Segregation and defluidized zones in liquid-solid and gas-liquid-solid fluidized beds.

摘要

This study applies a collision measurement technique to the prediction of solid circulation patterns, fluidization regimes, minimum fluidization velocity complete fluidization velocity, complete mixing velocity, segregation and defluidized zones in liquid-solid and gas-liquid-solid fluidized beds. A method was developed to make the solid phase (glass beads) conductive for collision experiments. Sixteen electrodes were installed at various locations in the bed for detection of collisions. The collision frequency was observed to be decreasing with increasing fluid velocity due to bed expansion. Special experiments showed that maximum erosion occurs on the lower part of horizontal cylindrical probes, facing the distributor. The time history of collisions was used for determination of solid circulation patterns. Different circulation patterns were obtained for liquid-solid and gas-liquid-solid fluidized beds. The formation of mixing cells, defluidized zones and partial segregation affects the local collision frequencies in the bed. Therefore the information about the local collision frequencies is an effective tool for the prediction of particle behavior in the bed. The study of fluidization regimes resulted in determination of minimum and complete fluidization velocities for monosize and binary mixtures of 3 and 5 mm glass beads. The complete mixing velocity was obtained by calculation of the coefficient of variation of standard deviation of the time interval between successive collisions for binary mixtures of 3 and 5 mm glass beads.;The segregation of particles with different sizes, densities and shapes showed that the circulation of bed particles plays an important rule. The distributor jets also affected the mixing of light and/or small particles as they repel these particles from the distributor. Special probes were designed and installed on the distributor for the study of defluidized zones.;Different sand-papers were placed on the distributor to demonstrate the effect of distributor surface roughness on the formation of defluidized zones. It was found that the roughness of the distributor surface and the friction between the particles and distributor surface are important factors in the formation of a defluidized zone. A simple experiment was developed for the determination of a "friction factor" including all forces opposing the motion of particles in the defluidized zone. It is used with a model to predict the formation of defluidized zones on the distributor.