Characterisation of organic solvent nanofiltration membranes in multi-component mixtures: Membrane rejection maps and membrane selectivity maps for conceptual process design

In organic solvent nanofiltration (OSN), the transfer of results attained during the membrane characterisation and testing to industrial applications plays a crucial role in the development of new processes. Even though industrial applications typically include multi-component mixtures, most permeability and rejection experiments are only conducted with single solvents. Therefore, this paper presents a characterisation of two commercial polyimide-based membranes, Starmem™122 and Puramem™280, in multi-component solvent mixtures. The experimental investigation includes solvent flux and rejection measurements of five dissolved compounds in binary and ternary mixtures of toluene, n-hexane and 2-propanol. The experimental results demonstrate the existence of permeate flux minima and maxima in binary solvent mixtures. Moreover, a strong dependence of the solute rejections on membrane swelling, solute solubility parameter and solute size is shown. As the solvent choice has a significant impact on membrane performance, membrane rejection maps (MRM) and membrane selectivity maps (MSM) were developed. They can be used as decision tools in the early stages of conceptual process design by selecting either pure solvents or solvent mixtures that enhance the downstream processing in OSN and thus can replace costly and time-consuming membrane screening or OSN membrane modification. To address the relevance towards other solvents, the transferability of these results to classes of similar solvents was demonstrated.