Segregation direction reversal of gas-fluidized biomass/inert mixtures – Experiments based on Particle Segregation Model predictions

The tendency for a gas-fluidized binary mixture to segregate or mix as a function of the size and density of the two solids, despite the extensive research efforts motivated by its strong influence on reactor performances, is still not fundamentally and thoroughly understood. Recently, the Particle Segregation Model (PSM) (Di Maio et al., 2013) has been proposed as a theoretical tool to predict the segregation direction of gently fluidized mixtures. An important and previously overlooked result, also predicted by PSM, is the presence of peculiar conditions in which the flotsam and jetsam components, for a given solid pair, may invert their role upon changing bed composition. In the present work, the segregation behavior of biomass/inert mixtures is investigated with the specific aim to check for the occurrence of this peculiar segregation direction reversal. Crushed olive pits are used as model biomass (1.54 mm diameter) in combination with glass beads (0.25 mm diameter) and two sand cuts (0.34 and 0.16 mm diameter). The segregation tendency of an initially mixed biomass/inert bed, deduced by the measured steady-state composition profiles, turns out to confirm the presence of segregation direction reversal as a result of a change of the relative solid volume fractions. Although the critical “equilibrium” composition is quantitatively captured only for the first pair, the occurrence of the phenomenon appears remarkably in agreement with the PSM predictions in all the three cases.