降膜板结构及液体表面张力对降膜蒸发传质的影响

发布时间：2018-03-25 07:18

  本文选题：降膜流动　切入点：传质　出处：《天津大学》2015年硕士论文

【摘要】：降膜流动由于结构简单、动力消耗小且在低流量、小温差的情况下具有高传热传质系数等独特优势在工业领域得到广泛应用。本课题研究目的在于探究二组分溶液降膜蒸发过程中的传质分离效果,通过改进降膜支撑媒介的结构以及改变溶液表面张力等方法强化气液两相的传质过程,并对其应用前景进行了展望。本文设计了降膜蒸发装置对下降液膜进行研究,利用电加热板对光滑平板和翅片板上的下降液膜加热,观察液膜的流动形态,发现光滑平板上受热液膜流动面积明显减小且出现径向收缩;而翅片板上液膜由于得到再分布,其湍流程度增大,内外界面更新速度加快,受热后流动面积变化很小且在径向收缩不明显,这种壁面结构为降膜蒸发传质的强化提供了很好的依据。为了考察液膜在不同流动状态及受热状态下的降膜蒸发传质效果,本文通过控制变量法对乙醇-水溶液在不同流量、不同加热功率以及不同结构降膜流动板上的降膜蒸发过程进行了实验。对比不同流量及不同加热功率下塔顶乙醇出料的质量分数,发现随着流量的增大及加热功率的降低,塔顶乙醇质量分数增大。同时,对比不同结构降膜流动板上二组分溶液的分离效果,发现粗糙平板对乙醇-水溶液的分离程度高于光滑平板,而流经翅片板的塔顶乙醇质量分数高于粗糙板,由此证实了通过改进降膜支撑媒介的结构来优化降膜流动形态、增大液膜湍动程度来强化降膜蒸发传质的可行性。为了考察液膜表面张力对降膜蒸发效果的影响,本文通过测量加入一定量表面活性剂的溶液在不同温度下的表面张力,以及该溶液在翅片板上塔顶乙醇出料的质量分数,得出加入表面活性剂有利于降膜传热传质的结论。表面活性剂的加入降低了溶液的表面张力,在一定程度上抑制了表面波的形成,增加了有效传质面积,因此明显提高了降膜蒸发的效果。基于此,本文就强化降膜蒸发传质过程提出以下建议:优化降膜支撑媒介的结构,加入适当表面活性剂降低液膜表面张力,对降膜流动的流量和加热量进行优化。
[Abstract]:Falling film flow because of its simple structure, low power consumption and low flow rate, In the case of small temperature difference, the advantages of high heat and mass transfer coefficient are widely used in the industrial field. The purpose of this research is to explore the effect of mass transfer and separation in the process of falling film evaporation of two-component solution. By improving the structure of the falling film supporting medium and changing the surface tension of the solution, the mass transfer process of the gas-liquid two-phase was enhanced, and the prospect of its application was prospected. In this paper, the falling film evaporation device was designed to study the falling liquid film. The falling liquid film on smooth plate and fin plate was heated by electric heating plate, and the flow pattern of liquid film was observed. It was found that the flow area of heated liquid film on smooth plate obviously decreased and the radial contraction occurred, while the liquid film on fin plate was redistributed. The turbulence degree increases, the internal and external interface renewal speed accelerates, the flow area changes very little after heating, and the radial contraction is not obvious. This wall structure provides a good basis for the enhancement of falling film evaporative mass transfer. In order to investigate the effect of falling film evaporation mass transfer under different flow and heated conditions, the flow rate of ethanol-aqueous solution in different flow rates is controlled by variable method. Experiments were carried out on the falling film evaporation process with different heating power and different structure. By comparing the mass fraction of ethanol at the top of the tower with different flow rate and different heating power, it was found that with the increase of flow rate and the decrease of heating power, At the same time, compared with the separation effect of two component solution on different structure falling film flow plate, it was found that the separation degree of ethanol aqueous solution on rough plate was higher than that on smooth plate. The mass fraction of ethanol at the top of the column passing through the fin plate is higher than that of the rough plate, which proves that the falling film flow pattern can be optimized by improving the structure of the falling film supporting medium. In order to investigate the effect of liquid film surface tension on the effect of falling film evaporation, the surface tension of solution with a certain amount of surfactant was measured at different temperatures. The mass fraction of ethanol at the top of the column on the fin plate shows that the addition of surfactant is beneficial to the heat and mass transfer of the film. The addition of surfactant reduces the surface tension of the solution. To some extent, the formation of surface wave is restrained, the effective mass transfer area is increased, and the effect of falling film evaporation is obviously improved. Based on this, the following suggestions are put forward for strengthening the falling film evaporation mass transfer process: optimizing the structure of falling film supporting medium, Proper surfactant is added to reduce the surface tension of liquid film, and the flow rate and heat of falling film flow are optimized.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ051.62
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本文编号：1662089