Study On The Hydrodynamics Characteristics Of A Vertical Two-pass Circulating Fluidized Bed Evaporator With A Baffle

The vertical two-pass circulating evaporator with baffle is widely applied in the evaporation process of industrial production for Na₂S and lithium products.However,serious fouling problem exists in this type of evaporator,greatly resisting the production.So far this problem has not been solved effectively.The fluidized bed heat transfer and antiscaling technology can effectively enhance heat transfer as well as prevent and remove fouling.And the homogeneous distribution of particles in tube bundle is crucial to the application of this technology.Therefore,a transparent vertica l two-pass circulating fluidized bed evaporator with a baffle is designed and built by combining the fluidized bed heat transfer and antiscaling technology with the two-pass circulating evaporator with a baffle to solve the fouling problem.The visualization investigation is conducted by the charge coupling device(CCD)image measurement and acquisition system.And the computational fluid dynamics software FLUENT is also used to carry out the corresponding numerical simulation.The effects of operating and equipment parameters,including the circulation flow rate,amount of added particles,baffle height,liquid level in evaporation chamber,particle diameter and density on the hydrodynamics characteristics such as the fluidization and distribution of solid particles and pressure drop are investigated.Six different types of distributors are designed to improve the uniformity of particle distribution in the tube bundle of the down-flow bed.The variation trend of the suitable structure of the distributors with the operating and equipment parameters is also obtained.The research results show the particle distribution in the up-flow bed is homogeneous and slightly influenced by the operating and equipment parameters.However,the particle distribution in the down-flow bed is uneven and greatly influenced by the operating and equipment parameters.The increase in circulation flow rate reduces the difference of solid holdups in the evaporation chamber between the two sides of the baffle,thereby resulting in a more even particle distribution in the down-flow bed.The increase in amount of added particles is not conducive to the uniform distribution of particles in the down-flow bed at low circulating flow rate,but slightly influences the uniformity of particle distribution at high circulating flow rate.The increase in baffle height increases the difference of solid holdups in the evaporation chamber between the two sides of the baffle,and is not conducive to the uniform distribution of particles in the down-flow bed.An appropriate increase in liquid level in the evaporation chamber is beneficial to the homogeneous distribution of particles in the down-flow bed.However,excessive liquid level leads to a contrary action.The increase in particle diameter and density leads to a great difference of solid holdups in the evaporation chamber between the two sides of the baffle.It decreases the solid holdup in the down-flow bed and results in an uneven distribution of particles.The uniformity of particle distribution can be obviously improved by adding the distributor with suitable structure.Within the range of the experiments,the non-uniform degree of the particle distribution could be reduced by up to 90%.However,the increased pressure drop introduced by the distr ibutor is no more than10%.The suitable structure of distributor is affected by the operating and equipment parameters such as circulation flow rate,baffle height and the amount of particles added,among which the circulation flow rate and baffle height have a greater effect,but the effect of the amount of added particles is less.The mesh of complex flow structure in the evaporator and the numerical model of liquid-solid two-phase circulation flow are established by using the computational fluid dynamics software FLUENT.The hydrodynamics characteristics such as the fluidization and distribution of particles and pressure drop in the evaporator are simulated under different circulation flow rate,amount of particles added,particle size and density.The simulation results are in good agreement with the experimental data.The results of this paper are helpful for deeply understanding the hydrodynamic characteristics of the vertical two-pass circulating fluidized bed evaporator with baffle,grasping the design principle of the structure and characteristic parameters of distributors,and exploring the numerical model and calculation method to the circulating fluidized bed evaporator with complex flow structure.The research also can advance the industrial application of fluidized bed heat transfer and antiscaling technology.