Mass transfer operations_size reduction.pptx

Description on size reduction techniques

1. Mass transfer operations

2. Size reduction: In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area. I n most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. The rate of combustion of solid particles is proportional to the area presented to the gas. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. In the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance the moisture must travel within the particles in order to reach the surface. In this case, the capillary forces acting on the moisture are also affected.

3. Size reduction: First mode called as Transitory mode. Second mode is characteristic of material and called as persistent mode. Grinding limit.

4. The method of application of the force to the particles may affect the breakage pattern . There are four basic patterns identified , though it is sometimes difficult to identify the dominant mode in any given machine. Impact —particle concussion by a single rigid force. Compression —particle disintegration by two rigid forces. Shear —produced by a fluid or by particle–particle interaction. Attrition —arising from particles scraping against one another or against a rigid surface .

5. Energy law: Three rate law: Rittinger law, Kick’s Law and Bond law.

6. Energy law: Neither of these two laws permits an accurate calculation of the energy requirements. Rittinger’s law is applicable mainly to that part of the process where new surface is being created. It holds most accurately for fine grinding where the increase in surface per unit mass of material is large. Kick’s law, more closely relates to the energy required to effect elastic deformation before fracture occurs. It is more accurate than Rittinger’s law for coarse crushing where the amount of surface produced is considerably less .

7. Energy law: Bond suggested law intermediate between Rittinger and Kicks law by putting p=-3/2. If L 1 and L 2 are in mm, E i is called as work index. It is expressed as the amount of energy required to reduce unit mass of material from an infinite particle size to a size L 2 of 100 μm , that is q =∞. The size of material is taken as the size of the square hole through which 80 per cent of the material will pass.  

8. What will be energy required to crush limestone from feed (of which 80% passes through 2 inch screen) to product (of which 80% passes through 1/8 inch screen). Work index of limestone is 12.74.

9. Energy utilization: Energy supplied to a crusher is utilised as follows: In producing elastic deformation of the particles before fracture occurs. In producing inelastic deformation which results in size reduction. In causing elastic distortion of the equipment. In friction between particles, and between particles and the machine. In noise, heat and vibration in the plant, and In friction losses in the plant itself. Only about 10 per cent of the total power is usefully employed .

10. Breakage distribution function: Here, n(v) is the number density function v and v’ are particle size S(v) is the specific rate of breakage β ( v,v ’) is the number-based breakage distribution function.

11. Size reduction equipment:

12. Hammer mill: The hammer mill is an impact mill employing a high speed rotating disc, to which are fixed a number of hammer bars which are swung outwards by centrifugal force.

13. Roll crusher: The material is crushed by compression and shearing between the two surfaces. It is used extensively for crushing coal .

14. Ball mill: In its simplest form, the ball mill consists of a rotating hollow cylinder, partially filled with balls, with its axis either horizontal or at a small angle to the horizontal. The material to be ground may be fed in through a hollow trunnion at one end and the product leaves through a similar trunnion at the other end.

15. Reference: Unit Operations Of Chemical Engineering By Julian Smith,warren Mccabe,peter Harriott . Coulson And Richardson’s Chemical Engineering Volume 2 Particle Technology And Separation Processes.