Fractionation and purification of lignin fraction in a bio-refinery: Membrane-approaches as an economic and environmental alternative to classic separation techniques

Producing biobased chemicals and substituting energy-intensive separation processes are integral parts of a sustainable chemical industry. However, the downstream separation and purification of lignin-based aromatic biochemicals are challenging. Therefore, this research explores the benefits that organic solvent nanofiltration can bring to the downstream separation process of these aromatic compounds.

Different nanofiltration membranes were investigated on their solvent resistance and separation performances for chemicals found in birch wood lignin oil. Only commercially available nanofiltration membranes were considered to ensure direct relevance to a future biorefinery. Additionally, only green solvents were selected to increase the environmental friendliness of the downstream process. The combined effects of two solvents with seventeen nanofiltration membranes were investigated.

As membrane-solvent-solute affinities play a deciding role in the separation performance, the polar properties of the membrane materials, solutes and solvents were characterized. Hansen solubility parameters were revealed to be an important tool for understanding the affinity-based transport behavior, often more relevant than size-based sieving effects.

The oligomers and monomers in this oil were successfully separated using organic solvent nanofiltration. The most performant systems were nonpolar, silicone-based membranes combined with ethyl acetate, the less polar solvent. Diafiltration and a two-step filtration were shown to improve the separation. However, fractionating the monomer fraction proved more challenging, as the model monomers have nearly identical sizes and polar properties. The most effective separations were achieved using the most polar PA- and PI-based membranes combined with the polar solvent methanol. Simulations showed that a small multi-stage membrane cascade is required to produce a pure monophenol stream. Still, the optimal cascade design heavily depends on the requirements of the product stream. Finally, this research discovered key influencing affinity parameters that allow the purposeful design of better organic solvent nanofiltration membranes and systems.