Serpil Awdry

The palladium catalysed oxidation of methane: reaction kinetics and the effect of diffusion barriers

Abstract The combustion of methane on a palladium catalyst was examined in a monolith reactor. The rate equation was determined and showed an approximately first order dependence in methane concentration and zero order dependence on oxygen concentration. Significant inhibition by water was observed, and inhibition by carbon dioxide was negligible. At high water concentrations the order with respect to water is approximately minus one. A significant reduction in both activity and activation energy was observed above temperatures of approximately 820 K with a dry feed. Significant diffusion limitation in the washcoat was observed. The intrinsic volumetric rate constant was found to be directly proportional to the palladium loading of the washcoat. The effect on the reaction rate of layers of inert washcoat placed on top of the active catalyst was investigated. These diffusion barriers reduced the reaction rate. The reactor performance was modelled using a two-dimensional finite element single channel model that included washcoat diffusion. The effect of diffusion barriers was compared to the effect of using a less active catalyst for steady state and transient modes of operation at values of the Lewis number. At low Lewis number the diffusion barrier was effective at reducing the temperature rise at the entrance to the reactor for large inlet reactant concentration.

An initial assessment for the use of seawater as a method to remove metabolically-produced carbon dioxide from a submersible atmosphere

Traditional CO2 absorption methods that are integrated into underwater life support systems use alkali metal hydroxide chemical beds—mostly calcium hydroxide—which have been shown to have poor absorption efficiencies at cold temperatures, and must be replaced at considerable trouble and expense on a frequent basis. With chemical utilizations as low as 20% in water temperatures of 2 °C, these hydroxides do not lend themselves to applications requiring extended durations due to the inability to carry sufficient quantities of these expendables. Experimental evidence suggests that seawater, a readily accessible medium during submersible operations, could be a highly efficient scrubbing medium to remove metabolically-produced carbon dioxide, particularly in cold water missions. This paper describes a feasibility assessment for using seawater to remove metabolically-produced carbon dioxide from a submersible atmosphere and proposes a multi-path scrubber design for use as an underwater life support system.

Hyperbaric testing of an alternative approach to remove carbon dioxide from underwater life support equipment

Traditional CO2 absorption methods for underwater life support equipment use alkali metal hydroxide chemical beds — mostly calcium hydroxide — that have been shown to have poor absorption efficiencies at cold temperatures, and must be replaced at considerable trouble and expense on a frequent basis. With chemical utilizations as low as 20% in water temperatures of 2°C, these hydroxides do not lend themselves to applications requiring extended durations in cold water due to the inability to carry sufficient quantities of expendables. A joint research effort between Duke University and the University of Bath has verified the feasibility in laboratory trials of an alternative carbon dioxide removal method that intimately mixes seawater with breathing circuit gases within a packed bed of Dixon rings. Based on the results of these laboratory trials, two multi-path scrubber prototypes were designed and fabricated for unmanned testing. In March 2013, the hyperbaric performance of these prototype scrubbers was characterized over a wide range of gas and water flow rates when operating the scrubbers in counter-current (water flowing in the opposite direction as gas flow) and co-current (water flowing in the same direction as gas flow) fashion. Significant findings from these tests included the following:• Both scrubber prototypes were found to be capable of delivering exit CO2 levels below 0.5 vol% (surface equivalent) at respiratory rates up to 22.5 liters per minute and at depths ranging from 0 to 40 meters of seawater (MSW).• Negligible collateral O2 absorption was observed at surface pressure (exit O2 levels were typically above 20.2 vol%), and exit O2 levels were typically above 18.4% during testing at 10 MSW.• At surface pressure, both prototypes had significantly lower breathing resistances than design goals established by the U. S. Navy.© 2014 ASME