The use of integrated countercurrent nanofiltration cascades for advanced separations

Abstract

BACKGROUND: The concept of integrated countercurrent nanofiltration cascades is derived from fractional distillation and results in an enriched and a depleted stream, the volume and purity of which can be controlled by changing the operating parameters. The aim of this study was to assess the potential of integrated countercurrent nanofiltration cascades for advanced separation of individual organic components in aqueous solution by means of single-stage filtration experiments using xylose and maltose and cascade simulations. The influence of module recovery, membrane characteristics, recycled fraction and the number of modules was evaluated. RESULTS: It was found that, in order to obtain a high product purity with a limited number of modules, there has to be a sufficiently high rejection difference between the components. Membranes with high rejections for the studied components (e.g. NF70 and UTC20) allow one to obtain a good separation factor but a low product recovery in a cascade configuration. A higher product recovery but lower separation factors are obtained for a cascade consisting of membranes with low rejections (e.g. NFPES10). Independently of the difference in rejection between the components, the number of modules, the recycled fraction and the module recovery determine the separation to a large extent. CONCLUSION: The optimal number of modules depends on the trade-off between a high selectivity and a high xylose recovery; the module recovery should be as high as (practically) possible. Higher selectivities and higher product recoveries are also obtained when the retentate is recycled to the previous module. This is different from the traditional cascade approach where retentates are recycled to the next module. (C) 2008 Society of Chemical Industry