Experimental and theoretical considerations on breakthrough pressure in membrane-based solvent extraction: Focus on citrus essential oil/hydro-alcoholic solvent systems with low interfacial tension
Résumé
The aim of this work was to select materials to facilitate interface stabilisation in membrane contactors in order to apply membrane-based solvent extraction to systems with low interfacial tension. For this purpose, polymeric and/or ceramic porous membranes able to achieve a robust stabilisation of the liquid-liquid interface were selected (breakthrough pressure higher than 0.5 bars). The case study considered was the recovery of highly valuable oxygenated terpenes from lemon essential oil in an aqueous solvent containing up to 80% v/v ethanol. Wetting of membranes and contact angle measurements were performed in order to select materials with a minimal affinity for the hydro-alcoholic mixtures. Eight flat sheet membranes differing as to type of material and structure were investigated at 25 degrees C. The membrane-solvent combinations were carefully chosen since properties of both liquids and membrane materials determine the breakthrough pressure Delta P(c), which could be interpreted using the Young-Laplace equation. Experimental determination of Delta P(c) under process conditions showed that very hydrophobic membranes (polytetrafluoroethylene or grafted ceramic) with a maximum pore size of 160 nm led to satisfying breakthrough pressures of up to 0.98 bars for solvents containing 40% v/v ethanol. This study identified the requirements in terms of both the type of material and the structure of membranes necessary to increase the interface stability, thus: (i) making it possible to recover target molecules from complex systems such as essential oils; (ii) extending the range of solvents that can be used; and (iii) making process control much easier and more robust. (C) 2011 Elsevier B.V. All rights reserved.