Adsorption of Gases on a Carbon Molecular Sieve Used for Air Separation:  Linear Adsorptives as Probes for Kinetic Selectivity

The adsorption of oxygen, nitrogen, and a series of gases/vapors with linear structures (carbon monoxide, carbon dioxide, nitrous oxide, acetylene and carbon disulfide) on a carbon molecular sieve typical of the materials used for air separation were studied over a range of temperatures as a function of pressure in order to understand further the mechanism of air separation. The ratios of the rate constants ( k O 2 / k N 2 ) were typically 25 for the carbon molecular sieve used thereby demonstrating the molecular sieving characteristics. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for most of the gases and experimental conditions studied. However there were deviations from the LDF model for carbon dioxide and nitrous oxide. In the case of the former, the kinetic models ranged from LDF through a combined barrier resistance/diffusion model to Fickian diffusion depending on the experimental conditions. The adsorption rate constants increase exponentially with increasing surface coverage for the molecules which follow the LDF model. The barrier resistance kinetic constants derived from the combined barrier resistance/diffusion model also have an exponential dependence on surface coverage. The results are discussed in terms of the mechanism of gas separation using carbon molecular sieves.