Membrane air-stripping of aroma compounds

The selective extraction of aroma compounds from a highly diluted aqueous feed by membrane air-stripping (MAS) was studied. The pilot-scale apparatus used for this study was made of a hollow fiber membrane contactor coupled with a multi-step condensation unit. The aqueous feed model solution containing 10 aroma compounds (at ∼2 × 10−2 kg m−3 each) cross-flowed on the shell side of the membrane and air flowed in the lumen of the hollow fibers. The impact of operating conditions (hydrodynamic conditions of aqueous feed and stripping air phases) on mass transfer was investigated and modeled. The limiting step of mass transfer was identified: the transfer in the gas boundary layer always represents the major resistance to mass transfer. However, its contribution depends on the volatility of the aroma compound (from 68% to 100% for dimethyldisulfide, where H = 65.2 × 10−3 at 298 K, and from 98% to 100% for hexanol, where H = 1.15 × 10−3 at 298 K, depending on the operating conditions). The global efficiency of the process was then evaluated. It showed that MAS coupled with multi-step condensation made it possible to extract aroma compounds of intermediate volatility (Henry's law constant, H ∼3 × 10−2) with a high recovery yield (>90%) and a high selectivity over water (>1500). Moreover, molecules with lower volatilities (H < 1 × 10−3) are almost not ever extracted (recovery yield <5%), which makes this technology highly effective for the selective extraction of aroma compounds with intermediate volatility from aqueous streams.