【作者】 李娜；

【导师】 张裕卿；

【作者基本信息】 天津大学 ， 化学工程， 2019， 博士

【摘要】 在硫化碱和锂行业等工业生产的蒸发过程中,普遍采用具有隔板的竖直双管程循环型蒸发器,但是该类型的蒸发器存在着严重的结垢问题,是制约生产的瓶颈,尚未得到有效解决。而流化床换热防垢节能技术可以有效地解决换热设备的在线强化传热和防、除垢问题。在该技术的应用中,惰性固体颗粒在管束中的均匀分布是保证技术顺利实施的关键子技术之一。因此,为解决硫化碱等行业中此类型蒸发器的结垢问题,本文将流化床换热防垢节能技术和该类型的蒸发器相结合,构建了一套具有隔板的透明竖直双管程循环流化床蒸发装置。利用CCD图像测量和数据处理系统开展了可视化研究,并采用计算流体力学软件FLUENT进行了相应的数值模拟,以考察在循环流量、颗粒加入量、隔板高度、蒸发室液位、颗粒粒径和类型等不同的操作和设备参数下,该蒸发装置中颗粒的流化、分布和压降等流体力学性能。并在此基础上,设计和构建了六种不同结构型式的分布板来改善颗粒的分布,获得了分布板的适宜结构型式随操作和设备参数的变化趋势。研究结果表明:上行床中颗粒的分布较为均匀,受各种操作和设备参数的影响较小。然而,颗粒在下行床中分布不均匀,受操作和设备参数的影响较大。随着循环流量的增加,颗粒在隔板两侧的蒸发室中固含率的差异减小,在下行床中分布得更加均匀。循环流量较低时,颗粒加入量的增加不利于下行床中颗粒的均匀分布;而循环流量较高时,颗粒加入量的增加对下行床中颗粒分布不均匀度的影响不大。隔板高度的增加,增大了隔板两侧蒸发室中固含率的差异,不利于颗粒在下行床中的均匀分布。蒸发室液位的适当升高有利于下行床管束中颗粒的均匀分布,但过高则不利于颗粒的均匀分布。颗粒粒径和密度的增加会导致颗粒在隔板两侧蒸发室中固含率的差异增大,使得下行床中的固含率降低,颗粒的分布变得更加不均匀。引入适宜结构型式的分布板可以明显地改善颗粒在下行床中的分布。在实验范围内,颗粒分布不均匀度最高可降低90%,且由分布板引入而增加的压降不超过10%。分布板的适宜结构型式受循环流量、隔板高度和颗粒加入量等参数的影响,其中循环流量和隔板高度的影响较大,而颗粒加入量的影响较小。采用流体力学软件FLUENT构建了蒸发器中复杂流动结构的网格,建立了液-固两相循环流动的数学模型。模拟了在不同的循环流量、颗粒加入量、颗粒粒径和密度等条件下,蒸发器中的颗粒流化、分布和压降等流体力学性能。模拟结果与实验数据吻合良好。本文的研究结果有助于深入了解和掌握具有隔板的竖直双管程循环流化床蒸发器中的流体力学性能,确定适宜分布板结构型式和参数的设计原则,探索适合于具有复杂流动结构的循环流化床蒸发器的流体力学模型和数值模拟方法,促进该类型蒸发器在硫化碱和锂行业等工业蒸发过程中的应用,促进流化床换热防垢节能技术的工业推广。更多还原

【Abstract】 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.更多还原