Kinetic rate data are needed to design all chemical processes. Specifically, they are needed to determine such things as ignition delay times, combustion times, and explosion limits in engine combustors. Previously, chemical kinetic rate data have been determined experimentally by use of reactors at the desired operating conditions. However, such data are often difficult to obtain.
A scheme for relating chemical kinetic rates to thermodynamic-property data has been developed. The scheme is based on the discovery that the rate, r, of a chemical reaction is related to the Gibbs gradient with respect to the extent of conversion, X, by
where the single constant D is used for all reactions, R is the universal gas constant, T is the absolute temperature, and G is the Gibbs energy. Using this equation, the chemical rate constants for each reaction can be related to the equilibrium constant for that reaction, and the net chemical rate can be predicted.
Ordinarily, energy levels from the thermodynamic data are used to calculate equilibrium compositions and not dynamic rates. The equilibrium condition for a given reaction is a balance between the forward and reverse rates for that reaction. When the system of reacting species is not in equilibrium, G is at a value higher than its minimum, and the rate is related to the difference between the actual and minimum values.
Data on the heat capacities, standard-state enthalpies, and Gibbs energies under equilibrium conditions were previously calculated for 1,130 chemical species and tabulated. These data can now be used for each chemical-reaction step to determine its rate. Using the present method, kinetic rates that have not yet been determined can be computed, enabling prediction of alternative chemical processes.
For comparison of experimental data with predictions by the present thermodynamic method, the Lewis General Chemical Kinetics and Sensitivity Analysis (LSENS) computer code was modified to incorporate the present method. The method was tested on the H/Br system with excellent results. In a similar test on the H/O system, the deviation between the predictions and the experimental data was greater for the present method than for the classical method. However, the disadvantage of the greater deviation in the present method may be offset by the advantage that one can use only the single constant D for all reactions, whereas in the classical method, one must use at least two constants (the preexponential factor and the activation energy) for each reaction to describe a given chemical system.
It will be necessary to demonstrate the present method on other chemical systems. One difficulty lies in the inclusion of small quantities of intermediate species, the properties of which may not yet have been tabulated. However, thermodynamic properties can be accurately estimated, given molecular structures. More work is being done to test the limits of the present thermodynamic method for kinetics. When completely verified, this method will have a widespread effect on the chemical industry.
This work was done by C. John Marek of Lewis Research Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 176 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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