U.S. patent application number 10/514409 was filed with the patent office on 2005-11-17 for process for the manufacturing of cellulose pulp.
Invention is credited to Lindstrom, Mikael, Olsson, Krister, Snekkenes, Vidar.
Application Number | 20050252625 10/514409 |
Document ID | / |
Family ID | 20288056 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050252625 |
Kind Code |
A1 |
Lindstrom, Mikael ; et
al. |
November 17, 2005 |
Process for the manufacturing of cellulose pulp
Abstract
The invention concerns a method for the production of cellulose
pulp. A wood raw material that principally comprises softwood,
preferably in the form of cut wood chips, is treated in several
stages in various treatment steps. One of the stages is cooking the
material in an alkali cooking fluid, with the aim of obtaining
improved quality with respect to tensile strength, and of obtaining
higher yield. A controlled addition of wood raw material in the
form of hardwood at an amount corresponding to 1-20% of the amount
of softwood, preferably 3-15%, and more preferably at least 5%,
takes place.
Inventors: |
Lindstrom, Mikael;
(Stockholm, SE) ; Snekkenes, Vidar; (Karlstad,
SE) ; Olsson, Krister; (Karlstad, SE) |
Correspondence
Address: |
FASTH LAW OFFICES (ROLF FASTH)
26 PINECREST PLAZA, SUITE 2
SOUTHERN PINES
NC
28387-4301
US
|
Family ID: |
20288056 |
Appl. No.: |
10/514409 |
Filed: |
November 15, 2004 |
PCT Filed: |
May 2, 2003 |
PCT NO: |
PCT/SE03/00712 |
Current U.S.
Class: |
162/57 |
Current CPC
Class: |
D21C 3/228 20130101;
D21C 3/02 20130101; D21C 3/22 20130101; D21C 5/00 20130101 |
Class at
Publication: |
162/057 |
International
Class: |
D21C 003/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2002 |
SE |
0201679-8 |
Claims
1. A method for the production of cellulose pulp, comprising:
providing an amount of wood raw material that principally consists
of softwood, cooking the material in an alkali cooking fluid, with
the aim of obtaining improved quality with respect to tensile
strength and of obtaining higher yield, and in a controlled manner
adding hardwood in an amount corresponding to 1-20% of the amount
of softwood.
2. The method according to claim 1 wherein the hardwood is added
and mixed with the said softwood that is provided as wood
chips.
3. The method according to claim 2 wherein the mixing takes place
in a chip bin.
4. The method according to claim 1 wherein the hardwood is added to
and mixed with the said softwood, the hardwood is provided as a
finely divided fraction, the finely divided fraction is added after
an impregnation stage.
5. The method according to claim 4 wherein the cooking takes place
continuously and the finely divided fraction is added at a top of a
continuous digester such that mixing is achieved before entry to a
first cooking zone.
6. The method according to claim 4 wherein the cooking takes place
continuously and the finely divided fraction is added to a
continuous digester by means of at least one fluid flow that adds
and/or recirculates fluid to a cooking zoner such that mixing is
achieved after entry to a first cooking zone.
7. The method according to claim 6 wherein at least parts of the
finely divided fraction are added by means of a flow of white
liquor addition.
8. The method according to claim 6 wherein the flow is delivered at
an upper part of the digester.
9. The method according to claim 1 wherein the hardwood is added to
and mixed with the softwood the hardwood is provided as logs after
which the wood raw material is converted to chips.
10. The method according to claim 1 wherein the hardwood is
pre-treated with alkali before the hardwood is added to and mixed
with the softwood.
11. The method according to claim 10 wherein the pre-treatment
takes place at an elevated temperature of 80-160.degree. C.
12. A cooking system for the production of cellulose pulp, in which
a wood raw material that primarily consists of softwood is treated
in several stages, comprising: a raw material preparation unit (1),
a raw material storage (10; 7) being downstream of the raw material
preparation unit, a cooking plant (15, 19) being downstream of the
raw material storage, and a control system (6) being upstream of
the raw material preparation unit, the control system regulating an
inflow of hardwood and softwood into the raw material preparation
unit so that an outflow of cellulose pulp from an outlet of a
digester contains 1-20% of hardwood fibers and 80-99% of softwood
fibers.
13. The cooking system according to claim 12 wherein the control
system (6) is arranged to control the inflow to the raw material
preparation unit (1), so that the fibre so that fiber raw materials
stored in the raw material storage (10) contain 1-20% hardwood and
80-99% softwood.
14. The cooking system according to claim 12, wherein the raw
materials storage (10; 7) has at least two separate storage spaces
(10, 7; 1A', 1B') so that one storage space is for hardwood raw
material and one storage space is for softwood raw material, the
control system (6) is in operative engagement with the raw material
preparation unit and regulates a flow from the said storage spaces
such that 1-20% controlled addition of hardwood takes place before
and/or during a cooking stage.
15. The cooking system according to claim 14, wherein the cooking
system has means for ensuring that the flow of hardwood is
delivered to a chip chute (11).
16. The cooking system according to claim 14, wherein the cooking
system has means (9) for ensuring that the flow of hardwood is
delivered at a top (18) of the digester (19).
17. The cooking system according to claim 14, wherein the cooking
system has means (8) for ensuring that the flow of hardwood is
delivered to a fluid line (21) arranged at the digester (19).
Description
[0001] The present invention concerns a method and a system for the
production of cellulose pulp in which wood raw material that
principally consists of softwood, preferably in the form of cut
wood chips, is treated in several stages in various treatment
steps, whereby one of the steps comprises the cooking of the
material in an alkali cooking fluid, with the goal of obtaining
improved quality with respect to tensile strength and increased
yield.
THE PRIOR ART
[0002] The yield calculated with respect to wood raw material is
very significant when cooking, and it normally lies around 45% for
softwood and approximately 50% for hardwood. An increase of a
single percentage point results in a production plant of normal
size, 1,500 tonnes of pulp per day (approximately 3,000 tonnes of
wood raw material per day), giving an increased production of 30
tonnes. With a pulp price of approximately USD 700 per ADT, this
gives an increase in revenue of at least USD 21,000 per day. An
increase in marginal production gives essentially a net profit.
Furthermore, the load on the recycling plant is decreased, if a
lower fraction of cellulose is sent to evaporation and combustion
in the soda furnace, something that in turn could enable a capacity
increase. The soda furnace is often the limiting production
resource (bottleneck), which means that an increase in total
capacity of an order of magnitude of 2% could be obtain, and
according to the example given above, thus enabling a total
increase of capacity of 60 tonnes per day.
[0003] Thus it can be realised that attempts have been made for a
long time to increase the yield, without relinquishing requirements
on quality or other factors that affect the cost. Many different
methods with this aim are thus known.
[0004] One known method for increasing yield, for example, has been
the addition of polysulphide during the cooking, such as known
through, for example, WO 95/32331. However, a problem associated
with this method is that polysulphide is subject to thermal
decomposition, which leads to a major part being broken down by the
high cooking temperature before the effect of increasing the yield
of pulp is obtained.
[0005] Another type of known principle to solve the problem is
based on the use of black liquor as impregnation fluid in an
impregnation zone before the cooking stage. The document U.S. Pat.
No. 5,080,755 reveals a system with black liquor in the feed
system. U.S. Pat. No. 5,053,108 reveals a variant in which black
liquor withdrawn from the digester is returned to the high-pressure
feeder where it is to form a major part of the treatment fluid in
the transfer flow to the digester. EP 477,059 reveals a modified
variant in which wood chips impregnated with black liquor are
raised to cooking temperature before the principal addition of
white liquor takes place. These examples are only some of a number
of various known methods that demonstrate that many different
suggestions for processes have been studied with the aim of
improving yield while maintaining pulp quality.
[0006] Known methods demonstrate either disadvantages related to
cost and/or do not provide sufficient quality with respect to
beatability or tensile strength.
AIM AND PURPOSE OF THE INVENTION
[0007] One aim of the invention is to increase the yield when
producing cellulose pulp, where wood raw material that principally
consists of softwood, preferably in the form of cut wood chips, is
treated in several stages in various treatment steps, whereby one
of the steps comprises the cooking of the material in an alkali
cooking fluid, through a controlled addition of wood raw material
in the form of hardwood at an amount corresponding to 1-20% of the
amount of softwood, preferably 3-15%, more preferably at least 5%.
A second aim of the invention is to be able to produce pulp with
improved properties with respect to tensile strength and/or
beatability.
[0008] It has been discovered, somewhat surprisingly, that the
invention allows the yield to be increased, often to a significant
degree, with essentially maintained, or even improved, properties
for the pulp, something that means that significant economic
advantages can be obtained. Most significantly, it has turned out
that pulp produced according to the invention demonstrates improved
properties with respect to tensile strength and beatability,
something that is of major significance for many paper products. It
is important that the addition of hardwood takes place in a
controlled manner, such that the improved quality can be maintained
on an even and high level, from the digester, whereby subsequent
bleaching and/or treatment stages can be opimised.
[0009] The invention can be applied both on steam phase digesters
and hydraulic digesters; with inverted top separators, with top
separators that feed downwards and with types that lack a top
separator.
[0010] The invention can also be applied for batch-cooking, in
which the chips are fed into a vessel where treatment subsequently
takes place in sequence with the chips held stationary in the
vessel.
DESCRIPTION OF DRAWINGS
[0011] The invention will now be described in more detail with
reference to the attached drawings, where:
[0012] FIG. 1 shows a diagram comparing tensile strength (as a
function of the degree of beating) between pulp from softwood
(Pinus radiata), produced according to the invention with the
inclusion in the mixture of 5% eucalyptus, with a pulp from
softwood (Pinus radiata) prepared according to a conventional
method.
[0013] FIG. 2 shows one preferred embodiment of a system, shown
schematically, according to the invention.
[0014] FIG. 3 shows a modified design of the system according to
FIG. 1.
[0015] FIG. 4 shows a further modification according to the
invention.
[0016] FIG. 5 shows a second diagram with a comparison of tensile
strength (as a function of the degree of beating) between a
conventional pulp from softwood (Pinus sylvestris), produced
according to the invention with the inclusion in the mixture of 15%
Scandinavian birch, with a pulp produced according to a
conventional method, and
[0017] FIG. 6 shows the increased yield from cooking according to
the invention with the pulp from FIG. 5.
DETAILED DESCRIPTION
[0018] Comparative tests have been carried out with the aim of
evaluating the invention. The same cooking conditions have been
used in the comparative tests and the same original material with
respect to softwood has been used. A conventional method was used
in one cooking experiment, whereby the wood raw material was 100%
constituted by softwood. Softwood was used for 97% of the raw
material in the method according to the invention, while 3% was
constituted by pin chips of birch.
1TABLE 1 Comparison between conventional cooking and the invention
Pulp: SW Conifer SW + 3% pin chips of birch Cooking run number: CK
2434 CK 2439 Kappa 23 24 Viscosity, dm.sup.3/kg: 1146 1170 Weighted
mean fibre length, mm 2.38 2.34 Zero Span, Nm/g: 145 144 Yield, %:
45.7 46.3 (47.8) Interpolated properties at tensile strength 80
kNm/kg: Revolutions, PFI 350 100 Slowness, .degree. SR: 15.0 15.0
Density, kg/m.sup.3: 655 655 Air resistance, sec./100 ml: 5.2 5.6
Burst index, MN/kg: 5.7 5.4 Tear index, Nm.sup.2/kg: 17.1 16.0
Tensile stiffness index, MNm/kg: 6.5 6.3
[0019] As the comparative experiments make clear, a significantly
improved yield was obtained according to the invention, 2
percentage points when calculated only with respect to the
softwood, and as much as 0.6% when calculated relative to the total
amount of wood, including the birch chips. Furthermore, it is shown
that a considerably improved tensile strength (80/100) is obtained
with a pulp according to the invention.
[0020] FIG. 1 shows the tensile strength (along the y-axis) as a
function of the degree of beating (along the x-axis), comparing a
pulp produced according to the invention (the upper curve with 5%
hardwood in the form of eucalyptus) with a pulp produced according
to a conventional method (lower curve), i.e. using solely softwood.
The same cooking conditions and cooking process (CoC.TM., Compact
Cooking, which has been developed by Kvaerner Pulping AB) have been
used in all experiments. It is clear that the pulp produced
according to the invention demonstrates a tensile strength that is
improved by 10-20%, which is of major significance for the
production of certain paper products.
[0021] A probable explanation of the considerable improvement in
tensile strength is that an increase in yield principally due to
xylan is obtained with a method according to the invention. Xylan
is a type of hemicellulose, and it can give improved beatability,
which in turn means an improvement in strength. To put it another
way: a pulp is obtained according to the invention that is easier
to beat, which means that the desired tensile strength can be
achieved at a lower degree of beating (fewer revolutions) than that
obtained with a pulp produced by conventional methods.
[0022] FIG. 2 shows schematically a preferred plant according to
the invention. A unit 1 for the preparation of raw material is
shown, containing a chip mill. Raw materials in the form of both
softwood logs 2 and in the form of hardwood logs 3 are supplied to
the raw materials preparation unit 1. A control unit 6 is arranged
to control the flow of hardwood logs 3 and that of softwood logs 2.
The control unit 6 can use any form of prior art technology to
control the amounts that are supplied to the chip mill in the raw
materials preparation unit 1 in the manner desired. It can, for
example, be the weight that is used, or it can be physical sensors
and/or optical sensors, etc. If physical or optical sensors are
used, the control unit 6 ensures that the desired quantities of
logs of each type of wood raw material are supplied to the chip
mill. If a mixing ratio of approximately 3% is desired, the control
unit 6 in this case ensures that, for an input flow of 100 logs,
softwood constitutes 97 of the logs and hardwood 3 of the logs, on
the condition that the logs are approximately the same size. The
control unit 6 in its simplest form can take the form of a supply
that is manually monitored, by using, for example, a conventionally
used manually controlled grapple loader that can take, when fully
loaded, for example 80 logs of normal size. In order to obtain a
continuous mixture at a level of 5%, thus, the supply can be
controlled with the aid of the manually controlled loading machine
in such a manner that four logs of hardwood are added after and
between each full load of softwood. Another simple alternative
solution, according to the same principle, is that the grapple
loader is allowed to load a maximum of 20 logs (for example, by
adaptation of the gripper), and that one log of hardwood is loaded
between each full load of softwood, whereby a mixture containing
approximately 5% hardwood is obtained. The chips are fed after the
chip mill, which provides a mixture of softwood chips and hardwood
chips according to what is desired, to storage, appropriately via a
transporter 4, which in the case shown leads to a chip bin 10.
[0023] The invention will be described below in association with
the use of a specially preferred form of continuous cooking. Chips
are fed in this case down into the chip bin 10 where the chips are
warmed in a known manner using steam, St, with the extraction of
weak gases, Gas. The chips warmed in this manner are subsequently
led to a chip chute 11 where the chips are mixed to a slurry to
give a suitable fluid/wood ratio, appropriately through the
addition of white liquor, WL, possibly in combination with the
addition of a certain amount of black liquor (not shown in the
drawing). The chips are fed onwards through a sluice feeder at the
bottom of the chip chute 11 using a high-pressure feeder 12 through
a transfer flow 13a, 13b to a pressurised treatment vessel 15 for
black liquor impregnation. The fluid that is added to the chip
chute 11 and that accompanies the chips along the flow 13a is
separated from the chips to a large extent with a top separator 14,
and is returned to the high-pressure tap 12 via the return flow
13b.
[0024] The addition of white liquor in the chip chute 11 ensures
that a relatively short retention time is obtained at an
intermediate temperature, in the region of 50-140.degree. C. for
approximately 2-60 minutes, preferably 2-10 minutes, and this is
the reason that a high alkali level, if present, does not have time
to influence the strength of the pulp.
[0025] An impregnation with black liquor takes place in the
treatment vessel 15, the black liquor being added via the flow 31
that has been withdrawn from the cooking stage via the withdrawal
strainer 20. The level of residual alkali in the flow 31 normally
lies in excess of 10 g/l, preferably 15-25 g/l.
[0026] Consumption of residual alkali takes place in the treatment
vessel. Consumed black liquor, having a level of residual alkali
below 10 g/l, is withdrawn from the strainer 16 for transport
onwards to recovery 32.
[0027] The chips, following treatment with black liquor in the
vessel 15, are fed to the digester 19 and it is appropriate that
the alkali-rich fluid 30 is added to the chips before the cooking
stage in association with the output 17 from the treatment vessel
15. The initial material that has been pre-treated in this manner
is fed continuously to the top of the digester 19. Following
cooking in, for example, a first concurrent flow and a final
countercurrent flow, the cooked pulp is fed out from the bottom of
the digester and onwards to washing equipment (not shown in the
drawing), where the lignin that has been released during the
cooking is washed out in order to obtain a cellulose pulp with a
kappa value with respect to the conifer pulp with a value of
approximately 30, preferably always under 40 and preferably between
35 and 25.
[0028] It will be realised that FIG. 2 shows the principle
schematically. It is to be understood, for example, that there may
be several heating flows and several withdrawal positions both from
the impregnation vessel 15 and from the digester 19. In the same
way, several other flows, additions of fluid and washing
arrangements can be arranged at various locations. It must also be
realised that the invention can be used, in principle, with all
known conventional cooling processes.
[0029] During the actual cooking stage, which is here shown as two
zones (concurrent flow and countercurrent flow), the temperature
lies in the region 150.+-.20.degree. C. The normal retention time
lies in the interval 40-240 minutes, and preferably approximately
120.+-.20 minutes in each cooking zone.
[0030] FIG. 3 shows a modified design according to the invention.
The same type of cooking plant as that shown in FIG. 1 is shown
here, for the sake of simplicity. On the other hand, two separate
raw material preparation units 1A, 1B are used. Thus, chips from an
initial material comprising softwood are manufactured in the first
wood preparation unit 1A, and solely softwood 2 in the form of logs
is added to this unit. Solely hardwood logs 3 are added to the
second unit 1B. Furthermore, it is indicated that raw materials
storage spaces 1A', 1B', are located in direct association with the
wood preparation units 1A, 1B for storage of each type of chips in
separate spaces. A chip transporter 4 from the first unit 1A is
available that feeds the chips towards the chip bin 10, while a
second transporter 5 is available from the second unit 1B that
feeds hardwood chips towards the same chip bin 10. It is ensured
that the desired mixture of hardwood chips and softwood chips is
achieved, either just before or in connection with input into the
chip bin 10. This takes place through arranging a control unit 6
that ensures in a suitable manner that a controlled, desired mixing
of the different sorts of chip takes place before input to the chip
bin 10. It is appropriate that this takes place through the volume
of flow out from each unit 1A, 1B being regulated by the control
unit 6, such that a desired distribution of the percentages of each
type of wood is obtained at the input to the chip bin 10.
[0031] FIG. 4 shows a further modified method according to the
invention. In contrast to the designs that have been previously
described, sawdust 3A is used here as initial raw material for the
addition of hardwood, instead of chips. The sawdust 3A is thus
stored in a separate container 7. The principle wood raw material
in the form of softwood is added as in previous designs in the form
of logs to a raw materials preparation unit 1 in which the chips,
when ready, are transported by a transporter 4 to the chip bin 10.
A control unit 6 receives information from this flow of chips and
controls the addition of sawdust 3A to the digester 19 based on
this input information, such that the desired amount of hardwood
sawdust 3A is added to the digester 19. This addition can take
place at a number of locations in the digester, for example, to a
cooking flow 21 via a line 8 that is delivered at the cooking flow
21. Alternatively, or in addition, the sawdust 3A can be added by
means of a line 9 at the top 18 of the digester 19, or it can be
pumped to the input, the preimpregnation vessel, the transfer, or
other suitable position.
[0032] It may be an advantage in certain applications to pre-treat
the hardwood such that the xylan is released before the hardwood
raw material is added to the softwood raw material. This can be
achieved, for example, by pre-treating the hardwood at a
temperature of 100-140.degree. C. in an alkali fluid (preferably in
white liquor) before it is, for example, added to the impregnation
stage and/or the cooking stage.
[0033] FIG. 5 shows the tensile strength (along the y-axis) as a
function of the degree of beating (along the x-axis), comparing a
pulp from softwood (principally Pinus sylvestris), prepared
according to the invention (upper curve) with 15% hardwood in the
form of Scandinavian birch, with a softwood (Pinus sylvestris) pulp
prepared according to a conventional method (lower curve), i.e.
using solely softwood. The same cooking conditions and cooking
process (CoC.TM., Compact Cooking, which has been developed by
Kvaerner Pulping AB) have been used also in this experiment. The
figure shows that the pulp produced according to the invention
demonstrates a tensile strength that is improved by around 10%.
[0034] FIG. 6 shows also the improved yield obtained from the
process according to the invention, with 15% hardwood in softwood
(Pinus sylvestris).
[0035] One effect of the mixing in of the hardwood is a certain
decrease in the average fibre length from 2.49 mm to 2.11 mm, while
fibre curl increases somewhat, from 11.8% to 12%.
[0036] Certain experiments have also been carried out with a larger
fraction of spruce in the softwood. In these experiments, a
softwood mixture with 70% spruce and 30% pine has been mixed with
15% hardwood. An increase in tensile strength of 5-10% was
obtained.
[0037] If a batch cooking system is used for the production of
cellulose pulp, continuous addition of hardwood sawdust cannot
normally be achieved, and for this reason one of the principles
according to FIG. 2 or 3 should be used.
[0038] It is appropriate that the controlled addition of hardwood
raw material takes place such that at least 1-20% of hardwood is
present in the cooking stage at all times during the process. The
control can thus be adjusted such that a controlled mixing takes
place in the flow of raw materials such that the said fraction of
hardwood is established for a pre-determined amount of wood raw
materials. It is appropriate that this amount corresponds to
preferably less than 50% of the amount that at each moment is
present in a digester 15, and even more preferably less than 25% of
the volume of the digester for wood raw material.
[0039] The control is to be adapted such that the desired amount of
hardwood is present in a controlled manner during the cooking stage
and such that this has sufficient time to release hemicellulose,
and to distribute during the cooking stage the released
hemicellulose essentially throughout the complete digester volume.
It is normally desired that an even mixture of hardwood is obtained
throughout the digester, although the hardwood in one extreme
variant is concentrated to a number of locations in the digester.
The variant shown in FIG. 4 provides the possibility of obtaining a
very even mixing throughout the digester, while at the same time
using a waste product (sawdust) from the raw material preparation
that otherwise would have been sent for incineration.
[0040] A targeted control of the supply of raw materials according
to the invention thus allows a stable, high yield and improved
tensile strength to be obtained for the cooked pulp.
[0041] The invention can be modified in a number of ways within the
framework of the claims. For example, the invention can also be
used in various types of combination according to the principles
shown in FIGS. 2, 3 and 4; for example, in such a manner that a
certain fraction of the hardwood raw material is added at the chip
preparation while another part is added in the form of sawdust
directly to a continuous digester. Furthermore, it will be realised
that various forms of intermediate storage of wood chips and
sawdust can take place, for example, with chips between the wood
preparation unit 1 and the chip bin 10, or in close association
with chip bin 10.
* * * * *