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Ind. Eng. Chem. Res. 1997, 36, 3978-3981
Absorption of Inorganic Halides Produced from Freon 12 by Calcium Carbonate Containing Iron(III) Oxide Seiichiro Imamura,* Yoichi Matsuba, Etsu Yamada, Kenji Takai, and Kazunori Utani Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Inorganic halides produced by the catalytic decomposition of Freon 12 were fixed by calcium carbonate, which is the main component of limestone. Iron(III) oxide, which is present as a contaminant in limestone, promoted the absorption of the halides by calcium carbonate at low temperatures. The supposed action of iron(III) oxide was to first react with inorganic halides, forming iron halides, and, then, transfer them to calcium carbonate to replace carbonate ion in a catalytic way. Thus, calcium carbonate containing iron oxides (limestone) can be used as an effective absorbent for the inorganic halogens produced during the decomposition of Freons. Introduction
Experimental Section
Although emission of chlorofluorocarbons (CFC’s) has been diminishing since the United Nations Environmental Protection Protocol for CFC regulation was adopted in Montreal, Canada, in 1987, some of them are still being used now and the amount of preserved CFC’s is enormous. Thus, efficient technologies to detoxify CFC’s must be developed. The technologies ever tested are reduction by use of sodium naphthalenide (Oku et al., 1988, 1989), laser-induced decomposition (Hudgens, 1977; Zitter et al., 1990), catalytic hydrogenolysis (Witt et al., 1981), combustion (Graham et al., 1986), biological degradation (Denovan and Strand, 1992; Lesage et al., 1992), and so on. Catalytic decomposition has also been examined (Imamura et al., 1990, 1991a,b, 1993a,b: Aida et al., 1990; Jacob, 1990; Miyatani et al., 1992; Okazaki and Kurosaki, 1989, 1991). There will, however, arise one problem when efficient catalytic decomposition, thermal decomposition, or incineration process is achieved; that is, how to dispose of the resultant inorganic halides. Recently, a new technology was attempted, in which Freons are decomposed directly in a cement kiln at a temperature of about 1400 °C and the resultant inorganic halides are trapped simultaneously as calcium halides (Ueno et al., 1996). Although this process seems attractive, the concentration of chlorine in the product cement must be kept below 200 ppm in order to maintain its quality, and, thus, the amount of CFC’s to be treated is limited. Limestone which is used for the cement production process is mainly composed of calcium carbonate and, in addition, contains oxides of iron, aluminum, magnesium, and other minerals. Their contents differ greatly depending upon the kinds of limestones obtained (Motoki, 1989), and those with too much contaminants are discarded as a waste. In this work the authors attempted to utilize low-grade limestone as a trapping agent for inorganic halides which are produced by the decomposition of Freons; absorption of inorganic halides by calcium carbonate containing iron(III) oxide was investigated at a much lower temperature region than that employed in the cement kiln technique. This process will recover a large amount of useful inorganic halogens as calcium halides, which can be utilized as a resource.
Materials. Calcium carbonate, calcium oxide, γ-alumina, iron(III) oxide, and other reagents were used as obtained commercially. A known amount of iron(III) oxide or γ-alumina was mixed thoroughly with calcium carbonate in a mortar. The mixture was pressed into a disk under a pressure of 30 MPa and was cut into granules of 8-14 mesh size. Calcium oxide was also used as an absorbent. BPO4, a Freon 12 decomposition catalyst, was prepared by the reaction of boric acid with phosphoric acid and was calcined at 600 °C in air for 3 h (Imamura et al., 1991; Ozaki, 1980). Apparatus and Procedure. Reactions were carried out with two kinds of reactor systems. One was composed of a single reactor tube made of alumina (outer diameter of 8 mm and inner diameter of 5 mm), in which 1 mL of the absorbents was charged. The other was equipped with two alumina tube reactors arranged in a series, and 2 mL of BPO4 was charged in the first reactor where Freon 12 was completely decomposed. The second reactor containing the absorbents (2 mL) trapped inorganic halides which were produced in the first reactor. A reaction mixture (Freon 12, 0.6%; O2, 22.3%; He, 77.1%) was fed into the reactor at a rate of 100 mL min-1. The effect of water was investigated as well because it is produced in the decomposition of some hydrogen-containing Freons and also because it is generally added in the catalytic combustion system in order to regenerate the catalyst surface by eliminating the accumulated inorganic halides (Imamura et al., 1993a,b). Water was introduced by passing O2 through a saturator kept at prescribed temperatures; in this case the concentration of oxygen was decreased by the amount corresponding to the water added. The exit gas was introduced into a NaOH trap to determine inorganic halides which escaped the absorbent reactor. Analyses. Freon 12 was determined with a Shimadzu GC-8A gas chromatograph equipped with a flame ionization detector on a chromosorb 101 column (1 m) at 120 °C. After CO2 and CO were separated on an activated charcoal column (1 m) at 120 °C with the same gas chromatograph, they were converted into methane with a Shimadzu MNT-1 methanizer and were determined. Fluoride ion trapped in a NaOH solution was determined with an Orion 811 pH meter by the use of a fluoride ion electrode. Chloride ion in the NaOH solution was converted into silver chloride by an addition of aqueous silver nitrate (0.5 wt %). After silver
* Author to whom correspondence is addressed. Phone: 8175-724-7534. Fax: 81-75-724-7580. E-mail:
[email protected]. ac.jp. S0888-5885(97)00026-2 CCC: $14.00
© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3979
Figure 1. Decomposition of Freon 12: (2) no catalyst, (b) CaCO3, (O) CaCO3 + 2% H2O, (0) CaCO3 + 4% H2O. Freon 12 ) 0.6%, O2 ) 22.3%, He ) 77.1%, catalyst ) 1 mL, SV ) 6000 h-1. When water was added, O2 was decreased by that amount.
Figure 2. Decomposition of Freon 12: (b) CaCO3, (O) CaCO3 + γ-Al2O3 (5 wt %), (0) CaCO3 + Al2O3 (5 wt %) + 2% H2O, ([) CaCO3 + Al2O3 (5 wt %) + 4% H2O. The reaction condition is shown in Figure 1.
Table 1. Decomposition of Freon 12 on CaCO3a
Table 2. Amount of F Trapped during the Decomposition of Freon 12 for 2 ha
absorbent
H2O (vol %)
T50 (°C)
none CaCO3 CaCO3 CaCO3 CaCO3 + Fe2O3 (1 wt %) CaCO3 + Fe2O3 (1 wt %) CaCO3 + Fe2O3 (10 wt %) CaCO3 + Fe2O3 (10 wt %)
0 0 2 4 0 2 0 2
883 814 800 802 800 812 781 790
a Freon 12 ) 0.6 vol %; O ) 22.3 vol %; He ) 77.1 vol %. CaCO 2 3 (plus Fe2O3) ) 1 mL. Space velocity ) 6000 h-1. When water was introduced, oxygen was decreased by that amount.
chloride was filtered with a membrane filter, chloride ion on the filter was determined with a Rigaku Denki 3270 energy-dispersive fluorescence X-ray spectrophotometer. The DTA-TG analysis was carried out with a Rigaku Denki TAS-100 thermal analyzer, and X-ray diffraction patterns were obtained with a Rigaku Denki Geigerflex 2012 X-ray analyzer.
F trapped (%) absorbent
500 °C
600 °C
700 °C
800 °C
CaCO3 CaO CaCO3/Fe2O3b
32.0 98.2 98.7
90.7 97.8 97.8
98.4 98.4 98.1
95.1 95.1 99.8
a Freon 12 ) 0.6 vol %; O ) 20.3 vol %; He ) 77.1 vol %. 2 Absorbent ) 2 mL. Space velocity ) 3000 h-1. Freon 12 was completely decomposed in the first reactor. The amount of F trapped by the absorbents was calculated on the basis of the total amount of F trapped in an alkaline solution without the absorbents in the second reactor (see the text). b Fe2O3 ) 10 wt %.
Table 3. Amount of Cl Trapped during the Decomposition of Freon 12 for 2 ha Cl trapped (%) absorbent
500 °C
600 °C
700 °C
800 °C
CaCO3 CaO CaCO3/Fe2O3b
4.0 97.4 79.8
82.7 100 94.4
100 100 100
100 100 99.7
a Reaction condition is the same as shown in Table 2. b Fe O 2 3 ) 10 wt %.
Results and Discussion Decomposition of Freon 12 was carried out on CaCO3 and CaCO3 containing Fe2O3 or Al2O3 by the use of a single reactor system. Typical curves for Freon 12 conversion vs temperature are shown in Figure 1. The reaction proceeds at a little lower temperature in the presence of CaCO3 compared with the reaction in its absence. As DTA-TG analysis revealed that CaCO3 began to decompose above 650 °C and was completely converted at 826 °C, the reaction below 800 °C is assisted mainly by CaCO3 and CaO accelerated the reaction above this temperature. Water seems not to have much accelerating effect. Table 1 summarizes the results of the effects of CaCO3, Fe2O3, and water. CaCO3 (or CaO if produced) shows some effect as is also shown above. However, Fe2O3 seems to have little effect; as much as 10 wt % of it decreased T50 only by about 30 °C (in the absence of water) from T50 of 814 °C in the presence of CaCO3. The effect of water is also trivial; although it increased the activity of CaCO3, it exhibited, on the contrary, a detrimental effect on the reaction in the presence of CaCO3 plus Fe2O3. These results show that CaCO3(CaO), Fe2O3, and water all are not effective in decomposing Freon 12.
Shown in Figure 2 is the effect of γ-Al2O3, another impurity in limestone. In this case temporary acceleration of the reaction occurred at a low-temperature region (below 800 °C), but the acceleration soon stopped and the decomposition curves were constringed to that in the presence of only CaCO3. Al2O3 seems to decompose Freon 12 in a stoichiometric reaction. As CaCO3 was not so effective in the decomposition of Freon 12, it was used as an abosrbent for inorganic halides produced in the decomposition of Freon 12. A two-reactor system was used, and the first reactor was charged with 2 mL of BPO4 (Imamura et al., 1991) and was maintained at 600 °C, where Freon 12 was completely decomposed. A total of 2 mL of the absorbent (CaCO3, CaO, or CaCO3 plus Fe2O3) was charged in the second reactor, and the efficiency of the absorption of inorganic halides was examined as a function of the reactor temperature. After the BPO4 reactor and the absorbent reactor were heated to the prescribed temperatures in a flow of oxygen, Freon 12 in He was introduced and the reaction was started. Tables 2 and 3 show the amount (%) of fluoride and chloride ions trapped during the reaction for 2 h. The material
3980 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 4. Change in the state of CaCO3 plus Fe2O3 (10 wt %) in the reaction at 500 °C for 3 h before (A) and after (B) the reaction. The reaction condition is shown in Figure 3. XRD analysis condition: Cu KR, 35 kV/15 mA, graphite monochromator.
Figure 3. Elution of CO2 from the bed of (A) CaCO3 and (B) CaCO3 plus Fe2O3 (10 wt %): (b) 500 °C, (O) 600 °C, (9) 700 °C, (]) 800 °C. Freon 12 ) 0.6%, O2 ) 22.3%, He ) 77.1%, absorbent ) 2 mL, SV ) 3000h-1. Freon 12 was completely decomposed in the first reactor.
balance for F was poor even when the second reactor was not charged with the absorbent; F was trapped inside the wall of the reactor or Teflon pipe leading to the NaOH trap. Thus, the amount of F trapped (%) was calculated on the basis of the mean amount of F which escaped the second reactor without the absorbents (66% based upon the Freon 12 decomposed), and, thus, the data shown in Table 2 are rather qualitative. On the other hand, Cl was scarcely absorbed by the reaction line. CaO with a basic nature can absorb both F and Cl efficiently as low as 500 °C. CaCO3 absorbs 32% of F and 4% of Cl at 500 °C and 90.7% of F and 82.7% of Cl at 600 °C. Although TG-DTA analysis indicated that CaCO3 does not decompose at these temperatures, the fact that the halides were absorbed indicates that they accelerate the decomposition of CaCO3 and replace the carbonate ion. The amount trapped was larger for F than Cl, F being more reactive toward CaCO3 than Cl. Interestingly, Fe2O3 increases the ability of CaCO3 to absorb the halides: although CaCO3 alone absorbs only 4% of Cl at 500 °C, as much as 79.8% was trapped at the same temperature in the presence of 10 wt % of Fe2O3. F was also more effectively trapped in the presence of Fe2O3 (98.7% at 500 °C) than in its absence (32%). The elution of CO2 from the absorbent reactor was monitored (Figure 3). Yield of CO2 was based on the amount of CO2 produced from Freon 12 which was decomposed completely at the BPO4 reactor. As CaCO3 was decomposed by absorbing the halides, the yield of CO2 naturally exceed 100%. A large amount of CO2 evolved at the start of the reaction at 800 °C from both CaCO3 and CaCO3/Fe2O3, showing that transformation to CaO occurred rapidly and absorption of the halides occurred simultaneously. The difference between the behavior of CaCO3 and that of CaCO3/Fe2O3 is seen in
the result carried out at lower temperature from 500 to 700 °C; the amount of CO2 evolved is larger in the presence of Fe2O3. This phenomenon is in accord with the fact that more halides were absorbed in CaCO3 in the presence of Fe2O3; Fe2O3 promotes the replacement of CO32- ion by the halide ions. X-ray analyses were carried out to clarify the function of Fe2O3. It was found that the decomposition of CaCO3 itself in air was scarcely affected in the presence of Fe2O3; temperatures of complete decomposition were 825.9 °C for pure CaCO3 and 818.3 °C for CaCO3 with 10 wt % of Fe2O3 as determined by DTA-TG analysis. Figure 4 shows X-ray diffraction patterns of CaCO3 with 10 wt % of Fe2O3 before and after reaction at 500 °C for 3 h. CaClF (McClune, 1983a) and CaF2 (Smith, 1967a) were produced but CaCl2 was not, indicating the higher reactivity of F. It is interesting to note that Fe2O3 (McClune, 1988) does not seem to be changed remarkably. There is a possibility that Fe2O3 suffers temporary halogenation but transfers the halide ions rapidly to CaCO3 to recover its original state. In order to examine this, HCl (1% in He) was passed over Fe2O3 at 500 °C for 3 h and the change of its state was monitored. Although no pattern change of Fe2O3 was observed by an XRD technique, a white-yellow precipitate was found to adhere on the wall of the exit of the reactor; an X-ray fluorescence analysis indicated that it contained F and Fe. As iron chlorides have low melting points [FeCl3, 304 °C; FeCl2, 677 °C] (Dean, 1985), they may have vaporized and escaped the catalyst bed. Next the possibility of the conversion from FeCl2 to iron oxides was examined in the presence of CaCO3; a mixture of CaCO3 and FeCl2 was calcined in air at 500 °C for 3 h (Figure 5). The intensity of the main peak of CaCO3 (2Θ of 29.7°) decreased remarkably, and the peak of FeCl2‚2H2O (McClune, 1984a) disappeared. In turn, CaCl2 (McClune, 1983b), CaCl2‚6H2O (McClune, 1984b), FeO (Smith, 1967b), Fe3O4 (McClune, 1979), and Fe2O3 (McClune, 1988) seem to have been formed, although not all of the peaks were clearly assigned. CaCO3 without FeCl2 did not change at all by the same treatment. Thus, the possible action of Fe2O3 is to first react with halide ions to form partially halogenated irons and, then, transfer them to CaCO3, forming
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3981
Figure 5. Calcination of a mixture of CaCO3 and FeCl2 (2:1 wt ratio) at 500 °C in air for 3 h before (A) and after (B) calcination. The XRD analysis condition is shown in Figure 4.
calcium halides and evolving CO2. In the latter reaction Fe2O3 should be recovered as deduced from the result shown in Figure 4. Fe2O3 accelerates the fixation of halides by CaCO3 in a catalytic way. The results obtained here indicate that limestone containing iron oxides works as an effective absorbent to trap inorganic halides produced from Freons at relatively low temperatures. Chlorine and fluorine can be recovered and re-used. The fact that alumina can decompose Freon may be an asset when decomposition of Freons and simultaneous fixation of inorganic halides are aimed at in a cement kiln process. Acknowledgment This work was supported by a research grant from Iwatani Naoji Foundation. Literature Cited Aida, T.; Higuchi, R.; Niiyama, H. Decomposition of Freon 12 and Methyl Chloride over Supported Gold Catalysts. Chem. Lett. 1990, 1147. Dean, J. A., Ed. In Lange’s Handbook of Chemistry; McGrawHill: New York, 1985; p 4-63. Denovan, B. A.; Strand, S. E. Biological Degradation of Chlorofluorocarbons in Anaerobic Environments. Chemosphere 1992, 24, 935. Graham, J. L.; Hall, D. L.; Dellinger, B. Laboratory Investigation of Thermal Degradation of a Mixture of Hazardous Organic Compounds. 1. Environ. Sci. Technol. 1986, 20, 703. Hudgens, J. W. In Situ Studies of Infrared Multiple Photon Laserinduced Decomposition of CF2Cl2 and CFCl3. J. Chem. Phys. 1978, 68, 777. Imamura, S.; Shiomi, T.; Ishida, S.; Utani, K.; Jindai, H. Decomposition of Dichlorodifluoromethane on TiO2/SiO2. Ind. Eng. Chem. Res. 1990, 29, 1758. Imamura, S.; Imakubo, K.; Fujimura, Y. Catalytic Decomposition of DichlorodifluoromethanesA Study on the Catalyst Durable Against Fluorine. Nippon Kagaku Kaishi 1991a, 645.
Imamura, S.; Imakubo, K.; Furuyoshi, S.; Jindai, H. Decomposition of Dichlorodifluoromethane on BPO4 Catalyst. Ind. Eng. Chem. Res. 1991b, 30, 2355. Imamura, S.; Higashihara, H.; Jindai, H. Reactivating Effect of Water on Freon 12 Decomposition Catalysts. Chem. Lett. 1993a, 1667. Imamura, S.; Shimizu, H.; Haga, T.; Tsuji, S.; Utani, K.; Watanabe, M. Decomposition of Dichlorodifluoromethane on PO4-ZrO2 Catalyst. Ind. Eng. Chem. Res. 1993b, 32, 3146. Jacob, E. Method for Converting Gaseous Organic Halocompounds to Carbon Dioxide and Hydrogen Halides. Ger. DE3841847, 1990; Chem. Abstr. 1990, 113, 196942c. Lesage, S.; Brown, S.; Hosler, K. R. Degradation of Chlorofluorocarbon-113 under Anaerobic Conditions. Chemosphere 1992, 24, 1225. McClune, W. F., Ed. In Powder Diffraction File, Sets 19-20; International Centre for Diffraction Data: Swarthmore, PA, 1979; p 197. McClune, W. F., Ed. In Powder Diffraction File, Sets 23-24; International Centre for Diffraction Data: Swarthmore, PA, 1983a; p 550. McClune, W. F., Ed. In Powder Diffraction File, Sets 23-24; International Centre for Diffraction Data: Swarthmore, PA, 1983b; p 563. McClune, W. F., Ed. In Powder Diffraction File, Sets 25-26; International Centre for Diffraction Data: Swarthmore, PA, 1984a; p 363. McClune, W. F., Ed. In Powder Diffraction File, Sets 25-26; International Centre for Diffraction Data: Swarthmore, PA, 1984b; p 888. McClune, W. F., Ed. In Powder Diffraction File, Sets 31-32; International Centre for Diffraction Data: Swarthmore, PA, 1988; p 666. Miyatani, D.; Shinoda, K.; Nakamura, T.; Ohta, M.; Yasuda, K. Catalytic Decomposition of CFC-112 and CFC-113 in the Presence of Ethanol. Chem. Lett. 1992, 795. Motoki, Y., Ed. In Handbook of Ceramic Technology; The Ceramic Society of Japan; Tokyo, 1989; p 836. Okazaki, S.; Kurosaki, A. Decomposition of Chlorofluorocarbons by the reaction with Water Vapor Catalyzed by Iron Oxide Supported on Activated Carbon. Chem. Lett. 1989, 1901. Okazaki, S.; Kurosaki, A. A Process for the Catalytic Decomposition of Halofluorocarbons. Eur. Pat. Appl. EP 412456, 1991; Chem. Abstr. 1991, 115, 135502d. Oku, A.; Kimura, K.; Sato, M. Chemical decomposition of Chlorofluorocarbons by Reductive Dehalogenation Using Sodium Naphthalenide. Chem. Lett. 1988, 1789. Oku, A.; Kimura, K.; Sato, M. Complete Destruction of Chlorofluorocarbons by Reductive Dehalogenation Using Sodium naphthalenide. Ind. Eng. Chem. Res. 1989, 28, 1055. Ozaki, A., Ed. In Shokubai Chousei Kagaku; Kodansha: Tokyo, 1980; p 258. Smith, J. V., Ed. In X-Ray Powder Data File, Sets 1-5; American Society for Testing and Materials: Philadelphia, PA, 1967a; p 590. Smith, J. V., Ed. In X-Ray Powder Data File, Sets 6-10; American Society for Testing and Materials: Philadelphia, PA, 1967b; p 127. Ueno, H.; Tatsuichi, S.; Soufuku, M.; Iwasaki, Y.; Oiwakawa, Y.; Sasaki, Y.; Miyakoshi, T. Decomposition of Chlorofluorocarbon 12 in Cement Kiln. J. Jpn. Soc. Atmos. Environ. 1996, 31, 88. Witt, S. D.; Wu, E. C.; Loh, K. L.; Tang, Y. N. Heterogeneous Hydrogenolysis of Some Fluorocarbons. J. Catal. 1981, 71, 270. Zitter, R. N.; Koster, D. F.; Choudhury, T. K.; Cantoni, A. Kinetics and Mechanisms of the CO2 Laser Induced Decompositions of CFCl3 and CF2Cl2. J. Phys. Chem. 1990, 94, 2374.
Received for review January 2, 1997 Revised manuscript received April 15, 1997 Accepted May 4, 1997X IE9700268
X Abstract published in Advance ACS Abstracts, July 1, 1997.
