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 dissolution gas dispersion reaction, including bioreactions solids leaching crystallization

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

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)

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

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

Impellers: Radial Axial

Reynolds Number, Re Reynolds number is more general than viscosity Limits for an impeller: laminar flow if Re<200-400 - special eqmt AVOID 400<Re<20000 turbulent if Re>2x104 D=impeller diameter N=rotational speed, rps

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

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

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

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

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

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

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

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. 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!!!

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