WO2011096857A1

WO2011096857A1 - Continuous digester with improved heating circulation

Info

Publication number: WO2011096857A1
Application number: PCT/SE2010/050128
Authority: WO
Inventors: Anders Samuelsson
Original assignee: Metso Paper Sweden Ab
Priority date: 2010-02-02
Filing date: 2010-02-02
Publication date: 2011-08-11

Abstract

The present invention concerns an improvement of the heating circulation in a continuous digester for the production of cellulose pulp. The heating circulation withdraws treatment liquor from the vessel via a strainer section 61 arranged in the wall of the digester and after passing the withdrawn treatment liquor trough a heating device 40 is the heated treatment liquor added into the centre of the vessel via a central pipe 21a. According to the inventive arrangement and method is a part flow of the heated treatment liquor also added via nozzles arranged in the wall of the vessel close to the strainer section 61. A more uniform heating over the entire cross section of the vessel is thus possible in especially overloaded digesters.

Description

Continuous Digester with improved heating circulation

Technical Area

The present invention concerns an improvement of the heating circulation in a continuous digester for the production of cellulose pulp according to the preamble of arrangement claim 1 and method claim 10.

Description of Drawings

Figure 1 shows a conventional design of a continuous digester;

Figure 2 shows a typical prior art heating circulation;

Figure 3 shows the temperature profile obtained by the prior art heating circualtion;

Figure 4 shows a first embodiment of the invention where a part of the heated treatment liquor is added above the upper screen;

Figure 5 shows a second embodiment of the invention where a part of the heated treatment liquor is added below the lower screen;

Figure 6 shows a third embodiment of the invention where a part of the heated treatment liquor is added via blind plates in a screen row;

Figure 7 shows the temperature profile obtained by the inventive heating circulation.

The Prior Art

Figure 1 shows a typical design of a conventional continuous digester. The cellulose material CH|_N is fed to the top of the digester vessel with additional charge of cooking chemicals WL. Excess liquor LIQ_RET is withdrawn in a top separator via a first strainer section 60 and a pump 30.

In a first cooking circulation comprising a strainer section 61 , pump 31 , heating device 40 and a central pipe 21a is the cellulose material heated to the necessary cooking temperature and cooking chemicals WL is added.

The cellulose material is thereafter moving down in a plug flow trough the digesting zone until it reaches an extraction circulation which terminates the cooking zone.The extraction circulation comprises a strainer section 62, here with 2 screen rows, pump 32, and a central pipe 22a. A larger part of the withdrawn and used treatment liquor is extracted from the digester and sent to recovery REC, or alternatively sent to any black liquor impregnation vessel preceding the cooking vessel.

Below the strainer section 62 of the extraction circulation is a counter current washing zone established, and the cellulose material is moving down in a plug flow trough the washing zone until it reaches a wash circulation which

terminates the washing zone. The wash circulation comprises a strainer section 63, here with 1 screen row, pump 33, and a central pipe 23a. As indicated in this drawing could also the wash circulation be complemented with a heating device 41 and addition of cooking chemicals WL, which then modifies the washing zone to a cooking zone.

Finally, dilution liquid DL is added to the bottom of the digester to facilitate out feed of digested cellulose material CHOUT-

While figure 1 only discloses the basic features of conventional continuous digester, it is to be understood that the system could be modified in a number of ways. For example, the number of circulations could be more than those shown in figure 1. The heating devices 40,41 could either be heaters using direct heating with steam, i.e. mixing steam into the circulation, or heat exchangers.

Figure 2 shows the first cooking circulation according to conventional prior art in more detail. The strainer section 61 could have 3 screen rows 61a, 61b and 61c. As shown for the upper screen row the withdrawn treatment liquor is first collected behind each screen in a chamber 70, which in turns drains to a header 73 via a flow channel 71. The pump 31 evacuates the withdrawn treatment liquor via pipe 74 and control valve 75 and feeds the treatment liquor to the heating device 40 and further via pipe 21 to the central pipe 21a. The amount of heating is controlled by a flow control valve FC in a conventional manner. The treatment liquor is thus heated in this circulation and establishes a radial displacement flow of heated treatment liquor, from the outlet of the central pipe and towards the screens 61 -61c. The radial heating profile of the pulp column descending down in the vessel is shown in principle in figure 3. The highest temperature is established close to the outlet of the central pipe 21 in a zone T1 , with successively lower temperatures in zones T2, T3 and T4 respectively. When it is desired to increase the production capacity of the digester, i.e. to increase the number of tonnes of digested pulp per day, the speed of the cellulose material, typically wood chips, in form of a column of pulp down through the digester increases, while it is necessary at the same time to withdraw a greater amount of treatment liquor per unit of time for heating from the strainer section. As a production increase also leads to reduced retention time for the cellulose material in the cooking zone, must the cooking

temperature be increased in order to compensate for reduced retention time, so that similar H-factor is established in the process.

US 5,236,554 reveals a strainer design with which it is desired to obtain enhanced uniformity of treatment of cellulose material in the digester. Here is new cooking fluid enriched with chemicals and heated added via a central pipe as well as via one of four sections arranged at the periphery of the digester wall around the column of chips. Simultaneously is cooking fluid withdrawn from an opposite sector. The particular addition sector and the particular withdrawal sector of these four sectors are varied over time, such that it possible to reduce radial temperature gradients and obtain an even digestion of the chips over the cross-section of the column of chips. The addition sectors can be designed as wall sections lying next to strainer surfaces, with nozzles arranged in these wall sections. However, this technology suffers from the disadvantage that only 25% of the strainer surface seen in the direction of the circumference of the digester is actively used as withdrawal strainer at any moment in time. The technology is not suitable for withdrawal sections in which there is instead a very high demand placed on the strainers (i.e. a large volume of withdrawn cooking fluid per unit of strainer area) around the complete digester, as is the case for overloaded digesters. Another disadvantage is that this intended uniformity of treatment is not at all established over the entire passing pulp column, as addition of heated treatment liquor is only made at 25% of the circumference of the descending plug movement. When a shift to another sector for addition of treatment liquor is activated will only the following parts of the descending plug movement be added with new treatment liquor.

The technology with crosswise or alternating addition and withdrawal around the wall of the digester is a technology that is revealed also in SE 145,257 (dated 1952).

US 6, 23,808 describe another variant of addition of dilution liquors directly under a withdrawal screen. This is an alternative to prior art dilution liquor supply as shown in figure 2 of this patent, where dilution liquor is added both via a bottom scraper, vertical dilution nozzles and horizontal nozzles via feed pipes 32, 28 and 27. None of these solutions intended for solving the problem of heating the pulp.

The Aim of the Invention

The primary aim of the invention is to improve the heating capacity of a heating circulation in continuous digesters.

A second aim is that of being able to increase the production of existing digesters without requiring installation of additional screen rows for withdrawing and circulating an increased volume of treatment liquor. The costs for an additional screen row is very high, and the invention avoid the essential part of these costs.

A further aim is to reduce the temperature profile in the radial direction of the vessel, and establishment of a more continuous and uniform temperature profile.

Brief Description of the Invention

The inventive arrangement is characterised in more detail by the characteristics described in claim 1 , and the inventive method is characterised in more detail by the characteristics described in claim 10.

In general the inventive arrangement for heating the treatment liquors in a continuous digester comprises an upright vessel having an inlet for cellulose material in the top and an outlet for delignified cellulose material in the bottom of the vessel. Said vessel having a strainer section in the wall of the digester between the top and the bottom of the vessel connected to a circulation pump for withdrawing treatment liquor from the vessel. The withdrawn treatment liquor is circulated trough a heating device and back to the digester via a central pipe having an outlet in the centre of the vessel at about the level of the strainer section. According to the invention is a first part of the circulated treatment liquor diverted in a first branch line after having passed the heating device and said first branch line is connected to nozzles arranged in the wall of the digester at about the level of the strainer section. Said strainer section further having screening surfaces distributed evenly around the entire circumference of the digester wall and connected to said pump for withdrawing treatment liquors .

In a further optional embodiment could also a second part of the circulated treatment liquor be diverted in a second branch line before having passed the heating device, and wherein said second branch line is connected to an extraction line sending the second part of the circulated treatment liquor away from the vessel. This optional embodiment is used when the heating circulation implemented in a position of the digester where a withdrawal of spent treatment liquor, i.e. black liquor, is needed.

In a first embodiment of the invention are the nozzles arranged at a vertical distance above the strainer section. This embodiment is preferably, but not exclusively, implemented in a position of the digester where the treatment zone above the strainer section is operating in a con current treatment zone, i.e. where the net volume of treatment liquor is flowing downwards trough the column of pulp descending down the digester.

In a second embodiment of the invention are the nozzles arranged at a vertical distance below the strainer section. This embodiment is preferably, but not exclusively, implemented in a position of the digester where the treatment zone below the strainer section is operating in a counter current treatment zone, i.e. where the net volume of treatment liquor is flowing upwards trough the column of pulp descending down the digester.

In a third embodiment of the invention are the the nozzles arranged in the strainer section. This embodiment is preferably, but not exclusively, implemented in a heating circulation using checkered screen rows.

In the above mentioned first and second embodiments could the vertical distance between the strainer section and the nozzles be less than the diameter of the vessel at the height position of the strainer section in the vessel. Mounting the nozzles at some distance from the strainer section reduce any immediate withdrawal from the strainer section of the additional treatment liquor added via said nozzles. Said distance preferably being at least 10-30 centimeters.

In the inventive arrangement according to any of first to third embodiments mentioned above are preferably more than 4 nozzles are arranged at equidistant circumferential positions around the vessel. And more preferably are the nozzles present in such a number that the distance between neighbouring nozzles in the direction of the circumference does not exceed 2 meters.

In the above mentioned third embodiment could preferably the strainer section comprise of at least one row of screens with several screen sectors where the screen sectors have imperforate wall sections without screen surfaces between them, and wherein the nozzles are located in the imperforate wall sections of the strainer section.

In general the inventive method for heating the treatment liquors is implemented in a continuous digester comprising an upright vessel having an inlet for cellulose material in the top and an outlet for delignified cellulose material in the bottom of the digester. Said vessel having a strainer section in the wall of the digester between the top and the bottom of the vessel connected to a circulation pump for withdrawing treatment liquor from the vessel. The withdrawn treatment liquor is circulated trough a heating device and a first part of the heated treatment liquor is added back to the digester via a central pipe having an outlet in the centre of the vessel at about the level of the strainer section. In the inventive method is a second part of the heated treatment liquor having passed the heating device added in the periphery of the vessel at about the level of the strainer section while simultaneously withdrawing the treatment liquor to be heated evenly around the entire circumference of the digester wall.

In an optional mode of the inventive method could preferably a part of the withdrawn treatment liquor be extracted from the digester before being heated in the heating device. This mode of operation is preferred if the heating circulation is implemented in a position of the digester where an amount of spent cooking liquor is to be withdrawn from the digester, in order to implement a specific profile of the treatment liquor.

In a first embodiment of the inventive method is the addition of the heated treatment liquor made at a distance above the strainer section.

In a second embodiment of the inventive method is the addition of the heated treatment liquor made at a distance below the strainer section.

In a third embodiment of the inventive method is the addition of the heated treatment liquor made in the strainer section.

According to the first and second embodiments mentioned above of the inventive method is preferably the distance between the strainer section and the location of addition of the heated treatment liquor made less than the diameter of the vessel at the height position of the strainer section in the vessel.

In a preferred mode of operation of the inventive method are the additions of the heated treatment liquor made at more than 4 locations at equidistant circumferential positions around the vessel. And more preferably are the additions of the heated treatment liquor made in such a number of positions that the distances between locations for additions of the heated treatment liquor in the direction of the circumference do not exceed 2 meters.

According to the third embodiment mentioned above of the inventive method is preferably the strainer section comprising at least one row of screens with several screen sectors where the screen sectors have imperforate wall sections without screen surfaces between them, and wherein the locations for additions of the heated treatment liquor are located in the imperforate wall sections of the strainer section.

The inventive arrangement and method could be modified in a number of ways, such as;

• Number of screen rows could be any number between 1-10;

• Flows from individual screen rows in a strainer section could be handled separately, but the total effect from the strainer section should be that a liquid volume is withdrawn from at least one screen row and added to central pipe or nozzle, while a liquid volume withdrawn from at least any other screen row is added to nozzle or central pipe, i.e. the other way around.

• Heating could be made with direct addition of steam to the circulated treatment liquors, or using heat exchangers.

• Additional treatment liquors, such as conventional white liquor, oxidised white liquor, polysulfide, dilution liquors, or cooking additives such as anthraquinone could also be added to the heated treatment liquors.

• The treatment liquor withdrawn and heated could either be impregnation liquor or cooking liquor.

• Applied to all types of cooking processes using cooking liquors such as kraft, soda, sulfite or alcohols or mixtures thereof.

Detailed Description of the Invention

Figure 4 shows a first embodiment of the inventive heating circulation. The strainer section 61 in this example has 2 screen rows 61a, 61b, each having screening surfaces distributed evenly around the entire circumference of the digester wall. As shown for the upper screen row the withdrawn treatment liquor is first collected behind each screen in a chamber 70, which in turns drains to a header 73 via a flow channel 71. In the figure is shown an internal header, but the invention could equally well be applied when using external headers. The pump 31 evacuates the withdrawn treatment liquor via pipe 74 and control valve 75 and feeds the treatment liquor to the heating device 40 and further via pipe 21 to the central pipe 21a. The treatment liquor is thus heated in this circulation and establishes a radial displacement flow of heated treatment liquor, from the outlet of the central pipe and towards the screens 61a-61b. All these features mentioned are according to prior art, and the inventive heating circulation further comprises a first branch line 80. A part of the circulated and heated treatment liquor is diverted in this first branch line 80, after having passed the heating device 40. The first branch line 80 being connected via a flow control valve 81 to nozzles 83 arranged in the wall of the digester via a header 82. The nozzles in this embodiment are arranged at a vertical distance above the uppermost screen row 61a in strainer section 61. The vertical distance between the strainer section 61 and the nozzles 83 is preferably less than the diameter of the vessel at the height position of the strainer section in the vessel.

A control system(not shown) is preferably used, which distribute the heated treatment liquor to the central pipe 21a and nozzles 83 respectively by controlling the flow control valves FC in proportions such that the intended heating of the central parts, via the central pipe 21a, and outer parts, via nozzles 83, are obtained.

Preferably are more than 4 nozzles 83 arranged at equidistant circumferential positions around the vessel, and more preferably are nozzles present in such a number that the distance between neighbouring nozzles in the direction of the circumference does not exceed 2 meters.

As indicated in figure 4 could a part of the circulated treatment liquor be diverted in a second branch line REC before having passed the heating device 41 , said second branch line being connected to an extraction line sending the part of the circulated treatment liquor away from the vessel, i.e. to recovery or to a preceding impregnation vessel.

The radial heating profile of the pulp column descending down in the vessel as established with the inventive heating circulation is shown in principle in figure 7. The highest temperature is established close to the outlet of the central pipe 21 in a zone T1, with successively lower temperatures in zones T2, T3 and T4 respectively, but zones with higher temperature, T1 , is also established close to the wall of the digester wall, with secondary zones T2 and T3 with successively decreasing temperature in these peripheral regions. The zones with lowest temperature T4 is thus reduced considerably to small regions between nozzles 83. Figure 5 shows a second embodiment of the inventive heating circulation. Most details in this embodiment are similar to those of figure 4, and in this

embodiment is instead a part of the circulated and heated treatment liquor diverted in a first branch line 80, after having passed the heating device 40. The first branch line 80 being connected via a control valve 81 to nozzles 83 arranged in the wall of the digester via a header 82. In contrast to the

embodiment in figure 4 are nozzles in this embodiment arranged at a vertical distance below the lowermost screen row 61b in strainer section 61. The vertical distance between the strainer section 61 and the nozzles 83 is preferably less than the diameter of the vessel at the height position of the strainer section in the vessel.

Figure 6 shows a third embodiment of the inventive heating circulation. Most details in this embodiment are similar to those of figures 4 and 5, but in and this embodiment is instead a part of the circulated and heated treatment liquor diverted in a first branch line 80, after having passed the heating device 40. The first branch line 80 being connected via a control valve 81 to nozzles 83 arranged in the wall of the digester via a header 82. In contrast to the embodiments in figure 4 and 5 are nozzles in this embodiment arranged in the strainer section in the lowermost screen row 61b in strainer section 61. The screen rows in this embodiment consist of several square strainer sectors and imperforate wall sections 61 x, i.e. blind plates without screen surfaces between them, and arranged in a pattern that forms a chessboard pattern around the circumference of the digester (an arrangement known as staggered screens), with the nozzles 83 mounted in the centre of each blind plate. If necessary could also nozzles 83 be mounted in blind plates of more than one screen row.

The invention can be modified in a number of ways within the framework of the claims.

Claims

1. An arrangement for heating the treatment liquors in a continuous digester comprising an upright vessel having an inlet for cellulose material in the top and an outlet for delignified cellulose material in the bottom of the vessel, said vessel having a strainer section (61) in the wall of the digester between the top and the bottom of the vessel connected to a circulation pump (31) for withdrawing treatment liquor from the vessel and circulating the withdrawn treatment liquor trough a heating device (40) and adding a first part of the heated treatment liquor back to the digester via a central pipe (21a) having an outlet in the centre of the vessel at about the level of the strainer section (61) characterised i n that a second part of the circulated treatment liquor is diverted in a first branch line (80) after having passed the heating device (40) and said first branch line being connected to nozzles (83) arranged in the wall of the digester at about the level of the strainer section, and said strainer section (61) having screening surfaces distributed evenly around the entire circumference of the digester wall and connected to said pump for withdrawing treatment liquors .

2. The arrangement according to claim 1, characterised in that a part of the circulated treatment liquor is diverted in a second branch line (REC) before having passed the heating device (41), said second branch line being connected to an extraction line sending the second part of the circulated treatment liquor away from the vessel.

3. The arrangement according to claim 1 or 2, characterised in that the nozzles (83) are arranged at a vertical distance above the strainer section (61).

4. The arrangement according to claim 1 or 2, characterised in that the nozzles (83) are arranged at a vertical distance below the strainer section (61).

5. The arrangement according to claim 1 or 2, characterised i n that the nozzles are arranged in the strainer section (61).

6. The arrangement according to claim 3 or 4, characterised i n that the vertical distance between the strainer section (61) and the nozzles (83) is less than the diameter of the vessel at the height position of the strainer section in the vessel.

7. The arrangement according to any of preceding claims, cha racterised in that more than 4 nozzles (83) are arranged at equidistant circumferential positions around the vessel.

8. The arrangement according to claim / cha racterised in that the nozzles (83) are present in such a number that the distance between neighbouring nozzles in the direction of the circumference does not exceed 2 meters.

9. The arrangement according to claim 5, characterised in that the strainer section (61) comprises of at least one row of screens (61a, 61b, 61c) with several screen sectors where the screen sectors have imperforate wall sections (61x) without screen surfaces between them, and wherein the nozzles (83) are located in the imperforate wall sections of the strainer section.

10. A method for heating the treatment liquors in a continuous digester comprising an upright vessel having an inlet for cellulose material in the top and an outlet for delignified cellulose material in the bottom of the digester, said vessel having a strainer section (61) in the wall of the digester between the top and the bottom of the vessel connected to a circulation pump (31) for withdrawing treatment liquor from the vessel and circulating the withdrawn treatment liquor trough a heating device (40) and adding a first part of the heated treatment liquor back to the digester via a central pipe (21a) having an outlet in the centre of the vessel at about the level of the strainer section (61) cha racterised i n that a second part of the heated treatment liquor having passed the heating device is added in the periphery of the vessel at about the level of the strainer section while simultaneously withdrawing the treatment liquor to be heated evenly around the entire circumference of the digester wall.

11.The method according to claim 10, characterised i n that a part of the withdrawn treatment liquor is extracted from the digester before being heated in the heating device (41).

12. The method according to claim 10 or 11, characterised in that the addition of the heated treatment liquor is made at a distance above the strainer section (61).

13. The method according to claim 10 or 11, cha racterised i n that the addition of the heated treatment liquor is made at a distance below the strainer section (61).

14. The method according to claim 10 or 11, characterised i n that the addition of the heated treatment liquor is made in the strainer section (61).

15. The method according to claim 12 or 13, c ha racterised i n that the distance between the strainer section (61) and location of addition of the heated treatment liquor is made less than the diameter of the vessel at the height position of the strainer section in the vessel.

16. The method according to any of preceding claims, characte rised i n that additions of the heated treatment liquor are made at more than 4 locations at equidistant circumferential positions around the vessel.

17. The method according to claim 16 cha racterised i n that additions of the heated treatment liquor are made in such a number of positions that the distance between locations for additions of the heated treatment liquor in the direction of the circumference does not exceed 2 meters.

18. The method according to claim 14, c h a ra cte ri se d i n that the strainer section (61) comprises of at least one row of screens (61a, 61b, 61c) with several screen sectors where the screen sectors have imperforate wall sections (61x) without screen surfaces between them, and wherein the locations for additions of the heated treatment liquor are located in the imperforate wall sections of the strainer section.