Mixing from Fickian diffusion and natural convection in binary non-equilibrium fluid phases



A two-dimensional (2-D) model that describes mass transport between non equilibrium gas and liquid phases of a binary non polar mixture in a closed system of fixed volume and temperature is presented. Diffusion, convective mass transport due to compressibility and non ideality, and the motion of the interface upon evaporation and dissolution are accounted. Natural convection in both phases is incorporated, which allows to study the effect of density increase in the liquid phase from gas dissolution. The Peng-Robinson equation of state is used to calculate the densities and the fugacities needed to find the interfacial composition consistent with local chemical equilibrium. The results obtained with a one-dimensional model was compared to our 2-D results, showing that natural convection influences the mixing time drastically. In the liquid bulk phase, convective flux is much higher than diffusive flux. Across the interface, diffusive flux is, however, the dominating flux, which allows accurate measurement of diffusion coefficients at high pressure in 2-D domains. © 2011 American Institute of Chemical Engineers AIChE J, 2012