Fractionation of hardwood using steam explosion and hydrotropic extraction : Process development for improved fractionation

Abstract: The utilisation of biomass instead of fossil resources is an important alternative for the transition into a sustainable society. Biomass, owing to its primary constituents—cellulose, hemicelluloses and lignin—has the potential to replace many products that are produced from fossil resources today, including plastics, textile fibres and fuels. To fully exploit this potential, all of the components in the biomass matrix need to be used. To this end, efficient and environmentally friendly fractionation processes must be developed.In the work that is described in this thesis, a two-stage process was examined for the fractionation of hardwood chips into cellulose, hemicelluloses and lignin. This process consists of an initial stage of steam explosion, in which hemicelluloses are recovered in the liquid fraction, and an extraction step, whereby lignin is removed from the solid fraction using a hydrotropic agent. Sodium xylene sulfonate, an environmentally friendly chemical that is commonly used in everyday products, such as shampoos and soaps, was used as the hydrotrope.During the hydrotropic extraction, lignin was extracted into the liquid fraction, generating a solid fraction that was enriched in cellulose. Next, the lignin was recovered from the liquid fraction by diluting the solution with water, causing the lignin to precipitate. Over 70% of the hemicelluloses and approximately 50% of the lignin were recovered from the liquid fractions after steam explosion and hydrotropic extraction respectively. More than 90% of the cellulose was recovered in the solid fraction after hydrotropic extraction.Next, the influence of process conditions during steam explosion and hydrotropic extraction was studied. Based on the findings, hydrotropic extraction could be performed even at ambient temperature with a short residence time, provided that the feedstock had been steam-pretreated prior to the hydrotropic extraction. The recycling of sodium xylene sulphonate was examined, and the results demonstrated that it could be reused several times without losing its efficiency. This property could be advantageous, because the regeneration of the used hydrotropic agent can be expensive and energy-demanding. Furthermore, it was shown that nanofiltration could be used to recover the diluted sodium xylene sulphonate solution, thus reducing the heat-energy demand of evaporation, which is commonly used for concentration.