Study On The Dynamic Mixing Property Of Dissimilar Particles In Fluidized Bed Using Capacitance Probe Method

Fluidized beds are very popular in industries because of the excellent heat and mass transfer efficiency.The treatment of dissimilar particles is generally needed for various purposes.Good mixing between dissimilar particles helps to increase the contact density of the solid-phase reaction system,and produce uniform concentration and temperature fields,thus speeding up the chemical reaction.Current experimental studies on the mixing properties of dissimilar particles in fluidized beds mainly focus on the mixing degree distribution in the stable mixing state and the kinetic properties of tracer particles during the mixing process.However,a comprehensive understanding of the dynamic mixing process of dissimilar particles is still lacked,which limit the further development of fluidized bed technology.In this paper,a new measurement system for the mixing characteristics is developed.The measuring principle is that the relative dielectric constant of binary mixture changes with the mixing ratio.This paper solves the problem that it is difficult to quantitatively obtain the microscopic mixing characteristics of dissimilar particles in non-stationary mixing process by conventional methods.In this paper,based on the capacitance probe heterogeneous particle mixing characteristics test system and the image acquisition system as an auxiliary test method,a comprehensive experimental test platform of fluidized bed heterogeneous particle non-stationary mixing characteristics is built.The verification experiment shows that the capacitance probe method proved to be a reliable measurement method in mixing and segregation of binary mixtures.A modified mixing index-based method of mixing entropy is used to describe the variation of the mixing extent in the transition from segregation to mixing at different lateral positions.First,the grid-scale steady-state mixing properties in fluidized bed with dissimilar particles are studied.The flow properties of gas-solid two-phase flow in fluidized bed and the internal relationship between the fluctuation of grid-scale mixing characteristics and the movement of particles/particle groups in different positions of fluidized bed are analyzed.Results show that,in the bubbling fluidized bed,the bubble motion characteristics determine the transfer characteristics of the bed,and particle circulation is the main form of particle mixing.The fluctuation of the volume fraction of deposited components at the bottom of the fluidized bed is obviously smaller than that at the central axis and top of the fluidized bed,and the particle mixing entropy at the central axis and top of the fluidized bed increases logarithmically with time,which is obviously higher than that at the bottom and wall of the fluidized bed;In the spouted bed,the unstable changes in the injection zone lead to a step-like distribution of the volume fraction of deposited components in the mixing lag zone,and can promote the particle mixing in the mixing lag zone to a certain extent.Second,in order to study the non-uniform transverse mixing of dissimilar particles in fluidized bed,the effects of bubble movement,particle circulation and fluidization gas velocity on the transverse mixing properties of dissimilar particles were studied,and the variation law of mixing index in the unsteady mixing process was obtained.The results show that the mixing velocity of particles in fluidized bed is positively correlated with the distance from the central axis when the bed height is high;The initial bed distribution has a great influence on the lateral mixing properties of dissimilar particles.The influence of fluidizing gas velocity on the transverse mixing properties of fluidized bed with dissimilar particles is mainly reflected in the mixing rate,while the mixing rate is different in the stable mixing stage.The degree of mixing at different fluidized gas velocities in the stable mixing stage is basically the same.Third,in order to study the dynamic mixing of particles along the axis of fluidized bed in different stages of mixing process,the flow,mixing and separation properties of dissimilar particles fluidized bed were analyzed.The results show that the axial mixing rate of particles is faster in the initial mixing stage.The influence of fluidized gas velocity on axial mixing is mainly reflected in mixing velocity,and increasing fluidizing gas velocity can significantly improve the mixing velocity of particles.When the bed is low and high,high fluidization gas velocity will promote the mixing degree of particle,but when the bed is high,increasing fluidization gas velocity will promote the particle separation phenomenon;The fluctuation of the volume fraction of deposited components in fluidized bed with large size dissimilar particles is large,but the final value of the mean curve of the volume fraction of deposited components in fluidized bed with heterogeneous particles of different particle sizes remains basically the same.