1. Agitation Equipment - Table 4.16
Short residence time
pumps, inline mixers
pipes
static mixers
extruders
T-junctions
valves and orifices
homogenizers
Good design requires expertise
Long residence time
stirred tanks
(very long pipes)
Design is generally more forgiving

2. Stirred Tanks are used for:
liquid blending
liquid liquid dispersion (drop break up)
solids suspension and dissolution
gas dispersion
reaction, including bioreactions
solids leaching
crystallization

3. Generic Tank Drive Seal Shaft+impeller Internals baffles dip pipe
coils/jacket

4. Typical dimensions Tank height per impeller<1.2T
T=tank diameter 0.5m<T<20m
H=total fluid height
Impeller diameter (0.2T<D<0.5T)
Impeller off bottom clearance (0.25T<C<0.4T; up to 0.5T for radial)
4 Baffles
baffle width=T/10 (m) or T/12 (feet)

5. The Importance of Baffles
Baffles convert swirl to axial circulation and macro-scale mixing

6. The Importance of Dip Pipes
Feed at the impeller for intense, rapid mixing and fast reaction
Feed below the impeller (ALWAYS!) for gas dispersion
Feed at the impeller for consistent product quality

7. Impellers: Radial Axial

8. Reynolds Number, Re Reynolds number is more general than viscosity
Limits for an impeller:
laminar flow if Re< special eqmt
AVOID 400<Re<20000
turbulent if Re>2x104
D=impeller diameter
N=rotational speed, rps

9. Turbulent Power Number, Np
Used to calculate power draw, P
Np=3.5 for the “best practice” radial impeller, the CD6 by Chemineer
Np=1.3 for the generic axial impeller, the PBT
Np=0.3 for high efficiency axial impellers

10. Sizing: Shortcut Design Ulrich
Table 4.16 in Ulrich gives ranges of power required for various applications
eg liquid liquid dispersion, mild agitation, P(kW)=0.25V0.8 where V is in m3
Ulrich scales
propellers as PV
axial impellers as PV0.8
radial impellers as PV
All of these refer to the tank volume

11. How accurate is short cut design?
Equation for blending in Ulrich
Current best practice equation (Grenville, 1995)
Where will Ulrich fail?

14. Sizing: Shortcut Design Kresta
Maintain geometric similarity (D/T, C/T etc) on scale-up
Scale with constant P/Vtank for ALL impellers; use P values from Ulrich
Call me for info. on detailed designs!
Coming soon: The Industrial Mixing Handbook

15. Short Cut Geometry PBT impeller, Np=1.3 (T=2m); D=T/3; C=T/3
Standard gear boxes give output speeds of 25, 30, 45, 56, 68, 84, 100 and 125 rpm, based on a 1750 rpm motor
A variable speed drive costs more - but is cheap insurance if you don’t know the real answer

16. Critical Design Variables
Tank size is a function of process scale and residence time: this is the key cost variable for the vessel
The drive cost is fixed by the power and torque: Ulrich considers only the power
Three variables define the mixing operation in the detailed design stage:
Impeller geometry
Impeller diameter, D
Rotational speed, N

17. Costing: The Agitator
Figure 5.42: the drive, coupling, shaft and impeller
Cost is a strong function of the seal
open tank<stuffing box<mechanical seal
CBM=CpxFBM
all data for the agitator is on one figure

18. Costing: The Tank For No Heat TF 98% vertical Size by height Figures
size (CP)
pressure (FP) and material (FM)
FBM=f(FMxFP)
CBM=CPxFBM
With Heat TF
Size by volume
Figure 5.23
All factors in one figure

19. Recap: Tanks vs. Pipes
Tanks are flexible where there is uncertainty in design
Tanks allow long residence times
Stirred tanks are cheap
Caveat: A poor mixing design lead to start up problems which cost many times more than the equipment itself!!!

20. Mixing Equipment - free gift
Recommend
stirred tanks
static mixers
T- mixers
jet mixers
generic PBT for easy jobs
wide range of well designed impellers for a variety of difficult jobs
Avoid if possible
bubble columns
glass lined vessels
propellers
retreat blade impellers
in-line mixers
side mounted mixers