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 (AtmerTM 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
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