THE UNSTEADY STATE DIFFUSION OF GASES FROM COALS

Special Research Report SR-42 The micropore structure of coals was investigated by measuring unsteady-state diffusion of different gases out of the coals. It was found that if diffusion from coals was measured at or above room temperature with nitrogen or argon, the diffusion coefficient was independent of pressure. Diffusion of gases of molecular dimension equal to or higher than argon from coals was found to be activated. Argon diffusion from 11 samples of ( 200x325 mesh) coal of volatile matter ranging from 4. 5 to 45. 5 per cent was studied. The activation energy of argon diffusion showed a maximum in the region of good coking coals indicating that the critical dimension of the micropores was narrowest in the coking coal range. Diffusion of nitrogen and carbon dioxide from coals was measured in the same temperature range. The activation energy values of carbon dioxide diffusion were found to be lower in all cases than those of nitrogen. From this it can be inferred that true micropore surface areas of coals can be obtained from carbon dioxide adsorption at -78° C. Diffusion of gases of different molecular dimensions was measured from Linde 3A molecular sieve as well as from an anthracite. In both cases it was found that the activation energy of diffusion increased with an increase of the molecular dimension of the diffusing gas. From these results it is shown that anthracite possesses some molecular sieve properties. Diffusion of argon from heat-treated anthracites showed that the critical size of the micropores was reduced and selec- tivity increased upon heat - treatment.