O. Ecenarro

Thermogravitational measurement of the Soret coefficient of liquid mixtures

We have designed and constructed a thermogravitational column for determining the Soret coefficient of liquid mixtures. The excellent agreement of the results obtained for toluene and n-hexane mixtures with recent accurate measurements performed by optical methods shows the validity of the thermogravitational method when a column with the correct geometry is used. We have applied this technique to determine the Soret coefficient of the binary systems benzene-n-heptane and carbon tetrachloride with cyclohexane, methanol and ethanol. Comparison with the literature data allows us to determine the reliability of the existing measurements of thermal diffusion for these mixtures.

Determination of the thermodiffusion coefficient in three binary organic liquid mixtures by the thermogravitational method (contribution of the Universidad del País Vasco, Bilbao, to the benchmark test)

Within the framework of an international benchmark test, the thermodiffusion coefficients D T of the three binary systems formed with dodecane, isobutylbenzene and 1,2,3,4-tetrahydronaphthalene (with a mass fraction of 0.5 for each component at a temperature of 25C) have been measured. Convective coupling in cylindrical thermogravitational columns has been used to determine D T based on the Furry-Jones-Onsager theory. For each system, several columns with different gaps and heights were employed. Our results for these three coefficients are in good agreement with those obtained by other benchmark tests; the maximum deviation from the proposed benchmark values is 8.5%, which is not too bad owing to the difficulties of measuring a thermodiffusion coefficient.

Validity Limits of the FJO Thermogravitational Column Theory

We have solved numerically in the formulation vorticity-stream function the equations that govern the separation process in a thermogravitational cell. The numerical values for the steady separation have been compared with the ones given by the Furry, Jones and Onsager theory for different values of the relevant parameters, namely the Grashof and Schmidt numbers and the aspect-ratio A. The obtained results show that with the exception of small regions near the cell ends where the vertical concentration gradient increases considerably, the FJO theory is accurate by more than 1% in the range Gr Sc ≲ 1000 A. Therefore, inside this range the standard formulation can be used with confidence to determine the Soret coefficient from steady separation measurements in a thermogravitational cell.

Fickian Diffusion Coefficient of Binary Liquid Mixtures in a Thermogravitational Column

Abstract By measuring the mass fraction difference between the top and the bottom of a thermogravitational column as a function of time, we show that this transient evolution of the separation toward its steady value gives the isothermal mass diffusion coefficient, at least in the validity limit of the Furry–Jones–Onsager theory, whereas the final steady separation produces the thermodiffusion coefficient. The following mixtures have been considered: water–ethanol (39.12 wt% ethanol), toluene–hexane (51.7 wt% toluene), and the three systems of the so-called “benchmark of Fontainebleau”, which are the three binaries composed of isobutylbenzene and/or dodecane and/or 1,2,3,4 tetrahydronaphthalene (50 wt% in each component for each case). The obtained results indicate that reliable values of the isothermal diffusion coefficient can be determined by using the thermogravitational method.