Vacuum evaporation of surfactant solutions and oil-in-water emulsions

Vacuum evaporation of surfactant solutions and oil-in-water (O/W) emulsions at laboratory-scale set-up was investigated. Experiments were performed with surfactant solutions and model emulsions formulated with a base oil (85–15% (w/w) mixture of a synthetic poly-α-olefin and a trimethylol propane trioleate ester, respectively) and the following surfactants: Brij-76 (polyethylene glycol octadecyl ether, non-ionic), CTAB (hexadecyltrimethyl ammonium bromide, cationic), or Oleth-10 (glycolic acid ethoxylate oleyl ether, anionic). Evaporation rates are strongly influenced by operating pressure and temperature. Surfactants enhance oil emulsification in water and increase the evaporation temperature and the water evaporation rate, especially at low pressures. Surfactant concentration effects depend on the type of surfactant. For surfactant solutions the evaporation rate is mainly controlled by the boundary layer which is formed at the solution surface, primarily by surfactants. For the O/W emulsions, the transfer of water to the liquid–vapour interface and the development of an oil boundary layer at the emulsion surface are also controlling steps. The chemical oxygen demand (COD) of the evaporation condensate was lower than 2% with respect to the original O/W emulsion/surfactant solution, which permits to recycle water in the closed-loop process.