Adsorption mechanisms of alcoholic additives in water at low pressure – An experimental study

Abstract

Alcoholic additives are used to enhance the heat and mass transfer of the absorber, which has been widely recognized as the bounding device of an absorption chiller. Small quantities of additives in the aqueous lithium bromide solution reduce its surface tension, leading to a better wetting of the tube bundle of the absorber and triggering the Marangoni convection. Nevertheless, the enhancement mechanism relies on many parameters which have not been yet studied in details. In this work, the adsorption mechanisms of surfactants at the interface of water drops are experimentally investigated. 2-ethylhexanol, 1-octanol, 3,5,5-trimethyl-1-hexanol and 3-phenyl-1-propanol are chosen as additives and are added to water in different concentrations. The static and dynamic surface tension is measured according to the pendant drop method and the surface excess concentration is hereafter calculated with the Gibbs isotherm. Drops are generated in a saturated atmosphere without inert gases at low pressure, which represents the real working condition of an absorber. The results show that 2-ethylhexanol, 1-octanol and 3,5,5-trimethyl-1-hexanol lead to high surface tension reduction at high concentrations, and they are characterized by a fast diffusion and adsorption. Contrary, 3-phenyl-1-propanol presents a slow adsorption kinetic and is not effective in reducing the surface tension of water.