Studies on fluidised-bed electrode

Abstract

Flowing electrolyte

The mass transfer coefficient k between the flowing electrolyte and the outer wall of an annulus in streamline flow varies as … The mass?transfer data obtained in streamline flow in this investigation with the annulus radius ratio of 0.57 and single concentration of maleic acid, 10 g/L, are correlated by the following dimensionless equation:

Sh = 4.92 (Re · Sc)^0.5

Inert material fluidized?bed

The presence of a fluidized?bed of inert particles (glass beads) increases the current supported by the electrode and hence the mass?transfer coefficient, the maximum being about 1.5?fold for the same superficial velocity. The mass?transfer coefficients for this particular cell configuration are correlated with the modified Reynolds number by the following dimensionless equation:

j = 0.062 (Re’)^0.15 (… )^1/3

Fluidized?bed electrode

Under identical operating conditions the current supported by

the solid electrode (feeder only) with flowing electrolyte, solid electrode (feeder) in the presence of inert material fluidized?bed, and fluidized?bed electrode

are approximately in the ratio 1 : 1.5 : 4 (maximum) for the same average liquid velocity.

Under identical operating conditions the current supported by the solid electrode (feeder) in the presence of inert material fluidized?bed and the active fluidized?bed electrode is approximately in the ratio 1 : 3 for the same bed expansion.

For the expansions of 10 to 50% studied, the total superficial area of the fluidized?bed electrode is 200 to 150 times greater than the area of the feeder electrode. But the current supported is only 2.5 to 2 times that of the solid (feeder) electrode.

The data on solution and metal potential profiles in the radial direction of the bed indicate that the active volume of the electrode is essentially confined to a narrow region adjacent to the feeder electrode.

In the activation?control region the cell current increases with bed expansion up to 30%, whereas in the activation?and?diffusion (mixed) control region such an increase was observed for the entire range of expansions up to 50%, the control potential being same in all cases.

At very low current densities the whole bed, except the first layer of particles adjacent to the feeder which is more active, exhibits a flat potential profile (compared to the one at high current densities).

The variation of potential in the radial direction near the bed support?cum?fluid distributor is less pronounced due to more solid concentration and hydrodynamic effects.

The variation of potential in the axial direction of the bed is not significant compared to the variation in the radial direction.

The value of effective diffusion?layer thickness, ?, is of the same magnitude as that in the case of solid electrode in the presence of fluidized?bed of inert particles. This shows the existence of considerable turbulence in the fluidized?bed even under moderate flow conditions.

The mathematical expressions deduced to generate the polarisation curves and solution and metal potential profiles based on the one?dimensional model for porous electrode by Newman and Tobias and the one?dimensional model for plane?parallel fluidized?bed electrode by Fleischmann et al., give results which agree reasonably well with the experimental data of fluidized?bed electrode of concentric configuration used in this investigation.

The effective specific resistivities of the discontinuous metal phase, ?m, computed from the solution and metal potential profiles, vary from 17.5 to 40.5 ohm?cm (reading a minimum value of 15 ohm?cm at 20% bed expansion) with bed expansions 10% to 50%. These values are higher than the corresponding values of the effective specific resistivities of the solution phase, ?s, which vary from 7.07 to 4.66 ohm?cm with bed expansions 10% to 50%.