Model Study Of Mass Transfer At Gas-Liquid Interface

The interfacial mass transfer of gas-liquid two-phase widely exists in the fields of petrochemical,biopharmaceutical,food processing,metallurgical refining,energy saving and environmental protection,and it often appear in the chemical basic unit operations such as distillation and absorption.It is of great significance for optimizing and improving the gas-liquid mass transfer equipment to understand the interfacial mass transfer process of gas-liquid two-phase.Due to the complexity and uncertainty of the flow field near the bubble surface,the current study on the mass transfer model of gas-liquid two-phase interface is not perfect.It has always been a research focus and difficulty around the world to build a reasonable mass transfer model to reveal the mechanism of gas-liquid interfacial mass transfer process.In this paper,based on the in-depth summary and analysis of the previous mass transfer models,it is considered that the existing models assume that"The influence of tangential molecular diffusion and convection on the gas-liquid mass transfer process on the bubble surface can be ignored",which is not universal,especially for the gas-liquid mass transfer process with high turbulence.Therefore,this paper,from the conservation equation of solute concentration in a single fluid,using Volume Of Fluid?VOF?to trace the gas-liquid two-phase interface,established solute concentration transport equations in the continuous phase and the dispersed phase,respectively,and applied Henry's law to correlate the solute concentration on both sides of the gas-liquid interface,then,using the concentration step condition at the interface,established a solute concentration transport equation in the cylindrical coordinate system,which considered both the normal and tangential concentration gradients at the interface,finally simplified the solute concentration equation.In this paper,the finite volume method was used to discretize the control equation of mass transfer and transportation,and the PIMPLE algorithm was used to calculate the pressure-velocity coupling equation.Then,the code of the solver is written to simulate the surface mass transfer process of bubbles in the two-dimensional axisymmetric oxygen water system in the water flowing from top to bottom for the force balance in the"suspension"state.It was found that the grid resolution of 12.5um can not only ensure the accuracy of numerical simulation results,but also ensure the calculation efficiency.The simulation results show that the oxygen concentration reduction in the small-sized bubble?1mm?is more than that in the large-sized bubble?4mm?in the same period of time,and the average oxygen concentration in the 4mm bubble is 27.29mol/m³ at 0.3s,while the average oxygen concentration in the 1mm bubble is only 2.22mol/m³.This is because the small-sized bubble has a large mass transfer specific surface area of phase interface,and the small-sized gas has a large specific surface area of phase interface and the small bubble is not easy to have a deformation,which is more conducive to the mass transfer process.With the relative velocity decreasing between bubble and water flow resulted from the density increase of continuous phase and dispersed phase,the turbulence of bubble surface and the surface renewal weakens,therefore,the mass transfer effect becomes worse.When the viscosity of continuous phase is increased,the resistance of bubble movement increases.And,when the viscosity of dispersed phase is increased,the resistance between oxygen molecules increases.Both above will reduce the mass transfer efficiency.The continuous phase surface tension will seriously affect the shape of the bubble.With the surface tension increasing,the smaller the deformation degree of the bubble is,the larger the mass transfer area of the bubble is,and the tension gradient between the two phases resulted from the increase of the surface tension increases.The mass transfer coefficient increased from9.07×10^-4m/s?0.00879N/m?to 2.61×10^-3m/s?0.0879N/m?.At last,this paper studied the influence of buoyancy,and the better mass transfer effect can be obtained with larger buoyancy.In addition,this paper made a comparison among the numerical simulation results of different mass transfer models.It is found that it is necessary to consider the tangential concentration gradient caused by tangential molecules'convection and diffusion at the phase interface.