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Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions
Journal Pre-proof Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions Pramod P. Kothmire, Yogesh J Bhalerao, Vijay M Naik, Rochish M Thaokar, Vinay A Juvekar
PII:
S0263-8762(19)30579-9
DOI:
https://doi.org/10.1016/j.cherd.2019.12.012
Reference:
CHERD 3935
To appear in:
Chemical Engineering Research and Design
Received Date:
1 March 2019
Revised Date:
1 November 2019
Accepted Date:
8 December 2019
Please cite this article as: Pramod P. Kothmire, Yogesh J Bhalerao, Vijay M Naik, Rochish M Thaokar, Vinay A Juvekar, Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions, (2019), doi: https://doi.org/10.1016/j.cherd.2019.12.012
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Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions Pramod P. Kothmirea,b , Yogesh J Bhaleraob , Vijay M Naika , Rochish M Thaokara , Vinay A Juvekara,∗ a Department
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of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai 400 076, India b School of Mechanical Engineering, MIT Academy of Engineering, Alandi, Pune 412105
Abstract
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The rate of coalescence in an electrocoalescer, which governs the residence time, sensitively depends upon the electrostatic conditions as well as the electrodes
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arrangement. An experimental study of a cylindrical electrocoalescer vessel for two different modes of operation is reported in this work. The design of the
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coalescer is optimized for the residence time and water separation. To justify the experimental findings and to unravel more physical insights, the single phase electrostatics is simulated using COMSOL Multiphysics software. The volume of
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the emulsion under the influence of 0.3 - 1.0 kV/cm electric field, calculated using COMSOL correlates well with the water separation. Moreover, the simultaneous measurement of current suggests an ordering, that leads to an increase in size of the droplets and settling of drops in the normal operation. On the other hand, chaining and shorting leads to surge in current at high fields and seems
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to prevent water separation.
Keywords: Electric field, Electrocoalescer, Electrode configuration, Electrostatic, COMSOL simulation
∗ Corresponding
author Email address: [email protected] (Vinay A Juvekar)
Preprint submitted to Chemical Engineering Research and Design
November 1, 2019
1. Introduction There are various techniques used for separation of water from a waterin-oil emulsion such as gravity sedimentation, centrifugation [1, 2], chemical demulsification [3, 4] ,pH adjustment and heat treatment [5, 6] etc. The most popular method used in an oil refinery to coalesce the water drops in commonly
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encountered crude-water (brine) emulsions is by the use of an electric field [7, 8]. The basic principle of an electrocoalescer is the coalescence of water droplets,
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resulting in bigger sized drops with higher settling velocity, thereby leading to faster separation of water from the emulsion. Several possible mechanisms for
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electrocoalescence have been identified [9], based on the dipolar attraction of drops in electric field.
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Coalescence between two droplets can be improved by the force generated by electric fields. Primarily, there are three types of electrostatic forces between droplets (i)dipolar attraction (ii) electrophoresis and (iii)dielectrophoresis. Dipo-
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lar attraction is the electrostatic attraction force between two polarized water droplets while electrophoresis is the motion of a charged droplet (where the charge is often acquired after contact with an electrode) under a uniform electric
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field. Dielectrophoresis is the movement of polarized, uncharged water droplets in a non-uniform electric field with the movement towards the direction of convergence of the field. There are several types of spatio-temporal electric fields that may be used to enhance the coalescence of water droplets. These include the AC (alternating current) field, DC (direct current) field or a combination of
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AC and DC fields. The DC electric field [10] is commonly used to treat emulsions with low water content [11]. Unlike with the DC field, the functioning of an electrocoalescer is safe even at high water content under an AC electric field [12]. AC fields are also reported to be more effective in increasing the coalescence rate than pulsed DC fields [13].
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The presence of an electric field promotes contact between drops, enhancing drop-drop and drop-interface coalescence [14, 15]. The spatial electric field distribution in an electrocoalescer [16] and electrode configuration plays an important role in effective coalescence. The phenomena such as water chain formation [17] and short circuiting [11] may occur if electrodes are not properly configured
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inside an electrocoalescer. Taylor[18] proposed a two-step mechanism of chain formation and coalescence for water-in-oil dispersions, influenced by a number
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of factors such as oil phase viscosity, disperse phase volume and the applied
electric field strength. Under high electric field and insufficient spacing between the electrodes, or between the electrodes and the vessel, there is often observed
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formation of chains of water droplets, aligned in the direction of maximum field strength. Moreover, the chains do not always start or finish at an electrode and
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may not be in contact with the electrodes. Pearce[19] suggested that chains are formed by forces due to the potential difference between drops as a result of
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their induced charges. High aqueous phase emulsions tend to short-circuit the electrodes and significantly reduce the electric field[11]. Major stress in the literature is on interaction between two drops under
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electric field leading to coalescence or non-coalescence [20–23]. Researchers have also studied effect of specific design and operating parameters such as type of electric field (AC/DC), intensity of electric field, frequency, waveform, geometry of electrodes[10–13], salinity, temperature, water content, and ph [24]. When all these factors comes together, the complexity of the problem doesn’t allow one
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to estimate the efficiency of coalescer. Numerical simulation allow us to take into account the various complexity in actual coalescer and helps to estimate the distribution of electric field and the rate of water separation. It will act as a good tool to design provided we can correlate the simulation results with actual performance of coalescer. The simulation can then act as a predictive
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as well as optimization tool for design of coalescer. Such kind of approach which relates electric field obtain from simulation to rate of water separation has not been reported so far in the literature. This form the focused of the present work. In this work, we have optimised the electrocoalescence process with respect to i) electrode configuration (spacing between the electrodes and
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depth of immersion of the electrodes in the emulsion) ii) the electric state of the vessel and electrodes (grounded vs insulated). In general, there exists an optimal
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range of electric field in which the rate of coalescence is sufficiently high. If the
field is lower than the lower limit of the optimal range, the rate of coalescence is poor due to weak electrostatic forces among the droplets. On the other hand,
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if the field is above the upper limit of the optimal range, the coalescence is poor because of the onset of the phenomenon known as non coalescence. Since
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in an electrocoalescer the electric field varies with location, there exists two regions, one in which the field is in the optimal range and the remaining region
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in which the field is either above the upper limit or below the lower limit of the optimal range. In this work we have experimentally determined the optimal range of the electric field in which coalescence rate is reasonable high. We have
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also found using COMSOL Multiphysics simulations, the regions within the electrocoalescer vessel in which electric field lies in the optimal range. We have then correlated experimentally obtained rate of water separation and the volume of region of the optimal electric field from simulations. This work will provide us a design procedure for electrocoalescer to optimize the electrode configuration
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and electrostatics so as to maximize the region of optimal electric field and thereby maximize the rate of water separation. We have also measured the variation in current during the experiment and
related it with progress of coalescence. Chaining of the drops is a prelude to their a coalescence. Chaining results in increase in the current because it provides
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a conducting path. Therefore measurement of current allow us to quantify the extent of chaining. If current is low chaining is poor and which also result in low rate of coalescence. On the other hand if the current is high it implies severe chaining which result in short circuiting and drop in the field which results in poor coalescence. We have therefore shown that the current in the system can
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be used as a diagnostic tool to study the progress of coalescence as well as to
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quantify the efficiency of coalescence.
2. Experimental
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2.1. Preparation of emulsion
500 ml water-in-sunflower oil emulsion was prepared by mixing 90% sun-
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flower oil (the continuous phase) and 10% deionized water (the dispersed phase) using a homogenizer (ultraturrex) at 10000 rpm for 5 minutes without adding any surfactant or demulsifier. The stator and rotor were immersed at a depth
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of 9.5 cm in the emulsion to avoid air suction. Table 1 summarizes the physical properties of the oil used in the experiment. Liquid viscosity was measured using the Physica MCR 301, Anton Paar Rheometer, while the density was
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measured using a DDM 2909 densitometer. The conductivities of dispersed and continuous phases were measured using a conductivity meter. The emulsions were judged to be stable when no changes were observed for 4 hours. The initial emulsion drop size distribution is shown in Figure 1. The Figure shows that most of the drops are segregated from each other. Moreover, they are small in
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size and uniformly distributed. The drop size distribution is in the range of 5 to 10 µm as shown in Figure 1. The droplet diameter was measured by Image J software. The same mixture was reemulsified to conduct three independent experimental runs. For the fourth trial, a fresh emulsion was prepared using the same proportion and technique. The room temperature throughout the
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experiment was controlled at 20±2◦ C using a room air-conditioner. 2.2. Experimental setup The electrocoalescer was a cylindrical borosilicate glass vessel of diameter 8 cm and the height of 10 cm as shown in Figure 2. The vessel top was covered
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with a circular acrylic plate. The circular plate was provided with two holes, slightly greater than the electrode diameter. The top plates with different spac-
ing between the holes were used for varying the spacing between the electrodes.
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The electrodes were two copper rods of 0.2 cm diameter and 14 cm length. 50Hz sinusoidal, AC electric field was applied using a custom made single phase trans-
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former with a rating 0-20 kV and 0-50 mA. The experiments were performed by using two modes. In the first mode of operation, the vessel was insulated
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with one energized electrode and the other grounded electrode. Insulated vessel was realized by keeping the boro-silicate glass vessel (an insulator) on a wooden
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table. This mode will be henceforth termed as ”insulated mode”. In the second mode of operation, a grounded metallic wire mesh was inserted around the inner wall of the circumference of the vessel and both the electrodes were energized.
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This mode will be henceforth termed as ”grounded mode”. In case of insulated mode, the nominal electric field is given by E= ∆V/X and E= ∆V/((diameter of the vessel-X)/2) in case of grounded mode where V is applied voltage and X is the electrode spacing. In both the modes of operation, the experiments were performed by changing the spacing between the two electrodes (X), the
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depth of electrodes immersed in the emulsion (Y) as shown in the Figure 2b and the electric field. The estimated depth of the oil-water interface, after complete separation of water was 9 cm from the top liquid surface. Hence the metallic wire mesh was inserted in such a way that the lower end of the wire was 8cm from the top liquid surface so that the separated pool of water will never touch the wire mesh. Current was measured using a Keithley multimeter Model 8605
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connected in series with the vessel on the lower voltage side. 2.3. COMSOL Simulation The electrostatics in the electrocoalescer vessel was simulated by using the Electrostatic AC/DC module in COMSOL multiphysics software (version 3.5a)
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for two different modes of operation by solving the relevant electrostatic equations with appropriate boundary conditions (Figure 3). The COMSOL Mul-
tiphysics software package is based on multidimensional (three-dimensional, in
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this case) finite element method. Although this technique is computationally
more intensive than the other techniques such as finite volume and finite dif-
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ference methods (because of the higher dimensionality of the problem), the automatic mesh generation program, powerful equation solvers and the graphi-
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cal interface of the software makes it easy to solve the problem and also assess various details of the solution. In each mode of operation, the electric field dis-
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tribution throughout the vessel was determined by solving the Gauss equation and the resulting profile of electric field distribution was regarded time variant and stored for later use. The boundary conditions determine the electrical vari-
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ables on the boundaries of the physical model. The Initial Values node adds an initial value for the electric potential V that can serve as an initial condition for a transient simulation or as an initial guess for a nonlinear solver. The value or expression for the initial value of the electric potential is given by V (SI unit: V) with a default value of 0 V. The Zero Charge node adds the condition that
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there is zero charge on the boundary so that n D = 0. This boundary condition is also applicable at symmetry boundaries where the potential is known to be symmetric with respect to the boundary. This is the default boundary condition at exterior boundaries. At interior boundaries, it means that no displacement field can penetrate the boundary and that the electric potential is discontinuous across the boundary. The Ground node implements ground (zero potential) as
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the boundary condition V = 0, implying zero potential on the boundary. This boundary condition is also applicable at symmetry boundaries where the potential is known to be anti symmetric with respect to the boundary. Boundary conditions on upper acrylic circular plate was kept floating (no charge). Since the wire mesh was kept 1 cm above the oil-water interface hence boundary con-
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dition to the oil-water interface was also kept floating. For simulations using COMSOL, it was more convenient to cast the problem in the cylindrical coordi-
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nates. We consider a cylindrical region of radius R and height H, enclosing the
cylindrical electrodes with different spacing (X) and the depth of an electrode (L) immersed in the main cylinder as shown in Figure 3. The curved surface
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of the cylinder forms a boundary of the domain. The dimensionless potential
coordinates ∇2 φˆ= 0.
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in the electrolyte, denoted by φ satisfies the Laplace equation in cylindrical
The geometry was discretized into simple triangular finer grids to ensure
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that results were independent of the mesh size. Electric potential was applied between the vessel and the electrodes as per the modes of operation discussed in the experimental set up. The nominal applied electric fields in the simula-
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tions were kept at values between 0.1 to 1.0 kV/cm at intervals of 0.1 kV/cm. The simulation model so developed was used to determine the effective volume under different values of an electric field for both the configuration either by changing the spacing between the electrodes or the depth of an electrodes inserted in the emulsion. The simulation results were then related to the ex-
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perimental findings. It should be noted that the electrostatic calculation was only for the oil as medium and the contribution of the dispersed aqueous phase was completely ignored. Thus the water droplets were not considered in the electrostatics calculations.
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3. Results and discussion We studied the performance of the electrocoalescer for two modes of operations (insulated and grounded). In all operations the nominal electric field was kept constant at 1 kV/cm. Figure 4(a) shows the percentage of water separation with respect to time for different depths of immersion of electrodes (Y)
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for grounded mode. During the initial period, the water separation increases linearly with time until it reaches a plateau beyond which there was no further
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water separation. Reasons for these trends are discussed later in conjunction with COMSOL simulations. As the depth of immersion of electrodes was in-
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creased from 3 cm to 8 cm in steps of 1 cm, the maximum attainable water separation increases from 15% to 95%. Moreover, the rate of water separation
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and the maximum attainable water separation increases linearly with respect to an increase in the depth of immersion of the electrodes as seen in Figure 4(b). At depth of 8 cm, as seen from the Figure 4a, the water separation was high-
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est and the separation time was lowest. Similar observations were made when the experiments were conducted in the insulated mode but are not shown for brevity.
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When the depth of immersion of electrodes was 9 cm, there was less water
separation of around 50% and 60% in case of insulated and grounded mode respectively. Moreover, fluctuations in current and visible arcing inside the vessel was observed (discussed later) rendering the operation unsafe. Based on the calculation of the interface level, we found that, at a depth of immersion of
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electrodes around 9 cm, the tip of the electrode would touch the oil-water interface after complete separation of water. Hence the depth of immersion of the electrodes in the emulsion for the configuration designed for all the experiments were limited to 8 cm. The electrode configuration was then optimized for the spacing between
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them. During initial period, the water separation increases linearly with time until it reaches a plateau beyond which there was no water separation. In the first mode with insulated vessel, with one electrode energized and the other grounded, as the spacing between the electrodes increases from 2 cm to 6 cm, the maximum attainable water separation increases from 10% to 95% and the time
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required for separation of water from the emulsion decreases from 15 minutes to around 8 minutes as seen in the Figure 5a. This indicates that the separation
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improves as the spacing between the electrodes increases. Also the rate of water separation and maximum attainable water separation increases linearly when we increase the spacing between the electrodes in case of insulated vessel as
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seen in the Figure 5b. On the other hand, for the second mode of operation with grounded vessel, as the spacing between the electrodes decreases from 6
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cm to 2 cm, the separation improves as shown in the Figure 6a. Also the rate of water separation and maximum attainable water separation increases with
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decrease in the spacing between the electrode as seen in the Figure 6b. Hence the optimum spacing for insulated modes was 6 cm and grounded mode was 2 cm. Reasons for these trends are discussed later in conjunction with COMSOL
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simulations.
Figure 7a and b show the spatial variation of electrode potential for both the
modes of operation (insulated as well as grounded) as obtained from COMSOL simulations. Figure 8a and b show the spatial variation of electrode potential for the depth of immersion of the electrodes of 3 cm and 8 cm for grounded mode.
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In the case of insulated mode, the electric potential calculated using COMSOL simulations is seen to vary gradually in the region between the energized and the ground electrode. In the case of grounded mode where both the electrodes were energized, the electric potential varies between the electrodes and the vessel. We define the effective volume under electric field (V ef f ) as the volume of
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emulsion in the vessel which contains dispersed water droplets which are exposed to a predefined range of electric field. This predefined range of electric field is essential for coalescence of minute droplets into bigger one so as to settle down with faster rate. The upper limit of the electric field was set as 1 kV/cm (nominal electric field used in all experiments) and the lower limit was set as
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0.3 kV/cm (the reason for this is explained later). This effective volume under an electric field (V ef f ) was obtained using the COMSOL simulations, and is
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used as a measure to correlate the water separation observed in experiments.
The correlation between V ef f and water separation is discussed below for the two modes of operation as a function of depth of immersion of electrodes and
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electrode spacing.
The electrocoalescence between droplets is known to be effective in a specific
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range of electric field. Too low fields result in very small rate of coalescence whereas very strong fields lead to non-coalescence of drops [15, 25]. In the
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present case the nominal electric field used was well below the non-coalescence limit. To quantify the relationship between the effective Volume of emulsion exposed to electric field (V ef f ), calculated using COMSOL simulations, with
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the experimental observation of separation of water from the emulsion, the lower limits on the electric field while calculating V ef f needs to be specified. Towards
this we varied the lower limit of the electric field from 0.1 kV/cm till 0.9 kV/cm. In COMSOL simulations, the V ef f was estimated and plotted for both the
modes of operation. For grounded mode, the V ef f for depth of immersion of
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electrodes in the liquid and spacing between the electrodes are shown in the Figures 9a and b. The figures show that a lower threshold of around 0.3 kV/cm gives a good correlation between the volume of emulsion exposed to electric field and the water separation. The similar correlation was found with the same lower limit of electric field for insulated vessel but is not shown for brevity.
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These results indicates that the minimum electric field require for significant rate of water separation can be assumed to be 0.3 kV/cm. To check the grid independency of the results, two different grids i.e. coarser and finer were tested and compared. The resulting value of V ef f was practically same in both the cases as shown in the Figure 9c.
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As an alternative measure to quantify the rate of water separation, we have
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propose the volume average electric field Eef f , defined by the equation | Eef f R | = (1/ V ef f ) V ef f EdV . The advantage of this measure over the V ef f is that it doesn’t need the lower threshold of the electric field for its estimation. The rate of water separation and maximum attainable water separation with
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respect to Eef f was plotted in the Figure 10a and b. Comparison of the two methods is shown in the Figure 11a, b and c. As seen from the Figure 11b
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which corresponds to insulated mode (Figure 3a), the electric filed is present in the space between the electrodes but there is practically no field between the
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electrodes and vessel. Hence with increase in the spacing between the electrodes, the region exposed to the field increases. Although the intensity of field decreases the overall effect is increase in Eef f with increase in the electrode spacing as
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shown in the Figure 11b. In the case of grounded mode(Figure 3b), the field is present between the electrodes and the vessel. Hence as electrodes spacing decreases the region exposed to the field increases. Although the intensity of field decreases, the overall effect is increase in Eef f with decrease in the electrode spacing as shown in the Figure 11c. It is seen that both the methods are
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promising and a fairly reasonable correlation of the water separated during dehydration with V ef f as well as Eef f was observed. Thus the electrostatic
calculations for various electrode configurations can be conducted to predict the coalescer performance. To understand the mechanism of shorting at low and high separation for the
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insulated and grounded cases respectively, experiments with a constant applied electric field equal to 1 kV/cm were then conducted in the two configurations. The variation in current with respect to time was observed by changing the spacing between the electrodes and its effect on separation time of water from water in oil emulsion.
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Figure 12a and b show the transient change of current at fixed electrode spacing of 6 cm and 2 cm in the insulated vessel configuration. After application
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of electric field to the emulsion, till about 3 minutes, a rise in the current to around Imax = 0.812 milli amperes was seen during which the water droplets get polarized and align themselves in the direction of the electric field as shown in
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the Figure 12a. After 3 minute and up to 4 minutes, a fall in current up to 22.8% of Imax was observed, the smaller droplets coalesce to form bigger droplets due
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to dipolar attraction. Droplets, that grow into a bigger size then settle down in the vessel due to gravity. The volume of water separated rises to around 80%
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in this regime. The corresponding fall in current can be attributed to the low water fraction in the oil in the coalescer. Another event of drop of current and subsequent water separation was observed and three such events were noted.
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Between 3 minutes to 10 minutes, the coalescence of bigger droplets and their subsequent gravity settling leads to complete separation of water. The current falls abruptly to 0.136 milli amperes indicating separation of water. After 10 minutes the current value remains almost constant at about 0.136 milli amperes and can be assumed to correspond to the conductivity of the de-watered oil. The
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occasional increase in the current in this period was characterized by no further discernible separation indicating chain formation that prevents coalescence and settling. For a configuration with spacing of 2 cm between the electrodes, the water
droplets that were polarized on application of electric field form a bridge between
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the two electrodes. Figure 12b shows that after applying electric field, there was a rise in current to a value of around Imax = 0.913 milliamperes in about 10 minutes, indicating aligning of water droplets, possible coalescence. Within a minute the current falls to 91.9% of Imax with corresponding water separation of around 10%. This was followed by a plateauing of current and separation for
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further 1 minute. We conjecture that since the electrodes were held very closed to each other, water chain formation takes place which leads to fluctuations in
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the current. The total volume of water separated was very less and even after 20 minutes the current value continues to be high and fluctuates in the same
range for almost next 15 minutes. The experiments were thereafter abandoned.
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Very similar results were observed in the grounded vessel configuration but
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are not shown for brevity.
4. Conclusions
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Experiments indicate that a maximum water separation can be obtained in minimum time when the depth of immersion of the electrodes was increased. However, for safe operation, there has to be a minimum gap of 1 cm between
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the electrodes and the oil-water interface since it leads to sparking otherwise. Also the study does not consider the effect of flow and it can differ the results. Comparing the performance of two modes of operations studied, the time
required for maximum water separation was less in the case of a grounded vessel. This was reassuring since most commercial designs use electrically grounded
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vessel configuration for treatment of water in crude oil emulsion. However, since most lab experiments are often conducted in glass and plastic vessels the results should be extrapolated to commercial electrocoalescers with caution. To achieve maximum water separation, the intensity of the applied nominal
field should be above a threshold of 0.3 kV/cm. However, at high applied
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field above 1.5 kV/cm, sparks are observed due to short-circuiting. Therefore, an electrocoalescer can be safely operated for fields in a range of 0.3 kV/cm to 1 kV/cm. Since our study shows that the water separation correlates well with the effective volume under electric field and the volume average electric field, this approach can enable exploring various electrode configuration using
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electrostatic calculation. Although, caution should be exercised with respect to water chaining.
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This study suggesting use of Eef f and V ef f can thus form the basis for designing electrostatic fields in large commercial electrocoalescers and possible reduction in number of expensive experiments required to optimize the water
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separation.
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Acknowledgments
The authors would like to acknowledge Department of Science and Technol-
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References
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ogy (DST), India for their financial support to this study.
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[23] Z. Liu, H. Wyss, A. Fernandez-Nieves, H. Shum, Dynamics of oppositely charged emulsion droplets., Physics of Fluids, 27 (2015) 082003 (2015).
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[24] M. Fortuny, C. Oliveira, R. Melo, M. Nele, R. Coutinho, A. Santos, Effect of salinity, temperature, water content, and ph on the microwave demulsi-
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fication of crude oil emulsions., Energy Fuels, 21 (2007) 1358–1364 (2007). [25] V. Anand, S. Roy, V. M. Naik, V. A. Juvekar, R. M. Thaokar, Electrocoalescence of a pair of conducting drops in an insulating oil., Journal of Fluid
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Mechanics, 859 (2019) 839–850 (2019).
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Table 1: Properties of the emulsion used in this work. Sunflower oil 922 46.5 4.9 7.62 x 10−5
Deionised water 1000 1 80 5.49
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Properties API/density(kg m−3 ) Kinematic Viscosity(mPa s) Dielectric constant Conductivity(µS m−1 )
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Figure 1: Size distribution of water-in-sunflower oil emulsion after homogenization where (a) Photograph of the initial emulsion before applying electric field (b) Initial drop size distribution before applying electric field.
Figure 2: Experimental set up is shown in figure a where 1. Single phase transformer 2. Vessel 3. Keithley multimeter and details of 2 in a is shown in more details in b
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Figure 3: Boundary conditions in the simulation for the defined geometry for different modes of operation a) Insulated vessel and b) Grounded vessel. In the figure φ represent electric potential and V is the potential of energized electrode.
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(a)
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Figure 4: Grounded vessel configuration: (a) Water separation with respect to the time at different depth of immersion of the electrodes in the emulsion at 1 kV/cm, where filled : 3 cm depth, filled : 4 cm depth, filled 4: 5 cm depth, filled 5: 6 cm depth, filled : 7 cm depth, filled /: 8 cm depth (b) Rate of water separation and maximum water separation with respect to different depth of immersion of the electrodes in the emulsion at 1 kV/cm.
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(b)
Figure 5: Insulated vessel configuration: (a) Water separation with respect to the time at different spacing between the electrodes immersed in the emulsion where filled : 2 cm spacing, filled : 3 cm spacing, filled 4: 4 cm spacing, filled 5: 5 cm spacing, filled : 6 cm spacing (b) Rate of water separation and maximum water separation with respect to different spacing between the two electrodes with 8 cm depth at 1 kV/cm. 22
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(b)
Figure 6: Grounded vessel configuration: (a) Water separation with respect to the time at different spacing between the electrodes immersed in the emulsion where filled : 2 cm spacing, filled : 3 cm spacing, filled 4: 4 cm spacing, filled 5: 5 cm spacing, filled : 6 cm spacing (b) Rate of water separation and maximum water separation with respect to different spacing between the two electrodes with 8 cm depth at 1 kV/cm.
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Figure 7: Boundary conditions in the simulation for the defined geometry from front view for different modes of operation a) Insulated vessel and b) Grounded vessel. In the figure φ represent electric potential and V is the potential of energized electrode.
Figure 8: Grounded configuration: 3D Electric potential Distribution obtained using COMSOL simulations for depth of immersion of electrodes a) 3 cm depth and b) 8 cm depth.
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Figure 9: Grounded vessel configuration: (a) Effective volume of emulsion under electric field (V ef f ) at different depth of immersion of the electrodes immersed in the emulsion with 2 cm as the spacing between the electrodes. (b) Effective volume of emulsion under electric field (V ef f ) at different spacing between the electrodes immersed in the emulsion at 8 cm depth of immersion of the electrodes in the emulsion. (c) Comparison between coarser and finer grids, where dotted line indicates coarser grid derived values and solid line indiactes finer grid derived values. Water separation obtained in experiments is shown using the symbols.
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Figure 10: Grounded vessel configuration: (a) Rate of water separation with respect to the volume average electric field (Eef f ) at different depth of immersion of the electrodes in the emulsion with 2 cm as the spacing between the electrodes. (b) Rate of water separation with respect to the volume average electric field (Eef f ) at different spacing between the electrodes at 8 cm depth of the electrodes immersed in the emulsion. 26
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(b)
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Figure 11: Comparison of two methods with water separation (a) V ef f and Eef f at different depth of immersion of the electrodes for both the modes with water separation. (b) V ef f and Eef f at different spacing between the electrodes for insulated mode with water sepa27 spacing between the electrodes ration. (c) V ef f and Eef f at different for grounded mode with water separation.
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Figure 12: Insulated vessel configuration: (a) RMS current and water separation for spacing 6 cm and depth 8 cm with respect to water separation time at 1 kV/cm electric field with Imax = 0.812 mA and Imin = 0.15 mA. (b) RMS Current drop and water separation for spacing 2 cm and depth 8 cm with respect to water separation time at 1 kV/cm electric field with Imax = 0.913 mA and Imin = 0.141 mA.
28
PII:
S0263-8762(19)30579-9
DOI:
https://doi.org/10.1016/j.cherd.2019.12.012
Reference:
CHERD 3935
To appear in:
Chemical Engineering Research and Design
Received Date:
1 March 2019
Revised Date:
1 November 2019
Accepted Date:
8 December 2019
Please cite this article as: Pramod P. Kothmire, Yogesh J Bhalerao, Vijay M Naik, Rochish M Thaokar, Vinay A Juvekar, Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions, (2019), doi: https://doi.org/10.1016/j.cherd.2019.12.012
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions Pramod P. Kothmirea,b , Yogesh J Bhaleraob , Vijay M Naika , Rochish M Thaokara , Vinay A Juvekara,∗ a Department
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of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai 400 076, India b School of Mechanical Engineering, MIT Academy of Engineering, Alandi, Pune 412105
Abstract
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The rate of coalescence in an electrocoalescer, which governs the residence time, sensitively depends upon the electrostatic conditions as well as the electrodes
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arrangement. An experimental study of a cylindrical electrocoalescer vessel for two different modes of operation is reported in this work. The design of the
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coalescer is optimized for the residence time and water separation. To justify the experimental findings and to unravel more physical insights, the single phase electrostatics is simulated using COMSOL Multiphysics software. The volume of
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the emulsion under the influence of 0.3 - 1.0 kV/cm electric field, calculated using COMSOL correlates well with the water separation. Moreover, the simultaneous measurement of current suggests an ordering, that leads to an increase in size of the droplets and settling of drops in the normal operation. On the other hand, chaining and shorting leads to surge in current at high fields and seems
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to prevent water separation.
Keywords: Electric field, Electrocoalescer, Electrode configuration, Electrostatic, COMSOL simulation
∗ Corresponding
author Email address: [email protected] (Vinay A Juvekar)
Preprint submitted to Chemical Engineering Research and Design
November 1, 2019
1. Introduction There are various techniques used for separation of water from a waterin-oil emulsion such as gravity sedimentation, centrifugation [1, 2], chemical demulsification [3, 4] ,pH adjustment and heat treatment [5, 6] etc. The most popular method used in an oil refinery to coalesce the water drops in commonly
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encountered crude-water (brine) emulsions is by the use of an electric field [7, 8]. The basic principle of an electrocoalescer is the coalescence of water droplets,
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resulting in bigger sized drops with higher settling velocity, thereby leading to faster separation of water from the emulsion. Several possible mechanisms for
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electrocoalescence have been identified [9], based on the dipolar attraction of drops in electric field.
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Coalescence between two droplets can be improved by the force generated by electric fields. Primarily, there are three types of electrostatic forces between droplets (i)dipolar attraction (ii) electrophoresis and (iii)dielectrophoresis. Dipo-
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lar attraction is the electrostatic attraction force between two polarized water droplets while electrophoresis is the motion of a charged droplet (where the charge is often acquired after contact with an electrode) under a uniform electric
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field. Dielectrophoresis is the movement of polarized, uncharged water droplets in a non-uniform electric field with the movement towards the direction of convergence of the field. There are several types of spatio-temporal electric fields that may be used to enhance the coalescence of water droplets. These include the AC (alternating current) field, DC (direct current) field or a combination of
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AC and DC fields. The DC electric field [10] is commonly used to treat emulsions with low water content [11]. Unlike with the DC field, the functioning of an electrocoalescer is safe even at high water content under an AC electric field [12]. AC fields are also reported to be more effective in increasing the coalescence rate than pulsed DC fields [13].
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The presence of an electric field promotes contact between drops, enhancing drop-drop and drop-interface coalescence [14, 15]. The spatial electric field distribution in an electrocoalescer [16] and electrode configuration plays an important role in effective coalescence. The phenomena such as water chain formation [17] and short circuiting [11] may occur if electrodes are not properly configured
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inside an electrocoalescer. Taylor[18] proposed a two-step mechanism of chain formation and coalescence for water-in-oil dispersions, influenced by a number
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of factors such as oil phase viscosity, disperse phase volume and the applied
electric field strength. Under high electric field and insufficient spacing between the electrodes, or between the electrodes and the vessel, there is often observed
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formation of chains of water droplets, aligned in the direction of maximum field strength. Moreover, the chains do not always start or finish at an electrode and
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may not be in contact with the electrodes. Pearce[19] suggested that chains are formed by forces due to the potential difference between drops as a result of
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their induced charges. High aqueous phase emulsions tend to short-circuit the electrodes and significantly reduce the electric field[11]. Major stress in the literature is on interaction between two drops under
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electric field leading to coalescence or non-coalescence [20–23]. Researchers have also studied effect of specific design and operating parameters such as type of electric field (AC/DC), intensity of electric field, frequency, waveform, geometry of electrodes[10–13], salinity, temperature, water content, and ph [24]. When all these factors comes together, the complexity of the problem doesn’t allow one
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to estimate the efficiency of coalescer. Numerical simulation allow us to take into account the various complexity in actual coalescer and helps to estimate the distribution of electric field and the rate of water separation. It will act as a good tool to design provided we can correlate the simulation results with actual performance of coalescer. The simulation can then act as a predictive
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as well as optimization tool for design of coalescer. Such kind of approach which relates electric field obtain from simulation to rate of water separation has not been reported so far in the literature. This form the focused of the present work. In this work, we have optimised the electrocoalescence process with respect to i) electrode configuration (spacing between the electrodes and
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depth of immersion of the electrodes in the emulsion) ii) the electric state of the vessel and electrodes (grounded vs insulated). In general, there exists an optimal
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range of electric field in which the rate of coalescence is sufficiently high. If the
field is lower than the lower limit of the optimal range, the rate of coalescence is poor due to weak electrostatic forces among the droplets. On the other hand,
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if the field is above the upper limit of the optimal range, the coalescence is poor because of the onset of the phenomenon known as non coalescence. Since
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in an electrocoalescer the electric field varies with location, there exists two regions, one in which the field is in the optimal range and the remaining region
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in which the field is either above the upper limit or below the lower limit of the optimal range. In this work we have experimentally determined the optimal range of the electric field in which coalescence rate is reasonable high. We have
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also found using COMSOL Multiphysics simulations, the regions within the electrocoalescer vessel in which electric field lies in the optimal range. We have then correlated experimentally obtained rate of water separation and the volume of region of the optimal electric field from simulations. This work will provide us a design procedure for electrocoalescer to optimize the electrode configuration
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and electrostatics so as to maximize the region of optimal electric field and thereby maximize the rate of water separation. We have also measured the variation in current during the experiment and
related it with progress of coalescence. Chaining of the drops is a prelude to their a coalescence. Chaining results in increase in the current because it provides
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a conducting path. Therefore measurement of current allow us to quantify the extent of chaining. If current is low chaining is poor and which also result in low rate of coalescence. On the other hand if the current is high it implies severe chaining which result in short circuiting and drop in the field which results in poor coalescence. We have therefore shown that the current in the system can
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be used as a diagnostic tool to study the progress of coalescence as well as to
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quantify the efficiency of coalescence.
2. Experimental
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2.1. Preparation of emulsion
500 ml water-in-sunflower oil emulsion was prepared by mixing 90% sun-
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flower oil (the continuous phase) and 10% deionized water (the dispersed phase) using a homogenizer (ultraturrex) at 10000 rpm for 5 minutes without adding any surfactant or demulsifier. The stator and rotor were immersed at a depth
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of 9.5 cm in the emulsion to avoid air suction. Table 1 summarizes the physical properties of the oil used in the experiment. Liquid viscosity was measured using the Physica MCR 301, Anton Paar Rheometer, while the density was
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measured using a DDM 2909 densitometer. The conductivities of dispersed and continuous phases were measured using a conductivity meter. The emulsions were judged to be stable when no changes were observed for 4 hours. The initial emulsion drop size distribution is shown in Figure 1. The Figure shows that most of the drops are segregated from each other. Moreover, they are small in
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size and uniformly distributed. The drop size distribution is in the range of 5 to 10 µm as shown in Figure 1. The droplet diameter was measured by Image J software. The same mixture was reemulsified to conduct three independent experimental runs. For the fourth trial, a fresh emulsion was prepared using the same proportion and technique. The room temperature throughout the
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experiment was controlled at 20±2◦ C using a room air-conditioner. 2.2. Experimental setup The electrocoalescer was a cylindrical borosilicate glass vessel of diameter 8 cm and the height of 10 cm as shown in Figure 2. The vessel top was covered
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with a circular acrylic plate. The circular plate was provided with two holes, slightly greater than the electrode diameter. The top plates with different spac-
ing between the holes were used for varying the spacing between the electrodes.
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The electrodes were two copper rods of 0.2 cm diameter and 14 cm length. 50Hz sinusoidal, AC electric field was applied using a custom made single phase trans-
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former with a rating 0-20 kV and 0-50 mA. The experiments were performed by using two modes. In the first mode of operation, the vessel was insulated
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with one energized electrode and the other grounded electrode. Insulated vessel was realized by keeping the boro-silicate glass vessel (an insulator) on a wooden
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table. This mode will be henceforth termed as ”insulated mode”. In the second mode of operation, a grounded metallic wire mesh was inserted around the inner wall of the circumference of the vessel and both the electrodes were energized.
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This mode will be henceforth termed as ”grounded mode”. In case of insulated mode, the nominal electric field is given by E= ∆V/X and E= ∆V/((diameter of the vessel-X)/2) in case of grounded mode where V is applied voltage and X is the electrode spacing. In both the modes of operation, the experiments were performed by changing the spacing between the two electrodes (X), the
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depth of electrodes immersed in the emulsion (Y) as shown in the Figure 2b and the electric field. The estimated depth of the oil-water interface, after complete separation of water was 9 cm from the top liquid surface. Hence the metallic wire mesh was inserted in such a way that the lower end of the wire was 8cm from the top liquid surface so that the separated pool of water will never touch the wire mesh. Current was measured using a Keithley multimeter Model 8605
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connected in series with the vessel on the lower voltage side. 2.3. COMSOL Simulation The electrostatics in the electrocoalescer vessel was simulated by using the Electrostatic AC/DC module in COMSOL multiphysics software (version 3.5a)
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for two different modes of operation by solving the relevant electrostatic equations with appropriate boundary conditions (Figure 3). The COMSOL Mul-
tiphysics software package is based on multidimensional (three-dimensional, in
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this case) finite element method. Although this technique is computationally
more intensive than the other techniques such as finite volume and finite dif-
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ference methods (because of the higher dimensionality of the problem), the automatic mesh generation program, powerful equation solvers and the graphi-
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cal interface of the software makes it easy to solve the problem and also assess various details of the solution. In each mode of operation, the electric field dis-
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tribution throughout the vessel was determined by solving the Gauss equation and the resulting profile of electric field distribution was regarded time variant and stored for later use. The boundary conditions determine the electrical vari-
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ables on the boundaries of the physical model. The Initial Values node adds an initial value for the electric potential V that can serve as an initial condition for a transient simulation or as an initial guess for a nonlinear solver. The value or expression for the initial value of the electric potential is given by V (SI unit: V) with a default value of 0 V. The Zero Charge node adds the condition that
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there is zero charge on the boundary so that n D = 0. This boundary condition is also applicable at symmetry boundaries where the potential is known to be symmetric with respect to the boundary. This is the default boundary condition at exterior boundaries. At interior boundaries, it means that no displacement field can penetrate the boundary and that the electric potential is discontinuous across the boundary. The Ground node implements ground (zero potential) as
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the boundary condition V = 0, implying zero potential on the boundary. This boundary condition is also applicable at symmetry boundaries where the potential is known to be anti symmetric with respect to the boundary. Boundary conditions on upper acrylic circular plate was kept floating (no charge). Since the wire mesh was kept 1 cm above the oil-water interface hence boundary con-
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dition to the oil-water interface was also kept floating. For simulations using COMSOL, it was more convenient to cast the problem in the cylindrical coordi-
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nates. We consider a cylindrical region of radius R and height H, enclosing the
cylindrical electrodes with different spacing (X) and the depth of an electrode (L) immersed in the main cylinder as shown in Figure 3. The curved surface
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of the cylinder forms a boundary of the domain. The dimensionless potential
coordinates ∇2 φˆ= 0.
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in the electrolyte, denoted by φ satisfies the Laplace equation in cylindrical
The geometry was discretized into simple triangular finer grids to ensure
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that results were independent of the mesh size. Electric potential was applied between the vessel and the electrodes as per the modes of operation discussed in the experimental set up. The nominal applied electric fields in the simula-
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tions were kept at values between 0.1 to 1.0 kV/cm at intervals of 0.1 kV/cm. The simulation model so developed was used to determine the effective volume under different values of an electric field for both the configuration either by changing the spacing between the electrodes or the depth of an electrodes inserted in the emulsion. The simulation results were then related to the ex-
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perimental findings. It should be noted that the electrostatic calculation was only for the oil as medium and the contribution of the dispersed aqueous phase was completely ignored. Thus the water droplets were not considered in the electrostatics calculations.
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3. Results and discussion We studied the performance of the electrocoalescer for two modes of operations (insulated and grounded). In all operations the nominal electric field was kept constant at 1 kV/cm. Figure 4(a) shows the percentage of water separation with respect to time for different depths of immersion of electrodes (Y)
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for grounded mode. During the initial period, the water separation increases linearly with time until it reaches a plateau beyond which there was no further
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water separation. Reasons for these trends are discussed later in conjunction with COMSOL simulations. As the depth of immersion of electrodes was in-
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creased from 3 cm to 8 cm in steps of 1 cm, the maximum attainable water separation increases from 15% to 95%. Moreover, the rate of water separation
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and the maximum attainable water separation increases linearly with respect to an increase in the depth of immersion of the electrodes as seen in Figure 4(b). At depth of 8 cm, as seen from the Figure 4a, the water separation was high-
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est and the separation time was lowest. Similar observations were made when the experiments were conducted in the insulated mode but are not shown for brevity.
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When the depth of immersion of electrodes was 9 cm, there was less water
separation of around 50% and 60% in case of insulated and grounded mode respectively. Moreover, fluctuations in current and visible arcing inside the vessel was observed (discussed later) rendering the operation unsafe. Based on the calculation of the interface level, we found that, at a depth of immersion of
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electrodes around 9 cm, the tip of the electrode would touch the oil-water interface after complete separation of water. Hence the depth of immersion of the electrodes in the emulsion for the configuration designed for all the experiments were limited to 8 cm. The electrode configuration was then optimized for the spacing between
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them. During initial period, the water separation increases linearly with time until it reaches a plateau beyond which there was no water separation. In the first mode with insulated vessel, with one electrode energized and the other grounded, as the spacing between the electrodes increases from 2 cm to 6 cm, the maximum attainable water separation increases from 10% to 95% and the time
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required for separation of water from the emulsion decreases from 15 minutes to around 8 minutes as seen in the Figure 5a. This indicates that the separation
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improves as the spacing between the electrodes increases. Also the rate of water separation and maximum attainable water separation increases linearly when we increase the spacing between the electrodes in case of insulated vessel as
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seen in the Figure 5b. On the other hand, for the second mode of operation with grounded vessel, as the spacing between the electrodes decreases from 6
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cm to 2 cm, the separation improves as shown in the Figure 6a. Also the rate of water separation and maximum attainable water separation increases with
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decrease in the spacing between the electrode as seen in the Figure 6b. Hence the optimum spacing for insulated modes was 6 cm and grounded mode was 2 cm. Reasons for these trends are discussed later in conjunction with COMSOL
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simulations.
Figure 7a and b show the spatial variation of electrode potential for both the
modes of operation (insulated as well as grounded) as obtained from COMSOL simulations. Figure 8a and b show the spatial variation of electrode potential for the depth of immersion of the electrodes of 3 cm and 8 cm for grounded mode.
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In the case of insulated mode, the electric potential calculated using COMSOL simulations is seen to vary gradually in the region between the energized and the ground electrode. In the case of grounded mode where both the electrodes were energized, the electric potential varies between the electrodes and the vessel. We define the effective volume under electric field (V ef f ) as the volume of
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emulsion in the vessel which contains dispersed water droplets which are exposed to a predefined range of electric field. This predefined range of electric field is essential for coalescence of minute droplets into bigger one so as to settle down with faster rate. The upper limit of the electric field was set as 1 kV/cm (nominal electric field used in all experiments) and the lower limit was set as
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0.3 kV/cm (the reason for this is explained later). This effective volume under an electric field (V ef f ) was obtained using the COMSOL simulations, and is
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used as a measure to correlate the water separation observed in experiments.
The correlation between V ef f and water separation is discussed below for the two modes of operation as a function of depth of immersion of electrodes and
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electrode spacing.
The electrocoalescence between droplets is known to be effective in a specific
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range of electric field. Too low fields result in very small rate of coalescence whereas very strong fields lead to non-coalescence of drops [15, 25]. In the
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present case the nominal electric field used was well below the non-coalescence limit. To quantify the relationship between the effective Volume of emulsion exposed to electric field (V ef f ), calculated using COMSOL simulations, with
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the experimental observation of separation of water from the emulsion, the lower limits on the electric field while calculating V ef f needs to be specified. Towards
this we varied the lower limit of the electric field from 0.1 kV/cm till 0.9 kV/cm. In COMSOL simulations, the V ef f was estimated and plotted for both the
modes of operation. For grounded mode, the V ef f for depth of immersion of
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electrodes in the liquid and spacing between the electrodes are shown in the Figures 9a and b. The figures show that a lower threshold of around 0.3 kV/cm gives a good correlation between the volume of emulsion exposed to electric field and the water separation. The similar correlation was found with the same lower limit of electric field for insulated vessel but is not shown for brevity.
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These results indicates that the minimum electric field require for significant rate of water separation can be assumed to be 0.3 kV/cm. To check the grid independency of the results, two different grids i.e. coarser and finer were tested and compared. The resulting value of V ef f was practically same in both the cases as shown in the Figure 9c.
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As an alternative measure to quantify the rate of water separation, we have
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propose the volume average electric field Eef f , defined by the equation | Eef f R | = (1/ V ef f ) V ef f EdV . The advantage of this measure over the V ef f is that it doesn’t need the lower threshold of the electric field for its estimation. The rate of water separation and maximum attainable water separation with
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respect to Eef f was plotted in the Figure 10a and b. Comparison of the two methods is shown in the Figure 11a, b and c. As seen from the Figure 11b
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which corresponds to insulated mode (Figure 3a), the electric filed is present in the space between the electrodes but there is practically no field between the
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electrodes and vessel. Hence with increase in the spacing between the electrodes, the region exposed to the field increases. Although the intensity of field decreases the overall effect is increase in Eef f with increase in the electrode spacing as
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shown in the Figure 11b. In the case of grounded mode(Figure 3b), the field is present between the electrodes and the vessel. Hence as electrodes spacing decreases the region exposed to the field increases. Although the intensity of field decreases, the overall effect is increase in Eef f with decrease in the electrode spacing as shown in the Figure 11c. It is seen that both the methods are
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promising and a fairly reasonable correlation of the water separated during dehydration with V ef f as well as Eef f was observed. Thus the electrostatic
calculations for various electrode configurations can be conducted to predict the coalescer performance. To understand the mechanism of shorting at low and high separation for the
12
insulated and grounded cases respectively, experiments with a constant applied electric field equal to 1 kV/cm were then conducted in the two configurations. The variation in current with respect to time was observed by changing the spacing between the electrodes and its effect on separation time of water from water in oil emulsion.
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Figure 12a and b show the transient change of current at fixed electrode spacing of 6 cm and 2 cm in the insulated vessel configuration. After application
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of electric field to the emulsion, till about 3 minutes, a rise in the current to around Imax = 0.812 milli amperes was seen during which the water droplets get polarized and align themselves in the direction of the electric field as shown in
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the Figure 12a. After 3 minute and up to 4 minutes, a fall in current up to 22.8% of Imax was observed, the smaller droplets coalesce to form bigger droplets due
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to dipolar attraction. Droplets, that grow into a bigger size then settle down in the vessel due to gravity. The volume of water separated rises to around 80%
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in this regime. The corresponding fall in current can be attributed to the low water fraction in the oil in the coalescer. Another event of drop of current and subsequent water separation was observed and three such events were noted.
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Between 3 minutes to 10 minutes, the coalescence of bigger droplets and their subsequent gravity settling leads to complete separation of water. The current falls abruptly to 0.136 milli amperes indicating separation of water. After 10 minutes the current value remains almost constant at about 0.136 milli amperes and can be assumed to correspond to the conductivity of the de-watered oil. The
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occasional increase in the current in this period was characterized by no further discernible separation indicating chain formation that prevents coalescence and settling. For a configuration with spacing of 2 cm between the electrodes, the water
droplets that were polarized on application of electric field form a bridge between
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the two electrodes. Figure 12b shows that after applying electric field, there was a rise in current to a value of around Imax = 0.913 milliamperes in about 10 minutes, indicating aligning of water droplets, possible coalescence. Within a minute the current falls to 91.9% of Imax with corresponding water separation of around 10%. This was followed by a plateauing of current and separation for
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further 1 minute. We conjecture that since the electrodes were held very closed to each other, water chain formation takes place which leads to fluctuations in
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the current. The total volume of water separated was very less and even after 20 minutes the current value continues to be high and fluctuates in the same
range for almost next 15 minutes. The experiments were thereafter abandoned.
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Very similar results were observed in the grounded vessel configuration but
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are not shown for brevity.
4. Conclusions
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Experiments indicate that a maximum water separation can be obtained in minimum time when the depth of immersion of the electrodes was increased. However, for safe operation, there has to be a minimum gap of 1 cm between
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the electrodes and the oil-water interface since it leads to sparking otherwise. Also the study does not consider the effect of flow and it can differ the results. Comparing the performance of two modes of operations studied, the time
required for maximum water separation was less in the case of a grounded vessel. This was reassuring since most commercial designs use electrically grounded
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vessel configuration for treatment of water in crude oil emulsion. However, since most lab experiments are often conducted in glass and plastic vessels the results should be extrapolated to commercial electrocoalescers with caution. To achieve maximum water separation, the intensity of the applied nominal
field should be above a threshold of 0.3 kV/cm. However, at high applied
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field above 1.5 kV/cm, sparks are observed due to short-circuiting. Therefore, an electrocoalescer can be safely operated for fields in a range of 0.3 kV/cm to 1 kV/cm. Since our study shows that the water separation correlates well with the effective volume under electric field and the volume average electric field, this approach can enable exploring various electrode configuration using
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electrostatic calculation. Although, caution should be exercised with respect to water chaining.
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This study suggesting use of Eef f and V ef f can thus form the basis for designing electrostatic fields in large commercial electrocoalescers and possible reduction in number of expensive experiments required to optimize the water
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separation.
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Acknowledgments
The authors would like to acknowledge Department of Science and Technol-
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References
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ogy (DST), India for their financial support to this study.
References
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Table 1: Properties of the emulsion used in this work. Sunflower oil 922 46.5 4.9 7.62 x 10−5
Deionised water 1000 1 80 5.49
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Properties API/density(kg m−3 ) Kinematic Viscosity(mPa s) Dielectric constant Conductivity(µS m−1 )
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Figure 1: Size distribution of water-in-sunflower oil emulsion after homogenization where (a) Photograph of the initial emulsion before applying electric field (b) Initial drop size distribution before applying electric field.
Figure 2: Experimental set up is shown in figure a where 1. Single phase transformer 2. Vessel 3. Keithley multimeter and details of 2 in a is shown in more details in b
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Figure 3: Boundary conditions in the simulation for the defined geometry for different modes of operation a) Insulated vessel and b) Grounded vessel. In the figure φ represent electric potential and V is the potential of energized electrode.
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Figure 4: Grounded vessel configuration: (a) Water separation with respect to the time at different depth of immersion of the electrodes in the emulsion at 1 kV/cm, where filled : 3 cm depth, filled : 4 cm depth, filled 4: 5 cm depth, filled 5: 6 cm depth, filled : 7 cm depth, filled /: 8 cm depth (b) Rate of water separation and maximum water separation with respect to different depth of immersion of the electrodes in the emulsion at 1 kV/cm.
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Figure 5: Insulated vessel configuration: (a) Water separation with respect to the time at different spacing between the electrodes immersed in the emulsion where filled : 2 cm spacing, filled : 3 cm spacing, filled 4: 4 cm spacing, filled 5: 5 cm spacing, filled : 6 cm spacing (b) Rate of water separation and maximum water separation with respect to different spacing between the two electrodes with 8 cm depth at 1 kV/cm. 22
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Figure 6: Grounded vessel configuration: (a) Water separation with respect to the time at different spacing between the electrodes immersed in the emulsion where filled : 2 cm spacing, filled : 3 cm spacing, filled 4: 4 cm spacing, filled 5: 5 cm spacing, filled : 6 cm spacing (b) Rate of water separation and maximum water separation with respect to different spacing between the two electrodes with 8 cm depth at 1 kV/cm.
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Figure 7: Boundary conditions in the simulation for the defined geometry from front view for different modes of operation a) Insulated vessel and b) Grounded vessel. In the figure φ represent electric potential and V is the potential of energized electrode.
Figure 8: Grounded configuration: 3D Electric potential Distribution obtained using COMSOL simulations for depth of immersion of electrodes a) 3 cm depth and b) 8 cm depth.
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Figure 9: Grounded vessel configuration: (a) Effective volume of emulsion under electric field (V ef f ) at different depth of immersion of the electrodes immersed in the emulsion with 2 cm as the spacing between the electrodes. (b) Effective volume of emulsion under electric field (V ef f ) at different spacing between the electrodes immersed in the emulsion at 8 cm depth of immersion of the electrodes in the emulsion. (c) Comparison between coarser and finer grids, where dotted line indicates coarser grid derived values and solid line indiactes finer grid derived values. Water separation obtained in experiments is shown using the symbols.
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Figure 10: Grounded vessel configuration: (a) Rate of water separation with respect to the volume average electric field (Eef f ) at different depth of immersion of the electrodes in the emulsion with 2 cm as the spacing between the electrodes. (b) Rate of water separation with respect to the volume average electric field (Eef f ) at different spacing between the electrodes at 8 cm depth of the electrodes immersed in the emulsion. 26
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Figure 11: Comparison of two methods with water separation (a) V ef f and Eef f at different depth of immersion of the electrodes for both the modes with water separation. (b) V ef f and Eef f at different spacing between the electrodes for insulated mode with water sepa27 spacing between the electrodes ration. (c) V ef f and Eef f at different for grounded mode with water separation.
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Figure 12: Insulated vessel configuration: (a) RMS current and water separation for spacing 6 cm and depth 8 cm with respect to water separation time at 1 kV/cm electric field with Imax = 0.812 mA and Imin = 0.15 mA. (b) RMS Current drop and water separation for spacing 2 cm and depth 8 cm with respect to water separation time at 1 kV/cm electric field with Imax = 0.913 mA and Imin = 0.141 mA.
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