Experiment Study On The Effect Of Additive Component In Gas Phase On The Interfacial Convection Of CO₂-water Dissolution Process By LIF/PIV Technique

In gas-liquid mass transfer process,the concentration difference between interface and liquid bulk can induce interfacial instability and then generate interfacial convection,including Marangoni convection induced by surface tension gradient and Rayleigh convection driven by density gradient.The interfacial convection can have significant effect on the process of gas-liquid mass transfer.And therefore,a further study of interfacial convection is of great importance for understanding the gas-liquid mass transfer phenomena,and for improving,enhancing mass transfer processes such as absorption and distillation in industry.In the gas-liquid mass transfer process,surface tension change can change the interfacial property and thus affect the interfacial convection and then the interfacial mass transfer.In this work,the effect of the surface tension change on the interfacial convection and mass transfer was investigated employing the process of dissolution of CO₂ into water.Second components of low surface tension were added in the gas phase to change the surface tension.Laser induced fluorescence?LIF?and Particle Image Velocimetry?PIV?techniques were used to investigate the effect of the additions of the low surface tension components on the concentration and velocity distribution respectively in the CO₂ dissolution process.Two components,ethanol and dichloromethane respectively,were used as the low surface tension components.Then understandings on the influence of the surface tension and density change on the interfacial mass transfer were discussed.LIF experiments and analysis showed that the additive component of low surface tension in gas phase can cause changes of interface physical properties including the distribution of surface tension and the change of vertical density gradient.When ethanol was added,the surface tension gradient along horizontal direction caused by the uneven distribution of ethanol led to the local Marangoni effects,and as a result the Rayleigh convection process was enhanced.But with the increase of ethanol concentration in the gas phase,local Marangoni effects decreased gradually,and with the increased dissolution of ethanol into water,the density gradient at the interface was changed to the opposite direction.As a result,Rayleigh convection was inhibited and the dissolution rate of CO₂ decreased.When dichloromethane was added,the experimental results showed that the mass transfer process was greatly enhanced with the increase of dichloromethane concentration in the gas because of the increase of the interface density gradient on the liquid surface.With the concentration of dichloromethane at the interface approaching to saturation,the interface density gradient got stabilized,mass transfer enhancement effect tended to be constant.PIV experiments showed that with the increase of dichloromethane concentration,the onset time of the Rayleigh convection was gradually advanced and finally tended to be stable.At the same time,the average velocity and vorticity at any cross sections of the liquid phase were much larger than that of the pure CO₂ absorption.The penetration depth for mass transfer with dichloromethane vapor was also much lager than that of pure water-CO₂ dissolution process.These indicate that not only the turbulence of the interface was enhanced,but also the disturbance of the liquid bulk was enhanced.Gas phase with dichloromethane greatly intensified Rayleigh convection mass transfer and accelerated the dissolution of CO₂ into water.