MXPA06008779A - Process for production of chlorine dioxide - Google Patents

Abstract

The invention relates to a process for continuously producing chlorine dioxide comprising the steps of:feeding to a reactor an acid, a reducing agent and alkali metal chlorate;reacting the alkali metal chlorate with the acid and the reducing agent to form a product stream containing chlorine dioxide and the alkali metal salt of the acid;and, bringing said product stream from the reactor to an absorption tower, where it is contacted with a flow of water to form an aqueous solution containing chlorine dioxide. The invention also relates to an apparatus to produce chlorine dioxide. The invention further relates to a novel aqueous solution comprising of chlorine dioxide.

Description

(84) Designated States (unless indicated for the purpose of two-letter codes and oiher abbreviations, refer to the "Guidkind of regional proiection available.): ARY (BW, GH. Ance Notes on Codes and Abbrevialions" appearing at the begin - GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM, nm 'g ofeach regular issue of the PCT Gazeue, ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, H, FR, GB, GR, HU, IE, IS, IT, LT, LU , MC, NL, PL, PT, RO, SE, SI, SK, TR), OAPI (BF, BJ, CF, CG, Cl, CM, GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG). Published: - with inlernalional search repon PROCESS FOR PRODOCCIOM PE CHLORINE DiOXfPO Field of La. { invention The present invention relates to a process for producing carbon dioxide from alkali metal keto acid and a reducing agent.

Background of the Invention Chlorine dioxide is used in various applications such as pulp bleaching, grease bleaching, water purification and removal of organic materials from industrial waste. Since the carbon dioxide is not stable in storage, it must be produced on site. Chlorine dioxide is usually produced by reacting an alkali metal chlorate with a reducing agent in an aqueous reaction medium. The chlorine dioxide can be removed from the reaction medium as a gas. as in the processes described by the patents of the United States of America 5091 66, 5091167 and EP 612686. The chlorine dioxide gas can then be absorbed in water to form an aqueous solution thereof. Preferably, these are large-scale processes that require extensive process equipment and instrumentation.

For the production of chlorine dioxide in small-scale units, such as purification applications or small bleaching plants, it is favorable not to separate the chlorine dioxide from the reaction medium but to recover a solution containing chlorine dioxide directly from it. reactor, optionally after dilution with water. Such processes are described in United States of America patents 2833624, 4534952, 5895638, 6790427, in WO 00/76916, and in the patent applications of the United States of America Publication No. 2004/0175322 and No. of Publication 2003/003 1621, and have been marketed in recent years. The equipment and instrumentation of required processes is considerably less expensive than in the large-scale processes described above. However, there is still a need for additional improvements. In smaller processes it has been difficult to obtain solutions with such a high concentration of chlorine dioxide as desired for many applications, such as bleaching recycled paper, bagasse bleaching, or small-scale bleaching of pulp. Another problem with existing small-scale processes for the production of chlorine dioxide is that the concentration of the product's carbon dioxide may fluctuate depending on the rate of production of the chlorine dioxide.

Compendium of the Invention It is an object of the invention to provide a process for the production of chlorine dioxide that allows the direct production of chlorine dioxide in an aqueous solution with a high concentration of chlorine dioxide. It is another object of the invention to provide a process for the production of chlorine dioxide that allows the direct production of chlorine dioxide in an aqueous solution with a concentration of chlorine dioxide that can be maintained constant regardless of the rate of production of the dioxide of chlorine. chlorine. It is another object of the invention to provide an apparatus for executing the process. It is a further object of the invention to provide a novel high concentration chlorine dioxide solution.

Brief Description of the Invention It has surprisingly been found that it is possible to fulfill those objects by providing a process for the continuous production of chlorine dioxide comprising the steps in continuous form of: feeding a reactor, an acid reduction agent and an alkali metal chlorate to a reactor.; reacting the alkali metal chlorate with the acid and reducing agent to form a process stream containing chlorine dioxide and the alkali metal salt of the acid; and bringing the product stream from the reactor to an absorption tower, where it is contacted with a stream of water to form an aqueous solution containing chlorine dioxide. It has been found that by bringing the product stream to an absorption tower it is possible to obtain an aqueous solution with a high concentration of chlorine dioxide, preferably of more than about 3 g / liter, more preferably of more than about g / liter, without first separating the chlorine dioxide gas from the reaction medium and then absorbing it in water. Any soluble species such as the alkali metal salt of the acid and the unreacted fed chemical agents are also absorbed, in the absorption tower. The flow velocity of the water to the absorption tower, whether cooled or not, is preferably adjustable so that the concentration of chlorine dioxide can be kept constant regardless of the production rate. The aqueous solution obtained in the absorption tower can have a chlorine dioxide concentration of a wide range, for example from about Q.1 g / titre to about 12 g / liter, preferably from about 3 g / liter to about 10 g / liter. g / liter, more preferably from about 4 g / liter to about 8 g / liter. The concentration of unreacted chlorate in the aqueous solution, which depends on the degree of conversion. is suitably below about 0.33 mole / mole ClO2, preferably below about 0.11 mole / mole ClOz, more preferably below about 0.053 mole / mole CIO2. The concentration of the alkali metal salt depends on the concentration of the chlorine dioxide and suitably is about 0.74 mmols / liter to about 59 mls / liter. The pH of the aqueous solution can vary depending on a wide range, depending partly on the concentration of chlorine dioxide, from about 0.1 to about 1, preferably from about 0.2 to about 0.8. By the term "absorption tower" as used herein, any column or tower or the like is represented wherein the gas is brought into contact with a flow of liquid to absorb the soluble compounds therein. Preferably, the gas and the liquid flow counter-current. Devices such as plates or packaging elements are preferably placed within the absorption tower to provide interfacial surfaces where the mass transfer between the gas and the liquid can take place. Examples of useful packaging elements include Rasching rings, Berl supports, Intaiox supports, etc. Examples of plates that can be used are screen plates and bubble cap plates. Preferably, a device that creates a pressure. subatmospheric s connected to the absorption tower that carries the product stream, including any liquid, foam and gas in it, to flow into the absorption tower. The non-absorbed gas is removed from the absorption tower by means of said device. Any commonly used devices such as fans, ejectors, etc., can be employed, preferably an ejector. In the latter case the ejector is fed with motor water, which can be supplied from a separate storage tank and a pump that only serves the ejector. The storage tank is preferably vented so that the non-absorbed process gas can be removed. The alkali metal chlorate is suitably fed into the reactor as an aqueous solution, and the alkali metal may be, for example, sodium, potassium or mixtures thereof, of which sodium is most preferred. The acid is preferably a mineral acid such as sulfuric acid, hydrochloric acid, nitric acid, percyrnic acid or mixtures thereof, of which sulfuric acid is most preferred. Various reducing agents can be used for example hydrogen peroxide, methanol, chlorine ions, etc., of which hydrogen peroxide is most preferred. In the latter case, when hydrogen peroxide is used, the molar ratio H2O2 to ClO3 fed to the reactor suitably is from about 0.5: 1 to about 1.5: 1, more preferably from about 0.5: 1 to about 1: 1. Alkali metal chlorate always contains al, gs > of chlorine as an impurity, although it is also quite possible to feed more chlorine to the reactor, such a metal chlorine or hydrochloric acid. However, in order to minimize the formation of chlorine, it is preferred to keep the amount of chlorine ions fed to the reactor low, suitably below about 1 moi%, preferably below about 0.1 mole%. more preferably less than about 0.05 moi%, more preferably less than about 0.02 mole% Cl to C10a. In case the sulfuric acid is used as a feed for the reactor, it preferably has a concentration of from about 60 to about 98% by weight, more preferably from about 70 to about 85% by weight and preferably at a temperature from about 0 to about 80 ° C, more preferably from about 20 to about 60 ° C, as may then be possible to operate the process substantially adiabatically. Preferably, from about 2 to about 7 kg of H2SO4, more preferably from about 3 to about 5 kg of H2S4 per kg of produced CIO2 are fed. In order to use the high concentration sulfuric acid, a dilution and cooling scheme is preferably applied as described in the patent application of the United States of North America No. of Publication 2004/0175322. In a particularly preferred embodiment the alkali metal chlorate and the hydrogen peroxide are fed to the reactor in the form of a pre-mixed aqueous solution for example a composition as described in WO 00/76916, which is incorporated in the present reference medium . Said composition may be an aqueous solution comprising from about 1 to about 6.5 mols / liter, preferably from about 3 to about 6 mols / liter of alkali metal chlorate, from about 1 to about 7 mols / liter, preferably from about 3 to about 5 mols / liter of hydrogen peroxide and at least one of a protective colloid, a radical scavenger or a complexing agent based on phosphoric acid, wherein the pH of the aqueous solution is suitably from about 0.5 to about 4, preferably from about 1 to about 3.5, more preferably from about 1.5 to about 3. Preferably, at least one complexing agent based on phosphoric acid is present, preferably in an amount from about 0.1 to about about 5 mmols / liter, more preferably from about 0.5 to about 3 mmols / liter. If a protective colloid is present, its concentration is preferably from about 0.001 to about 0.5 mols / liter, more preferably from about 0.02 to about 0.05 mols / liter. If a radical scavenger is present, its concentration is preferably from about 0.01 to about 1 mols / liter, more preferably from about 0.02 to about 0.2 mols / liter. Particularly preferred compositions comprise at least one complexing agent based on phosphoric acid selected from the group comprising 1-hydroxyetiifen-1,1-diphosphoric acid, 1-aminoethane-1,1-difiphosphoric acid, amtrtotrtmet? lerafosfÓF? acid, ethylene diamin tetramethylene phosphoric acid, hexamethylene diamine tetra methylene phosphoric acid). diethylene dihydroxymethylene methylenphosphonic acid), 1-aminoalkan-1, 1-diphosphoric acids (such as morpholinomethane diphosphoric acid, NEW-dimethylamotrimetimethyphosphoric acid, aminomethyl diphosphoric acid), reaction products and salts thereof, preferably sodium salts. Useful protective colloids include tin compounds, such as alkali metal stannate, in particular sodium stearate (Na2 (Sn (OH) 6). Useful radical scavengers include pridin carboxylic acids, such as 2,6-pridin dicarboxylic acid Suitably the amount of chlorine ions is below about 300 mmols / liter, preferably below about 50 mmols / liter, more preferably below about 5 mmols / liter, more preferably below of approximately 0.5 mmols / liter.

The reduction of the alkali metal chlorate to chlorine dioxide results in the formation of a product stream in the reactor, which normally comprises liquid and foam, and which contains chlorine dioxide, the alkali metal salt of the acid and, in most In some cases, some chemical agents fed without reacting. If hydrogen peroxide or reducing agent is used, the product stream also contains oxygen. Chlorine dioxide and oxygen can be present as they dissolve in the liquid and as gas bubbles. If sulfuric acid is used, the alkali metal salt is a sulfate salt. It has been found that it is possible to achieve a degree of conversion of alkali metal chlorate to chlorine dioxide from about 75% to 100%, preferably from about 80 to 100%, more preferably from about 95 to 100%. The temperature in the reactor is suitably maintained below the boiling point of the reagents and the product stream at the prevailing level, preferably from about 20 to about 80 ° C, more preferably from about 30 to about 60. ° C. The pressure maintained within the reactor is suitably slightly subatmospheric, preferably from about 30 to about 100 kPa absolute, more preferably from about 65 to about 95 kPa absolute. The reactor may comprise one or more containers, for example placed vertically, horizontally or inclined. The reagents can be fed directly into the reactor or through a separate mixing device. Suitably the reactor is a substantially tubular direct flow vessel or vessel, most preferably comprising means for mixing the reactive coughs in a substantially uniform manner. Such means for mixing are described for example in US 6790427 and U.S. Patent Application Publication No. 2004/0175322. The feeds, which include acid, alkali metal chlorate and reduction agent, are preferably fed near one end of the reactor and the product stream is preferably withdrawn at the other end of the reactor. The length . { in the main flow direction) of the reactor used is preferably from about 150 to about 1500 mm, more preferably from about 300 to about 900 mm. it has been found favorable to use a substantially tubular reactor with an internal diameter of from about 25 to about 300 IBHI, preferably from about 50 to about 150 mm. It is particularly favorable to use a substantially tubular reactor having a preferred length to internal diameter ratio of from about 12: 1 to about 1: 1 B more preferably from about 8: 1 to about 4: 1. A dwell time Suitable average in the reactor is in most cases from about 1 to about 60 seconds, preferably from about 3 to about 20 seconds. The process of the invention is particularly suitable for the production of chlorine dioxide in small scale, for example from about 0.5 to about 200 kg / hr, preferably from about 10 to about 150 kg / hr. A common small-scale production unit normally includes only one reactor, although it is possible to place several, for example up to about 15 or more reactors in parallel, for example as a bundle of tubes. If more than one reactor is used then it is optional if each reactor is connected to a separate absorption tower and a separate device that creates a subatmospheric pressure or if all the reagents are connected to an individual absorption tower and a device that creates a pressure subatmospheric. The invention further relates to a novel aqueous solution containing carbon dioxide that could be produced through the process as described above. The concentration of chlorine dioxide in the novel aqueous solution is from about 4 to about 12 g / liter, preferably from about 4 to about 8 g / liter, more preferably from about 4 to about 6 g / liter. The pH of the novel aqueous chlorine dioxide solution is from about 0 to about 1, preferably from about 0.2 to about 0.8. The concentration of sulfate in the aqueous solution is from about 1.1 mols / mol CIO2 to about 3.8 mois / mol C02, preferably from about 1.1 mols / mol GIG2 to about 3.2 mols / mol CIO2- The concentration of residual chlorate in the aqueous solution is suitably below about 0.33 mols / mol CIO2, preferably below about 0.1 1 mols / mol CIO2E more preferably below about 0.053 mols / oi CfO2- The invention also relates to an apparatus to produce chlorine dioxide according to the process described above. The apparatus comprises a reactor provided with one or more feed lines for alkali metal chlorate, hydrogen peroxide and acid, the reactor being connected to an absorption tower. The apparatus further comprises a device for creating a subatmospheric pressure in the reactor and the absorption tower. Preferably said device is an ejector fed with motor water. The process of the invention makes possible the production of an aqueous solution with a high concentration of chlorine dioxide, ie of more than about 3 g / liter, preferably of more than about 4 g / liter, with equipment that is simple and easy to operate.

The preferred embodiments of the apparatus are apparent from the foregoing description of the process and the description that follows relates to the drawings. However, the invention is not limited to the embodiments shown in the drawings and covers many other variants within the scope of the claims.

Brief Description of the Drawing The figure shows a schematic process diagram for one embodiment of the present invention.

Detailed Drawing Description Referring to the figure, a substantially direct tubular reactor 1 is supplied with sulfuric acid through a feed line 2 and a pre-mixed aqueous solution of sodium chlorate and hydrogen peroxide through the feed line 3. In reactor 1 the feed streams are mixed and reacted to form a product stream of liquid, foam and gas comprising chlorine dioxide, oxygen, sodium sulfate and some remaining sulfuric acid and sodium chlorate. The product stream is brought to the lower end of an absorption tower 4 which is fed with water in the upper part 6. the chlorine dioxide is absorbed in the water to form a product solution which is removed from the absorption tower in the lower part 5. To create a subatmospheric pressure in the reactor 1 and the absorption tower 4 an ejector 7 is connected to the absorption tower. The ejector 7 is fed with motor water which is recirculated through a storage tank 8 and is then pumped through the ejector by means of a pump 9. The motor water storage tank is vented so that any product gas Not absorbed in the absorption tower, such as oxygen, can be removed.

Claims (10)

1. A process for continuously producing chlorine dioxide comprising the steps in continuous form of: feeding an acid, a reducing agent and an alkali metal chlorate to a reactor; reacting the alkali metal chlorate with the acid and reducing agent to form a product stream containing chlorine dioxide and the alkali metal salt of the acid; and bringing the product stream from the reactor to an absorption tower, where it is contacted with a stream of water to form an aqueous solution containing chlorine dioxide.

2. A process according to claim 1, characterized in that the aqueous solution containing chlorine dioxide has a concentration thereof from about 4 to about 12 g / L

3. A process according to any of claims 1 to 2, characterized in that the concentration of C1O2 in the aqueous solution is maintained substantially constant regardless of the production rate of chlorine dioxide when adjusting the water flow for the absorption tower.

4. A process according to any of claims 1 to 3, characterized in that the gas not absorbed from the absorption tower is removed by an ejector, creating a subatmospheric pressure in the reactor and the absorption tower.

5. A process according to any of claims 1 to 4, characterized in that the reactor is operated at a pressure from about SO to about 100 kPa.

6. A process according to any of claims 1 to 5, characterized in that the acid is sulfuric acid.

7. A process according to any of claims 1 to 6, characterized in that the reducing agent is hydrogen peroxide.

8. A process according to claim 7, characterized in that the alkali metal chlorate and the hydrogen peroxide are fed to the reactor in the form of a pre-mixed aqueous solution.

9. A process according to any of claims 1 to 8, characterized in that the reactor is a direct flow vessel or pipe. A process according to any of claims 8 or 9, characterized in that the acid, the alkali metal chlorate and the reducing agent are fed near one end of the reactor while the product stream is withdrawn in the other. end of the reactor. 1. A process according to any of claims 1 to 10, characterized in that the product stream from the chlorine dioxide-containing reactor comprises liquid, foam and gas. 12. Apparatus useful for continuously producing chlorine dioxide according to any of claims 1 to 1 comprising a reactor which is a direct flow vessel or pipe supplied with feed lines for alkali metal chlorate, acid and a reducing agent and connected to an absorption tower, the absorption tower that is connected to a device that creates a subatmospheric pressure in the reactor and the absorption tower. 13. an aqueous solution comprising from about 4 to about 12 g / liter of chlorine dioxide and from about 1.1 to about 3.8 mols of sulfate per mol CIO2, wherein the pH of the aqueous solution is from about 0.1 to about 1.