Flow Characteristics In The Riser Of A Circulating Fluidized Bed

A plexiglass circulating fluidized bed (CFB) cold model apparatus, which mainly consists of a riser (0.08m ID and5.5m height) and solids feeding device of L-type valve, is designed and built in this thesis. The flow characteristics of Geldart B particle (quartz sand) are then investigated in the CFB riser by using the pressure detector and PV-5A optical fiber probe. The effects of operating conditions on solids circulation rate are further explored.It is found that the axial particle concentration of quartz sand is high in the bottom and top regions of the riser whilst is low in the middle part of the riser, showing a C-type distribution. With decreasing the superficial gas velocity or increasing the solids circulation rate, particle concentration increases in the axial direction of the riser. The concentration differenence and non-uniform of particle content aggravate. In the radial direction of the riser, particle concentration is lower in center and higher near the wall. In the present experimental range, particle concentration is insensitive to superficial gas velocitv/solids circulation rate in the riser center whist shows more sensitive in the near-wall region. Axial particle velocity heavily depends on the superficial gas velocity and the solid circulation rate, and it increases with increasing the superficial gas velocity. Particle velocity decreases with increasing the solids circulation rate in the acceleration stage while keepes almost unchanged in the fully developed section.Experiments shows that increasing the superficial gas velocity in storage tank is beneficial to the solids circulation. Solids circulation rate first increases, reaches upto maximum at the minimum fluidization velocity(umf) and then slightly decreases. For a great static height of storage tank (2m), soild particles circulate smoothly. For a low static height of storage tank(1m), solids circulation rate fluctuates obviously as the gas velocity is higher than umf. This fact is attributed to the movement of big bubble even slugging in the storage tank. At low gas velocity, solids circulation rate increases monotonously. For the high gas velocity, however, solids circulation rate first increases and levels of with increasing the static height of storage tank.Solids circulation rate increases with increasing the amount of main gas in the L-valve. In the bend of the L-valve, solids circulation rate first increases, reaches up to maxmium and then decreases. A low gas velocity in the connection pipe is helpful for particle circulation whilst a high gas velocity is harmful for the solid circulation.