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Caking of Powders- a common process
problem
Guest article by Richard J. Farnish, The Wolfson Centre for Bulk Solids Handling
Technology
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Many processes that are applied in industry
utilize heating, drying or crystallization as a principle mechanism in the production of
bulk solids. These processes can produce quantities of material at rates ranging from a
few grams per second to many tons per hour.
The need to improve the quality of the
product to retain and improve a competitive edge over competitors (that may produce a
similar product but with lower overheads) has resulted in the focussing of much attention
upon the application of technology into the production process. However, the ever present
need to maintain product quality is often thwarted once the material is handled and stored
prior to packing or outloading. The post processing stage in many cases presents the
biggest source of reduction of quality for materials that are prone to caking or
agglomeration (see Figure 1).
There are four main mechanisms by which
caking (or agglomeration) of product can occur. They are:
-Mechanical caking
-Plastic flow caking
-Latent chemical reactions
-Electrostatic charging.
Figure 1. An example of a typical caked
product
The effects of these mechanisms are
invariably time related and can be minimized by careful handling and design of storage
equipment.
The most commonly encountered problems are
usually associated with moisture migration between particles in storage (and this can
manifest itself either within a plant or when material is inspected at an end users
plant). In these instances it is often the case that warm (or above ambient temperature)
materials are introduced into storage vessels which have not been designed to offer
"first in first" out stock rotation, but which give "last in first
out" product flow (i.e. Fig 2, core flow).
Figure 2. Core flow discharge
Thus, if warm material is loaded into the
silo prior to outloading operations, it will be this material which will be discharged
first of all. This situation can result in containers of free flowing material being sent
to clients, but due to latent maturing, moisture migration and settling of the particles
it is quite possible that the end user may receive a solid "tombstone" or lumps
of material.
If material is being loaded into the silos
after having come from a process, it is most important that the particles are given
sufficient time for thorough drying or for any residual reactions to take place in a
controlled manner. The use of flash dryers can (in the case of some particle types and
operational conditions) serve to drive off moisture from the surfaces of particles, but
leave a core of moisture locked in the center. Once these particles have left the process,
this core of moisture can be drawn to the drier outer layer. If there is a mass of
particles, this moisture can migrate across particles and deposit a small amount of solid
as it does so.
Variations in ambient temperature (such as
overnight cooling and subsequent warming during the day) can generate several reversals of
this moisture migration, each of which deposits solids resulting in a stronger solid
bridge forming between the particles - and ultimately a cake to form.
Figure 3. Mass flow discharge
The correct design of silo for these types
of materials is the mass flow design (see Figure 3.), which will discharge evenly across
the entire cross section of the silo. The most important benefit of this mode of flow is
that the material which is loaded into the silo first actually comes out first, and the
most recently loaded material is allowed time in residence to cool or mature before it is
discharged. The other important feature is that since the entire mass of material moves
upon discharge the presence of stale or aged material is no longer a factor.
One approach to controlling the problem of
caking (where moisture migration has been identified as the problem) is to use a mass flow
design of silo in which one (or more) plenums are installed in the cone area. The exact
number of plenums required is dependant upon the determination of the safe upward air
velocity that can be employed to condition the material without the risk of fluidization
occurring (this data would be derived from trials using a fluidizing test rig). The
plenums take the form of an annular air inlet which is stepped back from the walls of the
cone section (an important feature since any intrusion into the cone i.e. air injection
points, weld beads, etc. runs the risk of disturbing the flow pattern by virtue of
providing points at which material can be supported and hence an irregular shear plane to
be created).
A mass flow silo with conditioning
plenum(s) can be operated equally well in either continuous or batch mode (subject to time
consolidation considerations), although it is often preferable to incorporate a
re-circulatory system for some materials (which will also provide the benefit of
eliminating the stress field generated within the material during initial filling - and
thus facilitate the use of a smaller outlet than would otherwise be the case for reliable
discharge). It is important to remember that a conditioning silo will need to be larger in
capacity than a conventional silo as the volume of the silo will be dictated by the rate
of process output multiplied by the length of time required for conditioning purposes.
Caking represents a widespread problem for
a large number of manufacturers, but is not an insoluble problem. As with any powder
handling problem it is essential to understand the flow characteristics and storage
requirements of the material being handled and to design the process equipment to take
these factors into account.
For more information contact:
Mr. Richard J. Farnish
The Wolfson Centre for Bulk Solids Handling Technology
University of Greenwich, London, UK
Tel: +44 (0)20 8331 8646
Fax: +44 (0)20 8331 8647
Email: R.J.Farnish@gre.ac.uk
Web site: http://www.gre.ac.uk/directory/wolfson
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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.
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