METHOD OF DECOMPOSING GASEOUS HALOCARBONS

Abstract: A method of completely decomposing gasesous halocarbons at temperatures of between about 500 C and about 700 C, preferably about 600 C. The gaseous halocarbons may initially pass through a vaporizer tube (14), before entering the reaction tube (13). The reaction and vaporizer tubes are heated by ceramic fiber heaters (15 and 16). Temperatures were monitored and controlled by a thermocouple inserted into the heaters. Gas flows are monitored with two flowmeters. This sytem for decomposing gaseous halocarbons has the advantage of mixing the halocarbon gas with oxygen and maintaining the temperature at 600 C for a length of time sufficient to eliminate all halocarbons from the gas.

Description: METHOD OF DECOMPOSING GASEOUS HALOCARBONS BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates generally to methods of decomposing gaseous halocarbons, and more particularly to a method of completely decomposing halocarbons in a gas stream at temperatures of about 600°C. Description of the Prior Art: A variety of halocarbon gases are produced during common commercial processes. In the prior art, it is known to combust these waste gases at temperatures on the order of 1200°C and higher in order to thermally decompose the gases to more environmentally benign products. Unfortunately, the high temperature required for these prior art processes reduces the commercial applicability of prior art methods and precludes their employment in many commercial applications. Recently, a variety of factors have increased the desirability of removing all halocarbons from an exhaust gas stream. In some circumstances the cost of producing halocarbon emissions has increased, while in other circumstances such emissions are prohibited altogether. A need has therefore remained for a method of decomposing gaseous halocarbons in an exhaust gas stream so that no halocarbons remain in the stream. The present invention addresses this need. SUMMARY OF THE INVENTION Briefly describing the present invention, there is provided a method of completely decomposing gaseous halocarbons at temperatures of about 600°C. In one aspect of the present invention, a halocarbon gas is mixed with oxygen and heated to 600°C to eliminate all halocarbon from the gas. In a preferred embodiment, CF. and oxygen are mixed at a 1:2 molar ratio and are heated to 600°C to eliminate all fluorocarbon from the gas. In another embodiment, CC1. is completely removed from a gas by mixing the gas with oxygen at a 1:7 molar ratio and heating the mixture to 600°C. One object of the present invention is to provide a method of decomposing gaseous halocarbons at temperatures of about 600°C to completely eliminate all halocarbon from the gas- Further objects and advantages of the present invention will be apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one apparatus used to test the method of the present invention according to one preferred embodiment. FIG. 2 is a schematic diagram of a second apparatus used to test the method of the present invention according to one preferred embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the method, and such further applications of the principles of the invention being contemplated as would normally occur to one skilled in the art to which the invention relates. The present invention relates to a method of completely decomposing gaseous halocarbons at temperatures of about 600°C. In one aspect of the invention, a gaseous halocarbon is contacted with an amount of oxygen effective to eliminate all halocarbons when the mixture is heated to a temperature of between about 500°C and about 700°C. The gaseous halocarbons of the present invention are lower halocarbons having between one and about six carbon atoms in the chain. The halocarbon is substituted with one or more halogens, i.e., with chlorine, bromine, fluorine or iodine, either alone or in combination. Further, any one or more of the carbon atoms may be halogenated. Examples of typical halocarbons which may be decomposed by the method of the present invention include CF. , CHF_, CC1 4 , CHC1„, CF_Br and the like. The halocarbons of the present invention may or may not include hydrogen atoms. In this description of the invention the term "halocarbon" is intended to include compounds having one or more halogens attached to one or more carbon atoms, as described above, regardless of whether hydrogen is present in the compound. These compounds therefore comprise halogens, carbon and possibly also hydrogen. The oxygen used to contact the gaseous halocarbon may be either pure 0„ or oxygen present in a gas such as air. In all embodiments the amount of oxygen provided is at least the - D - stoichiometric amount. In preferred embodiments, the molar ratio of oxygen to halocarbon present for the decomposition is between about 1.5:1 and about 10:1. The precise molar ratio depends on the halogenated compounds being decomposed, the reaction temperature, the contact time, the flow rate and other reaction parameters as can be appreciated by one skilled in the art. Precise ratios of oxygen to halocarbon gas may be determined for any commercial application without undue experimentation. In the process of the present invention, the oxygen and halocarbon are heated for a time of between about five seconds and about 60 seconds or more. It is to be appreciated that the precise contact time will depend on the halocarbons being decomposed, the temperature used in the reaction, etc. Parameters such as contact time can be optimized for a particular commercial process by those skilled in the art without undue experimentation. The halocarbon gas used in the present invention may include only a single halocarbon or a mixture of halocarbons. Contact times, reaction temperatures and ratios of halocarbon to oxygen may be adjusted for a particular application by one skilled in the art without undue experimentation. Reference will now be made to specific examples using the process described above. It is to be understood that these examples are provided to more completely describe and explain the preferred embodiments, and that no limitation to the scope or breadth of the invention is thereby intended. Experimental Procedure One experimental apparatus is shown in FIG. 1. Halocarbon waste gas 11 and 0„ 12 are provided to reaction tube 13, a 48" x 1/2" I.D. Inconel 600 tube. Waste gas 11 may initially pass through vaporizer tube 14 before entering the reaction tube 13. In experiments to date, the vaporizer tube 14 was a 12" x 1/2" I.D. Inconel 600 tube. The reaction and vaporizer tubes were heated by ceramic fiber heaters 15 and 16, respectively. Temperatures were monitored and controlled by a thermocouple inserted into the heaters. Gas flows were monitored with two flowmeters, both calibrated by a soap-film calibrator. After reaction, the gases are washed in a solution 17 of dilute base, such as dilute KOH, and then a solution 18 of Na„S_0„. The product gases are recovered, dried in dryer 19 and analyzed by gas chromatography. Another experimental apparatus is shown in FIG. 2. Chlorocarbons 21 (such as CC1. and CHC1„) and 0„ 22 are provided to reaction tube 23. In experiments to date, the reaction tube 23 was a 48" x 1/2" I.D. Inconel 600 tube. Both gases may initially pass through vaporizer 24 before entering the reaction tube. In experiments to date, the vaporizer tube 24 was a 12" x 1/2" I.D Inconel 600 tube. The reaction and vaporizer tubes were heated by ceramic fiber heaters 25 and 26, respectively. Temperatures were monitored and controlled by a thermocouple inserted into the heaters. Gas flows were monitored with two flowmeters, both calibrated by a soap-film calibrator. After reaction, the gases are washed in a solution 27 of dilute base, such as dilute KOH, and then a solution 28 of Na„S_0_. The product gases are recovered, dried in dryer 29 and analyzed by gas chromatography. In all experiments, two gas chromatography analyzers are used. One G.C. is used to separate the input gas from 0„, CO_ and decomposed by-product. The other G.C. is used to show the content of O.., CO- and decomposed by-product. The G.C. were operated under the following conditions. G.C. #1: Column: 3% SP-1500 on carbopack B, O.D. 5 mm x L. 3.1m, glass Detector temperature: 150°C Detector type: TCD @ lOOmΛ Injector temperature: 100°C Program: 30°C/4 min / 15°C/min / 110°C/3 min G.C. #2: Column: Carbosieve G, O.D. 5mm x L. 3.1m, glass. Detector temperature: 180°C Detector type: TCD @ 100mA Injector temperature: 180°C He: 40 mL/min Program: 30°C/3 min / 10°C/min / 120°C/8 min The reaction contact time (θ) is calculated as follows θθ == RReeaaccttoorr vvoo]lume (155 mL) / Total flow of 0„ and halocarbon gas EXAMPLE 1 Decomposition of CF_Br. This example was performed with the apparatus shown in FIG. 1. Experiments were carried out by combining 0„ and CF-.Br (Halon 1301) at the desired flow rates and temperature for one hour. The outlet gas was then sampled and analyzed by gas chromatography. Results are summarized in Table 1. It can be seen that CF„Br may be completely decomposed at 600°C so that no halocarbon remains. The reaction is effectively performed with a contact time of 30 seconds when the molar ratio of O " :CF_Br is 2:1. The effect of molar ratio can be seen by comparing entries six through nine. In this Example, a molar ratio of 1.5:1 was not effective to completely decompose CF_Br, while a molar ratio of 2:1 provided complete decomposition. TABLE 1 DECOMPOSITION OF CF3Br Halon 1301 (CF 3 Br) The effect of contact time can be seen by comparing entries one through four. In this Example, a reaction contact time of 20 seconds was not effective to completely decompose CF_Br, while a reaction contact time of 30 seconds provided complete decomposition. The effect of temperature can be seen by comparing entry 3 with entry 6, entry 7 with entry 11 and entry 9 with entry 10. As would be expected, the higher the temperature the better the decomposition. EXAMPLE 2 Decomposition of CF_C1. This experiment was performed with the apparatus shown in FIG. 1. CF_C1 and 0„ were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 2. It can be seen that CF„C1 is completely decomposed by mixing with 0 2 at a 2:1 molar ratio (0_:CF„C1) and heating at 600°C for 30 seconds. At a molar ratio of 1.5:1 the decomposition was not complete. When the reaction (contact) time was only 20 seconds the decomposition was not complete. EXAMPLE 3 Decomposition of CHF_ . This experiment was performed with the apparatus shown in FIG. 1. CHF_ and 0„ were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 3. It can be seen that CHF-, is completely decomposed by mixing with 0_ at a 2:1 molar ratio (0_:CHF„) and heating at 600°C for 30 seconds. At a molar ratio of 1.5:1 the decomposition was not complete and CHF„ remained in the TABLE 2 DECOMPOSITION OF CF3CI F-13 (CF3CI) F-14 (CF 4 ) M h-* F-23 (CF3H) exhaust gas. At a molar ratio of 1:2 the decomposition was not complete and both CHF„ and CF. remained in the exhaust gas. EXAMPLE 4 5 Decomposition of CF.. This experiment was performed with the apparatus shown in FIG. 1. CF 4 and 0„ were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 4. 0 It can be seen that CF. is completely decomposed by mixing with 0„ at a 2:1 molar ratio (0_:CF 4 ) and heating at 600°C for 30 seconds. Further, at a molar ratio of 1.5:1 the decomposition still goes to completion and no halocarbon remained in the exhaust gas. At a molar ratio of 5 1:1 the decomposition was not complete and CF. remained in the exhaust gas. EXAMPLE 5 Decomposition of CF_BrCl. This experiment was performed with the apparatus shown in FIG. 1. CF_BrCl and o 0-*, were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 5 It can be seen that CF„BrCl is completely decomposed by mixing with 0 at a 2:1 molar ratio (0_:CF BrCl) and 5 heating at 600°C for 30 seconds. Further, at a molar ratio of 1.5:1 the decomposition still goes to completion and no halocarbon remained in the exhaust gas. At a molar ratio of 1.2:1 the decomposition was not complete and CF_BrCl remained in the exhaust gas. TABLE 4 DECOMPOSITION OF CF 4 Flow(mL/min) Ratio Outlet Gas Conten (%) Temp.(°C) Time(sec) F-1 * θ2 02:F-14 O2+CO2 F-14 596 - 608 30 103 207 2 100 0 30 124 186 1.5 100 0 30 155 155 1 89.51 10.49 F-14 (CF 4 ) * Halon 1211 (CF 2 BrCl) F-13:4.217o, 1301:0.0247o, F-14:1.48% EXAMPLE 6 Decomposition of CHF„Br. This experiment was performed with the apparatus shown in FIG. 1. CHF„Br and 0- were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 6 Again, the CHF_Br is completely decomposed by mixing with 0„ at a 2:1 molar ratio (0_:CHF_Br) and heating at 600°C for 30 seconds. Also, as was the case with CF. and CF„BrCl, the decomposition still goes to completion at a molar ratio of 1.5:1. At a molar ratio of 1:1 the decomposition was not complete and CHF_Br remained in the exhaust gas. EXAMPLE 7 Decomposition of Trifluoropropane. This experiment was performed with the apparatus shown in FIG. 1, except that since trifluoropropane (TFP) is a flammable gas, the tests were performed with air instead of 100% 0„. TFP and 0_ in air were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 7. The TFP is completely decomposed by mixing with 0_ in air at a 4:1 molar ratio (air:trifluoropropane) and heating at 600°C for 10 seconds. The higher molar ratio is required due to the dilution effect of air as an 0„ carrier. EXAMPLE 8 Decomposition of CC1 4 . This experiment was performed with the apparatus shown in FIG. 2. CC1 4 and 0„ were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 8. TABLE 6 DECOMPOSITION OF CHFBr 2 Flow(mL/min) Ratio Outlet Gas Content(7.) Temp.(°C) Time(sec) FM-100 0 2 02:FM-100 02+C0 2 FM-100 by-product 596 - 608 30 2:1 100 0 0 30 1.5:1 100 0 0 30 1:1 99.31 0.69 σ*. F-13:0.0577o, 1301:0.0377o, F-14:0.5977o TABLE 7 DECOMPOSITION OF TFP Flow(mL/min) Ratio Outlet Gas Content(7.) Temp.(°C) Time(sec.) TFP Air Air:TFP O2+CO2 TFP by-product 596 - 608 iθ 85 845 10:1 100 0 0 10 155 775 5:1 100 0 0 10 186 744 4:1 100 0 0 10 233 698 3:1 Explosion danger -j 1 TABLE 8 CC1 4 was not completely decomposed by mixing with 0„ at a 2:1 molar ratio (0 2 :CC1.) and heating at 600°C for 30 seconds. In fact, a molar ratio of about 7:1 was required to obtain complete decomposition. Further, contact times of 5 about 60 seconds were used. EXAMPLE 9 Decomposition of CHC1„. This experiment was performed with the apparatus shown in FIG. 2. CHC1- and 0_ were combined at the desired flow rates and a 0 temperature of about 600°C for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 9. CHCl- j was not completely decomposed by mixing with 0_ at a 2:1 molar ratio (0 2 :CHC1 3 ) and heating at 600°C for 5 30 seconds. Here, a molar ratio of about 4.5:1, with a contact time of about 60 seconds, was required to obtain complete decomposition. EXAMPLE 10 Decomposition of CC1. with air. This experiment was o performed with the apparatus shown in FIG. 2. CC1 4 and 0_ in air were combined at the desired flow rates and temperatures for one hour. The outlet gas was then sampled and analyzed by gas chromatography. The results are summarized in Table 10. 5 CC1 4 was not completely decomposed by mixing with 0_ in air at a 12:1 molar ratio (air:CCl 4 ) and heating at 600°C for 30 seconds. However, at a molar ratio of about 12.5:1 and a contact time of 45 seconds complete decomposition was obtained. TABLE 10 DECOMPOSITION OF CCI4 WITH AIR Feed Rate Ratio Outlet Gas Content CC14 Entry Time Air CC14 Air:CC14 Air+Cθ2 CC14 Others in water (sec.) (mL/min) (g/min) (7») (ppm) 1 45 192 0.105 12.5:1 100 0 0 0 2 30 286 0.163 12:1 99.09 0.890 0.019 - , EXAMPLE 11 Repetition of the foregoing examples with other lower halocarbons, e.g. halocarbons having one to six carbon atoms, and having combinations of halogens, e.g. chlorine, bromine, fluorine, iodine, and combinations thereof, yields similar results. Advantageously, the various halocarbons are eliminated from a gas when combined with a sufficient amount of oxygen and upon being maintained at a suitably high temperature between 500°C and 700° for a determinable period of time adequate to provide for full thermal decomposition of the halocarbons. The prior art has failed to recognize that such total decomposition of gaseous halocarbons can be accomplished at such relatively low temperatures by the use of adequate amounts of oxygen and suitable periods of reaction time. It is to be appreciated that the method of the present invention allows the recovery of 0„ used in the decomposition process. Depending on the halocarbon being decomposed, rates of 0„ recovery may be as high as 100%. Rates of recovery for typical decompositions according to the method of the present invention are shown in Table 11. While the invention has been illustrated and described in detail in the foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. TABLE 11 Required 02/CFC moles and resulting 0 2 recovery. CF4 + 2 02 —> C02 + 2 OF2 2 H20 + 2 OF2 —> 4 HF + 2 02 1DO% 0 mol C2F6 +7/2 02 —>2 C02 + 3 OF2 3 H20 + 3 OF2 —> 6 HF + 3 02 86% 0.5 mol C2F4 + 3 02 —>2 C02 + 2 0F2 H20 + 2 OF2 —> 4 HF + 2 02 67% 1 mol CF3Br + 2 02 —> C02 + 3/2 0F2 + 1/2 0Br2 2 H20 + 3/2 0F2 -I- 1/2 0Br2 > HBr + 3 HF + 2 02 100% 0 mol CHF3 +3/2 02 —> C02 + OF2 + HF 2 H20 + 0F2 + HF > 3 HF + 1/2 02 CH2F2 + 02 —> C02 + 2 HF CH3F +3/2 02 —> C02 + HF + H20 C3H3F3 + 3 02 —>3 C02 + 3 HF

Claims: CLAIMS What is claimed is: 1. A method of eliminating gaseous halocarbons from a gas containing such compounds, comprising (a) providing a mixture of air and of the gas containing one or more gaseous halocarbons, the mixture including oxygen in an amount effective to thermally decompose all of the halocarbons in the mixture when the mixture is maintained at a temperature of between about 500°C and about 700°C; (b) heating the gas and oxygen to a temperature of between about 500°C and about 700°C; and (c) maintaining the mixture of gas and oxygen at about 500°C to about 700°C for a time sufficient to provide elimination of the halocarbons. 2. The method of claim 1 wherein the mixture of oxygen and gas containing halocarbon includes an amount of oxygen effective to eliminate all halocarbons from the gas when the gas and oxygen mixture are maintained at a temperature of about 600°C, and wherein each of said heating and said maintaining is at a temperature of about 600°C. 3. The method of claim 1 wherein the molar ratio of oxygen to halocarbon is between about 1.5:1 and about 10:1. 4. The method of claim 1 wherein said maintaining is for a time of between about 5 seconds and about 90 seconds. 5. The method of claim 1 wherein said gas consists essentially of fluorocarbons. 6. The method of claim 5 wherein the molar ratio of oxygen to halocarbon is at least about 2:1. 7. The method of claim 6 wherein said maintaining is for a time of at least about 30 seconds. 8. The method of claim 1 wherein said gas consists essentially of chlorocarbons. 9. The method of claim 8 wherein the molar ratio of oxygen to halocarbon is at least about 7:1. 10. The method of claim 9 wherein said maintaining is for a time of at least about 60 seconds. AMENDED CLAIMS [received by the International Bureau on 27 January 1994 (27.01.94); original claim 1 amended; other claims unchanged (1 page)] 1. A method of eliminating gaseous halocarbons from a gas containing such compounds, consisting essentially of (a) providing a mixture of air and of the gas containing one or more gaseous halocarbons, the mixture including oxygen in an amount effective to thermally decompose all of the halocarbons in the mixture when the mixture is maintained at a temperature of between 500°C and 700°C; (b) heating the gas and oxygen to a temperature of between 500°C and 700°C; and (c) maintaining the mixture of gas and oxygen at 500°C to 700°C for a time sufficient to provide elimination of the halocarbons. 2. The method of claim 1 wherein the mixture of oxygen and gas containing halocarbon includes an amount of oxygen effective to eliminate all halocarbons from the gas when the gas and oxygen mixture are maintained at a temperature of about 600°C, and wherein each of said heating and said maintaining is at a temperature of about 600°C. 3. The method of claim 1 wherein the molar ratio of oxygen to halocarbon is between about 1.5:1 and about 10:1. 4. The method of claim 1 wherein said maintaining is for a time of between about 5 seconds and about 90 seconds. 5. The method of claim 1 wherein said gas consists essentially of fluorocarbons. 6. The method of claim 5 wherein the molar ratio of oxygen to halocarbon is at least about 2:1.

Inventors: IIKUBO YUICHI, QIAN JOHN C, IIKUBO, YUICHI, QIAN, JOHN, C.
Applicants: GREAT LAKES CHEMICAL CORP, GREAT LAKES CHEMICAL CORPORATION
Application year: 1993
Main classification: B01D53
IPC codes: A62D3, B01D53