TITLE
Flow of Powder
Flow of Powder
INTRODUCTION
The flow
of a powder is very important in the production of tablets and capsules.
Different powder of a material having a different flow. A simple definition of
powder flow ability is the ability of a powder to flow. By this definition, flow ability is sometimes
thought of as a one-dimensional characteristic of a powder, whereby powders can
be ranked on a sliding scale from “free-flowing” to “non-flowing”. The same
powder may flow well in one hopper but poorly in another; likewise, a given
hopper may handle one powder well but cause another powder to hang-up. Therefore, a more accurate definition of powder
flow ability is the ability of a powder to flow in a desired manner in a
specific piece of equipment. The
specific properties of a powder that affect its flow are known as flow
properties. Examples of flow properties
include bulk density, permeability, cohesive strength, and wall friction. These flow properties arise from the
collective forces acting on individual particles, such as van der Waals,
electrostatic, surface tension, interlocking and friction.
EXPERIMENTAL METHODS
1. 5 'hoppers' of various sizes are taken.
2. Get
3-4 sand of various sizes.
3. The
opening of 'hopper' is covered by fingers and 100g of sand is added into the
hopper.
4. The
'opening' is opened and the sand is allowed to flow out.
5. Time
needed until all sand has flowed out is taken.
6. The above test is repeated with the sand
and the 'hopper' of different sizes.
RESULTS
150
|
355
|
500
|
850
|
Variables
|
|
0.6cm
|
2 sec
|
10 sec
|
10 sec
|
12 sec
|
2 sec
|
0.8cm
|
2 sec
|
7 sec
|
7 sec
|
7 sec
|
6 sec
|
1.0cm
|
5 sec
|
4 sec
|
4 sec
|
4 sec
|
3 sec
|
1.4cm
|
2 sec
|
3 sec
|
3 sec
|
3 sec
|
3 sec
|
1.6cm
|
2 sec
|
4 sec
|
4 sec
|
4 sec
|
3 sec
|
QUESTIONS
1. What are the factors that affect the flowing of the
powders?
Factors that affect the flowing of the powders are the
size of the sand and the opening size of the hoppers. A given hopper may handle
one powder well but cause another powder to hang up. Another factor is the
gravity.
2. Based
on the experiments on the sand and hoppers, which sizes show the best flowing
ability?
Based on the experiments, hopper which size 1.4cm and
sand 150µm show the best flowing ability.
3. What are the methods to be used to improve the flowing of
the powders?
Shake the hopper. Besides that, the hopper walls must be
sufficiently smooth (having low friction) and steep to allow the material to
slide along them. In addition, the orifice
must be large enough to prevent a stable arch from forming. Both the required hopper angle and the
minimum outlet size can be determined by analyzing flow properties test
results.
DISCUSSION
Powder behaviour and flow
characteristics 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, low flow characteristics, permeable and will
probably fluidize and will have low compressibility and relatively low shear
strength. Conversely, fine powders (<10µm), are likely to be cohesive,
compressible, contain much entrained air and yet have poor aeration
characteristics. Generally they have high shear strength, high flow energy, and
low permeability and are very affected by being consolidated when entrained air
is excluded.
The affection on flow rate
of powder from the hopper include if the semi-vertical angle smaller, the flow
type was whole flow (easier to flow out of the opening), the relation between
way dimension of feed opening and flow is nonlinearity. At the same time, it
would be one effective indirect measurement method by measuring the way
dimension of feed opening under special condition.
Process conditions
relevant to flow ability need to be determined. These might include the level
of static and dynamic head produced in a storage hopper, the amount of aeration
that occurs, the opportunity to adsorb moisture, become electrostatically
charged or be consolidated due to vibration. Other factors could be segregation
and attrition that may cause fines to collect, rounding of particles and so on.
Some errors can be
detected to be occurred during the experiment. Firstly, the hopper is shaken
during the process of powder flow from the hopper. This will affect the result
a lot as the hopper must be static and free from other external factors that
can make movement on it so that powder will flow evenly through the hopper. To
prevent this from happening, we can use other equipment such as retort stand that
can fix and provide static characteristic to the inverted position of the
hopper during powder flow.
Secondly, the internal
surface of the hopper might be irregular which also can affect powder flow.
Therefore, it is important to make sure to use an even and regular of internal
surface of hopper so that the powder flow can be improved.
When a powder flows out of
a hopper, bin or a container, one of two flow patterns will develop: funnel
flow or mass flow. Funnel flow occurs when the hopper walls are too shallow and
too frictional for material to slide along the. As a result, the material along
the walls remains stagnant, while material flows preferentially through a
funnel-shaped channel directly above the outlet. Funnel flow results in a
first-in-last-out flow sequence which often leads to particle segregation. Funnel
flow is suitable only for sufficiently coarse, free-flowing powders, where
segregation is not important. This rule out the vast majority of pharmaceutical
products.
Mass flow, is
characterized by a first-in-first-out flow sequence in which all of the
material is in motion whenever any is withdrawn from the container. This
eliminates ratholing and provides a reliable discharge. Mass flow generally
minimizes segregation. It also provides uniform bulk density during discharge.
Although mass flow designs can overcome a number of potential flow problems, it
should be noted that adverse two-phase (powder and interstitial gas) flow
effects can still remain. To achieve mass flow, the hopper walls must be
sufficiently smooth and steep to allow the material to slide along them. An understanding of flow patterns
provides a valuable insight in analyzing flow problems and determining
solutions.
Figure 1: The two primary flow patterns that occur in gravity discharge
are funnel flow and mass flow.
Figure 2: Stagnant, cohesive powders can
form a stable rathole in funnel flow.
CONCLUSION
The flowing of the powders is depends on
the size of the powder and the opening of the hopper , the shape of the
particles and also the gravity. Different powders have difference flowing.
Shaking the hopper will improve the flowing of the powders.
REFERENCES
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