Bubble and Elutriation Control in Fluidized Beds with Electric Fields

Publication Reference: 
24-06
Author Last Name: 
Colver
Authors: 
G M Colver, J S Wang, F Hossain, S El-Khabir
Report Type: 
FRR
Research Area: 
Powder Flow
Publication Year: 
1997
Publication Month: 
05
Country: 
United States

Executive Summary Theoretical and experimental studies have demonstrated that ac electric fields are effective in suppressing bubbles and promoting expansion of gas fluidized beds of fine powders (< 125 µm). In addition electric fields are effective in controlling elutriation of fines from the bed. A unified theory to explain bubble control and various bed phenomena was proposed and researched. Parametric studies of several variables affecting bubble control (temperature, particle size, particle material, fluidizing gas material, electric field strength, field frequency) were carried out over the last 5 years. A prototype 18 inch cold flow bed was built, tested, and instrumented during the 6th year.

Theoretical work included ac and dc interparticle force modeling, application of a perturbation theory for evaluating the bed elasticity modulus, and the utilization of a two-dimensional numerical code (K-Fix) incorporating electric fields to demonstrate real time bubble control. The use of correlations with electric fields represented an alternative approach to quantifying bubble control and bed expansion including the effects of bed voidage, superficial velocity, particle diameter, and electric field strength for glass spheres.

A total of five test facilities were constructed to verify bubble and elutriation control utilizing both rectangular and cylindrical beds. A radiant heater quartz bed with feedback temperature control was used to achieve temperatures up to 530 C for bubble control studies and to 500 C for elutriation control along with computer aided data acquisition. For our elutriation studies a unique electrode-from-below bed was designed to verify our theory of electric fields. A special sampling Faraday cage was designed to measure the charge of fines in the freeboard. A new real time laser monitoring system of particle concentration was developed allowing us to evaluate elutriation constants directly and with high accuracy. Previous researchers have emptied the bed and restarted experiments to achieve time dependent measurements.

For bubble control our studies confirm bed expansions up to 15% while maintaining control of bubbling up to 530 C for FCC Various equations have been developed such as the linear relationship between bed elasticity modulus “and electric field strength and an inverse relationship with field frequency. For elutriation control with fields, our experiments confirm a direct relationship with bubble control. Significant reductions in elutriation of fine particles up to 96 % were measured, The mechanism of retention of fines to parent particles in the bed with electric fields remains speculative.