Thermochemical and Chemical Kinetic Data
for Fluorinated Hydrocarbons
D.R.F Burgess, Jr., M.R. Zachariah, W. Tsang
Chemical Science and Technology Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899-0001
and
P.R. Westmoreland
Department of Chemical Engineering
University of Massachusetts
Amherst, MA 01003-3110
ABSTRACT
A comprehensive, detailed chemical kinetic mechanism was developed and is presented for C1 and C2 fluorinated hydrocarbon destruction and flame suppression. Existing fluorinated hydrocarbon thermochemistry and kinetics were compiled from the literature and evaluated. For species where no or incomplete thermochemistry was available, these data were calculated through application of ab initio molecular orbital theory. Group additivity values were determined consistent with experimental and ab initio data. For reactions where no or limited kinetics was available, these data were estimated by analogy to hydrocarbon reactions, by using empirical relationships from other fluorinated hydrocarbon reactions, by ab initio transition state calculations, and by application of RRKM and QRRK methods. The chemistry was modeled considering different transport conditions (plug flow, premixed flame, opposed flow diffusion flame) and using different fuels (methane, ethylene), equivalence ratios, agents (fluoromethanes, fluoroethanes) and agent concentrations. This report provides a compilation and analysis of the thermochemical and chemical kinetic data used in this work.
5. Reaction Set
The reaction set listing (Table 5) is divided into sets of similar reaction types (e.g., Fluoromethanes: thermally and chemically activated decompositions, Fluoromethanes: atom abstraction and metathesis, Fluoromethyls: oxidation, etc.). For each reaction, the reaction number and reaction are given and followed by the Arrhenius parameters. The listing is essentially a CHEMKIN II reaction input file (Kee et al., 1989).
The reactions are numbered according to the following scheme.
HO-xx Hydrogen/Oxygen Chemistry
HC-xx Hydrocarbon Chemistry
HF-xx Hydrogen/Oxygen/Fluorine Chemistry
MD-xx Fluoromethanes: Thermal and Activated Decompositions
MA-xx Fluoromethanes: Abstractions
NN-xx Fluoromethyl, Fluoromethylene, Fluoromethylidne Chemistry
PP-xx Carbonyl Fluoride and Fluoromethoxy Chemistry
ED-xx Fluoroethanes: Thermal and Activated Decompositions
EC-xx Hot Fluoroethanes & Fluoroethyls: Fluoromethylene Reactions
EA-xx Fluoroethanes: Abstractions by X (H, O, OH)
ER-xx Fluoroethanes: Abstractions by R (CxHy)
GG-xx Fluoroethyl Chemistry
JD-xx Fluoroethylenes: Thermal and Activated Decompositions
JA-xx Fluoroethylenes: Additions and Abstractions
JO-xx Fluoroethylenes & Fluorovinyls: Oxidations
KK-xx Fluoroethyne & Fluoroketene Chemistry
CF-xx H Atom Abstractions by F
The symbol "=" in the reaction indicates a reversible reaction and the symbol "=>" indicates an irreversible reaction. For reference purposes, the heat of reaction (in kcal/mol) is also given for a number of the reactions (but not all). In addition, a notation and references are given to provide traceability on each rate expression. A detailed legend for the notation given for each reaction is at the end of Table 5. For example in the listing, CH3F + H = CH2F + H2 (reaction MA-13) has Arrhenius parameters A=2.70E03, b=3.00, E/R=2667., where the rate expression is k = A*Tb*exp(-E/RT). The units are A1=mol/s, A2=mol/cm3/s, A3=mol/cm6/s (for first, second, and third order reactions, respectively), T=K, E=kJ/mol, and R=8.314 J/mol/K or 1.987 cal/mol/K). Please note 1 cal = 4.184 J (for conversion from SI units). For this reaction example, the notation and references "xf", "75WES/DEH", and "nist" indicate that the rate expression is our fit to the experimental data of Westenberg and deHaas (1975).
A number of the unimolecular reactions have rate expressions with third-body efficiencies and/or low pressure fall-off parameters. For example, H + O2 => HO2 + M (reaction HO-13) has explicit third-body efficiencies for M = H2O, CO2, H2, CO, and N2. An example, of a rate expression with low pressure fall-off parameters is the reaction CH3 + CH3 => C2H6 + M (reaction HC-16), where "LOW" and "TROE" are low pressure and Troe fall-off parameters. The reader is referred to Kee et al. (1989) for more details on third-body efficiencies and fall-off parameters.
For reference purposes at the end of Table 5, the experimental rate expression (A=1.80E13, b=0.00, E/R=4803.) and the temperature range (T=600-900K) from this work (75WES/DEH) are also given. In some cases, where rate expressions were estimated relative to a reference reaction, the A-factor or activation energy were adjusted. For example in the listing, the reaction CH3F + C2H3 = CH2F + C2H4 (reaction MA-20) has the notations "r CH3" and "E*0.9". These indicate that an activation energy was used that was 90% of the activation energy for the analogous abstraction by CH3, a reaction that is slightly more exothermic.
In order to reduce (slightly) the number of species in the reaction set, the isomers CHF=CHF(E), CHF=CH(E), and CHF=CF(E) were excluded (retaining the Z isomers). The differences in energies and chemistries are sufficiently small that this is justified. We note that a number of the fluoroethanes and fluoroethyl radicals have both trans and gauche forms. In this work, we used thermochemistry for the most stable conformational isomers.
The hydrogen/oxygen and hydrocarbon reaction subsets of the mechanism are derived from the Miller-Bowman mechanism (Miller and Bowman, 1989) and consists of about 30 species and 140 reactions. Any other hydrocarbon mechanism could be used instead. For example, the GRIMECH set (Bowman et al., 1995) is a recent hydrocarbon mechanism that accurately reproduces flame speeds for methane mixtures.
In this work, some modifications to the Miller-Bowman mechanism were made. All nitrogen-containing species and reactions were removed. A number of the rich species (e.g., C2H, C4H2) were eliminated from the mechanism in order to keep the number of species in the mechanism to a manageable level. A number of species (e.g., CH3OH) were also added to the mechanism. In addition to these addition and deletions, a number of rate constants for a number of reactions (e.g., CH3+OH) were adjusted to provide correct fall off and product channel ratios. In this section of the reaction set, the notation for the reference is slightly different. For example, for CH4 + H = CH3 + H2 (reaction HC-1), the notation "73CLA/DOV MBA004" means that this rate expression was determined by Clark and Dove (1973) and was reaction #4 in table A of the Miller-Bowman mechanism (Miller and Bowman, 1989). Where only the Miller-Bowman reference is given, either the expression is directly attributable to that work or the origins/traceability of the expression is not clear.
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