Quantitative evaluation of inert solids mixing in a bubbling fluidized bed
Paper in proceedings, 2012
This paper presents a method to evaluate the lateral mixing processes of bed material in bubbling fluidized beds. The method combines experiments and mathematical modeling and has the aim to constitute a tool for the investigation of the complex solids mixing phenomenon in fluidized bed units. The experimental method used is based on indirect measurements of a tracer agent. A batch of tracer is fed in one corner of the bed and the amount of tracer agent which traversed the bed is measured over time in a corner, diagonally opposite to the tracer feed point. The mathematical model is based on solving a diffusion-like partial differential equation describing the transient lateral dispersion of particles. From this, values of the lateral dispersion coefficient can be obtained.
The method is applied to evaluate the lateral solids mixing in a fluid-dynamically downscaled 3-dimensional cold model with cross-sectional dimensions of 0.3 m x 0.3 m. The cold model can be operated with a variable bed height up to 0.16 m. Since the cold model is designed according to Glicksman’s full set of scaling laws the fluid dynamics is assumed to resemble that of an industrial-scaled bubbling fluidized bed operated at 900°C with cross-sectional dimensions of 1.5 m x 1.5 m and bed heights up to 0.8 m. The results show good qualitative agreement between experimental results and the mathematical modeling and it is concluded that the macroscopic lateral solids mixing behavior in the bed geometry investigated can be described by a diffusion-like partial differential equation. Four superficial gas velocities are investigated and the lateral dispersion coefficients obtained are found to increase steadily over the range of the superficial velocities investigated.