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

Stirred Tanks are used for: liquid blending liquid liquid dispersion (drop break up)solids suspension and dissolutiongas dispersion reaction, including bioreactionssolids leachingcrystallization

Generic TankDriveSealShaft+impeller Internals

baffles dip pipe coils/jacket

Typical dimensionsTank 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)

The Importance of BafflesBaffles convert swirl to axial circulation

and macro-scale mixing

The Importance of Dip PipesFeed at the impeller for intense, rapid

mixing and fast reactionFeed below the impeller (ALWAYS!) for

gas dispersionFeed at the impeller for consistent

product quality

Impellers:Radial Axial

Reynolds Number, ReReynolds number is more general than

viscosityLimits for an impeller:

laminar flow if Re<200-400 - special eqmt AVOID 400<Re<20000 turbulent if Re>2x104

D=impeller diameterN=rotational speed, rps

2ND

Re

Turbulent Power Number, NpUsed 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

53DN

PNp

Sizing: Shortcut Design UlrichTable 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 PV axial impellers as PV0.8

radial impellers as PVAll of these refer to the tank volume

How accurate is short cut design?Equation for blending in Ulrich

Current best practice equation (Grenville, 1995)

Where will Ulrich fail?

5.1

3

5.0

m1

V

P

V12000

2

3/1p D

T

NN

2.55

Extrapolation fails: 40x to 500x to 10000x

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+01 1.E+03 1.E+05 1.E+07 1.E+09

Re

Res

iden

ce t

ime

(s)

PBT/water/2m tank

A310/water/2m tank

RT/water/2m tank

PBT/20m tank

PBT/large impeller

PBT/high viscosity

PBT/0.24m tank

Ulrich

Normal Range of Operation: 4x

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05 1.E+06 1.E+07

Re

Res

iden

ce t

ime

(s)

PBT/water/2m tank

A310/water/2m tank

RT/water/2m tank

PBT/large impeller

Ulrich

Sizing: Shortcut Design KrestaMaintain geometric similarity

(D/T, C/T etc) on scale-upScale with constant P/Vtank for ALL

impellers; use P values from UlrichCall me for info. on detailed designs!Coming soon:

The Industrial Mixing Handbook

Short Cut GeometryPBT impeller, Np=1.3 (T=2m); D=T/3; C=T/3Standard 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

Critical Design VariablesTank 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

Costing: The AgitatorFigure 5.42: the drive, coupling, shaft

and impellerCost is a strong function of the seal

open tank<stuffing box<mechanical seal

CBM=CpxFBM

all data for the agitator is on one figure

Costing: The Tank

For No Heat TF 98% vertical Size by height Figures

5-44 - size (CP)

5-45 - pressure (FP) and material (FM)

5-46 - FBM=f(FMxFP)

CBM=CPxFBM

With Heat TF Size by volume Figure 5.23 All factors in one

figure

Recap: Tanks vs. PipesTanks are flexible where there is

uncertainty in designTanks allow long residence timesStirred tanks are cheap

Caveat: A poor mixing design lead to start up problems which cost many times more than the equipment itself!!!

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