Process development for biomass delignification using deep eutectic solvents: Conceptual design supported by experiments

Deep eutectic solvents (DES) have been proposed as solvents for biomass delignification. This paper describes a conceptual process design for the delignification of Eucalyptus globulus using a DES comprised of 30 wt % choline chloride and 70 wt% lactic acid. In this design, the lignin and hemicellulose by-products are recovered by liquid–liquid extraction using 2-MTHF as solvent. Material and energy balances were made and the energy usage of the process was optimized with additional experiments. The amount of DES was reduced to the minimal amount required to fill the porous biomass (5 kg per kg wood), which only reduced the delignification from 94% to 87% and increased the yield from 57 to 59%. Direct recycling of lignin-in-DES mixtures without lignin removal by liquid–liquid extraction to the delignification stage may save energy, but increased repolymerization increases the lignin's molar weight, which decreases its value and makes recovery by liquid–liquid extraction more difficult. After optimization, the total heat duty of the proposed process is 7.9 GJ/t pulp, which is 28% lower than the kraft process. The main benefit of DES based delignification processes is the possible valorization of byproducts, such as lignin and furans from hemicellulose.