Convex bladed mixed flow (CBMF) impeller: A high performance agitator for mixing slurries

Abstract

Studies on the utility of the convex bladed mixed flow (CBMF) impeller to mix and aerate pig slurry are described. The power consumption characteristics were determined in aerated and unaerated pig slurry, on the laboratory scale (tank diameter = 0·29 m, impeller diameter = 0·098 m); these characteristics were compared with experimental data for a Rushton turbine and a pitched-bladed turbine of the same diameter. In farm scale experiments (tank diameter = 2·5 m, impeller diameter = 0·83 m), the power consumption and solid suspension characteristics of the CBMF impeller were studied using unaerated pig slurry: the performance of the CBMF impellers was compared with earlier data for a Rushton turbine and a pitched-bladed impeller in a tank of the same size. The mixing performance was characterized by defining the mixing index, M = 1 X f 2 ( s − 1 ) ∑ i = 1 s ( X f − X i ) 2 where Xf is the concentration of the solids in the slurry when it is fully mixed, Xi is the local concentration and s is the number of samples. The CBMF impeller effectively maintained a high level of homogeneity in the slurry. To maintain the mixing index at a typical value of 2·5 × 10−2, the CBMF impeller consumed 13% of the power reported for the pitched-bladed turbine and only 1·5% of that reported for the Ruston turbine. It should be said that there was a 2 year interval between the earlier tests carried out using the Rushton turbine and pitched-bladed turbine, and the present work. These comparisons must therefore be interpreted in the light of the complexities associated with the characterization of the two slurries: the slurry used to test the CBMF impeller, and that used by the earlier workers to test the performance of the pitched-bladed turbine and the Rushton turbine. Notwithstanding marginal differences between some of their properties, the slurries could be regarded as being similar in all respects, thereby justifying the comparisons. Both the slurries were drawn from the same piggery and their properties, especially those relevant to mixing—density, rheological behaviour, concentration of solids and their size distribution—were not significantly different.