Study On Residence Time Distribution Of Solid Particles And Drying Characteristics In Fluidized Bed With Spiral Internals

Fluidized bed has been widely utilized in various industrial processes.Residence time of particles is an important parameter for both fluidized bed dryers and reactors in engineering application.The homogeneity of solid particle residence time distribution(RTD)directly affects the reaction extent and product quality in the fluidized bed reactor,as well as the drying moisture uniformity and product quality in the fluidized bed dryer.The existence of spiral internals in the fluidized bed can improve the particle motion and effectively restrain the particle back-mixing,so it is of great significance to investigate the particle RTD in a fluidized bed with spiral internals.RTDs of uniform particles and light large particles in the fluidized bed with spiral internals are investigated in this paper,with drying characteristics further explored.This study may provide an experimental basis for scaling-up design and industrial application of fluidized beds with spiral internals.A large number of repeated experiments with pulse tracing method were carried out to investigate the particle RTD in the cold bubbling fluidized bed with spiral internals.The results showed that spiral internals had a significant effect on the RTD of uniform particles.With spiral internals,the average residence time of particles decreased,the dimensionless variance decreased significantly,and the particle flow tended to be plug flow.When the solid feeding rate was nearly doubled,the residence time of particles was reduced by approximately 50%and the dimensionless variance of particle RTD also declined.With the increase of bed height,both average residence time and dimensionless variance of particles increased,and the particle movement approached complete mixing flow.As the particle size increased,the residence time of particles in fluidized bed become longer,while the dispersion degree of particle RTD hardly changed.The average residence time of particles extended with the fluidized gas velocity increasing within the range in experiment and RTD curve had an obvious trail.The dimensionless variance also rose with the increasing of fluidized gas velocity,and the flow state of particles was far from plug flow.In order to explore the characteristics of RTD of light large particles in a fluidized bed with spiral internals,millet and sesame particles were used as tracers.Effects of large particle type,fluidized gas velocity and spiral internals on the RTD of light large particles were analyzed.The results showed that the minimum times for 90%of two kinds of light large particles to reach the discharge port in fluidized bed were shortened by about 50%with the existence of spiral internals.The spiral internals in fluidized bed weaken the back-mixing of large particles,making the movement of big particles more inclined to plug flow.It was found that laminar sesame particles with larger size and less density had a longer residence time and a larger dimensionless variance in the fluidized bed than millet particles with smaller size and higher density.And the movement of sesame particles was more complicated and the back-mixing was more pronounced,compared with the movement of millet particles.With the increase of fluidized gas velocity,the residence times of two kinds of light large particles gradually increased.When the gas velocity changed from 0.28 m/s to 0.32 m/s,the dimensionless variance nearly doubled.However,when the air velocity continued increasing to 0.39 m/s,the variation of dimensionless variance was not obvious.By adding an electric heater to the above-mentioned cold test system and arranging the corresponding temperature and humidity measuring points,a test bench for studying the drying characteristic in the fluidized bed with spiral internals was constructed.After adequately analyzing the heat and mass transfer process in the fluidized bed dryer,the drying model in the continuous drying fluidized bed was introduced.According to mass conservation and energy conservation equations,the fluidized bed drying characteristics can be calculated by measuring the differences in moisture content and temperature between inlet air and outlet air in the fluidized bed.The results showed that during the movement of particles along the helical channel to the discharge opening,the drying rate decreased and the drying energy consumption decreased.The higher the inlet air temperature was,the higher the drying rate in the first half of the spiral channel become,and the lower the drying rate in the second half of the spiral channel was.With the fluidized air inlet temperature increasing,the temperature difference between the inlet air and outlet air as well as the drying energy consumption increased.In the range of air velocity in experiment,the increase of air velocity had little effect on the drying rate.The higher the initial moisture content of the particles was,the higher the drying rate was and the more energy the drying consumed.