1. Chemical Engineering Department Government Engineering College
MIXING & AGITATION
B.E. (Chemical)- Sem- V
Subject : MO
Subject Code:
Guided By:
Prof. S.J.Solanki
Chemical Engineering Department
Prepared By:
Meet Somani
Chemical Engineering Department
Government Engineering College
Bhuj

2. Batching and Mixing
Batch feed always involves mixing, objective: - high uniformity, high reliability (more judgment (experience) rather science)
Before mixing: action of feeding; next few graphs showing some common equipments, goal: good powder flowability.
Most ideal state of mixing: random homogeneous mixture RHM

6. Properties of Mass of Particles
Pressure is not the same in all directions: one applied pressure will create some pressures in other directions, but always smaller; related to particle shape and packing; define K’ = normal pressure/applied pressure
Shear applied at surface will be transmitted through a static mass of particles
Density of mass will vary
Before flow, mass of particles will increase its volume first (dilation)
When angular solids are piled up on a flat surface, there will be an angle of repose; (free flowing solids: this angle is between 15 and 30o)

7. Angle of friction: influenced by particle size, shape, or even water content, it often increase cohesive force between particles  increase angle of friction  more difficult to free flow
Abrasion: another possible problem with ceramic particles during transportation

8. Powder Mixing More art than science;
Can never achieve perfect mixing like that in fluid phase;
Complete mixing: often refers to specific structure, not attainable from a random process;
 Characterization of mixture: I.e. degree of homogeneity (1) a statistical problem; (2) sample size (scale of scrutiny) consideration – need to be “proper”, too large or too small: little value; e.g. sintering after mixing – then consider diffusion distance during sintering, choose appropriate size for sampling，can be considered as single sample within that size.

9. Important Parameters Mixing effect affected by:  type of equipment
 energy input
 flowability and composition of sampe (size, shape, density, surface characteristics)
For complete description of mixing, one need:
 sample variance (intensity of segregation)
 scale of segregation (to micro-scale)
 long range structure

10. Microscale and Macroscale Mixedness
Microscale analysis  provide information on microscale mixedness
Macroscale mixedness  can be analyzed by many technqiues (chemical or physical)
Sampling and analysis error should be kept to a minimum
uncertainty in the standard deviation (s) become low when a large number of samples are taken

11. Taken from JS Reed, 1995
Commercial mixers: usually with two or more mixing elements to produce: high shear mixing in a local region & low shear bulk mixing

12. One example of two mixing elements
To avoid vortex, we may add baffles
Turbulence and cavitation – for diffusion (micro-scale mixing)

13. Mixing and EMI Performance
* ABS + Ni powder or fiber  mixing (Barbender mixer or dry mixing) composite for EMI measurements

14. Dry mixing produced better shielding effect than Barbender mixer (in terms of low threshold value)

15. Barbender mixer produced perfect mixing, not necessary good for EMI purposes; dry mixing produced macro-scale uniformity, not micro-scale uniformity
(a) Barbender mixer (powder; 20%); (b) dry mixing (powder 7%); (c) dry mixing (filament, 7%)

16. Agitator
Bioreactor: device, usually a vessel, used to direct the activity of a biological catalyst to achieve a desired chemical transformation.
Fermenter: type of bioreactor in which the biocatalyst is a living cell.

17. Mixing method: Mechanical agitation
Baffles are usually used to reduce vortexing
Applications: free and immobilized enzyme reactions
High shear forces may damage cells
Require high energy input

18. Batch operation with stirring
A foam breaker may be installed to disperse foam
A batch bioreactor is normally equipped with an agitator to mix the reactant, and the pH of the reactant is maintained by employing either buffer solution or a pH controller
Batch operation with stirring
Change of Cs with time, t

19. Continuous operation without stirring
Plug-flow mode
An ideal plug-flow reactor can approximate the long tube, packed-bed and hollow fiber or multistaged reactor
In a plug-flow reactor, the substrate enters one end of a cylindrical tube with is packed with immobilized enzyme and the product steam leaves at the other end.
F, Cs
F, Cs0 V
Residence time
t = 0
Continuous operation without stirring

20. Temperature Control (Heat Load)
Heat load: Heat load is determined by energy balances
Heat production rate:
Popular method
: heat production rate, kcal/ls
V: reactor liquid volume, l
: specific growth rate, s-1
C: biomass concentration (g/l)
Ykcal: a yield coefficient given as grams of cells formed per kcal energy released, g cells/kcal

21. Agitation (gas transfer)
1. Biological reactions almost invariably are three-phase reactions (gas-liquid-solid). Effective mass transfer between phases is often crucial. For example, for aerobic fermentation, the supply of oxygen is critical.
The equation governing the oxygen transfer rate is:
Agitation:
Mechanical stirring (for small reactors, and/or viscous liquids, low reaction heat)
Air-driven agitation (for large reactors and/or high reaction heat)