Carbon molecular sieves for the concentration of oxygen from air

Abstract

The micropore structure of a number of American coals varying in rank from LVB to lignite has been modified by two methods: 1. (1) heat treatment in nitrogen to 800–900 °C, and 2. (2) air oxidation near the ignition temperature followed by heat treatment to 900–1000 °C in nitrogen. The second method of treatment is a necessity for coking coals. The chars from HVC coal using method (1) and from LVB coal using method (2) showed selective oxygen adsorption from air. Preferential oxygen uptake is attributed to an initial rate of oxygen adsorption that is higher than that for nitrogen owing to molecular sieve structure. Carbons obtained from coconut shells and some highly crosslinked synthetic polymers (for example, polydivinyl benzene) also showed selective oxygen adsorption. Adsorption of both gases on these carbons is primarily physical, that is, adsorption and desorption cycles can be performed by pressure swing with no significant loss of capacity. The slope of the oxygen isotherm is higher than that of the nitrogen isotherm, indicating that oxygen adsorption is enhanced over that of nitrogen at higher pressures. Operating at pressures around 6.3 MPa and 25 °C with air, oxygen concentration in the desorption product stream can be increased to about 36%. Using three beds of carbon in series, the final product gas can be concentrated to about 80% oxygen. The fraction, by volume, of the inlet gas which is realized as product gas depends upon a number of variables.