[0001] The present invention relates to a method and an arrangement for producing chemical
pulp at a continuous digester plant of a chemical pulp mill.
[0002] Prior art utilizes a fiberline system with a chip bin, where wood chips or other
cellulosic material is steamed and liquid is admixed therein for forming a slurry,
after which the slurry is pressurized (this section is also referred to as a feed
system), fed into a treatment vessel or treatment vessels (which may be an impregnation
vessel, a pre-hydrolysis process or other vessels), whereafter follows a digester.
At present, at least one black liquor stream is extracted from the cooking system
(typically at a temperature of 120-180°C). The extracted black liquor stream or streams
can be used as a heat source for preheating white liquor, other black liquor streams
and/or other liquid streams being led to feed and cooking systems. The extracted black
liquor stream (or streams) can then be led to a pre-evaporation system, i.e. to one
or several flash tanks, wherein steam is generated from hot black liquor as it is
cooled, typically to a temperature of approximately 105-125 °C. After optional additional
cooling to 90-95 °C the black liquor is led to an evaporator system of the recovery
zone. The thus generated flash steam can be used at another location in the pulping
process. Flash steam can e.g. be used for direct preheating of chips prior to cooking.
[0003] The above described flashing process has the disadvantage that the generated steam
contains volatile compounds, including sulfur compounds, which are not desirable in
wood chip presteaming. Typically, wood chips are steamed at atmospheric or slightly
higher pressure so that residual gases are not absorbed into the wood chips, but they
are collected and treated. The treatment is typically combustion in the non-condensable
gas (NCG) system of the mill. This collection and treatment system becomes especially
significant when the used steam contains volatile compounds, including sulfur compounds,
which have a disadvantageous impact for the environment, including detrimental odor.
Therefore, it is advantageous to use such a heat source that minimizes or eliminates
the introduction of volatile compounds into the steaming process. In addition, an
explosion risk exists, if concentrated gases entrained in flash steam are introduced
into low concentration gases. Therefore, it is advantageous to find for flash steam
other objects of use than pretreatment, such a steaming, of cellulosic material being
fed into digestion.
[0004] Processes have been presented for producing clean steam for pretreating cellulosic
material.
US-patent 6,722,130 discloses a system for producing clean steam from black liquor. First the pressure
of the black liquor is decreased for producing a second black liquor having a higher
concentration and black liquor vapor, which is condensed to condensate. This condensate
is heated with the first black liquor and steam-flashed for producing clean steam
to be used in a chip bin.
[0005] WO 2007073333 discloses a system and a method for producing steam at a digester plant of a chemical
pulp mill. The pressure of hot pressurized black liquor obtained from a digester is
decreased in a first stage for producing black liquor steam that is used for chip
pretreatment in a second preheating stage. Clean steam for pretreating chips in the
first preheating stage is generated by re-heating the black liquor, the pressure of
which was decreased prior to the final subsequent pressure decrease, where the increased
black liquor steam volume is led into a steam converter for producing clean steam.
[0006] Known solutions offer various systems for using flash steam and for producing cleaner
steam for heating needs of a digester plant and for improving the energy economy of
a chemical pulp mill.
[0007] Steam is also needed for heating the fibrous material to cooking temperature in a
digester, typically in a vapor phase of the digester. In known systems, live steam
obtained from the turbine plant of the mill is typically used for this purpose. The
steam is condensed in the fibrous material, and the thus generated condensate cannot
be recovered However, the condensate of live steam is clean, and thus it could be
recovered for reuse.
US-patent 6346166 presents that the volume of flash steam obtained by flash evaporation of black liquor
is increased by operatively connecting a jet ejector to a steam outlet of the flash
tank. Steam at a higher pressure is introduced to the jet ejector from another source,
whereby a mixture of flash steam and another steam is formed, which can be used as
a steam source for heating fibrous material in a vapor phase digester. In this solution,
too, live steam is consumed in the digester for heating the fibrous material.
US-patent publication 8512514 discloses a process, in which a black liquor stream is extracted from the digester,
which stream is evaporated in an evaporation device using live steam as an indirect
heating medium. The steam generated from black liquor in the evaporation is used for
heating the fibrous material at the top of the digester, whereby live steam requirement
is decreased.
[0008] US 2010/0236733 A1 relates to a method for generating steam in a digester plant of a chemical pulp mill,
which includes the generation of vapor from black liquor by evaporating the black
liquor due to heating with fresh steam.
WO 2008/057040 A1 describes a method of producing cellulose pulp in a continuous digester system, wherein
black liquor is withdrawn from the digester vessel and led to the bottom of an impregnation
vessel containing impregnated chips and impregnation fluid.
US 2010/0224335 A1 relates to a method for heating (a mixture of) white liquor for use in a digester
system of a chemical pulp mill, wherein black liquor extracted from the digester system
is flashed to generate flash vapor that is then directed through a heat exchanger
to heat a white liquor.
[0009] WO-A1-00/11263 discloses a method of producing chemical pulp in a continuous digester plant according
to the preamble of claim 1.
[0010] An object of the present invention is to avoid the above mentioned problems and to
provide an improved and alternative method of heating fibrous material at the top
of the digester by utilizing the heat of black liquor as efficiently as possible.
One object is also to improve the overall energy economy of a chemical pulp mill.
Another object is to utilize flash steam of the black liquor at a location that is
advantageous in view of its composition.
[0011] The present invention provides a new method, in which the use of live steam for heating
fibrous material in a digester can be decreased. The new method comprises the following
steps:
- a) a slurry formed of comminuted cellulosic fibrous material and liquid is led into
a digester, heated, and the material is treated with a treatment chemical at a temperature
of over 130 °C for producing treated material and black liquor;
- b) at least one black liquor stream is extracted from the digester and the black liquor
is led into an indirect heat exchange relationship with steam in a heat exchanger
for heating the black liquor;
- c) the heated black liquor is led into a flashing vessel, wherein flash steam and
flashed black liquor are generated; and
- d) flash steam from step c) is used for heating the fibrous material in the digester
in step a).
[0012] According to an embodiment, the method comprises a step, wherein the flashed black
liquor from step c) is led into subsequent heat recovery stages, e.g. into a second
flashing vessel, whereby a second flash steam and a second flashed black liquor are
generated. Heat can be recovered from the flashed black liquor also or alternatively
e.g. in a kettle reboiler or in a black liquor cooler. The first mentioned allows
to produce as known per se steam from a clean liquid for pretreating fibrous material
and the latter allows heating water fractions of the mill.
[0013] A central feature of the new method is that extraction liquor from the digester is
led through an indirect heat exchanger, wherein its heat is increased with live steam,
typically with intermediate pressure steam, the pressure of which is typically 6-17
bar (g). By means of intermediate pressure steam, depending on its pressure, the temperature
of the black liquor is typically increased into a level of approximately 160-190 °C,
e.g. to 175-185 °C. Most advantageously the invention is utilized in a vapor phase
digester. At a higher temperature, more flash steam is obtained from the same amount
of liquor, but then a larger heat exchange area is needed in the heat exchanger, which
can increase the equipment costs. After this the heated black liquor is led into a
flashing vessel, where it is allowed to flash into steam that is used at the top of
the digester for heating the fibrous material instead of live steam.
[0014] The flashed black liquor can be led from the first flashing vessel into a subsequent
flashing vessel, typically with other extraction liquor or other extraction liquors
extracted from the digester, and the black liquor is flashed therein again. The flash
steam of this second flashing vessel can be used in a vapor reboiler for producing
clean steam, which can be used for steaming the fibrous material, such as wood chips,
prior to the digester.
[0015] Black liquor is extracted from the digester at one or several levels depending on
the cooking process, e.g. between cooking and washing zones. Extraction liquor from
the digester, which is heated in a heat exchanger, can be either upper extraction
liquor, lower extraction liquor, or if e.g. there are several lower extraction screens,
extraction liquor from the uppermost screen row. Black liquor can be led into the
heat exchanger also from two or more screen levels. According to a preferred embodiment,
black liquor at the highest temperature is heated for optimizing the consumption of
heating medium, steam, in the heat exchanger. The temperature of the black liquor
stream extracted from the digester is dependent on the cooking mode. If there are
more than one black liquor stream, the black liquor that is extracted at the end of
the cooking stage from the lower part of the digester typically has the highest temperature.
[0016] According to an embodiment, in step b) at least two black liquor streams are extracted
from the digester, which streams are at different temperatures. The black liquor stream
at a higher temperature is heated in the heat exchanger, and the second black liquor
stream at a lower temperature is led to heat recovery, such as into a second heat
exchanger or a second flashing vessel.
[0017] The present invention can be applied in connection with both a single vessel digester
and a two-vessel digester. Advantageously the digester is a vapor phase digester,
whereby the flash steam is led into a gas space at the top of the digester for heating
the fibrous material. The flash steam can also be fed into the upper part of a hydraulic
digester with a steam injector, as described in
WO-publication 2016/038251. An essential feature of the invention is heating of the extraction liquor with subsequent
flashing, whereby the digester does not necessary need steam from outside the balance
area, but the steam originates from internal circulation. Further, in an indirect
heat exchanger the condensate of live steam can be recovered and led back to the boiler
plant as boiler water.
[0018] By indirectly heating the liquor being led from the digester into a flash tank with
steam, typically with medium pressure steam, the amount of generated flash steam can
be increased (compared to a situation where the black liquor is not heated) and thus
produce economically an adequate amount of heating steam the digester needs and decrease
the use of direct live steam in the digester for heating the fibrous material. Flashed
liquor can also be recirculated back via the heat exchanger into a flashing vessel,
whereby the amount of flash steam can be further increased.
[0019] Easily volatile sulfur compounds entrained in flash steam are returned from the black
liquor into the upper part of the digester, which compounds may to some extent be
accumulated in the digester. This is not necessarily a negative phenomenon, but sulfur
compounds may have impacts that accelerate the cooking reaction and improve the yield.
[0020] Description of the drawing:
Figure 1 illustrates schematically an embodiment according to the invention, and
Figure 2 illustrates schematically another embodiment according to the invention,
Detailed description of preferred embodiments
[0021] A slurry formed of comminuted cellulosic fibrous material, such as wood chips, and
liquid is led into the top of the digester 1 via line 2. Prior to the digester the
slurry can alternatively be fed for treatment into a treatment vessel or vessels (can
be an impregnation vessel, pre-hydrolysis vessel or other vessels, not shown). The
figure illustrates only those components that are important in view of the invention.
In this case the digester is a vapor phase digester. Also steam is added to the top
of the digester via line 3.
[0022] The digester 1 comprises at least one or more screen arrangements 4, 5 and 6, via
which black liquor can be extracted. Black liquor having a temperature of typically
120-180 °C and a dry solids content of 12-17 %, is extracted via a screen 4 and line
7 and led into a flashing vessel 9. Line 7 is provided with a heat exchanger 8, in
which the black liquor is heated indirectly with steam, typically medium pressure
steam from line 24. The black liquor has typically been pressurized in the digester
to a pressure of 5 - 15 bar (abs) and its temperature corresponds to the temperature
of cooking treatment (approximately 130 - 180 °C). According to a preferred embodiment,
black liquor at the highest temperature is heated for optimizing the consumption of
heating medium, steam, in the heat exchanger 8. Live steam is condensed in the heat
exchanger 8, and the thus generated live steam condensate in line 25 is clean. The
condensate can be led into a recovery boiler plant of the mill, where it can be used
as feed water.
[0023] In the flashing vessel 9 the pressure of the black liquor is decreased for producing
flash steam and for producing flashed black liquor, the dry solids content of which
typically increases in the flashing. The construction of the flashing vessel 9 is
substantially conventional and it has a feed line 10 for hot black liquor, an outlet
11 for flash-evaporated vapor, and an outlet 12 for cooled black liquor at a lower
pressure. Flashing vessels are typically operated so that the pressure prevailing
in them is lower than the pressure of black liquor led thereto, but higher than the
pressure at the top of the digester, typically approximately 4-7 bar (abs). Flashing
vessels are containers designed for this purpose, which facilitate the pressure decrease
of hot, pressurized black liquor, whereby liquid, typically water, is vaporized from
the black liquor very quickly, into steam, whereby also the concentration of spent
cooking chemical and the products of a cooking reaction (dissolved solids) is increased.
The steam produced in this quick evaporation is discharged via a steam discharge opening
11 into line 3 at a pressure prevailing in the vessel 9, e.g. 4-7 bar (abs) and at
a saturation temperature corresponding to the prevailing pressure, e.g. approximately
140 - 180 °C. After flash evaporation the remaining liquid, which typically also is
at a temperature of approximately 160 °C settles onto the bottom of the vessel 9,
from where it is discharged from an outlet 12 via line 13.
[0024] The flash steam from flashing vessel 9 is led via line 3 to the top of the digester
1 to be used as heating steam. The temperature of the steam, e.g. 140-180 °C,140-180
°C, is typically higher than the cooking temperature, so that the steam that is led
via the steam inlet of the digester, which inlet is in contact with the flashing vessel,
heats the fibrous material to the cooking temperature. The flashed black liquor is
led via line 13 to another flashing vessel 15 of the mill, where the black liquor
is further flashed. A portion of the black liquor can be circulated via line 14 and
heat exchanger 8 to flashing, and thus it is possible to further increase the amount
of steam being generated in the flashing vessel 9.
[0025] Black liquor is supplied into the second flashing vessel 15 also from the digester
from digester screens, in this case from washing screen 6 via line 16. The liquid
remaining after the flashing, which typically is at a temperature of approximately
100 - 140 °C, settles on the bottom of vessel 15, from where it is discharged via
line 17. The discharge line 17 can be provided with a black liquor cooler 18, in which
heat can be recovered from the black liquor. Thereby water in line 26 is typically
heated for producing hot water, which discharges via line 28. The flashed, cooled
black liquor is led via line 27 to the evaporation plant of the mill, where the black
liquor is concentrated to a high solids content prior to combustion in a recovery
boiler.
[0026] The flash steam from the second flashing vessel 15 is led via line 19 into a reboiler
20, wherein the energy contained therein is recovered. The steam is set in an indirect
heat exchange contact with "clean" liquid free of volatile compounds for heating the
liquid to a temperature exceeding the boiling temperature for producing clean steam.
The reboiler is preferably a falling film tube reboiler. Flash steam is condensed
inside the tubes, while clean liquid boils on the outer surface of the tubes for producing
clean steam. Clean steam in line 22 is typically used for pretreating the fibrous
material to be fed into the digester, such as for steaming wood chips.
[0027] The clean liquid is led via line 21 into the reboiler 20. It can typically comprise
condensate from an evaporator, demineralized water, or adequately clean water fraction,
such as e.g. hot water of the mill. The clean steam produced in the reboiler contains
a substantially smaller amount of non-condensible gases than steam produced by direct
flashing of black liquor. Clean steam in line 22 is preferably used for wood chips
preheating by means of steam. Flash steam from the black liquor flashing vessel 15
contains volatile compounds, such a sulfur compounds. These compounds enter the foul
condensate and into the stream of concentrated non-condensible gases, CNCG, generated
in the reboiler. The foul condensate is sent from the reboiler via line 23 to the
evaporation plant where it is treated in a manner known per se. Heat can be recovered
from the flashed black liquor exiting from the first flashing vessel also in other
ways than additional flashing.
[0028] The produced pulp is discharged from the bottom of the digester via line 29.
[0029] Figure 2 illustrates an alternative solution for leading black liquor from the digester
into the first flashing vessel 9. Unlike the solution of figure 1, black liquor is
led into the first flashing vessel also from a second screen. The digester screen
6 comprises two screen levels 6a and 6b. At the end of the cooking zone black liquor
is extracted via the upper screen row 6a into line 30 and via line 31 further into
the flashing vessel 9. The temperature of this black liquor stream can be already
adequately high so that it does not need heating prior to flashing. However, preferably
the black liquor stream is led into heat exchanger 8 via lines 30 and 32.
[0030] Through a lower row of screens 6b black liquor is extracted via line 33 to heat recovery,
e.g. flashing.
Example
[0031] In a prior art solution, where live steam is used for heating fibrous material, intermediate
pressure steam is supplied to the top of the digester e.g. approximately 320 kg/adt.
From the end of the cook, one extraction liquor fraction is taken from an extraction
screen, which is e.g. 5,5 m
3/adt at a temperature of 146 °C. Heat is recovered from this black liquor e.g. by
flashing and the flash steam is used for steaming the chips.
[0032] In the solution according to the invention, the extraction liquor fraction taken
from the end of the cook is divided by means of two separate screens into two streams.
The amount of one of the streams is e.g. 3,0 m
3/adt at a temperature of 169 °C and the amount of the other stream is 2,5 m
3/adt at a temperature of 120 °C. The cooler stream of these is taken to conventional
heat recovery. The hotter stream is taken into a heat exchanger, where it is heated
indirectly with intermediate pressure steam to a temperature of 180 °C, whereby steam
is consumed in the amount of approximately 50 kg/adt. After this the heated liquor
is taken into a flash tank, where the pressure of the liquor decreases to a pressure
of 4,5 bar(g) and the temperature to a level of 158 °C. Thereby flash steam is generated
in the amount of approximately 110 kg/adt, which is taken to the top of the digester,
where the pressure level is approximately 4 bar(g). Thus, in this example the solution
according to the invention provides a saving of 110 kg/adt of direct intermediate
pressure steam at the top of the digester by using 50 kg/adt of indirect intermediate
pressure steam in the heat exchanger. Simultaneously, black liquor is evaporated by
100 kg/adt reducing the evaporation need of the evaporation plant and hot condensate
of intermediate pressure steam is recovered 50 kg/adt.
[0033] The present invention provides at least the following advantages:
- the total consumption of steam in the digester plant is decreased;
- live steam condensate is recovered when flash steam can be used in the digester instead
of live steam;
- the equipment that is needed is simple.
1. A method of producing chemical pulp in a continuous digester plant, which method comprises
the following steps:
a) leading a slurry formed of comminuted cellulosic fibrous material and liquid into
a digester (1) and treating the material with a treatment chemical at a temperature
of over 130 °C for producing treated material and black liquor;
b) extracting at least one black liquor stream from the digester (1) and leading the
black liquor into an indirect heat exchange relationship with steam in a heat exchanger
(8) for heating the black liquor; and
c) leading the heated black liquor into a flashing vessel (9), wherein flash steam
and flashed black liquor are generated,
characterized by the step of
d) using flash steam from step c) for heating the fibrous material in the digester
(1) in step a).
2. The method according to claim 1, in which a second black liquor stream is extracted
from the digester (1), and the second black liquor stream is combined to the first
black liquor stream prior to the heat exchanger (8) in step b).
3. The method according to claim 1 or 2, in which in step b) live steam is used that
is intermediate pressure steam having a pressure of 6-17 bar (g).
4. The method according to claim 1, 2 or 3, in which the flashed black liquor from step
c) is led into a second flashing vessel (9), whereby a second flash steam and a second
flashed black liquor are generated.
5. The method according to any of the preceding claims, in which flashed black liquor
from the first flashing vessel (9) is circulated via the heat exchanger (8) back to
flashing.
6. The method according to any of the preceding claims, in which in step b) at least
two black liquor streams at different temperatures are extracted from the digester
(1), and the black liquor stream at a higher temperature is heated in the heat exchanger
(8), and the second black liquor stream at a lower temperature is led to heat recovery.
1. Verfahren zum Herstellen von Zellstoff in einer kontinuierlichen Zellstoffkocheranlage,
wobei das Verfahren die folgenden Schritte umfasst:
a) Leiten einer aus zerkleinertem zellulosehaltigem Fasermaterial und Flüssigkeit
gebildeten Aufschlämmung in einen Zellstoffkocher (1) und Behandeln des Materials
mit einer Behandlungschemikalie bei einer Temperatur von über 130 °C zum Herstellen
von behandeltem Material und Schwarzlauge;
b) Abziehen mindestens eines Schwarzlaugestroms aus dem Zellstoffkocher (1) und Leiten
der Schwarzlauge in eine indirekte Wärmetauschbeziehung mit Dampf in einem Wärmetauscher
(8), um die Schwarzlauge zu erwärmen; und
c) Leiten der erwärmten Schwarzlauge in einen Entspannungskessel (9), wobei entspannter
Dampf und entspannte Schwarzlauge erzeugt werden,
gekennzeichnet durch den Schritt des
d) Verwendens von entspanntem Dampf aus Schritt c), zum Erwärmen des Fasermaterials
in dem Zellstoffkocher (1) in Schritt a).
2. Verfahren nach Anspruch 1, wobei ein zweiter Schwarzlaugestrom aus dem Zellstoffkocher
(1) abgezogen wird, und der zweite Schwarzlaugestrom vor dem Wärmetauscher (8) in
Schritt b) mit dem ersten Schwarzlaugestrom vereinigt wird.
3. Verfahren nach Anspruch 1 oder 2, wobei in Schritt b) Frischdampf verwendet wird,
bei dem es sich um Zwischendruckdampf handelt, der einen Druck von 6-17 bar (g) aufweist.
4. Verfahren nach Anspruch 1, 2 oder 3, wobei die entspannte Schwarzlauge aus Schritt
c) in einen zweiten Entspannungskessel (9) geleitet wird, wobei ein zweiter entspannter
Dampf und eine zweite entspannte Schwarzlauge erzeugt werden.
5. Verfahren nach einem der vorstehenden Ansprüche, wobei entspannte Schwarzlauge aus
dem ersten Entspannungskessel (9) mittels des Wärmetauschers (8) zum Entspannen zurück
zirkuliert wird.
6. Verfahren nach einem der vorstehenden Ansprüche, wobei in Schritt b) mindestens zwei
Schwarzlaugeströme bei unterschiedlichen Temperaturen aus dem Zellstoffkocher (1)
abgezogen werden, und der Schwarzlaugestrom bei einer höheren Temperatur in dem Wärmetauscher
(8) erwärmt wird, und der zweite Schwarzlaugestrom bei einer niedrigeren Temperatur
zur Wärmerückgewinnung geleitet wird.
1. Procédé de production de pâte chimique dans une installation de lessiveur en continu,
ledit procédé comprend les étapes suivantes :
a) l'acheminement d'une suspension formée d'une matière fibreuse cellulosique broyée
et d'un liquide dans un lessiveur (1) et le traitement de la matière avec un produit
chimique de traitement à une température de plus de 130 °C pour produire une matière
traitée et une liqueur noire ;
b) l'extraction d'au moins un flux de liqueur noire depuis le lessiveur (1) et l'acheminement
de la liqueur noire dans une relation indirecte d'échange de chaleur avec de la vapeur
d'eau dans un échangeur de chaleur (8) pour chauffer la liqueur noire ; et
c) l'acheminement de la liqueur noire chauffée dans une cuve de vaporisation éclair
(9), dans lequel une vapeur de vaporisation éclair et une liqueur noire vaporisée
sont générées,
caractérisé par l'étape de
d) l'utilisation d'une vapeur de vaporisation éclair de l'étape c) pour chauffer la
matière fibreuse dans le lessiveur (1) à l'étape a).
2. Procédé selon la revendication 1, dans lequel un second flux de liqueur noire est
extrait du lessiveur (1), et le second flux de liqueur noire est combiné au premier
flux de liqueur noire avant l'échangeur de chaleur (8) à l'étape b).
3. Procédé selon la revendication 1 ou 2, dans lequel, à l'étape b), il est utilisé une
vapeur vive qui est une vapeur de pression intermédiaire ayant une pression de 6 à
17 bars (g).
4. Procédé selon la revendication 1, 2 ou 3, dans lequel la liqueur noire vaporisée de
l'étape c) est acheminée dans une seconde cuve de vaporisation éclair (9), selon lequel
une seconde vapeur de vaporisation éclair et une seconde liqueur noire vaporisée soient
générées.
5. Procédé selon l'une quelconque des revendications précédentes, dans lequel une liqueur
noire vaporisée provenant de la première cuve de vaporisation éclair (9) circule via
l'échangeur de chaleur (8) de retour à la vaporisation éclair.
6. Procédé selon l'une quelconque des revendications précédentes, dans lequel, à l'étape
b), au moins deux flux de liqueur noire à des températures différentes sont extraits
du lessiveur (1), et le flux de liqueur noire à une température supérieure est chauffé
dans l'échangeur de chaleur (8), et le second flux de liqueur noire à une température
inférieure est acheminé vers une récupération de chaleur.