**2.2.2 Gas injector**

52 Electrochemical Cells – New Advances in Fundamental Researches and Applications

T4, with inner diameters of 1, 2, and 4 mm, respectively, are placed 5 cm below the probe.

At low gas volumetric flow rates, the effect of the regular passage of bubbles close to the electrochemical probe on the diffusion limit current (or the velocity gradient) is studied. The stability of the bubble frequency is shown on the signal by regularly spaced peaks (fig.5). Signals similar to this are obtained for all positions of the front of the spherical probe (active surface) exposed to bubbles (0° < < 120°). The frequency spectrum clearly shows a

Fig. 5. History records of the velocity gradient at the wall: Effect of the regular passage of

0 2 4 6 8 10 12

Knowing the volumetric flow rate, Qg, we can obtain the bubble volume, Vb by the relation:

*b*

Assuming spherical bubbles, we can deduce the bubble diameter: db = (6Vb/)1/3.

*g*

*<sup>V</sup> <sup>f</sup>* (9)

*Q*

A comparison at low gas flow rates is achieved by video recording when the bubble emission frequency and volume are obtained. Excellent agreement is observed, proving that the two methods give exactly the same frequency and the same average bubble diameter

T2(do=2 mm): Ul=2.7 cm/s, Qg=0.33 cm3/s <sup>3</sup>

Fig. 6. Power spectral density function (Ps/s2): Effect of the regular passage of bubbles near

048 11222 **f(Hz)**

*b*

The probe is placed 30 cm above the liquid distributor and in the column center.

single peak representing the bubble frequency, fb (fig.6).

bubbles near the electrochemical probe.

within 3%.

the electrochemical probe.

In the case of a gas injector consisting of two concentric circular tubes with regularly spaced holes, the average frequency is used (<f>) to evaluate the bubble diameter in the bubble column for the homogeneous bubbly flow. The bubble diameter is estimated by the following expression (Essadki et al., 1997):

$$d\_b = 1.5 \frac{\mathcal{U}\_\mathcal{S}}{\langle f \rangle} \tag{10}$$

The possibility of measuring bubble size by electrochemical probe is possible for the homogeneous bubbling regime in any conducting medium.
