Thermal diffusion near a binary liquid azetrope

[摘要] Induction of a mass flux by a temperature gradient is known as the thermal diffusion or Soret effect, and is characterized by the thermal diffusion factor. A thermogravitational column’was used to obtain thermal diffusion factors and steady state separations in the ethanol-water mixture at its azeotropic composition. The thermogravitational diffusion column was calibrated using the value of the thermal diffusion factor of the cyclohexane-benzene mixture, which had been previously determined at 25°C. The temperature dependence of the thermal diffusion factor was determined for the ethanol-water system within .5°C of the azeotropic point and in the range of 25° to 77.7°C. Within 1°C of the azeotropic point, the thermal diffusion factor shows an anomalous behavior. It appears to increase rapidly as the azeotrope temperature is approached. In addition to the thermal diffusion factor, the heat of transport and thermal diffusion coefficient were determined for the ethanol-water system as functions of temperature, based on the assumption of Onsager Reciprocity. These parameters also exhibit a divergent character as the boiling point is approached. Further studies are needed to determine the significance of this effect in terms of molecular models. Ethanol enriched fluid from the thermogravitational column operating in the continuous mode was used as feed to a packed distillation column. Further separation occurred via distillation. Thus, thermogravitational columns offer a convenient small scale intermediate to break the azeotropes which often arise in the separation of liquid mixtures by distillation.

[发布机构] Rice University