PRACTICAL 3: POWDER FLOW

Objective:

i.                    To evaluate powder flowability of various sizes of sand using hoppers with different size and orifice.

ii.                  To study the factors that influence powder flowability.

Introduction:

Powder flowability is defined as the ease of flow and relates to the change of mutual position of individual particles forming the powder bed. Powder flowability is very important in tablet and capsule manufacturing. Each powder of certain material has distinct flowability. Powder flowability is important because it ensures uniform feeding from container or hopper to equipments thus uniform tablet and capsule weight. This experiment is conducted to observe the flow of different sizes sand through hopper of various openings and orifice to understand the flowability.

Apparatus: Hoppers with size: 8mm, 10mm, 11mm, 13mm and 16mm.

Material: Sand with size: 335µm, 500µm, 850µm and various size.

Procedure:

1.      5 hoppers with various sizes such as 8mm, 10mm, 11mm, 13mm, and 16mm were taken.

2.      Sand with 335 µm, 500 µm, 850 µm and various size were collected to be used during the experiment.

3.      The orifice of the hopper was closed and 100g of the sand was poured into the hopper.

4.      The orifice of the hopper was then opened and the sand was allowed to flow out.

5.      Time required for all the sand to flow out from the hopper was taken.

6.      The steps were repeated using different sand and different size of hoppers.

Result:

Time required for all the sands to flow out from the hopper of various size (s)
Hopper orifice size Sand size	8mm	10mm	11mm	13mm	16mm
335 µm	26.63	11.49	7.90	3.98	3.56
500 µm	21.15	9.36	5.98	3.26	2.25
850 µm	18.97	8.20	5.73	2.89	1.78
Various size	10.66	10.41	7.94	3.45	3.21

Discussion:

The experiment above allowed us to study about hopper flow rate. Hopper flow rate is the rate at which powder discharges from hopper. Hopper flow rate is the simplest method of determining powder flowability directly. By dividing the discharged powder mass by this time, a flow rate is obtained which can be used for quantitative comparison of different powders.

            Based on the results obtained, sand size with 850 µm required the shortest time to flow out completely form the hopper among other size of sand. From this, we know that the bigger the particles size, the better the flowability. Meanwhile, shortest time was required for the sand to flow out from hopper with orifice size of 16mm diameter.  This means that bigger the size of hopper orifice, faster the sand flow out from the hopper. Although various size of sand flowed out from hopper with orifice size of 8mm diameter was with the fastest rate, but the flowability of various size of sand vary with its content. If the sample of  various size of sand taken has big proportion of large particles, then it will flow smoother compared to those with big proportion of fine particles. However, some particles may stuck at the orifice of hopper and slow down the flow rate. This made the time taken for sand to flow out completely inaccurate.

Questions:

1.      What are the factors that affect the powder flowability?

Those factors are cohesion and adhesion force of particle, particle size effects, particle shape, particle density and packing geometry.

2.      Based on the experiment above, which sand and size of hopper give the best powder flowability?

Based on the result, sand with 850µm and hopper with 16mm orifice diameter give the best powder flowability.

3.      What methods can be used to improve powder flowability?

These methods are through alteration of particles size and size distribution by manipulating proportion of coarser and finer particles. By altering particle shape or texture through process spray drying and temperature-cycling crystallization can improve the flowability. Besides that, flowability can be improved by reducing electrostatic forces and moisture content. Addition of glidants such as magnesium oxide and silicon treated powders increases the flowability. Alteration of process conditions to reduce frictional contacts by the usage of vibrantion-assisted hoppers and force feeders are also measures to improve power flowability.

Conclusion:

As a conclusion, the time required to allow sand to flow out of the hopper is dependent on the diameter of the hopper orifice and the size of sand particles. Bigger particle size has higher flow rate. The time required for the sand to flow out of hopper is inversely proportional hopper rate flow.