Chemical Engineering September 2020 - 59

Solids Processing
Reliable and Cost-Effective Solids Feeding
A holistic approach to bulk-solids-feeding installations can improve the effectiveness
and efficiency of operations involving solids handling
Richard Farnish
University of Greenwich
M
any chemical processes
involve the extraction of
bulk solid materials at
a controlled rate from a
storage vessel for delivery into the
process. The extraction of solid
materials usually requires the use
of technology that can deliver the
solids into the process at the required
feedrate and with minimal
variation in rate when in operation.
For every industrial installation, bulk
solid material type and set of process
requirements, a different optimal
technology exists. This article
presents some of the key considerations
involved in obtaining reliable
and cost-effective extraction from
storage vessels.
One of the most important aspects
of efficient feeder installation
is to understand that adopting a
holistic approach is required to optimize
any type of feeder and solidsfeeding
process. Such an approach
considers the storage vessel, the
interface between the storage vessel
and the feeder along with the
feeder itself as a single project entity
whereby each element of the
Flow from top of static
material
process must be specified and designed
to work together. For " clean
sheet of paper " projects, this approach
is usually more easily realized.
However, in many cases, engineers
are tasked with trying to make
the most of retrofits and upgrades
where budgets are rarely adequate
to support such an ideal approach.
In these real-world projects, the
need to compromise is important in
the delivery of effective solutions.
Sliding
Consistent volume delivery
A fundamental requirement for most
types of solids-feeding installations
is that the delivery of solids into the
process should proceed with consistent
volumes or weights of the
solids introduced into the process
without any interruptions in feeding.
These may sound like fairly undemanding
requirements, but in too
many solids-feeding cases, they
remain elusive for many end users.
Success in meeting these requirements
is driven by the design of the
feeder and the flow-channel profile
that it develops, but is also driven
by the inherent operational characteristics
of the storage vessel.
Achieving any degree of consistency
in the delivery of solids into
a process requires a correctly designed
and
installed feeder and
Static material
consistent bulk-material properties
at the outlet (interface with the storage
vessel). To do this, it is clearly
important to consider the conditions
of the bulk material that are
prevalent at the outlet, and the influence
of the storage vessel on those
conditions.
Discharge
through
central
" core "
Figure 1. The illustration shows the concept of
funnel/core-flow discharge
Depending upon the vessel geometry,
internal surface finishes and
material characteristics, two basic
flow patterns can develop in storage
vessels: mass flow and funnel
(core) flow. Simplified illustrations of
these two operational behaviors appear
in Figures 1 and 2.
Mass flow is characterized by
ChemiCal engineering www.Chemengonline.Com September 2020
on wall of
converging
section
All material in motion
during discharge
Figure 2. The diagram illustrates the concept of
mass flow discharge
the friction at the walls of the storage
vessel (this is a function of the
convergent angle of the vessel, the
surface finish of the vessel walls and
the interaction of these variables
with a specific bulk solid) being
sufficiently low to allow material to
slide downward when the outlet
cross-sectional area is fully active.
Because the designed geometry is
able to mobilize the friction at the
wall, the flow of material toward
the outlet approximates a crosssectional
plane drawn down. This
flow pattern endows the vessel with
the (very desirable) characteristic
of operating on a " first-in, first-out "
stock rotation - without any stagnant
zones or risk of solids retention
when the material drains down.
A very beneficial effect of establishing
mass flow discharge is
that the bulk density at the outlet
is largely independent of the head
of the material present above the
outlet. Similarly, where segregation
(by particle size, shape or density)
occurs in radial or lateral forms, the
cross-sectional drawn down serves
to effectively recombine distributed
59
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Chemical Engineering September 2020

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