A New Gas Dispersion Impeller with Vertically …

1 A New Gas Dispersion Impellerwith Vertically asymmetric BladesAndr BakkerKeywords: Mixing, Gas Dispersion , asymmetric blade Impeller , Stirred TankPublished in The Online CFM Book at (c) 2000 Andr BakkerAcknowledgmentuFluent Inc.: Lanre Oshinowo Liz MarshalluChemineer, Inc.: Mark Reeder Julian FasanouUniversity of Dayton: Kevin Myers Aaron ThomasAbstractFor several decades the so-called Rushton turbine was the standard Impeller for gas dispersionapplications. It features six flat blades mounted on a disk. John M. Smith and coworkersintroduced the concept of using concave blades. They explained the improved performance of theconcave blades compared to flat blades in terms of reduced cavity formation behind the with a semi-circular blade shape are now common in the industry. Relatively recent,new blade designs with a deeper concavity have been proposed by other researchers. Under mostconditions with these deeper blades the gas is being dispersed from the inside of the blade , insteadof from large cavities behind the of the disk-style gas Dispersion impellers studied in the literature so far haveblades that are symmetric with respect to the plane of the disk.

2 This is not necessarily optimal, asthe gas usually enters from the bottom, causing a distinctly asymmetric flow pattern. This paperdiscusses the performance of a new gas Dispersion Impeller with Vertically asymmetric blades. Thenew Impeller is designed to accommodate the different flow conditions above and below theimpeller disk. The blade shape was optimized in a comparative study of more than twenty paper discusses the performance of a gas Dispersion Impeller with blades that arevertically asymmetric ; the blade shape above the disk is different from the shape below thedisk. It is shown that this Impeller has a gassed power curve that is flatter than that of otherimpellers. Furthermore, it can disperse more gas before flooding than the impellers withsymmetric blades. Both experimental data and the results of advanced CFD simulations usingFLUENT 5 will be Dispersion Impellers: 1950 su Rushton Turbine Radial flow Impeller Six flat blades on a disk Suitable for gas Dispersion and liquid-liquiddispersion Turbulent power number range: Trailing vortices in single-phase flow Cavitation in gas-liquid flowGas Dispersion Impellers: 1980 su Chemineer CD-6 Radial flow Impeller with six concave blades ona disk.

3 Design based on prior research by Smith andco-workers. Lightnin R130, Philadelphia s Smith Turbine. Turbulent power number range: Impeller Systems - 1980 s CD-6/HE-3 systems mostwidely used system inlarge scale can be either downor up pumping. Over100,000 kW (130,000 HP)installed. Systems with solelydown pumping axial flowimpellers were notsuccessful. Solely uppumping impellers workbetter than down pumping. Gas Dispersion Impellers: 1988-1993u Proprietary Scaba and ICI Impellers Deeper concave blades Sharp back edge of blade with and without disksUS Patent 4,779,990 (1988)US Patent 5,198,156 (1993)Gas Dispersion Impellers: 1998uChemineer BT-6 asymmetric blade technology: Gas flow isasymmetric so why would blade shape besymmetric? blade curvature is different on top and bottomand the blade is longer on top. Rising gas is captured bylonger upper portion ofblade and dispersed frominside the deep Performance CharacteristicsuLow turbulent power number Po = number is constant for impellerReynolds numbers greater than 1, power draw curve under gas Dispersion Flow PatternuStudied extensively using theFluent 5 unstructured tetrahedralmeshes with up to 500,000nodes, created mesh RANS and LESturbulence models GriduA very fine meshwas used at theimpeller is needed inorder to captureimportant Pattern in VesselFlow Around Impeller BladesImpeller Flow PatternVorticity magnitude on a surface of constant velocity ( m/s).

4 Turbulence in Impeller RegionVelocity magnitude on a surface of constant turbulent kinetic energy ( m2/s2).Vorticity MagnitudeVorticity magnitude on the Impeller , tank wall, and Power Number ComparisonGas Dispersion Retrofit Comparison Rushton CD-6 BT-6 Gas flow rate 13 vvm (vsg= ).Gassed Power Draw ComparisonComparison with Up-Pumping HydrofoilAPV-B2 up-pumping wide blade hydrofoil data from:Hari-Prajitno et al., CJChE, December 1998BT-6/Maxflo Multiple Impeller SystemsFroude Number = ; D = down pumping; U = up pumping; equal power split)Torque Required for Complete DispersionPower Required for Complete Dispersion Gas Dispersion Capability0123456 RushtonCD-6PD-6BT-6 Retrofit comparison. Average of all available PD-6 is a concave blade Impeller with deep symmetric Gas Dispersion CapabilityConstant D/T comparison.

5 Average of all available PD-6 is a concave blade Impeller with deep symmetric Transfer Coefficient0 Mass Transfer Coefficient (1/s)0 Superficial Gas Velocity (m/s)BT-6CD-6D-6 Retrofit at Pu/V = kW/m3BT-6 Installation ExamplesuNumerous successful industrial installations: BT-6 with down-pumping HE-3 impellers in 750 kW(1000HP) fermenters. BT-6 with up-pumping Maxflo-Y impellers in 162 kW (215HP)fermenters. Multiple BT-6 Impeller systems in gassed staged columns. BT-6/PBT combinations in BT-6 ConclusionuThe Chemineer BT-6 is the latest innovation inradial flow gas Dispersion Impeller data was gathered with high gasflow rates in tanks with diameters up to : The patented Impeller disperses more gas than all otherdesigns. The effect of gas flow rate on power draw is reducedcompared to all preceding flow pattern was studied in detail using theFLUENT 5 unstructured CFD , installations range up to 750kW(1000HP).