Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to a process for bleaching
cellulosic material, in particular pulp, by means of
ozone ( 03 ) -
When bleaching cellulose with ozone, an ozonic gas
is initially produced by means of an ozone generator.If one departs from oxygen, using corona to form ozone,
oxygen concentrations of 100 to 130 g 03/Nm may be
attained, the current demand increasing with the 03
content. The ozonic gas in a mixer is admixed to the
pulp to be bieached or is employed in the form of an
aqueous ozonic solution.
If a cellulose consistency of 1 to 20 ~ is
applied, one speaks of low or medium consistency
bleaching, if cellulose is used at a higher consistency
of more than 20 %, this is termed high consistency
- bleaching.
When applying medium consisteny bleaching at
consistencies of about 10 ~, which is done most
frequently, it is not possible with ozone bleaching to
employ any desired amount of 03 per ton of cellulose,
not even if the bleaching gas is fed to the mixer under
pressure; the maxium amount of ozone used is 3 to 4 kg
03/t cellulose, because the amount of gas to be admixed
to the cellulose in a mixer is limited.
As the ozonic bleaching gas acts upon the
materials to be bleached, it is not oxygen that reacts,
but only the ozone contained therein. The oxygen leaves
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the bleaching vessel at the end of the bleaching
procedure, containing various impurities, such as
carbon monoxide, carbon dioxide as well as hydrocarbons
and other undesired components. In this contaminated
form, the oxygen comin~ from the bleaching vessel no
longer can be used to recover ozone, but either must be
discarded or must be subjected to expensive
purification procedures.
The mode of operation described involves high
oxygen and current demands, the bleaching effect
obtained is not always satisfactory, either.
~ he invention aims at avoiding the disadvantages
and difficulties described and has as its object
1) to control the overall process: ozone production
and bleaching in a manner that the oxygen from which
the 2~3 mi~ture i5 produced, does not get into
contact with the bleaching liquor such that it can be
used for another ozone production immediately upon
drying;
2) to reduce the amount of oxygen used considerably
as compared to the conventional mode of operation; and
3) to increase the amount of ozone to be introduced
into the bleaching liquor per ton of cellulose, if
desired.
In this connection, also the bleaching effect is to be
improved. In addition, the proc~ss is to be applicable
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both to low and to medium and high consistency
bleaching.
In accordance with the invention, these objects
are achieved by a combination of the following
measures:
a) that an ozonic gas is produced in an ozone
generator by corona in air or oxygen,
b~ that the ozonic gas is compressed and an aqueous
solution enriched with oxygen (strong water) is
recovered in an absorption vessel under pressure,
c) that the strong water is depressurized in a
depressurizing zone, thus releasing a gas having a high
03 content (strong sas),
d) that the strong gas is diluted with an ozone-free
or oxygen-poor gas,
e) that the gas obtained is mixed with the materials
to be bleached and is reacted in a bleaching zone,
f) that the material suspension bleached is degassed,
the solid material is washed and processed.
According to the invention, the ozone content in
the 02/03-mixture produced under a) may amount to 30 to
130 g 03/Nm . This content depends on the bleaching
effect sought and on economic considerations. I~ the
amount of ozone to be admixed to the cellulose is to be
25 increased to 4.5 to 8 kg 03/t cellulose, based on the
ozone added so far, an ozonic gas will be obtained in
the ozone generator at the upper limit of 130 g 03/Nm .
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If it suffices to keep the addition of ozone low, a gas
will be produced in the ozone generator at the lower
limit of the range, which has the advantage of
requiring less energy.
The compression of the ozonic gas and the
preparation of the aqueous solution (strong water) in
an absorption vessel may be effected at a pressure of 7
to 10 bar. The strong water may contain 200 to 350 g
03/m3 with a temperature-dependency being involved.
The gas obtained in the depressurizing zone
according to c) and having an elevated 03-content
contains more than lO0 g 03/m3, usually between 200 and
250 g 03/m3; it is diluted with an ozone-free or
oxygen-poor gas, an oxygen-poor gas being a gas that
does not contain more oxygen than air. In any event,
the dilution must be such that the risk of
decomposition, which would occur above 50 g 03/Nm , is
avoided; as a rul~, the gas is adjusted to a content of
100 to 180 9 03/Nm , pre~erably of 100 to 150 g 03/Nm .
By the process of the invention, it has become
possible to increase the amount of ozone employed in
medium consistency bleaching using mechanic mixers and
gaseous ozone, to 4 to 8 k~ 03/~ of the material to be
bleached.
An embodiment of the process according to the
invention, which, in the first place, is suitable for
consistencies of from 1 to 20 % still pumpable, will be
-- 5
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explained in more detail by way of the diagram of Fig.
1.
A modified process for the production of
cellulosic material, in particular pulp, to be employed
in high consistency bleaching, which is preferred with
consistencies of more than 20 %, is characterized by
the following measures:
a) that an ozonic gas is produced in an ozone
generator by corona in air or oxygen,
b) that the ozonic gas is compressed and an aqueous
solution enriched with ozone (strong water) is obtained
in an absorption vessel under pressure,
c) that the strong water is depressurized in a
depressurizing zone, gas having a high 03 content
(strong gas) being released,
d) that the strong gas is diluted with ozo~e-free or
oxygen-poor gas, preferably to an ozone content of 40
to 60 g 03/Nm ,
e) that the gas obtained is mixed with the material
suspension to be bleached having a consistency of more
than 20 % and is reacted in a bleaching zone,
f) that the offgas from the bleaching zone is
recycled to the depressurizing zone and ~s used as a
dilution gas,
g) that the bleached material suspension is washed and
processed.
2~8~
Measures a) to e) are identical with those of the
first embodiment; however, the 03-content is reduced
more intensively in the dilution in order to avoid
damage to the fibers. Also, there is no separate
degassing station after bleaching, but the offgas from
bleaching is recycled directly as a dilution gas. ~his
process variant is illustrated in detail in the diagram
of Fig. 2.
~ he invention also relates to a plant for carrying
out the process, comprising an ozone generator, an
absorption column and a drier connected in terms of gas
ducts, a depressurizing vessel including a liquid
volume and a gas volume, the liquid volume being
connected with the absorption column by an ozonic
solution feed duct and by a return duct for recycling
return water, a bleaching tower or bleaching reactor
provided with a supply duct for supplying the material
to be bleached and with a discharge duct for the
bleached material, which plant is characterized in that
the bleaching tower or bleaching reactor is connected
with the gas volume of the depressurizing ~essel by an
offgas recirculation, through which ozone-free or
oxygen-poor gas is feedable to the gas volume as a
dilution gas.
According to a preferred embodiment, the bleaching
reactor is connected with a degassing and pressure
control station.
-- 7 --
2~7~
The invention will be explained in more detail
with reference to the accompanying diagrams (Figs. 1
and 2~.
In the diagram according to Fig. 1, in which the
gas-feed ducts are entered in broken lines and the
ducts conducting the aqueous solutions or suspensions
are entered in full lines, oxygen is fed to an ozone
generator 1 through a duct 2. An O2/O3-mixture obtained
and having a content of, for instance, 50 to lOO g
O3/Nm oxygen, is compressed to a pressure of 7 to lO
bar and is conducted into the absorption (washing)
column 4 through a duct 3. There, the ozone is washed
out of the oxygen by means of circulating water and is
dissolved.
The non-dissolved oxygen is supplied to the drier
6 through a duct- 5 and from there is returned to the
ozone generator through a duct 7. Thus, oxygen in this
circuit of the process gets into contact with pure
water only and, therefore, will not be contaminated.
The aqueous ozone solution produced under pressure
(strong water) is supplied through a duct 8 from the
absorption column 4 to the depressurizing vessel 9
working without pressure. Since the solubility of ozone
is lower in the pressureless state than under pressure,
ozone and oxygen are exhaled from the solution~
Since the solubility of oxygen is lower than the
solubility of ozone in water, a highly concentrated
20~784~
ozone gas is formed in the gas volume 9a of the
depressurizing vessel as can not be produced by means
of presently available ozone generators. The ozone
content within the gas volume 9a of the depressurizing
vessel may amount up to 250 g 03/Nm . However, above
this c~ntent there is the risk of an explosive
decomposition such that it must be seen to it that a
dilution gas is fed into the gas volume 9a of the
depressurizing vessel thIough duct 11 in order to
reduce the ozone content, for instance, to a content of
200 g/Nm or less. In any event, the ozone content must
amount to more than 100 g/Nm3 in order to be able to
realize ~he desired bleaching action. Return water from
the depressurizing zone 9 is recycled into the washing
column 4 through duct 10.
- The diluted gas- is withdrawn from the
depressurizing zone 9 through duct 12 and is fed to a
mixer 13. A duct 14 for the material suspension to be
bleached leads to this mixer 13, which material
suspension is premixed in a mixer 15 and is adjusted to
an acidic pH of 4 or less. This premixed suspension is
mixed in the mixer 13 with the bleaching gas comins
from duct 12. Advantageously, the bleaching gas, at
that stage, has a concentration of 100 to 150 g 03/Nm3.
~leaching proper, i.e., the reaction of the
bleaching gas with the material suspension, takes place
in a bleaching tower 16 at a reaction time of 5 to 10
2~7~
minutes. From the bleaching tower 16, the suspension,
i.e., the mixture of material suspension and bleaching
gas, via duct 17, gets to the pressure control zone 18,
in which ozone-free or ozone-poor gas is released. In
this degassing zone, air is introduced through duct 19.
The air/oxygen mixture is fed to the gas volume
9a of the depressurizing zone 9 through ducts 20 and 11
as a dilution gas. In this manner, the oxygen content
of the bleaching gas is lowered in the circuit between
depressurizing zone, bleaching tower and offgas
recirculation; the oxygen saved may amount up to 80
as compared to earlier modes of operation.
The cellulose suspension withdrawn from the
pressure control zone is washed in the washing filter
21, the waste water 22 forming is supplied to the duct
- 17 conducting the bleached suspension prior to reaching
the pressure control zone as a dilution water through
duct 23. The bleached cellulose is discharged through
duct 24.
In the process diagram according to Fig. 2,
identical plant parts are denoted by the same reference
numerals, thus, the ozone generator by 1, the oxygen
supply duct by 2, the 2/3 duct by 3, the washing
column by 4, the oxygen return duct by 5 and the drier
by 6 such that this circuit corresponds to that of Fig.
1. Likewise, the ozone solution coming from the washing
-- 10 --
2~67~4
column 4 is denoted by 8, the depressurizing zone by 9
and the gas volume of this zone by 9a.
In the same manner as described in respect of Fig.
1, the return water is recycled from the depressurizing
zone to the washing column through duct 10. Also this
circuit corresponds to that described in Fig. 1.
In the gas space 9a of the depressurizing zone,
the exhaled ozone is diluted, the dilution gas corning
from duct 11, and is conducted directly into the
bleaching reactor 16 through duct 12. As in the first
embodiment, oxygen is again saved by supplying a
dilution gas.
The path of the material to be bleached leads
through a duct 25 to a press 26, where the material is
adjusted to the desired concentration, e.g., to a
consistency of above 20 ~. Subsequently, the material
is supplied to a fluffer 27 to produce fluffs. These
fluffs continue to pass through duct 25 into the
bleaching reactor, which is provided with appropriate
inlet and outlet openings.
The bleached material leaving the bleaching
reactor is diluted at the washing filter 21 and
conducted to cellulose washing. Air 19 may additionally
be fed to the systern via a pressure control system
connected to the bleaching reactor 16. However, it is
also possible to remove offgas from the system in case
the gas required for dilution becomes less than the gas
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incurred. Offgas from the bleaching reactor is supplied
as a dilution gas through duct 20 and duct 11.
In high consistency bleaching as described with
reference to the diagram of Fig. 2, it is usually
operated at a lower ozone content of the bleaching gas,
preferably at an ozone content of 40 to 60 g O3/Nm
oxygen, in order to keep any damage to the fibers low.
When carrying out the process according to the
invention, the following quan~titative data are
typically obtained:
lS,000 g oxygen are required to produce l,000 g
ozone having a concentration of 100 g 03/Nm . Taking
into account small losses, 15,500 g oxygen are actually
required to produce the 2/3 mixture in the 03-
generator in order to produce 1,000 g ozone, the amount
- of oxygen being constantly supplemented.
From the absorption column, strong water
containing 200 g 03/m is obtained. At the same time,
1,300 g oxygen/m3 dissolve in the strong water under
the conditions indicated. With the plant operated
continuously, the dissolved gases emerge from the
solution during depressurization of the strong water in
the depressurizing zone, l,000 g ozone and 1,300 g
oxygen thus being released, which corresponds to a
;~ 25 total of 2,300 g oxygen.
In recycling ~o the ozone generator, the oxygen
supplied amounts to 15,500 g and the oxygen consumed
- 12 -
2~7~
amounts to 2,300 g. Thus, 13,200 g oxygen remain to be
recycled. Since small losses still must be taken into
account in processing the oxygen, the amount will be
reduced by 5 ~ such that the recycled amount of oxygen
is 12,540 g or 80.9 % of the amount supplied.
During continuous operation, 2,960 g fresh oxygen
are supplied for the production of 1,000 g ozone. In
the depressurizing column, a gas mixture forms which
consists of 1,000 g ozone and 1,300 g oxygen. By
supplying air or offgas, the ozone content of the gas
is adjusted to the desired ozone concentration value.
The oxygen recycled to the ozone generator only needs
to be dried, because otherwise it does not contain any
impurities.
