FI121790B - Continuous boiler and system for a continuous boiler - Google Patents

Description

STRAINER AND STRAINER SYSTEM The parent applications of this application provide a critical 5 advancement: controlling the level of dissolved organic material (DOM) to increase pulp strength and bleach with lower alkali consumption and lower H-factor. DOM contains mainly dissolved lignin, hemicellulose, cellulose 10 and other substances separated from wood in the kraft pulping process. The DOM content is typically maintained at 100 g / l or less (e.g., about 50 g / l or less) substantially throughout the cooking, the hemicellulose content is typically maintained at or below 15 g / l (e.g., about 10 g / l). and below), and lignin content typically at or below 50 g / L (e.g., about 25 g / L or less). This is accomplished by removing the liquid having a high DOM content from the vertical digester and / or the feed or circulation line within the digester (typically the digester) and replacing the high DOM high fluid with the low effective DOM high fluid. As used herein, the term "effective DOM" refers to the amount of DOM that affects pulp strength, H-factor, effective alkali consumption, and / or ... 25 bleaching. Low Effective DOM Liquid • · typically consists of water, essentially DOM-free white liquor, washing filtrate, · · ·: · cold-shot filtrate, pressure-heated black liquor, and · 1 · 1: combinations thereof.

: · .1 30 • ♦ · ... In accordance with the present invention, various steaming systems and components and methods for treating finely divided cellulosic fibrous material are provided, which facilitate the DOM level of strain applications: 1 : 35 putting regulation into practice. According to the invention ·· «*. inexpensive results are achieved, some of which are achieved using new technical methods and some using traditional tools in new ways.

In DOM controlled pulping systems, the addition of white liquor at multiple points and levels in the pan and the addition of a low DOM dilution solution means less liquid is left for transporting the chips during or after impregnation, i.e. transporting from the high pressure transfer device to the digester. For example, the amount of alkali fed to the D0M regulating digester feed system may be only 40 to 55% (e.g. less than 55%). To remedy this, the liquid is removed from the circulation line of the top of the digester and introduced into the bottom of the impregnation vessel, cooled, and fed to the high pressure feed line of the high pressure transfer device.

In accordance with one aspect of the present invention, there is provided a two-pan dispenser system for cooking cellulosic fibrous material comprising: *** an impregnation vessel comprising a top and bottom, a slurry of a cellulosic fibrous material to be cooked:. at the top of the impregnation vessel, at the bottom of the sludge drainage • · '.'V 25 hole and at the top of the impregnation vessel for the circulation * outlet; A circulation system for feeding slurry to the top of the impregnation vessel and for receiving a circulation solution from the top of the impregnation vessel; 30 - Dispenser, comprising upper and lower part, slurry inlet for cookable cellulosic fibrous material: '·. in the upper part of the digester, in the lower part of the cooked pulp outlet kitchen1 · 1, in the upper part of the circulating fluid outlet of the digester; • 1 * · [· 35 - first set of impregnation vessel 1 ie step between suction port and digester sludge inlet and second unit between kitchen top recirculation outlet and bottom of impregnation tray ; - at least one DOM reducing screening group between the top and bottom of the digester; A means for removing and transporting for recovery each first dose of fluid passing through the DOM reducing screening group, leaving a second dose of fluid; means for adjusting the low-effective DOM-containing liquid li-10 to each second dose to increase the second dose; means for adding the cooking solution to at least one second portion; - a means for circulating the magnified second portions into the interior of the 15 digesters; and - as a novelty of the invention, a means for increasing the fluid flow of the circulating system, the means including at least a means for removing fluid from the other, a means for cooling the removed fluid and a means for introducing the cooled removed fluid into the circulation system.

The circulation system typically comprises at least a ♦ 1 · high pressure transfer device and a high pressure pump with · ”·. the inlet and outlet, and the high pressure pump is • • connected to the high pressure pump outlet. The means for introducing the cooled, withdrawn liquid into the recirculation systems · · 1 'comprises at least a means for introducing the cooled, discharged * · 1 liquid into a connection connected to the high pressure pump inlet. Preferably, the system also includes at least a means for adding the cooking solution to the removed liquid, e.g. means for cooling the liquid.

.:: 1 The means for cooling the discharged liquid preferably comprises at least a heat exchanger or evaporative cooler.

• · · · · · · • • • • • • • • 4

Preferably, the digester also comprises a separation screen group, and at least one of the DOM reducing screening groups comprises a control screen group located near the top of the digester and a cooking screen group located between the control screen group and the separation screen. Preferably, the digester also includes a wash screen set between the lower part of the digester and the separation screen, and a means for adding cooking liquor to the 10 wash screen sets. The digester is provided with a blind recycle tube at the separation screen so that the separation screen can be easily converted to comprise a recycling screen.

Alternatively, the digester comprises a combined recycling and separating screen group attached to the top of the digester, and at least one of the DOM reducing screening groups preferably comprises a first screening group located below the combined recycling and screening screen group and a second screening group located below the first screening group. The digester may also include a washing screen set located between the lower part of the digester and another screen set, and a means for adding the cooking solution to the screen screen set.

According to another aspect of the invention, a method may be carried out using a device of the above type. The method involves at least the following steps: (a) Ain j At least one point between the top and bottom of the digester · 1 · 1: DOM-containing liquid is removed from the digester, · · • '. <T is distributed to the first and second streams and conducted. ; ·. 30 first stream for recovery, (b) Add a second stream of low DOM-containing liquid, e.g. white liquor, and ... recycle another stream back into the digester; and • · · (c) adding a stream of liquid in the recycling system to remove the liquid from one outlet, cooling the removed liquid, and introducing the cooled removed liquid into the recycling system.

• • • • • • • • • • 5

Preferably, the recirculation system includes a high pressure transfer device and a high pressure pump having an inlet and an outlet connected to the high pressure transfer device, wherein step (c) is performed by introducing the cooled drained liquid into the high pressure pump inlet and . by adding cooking liquor to the drained liquid before cooling the drained liquid). Typically, the amount of white liquor introduced into the material prior to the boiler is about 55% or less of the total amount of white liquor added. The cooling in step (c) can be carried out by heat exchange or evaporative cooling.

15

The method typically also comprises the further steps of removing and circulating the liquid in the lower portion of the digester and adding white liquor to the washing cycle, separating the liquid from the digester and passing it to a recovery separate from the washing cycle and leading to a first batch of removed liquid. Generally, steps (a) and (b) are performed at several different levels of the digester.

According to the invention, it is also possible to provide a special device and procedure for controlling the liquid level in a vapor phase cooker U1. Contrary to traditional knowledge, it is not advantageous to separate the lye immediately after impregnation. During cooking, in accordance with this aspect of the invention, the separation from the top of the step may be operatively connected to the equalization tank, which is connected to the digester feed system. The difference can be either directly connected to the connection to the equalization tank • · * ··· 1 in a single vessel vapor phase system or in a high pressure: 35 transfer rinse stream using a dual vessel vapor phase system.

• • • • • • • • • • 6

In other words, according to this aspect of the invention, there is provided a vapor phase digester assembly comprising at least the following components: - a spout having a top and bottom, comprising a fine cellulosic fibrous material inlet at the top and a pulp outlet at the bottom; - at least one DOM reducing screening group for removing the liquid containing the first DOM content so that the DOM content decreases to a second concentration much lower than the first, then reintroducing the liquid containing the second DOM content into the digester; a feed system for supplying finely divided cellulosic fibrous material 15 to the upper part of the digester comprising at least a high pressure transfer device, a chop hopper connected to a high pressure transfer device, a high pressure pump connected to a high pressure transfer device, a high pressure transfer device inside the digester, which means for coarsely adjusting the liquid level ··· at the top of the digester to maintain the vapor phase at the top of the digester; Characterized by a means for fine-adjusting the liquid level in the upper part of the digester to maintain the vapor phase, which means includes a level regulating strainer 30 underneath the liquid level and above the other strainers and , which starts from the level control sieve and divides into the first and second branches, the first of which is Γ *. The other recirculates the liquid into the digester and the other is · *** operatively connected to the equalization tank for easy control of the liquid. *. *. * in the digester.

: · '· 35 • »*

The group typically also comprises means for cooling the liquid in the second branch between the digester and the level control tank and a third branch which leaves the second branch and leads to recovery. The valve may also be provided to adjust the relative fluid flow through the second and third arms.

5

It is also possible to use a two-vessel system having an impregnation vessel for circulation between the high pressure transfer device and the digester, and comprising a flushing port. In this case, the second leg is connected to the flushing connection.

The invention also relates to a method for treating finely divided cellulosic fibrous material using a system as described above, comprising at least 15 trays with upper and lower portions having an upper opening with a fine cellulosic fibrous material inlet and a lower pulp outlet. The system involves the following steps: - At least one point in the digester provides a reduction in the DOM level of the liquid in the kitchen by removing the liquid containing the first DOM level by lowering the DOM level, ··· the first concentration to a much lower second ** and followed by another mouthful of DOM-containing *! *** inside the digester.

• · 25 - Feeding a slurry of finely divided cellulosic fibrous material material into the upper part of the digester, at which step at least a high pressure transfer device, a high pressure transfer hopper, a high pressure transfer device and a high pressure transfer device connected to the high pressure transfer device rotary connections and a leveling tank operatively connected to the chipping hopper.

*. ·; - Perform a coarse liquid level control at the top of the digester to maintain the vapor phase in the upper part of the digester.

• # ·

Perform a fine liquid level adjustment at the top of the digester to maintain the vapor phase at the top of the digester by removing liquid from the top of the digester below the vapor phase but near the top of the vessel, distributing the discharged liquid to the first and second branches, circulating the liquid in the second branch to the equalization tank to facilitate fine adjustment of the digester level.

The method may also include an additional step of cooling the liquid in the second leg between the digester and the level control tank 10. If a third branch leaves a second branch, the method may comprise the further steps of leading the third branch to recovery and adjusting the relative amount of fluid flowing in the second and third branches.

15

According to another aspect of the invention, the separation is taken from the discharge of the refill liquor pump in the feed system. This allows for the removal of a lot of D0M-containing lye from the feed system (which is particularly advantageous because it implies a lower DOM level at the start of cooking), but leaves a finer or stick chips that may be present in the lye. The on-line dehumidifier • · · can be used in single or dual-vessel systems • · · · and in hydraulic or steam phase cookers.

According to the invention, it is also important to design a separation / diff. . filtration screens so that mixing of different DOM-level liquids can be minimized, but at the same time the flow direction of the chips and liquids in the vicinity of the sieves 30 can be controlled. This is achieved by providing first and second sieves inside the digester and at least one liquid. : · Re-import connection through separation / dilution screens-: * · * · To reintroduce the liquid into the digester close to the outlet where the liquid inlet port is in. The first and second · 35 screens are vertically relative to each other and their nearest parts are: V: at a distance from each other The re-import port has a 9-inlet liquid closest to the second strainer and is located a second distance from the nearest part of the second strainer, the first being 0 to 3 meters (0 to 10 feet). (preferably about 1.2-1.8 meters) (4-6 ft-5 ft) and another distance 0-3 meters (0 to 10 feet) (preferably about 0.9 to 1.8 meters) (3 to 6 feet). In countercurrent situations, the second screen is vertically above the first screen, and the opening of the re-import link is above the second screen. For mode-10 roads with parallel current, the second screen is vertically below the first screen and the re-import port opening is below the second screen.

Typically, one or both of the screens is connected to the expansion vessel via an outlet, and at least one of the screens is connected to a refill port. The digester typically also comprises at least one screening group for removing a large amount of D0M, which divides the liquid into the first and second streams and leads to recovery of the first stream, and adds a low D0M-containing liquid to the second stream and returns the second stream to the digester DOM • · · to reduce the content in the kettle.

It is a primary object of the present invention to provide various systems and methods for increasing the diversity and practicality of kraft pulping, taking into account and maintaining the DOM content. typically below 100 g / L throughout cooking. This and other aspects of the invention will be explained in the detailed description of the invention and in the appended claims.

• · · ♦ • «· • · • ·

Fig. 1 is a schematic view of a first exemplary embodiment of a dual-vessel 35-stage hydraulic cooker or steam cooker system according to the invention, taking into account the DOM level and? · 10 ensure the correct amount of fluid flow to the feeders;

Fig. 2 is a view similar to Fig. 1 with only 5 removed feed system and impregnation vessel, with modified digester rotations;

Figure 3 is a schematic view similar to Figure 1 of another embodiment of the digester; 10

Fig. 4 is a view similar to Fig. 1 of a single-vessel vapor-phase digester system illustrating fine-tuning of the plane according to the invention; Fig. 5 is a view similar to Fig. 4 of a two-pot digester system showing a fine-tuning of the level and an in-line dewatering apparatus;

Fig. 6 is a longitudinal cross-sectional view of the fluid removal device in the line of Fig. 5; ; ***: Fig. 7 is a schematic detailed view of a separation / dilution screening solution according to the invention to minimize mixing of liquids containing DOM contents when the liquid and chips flow in parallel; Fig. 8 is a view similar to Fig. 7 of a group having a counter current; and 30

Figures 9a-9d are schematic representations of single-vessel digesters with different screening solutions to achieve optimization of the DOM content profile based on raw materials and desired pulp properties.

35 ··· • · · · · · · · · · · · · · · ··· · · · · · · · · · · · · · ·· ·

Figure 1 illustrates an exemplary dual-hydraulic hydraulic digester system 10 according to the invention, which includes a rotation to increase the flow to the feed system of the digester system. However, this aspect of the invention can also be applied in a corresponding manner to a two-phase vapor phase system.

The system 10 comprises at least a high pressure transfer device 11 connected to a chipping hopper 12, which feeds the chips (or other fine cellulosic fibrous material) into the low pressure inlet of the device 11. The outlet 13 of the high pressure pump 14 is connected to the high pressure inlet of the device 11. The outlet of the device 11 comprises a first one 15 which conveys a slurry 15 from the outlet of the device 11 to the top of the impregnation vessel 16. The inlet of the pump 14 comprises a second one 17 which circulates the liquid separated from the slurry at the top of the vessel 16. The vessel 16 preferably comprises a central strainer 18 which removes fluid from inside the vessel 16 and conveys it through the condenser 19 before being fed to the second conduit 17. The white liquor may be added in line 20: *** · just before the condenser 19.

• · · • · ·:. ·. The second vessel of system 10 comprises a vertical flow digester 21 having an upper portion 22 and a lower portion 23. The slurry from the lower portion of the impregnation vessel 16 passes first. the second portion 25 returns the liquid separated from the upper portion 22 (via the screen 26) to the lower portion of the vessel 16, typically through the bottom circulation heaters 30 26.

The digester 21 undergoes multiple withdrawals and cycles: T to maintain a DOM concentration below 100 g / L throughout cooking. In the embodiment of Figure 1, the control screen set 28 *. * 35 is located near the top portion 22 and includes an outlet connection * * ···. 29, pump 30, heater 31, and reintroduction 32.

The outlet port 29 comprises a first leg 33 which is connected to the outlet 32 and a second leg 34 which leads to a recovery (e.g. an expansion vessel) and removes the DOM-rich fluid from the digester. The liquid stream in branch 33 is added with a low effective DOM inside-5 interval with the liquid, in the embodiment of Fig. 1 by a combination of cold buffalo filtrate and white liquor.

The cooking screen set 35 is located below the screening group 28 and comprises components comparable to the group 28, and below the group 35 is a separation screen 36 comprising an outlet 37 for recovery (e.g., an expansion vessel). The blind 36 is provided above the screen 36. arranged so as to allow flexibility in the operation of the digester 21. The sieve 36 can thus, if desired, be assembled up as the strainers 28, 35. The washing cycle 39 is provided at the bottom of the vessel 21 and can be supplemented with white liquor in line 40 for further modified cooking.

20

When using system 10 as above, *** · about 45-60% of white liquor is added to circuits in kitchen 21, leaving an insufficient amount of liquid in the feed system · 1] · system including the transfer device 11, and • · the invention * is added to provide the feed. 1 according to the flow of the circulation system by removing the wash • · \ 'from line 25 in line 41 and passing it to line 42, e.g.

Since the temperature of the liquid in line 41 is high enough to cause the device 11 to expand to a part of the fluid and thus also to produce pressure shocks, the liquid is preferably transported: 1 ·. through the condenser 43. The condenser 43 may be an indirect heat exchanger or a evaporative condenser. The white liquor .1 · is preferably added in line 42 before the cooler 43.

'·' · 35 The amount of white liquor in lines 42 and 20 is about 40% to 55% '··· 1: 1 of the total effective alkali needed for kraft yell in the digester 21. The current in line 17 can be increased by 13 outlets. through the screen 18 of the container 16 as shown in Figure 1.

Fig. 2 shows an embodiment similar to Fig. 1, except for special rotations of the digester 21. In the embodiment of Figure 2, components identical to those in Figure 1 are denoted by the same reference numerals. In this embodiment, the upper circulation 44 connected to the upper screen 45 comprises a side port 46 through which part of the DOM-rich liquid is led to recovery and the remainder recycled. The screening group 47 is located between the groups 35, 39 and is similar to the screening group 28 (and its components) of Figure 1. The impregnation vessel, line 41, condenser 43, and other components in this embodiment are the same as in the embodiment of Figure 1.

Figure 3 shows an embodiment of the digester 11, which is

similar to U.S. Patent Application 08 / 127,548 (FI

955247), except that line 25 is connected to an impregnation vessel.

Line 41, condenser 43, and other components are as .1 "· in the embodiment of Figure 1, with the addition of a screening group 48.

·· «

To the liquid to be recycled through the group 48 is added [·, White lye and low-effective D0M-containing liquid before recycling. This creates an upstream wash zone of 1, and provides a new level of wash zone in the wash zone.

• · • «

In the embodiment of Fig. 4, the single-vessel vapor-phase cooker system 30 is generally designated by reference numeral 50, and the components of this embodiment are comparable to those of Figs. is for components of 1 embodiment having the same .1 · 1 reference numerals. The digester 51 is a vapor phase digester having a.,., Liquid level around the top portion 22 as shown in the formula at 52 in Fig. 4, and the vapor is above the level 52 .... Typically, the liquid level 52 around the upper part of the digester 51 is primarily controlled by a separating current through a line through a separating strainer 66. The feed system 50 of the digester system 50 includes at least an equalization tank 53 connected to an in-line fluid drain 54 (which in turn is connected to the chips 12), 5 auxiliary liquor pump 55 and a white liquor source.

While the separation through line 67 allows for good coarse control of level 52, it is also possible according to the invention to fine-tune and simultaneously control D0M throughout the cooking process. This is accomplished by using a level control screen 56 located below the level 52 (but the first screening group of the digester 51 is above the other screens). The unit 57 starts from the screen 56 and is divided into 15 first legs 58 and a second leg 59. The first leg 58 is rotated inside the digester 51, and the second leg 59 is operatively connected to the equalization tank 53 and preferably leads to a cooler 60 (such as cooler 43). Preferably, a third branch 20 61 is also provided which ultimately leads to an expansion vessel 62 (recoveries), and a valve 63 may be provided at the intersection of the branches 59, 61 (e.g., equalization vessel, 53 or recovery 62). to control the amount of relative liquid V * · flowing.

25 · · · The kettle 51 also comprises other different cycles, e.g.

• · • * cycle 64, which delivers dilution solution to reduce effective • · *. * DOM content in digester 51, wash cycle 65 and separation screen 66.

30

Fig. 5 shows an embodiment similar to Fig. 4 with the exception of a two-vessel vapor phase system having at least an impregnation vessel 68, a recycling line 69 from vessel 68 to a high pressure transfer device 11, and a flow line from * 70 to 11 vessels 68. the upper part. The flushing stream ···: ... the backing line 70 'is arranged between the impregnation vessel 68 and the digester 51. In this embodiment, the current from line 59 · · ♦ · · · 15 preferably passes to flush flow line 70 ′, although it may also lead to tank 53 or line 70. Figure 5 also shows another device for adjusting the DOM level in the cooking process including 5 at least conventional an in-line dewatering device 71, an inlet feed line 72 and an outgoing drain line 73 therein, as well as a fluid outlet line 74 that conveys a lot of D0M-containing liquid to the expansion tank 62. The feed line 72 is preferably connected to the outlet 55 of the solution solution 10. Line 73 is a fluid line connected to the top of vessel 68.

The on-line fluid removal device 71 is conventional and is illustrated in more detail in Figure 6. It includes an internal strainer 75 that effectively separates a portion of the high D0M fluid entering line 72 but not the fines or chips remaining in circulation through lines 72, 73.

20 Using the apparatus and performing methods of adjusting the DOM level during cooking is highly desirable. ··· Ways to prevent mixing of fluid streams with substantially different DOM levels. In the past, it has not been necessary to specifically try to prevent mixing of different * * '25 currents in the renters. However, since it is desirable to adjust the DOM levels according to the invention, it is highly desirable to minimize noise. 7 and 8 using the screening groups shown in Figures 7 and 8.

30

Figure 7 schematically illustrates a digester 75 comprising at least one separation / dilution screening group 76, for example: ·; · vertically spaced first screen 77 and second screen 78. Screen 77, 78 are kitchen V. 35 between the top and bottom, typically in the middle of it. The first outlet line 79 leaves the screen 77, and the second outlet line 80 leaves the screen 78, which removes 16 liquids from the stove 75 of the digester 75. At least one liquid re-introduction nozzle 81 is disposed in a liquid re-introduction fluid from one of the two strainers 77, 78 (e.g., from line 80) into a digester 75 close to the stopping head. The assembly 81, typically at its lower end, has a fluid inlet 82.

To minimize mixing of liquids containing different DOM levels, strainers 77, 78 are spaced within the first 10 vertical distances X, which depends on the species to be cooked, the diameter of the digester 75, and many other variables. The distance X is 0 to 3 meters (0 to 10 feet), preferably about 1.2 to 1.8 meters (4 to 6 feet). Also, to minimize mixing, a second distance ("vertical clearance") is provided between the opening 82 and the nearest portion of the second screen 78. The distance d is also very dependent on soup and digester variables and is 0-3 meters (0-10 feet), typically about 0.9-1.8 meters (3-6 feet).

20

Fig. 7 shows an embodiment having a parallel flow of chips and liquid, ·· * tea, with opening 82 located vertically below the second screen 78. In the embodiment of Figure 8, a counter-flow of liquid and chips is provided. In the embodiment of Fig. 8, all components are the same as in the embodiment of Fig. 7, with the exception that% 82 * has an opening 82 vertically above the second screen 78 (which in this embodiment is above the first screen 77), and typically the exact connections of the discharge connections 79, 80 30 are different.

Figures 9a-9d schematically illustrate other embodiments of the one-pot hydraulic cookers of the invention having different separation and dilution sites. These «* '·' · 35 allow for a wide variety of cooking materials, • bleaching and bleaching requirements, etc. All variations of · · · *. 'In Figures 9a-9d are variants of US base • · 9 t »•» 17 of the basic structure shown in Fig. 20 of 08 / 127,548 (EN 955247).

The basic difference between the structure of Fig. 9a and the structure of Fig. 20 5 of the base application is the use of one cooking cycle 85 in the cooker 86 instead of two cooking cycles. The embodiment of Figure 9a is for a raw material which is easier to cook, for example hardwood, and is more economical than a two-cycle machine. The separation strainer 87 10 is below the cooking cycle 85, the modified cooking cycle 88 with white liquor supplements is below it, and the washing cycle 89 with white liquor supplements (in the extended modified cooking dish) is located near the bottom in the EMCC® cooker 86. Each cycle 85, 88 and 89 comprises 15 pumps, an indirect (steam) heater and similar conventional components, although for simplicity this is not shown in Fig. 9a. The stove 86 is provided with two separations.

In the embodiments of Figures 9b-d, components similar to those of Figure 9a are denoted by the same reference numerals.

The embodiment of Fig. 9b is the same as the embodiment of Fig. 9a, except that an additional separation 90 and an additional dilution 91 are provided in cycle 88 to supplement the separation to obtain a DOM concentration. further reduced *. . and / or in a more controlled manner than in the embodiment of Figure 9a.

30

The embodiment of Fig. 9c is the same as the embodiment of Fig. 9a except that the separating sieve 87 is below the modified leach cycle 88, that no dilution solution is added to the cooking cycle 85, and that the dilution solution 35 is added to the white liquor modified leachate. Rossa 88. The embodiment of Fig. 9d is similar to the embodiment of Fig. 9a except that the separation target 18 and 87 and the modified free circulation cycle 88 are only reversed and the diluent is reversed. added to cycle 88.

The use of alternative embodiments of Figs. 9a-d 5 allows for optimization of the DOM content profile based on the raw material and desired pulp properties (e.g., strength and bleachability) in a one-pot system and is more energy efficient than the strain application of Fig. 20. Although these embodiments are shown in connection with a one-pot hydraulic cooker 86, they can also be used in one-pot steam-phase cookers.

Thus, in accordance with the present invention, advantageous methods and systems have been developed that allow for efficient and versatile control of the DOM level during kraft cooking. Although the invention has been described herein according to the present state of the art in the most practical and advantageous embodiment, it will be apparent to those skilled in the art that many modifications may be made within its scope, which should be given the widest possible interpretation. covers corresponding systems and processes.

»· · • ♦ 1 1 1 1 1 1 1 F 1 1 · F F • F 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 F 1 1 1 F. · • ♦ • • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 «» 1 1 1 1 1 1 1

Claims (16)

A continuous digester comprising at least - a substantially vertical vessel having an upper portion and a lower portion, wherein a slurry of finely divided cellulosic fibrous material to be boiled is measured in the vicinity of the upper portion and mass is measured in the vicinity of the lower portion. - a countercurrent area for continuous boiling arranged on the first level of the digester and comprising at least a first screen for removing lye from the boiling area and a return circulation for boiling connected to the first screen for feeding the removed lute into the digester in the vicinity of the first screen; a means for supplying the cooking liquor to the return circulation for boiling; and - a separating screen and its separating tube at the second vertical level of the vessel, below the first level; characterized in that it further comprises a means for adding in the return circulation for boiling also a diluent solution having a lower content of dissolved organic matter than the removed liquor in the return circulation for boiling. · · · · · · · · · · 2. A boiler according to claim 1, characterized in that it comprises at least a second boiler circulation having at least a second strainer for the boiler circulation and is arranged. 'at the boiler's third vertical level, above the first vertical level. • · · • Cooker according to claim 1 or 2, characterized in that it comprises at least one washing circulation, where white liquor is added to the fourth level of the cooker, below the second vertical level. • · • · · · · · · · · · · · · · · · · · · · · · 24 The kettle according to claim 1, characterized in that it comprises at least one washing circulation, in which white liquor is supplied to the fourth level of the kettle, below the second vertical level. The kettle according to claim 2, characterized in that it comprises at least one separating tube connected to the second kettle circulation. Cooker according to claim 5, characterized in that it comprises at least one means for adding diluent in the second cooker circulation, and a wash circulation, where white liquor is supplied to the fourth level of the cooker, below the second vertical level. Boiler according to any of the preceding claims, characterized in that water or filtrate is added with the agent intended to add diluent to be used as diluent. An electric cooker according to any of the preceding claims, characterized in that the continuous cooker is a single-vessel hydraulic continuous cooker. ··· · · · · · · · · 9. A system with a continuous two-vessel boiler which includes at least • a · · · · · - an impregnation vessel; • · *. 1 - a substantially vertical boiler having an upper portion and a lower portion, wherein a slurry of finely divided cellulose-containing material, which comes from the impregnation vessel and which is to be boiled, is measured in the vicinity of the upper portion. the portion and where the mass is measured in the vicinity of the lower portion; A countercurrent area for continuous cooking located on the first level of the cooker and comprising at least a first strainer for removing liquor from the cooking area and a return circulation for boiling connected to the first screen for re-feeding the removed liquor into the digester in the vicinity of the first screen; a means for supplying the cooked liquor to the return circulation for boiling; and - a separating screen provided with a separating tube on the second vertical level of the vessel, below the first level; characterized in that it further comprises a means for adding in the return circulation for boiling also a diluent solution having a lower amount of dissolved organic matter than the removed liquor in the return circulation for boiling. 10. A digester system according to claim 9, characterized in that it comprises a second digester circulation having at least a second digester for the digester circulation located on the third level of the digester 20, above the first vertical level. 11. System with a continuous boiler with two vessels as. ** ·. includes • · • · · - an impregnation vessel; • '25 - a substantially vertical boiler which has an upper part and a lower part and a slurry of finely divided cellulosic content. 1 material, which comes from the impregnation vessel and which is to be boiled, measured in the vicinity of the upper part and the mass of the measured out. 30. a first sieve group located within the digester for removing liquid from the digester closer to its upper portion than its lower portion; and · · ·:: for directing the removed liquid to heat recovery; 26 - a second strainer group located vertically within the digester below the first strainer group within the digester area to remove liquid from the digester area and return the removed the liquid in the return circulation for boiling into the digester in the vicinity of the second screen group; characterized in that it comprises a further means for supplying a diluent solution to the second screen group and the return circulation for boiling, which dilution liquid has a lower volume of dissolved organic matter than the liquid removed in the first screen group; and a means for supplying boiling liquid to the second screen group and the return circulation for boiling. The digester system according to claim 11, characterized in that it comprises at least a third screen group located vertically in the digester below the second screen group for removing liquid from the digester and for conduction thereof for heat recovery, and the digester is a hydraulic digger. The kettle system according to claim 11, characterized in that it comprises at least one washing screen group located in the kettle in the vicinity of its lower part and below the second screen group. ·· ♦ • · • · ··· The kettle system according to claim 11, characterized in that it is. The second screening group and the return circulation for boiling also include an outlet tube for discharging liquid from the · I. 1 return circulation for boiling before dilution is supplied. · · Mm · the return circulation for boiling with the agent intended for · · · * · 1 'addition of diluent. 30 · · · The digester system according to claim 11, characterized in that the ··· ϊ ί first sieve group also includes a return circulation for boiling to return the removed liquid into the digester in the vicinity of the first sieve group, and that the digester is a • · *** 35 hydraulic boiler. • · · ♦ · ♦ • · ♦ · 27 Cooking system according to any one of claims 11 to 15, characterized in that the diluent liquid addition agent comprises at least one tube connected to a cold-blowing filtrate coolant. · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·