DIGESTER FOR CONTINUOUS COOKING OF FIBRE MATERIAL

Abstract: The present invention relates to a digester for continuous cooking under raised pressure and temperature of fibre material in a vertical digester (1), where input of fibre material and cooking liquid takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one digester screening arrangement (1D) between the top and the bottom of the digester, and fibre material is fed out from the bottom (1C) of the digester, wherein at least one of said screening arrangements (1, 2) has at least one screen element (2A) of which the main configuration is circular and which preferably is assembled by means of welding, and which is fitted into the digester shell by means of welding.

Description: Digester for continuous cooking of fibre material. The environmental authorities are placing ever more stringent demands on the pulp industry to decrease the use of chemicals which can be damaging to the envi¬ ronment, such as, for example, chlorine. Thus, permitted discharges of organic chlorine compounds in the waste water from bleaching plants, following on from the cook¬ ing process, have been decreased progressively and are now at such a low level that pulp factories have in many cases stopped using organic chlorine compounds as bleach¬ ing agents. In addition, market forces are tending pro¬ gressively to increase the demand for paper products which are not bleached with chlorine. The pulp industry is therefore searching for methods which allow bleaching of pulp without using these chemicals. The lignox method (see SE-A 8902058) , in which, inter alia, bleaching is carried out with hydrogen peroxide, may be mentioned as an example of such a method. Ozone is another interesting bleaching chemical which is also gaining increased application. It is thus possible, using bleaching chemicals of this nature, to achieve those brightnesses which are required for marketable pulp, i.e. 89 ISO and greater, without using chlorine-containing bleaching agents. There is, however, a problem in using presently- known bleaching procedures with these bleaching chemicals which do not contain chlorine, namely that they have a relatively large effect in diminishing the quality of the pulp fibres. By means of experiments which have been conducted under the auspices of Kamyr AB, it has been found, sur¬ prisingly, that extremely good results, with regard to delignification and strength properties, can be obtained if the pulp is cooked at the same temperature level in principally the whole of the digester, i.e. if essential¬ ly the same temperature is maintained in all cooking zones, and if a certain quantity of alkali is also sup¬ plied to the lowest zone in the digester, which zone is normally used for counter-current washing. Owing to the fact that essentially the same temperature level is maintained in virtually the whole of the digester, very extensive delignification can be achieved at a relatively low temperature. Besides this, it has been found that the strength properties are affected in a particularly favourable manner, that a higher yield of the crude fibre product is obtained and that the quantity of reject material decreases. These advantages are most clearly apparent from the diagrams shown in the Figures 1 and 2, which show comparative values between pulp (softwood) which has been cooked using a conventional, modified cooking technique and pulp which has been cooked using the process according to the invention, (in a similar digester, i.e. with a concurrent upper cooking zone, a central counter-current cooking zone and a bottom counter-current washing zone) in which a constant temper¬ ature level of about + 155°C has been maintained in the whole digester. The invention especially relates to (but not exclusively) an advantageous arrangement.of a set of apparatus for achieving a cooking according to the new process, in particular with regard to digesters built according to an older principle and consisting of an upper concurrent cooking zone and a lower counter-current washing zone. Such an arrangement is necessary since certain practical problems arise as a consequence of an isothermal cooking process. The first such problem is the difficulty of efficiently reaching and maintaining the temperature in the lower part of the digester, i.e. that part which is normally employed for washing. The main object is to create a more efficient screening means in order to improve the circulation and as a consequence also the temperature distribution in the digester. In this context it has been found to be advantageous to use digester screening arrangements consisting of circular screens, especially in connection with converting existing digesters, both of the modified type and the older type, for operation according to the new process, but also in connection with building of new digesters. Short description of the figures In Figure sheet 1, a comparison is made in three diagrams between isothermal cooking and so-called modified conventional cooking (MCC) . Figure sheet 2 shows a diagram which describes degree of delignification and viscosity (the viscosity is normally regarded as indicat¬ ing the strength properties of the pulp) , and Figures 3A, B and C shows how, in a preferred manner, an existing digester can be converted, using circular screens, to be operated according to the novel process. Figure 4 shows a cross sectional perspective view of a preferred embodiment of a circular screen according to the invention, figure 5 shows a screen of figure 4 seen from the inside of the digester vessel, figure 6 is a vertical cross sectional view of the same and figure 7 is a horizontal cross sectional view of said preferred screen. Detailed description The first figure page shows three diagrams which compare different results obtained with isothermal cook¬ ing and conventional modified cooking (MCC) . These sur¬ prisingly positive results show, according to the upper diagram, that, with a given amount of added alkali, substantially lower kappa numbers are obtained using isothermal cooking. Furthermore, the second diagram shows that manifestly improved strength properties are obtained when cooking down to the same kappa number. In addition, the third diagram shows that there is also the advantage that the quantity of reject wood (shives) decreases. If the fact is also taken into account that overall substan¬ tial energy savings are made when the temperature level is kept constant, it is evident that the results may be regarded as being surprisingly positive. Figure 2 additionally demonstrates that, using the method according to the invention, very low kappa numbers are reached while at the same time retaining good pulp strength (viscosity round about 1000) after oxygen delignification. Thus, when employing the method according to the invention, so-called environmentally friendly bleaching chemicals, such as peroxide and ozone, can be employed in subsequent bleaching stages without risking too low a strength for bleaching up to the level of brightness, and therewith also the level of purity, which the market demands. Figure 3A shows the lower part of a digester 1, which is intended to represent an existing digester shell on which has been arranged a new digester screening arrangement 2 in order to be able to raise the temperature in the counter-current zone. The digester is of the type which has an upper concurrent part and a lower counter-current part. In such a digester, full cooking temperature is normally maintained in the con¬ current zone (i.e. about 162°C for hardwood and about 168°C for softwood) while in the counter-current part, which in the main is a washing zone, the temperature is about 135°C on a level with the lower screen. In the following text, the counter-current zone of the digester which has been fitted with a further screening arrangement will be referred to as a cooking zone, even if it is to be considered as a washing zone according to conventional operation. The new digester screening arrangement 2 (in figure 3A) consists of a number of circular screens 2A for withdrawal 3 of cooking liquid in the lower part of the digester and is arranged immediately above the lower screening arrangement IB, preferably at most 1.5 metres above and more preferably at most 1 metre above, measured from the upper edge of the lower digester screening arrangement to the lower edge of the newly fitted digester screening arrangement. Wash liquor is supplied to the lower part of the digester through an inflow arrangement 4 attached in the vicinity of the bottom IA of the digester and cooking liquid (alkali addition) through the central pipes 5A, 5B. The cooked pulp is taken out from the bottom of the digester via a conduit IE. One of these central pipes, 5A, which belongs to the original system of the digester, penetrates down to the lower screening arrangement IB of the digester, after which the liquid, after heating via the first heat exchanger 6A, discharges through the said pipe on a level with the latter digester screening arrangement. Subsequently, a part of the liquid flows in a counter- current direction upwards towards the newly fitted digester screening arrangement 2. The liquid withdrawn from this system passes through the said conduit arrange¬ ment 3 and is heated via a heat exchanger 6B to the desired temperature before it discharges, via a second, newly fitted central pipe 5B, immediately above the newly fitted digester screening arrangement 2. A part of the cooking liquid supplied in this manner, which liquid has thus reached the desired temperature (e.g. 158°C) , chemical strength and distribution (spreading) over the whole of the cross-section of the digester, continues to flow upwardly in the digester. In a central digester screening arrangement ID, the spent cooking liquid, together with undissolved wood material, is drawn off for further treatment. The surface of each screening element 2A is made relatively small, preferably less than 0.3 m 2 . An advantage of screening elements of small area is that efficient back flushing can be achieved, which is often of great importance if the circulation flow is to func¬ tion efficiently. The new screening arrangement 2 is pre¬ ferably fitted with ring pipes 2C from which an indi¬ vidual conduit goes to each and every one of the screen¬ ing elements 2A. Using such a construction, and a valve arrangement belonging to it, a limited number (for example 4) of screening units 2A can be efficiently back- flushed at a time. Owing to the relatively small total screening surface which is back-flushed under these cir¬ cumstances (for example 0,5-1 m 2 ) , a very efficient back- flushing which cleans the screens is obtained, thereby ensuring that the circulation is highly efficient. In figure 3B it is shown a first embodiment of how such a back flushing system can be arranged. The back flushing liquid is collected via a branch conduit 7 (the main conduit for back flushing) from the liquid which circulates from the circular screens 2A via conduit 3 and out through central pipe 5B. The liquid which is fed into the main back flushing conduit 7 is there after sequentially fed to the different screens 2A by means of a number of valves 8, 9 (see enlarged part of figure 3B) . Beside the two valves needed for each screen 2A for providing the back flushing there is also provided a main valve 10 which provides for the possibility of shutting off the liquid supply from and to a screen totally. The liquid is withdrawn from the screen element 2A via a ring pipe 2C (and further via main pipe 3) and accordingly the main valve 10 and withdrawal valve 9 would then be opened whereas the back flushing valve 8 would then be closed. During back flushing the main valve 10 is opened, the withdrawal valve 9 is closed and the back flushing valve 8 opened. Preferably this is performed in a sequential manner so that four screens are closed for back flushing (e.g. all four at the same time) meanwhile the remaining screens, e.g. 20, would withdraw liquid. Hence preferably the pressure in the main conduit for back flushing 7 would be substantially equal. Instead of back flushing all four screens at the same time it is possible to back flush them two and two in order to increase the flow over each screen. In figure 3C it is shown a preferred embodiment of how to arrange a back flushing system. Also here there is a main conduit 3 for withdrawal of a liquid and main pipe 7 for the supply of back flushing liquid. Two screen elements 2A are interconnected with each other via a conduit forming a loop. This loop has an upper part 13A interconnected with the back flushing conduit 7 via branch conduit 7A. A valve 11 is arranged in this branch conduit 7A. The lower part of the loop 13B is interconnected with a branch conduit 3A which is joined with the withdrawal conduit 3. A valve 12 is fitted in the withdrawal branch conduit 3A. During withdrawal the valve 11 in the upper branch conduit 7A would be closed whereas the withdrawal valve 12 would be opened. Liquid will then be withdrawn from both of the screens 2A via the lower part of the loop 13B and the branch conduit 3A and further into the withdrawal conduit 3. During back flushing, which is performed sequentially, the upper valve 11 will open and the lower valve 12 will close and the back flushing liquid will then be introduced via branch pipe 7A through the upper part of the loop 13A into both of the screens 2A in order to rinse the screen faces. The advantage with the latter described embodiment is that the number of valves required is reduced, in relation to a conventional arrangement. In figures 4-7 the exact design of a preferred screen element 2A is shown. The screen 2A is shown fitted onto the digester wall 1. The screen is of the rod screen type, wherein rods 14 are used to form the screen face 15. The rods are supported by, preferably horizontal bars 16, which preferably would be made of stainless steel having a very high quality whereby preferably RP would exceed 200 MPa (more preferred 300 MPa) . The rods 14 are welded onto said bars 16. Furthermore the screen 2A also consists of an annulus 17 which preferably consists of a bent and welded plate. At the top and the bottom of this annulus 17 there are recess slots 18, so that an inwardly facing edge 19 is formed against which the top and bottom respectively of a rod 14 can rest. The bars 16, preferably at least two or three are welded within said annulus 17 so that its inwardly facing edge is levelled with said facing edge 19 of the annulus 17. Accordingly the rods 14 are supported not only by the bars 16 but also by said edge 19 of the annulus 17. An advantage with this arrangement is that the screen faces then can be installed in a manner to avoid edges which could cause the downwardly moving pulp to hang. The annulus with the rod 14 and bar 16, is fitted within a hollow element 21, which preferably consists of a forged cylinder 21. The cylinder 21 has a groove 21A (preferably turned) which is intended to receive the annulus 17, so that the annulus 17 can rest on the inwardly facing edge 2ID of said groove 21A. Furthermore the cylinder 21 is provided with a seal plate 2IB through which the inlet and outlet pipe 22 protrudes. The lower inner part of the cylinder 21 has a large tapered portion 21C in order to further eliminate possible risks of hanging of the pulp. A further object of this tapered portion 21C is to provide for attachment of the annulus 17 within said cylinder 21 by means of a weld 23A in the bottom region, away from the screen face 15 without creating any edges, which could cause hanging. This weld 23A can be performed in one piece, thanks to the rods 14 being positioned in slots 18 in the annulus 17. In figure 5 it is shown that the screen preferably is fitted within the cylinder 21 by means of welds 23, preferably four welds 23. One weld at the bottom 23A and one at the top 23B and two on each side 23C. It should be noted that the weld at the top 23B need not to be fitted within a large tapered portion (but possibly in a grinded groove) since the possible disturbing edges caused by this weld can be eliminated by means of grinding. The two welds at the sides will have no or little effect to the moving pulp since they are arranged along the part of the circle where they are almost vertical. This is important since otherwise the screen element would cause hanging of the pulp. In figure 6 a cross sectional along the vertical line shows that the screen 2A has an outlet and an inlet device 22 with fittings 22A in order to enable withdrawal of liquid as well as supply of back flushing liquid. In figure 7 there is shown a cross sectional view along a horizontal line of a preferred screen element 2A. It should be noted that not only the annulus 17 rests upon the inwardly facing edge 2ID of the groove 21A in the cylinder 21. Also the outer edge portions of the bars 16 rest on said edge 2ID. Accordingly it is important that when the different parts 14, 16 and 17 forming the screen face have been assembled, the outer periphery of the back of said assembly is in alignment in order to rest evenly on said edge 2ID. Preferably said assembly 14, 16, 17 is cut in order to obtain said aligned cylindrical surface. Furthermore it should be noted that all attachments are made by welding. As shown in figure 4 also the cylinder 21 is welded onto place within the digester shell 1. It is advantageous to use welding (compared to bolts) since sealing problems are avoided. Normally a screen element 2A according to the invention would first be assembled by welding the bars 16 into the annulus 17. The cylinder 21 with its sealing plate 2IB would preferably be assembled thoroughly so that the pipe 22 with fittings 22A would also be put on place. Finally the screen face 15 is positioned within the cylinder 21 by means of welds, preferably as described above. When the screen face must be exchanged this can be performed from the inside of the digester vessel by eliminating (e.g. grinding) the welds. 23 and thereafter taking out the screen face (parts 14, 16, 17) and replace it with a new one. During production of the screens the cylindrical form is advantageous since many operations could then be turned, etc. , etc. A further advantageous is that the hole and the weld in the digester vessel will have an annular form which is an advantage concerning the structural strength of the vessel. When the screen face is changed there is also a major advantage residing in the fact that the piping (on the outside) need not to be dissembled. The invention is not limited by that which has been described above, but can be varied within the scope of the subsequent patent claims. Thus, an existing digester of the MCC type can also be arranged in accordance with the invention, where, therefore, the digester has an upper concurrent part, a central, mainly counter-current, part and a lower counter-current part, where addition of a part of the cooking liquid takes place in the said lower counter-current part, the so-called high-heat zone. A digester of the so-called hydraulic type, with a lower temperature in the upper part (the impregnation zone) , may also advantageously be fitted with a digester screening arrangement according to the invention for cooking according to the invention, so-called iso- thermally. Additionally the preferred method may be used in connection with all types of cooking liquid, even if the method is principally intended for producing sulphate pulp. In addition, it is obvious to the person skilled in the art that the invention is not limited to the above mentioned exemplifying temperature levels. In this connection, however, it is important that the average temperature level in the digester preferably exceeds +150°C but is lower than +165°C, and preferably is between 150-155°C for hardwood and between 160-165°C for softwood, and furthermore that the average temperature in the cooking zone/zones is preferably about +151°C +-1°C, when the wood is hardwood, and that the average tempera¬ ture in a digester is +159°C +-1°C, when the wood is softwood. In addition, it is understood that screens deviating from a purely circular form, for example oval screens, may also be used, whereby, for technical reasons related to the construction, the smallest radius of curvature should preferably not fall below 0.2 m. Further it is stressed that both old and new digesters can be fitted with screens according to the invention. Further it should be noted that the basic design concept could also be used together with other screen faces than the rod-type, e.g. slotted screen faces. Moreover it should not be excluded that all screens are made of this circular type. Finally it is possible to only let the outer casing 21 be circular and to have an angular screen face assembly (e.g. rectangular) positioned therein. Instead of letting the screen face assembly rest on an edge formed by a groove 21A, it could rest on non- integral parts, such as screws, etc.

Claims: PATENT CLAIMS 1. Digester for continuous cooking under raised pressure and temperature of fibre material in a vertical digester (1) , where input of fibre material and cooking liquid takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one digester screening arrangement (ID) between the top and the bottom of the digester, and fibre material is fed out from the bottom (1C) of the digester, and at least one screening arrangement (2) in the lower half of the digester, characterised in at least one of said screening arrangements (1, 2) has at least one screen element (2A) of which the main configuration is circular and which preferably is assembled by means of welding, and which is fitted into the digester shell by means of welding. 2. Digester according to Claim 1, characterised in said screen element (2C) comprising a screen face assembly (14, 16, 17) which is supported by inwardly facing edges (21D) of a hollow part (21) having a circular outer form. 3. Digester according to Claim 2, characterised in that said edges (21D) are integral with said hollow member (21) . 4. Digester according to Claim 2, characterised in that said screen assembly (14, 16, 17) is welded into place within said hollow member (21) . 5. Digester according to Claim 1, characterised in that said hollow member (21) is arranged with sealing plate (2IB) through which an inlet and outlet pipe protrudes (22) . 6. Digester according to Claim 3, characterised in that said welds (23) are positioned so that hanging of the pulp is avoided. 7. Digester according to Claim 6, characterised in that at least two welds (23C) are used which are positioned substantially vertical. 8. Digester according to Claim 1, characterised in that the said digester screening arrangement (2) consists of a number of screen (2A) designed to withdraw displaced liquid for supply to a central pipe (5B) , which preferably discharges immediately above said screening arrangement (2) . 9. Digester according to Claim 8, characterised in a further lowermost screening arrangement (IB) and that the distance between the upper edge of the lowest digester screening arrangement (IB) and the lower edge of the second lowest digester screening arrangement (2) is less than 5 m, preferably less than 2 m and most preferred less than 1 m. 10. Digester according to Claim 8, characterised in that the temperature of the fibre material and upwardly flowing liquid which is located at or above the upper edge of the upper digester screening arrangement (2A) deviates from the temperature in the remaining cooking zone or cooking zones by at most 4°C, preferably 2°C, most preferred 1°C.

Inventors: BACKLUND AAKE, BELLSTROEM KENNETH, OULIE FINN, SOEDERQVIST SOEREN, SVANBERG JOHANNA, BACKLUND, ANGSTROM KE, BELLSTROEM, KENNETH, OULIE, FINN, SOEDERQVIST, SOEREN, SVANBERG, JOHANNA
Applicants: BACKLUND AAKE, BELLSTROEM KENNETH, KAMYR AB, OULIE FINN, SOEDERQVIST SOEREN, SVANBERG JOHANNA, BACKLUND, ANGSTROM KE, BELLSTROEM, KENNETH, OULIE, FINN, SOEDERQVIST, SOEREN, SVANBERG, JOHANNA
Application year: 1993
Main classification: D21C3
IPC codes: D21C3, D21C7