Characterization of downflowing high velocity fluidized beds.

摘要

It has been proposed that gas-solid cocurrent downflowing high velocity fluidized beds would have advantages for very short residence time reactions over the conventional upflow riser by minimizing backmixing. However, present databases do not supply sufficient information for a process development. In this work, a downer-riser circulating high velocity fluidization apparatus has been developed to study experimentally the fundamentals of gas-solid particle mixture downflow. The downer consists of a Plexiglas tube with 12.7cm ID and a length of 4.6m. A single gas-solid entrance design is employed, which is the simplest configuration that can be applied commercially. A pressure balance is maintained between the downer and the return riser during the operation. Fast response pressure transducers hardwired to a computerized data acquisition system are used to record the instantaneous pressure distribution around the loop. An x-ray chordal absorptometry technique is used to provide flow visualization and to measure instantaneous solid fractions and its radial and axial distributions. The local solids flux profiles are obtained by an aspirating probe device. Gas lateral and back mixing properties are studied by using helium tracer detected by an on-line analyzer.; Experimental data show that the acceleration and deceleration of solids in the downer due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an x-ray imaging system show the existence of a core-annulus flow with a dilute core surrounded by a denser wall region. The local solids flux profiles and the solid velocity profile obtained from these two measured quantities confirm that the majority of solids aggregates in the wall region but flows faster than in the core region.; Measurements show that gas backmixing is very limited in the downflow high velocity fluidized bed, and the lateral mixing is comparable with that in the riser. A mathematic model is applied to characterize the behavior of the gas dispersion in the two-phase downflow.; The downer reactor furnishes significant advantages over the conventional riser reactor particularly for short residence time reactions due to its specific flow pattern, reduced backmixing, shorter flow developing region and capability of handling very high solids/gas loading ratios.