A continuous process for fractionation of lignocellulosic biomass having a lignin content of less than 12%, comprising the steps of: a) exposing the lignocellulosic biomass to steam and an acid catalyst in a reaction vessel at a preselected temperature and a preselected reaction pressure, for a preselected exposure time, and at a selected pH value for removing a hemicellulose fraction of the lignocellulosic biomass and activating a cellulose fraction of the lignocellulosic biomass to obtain a prehydrolyzed lignocellulosic biomass; wherein the pH value is adjusted using an acid catalyst, wherein the acid catalyst is all or in part acetic acid released from the breakdown of the hemicellulose fraction of the lignocellulosic biomass, wherein a severity index of 3.5 to 4.0 is maintained during the exposing step, the severity index being calculated according to the equation: Log Severity Index = Log [Exp {(Temperature °C - 100)/14.75} x Retention Time (min)], and wherein the steam pretreatment is carried out for less than 90 min at a pH value of 3.0 to 4.0; b) purging liquid condensate and vapor generated during the exposure step to remove and collect a first liquid stream with water soluble compounds and a first vapor stream with volatile chemicals; c) extracting and removing from the prehydrolysed lignocellulosic biomass, under pressure and prior to explosive decompression, a hemicellulose degradation stream containing solubilized degradation byproducts of hemicellulose created in the exposing step, wherein acetic acid in the first vapor stream is collected, or acetic acid in the first vapor stream and in the hemicellulose degradation stream is collected; d) rapidly releasing the reaction pressure after the extracting step to afford explosive decompression of the prehydrolyzed lignocellulosic biomass into fibrous solids, vapor and condensate; e) collecting the vapor and condensate from the explosive decompression for separation and recovery of byproducts; and f) refining the hemicellulose degradation stream by vacuum evaporation, solvent extraction, adsorption, or ion-exchange precipitation to increase the concentration and to simultaneously remove volatile compounds and acid catalysts. a) exposing the lignocellulosic biomass to steam and an acid catalyst in a reaction vessel at a preselected temperature and a preselected reaction pressure, for a preselected exposure time, and at a selected pH value for removing a hemicellulose fraction of the lignocellulosic biomass and activating a cellulose fraction of the lignocellulosic biomass to obtain a prehydrolyzed lignocellulosic biomass; wherein the pH value is adjusted using an acid catalyst, wherein the acid catalyst is all or in part acetic acid released from the breakdown of the hemicellulose fraction of the lignocellulosic biomass, wherein a severity index of 3.5 to 4.0 is maintained during the exposing step, the severity index being calculated according to the equation: Log Severity Index = Log [Exp {(Temperature °C - 100)/14.75} x Retention Time (min)], and wherein the steam pretreatment is carried out for less than 90 min at a pH value of 3.0 to 4.0; b) purging liquid condensate and vapor generated during the exposure step to remove and collect a first liquid stream with water soluble compounds and a first vapor stream with volatile chemicals; c) extracting and removing from the prehydrolysed lignocellulosic biomass, under pressure and prior to explosive decompression, a hemicellulose degradation stream containing solubilized degradation byproducts of hemicellulose created in the exposing step, wherein acetic acid in the first vapor stream is collected, or acetic acid in the first vapor stream and in the hemicellulose degradation stream is collected; d) rapidly releasing the reaction pressure after the extracting step to afford explosive decompression of the prehydrolyzed lignocellulosic biomass into fibrous solids, vapor and condensate; e) collecting the vapor and condensate from the explosive decompression for separation and recovery of byproducts; and f) refining the hemicellulose degradation stream by vacuum evaporation, solvent extraction, adsorption, or ion-exchange precipitation to increase the concentration and to simultaneously remove volatile compounds and acid catalysts.