PULP COOKING WITH PARTICULAR ALKALI PROFILES

• US 20010032711A1
• Filed: 10/26/1998
• Published: 10/25/2001
• Est. Priority Date: 10/26/1998
• Status: Abandoned Application

• Current Assignee: Andritz Incorporated
• Original Assignee: Andritz Incorporated
• Inventors: STROMBERG, C. BERTIL, HENRICSON, KAJ O., KETTUNEN, AUVO K., JIANG, JIAN E.
• Application Number:
  US09/178,512
• Publication Number:
  US 20010032711A1
• Time in Patent Office: Days
• US Class Current: 162/17
• CPC Class Codes: D21C 3/02 with inorganic bases or alkaline reacting compounds, e.g. sulfate processes
• CPC Class Codes: D21C 3/26 Multistage processes
• CPC Class Codes: D21C 7/10 Heating devices
• CPC Class Codes: D21C 7/14 Means for circulating the lye

Abstract

Yield, particularly when treating hardwood chips, can be improved by at least 1-2% in a kraft cellulose pulping process by keeping the temperature and effective alkali (EA) low during impregnation, and by keeping the EA low in at least a first cook stage. After cooking, the pulp is subjected to cooling low EA liquor, e.g. to reduce its temperature to below 120° C. (preferably below 100° C.) with an EA below about 5 g/L (expressed as NaOH). Continuous treatment in a continuous digester system is preferred, with the EA below about 20 g/L during impregnation and the first cook.

Claims

1. A method of treating comminuted cellulosic fibrous material to produce cellulose pulp, comprising:

(a) treating the material with a first alkaline liquid having a first effective alkali concentration and at a temperature less than 120°

C.;

(b) treating the material with a second alkaline liquid having a second effective alkali concentration while heating the material to a temperature above 120°

C.;

(c) treating the material with a third alkaline liquid having a third effective alkali concentration at a temperature greater than 140°

C. to delignify the material; and

(d) treating the material with a fourth liquid to cool the material to a temperature less than 120°

C.;

wherein the first, second, third initial effective alkali concentrations are all less than 30 g/L, and wherein the second and third effective alkali concentrations are about 25 g/L or less.

2. A method as recited in

claim 1

wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 25 g/L.

3. A method as recited in

claim 1

wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 20 g/L.

4. A method as recited in

claim 1

further comprising (e), between (c) and (d) treating the material with an alkaline liquid having a fourth effective alkali concentration at a temperature greater than 140°

C.

5. A method as recited in

claim 4

wherein (e) is practiced at a fourth initial alkali concentration of greater than 30 g/L.

6. A method as recited in

claim 4

wherein (e) is practiced at a fourth initial alkali concentration of less than about 15 g/L.

7. A method as recited in

claim 4

further comprising (f), between (e) and (d), treating the material at a temperature of greater than 140°

C. at a fifth initial alkali concentration of less than 25 g/L.

8. A method as recited in

claim 2

wherein (a)-(d) are practiced using hardwood chips as the comminuted cellulosic fibrous material.

9. A method as recited in

claim 2

wherein the second effective alkali concentration is obtained by adding dilution liquor having a low or substantially zero alkali concentration.

10. A method as recited in

claim 3

wherein (b) is practiced to heat the material so that its temperature gradually increases, and so that the second EA concentration gradually increases, but stays below about 20 g/L.

11. A method as recited in

claim 10

wherein (c) is practiced so that the EA concentration gradually decreases while the temperature is maintained substantially constant.

12. A method as recited in

claim 11

wherein (d) is practiced with a cooling liquid having an EA of less than 5 g/L so that the EA is gradually reduced to a final level below 5 g/L while the temperature is gradually reduced.

13. A method as recited in

claim 12

wherein (a) is practiced so that the EA gradually decreases while the temperature remains substantially the same.

14. A method as recited in

claim 1

wherein (a)-(d) are practiced continuously.

15. A method as recited in

claim 7

wherein (a)-(f) are practiced continuously in the same vessel.

16. A method as recited in

claim 1

wherein (a)-(d) are practiced so as to increase yield by at least 1% compared to practicing (a) at a temperature of greater than about 120°

C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.

17. A method as recited in

claim 3

wherein (a)-(d) are practiced using hardwood chips as the cellulosic material, and so as to increase yield by at least 2% compared to practicing (a) at a temperature of greater than about 120°

C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.

18. A method of continuously treating hardwood chips to produce kraft pulp, using a continuous digester system in which a hardwood chip slurry primarily flows downwardly during treatment, comprising:

(a) impregnating the hardwood chips of the slurry in a first stage using a first alkaline liquid with an initial EA at the start of the first stage of about 25 g/L or less, and at a temperature of between about 90-110°

C., the EA gradually diminishing by at least 10 g/L during the first stage, and so that at the end of the first stage the EA is about 10 g/L or less;

(b) gradually heating the hardwood chip slurry to a cooking temperature of about 140-180°

C. as the slurry continuously moves through a second stage substantially contiguous with the first stage, by treating the slurry with a second alkaline liquid, the EA of the slurry starting at the beginning of the second stage at less than 15 g/L, and increasing at least about 5 g/L during the second stage, but not exceeding about 25 g/L;

(c) cooking the hardwood chip slurry in a third stage, using a third alkaline liquid, at a temperature that remains substantially constant and is between 140-180°

C. and at an initial EA at the start of the third stage of below 25 g/L, and gradually decreasing by at least about 5 g/L and so that the EA at the end of the third stage is below 20 /L;

(e) optionally subjecting the hardwood chips to at least a second cooking in a fourth stage at approximately the same substantially constant temperature in the third stage, using a fourth alkaline liquid; and

(d) in a last stage, using a last alkaline liquid, gradually cooling the hardwood chips slurry to a temperature less than about 110°

C. and so as to reduce the EA of the slurry at least about 5 g/L from the beginning to the end of the last stage, and so that the slurry has a final EA of less than about 5 g/L;

wherein (a)-(d) are practiced so as to increase yield of pulp produced by at least 2% compared to practicing (a) at a temperature of greater than about 120°

C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.

19. A method as recited in

claim 18

wherein (e) is practiced, and so that the EA increases from the beginning to the end of the fourth stage by at least 10 g/L.

20. A method as recited in

claim 19

wherein (a) is practiced at least in part by extracting liquor from the slurry at approximately the interface between the first and second stages;

and wherein (b) is practiced at least in part by adding a combination of heated cooking and dilution liquor at approximately the interface between the second and third stages so that the heated liquor flows substantially countercurrent to the chips slurry; and

wherein (c) is practiced at least in part by extracting liquor at approximately the interface between the third and fourth stages; and

wherein (e) is practiced at least in part by adding a combination of heated cooking and dilution liquid below the extraction at approximately the end of the fourth stage, to flow substantially countercurrent to the hardwood chips slurry; and

wherein (d) is practiced by introducing dilution liquor at a temperature below about 110°

C. adjacent a discharge of pulp from the digester system.

21. A continuous digester system comprising:

at least one substantially upright digester vessel having first, second, third, fourth, and last consecutive stages, each stage substantially contiguous with the previous stage and having an interface therewith;

an inlet adjacent the top of the first stage;

a first liquor extraction device at approximately the interface between the first and second stages, including an extraction means;

a first withdrawal and recirculating system at approximately the interface between the second and third stages, including a first recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a recirculation pipe at approximately the level of said first recirculation screen;

a second extraction screen at approximately the interface between the third and fourth stages;

a second withdrawal and recirculating system at approximately the interface between the fourth and next consecutive stage, including a second recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a second recirculation pipe at approximately the level of said second recirculation screen;

cooling liquor introducing devices adjacent the bottom of the digester vessel, at the bottom of the last stage; and

a pulp discharge from the last stage, adjacent the bottom of the digester vessel.

22. A continuous digester system as recited in

claim 21

wherein the second recirculation screen is substantially at the interface between the fourth and last stages.

23. A continuous digester system as recited in

claim 21

wherein all of the first through last stages are in a single upright vessel, said at least one vessel consisting essentially of a single vessel.

24. A continuous digester system as recited in

claim 21

further comprising a fifth stage between said fourth and last stages, and another recirculation system at the interface between said fourth and fifth stages.