Method For Producing Cellulose Pulp In A Continuous Digester In An Energy-efficient Manner

Abstract

The method is for producing cellulose pulp in a continuous digestion system in an energy-efficient manner. Cellulose chips are fed continuously with a transfer liquid in a transfer line to a top separator arranged at the top of a digester, for the separation of free liquid from the cellulose chips. The top separator has a withdrawal space in which free liquid collects. At least a fraction of the free liquid that has separated, with a temperature T.sub.out, is led in a return line from the withdrawal space of the top separator to a position to be used as transfer liquid. At least one pressurised withdrawal of black liquor is carried out in the digester essentially at the cooking temperature of at least 135.degree. C. An additive liquid with a temperature T.sub.in is added at the withdrawal space of the top separator, where the temperature T.sub.in is greater than the temperature T.sub.out.

Description

PRIOR APPLICATION

[0001] This patent application claims priority from Swedish patent application number SE 0602675-1, filed 13 Dec. 2007, now Swedish Patent number 529 573.

TECHNICAL AREA

[0002] The present invention concerns a method for producing cellulose pulp in a continuous digester in an energy-efficient manner.

BACKGROUND AND SUMMARY OF THE INVENTION

[0003] In conventional continuous digesters, chips are impregnated with impregnation liquid in an impregnation vessel. The impregnated chips are subsequently cooked in a digester, whereby the impregnation vessel and the digester are coupled to each other by means of a transfer circulation. The transfer circulation consists not only of a transfer line that feeds the chips from the outlet end of the impregnation vessel to the top of the digester for the separation of free liquid in a top separator, but also of a return line that feeds separated liquid from the top of the digester to the outlet end of the impregnation vessel in order to be used as transfer liquid for the impregnated chips.

[0004] In a steam phase digester steam is added at the top of the vessel, in order to heat the chips before the cooking is begun. This steam requires a great deal of energy and several inventions for this reason have been directed towards reducing the requirement for steam that is added to the digester.

[0005] SE 518 957 reveals an invention in which one of the aims is to reduce the requirement for the supply of steam to the top of the digester. According to this patent, a fraction of the black liquor withdrawn from the digester is added at the beginning of the transfer flow at what is essentially the withdrawal temperature T.sub.av, which the liquor has retained, with the aim of raising the temperature of the chips mixture in the transfer circulation. A fraction of the withdrawn liquid from the top separator of the digester is returned to the impregnation vessel such that it there can act as impregnation liquid. A heating of the chips is obtained with this invention, which ensures that the consumption of steam at the top of the digester can be reduced, and the steam can therefore be used for another purpose. One limitation of this method is that the temperature of the black liquor that is added should not be too high, since this would then introduce the risk of steam implosion.

[0006] SE 511 850 reveals a way of simplifying the digestion plant with respect to the withdrawal of liquid from the digestion system before the initial phase of the cooking, such that the design of the impregnation vessel can be simplified. In one preferred embodiment of the invention, cooking liquid is added to the top separator after the separation of liquid in order to be mixed with the fibre material that has a low liquid content under the influence of a screw in the top separator that feeds material in an upward direction.

[0007] The top separator of SE 511 850 has a rotating screw that is arranged vertically. The top separator has also a lower sieve section that is surrounded by a wall in order to form an intermediate liquid chamber for the collection of liquid, which liquid is pressed out through the sieve section under the influence of the screw. A ring-shaped supply pipe is arranged around the screw. Holes are arranged in the ring-shaped supply pipe for the supply of white liquor and possibly also other liquid to the chips. The return line is connected to the liquid chamber. Steam at intermediate pressure is added through a line to the upper steam space of the digester at the top of the digester in connection with the top separator in order to heat the chips that are fed in by the screw and that fall down over the edge. There is no connection between the withdrawal space and the ring-shaped supply line.

[0008] One risk with this solution is that the liquid that is added in the supply line can give rise to blockage at the top of the digester since the added liquid can risk flushing the chips in a backwards direction.

[0009] A first object of the present invention is to reduce the requirement of heating with steam at the top of the digester.

[0010] A second object of the present invention is to be able to heat the chips before the cooking process using a temperature of the hot liquid that is higher than that which is possible when the heating is carried out at the transfer flow.

[0011] A third object of the present invention is to avoid clogging at the top separator in a steam phase digester.

[0012] The concept of the present invention is to achieve a method for producing cellulose pulp in a continuous digester system that is efficient in its use of energy. In a conventional continuous digester, chips are fed continuously in a transfer liquid to a top separator arranged at the top of a digester.

[0013] Separation of free liquid from the cellulose chips into a withdrawal space where the liquid is collected takes place in the top separator. A fraction of the free liquid is drawn from the withdrawal space in a return line, to a position at which it can act as transfer liquid.

[0014] The present invention is characterised in that a hot additive liquid is added to the withdrawal space of the top separator. The temperature of the hot additive liquid that is added to the withdrawal space is greater than the temperature of the withdrawn liquid in the free liquid in the return line. The following advantages are achieved in that the hot liquid is added to the withdrawal space [0015] Any heating that takes place in the transfer circulation can be reduced or totally eliminated, which ensures that the heating of the chips can be carried out by a liquid at a higher temperature than that which is possible in, for example, the invention described in SE 518 957. [0016] The amount of steam that is added at the top can be reduced. [0017] The hot additive liquid that is added at the withdrawal position does not flush the chips against the direction of motion, and this means that the risk of blockage at the top of the digester is reduced. [0018] This method ensures that the withdrawal space is filled with liquid all the time, which prevents blockage of the top separator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 shows a first preferred embodiment of the present invention.

[0020] FIG. 2 shows a second preferred embodiment of a digester system according to the present invention.

[0021] FIG. 3 shows a third preferred embodiment of the present invention.

[0022] FIG. 4 shows a top separator arranged at the top of a digester according to the method of the present invention.

DETAILED DESCRIPTION

[0023] FIGS. 1 and 4 show a first preferred embodiment of a method for producing cellulose pulp in a continuous digester system in an energy-efficient manner. Cellulose chips are fed continuously to an impregnation vessel 10 in order to be impregnated in an impregnation liquid in the impregnation vessel 10.

[0024] Once the impregnation is finished, the impregnated chips are fed with a transfer liquid in a transfer line 11 to a top separator 20 arranged at the top of a digester 30, for the separation of free liquid from the cellulose chips, where the digester 30 is a steam phase digester.

[0025] The transfer line 11 is connected to the bottom of the top separator 20 and the chips are thus fed upwards through the top separator.

[0026] In accordance with FIG. 4, the top separator 20 has a rotating upwardly feeding screw 22 that is arranged vertically. The top separator 20 has also a screening surface 23, which is surrounded by an impenetrable wall 24 for the formation of an intermediate withdrawal space 21, for the collection of free liquid that is pressed out through the screening surface 23 under the influence of the screw 22. The withdrawal space 21 extends over at least 50%, preferably at least 75%, of the threaded height of the screw 22.

[0027] At least a fraction of the free liquid that has separated is led in a return line 32 from the withdrawal space of the top separator to the outlet end of the impregnation vessel in order to be used as transfer liquid. The return line 22 is connected to the lower part of the withdrawal space 21 and the temperature of the withdrawn liquid in the return line 32 is T.sub.out. A means 35 of creating pressure, preferably in the form of a pump, is arranged in the return line 22.

[0028] At least one pressurised withdrawal of black liquor is carried out in the digester 30 essentially at the cooking temperature of at least 135.degree. C. A fraction of this pressurised withdrawal of black liquor can be led to the bottom of the impregnation vessel along a black liquor line 34.

[0029] An additive liquid with a temperature T.sub.in is added through an addition line 33 connected to the upper part of the withdrawal space, where this additive liquid consists of at least part of the black liquor withdrawal. The additive liquid is mixed with the chips under the influence of the screw 22 that feeds material upwards. The relationship between the temperature of the black liquor withdrawal that is added to the withdrawal space and the temperature of the free liquid withdrawn from the withdrawal space is such that T.sub.in>T.sub.out. The additive liquid that is led in the addition line 33 is added to the withdrawal space 21 downstream of the return line 32, when seen in the direction of motion of the chips.

[0030] The liquid/wood ratio in the transfer line 11 is .gtoreq.7, the remaining liquid in the chips after they have passed the top separator lies at a liquid/wood ratio of .gtoreq.3 and the amount of additive liquid in the addition line 33 that is added to the withdrawal space 21 lies at a liquid/wood ratio .gtoreq.1.

[0031] The level of COD (Chemical Oxygen Demand) in the additive liquid that is added at the top separator 20 may be either lower or higher than that of the free liquid that is withdrawn at the top separator 20 in the return line 32.

[0032] FIG. 2 shows a second preferred embodiment according to the method according to the invention. Instead of using the pressurised black liquor withdrawal as additive liquid, at least a part of the free liquid that is withdrawn at the withdrawal space 21 is here used. This withdrawal is heated in a heat exchanger 40, before it is led in the addition line 33 to the withdrawal space 21 as additive liquid, where the temperature of the additive liquid T.sub.in is higher than T.sub.out. The heating agent with which the additive liquid exchanges liquid in the heat exchanger 40 may be constituted of the pressurised black liquor withdrawal. Other types of heating are possible instead of that of exchanging heat with liquids, such as, for example, heating by steam. This embodiment otherwise is identical with the first preferred embodiment.

[0033] FIG. 3 shows a third preferred embodiment according to the method according to the invention. The additive liquid in this embodiment is constituted by two liquids: partly the pressurised black liquor withdrawal according to the first preferred embodiment, and partly the heated free liquid that is withdrawn at the withdrawal space according to the second preferred embodiment. This embodiment otherwise is identical with the first preferred embodiment.

[0034] The following positive advantages are achieved with the present invention: [0035] Any heating that takes place in the transfer circulation can be reduced or totally eliminated, which ensures that the heating of the chips can be carried out by a liquid at a higher temperature than that which is possible in, for example, the invention described in SE 518 957. [0036] The amount of steam that is added at the top can be considerably reduced. [0037] The hot additive liquid that is added at the withdrawal position does not flush the chips against the direction of motion, and this means that the risk of blockage at the top of the digester is reduced. [0038] This method ensures that the withdrawal space is filled with liquid all the time, which prevents blockage of the top separator. [0039] The hot additive liquid does not noticeably affect the temperature in the transfer flow.

[0040] Several embodiments than those described above are possible within the framework of the attached patent claims. It is not necessary, for example, that the chips be impregnated in an impregnation vessel before they are fed in the transfer liquid up to the top separator. The chips instead can be subject to steam pre-treatment in a steam pre-treatment vessel, or they may be treated by some other method.

[0041] It is thus fully without significance for the innovative concept of the present invention how the chips have been treated before they are fed into the top separator. The important point is that a liquid is added to the withdrawal space, which liquid has a temperature that is higher than the temperature that is withdrawn in the return line from the withdrawal space.

[0042] The additive liquid can, as in the third preferred embodiment, be constituted by two liquids, and it can also be constituted by more than two liquids. The only point that is important for the innovative concept of the invention is that the temperature of the additive liquid is higher than the temperature of the withdrawal that is drawn in the return line.

[0043] While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.

Claims

1. A method for producing cellulose pulp in a continuous digestion system in an energy-efficient manner, comprising: continuously feeding cellulose chips with a transfer liquid in a transfer circulation to a top separator arranged at a top of a digester, the top separator having a withdrawal space defined therein, collecting free liquid in the withdrawal space; separating at least a fraction of the free liquid from the cellulose chips, the separated free liquid having a temperature T.sub.out, leading the separated free liquid in a return line extending from the withdrawal space of the top separator to a position, at the position, using the free liquid as the transfer liquid to transfer cellulose chips to the top separator; carrying out at least one pressurised withdrawal of black liquor in the digester essentially at a cooking temperature of at least 135.degree. C.; and adding an additive liquid, having a temperature T.sub.in, at the withdrawal space of the top separator, so that the temperature T.sub.in is greater than the temperature T.sub.out.

2. The method according to claim 1, wherein the cellulose chips have been impregnated with impregnation liquid in an impregnation vessel before the cellulose chips are fed in the transfer liquid to the top separator.

3. The method according to claim 2, wherein the free liquid that has been separated is led in the return line from the withdrawal space of the top separator to an outlet end of the impregnation vessel in order there to be used as the transfer liquid.

4. The method according to claim 1 wherein the additive liquid is added at the withdrawal space downstream of the return line, as seen in a direction of motion of the cellulose chips.

5. The method according to claim 1 wherein the additive liquid is mixed with the cellulose chips under an influence of a screw that feeds material upwardly in the top separator, and wherein the digester is a steam phase digester.

6. The method according to claim 1 wherein the additive liquid is constituted by a liquid that has been heated.

7. The method according to claim 6, wherein the liquid that has been heated is constituted at least partially by a liquid withdrawn from the top separator.

8. The method according to claim 1 wherein the additive liquid is constituted at least a part of the pressurised black liquor.

9. The method according to claim 1 wherein the additive liquid is constituted by at least a part of the pressurised black liquor and at least a part of a withdrawal from the top separator that has been heated.

10. The method according to claim 1 wherein a liquid/wood ratio in the transfer line is greater than or equal to 7, and a residual liquid in the cellulose chips after the top separator lies at a liquid/wood ratio that is greater than or equal to 3, where an amount of the additive liquid that is added lies at a liquid/wood ratio greater than or equal to 1.

11. The method according to claim 1 wherein the withdrawal space extends over at least 50% of a threaded height of a screw in the top separator.

12. The method according to claim 1 wherein the additive liquid that is added to the top separator has a lower COD level than a liquid that is withdrawn from the top separator.

13. The method according to claim 1 wherein the additive liquid that is added to the top separator has a higher COD level than a liquid that is withdrawn from the top separator.

14. The method according to claim 1 wherein the additive liquid is added to the upper part of the withdrawal space through an addition line, and the return line is connected to a lower part of the withdrawal space.