Characterization Of Apparent Viscosity Of Solid Particles And The Prediction Of Defluidization In The Fluidized Bed

Fluidized-bed reactors are well known as widely used in metallurgy,chemical and other industrial fields.The agglomeration of particles and subsequent defluidization occurred in the fluidized bed inhibited its widespread commercial.Theoretically,agglomerates formed when the cohesive force is larger than drag force.Numerous researches have been peformed on the drag force.However,few researches have been reported focused on the cohesive force.In order to intensively understand the interacting forces between solid particles,apparent viscosity is introduced to characterize the resistance of particles against movement.Based on the energy dissipation theory and kinematic equation of particles,a measuring method is developed.A new mathematical model based on the apparent viscosity is developed to predict the defluidization behavior of particles in fluidized beds.In the end,this study focus on the apparent viscosity of bubble fluidized bed and circulating fluidized bed,and an online system is developed to characterize the progress of reduction and rheology of the fluidized bed.The main research conclusions are as follows:(1)Compared with the rheology of fluid,the solid particles have the similar properties.The apparent viscosity was introduced to characterize the comprehensive results of interaction between particles.Based on the energy dissipation theory and kinematic equation of particles,a measuring method is developed,through which the torque exerted on a rotating blade inside a solidparticle layer was measured.When the blade rotating in the particles,the apparent viscosity of particles can be derived as:?=A·T/?NrD Here,?,T,Nr and D are the apparent viscosity(Pa·s),the torque exerted on the blade(N·m),blade stirring speed(s-1)and the blade diameter(m).A is a constant and depend on the blade shape.(2)According to the measurement method proposed above,the apparent viscosities of Fe and Cu particles are measured.The results indicated that at room temperature,the particle shape gives much more important effect on the apparent viscosity.The apparent viscosity of spherical particles is smaller than that of irregularly shaped particles.Under higher temperature,particle size tends to play a crucial role.The apparent viscosity of iron particles increases with particle size decreasing.The apparent viscosity logarithms of 75?m iron particles have a linear relaitionship with the reciprocal of temperature at various temperatures.(3)A new mathematical model based on force balance is proposed to predict the defluidization behavior of particles in fluidized beds.The fluidization behavior of copper particles including the minimum fluidization velocity and the defluidization temperature was examined in a laboratory's fluidized bed reactor.Comparing with the experimental data,the results predicted by the model represented a good agreement and the relative error within 15%.(4)The nano-SiO2 particles are added to the iron particles to improve the particles flow ability.The apparent viscosity of Fe particles with nano-SiO2 particles decreased from 90 Pa·s to 42 Pa·s with the nano-SiO2 particles mass rate increasing from 0 to 1.5%for rotation speed is 200r/min.In fluidized bed,the defluidization temperature increased from 520? to 870 ? with the nano-SiO2 particles mass rate increasing from 0 to 1.5%.The results shown that coating nano-SiO2 particles on iron particles can improve the flow ability of iron particles and inhibit the defluidization occured in the fluidized bed.(5)The apparent viscosity of dilute solids suspension flows has been inferred from the experimentally measured frictional pressure drop in a circulating fluidized bed.The results show that the apparent viscosity of gas-solid suspension flow increases with the increment of the solids holdup,and is,in the transition region only,inversely proportional to the gas velocity.As the influence of gas velocity to the gas-solid suspension flow becomes negligible in the turbulent region.The proposed correlations for the apparent viscosity of the gas-solid suspension flow in straight pipelines are:?s = 1.81 × 10-5 ×(8 × ?s-1)(For turbulent region)?s = ?s ×(4.87 × 10-3 × e-v/2.5 + 3.39 × 10-4)+ 1.03 × 10-7×v/e0.64-2.01 × 10-4(For transition region)Here,?s,?s and V are the apparent viscosity(Pa·s),the solids holdup and the gas velocity(m/s).(6)Based on the above study,an online system is developed to measure the apparent viscosity of fluidized bed during the reduction process.The apparent viscosity of fluidized bed with the reduction of Fe2O3 can be measured association with the reaction gas analyzed by mass spectrometer.According to the apparent viscosity,pressure drop of fluidized bed and the reaction gas composition,the online system can characterize the progress of reduction process and rheology of the fluidized bed.