Keynote Talk: Development of a New Methodology for Measurements of Particle Stresses in Fluidised Beds

Gas-solid fluidised beds are commonly used in industrial chemical processes such as fluid catalytic cracking, coal gasification and granulation [1]. In fluidised beds, large fluctuations in the solid volume fraction and phase velocities at a wide range of length and time-scales are observed. Researchers have investigated these flow behaviours in fluidised beds through experimental studies for a few decades. Nevertheless, accurate measurements of the particle stresses in fluidised beds are still not available [2]. Colafigli et al., [3] developed a device that allowed measurements of the bulk viscosity in a homogeneous gas-fluidised bed using silica particles. The device measurement principle is based on the Coaxial-Cylinder-Viscometer for liquids (CCV). The CCV is a well-known device used to evaluate the rheological characteristics of fluids. The 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 the dynamic viscosity and the shear stress of the fluid [4]. The main objective of the present work is to develop a new instrument to measure particle stresses in fluidised beds, based on the same rheological measurement principles employed in a CCV for liquids.

In our work, the Freeman FT4 Powder Rheometer, operated in its aeration mode 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 the purpose of the present 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 a Virtual Couette Cell (VCC) with a 7 mm gap [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. A new device called FBR (Fluidized-Bed-Rheometer- Ruhr-Universität, Bochum) was sized and developed in parallel with this. It is based on the same measurement principle of the Coaxial-Cylinder-Viscometer. Like the CCV, the FBR is composed of a rotating inner cylinder measuring 450 mm in length and 80 mm in diameter, while the outer cylinder is larger in both respects having a 600 mm length and 109 mm diameter. Each cylinder is composed of a different material with carbon fibre forming the inner cylinder and glass forming the outer. A fluidised bed is generated in the 14.5 mm gap between the cylinders, and the torque needed to rotate the bob is recorded. In order to avoid particles sliding on the rotating cylinder, the latter is coated with particles as the fluidised ones (glass beads).

Both technologies (FBR and VCC) were operated using mono (500 mm diameter) and poly (400 – 600 mm) dispersed spherical glass beads. The corresponding torque was recorded as the shear rate was raised from 1.5 to 32 s^-1. The air velocity was set to 0 m.s^-1 for the VCC and 1.6 m.s^-1 for the FBR. The registered torque was then converted to shear stress t 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 device's height. 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 h is also evaluated as the ratio of the shear stress to the shear rate, and the obtained results are in good agreement with the ones obtained by Marchal et al., [5], using the same 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 (t = t₀ + hgwhere t₀ is the yield stress); this result applies to both the VCC and the FBR. Further measurements, including different air velocities and different particles varying in size and shape, will be performed in order to confirm the preliminary 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, 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, 2008