Chemical and Biomolecular Engineering - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemUnimolecular Reactions of Resonantly Stabilized Radicals in Combustion( 2021)Despite years of concerted effort, the chemical mechanisms and kinetics detailing PAH formation are yet to be fully characterized. Resonantly Stabilized Radicals (RSRs) are of special consideration among PAH progenitors in flame chemistry due to their relative stability which leads to their build-up to comparatively higher concentrations and their ability to regenerate in hydrocarbon flames makes them available for molecular growth reactions. This thesis has applied fundamental chemical theories viz ab initio calculations and master equation simulations to gain more understanding of selected RSR reactions. Recent experimental data reveals that the resonantly stabilized c-C5H5 and l-C5H5 radicals can fragment to C5H4 + H aside from the traditionally reported C3H3 + C2H2 dissociation pathways. This makes it important to include these new dissociation channels in kinetic models for better flame predictions and to keep the reactions of c-C5H5 and l-C5H5 RSRs up to date with experimental findings, in kinetic models. As such, Chapter 4 of this thesis investigated the C5H5 potential energy surface and performed RRKM/Master Equation simulations to obtain kinetic data for the C5H4 + H reactions. It was observed that the C3H3 + C2H2 is the dominant fragmentation pathway with the C5H4 + H becoming important at temperatures between 900 – 2000 K. The developed rate parameters, coupled with recent literature kinetic data for relevant reactions, were used to update a kinetic model for toluene flame. The predicted results, especially for the resonantly stabilized cyclopentadienyl (c-C5H5) radical, were in good agreement with experimental data obtained from literature, for a low-pressure toluene premixed flame. The developed and presented detailed kinetic model will aid the development and inclusion of 1-vinylpropargyl (l-C5H5) and other C5Hx reactions in combustion studies. Chapter 5 of this thesis proposes a pathway to the formation of a recently detected C7H7 isomer, 3-ethynylcyclopentenyl (3ecpr), through the l-C5H5 + C2H2 reaction, using quantum chemistry techniques and RRKM master equation modeling. The calculated ionization energy for the formation of the first triplet state is in the range of 9.1 – 9.3 eV, which matches the appearance energy of a mystery C7H7 isomer seen in VUV photoionization experiments on the C3H3 + C2H2 reaction cascade. The low appearance energy for the ground state singlet cation (6.9 eV), and sharp photoionization onset apparent in Franck-Condon simulations, explain the absence of this state in these experiments. The C3H3 + C2H2 reaction produces the l-C5H5 isomer in experiments at 800 K and 8 Torr, and the master equation kinetic modeling done in this thesis illustrates that under these conditions the further addition of acetylene to l-C5H5 produces stabilized 3ecpr as a dominant reaction product. Simulations carried out at combustion relevant temperatures and pressures demonstrate that the l-C5H5 + C2H2 reaction will produce both 3ecpr and ethynylcyclopentadiene + H. The present work now allows for further development of l-C5H5 and C7H7 chemistries and their adequate characterization in detailed chemical kinetic models. The oxidation and molecular growth reactions of a larger resonantly stabilized radical (RSR), alpha-styryl (C6H5CCH2), were studied in Chapters 6 and 7 respectively. Regarding the addition of O2 to C6H5CCH2 reaction, it was predicted to proceed to multiple product channels (e.g., benzoyl + CH2CO, phenacyl radical + O, benzyl + CO2, benzoxyl radical + CO, benzaldehyde + HCO, isobenzofuranone + H) and it is pressure-dependent at temperatures below around 1300 K. This is contrary to what is obtainable in current kinetic models where the reaction is represented by an estimated high-pressure rate coefficient with a single product channel, benzoyl + CH2CO. Although the reaction population is dominated by benzoyl + CH2CO, phenacyl radical + O is predicted to be the main product at T > ~2650 K. The decomposition reactions of selected products of the C6H5CCH2 + O2 was also studied to provide kinetic data for their reactions in combustion studies. Phenacyl was found to predominantly dissociate to benzyl + CO while isobenzofuranone and benzofuranone have benzaldehyde and quinone methide has main decomposition products respectively. The newly calculated rate coefficients were tested in a styrene flame and were found to improve the concentration predictions of benzyl, phenol, and ortho-benzyne. Similar to the C6H5CCH2 + O2 reaction, the C6H5CCH2 + C2H2 is described with ‘simple’ high-pressure limit estimates in combustion models with naphthalene + H as the only products. The kinetic simulations performed in this work predict that at least two other product channels – methyleneindene + H and azulene + H are feasible. Furthermore, methyleneindene + H is predicted to be the dominant product and not naphthalene + H. Following the testing of the obtained rate data in a rich styrene flame, it was identified that reactions of methyleneindene are currently scarce in kinetic models, thereby inhibiting the adequate interplay of methyleneindene with other species in kinetic models. This makes it important to investigate other reactions of methyleneindene e.g., interconversion with other C10H8 species, molecular growth pathways, and oxidation reactions.