Experimental Investigation of the Scale up of Turbula Mixers on the Basis of Kinematic and Dynamic Similarities

[1] W. Entrop, 1978, International Syposium on Mixing, Mons, D1, pp 1-14[2] Albert Alexander, Troy Shinbrot, and Fernando J. Muzzio, 2002, Scaling surface velocities inrotating cylinders as a function of vessel radius, rotation rate, and particle size.PowderTechnology, 126(2) :174–190[3] Y. L. Ding, R. N. Forster, J. P. K. Seville and D. J. Parker, 2001, Scaling relationships forrotating drums. Chemical engineering science, 56:3737-3750[4] Wohlhart, K., 1981, A Dynamic analysis of the Turbula, International Symposium on Gearing& Power Transmissions, Tokyo.[5] P. Danckwerts, 1952, The definition, measurement and some characteristics of mixtures,Applied scientific research. Section A, 3, 279-296Many process from a wide range of industries (cosmetic, pharmaceutical, food) use powder asraw 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 thebioavailability of a drug. Thus, in order to meet the specifications of the product the choice of themixer and operating parameters is a key point for the process.However, the complexity of granular materials leads to difficulties in predicting the behavior ofpowders inside a blender. Simulation could be used to understand, (for a perfect mixture, as glasssphere) mixing mechnanisms inside the blender in oder to improve design or operations. Butthese data are difficult to transpose to a real material, where shapes and sizes of particles arewidely dispersed. Therefore the analysis and the optimization of a process mixing can not dowithout an experimental work for a given mixture. The cost and number of raw materials neededto conduct these studies imply that such work is often done at the laboratory scale. The mainchallenge then lies in the extrapolation of results from lab scale to pilot or industrial scale withthe minimum tests in order to optimize cost. As systems of powder can not be accuratelydescribed by adimentionnal number scaling-up is based rather on the principle of similaritieswhich 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 inindustry and research.. The efficiency of the TURBULA® is achieved by the interaction ofrotation, translation and inversion as per the geometric theory according to Schatz. Thiscombination results in a three-dimensional chaotic motion to obtain homogeneous mixturerapidly for many granular systems.[4]On the one hand, geometric, kinematic and dynamic studies was performed thanks to aSolidwoks 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 aconstant ratio between two key lengths for the 3 Turbula sizes. Amplitude velocities andacceleration 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 powdersystems were conducted in the three Turbula sizes. Intensity of segregation [5] was compared forkinematic or dynamic similarities (i.e equal velocities or acceleration for a point at the botoom ofthe tank). The dynamic criterion appears to lead to a better correlation between results obtained interms of homogeneity of powder mixtures, in the three Tubula sizes