3.5. Effect of distance between impellers in the multi-impeller reactor

To increase the active area of the reactor and its space time yield, a multi-impeller reactor should be used. To  study    the

effect of distance between the impellers on the mass transfer coefficient, a “3 impeller” reactor was used. Fig. 11 shows the effect of the distance between impeller on the mass transfer coefficient, the  data  show  that  the  impeller  separation has a little effect on the  rate  of  mass  transfer.  At  extremely low impeller separations the rate of mass transfer tends to decrease. This decrease attributed to the fact that the narrow zone between the impellers becomes depleted in the reactant where the rate of its consumption by the reaction becomes higher than the rate of its arrival by convective diffusion. Attenuation of convective diffusion in the narrow zone between impellers arises as a result of eddy damping caused by friction with rotating blades. Fig. 11 shows also that for a given set of conditions the average mass transfer coefficient of the multi-impeller electrode agrees fairly with the value of a single impeller electrode. It shows also that a small impeller separation such as 1.5 cm is reasonable in order to increase the area per unit volume  of  the  multi-impeller  reactor  as far as possible. The present result concerning the impeller separation is consistent with the finding of Despic et al. (1977) who studied the effect of the distance between successive horizontal parallel rotating discs (mounted on the same shaft) on the rate of mass transfer at the rotating discs, the authors represented their data in a mannar similar to Fig. 11 of the present study.

Fig. 10 – effect of Ref  of the superimposed axial flow on the mass transfer coefficient at different rotational speed.

Fig. 11 – Effect of the distance between impellers on the mass transfer coefficient at a multi-impeller electrode.

Table 5 – Effect of concentration of immiscible solvent (Toluene) on the % reduction in (y) at different impeller speeds in presence of baffles.

RPM μ for 10% toluene μ for 20% toluene μ for 30% toluene

360 13.64 22.73 27.27

400 14.29 21.43 27.14

440 13.70 21.92 24.66

480 10.53 21.05 25.00

520 12.50 21.25 25.00

560 13.10 21.43 25.00

3.6. Effect of drag reducing polymers

Drag reducing polymers have the potential of being us in the agitated vessels to reduce the mechanical energy consumed in rotating the impeller, this is attributed to the ability of the polymer molecules to damp the small scale high intensity energy dissipating eddies which prevails in the buffer zone of the turbulent hydrodynamic boundary layer formed at the solid surface (Sellin et al., 1982; White and Mugel, 2008). Since these eddies enhance the rate of mass transfer it is expected that the presence of the drag reducing polymer in the solu- tion would reduce the rate of mass transfer. Fig. 12 and Table 6 show that the presence of polyox WSR-301 drag reducing poly- mer reduces the rate of mass transfer at a single impeller by a value ranged from 3.3 to 15.8% in a baffled agitated vessel. Table 5 shows that the % reduction in mass transfer increases with increasing the polymer concentration and decreases with increasing Re because of the polymer degradation at high shear stress (Sellin et al., 1982; White and Mugel, 2008). Fig. 12 and Table 6 show that the ability of 300 ppm polyox to reduce K at relatively high Re is more pronounced compared to the lower concentrations of polymer because the high polymer concen- tration makes up for the shear degraded polymer molecules. The modest decrease in the rate of mass transfer in the present case compared to other cases where a turbine impeller was used (5–49%) (Sedahmed et al., 1998) indicates that the amount of small scale high frequency eddies (polymer sensitive) gener- ated by the present impeller is not as high as that generated by the turbine impeller. The modest decrease in the rate of mass transfer in diffusion controlled reactions encourages the use