unit operation solid handling lecture one

unit operation solid handling lecture one

1. Unit Operations in Chemical Engineering Course No. ChE 3107 Course Code: Unit Operations in Chemical Engineering Lecture No. 01 Maisa Rahman Lecturer Department of Chemical Engineering Rajshahi University of Engineering & Technology Email: [email protected] Contact No. 01926997916

2. Reference Books Unit Operation – I (fluid flow and mechanical operations) by K A Gavhane Drive Link: ( https://drive.google.com/file/d/1716Tdmn-suLEHqJkpo4h0WIoNJrBhyz9/view?usp=sharing ) Unit Operations of Chemical Engineering, 5 th edition, by McCabe & Smith Drive Link: ( https://drive.google.com/file/d/1QLU3MtEZDx7vMEHud3IyOLwskMuxvfjm/view?usp=sharing ) Maisa Rahman, ChE, RUET

3. Course Contents Maisa Rahman, ChE, RUET

4. Size Reduction Maisa Rahman, ChE, RUET What is the meaning of size reduction ? Size refers to physical Dimension of an object. Reduction refers to decrement or the process of decreasing the size. Size reduction is the operation carried out for reducing the size of bigger particles into smaller one of desired size and shape with the help of external forces. Definition: Size reduction or ‘comminution’ is the unit operation in which the average size of solid pieces is reduced by the application of grinding, compression or impact forces. It involves creating smaller mass units from larger mass units of the same material.

5. Size Reduction Maisa Rahman, ChE, RUET

6. Size Reduction Objectives of Size Reduction are – Increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Break a material into very small particles in order to separate the valuable amongst the two constituents. Achieve intimate mixing. To dispose solid wastes easily . To improve the handling characteristics. To mix solid particle more intimately. Maisa Rahman, ChE, RUET

7. Size Reduction - Advantages Uniformity of the contents Uniform flow Effective drying Uniform mixing and drying Improves rate of absorption . Smaller the particles greater is absorption. Improves dissolution rate. Increases the specific surface (surface-area-to-volume ratio); this facilitates several processes, such as heat exchange, extraction, chemical and biological reactions Maisa Rahman, ChE, RUET

8. Size Reduction - Disadvantages Increased energy requirement (e.g., the finer the size reduction, the higher the energy requirement). The cost of equipment increases significantly with the size reduction and the cutting precision. Quality degradation, like food, drug, etc. Higher Contamination chances. Maisa Rahman, ChE, RUET

9. Size Reduction - Mechanism Maisa Rahman, ChE, RUET Fig. Stress vs Strain Diagram for various materials Here, E, elastic limit; Y, yield point; B, breaking point; O-E, elastic region; EY, inelastic deformation; Y-B, region of ductility. & Hard, strong, brittle material. Hard strong ductile material. Soft, weak, ductile material. Soft, weak, brittle material.

10. Size Reduction - Mechanism 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. Larger particles 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. Maisa Rahman, ChE, RUET

11. Size Reduction - Principle To bring about a required reduced size of a solid substance, the larger mass units need to be subjected to stress by the application of force. Three types of force may be applied, i.e.: i. Compression: 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. Impact : Impact forces are used to mill a wide variety of materials, including fibrous types of solid materials. Shear force: Shear forces are best applied to relatively soft materials, again including fibrous types of solid materials. 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 mills. Maisa Rahman, ChE, RUET

12. Size Reduction - Principle The main kind of forces applied in size reduction are- Compression Shear or attrition Impact Cutting The application of forces applied: Pressure and/or friction on material placed between tool surfaces Shear force on material Collision between particles or impact between particles and tools Friction through medium surrounding the particles Maisa Rahman, ChE, RUET

13. Size Reduction - Mechanisms Impact —particle concussion by a single rigid force (hammer). Compression —particle disintegration by two rigid forces (nutcracker). Shear —produced when the particle is compressed between the edges of two hard surfaces moving tangentially. Attrition —arising from particles scraping against one another or against a rigid surface (a file). Maisa Rahman, ChE, RUET

14. Size Reduction – Machines Maisa Rahman, ChE, RUET

15. Size Reduction – Machines Ball Mill: Maisa Rahman, ChE, RUET

16. Size Reduction – Machines Maisa Rahman, ChE, RUET Fig. Schematic diagram of ball mill

17. Size Reduction – Machines Hammer Mill: Size reduction is achieved by Impact. Feed the material into the hammer mill through the hopper and press the start button. The Hammer assembly starts to rotate (with 8000 to 15000 RPM speed), and particle size reduces into small particles due to dynamic impact. After the reduction of particles falling within the container through the screen. Hammer mills are of two types , Horizontal shaft (Moves horizontally)and vertical shaft (Moves Vertically) types. They differ only by moving conditions, although the remaining things are the same . Maisa Rahman, ChE, RUET

18. Size Reduction – Machines Maisa Rahman, ChE, RUET Fig. Schematic diagram of hammer mill

19. Size Reduction – Machines Advantages of hammer mill It produced rapid action in grinding Hammer also useful for hard particles East and Fast to use Required less space and is easy to dismantle, and clean Disadvantages of hammer mill Produced more sound. Be careful to use heat-sensitive materials because the hammer produced more heat. Can’t be used to grind fiber materials Maisa Rahman, ChE, RUET

20. Size Reduction – Machines Roller Mill: Maisa Rahman, ChE, RUET

21. Size Reduction – Machines Attrition Mill: The mill is equipped with an outer stationary milling disc and an inner rotating milling disc that adjusts to vary the gap between the discs. Material enters the gap through the center of the outer disc, and is ground between the discs as it travels radially by centrifugal force created by the rotating disc, with greater reductions achieved by reducing the gap. Precise machining and alignment of the discs, together with constant spring pressure, maximize size uniformity of the reduced material. Plate patterns are selected to optimize the reduction for specific types of materials. Capacity is generally dependent on mesh size of the reduced material, with coarser products produced at higher rates. Quick-release grinding disc separation allows for thorough wash down between batches. Single runner mills come equipped with access doors for clean out and maintenance accessibility. Maisa Rahman, ChE, RUET

22. Size Reduction – Machines Maisa Rahman, ChE, RUET

23. Factors Affecting Size Reduction 1. Hardness: It is a surface property of the material. It is frequently confused with a property named strength. Thus, it is possible for a material to be very hard, but if it is brittle also then size reduction may present no special problems. An arbitrary scale of hardness has been devised known as Moh’s Scale; Moh’s Scale = 1 is for graphite Moh’s Scale < 3 is for soft material Moh’s Scale > 7 is for hard material Moh’s Scale = 10 is for diamond The harder the material the more difficult it is to reduce in size Maisa Rahman, ChE, RUET

24. Factors Affecting Size Reduction 2. Material structure: Some substances are homogeneous in character. Mineral substances may have lines of weakness along which the materials splits to form flake-like particles. 3. Abrasiveness: Abrasiveness is a property of hard materials (particularly those of mineral origin). It may limit the type of machinery that can be used. During the grinding of some very abrasive substances the final powder may be contaminated with more than 0.1 percent of metal worn from the grinding mill. Maisa Rahman, ChE, RUET

25. Factors Affecting Size Reduction 4. Softening temperature: During size reduction process sometimes heat is generated which may cause some substances to soften, and the temperature at which this occurs can be important. Waxy substances, such as stearic acid, or drugs containing oils or fats are examples that may be affected. Some methods can be used to overcome this like cooling the mill, either by a water jacket or by passing a stream of air through the equipment. Maisa Rahman, ChE, RUET

26. Factors Affecting Size Reduction 5. Moisture content: It is found that materials do not flow well if they contain between about 5 and 50 per cent of moisture. Under these conditions the material tends to cake together in the form of balls. 6. Crushing strength: The power required for crushing is almost directly proportional to the crushing strength of the material. Maisa Rahman, ChE, RUET

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