Study On The Pressure Gradient And Flow Behaviors In A Gas-Solid CFB Riser

Circulating fluidization, as an efficient and bubble-free contacting technology for gas and solids, has been one of the most active fields in fluidization research. Based on a large amount of experimental data of pressure gradient △P/△Z and local solids holdup of air-FCC two phase flow in a 1 6m long CFB riser, with superficial gas velocity U_g2.O～O.3_ms~-1 and solids circulating rate G_S=5.4～230kgm~-2s~-1 this work carried out a systematic study on the axial distribution of △P/△Z and its variations with operation conditions, especially the influences of the riser height on the distribution of API Az and the transition of flow regime from fast fluidization to dense pneumatic transport.The results show that the tiP I Az in riser assumes a non-uniform axial distribution as a result of solids acceleration. With increasing G~ or decreasing Ug, the solids acceleration region becomes longer and tiP I Az goes up at all axial locations, but the variation Wends of Al?I Liz in the upper section of riser are quite different from those in the lower section. With increasing G,, the ratio of Al?/ Liz in the upper section to that in the lower section keeps going down, suggesting that API Liz becomes less uniform along the riser. With increasing Ug, however, thiS ratio keeps going up not as steadily as usually taken for granted. For the condition of high G5, the ratio will always undergo a descending process with increasing big until it finally turns to going up at some higher Ug, indicating that the axial pressure gradients donalways keep approaching a uniform distribution with increasing Ug.The riser height is found to have significant effects on the distribution of Al?I Liz and the transition of fast fluidization to dense pneumatic transport. For given particles and operating conditions, the increased riser height leads to a smaller and a more uniform pressure gradient along the whole riser, and also a higher saturation carrying capacity G for a given Ug. As higher riser height needs more energy to support the gas-solid suspensions, the transition of flow regime corresponding to a given Ug will occur at a lower solids circulating rate and bed concentration. To predict the transition from fast fluidization to dense pneumatic transport, a new correlation relating G5 to big at transition points was developed.It is also found that the errors between the apparent solids holdups and the actual ones are minute in the upper section of riser, but, in the lower section, obvious errors exist due to solids accelerating and clustering, so that they must be taken into account when the pressure gradient is used to infer the solids holdup.