Note: Descriptions are shown in the official language in which they were submitted.
CA 02307818 2008-04-04
TITLE OF INVENTION
METHOD OF CHLORINE DIOXIDE PRODUCTION
FIELD OF THE INVENTION
The present invention relates to the production of chlorine dioxide,
particularly by an improved method involving the reaction between chlorate
ions
and hydrogen peroxide, carried out in a strongly acidic reaction medium.
BACKGROUND TO THE INVENTION
The reaction for the production of chlorine dioxide by reaction of chlorate
ion with hydrogen peroxide can be depicted by the following equation:
2003- + H2O2 + 2H+ --> 2002 + O2 + 2H2O (1)
Atmospheric type processes based on the above mentioned reaction were
described thoroughly in the prior art. For example, U.S. Patent 2,332,181
(Soule)
describes a highly efficient chlorine dioxide generation process based on
reaction
(1), carried out either batchwise or continuously, both in the absence and in
the
presence of chloride ions.
U.S. Patent 2,833,624 (Sprauer) discloses a process similar to that
described in USP 2,332,181 but performed at elevated temperatures of above
70 C. According to Sprauer, the presence of chloride ions in the reaction
medium
may have a beneficial catalytic effect on the kinetics of reaction (1).
Sprauer
suggests the use of a tubular reactor and shows that, under optimum
conditions,
90% or better conversion of chlorate to chlorine dioxide is possible.
Sprauer further suggests the possibility of premixing the feedstocks,
particularly chlorate, chloride and hydrogen peroxide. Both the specification
and
examples of Sprauer indicate that the premixing involves the use of solutions
of
separate feedstocks.
CA 02307818 2000-05-08
2
In recently published European Patent Application EP 866,031 (Tenney) it
was shown that the tubular reactor of the type described in Sprauer can be
used to
achieve chlorate conversion to chlorine dioxide of above 75% which is below
the
at least 90% value disclosed in Sprauer.
One of the deficiencies of all the above described processes is related to
the limited range of concentrations of chemicals (chlorate and hydrogen
peroxide)
achievable in the reaction mixture prepared by mixing of the corresponding
feedstocks in a ratio matching the requirements of the chlorine dioxide
generator.
Such a limitation on chemicals concentrations affects, in a negative way, both
the
kinetics of reaction (1) and the process yield resulting therefrom.
There is a need, therefore, to develop a highly efficient chlorine dioxide
generation process based on reaction (1) which is not affected by the kinetic
limitations characteristic for the prior art processes.
SUMMARY OF INVENTION
It was surprisingly found that by using a concentrated, mixed feed of
chlorate and hydrogen peroxide, in a form more concentrated with respect to at
least chlorate than is achievable by combining saturated chlorate solution
with
commercially-available hydrogen peroxide solution, the kinetic limitations
characteristic of the prior art processes can be overcome, resulting in a
chlorine
dioxide generation process having a significantly improved performance.
Accordingly, in one aspect of the present invention, there is provided an
aqueous solution of hydrogen peroxide and chlorate for use as a feed to a
chlorine
dioxide generating process which is more concentrated in at least chlorate
than a
solution prepared by mixing saturated chlorate solution and commercially-
available hydrogen peroxide solutions.
Commercially-available hydrogen peroxide solutions contain typically up
to 70 wt% H2O2. Routine manufacture of more concentrated hydrogen peroxide
solutions, such as about 90 wt% solution, has been discontinued sometime ago.
Nevertheless, the present invention constitutes an improvement even with
respect
to the latter solution.
The aforementioned concentrated, mixed feed can be prepared, for
example, by dissolving crystalline alkali metal chlorate, preferably sodium
CA 02307818 2000-05-08
3
chlorate, in a hydrogen peroxide solution of any desired concentration,
generally
from about 5 wt% to about 70 wt%, preferably about 10 wt% to about 50 wt%,
and most preferably from about 10 wt% to about 30 wt%. The chlorate is added
preferably to substantially saturate the aqueous hydrogen peroxide solution.
The present invention extends to a method of forming chlorine dioxide by
reacting chlorate with hydrogen peroxide in an aqueous acid reaction medium
according to the equation:
2C1O3- + H202 + 2H+ -+ 2C1O2 + 02 + 2H20
wherein a feed to the reaction is the aqueous solution of hydrogen peroxide
and
chlorate provided herein.
BRIEF DESCRIPTION OF DRAWING
Figure 1 is a graphical representation of the relationship of maximum
sodium chlorate concentration in chlorate/hydrogen peroxide mixtures prepared
by mixing saturated sodium chlorate solution with commercially-available
hydrogen peroxide solution (50 wt% chain line; 70 wt% solid line) at different
weight ratios.
GENERAL DESCRIPTION OF THE INVENTION
It is usually desirable to prepare a concentrated, mixed feed in which the
ratio of hydrogen peroxide to chlorate matches the requirements of the overall
chlorine dioxide generating process. These requirements are usually determined
by many factors, such as generator design, desired production rate, flow
characteristics, residence time, reaction kinetics and efficiency.
Upon selecting a specific H202 concentration within the aforementioned
most desirable range of about 10 wt% to about 30 wt%, such as for example an
about 18 wt% solution of H202, it is preferred to nearly saturate the about 18
wt%
H2O2 solution with sodium chlorate, resulting in a concentrated, mixed feed
containing approximately 620 g/L NaC1O3 and 155 g/L H2O2, corresponding to a
weight ratio of H202 to NaC1O3 of 0.25 g H202/g NaC1O3. Such a high
concentration of NaC1O3 in a NaC1O3/H2O2 mixture cannot be achieved by the
prior art method of mixing of separate solutions, even if a 90 wt% of H2O2 is
used
(in the latter case, the NaC1O3 concentration in the resulting mixture would
have
been below 550 g/L).
CA 02307818 2000-05-08
4
The weight ratio of H202 to NaC1O3 in mixtures useful for chlorine
dioxide generation can vary in a wide range of between about 0.16 to about
0.30 g
H202/g NaC1O3. The concentrations of the starting solutions of hydrogen
peroxide used for the preparation of a feed mixture with a given H2O2/NaC1O3
weight ratio vary accordingly. For example, in order to prepare a mixture
having
a weight ratio of H202 to NaC1O3 of about 0.17, the concentration of H202 in
the
starting solution should be about 15 wt%. Upon saturation of such solution
with
NaC1O3, a mixture containing approximately 11Og/L H202 and 635 g/L NaC1O3 is
formed.
Alternatively to the use of crystalline alkali metal chlorate for the
dissolution in the hydrogen peroxide solutions, it is possible to employ a
slurry of
alkali metal chlorate or even a concentrated alkali metal chlorate solution. A
resulting mixture is then concentrated, for example, by evaporation of water
therefrom, preferably at a reduced pressure, in order to lower the temperature
of
the evaporation step.
Regardless of the mode of preparation of the concentrated, mixed feed, the
concentration of at least chlorate contained therein exceed that achievable by
the
prior art methods, i.e. by mixing of the solutions of corresponding
feedstocks,
without the subsequent concentrating step. The maximum NaC1O3 concentration
levels in the NaC1O3/H2O2 mixtures achievable according to the prior art
processes are graphically depicted in Figure 1.
The concentrated, mixed feed of alkali metal chlorate and hydrogen
peroxide can be used to generate chlorine dioxide in any type of a generator.
For
example, a tubular generator of the type described in USP 2,833,624 (Sprauer)
and substantially reproduced in EP 866,031 (Tenney) can be employed.
Generators of other shapes and forms, for example, a conical or dual-conical
shape can be employed, if desired. A suitable generator can operate in a
single
pass mode or with the recirculation of chemicals. In the latter case, it is
preferred
to separate the desired product, chlorine dioxide, from the unreacted
chemicals,
i.e. chlorate, acid and hydrogen peroxide, before return to the reactor.
In addition to being used in the aforementioned type generators, typically
employed in the smaller scale, water treatment applications, the concentrated,
CA 02307818 2000-05-08
mixed feed can be utilized for the generation of chlorine dioxide in large
scale
applications, such as pulp bleaching. Conventional generators, both
atmospheric
and subatmospheric, operated in both crystallizing and non-crystallizing modes
of
operation can be utilized. A conventional atmospheric type generator utilizing
the
5 chemistry of reaction (1) is described, for example, in US Patent 5,380,517
(Sokol). The latter generator utilizes a design identical to that known for
the
Solvay or Mathieson reactor.
A subatmospheric, single vessel process (SVP) utilizing the chemistry of
reaction (1) is described, for example, in US Patents 5,091,166 and 5,091,167
(Engstrom et al).
It is noted that the use of a premixed feed of chlorate and hydrogen
peroxide, whether concentrated or not, has some additional benefits related to
the
simplification of the equipment required (less pumps required) and the
avoidance
of problems related to the inadequate and potentially unsafe control of
separate
feeds.
EXAMPLE
In order to illustrate the beneficial effect related to the use of
concentrated,
mixed feed of chlorate and hydrogen peroxide, two experimental runs were
conducted. An experimental run (run 2) carried out according to the preferred
embodiment of the invention involved the use of a concentrated, mixed feed
containing approximately 620 g/L NaC1O3 and 155 g/L H202 prepared by
saturating an about 18 wt% H202 solution with sodium chlorate, by dissolving
NaC1O3 crystals in the H202 solution.
A comparative example (run 1) involved the use of a mixed feed
containing approximately 480 g/L sodium chlorate and 120 g/L hydrogen
peroxide prepared conventionally by mixing the required amounts of a 50 wt%
H202 solution and a saturated NaC1O3 solution containing 610 g NaC1O3/L.
Both runs were conducted under similar conditions in terms of acid and
peroxide weight feed ratios (H2SO4/NaC1O3 and H2O2/NaC1O3) and operating
pressure. Both runs were conducted in the same, conical reactor of 0.81 L
capacity.
CA 02307818 2000-05-08
6
After the feed rates were set, the necessary time to reach steady
temperature was allowed before samples were taken. Steady state samples were
collected every 20 to 25 minutes for a total of 70 to 90 minutes before
considering
each run completed.
Chlorine and chlorine dioxide analysis were done by the Neutral/Acid
(KI/Thiosulphate) test performed on KI solution used to trap C1O2/C12 stripped
out
immediately after taking the process discharge sample.
The process conditions maintained during the runs were:
Run 1 Run 2
a) Pressure (mmHg) 480 480
b) (H2O2/NaClO3)Feed(g/g) 0.247 0.248
c) (H2SO4/NaC1O3)Feed(g/g) 2.489 2.222
d) [H2SO4]Feed(N) 27.3 27.3
e) [NaClO3]Feed(g/L) 480.5 618.4
f) Discharge Flow Rate (L/min) 41.86 42.13
g) Chlorate/Peroxide Feed Rate (L/min) 0.149 0.108
h) Acid Feed Flow Rate (L/min) 0.134 0.111
Results
Run 1 Run 2
C102 Yield from NaClO3(%) 92.4 98.5
Chemical Efficiency(%) 99.0 99.5
H202 Consumption(g/g C102) 0.421 0.398
H2SO4 Consumption(g/g Cl02) 4.25 3.560
NaC1O3 Consumption(g/g C102) 1.708 1.602
Discharges
i) [C102](g/L) 1.004 0.987
J) [C12](mg/L) 5.6 3.3
k) [NaC103](mg/L) 28.4 6.8
1) [H2SO4](g/L) 4.27 3.62
m) [C104](mg/L) 1.1 0.1
CA 02307818 2000-05-08
7
The above comparison clearly shows the superior performance of the
process of the invention (run 2) in terms of yield, efficiency, chemical
consumption for all feedstocks, product purity and the reduced losses of
chemicals
in the discharges (effluents).
SUMMARY OF DISCLOSURE
In summary of this disclosure, an improved process for the production of
chlorine dioxide uses a concentrated, mixed feed of alkali metal chlorate and
hydrogen peroxide. Modifications are possible within the scope of this
invention.
