On the thermodiffusion effect in vertical plate heat exchangers.

The subject of the transport phenomena has a pivotal role in the performance of aqueous lithium bromide falling film absorbers. Although the thermodiffusion phenomenon appears inside the solution, very little is known about its influence on the mass transport. This study numerically analyses the influence of the thermal mass transfer mechanism on a vertical plate-type heat exchanger for different Reynolds numbers ([Formula: see text]). Results indicate that, in general, the mass transfer caused by the temperature gradient enhances the total absorption because of the negative Soret coefficient of the salt solution. This improvement is found to be higher at higher Reynolds numbers. Furthermore, the concentration and temperature profiles and, therefore, the absorption change significantly along the flow length. Overall, this analysis assists the understanding of the role of the thermal mechanism in this type of absorbers and it demonstrates that it should be considered in future studies.

Soret coefficient of the n-dodecane-n-hexane binary mixture under high pressure.

In the present work, the Soret coefficient has been determined at high pressure for a binary hydrocarbon mixture by combining the thermogravitational column and the dynamic near-field imaging techniques. The analyzed mixture is an iso-massic n -dodecane-n -hexane mixture at 298.15K. The molecular diffusion coefficient has been measured up to 20MPa by means of the dynamic analysis of the light scattered by non-equilibrium concentration fluctuations. With a cylindrical thermogravitational column the thermodiffusion coefficient was determined from 0.1MPa to 10MPa. Density, as well as, mass expansion and thermal expansion have been measured with a high pressure densimeter. Dynamic viscosity at up to 20MPa has been determined with a high pressure viscometer. This work shows the decreasing tendency of both the molecular diffusion and the thermodiffusion coefficient with increasing pressure.

Thermodiffusion, molecular diffusion and Soret coefficient of binary and ternary mixtures of n-hexane, n-dodecane and toluene.

In this study, the thermodiffusion, molecular diffusion, and Soret coefficients of 12 binary mixtures composed of toluene, n-hexane and n-dodecane in the whole range of concentrations at atmospheric pressure and temperatures of 298.15 K and 308.15 K have been determined. The experimental measurements have been carried out using the Thermogravitational Column, the Sliding Symmetric Tubes and the Thermal Diffusion Forced Rayleigh Scattering techniques. The results obtained using the different techniques show a maximum deviation of 9% for the thermodiffusion coefficient, 8% for the molecular diffusion coefficient and 2% for the Soret coefficient. For the first time we report a decrease of the thermodiffusion coefficient with increasing ratio of the thermal expansion coefficient and viscosity for a binary mixture of an organic ring compound with a short n-alkane. This observation is discussed in terms of interactions between the different components. Additionally, the thermogravitational technique has been used to measure the thermodiffusion coefficients of four ternary mixtures consisting of toluene, n-hexane and n-dodecane at 298.15 K. In order to complete the study, the values obtained for the molecular diffusion coefficient in binary mixtures, and the thermodiffusion coefficient of binary and ternary mixtures have been compared with recently derived correlations.

Determination of thermal diffusion coefficient in equimolar n-alkane mixtures: Empirical correlations.

In this article we determined the thermal diffusion coefficient (D(T)) in equimolar mixtures of n-alkanes nC(i)-nC(12) (i=5,6,7,8,9,17,18), nC(i)-nC(10) (i=5,6,7,15,16,17,18), and nC(i)-nC(6) (i=10,12,14,16,18) at 25 degrees C and at atmospheric pressure using the thermogravitational technique. The results obtained from this study together with the previously published ones in the series of nC(i)-nC(18) (i=5,6,7,8,9,10,11,12,13) show that the main parameter that determines D(T) in each series is in association with the molecular weights of the mixture's components. However, an empirical quantitative correlation has been obtained between D(T), the molecular weights of the components, the viscosity, and the thermal expansion coefficient of the mixtures. This is the first report of a closely accurate correlation for D(T) in liquid mixtures.