Continuous digester system
In a continuous digester system the digester system is greatly simplified by using a single vertical atmospheric vessel, replacing the conventional chip bin, steaming vessel, chip chute, high pressure pumping or sluice feeders, impregnation vessels and top separator. Chips are simply fed to the top of the atmospheric vessel, and a chip level is established in the vessel. Treatment liquids are added to the vessel such that a total liquid volume (Z 1 +Z 2 ) with a liquid level (LIQ LEV) is established under the chip level (CH LEV). Impregnation stage and subsequent cooking stages are implemented in the atmospheric vessel at successively increasing temperature and depths into the total liquid volume, thus preventing boiling in the stages and preferably reducing steam blow trough of the chip surface in the top of the vessel.
1. A continuous digester system, comprising:
a vertically oriented atmospheric vessel having a top and a bottom;
the top of the vessel having means for establishing a pressure in a range of +0.5 bar and −0.5 bar;
a rotor disposed on top of the vessel and in operational engagement therewith;
an inlet chute disposed directly on top of the rotor;
the vessel having a first level (CH LEV) of fibrous comminuted cellulose raw material;
the vessel having a second level (LIQ LEV) of a liquid disposed below the first level;
the vessel having a concurrent impregnation zone disposed immediately below the second level;
the vessel having a first screen disposed below the concurrent impregnation zone;
the vessel having a first concurrent cooking zone disposed below the first screen;
the vessel having a second screen disposed below the first concurrent cooking zone;
an impregnation liquor conduit extending from the second screen to the concurrent impregnation zone;
a recirculation conduit extending from the first screen to a bottom of the impregnation zone;
a fresh cooking liquor conduit in fluid communication with the recirculation conduit, the fresh cooking liquor conduit containing fresh cooking liquor; and
a heater in operative engagement with the recirculation conduit.
2. The digester system according to claim 1 wherein the means for heating the cooking liquids comprises a first liquid circulation conduit having a screen (S 2 ,S 2 ″) in the wall of the vessel in first end of the circulation conduit and an outlet pipe (CP 2 ,CP 2 ″) in the center of the vessel at the second end of the circulation conduit, and a pump (P 2 ,P 2 ″) in the circulation conduit, wherein the liquid in the circulation conduit is passing a heater (HE) for heating the liquid circulated in the circulation conduit and wherein the first and second end of the first circulation conduit is located in the second lower volume of liquid (Z 2 ).
3. The digester system according to claim 2 wherein the means for supplying cooking liquids (WL M ), comprises a second liquid circulation conduit having a screen (S 2 ,S 2 ′) in a wall of the vessel in first end of the circulation conduit and an outlet pipe (CP 2 ,CP 2 ′) in the center of the vessel at the second end of the circulation conduit, and a pump (P 2 ,P 2 ′) in the circulation conduit, wherein the liquid in the circulation conduit is passing a mixer for adding fresh cooking chemicals (WL M ) to the liquid circulated in the circulation conduit and wherein the first and second end of the second circulation conduit are located in the second lower volume of liquid (Z 2 ).
4. The digester system according to claim 3 wherein the means for heating the cooking liquids includes a heater in form of an indirect heat exchanger (HE), where the heating medium used is steam.
5. The digester system according to claim 4 wherein the means for supplying impregnation liquids use as a liquid source at least partly a liquid withdrawn from the cooking zone in the second lower volume of liquid (Z 2 ).
6. The digester system according to claim 5 wherein the means for supplying impregnation liquids use as a liquid source at least partly fresh cooking chemicals (WL S ).
7. The digester system according to claim 6 wherein the vessel has a screen disposed in the impregnation zone for withdrawing spent impregnation liquids.
8. The digester system according to claim 6 wherein the vessel has a screen adjacent to the liquid level (LIQ LEV) for withdrawing liquid.
9. The digester system according to claim 1 wherein the first upper volume of liquid (Z 1 ) containing the impregnation zone have a height of at least 17 meter.
10. The digester system according to claim 9 wherein the second lower volume of liquid (Z 2 ) containing the cooking zone has a height of at least 30 meters.
11. The digester system according to claim 10 wherein the total height of the vessel, containing the impregnation and cooking zones is at least 70 meter high.