INTRODUCTION TO BULK SOLID

CHAPTER 1

PROPERTIES OF BULK SOLID

1.1.3

1.2.1 Aerated bulk density, ρabd

• The particles are separated from each other by a film of air and are not in direct contact with each other.

• Bulk density after the powder has been aerated.• Some of the methods in obtaining the aerated

density:

1.2.1.1 Poured bulk density, ρpbd

• Powder is pored into a container• Height of fall is fixed• The powder in the container must not be disturbed• Mass of powder is determined• Volume of powder is the volume inside the cup.

1.2.1.2 Tap bulk density, ρtbd

• Bulk density of a powder, which has been compacted by tapping or vibration following a specific procedure.

1.3 Voidage, or porosity• The volume of the voids within the bed. i.e. the

volume occupied by air divided by the total (overall) volume of the bed.

• Void volumes – includes the pores within the particles if they are porous.

1.4 Powder Flowability Determination

• No flowability test is universally applicable. • Thus series of powder flow behaviour must be

tested experimentally according to the need of industrial application such as powder streams in industrial storage, transport and processing, in terms of their range of applicability, cost and difficulty of measurement.

• Since the state of the powder varies for different types of applications, the test chosen has to reflect the state of powder in the actual process.

• Below are some of the tests commonly required in determining flow behaviour in selected process plant

1.5 Compressibility (from bulk density)

• Compression tests are mainly used for quality control, product evaluation and to measure the tendency for caking.

• One of the way to express differences between tap density and aerated density:

1.6 Angle of Repose

• Definition: the angle of the free surface of a pile of powder to the horizontal plane.

• Whenever a heap of powder is being formed, the angle of inclination of the free surface to the horizontal can take any value up to a maximum.

• Any attempt to build up a heap with steeper sides results in its collapse.

• Depending on the condition under which the pile has been poured and how the angle is measured, different values of angle can be obtained for the same powder.

• When a mixture of uniformly sized granular particles consisting of components with different angle of repose is poured on a heap, the particles having steeper angle of repose tend to concentrate in the center of the heap.

1.7 Hausner Ratio, HR

• The ratio of ρtbd/ ρabd is called the Hausner ratio, HR (Grey and Beddow 1968/1979).

• If the ratio:– HR >1.4 : Group C– 1.25 < HR < 1.4 : Transition AC– HR < 1.25 : Group A, B or D

• Other available types of test such as:– Funnel/discharge est– Fluidization index, deaeration, permeability– Floodibility test– Dispersibility– Pick up velocity– Pseudoviscosity and suspension-flow behaviour index etc.

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