Mixing of Large Solids in Fluidized Beds

Abstract: Fluidization is a technology that is widely used in systems in which particulate solids are to be transported, mixed, and/or reacted with gases. In fluidized bed applications, the lateral mixing rate of the solids and the heat and mass transfer with their surroundings play important roles in process performance. These transport mechanisms are affected by the solids axial mixing, as particles immersed in the dense bed will experience higher levels of heat transfer, lower mass transfer, and lower rates of lateral mixing than they would if floating on the bed surface. However, there is a lack of knowledge regarding the effects of the solids properties and operating conditions on the solids mixing. As a consequence, there is a lack of predictive tools that can be used for optimizing the design and operation of fluidized beds. This work focuses on advancing the current understanding of the mixing of large solids (typically fuels) in fluidized beds, with the aims of promoting the design of new applications and improving the scale-up and operation of commercial units. While a generic approach is adopted in terms of considering a wide range of solid particle properties (size and density), the focus is on biomass particles, for which thermochemical conversion fluidized beds are especially suited, due to their: high-level fuel flexibility (being able to convert efficiently low-grade fuels); ability to control emissions with in-bed methods; and inherent capability to capture CO2 with looping dual fluidized bed systems. This work combines semiempirical modeling with experiments that apply magnetic particle tracking in a fluid-dynamically downscaled bed, enabling the closure as well as the validation of the model. By deriving a mechanistic description of the motion of a spherical object, the model identifies key parameters that are crucial for describing the mixing. Among these, the effective drag of the bed emulsion acting on the fuel particle is further studied in dedicated experiments with falling and rising tracers in various types of beds at minimum fluidization. The stress patterns observed in these rheological experiments reveal a non-Newtonian behavior of the drag between the bed emulsion and immersed larger objects. This is then implemented in the model for further upgrading of the mechanistic description. The model is shown to describe ably both axial mixing and the lateral mixing of different fuel types under conditions applicable to industrial-scale hot units. The combination of modeling and experimental work shows that while axial mixing is fostered by increasing the fluidization velocity, bed height, distributor pressure drop, or fuel particle density and decreasing the fuel particle size, only a higher fluidization velocity exerts a clear influence on the lateral dispersion. This can be explained in terms of the influence of the fluidization velocity on the width of recirculation cells, which are found to play a major role in the lateral mixing of fuel particles and warrant further study.