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Hydrochlorofluorocarbon concentration in exit gas from a heated reactor
Microchemical Journal 71 Ž2002. 15᎐20
Hydrochlorofluorocarbon concentration in exit gas from a heated reactor E. Steib a , K. Gauthreaux a , J.N. Beck b,U a
b
St. James High School, St. James, LA 70086, USA Department of Physical Sciences, Nicholls State Uni¨ ersity, Thibodaux, LA 70310, USA
Received 30 June 2001; received in revised form 14 July 2001; accepted 1 August 2001
Abstract Hydrochlorofluorocarbons ŽHCFCs. have been shown to be harmful to the ozone layer because they provide free chlorine atoms that destroy ozone ŽO 3 .. The purpose of this research was to devise a method to decompose hydrochlorofluorocarbon molecules efficiently and inexpensively without producing harmful by-products. Monochlorodifluoromethane ŽCHClF2 . was selected for this study because it is a synthetic and frequently used refrigerant. A conversionrreactor tank was designed that consisted of four or five layers of sodium oxalate enclosed in the reaction tank, resting on glass wool and aluminum screens. Monochlorodifluoromethane enters above the layers of sodium oxalate and exits after passing through the last layer through a hole placed perpendicular to the entrance hole. Using the Argentometric method for chloride analysis, results showed that this device decomposed the HCFC molecules to chlorides, fluorides, and carbon dioxide. The method was tested under varying temperature, flow rates, and structural design conditions to obtain optimum efficiency. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrochlorofluorocarbon reaction; Oxalates; Chlorofluorocarbons
1. Introduction M onochlorodifluorom ethane Ž C H C lF 2 . Žhereafter simply named HCFC. was used to develop a reaction vessel that would decompose hydrochlorofluorocarbon molecules making them harmless to the ozone layer w1x. Monochlorodifluoromethane is a synthetic refrigerant used in air conditioning units and in household refrigeraU
Corresponding author. Tel.: q1-504-448-4500; Fax: q1504-448-4927. E-mail address: [email protected] ŽJ.N. Beck..
tors. It has a boiling point of y40.6⬚C, a normal head pressure at 30⬚C of 1184 kPa, and is stable, non-toxic, non-corrosive, non-irritating, and nonflammable w2x. The monochloro form was chosen to limit the number of chlorine molecules to control variability in the stoichiometry of the decomposition. Chlorine atoms found in HCFC are readily released from this molecule as very reactive free radicals. Once these molecules are released into the atmosphere and migrate to the stratosphere, the released chlorine atoms are able to react with ozone molecules. When a chlorine atom encoun-
0026-265Xr02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 1 1 1 - 4
16
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
ters an ozone molecule, the following reactions occur w2x. CHClF2 q h ª CHF2⭈q Cl
Ž1.
Cl⭈q O 3 ª ClO ⭈q O 2
Ž2.
ClO ⭈q O ⭈ª Cl⭈q O 2
Ž3.
Cl⭈q O 3 q ClO ⭈q O ⭈ª ClO ⭈q Cl⭈q 2O 2
Ž4.
Chlorine free radicals pull an oxygen atom from the O 3 molecule Ž2., forming chlorine monoxide, ClO, and an O 2 molecule. The newly formed ClO free radical readily reacts with an oxygen atom Ž3., yielding an O 2 molecule and a Cl free radical w3x. A single Cl free radical is capable of destroying more than 100 000 O 3 molecules before it is carried back to the lower atmosphere by winds. This work reports on the analytical results obtained during the testing and development of a reaction vessel constructed to decompose HCFC molecules. Pinholster w4x reported on a technique developed by Burdeniuc and Crabtree w5x to destroy CFCs using sodium oxalate at high temperatures. Dagani w6x also has reported on the CFCdestroying reaction using sodium oxalate. However, this work describes the results of trial runs involving HCFCs using a heated reactor system. Tests were conducted by varying flow rates, temperature, and structural design conditions to achieve optimum conditions for the destruction process.
2. Experimental 2.1. Reaction ¨ essel construction The reaction vessel was constructed using a 20-cm-diameter = 38 cm length = 0.63-cm-thick iron pipe, sealed on both ends by 0.63-cm-thick iron sheeting bolted to the iron pipe with a Teflon gasket to form an air-tight seal. An iron mesh screen of sufficient rigidity, cut to the desired size
to fit the curvature of the reaction vessel at four heights, was used to support the desired amount of sodium oxalate reagent and the bed-liner. Aluminum screen and Pyrex glass wool were used to form a porous bed-liner to allow passage of HCFC and provide support for the sodium oxalate. Four equal sections of aluminum screen covered by glass wool were cut to fit the iron mesh screens. The remaining two equal size layers of screen were placed on top of the glass wool and the screens were compressed until close enough to staple all four edges together to form a bed-liner. 2.2. Sample collection A sufficient amount of sodium oxalate was placed on top of each bed-liner to completely cover the screen to a depth of 1 cm. A reagent bottle containing sodium oxalate was weighed before and after placing the needed amount on the bed-liner. Each mass was recorded and the prepared bed-liners placed on each appropriate screen in the vessel. The lid of the vessel was then bolted shut. A propane gas tank was attached to the heating system connected to the vessel, and the burner lit to preheat the reaction vessel. A hose attached to the rear side allowed fumes to freely exit. The weighed canister of HCFC was then attached to its inlet on the vessel and the HCFC allowed to flow through the system. After the reaction was complete, the canister of HCFC was reweighed to determine the total amount used. 2.3. Reactor operation The vessel was heated, using specially constructed propane burners, for approximately 6 min, to 275 " 5⬚C measured on each reactor plate. Monochlorodifluoromethane was allowed to flow through the vessel at a constant flow rate for 2 min, while still being heated. While HCFC was flowing through the reactor, exit gas was trapped in 10-ml gas sampling vials after 10 s, 1 min, and 2 min. The collection bottles were immediately sealed with a crimp type seal and septa and labeled appropriately. Samples of HCFC passing
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
through the unheated and heated reactor were collected as above to allow collection of calibration standards of undecomposed gas. After 2 min, the burner and the canister of HCFC were turned off and the vessel allowed to cool for 15᎐20 min. The lid of the vessel was unbolted and the inside of the vessel was allowed to cool for 5᎐10 min. Each bedliner was removed and placed in a labeled container. 2.4. Procedure 2.4.1. Chloride determination Chloride resulting from the destruction of the HCFC was determined using the Argentometric method w7x. All reagents used in this study were ACS certified reagent grade. The chromate indicator solution was prepared by dissolving 50 g of potassium chromate in distilled water. Silver nitrate was added until a definite red precipitate formed. The solution was allowed to stand for 12 h, filtered, and diluted to 1 l using deionized water. Silver nitrate titrant was prepared by dissolving 23.95 g of silver nitrate in deionized water and diluting to 1 l. The sodium chloride standard solution was prepared by dissolving 0.824 g of dry sodium chloride in deionized water and diluting to 1 l. Potassium aluminum hydroxide was prepared by adding 25 g of aluminum potassium sulfate to 1 l of deionized water. The solution was warmed to 60⬚C while continuously stirring and adding 55 ml of ammonium hydroxide. This solution was allowed to stand for 1 h w7x. After removing the bed-liners from the reaction vessel, each was washed with deionized water and the collected supernatant from each bed-liner was diluted to 1 l and labeled. One hundred milliliter samples were taken from each supernatant solution, 3 ml of aluminum hydroxide added and the pH was between 7 and 10. One milliliter of potassium chromate Ž50 grl. and five drops of phenolphthalein were added to each sample. Each sample was titrated against the silver nitrate until the sample turned a pinkish-yellow color. The volume of silver nitrate required to neutralize the solution was recorded. The silver nitrate was also titrated against chloride-free wa-
17
ter and the resulting color was observed and recorded. 2.4.2. Fluoride determination An aliquot of the prepared supernatant solutions used in the chloride analysis was used for fluoride determination. A 50-ml sample from each supernatant solution was taken. The samples were placed in appropriate cups and fluoride concentrations determined using a fluoride meter ŽATI Orion 960 Autochemistry System.. A buffer ŽTISA Buffer 3. was added to the samples so that the proper pH was obtained. The sample was stirred while being titrated against sodium fluoride Ž0.1000 M. to obtain fluoride concentrations. 2.4.3. Exit gas analysis A Varian 䊛 3800 Gas ChromatographrSaturn 2000R Ion Trap Mass Spectrometer was used to analyze exit gases. A gas-tight syringe was used to obtain 1 ml of gas from each vial. The exit gas sample was injected into the gas chromatograph and passed through a 60-m DB-5 medium bore column. The instrument was allowed to perform the analysis using an adjusted method that required approximately 7.5 min. A library search was performed on the GCrMS spectra to identify and quantify the HCFC. Using the peak area obtained from the mass spectra, relative amounts of HCFC in the exit gas were determined. Samples of unreacted HCFC collected prior to heating the reactor vessel were also run to allow a calibration of the method.
3. Results and discussion 3.1. Explanation of chloride and fluoride determinations The following is a brief explanation of the experiments conducted. Trial 1 used only one screen, trial 2 used two screens, trial 3 used three screens and trial 4 used four screens. The same temperature Ž275⬚C. and flow rate Ž5.3 m3rmin. were used for trials 1᎐4. In trial 5, a higher flow rate Ž7.7 m3rmin. was used, but chloride con-
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
18
Table 1 Calculated masses of monochlorodifluoromethane reacted using chloride analysis for varying amounts and types of reactants, temperatures, and flow rates Trial
Actual CHClF2 mass used
CHClF2 reacted Žscreen 1.
CHClF2 reacted Žscreen 2.
CHClF2 reacted Žscreen 3.
CHClF2 reacted Žscreen 4.
1 2 3 4 5 6 7 8 9 10 11
NrA NrA NrA NrA 86.7 g 64.1 g NrA 34.1 g 59.9 g 75.6 g 55.1 g
16 g 18 g 15 g 15 g Contaminated 27.8 g 52.6 g 1.9 g 2.0 g NrA 14.9 g
NrA 9g 13 g 26 g Contaminated 16.0 g 14.5 g 1.8 g 0.8 g 24.0g 23.7 g
NrA NrA 9g 21 g 1.6 g 18.3 g 29.4 g 32.6 g 1.1 g 33.4 g 18.6 g
NrA NrA NrA 0.5g No chlorides found No chlorides found No chlorides found No chlorides found 10.2 g No chlorides found 2.1 g
tamination occurred, so trial 6 was a repeat of trial 5. A lower flow rate than normal Ž3.1 m3rmin. was used in trial 7. In trials 8 and 9, higher Ž330⬚C. and lower Ž215⬚C. temperatures were used, respectively. Trial 10 was a repeat of trial 3, in which only three bedliners were used. In trial 11, five bedliners were used instead of the normal four, and no chlorides were found on the fifth screen. Table 1 shows that the amount of HCFC destroyed increased as the number of bedliners increased at the same temperature and flow rates. This was to be expected; however, when five bedliners were used, no chlorides were produced on the bottom screen suggesting that all introduced HCFC had been destroyed on the first four screens. It may also be possible that since the fifth bedliner was located close to the exit gas hole, contact time with the fifth screen was too short allowing most of the HCFC to exit the reactor. Varying the flow rate under a constant temperature revealed that a lower flow rate provided more efficient results. Temperature variation trials indicated that higher temperatures appeared to work more efficiently than lower temperatures when using the same flow rate. The HCFC recorded in grams on screens 1᎐4 given in Table 1 were calculated from the recovered NaCl on each screen assuming a stoichiometry of 1:1 for CHClF2 to HCl. The mass balance
for HCFCs decomposed in the reactor based on the total recovered NaCl on all screens when compared to the original amount of HCFC flowing through the reactor was excellent for trials 6, 8, 10, and 11. The percent recovery for HCFC in each of these trials ranged from 89.2 to 107.6%, with an overall average recovery of 101.3%. The percent recovery for trial 9, the experiment done at low temperature was only 23.5%, suggesting incomplete decomposition of HCFC. For trials 1᎐4 and 7 the original masses of HCFC introduced into the reactor were not recorded so mass balances could not be determined. Results of the fluoride analysis performed using the ATI Orion 960 Analyzer are provided in Table 2 for trial 8. This analysis was performed to confirm that sodium fluoride was indeed one of the by-products. As can be seen in Table 2, the amount of fluoride found on the screens decreased as the screens approached the vessel bottom.
Table 2 Results of fluoride analysis for trial 8 of monochlorodifluoromethane reaction Screen 1 Screen 2 Screen 3 Screen 4
1.42 g CHClF2 1.04 g CHClF2 0.35 g CHClF2 0.15 g CHClF2
reacted reacted reacted reacted
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
19
Monochlorodifluoromethane in the exit gas was analyzed for two of the previously mentioned trials. The higher amount of HCFC in the exit gas for trial 10 can probably be accounted for as due to shorter contact time with the sodium oxalate. It is clear in Fig. 1 that a greater than 95% decrease in HCFC in the exit gas occurred after only 10 s. However, after 60 s, HCFC increased in the exit gas suggesting reduced decomposition of the HCFC. This slow increase may result from the reduction of sodium oxalate with time or a reduction in non-reacted surface area of the oxalate as it becomes covered with sodium chloride. In either case, there would be an expected reduction in removal efficiency with time. Monochlorodifluoromethane was determined in trial 9 only at 60 s after the reaction started due to an error in sample collection.
4. Conclusion
Fig. 1. HCFC peak area concentrations in exit gas samples after conversion in a reactor vessel for experimental trials 3, 9 and 10. The peak areas reported were determined using GCrMS spectrometry.
3.2. Exit gas results Fig. 1 shows the relative amount of HCFC remaining in the exit gas collected after 10 s, 1 min, and 2 min, respectively, after gas passed through the reactor. The data plotted at time zero represented pure HCFC passing through the unheated reactor and may be considered the maximum amount in the exit gas. Peak areas were used to determine the relative amount of HCFC remaining in the exit gas. NIST libraries were used to identify the gases in the samples collected from the reactor. The peak identified by the computer match as monochlorodifluoromethane had a 91᎐98% probability of being correctly identified as the compound introduced to the reactor. Earlier trial runs using pure HCFC indicated a range in peak areas of 32᎐35 kCount for the peak identified as HCFC.
Fluoride and chloride analyses indicated that HCFC reacted with sodium oxalate to produce sodium chloride and sodium fluoride. The analyses performed for temperature variations under constant flow rates yielded consistent decomposition rates. Results from trials 8 and 9 showed an obvious need for higher temperatures to maximize reaction rates using sodium oxalate. Results obtained for trials 1᎐4 and 11 indicated that increasing the number of screens, and thus the amount of sodium oxalate, increased HCFC destruction. Additionally, results from trials 6 and 7 showed that, if the temperature was held constant, decreasing the flow rate of HCFC through the reactor increased the amount reacted.
Acknowledgements The authors would like to thank Nicholls State University and St. James High School for the use of equipment and guidance provided. In particular, the authors would like to thank Clyde Cooper, Lane Beard, and the St. James High Agriscience
20
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
Department for their invaluable assistance in the construction of the reaction vessel. References w1x European Environment Agency, EEA, Ozone depleting substances, Information abstracted from EnvironmentExpert.com Žhttp:rrwww.environment-expert.com rarticlesrarticle129.htm . and the European Environment Agency website Žwww.eea.eu.int.. w2x A. Althouse, A. Bracciano, C. Turnquist, Modern Refrigeration and Air Conditioning Ž1988. 286.
w3x D. Bunce, A. Schwartz, R. Silberman, C. Stanitski, W. Stratton, A. Zipp, Chemistry in Context Ž1994. 48᎐49. w4x G. Pinholster, Rhubarb to the Rescue, Discover Magazine; December, 1996, p. 16. w5x J. Burdeniuc, R.H. Crabtree, Science 271 Ž5247. Ž1996. 340. w6x R. Dagani, Chem. Eng. News 74 Ž6. Ž1996. 6. w7x American Public Health Association, American Water Works Association, Water Environment Federation. Standard Methods for the Examination of Water and Wastewater. 1992, pp. 4᎐49.
Hydrochlorofluorocarbon concentration in exit gas from a heated reactor E. Steib a , K. Gauthreaux a , J.N. Beck b,U a
b
St. James High School, St. James, LA 70086, USA Department of Physical Sciences, Nicholls State Uni¨ ersity, Thibodaux, LA 70310, USA
Received 30 June 2001; received in revised form 14 July 2001; accepted 1 August 2001
Abstract Hydrochlorofluorocarbons ŽHCFCs. have been shown to be harmful to the ozone layer because they provide free chlorine atoms that destroy ozone ŽO 3 .. The purpose of this research was to devise a method to decompose hydrochlorofluorocarbon molecules efficiently and inexpensively without producing harmful by-products. Monochlorodifluoromethane ŽCHClF2 . was selected for this study because it is a synthetic and frequently used refrigerant. A conversionrreactor tank was designed that consisted of four or five layers of sodium oxalate enclosed in the reaction tank, resting on glass wool and aluminum screens. Monochlorodifluoromethane enters above the layers of sodium oxalate and exits after passing through the last layer through a hole placed perpendicular to the entrance hole. Using the Argentometric method for chloride analysis, results showed that this device decomposed the HCFC molecules to chlorides, fluorides, and carbon dioxide. The method was tested under varying temperature, flow rates, and structural design conditions to obtain optimum efficiency. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrochlorofluorocarbon reaction; Oxalates; Chlorofluorocarbons
1. Introduction M onochlorodifluorom ethane Ž C H C lF 2 . Žhereafter simply named HCFC. was used to develop a reaction vessel that would decompose hydrochlorofluorocarbon molecules making them harmless to the ozone layer w1x. Monochlorodifluoromethane is a synthetic refrigerant used in air conditioning units and in household refrigeraU
Corresponding author. Tel.: q1-504-448-4500; Fax: q1504-448-4927. E-mail address: [email protected] ŽJ.N. Beck..
tors. It has a boiling point of y40.6⬚C, a normal head pressure at 30⬚C of 1184 kPa, and is stable, non-toxic, non-corrosive, non-irritating, and nonflammable w2x. The monochloro form was chosen to limit the number of chlorine molecules to control variability in the stoichiometry of the decomposition. Chlorine atoms found in HCFC are readily released from this molecule as very reactive free radicals. Once these molecules are released into the atmosphere and migrate to the stratosphere, the released chlorine atoms are able to react with ozone molecules. When a chlorine atom encoun-
0026-265Xr02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 1 1 1 - 4
16
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
ters an ozone molecule, the following reactions occur w2x. CHClF2 q h ª CHF2⭈q Cl
Ž1.
Cl⭈q O 3 ª ClO ⭈q O 2
Ž2.
ClO ⭈q O ⭈ª Cl⭈q O 2
Ž3.
Cl⭈q O 3 q ClO ⭈q O ⭈ª ClO ⭈q Cl⭈q 2O 2
Ž4.
Chlorine free radicals pull an oxygen atom from the O 3 molecule Ž2., forming chlorine monoxide, ClO, and an O 2 molecule. The newly formed ClO free radical readily reacts with an oxygen atom Ž3., yielding an O 2 molecule and a Cl free radical w3x. A single Cl free radical is capable of destroying more than 100 000 O 3 molecules before it is carried back to the lower atmosphere by winds. This work reports on the analytical results obtained during the testing and development of a reaction vessel constructed to decompose HCFC molecules. Pinholster w4x reported on a technique developed by Burdeniuc and Crabtree w5x to destroy CFCs using sodium oxalate at high temperatures. Dagani w6x also has reported on the CFCdestroying reaction using sodium oxalate. However, this work describes the results of trial runs involving HCFCs using a heated reactor system. Tests were conducted by varying flow rates, temperature, and structural design conditions to achieve optimum conditions for the destruction process.
2. Experimental 2.1. Reaction ¨ essel construction The reaction vessel was constructed using a 20-cm-diameter = 38 cm length = 0.63-cm-thick iron pipe, sealed on both ends by 0.63-cm-thick iron sheeting bolted to the iron pipe with a Teflon gasket to form an air-tight seal. An iron mesh screen of sufficient rigidity, cut to the desired size
to fit the curvature of the reaction vessel at four heights, was used to support the desired amount of sodium oxalate reagent and the bed-liner. Aluminum screen and Pyrex glass wool were used to form a porous bed-liner to allow passage of HCFC and provide support for the sodium oxalate. Four equal sections of aluminum screen covered by glass wool were cut to fit the iron mesh screens. The remaining two equal size layers of screen were placed on top of the glass wool and the screens were compressed until close enough to staple all four edges together to form a bed-liner. 2.2. Sample collection A sufficient amount of sodium oxalate was placed on top of each bed-liner to completely cover the screen to a depth of 1 cm. A reagent bottle containing sodium oxalate was weighed before and after placing the needed amount on the bed-liner. Each mass was recorded and the prepared bed-liners placed on each appropriate screen in the vessel. The lid of the vessel was then bolted shut. A propane gas tank was attached to the heating system connected to the vessel, and the burner lit to preheat the reaction vessel. A hose attached to the rear side allowed fumes to freely exit. The weighed canister of HCFC was then attached to its inlet on the vessel and the HCFC allowed to flow through the system. After the reaction was complete, the canister of HCFC was reweighed to determine the total amount used. 2.3. Reactor operation The vessel was heated, using specially constructed propane burners, for approximately 6 min, to 275 " 5⬚C measured on each reactor plate. Monochlorodifluoromethane was allowed to flow through the vessel at a constant flow rate for 2 min, while still being heated. While HCFC was flowing through the reactor, exit gas was trapped in 10-ml gas sampling vials after 10 s, 1 min, and 2 min. The collection bottles were immediately sealed with a crimp type seal and septa and labeled appropriately. Samples of HCFC passing
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
through the unheated and heated reactor were collected as above to allow collection of calibration standards of undecomposed gas. After 2 min, the burner and the canister of HCFC were turned off and the vessel allowed to cool for 15᎐20 min. The lid of the vessel was unbolted and the inside of the vessel was allowed to cool for 5᎐10 min. Each bedliner was removed and placed in a labeled container. 2.4. Procedure 2.4.1. Chloride determination Chloride resulting from the destruction of the HCFC was determined using the Argentometric method w7x. All reagents used in this study were ACS certified reagent grade. The chromate indicator solution was prepared by dissolving 50 g of potassium chromate in distilled water. Silver nitrate was added until a definite red precipitate formed. The solution was allowed to stand for 12 h, filtered, and diluted to 1 l using deionized water. Silver nitrate titrant was prepared by dissolving 23.95 g of silver nitrate in deionized water and diluting to 1 l. The sodium chloride standard solution was prepared by dissolving 0.824 g of dry sodium chloride in deionized water and diluting to 1 l. Potassium aluminum hydroxide was prepared by adding 25 g of aluminum potassium sulfate to 1 l of deionized water. The solution was warmed to 60⬚C while continuously stirring and adding 55 ml of ammonium hydroxide. This solution was allowed to stand for 1 h w7x. After removing the bed-liners from the reaction vessel, each was washed with deionized water and the collected supernatant from each bed-liner was diluted to 1 l and labeled. One hundred milliliter samples were taken from each supernatant solution, 3 ml of aluminum hydroxide added and the pH was between 7 and 10. One milliliter of potassium chromate Ž50 grl. and five drops of phenolphthalein were added to each sample. Each sample was titrated against the silver nitrate until the sample turned a pinkish-yellow color. The volume of silver nitrate required to neutralize the solution was recorded. The silver nitrate was also titrated against chloride-free wa-
17
ter and the resulting color was observed and recorded. 2.4.2. Fluoride determination An aliquot of the prepared supernatant solutions used in the chloride analysis was used for fluoride determination. A 50-ml sample from each supernatant solution was taken. The samples were placed in appropriate cups and fluoride concentrations determined using a fluoride meter ŽATI Orion 960 Autochemistry System.. A buffer ŽTISA Buffer 3. was added to the samples so that the proper pH was obtained. The sample was stirred while being titrated against sodium fluoride Ž0.1000 M. to obtain fluoride concentrations. 2.4.3. Exit gas analysis A Varian 䊛 3800 Gas ChromatographrSaturn 2000R Ion Trap Mass Spectrometer was used to analyze exit gases. A gas-tight syringe was used to obtain 1 ml of gas from each vial. The exit gas sample was injected into the gas chromatograph and passed through a 60-m DB-5 medium bore column. The instrument was allowed to perform the analysis using an adjusted method that required approximately 7.5 min. A library search was performed on the GCrMS spectra to identify and quantify the HCFC. Using the peak area obtained from the mass spectra, relative amounts of HCFC in the exit gas were determined. Samples of unreacted HCFC collected prior to heating the reactor vessel were also run to allow a calibration of the method.
3. Results and discussion 3.1. Explanation of chloride and fluoride determinations The following is a brief explanation of the experiments conducted. Trial 1 used only one screen, trial 2 used two screens, trial 3 used three screens and trial 4 used four screens. The same temperature Ž275⬚C. and flow rate Ž5.3 m3rmin. were used for trials 1᎐4. In trial 5, a higher flow rate Ž7.7 m3rmin. was used, but chloride con-
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
18
Table 1 Calculated masses of monochlorodifluoromethane reacted using chloride analysis for varying amounts and types of reactants, temperatures, and flow rates Trial
Actual CHClF2 mass used
CHClF2 reacted Žscreen 1.
CHClF2 reacted Žscreen 2.
CHClF2 reacted Žscreen 3.
CHClF2 reacted Žscreen 4.
1 2 3 4 5 6 7 8 9 10 11
NrA NrA NrA NrA 86.7 g 64.1 g NrA 34.1 g 59.9 g 75.6 g 55.1 g
16 g 18 g 15 g 15 g Contaminated 27.8 g 52.6 g 1.9 g 2.0 g NrA 14.9 g
NrA 9g 13 g 26 g Contaminated 16.0 g 14.5 g 1.8 g 0.8 g 24.0g 23.7 g
NrA NrA 9g 21 g 1.6 g 18.3 g 29.4 g 32.6 g 1.1 g 33.4 g 18.6 g
NrA NrA NrA 0.5g No chlorides found No chlorides found No chlorides found No chlorides found 10.2 g No chlorides found 2.1 g
tamination occurred, so trial 6 was a repeat of trial 5. A lower flow rate than normal Ž3.1 m3rmin. was used in trial 7. In trials 8 and 9, higher Ž330⬚C. and lower Ž215⬚C. temperatures were used, respectively. Trial 10 was a repeat of trial 3, in which only three bedliners were used. In trial 11, five bedliners were used instead of the normal four, and no chlorides were found on the fifth screen. Table 1 shows that the amount of HCFC destroyed increased as the number of bedliners increased at the same temperature and flow rates. This was to be expected; however, when five bedliners were used, no chlorides were produced on the bottom screen suggesting that all introduced HCFC had been destroyed on the first four screens. It may also be possible that since the fifth bedliner was located close to the exit gas hole, contact time with the fifth screen was too short allowing most of the HCFC to exit the reactor. Varying the flow rate under a constant temperature revealed that a lower flow rate provided more efficient results. Temperature variation trials indicated that higher temperatures appeared to work more efficiently than lower temperatures when using the same flow rate. The HCFC recorded in grams on screens 1᎐4 given in Table 1 were calculated from the recovered NaCl on each screen assuming a stoichiometry of 1:1 for CHClF2 to HCl. The mass balance
for HCFCs decomposed in the reactor based on the total recovered NaCl on all screens when compared to the original amount of HCFC flowing through the reactor was excellent for trials 6, 8, 10, and 11. The percent recovery for HCFC in each of these trials ranged from 89.2 to 107.6%, with an overall average recovery of 101.3%. The percent recovery for trial 9, the experiment done at low temperature was only 23.5%, suggesting incomplete decomposition of HCFC. For trials 1᎐4 and 7 the original masses of HCFC introduced into the reactor were not recorded so mass balances could not be determined. Results of the fluoride analysis performed using the ATI Orion 960 Analyzer are provided in Table 2 for trial 8. This analysis was performed to confirm that sodium fluoride was indeed one of the by-products. As can be seen in Table 2, the amount of fluoride found on the screens decreased as the screens approached the vessel bottom.
Table 2 Results of fluoride analysis for trial 8 of monochlorodifluoromethane reaction Screen 1 Screen 2 Screen 3 Screen 4
1.42 g CHClF2 1.04 g CHClF2 0.35 g CHClF2 0.15 g CHClF2
reacted reacted reacted reacted
E. Steib et al. r Microchemical Journal 71 (2002) 15᎐20
19
Monochlorodifluoromethane in the exit gas was analyzed for two of the previously mentioned trials. The higher amount of HCFC in the exit gas for trial 10 can probably be accounted for as due to shorter contact time with the sodium oxalate. It is clear in Fig. 1 that a greater than 95% decrease in HCFC in the exit gas occurred after only 10 s. However, after 60 s, HCFC increased in the exit gas suggesting reduced decomposition of the HCFC. This slow increase may result from the reduction of sodium oxalate with time or a reduction in non-reacted surface area of the oxalate as it becomes covered with sodium chloride. In either case, there would be an expected reduction in removal efficiency with time. Monochlorodifluoromethane was determined in trial 9 only at 60 s after the reaction started due to an error in sample collection.
4. Conclusion
Fig. 1. HCFC peak area concentrations in exit gas samples after conversion in a reactor vessel for experimental trials 3, 9 and 10. The peak areas reported were determined using GCrMS spectrometry.
3.2. Exit gas results Fig. 1 shows the relative amount of HCFC remaining in the exit gas collected after 10 s, 1 min, and 2 min, respectively, after gas passed through the reactor. The data plotted at time zero represented pure HCFC passing through the unheated reactor and may be considered the maximum amount in the exit gas. Peak areas were used to determine the relative amount of HCFC remaining in the exit gas. NIST libraries were used to identify the gases in the samples collected from the reactor. The peak identified by the computer match as monochlorodifluoromethane had a 91᎐98% probability of being correctly identified as the compound introduced to the reactor. Earlier trial runs using pure HCFC indicated a range in peak areas of 32᎐35 kCount for the peak identified as HCFC.
Fluoride and chloride analyses indicated that HCFC reacted with sodium oxalate to produce sodium chloride and sodium fluoride. The analyses performed for temperature variations under constant flow rates yielded consistent decomposition rates. Results from trials 8 and 9 showed an obvious need for higher temperatures to maximize reaction rates using sodium oxalate. Results obtained for trials 1᎐4 and 11 indicated that increasing the number of screens, and thus the amount of sodium oxalate, increased HCFC destruction. Additionally, results from trials 6 and 7 showed that, if the temperature was held constant, decreasing the flow rate of HCFC through the reactor increased the amount reacted.
Acknowledgements The authors would like to thank Nicholls State University and St. James High School for the use of equipment and guidance provided. In particular, the authors would like to thank Clyde Cooper, Lane Beard, and the St. James High Agriscience
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