Hydrodynamics and Bubble Wake Modelling of Three-Phase Semifluidized Beds


Hydrodynamics and Bubble Wake Modelling of Three-Phase Semifluidized Beds

Schubert, M.; Larachi, F.

Three-phase fluidized beds are widely encountered in the chemical, petrochemical, and biochemical industry. Traditionally, they have been employed for hydrogenation and hydrodesulfurization of heavy oil and petroleum residuum in hydrotreating and upgrading processes, for the Fischer-Tropsch synthesis, for coal liquefaction and gasification, but also for bio-oxidation processes for wastewater treatment and for the production of pharmaceuticals. However, the field of application is widespread, including also physical processing such as filtration, particle collection, air cooling, and (de)humidification, as well as three-phase transport.
The most common operation mode is the cocurrent gas and liquid upflow fluidized bed system with liquid as the continuous phase. The semifluidization phenomenon is rather characterized by a combination of a fluidized bed and a fixed bed in series in a single containing column built by expansion of the fluidized bed and then compressing the solid particles against a porous retaining grid at the top that constrains the bed. A semifluidized bed is claimed to overcome some drawbacks of fluidized beds such as solids backmixing, attrition of particles, and erosion of surfaces, and is characterized by uniform bed temperature profiles and flow distribution which may not be as easily achievable as in the case of fixed beds.
For full hydrodynamic characterization of the semifluidized bed all phase holdups in both sections must be known. While the total pressure drop of the semifluidized bed was reasonably well predicted by ad hoc model equations proposed for each section, so far individual holdups were not directly measured for the fixed bed section (Chern et al. 1984). The gas holdup in the upper fixed bed section was only calculated on the basis of the pressure drop measurements and on using separate flow model equations. Furthermore, knowledge about the porosity is required which was proposed to take a constant value. While this assumption is valid for isolated packed beds, it is not evident that it holds in a semifluidized system, especially for industrial porous catalyst particles.

  • Poster
    19th International Congress of Chemical and Process Engineering CHISA 2010 and the 7th European Congress of Chemical Engineering (ECCE-7), 28.08.-01.09.2010, Prague, Czech Republic

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