WO1996007786A1

WO1996007786A1 - Continuous kraft cooking with black liquor pretreatment

Info

Publication number: WO1996007786A1
Application number: PCT/US1995/007074
Authority: WO
Inventors: Kaj O. Henricson; Pekka Tervola
Original assignee: Kamyr Inc
Current assignee: Kamyr Inc
Priority date: 1994-09-02
Filing date: 1995-06-02
Publication date: 1996-03-14
Anticipated expiration: 1997-03-02
Also published as: ZA956633B; AU2697295A

Abstract

A method and continuous digester system provide for a high Na2S concentration in the beginning of the cook of comminuted cellulosic fibrous material to produce cellulose pulp, providing a more selective cook and potential to lower cooking kappa. Two different streams of spent (e.g. 'sulphurous' and 'black') liquor are removed from different screens associated with continuous digesters. The spent liquor removed from the uppermost screen has a relatively high effective alkali and Na2S concentration. The second spent liquor, removed from the second screen, has effective alkali and Na2S concentrations which are at least 25 % less than for the first spent liquor. The first spent liquor is circulated to an impregnation vessel, or other part of a chip feed system, to enhance the amount of sulphur present at the beginning of the cooking stage in the digester. The second spent liquor may be used in a number of ways, including - with or without first passing it through a flash tank - circulating it to a chip slurrying system for slurrying the chips so that they can be fed to the digester. The steaming and slurrying system may include only an unpressurized chip bin, chip feeder, slurrying vessel, and high pressure feeder, thus being simpler than conventional constructions.

Description

CONTINUOUS KRAFT COOKING WITH BLACK LIQUOR

PRTϋT TaATMTSNT

BACKGROUND AND SUMMARY OF THE INVENTION

In kraft cooking, wood is delignified by a cooking liquor where the active components are Na₂S and NaOH. Na₂S is preferably primarily active at the beginning of the cook, and NaOH is preferably active at the end of the cook. The invention relates to a method of increasing Na₂S concentration in the beginning of the cook. A high Na₂S concentration in the beginning of the cook gives a more selective cook and the possibility of lowering pulp kappa.

Spent kraft cooking liquor, or black liquor, has been re- circulated for re-use in various fashions in prior art continuous kraft cooking systems. For example in U.S. patent 3,802,956, black liquor is added to the feed system of a continuous digester to aid in flushing chips from the high pressure transfer device (i.e., the high pressure feeder) to the top of the impregnation vessel. In U.S. patents 5,080,755 and 5,192,396 black liquor is used to supplement the liquor extracted in the impregnation vessel. These patents disclose a method of introducing co- and counter-current chip impregnation in an impregnation vessel by extracting liquor at a midpoint in the vessel. The re-circulated black liquor is used to increase the liquor volume (i.e., the liquor-to-wood ratio) to limit the alkali concentration increase caused by the extraction. The black liquor also improves the chip column movement.

In the early 1980s, based upon work performed by Sjoblom et al at the Swedish Royal Institute of Technology, it was recognized that the presence of sulfides in the early stages of kraft cooking can improve the strength of the resulting pulp. Attempts have been made to use the sulfides present in black liquor to provide the sulfide desired. U.S.patents 5,053,108 and 5,236,553 disclose a method of continuous kraft cooking in which black liquor is re-circulated to the feed system to treat the wood chips. In the '108 patent this treatment takes place in the chip chute and is referred to as "sulfonation". In the '553 patent, the black liquor is re-circulated to a chip chute/slurrying vessel. In both patents a single source of black liquor is used. Recent mathematical modeling pursuant to the invention suggests that a preferred method of pre-treating chips is by using two forms of black liquor, one of relatively weak sulfide concentration and one of relatively strong sulfide concentration. The invention provides such two different streams in a practical manner. According to one aspect of the present invention, a method of continuously kraft cooking comminuted cellulosic fibrous material utilizing an upright continuous digester having first and second extraction screens, a top, and a bottom, is provided. The method comprises the steps of continuously: (a) Feeding comminuted cellulosic fibrous material slurry to the top of the digester, (b)

Cooking the material in the digester as it passes downwardly therein, at a cooking temperature of about 150-170° C, producing spent liquor during cooking while digesting the material, (c) Using the first extraction screen, withdrawing a first spent liquor from the digester having a first amount of effective alkali and a first Na^ concentration, (d) Using the second extraction screen, withdrawing a second spent liquor from the digester having a second amount of effective alkali and a second Na₂S concentration, each of which are at least 25% less than the first amount of effective alkali and first NaaS concentration, (e) Combining at least some of the first spent liquor with the material prior to step (b) to enhance the amount of sulfur present at the beginning of step (b). And, (f) withdrawing pulp from the bottom of the digester.

In the practice of the method described above, step (c) is typically practiced to extract as the first spent liquor a Uquor having an effective alkali concentration of about 10-50 g/1 and an Na₂S concentration (assuming original sulfidity of the cooking Uquor of greater than about 25%) of greater than about 35 g/1. The first Uquor is also typically at a temperature of about 140-170° C and in an amount of about 2-6 m³ p.

Step (d) of the above method is typicaUy practiced to extract as the second spent Uquor a Uquor having an effective alkaU concentration of about 5-20 g/1 (typicaUy half or less of the concentration of the first Uquor), and an Na₂S concentration of less than about 20 g 1 (typicaUy less than half of that of the first Uquor). The second Uquor typically has a temperature of about 120- 160° C and is extracted in an amount of about 2-6 m³/tp.

The digester may comprise a two vessel hydrauUc system including a first impregnation vessel, in which case step (e) may be practiced to introduce the first spent Uquor into the bottom of the impregnation vessel to flow countercurrently to the material therein, or the digester may comprise a single vessel hydrauUc digester in which case the first Uquor may be introduced into the slurry any time prior to the practice of step (a). The second spent Uquor may be flashed to steam in a flash tank, and a more concentrated second spent Uquor which is withdrawn from the bottom of the flash tank may be used to slurry the comminuted ceUulosic fibrous material prior to step (a). For example where a high pressure transfer device is provided (high pressure feeder) having a feed circulation loop, which transfer device feeds slurry to the top of the top of the digester (either directly or through an impregnation vessel), the more concentrated second spent Uquor may be introduced into contact with the material in the feed circulation loop of the high pressure transfer device, i.e. slurrying the material. The first Uquor may also be flashed if desired, before adding it to the cellulosic material upstream of the digester cooking zone.

In order to enhance further the amount of sulphur present at the beginning of the cook, there may be the further step of supplementing the first spent Uquor with just above 0 to about 2 m³/ton of pulp of green Uquor.

According to another aspect of the present invention a method of continuously digesting comminuted ceUulosic fibrous material to produce cellulose pulp comprises the steps of substantially consecutively and continuously: (a) Slurrying the material with Uquor. (b) Treating the material with a first sulphurous Uquor having an effective alkali concentration of about 10-50 g/1 and an Na₂S concentration of at least about 35 g/1 (e. g. 40-60 g/1). (c) Cooking the material with cooking Uquor having an effective -.llral. concentration of over 100 g 1 and a sulfidity of at least about 25% at a cooking temperature of about 150-170° C to produce pulp, (d) Separating the first sulphurous Uquor from the pulp, (e) Separating a second Uquor from the pulp having a different effective a ka i concentration and Na₂S concentration than the first Uquor. And, (f) washing the pulp.

In the practice of the above method, step (a) may be practiced, at least in part, using the second Uquor from step (e). The first and second Uquors preferably have the temperature ranges and volumes described above with respect to a first aspect of the present invention. The second Uquor typicaUy has an effective alkali concentration of about 5-20 g/1 (e.g. about 10 g/1) and less than the effective alkali concentration of the first Uquor, and has an Na₂S concentration of less than about 20 g/1 (e.g. about 10-15 g 1). According to another aspect of the present invention a continuous digester system is provided. The continuous digester system comprises the foUowing elements: An upright digester vessel having a top and a bottom. A chip slurry feed inlet adjacent the top of the vessel. A chip feed system connected to the chip feed inlet. A pulp outlet adjacent the bottom of the vessel. A separating device adjacent the top of the vessel for separating some Uquor from chips fed into the chip feed inlet and returning it to the chip feed system. At least one upper screen in the vessel distinct from the separating device. A first extraction screen in the vessel below the at least one upper screen, and for extracting a first spent Uquor. A second extraction screen in the vessel below the first extraction screen and for extracting a second spent Uquor distinct from the first spent Uquor. And, a first conduit for circulating the first spent Uquor to the chip feed system.

In the continuous digester system described above, a chip slurrying system is also preferably provided connected to the chip feed system opposite the digester. A second conduit is also provided for circulating Uquor from the second extraction screen to the slurrying system. The second conduit may be connected directly to the slunying system, or through one or more flash tanks. The invention also may include a simplified steaming and slurrying system associated with the digester. For example instead of utilizing a chips bin, chip meter, chip feeder, horizontal steaming vessel, slurrying vessel, and high pressure feeder, the steaming and slurrying system may consist essentiaUy of only a chip bin, chip feeder, slurrying vessel (e.g. chute), and high pressure feeder. The continuous digester system may comprise any of a wide variety of conventional digester systems including digesters available from Kamyr, Inc. of Glens FaUs, New York and sold under the trademarks MCC®, EMCC®, and LO-SOLIDS™; the continuous digester system may include a single vessel hydrauUc system, a two vessel hydrauUc system (with an impregnation vessel in addition to the digester), or other conventional systems. Where an impregnation vessel is utiUzed, the first conduit may be connected to the impregnation vessel to introduce the first spent Uquor into the impregnation vessel. In the impregnation vessel the first, sulphurous, Uquor typically flows countercurrently to the ceUulosic material, but may instead be directed to flow co-currently.

It is the primary object of the present invention to provide two or more different streams of spent cooking Uquors so as to provide a high Na₂S concentration in the beginning of a kraft cook, resulting in a selective cook and the potential to lower cooking kappa (i.e. easily below 20), and a simplified chip feeding system. This and other objects of the invention wiU become clear from an inspection of the detailed description of the invention and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic view of exemplary apparatus for practicing the method of continuous kraft cooking with black Uquor pretreatment according to the present invention;

FIGURE 2 is a view like that of FIGURE 1 utiUzing a conventional Kamyr® two vessel hydrauUc digester system with co- current Uquor flow in the top of the digester, FIGURE 3 is a view like that of FIGURE 2 only not showing the chip bin and other conventional components, and showing a countercurrent Uquor flow in the top of the digester;

FIGURE 4 is a view like that of FIGURE 2 for a single vessel hydrauUc EMCC® digester system;

FIGURE 5 is a schematic view of a conventional prior art chip feeding system for a continuous digester, and

FIGURE 6 is a schematic side view of a modified chip feeding system for a continuous digester that may be utiUzed in the practice of the present invention.

DETAILED DESCRD7TION OF THE DRAWINGS

Exemplary apparatus for practicing an exemplary method according to the present invention is shown schematicaUy in FIGURE 1. In the description of FIGURE 1, and the other figures that foUow, the comminuted ceUulosic fibrous material that is digested to produce ceUulose pulp wiU be referred to as "chips", since wood chips are typicaUy used in the production of ceUulose pulp. However it is to be understood that a wide variety of different types of ceUulosic material can be utiUzed besides wood chips. Presteamed chips in line 10 are fed to a first, impregnation, vessel 11. The chips 10 are presteamed and pressurized in a slurrying and steaming system shown in FIGURES 2 or 3. From the vessel 11 the chips are transported via line 12 to the digester 13. Cooking Uquor, typicaUy white Uquor (WL), primarily comprising Na₂S and NaOH as the active ingredients, is added to the lower portion 14 of vessel 11 and upper portion 15 of vessel 13. Additional cooking Uquor may be added to even out the alkaU profile according to cooking methods such as those using MCC®, EMCC® or LO- SOLIDS™ digesters. As an example, WL is added in FIGURE 1 to the bottom 16 of vessel 13.

Initially the chips are steamed to remove air and to aUow ready penetration by impregnating and cooking Uquors, as is conventional. The actual cooking takes place in upper parts of digester 13. During the end and later parts of the cook the NaOH concentration decreases. According to the invention, the cooking vessel 13 is equipped with two separate extraction screens 17 and 18. With the first extraction screen 17 is extracted Uquor surrounding the chips which Uquor stiU contains active cooking chemicals NaOH and Na₂S. This first, spent, extracted Uquor in conduit 19 is, due to the cooking kinetics, rich in Na₂S and very useful in the beginning of the cook (e.g. at the top of vessel 13), but not as useful where it is present at the end of the cook where primarily NaOH is needed. This first, spent Uquor, which may be caUed sulphurous Uquor, is thus separated to be used in earUer phases of the cook. Typical properties of the sulphurous Uquor in line 19 are ("tw" means "ton of wood"): temperature 155° C (140-170° C) effective alkali 20 g/1 (10-50 g/1) amount 4 m³ tw (2-6 mVtw) NaaS >35 gΛ (e.g. 40-60 gΛ)

[Note that aU chemical concentrations are based upon equivalent

NaOH].

After the useful sulphurous Uquor has been separated, the cook continues for a while after which a second, spent residual Uquor, caUed black Uquor, is separated by second screen 18 into second conduit 20. The black Uquor in conduit 20 contains residual cooking chemicals and dissolved lignin and wash Uquid introduced into conduit 21 used to wash the pulp. Typical properties of the black Uquor in conduit 20 are: temperature 150° C (120-160° C) effective alkaU 10 g 1 (5-20 g/1) amount 4 m³/tw (2-6 m³ tw)

NaaS <20 gΛ

The second spent Uquor (black Uquor) in conduit 20 is so depleted in cooking chemicals that it has no significant potential as a cooking Uquor. However it can be utiUzed in a number of different ways before it is ultimately passed to a conventional recovery system. For example, as iUustrated in FIGURE 1, it may pass to a flash tank 22 to produce steam 23 that is used in presteaming the chips 10, with a more concentrated second (black) Uquor removed from the flash tank in line 24.

Note that the first and second extraction screens 17, 18 respectively are toward the end of the cooking zone in the digester 13. In the exemplary embodiment iUustrated in FIGURE 1 these screens are located near the middle of the digester 13, but below at least one upper screen 25, the upper screen 25 iUustrated in FIGURE 1 associated with the conventional recirculation loop 26, to which white Uquor may be added if desired. Typical white Uquor that is used wiU have, during cooking, an active alkali content of at least about 100 g 1, and a sulfidity of at least about 25%. The sulfidity may vary depending upon where the white Uquor is introduced. For example using known "spUt sulfidity" techniques, the Uquor introduced initiaUy (e.g. at 14, 15) may have higher sulfidity (e.g. above 40%), while white Uquor introduced later on (e.g. at 16) lower 10

sulfidity (e.g. below 30%). The temperature during cooking is around 160° C, preferably about 150-170° C.

The first spent Uquor from conduit 19 is mixed with the presteamed chips in line 10 prior to introduction into the top 15 of the digester 13. [ The first Uquor may also be flashed to recover steam, if desired, before being mixed with the presteamed chips in line 10.] This is preferably accomplished - as iUustrated in FIGURE 1 — by introducing it into the recirculatory loop 28 at the bottom of impregnation vessel 11. In this case the first, sulphurous, Uquor flows upwardly in the impregnation vessel 11 countercurrent to the flow of chips (downward) adjacent the bottom of the vessel 11. The temperature at the circulation 28 is typicaUy about 150° C (130- 160° C). Here the sulphurous Uquor reacts with the wood and sulphur diffuses into the chips. Typical reaction and diffusion times are about 30 minutes (e.g. 20-40 minutes).

Preferably - as also illustrated in FIGURE 1 - the second spent Uquor (black Uquor) from conduit 24 is introduced into the vessel 11 adjacent the circulation 30, slurrying the presteamed chips in conduit 10. Here the temperature is about 90-120° C. The black Uquor flows downwardly with the chips in the vessel 11 until the intermediate extraction screen 31 is reached.

The extraction screen 31 extracts "final" black Uquor. For example two different streams may be provided, a first stream 32 which is recirculated back to the line 24 for introduction at 30 to slurry the chips, and a second portion 33 which is fed to the flash tank 34. Steam 35 produced in flash tank 34 is fed to presteam the chips in line 10, and a first portion of the more concentrated black Uquor which is extracted from the bottom of the flash tank 34 may flow in line 36 back to the line 32 to slurry the chips. The majority of the concentrated black Uquor from flash tank 34 passes in line 37 to evaporators in a conventional chemical recovery loop for a kraft miU. After or during addition of the cooking Uquor, the temperature of the slurry is raised to cooking temperature which is about 160° C (150-170° C). Ultimately the chips are typicaUy washed in the bottom of the digester 13 with the wash Uquor introduced in line 21, whether an MCC®, EMCC®, or a LO-SOLIDS™ digester is utiUzed, and the pulp produced is withdrawn in line 39 from adjacent the bottom of the digester 13. FIGURE 2 schematicaUy iUustrates a second form of the invention in which the teachings of the invention are employed with a conventional two vessel hydrauUc KAMYR® continuous digester system. In this embodiment structures comparable to those in the FIGURE 1 embodiment are shown by the same reference numeral. In the FIGURE 2 embodiment the conventional upper and lower extraction screens correspond to the first and second extraction screens 17, 18 according to the present invention. The first spent (sulphurous) Uquor withdrawn in conduit 19 is used to — as in the FIGURE 1 embodiment - treat the chips prior to cooking, and therefore is introduced into the bottom of the impregnation vessel 11 as indicated generaUy at 40 in FIGURE 2 - or in the recirculation line 41 between the top 15 of the digester 13 and the bottom of the impregnation vessel 11, typicaUy right before the heaters 42, or — alternatively or in addition - after the heaters 42, as iUustrated at 43 in FIGURE 2. A conventional separating device — shown schematicaUy at 44 in FIGURE 2 — such as a screen assembly, or alternatively a "stilling weU", or a conventional top separator, is used to separate some of the slurrying Uquid from the introduced chips to be fed to the line 41. In the FIGURE 2 embodiment at least some of the second spent Uquor (black Uquor) removed via screen 18 into conduit 20 is used to slurry the chips. In the embodiment iUustrated in FIGURE 2 some of the black Uquor in line 20 passes to the flash tanks 22, 22' and this concentrated black Uquor is then passed to evaporators and to other conventional chemical recovery system components. However some of the black Uquor - in line 45 - is used to slurry the chips, for example being introduced into the recirculation loop 46 associated with a high pressure transfer device (feeder) 47, as indicated at 48 in FIGURE 2. The high pressure transfer device 47 and loop 46 associated therewith are conventional in two vessel hydrauUc systems for feeding presteamed chips in line 10 to the top of the impregnation vessel 11, and the black Uquor introduced at 48 is introduced in the recirculatory line 49 from the top of the impregnation vessel 11 to the high pressure pump 50 associated with the high pressure feeder 47.

FIGURE 2 also iUustrates a conventional steaming system for producing the steamed chips, which are slurried before passing into conduit 10 and before being introduced to the top of impregnation vessel 11. FIGURE 2 iUustrates a pressurized chip bin 51 with a chip meter 52, low pressure feeder 53, horizontal steaming vessel 54, and vessel — chute - 55 in which a Uquid level is established for Uquid that wiU slurry the steamed chips discharged from horizontal steaming vessel 54. In the FIGURE 2 embodiment, a valve 56 may be provided, if desired, between the first and second conduits 19, 20, to make minor adjustments in the amount of spent Uquor flowing in each of the conduits 19, 20 if more Uquor is needed in one conduit than the other. The valve 56 is controUed automaticaUy as is conventional. FIGURE 3 iUustrates a system siπnlar to that in FIGURE 2 and the same components are iUustrated by the same reference numerals. In this embodiment, however, there is a countercurrent Uquid flow in the top of the digester 13, as indicated by the arrow 57. A bottom circulation screen iUustrated schematicaUy at 58 acts as a top separator in FIGURE 3. In this case, then, the "first screen" for withdrawing the first spent Uquor (sulphurous Uquor) is either the trim screen, iUustrated schematicaUy at 59, or the bottom circulation screen (e. g. 58), or a combination of both, both screens 58, 59 being conventional in a two vessel hydrauUc system, and associated with the recirculation line 41, and having the conventional pumps 60, 61, respectively, associated with screens 58, 59. Therefore in this embodiment the sulphurous Uquor is returned to the bottom of the impregnation vessel 11, to flow upwardly therein, by the conventional recirculation line 41.

FIGURE 4 iUustrates the appUcation of the teachings of the invention to a conventional single vessel hydrauUc digester system. The FIGURE 4 system is very similar to that of FIGURE 2, and comparable components to the FIGURE 2 embodiment are shown by the same reference numeral, except that there is no impregnation vessel.

In the FIGURE 4 embodiment, a conventional cooking circulation loop 65, including an upper screen set 66, is associated with the digester 13 above the first and second extraction screens 17, 18, but below top separator 44. A second cooking recirculatory loop 67, with associated screens 68, also may be provided. In the embodiment of FIGURE 4 the bulk of the impregnation of the chips with cooking Uquor takes place in the transfer line 12 and adjacent the top 15 of the digester 13, while noπnaUy cooking takes place from about the level of screen 66 down to the extraction screens 17, 18. In the FIGURE 4 embodiment the first spent Uquor (sulphurous Uquor) in conduit 19 may be introduced into the chips in the transfer line 12, as indicated at reference numeral 70 in FIGURE 4, while the second spent Uquor (black Uquor) in line 45 may be introduced into the conduit 71 associated with the slurrying vessel/chute 55 which suppUes slurried steamed chips to the high pressure feeder 47.

Alternatively, in a single vessel hydraulic system as seen in FIGURE 4 the strong (first) spent Uquor may pass counter-currently in situ as in the counter-current mode at the top of the digester 13 in FIGURE 3. In this case an extraction must be taken from one of the upper digester screens in FIGURE 4 so that a counter-current flow of Uquor results below the screen, yielding a higher sulfidity Uquor during the early stages of cooking. The extraction from the upper screen would then typicaUy be taken to one or more flash tanks, and to conventional chemical recovery.

The sulphur content of the sulphurous Uquid in conduit 19 may be enhanced by using green Uquor as a source of sulfide. Green Uquor is an aqueous solution of primarily sodium carbonate and sodium sulfide. In conventional practice the carbonate is causticized to NaOH to produce white Uquor. The sulfide in green Uquor can perform the same function as sulfide of the first spent Uquor in line 19 . Green Uquor may be added to either of the first or second spent Uquors in lines 19, 20 to enhance sulfidity, although typicaUy any green Uquor added would be added to the first spent Uquor in line 19, as iUustrated at 73 in FIGURE 4 (green Uquor may also be used in the FIGURES 1-3 embodiments too, typicaUy added to line 19 although it could be added elsewhere). When used, the added green Uquor volume may be from just above 0 to about 2 Vton of pulp. WhUe the embodiments iUustrated in FIGURES 2 through 4 illustrate a number of different embodiments it is to be understood 15

that they are only exemplary and that the teachings of the invention in which two different spent Uquor streams having effective alkali and Na₂S concentrations which both differ by at least 25% (and preferably by at least 50%) are utiUzed, can be appUed to almost any conventional continuous digester system.

In a single extraction black Uquor recirculation described in the prior art the sulfide concentration is dUuted by the combined extraction of spent cooking Uquor and washing Uquor. However in the present "double-extraction" process the two Uquors are separated and the stronger Uquor is not diluted by wash Uquor. For example, for cooking systems with white Uquor having typical values of 140 g/1 active alkaU (at least 100 g/1) and about 35% sulfidity (typicaUy at least about 25% sulfidity, although spUt sulfidity streams can be used) the relative sulfide concentrations of the two different Uquors is at least 25%, and typicaUy more on the order of about 4:1. One particular example, where the white Uquor has 140 g/1 active al ali and 35% sulfidity, is provided in Table I below. In Table I what is referred to as "strong" Uquor corresponds to the first spent Uquor in line 19 in the exemplary embodiments of FIGURES 1 through 4, while what is described as "weak" black Uquor is the second spent Uquor in conduit 20 iUustrated in the drawings and as described above.

Table I

Comparison of Sulfide Concentrations of Extracted liquors

Assume white Uquor sulfidity is 35% and active alkali is 140 gΛ.

Assume weak Uquor extracted contains 30% of the total volume of added white Uquor, which is typical for MCC® and EMCC® style digesters.

Components of Prior Art Dual Extraction of Extracted Liquor Sinεrle Extraction Invention (m³/ton of wood)

Weak Strone

White Liquor 2.0 0.6 1.4

Wash Liquor 1.0 1.0 0.0 Total Extracted liquor 3.0 1.6 1.4

Na₂S Concentration in

Extracted Liquor (gΛ) 32.7 18.4 49.0

Utilizing the teachings of the present invention it is also possible to simplify the steaming/slurrying system associated with the digester 13 and/or impregnation vessel 11. FIGURE 5 schematicaUy shows a conventional prior art system, similar to that iUustrated in FIGURE 2, in which a pressurized chip bin 51, chip meter 52, low pressure feeder 53, horizontal steaming vessel 54, and slurrying vessel/chute 55 are associated with the high pressure feeder 47 to steam and slurry the chips. The use of the steaming vessel 54 typicaUy results in the temperature at the top of the impregnation vessel 11 being above 100°C, usuaUy about 120°C, and the low pressure feeder 53 is necessary to isolate the significantly different pressures of the chip bin 51 and chip meter 52 from the steaming vessel 54. However according to the present invention, a lower temperature can be held at the top of the impregnation vessel 11. Thus the feed/slurrying system iUustrated in FIGURE 6 can be utiUzed, in which just the chip bin 51', chip meter 52, and slurrying vessel/chute 55 may be utilized. If the temperature in the top of the impregnation vessel 11 is sUghtiy below 100° C, which is possible in some instances (although not aU), the chips are presteamed to only about 100° C in the unpressurized chip bin 51' [sHghtly different than the pressurized chip bin 51, and simpler].

It wiU thus be seen that according to the present invention an advantageous method and system are provided for optimizing the production of kraft pulp. Utilizing the method and apparatus of the invention it is possible to provide a more selective cook, with the potential to cook to a lower kappa (easily below 20).

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

WHAT TS ΠT.ATMP,T> TS-

1 1. A method of continuously kraft cooking comminuted

2 ceUulosic fibrous material utilizing an upright continuous digester

3 having first and second extraction screens, a top, and a bottom,

4 comprising the steps of continuously:

5 (a) feeding comminuted ceUulosic fibrous material slurry to the

6 top of the digester,

7 (b) cooking the material in the digester as it passes

8 downwardly therein, at a cooking temperature of about 150-170° C,

9 producing spent Uquor during cooking while digesting the material;

10 (c) using the first extraction screen, withdrawing a first spent

11 Uquor from the digester having a first amount of effective alkqli and

12 a first N-tjS concentration;

13 (d) using the second extraction screen, withdrawing a second

14 spent Uquor from the digester having a second amount of effective

15 alkaU and a second Na₂S concentration, each of which are at least

16 25% less than said first amount of effective alkaU and first NaaS

17 concentration;

18 (e) combining at least some of the first spent Uquor with the

19 material prior to step (b) to enhance the amount of sulfur present at

20 the beginning of step (b); and

21 (f) withdrawing pulp from the bottom of the digester.

1 2. A method as recited in claim 1 wherein step (c) is practiced

2 to extract as the first spent Uquor a Uquor having an effective alkali

3 concentration of about 10-50 g/1 and an Na₂S concentration of greater

4 than about 35 g/1. 3. A method as recited in claim 2 wherein step (d) is practiced to extract as the second spent Uquor a Uquor having an effective alkaU concentration of about 5-20 g/1, and an Na₂S concentration of less than about 20 g/1.

4. A method as recited in claim 2 wherein the digester comprises a second vessel in a two vessel hydrauUc continuous digester system, also including a first, impregnation, vessel; and wherein step (e) is practiced to introduce the first spent Uquor into the bottom of the impregnation vessel to flow counter-currently to the material therein.

5. A method as recited in claim 2 wherein the digester comprises a single vessel hydrauUc digester, and wherein step (e) is practiced to introduce the first Uquor into the slurry prior to the practice of step (a).

6. A method as recited in claim 2 wherein step (c) is further practiced to extract the first Uquor having a temperature of about 140-170° C and in an amount of about 2-6 m³/ton of wood, and wherein the second Uquor has effective alkali and Na₂S concentrations of at least 50% less than those of the first Uquor.

7. A method as recited in claim 3 wherein step (d) is further practiced to extract the second Uquor having a temperature of about 120-160° C and in an amount of about 2-6 m³/ton of wood.

8. A method as recited in claim 2 comprising the further steps of (g) flashing the second spent Uquor to produce steam and more concentrated second spent Uquor, and then (h) using the more concentrated second spent Uquor to slurry the comminuted ceUulosic fibrous material prior to step (b).

9. A method as recited in claim 8 wherein a high pressure transfer device, having a feed circulation loop, feeds slurry to the top of the digester; and wherein step (h) is practiced to introduce the more concentrated second spent Uquor into contact with the material in the feed circulation loop of the high pressure transfer device.

10. A method as recited in claim 2 comprising the further step of supplementing the first spent Uquor with just above 0 to about 2 m³/ton of pulp of green Uquor.

11. A method of continuously digesting comminuted ceUulosic fibrous material to produce ceUulose pulp comprising the steps of substantiaUy consecutively and continuously: (a) slurrying the material with Uquor, (b) treating the material with a first sulphurous Uquor having an effective al aU concentration of about 10-50 g 1 and a Na^ concentration of at least about 35 g/1; (c) cooking the material with cooking Uquor having an effective alkali concentration of over 100 g/1 and a sulfidity of at least about 25% at a cooking temperature of about 150-170° C to produce pulp; (d) separating the first sulphurous Uquor from the pulp; (e) separating a second Uquor from the pulp having a different effective alkali concentration and Na₂S concentration than the first Uquor, and (f) washing the pulp.

12. A method as recited in claim 11 wherein step (a) is practiced, at least in part, using the second Uquor from step (e).

13. A method as recited in clam 11 wherein step (d) is further practiced to extract the first Uquor having a temperature of about 140- 170° C and in an amount of about 2-6 m³/ton of wood, and with an Na₂S concentration of about 40-60 g/L

14. A method as recited in claim 13 wherein step (e) is further practiced to extract the second liquor having a temperature of about 120-160° C and in an amount of about 2-6 m³/ton of wood.

15. A method as recited in claim 11 wherein step (e) is practiced to extract as the second Uquor a Uquor having an effective alkali concentration of about 5-20 g/1, and less than the effective alkali concentration of the first Uquor, and having an Nsβ concentration of less than about 20 g/1.

16. A continuous digester system comprising: an upright digester vessel having a top and a bottom; a chip slurry feed inlet adjacent said top of said vessel; a chip feed system connected to said chip feed inlet; a pulp outlet adjacent said bottom of said vessel; a separating device adjacent the top of said vessel for separating some Uquor from chips fed into said chip feed inlet and returning it to said chip feed system; at least one upper screen in said vessel distinct from said separating device; a first extraction screen in said vessel below said at least one upper screen, and for extracting a first spent Uquor; a second extraction screen in said vessel below said first extraction screen and for extracting a second spent Uquor distinct from said first spent Uquor, and a first conduit for circulating the first spent Uquor to said chip feed system.

17. A continuous digester system as recited in claim 16 further comprising: a chip slurrying system connected to said chip feed system opposite from said digester, and a second conduit for circulating the second spent Uquor from said second extraction screen to said slurrying system.

18. A continuous digester system as recited in claim 17 wherein said second conduit is connected to a flash tank, and comprises a concentrated Uquor conduit from said flash t-anlr

19. A continuous digester system as recited in claim 16 wherein said chip feed system includes an upright impregnation vessel; and further comprising a steaming and slurrying system for feeding chip slurry to the top of said impregnation vessel; said steaming and slurrying system consisting essentiaUy of an unpressurized chip bin, a chip feeder, a slurrying vessel, and a high pressure feeder.

20. A continuous digester system as recited in claim 16 wherein said chip feed system includes an upright impregnation vessel; and wherein said first conduit is connected to said impregnation vessel to introduce the first spent Uquor into said impregnation vessel to flow countercurrentiy to chips flowing therein.