Transcript Properties, Handling and Mixing of Particulate Solids By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore.

Properties, Handling and Mixing
of Particulate Solids
By
Sidra Jabeen
Department of Chemical Engineering,
University of Engineering & Technology Lahore
You will study…
 Properties of particulate masses
 Types of particulate solids
 Storage of solids
 Flow out of bins
 Pressures in bins and silos
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States of Matter
Gas
o Particles can
move past one
another
o Assumes the
shape and volume
of container
o Compressible
o Flows easily
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Liquid
o Particles can move
past/slide past one
another
o Assumes the shape
of container which
it occupies
o Not easily
compressible
o Flows easily
Solid
o Particles locked
into the place
o Retains the fixed
volume and
shape
o Not easily
compressible
o Does not flow
easily
Properties of Particulate Masses / Solids
1. Resist distortion
The particles interlock and can not slide past one another that is why
they resist distortion when subjected to moderate force. When the
force is large enough, failure may occur and one layer of particle slide
over other.
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Properties of Particulate Masses / Solids
2. Non-uniform pressure distribution
Pressure is not same in all directions. Pressure applied in one
direction may create some pressure in other, but it is always smaller
than the applied pressure. It is min. in the direction perpendicular to
the applied pressure.
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Properties of Particulate Masses / Solids
2. Non-uniform pressure distribution
Pressure is not same in all directions. Pressure applied in one
direction may create some pressure in other, but it is always smaller
than the applied pressure. It is min. in the direction perpendicular to
the applied pressure.
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Properties of Particulate Masses / Solids
3. Shear stress in solids
Shear stress in solids is transmitted throughout the static mass of the
particles unless failure / distortion occurs.
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Properties of Particulate Masses / Solids
4. Solids density
Bulk Density: The mass of the particles divided by the volume they
occupy. Density of mass vary depending upon the degree of packing
of the grains. It is min. when the mass is loose and rises to maximum
when particles are packed under pressure.
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Properties of Particulate Masses / Solids
5. Dilation required for flow
The tightly packed solid particles must expand or increase in volume
to permit interlocking grains to move past one another to start flow.
Without dilation, flow is not possible.
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Types of particulate solids
Depending on flow properties, particulate solids are of two classes

Cohesive
For example; wet clay

Non-cohesive
For example; dry sand, sugar crystals, grains etc.
Storage of solids

Bulk storage

Bin storage
BULK STORAGE

Coarse solids like coal and gravels ……. Outside in large
piles

Unprotected from the weather

Most economical for large amount of material

Solid can be removed from pile by dragline or tractor shovel
and delivered to conveyer or process

May lead to environmental problems

Dusting - avoided by providing protective cover

Leaching - avoided by providing protective cover or
shallow basin
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Angle of Repose & Angle of Internal Friction
Angle of Repose:
The angle measured from the horizontal at which a heap of loose
solids will stand without sliding.
Angle of Internal Friction:
A measure of frictional forces between particulate masses.
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Angle of Repose
Angle of repose determines the flow
characteristics of particulate masses.
Smaller angle  Free flowing solid
Larger angle  Poor flow
characteristics
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BIN STORAGE

Valuable or soluble solids

cannot exposed in outdoor piles

That solids are stored in bins, silos and hoppers.

These are cylindrical or rectangular vessels of concrete or metal

SILO: tall and relatively small in diameter

BIN: not so tall and usually fairly wide

HOPPER: small vessel with a sloping bottom for temporary
storage before feeding
All these vessels are loaded from top and discharged
from bottom
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FLOW OUT OF BINS
Solids tend to flow out of any opening in
bottom of storage vessel.

Side opening

Base opening
Its not possible to open entire bottom for
discharge. Rather a bottom section is set at
the bottom of a bin for discharge;

Conical bottom

Pyramidal bottom
Bottom Section
Conical
Pyramidal
FLOW PATTERNS
 Mass flow
 Funnel flow
Flow pattern depends on

Steepness of the wall in the bottom section

Friction between solids and the wall
MASS FLOW

All the material moves downward uniformly

It occurs in cone-bottomed bins with sufficient
steep and/or smooth discharge
Characteristics of Mass flow

Flow is uniform and well controlled

Constant density flow

Sifting or segregation is minimized

No dead zones

Described as : FIFO (first in, first out)

Free flowing or non cohesive solids follow this pattern
FUNNEL FLOW

Some material is stationary and rest is moving.

A vertical column of solids above the opening
moves downward without disturbing material at
the sides.

It occurs when the bottom section is pyramidal or
shallow cone angle

Lateral flow
Characteristics of Funnel flow

Particles segregate and remain segregated

Flow is irregular and can cause flooding

Density of flow can vary

dead zones

Described as : LIFO (last in, first out)

Cohesive solids follow this pattern
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Flow out of a bin
The rate of flow of granular solids through opening in
the bottom of the bin depends upon the following
factors;
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
Diameter of discharge opening

Properties of the solids
Discharge opening

Not too small - clogging

Not too large – difficult to control and shut down

Small enough – to be readily closed during solid
discharge
Properties of the solids
 With cohesive solids, its hard to start flow. They
adhere to the vessel walls and support a plug flow over
the discharge opening.
 Vibrators on bin walls are used to start the flow.
 Flow is easier for non cohesive solids.
Pressures in bins and silos

Pressure exerted by solids on the floor of the container is small
because of the strong frictional forces between the particles
and walls.

In granular solids, high pressure does not increase the
tendency of the material to flow instead it often packs the
grains more tightly and makes flow difficult.

In extreme cases, frictional forces cause the particles to arch or
bridge, so that they don’t fall as the material below them is
removed.  arch breakers are provided near bottom to avoid
particles bridging.
Arch Breaker
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