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

Abstract

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.