The determination of diffusion coefficients in activated carbons made from bituminous coal

The gas chromatographic method has been used to measure the effective diffusion coefficients D_e or the diffusion parameters $\text{D}{}_{\mathrm{e}}\text{d}{}^2$. Nitrogen was used as a carrier gas, krypton-85 as test gas. The tests were carried out on different activated carbons made from air-preoxidized bituminous coals whose pore structure as a function of the burn-off was known. The diffusion parameters for krypton were found to increase exponentially with the mean micropore diameter of the activated carbons which was varied in our tests between 5.8 and 7.0 Å. For activated carbon with the narrowest pores the activation energy of krypton diffusion was found to be 2.8 kcal/mol. In case of such a product the diffusion time is not rigidly dependent on the particle diameter as would have to be expected theoretically. With increasing micropore diameter this dependency is attenuated very rapidly, and in carbons with a mean micropore diameter above ca. 6.8 Å it exists no longer at all. To check the results, gravimetric studies on the adsorption kinetics of krypton in vacua and on the desorption kinetics using the gas flushing method were made. It was found that the results obtained by the gas chromatographic method and the gas flushing method were in good agreement, whereas the diffusion parameters obtained by measurements in vacuo were by one order of magnitude lower. The findings are discussed in relation to the pore structure of the adsorbent and the properties of the sorbate. The limits of the different measuring methods are shown.