In continuous processes – for instance in construction chemicals, the food industry, plastics compounding, powder coating or battery material production – it is frequently necessary to mix large mass flows to a high degree of homogeneity in a very short time. A central design parameter here is the residence time, that is to say the average time a particle spends in the mixing chamber. The shorter the residence time, the higher the energy acting on the small process volume generally has to be. This also increases the importance of highly accurate dosing, since fluctuations have an immediate effect on the mixing quality where residence times are very short.

Important mixer concepts

The continuous mixers that can be designed for short residence times and high homogeneity include ring-layer mixers, highly dynamic turbulence or centrifugal mixers, continuous ploughshare mixers, twin-shaft paddle mixers, pin mixers, rotor-stator mixers, static mixers as well as twin-screw mixers and twin-screw extruders. Tubular mixers are particularly suitable, since the mix is transported from the inlet to the outlet and a comparatively defined residence time thereby results. The more cubic a mixing chamber is, the broader the residence time spectrum generally is. Such geometries are therefore often less well suited to very short and precisely controlled processes.

Significance of the residence time

Short residence times are particularly important where thermal or mechanical loads are to remain low, hold-up quantities have to be small or rapid product changes are required. At the same time, the mixing intensity has to be high enough to achieve good homogeneity.

Practical selection

Where short residence times with solids are the priority, ring-layer mixers, turbulence mixers or pin mixers are particularly well suited. For more flexibility with bulk materials, ploughshare mixers or twin-shaft paddle mixers suggest themselves instead.

Design parameters

For the selection, the residence time and the residence time distribution, the fill level, the mass throughput, the specific energy input and the desired mixing character are decisive above all. Distributive mixing describes the spatial distribution of the components, dispersive mixing the breaking up of agglomerates or droplets. Since continuous mixers have only a small buffer capacity, very constant dosing, generally gravimetric, is particularly important.

The final design should always be secured through pilot plant trials with the original product and through a measurement of the residence time distribution.