Experimental Investigations On Solids Mixing In An Internally Circulating Fluidized Bed By A Single Particle Tracer

Internally circulating fluidized bed (ICFB) is composed of two connected fluidized beds with different fluidization gas, which are separated by a partition or a tube with an orifice. Solids are transported through the orifice and the region above the bed, forming regular internal circulation between two beds. ICFB not only possesses advantages of good heat and mass transfer that general fluidized beds possess, but also can effectively regulate and control the macroscopic motion of particles in the bed and residence time in different regions, thereby enabling the hierarchical control of reaction. ICFB is widely used in industries due to its special internal structure and prominent advantagesSolids mixing is one of the important basic researches in fluidization, which is in relation to the heat distribution and gas-solid reaction in the bed. In the internally circulating fluidized bed, solids mixing includes two aspects:the lateral and vertical solids mixing in one side bed and the macroscopic solids transport between two beds, namely internally circulating solids flow. In this paper, results are obtained through the analysis of particle trajectories, velocity distribution field, velocity statistic distribution, residence time and cycle time by tracing fluorescent particles and processing images, which qualitatively reveals the internally circulating mixing characteristics. When the gas velocity in up-flow bed is above 4 times the initial fluidization velocity, bad flow region obviously exists in the down-flow bed, mainly near the partition area and areas under the orifice. Solids velocity distribution is mainly influenced by fluidization gas, which distinguishes obviously in the lateral and vertical direction. The solids velocity varies in 0-0.2m/s in the down-flow bed and 0-0.8m/s in the up-flow bed. In the down-flow bed, more than 50%solids velocities were below zero, indicating a strong down flow of solidsSolids internally circulating flow rate directly affects the heat transfer, mass transfer and reaction efficiency in the reactor, which is focused by researchers. In this paper, solids flow near the orifice is studied and solids flow rate is estimated by tracking a particle tracer. A 40mm long square tube is set at the orifice for better measurement. The effects of design parameters and operating parameters on the circulation flow rate are investigated, including the orifice size, solids size, fluidization gas in down-flow bed, fluidization gas in up-flow bed and static bed height. Correlation between the circulation rate and parameters is also put forward. It is found that the pressure drop across the orifice is the main driving force of solids flow from down-flow bed to up-flow bed. The increasing of static bed height, fluidization velocity and the orifice area are in favor of solids flow near the orifice, circulating flow rate varying in 10-60 Kg·m-2·s-1. Increasing solids size (A, B and D particles) leads to the flow rate increasing due to enhancement of flow resistance.In addition, hydrodynamics model is established to calculate the flow rate under the known conditions, namely bed geometry, bed material characteristics, bed material mass and fluidization gas in two beds. The model is validated compared with the experimental data. Finally, the influence of various parameters on the internal circulation is studied by this model. The operating region of fluidization gas and bed material parameters is given.