Unit operations and unit processes
• At the end of this unit you should be able to: a. List the various reasons for undertaking size reduction and enlargement in the chemical industry b. Describe the operation principles of some size reduction equipment and size enlargement equipment c. Explain how industrial materials can be separated on the basis of their magnetic, electrostatic, hydrophobic and volatility differences respectively d. Discuss various organic unit processes including polymerization, alkylation, and hydrolysis and their application in the production of organic chemicals
Unit operations involve physical transformations while unit processes consist of chemical conversions. In this unit, we want to study the purposes and operating principles of common unit operations and unit processes, The Learning Activity includes: Size reduction and size enlargement, magnetic and electrostatic separation, froth flotation, fractional distillation, other unit operations, polymerization, alkylation, hydrolysis and other unit processes.
• Deals with size reduction, size enlargement and separation of mixtures- look/understand the operation principles of equipment in these unit operations and their application in the chemical industry.
Size reduction refers to all the ways in which particles are cut or broken into smaller pieces. The objective is to produce small particles from big ones for any of the following reasons: 1. To reduce chunks of raw materials to workable sizes e.g. crushing of mineral ore. 2. To increase the reactivity of materials by increasing the surface area. 3. To release valuable substances so that they can be separated from unwanted material. 4. To reduce the bulk of fibrous materials for easier handling. 5. To meet standard specifications on size and shape. 6. To increase particles in number for the purpose of selling. 7. To improve blending efficiency of formulations, composites e.g. insecticides, dyes, paints
Principles of size reduction Most size reduction machines are based on mechanical compression or impact. When a solid is held between two planes and pressure is applied on one plane, the solid is fractured and breaks into fragments when pressure is removed. The fragments formed are of different sizes. E.g a jaw crusher. Impact is the breaking up of material when it is hit by an object moving at high speed. The product contain coarse and fine particles. A ball mill is based on impact.
Ball mill Jaw Crusher
ball mill in a Cement Plant
Size Enlargement (Agglomeration)
Size enlargement, also referred to as agglomeration, is carried out when particles are too small for use in a later stage of the process. For example in metal extraction, some particles may be too fine to be fed into a blast furnace.
Purposes of size enlargement
The following are some of the purposes of size enlargement in various industries: 1. Reduce dusting losses 2. Reduce handling hazards particularly with respect to irritating and obnoxious powders. 3. Render particles free flowing. 4. Densify materials. 5. Prevent caking and lump formation 6. Provide definite quantity of units suitable for metering, dispensing and administering
Purposes of size enlargement
7. Produce useful structural forms 8. Create uniform blends of solids which do not segregate 9. Improve appearance of products 10. Permit control over properties of finely divided solids e.g. solubility, porosity, surface volume ratio, heat transfer 11. Separate multicomponent particle size mixtures by selective wetting and agglomeration 12. Remove particles from liquids In size enlargement, small particles are gathered into larger
Separation Of Materials
how differences in the physical properties of materials are used to separate mixtures in the chemical industries. • Magnetic Separation If a mixture containing magnetic materials and nonmagnetic materials is subjected...
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