lumped kinetic scheme for the liquid-phase oxidation of p-xylene to terephthalic acid catalyzed by cobalt naphthenate is developed. It is assumed that all the reactions are. zeroth-and first-order with respect to gaseous and liquid reactants, respectively. The reliability of the developed kinetic model is investigated by comparison with experimental data obtained in a semibatch oxidation reactor where the gas phase and the liquid phase are well mixed. The experiments included different values of the initial concentration of the liquid reactants, two gaseous reactants (i.e. pure oxygen and air) and temperature values in the range IO&130°C. INTRODUCTION Liquid-phase catalytic oxidation of organic compounds by molecular oxygen, which can be either homolytic or heterolytic depending upon the mechanism of oxygen activation, plays an important role for a wide variety of industrial processes. In particular, homolytic oxidation processes (e.g. cyclohexanone and cyclohexanol from cyclohexane, benzoic acid form toluene, terephthalic acid from p-xylene, etc.) occur through chain elementary reactions, which involve a very large number of radical as well as molecular species (Sheldon and Kochi, 1981). Accordingly, we need to consider a suitably complex reaction scheme when developing a kinetic model to reproduce the overall rate of oxygen consumption and the distribution of the various intermediate products, such as alcohols, aldehydes, ketones and carboxylic acids. Indeed, in simulating the behavior of gas-liquid reactors, detailed kinetic schemes which describe all the involved elementary reactions and all the intermediate species, cannot be used for at least two basic reasons.
The first is related to the reliable estimate of the kinetic parameters of all the involved elementary reactions. This requires specific experimental studies performed in a semibatch reactor where the evolution of all participating species, including highly reactive radicals, is monitored as a function of time. On the other hand, theoretical methods for estimating elementary reaction rates in the gas phase are not reliable for reactions in condensed phases. To over-*To whom correspondence should be addressed.