A systematic investigation of transport phenomena in organic solvent nanofiltration

In several industries organic solvent nanofiltration (OSN) offers a high potential as energy-, product- and waste-saving alternative compared with commonly used unit operations. Thus, organic solvent nanofiltration gained importance in recent decades. This technology separates complex organic mixtures under mild conditions. To use the full potential of OSN, a thorough understanding of the separation mechanism is essential. This work systematically investigates solvent as well as solute transport through adense PDMS-based polymeric OSN membrane. Experiments with four different solute classes, namely polyethylene glycol (194 - 820 Da), linear carboxylic acid (228 - 340 Da), polystyrene (162 - 2000 Da) and n alkanes with different chain length (142 - 339 Da) were performed. These solutes are dissolved in n-hexane, toluene, isopropanol or in methanol. Furthermore the transport of n alkanes with different chain length (142 - 339 Da) is comprehensively quantified not only in pure solvents but also in binary mixtures of isopropanol/toluene, methanol/toluene and methanol/isopropanol. Negative retentions were observed using isopropanol or methanol as pure solvent and n alkanes or linear carboxylic acids as solutes, whereas positive retentions were measured with polyethylene glycol as solute. In contrast, using toluene or n-hexane as solvent, all observed retentions were positive. A significant minimum in retentions was found for all n alkanes with different chain length in the binary mixture of toluene and methanol. Furthermore salt permeation and retention in water/methanol mixtures were investigated. Membrane performance varied depending on the composition ranging from aqueous to methanol solutions. Ion pair formation or ion dissociation correlates with a different salt retention. So, depending on the solvent or solvent mixtures and the solute, there exist considerable differences in the retention behaviour due to different membrane-solvent-solute interactions. This significantly changes flux and retention. Also negative retentions were observed, opening up new possibilities for further applications of organic solvent nanofiltration