Organic Solvent Nanofiltration (OSN) for Energy-Efficient Processes

Abstract:
Nanofiltration is a pressure driven membrane separation process with a separation capability from 200 to 1000 Da between those of ultrafiltration and reverse osmosis. In recent years, Nanofiltration has found applications in separation problems with organic solvents like alkanes, ketones, or aromatics. Organic Solvent Nanofiltration (OSN) is a promising membrane technology and regarded advantageous in terms of energy saving or product recovery. The key limitation of OSN applications arises from the stability of membranes in organic solvents. In addition to the development of solvent-stable membranes, the production of tailor-made nanofiltration membranes is of great importance for the application of OSN in chemical processes. By selective modification of the chemical coating, a highly selective, solvent-resistant membrane can be produced with moderate effort to match the requirements of the respective separation problem. OSN can thus be easily available for a wide range of applications. The aim of this project is to develop a tailor-made membrane, offering stability in a variety of solvents as well as in long-term operation and having a sharp and adjustable cut-off. To classify these membranes, commercially available benchmark membranes are investigated. As little is known about the solute behaviour in OSN membranes under various conditions a systematic investigation of transport properties is crucial. Based on the experimental results, different mathematical models will be validated. To characterize tailor-made membranes, solvent flux and solute retention are measured. The solvent flux strongly depends on the solvent type due to differences in polarity and viscosity. The retention of solutes depends on parameters like e. g. pore size, solute size, and solute shape. Both the flux and the retention depend on transmembrane pressure and membrane swelling. A new characterization method to investigate the retention behaviour of the membrane will be developed using polyethylene glycols with low molecular weight as a standard. The polymer is dissolved in selected solvents and separated by a membrane. The corresponding retention is displayed as a function of molecular weights so that the MWCO at 90% retention can be determined as a characteristic measure of the membrane. The dependency of flux on solvent type is demonstrated by using three different solvents in this study: hexane as a long-chain hydrocarbon, methanol as an alcohol and toluene as a cyclic hydrocarbon. In order to develop new membranes systematically, there is also a great need to understand the transport phenomena in the membrane and to generate a suitable mathematical model. Several transport models have been proposed in the literature but basically two models are used to describe mass transport through OSN membranes: the Solution-Diffusion Model and the Pore-Flow Model. Both of these models will thus be validated with experimental data generated in this project

Keywords:
Organic Solvent Nanofiltration