Objective:
To analysis the flow of sand of different sizes using hopper of different sizes and opening.
Introduction:
Powders are a mass of solid particles or granules. In fact these particles are usually surrounded by air (or other fluid) and it is the solids plus fluid combination that largely determines the bulk properties of the powder. It is perhaps the most complicating characteristic because the amount of fluid can be so variable.
Powders are probably the least predictable of all materials in relation to flowability because of the large number of factors that can change their rheological properties. Physical characteristics of the particles, like size, shape, angularity, surface texture, porosity and hardness will all affect flow properties. External factors such as humidity, conveying environment, vibration and perhaps most importantly, aeration, will compound the problem. Another characteristic of powders is that they are often inherently unstable in relation to their flow performance. This instability is most obvious when a free flowing material ceases to flow. This transition may be initiated by the formation of a bridge in a bin, by adhesion to surfaces or by any event that may promote compaction of the powder. The tendency to switch in this way varies greatly from one powder to another, but can even be pronounced between batches of the same material.
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. Conversely, fine powders <10µm say, 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, low permeability and are very affected by being consolidated when entrained air is excluded.
There are many exceptions to the above – for example toner used in printers and copying machines are fine powders with an outstanding fluidisation characteristic. A small amount of aeration is sufficient to transform a consolidated powder into one with fluid like rheology. Another broad generalisation is that under forced flow conditions, where powders are made to move other than by gravity, fine powders can behave more like a fluid. They are able to extrude round corners or through holes, unlike coarse powders that are more likely to become solid like as particles realign and lock together and become very resistant to flow.
The nature of powders therefore is such that an adverse combination of environmental factors can cause an otherwise free flowing powder to block or flow with difficulty. Conversely, a very cohesive powder may be processed satisfactorily if the handling conditions are optimised.
Ordinary sand is a granular mate¬rial with interesting properties. Individual sand grains are solids, yet sand as a whole can flow through a narrow opening as in the case of an hourglass. Unlike fluids, the average flow is almost constant through a hole and does not depend on the height of the sand column.
Apparatus and Materials:
5 hoppers (8mm, 10mm, 11mm, 13mm and 16mm), 4 types of sands (355μm, 500μm, 850μm and variable diameter), newspapers, electronic weighing balance, stopwatch
Procedures:
1. 5 hoppers with different opening sizes were prepared. (8mm, 10mm, 11mm, 13mm and 16mm)
2. 4 types of sands with different sizes and behaviour were prepared for this experiment. (355μm, 500μm, 850μm and variable diameter)
3. The opening of the hopper with 8mm diameter was closed and 100g of sands with 355μm diameter were put into the hopper.
4. The opening was let opened and the sands were released so they can flow out.
5. The duration needed for all the sands to flow out was recorded.
6. The experiment was repeated with sands of diameter 500μm, 850μm and various diameter and 10mm, 11mm, 13mm and 16mm openings of the hoppers.
Results:
Duration of sand flow
Discussion:
The powder flow in this experiment is mass flow. Mass flow which is also known as mass transfer and bulk flow is the movement of material matter. Mass flow involves powder that discharged freely when first in first out. 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.
From this experiment, we can see that mass flow is influenced by both diameter of hopper orifice and also the particle size (diameter of sand). From the result obtained, the sands of different particle sizes flow through hopper with 16 mm orifice diameter with an average of 3.61 s, which is relatively fast compared to hopper with orifice diameter 13 mm (4.79 s), 11 mm (6.44 s), 10 mm (9.41 s) and 8 mm (11.03 s). Sand flows through the hopper with the smallest hopper orifice in this experiment (8 mm) with the highest average duration that is 11.03 s. This shows that hopper with larger orifice gives a better particle flow, in this case, sand flow while hopper with small orifice causes a slower flow. This can be explained as the larger the hopper orifice, the less the contact surface area of the particle (sand) to the wall of hopper. This causes reduction in friction and therefore the particle (sand) can move smoothly and faster.
Another factor affecting powder flow in this experiment is size of particle, in this case, diameter of the sand. From the result obtained, sand of various diameters has a better flow than that of diameters 355 μm, 500 μm and 850 μm. This can be explained by strong cohesion between sand of different sizes as the sand with smaller size filling up the spaces in between sand with larger size, indicating larger contact surface area of and particle. This is followed by sand of diameter 850 μm, 500 μm and then 355μm. This shows that the larger the particle size, the faster the particle flow. This is due to larger particle has larger gravitational force which will pull down the particle faster towards the ground.
There is some inconsistency of data obtained from the experiment where the duration of sand flow fluctuated. This might be caused by external forces accidentally applied by person holding the hopper, different shallow hopper angle and also hopper with different diameter. To overcome these problems, a distort stand can be used instead of using bare hand to hold the hopper when sand flow out from it. This can also ensure a constant height where the sand flows out of the hopper. Hoppers with same shallow hopper angles and also diameter should be used although the orifice diameter is difference. This can ensure more accurate results and prevent other factors affecting the experiment. There are many other factors that will influence flow property of powder 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 are the factors that affect powder flow?
Factors that influence the flow property of powder including shaking, presence of water and moisture, cohesion and adhesion force of particle, particle shape, particle density and particle packing geometry.
2. Based on the experiment above, what is the size of powder and hopper give the best powder flow?
Size of powder that gives the best powder flow is sand of various sizes. Hopper with orifice diameter 16 mm gives the best powder flow.
3. What are the methods that can be used to help the flow of certain powder?
Firstly, alteration of particle size and size distribution can be carried out by manipulating proportion of coarser and finer particles for example granulation. Alteration of particle shape or texture also can be carried out by making more spherical particles using spray drying. Surface forces also can be altered by reducing electrostatic forces through earthing and also reducing moisture content. Flow activators such as colloidal silicon dioxide and magnesium oxide help to reduce adhesion and cohesion thus improve powder flow. Process condition also can be altered for example using vibration-assisted hoppers and force feeders.
Conclusion:
Powder flow depends on the orifice diameter of the hopper and also particles size of powder a. Powder flow better and faster with larger hopper orifice diameter and larger particle size of powder while powder flows slower with smaller hopper orifice diameter and smaller particle size.
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