Experimental Investigation of the Scale up of Turbula Mixers on the Basis of Kinematic and Dynamic Similarities
Résumé
Many process from a wide range of industries (cosmetic, pharmaceutical, food) use powder as
raw materials. In most of them mixing or premixing dry particles is a critical step in the process.
Indeed the rate of mixing is known to have an influence on the taste of a food product or the
bioavailability of a drug. Thus, in order to meet the specifications of the product the choice of the
mixer and operating parameters is a key point for the process.
However, the complexity of granular materials leads to difficulties in predicting the behavior of
powders inside a blender. Simulation could be used to understand, (for a perfect mixture, as glass
sphere) mixing mechnanisms inside the blender in oder to improve design or operations. But
these data are difficult to transpose to a real material, where shapes and sizes of particles are
widely dispersed. Therefore the analysis and the optimization of a process mixing can not do
without an experimental work for a given mixture. The cost and number of raw materials needed
to conduct these studies imply that such work is often done at the laboratory scale. The main
challenge then lies in the extrapolation of results from lab scale to pilot or industrial scale with
the minimum tests in order to optimize cost. As systems of powder can not be accurately
described by adimentionnal number scaling-up is based rather on the principle of similarities
which can be, geometric [1], kinematic [2] or dynamic (use of Froude number) [3].
The work presented here concerned with the scaling-up of Turbula mixer, commonly used both in
industry and research.. The efficiency of the TURBULA® is achieved by the interaction of
rotation, translation and inversion as per the geometric theory according to Schatz. This
combination results in a three-dimensional chaotic motion to obtain homogeneous mixture
rapidly for many granular systems.[4]
On the one hand, geometric, kinematic and dynamic studies was performed thanks to a
Solidwoks simulation of three Turbula scales (Laboratory Scale – Turbula T 2F, Pilot scale –
Turbula T 10 B, Industrial scale – Turbula T 50 A). The geometric study showed that there is a
constant ratio between two key lengths for the 3 Turbula sizes. Amplitude velocities and
acceleration for a point at the bottom of the tank was extracted from simulations.
On the other hand, measurment campaigns of the mixture homogenity of two different powder
systems were conducted in the three Turbula sizes. Intensity of segregation [5] was compared for
kinematic or dynamic similarities (i.e equal velocities or acceleration for a point at the botoom of
the tank). The dynamic criterion appears to lead to a better correlation between results obtained in
terms of homogeneity of powder mixtures, in the three Tubula sizes.
Domaines
Sciences du Vivant [q-bio]Origine | Fichiers produits par l'(les) auteur(s) |
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