Carbon dioxide

Exposure to high CO2 levels in enclosed environments may result in adverse health impacts. To provide a safe breathing environment, the exhaled gases must be removed. Currently, NASA uses a multi-bed system known as the Carbon Dioxide Removal Assembly (CDRA) for CO2 removal. The process involves cyclic adsorption-desorption using zeolite-5A molecular sieves. Owing to the presence of a wet gaseous mixture and the hydrophilic nature of zeolite-5A, the removal of CO2 and water vapor must be conducted in two separate vessels, resulting in additional costs. Therefore, the objective of this study was to integrate and intensify the process utilizing amine-grafted silica. Adsorbent performance was gauged on equilibrium CO2 uptake and kinetics, activation temperature, CO2 desorption temperature, and consecutive cycling in the presence of 1 vol.% CO2 in N2 at 25 °C. Aminosilica outperformed 5A and achieved similar equilibrium CO2 uptake while exhibiting faster kinetics, and lower desorption and regeneration temperature requirements.

This project was designed to investigate the effects of carbon dioxide (CO2) levels on the
growth and pigment ratios (chemotaxonomy) of freshwater algal species typical to the
south Florida surface waters. Green algae, diatoms, and cyanobacteria were cultured
under 400 or 800 ppm CO3 in air for several weeks. Growth monitoring used a cell
counter, hemocytometer, and chlorophyll fluorescence. Pigments were analyzed using
HPLC-PDA. Experiments with certified CO2 concentrations (400, 600, 800, 1200 ppm)
were conducted with helium degassed ultrapure water and each of three culture media.
Theoretical and experimental pH values with water matched exactly. However, each
culture media proved to exhibit significant buffer capacity. Cell growth monitoring was
problematic except for the cyanobacterium Microcystis aeruginosa. That species
responded to increased CO2 (800 ppm) with increased growth rates as predicted. The
other species gave erratic results mainly due to difficulties in obtaining valid consistent
cell counts.

This research explores carbon dioxide transport in life support helmet annular space using new theoretical and experimental techniques. Increased transport from next generation helmets is necessary to allow reduction of fresh gas flow and associated noise. Conventional helmet noise interferes with communications and some underwater helmets even approach hearing threshold shift levels. Helmet flow is three dimensional, unsteady, and turbulent; this research is the first known effort to identify the fundamental mechanisms of CO2 transport. An analytical model is developed which predicts average inhaled CO2 concentration for generic helmet geometry using a mixing volume approach. The model includes sensitivity to supply flow, breath rate, metabolic CO2 production, inhalation and exhalation mixing volumes, and breathing symmetry. Numerical sensitivity analysis using the model indicates optimum design paths. Nominal head-helmet-lung geometry is identified. An experimental nominal model was developed which supports inhaled concentration measurements with air-CO2 or water-dye as working fluids. Water modeling provides flow visualization which is used to identify complex convective and turbulent CO2 transport mechanisms. Correlation of water-dye and air-CO2 results indicates conditions when molecular diffusion of CO2 is significant. The research was directed primarily toward diving helmets but is applicable to spacesuit and firefighter helmets, as well as any situation involving mass transport in a periodic mixing chamber. New analytical and experimental models are substantially more accurate than the conventional steady state helmet mixing model, and provide direction for improved helmet design.