CA3060754A1

CA3060754A1 - Chlorine dioxide system with improved efficiency

Info

Publication number: CA3060754A1
Application number: CA3060754A
Authority: CA
Inventors: Ganapathy RAMASUBBU; Frederic Muret
Original assignee: Chemetics Inc
Current assignee: Chemetics Inc
Priority date: 2019-10-30
Filing date: 2019-10-30
Publication date: 2020-02-06

Abstract

In the production of chlorine dioxide solution in a conventional integrated chloride dioxide system, it has been discovered that sodium sulfate present in the chlorate solution cycling through the system is not benign to the generation of chlorine dioxide. Instead, a higher sulfate content results in a higher residual hydrochloric acid content following the generation of chlorine dioxide. In turn, the higher hydrochloric acid content reduces the efficiency of the system. Reducing the sulfate content then, either in the makeup brine supplied to the system and/or in the recycling weak chlorate solution in the system, leads to an increase in system efficiency.

Description

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 Docket no.: Chemetics021-CA
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.

2 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

3 Docket no.: Chemetics021-CA
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.

4 Docket no.: Chemetics021-CA
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

5 Docket no.: Chemetics021-CA
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

6 Docket no.: Chemetics021-CA
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.

7 Docket no.: Chemetics021-CA
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.

8 Docket no.: Chemetics021-CA
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.

9

Claims

1. A method for producing chlorine dioxide solution in an integrated chloride dioxide system, the system comprising 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 comprising:
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;
characterized in that 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.

2. The method of claim 1 wherein the step of reducing the sulfate content in the recycled weak sodium chlorate solution comprises removing sulfate from the recycled weak sodium chlorate solution while providing the recycled weak sodium chlorate solution to the sodium chlorate production subsystem.

3. The method of claim 1 wherein the step of reducing the sulfate content in the recycled weak sodium chlorate solution comprises removing sulfate from the makeup sodium chloride brine prior to providing the makeup sodium chloride brine to the sodium chlorate production subsystem.

4. The method of claim 1 whereby the step of reducing the sulfate content in the recycled weak sodium chlorate solution increases the efficiency in the production of chlorine dioxide solution.