Effect of additive concentration on falling film absorption

Certain alcohols enhance the absorption of water vapor in aqueous solutions of lithium bromide. The mechanism behind the enhancement, however, is not clearly understood, which reduces the search for new additives to a trial and error experimental approach. Two main theories have been advanced to explain the mechanism. The first theory is that surface tension gradients induced by the additive cause the enhancement. This effect peaks when the excess additive concentrates at the surface, yielding two separate liquid phases. A second theory states that the additives facilitate the association of water molecules to lithium ions at high brine concentrations, and hence two liquid phases are not required for enhancement to occur. In addition, this second theory advances that branched additives tend to be more effective than linear ones. The present study investigates the effect of branched and linear additives on absorption for varying additive and brine concentrations. The additive concentrations were varied, from ten to one thousand parts per million, in a vertical falling film absorber. The enhancement measurements show that 2-ethyl-1-hexanol and 1-heptanol enhance absorption at concentrations below their solubility limits. These results are significant, in that the presence of two liquid phases is not necessary for enhancement to occur. It was also found that enhancement can be obtained at both low and high brine concentrations. Hence, the present work yields experimental information that partially supports both theories, but shows that new approaches are needed to explain the observed behavior.