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Studies on various mode of electrochemical reactor operation for the treatment of distillery effluent
Journal of Environmental Chemical Engineering 1 (2013) 552–558
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Studies on various mode of electrochemical reactor operation for the treatment of distillery effluent Modepalli Susree a, P. Asaithambi a, R. Saravanathamizhan b, Manickam Matheswaran a,* a b
Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, India Department of Chemical Engineering, SSN college of Engineering, Rajiv Gandhi Salai, Kelambakkam, Chennai 603110
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 March 2013 Received in revised form 18 June 2013 Accepted 24 June 2013
The performance of different electrochemical reactor configurations of batch, batch with recirculation and single pass were investigated based on the COD removal, current efficiency and power consumption for the treatment of distillery effluent. The effect of various operating parameters such as current density, electrolysis time, supporting electrolyte concentration and recirculation flow rate on the oxidation of pollutant has been studied. The removal of COD was found to have increased with increase in current density and electrolyte concentration in the batch electrochemical reactor. The maximum percentage COD removal was found to be 83.2% with an energy consumption of 9.77 kWh/kg of COD. For the singlepass operation 52.94% COD removal with an energy consumption of 44.76 kWh/kg of COD has been observed. Continuous systems were found to be better than batch systems in terms of energy utilization with comparable COD removal and it was better to treat huge volume of the effluent. The single-pass operation shows less completion of the process due to the lower residence time. The kinetics of electrochemical reaction of various modes of operations has also been studied. The dispersed plug flow model value was compared with experimental value. ß 2013 Published by Elsevier Ltd.
Keywords: Electrochemical reactor Distillery effluent Kinetics Modeling COD removal.
Introduction Global industrialization and urbanization have led to the increasing pollution in water bodies, thereby requiring the need to treat the industrial wastewater before letting out to the environment. The large volume of hazardous effluents contains many bio-recalcitrant compounds generated due to industrial activity. Various organic pollutants create the colour and odour problem. These are responsible for being toxic and deteriorating to the environment. The different industrial effluents like distillery, agrochemical, kraft-bleaching, pulp and paper, textile dyehouse, oilfield and metal-plating wastes contain biologically recalcitrant organic pollutants and are not easily removed by biological treatment. So the conventional techniques of physical and/or chemical process are applied for the treatment of wastewaters to enhance their biodegradability and allow further applicability of biological treatment. The conventional treatment techniques such as coagulation, membrane separation, and adsorption result in phase transfer of pollutants and also result in secondary pollution. However, these processes are not yet completely adequate in treating effluents or because pollutants are refractory to chemical
* Corresponding author. Tel.: +91 4312503120; fax: +91 4312503102. E-mail address: [email protected] (M. Matheswaran). 2213-3437/$ – see front matter ß 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jece.2013.06.021
oxidation in aqueous medium or due to production of partially oxidized reaction products having greater toxicity [1]. One of the processes for treating such water containing toxic and persistent organic pollutants that has been increasingly used in recent years is the advanced oxidation process (AOP), a powerful tool applied to degrade this pollutant [2–5]. AOPs includes following processes such as treatment with H2O2, H2O2/UV, UV, photocatalysis, ozonation, ultrasound, wet air oxidation, electrochemical oxidation, Fenton and photo-Fenton. Electrochemical oxidation is one of the major and rapidly emerging AOPs and can be considered as an alternative treatment technology. Electrochemical techniques have been receiving greater attention in recent years due to their typical advantages such as environmental compatibility, selectivity, ease of automation, ease of scale-up and most importantly, versatility [6,7]. The mechanism and application of electrochemical process for the treatment of different industrial effluents have been reported by several investigators [8–11]. The electrochemical process using chloride as the supporting electrolyte was reported for the treatment of different industrial wastewaters such as electroplating [12], oil mill [13], heavy metal laden [14], nitrite effluent [15], defluoridation [16], arsenic removal [17], textile dyes [18], landfill leachate [19], restaurant [20], laundry [21], surfactants [22], agro-industry effluent [23], etc. Electrochemical processes are based on the electrochemical reaction which occurs at the electrodes in the reactor. The design and development of an electrochemical cell is aimed at minimizing
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power losses due to poor current distribution and voltage drops and making the processes more competitive in energy consumption than the conventional techniques [24]. For an effective design of the electrochemical cell, understanding the various modes of its operation is crucial. The different possible reactor arrangements include batch, batch with recirculation, recycle and single-pass flow. The choice of batch or continuous operation for industrial application depends on the nature and toxicity of the effluent, simplicity of the process required, and the process economics [25]. In the present investigation, various modes of operation in the electrochemical cell have been tested for the treatment of industrial effluent to understand kinetics mechanism, pollutant removal and current efficiency. Effluent originating from distilleries is densely coloured due to a dark brown pigment called melanoidin which is produced during the processing of sugarcane molasses for alcohol production, and this further ads to the bio-recalcitrance of the distillery effluent. Few investigators have studied the treatment of distillery effluent for the removal of colour and Chemical Oxygen Demand (COD) by different methods like biological [26–30] and electrochemical [31– 34] methods. Krishna Prasad et al. have used electrochemical degradation and electrocoagulation for the treatment of spent wash from distilleries and observed the removal of COD and colour using ruthenium oxide coated titanium anode [31]. Manisankar et al. have studied treatment of distillery effluent in a static electrochemical cell employing two different kinds of anodes viz., graphite and titanium anodes under varying operating conditions and they have observed a maximum COD and BOD removal of 92% and 98.1%, respectively, with complete decolourization. The present study focuses on the electrochemical reactor in various mode of operation such as batch, batch with recirculation and single-pass flow reactors. The effect of various experimental parameters such as current density, electrolysis time, supporting electrolyte concentration and recirculation flow rate on the percentage COD removal, current efficiency and power consumption of these systems have been studied. The batch with recirculation mode has been suitably modeled to represent a dispersed plug flow behavior. Experimental details Materials The effluent was collected from a nearby distillery industry. The physicochemical characteristics of the effluent were determined before treatment. The main characteristics of the effluent are shown in the Table 1: and the effluent characteristics were determined according to the standard methods [35]. The dilution of effluent was carried out with deionised water from a Millipore purification system. The initial pH of the solution was measured using pH meter (Elico; Model LI120 and the accuracy is 0.01). The pH of the solution was adjusted using NaOH and H2SO4 solutions. All the chemicals were analytical reagent (AR) grade obtained from Merck and used without further purification. Electrochemical experiments were carried out using Mixed Metal Oxide (MMO) and
pH Chemical Oxygen Demand (COD) Biological Oxygen Demand (BOD) Total Suspended Solids (TSS) Total Dissolved Solids (TDS) Colour Odour
stainless steel (SS) as an anode and a cathode mesh type electrode respectively. The MMO anode (Ti/Ti0.7O2-Ru0.05O2-Ir0.25O2) has the composition of 70% TiO2, 5% RuO2 and 25% IrO2 (wt%). The electrodes were obtained from Titan Anode Fabricators Private Limited, Chennai. The MMO electrodes are having good electronic conductor at anode potential, high surface roughness and a large number of electro-active sites. Hence, these exhibit a high electrocatalytic activity for the oxygen per geometrical oxide [36]. Experimental method Batch operation The experimental set-up of the electrochemical reactor operated in a lab-scale batch mode of operation is shown in Fig. 1a. It consists of a glass beaker with a volume of 700 ml a pair of electrodes and DC regulated power supply. The anode and cathode has the dimensions of 7.25 cm 6.0 cm were placed vertically and parallel to each other with an inter-electrode distance of 2 cm. The void fraction of the mesh type anode accounts 20% by area. Hence, the available effective electrode area is 34.8 cm2 for anodic reactions. The electrode plates were cleaned manually by washing in distilled water prior to every run. The volume of effluent taken was 500 ml in the electrochemical reactor and the electrodes were connected to DC power supply (APLAB Ltd; Model L1606). The ionic conductivity of the wastewater was increased by adding 2–10 g L1 supporting electrolyte concentration of NaCl. The temperature of the reactor was maintained constant 2 8C by external cooling system with recirculation of water. The solution was continuously stirred at a constant speed using a magnetic stirrer. The cell voltage was periodically noted. The samples were collected at regular interval of time from the reactor and filtered using Whatmann 42 filter paper. The COD was determined to investigate the behaviour of electrochemical oxidation of distillery effluent in the batch reactor. The sample COD was determined using the dichromatic closed reflux method strictly following the APHA [35]. The performance of the batch electrochemical reactor was studied under various conditions of current density and electrolyte concentration of NaCl. The percentage COD removal, current efficiency (CE) and power consumption (EC) were studied at different current densities ranging from 1 to 5 A dm2 and NaCl concentration was varied from 2 to 10 g L1. The CE and EC of batch operation can be calculated using following equations [37]: CE ¼
EC ¼
V R DCOD 16It 2F
100
VIt
DCOD V R
(1)
(2)
where difference in Chemical Oxygen Demand (DCOD = CODinitial – CODfinal) is in g L1; applied current (I) in Ampere; electrolysis time (t) in hr; applied cell voltage (V) in volts, volume of the reactor (VR) in liters and Faraday’s constant (F) in C/mol. Batch recirculation operation
Table 1 Characteristics of the distillery effluent. Parameters
553
Range 4.1–4.3 80,000–90,000 (mg L1) 7000–8000 (mg L1) 15.44 (g L1) 5550–5750 (mg L1) Dark brown Burnt sugar
A schematic representation of the experimental set-up of the lab-scale batch recirculation and single-pass mode of operation is represented in Fig. 1b. The electrochemical reactor is made of filter press type having a volume of 250 ml with an inlet at one side at the bottom and an outlet in the other side at the top. The electrolytic flow reactor consisted of an anode and cathode with inter-electrode distance of 2 cm. The DC power supply was used for electrical connections to constitute an electrolytic cell with
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
554
(a)
The CE and EC of flow operation can be calculated using following equations [37]:
Amp
Volt
-
+
DC power supply
CE ¼
EC ¼
Q DCOD 16I 2F
100
VI
DCOD Q
(3)
(4)
Single-pass operation Anode
Cathod e
Cooling water
Magnetic stirrer
(b) 7
8
9
+ 3
A
The effluent with desired initial COD concentration containing 6 g L1 of supporting electrolyte was filled in the reservoir. The required flow rate through the reactor was established by pumping and adjusting the valves (i.e. stream 10 in closed condition). A DC power supply was connected to the electrodes maintaining a constant current density of 3 A dm2. The recirculation mode of operation was changed over to the once-through mode by closing the connection of the recirculation pipe to the reservoir and directly collecting the samples at the outlet. The effluent flow rate through the electrochemical reactor (Q) was varied (0.6, 0.9, 1.2, 1.7, 2.1 L h1) and NaCl electrolyte concentration also varied (2– 10 g L1), each experiment was run to reach steady state. The samples were collected after reaching the steady state at the outlet stream was used in determining COD concentration.
5 V
Result and discussion Batch reactor
6
4
10
3
1 2 Fig. 1. (a) Schematic diagram of batch electrochemical reactor. Fig. 1b Schematic diagram of batch electrochemical reactor with recirculation (1. Reservoir, 2. Centrifugal pump, 3. Control valve, 4. Rotameter, 5. DC power Supply, 6. Filter press type electrochemical reactor, 7. Anode, 8. Cathode, 9. Single-pass mode operation, 10. Recirculation mode operation).
maintaining a constant current density of 3 A dm2. The other components of the set-up were a 1000 ml reservoir, and a magnetically driven self-priming centrifugal pump, connected using silicone rubber tubes. The reservoir was filled with 500 ml of effluent with desired initial COD concentration containing 6 g L1 supporting electrolyte concentrations. The required flow rate through the reactor was established by pumping and adjusting the valves. The effluent recirculation flow rate (Q) into the reactor was varied (25, 30, 35, 40 L h1) using a calibrated rotameter. At a fixed flow rate, the effect of NaCl electrolyte concentration changing from 2 to 10 g L1 on the performance of the electrolytic cell was also studied. The cell voltage was periodically noted. The samples were collected periodically from the reservoir and filtered using Whatmann 42 filter paper. The COD was determined to investigate the behaviour of batch recirculation reactor.
The batch electrochemical reactor studies of various operating parameters were reported by Asaithambi et al. [25]. The effect of current density and supporting electrolyte concentration on COD removal efficiency and power consumption were considered for compare the different mode electrochemical cell operation of batch recirculation and single pass. The batch result reported that increasing the current density (1 to 5 A dm2) of the electrochemical cell follows production of more electrons, which results in increasing the percentage COD removal. But at higher current densities, production of extra electrons may contribute to undesirable side-reactions such as parasitic loss reactions leading to the depletion of hypochlorite concentration [38]. However, increasing current density beyond 3 A dm2, it was also observed that the loss of electrical energy occurs in the form of heat and more unwanted side reactions like water oxidation. 2H2 OðlÞ ! 2H2 ðgÞ þ O2 ðgÞ
(5)
The power consumption was increased from 7.19 to 21.60 kWh/ kg of COD with correspondingly the current efficiency decreasing from 95% to 54% for increasing the current density from 1 to 5 A dm2. This can be led to increase in the operating cost of the treatment process due to increase in power consumption with decreasing the current efficiency. The effect of supporting electrolyte concentration of NaCl on the percentage COD removal has been reported elsewhere that sodium chloride is very effective in the destruction of organics present in the effluent and decolourization can be completely achieved [39]. The COD removal is increased with increasing electrolyte concentration from 2 to 6 g L1. Further increase in NaCl concentration up to 10 g L1 did not bring about any meaningful improvements. The addition of the supporting electrolyte which allows the removal efficiency increases and a degradation of pollutants occurs due to the participation of active chlorine through the possible ‘‘direct’’ and ‘‘indirect’’ roles for the chloride anion in the electrochemical reaction. In indirect electro-oxidation, sodium chloride is added to
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
increase the conductivity and generates hypochlorite ions. The anodic reaction is given as: Anode Reaction k1
2Cl !Cl2 þ 2e k2
Cl2 þ H2 O!Hþ þ Cl þ HOCl k3
HOCl ! Hþ þ OCl 0
Recirculation flow (L h1)
Cell voltage (V)
COD removal (%)
Current efficiency (%)
Power consumption (kWh/kg of COD)
(7)
25 30 35 40
3.1 3.3 3.4 3.6
58.77 66.75 73.40 76.72
16.45 18.68 20.55 21.48
43.48 37.09 31.73 30.19
(8)
k4
(9)
The generated hypochlorite ions act as the main oxidizing agent in the pollutant degradation. In the indirect electro-oxidation rate of organic pollutants, a reaction sequence of chloride–chlorine– hypochlorite–chloride takes place in the bulk [40]. The maximum percentage COD removal is reported as 83.20% with a current efficiency of 82.69% and power consumption of 9.77 kWh/kg of COD, at an electrolyte concentration of 10 g L1 and an initial COD concentration of 10 g L1. The current efficiency is increased and power consumption is decreased for the increasing NaCl electrolyte concentration from 2 to 10 g L1. Batch with recirculation reactor The effect of supporting electrolyte concentration on the percentage COD removal is shown in Fig. 2 with fixed initial COD concentration of 10 g L1 at recirculation flow rate of 35 L h1. It is observed from the figure that the percentage of COD removal increased with increasing the electrolyte concentration from 2 to 10 g L1. Increase of NaCl concentration increases the conductivity of the effluent results decrease the cell voltage. It is found from Fig. 2 that COD removal increases from 64.05% to 86.50% with increase in electrolyte concentration from 2 to 10 g L1. Hence, the power consumption is decreased and current efficiency is increased. It can be observed that the batch recirculation system is superior in both completion of colour and COD removal. Thus, even though the energy consumption is marginally lower, the overall performance of the batch recirculation system is superior to the batch system. Also, 100% colour removal has been observed in the effluent after about 90 min of treatment in each case. The effect of recirculation flow rate on percentage COD removal is shown in the Table 2 at fixed current density 3 A dm2, supporting electrolyte concentration 6 g L1 and initial COD
100 80
COD removal (%)
Table 2 Effect of flow rate on the performance of batch electrochemical reactor with recirculation (Conditions: Initial COD concentration: 10 g L1; Electrolysis time: 6 h; Current density: 3 A dm2; Electrolyte concentration: 6 g L1).
(6)
k3
Organic þ OCl !CO2 þ H2 O þ Cl
555
concentration 10 g L1. The percentage of COD removal increases with increasing the recirculation flow rate. The Table 3 shows that the percentage of COD removal strongly depends on the reaction time. The COD removal percentage fixed time at 6 h of operation is found to be 58.77% to 76.72% with increasing circulation flow rates from 25 to 40 L h1, respectively. The rate of organic pollutant degradation is dependent on its bulk concentration. At higher flow rates, the rate of COD removal is significantly higher. This may be due to the increased generation of oxidants in the process. The transport of the Cl ion from the bulk to the electrode surfaces increases due to increase in electrolyte circulation flow rate. This would facilitate Cl2 generation, as well as the dissolution of Cl2 to form OCl for reaction which helps the increased removal of COD [40]. The current efficiency (16.45–21.48%) and power consumption (43.48–30.19 kWh/kg of COD) were considerably improved with increasing the effluent recirculation flow rate from 25 to 40 L h1. Single-pass flow reactor The effect of flow rate on the percentage COD removal, power consumption and current efficiency is shown in Fig. 3. It is observed from the figure, that the percentage of COD removal decreases with increase in the flow rate of effluent. This is due to the fact that residence time decreases with increase in flow rate. The percentage of COD removal of the effluent is under reaction control. On the other hand, the current efficiency increases and power consumption decrease with increase in flow rate. The effect of supporting electrolyte concentration on the percentage of COD removal, current efficiency and power consumption is shown in the Table 3. It is found from the table that COD removal increases from 49.01% to 61.65% with increase in electrolyte concentration from 2 to 10 g L1. This is because with increase in NaCl concentration, production of oxidizing agent was increased thus increasing the conductivity of the solution, and hence, reduction resistance in the cell voltage. This results in a decrease in power consumption from 65.88 kWh/kg of COD to 33.61 kWh/kg of COD and increase in current efficiency from 40.20% to 44.82%.
60 2 gL-1 4 gL-1 6 gL-1 -1 8 gL -1 gL 10
40
20 0
0
1
2
3 Time (h)
4
5
Table 3 Effect of electrolyte concentration on the performance of a single-pass flow electrochemical reactor (Conditions: Initial COD concentration: 10 g L1; flow rate: 1.2 L h1; Current density 3 A dm2; Initial pH: 6.1).
6
Fig. 2. Effect of supporting electrolyte concentration on the percentage COD removal in batch electrochemical reactor with recirculation (Conditions: Initial COD concentration: 10 g L1; recirculation flow rate: 35 L h1; Initial pH: 6.1; Electrolysis time: 6 h).
Electrolyte concentration (g L1)
Cell voltage (V)
COD removal (%)
Current efficiency (%)
Power consumption (kWh/kg of COD)
2 4 6 8 10
3.8 3.5 3.7 3.2 3.0
49.01 52.94 55.64 58.41 61.65
40.20 43.41 44.32 44.62 44.82
65.88 44.76 42.16 37.53 33.61
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
80
80
60
60
40
40
20
20
COD removal Power consumption
0
limiting oxidizing agent to another limiting condition such as limiting organic species due to addition of more salt [31]. In the case of current density, similar result was observed with the heterogeneous rate constant, which increases from 2.1 104 to 9.5 104 cms1 (the data not showed). In the batch electrochemical reactor with recirculation, the reservoir is assumed to be a continuous stirred tank. The flow is in axial direction in the reactor which is assumed to be an approximate plug flow reactor. The reaction rate for the removal of COD in a batch recirculation reactor under steady state can be expressed as [37]:
Power consumption (kWh/kg of COD)
COD removal (%)
556
@C Q @C ¼ kh aC Ae @x @t
0 1
0.5
1.5 Flow rate (Lh-1)
2
2.5
Fig. 3. Effect of flow rate on the performance of a single-pass flow electrochemical reactor (Conditions: Initial COD concentration: 10 g L1; Electrolyte concentration: 6 g L1; Current density 3 A dm2; Initial pH: 6.1).
At the steady state condition the solution of Eq. (11) is C Ae ¼ exp kh Co Q
In the batch electrochemical reactor, the COD concentration change with respect to time will remain constant under a given set of experimental conditions. However, the rate of COD removal is strongly dependent on current density and concentration of the electrolyte. The rate of COD removal in the batch electrochemical reactor can be expressed as [40]: C ¼ C 0 expðkh atÞ
(10)
where Co and C represents initial and at time t of COD concentration of the effluent in the batch reactor, kh is heterogeneous electrochemical rate constant, a is specific electrode area (Ae/VR), Ae is electrode area, VR is reactor volume. The heterogeneous rate constant with respect to the variation of current density and supporting electrolyte concentration was studied in the batch operation. The rate constant calculated from the slope of the plot ln(Co/C) vs t. in Fig. 4 shows the kinetic plot of batch electrochemical reactor at different electrolyte concentrations. The heterogeneous rate constant was increased from 3.3 104 to 9.1 104 cms1 with increasing the electrolyte concentration from 2 to 10 g L1. Especially at low salt concentration, the generation of oxidizing agent is more due to additional amount of salt added. The process might have changed from condition of 2
(13)
Solve the above mass balance equation after substituting the expression of COD concentration under steady state, knowing the initial COD of effluent in the reservoir. The resultant equation of COD variation in the batch recirculation reactor can be expressed C t Ae ¼ exp 1 exp kh (14) Co t Q where t is the residence time in the reservoir (Vres/Q). The heterogeneous rate constant kh can be calculated from the plot ln(C/Co) vs t having the slope ((1 exp(kha))/t). The rate constant and performance of the process are mainly dependent on the current density, electrolyte concentration and recirculation flow rate. Fig. 5 shows the kinetics plot of batch recirculation electrochemical reactor. The heterogeneous rate constant varies from 9.9 104 to 16.1 104 cms1 with increasing the recirculation flow rate at current density of 3 A dm2 with an electrolyte concentration of 6 g L1. The enhancement of heterogeneous rate constant at higher flow rates is due to increasing ionic conductivity due to bulk movement and decreases the resistance on the electrode surface. For the single pass or once through approach the effluent enters and leaves the reactor continuously. The rate of COD change
25 Lh-1
30 Lh-1
1.2
35 Lh-1
-ln(C/Co)
-ln(C/Co)
1.2
dC ¼ Q ðC C t Þ dt
1.6
2 gL-1 -1 4 gL -1 gL 6 -1 gL 8 -1 gL 10
1.6
(12)
The material balance can be written as around the reservoir is V res
Kinetics of COD removal
(11)
40 Lh-1
0.8
0.8
0.4
0.4 0
0
0
1
2
3 4 Time (h)
5
6
7
Fig. 4. Kinetics plot of ln(C/Co) vs t for the batch electrochemical reactor, at Current density: 3 A dm2; Initial pH: 6.1; Initial COD concentration: 10 g L1; Electrolysis time: 6 h.
0
1
2
3 4 Time (h)
5
6
7
Fig. 5. Kinetics plot of ln(C/Co) vs t for the batch electrochemical reactor with recirculation, at Initial COD concentration: 10 g L1; Current density: 3 A dm2; Initial pH: 6.1; Electrolysis time: 6 h.
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
1.5
1
1.2
0.8
0.9
0.6
557
C/Co
-ln(C/Co)
Model Value
0.6 0.3
Experimental Value
0.4 0.2
0
0
0.4
0.8 1.2 1/Q (hL-1)
1.6
0
2
0
1
2
3 Time (h)
4
5
6
Fig. 6. Kinetics plot of ln(C/Co) vs 1/Q for the single-pass electrochemical reactor, at Initial COD concentration: 10 g L1; Electrolyte concentration: 6 g L1; Current density 3 A dm2; Initial pH: 6.1.
Fig. 7. Comparison of experimental and model value in the batch electrochemical reactor with recirculation at Initial COD concentration: 10 g L1; Current density: 3 A dm2; Initial pH: 6.1.
expression can be written as [40]:
experimental result. Similar result has been observed for the other flow rates also.
k Ae C ¼ C 0 expðkh at R Þ ¼ C o exp h Q
(15) Conclusions
Hence, C k Ae ln ¼ h Co Q
(16)
The heterogeneous rate constant kh can be computed from the slope of the plot ln(C/Co) vs 1/Q. Fig. 6 shows the kinetic plot of single-pass electrochemical reactor at initial concentration of 10 g L1, current density of 3 A dm2 and electrolyte concentration 6 g L1. The value of rate constant obtained is 1.0 104 cms1 from the slope of the plot. Modeling Electrolyte with recirculation system, the reactor can be assumed as (continuous Stirred Tank Reactor) CSTR model and dispersed plug flow model. In the previous section, the PFR (Plug flow reactor) model equation (Eq. (14)) has been derived and compared with the experimental result. The model simulation does not match with the experimental result. Then the reactor assumed to be a DPFR (Dispersed Plug Flow Reactor) model. When the effluent is passed between the electrodes in the filter press type reactor set-up, some degree of back mixing or intermixing takes place in the reactor. The D/UL value of the residence time distribution study of filter press type reactor found to be less than 0.01 reveals that smaller deviation from the dispersed to plug flow behaviour. [41] The flow behaviour in the filter press type reactor takes a dispersed plug flow. The material balance is drawn around the reactor and reservoir, the final equation becomes [42] " ( #) 4qexp Pe C t 2 ¼ exp 1 Co t ð1 þ qÞ2 exp q Pe ð1 qÞ2 exp q Pe 2
(17)
2
tk 0:5 and Pe ¼ UL where, q ¼ 1 þ 4Pe D D is the dispersion coefficient (m2/s); U is the velocity (m/s); L is the length of the reactor (m); k is the heterogeneous rate constant (m/s); t is the residence time (min); and Pe is the peclet number. The above model equation (17) is used to find the theoretical C/C0 value after substituting all the values. The model simulation is compared with experimental values. Fig. 7 shows the model comparison with the experimental value for the effluent flow rate of 25 L h1 and the D/UL value of 0.01. It is observed from the figure that the dispersed plug flow model satisfactorily matches with the
Electrochemical oxidation of distillery effluent was investigated using a mixed metal oxide electrode in various types of reactor configurations, i.e. batch, batch with recirculation and single-pass systems. The effect of operating parameters like current density, electrolyte concentration, electrolysis time and recirculation flow rate on the percentage COD removal, power consumption and current efficiency were investigated. The COD removal and power consumption increased with increasing current density in the batch electrochemical reactor. In the recirculation mode of operation, increase in flow rate increases the COD removal. For the single-pass operation, COD removal has increased with decreasing flow rate. Batch system is better in terms of large amount of COD removal compared to continuous system. But continuous systems were found to be better than batch systems in terms of energy utilization with comparable COD removal and it was better to treat huge volume of the effluent. The single-pass operation shows less completion of the process due to the lower residence time. The kinetics study of all the three modes of operation has been carried out and the heterogeneous rate constant also reported. Then dispersed model has been proposed for the filter press type electrochemical reactor. The model results were well in accordance with the experimental results. References [1] M.A. Oturan, A. Lahkimi, N. Oturan, M. Chaouch, Removal of textile dyes from water by the electro-Fenton process, Environmental Chemistry Letters 5 (2007) 35–39. [2] J.R. Bolton, K.G. Bircher, W. Tumas, C.A. Tolman, Figures-of-merit for the technical development and application of advanced oxidation processes, Pure and Applied Chemistry 73 (2001) 627–637. [3] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Advances in Environmental Research 8 (2004) 501–551. [4] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Advances in Environmental Research 8 (2004) 553–597. [5] M.P. Titus, V. Garcia-Molina, M.A. Ban˜os Jaime Gime´nez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Applied Catalysis B: Environmental 47 (2004) 219–256. [6] K. Juttner, U. Galla, H. Schmieder, Electrochemical approaches to environmental problems in the process industry, Electrochimica Acta 45 (2000) 2575–2594. [7] G. Chen, Electrochemical technologies in wastewater treatment, Separation and Purification Technology 38 (2004) 11–41. [8] D. Pletcher, F.C. Walsh, Industrial Electrochemistry, 2nd ed, Chapman and Hall, London, 1990.
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Contents lists available at SciVerse ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Studies on various mode of electrochemical reactor operation for the treatment of distillery effluent Modepalli Susree a, P. Asaithambi a, R. Saravanathamizhan b, Manickam Matheswaran a,* a b
Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, India Department of Chemical Engineering, SSN college of Engineering, Rajiv Gandhi Salai, Kelambakkam, Chennai 603110
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 March 2013 Received in revised form 18 June 2013 Accepted 24 June 2013
The performance of different electrochemical reactor configurations of batch, batch with recirculation and single pass were investigated based on the COD removal, current efficiency and power consumption for the treatment of distillery effluent. The effect of various operating parameters such as current density, electrolysis time, supporting electrolyte concentration and recirculation flow rate on the oxidation of pollutant has been studied. The removal of COD was found to have increased with increase in current density and electrolyte concentration in the batch electrochemical reactor. The maximum percentage COD removal was found to be 83.2% with an energy consumption of 9.77 kWh/kg of COD. For the singlepass operation 52.94% COD removal with an energy consumption of 44.76 kWh/kg of COD has been observed. Continuous systems were found to be better than batch systems in terms of energy utilization with comparable COD removal and it was better to treat huge volume of the effluent. The single-pass operation shows less completion of the process due to the lower residence time. The kinetics of electrochemical reaction of various modes of operations has also been studied. The dispersed plug flow model value was compared with experimental value. ß 2013 Published by Elsevier Ltd.
Keywords: Electrochemical reactor Distillery effluent Kinetics Modeling COD removal.
Introduction Global industrialization and urbanization have led to the increasing pollution in water bodies, thereby requiring the need to treat the industrial wastewater before letting out to the environment. The large volume of hazardous effluents contains many bio-recalcitrant compounds generated due to industrial activity. Various organic pollutants create the colour and odour problem. These are responsible for being toxic and deteriorating to the environment. The different industrial effluents like distillery, agrochemical, kraft-bleaching, pulp and paper, textile dyehouse, oilfield and metal-plating wastes contain biologically recalcitrant organic pollutants and are not easily removed by biological treatment. So the conventional techniques of physical and/or chemical process are applied for the treatment of wastewaters to enhance their biodegradability and allow further applicability of biological treatment. The conventional treatment techniques such as coagulation, membrane separation, and adsorption result in phase transfer of pollutants and also result in secondary pollution. However, these processes are not yet completely adequate in treating effluents or because pollutants are refractory to chemical
* Corresponding author. Tel.: +91 4312503120; fax: +91 4312503102. E-mail address: [email protected] (M. Matheswaran). 2213-3437/$ – see front matter ß 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jece.2013.06.021
oxidation in aqueous medium or due to production of partially oxidized reaction products having greater toxicity [1]. One of the processes for treating such water containing toxic and persistent organic pollutants that has been increasingly used in recent years is the advanced oxidation process (AOP), a powerful tool applied to degrade this pollutant [2–5]. AOPs includes following processes such as treatment with H2O2, H2O2/UV, UV, photocatalysis, ozonation, ultrasound, wet air oxidation, electrochemical oxidation, Fenton and photo-Fenton. Electrochemical oxidation is one of the major and rapidly emerging AOPs and can be considered as an alternative treatment technology. Electrochemical techniques have been receiving greater attention in recent years due to their typical advantages such as environmental compatibility, selectivity, ease of automation, ease of scale-up and most importantly, versatility [6,7]. The mechanism and application of electrochemical process for the treatment of different industrial effluents have been reported by several investigators [8–11]. The electrochemical process using chloride as the supporting electrolyte was reported for the treatment of different industrial wastewaters such as electroplating [12], oil mill [13], heavy metal laden [14], nitrite effluent [15], defluoridation [16], arsenic removal [17], textile dyes [18], landfill leachate [19], restaurant [20], laundry [21], surfactants [22], agro-industry effluent [23], etc. Electrochemical processes are based on the electrochemical reaction which occurs at the electrodes in the reactor. The design and development of an electrochemical cell is aimed at minimizing
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power losses due to poor current distribution and voltage drops and making the processes more competitive in energy consumption than the conventional techniques [24]. For an effective design of the electrochemical cell, understanding the various modes of its operation is crucial. The different possible reactor arrangements include batch, batch with recirculation, recycle and single-pass flow. The choice of batch or continuous operation for industrial application depends on the nature and toxicity of the effluent, simplicity of the process required, and the process economics [25]. In the present investigation, various modes of operation in the electrochemical cell have been tested for the treatment of industrial effluent to understand kinetics mechanism, pollutant removal and current efficiency. Effluent originating from distilleries is densely coloured due to a dark brown pigment called melanoidin which is produced during the processing of sugarcane molasses for alcohol production, and this further ads to the bio-recalcitrance of the distillery effluent. Few investigators have studied the treatment of distillery effluent for the removal of colour and Chemical Oxygen Demand (COD) by different methods like biological [26–30] and electrochemical [31– 34] methods. Krishna Prasad et al. have used electrochemical degradation and electrocoagulation for the treatment of spent wash from distilleries and observed the removal of COD and colour using ruthenium oxide coated titanium anode [31]. Manisankar et al. have studied treatment of distillery effluent in a static electrochemical cell employing two different kinds of anodes viz., graphite and titanium anodes under varying operating conditions and they have observed a maximum COD and BOD removal of 92% and 98.1%, respectively, with complete decolourization. The present study focuses on the electrochemical reactor in various mode of operation such as batch, batch with recirculation and single-pass flow reactors. The effect of various experimental parameters such as current density, electrolysis time, supporting electrolyte concentration and recirculation flow rate on the percentage COD removal, current efficiency and power consumption of these systems have been studied. The batch with recirculation mode has been suitably modeled to represent a dispersed plug flow behavior. Experimental details Materials The effluent was collected from a nearby distillery industry. The physicochemical characteristics of the effluent were determined before treatment. The main characteristics of the effluent are shown in the Table 1: and the effluent characteristics were determined according to the standard methods [35]. The dilution of effluent was carried out with deionised water from a Millipore purification system. The initial pH of the solution was measured using pH meter (Elico; Model LI120 and the accuracy is 0.01). The pH of the solution was adjusted using NaOH and H2SO4 solutions. All the chemicals were analytical reagent (AR) grade obtained from Merck and used without further purification. Electrochemical experiments were carried out using Mixed Metal Oxide (MMO) and
pH Chemical Oxygen Demand (COD) Biological Oxygen Demand (BOD) Total Suspended Solids (TSS) Total Dissolved Solids (TDS) Colour Odour
stainless steel (SS) as an anode and a cathode mesh type electrode respectively. The MMO anode (Ti/Ti0.7O2-Ru0.05O2-Ir0.25O2) has the composition of 70% TiO2, 5% RuO2 and 25% IrO2 (wt%). The electrodes were obtained from Titan Anode Fabricators Private Limited, Chennai. The MMO electrodes are having good electronic conductor at anode potential, high surface roughness and a large number of electro-active sites. Hence, these exhibit a high electrocatalytic activity for the oxygen per geometrical oxide [36]. Experimental method Batch operation The experimental set-up of the electrochemical reactor operated in a lab-scale batch mode of operation is shown in Fig. 1a. It consists of a glass beaker with a volume of 700 ml a pair of electrodes and DC regulated power supply. The anode and cathode has the dimensions of 7.25 cm 6.0 cm were placed vertically and parallel to each other with an inter-electrode distance of 2 cm. The void fraction of the mesh type anode accounts 20% by area. Hence, the available effective electrode area is 34.8 cm2 for anodic reactions. The electrode plates were cleaned manually by washing in distilled water prior to every run. The volume of effluent taken was 500 ml in the electrochemical reactor and the electrodes were connected to DC power supply (APLAB Ltd; Model L1606). The ionic conductivity of the wastewater was increased by adding 2–10 g L1 supporting electrolyte concentration of NaCl. The temperature of the reactor was maintained constant 2 8C by external cooling system with recirculation of water. The solution was continuously stirred at a constant speed using a magnetic stirrer. The cell voltage was periodically noted. The samples were collected at regular interval of time from the reactor and filtered using Whatmann 42 filter paper. The COD was determined to investigate the behaviour of electrochemical oxidation of distillery effluent in the batch reactor. The sample COD was determined using the dichromatic closed reflux method strictly following the APHA [35]. The performance of the batch electrochemical reactor was studied under various conditions of current density and electrolyte concentration of NaCl. The percentage COD removal, current efficiency (CE) and power consumption (EC) were studied at different current densities ranging from 1 to 5 A dm2 and NaCl concentration was varied from 2 to 10 g L1. The CE and EC of batch operation can be calculated using following equations [37]: CE ¼
EC ¼
V R DCOD 16It 2F
100
VIt
DCOD V R
(1)
(2)
where difference in Chemical Oxygen Demand (DCOD = CODinitial – CODfinal) is in g L1; applied current (I) in Ampere; electrolysis time (t) in hr; applied cell voltage (V) in volts, volume of the reactor (VR) in liters and Faraday’s constant (F) in C/mol. Batch recirculation operation
Table 1 Characteristics of the distillery effluent. Parameters
553
Range 4.1–4.3 80,000–90,000 (mg L1) 7000–8000 (mg L1) 15.44 (g L1) 5550–5750 (mg L1) Dark brown Burnt sugar
A schematic representation of the experimental set-up of the lab-scale batch recirculation and single-pass mode of operation is represented in Fig. 1b. The electrochemical reactor is made of filter press type having a volume of 250 ml with an inlet at one side at the bottom and an outlet in the other side at the top. The electrolytic flow reactor consisted of an anode and cathode with inter-electrode distance of 2 cm. The DC power supply was used for electrical connections to constitute an electrolytic cell with
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554
(a)
The CE and EC of flow operation can be calculated using following equations [37]:
Amp
Volt
-
+
DC power supply
CE ¼
EC ¼
Q DCOD 16I 2F
100
VI
DCOD Q
(3)
(4)
Single-pass operation Anode
Cathod e
Cooling water
Magnetic stirrer
(b) 7
8
9
+ 3
A
The effluent with desired initial COD concentration containing 6 g L1 of supporting electrolyte was filled in the reservoir. The required flow rate through the reactor was established by pumping and adjusting the valves (i.e. stream 10 in closed condition). A DC power supply was connected to the electrodes maintaining a constant current density of 3 A dm2. The recirculation mode of operation was changed over to the once-through mode by closing the connection of the recirculation pipe to the reservoir and directly collecting the samples at the outlet. The effluent flow rate through the electrochemical reactor (Q) was varied (0.6, 0.9, 1.2, 1.7, 2.1 L h1) and NaCl electrolyte concentration also varied (2– 10 g L1), each experiment was run to reach steady state. The samples were collected after reaching the steady state at the outlet stream was used in determining COD concentration.
5 V
Result and discussion Batch reactor
6
4
10
3
1 2 Fig. 1. (a) Schematic diagram of batch electrochemical reactor. Fig. 1b Schematic diagram of batch electrochemical reactor with recirculation (1. Reservoir, 2. Centrifugal pump, 3. Control valve, 4. Rotameter, 5. DC power Supply, 6. Filter press type electrochemical reactor, 7. Anode, 8. Cathode, 9. Single-pass mode operation, 10. Recirculation mode operation).
maintaining a constant current density of 3 A dm2. The other components of the set-up were a 1000 ml reservoir, and a magnetically driven self-priming centrifugal pump, connected using silicone rubber tubes. The reservoir was filled with 500 ml of effluent with desired initial COD concentration containing 6 g L1 supporting electrolyte concentrations. The required flow rate through the reactor was established by pumping and adjusting the valves. The effluent recirculation flow rate (Q) into the reactor was varied (25, 30, 35, 40 L h1) using a calibrated rotameter. At a fixed flow rate, the effect of NaCl electrolyte concentration changing from 2 to 10 g L1 on the performance of the electrolytic cell was also studied. The cell voltage was periodically noted. The samples were collected periodically from the reservoir and filtered using Whatmann 42 filter paper. The COD was determined to investigate the behaviour of batch recirculation reactor.
The batch electrochemical reactor studies of various operating parameters were reported by Asaithambi et al. [25]. The effect of current density and supporting electrolyte concentration on COD removal efficiency and power consumption were considered for compare the different mode electrochemical cell operation of batch recirculation and single pass. The batch result reported that increasing the current density (1 to 5 A dm2) of the electrochemical cell follows production of more electrons, which results in increasing the percentage COD removal. But at higher current densities, production of extra electrons may contribute to undesirable side-reactions such as parasitic loss reactions leading to the depletion of hypochlorite concentration [38]. However, increasing current density beyond 3 A dm2, it was also observed that the loss of electrical energy occurs in the form of heat and more unwanted side reactions like water oxidation. 2H2 OðlÞ ! 2H2 ðgÞ þ O2 ðgÞ
(5)
The power consumption was increased from 7.19 to 21.60 kWh/ kg of COD with correspondingly the current efficiency decreasing from 95% to 54% for increasing the current density from 1 to 5 A dm2. This can be led to increase in the operating cost of the treatment process due to increase in power consumption with decreasing the current efficiency. The effect of supporting electrolyte concentration of NaCl on the percentage COD removal has been reported elsewhere that sodium chloride is very effective in the destruction of organics present in the effluent and decolourization can be completely achieved [39]. The COD removal is increased with increasing electrolyte concentration from 2 to 6 g L1. Further increase in NaCl concentration up to 10 g L1 did not bring about any meaningful improvements. The addition of the supporting electrolyte which allows the removal efficiency increases and a degradation of pollutants occurs due to the participation of active chlorine through the possible ‘‘direct’’ and ‘‘indirect’’ roles for the chloride anion in the electrochemical reaction. In indirect electro-oxidation, sodium chloride is added to
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
increase the conductivity and generates hypochlorite ions. The anodic reaction is given as: Anode Reaction k1
2Cl !Cl2 þ 2e k2
Cl2 þ H2 O!Hþ þ Cl þ HOCl k3
HOCl ! Hþ þ OCl 0
Recirculation flow (L h1)
Cell voltage (V)
COD removal (%)
Current efficiency (%)
Power consumption (kWh/kg of COD)
(7)
25 30 35 40
3.1 3.3 3.4 3.6
58.77 66.75 73.40 76.72
16.45 18.68 20.55 21.48
43.48 37.09 31.73 30.19
(8)
k4
(9)
The generated hypochlorite ions act as the main oxidizing agent in the pollutant degradation. In the indirect electro-oxidation rate of organic pollutants, a reaction sequence of chloride–chlorine– hypochlorite–chloride takes place in the bulk [40]. The maximum percentage COD removal is reported as 83.20% with a current efficiency of 82.69% and power consumption of 9.77 kWh/kg of COD, at an electrolyte concentration of 10 g L1 and an initial COD concentration of 10 g L1. The current efficiency is increased and power consumption is decreased for the increasing NaCl electrolyte concentration from 2 to 10 g L1. Batch with recirculation reactor The effect of supporting electrolyte concentration on the percentage COD removal is shown in Fig. 2 with fixed initial COD concentration of 10 g L1 at recirculation flow rate of 35 L h1. It is observed from the figure that the percentage of COD removal increased with increasing the electrolyte concentration from 2 to 10 g L1. Increase of NaCl concentration increases the conductivity of the effluent results decrease the cell voltage. It is found from Fig. 2 that COD removal increases from 64.05% to 86.50% with increase in electrolyte concentration from 2 to 10 g L1. Hence, the power consumption is decreased and current efficiency is increased. It can be observed that the batch recirculation system is superior in both completion of colour and COD removal. Thus, even though the energy consumption is marginally lower, the overall performance of the batch recirculation system is superior to the batch system. Also, 100% colour removal has been observed in the effluent after about 90 min of treatment in each case. The effect of recirculation flow rate on percentage COD removal is shown in the Table 2 at fixed current density 3 A dm2, supporting electrolyte concentration 6 g L1 and initial COD
100 80
COD removal (%)
Table 2 Effect of flow rate on the performance of batch electrochemical reactor with recirculation (Conditions: Initial COD concentration: 10 g L1; Electrolysis time: 6 h; Current density: 3 A dm2; Electrolyte concentration: 6 g L1).
(6)
k3
Organic þ OCl !CO2 þ H2 O þ Cl
555
concentration 10 g L1. The percentage of COD removal increases with increasing the recirculation flow rate. The Table 3 shows that the percentage of COD removal strongly depends on the reaction time. The COD removal percentage fixed time at 6 h of operation is found to be 58.77% to 76.72% with increasing circulation flow rates from 25 to 40 L h1, respectively. The rate of organic pollutant degradation is dependent on its bulk concentration. At higher flow rates, the rate of COD removal is significantly higher. This may be due to the increased generation of oxidants in the process. The transport of the Cl ion from the bulk to the electrode surfaces increases due to increase in electrolyte circulation flow rate. This would facilitate Cl2 generation, as well as the dissolution of Cl2 to form OCl for reaction which helps the increased removal of COD [40]. The current efficiency (16.45–21.48%) and power consumption (43.48–30.19 kWh/kg of COD) were considerably improved with increasing the effluent recirculation flow rate from 25 to 40 L h1. Single-pass flow reactor The effect of flow rate on the percentage COD removal, power consumption and current efficiency is shown in Fig. 3. It is observed from the figure, that the percentage of COD removal decreases with increase in the flow rate of effluent. This is due to the fact that residence time decreases with increase in flow rate. The percentage of COD removal of the effluent is under reaction control. On the other hand, the current efficiency increases and power consumption decrease with increase in flow rate. The effect of supporting electrolyte concentration on the percentage of COD removal, current efficiency and power consumption is shown in the Table 3. It is found from the table that COD removal increases from 49.01% to 61.65% with increase in electrolyte concentration from 2 to 10 g L1. This is because with increase in NaCl concentration, production of oxidizing agent was increased thus increasing the conductivity of the solution, and hence, reduction resistance in the cell voltage. This results in a decrease in power consumption from 65.88 kWh/kg of COD to 33.61 kWh/kg of COD and increase in current efficiency from 40.20% to 44.82%.
60 2 gL-1 4 gL-1 6 gL-1 -1 8 gL -1 gL 10
40
20 0
0
1
2
3 Time (h)
4
5
Table 3 Effect of electrolyte concentration on the performance of a single-pass flow electrochemical reactor (Conditions: Initial COD concentration: 10 g L1; flow rate: 1.2 L h1; Current density 3 A dm2; Initial pH: 6.1).
6
Fig. 2. Effect of supporting electrolyte concentration on the percentage COD removal in batch electrochemical reactor with recirculation (Conditions: Initial COD concentration: 10 g L1; recirculation flow rate: 35 L h1; Initial pH: 6.1; Electrolysis time: 6 h).
Electrolyte concentration (g L1)
Cell voltage (V)
COD removal (%)
Current efficiency (%)
Power consumption (kWh/kg of COD)
2 4 6 8 10
3.8 3.5 3.7 3.2 3.0
49.01 52.94 55.64 58.41 61.65
40.20 43.41 44.32 44.62 44.82
65.88 44.76 42.16 37.53 33.61
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
80
80
60
60
40
40
20
20
COD removal Power consumption
0
limiting oxidizing agent to another limiting condition such as limiting organic species due to addition of more salt [31]. In the case of current density, similar result was observed with the heterogeneous rate constant, which increases from 2.1 104 to 9.5 104 cms1 (the data not showed). In the batch electrochemical reactor with recirculation, the reservoir is assumed to be a continuous stirred tank. The flow is in axial direction in the reactor which is assumed to be an approximate plug flow reactor. The reaction rate for the removal of COD in a batch recirculation reactor under steady state can be expressed as [37]:
Power consumption (kWh/kg of COD)
COD removal (%)
556
@C Q @C ¼ kh aC Ae @x @t
0 1
0.5
1.5 Flow rate (Lh-1)
2
2.5
Fig. 3. Effect of flow rate on the performance of a single-pass flow electrochemical reactor (Conditions: Initial COD concentration: 10 g L1; Electrolyte concentration: 6 g L1; Current density 3 A dm2; Initial pH: 6.1).
At the steady state condition the solution of Eq. (11) is C Ae ¼ exp kh Co Q
In the batch electrochemical reactor, the COD concentration change with respect to time will remain constant under a given set of experimental conditions. However, the rate of COD removal is strongly dependent on current density and concentration of the electrolyte. The rate of COD removal in the batch electrochemical reactor can be expressed as [40]: C ¼ C 0 expðkh atÞ
(10)
where Co and C represents initial and at time t of COD concentration of the effluent in the batch reactor, kh is heterogeneous electrochemical rate constant, a is specific electrode area (Ae/VR), Ae is electrode area, VR is reactor volume. The heterogeneous rate constant with respect to the variation of current density and supporting electrolyte concentration was studied in the batch operation. The rate constant calculated from the slope of the plot ln(Co/C) vs t. in Fig. 4 shows the kinetic plot of batch electrochemical reactor at different electrolyte concentrations. The heterogeneous rate constant was increased from 3.3 104 to 9.1 104 cms1 with increasing the electrolyte concentration from 2 to 10 g L1. Especially at low salt concentration, the generation of oxidizing agent is more due to additional amount of salt added. The process might have changed from condition of 2
(13)
Solve the above mass balance equation after substituting the expression of COD concentration under steady state, knowing the initial COD of effluent in the reservoir. The resultant equation of COD variation in the batch recirculation reactor can be expressed C t Ae ¼ exp 1 exp kh (14) Co t Q where t is the residence time in the reservoir (Vres/Q). The heterogeneous rate constant kh can be calculated from the plot ln(C/Co) vs t having the slope ((1 exp(kha))/t). The rate constant and performance of the process are mainly dependent on the current density, electrolyte concentration and recirculation flow rate. Fig. 5 shows the kinetics plot of batch recirculation electrochemical reactor. The heterogeneous rate constant varies from 9.9 104 to 16.1 104 cms1 with increasing the recirculation flow rate at current density of 3 A dm2 with an electrolyte concentration of 6 g L1. The enhancement of heterogeneous rate constant at higher flow rates is due to increasing ionic conductivity due to bulk movement and decreases the resistance on the electrode surface. For the single pass or once through approach the effluent enters and leaves the reactor continuously. The rate of COD change
25 Lh-1
30 Lh-1
1.2
35 Lh-1
-ln(C/Co)
-ln(C/Co)
1.2
dC ¼ Q ðC C t Þ dt
1.6
2 gL-1 -1 4 gL -1 gL 6 -1 gL 8 -1 gL 10
1.6
(12)
The material balance can be written as around the reservoir is V res
Kinetics of COD removal
(11)
40 Lh-1
0.8
0.8
0.4
0.4 0
0
0
1
2
3 4 Time (h)
5
6
7
Fig. 4. Kinetics plot of ln(C/Co) vs t for the batch electrochemical reactor, at Current density: 3 A dm2; Initial pH: 6.1; Initial COD concentration: 10 g L1; Electrolysis time: 6 h.
0
1
2
3 4 Time (h)
5
6
7
Fig. 5. Kinetics plot of ln(C/Co) vs t for the batch electrochemical reactor with recirculation, at Initial COD concentration: 10 g L1; Current density: 3 A dm2; Initial pH: 6.1; Electrolysis time: 6 h.
M. Susree et al. / Journal of Environmental Chemical Engineering 1 (2013) 552–558
1.5
1
1.2
0.8
0.9
0.6
557
C/Co
-ln(C/Co)
Model Value
0.6 0.3
Experimental Value
0.4 0.2
0
0
0.4
0.8 1.2 1/Q (hL-1)
1.6
0
2
0
1
2
3 Time (h)
4
5
6
Fig. 6. Kinetics plot of ln(C/Co) vs 1/Q for the single-pass electrochemical reactor, at Initial COD concentration: 10 g L1; Electrolyte concentration: 6 g L1; Current density 3 A dm2; Initial pH: 6.1.
Fig. 7. Comparison of experimental and model value in the batch electrochemical reactor with recirculation at Initial COD concentration: 10 g L1; Current density: 3 A dm2; Initial pH: 6.1.
expression can be written as [40]:
experimental result. Similar result has been observed for the other flow rates also.
k Ae C ¼ C 0 expðkh at R Þ ¼ C o exp h Q
(15) Conclusions
Hence, C k Ae ln ¼ h Co Q
(16)
The heterogeneous rate constant kh can be computed from the slope of the plot ln(C/Co) vs 1/Q. Fig. 6 shows the kinetic plot of single-pass electrochemical reactor at initial concentration of 10 g L1, current density of 3 A dm2 and electrolyte concentration 6 g L1. The value of rate constant obtained is 1.0 104 cms1 from the slope of the plot. Modeling Electrolyte with recirculation system, the reactor can be assumed as (continuous Stirred Tank Reactor) CSTR model and dispersed plug flow model. In the previous section, the PFR (Plug flow reactor) model equation (Eq. (14)) has been derived and compared with the experimental result. The model simulation does not match with the experimental result. Then the reactor assumed to be a DPFR (Dispersed Plug Flow Reactor) model. When the effluent is passed between the electrodes in the filter press type reactor set-up, some degree of back mixing or intermixing takes place in the reactor. The D/UL value of the residence time distribution study of filter press type reactor found to be less than 0.01 reveals that smaller deviation from the dispersed to plug flow behaviour. [41] The flow behaviour in the filter press type reactor takes a dispersed plug flow. The material balance is drawn around the reactor and reservoir, the final equation becomes [42] " ( #) 4qexp Pe C t 2 ¼ exp 1 Co t ð1 þ qÞ2 exp q Pe ð1 qÞ2 exp q Pe 2
(17)
2
tk 0:5 and Pe ¼ UL where, q ¼ 1 þ 4Pe D D is the dispersion coefficient (m2/s); U is the velocity (m/s); L is the length of the reactor (m); k is the heterogeneous rate constant (m/s); t is the residence time (min); and Pe is the peclet number. The above model equation (17) is used to find the theoretical C/C0 value after substituting all the values. The model simulation is compared with experimental values. Fig. 7 shows the model comparison with the experimental value for the effluent flow rate of 25 L h1 and the D/UL value of 0.01. It is observed from the figure that the dispersed plug flow model satisfactorily matches with the
Electrochemical oxidation of distillery effluent was investigated using a mixed metal oxide electrode in various types of reactor configurations, i.e. batch, batch with recirculation and single-pass systems. The effect of operating parameters like current density, electrolyte concentration, electrolysis time and recirculation flow rate on the percentage COD removal, power consumption and current efficiency were investigated. The COD removal and power consumption increased with increasing current density in the batch electrochemical reactor. In the recirculation mode of operation, increase in flow rate increases the COD removal. For the single-pass operation, COD removal has increased with decreasing flow rate. Batch system is better in terms of large amount of COD removal compared to continuous system. But continuous systems were found to be better than batch systems in terms of energy utilization with comparable COD removal and it was better to treat huge volume of the effluent. The single-pass operation shows less completion of the process due to the lower residence time. The kinetics study of all the three modes of operation has been carried out and the heterogeneous rate constant also reported. Then dispersed model has been proposed for the filter press type electrochemical reactor. The model results were well in accordance with the experimental results. References [1] M.A. Oturan, A. Lahkimi, N. Oturan, M. Chaouch, Removal of textile dyes from water by the electro-Fenton process, Environmental Chemistry Letters 5 (2007) 35–39. [2] J.R. Bolton, K.G. Bircher, W. Tumas, C.A. Tolman, Figures-of-merit for the technical development and application of advanced oxidation processes, Pure and Applied Chemistry 73 (2001) 627–637. [3] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Advances in Environmental Research 8 (2004) 501–551. [4] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Advances in Environmental Research 8 (2004) 553–597. [5] M.P. Titus, V. Garcia-Molina, M.A. Ban˜os Jaime Gime´nez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Applied Catalysis B: Environmental 47 (2004) 219–256. [6] K. Juttner, U. Galla, H. Schmieder, Electrochemical approaches to environmental problems in the process industry, Electrochimica Acta 45 (2000) 2575–2594. [7] G. Chen, Electrochemical technologies in wastewater treatment, Separation and Purification Technology 38 (2004) 11–41. [8] D. Pletcher, F.C. Walsh, Industrial Electrochemistry, 2nd ed, Chapman and Hall, London, 1990.
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