Note: Descriptions are shown in the official language in which they were submitted.
20~6717
Docket No. 1197-IR-PA
METHOD AND APPARATUS FOR TREATING FIBROUS
MATERIALS WITH A GASEOUS REAGENT
BACKGROUND OF THE INVENTION
This invention relates generally to pulp
processing for papermaking and more particularly to
pulp bleaching using a gaseous bleaching reagent.
Bleaching of wood pulp using gaseous reagents
05 such as chlorine dioxide or ozone involves a large
volume percentage of carrier gas along with the
reactive gas. This results from the methods of
preparation of such gaseous bleaching reagents.
Ozone is generated by passing air, or oxygen,
l~ through an electrical field of sufficient intensity
to generate a corona discharge which converts a
portion of the oxygen into ozone. For example,
passing oxygen through the corona, in some ozone
generators, will result in conversion of as much as
six percent of the oxygen into ozone. The remaining
oxygen passes into the bleaching process with the
ozone as a "carrier gas". When reacted with the
pulp, the ozone fraction will be substantially
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Docket No. 1197-IR-PA
consumed and the oxygen must be removed from the
pulp as it is not reactive under the bleaching
conditions used and its recovery is important to the
economics of ozone generation.
05 One currently available ozone bleaching
apparatus treats pulp at a high consistency of 20 to
50 percent. The pulp is fluffed by a mechanical
fluffer in the presence of the reacting gas, is
transported by the gas through a conduit; and is
dropped onto a porous bed of fluffed pulp which
continuously moves downward through a cylindrical
reaction tower toward an expanded section which acts
as a gas separation chamber. The ozone/carrier gas
mixture is initially mixed with the pulp in the
fluffer to form a gas suspended mixture for
transport and initiation of reaction, which then
passes through a conduit to the top of the tower.
The ozone and carrier gas flows through the porous
bed at a substantially higher velocity than that at
which the bed moves down through the reaction
tower. The carrier gas leaves the pulp bed
peripherally into the gas separation chamber from
which it is recycled to the ozone generator.
Bleaching in this type reactor frequently results in
a mottled appearance due to the variable
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Docket No. 1197-IR-PA
permeability of the fluffed pulp agglomerations.
This variability is also attributed to non-uniform
compaction of the porous pulp bed which results from
localized impact of the falling pulp. Greater local
05 depth of the pulp bed also increases local
compaction. These compacting factors are further
aggravated by the pressure drop of the ozone and
carrier gas through the pulp bed. Since the
pressure drop is proportional to the compacted
density of the pulp bed, the aggravating character
of the gas flow is clear. In addition, the
exothermic bleaching reaction produces higher
temperatures in the zones of lower density and thus
higher bleaching rates in those zones. Since the
ozone/pulp reaction is very fast, and since the rate
decreases rapidly with concentration decrease, the
non-uniform bleaching effects of prior art reactors
have been further aggravated because a large
fraction of the ozone is consumed before it has an
opportunity to reach the fibers in more densely
packed zones of the pulp bed.
One important characteristic of the porous bed
type reactor is that it has proved highly efficient
in removing the ozone from its oxygen carrier gas.
Measurements of the gas leaving the porous bed into
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Docket No. 1197-IR-P~
the gas separation chamber have shown a very low
residual ozone content, for example 30 parts per
million (when the feed gas contained 4% ozone in
oxygen at the top of the reactor). This high
05 utilization of the ozone gas is important to the
economics of ozone bleaching as ozone is a
relatively expensive chemical. It is believed that
this high efficiency in ozone utilization is the
result of the fact that the bulk density of the pulp
bed is substantially increased near the gas
separation zone by the combined effects of weight of
the pulp above and pressure drop of the gas flowing
through the entire height of the bed. For example,
in a reactor operating with eight feet of pulp bed
above the separation zone, and a gas velocity in the
upper portion of the porous bed reactor of about 40
ft./minute, the bulk density of the pulp at the gas
separation zone will have risen to about 40
lbs./cu.ft. and the porosity will have been reduced
to about 36% of the bulk volume.
In this zone of reduced porosity, the ozone gas
is forced into intimate contact with the pulp
fibers, and the last traces of ozone are consumed in
bleaching rather than passing through the porous bed
and being destroyed in the recycling system used to
recover the oxygen carrier gas.
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Docket No. 1197-I~-PA
The foregoing illustrates limitations known to
exist in present gaseous reagent pulp bleaching
reactors. Thus, it is apparent that it would be
advantageous to provide an alternative directed to
05 overcoming one or more of the limitations set forth
above. Accordingly, a suitable alternative is
provided, including features more fully disclosed
hereinafter.
SUMMARY OF THE INVENTION
In one aspect of the present invention, this is
accomplished by providing an apparatus for treating
fibrous material with a gaseous reagent having
provision for fluffing the fibrous material in the
presence of the gaseous reagent to form a gas
suspension of the fibrous material and a device for
receiving and agitating said gas suspension of
fibrous material to mechanically maintain the
fibrous material in suspension in the gaseous
reagent until the gaseous reagent has substantially
reacted with the fibrous material. The mixture is
then dispensed into-a porous bed reaction and
degasification chamber wherein the reaction is
completed and the remaining reaction gases are
removed from the fibrous material.
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Docket No. 1197-IR-PA
The foregoing and other aspects will
become apparent from the following detailed
description of the invention when considered
in conjunction with the accompanying drawing
05 figures.
BRIEF DESCRIPTION OF T~E DRAWING FIGURES
Fig. 1 is a schematic partially sectional view
of an apparatus which embodies the features of the
present invention;
Fig. 2 is a fragmentary partially sectional
view of the apparatus as seen from line 2-2 of
Fig. l;
Fig. 3 is a graphic presentation of the
reaction rate of ozone with wood pulp in a porous
bed reactor; and
Fig. 4 is a fragmentary partially sectional
view of the apparatus of another embodiment of the
present invention in which the reagent gas is
introduced into the feed hopper of the mixing
chamber rather than into the fluffer.
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Docket No. 1197-IR-PA
DET~ILED DESC~IPTION
Fig. 1 shows the preferred embodiment of a
gas/solid reactor system 20 of the present
invention. It consists principally of a
fluffer/blower 60, which forms the gas suspended
05 mixture of the fibrous solid as previously
described, a mixing chamber 40, and a porous bed
reactor 30. The fluffer/blower 60 and
porous bed reactor 30 are well-known in the art, but
are described here in order to illustrate the
function of the present invention within the system.
Fluffer/blower 60 receives high consistency
pulp 5 (20% to 50% consistency) from a dewatering
pres-c, not shown, through conduit 66. It also
receives a gas mixture 71 of feed gas 70 and excess
15 blowing gas 76 through gas loop 50. Feed gas 70 is
composed of a gaseous reagent and, depending upon
the reaction desired, the gaseous reagent may be
mixed with any appropriate substantially
non-reactive carrier gas. For purposes of this
description, a wood pulp bleaching operation using
ozone as the gaseous reagent and oxygen as the
carrier gas will be used. Feed gas mixture 70 is
injected through valve 77 into gas loop 50 where it
Docket No. 1197-IR-PA
mixes with excess blowlng gas 76 and forms gas
mixture 71. It enters fluffer/blower 60 where it is
mixed with pulp 5 to form a gas suspended pulp fluff
mixture and provides the mass flow necessary to
05 transport gas/fluff mixture 54 which is discharged
through conduit 51 into fluff discharge cyclone 52.
From there pulp 5 falls into mixing chamber feed
hopper 46 and is fed into mixing chamber 40 by auger
43.
Within mixing chamber 40, the gas suspended
pulp fluff mixture is mechanically agitated by the
intense action of paddle blades 44 on rotor shaft 42
which is driven by motor 41. The velocities of the
fluffed pulp and the contacting gas 69, which is
composed of the remaining gaseous reagent in
combination with the non-reactive carrier gas plus
the developing gases resulting from the progressive
bleaching reaction, through the mixing chamber are
not the same. Usually the gas is traveling faster.
The intense mechanical action of the paddle blades
maintains the pulp fluff in suspension in the
contacting gas 69 and, thus, ensures intimate
contact between the contacting gas 69 and the
fluffed pulp so that no portion of the fluffed pulp
leaves the mixing chamber without contacting the
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Docket No. 1197-IR-PA
gaseous reagent. This ensures uniform bleaching
during the high rate portion of the bleaching
reaction. The mixture is discharged from the mixing
chamber 40 through discharge neck 45 into the porous
05 bed reactor 30. The action of the paddle blades 44
at the entrance to discharge neck 45 prevents
plugging of the neck by the pulp exiting the mixing
chamber 40.
The fluffed pulp, having formed a porous bed,
moves slowly downward through the porous bed reactor
30 and becomes more compacted toward the bottom as
previously described.
The contacting gas 69 passes down through the
permeable pulp bed 7, since lt has a much higher
velocity than that of the pulp through the reactor
30, until it reaches gas exit 37 where it escapes
peripherally to the annular gas exit chamber 32.
The contacting gas 69, by this time stripped of all
but a trace of ozone and predominantly composed of
oxygen plus yases produced as a by-product of the
bleaching reaction, is vented through nozzle 75 and
recycled to the ozone generator system. Bleached
pulp 10 at the bottom of porous bed reactor 30 is
diluted with recycled filtrate through dilution
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Docket No. 1197-IR-PA
nozzle 34 and is discharged through pulp discharge
36 as dilute bleached pulp 15.
Fig. 2 shows a partially sectioned schematic
view from line 2-2 of Fig. l of fluffer/blower 60
05 and its associated equipment. Pulp 5 from the
dewatering press enters compression screw feeder 62
through conduit 66 and from there is fed into
fluffer/blower 60. The compression screw feeder
compacts the pulp into a substantially impermeable
plug 99 which prevents backward flow of the mixed
gas from reactor system 20. Mixed gas 71 is fed
into fluffer/blower 60 through gas loop 50. It is
mixed with the fluffed pulp which it conveys as a
gas suspension of fluffed pulp by entrainment into
pulp and gas discharge conduit 51.
Referring to Figs. l and 2, the novel features
of the invention can best be understood. The
functions of the fluffer/blower 60 and its related
equipment, as well as the porous bed reactor 30, are
well-known in the art. Despite the shortcomings of
the prior art porous bed reactor, already described,
its high efficiency of ozone removal makes it a
desirable element, in combination with the present
invention, as explained below. The bleaching
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Docket No. 1197-IR-PA
reaction of ozone with pulp is very fast. Fig. 3
shows a graphic representation of the concentration
of ozone (in percent) versus time (in seconds) in
contact with fluffed pulp under conditions such as
05 those previously described for a porous bed
reactor. The concentration of ozone in oxygen
coming out of an ozone generator is shown as being
approximately 4%. It can be seen, that for a
porous bed reactor, approximatel~ 60% of the ozone
is consumed in less than one second of contact with
the pulp, while at six seconds, over 9G% of the
ozone is consumed. For this reason, thorough mixing
of the ozone with the pulp fibers is imperative in
order to avoid wide variations in bleached
lS brightness of the pulp. Thus, mechanical mixing
chamber 40 is provided between fluffer/blower 60 and
porous bed reactor 30.
The fluffed pulp/gas mixture 54 is injected
through fluff discharge cyclone 52 and mixing
chamber hopper 46 into auger 43 which conveys it to
the mixing paddles 44 mounted on rotor shaft 42.
The feed rate is maintained so that the bulk density
of the intensely agitated pulp in the substantially
horizontal mixing chamber is the maximum possible
which will not result in plugging of the chamber.
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Docket No. 1197-IR-PA
This ensures that all of the fluffed pulp passing
through chamber 40 is exposed, to the maximum
possible extent, to the gaseous reagent in the
chamber. Paddles 44 provide intensive mixing action
05 between the pulp fluff and the gas to maintain the
gas suspension of the pulp fluff which was formed in
the fluffer/blower 60 during the dwell time in the
mixing chamber 40. The mixing paddles closest to
discharge neck 45 prevent plugging of the neck by
the pulp exiting the mixing chamber 40 and dropping
into reactor 30 on top of pulp bed 7. The
contacting gas 69 which, after six seconds mixing,
contains less than half its initial ozone, continues
downward through the porous pulp bed until it
reaches gas exit 37 at which time its concentration
is typically less than 30 parts per million ozone.
The mixing chamber provides a pulp fluff dwell
time in the range of approximately 10 seconds while
maintaining the pulp in suspension in the contacting
gas 69 through intimate intensive mechanical mixing
of the gas and pulp fluff. This ensures the maximum
practical reaction extent prior to discharge of the
mixture into the porous bed reactor. Since the
ozone concentration entering the porous bed reactor
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Docket No. 1197-IR-PA
is suhstantially less than half its initial
concentration, the reaction rate is considerably
slower, and non-uniformity of bleaching due to
variable porosity of the pulp bed is further
05 mitigated.
Fig. 4 shows an alternative em~odiment of the
present invention which is similar to that of Fig.
1, except that, here, fluffer/blower 60 has been
replaced by fluffer 90 which, without any blowing
action, deposits fluffed pulp fibers directly into
mixing chamber hopper 46. Gas loop 50 has also been
eliminated in this embodiment, since the reagent gas
mixture is introduced in the mixing chamber hopper
rather than in the fluffer.
This invention, in its preferred embodiment,
provides the advantage of attaining uniform
bleaching reaction results in the example considered
without wasting expensive ozone or other gaseous
reagent. By providing the mechanical mixing chamber
prior to the porous bed reactor chamber, the desired
gas/pulp contact time is achieved while intimate
mixing is maintained. Thus, the invention provides
uniform substantially complete reaction under
commercially practical conditions over a wide range
of capacities.
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Docket No. 1197-IR-PA
The ozone/wood pulp reaction is used as an
example to illustrate the features of this
invention. Any gas/solid reaction in which the
reaction kinetics are similar to those of
05 ozone/wood pulp and in which the solid has fluffy
fibrous characteristics similar to wood pulp is an
appropriate application for this invention.
14
