Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS FOR THE PRODUCTION OF CHLORINE DIOXIDE
The present invention relates to a process for the continuous production of
chlorine dioxide under non-crystallising conditions in at least two reaction
vessels:
Chlorine dioxide used in aqueous solution is of considerable commercial
interest,
mainly in pulp bleaching, but also in water purification, fat bleaching,
removal of phenols
from industrial wastes etc. It is therefore desirable to provide processes in
which chlorine
dioxide can be efficiently produced.
There are numerous different processes for chlorine dioxide production. Most
large
scale processes in commercial use involve continuous reaction of alkali metal
chlorate in an
acidic reaction medium with a reducing agent such as hydrogen peroxide,
methanol,
chloride ions or sulfur dioxide to form chlorine dioxide that is withdrawn as
a gas from the
reaction medium. Generally, the acidity is mainly provided by addition of
sulfuric acid and the
sulfate is withdrawn as a by-product in the form of solid alkali metal sulfate
or dissolved in
depleted reaction medium.
In one kind of processes the reaction medium is maintained in a single
reaction
vessel under boiling conditions at subatmospheric pressure, wherein alkali
metal. salt of the
acid is precipitated and withdrawn as a salt cake. Such processes are
described in e.g. US
patents 5770171, 5091166 and 5091167.
In another kind of processes the reaction medium is maintained under non-
crystallising conditions, generally at substantially atmospheric pressure. In
most cases
depleted reaction medium from a first reaction vessel is brought to a second
reaction vessel
for further reactions to produce chlorine dioxide. Early examples of such
processes are the
Mathieson and Solvay processes using sulfur dioxide and methanol,
respectively, as
reducing agents. Attempts to modemise these processes by at least partly using
hydrogen
peroxide as reducing agent are described in e.g. JP Laid Open Applications,
Laid Open No.
1988-008203, 1991-115102 and WO 01/077012 but have been commercialised only to
a
very limited extent. A break-through came with the process disclosed in EP
612686 using
hydrogen peroxide as reducing agent in both the first and the second reaction
vessels. Such
a process has been commercialised under the trademark HP-A and is easy to
operate and
enables high capacity production of chlorine dioxide with high yield in simple
equipment.
In the non-crystallising processes depleted reaction medium withdrawn from the
final reaction vessel contains acid, alkali metal salt of the acid and
unreacted alkali metal
chlorate which thus is lost. It has then been believed that the process should
be operated
with as low chlorate concentration as possible in the final (normally the
second) reaction
vessel to minimise the losses. On the other hand, it has been found that if
the chlorate
concentration is too low, the corrosion of the process equipment (generally at
least partly
made of titanium) increases. However, it has now surprisingly been found that
in a
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process using hydrogen peroxide as reducing agent it is possible to operate
with a higher
chlorate concentration than previously believed without significantly
increasing the loss of
chlorate.
The invention thus concerns a process for the continuous production of
chlorine
dioxide under non-crystallising conditions in at least two reaction vessels
comprising the
steps of feeding to a first reaction vessel alkali metal chlorate, a mineral
acid and
hydrogen perokide to form an acidic reaction medium maintained in said first
reaction
vessel, reacting alkali metal chlorate, hydrogen peroxide and mineral acid in
said reaction
medium to form chlorine dioxide and alkali metal salt of the mineral acid,
withdrawing
chlorine dioxide from said reaction medium in said first reaction vessel as a
gas,
withdrawing depleted reaction medium comprising mineral acid, alkali metal
chlorate and
alkali metal salt of the mineral acid from said first reaction vessel and
feeding it to a
second reaction vessel, feeding hydrogen peroxide to the reaction medium in
said
second reaction vessel and maintaining said reaction medium therein at a
concentration
of alkali metal chlorate from about 9 to about 75 mmoles/litre, preferably
from about 14 to
about 56 mmoles/litre, most preferably 20 to about 47 mmoles/litre, reacting
alkali metal
chlorate, hydrogen peroxide and mineral acid in said reaction medium to form
chlorine
dioxide and alkali metal salt of the mineral acid, withdrawing chlorine
dioxide from said
reaction medium in said second reaction vessel as a gas, and withdrawing
depleted
reaction medium comprising mineral acid and alkali metal salt of the mineral
acid from
said second reaction vessel.
The reactions taking place in the reaction vessels are complex and not fully
known in every detail. The main products are chlorine dioxide, oxygen and
alkali metal
salt of the mineral acid. Under certain circumstances some of the chlorate is
converted to
chloride as end product instead of chlorine dioxide. It was found that the
amount of
chloride obtained as end product could be lowered by increasing the chlorate
concentration in the second reaction vessel. Thus, the lower amount of
chloride in the
depleted reaction medium withdrawn from the second reaction vessel to a great
extent
compensates for the loss through the higher chlorate concentration.
Preferably inert gas is blown through the reaction vessels to increase the
agitation and dilute the chlorine dioxide to a safe concentration. It is also
possible to
introduce some inert gas above the liquid level in the reaction vessels. Any
available inert
gas such as nitrogen or oxygen can be used, but for cost reasons it is usually
preferred to
use air.
The chlorine dioxide and oxygen formed in the reaction vessels are withdrawn
as
a gas together with any inert gas blown through the vessels. The gas is
preferably
brought to an absorber where it is contacted with water to dissolve the
chlorine dioxide
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while the main part of the oxygen and other non-soluble gases pass through.
The
chlorine dioxide water can then be collected in a storage tank and be used for
any
desired purpose such as bleaching of pulp.
The depleted reaction medium withdrawn from the second reaction vessel is
preferably brought to a stripper fed with inert gas to blow off chlorine
dioxide and other
gaseous species still remaining therein. The gas from the stripper can then be
brought to
the absorber together with the gas from the reaction vessels. The stripped
depleted
reaction medium, also referred to as waste acid, may in many cases be used for
pH
adjustments and/or a source of sulfur in a pulping process. It is also
possible to
electrochemically increase its acidity it in a cell as described in, for
example, US patents
5487881 and 6322690, and optionally recycle it fully or partly to the first
reaction vessel
where it then constitutes at least part of the mineral acid feed.
Usually chlorate of sodium, potassium or a mixture thereof is used, but also
other alkali metals may come into question. The alkali metal chlorate is
usually fed in the
form of an aqueous solution, preferably of high concentration, for example
from about 3
moles/litre up saturation. In most cases it is not necessary to feed any
chlorate to the
second reaction vessel apart from what is contained in the depleted reaction
medium
from the first reaction vessel.
Alkali metal chlorate usually contains small amounts of chloride as an
impurity,
but it is preferred if this amount is as low as possible which decreases the
formation of
chlorine as a by-product. It is preferred that the amount of chloride in
alkali metal chlorate
feed is less than about 1 mole %, more preferably less than about 0.5 mole %,
most
preferably less than about 0.05 mole %, particularly most preferably less than
about 0.02
mole %.
The mineral acid is preferably a halogen free acid such as sulfuric acid or
phosphoric acid, of which sulfuric acid is most preferred, for example at a
concentration
from about 60 to about 98 wt%. Also mixtures of mineral acids may come into
question. In
most cases it is not necessary to feed any mineral acid to the second reaction
vessel
apart from what is contained in the depleted reaction medium from the first
reaction
vessel.
It is preferred that substantially no chloride except the impurities in the
alkali
metal chlorate is fed to the process. However, small amounts of chloride may
be present
also in other feed streams, such as the mineral acid. Preferably the total
amount of
chloride fed to the process, including impurities in the alkali metal
chlorate, is less than
about 1 mole %, more preferably less than about 0.5 mole %, most preferably
less than
about 0.05 mole %, particularly most preferably less than about 0.02 mole %
chloride of
the alkali metal chlorate feed.
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Hydrogen peroxide is used as reducing agent in both the first and the second
reaction vessel and is usually fed as an aqueous solution, preferably with a
concentration
from about 10 to about 70 wt%, most preferably from about 25 to about 60 wt%.
Preferably the amount of hydrogen peroxide fed is from about 0.5 to about 2
moles per
mole alkali metal chlorate fed, most preferably from about 0.5 to about 1 mole
per mole
alkali metal chlorate fed, particularly most preferably from about 0.5 to
about 0.6 moles
per mole alkali metal chlorate fed. Preferably from about 50 to about 99.9 %,
most
preferably from about 85 to about 99.5 % of the total amount of hydrogen
peroxide is fed
to the first reaction vessel. Apart from the small amount of chloride present
as an impurity
in the chlorate hydrogen peroxide is preferably the only added reducing agent,
although it
is fully possible to also add other reducing agents such as methanol,
formaldehyde, formic
acid, sugar alcohols, sulfur dioxide and chloride. In the case of other
reducing agents being
added, the amount of hydrogen peroxide added may be lowered.
The reactants may be fed as separate or pre-mixed feed streams. Particularly
it
is possible to pre-mix hydrogen peroxide and alkali metal chlorate into a
common feed
stream, while it is preferred to feed the mineral acid separately.
The temperature of the reaction medium in the reactions vessels is preferably
maintained from about 30 to about 100 C, most preferably from about 40 to
about 80 C.
The temperature may be substantially the same in the first and the second
reaction
vessels, but it is also possible to operate with different temperatures.
Preferably the
temperature of the reaction medium in the first and second reaction vessels is
below the
boiling point at the prevailing pressure. Depending on the ambient
temperature, the
temperature of the feed streams, the rate of inert gas blowing and other
process
conditions, it may be necessary to heat or cool the reaction vessels in order
to maintain
the desired temperature.
The absolute pressure maintained in the reaction vessels is preferably from
about 50 to about 120 kPa, most preferably from about 80 to about 110 kPa,
particularly
most preferably at about atmospheric pressure. The pressure is usually but not
necessarily substantially the same in the first and the second reaction
vessels.
The acidity of the reaction medium in the first and the second vessels is
preferably maintained from about 4 to about 14 N, most preferably from about 6
to about
12 N. In most cases there is only a minor difference in acidity between the
first and
second reaction vessels, preferably less than about 15%, most preferably less
than about
10%.
The concentration of alkali metal chlorate in the reaction medium in the first
reaction vessel is preferably maintained from about 0.05 to saturation, more
preferably
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from about 0.075 to about 2.5 moles/litre, most preferably from about 0.1 to
about 1
mole/litre.
In a preferred embodiment the reaction medium in the first reaction vessel is
preferably maintained at an alkali metal chlorate concentration from about
0.05 to about
5 2.5 moles/litre, an acidity from about 6 to about 12 N, a temperature from
about 40 to
about 80 C and an absolute pressure from about 80 to about 110 kPa, while the
reaction
medium in the second reaction vessel is preferably maintained at an alkali
metal chlorate
concentration from about 14 to about 56 mmoles/litre, an acidity from about 6
to about 12
N, a temperature from about 40 to about 80 C and an absolute pressure from
about 80
to about 110 kPa.
The same type of reaction vessels and other process equipment as in other non-
crystallising processes (e.g. Mathieson, Solvay and HP-Ae) can be used.
Process
equipment in contact with the reaction medium, inciuding the reaction vessels,
is suitably
made from or lined with a material resistant to the chemicals therein.
Preferred materials
are titanium and other metals or alloys with capability of forming and
maintaining a
protective oxide layer in contact with the reaction medium, although part of
the equipment
may be made of other resistant materials such fluoro plastics or other
polymeric
materials. Preferably at least part of the equipment in contact with the
reaction medium,
including the second reaction vessel, is made of or lined with titanium.
The invention is further illustrated by means of the following example:
EXAMPLE: Chlorine dioxide was continuously produced in a generator comprising
a first reaction vessel (Primary reactor) and a second reaction vessel
(Secondary reactor).
Sodium chlorate (containing about 0.01 wt% sodium chloride as impurity),
sulfuric acid and
hydrogen peroxide were fed to the first reaction vessel. An overflow of
reaction medium from
the first reaction vessel was brought to the second reaction vessel to which
also hydrogen
peroxide was fed. An overflow of reaction medium from the second reaction
vessel was
withdrawn as waste acid after having passed a stripper. Air was blown through
the reaction
medium in both the reaction vessels diluting the chlorine dioxide gas
withdrawn therefrom.
Both the reaction vessels were maintained at atmospheric pressure and a
temperature of
57 C. Data were collected at several occasions under steady state conditions.
The results
are shown in the table below:
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Primary NaCIO3 103.3 90.2 112.7 131.5 171.0 129.6 156.9 242.4 131.5
Reactor mmol/I
Primary Acidity 10.0 9.7 9.7 9.8 9.7 9.6 9.3 9.5 9.8
Reactor (N)
Secondary NaCIO3 4.7 25.4 15.0 10.3 45.7 29.1 45.3 36.6 12.5
Reactor mmol/1
Secondary NaCI 29.1 12.0 13.7 24.0 8.6 12.0 8.6 14.4 25.8
Reactor mmol/1
Secondary Acidity 10.3 9.6 8.9 9.6 9.6 9.5 9.3 9.5 9.6
Reactor (N)
It appears that a decrease of the sodium chlorate concentration in the second
reaction
vessel leads to an increase in the sodium chloride concentration. Since this
increase is
the result of chlorate being converted to chloride as end product this also
represent a loss
through the waste acid. Thus, the loss of unconverted chlorate by increasing
the
concentration thereof in the secondary reaction vessel is at least partially
compensated
for by lower loss through chloride formation. Even if the total loss in some
cases may be
higher, it is still within acceptable levels considering the advantage of less
corrosion of
the process equipment.
