed at both ends of the droplets, which results in the deformation of the droplets. When the electrostatic pressure Fe and capillary pressure Fc are balanced, the droplet deformation reaches equilibrium. The ratio of electrostatic pressure to capillary pressure is characterized by the electric capillary number Ca [33]. (7)
γ r
(8)
Fc ∝ Ca =
ε0 εc E 2r γ
(10)
It can be seen that when the pressure difference △Pd-b > 0 (i.e. β < 45°), the liquid in the droplet extrudes the liquid bridge under the action of the pressure difference, and the Db increases gradually until the droplet coalesces and fuses. When the pressure difference △Pdb < 0 (i.e. β > 45°), the pressure at the position of the liquid bridge where the two droplets contact is greater than the pressure inside the droplet, which causes the liquid at the liquid bridge to extrude into the droplet, and the Db decreases gradually until the droplet is separated [36,37]. As shown in the red dotted line elliptical region of Fig. 8(d), When the electric field and magnetic field are distributed vertically and synchronously, ions are enriched at both ends of the droplet under the influence of Lorentz force, resulting in less ion distribution at the liquid bridge position where the droplet contacts and coalesces, thus weakening ion exchange, reducing the repulsion force Fr between the same charges, increasing the electric capillary number Ca, intensifying the coalescence and fusion of the Taylor cone of droplets, and inducing the coalescence of droplets, resulting in the coalescence of small droplets. Effective coalescence of droplets into large droplets promotes the separation of dispersed phases from emulsions [26].
Fig. 7. Performance of synergetic field with AC electric field as reference for improving separation efficiency.
Fe ∝ ε0 εc E 2
2γb ( cot β − 1) Db
4. Conclusions The spatial distribution of the electric field and magnetic field has a significant influence on the separation performance, and the oil-water separation efficiency is the highest when they are vertically distributed. The action order of electric field and magnetic field has a significant effect on the separation performance, and the oil-water separation efficiency is the highest when they are synchronized and synergistic. The theory of ion enrichment based on electromagnetic synergism was proposed. It is considered that the polarization-electromagnetic force and ion enrichment effect are the main reasons for the higher separation efficiency of emulsion treated by the electromagnetic synergistic field with vertical distribution and synchronization. Among them, the polarization-electromagnetic force affects the dynamic behavior of droplets in the direction perpendicular to the electric field, while the ion enrichment effect affects the distribution of droplets in the electric field and the forces inside and outside the liquid bridge.
(9)
Ristenpart et al. [34] considered that the pressure difference between the inner part of the droplet and the liquid bridge was the decisive factor to decide whether the droplet would coalesce or not. The pressure difference between droplets and bridges can be expressed as follows [35].
Fig. 8. Schematic diagram of ion enrichment theory. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.) 7
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[8]
[9]
[10] [11] [12]
[13] [14] [15]
[16]
[17] [18]
[19]
[20]
[21]
Fig. 9. Characteristic parameter diagram of droplet coalescence. (a) Schematic diagram of critical contact angle. (b) Schematic diagram of liquid bridge and contact angle.
[22] [23]
Declaration of Competing Interest
[24]
The authors declare no competing financial interest.
[25]
Acknowledgments
[26]
This work is supported by the National Natural Science Foundation of China (Grant No. 51674281), the Science and Technology Planning Project of Gansu Province of China (Grant No. 18JR3RA157), the Scientific Research Projects of Universities of Gansu Province of China (Grant No. 2018A-015).
[27] [28] [29] [30]
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Contents lists available at ScienceDirect
Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Experimental study on the effect of spatial distribution and action order of electric field and magnetic field on oil-water separation Kai Guoa,b,1, Yuling Lva, a b
⁎,1
T
, Limin Hea, Xiaoming Luoa, Jie Zhaoa
College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, 266580, PR China College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Spatial distribution Action order Separation Magnetic field Ion enrichment
Electromagnetic synergistic promotion of emulsion separation is a new oil-water separation technology under the background of interdisciplinary and multi-field coupling. However, the lack of research on the influence of characteristic parameters on separation efficiency limits the development of this technology. In this paper, the effect of the spatial distribution and the action order of electric field and magnetic field on oil-water separation was investigated by using the electromagnetic synergistic oil-water separation experimental system. The experimental results show that the spatial position and action order of electric field and magnetic field have a significant influence on the separation performance, and the separation efficiency is the highest when they are vertically distributed and synchronously operated. The theory of ion enrichment based on electromagnetic synergism was proposed. It is considered that the polarization-electromagnetic force and ion enrichment effect are the main reasons for the higher separation efficiency of emulsion treated by the electromagnetic synergistic field with vertical distribution and synchronization. Among them, the polarization-electromagnetic force affects the dynamic behavior of droplets in the direction perpendicular to the electric field, while the ion enrichment effect affects the distribution of droplets in the electric field and the forces inside and outside the liquid bridge.
1. Introduction The transmission friction, the secondary emulsification probability of crude oil and the corrosion probability of pipeline are increased by the water in crude oil [1–3]. Therefore, efficient oil-water separation and deep dehydration are of great significance to the treatment and transportation of crude oil. However, in the process of oil recovery, multi-chain chemical agents with complex polar groups such as surfactants and polymers promote the formation of a complex cross-linking structure at the oil-water interface [4–6], which makes the oil-water separation more difficult. The electrostatic treatment has become the most widely used method in many separation technologies due to its low energy consumption and high separation efficiency [7–12]. However, some experimental results show that when the electric field strength exceeds the critical field strength, the water droplets break up into smaller droplets due to electrical dispersion, that is, when the oil-water emulsion is treated by electrostatic technology, the separation efficiency has an upper limit [13–15]. Therefore, it is difficult to achieve deep
dehydration through single electrostatic treatment. With further research, synergistic technology based on electrostatic treatment has been proposed, such as heating-electrostatic technology, centrifugal-electrostatic technology, chemical-electrostatic technology, filtration-electrostatic technology, etc [11,16]. The electromagnetic cooperative processing technology (referred to as electromagnetic synergy) is produced under this background [17]. Although electromagnetic collaboration has not been applied in actual production, the effectiveness and efficiency of this technology have been proved by experimental results. Campos et al. treated the emulsion with an average particle size of 400 μm in an electromagnetic synergistic device with a voltage of 650 V and a magnetic field intensity of 0.455 ( ± 0.009 T), the particle size statistics showed that the average particle size of the dispersed phase under electromagnetic synergism was significantly larger than that under electric field [18]. The separation efficiency was tested under an external field, the results show that the treatment effect of the synergistic field of magnetic field and AC electric field is better than that of single AC electric field [19], the influence of magnetic field intensity on dehydration performance was explored, and
⁎
Corresponding author at: College of Pipeline and Civil Engineering, China University of Petroleum (East China), No. 66 Changjiang West Road, Qingdao, Shandong, 266580, PR China. E-mail address: [email protected] (Y. Lv). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cep.2019.107658 Received 8 July 2019; Received in revised form 20 August 2019; Accepted 9 September 2019 Available online 21 September 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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Nomenclature
Ca ε0 εc E r γ △Pd-b
φ Water content, % ηw Dehydration efficiency, % mw,untreated Total mass of water in emulsion, g mo,untreated Total mass of oil in emulsion, g mem,untreated Total mass of untreated emulsion, g mem,treated Total mass of treated emulsion, g mem,loss Mass loss in emulsion transfer, g Vw,treated Volume of separated water, mL ρw Density of water, g/cm3 δ Separation efficiency difference, % ε Performance enhancing ratio ηw, AC Dehydration efficiency in AC, % α contact angle, ° β critical contact angle, ° Db Length of liquid bridge, m Fe Electrostatic pressure, N Fc Capillary pressure, N
Electric capillary number Electrical constants, F·m−1 Relative permittivity, F·m−1 Electric field intensity, V/m Radius of a droplet, m Interfacial tension, N·m−1 Pressure difference
Subscripts o Oil w Water b Bridge d Droplet AC AC electric field em Emulsion loss Mass loss in emulsion transfer treated Treated by external fields untreated Untreated by external fields
field and magnetic field, lacking research on the effect of the spatial distribution and the action order of electric field and magnetic field. In this study, the effect of the spatial distribution and the action order of electric field and magnetic field on oil-water separation was investigated by using the electromagnetic synergistic oil-water separation experimental system. Based on the experimental results, the theory of ion enrichment is proposed, which is of great significance to improve the influence of electromagnetic field characteristic parameters on oil-
the optimal magnetic field intensity range in was [0.269, 0.355] T [20]. The mechanism of enhancing water droplet coalescence by electromagnetic synergy and the influence of electromagnetic field characteristic parameters on separation efficiency are the theoretical basis of strengthening coalescence device design and operation management. However, the application of electromagnetic synergistic enhanced coalescence technology in crude oil dewatering is mostly focused on the macro-evaluation of oil-water separation efficiency under the electric
Fig. 1. The sketch of the experimental setup.① Oscilloscope; ②Arbitrary waveform generator; ③ High voltage source;④ Intelligent DC power supply; ⑤ Electromagnet; ⑥ Sample chamber; ⑦ Bracket. I: Voltage monitor port; II: Current monitor port; III: Signal input; IV: High voltage output; V: Grounded. 2
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only 2%. C45+ is mainly composed of gum, asphaltene and polycyclic aromatic phenols with a mass fraction of 29.55%. The interfacial energy of droplets was lowed and the structure of the interfacial film was stabilized by the natural surfactants in crude oil. The strength of the interfacial film was enhanced the aggregation of droplets was hindered by C45 + . Therefore, the enhanced coalescence effect of external fields was hindered and the dehydration efficiency was reduced by C45+ and various impurities [21,23–25]. The total mass of each emulsion was 50 g. According to mass moisture content, water and oil were weighed by the electronic scale and transferred to the agitating tank. The emulsion was prepared by stirring the fixed time in a stirring emulsifier. The agitator is a fourblade impeller agitator. The length and diameter of the connecting rod are 180 mm and 5.5 mm respectively. The length, thickness and maximum surface height of the single blade are 25 mm, 1 mm and 3 mm respectively. The structure of the agitating tank is cylindrical. The diameter and height of the tank are 60 mm and 90 mm, respectively. The position of the agitator is fixed, the distance from the lowest point of the blade to the bottom of the agitating tank is 7 mm, and the distance from the highest point of the blade to the liquid surface is 10 mm. The temperature of the stirred medium is about 65 ℃.
water separation. 2. Experiments 2.1. Experimental test rig The experimental system and sample chamber used in this study are similar to the ones used by Guo et al. [19] and are depicted in Fig. 1. The experimental setup can be divided into 3 parts: an electric field system, a magnetic field system, and a sample chamber. The sample chamber was made with Perspex, and the electrodes were polished brass plates. The models of experimental devices are listed in Table 1. The dimensions of the electrode plate, electromagnet and sample chamber were measured. The geometric size of the electrode plate is 40*40*1 mm and the material is 304 stainless steel. The geometric size of the electromagnet is 1450*500*600 mm, the maximum diameter of the magnetic pole is 120 mm and the working pole is 100 mm, the spacing of magnetic poles is continuously adjustable in the range of 0–680 mm. The geometric size of the sample chamber is 40*40*40 mm and the material is plexiglass. Experimental steps: (1) the emulsion was prepared by a propeller agitator and the particle size distribution of untreated emulsion was tested by the particle size distribution evaluation system. (2) The prepared emulsion was transferred to the sample chamber and treated by external fields. (3) The treated emulsion was placed in a constant temperature water bath for thermal deposition after testing the particle size distribution. The volume of water separated during sedimentation was recorded to calculate the dehydration rate. The emulsion treated by external fields was transferred to the centrifugal tube, and then the centrifugal tube was placed in a constant temperature tank at 65 ℃ for thermal settlement. The initial time was recorded while the emulsion was put into the thermostat, then the sequence time and the volume of the separated water were recorded. The dimensions of the thermostat and tip-bottom centrifugal tube were measured. The diameter and height of the thermostat are 300 mm and 270 mm respectively. The type of centrifugal tube is 50 mL, which consists of a column and a cone. The volume and height of the column section are 35 mL and 50 mm respectively, and the volume and height of the cone section are 15 mL and 50 mm respectively.
2.3. Outfield parameters and particle size distribution of dispersed phase The separation performances of blank(BS), alternating current field (AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic field-alternating current vertical distribution synergetic field(DM&AC⊥) and electric field after magnetic field (DM@AC⊥) are compared. The frequency of the alternating current is 50 Hz. The electric field is perpendicular to the magnetic field when exploring the effect of the sequence of synergistic field action. Outfield parameters and control parameters are shown in Table 3. The application of electric field after magnetic field: The emulsion is placed in the sample, and the DC power supply supplied to the electromagnet is turned on for 10 s, then the DC power supply is turned off and the AC power supply supplied to the electrodes is switched on immediately. The electric field action time is 10 s. The particle size of the dispersed phase has a great influence on the speed and efficiency of oil-water separation [26]. When the properties of the emulsion are the same, the experimental results can be evaluated. The consistency of the properties of the emulsions to be treated by different fields is demonstrated by the particle size evaluation system. The particle size evaluation system was composed of a microscope (XSP-17CE, Shanghai Changfang Optical Instruments Co., Ltd.), an SLR camera (Nikon D7200) and a control computer. Photographs were processed with Image-Pro plus 6.0 to obtain droplets size, then the average particle size was calculated and the particle size distribution was fitted. The particle size of the dispersed phase was measured and counted before the emulsion was treated by external fields. The emulsion with 20% water content was prepared by agitating emulsifier at 500 r/min for 5 min, and the emulsion with 30% water content was prepared at 500 r/min for 8 min. The dispersed phase in the prepared emulsion was photographed and measured by the particle size distribution evaluation system. The results are shown in Fig. 3. In Fig. 3, pk is the probability function of particle size xk, and
2.2. Materials In this study, the crude oil (provided by Shengli Oilfield, Sinopec) and the water (the Institute of Chemical Engineering, China University of Petroleum (East China)) were used to prepare emulsions. The main physical properties of experimental materials are presented in Table 2. Wax molecules in crude oil are aligned in a magnetic field, thus the physical properties of the oil are changed [21,22]. Therefore, the wax content in the test medium is of great significance to the repetition of experimental results. It was measured by differential scanning calorimeter (DSC 821e, Metler-Toledo), and the value was 11%. The chemical compositions of crude oil were detected according to the simulated distillation method (ASTM D2887-08 & NB/SH/T 0558-2016). The test results are presented in Fig. 2. As shown in Fig. 2, the mass fraction of wax (C15–C18) is 10.78%, and the error between this value and the result by the DSC method is Table 1 The models of experimental devices. System
Device
Brand
Model
Function
Electric field system
Waveform generator Power amplifier Oscilloscope Intelligent DC power Electromagnet
Rigol Trek Tektronix Litian Litian
DG2041A 20/20C TDS2002B HCP20-130 PEM-5005H
Setting up signal Amplifying input signal Monitoring signal Power of electromagnet Generating magnetic field
Magnetic field system
3
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Table 2 Physical properties of oil and water. Water
Crude oil
Parameter Unit value Parameter Unit value Parameter unit value
Density ρ/kg·m−3 998 Density(65 ℃) ρ/g·cm−3 868 Condensation point ℃ 30
Dynamic viscosity μ/mPa·s 1.02 Colloid % 16.62 Wax precipitation ℃ 49
Surface tension γ/mN·m−1 72.41 Asphaltene % 1.58 Abnormal point ℃ 40
Conductivity σ/S·m−1 3.86×10−2 Conductivity σ/S·m−1 4.5×10−9 Dynamic viscosity(65 ℃) μ/mPa·s 67.5
Relative permittivity εr 78 Relative permittivity εr 2.44 Dynamic viscosity(70 ℃) μ/mPa·s 80
Notes: except for the notes in parentheses, the medium temperature is 25℃.
ηw =
ρw Vw,treated × 100% m w,untreated
(2)
The separation efficiency difference δ is defined as the D-value between the separation efficiency of the emulsion after an external field and that of the emulsion after AC field.
δ = ηw − ηw,AC
(3)
Separation performance improvement rate ε is defined as the ratio of separation efficiency under an external field to that under the AC field. ε can be calculated as follows.
ε= Fig. 2. The chemical compositions of crude oil.
2
δY =
2
2
2
⎛ ∂Y δx1 ⎞ + ⎛ ∂Y δx2 ⎞ + ⎛ ∂Y δx3 ⎞ +⋯+⎛ ∂Y δx n ⎞ ⎝ ∂x1 ⎠ ⎝ ∂x2 ⎠ ⎠ ⎝ ∂x n ⎝ ∂x3 ⎠ ⎜
⎟
⎜
⎟
⎜
⎟
⎜
⎟
(5)
According to Eq. (5), the uncertainty of water content φ, separation efficiency ηw, and separation performance-enhancing ratio ε are 2.91%, 4.42%, and 4.71%, respectively. Mass loss due to wall-sticking effect occurs when the prepared emulsion is transferred from the stirring tank to the sample chamber. It is necessary to test and verify the mass conservation performance of the experimental system in order to evaluate the effect of mass loss on the experimental results. The mass conservation equation is defined as follows.
me,untreated = me,treated + me,loss
2.4. Performance undices, uncertainty analysis and results validation
(6)
The mass loss during the transferring process of emulsions with the water content of 30% was measured, and the validation results are illustrated in Fig. 4. Almost all of the absolute differences in mass are less than 6%. The relative effect of mass loss on separation efficiency is 0.0638%, the rationality of the experimental system is demonstrated. 16 experiments were carried out to verify the mass conservation properties of the system. The total mass of the emulsion in the stirred tank was 50 g in each experiment. In Fig. 4, the Y-axis represents the mass of the emulsion poured out from the agitating tank. The red hollow square represents the results of the first to eighth experiments, which serial numbers are listed in the bottom abscissa. The blue hollow
Three indicators for characterizing oil-water separation performance are defined, the separation efficiency ηw, separation efficiency difference δ, and the separation performance-enhancing ratio ε. The water content of emulsion is defined as follows.
m w,untreated × 100% m w,untreated + mo,untreated
(4)
The uncertainties in this study were calculated according to the uncertainty propagation method [28].
cumulative probability F(x) is the sum of the probability of xk with X≤x [27]. The particle size distribution of the dispersed phase of the emulsion with 20% water content is shown in Fig. 3(a). The average particle size of the emulsion treated by five kinds of field is 86.76 μm, 86.30 μm, 87.70 μm, 84.73 μm and 84.37 μm, respectively. The probability function and cumulative probability distribution curve of the droplet size are basically the same. The particle size distribution of the dispersed phase of the emulsion with 30% water content is shown in Fig. 3(b). The same rule as Fig. 3(a) can be obtained from Fig. 3(b). The stirring time of the emulsion in Fig. 3(a) is 5 min, and that in Fig. 3(b) is 8 min. Compared with Fig. 3(a), the size of emulsion is smaller with longer stirring time.
φ=
ηw ηw,AC
(1)
The separation efficiency ηw is defined as the ratio of the quality of separated water to the total mass of water in untreated emulsions. Table 3 Control parameters of experimental conditions. External field
Spatial distribution ∥/⊥
Action order &/@
Type of electric field AC/DC
Voltage kV
Magnetic field intensity T
Action time s
BS AC DM&AC∥ DM&AC⊥ DM@AC⊥
– – ∥ ⊥ ⊥
– – & & @
– AC AC AC AC
– 10 10 10 10
– – 0.355 0.355 0.355
– 10 10 10 10
Notes: ∥-parallel; ⊥-vertical; &-synchronization; @-first magnetism, then electricity. 4
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Fig. 3. Particle size distribution of dispersed phase. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content.
pentagon represents the results of the ninth to sixteenth experiments, which serial numbers are listed in the top abscissa. 3. Results and discussion 3.1. Effect of spatial distribution of electric field and magnetic field on separation efficiency The effect of the spatial distribution of the electric field and magnetic field on separation efficiency is studied. Emulsions treated by blank(BS), alternating current field(AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic fieldalternating current vertical distribution synergetic field(DM&AC⊥) are deposited in a constant temperature tank. The oil-water separation efficiency of emulsions with 20% and 30% water content is shown in Fig. 5(a) and (b), respectively. As shown in Fig. 5, it can be seen that the separation efficiency of the emulsion under the magnetic field-AC parallel distribution synergistic field is basically the same as that of the emulsion under AC electric field, while the separation efficiency of the emulsion under the magnetic field-AC vertical distribution synergistic field is the highest. Therefore, when the spatial relationship between the magnetic field and AC electric field is vertical distribution, the electromagnetic synergistic field has the effect of promoting oil-water separation. According to the proposed theory of polarized electromagnetic force [19], when the magnetic field and electric field are vertically distributed, the droplets in the AC electric field are subjected to the polarized electromagnetic force, which belongs to the special ampere force of the current element. Therefore, when the magnetic field is
Fig. 4. Mass conservation of the experimental system. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
5
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field. For the electric field after the magnetic field, the emulsion is magnetized for a period of time and then treated by an electric field. Because the magnetization time is short and the medium temperature is higher than the wax precipitation point, the magnetization has no obvious effect on promoting oil-water separation [22].
3.3. Theoretical analysis The separation efficiency difference δ and the separation performance-enhancing ratio ε are used to characterize the performance of the synergistic field with AC electric field as a reference. The δ and ε of emulsion treated by alternating current field(AC), magnetic field-alternating current parallel distribution synergetic field(DM&AC∥), magnetic field-alternating current vertical distribution synergetic field (DM&AC⊥) and electric field after magnetic field(DM@AC⊥) are shown in Fig. 7. As can be seen in Fig. 7, the spatial position and action order of electric field and magnetic field have a great influence on the performance of oil-water separation. When the electric field and magnetic field are vertically distributed and the electric field and magnetic field are synchronized, the synergistic field can effectively promote oil-water separation. A theory of ion enrichment based on electromagnetic synergism is proposed. The analysis shows that when the vertical electric field and magnetic field work synchronously, oil-water separation is promoted through two functions: first, the polarization-electromagnetic force theory [19], the droplet is subjected to the electric field force and polarization electromagnetic force in the electromagnetic synergistic field,
Fig. 5. Effect of spatial distribution of electric field and magnetic field on separation efficiency. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content.
parallel to the electric field, there is no polarized electromagnetic force. According to this theory, when the magnetic field is parallel to the AC electric field, the oil-water separation performance is not different from that of the single AC electric field. When the magnetic field is vertical to the AC electric field, the oil-water separation performance is better than that of the single AC electric field. This conclusion is consistent with the experimental results. 3.2. Effect of action order of electric field and magnetic field on separation efficiency The effect of action order of electric field and magnetic field on separation efficiency is studied. Emulsions treated by blank(BS), alternating current field(AC), magnetic field-alternating current vertical distribution synergetic field(DM&AC⊥) and electric field after magnetic field(DM@AC⊥) are deposited in a constant temperature tank. The oilwater separation efficiency of emulsions with 20% and 30% water content is shown in Fig. 6(a) and (b), respectively. As shown in Fig. 6, it can be seen that the separation efficiency of the emulsion under the electric field after the magnetic field is basically the same as that of the emulsion under AC electric field, the separation efficiency of the emulsion by electromagnetic synchronization is the highest. Therefore, when the sequence of action between the magnetic field and AC electric field is synchronous synergistic, the electromagnetic synergistic field has the effect of promoting oil-water separation. The dispersed phase droplets are subjected to electric field force and polarized electromagnetic force in the synchronous electromagnetic
Fig. 6. Effect of action order of electric field and magnetic field on separation efficiency. (a) Emulsions with 20% water content. (b) Emulsions with 30% water content. 6
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ΔPd-b = Pdroplet − P bridge ≈
and the polarization electromagnetic force makes the droplet oscillate and coalesce in the direction perpendicular to the electric field, thus improving the probability of droplet collision. Secondly, the ion enrichment theory. The theory of ion enrichment is shown in Fig. 8. As shown in Fig. 8(a), ions with different charges are distributed on both sides of the droplet under an electric field. As shown in Fig. 8(b), electrostatic attraction force Fg is generated between heterogeneous charge layers, which induces droplets to approach each other. As shown in Fig. 8(c), ions are enriched at both ends of the droplet. Droplets are close to each other under the action of electrostatic attraction force Fg, and a very small liquid bridge is formed. Under the action of the electric field, the exchange of positive and negative ions through the liquid bridge will lead to the Coulomb repulsion force Fr between the two droplets to prevent the two droplets from coalescing [29]. As shown in Fig. 8(d), when the electric field (E) and magnetic field (B) are distributed vertically and synchronously, the ions on the droplet wall in the direction of electric field are enriched by Lorentz force to the two ends of the droplet perpendicular to the electric field [30–32], resulting in fewer ions in the contact part of the droplet, thus reducing the ion transfer probability and the repulsion force caused by the same charge ions between droplets. Under the dual effects of polarization-electromagnetic force and ion enrichment, the separation efficiency of emulsion under the electromagnetic field with vertical distribution and synchronous operation is obviously higher than that under other external fields. As shown in Fig. 9(a), half of the contact angle of droplets in the critical state of coalescence and separation is defined as the critical contact angle β. The contact angle α and liquid bridge diameter Db during coalescence are shown in Fig. 9(b). Droplets are polarized in an electric field, and charges with opposite polarity are distributed at both ends of the droplets, which results in the deformation of the droplets. When the electrostatic pressure Fe and capillary pressure Fc are balanced, the droplet deformation reaches equilibrium. The ratio of electrostatic pressure to capillary pressure is characterized by the electric capillary number Ca [33]. (7)
γ r
(8)
Fc ∝ Ca =
ε0 εc E 2r γ
(10)
It can be seen that when the pressure difference △Pd-b > 0 (i.e. β < 45°), the liquid in the droplet extrudes the liquid bridge under the action of the pressure difference, and the Db increases gradually until the droplet coalesces and fuses. When the pressure difference △Pdb < 0 (i.e. β > 45°), the pressure at the position of the liquid bridge where the two droplets contact is greater than the pressure inside the droplet, which causes the liquid at the liquid bridge to extrude into the droplet, and the Db decreases gradually until the droplet is separated [36,37]. As shown in the red dotted line elliptical region of Fig. 8(d), When the electric field and magnetic field are distributed vertically and synchronously, ions are enriched at both ends of the droplet under the influence of Lorentz force, resulting in less ion distribution at the liquid bridge position where the droplet contacts and coalesces, thus weakening ion exchange, reducing the repulsion force Fr between the same charges, increasing the electric capillary number Ca, intensifying the coalescence and fusion of the Taylor cone of droplets, and inducing the coalescence of droplets, resulting in the coalescence of small droplets. Effective coalescence of droplets into large droplets promotes the separation of dispersed phases from emulsions [26].
Fig. 7. Performance of synergetic field with AC electric field as reference for improving separation efficiency.
Fe ∝ ε0 εc E 2
2γb ( cot β − 1) Db
4. Conclusions The spatial distribution of the electric field and magnetic field has a significant influence on the separation performance, and the oil-water separation efficiency is the highest when they are vertically distributed. The action order of electric field and magnetic field has a significant effect on the separation performance, and the oil-water separation efficiency is the highest when they are synchronized and synergistic. The theory of ion enrichment based on electromagnetic synergism was proposed. It is considered that the polarization-electromagnetic force and ion enrichment effect are the main reasons for the higher separation efficiency of emulsion treated by the electromagnetic synergistic field with vertical distribution and synchronization. Among them, the polarization-electromagnetic force affects the dynamic behavior of droplets in the direction perpendicular to the electric field, while the ion enrichment effect affects the distribution of droplets in the electric field and the forces inside and outside the liquid bridge.
(9)
Ristenpart et al. [34] considered that the pressure difference between the inner part of the droplet and the liquid bridge was the decisive factor to decide whether the droplet would coalesce or not. The pressure difference between droplets and bridges can be expressed as follows [35].
Fig. 8. Schematic diagram of ion enrichment theory. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.) 7
Chemical Engineering & Processing: Process Intensification 145 (2019) 107658
K. Guo, et al.
[8]
[9]
[10] [11] [12]
[13] [14] [15]
[16]
[17] [18]
[19]
[20]
[21]
Fig. 9. Characteristic parameter diagram of droplet coalescence. (a) Schematic diagram of critical contact angle. (b) Schematic diagram of liquid bridge and contact angle.
[22] [23]
Declaration of Competing Interest
[24]
The authors declare no competing financial interest.
[25]
Acknowledgments
[26]
This work is supported by the National Natural Science Foundation of China (Grant No. 51674281), the Science and Technology Planning Project of Gansu Province of China (Grant No. 18JR3RA157), the Scientific Research Projects of Universities of Gansu Province of China (Grant No. 2018A-015).
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