搅拌釜反应器英文文献和中文翻译(6)

菜单

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.