Because the separation obtainable in a mass diffusion stage is even smaller than in a gaseous diffusion stage, a practical degree of separation requires either a multistage cascade, such as the 48-stage cascade used by Hertz [H3] to separate neon isotopes, or a mass diffusion column.

The mass diffusion column, devised by Benedict [ВІЗ, B15] to provide a greater degree of separation than attainable from a single mass diffusion stage, is shown in Fig. 14.34. The main differences from a mass diffusion stage are as follows: (1) Separating agent is charged to the light stream at a uniform rate over the entire length of the column, instead of at one end of a stage. (2) Separating agent is condensed at a uniform rate over the entire length of the column instead of just from the streams leaving the column. To permit uniform charging and condensation of separating agent, this mass diffusion column contains four compartments instead of the two used in the stage type. These may take the form of cylindrical shells (Fig. 14.34) or parallel ducts.

In Fig. 14.34 the innermost chamber carries separating agent and distributes it at uniform rate over the length of the column; the second chamber carries the light stream; the third chamber, the heavy stream; and the fourth chamber, cooling water. Separating agent vapor flows radially through holes in the central tube and diffuses through the chambers carrying light and heavy streams to the cooling surface, where it is condensed.

The mass diffusion screen divides the second chamber, carrying the light stream, from the third chamber, carrying the heavy. As the light stream flows up, it is progressively enriched in the light isotope, which diffuses through the screen against the separating agent. As the heavy stream flows down, it is progressively enriched in the heavy isotope, which is carried through the screen with the separating agent.

By proper control of flow rates of separating agent and light — and heavy-stream feed rates, it is possible to make the molar velocity of light component inward just equal to the molar velocity of heavy component outward, a preferred condition for isotope separation in this equipment.