Bubble Size Reduction in a Fluidized Bed by Electric Fields

Abstract

The reduction of the size of bubbles can improve both selectivity and conversion in gas-solid fluidized beds. Results are reported of the reduction of bubble size by the application of electric fields to uncharged, polarizable particles in fluidized beds. It is shown how average bubble diameters can be drastically reduced, with little change of the bed expansion. A literature review shows that to maintain smooth fluidization, electric fields in the direction of the gas flow, with a relatively low alternating frequency, are optimal. To measure average bubble diameters, a spectral decomposition technique of pressure fluctuation time series is used. Using this method, based on non-intrusive measurements, a characteristic length scale for bubble diameters can be found. It is shown experimentally, using video analysis, that this length scale is of constant proportionality for a given bed material and bed dimensions. The proportionality of the length scale to bubble diameter is independent of measuring height or gas velocity. With this, we have a tool for measuring bubble diameters in both 2-D and 3-D fluidized beds. Electric fields were applied to fluidized beds using thin wire electrodes placed inside the column. Both 2-D and 3-D columns were tested over a range of frequencies and field strengths. For Geldart A glass beads, an optimal range was determined at 5-20 Hz and 400-1600 V/cm fields. The reduction of bubble diameter was measured to be up to 25% for this system. Larger Geldart B glass particles show a larger reduction of bubble diameters -up to 85%. For these particles, the optimal frequency was at a higher range, 20-70 Hz. At higher frequencies (up to 100 Hz), bubble size reduction is less, but still substantial. Experiments in the 3-D column using Geldart A particles show a similar reduction in bubble diameters.

Effects of external electric field on bubble and charged particle hydrodynamics in a gas–solid fluidized bed

“…The bubble basically kept the original shape as it developed. This result was similar to the report of van Willigen et al [5]. They found the high frequency AC electric field exerted weak influence on the bubble size.…”

“…By this way, the bubble size and bubble spatial distribution can be controlled, which facilitates the reactor scale-up and increases the reaction conversion and selectivity [4]. For example, by applying the low-energy (50 W/m 3 ) alternating electric fields, the interactions between the charged particles could be changed, leading to smaller bubble size in the fluidized bed [5], further improving the gas-solid contacting and mixing efficiency. In order to control the fluidization characteristics and optimize the reactor operation by adding external electric field, it is of great importance to understand the particle charging and its effects on the two-phase hydrodynamics.…”

“…After reviewing the influences of different electric fields on the fluidized bed hydrodynamics, van Willigen proposed that the low frequency AC fields with strengths of 2-5 kV/cm were optimal for decreasing bubble number in the bed, while the DC fields had more effects but often led to spouting, stringing and cohesion of particles [20]. He then measured the external electric field influences on the bubble size in the fluidized bed by pressure fluctuation method, and proposed the optimized external electric field parameters [5,21]. The bubble size can be efficiently reduced by the external electric field with frequency of 5-20 Hz for smaller particles (77 lm), and with frequency of 20-70 Hz for larger particles (700 lm).…”

Measuring the Gas-Solids Distribution in Fluidized Beds -- A Review

“…The incoherent part of the power spectrum can be converted into an incoherent standard deviation, which is proportional to the average bubble diameter. This proportionality constant is independent of gas velocity and measuring height, but varies with bed diameter and bed material , Kleijn van Willigen et al, 2003. This makes the method suitable to assess changes in bubble size due to, for example, electric fields and gas injection (Kleijn van Willigen et al, 2005).…”

Two dimensional fluidised bed reactor: Performance of a novel multi-vortex distributor

“…Wong & Baird [12] and Pence & Beasley [13] altered the dynamics of the gas supply by applying continuous periodic variations and found a significant improvement in the quality of fluidisation which is a measure of the phase distribution in the bed. Kleijn van Willigen et al [14] successfully reduced the bubble sizes in a FBR by manipulating the solid phase dynamics by applying an alternating electric field to the FBR. This polarised the particles and increased the interparticle forces in the bed.…”