METHOD AND APPARATUS FOR PRODUCING CHLORINE DIOXIDE USEFUL IN WATER PURIFICATION PLANTS
Technical Field
The present invention relates to a method and apparatus for producing chlorine dioxide useful in water purification plants, and more particularly, to a method and apparatus for continuously producing chlorine dioxide of high purity by automatically injecting chlorite or a mixture of chlorite and hypochlorite in an amount required according to a volume of dilution water, the quality or flow rate of water to be treated before an ejector nozzle so that the reactants can be uniformly mixed and side effects are inhibited at most.
Background Art
Chlorine dioxide is recognized as a preferable disinfectant because it has strong oxidizing, sterilizing and deodorizing effects but does not produce byproducts of clorination such as trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs) and organic chlorides. However, it has a defect that cannot remove ammoniacal nitrogen or amine. Such problem can be dissolved by combined treatment with chlorine dioxide and chlorine in treatment of water while obtaining synergic effects of removing ammonia or amine as well as offensive taste and odor. The process including such pretreatment is disadvantageous due to high cost. Therefore, there is need to produce chlorine dioxide in an economically useful method.
Methods for producing chlorine dioxide are already well-known from long ago and are largely divided into methods by reduction of chlorates and methods by oxidization of chlorites. The former methods which require
several hundreds tones of chlorine dioxide per day are plant industrial methods needing large scale equipments such as large scale preparation equipments of raw materials, generation equipments of chlorine dioxide, separation equipments of by products and safety apparatuses, and a grand investment. On the other hand, the latter employ an oxidization method using a chlorite as a starting material. These methods are more economical and safer than the former methods, since production is possible only by a simple equipment and small investment. The methods using a chlorite as a starting material include generating chlorine dioxide by various oxidization, followed by conversion into chlorine dioxide solution, which is prepared and used in situ where needed.
The oxidization methods of chlorites can be classified as follows;
1) Oxidation with inorganic acid (HC1, H2S04)
5NaC102 + 4HC1 → 4C102 + 5NaCl + 2H20
5NaC102 + H2S04 → 4C102 + 2Na2S04 + NaCl + 2H20
2) Oxidation with hypochlorite and inorganic acid
2NaC102 + NaCIO + 2HC1 → 2C102 + 3NaCl + H20
3) Oxidation with chlorine
2NaC102 + Cl2 → 2C102 + 2NaCl
In conducting these methods of three types, reactants and production levels of by-products including C102 ", C10 ", Cl", Cl2 etc. are different according to the used method and the yield and purity of the product are largely affected
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according to how much the by-products remain in the solution. Upon researching and comparing the conventional method for producing chlorine dioxide by oxidation, it was found that the reaction progresses so rapidly that the reaction cannot be controlled and side reactions occur, which makes it difficult to obtain a desired yield and purity.
Firstly, a chlorite is reacted with an acid to produce chlorous acid.
2NaC102 + 2HC1 →2HC102 + 2NaCl
The chlorous acid thus obtained is very unstable and rapidly undergoes heterolysis to produce stable chloric acid and unstable hypochlorous acid.
2HC102 →HC103 + HCIO
Also, complex reactions continuously occur, as follows, according to acidity, concentration and temperature.
HCIO + HCI <→ C12 + H20 3HC10 (Temperature) →2HC1 + HC103 (Heterolysis) HC10 + HC102 ^HC103 + HC1
These reactions can produce high purity, high yield chlorine dioxide only when the above side reactions are prohibited by satisfying proportions and concentrations of reactants, by rapidly accomplishing uniform mixing of the reactants, and by adjusting a proper acidity.
According to the facts known up to date, as shown in Fig. 1, to prepare
chlorine dioxide, the pressure of dilution water is raised in a booster pump 1 and the highly pressurized dilution water (about 3 to 6 atm) is ejected through a narrow nozzle of an ejector 2 (20 m/sec or higher) and ejected. As a result, a pressure reduction phenomenon occurs in the ejector and a chlorite solution and chlorine are simultaneously injected through inlets 3,4 disposed at one side of the ejector by the pressure reduction. Since this method uses common water as a injection water, two or three kinds of reactants are mixed and reacted with water in an ejector.
However, in this method, when a high pressure injection water from the nozzle causing decompression, a reactant (NaC102 solution) and Cl2 gas sucked from the side impact with each other, generating heavy noise and vibration. Such heavy noise and vibration may exert a vast effect on the entire chlorine dioxide generator system upon mass production, thereby causing deterioration of the quality and safety of the system. Also, when rapid and uniform mixing of the high-pressure nozzle injection water with the chlorite solution and chlorine gas to be sucked is not normally accomplished, a low purity, low yield chlorine dioxide solution is produced. In this case, it is necessary that the chlorite solution and chlorine should be introduced in small amounts so that the pressure reduction in the ejector can be maintained a few more. Otherwise, when the chlorite solution and chlorine are introduced in large amounts, back current phenomena occur at a chlorite solution inlet or a chlorine inlet, leading the generator system in a very dangerous condition. Consequently, low yield, low concentration chlorine dioxide is produced even when an equal amount of injection water is used.
Disclosure of Invention
Therefore, the present inventors have been extensively studied in order to
solve the above drawbacks involved in the conventional processes, and as a result, found a generator system which can continuously produce high purity chlorine dioxide by injecting a solution of a chlorite or a mixture of a chlorite and a hypochlorite to injection water before the injection water reaches an ejector nozzle so that the injection solution is alkalified, and can be effectively installed and operated in a water purification plant by controlling concentrations and amounts of reactants by means of an automatic metering pump, where needed, and completed the present invention.
It is therefore an object of the present invention to provide a method and apparatus for continuously producing high purity chlorine dioxide which can improve the problems involved in the prior art, uniformly mix reactants and inhibit side reactions at most
Brief Description of the Drawings
Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic diagram of a chlorine dioxide generator of the prior art;
FIG. 2 is a schematic diagram of a chlorine dioxide generator according to one embodiment of the present invention; and
FIG. 3 is a schematic diagram of a chlorine dioxide generator according to another embodiment of the present invention.
Best Mode for Carrying Out the Invention
According to one aspect of the invention, there is provided a method for producing chlorine dioxide comprising reacting a chlorite or a mixture of a chlorite and a hypochlorite, high pressure injection water from nozzle and chlorine gas, in which the chlorite or the mixture of a chlorite and a hypochlorite is incorporated into the injection water before the injection water reaches an ejector nozzle so that an alkalified injection water is used as an ejector injection solution, and an apparatus performing the method.
Hereinafter, the present invention will be described in further detail.
It is important that the method and apparatus used in the present invention have a construction adapted to manually or automatically inject a chlorite solution or a solution of a mixture of a chlorite and a hypochlorite in an amount needed according to a flow rate of dilution water before a booster pump, as shown in Fig. 2 and Fig. 3.
In the prior art, hydrochloric acid solution was reacted with a hypochlorite solution to produce chlorine (aqueous solution and gas), which was then mixed and reacted with chlorite solution at a high concentration to produce chlorine dioxide (aqueous solution and gas). The produced chlorine dioxide gas is absorbed or dissolved into water by means of reduced pressure resulting from ejection of pure water through an ejector to prepare chlorine dioxide aqueous solution. According to the above-described techniques, chlorine dioxide is already produced before the reactants reach an ejector nozzle and an ejector acts to dilute chlorine dioxide in water.
On the other hand, in the present invention, a chlorite solution or a solution of
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a mixture of a chlorite and a hypochlorite is mixed with water to form injection water of the ejector before they reach the ejector. The diluted solution is ejected at a high pressure into the ejector (through a high pressure nozzle), in which it is instantaneously contacted and reacted with chlorine gas, which has been already introduced into the ejector by reduced pressure occurring when the diluted solution is ejected at a high pressure into the ejector, to produce chlorine dioxide. That is, the present invention newly employs a method to induce an instantaneous (10" seconds) reaction by alkalifying high-pressure injection water so that the injection water in the form of an aqueous solution can completely react with acidic chlorine gas, and eliminates secondary reactions and side reactions, whereby it is possible to readily produce chlorine dioxide at high yield without technical difficulty. Also, the present invention directly utilizes an ejector as a vessel for contact reaction, whereby it is possible to produce chlorine dioxide in the form of an aqueous solution at a high concentration (typically 5,000 ppm to 8,000 ppm) without problems involved in the prior art, including that chlorine dioxide at a high concentration may be exploded, because chlorine dioxide (aqueous solution and gas) is prepared by mixing and reacting chlorine (an aqueous solution and gas), produced by a primary reaction of a hypochlorite and hydrochloric acid solution, with a chlorite solution before reaching an ejector and thus, chlorine dioxide should be produced at a low concentration (typically 2,000 rpm).
Therefore, it should be understood that apparatuses for producing chlorine dioxide constructed on the basis of the above concepts and methods applying such a technical principle may fall in the scope of the present invention.
In the present invention, a separate apparatus for supplying chlorite to be mixed with dilution water before a booster pump is preferably provided to
adjust a supplied amount of chlorite according to flow rate of the dilution water. This apparatus system is already known to the art and is preferably equipped with a sensor (S), a controller (C), a electronic servo unit (EM), an induction motor (IM) and a NaClO2 solution tank (T).
Also, the injected chlorite or mixture of a chlorite and a hypochlorite is mixed with dilution water by a propeller of the booster pump 1 and pressed to a high pressure (about 3 to 6 atm), and the resulting diluted solution of the chlorite or mixture of a chlorite and a hypochlorite is ejected through a nozzle of an ejector 2 (ex. at least 20 m/sec). At this time, decompression occurs in the ejector and chlorine gas in a quantified amount (preferably a half of the chlorite injection water) is absorbed through a chlorine gas inlet 3 disposed on one side of the ejector. Since the chlorite solution (i.e. the diluted chlorite solution) ejected at a high pressure from the nozzle is alkaline and the chlorine gas sucked by decompression is acid, instantaneous mixing is uniformly progressed and the mixture is smoothly introduced into a chlorine dioxide generator 5 substantially without noise and vibration. The introduced reactants complete the reaction in the generator comprising a double tube, and continuously producing high purity chlorine dioxide solution in a high yield through an outlet 6. The double tube is used to extend the reaction time by lengthening residence time for extension of reaction time.
According to the present invention, it is preferred that the apparatus for the reaction is designed so that the chlorite is mixed with chlorine gas in a molar ratio of 2:1. Therefore, the apparatus according to the present invention is preferably constructed so that the mixture of respective materials is injected according to flow rate of dilution. Particularly, the apparatus may be constructed so that the chlorite or the mixture of a chlorite and a hypochlorite is ideally reacted with chlorine gas or hydrochloric acid solution (gas), which
is introduced later, according to the stoichiometrical equation. Therefore, it is apparent that those skilled in the art may suitably adjust the amounts of a chlorite or a mixture of a chlorite and a hypochlorite to be supplied to dilution water and the amount of chlorine gas or hydrochloric acid solution so that the chlorite and the chlorine gas react in a molar ratio of 2: 1.
Also, if needed, it is possible to suitably adjust concentrations and amounts of chlorine and chlorine dioxide which are produced in proportion to the introduced amount of the chlorite by introducing a chlorite only in a needed amount, which can be used for a desired application. Thus, it is possible to efficiently conduct purification of water by adjusting amounts of produced chlorine dioxide and chlorine, as needed, and immediately using them in combination in pre-treatment of water from a water supply source. This forms a part of features of the present invention.
Therefore, in accordance with an additional aspect, there is provided a method for purifying water comprising: incorporating a chlorite or a mixture of a chlorite and a hypochlorite to injection water before the injection water reaches an ejector nozzle so that alkalified injection water is used as an ejector injection solution, in which the chlorite or the mixture of a chlorite and a hypochlorite and chlorine are incorporated in amounts corresponding to desired concentrations of final products, in a manner that when the sum of an amount of chlorine to react with chlorite and a target chlorine amount is incorporated chlorine dioxide and chlorine are produced in final target amounts; successively preparing chlorine dioxide and chlorine at a desired proportion in a chlorine dioxide generator; and combinedly using the produced chlorine dioxide and chlorine in pre-treatment of water from water supply source.
In other words, when the number of ejector nozzles needed for decompression in one chlorine dioxide generator is determined, a solution of a chlorite or a mixture of a chlorite and a hypochlorite in a needed amount proportional to the determined number is automatically incorporated through a metering injector and chlorine or hydrochloric acid gas in a needed amount proportional to the incorporated amount of the solution is sucked into the ejector.
For example, a tubing metering pump is disposed in front of a booster pump to pressurize 4.8 It I min of water to 4 atm and 25% chlorite solution is injected. The injected chlorite is uniformly mixed with water in the booster pump. The uniformly mixed chlorite dilution was pressurized to 4 atm and rapidly ejected from a nozzle an ejector 2, upon which decompression occurs in the ejector. Chlorine is sucked through a chlorine gas inlet 3 disposed on one side to be instantaneously mixed with the chlorite solution. The mixture solution complete the reaction in a chlorine dioxide generator 5 and the produced chlorine dioxide solution is continuously exhausted from an outlet 6.
By this method, it is possible to prepare a chlorine dioxide solution of 0.03 to 0.5 vol% (a higher concentration is possible).
In the prior art ejectors, common water is generally used as a injection water to generate decompression by sucking and diluting chlorine dioxide. In the present invention, however, the injection water of an ejector is used as a direct reactant so that the injection water is ejected through a high pressure nozzle while directly contacting with the other reactant at the same time to induce a reaction. Therefore, the reactants are simplified as two types, i.e. an acid reactant and an alkali reactant, the reaction can be naturally induced without
side reactions and the contact efficiency, that is, reaction between the reactants can be maximized.
Also, in the method of the prior art, a process includes a reaction of two or more steps and three or more kinds of reactants including water. However, in the present invention, since a solution of a chlorite or a mixture of a chlorite and a hypochlorite, i.e. an alkali solution is used as injection water and chlorine or hydrochloric acid gas, i.e. an acid gas is employed as another reactant, the ejected liquid and the gas can instantaneously contact and react with each other, and contact efficiency for interaction is thus increased.
Also, by using an instantaneous reaction (10" second) of an acid and an alkali, chlorine dioxide can be prepared in a high yield.
Most of surface water should be purified for use as drinking water. The surface water has BOD, COD, total nitrogen content and total phosphorus content increased from the upstream of a river to downstream. In dry seasons of spring and fall, diatoms prosperously propagate in the raw water, due to deterioration in quality of water itself and effects of weather, and generate offensive taste and odor. Such offensive taste and odor which are fish smell generated when metabolic products and dead bodies of algae are decomposed and interfere with functions of a filtering paper. Therefore, the filtering paper often is subjected to back wash, which is one of main factors reducing productivity of a water purification system. Also, in hot summer, blue-green algae and green algae prosperously propagate in water, producing toxic substances and inducing offensive taste and odor.
Therefore, when the method for producing chlorine dioxide according to the present invention is applied in such water purification plants, it is possible to obtain synergic effects able to effectively remove ammonia and amine as well
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as offensive taste and odor from raw water without spending a high cost by combinedly employing chlorine dioxide and chlorine in a pre-treatment of the raw water.
The chlorine dioxide generator system can not only produce pure chlorine dioxide but also control amounts of produced chlorine dioxide and chlorine at will. Therefore, it has synergic effects able to readily remove offensive taste and odor, oxidants, algae, ammoniac nitrogen, amine, phenol, iron, manganese, bacteria and viruses which cannot be treated with chlorine alone, when suitably used in a pre-treatment of water, as needed. Further, as the quality of the raw water becomes improved and chlorine dioxide is not needed, the system can be economically operated by incorporating only chlorine into the generator system for pre-treatment of the raw water.
Examples
The present invention will hereinafter be described in further detail by way of the examples. However, it should be understood that the examples are for the illustrative purpose and the present invention is not limited to thereto.
Example 1
25% chlorite solution was injected at 106.4 mC/min through a tubing pump before a booster pump through which 4.8 It per minute of water is ejected at 4 atm and the injected chlorite was uniformly mixed in the booster pump. By the pressurized solution, decompression occurred in an ejector, upon which chlorine was sucked at 3.9 It /min through a chlorine supply pipe disposed at a side. The solution was then injected into a chlorine dioxide
generator comprising a double tube. As a result, chlorine dioxide of a purity 98 vol% was continuously produced through a chlorine dioxide outlet.
Example 2
While the same pressure and amount of water as in Example 1 were used, 25% chlorite solution was injected at 6.4 mi/mm through a tubing pump and chlorine gas was sucked into an ejector at 23.9 cπf/min. As a result, the mixture solution was injected into a chlorine dioxide generator comprising a double tube, in which the reaction was then completed. The produced chlorine dioxide has a purity of 96 vol%.
Example 3
While the same pressure and amount of water as in Example 1 were used, 25% chlorite solution was injected at 63.8 mϋVmin through a tubing pump and chlorine gas was sucked into an ejector at 2.39 It /min. As a result, the mixture solution was injected into a chlorine dioxide generator comprising a double tube, in which the reaction was then completed. The produced chlorine dioxide has a purity of 97 vol%.
Example 4
While the same pressure and amount of water as in Example 1 were used, 25% chlorite solution was injected at 57 mβ/min through a tubing pump and chlorine gas was sucked into absorbed an ejector at 2.39 It /min. The mixture solution was injected into a chlorine dioxide generator comprising a
double tube, in which the reaction was then completed. The produced chlorine dioxide has a purity of 97 vol% and chlorine was 5.8 vol%.
Example 5
3 It of water collected from Jamshil underwater dammed pool (containing 0.2 mg/ It ammonia) was mixed with the chlorine and chlorine dioxide solution prepared in Example 4 and left for 1 hour in a dark place. After treatment, it was found the chlorophylla was reduced by 85 to 90% and no ammonia was detected.
Example 6
While the same pressure and amount of water as in Example 1 were used, a mixture of 7.748 kg of 25% chlorite solution and 6.271 kg of l2% NaC10 solution was injected at 11.6 ml/min through a tubing pump and chlorine gas was sucked into an ejector at 478 cnf/min. The mixture solution was injected into a chlorine dioxide generator comprising a double tube, in which the reaction was then completed. As the result, the produced chlorine dioxide has a purity of 95 vol%.
Industrial Applicability
Conventional methods for generating chlorine dioxide by oxidizing a chlorite with chlorine have problems of risk caused by counter current at a chlorine inlet port, low productivity and generation of chlorine dioxide at a low- concentration. Also, commercially available chlorine dioxides αo not produce by-products but are very expensive. On the contrary, the chlorine
dioxide generator system according to the present invention can not only produce pure chlorine dioxide but also produce chlorine dioxide and chlorine in controlled amounts. Therefore, when the generator system is suitably used in pre-treatment of raw water, it has synergic effects able to readily remove offensive taste and odor, oxidizing substance, algae, ammoniac nitrogen, amine, phenol, iron, manganese, bacteria, viruses and the like. Further, when chlorine dioxide is not needed since the raw water has better quality, only chlorine can be absorbed to water for pre-treatment of water using the same generator. Therefore, the system is an economical, useful and improved method. Of all things, according to the present invention, it is possible to conveniently, economically and continuously produce chlorine dioxide in a purity of 94% or more by adjusting concentrations and amounts of reactants as needed.