What change blindness can teach us about skilled observation : a law enforcement and student comparison
Abstract
Despite known advantages of up-pumping wide-blade axial-flow impellers for gas dispersion, little is known about the design guidelines of these impellers. In the present study, the gas dispersion capabilities of the up-pumping axial-flow Maxflo W impeller in low-viscosity liquids have been characterized to incorporate effects of scale and system geometry. Further, a comparison is made with the dispersion performance of an up-pumping pitch-blade turbine and a radial-flow CD-6. This comparison showed that the up-pumping Maxflo W impeller have dispersion speed, torque and power requirements relatively independent of the gas flow rate, providing the ability to handle varying process conditions.Testing at three scales (tank diameters of 0.29 m, 0.44 m and 0.60 m) indicates that the dispersion capabilities of the up-pumping Maxflo W can be described in terms of a scale-independent aeration number - Froude number relationship. Although a minimum impeller rotational speed is required to disperse low gas flows, the increase in dispersion speed is small with increasing gas flow rate. The effects of geometric parameters such as impeller to tank diameter ratio, sparger size and ungassed liquid height on flooded to dispersed conditions have been investigated. Unlike radial-flow gas dispersion impellers that are not strongly affected by some geometric parameters, the up-pumping Maxflo W dispersion performance is strongly dependent on system geometric parameters. Additionally, small impeller to tank diameter ratios of Maxflo W impellers have been found to perform poorly in gas dispersion operations because of their highly axial discharge flow.It was found that the up-pumping pitched-blade turbine exhibits time-dependent dispersion behavior that can permanently fail to disperse gas after appearing to be capable of dispersion for a reasonably long period of time. For this reason, collection of design data required extended and meticulous testing.
