Saturday 20 December 2014

Practical 1 - Powder flow (Angle of repose)

Introduction:
                Powder can be defined as solid particles that either have the same or different chemical composition. It is mainly used in production of tablet and capsule. Every powder has its own flowability. Flowability is the capability of a loose particulate solid to move by flow. Angle of repose is a term used to describe the maximum angle, measured upwards from the horizontal, at which a pile of a particular granular material will remain stable without any of the material sliding downward. It is useful in designing storage and transportation machinery for granular materials as it can give an engineer insight into the appropriate size and shape of such devices. The angle of repose for a particular powder can be used to determine the flowability and the flow characteristics of a powder. The value is in the range of 0° to 90°. There are various methods available to measure this value. In this experiment, we are given sand of various sizes and to determine the angle of repose and the factors that may influence it.


Objective:
1.       To determine the angle of repose of various sizes of powder.
2.       To determine the factors that may influence the angle of repose.


Apparatus:
1.       Cylinder
2.        Rubber stopper
3.        Newspaper
4.       Weigh balance
5.        Spatula
6.       Weighing boats


Materials:
1.       100g of 150 micron sand mixture
2.       100g of 355 micron sand mixture
3.       100g of 500 micron sand mixture
4.       100g of 850 micron sand mixture
5.       100g of various sizes sand mixture
6.       Magnesium stearate


Procedure:
1.       100g of 150 micron sand mixture is prepared.
2.       The sand is placed in a cylinder with a rubber base at the bottom.
3.       The cylinder is then removed and the sand is allowed to flow out and heap is formed.
4.       The height, slope and diameter of the heap is measured.
5.       The angle of repose is then calculated.
6.       The experiment is repeated using other sand or with addition of glidant (magnesium stearate).





Results:

1.      Sand without Glidant

Size of particles of sand (micron)
Height (cm)
Width (cm)
Angle of repose
150
2.2
2.4
42.510
355
2.1
2.4
41.190
500
1.9
2.4
38.370
850
1.8
2.4
36.870
Various Size
2.0
2.4
39.810


2.      Sand is mixed with Glidant

Size of particles of sand (micron)
Height (cm)
Width (cm)
Angle of repose
150
2.1
2.4
41.190
355
2.0
2.4
39.810
500
1.9
2.4
38.370
850
1.7
2.4
35.310
Various Size
1.9
2.4
38.370


Discussion:
In this experiment, we had measured an angle of repose of 150 mic, 355mic, 500mic, 850mic and various sizes of sands with and without the presence of glidant. Magnesium Sterate is used as a glidants.

The angle of repose (α) was calculated, from the following equation:


Height = height of the cone of sand

          The results show that with the addition of glidants, the angle of repose will increase. This is because a glidant is a substance that is added to a powder to improve its flow ability by reducing friction between particles. The magnesium compounds works by mopping up the excess moisture keeping the granules dry and free flowing. But there are several other factors that will affect the effectiveness of glidants. Firstly, the flow of the granules depends on the shape and size of the particles of the glidants and the granules. Secondly, a glidant will only work at a certain range of concentrations. Above a certain concentration, the glidant will in fact function to inhibit flow ability.



Conclusion:
The larger and more irregular the granular material grains, the higher the angle of repose.
The sands angle of repose decrease with the addition of glidants.


Questions:
1. What is the angle of repose for each materials?

Size of particles of sand (micron)
Angle of repose
(Without Glidant)
Angle of repose
(With Glidant)
150
42.510
41.190
355
41.190
39.810
500
38.370
38.370
850
36.870
35.310
Various Size
39.810
38.370


2. What other factor that will influence angle of repose for the materials?

       Firstly, internal factor that will influence angle of the repose of the sand that we use is the particle size, coarser particles have high angles of repose than fine particles. The next factor is the particle shape. Another factor is cohesiveness, fine particles may reveal cohesiveness owing to spherical particles having a greater tendency to roll. Water content also affects the cohesiveness of particles. If water is added to particles such as sand, water coating the grains would tend to bind them together by its surface tension, giving rise to greater internal cohesion, and therefore shear strength. Thus, moist sand has a much higher angle of repose than dry sand.
       Secondly, the external factors which is the presence of other components such as glidants. Next, the moisture of the sand. Angle of repose of loose dry powder increases by compacting as well as by introducing by moisture. Moist sand has a much higher angle of repose than dry sand. The last factors is the methods of measurement, Ledge and erater method give higher angle of repose than from the heap formation methods.


3. What other method can be used to calculate the angle of repose for the powder?
Ledge Method
Where the powder is initially charged into a rectangular box.A slot at the base of one vertical wall is closed by a board.The closure board is then removed to allow the material to flow slowly through the narrow slot.The angle with the horizontal plane of the surface of the powder equilibrium when the flow stops is calculated as the angle of repose.

Tilting box method
 This method is appropriate for fine-grained, non-cohesive materials, with individual particle size less than 10 mm. The material is placed within a box with a transparent side to observe the granular test material. It should initially be level and parallel to the base of the box. The box is slowly tilted at a rate of approximately 0.3 degrees/second. Tilting is stopped when the material begins to slide in bulk, and the angle of the tilt is measured.

Fixed funnel method
The material is poured through a funnel to form a cone. The tip of the funnel should be held close to the growing cone and slowly raised as the pile grows, to minimize the impact of falling particles. Stop pouring the material when the pile reaches a predetermined height or the base a predetermined width. Rather than attempt to measure the angle of the resulting cone directly, divide the height by half the width of the base of the cone. The inverse tangent of this ratio is the angle of repose.

Revolving /Rotating cylinder method
Where a sealed hollow cylinder half full of powder surface is rotated until the powder surface shows its maximum angle with the horizontal.The material is placed within a cylinder with at least one transparent face. The cylinder is rotated at a fixed speed and the observer watches the material moving within the rotating cylinder. The effect is similar to watching clothes tumble over one another in a slowly rotating clothes dryer. The granular material will assume a certain angle as it flows within the rotating cylinder. This method is recommended for obtaining the dynamic angle of repose, and may vary from the static angle of repose measured by other methods. When describing the angle of repose for a substance, always specify the method used.

Crater Method /Discharge Method
Where circular tube is placed vertically on a plate with an orifice in the center.The height of the remaining powder against the wall of the tube is measured at eight equidistant points around the circumference to determine the angle of repose.

Dynamic Angle of Repose
Determine in the apparatus consisting of a drum with a roughened internal surface that is half filled with powder and slowly rotated around its horizontal axis.Within a certain range of rotation speeds (usually from 2.5 to 6 rpm) the surface of the powder in the drum comes to a sufficient steady condition.The maximum angle of bed inclination just before slump occurs is designated as the dynamic angle of repose.


References:

2 comments:

  1. Thanks, for such a great post. I have tried and found it really helpful. For more details to visit glidant used in tablets

    ReplyDelete