The Study On Bubbling Behaviors In The Fluidized Bed With Binary Particles

Due to such advantages as larger phase contact area, higher heat-mass transfer rate and uniform temperature distribution, gas-solid bubbling fluidized bed has been widely employed in industry processes, such as pharmaceutical, food and chemical industries. Bubble size is a key factor that affects heat and mass transfer of gas and solids in the fluidized bed, and then reactor efficiency. So, there is an urgent need to study the characters of the bubble behaviors in the bi-dispersed particles fluidized bed for optimizing operating conditions and designing fluidized bed reactor.Experimental was carried out in a 2-D fluidized bed, which is made of Perspex with dimensions equal to 200mm(width)×20mm(depth)×1500mm(height). A digital camera was set nearby 2D fluidized bed and 100 W halogen bulbs were used as the light source to illuminate the back side of the bed. Air was used as the fluidizing gas, dolomite particles were used as the heavy component and silica gel particles were used as the light component. Digital image analysis technique was employed and the program was developed with Matlab in this work. The program could get the bubble information, such as bubble diameter and bubble velocity.Experimental results show the existence of two characteristic fluidization velocities, i.e., initial fluidization velocity of light component Uif and that of heavy component Uff. Three fluidization states that is segregation, partial mixing and fully mixing of binary particles in the fluidized bed, can be identified by mixing index, which corresponds to the different bubble behaviors. When the binary particles are segregated or partial mixing, the phase interface between the lower region in which the heavy particles accumulate and the upper region in which the light particles concentrates is present. The bubble will break out when it raises through the phase interface, which results in greater bubble number, smaller bubble size and lower bubble velocity. When the binary particles are the fully mixing, the phase interface is not present. Under this fluidization state, bubble size and bubble velocity decrease with increasing jetsam concentration. Bubble size will also decrease with enlarging the difference of particle size of binary particles. Bubbles with same size decrease bubble velocity with the increase of jetsam concentration.Correlations for predicting bubble size, rising velocity have been developed based on the experimental data. As particle properties and mixing state of particles were taken into account, the empirical expressions are different to the different mixing degree of binary particles. The predicting results agree well with experimental data.