Correction for Finite Quenching Rates in Shock-Induced Methane Pyrolysis

Abstract

N this study, the feasibility of using a simple shock tube, without modifications to enhance the quenching rate, is explored. Such a simple shock tube is used to study the pyrolysis of methane as a step in studying reaction rates of hydrocarbons in, processes involving combustion and formation of air pollutants. The shock tube was operated in the tailored interface mode, and the reaction temperature ranged from 1,650 to 2,100 K, with a total reaction pressure up to 6.4xlO5 Pascal.The chemical reaction occurred behind the reflected shock wave, near the end wall of the test section. A sampling system was developed to allow a sample of product gas to be withdrawn from the test section before it was contaminated by the driver gas. A correction for finite quenching rate was developed and applied in this study as well as an error analysis. Contents The driven section of the shock tube was a rectangularshaped aluminum duct 8.54 m long. The inside dimensions were 7.62 x 15.22 cm. The driver section was a cylindrical tube 1.83 m long with an inside diameter of 17.8 cm. Further details of this shock tube are included in Ref. 1. The rate constants for the methane pyrolysis reaction reported in the literature differ from one another significantly. 2-6 The most recent study6 of this reaction in the shock tube was accomplished using optical methods to follow the progress of the reaction. The rate constant from that study is nearly an order of magnitude smaller than that reported in earlier studies in which the direct sampling of the reaction products was used. To summarize the results known to data on the pyrolysis of methane, it is clear that the reaction follows first-order kinetics with an apparent activation energy of 398±42 x 10 3 J/mol (95 ± 10 kcal/mol) and a frequency factor of 2.5±1 x 1013. There does appear to be some pressure effect on the reaction rate, although there is not complete agreement on the extent. A satisfactory reaction mechanism has yet to be established, particularly at temperatures below 2,100 K. Sampling System: The progress of reactions studies in this investigation followed by extracting from the shock tube a quenched sample of the reaction gas mixture at a certain time and analyzing it by means of a gas chromotograph to determine its composition. The sampling system to accomplish this