A comparative study on hydrodynamics of circulating fluidized bed riser and downer

Hydrodynamics in a 76mm i.d., 10.2m high circulating fluidized bed (CFB) riser and a 76mm i.d., 5.8m high CFB downer were studied for superficial gas velocities ranging from 2 to 5m/s and solid circulation rates up to 100kg/(m²s). Solid holdup, particle velocity and solid flux profiles in the radial and axial positions were presented. Under these operating conditions, axial solid holdup profiles in the riser and the downer could be approximated by an exponential decay function. The radial gradients of the solid holdup profiles in the riser were much higher than those in the downer, showing that the downer had much more uniform solid distribution. The average solid holdup for the entire riser was about 1.5 times higher than the predicted value from Gₛ/(ρₚUg). However, for the downer reactor this ratio dropped to 0.45–0.98 which increased with increasing superficial gas velocity and decreasing solid circulation rate. Solid flow developed much slower in the riser than in the downer. Negative particle velocity was observed in the near-wall region for nearly the entire height of the riser. The average particle velocity for the entire riser was 0.8–0.96 times higher than the superficial gas velocity, and increased with increasing superficial gas velocity and decreasing solid circulation rate. However, in the downer the average particle velocity was 1.13–2.13 times higher than the superficial gas velocity, and increased with both superficial gas velocity and solid circulation rate. High local solid fluxes were observed in the near wall region of the CFB riser and downer reactor. Over-estimation of the calculated cross-sectional average solid flux over the solid circulation rate in the CFB riser was attributed to the fluctuations of the solid holdup and particle velocity.