Control Of The Residence Time Of Particles With Wide Size Distribution In Fluidized Beds

Fluidized beds have been extensively applied for multi-size solids processing,such as mineral roasting,iron ore direct reduction,etc.For the wide size distribution(WSD)particles,different size particles require different residence times for a complete conversion.However,in conventional fluidized beds,it is difficult to match the residence times of the different size particles with their respective complete conversion times.It is therefore of great significance to adjust the residence times for particles with different sizes to match their complete conversion times to achieve synchronously complete conversion in a fluidized bed,which would greatly improve the conversion efficiency.To study the residence time of WSD particles,three different size glass beads with a size ratio(1:2:4)were mixed to simulated the WSD particles.The residence time of these particles was studied in a continuous fluidized bed.The influence of operation parameters on the mean residence time(MRT)difference of each size particles for the WSD particles was analyzed,and ways to adjust the MRT difference were also explored.Furthermore,a mathematics model was developed to predict the MRT of each size particles.The main results and major conclusions are as follows:(1)The effect of the feed rate,particle size distributions and gas velocity on MRT of WSD particles were studied.The MRT difference for each size particles increases with increasing the gas velocity.However,the feed rate and particle size distributions have minor influence on the MRT difference for different size particles.For the WSD particles with a maximum diameter ratio of 4,the MRT difference cannot reach 4 by only changing the gas velocity.It was found that the MRT difference is caused by the discharge mode difference of coarse and fine particles at high gas velocities,where a dilute zone formed above the bed surface,due to the fact that the most of ejected fine particles can be discharged from the bed while the ejected coarse particles often return back to the bed.(2)According to the mechanism of MRT difference,it was proposed to create multiple dilute zone in a fluidized bed through horizontal internals to enlarge the MRT difference of the coarse and fine particles.Consequently,the effect of baffle numbers,baffle position,baffle structural parameters(free area;hole diameter),and baffle types on the MRT difference for each size particles were studied.Compared with other type of baffles,the perforated baffle can significantly increase MRT difference for each size particles.Moreover,perforated baffles with a low free area or a small hole diameter are more effective to adjust the MRT difference.It was also found that no obvious segregation was found within each layer,while it showed that the concentration of the coarse particles decreases with increasing the number of the layer from the bottom.(3)Referring to the theory of gas-liquid distillation,the elutriation degree and relative elutriation degree were proposed for determining the particles concentration distribution on each layer.Combining with the hold-up prediction,a mathematic model was developed to determine the particle concentration in each layer,and to determine the MRT of each size particle.It showed that the model could well predict the MRTs of each size particles on the multi-baffle fluidized bed.Compared to previous studies about the prediction of the MRT for the particles using the average size,the proposed model could accurately predicted the MRT of each size particles.Besides,the model could predict the operation effects on the MRT difference,such as the optimum gas velocity,baffle number and baffle structural parameters.(4)It was attempted to study the MRT adjustment and the scale-up effect through CFD simulation for WSD particles.The Euler-Euler multi-fluid model with a structural drag model was used to simulate the fluidization of WSD particles in multi-baffle fluidized beds.Then the function of the average size was used to estimate the particles concentration in each layer,which can be converted to the MRT difference for each size particles.It found that the axial difference of the average size decreased slightly in scale-up of the fluidized beds(bed diameter,100 mm,190 mm and 400 mm),indicating that the MRT difference of WSD particles varies slightly with increasing the bed size.