Method for continuous cooking of chemical pulp to improve heat economy

1. A method for the continuous cooking of chemical pulp to improve heat economy in the digester system, comprising:

providing an impregnation vessel having an inlet defined therein, the impregnation vessel being in fluid communication with a digester via a transfer system, the digester having an outlet defined therein;

feeding a mixture of cellulose chips and process fluid into the inlet of the impregnation vessel; impregnating the cellulose chips in the impregnation vessel at a predetermined impregnation temperature T imp ;

feeding the impregnated cellulose chips to the digester through the transfer system; cooking the impregnated cellulose chips at a predetermined temperature T cook ;

feeding pulp dissolved in the digester out through the outlet of the digester:

withdrawing black liquor from the digester via a cooking withdrawal after partial or complete cooking of the cellulose chips; leading the withdrawn black liquor to the transfer system at a bottom of the impregnation vessel;

mixing the withdrawn black liquor with the impregnated cellulose chips in the transfer system for transport onwards to a top of the digester;

adding a part of the black liquor from the black liquor withdrawal to an addition point of the impregnation vessel without cooling the withdrawn black liquor, the black liquor having a withdrawal temperature T av , to a beginning of the transfer system, the black liquor having maintained essentially the temperature T av , to raise a temperature of a chips mixture in the transfer system;

a transport fluid being continuously withdrawn from the chips mixture at an end of the transfer system, the transport fluid maintaining a transport temperature T transp ; and

returning the transport fluid to the impregnation vessel without heating at a location that is upstream of the transfer system and the addition point of the black liquor wherein the black liquor from the black liquor withdrawal maintains a temperature of Tav that exceeds the temperature Ttransp of the transport fluid that is withdrawn from the top of the digester by at least 50°C., such that T av −T transp ≧5° C.

2. The method according to claim 1 , wherein the fraction of black liquor from the black liquor withdrawal that is led to the transfer system constitutes at least 25% of the total amount of black liquor that is withdrawn from the digester.

3. The method according to claim 1 wherein the fraction of withdrawn transport fluid that is returned to the impregnation vessel constitutes at least 10% of the total amount of the transport fluid withdrawn from the top of the digester.

4. The method according to claim 3 , wherein the black liquor from the black liquor withdrawal that is led to the transfer system maintains a withdrawal temperature T av that is essentially equivalent to the cooking temperature T cook and does not deviate from the cooking temperature by more than a maximum of 5° C.

5. The method according to claim 4 , wherein the fraction of transport fluid withdrawn, which maintains a transfer temperature of T transp , is returned to the impregnation vessel with an essentially maintained transfer temperature T transp in at least one location in the impregnation vessel.

6. The method according to claim 3 , the transport fluid that has been withdrawn and that is returned to the impregnation vessel is given a period as impregnation fluid in the impregnation vessel for at least 40% of the total retention time t imp of the chips in the impregnation vessel.

7. The method according to claim 1 wherein the black liquor from the black liquor withdrawal is added to a final mixing zone (Z 2 ) in the impregnation vessel.

8. The method according to claim 1 wherein the black liquor from the black liquor withdrawal is added at an outlet from the impregnation vessel.

9. The method according to claim 8 , wherein the final mixing zone (Z 2 ) in the impregnation vessel corresponds to a retention time (t 2 ) for the chips of a maximum of 25% of the total retention time (t imp ) for the chips in the impregnation vessel.

10. The method according to claim 9 , wherein the final mixing zone (Z 2 ) in the impregnation vessel is a zone of concurrent flow.

11. The method according to claim 1 wherein a fraction of the black liquor from the black liquor withdrawal is added at a high-pressure location in the transfer system, where this high-pressure location is located in association with the inlet for transport fluid on the high-pressure side of a high-pressure feeder.

12. The method according to claim 11 , wherein the black liquor from the black liquor withdrawal that is added at the inlet for transport fluid on the high-pressure side of the high-pressure feeder is constituted by the complete amount that is required in order to expel the chips from the high-pressure feeder in the high-pressure location.

13. The method according to claim 12 , wherein the complete amount of black liquor from the black liquor withdrawal that is led to the transfer system is added at the inlet for transport fluid at the high-pressure side of a high-pressure feeder.