Conventional single-shaft mixers such as propellers and […]
Conventional single-shaft mixers such as propellers and turbines can be capable of preparing high solids formulations in the low viscosity range. These are economical machines suitable for solids that readily incorporate into liquid with only gentle agitation or stirring action. However, for solids that resist wet-out, for instance hydrophobic powders added into water, low-speed agitators become very inefficient. To accelerate dispersion or dissolution of such solids, higher energy devices such as saw-tooth dispersers and rotor/stator mixers enable shorter lead times.
Running at tip speeds around , a properly designed saw tooth disc style disperser creates vigorous flow within the mix vessel. It generates a vortex into which solids can be added for quick incorporation into the batch. Dispersers are typically used in applications up to around in viscosity.In comparison, a rotor stator mixer typically features a four-blade rotor running at tip speeds in the range of min within a close tolerance fixed stator.
This type of device creates mechanical and hydraulic shear by continuously drawing product into the rotor and expelling it radially through the openings in the stator. Because of the restriction provided by the stator, this mixer offers higher shear but less pumping capacity than an open-disc saw-tooth disperser blade. For the same reason, its viscosity limit is lower The rotor stator mixer is available in both batch and inline designs.In earlier powder injection systems, a pump would propel the liquid stream into an eductor, creating a vacuum. Powders fed through an overhead tube would be drawn by this vacuum into the eductor where it joined the liquid flow.
The resulting ‘pre-mix’ moved on to a rotor/stator mixer which then applied shear and mixing action, breaking down agglomerates and transporting the mixture downstream. In its day, this system offered a useful tool for powder induction. The inline system eliminated the floating solids problem of batch systems, and it offered a more precise control over the mixing process. But this set-up also presented some serious limitations. With three separate devices in series, maintenance in terms of labor, required expertise and spare parts — was intensive. Balancing the performance of the pump, educator, and mixer was often difficult, and in many applications, downtime was quite high.
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