Experiments and model development are performed to understand fundamental reaction processes of oxygenated hydrocarbons: alcohols, ethers and esters important to combustion and gasoline octane blending. Detailed mechanisms are developed to allow optimization and trend prediction in engine performance and emission reduction.
Oxygenates, such as dimethyl ether, methanol and ethanol, are considered for use as additives in required oxygen containing fuels in many States. They also may serve as octane additives or alternative motor fuels. Methyl tertbutyl ether (MTBE), is widely used as an anti-knock component and oxygenate additive in gasolines. Experimental data on effects of operation parameters from fuels using oxygenates are needed to test and validate models. A model based on fundamental principles and tested against available experimental data should allow calculation of trends toward optimal fuel blends, performance, and emission characteristics for further testing and optimization.
Experimental: Gas mixtures are reacted in a variable pressure, high temperature tubular flow reactor. Reactor effluent is analyzed for products as a function of temperature, residence time, and fuel equivalence ratio. Analysis is performed with on-line gas chromatography (GC), flame ionization detection (FID), Fourier Transform Infrared (FTIR) and GC / Mass Spectrometry.
Modeling: Reaction mechanisms are built from elementary reactions with rate constants based upon fundamental principles of thermochemical kinetics, statistical mechanics and transition state theory. Quantum Rice-Ramsperger-Kassel theory for k(E), modified strong collision treatment for fall-off and thermodynamic properties are used in chemical activation and unimolecular reactions for rate constants. Ab initio and density functional calculations at the G2, CBS-Q, CBS-q and QCISD(t)/6-311+g(d,p)//MP2/6-31+G(d,p) or //B3LYP/6-31+G(d,p) levels of theory are used to determine thermodynamic properties and transition state structures and energies.
Relatively high level calculation theory (see above) for determining thermodynamic properties of molecules and transition states is implemented. Calculations now use ab initio and density functional theory in place of semi-empirical theory. A Number of complex reaction systems for hydrocarbon radical oxidation have been studies at CBS-Q, CBS-q and G2 levels of theory. These include methyl-tertbutyl, dimethyl ether, tertbutyl, isobutenyl and isobutyl radical reactions with oxygen.
Oxidation and pyrolysis experiments on MTBE oxidation from one to ten atmospheres pressure, fuel equivalence ratios 0.7 to 1.5 are complete. Experiments on dimethyl ether oxidation at varied fuel equivalence ratios, 1 atm pressure are complete. A thermodynamic database and a pressure dependent elementary reaction mechanism has been assembled for MTBE oxidation and evaluated against experimental data. Sub-models for oxidation of neopentane, isobutane, and isobutene oxidation are developed and model data compare well with experiment. A pressure dependent reaction mechanism for oxidation of dimethyl ether is developed and tested on flow reactor experiments.
Experiments are complete. The mechanisms are first generation and are partially validated. They are undergoing further testing and development.
Calculation capabilities for thermodynamic and kinetic properties are now higher level than proposed.
Chiung-ju Chen, Takahiro Yamada, Chad Sheng
John Seinfeld, Caltech; Jack Howard, MIT