U.S. patent application number 11/099144 was filed with the patent office on 2005-10-13 for method and device for handling cellulose pulp.
Invention is credited to Gustavsson, Lennart, Olsson, Goran, Saetherasen, Jonas, Snekkenes, Vidar.
Application Number | 20050224198 11/099144 |
Document ID | / |
Family ID | 32173711 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050224198 |
Kind Code |
A1 |
Snekkenes, Vidar ; et
al. |
October 13, 2005 |
Method and device for handling cellulose pulp
Abstract
The invention concerns a method and a device for the dilution of
dewatered cellulose pulp that maintains a consistency of 30% or
greater. By shredding of the pulp to a finely divided dry
granulate, dilution to a homogeneous consistency in the medium
consistency range can take place exclusively through hydrodynamic
effects from the addition of dilution fluid. The dilution fluid is
added to the granulate at a position at which the granulate is in
free fall in a stand pipe 22, 40 and above a level Liq.sub.LEV of
diluted pulp in the stand pipe. A number of nozzles are arranged
around the periphery of the stand pipe, directed in towards the
centre of the stand pipe, obliquely downwards in the direction of
fall of the granulate. It is possible through this simplified
procedure to avoid completely the conventional dilution screws, and
this reduces the investment costs and is operating costs, while at
the same time unnecessary mechanical influence of the pulp fibres
can be avoided.
Inventors: |
Snekkenes, Vidar; (Karlstad,
SE) ; Gustavsson, Lennart; (Karlstad, SE) ;
Saetherasen, Jonas; (Karlstad, SE) ; Olsson,
Goran; (Karlstad, SE) |
Correspondence
Address: |
FASTH LAW OFFICES (ROLF FASTH)
26 PINECREST PLAZA, SUITE 2
SOUTHERN PINES
NC
28387-4301
US
|
Family ID: |
32173711 |
Appl. No.: |
11/099144 |
Filed: |
April 5, 2005 |
Current U.S.
Class: |
162/17 ; 162/18;
162/19; 162/261 |
Current CPC
Class: |
D21F 1/74 20130101 |
Class at
Publication: |
162/017 ;
162/019; 162/018; 162/261 |
International
Class: |
D21C 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2004 |
SE |
0400940-3 |
Claims
1. A method for the removal of dewatered cellulose pulp from a
dewatering press, comprising: applying cellulose pulp to an outer
surface of two counter-rotating dewatering drums of a dewatering
press, the cellulose pulp having an initial consistency in a range
of 4-12%; feeding out the cellulose pulp after a final dewatering
nip of the dewatering press in a form of a continuous dewatered mat
that maintains a consistency of 30% or higher; feeding the
dewatered mat perpendicularly to a shredder screw having an axis of
shredding that is arranged essentially parallel to axes of rotation
of the dewatering drums and the shredder screw having an outlet
defined therein at one end of a surrounding outer cover; shredding
the dewatered mat in the shredder screw to produce a granulated
pulp having a normalized granulate size distribution in a range of
5-40 mm; feeding out the granulated pulp from the outlet of the
shredder screw to fall freely in a stand pipe connected to an
outlet end of the shredder screw; adding a dilution fluid under
pressure into the stand pipe through a number of fluid jets
arranged around a periphery of an upper end of the stand pipe and
above a surface liquid level (Liq.sub.LEV) of diluted granulated
pulp established in LOWER END OF the stand pipe, the added dilution
fluid establishing a consistency of the diluted granulated pulp in
a range of medium consistency 8-16%; more than 75-90%, of the added
dilution fluid being added through the fluid jets arranged above
the surface liquid level (Liq.sub.LEV) established in a lower end
of the stand pipe; exclusively diluting the granulated pulp under
an influence of hydrodynamic effect from the addition of the
dilution fluid through the fluid jet to reduce the consistency of
the granulated pulp from 30% or higher at an upper end of the
standpipe to a medium consistency of 8-16% at the lower end of the
standpipe without using any mechanical agitators in the standpipe;
and feeding the diluted granulated pulp onwardly to subsequent
treatment steps for the diluted granulated pulp by pumping the
diluted granulated pulp from the lower end of the stand pipe.
2. The method according to claim 1 wherein the addition of dilution
fluid from the fluid jets takes place in a form of pressurized
fluid jets directed obliquely downwardly in a fall direction of the
granulated pulp in the stand pipe
3. The method according to claim 1 wherein the fluid jets are
directed at an angle of 45.+-.15.degree. relative to a vertical
direction and a fall direction of the granulated pulp.
4. A device for the removal of dewatered cellulose pulp from a
dewatering press comprising: a first rotatable drum in operative
engagement with a second rotatable drum so that a dewatering nip is
formed therebetween; a dewatered mat disposed in the dewatering
nip; a shredder screw having an axis of shredding parallel to an
axis of rotation of the rotatable drums, the shredder screw having
an inlet defined therein for receiving the dewatered mat, the
dewatered mat extending from the dewatering nip into the inlet of
the shredder screw; the shredder screw having means for shredding
the dewatered mat into granulated pulp; the shredder screw having
an outlet defined therein; a vertical stand pipe having an upper
end connected to the shredder screw and in operative engagement
with the outlet of the shredder screw, the standpipe having a lower
end; the upper end of the vertical standpipe having a pressurized
chamber device arranged around a periphery of the standpipe, the
pressurized chamber device containing a dilution fluid; a plurality
of nozzles extending through a wall of the pressurized chamber
device so that an inside of the pressurized chamber device is in
fluid communication with the stand pipe; the lower end of the stand
pipe having diluted granulated pulp disposed therein, the nozzles
being directed towards the granulated pulp to dilute the granulated
pulp with the dilution fluid above a level of diluted granulated
pulp established in the lower end of the stand pipe; and a pump
arranged at the lower end of the stand pipe adjacent to a bottom of
the stand pipe for forwarding the diluted granulated pulp to
subsequent treatment steps.
5. The device according to claim 4 wherein at least four nozzles
are arranged around the periphery of the stand pipe.
6. The device according to claim 5 wherein each nozzle is directed
g towards a center of the stand pipe and obliquely downwards at an
angle 45.+-.15.degree. relative to a vertical direction and a fall
direction of granulated pulp.
7. The device according to claim 6 wherein all nozzles are
connected to a common distribution chamber for the dilution fluids
which is pressurized with a pressure-raising device means.
Description
[0001] The present invention concerns a method according to the
preamble of claim 1 and a device according to the preamble of claim
4.
THE PRIOR ART
[0002] In association with either one of the bleaching and the
delignification of cellulose pulp in bleaching lines, the pulp
passes between different treatment steps in which the pulp is
subjected to bleaching or the delignifying effect of various
treatment chemicals. The treatment typically alternates between
alkaline and acidic treatment steps in which typical sequences may
be of ECF type (elemental chlorine-free, Cl, in which chlorine
dioxide may be used) such as O-D-E-D-E-D, O-D-PO or sequences of
TCF-type (totally chlorine-free) such as O-Z-E-P. Other bleaching
steps, such as Pa steps and H steps may be used.
[0003] The treatment steps may take place either at medium
consistency (8-16%) or at high consistency (.gtoreq.30%), but it is
vitally important to wash out after each treatment step degradation
products and lignin precipitated during the treatment step and to
reduce to a minimum the remaining fraction of fluid, since the
latter will otherwise lead to an increased requirement for
pH-adjusting chemicals for the subsequent treatment steps and
transfer of precipitated lignin and other degradation products,
which subsequent step generally takes place at a completely
different pH.
[0004] Simple vacuum filters with dewatering drums that are
partially (typically 20%-40%) immersed in the pulp suspension that
is to be dewatered were used in certain older types of washing step
after a bleaching step or a delignification step. In these vacuum
filters, a bed of pulp forms spontaneously against the outer
surface of the drum under the influence of a negative pressure in
the interior of the drum, and the pulp bed is drawn up from the
pulp suspension by the rotation of the drum and is scraped off with
a scraper on the side of the drum that is moving downwards. A
consistency higher than 8-14% is generally never achieved for the
pulp bed that has been dewatered, due to the limited degree of
dewatering that is achieved, and the dewatered pulp that is scraped
of can be readily formed to a slurry with a low consistency again
in a subsequent collecting trough. The technique used here is a
lower degree of dewatering followed by slurry formation with a
cleaner filtrate, and this takes place in a series of vacuum
filters in order to achieve the required washing effect. For this
reason, it is attempted to achieve as high a degree of dewatering
as possible before the dewatered pulp is again formed to a slurry
with cleaner filtrate before the subsequent treatment stage.
[0005] A dominating washing machine on the market for bleaching
lines is the conventional dewatering press, or thickening press, in
which pulp is applied to at least one outer surface of the
dewatering drum and subsequently passes a nip between the drums and
acquires a consistency of 30% or greater after the nip. A practical
upper limit lies at 35-40%, where a higher degree of dryness cannot
be achieved without affecting the strength properties of the fibres
negatively. A representative washing press of this type is
disclosed in the patent U.S. Pat. No. 6,521,094.
[0006] The dewatered mat of cellulose pulp that is fed out from the
washing machine's nip must first be shredded due to the high degree
of dewatering, which shredding takes place in a shredder screw.
[0007] The purpose of the shredder screw has been exclusively to
break up the mat of dewatered cellulose pulp and feed it onwards to
equipment in which the cellulose pulp is rediluted to a consistency
that makes it possible to pump it onwards to the next treatment
step.
[0008] The redilution thus preferably takes place in association
with adjustment of the pH, which after an alkaline wash normally
involves the addition of powerful acidifiers, or the addition of
acidic return water/filtrate from subsequent process steps, before
the subsequent acidic treatment step. These acidic conditions have
involved the dilution in general being held well separated from the
previous alkaline wash as well as the associated shredder screw,
since the alkaline wash can be built from simpler material than
that which is normally required for washing machines that resist
acidic conditions. Acidic conditions require material that can
resist acids, and this is significantly more expensive that other
material.
[0009] Since the pulp on output from the shredder screw has an
extremely high degree of dryness, a consistency of 30% or higher,
redilution has generally been carried out in at least one separate
dilution screw arranged after the shredder screw where the dilution
fluid is added under intensive agitation from the dilution screw in
order to achieve a suitable homogeneous consistency that makes
pumping to the next step possible. The diluted pulp that is
achieved after the dilution screw is fed to a stand pipe in the
bottom of which a pump is arranged.
[0010] The very high consistency of the pulp after the dewatering
step has given rise to the belief that dilution to a homogeneous
medium consistency cannot take place unless the dilution takes
place under the influence of intensive agitation from the dilution
screw. A consistency of 30% or higher of the cellulose pulp is
experienced as dry and compact. It can be mentioned for the sake of
comparison that medium-consistency pulp is so compact that it is
just about possible to walk on this pulp, when it is at the upper
part of the consistency range.
[0011] The use of a dilution screw at this position, however,
increases the requirement for energy, it increases investment
costs, it raises the requirement for maintenance and it involves a
further mechanical treatment of the pulp which has a negative
influence on the strength properties of the pulp.
AIM AND PURPOSE OF THE INVENTION
[0012] The present invention is intended to remove the
above-mentioned disadvantages and is based on the surprising
insight that even if the pulp has been dewatered to give a very
high consistency, 30% or more, no mechanical agitation at all is
required during the dilution provided that the pulp bed has been
shredded to give small granules of a suitable size, and provided
that the dilution fluid is added evenly over a flow of the
granulated pulp.
[0013] It has become apparent, surprisingly, that the granulated
pulp, despite its high consistency displays the properties of a
sponge, and provided that the dilution fluid is added evenly to a
passing flow of granulated pulp, that is not tightly packed, a
primary homogenised dilution of the pulp takes place that is fully
adequate in order for it to be capable of being pumped onwards to
the next bleaching step.
[0014] It is sufficient in laboratory experiments with small
quantities of well-granulated pulp with a consistency around 30-35%
to pour the required amount of fluid to obtain the required
consistency into a container with granulated and non-compressed
pulp, and the complete mixture has been homogenised to an even
consistency after the addition of the fluid totally without
mechanical agitation. Observation of the granulated pulp has shown
that there lie cavities between the granules, and the fluid rapidly
penetrates between the granules through the complete volume of the
granules, after which the granules absorb the fluid as sponges.
[0015] This primarily homogenised pulp is fully sufficient such
that it can be pumped by a subsequent pump, in which a secondary or
supplementary homogenisation takes place, and together this ensures
that the same degree of homogenisation can be obtained for the
subsequent treatment step completely without a dilution screw.
[0016] The principal aim of the invention is thus to redilute pulp
from a high consistency of 30% or higher without the use of a
dilution screw and without intensive mechanical agitation, which
reduces losses in the strength of the pulp.
[0017] A second aim is to reduce operating costs and maintenance
costs for the process equipment in the redilution, since no
operation of dilution screw is necessary.
[0018] A further aim is to reduce the investment cost of the
process equipment. A reduction of both operating costs and
investment costs in the process equipment entails a reduction in
the cost of manufacturing bleached pulp to an equivalent degree,
and this saving is multiplied by the number of washing machines
that are used in the bleaching line. No less than six washing
machines are included in an O-D-E-D-E-D sequence, and thus the
reduction in costs can be significant.
[0019] Approximately 50 kW is required solely for the operation of
one dilution screw, and the investment cost is approximately SEK
500,000 (depending to a certain extent on requirements on
materials, i.e. whether it needs to be acid-resistant or not).
[0020] The operating costs per year in an O-D-E-D-E-D bleaching
line will be:
6*50 kW*SEK 0.20 (the price for an operator in Sweden)*24 hours*350
days (the number of operating days per year, excluding
stoppages)=SEK 500,000 SEK per year;
[0021] and the investment cost will be:
6*SEK 500,000=SEK 3,000,000.
[0022] The capital cost for an interest rate of 5% gives an annual
cost of SEK 150,000.
[0023] In summary, implementation of the invention involves a total
annual saving that approaches SEK 650,000-1,000,000 SEK including
maintenance costs and building space (frameworks, etc.) in a
bleaching line with a capacity of 1,000 tonnes per day.
[0024] Furthermore, availability of the mill increases since six
machines can be removed, each of which has an MTBF (mean time
between failure).
[0025] A further aim is to remove a treatment step between the
washing machine and the subsequent pumping, which makes possible a
more compact mill and opportunities to place the washing machines
at a lower height over the ground in the mill. The washing machines
are normally placed at a great height over the ground, and the pulp
falls downwards after being washed in the washing machine while it
passes through various conditioning steps. If one of these
conditioning steps (such as the dilution screw) becomes
unnecessary, the building height can be reduced, which in turn
gives a saving.
[0026] With these aims, the invention is characterised by the
characteristics of claim 1 with respect to the method according to
the invention, and by the characteristics of claim 4 with respect
to the device according to the invention.
DESCRIPTION OF DRAWINGS
[0027] FIG. 1 shows a typical treatment step for the pulp in a
reactor with a subsequent washing press according to the prior
art;
[0028] FIG. 2 shows part of the system in FIG. 1 (prior art);
[0029] FIG. 3 shows a dilution system according to the
invention;
[0030] FIG. 4 shows a detail of FIG. 3; and
[0031] FIG. 5 shows a view seen from underneath in FIG. 4, seen at
the level of the section A-A.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0032] FIG. 1 shows a conventional treatment step for cellulose
pulp, hereafter denoted "pulp". The pulp is fed by the pump 1 to a
mixer 2 in which necessary treatment chemicals are added. These
treatment chemicals can be, for example, oxygen gas, ozone,
chlorine dioxide, chlorine, peroxide, pure acid or a suitable
alkali for an extraction step, or a mixture of these, and possibly
other chemical or additives such as a chelating agent. The pulp is
transported after the addition of the necessary chemicals by the
mixer 2 to a reactor system 3, here shown in the form of a
single-vessel tower 3 of upwards flow. The reactor system can,
however, be constituted by simple pipes or by one or several
reactors in series, and possibly with the batchwise addition of
chemicals between the towers in those cases in which the bleaching
processes are compatible and do not require washing between the
towers. The treated pulp is fed after treatment in the reactor
system 3 to a pulp chute/stand pipe 4, which establishes the buffer
volume and static pressure required, to a pump 5 arranged at the
bottom of the pulp chute.
[0033] The pulp is fed from the pump 5 to a washing machine 7,
shown here in the form of a washing press with two drums 7a, 7b.
The pulp is applied to the drums, here at the 12 o'clock position,
and is led by convergent pulp collectors during the addition of
washing fluid (not shown in the drawing) to a final dewatering nip
between the drums, from where a mat of dewatered pulp is fed
upwards to a shredder screw 8.
[0034] The drums in FIG. 1 rotate in opposite directions, and the
pulp web is dewatered through the outer surface of the drum while
the pulp is led approximately 2700 around the perimeter of the drum
to the nip.
[0035] It is an advantage if the washing press is equivalent to
that shown in the patent U.S. Pat. No. 6,521,094. However, any
other type of dewatering press or washing press with a drum or
drums can be used in which a consistency of 30% or greater is
achieved, for example a washing press with a single dewatering drum
and a resistance roller, or another types of washing press with two
dewatering drums.
[0036] The pulp is fed upwards from the nip in the form of a highly
compressed mat 20 to a shredder screw 8, whose axis of shredding is
arranged essentially parallel with the axes of rotation of the
drums. A small oblique mounting of a maximum of 5-100 may, for
example, be present if a conical shredder screw is used, where the
mat is fed to an inlet slit in the outer casing of a conical
shredder screw, where the inlet slit lies parallel with the axes of
the drums. The fragmented pulp is led after this shredder screw 8
out from an outlet in the casing of the shredder screw in the flow
21 to a dilution screw 30 that is driven by a motor 31. The
dilution screw exposes the pulp to continuous tumbling during the
addition of dilution fluid Liq.sub.2, and the pulp is subsequently
fed to a stand pipe 40 at its finally conditioned consistency.
[0037] The pulp can subsequently be pumped from the stand pipe 40
to the next treatment step of similar type in the bleaching
line.
[0038] FIG. 2 shows a part of the same process in a different view
in which the shredder screw 8 is oriented in the same direction as
the dilution screw 30. In this case it is seen more clearly how the
highly compressed mat 20 of dewatered pulp is fed into the shredder
screw 8. The shredder screw contains a threaded screw 8a that is
driven by a motor 8c, and which may be equipped with a number of
impact pegs 8b at the end of its outflow, which pegs further beat
and break up the shredded pulp. The fragmented flow 21 of pulp
particles is fed to fall under its own weight to the subsequent
dilution screw 30.
[0039] FIG. 3 shows the dilution system according to the invention
in a treatment step that is otherwise equivalent to that shown in
FIG. 1. The dewatered web of pulp, which has a consistency of 30%
or greater, is fed in this case in to the shredder screw 8 in the
same way as shown in FIGS. 1 and 2.
[0040] However, dilution occurs in the outlet from the shredder
screw according to the invention in a significantly simplified
manner. It is important that the web/mat 20 of pulp, which has a
consistency of 30% or greater, is first fragmented by the shredder
screw such that the mat 20 is granulated to a size of granule that
has a normal distribution around a dimension in the range 5-40 mm.
The granulated pulp is then fed from the outlet of the shredder
screw to fall under its own weight in a stand pipe 22 connected to
the outer cover of the shedder screw at the end of its outlet. The
dilution fluid Liq.sub.DIL is subsequently added under pressure
into the stand pipe through a number of fluid jets arranged around
the periphery of the stand pipe and above a level Liq.sub.LEV of
diluted cellulose pulp established in the stand pipe. In the
embodiment shown in FIG. 3, the upper connection 22 of the stand
pipe to the outer casing of the shredder screw has a smaller
diameter than the lower part 40' that lies below. The principle is
that the pulp falls under the influence of gravity down through the
parts 22, 40' of the stand pipe, and its lower part 40' is given a
larger diameter in order to be able to establish a suitable buffer
volume before the pumping with the pump 41' at a given level of
pulp Liq.sub.LEV in the stand pipe 22, 40'.
[0041] The amount of dilution fluid Liq.sub.DIL added establishes a
consistency of the cellulose pulp within the range of medium
consistency 8-16%, which is a consistency that allows the pulp to
be sent onwards using an MC pump.
[0042] The amount of dilution fluid that is required in order to
establish the consistency at which the pulp is subsequently pumped
is constituted to more than 75-90% of the fluid that is added at
the said nozzles arranged above the level that has been established
in the stand pipe. A certain amount of chemicals such as
acidifiers/alkali or chelating agents may be added at the bottom of
the stand pipe 22/40', but the principal dilution takes place with
the dilution fluid above the pulp level established in the stand
pipe.
[0043] The cellulose pulp at this medium consistency is fed by the
pump 41 onwards from the lower end of the stand pipe to subsequent
treatment steps for the cellulose pulp.
[0044] The dilution of the pulp from high consistency of 30% or
greater at the upper part of the stand pipe to a medium consistency
of 8-16% before the pumping from the lower part of the stand pipe
takes place in this manner exclusively under the influence of the
hydrodynamic effect from the addition of the dilution fluid through
the said nozzles.
[0045] FIG. 3 and FIG. 4 show an embodiment of the manner in which
addition of the dilution fluid can be realised. The dilution fluid
is added by a pump to a distribution chamber 60 that is arranged
concentrically around the stand pipe 22. The pump pressurises the
fluid to a suitable level, an excess pressure of approximately
0.1-0.8 bar. Alternatively, high-pressure nozzles can be used,
which finely distribute the dilution fluid in the form of fanned
plumes of fluid, oriented at a suitable angle relative to the
vertical, a suitable angle being 30-90.degree..
[0046] A number of nozzles 62 are arranged at the bottom of the
distribution chamber oriented obliquely downwards, in the direction
of flow of the granulate, and inwards towards the centre of the
flow. The amount of obliqueness in the mounting is appropriately
45.+-.15.degree. relative to the vertical. The oblique orientation
downwards is favourable for achieving an ejecting influence on the
granulate flow, and for avoiding the risk that the dilution fluid
splashes upwards in the stand pipe.
[0047] A number of nozzles, at least four, are arranged around the
stand pipe 22/40', preferably with equal distances between them.
With a stand pipe 22 having a diameter of 800-1,500 mm, it is
appropriate that 10-40 nozzles are arranged around the periphery of
the stand pipe. It is appropriate that the distance between
neighbouring nozzles is less than 50-300 mm. If high-pressure
nozzles with fan-shaped plumes are used, the nozzles can be
arranged with a greater distance between neighbouring nozzles. It
is important that the dilution fluid is added evenly around the
complete circumference of the flow of granulate and at a
sufficiently high pressure in order to penetrate to the centre of
the granulate flow. The selection of the pressure is a setting
carried out on engineering grounds, considering the properties of
the actual nozzles used.
[0048] The invention can be modified in a number of ways within the
scope of the claims. The nozzle 62 for the addition of dilution
fluid can, for example, be realised as a simple drilled hole
through a thick-walled sheet, of thickness at least 8-10 mm.
However, specially designed nozzles are preferred for optimal
penetration of the flow of granulate and for even distribution
around the complete circumference of the flow, which nozzles
preferably generate fan-shaped plumes of fluid. Addition of
dilution fluid can also take place at a sufficiently high pressure
that the dilution fluid more forms a very finely divided mist in
the region that the granulated pulp passes.
[0049] Addition of dilution fluid takes place in the preferred
embodiment in association with an increase in the area of the stand
pipe 22 to a lower part 40' of the stand pipe having a larger
diameter, but it is not necessary that the addition takes place in
association with an increase in area.
[0050] A small amount may also be added at the outlet end of the
shredder screw, with the addition flow directed down towards the
stand pipe. But the dilution is to take place principally through
the hydrodynamic mixing effect from the addition of the dilution
fluid into the flow of granulate.
* * * * *