Characterization and Development of Packed-Fluidized Bed Reactors

Doktorsavhandling, 2024

Packed- or confined fluidized beds utilize fixed packings inside a bed of fluidizing particles. This novel concept can have advantages with respect to gas-solid contact and solids flow patterns. This could be of importance for many fluidized bed conversion technologies, such as chemical-looping. In this thesis, different aspects of packed fluidized beds are explored, using experimental and modeling tools. The study includes an investigation of the effects of packings on heat and mass transfer, as well as solids flow patterns. For this purpose, different types of packings are selected and assessed.

Initially, the heat transfer coefficient to a horizontal tube submerged in a packed-fluidized bed, pressure drop, and vertical solids segregation are experimentally evaluated for bed temperatures ranging from 400°C to 900°C and superficial gas velocities from 0.04 m/s to 0.411 m/s. Subsequently, the conversion of gaseous fuels during CLC is investigated. CLC experiments are conducted using CH4, CO, and syngas (50/50% H2/CO) as fuels, at temperatures between 840°C and 940°C. Next, gas-solid mass transfer in a bubbling packed-fluidized bed is studied. This is achieved through targeted experiments using a bed of moist silica gel, where the rate of H2O desorption is monitored during the experiments. A detailed mass transfer analysis is performed using a model that accounts for various steps in the mass transfer chain. Finally, the residence time distribution of bed material in a bubbling packed-fluidized bed with throughflow is examined. The cold-flow reactor setups allow for continuous cross-current and counter-current flow of particles and fluidizing gas. The effects of packing type, fluidization number, bed height, and solid throughflow rate are analyzed. The axial dispersion and tank-in-series models are utilized to categorize flow patterns of particles in packed-fluidized beds.

The results show that the nature of the packings have significant impact on the behavior of bubbling-fluidized beds. Packings with low void factor such as ASB have a lower heat transfer coefficient, higher pressure drop, and more significant vertical segregation, compared to a bubbling bed without packings. However, packings with high void factors such as RMSR showed an improvement in heat transfer coefficient (up to 1243 W/m2K) at higher gas velocities compared to a bubbling bed with no packings (up to 1124 W/m2K). Also, beds with RMSR and Hiflow packings had lower pressure drop, lower vertical segregation, and higher fuel conversion in CLC compared to a bubbling bed with no packings. Furthermore, the inhibition of bubble formation and growth in the packed-fluidized bed enhances the emulsion-bubble mass transfer by up to 23% compared to a bed without packings. As part of this work, it was also shown that low void factor packings can alter the mixing behavior of solids in BFBs. Without packings, the fluidizing solids are well mixed, similar to a continuous stirred tank reactor (CSTR), while packings could transmit the flow pattern to be more similar to a plug flow reactor (PFR). This was confirmed by the reactor modeling, with the tank-in-series model resulting in an increase in the number of reactors in series from 1 up to 10 reactors when using packings. Furthermore, the vessel dispersion number can be reduced up to tenfold in such packed-fluidized beds.

This work demonstrates that the use of packed-fluidized beds or confined fluidization has a significant effect on important phenomena in a bubbling fluidized bed. This could be of significance in a number of new and novel technologies, including chemical-looping combustion or gasification and hydrogen production systems.