Claims
- 1. A method of continuously producing cellulose pulp from comminuted cellulosic fibrous material, utilizing an upright digester having a top and a bottom, comprising the steps of:
- (a) continuously introducing comminuted cellulosic fibrous material entrained in cooking liquor into the top of the digester so that the material continuously moves downwardly in the digester;
- (b) continuously withdrawing cellulose pulp from the bottom of the digester; and
- (c) establishing a countercurrent flow between cooking liquor and material throughout the entire height of the digester between the material introduction at the top and the pulp withdrawal at the bottom, including by continuously withdrawing liquid from the digester at a plurality of different points along the height of the digester, and reintroducing the withdrawn liquid; and wherein step (c) is practiced in part by continuously withdrawing liquid from the top of the digester through a top extraction, and recirculating a part of the liquid, with any entrained material, back to the digester.
- 2. A method as recited in claim 1 wherein step (c) is practiced, in part, by recirculating the liquid back to the digester through an impregnation vessel connected to the digester, and by continuously removing a part of the withdrawn liquid from the digester-impregnation vessel loop.
- 3. A method as recited in claim 2 comprising the further step (d) of withdrawing some liquid from adjacent the top of the digester but below the top extraction, and then cooling and reintroducing the liquid into the digester so as to prevent hot cooking liquors from entering the top extraction.
- 4. A method as recited in claim 2 wherein step (c) is further practiced at the top extraction utilizing an in-line drainer to remove part of the withdrawn liquid.
- 5. A method as recited in claim 4 wherein liquid removed from the in-line drainer is passed to flash tanks, and wherein liquid with entrained material is passed through a pressure screen, to remove fines, and then is recirculated to the digester.
- 6. A method as recited in claim 5 comprising further steps of treating the removed fines with white liquor, then digesting these fines in a mini digester to produce pulp.
- 7. A method as recited in claim 5 comprising the further steps of treating the removed fines with white liquor and returning those fines to the digester for further treatment.
- 8. A method as recited in claim 4 comprising the further step (d) of withdrawing some liquid from adjacent the top of the digester but below the top extraction, and then cooling and reintroducing the liquid into the digester so as to prevent hot cooking liquors from entering the top extraction.
- 9. A method as recited in claim 2 wherein step (c) is practiced by withdrawing liquid at one or more points in the digester besides the top extraction; and heating the withdrawn liquid and then reintroducing the heated withdrawn liquid directly into the digester adjacent the one or more points from which it was withdrawn.
- 10. A method as recited in claim 9 wherein step (c) is practiced by withdrawing liquid at a mid-point extraction and a bottom extraction and comprising the further steps of: (e) periodically terminating continuous removal of some of the liquid withdrawn from the digester at the top extraction, and substantially simultaneously with step (e), (f) withdrawing liquid from an upper mid-point extraction, between the mid-point extraction and the top extraction, the liquid withdrawn from the upper mid-point extraction not being re-introduced into the digester; and (g) continuing the practice of steps (e) and (f) for a first period of time.
- 11. A method as recited in claim 10 comprising the further step, during the first period of time when step (g) is being practiced, of (h) introducing cooking liquor into liquid being withdrawn and re-introduced from a bottom extraction.
- 12. A method as recited in claims 11 wherein step (h) is practiced only during the first period of time when step (f) is being practiced.
- 13. A method as recited in claim 10 wherein steps (e) and (f) are practiced by closing a valve from the top extraction, and opening a valve from the upper mid-point extraction.
- 14. A method as recited in claim 10 wherein step (f) is also practiced by introducing the liquid withdrawn from the upper mid-point extraction into a flash tank.
- 15. A method as recited in claim 2 comprising the further step of introducing cooking liquid into the liquid being withdrawn and reintroduced from the mid-point extraction.
- 16. A method as recited in claim 2 wherein the removed liquid from the top extraction is passed to a flash tank.
- 17. A method as recited in claim 1 wherein the cooking liquor is selected from the group consisting essentially of sulphate liquor and sulfite liquor.
- 18. A method as recited in claim 1 further comprising effecting three-stage heating of the material in the upper part of the digester to minimize the temperature gradient the material experiences, and thus minimizing fiber damage.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 07/931,983, filed Aug. 19, 1992 (now abandoned).
US Referenced Citations (6)
Foreign Referenced Citations (1)
Number |
Date |
Country |
0476230 |
Mar 1992 |
EPX |
Non-Patent Literature Citations (2)
Entry |
Sven Rydohlm, "Continuous Pulping Processes", Tech. Assoc. of Pulp & Paper Ind., No. 7, 1970, pp. 124-133. |
"CPPA Tech '91" Kraft Pulping Course, Continuous Digesters Section III-MCC, Greenwood, B., Kamyr, Inc., Apr. 1991, 3.1-3.4. |
Continuation in Parts (1)
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Number |
Date |
Country |
Parent |
931983 |
Aug 1992 |
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