Countercurrent transport of organic and water molecules through thin film composite membranes in aqueous–aqueous extractive membrane processes. Part I: experimental characterisation

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This paper presents an experimental study on organic and water mass transfer through thin film composite (TFC) membranes, made of a 2 μm thick polydimethylsiloxane (PDMS) layer coated onto a microporous support of polyethersulfone. These were used for the extraction of various organic molecules from aqueous solutions with different ionic strengths. Independent measurements of organic and water transfer rates were performed using lab-scale single- and multi-tube mass exchangers. The rate of organic transfer across TFC membranes is shown to be independent of the affinity of the organic molecules for the PDMS layer, owing to negligible mass transfer resistance in this thin layer. As a consequence, the overall mass transfer coefficient measured for aqueous–aqueous extraction of hydrophilic compounds (i.e. with low affinity for PDMS) was up to one order of magnitude larger for TFC membranes than for 350 μm thick PDMS membranes operated under the same hydrodynamic conditions. The effect of osmotic pressure on water transport through TFC membranes was assessed by liquid–liquid extraction experiments using saline solutions. Water flux increases with increasing difference in salt concentration across the membrane, due to increased activity gradient. For the range of salt concentration and PDMS layer thickness used in this work, the osmotic water flux across the membrane has only a marginal effect on the rate of organic extraction. On the other hand, the ionic strength of feed and extractive phases has a direct influence on the organic transfer, due to the dependence of transport and equilibrium properties (i.e. diffusion and partition coefficients) on salt concentration. As a result, TFC membranes exhibit an asymmetric behaviour depending on operating mode. For the same hydrodynamic conditions and ionic strength, organic flux is higher when a saline solution is on the coated side than when it is on the porous side of the membrane. This has obvious implications in the design and operation of TFC membrane processes for the extraction of organics from saline solutions.