SO₂-Ethanol-Water (SEW) Pulping: I. Lignin Determination in Pulps and Liquors

Abstract

Quantitative determination of lignin in SO₂-ethanol-water (SEW) pulps and spent liquors is described. The methods developed for conventional sulfite pulping are successfully applied to the SEW process. Linear correlations between Klason/total lignin content and kappa number are found over a wide pulp yield range for spruce, beech, and wheat straw. Lignin content of the spruce spent SEW liquors is determined using either hydrogen peroxide to remove SO₂ and dilution by 3% sulfuric acid or simply by dilution with 0.1M sodium hydroxide. The recommended wavelength is 280 nm. The experimentally found values for the extinction coefficient of dissolved lignin in 3% sulfuric acid and in 0.1M NaOH are 19 and 23 L/(g·cm), respectively. The interference of furanic compounds is eliminated by reduction with sodium borohydride.

Keywords:

• Acid-soluble lignin
• biorefinery
• delignification
• ethanol
• kappa number
• Klason lignin
• lignin determination
• organosolv pulping
• SEW pulping
• sulfur dioxide

Acknowledgments

We thank Dr. Herbert Sixta and Anja Leponiemi for providing the beech wood and the straw, and for their advice related to pulping. Maarit Niemi, Minna Rakkolainen, Mikaela Trogen, and Kati Vilonen are acknowledged for analyzing furfural and hydro- xymethylfurfural in the spent liquors. Financial support of TEKES through the FiDiPro program is greatly appreciated.