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

It is known that impellers operating in boiling or near boiling liquids can develop cavities similar to those observed in gas-liquid systems at ambien t temperatures. Considerable reductions in the power demand of traditional impellers operating in unsparged boiling liquids compared with that at ambi ent temperature have previously been reported and linked to a submergence b ased agitation cavitation (Smith) number. The performance of high performan ce 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 modem impeller designs (Chemineer CD-6 and BT- 6, Lightnin A315 and an impeller based on the ICI Gasfoil design) working i n boiling liquids is reported, together with updated information about conv entional Rushton and pitched blade turbines. In boiling liquids the power d raw characteristics of the new designs are quite different from those of th e traditional impeller types. The modem impellers are all efficient at hand ling high loadings of inert gases. In boiling liquids they maintain high le vels 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 second ary nucleation, or the degradation of a particulate product, might be expec ted to follow cavitation. (C) 2001 Elsevier Science B.V. All rights reserve d.