Experimental study on evaluation and optimization of tilt angle of parallel-plate electrodes using electrocoagulation device for oily water demulsification

Accepted Manuscript Experimental study on evaluation and optimization of tilt angle of parallel-plate electrodes using electrocoagulation device for oily water demulsification Yang Liu, Wen-ming Jiang, Jie Yang, Yu-xing Li, Ming-can Chen, Jian-na Li PII:

S0045-6535(17)30474-5

DOI:

10.1016/j.chemosphere.2017.03.141

Reference:

CHEM 19100

To appear in:

ECSN

Received Date: 11 January 2017 Revised Date:

22 March 2017

Accepted Date: 25 March 2017

Please cite this article as: Liu, Y., Jiang, W.-m., Yang, J., Li, Y.-x., Chen, M.-c., Li, J.-n., Experimental study on evaluation and optimization of tilt angle of parallel-plate electrodes using electrocoagulation device for oily water demulsification, Chemosphere (2017), doi: 10.1016/j.chemosphere.2017.03.141. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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CHEM44806

Experimental study on evaluation and optimization of tilt angle of parallel-plate electrodes using Electrocoagulation device for oily water demulsification

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Yang LIUa,b, Wen-ming JIANGa,b*, Jie YANGa,b, Yu-xing LIa,b, Ming-can CHENa,b, Jian-na LIc

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(a． Shandong Provincial key laboratory of Oil & Gas Storage and Transportation Safety, Qingdao Key Laboratory of Circle Sea Oil & Gas Storage and Transportation Technology, Qingdao 266580, China； b． College of pipeline and civil engineering, China university of Petroleum, Qingdao 266580, China; c. Thermal Energy Engineering， Xi'an jiaotong university， Xi'an 710049, China)

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Abstract

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Tilt angle of parallel-plate electrodes (APE) is very important as it improves the economy of diffusion controlled Electrocoagulation (EC) processes. This study aimed to evaluate and optimize APE of a self-made EC device including integrally rotary electrodes, at a fixed current density of 120 Am-2. The APEs investigated in this study were selected at 0o, 30o, 45o, 60o, 90o, and a special value (α(d)) which was defined as a special orientation of electrode when the upper end of anode and the lower end of cathode is in a line vertical to the bottom of reactor. Experiments were conducted to determine the optimum APE for demulsification process using four evaluation indexes, as: oil removal efficiency in the center between electrodes; energy consumption and Al consumption, and besides, a novel universal evaluation index named as evenness index of oil removal efficiency employed to fully reflect distribution characteristics of demulsification efficiency. At a given plate spacing of 4 cm, the optimal APE was found to be α(d) because of its potential of enhancing the mass transfer process within whole EC reactor without addition, external mechanical stirring energy, and finally the four evaluation indexed are 97.07%, 0.11 g Al g-1 oil, 2.99 kwhkg-1 oil, 99.97% and 99.97%, respectively.

1. Introduction

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Electrocoagulation; Tilt angle; Rotary electrodes; Oily water; Demulsification

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On account of the increasing large volumes of oily water produced by oil extraction every day, a growing attention has been directed to the demulsification of oil in water emulsions for recycle or direct discharge. As reported by Ferro and Smith (2007), indicated that global oily wastewater associated with oil and gas fields was estimated at around 250 million barrels per day compared with around 80 million barrels per day of oil recovery. In addition, the oily water is characterized by high oil concentration, high salinity and micron sized oil droplets with high stability. Thus, untreated oily water not only causes pollution of the environment but also jams rock of formation with a consequent affecting subsequent recovery. Conventional technologies were unfit for demulsification of O/W emulsions due to complex control, high cost, low efficiency, limits and a lot of sludge generation (Asselin et al., 2008; Karhu et al., 2012). Nevertheless, electrocoagulation (EC) is receiving an increasing acceptance by industry in view of its advantages compared to other methods. In essence an EC reactor is an electrochemical cell, wherein chemical reactions occurring as seen in Eqs. (1) - (5) (Chen et al., 2004). For anode: Al-3e→Al3+ (1) 1

ACCEPTED MANUSCRIPT At alkaline conditions: Al3++3OH-→Al(OH)3(s)

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At acidic conditions:

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Al3++3H2O→Al(OH)3(s)+3H+ (3) In addition, there is oxygen evolution reaction: 2H2O-4e→O2↑+4H+ (4) The reaction at the cathode is: 2H2O+2e→H2↑+2OH(5) Aluminum cations and their hydrolysed derivatives are the efficient coagulant. Thus, as Eqs.(1)-(3), a sacrifical metal anode, usually aluminium, used to supply ions into the emulsion within the EC reactor (Chen et al., 2000). The generated Al3+ ions combine with water and OH- ions to immediately urdergo further spontaneous hydrolysis reactions to form various monomeric species as Al(OH)2+, Al(OH)2+, and Al(OH)4− , polymeric species as Al2(OH)24+ and Al2(OH)5+, which finally transform into amorphous Al(OH)3(s) and less soluble species as and Al2O3 in terms of many complicated processes (Holt et al., 2002), as Eqs. (6). Al3+→Al(OH)3(3-n)→Al2(OH)24+→Al13complex→Al(OH)3(s) (6) Thus, in an EC process the coagulating ions are produced ‘in situ’ for breaking of emulsion, and demulsification mechanism which depends on factors such as pH and coagulant dosage, may be summarized as two points (Holt, 2003): (i) A sorption coagulation mechanism postulated; probably charge neutralization that quickly aggregates pollutants particles forming open structured aggregates as indicated by the low fractal dimension. (ii) The amorphous hydroxide precipitate Al(OH)3(s) resulting in coagulation by an enmeshment mechanism (sweep coagulation). Consequently compact aggregates are formed, indicating by high fractal dimension. Furthermore, when direct current is applied to water through a pair of electrodes, the energy barrier is overcome, and water is electrolyzed (Yang et al., 2007). Thus, it could be obviously observed that the hydrogen bubbles evolved from the cathode (Eqs. (5)). Oxygen bubbles evolution is also possible on the anode (Eqs. (4)), although this was not detected by Przhegorlinskii et al. (1987). As reported by Khemis et al. (2006), analysis of collected gas by chromatography revealed the exclusive presence of hydrogen. Thus, these small bubbles with fine size can help in the removing the oil from the wastewater by flotation. Ultimately, these particles can be easily collected and removed. Therefore, EC processes are an effective pretreatment technology to remove the smallest colloidal particles which have a greater probability of being coagulated with the electric field that sets them in motion (Pouet et al., 1995). Despite more than a century’s worth of applications, many of them deemed successful, the science and engineering behind EC reactor design is still largely empirical and heuristic. This is a reason that lots of researchers have mainly focused on type of pollutant removed and the operating parameters, as current density, PH, material of construction of the electrodes, rate of flow and conductivity of the solution not to explore the fundamental mechanisms involved in the EC process (Emamjomeh et al., 2009; Khandegar et al., 2013). EC has generally been studied as a pollutant-centric technology, without specific attention to the underlying mechanism of mass transfer, electrochemistry and adsorption process (Lu et al., 2015). Recently, researchers not only investigate the composition and structure of the floc and the sludge of EC process to find the interdependent relationship between the electrochemical

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operating parameters and EC performance (Genuchten et al., 2012; Genuchten, 2013), but also study arrangement and shape of electrodes to improve the mass transfer process. For example, the effects of the orientation of electrodes between horizontal and vertical on the performance of EC cell were investigated (Fouad et al., 2009), and cell with array of horizontal cylindrical anodes in which oil separation efficiency was 99.8%, was found to have a higher mixing efficiency and higher floating ability than the traditional vertical parallel plate cell. Meanwhile, the array of closely packed screens as anode was proved to be more effective than single screen anode to remove phenol from oil refinery waste effluent in EC experiment, using three indexes (phenol concentration, energy consumption and aluminum electrode consumption) (Abdelwahab et al., 2009). Further, the effects of anode rotational speed and surface roughness on removal efficiency of Cr(VI) were explored, and a result was found that the performance of a cell with vertically oriented electrodes is superior to that of a cell with horizontal electrodes (Khalaf et al., 2016). Although significant researches have been conducted to investigate different shapes of electrodes including horizontally orient electrodes and rotating electrodes et al, the traditional vertical parallel plate cell is still conducted in majority of EC studies. This is may be attributed to the lack of universality in homemade EC reactor and additional energy consumption on the basis of local region in homemade EC reactor for the research. In order to develop maximum potential of the EC technology (sedimentation and flotation), the overarching aim of present work is to evaluate the performance of a EC cell with tilt angle of parallel-plate electrode (APE), which is novel for demulsification of O/W emulsion and is also beneficial to standardized design of EC device. Three of the evaluation indexes were employed to evaluate, as oil removal efficiency in the center between electrodes; energy consumption and Al consumption. Besides, on account of mass-transfer characteristic which directly caused removal efficiency with the great difference throughout EC reactor, a new universal evaluation index named as evenness index of oil removal efficiency (EIOR) was proposed to fully reflect distribution characteristics of separation efficiency in two cross sections (vertical and horizontal) within EC reactor in this study on the base of statistical bias and was expressed as:

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2 1 n (η j − η ) R = 1− ∑ 2n j =1 η

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where R, n, η,η are EIOR at a range from zero to 1, number of sample location in same cross section;

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oil removal efficiency(%); average oil removal efficiency in same cross rate(%), respectively. Noted that the closer that evenness index of oil removal efficiency is to 1, the better uniformity obtained on cross section in the entire installment of EC. After the APEs were evaluated comprehensively and systematically using above four indexes, the optimal APE was found and suggested.

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2. Experimental

2.1 Simulated oily wastewater Since real emulsion usually contain some surfactant for emulsification and homogenization, 1 g of anionic surfactant as sodium dodecyl benzene sulfonate (LAS) widely used in the petrochemical industry was slowly added to the 1.3 L of tap water. Subsequently, the solution was added diesel of 3 g supplied from Sinopec, and meanwhile mixed with high shear emulsifying machine for 10 min. After the solution had been retained in separating funnel, most of the oils in the form of emulsified oil were pulled out from the solution leaving the floatable oil. In order to increase conductivity of oily water, NaCl, about 2

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gL-1, was used as electrolyte. As suggested (Garmona et al., 2006), the characteristics of this emulsion are not affected by the addition of supporting NaCl at low or moderate concentrations. Moreover, initial pH was attained 7.0±0.2 by using 0.1 M HCl and 0.1 M NaOH and the pH was monitored with a pH meter (Rex electric chemical pHSJ-4F, Shanghai, China) before each run. For the case of using aluminum electrodes (Adhoum et al., 2004), the neutral pH values provides the highest EC yield due to the amphoteric nature of aluminum hydroxide. After adjustment of conductivity and pH, the final oily water was prepared. Characteristic of experimental water, as the initial size distribution of emulsified oil, was measured by laser particle analyzer (Malvern instrument Mastersizer 2000, Melvin, England), following standard methods (International Organization for Standardization's (ISO) 13320: 2009). Fig. SM-1 shows oil particle size distribution before the tests. The bulk of the mass (91.47%) exists as oil (particles size at range from 0 µm to 2 µm) and the maximum as 16.47% of total oil is with 1.06 µm. The mass of particles greater than 2 µm is virtually insignificant. 2.2 Experimental apparatus and methods

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c Fig. 1 The schematic design of EC reactor and rotary electrodes In EC experiments, a batch process design is preferable for research and is chosen for this study. The reason is that the batch process is going to promote more particles (and so a higher charged surface) and also more polymeric hydroxocationic species (Canizares et al., 2007). This means that the

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aluminium is more efficiently used in the batch reactor. Rectangular reactor tested is constructed of perspex and has effective volume of 1.2 L. The electrodes composed of one anode and one cathode, were both made of aluminum with a dimension of 10*10*0.3 cm. The anode and cathode were, respectively connected to the positive and negative outlets of a DC constant-current power source (Long Wei instrument PS-305DM, Hongkong, China) with a maximum current rating of 5 A at an open circuit potential of 30 V, as shown in Fig. 1a, and Fig. 1b. Both ends of six polyethylene rods of with a desired length were inserted into six holes on the anode and the cathode, respectively, in order to regulate plate spacing and they was shown as Fig. 1c. By using different lengths of rods, plate spacing was adjusted from 2 to 4 cm, at which the demulsification effect is outstanding through previous experiments (Bensadok et al., 2008; Sahu et al., 2014). The electrodes were fixed in grooves on inner-wall of EC reactor by a longer rod in the center of the electrodes. In order to investigate the impacts of different APEs on the performance of EC, the APEs were selected at 0o, 30o, 45o, 60o, 90o and a special value (α(d)) when the upper end of anode and the lower end of cathode is in a line vertical to the bottom of reactor was first found and proposed. The anode horizontally placed above the cathode is defined as 0o, and then others APEs were obtained by counter-clockwise rotating electrodes under the premise that relative position was remained unchanged, at a fixed current density of 120 Am-2. Note that decreasing plate spacing from 4 to 2 cm is helpful to increase in this special angle, and thus this α(d) is inversely proportional to electrode spacing. 2.3 Measurements All EC experiments were conducted under standard conditions (electrolysis time of 20 min and current density of 120 Am-2) determined through previous experiment. In order to assess the distribution characteristic of demulsification efficiency for a batch EC reactor, the emulsion is evenly divided into three sections on vertical direction and horizontal direction in the center of electrodes, respectively. Samples, 5 mL, were drawn at 4 min intervals during the experiment, and then hold for 1 h in separating funnel before inspection. As noted, since the absorbance of undiluted samples was too high to measure reliably (Zawadzki et al., 2007), after standing, the samples were taken out and then diluted with petroleum. Ultimately, oil content was measured using ultraviolet spectrophotometer (Aoe insturment A360, Shanghai, China) at 210 nm, according to the Beer–Lambert Law, a linear relationship between absorbance and oil concentration can be expected from a real solution (Yang, 2007). To avoid blockage of the Al surface by formation of a thick solid layer, the electrode should be dipped into acetone solution and 5% (v/v) hydrochloric acid solution for 10 min successively, and rubbed with a sponge and then rinsed with tap water 3 times before used. In order to ensure the consistency of each test, the cell was cleaned in 5% (v/v) hydrochloric acid for 15 min. For the best accuracy of measurement, all the tests were repeated three times, and the experimental error was below 5%. The average values were reported.

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3.Results and discussion

3.1 Oil removal efficiency in section B Oil removal efficiency in center section between electrodes is most important and commonest evaluation index for assessing demulsification efficiency of EC. In this study, oil removal efficiency in center section is calculated as Equation 8 (Ahmadi et al., 2013). η (%) =

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C0 − Ct × 100 % C0

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In the equation 8, C0 is the concentration of oil before processing in water (mgL-1); Ct is the 6

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concentration of oil with electrolysis time in water (mgL-1). In each run, current density was kept constant at 120 A/m2. As electrolysis time is the key operational parameter that may influence not only demulsification process but also cost of EC technology, the effect of electrolysis time on oil removal efficiencys were investigated with six APEs of electrodes as 0o, 30o, 45o, 60o, α(d), 90o, at a fixed plate spacing of 4 cm (Abdel-gaward et al., 2012). Figure 2 shows that, for a given APE, in general increasing the electrolysis time obviously raises demulsification efficiency. This is ascribed to the fact that electrolysis time not only determines the amount of electro-generated Al3+, OH- and babble of H2, but also strongly influences the depth of hydrolysis reaction with a consequent impacting formation of floc (Ghernaouta et al., 2011). At the beginning of electrolysis, less electrolysis time results to the hydrolyzates mostly in the form of Al(OH)2+, Al(OH)2+ and Al(OH)3 with a small amount of polymer, with a consequent forming flocs with porous structure (Holt et al., 2002). The change of oil concentration is slow as main demulsification mechanism is electrostatic neutralization. As time progresses, demulsification process begins to be speeded up from 8min this may be attributed to the following effects: (i) The increment of electrolysis time results in the increment of the amount of electro-generated Al3+ and H2 babbles in the emulsion according to Faradays law. (ii) Flotation is clearly favoured here by the higher bubble density. (iii) Macro convection not only improves anode concentration polarization with a consequent quickening hydrolysis reaction. Higher coagulant doses obtained by hydrolysis tend to increase both floc size and growth rate, as there are more particles available for aggregation (Lee et al., 2016).

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Fig. 2 Effect of tilt angle of electrode on oil removal with electrolysis time (current density is 120Am-2; plate spacing is 4 cm) In emulsion, 90% amorphous Al hydroxide particles formed with compact structures can directly absorb more than one neutralized oil droplets at the same time (Canizares et al., 2008). Thus, a higher electrolysis time results in neutralized oil droplets quickly removed through coagulation and flotation. At the later stage of electrolysis, it could not be neglected that there is a stable stage for which further addition of electrolysis time results in little change after treatment. This may be explained by the following facts: (i) Soluble aluminum hydroxides existed with excess Al3+ results in the trouble as the proportion of [Al(OH)4]- is considerable (Lu et al., 2015). (ii) Mixing at greater intensities tends to impede the adhesion process between pollutants and flocs and babbles with dramatical increment of H2 bubble production rate.(iii) Too many bubbles generated from the anode decrease the available surface

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area and the reaction becomes very energy dependent, with the electrolysis time increasing (Abdelwahab et al., 2009). Fig. 2 also shows that APE has also significant impacts on the process of removal oil in section B at plate spacing of 4 cm. In order to thoroughly explore the relevance between APE and final demulsification efficiency in same area, three plate spacing as 2 cm, 3 cm and 4 cm were used. Fig. 3 shows that it is concluded that for a given plates spacing, in general the increment of APE results in the decrement of final oil removal efficiency, but except α(d) APE. It is well known that APE determines the distribution of H2 babbles over the cross-section of the reactor with a consequent affecting mixing condition at the anode surface (Found et al., 2009). In other words, better distribution homogeneity of bubbles over the cross section promotes convection with a consequent increasing the rate of diffusion of Al3+ dissolved from anode to the emulsion this may be attributed to the following effects: (i) The scope of anode surface stirred by rising H2 bubbles turns into bigger with the increment of distribution homogeneity. As the solution entrained by the rising H2 bubbles swarm is recycled only to the upper part of anode with the vertical electrodes, while whole area of anode surface is evenly disturbed with the horizontal plats. (ii) Diffusion layer is further thinned by the radial momentum and the eddies are induced by the rising bubbles, which increases the flux of the dissolved Al3+ (Sedahmed et al., 1985).

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Fig. 3 Effect of tilt angle of electrode on oil removal with plate spacing (current density is 120Am-2; electrolysis time is 20min) Therefore, concentration polarization would be improved with more uniform raising bubbles. In addition, since emulsions including cathodically evolved OH- are carried by rising swarm of H2 bubbles, smaller angle results in more uniform decrement of the migration time of OH-, with consequent quickening hydrolysis reactions more uniformly. As expected, the cell with horizontal electrodes has greatest separation efficiency of section B at fixed plate spacing from 2 to 3 cm, as a result of the uniform distribution of the rising bubbles allover the cell cross-section (Fouad et al., 1972). Yet, in same cases, the removal efficiency with horizontal electrodes decreases as the plate spacing continues to increase, and finally plate spacing of 3 cm was optimal, which is just efficient enough to provide sufficient and appropriate turbulence to mass transfer and the swirling velocity of the liquid medium between the electrode may be reduced with spacing

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more than 3 cm (Sahu et al., 2014). Note that the removal efficiency in section B between electrodes with α(d) is inconsistent with above-said patterns. The oil removal efficiency rises sharply, and this may mainly reveal the synergetic effect of: (i) The cell with α(d) APE reduces accumulation of Al3+ and H2 bubbles nucleates at the

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3.2 Evenness index of oil removal efficiency (EIOR)

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Although some works have been conducted to compare between horizontal electrodes and vertical electrodes using oil removal efficiency in a region on the basis of assuming uniformity distribution of treatment efficiency, this evaluation index on behalf of local demulsification efficiency has its limits. As noted, demulsification processes are actually inconsistent in different locations within EC reactor, as results of synergy in EC process, for instance, anodic oxidation, cathodic reduction, electrophoretic migration, coagulation and flotation. Thus, evenness index of oil removal efficiency (EIOR) first proposed is very crucial to reflect the whole demulsification performance of EC fully and directly. For again, the closer EIOR tends to 1, the higher distribution uniformity of demulsification efficiency is. Three sections of emulsion, including section A, B, C, in vertical direction between electrodes with the increment of the emulsion depth were divided to investigate, as shown in Fig. 1b. The oil removal efficiency of three sections was measured, and then EIOR in the vertical cross-section could be calculated using Equation (7).

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electrode surfaces. (ii) As flotation and sedimentation occur simultaneously in EC process, settle is also reliant on adequate coagulant dispersion followed by still conditions for settling (Holt et al., 2003). This special design with α(d) APE may help to increase sedimentation ability of flocs with compact structure and big size away from the anode, instead of depositing on the surface of both anode and cathode, with a consequent increment of the amount of Al3+ dissolved and H2 bubbles released. (iii) The flotation of neutralized oil drops was quickened to reach the top of the emulsion with the absence of obstructions (Rajeshwar et al., 2003). Thus, cell with α(d) APE is a novel design that more fully brings out electrodes potentiality with a consequent improving performance of EC through co-precipitation and flotation. Ultimately, 97.07% emulsified oil in section B between electrodes was removed at fixed plate spacing of 4 cm, while only demulsification efficiencies of 96.60% and 93.97% were obtained in the cell with horizontal and vertical electrodes, respectively.

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Fig. 4 Effect of tilt angle of electrodes on evenness index of oil removal in the vertical cross-section 9

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(current density is 120Am-2; electrolysis time is 20min) Fig. 4 shows that in vertical cross-section, in general the cell with horizontal plates has lowest distribution uniformity of treatment efficiency, at both given plate spacing. This behavior is attributed to the fact that mixing condition determines the rate of hydrolysis reaction in emulsion with a consequent affecting demulsification process. As known to all, turbulence above cathode is sufficient by macro convection induced by the rising swarm of H2 bubbles, whilst the emulsion under the cathode suffers from gentlest disturbance. Besides, this result has shown a satisfactory agreement with the experiment phenomena that the emulsion below cathode is still turbid. On the contrary, for a given plate spacing, the cell with α(d) APE has greatest EIOR in same cross-section. This may be attributed to the fact that migration path of coagulant and H2 bubbles has significant effects on sedimentation and flotation capacity (Sahu et al., 2014). APE determines the distribution characteristics of rising bubbles generated at the electrodes. The cell with α(d) APE reduces the coalescence frequency of bubbles besides the electrode. As reported (Sedahmed et al., 2007), bubble collision and coalescence ant the attendant conversion of surface energy to kinetic energy. Thus, bubbles have longer retention time of rising with smaller kinetic energy. Rising fine bubbles can adhere to aggregated oil droplets in upper part of emulsion instead of nucleating to accumulate at the surface of electrodes, and ultimately directly arrive to the surface of the reactor without obstacle. As a consequent, better distribution uniformity of oil removal allover EC reactor with a consequent the potential of saving the mechanical stirring energy. On the other hand, compact amorphous Al hydroxide particles as primary species (Canizares et al., 2007) can settle adown easily, and meanwhile attach more than one oil droplets in the bottom of the reactor. Ultimately, increment of contact time of bubbles and coagulants will significantly increase in demulsification efficiency of upper and lower emulsion, respectively. In addition, although emulsion entrained by the rising bubbles is recycled only to the upper part of anode, the cell with vertical electrodes has higher EIOR as results of turbulence induced by current density of 120 Am-2 for sufficient mixing to vertical cross-section between the electrodes through generation of more bubbles and Al3+ (Sahu et al., 2014). Ultimately, Fig. 4 shows that for a given plate spacing, in general the APE has little influence on the distribution characteristic of separation efficiency besides the horizontal electrodes. The differences between the minimum and maximum values were only 0.86% and 0.14% at 2 cm and 4 cm, respectively.

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Fig. 5 Effect of tilt angle of electrodes on evenness index of oil removal in horizontal cross-section (current density is 120Am-2; electrolysis time is 20min) In horizontal cross-section, three sections of emulsion divided including D, B, E section, have been used to investigate the effect of APE on EIOR. From Fig. 5, it is obvious that the APE has important influence on the distribution uniformity of oil removal efficiency in horizontal section. The cell with vertical plates has nonuniform demulsification efficiency. This may be attributed to the fact that H2 bubbles in the form of a curtain beside the vertical cathode increase in thickness of bubble layer along the vertical electrode, and as a result that the emulsion outside the electrodes is not suffered effective mixing (Fouad et al., 2009). APE also determines the mass transfer of the whole EC reactor. Bubbles of gas, after being detached from the electrode, move upward parallel to the electrode along with the solution adjacent to the electrode in the form of a gas-liquid dispersion (Fouad et al., 1972). When they reach the free surface of the solution, part of them disintegrates into the atmosphere while the other part moves horizontally with the displaced solution till they meet the electrodes where they are reflected in a downward stream parallel to the electrodes, thereby the formation of a eddy between the electrodes. The eddy in the cell with α(d) APE not only assists in mass transfer of coagulant from the surface of the electrode to the bulk, but also disturb the solution between the plates, thereby an significant influence on the solution outside the plates. As a consequent, discharge capacity improved of bubbles benefits greatly mass transfer process of coagulant allover EC reactor, with a consequent the potential of saving the mechanical stirring energy.

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For the application of a wastewater treatment technology, cost consumption should be also considered. Thus, it is necessary to investigate the relationship between cost consumption (Al consumption and energy consumption) and APE with electrolysis time, which is novel to EC. As noted, in order to evaluate the overall performance of EC more accurately, cost consumption is calculated with average concentration of oil. The aluminum electrode consumption (Al Consumption) having a unit of g Al g−1 of removal oil is calculated from Faraday’s law from the following relation (Abdelwahab et al., 2009):

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the cost consumption of whole reactor more accurately. Fig. 6 represents the interrelation between the Al consumption and the APE with electrolytic time, when the current density was fixed at 120 Am-2. During the electrolysis, for a given the electrode spacing of 4 cm, the cell with vertical electrodes has obvious effect on the electrode consumption, with greatest attenuation rate at range from 0.49 to 0.12 g Al g-1 oil, because increasing thickness of the bubble layer along the vertical electrode increases in the cell resistance with a consequent increment of nonuniformity of current distribution (Fouad et al., 2009). In contrast, the cell with α(d) APE has gentlest decline of Al consumption at a range from 0.18 to 0.11 g Al g-1 oil with electrolysis time and ultimately has least Al consumption at 20 min.

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Fig. 6 Effect of tilt angle of electrodes on Al consumption with electrolytic time (current density is 120Am-2; plate spacing is 4 cm) Energy consumption is also main cost of operations except Al consumption and is expressed as equation (10) (Abdelwahab et al., 2009):

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where E, I, U, t and V are energy consumption (kwhkg-1 oil), applied current (A), voltage (V), electrolysis time (min) and the volume of the solution (L), respectively. Fig. 7 shows the effect of the APE on the energy consumption with time, when the current density was fixed at 120 Am-2. During the electrolysis, for a given the electrode spacing of 4 cm, decline of energy consumption from 13.83 to 3.41 kwhkg-1 oil is still obvious with the vertical electrodes as opposed to less attenuation rate with α(d) APE, from 4.62 to 2.99 kwhkg-1 oil. As noted that the cell with horizontal electrodes has greatest energy consumption. This may be due to the fact that location of anode above cathode hinders upward movement of bubbles, with a consequent decrement in the

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velocity of rise: this improves H2 bubbles coalesce to grow (Abdelwahab et al., 2009). Yet, the formation of bigger coalesced bubbles causes a rapid increase in voltage as results of increment of resistance. This result is consistent with experimental phenomena that a bigger bubble broke the surface of the emulsion intermittently during electrolysis, meantime the voltage showed with a steep rise and then decreased. 0

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Fig. 7 Effect of tilt angle of electrodes on energy consumption with electrolytic time ( current density is 120Am-2; plate spacing is 4 cm ) In addition, the experiments have found that gentle increasing APE can greatly reduce fluctuation of voltage. Ultimately, the cell with horizontal electrodes has greatest energy consumption of 4.45 kwhkg-1 oil, at electrolysis time of 20 min.

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The removal of emulsified oil from oil-in-water emulsion in bath EC reactor with APE was evaluated in this work using four indexes as oil removal efficiency in the center between electrodes, energy consumption and Al consumption and evenness index of oil removal efficiency. The results were drawn as followed: 1. The present results have shown that the cell with α(d) APE, has best demulsification efficiency in the center section, because of reducing the deposition of floc on the surface of electrodes and quickening flotation velocity of coalesced oil without any obstacles. The optimal oil removal efficiency in the center is 97.07% with α(d) APE at 4 cm. Lesser APE increases the oil removal efficiency in the center except the α(d) APE as a result of the better distribution uniformity of rising H2 bubbles. In same cases, the emulsified oil up to 96.6% in the center is removed with horizontal electrode. 2. The cell with horizontal and vertical electrodes has least EIOR of 85.01% and 97.18% on the vertical and horizontal cross-section at 2 cm, respectively, as a result that the emulsion outside the plates is suffered slight mixing. In contrast, the cell with α(d) APE has higher EIOR of 99.89% and 99.76% on the vertical and horizontal cross-section, respectively, as a result that migration path of floc and H2 bubbles could improve mass transfer of whole EC reactor. 3. The cost consumption at 20 min is greatest with horizontal electrodes when the current

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density is 120 Am-2, as Al consumption of 0.13 g Al g-1 oil and energy consumption of 4.45 kwhkg-1 oil, because locating anode above cathode promotes coalescence of H2 bubbles with a consequent voltage fluctuation. The cell with vertical electrodes has greater cost consumption as a result of nonuniformity distribution of current density, as Al consumption of 0.12 g Al g-1 oil and energy consumption of 3.41 kwhkg-1 oil. In general, the cell with α(d) APE has least cost consumption, as Al consumption of 0.11 g Al g-1oil and energy consumption of 2.99 kwhkg-1 oil. It can be concluded from this study that due to greater separation efficiency, higher EIORs and economy, the EC cell with α(d) APE, a optimal design of electrode has the potential of saving the mechanical stirring energy for demulsification.

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This study was supported by the National Natural Science Foundation of China (Grant No. 51406240), the Natural Science Foundation of Shandong Province (Grant No. ZR2014EEQ003) and the Fundamental Research Funds for the Central Universities (Grant No. 14CX02211A and No. 12CX04070A).

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References

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1. Tilt angles of parallel-plate electrodes first proposed was evaluated and optimized. 2. A novel universal evaluation index named as the evenness index of oil removal rate was employed to fully reflect distribution characteristics of demulsification efficiency within the

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electrocoagulation reactor. 3. A special value, namely α(d), is the optimal orientation of electrode, when the upper end of

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anode and the lower end of cathode is in a line vertical to the bottom of reactor.