Environ. Sci. Technol. 1999, 33, 1691-1696
Reaction between Chlorocarbon Vapors and Sodium Carbonate J. W. PARRETT, JR., J. P. SUMNER, AND T. C. DEVORE* Department of Chemistry, James Madison University, Harrisonburg, Virginia 22807
The kinetics of the reactions between tetrachloromethane (CCl4), 1,2-dichloroethane (C2H4Cl2), or chlorobenzene (C6H5Cl) and sodium carbonate were investigated using evolved gas analysis-Fourier transform infrared spectroscopy. Sodium carbonate reacted with CCl4 between 600 and 900 K to form over 90% carbon dioxide (CO2) and less than 10% tetrachloroethene (C2Cl4). This reaction followed the threedimensional diffusion mechanism and had an activation energy of 105 ( 10 kJ/ mol and a steric factor of 5000 ( 3000 min-1. The reaction between C2H4Cl2 and sodium carbonate produced CO2, ethanal (C2H4O), water (H2O), vinyl chloride (C2H3Cl), ethene (C2H4), and ethyne (C2H2) between 600 and 900 K from at least two different pathways. The product temperature profiles indicated that CO2, C2H4O, and C2H3Cl were formed initially and that approximately 10% of the product is C2H4 at 900 K. The reaction kinetics followed the Ginstling-Brounshtein diffusion mechanism and had an activation energy of 100 ( 10 kJ/ mol and a steric factor of approximately 104 min-1. Benzene was produced from the reaction between chlorobenzene and sodium carbonate at temperatures above 800 K. This reaction followed the three-dimensional diffusion mechanism and had an activation energy of 80 ( 10 kJ/mol and a steric factor of approximately 500 min-1.
Introduction Chlorocarbons are widely used in industry as solvents, have a high stability, and tend to accumulate in the environment. Investigations into the toxicity and the carcinogenic properties of these compounds have raised awareness about the need to find ways to properly dispose of these compounds (1). Incineration of chlorocarbons in high-temperature furnaces is currently the most commonly used disposal method, but it is expensive because it requires temperatures in excess of 1000 K to ensure complete combustion of the material (2, 3). Fuel costs for incineration can be 40% of the total cost of the plant operation (4). Incomplete combustion can also generate undesirable products which complicate the remediation effort. Consequently, there is considerable interest in developing a lower temperature method that will effectively destroy the chlorocarbons without generating incomplete combustion products (5). Two possible methods have been advanced. The more thoroughly investigated method is catalytic combustion of the chlorocarbons (5-7). While several metal oxide or supported metal oxide catalysts have shown promise for remediating a variety of chlorocarbons, a general catalyst that will remediate all chlorocarbons has not yet been found. The second method suggested is to react the chlorocarbon with alkaline salts. For example, sodium oxalate is reported to react quantitatively with chlorofluorocarbons at 550 K in circulating bed 10.1021/es980077b CCC: $18.00 Published on Web 04/02/1999
1999 American Chemical Society
systems (8, 9). Because sodium oxalate thermally decomposes to form sodium carbonate in this temperature region (10, 11), sodium carbonate may have contributed to the reaction and may also react with chlorofluorocarbons in this temperature range. Molten sodium carbonate has been reported to react with chlorocarbons (12) and to remove fluorine from dimethylperfluoro-3,6-dioxa1,7-1,8octanedioate (13). Thermodynamic calculations for selected reactions between sodium carbonate and tetrachloromethane (CCl4), trichloromethane (CHCl3), 1,2-dichloroethane (C2H4Cl2), or chlorobenzene (C6H5Cl) are presented in Table 1. The aliphatic chlorocarbon reactions have very favorable Gibbs free energies of reaction at 298 K (∆rxnG(298)) and at 750 K and should react nearly completely once the reaction is initiated. ∆rxnG(750) is not favorable for chlorobenzene unless oxygen is present. The reaction in oxygen is favorable, but this reaction would probably require incineration-type temperatures to completely destroy the chlorobenzene. Because alkaline substances are already used in the petroleum industry as HCl scavengers to reduce acidic emissions (14), they would have the advantage of also removing the HCl that is produced during the catalytic combustion of many chlorocarbons. Because the thermodynamics of these reactions are favorable, the kinetics will determine the effectiveness of these processes for destroying chlorocarbon vapors. The kinetics of the reactions between sodium carbonate and tetrachloromethane, 1,2-dichloroethane, or chlorobenzene have been investigated using evolved gas analysis-Fourier transform infrared spectroscopy. The products formed and the Arrhenius parameters were established for each reaction.
Experimental Section Reagents. The tetrachloromethane, 1,2-dichloroethane, and chlorobenzene were Fisher ACS. The sodium carbonate was from Mallinckrodt. All chemicals were used as received. Apparatus. The apparatus used in this investigation has been described in detail previously (15-17). Briefly, it was constructed by attaching KBr windows to two opposing arms of a stainless steel four-way cross-vacuum flange (MDC, Inc.). One of the remaining arms was connected to the vacuum line and the other was attached to a 25-cm-long piece of 9 mm glass tube that served as the reactor. The open end of this tube was attached to the chlorocarbon container. The flow of the chlorocarbon into the reaction vessel was controlled using a tetrafluoroethene needle valve, and the intensity of the IR bands provided a convenient measurement of the amount of chlorocarbon flowing through the cell. The IR absorbance was measured for the chlorocarbon vapor in equilibrium with the liquid at 298 K. The ratio of the IR absorbance during the experiment to this absorbance multiplied by the known vapor pressures at 298 K (15 kPa for CCl4, 11 kPa for C2Cl4, and 1.6 kPa for chlorobenzene) (18) gave the chlorocarbon vapor pressure during the experiment. A 12 mm Vycor tube was wrapped with Nichrome resistance heating wire and slipped over the reactor to heat the sample. A Perkin regulated power supply provided the power for this furnace. The temperature of the sodium salt bed was measured using a chromel-alumel thermocouple connected to a Keithly digital thermometer. The temperature was measured to ( 3 K over the temperature range (298-900 K) that could be obtained using this apparatus. IR spectra were obtained using a Nicolet Magna-750 FTIR. This spectrometer collected and stored one spectrum every two seconds for reaction times up to 1 h. Spectra were obtained from 4000 to 400 cm-1 with 4 cm-1 resolution. The VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1691
FIGURE 1. Infrared spectra observed for the reaction between Na2CO3 and CCl4. The reaction temperature is given in the figure. The initial CCl4 pressure was 5 kPa. The scale is expanded to show the product features.
TABLE 1: Thermodynamics (in kJ/mol) for the Most Likely Reactions Between Some Sodium Salts and Some Simple Aliphatic Chlorocarbons (Calculated Using Data from Ref 21 and 22) reaction
∆rxnH298
∆rxnG298
∆rxnG750
2Na2CO3 + CCl4 f 4NaCl + 3CO2 3Na2CO3 + 2HCCl3 f 6NaCl + 4CO2 + H2O + C Na2CO3 + C2H4Cl2 f 2NaCl + CO2 + C2H4O Na2CO3 + C2H4Cl2 f 2NaCl + 2NaOH + CO2 + CO + H2O + 2C 3Na2CO3 + C6H5Cl f NaCl + 5NaOH + + 2CO2 + 7C 8Na2CO3 + C6H5Cl + 5H2O f NaCl + 15NaOH + 7CO2 + 7C 3Na2CO3 + C6H5Cl + 7O2 f NaCl + 5NaOH + 9CO2
-462 -684 -121 -31 +112 +751 -2642
-570 -836 -173 -122 +86 +697 -2673
-734 -1068 -252 -261 +47 +615 -2719
TABLE 2: Products Observed for the Reactions of Simple Chlorocarbons with Sodium Carbonate in the Flow System (Relative Concentrations for
