Patent ID: 7976675

Claim:
A method for producing a digested raw material, comprising: providing a single vertically oriented atmospheric vessel having a top and a bottom, an inlet chute disposed on top of the vessel; continuously feeding cellulose fibrous raw material into the inlet chute; establishing a pressure in a range of +0.5 bar and −0.5 bar at the top of the vessel in an atmospheric gas phase; establishing a first level (CH LEV) of the cellulose fibrous raw material in the vessel; establishing a second level (LIQ LEV) of a liquid disposed below the first level by adding liquid into the vessel; establishing a concurrent impregnation zone disposed immediately below the second level; in a slurrying zone below the atmospheric gas phase, slurrying the cellulose fibrous raw material; in the concurrent impregnation zone below the second level (LIQ LEV) of the vessel, impregnating the cellulose fibrous material with an impregnation liquid; providing a first screen in the vessel disposed below the concurrent impregnation zone and a first concurrent cooking zone disposed below the first screen; heating a cooking liquor; adding the heated cooking liquor to the first concurrent cooking zone disposed below the concurrent impregnation zone; cooking the cellulose fibrous raw material in the first concurrent cooking zone until a slurry of a digested raw material having a kappa number below 100 is formed; providing a recirculation conduit extending from the first screen; adding fresh cooking liquor to the recirculation conduit; and continuously withdrawing the slurry of the digested raw material from the vessel.