DE69800767T2 - METHOD AND DEVICE FOR CONTINUOUS CELLULAR COOKING - Google Patents

Abstract

This invention relates to a new and improved way of continuously cooking fibre material, wherein temperatures and alkaline levels are controlled to be maintained within specific levels in different zones of the digesting process in order to optimise chemical consumption and heat-economy and at the same time achieve very good pulp properties. The fibre material is heated and impregnated by the aid of black liquor to a temperature less than 140 DEG C and passed in con-current in an impregnation zone (A). The heated and impregnated fibre material is transferred to a con-current cooking zone (B) in the upper part of a digester (6). At least 60 % of the quantity of liquor (F) required for the cooking reaction is supplied in the up-stream end of the con-current cooking zone (B) in order to obtain a level of effective alkali exceeding 35 g/l. The cooking temperature in the con-current cooking zone (B) is lower than 160 DEG C. Spent liquor (E), with an amount of effective alkali of at least 13 g/l, is withdrawn from a strainer girdle (8). A substantial part of the spent liquor is supplied to the impregnation zone.

Description

Technical area

The present invention relates to a new process for producing pulp, preferably sulphate cellulose, using a continuous cooking process. The process is aimed at exploiting the advantages associated with soaking the chips with hot black liquor in both steam/liquid phase cookers and hydraulic cookers.

State of the art

Environmental requirements in this area of technology have inevitably led to the development of improved cooking and soaking processes. A breakthrough was recently achieved with the company's own ITC~ concept, developed in 1992-1993. The ITCTTM concept is described in WO-A-9411566, which shows that very good results in terms of pulp quality were achieved. The ITCTM concept is mainly based on using practically the same temperature (which is relatively low compared to the state of the art) in all cooking zones in combination with moderate alkali levels. The ITCTM concept not only relates to the equalization of temperatures between different cooking zones, but also makes a significant contribution in that it also enables an equalized alkali profile in the lower part of the countercurrent cooking zone.

In addition, it is known that impregnation with black liquor can improve the strength properties of the fibres in the pulp produced. The impregnation serves primarily to thoroughly soak all the chips so that they are exposed by penetration and diffusion to the active cooking chemicals which are present in the sulphate cellulose. consist mainly of sodium hydroxide and sodium sulfide.

If a large proportion of white liquor is added in connection with the impregnation, as is common in the art (see e.g. US-A-5080755), there is no clear boundary between impregnation and cooking. This leads to difficulties in optimizing the conditions in the transfer zone between impregnation and cooking. In addition, the addition of white liquor during impregnation can sometimes lead to a deterioration in the strength properties of the pulp produced.

Two other cooking processes are known from WO-A-9518261 and WO-A-9607786, in which hot black liquor is fed to the impregnation zone. However, in both of these processes the use of countercurrent impregnation zones is proposed. To date it has not been possible to achieve positive results using a countercurrent impregnation zone, which is probably due to the fact that the precise control and optimization of the conditions in a countercurrent vessel is extremely difficult.

Since of these last two prior art documents WO-A-9607789 appears to show the closest prior art, the independent claims have been delimited with regard to the prior art described therein. The method according to WO-A-9607786 is also disadvantageous in that an insufficient amount of the withdrawn black liquor is fed to the impregnation tank and that it is fed to a location which is undesirable for achieving optimum conditions during impregnation if the withdrawn black liquor has a high residual alkali content. As a result, the consumption of chemicals may be undesirably high and/or the quality of the pulp may be lower than desired. In addition, WO-A-9607786 does not show how a clear boundary between impregnation and cooking can be achieved, which leads to further difficulties. in achieving low chemical consumption and/or high quality pulp. Furthermore, WO-A-9607786 does not teach a desirable solution regarding preferred temperature ranges or alkali contents, nor where to take the spent liquor in order to be able to carry out high alkaline cooking in an optimized manner.

Solution and benefits

It has now been found that, according to the invention, surprisingly good results can be achieved if:

the entire impregnation zone is a co-current zone, a substantial part of the withdrawn black liquor is fed near the upstream end of the impregnation zone with respect to the liquid flow of the co-current impregnation zone, the amount of black liquor fed near the upstream end of the impregnation zone (A) is more than 6 m³/ADT, so that the liquid/wood ratio in the impregnation zone (A) is between 5/l and 10/l, a substantial liquid exchange is carried out between the impregnation zone (A) and the co-current cooking zone (B), wherein after the separation and withdrawal of free liquid in the slurry no more than 1.5 m³/ADT remains as free, non-exchanged liquid,

waste liquor in an amount exceeding 4 m³/ADT, taken from the free liquid stream (D) separated at the end or after the end of the impregnation zone (A), is recycled,

and by the combination that the cooking temperature (T1) in the direct current cooking zone (B) is below 160ºC, the effective alkali content (EA1) is above 35 g/l and the amount of effective alkali in the withdrawn black liquor is at least 13 g/l.

This new process leads to a reduced H-factor requirement, reduced consumption of cooking chemicals and better heat economy. In addition, the new process leads to the production of high quality pulp with very good bleachability, which means that a wider variety of bleaching chemicals and processes can be selected than before to achieve the desired quality objectives (brightness, yield, tensile strength, viscosity, etc.) of a final bleached pulp.

The invention also relates to a preferred embodiment of a two-vessel digestion system for carrying out the new process. Even in conjunction with single-vessel type digesters (without a separate impregnation vessel), surprisingly good results are achieved when the basic principles of the present invention are applied.

Furthermore, potentially expensive equipment can be eliminated, e.g. knot catchers in the watering tank, hanging central pipes, etc., which makes a device much simpler and significantly cheaper.

Further advantages of the invention will become apparent from the following description.

Description of the characters

The following refers to the attached figures. They show:

Fig. 1 shows a preferred embodiment of a steam/liquid phase cooker arrangement according to the invention with two containers,

Fig. 2 shows a preferred head separator for use in a steam/liquid phase cooker according to the invention,

Fig. 3 shows a preferred head separator for use in a hydraulic cooker according to the invention,

Fig. 4 shows a preferred embodiment of a hydraulic single-container cooker according to the invention,

Figure 5 is a diagram showing the advantages regarding the H-factor when applying the present invention,

Fig. 6, which conditions were used in the laboratory for one of the ITC references and one of the cookings according to the invention (so-called modified ITC),

Fig. 7 is a schematic flow diagram of a second steam/liquid phase cooker system according to the invention with two vessels,

Fig. 8 is a schematic flow diagram of a steam/liquid phase cooker system according to the invention with a container,

Fig. 9 is a schematic flow diagram of a hydraulic digester system according to the invention with two containers,

Fig. 10 is a schematic flow diagram of a further hydraulic cooker system according to the invention with a container,

Fig. 11 Test data relating to peroxide consumption and whiteness for the inventive process compared to a conventional process,

Fig. 12 Test data relating to the tensile strength index and the tear strength index for unbleached pulp according to the process in question compared to a conventional process,

Fig. 13 Test data relating to the tensile strength index and the tear strength index for unbleached pulp according to the process in question compared to a conventional process,

Fig. 14 Test data relating to C1 loading and whiteness for the process according to the invention compared to a conventional process,

Fig. 15 shows a further embodiment according to the invention,

Fig. 16 shows a further embodiment of the invention,

Fig. 17 shows a further embodiment according to the invention,

Fig. 18 shows a further embodiment of the invention,

Fig. 19 shows a further embodiment according to the invention,

Fig. 20 shows a further embodiment of the invention,

Fig. 21 shows a further embodiment of the invention,

Fig. 22 shows a further embodiment of the invention, and

Fig. 23 a preferred embodiment of the inlet of a hydraulic tank according to the invention.

More detailed description

Fig. 1 shows a preferred embodiment of a steam/liquid phase digester according to the invention with two containers for producing pulp. The digestion system has an impregnation container 1 and a steam/liquid phase digester 6 as main components.

The impregnation tank 1, which is normally completely filled with liquid, has a conveyor device 2 of a conventional type at the top, i.e. a top separator with a screw conveyor device, which simultaneously transports the chips downwards while removing the transport liquid. At the bottom, the impregnation tank has a discharge device 3 with a bottom scraper. In addition, there is also a line 17 for supplying hot black liquor. As can be seen, the black liquor is preferably supplied at the top of the impregnation tank. In contrast to conventional impregnation tanks, there is no discharge screen on the impregnation tank. The chips are supplied from the chip silo 20A via the steaming tank 20B and the chip shaft 20C and finally by a conveyor device, preferably a high-pressure conveyor device 19, via a line 18 fed to the head of the impregnation tank 1. The conveying device 19 is arranged in a known manner to form a shaft and is connected to the necessary liquid circulation and refilling.

A line 21 for transporting chips leads from the bottom of the impregnation tank 1 to the head 5 of the cooker 6 with a steam space, the liquid level being indicated by the dashed line. A steam supply line at the head ensures that the steam space is heated. The line 21 opens into the bottom of the head separator 7, which conveys upwards with the aid of a screw. The sieve of the separator is used to withdraw the liquid D (which is returned in line 15), together with which the chips are transported to the head. An integrated annular ring 23 is arranged on the upper edge of the sieve (over which the chips fall). The annular ring 23 is connected to a line 24 which leads (preferably via a heat exchanger 13A) to a white liquor tank (not shown). A sieve belt section 8 is arranged in connection with a projection approximately in the middle of the digester 6. The discharge from this sieve belt section 8 can be fed directly to the impregnation tank 1 via line 17. Preferably, however, the black liquor is discharged via line 28 to a first flash cyclone 9.

At the bottom 10 of the cooker there is a discharge device with a scraper element 22.

According to a preferred embodiment, "cold blowing" is carried out, whereby the temperature of the pulp at the bottom of the digester is reduced by means of relatively cold (preferably 70-80ºC) liquid (washing liquid) which is fed at the bottom by means of the scraper element 22 and/or other liquid feed devices 25 (preferably ring-shaped pipes) and then subsequently fed upwards in countercurrent. With regard to the production of pulp of high To produce low and equal kappa quality fibres, it is essential that the chemicals and heat are evenly distributed throughout the digester so that all fibres in the column are treated under the same conditions. This is achieved by means of a lower circulation 11, 12, 13, 14, a so-called ITCTM circulation. This lower circulation consists of a screen belt section 12 (which in the embodiment shown consists of three rows) located high enough above the lower liquid feed point 22 and/or 25 to enable a desired flow to be achieved from the latter liquid feed point to the screen section 12. The effluent from the screen belts 12 is returned to the digester (for displacing black liquor countercurrent to the discharge screen 8) by means of a central pipe 14 (or alternatively a standpipe from the bottom of the digester), the central pipe 14 opening approximately level with the screen belt section 12. In the line 11 connecting the sieve belt 12 to the pipe 14, a heat exchanger 13 for temperature regulation (which increases the temperature of the liquid fed back in) and a pump are also arranged. The return loop 11 is also connected to the white liquor supply via a branch line 27 so that fresh alkali can be supplied and the kappa number can be further reduced in the form of countercurrent cooking. The cooker structure described is characterized by the absence of several central pipes arranged from above and hanging down, as well as feed lines connected to them and other parts necessary for the circulation.

A preferred device according to the invention functions as follows. The chips are fed in the usual way to a chip silo 20A, then steamed 20B and then fed to a shaft 20C. A high pressure conveyor 19 (which in a known way is used for lubrication with a small amount of white liquor (~5% of the total amount) fed), with the aid of which the chips are fed together with transport liquid to line 18. The slurry of chips and liquid fed in this way to the top of the impregnation tank has a temperature of approximately 110 to 120ºC when it enters the impregnation tank (excluding recirculated transport liquor).

In addition to the actual fibers in the wood, the latter also transports its own moisture (the wood moisture), which normally amounts to about 50% of the original weight, into the impregnation tank. In addition, some condensate from steaming is also present, i.e. at least part of the steam (mainly low-pressure steam) that was fed to the steaming tank 20B is cooled to such an extent that it condenses and is then recovered as a liquid together with the wood and the transport liquid.

In the head of the impregnation tank there is a screw conveyor 2 which pushes the chips from top to bottom. As usual, no liquid is recirculated in the impregnation tank. Instead, liquid is fed from a point after the first flash 9.

The chips discharged from the bottom of the head sieve 2 then move slowly downwards through the impregnation tank 1 in a plug flow, the liquid/wood ratio being 5/1 to 10/1. Hot black liquor drawn off from the first flash 9 is fed via line 17 to the top of the impregnation tank 1. The high temperature of the black liquor (100 to 160ºC), which is preferably above 130ºC and particularly preferably between 130 and 160ºC, ensures rapid heating of the chips. In addition, the relatively high pH value of the black liquor, which is above pH 10, neutralizes acid groups in the wood and any acid condensate accompanying the chips, thereby counteracting the formation of incrustations, the so-called stone formation. A further advantage of the process is that the black liquor fed to the impregnation tank has a high residual alkali content (EA as NaOH) of at least 13 g/l, preferably about 16 g/l or more and most preferably 13 to 30 g/l at the top of the impregnation tank. This alkali comes mainly from the black liquor due to the high alkali content in the co-current zone of the digester. Furthermore, the strength properties of the fibers are positively influenced by the impregnation due to the high amount of sulfide. The majority of the black liquor is fed directly (or via a flash) to the impregnation tank 1. A small amount of black liquor can be used to convey the chips from the high pressure conveyor to the inlet of the impregnation tank. This small stream must then be cooled (not shown) before it enters the conveyor. The two black liquor streams are preferably used to regulate the temperature in the impregnation zone, which would normally not exceed an average temperature of 140ºC. The total supply of black liquor to the impregnation tank is more than 80% of the amount withdrawn by the withdrawal knot catchers 8, preferably more than 90% and optimally about 100% of the total flow, which is normally about 8 to 12 m3/ADT.

The chips, thoroughly soaked and partially delignified in the impregnation tank, are fed to the top of the cooker 6 and conveyed into the upwardly conveying top separator 7. The chips are thus guided upwards through the sieve, while free transport liquid is drawn out through the sieve, and finally fall over the edge of the sieve and down through the steam space. Before or during their free fall, the pieces of chips are dewatered with cooking liquor, which is fed with the aid of the top separator 7. The white liquor is preferably heated with the aid of a heat exchanger 13A, which is preferably supplied with hot steam from the flash tank 9. The here The amount of white liquor added will depend on how much white liquor may be added elsewhere, but the total amount will be the amount of white liquor required to achieve the desired delignification of the wood. Preferably a large proportion of it is added here, i.e. more than 60%, which also improves the rate of diffusion as this increases with the concentration difference (chips - surrounding liquid). The thoroughly soaked chips absorb the active cooking chemicals particularly quickly by diffusion as the alkali concentration (EA as NaOH) is relatively high, amounting to at least 20 g/l, preferably between 30 g/l and 50 g/l, and most preferably about 40 g/l. The chips then move cocurrently downwards through the cooker 6 at a relatively low cooking temperature, i.e. between 130 and 160ºC, preferably about 140 to 150ºC. Most of the delignification takes place in the first direct current cooking zone.

The dwell time in this first cooking zone should be at least 20 minutes, preferably at least 30 minutes and most preferably at least 40 minutes. The liquid/wood ratio should be at least 5/l and less than 7/l. (The liquid/wood ratio in the countercurrent cooking zone should be approximately the same as that in the cocurrent cooking zone.)

The cooking liquid mixed with released lignins etc. is drawn off at the discharge screen 8. As already mentioned above, liquid is finally also introduced into the lower part of the digester, which moves in countercurrent. This can be described as the pipe 14 displacing it from the wood upwards to the discharge screen 8. This consequently leads to a prolongation of the delignification in the digester 6. The temperature in this lower zone C is preferably higher than in the cocurrent zone B in order to dissolve residual lignin, ie it is preferably more than 140ºC, preferably about 145 to 165ºC. The alkali content in the lowest part of the countercurrent cooking zone should preferably be below the alkali content at the beginning of the cocurrent zone and be more than 5 g/l but less than 40 g/l. Preferably less than 30 g/l and particularly preferably between 10 and 20 g/l. In the preferred case, a temperature difference of about 10ºC between the cooking zones is to be achieved. The lower circulation 11, 12, 13, 14 is expediently charged with about 5 to 20%, preferably 10 to 15%, white liquor. The temperature of the liquid recirculated via pipe 14 is regulated by means of a heat exchanger 13 so that the desired cooking temperature is obtained in the lowest part of the countercurrent cooking zone.

In the preferred case, "cold blowing" is used, where the temperature of the pulp in the outlet line 26 is less than 100°C. Accordingly, low temperature washing liquid, preferably about 70 to 80°C, is supplied in a known way using the scraper element and an outer annular line 25 arranged at the bottom. This liquid consequently displaces the boiling hot liquor in the pulp upwards in countercurrent and thus gives the remaining pulp a temperature at which cold blowing, i.e. relaxation and disintegration without real loss of strength, is possible.

Fig. 2A shows a preferred embodiment of a separator to be used in conjunction with one of the embodiments of steam/liquid phase digester systems described herein. For a steam/liquid phase digester, it is often preferred to have an upwardly conveying top separator. The separator may include a screen basket 61 in which a rotatable screw conveyor 62 is positioned. The screw conveyor is fixedly attached to a shaft 63 which is operatively connected at its upper end to a drive unit 30.

The drive unit 30 is attached to a plate 65 which is attached to the cooker shell 6.

A liquid collection chamber 67 is arranged around the screen basket 61, which can be connected to the return pipe circulation 15. Above the liquid collection chamber 67, a liquid supply chamber or a liquid supply opening 23 is also arranged around the screen basket 61, which is connected to the supply line 24 for supplying white liquor (F). The separator has several inlet openings 37 formed therein for treating the fiber chips with white liquor. The total area of the inlet openings is preferably more than 400 mm2. The total area of the inlet openings is particularly preferably at least 500 mm2. The total area of the inlet openings is very particularly preferably more than 600 mm2 in order to achieve a sufficient inflow into the chip pile. Between the outer peripheral wall 66 of the liquid collection chamber 67 or the liquid supply chamber 34 and the cooker casing 6 at the head there is an annular chamber 70 which opens downwards into the upper part of the cooker 6. The functioning of the head separator can be described as follows.

The thoroughly heated and soaked chips are introduced into the bottom part of the sieve basket 61 via the feed line 21. Here, the screw conveyor transports the chips upwards, while at the same time the transport liquid D is separated from the chips by being drawn out through the sieve basket 61 and further out of the cooker via the return line 15. As the chips are transported in the sieve basket 61, more and more liquid is drawn off the chips. Finally, the chips reach the height of the feed chamber 23. Here, the desired amount of cooking liquor, preferably white liquor, is fed via the feed chamber 23 and the openings 37 with a temperature according to the invention and an effective alkali content according to the invention.

To eliminate the risk of backflow of the liquid fed from the feed space 23 into the liquid collection space 67, a small amount of free liquid (at least about 0.5 m3/ADT) should remain together with the chips, which free liquid is then mixed with the input cooking liquor. Preferably, about one m3/ADT should remain together with the pulp. In addition, the white liquor should be injected at a point downstream of the flow of the suspension of pulp and free liquid fed by the screw element.

At the top of the strainer basket 61, the chips and the cooking liquor flow over its upper edge and fall into the steam/liquid space 70 and further onto the chip pile in the cooker, where the direct current cooking zone (B) begins.

A major advantage of the separators is that they ensure that a pronounced zone change occurs (they allow a virtually complete exchange of free liquid at this point), which means that in a two-vessel system one can easily set the desired conditions at the beginning of the cocurrent zone (B).

In Fig. 2B, another embodiment of a separator for use in connection with a steam/vapor phase cooker as described in Fig. 1 is shown. The separator according to Fig. 2B is almost identical to the separator according to Fig. 2A, but with the exception that a further feed chamber 25 is provided which is located below the first feed chamber 23. This further feed chamber 25 serves to enable the supply of a further liquid to the upwardly moving chip pile. In particular, it enables the supply of black liquor to ensure a minimum amount of free black liquor which remains in the chip pile after flows above, to prevent backflow of the cooking liquor fed above at 23.

In Fig. 3 a preferred embodiment of a separator for use in connection with a hydraulic digester is shown. Only a part of the head of the digester (6) is visible. The (optionally pre-soaked) slurry pulp is fed to the head of the digester via a transfer line 21 and enters an inlet space (30) of a screw conveyor (31). The screw conveyor (31) is attached to a shaft (32) which is connected to a drive unit (33) which is attached to a mounting plate (34) on the head of the digester shell (6). The drive shaft (32) is rotated so that the screw is forced to convey downwards. The screw conveyor (31) is surrounded by a cylindrical screen basket (35). The strainer basket (35) is arranged in the digester shell (6) so that a liquid collection space (36) is formed between the digester shell and the outer surface of the strainer basket (35). The liquid collection space (36), which is preferably annular, communicates with a line (17) for withdrawing liquid from the liquid collection space (36) which in turn is replenished with liquid from the slurry in the strainer basket (35). Most of the free liquid in the slurry entering the strainer basket is withdrawn into the liquid collection space (36), but a small amount of free liquid of at least 0.5 m3/ADT should not be withdrawn from the slurry. Near the outlet end of the screen basket (35) there is a liquid supply device (37) which preferably has an annular distributor ring which opens into the chip pile for supplying liquid to the pulp moving downwards in the digester (6). The liquid supply device (37) is refilled via line 38, in which a desired liquid is supplied. If it is a two-vessel digester system the liquid supplied by the liquid supply device (37) would be hot cooking liquor with a relatively large amount of effective alkali to enable a direct current cooking zone (B) to be set at a desired temperature of about 145 to 150ºC and a desired effective alkali content, e.g. about 45 g/l. The invention is also suitable for use in connection with single vessel cookers of both types, as will be demonstrated by examples below.

A major advantage of both types of separators shown is that they provide a pronounced zone change (they allow a virtually complete exchange of free liquid at this point), which means that in a two-vessel system one can easily set the desired conditions at the beginning of the cocurrent zone (B).

In Fig. 4 a preferred embodiment for applying the invention to a hydraulic digester with a tank is shown. Since the same type of basic devices as shown above and in connection with the high pressure conveyor in Fig. 1 are used here, these will not be described in detail here. Furthermore, the head of the digester 6 has attached to it essentially the same type of head separator arrangement as in Fig. 3. In the middle part of the digester there are arranged withdrawal knot catchers (8). The lowest part of the digester is in principle similar to the digester shown in Fig. 1 and has a feed line (25) for washing liquid and a blow line (26) for the digested pulp. At a short distance above the bottom there is a knot catcher arrangement (12) for withdrawing liquid which is heated and mixed with some white liquor, preferably about 10% of the total, and recirculated by means of a standpipe (39) which opens at about the same height as the lowest knot catcher belt (12). In the upper part of the Two further sets of knot catcher (40, 41) are arranged in the digester. The upper knot catcher (40) is arranged in such a way that liquid which has passed through the impregnation zone (A) is drawn off. Part of the drawn off liquid (D) is discharged via line 46 into a flash tank 47. The other part of the drawn off liquid is recirculated for re-introduction of drawn off liquid by means of a central pipe (42A) which opens at a height close to the knot catcher (40). Before re-introduction, the liquid drawn off from the knot catcher (40) can be mixed with white liquor via a feed line (43A), after which the liquid is heated to the desired temperature by means of the heat exchanger (44A).

The second knot catcher (41), which is located between the upper knot catcher (40) and the discharge knot catcher (8), also forms part of a recirculation. The withdrawn liquid is recirculated for re-introduction using a central pipe (42B) which opens at a height close to the knot catcher (41). Before being re-introduced into the main part, the liquor withdrawn from the knot catcher (41) is mixed with white liquor via a supply line (43B), after which the liquid is heated to the desired temperature using the heat exchanger (44B).

The digestion takes place in a digester according to Fig. 4 as follows. The slurry of chips and transport liquid is fed to the top of the digester, for example with the help of a high-pressure conveyor in the feed line (21), where it is fed into the top of the sieve basket (35) of the separator, in which the majority of the transport liquid is separated from the chips. At the lower end 37 of the separator, impregnating liquor E is fed via line 38. The impregnating liquor is hot black liquor, which is fed from the discharge sieve (8) via a flash tank 9 with the help of a feed line (38). If the entire desired quantity cannot be withdrawn via line 46 to the flash tank 47, provision is also made for withdrawal from the outlet of the first flash tank 9 via line 45. A small quantity of the black liquor withdrawn from the flash tank 9 can be used to convey the chips from the high pressure conveyor to the inlet of the digester 6. This by-stream must be cooled in a cooler 80 therein before entering the conveyor. The two black liquor streams are preferably used to regulate the temperature in the impregnation zone, which should normally not exceed an average temperature of 140ºC.

The amount of effective alkali in the black liquor E supplied is relatively high and is at least 13 g/l, preferably about 20 g/l, which ensures that the soaking zone (A) is established without any significant additional supply of white liquor at this point. The chips are then soaked and heated as they move downwards to the upper screen (40) where waste liquor (D) is withdrawn and fed via a line (46) to a flash tank (47). The heating of the chips and the supply of alkali are carried out by means of the cooking circulations (40, 42A, 43A, 44A, 41, 42B, 43B, 44B) described above to achieve the desired cooking conditions. In a preferred mode of operation, the temperature at the start of the direct current zone is about 145 to 160ºC for softwood and about 140 to 155ºC for hardwood with an alkali content of about 30 to 50 g/l. Due to the exothermic reaction of the chemicals, the temperature rises a little further as the pulp moves downwards in the direct current cooking zone (B).

A liquid with a relatively high content of effective alkali is added to the knot catchers (8) located near the centre. The alkali content of this extracted waste liquor (E) would normally be above 15 g/l.

In addition, lye is also removed from the countercurrent zone (C) at this knot catcher (8), since the lye supplied with the help of the standpipe (39) moves upwards in the countercurrent and finally reaches this knot catcher (8).

In the countercurrent zone (C) a higher temperature is preferably maintained than in the cocurrent zone (B). To this end, the liquid withdrawn from the lower discharge node catcher (12) is heated in a heat exchanger (51) before being introduced through the standpipe (39). In the preferred case, a small amount, about 10 to 15% of the total amount, of white liquor is also fed to this recirculation line in order to achieve the desired alkali concentration in the countercurrent cooking zone (C).

The pulp is cooled in the usual way using washing liquid (25) which is fed into the bottom of the digester and moves upwards in countercurrent and is then removed in the knot catcher (12). The cooled, fully digested pulp is then discharged from the digester into the blow line (26).

As already mentioned, pulp produced in this way is of higher quality and has better bleachability than pulp produced by known processes. In laboratory-scale tests it was found that in order to achieve full whiteness (about 90% ISO) about 10 kg of active chlorine can be saved compared to pulp cooked in the conventional way.

Fig. 5 shows a diagram comparing the H-factor for pulp produced by conventional ITCTM cooking and pulp produced according to the invention. The H-factor is a function of time and temperature in relation to the delignification process (degree of delignification) during cooking. The H-factor is used to control the delignification process of a digester, ie maintaining a certain H-factor leads in principle to the same Kappa number of the pulp produced (residual lignin content of the pulp) regardless of temperature variations during cooking. From Fig. 5 it can be seen that the H-factor for pulp produced according to the invention is very much lower (about 40 to 50% lower) than for pulp produced according to ITCTM. This means that for the same residence time much lower temperatures can be used to achieve a certain degree of delignification (Kappa number) and/or smaller containers can be used for cooking in a continuous digester and/or with the same type of basic equipment a lower Kappa number can be achieved and/or a higher production rate can be obtained.

The low H-factor requirement is achieved by a high alkali concentration and a low cooking temperature in the cocurrent cooking zone, which is shown in Figure 6, which represents a reference ITC" cooking (ITC 1770) and a cooking according to the invention (modified ITC* 1763). As can be seen, the temperature in the countercurrent cooking zone according to the invention is higher than in the cocurrent cooking zone, but still lower than the temperature in the countercurrent zone in the ITC reference.

Fig. 7 shows a preferred second embodiment of a two-vessel steam/liquid phase digester system according to the invention for producing pulp, particularly in relation to a retrofit of an MCC digester. The main components of the digestion system consist of an impregnation vessel 1b and a steam/liquid phase digester 6b. Some of the more important differences are described here.

A first sieve belt section 8b can be arranged on the upper middle part of the cooker 6b. If the cooker 6 is an MCC cooker, this screen section can be used to withdraw waste liquor which is fed to a recovery unit. According to the invention, the effluent from this screen belt section 8b can be fed via line 17b directly to the impregnation tank 1b. A second screen belt section 104b can be arranged below the first screen belt section 8b (in an MCC cooker the screen belt section 104b would normally be referred to as the MCC screen). A second co-current cooking zone C is defined between sections 8b and 104b. effluent from the second screen section 104b, such as waste liquor, ie black liquor, can be fed via line 106b to a first flash tank 108b for recovery of water vapor and depressurization and then to a recovery unit 110b. Preferably, the waste liquor is also passed via line 114b through a second flash tank 112b to further reduce the pressure and temperature of the waste liquor prior to feeding to the recovery unit 110b. In the preferred second embodiment, a line 124b carries the waste liquor from the return line 15b (preferably at least 4 m3/ADT; more preferably at least about 5 m3/ADT) to the second flash tank 112b. The waste liquor from the two flash tanks 108b and 112b is then fed to the recovery unit 110b via a line 126b. The flash tanks 108b and 112b may be connected to lines 128b and 130b, respectively, to supply steam to the chip silo 20A and the steam vessel 20B.

A third lower sieve belt section 12b is arranged at the bottom 10b of the cooker 6b. A countercurrent cooking zone D is defined between the sections 12b and 104b. The belt section 12b can, for example, have three rows of sieves for withdrawing liquid, which is heated and mixed with some white liquor, preferably about 10% of the total amount of white liquor in line 24b, via a branch line 117b and then by means of a central pipe 123b, which ends at approximately the same height as the lowest knot-catcher belt 12b.

The offtake from the screen belts 12b and the white liquor from the branch line 117b are preferably passed through a heat exchanger 120b to the bottom 10b of the cooker 6b. The temperature of this offtake is slightly below the temperature in the cooking zone D (e.g. about 140ºC) since the liquid is a mixture of wash liquid and black liquor. The white liquor is fed to the screen belt section 12b in a countercurrent direction via the central pipe 123b. The white liquor provides fresh alkali and, in the form of countercurrent cooking, a further reduction in the kappa number. A line 122b is connected to the high pressure steam line 102b and supplies the heat exchanger with steam to regulate the temperature of the liquid fed via the standpipe 123b. The bottom 10b of the digester is connected to a blow line 26b for discharging the digested pulp from the digester 6b.

A selected part of the device according to the invention according to Fig. 7 can function as follows. Some of the most important differences from the embodiment according to Fig. 1 are described below. According to the preferred second embodiment, the temperature of the black liquor in the impregnation tank should be between about 140°C and about 160°C. The temperature is particularly preferably between about 140°C and about 155°C. The temperature is most preferably between about 140°C and about 150°C.

The residence time in the impregnation zone A should be at least 20 minutes, preferably at least 30 minutes and particularly preferably at least 40 minutes. However, a residence time shorter than 20 minutes, such as 15 to 20 minutes, is also possible. The volume of the impregnation container 1b can be greater than 1/11, preferably greater than 1/10, of the volume of the cooker 6b. Furthermore, after the preferred embodiment, the volume V of the impregnation container 1 is a factor of 5 above the value of the square of the maximum cooker diameter, ie V = 5D², where D stands for the maximum diameter of the cooker 6b.

After passing through the head separator, the cutlets move downwards through the cooker 6b into the direct current zones B and C at a relatively low cooking temperature, i.e. between 130ºC and 160ºC, preferably about 140ºC to 150ºC. The majority of the delignification takes place in the first and second direct current cooking zones B and C respectively.

A further modification could be to design cooking zone C as a countercurrent zone or a mixed direct current/countercurrent zone.

The temperature in the countercurrent zone D is preferably higher than in the cocurrent zones B and C for the purpose of dissolving residual lignin, i.e. it is preferably more than 140ºC, preferably about 145ºC to 165ºC. The alkali content in the lowest part of the cocurrent cooking zone C should preferably be below the alkali content at the beginning of the cocurrent zone B and be more than 5 g/l but less than 40 g/l. Preferably less than 30 g/l and particularly preferably between 10 and 20 g/l. In the preferred case, a temperature difference of about 10ºC between the first and second cocurrent cooking zones is to be achieved. The line 116b can conveniently be fed via the line 117b with about 5 to 20%, preferably 10 to 15%, white liquor from the line 24b. Below the discharge screen section 104b is the countercurrent zone D, which is defined between the screen belt section 104b and the screen belt section 12b.

The temperature of the liquid recirculated via line 123b to the sieve belt section 12b is regulated by means of the heat exchanger 120b so that the desired cooking temperature is maintained in the lowest part of the countercurrent cooking zone D.

In the lowest part of the digester, cold washing liquid is added to displace hot liquid in countercurrent, which is then drawn off at the lowest sieve belt 12b.

Fig. 8 shows a preferred embodiment of a steam/liquid phase cooker system according to the invention with a container. Some of the most important differences from Figs. 1 and 4 are described below. The chips suspended in a transport liquid D are fed from a high-pressure conveyor 19h via a line 18h to the head of a cooker 6h.

The line 18h extends from the conveyor 19h to a head 5h of the cooker 6h. The line 18h may open at the bottom of a head separator 7h which conveys upwards by means of a screw. The separator 7h is preferably identical to or very similar to the head separator 7 shown in Fig. 1 and discussed in detail above. The sieve of the separator can be used to withdraw the transport liquid D (which is then returned in a return line 15h) with which the chips are transported from the conveyor 19h to the head 5h of the cooker 6h.

A first sieve belt section 8h can be arranged directly below the separator 7h or in its vicinity. Lye can be withdrawn via a recirculation line 17h and returned to a space defined between the first sieve belt section 8h and the separator 7h. The recirculation improves the distribution of the lye in the digester.

A second screen belt section 51h is arranged below the first screen belt section 8h, so that an impregnation zone A is defined between the screen belt sections 8h and 51h. Evidence has been found that it is desirable to maintain the alkali content in the impregnation zone A with respect to black liquor at more than at least 2 g/l, preferably more than 4 g/l, which normally corresponds to a pH value of would correspond to about 11. It appears that otherwise dissolved lignin precipitates and even condenses. Waste liquor can be withdrawn from the upper screen of section 51h and fed to a second flash tank 112h via a line 11h. Waste liquor is withdrawn from a lower screen of section 51h via a line 109h and returned to the space defined above the first screen belt section 8h so that the waste liquor can be returned to the lower screen of the second screen belt section 51h via a central pipe 105h. The temperature of the waste liquor can be regulated by a heat exchanger 13h. The heat exchanger 13h is in operative engagement with a high pressure steam line 102h via a line 122h.

A cooking liquor line 24h is operatively connected to line 109h for supplying a cooking liquor, such as white liquor, to line 109h. The liquor in line 109h contains at least about 35 g/l, more preferably at least about 40 g/l, and most preferably between about 45 g/l and about 55 g/l of effective alkali.

Approximately 95% of the total white liquor supply is fed into line 24h, while the remaining 5% is fed via line 132h and line 134h to high pressure conveyor 19h for lubrication.

A third screen belt section 104h may be arranged below the second screen belt section 51h so that a co-current cooking zone B is defined between the screen belt sections 51h and 104h. Effluent from the third screen section 104h, such as waste liquor, ie black liquor, may be returned to line 17h via a line 106h. The waste liquor carried in line 106h contains about 13 g/l or more of effective alkali. A small portion of the black liquor in line 106h may be fed to a first flash tank 108h via a line 107h in order to cool the waste liquor before feeding it to a recovery unit 110h. Preferably, the waste liquor is also fed via a line 114h is passed through a second flash tank 112h to further reduce the temperature and pressure of the waste liquor before feeding it to the recovery unit 110h. The waste liquor from the two flash tanks 108h and 112h is then fed to the recovery unit 110h via a line 126h. The flash tanks 108h and 112h may be connected to lines 128h and 130h, respectively, to supply steam to the chip silo 20A and the steam vessel 20B.

A discharge device with a scraper element 22 h is provided at the bottom 10 h of the cooker 6 h. A fourth lower sieve belt section 12 h is arranged at the bottom 10 h of the cooker 6 h, so that a co-current cooking zone C is defined between the sections 104 h and 12 h. The belt section 12 h can, for example, have three rows of sieves for withdrawing liquid, which is heated and mixed with some white liquor, preferably about 10% of the total amount of white liquor in line 24 h, via a branch line 117 h and then recirculated by means of a central pipe 123 h which opens at about the same height as the lowest knot catcher belt 12 h.

The offtake from the screen belts 12h and the white liquor from the branch line 117h are preferably returned to the bottom 10h of the digester 6h via a heat exchanger 120h. The line 122h is connected to the heat exchanger 120h and supplies it with steam to regulate the temperature of the white liquor in the line 116h. The temperature of this offtake is normally about 130ºC to 140ºC. The white liquor is fed to the screen belt section 12h in a countercurrent direction via the central pipe 123h. The white liquor provides fresh alkali and, in the form of countercurrent cooking, a further reduction in the kappa number. The bottom 10h of the digester is connected to a blow line 26h for discharging the digested pulp from the digester 6h.

A preferred device according to the invention according to Fig. 8 can be described as follows. The chips are fed to the chip silo 20A, thereafter steamed in the tank 20B and then fed to the shaft 20C. The high pressure conveyor 19h, which is charged with a small amount of white liquor (approximately 5% of the total amount for lubrication of the conveyor), feeds the chips together with the transport liquid to the line 18h. The slurry of chips and liquid is fed to the head of the digester 6h and has a temperature of about 110 to 120°C when it enters the digester 6h (excluding recirculated transport liquor).

In the head of the cooker 6h there is the head separator 7h, which pushes chips upwards through the separator, after which the chips then move slowly downwards through the impregnation zone A in a plug flow, with the liquid/wood ratio being 5/l to 10/l. The liquor withdrawn from the sieve belt section 8h can be recirculated via the line 17h into the space below the head separator 7h. The chips are then thoroughly impregnated in the impregnation zone A.

At the upper segment of the screen section 51h, a waste liquor is drawn off and fed to the second flash tank 112 h. At the lower segment of section 51h, a waste liquor is also drawn off and fed back in via the central pipe 105 h with the addition of white liquor via line 24h.

The cutlets move downwards through the cooker in the DC zone B for 6h at a relatively low cooking temperature, i.e. between 130 and 160ºC, preferably around 140-150ºC. Most of the delignification takes place in the first DC cooking zone B.

The temperature in the lower countercurrent zone C is preferably higher than in the cocurrent zone B in order to dissolve residual lignin, ie it is preferably more than 140ºC, preferably about 145ºC up to 165ºC. The alkali content in the lowest part of the countercurrent cooking zone C should preferably be below the alkali content at the beginning of the cocurrent zone B and be more than 5 g/l but less than 40 g/l.

Preferably less than 30 g/l and particularly preferably between 10 and 20 g/l. In the preferred case, a temperature difference of about 10°C is to be achieved between the cocurrent zone B and the countercurrent cooking zone C. The line 116h can conveniently be fed via the line 117h with about 5 to 20%, preferably 10 to 15%, white liquor from the line 24h.

The temperature of the liquid recirculated via pipe 123h into the sieve belt section 12h is regulated by means of the heat exchanger 120h so that the desired cooking temperature is maintained in the lowest part of the countercurrent cooking zone.

Fig. 9 shows a preferred embodiment of a two-vessel hydraulic digestion system according to the invention. This embodiment is very similar to the embodiment shown in Fig. 7, except that the digester 6d is a hydraulic digester so that the digester has a downward-feeding overhead separator as shown in Fig. 3. The rest of the digestion system is virtually identical to the other embodiment. If the digester 6d is an MCC digester, the screen section 8d can be used to withdraw spent liquor which is fed to a recovery unit. Discharge from this screen belt section 8d can also be fed directly to the impregnation vessel 1d via line 17d. Below the first screen belt section 8d there can be a second screen belt section 104d (in an MCC digester the screen belt section 104d would normally be referred to as the MCC screen). Discharge from the second screen section 104d, such as waste liquor, ie black liquor, can be fed via a line 106d to a first flash tank 108d for recovering water vapor and discharging pressure and then fed to a recovery unit 110d as described in connection with Fig. 7.

Fig. 10 shows a further embodiment of the digestion system according to the invention. In detail, the digestion system is a hydraulic digester system with a container. Only the significant differences between this embodiment and the embodiments described above are listed below.

A high pressure conveyor 19e carries the chips suspended in a transport liquid D via a line 18e into the head of a cooker 6e. The line 18e can open at the head of a head separator 7e, which conveys downwards with the aid of a screw. The separator 7e is preferably identical to or very similar to the head separator 7s shown in Fig. 3 and discussed in detail above. The screen of the separator can be used to withdraw the transport liquid D (which is then returned in a return line 15e), together with which the chips are transported from the conveyor 19e to the head 5e of the cooker 6e. A first screen belt section 8e can be arranged in connection with a projection approximately in the middle of the cooker 6e. The waste liquor extracted from a lower part of the sieve belt section 8e can be fed via the line 17e to an impregnation zone A defined between the sieve belt section 8e and the head 5e of the digester 6e. The waste liquor extracted from an upper part of the sieve belt section 8e can be fed via a line 111e to a second flash tank 112e.

A cooking liquor line 24e is operatively connected to line 17e for supplying a major portion of the cooking liquor, such as white liquor, to line 17e. The liquor in line 17e contains at least about 35 g/l, more preferably at least about 40 g/l, and most preferably between about 45 g/l and about 55 g/l effective alkali. Approximately 95% of the total white liquor supply is conducted in line 24e, while the remaining 5% is fed via line 132e and line 134e to the high pressure conveyor 19e for lubrication.

Below the first screen belt section 8e, a second screen belt section 104e can be arranged. Discharge from the second screen belt section 104e, such as waste liquor, i.e. black liquor, can be returned to a head of the impregnation zone A via a line 106e. The waste liquor conducted in line 106e contains about 10 to 20 g/l of effective alkali. Part of the black liquor in line 106e can be fed to the flash tanks as described in the previous embodiments. At the bottom of the digester there is a screen belt section 12e for withdrawing a waste liquor. The temperature of this discharge is about 130 to 150°C. The temperature can depend on how much washing liquid that has penetrated to the screen is withdrawn. The white liquor is fed in countercurrent direction via a central pipe 123e to the screen belt section 12e.

A major portion of the black liquor may be fed directly (or via a flash tank) to impregnation zone A. The total feed of black liquor to impregnation zone A may be more than 80% of the amount withdrawn from a withdrawal screen belt section 104e, preferably more than 90% and optimally about 100% of the total flow, which is normally about 8 to 12 m³/ADT.

The cutlets then move downwards through the cooker 6e in the DC zone B at a relatively low cooking temperature, i.e. between 130 and 160ºC, preferably about 140 to 150ºC. Most of the delignification takes place in the first DC cooking zone B.

The temperature in the lower countercurrent zone C is preferably higher than in the cocurrent zone B in order to dissolve residual lignin, ie it is preferably more than 140ºC, preferably about 145ºC to 165ºC. The alkali content in the lowest part of the countercurrent cooking zone C should preferably be below the alkali content at the beginning of the cocurrent zone B and be more than 5 g/l but less than 40 g/l.

Preferably less than 30 g/l and particularly preferably between 10 and 20 g/l. In the preferred case, a temperature difference of about 10°C is to be achieved between the cocurrent zone B and the countercurrent cooking zone C. The line 116e can conveniently be fed via the line 117e with about 5 to 20%, preferably 10 to 15%, white liquor from the line 24e.

Fig. 11 shows results of TCF bleaching using the cooking process according to the invention (so-called "new concept") compared to a conventional reference cooking process. The present invention provides a TCF bleached pulp with extremely good bleachability, a higher brightness is achieved compared to the conventional process for the same amount of peroxide consumption and a higher brightness upper limit is also obtained.

Fig. 12 shows the tear strength index in relation to the tensile strength index. The test data for the "new concept" uses the cooking process according to the invention and the conventional cooking process uses a reference ITC cooker.

Similarly, Fig. 13 shows test data for the "new concept" (the present invention) and a reference ITC cooker. The present invention is characterized by an improved tensile strength index compared to the conventional method.

Fig. 14 shows the brightness level when applying the present invention (new concept) and for digested reference pulp (conventional process). As can be seen, the cooking process according to the invention produces a pulp that is much is easier to bleach, ie fewer chemicals are used for a given degree of whiteness.

When cooking according to the "new concept", the following significant advantages are achieved compared to conventional ITC CookingTM.

When cooking softwood:

- 10% increased tear resistance

- Unchanged grindability

- Viscosity improved by 40 units with a kappa number of 22

- Whiteness of unbleached pulp increased by 2 ISO units

- Whiteness limit increased by 0.5-1% ISO unit

- H₂O₂ consumption reduced by 15-20%

- Knot content reduced by 60%

- Splinter content reduced by 55%

- Medium pressure steam consumption reduced by 10-15% in the cooker.

When cooking hardwood:

- Tensile index improved by 8% at 500 PFI revolutions

- Tensile stiffness improved by 8% at 500 PFI revolutions

- Viscosity improved by 50 units at a kappa number of 15

- Increased whiteness limit

- H₂O₂ consumption reduced by 15%

- Knot content reduced by 55%

- Splinter content reduced by 50%

- Alkali loading reduced by 10%.

Fig. 15 shows a possibility of carrying out the process in connection with an overloaded steam/liquid cooker with two vessels. Due to the overload (normally more than 30 ADMT pulp/hour and area in square meters in the final cooking zone (C)) it is usually difficult to obtain a pronounced upward flow in the so-called countercurrent cooking zone in such a cooker. The final cooking zone C is therefore designed in the form of a Direct current final cooking zone (C). For operation according to the invention, this requires the presence or retrofitting of a drain knot catcher (12) near the bottom and the retrofitting of a drain line 99 leading from the drain knot catcher 12. As can be seen, Fig. 15 is almost identical to Fig. 1 (no retrofitting), with the exception of the drain line 99 which supplies liquid withdrawn from the lowest screen 12 to the recovery. A larger part (more than 50%) of the liquor for recovery is, however, taken from the return line D. The process results in a higher wood/liquid ratio in the impregnation zone A than in the co-current cooking zone B which in turn has a higher wood/liquid ratio than the final cooking zone C, ie w/l A > w/l B > w/l C. To achieve cold blowing (below 100 C in blow line 26) a sufficient amount of cold wash liquor 25 must be added near the bottom of the cooker. The wash liquor is usually supplied via nozzles and sometimes preferably also through the scraper 22. The wash liquor flows partly upwards and displaces the hot cooking liquor in the pulp moving downwards below the screen 12 and partly exits in the blow line 26 together with the pulp. If necessary, some wash liquor can also be added via the central pipe 14 in order to radially displace hot cooking liquor. The latter can alternatively be achieved by means of a standpipe. This method can also be used in conjunction with non-overloaded digesters where problems arise in maintaining sufficient upward flow (for which a pulp dilution factor of at least 1.5 m3/ADMT is normally recommended for countercurrent cooking) in the final cooking zone C.

Fig. 16 describes another embodiment of a hydraulic cooker according to the invention with two containers. White liquor is fed via a line 24e to a.

Return line 15e. The temperature of the white liquor can be regulated with a heat exchanger 13e, which is designed to absorb water vapor from a high-pressure steam line 102e. The return line 15e ends at a liquid exchanger 31e, which performs the same function as the head separator, i.e. it ensures a very clear change of treatment zones by separating a first liquid practically completely from the chips and then adding a second liquid, so that any undesirable mixing is avoided. The liquid exchanger 31e in turn has a middle part, which is connected to a bottom part of an impregnation container 1e via a return line 33e. A slurry of the chips and transport liquid can be fed from the bottom part of the impregnation tank 1e via a line 35e to a bottom end of the liquid exchanger 31e; after exchange of liquid, the chips are transported in a line 21e to the head of the digester 6e. A portion of the waste liquor in the return line 33e is branched off and fed via a line 137e to a second flash tank 114e for recovery.

Black liquor is withdrawn from a belt section 8e of the digester 6e and returned via a line 17e to a head section of the impregnation tank 1e. Waste liquor is also withdrawn from a sieve belt section 104e and fed via a line 106e to a first flash tank 108e.

Fig. 17 shows an embodiment of the present invention similar to Fig. 16. Only some of the most important differences will be described. This embodiment has a digester 6f which has no screen belt section at the top of the digester. Most of the spent liquor is therefore withdrawn from the digester 6f at a screen belt section 104f and part of the withdrawn black liquor is fed via a line 17f to an impregnation tank 1f. The remaining part of the waste liquor is fed to a first flash tank 108f via a line 106f.

Fig. 18 shows an embodiment of the present invention which is similar to the embodiment shown in Fig. 17. Only some of the most important differences will be described. A chip slurry is fed from a high pressure conveyor 19g into a bottom portion of a liquid exchanger 31g. This liquid exchanger serves to ensure safe operation of the high pressure conveyor while maintaining a high temperature (e.g. 130°C) at the head of an impregnation vessel 1g, which is achieved by supplying hot black liquor to a line 21g via a line 17g. After liquid exchange, the slurry is further conveyed via a line 18g to a head of the impregnation vessel 1g. Relatively cold transport fluid is returned to the high pressure conveyor 19g via a line 23g. The temperature of the transport fluid can be kept low thanks to the complete exchange of free fluid.

Fig. 19 shows another embodiment of a hydraulic digester system according to the invention with a container. The chips are fed from a chip silo 20A through a steaming container 20B to a chip chute 20C. A feeding device, preferably a high pressure conveyor 19h, feeds the chips suspended in a transport liquid via a line 18h into the head of a digester 6h. The conveyor 19h cooperates with the chute 20Ch and is connected to the necessary liquid circulations and refilling.

The line 18h extends from the conveyor 19h to a head 5h of the digester 6h. The line 18h may open at the bottom of a head separator 7h which conveys downwards by means of a screw. The separator 7h is preferably equipped with the device shown in Fig. 3 and described above. The separator screen may be identical or very similar to the head separator 7 discussed in detail. The separator screen may be used to withdraw the transport liquid D (which is then returned in a return line 15h) with which the chips are transported from the conveyor 19h to the head 5h of the cooker 6h. A first screen belt section 8h may be arranged in conjunction with a projection approximately in the middle of the cooker 6h. The waste liquor withdrawn from this screen belt section 8h may be fed via line 17h to an impregnation zone A defined between the screen belt section 8h and the head 5h of the cooker 6h. Part of the waste liquor withdrawn from the screen belt section 8h may be fed via a line 111h to a second flash tank 112h.

A cooking liquor line 24h is operatively connected to line 17h for supplying a major portion of the cooking liquor, such as white liquor, to line 17h. The white liquor and spent liquor may be heated to a suitable temperature by a heat exchanger 13h before the liquor enters head 5h. Heat exchanger 13h may be operatively connected to a high pressure steam line 102h. The liquor in line 17h contains at least about 35 g/l, more preferably at least about 40 g/l, and most preferably between about 45 g/l and about 55 g/l of effective alkali. Approximately 95% of the total white liquor supply is fed into line 24h, while the remaining 5% is fed via line 132h and line 134h to high pressure conveyor 19h for lubrication.

Below the first screen belt section 8h, a second screen belt section 104h can be arranged. Discharge from the second screen section 104h, such as waste liquor, ie black liquor, can be returned via a line 106h to a head part of the impregnation zone A. The waste liquor conducted in the line 106h contains about 10 to 20 g/l of effective alkali. A part of the black liquor in the line 106h can be returned via a line 107h to a first flash tank 108h to cool the waste liquor before feeding it to a recovery unit 110h. Preferably, the waste liquor is also passed through a second flash tank 112h via a line 114h to further reduce the temperature and pressure of the waste liquor before feeding it to the recovery unit 110h. The waste liquor from the two flash tanks 108h and 112h is then fed to the recovery unit 110h via a line 126h. The flash tanks 108h and 112h can be connected to the lines 128h and 130h, respectively, to supply steam to the chip silo 20Ah and the steam tank 20Bh.

A discharge device with a scraper element 22 h is provided at the bottom 10 h of the cooker 6 h. A third lower sieve belt section 12 h is arranged at the bottom 10 h of the cooker 6 h. The belt section 12 h can, for example, have three rows of sieves for withdrawing liquid, which is heated and mixed with some white liquor, preferably about 10% of the total amount of white liquor in line 24 h, via a branch line 117 h and then recirculated with the aid of a central pipe 123 h which opens at about the same height as the lowest knot catcher belt 12 h.

The offtake from the sieve belts 12h and the white liquor from the branch line 117h are preferably passed through a heat exchanger 120h to the bottom 10h of the cooker 6h. The high pressure steam line 102h is connected to the heat exchanger 120h and supplies it with steam to regulate the temperature of the white liquor in the line 116h. The temperature of this offtake is normally about 130ºC to 150ºC. The temperature may depend on how much washing liquid which has penetrated to the sieve is being removed. The white liquor is fed to the sieve belt section 12h in a countercurrent direction via the central pipe 123h. The white liquor provides fresh alkali and, in the form of countercurrent cooking, a further Reduction of the kappa number. The bottom 10h of the digester is connected to a blow pipe 26h for discharging the digested pulp from the digester 6h.

A preferred device according to the invention as shown in Fig. 19 can be described as follows. The chips are fed to the chip silo 20Ah, thereafter steamed in the tank 20Bh and then fed to the shaft 200h. The high pressure conveyor 19h, which is charged with a small amount of white liquor (approximately 5% of the total amount for lubrication of the conveyor), feeds the chips together with the transport liquid to the line 18h. The slurry of chips and liquid is fed to the head of the digester 6h and may have a temperature of about 110 to 120°C when entering the digester 6h (excluding recirculated transport liquor).

In the top of the cooker 6h there is the top separator 7h which pushes chips downwards, after which the chips then move slowly downwards through the impregnation zone A in plug flow, the liquid/wood ratio being 5/l to 10/l. The hot black liquor withdrawn from the screen belt section 104h can be fed via line 106h to the top of the impregnation zone A of the cooker 6h. The black liquor can also be fed to other sections of the cooker, such as an intermediate part thereof. The high temperature of the black liquor (100 to 160ºC), which is preferably above 130ºC and more preferably between 130 and 160ºC, ensures rapid heating of the chips flowing through the impregnation zone A. In addition, the relatively high pH value of the black liquor, which is above pH 10, neutralizes acidic groups in the wood as well as any acidic condensate accompanying the chips, thereby counteracting the formation of incrustations, the so-called stone formation.

A further advantage of the process according to the invention is that the impregnation zone A The black liquor supplied has a high residual alkali content (effective alkali EA as NaOH) of at least 13 g/l, preferably about 16 g/l or more and particularly preferably 13 to 30 g/l at the top of the impregnation zone A. This alkali comes mainly from the black liquor due to the high alkali content in the cocurrent zone 3 of the digester 6h. Furthermore, the strength properties of the fibers are positively influenced by the impregnation due to the high amount of sulfide. The majority of the black liquor is supplied directly (or via a flash tank) to the impregnation zone A.

The total supply of black liquor to the impregnation zone A is more than 80% of the amount withdrawn from the screen belt section 104h, preferably more than 90% and optimally about 100% of the total flow, which is normally about 8 to 12 m³/ADT.

The residence time in the impregnation zone A should be at least 20 minutes, preferably at least 30 minutes and particularly preferably at least 40 minutes. However, a residence time shorter than 20 minutes, such as 15 to 20 minutes, is also possible. The volume of the impregnation zone A can be greater than 1/11, preferably greater than 1/10, of the volume of the cooker 6h. Furthermore, according to the preferred embodiment, the volume V of the impregnation zone A should be a factor of 5 above the value of the square of the maximum cooker diameter, i.e. V = 5D², where D is the maximum diameter of the cooker 6h.

The chips thoroughly soaked and partially delignified in the soaking zone A can be fed to the top of the cooker 6h and conveyed into the downward-conveying head separator 7h. The chips are thus guided upwards through the sieve, while free transport liquid is drawn out through the separator sieve, and finally fall downwards into the cooker 6h. Before or during their free fall, the pieces of chips are dewatered with cooking liquor, such as white liquor, which is fed to the head separator 7h.

The amount of white liquor added at the head separator 7h depends on how much white liquor may be added elsewhere. The thoroughly soaked chips absorb the active cooking chemicals particularly quickly by diffusion because the alkali concentration (EA as NaOH) is relatively high and amounts to at least 20 g/l, preferably between 30 g/l and 50 g/l and most preferably about 40 g/l.

The cutlets then move downwards through the cooker for 6h in the direct current zone B at a relatively low cooking temperature, i.e. between 130 and 160ºC, preferably about 140 to 150ºC. The majority of the delignification takes place in the first direct current cooking zone B.

The liquid/wood ratio should be at least 5/l and less than 7/l. (The liquid/wood ratio in the countercurrent cooking zone should be approximately the same as that in the direct current cooking zone.

The temperature in the lower countercurrent zone C is preferably higher than in the cocurrent zone B in order to dissolve residual lignin, i.e. it is preferably more than 140ºC, preferably about 145ºC to 165ºC. The alkali content in the lowest part of the countercurrent cooking zone C should preferably be below the alkali content at the beginning of the cocurrent zone B and should be more than 5 g/l but less than 40 g/l. Preferably less than 30 g/l and particularly preferably between 10 and 20 g/l. In the preferred case, a temperature difference of about 10ºC between the cocurrent zone B and the countercurrent cooking zone C is to be achieved. Line 116h can conveniently be fed via line 117h with about 5 to 20%, preferably 10 to 15%, white liquor from line 24h.

The temperature of the liquid recirculated via pipe 123h into the sieve belt section 12 h is regulated by means of the heat exchanger 120 h so that in the lowest part of the countercurrent cooking zone the desired cooking temperature is achieved.

In laboratory-scale tests, there is evidence that it is desirable to maintain the alkali content in the A impregnation zone in conjunction with black liquor at more than at least 2 g/l, preferably more than 4 g/l, which would normally correspond to a pH of about 11. It appears that otherwise dissolved lignin precipitates and even condenses.

Fig. 20 shows an embodiment of the present invention which largely corresponds to the embodiment shown in Fig. 19. Chips and a transport fluid are pumped up in a line 18i and a line 1991 via a liquid exchanger 331 to a head section 51 of a cooker 61. The function and operation of the liquid exchanger 31g are similar to the liquid exchanger shown in Fig. 18. As already described above, liquid is exchanged in the liquid exchanger 331 before the chips enter the head section 51 of the cooker 61.

A portion of the transport liquid may be returned in the return line 151, which leads from the head section 51 to a middle section of the liquid exchanger 331 and then via a line 25i to a conveyor 19i. Via line 106i, the waste liquor withdrawn from a screen belt section 104i is directed to the liquid from 117i and to line 151. A portion of the liquor in line 106i may be fed to a flash tank 108i.

Fig. 21 also shows an embodiment of the present invention which is similar to the embodiment shown in Fig. 19. Here some of the more important differences are described. The eleventh embodiment comprises a cooker 6j with a return line 15j attached to a head portion 5j of the cooker 6j. A recirculation line 101j is in communication with the return line 15j. so that a portion of the liquid in the return line 15j can be returned to the header 5j via the line 101j. The temperature of the liquid in the recirculation line 101j can be regulated by a heat exchanger 113j which is in operative engagement with a high pressure steam line 102j. The recirculation line is used to heat the liquid from the return line 15j before the liquid is introduced. The temperature in the return line 15j must not exceed about 100ºC so that undesirable flashing does not occur in the high pressure conveyor.

Similar to the embodiments described above, a flash tank 108j is in fluid communication with a screen belt section 104j via a line 106j so that waste liquor from section 104j can be fed to the flash tank 108j. A bottom portion of the flash tank 108j has a line 103j connected thereto through which a portion of the waste liquor can be returned to a line 134j which carries some white liquor from the cooking liquor line 24j.

Fig. 22 shows an embodiment of the present invention which is similar to the embodiment shown in Fig. 21, but which lacks the recirculation line 101j associated with the return line. Instead, the embodiment includes a cooker 6k with an additional screen belt section 200k which is located immediately below a head section 5k. The belt section 200k has a recirculation line 201k in fluid communication therewith. Cooking liquor is withdrawn from the belt section 200k via the recirculation line 201k and returned to a point immediately below a head separator 7k located within the head section 5k. The temperature of the liquor in the line 201k can be regulated by a heat exchanger 203k which is in operative engagement with a high pressure steam line 102k. The main reason for using the recirculation line 201k consists in improving the distribution of the white liquor withdrawn from the belt section 200k. The process of recirculating the cooking liquor is often referred to as quench circulation.

Fig. 23 shows a preferred embodiment of a separator for use with a hydraulic digester or soak tank forming part of a digester system such as the digester system shown in Fig. 1 where a thermal seal is required. The advantage of locating the thermal seal near the separator is that hot black liquor (above 100°C) can be introduced into the head of the tank without risk of operating the high pressure conveyor at too high a temperature. The thermal seal reduces or even eliminates the risk of inadvertent recirculation of hot liquor through the head separator to the high pressure conveyor, which can cause damage to the conveyor. The separator can also be used in a hydraulic digester with a tank if desired. Only a head part of such an impregnation vessel 1 or a digester is shown. The non-impregnated slurry pulp is fed to the head of the impregnation vessel or digester via the transfer line 321 which is attached circumferentially to the vessel wall 1 and opens into an inlet space 330. The screw conveyor is attached to a shaft 332 which is connected to a drive unit 333 which is attached to a mounting plate 334 on the head of the vessel shell 1. The drive shaft 332 is rotated so that the screw is forced to convey the chips and the transport fluid downwards.

The new design eliminates the previous problem associated with the mounting plate, which previously had to be shaped because of the inlet passing through it. Due to the presence of a tangentially arranged feed 321 for the chips the plate 334 may be made of any suitable standard steel (which is much cheaper). In the inlet space 330 between the head of the strainer 335 and the plate 334 the rotor 332 would not normally be provided with screw blades. According to the embodiment shown the head of the strainer 335 is formed integrally with a cylindrical shell 338 with a flange 339 resting on the head of the container. However, since large forces are generated, the head part 341 of the strainer 335 is also supported by the container 1, but only radially, that is to say by means of a snug fit of a collar 341 in a support ring 342. In the lowest part of the strainer there is a similar support device 343. This end of the strainer must, however, be adjustable in order to arrange it coaxially with the screw. For this purpose an adjustable (screw/nut element) sliding device 344 is provided. It is a great advantage that this adjustment can be made from underneath the screen (standing on a platform in the tank). This support 343 also secures the lower part of the screen against lifting by means of projecting pieces 345 (four in number) which rest against the "sliding ring" 343. (Normally these parts are welded together after fitting, the welds being ground down before disassembly).

To prevent the screen 335 from rotating, four U-beams 346 are provided (in addition to welds on the bottom). Recesses (dashed lines) in the beam 346 interact with a protruding piece 347 of the collar 341.

The strainer basket 335 is arranged in the container shell 1 in such a way that a liquid collecting space 336 is formed between the cooker shell and the outer surface of the strainer basket 335. The liquid collecting space 336, which is preferably ring-shaped, is connected to a line for withdrawing liquid from the Liquid collection space 336 which in turn is refilled with liquid from the slurry in the screen basket 335. Most of the free liquid in the slurry entering the screen basket is withdrawn into the liquid collection space 336, but a small portion of the free liquid of at least 0.5 m³/ADT should not be withdrawn from the slurry.

The diameter (d) of the head portion of the container 1 is preferably smaller than a diameter (D) of the container in a middle part and bottom part thereof, preferably d D/2. Furthermore, the length/height of the head portion (with d) must be significantly greater than d. Preferably, the distance h between the lowest part of the strainer basket 335 and the transition zone 65 (cylindrical/truncated cone) is greater or equal to d, more preferably h>1.2d. The diameter (d) is small in order to reduce or even avoid any heat transfer to the T-C lines, so that the T-C lines can maintain a maximum temperature which is at most slightly above 100ºC, but normally below 100ºC. In this way, a heat seal zone is formed between the liquid supply devices (for supplying hot black liquor) and the liquid collection space 336.

There are also many possibilities to further optimize the conditions, e.g. with new online measuring systems (for example using NIR spectroscopy) it is possible to precisely measure specific contents of the pulp and the liquids entering the digestion system, which enables more precise determination and control of the supply/addition of specific fluids/chemicals and also their removal to set optimized conditions. The use of different types of additives can be very advantageous, especially for example polysulfide, which has a better effect in a low temperature environment than at high temperatures.

In addition, anthraquinone (AQ) would also be very advantageous, as it combines very well with highly alkaline environments.

Furthermore, there are numerous alternatives to uniformly impregnating the chips with white liquor at the head of the cooker. For example, one can design a centrally located inlet (according to WO- with a scattering device that provides a mushroom-like liquid film in a known manner, as well as a centrally located sprinkler element or an annular pipe with slots, etc.

Furthermore, it is readily apparent to the person skilled in the art that the number of sieve belts shown does not limit the invention in any way, but rather the number can be varied according to different requirements. It is also obvious that the invention is in no way limited to a particular sieve configuration and it is understood that bar sieves can be replaced by sieves with slots cut out of sheet metal, for example. In addition, in some devices, movable sieves are preferred. Furthermore, it is readily apparent to the person skilled in the art that in order to improve heat economy, measures can be taken to reduce heat losses from the cooker, for example insulating the cooker shell and/or maximizing the volume in relation to the surface exposed to the outside, i.e. increasing the cross-sectional area.

The system shown in front of the digester does not limit the invention in any way, for example the steaming vessel can be omitted and a direct connection can be provided between the chip silo (for example a partially filled normal pressure vessel) and the chip shaft. Furthermore, other types of conveying systems than a high-pressure conveyor can also be considered, e.g. DISCFLOTM pumps.

To improve the distribution of the white liquor added at the head, a so-called Install a "quench circulation" that recirculates a desired amount of liquid from below the head sieve 7 to the ring pipe. Ordinary sieves are not absolutely necessary for this purpose. Finally, it is understood that the basic principle of the invention can also be applied to the impregnation tank in combination with a circulation (knot catcher and pipes), even if this of course reduces the cost advantage.

Furthermore, it will be understood that some advantages of the present invention can also be achieved in a two-zone cooker, even if almost the same temperature is maintained in the direct current and countercurrent cooking zones.

Claims (24)

1. Process for the continuous production of pulp, in which finely divided fiber material, preferably wood chips, is mixed with liquid to form a slurry, the slurry is fed to an impregnation zone (A) in which cooking pressure essentially prevails, the fiber material is heated with hot black liquor to a temperature (T0) of less than 140°C and impregnated and fed through the impregnation zone (A), a substantial part of the free liquid (D) is separated from the slurry and drawn off, the heated and thoroughly impregnated fiber material is fed to a direct current cooking zone (B) arranged in the upper part of a cooker (6) with an upstream and a downstream end, at least 60%, preferably more than 70%, of the amount required for the cooking reaction is fed into the upstream end of the direct current cooking zone (B). of liquor (F), whereby a first content of effective alkali (EA1) is obtained at the beginning of the direct current cooking zone (B), black liquor (E) which has passed through at least the direct current cooking zone (B) is withdrawn from at least one knot catcher belt (8) and pulp is removed from the bottom of the cooker (6), whereby a higher temperature (T1) is maintained at the beginning of the direct current cooking zone (B) than in the impregnation zone (A), characterized in that the entire impregnation zone is a direct current zone, a substantial part of the withdrawn black liquor is fed near the upstream end of the impregnation zone (A) with respect to the liquid flow of the cocurrent impregnation zone, the quantity of black liquor supplied near the upstream end of the impregnation zone (A) is more than 6 m³/ADT, so that the liquid/wood ratio in the impregnation zone (A) is between 5/l and 10/l, a substantial exchange of liquid is carried out between the impregnation zone (A) and the cocurrent cooking zone (B), whereby after separation and removal of free liquid in the slurry no more than 1.5 m³/ADT remains as free, non-exchanged liquid, waste liquor in an amount exceeding 4 m³/ADT, taken from the free liquid stream (D) separated at the end or after the end of the impregnation zone (A), is recycled, and by the combination that the cooking temperature (T1) in the direct current cooking zone (B) is below 160ºC, the effective alkali content (EA1) is above 35 g/l and the amount of effective alkali in the withdrawn black liquor is at least 13 g/l. 2. Process according to claim 1, characterized in that a substantial part of the black liquor fed to the impregnation zone has a temperature of more than 100°C, preferably between 120 and 160°C and particularly preferably between 130 and 150°C. 3. Process according to claim 1, characterized in that the black liquor is passed through a first flash tank (9) before being fed to the impregnation zone. 4. Process according to claim 1, characterized in that a countercurrent cooking zone (C) is also present and the temperature (T2) in the lowest part of the countercurrent cooking zone (C) considerably exceeds the temperature (T1) at the beginning of the direct current cooking zone (B), so that the temperature difference (ΔT = T2-T1) is more than 5ºC, preferably between 5 and 20ºC and particularly preferably between 7 and 15ºC. 5. Process according to claim 4, characterized in that a certain amount of effective alkali is added near the bottom part of the cooker, a second content of effective alkali (EA2) which is lower than the first effective alkali content (EA1) and an effective alkali difference (ΔEA = EA1-EA2) of at least 20 g/l, preferably more than 25 g/l and particularly preferably between 30 and 50 g/l. 6. Process according to claim 1, characterized in that it relates to a two-vessel digestion system with an impregnation vessel (1) and a digester (6) and a part of the drawn off black liquor (E) is fed near the inlet end of the vessel (1), wherein at least 70%, preferably at least 80% and particularly preferably 90 to 100%, of the drawn off black liquor is fed to the impregnation vessel (1). 7. Process according to one of the preceding claims, characterized in that the amount of black liquor supplied to the impregnation zone (A) is over 7 m³/ADT and preferably between 8 and 12 m³/ADT. 8. Method according to one of the preceding claims, characterized in that the liquid/wood ratio in the impregnation zone (A) is between 5/l and 8/l. 9. Process according to claim 6, characterized in that a substantial liquid exchange is carried out between the impregnation zone (A) and the cocurrent cooking zone (B), whereby at most 1 m³/ADT, preferably at most 0.5 m³/ADT, remains in the slurry as free, non-exchanged liquid. 10. Process according to claim 6, characterized in that the cooker is of the steam/steam phase type with a top separator (7) with an upwardly conveying screw (62) and the separator is used both for separating the waste liquor (D) and for supplying the cooking liquor (F). 11. A two-vessel digestion system for carrying out the process according to claim 1, comprising: a drinking container (1) with an inlet part and an outlet part (3); a cooker (6) which is in fluid communication with the impregnation tank (1) via a cooker line between the cooker and the impregnation tank; a first transfer line (21) between the impregnation tank and the head of the digester for transporting fibre material to the digester, a separator (23) arranged in connection with the first transfer line (21) either at the head of the digester or in an intermediate position of the transfer line with a discharge chamber for separating a transport liquid from a fibrous material; a first knot catcher belt section (8) which is arranged in the cooker for withdrawing a quantity of black liquor from the cooker and such that a first direct current cooking zone (B) is defined above the first knot catcher belt section; a second transfer line (17) in fluid communication with the impregnation tank for supplying black liquor withdrawn from the first knot catcher belt section (8) to the inlet part of the impregnation tank (1); a supply line (24) in fluid communication with a supply chamber near the head of the cooker for supplying a cooking liquor, a return line (15, 33) attached to the separator (23) and the outlet part (3) of the impregnation container (1) for returning the transport liquid from the separator to the first transfer line (21), characterized in that a recovery line (24) is attached to the return line for withdrawing a substantial amount of waste liquor from the transport fluid and supplying it to a recovery plant, the impregnation tank (1) is sieveless and the second transfer line (17) is for supplying a larger amount of the black liquor withdrawn from the first knot catcher belt section (8) to the inlet part of the The impregnation tank is dimensioned so that a direct current impregnation zone (A) can be established throughout the entire impregnation tank. 12. Two-container digestion system according to claim 11, characterized in that the volume of the impregnation container (1) is greater than 1/11, preferably greater than 1/10, of the volume of the digester (6). 13. Two-vessel digestion system according to claim 11, characterized in that the volume V of the impregnation vessel (1) is a factor of 5 above the value of the square of the maximum digester diameter, i.e. V = 5D², where D stands for the maximum diameter of the digester (6). 14. Two-container digestion system according to claim 11, characterized in that the separator is a so-called liquid exchanger (7, 31) with an upwardly conveying screw (62). 15. Two-vessel digestion system according to claim 14, characterized in that the liquid exchanger (7) is arranged in the head of the digester. 16. Two-vessel digestion system according to claim 15, characterized in that the liquid exchanger has a feed chamber (23) which is arranged near the discharge chamber (67) and above it. 17. Two-vessel digestion system according to claim 16, characterized in that the total area of the passage between the feed space (23) and the interior of the liquid exchanger is over approximately 400 mm², preferably over 600 mm², so that a sufficient flow rate is achieved. 18. Two-container digestion system according to claim 17, characterized in that it has a further feed chamber (25) which is preferably arranged between the first feed chamber (23) and the discharge chamber (35). 19. Two-tank digestion system according to claim 11, characterized in that the recovery line for reducing the temperature of the waste liquor in the recovery line before feeding the waste liquor to the Recovery system with a flash tank is in operation. 20. Two-container digestion system according to claim 11, characterized in that the head of the impregnation container (1) is provided with a heat sealing device. 21. Two-vessel digestion system according to claim 18, characterized in that the heat sealing device comprises a substantially cylindrical head section with a first diameter (d) and a first part with a downwardly conveying separator (7) and a second part comprising a void, wherein the void preferably has a height (h) which is greater than (d) and the first diameter (d) is substantially smaller than the diameter (D) of the lowermost part of the imbibing vessel. 22. A method according to claim 1, characterized in that it relates to a cooker without countercurrent cooking but with a second cocurrent cooking zone (C), all or substantially all of the liquid withdrawn after the first cocurrent cooking zone (B) is fed to the start of the soaking zone, a practically complete exchange of free liquid is carried out between zones (A) and (B), liquid for recovery is taken from the liquid which has passed through zone (A) and a further quantity of liquid is taken for recovery from a lowermost knot-catcher belt section 12 after the second cooking (C). 23. Process according to claim 22, characterized in that the amount of liquid removed for recovery after the impregnation zone (A) is greater than the liquid removed for recovery after the second cooking zone (C) and the liquid/wood ratio in the various zones is as follows: 1/w (A) > 1/w (B) > 1/w (C). 24. Method according to claim 1, characterized by the combination that the cooking temperature in the direct current cooking zone (B) is below 155ºC and particularly preferably between 140 and 150ºC, the content of effective alkali (EA1) is above 40 g/l and particularly preferably between 45 and 55 g/l and the amount of effective alkali in the withdrawn black liquor is preferably at least 16 g/l, preferably more than 18 g/l and particularly preferably about 20 g/l.