Modelling and Simulation of Nanoﬁltration Membranes

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Nanofiltration (NF) is a relatively recent membrane process and offers a plethora of application areas due to its selective remov...

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Nanofiltration (NF) is a relatively recent membrane process and offers a plethora of application areas due to its selective removal for ions and removal of organic matter above 200 g/mol molar mass. Application fields enlarged substantially in the last 25 years. Accordingly, there is an increasing need for process design and optimisation tools. Therefore, current research studiesfocus on a better understanding of mass transport phenomenon as well as the application and enhancement of the existing models to the real process streams.  In this thesis work, characterisation of NF membranes and investigations of their mass transport phenomenon through both experimental and modelling studies were of concern. For these purposes, flat sheet samples of four commercially available membranes were selected. Since the performance of a NF membrane is related to its structural and charge properties, membrane characterisation studies by microscopy, contact angle and electrokinetic measurements were conducted. A systematic experimental program was applied covering a wide spectrum of feed streams concerning both charged and uncharged solutes. Particularly charged streams regarding single salts and their mixtures were emphasised. Moreover, the effects of the operating conditions and the solution pH on membrane performances were investigated. Standard performance characterisation experiments, consisting of pure water permeability, organic and single salt rejection measurements, gave initial information on the membrane charge and performance characteristics. In mixture solutions of salts, distinctive behaviours of membranes to different ion types at different combinations were observed. Furthermore, artificial seawater experiments were conducted in order to determine the convenience of the considered NF membranes for this recently recognised process as an intensification step in the pre-treatment of seawater.  A practical tool was developed for the performance description and prediction purposes. For this purpose, currently available models were scrutinized. A physical model, based on extended Nernst-Planck equation describing mass transport through membrane active layer in conjunction with different partition equations at membrane and bulk interfaces, was built. Some modelling parameters were obtained either directly from the characterisation data or by applying some other straightforward models using experimental data. For the latter purpose, Fortran 90 programming code was used. As the programming efforts increase with the number of components in the feed stream, an equation based software Comsol Multiphysics was utilised. Herewith, model could be applied straightforwardly to multicomponents streams. Simulations were conducted systematically started from single salts, extended to salt mixtures and finally for seawater. A good agreement between experimental and simulation results were obtained. Suggestions were made to improve both the predictive ability and the reliability of the model. Incorporating the predictive model with system economics in a case study indicated that such an approach enables determining the optimum operating conditions and selecting the most appropriate membrane characteristics for the regarding process.