PRESSURIZED CATALYTIC PRODUCTION OF DIOXIDE SPECIES

Abstract

A packed bed catalyst in a pressurized vessel / reactor during contact with a dioxide species precursor improves the catalytic conversion of the precursor to dioxide species, compared to the same catalytic conversion performed in a non-pressurized vessel / reactor.

Description

PRESSURIZED CATALYTIC PRODUCTION OF DIOXIDE SPECIES FIELD OF THE INVENTION The present invention relates to the generation of aqueous chlorine dioxide from chlorine dioxide precursors. In particular, the present invention relates to the catalytical conversion of one or both of aqueous chlorous acid and aqueous chloric acid into aqueous chlorine dioxide. BACKGROUND OF THE INVENTION Chlorine dioxide (molecular formula ClO2) is a well-known disinfectant and cleaner that can be generated using chlorous acid as a starting material. Chlorous acid (molecular formula HClO2) is produced when there is an essentially complete substitution of the anionic chlorite countercation (ClO2) with the hydrogen ion (H+). Chloric acid (molecular formula HClO3) is produced when there is an essentially complete substitution of the anionic chlorate countercation (ClO3) with the hydrogen ion (H+). The generation of aqueous chlorous acid by acidifying an aqueous chlorite salt solution (sometimes incorrectly called stabilized aqueous chlorine dioxide solution) is well known. In fact, whether an aqueous solution contains a chlorite salt or chlorous acid depends on the solution's pH, with chlorous acid being present almost exclusively at a sufficiently low pH, e.g., below 1.7, and the chlorite salt being present exclusively at a sufficiently high pH, ​​e.g., around 8.5. A mixture of chlorous acid and chlorite salt is present at intermediate pH values. Below pH 4, chlorous acid predominates, and above that pH, chlorite is the predominant species. See Gilbert Gordon, The Chemistry of Chlorine Dioxide, Progress in Inorganic Chemistry: Volume 15, Ed. SJ Lippard, 1972, 201-286 (Gordon), whose description is incorporated as if stated in full in the present description. The rate of the catalytic reaction to chlorine dioxide depends on the ratio of chlorous acid to chlorite in the aqueous solution. It is also known that, over time, aqueous chlorous acid slowly converts to chlorine dioxide. This slow conversion predominates in solutions containing low concentrations of acid and high concentrations of chlorite, making the reaction difficult to control, especially in highly alkaline water supplies. It is also known that in an oxidizing environment, such as in the presence of chlorine or an anode, chlorine dioxide can be generated from chlorous acid. U.S. Patent No. 7,087,208 (hereinafter “US’208”), the description of which is incorporated by reference as if fully stated herein, teaches packing a reaction vessel with water-insoluble catalytic particles, continuously passing a stream of aqueous chlorous acid into the vessel and through the catalytic particles, thereby catalytically converting the chlorous acid in the stream to chlorine dioxide, and then continuously removing the stream of aqueous chlorine dioxide (generated) from the vessel. BRIEF DESCRIPTION OF THE INVENTION Accordingly, the present invention provides an improvement on the aforementioned method of catalytic generation of chlorine dioxide according to US '208. More precisely, it has been surprisingly discovered that pressurizing the reaction vessel, either continuously or intermittently, while the catalytic particles are flooded with a chlorous acid / chloric acid solution, significantly increases the rate of conversion to chlorine dioxide.Therefore, the present invention provides a process for generating aqueous chlorine dioxide comprising the steps of establishing or providing a pressurizable reaction vessel containing a packed bed of water-insoluble porous catalytic particles having a vessel inlet and a vessel outlet, continuously or intermittently introducing an aqueous solution containing at least one of chlorous acid and chlorotic acid through the vessel inlet until it comes into contact with the packed bed of water-insoluble porous catalytic particles under continuous or intermittent pressurization to produce an aqueous chlorine dioxide solution, and continuously or intermittently removing the aqueous chlorine dioxide solution thus produced from the vessel through the vessel outlet. Pressurizing the reaction vessel applies pressure to the packed bed flooded with chlorous acid / chloric acid solution, resulting in little or no fluidization of the packed bed. Applying pressure to the acid feed or the packed bed separately before contacting the bed with the solution would not have the desired effect. Pressure, whether continuous or intermittent, must be applied to the packed bed when it is flooded with the acid solution to improve (increase) the catalytic conversion rate of aqueous chlorous acid to aqueous chlorine dioxide, among other things, as taught in US 208. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an exploded elevation view of a plastic tube (container) used in Examples 1 and 4. Figure 2 shows an exploded elevation view of a plastic tube and ball valve used in Examples 2 and 5. Figure 3 shows an exploded elevation view of a plastic tube and return pressure regulator used in Examples 3 and 6. DETAILED DESCRIPTION OF THE INVENTION ινΐΛ / a / zuz ι / u io As used in this description, the following terms shall have the meanings indicated. The term “chlorous acid” refers to a solution in which the anionic chlorite countercation (ClO2+) has been essentially completely replaced by the hydrogen ion (H+) (“aqueous chlorous acid solution,” “aqueous chlorous acid,” and “chlorous acid solution” are used synonymously in this description). According to Gordon, chlorous acid exists predominantly (over chlorite) in solution at a pH below 4. The term “doric acid” refers to a solution in which the anionic chlorate countercation (ClO2+) has been essentially completely replaced by the hydrogen ion (H+) (“aqueous doric acid solution,” “aqueous doric acid,” and “doric acid solution” are used synonymously in this description).Similar to chlorous acid, an aqueous solution of dichlorous acid exists predominantly (over chlorate) at a pH below 4. The term “water-insoluble” means a substance incapable of dissolving. The term “packed bed of particles” means water-insoluble particles bound together in constant contact with one another, such as, for example, contained in a tube, pipe, or other container filled (completely) with the particles. The term “acid solution” refers to a solution of chlorous acid and / or dichlorous acid. According to the present invention, the feeding of the acid solution and the removal of the chlorine dioxide solution are carried out at the same rate and at the same regular or irregular intervals. Feeding and removal can be performed at intervals as short as approximately 1 second or as long as approximately 1 week; however, feeding and removal are preferably carried out continuously. The feeding and removal rates of the solutions fed into and removed from the reaction vessel containing the catalytic particles will depend at least in part on the size of the reaction vessel and the related equipment and can be readily determined by those skilled in the art.The pressure applied to the reaction vessel containing the catalytic particles and the chlorous and dichlorous acid solution to be converted should ideally be within the range of approximately 5 psi to approximately 250 psi, and preferably between approximately 25 psi and 60 psi. The water-insoluble, porous catalytic particles generally have particle sizes ranging from approximately 4 to approximately 50 US mesh, preferably from approximately 4 to approximately 40 US mesh. The catalytic particles must be water-insoluble to ensure that the bed remains packed. Water-insoluble porous catalytic particles are either made entirely of one or more water-insoluble catalysts, or made of water-insoluble porous inorganic particles with one or more water-insoluble catalysts deposited on them in a water-resistant manner. A person skilled in the art will readily understand how to obtain both particles made entirely of one or more water-insoluble catalysts and those made of water-insoluble porous inorganic particles with one or more water-insoluble catalysts deposited on them in a water-resistant manner. Illustrative catalysts include platinum group metals, platinum group metal oxides, transition group metals, and transition group metal oxides. Preferred catalysts include platinum, palladium, manganese dioxide, carbon, and ion-exchange material.Suitable commercially available catalysts include inorganic cation resin in the form of hydrogen with a platinum catalyst placed on the surface of the inorganic cation resin sold by ResinTech Inc. at 160 Cooper Rd, West Berlin, NJ under the name Resintech SIR-600 and inorganic clay with a platinum catalyst placed on the surface of the inorganic clay sold by Wateropolis Corp. at 12375 Kinsman Rd, Newbury Township, OH under the name Ceralite-A. Referring now to Figure 1, a basic container used to hold a packed-bed catalytic material is shown. The catalytic material can be packed in such a way that the particles are compacted and in contact with each other. This packed bed is contained within the basic container, such that the inlet and outlet prevent the packed material from escaping the container. In the configuration shown in Figure 2, the packed vessel is pressurized and maintained using a ball valve. This pressure on the packed catalytic material contained within the vessel can be produced using any means to ensure that constant pressure, either continuous or intermittent, is applied to the packed vessel. In the embodiment shown in Figure 3, a catalytic material is contained within a packed vessel, and a constant pressure is maintained on the vessel by means of a back pressure regulating valve. The packed vessel can be kept under pressure, either continuously or intermittently, during the generation of aqueous chlorine dioxide by any means that maintains such pressure on the packed vessel constant or nearly constant. The plastic tubes used to carry out the tests set out in Examples 1-6 below are shown in Figure 1, Figure 2, and Figure 3, where the numbers in the drawings refer to similar parts throughout the document. The plastic test tube 100 includes a generally cylindrical body 102 having a conventional connection closure mounted at each end thereof in the form of an inlet upper end connection 104 and an outlet lower end connection 106. PVC (polyvinyl chloride) screens 108 fitting over the inside diameter of the cylindrical tube 102 are glued at each end between the end of the cylindrical tube 102 and the end closures 104 and 106 to act as a support for the packed bed filling. Outlet tube 112 comes out from outlet end connection 106 (see Figures 2 and 3).A true union ball valve, model 200, manufactured by Spears at 15853 Olden St, Sylmar, CA, connects to the tubing downstream of packed vessel 102 to start and stop the removal of the chlorous acid stream. A backpressure valve, model M 300, manufactured by Griffco Valve, Inc. at 188 Creekside Drive, Amherst, NY, is set to 25 psi downstream of the pressure vessel to apply back pressure to the solution. Precursor Solution: In Examples 1–6, a chlorite precursor solution is used for each set of Examples. The chlorite precursor solution is prepared by diluting a 25% aqueous solution of active sodium chlorite with reverse osmosis water to a concentration of 1250 ppm. Before beginning each of the following Examples, the chlorite precursor solution is converted to chlorous acid by appropriate acidification. In Examples 7–8, a chlorate precursor solution is used for each set of Examples. The chlorate precursor solution is prepared by dissolving powdered sodium chlorate in reverse osmosis water to achieve a concentration of 1250 ppm. A powdered sodium bisulfite solution is then mixed into the chlorate precursor solution. The powdered sodium bisulfite solution weighed 1.33 times more than the powdered sodium chlorate. Example 1 Gravity flow with SIR catalyst Chlorine dioxide is generated by gravity-fed chlorous acid through a 30 mL plastic tube (102) as shown in Figure 1. The tube is packed with the commercially available catalyst Resintech SIR-600 described above, so that the tube is full. A 10 mL sample of the chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then poured into the plastic tube at atmospheric pressure and collected at atmospheric pressure as the solution flows out. The sample flow takes only a few seconds. The 10 mL sample is taken from outlet connection 106, and a Hach spectrophotometer is used to measure the chlorine dioxide concentration immediately after collection. The above test was repeated with four additional 10 mL samples.Table 1 records the conversion of the chlorine dioxide concentration as measured in each of the 5 trials. ινΐΛ / a / zuz ι / u io Example 2a Static contact time test with SIR catalyst A 30 ml plastic tube with a 200 ball valve on the downstream side of tube 112, as shown in Figure 2, is attached to a wall with tube clips. The tube is packed with the same Resintech SIR-600 catalyst as in Example 1, so that the tube is full. Chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then fed into the plastic tube at atmospheric pressure, and once the packed bed is flooded, the 200 ball valve is closed. The solution remains within the catalyst of the packed bed open at atmospheric pressure for five minutes. The chlorous acid solution is then removed at atmospheric pressure and collected, and a Hach spectrophotometer is used to measure the chlorine dioxide immediately after collection. The conversion of the chlorine dioxide concentration is recorded in Table 1. Example 2b Static pressure test with SIR catalyst A 30 ml plastic tube with a 200 ball valve on the downstream side of tube 112, as shown in Figure 2, is attached to a wall with tube clips. The tube is packed with the same Resintech SIR-600 catalyst as in Examples 1 and 2a, so that the tube is full. Chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then fed into the plastic tube. Once the packed bed is flooded with the chlorous acid solution, the ball valve is closed, and the inlet pressure is increased to 60 psi. The solution remains under static pressure at 60 psi in the plastic tube for five minutes. The chlorous acid solution is then removed and collected, and a Hach spectrophotometer is used to measure the chlorine dioxide immediately after collection. The conversion of the chlorine dioxide concentration is recorded in Table 1. Example 3 Dynamic pressure test with SIR catalyst A 30 ml plastic tube with a 300 backpressure regulator on the downstream side of tube 112, as shown in Figure 3, is attached to a wall with tube clips. The tube is packed with the same Resintech SIR-600 catalyst as in Examples 1, 2a, and 2b, so that the tube is full. The 300 regulator continuously applies backpressure at a constant pressure of 25 psi. Chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then continuously fed into the packed tube and removed at the same rate as it is fed. Five samples are taken every 5 minutes. These samples are measured for chlorine dioxide using a Hach spectrophotometer immediately after collection. The conversion of the chlorine dioxide concentration is recorded in Table 1. Example 4 Gravity flow with clay catalyst The five trials described in Example 1 are repeated here, except that the 30 mL plastic tube is packed with the commercially available Ceralite-A inorganic clay catalyst described above, so that the tube is full. A 10 mL sample of the chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then poured into the plastic tube at atmospheric pressure and collected at atmospheric pressure as it flows out. A Hach spectrophotometer is used to measure the chlorine dioxide immediately after collection. Table 2 records the conversion of the chlorine dioxide concentration as measured in each of the five trials. Example 5a Static contact time test with clay catalyst The test set out in Example 2a is repeated here, except that the 30 ml plastic tube is packed with the same Ceralite-A catalyst used in Example 4, so that the tube is full. The chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then fed into the plastic tube at atmospheric pressure, and once the packed bed is flooded, the 200 ball valve is closed. The solution remains within the catalyst of the open packed bed at atmospheric pressure for five minutes. The chlorous acid solution is then removed at atmospheric pressure and collected; a Hach spectrophotometer is used to measure the chlorine dioxide immediately after collection. The conversion of the chlorine dioxide concentration as measured is recorded in Table 2. Example 5b Static pressure test with clay catalyst The test set out in Example 2b is repeated here, except that the 30 ml plastic tube is packed with the same Ceralite-A catalyst used in Example 4, so that the tube is full. The chlorous acid solution at pH 1.8, as converted from the sodium chloride precursor solution, is then fed into the plastic tube, and once the packed bed is flooded, the ball valve 200 is closed, and the inlet pressure is increased to 60 psi. The solution remains under static pressure at 60 psi in the plastic tube for five minutes. The chlorous acid solution is then removed and collected, and a Hach spectrophotometer is used to measure the chlorine dioxide immediately after collection. The conversion of the chlorine dioxide concentration as measured is recorded in Table 2. Example 6 Dynamic pressure test with clay catalyst The test set up in Example 3 is repeated here, except that the 30 ml plastic tubing with a 300 back pressure regulator on the downstream side, as shown in Figure 3, is packed with the same Ceralite-A catalyst used in Examples 4, 5a, and 5b so that the tubing is full. Back pressure is continuously applied at a constant pressure of 25 psi. Chlorous acid at pH 1.8, as converted from the sodium chloride precursor solution, is then continuously fed into the packed tubing and removed at the same rate as it is fed. Five samples are taken every 5 minutes. These samples are measured for chlorine dioxide using a Hach spectrophotometer immediately after collection. Table 2 records the conversion of the chlorine dioxide concentration as measured in each of the five samples. Example 7 Gravity flow test with SIR catalyst A 30 mL plastic tube with a ball valve on the downstream end, as shown in Figure 2, is attached to a wall with tube clips and packed with the same Resintech SIR-600 catalyst used in Examples 1-3, so that the tube is full. A 10 mL sample of the chlorous acid solution at pH 1.8, as converted from the chlorite precursor solution, is then poured into the plastic tube and collected as it flows out. The 10 mL sample of solution is taken, and a Hach spectrophotometer is used to measure the chlorine dioxide concentration immediately after collection. The above test is repeated eleven more times at five-minute intervals. Table 3 shows the average concentration of the twelve (12) samples over a period of 55 minutes. Example 8 Dynamic pressure test with SIR catalyst A 30 ml plastic tube with a back pressure regulator on the downstream side, as shown in Figure 3, is attached to a wall with tube clips and packed with the same Resintech SIR-600 catalyst used in Examples 1-3 and 7, so that the tube is full. Back pressure is continuously applied at a constant pressure of 25 psi. The chlorous acid solution, as converted from the chlorite precursor solution, is then continuously fed into the packed tube, and the existing solution is removed at the same rate. Samples are taken every 5 minutes for one hour for a total of 12 samples. These 12 samples are measured for chlorine dioxide using a Hach spectrophotometer. The average is shown in Table 3. Example 3 | 5ώ conversion or co 0 co 0 co 0 00 CIO Concentration; 748 | 764 | I 6“ I o Π i- O 0 LO Example 2b | % conversion 79 8 | CIO Concentration; 732 | Assay - | ExampleZa | % conversion I θ5.4 | CIO Concentration: 599 0 & ε LU - | Example 1 | % conversion 35.9 60.3 III zr9 III CIO Concentration: 329 553 II 593 I «s I Assay - rw 10 Example £ | ¾ conversion S so Concentration of CIO; 823 1 «8 822 608 o II OO £ Example 5b | ¾ conversion o so Concentration of CIO; 1 «Z | Assay - | Example 5a | ϋ conversion 2 Concentration of CIO; ιςς I Assay o Ó E iñ' ¾ conversion 1 1 1 1 £ 46.8 1 1 Concentration of C IO?a 431o □99f °6zr o ó l / l UJ O a 04 oo Λ Average concentration of CIO during 1 hour. Example 7: 5 mg / L. Example 8: 32 mg / L. % increase: 540%. Table 3 IVIA / a / ZU¿ I / U 14 / IO

Claims

1. A process for generating aqueous chlorine dioxide comprising the steps of: - providing a pressurizable vessel containing a packed bed of water-insoluble porous catalytic particles capable of converting chlorous acid and chloric acid into chlorine dioxide, said vessel having an inlet and an outlet; - continuously or intermittently feeding an aqueous solution containing at least one of chlorous acid and chloric acid at a pH less than 4 through the inlet of the vessel to come into contact with the packed bed of water-insoluble porous catalytic particles under continuous or intermittent pressurization to produce an aqueous chlorine dioxide solution; and - continuously or intermittently removing said produced aqueous chlorine dioxide solution from the packed bed and the vessel through the outlet of the vessel.

2. The process of claim 1 wherein each of the feeding, disposal, and pressurization is continuous.

3. The process of claim 1 wherein each of the feeding and disposal is continuous and the pressurization is intermittent.

4. The process of claim 1 wherein each of the feeding and disposal is intermittent and the pressurization is continuous.

5. The process of claim 1 wherein each of the feeding, disposal, and pressurization is intermittent.

6. The process of claim 1, wherein the catalytic particles are selected from the group consisting of platinum group metals, platinum group metal oxides, transition group metals, and transition group metal oxides.

7. The process of claim 6, wherein the catalytic particles are selected from the group consisting of platinum, palladium, manganese dioxide, carbon, and ion exchange material.

8. The process of claim 1, wherein the continuous or intermittent pressurization of the catalytic particles is between approximately 5 psi and approximately 250 psi.

9. The process of claim 8, wherein the continuous or intermittent pressurization of the catalytic particles is between approximately 25 psi and approximately 60 psi.