Crop Process Engineering

Lesson 13. Principle of Size Reduction

 13.1 Necessity of size reduction

In process industries, this operation is usually carried out in order:

a) To increase the surface area to enhance the rate of a physical or chemical process. In most of the reactions and unit operations (e.g., leaching) involving solid particles, the rate increases by increasing the area of contact between solid and second phase     since the rate is proportional to the area of contact between the phases involved. In leaching, the of extraction increases because of the increased area of contact between solid and the solvent.

b) To effect the separation of two constituents in cases where one is dispersed in small isolated pockets.

c) To meet stringent specifications regarding the sizes of commercial products.

d) To accomplish intimate mixing of solids in a solid-solid operation since the mixing is more complete if the particle size is small.

e) To improve dissolution rate, solubility, binding strength and dispersion properties.

 13.2 Size reduction procedures

 In comminution of food products, the reduction mechanism consists of deforming the food piece until it breaks or tears and such breaking may be achieved by applying diverse forces. The types of forces commonly used in food processes are compression (crushing), impact, attrition or rubbing, cutting and shearing. In a comminution operation, more than one type of force is usually acting. For example, crushing, grinding, and milling take place in powdered sugar, flour, mustard, and cocoa production.

Table 13.2 Types of forces used in size reduction equipments

Crushing: when an external force applied on a material excess of its strength, the material fails because of its rupture in many direction.  The particles produced after crushing are irregular in shape and size. The type of material and method of force application affects the characteristics of new surfaces and particles. For examples:  Food grain flour, grits and meal, ground feed for livestock are made by crushing process. Extraction of oil from oilseeds and juice from sugarcane are also by crushing process.

 Impact: When a material is subject to sudden blow of force in excess of its strength, it fails. For example, cracking of nut with help of a hammer.

 Cutting: Size reduction is accomplished by forcing a sharp and thin knife through the material. In this process, minimum deformation and rupture of the material results and the new surface created is more or less undamaged. For example: Cutting of fruits and vegetable by sharp knife to reduce the size.

 Attrition:  Rubbing away or wearing down by friction. The material is pulverised between two toothed metal disks rotating in opposite directions.

 Shearing: It is a process of size reduction which combines cutting and crushing. The shearing unit consists of a knife and a bar. If the edges of knife or shearing edge is thin enough and sharp, the size reduction process nears to that of cutting, whereas a thick and dull shearing edge performs like a crusher. In a good shearing unit, the knife is usually thick enough to overcome the shock resulting from material hitting. In an ideal shearing unit the clearance between the bar and the knife should be as small as practicable and the knife as sharp and thin as possible.

 In general, compression is used for the coarse reduction of hard solids (to yields relatively few fines), impact gives coarse, medium or fine products, attrition gives very fine products from soft, non-abrasive materials and cutting produces a product of a definite particle size and sometimes a definite shape, with few or no fines.

 In Food processing industry, size reduction operation is carried out for sugar, spices, grains etc.

 13.3 Mechanical Resistance Involved in Size Reduction

 Mechanical resistance refers to all the properties that describe the behaviour of a solid material as it deforms and breaks under the influence of an applied stress (Loncin and Merson, 1979). The deformation of a certain food material can be elastic when the applied stress remains below a limiting value, or inelastic. The material experiences elastic deformation when it returns to its original shape when the force is removed. If the stress exceeds the elastic limit, the material undergoes permanent (inelastic) deformation until it reaches the yield point when it begins to flow (region of ductility) under the action of the applied stress until it finally breaks. This process defines the elastic stress limit, yield stress, breaking stress, and the region of ductility.

E is elastic limit, Y is yield point, B is breaking point, OE is elastic region, EY is elastic deformation, and YB is region of ductility. Different curves are represented for different types of material depending on their mechanical behaviour: material (1): hard, strong, and brittle; material (2): hard, strong, and ductile; material (3): hard, weak, and brittle; material (4): soft, weak, and ductile; material (5): soft, weak, and brittle. (Loncin and Merson, 1979)

The breaking stress or ultimate stress is a property of the material. Breaking occurs along cracks or defects in the piece structure. A large piece with many defects can be broken with a small stress with very little deformation. Smaller pieces have fewer defects remaining and will need a higher breaking strength. In the limit of very small particles, purely intermolecular forces must be overcome. This is why grinding is so difficult to achieve below a certain size. For example, fine grinding of roasted coffee (e.g., to less than 50 μm) is best recommended under cryogenic conditions (i.e., subzero temperatures) in order to accomplish the desired grinding efficiency. The elastic state is described by Hooke’s law ([stress] = E · [strain]). The most important characteristic is the modulus of elasticity E, which is the stress causing a unit change in length in the same direction as the applied force. The Poisson coefficient or bulk modulus permits prediction of the transverse contraction or expansion that occurs when a stress is applied longitudinally. Inelastic behavior is defined for stresses greater than the elastic limit but smaller than the breaking limit. The behavior is described by visco-elastic models (such as the Maxwell model or the Kelvin model), which combine elements of inelastic behaviour and elements of viscous flow. Stress can be applied to the particle in a variety of ways such as traction, compression, or shear (or combination of those methods). Although the stress limits are not the same for these different modes, there is clearly a relation among them. Consequently, it is often possible to use a compression test as an indication of the breaking load under tension. Since breaking occurs along cracks, in some materials the breaking point measured by compression is usually higher than when measured by traction; tension enhances the cracks, whereas compression tends to close them up.

 Fig. 12.1 represents the stress characteristics of materials. A food may be hard or soft; increased hardness is correlated with an increase in the modulus of elasticity. A strong material possesses a high elastic stress limit; and a weak material has a low elastic limit. Brittleness is a measure of the size of the region of ductility, and a brittle material breaks soon after the stress exceeds the yield stress. Conversely, a ductile material can deform considerably without breaking. A further property is toughness. A tough material has the ability to resist the propagation of cracks. Fibres impart toughness by relieving stress concentrations at the end of cracks. The opposite of toughness is fragility.

 13.4 Properties of comminuted products

 The structure and composition of the material to be processed greatly influence the size reduction mechanisms that can be employed and the equipment used. For example, a crystalline structure (such as sugar) will break along fracture planes which require compression (using a crushing technique) to bring about size reduction. If there are no fracture planes present then new cracks must be developed using impaction. On the other hand a fibrous structure, such as vegetable matter, suggests the need for cutting or shredding. Similarly, cutting is appropriate for ductile materials such as flesh foods such as meat.

 The presence of moisture can present problems in size reduction operations. Even small quantities of moisture on the surface of fine particles inevitably leads to the agglomeration of fines and therefore a size increase, although such agglomerates will be weak. More seriously, too high moisture content may lead to the rapid blockage of a mill. Equally, moisture can be useful in suppressing dust and preventing dust explosions and this is exploited in wet milling techniques for example in the milling of corn.

 In a comminution operation of food materials more than one type of the above-described forces is actually present. Regardless of the uniformity of the feed material, the product always consists of a mixture of particles covering a range of sizes. Some size reduction equipment is designed to control the size of the largest particles in its products, but the fine sizes are not under control. In spite of the hardness of the comminuted materials, the above-mentioned shape of produced particles would be subjected to attrition due to inter-particle and particle—equipment contacts within the dynamics of the operation. Thus, particle angles will smooth gradually, with the consequent production of fines. In actual practice, any feed material will possess an original particle size distribution while the obtained product will end with a new particle size distribution having a whole range finer than the feed distribution.

 In comminution practice, particle size is often referred to as screen aperture size. The reduction ratio, defined as the relation between average size of feed and average size of product, can be used as an estimate of the performance of a comminution operation. The values for average size of feed and product depend on the method of measurement, but the true arithmetic mean, obtained from screen analyses on samples of the feed and product streams, is commonly used for this purpose. Reduction ratios depend on the specific type of equipment. As a general rule, the coarser the reduction, the smaller the ratio. For example, coarse crushers have size reduction ratios of below 8:1, while fine grinders may present ratios as high as 100:1. However, large reduction ratios, such as those obtained when dividing relatively large solid lumps to ultra-fine powders, are normally attained by several stages using diverse crushing and grinding machines. A good example of this is the overall milling of wheat grain into fine flour, in which crushing rolls in series of decreasing diameters are employed.