Model Development And Numerical Simulation Of Mass Transfer Process At Two-phase Interface

Two-phase(such as gas-liquid,liquid-liquid)interface mass transfer process widely exists in petrochemical,food processing,biopharmaceutical,energy-saving and environmental protection,metallurgy and nuclear energy.The dispersed phases in the two-phase flow are dispersed in the flow field in the form of bubbles or droplets,so the surface mass transfer process of a single fluid particle(hereinafter referred to as fluid particles)actually forms the basis of the mass transfer process at the two-phase interface.Because of the complexity and uncertainty of the flow field around the fluid particles,how to deeply understand the mechanism of mass transfer process on the surface of fluid particles and establish a reasonable theoretical model has always been the focus and difficulty of international research.In this paper,first of all,the mass transfer model proposed by predecessors is analyzed in depth,and it is concluded that the assumption that the contribution of molecular diffusion on the surface of flowing particles to mass transfer process can be ignored in the existing models is not universal.Especially for the mass transfer process of strong tangential flow(tangential molecular diffusion and rapid surface renewal),it is necessary to consider the tangential solute concentration gradient caused by tangential molecular diffusion at the interface.Based on the above analysis,a new mass transfer model based on the local transient concentration conservation equation for single-phase flow is proposed in this paper.By constructing the concentration transport equations in continuous phase and dispersed phase,considering the concentration step condition at the phase interface and the concentration distribution relationship between the two sides of the phase interface,a mass transfer model considering both tangential and normal concentration gradients at the phase interface is established.Based on this new mass transfer model,a numerical simulation solver was built to simulate the mass transfer process on the surface of a single droplet in n-butanol-succinic acid-water system.The simulation results show that small droplets(1.38mm)have larger specific surface area than larger droplets(1.56 mm,1.76mm),and smaller droplets provide larger interphase mass transfer area.And the terminal rising velocity(2.79cm/s)of smaller droplets in continuous phase is smaller than that of larger droplets(2.81 cm/s and 2.92 cm/s),so the residence time in continuous phase is longer than that in continuous phase,which isbeneficial to mass transfer process.The smaller the surface tension of the continuous phase,the smaller the tension gradient between the two phases,the decrease of turbulence on the surface of the flowing particles,the weakening of the surface renewal,the increase of mass transfer resistance and the deterioration of the mass transfer effect.In the continuous phase with higher viscosity,the drop motion resistance increases and the surface movement decreases,thus the surface renewal weakens.Compared with the continuous phase with lower viscosity,the mass transfer process is more favorable to the mass transfer process.On the basis of numerical simulation,the mass transfer model is verified in this paper.The numerical simulation results of the single droplet static diffusion mass transfer are compared with those of the analytical solution.The results show that the concentration of droplet solute is in good agreement with the analytical results.In addition,the numerical simulation results of mass transfer process of n-butanol-succinic acid-water,methyl isobutyl ketone-acetic acid-water extraction system were compared with the experimental data.The time-varying trend of mass transfer coefficient predicted by the model is basically consistent with the experimental results,which shows that the model has a certain degree of reliability.In addition,numerical simulation is carried out in different dimensional coordinate systems.The results show that the movement of dispersed phase in three-dimensional space is less affected by wall effect,and the numerical simulation results can reflect the flow mass transfer process more accurately and intuitively.Finally,the numerical simulation results of different mass transfer models are compared,and it is considered that it is necessary to consider the tangential concentration gradient caused by tangential molecular diffusion reasonably,especially in the mass transfer process with large concentration difference between two phases.