Patent Application
20060034750
1. Field of the Invention
The present invention relates to generators for chlorine dioxide, and, more particularly, to a chlorine dioxide generator that is modular and scalable.
2. Description of Related Art
Chlorine dioxide is a strong oxidant that has been receiving increased attention as an alternative to chlorine for the disinfection and taste/odor (T/O) control of water and wastewater. The molecular formula of chlorine dioxide is expressed as CO2. As implied from its chemical formula, it has the disinfecting properties of both chlorine and oxygen. Moreover, chlorine dioxide exhibits good disinfection performance without the disadvantages of forming large quantities of undesirable chlorinated byproducts, since it does not react with hydrocarbons to form chlorinated hydrocarbons.
Chlorine dioxide (CIO2) first was discovered in 1811 in the form of a greenish-yellow gas by Sir Humphrey Davy, by reacting potassium chlorate (KClO3) with hydrochloric acid (HCl). It later was found that ClO2 could be used in a dilute acetic acid (CH3COOH) solution for the bleaching of paper pulp. Even though the outstanding disinfecting properties of chlorine dioxide have been consistently noted, its practical application has been hampered due to the lack of a safe and economical way of synthesizing it. In the 1930s, the Mathieson Alkali Works developed the first commercial process for making ClO2, from sodium chlorate (NaClO3) via sodium chlorite (NaClO2).
In the 1990s, the U.S. Environmental Protection Agency recommended that, as a part of the reauthorization of the Clean Water Act, a study should be undertaken to develop a strategy to prohibit, reduce, or find substitutes for the use of chlorine and chlorinated compounds. In recent years, free chlorine (Cl2) has been criticized by environmentalists, even though it is one of the most heavily used chemicals in various chemical and environmental applications. The disadvantages associated with using free chlorine can be summarized as follows:
(1) Chlorine is quite reactive with various substances, including water, ammonia, and hydrocarbons, having a very strong tendency to chlorinate organic chemicals, including phenols and amines to produce chlorinated organics such as chlorophenols and chloroamines;
(2) Even with water, it reacts to produce hydrochloric acid and hypochlorous acid;
(3) Solubility in water is relatively low, making it difficult to adequately disinfect without affecting the vapor space above;
(4) Chlorine is not effective in taste and odor (T/O) control, due to its low water solubility, pungent odor, and acidic reaction; and
(5) It is produced only as a bulk chemical commodity. A small batch capability does not exist, because on-site generation of chlorine is commercially unattractive, making chlorine unsuitable for wastewater treatment.
For at least these reasons, the replacement of chlorine with other chemicals such as chlorine dioxide has been of interest in recent years.
Chlorine dioxide is known to be an excellent disinfectant as well as a strong oxidizing agent. Its bactericidal, fungicidal, algicidal, bleaching, and deodorizing properties are well documented in the literature. Chlorine dioxide is soluble in water at room temperature (20° C.) to about 2.9 grams ClO2 per liter of water at 30 mmHg partial pressure of ClO2, or 8 grams per liter at 80 mmHg partial pressure. ClO2 is approximately 5 times more soluble in water than chlorine gas (Cl2). ClO2 is much more soluble in water than oxygen (O2) which only has 9.2 mg solubility per liter of water. The presence of chlorine dioxide in water is very easily detected by a color change from yellowish-green to orange-red as the concentration of ClO2 increases in water. At low temperatures, chlorine dioxide dissolves in water to a substantially greater extent due to lower vapor pressure, e.g., 12 g/L at 60 mmHg of partial pressure and 10° C.
The boiling point (b.p.) of the liquid form ClO2 is 11° C., and the melting point (m.p.) is −59° C. Gaseous ClO2 has a density of 2.4 (taking air as 1.0), and its molecular weight is 67.45 g/mol; i.e., it is a heavier gas than air. If chlorine dioxide is leaked into the air, it will tend to stay low, near the ground, and then gradually diffuse into the atmosphere.
Chlorine dioxide (ClO2) differs from Cl2 in that ClO2 does not react with water or ammonia. Also, unlike chlorine, ClO2 does not produce chlorinated hydrocarbons after reacting with hydrocarbons. In general, ClO2 is less corrosive to most metallic and nonmetallic substances than chlorine, which is an important advantage.
Conventional chlorine dioxide solutions prepared using methods disclosed in the prior art suffer from the drawback that they produce undesirable by-products. Some prior art methods, for example, use either strong acids, which are environmentally unfriendly, or chlorine gas, leading to the formation of a variety of chlorine-containing by-products via complex reaction pathways. Further, known methods are also believed to produce low concentrations of chlorine dioxide.
Thus there has existed a need to provide an economic and efficient method and apparatus for producing chlorine dioxide that does not also produce hazardous by-products (e.g., chlorine or chlorous acid), as well as substantial amounts of unusable salts (e.g., sodium chloride, sodium lactate). There has also existed a need for a method and apparatus for producing chlorine dioxide that does not suffer from the aforementioned disadvantages.
These needs have been solved by the apparatus and method of commonly owned U.S. Pat. Nos. 5,855,861 and 6,051,135, the contents of which are incorporated herein by reference. A particular drawback in certain applications of the inventions of these patents, however, resides in space and scalability considerations. Therefore, it would also be desirable to provide a chlorine dioxide generator having a smaller footprint and ease of scalability.
The present invention is directed to an apparatus for making chlorine dioxide that is easy to maintain, has a smaller footprint that prior known devices, has increased energy efficiency, and enables greater throughput. In addition, the present invention permits better conversion efficiency of raw materials, reduces process waste, and is less expensive to operate. The apparatus is highly scalable, and is capable of producing 2-1000 gal/h chlorine dioxide at 10,000 ppm. A final product of the apparatus and method comprises an aqueous chlorine dioxide solution that is substantially pure and substantially free of byproducts. All these features are believed to represent significant improvements over the prior art.
The apparatus in one embodiment comprises a reactor for reacting an aqueous reaction solution including an aqueous acid solution and a solution of an alkali metal salt of a chlorite ion to form a product solution. The reactor comprises a substantially cylindrical inner column for receiving the aqueous reaction solution and a substantially cylindrical outer column positioned in coaxial surrounding relation to at least a portion of the inner column. The outer column is for containing a temperature-controlling fluid for maintaining solution flowing through the inner column at a predetermined temperature. Also provided is a means for maintaining the fluid in the outer column at the predetermined temperature.
A stripper is in fluid communication with an outlet of the inner column for stripping chlorine dioxide from the product solution into a gas to provide a product gas and a stripped product solution. An absorber is provided for absorbing chlorine dioxide from the product gas to provide an aqueous chlorine dioxide solution.
Another embodiment of the present invention includes a modularized apparatus wherein the three columns (the reactor, the stripper, and the absorber) are provided in subdivided sections, typically 2, for ease of delivery. Such a portable unit would be appropriate, for example, for providing chlorine dioxide solution for small municipalities, swimming pools, parks, spas, lagoons, food treatment plants, utilities, remote sites, war and disaster areas, and demonstrations. This unit is designed to deliver 2 gal/h at 10,000 ppm, although this is not intended as a limitation.
The invention further includes a method of making chlorine dioxide, which comprises the steps of reacting an aqueous reaction solution comprising an aqueous acid solution and a solution of an alkali metal salt of a chlorite ion to form a product solution within a substantially cylindrical inner column. Solution within the inner column is maintained at a predetermined temperature. Chlorine dioxide is stripped from the product solution into a gas to provide a product gas and a stripped product solution. Chlorine dioxide is then absorbed from the product gas to provide an aqueous chlorine dioxide solution.
A method of disinfecting a target such as water, wastewater, or a surface comprises the steps of producing chlorine dioxide as above, and using the product solution on the target, such as by introducing the solution into a fluid or applying the solution to a surface, for example.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.
A description of the preferred embodiments of the present invention will now be presented with reference to
One aspect of the present invention is directed to an apparatus for making chlorine dioxide. A first embodiment of such an apparatus 10, illustrated in
Each of these tanks 13, 15, 17 is in fluid communication with a respective pump 18, 19, 20 for pumping the tank contents 12, 14, 16 into a line 21 leading to an inlet 22 of the reactor 11. Preferably pH adjustment 23 is also provided downstream of the reactor inlet 22, the pH level preferably ≦4.0. Dilution of the acid 12—sodium chlorite 14 mixture 24 is accomplished by adjusting the amount of water introduced into the line 21.
In a preferred embodiment the reactor 11 comprises a substantially cylindrical inner column 25 having an inlet 22 and an outlet 26. The inner column inlet 22 receives the aqueous reaction solution 24 from line 21, the aqueous reaction solution proceeding upward through the inner column 25, thereby controlling the bubbling of ClO2. In prior systems that included a coiled passageway, such bubbling can block the flow of liquid, and the reaction mixture can be premixed unnecessarily, which has a deleterious effect on the conversion process. The contents of the aqueous reaction solution 24 react to form a product solution 27 containing chlorine dioxide by the time the reactants reach the inner column outlet 26.
A substantially cylindrical outer column 28 having an inlet 29 and an outlet 30 is positioned in coaxial surrounding relation to at least a portion of the inner column 25. The outer column 28 is for containing a temperature-controlling fluid 31 for maintaining solution flowing through the inner column 25 at a predetermined temperature. The temperature of the fluid 31 is controlled by a temperature-controlling unit 32 and circulating pump 33 substantially continuously cycling fluid 31 through the outer column 28. In a preferred embodiment, the predetermined temperature is approximately 40° C. The flow of fluid 31 may be parallel or antiparallel to the flow of the aqueous reaction solution 24 and product solution 27 through the inner column 25.
A stripper column 34 having a fluid inlet 35 and a gas outlet 36 is in fluid communication via line 37 with the inner column outlet 26 for stripping chlorine dioxide from the product solution 27 into air flowing in countercurrent fashion, the air provided by air injector 38 at an opposite end 39 of the stripper 34 to provide a product gas 40 and a stripped product solution 41.
As the stripped product solution 41 typically will contain some unreacted elements, a recycle pump 42 is provided at the stripper's fluid outlet 43 for returning such elements to the reactor's inlet 22. Alternatively, the stripped product solution 41 may be pumped 43 to a drain 44. If the recycling option is taken, the pH of the mixture rises faster than in the case wherein only fresh reactant is used. Thus the final product concentration that is achievable is lower, but the pH is still maintained, since the fresh reaction feed typically has a pH of approximately 2.7-2.9, while the product pH is around 4.0.
The product gas 40 is then channeled via line 45 to a gas inlet 46 of an absorber column 47. The absorber 47 is in fluid communication with a water injector 48 injecting water at a fluid inlet 49 adjacent an opposite end 50 from the gas inlet 46 to achieve countercurrent flow against flow of the product gas 40. It is preferred that the time during which the CO2-rich remains a gas be minimized in order to prevent decomposition. The absorber 47 is adapted to absorb chlorine dioxide from the product gas 40 to provide an aqueous chlorine dioxide solution 51 at the absorber fluid outlet 52. Scrubbed air is vented from gas outlet 53 adjacent the absorber fluid inlet 49.
In this and other embodiments the stripper and absorber columns are preferably packed with packing materials such as, but not intended to be limited to, ceramic saddles/rings, glass beads, zirconia beads, etc. Unpacked columns may also be used.
The aqueous chlorine dioxide solution 51 is pumped 54 from the absorber fluid outlet 52 to a storage tank 55, from which it may be dispensed to disinfect a target as discussed above.
In a second embodiment 60, illustrated in
In a third embodiment 70 illustrated in
In a fourth embodiment 90, illustrated in
A fifth embodiment, illustrated in
The reactor 101, illustrated in more detail in
A top arm 114 of a second tee 115 similarly forms the closed top of the second outer column 116 via second adapter 117 linking to the top of the second inner column 118. The side arm 119 of the second tee 115 forms an inlet for temperature-controlling fluid 111 entering the second outer column 116 from the controller 113.
The top arm 121 of the first tee 108 and the bottom arm 122 of the second tee 115 connect, respectively, to first and second jacketing pipes 123, 124, which form the inner sectors of the first and second outer columns 109, 116.
A third 125 and a fourth 126 tee are provided at the top 127 and bottom 128 ends, respectively, of the first and second pipes 123, 124, connected thereto at their bottom 129 and top 130 arms, respectively. Adapters 131, 132 at the third tee's top arm 133 and the fourth tee's bottom arm 134 link to and close the junction with the first 105 and second 118 inner columns, respectively. Their side arms 135,136 provide a path via first and second tubing 137, 138 for bypassing a junction between the inner columns 105, 118 and are joined at a bypass junction 139 to permit temperature-controlling fluid flow.
The first and the second inner columns 105,118 are joined at a junction 140, the first inner column's bottom end serving as the reactor inlet 104, the second column's top end serving as the reactor outlet 141.
In a particular embodiment the tees comprise 1.5-in. tees; the jackets comprise 1.5-in. PVC Sch 40 tubing; and the adapters comprise ¾×1.5-in. adapters, although these specifications are not intended as limitations. Other possible polymeric tubings may include PVDF, CPVC, TTE-lined polyethylene, etc., although these are not intended as limitations.
This apparatus 100 further includes a first and a second stripper 142, 143 positioned collinearly to form the stripper 102 and a first and a second absorber 144, 145 positioned collinearly to form the absorber 103, these being joined at junctions 146, 147.
ClO2-laden air is channeled via line 148 from the stripper 102 to the absorber 103. Preferably, the ClO2-laden air should not remain in a gaseous state for very long.
This embodiment 100 is compact and easily transportable, having in a particular embodiment a footprint of only 5×3 ft.
It may be appreciated by one skilled in the art that additional embodiments may be contemplated, including additional modules among any or all of the columns, connected in series or in parallel.
In the foregoing description, certain terms have been used for brevity, clarity, and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for description purposes herein and are intended to be broadly construed. Moreover, the embodiments of the composition and associated methods described herein are by way of example, and the scope of the invention is not limited to the exact details disclosed.
