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 ifs 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(F)a(nu) 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(f)a(nu), a mass transfer coefficient that represents the axial dispersion in the packed bed k(z)a(nu) and a mass transfer coefficient that represents the intraparticle diffusion through micro pores in the adsorbent particle beta k(s)a(nu). The value of beta k(s)a(nu) is confirmed to be 1 to 50 s(-1), which decreases with an increase of hydrogen partial pressure, and the rate-controlling step is beta k(s)a(nu) when the hydrogen partial pressure is higher than several hundred pascals, and k(z)a(nu) becomes the rate-controlling step when the hydrogen partial pressure is low and gas velocity is slow. The dependence of K(F)a(nu) on hydrogen isotopes and adsorbents appears to be small under the current experimental conditions.