Osmotic evaporation: a new technique for fruit juice concentration

International audienceOsmotic evaporation is a membrane process allowing concentration of aqueous solutions at ambient temperature and under atmospheric pressure. The aqueous solution flows along one side of a porous hydrophobic membrane while a hypertonic solution circulates along the other side. A gaseous phase is entrapped inside the membrane pores and is separated from the non wetting liquids by a gas-liquid interface. Volatile components diffuse through the gas membrane, provided their chemical potentials in the two liquid phases are different.In this paper osmotic evaporation is presented as a gentle technique to extract water from fruit juices. A highly concentrated salt solution is used to maintain a concentration gradient with the juice, thus inducing at the gas-liquid interfaces a vapour pressure gradient which constitutes the driving force of the gas transfer through the membrane.The integrity of the gas membrane is guaranteed by the hydrophobicity of the polymer that prevents the liquids from penetrating into the pores. The wettability of the membrane depends on structural parameters like pore size and distribution or surface energy of the polymer but also on process parameters like surface tension of the solutions and pressure across the membrane.The vapour flux is related to the membrane porosity and thickness but depends mainly on operating parameters : the higher the concentration of the salt solution, the more efficient the water extraction ; an exponential type of dependance on temperature is observed. The hydrodynamic conditions can also affect the evaporative flux, i.e. velocity of the fluids, geometry of the module, rheology of the solutions etc.A sucrose solution and an orange juice have been successfully concentrated by osmotic evaporation up to 60 and 65 Brix respectively, with a mean evaporative flux of 2,2 kg.h-1.m-2.The effect of a concentration polarisation is discussed to explain part of the flux decay at high concentration levels