Theoretical Analysis Of Interfacial Convection Enhancing Mass Transfer

The phenomena of interfacial convection,including Rayleigh convection driven by the density gradient and Marangoni convection caused by the difference of surface tension,have been investigated by many researchers due to its significance to mass transfer and separation processes such as absorption/desorption,extraction,distillation and so on.Researchers have utilized many methods,like qualitative or quantitative experiments,numerical simulations and theoretical analysis,to further explore the characteristics of interfacial convection and obtained the general conclusion that it enhances the process of interphase mass transfer.The work of this thesis is to investigate the mechanism of convection enhancing mass transfer from the viewpoint of non-equilibrium thermodynamics and the process of mass transfer respectively,for the objective of controlling the process and improving the mass transfer performance.In this article,a gas-liquid mass transfer apparatus and a quantitative Schlieren system were built based on predecessor's work to visualize Rayleigh and Marangoni convection generated in the process of CO₂ absorbed into ethanol and desorbed from saturated ethanol in a direction of perpendicular to the gas-liquid interface.The conversion of energy and the dissipated available energy caused by mass transfer among the whole evolution process was analyzed at two different levels of local and global entropy generation rate.The results of local entropy generation indicated that local disturbances due to the increasing gravitational potential energy or surface energy,led to the instability of the system and even induced the interfacial convection when those energies overcome the viscous friction.The critical entropy generation provided an easier and more precise method to predict the onset of interfacial convection,which was dependent on the physical properties and boundary conditions for different gas-liquid mass transfer systems.Simultaneously,the conclusion was verified by the variation of global entropy generation that interfacial convection changed the irreversible route to enhance mass transfer performance.Additionally,from the process of mass transfer,the analysis of convective-diffusive equation in unstable state interpreted the internal mechanism of interfacial convection enhancing the mass transfer was embodied in the field synergy principle that the velocity vector cooperated with the concentration gradient.The mass entransy dissipation rate can identify the irreversibility caused by the losses of mass transfer capacity.The local mass entransy dissipation induced by convection indicated that they had the similar variation with the synergy effects after convection occurring for fixed surface concentration condition.Compared with the thermodynamic properties,the result could be concluded that the losses of mass transfer capacity caused by mass transfer resistance was identical to the dissipated available energy,which represented the different viewpoint for the exploration of interfacial convection.The innovations of the thesis are mainly embodied in three aspects.Firstly,entropy generation analysis is introduced to the transient state of unstable interfacial convection with the quantitative Schlieren experiment,which provides a new and more precise method to predict the onset of interfacial convection.Secondly,the conversion of energy is analyzed in detail with the profile of local entropy generation and the dissipation of available energy is identified with the possibility of optimizing mass transfer performance and improving the efficiency of energy utilization.Thirdly,the relationship between the reinforcement of mass transfer and the synergy of velocity and concentration gradient vector is built and mass entansy dissipation is introduced to assess the effects of field synergy,as a result of interpreting the internal mechanism of interfacial convection enhancing mass transfer.