Activation energy variation with pressure

Fig. 13. Plot of variations of activation energy ( /kJ mole"1) with water vapour pressure (PHjO/Torr) for dehydration of calcium sulphate. Data from Ball et al. [281,590, 591] who discuss the significance of these kinetic parameters. Dehydrations of CaS04 2 H2O, nucleation ( ), boundary (o) and diffusion (e) control Q-CaSC>4 5 H2O, diffusion control, below (X) and above (+) 415 K j3-CaS04 5 H20, diffusion control ( ).

Thus, the elementary cellular structure could be regarded as an intrinsic characteristic of fhe detonation in a mixture at given initial composition, temperature, and pressure. The dimension of X is of fhe order of magnitude of millimeters or less for gaseous mixfures with oxygen, but several centimeters for less sensitive mixtures (even larger, for methane/air af afmospheric pressure). It decreases when the initial pressure increases. Its variation with the initial temperature is more complicated and depends on the value of fhe reduced activation energy of fhe chemical reactions. The value of... 

The Mallard-Le Chatelier development for the laminar flame speed permits one to determine the general trends with pressure and temperature. When an overall rate expression is used to approximate real hydrocarbon oxidation kinetics experimental results, the activation energy of the overall process is found to be quite high—of the order of 160kJ/mol. Thus, the exponential in the flame speed equation is quite sensitive to variations in the flame temperature. This sensitivity is the dominant temperature effect on flame speed. There is also, of course, an effect of temperature on the diffusivity generally, the dif-fusivity is considered to vary with the temperature to the 1.75 power. 

The experimental data of Kowalsky on 2H2 + O2 mixtures have been analyzed by Semenov [59] on the basis of reactions (i)—(v), together with surface destruction of H atoms. Because of the lower activation energies of reactions (i) and (hi) compared with reaction (ii), the concentrations of OH and O were assumed to be small compared with H. The variation of H atom concentration could thus be deduced by the method of partial stationary state concentrations [60], giving the net branching factor 0 at pressure p as... 

The variation of R with total pressure for the standard mixture at 500 °C is given approximately by f ocP . Addition of nitrogen causes an increase in rate, but the increase is substantially less than with salt coated vessels, e.g. 200 % addition of Ni only increases the rate by about 50 %. The effect is least marked at low x. Over the temperature range 470—540 °C, the log R versus 1/T plot is closely linear, and gives an activation energy of 55.8 0.7 kcal. mole . All these properties contrast sharply with the behaviour in porcelain and salt coated vessels described earlier, with which, for example, the activation energy is 100 kcal. mole or greater. 

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

---