Environ. Sci. Technol. 1996, 30, 2790-2799
Hydrothermal Processing of Chlorinated Hydrocarbons in a Titanium Reactor BERNARD R. FOY,* KURT WALDTHAUSEN, MICHAEL A. SEDILLO, AND STEVEN J. BUELOW Chemical Science and Technology Division (CST-6), MS J567, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Experiments are reported on the oxidative hydrothermal destruction of chlorinated organics in a corrosion-resistant titanium reactor. Oxidation reaction conditions were 250-500 °C near 650 bar and reaction times of 30-100 s in a continuous-flow reactor. Trichloroacetic acid, trichloroethylene, and 1,1,1trichloroethane behaved similarly. The organic concentration was ∼1.5 wt %; hydrogen peroxide was the oxidizer; sodium bicarbonate was added to achieve neutral pH. Hydrolysis occurs at low temperature, producing chloride ion and secondary organics. Carbon dioxide is the sole carbon product at 500 °C. Sodium nitrate and sodium nitrite were also found to be effective oxidizers. Corrosion of the titanium was found to be slight (99.6%. The last entry in the table shows that most of the sodium bicarbonate pumped into the reactor is converted to sodium chloride and CO2 by the HCl formed in the reaction. The intensity of the TCE bands in the FTIR spectrum can be used to estimate the destruction of the starting compound. A reference spectrum at known concentration and path length was obtained from Infrared Analysis, Inc.
residual nitrate and nitrite to form N2 and N2O (13). Here, N2O has a maximum yield of about 10%. Hydrogen gas was produced at a concentration of 0.01-0.04 M (the units refer to moles of gas per liter of liquid effluent). If trichloroethylene were the only possible source of H2, this would correspond to a yield of 18-73%. Hydrolysis and pyrolysis, however, can also lead to H2 formation, as discussed above, so the actual “yield” is subject to definition. A small amount of CO was detected only at the earliest residence time, after which it reacts to form CO2. The removal of nitrate was 70-80%, so it was present in stoichiometric excess for the oxidation reaction. On average, 1.2 mol of nitrate was consumed per mole of trichloroethylene. The pH of the effluent was 6.7. These data can be written approximately as follows (neglecting minor products):
FIGURE 6. Products of reaction of trichloroethylene with sodium nitrate at 500 °C and 650 bar. CO was below detection limits at 50 and 120 s. The residence time is only an approximate measure, as explained in the text. Initial TCE concentration was 0.11 M.
(32). Comparison with the reference spectrum yields a TCE concentration in our sample of 3.7 × 10-5 mol/L, which corresponds to a TCE destruction efficiency of 99.96% at 450 °C. The calculation assumes that all of the TCE is transferred to the gas phase by gas sampling under vacuum and that no amount is lost by sample handling in the stainless steel sample cylinder or in the FTIR cell. A calculation using the known Henry’s law coefficient (31) indicates that 94% of the TCE is transferred to the vapor phase. (The liquid phase was not analyzed specifically for TCE, but only for total organic, which has an uncertainty too large to verify removal of the volatile compound.) The estimate of 99.96% destruction of TCE was obtained at 450 °C and 60 s reaction time (5.8 g/min flow rate). The same analysis gives an upper limit on the amount of chlorinated byproduct gases produced. For 1,1-dichloroethylene, for example, we estimate an upper limit of 1.9 × 10-5 mol/L. This translates to a conversion of 99.8%, similar to the results with nitrate. Correcting for the incomplete mass balances in Table 4, the following reaction expression may be written:
C2HCl3 + 2NaNO2 + 3NaHCO3 f 3CO2 + 2NaHCO3 + 3NaCl + 0.7N2 + 0.2N2O + 0.5H2 + 0.2NH3 Trichloroethane + H2O2. The decomposition of trichloroethane was very similar to trichloroethylene. Figure 8 shows the results of the oxidation of 1,1,1-trichloroethane with hydrogen peroxide and sodium bicarbonate at 400500 °C. A logarithmic scale is again used to show the products more clearly. The initial reactant concentrations were 1.3 wt % trichloroethane (0.10 M, 2420 ppm TOC), 0.30 M NaHCO3, and 0.5 M H2O2. Sodium is recovered quantitatively in the aqueous effluent, and chlorine is
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TABLE 3
Feed and Effluent Concentrations for Reaction of TCE + NaNO3 at 500 °Ca TCE
NO3-
HCO3-
CO2
Cl-
NO2-
N2
N2O
H2
feed 0.11 0.18 0.33 effluent 99% at 450500 °C, and 98% at 400 °C. Trichloroethane + NaNO3. The oxidation of trichloroethane by sodium nitrate is similar to the reaction of trichloroethylene with sodium nitrate. The products of this reaction are shown in Figure 9 for the reaction at 400, 450, and 500 °C. Data are listed in Table 6. Reactant concentrations were 1.3 wt % trichloroethane (0.10 M, 2420 ppm TOC), 0.30 M NaHCO3, and 0.21 M NaNO3. Chlorine is quantitatively converted to chloride even at the lowest temperature (the residence time is ∼65 s). The pH of the effluent was close to neutral (pH ) 6.8-7.1). Sodium was quantitatively recovered in the liquid; the point at 500 °C, showing ∼80% recovery, is probably in error. The total organic carbon in the liquid was not completely removed until 500 °C. At 400 °C, a trace of 1,1-dichloroethylene was detected in the gas product. Nitrogen (N2) was the major nitrogen product, with some nitrous oxide, ammonia, and nitrite also formed. Hydrogen (H2) was produced at concentrations of 7 mM at 400 °C, 5 mM at 450 °C, and not detected (99.6% at 450-500 °C. A more precise measurement of the destruction efficiency will require an improved TOC measurement. Reactor residence times were not very long, since the reactor tubing was chosen at a convenient length for bench-scale work. In a larger scale reactor, it would be relatively easy to achieve longer residence times, and the destruction efficiency is expected to be higher. Our experiments were carried out at 650 bar pressure (9500 psi). At this pressure, the sodium chloride product stream is probably a two-phase, liquid-vapor, system. This does not appear to hinder the oxidation reaction. It is possible that most or all of the organic component is in the vapor phase with most or all of the oxygen and that the sodium chloride settles out predominantly into a brine phase with other salts. The sodium bicarbonate that was added to provide neutralization of the HCl product may be present as a mixture with the sodium chloride product, but measurements would have to be made to confirm this. Small amounts of partially reacted gases are produced in these reactions. Carbon monoxide, hydrogen, and methane were detected under some conditions, and trace levels of the residual starting compound (0.03%) were observed at 450 °C. Because of this, it may be best for the process to be carried out either at longer reaction time or higher temperature to eliminate these products. At 450 °C,
TABLE 4
Feed and Effluent Concentrations for Reaction of TCE + NaNO2 at 500 °Ca TCE
NO2-
HCO3-
CO2
Cl-
N2
N2O
H2
NH3
feed 0.12 0.25 0.33 effluent
