Note: Descriptions are shown in the official language in which they were submitted.
CA 02277931 1999-07-14
WO 98J35092 PCT1SE98/(IOZ24
METHOD AND DEVICE FOR THE CONTINUOUS COOKING OF
CHEMICAL PULP
Technical field
The present invention relates to a novel top separator
and a method for producing pulp, preferably sulphate
cellulose, with the aid of continuous digester systems.
Background and summary of the invention
Environmental demands has forced our industry to
develop improved cooking and bleaching methods. One recent
breakthrough within the field of cooking is ITCH'', which was
developed in 1992-1993. ITf~ is described in WO-9412566,
which shows that very good results concerning the pulp
quality may be achieved. ITCH is mainly based on using
almost the same temperature (relatively low compared to
prior art) in all cooking zones in combination with
moderate alkaline levels. The ITCH-concept does not merely
relate to the equalization of temperatures between
different cooking zones, but a considerable contribution of
the ITCrM-concept relates to enabling an equalized alkaline
profile also in the lower part of the counter-current
cooking zone.
Moreover, it is known that impregnation with the aid
of black liquor can improve the strength properties of the
fibers in the pulp produced. The aim of the impregnation
is, in the first place, to thoroughly soak each chip so
that it becomes susceptible, by penetration and diffusion,
to the active cooking chemicals which, in the context of
sulphate cellulose, principally consist of sodium hydroxide
and sodium sulphide.
If, as is customary according to prior art, a large
proportion of the white liquor is supplied in connection
with the impregnation, there will exist no distinct border
between impregnation and cooking. This leads to
difficulties in optimizing the conditions in the transition
zone between impregnation and cooking.
CONFIRMATION
coPY
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2
Now it has been found that surprisingly good results can be
achieved when:
1. Keeping a low temperature but a high alkali
content in the beginning of a concurrent cooking zone of
the digester;
2. Withdrawing a substantial part of a highly
alkaline spent liquor that has passed through at least the
concurrent cooking zone; and
3. Supplying a substantial portion of the
withdrawn spent liquor that has a relatively high amount of
rest-alkali, to a point that is adjacent the beginning of
an impregnation zone.
This leads to a reduced H-factor demand, reduced
consumption of cooking chemicals and better heat-economy.
Additionally, the novel method leads to production of pulp
that has a high quality and a very good bleachability,
which means that bleach chemicals and methods can be chosen
with a wider variety than before for reaching desired
quality targets (brightness, yield, tear-strength,
viscosity, etc.) of the finally bleached pulp. This novel
process is defined in more detail in our co-pending
application PCT/SE97/00192.
The present invention relates to a preferred method and
device for practising the above. In connection with the
continuous cooking of cellulose containing fibre material,
it comprises impregnation of the fibre material with an
impregnation liquid in an impregnation vessel and cooking
of the impregnated fiber material in a digester, the
impregnation vessel and the digester being connected to
each other by a transfer circulation, which, via a feed
line, feeds the fibre material from an outlet end of the
impregnation vessel to the top of the digester, which feed-
line comprises a separator for separation of free liquid
from the fibre material and, which via a return line, feeds
separated liquid back to the outlet end of the impregnation
vessel for use as transfer liquid for the impregnated fibre
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wo son pcrisE9srooiZa
3
material. Cooking liquid, is added to the fibre material
after separation of the free liquid, preferably in
connection with the top of the digester, downstream said
separation of liquid.
According to conventional technique for withdrawal of
liquid from the cooking system, this is normally done
directly from the withdrawal strainer of the digester
itself. Alternatively in connection with a two vessel
system the impregnation vessel, may be supplied with fresh
cooking liquor and equipped with a screening device, from
which some of the withdrawn liquid is transferred to a
recovery plant, possibly after first having passed a flash
cyclone. The use of such a screening device involves a
considerable cost, due to a special construction of the
impregnation vessel being necessary, assembly of conduits
and installation of screens, blind plates, nozzles, a
possible central line and different instruments in addition
to labour for assembling, welding etc. In addition to this
there are difficulties in optimizing the withdrawal at this
point. Moreover the operating costs of such a screening
device is not neglectable. Furthermore the addition of
white liquor (fresh cooking liquid) within the impregnation
vessel or in the transfer circulation line leads to
difficulties in optimizing the process. Firstly when
supplying to the impregnation vessel it can be difficult to
achieve sufficient mixing of the added white liquor in the
impregnation vessel, leading to varying levels of alkaline
in different parts. Secondly different kind of wood chips
may consume varying amounts of alkaline, making it more
difficult to optimize the conditions in the impregnation
vessel. It is even claimed that the above might have a bad
influence on cost and the quality of pulp, since if a too
high amount of alkaline exists in connection with the
mechanical action of the outlet scraper might deteriorate
fibre strength.
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The object of the present invention is to improve
and simplify the cooking department with respect to
withdrawal and supply of liquid from the cooking system.
This is achieved by the use of a new method in connection
with a new separator, also leading to a simplified
construction of the impregnation vessel with resulting
savings in material and costs and to a better way of
optimizing withdrawal and supply of liquid thereby also
creating conditions for a better utilisation of the cooking
liquid.
According to one aspect of the present invention,
there is provided an apparatus for continuous cooking of
cellulose containing fiber material, comprising: an
impregnation vessel for impregnating the fiber material with
an impregnation liquid; a digester; a separator disposed in
the digester for separating a free liquid from the fiber
material, the separator defining a liquid chamber for the
separated free liquid, the separator further comprising a
solid cylindrical part being disposed above a cylindrical
screen part adjacent to the liquid chamber; a transfer
circulation comprising a feed line extending from an outlet
end of the impregnation vessel to a top of the digester for
feeding the fiber material from the impregnation vessel to
the digester and a return line extending from the digester
to the impregnation vessel for recirculation a first portion
of the separated free liquid to the impregnation vessel; a
supply line for providing a cooking liquor to the top of the
digester, the supply line being connected to a ring shaped
conduit disposed in the separator, the ring shaped conduit
surrounding the solid cylindrical part; a branch line in
fluid communication, at one end, with the liquid chamber and
the return line; the branch line being in operative
engagement at another end with a recovery unit; the branch
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line being for withdrawing a second portion of the separated
free liquid from the return line; an upwardly feeding screw
in operative engagement with the cylindrical part and the
cylindrical screen part; a plurality of holes defined in the
cylindrical part and the ring shaped conduit; and an inside
chamber defined inside the upwardly feeding screw and the
cylindrical screen part, the inside chamber being in fluid
communication with the plurality of holes for faciliting an
even addition and thorough mixing of the cooking liquor with
the fiber material.
According to another aspect of the present
invention, there is provided an apparatus for continuous
cooking of cellulose containing fiber material, comprising:
an impregnation vessel for impregnating the fiber material
with an impregnation liquid; a digester; a separator
disposed in the digester for separating a free liquid from
the fiber material, the separator defining a liquid chamber
for the separated free liquid, the separator further
comprising a solid cylindrical part being disposed above a
cylindrical screen part adjacent to the liquid chamber; a
transfer circulation comprising a feed line extending from
an outlet end of the impregnation vessel to a top of the
digester for feeding the fiber material from the
impregnation vessel to the digester and a return line
extending from the digester to the impregnation vessel for
recirculation a first portion of the separated free liquid
to the impregnation vessel; a first supply line for
providing a cooking liquor to the top of the digester, the
first supply line being connected to a first ring shaped
conduit disposed in the separator, the first ring shaped
conduit surrounding the solid cylindrical part; a branch
line in fluid communication, at one end, with the liquid
chamber and the return line; the branch line being in
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4b
operative engagement, at another end, with a recovery unit;
the branch line being for withdrawing a second portion of
the separated free liquid from the return line; an upwardly
feeding screw in operative engagement with the cylindrical
part and the cylindrical screen part; a second supply line
connected to a second ring shaped conduit disposed in the
separator below the first ring shaped conduit for providing
a free liquid into the separator; a plurality of holes
defined in the cylindrical part and each of the ring shaped
conduits and an inside chamber defined inside the upwardly
feeding screw and the cylindrical screen part, the inside
chamber being in fluid communication with the plurality of
holes for faciliting an even addition and thorough mixing of
the cooking liquor with the fiber material.
The device according to the invention is
characterised by a separator for wood chips disposed in the
feed line between an impregnation vessel and a digester
comprising:
a screw feeder having an inlet end and an outlet
end for feeding wood chips in an upward direction from the
inlet end towards the outlet end of the screw feeder;
a rotatable shaft in operative engagement with the
screw feeder;
a drive unit secured to the rotatable shaft for
rotating the rotatable shaft;
a cylindrical screen basket enclosing the screw
feeder; and
a liquid collecting space enclosing the
cylindrical screen basket for separating a substantial
portion of a free liquid, the liquid collecting space being
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4c
in fluid communication with a return line connected to the
outlet of the impregnation vessel; and
a distribution means for supplying a cooking
liquid to the fiber material, said distribution means being
positioned downstream of the collecting space in relation to
the flow of the chips.
According to a further aspect of the invention,
less than 100%, preferably less than 95o and more preferred
less than 90% of the liquid which is separated from the
fiber material in the transfer circulation is recirculated
to be used as transfer liquid for the impregnated fiber
material
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WO 98/35092 PCT/SE98/00224
and/or to be re-used in connection with the impregnation
vessel, either as a transfer liquid or as an impregnation
liquid.
5 Further the apparatus according to the invention is
characterised in that it comprises a connection, which
stretches from the liquid chamber of the separator to a
recovery plant, for withdrawal of a second part of the
liquid which is separated by the separator from the cooking
system.
According to a preferred embodiment of the invention, the
mixture of fibre material and impregnation liquid is fed
through the entire impregnation vessel, without liquid
being withdrawn from the cooking via the impregnation
vessel, besides which a second part of the liquid which is
separated in the separator is transferred to recovery.
It is preferred that the second part of the withdrawn
liquid is allowed to flash before the recovery.
The second part of the withdrawn liquid may suitably
constitute at most 20 m3/ADMT of pulp and at least 0.5
m3/ADMT of pulp, preferably at least 2 m3/ADMT of pulp and
more preferred at least 4 m3/ADMT of pulp. It is suitable
that liquid is separated from the fibre material in a
controlled amount, so that the fibre material contains at
least 0.5 m3 free liquid/ADMT of pulp.
According to yet another preferred embodiment of the
. invention, cooking liquid is added to the separator after
separation of liquid in order to be intimately mixed with
the fibre material which is poor in liquid, by influence of
an upwards feeding screw in the top separator.
The said second part of liquid is suitably withdrawn from
said return line via a branch line, directly or indirectly,
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6
outside the fibre material to the recovery without any
essential part thereof being recirculated to the digester.
The invention will in the following be further explained by
an example, with reference to the drawings.
Fig. 1 shows schematically a preferred two vessel
digester according to the invention.
Fig. 2 shows a preferred embodiment of a separator
positioned in the upper part of the digester according to
figure 1.
Fig. 3 is a cross-sectional view of a further
embodiment of a separator according to the present
invention;
IS Fig. 9 is a schematic flow diagram of a one vessel
steam-liquid digester using the novel process concept;
Fig. 5 shows a diagram presenting the advantages
related to the H-factor when using the invention;
Fig. 6 shows which conditions were used in the
laboratory for one of the ITC-references and one of the
cooking methods according to the invention (so called
modified ITC);
Fig. 7 shows test data related to peroxide consumption
and brightness for the present (compact) method compared to
a conventional process;
Fig. 8 shows test data related to tensile index and
tear index for the present (compact) method compared to a
conventional process;
Fig. 9 shows test data related to tensile index and
tear index for the present compact method compared to a
conventional process;
Fig. 10 shows test data related to C1 charge and
brightness for the present (compact) method compared to a
conventional process;
Fig. 11 shows a slight modification of the embodiment
shown in figure 9, whereby some of the features of this
invention and the novel process is used in a single vessel
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7
digester system.
Fig. 12 shows a further embodiment of a one vessel
digester using the novel process.
S The invention is described in connection with the
production of sulphate pulp with wood chips as raw
material, but it is of course applicable for production of
other types of pulp and with any type of suitable raw
materials consisting of cellulose containing fibre
material, e.g. bagasse, saw dust, etc.
The apparatus which is schematically shown in figure 1
comprises a vertical steaming vessel 1, a horizontal
steaming vessel 2, a vertical impregnation vessel 3 and a
IS vertical digester 4, which operates according to the steam-
liquid phase principle. The horizontal steaming vessel may
be excluded if wished. The chips are fed through a line 5
to the vertical steaming vessel 1, to which low pressure
steam or alternatively flash steam is added through a line
6 for heating of the chips and decreasing their content of
air. Separated air can be removed through a line 7, which
is connected to the horizontal steaming vessel 2. This pre-
steaming is conducted at atmospheric pressure. The heated
chips are measured with a chip meter, which is arranged in
a connection 8 between the two steaming vessels 1, 2, which
connection 8 also comprises a low pressure feeder 9, which
sluices the chips into the horizontal steaming vessel 2, in
which the pressure is 1-1.5 bar overpressure. The chips
fall from the pressurised steaming vessel 2 into a chute
I0, which has a high pressure feeder 11 arranged in its
. lower part. A certain level of liquid is maintained in the
chute 10.
Between the high pressure feeder 11 and the impregnation
vessel 3, there is a top circulation, which comprises a
feed line 12 for a mixture of chips and impregnation
liquid, and a return line 13 for separated impregnation
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8
liquid. A downwards feeding top separator 14 is arranged in
the top of the impregnation vessel 3 for feeding of the
chips into the impregnation vessel at the same time as a
part of the impregnation liquid is separated off and is
pumped with a pump 15 through the return line 13, back to
the high pressure feeder 11. The high pressure feeder 11 is
equipped with a rotor with pockets, whereby one pocket
always is in low pressure position, to be in open
connection with the steaming vessel 2 and one pocket
always, at the same time, is in high pressure position, to
be in open connection with the impregnation vessel 3 via
the feed line 12, which is connected to the top of the
impregnation vessel. When a rotor pocket, which is filled
with chips, arrives in high pressure position, that is in
direct connection with the top circulation, it is flushed
clean by the liquid from the return line 13, and the
suspension of chips and impregnation liquid is fed into the
top of the impregnation vessel 3 via the feed line 12.
Liquid, in a circulation loop 17, which is equipped with a
pump 16, is at the same time feeding chips from the chute
10 into one of the pockets of the high pressure feeder so
that this pocket is filled with chips. The circulation loop
17 is, via a line 18, connected with a level tank 19, which
in its turn, via a line 20, is connected to the return line
13 of the top circulation.
Suitable impregnation liquid, which may comprise black
liquor and white liquor and optionally other chemicals, is
added to the top circulation. Black liquor is added through
a line 21 and white liquor through a line 22, which two
lines are connected to the return line 13, via the line 20.
The impregnation vessel 3, itself, is, in accordance with
the present invention, in the shown embodiment, completely
free from an arrangement for withdrawal of liquid from the
impregnation phase of the cooking system, at a location
between the inlet 23 and the outlet 24 of the impregnation
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9
vessel. Consequently, the impregnation vessel 3 presents a
longish cylindrical tube, which is completely free from a
cost increasing withdrawal screen for withdrawal of liquid
from the impregnation phase and removal of this liquid from
the cooking system.
Between the impregnation vessel 3 and the digester
4, there is a transfer circulation, which comprises a feed
line 25 for the mixture of impregnated chips and liquid and
a return line 26 for separated liquid. The feed line 25 is,
by one of its ends, connected to an outlet end 27 of the
impregnation vessel 3, which outlet end 27 thus comprises
said outlet 24, and by its other end, to a top separator 28,
which is arranged in the top of the digester 4 for
separation of liquid from the chip-liquid mixture that has
been fed in.
As is more readily apparent from figure 2 in
combination with figure 1, the top separator 28 has a
vertically arranged screw 29, which is driven by a motor 30,
and a cylindrical body, in which the screw 29 rotates and
which has a lower screen part 31 and a thereby following,
upper part 32 which is not broken through and presents a
free upper edge 33. The screen part 31 is surrounded by a
concentric wall 34, which is not broken through, for
formation of a liquid chamber and/or withdrawal space 35,
therebetween for collection of liquid, which is pressed out
through the screen part 31 under influence of the screw 29.
The lower screen part 31 is preferably designed in
accordance with our design described in PCT/SE94/00315, i.e.
by the use of rigid vertically arranged rods, which are
welded onto support rings so as to form gaps of about
3-10 mm, preferably about 4-7 mm, therebetween.
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A ring shaped supply space 36 is arranged around
the screw 29 within the area of the part 32, which is not
broken through. Holes forming inlet apertures 37 are
arranged in the supply space 36 and the part 32 which is
5 otherwise not broken through for addition of white liquor
and possibly other liquid to the chips, which moves upwards
in the screw room 38 and from which a large part of the free
liquid has been pressed out through the screen part 31, just
before. The supply space 36 and the liquid chamber and/or
10 withdrawal space 35 are separated in a sealed manner. In
the preferred case the distance between the supply space 36
and the upper edge of the screen 31 is less than the
diameter (Ds) of the screw 29. According to the alternative
shown in fig. 2 they are positioned directly on top of each
other, which is achieved by means of a concentric ring plate
69, e.g. by the use of welding. Also according to the shown
embodiment the outer wall 34 of the liquid chamber and/or
withdrawal space 35 may be integral with the outer wall of
the supply space 36.
The feed line is connected to the bottom of the
top separator 28. The return line 26 is connected to the
liquid chamber and/or withdrawal space 35. Medium pressure
steam may be added via a line 39, to the upper steam room of
the digester in the top of the digester 4 in connection with
the top separator 28 in order heat the chips (and free
liquid) that are fed in by the screw 29 and which fall down
over the free edge 33 of the part 32, which is not broken
through.
The digester 4 has, within its middle part, a
withdrawal screen 40 for withdrawal of black liquor via a
line 41, that is connected to a first flash cyclone 42,
which is in connection with a second flash cyclone 43 via a
line 44. Effluent from the second flash cyclone 43 is led
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l0a
via a line 45, completely or partly, to a recovery plant
(not shown). The steam which is formed in the flash
cyclones 42, 43 can be used in different locations in the
cooking process, for example for the steaming in the
steaming vessels 1, 2. In the digester there is, in
addition to top and middle circulations, a bottom
circulation, which comprises a withdrawal screen 46 and a
circulation line 49, which is equipped with a pump 47 and a
heat exchanger 48, and which comprises a central line 50
that mouths at the withdrawal
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11
screen 46. Wash liquid is added to the bottom part of the
digester 4 via a line 51. The digested chips are fed out
through an outlet in the bottom of the digester 4 and are
led away through a line 52 for further treatment.
The top separator 28 is further, with its liquid chamber
35, connected with the second flash cyclone 43 via a
connection 53 which, in the embodiment shown, comprises the
return line 26 and a branch line 54 to the same. A
prechosen amount of liquid from the cooking system is
withdrawn through the connection 53, which thus takes place
with an existing screen device, that is, the top separator
28 in the digester which thereby achieves yet another
function when it takes over the function of the
conventional withdrawal screen in the impregnation vessel.
In an alternative embodiment, the withdrawn liquid is led
directly to recovery, without passing the flash cyclone.
White liquor is added to the top of the digester 4 , via a
line 55 which passes a heat exchanger 56. This heat
exchanger can alternatively be excluded. A line 57 connects
the return line 26 with the line 55 for white liquor for
addition of withdrawn liquid from the top separator 28,
when wished. This line 57 can alternatively be excluded. A
line 58 is, further, connected to the line 55 for white
liquor, for addition of wash liquid when wished. The heat
exchanger 56 may work with low pressure steam, medium
pressure steam or flash steam.
Instead of withdrawing liquid from a screen section in the
impregnation vessel, necessary withdrawal of liquid is thus
conducted on the liquor side of the transfer circulation.
An advantage of the invention is that the transfer
circulation does not need to be heated, which means that
chips which are fed out from the impregnation vessel 3 can
keep a lower temperature than before, for example 130°C as
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12
compared to previous 145°C, which in its turn has a
beneficial effect on the pulp quality. The lower
temperature in the transfer circulation will additionally
decrease the risk of the problems which may occur in the
top separator at the previously used high temperatures.
Hy adding the white liquor to the fibre material in
connection with the top separator 28 downstream the
location for the separation of the liquid, that is
downstream the screen part 31, this addition of white
liquor becomes completely separated from the transfer
circulation so that the entire amount of white liquor
normally can be used in the digester 4. The inlet for the
white liquor is preferably situated inside the top
separator in a blind zone 32, which surrounds the screw 29
and which is located above the screen part 31 itself. A
good mixing of chips and white liquor is thereby secured by
means of the influence of the screw 29, before the chips
and the white liquor are fed out from the screw and fall
down into the steam room of the digester. It is beneficial
that the chips contain at least a small amount of free
liquid when they leave the screen part 31 and are fed up
into the supply space 36, in order to thereby prevent that
white liquor is drawn down into the screen part and is
pressed out into the liquid chamber.
The relation between liquid and wood at the inlet of the
impregnation vessel can, for example, be 3.5/1, but the
invention makes it possible to use larger amounts of
liquid, as for example up to 6/1 and above. The pressure in
the impregnation vessel can, for example, be 10 bar
overpressure and the temperature can, for example, be kept
at 115-120°C at the top or lower for example 90-100°C. Any
displacement of liquid by withdrawal of liquid from the
3.5 cooking system does thus not take place in the impregnation
vessel.
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White liquor is added to the top of the digester in an
amount which is enough to obtain the wished delignification
of the chips. The impregnated chips avail the white liquor
through diffusion. Steam is added to adjust the cooking
temperature to the wished level, for example within 140-
170°C .
The liquid which is pressed out from the screw 29 and is
collected in the liquid chamber 35 can be distributed in a
suitable way with respect to transfer liquid, Which is fed
to the impregnation vessel via the return line 26, liquid
which i_s complementary to the white liquor which is
withdrawn through the line 57, and liquid which is
withdrawn from the cooking.system via' the connection 53,
IS that is, the line 26 and the branch line 54. The relation
can, in the order given, be 20-30 m3/ADMT of pulp (to the
impregnation vessel), 0-4 m3/ADMT of pulp (via the line 57)
and 0.5-10 m3/ADMT of pulp (via the branch line 54), or
sometimes even as much as 12-15 m3/ADMT. By attaching a
line between the withdrawal screen 90 and the top of the
impregnation vessel 3 the system shown in Fig. 1 may easily
be connected to run according to the novel process, which
process is described in more detail in our co-pending
application PCT/SE97/00192 and also in connection with
figures 4 and 11 herein.
In Figure 3 there is shown a further embodiment of a
separator to be used in connection with a steam/vapour
phase digester, as described in figure 2. The separator of
Fig. 3 is almost identical with the one shown in Fig. 2
except for the existence of a furtherintermediate,supply space 25,
being positioned below the first supply space 36. This
further supply space 25 has as its object to provide for
the possibility of supplying a further liquid to the up
moving chip pile. Especially for the possibility of
supplying black liquor in order to secure a minimum amount
of free liquid flowing upwardly in the chips pile, to
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14
eliminate back flow of the cooking liquor supplied above, in
23.
As in fig. 2 circumjacent the screen basket 61
there is arranged a liquid collecting space 67, which may be
connected to the return line 13. Above the liquid
collecting space 67, also circumjacent the screen basket 61,
there is arranged a liquid supply space or opening 23 which
is connected to the supply line 55 that supplies white
liquor (F). The separator also has a plurality of inlet
apertures 37 defined therein to subject the fiber chips with
white liquor. The inlet apertures preferably has a total
area that exceeds 400 mm2. More preferred, the total area of
the inlet apertures is at least 500 mmz. Most preferred, the
total area of the inlet apertures exceeds 600 mm2 to achieve
a sufficient flow into the chip pile. Between the outer
peripheral wall 66 of the liquid collecting space 67 and the
liquid supply space 23 respectively, and the digester shell
6 at the top, there exist a steam liquid space 70 which
opens up down into the upper part of the digester 4. The
functioning of the top separator may be described as
follows.
The thoroughly heated and impregnated chips are
transferred by means of the supply line 12 into the bottom
portion of the screen basket 61. Here the screw feeder 62
moves the chips upwardly at the same time as the transport
liquid D is separated from the chips, by being withdrawn
outwardly through the screen basket 61 and further out of
the digester through return line 13. More and more liquid
will be withdrawn from the chips during their transport
within the screen basket 61. First the chips will reach the
level of the first supply space 25 where a desired liquor,
for instance black liquor, is supplied. Eventually, the
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chips will reach the level of the supply space 23. Here the
desired amount of cooking liquor, preferably white liquor,
is added through the supply space 23 and the inlet apertures
37, having a temperature and effective alkaline content in
accordance with the invention.
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In order to eliminate the risk of back flowing of the
supplied liquid from the supply space 23 into the liquid
collection space 67, a minor amount of free liquid (at
least about 0.5 m3/ADT) should be left together with the
5 chips, which free liquid will then be mixed with the
supplied cooking liquor. As explained above this may also
be achieved by means of supply of free liquid through the
intermediate supply space 25. Preferably, about one m3/ADT
should be left together with the fiber material.
10 Additionally, the white liquor should be provided at a
point that is downstream of the flow of the suspension of
the fiber material and the free liquid that is being fed
through the screw member.
At the top of the screen basket 61, the chips and the
15 cooking liquor may flow over the upper edge thereof and
fall into the steam liquid space 70 and further on to the
top of the chips pile within the digester, where the
concurrent cooking zone starts.
A major advantage of the separation device is that
they provide for establishing a distinguished change of
zones (they enable almost a total exchange of free liquid
at this point), which means that for a two vessel system
the desired conditions in the beginning of the concurrent
zone can easily be established.
Fig. 4 illustrates a single vessel steam/liquid phase
digester system, wherein a conventional type of top
separator 7h is used. The chips are fed from a chip bin
20A, through a steaming vessel 20B and a chip chute 20C. A
feeding device, preferably a high-pressure feeder 19h feeds
the chips suspended in a transport liquid D via a conduit
18h to the top of a digester 6h. The feeder 19h is co-
operating with the chute 20C, and is connected to the
necessary liquid circulations and replenishment.
The conduit 18h extends from the feeder 19h up to a
top 5h of the digester 6h. The conduit 18h may open up at
the bottom of the top separator 7h that feeds by means of a
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16
screw in an upwardly moving direction. The separator 7h has
no supply space 36 as shown in Fig. 2. The screen of the
separator may be used to draw off the transport liquid D
(which is then returned in a return line 15h) together with
which the chips are transported from the feeder 19h up to
the top 5h of the digester 6h. A first screen girdle
section 8h may be disposed immediately below or adjacent
the separator 7h. A recirculation line 17h withdraws liquor
and brings it back to a space that is defined between the
first screen girdle section 8h and the separator 7h at the
same time as withdrawn black liquor is added. This
recirculation improves the distribution of the black liquor
withdrawn from screen section 104 h and added to the
impregnation zone A, in order to run the digester according
to the novel process.
A second screen girdle section 51h is disposed below
the first screen girdle section 8h so that an impregnation
zone A is defined between the screen girdle sections Bh and
51h. We have found indications that it is desirable to keep
the alkaline level at above at least 2g/1, preferably above
9g/1, in the impregnation zone A in connection with black
liquor, which would normally correspond to a pH of about
11. If not, it appears that dissolved lignin precipitate
and even condense. Spent liquor may be withdrawn from the
upper screen of the section 51h and conducted with a
conduit lllh to a second flash tank ll2h. Spent liquor is
withdrawn via a conduit 109h from a lower screen of the
section 51h and conducted back to the space defined above
the first screen girdle section 8h so that the spent liquor
may be reintroduced back to the lower screen of the second
screen girdle section 51h via a central pipe 105h. The
temperature of the spent liquor may be controlled by a heat
exchanger 13h. The heat exchanger 13h is in operative
engagement with a high pressure steam line 102h via a
conduit 122h.
A cooking liquor conduit 24h is operatively attached
to the conduit 109h to supply a cooking liquor, such as
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17
white liquor, to the conduit 109h. The effective alkali of
the liquor in the conduit 109h is at least about 13 g/1;
more preferably at least about 16 g/1; and, most preferably,
between about 13 g/1 and about 30 g/l.
Approximately 95% of the total supply of the white
liquor is conducted in the conduit 24h and the remaining 5%
is supplied to the high pressure feeder 19h via a conduit
132h and a conduit 134h to lubricate the high pressure
feeder 19h.
A third screen girdle section 104h may be arranged
below the second screen girdle section 51h so that a
concurrent cooking zone B is defined between the screen
girdle sections 51h and 104h. Draw-off from the third
screen section 104h, such as spent liquor, i.e., black
liquor, may be conducted via a conduit 106h back to the
recirculation line 17h. A portion of the black liquor in
the conduit 106h may be conducted to a first flash tank 108h
via a conduit 107h to cool the spent liquor before the
liquor is conducted to a recovery unit 110h. Preferably,
the spent liquor is also conducted through a second flash
tank 112h via a conduit 114h to further reduce the
temperature and pressure of the spent liquor before the
liquor is conducted to the recovery unit 110h. The spent
liquor from both flash tanks 108h, 112h are then conducted
with a conduit 126h to the recovery unit 110h. Conduits
128h and 130h may be connected to the flash tanks 108h,
112h, respectively, to provide steam that is sent to the
chip bin 20A and the steaming vessel 20B.
At a bottom lOh of the digester 6h, there is a
feeding-out device including a scraping element 22h. A
fourth lower screen girdle section 12h is disposed at the
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17a
bottom lOh of the digester 6h so that a counter-current
cooking zone C is defined between the sections 104h and 12h.
The girdle section 12h may, for example, include three rows
of screens for withdrawing liquid, which is heated and to
which some white liquor, preferably about 10% of the total
amount of the white liquor in the conduit 24h, is
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18
added via a branch conduit 117h before it is recirculated
by means of a central pipe 123h, which opens up at about
the same level as the lowermost strainer girdle 12h.
The draw-off from screen girdles 12h and the white
liquor from the branch conduit 117h are preferably
conducted via a heat exchanger 120h back to the bottom lOh
of the digester 6h. The conduit 122h is connected to the
heat exchanger 120h to provide the heat exchanger 120h with
steam to regulate the temperature of the liquor in the
conduit 116h. The white liquor is supplied in a counter-
current direction via the central pipe 123h to the screen
girdle section 12h. The white liquor provides fresh alkali
and, in the form of counter-current cooking, further
reducing the kappa number. A blow line 26h may be connected
1S to the bottom lOh of the digester for conducting the
digested pulp away from the digester 6h.
The installation, as shown in Fig. 9, using our novel
process (but not the specific invention presented herein)
may be described as follows. The chips are fed into the
chip bin 20A and are subsequently steamed in the vessel 20B
and, thereafter, conveyed into the chute 20C. The
high-pressure feeder 19h, which is supplied with a minor
amount of white liquor (approximately 5~ of the total
amount to lubricate the feeder), feeds the chips into the
conduit 18h together with the transport liquid. The slurry
of chips and the liquid are fed to the top of the digester
6h and may have a temperature of about 110-120°C when
entering the digester 6h (excluding recirculated transport
liquor) .
Inside the top of the digester 6h, there is the top
separator 7h that pushes chips in an upward direction
through the separator and then the chips move slowly
downwards in a plug flow through the impregnation zone A in
a liquid/wood ratio between 2/1-10/1 preferably between
3_S 3/1-8/1, more preferred of about 4/1-6/1. The liquor, which
is drawn off from the screen girdle section 8h, may be
recirculated via the conduit 17h to the space below the top
CA 02277931 1999-07-14
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19
separator 7h. The chips are then thoroughly impregnated in
the impregnation zone A.
The retention time in the impregnation zone A should
be at least 20 minutes, preferably at least 30 minutes and
more preferred at least 40 minutes. However, a shorter
retention time than 20 minutes, such as 15-20 minutes may
also be used. The volume of the impregnation zone A may be
larger than 1/11, preferably larger than 1/10 of the volume
of the digester 6h. Additionally, in the preferred
embodiment, the volume V of the impregnation zone A should
exceed 5 times the value of the square of the maximum
digester diameter, i.e., V - 5D2, where D is the maximum
diameter of the digester 6h.
The chips, which have been thoroughly impregnated and
partially delignified in the impregnation zone A, are then
passed into the concurrent cooking zone H.
A spent liquor is withdrawn at the upper segment of
the screen section 51h and conducted to the second flash
tank 112h. A spent liquor is also withdrawn at the lower
segment of the section 51h and reintroduced via the central
pipe 105h with the addition of white liquor supplied by the
conduit 24h.
The chips move down in the concurrent zone B through
the digester 6h at a relatively low cooking temperature,
i.e., between 130-160°C, preferably about 140-150°C. The
major part of the delignification takes place in the first
concurrent cooking zone B.
The liquid-wood ratio should be at least 2/1 and
should be below 7/1, preferably in the range of 3/1-5.5/1,
more preferred between 3.5/1 and 5/1. (The liquid
wood-ratio in the counter-current cooking zone should be
about the same as in the concurrent cooking zone.)
The temperature in the lower counter-current zone C
may in connection with some installation be higher than in
the concurrent zone B. The alkali content in the lowermost
part of the counter-current cooking zone C may in such
installations preferably be lower than in the beginning of
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the concurrent zone B. Expediently, the conduit 116h may be
charged with about 5-20~, preferably 10-15$, white liquor
from the conduit 24h via the conduit 117h.
The temperature of the liquid which is recirculated
S via the pipe 123h up to the screen girdle section 12h is
regulated with the aid of the heat exchanger 120h so that
the desired cooking temperature is obtained at the
lowermost part of the counter-current cooking zone.
10 In Fig. 5, there is shown a diagram comparing the H-factor
for pulp produced according to a conventional ITCTM-cooking
process and according to the cooking process of the present
invention. The H-factor is a function of time and
temperature in relation to the delignification process
15 (degree of delignification) during the cooking process.
The H-factor is used to control the delignification process
of a digester, i.e., maintaining a certain H-factor
principally leads to the same Kappa number of the produced
pulp (remaining lignin content of the fiber material)
20 independent of any temperature variations during the
cooking process.
In Fig. 6, it is shown that the H-factor for pulp
produced according to the present invention is extremely
much lower (about 40-50$ lower) compared to pulp produced
according to ITC. This means that much lower temperatures
may be used for the same retention time in order to reach a
certain degree of delignification (Kappa number) and/or
that smaller vessels for the cooking within a continuous
digester can be used and/or that a lower Kappa number may
be achieved with the same kind of basic equipment and/or
that higher rate of production can be obtained.
The lower H-factor demand is achieved by a high alkali
concentration and a low cooking temperature in the
concurrent cooking zone, which presents one reference
ITC-cook (ITC 1770) and one cook according to the present
invention (modified ITC* 1763). As shown, the temperature
in the counter-current cooking zone, according to the
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. 23402-155
21
present invention, is higher than in the concurrent zone but
still lower than the temperature in the counter-current zone
in the ITC-reference.
Fig. 7 shows results from TCF bleaching using the
novel cooking process (so called "new concept") of the
present invention compared to a conventional reference
cooking process. The present invention provides a TCF-
bleached pulp having extremely good bleachability, i.e. a
higher brightness is achieved compared to the conventional
process for the same amount of peroxide consumption, and
also a higher brightness ceiling is obtained.
Fig. 8 shows the tear index relative to the
tensile index. The test data compares results obtained by
the novel cooking process ("new concept") of the present
invention with a conventional cooking process ("ITC-
reference").
Similarly, Fig. 9 compares test data for the novel
process with those from a conventional process. As can be
seen the present invention exhibits better tensile index
compared to the conventional method.
Fig. 10 shows the brightness level by using the
novel process ("new concept") with reference cooked pulp.
The novel cooking process of the present invention exhibits
a higher brightness compared to the conventional cooking
process.
Fig. 11 shows principally the same system as
described in fig. 4 and uses the same reference numbers on
identical parts. In contrast to fig. 4, however, a top
separator 7i according to the invention is used.
Accordingly this top separator 7i is arranged with one (or
more) supply space 36 as described in detail in relation to
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22
fig. 2. This arrangement does eliminate the need of any
circulation in the top of the digester (see screen 8h and
re-circulation line 17h, shown in fig. 4). Instead hot
black liquor may be supplied to the beginning of the
impregnation zone by means of line 199 which withdraws
liquor from withdrawal strainer 104h, without risque for
getting a too high temperature in the return line.
Consequently the use of this specific invention adds further
advantages than described in conjunction with fig. 4 in
connection with running of our novel process.
Fig. 12 illustrates an embodiment of a one vessel
hydraulic digester wherein the novel process is used but
without the use of the present invention, in order to
demonstrate the simplifications rendered thereby. Since the
digester is hydraulic it has a downwardly feeding top
separator 5i. In order to achieve a desired/sufficient
exchange of liquid, a liquid exchanger 33i is arranged in
the transfer line comprising a conduit 18i and a conduit
1191. This liquid exchanger may be designed in accordance
with prior art exchangers, having the separation unit at the
bottom, a very long screw to feed the chips all the way
through it and a separate supply space positioned far above
the separation unit. Chips and a transport fluid is pumped
up in the conduit 18i and the conduit 1191 to a top
separator 5i of a digester 6i via the liquid exchanger 33i;
wherein liquid is exchanged before the chips enter the top
section 5i of the digester 6i.
A portion of the transport liquid may be returned
in return line 15i that leads from the top portion 5i to a
mid-section of the liquid exchanger 33i and then back to a
feeder 19i via a conduit 25i. The conduit 1061 conducts the
spent liquor withdrawn from a screen girdle section 104i to
the liquid from conduit 1171 and to the conduit 15i. A
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22a
portion of the liquor in the conduit 1061 may be sent to a
flash tank 1081. Figures 4, 11 and 12 use the same
reference numbers on identical parts. When comparing
figs. 12 and 11 the enormous advantages of the invention are
apparent, i.e. by the use of the invention only one
separator is needed and since the liquid exchanger may be
eliminated all its circulation lines, pump, valves, etc. are
also eliminated implying a considerable cost reduction not
only investment wise but also from a maintenance
perspective.
According to the novel process the black liquor
supplied into the impregnation zone A has a high content of
rest alkali, (effective alkali EA as NaOH), at least 13g/1,
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23
preferably about or above 16g/1 and more preferred between
13-30g/1 in the top of the impregnation zone A. This alkali
mainly comes from the black liquor due to the high amount
of alkali in the concurrent zone B of the digester 6h.
Furthermore, the strength properties of the fibers are
positively affected by the impregnation because of the high
amount of sulphide. A major portion of the black liquor may
directly (or via one flash tank) be fed into the:
impregnation zone A. The total supply of black liquor to
the impregnation zone A may exceed 80$ of the amount drawn
off from the draw-off screen girdle section 104h,
preferably more than 90~ and optimally about 100$ of the
total flow, which normally is about 8-12 m3/ADT.
The retention time in the impregnation zone A
1_S should be at least 20 minutes, preferably at least 30
minutes and more preferred at least 40 minutes. However, a
shorter retention time than 20 minutes, such as 15-20
minutes may also be used. The volume of the impregnation
zone A may be larger than 1/11, preferably larger than 1/10
of the volume of the digester 6i. Additionally, in the
preferred embodiment, the volume V of the impregnation zone
A should exceed 5 times the.value of the square of the
maximum digester diameter, i.e., V = 5D2, where D is: the
maximum diameter of the digester 6i.
The invention is not limited to what is described above,
but can vary with the scope of the appendant claims. For
example, the invention may also be performed in connection
With an impregnation vessel having the inlet at the bottom
and which accordingly has an upward flow of the chips.
Furthermore it is understood that instead of an annular
distribution ring for supply of cooking liquor, a number of
nozzles may be used, or even spray nozzles as described in
PCT/SE94/01230.
