The Study Of Interfacial Convection In The Process Of Gas-liquid Mass Transfer

Rayleigh convection and Marangoni convection usually occurs in the process ofinterphase mass transfer because of the change of physical properties of the fluid atinterface. Rayleigh convection is driven by the density gradient, while Marangoniconvection is driven by the interfacial tension gradient. These two interfacialconvection can enhance the process of interfacial mass transfer. Therefore, furtherstudy of this phenomena is significant for mass transfer and separation process suchas absorption, desorption, extraction and distillation.In this article, a gas-liquid mass transfer apparatus and a schlieren system werebuilt to visualize Rayleigh convection and Marangoni convection generated in theprocess of CO2absorption into ethanol and CO2desorption from ethanol in a directionperpendicular to the interface. The whole development and evolution process ofinterfacial convection was observed.The results of schlieren observation indicate that both the absorption anddesorption processes experienced two stages, dominated by molecular diffusion andconvection diffusion, respectively. The interface is relative stable at the beginning ofthe absorption. When the interfacial density gradients develop to a certain degree, theinterface become unstable, and some small-scale Rayleigh convection occur atinterface. This kind of small-scale Rayleigh convection tend to move horizontallywhile developing downward, making the small-scale Rayleigh convection merge tobigger ones. The Rayleigh convection gradually forms to a feathery convectionstructure and then became chaotic at the end of mass transfer. The interface wasrelative stable during initial stage of desorption. When the interfacial tension gradientsdevelop to a certain degree, the interface become unstable and an even-distributedgyrate convection patterns was formed. These convection patterns also tend todevelop and grow over time. However, Marangoni convection always develop andevolve near interface because of the effect of buoyancy.In addition, information such as concentration distribution and rate of masstransfer in the mass transfer process was studied through quantitative schlierenmethod. The quantitative results indicate that the rate of absorption and desorptionprocess is very slow. In the convection mass transfer process, the rate of mass transfer increased firstly and then decreased. In the absorption process, the larger densitylocated at the bottom of Rayleigh convection, driving the convection to movedownward, making the concentration at the center of the bottom of Rayleighconvection larger. In the desorption process, the larger interfacial tension gradientlocates at the edge and center of convectional cell type, driving the movements of theinterfacial fluid, and coupled with the buoyancy effect, forms the near interfacecirculating flow. This circulating flow leads to a concentration distribution which hasa higher value at the edge of the convectional cell and lower value inside. Thisaccelerates the renewal rate of the interfacial fluid element and thus the mass transferprocess was enhanced.