| dc.description.abstract | Gas processing operations conducted at high velocities (e.g., filtration and catalytic reaction) require specialized media and structure in order to achieve the highest possible level of filtration and/or reaction per unit of pressure drop. A novel catalyst structure, microfibrous entrapped catalyst (MFEC), has potential to achieve high conversion with negligible pressure drop in cases requiring high mass flow rates, compared to other media, e.g., wash-coated monoliths and catalyst particulates in packed bed reactors. These composite structures provide distinctive physical properties in terms of void volume, surface area, porosity, permeability, conductivity, ease of pleating, uniform structure, etc. MFECs are prepared by entrapping small catalyst/support particles into the microfibrous materials by wet lay paper making process followed by sintering in hydrogen. These materials are in the form of thin flexible sheets. In this work, pleated structures (e.g., flat, V-shaped, and W-shaped structure) of MFECs, containing mixed metal oxide catalyst, were investigated systematically for VOC removal (e.g., ethanol, toluene, and n-hexane) at various face velocities and low temperatures. The optimized structure of MFEC resulted low intra-layer residence time with significantly low pressure drop. The study showed that low intra-layer residence times had significant effect on the catalytic reaction which followed Mars-Van Krevelen mechanism in this unique velocity controlled region. The overall rate of reaction for VOC decomposition was limited by the surface reaction regardless to the tested catalysts (Chapter III).
In this study, the performance of pleated MFECs was calculated in terms of heterogeneous contacting efficiency (ηHCE) which relies on the logarithmic removal of reactant concentrations per unit of the pressure drop (ηHCE = log(C/Co)/ΔP). The critical pressure drop denominator in the above noted expression was determined and compared theoretically, computationally and experimentally for both flat as well as pleated structures. For theoretical modeling, porous media permeability (PMP) equation, which is a modified Ergun equation, was altered by a factor for dealing with the dimensional heterogeneity created by the fibers and particles, Reynolds number and shape factor dependent form drag co-efficient, and a pleat factor for estimating decreased face velocities inside the media of the pleated geometry. This developed model was segregated into the inertial loss and the viscous loss, which together address the friction factor and the form drag factor. For computational modeling, simulations were performed by ANSYS FLUENT using a turbulence flow model. This study explicated solution methods, and discretization techniques for determining pressure drops and fluid flow characteristics across the pleated media. In Chapter IV, a detailed discussion on comparative pressure study has been presented.
Furthermore, head-to-head theoretical and experimental performance comparisons for flow through pleated MFEC structures were made with conventional packed beds of various particle sizes and wash-coated monolith of different CPSI. This study showed that while packed bed had resulted higher pressure drops and monolith had caused low fluid-solid mass transfer rates, pleated MFEC had shown significantly improved performance in VOC removal in terms of conversion along with a significant reduction in pressure drop. Small particles in MFEC enhanced the intra-particle and inter-phase mass transfer rates and the flexibility of pleating lowered the effective velocity inside the media that resulted lower pressure drop and higher conversion. To verify the theoretical comparison, experimental pressure drops and VOC conversions at various flow rates were measured using pleated MFEC and wash-coated monolith. Furthermore, a reaction kinetic model was developed for pleated MFEC considering the Peffer’s model to substantiate the experimental results in the velocity controlled region (Chapter V).
Moreover, catalyst characterization of mixed metal oxide was performed in this study using different techniques to investigate the effect of sintering on the support and to evaluate the critical parameters of the mixed metal oxides, i.e., active metal sites, metal dispersion, metal phase. In Chapter VI, all the techniques are discussed in details.
This study has demonstrated the potential advantages of MFEC as heterogeneous contacting systems for the use in high throughput applications as well as for applications requiring multi-log-removal capability. This will assist to design better and more practical catalytic reactor systems through the use of optimized pleated MFEC configurations. Furthermore, this may help to extend the knowledge of high single pass removal efficiency with minimum energy penalty across the contacting system. | en_US |
