Laboratory and industrial risers are equipped with exits of many different layouts, and numerous publications discuss the influence of riser exit geometry on local and overall solids hydrodynamics in the riser. The pr...Laboratory and industrial risers are equipped with exits of many different layouts, and numerous publications discuss the influence of riser exit geometry on local and overall solids hydrodynamics in the riser. The present paper reviews literature findings--mostly based upon indirect experimental techniques and often somewhat contradictory, Direct measurement of particle velocity and particle occupancy near and in the riser exit provide a better indication of the effect of riser exit geometry. Positron Emission Particle Tracking （PEPT） was used in this work for the first time to investigate the exit region of the riser, An abrupt or sharp exit causes particles to be knocked out of the gas flow, so forming a recirculation or reflux region in the upper part of the riser. This is much less pronounced with a curved or gradual exit.展开更多
Riser operating modes are vital to designing a circulating fluidized bed （CFB） reactor for a required process of either a gas-solid or a gas-catalytic nature. Different operating modes provide different solids＇ res...Riser operating modes are vital to designing a circulating fluidized bed （CFB） reactor for a required process of either a gas-solid or a gas-catalytic nature. Different operating modes provide different solids＇ residence times and mixing behaviors, which define the reactions＇ efficiency and yield. The literature demonstrates distinct operating modes resulting from observed differences in slip factors and the range of particle velocities and their associated residence time distribution. The present research uses positron emission particle tracking （PEPT） in a riser of B-type bed material to determine the different operating modes by measuring （i） particle velocities and residence time distribution, （ii） population densities of these particles in the cross-sectional area of the riser, and （iii） solids flow pattern at the bottom of the riser, Data treatment defines four distinct solids hold-up regimes in the riser and proposes a ＂phase diagram＂ depicting the existence of the different operating modes （dilute, dense, core-annulus and combined） as a function of the superficial gas velocity and solids circulation flux in the riser. The delineated regimes have good agreement with available literature data and known industrial operations. Comparison with literature data for risers using A-type powders is also fair. The diagram enables CFB designers to better delineate operating characteristics.展开更多
文摘Laboratory and industrial risers are equipped with exits of many different layouts, and numerous publications discuss the influence of riser exit geometry on local and overall solids hydrodynamics in the riser. The present paper reviews literature findings--mostly based upon indirect experimental techniques and often somewhat contradictory, Direct measurement of particle velocity and particle occupancy near and in the riser exit provide a better indication of the effect of riser exit geometry. Positron Emission Particle Tracking （PEPT） was used in this work for the first time to investigate the exit region of the riser, An abrupt or sharp exit causes particles to be knocked out of the gas flow, so forming a recirculation or reflux region in the upper part of the riser. This is much less pronounced with a curved or gradual exit.
文摘Riser operating modes are vital to designing a circulating fluidized bed （CFB） reactor for a required process of either a gas-solid or a gas-catalytic nature. Different operating modes provide different solids＇ residence times and mixing behaviors, which define the reactions＇ efficiency and yield. The literature demonstrates distinct operating modes resulting from observed differences in slip factors and the range of particle velocities and their associated residence time distribution. The present research uses positron emission particle tracking （PEPT） in a riser of B-type bed material to determine the different operating modes by measuring （i） particle velocities and residence time distribution, （ii） population densities of these particles in the cross-sectional area of the riser, and （iii） solids flow pattern at the bottom of the riser, Data treatment defines four distinct solids hold-up regimes in the riser and proposes a ＂phase diagram＂ depicting the existence of the different operating modes （dilute, dense, core-annulus and combined） as a function of the superficial gas velocity and solids circulation flux in the riser. The delineated regimes have good agreement with available literature data and known industrial operations. Comparison with literature data for risers using A-type powders is also fair. The diagram enables CFB designers to better delineate operating characteristics.
