Kinetics of the gas-phase photochlorination of dichloromethane in a tubular photoreactor.

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The gas-phase photochlorination of dichloromethane was conducted at a low extent of conversion from 9 °C to 100 °C in a tubular flow reactor irradiated by means of a circular array of low-pressure mercury lamps. The kinetics were studied with due consideration taken of the radial variation in light intensity across the reactor and with the proper selection of kinetic equations, including the recombination of dichloromethyl radicals as the dominant termination step. The dependence of the absorbed radiant energy on the chlorine concentration was well simulated by the use of the radial-light and line-source model. The predominance of the observed production rate of hydrogen chloride over that of chloroform was also reproduced well by the appropriately selected kinetic expressions, without any use of the long-chain approximation. The recombination rate constant of dichloromethyl radicals was determined to be 1013.5 cm3 mol−1 s−1; this is larger than the previously reported value by the factor of 12.6, and is in fair agreement with the theoretical value, 1013.3−13.4, calculated from the statistical factor, and with the collision rate constant based on the hard-sphere model.
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