Rayleigh Convection And Its Effect On Interfacial Mass Transfer Based On Lattice Boltzmann Method

In an interfacial mass transfer process, the concentration gradient of thetransferred component near the interface may generate Rayleigh instability if theconcentration gradient leads to a density gradient that directs oppositely to thedirection of the gravity, and Rayleigh convection may occur if disturbance exists atthe interface. The convection has an important effect on the interfacial mass transfer.In the present study, a2D time-dependent Lattice Boltzmann Method （LBM） with adouble distribution model is established for simulating the liquid-phase Rayleighconvection in the gas-liquid interfacial mass transfer process, and in conjunction withqualitative observation by Schlieren and quantitative measurement by Particle ImageVelocimetry（PIV） were used to investigate interface instability, characterization ofRayleigh convection and its effect on interfacial mass transfer.Firstly, the established method is used for the simulation of two-dimensionalRayleigh convection of liquid phase in the process of CO₂absorption into liquidethanol, which is induced by discrete and continuous diffusion sources of CO₂on thegas-liquid interface. The simulated results show that the concentration contours agreequalitatively with the experimental observation results by Schlieren. The effects ofRayleigh convection on the transport mechanisms are investigated by analyzing theRayleigh convection and concentration contour patterns. The experimental device forgas-liquid mass transfer is established for Rayleigh convection induced by singlediffusion source of CO₂, and the simulated results on velocity distributions areexperimentally verified by PIV measurement.Secondly, in the gas-liquid mass transfer process of CO₂absorption intohorizontal and initially quiescent liquids, random disturbance model is proposed todescribe the interfacial perturbation and it is used to investigate the critical parametersof the onset of Rayleigh convection. The simulated results show that P and CDdenoting respectively the possibility and the magnitude of concentration perturbationfor random concentration disturbance model have been demonstrated to have asuitable scope (0.05≤P<0.3and0<CD≤10^-9kg·m^-3) in which the simulated criticaltimes, the maximum penetration depths and maximum transient Rayleigh numbers of the onset of Rayleigh convection are in good agreement with the theoretical predictedvalue, and model parameters have no influence on concentration distribution patternsand interfacial mass transfer. In order to further verification of the validity of thismodel, the experimental device for time-dependent absorption is established. Thesimulated critical onset times of Rayleigh convection are in agreement withexperimental observation results by Schlieren in the mass transfer process of CO₂absorption into ethanol.Finally, considering for the wall shear stress, the random concentrationdisturbance model with P=0.1and CD=10^-12kg·m^-3is used for the simulation of thecharacterization of Rayleigh convection and its effect on interfacial mass transfer inthe processes of CO₂absorption into ethanol and isopropyl alcohol desorption fromwater and acetone desorption from ethyl acetate. The characterizations of Rayleighconvection are experimentally verified. The results indicate that Rayleigh convectionhas effect mainly on the liquid bulk in which has larger turbulent velocity (10^-410^-3m·s-1). There are many circulation flows and vortexes in the liquid bulk, which impelexchange of the liquid between the interfacial vicinity and the liquid bulk and promotethe renewal of interfacial liquid. Plume convection patterns are dissipative structurewith the continuous renewal feature, which experience the development from order todisorder. Mass transfer enhancement factor indicate that the Rayleigh convection caneffectively promote the mass transfer rate for absorption and desorption process andintensify interfacial mass transfer process. Mass transfer models accompanied byRayleigh convection were analyzed. The width of plume convection pattern at theinterfacial vicinity is considered as characteristic scale, which was used to calculatethe surface renewal time. Furthermore, the mass transfer coefficients are calculated bythe surface renewal time and agree well with the LBM simulation results.