DESCRIPTION

Technical field

The present invention relates to a method for continuously cooking cellulose-containing fibre material, preferably by the kraft process, in a single-vessel system, with the fibre material being impregnated with black liquor at the start of the cooking process, chiefly for the purpose of achieving good pulp strength, but in a preferred embodiment also for the purpose of achieving relatively low energy consumption.

State of the art and problems

In existing kraft digesters, black liquor is used only in a limited amount, i.e. often in amounts which are substantially less than 50% of the total liquid content in the impregnation zone of the digester. The remainder of the externally added liquid usually consists of white liquor in the main. It has been found that this large addition of white liquor at such an early stage in the cooking process has an adverse effect on the tear resistance of the fully cooked fibres.

The document US 3 303 088 (Gessner) discloses a method for continuously cooking cellulose-containing fibre material in a single-vessel system, with chips being fed in at a first end of the digester, white liquor being added at a position at the said first end, the chips being impregnated in a concurrent impregnation zone, the chips being cooked in a cooking zone downstream of the impregnation zone, hot black liquor being

extracted from at least one screen section, black liquor being added to the said impregnation zone, and cooked pulp being discharged at the other end of the digester. Gessner further shows that the extracted liquor from the first screen section, which is arranged downstream of the position of addition of the hot black liquor, is returned to the digester by first being conveyed to a container in which white liquor and extracted impregnation and cooking liquid are mixed. Due to this recirculation, a high content build-up of, inter alia, volatile sulphur and terpene compounds is obtained in the impregnation and cooking liquid. In addition, a procedure according to Gessner does not permit sufficiently rapid heating of the cooking liquid to achieve optimal process conditions. It is also evident that Gessner's procedure for continuously cooking cellulose-containing material does not include process parameters which are necessary to achieve optimal conditions, such as, for example, the correct liquor-to- wood ratio for obtaining the desired movement of the chip column in the digester.

Solution and advantages

The object of the present invention is to provide an improved method for continuously cooking fibre- containing cellulose material, which method eliminates the abovementioned disadvantages, characterized in that black liquor is added to the said impregnation zone in an amount in excess of 40%, preferably 50%, and more preferably 60%, of the total amount of liquid, in that the liquor-to-wood ratio in the said impregnation zone exceeds 3:1, preferably 3.5:1, and is more preferably equal to or greater than 4:1, and in that the extracted liquor from the first screen girdle, which is arranged downstream of the position of addition of the said black

liquor, is largely removed from the digester, i.e. to an extent in excess of 50%, preferably in excess of 70%, and more preferably in excess of 90%.

A further aspect according to the invention is that at least one cooking circulation is arranged down¬ stream of the said first extraction screen, as a result of which a predetermined heating of the recirculated cooking liquid is obtained, and preferably also a predetermined addition of white liquor, so that optimal preconditions for the cooking can be achieved.

A further aspect according to the invention is to ensure that the distance between the lower edge of the said screen girdle and the upper edge of the next screen girdle, included in a cooking circulation, in the direction of feeding of the chips, is less than 5 metres, so that it is possible quickly to establish optimal conditions for the cooking.

Detailed description The invention will be explained in greater detail hereinbelow with reference to the drawing, which is a diagrammatic representation of a preferred flow plan for continuous cooking of fibre material in accordance with the present invention. The installation shown in the figure includes a chip bin (A) , a horizontal steaming vessel (B) and a digester (1). The broken-up fibre material, which preferably consists of wood chips, is fed from the chip bin (A) in a known manner through the steaming vessel (B) to a high-pressure feeder (C) , with the aid of which the chips are fed through a line (2) up to the digester top (3) (see for example SE-B-468053) . At the digester top (3) there is a screen for separating off a certain quantity of the liquid with which the chips are

transported up to the top. This liquid is returned and recirculated via the high-pressure feeder in a known manner.

The preferred embodiment according to the invention, as shown in the figure, involves the use of a hydraulic digester which, in contrast to a steam/liquor phase digester, is thus filled hydraulically with liquid and therefore uses a downward feeding screw in the top screen for discharging the chips. The chips then move slowly downwards with the chip column in a liquor-to-wood ratio which is approximately 2.0:1 to 4.5:1, preferably between 3:1 and 4:1. The temperature in this upper part

(3) of the digester is normally approximately 110°-120°C, but sometimes up towards 135°C. In this upper part, the liquid moves in concurrent in relation to the chip column. After some time, the chips have moved with the chip column down to a level at which a first central pipe (11) opens out (11A) . This central pipe (11) is connected to a circulation loop (8A) , (7) , (12) , (11) which extracts hot black liquor from the extraction screen section (8A) , (8B) , some of which is fed via the line (7) to a first flash cyclone (18) , and the remainder of which is fed with the aid of a pump (12) onwards to the central pipe (11) . Hot black liquor is therefore supplied in concurrent. The black liquor has a temperature of approximately 155°-165°C and is supplied in such an amount that the liquor-to-wood ratio preferably increases by at least ^s a unit, preferably by 1 unit, and in some cases by as much as 1 units. According to the most preferred embodiment of the invention, a sufficiently large addition of hot black liquor is made to obtain a liquor- to-wood ratio of between 4:1 and 5:1. At 4:1, the liquid consists of just under one part white liquor, one part

wood liquor and just over two parts black liquor, in accordance with a preferred example.

The temperature which is obtained in this case in the impregnation zone is approximately 120°-140°C. At a certain distance from the mouth (11A) of the said central pipe (11) , viewed in the direction of flow, there is a first extraction screen (14) . The screen (14) is placed, in the preferred case, sufficiently far from the mouth (11A) to obtain a dwell time at least in excess of 20 minutes for the chips to move from the level of the mouth

(11A) to the upper edge of the screen (14) . This means in practice that the distance is preferably in excess of 4 metres, preferably in excess of 5 metres, and more preferably in excess of 6 metres. At this first screen girdle (14) , such an amount of impregnation liquor is extracted liquored (13) that the desired liquor-to-wood ratio after addition of white liquor is obtained. In order fully to minimize the build-up of released material, all this extracted liquor (13) is led off, according to the preferred embodiment shown, to a second flash cyclone (20) from which the liquor (21) is taken to recovery. The steam from the second flash cyclone (20) is used, as is normal, at another point in the system.

After having passed the first screen girdle (14) , the chip column continues down and encounters, immediately below this first screen girdle (14) , a first cooking circulation (15) . The purpose of the cooking circulation (15) is to increase the temperature of the cooking liquid up to a suitable cooking temperature, i.e. preferably in excess of 150°C, more preferably in excess of approximately 155°C. In most cases, it is necessary to have at least two such cooking circulations (15) , (16) in order to achieve, with sufficiently good distribution, the desired temperature in the chip column. The first

cooking circulation (15) is placed quite near, i.e. immediately below, the said first extraction screen girdle (14) . The distance between the lower edge (14) of the extraction screen and the upper edge of the digester screen should be less than 5 metres, more preferably 3 metres, and even more preferably 1.5 metres, in order to attain the desired temperature sufficiently quickly.

The extracted liquor from the digester screen (15) is recirculated in a known manner by means of a pump (15A) pumping the cooking liquid through a heat exchanger

(15B) , where the desired heating is obtained, and is reintroduced into the digester preferably together with newly added white liquor by means of a central pipe whose mouth (15C) opens out approximately level with the actual screen girdle (15) . The second cooking circulation (16) , (16A) , (16B) , (16C) which is shown in the figure functions in a corresponding manner. In the preferred case which is shown, it has been chosen to use two digester screens. The chip column and its surrounding liquid have then reached the desired cooking temperature, whereupon it enters a cooking zone (16) and continues to move downwards. After a fairly long distance corresponding to a dwell time of approximately two hours, the chips have moved down to be level with a second set of extraction screens, which have already been mentioned above. These extraction screens correspond to the sort of extraction screen which is normally always found on a continuous digester. The greater part of this extracted liquor is thus conveyed to the first flash cyclone (18) and is thereafter conveyed onwards to recovery. Below the level of the extraction screens (8A) and (8B) , the chip column enters a countercurrent cooking zone. The chips here encounter cooking liquid which has been extracted at the lower screen (17) , has been heated in a lower heat

exchanger (17B) and with the aid of a pump (17A) has been recirculated via a central pipe whose mouth (17A) opens out level with the screen (17) .

According to the invention, white liquor is added at at least two positions, on the one hand at the digester top (3) , and on the other hand in at least one cooking circulation. In the case where there are two cooking circulations, it can be added to one cooking circulation or to both cooking circulations. It is of course also possible to add white liquor in the lower circulation (17) , so that the alkali concentration is increased in the counter-current zone, and approximately the same temperature is expediently maintained in all the cooking zones so that our patented method ITC™ is used. Wash liquor is added at the lower end (10) of the digester, which wash liquor thus moves in a conventional manner in countercurrent and displaces hot liquor from the fibre material, which permits a subsequent cold blow. The pulp is then fed through a feeding arrangement known per se and is conveyed out through a line for further treatment (9) .

The person skilled in the art will appreciate that the invention is not limited by what has been shown above, but can instead be varied within the scopes of the patent claims which follow. Thus, for example, it is possible to equip a digester according to the method described above with a further circulation down at the bottom, for example a so-called ITC™ circulation, in order to cook to an even lower kappa number, if so desired, cf. our own patent application SE 9203462. An MCC design is of course also conceivable to the person skilled in the art. In addition, the person skilled in the art will appreciate that a number of modifications can be made within the scope of the invention, such as.

for example, the choice of the exact temperature and alkali concentrations, etc.

Also, instead of pumping hot black liquor directly from the extraction screen (8) to the impregnation zone (5) , it is possible to pump the hot black liquor which is collected from the first flash cyclone (18) up to the impregnation zone (5) . The temperature of the black liquor is then lower, but the advantage obtained is that the black liquor contains less air, which can be a great advantage in connection with eliminating foaming problems in the digester. In addition, it is possible, in certain existing digesters, to use the existing screen arrangement and to lead off only some of the extracted liquor from the upper screen girdle and, in the same way as in a conventional cooking circulation, to recirculate the remainder and at the same time also expediently to heat and add white liquor. It is of course also possible for this last-mentioned principle to be used in connection with the erection of new digesters. A single-vessel steam phase digester can also be used.