Equilibrium molecular dynamics simulations were reported of oxygen and nitrogen molecules confined in graphite slit pores. Self- and collective diffusion coefficients were calculated as a function of pore width, temperature and density for each pure component in the pore space. The aim of this study was to elucidate the mechanism by which oxygen and nitrogen were kinetically separated when air was passed over an adsorbent bed consisting of molecular sieving carbon in the commercial production of oxygen. It was found that a critical pore width existed for each species at which there was a sharp drop in the rate of diffusion (both self- and collective diffusion) of each fluid. The critical pore width was one for which the individual molecules were prevented from rotating freely about one of their axes. The greater length of a nitrogen molecule means that the critical pore width was higher for this species than for oxygen. Consequently, oxygen molecules diffuse substantially faster than nitrogen molecules in the vicinity of the nitrogen critical pore width. From an analysis of correlation functions and their corresponding power spectra it was shown that the restricted rotations, which occurred at or below the critical pore width, cause a decoupling of translational and rotational modes, with the net result being a lowering of translational diffusion. The nitrogen critical pore width lay within the range of the mean pore size of most commercial molecular sieving carbons, and so this mechanism may help to explain the high oxygen selectivities reported in the literature.

Computer Simulation Investigation of Diffusion Selectivity in Graphite Slit Pores. Travis, K.P.: Molecular Physics, 2002, 100[14], 2317-29