Partical Technology in Chemical Engineering

motivate Your self withb partical knowledge

1. Particle Technology (ChE-203) Lecture No. 11 1

2. Contents Mixing of Solids Types of mixtures Mixing Mechanism Types of Mixers 2

3. Mixing Random distribution, into and through one another, of two or more initially separated phases (components). 3 Segregation (separation) Action of setting something apart from others. One or more key components congregates or it condensed locally in microscopic sense. The mechanism by which mixing & segregation occur are identical in principle.

4. Segregation Desired - Separation Undesired - avoided when homogeneity is required In chemical, pharmaceutical, agricultural & smelting industries segregation is undesirable, as it causes serious problems such as: Uneven quality of products Fluctuation in weight Low mechanical strength Poor refractory material Low rate of contact & reaction In screening or other separations, segregation is favourable. 4

5. Mixing of Solids Mixing of solids is somehow similar to low viscosity liquids. Liquids blending depends on the creation of flow currents which transport the unmixed material to the mixing zone. Solids – no such currents, so mixing is done by other means. That's why large power requirement for pastes & dry solids (sand, cements, gravels etc.) Liquid – well-mixed product (homogeneous) Solids – identifiable phases 5

6. Types of Mixtures Perfect mixture (green sand+ white salt) Random mixture Segregated mixture 6

7. Perfect mixture In which a group of particles taken from any position in the mixture will contain the same proportion of each particle as the proportion present in whole mixture. 7

8. Random mixture A mixture in which the probability of finding a particle of any component is the same at all locations and equal to the proportion of that component in the mixture as a whole. 8

9. Segregated mixture Particles of one component have a greater probability of being found in one part of the mixture and so a random mixture cannot be achieved. 9

10. Mixing Mechanism Convective mixing (macro mixing) in which group of particles are moved from one position to another. Inversion of powder bed using blade/screw in which large mass of material moves from one place to another. Diffusive mixing (micro mixing ) where particles are distributed over a freshly developed interface. Involve the random motion of particle within powder bed, thereby particles change their position relative to one another. Shear mixing where slipping planes are formed. Force of attraction are broken down so that each particle moves on its own between region of different component and parallel to the surface. Shear and convective mixing can produce the rough mixtures but groups of particles remain unseparated unless subject to diffusive mixing. 10

11. Properties affecting solid mixing Density Particle shape Surface characteristics Flow characteristics Friability ( describes the tendency of a solid substance to break into smaller pieces under duress or contact, especially by rubbing) State of agglomeration Moisture or liquid content of solid Temperature limitation of ingredients 11

12. A well-mixed product Is the one that does what is required and has the necessary property such as visual uniformity, high strength, uniform burning rate, or other desired characteristics. A good mixer is the one that produces this well-mixed product at the lowest overall cost. 12

13. Stages during mixing (cake formation) 13 Addition of liquid As a result of reaction First step Product

14. Selection of mixers Depends upon the type of solids: Cohesive solids (Pastes, rubbers and plastic materials) ( honey+moblile oil) Dry/non-cohesive solids (dry powders, free-flowing powders) Intermediate properties of the material such as stiffness, tackiness (sticky) and wettability are as significant as viscosity and density. Mixers for pastes and plastic masses must be versatile. Mixer chosen must handle the material when it is in its worst condition. 14

15. Types of Mixers 15

16. For Dry Solids Mixing is by slow speed agitation of the mass with an impeller: By tumbling (fall, rise, beat) By centrifugal smearing and impact Mixers for dry solids are: Light in construction Moderate power consumption per unit mass of material mixed 16

17. For Cohesive Solids Cohesive solids – most difficult of all mixing problems Appropriate contact between material and agitator is required. Mixing is by combination of low speed: Shear Smearing Wiping Folding Stretching Compressing Mixers require high power consumption and cooling. 17

18. High viscosity – Different and powerful equipment Mixing elements cannot generate flow currents Shear, fold, stretch and compress the material to be mixed Change-Can mixers Kneaders Dispersers Masticators Continuous kneaders Mixer-extruders Mixing rolls Mullers and pan mixers Pugmills 18

19. Change-Can Mixers For viscous liquids and light pastes as in: Food processing Paint manufacturing A small removable can 5 to 100 gallons in size Change-Can mixers are of two types: Pony mixer Beater mixer 19

20. Pony Mixer Agitator consist of vertical blades held on rotating head positioned near the wall of the can. Blades are slightly twisted. Agitator is mounted eccentrically. Can rest on a turn table driven in direction opposite to that of agitator. All the material in the can is brought to the blade to be mixed. When mixing is completed – cleaning of blades are performed and can is replaced. 20

21. Change-Can Mixers (Pony Mixer)

22. Beater Mixer Can or vessel is stationary. Agitator has a planetary motion. Visits all parts of vessel. Beaters are shaped to pass with close clearance over the side & bottom of mixing vessel. 22

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24. Recommended Books Text Book: W. L. McCabe, J. C. Smith and P. Harriott , Unit Operations of Chemical Engineering, 5th Ed. McGraw-Hill: 2004. (Section V: Chapter28) Reference Books: J. F. Richardson, J. H. Harker and J. R. Backhurst , Coulson and Richardson’s Chemical Engineering-Vol 2: Particle Technology & Separation Processes, 5th Ed. Butterworth-Heinemann: 2002. (Chapter 1) W. L. McCabe, J. C. Smith and P. Harriott , Unit Operations of Chemical Engineering, 7th Ed. McGraw-Hill: 2004. 24