Transient-state method for coupled evaluation of Soret and Fick coefficients, and related tortuosity factors, using free and porous packed thermodiffusion cells: Application to CuSO4 aqueous solution ( 0.25M)

Abstract

The measurement of Soret coefficients in liquids is not easy and usually not very precise because the resulting concentration gradient is small and moreover can be perturbed by undesired convection currents. In order to suppress, or to drastically reduce these convection currents, the use of a porous medium is sometimes suggested. The question arises as to whether the Soret coefficient is the same in free fluid and in porous medium. This is the aim of this paper. To this end, for a given liquid mixture, the time evolution of the vertical concentration gradient is experimentally measured in the same thermodiffusion cell filled first with the free liquid and next with a porous medium followed by saturation by the liquid mixture. Both the isothermal diffusion (Fick) coefficient and the Soret coefficient can be deduced, providing that a correct working equation is used. The proposed equation results from integration of the general mass conservation equation with realistic boundary conditions (zero mass flux at the boundaries) and some simplifying assumptions rendering this equation more tractable than the one proposed some decades ago by Bierlein (J.A. Bierlein, J. Chem. Phys. 23, 10 (1955)). The method is applied here to an electrolytic solution (CuSO4, 0.25 M) at a mean temperature of 37 degrees C. The Soret coefficients in free and porous medium (zircon microspheres in the range of 250-315 x 10(-6) m) may be considered to be equal ( S(T) = 13.2+/-0.5 x 10(-3) K(-1)) and the tortuosity factors for the packed medium are the same relative to thermodiffusion and Fick coefficients (tau = 1.51+/-0.02).

Thermal diffusion of water vapour in porous materials: Fact or fiction?

“…The thermal permeability δ T is thus of the same order of magnitude as δ p , but negative, indicating thermal diffusion transpiring from low to high temperatures. [23] corroborates that the thermal diffusion factor α remains valid when considering porous materials, so such ratio's are equally valid for vapour diffusion in porous materials.…”

Non-equilibrium molecular dynamics simulations of the transient Ludwig-Soret effect in a binary Lennard-Jones/spline mixture

“…al. [32], and Mialdun and Shevtsova [17] The analysis shown here is based on measuring the composition and temperature as function of distance from the heat source for fixed time. In an experimental setup, it may be more convenient to measure the composition as function of time for a fixed distance from the heat source.…”

Separation in an inclined porous thermogravitational cell

“…inaccuracies in measuring the porosity and other thermophysical parameters). The authors who have conducted experimental studies of species separation in vertical thermogravitational columns [19,20] reported errors of the same order of magnitude. …”

“…The first experiment was with (Cell 1 ) using an aqueous solution of CuSO 4 whose concentration expressed as mass fraction was 0.0733. This solution has been studied previously [18,19]. For the presented experiment, we chose an inclination of 75°from the vertical with the cold wall at 10°C and the hot wall at 30°C (Le = 130 ± 8, Ra = 1.500 ± 0.075 and w = 0.40 ± 0.05).…”