Ozone Removal at Microsecond Contact Time using Microfibrous Entrapped Catalysts

Abstract

Microfibrous Entrapped Catalysts (MFEC) manufactured from 8μm diameter nickel fibers were engineered into pleated heterogeneous catalytic reactors to improve catalytic performance from conventional packed bed and monolith reactors at high volumetric flows, since microfibrous materials can entrap small particles (150-200μm) which significantly improves inter-phase mass transfer. Conventional reactors usually operate with contact time less than 1s. MFEC reactors were able to reduce the contact time to micro seconds while maintaining similar catalytic performance. This gives MFEC a huge advantage in terms of weight and volume saving, since the conventional meter-long reactors were shortened to millimeter-thick material sheets. These unique reactors were targeted at ozone, which was widely recognized as the No.1 aircraft cabin air pollutant.

In this research, MFEC reactors were investigated under turbine bleed air conditions of high temperature (100-200 ºC) and high face velocity (10-40 m/s) resulting in an interlayer contact time of 67-200μsec. Precious metal (Pd, Ag) and transition metal (Mn) catalysts were impregnated on entrapped particles (e.g. γ-Al2O3) using incipient wetness method. Ozone test concentration was set at a high-demanding 1.5 ppmv. Results showed that a high level of ozone decomposition was achieved with a significant reduction of catalyst consumption. Compared with conventional aircraft filters, this reduction can be a huge advantage in terms of material cost and labor. Reaction kinetics analyses were compared for different catalysts. Results showed that precious metal catalyst performs better at a higher temperature while transition metal catalyst maintains similar conversion when changing temperature. Tests such as XPS, TPR and TGA have been used to evaluate performance of catalysts and modify catalysts to improve conversion rate. Also catalyst aging tests were conducted according to the frequency of commercial aircraft ozone reactor replacement, which evaluated long-term performance for actual MFEC reactor usage. In various applications, high volumetric flow application of MFEC encounters a high pressure environment; catalytic performances of MFEC at these conditions are expected to be different from their low pressure counterpart, since key physical characteristics, including density, effective diffusivity and reaction rate changed at these conditions. A small scale experiment setup is constructed to further evaluate the performance of MFEC at higher pressures. Various heat and mass transfer parameters have been compared by a model constructed by Kalluri et al.

In addition, a comprehensive CFD pressure drop has been established based on the physical characteristics of the fiber material and entrapped particles acquired by SEM imaging. A numerical solution of the Navier-Stokes equation within the fiber material has been done to prove that pressure drop of the whole material sheet can be modeled using a small disassembled part of the whole area. A large number of random areas have been sketched and tested for position dependence of the pressure drop. Simulation results showed good estimation of the total pressure drop of flat MFEC sheets. The accuracy of model has been improved by considering the compression effect caused by the large pressure drop across the material sheet. Pleated structure pressure drop has also been conducted using the micro scale data, which showed very accurate estimation of pressure drop across MFEC.

Heat transfer phenomenon of MFEC at high volumetric flows has also been studied in this research. Ensemble average method is used to evaluate radial and axial heat transfer coefficient at forced convection conditions. Temperature distribution within the MFEC is acquired by numerical solution of heat transfer equation at high volumetric conditions with different boundary conditions. Temperature distribution is also acquired experimentally by using an Omega® multi-point thermocouple. Two temperature profiles are compared for model accuracy. Nusselt number has been used to evaluate the degree of convection at different velocities. Moreover, heat transfer at higher pressure condition is also conducted as an evaluation of applying MFEC at high volumetric flow and high pressure environment.