Note: Descriptions are shown in the official language in which they were submitted.
~ Docket No. 1958-IR-PA
METHOD FOR RAPID OZONE BLEACHING OF WOOD PULP
BACKGROUND OF THE INVENTI02T
This invention relates generally to bleaching of
wood pulp and more particularly to rapid rate bleaching
of pulp at more than twenty percent consistency, using
gaseous ozone as the bleaching reagent.
Wood pulp bleaching with ozone and other relatively
high reaction rate gases promises a significant reduction
of objectionable pulp mill effluents to streams and to
the environment. It is to be expected that great
economic and ecological benefits can be achieved by
elimination of chlorine compounds from the bleaching
sequence. However, incorporation of these noncholorine
bleaching reagents can impose significant capital costs
on the mills due to the requirement for large complex
mechanical contactors and reaction vessels. Because of
these high capital equipment costs, virtually no
production scale experiments have been done, despite the
availability of such large reaction vessels.
Aside from its high cost, another deterrent to
application of bleaching~with ozone gas has been the
difficulty in obtaining uniform bleaching results. This
is primarily attributable to the rapid reaction rate of
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ozone with wood pulp, which results in overbleaching of -
some fibers which are first contacted by the ozone
bearing carrier gas, and underbleaching of other fibers
which are contacted after the ozone concentration in the
carrier gas has been severely reduced. It is obvious
that, if each pulp fiber were individually presented to
the bleaching atmosphere, all such fibers would be
uniformly bleached. To this end, the pulp industry has
directed intense efforts toward ever more vigorous
comminution to achieve finer fiber bundle particle sizes
in high consistency pulp in order to achieve greater
uniformity of contact between pulp fibers and ozone
bleaching gas. In addition, numerous machinès of varying
degrees of complexity have been employed in attempts to
improve the uniformity of bleaching by agitating the pulp
in the presence of the bleaching gas. These efforts have -
been directed toward elimination of channelling of the ~-~
bleaching gas through the pulp bed and increasing the
contact of unbleached fiber with the ozone gas.
Mixing reactors, however, frequently aggravate the
nonuniformity of bleaching by causing compaction of some
of the pulp into relatively impervious masses which are
not amenable to penetration by the bleaching gas. This ~-
compaction also causes increased power consumption,
thereby further reducing its economic viability. Of
~ Docket No. 1958-IR-PA
course, the longer such intense agitation is required for
completing the bleaching reaction, the greater will be
the size of the agitating reactor required for a given
pulp production rate, and the greater will be the
required operating power. It is clear that currently
available agitating reactors are not capable of avoiding
the problems described above.
The foregoing illustrates limitations known to exist
in present devices and methods. Thus it is apparent that
it would be advantageous to provide an alternative
directed to 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 o~ the present invention, this is
accomplished by providing a method for a rapid bleaching
of wood pulp at a consistency of more than 25% using
ozone gas, including the steps of introducing ozone in a
carrier gas, at a concentration of five percent to
fifteen percent, by weight, into a bleaching reactor at a
pressure of at least one and one-half atmospheres;
retaining the pulp and ozone gas in the bleaching reactor
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for a required bleaching time; and discharging bleached
pulp to a receiving vessel for further processing.
The foregoing and other aspects will become apparent
from the following detailed description of the invention
when considered in conjunction with the accompanying
drawings.
:
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic block diagram illustrating a
preferred embodiment of the present invention;
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Fig. 2 is a graphical representation showing the
relative ozone consumption rates for two different ozone
feed gas concentrations; and -~
Fig. 3 is a graphical illustration of the effect of
pressure on ozone consumption rate in a cocurrent flow
pulp bleaching reactor at a twelve weight percent ozone
feed gas concentration.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 illustrates~a portion of a pulp processing
line in which the present invention is embodied. Pulp
from conduit 5 enters pulp thickener 10 and is thickened
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Docket No. 1958-IR-PA
to a consistency of twenty percent or greater before
being discharged through conduit 15 to a fluffer 20.
After being fluffed, the pulp is transported through
conduit 25 to contacting reactor 300 in which the fluffed
pulp particles are mechanically tossed in the presence of
ozone in a carrier gas which is introduced through
conduit 67. After the required bleaching time in the
contacting reactor, the pulp, together with the reaction
gases, is discharged through conduit 35 to receiving
vessel 40. Here the gas is separated from the pulp and
discharged through conduit 45 to a gas purifier 60, in
which reaction products and other impurities are removed
from the carrier gas. The pulp, having been separated
from the gas in receiving vessel 40, is discharged
through conduit 55 to further processing. The carrier
gas from purifier 60 is discharged into conduit 62 where
it mixes with fresh ozone from ozone generator 100, which
is introduced through conduit 105. This gas mixture
enters gas compressor 65 in which the gas mixture may be
compressed to as much as fifteen atmospheres pressure.
The pressurized gas is metered through conduit 67 to
contacting reactor 300.
' The process just described is continuous and is made
possible by formation of a pulp plug at the juncture of
conduit 15 and pulp fluffer 20 and another plug at the
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Docket No. 1958-IR-PA
juncture of conduit 55 and receiving vessel 40. These
pulp plugs are being continuously added to at their
rearward faces and continuously subtracted from at their
forward faces, thereby providing continuous throughput of
pulp while retaining higher than atmospheric pressure in
the system between pulp fluffer 20 and receiving vessel
40. This permits operation of contacting reactor 300 at
higher than atmospheric pressure which increases the rate
of the pulp bleaching reaction. The rate of reaction of
lo ozone with wood pulp is directly proportional to the
ozone concentration and the pressure at which the
reaction is carried out. Thus, it is clear that by
maintaining the plugs previously described, it is
feasible to conduct a continuous reaction at elevated
pressure in which pulp is introduced at one end and
discharged at the other end without a loss in pressure
which would upset the reaction. In the past, the maximum
ozone concentration which was economically available from
standard ozone yenerators was approximately six percent.
Recently, however, it has become possible to generate
ozone at concentrations approaching fifteen percent.
. .
Figs. 2 and 3 graphically illustrate consumption of
ozone in wood pulp bleaching as a function of time of
reaction. In ~~ig. 2 the effect of ozone concentration is
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Docket No. 1958-IR-PA
shown, and as predicted, the consumption rate of ozone at
twelve percent is considerably higher than at three
percent. Here, also, it should be noticed that
consumption rates for cocurrent and countercurrent flows
are significantly different at a yiven ozone
concentration. In the case of a high intensity mixed
reactor, the time for 100% consumption of the ozone
applied to the pulp can be reduced by as much as 50%.
The effect of pressure on the ozone pulp reaction
rate is indicated in Fig. 3. Again, as expected, the
ozone consumption rate in the pulp bleaching reaction is
significantly increased by increased pressure. It
follows that combining high concentration ozone as in
Fig. 2 with high pressure ozone as in Fig. 3 will produce
a pulp bleaching reaction which is significantly faster
than was previously possible. This makes it possible to
accomplish the same pulp production in a significantly
smaller reactor.
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