465668 Impact of Particle and Column Electrical Properties on Entrainment from Gas-Solid Fluidized Beds
Impact of particle and column electrical properties on entrainment from gas-solid fluidized beds
Farzam Fotovat, John R. Grace, Xiaotao T. Bi
Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, Canada V6T 1Z3
Entrainment of fine particles is a common pitfall of fluidized beds working at high gas velocities resulting in loss of bed inventory such as precious catalysts, and potential release of fine particulates into the atmosphere. To reduce the amount of the elutriated particles, gas-solid separators like cyclones and filters are installed downstream of the fluidized beds. However, optimal design and operation of these devices greatly depends on estimating the extent of entrainment in the freeboard. Despite the numerous studies conducted on the entrainment process 13, it has not been well understood, due to the complexity of gas-solid flows, particularly with respect to inter-particle cohesion and hydrodynamic clustering effects.4 A handful of correlations have been developed to predict the entrainment rate of fluidized beds involving a variety of particles entrained at different operating conditions. In the absence of a thorough understanding of the underlying mechanisms of entrainment, there are huge discrepancies in the elutriation rates predicted by the existing empirical correlations and semi-empirical relationships.
The nature of gas-solid fluidization processes produces continuous motion and rubbing among bed particles such that the generation of electrostatic charges is inevitable. The electrostatic charges in fluidized systems can interfere with the normal hydrodynamics of the bed, resulting in particle-wall adhesion, inter-particle cohesion, electrostatic discharges, wall sheeting and even explosions, all of which can affect plant safety and economics.5,6 The particles entrained from the bed are also subject to the electrostatic forces, which are likely to affect the entrainment process by influencing whether particles travel individually or as aggregates/clusters. Although the significant impact of the electrostatics on the particle entrainment from gas-solid fluidized beds has been observed by a number of researchers79, its contribution to the correlations predicting the entrainment rate has been overlooked; an oversight that may well be responsible for the huge discrepancies in predicted entrainment rates.
To allow for the effect of the electrostatic interactions on entrainment, it is imperative to determine the parameters influencing the electrostatic forces exerted on the particles in the freeboard. To this end, focus of this work is exploring the impact of electrical properties of particles and the column wall on entrainment. Outcomes of this study may clarify how choosing appropriate materials can be helpful in controlling electrostatics and entrainment in gas-solid fluidized beds. Furthermore, this study may shed light on the discrepancies observed in the entrainment rates that cannot be explained by hydrodynamics.
A number of dielectric fines (such as glass, alumina, cork and, porcelain) as well as, conductive fines (such as silver-coated glass, copper, and stainless steel) were elutriated from a stainless steel column with an inner diameter of 0.15 m and a height of 2.0 m. To triboelectrically charge fines in the column, a binary fines and coarse glass beads were mixed before each test and the column was then loaded with the mixture to a total depth of 0.4 m. Each mixture contained 90 wt. % of the coarse material, with the fine species making up the balance. The entrainment rate of fines and their mass charge density were simultaneously measured by means of a sampling vessel into which the elutriated fines were diverted after being recycled by an external cyclone. The average entrained mass divided by the sampling time with at least five repetitions was taken as the entrainment rate for each run. The charge density of entrained particles was measured by a sampling device incorporating the Faraday cup principle developed and described by Alsmari et al.10 The relative humidity of the bed was set at ~10% for all experiments using a refrigerating unit and a vapor removal filter. Pressure and temperature were maintained constant at 205 kPa and 20°C, respectively. The superficial gas velocity was varied from 0.2 to 0.8 m/s. For the purpose of exploring the impact of the electrical properties of the column wall on entrainment of fines, the stainless steel column will be replaced with a non-conductive acrylic column and all the above-mentioned tests will be carried out in this column.
3. Preliminary Results
Fig. 1 shows the entrainment flux (Ws) vs. gas velocity in excess of the terminal velocity, (Ug-Ut) for each fine material elutriated from the stainless steel column. In general, the elutriation flux of the conductive fine particles (silver-coated glass, copper and stainless steel) was larger than for the non-conductive fine materials (glass, alumina, cork and porcelain), especially for (Ug-Ut)>0.3 m/s. As (Ug-Ut) approached 0.6 m/s, the entrainment rate of the silver-coated fine glass beads became more than six times larger than that of the (uncoated) fine glass beads at the corresponding (Ug-Ut), despite the close similarity of the physical properties (density, size, shape) of these two types of particles.
Fig. 1. Entrainment flux of particles as a function of (Ug-Ut). Open symbols: dielectric particles; filled symbols: conductive particles. Error bars show the standard deviations of the experimental measurements.
Assessment of the hydrodynamic forces such as gravity and drag exerted on the fine particles in the freeboard illustrates that these forces are not responsible for the higher entrainment rate of the conductive particles relative to the dielectric fines. On the other hand, Fig. 2 shows that the Fe/Fg (electrostatic to gravity force) ratios are significantly smaller for conductive than for dielectric species. By comparing regular and silver-coated fine glass beads (open and filled circle symbols), Fig. 2 indicates that changing the conductivity of particles can profoundly influence the magnitude of the entrainment flux.
This observation can be attributed to the intensification of electrostatic inter-particle forces for non-conductive particles due to the non-uniform electrical charge distribution over their surfaces. Moreover, dominance of the attractive forces among the dielectric particles is likely in the freeboard region, promoting formation of aggregates or clusters which reduces the entrainment rate. On the other hand, repulsive electrostatic forces between pairs of touching conductive particles can cause these particles to act independently, augmenting their entrainment. This study suggests that decreased electrical conductivity of particles can assist significantly in reducing the entrainment of fine particles from gas-solid fluidized beds.
Fig. 2. Normalized entrainment flux vs. electrostatic-to-gravity-force ratio of entrained particles. Open symbols: dielectric particles, filled symbols: conductive particles. Error bars show the standard deviations of the experimental measurements.
The plausible influence of the electrical properties of the column wall on entrainment of the fines will be discussed in the final version of this study.
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