Note: Descriptions are shown in the official language in which they were submitted.
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PATENT APPLICATION
TITLE
Displacement Heating in Continuous Digesters
TECHNICAL FIELD OF THE INVENTION
- The present invention relates to improvements in
apparatus and methods for the fiber liberating digestion of
continuously fed comminuted cellulosic fiber material by
cooking liquor and subsequent washing of the liberated fiber
material. More particularly, the invention relates to an
improved apparatus and process utilizing spent black liquor
for heating the chips to effect savings in thermal energy,
and for accomplishing high sulfidity cooking without changes
to the overall sulfide balance of the system to accomplish a
pulp having improved mechanical characteristics, to improve
pulp yield, and to achieve extended delignification.
BACKGROUND OF THE INVENTION
In producing chemical wood pulps, it has become the
practice to use cooking liquors cont~;n;ng various cooking
chemicals for liberating the pulp fibers. The so-called
kraft or sulphate pulp is produced by cooking the raw
chipped wood in a liquor wherein materials such as sodium
hydroxide and sodium sulfide serve as the essential fiber
liberating chemicals. The so-called soda pulp derives its
name from the caustic soda-cont~;n;ng cooking liquor which
is produced, namely a liquor cont~;n;ng principally caustic
soda as the active pulping chemical. There are
modifications of these processes based on the use of liquors
cont~in;ng caustic soda and sodium sulphite or containing
caustic soda, and sodium sulphide. All of these processes
are, however, performed similarly with respect to the
cooking being effected with an amount of liquor over a
period of time requiring the addition of heat to maintain
the process at the proper cooking temperature, approximately
170C.
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Two basic processes have been used for performing the
chip cooking. The first is batch cooking in which the chips
are placed in a digester, liquor is added, the temperature
and pressure are raised and the "batch" is maintained at the
elevated temperature and pressure to reach the desired stage
of delignification. The digester is then emptied, and a
subsequent fill is started for another batch. In continuous
digesting, the second basic process, a chip column
continuously moves through the digester with hot liquor
circulating therethrough. Process conditions are controlled
such that the desired stage of delignification has occurred
when the chips flow out of the digester.
Various advances have been made in batch cooking
processes utilizing spent liquor or black liquor in
transferring heat to the chips, but effective heat
conserving processes for continuous cooking have not been
developed to an advanced stage for attA;n;ng m~x;mum heat
energy conservation.
In conventional continuous digesters, the spent liquor
is allowed to flash and steam is generated. The steam is
normally utilized to pre-steam the chips and to generate hot
water. The heat and cooking chemicals could be utilized
more efficiently if the spent liquors were used to preheat
and precondition the chips and to preheat the cooking
chemicals such as white liquor in kraft processes which
enters the process.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to
provide a continuous digestion cooking process wherein an
improved saving in thermal energy is effected.
A further object of the invention is to provide a
continuous digestion process wherein the used black cooking
liquors and the white liquors are utilized in a unique
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WO91/05103 (3) PCT/US90/05443
manner in the process so as to obtain a saving in thermal
energy, and to effect an improvement in the resultant
qualities of the pulp which is produced.
Yet another object of the present invention is to
provide a process for achieving high sulfidity cooking
without significant changes to the overall sulfide balance
of the cooking system, and for achieving extended
delignification in a continuous digesting system while
improving pulp yield over existing continuous digesting
systems.
A feature of the invention is the provision of a
continuous digestion process wherein a plurality of
containers are used in sequence, with the first container
receiving preconditioned chips and the chips being
preimpregnated and heated in the first container with a low
temperature black liquor. Further heating of the chips is
accomplished with higher temperature liquors in the second
and subsequent containers, and after the chips are brought
up to the desired elevated temperature, a white liquor is
circulated through the chips at the digestion temperature
and pressure. The chips are continuously fed from the last
of the preimpregnation containers to the digester for
cooking. The cooked delignified chips are removed as pulp
from the bottom of the digester and circulated to final
washing. The wash liquor from the washer is utilized in two
or more stages to displace from the digester the free liquor
and liquor within the chips.
In accordance with the principles of the present
invention, the hot spent liquor is utilized to heat the
incoming materials for the continuous digester. The spent
liquor, having been extracted from the digester, is
accumulated in pressure vessels substantially at digester
temperature. Lower temperature liquors from final
displacement stages are also accumulated. The incoming
chips are first exposed to the lower temperature liquors and
then the higher temperature liquors. The white liquor is
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W O 91/05103 PC~r/US90/0~443
preheated in a heat exchanger, utilizing a portion of hot
spent liquor. The white liquor can be stored in a hot white
liquor accumulator whereas the hot spent liquor, after
having given energy to the white liquor, goes to the low
temperature accumulator.
Practically, the process can be accomplished wherein a
first initial chamber or vessel is utilized, being fed by a
screw conveyor feeder to press the chips downwardly. The
vessel may be employed with an extraction screen at the top
and with a recirculation screen further down the vessel,
with the recycled liquor flowing through a central pipe
ending at the screen level. Low temperature liquor is fed
from a low temperature tank to a circulation pump, and the
liquor is evenly spread over the vessel area. By extracting
liquor from the top screen, a portion of the added liquor
flows in a countercurrent path to the movement of the chips,
thus transferring the heat and residual chemicals in the
liquor to the chips. The duration of time that the chips
are in that zone, and the liquor flow rate will determine
the efficiency of the heat transfer. The amount of liquor
extracted from the top screen is an amount that generally
corresponds to the white liquor charge, wood moisture and
the dilution factor. The extracted liquor goes to the
evaporators. The rem~in;ng part of the liquor goes with the
chips downwardly in the chamber.
The treatment with hot spent liquor takes place in a
vessel at principally digester pressure. Preferably, this
high pressure vessel is located underneath the first vessel,
and the transfer of material takes place in utilizing a high
pressure feeder. The arrangement for heat exchange is, in
principal, essentially the same as in the first lower
temperature vessel. The hot spent liquor is introduced in a
recirculation circuit, and a portion of the liquor flows
countercurrent to the chips, being extracted from a top
screen. Thus, the low temperature liquor is being displaced
and substituted by a hot spent liquor. The low temperature
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liquor which leaves the top screen is conveyed back to the
low temperature tank.
A portion of the hot spent liquor is utilized to
preheat the white liquor that is introduced in the bottom of
the high pressure vessel. In the event a mechanical device
is utilized to feed chips out of the high pressure vessel,
white liquor should be added after that mechanical device to
avoid pulp deterioration. This can be done by utilizing a
vessel which forms part of the continuous digester and is
integrated into the digester apparatus itself.
Other objects, advantages, and features will become
more apparent with the teaching of the principles of the
invention in connection with the disclosure of the preferred
embodiments thereof in the specification, claims, and
drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic drawing of a process for
operating in accordance with the principles of the present
invention.
Figure 2 is a schematic drawing of a modification of
the process shown in Figure 1, wherein the high pressure
preimpregnation vessel forms a part of the continuous
digester.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated in Figure 1, wood chips are delivered to
a first chamber or container 10 through a screw delivery
mechanism 11. The chips may be preconditioned, such as by
being heated by steam, in a supply container 12.
In the first container, which is a warm liquor
preimpregnator, the preheating occurs by a supply of low
temperature black liquor supplied from a low temperature
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black liquor tank 13 through a supply line 14. The black
liquor is circulated through the chips through a
recirculating mechanism including a recirculating line 15
and a recirculating pump 16.
Excess black liquor, after having spent its heat energy
and residual chemicals, is removed near the top of the
chamber 10 through a line 17 controlled by a valve 18 to
flow to an evaporator 19 where the black liquor is
reprocessed and reclaimed in the manner which will be
recognized and understood by those versed in the art.
Screens lOa and lOb are provided for the liquor outlets to
the evaporator 19 and the recirculating line 15,
respectively.
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In the process depicted in Figure 1, preheated woodchips are forcibly conveyed downwardly to a second container
21, which is a hot liquor preimpregnator in which the wood
chips are subjected to liquor for a predetermined time at a
higher temperature and high pressure than in the first
container. For delivering the wood chips to the second
container 21, a mechanism such as a rotary delivery valve 20
is employed. Such valves are well-known to those skilled in
the art and will not be described further herein.
To preheat the chips in the hot liquor preimpregnator
to approach the digestion temperature, hot black liquor is
delivered to the second container by line 23, which receives
hot black liquor from a hot black liquor tank 22 delivered
by a pressure pump 22a. The hot black liquor is
recirculated through the moving chips by recirculation line
24, with the liquor being circulated by a ~ 24a. A
~portion of the liquor is removed through a line 24b,
controlled by a valve 24c, to be delivered back to the low
temperature black liquor tank 13, preferably upstream of a
delivery pump 13a which pumps the low temperature black
liquor to the first container 10.
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~ Arranged in the second container 21 are screens 21a,
21b, and 21c, which allow for the removal of the liquor,
with the screen 21a facilitating the removal of the excess
black liquor through the line 24b, and the screens 21b and
21c facilitating the recirculation of the high temperature
black liquor in two recirculation paths.
For the digestion process, white liquor is delivered to
the second container 21 through a line 25. Alternatively,
the white liquor can be supplied through a line 25a, shown
by dotted line in Figure 1, as the chips leave the second
cont~iner.
The white liquor is obtained from a high temperature
white liquor tank 27, being delivered therefrom by a pump
27a. The white liquor is preheated before delivery to the
second container 21 in a heat exchanger 28, with the white
liquor being supplied to the heat exch~nger from a supply
source not shown and a supply line 29. The heat exchanger
is heated by hot black liquor supplied through a line 30
leading from the high temperature black liquor tank 22, and,
after passing through the heat exchanger, the black liquor
flows through a line 31 to the low temperature black liquor
tank 13.
The preimpregnated chips and cooking liquor exit the
bottom of the second cont~;ner 21 through a discharge line
32, which connects the bottom of the second container 21 to
the upper end of the digester 33. Cooking liquor is
recirculated at the top of the digester through a circuit 34
having a recirculation pump 34a and extraction screen 34b
therein. Temperature adjustment of the cooking liquor may
be achieved with a trim heat exchanger 34c heated by steam
from a steam source 34d. A portion of the cooking liquor is
removed from the digester through a line 38 by a pump 38a,
and is delivered through a line 39 to the location where the
chips exit the second container. This recirculated cooking
liquor further dilutes the chips and liquor exiting the
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WO9l/05103 PCT/US90/05~3
second container to facilitate transport of the chips to the
digester.
In the digester 33, the chips move continuously
downward, and are cooked to the desired level of
delignification. Recirculation takes place through an
extraction line 35, an extraction screen 35a, a
recirculation line 36, and a pump 36a. A portion of the
recirculated liquor is directed to the hot liquor tank 22,
controlled by a valve 36b.
In the lower part of the digester, washer filtrate from
a line 50 is recirculated through a circuit 52 by a pump 54,
to el;m;n~te temperature and spent liquor concentration
gradients. The filtrate is added between an extraction
screen 56 and the pump 54, so that the extracted liquor
volume is less than the flow into the digester through
circuit 52, causing an upward flow of filtrate in the bottom
of the digester. A second recirculation circuit 60,
including a pump 62, is provided to extract a portion of the
upward flowing filtrate at an extraction screen 64, together
with rem~;n;ng hot spent black liquor. A portion of the
extracted filtrate and liquor is directed to the low
temperature tank 13, through line 66 controlled by a valve
68.
A blow line 80 is provided for removing pulp from the
digester to a washer. The m~nner in which the pulp is moved
from the digester, including any secondary dilution, is
well-known in the art of continuous digesters and will not
be described further herein.
In operation, preconditioned pulp is delivered via a
screw conveyor 11 into a first chip preheating container 10
where it is heated by low temperature black liquor obtained
from a low temperature black liquor tank 13. The preheated
chips pass downwardly through a rotary delivery valve 20 to
a second chamber 21, where the chips are further preheated
WO 91/05103 (9) 2066 1 8~T~Us9O~05443
by high temperature black liquor received from a high
temperature black liquor tank 22.
The high temperature and low temperature black liquors
are obtained from the pulp washer with the high temperature
liquor also being utilized for heating the white liquor
through the heat exchanger 28. White liquor delivered
through the screen 21c from a white liquor supply line 25,
is added to the chips before the chips enter digester 33.
In the digester, delignification takes place as the
chip column and liquor move downwardly. The volume of
filtrate added through supply line 50 should be sufficient
to displace the free hot black liquor extracted through
screen 35a and the warm liquor extracted through screen 64.
The countercurrent flow of liquor in the area between screen
35a and screen 64, and between 64 and screen 56 creates a
condition in which liquor held by the chips is displaced and
removed, so that the chips leaving through blow line 80 are
substantially free from cooking liquor.
The region of the countercurrent flow between screen 64
and screen 35a should be sufficiently long that the filtrate
is heated by the chips substantially to cooking temperature,
and the liquor and filtrate removed through line 35a are at
or near cooking temperature.
It is known that the sulphide ions in cooking liquors
that are absorbed by the chips prior to cooking are
liberated from the wood chips later in the cooking process,
so that approximately 90 percent of the sulphide is left in
the spent liquors. It is critical to cooking selectivity to
precondition chips with sulphide prior to bulk
delignification. In the present invention, liquor leaving
the top of the first chamber contains essentially the same
amount of sulphide as conventional spent liquors going to
evaporation. The sulphide concentration in the low
temperature tank is even higher than that coming from the
first chamber, and the sulphide concentration in the hot
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WO91/05103 ) PCT/US90/05443
black liquor tank is even higher. These concentrations, in
combination with the elevated temperatures, give an
efficient preconditioning of the wood chips with sulphide
prior to cooking.
It should be recognized that the various components of
the present process can be rearranged to achieve the desired
preimpregnation and preheating, with appropriate
recirculation. Additional preimpregnation containers can be
utilized, joined as shown in Figure l, or by other suitable
means. The two container process described also can be
alternatively arranged. For example, Figure 2 illustrates,
in schematic format, a process in which the second chamber
21 is continguous with the digester. Corresponding parts of
the process depicted in Figure 2 have been numbered
similarly to Figure l. Thus, the various extraction
screens, recirculation circuits, pumps, and the like for
both the high pressure preimpregnation process and the final
digestion process are contained in the combined
preimpregnation and digester vessel.
Thus, it will be seen that I have provided an improved
method and apparatus for a continuous digestion process
which meets the objectives and advantages above set forth.
It will be recognized that various modifications of the
process and apparatus within the spirit and scope of the
invention may be employed without departing from the
principles of the invention.
