| We analyzed the traditional practice of lumping (species aggregation into pseudo-species) in terms of "early" and "late" strategies. Historical limitations in analytical chemistry forced reaction modeling of complex feedstocks (e.g., petroleum fractions) to be in terms of distillation ad solubility classes. Such "early" lumping obscured the fundamental reaction pathways and kinetics that are the key to process and product improvements. Recently, new questions of unprecedented molecular detail, such as the need for environmentally acceptable products with high performance properties, have provided the impetus for reaction modeling at a molecular level. This modeling approach retains and utilities complete molecular detail in reactants, reactions and products. In contrast to that in "early" lumped models, the lumping in molecular level models is performed "late" to access yields and properties of commercially relevant global product fractions.; The availability of molecular structures allows fundamental pyrolysis chemistry to be brought to bear in the molecular level reaction model. The Rice-Herzfeld formalism was utilized to simulate, a-priori, thermal reactions at an elementary step level. The concept of Linear-Free-Energy-Relationships was invoked to organize hundreds of elementary step rate parameters in terms of only four "Quantitative-Structure-Reactivity-Relationships" (QSRR's). Model compound information, from the literature and our own experiments, was used to parametrize the QSRR's. The use of QSRR's in the reaction model eliminated the need for hundreds of elementary step parameters; instead, we used only O(10) chemically significant parameters.; The computational demand of mechanistic simulations, however, necessitated many "early" approximations in the pyrolysis rate laws. Specifically, we deduced semi-empirical rate laws (SERL's) from detailed mechanistic rate laws. The mechanism derived SERL's were fashioned after Langmuir-Hinshelwood-Hougen-Watson rate laws in catalysis, and were phrased in terms of simple pure component, initiation, propagation and termination groups. The molecular level model incorporated SERL's to account for kinetic coupling interactions in complex hydrocarbon mixtures.; The molecular level reaction model provided the structural identity of thousands of molecules. Using thermodynamic lumping techniques, this information was phrased in terms of yields and properties of global solubility and distillation lumps. This step allowed a direct comparison of the predictions of the molecular model with the experimental data and, thus, provided a "late" lumping prediction to be compared against the "early" lumping predictions. Group contribution "structure-property" correlations were interfaced with the reaction model to address product quality indices (e.g., viscosity, water solubility, vapor pressure, etc.). Accomplishment of these goals makes product engineering a realistic target. |
