An Experimental and Theoretical Study of Dense Fluidized Bed Fluid Dynamics
Doctoral thesis, Peer reviewed
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Original versionRautenbach, C. An Experimental and Theoretical Study of Dense Fluidized Bed Fluid Dynamics. PhD Thesis, Telemark University College, 2012
The papers presented in the present study aim to elucidate the complex fluid dynamic phenomena in a cold dense phase fluidized bed reactor. Theoretical and experimental studies were conducted with this aim and several advances has been made within the field of fluidization during the course of the present study. Most of the experiments performed in the present study made use of spherical glass particles of different sizes and size distributions. The glass particles are inert when fluidized with air and thus no chemical nor thermal reactions were simulated or modelled. Three measurement system were employed during the course of the study, a Pressure measurement system (probes and detectors), an Electrical Capacitance Tomography (ECT) system and Time-resolved X-ray tomograph. Both of these tomographs are non-invasive and was used to measure different dynamic properties within fluidized bed reactors. In the theoretical investigations it was found that a powered addition procedure can be used to correlate the pressure drop in the transition region between a fixed and fluidized bed. The aim of these studies were to quantify and explain the physical meaning of the shifting parameter. The shifting parameter with a value of approximately 15 was found to produce the best correlation with experimental pressure drop data in the transition region between a fixed and fluidized bed. This value was independent of the particle size distribution and particle density. The exact physical meaning is still the subject of ongoing research. With the tomographic experiments numerous results were obtained. The experimental span of Time-resolved X-ray tomography was found to influence the outcome of measurements made regarding dynamic parameters. The influence of small particles in a wide particle size distribution was also investigated in various ways. It was found that small particles can generally improve the quality of fluidization but may increase the risk of partial defluidization and segregation. The two tomographic modalities were also compared to allow future studies to make an informed choice when it comes to choosing the appropriate tomograph. In particular the ECT tomograph was found to be adequate for diagnosing the quality of fluidization when making use of various statistical concepts. Some of the measuring techniques utilised in the present study was finally also used in a study that had direct implication in industrial applications. This industrial application was the gasification of biomass and is an important technology as it can play a role in the fight against global warming. ZrO and plastic particles with a density ratio of 6, were used in the experiments to simulate the bed material and the char-wood particles in a biomass gasifier. The aim of that work was to study the fluidization properties in a cold fluidized bed with different mixtures of particles. The experimental results showed that the minimum fluidization velocities strongly depended on the particle composition in the bed. In conclusion this study has widened the research communities understanding of fluidized bed reactors by investigating bubble activities and characteristics theoretically and experimentally. The performance of numerous reactors depend on the bubble activity in the dense bed region of a fluidized bed. With the advances made in the present study, the research community has more diagnostic tools and knowledge of non-intrusive measurement systems to understand bubble activity and thus further fluidization technology.
Papers C, D and G are not available in TEORA due to publisher restrictions
Has partsPaper A: Rautenbach, C., Melaaen, M.C. & Halvorsen, B.M. Investigation of the shifting-parameter as a function of particle size distribution in a fluidized bed traversing from a fixed to fluidized bed. In: Fluidization XIII. New paradigm in fluidization engineering (2010). Edited by Sang Done Kim, Yong Kang, Jea Keun Lee and Yong Chil Seo. New York: Engineering Conferences International.
Paper B: Rautenbach, C., Melaaen, M.C. & Halvorsen, B.M. Investigation of the shifting-parameter as a function of density in the fluidization of a packed bed. Paper presented at the the 7th International Conference on Multiphase Flow (ICMF) in Tampa, Florida in May/June 2010.
Paper C: Rautenbach, C., Jayarathna, C.K., Melaaen, M.C. & Halvorsen, B.M. Extension of a model for the distribution of voidage around bubbles in a fluidized bed. In: Computational Methods in Multiphase Flow VI (2011). p. 189-198. Southampton: WIT Press. Full text not available in TEORA due to publisher restrictions
Paper D: Rautenbach, C., Melaaen, M.C. & Halvorsen, B.M. Investigating the influence of fines in fluidized bed reactors using 3D ECT images. In: Computational Methods in Multiphase Flow VI (2011). p. 141-151. Southampton: WIT Press. Full text not available in TEORA due to publisher restrictions
Paper E: Rautenbach, C., Mudde, R.F., Melaaen, M.C. & Halvorsen, B.M. The influence of the experimental span of Time-resolved X-ray tomography on dynamic parameters in a fluidized bed. In: IFSA 2011. Industrial fluidization South Africa. The proceedings of a conference on fluidization held in Johannesburg, South Africa 16-17 november 2011.Johannesburg: South African Institute of Mining and Metallurgy, 2011. p. 193-202
Paper F: Rautenbach, C., Mudde, R.F., Yang, X., Melaaen, M.C. & Halvorsen, B.M. (2013). A comparative study between Electrical Capacitance Tomography and Time-resolved X-ray tomography. Flow Measurement and Instrumentation 30, 34-44. The published version is available at http://dx.doi.org/10.1016/j.flowmeasinst.2012.11.005
Paper G: Rautenbach, C., Melaaen, M.C. & Halvorsen, B.M. (2013). Statistical diagnosis of a gas-solid fluidized bed using Electrical Capacitance Tomography. International Journal of Multiphase Flow 49, 70-77. The published version is available at http://dx.doi.org/10.1016/j.ijmultiphaseflow.2012.10.002
Paper H: Rautenbach, C., Melaaen, M.C. & Halvorsen, B.M. Identification of size difference segregation using Electrical Capacitance Tomography and statistical analysis. Submitted to Advanced Powder Technology Journal. Full text not available in TEORA.
Paper I: Thapa, R.K., Rautenbach, C. & Halvorsen, B.M. Investigation of flow behaviour in biomass gasifier using Electrical Capacitance Tomography (ECT) and pressure sensors. In: Proceedings of the 11th International Conference on Polygeneration Strategies ICPS. Vienna: Vienna University of Technology, 2011. p. 97-106