Continuous countercurrent extraction of hemicellulose from pretreated woodresidues

Two-stage dilute acid pretreatment followed by enzymatic cellulose hydrolys is is an effective method for obtaining high sugar yields from wood residue s such as softwood forest thinnings. In the first-stage hydrolysis step, mo st of the hemicellulose is solubilized using relatively mild conditions. Th e soluble hemicellulosic sugars are recovered from the hydrolysate slurry b y washing with water. The washed solids are then subjected to more severe h ydrolysis conditions to hydrolyze approx 50% of the cellulose to glucose. T he remaining cellulose can further be hydrolyzed with cellulase enzyme. Our process simulation indicates that the amount of water used in the hemicell ulose recovery step has a significant impact on the cost of ethanol product ion. It is important to keep water usage as low as possible while maintaini ng relatively high recovery of soluble sugars. To achieve this objective, a prototype pilot-scale continuous countercurrent screw extractor was evalua ted for the recovery of hemicellulose from pretreated forest thinnings. Usi ng the 274-cm (9-ft) long extractor, solubles recoveries of 98, 91, and 77% were obtained with liquid-to-insoluble solids (L/IS) ratios of 5.6, 3.4, a nd 2.1, respectively. An empirical equation was developed to predict the pe rformance of the screw extractor. This equation predicts that soluble sugar recovery above 95% can be obtained with an L/IS ratio as low as 3.0.