Field of invention

The invention relates to an improved method to digest cellulose fibrous material in a continuous digester to produce pulp.

Background

The production of chemical pulp of cellulosic fibers is normally performed by cooking wood material, such as chips, in cooking liquor (or white liquor) comprising chemicals that loosen the fibers from the lignin. The most common cooking process of today is the kraft process (also known as sulphate cooking), wherein the active chemicals in the cooking liquor are sodium hydroxide and sodium sulphide. The cooking takes place in a pressurized vessel, usually designed as a large vertical cylinder, called the digester. In a continuous cooking process, wood chips and cooking liquor are fed into the top of the digester in a continuous stream while cooked pulp is blown out from the bottom of the digester. Thus, the chips/pulp continuously moves downwards in the digester. The cooking is interrupted by the introduction of cooler washing liquid as the pulp is being washed. Typically, continuous cooking is combined with counter- current washing of the cooked pulp in the same vessel. In such an arrangement, the upper part of the cooking vessel forms a cooking zone, where the pulp and the liquor move from the top downwards. The lower part of the cooking vessel forms a washing zone, where the pulp moves further downwards and cooking liquor is displaced with wash liquor and thus moves upwards, counter current up through the chip column. The residual liquors (spent cooking liquor and wash liquor) are drawn off through a screen at the end of the cooking zone and further treated in the evaporator plant.

One challenge related to the continuous cooking process is to maintain the washing efficiency when increasing the production volume. Normally, it is difficult to keep a high dilution factor in the digester when the production rate is increased. EP1764441 describes process of continuous cooking of pulp, which process is said to increase the washing efficiency at the bottom of a continuous digester. To solve this problem, an extra screen section is arranged above the lowermost screen section and a number of addition positions for washing liquid are distributed around the

circumference of the digester between the uppermost part of the lowermost screen section and the lowermost part of the extra screen section. However, in such an arrangement, the extra washing liquid will be added peripherally into to the digester, in-between two screen sections, whereby the displacement efficiency will be very low. Consequently, the effect on the washing efficiency by this arrangement will be very limited.

Thus, there remains a need to provide a method and arrangement to enhance the washing capacity of a continuous digester at an increased production rate.

Description of the invention

One object of the present invention is thus to maintain or increase the washing efficiency in a continuous pulp digester when increasing the production volume.

The invention discloses a method of continuous cooking cellulose fibrous material in a digester, said digester having a top and a bottom. The method of the invention comprises the steps of:

- feeding a slurry comprising cellulosic fibrous material and cooking liquor into the top the digester,

- feeding a first washing liquor into the bottom portion of the digester, which first washing liquor moves counter currently up through at least part of the digester,

- removing pulp from the bottom of the digester,

- feeding a second washing liquor into the center of the digester at least at the level of a first screen section,

- feeding a third washing liquor into the center of the digester at the level of a second screen section, and

- withdrawing the cooking liquor and the washing liquors from the first screen section and from the second screen section, which first screen section is arranged in a lower part of the digester and which second screen section is arranged below said first screen section.

Thus, in accordance with the invention, a third washing liquor is added to the center of digester at the level of an additional, second extraction screen section arranged in the lower part of the digester. The method of the invention ensures that the

delignification reaction takes place in as big volume as possible in the digester. The method further makes it possible to increase the production rate in the digester but still maintain or even increase the washing efficiency. The addition of said third, washing liquor to the center of the digester ensures a high displacement efficiency, which further enhances the washing efficiency.

In the context of this application, the expression that the washing liquor is fed "into the center" of the digester means that the washing liquor is fed into the flow of pulp/liquors from a point or from several points located inside the digester. Thus, the washing liquor is not added in positions distributed around the circumference of the digester. Preferably, the washing liquor is fed or added in the middle, or substantially in the middle, of the vertical flow of pulp/liquors.

The expression "at the level of the extraction screen section" means a level within the area defined by the extraction screen section and 1 meter above the top edge of said screen section.

The term "screen section" includes one screen row or several screen rows arranged sequentially onto each other in the digester in the direction of the main flow of the cellulose fibrous material slurry.

The term "top of the digester" is meant to include the top portion of the digester and the "bottom of the digester" is meant to include the bottom portion of the digester.

The term "lower part of the digester" means the part of the digester where washing of the pulp takes place.

The invention further relates to a continuous digester, having a top and a bottom, for the production of chemical pulp by cooking wood chips in cooking liquor, which digester comprises;

- an inlet arranged at the top of the digester for feeding the wood chips and the cooking liquor into the digester,

- an outlet arranged at the bottom of the digester for removal of the pulp,

- at least a first and a second screen section, which first screen section is

arranged in the lower part of the digester and said second screen section is arranged below said first screen section,

- means for feeding a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester,

- means for feeding a second washing liquor into the center of the digester at least at the level of the first screen section, and

- means for feeding a third washing liquor into the center of the digester at the level of the second screen section,

- means for withdrawal of the cooking liquor and the washing liquors from at least the first screen section and from the second screen section.

Said "means" for feeding said first, second and third washing liquor may be, e.g. an inlet pipe or a similar arrangement. Example

The invention is further described by way of an example.

Fig. 1 shows a schematic view of a digester in accordance with the invention.

The continuous digester shown in fig. 1 is in the form of a pressurized, essentially cylindrical, vessel (1 ) comprising a top and a bottom. Cellulose fibrous material, e.g. wood chips, and cooking liquor are continuously fed into the digester through an inlet (2). In the vessel, the chips form a porous chip column descending downwardly in the digester. The cooking liquor may e.g. be kraft cookling liquor comprising sodium hydroxide and sodium sulphide. Cooked pulp is continuously removed or blown from the bottom of the digester through an outlet (3). Along the vertical center line of the vessel (1 ), central pipes (5a, 5c) are arranged, through which pipes separate liquor entry flows may be conveyed to different levels of the vessel. Said central pipes (5) may comprise an upper pipe (5a) and a lower pipe (5c) as shown in fig. 1. The skilled person realizes however that these pipes may be arranged in several different modes, e.g. a first pipe having an outlet in a first level may be concentrically arranged in a second pipe, having an outlet in a second level above said first level.

One or several extraction screen structures (4) may be arranged in the vessel wall of the upper or middle part of the vessel, from which screen structures cooking liquor may be removed (4a) from the digester to be reprocessed at another location.

In accordance with the invention, a first and a second extraction screen section (6 and 7) are arranged in the lower part of the digester, which first extraction screen section (6) is arranged in the lower part of the digester and said second screen section (7) is arranged below, but at a relatively small distance, from the first extraction screen section (6). The vertical distance between the lowermost part of the first extraction screen section (6) and the uppermost part of the second extraction screen section (7) may be, e.g., 0, 1 - 15 m, preferably between 0,5 - 5 m. The first extraction screen section (6) may be arranged above said second screen section at a distance corresponding to a retention time of between 0 - 30 minutes, preferably of between 0, 1 - 20 minutes and even more preferably of between 5 - 15 minutes.

A first washing liquor (8a, 8b) is pumped into the bottom part of the vessel (1 ). Said first washing liquor may be added through a single inlet or through several inlets arranged in the bottom part of the vessel. The upwards-flowing washing liquid displaces the cooking liquor towards the extraction screens (6 and 7). A second washing liquor (9) is fed into the center of the digester, and into the middle of the descending chip column, through the upper pipe (5a) having an outlet (5b) at the level of the first extraction screen section (6). A third washing liquor (10) is fed into the center of the digester, and into the middle of the descending chip column, through the lower central pipe (5c) having an outlet (5d) at the level of the second extraction screen section (7). The first, second and third washing liquors may originate from one, single, liquor fed from an earlier washing stage (13).

Cooking liquor and washing liquor (1 1 , 12) are withdrawn from said first extraction screen section (6) and from said second extraction screen section (7) and further treated in a flash and evaporation plant (not shown). Part of the liquor (12a) may be circulated back to the digester in order to enhance the distribution and mixing in the digester.

Preferably, the flow (m3/ADT), or volume per produced ton of pulp, of added washing liquor at the level of each extraction screen section is equal or less than the flow of liquors withdrawn from said extraction screen section. In this way, an efficient displacement is achieved and it is possible to maintain a stable chip column movement at the extraction screen sections. Furthermore, concurrent flow of liquors in the middle part of the digester is maintained. Thus, the flow of the added second washing liquor (9) is equal to or less than the flow of the liquors (1 1 ) withdrawn from the first extraction screen section (6). The flow of the added third washing liquor (10) is equal to or less than the flow of the liquors (12) withdrawn from the second extraction screen section (7).

It is to be understood that the invention is not limited to the particular process steps and arrangements described herein. Further extraction screens and washing liquor addition points may e.g. be arranged in sequence above each other. The method of the invention may e.g. comprise drawing off cooking liquor in a third and/or a fourth extraction screen section/s and addition of wash liquor at the level of said third and/or fourth extraction screen section/s.