【作者】 陈庚；

【导师】 张卫东；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2009， 博士

【摘要】 膜吸收技术是将膜分离技术与传统气体吸收技术相耦合的新型分离技术。该技术使用多孔膜分隔气、液两相,两相在膜孔处间接接触进行传质,可有效避免传统吸收操作过程中的液泛、雾沫夹带等问题,并具有单位体积接触面积大、易于放大、操作弹性大等优点,在温室气体的捕集分离方面具有巨大的应用潜力。在膜吸收过程中,多孔膜本身不具有分离作用,只起到分隔两相和固定相界面的作用。膜吸收过程的传质模型多是基于阻力串联模型建立,将总传质阻力分为气相、液相和膜相分传质阻力,大多数研究者忽视了膜结构参数对膜两侧流体中传质的影响,认为膜结构参数仅对膜相传质阻力产生影响。本文对膜吸收过程的传质行为进行了模型化研究,主要分为以下几个方面:（1）选取纯CO₂气体-去离子水体系,对气液两相均静止的非稳态传质过程进行了模拟,深入分析了膜结构参数（主要是孔径和孔隙率）对液相侧近膜壁面处扩散传质的影响,对不同条件下的溶质浓度剖面进行了模拟。模型结果表明,膜的存在会对近膜壁面处溶质的浓度分布产生影响。当孔隙率较大或者孔径较小时,膜孔相距较近,溶质浓度剖面可以在短时间内覆盖整个膜表面,近膜壁面处浓度分布较为均匀,膜结构参数对传质影响较小。反之,当孔隙率较小或者孔径较大时,膜孔间距较大,溶质浓度剖面难以在短时间内覆盖整个膜表面,近膜壁面处浓度分布不均匀,此时膜结构参数对传质有显著影响。上述研究屏蔽了流体流动、化学反应等因素的影响,集中分析膜结构参数对近膜壁面附近传质的影响,为综合分析各因素对膜吸收过程传质的作用、建立包含膜结构参数影响在内的传质模型提供了理论支持。（2）从改善近膜壁面处浓度分布的不均匀性入手,提出了在液相中加入固相粒子对传质过程进行强化的方法。利用固体粒子将动量引入液相边界层内,使膜壁面处溶质浓度分布趋于均匀。实验结果表明,固相粒子的加入减小了不同孔隙率膜之间的传质效果差异,同时还强化了传质,且膜的孔隙率越小强化效果越大。（3）在板式膜接触器内,采用纯CO₂-去离子水和纯CO₂-NaOH溶液体系,建立了液相流动条件下的稳态传质模型,分析了膜结构参数、化学反应强度和流体流动状态等因素对膜吸收过程传质的影响,对不同条件下的溶质浓度剖面进行了模拟和分析,并通过实验进行验证。结果表明,膜结构参数对传质的影响与吸收剂pH和液相流速有关。当吸收剂pH值增大时,由于化学反应的消耗,延缓了CO₂浓度剖面覆盖整个膜表面的速度,近膜壁面附近CO₂的浓度分布变得不均匀,膜结构参数对传质的影响增大。当流速增大时,更多的新鲜吸收剂得以补充,同时,液相边界层减薄,更多的CO₂分子可以扩散进入液相主体,因此加剧了近膜壁面附近CO₂浓度的不均匀分布,膜结构参数对传质的影响增大。（4）传质机理研究表明:膜结构参数对传质过程影响的原因是由于溶质在近膜壁面处的切向和法向传质距离不同。溶质通过膜孔扩散进入液相后,在液相侧近膜壁面处同时沿切向和法向两个方向扩散。法向和切向两个方向上传质距离的不同导致了近膜壁面附近溶质浓度分布的差异。其中,法向传质距离即为液相侧浓度边界层的厚度,与体系物性、化学反应强度和流动状态等有关,切向传质距离为膜孔间距,由膜结构参数决定。当法向传质距离大于切向传质距离时,溶质扩散到达液相主体需要较长时间,溶质扩散通过浓度边界层前即可在切向上覆盖整个膜表面,近膜壁面处的浓度分布较为均匀。反之,当法向传质距离小于切向传质距离时,溶质浓度剖面在未覆盖整个膜表面前即可通过边界层到达液相主体,近膜壁面处的浓度分布不均匀。（5）选用实际应用更为广泛的中空纤维膜接触器,建立了CO₂-去离子水和CO₂-NaOH体系的传质模型,综合考察了膜结构参数、操作条件和膜器长度等因素之间的相互作用及其对传质的影响。结果表明,吸收剂pH值和液相流速的增大以及膜丝长度的减小会引起液相侧浓度边界层减薄,传质系数上升,同时,膜壁面附近CO₂的浓度分布变得不均匀,膜结构参数对传质的影响增大。膜的孔隙率还会作用于上述操作条件以及膜器结构参数对传质的影响。当孔隙率较小时,上述参数的改变不仅会减小液相边界层的厚度,也会使近膜壁面处浓度分布变得更加不均匀,引起有效传质面积的减小;当孔隙率较大时,近膜壁面处的浓度分布越均匀,上述各参数的改变不会引起有效传质面积的改变。因此,孔隙率越大,上述参数的变化对传质的增强效果越明显。综上,本文通过对膜吸收过程传质行为的模型化研究,分析了膜结构参数对传质的影响及其与其他因素的相互作用,为建立通用的膜吸收过程传质关联式奠定了基础,有助于膜吸收技术的研究和应用。更多还原

【Abstract】 Membrane absorption technology is novel separate technology that combines membrane separation technology and traditional gas absorption technology. Due to the advantages such as independent manipulation of gas/liquid flow, larger mass transfer area and flexibility to scale up, membrane absorption technology has been considered to be one of the most promising alternatives for the capture of green house gases.In most studies on membrane absorption process, resistance-in-series model is used to describe the mass transfer process, in which the overall mass transfer resistance is assumed to consist of three partial resistances: the liquid phase resistance, the membrane resistance and the gas phase resistance. As for the micro-porous membrane, it is merely considered to have influence on membrane phase mass transfer resistance. Little attention has been given to the effect of membrane micro-structure on mass transfer in gas or liquid phase, and the concerned viewpoints somehow remain confused. So, the deeply theoretical study on mass transfer behavior in membrane absorption process has important theoretical and practical value.In this work, theoretical model has been developed to describe the mass transfer behavior in mass transfer process.（1） CO₂ is absorbed by de-ionized water in unsteady-state membrane absorption processes. A theoretical model is developed to describe the diffusing near membrane surface, in which the effects of membrane structure characteristics are studied. The effect of membrane structure on mass transfer can be determined by concentration profile near membrane surface. When membrane porosity is big or pore size is small relatively, the concentration profile near membrane surface can get homogeneous instantly due to the short distance between adjacent pores. In this case, the existence of membrane has less effect on mass transfer. However, when membrane porosity is small or pore size is large relatively, the distance between adjacent pores is large, so the concentration profile near membrane surface becomes inhomogeneous during the absorption process. Therefore, the concentration profile can be influenced significantly by membrane structure characteristics, which means that membrane structure has significant effect on mass transfer in liquid.The study above analyzed the effect of membrane structure characteristics deeply by avoiding the influence of flow status and chemical reaction. It is helpful for develop the mass transfer model including all the factors.（2） In order to improve the concentration distribution near the membrane surface, solid particles are added into the absorbent liquid to enhance the turbulence in the boundary layer. The results show that the movement of particles in the liquid boundary layer makes the the concentration profile near membrane surface get more homogeneous. Consequently, the difference of mass transfer coefficient obtained by varied porosities declined.（3） A theoretical model, including the effect of membrane structure characteristics, liquid velocity and absorbent pH, has been developed to describe the mass transfer in steady-state membrane absorption process. It shows that the effect of membrane structure on mass transfer can be influenced by liquid velocity and absorbent pH. The increasing of absorbent pH or liquid velocity makes it hard for the solute concentration profile near membrane surface to reach homogeneous, which means the effect of membrane structure characteristics on mass transfer becomes significant. The model results agree well with the experimental data.（4） The effect of membrane structure on mass transfer can be explained by the difference between the mass transfer distances in the normal direction and tangential direction. The mass transfer distance in normal direction is the thickness of concentration boundary which is influenced by absorbent pH and liquid velocity. And the mass transfer distance in tangential direction is determined by the membrane structure. When the distance in normal direction is larger than that in tangential direction, solute concentration profile can overcast the whole membrane surface before diffusing into liquid bulk, so the concentration profile near membrane surface is homogeneous. Contrarily, solute concentration profile can not overcast the whole membrane surface before diffusing into liquid bulk, and the concentration profile near membrane surface is inhomogeneous relatively.（5） The mass transfer in hollow fiber membrane contactor is modeling studied when CO₂ is absorbed by de-ionized water and NaOH solution. The effects of membrane structure characteristics, operational conditions and fiber length are considered. The model results show that the thickness of concentration boundary layer in liquid side can be reduce by the changes as following: the increasing of absorbent pH, liquid velocity, inner radius of fiber or the decreasing of fiber length. In this case, the mass transfer coefficient increases and the effect of membrane structure becomes significant. Simultaneously, the membrane structure characteristics can influence the effects of the changes above. When porosity is small relatively, the changes above make the concentration profile near membrane surface become more inhomogeneous and lead to the decreasing of effective mass transfer area. When porosity is big, concentration profile is homogeneous relatively, so the changes above has little effect on effective mass transfer area. Therefore, the effects of operation conditions become more significant as the increasing of porosity.The above researches can be used for further theoretical study and practical application of membrane absorption technology.更多还原