**3.5.1 Reactor design**

60 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Different typical reactors applied for electrochemical technologies are explained by Chen

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

(a) (b)

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

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

The overall liquid circulation velocity in the riser ULr can therefore be predicted from an

2

KT coefficient taking into account the effects of pressure drop in the riser and the separator

*<sup>d</sup>*), and also from the dispersion height. Gas hold-up is defined as the ratio of

2

2( )

*Dr d*

 

(1 ) (1 )

*K K A A*

*T B r r d d* (15)

*<sup>r</sup>*) and the

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

liquid circulation results from the gas hold-up difference between the riser (

**3.5 Airlift reactors as innovative electrocoagulation cells** 

(2004).

downcomer (

energy balance using Equation 15 (Chisti, 1989):

*Lr*

*g h <sup>U</sup>*

section and *KB* accounts for pressure drop in the downcomer and the junction.

be observed in the riser.

configurations (figure 11 a –b).

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*)

Electrochemical Probe for Frictional Force and Bubble Measurements

phase: ULr = Ad /Ar ULd.

pHmeter (WTW, Germany).

maximum wavelength *A450* (

erosion, which means low ULd values.

SP8-400, UK).

current densities.

in figure 13.

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 63

record the tracer concentration resulting from the injection of 5mL of a saturated NaCl solution at the top of the downcomer using a data acquisition system based on a PC computer equipped with UEI-815 A/D converter. The distance between the probes is 90 cm. Liquid velocity is estimated using the ratio of the mean transit time between the tracer peaks detected successively by the two electrodes and the distance between the probes. The superficial liquid velocity in the riser (ULr) is deduced from a mass balance on the liquid

An example of experimental data provided by the conductivity tracer technique is reported

Fig. 13. Example of experimental data from conductivity tracer experiments in the

Experiments are carried out using a red dye solution consisting of a mixture of 2-naphthoic acid and 2-naphtol with a total concentration C0 = 20 mg/L. Synthetic solutions were prepared by dissolving the dye in tap water. Solution conductivity and pH are measured using a CD810 conductimeter (Radiometer Analytical, France) and a Profil Line pH197i

Dye concentration is estimated from its absorbance characteristics in the UV-Vis range at

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

At constant electrode geometry, the solution consists of an adequate selection of ULd resulting from a compromise between mixing and floc stability. This can be obtained first by optimizing the axial position of the electrodes using both the conductivity tracer technique for ULd estimation and turbidity measurements to estimate the amount of dispersed Al particles in the downcomer. Experimental results show that no liquid overall circulation can be detected when the electrodes were placed in the upper part of riser, for H1 approximately higher than 60 cm. For 7 cm <H1 <60 cm, *U*Ld decreases when H1 increases. Overall liquid velocities in the downcomer for two axial positions are reported in figure 14 at various

*max*=450 nm) using a UV-Vis spectrophotometer (Pye Unicam,

downcomer section when current density *j*=21.4 mA/cm².

between 5.1 and 51 mA/cm2 are investigated, which corresponds to current (*I* = *jS*) in the range 1.0–10 A.

Fig. 12. External-loop airlift reactor (1: downcomer section; 2: riser section; 3: conductivity probes; 4: junction column; 5: separator 6: conductimeter; 7: analog output/input terminal panel (acquisition system); 8: 50-way ribbon cable kit; 9: data acquisition system; 10: electrodes).

#### **3.5.2 Chemicals and methods**

The average liquid velocity in the downcomer (ULd) is measured using the conductivity tracer technique. Two conductivity probes placed in the downcomer section were used to

between 5.1 and 51 mA/cm2 are investigated, which corresponds to current (*I* = *jS*) in the

Fig. 12. External-loop airlift reactor (1: downcomer section; 2: riser section; 3: conductivity probes; 4: junction column; 5: separator 6: conductimeter; 7: analog output/input terminal panel (acquisition system); 8: 50-way ribbon cable kit; 9: data acquisition system; 10:

The average liquid velocity in the downcomer (ULd) is measured using the conductivity tracer technique. Two conductivity probes placed in the downcomer section were used to

range 1.0–10 A.

electrodes).

**3.5.2 Chemicals and methods** 

record the tracer concentration resulting from the injection of 5mL of a saturated NaCl solution at the top of the downcomer using a data acquisition system based on a PC computer equipped with UEI-815 A/D converter. The distance between the probes is 90 cm. Liquid velocity is estimated using the ratio of the mean transit time between the tracer peaks detected successively by the two electrodes and the distance between the probes. The superficial liquid velocity in the riser (ULr) is deduced from a mass balance on the liquid phase: ULr = Ad /Ar ULd.

An example of experimental data provided by the conductivity tracer technique is reported in figure 13.

Fig. 13. Example of experimental data from conductivity tracer experiments in the downcomer section when current density *j*=21.4 mA/cm².

Experiments are carried out using a red dye solution consisting of a mixture of 2-naphthoic acid and 2-naphtol with a total concentration C0 = 20 mg/L. Synthetic solutions were prepared by dissolving the dye in tap water. Solution conductivity and pH are measured using a CD810 conductimeter (Radiometer Analytical, France) and a Profil Line pH197i pHmeter (WTW, Germany).

Dye concentration is estimated from its absorbance characteristics in the UV-Vis range at maximum wavelength *A450* (*max*=450 nm) using a UV-Vis spectrophotometer (Pye Unicam, SP8-400, UK).

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.

At constant electrode geometry, the solution consists of an adequate selection of ULd resulting from a compromise between mixing and floc stability. This can be obtained first by optimizing the axial position of the electrodes using both the conductivity tracer technique for ULd estimation and turbidity measurements to estimate the amount of dispersed Al particles in the downcomer. Experimental results show that no liquid overall circulation can be detected when the electrodes were placed in the upper part of riser, for H1 approximately higher than 60 cm. For 7 cm <H1 <60 cm, *U*Ld decreases when H1 increases. Overall liquid velocities in the downcomer for two axial positions are reported in figure 14 at various current densities.

Electrochemical Probe for Frictional Force and Bubble Measurements

H1=47 cm H1=7 cm

downcomer (initial pH: 8.3, conductivity:

**Absorbance A450**

0

0.05

0.10

0.25

0.20

0.25

0.30

**3.5.3 Some results** 

**3.5.3.1 Synthetic dye** 

percentage and defined as:

parameters.

conductivity

literature.

dye.

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 65

Fig. 15. Evolution of absorbance *A450* of non-filtered samples after 10 min operation with 1 cm of floc thickness already formed as a function of superficial liquid velocity *ULd* in the

The efficiency of electrocoagulation/electroflotation in removing colour from synthetic textile wastewater by using aluminum and iron electrodes in an external-loop airlift reactor is presented. Disperse, reactive and the mixture are used to determine the optimized

The real textile wastewater is then used using the optimized parameters. Three effluents were also used: disperse, reactive and the mixture. Energy is determined for each kind of

For this synthetic dye, chemical oxygen demand (COD) is measured using the standard closed reflux colorimetric method. Initial COD was about 2500 mg O2/L. Initial pH is varied between 5 and 10 using minute addition of 0.1 M H2SO4 or NaOH solutions. The

addition in the range 1.0–29 mS/cm, which covers the range usually explored in the

COD, color removal and turbidity efficiencies (*YCOD*, *YCOL* and *YABS*) are expressed as

COD

Y

COL

Y

(i.e. the ionic strength) of dye solutions is adjusted by sodium chloride

 450 450

(18a)

(18b)

COD t 0 COD t

COD t 0 

A t0

450 A t0 A t

*YCOL* and *YABS* are obtained using the same equation (Eq.18b), but *YCOL* was based on absorbance measurements at 450 nm after filtration, while *YABS* is measured without

=2.4 mS/cm).

0 10 20 30 40 **Current density j (mA/cm2 )** 

Fig. 14. Influence of the axial position of the electrodes (*H1*) and current density (*j*) on the overall liquid recirculation *ULd* (*h*=14 cm; initial pH: 8.3; initial conductivity: =2.4 mS/cm).

Two-parameter model is used to fit the data. The results are given in equation (17):

$$\text{CL}\_{Ld} = 5.8 \cdot \left(\frac{h\_D}{h\_{D\text{max}}}\right) \cdot j^{0.20} \text{ (cm/s)}\tag{17}$$

in which *hDmax* is the maximum dispersion height corresponding to *H1*=7cm. This equation confirms the key role of the axial position of the electrodes on reactor hydrodynamics and mixing properties, and the weaker influence of current.

The corresponding turbidity data based on A450 without filtration is reported in figure 15 for an electrolysis time of 10 min and two axial positions of the electrodes. In all cases, flocs occupy nearly 1 cm thickness at the free surface of the disengagement section and no settling is reported in the junction and in the separator. Figure 15 shows however that turbidity rose when H1 =7cm for current density higher than 15 mA/cm2. This corresponds to ULd values in the range 9–10 cm/s in figure 15. Conversely, complete flotation is always observed for H1 = 47 cm, as A450 remains low, about 0.06. As a result, electrode position must be chosen in order to maintain ULd always lower than 9 cm/s, regardless of current density in the range of *j* studied (Fig. 15). Such a condition is achieved for H1 = 47 cm, as shown in figure 15, which corresponds nearly to mid-height in the riser. However, H1 = 47 cm is only a coarse approximation of the optimum electrode height, but the simple way to optimize mixing conditions does not consist in adjusting precisely H1 because it is easier from a practical point of view to adjust the clear liquid height *h*. Indeed, *h* affects simultaneously *h*D and *h*Dmax in Eq. (17), but its range (2 cm < *h* < 14 cm in this work) is usually far smaller than H1 (between 7 and 77 cm).

Fig. 15. Evolution of absorbance *A450* of non-filtered samples after 10 min operation with 1 cm of floc thickness already formed as a function of superficial liquid velocity *ULd* in the downcomer (initial pH: 8.3, conductivity: =2.4 mS/cm).
