(70g) 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