Braided silicone rubber membranes for organic extraction from aqueous solutions: I. Mass transport studies

A new class of composite tubular membranes, steel braided membranes, has been tested in high flow velocity membrane systems for the extraction of organic pollutants from dilute aqueous solution. These membranes consist of a tubular membrane made of rubber material (e.g. polydimethylsiloxane or ethylene propylene diene monomer) contained in a netlike shell (braid), made of stainless steel threads, which supports the rubber tube. Overall mass transfer coefficients have been measured through steel braided and bare PDMS membrane tubes, using air, distilled water and polypropylene glycol (PPG) as extractive phases under a range of hydrodynamic conditions. The transport mechanism has been described by a resistance-in-series model approach. This has allowed the contributions of the braided side of the membrane tubes to the overall mass transfer resistance to be elucidated, and has shown that in aqueous-aqueous extraction operations the braid is the main resistance to mass transport across the membrane. Mass transfer coefficients measured through bare membrane tubes (1.4×10−5 to 2.5×10−5 m s−1) were 40% higher than those measured through braided membrane tubes (1.0×10−5 to 1.5×10−5 m s−1) under similar hydrodynamic conditions. This limitation has been overcome by using an extractive phase with high affinity for toluene (PPG). This enables high mass transport rates (overall mass transfer coefficients ranging from 4.3×10−5 to 5.2×10−5 m s−1) and allows an efficient use of braided membranes for extraction in modules with high fluid velocity.