Liquid-liquid equilibrium compositions and global phase behavior for the lignin-acetic acid-water system at 70 and 95 °C

Liquid-liquid equilibrium (LLE) compositions were measured for the pseudoternary system lignin-acetic acid-water at 70 and 95 °C, conditions of interest for the Aqueous Lignin Purification using Hot Acids (ALPHA) process for the purification and fractionation of lignin. Other regions of phase behavior, including a narrow region of LLE at high acetic acid (AcOH) concentrations, as well as solid-liquid (SL) and solid-liquid-liquid (SLL) equilibria, were also identified so that the global phase behavior could be mapped out. For both temperatures, tie-lines encompassing the entire region of “ALPHA” LLE were measured, from near the liquid-liquid critical point in the AcOH-rich region to the solid-liquid-liquid (SLL) phase boundary in the water-rich region. This LLE region exhibited several interesting characteristics. Surprisingly, there was essentially no difference between the solvent compositions in the solvent-rich vs. the lignin (polymer)-rich phase, indicating that the solvent mixture is exhibiting the properties of a single liquid solvent with respect to the lignin polymer. Furthermore, a strong maximum was measured on the lignin-rich side of the binodal curve, which can occur when a solvent mixture consists of strongly associating fluids. The average molecular weight of the lignin in the lignin-rich phase was found to be an order-of-magnitude higher than in the solvent-rich phase. Acid-catalyzed lignin condensation reactions were found to occur at the highest AcOH concentrations and temperatures in the lignin-rich phase, but there was no impact on the measured phase compositions.