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Evaluation of Microwave and Conventional Heating for Electrostatic Treatment of a Water-in-Oil Model Emulsion in a Pilot Plant Troner Assenheimer,† Alessandro Barros,† Khalil Kashefi,† José Carlos Pinto,§ Frederico Wanderley Tavares,†,‡ and Márcio Nele*,†,‡ †

Programa de Pós-Graduaçaõ em Tecnologia de Processos Químicos e Bioquímicos, Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, CEP 21949-900 Rio de Janeiro, Rio de Janeiro, Brazil ‡ Departamento de Engenharia Química, Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, CEP 21949-900 Rio de Janeiro, Rio de Janeiro, Brazil § COPPE, Universidade Federal do Rio de Janeiro, CEP 21941-972 Rio de Janeiro, Rio de Janeiro, Brazil ABSTRACT: The efficiency of electrostatic coalescence coupled with microwave heating in separation of water-in-oil (W/O) model emulsions was evaluated. A series of experiments were performed in a continuous pilot plant where a W/O emulsion can be treated by application of electric field, microwave heating, conventional heating, or a combination of these techniques. The separation efficiencies in the two process configurations were evaluated by measuring the water content of treated model emulsion. The combination of electrocoalescence and microwave showed better separation results, in comparison to conventional heating combination with electrocoalescence. The influence of four operational variables on the water contents of the treated emulsions were studied: salt concentration, flow rate, the electric field between electrodes, and water cut at the inlet were evaluated. It was observed that a lower flow rate (higher residence time) helped in reducing the final water content and that a higher salt content resulted in worse separation efficiency. The microwave heating showed to be an attractive alternative to conventional heating, particularly when the electrocoalescer is under operational stress. viewpoints,8−10 to treat an W/O emulsion. The use of a large amount demulsifiers to modify interfacial properties to promote separation leads to additional problems to remove the demulsifiers from the oil and aqueous phases. The use of pH adjustment is not effective to break water-in-oil emulsions. Gravitational settlers are large, and for small droplets, they have low efficiency, leading to centrifugation, which has a high operational cost. The heat treatment reduces the viscosity of the oil, helping the water droplet fall faster. Nevertheless, heat treatment and chemical treatment are expensive and heating results in high fuel consumption.8 The treatment of a crude oil usually starts with heating the fluid using the conventional heating and the addition demulsifiers (chemical treatment), before pumping the emulsion to the separation tanks (gravitational separation) and to the electrostatic coalescers.6 The heating stage in the treatment of crude oil causes a reduction in viscosity, thickness, and cohesion of the film surrounding the water droplets accelerating the coalescence. Higher temperature also reduces the viscosity of continuous phase helping the coalesced water droplets to settle quicker.11 In the conventional heating method, using heat exchangers, the energy is transferred to the fluid through heat transfer mechanisms such as convection, conduction, and radiation from a hot surface. Resulting in a heating limited by the temperature of that surface in addition to the physical

1. INTRODUCTION In crude oil production, water is almost always present; therefore the formation of water-in-oil (W/O) emulsions is customary. Crude oils contain inorganic particles accompanied by interfacial active organic molecules (e.g., asphaltenes and naphthenic acids) that with the existence of water droplets can construct mechanically strong viscoelastic films resulting in a stable emulsions.1,2 Emulsions can be formed at different stages of petroleum production such as a reservoir, well string, chock valves, and pipelines due to high shear rates or changes in pressure and temperature. The presence of water−oil emulsions brings some operational and technical problems. In addition, the final products should have a low water content to meet the required specification. There are several methods to break those rigid viscoelastic films, for instance, chemical demulsification, gravity or centrifugal settling, pH adjustment, filtration, heat treatment, membrane separation, electrostatic demulsification, and the combination of electrocoalescing with other technologies such as hydrocyclones, low-frequency ultrasound, and microwave radiation.3−7 The most common method in the oil industry for the separation of W/O emulsions is an electrostatic treatment which consists of the application of an external electrical field to a crude oil W/O emulsion.3 By increasing polarization of water droplets, the electrical field can promote the coalescence of the aqueous phase to improve the separation. The last aim of electrostatic demulsification is to take full advantage of the Stokes’ law10 by the settlement of the large droplets. It is reported that the electrostatic dehydration technique is the most effective process, from the environmental and energy © XXXX American Chemical Society

Received: January 27, 2017 Revised: April 28, 2017

A

DOI: 10.1021/acs.energyfuels.7b00275 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Properties of the Fluids variable

system

units

Exxsol/water interfacial tensiona

Exxsol + deionized water Exxsol + brine (1.75 wt % NaCl) Exxsol + brine (3.5 wt % NaCl) Exxxol (0.5 wt % Span 80 and AOT) + deionized water Exxsol (0.5 wt % Span 80 and AOT) + brine (1.75 wt % NaCl) Exxsol (0.5 wt % Span 80 and AOT) + (3.5 wt % NaCl) Exxsol deionized water brine (1.75 wt % NaCl) brine (3.5 wt % NaCl) Exxsol deionized water Exxsol deionized water

mN/m

conductivitya

dynamic viscosityb densityb a

nS/m

mPa s g/cm3

42.54 36.20 36.72 5.26 0.32 0.02 0.14 1.28 21,500 39,000 3.16 0.59 0.81 0.99

Room temperature. bCalculated at 45 °C.

Figure 1. Pilot plant process flow diagram (T, tank; DC, electrostatic treatment cell; H, heater; MP, dynamic mixer; MW, microwave; SP, sampler; P, pump; CV, control valve; GB, globe valve; RV, check valve; TV, three-way valve; FIT, flow indicator transmitter; PCV. pressure control valve; PI, pressure indicator; PIC, pressure indicator controller; PIT, pressure indicator transmitter; TI, temperature indicator; TIT, temperature indicator transmitter).

compared to the oil phase, the dielectric heating in the emulsion occurs predominantly in the water droplets.12 Many works about microwave demulsification have shown that the selective heating of the water droplet can improve the separation of water-in-oil emulsions. Ferreira et al.6 observed that the separation efficiency of water using microwave heating is higher than with conventional heating, and claimed that a higher temperature is obtained in the water droplet, which reduces the strength of the interfacial layer and helps the drainage of the interfacial film, providing a better water−oil emulsion destabilization. In addition, the application of microwave heating has shown advantages over conventional heating in other studies. Recently Lam et al.17 have shown the microwave pyrolysis advantages over conventional pyrolysis techniques that use traditional thermal heat sources in transforming waste materials (e.g., diesel, palm oil, and engine oil) into potential fuel products.18,19

properties of the emulsion being heated such as the density, heat conductivity, and heat capacity.12,13 In contrast, microwave energy is delivered directly to the material through molecular interaction with the electromagnetic field.14 The heating effect arises from the interaction of electromagnetic wave with the molecular dipoles by a phenomenon known as dielectric heating, where the heating comes from friction due to rapid dipole reorientation in the irradiated material. By such effect, heat is generated within the target emulsion rather than from an external source, thereby giving potentially a more efficient heating process compared to conventional surface heating, with respect to a uniform distribution of the heat.16 In addition, high temperatures and heating rates can be obtained through microwave heating, and it shows a high conversion efficiency of electrical energy into heat (80−85%).12,13,15 Due to its high dielectric constant, B


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Energy & Fuels Eskandari et al.20 presented the benefits of microwave heating in enhanced oil recovery. Microwave heating has been considered as a method to be combined with electrocoalescence in order to perform the heating stage. Some researchers have studied microwave heating, and it has been compared with conventional thermal heating;5,7,14,21−33 however, all works were performed in bench scale. Besides, as far as we know, the combination of heating process (conventional or microwave) with electrocoalescence techniques in order to evaluate the dehydration efficiencies of different process arrangements has never been the subject of any study. In this study, with the application of pilot plant equipment, the efficiency of the electrocoalescence process in dehydration of water−oil model emulsions was evaluated. The pilot plant has the capabilities of making consistent emulsions continuously and controlling the temperature and pressure in wide ranges of operation. Two main process arrangements, first the conventional heating-electrocoalescence and then the microwave heating-electrocoalescence processes were employed to carry out this study. In addition, a systematic experimental design was proposed to investigate the influence of four variables as inlet water content, salt concentration, flow rate, and electric field intensity during the mentioned demulsification processes. The outlet water content was considered as the response value to investigate the dehydration efficiencies in two process arrangements.

The apparatus measures the transmitted and backscattered light that is related to the concentration and size of water droplets present in the sample tube. The measure is carried out in a cylindrical cuvette that is scanned along its height in function of time. Thus, it is possible to detect changes in particle size (flocculation and coalescence) and phase separation (sedimentation and clarification, in the case of the dispersed phase, are denser than the continuous phase). The response of the device is displayed via reflected light flux percentage (backscattering) as a function of the sample height of the tube (mm) curves.35 In this study, only the backscattered light profile was evaluated; the emulsions were opaque and presented zero light transmission. All emulsion stability tests were performed at 25 °C. The determination of the average droplet diameter was performed by optical microscopy. The images of the emulsions were obtained from an inverted optical microscope (Carl Zeiss Axiovert 40 MAT) equipped with a 1.4 megapixel camera (Axiocam MRc) and a computer (Axiovision software (4.8.1)). The technique used was the bright field with transmitted light, which does not use filters and the properties of light are not changed. The calculation of the average droplet size was carried out by ImageJ, which is a software for processing and analyzing the images, developed by Wayne Rasband at the National Institute of Mental Health, USA, in Java. By this software, it is possible to observe, edit, analyze, process, save, and print the 8-, 16-, and 32-bit images. In ImageJ, the calculation of areas is done by pixel count from the sections selected by the user or by a specific algorithm. In this work, to calculate the average diameter, at least 200 droplets were used and for each emulsion the average was calculated with three replicates. The water content in the oil phase was measured by volumetric Karl Fischer titration method on Karl Fischer equipment 836 Titrando, from Metrohm. The interfacial tension (IFT) of phases was measured in a tensiometer K100, manufactured by Kruss using the Wilhelmy plate method. Measurements were performed in triplicate at room temperature. The duration of experiments was set to 3 h and the mean IFT values were calculated from the interval of 1 h 40 min to 3 h. The viscosity and density of the fluids were measured in a SVM 3000 (Anton Paar) instrument. The measurements were performed in triplicate at 20, 40, and 60 °C. To calculate the density and the viscosity at 45 °C, the same procedure described by Bergman and Sutton36 was applied. The conductivity of the continuous and dispersed phases of the emulsions was determined by using two digital conductivity meters: one manufactured by Gehaka (model GC 2000) and the other by EMCEE Electronics (model 1152). The choice of equipment was related to the measurement range of conductivity. Measurements were made at room temperature (around 25 °C). Table 1 shows the properties of the fluids used in this series of experiments. 2.3. Pilot Plant. The pilot plant process has two storage tanks, one for water (T-01) and the other one for oil (T-02). Two piston pumps (high-pressure dosing pump, Omel) are employed to flow the water (P-01) and the oil (P-02) through the process. The water feed line is equipped with one Coriolis flowmeter (FIT-01) manufactured by Micro Motion, Emerson, to measure the water mass flow rate. Where the water and oil pipes join, there is another Coriolis flowmeter (FIT02) to measure the total mass flow rate (oil and water). Two electrical heaters are installed to heat up the water (H-01) and the oil (H-02). Then, with the aid of a dynamic mixer (MP-01), the emulsion is made. The rotation frequency of the dynamic mixer can be altered to apply the proper shear rate to the fluids. After formation of an emulsion with the application of dynamic mixer, there is a sampler (SP-01) where the emulsion sample can be taken to check the characteristic of the emulsion, e.g., water content and average droplet size. It is important to mention that the emulsion is prepared in the high-shear region in the mixer; outside this device the flow in the plant is laminar and the shearing is low, and it is not expected to alter the emulsion characteristics. Concerning the experiments in the absence and presence of a microwave, where the emulsion flows through different pathways and through a microwave reactor, the similarities in the observed average droplet size confirm this hypothesis. A three-way valve (TV-01) can be set to direct the flow to a continuous microwave 0.25 L tank reactor (MW-01), BatchSYNTH

2. EXPERIMENTAL SECTION 2.1. Materials. To prepare the water−oil model emulsions, Exxsol D130 was selected as the continuous phase. It is characterized as

Table 2. Experimental Design for the Tests Performed at the Pilot Plant run

H2O in (wt %)

FR (kg/h)

STC (wt %)

EF (kV)

1 2 3 4 5 6 7 8 9 (c) 10 (c) 11 (c)

5 15 5 15 5 15 5 15 10 10 10

5 5 10 10 5 5 10 10 7.5 7.5 7.5

0 0 0 0 3.5 3.5 3.5 3.5 1.8 1.8 1.8

1.5 4.5 4.5 1.5 4.5 1.5 1.5 4.5 3 3 3

paraffinic oil and has low aromatic content. The deionized water was used as the dispersed phase. NaCl was utilized to make a brine (99.9% of purity, Sigma-Aldrich). Two surfactants were chosen to stabilize the water-in-oil emulsions: SPAN 80 (sorbitan monooleate, nonionic surfactant, the molecular weight of 428.61 g/mol; Tedia) and AOT (dioctyl sulfosuccinate sodium, ionic, the molecular weight of 444.59 g/mol; Sigma-Aldrich). The mixture of surfactants with the weight ratio of 9 to 1 of SPAN 80 and AOT was added to the continuous phase (oil) since the two surfactants are lipophilic. The hydrophilic−lipophilic balances for SPAN 80 and AOT are 4.3 and 32, respectively. According to Bancroft’s law, the external phase is the best phase for the surfactants to be soluble;34 the resulting HLB is below 8, and it will produce a water-in-oil emulsion. 2.2. Emulsions and Fluids Characterization. The emulsion stability in a gravitational field was determined by observing the emulsion using laser light scattering profiling (Turbiscan LAB Expert). C


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Figure 2. Results of laser light scattering profiling as a function of time and tube length for five runs. temperature (TT) and then it flows to the electrostatic treatment cell. The electrostatic treatment cell has thermocouples located in the bottom and top of the cell to monitor the temperature, and the water level is controlled by a needle valve in the bottom of the cell. After performing the dehydration process in the electrostatic treatment cell, the dehydrated oil flows to a sampler (SP-02), where the treated oil samples were collected periodically (every 15−30 min, depending on the flow rate) from the top of the unit. The water fraction is measured by the Karl Fisher method until a steady state is reached (until the

from Milestone, before the emulsion flows to the electrostatic treatment cell (Interav) (DC-01), or bypass the microwave reactor. The microwave vessel is gently stirred (5 rpm) to avoid the formation of hotspots, as suggested by the manufacturer; due the characteristics of the microwave equipment, in particular, the small diameter (i.d. = 3.5 cm), microwaves could penetrate to the core of the emulsion as the penetration depth for a 15% water emulsion is estimated to be 2.2 cm. In the case of bypassing the microwave reactor, the emulsions first flow through the heaters (H-01 and H-02) to reach the treating D


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Energy & Fuels absolute error of the water content in the treated oil reaches a value lower than 5%). The steady-state water content value is reported for each condition. The average droplet sizes are also measured and analyzed by microscopy using the software ImageJ. In the case of working at high temperature, a heat exchanger (H-03) is available to cool the fluids before disposal. There is a control valve at the exit line of the pilot plant to regulate the system pressure (PIC-01). Figure 1 shows the pilot plant process flow diagram. As it can be seen, the pilot has the advantage of working continuously, which means it does not circulate the emulsions in a loop. 2.4. Experimental Design. In this study, inlet water content in the emulsion (H2O in), the flow rate (FR), the salt concentration (STC), and the electric field intensity between electrodes (EF) were selected as the manipulated variables in the experimental design. The treatment pressure, the surfactant concentration, the gap between the electrodes, and the temperature were fixed at 10 bar, 0.5 wt %, 37.5 mm, and 45 °C, respectively. A fractional factorial 24−1 experimental design was used to evaluate the effects of the mentioned variables. The values of these four experimental variables (H2O in, FR, STC, and EF) were normalized between −1 and +1 to calculate the effects of the variables (Table 2). The design has resolution IV, where no main effects are confounded or overlapped with other main effects or any other effect of second-order interaction. However, the second-order interactions are confounded. To evaluate the experimental error, three replicas were made at the central point.37 The temperature was selected at the microwave (or in the case of using conventional heating, at the heaters) in a manner in which the treating temperature (TT) at the electrostatic treatment cell remains constant at 45 °C during the entire experiment. The water contents in the inlet of Exxsol-in-water emulsion (H2O in) were 5, 10, and 15 wt %, the intensities of the electric field between electrodes were 0.4, 0.8, and 1.2 kV/cm (alternating current with 60 Hz), the flow rates were 5, 7.5, and 10 kg/h, and the salt concentrations were 0, 1.75, and 3.5 wt %.

3. RESULTS AND DISCUTION 3.1. Emulsion Stability. In order to show that all emulsions were prepared in the same conditions, Figure 2 presents the feed emulsion stability results for one to five runs; the other runs presented similar results for both conventional heating and microwave heating. All emulsions showed a slow sedimentation process at the inlet to all runs (Figure 2) observable by an increase of the backscattering in the bottom of the flasks (at the left part of the profiles) and a decrease in the top of the flask. Some sedimentation is expected, as the sizes of the droplets were large enough to overcome the viscous forces in the oil phase; the dynamic viscosity of the Exxsol is presented in Table 1. These results were similar to those obtained by França et al.28 and Balsamo et al.38 who performed similar stability tests. It is important to emphasize that the backscattering profiles are fairly similar for each pair of runs, with and without microwave heating, leading to some confidence that the emulsion feed is suitable for comparing the processes. Interestingly, emulsions from the runs 1 and 3 showed an increasing in the backscattering after 1 h indicating a relevant rate of Ostwald ripening, in addition to sedimentation. It is also possible to observe that, in run 5, when the salt was added, the stability increased as the highly scattered zone in the bottom of the tube is small. The calculated average droplet sizes from the backscattering profiles were very similar to the values obtained by microscopy (Figure 3). 3.2. Pilot Plant Demulsification Tests. As explained previously, the experiments were run in the pilot plant according to the fractional factorial experimental design (Table 2). The experimental error involved in the determi-

Figure 3. Micrographs of the emulsions for runs with microwave and conventional heating at the inlet and outlet of the electrostatic treater.

Table 3. Variance Analysis for the Replicas Obtained in the Central Point for Each Heating Methods design

no. of replicas

H2O outlet av (wt %)

variance

microwave conventional heating

3 3

6.913 7.22

0.0001 0.004

nation of the final water content of the dehydrated oil was evaluated using three independent replicas at the central point. Table 3 presents the mean water content and variances for each set of a central point for two sources of heating. The variances were very small. Table 4 presents the water content in the oil phase for both microwave and conventional heating, the average droplet diameters (evaluated from microscopy) for the feed emulsions and treated oils. It is possible to notice that the average droplet diameters in the inlet were similar for both sources of heating. It is also possible to observe in Figure 3, where the micrographs are presented, that the morphologies of the feed emulsions were similar, indicating the consistent emulsion formation with the pilot unit. Therefore, the difference in the appearance of the outlet water droplets (microscopies) and the conventional heating methods can be considered to be due to the action of the microwave heating/treating. Figure 4 shows the dependence of the water content at the outlet with the average droplet E


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Energy & Fuels Table 4. Results from Experimental Design (Water at the Outlet) to Each Heating Sources, Average Droplet Diameter at the Inlet and Outlet of Electrostatic Treatment Cell, and the Difference between Them microwave

conventional heating

av droplet diam (μm)

run 1 2 3 4 5 6 7 8 9 (c) 10 (c) 11 (c)

H2O outlet (wt %)

inlet

2.44 2.97 2.44 2.58 0.40 7.22 2.71 9.93 6.89 6.94 6.91

av droplet diam (μm)

diff in H2O outlet

outlet

H2O outlet (wt %)

inlet

outlet

(CHMW)

6.11 3.84 6.01 4.32 5.66 3.31 4.31 3.85 4.81 4.23

4.36 4.01 7.25 5.91 4.00 3.99 3.38 4.78 4.57 4.13

3.2 3.36 2.86 3.55 0.49 9.26 3.14 10.02 7.28 7.21

4.61 4.01 4.66 3.64 4.29 3.03 3.89 3.47 3.89 4.01

3.83 3.74 4.27 5.19 4.42 3.99 4.05 3.83 4.05 4.31

0.76 0.39 0.43 0.97 0.09 2.05 0.43 0.09 0.39 0.27

3.70

3.98

7.16

4.23

4.59

0.25

Figure 5. Water content (wt %) for the conditions tested in pilot plant comparing performances of the conventional heating and microwave.

temperature in the electrocoalescer was the same for both cases. Analyzing the difference of the water contents at the outlet (Figure 5) in runs 1, 4, 6, and 7 shows that the performances of microwave are higher than the other runs; this could be due to the fact that the electric field intensities in these runs were on the lower level, and thus the influence of microwave energy input33 is more apparent. In order to analyze quantitatively the effects of the various manipulated experimental variables in electrostatic coalescence process combined with different heating methods, a linear regression (eq 1) was selected:

diameter of the inlet and outlet for microwave and conventional heating. Figure 4b shows that average droplet diameter at the outlet with microwave heating was higher than the conventional treating. The action of microwave heating results in enhanced droplet coalescence; the same was observed by Binner et al.7 This behavior is attributed to the reduction of the interfacial tension of the emulsion.32 It is possible to conclude that emulsion treatment after microwave heating was superior to the conventional heating on almost all runs. It is worth emphasizing that the treatment

n

ye = a0 +

n

n

∑ aixi + ∑ ∑ aijxij + εi i=1

i=1 i=1

(1)

where a0 is the independent constant; xi are the independent variables (electrical field, amount of surfactant, salt concentration and flow rate) and xij are the combination of the independent variables (called secondary effect), distance between electrodes, and inlet water content); ye is the response

Figure 4. Dependence of the water content at the outlet (wt %) with the average droplet diameter of the inlet and outlet for microwave and conventional heating. F


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Figure 6. Predicted vs observed values of water in the outlet (microwave, (a) linear model and (b) quadratic model; conventional heating, (c) linear model and (d) quadratic model; difference between the two methods of heating, (e) linear model and (f) quadratic model).

by Suemar et al.,40 Mahdi et al.,41 and Abdul-Wahab et al.,42 can be presented. Therefore, a better explanation of the separation efficiency was obtained by means of a model containing a quadratic term which takes into account a possible nonlinear effect; the R2 was 0.99 to all cases. The quadratic coefficients are shown in Table 5, and Figure 6b,d,f presents the fitting of the equation containing the quadratic term to the experimental data. The values of the quadratic coefficient are now shown because it is not possible to assign which variable is responsible for the nonlinear effect, because of the confounding between the quadratic term and the linear coefficient (a0).39 Abdul-Wahab et al.42 explained this nonlinear effect, as a result from a quadratic behavior of initial water content. It can be seen in Table 5 that the values of coefficients related to the water cut of the inlet (H2O in) are larger, showing the greater effect of inlet water content among the others in both cases of heating. The variables, such as inlet water cut, flow rate, and the salt concentration, showed a direct influence on the response values (water content in the treated oil phase), which means the water content increased when those variables increased (it can be seen in Figure 7a−c). The only inverse effect observed was from the voltage between electrodes (electric field Figure 7d).

of the model, which is the amount of water at the outlet; ai is the calculated coefficient; aij is the combination of effects; and ϵi is the prediction error.28,39 Table 4 presents the coefficients of the linear regression based on the data that were previously standardized for each heating scenario and also for the difference between them. The regression equations were the purely linear one (eq 1) and an equation including a quadratic variable effect. The statistically significant variables are in bold, considering a confidence interval of 95%. The predicted outlet water concentration values with linear model vs the observed experimental values are shown in Figure 6a,c,e. The linear model was not able to describe the experimental data satisfactorily. The fit of the model was expressed by the determination of the coefficients and with R2 of 0.77, 0.83, and 0.91 (Table 5), indicating that 77%, 83%, and 91% of the variability in the response values can be obtained by the model. This represents that the model did not fit the experimental data with accuracy, even though all the variables were shown to be statistically significant. The central points, runs 9, 10, and 11, which are located in the middle of graphs in Figure 6a,c,e clearly stood off the fitting line, indicating that nonlinear effects which were also observed G


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1.23 ± 0.008 1.24 ± 0.02 0.015 ± 0.03

−0.1 ± 0.008 −0.3 ± 0.02 −0.4 ± 0.03

0.003 ± 0.008 −0.17 ± 0.02 −0.17 ± 0.03

1.67 ± 0.008 1.85 ± 0.02 0.18 ± 0.03

0.7 ± 0.008 0.45 ± 0.02 −0.23 ± 0.03

0.77 0.83 0.91

The dehydration efficiency improved (with lower water content in the outlet) as the strength of electric field (EF) was increased. This is in agreement with Eow et al.8 work. The flow rate has an important effect due to the fact that it has direct influence in the residence time of the fluid between the electrodes. When the flow rates were 10 kg/h (runs 3, 7, and 8), the separation was lower than the runs with the low flow rates, as it can be observed graphically in Figure 7b. In the case of higher flow rate, the droplet sizes at the outlet were bigger than the inlet, which means that the settling time was not sufficient to separate the water from the oil. Although the flow regime in all runs was laminar, the higher flow rate resulted in less separation, mainly due to the fact that the time that emulsion stays in the electrodes zone was not sufficient.43 In runs 4 and 7, the highest flow rate and lowest electrical field were applied (at different water contents); almost no separation was expected, but, still, there was noticeable dehydration with a better separation in the feed emulsion with higher water cut. A high water cut is beneficial to both electrocoalescence and microwave heating (Evdokimov and Losev33), but a larger effect of this variable was found in microwave heating, compared to conventional heating. The microwave effect is associated with the increase of microwave absorption by the emulsion as the water content increases. Salt concentration caused to increase the water content in the outlet (Table 4 and Figure 7c). The salt concentration results in an increase in the stability and interacts with the ionic surfactant, as Kini et al.44 mentioned. Suggesting that the dissolved ions act in the interface by changing the physical properties of the interfacial film buildup between the droplets and a continuous phase, which was described by Perles et al.45 The salinity of water has a strong effect on the interactions of surfactant with water, particularly in the case of ionic surfactants.43 In addition, the combination of the salt with the ionic surfactant plays an important role in the formation and stability of the emulsion by lowering the interfacial tension in 1 order of magnitude, when compared to the interfacial tension of this system in the case of no added salt (Table 1). This is reflected in the higher emulsion stability observed by laser light profiling (Figure 2, run 5). It is important to mention that the addition of salt has an immediate effect on the dielectric properties of the water phase and, therefore, it can be more easily be heated by microwave. When salt is added, an ionic conduction component to the loss mechanism is introduced, leading to more efficient heating compared with pure water. Furthermore, salt can have an effect on the emulsion stability by changing the surfactant partition in the water phase. Therefore, the effect of salt in a separation process involving microwave treatment is a compromise between improved heating characteristics of the salty water phase and modified emulsion stability due to the presence of salt. In the experiments presented herein, the presence of salt increased the final water content in the treated emulsion for both conventional and microwave heating showing that the enhanced emulsion stability due to salt dominated the salt concentration effect. In order to evaluate the main effects and compare the two heating scenarios, the algebraic difference between the final water content by different treatments was also analyzed. All variables, but salt concentration, were statistically significant. This occurs because the effect of the salt concentration, increasing emulsion stability, was similar (same coefficient, Table 5) in both microwave and conventional heating;

Data in bold are statistically significant. a

1.84 ± 0.008 2.06 ± 0.02 0.22 ± 0.03 4.67 ± 0.007 5.23 ± 0.02 0.56 ± 0.02 microwave conventional heating CH-MW

0.58 ± 0.008 0.41 ± 0.02 −0.17 ± 0.03

R2 1 × 4 and 2 × 3 interactions

1 × 3 and 2 × 4 1 × 2 and 3 × 4 EF (4) STC (3) FR (2) H2O in (1) a0 design

Table 5. Author: Please note that we do not allow emphasis in a table (see boldfacing of Table 5) without explanation. If you would like to discuss certain items, please use footnotes (provide in the proof corrections).Linear Multiple Regression Coefficients for Data Obtained for Each Heating Source and the Difference between Thema

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H


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Figure 7. Dependence of the water content at the outlet (wt %) with the four operational variables for microwave and conventional heating.

therefore the difference between these coefficients is close to zero. Furthermore, when a difference between experiments is used as experimental response, the experimental error is increased and the coefficient errors are larger as it can be observed for the calculated errors of the coefficients of the regression of the difference between treatments (CH-MW, Table 5) By evaluating the coefficients of the equation relative to the “difference between the two methods”, two noticeable facts appeared (Table 5). First, the most important variable was the electrical field, and it showed an inverse effect. This indicates that when the electric field is high, the difference in the

performances of the two heating methods decreases. In other words, at a higher electric field, the electrocoalescer is efficient enough to separate the emulsion without any possible aid from the microwave heating. The flow rate also had a negative effect difference between heating methods equations, meaning that at a higher flow rate the microwave is more efficient than conventional heating. Table 5 also presents the values for the pairwise interactions between main variables. Due to the nature of fractional experimental design, the interactions are confounded; therefore, it is not possible to know if the calculated effect arises from a given pair of variables without additional experiments. I


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the study of coupled microwave-electrocoalescence demulsification and Universidade Federal de Itajubá (UNIFEI) for building the pilot plant.

The confounded interactions are shown in Table 5. The strongest interaction arises from water cut and salt concentration ((1) and (3)) or between the flow rate and the electric field ((2) and (4)); since water cut and salt concentration have a strong effect, it is likely that the observed effect comes from an interaction between them. The observed interactions had high values, indicating once more the strong nonlinear effects were influencing the experiments.40 These two results suggest that when the electrocoalescer is bottlenecked, a microwave unit can be installed to aid the separation and debottleneck the dehydration process. Despite the fact the factorial design has flaws, the experiments showed a high potential of the combination of microwave and the electrical coalescence, and more experiments using crude oils with different densities are being made to confirm this tendency. This is an important finding, as a microwave unit can have a smaller footprint than a separator and can be used in oil rigs where space is usually scarce or as an alternative to the addition of demulsifiers to treat persistent emulsions.

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4. CONCLUSIONS This work presents a study on the electrostatic treating process performed in a continuous pilot plant comparing two methods of heating, microwave, and conventional heating. A W/O model emulsion with Exxsol as the oil phase was tested, and the investigated variables were the water content in the feed emulsion, salt concentration, flow rate, and electric field between electrodes. The electric field intensity regularly reduced the final water content in the treated oil. The lower flow rate (residence time) of the emulsion also helped in reducing the final water content. The salt content played as a stabilizer of the emulsion with decreasing the interfacial tension due to the combination with the surfactant. The comparison of the two process arrangements shows that the combination of electrocoalescence plus microwave heating has a better performance in all experimental conditions evaluated in this work, even in the case of salt presence, which makes the microwave heating less effective. The observed advantage of the microwave was higher in lower electrical fields and high residence times, conditions resembling a bottlenecked electrocoalescer. Therefore, microwave demulsification can be used as an alternative to the traditional sources of heating, particularly when the electrocoalescer is under operational stress.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frederico Wanderley Tavares: 0000-0001-8108-1719 Márcio Nele: 0000-0003-0106-8034 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Petrobras S/A, Coordenação de ́ Aperfeiçoamento de Pessoal de Nivel Superior (Capes), ́ Agência Nacional do Petróleo, Gás Natural e Biocombustiveis (ANP), Conselho Nacional de Pesquisa (CNPq), and Fundaçaõ Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for their financial support. We also thank Mrs. Raquel Coutinho (Petrobras) for proposing J


Article

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