Total permeate pressure influence on the selectivity of the pervaporation of aroma compounds

Pervaporation experiments with four aroma compounds (covering a wide range of physicochemical properties: from hydrophilic low-boilers to very hydrophobic high-boilers) diluted in model binary aqueous solutions, were conducted at 30 degrees C through three kinds of commercial organophilic membranes on a plate-and-frame module. A model, based on the estimation of transmembrane transfer coefficients (with a driving force expressed in terms of partial pressure difference between each side of the membrane), was used in order to represent the influence of the total permeate pressure on the selectivity of the pervaporation of the various studied aroma compounds. At first, a silicalite-filled silicone membrane set, best-suited for the selective extraction of small-size organic permeants, was used for the pervaporation of low-boilers (diacetyl and ethyl acetate). With these two molecules, the separation factor of the pervaporation operation was independent of the total permeate pressure. Moreover, the silicalite-filled membrane proved to be more selective than the vapour-liquid equilibrium characteristic of the aqueous feed containing the most hydrophobic low-boiler, i.e. ethyl acetate. A second series of experiments was carried out on unfilled membranes (selective layer composed of PDMS and PEBA) with high-boiling aroma compounds (S-methylthiobutanoate and gamma-decalactone). In both cases, selectivities were highly dependent of the total permeate pressure. The PDMS membrane displayed an unusual behaviour with gamma-decalactone, as it appeared that the lactone flux was not induced by a difference in partial pressure at each side of the membrane. Whatever the nature of the membrane-aroma compound associations, the transmembrane transfer coefficient model resulted in an accurate prediction of the selectivity pervaporation operation over a large range of total permeate pressure. (C) 1999 Elsevier Science B.V. All rights reserved.