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气-液-固三相搅拌槽反应器的数值模拟与实验研究
杨士芳
学位类型博士
导师杨超
2016-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词气-液-固三相 搅拌槽反应器 数值模拟 测量方法 反应器设计
摘要

气-液-固三相搅拌槽在化工、石化、冶金等工业生产过程中应用非常普遍。由于三相体系内同时存在着两种分散相,相间相互作用方式复杂,目前尚没有成熟的实验和理论研究工具,因此对其内部的多相流动、混合及传递机理的研究还远不成熟。本论文从实验和数值模拟两个方面对气-液-固三相搅拌槽反应器内的流体动力学特性进行了研究。首先,针对气-液-固三相搅拌槽反应器实验测量手段缺乏的问题,设计了一种新型的多相测量设备。利用这套仪器实现了对气-液、液-固以及气-液-固多相搅拌槽内分散相尺寸分布的测量。同时利用数值模拟方法定量分析了侵入式探头的尺寸和朝向对测量结果的影响。结果表明:当探头直径由0.05T增大至0.15T时,固含率的测量误差由4.3%增加至11.8%,因此在满足测量视野的前提下,需要进一步减小探头直径;当探头朝向与探头前端流体的合速度方向之间夹角大于90°时,探头对流场和相含率分布的影响较小。其次,采用基于Eulerian-Eulerian观点的“三流体”模型,建立了适用于三相体系模拟的数学模型,首次在三相搅拌槽的数值模拟中考察了第二种分散相存在对相间作用力的影响,同时在模型中耦合了气泡尺寸变化模型,使得数学模型与实际过程更加接近。利用该模型数值模拟了气-液-固三相搅拌槽反应器内的流场结构、相含率分布以及气泡尺寸分布。将模拟结果与利用改进取样法得到的局部相含率实验数据进行对比,结果表明利用该模型的计算结果与实验数据更加吻合。同时还发现,双层桨的加入可以明显加强反应器内的流场强度,促进混合效果,使得相含率分布更加均一,气泡尺寸明显减小,进而有利于提高反应器内的气液传质特性和反应器效率。最后,针对高径比大于1的低剪切搅拌槽生物反应器存在的气含率分布不均的问题,提出了分段式多级导流筒构型设计。实验测量了分段式多级导流筒反应器内的临界离底悬浮转速、功率消耗、气液体积传质系数kLa,并与单级导流筒的结果进行对比。当导流筒间距为3.5 cm和7.0 cm时,反应器内固体颗粒更易悬浮,临界离底悬浮转速低于单级导流筒的结果,同时反应器内的功率消耗明显降低, kLa也明显高于单级导流筒的结果。而当导流筒间距增大至10.5 cm时,反应器气液传质特性明显下降,甚至低于单级导流筒的情况。研究了搅拌转速、通气速率以及固体加入量对反应器内单位功耗下的气液体积传质系数η的影响发现,η随着搅拌转速的增大而降低,随着通气速率和固体颗粒加入量的增大而提高。在此基础上,建立了内置多级导流筒的气-液-固三相搅拌槽反应器的功率和气液体积传质系数的经验关联式,为该类型反应器的工程放大提供理论依据。

其他摘要

Gas-liquid-solid three-phase stirred reactors are widely employed in chemical, petrochemical and hydrometallurgical processes. Due to the complex interactions between the coexisting dispersed phases and the continuous phase, there is no mature measurement method and the researches for the multiphase flow, mixing and transport mechanism in the reactors are far from enough. The major objective of the present dissertation is to investigate the hydrodynamic characteristics in gas-liquid-solid three-phase stirred reactors both experimentally and numerically. In order to solve the problem of the lack of experimental techniques, a new multiphase measurement instrument is designed. The distributions of the dispersed phase size in solid-liquid, gas-liquid and gas-liquid-solid stirred reactors are measured. The influence of the diameter and orientation of an intrusive probe on the measurement results is investigated by numerical simulation. It can be found that, when the diameter of probe increases from 0.05T to 0.15T, the measurement error increases from 4.3% to 11.8%. So it is necessary to further reduce the diameter of the probe on the premise of meeting the requirement on the measurement field of view. Besides, when the angle between the direction of probe orientation and the fluid velocity at the measurement point is larger than 90°, the optical probe has less impact on the flow field and the error of local phase holdup measurement.An Eulerian-Eulerian multi-fluid model is developed for simulating the gas-liquid-solid flow. The interphase drag force model is modified for the first time to account for the effect of the second dispersed phase in a three-phase stirred reactor and a bubble size model is coupled to predict the bubble size distribution. The flow field, distribution of local phase holdup and bubble sizes are simulated and the results are compared with the experimental data measured by an improved sample withdrawal technique. The results show that the prediction by the present modified model is much closer to the experimental data. It is also found that the intensity of the flow field is significantly strengthened and the mixing efficiency is promoted when the double impellers are installed. Besides, the phase holdup distribution is much more uniform and the bubble sizes obviously decrease, which is beneficial to improving the gas-liquid mass transfer and reaction efficiency.A dual-stage draft tube is proposed to solve the problem of the uneven gas holdup distribution in a low-shear stirred bioreactor when the height-to-diameter ratio is greater than 1. The critical impeller speed, power consumption and volumetric gas-liquid mass transfer coefficient, kLa, are measured and the results are compared with that measured with a one-stage draft tube. It can be found that when the distance between two stages of draft tube is 3.5 cm or 7.0 cm, the solid particles is much more easily to be suspended and the critical impeller speed is smaller than that measured with a one-stage draft tube. Besides, the power consumption decreases and a greater value of kLa can be obtained. Nevertheless, when the distance is 10.5 cm, the overall efficiency decreases to below that with a one-stage draft tube. The impacts of the impeller speed, superficial gas flow rate and solid concentration on the gas-liquid mass transfer rate per energy dissipation, η, are evaluated. It is observed that η decreases with increasing impeller speed and increases with increasing gas flow rate and solid concentration. On the basis of this, the correlations for power consumption and kLa are established to provide theoretical basis for reactor design and scale up. 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/22900
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杨士芳. 气-液-固三相搅拌槽反应器的数值模拟与实验研究[D]. 北京. 中国科学院研究生院,2016.
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