Chemical Engineering Research & Design, Vol.82, No.9, 1073-1081, 2004
The versatility of up-pumping hydrofoil agitators
In the early 1980s, reports appeared showing that, counter-intuitively, up-pumping impellers were very effective for gas dispersion. Recently, this up-pumping concept has come of age, particularly with wide-blade, up-pumping hydrofoils that have the following advantages. From the turbulent to the transitional flow regime, they handle larger quantities of gas without flooding, with a minimal loss of power and negligible torque and flow instabilities. Also, with multiple impellers, the mixing time can be reduced by more than half compared with radial impellers. They are also very effective at other agitation-based processes. Firstly, they can suspend solids in three-phase systems at much lower power inputs than Rushton turbines or other radial flow impellers, and at high gas flow rates, with less power and with much more stable flow, torque and power characteristics than with wideblade, down-pumping hydrofoils. Secondly, they are the most effective impellers for foam breaking (SAFD) when placed close to the top surface of a fermentation broth. Thirdly, whether wide or narrow blade, they can ingest floating solids at the lowest power input under unsparged conditions (and with less tendency for associated surface gassing), or when sparging is used. Fourthly, they enable a significantly improved selectivity to be achieved in semi-batch reactors with competitive reactions when surface feeding is employed, similar to that obtainable with sub-surface feeding. In addition to the particular `process results' addressed above, on the basis of the present state-of-the-art, others mixing processes that might benefit from the use of up-pumping impellers are suggested.