November 30, 2011

Chew, J. W. , Roy Hays, John G. Findlay, S.B. Reddy Karri, Ted M. Knowlton, Ray A. Cocco, Christine M. Hrenya, Chemical Engineering Science 66, 4595–4604 (2011).

Experiments involving a gas–solid, pilot-scale circulating fluidized bed (CFB) have been carried out with a focus on species segregation measurements in a riser. Three mixtures were considered: (i) a binary mixture with particles of different sizes (dave) but with the same material density (rs), (ii) a binary mixture with particles of different material densities (rs) but with the same size (dave), and (iii) a continuous particle size distribution (PSD). Local measurements of the composition (i.e., species segregation) of each mixture were obtained over a range of operating conditions. Similar to previous works, the results show that the more massive species (i.e., greater dave or rs) preferentially segregate toward the wall in all cases. Several new trends were also observed. First, for the binary mixtures, composition of the more massive species increases with riser height at the wall under some operating conditions. The operating conditions that cause this phenomenon are mutually exclusive for the size-difference and density-difference systems. Second, for the continuous PSD, radial segregation is observed even when there is a net positive flux in the annular region, contrary to previous findings which indicated segregation only for conditions leading to a net downward flux in the annular region. Finally, two qualitative differences between the binary and continuous mixtures were noted: (i) a monotonic decrease in species segregation is observed for the binary mixtures with an increase in the solid loading (m), while a non-monotonic trend is observed for the continuous PSD, and (ii) while the shape of the radial segregation profile is flattest at the riser bottom for the binary mixtures, the flattest radial profile is at the riser top for the continuous PSD.