Executive Summary
The goal of developing a unifiedfield theory for bubble control in a fluidized bed is reviewed. Included in this unified theory is a single inter-particle force model that will handle the limiting cases of high as well vanishing field frequencies (ic. the ac and dc limits). The electrostatic bubble model utilizes a perturbation theory to relate force relationships at the particle level to the continuum equations of mass and momentum at the bulk bed level. An extension of the Davidson bubble model gives information on the local stresses induced around a bubble in terms of the far distant electric field strength.
For the first time in this IFPRI work we introduce a relationship between bubble control and elutriation control through an understanding of the various interparticle forces. Two mechanisms relating to electrostatics effects in elutriation are postulated by which the entrainment of fine particles can be diminished with electric fields: a) that bubble formation is inhibited with the application of electric fields, and (b) that the bonding strength of fine particles to bulk bed particles is increased with electric fields.
To elaborate on these two mechanisms, various permanent and electrostatic interparticle forces are evaluated. For the electrical forces, current constriction is shown to be the dominating cohesion force that influences both bubble control and elutriation control, with the force of induction charging found to bc less important. For the permanent forces, van der Walls and triboelectric particle forces contribute over different size ranges of fine particles depending on the surface roughness of the particle. Gravitational forces affect mainly bubble action through the larger bulk bed particles.
In a separate experiment it is now confirmed that elutriation control with electric fields is a consequence of field action within the bed and not simply a precipitator effect in the bed freeboard. The test was carried out on a specially designed fluidizcd bed in which the high voltage electrode entered through the bottom of the bed rather that from above so that the possibility of precipitator action was largely reduced or eliminated.
Our present correlations now include bed expansion, superficial velocity, particle diameter, and electric field strength for glass spheres Previously, particle diameter had not been included in these correlations. The new results should be useful in helping to predict scale-up of beds as well as for predicting the delay of minimum bubbling conditions with fields.
The future plans for this research call for scale-up studies utilizing a larger bed experiment for bubble control and for developing a new high temperature facility. Numerical modeling studies will also be introduced. Our present facility has provided data up to 500 degrees C.