Pulp cooking with particular alkali profiles

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.

What is claimed is:  
     
         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.