NOVELTY - Fractionating lignocellulosic biomass, involves: (a) in a digester, fractionating a feedstock comprising lignocellulosic biomass feedstock in the presence of a solvent for lignin, a hydrolysis catalyst, and water, to produce a liquor; (b) substantially separating the cellulose-rich solids from the liquor; (c) hydrolyzing the hemicellulose contained in the liquor to produce hemicellulosic monomers; (d) recovering the hemicellulosic monomers as fermentable sugars; (e) fermenting at least a portion of the fermentable sugars to a fermentation product; and (f) recovering the fermentation product.    USE - The process is useful for fractionating lignocellulosic biomass into cellulose, hemicellulose, and lignin. The hemicellulose is useful for producing a fermentation product including: an organic acid consisting of formic acid, acetic acid, oxalic acid, lactic acid, propionic acid, 3-hydroxypropionic acid, malonic acid, aspartic acid, fumaric acid, malic acid, succinic acid, glutaric acid, adipic acid, citric acid, itaconic acid, levulinic acid, ascorbic acid, gluconic acid, kojic acid, threonine, glutamic acid, proline, lysine, alanine, serine, and/or their isomers or derivatives; and an oxygenated compound consisting of ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, glycerol, sorbitol, propanediol, butanediol, butanetriol, pentanediol, hexanediol, acetone, acetoin, butyrolactone, 3-hydroxybutyrolactone, and/or their isomers or derivatives. The cellulose is useful for producing glucose, which is useful for the production of the fermentation product (all claimed).    ADVANTAGE - The mass and/or energy integrated process is capable of efficiently fractionating lignocellulosic biomass into cellulose, hemicellulose, and lignin with high yield in an economical manner.    DETAILED DESCRIPTION - Fractionating lignocellulosic biomass into cellulose, hemicellulose, and lignin, involves: (a) in a digester, fractionating a feedstock comprising lignocellulosic biomass feedstock in the presence of a solvent for lignin, a hydrolysis catalyst, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; (b) substantially separating the cellulose-rich solids from the liquor; (c) hydrolyzing the hemicellulose contained in the liquor to produce hemicellulosic monomers; (d) recovering the hemicellulosic monomers as fermentable sugars; (e) fermenting at least a portion of the fermentable sugars to a fermentation product having a relative volatility with water of less than 1; and (f) recovering the fermentation product, where the process includes process integration of mass and/or energy between at least two of steps (a)-(f).