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 hydroph obic high-boilers) diluted in model binary aqueous solutions, were conducte d at 30 degrees C through three kinds of commercial organophilic membranes on a plate-and-frame module. A model, based on the estimation of transmembr ane transfer coefficients (with a driving force expressed in terms of parti al pressure difference between each side of the membrane), was used in orde r to represent the influence of the total permeate pressure on the selectiv ity of the pervaporation of the various studied aroma compounds. At first, a silicalite-filled silicone membrane set, best-suited for the selective ex traction of small-size organic permeants, was used for the pervaporation of low-boilers (diacetyl and ethyl acetate). With these two molecules, the se paration factor of the pervaporation operation was independent of the total permeate pressure. Moreover, the silicalite-filled membrane proved to be m ore selective than the vapour-liquid equilibrium characteristic of the aque ous feed containing the most hydrophobic low-boiler, i.e. ethyl acetate. A second series of experiments was carried out on unfilled membranes (selecti ve layer composed of PDMS and PEBA) with high-boiling aroma compounds (S-me thylthiobutanoate and gamma-decalactone). In both cases, selectivities were highly dependent of the total permeate pressure. The PDMS membrane display ed an unusual behaviour with gamma-decalactone, as it appeared that the lac tone flux was not induced by a difference in partial pressure at each side of the membrane. Whatever the nature of the membrane-aroma compound associa tions, the transmembrane transfer coefficient model resulted in an accurate prediction of the selectivity pervaporation operation over a large range o f total permeate pressure. (C) 1999 Elsevier Science B.V. All rights reserv ed.