The stage type of mass diffusion was patented by Hertz [H4], who used this method to separate the isotopes of neon [H5, H6]. The means by which separation is effected in a mass diffusion stage are shown in Fig. 14.33, which illustrates the type of equipment used by Maier [Ml] to separate hydrogen from other gases.

The heart of this apparatus is the mass diffusion stage, in this case of cylindrical cross section, which is divided into two annular chambers by the mass diffusion screen. Feed gas is brought to the top of the inner compartment by a riser. As this stream flows down through the inner chamber, it gives up a portion of the feed, which diffuses through the screen into the outer chamber against the inward-diffusing separating agent. Because the light component of the feed diffuses at a higher speed than the heavy, the stream in the inner chamber is progressively depleted in the light component relative to the heavy.

Steam or other separating-agent vapor is admitted at the bottom of the outer chamber. As this stream flows upward it gives up separating agent to the heavy stream and picks up from it a portion of the feed, enriched in the light component.

After the light and heavy streams leave the diffusion stage, they are cooled to condense the separating agent. After separation of the condensate, they leave the apparatus as the light and heavy fractions.

For the mass diffusion screen, Maier used a variety of materials, such as plates perforated with 0.4-mm holes, fine-mesh wire screen, or alundum filter plates. Very fine holes, such as is needed in gaseous diffusion, are not required, although holes with diameter under 10 цтп are preferred because control of mass flow through the screen is easier. In the uranium isotope separation design example to be given in Sec. 7.4, electroformed nickel screen with holes 6.76 Mm in diameter and 30 percent free area was specified.

The main requirements of the separating agent are that it be selective, be readily separable from the components of the mixture to be separated, and be chemically inert to it. For isotopic mixtures in the form of a permanent gas, such as neon or methane, a readily condensible vapor such as steam or mercury has been used. For UF6 feed neither of these can be used because of chemical reactivity, and fluorocarbon vapor is specified. Selectivity is enhanced by using a separating agent of appreciably higher molecular weight than the components to be separated.

Table 14.21 gives examples of isotope separation reported for this type of apparatus.