Effect of operating conditions on water transport during the concentrationof sucrose solutions by osmotic distillation

A recent membrane technique, osmotic distillation (OD), is used to concentr ate binary water-sucrose solutions at ambient temperature under atmospheric pressure. The principle is based on the extraction of water vapour from a dilute aqueous solution, which is put in contact with a hypertonic salt sol ution by means of a macroporous hydrophobic membrane. The concentration dif ference between both solutions translates into a transmembrane vapour press ure drop, that constitutes the driving force for mass transfer. An experime ntal device is designed at laboratory scale for this study, allowing achiev ement of vapour fluxes of 10 kg m(-2) h(-1) under standard conditions, The effect of various operating parameters on vapour flux is studied. The solut e content results in the most influencing variable via water activity in br ine and via viscosity in sugar solutions. The effect of concentration polar isation on the brine side is not negligible and would have to be taken into account for process optimisation. This phenomenon could not be quantified on the sugar solution side due to pressure drop limits of the pilot rig. Ev entually, the vapour flux can be significantly increased by adding a temper ature difference to the transmembrane concentration difference, when pure w ater is evaporated. (C) 2000 Elsevier Science B.V. All rights reserved.