Note: Descriptions are shown in the official language in which they were submitted.
Docket no.: Chemetics021-CA
CHLORINE DIOXIDE SYSTEM WITH IMPROVED EFFICIENCY
Technical Field
The present invention pertains to methods and systems for producing chlorine
dioxide, such as in
integrated systems for producing chlorine dioxide. In particular, it pertains
to achieving higher efficiencies
by reducing sulfate content in the system.
Background
The demand for, and hence industrial production of, chlorine dioxide has grown
substantially over the
years. In great part, this is a result of environmental concerns about use of
chlorine as a bleaching agent
and worldwide regulations limiting this use. An aim of these regulations is to
limit pulp mill effluent of
absorbable organic halide and/or Total Organic Chlorides and further to carry
out the delignification and
bleaching of pulp without producing chloroform, furans and dioxins.
Chlorine dioxide is a toxic gas and can decompose explosively at higher
partial pressures and/or
temperature. It thus poses significant safety concerns with regards to
handling and storage. Usually it is
handled as a dissolved gas in water at low concentrations. Because its
solubility increases at lower
temperatures, chilled water is often used. As a result of the difficulties in
handling and storage, for
industrial uses such as the bleaching of pulp, it is preferred to generate
chlorine dioxide as required on site
and to handle it dissolved in chilled water.
Since chlorine dioxide is extremely explosive at high temperatures, the
reactions described hereinabove
have generally been carried out at relatively low temperatures. Furthermore,
in order to reduce still further
the danger of explosion, a nonreactive (inert) gas can be conducted into the
reaction vessel; the purpose of
the gas is to reduce the concentration of chlorine dioxide in the vessel to a
nonexplosive proportion.
A preferred method for producing chlorine dioxide involves preparation from an
aqueous solution of an
inorganic chlorate by the use of a reducing agent including an aqueous
solution of hydrochloric acid. An
elegant overall process based on this method is an integrated process which
can be preferred for
efficiency, environmental, and cost reasons. Such an integrated process and
associated system is disclosed
in US3607027. Therein, the reactants for preparing chlorine dioxide are
provided by an electrolytic
chlorate cell. Further, byproduct chlorine can be used for the production of
the reducing agent. Further
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still, the off-gases from the electrolytic chlorate cell can be used as a
source of hydrogen gas for the
reaction to form HC1.
As disclosed for instance in "Adopting The Integrated Chlorine Dioxide Process
For Pulp Bleaching, To
.. Comply With CREP Regulations", A. Barr et al., 1PPTA J. Vol. 21, No. 1, Jan-
March, 2009, page 121-
127 and elsewhere, integrated chlorine dioxide processes and systems offer
many advantages over other
alternatives. For instance, it provides a low cost method of producing
chlorine dioxide without the
requirement to import feedstock chemicals. By making chemicals in-situ, a
reliable supply of product is
provided while avoiding a dependence on the market, while eliminating the
costs, uncertainty, safety
.. issues and administration in importing and storing large quantities of
sodium chlorate, sulphuric acid,
methanol, and hydrogen peroxide. Since the only inputs are chlorine (typically
from an on-site chlor-alkali
plant), power, and water, the integrated process offers the lowest cost method
to produce chlorine dioxide,
with no salt cake for disposal. The chlorine dioxide solution has a low
chlorine content and can be used to
produce ECF-grade bleached pulp. Plants based on processes such as these can
have low maintenance
.. requirements and over several decades have proven to be safe, reliable,
efficient, and easy to operate.
While industrial processes for the manufacture of chlorine dioxide are quite
advanced, there still remains a
desire for improvements in efficiency, electrolyser lifetime, and cost
reduction. This is true generally,
including for the aforementioned integrated chlorine dioxide process,
notwithstanding its many benefits.
Summary
The present invention provides for greater efficiency in the production of
chlorine dioxide solution in
integrated chloride dioxide systems. Surprisingly, and contrary to previous
understanding, it has been
discovered that sodium sulfate present in the chlorate solution cycling
through such system can have a
significant effect on the reactions taking place in the chlorine dioxide
generation subsystem. A higher
sulfate content can result in a higher residual hydrochloric acid content
following these reactions. In turn,
this higher hydrochloric acid content reduces the efficiency of the system
because additional sodium
chlorate must be consumed to react therewith and without any useful production
of chlorine dioxide.
System efficiency can thus be improved by reducing the sulfate content present
in the system. This can be
accomplished for instance by reducing sulfate in the makeup brine supplied to
the system and/or in the
recycling weak chlorate solution in the system.
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Docket no.: Chemetics021-CA
In the present invention, a relevant integrated chloride dioxide system
comprises a sodium chlorate
production subsystem comprising an electrolyzer for producing sodium chlorate,
a hydrochloric acid
synthesis subsystem for producing hydrochloric acid, a chlorine dioxide
generation subsystem for
generating chlorine dioxide, and a chlorine dioxide absorption subsystem for
absorbing generated chlorine
dioxide into chilled water. The method for producing chlorine dioxide solution
comprises the usual steps
of:
providing a supply of makeup sodium chloride brine, electricity, and weak
chlorate solution
recycled from the chlorine dioxide generation subsystem to the sodium chlorate
production
subsystem,
producing sodium chlorate and hydrogen in the electrolyzer in the sodium
chlorate production
subsystem,
providing the hydrogen and the strong sodium chlorate solution produced in the
sodium chlorate
production subsystem to the hydrochloric acid synthesis subsystem and to the
chlorine dioxide
generation subsystem respectively,
providing a supply of demineralized water, a supply of chlorine gas, and
recycled chlorine gas
from the chlorine dioxide absorption subsystem to the hydrochloric acid
synthesis subsystem;
synthesizing hydrochloric acid solution in the hydrochloric acid synthesis
subsystem,
providing the hydrochloric acid solution synthesized in the hydrochloric acid
synthesis subsystem
to the chlorine dioxide generation subsystem,
generating chlorine dioxide in the chlorine dioxide generation subsystem
thereby producing a
weak chlorate solution from the strong chlorate solution,
providing a supply of chilled water and the chlorine dioxide generated in the
chlorine dioxide
generation subsystem to the chlorine dioxide absorption subsystem, and
absorbing the generated chlorine dioxide into the chilled water in the
chlorine dioxide absorption
subsystem, thereby producing the chlorine dioxide solution.
In the present invention however, the method further comprises reducing the
sulfate content in the weak
chlorate solution recycled from the chlorine dioxide generation subsystem to
the sodium chlorate
production subsystem. By so doing, the production efficiency of chlorine
dioxide solution in the system is
increased.
An exemplary method for reducing the sulfate content in the recycled weak
sodium chlorate solution is to
remove sulfate from the recycled weak sodium chlorate solution while providing
the recycled weak
sodium chlorate solution to the sodium chlorate production subsystem. This may
be accomplished via
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nanofiltration of the recycled weak sodium chlorate solution or liquor, via
precipitation of sulfate using
barium salt or calcium salt, or other methods known to those skilled in the
art.
An alternative exemplary method for reducing the sulfate content in the
recycled weak sodium chlorate
solution is to remove sulfate from the makeup sodium chloride brine prior to
providing the makeup
sodium chloride brine to the sodium chlorate production subsystem. This may
also be accomplished by a
variety of methods known to those skilled in the art including use of pure,
high grade (e.g. "vacuum
grade") sodium chloride salt, brine recrystallization and/or extra on site
salt washing methods,
nanofiltration, barium precipitation methods, and/or other known desulfation
methods using zeolites, ion
retardation and the like.
Brief Description of the Drawings
Figure 1 shows a schematic of a conventional integrated chlorine dioxide
system of the prior art.
Figure 2 plots the results of residual HC1 concentration following chlorine
dioxide generation at several
temperatures versus Na2SO4 concentration in the weak sodium chlorate liquor
from the small scale C102
generator testing in the Examples.
Detailed Description
Unless the context requires otherwise, throughout this specification and
claims, the words "comprise",
"comprising" and the like are to be construed in an open, inclusive sense. The
words "a", "an", and the
like are to be considered as meaning at least one and not limited to just one.
Figure 1 shows a schematic of a conventional integrated chlorine dioxide
system of the prior art. As
illustrated, integrated chlorine dioxide system 1 consists of three plant
areas to produce the two
intermediate products, sodium chlorate (NaC103) and hydrochloric acid (HC1),
and the final product,
chlorine dioxide (C102). Specifically, the three plant areas comprise areas
for sodium chlorate production
2, hydrochloric acid synthesis 3, and chlorine dioxide production 4. Further
as shown, the area for
chlorine dioxide production 4 comprises distinct areas for chlorine dioxide
generation 4a and for chlorine
dioxide absorption 4b.
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Sodium chlorate is produced in sodium chlorate production area 2 by passing an
electric current 2c
through a solution that contains sodium chloride (salt) to make strong sodium
chlorate liquor. The salt for
this reaction is supplied as a recycled by-product from chlorine dioxide
production area 4 via weak
chlorate liquor loop 2a with additional salt provided as required from brine
makeup supply 2b. In sodium
chlorate production area 2, hydrogen gas is co-produced with the sodium
chlorate, and is used as hydrogen
feedstock 3a for HCI synthesis in hydrochloric acid synthesis area 3.
In hydrochloric acid synthesis area 3, HCl is produced by burning chlorine gas
and hydrogen gas. As
mentioned above, hydrogen gas 3a comes from sodium chlorate electrolysis area
2. A supply of weak
chlorine gas 3b is obtained as a recycled by-product of chlorine dioxide
production area 4, which is
supplemented with chlorine gas makeup supply 3c prior to being burned with the
hydrogen gas.
Demineralized water 3d is also provided as required to hydrochloric acid
synthesis area 3.
In chlorine dioxide generation area 4a of chlorine dioxide production area 4,
chlorine dioxide gas is
produced, along with chlorine gas and sodium chloride (salt), by combining
strong chlorate liquor and
hydrochloric acid in a chlorine dioxide generator. As mentioned, the supply of
strong chlorate liquor 4c
comes from sodium chlorate production area 2. The supply of HCl 4d comes from
hydrochloric acid
synthesis area 3. In some embodiments (e.g. when a horizontal generator is
employed), air 4e may also
optionally be provided as required to chlorine dioxide generation area 4a.
In chlorine dioxide absorption area 4b, the generated chlorine dioxide gas is
absorbed in chilled water and
then stripped with air to remove residual chlorine, to produce a high-purity
chlorine dioxide solution for
use (e.g. in an ECF pulp mill bleach plant). As shown in Figure 1, a supply of
chilled water 4f is provided
to chlorine dioxide absorption area 4b and the high-purity chlorine dioxide
solution 4g is obtained
therefrom. An optional air stream may be provided to chlorine dioxide
absorption area 4b (not shown in
Figure 1). As mentioned above, chlorine gas by-product produced during
chlorine dioxide generation.
which is not absorbed, is recycled as weak chlorine gas supply 3b for use in
hydrochloric acid synthesis
area 3. The liquor leaving the chlorine dioxide generator contains =reacted
sodium chlorate and by-
product salt. This solution, called weak chlorate liquor, is recycled back to
the sodium chlorate electrolysis
area for reconcentration via weak chlorate liquor loop 2a.
As a result of the integration of these three plant areas, the key operating
costs are for makeup chlorine 3c,
and for electrical energy 2c that is consumed in sodium chlorate production
area 2. With these relatively
low-cost inputs, the integrated chlorine dioxide process offers much lower
production costs than
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competing processes that require the purchase of sodium chlorate, acids,
methanol, and/or hydrogen
peroxide. Aside from low production costs, system I of Figure 1 and its
associated processes offer the
following additional advantages. The chlorine dioxide product is of high
purity (e.g. is a low chlorine
product). No purchased chlorate, acid, methanol. or peroxide is required for
operation. There is thus a
security of supply for the feedstocks that are required. The system and
approach improves the balance of
chlorine/ caustic consumption in pulp mill applications. Further, no solids
handling is required and there
is no resulting salt cake for disposal.
Integrated chlorine dioxide systems like that shown in Figure 1 have proven to
be efficient and
commercially successful. Hitherto, it was previous understanding that sodium
sulfate did not take part in
and had no significant influence on the chemical reactions occuring inside the
chlorine dioxide generator.
Thus, it was not considered necessary to monitor and control the sodium
sulfate content present in the
system. Surprisingly however, we have recently discovered that there is a
direct link between sodium
sulfate content and residual HC1 in the weak chlorate stream in weak chlorate
liquor loop 2a. That is, the
higher the sodium sulfate content is, the higher is the residual HCl. Further,
it is well known in the art that
high residual HC1 content in the weak chlorate liquor will result in a
reduction in efficiency. Thus, higher
sodium sulfate content in the weak chlorate liquor will result in a reduction
in system efficiency, while
conversely reducing the sulfate content should reduce the residual HC1 present
and improve system
efficiency. There are various methods known to those skilled in the art which
can serve to reduce sulfate
content and any or all of them may be employed in the present invention.
The problem arising from high sodium sulfate content was discovered during
recent commissioning of an
integrated chlorine dioxide plant in the field. Reduced efficiency of chlorine
dioxide production was
observed compared to other similar historic plants. Chemical analysis was
performed to determine the
possible causes for the reduced efficiency. The weak chlorate stream in weak
chlorate liquor loop 2a was
found to have higher acidity due to higher residual HC1 coming from chlorine
dioxide generation area 4a.
This higher excess residual HC1 has to be reacted away using extra sodium
chlorate. This extra sodium
chlorate does not therefore produce any chlorine dioxide and results in an
inefficiency in the process.
Further investigations were made to determine the cause of the higher acidity
in the weak chlorate stream.
.. It was found that all aspects of plant operation and particularly the
chemical composition of the weak
chlorate steam were essentially very similar to all other historic chlorine
dioxide plants with the exception
of the sodium sulfate content present, which was significantly higher than
usual. It thus appeared that high
sodium sulfate content in the system somehow resulted in higher residual HC1
in the weak chlorate liquor
loop, which in turn is well known to result in a reduction in efficiency.
Laboratory testing confirmed that
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this was the case, namely high sodium sulfate content results in high residual
acidity (see Examples
below).
Based on this discovery, it is apparent that reducing the sodium sulfate
content in the system will improve
production efficiency by reducing the residual HCl content in the weak liquor
stream. There are numerous
methods known to those skilled in the art which maybe used to reduce sulfate
content and thus it is
expected that any or all of these various methods may be used to do so and
thereby improve plant
efficiency.
For instance, in one approach, sulfate can be removed from the recycled weak
sodium chlorate solution
while providing the recycled weak sodium chlorate solution to the sodium
chlorate production subsystem.
This may be accomplished in numerous ways, e.g. via nanofiltration or other
filtration of the recycled
weak sodium chlorate liquor, via precipitation of sulfate using barium salt or
calcium salt, via
crystallization methods, or via other methods known to those skilled in the
art.
Alternatively in another approach, methods may be employed to reduce the
ingress of sulfate into the
system, e.g. from the makeup sodium chloride brine prior to providing the
makeup sodium chloride brine
to the sodium chlorate production subsystem. This may also be accomplished by
a variety of methods
known to those skilled in the art including use of pure, high grade (e.g.
"vacuum grade") sodium chloride
salt, brine recrystallization and/or extra on site salt washing methods,
nanofiltration, barium precipitation
methods, and/or other known desulfation methods using zeolites, ion
retardation and the like.
The following Examples have been included to illustrate certain aspects of the
invention but should not be
.. construed as limiting in any way.
Examples
Tests were performed using a small size, laboratory chlorine dioxide generator
in order to study the effect
of excessive sulfate on the residual hydrochloric acid content in the weak
sodium chlorate liquor in
conventional chlorine dioxide systems. Samples of typical weak chlorate liquor
that had been spiked with
varying amounts of sodium sulfate were reacted in the laboratory generator
under typical commercial
operating conditions and the residual HCl content was measured thereafter.
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The weak chlorate liquor composition samples all comprised:
Sodium chlorate ¨ approx. 375 g/L
Sodium chloride ¨ approx. 150 g/L
Sodium dichromate ¨ approx. 5 g/L
Sodium sulphate ¨ approx. 10 g/L
This composition is typical of that of the weak chlorate liquor in an actual
integrated chlorine dioxide
system.
For testing purposes, a sample was prepared without any sodium sulfate, i.e. 0
g/L. Additional samples
were also prepared comprising several different amounts of sodium sulfate,
namely at concentrations of 10
g/L, 20 g/L, 30 g/L and 40 g/L.
The samples were then allowed to react in the laboratory generator under
conditions similar to those used
in actual commercial practice. For instance, in the field, vacuum type
generators are operated at 75 C
while horizontal type generators are operated at 90 to 95 C. Therefore,
samples were allowed to react at
various temperatures bracketing this range.
Specifically, the test method comprised the following steps. First, the
laboratory generator containing a
given sample was heated to 75 C. Then, 32% HCL solution was injected into the
generator to an
equivalent amount of 15 g/L. The generator temperature was maintained at 75 C
for 15 minutes to allow
for reaction of the acid with the chlorate present. A first sample of test
liquor was then taken from the
generator for analysis of 1-ICI content. The generator temperature was then
raised to 90 C and maintained
there for 20 minutes to allow for any further reaction, after which a second
sample was taken for HC1
content analysis. Then the generator temperature was raised once again to 105
C and maintained there
again for 20 minutes to allow for any further reaction, after which a third
sample was taken for HC1
content analysis. The HCl content in each sample was then measured and the
results appear in Table 1
below.
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Table 1. Acidity of weak chlorate liquor as a function of sulfate
concentration and temperature
HC1 content in g/L
with 0 g/L with 10 g/L with 20 g/L with 30 g/L
with 40 g/L
Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4
@ 75 C 7.3 11.7 12.4 13.1 15.3
@ 90 C 4.4 6.6 8.4 9.8 11.7
@ 105 C 3.3 4.7 6.2 8.0 9.1
Figure 2 plots these results of residual HCl concentration following chlorine
dioxide generation at the
three different temperatures versus Na2SO4 concentration.
This example demonstrates that the residual HCl content in the weak sodium
chlorate liquor in such
systems is a significant function of the sulfate concentration present. At all
temperatures, the higher the
sulfate concentration is, the higher the residual HCl concentration is
expected to be. And in turn, the lower
the system efficiency will be.
All of the above U.S. patents, U.S. patent applications, foreign patents,
foreign patent applications and
non-patent publications referred to in this specification, are incorporated
herein by reference in their
entirety.
While particular elements, embodiments and applications of the present
invention have been shown and
described, it will be understood, of course, that the invention is not limited
thereto since modifications
may be made by those skilled in the art without departing from the spirit and
scope of the present
disclosure, particularly in light of the foregoing teachings. Such
modifications are to be considered within
the purview and scope of the claims appended hereto.
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