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Is My Pneumatic Conveying
System Causing Segregation?
Guest article by Richard Farnish
The Wolfson Centre for Bulk Solids Handling Technology
Many plant engineers consider
that if a pneumatic conveying system has been included within a process
plant, and that there are problems with the quality of the powder or end
product being manufactured, then the most likely cause of the problems can
be associated with the pneumatic conveying system. It is often considered
that segregation of the powder, in terms of size distribution variations,
is as a direct consequence of the pneumatic conveying process itself.
However, it is evident that many bulk handling systems employ a multitude
of different components, all of which can effect the performance of a
process and quality of the end product or powder.
Figure1: Shows the basic
features of a positive pressure pneumatic conveying system.
The propensity for a blended
powder to segregate is dependent upon the size ranges of the constituent
powders or the variation in particle density for the blend components. In
general terms, if there is a particle size ratio of greater than 1:1.3
then segregation problems can be anticipated (the greater the size range,
the greater the propensity for segregation to occur).
Segregation of the powder or
blend can occur after several or even just one handling operation. The
most common area where segregation occurs in any powder handling plant is
in the filling, storage and discharge of hoppers, bins and silos. Thus,
segregation can occur irrespective of whether or not a pneumatic conveyor
forms part of a powder handling system.
There are several mechanisms
by which the segregation of particles can occur, but by far the common is
that of rolling segregation. As the powder mixture builds up an
ever-increasing inventory on an open stockpile or within a silo, the free
surface forms a conical shape.
As the powder lands upon the
apex of the free surface, the coarse particles tend to roll down the
surface due to their momentum. However, the fines, due to their small
size, tend to stay in the central area of the conical heap beneath the
filling point. Thus, the sizes of the particles tend to increase as we
move radially outwards from the centerline of the apex of the free surface
towards the hopper wall. Thus, the likelihood of segregation to occur in
even the most basic pneumatic conveying system (typical to that depicted
in Figure 1) is high because of the presence a feed hopper at the very
start of the system, and not because the pneumatic conveyor itself is
present!
Depending upon the design and
operational characteristics of the feed hopper, the effects of any
segregation present within the hopper can manifest itself in different
ways. In order to appreciate the importance of the design of the feed
hopper it is essential that the two basic flow patterns that can develop
is understood - these being “core flow” (Figure 2) and “mass flow”
(Figure 3).
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Figure 2: Core flow
discharge pattern |
Figure 3: Mass flow
discharge pattern |
Core flow key features:
-
“first in last out”
discharge
-
“dead” regions of
product
-
erratic discharge caused
by product on product shear during emptying
-
central discharge channel
-
exaggerates segregation
effects of particles
-
hopper half angle
typically greater than 25° (from vertical)
-
poor stock rotation
-
high storage capacity for
a given headroom
Mass flow key features:
-
“first in first out”
discharge
-
all storage capacity is
“live”
-
even discharge encouraged
by the reduced levels of shear generated as the product discharges
(shear takes place against the relatively smooth wall material - not
the static or slower moving product)
-
degree of remixing during
discharge minimizes segregation effects
-
hopper half angle
typically less than 25° (from vertical)
-
relatively low storage
volume for a given headroom (but all the product can be retrieved)
Hoppers are often overlooked
with respect to exaggerating segregation effects within a process, and the
technique applied to fill these vessels is itself quite often a
contributory factor in the segregation effects that develop. The issue of
filling applies equally to both the feed hoppers in a system and the
receiving hoppers at the end of a system.
Examples of the segregation
effects that result from different filling approaches appear as Figures 4
- 7. From inspection of the illustrations it can be appreciated that even
if the powder has not segregated prior to the pneumatic conveying pipeline
itself, the interface at the receiving hopper could give rise to
segregation effects itself.
The segregation effects
resulting from the filling techniques shown above are:
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| Figure 4:
Fines concentrated in the central region, lack of fines evident at
the periphery of the inventory - radial segregation. |
Figure 5:
Fines concentrated around the periphery, lack of fines in the center
(this effect is dependant upon particle characteristics and
velocities) - radial segregation. |
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| Figure 6:
Fines evident on one side of hopper, coarser material apparent on
opposite side (material build up through impact on opposite side
of hopper can also be evident for some types of particles) |
Figure 7:
Excess of fines in top layer of inventory caused by “blast”
effect of conveying line generating a cloud of fines that settle,
lack of fines in lower section of inventory - stratified
segregation |
Having briefly examined the
segregation effects caused by filing a vessel, we will now look at
segregation effects exacerbated by the emptying or interfacing to a
system.
Any type of feeder that draws
material preferentially from a vessel will revert that vessel to a core
flow discharge pattern (see Figures 8a & 9a). Thus, careful
consideration must be given to the overall scheme of the proposed
conveying system and associated hardware.
The interface diagrams show:
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| Figure 8a:
Direct rotary valve installation resulting in a preferential draw
(core flow) |
Figure 8b:
Rotary valve installed with stand pipe to allow flow channel
development to full outlet area (mass flow) |
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| Figure 9a:
Constant pitch screw giving preferential flow channel (core flow) |
Figure 9b:
Increasing capacity screw drawing from the full outlet area (mass
flow) |
It is very important to note
that the above examples will only give mass flow if the hopper to which
the discharger is installed has been design with appropriate geometry for
this flow pattern to develop for the material being handled.
In terms of impact of the
conveying line itself, the length and geometry of the conveying pipeline
does not have a significant effect upon the extent to which the quality of
a blended powder varies between the feed and discharge points of the
conveying pipeline.
Dense phase conveying systems,
by virtue of the pipeline being full of particles, are less likely to
cause segregation. Since the particles are closely packed, they have less
freedom being constrained by their neighboring particles and hence the
propensity for the fines to be separated from the coarse fractions is minimized.
With lean phase systems the
particles are widely dispersed and are free to move relative to their neighboring
particles, and hence segregation is more likely to occur. However, it is
the experience of the author that very few instances have been reported
whereby the act of conveying the powder has caused segregation to occur
within the pipeline itself.
It is hoped that this very
brief overview of segregation effects commonly encountered in industry
(and commonly associated rightly or wrongly with pneumatic conveying
systems), will enable engineers faced with quality related problems to
appreciated the holistic approach which is often required to address these
issues.
For further information or
advice on powder, contact:
The Wolfson Centre for Bulk
Solids Handling Technology
The University of Greenwich.
Tel: +44 (0)20 8331 8646
Fax: +44 (0)20 8331 8647
e-mail A.N.Pittman@gre.ac.uk
URL: www.bulksolids.com
Help others by posting your comments, suggestions and
experiences with bulk solids feeding or any other materials handling concerns you may have
on our On-Line Help Forum. For
past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.
Guest articles for the Ask Joe! Column are always welcome,
for more information please contact Joe Marinelli directly at his email address:
joe@solidshandlingtech.com.
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