(554b) Characterization of the Gas Dispersion Capabilities of the Bt-6 Impeller

The flooded-to-loaded transition in gas dispersion agitation has been well characterized for the Rushton turbine at lower gas flows. The scale-independent criterion for the impeller to effectively disperse gas is of the following well-known form

N_A < 30 N_Fr (D/T)^3.5

where N_A is the aeration number (Q_g/ND³) and N_Fr is the Froude number (N²D/g). The development of this equation and the associated study of the cavity structure of the Rushton turbine in gas-liquid systems led to a thorough understanding of the gas dispersion process and paved the way for the development of the next generation of gas dispersion impellers. These concave-blade impellers, such as the CD-6 and BT-6, can disperse more gas than the Rushton turbine, and their power draws do not drop appreciably upon gassing like that of the Rushton turbine. Because of the advantages of concave-blade impellers, the flat-blade Rushton turbine is not often used in industrial gas dispersion applications. Additionally, current practice frequently uses substantially higher gas flows than were studied in the development of the preceding Rushton turbine correlation.

Despite the widespread use of concave-blade turbines at high gas flows, very little information about their dispersion capabilities is available in the literature; however, this has been changing. The gas dispersion performance of the concave-blade Scaba 6SRGT has been compared to the Rushton turbine, while a detailed study of the gas dispersion capabilities of the Chemineer CD-6 has been published. These studies indicate that the concave-blade impellers have enhanced gas dispersion capabilities at high gas flows while the Rushton turbine does not.

The current work is a thorough examination of the gas dispersion capabilities of the BT-6 impeller at gas flows covering the spectrum of industrial applications. Results include an alternative flooded-to-loaded transition correlation for the BT-6 that is similar to that of the Rushton turbine; however, at high gas flows, the gas handling capabilities of the BT-6 increase dramatically in a manner similar to that found for other concave-blade impellers. The correlation has been tested at a number of scales and across the spectrum of impeller diameter to tank diameter ratios (D/T). Additionally, the torque and power required for effective gas dispersion are presented. Data for the Rushton turbine and CD-6 impeller are included, demonstrating the superiority of the asymmetric concave-blade BT-6 to the flat-blade Rushton turbine and the symmetric concave-blade CD-6.