A COMPARISON OF CAVERN DEVELOPMENT IN MIXING A YIELD STRESS FLUID BY RUSHTON AND INTERMIG IMPELLERS

Intermig impellers have been postulated as very efficient for mixing h ighly viscous non-Newtonian fluids (such as xanthan and mycelial broth s). However, no formal characterisation has been published and no fair comparisons have been made, based on accurate power drawn measurement s and using equal number of impeller stages and equal diameter, if com pared (for example) with the performance of Rushton turbines. Characte risation of the shape, size, and evolution of the well-mixed zones or ''caverns'' were correlated with power drawn, for single and dual Rush ton turbines and for one- as well as two-stage Intermig unslotted impe llers. Cavern evolution studies were carried out in a mixing tank (dia meter = 0.205 m, H/T = 1.6) equipped with an accurate air bearing dyna mometer. Carbopol 940 (0.25 wt.-%) was used as a model, transparent fl uid. Impeller to tank diameter ratio was 0.53 for both impellers. Cave rns were visualised by injecting methylene blue in the well-mixed zone s. A single Rushton turbine developed larger caverns if compared with one-stage Intermig of the same diameter under power drawn below 1.5 kW m(-3). At higher power drawn, both impellers behaved very similarly, reaching a limit in cavern volume of about 40% of the total liquid vol ume, even at very high (20 kW m(-3)) power drawn. A similar trend char acterised dual combinations: below 3 kW m(-3), dual Rushtons gave larg er cavern volume if compared with the performance of two-stage Intermi gs. In either case, power drawn higher than 3 kW m(-3) was sufficient to mix more than 90 % of the liquid volume. The presence or absence of the slot in the Intermig did not influence cavern development. Experi ments with a smaller Intermig (D/T = 0.47) showed that, under equal po wer drawn, caverns were smaller if compared with those obtained with t he larger Intermig (D/T = 0.53).