Continuous-Flow Process for the Purification and Fractionation of Alkali and Organosolv Lignins

Abstract

Hot acetic acid–water mixtures can be used to purify and fractionate alkali (kraft) and organosolv lignins via the Aqueous Lignin Purification with Hot Acids (ALPHA) process. However, condensation polymerization reactions can occur in the solvated, lignin-rich liquid fraction at the elevated temperatures (e.g., 60–100 °C) of operation, significantly increasing the lignin molecular weight. Thus, conversion of ALPHA to continuous-flow operation was investigated in order to determine the effect of reduced residence times on both the above reactions and on the extraction of metals from the solvated lignin to the solvent phase. All four lignins investigated could be converted to a homogeneous lignin–water slurry at ambient temperatures, facilitating continuous operation. When a static mixer is used for the mixing and equilibration device, reducing residence times to 20–30 s, condensation polymerization reactions are essentially eliminated. For example, the molecular weight of the lignin fraction remained uncha...