Mass transfer in osmotic evaporation: effect of process parameters

The influence of relevant parameters on the osmotic evaporation process, namely temperature, stirring rate and osmotic agent’s nature and concentration, is evaluated. The water flux is expressed as a function of the Reynolds number (Re), characterising the hydrodynamic conditions, and as a function of the water activity difference, which is the driving force of the process. It is our intention to emphasise the need of a systematic presentation of the results, in order to compare different conditions. Mass transfer is described by a series resistance model: the membrane resistance and the boundary layer resistance in both sides of the membrane. The contribution of the boundary layer resistance is determined for two osmotic agents (calcium chloride and glycerol), and a correlation for the mass transfer coefficient, Sh=1.63Re0.56Sc0.33, was obtained. Using as osmotic agents calcium chloride, sodium chloride and glycerol, it was noticed that, the flux increases with the osmotic solution concentration, and is not affected by the osmotic agent’s nature. The water flux increases with temperature according to an Arrhenius type equation and this increase is mainly due to an increase of the water vapour pressure.