Attrition of Sand Particles in a Cold MODEL RE-Circulating Fluidized BED (RCFB)

 

Attrition of bed material is a major cause of concern in any type of fluidized bed reactor as it affects the operational characteristics and fluidization properties. With attrition, the bed material will change in size, weight and shape. The main problem with attrition is the generation of fines and loss of valuable material. An experimental study was carried out on the attrition in a Re-Circulating Fluidized Bed (RCFB) using Indian standard sand (grade I, size between 2mm-1mm, average diameter 1.3 mm) for 50 hours checking for attrition at different time intervals. Furthermore, the influence of operational time on attrition and size distribution of sand particles was studied with a gas flow rate 8.15 m/sec and bed inventory of 8 kg of sand. From the cumulative sand particle size distributions (Fig. 1) for different attrition times, it was noticed that the mechanism of attrition of bed material in the reactor was abrasion not fragmentation. The reciprocal of harmonic mean varies approximately linearly with the operation time which describes the continuous attrition in RCFB. The larger particles were reducing in size due to vigorous attrition of particles and the fraction of attrited particles lower than 300μm were going off along with the fluidizing air. As the time progressed, there was a reduction in the downcomer bed height indicating that the attrition was significant and attrited mass was going out of the reactor with the fluidizing air. Fig. 2 shows variation in the fractional size distribution from the beginning to end of the operation at different time intervals. As observed, a change in the particle size of the sand is noticed at every interval of the operation time. Variations in the harmonic diameter, mean diameter, effective size of particle and average diameter were showing the decreasing pattern with different operation time periods. At the end of operation it was found that significant amount of fines were elutriated with the fluidizing air from the reactor.

Fig. 1 Cumulative sand particle size distributions

Fig. 2 Fractional size distribution of sand particles