Optimum Operating Frequency for Electrocoalescence Induced by Pulsed Corona Discharge.

Current oil-water separation methods require a significant power, a high processing time, and costly equipment, which typically yield low treatment efficiency. Pulsed direct current (dc) electric fields and recently nonuniform electric fields caught considerable attention in the petroleum industry research in order to address the most common oil-water separation issues such as chain formation, partial coalescence, and low efficiency in either the energy consumption or coalescence rate. Here, a contact-less charge injection method induced by corona discharge is utilized to investigate the impacts of nonuniform and pulsed dc electric fields on the coalescence of water droplets inside an oil medium. The operating process parameters were experimentally calibrated and optimized with the goal of increasing the effectiveness and energy consumption efficiency of the coalescence process. High-speed imaging and image processing techniques were used in order to investigate the effect of different active forces (i.e., dipole-dipole interaction, migratory coalescence, or electrophoresis, and dielectrophoresis) during the coalescence process. Different pulsed dc frequencies and pure dc waveforms were utilized and their impact on the coalescence of water droplets was investigated. An optimal coalescence recipe was proposed by continuous measurement of the distance, velocity, and acceleration of the coalescing water droplets. The results of this study suggest use of pulsed dc and pure dc electric fields for coalescence of water droplets in concentrated and dispersed emulsions, respectively.

Introduction

Many natural and synthetic products such as milk, petroleum, food products, drugs, paints, and many others are made from dispersing droplets of one fluid in another immiscible fluid, known as emulsions.[1] While emulsion formation is a challenging task in industries such as cosmetics and food, the separation process is important when there is a need to purify a liquid product, such as extracting water from crude oil and phase separation during solvent extraction.[2−4] Centrifugal separation, chemical demulsification, electrocoalescence, membrane separation/filtration, pH adjustment, and heat treatment are the most common separation processes that depend on the application.[5−8] However, processes that utilize high electric fields have been extensively used in water/oil separation, especially in the oil and gas industries. The importance of phase separation in these industries is that the emulsions are unwanted phases of extracts, which increase the processing costs, transportation-related expenses, catalyst malfunction, damage to the equipment, and degrade product quality.[9]

The focus of electrostatic demulsification processes has been on reducing the cost and energy while improving the quality of water/oil separation.[10,11] Although these methods are promising, the commercially available electrocoalescers are costly to operate because they utilize large vessels to provide enough residence time (∼30 min) needed for efficient separation of immiscible phases. More details of the currently available technology could be found elsewhere in the literature.[12−14] Providing a reliable and efficient design while meeting the compact dimension requirement in some industries is a difficult task, regardless of the target demulsification efficiency.[15,16] Most of the current electrocoalescer designs utilize alternating current electric fields [alternating current (ac) electric fields] with frequencies in the range of 50–60 Hz. However, using ac electric field has shown a limited response in later stages of the coalescence because of the increasing distance between water droplets and decreasing electrocoalescence forces. In addition, it has been shown that the ac electric field consumes more power by increasing frequency because of the capacitance effect of the oil and decreasing impedance of the oil subject to the electric field.[17] On the other hand, the direct current electric fields (dc electric fields) have been less investigated while it has been shown that the efficiency of the dc-based setups could be higher because they maintain high electric field for the entire duration of the processing and enhanced motion of the water droplets, which increases the chance of coalescence.[18−21] However, using dc electric field activates other undesired behavior like chain formation. Berg et al.[22] have demonstrated that the tiny water droplets dispersed in an oil medium form a stable chain aligned with the external electric field. The water droplets can form a long chain that can act as a bridge between two electrodes causing a short circuit or can form a shorter chain that can reduce the electrocoalescence efficiency.[17] In addition, using dc electric field exposes the anode to the solution leading to potential corrosion and subsequently increasing the downtime and maintenance time of the setup.[10] Bailes et al.[23] have introduced an alternative method that was implementing a pulsed dc electric field. They have found that the pulsed dc electric field has advantages of both ac and dc electric fields and can maintain the electrocoalescence performance for the entire separation process, reducing the chances of chain formation and enhancing the efficiency of the electrocoalescence. In the 1980s, they have investigated the different parameters affecting the efficiency of electrocoalescence in a pulsed dc electric field; they found that the pulse shape, frequency, and amplitude have major impacts on the coalescence events.[24] In addition, it has been reported that a protective coating layer on the electrodes might minimize the corrosion and enhance the electrocoalescence efficiency via optimizing process parameters; however, the cost of the coating and its lifecycle maintenance is considerable.[25,26]

Aside from the type of electrocoalescence processes, it is crucial to understand the nature and fundamentals of the process in order to fully utilize the benefits. The two main schemes of coalescence of water droplets are known to be dipole–dipole interaction (DDI) and migratory coalescence. DDI takes place as a result of the interaction between polarized and aligned water droplets in an electric field at a close distance. The adjacent water droplets exert an attractive force to each other and after reducing the distance, lead to the formation of larger water droplets.[27,28] On the other hand, migratory coalescence, which is electrophoresis (EP) in nature and is observed under dc or ac (with low frequency) electric fields, can lead to the formation of larger droplets. The water droplets receive free charges when they touch any electrodes or other charged droplets (noncoalescence events); then, the newly charged droplets move toward the opposite electrode in the unit cell, based on their distance. This motion can trigger coalescence due to collision between moving droplets or attraction forces between oppositely charged droplets. Finally, larger droplets form and they sediment faster, ultimately leading toward two separate phases.[29,30] However, these two mechanisms are not completely distinct and might act together in a process of electrocoalescence. This means that to an extent that the water droplets can acquire charge (closer to the electrodes), one mechanism triggers, and when the water droplets get farther from the electrodes, another one becomes the main acting mechanism.[31]

The performance of the DDI and the EP is correlated with strength of the electric field; overall, by intensifying the electric field the droplets coalesce faster. However, by increasing the strength of the electric field after some threshold, not only there is no increase in the coalescence event but also the breakup mechanisms are activated.[32,33] There are three main mechanisms of water droplet breakup that are introduced by Williams and many other researchers.[20,34] In the first one, the water droplet is polarized and elongated in the presence of the effect of the electric field and starts to disintegrate into smaller droplets (when the electric field is strong enough in that region). In the second one, water droplets can acquire free charges by coming into contact with electrodes and the charged droplets break up because of limitations on the amount of accumulated charge they can have (the Rayleigh limit) if the electric field is strong enough. Finally, water droplets might get stuck to the electrodes, which have higher potentials compared to the capacity of the water droplets. This high charge results in Taylor cone formation, which disperses tiny droplets of water into the oil, acting against the separation intended.[35]

Regardless of the distance to the electrodes, the uniformity of the electric fields is another important parameter that significantly affects the efficiency of electrocoalescence. The first implementation of nonuniform electric fields in electrocoalescence and water motion was conducted by Pearce.[36] Eow et al.,[37] in another study, have investigated the effect of electrode geometry and other dynamic parameters on the electrocoalescence of two water droplets in an oil medium. In addition, they have investigated the effect of the direction of the electric field relative to the water droplets. In another research by the same group, the effects of liquid flow, electric field strength, and fluid flow rate on coalescence rate were investigated, concluding that the coalescence rate could be easily enhanced or disturbed by different elements of experiments particularly electrode geometry.[38] Hoseini et al. have investigated the separation of oil droplets from the water medium in the presence of nonuniform ac and dc electric fields.[39,40] Nonuniform electric fields can be generated using different electrode configurations, which pin-to-plate is the most common one. However, the optimum geometry is still challenging because of the Joule heating effect on sharp edges and/or mechanical restrictions.[41,42] Although many other kinds of research may be found in the literature on the application of nonuniform electric fields in electrocoalescence, there are a few studies available in which the simultaneous effect of electric field nonuniformity and pulsed dc waveform was evaluated.[43] It is hypothesized that by combining these two features, the efficiency of coalescing performance increases. Here, this simultaneous effect was achieved utilizing a corona discharge.

Corona discharge is a type of cold plasma that could be useful in electrocoalescence. Thanks to their ability in ionization of gases and releasing radicals, cold plasmas have many different applications such as air purification and filtration, dust removal of airflows, bacteria removal in respirators, and electrohydrodynamic pumping.[44−47] Corona discharge is simply a high voltage discharge that reaches a point of differential energy that ionizes the air molecules. As a result, the corona discharge setups could be available in different forms of ac, dc, and pulsed dc.[48] Based on the orientation of the electrodes, the corona discharge could be named as either positive or negative corona discharge, from which the latter is utilized in most of the applications such as flow velocimetry and pressure sensors.[49] Corona discharge could be implemented in the field of electrocoalescence by charging the water droplets, build-up electric field, and consecutively, coalescence. The application of corona discharge in electrocoalescence can bring notable advantages compared to available methods, such as the contactless nature of corona discharge and its ability to form a nonuniform electric field. The performance of a dc nonuniform electric field induced by corona discharge for electrocoalescence was discussed in the literature.[50] The main role of corona discharge is to ionize the environment around the discharge (mostly air molecules) and form charged ions moving toward a ground electrode. In order to have the ionizing regime between the electrodes, it is important to keep the voltage gradient high by utilizing a sharp tip electrode (a small surface area). Ions are created in the area close to the tip and accelerate toward the opposite electrode, forming an ionic wind. The ions can penetrate the surface of a liquid medium if it is in their path and they can be transferred either by conduction or convection and building up a nonuniform electric field inside of the oil.[51] The presence of electric fields causes the electrocoalescence phenomenon in which the tiny water droplets in a W/O emulsion will coalesce and form a larger phase of water separated from the oil phase. As discussed earlier, increasing the strength of the electric field results in increasing the coalescence rate but an optimum range of the electric field should be utilized for the maximum efficiency. In addition, it is crucial to understand the different types of acting forces and coalescence acceleration/deceleration mechanisms.[52]

Coalescence (DDI or migratory) or break up (any of the three discussed above) mechanisms could be easily triggered by applying different experimental conditions. As a result, it is important to choose the best set of parameters in order to have the correct outcome with the highest efficiency. In this study, it is tried to have the coalescence of water droplets in a silicone oil medium under pure and pulsed dc electric fields. The experimental setup is schematically illustrated in Figure .

Figure 1

(A) Schematic showing various coalescence mechanisms of water droplets in 3000cst silicone oil, induced via corona discharge that is formed by applying a high voltage to a sharp conductive electrode. (B) Two water droplets dispersed in the center of the cuvette, to examine the effect of dipole–dipole interaction and the imposed electric fields on their coalescence. (C) Migratory coalescence (EP) by dispensing a second water droplet when the first one reaches the ground electrode. Note that the droplets are far away when compared to the case (B). (D) DEP force acting on a single water droplet, bouncing between the two electrodes under the corona discharge.

Results and Discussion

To study the impact of nonuniform electric field on the coalescence of W/O emulsions, two water droplets (10 μL) were placed in a silicone oil medium with a viscosity of 3000cst under pure and pulsed dc (1, 10, and 20 Hz) electric fields. The electrocoalescence behavior of the water droplets was monitored under various operating frequencies for both pure and pulsed dc electric fields. Figure A,B shows the normalized distance of two water droplets with specific process parameters of the distance between the droplets (d) over their initial distance (d0), and their approaching velocity, respectively. It is hypothesized that the DDI is the main component of the acting forces on adjacent water droplets during the coalescence process.[53] The DDI forces are results of the interaction between two adjacent and charged/polarized water droplets in tangential and radial directions. These forces could be calculated using the following equations (see Supporting Information)where a1 and a2 are droplet radii, ε0 is the permittivity of the free space, εc is the dielectric constant of the continuous phase (silicone oil in this case), Kc is the Clausius–Mossotti factor defined as (εd – εc)/εd + 2εc, ε is the dielectric constant of the dispersed phase (water in this case), θ is the angle between the dipole axis of water droplets and the distance-vector, d is the distance between the leading edge of water droplets, and finally E is the applied external electric field as shown in Figure . Depending on the angle of the droplets relative to each other and the distance between them, the magnitude of the force components changes. It is tried to have the two droplets dispersed in a way that they have the minimum angle between their vertical symmetry axis (ideally θ = 0°) maintaining a distance of d = 6 mm. Because the water droplets are polarized, a DDI force is formed between the close-enough water droplets. The DDI forces are the dominant reason for the coalescence of the water droplets in the initial steps of the process because the water droplets are initially close to each other. The DDI forces are in relation to the inverse of the fourth power of the distance between them (FDDI ∝ d–4) and they lose effect in larger distances. This correlation can be clearly seen from Figure A where the normalized distance of the water droplets is exponentially decreasing with time while the two water droplets get closer and the DDI forces become stronger. It is also shown that a decrease in the frequency of pulsed dc from 20 to 1 Hz reduces the coalesce time of the droplets. However, the coalescence time of the sample under 10 Hz pulsed dc is comparable to that of pure dc. This is taken as a reference when comparing the performance of different electric fields in the coalescence of water droplets. The lower pulsed dc and pure dc showed a similar behavior over the DDI experience without a significant difference in the coalescence time. To further examine the coalescence behavior of water droplets, the relative velocity of the two approaching droplets was plotted versus time in Figure B. The relative velocity of the approaching droplets significantly increases with time, experiencing its maximum value just before the coalescence. This verifies the finding that stronger DDI forces exist as the distance between the droplets decreases. The fluctuations observed in the relative velocity suggest the presence of forces resisting the coalescence and acting in the opposite direction of the DDI forces. It is hypothesized that these coalescences resisting hydrodynamic forces are drag and film-thinning forces that arise from the fluid friction and squeezing of the oil between approaching water droplets. These forces result in a local decrease in the relative velocity of the approaching droplets while the DDI forces continue to increase the overall trend of the relative velocity. The introduced damping forces are defined as[17]where 1 and 2 are droplet velocity and ηc is the dynamic viscosity of the continuous phase (i.e., oil). The interaction between these forces creates the fluctuation in water droplets’ motion. Similar to the normalized distance versus time plot, the relative velocity plot shows comparable results under the pure dc and the 10 Hz pulsed dc electric fields. The reason for the delayed increase of the relative velocity under the 20 Hz pulsed dc electric field is that the droplets are not exposed to such a strong electric force to trigger their motion. It took nearly 16 s for the droplets to get close enough and excite the motion toward each other. In addition, the low relative velocity observed under 20 Hz pulsed dc is accompanied by few fluctuations in the relative velocity, verifying the presence of the drag and the film-thinning forces with their strength depending on the velocity. The real-time sequences of the DDI coalescence of the two water droplets can be seen in Figure C,D (see also Video S2). The graph shows that the sample under 1 Hz (fastest coalescence of all in 3.3 s) coalesces more quickly than the one under 20 Hz (slowest of all coalesce in 18.9 s).

Figure 2

(A) Normalized distance of two nearby water droplets (10 μL) in 3000cst silicone oil vs time for coalescence, that is, induced by pure dc and pulsed dc (1, 10, and 20 Hz) corona discharge. (B) The relative velocity of the coalescing droplets vs their coalescence time under pure dc and pulsed dc (1, 10, and 20 Hz) electric fields. (C,D) Snapshots of coalescing droplets via DDI induced by pulsed dc electric field with 1 and 20 Hz frequency, respectively. All the experiments were conducted at +6 kV voltage.

Although it is discussed that the DDI forces act as the primary reason for enabling coalescence, increasing the distance of the water droplets diminishes the strength and effectiveness of the DDI forces in coalescence. However, when water droplets are relatively far, another type of force is responsible to increase the performance of the coalescer. These forces, which are commonly known as EP (migratory coalescence), play a role when water droplets acquire free charges. The EP force between two charged water droplets can be expressed as follows (see Supporting Information)while the EP forces act at larger distances when compared to those of the DDI forces, they are proportional to the inverse of the second power of the distance of the water droplets (EP ∝ d–2). Comparing this to the distance dependence of the DDI (DDI ∝ d–4), it is evident why lowering the distance leads to two times enhancement of the DDI forces, when compared to the EP forces. The EP forces occur naturally in dc electric fields or ac electric fields with low frequencies.[17] Migratory coalescence takes place because of the traverse of the charged water droplets between the two electrodes based on the relaxation time of the silicone oil as the medium. The relaxation time determines whether the water droplets are able to keep their charge to travel at least half of the distance between the electrodes.[51] The whole process of migratory coalescence occurs because of the Coulombic forces. Figure shows the migratory coalescence behavior of two far-away water droplets in silicone oil under pure and pulsed dc electric fields of +6 kV. The Coulomb force due to the electrophoretic phenomenon results in the initial attraction of the water droplets because the initial distance between the water droplets is further than what is needed to have effective DDI forces. Unlike close water droplets, the pure dc electric field is more effective in the attraction of faraway water droplets when compared to the pulsed dc electric fields. The highest efficiency of the coalescence is yielded under a pure dc electric field and for all the pulsed electric fields, the coalescence occurs slower (see also Video S3). Because the behavior of the coalescence process is not proportional to the frequency of the electric field, it is crucial to have the optimum working frequency while utilizing a pulsed dc electric field. As can be seen in Figure A, the coalescence efficiency of the sample under 10 Hz is the highest, while the ones under 1 and 20 Hz are less efficient. Despite the DDI experiences, there is less similarities between the coalescence time of pure dc and pulsed dc (with low frequency) electric fields; it seems that it is directly correlated with duty cycle of waveform (%100 compared to pulsed dc %50). The amount of electric field charges transferred during the touching process is the main factor to manipulate water droplets during migratory coalescence. From Figure B, it can be concluded that the fastest coalescence takes place under a pure dc electric field. The behavior of the samples under a pulsed dc electric field is different in a way that in all cases, the relative velocity increases gradually and does not reach the highest peak provided by the pure dc electric field. Similar to the trends in the normalized distance in Figure A it can be seen that there is a need to optimize the working frequency of the electric field in order to enhance the efficiency of coalescence. The coalescence time of the droplets under the 10 Hz pulsed dc is close to that observed under the pure dc electric field, while the ones under 1 and 20 Hz are noticeably slower. The real-time sequences of coalescence of the droplets treated under pure and pulsed dc (20 Hz) electric fields as the fastest and the slowest coalescence processes are presented in Figure C,D.

Figure 3

(A) Normalized distance of two far-away water droplets (10 μL) in 3000cst silicone oil vs their coalescence time, induced by pure and pulsed dc (1, 10, and 20 Hz) electric fields. (B) Relative velocity of the droplets in motion toward each other vs their coalescence time under pure and pulsed dc (1, 10, and 20 Hz) electric fields. (C) Migratory coalescence (EP) observed from snapshots of two far away droplets under 1 Hz pulsed dc electric field. (D) Migratory coalescence sequences of the same droplets in (C) under a 20 Hz pulsed dc electric field. All the experiments were conducted under +6 kV.

In comparison to the coalescence time of the pure dc-treated sample, it is obvious that the coalescence occurs nearly four times slower in the samples treated under a pulsed dc electric field (20 Hz). The real-time images are in accordance with the experimental results plotted in Figure . The slope of the relative velocity graph (Figure B) represents a relative water droplet acceleration. The slope in the sample treated under a pure dc electric field is the highest acceleration. This fact can also translate to the highest net force acting on the water droplets. Also, this high acceleration is reflected in the high relative velocity of the water droplets in the pure dc electric field at the early stages. Because the droplets are fully charged at the beginning of the coalescence process, they experience the highest EP forces and highest accelerations. However, the slope decreases over time because the water droplets are discharged in the dielectric (silicone oil) medium and the Coulomb force decreases. The increase of the relative velocity is limited because of the increase of opposite hydrodynamic forces (eq ). The relative velocity fluctuation in the samples treated under pulsed dc electric field is negligible through the beginning of the process to the three-quarters of the distance but after that, the fluctuation starts. At the endpoint of the graphs where the distance is close to zero, the coalescence regime transforms from the migratory coalescence to the DDI.

Figure compares acceleration between approaching water droplets in silicone oil under pure dc and pulsed dc electric fields and also under both DDI and EP experiments. In DDI cases (4A,B), the pulsed dc treated sample shows higher acceleration over the entire coalescence course. This force (DDI) increases by decreasing the distance and fluctuates as a result of resistance forces such as drag force in higher distances and film-thinning in final steps of coalescence. From Figure C,D, it can be seen that pure dc provides acceleration in the beginning, while the pulsed dc has a significantly lower acceleration and limited response, till it passes half of the course. This behavior can be explained by the small dc component of pulsed dc in comparison to that of pure dc. The acceleration declines gradually from the beginning to about half of the distance in pure dc and starts to increase in both cases nearly from the same point. This regime suggests that most of the free charges in the water droplets have vanished and the coalescence mechanism is changed from EP to a combination of EP and DDI in the lower distances. This transient point is important during the design of a coalescer and needs to be considered in a way that water droplets can have an opportunity to transit from the EP to the DDI-induced coalescence; otherwise, a low enhancement of coalescence efficiency can be seen by solely relying on the EP phenomena. Other than DDI and migratory coalescence mechanisms, it is possible to boost the motion of the water droplets in a medium under an external nonuniform electric field. The motion of the charged droplets toward the strongest pole of a nonuniform electric field because of a high dielectric constant is called dielectrophoresis (DEP), which takes place regardless of the distance of the adjacent charged droplets[54] and defined as

Figure 4

Relative acceleration of two nearby water droplets (10 μL) in 3000cst silicone oil coalescing by DDI under a pure dc electric field in (A) and under 1 Hz pulsed dc electric field in (B). The droplets experience higher acceleration under a low-frequency pulsed dc electric field, leading to faster coalescence. (C) Relative acceleration of two far-away water droplets approaching each other because of EP induced by pure dc and 10 Hz pulsed dc electric field in (C,D), respectively. In the case of pure dc, the relative acceleration is higher, and the coalescence is faster. Comparing all cases (A–D), coalescence is the best by EP under pure dc electric field.

The DEP occurs in only one direction even if the direction of the electric field is changed, that is, the only triggering component is the gradient of electric field strength (DEP ∝ E2) not the direction in which the charges move.[55,56] The electric field intensity in the oil medium is different based on the angle of the needle compared to the surface of the test cell in each point. The highest intensity of the electric field occurs exactly beneath the needle electrode where θ = 0° and it decreases as the angle continues to increase (see schematic in Figure ). The distribution of the electric field could be considered similar to a cone with the apex on the sharp conductive electrode (needle). Consequently, the nonuniform distribution of the current density is experienced and could be calculated using different equations such as Warburg’s correlation[57] leading to electric field presence expressed by Ohm’s law in electrohydrodynamics.[58] The nonuniformity of the electric fields results in weaker and stronger electric fields applied to the different regions of the continuous phase (i.e., silicone oil). This phenomenon pushes the water droplets toward the stronger regions of the electric field.[50,51,59] To further investigate the impact of nonuniform electric field and DEP phenomena, the motion of a single water droplet under pure and pulsed dc electric fields was investigated (Figure , see also Videos S4 and S5).

Figure 5

Velocity of one water droplet (10 μL) in 3000cst silicone oil vs its traveling time between two electrodes under pure and pulsed dc (1, 10, and 20 Hz) electric fields in a downward motion in (A) and in an upward motion in (B). It is believed that the DEP is responsible for difference in the droplet motion. (C,D) Snapshots of a droplet showing upward (tu) and downward (td) traveling time between the two electrodes under a pure dc electric field and 20 Hz pulsed dc electric fields, respectively. In all cases, the EP and DEP agree in the upward direction and opposite in a downward direction. The magnitude of DEP significantly decreases by increasing the frequency results to lower.

Figure A shows that the downward velocity of the droplet is not impacted by the frequency of the pulsed dc; however, switching to pure dc leads to an enhanced downward velocity. Under all applied pulsed dc voltages with various frequencies, the motion patterns of the droplets are comparable in the first 10 s. In the first 10 s of processing under pure dc, the relative velocity is less than those of the pulsed dc cases. However, after 10 s the velocity of the droplet under a pure dc field increases significantly when compared to the droplet under various pulsed dc electric fields. It is speculated that this is a result of the higher electric field intensity in the areas closer to the tip of the corona needle and weaker intensity in the lower areas. This phenomenon consequently results in higher and lower DEP forces in the upper and lower regions of the oil medium, respectively. Because DEP acts in the opposite direction, the velocity increases by increasing the distance relative to the oil surface and eventually, EP forces overcome DEP because of the decrease in the electric field intensity in the lower areas. At some point, the droplet velocity reaches its peak and starts to decline gradually. This point is where the water droplets lose all the accumulated charges and become less active under the electric field. Similar peak and decline in the droplet velocity observed under pulsed dc electric field as well but the rates of increase and decrease of the velocity and the peak values are different. There is a tiny difference in the traveling time of the different pulsed dc treated samples, which is caused by the frequency and the falling/rising potentials of the waves. The other similarity of the patterns is that in all of them, the final point has a sharp decrease in velocity (instantaneously drops to zero), which indicates the point that the water droplets touch the opposing electrode and the velocity direction starts to change (half of the traveling distance, tu = 0). Because of the agreement in the direction of EP and DEP in the reverse motion (Figure B), the pattern of the velocities is fully changed and the velocity values are nearly three times those values observed in the downward motion. In all cases, the peak is the point where the droplets become closer to the top surface of the test cell (the surface of the silicone oil) where the intensity of the electric field is the highest. The peak in velocity values is caused by continuously increasing DEP forces, which pull the droplets toward the surface. Passing the peak point, there is a relatively sharp decline in the velocity, which indicates the point where the droplets touched the top surface and released all their charges, obtained opposite charges, and started moving downward once more.

Figure C,D represents the DEP movement sequences of two water droplets under pure and pulsed dc (20 Hz) electric fields, respectively. Under pure dc, the upward motion takes 8.1 s, while the total transverse time in downward motion is equal to 34 s. The upward motion takes a less time because the velocity is higher (the effect of the DEP and EP forces are in one direction); however, in a downward motion, the velocity is less because the DEP and EP are in the opposite direction. This significant difference in the upward versus downward velocities takes place under the pulsed dc (1, 10, and 20 Hz) electric fields as well. However, the difference in the travel time in the pulsed dc samples is not significantly higher than that of the pure dc sample. The pulsed dc samples show a higher upward traveling (30 s in 20 Hz) time and a lower downward traveling time, which is the opposite of what happens under the pure dc (39 s in 20 Hz). By deeply considering the change in behavior, it is safe to conclude that the effect of DEP becomes weaker in the pulsed dc electric field. It is believed that this occurs because of the constant change in the potential of the electric field. Overall, understanding the movement of the charged particles helps to increase the chance of collision between the water droplets and enhance the coalescence process when multiple droplets are present. This behavior suggests that utilizing DEP enhances the efficiency of the coalescer, especially in dilute emulsions.

In order to find the best strategy to perform a coalescence process, it is needed to first understand and define the coalescence mechanisms based on the different experimental conditions, aiming to obtain an optimized recipe. Figure represents different comparisons of the upward/downward velocities of water droplets, velocities of water droplets in DDI and migratory coalescence, and the consumed power based on the frequencies of the processes. It should be noted that the 0 Hz frequency represents the case where pure dc electric fields were utilized. Figure A shows the velocity of a single water droplet in the upward and downward directions because of the DEP forces. Forces that act on a single water droplet are a combination of buoyancy, gravity, drag, DEP, and EP and are shown in eqs and 8 for the downward and the upward motions, respectively. The difference between eqs and 8 is approximately equal to two times of the DEP force (eq ). This assumption is valid because the only positive values in eq are the buoyancy force, which is small when compared to the other forces (ρwater = 1000 kg/m3 and ρoil = 967 kg/m3).

Figure 6

(A) Average velocity of a water droplet in 3000cst silicone oil in upward and downward motions because of the DEP forces under pure dc (0 Hz frequency) and various pulsed dc electric fields. (B) Average relative velocity of two approaching water droplets (10 μL) in 3000cst silicone oil under the DDI and the migratory coalescence conditions vs the frequency of the applied electric field. (C) ac, dc, and total power consumption at different frequencies.

As indicated earlier, in the upward motion the droplet has a higher velocity but as the frequency of electric field increases, the difference between velocities in the upward and downward diminishes to the point where they become identical at a frequency of 50 Hz. Thus, it is reasonable to design the coalescence setups in a way that the droplets mostly experience upward motions instead of a reciprocating one. Now that the direction of the motion is determined, it is better to understand and select the best fit for the type of motions. This is crucial where the water droplets need to gather in the surface of oil or the bottom of oil (higher or lower oil density compared to water density).

Figure B represents the average relative velocities of two approaching droplets moving due to DDI and EP forces under the various frequency of applied electric fields. As can be seen, the average velocity of the migratory coalescence scheme (EP) is higher than that of the DDI-based coalescence. In the pulsed dc electric fields, the efficiency tends to drastically decrease after passing the frequency of 10 Hz. However, the overall efficiencies of DDI and EP between the 1 and 10 Hz are in the same order in general. It should be mentioned that in dilute emulsions (higher water droplet distances), it is more efficient to use a pure dc electric field, while in highly concentrated emulsions (lower water droplet distances), the pulsed dc electric fields with the lowest possible frequency are the most efficient ones. Figure C represents the efficiency of the coalescence processes based on the consumed power in different electric field sources. The pulsed dc waveform was a combination of the ac and the dc waveforms. The power in all cases was calculated based on measuring current using a multimeter in both ac and dc modes. The total power was the addition of the values obtained in the ac and dc modes (PTotal = Pac + Pdc). As can be seen, the majority of the power go to the samples treated under pulsed dc electric field with a higher frequency because of an increase in the oil capacitance (higher current). Considering the effect of the frequency, it is obvious that the change in frequency from 0 Hz to higher values keeps the power consumption of the dc part constant; however, for the similar frequencies, the power consumption of the ac part rapidly increases from 10 to 15, and then 25 times of pure dc at frequencies of 10, 20, and 50 Hz, respectively. To finalize this discussion, it is important to consider the type, direction, and the power consumption of the coalescence process and select the best and the most efficient coalescence scheme based on emulsion types in order to achieve the highest efficiency.

Conclusions

In the current study, a series of experiments were conducted in order to investigate the effect of dc and pulsed dc electric fields on the performance of the electrocoalescence process initiated by unipolar charge injection using corona discharge. Three main coalescence forces, DDI, EP, and DEP, were considered and investigated under different electric field types (dc and pulsed dc electric fields). All the experiments were analyzed using digital image-processing techniques in order to capture the real-time values of the distance between water droplets, their acceleration and velocity in different modes of coalescence, in addition to the power consumption of each case, were monitored and recorded. Based on the concentration of the emulsions and size of the water droplets inside the oil media, different regimes of electrocoalescence trigger, which requires different processing conditions. As a result, there is no global solution to the emulsion separation process using a corona discharge-induced charge injection. Thus, a general and optimum strategy was introduced to address different emulsions with different water concentrations and conditions.

Pulsed dc with the lowest frequency shows a better performance in DDI cases and is recommended for the emulsions with a high water concentration.

Pure dc shows the best performance in EP, beyond the other cases (pulsed dc), and is recommended for the emulsions with a low water concentration.

For emulsions with moderate water concentrations (between the highest and the lowest concentrations), the use of the pure or pulsed dc strategies is outweighed against the power consumption and processing time in order to find the optimal process conditions.

The use of DEP forces was also investigated to address the best performance of coalescers for oil with different densities.

Finally, the authors believe that this strategy can be time-varying based on dynamic emulsion conditions.

Experimental Section

A desktop experimental setup was assembled and utilized in order to study the impact of corona discharge on the coalescence behavior of water droplets in the oil medium (Figure ). The setup has consisted of a high-voltage power supply with an amplifier, a sharp tungsten needle (purchased from Bovie Medical with a tip diameter of 65 μm) acting as the working electrode, a lab jack holding the two-phase medium (water droplets dispersed in oil) and acting as the ground electrode, a measurement unit with the ability to measure instantaneous currents. The experiments were monitored with a high-speed camera (Olympus TR i-speed) equipped with a photography lens (Tamron 90 mm 1:2.8), which was capable of imaging up to 10,000 frames per second (fps). The high-voltage amplifier (TREK 10/10B-HS) was equipped with a function generator (SDG 1032X) that was capable of providing different waveforms and frequencies. The amplifier was capable of generating positive/negative high-voltage dc and ac by a 1:1000 ratio with the highest frequency of 1000 Hz. However, in the experimental process, only the pure and pulsed dc waveforms were applied. For current measurement, a Keithley 2100 multimeter was connected to the high-voltage amplifier.

Different materials used in these experiments were mainly deionized water (Sigma-Aldrich) and silicone oil (HUDY). An automated Ramé-Hart dispenser (P/N 100-22 with a maximum accuracy of ±0.002 μL) was used to disperse small water droplets (a volume of 10 μL) with a controlled volume into the silicone oil medium. The experiment cell was a quartz cube with a dimension of 30 × 30 × 30 mm3, filled with silicone oil 3000cst up to 15 mm height (the maximum oil level). The quartz cube was equipped with a one-inch copper plate at the bottom surface that was connected to the grounding electrode (out of the experiment cell) through a tiny hole. With the help of a laboratory jack, the gap between the corona generating electrode (sharp tungsten needle) and the top surface of the oil in the cube was kept constant to a specific distance of t = 5 mm for these sets of experiments. Finally, water droplets added to the silicone oil were treated by +6 kV pure and pulsed dc electric fields under different frequencies of 1, 10, and 20 Hz. The recorded videos were then converted to images and analyzed via digital image-processing software packages (MIPAR).[60]Video S1 shows the original video and the postprocessed video. At first, the water droplets as a circle object were introduced to the software recipe, and second the circles were actively tracked over the corona discharge and all geometry data were recorded. All experiments were conducted under room conditions (a temperature of 25 °C and a pressure of 1 atm).