WO1995016817A1

WO1995016817A1 - Screen device

Info

Publication number: WO1995016817A1
Application number: PCT/SE1994/001045
Authority: WO
Inventors: Peter Olsson
Original assignee: Kvaerner Pulping Technologies Ab
Priority date: 1993-12-16
Filing date: 1994-11-08
Publication date: 1995-06-22
Also published as: SE9304177D0; AU1285595A; SE501243C2; SE9304177L

Abstract

The present invention relates to a screen device when displacing liquid through a chip bed, preferably in conjunction with the withdrawal of cooking liquid in a continuous digester for producing chemical pulp. The said screen device (5) preferably has a rectangular shape and is constructed from a multiplicity of rod profiles (6) which are fixed, parallel to each other, at a number of crossbars (7). The said rod profiles (6) are arranged with their longitudinal extent horizontal, and these rod profiles (6) together exhibit a wash-board-like surface facing towards the chip bed. This surface is produced by the surface of each rod profile (6) facing towards the chip bed being bevelled at a given angle, which can be optimized, so that a widening in the cross-sectional area of the digester occurs prior to each new rod, seen from above and downwards.

Description

SCREEN DEVICE

TECHNICAL FIELD

The invention described here is a screen device which is primarily intended to be used when displacing liquid through a chip bed. The preferable field of application is the withdrawal of cooking liquid when producing chemical paper pulp in a continuous digester.

STATE OF THE ART AND PROBLEMS

In today's society, one of the most important problems is that of the damage inflicted on the natural environment. The industry responsible for producing chemical pulp has realized the enormous importance of improving its production methods so that the discharge of environmentally damaging substances is minimized. Particularly significant progress has been made as regards the bleaching of chemical pulp. The novel, more environmentally friendly, bleaching methods make greater demands than was previously the case on the strength of each individual fibre in the pulp which is fed into the bleaching plant. The digester must therefore be designed so that the cooking process is as mild as possible for the fibres. One way to bring about mild cooking is to maintain profiles, of, for example, temperature and alkali content, which are as uniform as possible over the large cross-sectional area of the digester. In order to obtain sufficiently uniform cross-sectional profiles, great demands have to be placed on screen arrangements which make it possible to displace liquid through the chip bed in an efficient manner.

It has recently become increasingly apparent that large circulation flows, which pass through the screens, are required in order to obtain the desired uniformity in the process. As will be shown, these large flows signify problems for the current type of screens. It is therefore of the utmost importance to devise new, improved screen arrangements.

Present-day screens usually consist of rod screen sections which are often arranged in a chessboard-like pattern at the desired positions in the digester wall. Each "chessboard square" consists of a multiplicity of vertically arranged rods. Between each rod there is a gap through which liquid can be withdrawn. The rods are fixed in parallel at crossbars and the screen is provided with a frame of L iron (see Fig. 1) .

One of the problems with this known screen construction is that it has a relatively great tendency to become blocked by chip bits which become stuck in the gaps. This is due, to a large extent, to the fact that the liquid has a radial velocity, outwards through the screen, which is approximately 5 times greater than the velocity of descent of the chip bits. Typically, the velocity of the liquid is of the order of size of 10-15 mm/s, as compared with the 2-3 mm/s velocity of the chip column. Some of the chip bits accompany the larger radial liquid flow and then become stuck in the gaps between the screen rods. The problem is further aggravated by the difficulty in ensuring that the rods are exactly parallel in relation to each other. Thus, if a gap happens to be a little narrower at the bottom than at the top, it is easier for the chip bits to become stuck. Further chip bits can then come to lie on top of a chip bit which has become wedged, thereby blocking an even greater part of the screen. The effect is exacerbated by the fact that the loading on the area of the screen which remains open becomes greater the more of the originally open area is blocked, with increasing risk of occlusion as a consequence. Cleaning occluded screens is very laborious and is a procedure which is best avoided.

It is thus the case that screens as currently constructed are difficult to operate, and it is the view of many that the function of rod screens is a critical point in current KAMYR digesters.

SOLUTION AND ADVANTAGES The previously mentioned problems associated with using rod screens can be eliminated with the aid of the present invention.

The invention is based on the simple but ingenious idea of turning each screen part through 90° so that the rods become horizontal.

In order to be even more certain of avoiding the risk of occlusion, a preferred solution is proposed in which each rod profile is provided with a bevel, shaped so that a widening in the cross-sectional area of the digester is formed before the chip bits meet a new rod profile. This prevents the chip bits from directly encountering an edge when they leave the rod situated above during their journey downwards.

In order to ensure that the treatment of the fibres is as mild as possible, it is advantageous to have a gentle transition section at the upper edge of the bevel.

Thus, the great advantage of a screen element in accordance with the invention is that the digester, equipped with such a screen, will not be found to be so difficult to operate as is currently the case. By these means, the cooking process can be optimized so that the fibres, when departing from the digester, have such strength properties that they are able to survive the novel, environmentally friendly, bleaching methods.

An additional advantage resulting from this is that laborious and expensive cleaning work on the screens can be avoided.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below using the attached figures: Figure 1 shows a previously known screen element. Figure 2 shows the section A-A in Figure 1. Figure 3 shows a screen element which is preferred in accordance with the invention. Figure 4 shows the section B-B in Figure 3.

Figure 1 shows a known method of arranging a screen element. This screen element consists of a multiplicity of rod profiles which are fixed, vertically and parallel to each other, at a number of crossbars. The height of the screen element is typically in the order of size of 500 mm.

Figure 2 shows a cross-section indicating how the rods 2 in a known screen element 1 typically appear and how these rods are joined to the crossbars 3, which are in turn fixed to a frame of L iron 4. Steel is normally used in all parts and the method of assembly is welding. Note especially that the cross-sectional profile of the rods is symmetrical on each side of their vertical axis and that the surface of the screen facing in towards the centre of the digester is completely plane.

Figure 3 shows a preferred embodiment of a screen element according to the invention. The screen 5 is essentially rectangular and consists, in this case too, of a multiplicity of rod profiles 6 which are fixed, parallel to each other, at a number of crossbars 7, which expediently consist of flat bar steel. These crossbars are then, like the short sides of the rod profiles, fastened to a frame of L iron 8.

The method of joining is preferably welding and the parts included in the screen section are preferably manufactured from steel. The difference from the known embodiment is that the rods are arranged horizontally in the digester wall, that is perpendicular to the longitudinal axis of the digester.

The difference in the cross-sectional profile of the rods as compared with Figure 2 is clearly evident from Figure 4. At their surface facing in towards the centre of the digester, the rod profiles 6 according to the invention are bevelled at a given angle a^, which can be optimized, between the normal to the surface 9 and the horizontal plane, so that the screen assumes a wash-board-like appearance. The angle a^ is between 0 and 30°, preferably 10-20° and more preferably approximately 15°. As a result, the cross- sectional profile of the rods is not symmetrical. The surface of the screen is arranged so that an imaginary vertical line intersects the surface 9 of the rod profiles at essentially the same point on each rod.

In addition, the gap between the rods is such that it at least does not decrease, seen from the surface of the screen facing in towards the digester, and radially outwards. This is expediently brought about by the rod profiles being provided, at their surface facing in towards the chip bed, with a lip 10 at the bottom. The surface 9 of each rod profile is terminated in a sharp manner by the lower edge of the lip 10 being designed with a distinct angle a₂.

The transition section between the upper edge of the rod profiles 6 and the bevelled edge 9 can be rounded in order to provide a mild treatment for the chips. The radius is then at most equal to half the thickness t of the rod profile, preferably approx¬ imately 30% of the thickness t of the rod profile. This embodiment is shown in principle in the lowest rod profile in Figure 4.

The invention is not limited by the embodiment in the figure, but can be varied within the scope of the subsequent patent claims.

Claims

PATENT CLAIMS

1. Screen device when displacing liquid through a chip bed, preferably in conjunction with the withdrawal of cooking liquid in a continuous digester for producing chemical pulp. The said screen device (5) preferably has a rectangular shape and is constructed from a multiplicity of rod profiles (6) which are fixed, parallel to each other. The screen device is c h a r a c t e r i z e d i n that the said rod profiles (6) are arranged with their longitudinal extent horizontal, so that the chip column, during its passage downwards, essentially meets the rod profiles perpendicularly.

2. Screen device according to Claim 1, c h a r a c t e r i z e d i n that the said rod profiles (6) together exhibit a wash-board-like surface facing towards the chip bed.

3. Screen device according to Claim 2, c h a r a c t e r i z e d i n that the wash-board¬ like surface is produced by each rod-profile surface (9) , facing towards the chip bed, being bevelled at an angle (a ) so that a widening in the cross-sectional area of the digester occurs prior to each new rod, seen from above and downwards.

4. Screen device according to Claim 1, c h a r a c t e r i z e d i n that the said surface (9) of the rod profiles is intersected by an imaginary vertical line at essentially the same point.

5. Screen device according to Claim 1 or 3, c h a r a c t e r i z e d i n that the gap between the said rod profiles (6) at least does not decrease radially outwards.

6. Screen device according to Claim 5, c h a r a c t e r i z e d i n that the gap orifice between the said rod profiles (6) is made smaller at the surface of the screen facing in towards the chip bed by the rod profiles being provided with a lip (10) at the bottom.

7. Screen device according to Claim 6, c h a r a c t e r i z e d i n that the lower edge of the lip (10) is designed with a sharp and distinct angle (a₂) .

8. Screen device according to Claim 1 or 3, c h a r a c t e r i z e d i n that the transition section between the upper edge of the rod profiles (6) and the bevelled edge (9) is rounded, with a radius which at most is equal to half the thickness (t) of the rod profile, preferably approximately 30% of the thickness (t) of the rod profile.

9. Screen device according to Claim 1, c h a r a c t e r i z e d i n that the rod profiles (6) are fixed at a number of crossbars (7) .