Catalytic incineration of contaminants in air with a spiral heat exchanger-combustor
A process has been studied both experimentally and theoretically for the complete oxidation of contaminants in air. Complete oxidation of 500 ppm propane in air as a prototype contaminant was accomplished by heating the contaminated air to 870 K in the presence of a palladium/rhodium catalyst. One important application of the process is cleaning of air in confined spaces. The heating and subsequent cooling are accomplished in a double-spiral exchanger of ten or more turns whose surfaces are coated with catalyst. A small input of energy is supplied at the core in these experiments by an electrical heater. The objective of the heat exchanger is to raise the temperature of the air to a level at which catalytic oxidation of the contaminants will occur while expending a minimal amount of energy on heat losses and on raising the temperature of the exiting, purified air. Owing to the spiral geometry and in particular to the consequent transfer of heat through both the inner and outer surfaces, double-spiral exchangers exhibit unique thermal and fluid-mechanical behavior. The objective of this work was to develop a prototype spiral and to characterize mathematically the associated thermal and fluid-mechanical behavior. Simplified thermal models were developed and solved analytically to show that a maximum in thermal efficiency exists for a given set of operating conditions. Flow visualization experiments revealed an intense secondary fluid motion (as pairs of counter-rotating vortices) in the plane normal to the direction of flow. An axisymmetric finite-element approximation was developed for a single pair of vortices associated with a channel of infinite aspect ratio. Flow results were also derived for channels of aspect ratio equal 5, 12, and 16. Local coefficients of heat transfer were derived from solutions of the energy equation for thermal boundary conditions of various ratios of heat flux for inner and outer walls of the channel. Numerical solutions derived for the resulting overall thermal behavior are in semi-quantitative agreement with measurements.
Chemical engineering|Mechanical engineering
Targett, Matthew John, "Catalytic incineration of contaminants in air with a spiral heat exchanger-combustor" (1992). Dissertations available from ProQuest. AAI9308669.