Claims
- 1. A continuous kraft pulping process comprising:
(a) passing a slurry of wood chips and liquor in a digester sequentially through a screen set which includes first and second screen assemblies longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester; (b) extracting liquor from the first screen assembly of said set without recirculation to the digester to a location above the first screen assembly, while simultaneously (c) extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor from the second screen assembly to a discharge location within the digester which is above the second screen assembly and spaced below the first screen assembly by a distance which is less than one-half the digester diameter.
- 2. The process of claim 1, wherein dilution liquor is added to the liquor extracted from the second screen assembly and recirculated to the digester.
- 3. The process of claim 1, wherein at least a portion of the liquor extracted from the first screen assembly is reintroduced to the digester at a location below the second screen assembly.
- 4. The process of claim 1, 2 or 3, wherein there is at least one additional screen assembly set below the first mentioned screen assembly set, wherein said at least one additional screen assembly set includes third and fourth screen assemblies positioned in longitudinally spaced relationship by a distance which is greater than a diameter of the digester, and wherein said process includes:
(d) extracting liquor from the digester at the third screen assembly set without recirculation to the digester to a location above the third screen assembly, while simultaneously (e) extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor from the fourth screen assembly to a discharge location within the digester which is above the fourth screen assembly and spaced below the third screen assembly by a distance which is less than one-half the digester diameter.
- 5. The process of claim 4, wherein at least a portion of the liquor extracted from the first screen assembly is combined with the liquor extracted from the third screen assembly.
- 6. The process of claim 5, wherein step (c) is practiced by introducing a dilution liquor into the portion of liquor extracted from the second screen assembly and recirculated to the digester to a location between the first and second screen assemblies.
- 7. The process of claim 6, wherein said dilution liquor is selected from water, white liquor, wash liquor, bleach plant filtrate and mixtures thereof.
- 8. The process of claim 1, wherein step (c) is practiced by replacing some or all of the extracted liquor with a liquor containing a substantially lower effective dissolved organic material level than the extracted liquor.
- 9. The process of claim 8, wherein the digester has multiple other screen assemblies, and wherein liquor is extracted respectively at least at one of the multiple other screen assemblies and recirculated to the digester at a location above the at least one of the multiple other screen assemblies.
- 10. The process of claim 9, which comprises introducing a dilution liquor to the liquor extracted respectively at least at one of the multiple other screen assemblies and recirculated to the digester at a location above the at least one of the multiple other screen assemblies.
- 11. The process of claim 10, wherein the dilution liquor is selected from water, white liquor, wash liquor, bleach plant filtrate and mixtures thereof.
- 12. A continuous kraft pulping system comprising:
a digester having a screen assembly set which includes first and second screen assemblies which are longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester; a feed line for feeding a slurry of wood chips and liquor to a top of the digester above the screen assembly set so that the slurry can pass sequentially therethrough said first and second screen assemblies thereof; a first extraction line for extracting liquor from the first screen assembly of the set in the absence of recirculation to the digester to a location above the first screen assembly; a second extraction line for simultaneously extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor to a discharge location between the first and second screen assemblies of the set which is spaced below the first screen assembly of the set by a distance with is less than one-half the digester diameter.
- 13. The system of claim 12, comprising a dilution liquor line for adding dilution liquor to the liquor extracted from the second screen assembly and recirculated to the digester.
- 14. The process of claim 12, wherein at least a portion of the liquor extracted from the first screen assembly is reintroduced to the digester at a location below the second screen assembly.
- 15. The system of claim 14, further comprising:
at least one additional screen assembly set positioned within the digester below the second screen assembly of the first mentioned set, said at least one additional screen assembly set including third and fourth screen assemblies longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester, a third extraction line for extracting liquor from the third screen assembly in the absence of recirculation to the digester to a location above the third screen assembly; and a fourth extraction line for simultaneously extracting liquor from the fourth screen assembly and recirculating at least a portion of the extracted liquor to a discharge location between the third and fourth screen assemblies which is spaced below the third screen assembly by a distance with is less than one-half the digester diameter.
- 16. The system of claim 15, wherein at least a portion of the liquor extracted from the first screen assembly is combined with the liquor extracted from the third screen assembly.
- 17. The system of any one of claims 12-16, comprising multiple other screen assemblies, at least one other extraction line for extracting liquor from at least at one of the multiple other screen assemblies, and at least one other recirculation line for recirculating at least a portion of the liquor extracted to the digester at a location above the at least one of the multiple other screen assemblies.
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based on, and claims domestic priority benefits under 35 USC §119(e) from, U.S. Provisional Application Serial No. 60/348,367 filed on Jan. 16, 2002, the entire content of which is expressly incorporated hereinto by reference.
Provisional Applications (1)
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Number |
Date |
Country |
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60348367 |
Jan 2002 |
US |