Abstract
Circulating Fluidized Beds (CFB) are attracting increasing interest for both gas-solid and gas-catalytic reactions, although operating modes differ completely. To model the reactor, the knowledge of the solids residence time is an important parameter. Previous studies mostly assess operations at moderate values of the solids circulation rates (≤ 100 kg/m2s), whereas gas-catalytic reactions and biomass pyrolysis require completely different operating conditions. Positron Emission Particle Tracking (PEPT) is used to study the movement and population density of particles in the CFB-riser. Transformation of the PEPT results define (i) the vertical particle movement and population density in a cross sectional area of the riser; (ii) the required transport gas velocity (Utr) to operate in a fully established circulation mode; (iii) the overall particle movement mode (plug flow vs. core/annulus flow); and (iv) the particle slip velocity. Only in a plug flow mode can the particle slip velocity be estimated from the difference of superficial gas velocity and particle terminal velocity. The slip velocity is lower than U - Utr outside the plug flow mode. To operate in plug flow, the superficial gas velocity should exceed Utr by 1 m/s and the solids circulation rate should exceed 200 kg/m2s.