Regulation Of Hydrodynamics In The Fluidized Bed With Electrostatics

Electrostatics widely exist in the industrial processes involved in fluidized beds, such as polyolefin, coal combustion and biomass combustion. This kind of gas-solid fluidized bed reactor can be named as the electrostatic fluidized bed reactor. In electrostatic fluidized bed reactor, electrostatic forces on charged particles make both negative and positive effects on the performance of the reactor. On one hand, electrostatics may lead to particle cohesion and then make the appearance of wall sheeting, which inhibit the stability of the fluidized reactor severely. On the other hand, electrostatics is also able to improve the fluidization and then strengthen the mass and heat transfer in the electrostatic fluidized bed reactor, which enhance the performance of the reactor. Therefore, investigating how to regulate the hydrodynamics in the electrostatic fluidized bed by electrostatics and then changing the negative electrostatics into the positive attributes of electrostatic fluidized bed is of value.The external electric field and the self-induced electrostatic field are the most important methods to use the electrostatics in gas-solid systems. The conception of fluidized bed with external electric fields had been raised in the middle of the 20th century, but because of the poor understanding of the electrostatics and the lack of proper measurements, current research only studied the stability of the electric fields and the effects of electric fields on bubble behaviors preliminarily based on the polarization forces on particles. In recent years, investigations about the nanofluidization shows that the Coulomb forces on charged nanoparticles may affect the nanofluidization seriously. The nanofluidization is totally different with the normal fluidization in this work, but by considering both Coulomb forces and polarization forces on charged particles, the effects and effect mechanism of external electric fields on the hydrodynamics in the electrostatic fluidized bed will be revealed comprehensively. Besides, former researchers reported that the self-induced electrostatic field in the electrostatic fluidized bed had the possibility to reduce the fine particle elutriation, but the effects mechanism is confused. Therefore, based on acoustic emission technique, the pressure fluctuation detection and the charge-to-mass ratio measurement, effects of electric fields on meso-scale structures including bubbles and agglomerates, particle motions and particle cohesion in the electrostatic fluidized bed are studied. Besides, based on the simulation method combined the computational fluid dynamics (CFD) with discrete elements model (DEM), the effect mechanism of the self-induced electrostatic field on fine particle motion in the electrostatic fluidized bed is also revealed. The whole work has been carried out on the following four aspects:1. Effects and effect mechanism of DC & AC electric fields on the meso-scale structures (bubbles and agglomerates) in the electrostatic fluidized bed are studied. Results show that the competition between the Coulomb force and the polarization force is the basic effect mechanism of external electric field.In the DC electric field from the wall of the reactor to the center of the reactor, the negatively charged fluidized bed is compressed under the function of Coulomb force at lower field strengths, and then the mean voidage in the emulsion phase is decreased and the bubble size is increased. While at higher field strengths, the polarization forces on particles make the formation of agglomerates and lead to the decrease of bubble size. In the experimental system used in this work, the critical field strength of DC electric field is between 1.0 and 1.5 kV/cm.In the AC electric field, the polarization forces on particles also make the formation of agglomerates and decrease the bubble size at higher field strengths in theory. However, the Coulomb forces on particles make charged particles swing and then suppress the formation of agglomerates caused by polarization forces. So the critical field strength of AC electric field is enlarged. In this work, the AC electric fields with the frequency of 50 Hz do not affect the bubbles and agglomerates in the electrostatic fluidized bed at any field strength between 0 and 2.5 kV/cm.2. Effects and effect mechanism of AC & DC electric fields on particle motions in the electrostatic fluidized bed are investigated. Results show that the Coulomb forces on particles are the basic effect mechanism of the DC electric fields with lower field strengths and the polarization forces on particles are the fundamental mechanism of the DC electric field with higher field strengths. While in the AC electric field, because the Coulomb forces on particles will suppress the agglomeration caused by polarization forces, so the Coulomb forces is the basic effect mechanism at any field strength used in this work. So the effect of DC electric field on particle motion is related to its direction at the lower field strengths, but the direction of DC electric field does not affect its effect on particle motion any more at higher field strengths.In this work, DC electric fields from the wall of the reactor to the center of the reactor make the particle activity decrease and the particle-wall impact angle increase between 0 and 1.0 kV/cm, while the DC electric fields with opposite direction make the particle activity increase and the particle-wall impact angle decrease at the same field strengths. At any field strength higher than 1.5 kV/cm. DC electric fields with both directions make the particle activity decrease and the particle-wall impact angle increase in the middle or lower part of the fluidized bed. The AC electric fields always make the particle activity increase and the particle-wall impact angle increase at any field strength between 0 and 2.5 kV/cm.3. Effects of AC & DC electric fields on particles adhesion in the electrostatic fluidized bed is studied. Results show that particles adhesion under the external electric fields is control by the normal stress, the electric image force, the radial polarization force and the radial Coulomb force on particles.In the DC electric field, coordination and competition among these four forces is the basic effect mechanism. So in the DC electric field from the wall of the reactor to the center of the reactor, particle adhesion increases with the increasing field strength, while in the DC electric field with opposite direction, particle adhesion increases firstly and then decreases with the increasing field strength. In the AC electric field, the normal stress on particles is the most dominant. Thus with the increase of the field strength, the normal stress on particles is increased and then makes the particle adhesion decrease. So AC electric field is the preferred electric field to control the particle adhesion. In the experimental system used in this work, the particle adhesion was reduced by 57% under the effect of 2.5 kV/cm AC electric field.Besides, by investigating the effects of agglomerates on electrostatic behaviors in the electrostatic fluidized bed, a novel detection method of agglomerates based on the induced electrostatic potential is established. When a sheet gets away from the wall of the reactor and falls into the fluidized bed, the particle concentration distribution in the bed and the surface charges on particles are both changed. These two changes will make the appearance of V-shaped fluctuation of the induced electrostatic potential, and this typical fluctuation can be used to detect the falling sheet. In addition, small agglomerates which are fluidized in the lower part of the fluidized bed might move into the sensitive zone of the probe, and then make the polarity reversal of the induced electrostatic potential and further decrease the standard deviation of the induced electrostatic potential. These two typical changes can be used to detect small fluidized agglomerates and locate them.4. By CFD-DEM modelling, the multifactor effect mechanism of electrostatics on particle elutriation in the electrostatic fluidized bed is revealed. Electrostatic charges on particles may result in the increase of the fluidized bed height, the axial velocity of small particles in the upper part of the free board and then promote particle elutriation. But meanwhile, charges on particles will lead to agglomeration, decrease the concentration of small particles in the upper part of the free board and thus suppress particle elutriation. From the simulated results, agglomeration caused by electrostatic attractions is always the most dominant.