Chemical Engineering August 2012 - 44

Fast fluidization
Pneumatic transport
6
7
2
3
4
5
0.5
1
B
A
Aeratable
Sand-like
Channeling
Particulate
or smooth
fluidization
C
Cohesive
Geldart
C
20
50 100
2
dsv, µm
FIGURE 2. Geldart classiication for luidization
characteristics in air at ambient conditions [3]
tend to sink while the lighter ones will
tend to float to the surface of the bed.
The top surface of a fluidized bed will
be relatively horizontal (with an angle
of repose of zero) and the particles will
flow from a higher elevation to a lower
elevation. Bubbling beds also have similar
behavior as sparged liquid columns.
The behavior of the bed after minimum
fluidization depends on whether
the fluid is a liquid or a gas. For liquids,
as the upward flow of liquid is
increased, the packed bed continues to
expand uniformly and homogenously
with increasing interstitial void space
until the particles are eventually carried
away (elutriated) [9]. This type of
fluidization behavior is called particulate
or homogenous fluidization.
However, when the fluid is a gas,
the excess gas may manifest itself as
bubbles. The bubbles will result in two
distinct phases within the bed and give
an appearance of non-homogeneity.
Such a behavior is called aggregative
fluidization (Figure 1). Transition from
a packed bed to either particulate or
aggregative fluidization for gas-solid
systems depends on particle and gas
properties. To develop fluidized-bed
processes for gas-solid systems, the
ability to predict flow behavior and
calculate essential operating parameters
(such as minimum fluidization
velocity, flow regimes, bubbling characteristics
and more) is critical.
As shown in Figure 1, as the gas
flowrate is increased in a bubbling
gas-solid fluidized bed, the bubbles
coalesce and may grow larger as they
5
1,000
Geldart
A
Dbubble = Maximum bubble diameter
Dbed = Diameter of fluidized bed
FIGURE 3. Shown here are the typical transitions
between various luidization regimes
migrate to the bed surface (with coarse
or granular solids) or achieve a stable
size (with fine powders). The passage
of bubbles through the bed will result
in mixing and churning of the bed
material, which creates uniform conditions
within the bed. If the bubbles
get larger than two-thirds of the bed
diameter, slugging may result. Slugging
conditions can cause severe pressure
fluctuations and bed vibrations
along with significant reduction in
heat and mass transfer. Commercially
scaled units for fine powders tend not
to exhibit slugging. However, slugging
behavior may be observed with coarse
materials.
Further increase in gas flowrate
and the onset of a turbulent regime
is marked by the disappearance of a
distinct bed surface. At this point, the
void spaces and the particles form
co-continuous phases. At higher velocities,
the contents of the bed are
elutriated (carried out of the vessel),
thereby resulting in partial depletion
of the bed material. This fluidization
regime - also known as fast fluidization
- is characterized by an axial
concentration gradient, a subtle coreannulus
profile and recirculation at
the walls.
At even higher gas velocities, complete
removal of the bed material
can occur. The " bed " becomes divided
between a developing flow and fully
developed axial flow with a distinct
core-annulus radial profile. This flow
regime is referred to as the transport
flow regime.
Geldart classification
In 1973, Professor Derek Geldart proposed
a simple and elegant approach
for predicting the fluidization behavior
of particulate materials in a classic
paper [2]. He plotted the density difference
between the solid phase and
the fluid phase versus particle size on
a log-log plot, and proposed four major
classes (A, B, C and D) based on their
fluidization behavior (Figure 2). These
have come to be known as Geldart
particle classifications.
Since the density of gas is typically
three orders of magnitude smaller
than the density of particles, the particle
density dominates the y-axis.
The particle diameter on the x-axis
refers to the surface-volume diameter
(dsv) for uniform-sized particles and
the surface-volume mean diameter
(dsvm) when the particles are nonuniform
in size.
Typical characteristics of each of the
Geldart classes are summarized here:
Class A
* Fine powders
* Easy to fluidize; aeratable
* Exhibit homogenous or particulate
fluidization until bubbling
* The bubbling velocity is greater than
the minimum fluidization velocity
* Good mixing occurs during complete
fluidization
* Slow deaeration rate or long
de-fluidization time observed
* Examples include fluidized
catalytic cracking catalysts,
high-density polyethylene powders
and TiO2
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2012 39
Geldart
B
Fixed bed or packed bed
Geldart
D
Aggregative
or bubbling
fluidization
Slugging
Dbubble >
0.66 Dbed
Spouting
Spoutable
D
Turbulent fluidization
ρp-ρf, g/cm3
Increasing gas velocity
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