Diffusivities of the ternary liquid mixture 2-propanol—water—glycerol and three-component mass transfer in liquids

Abstract

The theoretical description of multicomponent diffusion in liquids is still at an early stage of development. The multicomponent diffusion coefficients strongly depend on the composition of the mixture. In order to calculate accurate mass transfer coefficients, extensive measurements of diffusivities over the whole range of concentrations are necessary. This article shows how the generalized Maxwell—Stefan (GMS) equations can be applied to diffusion in ternary liquid mixtures. In a first step, the ternary diffusion coefficients of the strongly nonideal liquid mixture 2-propanol—water—glycerol and the binary diffusion coefficients of the systems 2-propanol—glycerol and water—glycerol are measured by using the diaphragm cell method. As a second step, different methods of calculating liquid-side mass transfer fluxes are compared. Finally, the theoretical results are confirmed with the help of evaporation experiments. The experiments show that the ternary GMS diffusion coefficients are similar to the binary ones with the exception of the diffusion coefficients of the pair 2-propanol—water. In the binary mixture, this diffusion coefficient slightly decreases when the alcohol concentration rises. In the ternary mixture, however, it strongly decreases with increasing glycerol concentration which, according to the Stokes—Einstein relation, again leads to a higher viscosity. The calculation of mass transfer fluxes with the binary “viscosity-corrected” GMS diffusion coefficients is confirmed by the experimental results obtained from evaporation experiments. So-called “quasi-ideal” diffusion coefficients allow for a rapid estimation of how the real behavior of the liquid affects mass transfer in a ternary mixture. Thee quasi-ideal diffusion coefficients take the real behavior of the liquid into account. Their definition follows from the ideal formulation of the Maxwell—Stefan equations. With the help of quasi-ideal diffusion coefficients, the description of diffusion in ternary ideal gas mixtures can be applied to diffusion in liquids.