Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/24—Continuous processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C1/00—Pretreatment of the finely-divided materials before digesting

Definitions

• the present invention relates to a process for the continuous digestion of cellulosic fiber material, comprising impregnation with impregnating liquid consisting of at least one fresh digesting liquor, in a closed impregnating system and subsequent digestion with digesting liquid in a closed digester system, said impregnating liquid being added through a supply system, together with steamed fibre material, to an inlet end of the impregnating system, liquid being withdrawn from the impregnating system at a point located at a predetermined distance from said inlet end.
• 3 digester is normally 3.0 - 3.5 m per ton dry chips.
• 70-100% of the white liquor required is normally added to the supply system for the impregnating vessel via a high-pressure pump.
• the amount of liquid at the inlet to the impregnating vessel may
• Chips moisture (moisture content 50 %) 1.0 m
• black liquor can be added to the impregnating vessel through the supply system in order to
• the temperature of the black liquor should not exceed 100 C° . If too little liquid is added to the impregnating vessel, disturbances will result in the supply system due to the formation of steam in the down pipe located before the high pressure valve. The temperature of the liquid exceeds the temperature corresponding to the steam pressure in the pressure steaming vessel located upstream. Sinking of the chips in the impregnating vessel is also impeded. The quantity of free liquid, i.e. the portion of liquid which is not absorbed by the chips, becomes too low and the flow rate of the liquid is correspondingly reduced. The flow rate of the free liquid exerts a driving force on the column of chips.
• the liquid flow at the entrance to the digester must be controlled so that it does not become too great since this results in too great a consumtion of high-pressure steam in heat exchanger present in the transfer system and of direct steam to the top of the digester. Furthermore, the withdrawal strainer in the upper part of the countercurrent flow washing zone of the digester becomes overloaded.
• the object.of the present invention is to eliminate said problems and provide a simple and reliable method of maintaining a sufficiently high flow of liquid in at least the initial part of the impregnating vessel without the liquid flow in the digester becoming too high.
• the invention aims to solve the problems existing when black liquor cannot be used in order to provide a sufficiently high liquid to wood ratio in the impregnating vessel without such ratio being too high in the digester.
• the process according to the invention is characterised in that such a first quantity of liquid (F) is whithdrawn at said withdrawal point and recirculated directly to the supply system that a predetermined minimum ratio between liquid and fibre material is ' continuously maintained in the impregnating system prior to said withdrawal point, the liquid in said liquid to fibre material ratio being derived from original moisture (A) in the fibre material, condensates (B, C) from an initial steaming of the fibre material, and added fresh digesting liquor(s) (D and/or E) besides said withdrawn and recirculated quantity of liquid (F).
• the term "directly” as used herein means that the liquid is recirculated as such to the supply system, i.e. without being mixed with black liquor.
• the plant shown in the drawing comprises a vertical steaming vessel 1, a horizontal steaming vessel 2, a vertical impregnating vessel 3 and a vertical digester 4. Chips are fed through a pipe 5 to the vertical steaming vessel 1, to which low pressure steam is supplied through a pipe 6 in order to heat the chips and reduce their air content. This pre-steaming process is performed at atmospheric pressure.
• the heated chips are dosed by a chip meter disposed in a connection 7 between the two steaming vessels, said connection 7 also containing a lower-pressure valve 8 which locks through the chips into the horizontal steaming vessel 2.
• Low-pressure steam is added through a pipe 9.
• the pressure in this second steam vessel 2 is 1-1.5 bar over-pressure.
• the chips fall into a down pipe 10 with a high pressure valve 11 disposed at the lower end thereof.
• a liquid level is maintained in the down pipe 10.
• the high pressure valve 11 is provided with a rotor having pockets. One pocket is always in low pressure position to be in open communication with the pre-steaming vessel 1, and another pocket is always simultaneously in high pressure position to be in open communication with the impregnating vessel 3 via a supply pipe 12 connected to the top of the impregnating vessel 3.
• Liquid in a circulation loop 13 provided with a pump 20 feeds the chips from the down pipe 10 into the high pressure valve 11 and fills one of the rotor pockets.
• a screen is mounted in the housing of the high pressure valve 11 to prevent chips from accompanying the circulation liquid in the circulation loop 13.
• the liquid displaced by the chips in the high pressure valve 11 is withdrawn from the circulation loop 13 by means of a screen 14 disposed therein, from which the liquid withdrawn is conducted to a level tank 15 via a pipe 16.
• the level tank 15 is connected to the top of the impregnating vessel 3 by a pipe 17 containing a high-pressure pump 18 to pump the liquid withdrawn into the impregnating vessel 3.
• the high pressure valve 11 and impregnating vessel 3 are connected by means of a circulation loop comprising said supply pipe 12 which is connected to the top of the impregnating vessel 3, and a return pipe 21 which connects the top of the impregnating vessel to the high pressure valve 11 to return liquid separated off by a top separator disposed in the impregnating vessel 3.
• the liquid is caused to circulate by means of a pump 22 located in the return pipe 21.
• a pump 22 located in the return pipe 21.
• the flow rate of the return liquid is so high that it flushes the chips from the rotor pocket with it into the impregnating vessel 3.
• the pressure in the impregnating vessel 3 is normally 8-15 bar over-pressure. However, for certain processes it may be as high as 15-45 bar over-pressure.
• Fresh digesting liquor D from a first storage tank is fed to the top of the impregnating vessel 3 via a pipe 23.
• the pipe 23 is suitably connected to the pipe 17 from the level tank 15 so that the high-pressure pump 18 is utilized for pumping both the liquid withdrawn from the first circulation loop 13, and the digesting liquor D which thus constitutes fresh impregnating liquid.
• Fresh digesting liquor E is fed to the digester from a second storage tank via a pipe 24 containing a high-pressure pump 25.
• Some of the digesting liquor E may be used as impregnating liquid and be fed to the top of the impregnating vessel 3 via a branch pipe 26 provided with a valve.
• This branch pipe 26 is suitably connected to the pipe 17 from the level tank 15.
• the impregnated chips are transferred from the bottom of the impregnating vessel 3 to the top of the digester 4 via a pipe 27.
• a strainer is mounted at the top of the digester 4 to separate off a certain amount of liquid which is then returned to the bottom of the impregnating vessel 3 via a return pipe 28 containing a pump 29 to pump the chips to the digester 4 with the aid of the separated liquid.
• the return pipe 28 passes a heat exchanger 30 to heat the liquid passing it by the supply of high-pressure steam via a pipe 31.
• the supply pipe 27 and return pipe 28 form a transfer circulation for the suspension of impregnated chips and boiling liquid.
• a larger quantity of digesting liquor E is added to said transfer circulation in that the pipe 24 for the digesting liquor is connected to the return pipe 28 before pump 29.
• a smaller quantity of the digesting liquor may be added to a circulation pipe 33 of he digester, via a pipe 19 provided with a valve.
• the digester is provided with a strainer 32 for circulation of liquid through pipe 33 with the aid of a pump 34, the liquid being heated in a heat exchanger 35.
• the pipe 33 comprises a central tube disposed in the centre of the digester with its orifice at the strainer 32.
• the digested chips are washed in countercurrent flow in the lower part of the digester, using washing liquid supplied through a pipe 36 and pumped by a high-pressure pump 37 into the lower part of the digester in an amount which is regulated to ensure that the digester is kept filled with liquid.
• the washing liquid is heated indirectly by steam which is supplied to a heat exchanger 38 disposed in a pipe 39 for circulation of washing liquid by means of a pump 40.
• the washing liquid is removed through a strainer 41 and returned through a central pipe extending from the bottom of the digester and having its orifice at a strainer 41.
• the washing liquid heated in this manner is forced to flow in countercurrent up through the chips column as it moves slowly downwards, thereby displacing its content of spent digesting liquor which is withdrawn through a strainer 42 and conducted via a pipe 43 to a flash cyclone (not shown) and a recovery plant (not shown).
• the digested chips are fed out at the bottom of the digester using suitable scrapers, and is conducted through a pipe 44 for continued processing.
• indirect heating of digesting liquid and wood in said transfer circulation 27, 28 is also performed with steam supplied to the top of the digester through a pipe 45.
• the impregnating vessel is provided with a strainer 46 disposed at a predetermined distance from the top of the impregnating vessel as seen from the point where the chips are supplied, i.e. the inlet end.
• a return pipe 47 provided with a valve is connected between this strainer 46 and the top of the impregnating vessel in order to return a predetermined quantity of liquid to the start of the impregnating zone.
• the return pipe 47 contains a high-pressure pump 48 and passes through a heat exchanger 49, the other fluid side of which is connected to the pipe 24 for fresh digesting liquor E via branch pipes 50, 51.
• the flow to the heat exchanger 49 is regulated by a valve 52 in the pipe 24 for fresh digesting liquor E.
• the return pipe 47 is connected to the pipe 17 from the level tank 15 at a point located before the high-pressure pump 18.
• the impregnating liquid D is added in an amount of about 3 0.7 m per ton dry wood.
• Digesting liquor E in the form of white liquor is supplied to the digester 4 via pipes 24, 28, 27 and 19 in
• the temperature at the top of the impregnating vessel is generally about , 110-120 C° and at the bottom, i.e. in the transfer circulation 27, 28, about 130-160 C° .
• the liquid withdrawn through the strainer 46 has a temperature of about 120-130 C°, while the black liquor withdrawn from the digester through the strainer 42 has a temperature of about 150-170 C°.
• the method according to the invention for maintaining a sufficiently high flow of liquid in the upper part of the impregnating vessel can be utilized for various embodiments of the digesting process.
• the fresh digesting liquor D may consist of an alkaline solution with a high sodium sulphide content or of only sodium sulphide. Sodium sulphide is consumed to a relatively small extent during impregnation conditions and a sufficiently high sodium sulphide content can therefore be maintained even if impregnating liquid F is withdrawn and returned to the inlet of the impregnating vessel.
• the fresh digesting liquor E may consist of ordinary white liquor, i.e. a mixture of substantially sodium sulphide and sodium hydroxide, or a white liquor containing substantially sodium hydroxide.
• the liquid F withdrawn from the initial concurrent flow impregnating zone may be large enough to correspond to all the free liquid.
• the .liquid bound by the chips is about 1.8 m 3 per ton dry wood in the case of softwood and
• the temperature in the impregnating vessel 3 can advantageously be regulated to a suitable level by cooling the withdrawn impregnating liquid F in the cooler 49.
• the fresh digesting liquor D may also consist of an organic substance, such as methanol or ethanol, to impregnate the steamed chips, possibly in the presence of a small quantity of fresh digesting liquor E, e.g. white liquor.
• the rest of the fresh digesting liquor E is then added to the transfer circulation 27, 28 and/or one of the liquid circulations, e.g. 33, of the digester.
• the process according to the invention is also applicable to sulphite digestion in several steps, for instance acid sulphite digestion. Neither is it limited to a continuous digester with separate impregnating vessel, but may be used in a digester vessel for the continuous digesting with initial impregnating zone, such as a one-vessel hydraulic digester.
• the impregnating liquid F withdrawn may alternatively be returned to the supply system at a point in the pipe 17 located after the high-pressure pump 18. This reduces the need of pump energy. If the liquid F withdrawn is enriched with gas, which may cause operating disturbance, it may be suitable to flash the liquid to a low pressure for degassing before feeding it to the sypply system.

Landscapes

• Paper (AREA)
• Artificial Filaments (AREA)
• Treatments For Attaching Organic Compounds To Fibrous Goods (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20383529

Family Applications (1)

Country Status (11)

Cited By (1)

Families Citing this family (5)

Citations (2)

• 1991
  • 1991-08-23 SE SE9102427A patent/SE469078B/sv not_active IP Right Cessation
• 1992
  • 1992-08-11 AU AU24903/92A patent/AU2490392A/en not_active Abandoned
  • 1992-08-11 AT AT92918282T patent/ATE145025T1/de not_active IP Right Cessation
  • 1992-08-11 JP JP5504253A patent/JPH06510091A/ja active Pending
  • 1992-08-11 EP EP92918282A patent/EP0662169B1/en not_active Expired - Lifetime
  • 1992-08-11 WO PCT/SE1992/000546 patent/WO1993004232A1/en active IP Right Grant
  • 1992-08-11 DE DE69215110T patent/DE69215110T2/de not_active Expired - Fee Related
  • 1992-08-11 CA CA002115578A patent/CA2115578A1/en not_active Abandoned
  • 1992-08-21 ID IDP460392A patent/ID848B/id unknown
  • 1992-08-21 PT PT100799A patent/PT100799A/pt not_active Application Discontinuation
• 1994
  • 1994-02-22 FI FI940826A patent/FI940826A0/fi not_active Application Discontinuation

Patent Citations (2)

Also Published As

Similar Documents

Legal Events