Particle residence time distribution and axial dispersion coefficient in a pressurized circulating fluidized bed by using multiphase particle-in-cell simulation

The particle residence time distribution(RTD)and axial dispersion coefficient are key parameters for the design and operation of a pressurized circulating fluidized bed(PCFB).In this study,the effects of pressure(0.1-0.6 MPa),fluidizing gas velocity(2-7 m·s^(-1)),and solid circulation rate(10-90 kg·m^(-2)·s^(-1))on particle RTD and axial dispersion coefficient in a PCFB are numerically investigated based on the multiphase particle-in-cell(MP-PIC)method.The details of the gas-solid flow behaviors of PCFB are revealed.Based on the gas-solid flow pattern,the particles tend to move more orderly under elevated pressures.With an increase in either fluidizing gas velocity or solid circulation rate,the mean residence time of particles decreases while the axial dispersion coefficient increases.With an increase in pressure,the core-annulus flow is strengthened,which leads to a wider shape of the particle RTD curve and a larger mean particle residence time.The back-mixing of particles increases with increasing pressure,resulting in an increase in the axial dispersion coefficient.

Magnetic reduction of gas back-mixing in bubbling fluidized beds with Geldart-B magnetizable particles

This study investigated the performance of magnetic fields in reducing gas back-mixing in bubbling fluidized beds with Geldart-B magnetizable particles.The Peclet number(Pe)and axial dispersion coefficient(Da,g)were determined using the one-dimensional dispersion model.A weak magnetic field reduced gas back-mixing to a certain extent,while a moderate field resulted in minimal decrease.The performance of a strong magnetic field varied significantly depending on the operation mode.Under the magnetization-FIRST operation mode,gas back-mixing was significantly reduced.The corresponding Pe and Da,g were calculated as∼76 and∼3.6×10^(−4) m^(2)/s,indicating that the gas flow approached the ideal plug-flow manner.However,when the magnetization-LAST operation mode was used,the strong magnetic field failed to mitigate gas back-mixing.Therefore,the performance of magnetic fields in reducing gas back-mixing depended not only on their intensity but also on their application sequence to the gas flow field.