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Title:
IMPREGNATION OF FIBRE MATERIAL
Document Type and Number:
WIPO Patent Application WO/1997/003243
Kind Code:
A1
Abstract:
Method of cooking chips, which are impregnated with liquid in a vessel (3) in a cocurrent impregnation zone (A) and a subsequent countercurrent impregnation zone (B), whereby impregnation liquid is supplied in a mixture together with steamed chips through a feeding system, to the vessel (3), and liquid for recovery is extracted at a first site (41), situated in the lower part of the cocurrent impregnation zone, and additional liquid is supplied to the countercurrent impregnation zone. According to the invention is also liquid extracted at a second site (43), situated in the upper part of the countercurrent impregnation zone, and is caused to circulate in an impregnation circulation (44, 45), which opens out in the centre of the vessel (3) at a site located downstreams of the said first site (41) for extracting liquid for recovery, so that a flow of liquid is created from the centre of the vessel (3) in a direction which is radial. A first fraction of the said additional liquid is supplied in the lower part of the countercurrent impregnation zone in order to establish a countercurrent in the direction towards said first extraction site, and a second fraction is supplied to the said impregnation circulation. The said first fraction is maximized so that the movement of the chips downwards in the vessel is ensured.

Inventors:
SJOELUND LENNART (SE)
Application Number:
PCT/SE1996/000808
Publication Date:
January 30, 1997
Filing Date:
June 20, 1996
Export Citation:
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Assignee:
KVAERNER PULPING TECH (SE)
SJOELUND LENNART (SE)
International Classes:
D21C1/06; D21C3/24; (IPC1-7): D21C3/24
Domestic Patent References:
WO1996007787A11996-03-14
Foreign References:
US3802956A1974-04-09
US5080755A1992-01-14
Download PDF:
Claims:
PATENT CLAIMS
1. Method of continuously cooking cellulose containing fibre material which is impregnated with liquid in a vessel (3) in a cocurrent impregnation zone (A) and a subsequent countercurrent impregnation zone (B) , with impregnation liquid being supplied, in a mixture together with steamed fibre material, to the vessel (3) through a feeding system and liquid for recovery being extracted at a first site (41) which is situated in the lower part of the cocurrent impregnation zone (A) , and with additional impregnation liquid being supplied to the countercurrent impregnation zone (B), c h a r a c t e r i z e d i n that liquid is additionally extracted at a second site (43) which is located in the upper part of the countercurrent impregnation zone (B) and is caused to circulate in an impregnation circulation (44, 45) which opens out in the centre of the vessel (3) at a site which is located downstream of the said first site (41) for extracting liquid for recovery, so that a flow of liquid is created from the centre of the vessel (3) in a direction which is principally radial; in that a first fraction of the said additional impregnation liquid is supplied in the lower part of the counter¬ current impregnation zone (B) in order to establish a countercurrent in the direction towards the said first extraction site (41), and a second fraction is supplied to the said impregnation circulation; and in that the said first fraction is maximized so that the movement of the fibre material downwards in the vessel is ensured.
2. Method according to Claim 1, c h a r a c t e r i z e d i n that the impregnation liquid which is supplied to the feeding system consists of one or more of the liquids black liquor, white liquor, green liquor, another sulphidecontaining solution and another sulphurcontaining solution.
3. Method according to Claim 1 or 2, c h a r a c t e r i z e d i n that the said additional impregnation liquid which is supplied to the countercurrent impregnation zone (B) consists of one or more of the liquids black liquor, white liquor, green liquor, liquid from a transfer circulation between the impregnation vessel (3) and a digester (4) and wash liquor.
4. Method according to Claim 3, c h a r a c t e r i z e d i n that the said additional impregnation liquid which is supplied to the countercurrent impregnation zone (B) consists of black liquor.
5. Method according to Claim 1, in which black liquor is extracted from the digester (4) and trans¬ ferred to a multiplicity of flash cyclones (30, 31) which are coupled in series, c h a r a c t e r i z e d i n that the black liquor which is supplied through the said feeding system is a part of the black liquor which is extracted from the digester or of the effluent from one of the said flash cyclones (30, 31), preferably the penultimate flash cyclone (30) .
6. Method according to Claim 1, c h a r a c t e r i z e d i n that black liquor which is supplied to the said impregnation circulation is at a temperature of 120170°C.
7. Method according to Claim 1 or 6, in which black liquor is extracted from the digester (4) and transferred to a multiplicity of flash cyclones (30, 31) which are coupled in series, c h a r a c t e r i z e d i n that the black liquor which is supplied to the said impregnation circulation is a part of the black liquor which is extracted from the digester or of the effluent from one of the said flash cyclones (30, 31) .
8. Method according to any one of Claims 17, c h a r a c t e r i z e d i n that the impregnation liquids are selected, distributed and supplied in such a manner that the HSVOH" ratio in the feeding system is as high as possible.
9. Method according to any one of Claims 17, c h a r a c t e r i z e d i n that the impregnation liquids are selected, distributed and supplied in such a manner that the HSVOH" ratio in the feeding system is higher than in the impregnation circulation.
10. Method according to any one of Claims 19, c h a r a c t e r i z e d i n that the temperature in the cocurrent impregnation zone (A) is 100140°C, preferably 120130°C, and in the countercurrent impregnation zone (B) is 120160°C, preferably 130 150°C.
11. Method according to any one of Claims 110, c h a r a c t e r i z e d i n that the dwell time of the fibre material in the cocurrent impregnation zone (A) is at least 15 minutes and in the countercurrent impregnation zone (B) is at least 10 minutes.
Description:
Impregnation of fibre material

The present invention relates to a method of continuously cooking cellulose-containing fibre material which is impregnated with liquid in a vessel in a cocurrent impregnation zone A and a subsequent countercurrent impregnation zone B, with impregnation liquid being supplied, in a mixture together with steamed fibre material, to the vessel through a feeding system and liquid for recovery being extracted at a first site which is situated at the lower part of the cocurrent impregnation zone A, and with additional impregnation liquid being supplied to the counter¬ current impregnation zone B. Pre-impregnation of chips with sulphide-con¬ taining solutions accelerates the delignification and improves the selectivity in the subsequent sulphate cooking. In this connection, the cooking can be carried through to low kappa numbers without impairing the quality of the pulp. The strength properties, especially the tear strength of pulp which has been cooked in accordance with an impregnation of this nature, are substantially improved. The improvement in the strength properties also remains or is amplified during subsequent bleaching.

Pre-impregnation of chips in countercurrent is described, for example, in SE-359 331 and SE-468 053.

A problem which is associated with counter¬ current impregnation is that the movement of the chip column downwards towards the outlet of the impregnation vessel is impeded by the impregnation liquid which is supplied and which flows upwards towards the extraction screen. The quantity of liquid which is used for the countercurrent impregnation has therefore to be reduced, with the consequence that the advantages which are associated with an impregnation of this nature decrease to a corresponding degree. At the same time, the freedom to distribute the impregnation liquids to different sites in the impregnation vessel is decreased or is completely lost. There is also a decreased

possibility of controlling certain parameters during different stages of the impregnation, such as the HSVOH " ratio in order thereby to reduce the attack of the chemicals on the carbohydrates of the hemicellulose and cellulose and reduce the content of shives in the pulp after the cooking to a level which is even lower than that which has hitherto been possible, and such as the temperature in order thereby to improve the heat economy. The object of the present invention is to provide an improved method of impregnating the fibre material so that the abovementioned problems are at least substantially reduced.

The method according to the invention is characterized in that liquid is additionally extracted at a second site which is located in the upper part of the countercurrent impregnation zone B and caused to circulate in an impregnation circulation which opens out in the centre of the vessel at a site which is located downstream of the said first site for extracting liquid for recovery so that a flow of liquid is created from the centre of the vessel in a direction which is principally radial; in that a first fraction of the said additional impregnation liquid is supplied in the lower part of the countercurrent impregnation zone B in order to establish countercurrent in the direction towards the said first extraction site, and a second fraction is supplied to the said impregnation circulation; and in that the said first fraction is maximized so that that movement of the fibre material downwards in the vessel is ensured.

Black liquor which is supplied to the said impregnation circulation is expediently at a temperature of 120-170°C. In a suitable embodiment of the invention, in which black liquor is extracted from the digester and transferred to a multiplicity of flash cyclones which are coupled in series, the black liquor which is supplied through the said feeding system constitutes a

part of the black liquor which is extracted from the digester, or of the effluent from one of the said flash cyclones, preferably the penultimate flash cyclone. Under the same conditions, black liquor which is supp- lied to the said impregnation circulation can, in the same way, expediently constitute a part of the black liquor which is extracted from the digester, or of the effluent from one of the said flash cyclones.

According to the invention, it is advantageous for the impregnation liquids to be selected, distri¬ buted and supplied in such a manner that the HS " /0H " ratio in the feeding system is as high as possible and expediently higher than in the impregnation circu¬ lation. According to the invention, it is expedient for the temperature in the cocurrent impregnation zone A to be 100-140°C, preferably 120-130°C, and in the counter¬ current impregnation zone B to be 120-160°C, preferably 130-150°C. It is furthermore expedient, according to the invention, for the dwell time of the fibre material in the cocurrent impregnation zone A to be at least 15 minutes and that in the countercurrent impregnation zone B to be at least 10 minutes. In that which follows, the invention is described in more detail with reference to the drawings.

Figure 1 diagrammatically shows a flow diagram of a plant for the continuous cooking of cellulose-con- taining fibre material which is impregnated in accor¬ dance with a first embodiment of the present invention. Figure 2 shows a plant which is similar but which is modified for impregnating in accordance with a second embodiment. The plant which is shown diagrammatically in

Figure 1 includes a vertical steaming vessel 1, a hori¬ zontal steaming vessel 2, a vertical impregnation vessel 3 and a vertical digester 4. The fibre material, which consists of chips, for example, is fed through a

line 5 to the vertical steaming vessel 1, to which low- pressure steam is supplied through a line 6 for the purpose of heating the chips and decreasing their con¬ tent of air. Air which is driven off is removed through a line 7 which is connected to the horizontal steaming vessel 2. This preliminary steaming is carried out at atmospheric pressure. The heated chips are metered with a chip meter which is arranged in a connection 8 between the two steaming vessels, which connection 8 also contains a low-pressure feeder 9 which passes the chips into the horizontal steaming vessel 2, in which the pressure is 1-1.5 bar overpressure. From the pressure-steaming vessel 2, the chips fall down into a chute 10 in the lower part of which a high-pressure feeder 11 is mounted. A predetermined level of liquid is maintained in the chute 10. The high-pressure feeder 11 is provided with a rotor which possesses pockets, with one pocket always being in a low-pressure position, in order to be in open communication with the steaming vessel 2, and one pocket always simultaneously being in a high-pressure position, in order to be in open communication with the impregnation vessel 3 via a feeding line 12, which is connected to the top of the impregnation vessel 3. Liquid in a circulation loop 14 which is provided with a pump 13 feeds the chips from the chute 10 into the high-pressure feeder 11 so that one of the pockets of the rotor is filled.

A return line 15 connects the upper part of the impregnation vessel 3 to the high-pressure feeder 11 for the return of liquid, which is separated off by means of a top separator 19 which is arranged in the impregnation vessel 3. The feeding line 12 and the return line 15 form a feeding system with a loop for circulation of liquid with the aid of a pump 16 which is arranged in the return line 15. When a filled rotor pocket comes into the high-pressure position, i.e. in direct communication with the circulation loop 12, 15, it is flushed clean by the return liquid from the return line 15.

The circulation loop 14 is connected, via a line 17, to a level tank 18 which, in its turn, is connected to the return line 15 via a line 20.

At its bottom, the impregnation vessel 3 has an outlet 21 for the impregnated chips, from which outlet the chips are transferred to the top of the digester 4 via a feed line 22. A screen 23 is arranged at the top of the digester 4 for the purpose of separating a certain quantity of liquid, which liquid is returned to the bottom of the impregnation vessel 3 via a return line 24 which contains a pump 25 for pumping the chips to the digester with the aid of the separated liquid. A heat exchanger 55 is also present in the line 24. The feed line 22 and the return line 24 form a circulation for transferring the suspension of chips and cooking liquid.

The digester 4 has upper, central and lower extraction screens 26, 27, 28 for extracting liquor at different levels. The central extraction screen 27 is connected, by a line 29, to a first flash cyclone 30, which is connected to a second flash cyclone 31 via a line 32 and to the said level tank 18 via a line 33. Effluent from the second flash cyclone 31 is conducted via a line 34 to a recovery plant (not shown) . The steam which is formed in the flash cyclones 30, 31 is conducted through the line 35 and the line 6 to the chute 10 and the steaming vessel 1, respectively. The lower extraction screen 28 is connected to a line 36, which is provided with a pump 37 and heat exchanger 58 and which extends to the upper part of the digester in order to connect there with a central pipe 38 which opens out below the lower extraction screen 28. This circulation increases the rate of flow of the black liquor, thereby facilitating discharge of the cooked chips via an outlet 39 which is arranged at the bottom of the digester 4. The cooked pulp is conducted away for subsequent treatment through a line 40.

The impregnation vessel has a first extraction screen 41, which is arranged at the centre of the

impregnation vessel 3 or directly below the centre, for extracting liquid, which is conducted away, via a line 42, to the second flash cyclone 31. At some distance from the bottom of the impregnation vessel 3, and at a short distance below the first extraction screen 41, there is a second extraction screen 43 for extracting liquid in a circulation loop which consists of a line 44, which extends to the upper part of the impregnation vessel 3, and a central pipe 45 to which the line 44 is joined, with the line 44 containing a pump 46 for circulating liquid through the line 44 and the central pipe 45. The central pipe 45 opens out at the upper end of the extraction screen 43. The pumped liquid flows out of the central pipe at high speed and in the main radially outwards towards the screen surfaces of the extraction screen.

White liquor is supplied to the system, from a store, via a main line 47, which is firstly connected, by a line 48, to the line 36 for supplying a certain quantity of white liquor to the discharge circulation of the digester, is secondly connected, by a line 49, to the return line 24 for supplying a certain quantity of white liquor to the transfer circulation between the impregnation vessel 3 and the digester 4, is thirdly connected, by a line 50, to the lower part of the vessel 3 for supplying a certain quantity of white liquor to the final phase of the impregnation, and is fourthly connected, by a line 51, to the chip outlet of the high-pressure feeder 11, which outlet is connected to the feeding line 12.

Black liquor is fed to the feeding circulation from the penultimate flash cyclone 30, which is the first in the embodiment shown, through the line 33 to the level tank 18 and, from there, through the line 20 to the return line 15. In addition, black liquor is transferred, in a predetermined quantity, from the central extraction screen 27 of the digester to that part of the impregnation vessel 3 which is situated below the first extraction screen 41, which transfer

takes place via a line 52 which is provided with a pump 57 and which is coupled to the line 29, and which firstly has a branch 60, which is connected to the lower part of the vessel 3 close to the outlet 21, and secondly has a branch 61, which is connected to the circulation line 44.

The impregnation of the chips in the impreg¬ nation vessel 3 takes place firstly in cocurrent and then in countercurrent. The impregnation liquid which is fed in at the top consists of warm black liquor and a small quantity of white liquor. If so desired, warm green liquor, modified green liquor or another sul¬ phide-containing or sulphur-containing solution can also be included in the impregnation liquid. The liquid/wood ratio of the material which is fed in at the top is 2.5-4.0 or greater. The impregnation vessel 3 is divided into a cocurrent impregnation zone A and a countercurrent impregnation zone B. The dwell time for the chips is at least 15 minutes in the cocurrent impregnation zone A and at least 10 minutes in the countercurrent impregnation zone B, with it being pos¬ sible for the total dwell time to be at least 25 minutes. The temperatures in the two impregnation zones A and B can be the same or different and are within the intervals 100-140°C and 120-160°C, respectively. For reasons of heat economy, it is advantageous to have a higher temperature in the countercurrent impregnation zone B. At the end of the cocurrent impregnation zone A, liquid is extracted and transferred to the final flash cyclone 31 via the line 42.

White liquor and hot black liquor, which is transferred from the extraction screen 27 of the digester, are supplied, with the aid of the impreg¬ nation circulation, to the previously impregnated chips, which are moving downwards and part of whose liquid content has been extracted immediately before¬ hand. The impregnation circulation gives rise to an elevated flow of liquid through the chips, when circu¬ lated liquid, which is supplemented with hot black

liquor and white liquor, flows out in the centre of the impregnation vessel 3 on a level with the circulation screen 43, which liquid flow is in a direction which is in the main radial. The circulation flow, with such a radial displacement of liquid, serves to distribute and equalize the white liquor which is continuously added to the impregnation circulation, as well as the black liquor, which is supplied at the same time for con¬ tinued and final impregnation of the chips in the countercurrent impregnation zone B. This thereby ensures a very uniform alkali and temperature profile in the countercurrent impregnation zone B. A first fraction of the additional impregnation liquid, which, in the example shown in Figure 1, consists of black liquor and white liquor, is supplied to the lower part of the countercurrent impregnation zone B via the line 60 while the remaining, second fraction is supplied to the impregnation circulation via the line 61. The first fraction is adjusted to such a level that the chips are not impeded to a detrimental extent in their movement downwards when liquid in countercurrent is simul¬ taneously extracted via the extraction screen 43. Thus, there is a maximum limit for the said first fraction in each individual operation case. At the same time, there is nevertheless the opportunity of adding, via the impregnation circulation, the necessary quantity of additional impregnation liquid to the countercurrent impregnation zone in the form of the said second frac¬ tion. In the impregnation procedure which has been described, an elevated, favourable HS " /0H " ratio can be maintained in the cocurrent impregnation zone A. Having a high HS " content at the same time as the OH " content is low renders it possible to achieve maximum sorption of sulphide ions in the chips while the attack on the carbohydrates of the hemicellulose and the cellulose is kept at a low level. In the second stage of the impregnation, i.e. in the countercurrent impregnation zone B, alkali is added so that the HS " /OH " ratio is

decreased, thereby ensuring that the shives content of the pulp after the cooking is at a lower level than that obtained when no such regulation of the said ratio takes place. It is also possible to have different temperatures in the two zones when this procedure is used. The temperature can be low in the first stage, while the temperature in the lower zone B is raised with the aid of hot black liquor. The heat economy is also improved by directly heating the chips with hot black liquor in this way.

The plant which is shown diagrammatically in Figure 2 is similar to that in Figure 1 with the sole exception of the liquid which is supplied to the countercurrent impregnation zone. According to this second embodiment, a line 53 is coupled to the return line 24 and bifurcates to the line 44 and the bottom of the vessel 3 for supplying transfer liquid instead of black liquor to the impregnation circulation and the bottom of the vessel, respectively. The choice between the two embodiments depends on the heat economy requirements. The quantity of the liquid which is extracted through the screen 41 is greater than that of the free liquid in the cocurrent impregnation zone A in order thereby to secure a counterflow of liquid from the vessel space below this screen 41.