Size reduction and Screening of Solids

Size reduction and Screening of Solids

SHAHJALAL UNIVERSITY OF SCIENCE & TECHNOLOGY SYLHET-3114Department of Food Engineering and Tea Technology06-Jan-11Book NameFood Engineering Operations3rd EditionWriterJ.G.BRENNAN, J.R.BUTTERSN.D.COWELL, A.E.V. LILLEY

CHAPTER 4 SIZE REDUCTION AND SCREENING OF SOLIDS

4.1GENERAL PRINCIPLES4.1.1. INTRODUCTIONSize reduction is the breakdown of solid material through the application of mechanical forces, is a frequent requirement in many food processing operations. The reasons for size reduction are varied. Size reduction may aid the extraction of a desired constituent from a composite structure, e.g. flour from wheat grains or juice from sugar cane. Reduction to a definite size range. A decrease in particle size of a material leads to an increase in surface of the solid. This increase in surface is of assistance in many rate processes, e.g. The drying time for moist solids is much reduced by increasing the surface area of the solid. The rate of extraction of a desired solute is increased by increasing the contact area between solid and solvent. Process time required for certain operations---cooking, blanching etc. ---can be reduced by cutting, shredding or dicing the process material.Intimate mixing or blending is usually easier with smaller size ranges of particles.4.1.2 NATURE OF FORCES USED IN SIZE REDUCTIONThe types of force redominating in some of the mills in common use in the food industry are summarized as follows: Force Principle Machine

CompressiveCompression (nutcracker) Crushing rolls

ImpactImpact (hammer) Hammer mill

Shear (attrition)Attrition (grindstone) Disc attrition mill

Compressive forces are used for the coarse crushing of hard materials. Impact forces can be regarded as general purpose forces and are used for coarse, medium and fine grinding of a variety of food materials. Attrition or shear forces are extensively used in machines for the communication of softer, non-abrasive, materials in the smaller size ranges, i.e. in fine grinding.4.1.3. REDUCTION RATIO (R.R.)The ratio, Average size of feed/Average size of product

is known as the reduction ratio and can be of use in predicting the likely performance of a particular machine. Coarse crushers have size reduction ratios of below 8:1, but for fine grinding, ratios as high as 100:1 can be realized. Much depends on the particular machine and feed material. The values for average size of feed and product depend on the method of measurement. 4.2. SIZE REDUCTION EQUIPMENT Machines of various types and sizes are available for the size reduction of food materials. Larger types of coarse crushers such as aw crushers and gyratory crushers are not normally encountered in the food industry. The more common types of machines used are discussed below.4.2.1. CRUSHING ROLLSIn this machine, two or more heavy steel cylinders revolve towards each other (fig 2). Particles of feed are nipped and pulled through the rolls, experiencing a compressive force with crushes them. Its speed is usually 50-300 rpm. Size reduction ratios are low-usually below 5. The diameter of the rolls, speed of the rolls, and the nip, the spacing between the rolls can be varied.4.2.1.1 Angle of nipIf A is the angle of nip, Df the average diameter of feed particles, Dp the average diameter of the product particles, Dr the diameter of the rolls, it can be shown that, .. (1)For the limiting case when the particle is just pulled into the rolls by friction: . (2)where is the coefficient of fraction between the particle and the rolls.4.2.1.2. Capacity of crushing roilsFor a machine with roll diameter Dr m, and length of face l m, when the nip is Dp m and the roll speed is N rpm, the volumetric capacity (Q) is given by, .. (3)

4.2.2. HAMMER MILLThis type of impact, or percussion, grinder is common in the food industry (Fig: 3). A high speed rotor carries a collar bearing a number of hammers around its periphery. When the rotor turns, feed passes into the action zone where the hammers drive the material against the breaker plate. Reduction is mainly due to impact forces, although attrition forces can also play a part in the size reduction. The hammers are often replaced by cutters or by bars as in the beater bar mill. The hammer mill may be regarded as a general purpose mill, vegetable matter, sticky materials, etc. in the food industry it is extensively used for grinding peppers and spices, dried milk, sugars, etc. due to excessive wear, impact mills are not recommended for the fine grinding of very hard materials.4.2.3. DISC ATTRITION MILLSMills utilizing attrition or shear forces for size reduction play a major part in fine grinding. They are two types (Fig. 4(a) & (b)):4.2.3.1. Single disc millIn this device the feed stock passes into a narrow gap between a high speed, rotating grooved disc and the stationary casing of the mill. Intense shearing action results in comminuting of the feed. The gap is adjustable, depending on feed size and product requirements.4.2.3.2. Double disc millIn this modification the casing contains two rotating discs. The discs rotate in opposite directions giving a greater degree of shear. This type of disc attrition mill is widely used in cereal preparation, corn and rice milling.4.2.3.3. Buhr millThis is an older type of disc attrition mill, originally used in flour milling. Two circular stones are mounted on a vertical axis. The upper stone, which is often fixed, has a feed entry port (Fig. 5). The lower stone rotates. Feed material passes to the gap between the upper and lower stones. The material, after subjection to the shearing force developed between the stones, is discharged over the edge of the lower stone. In some models both stones rotate, in opposite directions. In modern machines, toughened steel stones are replacing the traditional natural or composition stones. This type of mill is still being used in the wet milling of corn for the separation of starch gluten from the hulls.

4.2.4. TUMBLING MILLSA mill extensively used in many process industries for fine grinding is the tumbling mill. Two basic types, the Ball mill and the Rod mill, are available.4.2.4.1. Ball millsIn the ball mill (Fig. 6) both shearing and impact forces are utilized in the size reduction. In this mill, a rotating shell contains a lot of balls. When feed is given in the rotating shell and rotate it, the collision between two balls and shell occur. Because of these processes the feed is reduced in size. Ball sizes are usually in the range 25-150 mm (1-6 in). Small balls give more point contacts but larger balls give greater impact.

4.2.4.2. Rod millsIn the rod mill, the balls are replaced by high carbon steel rods. Impact and attrition forces still play a part but the effect of impact forces is less pronounced.

4.3. MODES OF OPERATION OF SIZE REDUCTION PLANT4.3.1. OPEN CIRCUIT GRINDINGThis is the simplest method of operating a mill (Fig. 7). Feed enters the mill, passes through the action zone and is discharged as a product. No recycle of oversize particles is possible.

4.3.2. FREE CRUSHINGWith this method of operation residence time in the action zone is kept short. This is commonly accomplished by allowing the feed material to fall through the action zone under gravity.

4.3.3 CLOSED CIRCUIT GRINDINGThe method of operation is shown diagrammatically in (Fig. 7(c)). The residence time of material in the mill is kept short, either by gravity fall, or by rapid transport through the action zone in a gas or liquid stream. In these systems recycle of oversize particle is possible. 4.4 ENERGY REQUIREMENTS FOR COMMINUTION OF SOLIDSLittle fundamental work on the energy consumption in such disintegration operations as cutting, shredding and dicing has been carried out. Somewhat more is known of the breakdown of friable materials. Two stages of breakage are recognized:(1) Initial fracture along existing fissures or cleavage planes in the body of the material.(2) The formation of new fissures or crack tips, followed by fracture along these fissures.4.4.1. RITTINGERS LAW (1867)Rittinger considered that for the grinding of solids, the energy required should be proportional to the new surface produced and put n=2.Then,

Or, integrating

X1 is the average initial feed size, x2 is the average final product size. E is the energy per unit mass required for the production of this new surface and is usually measured in horsepower hour ton-1. K is called Rittingers constant and is a constant for a particular machine and material. Rittingers law has been found to hold better for fine grinding, where a large increase in surface results.4.4.2. KICKS LAW (1885)Kick considered that the energy required for a given size reduction was proportional to the size reduction ratio, which requires that n=1.Then, Or, X1/X2 being the size reduction ratio. Kicks law has been found to hold more accurately for coarser crushing where most of the energy is used in causing fracture along existing fissures. It gives the energy required to deform particles within the elastic limit. For many crushing operations the energy requirement suggested by Kicks law appears to be too low, whereas that required by Rittingers equation appears to be excessive.4.4.3. F.C. BOND (1952) - THIRD LAW OF COMMINUTIONIn Bonds work, n takes the value 3/2 giving Or, When x1 and x2 are measured in micrometers and E in kWh short ton-1 (907.16 kg), K=5EiWhere Ei is the Bond Work Index- the energy required to reduce unit mass of material from an infinite particle size to a size such that 80% passes a 100 micrometer sieve. The Bond Work Index is obtained from laboratory crushing tests on the feed material. The third theory holds reasonably well for a variety of materials undergoing coarse, intermediate and fine grinding.4.4.4. F.M.( Finess modulus)