The Green Group

Professor Green's research group focuses on the central problem of reactive chemical engineering: quantitatively predicting the time evolution of chemical mixtures. Accurate chemical kinetic models are extremely powerful and valuable, since they allow predictions about the impact of modifying a system; already many significant public policy and business decisions are made on the basis of kinetic model predictions. For example, the Montreal Protocol, which imposed a worldwide ban on certain halocarbons, was based on a kinetic model of the ozone layer.

For most technologically important systems, including combustion, pyrolysis, and atmospheric oxidation of organic compounds, it is very difficult to construct a reliable kinetic model. There are typically hundreds of reaction intermediates, and only a small fraction of the rate parameters are known experimentally. It is usually impossible to measure the concentrations of all the kinetically significant chemical species. Until recently, it was not feasible to numerically solve the large systems of differential equations that describe these systems, even if one did manage to measure or obtain good estimates of all the rate parameters. However, advances in computational chemistry, numerical methods, and computer hardware have now made it possible to construct and solve kinetic models capable of reliably modeling these complicated system. We are now beginning to extend these techniques to model spatially inhomogeneous reacting flows.

State-of-the art quantum chemistry techniques are used to estimate thermochemistry, barriers and transition state properties for specific reactions; these are then coupled with transition state theory, experimental data, and functional group additivity ideas to provide rate estimates for broad classes of reactions. Special treatment is required for reactions in solvents, on surfaces, and for fast gas-phase reactions (which do not thermalize). The computer is used to generate and solve the large reaction scheme implied by these rate estimates. The results are then validated by comparison with experiment, and predictions can be made for situations where no experimental data are available. In one recent case, the computer considered the impact of more than 100,000 reactions before selecting the few hundred kinetically dominant reactions needed to develop a numerically accurate kinetic scheme for an ethane steam-cracker.

The model predictions are tested against experimental data from the literature, and measurements made in our laboratory or by our collaborators. The group is currently using laser techniques to probe free radicals in the gas-phase and in solution, and a variety of analytical techniques to measure product yields in liquid phase and catalytic oxidations.

Uncertainties enter all along the process of constructing and experimentally validating a kinetic model, from the microscopic electronic structure calculations through the macroscopic measurements that are typically made on a complicated product stream. We are quantifying these uncertainties and developing improved methods for the steps, which introduce the largest errors. For example, the group is currently developing more accurate density functionals for quantum calculations, and we are examining methods for minimizing the number of chemical species, which must be treated exactly in a simulation.

The Green Group is developing this simulation technology to solve practical problems related to the atmospheric chemistry of organic pollutants, the conversion of natural gas to liquid fuels, the oxidation of organics in the gas and liquid phase, the removal of sulfur contaminants from gasoline, and the formation of carcinogenic pollutants in combustion.