Experimental Investigation of High-Temperature Fluidization Using Endoscopic Particle Image Velocimetry-Digital Image Analysis Technique (ePIV/DIA)

Enhanced mass and heat transfer, resulting in a more homogeneous heat and mass distribution, makes the use of fluidized bed reactors an interesting alternative to conventional configurations such as packed bed reactors, especially for processes operated at elevated temperatures. However, detailed studies on the hydrodynamics of fluidized bed reactors at high temperatures are still scarce, and a deeper understanding of the influence of the temperature on the fluidization behaviour is required for the modelling and optimization of high-temperature fluidized bed reactors.

The main objective of this work is to investigate high-temperature fluidization experimentally, using the novel non-intrusive ePIV/DIA technique. This technique combines endoscopic particle image velocimetry with digital image analysis, with which detailed information on both the gas and solid phases of the fluidized bed can be obtained simultaneously with high spatial and temporal resolution at the required high operating temperatures.

Using a fluidized bed consisting of zirconia particles of 400-600 mm diameter (Geldart B type), ePIV/DIA experiments have been carried out up to a temperature of 500 ºC, keeping the excess velocity constant using the measured minimum fluidization velocity at the respective temperature. From the experimental data, detailed information on the solids circulation patterns and bubble properties as function of the position in the bed at different temperatures have been obtained.

A very strong influence of the operating temperature has been observed. The effect of particle raining increases significantly at elevated temperatures, especially at 400-500 ºC, affecting considerably the hydrodynamics of the system. The increase in the extent of particle raining at higher temperatures results in a decrease in the average bubble diameter, but also in an increase in the number of bubbles recorded per image. The larger number of smaller bubbles at higher temperatures improves the fluidization behaviour, since it enhances the gas-solid contact and reduces the gas by-pass through the bubbles. However, it has also been observed that at elevated temperatures the solid fluxes in the bed are smaller, most probably also related to the increase in particle raining reducing the upwards solids movement. It should be noted that decreased solid fluxes could result in reduced solids mixing, consequently lowering the quality of the fluidization.

The increased particle raining at elevated temperatures can be caused by the decrease in gas density resulting in a decrease in the hydrodynamic forces, or because of the increase in the importance of interparticle forces, as suggested by different authors in the literature. In this work, the relative importance of the hydrodynamic and interparticle forces have been analysed using the obtained detailed hydrodynamic data as a function of temperature.

ACKNOWLEDGEMENTS

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 679933 (MEMERE project).

The present publication reflects only the author’s views and the European Union is not liable for any use that may be made of the information contained therein.