CA2149423C

CA2149423C - Process for continuous cooking of pulp

Info

Publication number: CA2149423C
Application number: CA002149423A
Authority: CA
Inventors: Ake Backlund; Johanna Svanberg
Original assignee: Kvaerner Pulping AB
Current assignee: Valmet AB
Priority date: 1992-11-18
Filing date: 1993-10-08
Publication date: 2004-03-16
Anticipated expiration: 2013-10-08
Also published as: JP3287848B2; JP3287849B2; AU4096693A; BR9307480A; NO951973L; DE69310929T2; NZ257659A; SE513746C2; US5827401A; ZA937959B; RU2126470C1; FI952387A; NZ252758A; NO951973D0; AU684759B2; EP0669998B2; DE69310929D1; DE69317670T2; SE9402412D0; FI952388A

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 strainer girdle (1D) between the top and the bottom of the digester, and fibre material is fed out from the bottom (1C) of the digester, wherein an existing digester (1) is fitted with an additional di-gester strainer girdle (2) above the lowest strainer girdle (1B) of the digester so that the temperature in all the participatory coo-king zones in the digester can he kept at essentially the same temperature level.

Description

VN~ 94/i1S66 PCT/SE93/00816 ~~~~~~~~
Presses far canta.nuaus caal~3.ng of pulp Technical field This invention relates to a method for continuous cooking of pulp, which method improves pulp quality so that sub-sequently none chlorine containing~bleaching processes ~.0 can be used in order to reach full brightness for a pulp having desired strength and quality properties.
' ~ Background and t~roble~a I5 The environmental authorities are placing ever more stringent demands on the gulp industry to decrease the use of chemicals which can be damaging to the envi-ronment, such as, for example, chlorine. Thus, permitted d~,scharges of organic chlorine compounds xn the waste 2o water from b3.eaching plants, f~llowing on from the cook-ing process, have been decreased pR ogressively and are now at such a low level that pulp factories have in many cases stopped using organic chlorine compounds as bleach-ihg agents. In addi~ion,,mar%~t forces are tending pro-25 gressively to increase the demand for paper Products which are nod bleached with chlorine or chlorine containing bleaching agents:
The pulp industry is therefore searehing,for methods 30 which allow bleaching o~f pulp without using these che~i-cals. The lignox method (set SB-A 8~Ox058~, in which, inter alia, bleaching 3s carried out with hydrogen peroxide; may be mentioned as an example of such a method. O~ene is another interesting bleaching chemical 35 which is also gaining increased application. It is thus possible, using bleaching,chemicals of this nature, to VVO 94111566 ~ ~ ~ ~ ~ , achieve those brightnesses which are required for market-able pulp, i.e. 89 ISO and_greater, without using chlo-rine 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 rela-tively large effect in diminishing the quality of the pulp (fibres.
From 1~P 476 230 there is known a method for continuous cooking under raised pressure and temperature of fibre material in a vertical digester, where input of fibre ma-terial and .cooking liquid takes place at the tap of the digester, withdrawal of sgent cooking liquor is carried out from at least one digester strainer girdle between the top and the bottom~f the digester, and (fibre mate-rial is fed out from the bottom of the digester, wash ~.iquidvis supplied at the bottom of the digester, which wish liquid is withdrawn~f~om a lowermost strainer girdle, whereby the digester comprises a digester strainer girdle between said lowest strainer girdle and said strainer girdle for spent liquor, which in-between strainer girdle is attached to a heating device.
This known method indicates that the quality of the pulp fa.bres~ after conta.nuous,cooking can be somewhat improved if cooking liquid is added to the wash circulation in the lower part of the digester and the temperature is raised.
In order to be able to bleach pulp to sufficient bright-, ness Without the use of ehlorirre containing bleaching agents, however,,th~is known method is not always suf f icient .
Solut~.on and advantages BY means of experiments wha.ah have been conducted under the auspices of lCamyr AB, it has been found, surpris-9~V~I'~~/11556 ~ ~ ~ ~ '~ ~ ~ PCT/SE93/00~16 ingly, that extremely good results, with regard to delig-nification and strength properties, can be obtained if the pulp is cooked at the same temperature level in prin-cipally the whole of the digester, i.e. if essentially the same temperature is maintained in all cooking zones, and if a certain quantity of alkali is also supplied to the lowest zone in the digester, which zone is normally used for counter-current washing. Owing to the fact that essentially the sameltemperature level is maintained in LO virtually the whole of the digester, very.extensive de-lignifacation can be achieved at a relatively low tem-perature, aesides this, it has been found that the - strength properties. are affected in a parta,cularly favourable manner, that a higher yield of the crude fibre 1.5 product is obtained and'that the quantity of reject mate-ra:al decreases: These advantages are most clearly apgar-en~ from the diagrams shown in the Figures 1 and 2, which show comparative values between pulp (softwood) which has been cooked using a conventional, modifiedcooking tech-2 0 pique and pulp which has been cooked using the process according to the invention, (in a similar digester, i.a.

with a concurrent upper cooking zone, a central counter-current cooking zone and a bottom countercurrent washing xon~) in which a constant temperature level of about 25 + x.55C has been maintained in the whole digester.

Tie invent~.on mainly relates to a method for achieving a cooking according to the new process, but also to a pre-feared arrangement f~r achieving a cooking according to tion iri 'pa~'ticular'with regagd to 'digesters nve 30 n the i lbuilt according to ~n older principle and consisting of an upper concurrent cpoking zone and a lower counter-current washing zone. Such an arrangement is necessary since certain practical problems arise as a consecsuence 35 of an isothermal cooking process. The first such problem the difficulty of efficiently reaching and maintaining i s the temperature in the lower part of the digester, i.e. that part which is normally employed for washing. The said problem is solved by creating a more efficient circulation and thus temperature distribution in the lower part (the high-heat or washing zone) of the digester. This is achieved by arranging a strainer girdle adjacent above the lowest strainer girdle, somewhere between the strainer girdle for withdrawal c>f spent liquor and said lowest strainer girdle, whereby said in-between strainer is positioned in such a manner that its lowest part is positioned less than 5 meters from the upper part of said lowest strainer and that cooking liquid is supplied to that liquid flow which has i.ts temperature controlled by means of the heating device, which flow discharges adjacent the level of said lowest. strainer in by means of a central pipe, so that the temperature in the cooking zone immediately above the said additional digester strainer girdle can be maintained at essentially the same temperature level as the remaining cooking zone or cooking zones of the digester.
In this context it has been found to be advantageous to use digester strainer girdle consisting of oval or circular strainers, in particular so-called manhole strainers, in connection with converting existing digesters, both of the modified type and the older type, for operation according to the new process.
According to one aspect of the present invention, there is provided a method for continuous cooking under raised pressure and temperature of fibre material in a vertical digester, having a top and a bottom, where input of fibre material and cooking liquor takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one strainer girdle for spent liquor 4a between the top and the bottom of the digester, and fibre material is fed out from the bottom of the digester, wash liquor is supplied at t:he bottom of the digester, wherein said method liquor is withdrawn from a lowest digester strainer girdle and said liquor, after heating via a first heat exchanger, by means of a first central pipe, discharges on a level with the said lowest digester strainer girdle, and wherein said method the digester comprises an upper digester strainer girdle between said lowest digester strainer girdle and said at least one strainer girdle for spent liquor, wherein t:he upper digester strainer girdle withdraws spent liquor, via a second heat exchanger, and a second central. pipe, arid discharges the spent liquor above the said upper digester strainer girdle, and wherein the digester comprises at least one conduit arrangement, for addition of cooking liquor :in connection with one of the said lowest digester strainer girdle and said upper digester strainer girdle, wherein the method the cooking is carried out isothermally, and said upper digester strainer girdle is positioned in such a manner that a lowest part of the upper digester strainer is positioned :Less than 5 metres from .an upper part of said lowest digester strainer girdle, wherein the method an efficient: circulation and temperature distribution is created towards the bottom of the digester and wherein the method discharged liquors from the first and second central pipes ara heated to such a temperature that the temperature in a cooking zone immediately above said upper digester strainer girdle is maintained at essentially the same temperature level as in remaining cooking zone or cooking zones of the digester.
Short description of tree figures In Figure sheet 1, a comparison is made in three diagrams between isothermal cooking and so-called modified 4b conventional cooking (MCC). Figure sheet 2 shows a diagram which describes degree of delignification and viscosity (the viscosity is normally regarded as indicating the strength properties of the pulp), and Figure 3 shows how,, W~ 94i115bb 1PC'I'1SE93140816 in a preferred manner, an existing digester can be con-vez°ted, using manhole strainers, to be operated according to the novel process.
Detai ed description The first.ffigure page shows three diagrams which compare different results obtained with isothermal cooking and conventional modafied cooking (MCC). These surprisingly positive results show, according to the upper diagram, that, with a given amount of added alkali, substantially lower kapga numbers are obtained using isothermal cook-ing. Furthermore, the second diagram shows that mani-festly improved strength properties are obtained when cooking down to 'the safe kappa number. Tn addition', the third daagram shows that there is also the advantage that the quantity of reject wood (skives) decreases. If the w fact is also taken into account that ~verall substantial energy savings are made when the temperature level is 2.(~ kept constant, it is evident hat the results may be re-garded as beia~g 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 2~ strength (viscosity round about 1000) after oxygen delig-nificati.on. Thus, when employing,the method according to the in~renti,on, so-called environmentally friendly bleach-ing chemicals, such as peroxide and ozone, can be employed in ~ubseguent bleaching stages without risking a ~ ~. strength ' f or vl~aching up ' tQ the level 'of too low 3 , I

brightness, and therewith also the level of purity, which the market demands.

Figure 3 showsthe. lower part of a digester 1, which is intended to represent an existing digester shell (which 35 definitely is higher than 2~ mitres normally higher than 30 anetres and usually about 40 metres and more and having i'ar~ 9x11 X566 ~'~ ~ r$ ~ ~ PCT/SE93/00~16 ys a diameter of 1/1a of the height)on which has been arranged a new digester strainer~girdle 2 in order to be able to raise the temperature in. the counter-currant zone. The da.gester has a lowest strainer girdle (1B) and one strainer girdle (1D) for withdrawal of spent liquor and 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 strainer.
- .In the text following, the counter-current zone of the digester which has been fitted with a further strainer ~ga~rdle will be referred to as a cooking zone, even if it is to be considered as a washing~zone according to con-ventional operation.
The new digester strainer girdle 2 consists of a number of so-called manhole strainers 2A for withdrawal 3 of cooking liquid in the lower past of 'the digester and is arranged immediately above the lowex° strainer girdle 1D, preferably at most S'metres above yore preferred 1,5 metres above and, more preferably at, most I metre above, measured from the upper edge of the lower digester strainer girdle to the lower edge of the newly fitted digester strainer girdle. 6~ash liquor is supplied to the lower Bart of the digester through an inflow arrangement 4 attached in the vicinity of the bottom 1A of the digester and cooking liquid (alkali addition) through the 3Q central pipes 5A,''58. The~coo~ed pulp is'taken out from' the bottom of the digester via a conduit 1E.
One of these central pipes, SA, which belongs to the or~.ginal system of the digester, penetrates down to the lower strainer girdle ~.8 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 strainer girdle. Subsequently, a part of the liquid flows in a counter-current direction upwards towards the newly fitted digester strainer girdle 2. The liguid withdrawn from this system passes through the said withdrawal line 3 and is heated via a heat exchanger 6H to the desired temperature before it discharges, via a second, newly fitted central pipe 5B, immediately above the newly fitted digester strainer girdle 2. A part of the cooking liquid supplied in this manner, which liquid has thus achieved the desired temperature, chemical strength and distribution (spreading) over the whole of the cross-section of the digester, continues to flow up-wards in the digester. In order to achieve even distribu-tion the flow within each cooking circulation, would nor-mally exceed 1 m3~h. In a central digester strainer girdle lo, the spent cooking liquid, together with undis-solved wood material, is drawn off for further treatment.
The surface of each strainer element 2A is made rela-tively small, preferably less than 0.3 m'. An advantage of strainer elements of small area is that efficient back flushing can be achieved, which is often of great impor-tance if the circulation flow is to function efficiently.
The new strainer girdle 2 is preferably fitted with ring pipes 2C from which an individual conduit goes to each and every one of the strainer elements 2A. Using such a construction, and a valve arrangement belonging to it, a limited number (for example 4) of strainer units 2A can be efficiently back-flushed at a time. owing to the rela-tively small total strainer surface which is back-flushed under these circumstances (for example 1 m2), a very ef-ficient back-flushing which cleans the strainers is obtained, thereby ensuring that the circulation is highly efficient.
The invention is not limited by that which has been de-scribed above, but can be varied within the scope of the subseguent patent claims. Thus, an existing digester of CVO 94/11566 ~; PCI'/SE93/00~1~6 the MCC type can also be arranged in accordance with the invention, where, therefore, the digester. has an upper concurrent part, a central,.~nainly 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 strainer girdle according to the invention for cooking according to the invention, so-called isothermally. Addi-tionally the method may be used in connection with all - types of cooking liquid, even if the method is princi-pally 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 exempli-fying temperature levels:- In this connection, however, it is important that the average temperature level in the digester preferably eatceeds +150°C but is lower than Zp +165°C, and preferably is between 150--1.55°C for hardwood and between x.60-165°C for softwood, and furthermore that the average temperature in the cooking zone/zones is preferably about +~51°C +-1°C, when.the wood is hard-wood, and that the average temperature in a digester is ~5 +159°C +-I°C, When the wood is softwood. In addition, it is understood that strainers deviating from a circular form, for example oval strainers, or even rectangular may also be used, whereby, for technical~reasons related to the construction, the smallest radius of curvature should 30 preferably not fall ~ below 0. 2 m.
Finally, it as pointed os~t that new digesters can naturally also bs fitted faith strainer girdles, and be operated, according to the invention. Furthermore it is 35 understood that moxe than one, strainer girdle can be arranged between the strainer girdle withdrawing spent PGT/SE93f~D0~16 W~ 94/1156b p v ' cooking liquor and the lowest strainer girdle. It is. also evident for the skilled man that more than one strainer girdle may bemused for withdrawing spent liquor.
This lowest strainer girdle is normally positioned within its lower edge about 1-2 metres above the welded seam that.joins the spherical bottom position with tine cylindrical digester shell, but in extreme designs the distance might be as short as 0.5 metres and as long as 5 metres.

Claims

CLAIMS:

1. Method for continuous cooking under raised pressure and temperature of fibre material in a vertical digester, having a top and a bottom, where input of fibre material and cooking liquor takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one strainer girdle for spent liquor between the top and the bottom of the digester, and fibre material is fed out from the bottom of the digester, wash liquor is supplied at the bottom of the digester, wherein said method liquor is withdrawn from a lowest digester strainer girdle and said liquor, after heating via a first heat exchanger, by means of a first central pipe, discharges on a level with the said lowest digester strainer girdle, and wherein said method the digester comprises an upper digester strainer girdle between said lowest digester strainer girdle and said at least one strainer girdle for spent liquor, wherein the upper digester strainer girdle withdraws spent liquor, via a second heat exchanger, and a second central pipe, and discharges the spent liquor above the said upper digester strainer girdle, and wherein the digester comprises at least one conduit arrangement, for addition of cooking liquor in connection with one of the said lowest digester strainer girdle and said upper digester strainer girdle, wherein the method the cooking is carried out isothermally, and said upper digester strainer girdle is positioned in such a manner that a lowest part of the upper digester strainer is positioned less than 5 metres from an upper part of said lowest digester strainer girdle, wherein the method an efficient circulation and temperature distribution is created towards the bottom of the digester and wherein the method discharged liquors from the first and second central pipes are heated to such a temperature that the temperature in a cooking zone immediately above said upper digester strainer girdle is maintained at essentially the same temperature level as in remaining cooking zone or cooking zones of the digester.

2. A method according to claim 1 wherein the second central pipe discharges immediately above the upper strainer girdle.

3. A method according to claim 1 or 2, wherein the said upper digester strainer girdle consists of a number of strainers having circular or oval form designed to withdraw displaced liquor for supply to the second central pipe.

4. A method according to any one of claims 1 to 3, wherein the distance between an upper edge of the lowest digester strainer girdle and a lower edge of the upper digester strainer girdle is less 2 m.

5. A method according to claim 4, wherein the distance is less than 1 m.

6. A method according to any one of claims 1 to 5, wherein the temperature of the fibre material and upwardly flowing liquor which is located adjacent above an upper edge of the upper digester strainer girdle deviates from the temperature in the remaining cooking zone or cooking zones by at most 4°C.

7. A method according to any one of claims 1 to 5, wherein the temperature of the fibre material and upwardly flowing liquor which is located adjacent above an upper edge of the upper digester strainer girdle deviates from the temperature in the remaining cooking zone or cooking zones by at most 2°C.

8. ~A method according to any one of claims 1 to 5, wherein the temperature of the fibre material and upwardly flowing liquor which is located adjacent above an upper edge of the upper digester strainer girdle deviates from the temperature in the remaining cooking zone or cooking zones by at most 1°C.

9. A method according to any one of claims 1 to 8, wherein the digester comprises two conduit arrangements.

10. A method according to any one of claims 1 to 9, wherein said upper digester strainer girdle is newly fitted to an existing digester shell.

11. A method according to claim 2 or 3, wherein said central pipe is newly fitted to an existing digester.