Electrocoagulation of Crude Oil From Oil-In-Water ...

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Electrocoagulation of Crude Oil From Oil-In-Water Emulsions Using a Rectangular Cell with a Horizontal Aluminium Wire Gauze Anode Y. O. Fouad

a

a

Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Accepted author version posted online: 10 Feb 2012.Published online: 30 Jan 2013.

To cite this article: Y. O. Fouad (2013): Electrocoagulation of Crude Oil From Oil-In-Water Emulsions Using a Rectangular Cell with a Horizontal Aluminium Wire Gauze Anode, Journal of Dispersion Science and Technology, 34:2, 214-221 To link to this article: http://dx.doi.org/10.1080/01932691.2012.657978

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Journal of Dispersion Science and Technology, 34:214–221, 2013 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932691.2012.657978

Electrocoagulation of Crude Oil From Oil-In-Water Emulsions Using a Rectangular Cell with a Horizontal Aluminium Wire Gauze Anode Y. O. Fouad Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt

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GRAPHICAL ABSTRACT

Separation of crude oil from oil-in-water emulsions in a square batch electrocoagulation cell, using an aluminum screen as the sacrificial anode was studied, the cathode was a rectangular aluminum plate placed on the cell bottom below the anode. The oil separation efficiency was insensitive to the sodium chloride electrolyte concentration. Increasing current density increased the rate of oil separation from the emulsion. Also it was found that increasing the number of screens per stack at the same current intensity, does not affect the efficiency of oil separation. The removal doesn’t depend on the initial pH in the range of pH 38. Keywords Electrocoagulation, oil=water emulsions, water treatment, woven screens

1. INTRODUCTION Pollution caused by oil-water emulsion is generated by several industrial activities such as refineries, machining shops, off shore platforms, automotive repair shops, and oil transportation, distribution, and storage facilities. Several forms of oil and grease present in wastewater are free, dispersed, or emulsified. The droplet size of oils is a major factor for their classification. Free oil has a droplet size larger than 150 mm. Dispersed oil is characterized by droplet sizes ranging from 120 to 150 mm. Oily water, with droplet size less than 20 mm, is classified as emulsified oil. Oily wastewater treatment can be classified into two categories: primary and secondary treatment systems. The Received 11 December 2011; accepted 3 January 2012. Address correspondence to Y. O. Fouad, Faculty of Engineering-ElHoreya Street, Alexandria, Egypt. E-mail: yasossama@ gmail.com

primary treatment is employed to separate floatable oils from water and emulsified oil. This system is suitable for oil or grease that is of lower specific gravity than water. Skimmers and gravity separators are the major treatment systems belonging to this group. A secondary treatment system’s goal is to treat or break emulsified oil and, then, remove oil from water. Various techniques used for separation of emulsified oil include chemical treatment, dissolved air flotation, membrane filtration, electrical process, hydrocyclone, and the novel technology of ultrasonic field application.[1,2] A growing interest in electrocoagulation has been spurred by the search for reliable, cost-effective water treatment processes. This technology delivers the coagulant in situ as the sacrificial anode corrodes, due to an applied potential, when aluminium electrodes are used, the aluminium dissolves at the anode and hydrogen gas is released at the cathode. The coagulating agent combines with the pollutants to form large size flocs. As the bubbles rise to

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SEPARATION OF CRUDE OIL FROM OIL-IN-WATER EMULSIONS

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the top of the tank they adhere to particles suspended in the water and float them to the surface.[3,4] Electrocoagulation possess many advantages that makes it an interesting option for water treatment: i. The only chemicals required are those used to control pH. ii. They have the ability to handle a wide range of pollutants, that is, they can process multiple contaminants—suspended and colloidal solids, heavy metals, free and emulsified oils, bacteria, and organics; iii. They can tolerate fluctuations in influent water quality. iv. The gas bubbles produced during electrolysis can float the pollutant to the top of the solution, where they can be more easily concentrated, collected, and removed;. v. They have the ability to recycle water for reuse; vi. The residue is reduced, that is, the system produces half to one-third of the residue in chemical coagulation. The amount of dried residue is 0.20–0.37 kg= kg COD removed. The residue tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides=hydroxides, vii. They integrate benefits of chemical precipitation, flotation, and settling in a much smaller footprint, viii. They are fully automated, minimal operator attention, ix. They have low power consumption; that is the power requirement is only 0.5 kWh=m3 under a set of typical operating conditions, x. Electrocoagulation is more efficient than chemical coagulation in turbidity removal. xi. The technique can be conveniently used in rural areas where electricity is not available, since a solar panel attached to the unit may be sufficient to carry out the process. Chemical coagulation is carried out by adding salts such as ferric sulphate or aluminium sulphate to the emulsion followed by a precipitation reaction. This method generates a high water-content sludge with attendant dewatering and disposal problems beside the high cost of the coagulating chemicals, mechanical methods such as ultrafiltration are limited in use because of the rapid fouling of the membranes used in ultrafiltration.[5] Although some work on electroflotation was conducted using cells with horizontally oriented screen electrodes[6] the majority of electrocoagulation studies were conducted using the traditional vertical parallel plate cell[7–12] despite the apparent merits of the horizontally oriented electrode cell. The aim of the present work is to examine the performance of a rectangular cell with horizontal electrodes. The cathode is made of a horizontal aluminium plate rested on the cell bottom while the anode was made of an aluminium woven screen with mesh number 10 (the single screen, the closely packed multiscreens and separated multiscreens

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were tested) placed above the cathode at a short distance from it. This design offers the following merits: i. The cathodically evolvedH2 bubbles are uniformly distributed over the whole cross sectional area of the cell, i.e., the floating ability of H2 bubbles is uniform as opposed to the vertical cell where H2 evolve in the form of a curtain beside the vertical cathode; besides, the thickness of the bubble layer increases along the vertical electrode with a consequent increase in the cell resistance and the nonuniformity of current distribution.[13] ii. Locating the dissolving Al anode above the H2 evolving cathode leads to improving the mixing conditions at the anode surface by virtue of the macro convection induced by the rising swarm of H2 bubbles.[14,15] As a consequence concentration polarization would decrease at the anode and dissolved Al3þ would be uniformly distributed in the emulsion. iii. The evolution of the hydrogen gas from the cathode through the aluminium screens with a high velocity increases the degree of mixing of Al3þ with the emulsion with a consequent increase in the frequency of collision of oil drops and the rate of their flotation. The effect of different operating parameters on the performance of the cell especially its ability to bring down oil content in the oily wastewater below the maximum permissible value of 10 ppm[16] was investigated. 2. EXPERIMENTAL TECHNIQUE Figure 1 shows the cell and electrical circuit used in the present work, the cell consisted of a rectangular plastic container with a 15  15 cm base and 25 cm height. The cathode consisted of a horizontal Al plate placed on the cell bottom;

FIG. 1. Cell and electrical circuit:1 ¼ horizontal rectangular aluminium cathode, 2 ¼ aluminium screen anode, 3 ¼ rectangular plastic container, 4 ¼ synthetically prepared emulsion, 5 ¼ level of the emulsion, 6 ¼ multirange ammeter, 7 ¼ voltmeter, 8 ¼ DC power supply.

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the back of the Al plate was insulated with epoxy resin. The anode was made of an aluminium array its mesh number (wire=in) was 10; three configurations of screens were tested (the single screen, the closely packed multiscreens and the separated multiscreens), the aluminum screen was held in position by two vertical Al strips welded to screen ends, the vertical Al strips acted as a current feeder to the anode. The vertical strips were insulated with epoxy resin. The cathode–anode separation was fixed at a distance of 1.5 cm; and in case of separated multiscreens the same distance was held between consecutive screens. The electrical circuit consisted of 20 V dc. Power supply with a voltage regulator, a multirange ammeter, all connected in series with the cell, a voltmeter was connected in parallel with the cell to measure its voltage when needed. Synthetic emulsions of initial oil concentration of 500 mg=L oil were prepared by mixing crude oil with saline water containing 0.15% of polyethylene oleate emulsifier in agitated vessel. Water with NaCl concentration of 3.5%, 2.7%, 1.5%, and 85 ppm were prepared to simulate the sea water, the brackish water and the tap water. Before each run the cell was filled with 3.5 L of oil–water emulsion, during electrolysis a sample of 10 cm3 was taken from the bulk of the emulsion every 5 minutes for oil analysis. Oil concentration in the emulsion was determined by means of a spectrophotometer using a wavelength of 580 nm. A calibration curve (absorbance vs. oil concentration) was used to determine oil concentration from the sample absorbance measured by the spectrophotometer. During electrolysis the cell was placed in a thermostated water bath to control its temperature. Results were expressed in terms of percentage of oil removal. % Removal ¼

C0  Ct  100 C0

½1

where C0 is initial concentration and Ct is final concentration. Screen area used to calculate the current density was calculated in terms of the wire diameter (dw) and mesh number (Nm) using the method of Amour and Cannon[17] as shown below; the specifications of the screen used in the experiments are presented in Table 1. Let (a) be the total screen surface area per total volume of one screen, cm2=cm3. TABLE 1 Specification of the screen used in the present study Mesh number (wire=in) Mesh number, Nm (wire=cm) Wire diameter, dw (cm) Screen Thickness (cm) Specific area (cm2=cm3) Screen porosity

10 3.937 0.071 0.142 12.843 0.772

The value of (a) can be calculated from the equation a ¼ pLN2m

½2

where "

1 L¼ þ d2w N2m

#12 ½3

where dw ¼ wire diameter, cm, Nm ¼ mesh number (number of wires=cm). The total screen area (A) is given by A ¼ anVs, where Vs is the volume of a single screen, n is the number of screens per array. In calculating the volume of screens; the thickness is taken as twice the wire diameter (2dw). The screen porosity can be calculated from the equation e¼1

pLN2m dw 4

½4

where e is screen porosity. For a further evaluation of the economic feasibility of electrocoagulation method, the energy consumption was calculated as follows:[18,19] EnergyConsumptionðkWh=kgoilremovedÞ ¼

VIt  103 60ðCo  Ct Þ  treatedvolumeðLÞ

½5

where V ¼ cell voltage in volts, I ¼ cell current in amperes, t ¼ electrocoagulation time in minutes, Co ¼ initial oil concentration in ppm, Ct ¼ oil concentration at time t in ppm. 3. RESULTS AND DISCUSSION 3.1. Effect of Different Parameters on the Percentage Oil Recovery In order to explain the obtained results it would be illuminating to outline the electrocoagulation mechanism: Electrocoagulation occurs via serial steps such as[20] 1. Electrolytic reactions at electrode surfaces, such as Al, Al ions form at the anode and hydroxyl ions are generated at the cathode, 2. In situ oxidation of Al ions and subsequent precipitation of aluminum hydroxide in aqueous phase, and 3. Adsorption of soluble or colloidal pollutants on coagulants which are removed by sedimentation or flotation.

SEPARATION OF CRUDE OIL FROM OIL-IN-WATER EMULSIONS

The main reactions are as follows Anode:  Al0ðSÞ ! Al3þ ðaqÞ þ 3e

½6

Cathode: Hydrogen evolution occurs at the cathode according to the following reactions: 3 3H2 O þ 3e ! H2ðgÞ þ 3OH ðaqÞ ðalkaline solutionÞ ½7 2 or

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3  3Hþ ðaqÞ þ 3e ! H2ðgÞ ðacidic solutionÞ 2

½8

When the anode potential is sufficiently high, secondary reactions, namely oxygen evolution, take place at the anode according to the equation.[21] 2H2 O ! O2 þ 4Hþ þ 4e

½9

Al3þand OH ions generated by electrode reactions [6 and [7 react to form different monomeric and polymeric species which transform finally into Al(OH)3(S).[18] þ Al3þ ðaqÞ þ 3H2 O ! AlðOHÞ3 ðSÞ þ 3HðaqÞ

½10

Freshly formed amorphous Al(OH)3(S) ‘‘sweep flocs’’ have large surface areas which is beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these flocs are removed easily from aqueous solution by sedimentation or floatation. On the other hand, at high pH values, both Al cathode and anode may be chemically attacked by OHions in view of the amphoteric nature of Al. 2Al0ðSÞ þ 6H2 O ! þ2OH ! 2AlðOHÞ 4aq þ 3H2 g

½11

It is also possible that electrophoresis, i.e., migration of the negatively charged oil drops towards the positively charged anode under the influence of electrical field contributes to the process of demulsification as a result of neutralization of the negatively charged oil drops at the anode surface.[1,21] The neutral Al hydrolysis products such as Al (H2O)3 (OH)3 contributes to the demulsification process by virtue of the preferential adsorption of oil drops on Al[(H2O)3 (OH)3].[22]The coalesced oil drops adhere to the cathodically generated small sized H2 bubbles and float to the top of the emulsion.[20.22] Accordingly, two major interaction mechanisms are being proposed namely: precipitation and adsorption, each one being suggested for a separate pH range. Flocculation

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in the low pH range is explained as precipitation while the higher pH range (>6.5) as adsorption.[20] It is also worth noting that during electrocoagulation treatment, some fine particles of hydroxides re-enter the suspension, which can slightly affect the TS, and TSS concentrations (e.g., increase in turbidity).[23] Al complexes acting as coagulants are adsorbed on the particles and thus neutralize the colloidal charges, resulting in destabilization of the emulsion. This phenomenon is similar to the action of chemical coagulants in the conventional chemical treatment. Hydrogen bubbles formed at the cathode can adsorb on the flocculated species and induce their flotation. The bubbles formed also reduce fouling of the cathode surface which could occur by the formation of deposits.[24] Figure 2 shows the effect of current density on the oil percentage removal vs. time, in general increasing current density increases the rate of oil separation from the emulsion, and this may be attributed to the following effects[22]: 1. The increase in Al3þ content of the emulsion according to Faradays law.[1,2,6] 2. The increase in the H2 discharge rate resulting from the increase in current density increases the upward velocity of the rising gas–liquid dispersion past the cylinder array anode with a consequent increase in the rate of Al3þ diffusion away of the anode surface; this reduces anode concentration polarization and increases its dissolution efficiency.[20,22,25] 3. Increasing the current density increases the rate of demulsification by electrophoresis.[20] 4. As the current density increases the floating ability if H2 bubbles increases as a result of increasing the number of bubbles and the reduction in bubble size; small sized bubbles rise slowly, thereby their chance of attaching themselves to neutralized oil drops is higher than the fast rising large bubbles. To study the effect of NaCl concentration four different NaCl concentrations were used namely 85 ppm (tap water),

FIG. 2.

The effect of current density on the oil percentage removal.

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1.5%, 2.7%, and 3.5%; the effect of the concentration of sodium chloride as an electrolyte can be interpreted in terms of two opposing effects. The first effect is that the presence of chloride ions remove the passive oxide layer form on electrode surface which tends to form on aluminum anodes at relatively high potential and limit aluminum dissolution,[25] hence it increases the availability of aluminum hydroxide in the solution and improves the efficiency of oil separation.[20] The second effect is that the degree of oil separation decreases as the NaCl content of the aqueous phase increases. This may be attributed to the fact that demulsification by electrophoresis decreases with increasing NaCl concentration because the competing chloride ion migrates to the anode surface in preference to the negatively charged oil drops. Despite the high rate of oil removal in emulsions with low salt content, a high voltage penalty is incurred as a result of the low solution conductivity.[22] The net result in our case as it is obvious from Figure 3 that the two effects are nulling each other, making the oil separation efficiency using aluminum screens electrocoagulation insensitive to the sodium chloride electrolyte concentration. However, higher concentrations are preferred, economically, to reduce the energy consumption. This Result agrees with the finding of Xinhu Xu.[26] To examine the effect of the initial pH of the emulsion the pH was adjusted to the desired values by aliquots of sodium hydroxide or hydrochloric acid. Figure 4 shows that pH has a little effect on the percentage oil removal within the pH range 3–8. For neutral or acidic pH, the effect is less significant on the removal whose value is higher than 90%. Solid precipitate of aluminium hydroxide formed at pH 6–7 is a precursor for oil removal by coagulation.[24]. In alkaline medium (pH > 7), the final pH does not vary very much and a slight drop was recorded. However

FIG. 3. The effect of NaCl concentration on the oil percentage removal.

FIG. 4. The effect of the initial solution pH on the oil percentage removal.

for initial pH over 10, ionic forms of Al-hydroxides predominate, which reduces the efficiency of the treatment by electrocoagulation.[22,24] Also amphoteric Al(OH)3 dissolves in the alkaline solution to form aluminates; as such it does not contribute to the process of oil coagulation and floatation. However as the time passes excess anodically dissolved Al3þ becomes available to enhance the rate of demulsification.[22] To examine the effect of number of screens per anode on the percentage oil removal, anodes composed of 1, 2, and 3 aluminum screens at the anode, were carried out. Results showed that increasing the number of electrodes has no effect on the efficiency of oil separation as shown in Figure 5. Figure 5 shows that increasing the no of closely packed screens per anode at a given current density tends to slightly reduce the percentage oil recovery probably because screens reduce the momentum of the rising H2-solution dispersion by

FIG. 5. The effect of the number of closely packed aluminum screens per anode on oil percentage removal.

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virtue of friction between the upward dispersion and the screens, accordingly the mixing efficiency of the gas-liquid dispersion decreases with a consequent decrease in the percentage oil recovery. Figure 6 shows that increasing the number of separated screens (as anode) from 1 to 2 to 3 screens increased the percentage of the removal at the beginning of the run but ended at the same percentage of removal this may be due to the fact that the rising swarm of H2 bubbles receives higher resistance from the additional anode with a consequent decrease in its rise velocity; this improves the floating ability of H2 bubbles and increases the degree of removal but due to the high performance of the single screen they both meet at the end of the run at the same high percentage oil removal. 3.2. Effect of the Different Parameters on Energy Consumption (EC) during Electrocoagulation To assist the economic feasibility of electrocoagulation in removing oil from oil-water emulsions compared to other methods, the effect of different variables on energy consumption was studied as follows: Figures 7–11 show the effect of current density, NaCl concentration, pH, no of closely packed screens per anode and no of separated screens per anode on the energy consumption which was calculated from Equation (5). Energy consumption which ranges from 0.48 to 12.75 kWh=kg oil removed increases with increasing current density (Figure 7) because of the increase of the ohmic drop (IR) and activation polarization at the cathode and the anode (g).[27] Energy consumption increases with decreasing the NaCl concentration (Figure 8) as a result of the increase of the cell resistance (R) and the ohmic drop (IR) which contributes to the total cell voltage (V) according to the equation: V ¼ Veq þ ganode þ gcathode þ IR

FIG. 7.

The effect of current density on the energy consumption.

Veq ¼ equilibrium cell voltage, g anode, g cathode ¼ activation polarization at the anode and cathode, IR ¼ ohmic drop. The pH has little effect on energy consumption (Figure 9) except for pH 11 which increases the energy consumption considerably because of the low percentage oil recovery at this pH. Figure 10 shows that increasing the number of closely packed screens per anode increases the energy consumption probably because horizontal screens lying above the cathode hinder the escape of the cathodically evolved H2 bubbles from the solution; as the holdup of the bubbles increases in the solution the solution resistance, R, increases[27] with a

½12

FIG. 6. The effect of increasing number of distant aluminum screens on the percentage of oil removal.

FIG. 8. The effect of NaCl concentration on the energy consumption.

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consequent increase in the IR drop, cell voltage, and the energy consumption. The power consumption increases with increasing the number of separated screens per anode as shown in Figure 11 this is maybe due to the higher resistance met by the hydrogen bubbles during their path upwards as is the case with the closely packed screens per anode. 4. CONCLUSIONS

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FIG. 9.

The effect of the initial solution pH on the energy consumption.

FIG. 10.

The effect of increasing number of aluminum screens at the anode on the energy consumption.

1. The present results have shown that cells with horizontal screen anode and horizontal sheet cathode electrodes are efficient in separating oil from oily wastewater by electrocoagulation. Cells with horizontal electrodes have a high mixing efficiency and higher floating ability thanks to the uniform distribution of the cathodically evolved H2 bubbles all over the cell cross-section. 2. Simultaneous electrocoagulation and electroflotation result in a rapid oil separation to a degree depending on current density, temperature, NaCl concentration and pH of the emulsion. Under the present range of experimental conditions most of the oil content of the emulsion separate within 10 minutes after the beginning of electrolysis; oil separation using aluminum screen electrocoagulation followed by filtration can reach  100% removal after 40 minutes electrocoagulation time. 3. The obtained results showed that increasing the current density increases the rate of oil separation from the emulsion, reducing the time required for required degree of removal; the oil separation efficiency using aluminum screens electrocoagulation was insensitive to the sodium chloride electrolyte concentration; however, higher concentrations are preferred, economically, to reduce the power consumption. Also it has been concluded that increasing the number of closely packed screens per anode at the same current intensity, hasn’t affected the efficiency of oil separation and increasing the number of separated screens per anode from 1 to 2 screens, 1.5 cm apart, has not affected the oil separation efficiency. REFERENCES

FIG. 11.

The effect of increasing the number of the anodes on the energy consumption.

[1] Comninellis, C. and Chen, G. (eds.). (2010) Electrochemistry for the Environment; New York: Springer Science DOI 10.1007=978-0-387-68318-8 1. [2] Wang, L.K., Hung, Y.T., and Shammas, N.K. (2006) Handbook of EnvironmentalEngineering; Totowa, NJ: The Humana Press, Vol 4, pp. 45–52. [3] Holt, P.K., Barton, G.W., Wark, M., and Mitchell, C.A. (2002) A quantitative comparison between chemical dosing and electrocoagulation. Colloids Surfaces A, 211: 233–248. [4] Emamjomeh, M.M. and Sivakumar, M. (2009) J. Environm. Manag., 90: 1663–1679. [5] Benito, M., Rois, G., Pazos, C., and Cosa, J. (1998) Trends Chem. Eng., 4: 203–231.

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