Note: Descriptions are shown in the official language in which they were submitted.
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_ METHOD FOR PRODUCING CHLORINE DIOXIDE USING METHANOL AND
HYDROGEN PEROXIDE AS REDUCING AGENTS
FIELD OF INVENTION
The present invention relates to a method for producing chlorine dioxide.
BACKGROUND OF THE INVENTION
Chlorine dioxide is employed in a wide variety of industrial applications,
including bleaching wood pulp for paper making, bleaching textiles, treating
water, and
abating odors. The use of chlorine dioxide for bleaching wood pulp has
increased
because chlorine dioxide is more environmentally friendly than chlorine or
hypochlorite)
which can leave larger quantities of chlorinated organic compounds in
bleaching effluent.
In a typical commercial processes for generating chlorine dioxide, sodium
chlorate is reacted with a reducing agent in a strongly acidic aqueous medium.
A metal
chloride salt, sulfur dioxide, methanol, or hydrogen peroxide is commonly used
as the
reducing agent. The typical acid used is sulfuric acid or hydrochloric acid,
generally to
obtain an acidity of between about 3 to 10 N for the reaction mixture.
The reduction of sodium chlorate with sodium chloride can be represented
by the following formula:
NaC103 + NaCI + HzS04 --------> C102 + '/a C12 + Na2S04 + H,O (1)
A principle disadvantage of this process is the formation of half a mole of
chlorine gas for each mole of chlorine dioxide produced. At one time this
chlorine gas
was used for bleaching pulp. This use, however, is now disfavored because of
environmental concerns.
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Less chlorine gas is generated if sodium chloride is replaced by sulfur
dioxide or methanol in this process. The methanol based process is termed the
Solvay
process, while the sulfur dioxide process is termed the Mathieson process.
However, the
reaction between chlorate and either sulfur dioxide or methanol is very slow,
resulting in
a low rate of chlorine dioxide generation. The following reactions occur
initially,
resulting in generation of chloride ion, which acts as a reducing agent:
C103- + 350, + 3H,0 ----- > CI- + 3HZSO4 (2)
2C103- + 3CH30H ---- > 2C1- + 2HCOOH + 4H0 + COZ (3)
The chloride ions reduce chlorates present in the reaction mixture according
to formula
(1) shown above, resulting in the production of chlorine gas. The chlorine gas
reacts
with the sulfur dioxide or methanol to regenerate chloride ions according to
the following
formula:
SO~ + Ch + 2H=O ----- > 2HCI + HZS04 (4)
CH30H + 3C12 + HBO ----- > 6HC1 + COZ (5)
The overall reaction using methanol as reducing agent is as follows:
9NaC103 + 2CH30H- + 6H~S0,, ----- > 9ClOz + (b)
3Na3H(S04)2 + 1/zCO~ + 1.SHCOOH + 7H,0
When using sulfur dioxide or methanol as reducing agent, however, at least a
small
amount of chlorine gas by-product is produced. Also, when using methanol as
reducing
agent at high chlorate concentrations and acidity, the chloride formed in
reactions 2-5 can
be consumed in the subsequent formation of chlorine dioxide faster than it is
generated.
When the chloride is exhausted, chlorine dioxide generation ceases and a
phenomenon
known as "whiteout" results, i.e., the reaction medium becomes clear. To
prevent this, it
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is often necessary to add a small amount of sodium chloride continuously, or
to avoid
certain high concentrations of chlorate and acid.
Hydrogen peroxide has been used as a reducing agent in chlorine dioxide
generation to eliminate production of chlorine. Using hydrogen peroxide also
results in a
significantly faster chlorine dioxide generation rate than other processes.
The reaction
using hydrogen peroxide is represented by the following formula:
2NaC103 + HZOz + HZS04 ------ > 2C10 + Na~S04 + 2H20 + OZ (7)
An important disadvantage of this process, however) is that hydrogen peroxide
is much
more expensive than methanol, sodium chloride, or sulfur dioxide. For this
reason, the
hydrogen peroxide based process is not used as commonly as the methanol and
sulfur
dioxide based processes.
There is therefore a need for a method of producing chlorine dioxide that is
efficient and economical, which does not generate substantial amounts of
chlorine, and
which reduces the possibility of a whiteout condition.
SUMMARY OF THE INVENTION
The present invention relates to a method for producing chlorine dioxide by
reacting alkali metal chlorate with reducing agents in an aqueous acidic
medium, wherein
the reducing agents are methanol and hydrogen peroxide. It has been determined
that
combining hydrogen peroxide and methanol causes an unexpectedly strong
enhancement
in the rate of chlorine dioxide generation.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a continuous process embodiment
of the invention.
Figure 2 is a graph showing chlorine dioxide generation using the process
of the invention at atmospheric pressure, 60' C, and 12N HZS04. The points on
the
graph shown with a " D " indicate chlorine dioxide generation rates for
combinations of
methanol and hydrogen peroxide reducing agent in the equivalent strength
ratios
indicated. The dotted line connecting the points for l00 mole % CH30H and 100
mole %
H20, shows the expected chlorine dioxide generation rate for a combination of
hydrogen
peroxide and methanol reducing agents.
Figure 3 is a graph showing chlorine dioxide generation using the process
of the invention at a sub-atmospheric pressure of 300 mm Hg, 60' C, and 10 N
H~S04.
The points on the graph shown with a " Cl " indicate the amount of chlorine
dioxide
generated for 100 % methanol, combinations of methanol and hydrogen peroxide,
and
100 % hydrogen peroxide. The dotted line shows the expected amount of chlorine
dioxide
generation for a combination of methanol and hydrogen peroxide reducing
agents. The
point on the graph shown with a "+" indicates the amount of chlorine dioxide
generated
for a combination of sodium chloride, methanol, and hydrogen peroxide in a
l0:80:10
equivalent strength molar ratio.
DETAILED DESCRIPTION OF THE INVENTION
We have determined that a synergism between methanol and hydrogen
peroxide reducing agents results in the generation of chlorine dioxide at a
higher rate than
would have been expected. This higher rate allows more chlorine dioxide
production
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from a given generator volume, thereby reducing capital cost. It also allows,
for
example, a smaller generator to be used to meet a given chlorine dioxide
demand. In
addition, by varying the ratio of methanol to hydrogen peroxide, one can
increase or
decrease the chlorine dioxide generation rate. This degree of flexibility in
choosing a
desired chlorine dioxide generation rate is unique to the present invention.
For example, it has been found) at atmospheric pressure, that by
substituting 10 % of the methanol in the methanol type process described above
with the
same amount, on an equivalent strength basis, of hydrogen peroxide) the
generation rate
of chlorine dioxide is doubled. The generation rate of chlorine dioxide
obtained using
hydrogen peroxide is known to be higher than that obtained using methanol. But
the
increase obtained by substituting only 10 % hydrogen peroxide in the methanol
process
represents about 40 % of the increase obtained if 100 % hydrogen peroxide is
used. Thus
the increase in chlorine dioxide generation that is obtained using a
methanol/hydrogen
peroxide mixture is unexpectedly high.
As noted above, the invention results in surprising benefits at atmospheric
pressure. Preferred pressures are between about 400 and 900 mm Hg. It has been
determined, however, that the benefits of the invention can also be enjoyed
when the
process is carried out at sub-atmospheric pressure, a preferred sub-
atmospheric pressure
being from about 100 mm Hg to 400 mm Hg. At sub-atmospheric pressure, when 10%
of methanol in the methanol-based process was substituted for an equivalent
amount of
hydrogen peroxide, generation of chlorine dioxide was increased by 20 % . When
30 % of
- methanol was substituted for by an equivalent strength of hydrogen peroxide,
the
generation of chlorine dioxide was increased by 68 % .
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The method of the invention is preferably carried out as a continuous
process. In one embodiment, depicted in a flow diagram in Figure l, sodium
chlorate is
reacted with hydrogen peroxide and methanol reducing agents in the presence of
concentrated sulfuric acid. The reactants can be introduced together, but
preferably are
introduced separately into a conventional reaction vessel. The reaction can be
carried out
at atmospheric pressure, with air, or other inert gas such as nitrogen,
circulating through
the reaction vessel. The reaction should be maintained substantially in a
steady state by
continuously feeding the reactants, and by ensuring that they are evenly
distributed in the
reaction medium. The chlorine dioxide gas that is generated can be collected
and
absorbed outside of the reaction vessel. Water vapor and other gaseous
byproducts
should also be continuously removed from the reaction vessel, and vented to a
chlorine
dioxide absorber. Reaction medium containing alkali metal salt (Na2SOd in
Figure 1},
unreacted chlorate, acid and reducing agents should also be continuously
removed (e.g.,
"HzS04 Effluent" in Figure 1 ). Sodium acid sulfate deposited in the reaction
at sub-
atmospheric conditions can be removed and subjected to a metathesis reaction
to form
neutral sodium sulfate and acidic aqueous solution.
Preferably, the reaction medium that is withdrawn from a reaction vessel
running at atmospheric pressure is cascaded into a second reaction vessel
operating at
sub-atmospheric pressure, such as a "single vessel process" (SVP''") reactor.
For the
preferred reactants, the withdrawn medium from a reaction vessel at
atmospheric pressure
contains largely sulfuric acid, with lesser amounts of chlorine dioxide,
sodium chlorate,
sodium sulfate, and hydrogen peroxide. The withdrawn medium, new reducing
agents,
sodium chlorate, and sulfuric acid are preferably added separately to the
second vessel,
and the second vessel kept at a sub-atmospheric pressure of between 100 and
400 mm
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Hg, preferably between 100 and 300 mm Hg. Chlorine dioxide gas is recovered
outside
of the first and second reaction vessels. In the cascade process run under
these
conditions, the only by-product generated in the second reactor can be a
neutral metal
salt, or acidic salt cake, depending on the acidity of the reaction medium.
If desired, catalysts that enhance the generation of chlorine dioxide are
added to the reaction as well. Such catalysts include, e. g. , silver nitrate,
manganese
sulfate, vanadium pentoxide, ruthenium oxide, rhodium oxide, and palladium
oxide.
The process is conducted in a temperature range of between about 20~ C
and about 140~ C, preferably between about 35~ C and 80~ C, and most
preferably
between about 50 ~ C and 75 ~ C.
Suitable acids for use in the reaction include, e.g., sulfuric acid,
hydrochloric acid, phosphoric acid, nitric acid, and chloric acid. Sulfuric
acid is
preferred. The acid normality is maintained in the aqueous reaction medium
between
about 1 N to 15 N, preferably between about 4 N and 12 N. Most preferably, the
normality is maintained at between about 7 N and 10 N for atmospheric
conditions, and
between about 4 N and 5 N or 7 N to 10 N for subatmospheric conditions.
Performance
of the process at 7 N to 10 N generates acid salt cake while performance at 4
N to 5 N
generates neutral saltcake.
Alkali metal chlorates that can be used in this process include, e. g. ,
sodium
chlorate and potassium chlorate. Sodium chlorate is preferred. The alkali
metal chlorate
concentration employed in the reaction is between about 0.01 M and saturation
concentration, preferably between about 0.01 M and 4 M. It is most preferably
between
about 0.05 M and 0.3 M at a pressure between about 400 and 900 mm Hg, and
between
about 0.3 M and 1.5 M at a pressure between about 100 and 400 mm Hg.
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The optimum percentage of hydrogen peroxide with respect to the total
amount of reducing agent used in the process, on an equivalent strength basis,
depends on
the chemical costs, chlorine dioxide demand, and byproduct demands. It can
vary from
1 % to 99 % , but for a typical paper mill is preferably less than 50 % , more
preferably less
than 30 % , and most preferably between about 5 and 10 % . The amount of total
reducing
agent consumed in the reaction is preferably from about 100 % to 120 % of the
stoichiometrically calculated amount.
If desired, the method of the invention can be practiced by adding
hydrogen peroxide to an existing chlorine dioxide generator that uses a
methanol reducing
agent. The method can also be practiced by adding methanol to an existing
generator that
uses a hydrogen peroxide reducing agent. Alternately, hydrogen peroxide can be
added
to an existing generator that uses methanol reducing agent, or a combination
of hydrogen
peroxide and methanol can be added to an existing generator that uses sulfur
dioxide
reducing agent.
The invention is illustrated by the following examples, which are intended
to merely exemplify the invention, not to limit its scope.
EXAMPLE 1: PARTIAL SUBSTITUTION OF METHANOL REDUCING AGENT
WITH HYDROGEN PEROXIDE AT ATMOSPHERIC PRESSURE
Sodium chlorate (27 grams/liter) and sulfuric acid {588 grams/liter) were
added to a laboratory reaction vessel equipped with a nitrogen sparger to
obtain a 600 ml
reaction mixture having a chlorate concentration of 0.25M and an acid
normality of 12 N.
The reactor was operated at atmospheric pressure and 60~ C.
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Methanol, methanol and hydrogen peroxide mixtures, or hydrogen
peroxide, were added to the reactor based on the stoichiometric amount that
was
equivalent to 0.15 moles sodium chloride. As shown in reaction formula { 1 )
above, only
one mole of salt is added for every mole of chlorate in the process for
producing chlorine
dioxide.
The amount of methanol that was equivalent to 0.15 moles of salt was
calculated by the following equation:
0.15 x 32/4.5 = 1.07 gram.
The "32" in this equation is the molecular weight of methanol. The "4.5" is
derived
from the reaction stoichiometry for producing chlorine dioxide with methanol
shown in
equation (6) above, in which one mole of methanol produces 4.5 moles of
chlorine
dioxide.
The amount of hydrogen peroxide that was equivalent to 0.15 moles of salt
was determined by the following equation:
0.15 x 34/2 = 2.55 grams
The "34" in this equation is the molecular weight of hydrogen peroxide. The
"2" is
derived from the reaction stoichiometry for producing chlorine dioxide with
hydrogen
peroxide shown in equation (7) above, in which one mole of hydrogen peroxide
produces
2 moles of chlorine dioxide.
Figure 2 shows the amount of chlorine dioxide (in grams) generated over
1.5 minutes when methanol was used alone, methanol was mixed with hydrogen
peroxide
in a 9:1 equivalent strength ratio, methanol was mixed with hydrogen peroxide
in a 7:3
equivalent strength ratio, and hydrogen peroxide was used alone. The rate of
chlorine
dioxide generation in this system was calculated by the following formula:
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ClOz generation = (grams of C102/67.5 grams/mole)/(0.6 liter x 1.5 min)
The amount of chorine expected to have been generated is shown by the dotted
line.
This substitution of 10 % of methanol reducing agent with hydrogen
peroxide resulted in a chlorine dioxide generation rate of 6.6 x 10-2
moles/(liter-minute),
which was surprisingly greater than the rate obtained using methanol alone of
3.1 x 10-Z
moles/(liter-minute). Use of 100% hydrogen peroxide resulted in a generation
rate of
12.2 X 10-Z moles/(liter-minute). Thus, substitution of IO% methanol increases
the
generation rate by about 39 % of the total increase in the chlorine dioxide
generation rate
achieved using 100% hydrogen peroxide.
The increase in chlorine dioxide generation that was obtained using a 7:3
equivalent strength ratio {i. e. , 30 % hydrogen peroxide) was 9.3 x 10-Z
moles/(liter-
minute). This amounted to about 68 % of the increase in chlorine dioxide
generation rate
that was produced using 100% hydrogen peroxide.
Thus, by substituting a relatively small amount of hydrogen peroxide for
methanol reducing agent, a surprisingly large increase in the chlorine dioxide
generation
rate was obtained.
EXAMPLE 2: PARTIAL SUBSTITUTION OF METHANOL WITH HYDROGEN
PEROXIDE AT SUB-ATMOSPHERIC PRESSURE
The experiment described in Example 1 was carried out as described
therein, except that the process was carried out at a sub-atmospheric pressure
of 300 mm
Hg and 490 grams/liter of sulfuric acid was used to obtain a reaction mixture
having an
acid normality of 10 N. The results of this experiment are shown in Figure 3.
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At a 9:1 equivalent strength ratio of methanol:hydrogen peroxide, (i.e., a
% equivalent strength of hydrogen peroxide) a chlorine dioxide rate of 7. b x
10'2
moles/(liter-minute) was obtained. This rate was surprisingly higher than the
rate
obtained using only methanol (6.8 x 10'z moles/(liter-minute)), and
represented about 20%
of the increase in the rate that was obtained when 100 % hydrogen peroxide was
used
{11.2 x 10-2 moles/(liter-minute)).
At a 7:3 equivalent strength ratio of methanol:hydrogen peroxide, (i.e.,
30% equivalent strength hydrogen peroxide), a chlorine dioxide generation rate
of 9.8 x
10-Z moles/(liter-minute) was obtained, which was significantly higher than
that obtained
using only methanol (6.8 x 10-Z moles/(liter-minute)). This amounted to about
68 % of the
increase in the chlorine dioxide generation rate that was obtained using 100 %
hydrogen
peroxide.
Thus, substitution of 10% or 30% of methanol with hydrogen peroxide
resulted in a surprising increase in chlorine dioxide generation.
As shown in Figure 3, use of a combination of sodium
chloride:methanol:hydrogen peroxide in a 10:80:10 equivalent strength ratio
also resulted
in a high chlorine dioxide generation rate. The use of chloride ion in
combination with
methanol and hydrogen peroxide reducing agents is a separately patentable
invention that
is the subject of copending application serial no. 08/720,087, entitled
"Method for
Producing Chlorine Dioxide using Methanol, Chloride, and Hydrogen Peroxide as
Reducing Agents", filed on the same date as the present application.
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