Decomposition of Propanal at Elevated Temperatures. Experimental and Modeling Study

Article abstract of DOI:10.1021/j100370a043

The thermal decomposition of propanal (CH3CH2CHO) highly diluted in argon was studied behind reflected shocks in a pressurized-driver single-pulse shock tube over the temperature range 970-1300 K and overall densities of ca. 2.5 * 1E-5 mol/cm³.Under these conditions the following decomposition products were obtained (in order of decreasing abundance): CO, C2H4, CH4, C2H6, C3H8, C3H6, C4H6, and C2H2.Mixtures containing 0.1percent propanal and 1percent toluene in argon showed a factor of ca. 3 decrease in the production rate of ethylene, indicating the involvement of a free-radical mechanism.Two dissociation reactions initiate the decomposition of propanal: (1) C2H5CHO -> C2H5 radical + CHO radical and (2) C2H5CHO -> CH3 radical + CH2CHO radical.It then proceeds via a free-radical chain mechanism where the most important reactions are C2H5CHO + R radical -> CH3CH2CO radical + RH, CH3CH2CO radical -> C2H5 radical + CO, and CH3CH2CO radical -> CH3 radical + CH2CO.The overall decomposition of propanal was computer modeled with a reaction scheme containing 22 species and 52 elementary reactions.A sensitivity analysis showed that the system is very sensitive to the two initiation reactions and to the two-channel dissociation of the radical CH3CH2CO.The concentration ratio / is highly dependent on k(C2H5 + CO)/k(CH3 + CH2=CO).A ratio of ca. 100 in the rate constants accounts for a ratio of ca. 30 in the concentrations of ethylene and methane.There are four major free radicals in the system, C2H5, CH3, and H, and HCO.The modeling studies show that they reach steady-state concentrations after several hundred microseconds.The rate constant for the overall pyrolysis of propanal over the temperatureand pressure ranges covered in this investigation is k_total = 1E16.39 exp(-73 * 1E3/RT) s^-1, where R is expressed in units of cal/(K mol).