APPARATUSES, METHODS AND SYSTEMS FOR YIELD INCREASE IN A KRAFT COOKING PLANT

Abstract

The APPARATUSES, METHODS AND SYSTEMS FOR APPARATUSES, METHODS AND SYSTEMS FOR YIELD INCREASE IN A KRAFT COOKING PLANT present synergies for at least two parallel cooking plants, e.g., one producing dissolving pulp in a prehydrolysis kraft process, and the other producing kraft pulp by kraft pulping process, which may facilitate increased cooking yield in the kraft cooking plant producing kraft pulp by recovering the hemicelluloses solubilized in the acidic hydrolysate of the prehydrolysis kraft process. In some implementations, there may be no cold caustic extraction step on the dissolving pulp line as the target pulp purity can be achieved by performing just a prehydrolysis step, with reutilization of an acidic hydrolysate stream in a kraft pulp line.

Claims

1. A process for increasing the pulp yield in a kraft cooking plant, comprising: utilizing a stream of acidic hydrolysate from an adjacent prehydrolysis kraft process producing dissolving wood pulp.

2. A process according to claim 1, wherein the stream of acidic hydrolysate, which contains dissolved hemicelluloses and is produced by prehydrolysis of wood chips in water phase, is reutilized in the kraft cooking plant.

3. A process according to claim 2, wherein the wood chips in the kraft cooking plant are pretreated and impregnated with the acidic hydrolysate prior to an alkaline cooking process.

4. A process according to claim 3, wherein the acidic impregnation is performed in a vessel preceding an alkaline digester.

5. A process according to claim 3, wherein the acidic impregnation is performed in the alkaline digester.

6. A process according to claim 3, wherein the chips are impregnated with the acidic hydrolysate for a duration of between 2 to 180 minutes.

7. A process according to claim 3, wherein the chips are impregnated with the acidic hydrolysate at a temperature of between 70 to 125° C.

8. The process according to claim 7, wherein the chips are impregnated with the acidic hydrolysate at a temperature of between 90 to 100° C.

9. A process according to claim 2, wherein the acidic hydrolysate is injected in the digester in an intermediate alkaline zone.

10. A process according to claim 9, wherein the acidic hydrolysate is injected in the last alkaline cooking zone.

11. A process according to claim 9, wherein the intermediate alkaline zone has a residual effective alkali concentration of 10 g/l or less.

12. A process according to claim 9, wherein the intermediate alkaline zone has a chip retention time of 30 to 90 minutes.

13. A method for producing kraft pulp, comprising: extracting an acidic hydrolysate from a prehydrolysis kraft process producing dissolving wood pulp; applying the acidic hydrolysate to a kraft cooking process in a kraft cooking plant.

14. The method of claim 13, wherein the acidic hydrolysate is produced by prehydrolysis of wood chips in a water phase.

15. The method of claim 13, wherein applying the acidic hydrolysate to the kraft cooking process further comprises: performing an acidic impregnation of wood chips in the kraft cooking plant with the acidic hydrolysate prior to an alkaline cooking process.

16. The method of claim 15, wherein the acidic impregnation of the wood chips is performed in a vessel preceding an alkaline digester.

17. The method of claim 15, wherein the acidic impregnation of the wood chips is performed in an alkaline digester.

18. The method of claim 17, wherein the acidic impregnation is performed in an acidic impregnation zone near a top portion of the alkaline digester.

19. The method according to claim 15, wherein the acidic impregnation is performed for a duration of between 2 to 180 minutes.

20. The method of claim 15, wherein the acidic impregnation is performed at a temperature of between 70 to 125° C.

21. The method of claim 20, wherein the acidic impregnation is performed at a temperature of between 90 to 100° C.

22. The method of claim 13, wherein applying the acidic hydrolysate to the kraft cooking process further comprises: injecting the acidic hydrolysate in an intermediate alkaline zone of an alkaline digester.

23. The method of claim 22, wherein the acidic hydrolysate is injected in the last alkaline cooking zone of the alkaline digester.

24. The method of claim 22, wherein the intermediate alkaline zone has a residual effective alkaline concentration of 10 g/l or less.

25. The method of claim 22, wherein the intermediate alkaline zone has a chip retention time of 30 to 90 minutes.

26. A system for producing kraft pulp, comprising: a kraft cooking plant; and an acidic hydrolysate source providing acidic hydrolysate to a kraft cooking process of the kraft cooking plant.

27. The system of claim 26, wherein the acidic hydrolysate source comprises a prehydrolysis kraft process producing dissolving wood pulp.

28. The system of claim 26, wherein wood chips of the kraft cooking plant are impregnated with the acidic hydrolysate prior to an alkaline cooking process.

29. The system of claim 28, wherein kraft cooking plant further comprises: an alkaline digester; and an impregnation vessel preceding the alkaline digester; wherein the acidic hydrolysate is provided to the impregnation vessel.

30. The system of claim 28, wherein the kraft cooking plant further comprises: an alkaline digester comprising an impregnation zone near a top portion of the alkaline digester; wherein the acidic hydrolysate is provided to the impregnation zone.

31. The system of claim 28, wherein the wood chips are impregnated with the acidic hydrolysate for a duration of between 2 to 180 minutes.

32. The system of claim 28, wherein the wood chips are impregnated with the acidic hydrolysate at a temperature of between 70 to 125° C.

33. The system of claim 32, wherein the wood chips are impregnated with the acidic hydrolysate at a temperature of between 90 to 100° C.

34. The system of claim 28, wherein the kraft cooking plant further comprises: an alkaline digester comprising an intermediate alkaline zone; wherein the acidic hydrolysate is provided to the intermediate alkaline zone.

35. The system of claim 34, wherein the intermediate alkaline zone has a residual effective alkaline concentration of 10 g/l or less.

36. The system of claim 34, wherein the intermediate alkaline zone has a chip retention time of 30 to 90 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying appendices and/or drawings illustrate various non-limiting, example, innovative aspects in accordance with the present descriptions:

[0040] FIG. 1 shows a flowsheet of a PHK cooking plant representing a 2-vessel continuous digester for dissolving grade pulp production, with side stream of acidic hydrolysate generation containing said dissolved hemicelluloses in one embodiment;

[0041] FIG. 2 shows a flowsheet of an embodiment of method A in Kraft cooking plant for kraft pulp, representing a 2-vessel continuous digester configuration with separate acidic chip impregnation vessel;

[0042] FIG. 3 shows a flowsheet of an embodiment of method A in Kraft cooking plant for grade kraft pulp, representing a 1-vessel configuration (continuous or batch digester);

[0043] FIG. 4 shows a flowsheet of an embodiment of method B in Kraft cooking plant (continuous or batch digester) for kraft pulp; and

[0044] FIG. 5 shows a comparison of cooking screened yield between both described methods A and B for kraft pulp and a reference cooking process over a wide range of kappa numbers in one embodiment.

[0045] Embodiments of the disclosed apparatuses, methods and systems include two parallel continuous cooking plants, one line producing a prehydrolysis kraft dissolving wood pulp (PHK) and the second line producing kraft pulp (KP) by a kraft process (KP). Alternative embodiments include two parallel lines where the dissolving wood pulp is produced in continuous PHK process while the kraft pulp is produced in a batch kraft process. The PHK process may comprise of one or several vessels.

[0046] FIG. 1 shows a configuration of a Prehydrolysis Kraft (PHK) continuous cooking plant for dissolving grade pulp production in one embodiment. In such embodiment, wood chips 101, water 105 and steam 110 are fed into a vessel where the prehydrolysis reaction is conducted 115. Water and/or evaporation plant clean condensate is added, e.g., in an amount of 0.5 to 5 m.sup.3/BDtw, or in the range of 1 to 3 m.sup.3/BDtw relative to the wood inlet flow. Prehydrolysis temperature inside the PHV vessel may be controlled by the steam flow to achieve a target prehydrolysis severity (P factor) for a given chip retention time, e.g., in the range of 140 to 175° C. for a P factor in the range 50 to 1000 units.

[0047] With the progressive degradation and solubilization of hemicelluloses, the liquid phase of the reactor is transformed into hydrolysate. In the conditions aforementioned, up to 5 m.sup.3/BDtw, or up to 2 m.sup.3/BDtw, of hydrolysate can be separated from the wood chips stream, e.g., via strainers on the prehydrolysis vessel 115, then sent to the parallel kraft pulp line for recovery. Wood chips are transferred to the second vessel (digester) 120 and cooked to produce dissolving grade pulp 125.

[0048] Embodiments of the disclosed apparatuses, methods and systems include implementation of methods A and/or B for reutilizing the hydrolysate in a second parallel production line, which may serve to reduce overall specific wood consumption.

[0049] One embodiment type of method A is represented in FIG. 2. Chips 201 and hydrolysate 205 may be continuously fed into a vessel 210 for an acidic impregnation time up to 180 minutes, or in the range of 40 to 100 minutes at a temperature of 70 to 125° C. In some implementations. hydrolysate may be added in an amount comprising up to 5 m.sup.3/BDt of wood, or in the range of 0.5 to 2 m.sup.3/BDtw, and may be cooled by flashing and/or in an indirect heat exchanger to reach a target impregnation temperature. From the vessel outlet the hydrolysate impregnated chips 215 are transferred to the sub-sequent digester 220 for continuous kraft cooking to produce wood pulp for paper grade 225.

[0050] Another variation of method A is represented in FIG. 3. In this embodiment, chips and hydrolysate 301 are fed to the top of the digester 305 directly, instead of to a separate vessel. In such embodiments, the acid impregnation occurs in the topmost zone of the digester, in the same mass quantities, impregnation time and temperatures as above mentioned. At the designed section to start the alkaline cooking process, excess hydrolysate can be extracted and replaced with white liquor 310 and/or other alkaline liquor to neutralize and alkalinize the acidic chips and remaining hydrolysate. After neutralization, the following digester cooking zones are typical for any kraft cooking processes and will not be described further.

[0051] Method A described above can be further derived to embodiments where the KP line comprises a batch cooking system. In such implementations, the hydrolysate can be, e.g.: [0052] Cooled and fed into the digester top simultaneously with the wood chips; or [0053] Injected in the digester bottom after the wood chips are loaded, and further displaced through the digester by injecting liquor, e.g., white liquor and/or other alkaline liquor to neutralize and alkalinize the acidic chips and remaining hydrolysate.

[0054] FIG. 4 shows an embodiment of method B. In a continuous cooking digester 401, comprising of multiple zones, hydrolysate is added 405, e.g., to the lowest (e.g., final) cooking zone in an amount, e.g., up to 2 m.sup.3/bdt relative to the dry wood inlet flow. A matching amount of black liquor 410 can be extracted from the digester so the liquor to wood flow ratio is not adversely affected by the hydrolysate addition. In this implementation, the retention time in the combined precipitation/cooking phase may be, e.g., in the range of 30 to 90 minutes, a residual effective alkali below 10 g/l as NaOH and typical temperatures (140-170° C.) for kraft cooking processes.

[0055] In an alternative implementation of method B for batch cooking systems, hydrolysate 405 can be added to the digester 401 in an intermediate time in the cooking phase, e.g., being mixed with the cooking liquor inside the digester and circulated by the remaining cooking duration, displaced through the digester (in systems without a circulation pump), and/or the like.

[0056] FIG. 5 shows a comparison of cooking screened yield 501 between implementations of methods A and B for kraft pulp and a reference cooking process over a wide range of kappa numbers 505 in one embodiment.

[0057] Table 2 shows the absolute increase in screened cooking yield for implementations of method A over a wide range of retention times and at comparable kappa numbers.

TABLE-US-00002 TABLE 2 Method A—Increase in screened cooking yield for different retention times Retention time, Yield increase, minutes %-units 2 0.3 15 0.4 45 1.1 70 1.1 90 0.9 100 1.1 120 0.6