**3.3.4 Power supply**

58 Electrochemical Cells – New Advances in Fundamental Researches and Applications

3600. ... <sup>1</sup> ( / pollutant) . <sup>3</sup>

using initial pollutant concentration *C0* (kg/m3), current intensity *I* (A), cell voltage *U* (V), electrolysis time *t* (h), liquid volume *V* (m3), molar weight of aluminum *MAl* = 0.02698

> <sup>3</sup> . 3600. . . *Al Al F*

*M I t*

This parameter depends upon the pH and the amount of other species present in solution,

The increase of the conductivity () by the addition of sodium chloride is known to reduce the cell voltage U at constant current density due to the decrease of the ohmic resistance of wastewater. Energy consumption, which is proportional to *U.I*, will therefore decrease. Chloride ions could significantly reduce the adverse effects of other anions, such as HCO3 – and SO4 <sup>2</sup>−, for instance by avoiding the precipitation of calcium carbonate in hard water that could form an insulating layer on the surface of the electrodes and increase the ohmic resistance of the electrochemical cell (Chen et al., 2004). Chloride anions can also be oxidized and give active chlorine forms, such as hypochlorite anions, that can oxidize pollutants. The

However, an excessive amount of NaCl (higher than 3 g/L) induces overconsumption of the aluminum electrodes due to "corrosion pitting" described above; Al dissolution may

pH is known to play a key role on the performance of EC. An optimum has to be found for the initial pH, in order to optimize the EC process. However, the pH changed during batch EC. Its evolution depended on the initial pH. EC process exhibits some buffering capacity because of

prevents high change in pH. The buffering pH seems just above 7: when the initial pH is above

this value, pH decreases during EC; otherwise, the opposite behavior is observed. The effect of pH can be explained as follows. The main reactions during EC are:

*<sup>M</sup> I t kgAl kg F VY <sup>C</sup>*

*Al*

constant *F* (96,487 C/mol*e*–) and the faradic yield

for example co-existing anions.

main mechanism is as follows:

become irregular.

**3.3.3 pH effect** 

Anode: Al0(s)→ Al3+ + 3e−

Cathode: 2H2O + 2e- → H2(g) +2OH-

Cl2 +2e−→ 2Cl<sup>−</sup>

Cl2 +H2O → Cl− +ClO− +2H+

the balance between the production and the consumption of OH-

efficiency.

*mth*: the experiments

**3.3.2 Conductivity** 

<sup>0</sup>

*Al* of Al dissolution, Y is the removal

(Chen et al., 2004), which

*Al Al*

(11)

*mexp* and the amount of aluminum consumed theoretically at the anode

exp

(12)

*m*

*Al*is estimated as the ratio of the weight loss of the aluminum electrodes during

From an energetic point of view, energy consumption during EC is known to vary as the product *UIt*. Energy requirements per kg of pollutant removed (*E*pollutant) to achieve a certain percentage of efficiency (Y) shows a continuous increase of *E*pollutant with *j*.

The specific electrical energy consumption per kg pollutant removed (*E*pollutant) is calculated as follows:

$$\mathbb{E}\left(\text{kWh} / \text{kg} \mid \text{pollutant}\right) = \frac{\text{U}.\text{I.t.}}{1000.\text{V.}\left(\text{C}\_{0}\text{Y}\right)}\tag{13}$$

#### **3.3.5 Temperature**

Few papers were investigated to show the effect of temperature on EC efficiency. The current efficiency of aluminum was found to be increased with temperature until about 60°C (Chen, 2004) where a maximum was found. Further increase in temperature results in a decrease of EC efficiency. The increase of temperature allows to a destruction of the aluminum oxide film on the electrode surface.

### **3.4 Design of electrocoagulation cell**

The position of the electrodes in the reactor can be optimized as a function of hydrodynamic parameters and current density (j). Complementary rules should include the influences of electrode gap (e) and operating conditions on voltage U (and consequently on energy consumption). The measured potential is the sum of three contributions, namely the kinetic overpotential, the mass transfer overpotential and the overpotential caused by solution ohmic resistance. Kinetic and mass transfer overpotentials increase with current density, but mass transfer is mainly related to mixing conditions: if mixing is rapid enough, mass transfer overpotential should be negligible. In this case, the model described by Chen et al., (2004) is often recommended for non-passivated electrodes:

$$
\Delta U = -0.76 + \frac{e}{k}j + 0.20 \text{ ln } (j) \tag{14}
$$

Electrochemical Probe for Frictional Force and Bubble Measurements

electrochemically-generated. This means that both *hD* and

the electrodes in the riser. At constant current density,

from Equations (15) and (16) that *ULdhD*.

erosion, which means low *ULd* values.

**3.5.1 Reactor design** 

dispersion height *hD* when electrode position is modified, provided

phase:

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 61

Ad: cross-sectional area of the downcomer (m2) and Ar: cross-sectional area of the riser (m2) The superficial liquid velocity in the riser (*ULd*) is deduced from a mass balance on the liquid

> *d Lr Ld r <sup>A</sup> U U*

In order to transform this reactor to an electrochemical one, the gas phase is not injected, but

The objective of complete flotation may be achieved only if hydrodynamic shear forces remain weak in the riser to avoid floc break-up and in the separator to limit break-up and

An external-loop airlift made of transparent plexiglas is used for this study. The reactor geometry is illustrated by figure 12. By definition, the riser is the section in which the gas phase is sparged and flows upwards. The diameters of the riser and the downcomer are respectively 94mm and 50 mm. Consequently, the riser-to-downcomer cross-sectional ratio (Ar/Ad) is about 3.5. This is a typical value when reaction takes place only in the riser section. Both are 147 cm height (H2 +H3) and are connected at the bottom by a junction of 50 mm diameter and at the top by a gas separator (also denoted gas disengagement section) of HS = 20 cm height. The distance between the vertical axes of the riser and the downcomer is 675 mm, which limits the recirculation of bubbles/particles from the riser into the downcomer. At the bottom, the curvature radius of the two elbows is 12.5 cm in order to minimize friction and avoid any dead zone. The liquid volume depends on the clear liquid height (*h*) and can be varied between 14 L and 20 L, which corresponds to a clear liquid level between 2 cm and 14 cm in the separator section. All the experiments are conducted at room temperature (20±1 ◦C) and atmospheric pressure in the semi-batch mode (reactor open to the gas, closed to the liquid phase). Contrary to conventional operation in airlift reactors, no gas phase is sparged at the bottom in the riser. Only electrolytic gases induce the overall gas recirculation resulting from the density difference between the fluids in the riser and the downcomer. Two readily available aluminum flat electrodes of rectangular shape (250mm×70mm×1 mm) are used as the anode and the cathode, which corresponds to *S* = 175 cm2 electrode surface area (Fig. 12). The distance between electrodes is *e* = 20mm, which is a typical value in EC cells. They are treated with a HCl aqueous solution for cleaning prior use to avoid passivation. The electrodes are placed in the riser, parallel to the main flow direction to minimize pressure drop in the riser and maximize the recirculation velocity. The axial position of the electrode can also be varied in the column. The distance (H1) between the bottom of the electrodes and the bottom of the riser ranged between 7 cm and 77 cm. EC is conducted in the intensiostat mode, using a digital DC power supply (Didalab, France) and recording potential during the experiments. The width of the electrodes is maximized by taking into account riser diameter and electrode inter-distance. Current density values (*j*)

*<sup>A</sup>* (16)

*<sup>r</sup>* depend on the axial position of

*<sup>r</sup>* should vary approximately as

*<sup>r</sup>* <<1. One can deduce

Different typical reactors applied for electrochemical technologies are explained by Chen (2004).

#### **3.5 Airlift reactors as innovative electrocoagulation cells**

Airlift reactors constitute a particular class of bubble columns in which the difference in gas hold-up between two sections (namely, the riser and the downcomer) induces an overall liquid circulation without mechanical agitation (Chisti, 1989). They have been extensively applied in the process industry to carry out chemical and biochemical slow reactions, such as chemical oxidation using O2, Cl2 or aerobic fermentation, but never as EC cells, as far as we know. Airlift reactors present two main designs: external-loop and internal-loop configurations (figure 11 a –b).

Fig. 11. Airlift reactors, (a) : internal loop airlift reactor, (b) : external loop airlift reactor.

External-loop airlift reactors offer the advantage to allow various designs of the separator section, which favors gas disengagement at the top of the reactor and maximizes consequently the overall recirculation velocity at the expense of more complex reactor geometries. Their hydrodynamics has also been extensively studied in two-phase gas-liquid and three-phase gas-liquid-solid flows. In airlift reactors, the driving force of the overall liquid circulation results from the gas hold-up difference between the riser (*<sup>r</sup>*) and the downcomer (*<sup>d</sup>*), and also from the dispersion height. Gas hold-up is defined as the ratio of volume occupied by the gas phase over the total volume of the corresponding section. Dispersion height (*hD*) corresponds to the distance from the surface in which a gas phase can be observed in the riser.

The overall liquid circulation velocity in the riser ULr can therefore be predicted from an energy balance using Equation 15 (Chisti, 1989):

$$\mathcal{U}\_{Lr} = \left| \frac{2 \, \text{g} \cdot \text{h}\_{\text{D}} \cdot (\varepsilon\_r - \varepsilon\_d)}{\left(\frac{K\_T}{\left(1 - \varepsilon\_r\right)^2} + \left(\frac{A\_r}{A\_d}\right)^2 \frac{K\_B}{\left(1 - \varepsilon\_d\right)}\right)} \right| \tag{15}$$

KT coefficient taking into account the effects of pressure drop in the riser and the separator section and *KB* accounts for pressure drop in the downcomer and the junction.

Ad: cross-sectional area of the downcomer (m2) and Ar: cross-sectional area of the riser (m2)

The superficial liquid velocity in the riser (*ULd*) is deduced from a mass balance on the liquid phase:

$$
\mathcal{U}L\_{Lr} = \frac{A\_d}{A\_r} \mathcal{U}\_{Ld} \tag{16}
$$

In order to transform this reactor to an electrochemical one, the gas phase is not injected, but electrochemically-generated. This means that both *hD* and *<sup>r</sup>* depend on the axial position of the electrodes in the riser. At constant current density, *<sup>r</sup>* should vary approximately as dispersion height *hD* when electrode position is modified, provided *<sup>r</sup>* <<1. One can deduce from Equations (15) and (16) that *ULdhD*.

The objective of complete flotation may be achieved only if hydrodynamic shear forces remain weak in the riser to avoid floc break-up and in the separator to limit break-up and erosion, which means low *ULd* values.
