The reduction of the size of bubbles can improve the performance of gas-solid fluidized beds significantly. For chemical processes, both the selectivity and the conversion may improve; for physical processes such as coating or granulation of particles, the control of bubble size may lead to better-defined product properties. However, the control of bubbles is difficult to achieve without methods that use a lot of energy and/or deteriorate the fluidization behaviour. We present results of a method that decreases the size of bubbles while maintaining the advantages of fluidized beds. It will be shown how the application of low-energy electric fields in fluidized beds can reduce the bubble size up to 80% while maintaining the free movement of particles so essential to fluidization. The electrical cost of applying these electric fields is as low as 50 W/m³.

Experimental results will be presented for Geldart A and B glass beads in the bubbling fluidization regime. A number of parameters influence the effect that applied electric fields have on the bubble reduction, such as the size and the dielectric properties of the particles, as well as the frequency and strength of the field. Larger particles show a more drastic decrease in bubble size than smaller particles: a maximum of 80 % bubble diameter reduction for 700 um glass beads vs. a reduction of 25% for 77 um glass beads. For different bed materials, shifts in optimal field strengths and frequencies are observed.

On a particle level, the electric fields cause an electric polarization of the individual particles – the net charge on the particles remains zero. However, the polarization results in particle-particle forces. We will show by discrete particle CFD simulations that this leads to the formation of structures on the particle level and the reduction of the bubble size. By experiments with ozone conversion in an electric field optimised fluidized bed, it is demonstrated that a bubble size reduction indeed leads to a significant increase of conversion in case of a mass-transfer-limited reaction.