Modeling Study On The Mass Transfer Behavior In Membrane Absorption Process

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.