A method to process cellulosic material in a treatment vessel including: introducing the cellulosic material to a processing chamber of the vessel; adding heat energy or pressure to the vessel to hydrolyze the cellulosic material in the processing chamber and dissolve hemi-cellulosic from the cellulosic material; compressing the cellulosic material in the processing chamber; extracting the dissolved hemi-cellulosic material through a screen from the processing section; draining the extracted hemi-cellulosic material from the vessel; and discharging the cellulosic material from the vessel separately from the extracted hemi-cellulosic material.