Detailed modelling of catalytic chemistry in short contact time reactors

Abstract

The current thesis presents a detailed modelling study of the selective oxidation of ethane over noble metal coated surfaces in short contact time reactors. Computational studies were performed featuring heated gas streams flowing through ceramic-foam catalysts coated with platinum and followed by a long inert section. The detailed chemical kinetic mechanisms, with coupled surface and gas-phase chemical reactions, were explored via extensive reaction path and sensitivity analyses to assess the relative contributions of the homogeneous and heterogeneous chemistries and to establish the key heterogeneous pathways driving the chemical processes. A comprehensively validated detailed chemical mechanism was used for the gas phase. The mechanism initially featured 44 chemical species and 271 reversible reactions and was later extended to 176 reactants with 993 reversible reactions. Heterogeneous models describing the surface chemistry were derived on the basis of classical kinetic collision theory and with energy barriers obtained from Density Functional Theory studies combined with the Unity Bond Index-Quadratic Exponential Potential method. The derived surface mechanisms account for differences in site occupation and surface bonding types and include four reaction classes (direct adsorption, adsorption on an adsorbate, surface reactions with adsorbed reactants and uni-molecular surface reactions including desorption) via 35 adsorbed chemical species and 284 reversible reactions. The complete chemistry was thoroughly evaluated by comparison with multiple sets of existing and new experimental data provided by industrial partners. Key modelling parameters in the process, such as streams velocities, temperature profiles, catalyst loading and pressure were critically examined. The reaction dynamics were validated with C2H6/O2/H2 mixtures with different initial hydrogen contents and with oxygen to carbon weight ratios ranging between 0.25 and 0.9. The major chemical pathways for the production of ethylene through the selective dehydrogenation of ethane, combined with the heterogeneous oxidative were identified.