Note: Descriptions are shown in the official language in which they were submitted.
CA 02156523 2004-06-O1
WOOD PULP PROCESSING APPARATUS AND METHOD
Background of the Invention
a. Field of the Invention
The present invention relates to an apparatus
and method far processing wood pulp, and more
particularly to such an apparatus and method which
is particularly adapted for use in a system
incorporating bleaching, and dewatering and/or
washing material such as wood pulp or the like.
b. Background Art
In the pulp and paper industry, it is common
to take the wood pulp from the digester and then
dewater and wash the wood pulp through several
cycles. Then the wood pulp is directed through a
number of bleaching stages, and between each
bleaching stage there is a dewatering process and
also washing to remove the residue resulting from
that particular bleaching step.
With regard to dewatering and washing, there
are in general three common methods of
accomplishing a dewatering and/or washing
operation. One method is to employ a rotating drum
which has a perforate cylindrical sidewall, where
the cylindrical sidewall on one side travels
downwardly into a bath of a pulp slurry and then
WO 94/19534 PCT/US94/01943
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travels upwardly to a location above the pulp
slurry bath. A suction is applied within the
drum, so that a portion of the pulp slurry adheres
to the surface of the drum. As the layer of wood
pulp on the drum is carried upwardly above the
pulp slurry bath, in a first path of travel a
dewatering operation is accomplished where the
liquid from the pulp slurry is drawn into the
interior of the drum. Then in a second part of
the travel of the pulp on the drum, a wash water
is deposited on the dewatered pulp mat to cause
displacement washing. Before the layer of pulp
material that has been dewatered and washed
travels back into the pulp bath, this layer is
removed from the drum by a doctor blade or the
like. The drum is sometimes enclosed in a
pressure chamber, and for practical reasons, the
pressure differential used in the drum-type
dewatering/washing operation is in the range of
about four to ten pounds per square inch.
A second method is to use a continuously
moving foraminous conveyor belt onto which a wood
pulp slurry is deposited. The conveyor belt
carries the wood pulp slurry sequentially over a
series of suction boxes which create a lower than
atmospheric pressure below the belt to apply a
differential pressure across the moving conveying
belt to perform first a dewatering operation, and
then a washing operation where wash water is
deposited on the layer of wood pulp. In this type
of operation, the pressure differential that can
be applied across the pulp layer is limited
because of the frictional force created between
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the moving belt and its underlying support
structure, and the pressure differential
limitations in such devices are generally in the
range of about two to three pounds per square
inch.
A third type of dewatering/washing operation
is to move the pulp between upper and lower
foraminous belts which are pushed toward one
another to squeeze the water from the pulp. Then
the pulp is mixed with a cleaning liquid, and the
liquid removal operation is again repeated by
again squeezing the pulp. This series of steps is
continued until the desired dewatering and washing
is accomplished.
To the best knowledge of the applicant, all
of the dewatering/washing systems that nave
actually been used commercially operate with
pressure differentials in the range of two to ten
pounds per square inch, but no higher.
With regard to prior art bleaching wood pulp,
for many years, chlorine and chlorine dioxide have
commonly been used as bleaching agents. However,
in recent years, for environmental reasons, there
has been greater effort to move toward the use of
other bleaching agents, such as oxygen (02), ozone
(03), hydrogen peroxide (H202), and possibly
others. Quite commonly, the bleaching agent is
introduced into a pulp slurry, which is then fed
into a processing tower where the pulp slurry
moves slowly upwardly or downwardly through the
Y
tower. As the pulp moves, the bleaching agent
reacts with the wood pulp. The height of the
tower and the rate of movement of the wood pulp is
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controlled so that the bleaching agent works on
the wood pulp for a predetermined period of time
s
(e. g. one half hour to several hours, depending
upon the type of bleaching agent and other
factors).
After the wood pulp slurry is removed from
the tower, more liquid is added to the wood pulp
to form it into a slurry of low consistency
(possibly one percent). It is then subjected to a
dewatering and a washing process to remove the
residue resulting from the immediately prior
bleaching step. The thickened slurry with the
next bleaching agent mixed with it is then
directed to another bleaching tower, where another
bleaching operation is accomplished possibly with
a different (or sometimes the same) bleaching
agent. This process is repeated until the
bleaching is completed.
Summary of the Invention
The method of the present invention is
arranged to combine effective dewatering and
washing of the pulp with the bleaching stages in a
manner to give a high degree of efficiency and
effective bleaching, with relatively little or no
net effluent leaving the system.
The method comprises dewatering and washing a
batch of pulp at a plurality of wash locations,
with the washing being accomplished by moving
successive quantities of wash liquid into and
through the batch of pulp while applying a
sufficient pressure differential across the pulp
to maintain the pulp at a consistency of at least
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about one part by weight of cellulose fiber to
four parts by weight of liquid.
After the dewatering and washing of the pulp
at each of the wash stations, the pulp is
delivered to a related one of a plurality of
bleaching locations where the pulp is treated with
a bleaching agent. The method is characterized in
that after the pulp is treated with the bleaching
agent at each of the bleaching locations, the pulp
is delivered to a subsequent washing location
where the pulp is dewatered and washed.
In the preferred form, the pulp is dewatered
and washed by delivering the pulp as a slurry into
a pressure chamber in a pressure vessel so as
positioned as a pulp layer over a pressure
differential table means, and the pressure
differential is applied through said pressure
differential table means. The preferred form is
that successive batches of pulp are delivered into
the pressure vessel at each washing location and
are removed from the pressure vessel while
maintaining above atmospheric pressure in the
pressure chamber.
The preferred form of discharging the pulp
from the pressure vessel is accomplished by
positioning the pulp at a discharge location in
the vessel while the pulp is subjected to above
atmospheric pressure in the vessel. A discharge
passageway is provided and opened to the discharge
location at a pressure below pressure in the
a
vessel to create a pressure differential to move
the pulp through the discharge passageway. The
discharge passageway leads to the subsequent
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bleach location, and the pulp is delivered from
the passageway to a treatment containing means at
the subsequent bleach location.
In at least one discharge passageway, a
bleaching agent is delivered into the passageway
and to pulp passing through that passageway.
Preferably, the pulp is moved through a venturi in
the passageway, and the bleaching agent is
delivered into the passageway at a location
adjacent to the venturi. Desirably, this
bleaching agent is a gaseous bleaching agent and
in one embodiment the bleaching agent is ozone.
The pulp is delivered through a plurality of
venturis in the passageway, and the passageway is
of sufficient volume and length whereby
substantial bleaching of the pulp by the ozone can
be accomplished in the passageway.
Also, at least one venturi is desireably
positioned in each passageway to accelerate the
pulp and cause shredding of the pulp into smaller
particles.
In the latter part of a washing cycle of at
least one of said wash locations a pulp treating
agent is entrained in a portion of wash liquid
which is moved into the pulp and remains in the
pulp after the pulp is delivered to a subsequent
bleaching location.
In a preferred form, the consistency of the
pulp during washing is maintained at a value at
least as great as about one part cellulose fiber
by weight to three parts liquid. More desireably,
the consistency is maintained at least as great as
A
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WO 94/19534 PCT/US94/01943
about one part cellulose fiber to two and a half
parts liquid.
Fresh water is used as a portion of the wash
liquid at a latter portion of washing cycle of at
least some of said wash locations. Desireably,
portions of wash water effluent from the pulp are
recycled in the washing locations, and a portion
of the effluent is delivered to an evaporating
recycling mean to remove dissolved solids from the
effluent to yield fresh water. This fresh water
is utilized as wash liquid in subsequent washing
cycles. Desirably, the pressure in each vessel is
maintained at least as high as about one bar above
atmospheric pressure, and the temperature in the
pulp vessel is maintained at least as high as
about 50 degrees C.
The pulp bleaching system of the present
invention comprises a plurality of pulp washers
arranged to dewater and move successive quantities
of wash liquid into and through a batch of pulp
while applying a sufficient pressure differential
across the pulp to maintain the pulp at a
consistency of at least about one part by weight
of cellulose fiber to four parts by weight of
liquid.
The system also comprises a plurality of
bleaching means each arranged to receive the pulp
from a related one of the pulp washers and to
treat said pulp with a bleaching agent.
The system is characterized in that after the
pulp is treated with the bleaching agent at each
of the bleaching systems, the pulp is delivered to
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a subsecruent pulp washer where the pulp is
dewatered and washed
In the preferred form, each pulp washer
comprises a pressure vessel having a pressure
chamber, a pressure differential table means
positioned in the chamber to receive a layer of
pulp slurry thereon, and arranged to create a
pressure differential at the pressure differential
table means to effect dewatering and washing of
the pulp.
Other features of the present invention will
become apparent from the following detailed
description.
Brief Descri~~ion of the Drawincts_
Figure 1 is a schematic drawing of a commonly
used prior art bleaching system;
Figure 2 is a side elevational view, taken
partly in section, showing a dewatering/washing
apparatus used in the system of the present
invention;
Figure 3 is a sectional view taken along line
3-3 of Figure 2;
Figure 4 is a top elevational view of only
the pressure vessel of the apparatus of Figure 2,
showing various locations of the openings therein;
Figure 5 is a side elevational view of the
pressure vessel of Figure 4, showing in addition
r
certain fittings;
Figure 6 is a side elevational view showing
only the headbox and manifold assembly which
provides the pulp for processing in the apparatus
of Figure 2;
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WO 94/19534 PCT/US94/01943
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Figure 7 is front elevational view of the
headbox and manifold assembly of Figure 6;
b
Figure 8 is a top elevational view thereof;
Figure 9 is a transverse sectional view
showing a portion of the table unit and a top
portion of one of the conduits to which it is
attached;
Figure 10 is a top plan view showing only the
recirculating tubes used in the apparatus of
Figure 2;
Figure 11 is an end view of the tubes of
Figure 10, taken at line 11-11 of Figure 10;
Figure 12 is a side elevational view of the
tubes of Figure 10, taken at the location of line
12-12;
Figure 13 is a perspective view of a portion
of the contact plate of the apparatus of Figure 2;
Figure 14 is a side elevational view of the
pulp discharge assembly of the apparatus of Figure
2;
Figure 15 is a top elevational view of the
discharge assembly of Figure 14;
Figure 16 is an isometric view of the
discharge assembly as shown in Figures 14 through
15;
Figure 17 is an isometric view of the
conveying belt of the apparatus, and showing also
the rollers associated therewith;
4
Figure 18 is a transverse cross-sectional
view of the contact plate;
Figure 19 is a sectional view similar to
Figure 14 showing a modified version of the pulp
discharge assembly of the apparatus;
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WO 94/19534 PCT/US94/01943
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Figure 20 is a view similar to Figure 19
showing the pulp having entered the housing of the
discharge assembly;
Figure 21 is a top plan view, partly in
section, of the modified version of Figures 19 and
20;
Figure 22 is a bottom plan view of a modified
form of the contact plate of the present
invention;
Figure 23 is a sectional view taken along
line 23-23 of Figure 22; and
Figure 24 is a sectional view taken along
line 24-24 of Figure 22;
Figure 25 is a schematic drawing of the
system of the present invention, utilizing the
dewatering/washing apparatus shown in Figures 2-
21;
Figure 26 is a schematic view similar to
Figure 22; but showing only a single processing
staiton and a washing station, including more of
the details thereof;
Figure 27a is a longitudinal sectional view
through a cellulose fiber, showing its surrounded
in liquid at 10% consistency;
Figure 27b is a sectional view transversely
to the cellulose fiber of Figure 27a;
Figure 27c is a sectional view the same as
Figure 27b, except showing the cellulose fiber to
a larger scale;
Figure 28a is a view similar to Figure 27a,
but showing the cellulose fiber in liquid at 25%
consistency;
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' Figure 28b is a sectional view taken
transversely across the fiber of Figure 28a;
f
Figure 28c is a view similar to Figure 28b,
but drawn to an enlarged scale;
Figure 29a is a graph showing curves
representing washing through a pulp layer in an
idealized situation, utilizing teachings of the
present invention;
Figure 29b is a graph similar to Figure 29a,
but showing operation of a prior. art Kemmi-Washer;
Figure 30a is a graph illustrating the
washing operation of the present invention in
actual operation;
Figure 30b is a graph similar to Figure 30a,
but showing the performance of a prior art Kemmi-
Washer;
Figures 31 thru 37 are seven figures
illustrating successive operating stations in a
system of the second embodiment of the present
invention;
Figure 38 is a table illustrating performance
of the system illustrated in Figures 31 thru 37;
Figure 39 is a schematic drawing of the
system of the second embodiment, and showing the
recycling of fresh water in the system.
Description of the Preferred Embodimeat
It is believed that a better understanding of
the function and advantages of the system of the
present invention will be obtained by first
discussing generally one of the commonly used
prior art bleaching systems used in the wood pulp
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WO 94/19534 PCT/US94/01943
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industry, after which the system of the present
invention will be described.
A. A Typical Prior Art Washiag and Blechiag
System
This prior art bleaching system is
illustrated somewhat schematically in Figure 1.
Initially, before any bleaching is accomplished,
the wood pulp that has initially been processed in
a digester is then dewatered and washed in a brown
stock washing apparatus. The slurry discharged
from this washer typically has a consistency of,
for example, 10-13%. This pulp slurry is then
diluted to a consistency of, for example, four
percent and chlorine gas is introduced into this
slurry. The slurry is then directed by a pump P-1
in the lower end of a chlorination tower T-1 where
it slowly moves upwardly to the top of the tower.
The travel time upwardly through the tower in a
typical system could be between about one half
hour to an hour. The slurry leaving the top of
the chlorination tower T-1 then goes to a mixer M-
1 where liquid recirculated from a washer W-1 is
mixed with the slurry in the mixer M-1 to bring
the consistency of the slurry down to about one
percent. This one percent slurry is then directed
into the washer W-1.
Typically, this washer W-1 is a drum washer
as previously described herein in the section on
background art, and sometimes more than one drum
washer is used to wash the pulp further. The pulp
from the washer W-1 can have steam added thereto
(primarily to raise the temperature of the pulp to
the desired level), and then this is directed by a
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pump P-2 into the lower end of a second tower T-2
which is an alkali extraction tower. The
retention time in this tower T-2 is typically
between one to two hours. The pulp leaving the
top of the alkali extraction tower T-2 is then
directed into a second washer W-2, and
substantially the same operation is accomplished
as described with regard to the washer W-1. More
specifically, the slurry is diluted by the mixer
M-2 and directed to the washer W-2 where it is
dewatered and washed. Then steam is added and the
slurry is then directed by a pump P-3 into the
bottom of a third tower T-3, which in this
instance is a chlorine dioxide tower. Chlorine
dioxide is mixed to the pulp at the bottom of the
tower.
After the bleaching step in the chlorine
dioxide tower T-3 is completed, typically in about
three to four hours, the pulp moves through a
third washer W-3 to the fourth tower T-4 (an
alkali extraction tower), then through the washer
W-4 to the fifth tower T-5 (which is another
chlorine dioxide tower). From the tower T-5, the
pulp is directed to the final washing stage at W-
5.
In this prior art bleaching operation, as
shown in Figure 1, fresh water is added at certain
locations, and also liquid from some of the
washing stages is recirculated back through the
"upstream" portions of the system. For example,
in the prior art system shown in Figure 1, fresh
water would be added into the last two stages of
the washing system (W-4 and W-5). Further, in a
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typical operation, a certain portion of the fluid
that is extracted from the washer W-5 would be
directed back to the washer W-3; also, a portion of
the liquid from the washer W-4 would be directed
back to the washer W-2; and further a portion of
the liquid from the washer W-3 would be directed
back to the washer W-1.
With the consistencies of the stock from the
washing stages generally being between ten to
fourteen percent, for each unit of pulp material
that is processed, there would possibly be
approximately twenty five or more units of liquid
that would be directed to a sewer as waste (this
varying, of course, depending upon the actual
consistency of the pulp entering and leaving the
various washers and washer efficiency.)
B. The Dewatering/Washing Apparatus of the
System of the Present Invention
In the preferred form of the present
invention, there is utilized a dewatering and
washing apparatus and method as described generally
in U.S. Patent 5,482,594 issued January 9, 1996.
Other embodiments described in the two
aforementioned earlier filed patent will not be
described, with the understanding that, of course,
such other embodiments or modifications thereof
could be used in the system of the present
invention. Then there will be a description of the
overall system of the present invention.
WO 94/19534
PCT/US94/01943
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As will become apparent from the following
detailed description, this dewatering and washing
apparatus has significant advantages when used in
the system o~f the present invention, and this will
be discussed later in this text.
There will now be a description of the
dewatering and washing apparatus 10 used in the
present invention, and then a description of how
this apparatus 10 is used in the various stages
of
the bleaching of the wood pulp in the system of
the present invention.
With reference to Figures 2 through 18,
the dewatering and washing apparatus 10 utilized
in the system of the present invention comprises
a
main pressure vessel 12 with front and rear ends
14 and 16. There is a single processing area 18
which is both a dewatering and washing processing
area, and which occupies a substantial portion of
the pressure vessel 12.
There is a head box 20 to contain the pulp
slurry to be processed, a pressure inlet 21, and
also a manifold system 22. Positioned within the
pressure vessel 12 is a conveying and pressure
differential table unit 24. As will be disclosed
more fully later herein, this table unit 24, in
conjunction with other components, accomplishes
both dewatering and washing operations
sequentially.
Also positioned within the pressure vessel 12
is an enclosure frame 26, which contains the pulp
slurry and wash water in a manner that there is
a
sealed area to accomplish the dewatering and
washing. In terms of function, the table unit 24
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and the enclosure frame 26 can 3~e considered to
J'
function as a table assembly..,2'7 that contains the
pulp slurry and accomplishes certain processing
functions.
There is a dewatering/washing plate assembly
28 which, during the latter part of the dewatering
operation and during the washing sequence, is
pressed against the top surface of the pulp slurry
being processed. Just above the plate assembly 28
is positioned a wash water dispensing assembly 30
which comprises a plurality of dispensing troughs
32. This trough assembly 30 provides a convenient
means for dispensing the wash water sequentially
onto the plate assembly 28 during the wash cycles.
The table unit 24 comprises a conveyor belt
34 which engages front and rear rolls 36 and 38.
The upper run of this conveyor belt 34 extends
along the top portion of the table unit 24.
At the rear end of the conveying belt 34
there is a pulp discharge assembly 40 which
receives the processed pulp mat directly from the
conveying belt 34 and moves the pulp mat through a
discharge passageway at which the pulp mat forms a
substantial seal. Then as the pulp mat is moved
into a discharge area, pressurized gas (e.g. air
or steam) from inside the vessel 12 moves the
processed pulp material through a valve to a
subsequent processing location outside the
6
pressure vessel 12. This will be described in
more detail later herein.
With reference to Figures 6 through 8, the
headbox 20 is made in the form of a tank to
contain pulp at a relatively high pressure which
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is moderately greater than the pressure within the
main pressure vessel 12 due to it's hydrostatic
head only. There is a pulp inlet pipe 42 having
an upper inlet end 44 and a lower outlet end 46
located within, and at the lower part of, the
headbox 20.
As will be described hereinafter, the pulp in
the headbox 20 is discharged rather quickly in a
batch into the processing area 18. During the
operation of the apparatus 10, wood pulp is
directed substantially continuously into the
headbox 20 and reaches an upper level 48, and
drops to a lower level 50 immediately after a
batch discharge of the pulp. The lower end 46 of
the pipe 42 is positioned moderately below the
lower level 50 so that splashing of the incoming
pulp slurry and a mixing with air is minimized.
There is a pressure equalizing conduit 52
which has an end elbow 54 that extends into the
headbox 20 and has an end opening 56 that is
located above the upper pulp level 48. This
equalizing tube 52 connects to a fitting 58 that
leads into the interior of the main pressure
vessel 12. Thus, as the liquid level of the pulp
slurry in the headbox 20 changes, gas is permitted
to flow between the headbox 20 and the interior of
the main pressure vessel 12 to equalize the
pressure in the headbox 20 and the interior of the
vessel 12.
The manifold system 22 has a central inlet 60
r
that connects to a lower outlet valve 62 of the
headbox 20. This inlet 60 leads into a main trunk
section 64 that extends forwardly and rearwardly
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from the inlet 60, with the forward and rear ends
of the trunk section 64 leading into two branch
lines 66, with each branch line 66 leading into a '
respective sub-branch 68. The sub-branches 68
each comprise lines 70 that are in turn connected
through respective valves 71 to outlet members 72,
each of which is positioned at the side of the
pressure vessel 12 a little above the top of the
enclosure frame 26.
More particularly, as can be seen in Figure
3, the outlet end 74 of each of the members 72 is
positioned moderately above the edge of the frame
26 so as to be slightly outward of the vertical
inside surface 76 of the enclosure frame 26.
Also, this outlet 74 is positioned laterally just
a short distance outside of the lateral edges of
the dewatering/washing plate assembly 28. Thus,
the pulp slurry is able to be discharged from the
outlets 74 onto the table unit 24 within the area
of the enclosure frame 26. The positioning of the
outlet 74 permits the plate assembly 28 to be
lowered downwardly to fit within the enclosure
frame 26 and press against the pulp on top of the
table unit 24. As can be seen in Figures 2, 4,
and 5, the pulp outlets 74 are positioned in pairs
on opposite sides of the pressure vessel 12 and
spaced longitudinally at even intervals for proper
distribution of the wood pulp onto the table unit
24.
The enclosure frame 26 comprises two
longitudinally aligned side members 78 that extend
along the lateral edge portions 80 of the belt 34,
and two transverse members 82 that join the front
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and rear ends of the side members 78. These
members 78 and 82 form a rectangular enclosure
b
frame that defines the processing area 18.
This enclosure frame 26 can be raised or
lowered, and to accomplish this, there is provided
a set of four hydraulic jacks 84. The upper
surfaces 86 of the side and end members 78 and 82
are slanted downwardly and inwardly toward the
processing area 18 so that if any of the wood pulp
happens to splash onto the surface 86, it will
tend to flow into the enclosed processing area 18.
The aforementioned table unit 24, as can be
seen in Figure 9, in addition to comprising the
conveyor belt 34 further comprises a pressure
differential plate assembly 88 comprising an upper
plate 90 and a lower plate 92 which are joined and
sealed at their edges by a perimeter strip 94.
The upper and lower plates 90 and 92 each have a
planar, rectangular configuration, and are spaced
a short distance vertically from one another to
provide an enclosed chamber 96 that extends
through substantially the entire area of the plate
assembly 88. These plates 90 and 92 extend
beneath the entire processing area 18, and the
side and end edges of these plates 90 and 92 are
positioned just below the perimeter frame members
78 and 82.
The upper plate 90 is formed with a plurality
of vertical through openings 98 positioned at
equally spaced intervals over substantially the
entire surface of the upper plate 90. The upper
run of the conveying belt 34 rests on top of the
plate 90 and the liquid from the pulp mat on the
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belt 34 flows through the openings 98 and into the
chamber 96 between the plates 90 and 92. The
spacers between the upper and lower plates 90 and
92 are conveniently provided in the form of round
wires 99 that extend transversely (e.g. about 1/4
inch in diameter) across the entire width of the
table unit 24, with these wires 99 being spot
welded in place at longitudinally spaced intervals
along the entire length of the plate assembly 88.
The wires 99 divide the chamber 96 into a
plurality of transversely extending subchambers
into which the openings 98 lead.
To provide the pressure differential in the
table unit chamber 96 and also to remove the
liquid collected in this chamber 96, there is
provided a plurality of longitudinally extending
tubes 100 that are positioned immediately below
the lower plate 92 of the table unit 24. The
lower plate 92 rests directly on the tubes 100 and
each tube 100 is provided with a plurality of
spaced through openings 104 that are aligned with
and directly adjacent to corresponding openings
102 in the lower plate 92. The plate and tube
perimeter edge portions around these openings 102
and 104 are joined by a circumferential weld 106
that forms a fluid tight and gas tight seal around
each pair of aligned openings 102 and 104. These
tubes 100 are spaced laterally across the
processing area so that these openings 102 and 104
provide adequate flow paths throughout the entire
Y
area of the chamber 96.
The rear ends of the tubes 100 are (see
Figure 11) formed with right angle sections 108,
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each of which extends laterally and connects with
a related outlet fitting 110 (see Figure 3)
mounted in the wall of the vessel 12. As shown in
Figure 3, each fitting 110 is connected through a
line 112 to a related outlet valve 114 which leads
through a line 116 to a selected one of several
tanks (to be described hereinafter). Also, one
line 112 that is connected to the tube 100 nearest
the centerline of the main pressure vessel 12 is
connected to an equalizing valve 118 which leads
back into the interior of the vessel 12 through a
line 120.
Each of the valves 114 and the valve 118 is
operated in a manner to either create a pressure
differential between the plate unit chamber 96 and
the interior of the vessel 12, or to equalize the
pressure in the chamber 96 and inside the vessel
12.
It is to be understood that the aligned
openings 102 and 104 are positioned not only along
the longitudinally aligned sections of the tubes
100, but also in the curved and right angle
sections 108 so that these openings pairs 102-104
are positioned throughout substantially the entire
surface of the table unit 26.
To describe in more detail the
dewatering/washing plate assembly 28, this
assembly comprises a main horizontal rectangular
contact plate 122 having a peripheral side wall
124 extending entirely around the side edges of
the plate 122. This plate 122 is lowered and
raised by a set of four pistons 123, two of which
are shown somewhat schematically in Figure 3.
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 ~ . , ~ PCT/US94/01943
. . ,.. r.
.. -, .: . . ~~~.
-22-
As can be seen in Figure 13, the contact
plate 122 is formed on its lower side with a
Y
plurality of longitudinally extending inverted "V"
shaped grooves 126, each groove 126 being formed
by slanting surfaces 127 that lead to an upper
apex line or strip 128, and each pair of adjacent
grooves forming a lower ridge line or strip 130
where the adjacent sides of adjacent grooves meet
one another. The contact plate 122 is formed with
a plurality of evenly spaced vertical through
openings 132 extending along the length of each
apex strip 128. Thus, when wash liquid is placed
on the top surface 134 of the contact plate 122,
the liquid flows through the openings 132 and into
the grooves 126 at the location of the apex strips
128.
At this point the function of the dewatering/
washing plate assembly will be described briefly,
but there will be a more detailed description of
its operation later herein. After the pulp slurry
is initially discharged from the headbox 20 and
manifold system 22 into the processing area 18
inside the enclosure frame 26 (which is in its
lower position), the plate assembly 28 is lowered
so that the contact plate 122 comes into contact
with the upper surface of the pulp slurry. At the
same time, there is a pressure differential
between the interior of the vessel 12 and the
pressure differential chamber 96 so that the
pressure applied to the top of the contact plate "
122 acts to push the plate 122 downwardly.
The pressure inside the vessel 12, in
addition to pushing on the upper surface 134 of
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ ~ PCT/US94/01943
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the plate 122, also exerts pressure through the
openings 132 to bear directly against the pulp
located at the opening 132. The pressure on top
of the contact plate 122 causes the liquid in the
pulp slurry to flow downwardly through the
conveyor belt 34, through the plate openings 102-
104 and into the pressure differential chamber 96
which is at this time at a low pressure level.
As the dewatering operation is just being
completed, the wash cycles begin. A first layer
of wash liquid is deposited on top of the contact
plate 122, and this wash liquid flows through the
openings 132 downwardly through the pulp mat to
cause a liquid displacement washing operation. At
the completion of the washing cycles the valves
114 and 118 are operated to equalize the pressure
in the pressure differential chamber 96 with the
pressure in the vessel 12, the dewatering/washing
plate unit 28 is raised, and the enclosure frame
26 is also raised. Then the conveying belt 34 is
moved to move the processed pulp mat through the
discharge assembly 40 to a location outside of the
pressure vessel 12.
To describe further the configuration and
operation of the contact plate 122, reference is
now made to Figure 18. First, with regard to the
dimension and configuration of the contact plate
- 122, in one embodiment which has been found to
operate effectively, the diameter of the openings
132 (indicated at "a") were made as 3/32 inch.
The lateral spacing of these openings 132 from one
apex line 128 to another is 1/2 inch (indicated at
"b" in Figure 19) and the longitudinal spacing
SUBSTITUTE SHEET RULE 26)
WO 94/19534 : ~ ~ PCT/US94/01943
2~~~~~~
-24-
along each apex line 128 is 1/2 inch. The total
depth dimension of the plate 122 (indicated at
"c") is 1/4 inch. The width of the apex line 130
(indicated at "d" in Figure 19) is 1/16 inch. The
angle of slant of the surfaces 127 relative to the
horizontal was about 30° (indicated at "e" in
Figure 18).
It is to be understood, of course, that the
dimensions and angles presented above are given
simply by way of example of a contact plate 122
that has been found to work effectively, and these
could be varied. With regard to the spacing and
the diameter of the openings 132, the total cross-
sectional area of these openings 132 should be
great enough to permit an adequate rate of flow of
the wash liquid through the openings 132 and into
the pulp mat. Yet present analysis indicates that
the size of the opening 132 should be sufficiently
small and the spacing of the openings sufficiently
great so that the contact plate 122 had adequate
area to maintain its capability to enhance the
dewatering operation.
It has been found that the use of this
contact plate 122 in the present invention
enhances both the dewatering and the washing
operation. It was found experimentally that when
the pulp slurry was dewatered to a certain
consistency (in the order of about 20 to 25
percent) without use of the contact plate 122,
further dewatering was not accomplished, because
as the pressure differential continued to be
applied, small air passageways (or steam
passageways, if steam is present in the vessel 12)
SUBSTITUTE SHEET (RULE 26~
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would form through the pulp mat, causing a
"fingering", a blow-by condition.
On the other hand when the pressure plate 122
was used in the manner indicated above, it was
found that the dewatering process could be
continued until (for the same type of pulp slurry)
the consistency of the pulp slurry was raised to
as high as 25 to 35 percent before further
effective dewatering could not be accomplished.
Also, as indicated above, the use of the
contact plate 122 enhances the wash operation.
There is what is called a "dilution factor" where
during each washing operation a certain portion of
the black liquor that is removed from the pulp mat
is diluted by fresh wash water in the system. If
a higher percentage of the black liquor is removed
during the dewatering process, then there is less
black liquor that needs to be removed during the
washing process. Thus, for the same dilution
factor (which in the preferred embodiment of the
present invention is one or less), there is less
black liquor to be removed during the washing
cycle and hence less wash water is needed for each
cycle of displacement washing cycle.
Attention will now be directed toward the
countercurrent liquid recirculating system of the
present invention. In the following description,
a three stage wash cycle is described. Further
analysis has indicated that this could
advantageously be a five stage wash cycle or
possibly more than five wash cycles. However, for
ease of explanation, only three wash cycles are
described.
SUBSTITUTE SHEET (RULE 26)
CA 02156523 2004-06-O1
-26-
As indicated previously, there are three wash
water discharge troughs 32 which are utilized to
accomplish (in the present embodiment) three wash
cycles. Each trough 32 has the configuration of
half of a cylindrical shell, where the cylinder has
been divided along a plane coinciding with the
center axis of the cylinder. Each trough 32 is
rotatably mounted about its center axis of
curvature 136 in a manner that by rotating the
trough 32 ninety degrees, the liquid in the trough
can be discharged by gravity flow onto the contact
plate 122. Each trough 132 is fed by a respective
inlet pipe 138. Each pipe 138 extends through the
pressure vessel and is connected to a respective
feed tube 140.
In the liquid flow system described more fully
in U.S. Patent No. 5,482,594 there are three liquid
receiving tanks. These are operatively connected
to the tubes 100 in a manner that the liquid in the
system is recirculated in a counterflow pattern as
described in U.S. Patent No. 5,482,594. Since this
precise pattern of recirculation is not necessary
for a proper understanding of the system of the
present invention, it will not be described in
detail herein.
The process of the initial dewatering and
washing accomplished by the apparatus 10 is started
by initially depositing a charge of wood pulp,
having a consistency of, for example, one and one
half to two and one half percent onto the conveying
belt 34 at the table unit 24, where it immediately
begins to flow laterally to form a
WO 94/19534 PCT/US94I01943
-27-
layer. As soon as about one quarter to one half
of the charge of the wood pulp is deposited on the
table unit 24, the valves 114 connected to some of
the collecting tubes 100 are opened (with the
equalizing valve 118 being closed at this time) to
connect these tubes 100 to a low pressure area,
which for dewatering could be the pressure at a
black liquor collection tank. This causes the
pressure in the table unit chamber 96 to drop to
create the pressure differential between the
interior of the vessel 12 and the lower pressure
in the table unit chamber 96, which in turn causes
liquid to flow from the pulp slurry into the
chamber 96, and thence into the collecting tubes
100.
As indicated previously, as soon as the full
charge of pulp has been deposited on the table
unit 24, the dewatering/washing plate assembly 28
is immediately lowered so that the contact plate
122 comes into engagement with the upper surface
of the pulp slurry. As described previously, the
pressure differential between the interior vessel
pressure above the plate and the lower pressure
below causes the plate 122 to press down on the
pulp slurry, and also gaseous pressure is exerted
through the openings 132. This enhances the
movement of the black liquor that is in the pulp
slurry out of the pulp, into the chamber 96 and
thence into the tubes 100.
When this initial dewatering step is just
being completed one of the troughs is rotated
ninety degrees to deposit the wash liquid in that
trough 32 onto the top of the contact plate 122.
SUBSTITUTE SHEET (RULE 261
WO 94/19534 PCT/US94/01943
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Since the pressure differential between the
interior of the vessel 12 and the chamber 96 r
remains, the wash liquid indicated at 164 is
caused to move into the pulp mat and begin a
displacement washing cycle.
When the batch of washing liquid has moved
into the mat, then the open valves 114 leading
from the related tubes 100 are closed, and the
other two valves 114 which connect to other tubes
100 are opened. At this second point the trough
32 is rotated ninety degrees and the next batch of
wash liquid is deposited on the contact plate 22.
The liquid that is now being removed from the wood
pulp is directed to a collection tank. This
process can be repeated to accomplish as many wash
cycles as desired.
It should be pointed out that as a final
step, the liquid that is finally moved into the
pulp mat, so as to remain in the pulp mat when the
pulp mat is moved from the apparatus 10, normally
has dissolved or suspended therein a chemical
compound or substance (e.g. a bleaching agent or a
chemical compound to change the pH or have some
other effect) to be used in a subsequent
operation.
The next step is to remove the processed pulp
mat 158 from the processing area 18 and also to a
location outside of the main pressure vessel 12.
To accomplish this, first the enclosure frame 26
and the dewatering/ ,
washing plate assembly 28 are both raised, and the
belt 34 is moved in a forward direction to carry
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 PCT/US94/01943
-29-
the processed pulp mat 58 to the discharge
assembly 40.
As can be seen in Figures 14, 15 and 16 the
discharge assembly 40 comprises a guide plate 180
that has two side plates 182 that defines with the
underlying belt 34 an inlet 184 which has a height
dimension moderately greater than the thickness
dimension of the processed pulp 158. The plate
180 slopes very moderately downwardly in a forward
direction so that the passageway 186 leading from
the inlet 184 and defined by the plates 180 and
182 and the upper run of the belt 34 tapers in
thickness in a forward direction. The effect of
this is that the pulp layer becomes squeezed to a
moderate extent in the passageway 186 as it
progresses forwardly.
The effect of so moving the pulp into the
passageway 186 is to substantially close the rear
end of the passageway 186 so that very little of
the gas (e.g. air or steam) within the vessel 12
passes through the passageway 186. The passageway
186 at its rear end terminates in a discharge
passageway portion 190 having a substantially
constant cross-sectional area. This forward
passageway section 190 leads into a discharge
chamber defined by a discharge housing 192. This
discharge housing 192 has a generally frusto-
conical configuration, having at its side a right
side 194 a smaller cross-sectional area and
extending laterally through the opposite side of
the vessel 12 where it has a more expanded area
195. At the outlet end, there is an outlet valve
196 that connects to a tube or other member that
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 PCTIUS94/01943
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leads to the next processing~'station. There is a
gas pressure line 198 leading into the right end
194, and this line 198~connects through a valve
200 to an elbow 202 that leads into the interior
of the pressure vessel 12 at 204.
There is a doctor blade 206 having a forward
edge 208 that removes the pulp mat from the belt
34. This doctor blade 206 defines the Lower
portion of the aforementioned passageway 190.
In operation, when the dewatering and washing
cycle has been completed, the belt 34 is moved to
in turn move the forward edge of the pulp mat
through the entryway 184 and into the passageway
186. When the pulp mat has moved forward to the
extent that it substantially encloses the
passageway 186, then the valve 196 is opened, thus
reducing the pressure in the discharge housing 192
to a lower level. This enhances the movement of
the pulp mat toward the discharge housing 192. As
the pulp mat begins to be discharged from the
forward passageway portion 190 into the housing
192, then the valve 200 can be selectively opened
to cause the high pressure gas in the vessel 12 to
pass into the inlet 204, through the valve 200 and
into the housing 192 to assist in blowing the pulp
material outwardly from the housing 192 and
through the valve 196. When the pulp mat has been
discharged, then these valves 200 and 196 are
closed.
To further describe the conveying belt 34,
reference is made to Figure 17. It can be seen
that the belt 34 has two edge strip portions 206
and four transverse strip portions 208 that are
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ PCT/ITS94/01943
-31-
made imperforate. These strips 206 and 208 engage
the lower edge portions of the enclosure frame 226
to make a seal. After the dewatered and washed
pulp mat is discharged from the upper run of the
belt 34, the lower run moves to become the upper
run of the belt for a second dewatering and
washing operation.
As indicated previously, one of the desirable
features of the apparatus 10 is that with the
entire process being performed within the
pressurized vessel 12, it becomes more practical
to operate with much higher pressure differential
across the pulp mat, and also to operate at higher
temperatures, if desired. This leads to a number
of advantages in the system of the present
invention.
With the foregoing detailed description of
the apparatus having been completed, the overall
operations of this apparatus 10 will now be
summarized. Initially, the vessel 12 is
pressurized to an above atmospheric operating
pressure. Generally, the pressurized gaseous
medium within the vessel 12 is air, steam, or a
combination of air and steam. Also, desirably the
temperature within the vessel 12 is maintained at
a level above ambient temperature, this depending
on operating pressures and other factors.
A pulp slurry is directed through the pipe
inlet 44 to flow into the headbox 20, until the
t 30 headbox 20 is filled to an adequate level (i.e. to
the level 48 as shown herein). Also the enclosure
frame 26 is lowered onto the table unit 24 and the
plate assembly 28 remains in its raised position.
SUBSTITUTE SHEET (RULE 26~
PCT/US94/01943
WO 94/19534
-32-
Then the valves 71 in the manifold outlet members
72 are opened to cause the pulp'slurry in the
headbox .20 to flow through the manifold 22 and
through the outlets 74 onto the portion of the
upper run of the conveyor belt 34 that is within
the enclosure frame 26. The pulp slurry typically
would have a consistency of one and one half to
two and one half percent, and at that consistency
it would readily flow across the table unit to
form a substantially uniform layer within the
enclosure frame 26.
While the pulp slurry is flowing onto the
table unit 24, as soon as this pulp slurry has
reached a sufficient depth so that the dewatering
process can begin (typically in the order of 3 to
4 inch), the valve 114 that leads from one set of
the conduits 100 is opened to lower the pressure
in the variable pressure chamber 96 in the plate
assembly 88. At the same time, the equalizing
valve 118 is closed (or remains closed). This
pressure differential causes the liquid in the
pulp slurry to begin flowing through the
foraminous conveying belt 34 and through the
openings 98 in the plate 90, thence into the
chamber 96, from which the liquid flows through
the openings 104 that lead into the conduits 100.
These conduits 100 in turn direct the liquid into
the tank.
As soon as the first batch of pulp slurry has
been completely discharged through the openings 74
into the processing area 18, then the contact
plate 122 is promptly lowered onto the upper
surface of the pulp slurry. As described in more
SUBSTITUTE SHEET RULE 26~
WO 94/19534 PCT/US94/01943
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detail previously herein, the gaseous pressure
above the plate 122 pushes downwardly on the plate
122 to cause it to press against the pulp slurry
and assist in the dewatering operation. At the
same time, gaseous pressure acts through the
openings 132 directly against the pulp slurry.
This pressure differential is maintained until the
desired amount of liquid has been removed from the
pulp slurry to form the pulp mat.
As soon as the dewatering step is completed,
then the troughs 32 are tipped sequentially to
deposit the wash liquid onto the pulp mat. The
wash cycles are accomplished as described above.
When the final wash cycle is completed, then
both the enclosure frame 26 and the contact plate
122 are raised, the valves 114 that are open are
then closed and the equalizing valve 118 is opened
to raise the pressure in the table unit chamber 96
to the level in the vessel 12. With the pressure
20_ being so equalized, the only force bearing on the
upper surface of the conveyor belt 34 is the
weight of the pulp mat. Then the conveyor belt 34
is moved to cause the pulp mat to move into the
discharge assembly 40. Then the discharge
assembly is operated as described previously
herein to move the pulp mat out of the pressure
chamber of the vessel 12.
A modified form of the discharge assembly 40
is illustrated in Figures 19 through 21. To
distinguish this modified form and the components
thereof, like numerical designations will be used
for components similar to the discharge assembly
SUBSTITUTE SHEET (BHLE 26~
PCTIUS94/01943
WO 94/19534 '
;° v
s.._
-34-
40, with an "a" suffix distinguishing those of the
modified form.
As in the original version of the discharge
assembly 40, the present assembly 40a has a
guideplate 180a positioned above the belt 34.
This plate 180a with the belt 34 defines an inlet
184a, and has a downward and forward slope. The
longer inlet passageway 186 as shown in the first
version described previously herein is
substantially eliminated, and the inlet 184a
actually is sufficiently short so that it does not
have a clearly defined inlet and outlet. It will
be noted that this inlet 184a leads substantially
directly into the chamber defined by the housing
192a. Further, the area 220 immediately below the
belt 34 immediately adjacent to the inlet 184a is
at the pressure level within the pressure vessel
so that the gaseous medium below the belt acts to
move the pulp from the belt 34 directly into the
interior of the housing 192a (this being
illustrated in Figure 20.) The doctor blade 206a
is positioned at a further forward location so
that the pulp mat is actually lifted off the more
forward part of the belt 34 when the pressure
differential is applied before it reaches the
doctor blade 206a.
With reference to Figure 21, it can be seen
that at the expanded portion 195 of the housing
192a, there is a venturi section 222 that leads
from the housing 192a and to the valve 196a. As
will be described later herein, this venturi
section 222 accelerates the pulp which is being
blown from the housing 192a through the throat
SUBSTITUTE SHEET (RULE 26~
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portion 224 of the venturi, after which it
recovers its static pressure. In the system of
the present invention, when a gaseous medium, such
as oxygen (02) or ozone (03) is used in subsequent
bleaching, this can be introduced into the system
at the location of the venturi 224 enhances the
mixing process and acts as a shredder for the
discharged pulp.
It will be noted that the aforementioned
valve 200 and pipe sections 198 and 202 of the
first version of the discharge assembly 40, are
eliminated in this modified version, since the
gaseous medium from within the pressure vessel
enters directly into the entry portion 184 to blow
the pulp toward the exit end where the valve 196a
is located.
A modified form of the contact plate 122 is
illustrated in Figures 22 through 24. This
modified plate 250 differs from the plate 122 in
that instead of providing V shaped grooves 126,
there is provided a plurality of conically shaped
recesses 252 arranged in a square pattern. The
plate 250 has a plurality of through openings 254
arranged in a regularly spaced squyare pattern,
extending from the upper plate surface 256, each
to an upper apex location 258 of a related recess
252. It can be seen that each recess 252 is
defined by a surface 260 having the configuration
of a truncated cone, with the lower edges of each
fusto-conical surface 260 overlapping with one
another, so that the juncture line to adjacent
surfaces 260 each define a crescent shaped lower
edge 262. Thus, it can be seen that liquid
SUBSTITUTE SHEET (RULE 26~
CA 02156523 2004-06-O1
-36-
flowing through the opening 254 is able to flow
down along each frusto-conical surface 260 in an
expanding downward pattern.
C. The First Embodiment of the System of the
Present Invention
This first embodiment of the system of the
present invention is described in an earlier U.S.
patent application on which this application is
based and is thus an earlier design. Since that
design was originated there have been further
modification and improvements, and these are
incorporated in the second embodiment which is
described in Section E of this text. While the
first embodiment described in this section is
believed to be a significant advance over the prior
art, the later embodiment described in Section E is
presently believed to be the preferred embodiment
of the present invention.
The system of the present invention is
illustrated somewhat schematically in Figure 25.
In the particular embodiment of the system shown
herein, there are seven washing stations, (each of
which has a dewatering/washing apparatus 10 as
described above) and six processing stations
interspersed with the washing stations. In the
particular arrangement shown herein, in the first
processing station, the pulp is treated with oxygen
(Oz); in the second and fifth processing station the
pulp is treated with ozone (03); and in the third,
fourth and sixth processing station, the pulp is
treated with hydrogen peroxide (HZ
WO 94/19534 ~ PCT/US94/01943
-37-
02). However, it is to be understood, of course,
that the number of washing stations and processing
stations, and also the particular treatment given
the pulp at any particular processing station
could be varied, depending upon the type of pulp
being processed, the end pulp product desired, and
other factors. Thus, the processing compounds
specified, the specific values specified and the
steps presented in this description of the system
of Figure 25 are intended to relate to one
preferred embodiment, and obviously modifications
could be made.
Also, in describing the system of Figure 25,
there will be given specific numerical values as
to the consistency of the pulp at various stages,
relative units of different portions of the liquid
and the pulp, certain pressure levels and
temperatures, and other detailed processing
information. This is done to present what is
presently believed to be a preferred embodiment
that is generally representative of the type of
system that would be contemplated for commercial
use, and is not intended to be limiting. Further,
these values are given so that a clearer
understanding of the operating advantages of the
system of the present invention over the prior art
can be obtained. However, further analysis and
development may indicate other numerical ranges or
values may be preferred.
To describe now the system of the present
invention as shown in Figure 25, the seven
dewatering and washing stations are designated W-1
through W-7, and the six processing stations are
SUBSTITUTE SHEET ~RU~.E 26~
PCTIUS94/01943
WO 94/19534 ~ ~ ~ ~ '~
-38-
designated P-1 through P-6. For convenience, the
dewatering .and washing stations~will simply be
referred to each as a "wash~ing~station", since the
major portion of the equipment (i.e. the apparatus
10) at each of these stations W-1 through W-7 is
normally referred to as "a washer". As will
become apparent, however, from the following
detailed description, at each washing station
there is accomplished a dewatering, and also the
introduction of the processing chemicals or
compounds and/or bleaching agents into the wood
pulp for treatment of the pulp during the
retention period at the processing station.
The pulp from a digester (not shown in Figure
25) is introduced into the initial washer 10-1, a
preferred embodiment of which is disclosed above
as the apparatus 10 and shown in Figures 2 through
24. In this particular embodiment, the
consistency of the pulp slurry being directed to
the washer 10-1 is specified as 1.75 (but this
could be varied). At such consistency, for every
one part of pulp fiber, there would be two parts
of liquid within the fibers themselves, and fifty
six parts liquid between and around the fibers.
This slurry is first dewatered and then subjected
through a number of wash cycles in the washer 10-
1, as described previously herein in the more
detailed description of the apparatus 10.
During the last wash cycle, the liquid that
is deposited on the pulp mat contains sodium
hydroxide. This liquid with sodium hydroxide
remains in the pulp mat, which is then discharged
from the washing apparatus 10-1 to be directed
SUBS1'lTUTE SH~ET RULE 26~
WO 94/19534 PCT/US94/01943
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toward the top of a first pressurized processing
vessel V-1. The sodium hydroxide raises the pH
level of the pulp mat (e.g. to a pH of 9-11) so
that the oxygen to be introduced reacts in the
desired manner r~ith the pulp.
The pulp being discharged from the washer 10-
1 then has the oxygen (02) introduced therein
(desirably in the form of liquid oxygen which
vaporizes and permeates throughout the discharged
pulp). This is desirably accomplished by placing
in the discharge conduit leading from the
discharge valve 196 of the discharge housing 192 a
tube having a venturi section which enhances the
mixing with the oxygen, and the oxygen is
introduced at the location of the venturi. This
pulp, containing the sodium hydroxide, and also
having the oxygen introduced therein is then
directed into the top of the pressure vessel V-1.
The pressure in the vessel V-1 is maintained
at a level moderately below that in the chamber of
the washer 10-1. For example, with the pressure
in the chamber of the washer 10-1 being nine bars
(i.e. nine times atmospheric pressure), the
pressure in the vessel V-1 could be about six
bars. Thus there would be a three atmosphere
pressure drop so as to drive the pulp from the
washer 10-1 to the vessel V-1. The temperature
_ maintained in the vessel V-1 is desirably at a
relatively high temperature (e. g. 160°C), this
being made possible by the relatively high
pressure maintained in the vessel V-1.
The pulp in the vessel V-1 is extracted on a
continuous basis from the lower part of the vessel
SUBSTITUTE SHEET (RULE 26)
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V-1, so that the pulp depositedin the top part of
the vessel V-1 gradually,descends toward the
bottom. The timing is such that the total
retention time in the vessel V-1 for any one
portion of pulp would be, for example, forty
minutes. Liquid from the washer 10-2 is
introduced into the lower part of the vessel V-1
to decrease the consistency of the pulp to, for
example, twelve and one half percent in the lower
part. Thus, for approximately the last ten
minutes of this forty minute retention period, at
twelve and one half percent consistency there is
one part pulp fiber, two parts liquid within the
fibers themselves, and five parts liquid
surrounding and between the fibers.
One of the main reasons for introducing this
liquid into the pulp in the lower part of the
vessel V-1 is that this will cause a dilution of
the liquid in the pulp fibers that carry the
oxidized residue therein. During the retention
period in the upper part of the vessel V-1, the
oxygen is reacting with the material in the
fibers, and since the consistency of the pulp is
very high in the upper portion of the vessel V-1,
for example twenty five percent, there is little,
if any, liquid surrounding the fibers to cause the
oxidized residue within the fibers to move out
from he fibers. However, when the liquid is added
' in the lower part of the vessel V-1 to lower the
consistency, the liquid within the fibers, having
a high concentration of the oxidized residue, has
the contained residue diffused outwardly through
the fibers into the surrounding liquid.
SUBSTITUTE SHEET (RULE 2G~
WO 94/19534 PCT/US94/01943
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The pulp which is discharged from the lower
part of the vessel V-1 is then mixed with liquid
circulated from the second washing apparatus 10-2
so that the consistency of the pulp drops to a
1.7~ level. This pulp in turn is introduced into
the headbox 20 of the second stage washing
apparatus 10-2.
It should be noted that a small amount of
liquid is being added to the washer 10-1 at the
location indicated at "a". This portion of liquid
is taken from the liquid outflow from the second
washer 10-2 and recycled in an "upstream"
direction (i.e. to the washer 10-1). The liquid
from the second stage washer 10-2 is cleaner than
the liquid being recirculated in the first stage
washer 10-1, and this flow from the second stage
washer 10-2 works to replenish the washed liquid
in the first stage washer 10-1, so as to keep the
impurities at a sufficiently low level. Also,
part of this portion of liquid being recirculated
at "a" in the washer 10-1 is liquid that is mixed
with the sodium hydroxide which is introduced into
the pulp in the very final washing stage. This is
accomplished so that if, for example, 1.5 units of
the liquid is recirculated from washer 10-2 to the
washer 10-1 for each unit of pulp, 0.5 units of
this liquid is introduced into the wash water, and
., the remaining 1.0 units of this liquid is
separately mixed with the sodium hydroxide so that
it moves into the mat at the very end of the
dewatering/washing process to be retained, in the
pulp mat, after which the pulp mat is moved from
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the washer 10-1 to be mixed with~:the oxygen and
then moved into the first stage~wessel V-1.
The consistency of the pulp which is
discharged from the first washer 10-1 is at about
twenty five percent, this being one part pulp
fiber, two parts liquid retained within the pulp
fibers, and one part of the liquid having the
sodium hydroxide outside and between the pulp
fibers.
As mentioned above, the pulp slurry
discharged from the vessel V-1 is mixed with
liquid from the washer 10-2 so as to be at about a
1.7~ consistency, and is introduced into the
second washer 10-2. The dewatering is
accomplished in the washer 10-2 as described
above, and at the end of the dewatering, a small
amount of fresh wash water is introduced into the
pulp mat. A second wash liquid containing fresh
water and sulfuric acid (H2 S04) and "EDTA" (an
abbreviation for ethylene diamine tetra acidic
acid) is then added. In this specific embodiment,
it is indicated that 1.5 units of fresh water are
added, with 0.5 of these units being introduced at
the very end of the dewatering cycle so as to be
introduced along with some of the displaced liquid
from the pulp into the first washer 10-1 at
location "a". The one part of this fresh water
(having the sulfuric acid and the EDTA) remains in
the pulp and is conveyed into the next processing
station P-2.
The sulfuric acid serves to adjust the pH of
the pulp to be compatible with the subsequent
SUBSTITUTE SHEET (RULE 26)
WO 94119534 PCTIUS94/01943
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ozone bleaching step. The "EDTA" is added to stop
the metal ions present from reacting with the
ozone. The pulp discharged from the second washer
10-2 has a consistency of about twenty five
percent (one part pulp fiber, two parts liquid
contained in the pulp fiber, and one part liquid
containing the sulfuric acid and the "EDTA"
outside and between the fibers). This.is mixed
with ozone (03) as the pulp is being discharged
from the discharge assembly 40 of the washer 10-2,
and this is in turn directed into the second
pressure vessel V-2. As the pulp in the vessel V-
2 descends gradually downwardly, the ozone reacts
with the pulp fibers.
The total dwell time in this second vessel V-
2 is about ten minutes. The pressure is at about
six bar and the temperature at 70°C. In the first
two minutes, the pulp is at twenty five percent
consistency, and in the second eight minutes fluid
from the third washer 10-3 is directed into the
lower part of the vessel V-2 to lower the
consistency to twelve and one half percent. As
indicated above, with reference to the first
vessel V-1, this is done so that the oxidized
residue in the pulp fibers can diffuse outwardly
into the liquid surrounding the pulp fibers. The
pulp discharged from the second vessel V-2 is
- treated in substantially the same manner as that
discharged from the first vessel V-1. One
difference is, however, that the additional liquid
introduced at the location "b" into the washer 10-
3 is from a corresponding location "b" which is
the discharge from the sixth washer 10-6. It
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-44-
should be noted that this liquid is derived from
the washer 10-6 as displaced liquid from the pulp
that has just been treated in the second ozone
processing vessel V-5.
The pulp from the second vessel V-2 is
diluted with liquid from the third washer and
directed into the third washer 10-3. The pulp in
the third washer 10-3 has sodium hydroxide and
hydrogen peroxide added thereto, and is then
directed into the top of the third vessel V-3.
The total dwell time in this third vessel is
approximately three hours and ten minutes; the
pressure six bar and the temperature 90°C. For
the first three hours, the pulp remains at a
twenty five percent consistency, and for the last
ten minutes liquid is introduced from the fourth
washer 10-4 to lower the consistency to twelve and
one half percent.
Essentially the same process as described
before is repeated with respect to the fourth,
fifth and sixth washing station (W-4 through W-6).
In the fourth washer 10-4, sodium hydroxide and
hydrogen peroxide are again added into the final
processing as in the washing station W-3, and the
liquid that carries these compounds along with
supplying some replenishing liquid (as indicated
above approximately 1.5 units of such liquid) is
derived from location "c" which is the discharge
from the fifth washer 10-5. The fourth vessel V-4
thus also is a peroxide bleaching vessel and has
the same processing conditions as the vessel V-3.
In the fifth washer 10-5, liquid is indicated
as being added at "d", and this is from the
SUBSTITUTE SHEET (RULE 26}
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WO 94/19534 PCT/US94/01943
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location "d" leading from the seventh washer 10-7.
Added to a portion of this water introduced at "d"
into the fifth washer 10-5 is sulfuric acid and
EDTA. The pulp from the fifth washer 10-5 is
directed into the fifth processing vessel V-5,
which is an ozone bleaching vessel, and is
processed as in the second vessel V-2.
Accordingly, ozone is introduced into the pulp
that is being discharged from the fifth washer 10-
5 and is going in to the fifth vessel V-5.
In the sixth washer 10-6, fresh water is
added, with one part of this fresh water having
sodium hydroxide and peroxide added thereto. The
pulp is then directed into the sixth pressure
vessel V-6. The processing conditions in this
vessel V-6 are substantially the same as in the
other two peroxide bleaching vessels V-3 and V-4.
The pulp from the sixth vessel V-6 is then
delivered to the final washer 10-7 of the washing
station W-7, where in the final stage, fresh water
is added, with S02 water being added in the final
wash cycle. A portion of the liquid from the
washer 10-7 is directed (as indicated at "d") into
the washer 10-5. The fully bleached pulp taken
from the final washer 10-7 is then directed to a
location for further processing.
To further illustrate certain aspects of the
. present invention, reference is made to Figure 26
which shows only a single washing station and a
single processing station. For convenience, the
washing station W-3 and processing station P-2
have been selected. Thus, there is shown the
washing unit 10-3 and the pressure vessel V-2.
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WO 94/19534 ~ PCT/LTS94/01943
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There is a first input line 300 which delivers one
unit of fresh water liquid having 'the sodium .
hydroxide and hydrogen peroxide contained therein.
There is a second input line 302 through which the
0.5 units of fresh water is added. As described
above, the 0.5 units of fresh water is introduced
into the pulp at the very end of the dewatering
process. Immediately thereafter, the one unit of
fresh water with the sodium hydroxide and hydrogen
peroxide is introduced in the pulp, and is
retained therein for delivery into the third
processing station P-3. For the other washers 10-
2 and 10-4 to 10-7, the liquid and treating
chemicals and/or compounds introduced will be
those designated in Figure 25.
There is a first discharge line 304 which
leads from the washer 10-3 and directs liquid from
the washer 10-3 into a holding tank 306. A line
310 leads from the holding tank 306 to a pump 312,
and a line 314 leads from the pump 312 to a pump
316 that directs liquid into the washer 10-3.
Another line 318 directs the pulp from the lower
end of the pressure vessel V-2 to join with the
liquid in the line 314 to be mixed therewith and
be directed to the pump 316 for introduction into
the washer 10-3.
The flow through the line 304 into the
holding tank 306 is intermittent, in accordance .
with the operation of the washer 10 as described
previously herein. On the other hand, the
operation of the pump 312 and the pump is
substantially continuous so that there is a
substantially constant flow of the pulp slurry at
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 PCT/US94/01943
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about 1.7 percent consistency into the headbox of
the washer 10-3.
Also, another line 320 leads from the holding
tank 306 to a pump 322, and this directs a smaller
portion of the liquid (i.e. one and one half units
of liquid for each unit of pulp) from the tank 306
to be discharged to the evaporator. For the other
washers 10-2 and 10-4 to 10-7 this 1.5 unit of
liquid would either be recirculated or discharged
for evaporation as indicated in figure 25.
Another line 324 leads from the washer 10-3,
and this is directed into a second holding tank
326. The liquid that is directed into this line
324 is the liquid which is 3.5 units of liquid
that flows through the pulp during the very last
portion of the dewatering of the pulp, along with
the 0.5 units of fresh water introduced through
the line 302. Thus, there is relatively clean
liquid in this holding tank 326, and this liquid
is drawn from the tank 326 by a pump 328 and
thence through a line 330 into the diffusion zone
332 of the pressure vessel V-2.
It can be seen with reference to Figure 25
that the only effluent which is discharged from
the system shown in Figure 22 is that which is
discharged at washing stations W-3 and W-4,
indicated at "e" and "f". Further, it can be seen
- that the amount of this effluent from each of the
washing stations W-3 and W-4 is 1.5 tons for each
ton of pulp, making a total of 3.0 tons of
discharged effluent for each ton of pulp that is
processed. This is a sufficiently small quantity
of effluent so that it can economically be
SUBSTITUTE SHEET (ROLE 26~
WO 94/19534 ~ ~ ~ ~ ~ PCT/US94/01943
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directed to an evaporator. Accordingly, there is
no net discharge of liquid e~.fLuent from the
system into a sewer line. ~~
In contrast to this, as discussed earlier
herein, in the prior art system shown in Figure 1,
the amount of discharged effluent is in the order
of twenty five or even as high as seventy five
tons, depending on how many bleaching stages there
are and the end brightness desired. In that prior
art system, it is not economically feasible to
evaporate the effluent. First, the amount of
effluent is sufficiently high so that the
evaporation cannot be done economically. Further,
since the bleaching agents in the prior art system
of Figure 1 are chlorine and chlorine dioxide, it
would be necessary to contain these in vessels or
conduits made of titanium. On the other hand, in
the present invention, the evaporation could take
place in stainless steel containers.
Also, with reference to Figure 25, it can be
seen that for each ton of pulp that is processed,
four and one half tons of fresh water are used.
Three tons of this fresh water are in the effluent
which goes to the evaporator. One ton of this
fresh water remains in the pulp that is discharged
from the final washer 10-7. Only one half ton of
this fresh water goes as recirculated liquid to
the brown stock washer 10-1 and is then discharged
with the black liquor to the evaporator. Thus,
there is from the first stage brown stock washer
10-1 only a dilution factor of 0.5.
D. Further Discussion of the Overall
Dewatering/Washing/Bleaching Operation
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WO 94/19534 PCT/US94/01943
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and Particularly the Washing Operation
Subsequent to the design of the first
embodiment of the system of the present invention
described in Section C of this text, further
analysis was conducted relative to the system of
the present invention. This led to the design of
the second presently preferred embodiment of the
system of the present invention which will be
discussed later in Section E. It is believed that
a better appreciation of some of the significant
features of the present overall invention will be
obtained by analyzing at this time in more detail
the dewatering/washing/bleaching operation and in
particular the washing operation accomplished by
the washing apparatus which was described in
Section B of this text. After that, the overall
system of the second embodiment will be described
in Section E, and also benefits derived from that
system.
In a pulp processing operation, the digester
pulp in the digester is treated with various
chemicals to break down various organic material,
and the resulting effluent from the digester is a
mixture of the black liquor and pulp. Typically,
the consistency in the digester would be about
twelve and one half percent, which means that
there is one part cellulose fiber by weight and
seven parts black liquor. The black liquor that
leaves the digester is a solution containing about
' 30 twenty five percent dissolved solids, with about
half of these solids being organic compounds and
the other half various chemicals used in the
digesting process. The organic compounds include
SUBSTITUTE SHEET (RUtE 26~
PCTlUS94/01943
W094/19534~~~~~~~
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. ..
,, . '
partially fragmented lignin, cellulose and hemi-
cellulose degradation products and their
solubilized derivatives, and other reaction by-
products such as sodium soaps. The inorganic
material in the black liquor is primarily cooking
chemicals in ionic form.
After the black liquor is separated from the
pulp, it is usually designated as a "weak black
liquor". This contains about fifteen percent
dissolved solids. Normally, the weak black liquor
goes to a recovery operation (i.e. an evaporator
recovery boiler, etc.) for reclamation of
chemicals and reformation into cooking liquor.
The organic material that is dissolved in the
black liquor is burned at the mill in a recovery
boiler to recover the energy contained therein and
the noncombustible portion is sent to chemical
recovery.
The pulp with the black liquor from the
digester is initially directed to the brown stock
washer which first "dewaters" the pulp to remove a
large percentage of the black liquor, and then
subjects the pulp to several washing steps to
separate a high percentage of the remaining
dissolved organic and inorganic chemicals from the
pulp fibers. The organic material that remains in
the pulp after dewatering and thus travels through
subsequent processing steps are sources of
biological oxygen demands, chemical oxygen demand,
and color in pulp mill effluent streams. In
addition, poorly washed pulp is a source of
precursors for dioxins and chlorinated furans
during the bleaching operation.
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 PCT/US94/01943
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Since the organic material in the black
- liquor is a source of energy and much of the
inorganic material in the black liquor is a source
of digesting chemicals, improving washing
efficiency improves energy and chemical recovery.
Also, organics that remain with the pulp require
increased amounts of chemicals to accomplish the
bleaching.
The pulp material itself is made up of
cellulose fiber which is a heterogeneous mass of
several layers of cellulose molecules (lamellae)
in a large central cavity (lumen). Cellulose
fibers also bond to each other, creating a network
of fibers and cavities. When suspended in water,
the fiber network swells, and movement of liquid
from one cavity to another is relatively easy.
The communication among the lumen cavity, the
liquor between the lamellae, and the liquor
surrounding the cellulose fiber is limited to what
can pass through the pores of the fiber wall.
Liquor outside the fiber can be displaced or mixed
with cleaner liquor by breaking up the fiber
network. However, the liquor inside the fiber can
only be affected by diffusion through the pores of
the fiber wall.
Diffusion occurs whenever there is a
difference in liquor concentration between the
inside and the outside of the fiber. Thus, when a
cleaner liquid is passed through the pulp, and the
- 30 liquor inside the pulp has a higher concentration
of solids, there is a diffusion to cause the
liquid in the fibers and surrounding the fibers to
move out (with the liquid outside the fibers
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 ~ ~, ~ ~ ~ ~ '3 PCT/US94/01943
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moving into the fibers) toward an equilibrium
where the liquid inside arid.~butside the fibers has
the same solids concentration.
The speed at which diffusion takes place is
dependent on a number of factors, namely:
(i). The difference in concentration
of solids between the inside and
outside of the fiber,
(ii). temperature,
(iii). turbulence in the liquor
surrounding the fiber, and
(iv). the size of the molecules that
are subject to diffusion.
The rate of diffusion is highest when the
difference in concentration is large, the
temperature is high, the turbulence is intense,
and the molecule undergoing diffusion is small.
The washing of the pulp fibers occurs by a
combination of displacement washing (moving a
cleaner liquid through the pulp mat to displace
the liquor that is outside of the fibers), and
diffusion processes (i.e. the migration of the
more concentrated liquor inside the fibers to a
location outside of the fibers.
An ideal displacement washing would have a
plug of wash liquid moved through the stationary
pulp pad, completely replacing the previous
solution. However, real washing processes exhibit
several departures from this ideal condition. For
example, there can be viscous fingering and axial
diffusion or back-mixing. Some of the solute will
be contained in stagnant areas between fibers, in
the fiber lumen, and within the fiber walls. This
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WO 94/19534 . PCT/US94/01943
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material must diffuse out of the stagnant areas
~ before it can become available for displacement.
Viscous fingering, axial diffusion and even
diffusion from stagnant areas between the fibers
all occur on a relatively short time scale.
There are typically two parameters that are
used to determine the performance of washing
systems. One is the dilution factor (DF), and the
other is the displacement ratio (DR). The
dilution factor is the amount of water over that
is required for total displacement. The equation
for the dilution factor is as follows:
DF = (Ws - Wm) /Wp
where
Ws = mass of shower liquor
Wm = mass of liquor in mat leaving washer
Wp = mass of dry pulp.
The displacement ratio is the ratio of the actual
solids reduction in a single stage to the maximum
possible reduction in that stage. The equation
for the dilution factor is as follows:
DR = ( Ce - Cm) / ( Ce - Cs )
where
Ce = liquor solids concentration entering
washer
Cs = shower liquor solids concentration
Cm = solids concentration in mat liquor
leaving washer
It was indicated earlier in this text that
the pulp fibres that are in the digester and are
then processed in the dewatering/washing apparatus
contain within the fibers two parts liquid by
weight for one part cellulose fibre. Thus, for
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PCT/US94/01943
WO 94/19534
-54-
one ton of cellulqs~.~~~ibre (this being the fibre
that would remain after it has been thoroughly
washed and oven dried), there are two tons of
liquid within the pulp fibres.
To illustrate this, reference is made first
to Figures 27A, 27B and 27C which illustrates a
single wood fibre 400 where the total batch of
pulp is at a ten percent consistency. This means
that for one part of fibre material, there are
nine parts liquid (including the material
dissolved in the liquid). Since each fibre 400
has within its outer shell 402 two parts liquid
404 for each part of fibre by weight, at ten per
cent consistency there are seven parts liquid
(indicated at 406) outside the fibre. The
currently commercially used types of dewatering
and washing apparatus are such that, the
consistency of the wood pulp after it is dewatered
is commonly between ten and thirteen percent, so
Figures 27A, B and C are reasonabily
representative of the pulp after dewatering in
accordance with the prior art.
Now let us examine what occurs during a
single washing step. As the wash liquid moves
into the layer of dewatered fibre, it displaces
that portion of liquid 406 that is outside of the
fibres. Let us assume in this instance that the
wash liquid is pure water with no dissolved solids
therein, and the liquid 404 and 406 inside and
outside the fiber 400 is black liquor have 25% .
dissolved solids therein. As soon as the first
wood fibers 400 at the top of the dewatered pulp
are surrounded by the fresh water, the liquid 404
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(with the dissolved solids) within the fibre 400
starts to diffuse outwardly into the surrounding
fresh water 406, to come nearer to equilibrium
(relative to the percentage of the dissolved
material) with the liquid 406 surrounding the
ffibre.
In this instance, if (as assumed) the liquid
within the fibre and initially surrounding the
fibre is black liquor at 25% dissolved solids
content (i.e. three parts liquid and one part
dissolved solids), and assuming that the liquid
surrounding the fibre is totally displaced with
fresh water (not having any dissolved liquids),
and if diffusion of the liquid within the
cellulose fibre 400 with the surrounding wash
water would be such so that complete equilibrium
is reached, then the liquid within each fibre 400
and outside the fibre 400 would have 5.5%
dissolved solids, and the liquid, surrounding the
fibers would have that same concentration of
solids.
Thus, since the dewatering/washing apparatus
that is commonly used in the prior art dewaters
the pulp material to approximately a ten percent
to a thirteen percent consistency (as indicated
above), the situation shown in figures 27A, B and
C is typical of what would occur during the
initial washing step at the topmost portion of the
pulp material if diffusion were complete and
complete equilibrium would occur.
Now let us turn out attention to Figures 28A,
B and C which represent what would occur in an
ideal situation during the washing of pulp fibres
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where pulp after dewatering i~.:~at twenty five
. 4'~,
percent consistency (three.p'arts total liquid,
w .
including dissolved solids,~to one part cellulose
fibre). Let us also assume that the liquid within
the fibre and that surrounding the fibre is black
liquor having three parts liquid to one part
dissolved solids (i.e. 25% dry solids content).
Since (as discussed above) the liquid within
the fibre 400 is twice the weight of the cellulose
fibre itself, there is one part by weight wood
fibre, two parts by weight liquid 404 (including
dissolved solids) inside the fibre 400, and one
part liquid at 406 surrounding the wood fibre.
Let us now assume that during an initial step
in the washing process, the liquid 406 surrounding
the fibre 400 is totally displaced with fresh
water (with no solids dissolved therein), this
occurring when the first portion of fresh wash
water encounters the uppermost fibres 400 in the
pulp mat. With the pulp being at twenty five
percent consistency there will be for each fibre
400 only one part of fresh water surrounding the
individual fibre. When two parts of the liquid
within the fibre 400 diffuses outwardly, and the
one part of the liquid outside the fibre diffuses
inwardly into the fibre 400, if complete
equilibrium is reached, there will be two 16 2/3%
dissolved solids throughout the liquid ((2 x
0.25)/3= 16 2/3).
To further pursue this analysis, reference is
now made to Figure 29A where there is shown
schematically a layer of pulp 408 which has been
dewatered in a brown stock washer of the present
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WO 94/19534 PCT/US94/01943
-57-
invention to a twenty five percent consistency
(i.e. three parts liquid for every one part
fiber), and that each unit of liquid (i.e. black
liquor) comprises twenty five percent dissolved
solids. Let us create a hypothetical situation
and assume that a quantity of pure wash water
(without any dissolved liquids) is placed on top
of the pulp mat 408, and for purposes of analysis,
let us divide the pulp mat 408 into ten layers
numbered one through ten.
Let us further assume that the pulp layer
comprises one ton of dry cellulose fiber and that
one ton of wash liquid is placed on top of the
pulp, after which the pressure differential is
applied to move the wash water into the pulp and
to displace the liquid that surrounds the pulp
fibers. Since the pulp is at twenty five percent
consistency, there is two tons of liquid inside
the pulp fibers and one ton of liquid outside of
the pulp fibers. Let us also assume that the
first one tenth of the wash water moves into the
top one tenth of the pulp mat.
For purposes of analysis, let's first assume
a further idealized situation where as soon as
that one tenth of that wash water enters the top
tenth of the pulp mat, there is complete diffusion
so that the liquid inside the pulp fibers has the
same percentage of dissolved solids as the liquid
outside of the pulp fibers. As indicated in the
prior analysis, this would mean that the liquid
inside and outside the pulp fibers would have
sixteen and two-thirds percent dry solids
dissolved in the liquids.
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Now let us assume this same first one tenth
of this wash liquid now at 16 2/3%~ dry solids
content moves downwardly into the second one tenth
thick layer of pulp. Again, the liquid inside the
pulp fibers has twenty five percent dry solids
content. Let us assume again that there is
immediately complete diffusion so that there is
complete equilibrium between the liquid inside the
fibers and outside the fibers. Analysis indicats
that the liquid and the percent of dissolved
solids in the liquid would be 22.2 percent
[(2x25)+(1x16 2/3)]. When this same first one
tenth of the wash water moves into the third layer
and assuming complete equilibrium is achieved with
the liquid inside the fibers in that third layer,
the percentage of dissolved solids in the wash
water would further increase.
[(2x25)+1x22.2)=24.1)].
It can readily be seen from the above
analysis that the first one tenth increment of the
wash water obtains a higher and higher percentage
of dissolved solids as it proceeds downwardly
through the layer of pulp fiber.
It is also apparent that when the second one
tenth increment of wash water moves into the top
one-tenth layer of pulp, that second increment of
wash water (without any dissolved solids therein)
is encountering the first layer of pulp which has
only 16 2/3% dissolved solids content. As this
second increment of wash water proceeds downwardly
through the pulp mat, its percentage of dissolved
solids increases, but not as rapidly as the first
one-tenth increment of wash water.
SUBSTITUTE SHEET (RULE 26~
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These percentage changes have been calculated
and are presented on the graph of 29A. It can be
seen that on the vertical axis, there is plotted
the pulp mat thickness in one-tenth increments.
On the horizontal axis, the percentage of dry
solids present in the liquid is given. It can be
seen that if only one half of a ton of wash water
is used per one ton of pulp, a fair percentage of
the dry solids have been removed from only the top
half of the pulp mat, but the wash water has only
progressed half way into the mat. When a full one
ton of wash water is used, then the second curve
indicates that removal of dry solids is
substantial in the top half of the mat, but
diminishes to almost zero when the wash water
reaches approximately the ninth increment of one-
tenth layers of pulp. However, when three tons of
wash water is used, it can be seen that there has
been a significant removal of the dry solids and
there is a displacement ratio of 0.898. If 3.3
tons of wash water is used, the displacement ratio
goes to 0.936. It is to be emphasized that this
analysis was done for a hypothetical idealized
situation.
In Figure 29B, this same analysis was
performed, but with the consistency of the pulp
mat being ten percent, which would be typical of a
prior art washer such as the Chemi-Washer. As
will be recalled from the analysis given
' 30 previously in this text, this would mean for every
one ton of pulp fiber, there are two tons of
liquid within the fibers, and seven tons in the
mat outside of the fibers. It can be seen that
SUBSTITUTE SHEET RULE 26~
WO 94/19534 PCTlUS94/01943
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when 9.7 tons of water is used, there is a
displacement ratio of only 0.8~4~.~ From the graph .
immediately above at 29A, it~,can b'e seen that with
the use of only three tons of water, there is a
displacement ratio of 0.898.
It should be understood that Figures 29A and
29B were calculated where an ideal situation was
assumed in which there is immediate and complete
diffusion of dry solids from inside the pulp
fibers to the surrounding filtrate, and pure wash
water was used in the initial washing step and the
wash water was assumed to ideally displace the
liquid outside the fibres without any mixing with
it. Further calculations were run to simulate
what would happen under actual operating
conditions. These are illustrated in Figures 30A
and 30B. The thick lines which are shifted
somewhat to the right in comparison with the thin
lines represent the percentage of dry solids in
the liquids inside the fibers, and the thin lines
somewhat to the left of the thick lines represent
the dry solids in the filtrate outside of the pulp
fibers.
The values given in Figure 30A represent what
computer analysis indicates would occur during a
complete washing using the washer described in the
text of this present application with five
countercurrent passes of the wash water through
the pulp mat after the dewatering. It is assumed
that 3.3 tons of wash water is used for each ton .
of pulp, and that the same wash liquid is recycled
(countercurrently four times for subsequent
washing cycles. To clarify the terminology in
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ ~ s PCT/US94/01943
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each washing cycle, there are five washing steps
where 3.3 tons of wash water is passed through one
ton of pulp five times. On a first washing cycle
the totally clean water would be passed through
the pulp mat on the very last (i.e. fifth) washing
step. The prior four washing steps for that cycle
would use dirtier wash water that was used in
prior washing cycles. Then in the next cycle
where the pulp mat is being washed, the recycled
water from the fifth step in the previous cycle
would now be used for the fourth washing step. In
this manner, the wash water is recycled
countercurrently through four washing cycles, and
as that particular batch of wash water becomes its
"dirtiest", it is used in the very first washing
step on the fifth cycle.
Figure 30B represents what a computer
analysis indicates would be achieved in the
washing operation using the prior art Chemi-
Washer. It is assumed that ten tons of fresh wash
water is used for one ton of pulp fiber. There
are shown seven washing steps for each cycle.
Also, in this instance, the fresh water is
countercurrently recycled for use on subsequent
cycles so that as the wash water has a higher
percentage of solids, it is used for a yet earlier
washing step in subsequent cycles.
' The values showing the performance of the
washer of the present invention and the prior art
' 30 Chemi-Washer are printed on the graphs of Figures
30A and 30H. It can be seen that for the washer
of the present invention, the dry solids content
for the liquid inside the fibers is at the average
SUBSTITUTE SHEET (RULE 26)
~1~~~23
WO 94/19534 ~ , . PCTIUS94/01943
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of 0.22 percent at the completion of the cycle,
while the dry solids content for the liquid
outside the fibers is 0.16 percent. For the prior
art Chemi-Washer, the dry solids content for the
liquid inside the fibers is 0.53 percent, while
the dry solids content for the liquid outside the
fibers is 0.13 percent.
It is believed the significance of the above
analysis will become more apparent from the
following description of the second embodiment of
the system of the present invention.
F. Second Embodiment of the System
of the Preseat Iaveation (Figures 31-39)
This second embodiment is similar to the
first embodiment in several respects. In this
second embodiment there is an initial brown stock
washer and six additional washers, each of which
is utilized after a related one of six bleaching
operations. There are six bleaching stations, and
these six bleaching stations utilize at each
station the same bleaching agents as in the first
embodiment. (In this description the washer
following each bleaching apparatus is simply
condsidered to be part of that bleaching station.)
To distinguish these components of the second
embodiment components from those of the first
embodiment, numerical designations will be used,
and letters will be added to distinguish different
locations in the system. The basic system in the
second embodiment is disclosed in seven drawings,
Figures 31-37. Figure 31 discloses the initial
brown stock washing station where there is a
digester 500 and a brown stock washer 502.
SUBSTITUTE SHEET ERULE 26)
WO 94/19534 ~ PCT/US94/01943
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Figures 32-37 disclose in sequence the six
bleaching stations. the components which appear
on Figure 32 will be given "a" designations; the
components in Figure 33 (the second bleaching
station) will be given "b" designations, with the
stations of Figures 34 through 37 being given
designations "c", "d", "e", and "f".
Throughout this description of the second
embodiment, various values of weight, percentages,
consistency and other values will be printed
directly on the drawings. It is believed that
this will add clarity to the overall description.
These values represent what current analysis by
the inventor herein indicates would be within an
optimized range in this embodiment. However, it
is to be understood that these values could be
modified depending on a number of factors.
Further, if this system of the second embodiment
is to be incorporated into an existing pulp
bleaching system, using some of the components
from that existing system and adding components of
this second embodiment, then this might require
further modifications. For example, in the
following detailed description, it will be pointed
out that certain effluent may be recirculated or
possibly directed to the digester, instead of the
evaporator, this depending to some extent on the
capacity of the existing evaporator.
In this second embodiment, sufficient
pressure is used in the washer 502 (e. g. 10 bar)
to obtain a consistency of the dewatered pulp of
28.5. This would mean that there is only one
half ton of liquid around the pulp fibers for each
SUBSTITUTE SHEET (RULE ~6)
WO 94/195~~ ~ ~ ~ ~ y , ~ PCT/US94/01943
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ton of pulp fiber, with two tons of liquid
remaining in the pulp fibers.
With reference to Figure 31, it can be seen
that the pulp in the digester 500 has a twelve and
one half percent consistency and that it contains
twenty five percent dry solids for each unit of
liquid. The pulp passes from the digester 500
through 504 to a pump 506. At the same time,
black liquor that has been removed from the washer
502 in the first dewatering step in the washer 502
and collected in a tank 508 is recirculated by
means of a pump 510 through 512 to be combined
with the pulp slurry from the digester 500 to flow
to the pump 506 to be directed into the washer 502
in the manner described previously in Section H of
this text.
In the particular embodiment disclosed
herein, a sufficient amount of the black liquor
(i.e. 51 tons of black liquor for each ton of
pulp) is recirculated from the tank, 508 so that
when this is added to the pulp slurry discharged
from the digester 500, the pulp directed by the
pump 506 into the washer 502 has the consistency
of 1.7 percent. As soon as the batch of pulp
slurry is deposited onto the processing screen in
the washer 502 as descrived in Section B of this
text and the dewatering is initiated, the black
liquor flows into the tank 508. Then there is a '
sequence of six washing steps which follow one
immediately after another, as described
previously. these washing steps are designated as
"A", "B", °C", "D", and "W" in Figure 31, and the
liquid for each of these washing operation flows
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 PCT/US94/01943
-&5-
from a different portion of a batch tank or from
r its related tank or container. As will be
described further in this text, each portion of
the wash liquid supplied to these tank portions
"A", "B", "C", "D" is derived from the washers at
different downstream locations in the system of
the present invention. For example, the wash
liquid B used in the second washing step in Figure
31 is derived from the effluent removed from the
pulp in the washer 502c (See Figure 34) during the
second washing step at the third bleaching
station.
Then the final step of the washing (indicated
at the two locations designated "W°) employs fresh
water. As shown herein, there is a first batch of
fresh water which is equal to one ton of water for
each ton of pulp, and then a second last portion
of fresh water which is equal to 0.5 tons of water
for each ton of pulp. The final 0.5 tons of wash
water contains fifteen kilograms of sodium
hydroxide (NaOH) per ton of pulp. This last 0.5
tons of fresh water moves into the pulp mat
(dispensing the free liquid already in the pulp
mat) and remains in the pulp mat when it leaves
the washer 502 to go to the first bleaching
station shown in Figure 32.
The four initial portions of recycled liquid
at A, B, C and D are designated 514, 516, 518 and
520, respectively, and the two clean water sources
are indicated 522 and 524, respectively.
When the first 2.5 tons of liquid from souce
"A" 514 moves into the tank and moves into and
through the pulp mat, the liquid that flows out of
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ ~ ~ ~ ~ PCT/LTS94/01943
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a. '
the pulp is collec~~d~in the tank 528. When the
liquid 516 is moved into the washer 502 and moves
through the pulp in its washing operation, the
displaced liquid from the pulp resulting from this
flows into the tank, 530. In like manner, the
remaining three tanks designated 532, 534 and 536
receive the liquid outflow from the pulp that
result from the liquid source C (518), D (520) and
W (522 and 524), respectively, with the flow
caused resulting from the two sources ~ (522 and
524) flowing into the tank 536.
It is to be understood that for most of the
locations where flow is indicated, there will be a
valve and/or a pump as needed to control the flow
and/or provide the pumping action. However, since
this is an obvious requirement and within the
skill of the art to provide these, such valves and
pumps are (for clarity of explanation) in large
part not shown in system shown in Figures 31
through 37. It is to be understood that the
presence of such valves and/or pumps are deemed to
be indicated in the drawings and explanation by
the indication in the drawings and/or in the text
of such flow.
It will be noted that an overflow from the
black liquor collecting tank 508 goes into the
next tank 528 which receives the effluent from the
first washing step. The effluent collected in the
tank 528 is directed by a pump 538 to an
evaporator 540 indicated at II in Figure 31.
Also, the overflow from the tank 530 flows into
the tank 528 to mix with the liquid therein and
thus is also directed to the evaporator 540.
SUBSTITUTE SHEET (RULE ~6~
WO 94/19534 ~ PCTlUS94/01943
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r
The outflow from the tank 532 is moved by the
- pump 542 and is directed back to the digester or
to the evaporator, and this outflow is indicated
at JJ and at 544. The outflow from the final tank
536 is directed by a pump 546 to the source E
(548). This outflow at E (548) is cycled back to
the washer 502b (see Figure 33) which is at the
second bleaching station. This effluent is used
for the liquid in the first washing step at the
washer 502b.
Finally, the dewatered and washed pulp mat
from the brown stock washer 502 is directed into a
venturi tube 550 where oxygen (02) is added to the
pulp and directed to the inlet 552 of the first
bleaching station (indicated at BB both in Figures
31 and 32).
As a further note of explanation, the
numerical designations that have "T/T" thereafter
indicate the toneage of liquid flow for each ton
of pulp fibre. The percentage designations
immediately adjacent to that T/T designation
indicate the percentage of dissolved solids in
that liquid. Thus, in Figure 31, it can be seen
that the flow A (514) for that particular washing
step uses 2.5 tons of liquid for each ton of pulp
fiber, and that particular liquid has 1.92 percent
dissolved solids content. Also it can be seen
that in the first dewatering/washing process in
the brown stock washer 502 in Figure 31, the
liquid in the pulp slurry entering the washer 502
has 25% dissolved solids therein, while the liquid
contained in and surrounding the pulp mat that is
finally discharged into the venturi 550 has 0.25%
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ ~ ~ PCT/US94/01943
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dissolved solids therein. -.Thus, (also as
indicated in Figure 31);~~frthe displacement ratio
achieved by the first brown stock washer 502 is
0.99. These are values calculated by computer
analysis, and it is presently believed that these
are close to values which can reasonably be
achieved.
In the following text, there will be
discussion of each of the six bleaching stages
with reference to Figures 32 thru 37. For clarity
of presentation, the following description will be
divided into six sections, one for each of the
stages shown in Figures 32 thru 37.
a. First Bleachiag Stage (Oxygea Stage -
Figure 32).
As indicated immediately above, the pulp
discharged from the brown stock washer 502 is
directed into a venturi 550. As it's name
implies, this venturi 550 comprises a
converging/diverging passageway through which the
flow rapidly increases in velocity through the
throat of the venturi and decreases as it flows
out of the venturi. A valve 554 is provided
downstream of the venturi 550 to control the flow
through the venturi 550.
As indicated in Figure 31, 20 kg of oxygen is
added to the pulp at (or adjacent to) the location
of the venturi 550. The flow of the pulp through .
the venturi 550 causes the pulp to be shredded
into smaller particles to facilitate the mixture
of the oxygen with the pulp particles, and also to
facilitate the movement of the pulp into and at
the next bleaching station.
SUBST~~E SHEET (RULE 26)
WO 94119534 PCT/US94/01943
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As can be seen in Figure 32, the pulp from
r the brown stock washer 502 that is passed through
the venturi 550 (with the oxygen therein) is
directed into an inlet 556a at the upper end of a
bleaching tank 558a. This inlet 556a is arranged
so as to discharge the pulp material into the
upper end of the tank 558 in a swirling circular
pattern so that the gaseous portion of the flow
(steam and/or air along with a portion of the
added oxygen) can be directed into the lower end
of a centrally positioned tube 560a. Some of this
flow is vented as at 562a (for example, the steam
would be vented), and there is an oxygen recovery
line 564a to recycle the oxygen back to a lower
portion 566a of the tank 558a. This recycled
oxygen is moved upwardly through the pulp
contained in the tank 558a to carry on the
bleaching process.
The operating conditions are indicated on
Figure 32, and it shows the desired pulp
temperature of 115C, the pressure at 6 bars and
the processing time 40 minutes. It can also be
seen that the pulp that is delivered to the
bleaching tank 558a has 0.5 tons of liquid per ton
of pulp outside of the cellulose fibres, and two
tons of liquid per ton of pulp inside the
cellulose fibres.
As the pulp collects in the bottom part 568a
of the tank 558a, liquid is introduced at 570a by
means of a pump 572a drawing liquid from a first
stage tank 508a. This is to add sufficient liquid
to the pulp to bring it to a 12.5% consistency so
that it can be moved out of the tank 558a. The
SUBSTITUTE SHEET (RULE 26~
WO 94/19534
PCT/US94/01943
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t
tank 558a is sufficient~ly,:,~.arge so that as
sequential batches of pulp are moved into the
upper end of the tank 558a, the pulp gradually
moves downwardly to the lower end of the tank,
this being done at a sufficient rate so that there
is a total dwell time of 40 minutes for each
portion of pulp brought into the tank 558a.
The pulp at a 12 1/2% consistency is moved
from the tank 568a through 504a to a pump 506a
into a washer 502a. Also, liquid derived from the
tank 508a is moved by a pump 510a up to 512a to
combine with the flow from the bleaching tank 558a
to bring the pulp slurry moved by the pump 506a
into the washer 502a to a 1.7% consistency.
There is a first dewatering step, by which
the pulp slurry is reduced from a 1.7% consistency
to a 28.5% consistency. It will be noted that as
indicated in the printing within the RKS Washer
502a that the dissolved solids in the liquid is at
2.25%, while (as indicated in figure 31), the
dissolved solids in the liquid remaining with the
pulp mat from the brown stock washer 502 is at
0.25% solids. This additional 2% dissolved solids
is attributable to the bleaching chemicals and to
the additional organic material which is broken
down in the pulp fibre during the bleaching
operation within the tank 558a.
The first portion of washed liquid F514a is
derived from a source indicated at the effluent
outlet at F in the fourth bleaching station shown
in Figure 35. The second wastewater source G 516a
is derived from the effluent indicated at G in
Figure 32 from the tank 536a that receives the
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 ~ ~ ~ ~ ~ ~ ~ PCTlLTS94/01943
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discharge resulting from the last washing step
using fresh water as the source liquid. Also, it
will be noted that the effluent indicated at "A"
and "C" in Figure 32 are both recycled to provide
the liquid sources at A514 and H516 in the brown
stock washer of Figure 31.
The final inflow of fresh water into the
washer 502a (0.5 tons of fresh water per ton of
pulp) has added thereto 5 kg (kilograms) of
sulpheric acid (H2S04) and 2 kg of DPTA (i.e.
diethylenetriaminepentaacetic acid) per ton of
pulp.
It will be noted that the sulpheric acid and
DPTA with the final 0.5 tons (per ton of pulp
fibre) of wash liquid remain in the pulp mat when
the pulp is discharged from the wsher 502a. As
indicated previously in the description of the
first embodiment, this sulphuric acid serves to
adjust the pH of the pulp to be compatable with
the ozone bleaching step. The DPTA is added to
stop the metal ions present from reacting with the
ozone. Also, as previously described herein, the
sodium hydroxide which is added in the final
washing step of the brown stock washer (as shown
in Figure 31), is to adjust the pH in the pulp to
be compatable with the oxygen bleaching step.
b. The Secoad Bleaching Station (Ozone Stage
- Figure 33).
The pulp that is discharged from the washer
502a is directed into a series of three venturis
550b, each of which is followed by a valve 554b.
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ ~ ~ ~ PCT/US94/01943
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t
As in the prior bleaching station. shown in Figure
32, each venturi 550b accelerates the flow ,
_-,.
therethrough as it passes through the necked
section of the venturi to shred the pulp. At the
same time, ozone is introduced into the pulp at
(and/or adjacent to) the location of the venturi
550b. Desireably, the concentration of the ozone
is 4 kg of ozone per ton of pulp at each of the
first two venturis to make a total of 8 kg per
ton. There is a pressure drop after the first
venturi 550b down to six bars. A second pressure
drop after the second venturi 550b brings the
pressure down to three bars, and the pressure drop
after the third venturi 550b brings the pressure
down to near atmospheric, at which time the pulp
enters into the second stage tank 558b. As in the
first stage, there is a gaseous discharge tube
560b, and the gaseous discharge of the remaining
ozone in the oxygen is moved to a recovery
location. Between the three venturies, 550b,
there are two sections of pipe 571b with enough
length and volume to provide space for the amount
of pulp blown in each cycle and to provide about
one minute reaction time for the ozone bleaching
to take place.
The recycling of liquid from the dewatering
tank 508b is accomplished in substantially the
same manner as described with reference to the
first bleaching stage in Figure 32 in that a
portion of the liquid from the tank 508b is
directed back to the lower part of the bleaching
SUBSTITUTE SHEET (RULE 2fi)
WO 94/19534 a PCT/US94/01943
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tower 558b, while most of the dewatering liquid is
directed back through 512b to dilute the flow from
the bleaching tower 558b to 1.7% consistency.
With regard to the liquid sources for the
washing operation, the initial washing source
E514b is derived from the effluent discharged from
the tank 536 in Figure 31. The second liquid
source at H516b is derived from the effluent
flowing out of the washer 502e in the first stage
of washing at the fifth bleaching station shown in
Figure 36. This fifth bleaching station also uses
ozone as the bleaching agent. The use of ozone as
a bleaching agent produces toxic material which is
desirably not directed to the main evaporator, but
is treated separately. Accordingly, this filtrate
flowing at H from the second ozone stage bleaching
(Figure 36) is directed back to the first ozone
stage bleaching station in Figure 33.
The effluent resulting from each of the
three washing steps from the liquid sources E514b,
and I518b are each directed to three tanks, 574b,
576b, 578b, respectively. The effluent in these
three tanks 574b, 576b and 578b is collected in
the middle tank 576b and directed by the pump 580b
to a separate evaporator KK582b where this
effluent is treated separately. The effluent
resulting from the last washing steps 520b and
522b using fresh water is directed at I back as
the third liquid source 518b in Figure 33.
SUBSTITUTE SHEET, tRULE 26)
WO 94/19534 PCT/US94/01943
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In the last part of the fresh water stage,
the 0.5 tons of fresh water per ton of pulp that _
is last moved into the pulp mat contains 10 kg of
sodium hydroxide per ton of pulp and 10 kg of
hydrogen peroxide per ton of pulp. These two
ingredients remain in the pulp as it is delivered
to the third bleaching station in Figure 34. The
sodium hydroxide adjusts the pH level, and the
hydrogen peroxide (H202) acts as a bleaching
agent.
The pulp that is discharged from the washer
502b at the completion of the washing is directed
through an outlet venturi 550b to pass through a
valve 554b and thence to the third bleaching
station shown in Figure 34.
c. Third Bleaching Station (Hydrogen
Peroxide Bleaching - Figure 34)
The pulp with the hydrogen peroxide therein
is directed into the bleaching tank 558c through
an inlet 556c and there is a central vent tube
560c. It can be seen from the printed material
appearing in Figure 34 that the temperature in the
reaction zone is 80 degrees C, the pressure six
bars, and the total bleaching time 200 minutes.
As each additional batch of pulp is delivered into
the top of the bleaching tank 558c and moves ,
downwardly, a relatively slow bleaching reaction
takes place. It can be appreciated that because y
of the long retention time in the tank 558c that
this tank 558c is made rather large.
SUgSTiTUTE SHEET (RULE 26~
WO 94/19534 PC'f/US94/01943
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The dilution of the pulp first in the
bleaching tank 558c and subsequently by recycling
the liquid from the dewatering tank 508c, the
dewatering and washing of the pulp in the washer
508c, and the delivery of the pulp from the washer
502c through the venturi 550c is accomplished in
substantially the same manner as described in
connection with the oxygen stage bleaching (the
first bleaching stage shown in Figure 32).
Accordingly, it is believed that a detailed
description is not needed herein of this third
stage bleaching of Figure 34. It can be noted
however that two portions of effluent indicated at
B and D in Figure 34 are directed back to the
brown stock washer and provide the liquid sources
at B516 and D520. The effluent at K from the
final fresh water washing stage is recycled to
provide the water source at K516c in the washer
502c.
The last portion of fresh water at 524c has
added thereto 4 kg of sodium hydroxide per ton of
pulp, and 5kg of hydrogren peroxide per ton of
pulp, and this remains in the pulp which is
discharged through the venturi 550c.
d. Fourth Bleaching Station (Second Stage of
Hydrogen Peroxide Bleaching - Figure 35)
The fourth bleaching stage illustrated in
Figure 35 is substantially similar to the first
hydrogen peroxide bleaching stage shown in Figure
34, so it is believed that a detailed description
SUBSTITUTE SHEET (RULE 26~
WO 94/19534 ~ ~ ~ ~ ~ PCT/US94/01943
,. , _76-
of the fourth stage of Figure 35 is not needed.
It can be seen that the processing conditions in
the bleaching tank 558d are substantially the same
as that in 558c, and the recycling of the liquid
from the dewatering tank 508d is substantially the
same as accomplished in the third stage of Figure
34.
The recycling of the effluent discharge from
the washer 502d in figure 35 is different than
shown in Figure 34. In the present bleaching
stage of Figure 35, the last three portions of
effluent indicated at N, M and L are recycled back
to the washer 502d as the source liquids at L514d,
M516d, and N518d. Then, the effluent at F from
the second effluent tank 528d is recycled back to
the first bleaching stage of Figure 32 to be
become the fluid source at F514a in Figure 32 for
the first bleaching stage which is an oxygen
bleaching stage.
e. Fifth Bleaching Station (Second Ozone
Stage - Figure 36)
The operation at the fifth bleaching station
(Figure 36) is in large part similar to that shown
in Figure 33. Accordingly, it is believed that a
detailed description is not needed for a proper
understanding of the apparatus and operation of ,
the second ozone stage.
Thus, there are the three venturis 550e, with
the ozone being injected into the flow passages at
SUBSTITUTE SHEET (RULE 2G)
WO 94/19534 ~ PCT/US94/01943
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the location of the first two venturis 550e.
However, only two kg of ozone are injected into
the pulp stream at each of the first two venturis
550b, which is half of the amount of ozone used in
the first ozone stage of Figure 33. Other than
that, the bleaching conditions are substantially
the same as shown in Figure 33.
The initial recycling of the dewatering
liquid from the tank 508e is substantially the
same as in the second bleaching stage shown in
Figure 33.
However, the recirculating pattern of the
effluent from the washer 502e is somewhat
different than with the washer at 502b (Figure
33). First, the effluent from the tank 574e is
directed at H to the liquid source H516b in the
first ozone stage of bleaching at Figure 33. The
next three effluent tanks 576e, 578e and 536e have
their effluent directed to source locations which
lead directly back to the washer 502e, these being
indicated at 0514e, P516e and Q518e.
The last portion of wash liquid has dissolved
therein 4 Kg of NaOH and 3Kg hydrogen peroxide
(H202) per ton of pulp, which remain in the pulp
mat as it is discharged from the washer 502e,
through the venturi 550e and a valve 554e to the
final bleaching station shown in Figure 37.
f. Sixth Bleaching Station (Third Hydrogen
Peroxide Stage - Figure 37)
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WO 94/19534 PCT/US94/01943
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This sixth and final bleaching station is
substantially the same as (and functions
substantially the same as)r~the second hydrogen
peroxide bleaching station shown in Figure 35.
Accordingly, there will be no detailed description
with regard to this final bleaching stage.
As in the fourth bleaching stage of Figure
35, the final three tanks of effluent from the
washer 502f (indicated at R, S and T) are recycled
directly back to the same washer 502f at that
station. The effluent from the second tank 528f
is indicated at J and is recycled to become the
first liquid source at J514c in the third
bleaching stage of Figure 34, which is also a
hydrogen peroxide bleaching stage.
The final portion of wash liquid has S02
(sulphur dioxide) dissolved therein, and this
remains in the pulp mat that is discharged through
the venturi 550f. The pulp from the venturi 550f
is shown being discharged at HH, and this pulp is
commonly dried and then bailed for future use,
such as being shipped to a paper mill.
Figure 38 presents a data summary of the
second embodiment shown in Figures 31 thru 37. On
the left hand column, there is first listed in the
first six columns the six types of dissolved
solids. The first organic solids is the organic
material which in the bleaching stages is broken
down into soluable organic material that dissolves
in the liquid. Then the two bleaching agents are
shown, namely the oxygen (02 and ozone) and the
SUBSTITUTE SHEET (RULE 26)
WO 94/19534 ~ PCT/iJS94/01943
_79_
hydrogen peroxide. Then the three treating agents
w are also shown (solidum hydroxide, sulpheric acid,
and DTPA).
Next, in column 1 is the carryover which
represents the weight of dissolved solids per ton
of wood fiber that is carried over from each
washer to the next bleaching stage, and the next
space indicates the dry solids. Below is the
fresh water, and this indicates the amount of
fresh water added at each bleaching station. In
the following six spaces in the first column,
there is shown the filtrates and the movement from
one washing station to the other as indicated by
the arrows. In the next space down (first
column), there is indicated the black liquor that
goes to the evaporator. In the next two columns
down in the first column, there is given the dry
solids initially in the liquid and then in the
liquid that is carried over to the next stage,
these being given as percentage values. In the
next row down, first column the displacement ratio
at each wash station is indicated. Finally, in
the last row, first column, there is indicated how
many washing stages there is for each washing
station.
Along the top row, there is indicated first
the brown stock washing station, and then the
subsequent six bleaching stations. The values
' given in colun indicate those for bleaching
station listed at the top of the column.
SUBSTITUTE SNE~T (RULE 26)
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It will be noted that there is a total of
only sixteen and one half tons of~~.fresh water
added for each ton of cellulose'fiber. Because of
the efficiency of the system.of.this invention, it
now becomes feasible to recycle the effluent
discharge from the system to recover fresh water,
and to utilize this fresh water in the washing
operation in accordance with the steps shown in
Figures 31 thru 37.
This particular feature of the present
invention is illustrated in Figure 39, where th
main components described with reference to
Figures 31 thru 37 are shown somewhat
schematically. It is indicated that fresh water
condensed from each of the evaporators 540, 544
and 582b can be condensed and recycled back to the
various washers 502 thru 502f to act as the source
of fresh wash water. Accordingly, it would be
possible to have a totally closed system where no
net effluent of wash water is discharged.
Usually, a bleaching plant is located near a
large body of water so that the substantial
guantities of effluent from the washers can be
discharged into (and thus become diluted in) the
large body of water, such as a river or the ocean.
However, with the growing environmental concerns,
the system of the present invention can provide
significant environmental benefits. This is not
simply in avoiding the polluting of nearby water '
sources, but also in the initial conservation of
water, since the present system could be
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substantially independent of a relatively large
source of water that is usually required for
bleaching plants. This could even permit the
bleaching plant to be located remote from water
sources. In such a closed system, at most only a
small amount of makeup water would be needed from
time to time due to the relatively small losses
that might be experienced in such a closed system.
Another significant benefit of the present
invention is that because of the efficiency of the
overall operation, less chemicals are required for
a given amount of pulp, since there is less
dissolved solids in the liquid remaining in the
pulp at the various bleaching and washing stages.
A comparison has been made with the values given
in Figure 38 with prior art systems, and in the
present system there is a significant decrease in
organic solids in the various locations in the
system, and a corresponding decrease in the use of
chemicals.
Another efficiency is realized in the present
system in that by bringing the consistency of the
pulp to a high percentage level, the diffusion
process in the washers is enhanced, and the
bleaching is also enhanced. As indicated
previously in this text, the speed at which
diffusion takes place is a function a number of
several factors, two of these being temperature
' and the turbulance in the liquid surrounding the
fibers. With regard to temperature, since
relatively high pressures are utilized, the
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operating temperature can be made higher. With
regard to turbulance, with the higher percentage
consistency of the pulp, and with the higher
pressures involved, the passageways are relatively
smaller, and there is a more rapid travel of the
liquid through the pulp mat. This results in an
increase in turbulance which in turn increases the
rate of diffusion.
With regard to the effectiveness of
bleaching, since there is less filtrate with
dissolved organic solids in the pulp at any one
time, the bleaching chemicals are not wasted in
reacting with organic material which would in any
event be discharged to the black liquor evaporator
in the present invention.
For example, while certain benefits can be
obtained by maintaining the pressure in the
pressure vessel 12 at least one bar, the preferred
embodiment maintains this pressure at 10 bar.
Thus, it is to be understood that, for example,
within the broad scope, the pressure could be at
2, 3, 4, 5 or 6 bar (at which good results can be
obtained), and also from 7, 8 and 9 bar, up to 10
bar. Further, it would be possible to maintain
the pressure above 10 bar to achieve certain
results. In like manner, while it is believed a
minimum temperature of which the pulp should be
maintained at 50 degrees C, this could be
increased to be at various increments of, for '
example, 5 degree C, such as 55C, 60C, 65C, 70C,
75C, 80C, 90C, 95C and 100C. With higher pressure
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in the vessel 12, it would be possible to maintain
higher pressure such as 105C, 110C, 115C, 120C,
and possibly higher.
It is to be understood that various
modifications could be made to the present
invention without departing from the basic
teachings thereof. Also, it is understood that
the description contained in this text is arranged
to show preferred designs, and when specific
procedures, dimensions and arrangements are
described, these are not intended to limit the
claims which define the scope of the present
invention and follow this text.
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