Chemical Engineering August 2012 - 43

Cover Story
A Primer on Gas-Solids
Fluidization
AGGREGATIVE FLUIDIZATION
To develop suitable
fluidized-bed processes
for gas-solids systems,
the ability to predict
behavior and calculate
essential operating
parameters is critical.
Some of the key concepts
and equations are
presented
Shrikant Dhodapkar
The Dow Chemical Company
Abdolreza Zaltash
Oak Ridge National Laboratory
George Klinzing
University of Pittsburgh
T
he earliest application of the fundamental
concepts of fluidization
dates back to 16th century when
a German scientist (Georgius
Agricola) described a process to upgrade
ores. However, it was not until the 1930s,
with the development of the Winkler coalgasification
process, that commercial use
of fluidized beds on an industrial scale
was recognized. The need for gasoline
during the World War II accelerated the
development and implementation of the
fluid catalytic cracking (FCC) process by
a consortium of U.S. petrochemical and
engineering companies.
During the past five decades, fluidization
technology has been extensively
applied to various chemical processes.
It provides better heat and mass transfer
between the fluid and the solid
compared to conventional packed- or
!
Packed
bed
Bubbling
regime
Slugging
Round Square-nose
Turbulent
regime
INCREASING GAS VELOCITY
FIGURE 1. Different luidization regimes in gas-solid systems
are demonstrated here (Image adapted from [5] and [7]
moving-bed unit operations. Some
common industrial processes using
fluidization technology include drying,
catalytic cracking, chemical synthesis,
adsorption-desorption, gasification,
pyrolysis, granulation, calcination,
combustion, coating, bioreaction, polymerization,
ore beneficiation and
coking. This article summarizes the
basic concepts of fluidized-bed technology
and provides a useful collection
of equations for gas-solids systems.
Background
When particulate matter or bulk solids
are poured into a vessel, the particles
arrange themselves into a random
configuration to form a fixed (or
packed) bed. The space between the
particles becomes filled with ambient
gas and forms a network of interconnected
voids. The volume occupied by
the packed bed is always greater than
the volume of the particulate material
itself. The ratio of void volume
to the total volume of packed bed is
called voidage. Sometimes " porosity "
is used to describe voidage of packed
beds, but this should not be confused
38 CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2012
with porosity within the particles.
When a fluid (gas or liquid) is introduced
uniformly at the bottom of the
packed or fixed bed, it percolates upwards
through the interstitial voids.
The drag of the gas on the particles
is counteracted by the pressure drop
across the bed or the weight of the bed
divided by cross-sectional area. The
packing configuration of the bed remains
unaltered as the fluid finds the
tortuous path through the packing in
the upward direction.
If the upward velocity of the fluid
is increased such that the weight of
the bed per unit cross-sectional area
is equal to the pressure drop across
the bed, then the particles begin to
suspend and particle-particle contact
is no longer assured. This condition is
called minimum fluidization. Further
increase in fluid velocity, will cause
the suspended bed to exhibit " fluidity "
or fluid-like behavior, creating the socalled
fluidized bed.
Much like fluids, the particles in a
fluidized bed can be stirred and discharged
from a lateral orifice in the
vessel. Particles of higher density will
Fast
fluidization
regime
Pneumatic
transport
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Chemical Engineering August 2012

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