Abstract
The difference of gas-solids flow between a circulating fluidized bed (CFB) downer and riser was compared by computational particle fluid dynamics (CPFD) approach. The comparison was conducted under the same operating conditions. Simulation results demonstrated that the downer showed much more uniform solids holdup and solids velocity distribution compared with the riser. The radial non-uniformity index of the solids holdup in the riser was over 10 times than that in the downer. In addition, small clusters tended to be present in the whole downer, large clusters tended to be present near the wall in riser. It was found that the different cluster behavior is important in determining the different flow behaviors of solids in the downer and riser. While the particle residence time increased evenly along the downward direction in the downer, particles with both shorter and longer residence time were predicted in the whole riser. The nearly vertical cumulative residence time distribution (RTD) curve in the downer further demonstrated that the solids back-mixing in the downer is limited while that in the riser is severe. Solids turbulence in the downer was much weaker compared with the riser, while the large clusters formation near the wall in the riser would hinder solids transportation ability.
Highlights
The circulating fluidized bed (CFB) [1], which is extensively applied in industrial production such as fluidized catalytic cracking (FCC) and coal combustion [2], is mainly composed of a riser in which the gas phases move upward and the solids phases move upward in the center and flow downward at the wall, as well as composed of a downer in which the gas-solids phases flow downward co-currently
CPFDprocess, Model the fluid phase is described by solving the continuity and momentum equation, and the momentum transfer of particles is described based on the multi-phase particle in cell (MP-PIC) formulation
The accuracy of the computational particle fluid dynamics (CPFD) model should be validated before its further usage in analyzing the hydrodynamics in the CFB riser and downer
Summary
The circulating fluidized bed (CFB) [1], which is extensively applied in industrial production such as fluidized catalytic cracking (FCC) and coal combustion [2], is mainly composed of a riser in which the gas phases move upward and the solids phases move upward in the center and flow downward at the wall, as well as composed of a downer in which the gas-solids phases flow downward co-currently. The above experimental studies summarized that the gas-solids flow behaviors in the downer was much more uniform compared to the riser, under low density operation and under high density operation These previous experimental studies indicated that the heat transfer and reaction performance in the downer and riser were closely associated with the corresponding gas-solids flow characteristics. The difference of solids holdup as well as some particle-scale information including solids velocity, solids residence time, solids dispersion, and granular temperature between the downer and riser was compared by CPFD approach in order to further analyze the difference of macro-flow behaviors, cluster behaviors, and solids back-mixing. This work is of great significance to provide basic knowledge about the design and operation of the two reactors
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