Kinetic Mechanism Reduction for Chemical Process Hazard Application

Despite considerable knowledge of the potential hazards associated with chemical process industries, explosion hazards continue to occur during hydrocarbon processing under partial oxidation conditions. Among the reasons is the change of operations that arise from process intensification, combined with an incomplete knowledge of the combustion characteristics of the processed materials. The ability to couple chemical kinetics with fluid dynamics and simulate these processes in reactive multi-dimensional flows would be a powerful process engineering tool that would constitute a significant advance in methodologies available to predict such hazards. Detailed combustion kinetic mechanisms for hydrocarbon oxidation may contain hundreds of chemical species and thousands of reactions, making them too computationally expensive to be solved in computational fluid dynamic (CFD) codes. By adopting formal mathematical procedures, more compact and computationally efficient models can be achieved by reducing the numbers of species and reactions from the detailed mechanisms, thus making the incorporation of chemical kinetic effects into CFD possible. In addition, reduced reaction mechanisms can be used to gain kinetic understanding and elucidate the effect of poorly known reaction rate parameters and thermochemistry. Currently, mechanism reduction can be achieved by running full models with multiple initial conditions in a non CFD-based environment, interpreting the results using local sensitivity methods, identifying and removing redundant species and reactions,and then testing the reduced mechanisms. Many hours can also be saved by automating these tasks using programming techniques. In this work we present automatic methods for removing species and reactions from comprehensive reaction mechanisms without significant detriment to model performance. The software has been applied to a range of chemical mechanisms covering different fuels, including propane, n-butane and cyclohexane which were generated using the EXGAS program. Reduced chemical models which can be used in higher dimensional simulations are obtained as output. A method for the automatic construction of closed vessel ignition diagrams is presented, with such diagrams used to evaluate the comprehensive and reduced models. The benchmark is set by the performance of the full scheme and the criteria for performance of the reduced models are matched to this. Kinetic investigations have been carried out using the reduced mechanism. Global uncertainty analysis methods were applied within the SAFEKINEX project to the reduced propane mechanism to establish the main contributors to uncertainties in output predictions. Results from the application of a Monte Carlo uncertainty analysis are reported in comparison with analysis of the main element fluxes within the scheme during oxidation. The kinetic foundation of the reduced n-butane mechanism was then analysed by formal numerical methods to trace the origins of the dramatic shifts in autoignition temperature as conditions are changed. One of the key factors in the chemistry that promotes autoignition at low temperatures (T < 700 K) is the transition from a cool flame to a second stage, within a two-stage ignition, driven by the formation and decomposition of hydrogen peroxide. A quantitative investigation into the kinetic origins of hydrogen peroxide in the transition stage has been performed. The quasi steady state approximation (QSSA) combined with reaction lumping was applied to the reduced cyclohexane mechanism to further eliminate a number of intermediate species whilst incurring little error to output ignition delay predictions. The QSS species are fast reacting species which locally equilibrate with respect to the slower species in the system. Thresholds were applied to the calculated instantaneous QSSA error for each species over all considered time points, thus providing an automatic way of identifying QSS species. Species were eliminated by the formulation of lumped reactions with corresponding lumped reaction rates. With the aid of element flux visualization software, analysis of the fluxes of carbon atoms during isothermal oxidation has been undertaken using the reduced cyclohexane mechanisms. This shows the major reaction pathways of fuel to oxygenated cyclic compounds, the breakage of the ring to form aldohydroperoxides and ketohydroperoxides and their subsequent decomposition.