Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02341490 2001-02-23
WO 00/11262 PCT/F199/00696
BLEACHING OF MEDIUM CONSISTENCY PULP WITH OZONE
WITHOUT HIGH SHEAR MIXING
Field of the invention
The invention relates to a method for bleaching medium consistency pulp with
an ozo-
ne-containing gas. In particular, the invention relates to the proper
utilization of the very
fast reaction of ozone, by providing efficient but pulp-preserving mixing
immediately
on introducing a substantial amount of ozone into the pulp.
Background of the invention
A number of methods for the bleaching of pulp with ozone is known in the art.
These
methods have developed towards carrying out the bleaching stage with medium
consis-
tency pulp, i.e. having a consistency of about 7 - 16 per cent.
Generally, ozone bleaching of medium consistency (MC) pulp according to
current
practice can be described as ozone generation followed by compression before
introdu-
cing the ozone containing gas into the the MC pulp flow. The gas-liquid-fiber
suspensi-
on is vigourously treated in one or several high shear mixers before the
suspension is
lead to a bleach tower. The ozone may be introduced at several points along
the pulp
stream. Vent gases must be treated because of excess ozone carried over.
2o The principle described may be a result of the application of oxygen
bleaching methods.
Oxygen, however, operates at a much slower rate, and the temperatures used are
signifi-
cantly higher than those employed in ozone bleaching.
Typical and frequent problems arise from the difficulty to keep the suspension
uniform.
Segregation into two-phase flow easily occurs, and the ozonisation rate drops
signifi-
cantly (to 1 or even 0.1 % of its optimum rate) This is a dominant problem,
which may
be reduced by using a higher quality ozone, resulting in less gas void and
consequently
less need for vigorous mixing. A typical solution in the present state of the
art is the use
of more than one mixer. This does not, however, eliminate the problem, and by
applying
more shear forces to the pulp, the strength properties of the resulting
product are severe-
ly affected.
A basic problem with such mixers is the short residence time, and if mixing
time is in-
creased, undesired backmixing may occur.
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After leaving the mixers, the gas-pulp suspension soon segregates into two-
phase flow
having a relatively small gas-liquid interface per unit volume. The chemical
consequen-
ces of this are low capacity and a non-uniform bleaching result. Obvious
evidence of
this phenomenon is the significant ozone surplus often remaining after the
bleaching
stage, representing both a hazard and an economical loss.
Description of the prior art
A pulp bleaching method comprising introduction of high pressure ozone in a
carrier
gas into a pulp stream with vigorous mixing and subsequent removal of carrier
gas is
lo disclosed in, e.g. EP-A 511 433. The major issue of this document is the
removal of gas
from the pulp after injection into the mixer; the reaction is said to take
place essentially
within ten seconds in a vertical reaction vessel situated immediately
following the flui-
dizing mixer. Gas at about 10-13 bar containing about 3-10 % ozone by weight
(6.8 vol
%) is used. Preferably, the gas-pulp mixture is carried in a horizontal path
following the
vertical reaction step to effect separation of the large amount of carrier gas
involved.
Austrian patent application 2203/92 describes a method wherein medium
consistency
pulp is treated with an ozone-containing gas comprising more than 120 g 03 /
normal
m3 gas (5.6 vol %) whereby the gas is introduced as fine bubbles with a low
differential
pressure (preferably less than 1 bar). It is considered that using gas with a
high ozone
content, a sufficient amount of ozone is can be suspended into the gas to
achieve the
desired bleaching. Further, AT 2203/92 discloses the use of mixers with or
without
fluidisation effects, and of an ozone reaction stage subsequent to the mixing
stage, as
well as additional ozone addition stages with degassing stages in between.
Characteris-
tically, the highly concentrated ozone is introduced in static mixers at
several points,
possibly removing the inert carrier gas (normally oxygen) between stages, and
the final
reaction between ozone and fiber takes place in a bleach reactor, typically of
the tradi-
tional upflow tower type.
3o A common feature of several other publications disclosing ozone bleach
processes for
medium consistency pulp is the use of fluidizing mixers in connection with the
injection
CA 02341490 2001-02-23
WO 00/11262 3 PCT/F199/00696
of ozone-carrying gas, and the use of subsequent, relatively extended reaction
stages and
gas separation.
In chemical process terms, MC ozonisation can be described as ozone molecules
in a
gas phase that must be transported to the vicinity of the fiber and react with
the fiber or
other substrates. The ozone must diffuse through the gas-liquid interface,
through the
liquid to the fiber. The applied mixing affects the size and the relative
velocity of the
gas bubbles, and also the amount of fiber-liquid interface. The rate limiting
step
completely dominating the interaction of ozone with the fiber material is the
transport of
lo ozone through the gas-liquid interface. The gas-liquid transfer rate in a
given volume is
heavily dependant on the bubble size, i.e. gas-liquid surface area m2 gas/m3
suspension,
and on the partial pressure of ozone. Other rate limiting steps, like
diffusion in the fiber
material itself, are determined by the nature and the consistency of the pulp,
which is
dominantly affected by the temperature.
Due to its dependency on mass transfer, the reaction rate of ozone is,
theoretically and
empirically, first order.
Consequently, efficient process solutions must be characterized by that
- the residence time distribution (RTD) must follow a plug-flow pattem (in
contrast,
backmixing commonly occurs in mixers), which requires special reactor geometry
to
avoid backmixing e.g appropriate turbine and baffles.
- mean residence time in transfer/mixer/reactor must match transport and
reaction
times for complete conversion of ozone; consequently reactor diameter, shape
and
rotation rate of a possible turbine must match flow rate.
- all ozone should be introduced in one step.
The high gas void, i.e. the low concentration of ozone generated by most
present ozone
generators, limits the possibilities to improve the situation. Reduced gas
void in subse-
quent generations of ozone generators will reduce the need for mixing and
reduce ener-
gy requirements as well as the size of the equipment. Higher ozone
concentrations will
also increase the ozonisation rate.
CA 02341490 2008-07-03
4
Disclosure of the invention
According to the method of the present invention, high-concentration, high
pressure
ozone is introduced into the pulp line, whereby conditions approaching plug
flow are
achieved, a high concentration of ozone is reached with a mass transfer area
in the
suspension which is sufficient for effective delignification.
In accordance with one embodiment of the present invention, there is provided
a method
for bleaching cellulose pulp of 7 to 16 % consistency, comprising introducing
into a pulp
stream a stream of ozone-containing gas at a pressure of at least 10 bar and
having an
ozone concentration of at least 20 % by weight, in essentially plug flow
conditions
without applying high shear mixing.
According to one preferred aspect of the present invention, the ozone is
introduced using
effective injection nozzles providing for the efficient dispersion necessary
for obtaining a
uniform distribution as well as sufficient mass transfer area to overcome the
rate-
delimiting mass transfer threshold present in methods according to the prior
art. Thus, the
need for fiber-destroying high shear fluidizing mixers is removed.
According to another preferred aspect of the present invention, a dynamic low
to medium
intensity mixer is provided in the pulp stream immediately downstream of the
ozone
injection site. Such a mixer delivers to the pulp stream amounts of energy
which are well
below fluidization energies, and does not mechanically affect the fiber.
With the aid of recent technology, as disclosed in e.g. WO 97/01507, ozone
with a
concentration of up to 18-20 % by volume may be generated. References to
concentrations as high as 300 g 03/Nm3 have been made in prior art
publications (e.g.
EP-A-426 652, priority 30.10.1989), but such concentrations have not been
technically
feasible until recently. Using a high ozone concentration (300 g per m3 and
higher) and at
high pressure (10 bars and higher) together with proper injection technique,
the reaction
between ozone and fiber is allowed to take place at such a rate that the
subsequent use of
an upflow bleach tower is not necessary. The gas pressure is obtained by using
precompressed oxygen, optionally mixed with other gases or liquids (e.g.
argon) to
maintain a suitable conductivity for ozone generation.
CA 02341490 2007-07-11
4a
Oxygen is the most common carrier gas used for ozone. Highly concentrated
ozone is
usually considered an explosion hazard. As the ozone generating technology has
developed, the accepted limit for stable oxygen-ozone mixtures has been
repeatedly
pushed upwards, and it appears that no absolute concentration limit for the
safe handling
of ozone has yet been established. Thus, use of very high ozone concentrations
may yet
be possible, which further facilitates use of methods according to the present
invention.
According to the present invention, the concentration of ozone in the gas
introduced to
CA 02341490 2001-02-23
WO 00/11262 5 PCT/F199/00696
the pulp stream is sufficient for achieving bleaching without any fiber-
destroying
mechanical impact.
The initial distribution of highly concentrated ozone into the pulp is of
importance, for
the selectivity, as the carbohydrate component itself may be attacked by ozone
if ex-
posed for an extended time. The absence of backmixing, as may occur in high
shear
mixers, and the presence of plug flow conditions counteract this phenomenon.
Description of preferred embodiments
1o Figure 1 shows a comparison between the changes in reaction rates against
time in a
prior art ozone pulp bleaching process using a medium consistency mixer, and a
process
according to the present invention.
Example 1
Ozone-carrying gas having a pressure of about 15 bar and an ozone
concentration 14 %
by volume is introduced into a medium consistency pulp line carrying 1000
tons/day via
a collar of radially arranged nozzles. Preferably, the nozzles are arranged to
direct the
gas radially into the pulp flow, essentially in a direction perpendicular to
the pulp flow.
A number of nozzles sufficient for distributing the gas evenly must be used.
On this
production scale, 186 nozzles with an inlet diameter of maximum 1 mm may be
used.
A sufficient mean residence time (10-40 seconds) must be allowed before any
other
disturbing action to the pulp.
Example 2
A medium intensity (low-shear) mixer is adapted into the pulp stream of the
previous
example, preferably immediately following the gas injection site. The mixer
turbine is
preferably a double or multiple screw with blade angles and rotation rate
balanced to
maintain the plug flow residence time distribution (RTD) and giving good
radial mixing
efficiency. The center blade has a steeper angle than the outer screw blade.
Alternative-
ly, porous metal injector devices for introduction of ozone can be arranged
peripherical-
ly or on the turbine.
CA 02341490 2001-02-23
WO 00/11262 6 PCT/F199/00696
Figure 1 shows a comparison between a system employing a traditional medium
consis-
tency mixer with a vet-y high capacity for a short interval dropping rapidly
to zero, com-
pared to a system according to the invention with a moderately high capacity
kept
constant for a long period. The dotted line represents state-of-the-art
traditional medium
consistency mixer technology. The first, steep section shows the effect of the
mixer with
high reaction and uniform distribution. The low rate section shows the effect
of the cor-
ruption of the gas-suspension interface. The reaction takes place with a
nonunifonn
distribution and the pulp is mechanically stressed by high shear mixing.
The solid line represents a system according to the invention. Throughout the
process, a
1o moderately fast reaction is taking place in a mildly stressed pulp and with
a uniform
distribution of ozone.
Table 1 shows a comparison in numbers between a typical conventional MC
bleaching
system, a state-of-the-art system and a system according to the present
invention.
Table 1
Conventional Modern Present invention
Calculus Base Units
Pulp production ton OD/day 1000 1000 1000
Consistency % 10 10 10
Ozone pressure bar 9 9 15
Ozone concentration w% 10 14 20
vol% 7 10 14
Ozone charge (3-5) kg/ton OD pulp 5 5 5
Ozone generator kg/h 208 208 208
Ozone volume flow m'/s 0,0146
Nozzle diameter m 0,001
Number of nozzles 186
Process
Process temperature C 40 40 40
Process pressure bar 7 7 15
Pulp Flow ton OD pulp /h 42 42 42
Volume Flow m'/h MC pulp 375 375 375
Ozone gas charge m'/h at actual press. 234 165 53
Gas void * `%) 38 31 12
Equipment Ozone compres- Ozone com- No ozone com-
sor pressor pressor
1-3 mixers 1+ mixers No mixer
Bleach tower Bleach tower Small bleach
reactor
* Note: Gas void is propoi-tional to process problems
