Thermal desorption and decomposition of hazardous organic compound I: Mass transfer of benzene and chlorobenzene in soil matrices II: Oxidation and pyrolysis of 1,1,1-trichloroethane in methane/oxygen/argon
Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical Engineering, Chemistry and Environmental Science
Joseph W. Bozzelli
Dana E. Knox
Jay N. Meegoda
I: Mass Transfer of Benzene and Chlorobenzene in Soil Matrices
Experimental measurements on several apparatus were used in conjunction with chromatographic theory to study the thermal adsorption and desorption of organic vapors on soil with different particle sizes for analysis mass transfer parameters and heat of adsorption. Benzene (C6H6) and chlorobenzene (C6H5Cl) were tested on soil with 0.55, 0.46, 0.36, and 0.225 mm average particle sizes. Sample injection volumes for organic compounds were studied to ensure linearity of gas chromatography. Equilibrium constants were strongly dependent on temperature but not on particle size. Heats of adsorption, determined from the slope of the plot of Van't Hoff's equation, were -16.08 for C6H6 and -19.48 kcal/mole for C6H5Cl. An analysis of second central moment showed that mass transfer resistance of larger molecules like C6H6 and C6H5Cl was not strongly dependent on temperature within the ranges of this study and for this soil matrix.
Results from continuous adsorption and desorption experiments showed that temperature and inlet flow rate affect effluent concentration. The temperature effect on the effluent concentration of C6H6 became less significant for temperatures above 220 °C.
The results from an analytical solution and numerical analysis using orthogonal collocation method showed excellent coincidence. The numerical approach was therefore chosen to model experimental results. This model includes axial dispersion coefficients, intraparticle diffusion coefficient, film mass transfer coefficients plus the adsorption equilibrium constant and shows good agreement at higher temperatures; but shows slower transfer through the bed for the effluent concentration of organics on soil at lower temperature.
H: Oxidation and Pyrolysis of 1,1,1-Trichloroethane in Methane/Oxygen/Argon
The thermal decomposition of 1,1,1-trichloroethane in methane/oxygen mixtures and argon bath gas was carried out at 1 atmosphere total pressure in tubular flow reactors. The thermal degradation of 1,1,1-trichloroethane and methane was analyzed systematically over the temperature range of 500 to 800°C, with average residence times of 0.05 to 2.5 seconds. Five reactant ratio sets, in three different diameter flow reactors, were studied.
It was found that the 99% decay of the 1,1,1-trichloroethane at 1 second residence time occurs at about 600°C for all the reactant ratio sets. The major products for 1,1,1-trichloroethane decomposition were 1,1-dichloroethylene and HCl. Oxygen (maximum 4.5%) had almost no effect on the initial decay rates of 1,1,1-trichloroethane in our study. Formation of CH2CCl2 as a major product from CH3CCl3 increased with increasing temperature to a maximum near 600°C at 1.0 sec residence time, and was independent of O2/CH3CCl3 reactant ratio from 0 to 9. It then drops quickly with increasing temperature and increased O2 partial pressure. Faster decay of compounds, such as C2H3CI, C2H2, C2H4 and C2HCl which were formed at lower temperatures, occurred when the reactor temperature was increased above 650°C, and higher oxygen levels were present in the mixture. At higher ratios of O2 to CH4, it was observed that lower temperatures were needed to form CO and CO2. The major products at temperature above 750°C are HCl, C2HCl, and non-chlorinated hydrocarbons C2H2, C2H4, CO and CO2.
Increasing the surface to volume ratio of the quartz reactor accelerated the decomposition of the reactants, but had no effect on distribution of major products.
A detailed kinetic reaction mechanism was developed and used to model the experimental results. The mechanism consists of 339 elementary reactions with both forward and reverse rate constants based on thermochemical principles. A sensitivity analysis of the model was done to show the most important reactions in the mechanism.
Rate constants obtained for initially important decomposition of 1,1,1-trichioroethane over the temperature range 500 -1000°C are:
A(sec-1) Ea (kcal/mole)
CH3CCl3 --> CH2CCl2 + HCl 3.35E+ 13 51.2
CH3CCl3 --> CH3CCl2 + Cl 4.59E+ 14 66.6
Transition state theory was utilized for isomerization reactions and Quantum Kassel theory was used to account for fall-off in unimolecular dissociation reaction and in reactions of adducts formed from addition, combination or insertion reactions.
Wu, Yo-Ping, "Thermal desorption and decomposition of hazardous organic compound I: Mass transfer of benzene and chlorobenzene in soil matrices II: Oxidation and pyrolysis of 1,1,1-trichloroethane in methane/oxygen/argon" (1992). Dissertations. 1165.