Abstract

Yield stress fluids are commonly encountered in the pharmaceutical, wastewater and bioprocess industries. On agitation of these fluids with an impeller, a zone of significant motion (cavern) is formed surrounded by stagnant regions. These inhomogeneous conditions are undesirable from a product quality standpoint. Therefore, to evolve a mixing system design that would eliminate these problems, experimental measurements of mixing time were obtained and combined with power consumption to provide a measure of mixing system efficiency. The effect of different parameters such as fluid rheology, impeller rotational speed, impeller type and impeller clearance on the mixing times was also investigated. In addition, using CFD, numerical mixing times were calculated and a comparison of the numerical and experimental mixing times were conducted to investigate the capability of the CFD tool to correctly predict the homogenization process in mixing tanks. In general, it was observed that the power characteristics of the different agitators were well reproduced by the computational package. In addition, CFD was able to correctly predict the effect of impeller rotational speed and fluid yield stress on the mixing times. However, the effect of impeller clearance on the mixing time was not correctly predicted by the CFD package when compared with experimental results obtained in this work as well as those obtained by other researchers. A comparison of the impellers used in this study (Pitched Blade Turbine (PBT), marine propeller and Lightnin A320) using the mixing time correlations available in the literature to fit the experimental data revealed that the PBT was superior to the other impellers in mixing yield stress fluids. In addition, the validated CFD model was used to measure the dimensions of the cavern formed around the impeller and it showed good agreement with the Elson's cavern model.

Highlights

  • Such fluids exhibit a high apparent viscosity at low shear rates, and since the shear rate decreases as the distance from the impeller increases, circulation problems can often be encountered when mixing such fluids (Hayes et ai., 1998)

  • A relationship between the results obtained in literature and those of this work can be established by noting that the pitched blade turbine generates two circulation loops or compartments (Paul et ai., 2004) in the mixing tank

  • According to Figure 6. 13, before the cavern reache the wall, the ratio H C i an average value Dc of 0.48 for both impeller type, which i within the range of the value of 0.55 ± 0.10 predicted by EI on (1998) and Solomon el al. (1981) for the pitched blade turbine but con iderably Ie than the value of 0.75 ± 0.05 predicted by El on (1990) for the marine propeller, which may be due to difference between the rheological propertie of the fluid u ed in thi work and that reported in the literature

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Summary

Introduction

Yield stress fluids contain structured networks of molecules that depend on the shear rate of the fluid. Such fluids exhibit a high apparent viscosity at low shear rates, and since the shear rate decreases as the distance from the impeller increases, circulation problems can often be encountered when mixing such fluids (Hayes et ai., 1998). A review of two important parameters for describing mixing system performance namely power consumption and mixing time is undertaken, and a summary of the important research involving non-Newtonian fluids in stirred tanks is conducted, after which the concept of cavern formation is presented especially with regards to the important information it provides on the extent of mixing in the mixing tank.

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