Elutriation control and charge measurement of fines in a gas fluidized bed with ac and dc electric fields

A study of elutriation control using ac and dc electric fields was carried out in an air-fluidized bed of sand in the bubbling regime. Reductions of sand fines concentration up to 96% in the bed freeboard were measured using a real time laser-optical technique together with a solids flux model. In this method, the bed is not shut down during the test. A unique submerged electrode fluidized bed was used to test the bed retention mechanisms of fines and to measure elutriation constants and electrostatic charge of the fines. Real time particle sampling using a Faraday cage in the bed freeboard confirmed that net charge (per mass) on particles leaving the bed first increases and then decreases over time. Together with electric field bubble control, this charge is thought to contribute to improved particle retention in the bed. The experimental variables studied for elutriation included electric field strength and frequency, superficial velocity, bed temperature, and distance of the (submerged) electrode below the surface of the bed. The importance of charge relaxation on particle retention in the bed is discussed.