METHOD OF DIGESTING CELLULOSE FIBROUS MATERIAL IN A CONTINUOUS DIGESTER

Abstract

The invention discloses a method of continuous cooking cellulose fibrous material in a digester, said digester having a top and a bottom. The method of the invention comprises the steps of:feeding a slurry comprising cellulosic fibrous material and cooking liquor into the top the digester,feeding a first washing liquor into the bottom portion of the digester, which first washing liquor moves counter currently up through at least part of the digester,removing pulp from the bottom of the digester,feeding a second washing liquor into the center of the digester at least at the level of a first screen section,feeding a third washing liquor into the center of the digester at the level of a second screen section, andwithdrawing the cooking liquor and the washing liquors from the first screen section and from the second screen section, which first screen section is arranged in a lower part of the digester and which second screen section is arranged below said first screen section. Thus, in accordance with the invention, a third washing liquor is added to the center of digester at the level of an additional, second extraction screen section arranged in the lower part of the digester. The method makes it possible to increase the production rate in the digester but still maintain or even increase the washing efficiency.

Claims

1. A method of continuous cooking cellulose fibrous material in a digester, said digester having a top and a bottom, comprising the steps of feeding a slurry comprising cellulosic fibrous material and cooking liquor into the top of the digester, feeding a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester, removing pulp from the bottom of the digester, feeding a second washing liquor into the center of the digester at least at the level of a first screen section, feeding a third washing liquor into the center of the digester at the level of a second screen section, and withdrawing the cooking liquor and the washing liquors at least from the first screen section and from the second extraction screen section, which first screen section is arranged in a lower part of the digester and the second screen section is arranged below said first screen section.

2. A method according to patent claim 1, wherein the digester comprises at least one central pipe and wherein the second and/or the third washing liquor is fed into the center of the digester through said central pipe.

3. A method according to claim 1, wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 0-30 minutes.

4. A method according to claim 1, wherein said first screen section is arranged above said second screen section at a distance of between 0.1-15 m.

5. A method according to claim 1, wherein cooking and wash liquor is withdrawn from the digester in an additional third screen section and wherein wash liquor is fed into the digester at the level of or close to said third screen section.

6. A continuous digester, having a top and a bottom, for the production of chemical pulp by cooking wood chips in cooking liquor, which digester comprises; an inlet arranged at the top of the digester for feeding the wood chips and the cooking liquor into the digester, an outlet arranged at the bottom of the digester for removal of the pulp, at least a first and a second screen section, which first screen section is arranged in the lower part of the digester and said second screen section is arranged below said first screen section, means for feeding a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester, means for feeding a second washing liquor into the center of the digester at least at the level of the first screen section, and means for feeding a third washing liquor into the center of the digester at the level of the second screen section, means for withdrawal of the cooking liquor and the washing liquors from at least the first screen section and from the second screen section.

7. A continuous digester, having a top and a bottom, for the production of chemical pulp by cooking wood chips in cooking liquor, which digester comprises; an inlet arranged at the top of the digester for feeding the wood chips and the cooking liquor into the digester, an outlet arranged at the bottom of the digester for removal of the pulp, at least a first and a second screen section, which first screen section is arranged in the lower part of the digester and said second screen section is arranged below said first screen section, a first inlet pipe arranged to feed a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester, a second inlet pipe arranged to feed a second washing liquor into the center of the digester at least at the level of the first screen section, a third inlet pipe arranged to feed a third washing liquor into the center of the digester at the level of the second screen section, and outlet pipes arranged for withdrawal of the cooking liquor and the washing liquors from at least the first screen section and from the second screen section.

8. A method according to claim 1, wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 0.1-20 minutes.

9. A method according to claim 1, wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 5-15 minutes.

10. A method according to claim 1, wherein said first screen section is arranged above said second screen section at a distance of between 0.5-5 m.

Description

EXAMPLE

[0028] The invention is further described by way of an example.

[0029] FIG. 1 shows a schematic view of a digester in accordance with the invention.

[0030] The continuous digester shown in FIG. 1 is in the form of a pressurized, essentially cylindrical, vessel (1) comprising a top and a bottom. Cellulose fibrous material, e.g. wood chips, and cooking liquor are continuously fed into the digester through an inlet (2). In the vessel, the chips form a porous chip column descending downwardly in the digester. The cooking liquor may e.g. be kraft cooking liquor comprising sodium hydroxide and sodium sulphide. Cooked pulp is continuously removed or blown from the bottom of the digester through an outlet (3). Along the vertical center line of the vessel (1), central pipes (5a, 5c) are arranged, through which pipes separate liquor entry flows may be conveyed to different levels of the vessel. Said central pipes (5) may comprise an upper pipe (5a) and a lower pipe (5c) as shown in FIG. 1. The skilled person realizes however that these pipes may be arranged in several different modes, e.g. a first pipe having an outlet in a first level may be concentrically arranged in a second pipe, having an outlet in a second level above said first level.

[0031] One or several extraction screen structures (4) may be arranged in the vessel wall of the upper or middle part of the vessel, from which screen structures cooking liquor may be removed (4a) from the digester to be reprocessed at another location.

[0032] In accordance with the invention, a first and a second extraction screen section (6 and 7) are arranged in the lower part of the digester, which first extraction screen section (6) is arranged in the lower part of the digester and said second screen section (7) is arranged below, but at a relatively small distance, from the first extraction screen section (6). The vertical distance between the lowermost part of the first extraction screen section (6) and the uppermost part of the second extraction screen section (7) may be, e.g., 0.1-15 m, preferably between 0.5-5 m. The first extraction screen section (6) may be arranged above said second screen section at a distance corresponding to a retention time of between 0-30 minutes, preferably of between 0.1-20 minutes and even more preferably of between 5-15 minutes.

[0033] A first washing liquor (8a, 8b) is pumped into the bottom part of the vessel (1). Said first washing liquor may be added through a single inlet or through several inlets arranged in the bottom part of the vessel. The upwards-flowing washing liquid displaces the cooking liquor towards the extraction screens (6 and 7). A second washing liquor (9) is fed into the center of the digester, and into the middle of the descending chip column, through the upper pipe (5a) having an outlet (5b) at the level of the first extraction screen section (6). A third washing liquor (10) is fed into the center of the digester, and into the middle of the descending chip column, through the lower central pipe (5c) having an outlet (5d) at the level of the second extraction screen section (7). The first, second and third washing liquors may originate from one, single, liquor fed from an earlier washing stage (13).

[0034] Cooking liquor and washing liquor (11,12) are withdrawn from said first extraction screen section (6) and from said second extraction screen section (7) and further treated in a flash and evaporation plant (not shown). Part of the liquor (12a) may be circulated back to the digester in order to enhance the distribution and mixing in the digester.

[0035] Preferably, the flow (m3/ADT), or volume per produced ton of pulp, of added washing liquor at the level of each extraction screen section is equal or less than the flow of liquors withdrawn from said extraction screen section. In this way, an efficient displacement is achieved and it is possible to maintain a stable chip column movement at the extraction screen sections. Furthermore, concurrent flow of liquors in the middle part of the digester is maintained. Thus, the flow of the added second washing liquor (9) is equal to or less than the flow of the liquors (11) withdrawn from the first extraction screen section (6). The flow of the added third washing liquor (10) is equal to or less than the flow of the liquors (12) withdrawn from the second extraction screen section (7).

[0036] It is to be understood that the invention is not limited to the particular process steps and arrangements described herein. Further extraction screens and washing liquor addition points may e.g. be arranged in sequence above each other. The method of the invention may e.g. comprise drawing off cooking liquor in a third and/or a fourth extraction screen section/s and addition of wash liquor at the level of said third and/or fourth extraction screen section/s.