New possibilities to optimize the sulfite process


Raghu Deshpande has in his PhD and postdoc works shown research results which open new opportunities to optimize the sulfite process. The results of seven years’ research were recently presented under the headlines “Sodium sulfite pulping of softwood” and “Lignin-carbohydrate complexes studies during dissolving pulps production” at MoRe Research.

“I have studied different process conditions in both one-stage and a two-stage sodium based acid sulfite process using spruce and pine as raw material for the production of dissolving pulp for textiles and regenerated cellulose applications,” says Raghu Deshpande. “Scope of the research study focused on many aspects; What happens during the cooking process, how are the kinetic conditions and how are the different wood components separated from each other? How do the kinetic conditions between spruce and pine differ in the sulfite cooking stages and the extent of side reactions are their consequences were investigated?

“Pulp made from pine is cheaper than that of spruce and it is therefore of interest to be able to use 100% pine in the sulfite process, which is difficult at low pH levels. That is why many cooking trials were conducted in MoRe’s pilot digester equipment with chips from spruce, pine and of pine heartwood. The trials showed that a two-stage sodium sulfite process works fine with spruce and pine chips; but with pine heartwood chips in one-stage sodium acid sulfite process resulted in lignin condensation.”

“In an industrial process, it is difficult to investigate what happens during the different phases of cooking time in the pulping digester. However, in the PhD work, the trials were done in the pilot digester and hence the two-stages could be carried out separately and analyzed after each stage and also during intermediate cooking time.”

“Thanks to the trials in MoRe’s pilot digester plant as we could dig deeper into the wood components separation investigations,” Raghu Deshpande continues. “We could simulate different cooking conditions and then analyze the lignin-carbohydrate bonding at Karlstad University and KTH. By using a special analysis technique at KTH (Royal Institute of Technology) I have been able to study how the bonding between lignin and hemicellulose are affected during different stages of the pulping processes.”

“In the sulfite cook, the hemicelluloses are broken down to different polysaccharides. By knowing the activation energies for hemicellulose and different polysaccharides, mainly glucomannan and xylose, as well as lignin, one can calculate backwards and get a good cellulose purity figure. In practical terms, it means you can adjust the cooking conditions to get pure cellulose as well as to optimize the volume of by-products.”

“The results of the PhD work summarize that both pine and spruce can be used in a two-stage sodium bisulfite process and that this method can get a renaissance as a cooking method for dissolving cellulose.”

After five years’ PhD studies between 2012 and 2017 the research continued as a postdoc position at MoRe Research and in cooperation with KTH, Domsjö Fabriker, MoRe and RISE Processum. The postdoc was financed by the Kempe Foundations and by the Knut and Alice Wallenberg Foundation.

Conclusions of the PhD work and the postdoc work are these:

  • Trials with industrial cooking liquid and that made in a laboratory do not give the same results as the industrial cooking liquor also contains components from the recycling process.
  • Pine heartwood versus pine wood:
    • Lignin condensation at lower temperature during acid sulfite cooking with pine heartwood.
    • Pine wood gave higher delignification than pine heartwood.
  • A higher temperature increases the thiosulfate formation rate.
  • Side reactions were higher in laboratory pulp experiments with pine.
  • The temperature can be used to control the hemicellulose composition and the R 18 value.
  • LCC bonds were found between lignin-xylan and lignin-glucomannan.
  • LCC bonds between lignin and wood polysaccharides might be a bottleneck in sulfite pulping.