Effects of Multiple Pulping Variables on the Molecular Weight and Molecular Weight Distribution of Kraft Lignin

The effects of pulping variables on the molecular weight and molecular weight distribution of dissolved lignin have been studied. Kraft lignins were prepared by cooking Slash pine (Pinus caribaea) wood chips in a pilot-scale batch circulation digester. Four pulping variables, effective alkali, sulfidity, cooking temperature, and time-at-temperature. were arranged in a very broad input space under a central composite experimental design, and their effects on the molecular weight of dissolved lignin were examined by means of fifteen-parameter quadratic models. It was found that, generally, the molecular weight of dissolved lignin increases in both bulk and final phases as delignification proceeds. Prolonged cooking at the end of final phase delignification causes degradation of lignin in the liquor and reduces the molecular weight of Kraft lignin. Discussions of pulping effects on the molecular weight of dissolved lignin are presented in detail.