Design and scale up of a mixing process for a novel automotive coating

The study reported here was performed as part of a larger project aiming to develop a high performance automotive coating and the associated manufacturing process to enable large scale manufacture of the new product. We report from the process development aspect of the project which aimed at establishing the comparative blending performance of two types of impeller- one currently in use and an alternative- at different scales. Mixing time measurements compared on the basis of volumetric power input were comparable for the sawtooth impeller and pitched blade turbine over the turbulent and transitional regimes at formulation (T= 0.13 m, ∼2 l) and pilot scales (T= 0.30 m, ∼ 20 l). At the largest scale of ∼ 160 l, it took a lot longer to blend when using the sawtooth impeller. Measured mixing time results were in agreement with predicted values using the well-established correlation for both impellers in T= 0.30 m and also for the PBT at large scale, For the sawtooth impeller the correlation underpredicted at large scale unless a higher value for the constant in the mixing time correlation (about twice) was used. This deviation in the performance at large scale has highlighted the complexity of design and scale up using a sawtooth impeller.