Blending

Blending
The objective in any blending action is to achieve the homogeneity required for the process throughout the volume of the vessel. Concentration and/or temperature gradients arise through feeding in product, mass transfer, chemical reactions or through supplying or removing heat. These situations can occur in single-phases and multiphase systems.
 
Turbulent blending
Extensive basic research by many authors together with measurements taken in industrial vessels have shown that the blend time characteristic n · q is a constant for stirred vessels fitted with baffles. This applies generally for turbulent flow regimes, but also for laminar flow induced by positive displacement impeller systems.
 
Laminar blending
To achieve a defined blending result it is a prerequisite that the entire volume of the vessel is involved in the laminar flow. The Reynolds number calculated with the impeller diameter represents only an inadequate description of the flow conditions in the entire vessel, particularly in the case of impellers with small zones of agitation. The dimensionless blend time in n x mixing time for these impellers therefore starts to increase at a point where the reducing Reynolds number theoretically still indicates turbulent flow.
 
Multiple impeller systems with a large diameter ratio, such as EKATO VISCOPROP impellers arranged one above another, are suitable for Reynolds numbers > 50.
 
With high viscosities, encountered in the polymerization of polyamides, polyesters or polystyrene (up to 1000 Pas), only positive displacement impellers such as the EKATO PARAVISC or helical ribbon agitators can insure a sufficient degree of homogeneity. Even at extremely low Reynolds numbers a circulating flow throughout the entire process space is achieved, which carries the product fed into the vessel to zones with high shear rates and reduces the thickness of streaks from the initial value.