EP1552053A1

EP1552053A1 - A continuous process for the cooking of cellulose-besed pulp and a pulp digester applicable for the same

Info

Publication number: EP1552053A1
Application number: EP03738162A
Authority: EP
Inventors: Ari-Pekka MÄÄTTÄNEN; Hannu Sonni
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2002-07-12
Filing date: 2003-07-14
Publication date: 2005-07-13
Also published as: WO2004007834A1; AU2003244682A1; FI20021369L; FI20021369A0; FI120505B

Abstract

The invention relates to a continuous method for cooking cellulose-based pulp and to a pulp digester suitable for the method. In continuously operating cooking, the fibrous pulp moves downwards in a vertical cooking vessel (1), whose upper part forms a cooking zone (2) and lower part forms a pulp counter-current washing zone (3) for cooked pulp, and is removed as cooked pulp from the lower end of the vessel to an exhaust pipe (9). In the invention, the liquid phase in the washing zone of the cooking vessel is separated and transferred as wash liquor to a location higher than the separation point (10) and also to the lower part of the cooking vessel to form diluting liquor reducing pulp consistency. The invention has the special purpose of reducing fibre agglomeration and scuffing, which damages fibres and deteriorates the strength properties of cooked pulp removed from the vessel.

Description

A continuous process for the cooking of cellulose-based pulp and a pulp digester applicable for the same

This invention relates to a continuous process for cooking cellulose-based pulp, in which the fibrous pulp moves downwards in a cooking vessel, whose upper part forms a cooking zone, where the cooking liquor moves downwards concurrently with the pulp, and the lower part forms a washing zone, where the pulp is washed with a wash liquor led upwards counter-stream relative to the pulp, and cooked pulp is removed from the lower end of the cooking vessel. In addition, the invention re- lates to a continuously operating pulp digester suitable for the method.

The production of chemical pulp consisting of cellulosic fibres is performed by cooking wood chips or any similar comminuted wood raw material, in which the cooking chemicals contained in the cooking liquor are dissolved in the lignin contained in the wood. The cooking chemicals and the lignin terminate in the residual liquor removed from the cooking vessel, which is evaporated in order to recover the material dissolved into it as a fuel. Continuous cooking, in which the cooking vessel is continuously supplied with wood raw material and cooking liquor and from which is also continuously removed cooked pulp and residual liquor, has typically been combined with counter-current washing of cooked pulp in the same vessel. Then the upper part of the cooking vessel forms a cooking zone, where the pulp and the cooking liquor move from the top downwards, and the lower part of the cooking vessel forms a washing zone, where the pulp moves further downwards and the cooking liquor moves upwards concurrently with pulp. At the zone boundary, the cooking liquor and the washing liquor join as residual liquor, which is discharged from the cooking vessel and led to an evaporation plant.

To enhance pulp washing, the washing zone of the cooking vessel is usually equipped with a "washing cycle", where a sieve or any similar separating means placed in the vessel is used to separate the liquid phase and transfer it above the separating point, where it is applied on the transverse surface of the vessel in order to equalise the flow and temperature profiles. By these means, irregular washing caused by ducting of the incoming washing liquor flow is avoided.

In current processes, cooked pulp is removed into an exhaust pipe, usually by means of a scraper mounted at the lower end of the vessel. Pulp agglomeration at the lower end of the vessel exerts stress on the scraper and hampers the expulsion of pulp into the exhaust pipe. It is particularly detrimental if the pulp is simultaneously subjected to strong scuffing by remaining fibre agglomerations corresponding chiefly to chip particles in the raw material being scuffed against each other. This causes damage to the fibres, appearing as crimped fibres with reduced strength. The technical properties of paper produced from this pulp deteriorate accordingly.

Fibre agglomeration problems could be eased by increasing the washing liquor flow supplied for counter-current washing of the pulp at the lower end of the cooking vessel. The washing liquor would then dilute the pulp so as to diminish scuffing and to ease pulp removal from the lower end of the vessel. However, this solution in- volves the problem of excess washing liquor terminating in the residual liquor, thus increasing the amount of this, so that its treatment would require additional evaporation capacity and would cause higher cost.

The purpose of this invention is to resolve problems caused by fibre agglomeration in a manner that is substantially more advantageous than the one described above. The method of the invention is characterised in the liquid phase being separated at the separation point in the washing zone of the cooking vessel, the liquid phase being then transferred as washing liquor upstream from the pulp above the separation point, and in the separated liquid phase being additionally transferred below the separation point to form a diluting liquor that reduces pulp consistency.

By means of the inventive solution, the cooked pulp formed of fibres is diluted, resulting in easier removal from the lower end of the cooking vessel, reduced scuffing and fibre damage entailed by this, and improved pulp strength properties. The essential feature of the invention is that dilution is performed in combination with the wash cycle known per se, by means of liquor transfer within the cooking vessel, raising the dilution factor at the lower end of the vessel without changing the overall flow degree of the cooking process; when the cooking vessel is not supplied with fresh diluting liquor, there will consequently be no increase in the amount of residual liquor to be evaporated.

Conducting diluting liquor to the lower end of the vessel in accordance with the in- vention can be carried out by branching off the washing cycle. Wash liquor circulation and diluting liquor transfer can be provided with the use of a common pump disposed in the circulating channel system.

In the wash zone, the separated diluting liquor is circulated in accordance with the invention so as to lift the counter-current at the lower end of the vessel, and it can be completely or partly combined with the wash liquor flow supplied to the lower end of the vessel, so that the diluting liquor returns to the vessel along with fresh wash liquor. With an increase of the counter-current amount, the diluting liquor will decrease pulp consistency between the lower end of the vessel and the diluting liq- uor separation point, which is at a higher location in the wash zone.

The continuous pulp digester of the invention, by means of which the method described above can be implemented, comprises as elements known per se a vertical cooking vessel, feed pipes for feeding ligno-cellulose material and wash liquor to the upper end of the vessel, a feed pipe for feeding wash liquor to the lower end of the vessel, an exhaust pipe for removing cooked delignified pulp from the lower end of the vessel, and an exhaust pipe for removing residual cook liquor from the area between the upper end and the lower end of the vessel. In accordance with the invention, the cooking vessel is characterised in that the wash zone of the digester is equipped with a means for separating the liquid phase, from where by-pass ducts are led to transfer the separated liquor as was liquor upstream from the pulp, above the separation point, and also below the separation point, to form a diluting liquor reducing pulp consistency.

The invention is explained in greater detail below by means of an example and with reference to the accompanying drawing (figure 1), which illustrates a continuous pulp digester of the invention comprising washing and counter-current washing zones.

The pulp digester illustrated in the drawing consists of a vertical, principally cylindrical cooking vessel 1 including associated flow pipes. In the cooking vessel 1, the cellulose-based pulp is continuously cooked in the cooking zone 2 at the upper end of the vessel and the cooked pulp is continuously counter-current washed in the wash zone 3 at the lower end of the vessel. The ligno-cellulose material forming raw material and the cooking liquor containing cooking chemicals are supplied to the upper end of the cooking vessel 1 through a common feed duct 4.

The pulp and the cooking liquor move downwards in the cooking zone 2 of the ves- sel, where fibres separate as cooked pulp when lignin is dissolved into the cooking liquor. The cooking liquor and the material dissolved by this terminate in the residual liquor, which is separated at the boundary between the cooking and wash zones 2, 3 with a sieve 5 and is removed to an exhaust duct 6 leading to an evaporation plant (not shown). For counter-current washing of cooked pulp in the washing zone 3 at the lower end of the cooking vessel, wash liquor is conducted to the lower end of the vessel as a continuous flow from the feed duct 7, the wash liquor being a filtrate in the following washing process. The pulp moves downwards in the washing zone 3 towards the lower end of the vessel, which is equipped with a scraper 8, which expels the cooked and washed pulp from the lower end of the vessel into the exhaust duct 9. The wash liquor flows from the washing zone 3 upwards against the pulp flow direction and joins the cooking liquor at the boundary between the cooking and washing zones 2, 3, and eventually terminates in the residual liquor to be discharged from the vessel into the duct 6.

The continuous process described above is suitable for e.g. hardwood or softwood Kraft cooking, in which wood chips and white liquor is brought from the feed duct 4 to the upper end of the cooking vessel 1 and wash water is brought from the duct 7 to the lower end of the vessel. The sulphate cellulose pulp obtained by cooking is discharged in washed state into the duct 9, e.g. for transfer to the following wash process, and the black liquor obtained as residual liquor is conducted through the duct 6 to the evaporation plant.

In the continuously operating digester described above, the ratio of liquor to fibres at the lower end of the vessel, i.e. the amount of liquor flowing upwards to the amount of liquor flowing downwards, is too low, and for this reason the fibres tend to agglomerate, resulting in mutual scuffing that damages the pulp and deteriorates its strength. In the digester of the invention described above, this problem has been overcome by diluting the pulp at the lower end of the cooking vessel 1 by means of diluting liquor conducted from a separation point 10 located higher in the digester. The cooking vessel comprises a sieve 10 disposed in the washing zone 3 and serving the internal wash cycle arranged in the wash zone. In this cycle, wash liquor separated from the pulp by the sieve 10 is transferred with a pump 11 through a circulating duct 12 to a level located downstream with the vessel relative to the wash liquor and upstream relative to the pulp, above the sieve 10, and by means of which the liquor transferred as a by-pass flow is evenly applied on the cross-surface of the vessel in order to equalise the wash liquor flow and the temperature of the wash liquor. A duct 13 has been branched off from the wash cycle 12 for circulating wash liquor separated by the sieve 10 by means of the pump 11, also concurrently with the pulp, to form diluting liquor at the lower end of the vessel. As shown in the drawing, the diluting liquor circulating duct 13 is bifurcated, joining the inlet branches 14, 15 of the wash liquor feed duct 7, with dilution directed partly to the scraper 8 at the lower end of the vessel and partly slightly above this. Regarding these circulating facilities, however, there are optional variations of the construction, which are obvious to those skilled in the art.

In other respects as well, it is obvious to those skilled in the art that the applications of the invention are naturally not limited to the example particularised above, but may vary within the scope of the accompanying claims. Thus, diluting liquor may be independently fed to the lower end of the vessel, independently of the wash liquor supply. The diluting liquor circulating duct 13 may be located, as shown in the drawings, outside the jacket of the cooking vessel 1, or also inside the jacket. The number of circulating ducts 13 may also vary and they may start from liquor separation points located at varied heights in the vessel.

Claims

1. A continuous method for cooking cellulose-based pulp, in which a fibrous pulp moves downwards in a cooking vessel (1), whose upper part forms a cooking zone (2), where cooking liquor moves concurrently with the pulp and the lower part forms a washing zone (3), where pulp is washed with a wash liquor conducted upwards counter-current relative to the pulp, and cooked pulp is discharged from the lower end of the cooking vessel, characterised in that, at a separation point (10) located in the washing zone (3) of the cooking vessel (1), liquid phase (12) is sepa- rated and transferred as wash liquor upstream relative to the pulp above the separation point, and that separated liquid phase (13) is additionally transferred below the separation point (10) to form a diluting liquor reducing the consistency of the pulp.

2. A method as defined in claim 1, characterised in that cooked pulp is removed from the cooking vessel (1) by means of a scraper (8) located at the lower end of the vessel.

3. A method as defined in claim 1 or 2, characterised in that at least part of the circulated diluting liquor (13) is combined with the wash liquor flow (7) fed to the lower end of the cooking vessel (1).

4. A method as defined in any of the preceding claims, characterised in that the circulation (13) of diluting liquor has been effected by branching off the washing cycle (12).

5. A method as defined in claim 4, characterised in that the washing cycle (12) directed upwards in the cooking vessel and the circulation of diluting liquor (13) directed downwards are maintained by means of a common pump (11) provided in the circulating duct system.

6. A method as defined in any of the preceding claims, characterised in that spent cooking liquor and wash liquor are removed from the cooking vessel (1) at the boundary between the cooking zone (2) and the subjacent washing zone (3).

7. A method as defined in claim 6, characterised in that the method is a Kraft cooking method, in which ligno-cellulose material such as wood chips, and white liquor are fed to the upper end of a cooking vessel (1), and in which spent cooking liquor and wash liquor join at the boundary between the cooking and washing zones (2, 3) to form black liquor (6) removed form the vessel.

8. A continuous pulp digester for implementing the method defined in any of the preceding claims, the digester comprising a vertical cooking vessel (1), feed pipes

(4) for feeding ligno-cellulose material and cooking liquor to the upper end of the vessel, a feed pipe (7) for feeding wash liquor to the lower end of the vessel, an exhaust pipe (9) for removing cooked delignified pulp from the lower end of the vessel, and an exhaust pipe (6) for removing residual liquor from the cook from the area between the upper and the lower ends of the vessel, characterised in that the washing zone (3) of the cooking vessel (1) is equipped with a means (10) for separating liquid phase, from where by-pass ducts (12, 13) have been conducted for transferring the separated liquor as wash liquor upstream relative to the pulp above the separation point, and also below the separation point (10) to form a diluting liq- uor reducing the consistency of the pulp.

9. A digester as defined in claim 8, characterised in that the means for separating the liquid phase is a sieve (10).

10. A digester as defined in claim 8 or 9, characterised in that the lower end of the cooking vessel (1) is equipped with a scraper (8) for expelling cooked pulp from the exhaust pipe (9).

11. A digester as defined in any of claims 8 to 10, characterised in that the by-pass duct (13) directed downwards from the separation point (10) joins the wash liquor feed pipe (7) so that at least part of the liquor fraction to be circulated as diluting liquor returns to the cooking vessel (1) along with the wash liquor.

12. A digester as defined in any of claims 8 to 11, characterised in that the by-pass ducts (12, 13) have been conducted externally of the cooking vessel (1) and equipped with a pump (11).