Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF TH INVENTION
1. Field of the Invention
This invention relates to bleaching or
delignification of cellulose pulp.
2. Description of the Prior Art
Canadian Patent Application 507,000, Lee,
Hornsey, Perkins and Davidson, filed April 18, 1986
contains a bibliography on the use of oxygen in
bleaching cellulose pulp slurry.
The Lee et al application discloses a process
and apparatus for bleaching pulp in the form of an
aqueous slurry having a consistency from 8% to 16~, by
introducing oxygen into the slurry, in the form of
minute bubbles, at the time the slurry is passing, under
pumping pressure, through a diffusing and mixing conduit
which confines the slurry to a cross section so that, at
the pumping pressure, it moves at a high velocity
effective to provide turbulent liquid flow. The
phenomenon of the liquid behavior of pulp slurry
confined under high velocity is explained in an article
entitle "Medium Consistency Technology" appearing in the
TAPPI Journal, Vol. 64, No. 6, June 1981, and in the Lee
et al application 507,000.
The slurry containing the bubbles is
immediately passed from the mixing conduit into and
through a reaction vessel which releases the slurry to a
cross section at which it assumes relatively slow plug
flow, to give the oxygen
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bubbles dispersed through it, time to react with the cellulose.
The treated slurry is recovered from the reaction vessel. Dis-
persing the bubbles within the turbulent liquid slurry is
accomplished by passing the oxygen, under pressure greater
than that of the slurry pressure, through a microporous wall
into contact with the slurry.
The slurry may be preheated by passing it through a
steam mixer, on the way to the pump, and the heated slurry
deposited in a storage vessel upstream of the pump.
The minuteness of the bubbles, produced initially by
diffusing the oxygen into the slurry through the porous wall,
favours good dispersion throughout the mass of the pulp and
consequently good bleaching or delignification contact with
the pulp fibers.
SUMMARY OF THE INVENTION
It is an aim of the present invention to improve con-
tact between the oxygen and the pulp still further so as to
achieve enhanced bleaching action with the same amount of
oxygen.
This is accomplished, according to the invention, by
injecting the oxygen into the diffusing and mixing conduit in
the way described above, but as a mixture of oxygen and steam.
Minute bubbles of the oxygen-steam mixture are formed momen-
tarily as the mixture passes from the minute pores of the
; 25 injector wall, but as each bubble meets the pulp, the steam
condenses and leaves the oxygen content as a smaller bubble.
The condensing steam imparts additional heat to the slurry.
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Preferably as much of the steam needed to bring the
slurry to reaction temperature as possible is injected, in one
stage, through the diffuser-mixer. Whether this can be done
depends on capacity of the diffuser-mixer. Therefore, addi-
tional steam may be injected, if required, in a steam mixerbringing it to a supply temperature in a preliminary stage,
through an injector between the pump and the oxygen-steam
diffuser-mixer to raise the temperature closer to reaction.
Steam may also be injected to preheat the slurry upstream of
the pump.
So, the invention contemplates:
a) preferably injecting all the steam, in the
diffuser-miser, in the form of an oxygen-
steam mixture, or
b) injecting some of the steam between the pump
and the diffuser-mixer to raise the pump to
an intermediate temperature, and
c) in combination with either a) or b) injecting
steam to preheat the pulp upstream of the
pump, to preheated supply temperature.
By injecting the oxygen into the liquid-behaving
slurry, in an oxygen-steam mixture, as compared with injecting
oxygen alone through the same microporous surface, under the
same conditions, the effective surface area of the oxygen, in
contact with the pulp, may be increased from twice to ten times,
with a preferred increase from three to five times. This bring
about a corresponding increase in the bleaching effect of the
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of the oxygen and a conse4uent saving of chemical bleaching
agent. Heating the slurry with the injected steam, at this
stage, is also advantageous.
For bleaching, the proportion of oxygen to steam may
range from 20 to 200:1, by weight, with 100 to 150:1 preferred.
Or, in terms of volume, the proportion of oxygen to steam may
range from 50 to 500:1 with 300 to 400:1 preferred. For
delignification, the oxygen may range from 80 to 800:1, by
weight, with 400 to 600:1 preferred. Or, by volume, the
oxygen may range from 200 to 2000:1, with 1200 to 1600:1 pre-
ferred.
The oxygen to pulp ratio may range from 10 lbs. to
40 lbs. per ton of pulp, measured on a dry basis. Then it is
desirable to add as much steam as possible with the oxygen-
steam mixture. Since the amount of steam which can be added
is limited by the capacity of the diffuser, it may be necessary
as well to add steam at an earlier stage, in order to inject
enough heat to bring the slurry to reaction temperature.
The invention provides for potential maximum utiliza-
tion of oxygen and steam in the delignification or bleachingprocess in the following way. The total amount of steam,
injected into the slurry, is a controlled amount that will
heat the slurry to reaction temperature in the reaction
vessel. Preferably, according to the invention, the propor-
tion of the total amount of steam injected with the oxygen-
steam mixture in the diffuser would be substantially the
maximum allowed by the capacity of the diffuser to inject it
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or a proportional selected amount close to this maximum, say 80%
or more. The remainder of the steam necessary to heat the
slurry to reaction temperature may then be injected into it up-
stream of the diffuser. The optimum proportion of oxygen to beinjected into the pulp for good delignification or bleaching is
determined by tests, as is well known in the art, and selected
for the particular plant and the particular conditions involved.
Oxygen injection is then controlled to feed the selected amount
continually to the slurry. The injection of steam, along with
the oxygen, results in the oxygen being injected in smaller
bubbles than if oxygen alone were being injected which, in turn,
provides a greater surface area of the oxygen in contact with
the pulp and better absorption for a given amount of oxygen.
The invention, therefore, provides several advantages, namely
that a large amount of steam used for heating is injected where
it is needed just prior to the reaction zone and, at the same
time, the injection of the oxygen along with the steam results
in greater surface area of the oxygen injected and better
reaction with the pulp for a given amount of oxygen. In this
way, the utilization of both oxygen and steam is substantially
at a maximum for the particular plant.
If all the steam needed to bring the slurry to re-
action temperature can be added at the diffuser control may be
exerted continuously by regulating the amount of steam fed in
the steam-oxygen mixture by the temperature of the slurry down-
stream of the oxygen-steam diffuser while keeping the supply of
oxygen constant or by control!ing it in keeping with some
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relevant variable. If it is necessary to add additional steam
upstream of the diffuser the temperature control will also
regulate such upstream supply.
The invention lends itself particularly to the incre-
mental dosage of the pulp slurry with oxygen. This is done byfeeding of a stream of cellulose pulp slurry under pressure,
from a supply, through a reaction path made up of a plurality of
successive stages, in each of which the stream is first confined
to a cross-section at which it has a velocity such as to fluidize
it to liquid flow and is then expanded to a cross-section at which
it has a velocity such as to provide plug flow. In each stage,
oxygen is injected into the liquid flow under pressure, in the
form of a mass of minute bubbles to disperse them throughout,
whereby most of the oxygen is reacted with the pulp in the plug
flow at that stage. The treated slurry is then recovered from
the final stage.
The invention also contemplates a preferred further
mixing of the slurry leaving the oxygen diffuser by passage
through a pulp fluidizing mixer. This can be located adjacent
to the diffuser or positioned some distance downstream. This
mixer provides the effective further mixing and dispersing of
any agglomeration of oxygen so that the oxygen remains in finely
divided form and is more intemately mixed with the slurry. A
preferred static mixer is made up of a pipe tapering down to a
choke, followed by a pipe that flares out to the diameter of the
pipe in which the pulp slurry is flowing. The velocity through
the choke will be about the same as that in the diffuser.
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The invention also contemplates means for coordinating
the flow of oxygen to the flow of pulp slurry so as to maintain
the proper oxygen dosage. This may be accomplished by an appara-
tus which includes a flow measuring device, for example, an
orifice plate within the path of the slurry, in advance of the
diffuser, the orifice plate being connected to a differential
pressure cell which, in turn, is connected to a controller.
The controller sends a signal to an oxygen flow con-
trol valve controlling the oxygen supply to the diffuser. There
is an orifice plate or other device in the oxygen supply line
that measures the oxygen flow. The control mechanism may include
a computer that maintains a constant, predetermined and set ratio
of oxygen to pulp so as to increase and decrease the dosage to
keep pace with the differing flow rates of the pulp slurry.
An apparatus, according to the invention, includes a
supply vessel for the slurry, a substantially unobstructed
injection and mixing conduit for confining the slurry to a
stream for substantially free flow therethrough, a reaction
vessel having a greater cross-section than said conduit and
connected therewith, means for continuously force-feeding the
slurry under pressure from the supply vessel to the conduit,
and means for regulating the feeding pressure so that the
slurry in the conduit has a velocity such as to provide a
turbulent liquid flow therethrough and the slurry in the
reactor assumes plug flow. There is diffusing means, includ-
ing an extensive microporous surface interfacing with the slurry,
in the conduit, to diffuse an oxygen-steam mixture into the
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slurry under pressure higher than the pulp feeding pressure
thereby to form a mass of minute oxygen bubbles and mix them
throughout the slurry in the conduit whereby they remain in
contact with the thick pulp throughout the plug flow to pro-
vide delignification or bleaching action. The control mechanismincludes a supply conduit for supplying a mixture of oxygen
and steam to the diffusing means and means for regulating the
flow of oxygen therethrough, a steam conduit leading from a
source of steam to said supply conduit and means for regulating
the flow of steam therethrough, the means for controlling the
steam supply being responsive to the temperature of the slurry
passing from the mixing conduit to the reaction vessel.
Means may also be provided for adding steam to the
slurry prior to the diffusing means. In this event, the means
for controlling the steam supply responsive to the temperature
of the slurry may also control the additional supply of steam.
In a preferred apparatus, the pulp slurry flows through
an initial oxygen diffuser and a plurality of reactors each pre-
ceded by an oxygen diffuser. Any or all may be passed into a
reaction tower to allow further extraction with caustic soda.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further explained by reference
to the accompanying drawings, in which:
FIG. 1 is a side elevation partly in section through
apparatus of the type currently used for
steam and oxygen addition to pulp in a
bleaching process;
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FIG. 2 is a similar side elevation partly in
section through a preferred apparatus
effective for carrying out a preferred
method, according to the invention; and
FIG. 3 is a side elevation, partly in section
through a preferred diffusing device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates a prior method of employing steam
and oxygen together in bleaching pulp.
Referring to Fig. 1, a pulp mixture, which is normally
a thick heavy mass, is continuously passed from a source of
supply through a steam mixer 15 which preheats it to a tempera-
ture from 100 to 150F to 170 to 200F and thence through a
pipe 16 to the top of a storage chute 17. From the storage
chute 17 the pulp, mixed with steam, is pumped, as a slurry,
by a pump 19, through another pipe 21 through an oxygen dis-
perser conduit 23 illustrated in more detail in Fig. 3. The
pressure at which the slurry is pumped is effective to cause
it to flow through the conduit 23 at a velocity such that it acts
as a turbulent liquid. This phenomenon is described in the Lee
et al application 507,000, referred to above. Steam is supplied
to the steam mixer 15 from a source of supply through a pipe 25.
The supply of steam is regulated by a temperature controller 27
connected to a valve 29 in the pipe 25.
Oxygen is supplied to a diffusing device within the
conduit 23, through a line 31 leading from a source of oxygen
supply through a pressure controller 33. The diffusing device
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has a microporous wall interfacing with the slurry.
Oxygen is thus continuously introduced into the
heated pulp slurry through the diffusing device in the form of
a mass of minute bubbles at a time when it acts as a turbulent
liquid, so that the oxygen bubbles are difFused into the slurry
and mixed throughout. The slurry containing the oxygen passes
through a pipe 35 into the bottom of a retention tube 37 pro-
vided with a back pressure control vaive 39. The slurry slows
down in the tube 37 to plug flow velocity at which it reassumes
its normal thick condition and is held under pressure so that it
gradually absorbs the oxygen whlch reacts with it.
The pulp is retained in the retention tube for suffi-
ciently long to allow the oxygen to react with the cellulose.
The parts of the preferred apparatus of Fig. 2 have
been given the same tens and digits as in Fig. 1 but have been
raised by 100.
Method of the Invention
In the apparatus of Fig. 2, illustrating a preferred
method, according to the invention, steam is added to the pipe
131 conducting the oxygen to the diffusing device A (see Fig. 3)
within the disperser conduit 123 to form an oxygen-steam mixture.
The steam supply is controlled by a temperature con-
troller 127 linked to the pipe 135 between the disperser 123
and the retention tube 137. Thus, steam is first mixed with
oxygen and the oxygen-steam mixture added to the slurry,
through the diffusing device A, in the form of a mass of minute
bubbles. When the oxygen-steam mixture contacts the pulp, the
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steam condenses to water vapor, imparting heat to the pulp, and
leaving the oxygen-gas within the pulp in the form of smaller
bubbles (than the oxygen-steam bubbles) which are dispersed
through the slurry by its turbulence. Better sub-division and
dispersion of the oxygen is achieved to provide a much greater
effective surface area of the oxygen interfacing with the
slurry than would occur without the steam. This increases
the oxygen transfer rate accordingly and less oxygen need be
employed, than otherwise, to obtain the same bleaching effect.
The pressure of the oxygen-steam mixture, as injected,
is about the same or slightly higher than the oxygen pressure as
in the method of Fig. 1.
In contrast to the prior method, steam is mixed with
the oxygen just before injection of the mixture into the
diffuser A and the control of the steam flow is carried out by
a temperature controller 127 sensing the temperature of the
pulp downstream of the disperser.
The pulp then passes into the pressurized retention
tube 137 where it assumes plug flow and moves at relatively
slow speed, under pressure, with the oxygen dispersed through-
out the plug in small bubbles to react with the cellulose.
In the case where several diffusers are used in series,
- as for example in the Lee et al Canadian Application 507,000,
steam is added to the first stage.
The diffuser through which the oxygen-steam mixture
is injected into the slurry in the diffuser A may take various
forms, some of which are shown in the Lee et al application
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referred to above all of which include a microporous wall which
interfaces with the slurry and through which the oxygen-steam
mixture is diffused. This is preferably a sintered stainless
steel element as described in Bulletin M 201 of Pall Trinity
Micro Corporation, a subsidiary of Pall Corporation and in U.S.
Paent 2,554,343. Such elements are generally used for filtering.
The pore size should be within the range from about 2 microns
to 100 microns, preferably less than about 10 microns. The
porosity should be substantially uniform with voids making up
about 40 to 50% of the volume of the wall, which has a thickness
of from about 1/32 of an inch to about 1/2 inch.
A preferred form of diffuser A is shown in Fig. 3. It
is made up of a cylinder 123 providing a passage for the pulp
slurry. The cylinder has flanges 63, 64 on its ends for
connecting into a pipeline. A tube 65 extends transaxially and
centrally across the passage 123 and is held in it by a suitable
socket 69 formed on the wall of the cylinder 123. The pipe 131
for the delivery of the oxygen-steam mixture to the tube 65 leads
to the diffusion tube 65.
The minute streams of oxygen-steam mixture, as the
mixture passes through the pores in the tube 65 wall, as they
leave the porour surface, are stripped therefrom by the slurry
as minute bubbles. The steam condenses from each bubble,
immediately it contacts the slurry and leaves a bubbles of
oxygen, having a fraction of the size it would have had oxygen
alone been injected. While there may be some coalescence,
generally speaking, the mass of oxygen bubbles are dispersed
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throughout the slurry by its turbulence as it passes with great
rapidity through the conduit 123 and the bubbles are carried in
the slurry into the reaction chamber 137 where it assumes rela-
tive slow plug flow. The oxygen therefore has time to react
with the pulp and is absorbed as the plug of slurry passes
relatively slowly through the vessel 137.
The Diffuser
The diffusing element may include a conduit or cylin-
der through which the slurry passes, the wall of the cylinder
defining the flow path. The diffusing element may form a part
of the wall of the cylinder, may take the form of a porous pipe
extending transaxially of the cylinder, or may take the form of
a porous pipe extending axially of the cylinder. In all cases,
the path of the slurry is relatively unobstructed. Where the
cylinder has a porous wall, the slurry passes directly through
the cylinder without any obstruction. In the case of a pipe
within the cylinder, it takes up a minor space so that the
major volume of the space in the cylinder is taken up by the
slurry moving through it in contact with the porous diffusing
surface.
Starting Material
The starting material for the present process will
depend on whether the method is being employed at the deligni-
- fication stage or the bleaching stage of the pulp treatment.
For delignification, the aqueous pulp slurry comes
from three or four washing stages following the brownstock
washing stage, with the pulp containing considerable lignin,
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as indicated by a Kappa number between 30 and 60, typically 40
to 50. It will contain 8 to 16%, preferably 10 to 12%, by weight,
solids. Caustic will be added in an amount from 2 to 4%, by
weight, typically 3%, prior to delignification according to the
present invention.
Bleaching is effected, according to the invention,
usually after the slurry has been washed, following the first
chlorination stage. Then, it will have a Kappa number from 3 to
8, typically 4 to 5. It will contain from 8 to 16%, preferably
10 to 12%, by weight, solids. Again, caustic will be added in
an amount of 2 to 4%, by weight, typically 3%, to provide the
starting material for bleaching, according to the invention.
Turbulent Liquid Flow
A typical diffusing and mixing conduit 123 may range
in cross-sectional area from about 4 to about 17 sq. inches.
To produce turbulent flow, the slurry is fed through the conduit
at a pressure from about 20 to about 200 psig. This gives the
slurry a velocity from about 0.5 to about 50 meters per second,
preferably from about 3 to about 10 meters per second.
The residence time in the conduit 123 will then range
from about 0.001 to about 0.120 seconds. The residence time in
the reactor tube 137 will range from about 1 to about 5 minutes.
Temperature
Steam serves both the function of heating the slurry
and as a vehicle for the injection of the oxygen so as to reduce
the size of the bubbles of the latter mixed into the slurry.
Typical preferred temperatures are:
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a) the starting pulp slurry prior to the mixer 15
will usually have a temperature below around 130F,
b) in the mixer 15 the temperature will be raised
from 130F to about 140F,
c) after the preliminary injection downstream of
the pump and prior to the oxygen-steam diffuser
mixer to an intermediate temperature between
140F and 160F,
d) after the diffuser mixer to a reaction tempera-
ture in the range from about 160F to 180F,
e) before entry into the retention vessel the pulp
should have a temperature within a range from
about 160F to about 180F.
Porous Surface
The surface of the porous wall interfacing with the
slurry may preferably have an area ranging from about 0.05 to
about 0.2 sq. feet covered with pores having a diameter within
the range from about 10 to about 100 microns.
Oxygen-Steam
The oxygen employed may be molecular oxygen, as commer-
cially available, containing 90% or more and preferably 98% or
more of oxygen. Or, the oxygen can be in the form of a gas con-
taining more than 50% oxygen. The starting pulp slurry may con-
tain about 10 to about 20% of air by volume of the pulp dispersed
which dilutes added oxygen. For a pulp containing little or no
air, the concentration of the added oxygen containing gas may be
at the lower end of the oxygen content range and for a pulp
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containing a lot of air, the added gas may contain oxygen toward
the upper end of the oxygen range.
In order to control the oxygen dosage to meet require-
ments, the oxygen content of off-gas leaving the vent of the final
treatment vessel is measured. From this it can be determined
whether good mixing is being achieved. Adjustments can be made
accordingly and the incremental dosage can be manipulated in
order to achieve the best results.
In a multi-stage process, as described, large total
amounts of oxygen can thus be added incrementally to a pulp
slurry, by adding small amounts at each stage according to the
formula:
Volume of oxygen + gas <
Volume (oxygen + gas + pulp + steam) 0.1
This formula is calculated at the operating pressure of
that particular stage.
The dosage may be proportioned evenly to each stage or
may be varied to suit varying conditions such as changes in the
nature of the pulp or otherwise.
The oxygen employed should have at least 90% oxygen
content by volume, preferably more than 98% measured under
standard conditions.
The steam should be saturated steam.
The oxygen-steam mixture injection pressure should be
greater than the pulp slurry pressure from about 15 to 150 psig.
The temperature of the oxygen-steam mixture, at injection,
should be from about 265F to about 390F.
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The ratejo-f injection of the oxygen-steam mixture
should be within the range from about 10 Ibs. of oxygen to about
50 lbs. of oxygen per ton of pulp measured on a dry b~sis.
As will be understood by one skilled in the art, the
vessels and piping should be insulated so as to retain heat.
