(754c) The Effects of Electric-Field Induced Forces on Particles in an Electric-Field Enhanced Fluidized Bed

Yao
Yang, Zhengliang Huang, Zuwei
Liao, Binbo Jiang, Jingdai
Wang and Yongrong Yang

State Key
Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou
310027, P.R. China

Abstract: As
electric
field
is applied to a fluidized bed, particles in the bed will undergo electric-field
induced forces, and this may change the
fluidized bed behaviors, such as reducing the
bubble size and increasing the bubble residence time. Understanding the effects
of these electric-field induced forces on particles is the key to the
successful design and application of the electric-field enhanced
fluidized beds.
Most previous investigations thought that the electric-field induced forces
only referred to the polarization force. While in most industrial processes,
particles in gas-solid fluidized beds are insulted and fluidized in an relative
dried environment, so electrostatic charges can generate and accumulate on the
surface of particles and then the particles will be bound to undergo another
kind of electric-field induced force, which is the Coulomb
force. So only considering the effect of the polarization force on particles
may limit the understanding of the electric-field enhanced fluidized beds.
This work was proposed to investigate the different effects of the polarization force
and the Coulomb force on particles in electric-field enhanced fluidized beds,
and thus comprehensively reveal the effects of  the electric field on the fluidized bed
behaviors.

In details, this work
was conducted in a three-dimension fluidized bed made of a Plexiglas
column with an inner diameter of 80 mm. The electric
field was supplied by a high voltage DC power (Dalian
Dingtong Technology Development Co., Ltd.), and the
electrodes consisted of a 3 mm copper wire fixed along the center axis of the
column and 1 mm aluminum foil
pasted on the inner wall of the column. So the electric field was cross-flow. Polyethylene particles (Sinopec, China) with an average
diameter of 666 µm and density of 918 kg/m³ were used as the fluidized particles
and the dried compressed air was used as the fluidizing gas. The commercial
liquid antistatic agent
(Atmer^TM 163) was used to eliminate the
electrostatics in this work and the amount added in each run of the experiment
was 0.1 mL. In order to detect the fluidized bed
behaviors, the pressure measurement system and the online AE system were used. The pressure measurement
system was composed of a pressure transducer (CYG
1219 type, Baoji Research Center of Transducer, Baoji, China), a data acquisition card (NI, USA/PCI-6071E) and a computer. The measuring range of the pressure
transducer is 0-2 kPa, and its relative accuracy is
±0.25% of full scale. The online AE system was developed
by UNILAB Research Center of Chemical Engineering in
Zhejiang University, and which includes an AE
sensor (AE 114S,Fuji
ceramics corporation, 140 kHz), a preamplifier, a main
amplifier, A/D conversion module, a
data acquisition card (NI, USA/PCI-6071E) and a computer. The gain of preamplifier is 40 dB
and the digital resolution of the capture card is
16-bit. To avoid the interferences
of the electric field on the AE sensors, waveguiders
were used to make the AE sensors away from the column. The waveguiders
used were cylindrical robs made up of Plexiglas, which was 8 mm in diameter and
150 mm in length. In each experiment, sampling frequencies for pressure fluctuant
signals and acoustic signals were 400 Hz
and 500 kHz respectively, and the sampling times were 20 min and 2 s/ 5 min. Charges
of particles were measured through a Faraday Cup and a electrometer(Monroe Electronics,NanoCoulomb Meter 284) by sampling method in this work. Particles flew into the Faraday Cup directly from the
sampling ports for charge measurement.

For studying the effect
of the polarization force on particles, electrostatic charges was eliminated to exclude the
interference of the Coulomb force. After the electrostatics was eliminated, the pressure
fluctuations and acoustic signals were compared in different electric field strengths.
The results shown that as the electric field was applied, the standard
deviations of pressure fluctuations and the total energies of acoustic signals
all decreased, and the larger the electric field strengths, the lower the
standard deviations of pressure fluctuations and the total energies of acoustic
signals. The V-statics analysis was further applied to these acoustic signals and
found that under the effect of the polarization force on
particles, agglomerates were appeared in high electric field strengths. And
the formation of agglomerates were the reason to the decrease of the standard
deviations of pressure fluctuations and the total energies of acoustic signals.
The effect of the polarization force on particles was
hard to be eliminated by some kinds of experimental methods. But based on the
conclusion raised above, the polarization force on particles would only make
the formation of agglomerates, while from the theoretical analysis, the Coulomb
force on particle would make the charged particles move in the horizontal
direction in a cross-flow electric-field enhanced fluidized bed and then change
the charges distribution. So in a bed without eliminating the
electrostatics, charge-to-mass ratios of
particles near the wall of the column were measured by sampling methods in
different electric field strengths. As the copper wire was grounded and the aluminum
foil was the high voltage electrode, the measured charge-to-mass ratios of
particles were always negative and more and more smaller as the electric field
strengths increased. While as the electric field direction turned over, the
measured charge-to-mass ratios of particles were more and more lager and
finally the reversal of polarity was appeared as the electric field strengths
increased. These results all proved the existences of the Coulomb force and the
horizontal migration of particles. Further analysis found that compared with the
situation when the electrostatics was eliminated, the hydrodynamic behaviors in
the bed also displayed different changes as electric field was applied. At some
low electric field strengths, the standard deviations of pressure
fluctuations and the total energies of acoustic signals became larger than
these before the electric field was applied. These results further confirmed
that the Coulomb force on particles could also affect
the fluidized bed behaviors. Above all, this work found that in a electric-field
enhanced bed without eliminating the electrostatics, the polarization force and
the Coulomb force could all make significant effects on particles and then
affect the fluidized bed behaviors, specially speaking, the former one always
led to agglomerates, and the latter always made the horizontal migrations of charged
particles. So when an electric-field enhanced fluidized bed was investigated,
considering both the effects of the polarization force and the Coulomb force on
particles is necessary.

Key words: electric-field enhanced
bed; polarization force; Coulomb force; agglomerates; horizontal migration