Field-flow Mechanism and Synergistic Heat Transfer Enhancement of Centrifugal Double Vortex Fast Pyrolyzer

Abstract

The downer is a promising pyrolyzer, but the low solid holdup restricts the improvement of heat transfer rate. In this regard, this paper proposed the concept of centrifugal double vortex fast pyrolyzer (CDVFP) to solve it. The spatial and temporal characteristics and heat transfer process of gas-solid two-phase in CDVFP were studied by computational fluid dynamics coupled discrete element method (CFD-DEM). The results show that the weakening of the double vortex “flow” by the centrifugal force “field” causes the reversal of gas-solid momentum exchange law. Above the vortex tail, the airflow structure is close to the Rankine vortex, and the airflow transfers momentum to particles; below the vortex tail, the gas velocity is lower than the particle velocity, and the direction of momentum transfer is reversed. The “field” gathers the particles on the wall of vessel, and the “flow” slows down the particles' downward velocity, thereby increasing local solid holdup. Although the uniformity of binary particle mixing in CDVFP is lower than that of conventional downer, the heat transfer rate is higher and the heat loss is less, so the equipment height can be reduced by about 3/4. The synergistic effect of centrifugal force “field” and double vortex “flow” enhances the heat transfer between binary particles in the descending bed.

Keywords

• fast pyrolyzer
• heat transfer
• centrifugal force
• double vortex airflow
• synergistic effect