Study On Gas-liquid Interface Mass Transfer Mechanism Under The Condition Of Turbulence

Current research about the theory of two phase interface mass transfer is not mature and perfect. In the actual industrial production process, the efficiencies of mass transfer still remain very low, and it is hard to meet production requirements, needed to be optimized to upgrade. Therefore, it is of virtual significance to deeply investigate the micro mechanism of interfacial mass transfer and develop a novel mathematical model for predicting the liquid-side mass transfer coefficient, which used to guide the design and optimization of unit equipment and strengthen mass transfer efficiency. Based on the problems of the existing models, this work aims to develop a novel mass transfer model based on fluid elements or eddies, wide spectrum distribution and random interface interaction by using the phenomenological method and the theory of turbulence structure.This thesis has two parts. The first one is the gas-liquid mass transfer model. In the literature, more attention has been paid to the models of mass transfer coefficient based on the penetration or surface renewal theory. The mass transfer was considered to be governed by molecular diffusion. But large eddy model and small eddy model presupposed mass transfer were controlled by a fluid structure of certain wave spectrum. However, the different scale of fluid elements structure contribution to mass transfer was considered within the scope of the full energy spectrum in this article. Therefore, the above assumption was no longer needed. This work focused on the interfacial mass transfer process of bubbles. Adopting the characteristic line method and concentration gradient of the thin-layer approximation, a novel mathematical model for predicting the liquid-side mass transfer coefficient has been developed. It was derived from the combination of the unsteady-state convection-diffusion equation of concentration and the interaction between bubble and eddy. In this model, only partial relative motions between bubble and eddy were considered to be effective to let eddy contact with bubble, and an arriving frequency density distribution function was derived theoretically based on a collision concept. This function can be used to account for the differences of the whole spectrum each wave fluid elements micro structure contribute to the mass transfer coefficient theoretically. Furthermore, the effect of size of bubbles, the deformation and oscillation of bubbles, and number density of eddies on mass transfer will also be considered. The results predicted by this model showed a good agreement with the reported experimental data.For the second part, firstly, the numerical method was employed to verify the rationality of the characteristic line and the thin layer approximation(concentration gradient layer thickness on the whole less than 3.18% of the eddy size), and proved that the mass transfer process should be regarded as a steady-state process even for the single eddy. The results of numerical calculation showed that eddy in dissipation area can take more solute under the same the contact time, so mass transfer efficiency was higher. Comparing the results of numerical solving the equation of unsteady concentration by the derived LBM model with the analytical model in this paper, we found the interface liquid-side tangential molecular diffusion has a hardly influence on mass transfer coefficients. When ignoring tangential molecule diffusion, the error of the mass transfer coefficient was less than 1%. It showed that leaving out this diffusion is reasonable.