Effects of operating parameters on countercurrent extraction of hemicellulosic sugars from pretreated softwood

In a previous study using a continuous countercurrent screw extractor for two-stage dilute-acid hydrolysis, which was focused on the effects of liquid-to-insoluble solids (L/IS) ratio, we demonstrated that by using low volumes of wash water soluble sugars can be recovered from first-stage pretreated softwood at high yields and also at high sugar concentrations. In this study, we investigated the effects of important operating parameters other than the L/IS ratio, such as the feed rates of water and pretreated biomass and the extractor inclined angle, on the performance of the extractor using first-stage pretreated softwood. As biomass and water feed rates increased at the same L/IS ratio, the recovery yield of soluble sugars decreased, probably owing to a reduced solids residence time in the extractor, which is related to the solid/liquid contact time. The sugar recovery yield was higher at a higher extractor inclined angle. This may be attributed to the effects of increased back mixing and a longer residence time for solids at a higher extractor angle. Countercurrent extraction was also carried out with other pretreated biomass having smaller particle sizes and poor drainage rates. The countercurrent screw extractor was found to be unsuitable for these fine materials due to the slow liquid drainage rate and filter-clogging problems. In a test for stability of soluble sugars in first-stage softwood hydrolysate, irrespective of the storage temperature and storage form, the sugar concentration slowly decreased with storage time. However, storage in slurry form showed higher sugar stability compared with that in liquor form at the same conditions.