Superfund Research Program
Wet Oxidation of Hazardous Chemicals in Sub- and Supercritical Water
Project Leader: Jefferson W. Tester
Grant Number: P42ES004675
Funding Period: 1995 - 2000
During the past year investigators focused on a theoretical analysis of methylene chloride hydrolysis, testing of our new reactor system, and on an in-depth investigation into the oxidation of benzene in SCW.
The analysis focused on explaining the experimental observation that significant reaction of methylene chloride (CH2Cl2) occurs in subcritical water while rates are unexpectedly low at supercritical conditions. A standard Arrhenius form of the rate expression cannot account for this observation. Observed low CH2Cl2 hydrolysis rates under supercritical conditions are explainable through an analysis of the transition state. The transition state complex formed from the reaction of CH2Cl2 and H2O is more polar than either of the two neutral reactants. Since water changes from a polar to a nonpolar solvent in the shift from sub- to supercritical conditions (as evidenced by a decreasing dielectric constant), this transition state is less stabilized, and consequently a slower reaction rate is expected relative to that in subcritical water liquid.
Ab initio predictions of the stereochemistry and physical properties of both reactants and their transition state are consistent with expectations. Simulations show an increase in the activation energy of the reaction and a changing reaction profile as the dielectric constant of water decreases. Due to these changes, the hydrolysis rate is predicted to decrease by about three orders of magnitude at 550°C. Applying a Kirkwood correction factor to the standard Arrhenius form of the rate constant properly captures the magnitude of the changes in the rate constant. A regressed global rate expression for the hydrolysis of CH2Cl2 incorporating was developed which predicts the trends in the experimental data and provides a model of the reaction rate over an extended temperature range of interest in engineering applications.
New Reactor System
As noted previously, a continuously fed stirred tank reactor (CSTR) was constructed which will allow greater flexibility and expanded capabilities in both the types of experiments allowable and the range of operating conditions that can be achieved. For example, researchers can now conduct experiments with contaminated soils or other solids, experiments that are not possible in our flow reactors. Additionally, project investigators can access longer residence times and operate at higher organic concentrations than was previously possible. Preliminary oxidation experiments in the CSTR with methanol indicate good agreement between reaction rates in the CSTR and our flow reactor.
Benzene Oxidation in Supercritical Water
Experiments on the oxidation of benzene in supercritical water are complete. Benzene was selected because it is a known human carcinogen and a ubiquitous hazardous waste. The aromatic ring of benzene is chemically stable and is likely to be a refractory intermediate in the SCWO of polyaromatic compounds. Using the bench-scale, tubular flow reactor, over 100 oxidation and hydrolysis (reaction in the absence of oxygen) experiments with benzene were performed. The hydrolysis experiments indicate that benzene reacts minimally, if at all, without oxygen at temperatures up to 600°C.
Investigators measured benzene oxidation rates over the following conditions: T = 475-590°C, [C6H6]o = 0.4-1.2 mM, = 3-7 s, P = 139-278 bar ( = 0.04 to 0.09 g/mL), and with 40% to 200% of stoichiometric oxygen demand. Temperature has the strongest influence on benzene conversion, with only minimal conversion occurring below 520°C. Conversion increases linearly with temperature from 520 to 570°C and asymptotically approaches 100% for temperatures above 575°C. The oxidation rate is dependent on the [C6H6]o at certain conditions. Benzene concentration was varied from 0.4 to 1.2 mM at 530, 540, and 550°C with P = 246 bar and a stoichiometric amount of O2. Benzene conversion decreases as [C6H6]o increases at 530 and 540°C, but at 550°C the researchers cannot discern, within experimental error, a difference in the conversion of benzene as [C6H6]o increases. Experiments varied the [O2]o/[C6H6]o ratio by changing [O2]o while keeping [C6H6]o = 0.6 mM and P = 246 bar and showed a dependence of benzene oxidation on this oxygen-to-fuel ratio. Experiments adding between 40% and 200% of the total oxygen demand were performed at 540 and 550°C, and showed that benzene conversion increases as [O2]o/[C6H6]o increases, indicating that oxygen participates in one or more of the rate limiting steps in benzene oxidation. The final set of experiments was to determine the effect of system pressure, or fluid density, on benzene conversion. Conversions were measured with pressures of 139-278 bar (=0.04-0.09 g/mL) while maintaining T = 540°C, [C6H6]o = 0.6 mM and a stoichiometric amount of O2. The experiments show that benzene conversion appears independent of pressure for subcritical pressures (Pc=221 bar), but increases with pressure for supercritical pressures. The conversion data from all of the benzene experiments were fit to a global rate expression using a non-linear regression algorithm. The regression yielded a global rate expression that adequately represents the data.
Project investigators used GC-FID and GC-TCD for the identification and quantification of benzene and its partial and final oxidation products. The partial and final oxidation products in the most significant quantities are phenol, carbon monoxide, carbon dioxide, and methane. Ethane, ethylene, acetylene and propylene are present in much lower amounts.