(85d) Impact of Particle and Column Electrical Properties on Entrainment from Gas-Solid Fluidized Beds

Authors: 
Fotovat, F., University of British Columbia
Grace, J. R., The University of British Columbia
Bi, X., University of British Columbia

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

  1.     
Introduction

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 1–3,
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 researchers7–9,
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.

  2.     
Experimental

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.

  4.     
References

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(3)      Tasirin,
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(5)      Cross,
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(8)      Briens,
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(9)      Alsmari,
T. A.; Grace, J. R.; Bi, X. T. Effects of particle properties on entrainment
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(10)    Alsmari,
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