Chemical Engineering September 2020 - 60

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Time, s
Figure 3. The graph shows the discharge characteristic for a standard
screw feeder and agitator system operating in funnel flow
elements at the solids outlet. Thus,
the most ideal conditions can be
generated at the outlet to support
feeder consistency and accuracy -
these being minimal variation in bulk
density and improved homogeneity
(also associated with bulk density
but often, more critically, associated
with good blend composition).
Designing for mass flow
Attaining mass flow through the
storage vessel is of critical importance
where the accuracy of feeder
operation is a main consideration.
Reliability of discharge is a function
of the outlet size required to prevent
rat-holing or bridging (arching)
of the solid material. This dimension
can also be calculated for a
given type of bulk solid, and should
always be incorporated into the
design and construction of massflow
vessels.
Storage vessels that have been
constructed without consideration
of the bulk-solid properties will by
default develop funnel (core) flow
(Figure 1). Inappropriately high effective
friction at the wall (caused
by shallow vessel geometry or by
high-friction surface finishes) means
that when discharge occurs, material
will be retained against the walls,
and a preferential flow channel will
propagate upward from the outlet.
When such channels develop, material
is drawn down from the top
surface of the solids inventory. This
type of flow behavior endows the
vessel with a range of characteristics
that, while not a major issue for
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some bulk solids, can
cause serious issues
for others (typically
cohesive or time-dependent
materials).
These issues include
retention, poor stock
rotation, cross-contamination
and exaggerated
segregation
effects. From the
perspective of feeder
performance, the
greatest drawbacks
are that the bulk density
at the outlet is
directly influenced by
the head of material,
and that segregation
effects also influence not only bulk
density, but also critically adversely
impact blend quality.
Where many feeder installations
fail is in the use of equipment that
is incapable of drawing down from
the cross-sectional area of the vessel
outlet. As a default, most feeder
types are what is termed fixedvolume
devices (including standard
screws, drag link, belts, rotary
valves, and so on), which has the
implication that if simply purchased
and bolted onto the underneath of
a vessel or bunker, a preferential
draw will develop though the outlet
of the vessel. The result of this is
that any vessel (even one designed
for mass flow) connected onto the
equipment will default to funnel flow
(which is the default for the majority
of vessels anyhow).
One of the challenges
for feeders
employed in applications
where good
repeatability is a key
objective, is that, if
funnel flow is present
(that is, for a
fixed-volume-type
feeder,
suboptimal
vessel geometry, or
both), the bulk density
developed in
the feeder will be a
reflection of the inventory
level at any
given moment (that
is, the bulk density
will oscillate as a
reflection of emp20
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Time,
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Figure 4. The graph shows the discharge characteristic for a modified
screw feeder with the agitator system removed operating in mass flow
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tying or filling cycles). Obviously, if
the range of inventory variation can
be controlled by feedback control
based on load cells or level probes,
then the variation in bulk density will
be lessened. This approach is one
of several reasons why it is not unusual
to find subhoppers used for
controlled feeding, where they are
fed into from larger-capacity vessels.
The less variation in bulk density
that occurs, the simpler (and
less expensive) the feedback control
can be on the feeder.
Figure 3 shows the typical fall off
in dose weight over the discharge
to empty a small vessel where the
geometry and feeder design factors
have created funnel flow. The trend
is very clear. By contrast, Figure 4
shows a modified version (to support
mass flow) of the vessel in use
with a modified screw. The stability
of the bulk density in the feeder
during the drain down to empty the
vessel contrasts very strongly with
the behavior of the funnel-flow system
in Figure 3.
Many small-scale dosing systems
employ agitators (usually as a default)
in their design - the purpose
of which is to prevent flow stoppage
(since the dimensions of the dosing
screws and their corresponding
flow-channel volumes are invariably
significantly below the rat-hole
or arching dimension for the powder
being handled). Such agitators
may support flow reliability, but can
also (in some designs) be responsible
for cyclic variability in density
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ChemiCal engineering www.Chemengonline.Com September 2020
Dose weight ,g
Dose weight, g
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Chemical Engineering September 2020

Table of Contents for the Digital Edition of Chemical Engineering September 2020
