(140d) Development, Validation and Application of a New Methodology for the Measurements of Particle Stresses in an Aerated Bed

Fluidised beds are one of the most widespread methodologies in the chemical engineering industry, they can be found in various applications such as fluidised bed reactors, combustors and dryers [1]. However, despite particle stresses are of high importance to understand the hydrodynamics of fluidised bed, methodologies that allow the accurate evaluation of the particle stresses are lacking [2]. Colafigli et al., [3] developed a device that allows for the evaluation of the particle stresses and thus the viscosity in a homogeneous gas-fluidised bed using silica particles. The measurement principle of such a device is based on the Coaxial-Cylinder-Viscometer for liquids. The Coaxial-Cylinder-Viscometer (CCV) is a well-known device used to evaluate the physical characteristics of fluids. This instrument consists of a rotating inner cylinder, the so-called bob, and a static outer cylinder or cup. The fluid is placed in the annular gap between the two cylinders. The apparent torque, which can be measured during the rotation of the inner cylinder, can be converted to dynamic viscosity and shear stress [4]. The main objective of this work is to confirm whether it is possible to measure particle stresses in fluidised beds using the same measurement principle as the one used in a CCV for liquids.

In our work, the Freeman FT4 Powder Rheometer aeration test was used as a smaller version of the CCV, where the original 48 mm rotating impeller was replaced with a new cylindrical cell designed for this study. This cell, which is 70 mm in height, 36 mm in diameter and composed of 6 blades is 3D printed using plastic material. It is placed inside the FT4 cylindrical cup and is analogous to the VCC with a 7 mm gap (Marchal et al., [5]). The cell is then rotated at different rotational velocities, and the torque needed to move the aerated powder placed in the cup is evaluated. In order to validate this new FT4-VCC combination, it was firstly tested with glycerol 99% which is a Newtonian fluid with a known viscosity. The corresponding torque was registered as the shear rate ý was raised from 0.45 to 48 s^-1. The torque was then converted to shear stress τ using the relations given by Marchal et al., [5], this while assuming a cylindrical shear zone in the VCC and a homogeneous stress distribution along the devices height. The obtained shear stresses appeared to increase proportionally with the imposed shear rate, following the equation, with being the viscosity of the fluid, which was found to be in good agreement with the known viscosity of glycerol 99%.

The VCC was then employed using monodispersed (500 mm diameter) spherical glass beads. The corresponding torque was registered as the shear rate ý was raised from 1.5 to 32 s^-1 and the air velocity was increased from 0 to 100 mm.s^-1. The first results obtained with the VCC with no aeration showed that the shear stress is independent of the increasing shear rate. The corresponding viscosity η is also evaluated as the ratio of the shear stress to the shear rate, and the obtained results are in good agreement with the one obtained by Marchal et al., [5], using the same type of particles. Furthermore, the results obtained with the air velocities used in this study showed that the glass beads shear stress increases with shear rate, showing a Bingham plastic behaviour (Ï„=Ï„₀+ηý where Ï„₀ is the yield stress). Further measurements including different air velocities and lower shear rates will be performed in order to confirm the first measurements and observations.

References

[1] M. S. Ray, “Chemical Engineering, Volume 2: Particle Technology and Separation Processes, 4th edn, by J.M. Coulson and J.F. Richardson. Pergamon Press, Oxford, UK. 1991. 968 pp. ISBN 0-08-037957-5,” Dev. Chem. Eng. Miner. Process., vol. 1, no. 2–3, pp. 172–172.

[2] R. Borghi and F. Anselmet, Turbulent Multiphase Flows with Heat and Mass Transfer. John Wiley & Sons, 2013.

[3] A. Colafigli, L. Mazzei, P. Lettieri, and L. Gibilaro, “Apparent viscosity measurements in a homogeneous gas-fluidized bed,” Chem. Eng. Sci., vol. 64, no. 1, pp. 144–152, Jan. 2009.

[4] D. S. Viswanath, T. Ghosh, D. H. L. Prasad, N. V. K. Dutt, and K. Y. Rani, Viscosity of Liquids: Theory, Estimation, Experiment, and Data. Springer Netherlands, 2007.

[5] P. Marchal, N. Smirani, and L. Choplin, “Rheology of dense-phase vibrated powders and molecular analogies,” J. Rheol., vol. 53, no. 1, pp. 1–29, Dec. 2008.