Autocatalytic oxidation of cyclohexane—modeling reaction kinetics

Abstract

Liquid phase oxidation of hydrocarbons is a major source of organic chemicals. This paper concerns itself with the oxidation of cyclohexane. The process is one in which gas absorption is accompanied by chemical reaction, but little information has been reported. A major factor has been safety considerations, the Flixborough explosion being a constant reminder. In this paper attention is given to the kinetics and to the interaction of kinetics and mass transfer, showing that in this autocatalytic system the dissolved oxygen level rises to saturation and falls as the rate of reaction increases. Initially zero order in oxygen, the reaction becomes first order at low oxygen levels. Oscillations and enhancement due to reaction within the boundary layer are noted. A kinetic model is developed based on simplification of the accepted free radical scheme and it is reasonably successful in accounting for the results reported. Experiments at temperatures up to 17OOC and 70 bar using uncatalyzed reaction are reported from three different reactors: a homogenous batch reactor, a flat interface reactor, and a sparged agitated bubbling reactor. A. K. Suresh, T. Sridhar, 0. E. PotterDepartment of Chemical Engineering Monash University Clayton, Victoria 3168, Australia lntroduc tionThe liquid phase oxidation of hydrocarbons is a major source of organic chemicals in the chemical industry. Most of the processes of industrial interest are carried out at pressures of 10-30 bar and temperatures of 353-523 K. In terms of the kinetic mechanisms that operate in liquid phase hydrocarbon oxidations, the liquid phase oxidation of cyclohexane is a typical example of this class of reactions. It is also a reaction of considerable industrial importance. The oxidation of cyclohexane supplies much of the raw materials needed for the production of nylon (both nylon-6 and nylon-6,6)-cyclohexanol, cyclohexanone, and adipic acid. Selective production of cyclohexanol and cyclohexanone requires the use of a low-conversion process with muliple stages. A number of catalysts for the reaction have been reported. Cyclohexanol and cyclohexanone are usually subsequently oxidized with nitric acid to adipic acid.The kinetic mechanisms that operate in hydrocarbon oxidations in general and cyclohexane oxidation in particular have been quite extensively studied, especially by Russian kineticists (Berezin et al., 1966;Emanuel et al., 1967). However, these mechanistic studies do not lead to rate expressions with con-A.K. Suresh is presently with Hindusthan Lever Research Center, Bombay, India.stants that can readily be used in the design of industrial reactors. Bearing in mind the way industrial processes operate, these reactions should be studied as gas-liquid reactions in which both mass transfer and reaction kinetics have a role to play. Few studies in the literature have accomplished this. The bulk of available literature (mainly patent literature) discusses the effect on product distribution of operating variables such as temperature, pressure, catalyst, and con...

Kinetic Modeling of Liquid Phase Oxidation of Cyclohexane

“…In Figures 2 to 4 these predicted values were compared with the corresponding observed data in the kinetic experiments at three temperatures conducted by Suresh et al (1988b). It would appear that the model presented herein that uses the general free radical mechanistic scheme could reproduce the basic trends of the observed data fairly well.…”

“…The predicted results have been compared with the experimental data presented by Suresh et al (1988b).…”

Kinetic modeling of liquid phase autoxidation of cumene

“…Suresh et al [16] have examined the kinetics of the uncatalyzed LPO of cyclohexane, conducted in a stainless steel semi-batch reactor (with continuous flow of diluted air). In order to interpret the apparent autocatalytic behaviour observed in their experimental data on the rate of oxygen absorption and dissolved oxygen concentration, they proposed a closed form kinetic model based on simplification of the classical free-radical scheme by lumping all manner of products into one.…”

Synthesis of heterogeneous copper complex catalyst for oxidation of cyclohexane using molecular oxygen

“…• C temperature), and low activity (Suresh et al, 1988a). An induction time, generally observed in the case of air oxidation of cyclohexane, is reduced by adding promoters or co-reactants such as acetaldehyde, cyclohexanone, cyclohexanol, and azobis (isobutyronitrile; AIBN; Wen et al, 1997).…”

“…The mechanisms (see Figure 1) suggested in the literature assume that cyclohexyl hydroperoxide (CHHP) is the intermediate formed in the presence of transition metal salts (Suresh et al, 1988b;Tolman et al, 1989;Wen et al, 1997) and is a multistage, free radical chain reaction, comprising of initiation, chain propagation, and chain termination step. Tolman et al (1989), Spielman (1964), and Alagy et al (1974) developed a reaction scheme consisting of 154 reactions which is impractical to analyze as it requires the determination of as many number of rate constants simultaneously with high accuracy.…”