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

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 degrees 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 condensation polymerization reactions are essentially eliminated. For example, the molecular weight of the lignin fraction remained unchanged with continuous processing but increased from 7900 to 13200 when batch processing was used for the same ALPHA conditions. Furthermore, the 10-fold reduction in metals content (e.g., sodium content reduced from 1400 to <100 ppm) was equal to or better than that obtained by single-stage batch processing. Thus, continuous ALPHA can be used to obtain an ultrapure, metals-free lignin of well-defined and controllable molecular weight.