Method for recovering hydrolysis products

A method for recovering carbohydrates in a prehydrolysis batch pulping process comprising the following stages:

• a) providing a digester having a top and bottom, and containing a column of lignocellulosic material having voids within the column of lignocellulosic material,
• b) heating the digester and its contents to a predetermined pre-hydrolysis reaction temperature by directly adding steam to the digester,
• c) providing a flow of hydrolysate liquid at a temperature essentially corresponding to the prehydrolysis reaction temperature and containing hydrolysis product from lignocellulosic material to the top of the column, creating a trickle-bed downflow of hydrolysate liquid wherein the liquid volume of the trickle-bed downflow within the digester facilitates the retention of voids within the column of lignocellulosic material, forming a liquid phase not filling the voids in the lignocellulosic material, but trickling uniformly through the column of lignocellulosic material towards the digester bottom,
• d) collecting a first fraction of the hydrolysate liquid resulting from said trickle-bed downflow from the digester bottom,
• e) adding liquid to the flow provided to the top of the column and continuing the trickle-bed downflow wherein the liquid volume of the trickle-bed downflow within the digester facilitates the retention of voids within the column of lignocellulosic material, collecting a second hydrolysate fraction and discharging said second hydrolysate fraction from the digester to a hydrolysate storage tank.