Osmotic distillation with propylene glycol, glycerol and glycerol-salt mixtures

Aqueous solutions of propylene glycol (PG), glycerol and glycerol-salt mixtures are investigated as alternative to calcium chloride, normally employed to concentrate aqueous mixtures by osmotic distillation (OD). These extractants can overcome the problems of corrosion and scaling associated with the use of brines. The proposed extractants are compared in terms of driving force available for mass transfer, flux achievable, viscosity, penetration pressure through the membrane pores, solvent entrainment. The water activities of the relevant solutions are calculated by predictive methods and, in the case of glycerol-salt mixtures, measured by an isopiestic technique. Propylene glycol and glycerol solutions exhibit similar extractive power: the concentration has to be limited to 70-75 wt.% in order to limit the viscosity, as a consequence these compounds are a bit less effective than highly concentrated CaCl2 solutions. Notwithstanding the good flux achievable, propylene glycol cannot be recommended as extractant for juice concentration, indeed, owing to the not negligible volatility, it diffuses toward the juice in considerable amount, in addition, the penetration pressure through the membrane pores is quite small. The ternary mixture glycerol-salt-water allows to obtain the same flux achievable with glycerol alone, but with a substantially lower viscosity. (C) 2003 Elsevier B.V All rights reserved.