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搅拌槽由于其具有操作条件灵活、传质传热效率高等优势，广泛应用于化工、冶金、环境和生物医药等工业过程。搅拌槽内液液体系的相间传质又是工业过程中最普遍的单元操作之一。近年来，不断有研究对搅拌槽内不互溶液液体系的分散过程进行探索，但主要集中在流动特性和混合特性等方面，没有考虑相间动态传质过程。另外，从可视化测量的角度出发，目前使用的示踪剂只溶解在分散相或者连续相中，不能综合反应分散相和连续相之间的相互作用对相间传质过程的影响。所以，对搅拌槽内发生的流动、宏观混合和相间传质等多过程耦合行为的可视化研究尚不清楚。有鉴于此，本文选用NaI溶液-辛醇构成不互溶液液体系，罗丹明B作为荧光示踪剂，使用平面激光诱导荧光法（Planar Laser Induced Fluorescence，PLIF）结合折射率匹配技术，可视化地监测搅拌槽内不互溶液液体系的相间动态传质过程，并通过测量相间传质平衡时间和计算传质系数，分析不同操作条件对不互溶液液体系相间传质的影响规律。首先，实验考察了搅拌转速、分散相体积分数、桨型（RDT、PBTU、PBTD和VBT）、桨离底高度和偏心搅拌对传质平衡时间的影响。实验结果表明：搅拌转速增大时两相间的传质效率增强，传质平衡时间减小；在相同转速下，RDT桨的相间传质效率比其他搅拌桨的效率高，VBT桨的分散和传质效果最差；对离底高度而言，当RDT桨从T/10增大到2T/5时，叶轮的双循环回路逐渐恢复使剪切程度增强，所以相间传质效率提高；而且，随着分散相（NaI溶液）体积分数从2%增加到50%，相间传质过程被抑制，传质平衡时间不断增加；当叶轮偏心率增大时，相应传质平衡时间增大，不互溶液液体系中传质平衡时间的变化趋势与单相混合时间随偏心率的变化趋势相反。 其次，使用相同的不互溶液液体系，将PLIF方法应用于测量恒界面池中搅拌转速和溶质浓度对相间传质系数的影响。研究发现：搅拌转速的增大使相界面处剪切速率和波运动幅度变大，进而使传质系数明显增大；另一方面，相间传质系数与溶质浓度之间呈现正相关关系。当转速较低（50 rpm）和在较高溶质浓度（60 μg/L-120 μg/L）范围内时，传质系数受浓度的影响更为明显；然而，在转速增加一倍（100 rpm）和低浓度（30 μg/L-60 μg/L）的情况下，传质系数随浓度增加的变化更为显著。最后，关联实验数据拟合得到此不互溶液液体系相间传质系数的经验关联式，传质系数的实验值与拟合公式计算值之间的平均相对误差为2.70%。;Stirred tanks are widely used in industrial processes, such as chemical, metallurgical, environmental and biomedical industries, because of its advantages of flexible operating conditions and efficient mass transfer/heat transfer. The interphase mass transfer of liquid-liquid systems in a stirred tank is one of the most common unit operations in industrial processes. In recent years, there have been many researches on the dispersion process in immiscible liquid-liquid systems in a stirred tank, but they mainly focused on the flow and mixing characteristics, and did not consider the interphase dynamic mass transfer process. In addition, from the perspective of visualized measurement, the tracer currently used is only dissolved in the dispersed phase or the continuous phase, and cannot comprehensively reflect the interaction between the dispersed and the continuous phasees on the interphase mass transfer process. Therefore, the visualization of multi-process coupling behaviors of the flow, macromixing and interphase mass transfer has not been found in the stirred tank.In view of this, NaI solution and 1-octanol are used to construct an immiscible liquid-liquid system. Rhodamine B is chosen as the fluorescence tracer. The planar laser-induced fluorescence (PLIF) method combined with the refractive index matching is used to visually measure the interphase mass transfer process in the immiscible liquid-liquid system in a stirred tank. By measuring the interphase mass transfer equilibrium time and calculating the mass transfer coefficient, it is convenient to analyze the influence of different operating conditions on the interphase mass transfer of the immiscible liquid-liquid system.Firstly, the effects of agitation speed, dispersed phase volume fraction, impeller types (RDT, PBTU, PBTD and VBT), impeller clearance and eccentric stirring on the mass transfer equilibrium time are investigated. The experimental results indicate that the interphase mass transfer efficiency increases and the mass transfer equilibrium time decreases with the increase of agitation speed. The mass transfer efficiency of RDT impeller is higher than that of other impeller types, and the dispersion and mass transfer efficiency of VBT impeller are the worst of them. For the impeller clearance, the double circulation loop of the RDT impeller is gradually recovered to enhance the shear intensity when the impeller clearance increases from T/10 to 2T/5, so the interphase mass transfer efficiency is enhanced. Furthermore, the interphase mass transfer process is dampened and the equilibrium time increases continuously when the volume fraction of NaI solution increases from 2% to 50%. When the impeller eccentricity increases, the mass transfer equilibrium time increases correspondingly and the trend of its change in the immiscible liquid-liquid system is contrary to that of the mixing time in the single phase.Then, the PLIF method is applied to measure the effect of the solute concentration and agitation speed on the interphase mass transfer coefficient in Lewis cell by using the same immiscible liquid-liquid system. It is found that the increase of agitation speed will enhance the shear rate and the amplitude of wave motion at the interphase, and the mass transfer coefficient will increase obviously. On the other hand, there is a positive correlation between the interphase mass transfer coefficient and the solute concentration. The mass transfer coefficient is more significantly affected by the concentration when the agitation speed is lower (50 rpm) and the solute concentration is in the higher range (60 μg/L-120 μg/L). However, in the case of the doubled agitation speed (100 rpm) and the lower concentration (30 μg/L-60 μg/L), the mass transfer coefficient changes more obviously with the increase of the solute concentration. Finally, an empirical correlation formula of mass transfer coefficient of this immiscible liquid-liquid system is obtained by fitting the experimental data, and the average relative deviation between the calculated and the experimental values of the mass transfer coefficient is as low as 2.70%.
|杜秀鑫. 搅拌槽内不互溶液液体系相间传质过程的实验研究化学工艺[D]. 中国科学院大学,2020.|
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