K = 2

w tc

(9)

Combining Eqs. (7) and (9) leads to the following equation:

K ˛ N0.5 (10)

3.2. Effect of baffles

The above equation was obtained in a cylindrical agitated ves- sel without baffles; Fig. 9 shows the effect of the presence of standard baffles on the mass transfer coefficient, the data show that the presence of baffles increases the mass transfer coefficient by an amount ranging from 17.9 to 30.4% depending on impeller speed. The increase in the rate of mass transfer in the presence of baffles is ascribed  to  the  ability  of baf- fles to convert the swirl component of the discharge stream velocity into the more effective radial and axial flow veloc- ity components (Oldshue, 1983). The increase in the rate of mass transfer in the presence of baffles lends support to the idea that recycled flow contributes to enhancing the rate of mass transfer at the impeller beside impeller rotation. The present enhancing effect of baffles is consistent with the find- ing of other investigators who studied rates of mass transfer at the wall of cylindrical agitated vessels (Askew and Beckmann, 1965; Mowena et al., 2013; Bourne et al., 1984) with and without baffles.

3.3. Effect of the presence of immiscible  liquids

Since the present reactor can be used to process immiscible liquid reactants as in the case of  electro-organic synthesis and catalytic organic synthesis, it would be of interest to investigate the effect of immiscible liquids on the rate  of mass transfer at the  rotating  impeller.  To  this  end toluene (p = 0.87 g/cm3) was chosen in view of its stability under the present conditions. Tables 3 and 4 show that under the present conditions toluene reduces the rate of mass transfer at the impeller by an amount ranging from 3.3 to 27.3% depending on the amount of toluene present (% by volume), impeller

speed and the presence of baffles. The decrease in the rate of mass transfer in presence of toluene is ascribed to the fact that toluene droplets obstruct the diffusion of ferricyanide ions to the electrode surface. Baffles increase the tendency of toluene droplets to decrease the rate of mass transfer owing to the disintegration of toluene drops to smaller droplets under the influence of radial and axial flow induced by baffles, accordingly the total area of toluene droplets increase with a corresponding equivalent increase in the obstruction effect. The present result is consistent with the results of previous studies on the electrolysis of emulsions composed of low den- sity (p < 1 g/cm3) inert organic liquid and an aqueous solution using different geometries and different hydrodynamic condi- tions (Moreland and Fenton, 1989). On the other hand authors who used emulsions containing high density (p > 1 g/cm3) inert organic liquids or organic liquids containing soluble reactant found an increase in the rate of mass transfer in stirred cells owing to the disruption of the diffusion layer by the organic droplets and the rapid transfer of the reactant dissolved in the organic droplets to the electrode surface when these droplets strike it (Moreland and Fenton, 1989; Dworak et al., 1979).

3.4. Effect of superimposed solution  flow

To examine the performance of the present reactor in continuous operation the effect of superimposed solution flow on the rate of mass transfer  of  the  rotating impeller was studied as shown in Fig. 10. The data  show  that the mass transfer coefficient increases slightly with  increasing Ref, reaches a maximum at Ref  of about 300 then starts to

Fig. 9 –  Effect of baffles on the mass transfer    coefficient.

decrease at higher Ref. The initial increase in K is attributed to the turbulent wake formation behind the rotating cylinder blades of the impeller in the direction of flow as it crosses the blades. The decrease in K after reaching a maximum at relatively high Ref is explained by the fact that axial flow sweeps away the turbulence generated by impeller rotation from the impeller zone. Within the present range of Ref (50–450) the slight variation of K does not invalidate Eq. (6) of the batch reactor. The decrease in mass transfer coefficient at relatively high superimposed solution velocities is consistent with the finding of other authors who studied mass transfer in agitated vessels with superimposed axial flow. Gabe and Walsh reported that a high axial solution velocity reduces the rate of mass transfer at rotating cylinders owing to the eddy removal from the rotating cylinder (Gabe and Walsh, 1983), Mowena et al. (2013) who studied rates of mass transfer at a fixed bed in an agitated vessel found that the rate of mass transfer at a fixed bed tends to decrease in the presence of superaxial flow especially at high impeller rotation speed.