Executive Summary
Bubble Control with electric fields in fluidized beds-Part I
New results are presented for both ac and dc electric field bubble control in gas fluidized beds tested to 125 “C. Modeling includes, (a) electrostatic forces at a bubble interface for dc fields, and (b) an ac theory for inter-particle capacitive forces. Correlations for bed expansion are presented that includes the electric field strength and superficial velocity based on an extension of two-phase fluidization theory. The experimental results reflect studies that were initiated last summer with the new Trek 10 kV high voltage power supply purchased with IFPRI funds. This supply has permitted us to undertake the study of charge relaxation, which is known to influence inter-particle forces, and also to study the effect of field strength and frequency on bubble control and bed expansion to greater extent than was possible before.
The experimental variables studied to date include bed temperature, electric field strength, and field frequency. Bubble control experiments were carried out on two sizes of glass microspheres, 44-77 urn and 77-144 pm., both glasses being Class A powders using Geldart’s quality of fluidization criteria. Significant effect was observed for bubble control with simultaneous bed expansion to 15% when optimized at a frequency of 3 Hz (compared to l-2% expansion for a field-free bed). An asymptotic drop-off in bed height with increasing frequency and improved bubble control with increasing temperature can be explained by ac modeling of inter-particle forces. The reason for the temperature dependence is not yet fully understood.
DC modeling for the mrtvimum electrostatic forces at the bubble interface follow an assumed spherical bubble shape, chosen to be consistent with the Davidson model used for the fluid dynamics. It is postulated that electrostatic forces must be of the same order of magnitude as particle-fluid forces for effective bubble control. Our dc model is successful in predicting the correct magnitude of electric field strength (E) required for the onset of bubble control showing that E - O(kV/cm) as observed experimentally. However, the model is deficient in that it does not show a dependence of bubble control on superficial velocity as observed experimentally with ac fields and to a lesser extend with dc fields.
Modeling of ac fields shows that inter-particle forces depend in addition (to dc force effects) on the frequency of the field, particle contact resistance, particle surface resistivity and the capacitance of adjacent particles. The ac model needs to be perfected to include the important effect of superficial velocity based on experimental evidence.
Future modeling will include non-spherical bubble shapes and the coupling between ac and dc particle forces with bed expansion (superficial velocity effect). Our numerical program K-Fix is still being developed by Forhad Hossain and is considered to be a long-range part of our modeling program for the electrostatics and hydrodynamics in fluidized beds. Future experiments will incorporate materials of industrially importance at elevated temperature using a quartz bed. These studies will also include particle diameter and relative humidity as variables.
Elutriation control with electric fields-Part II
Fines concentration reductions of up to 96% have been measured in the freeboard of a gas tluidized bed with ac and dc fields. Experimental studies were first reported at Harrogate for dc fields. An optical method employing a helium-neon laser was developed to measure real time particle concentration in the freeboard. A theory was then developed to interpret this data in terms of the elutriation constants (ki).
In the experiments, sand fines, 3.67 to 32 pm, were tested. with ac and dc fields in bulk sand having an average diameter of 300 pm. Elutriation constants were of the order of magnitude of ki = 0.02 set-l and varied with dc field strength depending on the temperature of the bed, whereas the elutriation constants remained relatively independent of tcmperaturc up to 500 “C. Additional data are needed to obtain correlations and to expand this research to include industrial materials of different kinds and shapes.
Modeling of electrostatic forces in the bed is needed to better understand the experimental results. Two approaches for modeling particle-surface forces arc suggested, (1) equilibrium and (2) stability analysis. A closed cell model will dctcrmine the macroscopic field in which fine particles are acted upon by electrostatic forces and surface currents.
The experimental aspect of our research, Parts I and II, is being carried out by J. S. Wang, a Ph.D. student in mechanical engineering
Electrostatic powder separation-Part Ill-a
This study concluded our first year IFPRI study of triboelectric charging in a circulating tluidized bed. It demonstrated that separation of the constituents in a powder mixture, in our case coal pyrite from the parent coal, is possible using selective triboelectric charging in a circulating lluidized bed. Analysis of data showed that the single most important parameter is the difference of charge received by individual constituents. The voltage difference in the precipitator and the superficial velocity were found to be of secondary importance.
Corona discharge-Part 111-b
Corona discharge will often accompany the application of high voltages, high temperatures, and surfaces exhibiting high curvature (e.g. fine particles). It is likely that corona breakdown takes place between particles in an electrofluidized bed under conditions of sufficiently high fields. In the presence of particles, the so called corona wind will literally blow particles toward walls.
Our first study in corona discharge was to investigate modeling of the corona wind and to measure its effect as drag on a glass plate in a low Reynolds number wind tunnel, Rex < 3.6x10^3 (x = length of glass plate ). Our model was successful in predicting the expected trend in drag phenomena over a flat plate using the Karman-Pohlhausen method. However, theoretical predictions of drag were two orders of magnitude too large compared to experiments with ac corona. The reason for the large discrepancy is thought to be that the model is essentially dc whereas the experiment was for an ac corona discharge, Also the effective ionic mobilities usad in the predictions were thought to be too large for the actual conditions.
There are no immediate plans to extend this work. However, corona discharge phenomena will likely need to be included or at least understood in our future modeling of high temperature fluidized beds as well as in the freeboard region if high voltage fields or highly charged particles present.