The characterisation of mixing quality in stirred tanks is not trivial, particularly when the vessel contents are opaque and simple flow visualisation tools are not applicable. Electrical Resistance Tomography (ERT) has been used previously to measure mixedness within stirred tanks and is by now a well-established technique, readily applicable to both single phase and multi-phase process operations. One aspect of industrial mixing in stirred tanks that has not yet received significant attention is the processing of non-Newtonian fluids, where the apparent viscosity of the fluid is a function of the applied shear rate. In such cases, a region of stagnant fluid can occur in remote regions of the tank where the shear rate is low, while a region of mobile fluid (a so-called cavern) persists near the rotating impeller. This paper describes the development and validation of a tomographic technique using ERT to identify the occurrence and spatial dimensions of caverns within non-Newtonian fluids agitated in stirred tanks previously published in Simmons et al (2009) published in American Journal of Chemical Engineers, Volume 55. The injection of a concentrated brine solution into the centre of a stirred tank containing the non-Newtonian fluid Carbopol 940 created a high conductivity region that was imaged using both 2D and 3D ERT. The brine was well mixed within the cavern, but diffusion of the brine across the cavern boundary and into the stagnant fluid bulk did not occur over the duration of the measurement. The ERT was validated with optical flow visualisation and Positron Emission Projection Imaging (PEPI) measurements. In the case of the optical visualisation, a coloured dye was used in place of the concentrated brine. For the PEPI work, a positron-emitting tracer fluid was used. Excellent agreement was found between the three techniques.
|Title of host publication
|6th World Congress in Industrial Process Tomography
|Published - 2010