Mechanistic modeling of autoxidation of ethyl benzene
Siddharth R. Jain a, Kaushik Basak b, R. Vinu c
a Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai – 600036, E-mail: firstname.lastname@example.org
b Shell Technology Center Bangalore (P&T), Shell India Markets Pvt. Ltd., RMZ Centennial Campus B, # 8B Kundalahalli Main Road, Bengaluru 560048, India, E-mail: email@example.com
c Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai – 600036, E-mail: firstname.lastname@example.org
Liquid phase autoxidation of hydrocarbons has industrial importance especially for alkyl benzenes as these form key building blocks for the production of phenol, acetone, polyethylene terephthalate, styrene and propylene oxide, etc. Autoxidation proceeds via free radical mechanism and chemical kinetics developed so far are empirical in nature. Increasing the yield and selectivity today even poses a scientific challenge owing to its complex nature. Oxidation of alkyl benzenes involves autocatalytic chain reactions, including the formation and consumption of alkyl peroxy, alkoxy and alkyl radicals in a series of initiation, propagation and termination reactions. The desired intermediates further react and form undesirable products, consequently drawing a sense of balance in keeping lower conversions and higher selectivity.
A mechanistic/microkinetic model for liquid phase autoxidation shall provide key pathways for the selective production of desired products and intermediates at higher conversions. The kinetic model was developed with heuristic approach with plausible reaction products for liquid phase oxidation of ethyl benzene (EB) at 400 K. Abridged mechanism of key reaction sequences are shown in Figure 1, where short lived alkyl peroxy radicals are formed by addition of molecular oxygen over the alkyl radicals, and the former abstracts hydrogen from the substrate to form the desired products. Mole balances of all the species involved in the process were coupled with batch, semi-batch and stirred tank reactor models to apprehend the details of the microkinetic modeling. The system of differential-algebraic equations was solved to understand the time evolution of intermediates and final products. Importantly, the rates of free radical species were also included (quasi steady state assumption for free radicals used usually was ridiculed) in the study. A majority of the rate coefficients of elementary steps were taken from the literature, while a few were numerically fitted to match the product profiles. The proposed mechanism was then validated with the experimental outcomes from a pressurized reactor vessel. As a part of intensifying the overall process selectivity, a number of catalysts (based on cobalt, nickel, barium and quaternary ammonium salts) were utilized to comprehend the competing reactions from the proposed mechanism to improve the selectivity of desired products like 1-phenyl-ethylhydroperoxide (ethylbenzene hydroperoxide, EBHP), acetophenone (methyl phenyl ketone, MPK), and 1-phenylethanol (methyl phenyl carbinol, MPC). Statistical design of experiments was carried out and the effects of temperature, initiator and passivator concentration at various levels were analyzed. Interesting results from the study shall be discussed in the presentation.
Figure 1. Key steps involved in the autoxidation mechanism of ethyl benzene.