Fractionating and Purifying Softwood Kraft Lignin with Aqueous Renewable Solvents: Liquid-Liquid Equilibrium for the Lignin-Ethanol-Water System

Abstract

Hot ethanol-water solutions can be used to simultaneously fractionate and purify softwood Kraft lignin through the Aqueous Lignin Purification with Hot Agents (ALPHA) process, using the regions of liquid-liquid equilibrium (LLE) that form at selected temperatures and solvent-to-lignin feed (S/F) ratios. Lignin, ethanol, and water compositions are measured for the solvent-rich (SR) and lignin-rich (LR) liquid phases in mutual equilibrium, as well as the lignin and metals mass distributions between the two phases. As depicted in quasi-ternary diagrams for clarity, both temperature and S/F ratio can be used to grow, merge, and even split the regions of LLE, giving significant control over both molecular weight (MW) and lignin purity. For example, a solvent comprising 45:55 EtOH/H₂ O at 75 °C and an S/F ratio of 6:1 enables recovery of an ultrapure (95 % of Na removed), higher MW (M_n =8400 Da) lignin fraction in the LR phase. On the other hand, 95:5 EtOH/H₂ O at 45 °C and S/F=3:1 enables recovery of an ultrapure, low MW (1500 Da) lignin in the SR phase.

Keywords: green chemistry; lignin; liquid-liquid equilibrium; phase diagrams; renewable resources.