Computational Fluid Dynamics Simulation Of Chemically Reacting Gas Flows Through Microfibrous Materials

Abstract

This dissertation presents results of research efforts to propose a design space for new microfibrous materials with enhanced chemical reactivity with optimum pressure gradient. These microfibrous materials material are a new class of patented materials, consisting of catalytic particles (50-300 micron in size) entrapped in a matrix of microfibers (2-10 micron in size), which show enhanced chemical reactivity compared to packed beds. Computational Fluid Dynamics (CFD) was used as a design tool to analyze the flow through microfibrous materials and to investigate the underlying mechanisms behind the enhancement in chemical reactivity. Numerical experiments were performed using different materials such as packed beds, frozen beds (particles frozen in space), and microfibrous materials with different geometric properties for two gas phase applications: (1) desulfurization, where a challenge gas of 2 vol.% H2S in H2 is used at a temperature of 400 0C and ZnO/SiO2 is used for desulfurization, and (2) removal of trace amounts of hexane (100 ppmv of C6H14) from air.

Pressure drops were predicted using CFD and are in good agreement with experiments, even with significant geometric approximations. The effects of residence time, dilution with void, clustering, fiber diameter, and fibers loading on chemical conversion are studied. Dilution with void and clustering showed a negative effect on chemical conversion compared with packed bed. Adding fibers enhanced the chemical reactivity by providing more uniform, plug flow like conditions, even in the presence of dilution. Microfibrous materials with more numerous smaller diameter fibers are required to have enhanced chemical conversion when pressure drop is not important with a maximum factor of increase in chemical reactivity for these cases (60-80%). Another design criteria is investigated by comparing the ratio of log reduction to pressure drop (using the same amount of catalyst) for different materials. This work suggests that new microfibrous materials with enhanced chemical reactivity for a given pressure drop should be designed with fewer, larger diameter fibers. Maxima increases in chemical reactivity per pressure drop of 8 and 6 were found for H2S and hexane, respectively, using 8 micron diameter fibers at 3% volume fraction with a total voidage of 80%.