FOOD ENGINEERING

Module 5. Food processing equipments and unit operations

Lesson 43
SIZE REDUCTION

43.1 Introduction

Size reduction of solids involves creating smaller mass units from larger mass units of the same material. To bring this about, the larger mass units need to be subjected to stress by the application of force. Three types of force may be applied, i.e. compression, impact and shear. Compressive forces are generally used for the coarse crushing of hard materials. Careful application of compressive forces enables control to be exercised over the breakdown of the material, e.g. to crack open grains of wheat to facilitate separation of the endosperm from the Bran. Impact forces are used to mill a wide variety of materials, including fibrous foods. Shear forces are best applied to relatively soft materials, again including fibrous foods. All three types of force are generated in most types of mill, but generally one predominates. For example, in most roller mills compression is the dominant force, impact forces feature strongly in hammer mills and shear forces are dominant in disc attrition

The extent of the breakdown of a material may be expressed by the reduction ratio, which is the average size of the feed particles divided by the average size of the products particles. In this context, the term average size depends on the method of measurement. In the food industry, screening or sieving is widely used to determine particle size distribution in granular materials and powders. In this case, the average diameter of the particles is related to the aperture sizes of the screens used. Size reduction ratios vary from below 8:1 in coarse crushing to more than 100:1 in fine grinding.

43.2 Objective

The breakdown of solid material through the application of mechanical forces is a frequent requirement is many food-processing operations. The reasons for size reduction are varied.

1. Size reduction may aid the extraction of a desired constituent form a composite structure, e.g. flour from wheat grains or juice from sugar cane.
2. Reduction to a definite size range may be a specific product requirement; e.g. as in the manufacture of icing sugar, in the preparation of spices and in chocolate refining.
3. A decrease in particle size of a given mass of material leads to an increase in surface of the solid. This increase in surface is of assistance in many rate process, e.g.

1. The drying time for moist solids is much reduced by increasing the surface area of the solid.
2. The rate of extraction of a desired solute is increased by increasing the contact area between solid and solvent.
3. Process time required for certain operation such as cooking, blanching, etc.-can be reduced by cutting, shredding or dicing the process material.
4. Intimate mixing or blending is usually easier with small size ranges of particles, an important consideration in the production of formulated packaged soups, cake mixes, etc.

The objective in many size reduction operations is to produce particles within a specified size range. Consequently, it is common practice to classify the particles coming from a mill into different size ranges. Again, screening is the technique most widely used for this purpose. To achieve a specified reduction ratio, it may be necessary to carry out the size reduction in a number of stages. A different type of mill may be used in each stage and screens employed between stages.

43.3 Process Description

When a solid material is subjected to a force, its behavior may be represented by a plot of stress versus strain, Some materials exhibit elastic deformation when the force is first applied. The strain is linearly related to stress (see curve 2 in Fig.43.1). If the force is removed the solid object returns to its original shape. Elastic deformations are valueless in size reduction. Energy is used up but no breakdown occurs. Point E is known as the elastic limit. Beyond this point, the material undergoes permanent deformation until it reaches its yield point Y.
Brittle materials will rupture at this point. Ductile materials will continue to deform, or flow, beyond point Y until they reach the break point B, when they rupture. The behaviour of different types of material is depicted by the five curves in Fig 43.1. and explained in the legend on the figure. The breakdown of friable materials may occur in two stages. Initial fracture may occur along existing fissures or cleavage planes in the body of the material. In the second stage new fissures or crack tips are formed and fracture occurs along these fissures. Larger particles will contain more fissures than smaller ones and hence will fracture more easily. In the case of small particles, new crack tips may need to be created during the milling operation. Thus, the breaking strength of smaller particles is higher than the larger ones. The energy required for particle breakdown increases with decrease in the size of the particles.
In the limit of very fine particles, only intermolecular forces must be overcome and further size reduction is very difficult to achieve. However, such very fine grinding is seldom required in food applications. Only a very small proportion of the energy supplied to a size reduction plant is used in creating new surfaces. Literature values range from 2.0% down to less than 0.1%. Most of the energy is used up by elastic and inelastic deformation of the particles, elastic distortion of the equipment, friction between particles and between particles and the equipment, friction losses in the equipment and the heat, noise and vibration generated by the equipment.

43.4 Energy Analysis

Mathematical models are available to estimate the energy required to bring about a specified reduction in particle size. These are based on the assumption that the energy dE required to produce a small change dx in the size of a unit mass of material can be expressed as a power function of the size of the material.Thus:

dE/dx = - K/xⁿ ……………. 1

Rittinger’s Law is based on the assumption that the energy required should be
proportional to the new surface area produced, i.e. n=2. So:

dE/dx = - K/x² …………… 2

or, integrating

E = K[1/x₂ – 1/x₁] ……….. 3

Where x₁ is the average initial size of the feed particles, x₂ is the average sizeof the product particles, E is the energy per unit mass required to produce this increase in surface area and K is a constant, known as Rittinger’s constant. Rittinger’s law has been found to hold better for fine grinding.
Kick’s Law is based on the assumption that the energy required should be proportional to the size reduction ratio, i.e. n = 1. So:

dE/dx = - K/x ………….. 4

or, integrating

E = - ln[x₁/x₂] ………. 5

Where is the size reduction ratio (see above). Kick’s law has been found to apply best to coarse crushing.
In Bond’s Law, n is given the value 3/2. So:

dE/dx = K/x^3/2 …………. 6

or, integrating :

E = 2 K[ 1/ (x ₂)^1/2 - 1/ (x ₁)^1/2 ]………… 7

Bond’s law has been found to apply well to a variety of materials undergoing coarse, intermediate and fine grinding.

43.5 Size-Reduction Equipment

a) Roller mill

A common type of roller mill consists of two cylindrical steel rolls, mounted on horizontal axes and rotating towards each other. The particles of feed are directed between the rollers from above. They are nipped and pulled through the rolls where they are subjected to compressive forces, which bring about their breakdown. If the rolls turn at different speeds shear forces may be generated which will also contribute to the breakdown of the feed particles .

b) Ball Mills

In the ball mill both shearing and impact forces are utilized in the size reduction. The unit consists of a horizontal, slow speed-rotating cylinder containing a charge of steel balls or flint stones. As the cylinder rotates balls are lifted up the sides of the cylinder and drop on to the material being comminuted, which fills the void spaces between the balls. The balls also tumble over each other, exerting a shearing action on the feed material. This combination of impact and shearing forces brings about a very effective size reduction. Ball sizes are usually in the range 1 – 6 inches. Small balls give more point contacts but larger balls give greater impact. As with all grinding mills, working surfaces gradually wear, so product contamination must be guarded against.
At low speeds of rotation the balls are not lifted very far up the walls of the cylinder. The balls tumble over each other and shear forces predominate. At faster speeds the balls are lifted further and the impact forces increase. Attrition and impact forces play a part in reduction. At high speeds the balls can be carried round at the wall of the mill under the influence of centrifugal force. Under these conditions grinding ceases. For efficient milling the critical speed should not be exceeded. This is defined as the speed at which a small sphere inside the mill just begins to centrifuge. It can be shown that the critical speed N_c in r.p.m., is given by:

N_c = 41.98 / Ö 2 R

Where, R is the radius of the mill, in m
In practice, the optimum operating speed is about 75% of the critical speed and should be determined under the plant operating conditions.

c) Impact Percussion Mill

When two bodies collide, i.e. impact, they compress each other until they have the same velocity and remain in this state until restitution of the compression begins. Then the bodies push each other apart and separate. If one of the bodies is held in position, the other body has to conform with this position for a short interval of time. During the very short time it takes for restitution of compression to occur, a body possesses strain energy which can lead to fracture. The faster the bodies move away from each other the more energy is available to bring about fracture.

d) Beater Bar mill

In this type of mill, the hammers are replaced by bars in the form of a cross. The tips of the bars pass within a small clearance of the casing. Beater bars are mainly used in small machines.

e) Comminuting Mill

Knives replace the hammers or bars in this type of mill. They may be hinged to the shaft so that the swing out as it rotates. Alternatively, they may be rigidly fixed to the shaft. Such mills are used for comminuting relatively soft materials, such as fruit and vegetable matter. In some designs, the knives are sharp on one edge and blunt on the other. When the shaft rotates in one direction the machine has a cutting action. When the direction of rotation of the shaft is reversed, the blunt edges of the knives act as beater bars .

f) Pin (Pin-Disc) Mill

In one type of pin mill a stationary disc and rotating disc are located facing each other, separated by a small clearance. Both discs have concentric rows of pins, pegs or teeth. The rows of one disc fit alternately into the rows of the other disc. The pins may be of different shapes; round, square or in the form of blades. The feed in introduced through the centre of the stationary disc and passes radially outwards through the mill where it is subjected
to impact and shear forces between the stationary and rotating pins. The mill may be operated in a choke feed mode by having a screen fitted over the whole or part of the periphery.

g) Fluid Energy Jet Mill

In this type of mill, the solid particles to be comminuted are suspended in a gas stream travelling at high velocity into a grinding chamber. Breakdown occurs through the impact between individual particles and with the wall of the chamber. The gases used are compressed air or superheated steam, which are admitted to the chamber at a pressure of the order of 700 kPa. An air-solids separation system, usually a cyclone, is used to recover the product. Particles up to 10 mm can be handled in these mills but usually the feed consists of particles less than 150 m. The product has a relatively narrow size range. Since there are no moving parts or grinding media involved, product contamination and maintenance costs are relatively low. However, the energy efficiency of such mills is relatively low.

h) Attrition Mills

The principle of attrition mills is that the material is rubbed between two surfaces. Both pressure and frictional forces are generated. The extent to which either of these forces predominates depends on the pressure with which both surfaces are held together and the difference in the speed of rotation of the surfaces.

i) Rod Mill

Grinding rods, usually made of high carbon steel, are used instead of balls in rod mills. They are 25–125 mm in diameter and may be circular, square or hexagonal in cross-section. They extend to almost the full length of the shell and occupy about 35% of the shell volume. In such mills, attrition forces predominate but impacts also play a part in size reduction. They are classed as intermediate grinders and are more useful than ball mills for milling sticky materials.

43.6 Selection Criteria for Size-reduction Equipment

a) Mechanical Properties of the Feed

Friable and crystalline materials may fracture easily along cleavage planes. Larger particles will break down more readily than smaller ones. Roller mills are usually employed for such materials. Hard materials, with high moduli of elasticity, may be brittle and fracture rapidly above the elastic limit. Alternatively, they may be ductile and deform extensively before breakdown. Generally, the harder the material, the more difficult it is to break
down and the more energy is required. For very hard materials, the dwell time in the action zone must be extended, which may mean a lower throughput or the use of a relatively large mill. Hard materials are usually abrasive and so the working surfaces should be made of hard wearing material, such as manganese steel, and should be easy to remove and replace.

b) Moisture Content of the Feed

The moisture content of the feed can be of importance in milling. If it is too high, the efficiency and throughput of a mill and the free flowing characteristics of the product may be adversely affected. In some cases, if the feed material is too dry, it may not breakdown in an appropriate way. For example, if the moisture content of wheat grains are too high, they may deform rather than crack open to release the endosperm. Or, if they are too dry, the bran may break up into fine particles which may not be separated by the screens and may contaminate the white flour. Each type of grain will have an optimum moisture content for milling. Wheat is usually ‘conditioned’ to the optimum moisture content before milling.
Another problem in milling very dry materials is the formation of dust, which can cause respiratory problems and fire and explosion hazard. In wet milling, the feed materials is carried through the action zone of the mill in a stream of water.

c) Temperature Sensitivity of the Feed

A considerable amount of heat may be generated in a mill, particularly if it operates at high speed. This arises from friction and particles being stressed within their elastic limits. This heat can cause the temperature of the feed to rise significantly and a loss in quality could result. If the softening or melting temperatures of the materials are exceeded the performance of the mill may be impaired. Some mills are equipped with cooling jackets to reduce these effects. Cryogenic milling involves mixing solid carbon dioxide or liquid nitrogen with the feed. This reduces undesirable heating effects. It can also facilitate the milling of fibrous materials, such as meats, into fine particles.