A turbulent film model for multicomponent mass transfer

Abstract

This paper analyses steady-state multicomponent mass transfer between an interface and a turbulently flowing fluid phase. The molecular diffusion contribution to the transfer process is modelled by using a matrix of diffusion coefficients D, including off-diagonal elements. A turbulent eddy diffusivity ε_m is used to describe the turbulent mass transfer contribution. For cases (e.g. pipe flow) in which the variation of ε_m in the boundary layer region can be estimated, an analytic expression is derived for the matrix of multicomponent mass transfer coefficients k^•. Appropriate correction factors to take account of the effect of high transfer fluxes on the transfer coefficients are derived in the analysis. The analysis shows that increase in the level of turbulence in the system, for example by increasing the Reynolds number, Re, for flow inside a conduit, results in a diminished influence of molecular diffusion coupling on the system the system transfer behaviour. This is illustrated by means of a numerical example involving mass transfer in the gaseous system acetone-benzene-helium in which, for the chosen conditions, increase in Re for flow along the interface alters the direction of transfer of acetone, a direct consequence of diminished diffusional coupling at increased turbulence levels; i.e. k₁₂/k₁₁ decreases as Re increases. Use of the multicomponent generalization of the Chilton-Colburn analogy, recommended in the literature, leads to the conclusion that the ratio k₁₂/k₁₁ is independent of Re; thus the relative importance of diffusional coupling is not affected by the turbulence level, an unlikely circumstance. It is concluded that the Chilton-Colburn analogy cannot be applied for describing mass transfer in strongly coupled multicomponent systems. For such systems a fundamental description of the mass transfer mechanism, e.g. of the relative contributions of molecular diffusion, turbulent diffusion, interfacial turbulence etc. is essential for the calculation of the individual fluxes.