The unsparged power demand of modern gas dispersing impeller in boiling liquids

Abstract

It is known that impellers operating in boiling or near boiling liquids can develop cavities similar to those observed in gas–liquid systems at ambient temperatures. Considerable reductions in the power demand of traditional impellers operating in unsparged boiling liquids compared with that at ambient temperature have previously been reported and linked to a submergence based agitation cavitation (Smith) number. The performance of high performance gas dispersing impellers operating in boiling liquids has not previously been reported, despite their widespread adoption for mixing and dispersion in chemical reactors. The power demand of selected modern impeller designs (Chemineer CD-6 and BT-6, Lightnin A315 and an impeller based on the ICI Gasfoil design) working in boiling liquids is reported, together with updated information about conventional Rushton and pitched blade turbines. In boiling liquids the power draw characteristics of the new designs are quite different from those of the traditional impeller types. The modern impellers are all efficient at handling high loadings of inert gases. In boiling liquids they maintain high levels of power input — even when operated with high impeller tip speeds that correspond to low cavitation numbers. Such cavitation as may occur clearly does not affect the power demand. The results are of particular relevance to the design and operation of forced circulation crystallisers when secondary nucleation, or the degradation of a particulate product, might be expected to follow cavitation.