Modeling industrial ethane steam cracking from first principles

Abstract

Many important processes such as combustion, atmospheric chemistry, polymerization, and steam cracking proceed via a complex gas phase radical reaction mechanism, often involving hundreds of important reactions. Building accurate models and determining the required kinetic and thermodynamic parameters for such processes is challenging. Using state-of-the-art quantum chemical calculations and methods from statistical mechanics, it has become possible to accurately predict kinetic and thermodynamic parameters for well-defined gas phase reactions. These techniques then make it possible to quantitatively model complex radical processes fully from first principle, i.e., without the input of experimental data. To illustrate that the accuracy that can be obtained with ab initio computational chemistry methods has become sufficient to describe a complex, high temperature gas phase radical process, the industrial steam cracking of ethane was modeled. Steam cracking is the most important industrial process for the production of light olefins, which are the building blocks for the chemical, plastic, and pharmaceutical industry. A relatively small reaction network of 150 reversible elementary reactions and 20 species was constructed and all the thermodynamic and kinetic parameters were obtained from ab initio CBS-QB3 and W1U calculations. After accounting for hindered internal rotations, quantum tunneling, variational corrections to transition state theory, and the pressure dependence of the rate coefficients, the predicted C 2H 6, C 2H 4, H 2 and CH 4 yields and reactor outlet temperature are within 10 % of experimental values. This study hence demonstrates the feasibility of constructing a predictive kinetic model for gas phase chemistry fully from first principles.