Study On Residence Time Distribution Of Particles In Fluidized Beds With Vertical Baffles

Fluidized bed reactors have been widely used in drying,comprehensive utilization of biomass gasification and combustion,minerals,food and other fields.The residence time of particles in reactors decides the reaction degree of materials.Therefore,the development of high efficiency and precision measurement technologies of particle residence time and the analysis of various factors influencing the particle residence time distribution of the research and engineering application of fluidized bed are of important significance.In view of this,a cold fluidized bed with vertical baffles was designed to analyze the residence time distribution(RTD)of uniform particles,in order to provide theoretical guidance for the control of the residence time of particles in the bed.Aimed at the limitations of RTD measurement technologies,the traditional dyed tracer technology was improved.Based on digital image processing,the RTD measuring method was proposed and was verified by using spectrophotometry experimental measuring.This method can efficiently measure the RTD and concentration of colored particles from bed material.The relative errors of concentration measurement are less than 5% with advantages of repeatability and high degree of automation.The effects of vertical baffles on particle RTD in fluidized bed were investigated.The differences of RTD were analyzed according to the number of side baffles,length of side baffles and number of upper baffles.It was found that vertical baffle arrangement in fluidized bed had a significant effect on reducing particle backmixing degree.With the increase of the baffle number,the particle flow state is closer to the ideal plug flow.But there is still a big gap between the particle flow and the ideal plug flow.The length of baffles determines the gap width of the particle flow channel in the bed.The increase of baffle length reduces the backmixing between the areas in the bed,and the flow state is closer to the plug flow.The relationship between the peak time and the length of side baffles is almost linear.In addition,the longer the length is,the more significant the change degrees of other residence time parameters are,such as variance and concentration degree.The regulation effect of the side baffle is better than that of the upper baffle with the same number of baffles.The RTD characteristics under different operating conditions were studied in a fluidized bed within two side baffles.It was concluded that the mass flow rate,fluidized number and particle size are the main factors on the RTD,while the bed volume is the secondary factor.The peak value of mass flow rate and residence time curve is positively correlated with concentration degree.And the change of average residence time with the increase and decrease of mass flow rate is the same as the change of design average residence time.The increase of fluidization number obviously leads to the advance of the peak exit time,the rapid increase of variance and the decrease of residence time concentration.And the particle flow state is closer to the full mixed flow.The concentration and mixing degree of residence time can be well controlled by adjusting the gas velocity,and the average residence time can be maintained in a relatively stable state.A larger particle size means smaller number of particles and weaker interaction between particles,which leads to a later peak time of RTD curve and a higher average residence time.The Euler-Euler two-fluid model was used to simulate the gas-solid flow behavior in a fluidized bed,and the particle RTD was calculated by the spicies transport equation.Compared with the experimental data,it is found that the bubble area ratio in the simulation is 15% lower than the experimental value.In terms of the shape of residence time curve,the position and size of peak,the simulation is in good agreement with the experimental results,and the error of peak emergence time and peak size is within 15%.In order to reduce the computational time,a reduced geometry bed was used for the study.The feasibility of the proposed method was verified by comparing the dimensionless RTD results of simulation and experiment.The results show that the linear effects of length and width on the mean residence time of particles in the bed are consistent in law and degree,while the linear effects of length and width on the variance were opposite in law.In addition,the variance of residence time almost linearly decreases with the length-width ratio increasing.Under certain conditions,the variance can be adjusted by changing the length-width ratio,and the variance of other beds can even be predicted by using the variance of particle residence time of beds with known size.