Chemical Engineering October 2011 - 74

Solids Processing
Characterizing
Powder Flow
Flow behavior is influenced by prevailing
process conditions. Efforts to assess
powder characteristics must be aligned with
the process itself
Tim Freeman
Freeman Technology Ltd.
T
o confidently design, operate
and troubleshoot solidshandling
processes, engineers
need to characterize powder
flow in ways that relate to in-plant
behavior. Universal powder testers
provide the multi-faceted characterization
that is necessary to reliably
predict operational performance. This
article examines the data that such
instruments generate, and discusses
their practical application.
While many process engineers
would express confidence in their ability
to effectively design and optimize
plants for liquid and gas processing,
even those with extensive experience
are often challenged by powder-handling
processes. The open literature
offers only limited data and the links
between actual process behavior and
defined powder properties often appear
tenuous and complex. However,
for effective processing and design, it
is essential to describe powder behavior
in a process relevant way.
Dynamic flow characterization
measures powders in motion, offering
engineers an intuitively rational approach
to characterization. The most
advanced dynamic powder testers incorporate
a range of methodologies,
characterizing conventional bulk and
shear properties to supplement dynamic
measurement. Since powders
can be measured in a consolidated,
conditioned, aerated or even fluidized
state, these universal powder testers
generate information that relates
closely to a wide range of processing
conditions. Such data provide a
firmer basis for design, and provide
insight that rationalizes operational
experience, enhancing the underlying
knowledge base.
One of the most important aspects
of powder behavior is flowability
under real-world conditions. Getting a
powder to flow smoothly through multiple
unit operations, at the required
flowrate, is often extremely difficult.
Here we consider why powders exhibit
variability, and addresses the issue of
powder flow in some detail. A particular
focus is the impact of applied force,
the contrast between extruded or
forced versus unconfined flow, and the
implications of these for processing.
Powder fundamentals
Key to understanding powders is an
appreciation of the reasons why behavior,
especially flow properties, can
change so radically under process
plant conditions. Unfortunately many
variables influence flowability. Some
relate to the solid itself - particle
size, shape, surface roughness or density,
for example - while others are
determined by the processing environment
or system.
Primary examples of system variables
include air content, electrostatic
charge and moisture level. Maintaining
consistency, during both measurement
and processing is challenging,
because it requires that all influential
variables be kept constant (or at least
controlled within limits).
Over time, the flow behavior of a
66 ChemiCal engineering www.Che.Com oCtober 2011
Figure 1. Using a powder rheometer
to investigate the extrusion behavior of
powders, a downward force on the piston
pushes material out of the orifice on the
right hand side. Extrusion is stopped
when a force limit is reached
powder that starts off its processing
life with well-defined flow characteristics
may change. For instance, as it
passes through the plant, the powder
may pick up or lose moisture, becoming
tribo-charged or segregated, suffer
from attrition or agglomeration
(thereby changing size and/or shape),
fluidize or become compacted. Each
change will influence flowability.
This is why it is so important to understand
the conditions to which the
powder will be exposed as it moves
through different unit operations. For
example, during transport, powders
may consolidate through vibration,
while those in bulk storage in a silo,
sack or hopper may consolidate under
their own weight. Conversely, discharge
from a container or sack usually
increases aeration, which encourages
the powder to flow freely - possibly
too freely - potentially increasing the
risk of flooding. If the moisture level of
the storage or transport environment
is not closely controlled, then this too
is likely to influence downstream processing
performance.
Knowing what demands the process
is going to place on the powder is, however,
only one side of the problem. The
other is understanding the powder's
likely response. This is where the use
http://www.Che.Com
Chemical Engineering October 2011

Table of Contents for the Digital Edition of Chemical Engineering October 2011
