Fluidization Properties In Micro-scale Fluidized Beds With Acoustic Intensification

Micro-scale fluidized beds(MFBs) are characterized excellent heat and mass transfer efficiency. Because of the small dimension of MFBs, the energy consumption and waste effluent is decreased greatly. With the intensification of acoustic wave, the effect of flow, heat and mass transfer rates in MFBs are increased significantly. It can be used as catalytic and none catalytic reactors. The fluidization properties are valuable for theoretical investigations and industrial applications in MFBs.The fluidization characteristics of quartz sand and FCC catalyst particles in 6 micro-scale fluidized beds with inner diameters(Dt) of 4.3, 5.5, 10.5, 15.5, 20.5, 25.5 mm are investigated. The effects of particles, gas properties, wall effect and bed voidage are examined, respectively. Experimental results show that the theoretical pressure drops of fixed bed in micro-scale fluidized beds deviate from the experimental values, and the deviation increases with decreasing bed diameter. When Dt is smaller than 15.5 mm, the theoretical pressure drops of fluidized are greater than the experimental values, then the theoretical pressure drops of fluidized beds show no obvious differences with the experimental values when Dt exceeds than 15.5 mm. The main reason is that smaller Dt causes incrased bed voidage. The pressure drop and minimum fluidization velocity correlations derived from large size fluidized beds fail to compute MFBs. The minimum fluidization velocity (umf) is increased with decreasing Dt. When the ratio of Hs to Dt ranging from 1 to 3, umf is expressed by linear relationship with Hs. A new equation was developed to predict umf in micro-scale fluidized beds under the present experimental conditions with its error being 15%.The pressure fluctuation signals of FCC catalyst and quartz sand particles in MFBs are tested. The signals are analysed by Daubechies 2-order wavelet using 5 scales. Then the flow regimes in MFBs are identified by analysing the standard deviations together with the pictures of the beds at various gas velocities. By reconstructing the 3 and 4 scale signals, the fluctuation signals caused by bubbles in MFBs can be separated effectively. When the bed diameter ranges form 4.3 mm to 20.5 mm, there are four different flow regimes: fixed bed, uniform fluidization, bubbling fluidization, and slugging fluidization. No turbulent fluidization regime is observed in MFBs. Finally, the minimum fluidization velocities evaluated by pressure fluctuation range accord well with the experimental values.The fluidization characteristics of quartz sand and FCC catalyst particles in an sound assisted MFBs with of 4.3 mm i.d. were investigated at various sound-pressure levels and frequencies. An acoustic field can decrease the bed voidage of MFBs and improve the fluidization quality of fine powders. At the same sound frequency, the minimum fluidization velocities decrease with increasing sound pressure level. The minimum fluidization velocities of FCC(83μm) and three kinds of quartz sand (51,67and83μm) have a minimum value when the sound frequencies are 142,91,101,133 Hz, respectively. With the assistance of acoustic wave, the minimum fluidization velocities in MFBs are reduced by 10~20%.