Screen Device For a Digester For Producing Pulp

Abstract

A central screen element being located substantially parallel with the vertical axis of a pulp digester is provided. In the vertical direction, the screen element comprises at least one zone including one screen segment for removing a liquid stream from the digester and at least one outlet for introducing a liquid stream to the digester. The zones may be adjacent or separated and liquid streams may be independently removed and introduced, as required by the properties and the stage of the cooking process. A screen element may be used in both continuous and batch digesters.

Description

FIELD OF THE INVENTION[0001]The present invention concerns a screen arrangement of a digester, and more precisely, a screen element being located substantially parallel with the vertical axis of the digester, in the middle of the digester, and process chemical being arranged to flow through said screen simultaneously either into the digester or out of the digester, depending on the determinations of the process and the stage of the process. The field of use comprises especially feeding of process chemicals to the digester and removal of those from the digester when producing pulp or paper stock from wood chips in a batch or continuously operating digester.PRIOR ART[0002]In the technique of prior art, process chemicals are moved with respect to the material to be processed, for example when digesting pulp from wood chips so, that mounted to the inner surface of the process vessel there is a screen with apertures of the kind that chemicals can flow through the screen surface, but the ...

AI Technical Summary

Benefits of technology

[0017]This kind of a screen element installed in the middle of the digester increases the screen area of the digester, thus improving the screen capacity. As the screen element has a plurality of zones together with multiple channels, the feed and removal of process chemicals can be optimized so, that the digestion is generally better. The optimizing of the feed and removal means firstly, that the chemical can be fed to the right points in the longitudinal direction of the digester, and also to the middle of the cooking chip column. This is to ensure that there is enough of the process chemical and that it is evenly distributed throughout the whole chip column. On the other hand, the feed and removal flows have influence on the movements of the chip column and on the flows of the whole digester, and in the changing digestion conditions the best possible flowing conditions for each digestion stage are provided by adjusting the places of the feeds and removals. This, for its part, improves the quality and homogeneity of the pulp.

Problems solved by technology

In the technique of prior art, process chemicals are moved with respect to the material to be processed, for example when digesting pulp from wood chips so, that mounted to the inner surface of the process vessel there is a screen with apertures of the kind that chemicals can flow through the screen surface, but the material to be processed is not able to pass through the screen surface.

This requires big circulation and expansion flows.

Even a partial clogging of for example expansion and circulation screens causes channelling and disturbed flows in the digester.

For this reason it is not always possible to maintain such digestion conditions, that the pulp to be processed would be strong, pure and homogenous.

This causes increased consumption of raw material, energy and chemicals, which, in turn, increases production costs and environmental load.

As a result of the clogging, the screens can also be broken, when the support constructions fail.

On average, this causes need for renovation of screens.

Correspondingly, one problem of the screens used for the digestion in prior art is the fact that they clog easily, when the chip particles cling to the slots.

When the process chemical flow sucks the chips, sticks and pre-digested pulp against the screens, this and the radial component of the pressure resulted from the chip column cause a resultant force pushing the particles to the slots of the screens said particles clogging superpositioned from bottom upwards causing blocking, which in turn disturbs the plug flow and causes channelling of the flow.

New sticks and chip particles cling more easily to these clung particles resulting in clogging of a wider screen area.

The clogging increases continuously as the screen area decreases.

As a result of the strong clogging, the flows slow down also on the backside of the screen (between the screen and the digester jacket), resulting in that heavier and heavier particles are carried along to the backside of the screen and block the flow area between the screen and the jacket.

Due to the location of the screens and the way of blocking, they are difficult to clean, and it is tried to be avoided as far a possible, for example by changing the construction types of the screens to the same place on the inner surface of the digester, on the casing of the pressure vessel.

Problems related to the correct alkali distribution, suitable temperature profile, adequate liquid amount and optimal flowing conditions still exist, and no solution has been suggested by locating the screens for example into the middle of the digester.

One of the biggest problems is to provide an even distribution of the process chemical to the whole chip column without disturbing the flows.

Problems are caused, however, by the location of the central screen with respect to the feeding point of the washing liquor.

As a result of that, the central screen disclosed in U.S. Pat. No. 3,475,271 can be easily clogged.

Its weakness is the limited possibility of its construction with respect to the multipurpose operation.

The location of the screen is unprofitable, as for having quickly homogenous digestion.

Thus, among others, digestion time is wasted.

Firstly, because the screens are located only to the inner surface of the vessel, it is difficult to have the chemical evenly distributed to the chip column in the whole vessel.

As a result, the quality of the product received with this method is easily uneven.

Secondly, as the process chemical flow is effected extending to the whole area of the screen, the flow can change uncontrollably, as the single chip pieces of the chip column to be processed are moving during the digestion for example due to settling.

Thirdly, these kinds of screen constructions attached to the walls of a process vessel and typically to those of a pressurized process vessel are expensive.

In order to increase the capacity of the digester, however, the increasing of the screen area is unavoidable.

And fourthly, if increasing of a screen becomes necessary afterwards for improving the process, the installing of a screen of prior art type into an existing process vessel is difficult, expensive and slow.

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