Control Variables

Particle size is a property that can relate to both individual particles and to the bulk, while shape is principally a particle property. Most bulk solids consist of many particles of different sizes, randomly grouped together to form a bulk. For some purposes a single linear dimension, as a representative value of particle size, may be all that is required to specify a material. In other cases some form of distribution may also be necessary in order to give some indication of the size range of the particles constituting the bulk material.

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A spherical particle is clearly defined by its diameter and this is a meaningful parameter. The general definition of particle size, however, is neither straightforward nor unique. Irregular particles may have a diameter defined in terms of a three-dimensional equivalence, such as:

• the diameter of a sphere having the same surface area,

• the diameter of a sphere having the same volume or mass,

• the size of a hole (circular or square) through which the particle will just pass.

Alternatively the equivalent diameter could be defined in terms of a two-dimensional equivalence, such as:

• the diameter of an inscribed circle,

• the diameter of a circumscribed circle,

• the diameter of a circle with the same perimeter.

There are also statistical diameters, such as:

• Feret‟s diameter, which is the distance between the tangents to extremities of the particle, measured in a fixed direction;

• Martin‟s diameter, which is the length of the line, in a fixed direction, that divides the particle seen in three dimensions into two equal areas.

A size distribution can be obtained by submitting a representative sample of a bulk solid to a particle size analysis. This relates the distribution of the particle size fractions that comprise the bulk. Two methods of presenting the data are commonly used. One is a cumulative plot and the other is a fractional plot. Both linear and logarithmic plots are also used for the particle size axis.

Materials and Method:

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The feedstock for cassava flash drying is dewatered cassava mash. This requires size reduction from the tuber to the mash and this is usually done by grating. The size reduction of cassava tubers by grating is affected by a lot of parameters which includes the height of the rasp above the rasp sheet, the number of rasps per unit area, and the clearance between the rasp sheet and the backing plate. Other parameters like the grating drum diameter, length, speed and feed pressure affects the throughput of the grating process (Otuu Obinna et al, 2009). This brings to the fore the problem of varied particle size as there is indeed no attempt at standardising these parameters.

This is important because large variations in the size distribution of the feedstock will alter the thermodynamic balance of the drying process and definitely the expected final product moisture content. If the distribution shifts to a predominantly lower particle size than the designed size, there will be over reduction of the moisture content which translates to a waste of energy and if the converse is the case the expected moisture reduction will not be achieved. In the light of the above, this paper shall determine the particle size of grated and dewatered mash by sieving.

After the material was rasped, it was then dewatered as a pre-drying operation, to reduce the moisture content from about 70% to between 30 - 40% before flash drying.

Cassava mash dewatering parameters were identified and the work evaluated the influence of cassava age on these parameters. They reported that the moisture content was reduced as the cultivar ages because of the presence of fibre which offers resistance to compression. Cassava cultivar TMS 4(2) 1425 has the best garification properties based on an IITA report (IITA, 1987). Cassava was pressed at a pressure of 48.3 kN/m² over a platen area of 0.0707 m² to arrive at a moisture content of 43.3%.

Garification process requires some level of moisture but in the case of flour the

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intention is to reduce the moisture so that drying can be more efficient. The dewatering should be carried as far as is possible and economically viable to reduce the moisture content as low as possible. This will reduce the moisture-load that the needs to be removed during flash drying. This also brings to the fore the arbitrariness in the design of dewatering presses. There is no information on the force per area required to reduce mechanically the moisture content of cassava (TMe 419) from values A% to B%. However, for the purpose of this work the mash was pressed to a pressure that enabled a change of the initial moisture content of the mash from 61% to 45% moisture content.

The pressed cake is consolidated by the pressure used in dewatering, and for the material to be fed into the sieve, the cake must be broken. This is achieved by passing the cake through the same grater that was used for size reduction. It is at this point that the particle size analysis was then carried out.

Sample Collection and Preparation.

The selected cultivar, TMe 419 was obtained from National Root Crop Research Institute, (NRCRI) Umudike, Abia state. This was for the accurate determination of the cultivar and its age. The cultivar was peeled, washed, grated and bagged for pressing. The sample was dewatered in a press by subjecting it to a pressure that reduced the moisture content of the consolidated cake from 61% to 45% moisture content. The consolidated cake was subsequently broken down by passing it through a grater. A particle size distribution was determined using the sieve method.