Objective:
To evaluate the powder flow of sand with different sizes by using
hoppers of different sizes and orifices.
Introduction:
Powder
flowability is defined as the ease with which a powder will flow under a
specified set of conditions. Some of these conditions included the pressure on
the powder, the humidity of the air around the powder and the equipment the
powder is flowing through or from. Powder flow is crucial in tablet and capsule
manufacturing. Each material has unique powder flow properties. This practical
allow us evaluate flow of sands of various sizes.
Powder behaviour will be very dependent upon particle size, the
variation of size and the shape of the particles. In general powders with large
particles (>100µm) will be non-cohesive, permeable and will probably
fluidise and will have low compressibility and relatively low shear strength. In the case of free flowing solids, the bin may be used to give a
nearly constant flow rate of solids. The flow rate obtained in such a case
however will depend on the use of hopper of different orifice size, as well as
the nature and size of powder particles.
Procedures:
1.
Five
hoppers with different sizes such as 8mm, 10mm, 11mm,13mm and 16mm were taken.
2.
Four
different types of sand which their particle sizes are 850µm, 500µm, 355µm and
various sizes were taken and 100g of sand were weighed by using electronic
weighing machine.
3.
The
orifice of the hopper was closed and 100g of sand was pour into the hopper.
4.
The
orifice was opened and the sand was let go so that it flow.
5.
The
time required for all the sand to flow out from the hopper was recorded.
6.
The
above test was repeated by using different types of hoppers and sand.
Results:
Particle
Size of Sand
(µm)
|
Time
of sand flow out from different diameter of orifice(s) of hopper
|
||||
8mm
|
10mm
|
11mm
|
13mm
|
16mm
|
|
Various
size
|
9.75
|
8.34
|
6.80
|
3.39
|
3.02
|
355
|
19.78
|
14.77
|
8.45
|
5.87
|
4.05
|
500
|
13.80
|
9.55
|
7.66
|
5.16
|
3.25
|
850
|
11.97
|
8.76
|
7.01
|
3.46
|
3.17
|
Powder flowability can be defined as capability of a liquid or
loose particulate solid to move by flow. It is important to ensure uniform
feeding from containers or hopper to equipments. Besides, it can be
reproducible filling of tablets dies and capsule dosators with improved weight
uniformity. Particulate systems will take the shape of the container they
occupy, exert pressure on container walls and flow through orifices. Solids can
sustain a shear stress, though the magnitude of the shearing stress at a point
is generally indeterminate while for liquids the shearing stress is
proportional to the normal load rather than to the rate of deformation.
This experiment is carried out to study the flow patterns of
discharge from bins and hoppers. The discharge rate depends mainly on the
geometry of orifice, the nature of the powder and independent of the height of
the powder above the exit and the vessel diameter.
This experiment undergoes powder flow process by vibrating the
hopper. Ratte at which powder discharges from hopper can be determined. During
the experiment, a simple shutter placed over the hopper outlet and hopper
filled with powder. Shutter then removed and time taken for powder to discharge
completely recorded. By dividing the discharged powder mass by this time, flow
rate is obtained for quantitative comparison of different powder.
The flow pattern of this
experiment is mass flow. Mass flow, also known as mass transfer and bulk flow is the movement of material matter. In mass flow, all of the powder is discharged freely when first in
first out sequence holds throughout the bed. It eliminate stagnant powder,
provides a steady discharge with a consistent bulk density, and yields a flow
that is uniform and well controlled. Powder leaves in near horizontal bands
which move down the hopper en masse. Mass flow also reduces the extent to which
some types of segregation affect the powder. Although all of the material is
moving, velocity profiles may still exist within the hopper.
Requirements for achieving mass flow include sizing the outlet
large enough to prevent an arch from forming and ensuring that the hopper walls
are steep and smooth enough to promote flow along them. Besides that, internal
friction values are important when characterizing the flow properties of a
powder. Such friction is caused by the solid particles flowing against each
other and is expressed as an angle of internal friction. In addition, wall
friction also occurs when particles sliding along a surface. This kind of
friction is known as adhesion force that also influences the flow properties of
a powder.
Based on the result obtained in the experiment, we know that hopper
with size of 16mm gives the best flow of powder particles because the time
needed for sand to flow out the hopper is the shortest compare to 8mm, 10mm,
11mm and 13mm. This mean that the flow
of the sand is smoother and faster when the orifice size of the hopper is
larger The larger orifice size of hopper have reduced the friction as the sand
flow. The lesser the friction, the
faster the powder flow and finally the time needed for sands to flow out the
hopper is shorter. Besides, the sand with particle size 850µm give the best
flow of powder particles because time needed for sand to flow out the hopper is
the shortest compare to 355µm, 500µm and various size sand particle. The flow
of sand is faster when the particle size is bigger. This is because the bigger
the particle size, the greater the gravity force faced by the particles and
thus gravity force will greater than frictional force and cause the faster sand
powder flow and the time required to flow out of hopper is shorter.
Conclusion:
The flow property of powder is depending on the particles size of
powder and the orifice size of the hopper. Powder flow the fastest with the
bigger particle size of powder and bigger the orifice size of hopper. However,
there are many other factors that influence flow property such as shaking,
presence of water and moisture, cohesion and adhesion force of particle,
particle shape, particle density and particle packing geometry.
Questions:
1)
What factors
that will affect powder flow?
Factors
that will affect the powder flow included shaking, presence of water and
moisture, diameter of hopper, cohesion and adhesion force of particle, particle
size of powder, particle shape, particle density and particle packing geometry.
2)
Based on the
experiment above, what the particle size of sand and orifice size of hopper
give the best powder flow?
The
larger the orifice size of hopper, the faster the powder flow. Moreover, the
smaller the size of sand, the faster the powder flow.
3)
What the method
that can be used to help the powder flow?
There
are a few methods to improve the powder flow process. Firstly, we need to
convert finer particles into larger aggregates by the process of wet
agglomeration. Binding agent such as can be to make particles stick together to
form spherical granules. This granulated mix flows better than a non-granulated
powder. Second, lubricant or glidant
material such as colloidal silica or magnesium stearate can be added to the
powder. It will reduce the friction between the powder and the walls of the
hopper, leading to better flow. Third, coat the hopper walls with anonsticking,
coating material will also reduce the friction between the powder particles and
the hopper walls and thus improving flowability. Fourth, try to carry out test
runs by setting the hopper's inclines at different angles. Study the powder
flow at each angle to find out the inclination at which the powder flows best.
Fifth, we can design a new hopper with hopper walls steep to reduce cohesion
and friction and keep the outlet hole big to allow sufficient space for the
powder to flow out smoothly.
No comments:
Post a Comment