【作者】 曹仲文；

【导师】 袁惠新；

【作者基本信息】 江南大学 ， 轻化工技术与工程， 2008， 博士

【摘要】 气体吸收在化工生产中有广泛地应用。传统的气液吸收设备结构复杂、造价高,而新型的旋转填料床又存在能耗较高、运动可靠性与稳定性等方面的限制,降低建设投资,发展简单、可靠的气液吸收设备的呼声很高。旋流器因结构简单、维护方便及投资成本较低,在离心沉降分离方面得到了广泛地研究和应用。实际上,旋流器中具有强化传递过程的流体力学环境,故可探索将旋流器应用于气液吸收过程。利用CFD软件对旋流场中的单相及两相流场进行了仿真研究。通过间接验证的方法,得到了本论文中旋流吸收器的流场模拟的比较合适的方案。CFD仿真的结果表明,在旋流场中,径向与轴向的速度的量级,相比于切向是较小的,且旋流场中的三个方向存在着速度梯度。旋流场内两相在径向方向存在相对速度,且相对速度随着流量的增加而增加,相对速度的存在对于加强两相接触作用,强化传质是有利的;而切向和轴向的相对速度在普通的旋流器中较小。此外,模拟结果还说明了本论文所研制的旋流吸收试验装置由于在侧壁进液,增大了轴向的相对速度,从而能够更好地增强旋流吸收器的传质效果,但是从两相的体积率的分布来看,尚需探索实际应用中的喷嘴的安装位置。对旋流场中分散相动力学特性进行了研究。在分析了诸多力对分散相运动的影响的基础上,建立了分散相径向运动方程以及旋流场中两相的相对运动方程。从分散相径向运动方程出发,得到了分散相径向速度在旋流器半径上的分布和解析解,并由此得到分散相径向运动特性时间t r。相对运动方程的求解结果与CFD模拟的结果一致,且切向和轴向的相对速度随着时间呈指数衰减。根据两相在轴向相对运动的无滑移条件,得到了分散相在轴向的运动特性时间t z;此外,将轴向无滑移条件与分散相径向速度的分布相联系,得到分散相的轨迹方程。而轨迹方程和分散相的运动特性时间,可计算旋流器的工作效率和指导旋流器的开发设计。应用量纲分析的方法,对旋流场中的应力进行了分析,并由此对分散相颗粒的变形和破碎进行了研究。量纲分析的结果表明:在湍流区域中,雷诺应力在量级上是粘性应力的倍数,且此倍数即为雷诺数,主要引起颗粒的破碎;而在其他区域,分散相颗粒则主要是变形和聚结,其中起主要作用的是粘性剪切应力,而在粘性剪切应力中起重要作用的则是旋流器中切向和径向剪切应力。在此基础上,推导得到了粘性剪切应力和湍动能引起液滴破碎的临界条件;通过韦伯数与相对速度的关系,得到了一定流量条件下,分散相液滴在特定区域中发生变形、破碎的情况。在流场仿真和理论分析的基础上,阐释了旋流吸收器中气液传质过程的特点。旋流场中不仅相对速度的存在强化了气液传质的过程,而且由于存在着速度梯度,增大了湍流扩散系数,并且产生了远强于湍流扩散的剪切弥散,从而极大地增强了旋流吸收器的传质能力。从对流传质方程出发,基于旋流场中分散相不同的运动形态,建立了旋流吸收器中伴有化学反应的气液吸收传质模型。该模型与Danckwerts的表面更新模型在表达形式上是完全一致的,并且克服了前人由于只考虑稳态的扩散传质,没考虑对流和非稳态传质而使模型具有的局限性。但该模型的计算需要测定一些特定的物理量,在应用上受到了限制。根据传质和湍流的相关理论,建立了基于旋流场中涡旋作用的气液吸收传质模型。该模型相比于其他模型,相关参数主要涉及到一些可控可测的操作、物性及结构等宏观方面的参数,计算简单,意义明确。从其相关参数的确定过程还可以看出,该模型还可推广应用于一般的自由界面的气液传质过程。由传质系数模型可得到扩散时间t D的关联式,根据旋流吸收器中t D << tr<<tz的要求,将扩散时间的关联式与分散相运动特性时间的表达式相联系,可以指导旋流吸收器应用于传质过程的设计与开发。设计了能够测定和计算出表征旋流吸收器中吸收特性参数的试验方法,研制了试验用的旋流吸收试验装置,搭建了整个试验系统以及相应的取样、分析测试设备。在此基础上,分别考察了处理量、液气比、气相分压、液滴的粒度以及结构参数对吸收过程的影响。试验结果表明,在本试验条件下,旋流吸收器的吸收速率与处理量、液气比、气相分压成正比,随液滴的粒度的下降而增大,但是如果液滴的粒度过小,由于表面张力的影响,反而可能不利于吸收速率的增加。测定和计算出的旋流吸收器的比相界面积与气相分压无关,可通过增大处理量、吸收剂量及减小液滴粒度的方法获得较大的比相界面积,但这三种方法都有所限制;结构尺寸较小的旋流吸收器较易获得较高的比相界面积。与文献中的其他吸收设备相比较,旋流吸收器的比相界面积的值居于前列。测定计算的吸收效率表明,在本试验条件下,旋流吸收器的吸收效率与液气比、处理量及液滴的粒度有关,而与气相分压无关,不过这可能与本试验的气相分压变化的幅度不大所致。根据试验数据计算得到的旋流吸收器的液相物理传质系数表明其值随气体处理量的增大而增大,与液滴的粒度及气相分压无关,在吸收剂量所占气液两相中的比例较低时,随着液气比的上升而下降。此结论不仅与Kolmogorov理论相一致,即能量耗散速度是湍流场中除物性以外,对扩散速率唯一的影响因素,而且与所建立的两个气液传质模型的理论结果吻合得相当好,说明所建立的气液传质模型有很好地应用性。理论探讨了温度对传质过程的影响,温度对于物理吸收过程的气相与液相的传质分系数的影响是不同的。而对于化学吸收过程,在一定温度范围内传质效果随温度的升高而升高;当高出这个温度范围的时候,温度的升高反而不利于吸收传质过程。计算了本试验条件下的各种热效应,最终证明了旋流器中的吸收试验基本上是等温的试验过程。最后建立的能量方程,可以更全面地表达试验过程中温度的变化。对旋流场及分散相运动特性的理论研究结果表明,旋流器中具有强化气液传质的特性,而比相界面积、传质系数以及扩散时间的试验结果也证明了旋流吸收器作为一种气液传质设备的应用可行性,如果再考虑其制造安装简单、维护方便等特点,且同时具有传递—分离一体化的效果,因而对它的推广应用具有实际的意义。更多还原

【Abstract】 The gas absorption has wider application in chemical production. Because of complex structure and high cost of traditional gas-liquid absorption equipment, while RPB (Rotating Packed Bed) as late model equipment for absorption is restricted for its high energy consumption and reliability stability of motion, reducing construction investment and developing easy and reliable equipment for gas-liquid absorption become more and more important. However, for simple structure、maintenance-friendly and the low cost of investment, cyclone has been widely researched and applied in centrifugal action separation field. Actually, there is hydromechanics environment in which transfer process can be strengthened.Simulation study for the single phase and multiphase flow in cyclone absorber was set forth by the Computational Fluid Dynamics(CFD)software. Through indirect methods of verification, the suitable scheme was received for CFD in the cyclone absorber in this present. The order of magnitude of tangential speed is larger than those of radial and axial speed, and there is the grade of velocity in swirling flow field from the result of CFD, in line with the description of related literature. It was verdicted that there be relative speed of radial of two phases which can strengthen the contact of two phases and is in favor of mass transfer, with the increased handling capacity、tangential relative speed and axial relative speed which are comparatively small. Besides, Because of entering the liquid on side wall, the axial relative velocity is augmented in the cyclone absorber manufactured in this work from the result of CFD, whereas it is needed that looking for a more sutitable situation for nozzle from the distributing of volume fraction of two phases.Through the dynamics analysis of dispersed phase in swirling flow field, the radial motion equation of particles and relative motion equations of two phases have been derived on the basis of the analysis of force. From the dispersed phase radial motion equation, the distribution of the radial velocity of dispersed phase particles to radius in cyclone and its analytic solution were obtained, furthermore , as a result, the radial motion time of particles t r in swirling flow field gained. Moreover, the relative speed of radial of two phases is consistent with that of CFD in this present, while tangential relative speed and axial relative speed have exponential decay with time according to relative motion equations of two phases. Under the condition of no slip between two phases in axial movement, the axial motion time of dispersed phase t zwas derived, and the trajectory equation of particles was established in relate to the distribution of the radial velocity of dispersed phase particles to radius, which could be used to count work efficiency of cyclone and be used for design of cyclone and its development.From the application of dimensional analysis method, stress in swirling flow field was analyzed, leading to the investigation into distortion and breakage of particles. According to the results of dimensional analysis, the conclusion is that Reynolds stress is larger than viscosity stress in turbulent flow, and the multiple is the Reynolds number, which induces into breakage of particles. In other regions,what can happen to the dispersed phase pellet mainly is distortion and coalescence, which plays the leading role is in cyclone viscosity sheering force, in which the most important part are tangential and radial viscosity sheering force. Based on this, critical condition for the breakage of particles under viscosity sheering force and turbulent energy was deuced. Through the relationship between Weber number and the relative speed, situation of distortion or breakage of dispersed phase particles in region-specific could be comprehended under the specific handling capacity.On CFD and theoretical analysis foundation, the peculiarities of mass transfer in cyclone absorber were set forth. The gas-liquid absorption process was intensified not only by the relative speed in cyclone, but also the grade of velocity in swirling flow field, owing to which, turbulent diffuse coefficient and dispersion coefficient that is much larger than diffuse coefficient in turbulent flow come into being in cyclone, intensifying the swirling flow field to aggrandize mass transfer enormously.A mathematic model of gas-liquid absorption and mass transfer with chemical reaction in a cyclone absorber has been established according to different dynamical characteristics of the dispersed phases in swirling flow field based on the species mass-conversation equations in this thesis, which is consistent with the Danckwerts’ Surface Renewal Theory. Not only considering the diffusive term and source term, but also the transient and convective term, this model could explain the reaction process in swirling flow field much more exactly. But because the model’s computation needs to determine some specific physical quantities, it is restricted for application.According to the theory of mass transfer for gas-liquid and turbulence, established based on vortex has been suitable for gas-liquid absorption in cyclone absorber mass transfer model. The model compared to other models, parameters mainly were related to some measurable and controlled parameters about operation、nature and structure in macro aspects, so its computation is simple and its significance is clear. From the process of making sure the relevant parameters, the model can also be applied to the general freedom of the interface of gas-liquid mass transfer.By the mass transfer coefficient model, the correlation of diffusion time t D was available. On the base of the demand for t D << tr<<tz in cyclone absorber, the relationship between diffusion time and motion time of dispersed phase could be used for development and design of cyclone absorber.The absorption test method has been designed that could notify the characteristic parameters of cyclone absorption. And two set of testing installment were manufactured, correspondingly, the sampling and the analysis of test instrument were structured. Based on this,the experiment was carried through, in which the handling capacity、the ratio of liquid/gas、differential pressure of gas and size of droplets along with configuration parameter were reviewed.The result of experiment proved that the rate of mass transfer augment with the handling capacity and the ratio of liquid/gas & differential pressure of gas, but it will decrease if the size of droplets is larger, however, this relationship is bound by the interval. But, if the size of droplets is extremly small, because of the enfluence of surface tense, conversely, it will take disadvantage of the rate of mass transfer.The interfacial surface per volume determined and calculated is related to the handling capacity、the ratio of liquid/gas and size of droplets, having nothing to do with differential pressure of gas, which has larger value in smaller cyclone absorber, compared to the other absorption equipment in the literature, the value of cyclone absorber living in the area of the forefront.The efficiency of absorption accounted in terms of test shows that it is related to the ratio of liquid/gas and the handling capacity and the size of droplets, independent of differential pressure of gas under this test condition, which perhaps is related to limited change range of differential pressure.According to test data calculated on the mass transfer coefficient in liquid side, it arised with the value of the increased handling capacity, however, the size of droplets and gas pressure were not related to that and it decreased with the increased ratio of liquid/gas if the ration of liquid/gas is just small. The conclusion not only is consistent with Kolmogorov theory that the mass transfer coefficient is only regarding on energy aerodynamic speed besides the nature, but also tallies with the two models established in this work, which indicates that the models established in this work have utility just well.The influence of temperature on mass transfer has been discussed by theory, and it was pointed out that there was different result between mass transfer coefficient in gas side and liquid side with temperature. For chemical absorption, the mass transfer efficiency elevates in certain temperature range along with the temperature arising, but the temperature would not be favor of absorption process forever. Has been calculated under this test condition each thermal reaction, finally it was proven that this experiment basically is the isothermal testing. The energy equation establishment in the end can express incomprehensively the temperature change in test.It could be concluded that there is a friendly envirmonent for gas-liquid transfer in a cyclone, moreover, the result of interfacial surface per volume、mass transfer coefficient and diffusion time confirmed that there is feasiablity for cyclone as a gas-liquid absorption equipment. If considering the simple strructure and maintenance-friendly of a cyclone, besides , it has coherently transfer-seipartion effect, therefore, there is actual meaning for prevail and application of a cyclone.更多还原