Continuous liquid–liquid extraction to recover lignin and furanics from lactic acid: Choline chloride deep eutectic solvent after cooking of spruce

Lactic acid: choline chloride mixed in a 10:1 ratio is known to be a suitable delignification solvent. This deep eutectic solvent (DES) can produce lignin and hemicellulose derivatives as byproducts to cellulose. Hemicellulose byproducts include furanic compounds like furfural and 5-HMF. Sustainable and economically viable fractionation of lignin and furanics from the DES is required. After removing the larger lignin (>2500 Da) through water precipitation, non-precipitated molecules such as smaller lignin molecules and furanics can be retrieved through liquid–liquid extraction (LLX). In a previous investigation on solvent extraction conducted in a batch mode, 2-MTHF and guaiacol have been identified as potential solvents. This study investigates the application of a continuous centrifugal contactor separator (CCCS) and Karr reciprocating plate column for LLX from aqueous DES-black liquor remaining after water-precipitation, was employed as the feed. Single stage CCCS operation exhibited extraction yields close to equilibrium yields, thus almost 100 % extraction efficiency. Performing a two-stage countercurrent LLX process via CCCS showed increased removal for furfural and 5-HMF extraction, while lignin extraction yield remained at about 90 %, comparable to single-stage extraction. Karr column countercurrent extraction with different solvent-to-feed ratios (0.5 and 1) yielded approximately 99 % furfural extraction. For 2-MTHF system, 5-HMF achieved 99 % extraction yield at a solvent-to-feed ratio of 1, while guaiacol yielded around 90 % at both solvent-to-feed ratios. However, 5-HMF extraction using 2-MTHF with a ratio of 0.5 required more stages. For guaiacol improvement of lignin extraction applying more stages was hindered by hydrophobic characteristic of residual lignin in raffinate. Conversely, 2-MTHF's multi-stage approach enhanced smaller lignin extraction influenced by increased lactic acid leaching into solvent. Both CCCS and Karr column methods proved viable for counter-current extraction with both solvents.