Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND ARRANGEMENT FOR DISCHARGE DILUTION
TECHNICAL FIELD
The present invention relates to pulp manufacturing processes in general, and
particularly to adaptations of discharge dilution to enable improvements in
screening in such processes.
BACKGROUND
Pulp processing (e.g. Kraft process or sulfite process) typically comprises
introducing wood chips or other lignocellulosic material into a digester or
cooker together with an impregnation liquid e.g. black liquor from a
displacement tank and the chips or other lignocellulosic material is preheated
and pre-impregnated. Black liquor is the waste product from the so called
Kraft process when pulpwood is digested into paper pulp removing lignin,
hemicelluloses and other extractives from the wood to free the cellulose
fibers.
Thereby, black liquor is an aqueous solution of lignin residues,
hemicellulose,
and the inorganic chemicals used in the process. Correspondingly, white
liquor is a strongly alkaline solution typically of sodium hydroxide and
sodium
sulfide which is used to break the bonds between lignin and cellulose.
Thereby, white liquor is the "pure" impregnation liquid whereas the black
liquor is recycled impregnation liquid. Subsequently, hot black and white
liquor is introduced into the digester and displaces the initially introduced
impregnation liquid back into the displacement tank. Due to the introduction
of hot liquor the fiber is heated and cooked during a period of maintaining
the
temperature at a desired level. The cooking dissolves or softens the lignin in
the material and the cooking time and temperature is dependent on the
material to be cooked as well as desired fiber properties after cooking. After
cooking, black liquor e.g. displacement liquor (from the displacement tank)
with a lower temperature is introduced into the digester to lower the
temperature and thereby terminate the cooking reactions. Finally the digester
is discharged to an atmospheric storage tower e.g. discharge tank from which
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the cooked pulp is transported via piping to e.g. a screening process in order
to deliver a finished pulp with predetermined properties e.g. predetermined
fiber fractions i.e. concentration of fibers in the pulp.
In this context, wood pulp can be defined as wood chips mechanically ground
and/or chemically digested at elevated temperatures to produce fibers for use
in manufacturing paper or fiber-based wood composites [1]. In the same
manner pulp can be manufactured from other lignocellulosic materials, e.g.
bagasse. Lignocellulose or lignocellulosic material refers to plant dry matter
(biomass), so called lignocellulosic biomass. So called virgin biomass
includes
all naturally occurring terrestrial plants such as trees, bushes and grass. So
called waste biomass is produced as a low value byproduct of various
industrial sectors such as agricultural (corn stover, sugarcane bagasse, straw
etc.), forestry (saw mill and paper mill discards). Energy crops are crops
with
high yield of lignocellulosic biomass produced to serve as a raw material for
production of second generation biofuel examples include switch grass
(Panicum virgatum) and Elephant grass.
Pulp consistency is defined as the mass concentration of fibers in water.
Consistency is calculated as (dry weight of sample/wet weight of sample)
x100%. Pulp consistency is roughly divided into three ranges:
Low consistency: <5%
Medium consistency: 5 - 15%
High consistency: >15%
The different ranges of consistency are found in different parts of the
pulping
and papermaking process, for example high consistency can be found in e.g.
mechanical pulping, reject refining, bleaching, storage, medium consistency
in e.g. cooking, bleaching, storage, re-pulping, low consistency in stock
preparation, cleaning, screening, beating, and blending.
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In present pulp manufacture processes, liquor from the displacement liquor
tank is utilized in order to dilute the pulp in the discharge sequence. The
dilution is necessary in order to enable emptying or discharging the pulp from
the digester and into the discharge tank. In the subsequent screening process
some of the process steps are fed with a pulp at low consistency and using
liquor for its function. A reject washer is often such a process step/ stage.
The
accept from these process steps, which is a low consistency pulp which
resembles pure liquor more than a thicker pulp, is subsequently fed to a
reject
tank and then fed to the discharge tank. The flow from the reject tank then
lowers the consistency in the system significantly. To reduce the consequences
of the reject tank flow, typically a so called delta thickener is added, which
removes most of the liquor from the pulp fed to the discharge tank
Consequently, the pulp fed to the discharge tank from batch cooking does not
have high consistency enough to handle all the liquor from the reject tank as
dilution in the discharge tank without lowering the screen feed consistency
more than desired. In many applications today, the consistency is increased
with a thickening device e.g. delta thickener or the like in order to enable
the
screening being fed at a normal consistency. Depending on the manufacturer
of the processing equipment, the thickening agent or device varies. The
thickening agent and/or machinery introduces both unwanted process steps
and material into the process, thus increasing both time and costs for the
pulping process.
Therefore, there is a need for an improved dilution method and arrangement
in the discharge sequence without the above mentioned disadvantages.
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SUMMARY
The present invention relates to improved discharge method and arrangement
in pulp processing.
In a first aspect the proposed technology discloses a method of providing a
diluted discharge pulp, comprising the steps of providing pulp with a first
consistency in a digester, which pulp comprises cooked lignocellulosic fibrous
material, diluting the provided pulp by feeding a reject liquor from a
screening
process into the digester to provide a diluted pulp in the digester, where the
reject liquor from the screening process is a pulp with a lower consistency
than the pulp with a first consistency, and feeding the diluted pulp through a
discharge pipe to a discharge tank to provide a diluted discharge pulp.
By means of the proposed technology the pulp can be diluted without addition
of any unwanted additives and reject liquor can be reused in the discharge
process.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best
be understood by referring to the following description taken together with
the
accompanying drawings, in which:
Fig. 1 is a flow chart of an embodiment of a method according to the
proposed technology;
Fig. 2 is a schematic block chart of a system according to the proposed
technology.
Fig. 3 is a schematic block chart illustrating prior art;
Fig. 4 is a schematic block chart illustrating the benefits of the
embodiments of the proposed technology;
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DETAILED DESCRIPTION
The present disclosure will be described in the context of a batch cooking
process. However, it is easily adapted for a case of continuous cooking.
Further, although the background and the description of the proposed
5 technology primarily describes the so called Kraft process, it is equally
applicable to sulfite processes or similar with appropriate adaptation of
processing terminology.
The inventors have identified the benefits of reusing reject liquor or low
consistency pulp, i.e. pulp with a low concentration of solid fibers, from the
screening process for diluting the pulp during the discharge sequence. In
other
words, instead of using liquor from the displacement liquor tank during the
discharge sequence, the same functionality can be met by using liquor from a
screening reject tank. As most of the flow from the reject tank will be used
in
the cooking discharge sequence, only a small part will be sent to the bottom
of the discharge tank to control the consistency. The dilution is then reduced
so that it is not necessary to add any thickening device to run screening in
normal consistency. Thereby, reusing low consistency reject liquor from the
screening process for dilution during the discharge sequence will reduce the
need for additives or machinery that increase the consistency of the feed into
the screening process.
With reference to Figure 1, embodiments of a method of providing a diluted
discharge pulp in a pulp processing plant will be described. In this context
the
pulp can comprise processed wood fiber [1] or other processed lignocellulosic
fiber. According to a first embodiment, the method includes the steps of
providing S 10 pulp with a first consistency in a digester, said pulp
comprising
digested or cooked wood chips or other lignocellulosic fibrous material at a
certain concentration. Subsequently, diluting S20 the provided pulp by
feeding a low consistency screening reject liquor, i.e. a reject liquor from a
screening process, into the digester to provide a diluted pulp in the
digester,
and finally feeding S30 the diluted pulp through a discharge pipe to a
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discharge tank to provide a diluted discharge pulp. As described above, the
low consistency screening reject liquor is in fact a pulp with low
consistency,
i.e. lower consistency than the pulp with a first consistency. Thus, the low
consistency screening reject liquor has a lower concentration of fibers than
the pulp with a first consistency.
According to a further embodiment, the diluting step S20 comprises feeding a
first fraction of the reject liquor from the screening process into the
discharge
pipe to further dilute the pulp.
According to yet another embodiment, the method includes the further steps
of diluting S40 the discharge pulp in the discharge tank with a second
fraction
of the reject liquor from the screening process, before (which is common for
all embodiments) feeding S50 the (diluted or undiluted) discharge pulp to the
screening process for screening S60.
According to a further embodiment, the method includes an intermediate step
of feeding the reject liquor from the screening process to a buffer, and
subsequently feeding the first and/ or second fraction from the buffer to the
digester and optionally also to the discharge pipe and/or to the discharge
tank.
In the above described embodiments, no mention is made to the consistency
or concentration of the diluted pulp. However, according to a further
embodiment, it is possible to control, in step S25, the dilution in order to
achieve a diluted pulp with a predetermined consistency or concentration of
fibers. This may be performed by determining the concentration of the pulp in
the digester and diluting the pulp in order to achieve a wanted concentration.
More commonly though is diluting the pulp just enough to enable discharging
the pulp from the digester.
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With reference to Figure 2 embodiments of a discharge dilution arrangement
100 according to the present disclosure will be described.
According to a first embodiment, the discharge dilution arrangement 100,
comprises a digester 1 configured for receiving at least a lignocellulosic
fibrous
material and a cooking liquor to cook into a pulp. Further, the discharge
dilution arrangement 100 includes a discharge tank 2 configured for receiving
said pulp from said digester 1 and for outputting said pulp to a screening
process. The digester 1 and the discharge tank 2 are connectable via a
discharge pipe 3, configured for feeding cooked pulp from the digester 1 to
the
discharge tank 2. Finally, the discharge dilution arrangement 100 includes a
digester dilution pipe 4 configured for feeding a reject liquor from the
screening
process into at least the digester tank 1 to dilute the pulp, where the reject
liquor from the screening process is a pulp with a lower consistency than the
pulp in the digester. Preferably, the screening reject liquor is fed into the
digester 1 at the bottom of the tank. However, it is also technically feasible
to
add the screening reject liquor to other parts of the tank as well.
According to a further embodiment, the discharge dilution arrangement 100
includes a first discharge dilution pipe 4' configured for additionally
feeding
the reject liquor from the screening process into the discharge pipe 3 to
further
dilute the pulp in the discharge pipe 3.
According to another embodiment, the discharge dilution arrangement 100
further includes a second discharge dilution pipe 5 configured for
additionally
feeding the reject liquor from the screening process into the discharge tank 2
as well to further dilute the pulp.
In order to improve the control of the dilution and ensure a sustainable and
even access to the reject liquor, it might prove beneficial to collect the
reject
liquor into a buffer tank from which the reject liquor is then distributed to
the
parts of the arrangements where dilution is required. According to a further
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embodiment, the discharge dilution arrangement 100 further comprises a
buffer tank 6 arranged to receive the reject liquor from the screening process
and to provide a first fraction of the reject liquor from the screening
process
to the digester dilution pipe 4 and optionally also a second fraction of the
reject
liquor from the screening process via the second discharge dilution pipe 5 to
the discharge tank 2, to provide a diluted discharge pulp.
For those cases where a dilution in the discharge pipe 3 is also required,
according to a further embodiment, the buffer tank 6 is further configured to
provide a third fraction of the reject liquor from the screening process via
the
first discharge dilution pipe 4' to the discharge pipe 3.
The above described arrangements can be implemented for both batch
digesters and continuous digesters.
In order to further explain the benefits of the above described embodiments,
an example of the impact of various concentrations of fibers in the pulp in
the
different process steps will be described with reference to Figure 3 and
Figure
4. The consistency and consistency intervals of the pulp are to be viewed as
non-limiting examples and are indicated merely to illustrate the benefits of
the
proposed technology.
In Figure 3, which illustrates prior art without the use of a thickening
device,
a digester 1 is disclosed with a capacity for processing 20 t/h of wood chips
or other lignocellulosic material. Impregnation and/or cooking liquid, e.g.
black and/or white liquor or similar depending on the chemical process used,
is added to achieve a concentration of fibers in the pulp, or first
consistency
of the pulp, of approximately 10%, corresponding to a total volume of 200
m3/h of cooked pulp. In order to enable emptying or discharging the digester
1, more liquor or similar is added to dilute the cooked pulp and to reduce the
consistency of the cooked pulp to e.g. 5%. In order to handle the processing
capacity of 200 m3/h from the digester 1, it is necessary to add 200 m3/h of
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dilution liquid e.g. liquor or similar, thereby resulting in a feed of 400
m3/h of
diluted cooked pulp from the digester 1 to the discharge tank 2. This results
in a doubling of the processed volume.
As discussed previously, it is then potentially necessary to further dilute
the
cooked pulp in order to enable emptying the discharge tank 2 into the
subsequent screening process. Accordingly, the discharge 2 is fed with
screening reject liquor to further lower the concentration of fibers to about
3%.
In this example the low consistency screening reject liquor is fed with a
capacity of 2 t/h and a concentration of fibers, or consistency, of 0.6%,
which
corresponds to 330 m3/h. Thereby, the discharge tank 2 outputs 22 t/h with
a consistency of 3% and a volume of 730 m3/h. Through the screening process
a screened pulp with a consistency of maximum 3% is provided to press feed
and about 10% of the production is provided to a screening reject wash.
With reference to Figure 4, the dilution in both the digester 1 and optionally
the discharge tank 2 is provided by adding the low consistency screening
reject
liquor, in contrast to the prior art. In a corresponding manner as in Figure
3,
a digester 1 is disclosed with a capacity for processing 20 t/h of wood chips
or other lignocellulosic material. Impregnation and/or cooking liquid, e.g.
black and/or white liquor or similar depending on the chemical process used,
is added to achieve a concentration or first consistency of approximately 10%,
corresponding to a total volume of 200 m3/h of cooked pulp. In order to enable
emptying or discharging the digester 1, a low consistency screening reject
liquor with a consistency of approximately 0.6% is added to dilute the cooked
pulp in the digester. In order to handle the processing capacity of 200 m3/h
from the digester 1, it is necessary to add 1.2 t/h of the low consistency
screening reject liquor with a consistency of 0.6%, thereby resulting in a
feed
of 400 m3/h of diluted cooked pulp with a concentration of 5.3% from the
digester 1 to the discharge tank 2. This results in a doubling of the
processed
volume, in a similar manner as in Figure 3.
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As discussed previously, with reference to Figure 3, it is then potentially
necessary to further dilute the cooked pulp. Accordingly, the discharge 2 is
also fed with the low consistency screening reject liquor. However, in
contrast
to the prior art solution in Figure 3 the addition of screening reject liquor
is
5 performed with 0.8t/h with a consistency of 0.6% corresponding to 130
m3/h.
Thereby, the discharge tank 2 outputs 22 t/h with a concentration of 4.1%
and a volume of 530 m3/ h.
In case of using some other liquid than the low consistency screening reject
10 liquor it is easy to see the benefits of the current embodiments. Using
a
diluting liquid with 0% fibers corresponds to an output volume of 730 m3/ h
from the discharge tank 2 as illustrated in Figure 3 (without a thickening
agent) to provide the necessary screening consistency of 3-4.5%. By utilizing
a diluting liquid with 0.6% fibers the processed volume can be reduced to 530
m3/ h, which is a significant improvement and saves both time and money.
As mentioned before, the above described fiber concentrations are merely
examples. The above mentioned first consistency is typically around 10%, but
can also vary within consistency intervals of for example around 9.5-10.5%,
9-11%, 8-12%. The low consistency screening reject liquor is potentially 0.6%,
but can also vary within concentration intervals of for example 0.3-1%. The
resulting diluted pulp is diluted to such an extent that it is possible to
empty
the digester with a minimum of dilution. As an example the diluted pulp could
be between 4-7%, or 4-6%, 4-5%, 4.5-5.5%. Basically, depending on the
material to be processed, the chemicals used, the cooking or digesting
parameters applied in the process, the type of screening and other variables
the specific concentrations can vary also outside the above given example
intervals.
With the presently proposed technology it is possible to achieve a desired
consistency of the pulp and at the same time avoiding the need for a
thickening agent or machinery to be introduced into the system.
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The embodiments described above are merely given as examples, and it should
be understood that the proposed technology is not limited thereto. It will be
understood by those skilled in the art that various modifications,
combinations and changes may be made to the embodiments without
departing from the present scope as defined by the appended claims. In
particular, different part solutions in the different embodiments can be
combined in other configurations, where technically possible.
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REFERENCES
[1] "Fact Sheet Agriculture and Natural Resources", F-85-13, Forest
Products Terminology, E. McConnell, N Irby, (2013) Ohio State University,
http: / / ohiowood.osu.edu/ images/ F 85_13_FP_Terminology_Fact_Sheet.pdf
[2] Chemical Pulping, 6A and 6B, J Gullichsen, C-J Fogelholm, Paper
Making Science and Technology, ISBN 952-5216-06-3, (1999) Jyvaskyla,
Finland
