I NTERNATIONAL J OURNAL OF C HEMICAL R EACTOR E NGINEERING Volume 9
2011
Article A39
Augmentation of the Performance of Batch Electrocoagulation Unit by Using Gas Sparging Ahmed Hassan Elshazly∗
∗
Alexandria University, elshazly
[email protected] ISSN 1542-6580 c Copyright 2011 The Berkeley Electronic Press. All rights reserved.
Augmentation of the Performance of Batch Electrocoagulation Unit by Using Gas Sparging∗ Ahmed Hassan Elshazly
Abstract The present work investigates the effect of gas sparging in improving the performance of a batch electrocoagulation unit used to treat wastewater generated from the dyeing industry. Monopolar cylindrical aluminum electrodes were used. Many variables were investigated such as superficial gas velocity, current density, initial dye concentration, area ratio (cathode/anode), time of operation and the effect of adding chemical coagulant as FeSO4 . The results show that the percentage of dye removal has been increased by a factor ranging from 2.52 to 5.14 by increasing the gas flow rate from 0.4 to 0.8 liter/min respectively and that about 93.5 percent of the dye can be removed within 60 minutes. Also it was found that using gas sparging is more efficient than adding chemical coagulant as ferrous sulfate for the removal of dye from wastewater. The power consumption for the unit was measured for different gas flow rates and different current densities; the results show that lower gas flow rate can improve the economy of the process. KEYWORDS: gas sparging, electrocoagulation, dye removal, wastewater treatment, decolorization
∗
Ahmed Hassan El-Shazly, Chemical Engineering Department, Faculty of Engineering, Alexandria University, Egypt; email: elshazly
[email protected].
Elshazly: Augmentation of the Performance of Batch Electrocoagulation Unit
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INTRODUCTION Effluent from the dyeing industry can be treated by traditional methods like physical and chemical processes. The biological methods are cheap and simple to apply, but cannot be applied to most textile wastewaters because most commercial dyes are toxic to the organisms used in the process. Electrocoagulation technology (EC) is considered to be a potentially effective tool for treatment of textile wastewaters with high removal efficiency. The removal efficiency was found to depend on the initial pH, the dye concentration, the applied current density, the electrolysis time in batch model, and the flow rate in continuous flow reactor (Emamjomeh and Sivakumar, 2009). Many attempts have been made for improving the performance of the EC process, depending on the orientation of the electrode plates, the electrocoagulation cell can be horizontal or vertical and in monopolar or bipolar mode for the water flow through the space between the plates (Daneshvar et al., 2004; Fan et al., 2004; Mollah et al., 2004a). For water treatment, a cylindrical design can be used. An alternative option of cylindrical design is that a venture is placed in the center of the cylinder with water and coagulants flowing inside it to achieve a good mixing (Chen, 2004)[2]. The electrocoagulation reactor can be operated in continuous as well as in batch operation (Mollah et al., 2001, 2004a, 2004b). Another improvement is a combined electrocoagulation and flotation process (Pouet and Grasmick, 1995a, 1995b; Ibanez et al., 1995; Jiang et al., 2002a, 2002b) in which the gas bubbles can carry the pollutant to the top of the solution where it can be more easily concentrated, collected, and removed. Others have also combined electrocoagulation and electroflotation for process improvement (Hosny, 1996; Chen et al., 2002; Ibrahim et al., 2001; Alexandrova et al., 1994). Electroflotation (EF) was proven to be an efficient tool for industrial wastewater treatment. The effective electroflotation obtained is primarily attributed to the generation of uniform and tiny bubbles. It is well known that the separation efficiency of a flotation process depends strongly on bubble sizes (Chen, 2004). This is because smaller bubbles provide larger surface area for particle attachment. The sizes of the bubbles generated by electroflotation were found to be log-normally distributed with over 90% of the bubbles in the range of 15–45µm for titanium-based, dimensionally stable anode (DSA) (Chen et al., 2002). Electroflotation clearly shows advantages over dissolved air flotation (DAF), settling, or impeller flotation (IF) (Hosny, 1996; Chen et al., 2002; Ibrahim et al., 2001; Alexandrova et al., 1994). Electroflotation is usually combined with electrocoagulation or chemical flocculation. In order for the chemical reagents to mix well with the pollutants before flotation, fluidized media have been used (Gvozdez and Ksenofontov, 1986). This design allows an intensive contact of the solid phase in the mixing chamber with coagulants to
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form suspension particle agglomerates and at the same time not to break up the flocculates formed. The most important parameter involved in electroflotation is the power consumption, especially when the conductivity is low; direct application of EF consumes large amounts of electricity. Adding NaCl salt, in addition to decreasing the distance between electrodes, was found helpful in reducing the power consumption of electrocoagulation unit (Ibrahim, 2001). On the other hand, gas sparging is widely employed in many industrial applications as in reactors or gas-liquid contactors. Typical examples can be found in oxidation, hydrogeneration, chlorination, and wastewater treatments. Bubble columns have the advantages of being geometrically simple, easily manufactured, free of moving parts, superior for gas-liquid interfacial mass and heat transfer, easily operated and cost effective (Guang et al., 2009; Deckwer et al., 1992). Ibl et al. (1975) tested the effect of gas sparging on the rate of mass transfer using a parallel plate electrochemical reactor and concluded that gas sparging is an effective way of stirring and is cheaper than mechanical stirring. Since then, gas sparging has been used to enhance the rate of liquid solid mass transfer with vertical plate electrodes (Sigrist et al., 1979) fixed bed electrodes (Sedahmed et al., 2001), vertical bubble columns (Nosier et al., 1997; Zarraa et al. 1994) and horizontal and vertical screens (Zaki et al., 2001). Hwang and Hsu (2009) found that for a bioreactor process, the existence of small bubbles in bubbly flow is appropriate for supplying a greater gas-liquid interface area for improving oxygen transfer. Cabassud et al. (2001) analyzed the influence of hydrodynamic parameters of gas-liquid flow on the cake characteristics in ultrafiltration hollow fibers. They claimed that the flux enhancement was controlled by the mixing or turbulence near the membrane surface. This paper investigates the possibility of improving the performance of batch cylindrical electrocoagulation unit used for textile waste treatment, especially for dye removal, using gas sparging by forcing air with different flow rates through the EC unit. A cylindrical aluminum electrode connected to the wall of the reactor was used as the cathode and an aluminum rod fixed at the center of the EC unit was used as the anode. 2. EXPERIMENTAL 2.1 Chemicals used
Terasil Yellow 4G, commercial pyridone dye (Ciba Co., Egypt), was chosen as the model compound, whose chemical structure is given in Figure 1.
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A 3.5% sodium chloride (NaCl) solution was used for depassivation of aluminum electrodes and increasing solution conductivity. Ferrous sulfate was used as a chemical coagulating agent. Carbon tetrachloride was used for electrode degreasing.
Figure 1. Terasil Yellow 4G chemical structure
D.C. Power supply
+
Ammeter
-
Voltmeter
A V
Aluminum anode Aluminum cathode
Valve
Rotameter Air compressor
Air
Figure 2. Experimental setup and electrical circuit
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2.2 Setup and procedure The experimental setup and electrical circuit used is shown in Figure 2. The setup consisted of a 1000 ml cylindrical glass container that was 15 cm in diameter and 15 cm in height, connected at the bottom to a sintered glass G4 type, with a 0.33 hp compressor connected to the bottom of the sintered glass and with a check valve for preventing solution from flowing downward. Two monopolar aluminum electrodes (cathode and anode) were placed vertically, with the anode (aluminum rod) at the center of the cell parallel to the cathode (aluminum cylinder) adherent to the wall, the anode diameter ranged from 1 to 3 cm with area ratio (cathode/anode) ranging from 15 to 5 respectively. Before each run, aluminum electrodes were immersed in diluted carbon tetrachloride solution for few minutes for removing any greases or oxides, washed with distilled water, and finally connected to a DC power supply (25 volts, 10 Ampere) fitted with a voltage regulator. A multirange ammeter was connected in series with the cell and a DC voltmeter was connected in parallel with the cell to measure its voltage. In each run 1000 ml of dye solution with initial concentration ranging from 10 to 40 ppm were placed at the electrolytic cell with 3.5 wt percent sodium chloride (constant concentration was used in all experiments), and then air was sparged into the solution at different flow rates ranging from 0.0 to 0.8 liter/min that have been measured by means of a Rotameter, for different time intervals ranging from 10 to 60 minutes. The dye concentration at any time was found by using a calibration curve prepared by measuring the absorbance for different concentrations of dye at wave length (λ) of 520 nm using a UV-VIS Spectrophotometer (this wave length was found out experimentally). The percentage of dye removal was evaluated by measuring the absorbance of the dye solution before and after each run. The following equation was used % Dye removal = 100(Co-C)/Co where Co is the initial dye concentration and C is the dye concentration at the end of electrolysis time. Many variables, such as air flow rate ranging from 0.5 to 0.8 liter/min, time of operation from 10 to 60 min, initial dye concentration from 10 to 40ppm, applied current density ranging from 1 to 3.28 mA/cm2, and the area ratio (cathode/anode) ranging from 3 to 15, were investigated for their effect on the removal efficiency of the gas sparged EC unit. Also the effect of using chemical coagulant as FeSO4 was examined with different concentrations ranging from 5 to 20 ppm under different gas flow rates. For economic evaluation of the unit performance the power consumed for the electrical circuit and for gas sparging was measured using a wattmeter under different conditions of gas flow rate and current density.
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Elshazly: Augmentation of the Performance of Batch Electrocoagulation Unit
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3. RESULTS AND DISCUSSIONS 3.1 Effect of inlet air flow rate As shown in Figure 3 the percentage removal efficiency of dye from waste solution has been increased by increasing the superficial air flow rate sparged to the EC unit. To clarify the role of gas sparging on the unit performance the following explanation has to be considered. For an EC unit with aluminum electrodes the following reactions take place. At the anode: Al → Al+3 +3e
(1)
Al+3 + 3OH- → Al (OH)3
(2)
At the cathode: 2H2O + 2e → H2 + 2OH-
(3)
With the overall cell reaction that: Al + 3H2O → Al (OH)3 + 3/2 H2
(4)
Accumulation of anodic and/or cathodic product on the electrode surfaces will certainly increase the resistance to mass transfer of Al+3 and/or H2 bubbles from the vicinity of the anode and cathode surfaces respectively to the solution bulk, which increases polarization on the electrode surfaces that reduces the amount of Al+3 and H2 bubbles generated and consequently reduces the unit performance. In addition, the increased polarization on both anode and cathode surfaces due to slow transfer of Al+3 and/or H2 bubbles will increase the power consumption of the unit according to equations (1, 2) E = Eo+ (ηa+ ηc)anode + (ηa+ ηc)cathode+ R
(5)
Where E is the required cell voltage, Eo is the equilibrium potential difference, ηa and ηc are activation and concentration polarization respectively, R is cell resistance due to solution and wire connections. Also higher cell potential will increase the possibility of anode passivation, which reduces the amount of Al+3 generated and decreases the unit performance. Gas sparging can ensure good mixing conditions inside the EC unit that reduces the thickness of diffusion layers in the vicinity of the anode and cathode
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electrodes that enhance the rate of mass transfer of Al+3 and H2 bubbles from the anode and cathode surfaces respectively to the solution bulk. In addition, the presence of air bubbles combined with H2 bubbles generated at the cathode will enhance the flotation rate and will improve the unit performance.
% Dye removal
2
70 60 50 40 30 20 10 0
Current density = 1.5 mA/cm , Operating time = 30 min, Area ratio(Cathode/Anode)=15, Intial dye concentration(ppm) 10 20 30 40
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Air flow rate(liter/min)
Figure 3. Percentage dye removal vs. air flow rate at different initial dye concentrations Initial dye concentration= 20 ppm, Time of operation = 30 min Area ratio (Cathode/Anode) = 15, Air flow rate (liter/min) 70
0.4 0.5
60
%Dye removal
0.6 50
0.7 0.8
40 30 20 10 0 1
1.5
2
2.5
3
3.5
4
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Current density(mA/cm )
Figure 4. Percentage dye removal vs. current density at different air flow rate
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3.2 Effect of applied current density As shown in Figure 4, the decolorization of dye was increased by increasing the applied current density up to 3 mA/cm2. Increasing current density above this limit will reduce the efficiency of dye removal. The above results can be attributed to the fact that increasing current density will increase the dissolution rate of aluminum electrode according to Faraday's law with the formation Al+3 and hence the formation of Al(OH)3 coagulant according to reactions 1 to 4. A higher rate of freshly formed amorphous Al(OH)3 having large surface area on which rapid adsorption of dye molecules takes place with a consequent removal of color from wastewater In addition the cathodically evolved H2 bubbles aided with the induced air sparging float the Al (OH)3 along with the adsorbed dye molecules to the upper surface of the solution. The decrease in the percentage of removal at current densities higher than 3 mA/cm2 may be ascribed to the potential passivation of the aluminum anode due to the higher current and potential applied, which decreases the dissolution rate of aluminum anode and the percentage of removal of dye. 3.3 Effect of initial dye concentration Figure 3 shows that the percentage of dye removal has been decreased by increasing the initial dye concentration. This can be ascribed to the fact that higher concentrations of dye will block adsorption sites of Al (OH)3 rapidly and Al(OH)3 will decrease its ability to adsorb more dye molecules. In addition increasing dye concentration would probably increase activation polarization via adsorption on the anode and cathode with a consequent decrease in the rate of aluminum dissolution at the anode and hydrogen evolution at the cathode that reduces the unit performance. 3.4 Effect of distance between electrodes The distance between electrodes was changed by increasing the anode diameter from 1 to 3 cm, thus the distance between electrodes changed from 14 to 12 cm respectively with a constant cathode diameter of 15 cm. As shown in Figure 5, the percentage of removal efficiency of dye decreased by increasing the distance between electrodes. The above result can be ascribed to the fact that increasing the anode diameter decreases the annular space available for gas flow; this increases the superficial gas velocity in the area between cathode and anode that ensures good mixing conditions in this area. This decreases the diffusion layer thickness and improves the transfer of anode and cathode product from the
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vicinity of electrodes area to the solution bulk, which improves the unit performance.
Figure 5. Percentage dye removal vs. distance between electrodes at different air flow rates 3.5 Effect of operating time The percentage removal efficiency of dye molecules was increased by increasing the operating time of the cell as shown in Figure 6. It is clear that for a solution of 40 ppm concentration of dye and a current density of 1.5 mA/cm2, about 45.9 to 93.5% of dye color was removed within 60 minutes with changing air flow rate from 0.4 to 0.8 liter/min respectively. These results show that gas sparging improved the unit performance by a factor ranging from 2.52 to 5.14 depending on gas flow rate. Note that gas sparged electrocoagulation of dye is faster than the biological methods, which require more duration for the same removal efficiencies; this would improve the capacity of the wastewater treatment plants.
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2
%Dye removal
Initial dye concentration=40ppm, Current density = 1.5 mA/cm Area ratio (Cathode/Anode)=15, Air flow rate(liter/min) 100 90 80 70 60 50 40 30 20 10 0
0.4 0.5 0.6 0.7 0.8
0
20
40 Time(min)
60
80
Figure 6. Percentage dye removal vs. time at different air flow rate 2
% Dye removal
Current de nsity = 1.5 mA/cm , Area ratio (Cathode/Anode)= 15 Ope rating time = 30 min, Initial dye concentration= 30 ppm, Air flow rate (liter/min) 0.4
80 70 60 50 40 30 20 10 0
0.5 0.6 0.7 0.8
0
5 10 15 20 Initial FeSO4 concentration(ppm)
25
Figure 7. Percentage dye removal vs. initial ferrous sulfate concentration at different air flow rate 3.6 Effect of adding chemical coagulant (FeSO4) The effect of adding chemical coagulant as ferrous sulfate was investigated using different concentrations ranging from 5 to 20 ppm. As shown in Figure 8 the results show that the percentage of removal of dye increased by increasing the amount of ferrous sulfate added, which can be attributed to the increased surface area of the coagulating agent due to ferrous sulfate added and Al (OH)3 generated
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from the electrocoagulation reactions. Note that increasing the amount of ferrous sulfate added has increased the percentage of dye removed by a factor ranging from 1.2 to 1.56 depending on the amount of FeSO4 added, while changing the gas flow rate has increased the percentage of dye removed by a factor ranging from 2.6 to 3.44 for solutions of 20 and 5 ppm ferrous sulfate added respectively. The above result shows that gas sparging is more efficient than adding chemicals such as ferrous sulfate for complete removal of dye. On the other hand, adding chemicals will produce more sludge and cause a solid waste problem, while using gas sparging does not. In addition, gas sparging is simple and cheap. 3.7 Electric power consumption The electric power consumption of the process was calculated per m3 of the waste solution using the equation that (2, 4): P = EIt/1000V where P is the specific power consumed (kWh/m3). E is the cell voltage in volt (V), I is the current in ampere (A), t is the time of electrocoagulation in hour (h) and V is the solution volume in cubic meter (m3). Time of operation was fixed at 60 minutes. As shown in Figure 8, it is clear that increasing the gas flow rate will increase both the power consumption and percentage removal efficiency of dye. To throw some light on the role of gas sparging in the economy of the process the ratio (percentage removal efficiency/power consumption) was plotted versus air flow rate for different current densities of 1.5 to 3mA/cm2. As shown in Figure 9, the ratio decreased by increasing both the gas flow rate and the current density. These results suggest that the economy of the process is favored by lowering gas flow rate and current density. It is important to note that the ratio has a slight increase when using the gas sparging up to 0.4 liter/min, which indicates that using lower gas flow rates can improve both the process economy and productivity.
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0.35
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0.3
60
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0.15
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Current density = 3 mA/m Initial dye oncentration=20ppm Operating time = 30 min %Dye removal
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Elshazly: Augmentation of the Performance of Batch Electrocoagulation Unit
power %redn
0 0
0.4
0.5
0.6
0.7
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Figure 8. Power consumption and percentage dye removal vs. air flow rate
%removal efficiency/power consumption
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Current density=3mA/cm2 Current density=1.5mA/cm2
500 400 300 200 100 0 0
0.2
0.4
0.6
0.8
1
Air flow rate(liter/min)
Figure 9. The ratio (percentage removal efficiency/power consumption) vs. air flow rate at different current density. CONCLUSION Gas sparging was used to augment the performance of a batch electrocoagulation unit composed of cylindrical aluminum electrodes, used for the removal of dye from waste solutions. The results show that the percentage of dye removal has
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been increased by a factor ranging from 2.52 to 5.14 by increasing the gas flow rate from 0.4 to 0.8 liter/min respectively and that about 93.5% of the dye can be removed within 60 minutes. In addition the power consumption for the unit was measured for different gas flow rates and different current densities; the results show that lower gas flow rate up to 0.4 liter/min can improve the economy of the process. The above results dictate that gas sparging can be used for improving the performance of electrocoagulation process, as it is considered to be simple, cheap, easy to operate and efficient tool for improving mass transfer operation. REFERENCES Alexandrova, L., Nedialkova, T. and Nishkov, I., 1994, “Electroflotation of metal ions in waste water”, Int. J. Miner. Process, 41, 285–294. Cabassud, C., Laborie, S., Bourlier, L.D. and Lainé, J.M. 2001, “Air sparging in ultrafiltration hollow fibers: relationship between flux enhancement, cake characteristics and hydrodynamic parameters”, J. Membr. Sci., 181, 57– 69. Chen, G., 2004, “Electrochemical technologies in wastewater treatment”, Sep. Purif. Technol., 38 (2004) 11–41. Chen, X., Chen, G. and Yue, P.L., 2002, “A novel electrode system for electroflotation of wastewaters”, Environ. Sci. Technol., 36 (4), 778–783. Daneshvar, N., Sorkhabi, A.H., Kasiri, M.B., 2004. “Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections”, J. Hazard. Mater., B112 (1–2), 55–62. Deckwer, W.D., 1992. Bubble Column Reactors. Wiley, New York, USA. Emamjomeh, M. M. and Sivakumar, M., 2009, “Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes”, J. Environ. Manag., 90, 1663–1679. Fan, L., Yang, F. and Yang, W., 2004. “Performance of the decolorization of an Azo dye with bipolar packed bed cell”, Sep. Purif. Technol., 34, 89–96. Guang, L., Xiaogang, Y. and Gance, D., 2009, “CFD simulation of effects of the configuration of gas distributors on gas–liquid flow and mixing in a bubble column”, Chem. Eng. Sci., 64, 5104–5116.
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Gvozdez, V.D. and Ksenofontov, B.S., 1986, “Waste water treatment in an electroflotation apparatus with a fluidized media”, Khimiya I Tekhnologiya Vody, 8, 4, 70–72. Hosny, A.Y., 1996. “Separating oil from oil–water emulsions by electroflotation technique”, Sep. Technol., 6, 9–17. Hwang, K.J. and Hsu, C.E. 2009, “Effect of gas–liquid flow pattern on airsparged cross-flow microfiltration of yeast suspension”, Chem. Eng. J., 151, 160–167. Ibanez, J.G., Takimoto, M. and Vasquez, R.C., 1995. “Laboratory experiments on electrochemical remediation of the environment: electrocoagulation of oily wastewater”, J. Chem. Educ., 72, 11, 1050–1051. Ibl, N., Kind, R. and Adam, E., 1975, “Mass transfer at electrodes with gas stirring”, An Quim, 71, 1008–1016. Ibrahim, M.Y., Mostafa, S.R., Fahmy, M.F.M and Hafez, A.I, 2001, “Utilization of electroflotation in remediation of oily wastewater”, Sep. Sci. Technol., 36, 16, 3749–3762. Jiang, J.Q., Graham, N., Andre, C., Kelsall, G.H., Brandon, N., 2002a. Laboratory study of electro-coagulation-flotation for water treatment”, Water Res., 36, 16, 4064–4078. Jiang, J.Q., Graham, N., Andre, C., Kelsall, G.H., Brandon, N.P., Chipps, M.J., 2002b. “Comparative performance of an electrocoagulation/flotation system with chemical coagulation-dissolved air flotation: a pilot-scale trial”, Water Sci. Technol.: Water Supply, 2, 1, 289–297. Mollah, M.Y.A., Morkovsky, P., Gomes, J.A.G., Kesmez, M., Parga, J., Cocke, D.L., 2004a. “Fundamentals, present and future perspectives of electrocoagulation”, J. Hazard. Mater., 114, 1–3, 199–210. Mollah, M.Y.A., Pathak, S.R., Patil, P.K., Vayuvegula, M., Agrawal, T.S., Gomes, J.A.G., Kesmez, M., Cocke, D.L., 2004b. “Treatment of orange II azo-dye by electrocoagulation (EC) technique in a continuous flow cell using sacrificial iron electrodes”, J. Hazard. Mater., 109, 1–3, 165–171. Mollah, M.Y.A., Schennach, R., Parga, J.R., Cocke, D.L., 2001. “Electrocoagulation (EC) – science and applications”, J. Hazard. Mater., 84, 1, 29–41.
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Nosier, S.A., El-Kayar, A., Farag, H.A., Sedahmed, G.H., 1997, “Solid-Liquid mass transfer at gas sparged fixed bed of Raschig rings”, Int. commun. Heat Mass Transfer, 24, 5, 733–740. Pouet, M.F. and Grasmick, A., 1995a, “Urban wastewater treatment by electrocoagulation and flotation”, Water Sci. Technol., 31, 275–280. Pouet, M.F. and Grasmick, A., 1995b, “Comparative performance of an electrocoagulation/flotation system with chemical coagulation-dissolved air flotation: a pilot-scale trial”, Water Sci. Technol.: Water Supply 2, (1), 289–297. Sedahmed, G.H., Abdo, M.S., Kamal, M.A., Fadaly, O.A., and Osman, H.M. 2001, “Effect of gas sparging on the rate of mass transfer during electropolishing of vertical plates”, Chem. Eng. Process., 40, 3, 195–200. Sigrist, L., Dessenbach, O. and Ibl, N. 1979, “On the conductivity and void fraction of gas dispersion in electrolyte solutions”, J. Appl. Electrochem., 22, 1393–2003. Zaki, M.M., Nirdosh, I. and Sedahmed, G.H. 2001, “Liquid solid mass transfer at horizontal woven screens with upward cocurrent gas-liquid flow”, Chem. Eng. Comm., 186, 1, 43–56. Zarraa, M.A, El-Abd, M.Z., Eltawil, Y.A., Farag, H.A. and Sedahmed, G.H. 1994, “Liquid solid mass transfer in a batch packed bubble column”, Chem. Eng. J. and Biochem. Eng. J., 54, 1, 51–56.
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