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Some effects of pseudoplasticity on hold-up in aerated, agitated vessels

The Chemical Engineering Journal
Publication Date
DOI: 10.1016/0300-9467(80)85078-7


Abstract The flow curves of four pseudoplastic sodium carboxy methyl cellulose (CMC) solutions are reported. Each solution was air sparged at rates up to 1 vvm (vvm is the volumetric flow rate of air per minute per volume of liquid in the vessel) in a 0.56 m diameter agitated vessel of Rushton dimensions and power inputs (up to 2.0 W/kg) and hold-ups have been measured. The ratio of gassed to ungassed power versus aeration number is compared with similar data from the agitation of Newtonian fluids and is correlated by an empirical equation proposed by Kudrna. The reported hold-ups are in the range 1.5 – 8% and, for equivalent sparging and agitation conditions, increasing pseudoplasticity leads to decreasing hold-up. This decrease is due to an increasing proportion of large bubbles which have a short residence time. For each CMC solution, hold-up, aeration rate and power input can be related by ϵ H ∝ ϵ 0.3 V s 0.7 The technique for measuring hold-up allows surface topographies to be determined. These topographies suggest that when the level of agitation is well matched to the sparge rate, the upper surface of the dispersion is relatively level.

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