CA2115578A1 - A process for the continuous digestion of cellulosic fiber material - Google Patents
A process for the continuous digestion of cellulosic fiber materialInfo
- Publication number
- CA2115578A1 CA2115578A1 CA002115578A CA2115578A CA2115578A1 CA 2115578 A1 CA2115578 A1 CA 2115578A1 CA 002115578 A CA002115578 A CA 002115578A CA 2115578 A CA2115578 A CA 2115578A CA 2115578 A1 CA2115578 A1 CA 2115578A1
- Authority
- CA
- Canada
- Prior art keywords
- liquid
- impregnating
- fibre material
- withdrawn
- digester
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
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
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- Paper (AREA)
- Artificial Filaments (AREA)
- Treatments For Attaching Organic Compounds To Fibrous Goods (AREA)
Abstract
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 supplied through a supply system, together with steamed fibre material, to an inlet end of the impregnating system, liquid being withdrawn from the impregnating system at a point (46) located at a predetermined distance from said inlet end. According to the invention such a first quantity of liquid (F) is withdrawn at said withdrawal point (46) and recirculated directly to the supply system that a predetermined minimum ratio between liquid and fibre material is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 (46), 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).
Description
W0~3/04232 1 PCT/SE92/00~6 A process for the continuous di~estion of c_llulosic fiber material The prese~t invention relates to a process for the continuous digestion of cellulosic fiber material, 5- comprising impregnation with impregnating liquid consisting o a~ least one fresh digesting liquor, in a closed impregnating system and subsequent dige~tion with digesting li~uid in a closPd digester system t said impregnating liqu~d being added through a supply system, together with~staam~d f~bre ma~erial, to a~ inlet end of the impregnating system, li~uid bPing withdrawn ro~ the impregnating system at a point located at a predetermined distance from said inlet end.
When ~eding wood chips, for instance, into a continuous digester a certain minimum guantity of liquid mus~ be added through the supply system together with the chips.
If too littl~ liguid is added in relation ~o the quan~ity of chips the temperature wi~l become too high in the supply system and the sinking rat~ of th~ chips in the 2D impregnating vPssel or digester will be disturbed. The requisite amount o liquid is normally obtained with the digesting liquor, e.g. white liquor~ added to the supply system for impregnating the chips. If ne essary spent , digesting liquid (black li~uor), the temperature of which has been lowered through relief in several steps or by means of indirect cooling, may be returned to the supply system, thus increasing the amount of liquid available.
In certain multi-step processes in whtch th~ first step ~;
is impregnatlon or digestion with a different chemical from that used in subsequent digestion step or where for technical reasons it is unsuitable to use fuIly spent digestlng liquid (blac~ liquor), the requirement for a certain minimum quantity of liquid must be fulf~lled by adding a sufficien~ amount o~ the digestiny liquid ~rom WO 93~04232 2 PCI'~SE92/00546 the f irst s~ep, possibly diluted with water . This entails several drawbacks such as p~orer heat economy due to the increased consumption of steam in ~ubsequent cook~ ng s~eps and/or increased need for evaporation iEor recovery 5 of the c:hemicals. Another draw~ack may be ~hat the . concentration of the cooking c:hemicals in first step be~s:)mes too low to achiPve th~ desired effect of the process. Addition of black liquor ~o the supply system is described in SE:-359 331 and Si3-S804578-6. In the 10 processQs according to said docum~nts liquid is withdra~
:Erom the impregnation ~;ystem at a point 1 ocated at - a -~ pr~det~rmined distan::e from the ia~let end, and the li quid `~
wi~hdrawn iæ mixed with black liquor~ -The liquid to wood ratio at the inlet to a continuous digester is no:rmally 3 D O - 3 ,. 5 m3 per ton dry chips ~ In :::omrentional sulphate cookin~a 70-100~ of the whi~e liquor required is normally added to the supply system for the impregnating ve-csel via a his~h-pressure pll~llpr The amount of liquid at the inlet to ~h~ impregnating vessel may then be within an inter~al oiE about 2. ~ - 2.8 m3 per ton ~ry chips, distributed as follows:
A Chips moisture ( moisture content 5f) % ) 1. O m~
B~C Steam c:ondensate 0 . 3 m3 6 White 7 iquor ( 70-100% ) 1.1-1~ 5 m3 As mentioned above, black liquor can b~3 added to the impregnating vessel through th~ supply 5ystem in order to increase th~ liquid to wood ratio, for instanc:e to 3 ., O m3 or above. The temperature of the black liquor should not ::
exceed ïoo co If too little li~uY d is added tc~ l:h~
impregnating vessel, di5tux~an ::es will result in the supply system due to the f ormation of steam is~ the down pipe locat~d before the high pres~:ure valve. The ~mperature of the llquid exceeds th~ ternperature corresponding to the steam pressure in the pres~ure W093/04232 3 PCT/SE92/00~6 steaming vessel located upstream. Sinking of the chips in the impregnating vessel is also impeded. The quantity o~
free llquid, i.e. the portion of liquid which is not absor~ed by the chips, becomes too low and the flow ra~e of the liquid is corr~spondingly reduced. The ~low rate of the free liquid exerts a driving force on the column of chips. It is important that this driving orce is great in the upper part of the impregnating vessel wh~rP
the chips are 5till not saturated with liquid but have a tendency to float. This requirement is of less importance in the lower part of the lmpregnating v~ssel since by this stage the`chips`are sub~tantially saturated with liquid and therefore tend to sink due to their own weight. When treatment is carried out in several steps using different digesting liquids in the various steps, there is the same need to malntain a sufficiently high :~
liquid to wood ratio in the first step, i.e. in the initial concurrent flow zone i.n the impregnatin~ vessel.
However, it may be that the process or operating economy requires that the dige~ting li.~uid of the first step has to be added in a limited ~uantity, in which case it may b~, diff~cult to achieve a sufficiently high liquid to wood ratio. At the ~ame time, the liquid flow at the entrance to the digester must be controlled so that it .~
25 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 o~ the digester. Furthermore, the withdrawal strainer in the upper part of the countercurrent flow washing zone of the digester becomes overloaded~
. .
~hs object of the prasent invention is to eliminate said problems and provide a 5imple and reliab~e method of maintaining a sufficiently high ~low of liquid in at l as~ the initial part of the impregnati ng vessel without the liquid flow in the digester becoming too h~gh. In other words, the inYention aims to solve the problems WO~3J04232 A PCT/SE92~00~6 exis-ting when black liquor cannot be used in order to provide a sufficiently high liquid to wood ratio in the impregnati~g vessel without such ratio being too high in the diges~er.
ThP process according to the invention is characterised in tha~ such a first quantity of liquid (F3 i5 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 10 the impregnating system prior to said withdrawal point, :~
the liquid in said liquid to fibre mat~rial ratio be~ng derived from original moisture (A) in the flbre mat~r~al, condensates ~ B, C~ ) from an initial ~teaming o the fibre material, and added fresh digesting liquor( s ) ( D and/or 15 E ) besides said withdrawn and recirculated quantity of li~[uid ( F ) . The term "di rectly" as used herein means that the liquid is recirculated as such to the supply system, i . e . without being mixed wit:h black llquor .
The invention will be described in mcsre detail in th~
20 following with referen::e to the drawing whit:~h shows ~chematically a flow diagram of a plant for the continuous digesting of cellulosic fiber material in the form of wood material which has been disintegrated to chips.
The plant shown in the drawing comprisi2s a vertical steaming vessel 1, a horizontal steaming vessel 7, a vertical impregnating vessel 3 and a vertical digester 4.
Chips are fed through a pipe 5 to th~ vertical steaming vessel ~, t~ which.low pressure steam is supplied through a pipe 6 in order to hieat the chipæ and reduce their air content. This pre-steaming process is performed at atmospheric pressurP. Tha heated Chip5 are dosed by a chip ~et~r disposed in a connection 7 between the two steaming vessels, said connection 7 also containing a W093/~4~32 5 PCT/SE92/00~6 lower-pressure valve 8 which locks through the chips into the horizontal steaming vessel 2~ Low-pressure ~team is added through a pipe 9. ~he pressure in this second steam vessel 2 is 1-1.5 bar over-pressure. From the pressure steaming vessel 2, the chips fall into a down pipe 10 . with a high pressure ~alve 11 dispos~d at the lower end th~reof. A liquid level is maintained in the down pipe 10. The high pressure valve 11 is provided with a rotor having pockets. One poc~et is always in low pressure position ~o be in open communication with the pre-steaming vessel 1, and another pocket is always ~simul~aneously in high pressure position to be in: open communication with the impregnating vessel 3 via a supp~y plpe 12 connected to the tOp of the impregnating vessel 3. Liquid in a circulation loop 13 provided with a pump 20 feeds the chips fro~ 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 Girculation loop 13. The liquid displaced by the chips in the high p~essure valv~ 1~ is withdrawn from the circulation loop 13 by means of a screen 14 disposed therein, from which the li~uid .
withdrawn is conducted to a level tank 15 via a pipe 16.
The le~el tank 15 is connec~ed to the top of the impregnating vessel 3 by a pip~ 17 containing a hiyh-pxessure pump 18 to pump ~he liguid withdrawn into the impregnating vessel 3.
The high pressure valve 11 and impregnating ~es el 3 are connected by means of a circulation loop compris~ng said supply .p~pe 12 which is connected to the top o~ the impregnating vessel 3, and a return pipe 21 which connect the top of the impregnating vessel to the high pressure val~e 11 to return l~uid separated off b~ a top eeparator disposed in the impregnating vessel 3. The liquid is caused to circulate by means of a pump 22 W0~3/04232 6 PCT/SE92~00~6 located in the re~urn pipe 21. When a filled rotor pocket arrives in the high pressure position, i.e. in dir~ct communication with the circulation loop 12, 21, it is rinsed clean by the return liquid in this circulation loopO The 1OW rate of the return liquid is so hlgh that it flushes ~he chips from the ~otox pocket with it into the impregnating vessel 3. The pressure in the impregnatiny vessel 3 is normally 8-15 bar over-pressure.
However, iEor certain processes it may be as high as 15-45 10 bar over-pressure.
Fresh digesting liquor D rom a first storage tank is fed to the top of the impregnating vessel 3 via a pipe 23.
The pipe 2-~ i5 suitably connected to th~ pipe 17 from the level tan}c 15 so that the high-pressure pump 18 is 15 utilized iEor pumping both the liquid withdrawn from the first circulation loop 13, and the digesting liquor D
which thus constitutes f resh impregnating liquid .
Fresh diges~ing liquor is fed to the digester from a second storage tank via a pip~ 24 containiny a 20 high-pressure pump 25. Soane of t:he digesting liquor E may be used as impregnating liquid and be fed to the top of the impresnating vessel 3 via a branch pipe 26 provlded with a val~e. This branch pipe 26 i5 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 .rekurned to the bottom of the impregnatlng ve5sel 3 via a return pipe 28 containing a pump 29 to pump thQ chips to the digester 4 with the aid of the separated liquid~ The return pipe 28 passes a heat exchanger 30 to heat the ~iquid passing it by the supply of high-pr~ssure st~3am ~ia a pipe 31. The supply pipe 27 and return pipe 28 form 35 a transfer circlllation for the suspension of impregnated W093~04232 PCT/SE92/00~6 chips and boiling liquid. A larger quantity of digesting liquor E is add~d to said transf er circulation in that the pipe 24 for the digesting liquor is connected to the return pipe 28 before pump 29. If desired, a smaller quantity of the digesting liquor may be added to a circulation p~pe 33 of-~ha digester~ ~ia 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 pip~ 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 dige.ster, using washing liquid supplied throug~ a pipe 36 and pumped by a high-pressure pump 37 into the lower part of the dige~ter in an amount which is regulated to ensure that the digester is kept filled with liquid. The washin~
liquid is heated indirectly by steam which is supplied to a heat e~changer 38 disposed .in a pipe 39 for circulation of washing liquid by means of a pump 40. The washing 11~uid is r~movsd through a strainer 41 and returned through a central pipe e~tending from the bottom o ~he digester a~d having its ori~ice at a strainer 41. The washing liquld 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 sui~able scrapers, and is conducted through a plpe 4g for continued processing. Besides indirect heating o~
digesting liquid and wood in said transfer circulation 27, 28, indirect heating is also performed with steam ~upplied to the top of the di~ester t~rough a pipe 45.
.. . .. . . , ......... ....... ~ . ..... .. . ... . . . .. . . . .
W0~3/04232 PCT/SE92/~0~46 According -to the present invention 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 5 supplied, i.e. the inlet end. AcGording to the present .
invention, furthermor~, a return pipe 47 provided with a valve is connected be-tween this strainer 46 and the top of the impregnating vessel in order to return a prede~Prmined quantity of liquid to the start of the ;;
impregnating zone. The return pipe 47 con~ains a high-pressure pump 48 and passes through a hea~ exchanger 49, ~he other fluid side of which is co~nected to the pipe 24 for fresh digesting liguor E via branch pipes 50, 51. The flow to the heat exchanger 49 is re~ulated by a valve 52 in the pipe 24 for fresh digesting liguor E~ The return pipP 47 is connected to the pipe 17 from ~he l~vel tank 15 at a point located before the high-pressure pump 18.
The following example further illu trates the invention.
The impregnating liquid r is added in an amount of about 0.7 m3 per ton d~y wood~ The quantity of liquid A from chips ~oisture is abo~t 1.8 m3 and from the s~eam cond~nsates B and C about 0.3 m per ton dry wood. These three sources of liquid thus supply about 2.0 m per ton dry wood to the top of the impregnatin~ vessel 3. Liguid F is withdrawn from the strainer 46 in the impre~nating vessel in an amount of about 1.0 m3 per ton dry wood. The liquid F withdrawn is cooled to a suitable te~p~ra~ure in the cooler 49 in order to avoid disturbances in the supply sys~em. The tot~l amount of the liquids A, B, C, D
and F is thus about 3.0 m per ton dry wood~ This quantity of li~uid is sufficient to provide a constant downward movemen~ of the chips column i~ the lmpregnating ~essel 3. The amount of li~uid F may be varied within ::
wide limi~s, depending on the other liquid flows to the W093/04232 PCT/SE92/00~6 impregnating vessel and on other operating conditions.
Digesting liquor E in the form of white liquor is supplied to the digester 4 via pipes 24, 28, 27 and 19 in an amount of about 1.5 m3 per ton dry chips. All together about 3.5 m liquid per ton dry chips is thus transferred to the digester~ iOe. the sum of the li~uid quantities A, B, C, D and E~ T~is givQs an acceptable flow of li~u~d in the digester 4 with respect to heat economy and operating reliability.
When the process i~ repeated without wi~hdrawing and returning liguid F (the valve in pipe 47 is closed), ~he amount of fresh digesting liq~or D must be increased from 0.7 to 1.7 m3 per ton dry Chipsr thus giving a total flow of li~uid to the digester of 4.5 m per ton dry chips.
However, such a run is unsuitable in practice since the large flow of liquid causes too high a consumption of high-pr~ssure steam in the heat exchanger 30 in the transfer syst~m and of direct steam to the top of the digester. The strainer in the upper part of the countercurrent flow washing zone of the digester also becomes o~erloaded.
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 maintainin~ a 3n suffi~iently high flow of liquid in the upper part of the impregnatin~ vessel can be utilized for various embodiments of the digesting process. The fresh diges~ing llquor D, for instance, may con5ist of an alkaline 801ution with a high sodium sulphide content or of only W093/~4~32 lO P~T/S~92/00546 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 mi~ture of substantially sodium sulphide and sodium hydroxide, or a white liquor containing substantially sodium hydroxideO
In the case the chips are impregnated with a part of the fresh digesting liquor E and no other fresh digesting liquor D is ~dded, a suf~icient flow of liquid will be obtained at the beginning of the impregnating vessel by withdrawing a sufficient quantity of liquid F from the impregnating vessel 3 and returning it via the supply system. The fresh digesting :Liquor E required for the rest of the process is added to the digester 4 via the ~xansfer circulat~on 27, 28 and/or the circulation 33 described or some other circul~tion of digesting liquid in the digester v~ssel.
The liquid F withdrawn from the initial concurrent flow impregna~ing zone may be large enough to correspond to all the free li~uid~ The liquid bound by the chips is about 1.8 m3 per ton dry wood in the case of softwood and ~25 about l.O-1.5 m3 per ton dry wood in the case of hardwood, dependin~ on the density of the chips.
Withdrawing additional liquid through strainer 46 will cause liquid to flow upwardly from the bottom of the impregnating vessel towards the strainer 46. In such an 30 operation some of the digestin~ liqour E which has been -;
added to the transer circulation will meet the chips in countercurrent flow, thereby resulting in improved impregnationO The additional liquid withdrawn from the lower countercurrent flow impregnatin~ zone ~ia the strainer 46 can be returned to the transfPr circula~ion WO 93/04~32 11 PCI/SE92/00546 27, 28 or be used in som other way. It is thus not included when calculating the amollnt of liquid F which is withdrawn and returned to the supply system.
Circulating a considerable proportion of the impregnating liquid back to the supply system increases the concentration of the fresh digesting liquor D when this is used, thus produc~.ng improved impregnating results, particularly when the fre~h digesting liquor D consists of a polysulphide solution or a solution containing anthra~uinone. As mentioned earlier, the temperature in the impregnating vessel 3 can advantageously be re~ula~ed to a suita~le 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 o 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 circul tions, 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 25 used in a digester vessel for the continuous digesting ~;
with initial impregnating zone, such as a one-vessel ~`
hydraulic digesterO
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 feedin~
it to the sypply system.
5 ~s ~ill be understood from the ~bove description and following claims no black liquor i5 added to the supply system of the impregnating system.
When ~eding wood chips, for instance, into a continuous digester a certain minimum guantity of liquid mus~ be added through the supply system together with the chips.
If too littl~ liguid is added in relation ~o the quan~ity of chips the temperature wi~l become too high in the supply system and the sinking rat~ of th~ chips in the 2D impregnating vPssel or digester will be disturbed. The requisite amount o liquid is normally obtained with the digesting liquor, e.g. white liquor~ added to the supply system for impregnating the chips. If ne essary spent , digesting liquid (black li~uor), the temperature of which has been lowered through relief in several steps or by means of indirect cooling, may be returned to the supply system, thus increasing the amount of liquid available.
In certain multi-step processes in whtch th~ first step ~;
is impregnatlon or digestion with a different chemical from that used in subsequent digestion step or where for technical reasons it is unsuitable to use fuIly spent digestlng liquid (blac~ liquor), the requirement for a certain minimum quantity of liquid must be fulf~lled by adding a sufficien~ amount o~ the digestiny liquid ~rom WO 93~04232 2 PCI'~SE92/00546 the f irst s~ep, possibly diluted with water . This entails several drawbacks such as p~orer heat economy due to the increased consumption of steam in ~ubsequent cook~ ng s~eps and/or increased need for evaporation iEor recovery 5 of the c:hemicals. Another draw~ack may be ~hat the . concentration of the cooking c:hemicals in first step be~s:)mes too low to achiPve th~ desired effect of the process. Addition of black liquor ~o the supply system is described in SE:-359 331 and Si3-S804578-6. In the 10 processQs according to said docum~nts liquid is withdra~
:Erom the impregnation ~;ystem at a point 1 ocated at - a -~ pr~det~rmined distan::e from the ia~let end, and the li quid `~
wi~hdrawn iæ mixed with black liquor~ -The liquid to wood ratio at the inlet to a continuous digester is no:rmally 3 D O - 3 ,. 5 m3 per ton dry chips ~ In :::omrentional sulphate cookin~a 70-100~ of the whi~e liquor required is normally added to the supply system for the impregnating ve-csel via a his~h-pressure pll~llpr The amount of liquid at the inlet to ~h~ impregnating vessel may then be within an inter~al oiE about 2. ~ - 2.8 m3 per ton ~ry chips, distributed as follows:
A Chips moisture ( moisture content 5f) % ) 1. O m~
B~C Steam c:ondensate 0 . 3 m3 6 White 7 iquor ( 70-100% ) 1.1-1~ 5 m3 As mentioned above, black liquor can b~3 added to the impregnating vessel through th~ supply 5ystem in order to increase th~ liquid to wood ratio, for instanc:e to 3 ., O m3 or above. The temperature of the black liquor should not ::
exceed ïoo co If too little li~uY d is added tc~ l:h~
impregnating vessel, di5tux~an ::es will result in the supply system due to the f ormation of steam is~ the down pipe locat~d before the high pres~:ure valve. The ~mperature of the llquid exceeds th~ ternperature corresponding to the steam pressure in the pres~ure W093/04232 3 PCT/SE92/00~6 steaming vessel located upstream. Sinking of the chips in the impregnating vessel is also impeded. The quantity o~
free llquid, i.e. the portion of liquid which is not absor~ed by the chips, becomes too low and the flow ra~e of the liquid is corr~spondingly reduced. The ~low rate of the free liquid exerts a driving force on the column of chips. It is important that this driving orce is great in the upper part of the impregnating vessel wh~rP
the chips are 5till not saturated with liquid but have a tendency to float. This requirement is of less importance in the lower part of the lmpregnating v~ssel since by this stage the`chips`are sub~tantially saturated with liquid and therefore tend to sink due to their own weight. When treatment is carried out in several steps using different digesting liquids in the various steps, there is the same need to malntain a sufficiently high :~
liquid to wood ratio in the first step, i.e. in the initial concurrent flow zone i.n the impregnatin~ vessel.
However, it may be that the process or operating economy requires that the dige~ting li.~uid of the first step has to be added in a limited ~uantity, in which case it may b~, diff~cult to achieve a sufficiently high liquid to wood ratio. At the ~ame time, the liquid flow at the entrance to the digester must be controlled so that it .~
25 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 o~ the digester. Furthermore, the withdrawal strainer in the upper part of the countercurrent flow washing zone of the digester becomes overloaded~
. .
~hs object of the prasent invention is to eliminate said problems and provide a 5imple and reliab~e method of maintaining a sufficiently high ~low of liquid in at l as~ the initial part of the impregnati ng vessel without the liquid flow in the digester becoming too h~gh. In other words, the inYention aims to solve the problems WO~3J04232 A PCT/SE92~00~6 exis-ting when black liquor cannot be used in order to provide a sufficiently high liquid to wood ratio in the impregnati~g vessel without such ratio being too high in the diges~er.
ThP process according to the invention is characterised in tha~ such a first quantity of liquid (F3 i5 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 10 the impregnating system prior to said withdrawal point, :~
the liquid in said liquid to fibre mat~rial ratio be~ng derived from original moisture (A) in the flbre mat~r~al, condensates ~ B, C~ ) from an initial ~teaming o the fibre material, and added fresh digesting liquor( s ) ( D and/or 15 E ) besides said withdrawn and recirculated quantity of li~[uid ( F ) . The term "di rectly" as used herein means that the liquid is recirculated as such to the supply system, i . e . without being mixed wit:h black llquor .
The invention will be described in mcsre detail in th~
20 following with referen::e to the drawing whit:~h shows ~chematically a flow diagram of a plant for the continuous digesting of cellulosic fiber material in the form of wood material which has been disintegrated to chips.
The plant shown in the drawing comprisi2s a vertical steaming vessel 1, a horizontal steaming vessel 7, a vertical impregnating vessel 3 and a vertical digester 4.
Chips are fed through a pipe 5 to th~ vertical steaming vessel ~, t~ which.low pressure steam is supplied through a pipe 6 in order to hieat the chipæ and reduce their air content. This pre-steaming process is performed at atmospheric pressurP. Tha heated Chip5 are dosed by a chip ~et~r disposed in a connection 7 between the two steaming vessels, said connection 7 also containing a W093/~4~32 5 PCT/SE92/00~6 lower-pressure valve 8 which locks through the chips into the horizontal steaming vessel 2~ Low-pressure ~team is added through a pipe 9. ~he pressure in this second steam vessel 2 is 1-1.5 bar over-pressure. From the pressure steaming vessel 2, the chips fall into a down pipe 10 . with a high pressure ~alve 11 dispos~d at the lower end th~reof. A liquid level is maintained in the down pipe 10. The high pressure valve 11 is provided with a rotor having pockets. One poc~et is always in low pressure position ~o be in open communication with the pre-steaming vessel 1, and another pocket is always ~simul~aneously in high pressure position to be in: open communication with the impregnating vessel 3 via a supp~y plpe 12 connected to the tOp of the impregnating vessel 3. Liquid in a circulation loop 13 provided with a pump 20 feeds the chips fro~ 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 Girculation loop 13. The liquid displaced by the chips in the high p~essure valv~ 1~ is withdrawn from the circulation loop 13 by means of a screen 14 disposed therein, from which the li~uid .
withdrawn is conducted to a level tank 15 via a pipe 16.
The le~el tank 15 is connec~ed to the top of the impregnating vessel 3 by a pip~ 17 containing a hiyh-pxessure pump 18 to pump ~he liguid withdrawn into the impregnating vessel 3.
The high pressure valve 11 and impregnating ~es el 3 are connected by means of a circulation loop compris~ng said supply .p~pe 12 which is connected to the top o~ the impregnating vessel 3, and a return pipe 21 which connect the top of the impregnating vessel to the high pressure val~e 11 to return l~uid separated off b~ a top eeparator disposed in the impregnating vessel 3. The liquid is caused to circulate by means of a pump 22 W0~3/04232 6 PCT/SE92~00~6 located in the re~urn pipe 21. When a filled rotor pocket arrives in the high pressure position, i.e. in dir~ct communication with the circulation loop 12, 21, it is rinsed clean by the return liquid in this circulation loopO The 1OW rate of the return liquid is so hlgh that it flushes ~he chips from the ~otox pocket with it into the impregnating vessel 3. The pressure in the impregnatiny vessel 3 is normally 8-15 bar over-pressure.
However, iEor certain processes it may be as high as 15-45 10 bar over-pressure.
Fresh digesting liquor D rom a first storage tank is fed to the top of the impregnating vessel 3 via a pipe 23.
The pipe 2-~ i5 suitably connected to th~ pipe 17 from the level tan}c 15 so that the high-pressure pump 18 is 15 utilized iEor pumping both the liquid withdrawn from the first circulation loop 13, and the digesting liquor D
which thus constitutes f resh impregnating liquid .
Fresh diges~ing liquor is fed to the digester from a second storage tank via a pip~ 24 containiny a 20 high-pressure pump 25. Soane of t:he digesting liquor E may be used as impregnating liquid and be fed to the top of the impresnating vessel 3 via a branch pipe 26 provlded with a val~e. This branch pipe 26 i5 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 .rekurned to the bottom of the impregnatlng ve5sel 3 via a return pipe 28 containing a pump 29 to pump thQ chips to the digester 4 with the aid of the separated liquid~ The return pipe 28 passes a heat exchanger 30 to heat the ~iquid passing it by the supply of high-pr~ssure st~3am ~ia a pipe 31. The supply pipe 27 and return pipe 28 form 35 a transfer circlllation for the suspension of impregnated W093~04232 PCT/SE92/00~6 chips and boiling liquid. A larger quantity of digesting liquor E is add~d to said transf er circulation in that the pipe 24 for the digesting liquor is connected to the return pipe 28 before pump 29. If desired, a smaller quantity of the digesting liquor may be added to a circulation p~pe 33 of-~ha digester~ ~ia 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 pip~ 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 dige.ster, using washing liquid supplied throug~ a pipe 36 and pumped by a high-pressure pump 37 into the lower part of the dige~ter in an amount which is regulated to ensure that the digester is kept filled with liquid. The washin~
liquid is heated indirectly by steam which is supplied to a heat e~changer 38 disposed .in a pipe 39 for circulation of washing liquid by means of a pump 40. The washing 11~uid is r~movsd through a strainer 41 and returned through a central pipe e~tending from the bottom o ~he digester a~d having its ori~ice at a strainer 41. The washing liquld 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 sui~able scrapers, and is conducted through a plpe 4g for continued processing. Besides indirect heating o~
digesting liquid and wood in said transfer circulation 27, 28, indirect heating is also performed with steam ~upplied to the top of the di~ester t~rough a pipe 45.
.. . .. . . , ......... ....... ~ . ..... .. . ... . . . .. . . . .
W0~3/04232 PCT/SE92/~0~46 According -to the present invention 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 5 supplied, i.e. the inlet end. AcGording to the present .
invention, furthermor~, a return pipe 47 provided with a valve is connected be-tween this strainer 46 and the top of the impregnating vessel in order to return a prede~Prmined quantity of liquid to the start of the ;;
impregnating zone. The return pipe 47 con~ains a high-pressure pump 48 and passes through a hea~ exchanger 49, ~he other fluid side of which is co~nected to the pipe 24 for fresh digesting liguor E via branch pipes 50, 51. The flow to the heat exchanger 49 is re~ulated by a valve 52 in the pipe 24 for fresh digesting liguor E~ The return pipP 47 is connected to the pipe 17 from ~he l~vel tank 15 at a point located before the high-pressure pump 18.
The following example further illu trates the invention.
The impregnating liquid r is added in an amount of about 0.7 m3 per ton d~y wood~ The quantity of liquid A from chips ~oisture is abo~t 1.8 m3 and from the s~eam cond~nsates B and C about 0.3 m per ton dry wood. These three sources of liquid thus supply about 2.0 m per ton dry wood to the top of the impregnatin~ vessel 3. Liguid F is withdrawn from the strainer 46 in the impre~nating vessel in an amount of about 1.0 m3 per ton dry wood. The liquid F withdrawn is cooled to a suitable te~p~ra~ure in the cooler 49 in order to avoid disturbances in the supply sys~em. The tot~l amount of the liquids A, B, C, D
and F is thus about 3.0 m per ton dry wood~ This quantity of li~uid is sufficient to provide a constant downward movemen~ of the chips column i~ the lmpregnating ~essel 3. The amount of li~uid F may be varied within ::
wide limi~s, depending on the other liquid flows to the W093/04232 PCT/SE92/00~6 impregnating vessel and on other operating conditions.
Digesting liquor E in the form of white liquor is supplied to the digester 4 via pipes 24, 28, 27 and 19 in an amount of about 1.5 m3 per ton dry chips. All together about 3.5 m liquid per ton dry chips is thus transferred to the digester~ iOe. the sum of the li~uid quantities A, B, C, D and E~ T~is givQs an acceptable flow of li~u~d in the digester 4 with respect to heat economy and operating reliability.
When the process i~ repeated without wi~hdrawing and returning liguid F (the valve in pipe 47 is closed), ~he amount of fresh digesting liq~or D must be increased from 0.7 to 1.7 m3 per ton dry Chipsr thus giving a total flow of li~uid to the digester of 4.5 m per ton dry chips.
However, such a run is unsuitable in practice since the large flow of liquid causes too high a consumption of high-pr~ssure steam in the heat exchanger 30 in the transfer syst~m and of direct steam to the top of the digester. The strainer in the upper part of the countercurrent flow washing zone of the digester also becomes o~erloaded.
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 maintainin~ a 3n suffi~iently high flow of liquid in the upper part of the impregnatin~ vessel can be utilized for various embodiments of the digesting process. The fresh diges~ing llquor D, for instance, may con5ist of an alkaline 801ution with a high sodium sulphide content or of only W093/~4~32 lO P~T/S~92/00546 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 mi~ture of substantially sodium sulphide and sodium hydroxide, or a white liquor containing substantially sodium hydroxideO
In the case the chips are impregnated with a part of the fresh digesting liquor E and no other fresh digesting liquor D is ~dded, a suf~icient flow of liquid will be obtained at the beginning of the impregnating vessel by withdrawing a sufficient quantity of liquid F from the impregnating vessel 3 and returning it via the supply system. The fresh digesting :Liquor E required for the rest of the process is added to the digester 4 via the ~xansfer circulat~on 27, 28 and/or the circulation 33 described or some other circul~tion of digesting liquid in the digester v~ssel.
The liquid F withdrawn from the initial concurrent flow impregna~ing zone may be large enough to correspond to all the free li~uid~ The liquid bound by the chips is about 1.8 m3 per ton dry wood in the case of softwood and ~25 about l.O-1.5 m3 per ton dry wood in the case of hardwood, dependin~ on the density of the chips.
Withdrawing additional liquid through strainer 46 will cause liquid to flow upwardly from the bottom of the impregnating vessel towards the strainer 46. In such an 30 operation some of the digestin~ liqour E which has been -;
added to the transer circulation will meet the chips in countercurrent flow, thereby resulting in improved impregnationO The additional liquid withdrawn from the lower countercurrent flow impregnatin~ zone ~ia the strainer 46 can be returned to the transfPr circula~ion WO 93/04~32 11 PCI/SE92/00546 27, 28 or be used in som other way. It is thus not included when calculating the amollnt of liquid F which is withdrawn and returned to the supply system.
Circulating a considerable proportion of the impregnating liquid back to the supply system increases the concentration of the fresh digesting liquor D when this is used, thus produc~.ng improved impregnating results, particularly when the fre~h digesting liquor D consists of a polysulphide solution or a solution containing anthra~uinone. As mentioned earlier, the temperature in the impregnating vessel 3 can advantageously be re~ula~ed to a suita~le 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 o 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 circul tions, 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 25 used in a digester vessel for the continuous digesting ~;
with initial impregnating zone, such as a one-vessel ~`
hydraulic digesterO
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 feedin~
it to the sypply system.
5 ~s ~ill be understood from the ~bove description and following claims no black liquor i5 added to the supply system of the impregnating system.
Claims (10)
1. 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 supplied 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 (46) located at a predetermined distance from-said inlet end, characterized in that such a first quantity of liquid (F) is withdrawn at said withdrawal point (46) 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(46), 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).
2. A process as claimed in claim 1, characterized in that said liquid to fibre material ratio is at least 2.5:1, preferably at least 3.0:1, calculated on dry fibre material.
3. A process as claimed in claim 1 or 2, characterized in that the fresh digesting liqor (E) added through the supply system consists chemically of the same fresh digesting liquor (E) supplied to the digester system.
4. A process as claimed in claim 1 or claim 2, characterized in that a fresh digesting liquor (D) having a different chemical composition from the fresh digesting liquor (E) added to the digester system, is added to the supply system.
5. A process as claimed in claim 4, characterized in that a part of the fresh digesting liquor (E) for the digester system is also added to the supply system.
6. A process as claimed in any of claims 1-5, characterized in that the temperature in the impregnating system is regulated by causing the first quantity of liquid (F), which is withdrawn and recirculated, to pass a heat exchanger (49) before being introduced into the supply system.
7. A process as claimed in claim 6, characterized in that the digesting liquid which is added to the digester system or water for the production of hot water is used as heat-exchange medium.
8. A process as claimed in claim 4 or 5, characterized in that a neutral or alkaline solution of sodium sulphide, a solution of an organic compound such as methanol and ethanol, a solution of polysulphide or a solution of anthraquinone is used as said digesting liquor (D) of different chemical composition.
9. A process as claimed in any of claims 1-8, characterized in that the quantity of liquid (F) withdrawn and recirculated to the supply system is equivalent to the quantity of free liquid in the impregnating system before the withdrawal point (46).
10. A process as claimed in claim 9, characterized in that a countercurrent flow is maintained downstream of the withdrawal point (46), seen in relation to the movement of the fibre material, by withdrawing a second quantity of liquid from the impregnating system over and above said free liquid (F), said second quantity of liquid constituting a part of a liquid which is introduced at the outlet end of the impregnating system to transfer impregnated fibre material to the digester system.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE9102427A SE469078B (en) | 1991-08-23 | 1991-08-23 | SET FOR CONTINUOUS COOKING OF CELLULOSIC FIBER MATERIAL |
| SE9102427-3 | 1991-08-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2115578A1 true CA2115578A1 (en) | 1993-03-04 |
Family
ID=20383529
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002115578A Abandoned CA2115578A1 (en) | 1991-08-23 | 1992-08-11 | A process for the continuous digestion of cellulosic fiber material |
Country Status (11)
| Country | Link |
|---|---|
| EP (1) | EP0662169B1 (en) |
| JP (1) | JPH06510091A (en) |
| AT (1) | ATE145025T1 (en) |
| AU (1) | AU2490392A (en) |
| CA (1) | CA2115578A1 (en) |
| DE (1) | DE69215110T2 (en) |
| FI (1) | FI940826A (en) |
| ID (1) | ID848B (en) |
| PT (1) | PT100799A (en) |
| SE (1) | SE469078B (en) |
| WO (1) | WO1993004232A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7297236B1 (en) | 2001-06-30 | 2007-11-20 | Icm, Inc. | Ethanol distillation process |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5413677A (en) * | 1993-04-05 | 1995-05-09 | Kamyr, Inc. | Method for producing chemical pulp from hardwood chips |
| SE506703C2 (en) | 1995-07-12 | 1998-02-02 | Kvaerner Pulping Tech | Impregnation of fiber material in co-current during continuous boiling |
| SE527058C2 (en) * | 2004-02-09 | 2005-12-13 | Kvaerner Pulping Tech | Continuous cooking process with improved heat economy |
| SE0602675L (en) * | 2006-12-13 | 2007-09-18 | Metso Fiber Karlstad Ab | Method for energy efficient production of cellulose pulp in a continuous cookery |
| US9644317B2 (en) | 2014-11-26 | 2017-05-09 | International Paper Company | Continuous digester and feeding system |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE359331B (en) * | 1970-03-17 | 1973-08-27 | Kamyr Ab | |
| SE468053B (en) * | 1988-12-20 | 1992-10-26 | Kamyr Ab | SET ON CONTINUOUS DISSOLUTION COOKING OF CELLULOSIC FIBER MATERIAL |
-
1991
- 1991-08-23 SE SE9102427A patent/SE469078B/en not_active IP Right Cessation
-
1992
- 1992-08-11 AU AU24903/92A patent/AU2490392A/en not_active Abandoned
- 1992-08-11 CA CA002115578A patent/CA2115578A1/en not_active Abandoned
- 1992-08-11 EP EP92918282A patent/EP0662169B1/en not_active Expired - Lifetime
- 1992-08-11 JP JP5504253A patent/JPH06510091A/en active Pending
- 1992-08-11 DE DE69215110T patent/DE69215110T2/en not_active Expired - Fee Related
- 1992-08-11 WO PCT/SE1992/000546 patent/WO1993004232A1/en active IP Right Grant
- 1992-08-11 AT AT92918282T patent/ATE145025T1/en not_active IP Right Cessation
- 1992-08-21 PT PT100799A patent/PT100799A/en not_active Application Discontinuation
- 1992-08-21 ID IDP460392A patent/ID848B/en unknown
-
1994
- 1994-02-22 FI FI940826A patent/FI940826A/en not_active Application Discontinuation
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7297236B1 (en) | 2001-06-30 | 2007-11-20 | Icm, Inc. | Ethanol distillation process |
| US7572353B1 (en) | 2001-06-30 | 2009-08-11 | Icm, Inc. | Ethanol distillation process |
Also Published As
| Publication number | Publication date |
|---|---|
| DE69215110D1 (en) | 1996-12-12 |
| FI940826A0 (en) | 1994-02-22 |
| SE9102427L (en) | 1993-02-24 |
| AU2490392A (en) | 1993-03-16 |
| SE9102427D0 (en) | 1991-08-23 |
| PT100799A (en) | 1994-05-31 |
| EP0662169A1 (en) | 1995-07-12 |
| ID848B (en) | 1996-08-01 |
| ATE145025T1 (en) | 1996-11-15 |
| EP0662169B1 (en) | 1996-11-06 |
| JPH06510091A (en) | 1994-11-10 |
| SE469078B (en) | 1993-05-10 |
| WO1993004232A1 (en) | 1993-03-04 |
| FI940826A (en) | 1994-02-22 |
| DE69215110T2 (en) | 1997-04-03 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |