1. CBE 465
4/19/2017
Heuristics
19 Oct 12

2. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Suspend solids
Disperse gases and liquids
Emulsify one liquid in another
Promote heat transfer
Blending two or more materials together
Overmixing maybe undesirable
in biological application, high shear may damage organisms
polymer molecules may be damaged by long mixing or high shear
For design or consideration of mixing process should understand:
mechanism of mixing
scale-up criteria
power consumption
flow patterns
mixing time/rates
types of equipment available
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

3. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Fluid mixing
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

4. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Fluid mixing: Baffles
Unbaffled mixing tanks often used:
in transition region
for sticky materials
where perfect cleaning is required
in large tanks where baffle effects are small
processes where it is not clear baffles have an effect on mixing performance
3 min
G.B. Tatterson., Fluid Mixing and Gas Dispersion in Agitated Tanks, McGraw-Hill, 1991
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

5. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Fluid mixing: Baffles
Fluid mixing: off-center
4:10 min
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

6. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Side mounted mixers.
Flow patterns for side-entering propeller
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.
Paul, et.al., Handbook of Industrial Mixing, Wiley, 2004

7. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Common Impellers
Figure 7.20 Commonly used impellers (a) Three-bladed propeller (b) Six-bladed disc turbine (Rushton turbine) (c) Simple paddle (d) Anchor impeller (e) Helical ribbon.
2 min
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

8. Agitators and Mixing Equipment
CBE 465
Agitators and Mixing Equipment
4/19/2017
Various Turbine Impellers
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

9. Various Impeller Types
CBE 465
Various Impeller Types
4/19/2017
Axial Flow Impellers
Hydrofoil Impellers
High-Shear Impellers
Radial Flow Impellers
Paul, et.al., Handbook of Industrial Mixing, Wiley, 2004

10. Various Impeller Types
CBE 465
Various Impeller Types
4/19/2017
R. Hesketh, mixing notes

11. Various Impeller Types
CBE 465
Various Impeller Types
4/19/2017

12. Agitators and Mixing Equipment
CBE 465
4/19/2017
Agitators and Mixing Equipment
Selecting Agitator Type
Used to make preliminary agitator selection based on tank volume and liquid viscosity.
Turbines, Pitched Blade Turbines, and Propellers are typically used at high Re and low viscosity.
Anchor, Helical Ribbon, and Paddle agitators are used for higher viscosity (more laminar-like Re) fluids.
1 cP = Pa*s = 1 mPa*s
1 Pa*s = 1 N*s/m2 = 1 kg/(m*s)
Coulson and Richardson’s Chemical Engineering Volume 1, 6th ed.

13. Flow Patterns for Various Impellers
CBE 465
Flow Patterns for Various Impellers
4/19/2017
Flat Blade Turbine = FBT
Pitched Blade Turbine = PBT
Paul, et.al., Handbook of Industrial Mixing, Wiley, 2004

14. Typical Dimensions for Mixing Equipment
CBE 465
Typical Dimensions for Mixing Equipment
4/19/2017
G.B. Tatterson., Fluid Mixing and Gas Dispersion in Agitated Tanks, McGraw-Hill, 1991

15. Typical Dimensions for Mixing Equipment
CBE 465
Typical Dimensions for Mixing Equipment
4/19/2017
G.B. Tatterson., Fluid Mixing and Gas Dispersion in Agitated Tanks, McGraw-Hill, 1991

16. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Consider geometry, fluid properties, flow patterns, power, and so on. Has been considered through dimensional analysis.
With:
For geometrically similar vessels, ratios of all terms to right of the Froude number are negligible.
The Froude number is only important when significant vortex develops (in unbaffled tanks); for baffled tanks the NP does not depend on the Froude number.
P = power dissipation to the fluid. W=J/s = N-m/s = kg-m^2/s^3
Power rating of the driver must exceed P, i.e. has to overcome losses of driver, seals, bearing, etc.
As R.O.T., estimate losses as 25 % of calculated P
Therefore: Pdriver = 1.25 * P Use 1.75 factor when solids might settle on propeller before it is started.
(info from Cheremisinoff)
Tatterson & Colson and Richardson.

17. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Consider low viscosity, unbaffled systems.
Colson and Richardson.

18. CBE 465
In-Class PS Exercise
4/19/2017
Consider a solution of sodium hydroxide with the properties listed below. It is agitated by a propeller mixer that is 0.5m in diameter in a 2.28m diameter unbaffled tank. The liquid depth is 2.28m. The impeller is located 0.5m above the bottom of the tank. If the propeller is rotated at 2 Hz, what power is required?
Colson and Richardson

19. CBE 465
4/19/2017
Heuristics
22 Oct 12

20. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Consider low viscosity, baffled systems.
Colson and Richardson.

21. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Consider low viscosity, baffled systems (wall baffles).
Curve 1 = Rushton impeller
Curves 2 & 4, open, flat blade impellers
Curve 5 is Backswept open impeller
Curve 6 is PBT (pitched blade turbine)
Figure Power correlations for turbine impellers in a tank with 4 baffles. [w, D, impeller width and diameter, respectively.]
Colson and Richardson.

22. CBE 465
In-Class PS Exercise
4/19/2017
Assume you are mixing a small amount of material into water in a standard configuration baffled tank. The diameter of the pitched blade turbine is 1 m and it is desired to operate at 84 RPM. Estimate the power required.

23. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Consider low viscosity, baffled systems (wall baffles).
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

24. Power Consumption and Scale-up in Mixing
CBE 465
Power Consumption and Scale-up in Mixing
4/19/2017
Propeller pitch:
Propeller pitch = (distance fluid (or propeller) travels in one full revolution) / (propeller diameter)

25. CBE 465
Other Terms in Mixing
4/19/2017
Pumping Capacity: discharge flowrate from an impeller:
Tip Speed of an impeller:
Torque: “twist” force acting on agitator shaft:
Power per unit volume:
Q = discharge flowrate, or pumping capacity.
For “square-pitched” propellers in turbulent region, take NQ = 0.5
W=J/s = N-m/s = kg-m^2/s^3
Blend time (estimation to within 5% desired concentration):

26. Discharge Coefficient
CBE 465
Discharge Coefficient
4/19/2017
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

27. CBE 465
Mixing Time
4/19/2017
Ni tm = dimensionless mixing time or the number of stirrer rotations to mix the solution to homogenization.
Blend time (estimation to within 10% desired concentration):
P.M. Doran, Bioprocess Engineering Principles, 2nd Ed., Academic Press 2012

28. CBE 465
Mixing Time
4/19/2017
Doran suggests that for turbulent mixing conditions, irrespective of the impeller type, that (baffled vessel, single impeller, H=T):
Verified under aerated conditions also (impeller not flooded) and for:
P.M. Doran, Bioprocess Engineering Principles, 2nd Ed., Academic Press 2012

29. CBE 465
In-Class PS Exercise
4/19/2017
A fermentation broth with properties as given below, is agitated in a 2.7 m3 baffled tank using a Rushton turbine with a diameter of 0.5 m and a stirred speed of 1 Hz. Estimate the mixing time.

30. Additional Plots for Non-Standard Mixing
CBE 465
Additional Plots for Non-Standard Mixing
4/19/2017
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

31. Additional Plots for Non-Standard Mixing
CBE 465
Additional Plots for Non-Standard Mixing
4/19/2017
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

32. Additional Plots for Non-Standard Mixing
CBE 465
Additional Plots for Non-Standard Mixing
4/19/2017
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

33. Additional Plots for Non-Standard Mixing
CBE 465
Additional Plots for Non-Standard Mixing
4/19/2017
N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, B-H, 2000

34. Heuristics in Mixing CBE 465 4/19/2017

35. CBE 465
Heuristics in Mixing
4/19/2017
Colson and Richardson.

36. CBE 465
In-Class PS Exercise
4/19/2017
A pilot-plant vessel that is 0.3 m in diameter is agitated by a six-bladed turbine impeller (Rushton) that is 0.1 m in diameter. With the impeller NRe at 104, the blending time of two miscible liquids is found to be 15 s. The power per unit volume is 0.4 kW/m3. The mixing vessel is to be scaled-up to a vessel diameter of 1.8 m. Assume that the new vessel is geometrically similar to the pilot-plant vessel. You may assume the NRe in the larger vessel is still 104 or larger.
For the scaled-up vessel, what is the power/volume required to keep the blending time the same (15 s)? Comment on your results.
For the scale-up, assume that the power/volume required is kept the same as the pilot-plant vessel. What will be the new blending time in the larger tank?

37. CBE 465
4/19/2017
Heuristics
Questions?