Modulating The Residence Time And Distribution Of Particles With Different Sizes In Fluidized Bed

The residence time distribution(RTD)of the particles with different sizes have an important influence on the conversion,selectivity and yield of the products in the fluidized bed reactors.Different size particles require different residence times to achieve the complete conversion.However,it is difficult to match the residence times of the different size particles with their respective complete conversion times,especially when a wide size distribution(WSD)is encountered.Therefore,how to increase the matching degree between the optimum reaction time and the residence time for each solid species would be the key issue to the high quality of product for the fluidized bed reactor.In accordance with this problem,two new types of the fluidized beds with vertical baffles and side outlet were designed in this paper to modulate the mean residence time(MRT)and its difference between the particles with different sizes.The flow of the particles with different sizes was simulated to investigate the reasons of the MRT difference of particles with different sizes in the new fluidized bed with a side outlet.The synchronous of the oxidation of ilmenite particles with different sizes has been studied in the new fluidized bed with a side outlet.The principal results and major conclusions are as follows:1)The residence time and distributions of the particles with different sizes were systematically investigated in the vertical baffles fluidized bed.The RTD difference of the particles with different sizes is small in the fluidized bed of vertical baffles,especially for the mixtures of the binary or multiple with narrow size distributions.The height of the baffles has a significant effect on the RTD of the particles.The backmixing decreases with reducing the height of the baffles,but the height of the baffles has a limited effect on the MRT difference.The gas velocity has a great effect on the MRT of the particles,the MRT of particles decreases with increasing the gas velocity.But the gas velocity has little effect on the MRT difference for the particles with different sizes in the vertical baffles fluidized bed.Although the MRT difference can be modulated for the particles with different sizes,the results cannot meet the requirements of synchronous of particles with different sizes.Therefore,the fluidized bed with vertical baffles cannot be used to modulate the MRT of particles with different sizes.2)The MRT difference of particles with different sizes was studied by changing the gas velocities and feed compositions in a new type of fluidized bed with a side outlet.With increasing the gas velocity,the MRT ratio of the coarse and fine particles firstly increases and then decreases,and the transition point is near the terminal velocity of the coarse particles.It is beneficial to increase the MRT ratio of the coarse and fine particles by increasing the proportion of coarse particles or increasing the size of the coarse particles in the feed.The size grading,one of the reasons for causing MRT difference,can be strengthened by adjusting the gas velocity,increasing the concentration of coarse particles and size ratio.So the MRT difference of the particles with different sizes can be increased in the new fluidized bed to meet the requirement of the chemical reaction control and ash diffusion control reaction for the synchronous of particles with different sizes.3)The flow of the particles with different sizes was simulated in the new fluidized bed with a side outlet via Fluent.The radial solid holdup of the coarse particles is obviously larger than that of fine particles at the same height,which is the direct reason for the MRT of the coarse particles being larger than that of the fine particles.The velocity difference between the coarse particles and the fine particles in dense phase region is very small,so that the cumulative effect of the velocity difference between the coarse and fine particles has little effect on the MRT ratio of particles with different sizes.The particles with different diameters can be separated at the side outlet,where the coarse particles flow out from the side outlet more easily and the fine particles have a greater probability of flowing out from the top outlet.This is the reason for the MRT difference between the coarse and fine particles increasing with increasing the gas velocity under the terminal velocity of coarse particles.4)The synchronous of particles with different sizes was realized by modulating the MRT difference of the ilmenite particles with different sizes in the new fluidized bed.The oxidation rate of ilmenite is affected by the interaction of chemical reaction control and ash layer diffusion control.By analyzing the kinetic data of oxidation and the variation of MRT ratio of particles with different sizes,it is concluded that the synchronous of ilmenite with the size ratio of 2.6 can be achieved at a temperature of 900? in a gas velocity of 0.34 m/s.