Estimation of the deprotonation degree of phenolic groups on kraft lignin

Paper in proceeding, 2015

Lignin is an abundant biomaterial, with potential as raw material feed stock for production of solid- and liquid fuels as well as phenolic components and carbon fibres. Lignin may be separated from various process streams in an integrated pulp production and wood-based biorefinery. Separation of Kraft lignin from black liquor can be made by the LignoBoost technology; it starts with precipitation of lignin by acidification of the black liquor and consequently protonation of the phenolic groups on lignin, in equilibrium with the surrounding liquid. The protonation degree of phenolic groups on kraft lignin has been identified as a key-property in controlling the precipitation yield. It has been investigated if the protonation degree of lignin in a black liquor system can be modelled using a modified Poisson-Boltzmann cell model which has been combined with the Pitzer method. It was found that the model gave reasonable results. Furthermore, the model indicated that the protonation degree is strongly dependent on the pH and temperature of the black liquor, however, the molecular weight of lignin has a minor influence and at the high ionic strength in black liquor a small change in ionic strength has only a very small influence on the protonation. Additionally, the predicted deprotonation degrees were compared with previously measured precipitation degrees at industrially relevant conditions.