Chemical Engineering August 2015 - 64

the optimization of operating parameters
for fluidization.
When designing pneumatic-conveying
systems, two parameters
are especially important: throughput
and pressure drop. Tests suggest
that a number of dynamic powder
properties and the bulk property of
permeability are all important when
determining these parameters. AZO
researchers are establishing closer
correlation between pneumaticconveying
performance and a number
of powder properties, enabling
them to strengthen design capabilities
in this area.
Correlation between process performance
and measured properties
Definition of design space
FIgure 4. Correlating measurable powder properties with processing performance enables the definition
of the " design space " - the identification of the powder characteristics and their values that ensure acceptable
process behavior
of the silo. For these powders, efficient
hopper discharge depends
on the use of mechanical aids,
such as vibrating devices or airinjection
systems. The new testing
regime rapidly and robustly identifies
powders that fall into this classification,
thereby helping to define
an optimal storage solution.
A further issue in powder storage
is the potential impact of consolidation
or " caking " over time. The compressing
load on a powder stored
under its own weight can have a
significant impact on its flowability.
Using dynamic powder testing,
researchers at AZO have directly
investigated this aspect of behavior
and used the results to guide operational
practice. For example, testing
helped determine the frequency with
which a silo must be emptied.
Powder discharge. During discharge
from a storage vessel, there
is potential for a powder to draw in air
and become fluidized, to the point of
uncontrollable flooding from the hopper.
Rotary valves are an option for
controlling powders that readily flood
62
in this way, while doping screws are
an alternative for more challenging
materials. Deciding on which system
to adopt is critical.
The AZO team has assessed
whether the ability of the powder
tester to directly quantify the
response of a powder to air (via
dynamic testing) can be exploited
to classify a powder's propensity
to fluidize in this way. Aeration
data have certainly proven to correlate
with discharge characteristics,
however, investigations have
shown that other powder properties
also influence this behavior,
further supporting the need for a
multifaceted test regime.
Pneumatic conveying. In pneumatic
conveying, powders may
be transported in a fluidized state
- a state that can be studied directly
using dynamic powder characterization.
Through appropriate
testing, it is possible to determine
whether a material can be fluidized
or not, and to measure the air velocity
needed to reach fluidization.
At AZO, such testing now supports
A dynamic industry
Developing, operating and optimizing
powder processes introduces
significant challenges, particularly in
the absence of tools, such as accurate
process simulation, that may
now be taken for granted in other
fields. However, powder-characterization
techniques and technologies
have advanced considerably
in the last decade or so, supporting
a move away from the inefficient
" trial-and-error " approach to powder
processing.
Experience suggests that by
combining test methods - most
productively dynamic, bulk and
shear-property measurement -
processors can characterize powders
in ways that successfully
elucidate, rationalize and control inprocess
behavior. By employing a
pragmatic approach, based on the
use of multiple powder properties
and operational experience, powder
processors can access new levels
of efficiency and meet industrial
requirements for more competitive,
leaner processing.
n
Edited by Scott Jenkins
Author
Jamie Clayton is the operations
director at Freeman Technology
Ltd. (1 Miller Court, Severn
Drive, Tewkesbury, Gloucestershire,
GL20 8DN U.K.; Phone: +44 1684
851551; Email: info@freemantech.co.uk;
Web: www.freemantech.co.uk).
He graduated from the
University of Sheffield with a degree
in control engineering and is
responsible for all daily activities of the company, including
overall management of the administration, production,
R&D, sales and customer support teams. Clayton
also works with the company's clients to provide
application-based support.
ChemiCal engineering www.Chemengonline.Com august 2015
Aerated energy
Specific energy
Aerated energy
Pressure drop
Specific energy
Pressure drop

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Chemical Engineering August 2015

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