Communication to the Editor

Comments on the Paper “Gas-Phase Pyrolysis of 2,2,3,3-Tetramethylbutane Using a Wall-less Reactor”

HUBERT DAVIS I ‘nirm (’arbidc Corporation. Chemicals and Plastics, South Charleston,

West Virginia 25503

The article on the above subject by Taylor and Milazzo [ 1) reports an energy of only 42,400 cal/mol versus about 69,000 reported by Tsang [2]. The discrepancy is blamed on the use of toluene in Tsang’s experiments. Still 42,400 seems a rather low value compared with the heat of breaking the central carbon-carbon bond in tetramethylbutane (the initiation re- action). Using heat of formation and heat capacity data given by Benson [: ( I , 1 estimate this to be about 62,000 cal/mol in the temperature range studied. Perhaps there are other possible explanations.

We recently discussed one such possibility in connection with the py- rolysis of propane, n-butane, and isobutane. When rates of highly prod- uct-inhibited reactions are compared a t variable temperatures, and con- stant reaction times, inhibition is higher a t the higher temperatures (more product), and E,,, comes out low (typically about 46,000 cal/mol for the large number of paraffins we looked at) . Study the reaction a t constant decomposition and variable temperatures, and much higher activation energies-comparable to the zero decomposition value -are obtained. In the case we studied the ratio E,,t (const. decomposition)lE,,, (const. time) was 1.68. It was constant enough for us to derive algebraically a useful kinetic expression. The effect appears to be independent of concentration f i r product-inhibited first-order pyrolyses. The concentration of the chain carrier, H or CH:,, is not a function of concentration, and inhibition depends only on the ratio of inhibiting products to substrate, not on the absolute concentration. Thus the finding may apply to Taylor and Milazzo’s data, since their comparison was made a t essentially constant reaction time.

More importantly, whether my proposed explanation of Taylor and Milazzo’s case fits or not, I suggest that any values of Arrhenius parameters derived for complex overall reactions a t moderate to high conversions should be surrounded with conditions and viewed with skepticism. A very current case in point may be found in the most recent issue of an industrial engineering quarterly [5]. The authors deduce the equation for propane pyrolysis

International .Journal of Chemical Kinetics, Vol. XI, 1131-1132 (1979) c 1979 John Wiley & Sons, Inc. 0538-8066/79/0011-11:31$01.00

h, = 2.63 X 1014 exp(-58.6/RT)

They generally dropped the reaction time as they raised the temperature, hu t not enough to keep decomposition constant.

From our experience the authors could have found anything from <46 kcal to >80 kcal, depending on how they took and interpreted their data. A would fall in line accordingly. Such a range of activation energies for propane pyrolysis is actually found in the literature.

Bibliography

I I I J. R. Taylor and '1'. S. Milazzo, In ( . J . Chrm. Kiriet., 10, 1245 (1978).

1:11 S. W. Henson, 'l'hc~rmochc~rnical Kinetics. (Wiley, New York, 1968). 1.11 H. (;. Davis and K. 1). Williamson, Preprints, Div. of Pet. Chem. Inc, Am. Chem. Soc.,

Mar. 1978. Advnnccs in ('hemistry ser., Thermal Hydrocarbon Chrmistry. A. G. Oblad, I<. 7'. Kddinger, and H. G. Davis, Eds., in press.

121 W. 'rsang, J . rhrm. f'hys.. 44,4283 (1966).

151 S. K . 1,ayokon and 1). H. Slater, Ind. Eng. Chem. Process. DPL,., 18,232 (1979).

Received February 26,1979 Accepted April 37,1979