Mass Transfer Coefficients in Cryosorption of Hydrogen Isotopes on Molecular Sieves or Activated Carbon at 77.4 K

The tritium bred in a deuterium-tritium fusion reactor is removed from its blanket by using helium sweep gas. From the viewpoint of adsorption capacity and pressure of tritium at release, a cryosorption bed, which uses molecular sieves or activated carbon at the temperature of liquid nitrogen, is attractive for the recovery of this tritium. The mass transfer coefficients required to predict the breakthrough curve are experimentally discussed. The overall mass transfer coefficient K_Fa_v in the cryosorption of hydrogen isotopes on molecular sieves or activated carbon at 77 K consists of a mass transfer coefficient that represents the transfer from the bulk gas flow to the surface of the adsorbent through the boundary layer k_fa_v, a mass transfer coefficient that represents the axial dispersion in the packed bed k_za_v and a mass transfer coefficient that represents the intraparticle diffusion through micro pores in the adsorbent particle βk_sa_v. The value of βk_sa_v is confirmed to be 1 to 50 s⁻¹, which decreases with an increase of hydrogen partial pressure, and the rate-controlling step is βk_sa_v when the hydrogen partial pressure is higher than several hundred pascals, and k_za_v becomes the rate-controlling step when the hydrogen partial pressure is low and gas velocity is slow. The dependence of K_Fa_v on hydrogen isotopes and adsorbents appears to be small under the current experimental conditions.